Participation of endogenous neuropeptide Y in the suppression of baroreceptor reflex response by locus coeruleus in the rat.

We evaluated the potential participation of endogenous brain neuropeptide Y (NPY) in the suppression of baroreceptor reflex (BRR) response by locus coeruleus (LC), using adult male Sprague-Dawley rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Bilateral microinjection of an antiserum against NPY (1:20, 20 nl) into the caudal one-third level of the nucleus tractus solitarii (NTS), the terminal site for baroreceptor afferent fibers, significantly reversed the suppressive effect of electrical or chemical activation of the LC on the BRR response. Treatments with NPY (4.65 pmol, 20 nl), normal rabbit serum, aCSF and heat-inactivated NPY or NPY antiserum, on the other hand, were ineffective. The LC-promoted inhibition of the BRR response was also attenuated by the alpha 1-adrenoceptor antagonist prazosin (50 pmol, 20 nl), either microinjected alone or in combination with NPY antiserum into the bilateral NTS. Mathematical treatment of our data revealed that the depressive effect on the BRR response of NPY or NE released at the NTS following LC activation manifested different time-course and magnitude. The one by endogenous NPY maximized at 40 min and amounted to no more than 20% of, whereas that by NE peaked at 10 min and contributed no less than 30% to, the suppression. These results suggest that both endogenous NPY and NE may participate in the suppression of BRR response by the LC at the NTS.