Brown adipose tissue thermogenesis contributes to fentanyl-evoked hyperthermia.

Mu-opioid receptor activation increases body temperature and affects cardiovascular function. In the present study, fentanyl was administered intravenously [100 mug/kg (300 nmol/kg) iv] and intracerebroventricularly [3.4 mug (10 nmol) in 10 microl icv] in urethane-chloralose-anesthetized, artificially ventilated rats. Increases in brown adipose tissue (BAT) sympathetic nerve activity (SNA) (peak, +326% of control), BAT temperature (peak, +0.8 degrees C), renal SNA (peak, +146% of control), and heart rate (HR; peak, +32 beats/min) produced by intravenous fentanyl were abolished by premamillary transection of the neuraxis but were mimicked by intracerebroventricular administration of fentanyl, which also increased arterial pressure (AP; peak, +12 mmHg). Pretreatment with the opioid antagonist naloxone (100 nmol in 10 microl icv) eliminated the intracerebroventricular fentanyl-evoked responses. Microinjection of glycine (0.5 M, 60 nl) to inhibit local neurons in the rostral raphe pallidus (RPa) selectively reversed the intracerebroventricular fentanyl-evoked increases in BAT SNA and HR, while the fentanyl-evoked excitation in RSNA, the pressor responses, and the tachycardic responses were reversed by inhibition of neurons in the rostral ventrolateral medulla (RVLM). Prior inhibition of neurons in the dorsomedial hypothalamus eliminated the intracerebroventricular fentanyl-evoked increases in BAT SNA, BAT temperature, and HR, but not those in RSNA or AP. These results indicate that activation of central mu-opioid receptors with fentanyl can elicit BAT thermogenesis and cardiovascular stimulation through excitation of the sympathetic outflows to BAT, kidney, and heart. Activation of neurons in the rostral RPa and RVLM are essential for the increases in BAT thermogenesis and renal sympathoexcitation, respectively, induced by activation of central mu-opioid receptors. BAT thermogenesis could contribute to fentanyl-evoked hyperthermia, particularly in infants where BAT plays a significant role in thermoregulation.