A COMPARISON OF HYPOTENSIVE AND NONHYPOTENSIVE HEMORRHAGE ON FOS EXPRESSION IN SPINALLY PROJECTING NEURONS OF THE PARAVENTRICULAR NUCLEUS AND ROSTRAL VENTROLATERAL MEDULLA

The protein, Fos, detected immunohistochemically, was used to identify neurons in the brain that were activated after hemorrhage in the cons cious rat. Spinally projecting neurons in the paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM) were identified by the presence of rhodamine-labeled latex beads which had been previously in jected into the upper thoracic spinal cord. On the experimental day, c onscious rats underwent either (1) withdrawal of 4 ml of blood from a carotid cannula (n = 8) which reduced mean arterial pressure from 96.6 +/- 2.7 to 42.7 +/- 7.1 mmHg, (2) withdrawal of 2 ml of blood (n = 4) which did not affect mean arterial pressure. Animals that were not he morrhaged were used as controls (n = 6). After the 4 ml hemorrhage, de nse concentrations of Fos-positive cell nuclei were found in the lamin a terminalis, supraoptic nuclei (SON), PVN and in the medulla. In cont rast, the density of Fos-positive cells in 2 ml-hemorrhaged rats was n ot different from controls except in the SON and in the medial PVN in 2 of 4 rats. After the 4 ml hemorrhage 14.4 +/- 1.2% of the spinally p rojecting neurons in the PVN and 22.7 +/- 6.1% in the RVLM expressed F os (P < 0.001 compared to control). After the 2 ml hemorrhage the prop ortion was 12.2 +/- 3. 1% in the PVN (P < 0.001 compared control) but only 5.4 +/- 2.2% in the RVLM (P > 0.05 compared to control). The resu lts suggest that spinally projecting neurons in the PVN and RVLM parti cipate in the reflex responses to hemorrhage. PVN-spinal neurons may r espond to changes in blood volume even when arterial pressure does not alter.