A 3-dimensional reconstruction of the hypothalamo-neurohypophysial system of the rat. The neurons projecting to the neuro/intermediate lobe and those containing vasopressin and somatostatin

A three-dimensional reconstruction of the rat's hypothalamo-neurohypophysial system, depicting the neurons that project to the neuro-intermediate lobe of the pituitary, has been made by using retrograde transport of horseradish peroxidase injected into the neuro-intermediate lobe and by using immunocytochemistry with antisera to vasopressin and somatostatin. The overall picture illustrates the neurons situated in the walls of a pair of ill-defined cone-shaped tunnels, the apices pointing anteriorly. Among the neuronal aggregates in the tunnel wall two, the paraventricular and forniceal nuclei, appear similar in shape but clearly separated by a gap of at least 150 micrometers. Many of the vasopressin-positive neurons lie in the same nuclear aggregates with two notable exceptions: the suprachiasmatic nucleus contains many vasopressin-positive cells but does not project to the pituitary, and the forniceal aggregate, which does project to the pituitary, contains no vasopressin-positive cells. Somatostatin-positive cells are situated close to the third ventricle and their size is intermediate between parvocellular and magnocellular. Cell counts show only half the cells in the system lying in the supraoptic and paraventricular nuclei, the rest being in "accessory nuclei".