Endogenous norepinephrine (NE) release is critical for pulsatile gonadotropin-releasing hormone (GnRH) secretion in the mediobasal hypothalamus (MBH; Endocrinology 124:891, 1989). The quantity of synaptically released NE that can reach the GnRH cell surface depends on the amount of presynaptic NE reuptake, a process that is mediated by specific NE transporter protein (NET). We hypothesize that alteration of endogenous NET activity changes NE reuptake, and therefore pulsatile GnRH secretion. We addressed this hypothesis by utilizing MBH microdialysis (fD) and high pressure liquid chromatography (HPLC) to monitor MBH-NE and MBH-GnRH release without or with MBH infusion of a NET antagonist, desipramine (DMI) in intact rhesus monkeys during the spontaneous follicular phase (FP) of the menstrual cycle. First, to establish the pulsatile NE/GnRH pattern without DMI, six FP monkeys were subjected to fD for 5-16 h. The results showed that spontaneous NE and GnRH pulses were released at a frequency of 1.12 q 0.25 and 1.35 q 0.21 pulses/h, respectively. Over 70% of these pulses occurred within 10 min of each other. Second, to examine changes in NE/GnRH release by a temporal disruption of NET activity, three doses of DMI, i.e., 0.5 mM (n=3), 2 mM (n=4) and 7.5 mM (n=4), were continuously infused for 1 h into the MBH via the fD probe following several hours of baseline fD collection. While baseline pulses of NE and GnRH were similar in the two groups (without or with NET), an increase in NE release was observed within 10 min of DMI at doses of 2 mM or higher. The rise of NE returned to baseline levels within 1 h. The increment of NE concentration was dose dependent. Similarly, an increase in MBH-GnRH was observed that paralleled the magnitude of NE increase. The specificity of NET on NE release was partially demonstrated in that dopamine did not change by NET infusion. Concentration of epinephrine, the other catecholamine, was not determined; however, we have shown previously that in the rabbit, NET stimulated NE and GnRH, but not dopamine or epinephrine release, in a dose-dependent manner. Collectively, these results suggest that the activity of NET at the presynaptic NE terminal may be an integral component of the GnRH pulse generator. Drugs such as cocaine and heroin that act in part through brain NET could directly influence reproductive function via the NE-GnRH neuronal network.
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