Thermoregulatory effects of centrally administered bombesin, bradykinin, and methionine-enkephalin

Bombesin (BO, 100 ng), bradykinin (BR, 10 μg), or methionine-enkephalin (EN, 10 μg) was administered intracerebroventricularly to adult male rats at an environmental temperature of 4°C, 22°C, or 35°C, and rectal (T_re) and tail-skin (T_sk) temperatures were monitored for 5 hours. At 4°C and 22°C BO-treated animals developed acute hypothermia (max ΔT_re = −3.25°Cand −2.71°C, respectively) which persisted for 2 hours (p<0.05). At 22°C and at 300 min post-injection, BO-treated animals became significantly (p<0.05) hyperthermic (ΔT_re = +1.28°C) when compared to controls. While BR had no effects at 22°C, EN-injected rats demonstrated a significant (p<0.05) hyperthermia from 180 min through 300 min (ΔT_re = +1.40°C). At 22°C both BO and, surprisingly, EN increased T_re (e.g. ΔT_re = +3.49°Cand +2.01°C at 60 min). At 35°C EN elicited hyperthermia which was significantly (p<0.05) increased from time 0 at all sampling times (mean ΔT_re = +1.85°C) and from control levels at 300 min (ΔT_re = +1.07°C, p<0.05). BO again caused a significant (p<0.05, BO vs control, 30 min) decrement (ΔT_re = −1.22°C followed by increments (p<0.05) from 120–300 min. We conclude that the hypothermic effect of BO is dependent upon environmental temperature, partially caused by vasodilation, and possibly biphasic in nature; EN treatment generally elicits hyperthermia under these conditions while BR produced no effects on thermoregulation.