Brown adipose tissue thermogenesis evoked by medial preoptic stimulation is mediated via the ventromedial hypothalamic nucleus
Experiments were designed to determine if a functional ventromedial hypothalamic nucleus was required for the activation of brown adipose tissue thermogenesis evoked by medial preoptic stimulation. Male, urethane-anesthetized Long–Evans rats, maintained at 37 °C, had temperatures (thermistor probes for gastrocnemius, Tm; intrascapular brown adipose tissue, 7IBAT; colonic, Tc; and tail, Tt), gastrocnemius electromyogram activity (via stainless steel recording electrodes), and systemic blood pressure and heart rate (via a femoral arterial catheter) measured before and after a series of unilateral medial preoptic electrical stimulations (monophasic 0.5-ms pulses of 300 μA at 50 Hz for 30 s). Measurements were made (i) after an initial control medial preoptic electrical stimulation, (ii) after medial preoptic stimulation was applied 1 min following an intracranial injection of 300 nL of sterile saline or buffered 2% Lidocaine into the ipsilateral posterior hypothalamic nucleus or the ipsilateral ventromedial hypothalamic nucleus, and (iii) after recovery medial preoptic stimulation 45 min after Lidocaine was injected into the ventromedial hypothalamic nucleus. TIBAT and blood pressure rose significantly (p < 0.05) above the corresponding prestimulation control values with all protocols, except when Lidocaine was injected into the ventromedial hypothalamic nucleus prior to medial preoptic stimulation. Shivering (electromyogram) activity was not evoked following medial preoptic stimulation and Tm and Tt did not significantly change from the corresponding prestimulation values. A recovery medial preoptic stimulation 45 min after Lidocaine treatment of the ventromedial hypothalamic nucleus again evoked significant increases in TIBAT above the core temperature, similar to the rise in TIBAT seen after the first control medial preoptic stimulation. Pretreatment with Lidocaine into the posterior hypothalamic nucleus before medial preoptic stimulation caused no suppression of blood pressure compared with treatments after the control medial preoptic stimulation; however, TIBAT was reduced (p < 0.05) from the marked rise in TIBAT seen after the control stimulation. Results indicate that a functional ventromedial hypothalamic nucleus is required for medial preoptic stimulation to activate brown adipose tissue thermogenesis.Key words: brown adipose tissue thermogenesis, medial preoptic stimulation, thermoregulation, heat production.