Ultrastructure of cerebrospinal fluid-contacting neurons immunoreactive to vasoactive intestinal peptide and properties of the blood-brain barrier in the lateral septal organ of the duck.

Immuno-electron-microscopic investigations of cerebrospinal fluid (CSF)-contacting neurons immunoreactive to vasoactive intestinal peptide in the duck lateral septum have revealed that this cell type gives rise to an adventricular dendrite terminating with a bulbous swelling in the lateral ventricle. The swelling bears a cilium and contains mitochondria and immunolabeled dense-core vesicles. Two types of processes emerge from the basal part of the perikaryon. The first has a large diameter, contains diffusely distributed immunoreaction, and receives synaptic input, indicating that this process is a basal dendrite. The other type is of a beaded appearance, displays immunolabeled dense-core vesicles, and represents the axon of the CSF-contacting neuron. VIP-immunoreactive terminal formations are located within the neuropil of the lateral septum and the nucleus accumbens. Some of them form synaptic contacts with immunonegative profiles. No VIP-immunoreactive terminal formations are seen in the perivascular spaces of the lateral septum. Tracer experiments with horseradish peroxidase have revealed that the blood-brain barrier is lacking in the lateral septal organ and nucleus accumbens of the duck. Capillaries, arterioles, and venoles of this region are coated by nonfenestrated endothelial cells connected by "leaky" junctions, allowing the tracer to penetrate from the lumen into the perivascular space and further into the intercellular clefts of the neuropil. Our immuno-electron-microscopic investigations show that VIP-immunoreactive CSF-contacting neurons of the lateral septum closely resemble CSF-contacting neurons occurring in other brain regions, e.g., the hypothalamus. The arrangement of VIP-immunoreactive terminal formations suggests that, in the lateral septum, the VIP-like neuropeptide serves as a neurotransmitter (-modulator). The lack of a blood-brain barrier in the lateral septal organ and the nucleus accumbens raises the possibility that this region is a window in the avian brain allowing exchange of information between the central nervous system and the bloodstream; it thus resembles a circumventricular organ.