Baclofen (beta-p-chlorophenyl-gamma-aminobutyric acid) enhances [3H]gamma-aminobutyric acid (3H-GABA) release from rat globus pallidus in vitro

PHARMACOLOGICAL TREATMENTS FOR TINNITUS: NEW AND OLD

Subjective tinnitus, the phantom ringing or buzzing sensation that occurs in the absence of sound, affects 12-14% of adults; in some cases the tinnitus is so severe or disabling that patients seek medical treatment. However, although the economic and emotional impact of tinnitus is large, there are currently no FDA-approved drugs to treat this condition. Clinical trials are now underway to evaluate the efficacy of N-methyl-d-aspartate (NMDA) and dopamine D(2) antagonists, selective serotonin reuptake inhibitors (SSRIs), γ-aminobutyric acid (GABA) agonists and zinc dietary supplements. Previous off-label clinical studies, while not definitive, suggest that patients with severe depression may experience improvement in their tinnitus after treatment with antidepressants such as nortriptyline or sertraline. A small subpopulation of patients with what has been described as "typewriter tinnitus" have been shown to gain significant relief from the anticonvulsant carbamazepine. Preliminary studies with misoprostol, a synthetic prostaglandin E1 analogue, and sulpiride, a dopamine D(2) antagonist, have shown promise. Animal behavioral studies suggest that GABA transaminase inhibitors and potassium channel modulators can suppress tinnitus. Additionally, improvements in tinnitus have also been noted in patients taking melatonin for significant sleep disturbances. Like other complex neurological disorders, one drug is unlikely to resolve tinnitus in all patients; therapies targeting specific subgroups are likely to yield the greatest success.

Gamma-aminobutyric acid and mefloquine-induced seizures in mice

The role of GABAergic mechanism in the convulsant effect of mefloquine was investigated in mice. Mefloquine dose dependently induced tonic seizures in mice. Aminooxyacetic acid, diaminobutyric acid and muscimol significantly protected mice against mefloquine-induced seizures by significantly delaying the onset and decreasing the incidence of the seizures. Bicuculline and picrotoxin significantly enhanced the seizure producing effect of mefloquine and also significantly antagonised the protective effect of muscimol against the seizures. Phenobarbitone and diazepam effectively protected mice against mefloquine-induced seizures. Phenytoin did not alter mefloquine-induced seizures. These data indicate that GABA mechanisms might be involved in seizures produced by mefloquine in mice.

Gamma-aminobutyric acid and glutamic acid receptors may mediate theophylline-induced seizures in mice

The effects of drugs affecting GABA and glutamic acid receptors on theophylline-induced seizures were investigated in mice. Theophylline elicited tonic seizures in mice in a dose dependent manner. Muscimol, DABA and AOAA significantly prolonged the onset and significantly decreased the incidence of theophylline-induced seizures. Baclofen significantly delayed the onset of the tonic seizures induced by theophylline. Bicuculline and picrotoxin significantly shortened the onset and significantly increased the incidence of seizures induced by a low dose of theophylline and also significantly antagonized muscimol-attenuating effect against theophylline seizures. N-methyl-DL-aspartic acid significantly shortened the onset and significantly increased the incidence of seizures elicited by a low dose of theophylline. D-(-)-2-amino-phosphonopentanoic acid effectively delayed the onset and significantly decreased the incidence of seizures elicited by theophylline and also significantly antagonized the potentiating effect of N-methyl-DL-aspartic acid on seizures induced by a low dose of theophylline. Dextromethorphan and ketamine profoundly shortened the onset of theophylline-induced seizures. Clonidine effectively prolonged the onset and significantly decreased the incidence of theophylline-induced seizures. These data indicate that GABA(A) and N-methyl-D-aspartic acid receptors may mediate theophylline-elicited tonic seizures in mice.

Involvement of GABAB receptors in the antinociception induced by baclofen in the formalin test

1. The effect of GABA receptor antagonists on baclofen-induced antinociception was examined in rats using the formalin test. Intraperitoneal (IP) administration of different doses of baclofen (2.5-10 mg kg-1) to rats induced antinociception in both phases of the test. 2. The response was dose-dependent and the maximum response was observed with 10 mg kg-1 of the drug. Intracerebroventricular (ICV) injection of baclofen (0.5-20 micrograms/rat) also induced dose-dependent antinociception in the second phase of the formalin test. 3. The GABAB antagonist, phaclofen (1 mg kg-1, IP) but not the GABAA antagonists picrotoxin (1 mg kg-1, IP) and bicuculline (1.5 mg kg-1, IP), decreased the antinociception induced by both ICV and IP administration of baclofen. 4. It is concluded that baclofen antinociception in the formalin test is mediated through GABAB receptor activation.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

GABAergic and dopaminergic systems may be involved in seizures induced by pyrimethamine in mice

1. The effects of some GABAergic and dopaminergic agents on pyrimethamine-induced tonic seizures were investigated in mice. 2. Pyrimethamine dose dependently induced seizures in mice. 3. Muscimol, AOAA and DABA significantly protected mice against pyrimethamine-induced seizures. 4. Bicuculline and picrotoxin effectively potentiated seizures elicited by pyrimethamine and significantly antagonized the protective effect of muscimol against the seizures. 5. Diazepam and phenobarbitone effectively protected mice against seizures elicited by pyrimethamine. 6. L-Dopa significantly potentiated pyrimethamine-induced seizures. 7. Apomorphine and pargyline significantly reduced the latency of seizures induced by pyrimethamine. 8. Haloperidol and pimozide effectively protected mice against pyrimethamine-elicited seizures and also significantly antagonized the potentiating effects of apomorphine and L-dopa on the seizures. 9. Disulfiram significantly potentiated seizures induced by pyrimethamine and also significantly enhanced the seizure-potentiating effect of L-dopa. 10. alpha-Methyl-p-tyrosine effectively protected against seizures induced by pyrimethamine. However, L-dopa significantly potentiated the seizures in alpha-methyl-p-tyrosine-pretreated animals. 11. Muscimol significantly attenuated the potentiating effect of L-dopa on pyrimethamine-induced seizures while bicuculline significantly enhanced the effect of L-dopa. Furthermore, haloperidol significantly potentiated the protective effect of muscimol against pyrimethamine-induced seizures. 12. These results suggest that both GABA and dopamine might be involved in the mechanism(s) of pyrimethamine seizures in mice.

Cimetidine-induced seizures in mice. Antagonism by some GABAergic agents

The effects of muscimol, aminooxyacetic acid (AOAA), diamino-n-butyric acid (DABA), baclofen, bicuculline, picrotoxin, strychnine, diazepam, phenobarbitone and phenytoin on cimetidine-induced seizures were studied in mice. Cimetidine (400-1000 mg/kg, i.p.) induced dose-dependent tonic convulsion. Muscimol, AOAA and DABA effectively protected mice against cimetidine-induced seizures. Bicuculline and picrotoxin significantly potentiated the seizures induced by cimetidine and effectively antagonized the protective effects of muscimol, AOAA and DABA against the seizures. Diazepam and phenobarbitone significantly protected the mice against cimetidine-induced seizures while phenytoin and strychnine did not significantly alter the seizures. These results indicate that the attenuation of central gamma-aminobutyric acid neurotransmission may underlie cimetidine-induced seizures in mice.

Effects of some GABAergic agents on quinine-induced seizures in mice

The effects of some GABAergic agents on seizures induced by quinine were studied in mice. Muscimol, AOAA, DABA and baclofen significantly protected mice against quinine-induced convulsions. Bicuculline effectively enhanced quinine-induced convulsions, and significantly attenuated the protective effects of muscimol, AOAA and DABA against convulsions induced by quinine. Diazepam and phenobarbitone significantly protected mice against convulsions induced by quinine. However, phenytoin did not affect quinine-induced seizures to any significant degree. These results indicate that the convulsant effect of quinine may be due to a disturbance in the status of the GABAergic system.

Involvement of GABAergic mechanisms in chloroquine-induced seizures in mice

1. The influence of some GABAergic agents on tonic seizures elicited by chloroquine was investigated in mice. 2. Chloroquine (45-100 mg/kg) elicited seizures in mice in a dose related manner. 3. Muscimol (1-2 mg/kg), DABA (8-16 mg/kg) and baclofen (4-16 mg/kg) profoundly delayed the onset of chloroquine (65 mg/kg)-induced seizures. The incidence of the seizures was also significantly reduced by muscimol (1-2 mg/kg), DABA (8 mg/kg) and baclofen (4-8 mg/kg). 4. AOAA (10 mg/kg) profoundly reduced the proportion of mice that convulsed while AOAA (20 mg/kg) completely protected mice against chloroquine (65 mg/kg)-induced seizures. 5. Bicuculline (5 mg/kg) and picrotoxin (0.5-1 mg/kg) significantly potentiated chloroquine (50 mg/kg)-induced seizures. The onset of seizures and the number of mice that convulsed were shortened and increased respectively. The onset of chloroquine (65 mg/kg)-elicited seizures was also profoundly shortened. Bicuculline (5 mg/kg) and picrotoxin (0.5 mg/kg) effectively antagonised the protective effects of muscimol (2 mg/kg), AOAA (10 mg/kg) and DABA (8 mg/kg) against chloroquine (65 mg/kg)-elicited seizures. 6. Diazepam (1 mg/kg) and phenobarbitone (20 mg/kg) significantly antagonised chloroquine (65 mg/kg) seizures. The onset of seizures was significantly delayed by both diazepam (0.25-1 mg/kg) and phenobarbitone (10-20 mg/kg). 7. These data suggest that enhancement and inhibition of GABAergic neurotransmission respectively attenuate and potentiate chloroquine seizures in mice.

Facilitation of amphetamine-induced hypothermia in mice by GABA agonists and CCK-8

1. Amphetamine-induced hypothermia in mice is facilitated by dopaminergic stimulation and 5-hydroxytryptaminergic inhibition. The present study was designed to investigate: (a) the involvement of other neuronal systems, such as the gamma-aminobutyric acid (GABA), the opioid and the cholecystokinin (CCK-8) systems; (b) the possible contribution of hydroxylated metabolites of amphetamine to the hypothermia; (c) the capacity of dopamine itself to induce hypothermia and its mechanisms, in order to clarify the resistance of amphetamine-induced hypothermia to certain neuroleptics. 2. Pretreatment with the GABA antagonists, bicuculline and picrotoxin, did not inhibit amphetamine-induced hypothermia. The GABAB agonist, baclofen (2.5 mg kg-1, i.p.) potentiated this hypothermia, whereas the GABAA agonist, muscimol, did not. gamma-Butyrolactone (GBL) (40 mg kg-1, i.p.) and the neuropeptide CCK-8 (0.04 mg kg-1, i.p.) also induced potentiation. The opioid antagonist, naloxone, was without effect. 3. Dopamine itself (3, 9, 16 and 27 micrograms, i.c.v.) induced less hypothermia than the same doses of amphetamine. Sulpiride did not block dopamine-induced hypothermia, but pimozide (4 mg kg-1, i.p.), cis(z)flupentixol (0.25 mg kg-1, i.p.) and haloperidol (5 micrograms, i.c.v.) did. The direct dopamine receptor agonist, apomorphine, did not alter the hypothermia. Neither the 5-hydroxytryptamine (5-HT) receptor blocker, cyproheptadine, nor the inhibitor of 5-HT synthesis, p-chlorophenylalanine (PCPA), modified dopamine-induced hypothermia. Fluoxetine, an inhibitor of 5-HT reuptake, had no effect, whereas quipazine (6 mg kg-1, i.p.), a 5-HT agonist, totally prevented the hypothermia. Hypothermia was unaffected by pretreatment with CCK-8. 4. These data indicate that the hypothermia induced by amphetamine involves not only dopaminergic and 5-hydroxytryptaminergic systems which are functionally antagonistic, but is also facilitated by direct or indirect GABA and CCK-8 receptor stimulation. This facilitation could result, in part, from modulation of dopaminergic neurotransmission. This may explain the apparent resistance of amphetamineinduced hypothermia to some neuroleptics, while dopamine-induced hypothermia is not resistant. The possible action of hydroxylated metabolites of amphetamine may also help to explain these differences.

Food intake suppressant effect of baclofen in rats

1. Baclofen given intraperitoneally (i.p.) to rats caused a dose-dependent decrease in food intake. 2. Bicuculline or picrotoxin (GABAA-antagonist) and methergoline (5-HT antagonist) decreased the anorectic effect of baclofen. 3. Pimozide (dopamine receptor blocker), phenoxybenzamine and propranolol (alpha and beta adrenergic blockers) did not diminish the baclofen effect, but even increased the anorexia induced by the drug. 4. It can be postulated that, at least partially, GABAA receptor mechanism, GABA-5HT receptor complex and/or 5-HT mechanism may be involved in baclofen induced anorexia.

GABA-B receptor activation and conflict behaviour

Baclofen and oxazepam enhance extinction of conflict behaviour in the Geller-Seifter test while baclofen and diazepam release punished behaviour in Vogel's conflict test. In order to investigate the possibility that the effect of the selective GABA-B receptor agonist baclofen is mediated indirectly via the GABA-A/benzodiazepine receptor complex, the effect of pretreatment of rats with baclofen on [3H]-diazepam binding to washed and unwashed cortical and cerebellar membranes of rats has been studied. Baclofen pretreatment increased Bmax in washed cerebellar membranes when bicuculline was present in the incubation mixture. No effect was seen in cortical membranes. The present results render it unlikely that the effect of baclofen on extinction of conflict behaviour and punished drinking is mediated via the GABA-A/benzodiazepine receptor complex.

GABAA-antagonists and baclofen analgesia

1. Intraperitoneal (i.p.) injection of different doses of baclofen (5, 7.5 and 10 mg/kg) induced analgesia in tail-flick test. The effect was dose-dependent. 2. The antinociception induced by baclofen (10 mg/kg, i.p.) was decreased in animals pretreated with bicuculline (1.5 mg/kg, i.p., 30 min), but not with naloxone (1.5 mg/kg, i.p., 30 min). 3. In picrotoxin (1 mg/kg, i.p., 15 min) pretreated mice, baclofen (5 mg/kg, i.p.) showed a significant analgesic effect. 4. Morphine (6 mg/kg, s.c.) induced analgesia which was antagonized by naloxone pretreatment (1.5 mg/kg, i.p.), while bicuculline or picrotoxin did not alter the morphine response. 5. These data suggest that a part of analgesic effect of baclofen may be mediated through GABAA receptor sites, and differs from that of morphine.

GABAA and GABAB receptor sites involvement in rat thermoregulation

1. Intraperitoneal (i.p.) injection of muscimol (MUS, 2-8 mg kg-1) decreased the core body temperature (BT) of the rats dose-dependently. 2. Intracerebroventricular (i.c.v.) injection of MUS (1 microgram/microliter/rat) also caused a fall in BT. 3. The hypothermia induced by MUS was inhibited by pretreatment of the animals with either bicuculline (BIC) or picrotoxin (PIC). 4. i.p. Injection of baclofen (BAC, 2.5-10 mg kg-1) induced hypothermia. Higher dose of the drug (20 mg kg-1) caused an initial fall followed by a marked increase in BT. 5. i.c.v. Injection of BAC produced a rise in BT. 6. The hypothermic effect of BAC was antagonized in animals pretreated with either BIC or PIC, while hyperthermic effect of the drug was potentiated with PIC pretreatment. 7. i.c.v. Injection of isoguvacine (ISO) induced hypothermia, which was attenuated in rats pretreated with either BIC or PIC. 8. It can be concluded that: activation of GABAA or GABAB receptor sites respectively may induce hypothermia or hyperthermia in rats.

Opposing roles of dopamine D1 and D2 receptors in nigral gamma-[3H]aminobutyric acid release?

This study examined the effects of dopamine D1 and D2 receptor agonists and antagonists on the spontaneous and calcium-dependent, K+-induced release of gamma-[3H]aminobutyric acid [( 3H]GABA) accumulated by slices of rat substantia nigra. SKF 38393 (D1 agonist) and dopamine (dual D1/D2 agonist) were without effect on [3H]GABA efflux by themselves (1-40 microM), or in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (0.5 mM), but potentiated evoked release in the presence of forskolin (0.5 microM), an adenylate cyclase activator. These increases in release were prevented by the D1 antagonist SCH 23390 (0.5 microM), but not by the D2 antagonist metoclopramide (0.5 microM). Higher concentrations of forskolin (10-40 microM) augmented stimulus-evoked [3H]GABA release directly, whereas dibutyryl cyclic AMP (100-200 microM) depressed it. Apomorphine, noradrenaline, and 5-hydroxytryptamine (1-40 microM) had no effect. The D2 stimulants lisuride, RU 24213, LY 171555, and bromocriptine dose-dependently inhibited depolarisation-induced but not basal [3H]GABA outflow. These inhibitory responses were not modified by the additional presence of SKF 38393 (10 microM) or SCH 23390 (1 microM), or by injection of 6-hydroxydopamine into the medial forebrain bundle 42 days earlier, but were attenuated by metoclopramide (0.5 microM). Higher amounts (10 microM) of SCH 23390, metoclopramide, or other D2 antagonists (loxapine, haloperidol) reduced evoked GABA release by themselves, probably by nonspecific mechanisms. These results suggest D1 and D2 receptors may have opposing effects on nigral GABA output and could explain the variable effects of mixed D1/D2 dopaminomimetics in earlier release and electrophysiological experiments.

Systemic baclofen stimulates gastric motility and secretion via a central action in the rat

Intravenous (0.5 mg kg-1) or subcutaneous (2-16 mg kg-1) administration of the gamma-aminobutyric acid (GABA) analogue baclofen resulted in a stimulation of gastric motility and secretion in the rat, anaesthetized with urethane. The motility response to subcutaneous injection was dose-related. This effect was abolished by vagotomy or atropine. There was no response to baclofen in decerebrate animals. These results indicate that systemic baclofen, probably acting at a site rostral to the brainstem, stimulates gastric motility and acid secretion by a vagally-dependent mechanism.

The depressant action of baclofen on the isolated spinal cord of the neonatal rat

Neurotransmission in isolated hemisected spinal cord preparations from immature rats was depressed by micromolar levels of baclofen (threshold 0.5 microM). The depressant action of baclofen was not antagonised by bicuculline and baclofen, unlike GABA, did not depolarize primary afferent fibres. Neurotransmission in isolated vas deferens, anococcygeus muscle and superior cervical ganglion of the rat was unaffected by baclofen (0.1-1 mM). Depolarization of motoneurones, as recorded in ventral roots of tetrodotoxin-blocked spinal cord preparations, induced by excitant amino acids, substance P, noradrenaline or carbachol was unaffected by baclofen (250 microM or higher). The depressant action of baclofen on spinal cord preparations was similar to that produced by the excitant amino acid antagonist alpha,epsilon-diaminopimelic acid. A structure-activity study showed that the (--)-isomer of baclofen was over 20 times more potent than the (+)-isomer as a spinal depressant. Also the position and nature of the halogen substitutent in the ring is critical with baclofen giving optimal activity. It is concluded that the depressant action of baclofen from depression of the presynaptic release of excitant amino acid transmitter(s).

GABA and baclofen potentiate the K+-evoked release of methionine-enkephalin from rat striatal slices

GABA potentiates the potassium-evoked release of methionine-enkephalin (ME) from slices of rat corpus striatum. This potentiation is observed only when a submaximal concentration (30 mM) of K+ is used to evoke release. The effect of GABA is dose-dependent between 100 and 100 micrometers. The basal release of ME is not altered by these concentrations of GABA. Before, but not muscimol, mimics the effect of GABA on the evoked release of ME. This effect is not stereoselective as both the (+)- and (-)-isomers of baclofen enhance ME release. Picrotoxin (100 micrometers) blocks the enhancement of ME release produced by both GABA and baclofen. Bicuculline methiodide (100 micrometers) does not block the effect of GABA. The effect of GABA on ME release may be mediated by an atypical GABA receptor which is activated by baclofen.

The effects of GABA-ergic drugs on enkephalin-induced motor seizure phenomena in the rat

1. The relationship between the effects of GABA-ergic drugs and D-ala2-met-enkephalinamide (DALA)-induced myoclonic contractions of inframandibular muscles has been studied in the rat. 2. GABA-ergic drugs altered enkephalin-induced myoclonic contractions in the following manner: (a) The GABA-mimetic drugs, muscimol, gabaculine and baclofen, decreased DALA-induced myoclonic contractions. (b) The GABA antagonist, bicuculline and the anticonvulsant substance, sodium valproate (dipropylacetic acid, DPA) potentiated DALA-induced myoclonic contractions. The potentiating effect of DPA is probably due to its opiate-like activity, since naloxone abolished this effect. 3. The modulatory effect of the GABA-mimetic drug on enkephalin-induced myoclonic contractions may give grounds for further study to test the possible use of other GABA-mimetic drugs and possibly opiate antagonists for the treatment of myoclonic syndromes.

Antinociceptive effects of baclofen and muscimol upon intraventricular administration

The effects of intraventricularly administered baclofen and muscimol were investigated on tail-flick responding and on vocalization and motor responses to nociceptive pinch. Baclofen (1 microgram) and muscimol (0.1 microgram) strongly reduced responding to pinch, particularly vocalization, without altering tail-flick responding. When given systemically, however, baclofen markedly attenuated tail-flick as well as pinch responding. Muscimol produced only weak antinociception by systemic administration, suggesting that it may have poor access to brain. At antinociceptive doses, i. vent. muscimol produced less apparent muscle relaxation than did baclofen. These results suggest that baclofen's antinociceptive action may be mediated in part by a supraspinal, GABAergic substrate, in addition to a spinal component which may not directly involve GABA.

An in vivo method for testing putative GABA-like compounds

A technique is described that enables compounds with GABA-ergic properties to be rapidly identified in vivo. Electrical stimulation of the neostriatum in the conscious rat evoked a contralateral head-turn. Evidence is presented that this easily timed motor response involves, at least in part, GABA-ergic mechanisms in the globus pallidus. GABA drugs were injected through a cannula into the ipsilateral globus pallidus and their effects on head-turning observed. Known GABA agonists including muscimol slowed the head-turn, whereas the GABA antagonist picrotoxin facilitated it. A number of drugs such as baclofen, diazepam, and pentobarbitone which have been attributed with GABA-like properties behaved like GABA agonists in the head-turn model following either intrapallidal or intraperitoneal injection. Other drugs, e.g. tranquillizers, with no known GABA-like properties, did not effect the head-turn time.

Effects of the unilateral nigral application of baclofen on dopamine release in the two caudate nuclei of the cat

"Encéphale isolé" cats were implanted with three push-pull cannulae. The two caudate nuclei (CN) were superfused continuously with 3H-tyrosine to measure the release of 3H-DA in serially collected fractions. Three hours after the beginning of the experiment, d,l-baclofen, d- or l-baclofen or gamma-hydroxybaclofen were introduced for 15 min in the medium used to superfuse the left substantia nigra. d,l-Baclofen (10(-6) M) stimulated 3H-DA release in the ipsilateral as well as in the contralateral CN but the effect was more pronounced in the ipsilateral CN. Similar effects were observed with l-baclofen. d-Baclofen and gamma-hydroxybaclofen were inactive. The results obtained are discussed in the light of effects previously observed during the nigral application of gamma-aminobutyric acid (GABA) and related compounds.

Effects of some anti-epileptic, neuroleptic and gabaminergic drugs on convulsions induced by D,L-allylglycine

The antagonism of various seizure and time-related components of the convulsions resulting after IV injection of D,L-allylglycine into male Wistar rats were assessed in a standard test procedure. Trimethadione and ethosuximide did not antagonize the seizure components, whereas clonazepam, phenobarbital, diphenylhydantoin, primidone, valproate sodium, aminoxyacetic acid, etomidate, acetazolamide, flunarizine, pipamperone and baclofen did. The allylglycine test may thus represent a relatively specific method of differentiating between drugs effective against partial or generalized convulsive seizures from those effective against absence seizures. The neuroleptics haloperidol and pimozide were completely inactive in contrast to their reported antagonism of bicuculine seizures. The spectra of the active substances are discussed with respect to Principal Component and Cluster Analysis. Noteworthy are the similarities between baclofen and etomidate; between aminoxyacetic acid, phenobarbital and valproate sodium; and between diphenylhydantoin and flunarizine.

Dopamine-dependent hyperactivity in the rat following manipulation of GABA mechanisms in the region of the nucleus accumbens

The effect of manipulation of GABA mechanisms in the region of the nucleus accumbens on dopamine-dependent locomotor hyperactivity in the rat has been studied. Two models of hyperactivity were used: (1) the injection of dopamine into the region of the nucleus accumbens in nialamide-pretreated animals and (2) the systemic administration of d-amphetamine. Both GABA and the GABA agonist 3-aminopropane sulphonic acid (3-APS) depressed hyperactivity in a dose-related manner. High concentrations of GABA (greater than 100 micrograms) were required to produce a significant effect and the response was short-lived possibly reflecting the efficient GABA inactivating mechanisms. 3-APS proved to be approximately 10 times more potent as compared to GABA in the dopamine-accumbens hyperactivity model. Conversely GABA receptor antagonism with low doses of either picrotoxin or bicuculline enhanced the mild locomotor response induced by a low dose of dopamine injected into the nucleus accumbens. However such results were difficult to evaluate fairly as higher doses of the GABA antagonists resulted in varying degrees of generalized seizures. Blockade of GABA uptake systems with cis-1, 3-aminocyclohexane carboxylic acid (ACHC), nipecotic acid or beta-alanine within the region of the nucleus accumbens produced dose-related depression of dopamine-dependent hyperactivity in both models. GABA uptake blockade (nipecotic acid) significantly enhanced the GABA-mediated depression of hyperactivity induced by bilateral injection of dopamine into the nucleus accumbens. The results demonstrate an inhibitory action of GABA and drugs facilitating GABA-ergic transmission on dopamine-dependent hyperactivity in the rat. Although open to criticisms of not being able to distinguish between true GABA effects and the results of non-specific neuronal depression the hyperactivity model underlines the potency of the GABA uptake blocking compounds and their possible potential for future clinical use.