It was accepted for a long time that in mammals there is only retinofugal neuronal connection between the eye and the pineal body (PB). In our previous paper we described that nerve cells were present in hamster PB and these neurons could establish a reverse connection with the retina through a transsynaptic pathway. In adult albino rats neuronal perikarya were not found. In this present experiment it was examined whether the lack of these nerve cells in the PB of adult rats is the result of an apoptotic phenomenon or the lack of migration during the fetal period. Green fluorescence protein expressing pseudorabies virus, spreading only in retrograde direction, was injected into the vitreous body of rats at various postnatal ages. Virus labeled cell bodies were not observed in the PB of adult rats; however, labeling with gradually decreasing number of cells was present in animals aged 3–6, 13–14, 20, 35 and 41 postnatal days. Injection of virus, spreading in anterograde direction (expressing red fluorescence protein), into the PB of young prepubertal animals resulted in labeling in the retina. This observation indicates that the pinealo-retinal connection in prepubertal period is active. Immunostaining revealed that some of the labeled neuronal perikarya showed activated caspase-3 (an apoptotic marker) immunoreactivity. Our results clearly show that the neurons migrate to the PB and later, during the prepubertal period, they disappear. Caspase-3 immnoreactivity indicates that these cells die off by apoptosis.
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