Thermal control of mother-young contact revisited: hyperthermic rats nurse normally

Morphine (MOR) is known to inhibit maternal behavior and induce hyperthermia; at appropriate doses, concurrent administration of naloxone (NAL) counteracts its disruption of maternal behavior but not the hyperthermia. We used these findings to evaluate the view that lactating rats terminate nursing due to intolerable hyperthermia. After a dam-litter separation of 4 h on Day 7 postpartum (PP), mother-litter interactions were observed continuously for 1 h. One hour before reunion, the dams received two injections (1 ml/kg ip each) of saline (SAL), MOR (20 mg/kg) and/or NAL (1 mg/kg) in the following combinations (n = 7 each): SAL + SAL, SAL + NAL, MOR + SAL or MOR + NAL. MOR profoundly disrupted maternal behavior, thereby preventing litter weight gains; these effects were completely counteracted by NAL, which alone had no discernible effects. In contrast, MOR-induced hyperthermia (approximately 0.7 degrees C increase in each hour, before and after reunion with pups) was not antagonized by NAL at the doses used. Thus, an additional 0.7-1.4 degrees C of body temperature (T) did not delay the onset or reduce the duration of nursing compared with SAL-treated controls. Further, there were no group differences in behaviors displayed both shortly before and after a nursing bout that included milk ejections or in the resumption of nursing. Together with earlier methodological and empirical criticisms of the thermal control theory, as well as knowledge about the somatosensory determinants of nursing, the present results suggest that nursing bouts in lactating rats are not limited by the mother's T.