strychnine and gaboxadol

The sedative and hypnotic agent 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-3-ol (THIP) is a GABA(A) receptor (GABA(A)R) agonist that preferentially activates delta-subunit-containing GABA(A)Rs (delta-GABA(A)Rs). To clarify the role of delta-GABA(A)Rs in mediating the sedative actions of THIP, we utilized mice lacking the alpha(1)- or delta-subunit in a combined electrophysiological and behavioural analysis. Whole-cell patch-clamp recordings were obtained from ventrobasal thalamic nucleus (VB) neurones at a holding potential of -60 mV. Application of bicuculline to wild-type (WT) VB neurones revealed a GABA(A)R-mediated tonic current of 92 +/- 19 pA, which was greatly reduced (13 +/- 5 pA) for VB neurones of delta(0/0) mice. Deletion of the delta- but not the alpha(1)-subunit dramatically reduced the THIP (1 mum)-induced inward current in these neurones (WT, -309 +/- 23 pA; delta(0/0), -18 +/- 3 pA; alpha(1) (0/0), -377 +/- 45 pA). Furthermore, THIP selectively decreased the excitability of WT and alpha(1) (0/0) but not delta(0/0) VB neurones. THIP did not affect the properties of miniature inhibitory post-synaptic currents in any of the genotypes. No differences in rotarod performance and locomotor activity were observed across the three genotypes. In WT mice, performance of these behaviours was impaired by THIP in a dose-dependent manner. The effect of THIP on rotarod performance was blunted for delta(0/0) but not alpha(1) (0/0) mice. We previously reported that deletion of the alpha(1)-subunit abolished synaptic GABA(A) responses of VB neurones. Therefore, collectively, these findings suggest that extrasynaptic delta-GABA(A)Rs vs. synaptic alpha(1)-subunit-containing GABA(A)Rs of thalamocortical neurones represent an important molecular target underpinning the sedative actions of THIP.

Topics: Analysis of Variance; Animals; Animals, Newborn; Bicuculline; Dose-Response Relationship, Drug; Electric Stimulation; Female; GABA Agonists; GABA Antagonists; Glycine Agents; In Vitro Techniques; Isoxazoles; Locomotion; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; Motor Activity; Neural Inhibition; Patch-Clamp Techniques; Receptors, GABA-A; Strychnine; Thalamus; Time Factors

Retinal levels of [Leu5]enkephalin-like immunoreactivity (LE-LI) increase during the light and decrease during darkness, in vivo15. Intravitreal injection of the GABA antagonist picrotoxin had no effect on the accumulation of LE-LI during the light, suggesting the absence of significant GABAergic control over LE-LI cells. However, injection of the glycine antagonist strychnine, prevented the light-induced increase of retinal levels of LE-LI during 6 h exposure to light, indicating the presence of glycinergic control over the LE-LI neurons. When applied during the dark, strychnine increased the depletion of LE-LI by 34% compared to vehicle-injected eyes, suggesting that the LE-LI neurons receive some glycinergic input during the dark as well. The release of LE-LI from retinas superfused in vitro is depressed by exposing the preparation to light. Superfusing isolated retinas with physiological buffer containing picrotoxin (100 microM), GABA (50 mM), or the GABA agonists muscimol (100 microM), (+)-baclofen (200 microM), or 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) (100 mM), had no effect on the efflux of LE-LI. Strychnine (100 mM) however increased the efflux of LE-LI by 64%, compared to the spontaneous efflux during the light. Glycine (15 and 50 mM) decreased the spontaneous efflux of LE-LI from retinas superfused in darkness by 44-48% and by 31% at 5 mM. These data are consistent with the results from pharmacological manipulations in vivo. We conclude that the LE-LI amacrine cells are under inhibitory control from glycinergic but not from GABAergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Baclofen; Chickens; Enkephalin, Leucine; GABA Antagonists; gamma-Aminobutyric Acid; Glycine; Isoxazoles; Muscimol; Neurotransmitter Agents; Picrotoxin; Radioimmunoassay; Retina; Strychnine

Pentobarbital and phenobarbital exhibited anticonvulsant effects against picrotoxin (10 mg/kg, IP) as well as against strychnine (4 mg/kg, IP). Pentobarbital was also effective against bicuculline whereas only hypnotic doses of phenobarbital provided some protection against bicuculline- (8 mg/kg, IP) induced convulsions. Diazepam as well as THIP, but not baclofen, were also effective against all the three chemoconvulsants. Baclofen or subeffective doses of diazepam or THIP, when combined with subeffective dose of pentobarbital exhibited anticonvulsant activity against all the chemoconvulsants studied. On the other hand, a combination of subeffective doses of these agents with subeffective doses of phenobarbital provided protection only against picrotoxin and strychnine. These observations indicate that pentobarbital is quite effective against convulsions caused by agents acting at picrotoxin site, GABAA receptor or glycine receptor whereas phenobarbital is effective only against agents acting at picrotoxin site and glycine receptor, and is very weak anticonvulsant against agents causing blockade of GABAA receptors. Furthermore, activation of GABAA receptors or benzodiazepine receptors also provide protection against agents acting at GABAergic system or glycine receptors. On the contrary, activation of only GABAB receptors is inadequate to provide the protective effect. However, the activation of GABAA as well as GABAB receptors facilitate the anticonvulsant effect of both the barbiturates. Furthermore, pentobarbital, but not phenobarbital, facilitates the anticonvulsant effect of benzodiazepines against chemoconvulsants acting at GABAergic site or glycine receptors.

Topics: Animals; Anticonvulsants; Baclofen; Bicuculline; Convulsants; Diazepam; Drug Interactions; Isoxazoles; Male; Pentobarbital; Phenobarbital; Picrotoxin; Rats; Rats, Inbred Strains; Receptors, GABA-A; Seizures; Strychnine

[3H]Strychnine binding to rat pons + medulla membranes was used as a measure of glycine receptors or glycine receptor-coupled chloride channels in vitro. A series of compounds structurally related to 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), which previously were shown to antagonize glycine responses in cat spinal cord, inhibited [3H]strychnine binding in micromolar concentrations. The most potent of these glycine antagonists, 5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol (iso-THAZ), was also the most potent inhibitor of [3H]strychnine binding, with a Ki of 1,400 nM. The Ki value for strychnine was 7.0 nM, whereas the Ki value for the mixed gamma-aminobutyric acid (GABA)/glycine antagonist 3 alpha-hydroxy-16-imino-5 beta-17-aza-androstan-11-one (RU 5135) was only 4.6 nM. Sodium chloride (1,000 mM) enhanced the affinity of strychnine, brucine, isostrychnine, and the nonselective GABA antagonist pitrazepin for [3H]strychnine binding sites, whereas the affinities of glycine, beta-alanine, and taurine were reduced. These sodium chloride shifts, however, were not predictive of antagonist or agonist properties, since the sodium chloride shift for the glycine antagonist iso-THAZ and of the other THIP-related antagonists were similar to those of the glycine-like agonists. The various sodium chloride shifts show that different groups of ligands bind to glycine receptor sites in different ways.

Topics: Animals; Anions; Binding, Competitive; Cell Membrane; Chlorides; Choline; GABA Antagonists; Glycine; Isoxazoles; Male; Medulla Oblongata; Oxazoles; Pons; Rats; Rats, Inbred Strains; Receptors, Glycine; Receptors, Neurotransmitter; Sodium Chloride; Strychnine

Sprague-Dawley rats were used to further characterize the anorexic action of orally-administered THIP, a GABA-analogue. The anorexic action of THIP (5 or 10 mg/kg) was antagonized by prior subcutaneous injection of bicuculline (1 mg/kg), but not by prior subcutaneous injection of bicuculline-methobromide (1.5 mg/kg), strychnine-SO4 (0.75 mg/kg), pentylenetetrazol (25 mg/kg), or picrotoxin (1 mg/kg). Orally-administered GABA (50-300 mg/kg), bicuculline (1-10 mg/kg) or picrotoxin (1-10 mg/kg) generally did not inhibit food intake. These results indicate that the anorexic action of THIP is mediated by central GABA-receptors.

Topics: Administration, Oral; Animals; Bicuculline; Eating; gamma-Aminobutyric Acid; Isoxazoles; Male; Oxazoles; Pentylenetetrazole; Picrotoxin; Rats; Rats, Inbred Strains; Receptors, Cell Surface; Receptors, GABA-A; Strychnine