Component Of Inhibitory Postsynaptic Potentials Research Articles

Altered Inhibition in Lateral Amygdala Networks in a Rat Model of Temporal Lobe Epilepsy

Clinical and experimental evidence indicates that the amygdala is involved in limbic seizures observed in patients with temporal lobe epilepsy. Here, we used simultaneous field and intracellular recordings from horizontal brain slices obtained from pilocarpine-treated rats and age-matched nonepileptic controls (NECs) to shed light on the electrophysiological changes that occur within the lateral nucleus (LA) of the amygdala. No significant differences in LA neuronal intrinsic properties were observed between pilocarpine-treated and NEC tissue. However, spontaneous field activity could be recorded in the LA of 21% of pilocarpine-treated slices but never from NECs. At the intracellular level, this network activity was characterized by robust neuronal firing and was abolished by glutamatergic antagonists. In addition, we could identify in all pilocarpine-treated LA neurons: 1) large amplitude depolarizing postsynaptic potentials (PSPs) and 2) a lower incidence of spontaneous hyperpolarizing PSPs as compared with NECs. Single-shock stimulation of LA networks in the presence of glutamatergic antagonists revealed a biphasic inhibitory PSP (IPSP) in both NECs and pilocarpine-treated tissue. The reversal potential of the early GABA(A) receptor-mediated component, but not of the late GABA(B) receptor-mediated component, was significantly more depolarized in pilocarpine-treated slices. Furthermore, the peak conductance of both fast and late IPSP components had significantly lower values in pilocarpine-treated LA cells. Finally, paired-pulse stimulation protocols in the presence of glutamatergic antagonists revealed a less pronounced depression of the second IPSP in pilocarpine-treated slices compared with NECs. Altogether, these findings suggest that alterations in both pre- and postsynaptic inhibitory mechanisms contribute to synaptic hyperexcitability of LA networks in epileptic rats.

In vitro study of afferent synaptic transmission in the rostral gustatory zone of the rat nucleus of the solitary tract

The synaptic responses of rostral nucleus of the solitary tract (rNST) neurons to electrical stimulation of the solitary tract (ST) fibers were investigated using whole-cell recordings in brain slices of adult rat medulla. Most neurons of the rNST (47%) responded to stimulation of the ST with excitatory postsynaptic potentials (EPSPs), 28% responded with mixed excitatory and inhibitory postsynaptic potentials (PSPs) and 25% responded with inhibitory postsynaptic potentials (IPSPs). The estimated reversal potentials for the EPSPs ( E EPSP) was −7 mV and for the IPSPs ( E IPSP) was −69 mV. The glutamate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) acting at the α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate receptor, either reduced or blocked all EPSPs tested. d-2-Amino-5-phosphonovalerate (APV), a selective N-methyl- d-aspartate (NMDA) receptor antagonist, also reduced the amplitude of the EPSPs. These results suggest that glutamate is released following stimulation of afferent fibers in the ST and acts on both AMPA/kainate and NMDA glutamate receptors. The IPSPs result from release of y-aminobutyric acid (GABA) since superfusion of the GABA A receptor antagonist, bicuculline reversibly blocked the IPSPs. The GABA, receptor antagonist, phaclofen, also reduced the IPSP components in some neurons, indicating that both GABAA and GABA, receptors are involved in inhibitory transmission in the rNST. When the morphology of the recorded neurons was examined by filling the neurons with biocytin and reconstructing the neurons, each morphological type of rNST neuron responded with excitatory and inhibitory PSPs following stimulation of the ST.

Clonazepam suppresses GABAB-mediated inhibition in thalamic relay neurons through effects in nucleus reticularis

1. Experiments were carried out using patch-clamp techniques in rat thalamic slices, maintained in vitro, to examine the effects of the benzodiazepine compound, clonazepam (CZP), on intrathalamic inhibition. Bath-applied CZP reduced the gamma-aminobutyric acid-B (GABAB) component of inhibitory postsynaptic potentials and currents (IPSPs and IPSCs, respectively) evoked in rat thalamic somatosensory relay neurons by stimulation of nucleus reticularis thalami (nRt), without consistently affecting the GABAA IPSP. Secondary IPSPs, which occur as a result of intrathalamic oscillations, were dramatically reduced. 2. Voltage-clamp experiments combined with local or bath perfusion of the GABAA antagonist bicuculline methiodide (BMI), demonstrated that nRt is a site of GABAA-mediated postsynaptic inhibition that affects inhibitory output onto relay neurons. BMI enhanced both GABAA and GABAB postsynaptic inhibition in relay neurons when applied to nRt. Focal applications in the ventrobasal relay nucleus near the recording electrode blocked the GABAA-mediated IPSP but had no effects on GABAB inhibitory potentials. 3. Results suggest that CZP acts to facilitate recurrent inhibition in nRt and decrease its inhibitory output onto relay neurons. Intra-nRt GABAA-mediated inhibition thus has an important role in controlling thalamic excitability and in the anti-absence actions of CZP.

GABA A receptor-mediated mechanisms contribute to frequency-dependent depression of IPSPs in the hippocampus

Intracellular recordings from in vitro guinea-pig hippocampal slices were obtained to investigate the mechanisms underlying activity-dependent depression of inhibitory postsynaptic potentials (IPSPs) in Ca1 pyramidal neurons. IPSPs were studied following blockade of glutamatergic transmission. Attenuation of both fast and slow IPSP components was apparent at stimulus rates of >0.2 Hz, but the late IPSP showed depression at even lower rates of stimulation. Overlap of these events made resolving response components during depression difficult. Fast IPSPs were isolated using CsCl-filled microelectrodes, blocking slow potassium-dependent IPSPs and reversing the chloride gradient. Under this condition repetitive stimulation did not induce IPSP depression. Other experiments showed slow IPSP depression at stimulus rates of <0.5 Hz was relieved in the presence of picrotoxin which antagonizes GABA A events. These results suggest that activity-dependent depression of the fast IPSP results largely from chloride accumulation as a consequence of repetitive activation. Depression of the slow IPSP appears to arise from at least two components: (1) a masking of the slow hyperpolarization resulting from an apparent increase in the depolarizing GABA response due to chloride accumulation; and (2) a novel process involving GABA A receptors, perhaps mediated through presynaptic inhibition.

Inhibitory transmission in the basolateral amygdala

1. Intracellular recording techniques were used to characterize synaptic inhibitory postsynaptic potentials (IPSPs) recorded from neurons of the basolateral nucleus of the amygdala (BLA). Bipolar electrodes positioned in the stria terminalis (ST) or lateral amygdala (LA) were used to evoke synaptic responses at a frequency of 0.25 Hz. 2. Two synaptic waveforms having IPSP components could be evoked by electrical stimulation of either pathway: a biphasic, excitatory postsynaptic potential (EPSP), fast-IPSP (f-IPSP) waveform, and a multiphasic, EPSP, f-IPSP, and subsequent slow-IPSP (s-IPSP) waveform. Expression of either waveform was dependent on the site of stimulation. ST stimulation evoked a similar number of biphasic (45%) and multiphasic (50%) synaptic responses. In contrast, stimulation of the LA pathway evoked mainly (80%) multiphasic synaptic responses. 3. Both the f- and s-IPSP elicited by ST stimulation could be reduced in amplitude in the presence of the glutamatergic, N-methyl-D-aspartate (NMDA) antagonist, (DL)-2-amino-5-phosphonovaleric acid (APV, 50 microM), and were abolished by the glutamatergic, non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). In contrast, a CNQX-resistant f-IPSP was evoked with LA stimulation and abolished by subsequent addition of bicuculline methiodide (BMI), a gamma-aminobutyric acid (GABAA) receptor antagonist, suggesting direct inhibition of BLA neurons by GABAergic LA interneurons. The sensitivity of the s-IPSPs and the f-IPSPs to glutamatergic antagonists suggests the presence of feed-forward inhibition onto BLA neurons. 4. The f-IPSP possessed characteristics of potentials mediated by GABAA receptors linked to Cl- channels, namely, a reversal potential of -70 mV, a decrease in membrane resistance (13.5 M omega) recorded at -60 mV, a block by BMI, and potentiation by sodium pentobarbital (NaPB). 5. The s-IPSP was associated with a resistance decrease of 4.5 M omega, a reversal potential of -95 mV, and was reversibly depressed (approximately 66%) by 2-hydroxy-saclofen (100 microM), suggesting activation of GABAB receptors. 6. The large resistance change associated with the f-IPSP, its temporal overlap with evoked EPSPs, and the development of both spontaneous and evoked burst firing in the presence of BMI suggests that the f-IPSP determines the primary state of excitability in BLA neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity‐induced decreased in early and late inhibitory synaptic conductances in hippocampus

The use dependence of inhibitory postsynaptic potentials (IPSPs) and their underlying conductances was studied in area CA1 of the hippocampal brain slice preparation, using a two-pulse paradigm in which paired activation of two separate synaptic inputs resulted in changes in the second, or "primed" response. In intracellular current-clamp recordings, the "primed" response, normally triphasic, exhibited a larger, wider excitatory PSP (EPSP) component and greatly reduced or absent IPSP components. Maximal widening occurred when the interval between synaptic stimuli was between 200 and 250 msec. Hyperpolarization of the postsynaptic cell reversed both the early IPSP and the direction of change of the width of the "primed" EPSP response, suggesting that the changes in the "primed" waveform were not due to the addition of an unidentified inward current(s). Furthermore, the reduction of the IPSPs during the "primed" response could not be accounted for by the fact that the membrane potential of the postsynaptic cell was hyperpolarized and therefore closer to IPSP reversal potential. Using single-electrode voltage-clamp techniques, we found that the early inhibitory conductance generally decreased by approximately 50%, with little if any change in reversal potential. The late inhibitory conductance also showed a priming-induced decrease of approximately 95%. Finally, "primed" four-pulse bursts of stimuli induced a larger depolarization in the postsynaptic cell than did unprimed bursts, also with an optimal interval of about 250 msec. We conclude that activation of certain synaptic pathways in the hippocampus results in a temporal window of 200-300 msec during which inhibitory synaptic activity is depressed and excitatory synaptic transmission is maximally effective, especially if the excitation occurs in short bursts. Such a mechanism would endow the inhibitory synaptic components of the hippocampus with a "gating" function to control long-term synaptic modification at excitatory synapses in the same region.

Inhibitory role of dentate hilus neurons in guinea pig hippocampal slice

1. Current and voltage-clamp recording of CA3/CA4 pyramidal neurons, hilar neurons, and granule cells or pairs of these neurons were used to study the generation of Cl-dependent and K-dependent inhibitory postsynaptic potentials (IPSPs) in the guinea pig hippocampal slice preparation. 2. A sequence of an early Cl-dependent and a late K-dependent IPSP was evoked in CA3 neurons by electrical stimulation from the stratum moleculare of the dentate gyrus, the hilus, and the stratum oriens/alveus. Blockade of glutamatergic excitation by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D(-)-2-amino-5-phosphonovaleric acid (APV, 30 microM) abolished IPSPs evoked from the stratum moleculare of the dentate gyrus, but IPSPs could still be evoked from the hilus and the stratum oriens/alveus. 3. Repetitive giant IPSPs, which consisted of Cl-dependent and K-dependent components, were evoked by bath application of 4-aminopyridine (4-AP, 10-50 microM) in CA3 neurons and in granule cells. Giant IPSPs were blocked by bath-applied tetrodotoxin (TTX). In addition, 4-AP hyperpolarized CA3 neurons in a Cl-dependent and picrotoxin-sensitive way. 4. Focal application of TTX to the dentate gyrus or the hilus considerably reduced the amplitude of giant IPSPs evoked by 4-AP in CA3 neurons. In hilar neurons, 4-AP evoked repetitive bursts, eventually, but not necessarily intermingled with giant IPSPs. Bursts were observed in hilar neurons in presence as well as absence of CNQX and APV. 5. In paired recordings, bursts in hilar neurons induced by 4-AP occurred simultaneously to giant IPSPs in granule cells and CA3 neurons, and giant IPSPs in granule cells occurred simultaneously to giant IPSPs in CA3 neurons. Blockade of glutamatergic excitation by CNQX and APV did not abolish this synchrony. 6. 4-AP-evoked Cl- and K-dependent IPSPs were, unlike electrically evoked IPSPs, not strictly coupled: some 20% of large IPSPs and up to 90% of small IPSPs were either Cl or K dependent. In granule cells K-dependent components either preceded or followed Cl-dependent components. 7. K-dependent IPSPs only could be evoked in CA3 neurons by focal application of 4-AP (1 mM) to the hilus, the stratum lacunosum moleculare or the stratum pyramidale. Wash out of Ca for 15-20 min blocked the Cl-dependent but not the K-dependent component of giant IPSPs evoked by bath-applied 4-AP.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular injection of apamin reduces a slow potassium current mediating afterhyperpolarizations and IPSPs in neocortical neurons of cats

Electrophysiologic effects of intracellularly injected apamin, a Ca 2+-dependent K + channel blocker, were investigated in neurons of the motor cortex of awake cats. Single-electrode voltage clamp techniques were used to measure changes in membrane currents including those that were synaptically activated. All changes occured within 2–4 min after pressure injection of apamin with partial recovery observed within 8–15 min. Apamin selectively abolished an outward current that mediated a slow afterhyperpolarization (AHP) following intracellular depolarizing current pulses and action potentials without influencing the time course of the action potentials or an associated fast AHP component. In addition apamin increased the number and frequency of spike discharges evoked by the depolarizing current pulses and produced a small increase in the rate of background firing activity. The baseline resting potential and input resistance were essentially unchanged by apamin. Apamin also diminished a late, slowly decaying component of inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) elicited by stimulation of the ventrolateral thalamus or the pyramidal tract. The apamin-induced changes were concomitant with a decrease of the decay time constant of both IPSPs and IPSCs and a positive shift in their reversal potential. The results suggest that the late, slowly decaying component of these inhibitory postsynaptic responses is generated by an apamin-sensitive Ca 2+-dependent K + conductance which is also responsible for the slow AHP.

Pharmacological dissection of pyloric network of the lobster stomatogastric ganglion using picrotoxin.

1. Picrotoxin (PTX) (10(-7)-10(-6) M) completely blocked most inhibitory synapses in the pyloric pattern generator of the lobster (Panulirus interruptus) stomatogastric ganglion. The sensitivity of synapses from most classes of identified neurons was examined. Blockade was at least partly reversible with prolonged washing. 2. The synapses from pyloric dilator (PD) neurons were the only inhibitory synapses that picrotoxin failed to block completely. 3. A correlation is derived that brief, fast-rise inhibitory postsynaptic potentials (IPSPs) are picrotoxin sensitive, whereas a slow rounded component of IPSPs from PD neurons is not picrotoxin sensitive. 4. Picrotoxin caused specific changes in the pattern of the motor rhythm produced by the 16-cell pyloric network. This sheds some light on the functional role of particular synapses in the pyloric generator. 5. The endogenously bursting neurons (PD and anterior burster (AB)), which drive the pyloric rhythm, kept a similar burst rate. 6. Under picrotoxin, the pyloric "follower" neurons all moved to later phase relative to the "driver" group. Some normally antagonistic cells, related by reciprocal inhibitor connections, became in-phase. These and other pattern changes could be related to blockade of particular synapses. 7. The pyloric rhythm was still quite recognizable under picrotoxin despite the drastically altered circuitry of the synaptic network. This supports the idea that periodic inhibition from the PD driver neurons plays a primary role in creating the pyloric pattern.

Penicillin-induced spinal seizures: Selective effects on synaptic transmission

We investigated possible alterations of synaptic transmission caused by penicillin in the generation of epileptiform activity in the mammalian central nervous system by observing postsynaptic potentials in spinal motor neurons before and after the topical application of penicillin to the spinal cord. We obtained intracellular recordings from 337 identified flexor and extensor motor neurons in 28 cats with intact neuraxis, maintained under light pentobarbital anesthesia. Monosynaptic excitatory postsynaptic potentials (EPSPs) from group Ia afferents and descending supraspinal fibers, and disynaptic Ia antagonist and Renshaw inhibitory postsynaptic potentials (IPSPs) were not significantly altered by convulsive doses of penicillin. In contrast, the EPSP components of mixed polysynaptic postsynaptic potentials from cutaneous afferents were markedly enhanced and often prolonged, and the IPSP components were smaller after penicillin. Cutaneous pathways alone appear to be instrumental in the initiation and synchronization of motor neuron bursting caused by penicillin. Within this pathway penicillin may be specifically altering afferent terminals, interneuron membranes, or synaptic transmission, but these elements appear to be unaffected in the monosynaptic and disynaptic linkages derived from muscle afferents.