Functional interactions between anteroventral periventricular kisspeptin neurons and gonadotropin-releasing hormone neurons in female mice

Kisspeptin neurons in the rostral hypothalamus are hypothesized to initiate preovulatory gonadotropin-releasing hormone (GnRH) surges by causing estradiol-dependent activation of GnRH neuron action potential firing and subsequent GnRH release. To determine if estradiol or ovarian cycle stage modulates functional connectivity in this circuit, we used optogenetics to photostimulate anteroventral-periventricular (AVPV) area kisspeptin neurons while recording electrical activity and/or evoked synaptic currents from preoptic area GnRH neurons in acutely-prepared mouse brain slices. Slices were prepared from mice in multiple hormonal states, including 2-days post ovariectomy (OVX) and OVX plus estradiol during the morning or afternoon, diestrus, proestrus and 1-week post OVX, and 6-weeks post OVX with or without 1 week of estradiol replacement. Photostimulation induced a sustained, frequency-dependent increase in GnRH neuron firing rate. This neuromodulatory-typical response was not different in diestrous vs proestrous mice but was blunted in 1-week OVX mice, suggesting ovarian steroids amplify this response. Neuromodulatory responses were infrequent in 6-week OVX mice even with 1-week of estradiol treatment. A minority of GnRH neurons exhibited a substantial and near-immediate increase in firing rate typical of fast synaptic transmission. Monosynaptic connectivity was low and stable across the hormone states tested and mediated by GABA. Interestingly, evidence of a monosynaptic connection was not a requirement for GnRH neurons to exhibit a sustained increase in firing rate, suggesting non-synaptic or volume transmission occurs in this system. Synaptic connectivity did, however, amplify the increase in firing rate observed in GnRH neurons from proestrous mice, indicating proestrous hormonal conditions can amplify this response.