Up-regulation of nitric oxide synthase messenger RNA in an integrated forebrain circuit involved in oxytocin secretion.

The hypothalamo-neurohypophysial system contains high levels of neuronal nitric oxide synthase and this increases further during times of neurohormone demand, such as that following osmotic stimulation. Using double in situ hybridization, we demonstrate here an increase in the expression of nitric oxide synthase messenger RNA by oxytocin neurons, but not vasopressin neurons, of the supraoptic nucleus at the time of lactation, when oxytocin is in demand due to another neuroendocrine stimulus, the milk-ejection reflex. In addition, using immunocytochemical retrograde tracing, we show that neurons of the subfornical organ, median preoptic nucleus and organum vasculosum of the lamina terminalis, which project to the supraoptic nucleus, contain nitric oxide synthase. These three structures of the lamina terminalis, together with the hypothalamo-neurohypophysial system, make up the forebrain osmoresponsive circuit that controls osmotically-stimulated release of oxytocin in the rat. The expression of nitric oxide synthase messenger RNA in the lamina terminalis was also shown to increase during lactation. The increases in nitric oxide synthase messenger RNA were not apparent during pregnancy. These results provide evidence for an integrated nitric oxide synthase-containing neural network involved in the regulation of the hypothalamo-neurohypophysial axis. The expression of nitric oxide synthase messenger RNA increases in this circuit during lactation and correlates with a reduction in the sensitivity of the circuit to osmotic stimuli also present in lactation but not pregnancy. As nitric oxide is believed to attenuate neurohormone release, it seems that the increased nitric oxide synthase messenger RNA expression detected here during lactation at a time of high oxytocin demand may be involved in reducing the sensitivity of the whole forebrain circuit to osmotic stimuli.