PLASMA HYPERNATREMIA INDUCES C-FOS ACTIVITY IN MEDULLARY CATECHOLAMINERGIC NEURONS

Experiments were done in conscious rats to investigate the effect of i ntravenous infusion of hypertonic saline on the induction of the phosp hoprotein Fos in brainstem catecholaminergic neurons. Fos induction, d etected immunohistochemically, was used as a marker for neuronal activ ation. Infusions of 165 mM or 1.4 M NaCl solutions into the jugular ve in resulted in Fos-like immunoreactivity in approximately the caudal t wo thirds of nucleus of the solitary tract (NTS), the caudal and rostr al ventrolateral medulla (VLM), and in the lateral aspects of the para brachial nucleus (PEN). Within caudal NTS a small number (7.9+/-1.8%) of Fos labelled neurons were found also to contain tyrosine hydroxylas e (TH) or dopamine beta-hydroxylase (DBH) immunoreactivity. In rostral NTS no Fos labelled cells were found to contain phenylethanolamine N- methyltransferase (PNMT) immunoreactivity, although a few (8.5+/-2.3%) were immunoreactive to TH. Similarly, in VLM, most of the Fos labelle d cells in caudal VLM (65.9+/-2.7%) contained either TH or DBH immunor eactivity, whereas in the rostral VLM, 32.2+/-4.6% of the Fos labelled cells were also immunoreactive to TH or DBH. However, no Fos cells we re found in either the caudal or rostral VLM that were immunoreactive to PNMT. Little or no Fos-like immunoreactive neurons were detected in the brainstem after intravenous infusions of physiological (143 mM) o r hypotonic (106 mM) NaCl solutions. These data suggest that noradrene rgic neurons of the caudal NTS and VLM are components of central circu its that are involved in osmoregulation and cardiovascular function.