Pharmacological characterization of (10bS)-1,2,3,5,6,10b-hexahydropyrrolo[2,1-a]isoquinoline oxalate (YSL-3S) as a new alpha(2)-adrenoceptor antagonist

Abstract

alpha(2)-Adrenoceptor antagonists, which can enhance synaptic norepinephrine levels by blocking feedback inhibition processes, are potentially useful in the treatment of disease states such as depression, memory impairment, impotence and sexual dysfunction. (10bS)-1,2,3,5,6,10b-Hexahydropyrrolo[2,1-a]isoquinoline oxalate (YSL-3S) was evaluated in several in vitro biological tests to establish its pharmacological profile of activities as an alpha(2)-adrenoceptor antagonist. Saturation binding assay revealed that [H-3]rauwolscine bound to the alpha(2)-adrenoceptors with a Kd value of 6.3 +/- 0.5 nM and a Bmax value of 251 +/- 39 fmol/mg protein in rat cortical synaptic membranes. Competitive binding assay showed that YSL-3S inhibited the binding of [H-3]rauwolscine (1 nM) in a concentration-dependent manner with a Ki value of 98.2 +/- 12.1 nM while it did not inhibit the binding of [H-3]cytisine (1.25 nM) to neuronal nicotinic cholinergic receptors. The Ki values of yohimbine, clonidine and norepinephrine for [H-3]rauwolscine binding were 15.8 +/- 1.0, 40.1 +/- 5.9 and 40.0 +/- 11.5 nM, respectively. In addition, the binding affinity of YSL-3S for a(2)-adrenoceptors was higher than that of its antipode and the racemic mixture. The functional activity of YSL-3S at the presynaptic alpha(2)-adrenoceptors was assessed using the prostatic portion of the rat vas deferens. Clonidine inhibited field-stimulated contractions of the vas deference in a dose-dependent manner. The presence of YSL-3S or yohimbine caused a parallel, rightward the dose-response curve of clonidine in a dose-dependent manner, indicating an antagonistic action at the presynaptic alpha(2)-adrenoceptors. The pA(2) values of yohimbine and YSL-3S were 7.66 +/- 0.13 and 6.64 +/- 0.18, respectively. The results indicate that YSL-3S acts as a competitive antagonist at presynaptic alpha(2)-adrenoceptors with a potency approximately ten times lower than yohimbine, but is devoid of binding affinity for neuronal nicotinic cholinergic receptors.