Somatostatin in Hypothalamus, Extrahypothalamic Brain, and Peripheral Tissues of the Rat*

Abstract

A radioimmunoassay (RIA) for somatostatin was developed using rabbit antiserum raised against cyclic somatostatin conjugated to thyroglobulin, radioiodinated [Tyr1]somatostatin and cyclic somatostatin standards. Damage to the tracer during the RIA incubation was prevented by the use of 0.25 M EDTA or lower EDTA concentrations plus Trasylol. The mean detection limit for 15 individual assays was 1.6 .+-. 0.4 pg (SD). Linear somatostatin showed 70% cross-activity in the assay but [Tyr1]somatostatin reacted to the same extent as the cyclic somatostatin standards. Acetic acid extracts (1.0 N) of rat hypothalamus, cerebral cortex, other brain areas, and a number of visceral organs assayed in serial dilutions gave inhibition curves parallel to those of cyclic somatostatin. Recovery of tissue immunoreactivity based on addition studies was 82.8 .+-. 9.0 (SEM [standard error of the mean])%. Gel filtration of rat hypothalamic, cerebral cortical, stomach and pancreatic islet extracts revealed 2 immunoreactive peaks, 1 corresponding to synthetic somatostatin, the other representing a larger molecular species postulated to be a somatostatin precursor, or a somatostatin complex. For this reason, results for tissue concentration represent both free and big hormone. Immunoreactive somatostatin concentration (nanograms per milligram of protein, not corrected for recovery) was highest in isolated pancreatic islets (786) and in the stalk median eminence region (248). In descending order of concentration in brain regions are hypothalamus (26.1), ventromedial hypothalamic nucleus (17.5), spinal cord (10.4), cerebral cortex (6.7), brain stem (5.1), olfactory lobe (1.07), cerebellum (0.43) and pineal gland (0.14). Somatostatin concentration in the pyloric region of the stomach was similar to that of the cerebral cortex (6.4), and other regions of the stomach and gut also had substantial concentrations of somatostatin. The specificity of these findings is suggested by the failure to demonstrate somatostatin in liver, lungs, kidneys and submandibular glands.