Involvement of α1B-adrenoceptors in the anti-immobility effect of imipramine in the tail suspension test

Imipramine is a tricyclic antidepressant inhibitor of norepinephrine and serotonin neuronal reuptake. The roles of specific α1-adrenoceptor subtypes that might be targeted by the increased synaptic levels of noradrenaline induced by imipramine are not well understood. This study investigates the α1-adrenoceptor subtypes involved in the anti-immobility effect of imipramine in the mouse tail suspension test. The anti-immobility effect of imipramine (32mg/kg, i.p.) was significantly antagonised by the non-subtype-selective α1-adrenoceptor antagonist prazosin (0.5 and 1.0mg/kg, i.p.). Neither the selective α1A-adrenoceptor antagonist 5-methyl-3-[3-[3-[4-[2-(2,2,2,-trifluroethoxy)phenyl]-1-piperazinyl]propyl]-2,4-(1H,3H)-pyrimidinedione (RS-100329, 0.5 and 1.0mg/kg) nor the selective α1D-adrenoceptor antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride, (BMY-7378, up to 1.0mg/kg, i.p.) affected the anti-immobility effect of imipramine. However, the anti-immobility effect of imipramine was significantly antagonised by the selective α1B-adrenoceptor antagonist (2S)-4-(4-amino-6,7-dimethoxy-2-quinazolinyl)-2-[[(1,1-dimethylethyl)amino]carbonyl]-1-piperazinecarboxylate (L-765,314). In addition, mice treated only with RS-100329 or BMY-7378, but not with L-765,314, showed reduced immobility times in comparison to mice treated with vehicle. These results indicate that the selective antagonism of α1A- and α1D-adrenoceptors results in antidepressant-like effects and that the α1B-subtype is the main target for the increased levels of noradrenaline caused by imipramine.