AORTIC BARODENERVATION UP-REGULATES ALPHA(2)-ADRENOCEPTORS IN THE NUCLEUS-TRACTUS-SOLITARIUS AND ROSTRAL VENTROLATERAL MEDULLA - AN AUTORADIOGRAPHIC STUDY

Earlier findings have shown that alpha(2)-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla modulate baroref lexes. The present study investigated whether attenuation of barorefle xes induced by surgical interruption of aortic baroafferents is relate d to an alteration of alpha(2)-adrenoceptor binding in these regions o f the brainstem. In vitro autoradiography was utilized to assess the d ensity and binding dissociation constant (affinity) of alpha(2)-adreno ceptors in the rostral ventrolateral medulla and in the middle and ros tral portions of the nucleus tractus solitarius of aortic-barodenervat ed and sham-operated rats. Compared to sham operation, aortic barodene rvation caused an acute rise in mean arterial pressure and heart rate and a significant reduction in baroreflex sensitivity. Two days later, mean arterial pressure and heart rate of conscious aortic-barodenerva ted rats subsided to sham-operated levels, whereas the baroreflex sens itivity remained significantly (P<0.05) reduced when measured by pheny lephrine (0.55 +/- 0.08 vs 1.26 +/- 0.07 ms/mmHg) or nitroprusside (0. 43 +/- 0.06 vs 1.01 +/- 0.09 ms/mmHg). Examination of brainstem corona l sections obtained from separate groups of rats 48 h after surgery an d preincubated with [H-3]rauwolscine (0.5-16 nM) revealed that labelin g of alpha(2) binding sites was saturable and of high affinity. Scatch ard analysis of the saturation isotherms obtained from the three brain areas of sham-operated rats showed an uneven distribution of alpha 2 binding sites; the rostral nucleus tractus solitarius exhibited the hi ghest density and lowest affinity. Aortic barodenervation caused regio n-dependent changes in the binding activity of alpha(2)-adrenoceptors. These changes comprised significant (P<0.05) increases in the density of alpha(2)-adrenoceptors in the middle nucleus tractus solitarius (4 36 +/- 60 vs 240 +/- 50 fmol/mg protein) and rostral ventrolateral med ulla (350 +/- 67 vs 194 +/- 35 fmol/mg protein) compared with sham-ope rated rats; no significant changes occurred in the rostral nucleus tra ctus solitarius. The affinity of alpha(2) binding sites was not change d by aortic barodenervation in any of the three brain regions. These f indings suggest that attenuation of baroreflexes produced by aortic ba rodenervation coincides with up-regulation of alpha(2)-adrenoceptors i n brainstem areas that play critical roles in the control of cardiovas cular functions. (C) 1997 IBRO.