Involvement of postsynaptic alpha 1- and alpha 2-adrenoceptors in the flexor reflex activity in the spinal rats.

Clonidine (0.2 mg/kg i.v.) and St 587 (1 mg/kg i.v.), selective agonists of alpha 2- and alpha 1-adrenoceptors, respectively, increased the flexor reflex amplitude in the spinal rat. Yohimbine and rauwolscine, alpha 2-adrenoceptor antagonists, inhibited dose-dependently the effect of clonidine but not of St 587. However, prazosin and clozapine, alpha 1-adrenoceptor antagonists, diminished the action of both agonists in a dose-dependent manner. After central chemosympathectomy caused by 6-OH-DA or DSP-4 the response to clonidine did not differ from that in the sham-operated animals. In 6-OH-DA treated rats yohimbine (1 mg/kg i.v.) antagonized the effect of clonidine to the same extent as it did in unlesioned animals. It is concluded that the results are functional evidence that alpha 1- and alpha 2-adrenoceptors are present in the rat spinal cord, and stimulation of each receptor increases flexor reflex activity.