DIRECTIONALLY SPECIFIC CHANGES IN ARTERIAL-PRESSURE INDUCE DIFFERENTIAL PATTERNS OF FOS EXPRESSION IN DISCRETE AREAS OF THE RAT BRAIN-STEM - A DOUBLE-LABELING STUDY FOR FOS AND CATECHOLAMINES

Although the nucleus tractus solitarii (NTS) has been established as t he primary site of synaptic integration for the baroreceptor reflex, t he higher-order pathways responsive to, and mediating, changes in vaso motor tone are not well characterized. We used immunohistochemistry to determine the distribution of cells expressing the Fos protein follow ing pharmacologically induced, directionally specific changes in arter ial pressure. The goal of this investigation was to determine if this immediate early gene product is differentially expressed in neurons of the rat brainstem following increased (presser) versus decreased (dep ressor) arterial blood pressure (AP). Because brainstem catecholaminer gic (CA) cell groups have been implicated in cardiovascular regulation , a double-labeling immunohistochemical procedure was used to examine the distribution of Fos in CA cells. Animals received continuous intra venous infusion of either a vasoconstrictor (l-phenylephrine hydrochlo ride), a vasodilator (sodium nitroprusside), or physiological saline. Extensive Fos-like immunoreactivity (FLI) was induced in both the pres ser and depressor conditions in the NTS, caudal ventrolateral medulla (CVLM), rostral ventrolateral medulla (RVLM), A5, locus coeruleus (LC) , Kolliker-Fuse, and parabrachial nucleus (PBN). These regions have al l been implicated in central cardiovascular regulation. There were dif ferences in the anatomical distribution of Fos-positive cells along th e rostrocaudal axis of CVLM in the presser and depressor conditions. S pecifically, increased AP induced significantly more FLI cells within the rostral aspects of CVLM, whereas decreased AP resulted in a signif icantly greater number of FLI cells within the caudal CVLM. This resul t suggests that selective vasomotor responses differentially engaged d iscrete subsets of neurons within this brainstem region. Overall, appr oximately 50% of CA-immunoreactive cells were also FLI (CA-FLI) in the A1, A5, and A7 regions. Interestingly, increased AP produced signific antly more CA-FLI double-labeled cells within the caudal than rostral Al compared with depressor and control groups. Additionally, increased AP yielded significantly less CA-FLI double-labeled cells within the caudal A2 region. This suggests that CA baronsensitive neurons in the CVLM/A1 and NTS/A2 regions are functionally segregated along the rostr ocaudal axis of these structures. While twice as many PNMT-FLI double- labeled neurons were found in the C1-C3 regions following vasomotor ch anges versus saline control, there were no differences in the numbers or anatomical locations of labeled cells between presser versus depres sor groups. The results of this study indicate that (1) tonic changes in AP induce robust Fos expression in brainstem cardiovascular areas a nd (2) neurons responsive to specific directional changes in arterial pressure are segregated in some brainstem regions. (C) 1994 Wiley-Liss , Inc.