Posterodorsal medial amygdala urocortin-3, GABA and glutamate mediate suppression of LH pulsatility in female mice

The posterodorsal subnucleus of the medial amygdala (MePD) is an upstream modulator of the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-adrenal (HPA) axes. Inhibition of MePD urocortin-3 (Ucn3) neurons prevents psychological stress-induced suppression of LH pulsatility while blocking the stress-induced elevations in corticosterone (CORT) secretion in female mice. We explore the neurotransmission and neural circuitry suppressing the GnRH pulse generator by MePD Ucn3 neurons and we further investigate whether MePD Ucn3 efferent projections to the PVN control CORT secretion and LH pulsatility. Ucn3-cre-tdTomato female ovariectomised (OVX) mice were unilaterally injected with AAV-ChR2 and implanted with optofluid cannulae targeting the MePD. We optically activated Ucn3 neurons in the MePD with blue light at 10Hz and monitored the effect on LH pulses. Next, we combined optogenetic stimulation of MePD Ucn3 neurons with pharmacological antagonism of GABAA or GABAB receptors with bicuculline or CGP, respectively, as well as a combination of NMDA and AMPA receptor antagonists, AP5 and CNQX respectively, and observed the effect on pulsatile LH secretion. A separate group of Ucn3-cre-tdTomato OVX mice with 17{beta}-estradiol (E2) replacement were unilaterally injected with AAV-ChR2 in the MePD and implanted with fibreoptic cannulae targeting the PVN. We optically stimulated the MePD Ucn3 efferent projections in the PVN with blue light at 20Hz and monitored the effect on CORT secretion and LH pulses. We reveal for the first time that activation of Ucn3 neurons in the MePD inhibits GnRH pulse generator frequency via GABA and glutamate signalling within the MePD, while MePD Ucn3 projections to the PVN modulate the HPG and HPA axes.