Muscarinic cholinoceptors in the ventromedial hypothalamic nucleus facilitate tail heat loss during physical exercise

The aim of this study was to evaluate the participation of ventromedial hypothalamic nucleus (VMH) muscarinic cholinoceptors in heat balance and central fatigue during treadmill exercise (24 m min−1, 5% inclination). The animals were anesthetized with pentobarbital sodium (50 mg/kg body weight i.p.) and fitted with bilateral cannulae into the VMH 1 week prior to the experiments. Tail skin (Ttail) and core body temperatures (Tb) were measured after the injection of 0.2 μL of 5 × 10−9 mol methylatropine (Matr) or 0.15 M NaCl solution (Sal) into the hypothalamus. Methylatropine injection into the VMH greatly increased heat storage rate (HSR) measured until fatigue (19.7 ± 4.6 cal min−1 Matr versus 9.7 ± 3.3 cal min−1 Sal; P < 0.05) and attenuated the exercise-induced tail vasodilation as seen by Ttail (23.98 ± 0.43 °C Matr versus 25.52 ± 0.85 °C Sal; at 6.5 min; P < 0.05), indicating inhibition of the heat loss process. The 2 min delay and the increased ΔTb, which triggered the heat loss mechanisms observed in Matr-treated rats, are associated with increased HSR and may be responsible for the decreased running performance of these animals (21.0 ± 2.9 min Matr versus 33.5 ± 3.4 min Sal; P < 0.001). In fact, a close negative correlation was observed between HSR and time to fatigue (r = −0.61; P < 0.01). In conclusion, VMH muscarinic cholinoceptors facilitate tail heat loss mechanisms, and a delay in this adjustment would lead to a decrease in physical exercise performance due to excess heat storage.