Hypothalamic PVN CRH neurons signal through PVN GABA neurons to suppress GnRH pulse generator frequency in female mice.

Corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) are central to the stress response. Chemogenetic activation of PVN CRH neurons decreases LH pulse frequency but the mechanism is unknown. In the present study, optogenetic stimulation of PVN CRH neurons suppressed LH pulse frequency in estradiol (E2)-replaced ovariectomised (OVX) CRH-cre mice and this effect was augmented or attenuated by intra-PVN GABAA or GABAB receptor antagonism respectively. PVN CRH neurons signal to local GABA neurons which may provide a possible indirect mechanism by which PVN CRH neurons suppress LH pulse frequency. Optogenetic stimulation of potential PVN GABAergic projection terminals in the hypothalamic arcuate nucleus (ARC) in OVX E2-replaced Vgat-cre-tdTomato mice via an optic fibre implanted in the ARC suppressed LH pulse frequency. To further determine whether PVN CRH neurons signal through PVN GABA neurons to suppress LH pulsatility, we combined recombinase mice with intersectional vectors to selectively target these neurons. CRH-cre::Vgat-FlpO mice expressing the stimulatory opsin, ChRmine in non-GABAergic CRH neurons alone or in combination with the inhibitory opsin NpHR3.3 in non-CRH expressing GABA neurons in the PVN were used. Optogenetic stimulation of non-GABAergic CRH neurons suppressed pulsatile LH secretion, however LH pulse frequency was not affected when CRH neurons were stimulated and PVN GABA neurons were simultaneously inhibited. Together, these studies demonstrate that suppression of LH pulse frequency in response to PVN CRH neuronal activation is mediated by GABAergic signalling intrinsic to the PVN and may incorporate PVN GABAergic projection to the hypothalamic GnRH pulse generator.