Dual Somatic Recordings from Gonadotropin-Releasing Hormone (GnRH) Neurons Identified by Green Fluorescent Protein (GFP) in Hypothalamic Slices

Gonadotropin-Releasing Hormone (GnRH) is a small neuropeptide that regulates pituitary release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These gonadotropins are essential for the regulation of reproductive function. The GnRH-containing neurons are distributed diffusely throughout the hypothalamus and project to the median eminence where they release GnRH from their axon terminals into the hypophysiotropic portal system (1). In the portal capillaries, GnRH travels to the anterior pituitary gland to stimulate release of gonadotropins into systemic circulation. GnRH release is not continuous but rather occurs in episodic pulses. It is well established that the intermittent manner of GnRH release is essential for reproduction (2, 3)

Integrating Network and Intrinsic Changes in GnRH Neuron Control of Ovulation

Infertility affects 15-20% of couples; failure to ovulate is a common cause. Ovulation is triggered when estradiol switches from negative feedback action on the pituitary and hypothalamus to positive feedback, initiating a surge of gonadotropin-releasing hormone (GnRH) secretion that causes a surge of luteinizing hormone (LH) release, which triggers ovulation. Our understanding of the neurobiological changes underlying the switch from negative to positive feedback is incomplete. High levels of estradiol are essential, and in rodents, the LH surge tends to occur at a specific time-of-day. GnRH neurons, however, do not express the estrogen receptor required for feedback, thus estradiol-sensitive afferents likely convey estradiol information to GnRH neurons. We hypothesized that GnRH neurons switch from negative to positive feedback by integrating multiple changes to their synaptic inputs and intrinsic properties. To investigate the neurobiological mechanisms that underlie surge generation, daily GnRH/LH surges can be induced by ovariectomy and estradiol replacement (OVX+E) in rodents. GnRH neuron activity and release are increased in the afternoon (positive feedback) and decreased in the morning (negative feedback). No time-of-day changes are observed in OVX mice that do not receive an estradiol implant. Previous studies using the daily surge model have elucidated multiple GnRH neuron intrinsic and fast-synaptic changes during the switch from negative to positive feedback. It is unclear which if any of these changes are necessary for increasing GnRH firing rate during positive feedback. We hypothesized that changes to GnRH neuron intrinsic properties culminate in an increase in excitability to current steps during positive feedback and a decrease in excitability during negative feedback. To our surprise, changes to GnRH neuron ionic conductances rendered GnRH neurons more excitable during positive feedback relative to all other groups, but changes to ionic conductances between OVX and negative feedback animals had no net effect on GnRH neuron excitability. A mathematical model using a novel application of a rigorous parameter estimation method predicted that multiple, redundant combinations of changes to GnRH intrinsic conductances can produce the firing response in positive feedback. Changes to two interdependent parameters that determine the kinetics of voltage-gated potassium channels accounted for the similar neural responses during negative feedback and in OVX mice. Although enhancing GnRH neuron excitability is expected to increase firing rate during positive feedback, it is unclear if this change is necessary or if the concomitant increase is fast-synaptic transmission is sufficient for increasing GnRH neural activity during positive feedback. To test this, we used dynamic clamp to inject positive feedback, negative feedback, and OVX postsynaptic conductance trains into cells from positive feedback, negative feedback, and OVX mice. Positive feedback conductance trains were more effective in initiating spiking in cells from all three animal models relative to negative feedback and OVX trains. However, the positive feedback train elicited twice the number of action potentials from positive feedback mice relative to those from all other groups. Lastly, we extended our previous work to measure changes to GnRH neuron excitability and GABAergic inputs during the estrous cycle. We demonstrated that GABA postsynaptic current frequency and GnRH neuron excitability are both increased during positive feedback (proestrus) relative to negative feedback (diestrus) and strikingly similar to changes observed in the daily surge model. Collectively, these studies demonstrate that GnRH neurons act to integrate and amplify multiple signals to increase firing rate during the preovulatory surge.PHDMol & Integrtv Physiology PhDUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155290/1/adamsce_1.pd