Comparative density of CCK- and PV-GABA cells within the cortex and hippocampus
Rats and golden hamsters show a differential feeding response to intracranial injections of cholecystokinin (CCK). Rats, but not hamsters reduce food intake after CCK injections into the hypothalamic paraventricular nucleus. In view of this species difference, we undertook an immunohistochemical study of the distribution of CCK-immunoreactivity in the hamster hypothalamus and remaining forebrain. CCK-immunoreactive perikarya were abundant in the neocortex, claustrum, hippocampal formation, amygdaloid complex, bed nucleus of the stria terminalis, nucleus of the lateral olfactory tract and in the magnocellular basal nucleus. CCK-immunoreactive neurons had a more restricted distribution in the diencephalon and were relatively rare in the preoptic area-hypothalamus. The only exception was the suprachiasmatic nucleus and adjacent medial anterior hypothalamus, in which CCK-immunoreactive neurons were numerous. CCK-containing perikarya were not observed in the hamster hypothalamic paraventricular and supraoptic nuclei, where they have been reported to occur in the rat. Groups of CCK-positive perikarya were also noted in the hamster thalamic paratenial and parafascicular nuclei. CCK-immunoreactive fibers/terminals were localized in the caudate and putamen, periventricular zones, dorsolateral geniculate, thalamic reticular nucleus and the superficial layer of the optic tectum. Fiber/terminal labeling was also present in those regions associated with CCK-immunoreactive perikarya. Our results indicate that the telencephalic distribution of CCK-containing neurons in the hamster appears to be similar to that reported in the rat. However, several differences occur in the diencephalon. Perhaps the most striking is that the hamster differs from the rat in having a large group of CCK-containing neurons in the suprachiasmatic nucleus, and in lacking the CCK-containing perikarya observed in the rat paraventricular and supraoptic nuclei. These differences may underly species differences in feeding responses to intracranial CCK injections and gonadal responses to short photoperiods. Our data further suggest that the distribution of neuropeptides and other neuroactive substances may not always be conserved during evolution.