Neural mechanisms of somatic sensory regulation of catecholamine secretion from the adrenal gland.

The reflex regulation of catecholamine secretion of the adrenal gland by various somatic afferent stimuli via sympathetic nerves in the efferent limb of the reflex has been of physiological interest since W.B. Cannon studied the subject. The reflex regulation is characteristically interfered with by emotional involvement which made it difficult to analyze the neural mechanisms in conscious subjects and delayed these studies. However, for the last several years, electrophysiological and endocrinological approaches have advanced with the aid of anesthesia. Recent studies on anesthetized animals have proven that the adrenal sympatho-medullary functions are influenced by various somatic afferent stimuli, either natural or electrical stimulation, and that the somatically induced adrenal sympathetic reflex responses result in changes in catecholamine secretions from the adrenal gland. Somato-adrenal sympathetic reflexes contain both spinal and supraspinal reflex components, the spinal component having a segmental reflex organization, whereas the supraspinal component is generalized and non-segmental. The reflexes are excitatory or inhibitory, depending on the stimuli and species of animals, e.g., excitatory by pinching and inhibitory by brushing in rats. The excitatory responses are mainly mediated by somatic unmyelinated C afferent fiber activation, whereas the inhibitory responses are carried by somatic myelinated A afferent activation. The recent finding that sympatho-adrenal medullary function increases at rest in normal rats during aging, and in SHR-SP even during their youth was reviewed. Although no neural mechanism governing the increase in adrenal sympathetic nerve activity for these cases has been determined, the reflex regulation of such activity by somatic afferent and arterial baroreceptor afferent stimulation is well maintained in aged normal rats. These facts suggest the possibility that the sympatho-adrenal medullary functions can somatically be regulated in the aged. The recent study demonstrating that i.c.v. administered CRF increases in sympatho-adrenal medullary functions was reviewed, because central CRF seems to play an important role in controlling the somatically induced sympatho-adrenal medullary reflex. Finally, all these findings from anesthetized animals seem useful in understanding the neural mechanism of the somato-adrenal sympathetic reflex in conscious subjects.