Development and Prenatal Androgenization Change the Intrinsic Properties of Gonadotropin-Releasing hormone (GnRH) Neurons

Abstract

Dynamic secretion patterns of gonadotropin-releasing hormone (GnRH) are critical for reproductive function. Disruptions in the patterns of GnRH and subsequent gonadotropin release impairs fertility, such as in polycystic ovary syndrome (PCOS), the leading cause of female infertility. A subset of PCOS patients has hyperandrogenemic PCOS. PCOS-like symptoms have also been observed in peripubertal girls, suggesting that the reproductive axis is susceptible to alterations even before reproductive maturity is reached. Despite decades of research, the etiology of PCOS remains largely unknown, but it is postulated that in utero exposure to elevated androgens reprograms the reproductive system and contributes to the development of a PCOS-like phenotype.

Various animal models have been instrumental in studying reproduction and how prenatal androgen exposure alters GnRH neurons. The work in this dissertation utilizes the prenatally androgenized (PNA) female mouse model, which recapitulates many neuroendocrine abnormalities resembling those seen in hyperandrogenemic PCOS patients, to test if the intrinsic properties of GnRH neurons change with age and/or PNA treatment. First, we tested if the excitability of GnRH neurons is altered in 3wk-old and adult control and PNA mice. In adulthood, GnRH neurons are more excitable, generating more action potentials in response to controlled depolarizing current input compared to 3wk-old females, regardless of PNA treatment. Age also drove changes in GnRH neuron action potential properties. Specifically, in adulthood the afterhyperpolarization potential of the action potential was larger and delayed compared to 3wk-old females; PNA treatment had no effect. Given that voltage-gated potassium currents play a role in shaping neuron firing activity and action potential properties we next tested if the properties of voltage-dependent potassium currents in GnRH neurons change with age and/or PNA treatment. Age and PNA treatment altered various properties of a transient and residual potassium current. Specifically, at 3wks of age, PNA treatment increased the density of a transient potassium current compared to 3wk controls. Furthermore, age and/or PNA treatment altered the voltage-dependent activation and inactivation of the transient potassium current and activation of a residual non-inactivating potassium current. Lastly, we used dynamic clamp to test how the intrinsic properties of GnRH neurons interact to shape their response to the same simulated GABAergic conductances. Neither development nor PNA treatment altered GnRH neuron responses to the smaller conductances tested. Age and/or PNA altered different properties of the response of GnRH neurons to the largest conductances tested. Few GnRH neurons from adults generated action potentials in response to the 10nS test conductances compared to 3wk-old females. Interestingly, an altered action potential waveform with different action potential properties than those measured from typical spikes was observed, but only in 3wk-old PNA females further providing evidence that at this age, PNA treatment disrupts the ionic conductances of GnRH neurons to alter how they respond to the input they receive.

Together, the work in this dissertation increases our understanding of how the properties of GnRH neurons in female mice change with age and whether or not these properties are susceptible to alterations when prenatal androgenization occurs. Importantly, this work could inform future therapeutic interventions for PCOS patients.