piperidines and methyl-6-7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate

We investigated the role of the NMDA receptor complex in DMCM (methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate)-induced seizures in mice. The seizure threshold of DMCM was evaluated using an i.v. infusion technique. Pretreatment with the non-competitive NMDA receptor antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cycloheptan-5,10-imine maleate) or phencyclidine (PCP) significantly increased the seizure threshold for DMCM. Furthermore, the seizure threshold of DMCM was increased by intracerebroventricular (i.c.v.), but not intrathecal (i.t.), pretreatment with MK-801. Moreover, 7-chlorokynurenic acid, a glycine site antagonist, also increased the seizure threshold of DMCM, whereas ifenprodil, a non-competitive polyamine site antagonist, did not. These findings indicate that the ion-channel binding site and the glycine binding site on the NMDA receptor complex in the brain may be involved in the expression of seizures induced by DMCM.

Topics: Animals; Carbolines; Cerebral Ventricles; Convulsants; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Infusions, Intravenous; Injections, Intraventricular; Injections, Spinal; Kynurenic Acid; Male; Mice; Mice, Inbred Strains; Phencyclidine; Piperidines; Receptors, N-Methyl-D-Aspartate; Seizures; Spinal Cord

We have previously characterized 5-(4-piperidyl)isoxazol-3-ol (4-PIOL) as a non-desensitizing partial agonist at GABAA receptors and shown that the responses are mediated by short-duration channel openings consonant with single-ligand gated openings of the Cl- channels. We presently investigate whether responses of cultured rat hippocampal neurones to 4-PIOL are modulated by benzodiazepine (BDZ) and barbiturate receptor ligands. Whole-cell patch-clamp recordings of maximal responses to 1 mM 4-PIOL were comparable in size to responses evoked by 10 microM of the full GABAA agonist, isoguvacine. The BDZ receptor inverse agonist, DMCM (1 microM) reduced responses to isoguvacine (to 65.7 +/- 11.0%) and 4-PIOL (to 69.3 +/- 3.5%) to a similar extent. The BDZ agonist, midazolam (0.1 microM) potentiated responses to both agonists, and resulted in responses with an early peak with later fading. Potentiation of the peak response to 4-PIOL (to 163 +/- 14%) was significantly less than for isoguvacine (215 +/- 11%). Pentobarbital (50 microM) caused a very marked, but variable, potentiation of the peak response to 4-PIOL (to 484 +/- 93%), which was significantly greater than the potentiation of the peak response to isoguvacine (to 304 +/- 46%), and induced fading. This suggests that a relatively larger number of the 4-PIOL-induced channel openings can be transformed to longer duration openings by pentobarbital. In conclusion, responses to 4-PIOL and isoguvacine are modulated by BDZ and barbiturate ligands in a qualitatively similar manner, but with a number of quantitative differences which cannot be readily explained by the kinetic model of Macdonald and Twyman (1992). Investigation of these responses at the single-channel level could provide further insight into the operation of the GABAA receptor-ionophore complex.

Topics: Animals; Barbiturates; Benzodiazepines; Carbolines; Cells, Cultured; Convulsants; Electrophysiology; GABA Agonists; GABA Modulators; GABA-A Receptor Agonists; Hippocampus; Hypnotics and Sedatives; Isonicotinic Acids; Isoxazoles; Membrane Potentials; Midazolam; Neurons; Patch-Clamp Techniques; Pentobarbital; Piperidines; Rats

Electrophysiological recordings from mouse neurones in tissue culture have been used to investigate how agents which interact with the benzodiazepine receptor modulate neuronal responses to gamma-aminobutyric acid (GABA) and its mimetics, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and piperidine-4-sulphonic acid (P4S). Experiments were performed in a physiological medium, pH 7.35 at 34-36 degrees C. gamma-Aminobutyric acid, THIP and P4S were applied by iontophoresis to neuronal somata. Responses were assessed by current-clamp or voltage-clamp recordings. Midazolam (an agonist at the benzodiazepine receptor) and the beta-carboline, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM; an inverse agonist at the BZ receptor), were applied by pressure ejection from blunt pipettes. The potency order of the agonists was GABA greater than P4S greater than THIP. Midazolam (10(-7)-10(-5) M) potentiated responses to all three agonists to a similar extent with a shift to the left of the dose-response curve. The drug DMCM (10(-6)-10(-5) M) decreased the responses to all three agonists to a similar extent. The DMCM-induced depression was of a non-competitive nature. It has previously been proposed that THIP is a partial agonist and P4S an antagonist at the GABA receptor coupled to the benzodiazepine receptor, or that the benzodiazepine-receptor-coupled and electrophysiological GABA receptors are distinct. In the present study, responses to the three agonists were modulated to a comparable extent following manipulation of the benzodiazepine receptor. It is therefore unnecessary to invoke the above explanations to account for these results.

Topics: Animals; Benzodiazepines; Carbolines; Cells, Cultured; Convulsants; Electrophysiology; Female; gamma-Aminobutyric Acid; Iontophoresis; Isoxazoles; Mice; Midazolam; Neurons; Oxazoles; Piperidines; Receptors, GABA-A