Tonic descending inhibition of the spinal cardio-sympathetic reflex in the cat

Abstract

Electrical stimulation of the left inferior cardiac nerve elicited a two-component reflex potential (spinal and supraspinal reflexes) in the ipsilateral white ramus T 3 from which recordings were made in chloralose-anaesthetised cats. Reversible interruption of all spinal pathways achieved by cooling the spinal cord at C 2/C 3 produced an enhancement of the spinal reflex and abolished the supraspinal reflex, the latter usually being the more prominent reflex potential prior to spinal cord block. The spinal cord block-induced increase in the amplitude of the spinal reflex was, however, less than the increase observed during stimulation of the somatic intercostal nerve T 4. Recordings of the afferent volley following cardiac nerve stimulation and analysis of the stimulus-reflex response relationship in neuraxis-blocked cats indicated that the spinal reflex as determined here was activated by Aδ afferent fibres. However, if stimulus strength was raised above C-fibre threshold, spinal cord block revealed in addition a late spinal reflex response. In some cases, the appearance of this late potential was accompanied by a secondary decline of the earlier spinal reflex potential, possibly indicating C-fibre-mediated afferent inhibition. Neither baroreceptor activation nor denervation had any effect on spinal reflex amplitudes. Pharmacologically, clonidine given i.v. to cats with a blocked neuraxis reduced the spinal reflex amplitudes to pre-block values, an action which could be antagonised by the subsequent administration of the α 2-adrenoceptor antagonist rauwolscine. When given to non-pretreated cats with intact neuraxis, however, neither rauwolscine nor its analog yohimbine were capable of inducing a persistent release from tonic inhibition. The results suggest that both purely visceral and somato-visceral reflexes are subject to tonic descending inhibition, but they do not support the hypothesis that a catecholamine is the responsible transmitter mediating this inhibition.