Acute changes in 3H-PAC and 125I-PYY binding in the nucleus tractus solitarii and hypothalamus after a hypertensive stimulus.

Activation of alpha-2-adrenergic and neuropeptide Y (NPY) receptors in the nucleus tractus solitarii (NTS) induces hypotension and bradycardia. On the contrary, activation of angiotensin II (Ang II) receptors leads to hypertension. Acute changes in binding parameters of alpha-2-adrenergic, NPY and Ang II receptors were evaluated in the NTS and paraventricular hypothalamic nucleus (PVN) of rats after a hypertensive stimulus employing quantitative receptor autoradiography. Saturation experiments showed a decrease in the number (Bmax) of alpha-2-adrenergic binding sites in the NTS 6 hours after coarctation-induced hypertension. Furthermore, the affinity of NPY receptors was diminished as seen by the increase in the KD value of 125I-PYY. Tyrosine hydroxylase and NPY immunoreactivities were increased in the NTS and ventral medulla. Binding of 125I-Ang II was not changed in the NTS. Binding of all ligands analyzed was not altered in the PVN. The results suggest an acute down-regulation of alpha-2-adrenergic and NPY receptors involved with hypotension in response to hypertensive stimulus, which might be related to an increased availability of catecholamines and NPY in the NTS.