Permissive role of spinal alpha 1-adrenoceptors in sudomotor efferents.
The electrodermal potential (EDP) recorded in the forepaws of anaesthetized cats in response to stimulation of the cholinergic-sympathetic nervous system at different levels was taken as a measure for sudomotor activity. Electrical stimulation of the hypothalamus with square wave pulses (1 ms duration, 0.5-64 Hz, 2 s train length) at intervals of 1 min induced rate-dependent EDPs which were inhibited at all rates of stimulation by intravenous (i.v.) injection of doses less than 40 micrograms/kg of the alpha 1-adrenoceptor blocking drug prazosin. However, prazosin (50-500 micrograms/kg i.v.) did not impair EDPs induced by preganglionic stimulation (0.5-64 Hz) or by injection of the nicotinic ganglion stimulant DMPP (50 micrograms/kg i.v.). Prazosin (50 micrograms/kg i.v.) inhibited EDPs induced by unilateral electrical stimulation (square wave pulses, 1 ms duration, 16 Hz, 2 s train length, 1 min intervals) of the spinal cord at C 1 in cats with an axotomy at the level of the medulla oblongata, thus indicating a spinal site of prazosin action and suggesting a permissive role of spinal catecholamines by activation of alpha 1-adrenoceptors. In spinal preparations pretreated with 250 micrograms/kg yohimbine i.v. to block inhibition by alpha 2-adrenoceptors of catecholamine reuptake, cocaine 2.5 mg/kg i.v. potentiated EDPs induced by spinal stimulation with 8 Hz. This effect could be antagonized by 50 micrograms/kg prazosin or 1000 micrograms/kg corynanthine i.v. In spinal preparations pretreatment with 5 mg/kg reserpine i.p. for depletion of catecholamines and 250 micrograms/kg yohimbine i.v., EDPs (16 Hz) were smaller than in undepleted preparations. Under these conditions injection of 100 micrograms/kg clonidine i.v. caused amplification of EDPs. This effect was antagonized by 50 micrograms/kg prazosin i.v. After i.v. pretreatment with 250 micrograms/kg yohimbine the i.v. injection of 2.5 mg/kg cocaine also potentiated EDPs which were induced by hypothalamic stimulation in intact cats. The results indicate that catecholaminergic neurons influence sudomotor activity by interaction with efferents of the cholinergic-sympathetic nervous system at the level of the spinal cord. Catecholamines seem to facilitate impulse transmission in non-catecholaminergic synapses by activation of alpha 1-adrenoceptors.
Rybarczyk
M C
MC
Walland
A
A
D000818:Animals; D002395:Catecholamines / Q000502:physiology; D002415:Cats; D003042:Cocaine / Q000494:pharmacology; D004558:Electric Stimulation; D005260:Female; D005712:Galvanic Skin Response / Q000187:drug effects; D005727:Ganglia, Spinal / Q000502:physiology; D007031:Hypothalamus / Q000502:physiology; D008297:Male; D009476:Neurons, Efferent / Q000187:drug effects*; D011224:Prazosin / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D012110:Reserpine / Q000494:pharmacology; D013116:Spinal Cord / Q000502:physiology; D015016:Yohimbine / Q000494:pharmacology
D002395:Catecholamines; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D012110:Reserpine; D003042:Cocaine; D011224:Prazosin