Comparison of the influence of rostral and caudal raphe neurons on the adrenal secretion of catecholamines and on the release of adrenocorticotropin in the cat.

Neuroendocrine and autonomic responses were assessed in chloralose-anesthetized cats after chemical stimulation of medial brain-stem regions, including those that influence nociceptive input to the medullary or spinal dorsal horn. Microinjections of L-glutamate (0.5 M, 160 nl) were directed at the following rostral and caudal raphe nuclei: the periaqueductal gray (PAG), the dorsal raphe nucleus (DR), the raphe magnus (RM), and the raphe obscurus/raphe pallidus (Ro/Rpa). Activation of DR neurons evoked a significant increase in the adrenal secretion of epinephrine (+2.6 +/- 1.1 ng/min, P less than 0.01) that returned towards prestimulus values by 6 min, whereas microinjections into other raphe nuclei had no consistent effect. Activation of Ro/Rpa neurons evoked an increase in the plasma concentration of adrenocorticotropin (ACTH, +47.9 +/- 12.3 pg/ml, P less than 0.01), whereas microinjections into other raphe nuclei did not affect ACTH. Arterial pressure increased significantly after activation of PAG (+7.5 +/- 2.1 mm Hg, P less than 0.01) or of DR (+4.8 +/- 2.0 mm Hg, P less than 0.05) neurons, whereas heart rate increased significantly (P less than 0.05) after stimulation of cells within the Ro/Rpa. Glutamate microinjections within the RM, a raphe nucleus that exerts a significant descending influence on nociceptive input to the medullary and to the spinal dorsal horns, had no consistent effect on any measured variable. No evidence was seen to suggest that chemical activation of neurons within raphe nuclei inhibited the adrenal secretion of catecholamines or inhibited the release of ACTH. The results indicated that glutamate activation of neurons within different raphe nuclei evoked non-uniform effects on neuroendocrine and autonomic function. Further, these data suggested that the neural substrate underlying the control of the adrenal secretion of catecholamines and of the release of ACTH in response to activation of raphe neurons is likely distinct from that which contributes to the descending influence on nociceptive input to the medullary and spinal dorsal horn.