Nitric oxide synthase in the hypothalamic paraventricular nucleus of the female rat; organization of spinal projections and coexistence with oxytocinor vasopressin

We investigated the distributions and interrelations of neuronal nitric oxi de (NO) synthase- (nNOS), oxytocin- (OT), and 8-arginine vasopressin- (AVP) immunoreactive (IR) neurons in the paraventricular nucleus (PVN), and the occurrence and distribution of nNOS spinally projecting neurons in the PVN of the female rat. Using double labelling immunohistochemistry, we mapped t he distribution of nNOS-, OT- and AVP-immunoreactive (IR) neuronal cell bod ies in the different parts of the PVN. About 80% of nNOS-IR cell bodies wer e magnocellular. About 30% of the nNOS-IR cell bodies were OT-IR, colocaliz ation being most frequent in the rostral parts. In comparison, only similar to3% of all nNOS-IR cell bodies were AVP-IR, evenly distributed throughout the PVN. True Blue (TB), administered unilaterally into the spinal cord, d isclosed that most spinally projecting cell bodies in the PVN were localize d in caudal parts. Combined TB tracing and nNOS immunohistochemistry showed that similar to 30% of spinally projecting neurons in the PVN were nNOS-IR , and that similar to 40% of these were magnocellular. Ipsilateral nNOS spi nal projections were about eight times more frequent than the contralateral nNOS projections. The study describes the detailed neuroanatomical organiz ation of nNOS neurons coexpressing OT or AVP, and of nNOS spinally projecti ng neurons within defined parts of the PVN. In contrast to the paraventricu lo-spinal system in general, we show that the nNOS paraventriculo-spinal pa thway to a large extent originates in magnocellular cell bodies. The result s suggest that NO is an important messenger in the paraventriculo-spinal pa thway that may in part act in concert with OT. (C) 2001 Published by Elsevi er Science B.V.