Simón Brailowsky

Epileptogenic γ-aminobutyric acid-withdrawal syndrome after chronic, intracortical infusion in baboons

We studied the effects of chronic (7 days) infusion of GABA (100 micrograms/microliter) applied intracortically into the fronto-rolandic (FR) area of baboons, via osmotic minipumps. In photosensitive animals, bilateral GABA application produced a complete blockade of the paroxysmal discharges and associated clinical signs induced by intermittent light stimulation. Unilateral administration had similar effects, although these developed more gradually. At the end of the infusion period, both photosensitive and non-photosensitive animals showed a transitory state (3-4 days) of cortical hyperexcitability (spontaneous epileptogenic activity) localized to the infused area. The data indicate a role of GABA both in the natural photosensitivity of the epileptic baboon and in the withdrawal syndrome consecutive to the sudden interruption of chronically enhanced GABA levels in the FR territories of this monkey.

Decrease of glutamate decarboxylase activity after in vivo cortical infusion of γ-aminobutyric acid

gamma-Aminobutyric acid (GABA) levels and the activity of glutamate decarboxylase were measured in homogenates of rat brain cortical tissue, at different times after chronic intracortical infusion of GABA in vivo during 2, 6 or 24 h. Cortical electrical activity was also recorded. As previously described, about 1 h after cessation of the infusion epileptic discharges were observed (GABA-withdrawal syndrome), which lasted for several days. At zero time after cessation of the infusion, before the appearance of seizures, GABA levels were increased 3-6-fold and glutamate decarboxylase activity was decreased 27-48% in the infused cortex, as compared to the contralateral cortex or to tissue from control intact rats. During epileptic discharges GABA levels gradually returned to normal values. In contrast, glutamate decarboxylase activity remained decreased during seizures and returned to normal only after recovery from the GABA-withdrawal syndrome. These results suggest that the persistent decrease in the activity of the decarboxylase is due probably to a lowered amount of the enzymatic protein, occurring as a consequence of a temporarily elevated intracellular GABA concentration. The decreased rate of GABA synthesis might be involved in the pathophysiology of the GABA-withdrawal syndrome.

Hyperexcitability induced by GABA withdrawal facilitates hippocampal long-term potentiation.

In some mammals, epileptic seizures have been induced in the cerebral cortex, hippocampus and other limbic structures after the sudden suppression of chronically infused GABA. This hyperexcitability state induced by the endogenous neurotransmitter resembles the withdrawal seizure-responses to other GABA(A) receptor agonists such as benzodiazepines, barbiturates and alcohol. Hyperexcitability induced by GABA withdrawal also persists in in vitro preparation. Hippocampal slices, obtained from rats with seizures induced by GABA-withdrawal showed field potential oscillations and paroxysmal activity in the Ammons horn region 1. During GABA-withdrawal hyperexcitability the threshold of hippocampal long-term potentiation (LTP) decreased to a point in which a brief frequency stimulation that normally failed to produce long lasting changes in synaptic strength, was now able to induce LTP. Facilitation of the LTP induction was associated with a decreased GABA(A)-mediated inhibitory activity, because the effect of the GABA(A) receptor antagonist, bicuculline, was occluded during hyperexcitability and the dose-response curve for bicuculline showed a 50% efficacy reduction with a shift in the effective concentration required for half-maximal activation from 4.5-1.1 microM relative to controls. Nevertheless, the dissociation constant of the antagonist did not change significantly. Our results support the idea that changes in hippocampal plasticity under altered inhibitory neurotransmission states, like those induced by withdrawal syndromes to anxiolytic, sedative or anticonvulsant drugs may be engaged during seizures.

Hippocampal hyperexcitability induced by GABA withdrawal is due to down-regulation of GABAA receptors

The sudden interruption of an intracortical instillation of exogenous gamma-aminobutyric acid (GABA) generates an epileptic focus in mammals. Seizures elicited by GABA withdrawal (GW) last for weeks. A similar withdrawal-induced hyperexcitability is also produced by several GABA(A) receptor agonists. This work reports a quantitative analysis of GW-induced hyperexcitability produced in the hippocampus in vitro. GW produced a left-ward displacement of the input/output (I/O) function, suggesting that the postsynaptic component is predominant to explain the hyperexcitability. A decrease in the inhibitory efficacy of the GABA(A) receptor agonist, muscimol, confirmed that inhibition was impaired. Binding saturation experiments demonstrated a decrease in [(3)H]-muscimol binding after GABA withdrawal showing a close correlation with the development of hyperexcitability. All these modifications coursed without changes in receptor affinity (K(D)) for muscimol or bicuculline as demonstrated by both binding studies and Schild analysis. It is concluded that, in the CA1 region of the hippocampus, it is the number of functional GABA(A) receptors, and not the affinity of the receptor, what is decreased during GW-induced hyperexcitability.

Allopregnanolone potentiates a GABA-withdrawal syndrome in the rat cerebral cortex.

We have studied the neuromodulatory effect of the neurosteroid 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone-3 alpha-5 alpha P-) in the GABA-withdrawal syndrome (GWS). This is a model of partial epilepsy consisting of an enduring paraoxysmal activity recorded at the site of GABA infusion that depends, for its induction, on GABA receptor activation. Rats were chronically implanted for frontal and occipital EEG recording with infusion cannulae fixed on the somatomotor cortical region. When the neurosteroid was infused after or concurrently with GABA, a potentiation of the GWS (i.e. shorter latency and prolonged duration) was observed. No modifications in EEG activity were detected when allopregnanolone was administered alone or prior to GABA administration. These results indicate a neuromodulatory effect of allopregnanolone, dependent on the presence of GABA at the receptor site.

Chronic infusions of GABA into the medial prefrontal cortex induce spatial alternation deficits in aged rats

It has been proposed that functions associated with the prefrontal cortex could change as a consequence of aging. Previous experiments in young rats have demonstrated that anatomical lesions or chronic GABA infusions into this area produce deficits in spatial delayed alternation tasks. The present study examines the effect of chronic (7 days) GABA or saline infusion into the prefrontal cortex on the performance of delayed alternation task in old rats (24 months). The results suggested that aged rats needed more sessions to acquire the delayed alternation task. GABA infusions into the prefrontal cortex produced deficits in spatial alternation tasks similar to those previously observed in young rats. Performance rapidly recovered after the infusion period. Histological analysis showed similar lesion size in both groups. The results suggest that aged prefrontal cortex and/or related areas participating in the acquisition of the delayed alternation task are more sensitive to aging processes. Furthermore, the prefrontal cortex is important for the retention of a previously learned spatial delayed alternation task. The structures involved in functional recovery from these deficits appear to be fully functional in aged rats.

Susceptibility to focal and generalized seizures in Wistar rats with genetic absence-like epilepsy

The susceptibility to develop cortically induced focal and generalized seizures was examined in Genetic Absence Epilepsy Rats from Strasbourg (GAERS), an inbred strain of Wistar rats with absence epilepsy. A GABA-withdrawal syndrome induced after suppression of a 2-h intracortical GABA infusion was used as a model of focal epileptogenesis: localized cortical discharges appear at the infusion site within 1 h. GAERS were more prone to develop a GABA-withdrawal syndrome than non-epileptic inbred controls and non-selected Wistar rats. After a transient suppression of absence seizures following GABA infusion in GAERS, generalized spike-and-wave discharges and focal spikes were recorded simultaneously in the cortex. GAERS also showed a higher incidence of systemic pentylenetetrazol-induced convulsions at the dose of 25 mg/kg. Higher doses had similar convulsant effects in all groups. In conclusion, the results confirm a genetic susceptibility in GAERS and/or resistance in inbred non-epileptic rats to focal and generalized seizures involving the cortex. Rats with absence epilepsy appear to be more prone to seizures elicited by cortical GABA deficiency.

Long-Lasting Effects of GABA Infusion Into the Cerebral Cortex of the Rat

In electrophysiological terms, experimental models of durable information storage in the brain include long-term potentiation (LTP), long-term depression, and kindling. Protein synthesis correlates with these enduring processes. We propose a fourth example of long-lasting information storage in the brain, which we call the GABA-withdrawal syndrome (GWS). In rats, withdrawal of a chronic intracortical infusion of GABA, a ubiquitous inhibitory neurotransmitter, induced epileptogenesis at the infusion site. This overt GWS lasted for days. Anisomycin, a protein synthesis inhibitor, prevented the appearance of GWS in vivo. Hippocampal and neocortical slices showed a similar post-GABA hyperexcitability in vitro and an enhanced susceptibility to LTP induction. One to four months after the epileptic behavior disappeared, systemic administration of a subconvulsant dose of pentylenetetrazol produced the reappearance of paroxysmal activity. The long-lasting effects of tonic GABAA receptor stimulation may be involved in long-term information storage processes at the cortical level, whereas the cessation of GABAA receptor stimulation may be involved in chronic pathological conditions, such as epilepsy. Furthermore, we propose that GWS may represent a common key factor in the addiction to GABAergic agents (for example, barbiturates, benzodiazepines, and ethanol). GWS represents a novel form of neurono-glial plasticity. The mechanisms of this phenomenon remain to be understood.

Neocortical hyperexcitability after GABA withdrawal in vitro.

The sharp interruption of the intracortical instillation of exogenous gamma-aminobutyric acid (GABA), generates an epileptic focus in mammals. Seizures elicited by GABA withdrawal last several days or weeks. The present work reports that GABA withdrawal-induced hyperexcitability can be produced in vitro: a sudden withdrawal of GABA (5 mM; 120 min) or benzodiazepine (60 microM flunitrazepam) from the superfusion, induced a gradual increase in the amplitude of the evoked population spike (PS) recorded on neocortical slices. PS enhancement reached 150% above the control value 2.5 h after GABA withdrawal. GABA withdrawal-induced hyperexcitability was facilitated by progesterone. PS enhancement induced by GABA withdrawal was associated with an impairment of GABA transmission occurring before epileptiform discharges were fully established. Paired pulse inhibition and evoked [3H]-GABA release appear decreased; suggesting that cortical hyperexcitability as a result of GABA withdrawal involves pre-synaptic changes. Specific muscimol binding decreased during GABA superfusion but recovered after GABA withdrawal. However, the sensitivity of the post-synaptic response to 3alpha-OH-5alpha-pregnan-20-one or allopregnanolone (alloP) was enhanced after GABA withdrawal, suggesting a functional change in the GABA(A) receptors. The changes described may be the cellular correlates of the withdrawal syndromes appearing after interruption of the administration of GABA(A) receptor agonists.

Effects of GABAB receptor antagonists on two models of focal epileptogenesis

The acute effects of two GABAB receptor antagonists (phaclofen and CGP-35348) were studied in two types of epileptogenic activity: that produced by intracortical injections of baclofen and that appearing after withdrawal of chronic intracerebral GABA infusion (the GABA-withdrawal syndrome, GWS). Intracortical baclofen induced two types of electrographic paroxysmal discharges: one consisting of single spike-and-wave (pattern I) and another of polyspike-and-wave patterns (pattern II). Both patterns showed similar latencies and temporal evolution of spike frequency discharges. Phaclofen, applied directly into the baclofen-induced epileptogenic focus, suppressed pattern II but was ineffective in modifying both pattern I and the GWS. CGP-35348, administered systemically, inhibited both patterns I and II. Intracortical microinjection of baclofen or phaclofen in rats showing a GWS had no effect, nor the systematically given CGP 35348. These results indicate a differential participation of GABAB receptors in GABA-related epileptic syndromes of cortical origin.