Glutamatergic neurokinin 3 receptor neurons in the median preoptic nucleus modulate heat-defense pathways in female mice

We have proposed that arcuate neurons coexpressing kisspeptin, neurokinin B, and dynorphin (KNDy neurons) contribute to hot flushes via projections to neurokinin 3 receptor (NK 3 R)-expressing neurons in the median preoptic nucleus (MnPO). To characterize the thermoregulatory role of MnPO NK 3 R neurons in female mice, we ablated these neurons using injections of saporin toxin conjugated to a selective NK 3 R agonist. Loss of MnPO NK 3 R neurons increased the core temperature (T CORE) during the light phase, with the frequency distributions indicating a regulated shift in the balance point. The increase in T CORE in the ablated mice occurred despite changes in the ambient temperature and regardless of estrogen status. We next determined whether an acute increase in ambient temperature or higher T CORE would induce Fos in preoptic enhanced green fluorescent protein (EGFP)-immunoreactive neurons in Tacr3-EGFP mice. Fos activation was increased in the MnPO but no induction of Fos was found in NK 3 R (EGFP-immunoreactive) neurons. Thus, MnPO NK 3 R neurons are not activated by warm thermosensors in the skin or viscera and are not warm-sensitive neurons. Finally, RNAscope was used to determine whether Tacr3 (NK 3 R) mRNA was coexpressed with vesicular glutamate transporter 2 or vesicular γ 3-aminobutyric acid (GABA) transporter mRNA, markers of glutamatergic and GABAergic neurotransmission, respectively. In the MnPO, 94% of NK 3 R neurons were glutamatergic, but in the adjacent medial preoptic area, 97% of NK 3 R neurons were GABAergic. Thus, NK 3 R neurons in the MnPO are glutamatergic and play a role in reducing T CORE but are not activated by warm thermal stimuli (internal or external). These findings suggest that KNDy neurons modulate thermosensory pathways for heat defense indirectly via a subpopulation of glutamatergic MnPO neurons that express NK 3 R.