Changes in thermoregulation and monoamine release in freely moving rats during cold exposure and inhibition of the ventromedial, dorsomedial, or posterior hypothalamus
The hypothalamus is critical for regulating thermogenesis, but the role of monoamines in specific hypothalamic subregions in thermogenesis is not thoroughly established. The purpose of this study was to confirm changes of body temperature (Tb) and thermoregulatory parameters upon inhibition of neural activity in hypothalamic subregions in freely moving rats. In addition, the pattern of monoamine release in these nuclei was measured during active thermoregulation using microdialysis. Tetrodotoxin (TTX) was perfused into the ventromedial hypothalamus (VMH), dorsomedial hypothalamus (DMH), or posterior hypothalamus (PH) at two different ambient temperatures (5 or 23 °C). Using telemetry, we continuously measured the Tb and the heart rate (HR) as an index of heat production as well as locomotor activity (Act). Tail skin temperature (Ttail) was also continuously measured as an index of heat loss. Although the perfusion of TTX into hypothalamic subregions had no effect on any of the measured thermoregulatory parameters at an ambient temperature of 23 °C, it induced significant Tb decrease under cold conditions only when perfused into the DMH and the PH. In contrast, the HR decreased only after perfusion of TTX into the PH during cold conditions, while the Ttail and Act remained unchanged. Serotonin (5-HT) in the DMH and dopamine (DA) metabolite 3,4-Dihydroxyphenylacetic acid in the PH, but not noradrenaline, increased significantly during exposure to cold temperatures. Our results indicate that the DMH and the PH, but not the VMH, are particularly involved in heat production under cold conditions. In addition, 5-HT in the DMH and DA in the PH may be involved in thermogenesis.