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 (NK3R)-expressing neurons in the median preoptic nucleus (MnPO). To characterize the thermoregulatory role of MnPO NK3R neurons in female mice, we ablated these neurons using injections of saporin toxin conjugated to a selective NK3R agonist. Loss of MnPO NK3R neurons increased the core temperature (TCORE) during the light phase, with the frequency distributions indicating a regulated shift in the balance point. The increase in TCORE 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 TCORE 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 NK3R (EGFP-immunoreactive) neurons. Thus, MnPO NK3R 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 (NK3R) mRNA was coexpressed with vesicular glutamate transporter 2 or vesicular γ-aminobutyric acid (GABA) transporter mRNA, markers of glutamatergic and GABAergic neurotransmission, respectively. In the MnPO, 94% of NK3R neurons were glutamatergic, but in the adjacent medial preoptic area, 97% of NK3R neurons were GABAergic. Thus, NK3R neurons in the MnPO are glutamatergic and play a role in reducing TCORE 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 NK3R.

Estradiol alters body temperature regulation in the female mouse.

Hot flushes are due to estrogen withdrawal and characterized by the episodic activation of heat dissipation effectors. Recent studies (in humans and rats) have implicated neurokinin 3 (NK3) receptor signaling in the genesis of hot flushes. Although transgenic mice are increasingly used for biomedical research, there is limited information on how 17ß-estradiol and NK3 receptor signaling alters thermoregulation in the mouse. In this study, a method was developed to measure tail skin temperature (TSKIN) using a small data-logger attached to the surface of the tail, which, when combined with a telemetry probe for core temperature (TCORE), allowed us to monitor thermoregulation in freely-moving mice over long durations. We report that estradiol treatment of ovariectomized mice reduced TCORE during the light phase (but not the dark phase) while having no effect on TSKIN or activity. Estradiol also lowered TCORE in mice exposed to ambient temperatures ranging from 20 to 36°C. Unlike previous studies in the rat, estradiol treatment of ovariectomized mice did not reduce TSKIN during the dark phase. Subcutaneous injections of an NK3 receptor agonist (senktide) in ovariectomized mice caused an acute increase in TSKIN and a reduction in TCORE, consistent with the activation of heat dissipation effectors. These changes were reduced by estradiol, suggesting that estradiol lowers the sensitivity of central thermoregulatory pathways to NK3 receptor activation. Overall, we show that estradiol treatment of ovariectomized mice decreases TCORE during the light phase, reduces the thermoregulatory effects of senktide and modulates thermoregulation differently than previously described in the rat.