CNS Relaxin activates spinally projecting and vasopressin containing cells in the hypothalamic paraventricular nucleus

Abstract

Relaxin (RLX), an ovarian hormone which is secreted in pregnancy, activates the subfornical organ (SFO) and hypothalamic regions associated with control of blood volume and sympathetic nerve activity (SNA). The current experiments phenotyped cells in the paraventricular nucleus (PVN) which are activated by RLX. Carotid and femoral artery and vein catheters were implanted in female virgin rats. Spinally projecting cells were retrogradely labeled by microinjection of fluorogold (FG) or cholera toxin‐b (CTb) (90nl) into the intermediolateral cell column. After 5 days human RLX 2 (1μg/hr) or saline (Sal, 1ml/hr) was infused (1.5 hr) into the forebrain circulation (intra‐carotid artery, ica) of conscious rats. RLX (n=3) increased mean arterial pressure (MAP, 9±2 mmHg) while Sal (n=5) had no effect (+1 ± 1mmHg). Rats were euthanized, perfused with 4% paraformaldehyde, brains sectioned (30 μm), and Fos‐, CTb‐, and vasopressin (VP‐) immunoreactivity (IR) were evaluated. Following RLX, cells in the lateral margins of the SFO expressed Fos‐IR, consistent with activation of PVN projecting neurons. Forty ±11% of VP‐IR cells (bregma −1.8mm) and 36±7% of spinally projecting cells (bregma −2.12mm) were activated (Fos‐IR) by ica RLX. In contrast, Fos‐IR was minimal in Sal treated rats. These data provide an anatomical substrate for a role of RLX in the adaptations in regulation of VP and SNA in pregnancy. HL091164(CMH)