Effect of a mu-selective opioid antagonist on CCK-8-induced changes in thermoregulation in the rat

We examined the effects of I.C.V. injection of CCK-8 alone and in combination with CTAP, a mu-selective antagonist, on brain surface temperature (Tb), oxygen consumption (VO2), and heat exchange (Q) in unrestrained male S-D rats at an ambient temperature of 20 +/- 0.5 degrees C. CCK-8 (300 ng, I.C.V.) produced hyperthermia (deltaTb: 0.86 +/- 0.22 degrees C) lasting for 30-60 min, which was associated with an increase in VO2 (1.42 +/- 0.28 ml/g/h). There was an increase in Q (1.87 +/- 1.2 cal/g/h) beginning 15 min after injection and lasting for 60 min. The CCK-8-induced hyperthermia was attenuated by a postinjection increase in Q. During the 60-120 min postinjection period, VO2 and Q returned to baseline followed by the return of Tb to preinjection levels. CCK-8-induced increases in Tb, Q, and VO2 were blocked by pretreatment with CTAP (0.75 nmol, I.C.V.) 15 min earlier. CTAP alone did not significantly affect deltaTb, deltaVO2, and deltaQ. These results suggest that, within the thermoneutral zone, the thermoregulatory effects of CCK-8 in the rat involve participation of mu-opioid receptors.