DOWN-REGULATION OF ENDOGENOUS NITRIC-OXIDE SYNTHASE IN LATE-PREGNANCYAND PARTURITION IN THE RAT HYPOTHALAMIC MAGNOCELLULAR NEURONS AND NEUROHYPOPHYSIS

Several recent lines of evidence suggest that nitric oxide (NO) may be an endogenous inhibitory regulator of the neurosecretory mechanism in magnocellular neurons of the paraventricular and the supraoptic nucle i in the hypothalamus. The NO synthase (NOS) system in the hypothalamo -neurohypophysial axis is regulated in an activity-dependent manner. T he present study examined NOS activity in the magnocellular neurons an d neurohypophysis during pregnancy and parturition by using the nicoti namide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistr y and assay of the specific NOS enzyme activity, respectively. In the paraventricular and supraoptic nuclei, the density and number of NADPH -diaphorase-positive cells decreased in late-pregnancy and parturition . The specific activity of NOS in the neurohypophysis also decreased i n late-pregnancy through parturition, and increased shortly afterward. Together with the ability of a NO donor to significantly delay the pr ogress of parturition when administered centrally in parturient rats, these observations suggest that this down-regulation of NOS activity i n the hypothalamo-neurohypophysial axis in late-pregnancy and parturit ion may be of physiological importance in the onset and/or progress of parturition. Copyright (C) 1996 Elsevier Science Ireland Ltd.