Cellular localization of somatostatin in endocrine-like cells and neurons of the rat with special references to the A1-cells of the pancreatic islets and to the hypothalamus.

With the indirect immunofluorescence technique somatostatin, a recently isolated and structurally characterized hypothalamic tetradecapeptide, which inhibits growth hormone release, has been traced in different tissues of the rat. Somatostatin or somatostatin-like immunoreactivity (SLI) was present in both neurons and endocrine-like cells. Somatostatin positive nerve cell bodies were observed in the periventricular region in the anterior parts of the hypothalamus and in probable nerve endings in the external and internal layer of the median eminence and in the pituitary stalk and in the ventromedial, arcuate and ventral premammillary nuclei and to a small extent in the periventricular region. Somatostatin positive nerve fibers were also found in the posterior pituitary, indicating the existence of a third neurosecretory hypothalamo-hypophysial system, and in nerves in different layers of the wall of the small and large intestine. These nerves may possibly be of sensory nature since some cell bodies of the spinal ganglia showed a weak immunofluorescence. Somatostatin or SLI was found in certain cells localized in the periphery of the pancreatic islets. In parallel studies with glucagon antibodies it could be established that the somatostatin positive cells and the glucagon positive cells were not identical but localized extremely close to each other. Furthermore, with the Hellman-Hellerström silver staining technique it could be shown that virtually all somatostatin cells are argyrophilic and vice versa. Somatostatin positive cells thus constitute the socalled A1-cells (D-cells). Glucagon positive cells, on the other hand, do in all probability not exhibit argyrophilia. The occurrence of probable somatostatin containing cells in the pancreatic islets is of special interest in view of several recent studies demonstrating an inhibitory action of this peptide on glucagon and insulin secretion. Somatostatin or SLI was found in a rather small number of cells in the thyroid gland with a parafollicular position. These cells exhibited a positive immunofluorescence also after pretreatment of the antiserum with calcitonin. Furthermore, endocrine-like cells in the stomach and in the intestine were also somatostatin positive. A large number of cells in various tissues, especially in the lamina propria of the gastro-intestinal tract, showed a strong fluorescence both after incubation with control serum and with FITC conjugated serum alone. The present results indicate that somatostatin or a somatostatin-like peptide(s), in addition to its inhibitory action on growth hormone release, may play a physiological role at many other levels of the organism.