Evoked Excitatory Postsynaptic Potentials Research Articles

Transplanted embryonic entorhinal neurons make functional synapses in adult host hippocampus

Grafts of embryonic entorhinal cortex (EC) or non-entorhinal cortex (NEC) were placed into the hippocampus of adult rats with transection of the perforant paths. Graft–host connectivity was investigated at 4–6 months post-transplantation by recording extracellular evoked responses in hippocampal slice preparations. Electrical stimulation of the grafts evoked excitatory postsynaptic potentials (EPSPs) in the outer molecular layer of the dentate gyrus, and the stratum lacunosum moleculare of CA1, CA3, and elicited population spikes in the granule cell layer and the pyramidal cell layer of CA1, but not CA3. While the latencies and the forms of these evoked response were similar to those in matched control slices from the normal animals, the amplitudes were smaller than normal controls. However, in the slices with NEC grafts, no such responses were recorded when stimulus was applied in similar position in the grafts. The findings suggest that grafted entorhinal neurons make viable synaptic connections with the host hippocampus.

Synaptic differentiation of single descending fibers studied by triple intracellular recording in the frog spinal cord.

Evoked excitatory postsynaptic potentials (EPSPs) were simultaneously intracellularly recorded in two lumbar motoneurons located in spinal segments 8-10 in response to intraaxonal stimulation of a descending fiber. Their mean amplitudes, paired-pulse facilitation, and short- and long-term posttetanic potentiation were compared to reveal possible functional differences among synapses formed by one axon on different postsynaptic targets. The mean amplitudes of EPSPs recorded in two motoneurons were significantly different in most experiments. This amplitude difference was related to the location of motoneurons in that it was twofold larger in motoneurons separated by >1 mm than in motoneurons located within 200 micron m of one another and also that the amplitude of EPSPs recorded in motoneurons located in the tenth segment was regularly smaller than the amplitude recorded in the ninth segment. The estimation of binomial model parameters suggests that the difference in mean EPSP amplitude was due mostly to differences in the maximal number of quanta prepared for release (binomial parameter N) and in mean release probability rather than to differences in quantal size. The ability of connections formed by a single axon on different motoneurons to undergo use-dependent synaptic modulations was different on scales of milliseconds, seconds, and tens of minutes as revealed by the measurements of effects of paired-pulse and tetanic stimulation. The difference in magnitude of short-term posttetanic potentiation in connections formed by a single descending axon was significantly correlated with the difference in mean probability of release in these connections. Thus our data revealed a functional nonuniformity of synapses formed by individual descending fibers on widely separated motoneurons, most likely innervating different muscles. This process can be one of the mechanisms by which a fine descending control of recruitment of motoneuronal populations is achieved.

Halogenated Aromatic Hydrocarbons Suppress CA1 Field Excitatory Postsynaptic Potentials in Rat Hippocampal Slices

Halogenated aromatic hydrocarbons (HAHs), such as polychlorinated biphenyls (PCBs) and dibenzo-p-dioxins (PCDDs), alter cognitive function and learning. The cellular basis of HAH-induced alteration of brain function is not well-understood. The hippocampus is a likely site of toxic action because of its well-known roles in learning and memory, as well as its propensity to accumulate environmental neurotoxicants. A hippocampal function that can be measured readily is evoked excitatory postsynaptic potentials (EPSPs), which are an index of excitatory synaptic function. In this study, effects of HAHs on EPSPs were characterized in hippocampal slices from adolescent to adult maleSprague—Dawley rats. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,4-TCDD were used because these HAHs are prototypical potent and weak aryl hydrocarbon (Ah) receptor agonists, respectively. 2,2′,5,5′-Tetrachlorobiphenyl (TCB) was used as a prototypical ortho-substituted PCB, which acts through Ah receptor-independent pathways. For each hippocampal slice, peak amplitudes of EPSPs during a 15-min recording period (1 recording/min) were averaged and used as baseline (100%). Subsequent EPSPs were expressed as percentage of baseline. TCDD and 1,2,3,4-TCDD did not alter EPSPs in slices from the middle third of the hippocampus. However, in ventral slices, TCDD significantly decreased EPSPs, whereas 1,2,3,4-TCDD was inactive. TCB decreased EPSPs in both middle and ventral slices at half-maximal stimulation. An unexpected reversal of inhibition was observed within 30 min of continuous application of TCDD or TCB. In ventral slices,l-type calcium channel blocker nifedipine blocked inhibition of EPSPs induced by TCDD but not EPSPs inhibited by TCB. These results suggest that, while TCB-induced inhibition of EPSPs occurs through an unknown mechanism, TCDD-induced inhibition of EPSPs was mediated byl-type calcium channel activity in a congener-specific manner.

Deafferentation weakens excitatory synapses in the developing central auditory system.

Decreased excitatory synaptic activity during development often leads to pre- and postsynaptic atrophy, as assessed anatomically. The present study considers the effect of decreased excitatory transmission on the maturation of synaptic strength. Towards this end, cochlear nucleus neurons, which project to the ipsilateral lateral superior olive (LSO), were denervated in gerbils at postnatal day 7, before the onset of hearing. This manipulation was intended to disrupt spontaneous glutamatergic transmission in the LSO while sparing the glycinergic afferents from the medial nucleus of the trapezoid body (MNTB). Afferent-evoked synaptic activity was assessed 1-6 days after ablation in a brain slice preparation using whole-cell current- and voltage-clamp recordings. In control animals, ipsilaterally evoked excitatory postsynaptic potentials (EPSPs) were present in 91% of neurons tested, but were observed in only 60% of neurons following cochlea removal. The maximum EPSP amplitude was significantly smaller in manipulated neurons compared with controls, and this was accompanied by a higher incidence of ipsilaterally evoked inhibitory postsynaptic potentials (IPSPs). To study the efficacy of excitatory synapses in greater detail, voltage-clamp recordings were made in the presence of strychnine and AP-5 [D(O)-2-amino-5-phosphonopentanoic acid]. The minimum excitatory postsynaptic current (EPSC) amplitude, presumed to reflect the efficacy of a single glutamatergic afferent, was approximately 40% smaller in manipulated neurons. In contrast, MNTB-evoked IPSPs were similar in neurons from control and ablated animals. However, manipulated neurons often exhibited a rebound depolarization after a hyperpolarizing current pulse or an afferent-evoked IPSP. In 70% of manipulated neurons, synaptically evoked rebound depolarizations were reduced, but not eliminated, by glutamate receptor antagonists. The glycine receptor antagonist strychnine did eliminate the IPSP-associated depolarization in these neurons. Collectively, these results suggest that functional denervation of excitatory afferents decreases their synaptic efficacy as result of both cell loss as well as decreased strength of individual surviving synapses.

Age-related change in short-term synaptic plasticity intrinsic to excitatory striatal synapses.

Aging disrupts the expression of synaptic plasticity in many central nervous system (CNS) structures including the striatum. We found age differences in paired-pulse plasticity to persist at excitatory striatal synapses following block of gamma aminobutyric acid (GABA)A and GABA(B) receptors, a property that was independent of the number of afferents activated. High Mg2+/low Ca2+ artificial cerebral spinal fluid (ACSF) reduced release probability and consequently the size of the evoked excitatory post-synaptic potential (EPSP). High Mg2+/low Ca2+ ACSF also increased the expression of paired-pulse facilitation and eliminated the age difference seen previously in normal ACSF. These data suggest that age differences in paired-pulse plasticity reflect an alteration in release probability at excitatory striatal synapses. In support of this hypothesis, we found age differences in another presynaptic form of plasticity referred to as synaptic augmentation. Examination of the synaptic depression that developed during the conditioning tetanus also revealed an age-related increase in synaptic depression. These data indicate that age-related changes in facilitation may be due in part to a reduction in the readily releasable pool of synaptic vesicles. Dendritic structure (spine density and dendritic length) was correlated with short-term synaptic plasticity, but these relationships depended upon the variance associated with age (hierarchical regression). Post-hoc within-age group regressions demonstrated relationship between spine density and paired-pulse plasticity. No other age-specific correlations were found. These findings imply an age-dependent association between altered dendritic morphology and changes in synaptic plasticity.

Excitatory connections between neurons of the central cervical nucleus and vestibular neurons in the cat.

The central cervical nucleus (CCN) of the cat receives input from upper cervical muscle afferents, particularly primary spindle afferents. Its axons cross in the spinal cord, and while in the contralateral restiform body give off collaterals to the vestibular nuclei. In order to study the connections between CCN axons and vestibular neurons, we stimulated the area of the CCN in decerebrate cats while recording intra- or extracellularly from neurons in the contralateral vestibular nuclei. CCN stimulation evoked excitatory postsynaptic potentials (EPSPs) or extracellularly recorded firing in the lateral, medial and descending vestibular nuclei. The latency of EPSPs (mean 1.6 ms) was on average 0.4 ms longer than the latency of antidromic spikes evoked in the CCN by stimulation of the contralateral vestibular nuclei (mean 1.2 ms), demonstrating that the excitation was typically monosynaptic. The results provide further evidence that the CCN is an important excitatory relay between upper cervical muscle afferents and neurons in the contralateral vestibular nuclei.

809 Layer-specific no release in the rat auditory cortex

Intracellular recordings were made from CA1 pyramidal neurones of hippocampus maintained in organotypic culture. Both spontaneous interictal and ictal epileptiform activity was observed. CNQX, an antagonist at kainate/quisqualate but not at N-methyl-d-aspartate (NMDA)-sensitive excitatory amino acid receptors depressed but did not abolish spontaneous epileptiform activity. Addition of the specific NMDA receptor antagonist d-2-amino-5-phosphonovalerate (d-APV) abolished the remaining activity. Similar effects were observed on electrically evoked excitatory post synaptic potentials (EPSPs). This suggests a role for endogenous excitatory amino acids acting at both kainate/quisqualate and NMDA sensitive excitatory amino acid receptors in the generation and maintainance of epileptiform activity within these organotypic cultures.

Muscarine receptor activation in the substantia gelatinosa of the spinal trigeminal nucleus of the guinea pig.

1. Intracellular recordings were made from slices of guinea pig spinal trigeminal nucleus pars caudalis (SG). 2. Muscarine [0.3-30 microM; half maximally effective concentration (EC50) = 2.9 microM] hyperpolarized 61% of SG neurons. The effect was mimicked by carbachol (0.3-30 microM; EC50 = 3.9 microM) and antagonized by pirenzepine (1 microM). Thirty-four percent of the neurons were depolarized by muscarine and carbachol (1-30 microM: EC50 = 5.7 microM), and the effect was antagonized by pirenzepine (100 nM). 3. In approximately 80% of recordings, muscarine (10-30 microM) evoked repetitive spontaneous inhibitory postsynaptic potentials (IPSPs) that were sensitive to bicuculline (10 microM). 4. Muscarine (1-30 microM; EC50 = 3 microM) decreased the amplitude of the majority of evoked excitatory postsynaptic potentials (EPSPs), and the effect was mimicked by carbachol and antagonized by pirenzepine (100 nM). 5. These results indicate that there are at least three mechanisms by which muscarine inhibits SG neurons: 1) hyperpolarization through activation of non-M1 receptors; 2) activation of gamma-amino-butyric acid-containing interneurons that mediate IPSPs in a subset of neurons; and 3) a decrease in evoked EPSP amplitude. Muscarine can also activate SG neurons via interaction with an M1-type receptor.

Cholinergic and GABAergic receptors on fly tangential cells and their role in visual motion detection.

1. To identify some of the neurotransmitters involved in the processing of visual motion information the pharmacology of transmitter receptors on motion-sensitive visual interneurons (VS and HS cells) was investigated in an in vitro preparation of the blowfly (Calliphora erythrocephala) brain. Cholinergic and GABAergic drugs were applied in the bath and iontophoretically while recording intracellularly from HS and VS cells. 2. Bath-applied carbachol (10 and 100 microM) leads to a depolarization in HS and VS cells. One micromolar nicotine also has a depolarizing effect. Both agonists are effective in 0 Ca2+/high Mg(2+)-saline, too, which isolates the cells synaptically. The muscarinic agonists pilocarpine and oxotremorine have no effects on the membrane potential. 3. Iontophoretic application of acetylcholine, carbachol, and nicotine depolarizes VS and HS cells. The iontophoretic carbachol response is antagonized by alpha-bungarotoxin (EC50 = 0.19 microM), mecamylamine (EC50 = 0.32 microM), d-tubocurarine (EC50 = 9.5 microM), and bicuculline but not by decamethonium and scopolamine. 4. Bath application of muscimol strongly hyperpolarizes VS cells in normal fly saline. The gamma-aminobutyric acid-C (GABAC)-receptor agonist cis-4-aminocrotonic acid (CACA) has no effects. The hyperpolarizing response to iontophoretic applied muscimol is present in 0 Ca2+/high Mg2+ saline as well as in Co(2+)-containing saline. The muscimol response is reduced in low chloride saline and thus chloride sensitive. The muscimol response is blocked by picrotoxinin (EC50 = 3.4 microM) but not by the GABAA receptor antagonist bicuculline. 5. Taken together the primary responses of the lobula plate tangential cells appear to be nicotinic cholinergic and GABAergic. 6. The pharmacology of natural synaptic input to VS cells was investigated by extracellular electrical stimulation of the medulla. Such evoked excitatory postsynaptic potentials (EPSPs) are blocked reversibly in 0 Ca2+/high Mg2+ saline. The nicotinic antagonists mecamylamine (1 microM) and d-tubocurarine (50-100 microM) abolish or diminish the EPSPs, respectively. 7. The pharmacological data are incorporated into a semicellular model of a visual motion detector favoring a role of lobula plate tangential cells in certain steps of visual motion processing. Cholinergic and GABAergic inputs are an ideal cellular implementation of a linear subtraction of the signals arising from local motion-sensitive elements with opposite preferred direction. Such a mechanism enhances direction-selectivity and, together with dendritic integration, increases the sensitivity of the tangential cells for wide-field motion.

Group I extensor afferents evoke disynaptic EPSPs in cat hindlimb extensor motorneurones during fictive locomotion.

1. Intracellular recording from extensor motoneurones in paralysed decerebrate cats was used to examine the distribution of short-latency non-monosynaptic excitation by group I afferents during fictive locomotion produced by stimulation of the mesencephalic locomotor region (MLR). 2. During the extension but not the flexion phase of fictive locomotion, stimulation of ankle extensor nerves at 1.2-2.0 times threshold evoked excitatory postsynaptic potentials (EPSPs) in motoneurones innervating hip, knee and ankle extensors. Disynaptic EPSPs were also evoked by selective activation of group Ia muscle spindle afferents by muscle stretch. 3. The central latencies of these group I-evoked EPSPs (mean, 1.55 ms) suggest their mediation by a disynaptic pathway with a single interneurone interposed between extensor group I afferents and extensor motoneurones. Disynaptic EPSPs were also evoked during periods of spontaneous locomotion following the cessation of MLR stimulation. 4. Hip extensor motoneurones received disynaptic EPSPs during extension following stimulation of both homonymous and ankle extensor nerves. Stimulation of hip extensor nerves did not evoke disynaptic EPSPs in ankle extensor motoneurones. 5. The appearance of disynaptic EPSPs during extension appears to result from cyclic disinhibition of an unidentified population of excitatory spinal interneurones and not postsynaptic voltage-dependent conductances in motoneurones or phasic presynaptic inhibition of group I afferents during flexion. 6. The reorganization of group I reflexes during fictive locomotion includes the appearance of disynaptic excitation of hip, knee and ankle extensor motoneurones. This excitatory reflex is one of the mechanisms by which group I afferents can enhance extensor activity and increase force production during stance.

Pharmacological properties of peripherally induced postsynaptic potentials in bulbar respiratory neurons of decerebrate cats

Intracellular recordings of bulbar inspiratory and post-inspiratory neurons, combined with extracellular iontophoresis of antagonists of putative neurotransmitters, were performed in decerebrate cats. Inhibitory postsynaptic potentials (IPSPs) evoked by stimulation of the superior laryngeal nerve or vagus nerve were depressed by bicuculline in all 22 neurons tested, but not modified by strychnine. The non- N-methyl- d-aspartate (NMDA) glutamate antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the neurally evoked excitatory postsynaptic potentials (EPSPs) in 23 out of 26 neurons tested, while the NMDA antagonist dizocilpine had no notable effect. The present results suggest that the peripherally induced IPSPs are mediated through γ-aminobutyric acid (GABA) A receptors and the EPSPs through non-NMDA glutamate receptors in bulbar respiratory neurons.

Block of glutamate decarboxylase decreases GABAergic inhibition at the crayfish synapses: possible role of presynaptic metabotropic mechanisms.

1. The cytosolic concentration of a neurotransmitter is believed to be an important factor determining its release. The effects of 3-mercaptopropionic acid (MP) and aminooxyacetic acid (AOAA), glutamate decarboxylase (GAD) blockers, on GABAergic postsynaptic and presynaptic inhibitory neurotransmission were examined in the crayfish (Procambarus clarkii) opener neuromuscular synapses. 2. Intracellular recordings of evoked excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) as well as loose macropatch clamp measurements of excitatory postsynaptic currents (EPSCs) and inhibitory postsynaptic currents (IPSCs) were used to evaluate the effects of the drugs, which were applied exclusively to the nerve bundle. 3. Under normal conditions, a stimulus train to the inhibitor preceding the excitor stimulation elicited a large reduction in EPSP amplitude in a time interval-dependent manner. This inhibition is effected by postsynaptic as well as presynaptic processes. 4. Treatment with MP or AOAA decreased the IPSP amplitude and its altered conductance but had no effect on the IPSP reversal potential or the resting potential of the cell. They did, however, slightly increase the Rin of the fiber. 5. Quantal analysis of single IPSCs revealed that GAD blockers increased the number of failures and thus reduced quantal content (m), diminished the probability of release (p), but did not affect the quantum current (q) or the statistical parameter (n), believed to be the number of available active zones. 6. Quantal analysis of EPSCs, released after interaction with the inhibitor, revealed a reduction in m without any effect on q. GAD blockers greatly reduced the efficacy of this inhibition without affecting the EPSC q. 7. GAD blockers increased the output of the excitor release sites by the following mechanisms: 1) increased EPSC, 2) increased EPSC facilitation, or 3) enhancement of spontaneous activity (miniature EPSCs). 8. Short time incubation with picrotoxin and CGP-35348 eliminated IPSCs and evoked inhibition. However, longer exposure (90 min) increased the excitor responses, similarly to the effects of GAD blockers. 9. Baclofen, a gamma-aminobutyric acid-B (GABAB) agonist, antagonized AOAA effects on evoked inhibition. 10. These results demonstrate that GAD blockers decrease postsynaptic and presynaptic inhibition by reducing both tonic and evoked release, most likely by diminishing p. 11. The reduction in GABA synthesis and release revealed a complex mechanism for GABAergic metabotropic regulation of inhibition efficacy and the release from the excitor glutamatergic terminals.

Shared calcium signaling pathways in the induction of long-term potentiation and synaptic disinhibition in CA1 pyramidal cell dendrites

1. Calcium signaling pathways were examined in the induction of long-term synaptic disinhibition following tetanization. Effects of tetanization on gamma-aminobutyric acid-A (GABAA receptor-mediated inhibitory responses were measured and compared with excitatory responses under experimental conditions previously used for examining induction mechanisms of N-methyl-D-aspartate (NMDA)-dependent long-term potentiation (LTP). Intracellular recordings were performed in current-clamp and discontinuous single-electrode voltage-clamp (dSEVC) modes in CA1 pyramidal cell apical dendrites in hippocampal slices of adult guinea pigs with the use of sharp electrodes. Test pulses and tetanic stimuli were applied to the Schaffer collateral fibers in stratum radiatum. 2. Under standard control conditions [3 M K Ac in the recording pipette and artificial cerebrospinal fluid as extracellular solution], tetanization-induced sustained increases of excitatory responses were accompanied by marked decreases of parameters of GABAA-mediated synaptic inhibition: at 40 min after tetanization [posttetanus 40 (PT 40)], orthodromically evoked excitatory postsynaptic potential (EPSP) peak amplitudes were on average 195 +/- 15% (mean +/- SE) and excitatory postsynaptic currents (IPSPs) were 166 +/- 10% of pretetanus controls. Peak amplitudes of orthodromically evoked inhibitory postsynaptic potentials (IPSPs) were 30 +/- 5% and inhibitory postsynaptic currents (IPSCs) were 21 +/- 4% at PT 40. Synaptic GABAA conductances (measured as chord conductances) were reduced to 22 +/- 4% at PT 40. Iontophoretic GABAA responses measured as conductance changes were 28 +/- 4% of pretetanus controls at PT 40. 3. A role of NMDA receptors in induction of long-term synaptic disinhibition was tested by preventing NMDA receptor activation 1) by pharmacological means and 2) by holding the membrane clamped at -80 mV (in dSEVC) during tetanization. In the presence of the NMDA-receptor antagonist D-2-amino5-phosphonopentanoic acid (D-AP5) 10-40 microM), orthodromically evoked EPSP amplitudes were 107 +/- 9%, EPSCs were 104 +/- 6%, GABAA-mediated IPSPs were 88 +/- 8%, IPSCs were 97 +/- 8%, synaptic GABAA conductances were 84 +/- 9%, and iontophoretic GABAA conductances were 102 +/- 13% at PT 40. In recordings in which the dendritic membrane potential was clamped at -80 mV during tetanization, orthodromically evoked peak amplitudes of EPSPs were 105 +/- 11%, EPSCs were 102 +/- 8, IPSPs were 103 +/- 4%, IPSCs were 102 +/- 5%, GABAA chord conductances were 101 +/- 9%, and iontophoretically evoked GABAA conductances were 105 +/- 5% at PT 40. 4. In recordings in which the intracellular pipette was preloaded with the Ca2+ chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N'N"-tetraacetic acid (BAPTA) (5mM), long-term changes of synaptic transmission (increases of excitation, decreases of synaptic inhibition) were prevented. At PT 40, EPSP peak amplitudes were 93 +/- 7%, EPSCs were 115 +/- 6%, IPSPs were 115 +/- 9%, IPSCs were 117 +/- 8%, and synaptic GABAA conductances were 108 +/- 17%. Iontophoretic conductances at PT 40 were 109 +/- 9% over pretetanus controls when recorded with BAPTA-containing electrodes. 5. In recordings in which the intracellular pipette was preloaded with cypermethrin, a potent and selective inhibitor of phosphatase 2B, respective long-term changes of synaptic transmission (increases of excitation, decreases of synaptic inhibition) were prevented. At PT 40, EPSP peak amplitudes were 98 +/- 6%, EPSCs were 105 +/- 10%, IPSPs were 99 +/- 5%, IPSCs were 104 +/- 7%, synaptic GABAA conductances were 97 +/- 6% and iontophoretic GABAA conductances were 113 +/- 18% over pretetanus controls in cypermethrin-containing recordings. 6. In conclusion, the data presented demonstrate shared cellular pathways in the induction of both LTP and long-term synaptic disinhibition in apical dendrites of CA1 pyramidal cells after tetanization of the Schaffer collaterals.

Chronic corticosterone treatment maintains synaptic activity of CA1 hippocampal pyramidal cells: acute high corticosterone administration increases action potential number.

The hypothalamic-pituitary-adrenocortical (HPA) axis controls the levels of plasma corticosterone (CT) in the rat and the levels of cortisol in man. CT is important in maintaining homeostasis and regulating energy production. Homeostasis is maintained by basal activation of the hippocampal-HPA axis. In response to stress CT secretion is increased. CT activation of receptors in the hippocampus provides feedback inhibition of the HPA axis to return the system to basal activity. There are two types of CT receptors: the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). CT has a 10-fold higher affinity for MR than GR. Normal basal levels of CT occupy the majority of the MR. During the diurnal surge of CT and following the presentation of a stressful stimulus, the MR and GR are both maximally occupied. To begin to understand how CT influences the hippocampal-HPA axis, intracellular recording techniques in the hippocampal brain slice preparation were used to determine how high concentrations of CT may alter cell characteristics and/or evoked synaptic activity. Two treatment groups were used, i.e., adrenalectomized (ADX) and ADX with CT pellet replacement (ADX+CT) that produced plasma blood levels equal to that seen in a normal rat in the morning. Acute administration of 100 nM CT decreased action potential threshold and the number of action potentials elicited by a depolarizing current pulse in cells from both the ADX and ADX+CT treated rats. The amplitude of the evoked excitatory postsynaptic potentials (EPSP) or inhibitory postsynaptic potentials (IPSP) declined in cells recorded from ADX animals and ADX rats acutely treated with high concentrations of CT (ADX/CT).(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression by memantine and amantadine of synaptic excitation intrastriatally evoked in rat neostriatal slices

The competitive N- methyl- d-aspartate (NMDA) receptor antagonists, dl-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849) and D(−)-2-amino-5-phosphonovaleric acid (APV), and the non-competitive NMDA antagonists, memantine and amantadine, which are used in the treatment of Parkinson's disease, were tested for their effects on intrastriatally evoked excitatory postsynaptic potentials (EPSPs) in rat neostriatal slices. Fast, non-NMDA receptor mediated synaptic excitation was not affected by any of the NMDA receptor antagonists. The NMDA component of the EPSPs was more prominent following reduction of the non-NMDA component of the EPSP by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5–10 μM). Memantine (30 μM) and amantadine (100 μM) had similar effects in reducing the NMDA component, but were not as effective as CGP 37849 (1–5 μM) or APV (10 μM). The data are compatible with a possible locus of action of memantine and amantadine in the neostriatum.

Altered desensitization produces enhancement of EPSPs in neocortical neurons

1. Neocortical brain slices were prepared from rats (35-50 days of age) and maintained in vitro. Intracellular recordings were obtained from neurons in cortical layers II/III. The effect of bath application of cyclothiazide (CYZ), a potent blocker of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor desensitization, on evoked synaptic activity and passive membrane properties was investigated. 2. Bath application of CYZ did not significantly affect resting membrane potential, input resistance, or repetitive firing. CYZ increased both the amplitude and duration of evoked excitatory postsynaptic potentials (EPSPs). Polysynaptic responses were also augumented. These effects persisted after the blockade of N-methyl-D-aspartate (NMDA) receptors with D-2-amino-5-phosphonovaleric acid (D-APV). The magnitude of these effects appeared to vary directly with stimulation intensity and presumably, amount of glutamate release. 3. Epileptiform activity was induced by bath application of bicuculline methiodide. The amplitude and duration of evoked paroxysmal discharges were increased by CYZ. Similar results were seen in presence of D-APV. 4. These results indicate that CYZ has significant effects on synaptic transmission. Desensitization of non-NMDA receptors may be an important mechanism for determining the time course of EPSPs and in curtailing epileptiform responses in the rat neocortex.

Segregation of cerebrorubral and cerebellorubral synaptic inputs on rubrospinal neurons of fetal cats as demonstrated by intracellular recording

Cerebrorubral and cerebellorubral inputs are localized to distal dendrites and somata of red nucleus neurons in adult cats, respectively. To examine if this segregation is established early in development, we performed intracellular recording from rubrospinal neurons of fetal cats aged from embryonic day 58 to 65. Stimulation of the contralateral cerebellar nuclei evoked excitatory postsynaptic potentials (EPSPs). EPSPs were also induced by stimulation of the ipsilateral pericruciate cortex but they were much slower in time course and smaller in amplitude compared to cerebellar ones. We suggest that cerebrorubral and cerebellorubral synapses are segregated on soma-dendritic membrane of rubrospinal neurons early in development.

Characteristics of spontaneous and evoked EPSPs recorded from dentate spiny hilar cells in rat hippocampal slices

1. Excitation of the spiny subtype of hilar neurons in the fascia dentata was characterized by intracellular recording from hilar cells in hippocampal slices. Stimulation of the outer molecular layer was used to activate the perforant path. Evoked responses were examined, as well as the large spontaneous excitatory potentials that are a distinctive characteristic of spiny hilar cells. 2. Excitatory potentials that occurred spontaneously, as well as those that occurred in response to outer molecular layer stimulation, were similar among the cells that were sampled, regardless of morphological variations such as the presence or absence of thorny excrescences. Spontaneous and evoked excitatory postsynaptic potentials (EPSPs) were complex depolarizations that often had several discrete peaks. Spontaneous EPSPs increased in amplitude slightly with hyperpolarization, and evoked EPSPs clearly increased with hyperpolarization. 3. Applications of selective antagonists of excitatory amino acid receptors were used to determine which excitatory amino acid receptor mediates EPSPs of these cells. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was used to block the receptor subtype selective for the agonists alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid (the "AMPA/kainate" receptor). 2-amino-5-phosphonovaleric acid (APV) was used to block receptors specific for the agonist N-methyl-D-aspartate (NMDA; the "NMDA" receptor). Perfusion with CNQX (5-25 microM) completely blocked all spontaneous and evoked excitation, even when activity was examined at relatively depolarized membrane potentials and a low concentration of extracellular magnesium (0.5 mM) was used. Under these conditions, APV (25-50 microM) had no detectable effect on spontaneous activity but did increase the stimulus strength required to elicit responses to outer molecular layer stimulation. 4. When extracellular magnesium was lowered to 0 mM (nominally), there was strong evidence for a contribution of NMDA receptors to spontaneous and evoked EPSPs. Thus, when cells were perfused with 0 mM extracellular magnesium and 5 microM CNQX, spontaneous depolarizations were present and EPSPs could be triggered by stimulation of the outer molecular layer. Both the spontaneous and evoked EPSPs were blocked by 25 microM APV. 5. Because gamma-aminobutyric acid (GABA)A receptors can cause depolarizations in hippocampal neurons, the GABAA receptor antagonist bicuculline was used to determine whether some of the EPSPs were mediated by GABAergic neurons that are normally activated by spontaneous release of excitatory amino acids. Bicuculline (5-25 microM) had no effect on spontaneous depolarizations, and led to an enhancement of evoked depolarizations. Therefore it does not appear that GABAA receptor-mediated depolarizations contribute to hilar cell depolarizations.(ABSTRACT TRUNCATED AT 400 WORDS)

Trigeminal excitation of dorsal neck motoneurones in the cat.

Excitation of dorsal neck motoneurones evoked by electrical stimulation of primary trigeminal afferents in the Gasserian ganglion has been investigated with intracellular recording from alpha-motoneurones in the cat. Single stimulation in the Gasserian ganglion ipsi- and contralateral to the recording side evoked excitatory postsynaptic potentials (EPSPs) in motoneurones innervating the lateral head flexor muscle splenius (SPL) and the head elevator muscles biventer cervicis and complexus (BCC). The gasserian EPSPs were composed of early and late components which gave the EPSPs a hump-like shape. A short train of stimuli, consisting of two to three volleys, evoked temporal facilitation of both the early and late EPSP components. The latencies of the gasserian EPSPs ranged from 1.6 to 3.6 ms in SPL motoneurones and from 1.6 to 5.8 ms among BCC motoneurones. A rather similar latency distribution between 1.6 and 2.4 ms was found for ipsi- and contralateral EPSPs in SPL and BCC motoneurones, which is compatible with a minimal disynaptic linkage between primary trigeminal afferents and neck motoneurones. Systematic transections of the ipsi- and contralateral trigeminal tracts were performed in the brain stem between 3 and 12 mm rostral to the level of obex. The results demonstrate that both the ipsi- and contralateral disynaptic and late gasserian EPSPs can be mediated via trigeminospinal neurones which take their origin in the nucleus trigeminalis spinalis oralis. Transection of the midline showed that the contralateral trigeminospinal neurones cross in the brain stem. Systematic tracking in and around the ipsilateral trigeminal nuclei demonstrated that the axons of ipsilateral trigeminospinal neurones descend just medial to and/or in the medial part of the nucleus. Spinal cord lesions revealed a location of the axons of the ipsilateral trigeminospinal neurones in the lateral and ventral funiculi. Interaction between the ipsi- and contralateral gasserian EPSPs showed complete summation of the disynaptic EPSP component, while the late components were occluded by about 45%. These results show that the disynaptic EPSPs are mediated by separate trigeminospinal neurones from the ipsi- and contralateral side, while about half of the late EPSPs are mediated by common neurones which receive strong bilateral excitation from commissural neurones in the trigeminal nuclei. Spatial facilitation was found in the late gasserian EPSP but not in the disynaptic gasserian EPSP by conditioning stimulation of cortico- and tectofugal fibres. Disynaptic pyramidal and tectal EPSPs, which are mediated by reticulospinal neurones, were facilitated by a single stimulation in the gasserian ganglion at an optimal interval of 2 ms.(ABSTRACT TRUNCATED AT 400 WORDS)

Remote Control of the Swimmeret Central Pattern Generator in Crayfish (Procambarus Clarkii and Pacifastacus Leniusculus): Effect of a Walking Leg Proprioceptor

ABSTRACT An isolated preparation of the crayfish nervous system, comprising both the thoracic and the abdominal ganglia together with their nerve roots, has been used to study the influence of a single leg proprioceptor, the coxo-basal chordotonal organ (CBCO), on the fictive swimmeret beating consistently expressed in this preparation. Both mechanical stimulation of the CBCO and electrical stimulation of its nerve were used. In preparations not displaying rhythmic activity, electrical or mechanical stimulations evoked excitatory postsynaptic potentials (EPSPs) in about 30% of the studied motor neurones with a fairly short and regular delay, suggesting an oligosynaptic pathway. Such stimulation could evoke rhythmic activity in swimmeret motor nerves. The evoked swimmeret rhythm often continued for several seconds after the stimulus period. When the swimmeret rhythm was well established, electrical and mechanical stimuli modified it in a number of ways. Limited mechanical or weak electrical stimuli produced a small increase in swimmeret beat frequency, while more extreme movements of the CBCO or strong electrical stimuli had a disruptive effect on the rhythm. The effect of low-intensity stimulation on existing swimmeret beating was phase-dependent: it shortened the beat cycle when applied during the powerstroke phase and lengthened it when applied during the retumstroke phase. Rhythmic mechanical stimulation of CBCO or electrical stimulation of the CBCO nerve entrained the swimmeret rhythm within a limited range in relative or absolute coordination. Electrical stimuli also produced systematic effects on the whole metachronal pattern of the swimmeret rhythm, perturbing the interganglionic coordination.