Increased expression of the N-methyl- d-aspartate receptor subunit, NR1, in immunohistochemically identified magnocellular hypothalamic neurons during dehydration

Abstract

N-Methyl- d-aspartate receptors are thought to be involved in synaptic signaling within the hypothalamo-neurohypophysial system, but the extent and nature of their involvement has not been determined. In this study, in the rat, we evaluated the effect of hyperosmotic stimulation on the N-methyl- d-aspartate receptor subunit, NR1, which confers function to N-methyl- d-aspartate receptor heteromers. Co-localization of immunoreactivity for NR1 and vasopressin- or oxytocin-associated neurophysin in magnocellular neurons of the supraoptic and paraventricular hypothalamic nuclei was accomplished using double-label immunohistochemistry. Our results show that vasopressin- and oxytocin–neurophysin-positive populations contained detectable levels of NR1 labeling. Using NR1 labeling as a measure of N-methyl- d-aspartate receptor density, we examined the effect of dehydration in these nuclei. Using computer-assisted densitometry, we found significantly greater NR1 labeling densities in the magnocellular regions of both the supraoptic and paraventricular nuclei of saline-treated rats than of control rats. This increase was not due to methodological factors, since no changes in NR1 labeling density were found in a nearby nucleus, the nucleus reuniens. Western blot analysis showed similar selective increases in NR1 labeling in homogenates from the supraoptic nucleus, paraventricular nucleus and in some cases from the anterior hypothalamic area. In both immunohistochemical and western blotting experiments we did not observe a dehydration-induced increase in NR1 in other brain areas examined. Our results showing an up-regulation of NR1-containing N-methyl- d-aspartate receptors during dehydration suggest that these receptors are involved in the regulation of body water and may represent an adaptive physiological response following activation of the hypothalamo-neurohypophysial axis. In addition, these results suggest that the functional expression of N-methyl- d-aspartate receptors is dynamic and may be modified according to the physiological state of the animal.