Discrete thalamic lesions attenuate winter adaptations and increase body weight.

The midline thalamus (e.g., the paraventricular thalamic nuclei and the reuniens nucleus) of Siberian hamsters and other mammals has been reported to contain specific binding sites for melatonin, a hormone that is essential for photoperiodically induced winter adaptations such as reproductive quiescence, loss of body weight, daily torpor, and the winter molt. The first experiment investigated whether the midline thalamus is necessary for these winter adaptations. Adult Siberian hamsters received discrete neurotoxic lesions of the paraventricular thalamic nuclei or the reuniens nucleus while under pentobarbital sodium-induced anesthesia. After recovery, the hamsters were monitored for winter adaptations while they were exposed to short photoperiods (10 h light/day) for 12 wk at 22 degrees C then for 60 days at 7 degrees C. Lesions of the reuniens nucleus, but not of the paraventricular thalamic nuclei, significantly inhibited short photoperiod-induced loss of body weight and tended to increase food consumption and decrease daily torpor. The second experiment showed that lesions of the reuniens nucleus increased body weight gain compared with that in controls during exposure to long photoperiods at 22 degrees C for 16 wk. In summary, these findings show that the reuniens nucleus is an important site for regulation of body weight and suggest that lesions of the reuniens nucleus may attenuate winter metabolic adaptations by causing an increase in body weight.