Changes in extracellular serotonin in rat hypothalamus affect thermoregulatory function

1 Department of Physiology and Institute of Neuroscience, National Yang-Ming University, Taipei, Taiwan 11221; 2 Department of Physiology, National Cheng-Kung University, Tainan, Taiwan 40101; and 3 Institute of Chinese Pharmaceutical Sciences, China Medical College, Taichung, Taiwan 70101, Republic of China Experiments were carried out to determine the effects of altering the serotonin (5-HT) levels in the hypothalamus on thermoregulatory function in unanesthetized restrained rats. Local perfusion of the hypothalamus with dialysis solution containing 5-hydroxytryptophan (a 5-HT precursor), fluoxetine (a 5-HT reuptake inhibitor), or high potassium significantly increased both colonic temperature (T co ) and the extracellular concentrations of 5-HT in the hypothalamus. Reciprocally, both extracellular concentration of 5-HT in the hypothalamus and T co were decreased with a dialysis solution containing tetrodotoxin (which blocks the voltage-dependent sodium channel), zero calcium concentration, or systemic administration of 8-hydroxy-2-(di- n -propylamino)tetralin (8-OH-DPAT, 5-HT 1A agonist). Intrahypothalamic administration of 8-OH-DPAT and (2,5-dimethoxy-4-iodophenyl)-2-aminopropane (a 5-HT 2 agonist) produced hypothermic and hyperthermic effects, respectively. The results indicate that elevating the 5-HT levels in the hypothalamus activates postsynaptic 5-HT 2 receptors and results in hyperthermic effects, whereas stimulation of presynaptic 5-HT 1A receptors in the hypothalamus reduces the endogenous 5-HT release and results in hypothermic effects. thermoregulation; microdialysis