Aging attenuates acquired heat tolerance and hypothalamic neurogenesis in rats

This study investigated age-dependent changes in heat exposure-induced hypothalamic neurogenesis and acquired heat tolerance in rats. We previously reported that neuronal progenitor cell proliferation and neural differentiation are enhanced in the hypothalamus of long-term heat-acclimated (HA) rats. Male Wistar rats, 5 weeks (Young), 10-11 months (Adult), or 22-25 months (Old) old, were subjected to an ambient temperature of 32°C for 40-50 days (HA rats). Rats underwent a heat tolerance test. In HA rats, increases in abdominal temperature (Tab ) in the the Young, Adult, and Old groups were significantly smaller than those in their respective controls. However, the increase in Tab of HA rats became greater with advancing age. The number of hypothalamic bromodeoxyuridine (BrdU)-immunopositive cells double stained with a mature neuron marker, neuronal nuclei (NeuN), of HA rats was significantly higher in the Young group than that in the control group. In Young HA, BrdU/NeuN-immunopositive cells of the preoptic area/anterior hypothalamus appeared to be the highest among regions examined. Large numbers of newborn neurons were also located in the ventromedial and dorsomedial nuclei, as well as the posterior hypothalamic area, whereas heat exposure did not increase such numbers in the Adult and Old groups. Aging may interfere with heat exposure-induced hypothalamic neurogenesis and acquired heat tolerance in rats.