Clonidine inhibits nicotinic effects in ganglia of the cholinergic-sympathetic system.
The electrodermal potential (EDP) recorded with surface electrodes between the palm and the shaven back of the forepaws of anaesthetized cats was taken as a measure of sudomotor activity in response to submaximal activation of cholinergic-sympathetic efferents with the nicotinic ganglionic stimulant DMPP. The intravenous injection of 30 micrograms/kg clonidine inhibited EDPs in despinalized cats and in vagotomized cats with chronic denervation of the stellate ganglion. In vagotomized cats clonidine (30 micrograms/kg i.v.) shifted the dose-response curve of DMPP (4-256 micrograms/kg i.v.) to the right. An inhibitory effect was also observed in despinalized cats upon topical application of 0.1 and 1 microgram clonidine to the stellate ganglion. In all preparations the inhibitory effect of clonidine could be antagonized by intravenous injection of 200 micrograms/kg yohimbine. As all substances used were also active when injected during arrest of blood flow to the foreleg by means of a tourniquet, actions at the level of the sweat gland could be excluded. The results lead to the conclusion that the activation of postsynaptic somadendritic alpha 2-adrenoceptors in sudomotor ganglia of the cholinergic-sympathetic nervous system inhibits the effects of nicotinic ganglionic stimulation. It is assumed that this action is due to the known ganglionic hyperpolarization induced by alpha 2-adrenoceptor stimulants.
D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002415:Cats; D003000:Clonidine / Q000494:pharmacology*; D003714:Denervation; D004246:Dimethylphenylpiperazinium Iodide / Q000494:pharmacology; D005260:Female; D005712:Galvanic Skin Response; D005725:Ganglia, Autonomic / Q000187:drug effects*; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D011978:Receptors, Nicotinic / Q000187:drug effects*; D013116:Spinal Cord / Q000502:physiology; D013997:Time Factors; D014628:Vagotomy; D015016:Yohimbine / Q000494:pharmacology
D011978:Receptors, Nicotinic; D015016:Yohimbine; D004246:Dimethylphenylpiperazinium Iodide; D003000:Clonidine