Evidence for modulation of GABAergic neurotransmission by nicotine

Bath-application of nicotine (800 microM) to mouse hippocampal slices resulted in an increase in the amplitude of the population spike and the appearance of multiple population spikes in the CA1 pyramidal cell layer. Similar effects were observed after perfusion of the GABAA antagonist bicuculline methiodide (2 microM) and the glutamate decarboxylase inhibitor L-C-allylglycine (4 mM). These apparently excitatory effects of nicotine (800 microM) could be reversed by bath-application of gamma-aminobutyric acid (GABA; 400 microM), as well as by the GABA uptake inhibitor nipecotic acid (5 mM) and the benzodiazepine flurazepam (4 microM). Nicotine did not alter binding of [3H]GABA or [3H]flunitrazepam to whole brain plasma membranes. The results are consistent with the hypothesis that the electrophysiological effects of nicotine on CA1 pyramidal cell excitability is mediated by disruption of GABAergic transmission.

Evidence for synergism between the antimyoclonic actions of 5-hydroxytryptophan and clonazepam in rats

The protective effect of the precursor of 5-hydroxytryptamine (5-HT), 5-hydroxytryptophan (5-HTP) against myoclonus induced in rats by picrotoxin and allylglycine was demonstrated. The inhibition by 5-HTP of picrotoxin-induced myoclonic movements was found to correlate well with an increased 5-HT release from the cerebral cortex. p-Chlorophenylalanine (PCPA) pretreatment aggravated the actions of both picrotoxin and allylglycine by shortening their myoclonic latencies. These findings suggest that there is an antimyoclonic effect of 5-HT in the brain. The protective effect of clonazepam against these two myoclonic models was found to be potentiated in 5-HTP-pretreated animals. Only a partial inhibition of its protective effect resulted from PCPA pretreatment. These data suggest that a beneficial synergism is likely to occur between 5-HTP and clonazepam for the inhibition of myoclonus and that a 5-HTergic mechanism does not play a significant role in the antimyoclonic action of clonazepam.

Haem synthesis during cytochrome P-450 induction in higher plants. 5-Aminolaevulinic acid synthesis through a five-carbon pathway in Helianthus tuberosus tuber tissues aged in the dark

Chlorophyll and haem synthesis in illuminated Jerusalem artichoke tuber tissues were very efficiently inhibited by gabaculine (3-amino-2,3-dihydrobenzoic acid). This inhibition seems to be due specifically to a blockade of the pathway for 5-aminolaevulinate biosynthesis which used glutamate as a substrate (the so-called C5 pathway) since we could not detect any inhibition of protein synthesis in the treated tissues and there was no effect of gabaculine on the glycine-dependent yeast 5-aminolaevulinate synthase used as a model. In dark-aged artichoke tissues, gabaculine also effectively blocked cytochrome P-450 induction, peroxidase activity and 5-aminolaevulinic acid synthesis, thus suggesting the involvement of a C5 pathway in cytoplasmic and microsomal haemoprotein synthesis in this higher plant. Allylglycine and (2-amino-ethyloxyvinyl)glycine, two olefinic glycine analogues which are potential suicide inhibitors of pyridoxal phosphate enzymes, were also demonstrated to be effective blockers of chlorophyll synthesis in artichoke tuber and Euglena cells exposed to light.

A new experimental model for drug studies: effects of phenobarbital and phenytoin on photosensitivity in the lateral geniculate-kindled cat

Photosensitivity was acquired as a result of kindling in the lateral geniculate body (GL), and the GL-kindled cat pretreated with DL-allylglycine showed a stable level of photosensitivity. To test the usefulness as a model for the evaluation of anticonvulsant drugs, the effects of phenobarbital (PB) and phenytoin (PHT) on photosensitivity were studied in the GL-kindled cat under DL-allylglycine. PB (5 and 10 mg/kg intravenously, i.v.) completely suppressed photically induced seizures in most subjects at plasma concentrations of 7-16 micrograms/ml, and this anticonvulsant action persisted for at least 4 h after the injection. PHT (15 mg/kg, i.v.) at plasma concentrations of 9-15 micrograms/ml produced toxic signs, e.g., pupil dilatation, hypersalivation, and tachypnea. At this dose, PHT was inactive against photically induced myoclonus but prevented the elicitation of a generalized tonic-clonic convulsion. From these results showing that the effects of anticonvulsant drugs on photically induced seizures can be assessed in relation to plasma concentration and acute neurologic toxicity, we suggest that the GL-kindled cat is a potentially useful animal model of epilepsy for testing the efficacy of anticonvulsant drugs.

Redistribution of transmitter amino acids in rat hippocampus and cerebellum during seizures induced by L-allylglycine and bicuculline: an immunocytochemical study with antisera against conjugated GABA, glutamate and aspartate

The effects of the convulsants L-allylglycine and bicuculline on the distribution of gamma-amino-butyric acid (GABA), glutamate and aspartate in rat brains were assessed immunocytochemically, using antisera raised against glutaraldehyde-protein conjugates of the respective amino acids. In accord with previous biochemical studies of GABA content, L-allylglycine treatment was followed by a decreased immunoreactivity for GABA in the hippocampus and cerebellum, whereas treatment with bicuculline led to an increased immunoreactivity in the hippocampus, but not in the cerebellum. Different cells and zones were affected differentially. With both convulsants the hippocampus showed the most pronounced changes in the neuropil of the pyramidal and granular cell layers. L-Allylglycine treatment led to a substantial decrease in the concentration of detectable GABA-immunoreactive bouton-like dots in the stratum oriens, radiatum and lacunosum-moleculare and in the deep hilar region, but did not produce statistically significant changes in this parameter in the outer and intermediate zones of the dentate molecular layer. In the cerebellum, the decrease in GABA immunoreactivity after L-allylglycine treatment was less in the basket cell terminals than in other GABA-containing elements. Neither convulsant altered the average staining intensity for aspartate or glutamate in the two regions studied, but L-allylglycine reduced the level of aspartate-like immunoreactivity in hippocampal hilar cells. All the changes described were evident after 20 min of seizure activity and were qualitatively similar after 60 min of seizure (animals paralysed and ventilated). Our results indicate that L-allylglycine or bicuculline given intravenously exerts specific effects on cerebral amino acid metabolism. The nature and magnitude of these effects show inter-regional variations and also differ among cellular compartments within each region. Amino acid immunocytochemistry may prove to be a valuable tool for the investigation of metabolic changes associated with epileptic seizures and should be particularly useful in regions showing heterogeneous changes that would tend to cancel each other in biochemical analyses.

Effects of allylglycine on photosensitivity in the lateral geniculate-kindled cat

The effects of DL-allylglycine, an inhibitor of GABA synthesis, on the responses to photic stimulation were studied in the cat kindled in the lateral geniculate body (GL). For 3 to 8 h after the injection of DL-allylglycine at a subconvulsant dose (30 or 40 mg/kg, i.v.), the kindled cat showed a stable level of photosensitivity without any toxic effects and responded with various degrees of myoclonus or a generalized tonic-clonic convulsion when photic stimulation was repeated at hourly intervals. The incidence of photically induced myoclonus reached its plateau during this period. Our results suggest that photosensitivity of the lateral geniculate-kindled cat is related to the modification of GABAergic mechanisms, and that when the GL-kindled cat is pretreated with DL-allylglycine it is a reliable model of photosensitive epilepsy.

The role of GABA and excitatory amino acids in the development of the leptazol-induced epileptogenic EEG

The developing epileptogenic electroencephalogram (EEG), seen during the slow intravenous infusion of leptazol, is sensitive to various anticonvulsant drugs, particularly those known to augment the function of gamma-aminobutyric acid (GABA), such as clonazepam and sodium valproate, which specifically prolong the earlier wave-like (pre-spiking) phases. Thus, whilst antagonism of GABA may be responsible for spiking, the early wave-like phases may be due to GABA released in the cortex as a feedback control to delay spiking. Intravenous infusion of the GABA antagonists, bicuculline and picrotoxin, produced a developing EEG with spiking the first abnormal feature noted and no wave-like phase, like that seen with leptazol. Cortical superfusion of GABA during the infusion of leptazol, enhanced kand prolonged the wave-like phase, whilst bicuculline reduced it. Cortical superfusion of leptazol, picrotoxin or larger concentrations of bicuculline produced spiking but no wave-like activity. When leptazol and GABA were superfused together they produced wave-like activity similar to that seen during infusions of leptazol. Of the excitatory amino acid antagonists, only those active at receptors for N-methyl-D-aspartate (NMDA) influenced the EEG changes induced by leptazol. It is suggested that leptazol produces waves in the EEG by stimulating subcortical pathways to release GABA in the cortex and that spiking occurs as the cortex is further stimulated by GABA antagonism and the release of excitatory amino acids.

Escape behavior produced by the blockade of glutamic acid decarboxylase (GAD) in mesencephalic central gray or medial hypothalamus

Microinjections into the mesencephalic central gray (CG) or the medial hypothalamus (MH) of three drugs (L-allylglycine, Semicarbazide or 4,5 dihydroxy-isophtalic acid) known to block glutamic acid decarboxylase (GAD) produced a dose-dependent behavioral activation accompanied by jumps. These effects are qualitatively similar to those produced by microinjections of SR 95103 (a GABA-A receptor antagonist) at the same site. These findings suggest that, at both the level of the CG and the MH, gamma-aminobutyric acid (GABA) tonically inhibits a neuronal substrate involved in the generation of flight reactions.

On GABAergic mechanisms in the optokinetic nystagmus of the frog: effects of bicuculline, allylglycine and SR 95103, a new GABA antagonist

In a monocular situation, an intravitreal injection of the GABA antagonists, bicuculline or SR 95103 provoked both the suppression of the optokinetic nystagmus (OKN) related to the injected eye and the appearance of a Nasal-Temporal (N-T) component in the OKN triggered by the contralateral non-injected eye (this N-T component being absent in control OKN). These two effects were added in a binocular condition. Similar results were obtained with L-C allylglycine which reduces the endogenous GABA level, but these effects were delayed when compared to those of GABA antagonists. All these data are roughly analogous to those previously obtained with picrotoxin (a non-competitive GABA antagonist) and thus confirm that GABA mechanisms are involved in the control of the frog OKN. Furthermore, SR 95103 acted in this model as a potent selective GABA antagonist, as has been demonstrated in another system.

Involvement of an "antizyme" in the inactivation of ornithine decarboxylase

DL-Allylglycine causes a marked increase in mouse brain ornithine decarboxylase (ODC) activity. The amount of immunoreactive enzyme protein increases concomitantly with the activity, but the enzyme protein decreases more slowly than that of the activity. The amount of immunoreactive ODC in brain is many hundred times that of the catalytically active enzyme. The fact that mouse brain cytosol contains high amounts of dissociable antizyme (an inactivating protein) indicates the existence of an inactive, immunoreactive ODC-antizyme pool. The total antizyme content does not change markedly, but instead there are significant changes in different antizyme pools. Putrescine concentrations start to increase 8 h after treatment with allylglycine and concomitantly with this increase, antizyme is released to inhibit enzyme activity. These results indicate the involvement of antizyme in the inactivation process of ODC.

Strychnine and L-allylglycine but not bicuculline and picrotoxin induce transsynaptic degeneration following transection of the inferior alveolar nerve in adult rats

The effects of the convulsants strychnine, bicuculline, picrotoxin and L-allylglycine on the transsynaptic destruction of medullary dorsal horn neurons were examined following transection of the inferior alveolar nerve in adult rats. Strychnine and L-allylglycine enhanced the transsynaptic effect of nerve transection and caused degeneration of many dorsal horn neurons, while bicuculline and picrotoxin did not. The removal of glycinergic and GABAergic postsynaptic inhibition appears to enhance the transsynaptic destructive activity which follows the peripheral nerve transection.

Changes in regional neurotransmitter amino acid levels in rat brain during seizures induced by L-allylglycine, bicuculline, and kainic acid

Changes in amino acid concentrations were studied in the cortex, cerebellum, and hippocampus of the rat brain, after 20 min of seizure activity induced by kainic acid, 47 mumol/kg i.v.; L-allylglycine, 2.4 mmol/kg i.v.; or bicuculline, 3.27 mumol/kg i.v. in paralysed, mechanically ventilated animals. Metabolic changes associated with kainic acid seizures predominate in the hippocampus, where there are decreases in aspartate (-26%), glutamate (-45%), taurine (-20%), and glutamine (-32%) concentrations and an increase in gamma-aminobutyric acid (GABA) concentration (+ 26%). L-Allylglycine seizures are associated with generalized decreases in GABA concentrations (-32 to -54%), increases in glutamine concentrations (+10 to +53%), and a decrease in cortical aspartate concentration (-14%). Bicuculline seizures, in fasted rats, are associated with marked increases in the levels of hippocampal GABA (+106%) and taurine (+40%). In the cerebellum, there are increases in glutamine (+50%) and taurine concentrations (+36%). These changes can be explained partially in terms of known biochemical and neurophysiological mechanisms, but uncertainties remain, particularly concerning the cerebellar changes and the effects of kainic acid on dicarboxylic amino acid metabolism.

Regional brain glucose metabolism in chemically-induced seizures in the rat

Measurement of regional brain glucose metabolism may give information concerning the mechanism of neuronal cell death developing after prolonged periods of epileptic activity. Regional brain glucose utilization was measured in paralyzed ventilated rats during seizures induced by L-allylglycine, kainic acid and bicuculline using the [14C]deoxyglucose method. Regional brain glucose concentration was measured in another series of rats, after similar periods of seizure activity, to permit a more accurate calculation of the lumped constant. In L-allylglycine-induced seizures regional brain glucose concentration did not vary from control values, so no correction of the lumped constant was necessary. Regional brain glucose utilization increased throughout the brain, the largest increase being in the hippocampus (control 36 +/- 6 mumol 100 g-1 min-1; seizure 120 +/- 12 mumol 100 g-1 min-1). In kainic acid-induced seizures, brain glucose concentration fell in the hippocampus, involving some correction of the lumped constant. Increases in glucose utilization were limited primarily to the hippocampus, with some involvement of the inferior colliculus. The ventral hippocampus showed the largest increase in glucose utilization (control 34 +/- 5 mumol 100 g-1 min-1; seizure 167 +/- 10 mumol 100 g-1 min-1). In bicuculline-induced seizures, in starved rats, brain glucose concentration fell in all regions investigated and no increase in regional glucose utilization was recorded. In L-allylglycine and kainic acid-induced seizures, the hippocampus, a region vulnerable to neuronal damage, shows the largest increase in glucose utilization. Studies involving bicuculline need further investigation, due to severe perturbation of brain and plasma glucose concentration.

Effects of DL-allylglycine, alone or in combination with morphine, on passive avoidance behaviour in C57BL/6 mice

In a first set of experiments the posttrial intraperitoneal acute administration of the GAD-inhibitor DL-allylglycine impaired the memory processes of C57BL/6 (C57) mice tested in a passive avoidance apparatus. Memory improvements were, on the contrary, observed following morphine treatment. These effects were dose- and time-dependent. They were, in particular, evident if mice were injected immediately, or 30 min, but not 60 min, after training in the apparatus, and were absent in the no-footshock groups, suggesting lack of proactive effects of the drugs on performance. In a second set of experiments, the posttraining administration of allylglycine, at a per se ineffective dose (5 mg/kg), antagonized the memory improvement exerted by the administration of morphine (0.5 or 1.0 mg/kg). The results support the hypothesis of an involvement of GABAergic mechanisms in the memory consolidation processes, and suggest the possibility that GABAergic systems play a role in the effects of morphine on memory in C57BL/6 mice.

Allylglycine affects acetylation of putrescine and spermidine in mouse brain

Administration of allylglycine to mice (.8 mmole/kg, i.p.) results in a depletion of GABA levels, and it is accompanied by a decrease in SAM-DC activity and spermidine and spermine levels (Pajunen et al., 1979). Here we describe a biphasic effect on the acetylation of putrescine and spermidine in mouse brain homogenate. There appears to be an inverse correlation between the initial decrease in spermidine levels at 2 hours and the increase in the acetylation of spermidine. This is suggestive of a conversion of spermidine, probably through N1-acetylspermidine to putrescine. The peak of putrescine acetylation observed by us at 4 hours may also reflect a conversion of putrescine, via acetylputrescine to GABA. The interconversion hypothesis is supported by the fact that putrescine levels remain essentially stable in spite of a significant depletion of spermidine and spermine. In addition, there is a decrease in putrescine and spermidine acetylation at 8 hours, which coincides with the increase in ODC activity and the increase towards control levels of GAD activity (Pajunen et al., 1979). Such inverse correlations suggest a mechanism for replenishment of polyamines once GAD activity returns to control levels.

Early changes in the rat hippocampus following seizures induced by bicuculline or L-allylglycine: a light and electron microscope study

Status epilepticus was induced in thirteen paralysed and ventilated rats by the injection of either bicuculline or L-allylglycine. After 1-2 h of seizure activity the animals were intracardially perfused with a 2% glutaraldehyde/3% paraformaldehyde solution. Hippocampal blocks from each rat were processed for light and electron microscopy. The effects of L-allylglycine were more severe than those of bicuculline. Changes include perivascular and perineuronal swelling of astrocytic processes, and neuronal alterations which were graded as follows: Grade I (least severe), neuronal cytoplasm appears slightly darker than usual; Grade II, condensed or dark neurons, usually with microvacuoles; and Grade III classical 'ischaemic cell change'--the cytoplasm and karyoplasm is dark and shrunken, with or without microvacuoles. Many of the microvacuoles originate from mitochondria. In a few cases swollen and disrupted mitochondria are also seen is distended basal dendrites of the CA3 and CA1 pyramidal neurons. Dentate granule cells appear unaffected. The hippocampal neuronal alterations induced by seizure activity include those of 'ischaemic cell change'. The pathogenetic factors common to hypoxia/ischaemia and status epilepticus remain to be identified.

Action of progabide in the photosensitive baboon, Papio papio

Progabide is a GABA agonist which is nontoxic and crosses the blood-brain barrier. Progabide was tested in naturally and allylglycine-induced photosensitive baboons (Papio papio). Intravenous injection of various doses (30 to 100 mg/kg) of progabide induced a protective effect against intermittent light stimulation (25 Hz) in both test situations. This protection lasted 10 to 120 min. This duration was dose-dependent and was generally shorter in allylglycine-treated than in naturally photosensitive baboons. At all doses, progabide reinforced rhythmic activities in the frontorolandic and occipital regions. At doses greater than 50 mg/kg, progabide produced a mild sedation with somnolence and myorelaxation, all effects which could be reversed by sensory stimulation. These data confirm the anticonvulsant properties of progabide in another animal model.

Characteristics and origin of frontal paroxysmal responses induced by light stimulation in the Papio papio under allylglycine

In the Papio papio, curarized and rendered photosensitive by injection of a subconvulsant dose of DL-allylglycine, single flashes induce frontal paroxysmal evoked responses on condition that they be preceded by trains of intermittent light stimulation (ILS). The characteristics of these responses have been compared to those of non-paroxysmal responses induced in the same cortical area by isolated flashes (not preceded by trains of ILS). The paroxysmal responses resemble spikes and waves and consist of one or two positive spikes followed by a slow negative wave. The intracortical distribution of these responses has been studied in the motor cortex. The non-paroxysmal responses are probably not generated at this level. On the other hand, observations made during paroxysmal responses show the existence of two cortical responses; this demonstration follows from the existence of an inversion in some response components, linked to a negativity and a local cellular activation. A generator, situated in the pyramidal cell layer, is active during the positive surface spikes; the other generator, situated in the more superficial cortical layers, is active at the beginning of the slow negative surface wave. The cortical and subcortical afferents likely to bring these generators into play are discussed.

[Modification of frontal and occipital cortical excitability provoked by trains of flashes in Papio papio (author's transl)]

The experimental conditions necessary for obtaining an evoked paroxysmal response from the frontal cortex were studied in the baboon Papio papio. The trigger stimulus was comprised of an isolated flash preceded by a train of intermittent light stimulation (SLI). Two conditions were necessary for the appearance of paroxysmal responses: a subconvulsant dose of DL-allylglycine had to be injected at least 3 h previous to recording, and a sufficient number of SLI trains had to be presented to the animal. The paroxysmal responses disappeared as soon as SLI trains were stopped. At the same time, modifications in the evoked occipital potential continue, although these do not become paroxysmal. These modifications appear either simultaneously with or previous to the paroxysmal frontal response.

[Effect of DL-allyglycine on amygdala kindling in the rat (author's transl)]

The effect of decreased cerebral GABA levels on the development of amygdaloid kindling in rats was investigated. DL-Allylglycine, an inhibitor of glutamic acid decarboxylase (GAD), administered for 2 or 9 days (100 mg/kg and 50 mg/kg respectively) was found to accelerate the kindling process. The results are discussed according to a possible role of GABA mechanisms in the kindling phenomenon.

Involvement of the GABAergic system on shock-induced aggressive behavior in two strains of mice

The effects of drugs that antagonize or potentiate the action of brain gamma-aminobutyric acid (GABA) on shock-induced aggressive behavior in mice were investigated. In previous studies it has been shown that in C57 BL/6 strain shock-induced aggressive behavior is absent up to the 10th week of age and rises to the highest intensity after the 20th week, while at the same ages aggressive responses are lowest or absent in DBA/2 strain. GABA antagonist, picrotoxin and glutamic acid decarboxylase (GAD) inhibitor, D, L-allylglycine induced aggressive responses in non-aggressive 10 week old C57 BL/6 and 20 week old DBA/2 mice. GABA agonist muscimol hydrobromide, and GABA-T inhibitor sodium n-dipropylacetate inhibited aggressive responses in 20 week old C57 BL/6 mice. These effects were not related to changes in shock sensitivity and motor activity. The results strongly suggest that the GABAergic system is involved in the control of shock-induced aggressive behavior in mice and that this control is related to developmental and genetic factors.