Barbara Esplin

Reversal of the activity-dependent suppression of GABA-mediated inhibition in hippocampal slices from γ-vinyl GABA (vigabatrin)-pretreated rats

The antiepileptic drug, gamma-vinyl GABA (GVG, vigabatrin), is an irreversible inhibitor of GABA-transaminase, the enzyme responsible for the breakdown of GABA. In hippocampal slices prepared from rats pretreated with either an anticonvulsant dose of GVG (1500 mg/kg) or saline, electrophysiological recordings were performed in order to examine the effects of GVG pretreatment on GABAergic neurotransmission. Although GVG had no effect on the effectiveness of GABA-mediated inhibition when elicited by a single stimulus, it reversed the activity-dependent depression of inhibition which is typically observed when inhibitory pathways are activated repetitively by a train of stimuli delivered at low frequency. Similarly, GVG pretreatment prevented the progressive decline in the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) during low-frequency stimulation of inhibitory interneurons. Thus, in slices from GVG pretreated rats, the amplitudes of both the fast and slow components of the last of a series of IPSPs evoked by a 5 Hz, 4 s train were maintained at 91.5 +/- 6.6% and 87.7 +/- 6.5%, respectively, compared to 61.1 +/- 3.9% and 57.1 +/- 5.0% in control slices. Finally, in slices from GVG pretreated rats, we observed a reduction in the ability of the GABA(B) receptor agonist, baclofen, to decrease the amplitude of monosynaptic inhibitory postsynaptic currents. These results suggest that GVG may produce its frequency-dependent actions by reducing the function of release regulating presynaptic GABA(B) autoreceptors. The frequency-dependent reinforcement of inhibition by GVG may importantly contribute to the anticonvulsant effectiveness of this compound.

Acute effects of γ-vinyl GABA (vigabatrin) on hippocampal GABAergic inhibition in vitro

The acute effects of γ-vinyl-GABA (GVG) on GABAergic inhibition were investigated in the hippocampal slice preparation using the paired-pulse test of inhibition during extracellular recordings. Superfusion of GVG (100–500 μM) for 60 min resulted in a concentration-dependent decrease in GABAergic inhibition. Slices superfused with higher concentrations of GVG (0.5–1 mM) were hyperexcitable as demonstrated by the appearance of multiple spikes. Binding studies showed that GVG (1 mM) had no effect on the binding of [3H]flunitrazepam or [3H]TBOB and displaced no more than 15% of specific [3H]GABA binding, which indicates that GVG-induced disinhibition is not mediated through an action at the GABAA receptor complex. Consistent with this suggestion is the finding that GVG (500 μM) had little effect on the inhibition of the orthodromically evoked CA1 population spike produced by the GABAA receptor agonist muscimol (10 μM), whereas this inhibition was considerably attenuated by the GABAA receptor antagonist, bicuculline methiodide (5 μM). The results of this study suggest that the acute actions of GVG on the GABAergic neurotransmitter system are not involved in its anticonvulsant effect.

Phencyclidine suppresses hippocampal long-term potentiation through stereospecific activation of phencyclidine receptors.

The effects of phencyclidine and the dioxolane enantiomers, dexoxadrol and levoxadrol, on long-term potentiation in the hippocampus were compared. Field potentials were evoked by stimulation of Schaffer collaterals and recorded from the CA1 region. Long-term potentiation was induced by stimulation with a single train of 25 pulses at 50 Hz. The drugs were delivered by pressure, 1 min before tetanization. Phencyclidine and its receptors ligand, dexoxadrol, abolished the induction of long-term potentiation. Levoxadrol which has very low affinity for the phencyclidine receptor was devoid of this action although it reduced the magnitude of long-term potentiation. These results indicate that phencyclidine blocks long-term potentiation by stereospecific activation of phencyclidine receptors.

Effect of phencyclidine on inhibition in the hippocampal slice.

SummaryThe effects of phencyclidine (PCP) on synaptic transmission were studied in the hippocampal slice. Population spikes evoked by orthodromic or antidromic stimulation were recorded from CAl pyramidal cells. Bath applied PCP (10−4 M) reduced moderately both the orthodromic and antidromic population spikes. Lower concentrations, 5×10−6 to 5×10−5 M of PCP, which did not depress the population spikes, reduced inhibition of the orthodromically evoked spike in a dose dependent reversible manner. Diazepam (10−6 to 10−5 M) restored the inhibition despite the continued presence of PCP. It is suggested that PCP-induced seizures and other signs of hyperexcitability could be a result of reduced inhibition.

Attenuation of hippocampal inhibition by a NMDA (N-methyl-d-aspartate) receptor antagonist

The effects of the competitive NMDA (N-methyl-D-aspartate) receptor antagonist, APV (2-amino-5-phosphonopentanoate; AP5), were examined in the hippocampal slice preparation. APV (50-100 microM) attenuated inhibition of the orthodromically evoked population spikes in the CA1 region produced by a conditioning stimulus to the alveus or to the stratum radiatum. This suggests that NMDA receptors contribute to synaptic activation of the inhibitory interneurons by a single afferent volley.

Effects of cyclobenzaprine on spinal synaptic transmission

Abstract The effects of cyclobenzaprine on spinal synaptic transmission were studied in spinal unanesthetized cats. The drug, in cumulative doses up to 20 mg/kg, depressed monosynaptic reflexes evoked by stimulation at 0.2 Hz to a moderate degree. Polysynaptic reflexes were depressed to a similar extent. Monosynaptic reflexes evoked repeatedly at 5 and 10 Hz were depressed even after 5 mg/kg of cyclobenzaprine. Maximum potentiation of monosynaptic responses after tetanization was not affected. Neither direct postsynaptic nor recurrent inhibition was influenced by the drug. Presynaptic inhibition was slightly reduced. It is suggested that direct effects of cyclobenzaprine at the spinal level do not represent the primary site of action responsible for its antispastic activity.

Frequency-dependent enhancement of hippocampal inhibition by GABA uptake blockers

The effects of GABA uptake inhibitors, SKF 89976A and SKF 100330A, on recurrent inhibition were studied in the rat hippocampal slice preparation by the antidromic-orthodromic stimulation test. Population spikes evoked orthodromically by stimulation of the stratum radiatum and recorded in the CA1 pyramidal cell body layer were inhibited antidromically by stimulation of the alveus by a single pulse or by a train of pulses, either at low or at high frequency. Low frequency train conditioning produced less inhibition than a single pulse. The uptake blockers had no effect or slightly enhanced the inhibition produced by single stimuli or low frequency trains. High frequency train conditioning produced more and much longer inhibition than a single pulse. This inhibition was further substantially enhanced and prolonged by the drugs. Frequency-dependent enhancement of inhibition may be responsible for suppression of epileptiform discharges by GABA uptake blockers.

Disinhibitory effect of phencyclidine in the hippocampus in vitro: PCP receptors implicated

The effects of phencyclidine (PCP) and two dioxolane stereoisomers, dexoxadrol and levoxadrol, on hippocampal inhibition were compared. Field potentials were recorded in the CA1 pyramidal cell layer in the rat hippocampal slices in vitro. Recurrent inhibition of the population spikes evoked orthodromically by stimulation of the Schaffer collaterals was induced by antidromic conditioning stimulation at appropriate time intervals before the orthodromic stimulation. The drugs were applied by micropressure ejection in concentrations which did not affect the unconditioned population spike. After PCP or dexoxadrol administration, the orthodromically evoked population spike was much less reduced by the antidromic conditioning stimulation than before, suggesting that the recurrent inhibition was diminished. Levoxadrol had only negligible effect. Since dexoxadrol has many PCP-like pharmacological properties but levoxadrol does not, we concluded that PCP attenuates hippocampal recurrent inhibition by activating the PCP receptors. It is suggested that this action results in depression of excitatory synaptic transmission from axon collaterals to the inhibitory interneuron with possible involvement of the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor.

Epileptogenic action of penicillin derivatives: structure-activity relationship

In the hippocampal slice preparation, perfusion with benzyl penicillin evokes multiple population spikes and spontaneous discharges. Doses of 0.25 to 2 mM of the drug produced this effect within 20-40 min. Cleavage products and analogues of benzyl penicillin, penicilloic acid, 6-aminopenicillanic acid, cephalexin, thioproline and penicillamine, were devoid of such action. It is concluded that the structural requirements for epileptogenic action of penicillin include not only the beta-lactam ring and side chain substitution on C-6, but also the thiazolidine ring.

The effects of benzodiazepines on spinal homosynaptic depression

Clonazepam (0.5 mg/kg, i.v.) changed the characteristic pattern of the exponential decline of the monosynaptic responses, the early tetanic rundown, evoked by trains of 10 stimuli (2, 5 or 10 Hz) applied to either the biceps-semitendinosus or triceps surae nerve, and recorded from the ventral root in spinal cats. In the case of the biceps-semitendinosus, clonazepam did not affect the first monosynaptic response or the last five monosynaptic responses forming the plateau, while the second monosynaptic response was markedly depressed, especially at the higher frequencies tested. The triceps surae reacted differently to the administration of clonazepam, in that the first response was increased and the amount of depression of the second response was lessened, with no change of the plateau. All the effects of clonazepam were reversed by the benzodiazepine antagonist, ethyl 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a] [1,4]benzodiazepine-3-carboxylate (Ro15-1788; 5 mg/kg, i.v.), which alone had no effect of its own on any parameters, suggesting that the effects of clonazepam were mediated by central benzodiazepine receptors. Diazepam (1.0 mg/kg, i.v.), caused the same changes in the homosynaptic depression of the biceps-semitendinosus pathway as did clonazepam, but increased the plateau instead of the second response in that of the triceps surae pathway.