GnRH Neuron Excitability and Action Potential Properties Change with Development But Are Not Affected by Prenatal Androgen Exposure

Abstract

Gonadotropin-releasing hormone (GnRH) neurons produce the final output from the brain to control pituitary gonadotropin secretion and thus regulate reproduction. Disruptions to gonadotropin secretion contribute to infertility, including polycystic ovary syndrome (PCOS) and idiopathic hypogonadotropic hypogonadism. PCOS is the leading cause of infertility in women and symptoms resembling PCOS are observed in girls at or near the time of pubertal onset, suggesting that alterations to the system likely occurred by that developmental period. Prenatally androgenized (PNA) female mice recapitulate many of the neuroendocrine phenotypes observed in PCOS, including altered time of puberty, disrupted reproductive cycles, increased circulating levels of testosterone, and altered gonadotropin secretion patterns. We tested the hypotheses that the intrinsic properties of GnRH neurons change with puberty and with PNA treatment. Whole-cell current-clamp recordings were made from GnRH neurons in brain slices from control and PNA females before puberty at three weeks of age and in adulthood to measure GnRH neuron excitability and action potential (AP) properties. GnRH neurons from adult females were more excitable and required less current to initiate action potential firing compared with three-week-old females. Further, the afterhyperpolarization (AHP) potential of the first spike was larger and its peak was delayed in adulthood. These results indicate development, not PNA, is a primary driver of changes to GnRH neuron intrinsic properties and suggest there may be developmentally-induced changes to voltage-gated ion channels in GnRH neurons that alter how these cells respond to synaptic input.

Keywords: GnRH neurons; development; electrophysiology; intrinsic properties; prenatal androgenization.

Figure 1.

Confirmation of PNA phenotype. A–C, Individual values and mean ± SEM for age of vaginal opening (VO; A), body mass at VO (B), and adult anogenital distance (AGD, mm; C). D, Representative estrous cycles over 14 d. P, proestrus; D, diestrus; E, estrus. E, Individual values and mean ± SEM days in each cycle stage over 14 d. Statistical parameters are in Table 1; ***p < 0.005, ****p < 0.0001.

Figure 2.

Recording quality parameters. A–D, Individual values and mean ± SEM for compensated series resistance (A), capacitance (B), input resistance (C), holding current (D). Statistical parameters are in Table 2.

Figure 3.

GnRH neuron excitability is increased, and action potential properties altered, in adult versus three-week-old mice. A, Representative membrane voltage responses (top) to depolarizing current injections (bottom); only three current steps are shown for clarity. B, Mean ± SEM # of action potentials (APs) fired as a function of current injection in age-combined groups. C, representative traces of the rheobase AP for each experimental group. D–K, Individual values and mean ± SEM for AP threshold (D), latency (E), rate of rise (F), AP amplitude (G), full width at half-maximum (FWHM; H), rheobase (I), afterhyperpolarization potential (AHP) time (J), and AHP amplitude (K). Statistical parameters are in Tables 3 and 4.

Figure 4.

Neither development nor PNA treatment alter the response of GnRH neurons to hyperpolarizing current. A, Representative membrane voltage (top) responses to hyperpolarizing current injections (bottom); only three steps are shown for clarity. Individual values ± SEM for sag (B), and GnRH neuron rebound following the hyperpolarizing current relative to baseline membrane potential (C). Statistical parameters shown in Table 5.