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

We investigated the distributions and interrelations of neuronal nitric oxide (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 the distribution of nNOS-, OT- and AVP-immunoreactive (IR) neuronal cell bodies in the different parts of the PVN. About 80% of nNOS-IR cell bodies were magnocellular. About 30% of the nNOS-IR cell bodies were OT-IR, colocalization being most frequent in the rostral parts. In comparison, only approximately 3% of all nNOS-IR cell bodies were AVP-IR, evenly distributed throughout the PVN. True Blue (TB), administered unilaterally into the spinal cord, disclosed that most spinally projecting cell bodies in the PVN were localized in caudal parts. Combined TB tracing and nNOS immunohistochemistry showed that approximately 30% of spinally projecting neurons in the PVN were nNOS-IR, and that approximately 40% of these were magnocellular. Ipsilateral nNOS spinal projections were about eight times more frequent than the contralateral nNOS projections. The study describes the detailed neuroanatomical organization of nNOS neurons coexpressing OT or AVP, and of nNOS spinally projecting neurons within defined parts of the PVN. In contrast to the paraventriculo-spinal system in general, we show that the nNOS paraventriculo-spinal pathway to a large extent originates in magnocellular cell bodies. The results suggest that NO is an important messenger in the paraventriculo-spinal pathway that may in part act in concert with OT.