Immunocytochemical and Ultrastructural Analysis of Opsin- and Vasoactive Intestinal Peptide (VIP)- like Immunoreactive Neurons in the Lateral Septum of the Pigeon

Abstract

Summary CSF-contacting neurons of the lateral septum are considered as a putative deep brain photoreceptor in the avian brain. By means of immunocytochemistry using antibodies to a visual pigment (RET-P1, a monoclonal antibody against opsin, Barnstable, 1980) and vasoactive intestinal peptide (VIP), I demonstrated clusters of immunoreactive small neurons in the lateral septum of the pigeon. Opsin-like immunoreactive neurons and VIP-like immunoreactive neurons have similar morphological features. Their perikarya accumulate in the subependymal regions of the ventromedial walls of both lateral ventricles. The labelled neurons are multipolar or bipolar with pyriform or spindle-shaped cell bodies. Immunoreactive CSF-contacting neurons contact the CSF via a process that penetrates the ependyma and terminates in a single knob-like swelling. Immunoreactive fibers originating in the group of cell bodies seem to give rise to dense terminal-like structures in the septal area. Immuno-electron-microscopic investigations of these neurons revealed an accumulation of VIP- and opsin immunoreactive dense-core vesicles (100-150 nm in diameter) in ventricular terminals, perikarya and neuronal terminal-like structure with VIP- and RET-P1-immunolabelling respectively. Based on these evidence it seems clear that VIP-and opsin-like immunoreactive neurons of this study are the same as the neurons that express both opsin- and VIP-like immunoreactivity in the ateral septum of the ring dove (Silver et al., 1988). In this study double immunolabelling using VIP and RET-P1 antibodies shows the coexistence of VIP and opsin in the same dense vesicles.