Cerebroventricular administration of somatostatin (SRIF): Effect on central levels of cyclic AMP

Localization and characterization of brain somatostatin receptors as studied with somatostatin-14 and somatostatin-28 receptor radioautography

The localization and characterization of receptors for somatostatin-14 (S-14) and somatostatin-28 (S-28) were studied in the rat brain using the iodinated agonists [Tyr0,D-Trp8]S-14 and [Leu8,D-Trp22,Tyr25]S-28 as tracers. Slide-mounted frozen sections were used for the radioautographic localization and biochemical characterization of somatostatin receptors. In the latter case counting was performed on scraped off serial sections from rostral regions of the brain. Specificity studies demonstrated that either tracer could be displaced with S-28, S-14 or their agonists. The N-terminus fragment (1-12) of S-28 as well as a number of unrelated peptides were unable to compete with either tracer, indicating that the binding capacity for ligand-receptor recognition is located in the C-terminal portion of S-28. Scatchard analysis of saturation curves gave a one-site interaction with Kd values of 0.42 +/- 0.09 nM and 0.32 +/- 0.04 nM for the S-14 and S-28 iodinated agonists, respectively. By radioautography, the distribution of receptors for both S-14 and S-28 appeared very similar with high levels of binding in the deep layers of the cortex, the cingulate cortex, the claustrum, the locus coeruleus and most structures of the limbic system. Treatment with cysteamine, which caused a somatostatin depletion in the brain, was required to observe labeling in the hypothalamus. In some caudal areas of the brain, especially in the cerebellar nuclei, the solitary tract nucleus and the nucleus of the vagus nerve, only labeling with the S-28 agonist could be detected. This S-28 binding could be displaced by native S-14 (10(-6) M). Generally, there was a correlation between the localization of somatostatin receptors and that of immunoreactive somatostatin, as evaluated by immunocytochemistry. However, in some areas, an inverse correlation between receptor and peptide concentrations was observed. These results are in agreement with previous data suggesting that somatostatin could act as a neurotransmitter or neuromodulator in several brain areas.

Somatostatin along the auditory pathway

Several structures associated with the auditory pathway have been examined by radioimmunoassay for their content of somatostatin. Of these, the medial geniculate body had the highest content, followed by the cochlear nucleus, inferior colliculus, auditory cortex and cochlea. Cochlear perilymph had no detectable somatostatin-like immunoreactivity.

Lack of behavioural effects following intraventricular infusion of somatostatin in the conscious goat

The effect of IV or intracerebroventricular (ICV) administration of somatostatin was studied on the behaviour of conscious goats. The doses of somatostatin infused IV were 100 and 300 microgram for 30 min and 600 microgram for 6 min. The doses infused ICV were 10 and 100 microgram for 30 min and 600 microgram for 6 min. In contrast to earlier reports on experiments with rats ,no behavioral effects whatsoever were seen in goat. IV infusion of 100 to 600 microgram and ICV infusion of 600 microgram of somatostatin caused a difinite reduction in the secretion of insulin and growth hormone, but had no effect on the concentration of blood glucose. The reason why neither IV nor ICV administration of somatostatin had any behavioural effects in the conscious goat, in contrast to the effects in rat, cannot be explained with certainty. This may be due to species specificity, to the amount of somatostatin reaching the central nervous system, or to some metabolic changes in rat but not in goat.