Altered central nervous system processing of baroreceptor input following hindlimb unloading in rats

The effect of cardiovascular deconditioning on central nervous system proce ssing of baroreceptor afferent activity was evaluated following 14 days of hindlimb unloading (HU). Inactin-anesthetized rats were instrumented with c atheters, renal sympathetic nerve electrodes, and aortic depressor nerve el ectrodes for measurement of mean arterial pressure, heart rate, renal sympa thetic nerve activity (RSNA), and aortic depressor nerve activity (ADNA). B aroreceptor and baroreflex functions were assessed during infusion of pheny lephrine and sodium nitroprusside. Central processing of baroreceptor affer ent input was evaluated by linear regression relating RSNA to ADNA. The max imum baroreflex-elicited increase in RSNA was significantly reduced in HU r ats (122 +/- 3.8 vs. 144 +/- 4.9% of baseline RSNA), whereas ADNA was not a ltered. The slope (-0.18 +/- 0.04 vs. -0.40 +/- 0.04) and gamma-intercept ( 121 +/- 3.2 vs. 146 +/- 4.3) of the linear regression relating increases in efferent RSNA to decreases in afferent ADNA during hypotension were signif icantly reduced in HU rats. There were no differences during increases in a rterial pressure. Results demonstrate that the attenuation in baroreflex-me diated increases in RSNA following HU is due to changes in central processi ng of baroreceptor afferent information rather than aortic baroreceptor fun ction.