Functional characterisation of alpha 1-adrenoceptor subtypes mediating noradrenaline-induced inositol phosphate...

In cross-chopped slices from rat thalamus and in the presence of 10 mM LiCl, noradrenaline stimulated the accumulation of [3H]inositol phosphates with [3H]inositol monophosphates ([3H]IP1) being the major product detected (86 +/- 2% of total [3H]inositol phosphates). Noradrenaline-induced [3H]IP1 accumulation was concentration-dependent and yielded and EC50 of 4.6 +/- 0.2 microM, maximum effect of 272 +/- 3% of basal formation and Hill coefficient (nH) of 1.6 +/- 0.1. The effect of 100 microM noradrenaline was inhibited by the alpha 1-adrenoceptor antagonists prazosin, (+)-niguldipine, 5-methylurapidil and WB-4101 (2-(2,6-dimethoxyphenoxyethyl) aminomethyl-1,4-benzodioxane). The inhibition curve for prazosin best fit to a single-site model whereas curves for (+)-niguldipine, 5-methylurapidil and WB-4101 best fit to a two-site model. The putative alpha 1D-adrenoceptor-selective antagonist BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8- azaspiro[4.5]decane-7,9-dione) showed low potency and efficacy to inhibit the response to noradrenaline. Pre-treatment of the slices with chloroethylclonidine (100 microM; 30 min) decreased by 64 +/- 4% the maximum response. Noradrenaline-induced [3H]IP1 accumulation was significantly reduced by Ca2+ removal (by 64 +/- 2%) and by the Ca(2+)-channel blockers Ni2+, Co2+ and nimodipine (inhibition of 56 +/- 6%, 54 +/- 5% and 41 +/- 5%, respectively). Taken together these results indicate that noradrenaline-induced inositol phosphate formation in thalamus slices is mainly mediated by the activation of both alpha 1B and alpha 1A subtypes of alpha 1-adrenoceptors.