Early Inhibitory Postsynaptic Potentials Research Articles

GABAB receptor- and metabotropic glutamate receptor-dependent cooperative long-term potentiation of rat hippocampal GABAA synaptic transmission.

Repetitive stimulation of Schaffer collaterals induces activity-dependent changes in the strength of polysynaptic inhibitory postsynaptic potentials (IPSPs) in hippocampal CA1 pyramidal neurons that are dependent on stimulation parameters. In the present study, we investigated the effects of two stimulation patterns, theta-burst stimulation (TBS) and 100 Hz tetani, on pharmacologically isolated monosynaptic GABAergic responses in adult CA1 pyramidal cells. Tetanization with 100 Hz trains transiently depressed both early and late IPSPs, whereas TBS induced long-term potentiation (LTP) of early IPSPs that lasted at least 30 min. Mechanisms mediating this TBS-induced potentiation were examined using whole-cell recordings. The paired-pulse ratio of monosynaptic inhibitory postsynaptic currents (IPSCs) was not affected during LTP, suggesting that presynaptic changes in GABA release are not involved in the potentiation. Bath application of the GABAB receptor antagonist CGP55845 or the group I/II metabotropic glutamate receptor antagonist E4-CPG inhibited IPSC potentiation. Preventing postsynaptic G-protein activation or Ca2+ rise by postsynaptic injection of GDP-beta-S or BAPTA, respectively, abolished LTP, indicating a G-protein- and Ca2+-dependent induction in this LTP. Finally during paired-recordings, activation of individual interneurons by intracellular TBS elicited solely short-term increases in average unitary IPSCs in pyramidal cells. These results indicate that a stimulation paradigm mimicking the endogenous theta rhythm activates cooperative postsynaptic mechanisms dependent on GABABR, mGluR, G-proteins and intracellular Ca2+, which lead to a sustained potentiation of GABAA synaptic transmission in pyramidal cells. GABAergic synapses may therefore contribute to functional synaptic plasticity in adult hippocampus.

Medullary reticulospinal tract mediating the generalized motor inhibition in cats: Parallel inhibitory mechanisms acting on motoneurons and on interneuronal transmission in reflex pathways

The present study was designed to elucidate the spinal interneuronal mechanisms of motor inhibition evoked by stimulating the medullary reticular formation. Two questions were addressed. First, whether there is a parallel motor inhibition to motoneurons and to interneurons in reflex pathways. Second, whether the inhibition is mediated by interneurons interposed in known reflex pathways. We recorded the intracellular activity of hindlimb motoneurons in decerebrate cats and examined the effects of medullary stimulation on these neurons and on interneuronal transmission in reflex pathways to them. Stimuli (three pulses at 10–60 μA and 1–10 ms intervals) delivered to the nucleus reticularis gigantocellularis evoked inhibitory postsynaptic potentials in α-motoneurons ( n=147) and γ-motoneurons ( n=5) with both early and late latencies. The early inhibitory postsynaptic potentials were observed in 66.4% of the motoneurons and had a latency of 4.0–5.5 ms with a segmental delay of more than 1.4 ms. The late inhibitory postsynaptic potentials were observed in 98.0% of the motoneurons and had a latency of 30–35 ms, with a peak latency of 50–60 ms. Both types of inhibitory postsynaptic potentials were evoked through fibers descending in the ventrolateral quadrant. The inhibitory postsynaptic potentials were not influenced by recurrent inhibitory pathways, but both types were greatly attenuated by volleys in flexor reflex afferents. Conditioning medullary stimulation, which was subthreshold to evoke inhibitory postsynaptic potentials in the motoneurons, neither evoked primary afferent depolarization of dorsal roots nor reduced the input resistance of the motoneurons. However, the conditioning stimulation often facilitated non-reciprocal group I inhibitory pathways (Ib inhibitory pathways) to the motoneurons in early (<20 ms) and late (30–80 ms) periods. In contrast, it attenuated test postsynaptic potentials evoked through reciprocal Ia inhibitory pathways, and excitatory and inhibitory pathways from flexor reflex afferent and recurrent inhibitory pathways. The inhibitory effects were observed in both early and late periods. The present results provide new information about a parallel inhibitory process from the medullary reticular formation that produces a generalized motor inhibition by acting on α- and γ-motoneurons, and on interneurons in reflex pathways. Interneurons receiving inhibition from flexor reflex afferents and a group of Ib interneurons may mediate the inhibitory effects upon motoneurons.

Role of GABA(A)-mediated inhibition in controlling the responses of regular spiking cells in turtle visual cortex.

The visual cortex of freshwater turtles contains pyramidal cells, which have a regular spiking (RS) firing pattern, and several categories of aspiny, inhibitory interneurons. The interneurons show diverse firing patterns, including the fast spiking (FS) pattern. Postsynaptic potentials (PSPs) evoked in FS cells by visual stimulation of the retina reach their peak amplitudes as much as 200 ms before PSPs in RS cells (Mancilla et al., 1998). FS cells could, consequently, control the amplitudes of light-evoked PSPs in RS cells by producing disynaptic, feedforward inhibitory postsynaptic potentials (IPSPs) that overlap in time with geniculocortical excitatory postsynaptic potentials (EPSPs). Since FS cells receive recurrent, excitatory inputs from RS cells, they could also control the amplitudes of light-evoked PSPs in RS cells via polysynaptic, feedback inhibition. The in vitro geniculocortical preparation of Pseudemys scripta was used to characterize the temporal relationships of EPSPs and IPSPs produced in RS cells by electrical activation of geniculate afferents and by diffuse light flashes presented to the retina. GABA(A) receptor-mediated inhibition was blocked using extracellular application of bicuculline (3.5 mM) or intracellular perfusion of picrotoxin (1 microM) in individual RS cells. Electrical stimulation of thalamic afferents produced compound PSPs. Blockade of GABA(A) receptor-mediated IPSPs with either bicuculline or picrotoxin provided evidence for both early and late IPSPs in RS cells. Analysis of the apparent reversal potentials of light-evoked PSPs indicated the existence of early IPSPs during the first 140-300 ms following light onset. Light responses of cells perfused with picrotoxin diverged from control light responses at about 300 ms after light onset and had maximum amplitudes that were significantly different from control light responses. These experiments indicate that the responses of RS cells to both electrical and natural stimulation of geniculate afferents are controlled by both early and late IPSPs, consistent with activation of both feedforward and feedback pathways.

GABA(B)-receptor-mediated inhibition in developing mouse ventral posterior thalamic nucleus.

Inhibitory postsynaptic potentials (IPSPs) generated by activation of the thalamic reticular nucleus (RTN) were recorded in neurons of the ventral posterior nucleus (VP) in vitro in slices from mice aged postnatal day (P)1-P17. An early IPSP peaking 41 +/- 2.5 (SE) ms after electrical stimulation of the internal capsule or RTN was found in 96% of VP neurons. This early IPSP was blocked by bicuculline, showing its dependence on gamma-aminobutyric acid-A (GABA(A)) receptors. A late IPSP peaking 357 +/- 27 ms after the stimulus was observed in 22% of VP neurons in control medium but was uncovered in 38% of neurons when bicuculline was added. The late IPSP was blocked by addition of a GABA(B) antagonist, 2-hydroxysaclofen, to the medium (n = 7); it had a reversal potential of -98 +/- 1.3 mV, 14 mV negative to the early component. In contrast to the early IPSP, whose reversal potential became more negative during postnatal development, the reversal potential of the late IPSP remained constant throughout the postnatal period studied. The most significant change in the late IPSP was shortening in duration, with reduction in latency-to-peak by >400 ms, between P1 and P10. No changes of comparable magnitude were observed in the duration of the earlier GABA(A) response. These results show that both GABA(A) and GABA(B) IPSPs are present very early in the postnatal thalamus and that their characteristics evolve along independent paths during postnatal development.

Physiology and pharmacology of corticothalamic stimulation-evoked responses in rat somatosensory thalamic neurons in vitro.

Whole cell current- and voltage-clamp recording techniques were employed in a rat thalamocortical slice preparation to characterize corticothalamic stimulation-evoked responses in thalamic neurons. Three types of corticothalamic stimulation-evoked responses were observed in thalamic neurons. Of thalamic neurons, 57% responded to corticothalamic stimulation with purely excitatory synaptic responses, whereas 27% had inhibitory synaptic responses and 16% had mixed excitatory/inhibitory responses. This suggested corticothalamic activation of multiple distinct synaptic circuits, presumably involving both nucleus reticularis thalami (NRT) and thalamus, because the rat ventrobasal complex is virtually devoid of GABAergic interneurons. Corticothalamic-stimulation-evoked excitatory postsynaptic currents (EPSCs) were predominantly slow rising currents that showed nonlinear voltage dependence, characteristics of an N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic current. These slow rising EPSCs were blocked by the NMDA antagonist 2-amino-5-phosphonovaleric acid (APV). A minority of corticothalamic EPSCs had faster kinetics, and were blocked by 6-cyano-7 nitroquinoxaline-2,3-dione (CNQX). Corticothalamic stimulation of varying frequency optimally activated burst responses in thalamic neurons at low frequencies (3-6 Hz). The optimal 3- to 6-Hz response was reduced by ethosuximide, by APV, and by detaching the neocortex from the thalamocortical slice, suggesting that T current, NMDA receptors, and neocortical properties all contributed to generation of this 3- to 6-Hz frequency preference. In contrast to corticothalamic EPSCs, medial-thalamic-stimulation-evoked responses consisted of fast CNQX-sensitive EPSCs that were predominantly voltage insensitive, with no 3- to 6-Hz frequency preference. In thalamic neurons in which corticothalamic stimulation evoked predominantly inhibitory synaptic responses, this inhibitory postsynaptic potential (IPSP) had early and late phases, often followed by a rebound burst. The early IPSP reversed at -95 mV and was bicuculline sensitive, whereas the late IPSP reversed at -113 mV and was blocked by the gamma-aminobutyric acid-B (GABA(B)) antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzy lphoshinic acid (CGP-55845A). In thalamic neurons in which corticothalamic stimulation evoked a mixed excitatory postsynaptic potential (EPSP)/IPSP response, repetitive corticothalamic stimulation rapidly reduced IPSPs and enhanced EPSPs at higher frequencies. This resulted in burst firing being triggered in these mixed response neurons at frequencies >6 Hz. Corticothalamic feedback onto thalamic relay neurons activated diverse responses due to differing relative activation of NRT and "feedforward" inhibitory responses. These multiple in vitro corticothalamic responses differ from responses encountered in other in vitro thalamic preparations lacking a synaptically connected neocortex, but are similar to results evident in thalamic neurons in response to cortical stimulation in vivo. In addition, the thalamocortical 3- to 6-Hz frequency preference was conserved, suggesting that many factors critical for this emergent property of the thalamocortical system are maintained in vitro.

Effects of interstimulus interval on somatosensory evoked magnetic fields (SEFs): a hypothesis concerning SEF generation at the primary sensorimotor cortex

Cerebral responses evoked by peripheral stimuli are known to depend critically on the interstimulus interval (ISI). Here we report on the effects of ISI on somatosensory evoked magnetic fields (SEFs) to right median nerve stimulation, obtained in 9 healthy adults with ISIs of 0.15, 0.3, 1, 3 and 5 s. At the contralateral (left) primary sensorimotor cortex (SMI), the first cortical response, N20m, was stable between the ISIs 0.3 and 5 s, but slightly attenuated at the shortest ISI of 0.15 s. In contrast, the P35m and P60m deflections were very sensitive to changes of the ISI, declining steadily with shortening of the ISI throughout the entire range. These deflections were frequently undetectable at the shortest ISI of 0.15 s. Concomitant with the reductions of P35m and P60m, an N45m deflection was enhanced toward the short ISIs. Responses from second somatosensory cortex (SII) and posterior parietal cortex (PPC) were seen only with ISIs of 1 s or greater, being strongest at the 5 s ISI. Based on known effects of the ISI on intracellular evoked potentials, we present the following tentative model for the generation mechanism of the SMI response: N20m represents early excitatory postsynaptic potentials (EPSPs), P35m early inhibitory postsynaptic potentials (IPSPs), N45m secondary EPSPs and P60m late IPSPs in pyramidal neurones of area 3b. For practical purposes, SEFs from SMI can be obtained with short ISIs, while responses from SII and PPC require an ISI of at least 1 s.

Effects of interstimulus interval on somatosensory evoked magnetic fields (SEFs): a hypothesis concerning SEF generation at the primary sensorimotor cortex.

Cerebral responses evoked by peripheral stimuli are known to depend critically on the interstimulus interval (ISI). Here we report on the effects of ISI on somatosensory evoked magnetic fields (SEFs) to right median nerve stimulation, obtained in 9 healthy adults with ISIs of 0.15 0.3, 1,3 and 5 s. At the contralateral (left) primary sensorimotor cortex (SMI), the first cortical response, N20m, was stable between the ISIs 0.3 and 5 s, but slightly attenuated at the shortest ISI of 0.15 s. In contrast, the P35m and P60m deflections were very sensitive to changes of the ISI, declining steadily with shortening of the ISI throughout the entire range. These deflections were frequently undetectable at the shortest ISI of 0.15 s. Concomitant with the reductions of P35m and P60m, an N45m deflection was enhanced toward the short ISIs. Responses from second somatosensory cortex (SII) and posterior parietal cortex (PPC) were seen only with ISIs of 1 s or greater, being strongest at the 5 s ISI. Based on known effects of the ISI on intracellular evoked potentials, we present the following tentative model for the generation mechanism of the SMI response: N20m represents early excitatory postsynaptic potentials (EPSPs), P35m early inhibitory postsynaptic potentials (IPSPs), N45m secondary EPSPs and P60m late IPSPs in pyramidal neurones of area 3b. For practical purposes, SEFs from SMI can be obtained with short ISIs, while responses from SII and PPC require an ISI of at least 1 s.

Changes in inhibitory neurotransmission in the CA1 region and dentate gyrus in a chronic model of temporal lobe epilepsy.

1. In this report we compare changes in inhibitory neurotransmission within the CA1 region and the dentate gyrus (DG) in a model of chronic temporal lobe epilepsy (TLE). Extracellular and intracellular recordings were obtained in combined hippocampal-parahippocampal slices > or = 1 mo after a period of self-sustaining limbic status epilepticus (SSLSE) induced by continuous hippocampal stimulation. 2. Polysynaptic inhibitory postsynaptic potentials (IPSPs) were induced by positioning electrodes to activate specific afferent pathways and evoking responses in the absence of glutamate receptor antagonists [D(-)-2-amino-5-phosphonovaleric acid (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)]. Polysynaptic IPSPs were evoked in CA1 pyramidal cells from electrodes positioned in stratum radiatum and in stratum lacunosum/moleculare. Polysynaptic IPSPs were evoked in DG granule cells from electrodes positioned over the perforant path located in the subiculum. Monosynaptic IPSPs were induced by positioning electrodes within 200 microns of the intracellular recording electrode (near site stimulation) and stimulating in the presence of APV and CNQX to block ionotropic glutamate receptors. Monosynaptic IPSPs were evoked in CA1 pyramidal cells with electrodes positioned in the stratum lacunosum/moleculare and stratum pyramidale. Monosynaptic IPSPs were evoked in DG granule cells with electrodes positioned in the stratum moleculare. 3. Population spike (PS) amplitudes were employed to assure that a full range of stimulus strengths, from subthreshold for action potentials to an intensity giving maximal-amplitude PSs, was used to elicit polysynaptic IPSPs in CA1 pyramidal cells in both post-SSLSE and control slices. In control tissue, polysynaptic IPSPs were biphasic, composed of early and late events. In post-SSLSE tissue, polysynaptic IPSPs were markedly diminished. The diminution of polysynaptic IPSPs was detected at all levels of stimulus intensity. Both early IPSPs [mediated by gamma-aminobutyric acid-A (GABAA) receptors] and late IPSPs (mediated by GABAB receptors) were diminished. Polysynaptic IPSPs were diminished with both stratum radiatum and with stratum lacunosum/moleculare stimulation. 4. Reversal potentials for either polysynaptic early or polysynaptic late IPSPs evoked in CA1 pyramidal cells by stratum radiatum stimulation were not different in slices from post-SSLSE animals as compared with control animals. Likewise, reversal potentials for either polysynaptic early or polysynaptic late IPSPs evoked by stratum lacunosum/moleculare stimulation did not differ in the two groups. These findings excluded changes in driving force as an explanation for the diminished amplitude of IPSPs in CA1 pyramidal cells in the post-SSLSE model.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular correlates of neuronal hyperexcitability in the vicinity of photochemically induced cortical infarcts in rats in vitro

Intrinsic membrane properties and synaptic responses of neocortical neurons located lateral to photochemically induced ischemic lesions were investigated using neocortical slice preparation. In comparison to neurons from control slices, these neurons had a significantly less negative resting membrane potential without any significant change in input resistance. In addition, γ-aminobutyric acid (GABA) mediated synaptic inhibition was found to be less efficient; the conductances of both the early and late inhibitory postsynaptic potentials (IPSPs) were significantly smaller, and the reversal potential of the early IPSP was shifted to a more positive value. In some of the neurons, ‘epileptiform’ postsynaptic potentials could be elicited, which were abolished after wash-in of the N-methyl- d-aspartic acid (NMDA)-receptor antagonist o-2-amino- 5-phosphonovaleric acid (AP-5). The results provide a possible explanation for the hyperexcitability found in the vicinity of cortical infarcts.

Relative contributions of thalamic reticular nucleus neurons and intrinsic interneurons to inhibition of thalamic neurons projecting to the motor cortex.

1. Intracellular responses to stimulation of the cerebral cortex (Cx) and cerebellum were analyzed in thalamocortical neurons (TCNs) in the ventroanterior-ventrolateral (VA-VL) complex of the thalamus and neurons in the thalamic reticular nuclei (RNs) of anesthetized cats, and the contribution of reticular nucleus neurons (RNNs) and thalamic interneurons (TINs) to cerebral and cerebellar inhibition of TCNs was determined. 2. Single TCNs projecting to area 4 or 6 received convergent monosynaptic excitatory and disynaptic inhibitory inputs from both the dentate nucleus (DN) and the interpositus nucleus (IN). These TCNs also received monosynaptic excitatory postsynaptic potentials (EPSPs) and disynaptic inhibitory postsynaptic potentials (IPSPs) from the pericruciate cortex (areas 4 and 6). Each TCN received the strongest excitatory and inhibitory inputs from the cortical area to which that TCN projected, and weaker inhibitory inputs from adjacent cortical areas. 3. RNNs were identified morphologically by intracellular injection of horseradish peroxidase (HRP). Stimulation of the brachium conjunctivum (BC) evoked disynaptic EPSPs with a long decay phase in RNNs in the anterior ventrolateral part of the RN. Single RNNs received convergent disynaptic excitatory inputs from both the DNA and the IN. Stimulation of the Cx produced monosynaptic long-lasting EPSPs with two different latencies in these RNNs: early EPSPs with latencies of 0.9-2.1 ms and late EPSPs with latencies of 1.8-3.5 ms. Collision experiments with BC- and Cx-evoked EPSPs in RNNs indicated that BC-evoked disynaptic EPSPs and Cx-evoked early EPSPs were produced by axon collaterals of TCNs to RNNs. The latencies of the Cx-evoked late EPSPs in RNNs were almost identical to those of Cx-evoked monosynaptic EPSPs in TCNs, indicating that corticothalamic neurons (CTNs) exert monosynaptic excitatory effects on RNNs and TCNs. 4. Stimulation of the Cx produced IPSPs in TCNs with short latencies of 1.8-2.7 ms and longer latencies of > or = 2.8 ms. The Cx-evoked early IPSPs with latencies of 1.8-2.7 ms were mediated by RNNs. The origin of Cx-evoked late IPSPs with latencies of > or = 2.8 ms in TCNs was twofold, Cx-induced early IPSPs in TCNs were facilitated by conditioning cortical stimulation that induced late IPSPs in the TCNs. The same conditioning cortical stimulation also facilitated BC-evoked disynaptic IPSPs. The time course of this facilitatation indicated that CTNs produce long-lasting excitation in TINs. These results indicated that Cx-evoked IPSPs with latencies of > 2.7 ms were mediated at least in part by RNNs and inhibitory TINs in the VA-VL complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction of GABA-mediated inhibitory postsynaptic potentials in hippocampal CA1 pyramidal neurons following oral flurazepam administration.

Oral administration of the benzodiazepine, flurazepam, for one week results in tolerance in vivo and in vitro and in a reduction in recurrent and feedforward inhibition in vitro in the CA1 pyramidal cell region of hippocampus. In the present study CA1 pyramidal cells were examined intracellularly in vitro in rat hippocampal slices (500 microns) from rats sacrificed two or seven days after cessation of oral flurazepam treatment. Following drug treatment, the membrane characteristics of CA1 pyramidal cells were not significantly different from control neurons. GABAA-mediated, early inhibitory postsynaptic potentials were significantly reduced in amplitude (60%) in pyramidal neurons from rats killed two days, but not in those killed seven days, after the end of drug administration. The decrease in early inhibitory postsynaptic potential amplitude was observed using just-subthreshold, threshold and supramaximal orthodromic stimulation as well as following antidromic activation. The magnitude of the decrease in the early inhibitory postsynaptic potential amplitude was similar in the presence of the GABAB antagonist, CGP 35348, and could not be attributed to differences in the strength of afferent stimulation between flurazepam-treated and control groups. The size of the GABAB-mediated, late inhibitory postsynaptic potentials was also significantly decreased (45%) in comparison to control cells. Reversal potentials for both the early (-72 mV) and late (-92 mV) hyperpolarizations were not significantly different between groups. Following high intensity orthodromic stimulation, in the presence of an intracellular sodium channel blocker (QX-314) which also blocks the GABAB-mediated late hyperpolarization, a bicuculline-sensitive late depolarizing potential was unmasked in neurons from FZP-treated rats, but never from control cells. Excitatory postsynaptic potential amplitude was significantly increased in flurazepam-treated neurons and the threshold for the synaptically-evoked action potential was significantly increased. Following depolarizing current injection, the duration and frequency of pyramidal cell discharges and the action potential threshold were not altered by oral flurazepam treatment. The amplitude of the fast afterhyperpolarization was also not changed. Overall, the findings indicate an impairment of transmission at GABAergic synapses onto hippocampal CA1 pyramidal cell neurons after chronic benzodiazepine treatment at a time when rats are tolerant to the anticonvulsant effects of the benzodiazepines in vivo.

The Neuroactive Steroid 5α‐Tetrahydrodeoxycorticosterone Increases GABAergic Postsynaptic Inhibition in Rat Neocortical Neurons in vitro

The neuroactive steroid 5 alpha-pregnane-3 alpha, 21-diol-20-one (5 alpha-tetrahydrodeoxycorticosterone; 5 alpha-THDOC) has been shown to potentiate GABA-induced chloride currents in cell cultures and subcellular preparations. In this study, we recorded from pyramidal neurons in an in vitro slice preparation of the adult rat frontal neocortex using intracellular microelectrodes. 5 alpha-THDOC (10 microM) increased and prolonged the inhibitory postsynaptic potential (IPSP). The mean maximal synaptic conductance of the early, GABAA receptor-mediated, IPSP was enhanced to more than 700%, the one at the maximum of the late, partially GABAB receptor-mediated, IPSP to approximately 400%. The progesterone/glucocorticoid receptor antagonist RU 38486 did not prevent the IPSP increase. At a concentration of 1 microM 5 alpha-THDOC increased only the early IPSP to about 125%. Responses to the iontophoretically applied specific GABAA receptor agonist muscimol but not to the specific GABAB receptor agonist L-baclofen were enhanced by 5 alpha-THDOC (10 microM). In the giga-seal whole-cell configuration when the GABAB receptor-mediated IPSP component was absent due to intracellular perfusion, 5 alpha-THDOC (10 microM) increased IPSPs to a similar extent as in the conventional microelectrode recordings. Excitatory postsynaptic potentials, resting membrane potential, input resistance and action potential amplitude were not affected by 5 alpha-THDOC (10 microM). These data demonstrate that in neocortical tissue of the rat 5 alpha-THDOC enhances GABAergic inhibition by interacting with postsynaptic GABAA receptors while synaptic excitation and parameters of electric excitability remain unchanged.

Participation of voltage-gated conductances on the response succeeding inhibitory synaptic potentials in the crayfish slowly adapting stretch receptor neuron.

1. We examined the contribution of voltage-gated conductances to inhibitory postsynaptic potential (IPSP) effects under current clamp in silent and spiking slowly adapting stretch receptor neurons (SN1s) in the slow receptor muscle of the crayfish Procambarus. The receptor exemplifies the simplest inhibitory neural circuit, with one presynaptic and one postsynaptic neuron. The effects of synaptic inhibition were compared with the outcome of hyperpolarizing current pulses. Because pulse effects were exclusively due to postsynaptic mechanisms, an estimation of the synaptic or extrasynaptic origin of the results of IPSP was possible. 2. Inhibition by single IPSPs increased gradually with the time elapsed from the preceding spike in 60% of the spiking SN1s. However, early IPSP arrivals were exclusively excitatory in the rest of the cases. Inhibition was restricted to a single expanded SN1 interspike interval, but the early excitation and the postinhibitory rebound lasted several intervals. Rebound was invariably present; it was the only consequence of IPSPs in silent receptors and could be extremely long lasting (> 25 s). 3. The membrane potential of the SN1 neuron was clamped at hyperpolarized values (greater than -65 mV) by prolonged IPSP barrages at high rate (> 20/s). A prominent depolarizing sag and a gradual reduction of the IPSP amplitude were observed with prolonged presynaptic stimulation. There were subthreshold IPSP amplitude oscillations consisting of gradual increases and decreases of the post-IPSP peak depolarization at lower presynaptic rates. IPSP amplitude variations (< or = 10 mV) were primarily due to larger local responses. 4. Essentially all IPSP effects were mimicked by hyperpolarizing pulses. Sag was also evoked by pulses and was accompanied by a gradual conductance increase preceded by a brief initial drop. Sag and rebound were markedly reduced by Cs+ (2 mM) and tetrodotoxin (1 microM) and less by Ba2+ (5 mM) or tetraethylammonium (25 mM) superfusion. Both were somewhat decreased by acetylcholine (30 microM), which also markedly depolarized and accelerated firings, results which were usually reduced by atropine (10 microM). 5. In conclusion, IPSP and hyperpolarizing pulse effects were essentially identical, implying that extrasynaptic membrane properties were decisive. Interestingly, net excitatory consequences were usual, effectively increasing sensitivity and reducing the sensory threshold. Pharmacological evidence is provided suggesting that the hyperpolarization-activated current, IQ, and also probably the K+ M-current, the A-current, and the low-threshold, persistent Na+ conductances participate in sag and rebound genesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Carbachol-induced synchronized rhythmic bursts in CA3 neurons of guinea pig hippocampus in vitro

1. Carbachol effects on CA3 hippocampal cells were studied in the absence of ionotropic glutamatergic and GABAergic transmission with intracellular and extracellular recordings from guinea pig septohippocampal slices. 2. In all experiments the perfusing solution contained ionotropic glutamate and gamma-aminobutyric acid (GABA) receptor blockers [6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-20 microM), 3-((+/-)-2-carboxypiperazin-4-il)propyl-1-phosphonic acid (CPP, 10-20 microM), and picrotoxin (50 microM), respectively]. Under these conditions, the excitatory and early inhibitory postsynaptic potentials, evoked in CA3 pyramidal cells by mossy fiber stimulation before the addition of the blockers, were completely suppressed. 3. Carbachol (50 microM) introduced via bath perfusion or pulse application elicited a series of rhythmic bursts with overriding action potentials. Each rhythmic burst lasted up to 30 s and repeated at intervals of 0.7-6 min. Rhythmic bursts were blocked by atropine (1 microM). 4. At membrane potentials more depolarized than -70 mV, carbachol also elicited a sustained depolarization associated with an increase in membrane input resistance and action-potential firing. This response was blocked by atropine (1 microM). 5. Carbachol can induce both rhythmic bursts and sustained depolarizations in the same cell. Rhythmic bursts were elicited when the membrane potential of the cell was more hyperpolarized than -70 mV; sustained depolarizing responses were activated by carbachol when the cell membrane potential was more depolarized than -70 mV. 6. Extracellular field potential responses in the CA3 region occurred simultaneously with rhythmic bursts, indicating the synchronization of the event in the CA3 field. Dual intracellular recordings confirmed that rhythmic bursts occurred simultaneously in CA3 hippocampal pyramidal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal circuits associated with the output of the dorsal cochlear nucleus through fusiform cells

1. Intracellular recordings were made from 21 anatomically identified fusiform cells in the dorsal cochlear nucleus (DCN) of mice in slices. The aim of the experiments was to dissect the synaptic responses to shocks of the auditory nerve to correlate functional characteristics with the different classes of synaptic inputs. 2. When depolarized from rest (-57 +/- 5 mV) with current pulses, fusiform cells fired regular, overshooting action potentials that were followed by two undershoots. The frequency of firing increased with the strength of injected current by between 100 and 300 spikes/s/nA. The current-voltage relationship rectified between 10 and 15 mV below the resting potential. The slopes of current-voltage relationships of fusiform cells in the range between the resting potential and 10 mV hyperpolarization indicated an average input resistance of 86 +/- 37 M omega. 3. In each of the labeled fusiform cells frequent, spontaneous inhibitory postsynaptic potentials (IPSPs) were recorded singly or in bursts. Some, but not all, IPSPs were preceded by a slowly rising excitatory postsynaptic potential (EPSP). The temporal association of spontaneous EPSPs and IPSPs suggests that they are driven by a common source, possibly granule cells. 4. Shocks to the auditory nerve evoked synaptic responses consisting of early (1 to approximately 10 ms) and late (approximately 10 to 100 ms) components. 6,7-Dinitroquinoxaline-2,3-dione (DNQX) at 20 to 40 microM eliminated all detectable excitation and all late IPSPs. Late bursts of IPSPs, therefore, are mediated through a polysynaptic pathway that includes a DNQX-sensitive stage. Strong shocks to the nerve root elicited single monosynaptic IPSPs, indicating that inhibitory interneurons have processes close to the auditory nerve. Strychnine at 0.5 microM eliminated all detectable inhibition. 6. Cuts through the posteroventral cochlear nucleus (PVCN), which severed the descending branches of auditory nerve fibers, eliminated early EPSPs and IPSPs leaving late, slowly rising EPSPs and bursts of IPSPs in responses to shocks of the auditory nerve. Late, slowly rising EPSPs and bursts of IPSPs, as well as monosynaptic IPSPs, could also be evoked by stimulating the anteroventral cochlear nucleus (AVCN). 7. Focal applications of glutamate evoked excitation and inhibition from many parts of a slice, with patterns varying among cells, indicating that fusiform cells receive inputs through several groups of interneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyperpolarizing synaptic potentials evoked in CA1 pyramidal cells by glutamate stimulation of interneurons from the oriens/alveus border of rat hippocampal slices. I. Electrophysiological response properties.

To examine the inhibitory postsynaptic potentials (IPSPs) elicited in pyramidal cells by interneurons situated at the stratum oriens/alveus border (O/A), glutamate was applied by micropressure to this area during intracellular recordings from CA1 pyramidal cells. Glutamate stimulation evoked IPSPs (glut-IPSPs) of small amplitude (4 mV), delayed peak latency (100-110 ms), and long duration (300-400 ms). Recurrent activation of interneurons via glutamate stimulation of pyramidal cells by local application in stratum pyramidale (PYR) evoked recurrent IPSPs (PYR glut-IPSPs) with similar amplitude and time course as O/A glut-IPSPs. The mean equilibrium potential of O/A glut-IPSPs (-77 mV) was significantly different from that of the PYR glut-IPSPs (-71 mV), however, neither equilibrium potential was significantly different from that of the electrically evoked early IPSP in the same cells. Glutamate-evoked IPSPs elicited from O/A displayed some response reversal (27% reversal) like those evoked from PYR (41% reversal). The early IPSP evoked by electrical stimulation displayed significantly more response reversal (67% reversal) than glut-IPSPs. Both types of glut-IPSPs (O/A and PYR) were associated with moderate increases in membrane conductance (5.9 and 6.6 nS, respectively), which were significantly less than the conductance change associated with the early IPSP (45.8 nS). In interneurons within PYR, glutamate stimulation in PYR readily elicited a flurry of excitatory postsynaptic potentials, whereas glutamate stimulation in O/A elicited IPSPs. The electrophysiological properties of IPSPs elicited in pyramidal cells by glutamate stimulation of interneurons in O/A were similar to those of recurrent IPSPs evoked from PYR. Given that both of these types of glutamate-evoked IPSPs were mostly mediated via GABAA receptor channels (Samulack DD, Lacaille J-C, 1993, Hippocampus 3:345-358), the small differences observed between equilibrium potentials, response reversals, and conductance changes could be due to a more electronically distant location from the soma of the synapses involved in O/A glut-IPSPs as compared to those of recurrent IPSPs elicited from PYR.

Monosynaptic GABA-mediated inhibitory postsynaptic potentials in ca1 pyramidal cells of hyperexcitable hippocampal slices from kainic acid-treated rats

To examine the mechanisms underlying chronic epileptiform activity, field potentials were first recorded to identify hyperexcitable hippocampal slices from kainic acid-treated rats. Intracellular recordings were then obtained from CA1 pyramidal cells in the hyperexcitable areas. Twenty-two of the 47 cells responded to electrical stimulation of the stratum radiatum with a burst of two or more action potentials and reduced early inhibitory postsynaptic potentials, and were considered hyperexcitable. The remaining 25 cells were not hyperexcitable, displaying a single action potential and biphasic inhibitory postsynaptic potentials after stimulation, like control cells ( n = 20). A long duration, voltage-sensitive component was associated with subthreshold excitatory postsynaptic potentials in the majority of hyperexcitable (12/15) and non-hyperexcitable (3/5) cells examined from kainic acid-treated animals, but not from cells (1/10) of control animals. Stimulation of stratum radiatum during pharmacological blockade of ionotropic excitatory amino acid synaptic transmission elicited biphasic monosynaptic inhibitory postsynaptic potentials in all hyperexcitable ( n = 9) and non-hyperexcitable ( n = 9) cells tested from kainate-treated animals, as well as in control cells ( n = 8). The mean amplitude, latency to peak, equilibrium potential, and conductance changes of early and late monosynaptic inhibitory postsynaptic potentials were not different between cells of kainic acid-treated and control animals. In seven hyperexcitable cells tested, the early component of monosynaptic inhibitory postsynaptic potentials was significantly reduced by the GABA A receptor antagonist bicuculline (100–200 μM). The late component was significantly decreased by the GABA B receptor antagonist 2-hydroxysaclofen (1–2 mM; n = 3). Comparable effects were observed on early and late monosynaptic inhibitory postsynaptic potentials in non-hyperexcitable cells ( n = 4) from kainic acid-treated animals and control cells ( n = 5). These results suggest that GABAergic synapses on hyperexcitable hippocampal pyramidal cells of kainate-treated rats are intact and functional. Therefore, epileptiform activity in the kainate-lesioned hippocampus may not arise from a disconnection of GABAergic synapses made by inhibitory interneurons on pyramidal cells. The hyperexcitability may be due to underactivation of inhibitory interneurons and/or reorganization of excitatory inputs to pyramidal cells since, in kainate-treated animals, pyramidal cells appear to express additional excitatory mechanisms.

Physiological evidence for two distinct GABA A responses in rat hippocampus

The γ-aminobutyric acid A (GABA A) receptor is a ligandgated ionophore involved in synaptic inhibition. Biochemical and molecular biological studies indicate that considerable receptor heterogeneity exists, but physiological differences between inhibitory GABA A synaptic responses have not been identified in the brain. The present report describes two anatomically segregated GABA A-mediated synaptic currents in the hippocampal CAI region that have distinct physiological, pharmacological, and functional properties. GABA A,fast enters at or near the cell body, decays rapidly (3–8 ms), is blocked by furosemide, and rapidly curtails the excitatory response. GABA A,slow enters far from the cell body, decays slowly (30–70 ms), is not blocked by furosemide, and underlies the conventionally recognized early inhibitory postsynaptic potential. The receptors producing these responses may represent subtypes of the GABA A receptor.

Inhibitory responses of rat basolateral amygdaloid neurons recorded in vitro

The purpose of the present study was to characterize the ionic and pharmacological basis of the actions of synaptically released and exogenously applied GABA in basolateral amygdaloid pyramidal cells in vitro. Stimulation of forebrain afferents to pyramidal neurons in the basolateral amygdala evoked an excitatory postsynaptic potential followed by early and late inhibitory postsynaptic potentials. The early inhibitory postsynaptic potential had a reversal potential near −70 mV, was sensitive to changes in the chloride gradient across the membrane and was blocked by the GABA A antagonists picrotoxin and bicuculline methiodide but not by the GABA B antagonists phaclofen or 2-hydroxysaclofen. In contrast, the late inhibitory postsynaptic potential had a reversal potential of approximately −95 mV and was markedly reduced or abolished by GABA B antagonists. Pressure application of GABA to the surface of the slice typically elicited a triphasic response in basolateral amygdaloid pyramidal neurons consisting of a short-latency hyperpolarization that preceded or was superimposed on a membrane depolarization followed by a longer latency hyperpolarization. Each of the responses was associated with an increase in membrane conductance. Determinations of the reversal potential, ionic dependency and sensitivity to pharmacological blockade of each component of the GABA-induced response revealed that the initial hyperpolarizing ( E rev approximately −70 mV) and depolarizing ( E rev approximately −55 mV) responses were mediated by a GABA A-mediated increase in chloride conductance, whereas the late hyperpolarizing response ( E rev approximately −82 mV) to GABA arose from a GABA B-mediated increase in potassium conductance. Experiments in which GABA was applied at various locations on the cell suggested that the short-latency hyperpolarization resulted from activation of somatic GABA receptors, whereas the depolarizing and late hyperpolarizing responses were generated primarily in the dendrites. In contrast to the complex membrane response profile elicited by GABA, pressure ejection of the GABA B agonist baclofen produced only membrane hyperpolarizations. Taken together, these results suggest that inhibitory responses that are recorded in basolateral amygdaloid pyramidal cells are mediated by activation of both GABA A and GABA B receptors. Consistent with findings elsewhere in the CNS, the early inhibitory postsynaptic potential and initial hyperpolarization and depolarizing response to local GABA application appear to involve a GABA A-mediated increase in chloride conductance, whereas the late inhibitory postsynaptic potential and the late hyperpolarizing response to GABA arise from a GABA B-mediated increase in potassium conductance.

Muscarinic responses of rat basolateral amygdaloid neurons recorded in vitro.

1. Intracellular recordings were obtained from pyramidal-type neurons in the basolateral amygdaloid nucleus (BLA) in slices of rat ventral forebrain and used to compare the actions of exogenously applied cholinomimetics to the effects produced by electrical stimulation of amygdalopetal cholinergic afferents from basal forebrain. 2. Bath application of carbachol depolarized pyramidal cells with an associated increase in input resistance (Ri), reduced the slow after-hyperpolarization (AHP) that followed a series of current-evoked action potentials and blocked spike frequency accommodation. All of these effects were reversed by the muscarinic antagonist atropine but not by the nicotinic antagonist hexamethonium. 3. Electrical stimulation of amygdaloid afferents within the external capsule evoked a series of synaptic potentials consisting of a non-cholinergic fast excitatory postsynaptic potential (EPSP), followed by early and late inhibitory postsynaptic potentials (IPSPs). Each of these synaptic potentials was reduced by carbachol in an atropine-sensitive manner. 4. Local application of carbachol to pyramidal cells produced a short-latency hyperpolarization followed by a prolonged depolarization. The hyperpolarization and depolarization to carbachol were blocked by atropine but not hexamethonium. 5. The carbachol-induced hyperpolarization was associated with a decrease in Ri and had a reversal potential nearly identical to that of the early IPSP. The inhibitory response was blocked by perfusion of medium containing tetrodotoxin (TTX), bicuculline or picrotoxin, while the subsequent depolarization was unaffected. On the basis of these data, it is concluded that the muscarinic hyperpolarization is mediated through the rapid excitation of presynaptic GABAergic interneurons in the slice. 6. The findings that the carbachol-induced depolarization was associated with an increase in Ri, often had a reversal potential below -80 mV, was sensitive to changes in extracellular potassium concentration and was blocked by intracellular ionophoresis of the potassium channel blocker caesium suggest that it resulted from a muscarinic blockade of one or more potassium conductances. 7. Repetitive stimulation of sites within the slice containing cholinergic afferents evoked a series of fast EPSPs followed by IPSPs. These non-cholinergic potentials were followed by a slow EPSP that lasted from 10 s-4 min. The slow EPSP was enhanced by eserine and blocked by atropine. It was also blocked by TTX or cadmium, indicating that it was dependent on spike propagation and calcium-dependent release of acetylcholine (ACh). 8. Stimulation of cholinergic afferents in the slice mimicked other effects produced by carbachol including blockade of the slow AHP and accommodation of action potential discharge and these actions were potentiated by eserine and blocked by atropine.(ABSTRACT TRUNCATED AT 400 WORDS)