An analysis of the α-adrenoreceptors of gigantocellular reticular nucleus concerned with cardiovascular regulation

In the present study an attempt was made to characterize the subtypes, define the location and delineate the function of α-adrenoceptors of gigantocellular reticular nucleus (GRN) in cats. Clonidine (150 and 450 ng), which has greater affinity for α₂ than α₁ adrenoceptors, induced dose-dependent hypotension and bradycardia in nanogram doses which could be blocked by prior treatment of the nucleus with yohimbine (5 μg) and guanethidine (20 μg) but not by phenoxybenzamine (5 μg). Similarly, noradrenaline (α₁=α₂) produced similar dose-dependent hypotension and bradycardia in microgram doses (5, 10 & 20 μg) which again could be blocked by yohimbine and guanethidine but not by phenoxybenzamine. Phenylephrine, which has a higher affinity for α₁ than α₂ adrenoceptors, did not produce any significant change in BP and HR up to 20 μg dose. These results clearly demonstrate that the adrenoceptors of GRN involved in cardiovascular regulation are of α₂ subtype, they are located presynaptically on catecholaminergic neurons and subserve an inhibitory function.