Use of a sympathetic-cholinergic system in the analysis of sympatho-inhibition produced by clonidine and some congeneric derivatives of clonidine

Clonidine inhibits electrodermal responses by an action on the spinal cord

Experiments were designed to determine the neural site of action for clonidine inhibition of sympathetic-cholinergic electrodermal responses (EDR) in anesthetized cats. Administration of clonidine (0.3-3.0 micrograms i.a.) directly to the stellate ganglion did not significantly decrease the amplitude of responses evoked by submaximal hypothalamic stimulation but did inhibit hypothalamic-evoked EDR when administered intrathecally at the C6 to T2 spinal levels. Administration of clonidine to the ganglion, however, did depress EDR evoked by the ganglionic stimulant, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP, 10 micrograms i.a.). Intravenous clonidine (1-30 micrograms) also reduced EDR amplitude evoked by single pulse stimulation of both the pre- and postganglionic sympathetic nerves with responses elicited from both sites depressed to an equal extent. Yohimbine (0.5 mg/kg i.v.) uniformly antagonized clonidine's depression of EDR regardless of the site or mode of activation. These results indicate that clonidine depresses centrally evoked sudomotor responses by activation of alpha 2-adrenoceptors in the spinal cord and to a limited extent by direct action at the neuroeffector junction. Although a possible DMPP-clonidine interaction appears to take place at the level of the sympathetic ganglion, it is unlikely that ganglionic blockade contributes significantly to clonidine inhibition of EDR evoked by electrical activation of the nervous system.

Pupillary dilation as an index of central nervous system alpha 2-adrenoceptor activation

In recent years there has been increasing evidence that some antihypertensive drugs like clonidine and alpha-methyldopa (after conversion in the brain to alpha-methylnorepinephrine) may decrease sympathetic tone by stimulating central nervous system (CNS) alpha 2-adrenoceptors. These same drugs also produce pupillary dilation in cats and rats. In this review, evidence is presented supporting the hypothesis that clonidinelike drugs act either directly or indirectly on CNS postsynaptic alpha 2-adrenoceptors to cause pupillary dilation by reduction of parasympathetic neural tone to the iris. It is further suggested that the underlying physiologic mechanism for this mydriatic action is activation of an ascending pathway that provides tonic inhibitory input by releasing norepinephrine on neurons in the Edinger-Westphal complex. Yohimbine-sensitive pupillary dilation in these species may provide a simple and effective model for quantitatively accessing CNS alpha 2-adrenoceptor activity.

A physiological and pharmacological analysis of the electrodermal response in the rat

Electrodermal responses (EDR) of the sympathetic-cholinergic sudomotor system were elicited in the footpads of the hindpaws of anesthetized rats. The most reactive CNS loci were caudal to the region of the posterior hypothalamus. Peripherally evoked responses were elicited by electrical stimulation of the decentralized tibial nerve. The amplitude of these evoked EDR was stable over time and both the centrally and peripherally elicited responses were frequency-dependent. Atropine (200 micrograms/kg) depressed the EDR elicited by both peripheral and central stimulation whereas hexamethonium (20 mg/kg) only inhibited the central EDR. Clonidine had no effect on the EDR evoked at the periphery but produced a significant dose-dependent depressant effect on the centrally evoked EDR; this effect was partially antagonized by yohimbine (0.75 mg/kg). It is suggested that the rat is a suitable species for the use of the sudomotor system in the investigation of adrenergic agents which are thought to have a central sympatho-inhibitory action.