Seasonal changes in methionine-enkephalin immunoreactivity in the brain of a hibernator, Spermophilus columbianus

Abstract

To identify the actual location of central endogenous opioid systems which may be involved in regulating the hyberbation cycle, differences in the pattern of central methionine-enkephalin (Met-EK) immunoreactivity were compared between hibernating (body temperature, T b = 7 °C) and non-hibernating ( T b = 37 °C) Columbia ground using the peroxidase-antiperoxidase technique. In non-hibernating animals, Met-EK-immunoreactive perikya were observed in telencephalic (putamen, caudate,, nucleus, medial septum-diagonal band complex, amygdala) and diencephalic (periventricular hypothalamic nucleus, lateral hypothalamic area) regions; whereas immunoreactive fibers were found in the lateral septum, stria terminalis nucleus, various hypothalamic areas, arcuate nucleus, median eminence, thalamic intralaminar, periventricular nucleus and lateral habenular nucleus. Compared to the non-hiberbnating animal, a marked increase in the number of Met-EK-immunoreactive fibers was found in the lateral septal nucleus, the periventricular nucleus, the intralaminar thalamus and the paraventricular hypothalamus of hibernating ground squirrels. Since these changes in immunoreactivity were not observed in the artificially induced hypothermic ground squirrels ( T b = 7 °C), it is unlikely that the dissimilarity in immunoreactivity between animals from different hibernating phases is due to differences in their T b. In combination with our previous studies, these results tend to suggest that hibernation maybe be brought about an increase in endogenous opioid activity, especially in the lateral septal region.