Centrally-acting drugs and the formation of brain gamma-aminobutyric acid

Central GABAergic systems and depressive illness

Clinical depression and other mood disorders are relatively common mental illnesses but therapy for a substantial number of patients is unsatisfactory. For many years clinicians and neuroscientists believed that the evidence pointed toward alterations in brain monoamine function as the underlying cause of depression. This point of view is still valid. Indeed, much of current drug therapy appears to be targeted at central monoamine function. Other results, though, indicate that GABAergic mechanisms also might play a role in depression. Such indications stem from both direct and indirect evidence. Direct evidence has been gathered in the clinic from brain scans or postmortem brain samples, and cerebrospinal fluid (CSF) and serum analysis in depressed patients. Indirect evidence comes from interaction of antidepressant drugs with GABAergic system as assessed by in vivo and in vitro studies in animals. Most of the data from direct and indirect studies are consistent with GABA involvement in depression.

The effects of vigabatrin on electrophysiology and visual fields in epileptics: a controlled study with a discussion of possible mechanisms

PURPOSE

To compare the visual electrophysiology and visual fields of patients taking vigabatrin to those of a control group of epileptics on other anti-epileptic drugs (AEDs).

METHODS

Fourteen epileptics treated with vigabatrin and 10 control patients treated with other AEDs underwent ERG and EOG. Goldmann visual fields were performed and analysed using standard software to measure areas contained within I4e isopters.

RESULTS

The cone and rod b-waves of the ERG, the oscillatory potential amplitudes and Arden indices were reduced in vigabatrin-treated subjects and the oscillatory potentials delayed. The Arden indices were reduced due to an increased dark trough. The areas contained within the I4e isopter of vigabatrin treated subjects were reduced compared to the control group and these areas correlated well with oscillatory potential amplitudes and b-wave amplitudes in the vigabatrin group only.

CONCLUSIONS

The use of vigabatrin is associated with a reduction of the ERG cone b-wave amplitude and oscillatory potentials which correlates with visual field loss. The Arden ratio is reduced in subjects taking vigabatrin but may recover after cessation. However, visual loss may persist in the presence of a recovered EOG. These findings suggest further effects of the drug than those mediated by GABA receptors, and support the contention that the cause of the field loss may be at least in part due to retinal effects. Possible mechanisms are discussed.

Pharmacology and function of imidazole 4-acetic acid in brain

1. Imidazole 4-acetic acid (IMA) is a naturally occurring metabolite in brain, although it is unclear what biochemical pathways are involved in its biosynthesis and breakdown. Some evidence, however, suggests that IMA is an oxidation product of histamine. 2. The compound has pronounced neuropharmacological properties, many of which are consistent with an activation of GABA(A) receptors. Indeed, IMA is able to displace [3H]GABA from GABA(A) sites in a potent manner. 3. IMA displays definite partial agonist characteristics as an enhancer of benzodiazepine binding to the GABA(A) receptor complex in membrane preparations. In addition, it has an affinity for GABA(C) receptors, where it seems to act as an antagonist, and perhaps as a weak partial agonist. A third recognition site for IMA in brain is the I1-imidazoline receptor. 4. Parenteral administration to experimental animals leads to a sleep-like state which can often be accompanied by seizures. In addition, central application of IMA has been associated with a dose-related reduction in arterial pressure and sympathetic nervous discharge. 5. No specific receptor site or uptake system for IMA has yet been discovered, adding uncertainty to its role in central nervous system function. Yet the possibility cannot be overlooked that IMA plays a role in regulating blood pressure.

Pharmacological investigation of convulsant gamma-aminobutyric acid (GABA) antagonists in amygdala-kindled rats

The convulsant potencies of several drugs that antagonize GABAergic neurotransmission were assessed in amygdala-kindled and control rats. The potencies of picrotoxin, pentylenetetrazol, and a convulsant barbiturate were increased by amygdala kindling. No significant potentiation, however, was observed with isoniazid, 3-mercaptopropionic acid, bicuculline, or the glycine antagonist strychnine. Destruction, by electrolytic lesion, of the amygdala focus did not reverse the kindling-induced potentiation of picrotoxin convulsions. The potentiation of picrotoxin convulsions was correlated with the pattern of kindling development but not with the number of stimulations or seizures. The present results are consistent with the hypotheses that (1) kindling potentiates only some convulsant agents, (2) kindling-induced convulsant potentiation takes place outside of the kindled focus, and (3) kindling is not produced by a failure of the GABA system.

Changes in regional brain levels of amino acid putative neurotransmitters after prolonged treatment with the anticonvulsant drugs diphenylhydantoin, phenobarbitone, sodium valproate, ethosuximide, and sulthiame in the rat

The effect of prolonged treatment (10 days) with the anticonvulsant drugs diphenylhydantoin (DPH), phenobarbitone, sodium valproate, ethosuximide and sulthiame, both singly and in combination, on regional rat brain amino acid neurotransmitter concentrations (GABA, glutamate, aspartate and taurine) were assessed. DPH had a major effect in the cerebellum and hypothalamus in that it significantly reduced cerebellar GABA, taurine and aspartate and hypothalamic GABA and aspartate. Sodium valproate significantly elevated GABA and taurine in most regions. Aspartate and glutamate were less affected. Phenobarbitone significantly elevated GABA concentrations in all brain regions, while taurine concentration was only elevated in the cerebral cortex. Ethosuximide induced changes were small compared to the other anticonvulsants while sulthiame produced complex changes. Anticonvulsant drugs administered in combination resulted in complex changes, suggesting that their mode of action is different.