Agonistic effects of the β-carboline DMCM revealed in GABAA receptor γ2 subunit F77I point-mutated mice

Affinity of the inverse agonist methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) to the benzodiazepine binding site of the GABAA receptor is abolished by a phenylalanine (F) to isoleucine (I) substitution at position 77 of the γ2 subunit. We tested the effects of DMCM in gene knockin γ2I77 mice carrying this mutation. Unlike in wild-type mice, DMCM was not able to reverse the GABA-induced reduction of the picrotoxin-sensitive t-butylbicyclophosphoro-[35S]thionate ([35S]TBPS) binding to GABAA receptor channels in the forebrain sections of γ2I77 mice. Accordingly, DMCM was not convulsant in the mutant mice even at doses 20-fold higher (60 mg/kg, i.p.) than those producing convulsions in wild-type littermate controls (3 mg/kg, i.p.). Neither did DMCM raise the c-Fos levels in γ2I77 mouse brain. DMCM additionally exhibits a less well described agonistic effect on GABAA receptors that is normally masked by its strong inverse agonist effect. DMCM agonistically enhanced the GABA-induced reduction in [35S]TBPS binding to the cerebellar granule cell layer in control and mutant mice. In vivo DMCM (20-60 mg/kg i.p.) produced modest anxiolytic-like effects in γ2I77 mice as assessed by elevated plus maze and staircase tests, but no motor impairment was found in the rotarod test. The results suggest only minor agonistic efficacy for the β-carboline DMCM.