The effects of desipramine (DMI) and alpha-2 adrenoceptor agonists on flexor reflex activity (FRA) in the spinalized and decerebrate rat.

In the spinalized and decerebrate rat, electrically evoked flexor reflex activity (FRA) of the right anterior tibialis muscle was facilitated by DMI (0.1-3 mg/kg i.v.). DMI (3 mg/kg i.v.) had not effect on twitches of the left ant. tib. muscle evoked by electrical stimulation of the distal stump of the severed sciatic nerve (directly evoked twitches). The alpha-1 adrenoceptor antagonist prazosin (300 micrograms/kg i.v.) and the alpha-2 adrenoceptor agonists clonidine (5 and 10 micrograms/kg i.v.) and BHT-920 (100 micrograms/kg i.v.) all suppressed FRA facilitated by DMI (3 mg/kg i.v.). The alpha-2 adrenoceptor antagonists yohimbine (100 micrograms/kg i.v.) and mianserin (30 micrograms/kg i.v.) antagonized the suppressant effect of clonidine (5 micrograms/kg i.v.) on DMI-facilitated FRA. Clonidine (10 micrograms/kg i.v.) had no effect on directly evoked twitches, and at 3-300 micrograms/kg i.v. had no effect on FRA facilitated by L-DOPA (100 mg/kg i.p.). It is suggested that DMI-facilitated FRA can be used as a test for alpha-2 adrenoceptor agonists which may act at presynaptic alpha-2 adrenoceptors in the spinal cord.