Long-term thermoregulatory changes following perinatal methadone exposure in rats

Offspring of female rats injected daily with methadone (5 mg/kg, IP) or saline were cross-fostered at birth to form drug groups exposed during gestation (G), lactation (L), or both gestation and lactation (G-L); controls were exposed only to saline. Rectal temperatures were taken on postnatal Days 20, 43, 57 and 75, and at 3-day intervals from Days 128-140 and 157-169. Ambient temperature was 21°C except from Days 131-134 (10°C) and 160-163 (33°C). Methadone-exposed rats tended to have lower rectal temperatures than controls at 21°C; this was significant on Days 57 and 160 for Group G, on Days 43 and 160 for Group L, and on Days 43, 57, 128, 160 and 166 for Group G-L. Relative to controls, Groups G and G-L became hypothermic during cold stress and hyperthermic during heat stress. Thermal deviations were unrelated to changes in food intake and body weight. These results indicate that perinatal methadone exposure in rats produces thermoregulatory changes that persist into adulthood.