Somatostatin in the brain of the cave salamander, Hydromantes genei (Amphibia, Plethodontidae): immunohistochemical localization and biochemical characterization

The distribution of somatostatin-like immunoreactivity in the brain of the cave salamander Hydromantes genei (Amphibia, Plethodontidae) was investigated by using two distinct antisera raised against somatostatin-14. Most somatostatin-positive cells were detected in the ependymal cell layer surrounding the ventricles. These cells possessed the typical morphological characteristics of tanycytes or radial glial cells. Double-labeling with an antiserum against somatostatin and a monoclonal antibody against glial fibrillary acidic protein showed that somatostatin-immunoreactive cells lining the ventricles also exhibited GFAP-like immunoreactivity. Injection of the neurotracer biocytin into the lateral ventricle revealed that neurons lining the ventricles did not contain somatostatin-like immunoreactivity. In the telencephalon, somatostatin-like immunoreactivity was confined to radial glial cells. In the diencephalon, in addition to somatostatin-immunoreactive cells in the ependyma, positive cell bodies were also found in the periventricular preoptic nucleus, the infundibular nucleus, the epiphysis, and the subcommissural organ. In the metencephalon, positive cell bodies were found in the auricula cerebelli, whereas in the rhombencephalon numerous somatostatin-immunoreactive cells were seen lining the ventricular cavity. Immunoreactive nerve fibers were observed in the hypothalamus-median eminence complex. In the pituitary, a discrete group of somatostatin-positive cells was found in the pars distalis. High-performance liquid chromatography analysis of brain extracts revealed that the immunoreactive material coeluted with somatostatin-14. The present results show that the somatostatin peptidergic system in the brain of the cave salamander has a more simple organization than those described in the brain of frog and other vertebrates. This feature is probably related to the expression of high pedomorphic characters in plethodontids. The distribution of somatostatin-like immunoreactivity suggests that, in the cave salamander, somatostatin may act as a neurotransmitter and/or neuromodulator, a central regulator of fluid homeostasis, and a hypophysiotropic neurohormone.