The magnocellular neurohypophysial neurons of the supraoptic and paraventricular nucleus are possibly the most studied and best described peptidergic neurons in the mammalian brain. Throughout the 1960’s and 1970’s, an extensive series of investigations began to characterize the activity patterns of the vasopressinergic and oxytocinergic neurons which comprise this system10. Such studies, which continue to this day, have revealed unique and characteristic activity patterns which are displayed by these cells and which are associated with particular secretion patterns for each peptide (reviewed in25,28). Thus, oxytocin neurons are known to respond to such stimuli as increased osmolarity with small increases in spontaneous firing rate, whereas during lactation, these same cells display explosive bursts of activity associated with release of a bolus of oxytocin into the circulation to cause contraction of mammary smooth muscle and milk letdown. In contrast, vasopressinergic neurons respond with increases of activity to both reductions in blood pressure and to increases in osmolarity; the overall increased level of activity is often associated with a very characteristic, phasic activity pattern. Although both oxytocin and vasopressin neurons appear to possess intrinsic sensitivity to osmotic stimuli1,24, it is apparent that these cells are synaptically regulated by afferents arising from a large number of limbic and sensory structures28,29,33.