Prepubertal Development of GABAergic Transmission to Gonadotropin-Releasing Hormone (GnRH) Neurons and Postsynaptic Response Are Altered by Prenatal Androgenization

Abstract

Gonadotropin-releasing hormone (GnRH) neurons regulate reproduction through pulsatile GnRH release. Women with polycystic ovary syndrome (PCOS) have persistently elevated luteinizing hormone release frequency, reflecting GnRH release; this exacerbates hyperandrogenemia and disrupted reproductive cycles that are characteristic of this disorder. Clinical evidence suggests that neuroendocrine features of PCOS may manifest peripubertally. Adult mice prenatally exposed to androgens (PNA) mimic several reproductive features of PCOS. GnRH neurons from these mice have increased firing activity and receive increased GABAergic transmission, which is excitatory. When changes emerge during development is unknown. To study the typical postnatal development of GABAergic transmission and the effects of PNA treatment and sex, whole-cell voltage-clamp recordings were made of GABAergic postsynaptic currents (PSCs) in GnRH neurons in brain slices from prepubertal through adult control and PNA female and male mice. GABAergic transmission was present by 1 week of age in females and males and increased in frequency, reaching adult levels at 3 and 4 weeks, respectively. GABAergic PSC frequency was elevated in 3-week-old PNA versus control females. PSC frequency in both controls and PNA mice was activity independent, suggesting that PNA induces changes in synapse organization. PNA also alters the functional response of GnRH neurons to GABA. GABA induced firing in fewer neurons from 3-week-old PNA than control females; membrane potential depolarization induced by GABA was also reduced in cells from PNA mice at this age. PNA thus induces changes during development in the presynaptic organization of the GABAergic network afferent to GnRH neurons as well as the postsynaptic GnRH neuron response, both of which may contribute to adult reproductive dysfunction. The central neuronal network that regulates reproduction is overactive in polycystic ovary syndrome (PCOS), a leading cause of infertility. Recent evidence of neuroendocrine dysfunction in midpubertal girls suggests that the pathophysiological mechanisms underlying PCOS may arise before pubertal maturation. Prenatal exposure to androgens (PNA) in mice mimics several neuroendocrine features of PCOS. GABAergic transmission to gonadotropin-releasing hormone (GnRH) neurons is important for reproduction and is increased in adult PNA mice. The typical development of this network and when changes with PNA and sex arise relative to puberty are unknown. These studies provide evidence that PNA alters prepubertal development of the GABAergic network afferent to GnRH neurons, including both the presynaptic organization and postsynaptic response. These changes may contribute to reproductive dysfunction in adults.

Mapping GABA and glutamate inputs to gonadotrophin‐releasing hormone neurones in male and female mice

“…Sex differences in GABA A receptor subunit expression at the postsynaptic level have been identified, 37,38 although spontaneous GABAergic transmission frequency to adult GnRH neurones is reportedly not significantly different between males and females. 25 Morphological plasticity has been reported previously in females at the time of the oestrogen positive-feedback-induced preovulatory GnRH/LH surge. Specifically, cFos positive GnRH neurones exhibit significantly increased spine density at the time of the surge.…”

“…The lack of significant sex differences in morphological GABA and glutamate contacts to the GnRH neurone soma and proximal dendrite is of interest when considering previous studies that might suggest the contrary, particularly for GABA input. Prenatal androgen exposure of female mice significantly elevates both GABA activity and GABA innervation to GnRH neurones. The lack of differences identified in the present study between males and females, using a similar morphological approach, suggest that prenatal androgenisation with the non‐aromatisable androgen dihydrotestosterone does not masculinise the GABA to GnRH neurone circuitry but instead results in an innervation pattern different to both male and healthy female controls.…”

“…Visualisation and quantification of apposed vesicular transporters is an effective and efficient approach for identifying morphologi- The lack of significant sex differences in morphological GABA and glutamate contacts to the GnRH neurone soma and proximal dendrite is of interest when considering previous studies that might suggest the contrary, particularly for GABA input. Prenatal androgen exposure of female mice significantly elevates both GABA activity 24,25 and GABA innervation [21][22][23] to GnRH neurones. The lack of differences identified in the present study between males and Sex differences have been identified for glutamate and GABA inputs to GnRH neurones originating from the anteroventral periventricular nucleus (AVPV), 35 an area that is sexually dimorphic and larger in females than in males.…”

“…However, because the balance of total inputs is not morphologically different, this suggests that equal and opposite dimorphic projections originate from other as yet unidentified brain regions. Sex differences in GABA A receptor subunit expression at the postsynaptic level have been identified, although spontaneous GABAergic transmission frequency to adult GnRH neurones is reportedly not significantly different between males and females …”

“…In the mouse, the preovulatory surge is linked to increased GnRH neurone spine density, 15 a morphological change typically associated with increased glutamatergic input in other neuronal systems. Increased GABA inputs, [21][22][23] GABA transmission 24,25 and GnRH neurone spine density 21,22,26 are also evident in females prenatally exposed to androgens that exhibit a hyperactive GnRH axis.…”