Gonadal Feedback Alters the Relationship between Action Potentials and Hormone Release in Gonadotropin-Releasing Hormone Neurons in Male Mice

Abstract

In vertebrates, the pulsatile release of gonadotropin-releasing hormone (GnRH) from neurons in the hypothalamus triggers secretion of anterior pituitary gonadotropins, which activate steroidogenesis, and steroids in turn exert typically homeostatic negative feedback on GnRH release. Although long-term episodic firing patterns of GnRH neurons in brain slices resemble the pulsatile release of GnRH and LHin vivo, neither the relationship between GnRH neuron firing and release nor whether this relationship is influenced by gonadal feedback are known. We combined fast-scan cyclic voltammetry and patch-clamp to perform simultaneous measurements of neuropeptide release with either spontaneous action potential firing or in response to neuromodulator or action-potential-spike templates in brain slice preparations from male mice. GnRH release increased with higher frequency spontaneous firing to a point; release reached a plateau after which further increases in firing rate did not elicit further increased release. Kisspeptin, a potent GnRH neuron activator via a Gq-coupled signaling pathway, triggered GnRH release before increasing firing rate, whether globally perfused or locally applied. Increasing the number of spikes in an applied burst template increased release; orchidectomized mice had higher sensitivity to the increased action potential number than sham-operated mice. Similarly, Ca2+currents triggered by these burst templates were increased in GnRH neurons of orchidectomized mice. These results suggest removal of gonadal feedback increases the efficacy of the stimulus–secretion coupling mechanisms, a phenomenon that may extend to other steroid-sensitive regions of the brain.SIGNIFICANCE STATEMENTPulsatile secretion of GnRH plays a critical role in fertility. The temporal relationship between GnRH neuron action potential firing and GnRH release remains unknown as does whether this relationship is influenced by gonadal feedback. By combining techniques of fast-scan cyclic voltammetry and patch-clamp we, for the first time, monitored GnRH concentration changes during spontaneous and neuromodulator-induced GnRH neuron firing. We also made the novel observation that gonadal factors exert negative feedback on excitation–secretion coupling to reduce release in response to the same stimulus. This has implications for the control of normal fertility, central causes of infertility, and more broadly for the effects of sex steroids in the brain.