Third ventricular subependymal oxytocin-like immunoreactive neuronal plexus in the rat.

In a previous retrograde tracing study, a dense subependymal neuronal plexus was found along the anterior ventral third ventricle that projects to the posterior pituitary. In the work reported here, the oxytocin-like immunoreactive neurons of this plexus were studied in detail. It has a population of about 650 cells with a great wealth of dendrites. The neurons are of magnocellular neurosecretory type with long straight dendrites running parallel to the ependyma. The plexus is composed of a dorsal and a ventral part. The dorsal pari consists of about 75% of the whole population and is most dense at the levels of the anterior and medial magnocellular paraventricular nuclei. Their dendrites appear vacuous in immunohistochemically stained sections and have a tendency to form fascicles. The ventral part is more sparse. The dendrites of the subependymal plexus are well organized so the anteriorly located ones tend to be directed rostrally and the posteriorly ones caudally. The functional significance of the plexus is discussed.