Gold-thioglucose-induced hypothalamic lesions inhibit metabolic modulation of light-induced circadian phase shifts in mice

The circadian clock located in the suprachiasmatic nuclei is entrained by the 24-h variation in light intensity. The clock's responses to light can, however, be reduced when glucose availability is decreased. We tested the hypothesis that the ventromedial hypothalamus, a key area in the integration of metabolic and hormonal signals, mediates the metabolic modulation of circadian responses to light by injecting C57BL/6J mice with gold-thioglucose (0.6 g/kg) which damages glucose-receptive neurons, primarily located in the ventromedial hypothalamus. Light pulses applied during the mid-subjective night induce phase delays in the circadian rhythm of locomotor activity in mice kept in constant darkness. As previously observed, light-induced phase delays were significantly attenuated in fed mice pre-treated with 500 mg/kg i.p. 2-deoxy-D-glucose and in hypoglycemic mice fasted for 30 h, pre-treated with 5 IU/kg s.c. insulin or saline, compared to control mice fed ad libitum. In contrast, similar metabolic challenges in mice with gold-thioglucose-induced hypothalamic lesions did not significantly affect light-induced phase delays compared to mice treated with gold-thioglucose and fed ad libitum. These results indicate that destruction of gold-thioglucose-sensitive neurons in the ventromedial hypothalamus prevent metabolic regulation of circadian responses to light during shortage of glucose availability. Therefore, the ventromedial hypothalamus may be a central site coordinating the metabolic modulation of light-induced phase shifts of the circadian clock.

Perinatal exposures to rotating magnetic fields 'demasculinize' neuronal density in the medial preoptic nucleus of male rats

Pregnant rats were exposed continuously for 3 days before to 3 days after birth to 0.5 rotating magnetic fields (RMF) whose intensities ranged between 1.5 and 3.0 mT or between 50 and 300 microT or to sham field conditions. When the male and female rats exposed to these perinatal conditions were about 100 days old, the numbers of neuronal soma and the numbers of nuclei for the three major types of glial cells were counted within the medial preoptic nucleus (MPO), suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMH) of the hypothalamus. The male rats but not the female rats that had been exposed to either intensity of the RMF showed a significant reduction (similar to normal females) in the numbers of neurons within the MPO; the differences accommodated one-third of the variance and were not reduced significantly when the cell densities of the VMH or SCN were covaried before the analyses. The results suggest that some sexually dimorphic structures may be permanently and differentially affected when exposed perinatally to relatively weak extremely low frequency magnetic fields.