Distribution of somatostatin receptors in the adult human brainstem

The neuropeptide somatostatin is widely distributed in the central nervous system of rat and human. Somatostatin-containing neurons are particularly abundant in the hypothalamus, the cerebral cortex and the limbic system. Somatostatin is also present in a number of discrete structures in the brainstem and spinal cord. The localization of somatostatin receptors provides valuable information regarding the possible roles of the peptide in the brain. In the present study, we have investigated the precise distribution of somatostatin binding sites in the human lower brainstem by quantitative autoradiography, using [125I- Tyr0,DTrp8]S14 as a radioligand. The tissues were collected from two individuals, aged 50 and 67 years, who had no antecedent of neurological disorders. The binding of the radioligand was visualized in 73 distinct anatomical regions of the medulla and pons and quantified by computer-assisted image analysis. Somatostatin binding sites were present in sensory nuclei, the highest densities being observed in the trigeminal complex (spinalis oralis and interpolaris) and in the nucleus (N.) tractus solitarii. Moderate to low densities of binding sites were detected in the N. vestibularis medialis and spinalis, and in the N. nervus trigemini sensibilis principalis. Many relay nuclei of the ascending somatosensory pathways contained moderate to high densities of binding sites: the inferior olivary complex, the N. arcuatus and the N. praepositus hypoglossi. Binding sites were also present in several motor nuclei such as the N. nervi hypoglossi, the N. dorsalis motorius nervi vagi, the N. nervi facialis and the N. nervi abducentis. Moderate to low concentrations of binding sites were detected in nuclei related to the reticular formation including the N. raphae pallidus, the N. parabrachialis and the N. supratrochlearis. The N. locus coeruleus exhibited a very high concentration of somatostatin binding sites in both individuals. The present data, together with previous studies on the distribution of somatostatin-immunoreactive fibers in the human brainstem, suggest that somatostatin may be involved in (i) sensory processes including vestibular sensitivity, somatosensoriality and proprioception, (ii) sleep-waking cycle and arousal and (iii) control of various neurovegetative functions including regulation of cardiovascular and respiratory activities as well as gastric acid secretion.