Nitric oxide (NO) and its downstream signaling cascades are critical to various cellular functions in the brain, including the neuroendocrine control of reproduction. NO is produced by neuronal- NOS (nNOS) expressing neurons found in the vicinity of GnRH containing perikarya in the preoptic region of the hypothalamus (POA), and has thus been implicated in the regulation of GnRH activity^1 2 . Additionally, the mapping of leptin responsive cells in the hypothalamus has revealed that many LepR neurons are present in the POA, while some of them are also NO-synthesizing neurons. Recent studies from our lab have demonstrated that leptin is able to induce an acute increase in P-nNOS expression in the OVLT and MEPO hypothalamic areas, which coincide with a rise in LH levels (Bellefontaine et al., submitted). With the aim to clarify the leptin- NO crosstalk dynamics in the POA we applied our results to a mathematical model describing the active nNOS neurons of the POA as a 3-dimensional array of NO-emitting spheres. Here we propose that leptin can induce the activation of nNOS neurons resulting to the “transformation” of NO into a “volume transmitter” able to regulate GnRH activity and hence induce the LH release. Since the biological actions of NO depend critically on its concentration, which is difficult to measure, we use a novel cGMP biosensor in combination with an ultrasensitive detector cell line in order to quantify the active concentration of NO being released in a mouse hypothalamic slice using live imaging techniques, under physiological conditions³ . Our results suggest that nNOS neurons within and in direct proximity to the OVLT, a site devoid of the blood brain barrier and to which GnRH neurons extend dendrites, may acutely sense changes in leptin levels and rapidly relay this information to GnRH neurons, which in turn stimulate LH release.