Distribution of somatostatin receptors in the brain of the frog Rana ridibunda: Correlation with the localization of somatostatin‐containing neurons
Authors: Laquerriere Annie , Leroux Philippe , Gonzalez Bruno J. , Bodenant Corinne , Vaudry Hubert , Benoit Robert 
Published: January 2005

The biochemical characterization and anatomical distribution of somatostatin binding sites were examined in the brain of the frog Rana ridibunda, and the distribution of the receptors was compared with the location of somatostatin immunoreactive neurons. The pharmacological profile of somatostatin receptors was determined in the frog brain by means of an iodinated superagonist of somatostatin, [125I‐Tyr0,DTrp8]S‐14. Membrane‐enriched preparations from frog brain homogenates were shown to contain high‐affinity receptors (KD = 0.78 ± 0.34 nM; Bmax = 103 + 12.7 fmoles /mg protein) with pharmacological specificity for [DTrp] substituted S14 and S28 analogs. The distribution of somatostatin‐binding sites was studied by autoradiography on coronal sections of frog brain. Various densities of somatostatin receptors were detected in discrete areas of the brain. The highest concentration of binding sites was observed in the olfactory bulb, in the pallium, and in the superficial tectum. Moderate binding was observed in the striatum, amygdaloid complex, preoptic area, and cerebellum.

Immunocytochemical studies of the distribution of somatostatin‐28 (S28) related peptides were also conducted in the frog brain. Two antisera that recognize distinct epitopes of the somatostatin molecule have been used for immunohistochemical mapping of the peptide. Antiserum SS9 recognizes both S28 and somatostatin‐14 (S14) and allowed the labelling of perikarya. Antiserum S320 recognizes the N‐terminal fragment (1–12) resulting from enzymatic cleavage of S28. This latter antiserum, which does not cross‐react with S28, stained mainly neuronal processes. At the infundibular level, however, both antisera stained cell bodies and fibers. Immunoreactive somatostatin‐related peptides were detected in many areas of the frog brain. In the diencephalon, a heavy accumulation of perikarya and fibers was seen in the preoptic nucleus, the dorsal and ventral infundibular nuclei, and the median eminence. Immunoreactive perikarya were also observed in the telencephalon, especially in the pallium and in thalamic nuclei. Immunostained processes were detected in many telencephalic areas and in the tectum.

There was good correlation between the distribution of somatostatin‐immunoreactive elements and the location of somatostatin‐binding sites in several areas of the brain, in particular in the median pallium, the tectum, and the interpeduncular nucleus. In contrast, mismatching was observed in the olfactory bulb, lateral pallium, and the cerebellum (which contained moderate to high levels of binding sites but virtually no somatostatin‐immunoreactive fibers) and the hypothalamic areas (preoptic area and infundibular nuclei), in which a low concentration of binding sites but a high density of immunoreactive perikarya and fibers were detected.

The present data provide the first pharmacological characterization and anatomical distribution of somatostatin‐binding sites in the brain of a non‐mammalian vertebrate species. The present results suggest that many of the major features of somatostatinergic systems described in mammals (e.g., the wide distribution of somatostatinergic neurons and somatostatin‐binding sites; pharmacological characteristics of somatostatin receptors) had already arisen in tetrapod ancestors. Such findings suggest that both neuroendocrine and extrahypothalamic somatostatinergic systems have important functions throughout the vertebrate phylum.