Participation of hypothalamic paraventricular nucleus in locus ceruleus-induced baroreflex suppression in rats

We evaluated the potential role of the paraventricular nucleus (PVN) of the hypothalamus in the suppression of baroreceptor reflex (BRR) response by locus ceruleus (LC), using adult male Sprague-Dawley rats anesthetized with pentobarbital sodium. Electrical stimulation of histologically verified sites in the LC elicited significant reduction in the BRR response. This suppressive effect of LC on BRR response was appreciably antagonized by bilateral electrolytic lesions of the PVN and reversibly blocked by bilateral microinjection of 2% lidocaine into similar hypothalamic loci. Local administration of the alpha 1-adrenoceptor antagonist, prazosin (50 pmol), or an antiserum directed against neuropeptide Y (NPY) (1:20) into the bilateral PVN also significantly attenuated the suppression elicited by LC on BRR response. In contrast, treatments with the alpha 2-adrenoceptor blocker, yohimbine (50 pmol), heat-inactivated NPY antiserum (1:20), artificial cerebrospinal fluid, or normal rabbit serum (1:20) produced no discernible effect. These results suggest that the PVN may constitute part of the efferent pathways through which LC elicits its inhibition on the BRR response. Furthermore, LC may activate this pathway via alpha 1-adrenoceptors and NPY receptors in the PVN.