Vasoactive intestinal peptide-immunoreactive cerebrospinal fluid-contacting neurons in the reptilian lateral septum/nucleus accumbens.

By means of immunocytochemical demonstration of vasoactive intestinal peptide (VIP) an accumulation of cerebrospinal fluid (CSF)-contacting neurons was found in a circumscribed region of the nucleus accumbens/lateral septum of eleven reptilian (chelonian, lacertilian, ophidian, crocodilian) species. Basal processes of these cells contribute to a subependymal plexus whose density displays considerable interspecific variation. VIP-immunoreactive nerve fibers occur also in the lateral septum and the nucleus accumbens where they encompass immunonegative cells in a basket-like pattern. The CSF-contacting neurons are surrounded by columnar ependymocytes frequently arranged in a pseudostratified manner. These specialized arrays of ependymal cells, however, occupy a more extended area than the VIP-immunoreactive CSF-contacting neurons and can be traced from the rostro-ventral pole of the lateral ventricle to the interventricular foramen. These observations suggest the existence of a telencephalic site of CSF-contacting neurons which may be more widespread than hitherto thought and which may participate in a circumventricular system of the lateral ventricle. Previous studies mainly performed with birds indicate that the VIP-immunoreactive CSF-contacting neurons of the nucleus accumbens might form a part of the "encephalic" (extraretinal and extrapineal) photoreceptor. However, further experiments are required to test this supposition since the VIP-immunoreactive neurons of the nucleus accumbens remained unlabeled by antibodies against bovine rodopsin and chicken cone-opsin in all eleven species analysed in this investigation.