New anticonvulsants: Schiff bases of gamma-aminobutyric acid and gamma-aminobutyramide.

Schiff bases of gamma-aminobutyric acid (gammaAbu) and gamma-aminobutyramide (gammaAbuNH2) were prepared and tested for anticonvulsant and gammaAbu mimetic activity. 4-[[(4-Chlorophenyl)(5-fluoro-2-hydroxyphenyl)methylene]amino]butanoic acid monosodium salt (4) and 4-[[(4-chlorophenyl)(5-fluoro-2-hydroxyphenyl)methylene]amino]butanamide (5) blocked bicuculline-induced lethality and convulsions and displaced [3H]gammaAbu from its membrane binding sites. In the rat dorsal root sensory ganglion, compound 4 exhibited gammaAbu agonist properties. Compounds 4 and 5 are thus anticonvulsants and directly acting gammaAbu mimetics.

Distribution and metabolism of L-3-O-methyldopa in rats.

1. After intraperitoneal administration of L-2-(14)C-3-O-methyldopa ((14)C-O-methyldopa) to rats, the amino-acid was distributed evenly in blood, brain, heart, adipose tissue and liver, whereas it accumulated more in the kidney and the pancreas. (14)C-O-methyldopa showed a biological half-life of about 12-13 h in blood, brain and heart.2. The concentration curve of (14)C-O-methyldopa in brain (after increasing doses of the amino-acid) was linear if measured 2 h after administration, but seemed to reach a plateau at the higher doses if determined after 16 h.3. The concentrations of (14)C-O-methyldopa metabolites (mainly homovanillic acid and 4-hydroxy-3-methoxyphenyllactic acid) were low, except in the kidney, and varied according to the tissue.4. Twenty-four hours after administration of (14)C-O-methyldopa, 33% of the injected radioactivity appeared in the urine. This radioactivity consisted of about 95% of metabolites (probably in the main (14)C-homovanillic acid and (14)C-4-hydroxy-3-methoxyphenyllactic acid) and of 5% of unchanged (14)C-O-methyldopa. In the faeces, 10% of the radioactivity appeared, mainly as metabolic end-products.5. It is concluded that (14)C-O-methyldopa easily penetrates from the blood into various tissues, including brain, and that the majority of the amino-acid undergoes a slow metabolism. The different shape of the concentration curves for (14)C-O-methyldopa in the brain after 2 and 16 h might indicate the presence of two tissue pools of the amino-acid.

Effects of clozapine on cerebral catecholaminergic neurone systems.

1. Clozapine, a dibenzodiazepine derivative claimed to possess antipsychotic properties in man without producing extrapyramidal disorders, greatly increased the turnover of cerebral dopamine in the rat.2. The drug itself was virtually devoid of cataleptigenic activity in rats; however, it antagonized prochlorperazine-induced catalepsy.3. It is proposed that clozapine causes a blockade of striatal dopamine receptors which is of the surmountable type in contrast to that produced by cataleptigenic neuroleptics. In addition, clozapine may also increase the turnover of cerebral noradrenaline.

Pharmacology of the GABAergic system: effects of progabide, a GABA receptor agonist.

Stimulation of GABA receptors (e.g. by progabide, a new GABA receptor antagonist, or by muscimol) enhances the liberation of norepinephrine in limbic forebrain areas of the rat and reduces 5-hydroxytryptamine turnover. On repeated administration, this latter effect is associated with an up-regulation of 5-HT2 receptors as it occurs after electroconvulsive shock. The monoaminergic changes induced by progabide, though dissimilar from those induced by tricyclics, are probably connected with the antidepressant action on the compound observed in double-blind clinical trials. In the basal ganglia, GABA receptor agonists reduce dopamine turnover and potentiate the cataleptogenic action of neuroleptics. They also antagonize the sterotypic behaviour induced by dopaminomimetics, indicating an additional action beyond the dopamine synapse. On repeated co-administration with neuroleptics, progabide antagonizes the tolerance to the cataleptogenic action, the supersensitivity to dopaminomimetics, and the increase in 3H-spiperone binding which are caused by sustained neuroleptic treatment. This appears to be the basis for the clinical action of progabide in neuroleptic-induced dyskinesia, L-dopa-induced involuntary movements, and possibly mania. GABA receptor agonists decrease cellular excitability in several animal models and antagonize seizures, whatever their origin (GABA-mediated or GABA unrelated mechanisms). Progabide has been shown to be effective in various forms of epilepsy in double-blind and long-term clinical trials. The compound exerts a therapeutic action in patients resistant to "classical" antiepileptic drugs, in the virtual absence of major side effects.

Increase in striatal acetylcholine levels by GABAergic agents: dependence on corticostriatal neurons.

Surgical lesion of the corticostriatal projections almost totally reduced the ability of GABA agonist agents but not apomorphine to increase rat striatal ACh concentrations. This suggests that in intact rats, the GABAergic inhibition of striatal cholinergic neurons depends, at least in part, upon the corticostriatal (possibly glutamatergic) tract whereas the action of the dopaminergic pathway on cholinergic cells is independent of corticostriatal neurons.

Selective antagonists of dopamine receptor subtypes differentially affect substance P levels in the striatum and substantia nigra.

Repeated administration to rats of SCH 23390, a specific antagonist of the D-1 dopamine receptor, produced an increase in the substance P immunoreactivity in the striatum but not in the substantia nigra, whereas similar treatment with sulpiride, a specific D-2 dopamine receptor antagonist, reduced the nigral but not the striatal content of the peptide. When the two antagonists were given together, the SCH 23390-induced increase in striatal substance P was significantly reduced. The SCH 23390-induced increase in striatal substance P was curtailed by concomitant administration of progabide, a selective gamma-aminobutyric acid (GABA) receptor agonist. These results suggest the existence in the nigro-striatal complex of two different substance P-containing neurons which are differentially regulated by the dopamine receptor subtypes and indicate a role of GABA in the action of SCH 23390.

[3H]Haloperidol labels brain dopamine receptors after its injection into the internal carotid artery of the rat.

Pulse injection of [3H]haloperidol (0.2 microCi; 0.003 microgram) into the internal carotid artery of the rat specifically labelled dopamine receptors in striatum and olfactory tubercle, as indicated by the kinetics of, and the effects of neuroleptic drugs on, the ligand disposition. The described method may prove useful for labelling brain receptors with ligands which readily cross the blood-brain barrier but which do not selectively mark their receptors if injected systemically.

Cortical modulation of striatal function.

The effect of bilateral section of the corticostriatal projections or of selective bilateral ablation of the frontal cortex on behavioral and biochemical parameters related to striatal function were investigated in the rat. Either lesion almost completely prevented the cataleptogenic action of haloperidol: this effect was observed as soon as 3 days and lasted for at least 3 months after surgery, paralleling a reduction in striatal glutamate uptake. Also, such lesions enhanced the apomorphine-induced stereotyped behavior (as measured 21 days after surgery). In the striatum, dopamine, dihydroxyphenylacetic acid, acetylcholine and substance P levels as well as choline acetyltransferase and glutamic acid decarboxylase activities were unaffected 10 or 21 days after either type of lesion. In the substantia nigra, substance P levels were unchanged 10 days following suction of the frontal cortex, but glutamic acid decarboxylase was reduced at 21 days postsurgery. Cortical lesions only partially prevented the reduction in striatal acetylcholine concentrations and did not affect the increase in striatal dihydroxyphenylacetic acid caused by haloperidol. Finally, lesions of the corticostriatal pathways failed to affect the apomorphine-induced increase in striatal acetylcholine levels, reduction of the potassium (20 mM) evoked [3H]acetylcholine release in striatal slices preloaded with [3H]choline and decrease of striatal dihydroxyphenylacetic acid concentrations. These findings indicate that the frontal cortex influences extrapyramidal function by a mechanism which--in behavioral terms--is antagonistic to dopamine-mediated events. As indicated by the biochemical data, this mechanism does not involve changes in striatal dopaminergic and cholinergic neuron activity. This mechanism may utilize: (1) corticostriatal glutamatergic neurons as suggested by the reduction in striatal glutamate uptake following lesions; and (2) GABAergic pathways as suggested by the reduction of nigral glutamic acid decarboxylase activity as well as by the finding that GABA receptor agonists reinstate haloperidol-induced catalepsy.

Lysergic acid diethylamide: evidence for stimulation of cerebral dopamine receptors.

In the rat, lysergic acid diethylamide (LSD) decreased the striatal and retinal content of homovanillic acid. LSD did not change the level of dopamine (DA), but delayed the a-methyl-p-tyrosine-induced disappearance of this amine in the teldiencephalon. In the cat, LSD diminished the DA output into the perfusate of the caudate nucleus. Furthermore, LSD increased the activity of adenylate cyclase in striatal homogenates of rat. These and other findings indicate that in the central nervous system LSD stimulates DA receptors which may be involved in LSD-induced phychosis.

Involvement of the D-2 dopamine receptor in the neuroleptic-induced decrease in nigral substance P.

Repeated treatment with, but not single administration of drugs which impair dopaminergic transmission produced a consistent reduction in substance P immunoreactivity in the rat substantia nigra. This effect appears to be related to the D-2 dopamine receptor function as the blockade of this receptor subtype by selective antagonists produced effects qualitatively similar to those produced by drugs lacking selectivity for different subclasses of dopamine receptors.

Progabide reverses the nigral substance P reduction induced by chronic impairment of dopaminergic transmission.

Repeated treatment with haloperidol or lesion of nigrostriatal dopaminergic neurons with 6-hydroxydopamine produced a reduction in substance P immunoreactivity in the rat substantia nigra. This reduction was reversed by the repeated administration of progabide, a selective GABA receptor agonist. As GABA inhibits substance P release, these results suggest that the reduction in nigral substance P levels was due to an increased liberation of the peptide probably related to deficient GABAergic function induced by impairment of striatal dopaminergic transmission.

Difluoromethyl ornithine protects against the neurotoxic effects of intrastriatally administered N-methyl-D-aspartate in vivo.

The neurotoxic effects of intrastriatally administered N-methyl-D-aspartate (NMDA) (250 nmol), as measured by reductions in striatal choline acetyl transferase activity and by increased binding of the glial marker [3H]PK 11195 10 days later, were reduced by coinfusion of the irreversible ornithine decarboxylase inhibitor difluoromethylornithine (250 nmol) in the rat. The data suggest a crucial role for the polyamines in NMDA receptor-mediated neurotoxicity.

Experimental basis for the antidepressant action of the GABA receptor agonist progabide.

gamma-Aminobutyric acid (GABA) receptor agonists (e.g. progabide) are effective in behavioral tests predictive of antidepressant drug action. Also, these compounds, by changing the firing rate of the corresponding neurons, accelerate norepinephrine turnover (without changes in postsynaptic receptor density) and decrease 5-hydroxytryptamine (5-HT) liberation (with up-regulation of 5-HT2 receptors). At variance, tricyclic antidepressants block monoamine reuptake and cause down-regulation of beta-adrenergic and 5-HT2 receptors. Progabide exerts an antidepressant action which is indistinguishable from that of imipramine. The different modes of action of GABA receptor agonists and tricyclics, as well as alterations of GABA-related parameters by tricyclics, challenge the classical monoaminergic hypothesis of depression and suggest that GABA-mediated mechanisms play a role in this disorder.

The gabaergic hypothesis of depression.

UNLABELLED: 1. GABAergic mechanisms have been generally ignored in the study of mood disorders and antidepressant drug (AD) action. Recently data have accumulated indicating that GABAergic mechanisms may be involved in both of these. 2. Mood disorders: GABA levels are reported to be low in the CSF and plasma of depressed patients and are related to mood changes. GABAB receptors are decreased in the frontal cortex in two rodent behavioral models of depression and GABA release is reported diminished in the hippocampus. GABAergic drugs (progabide, fengabine) reverse the behavioral deficits in the rodent models and exert clear therapeutic effects in depressed patients. 3. AD action: In behavioral models imipramine upregulates GABAB receptors only in those animals which respond behaviorally to the AD. In naive rats repeated administration of varied ADs upregulates GABAB receptors in the frontal cortex whereas non-ADs (including amphetamine) do not. Bicuculline inhibits the action of imipramine in the learned helplessness model. GABAA receptor stimulation enhances noradrenaline release in the ventral NA pathway. 4. CONCLUSIONS: GABAergic mechanisms likely play a role in the modulation of mood and increasing GABAergic tone exerts and antidepressant effect. Actions at GABA synapses appear to be a fundamental facet of ADs, perhaps together with beta-adrenoceptor mediated events.

"In vivo" release of endogenous neurotransmitters in cat limbic regions: effect of chlorpromazine and of electrical stimulation.

The effect of chlorpromazine (10 mg/kg i.v.) on the spontaneous release of endogenous dopamine (DA), noradrenaline (NA) and acetylcholine (ACh) within limbic areas perfused by means of the push-pull cannula was investigated in the gallamine-immobilized cat. Chlorpromazine increased the liberation of DA and NA in the nucleus accumbens septi, indicating blockade of the amine receptors. However, the drug did not change the output of ACh from this nucleus nor from ventral and dorsal hippocampal formations which receive a cholinergic input from the septum as indicated by several published findings and by the increased liberation of ACh after electrical stimulation of the homolateral nucleus medialis septi. These results seem to exclude that cholinergic neurons of some limbic areas mediate the effect of the blockade of DA (and of NA) receptors which is possibly involved in the antipsychotic action of neuroleptic drugs.

The potential of GABA-mimetics in the therapy of extrapyramidal disorders.

GABA mimetics inhibit extrapyramidal DA and ACh neurons and affect an unknown system beyond both DA and ACh receptors, which is involved in extrapyramidal motor outputs. Based on these data, the rationale is discussed for the clinical use of GABA mimetics in Huntington's chorea, parkinsonian tremor, L-DOPA or neuroleptic-induced dyskinesias.

Pharmacological and therapeutic actions of GABA receptor agonists.

GABA receptor agonists, e.g. progabide, modify the activity of several brain neuronal systems which are implicated in the pathogenesis of some neuropsychiatric disorders. Thus, alterations in noradrenergic and serotoninergic transmissions induced by progabide may be a mechanism involved in the antidepressant action of this drug in the clinic. The antagonism of the neuroleptic-induced increase in dopamine receptor sensitivity and the decrease in dopamine synthesis and release may be responsible for the effectiveness of the GABA receptor agonists in the treatment of neuroleptic- and L-DOPA-induced dyskinesia. This action of GABA receptor agonists also suggests their therapeutic potential in mania. Finally, decrease in cellular excitability induced by GABA receptor agonists, e.g. progabide, accounts for their efficacy in epilepsy.

The psychopharmacology of GABA synapses: update 1989.

Recent advances in the psychopharmacology of GABA synapses are reviewed. The usefulness of GABA mimetics in tardive dyskinesia and epilepsy has been confirmed, as has a dysfunction of GABA synapses in the etiopathology of these conditions. The antidepressant profile of GABA agonists in animal models for depression has been extended. The role of GABA receptors in the mechanism of action of antidepressants has been further delineated, with a parallelism occurring between the behavioral and biochemical response to antidepressant drug treatment in different animal models of depression.

GABA receptor agonists and extrapyramidal motor function: therapeutic implications for Parkinson's disease.

GABA receptor agonists display a dual action on DA-mediated events. One includes a decrease in DA release, reduction in DA receptor density, and decreased response of postsynaptic cells to dopaminergic stimulation; it results in antidopaminergic effects. The other consists of a reduction of striatal cholinergic activity resulting in a facilitation of dopaminergic effects. These two effects could be dissociated depending on the dose of GABA receptor agonists. This dual action probably explains the results of clinical trials showing either amelioration of parkinsonian symptoms with aggravation of L-DOPA-induced dyskinesia or improvement of dyskinesia without or with aggravation of parkinsonian symptoms.

Non-benzodiazepine anxiolytics: potential activity of phenylpiperazines without 3H-diazepam displacing action.

Four phenylpiperazine derivatives exhibited an activity similar to benzodiazepines and meprobamate in the 4-plate test. One of these (compound IV) demonstrated anxiolytic like activity in a step-down avoidance technique, in electroshock induced aggression and in the staircase test. In contrast to benzodiazepines, compound IV was not anticonvulsant, myorelaxant or sedative. Confirmation of the anxiolytic activity of compound IV in animal models was obtained in 3 separate clinical trials in anxious patients. The mechanism of action of these phenylpiperazines appears to be different from the benzodiazepines as they do not displace 3H-diazepam binding nor do they interact with other elements of the GABA receptor macromolecular complex. Instead, compound IV interacts with both dopaminergic and serotoninergic neuron systems. Thus, from this data it would appear that an activity at the benzodiazepine recognition site is not obligatory for anxiolytic activity in man or in animals models.