Neural connections of the suprachiasmatic nucleus and medial hypothalamus of the Syrian hamster (Mesocricetus auratus).

The neural connections of brain areas involved in the control of circadian rhythms were investigated in the Syrian hamster by observing the anterograde and retrograde neural labelling resulting from injections of HRP-WGA into the suprachiasmatic nucleus (SCN) and subparaventricular zone (SPZ). The neural connections of these 2 brain areas in the strongly photoperiodic hamster are very similar to those in the more weakly photoperiodic rat. Two differences were observed: (1) in the rat, but not in the hamster, the SCN projects to the ventral geniculate nucleus and intergeniculate leaflet of the thalamus; and (2) reciprocal contralateral neural connections appear to be more numerous in the hamster than in the rat SCN. These differences may relate to the differences in photoperiodic sensitivity of the 2 species.