Activation of neurokinin 3 receptors in the median preoptic nucleus decreases core temperature in the rat

Abstract

Estrogens have pronounced effects on thermoregulation, as illustrated by the occurrence of hot flushes secondary to estrogen withdrawal in menopausal women. Because neurokinin B (NKB) gene expression is markedly increased in the infundibular (arcuate) nucleus of postmenopausal women, and is modulated by estrogen withdrawal and replacement in multiple species, we have hypothesized that NKB neurons could play a role in the generation of flushes. There is no information, however, on whether the primary NKB receptor [neurokinin 3 receptor (NK(3)R)] modulates body temperature in any species. Here, we determine the effects of microinfusion of a selective NK(3)R agonist (senktide) into the rat median preoptic nucleus (MnPO), an important site in the heat-defense pathway. Senktide microinfusion into the rat MnPO decreased core temperature in a dose-dependent manner. The hypothermia induced by senktide was similar in ovariectomized rats with and without 17β-estradiol replacement. The hypothermic effect of senktide was prolonged in rats exposed to an ambient temperature of 29.0 C, compared with 21.5 C. Senktide microinfusion also altered tail skin vasomotion in rats exposed to an ambient temperature of 29.0 but not 21.5 C. Comparisons of the effects of senktide at different ambient temperatures indicated that the hypothermia was not secondary to thermoregulatory failure or a reduction in cold-induced thermogenesis. Other than a very mild increase in drinking, senktide microinfusion did not affect behavior. Terminal fluorescent dextran microinfusion showed targeting of the MnPO and adjacent septum, and immunohistochemical studies revealed that senktide induced a marked increase in Fos-activation in the MnPO. Because MnPO neurons expressed NK(3)R-immunoreactivity, the induction of MnPO Fos by senktide is likely a direct effect. By demonstrating that NK(3)R activation in the MnPO modulates body temperature, these studies support the hypothesis that hypothalamic NKB neurons could be involved in the generation of menopausal flushes.

Fig. 1.

Change from baseline in average core temperature in OVX + E₂ rats after microinfusion of senktide (18 or 90 pmol) or vehicle (300 nl of aCSF) in the rat MnPO (ambient temperature of 21.5 C, n = 5–8 microinfusions/group). Senktide microinfusion reduced the average core temperature in a dose-dependent manner. The horizontal line represents the average baseline value of all groups. #, Significantly different, 90 pmol senktide compared with vehicle.

Fig. 2.

Change from baseline in average core temperature after microinfusion of 90 pmol senktide or vehicle (aCSF) in the MnPO of OVX + E₂ and OVX rats (ambient temperature of 21.5 C, n = 6–8 microinfusions/group). Senktide resulted in a significant decrease in core temperature that was similar in OVX + E₂ and OVX rats. The horizontal line represents the average baseline value of all groups. #, Significantly different, senktide compared with vehicle, OVX + E₂ rats; *, significantly different, senktide compared with vehicle, OVX rats; +, significantly different from time 0 in OVX + E₂ vehicle-microinfused rats; five-pointed star, significantly different from time 0 in OVX vehicle-microinfused rats.

Fig. 3.

Change from baseline in average core temperature after microinfusion of 90 pmol senktide or vehicle in the MnPO of OVX + E₂ rats at ambient temperatures of 21.5 or 29.0 C (n = 6–8 microinfusions/group). The data from OVX + E₂ rats at the ambient temperature of 21.5 C is duplicated from Fig. 2 to facilitate comparisons. Senktide microinfusion resulted in a significant decrease in core temperature (compared with vehicle) in rats exposed to ambient temperatures of both 21.5 and 29.0 C. At the ambient temperature of 29.0 C, the hypothermia induced by senktide was prolonged. The horizontal line represents the average baseline value of all groups. #, Significantly different, senktide compared with vehicle, ambient temperature of 21.5 C; *, significantly different, senktide compared with vehicle, ambient temperature of 29.0 C; five-pointed star, significantly different, senktide at ambient temperature of 29.0 C, vs. senktide at ambient temperature of 21.5 C; +, significantly different from time 0 in vehicle-microinfused rats, ambient temperature of 21.5 C.

Fig. 4.

Change from baseline in average tail skin temperature in OVX + E₂ rats after microinfusion of 90 pmol senktide or vehicle into MnPO at ambient temperatures of 21.5 C (n = 5–6 microinfusions/group) (A) or 29.0 C (n = 6–7 microinfusions/group) (B). At 21.5 C, both senktide and vehicle-microinfused rats exhibited an acute reduction in tail skin temperature, indicative of tail vasoconstriction secondary to the microinfusion procedure. Acute tail skin vasoconstriction was also observed in the vehicle-microinfused animals at the ambient temperature of 29.0 C (B). In contrast, within 30 min after microinfusion of senktide at 29.0 C, there was no significant change in tail skin temperature. These rats exhibited a delayed decrease in tail skin temperature (compared with vehicle controls) from 65 to 90 min after senktide microinfusion. The horizontal line represents the average baseline value of all groups. Five-pointed star, Significantly different from time 0 in senktide and vehicle-microinfused rats, ambient temperature of 21.5 C (A); +, significantly different from time 0 in vehicle-microinfused rats, ambient temperature of 29.0 C (B); *, significantly different, senktide compared with vehicle, ambient temperature of 29.0 C (B).

Fig. 5.

A–C, Representative photomicrographs from a rat receiving a terminal microinfusion of fluorescent dextran (red). These photomicrographs were created by combining brightfield images with fluorescent images of the red dextran dye. The numbers in the upper right corner of each photomicrograph indicate the plate number from the Paxinos rat brain atlas (42) and distance from Bregma. Diffusion of fluorescent dextran was identified in the vertical limb of the diagonal band of Broca (A), the medial septal nucleus (A and B), septohypothalamic nucleus (B and C), and the MnPO (B and C). The guide cannula tract and a dense deposit of fluorescent dextran is identified in A. D and E, Representative photomicrographs of Fos immunohistochemistry in OVX + E₂ rats killed 90 min after microinfusion of either vehicle (D) or senktide (E) at the ambient temperature of 21.5 C. Microinfusion of senktide dramatically increased the number of Fos-ir cells in the MnPO (E). 3V, Third ventricle; ac, anterior commissure, LV, lateral ventricle; MS, medial septum, POA, preoptic area; SHy, septohypothalamic nucleus; oc, optic chiasm; VDB, vertical limb of the diagonal band of Broca. Scale bars, 500 μm (A–C) and 100 μm (D and E).

Fig. 6.

Representative photomicrographs of single-label NK₃R-immunofluorescence with TSA amplification in an OVX + E₂ rat. At low magnification, the MnPO is highlighted by NK₃R immunofluorescence (A). The box in A outlines the location of the small NK₃R-ir cell bodies and puncta in the MnPO shown at higher magnification in B. The arrow in A points to the location of NK₃R-ir cell bodies in the ventral septohypothalamic nucleus shown in higher magnification in C. ac, Anterior commissure; 3V, third ventricle. Scale bars, 100 μm (A), 10 μm (B), and 25 μm (C).