DISTRIBUTION OF SOMATOSTATIN RECEPTORS IN THE ADULT HUMAN BRAIN-STEM

The neuropeptide somatostatin is widely distributed in the central ner vous system of rat and human. Somatostatin-containing neurons are part icularly abundant in the hypothalamus, the cerebral cortex and the lim bic system. Somatostatin is also present in a number of discrete struc tures in the brainstem and spinal cord. The localization of somatostat in receptors provides valuable information regarding the possible role s of the peptide in the brain. In the present study, we have investiga ted the precise distribution of somatostatin binding sites in the huma n lower brainstem by quantitative autoradiography, using [I-125-Tyr(0) ,DTrp(8)]S14 as a radioligand. The tissues were collected from two ind ividuals, aged 50 and 67 years, who had no antecedent of neurological disorders. The binding of the radioligand was visualized in 73 distinc t anatomical regions of the medulla and pens and quantified by compute r-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.) tr actus solitarii. Moderate to low densities of binding sites were detec ted 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. praep ositus hypoglossi. Binding sites were also present in several motor nu clei such as the N. nervi hypoglossi, the N. dorsalis motorius nervi v agi, the N. nervi facialis and the N. nervi abducentis. Moderate to lo w concentrations of binding sites were detected in nuclei related to t he reticular formation including the N. raphae pallidus, the N. parabr achialis and the N. supratrochlearis. The N. locus coeruleus exhibited a very high concentration of somatostatin binding sites in both indiv iduals. The present data, together with previous studies on the distri bution of somatostatin-immunoreactive fibers in the human brainstem, s uggest that somatostatin may be involved in (i) sensory processes incl uding vestibular sensitivity, somatosensoriality and proprioception, ( ii) sleep-waking cycle and arousal, and (iii) control of various neuro vegetative functions including regulation of cardiovascular and respir atory activities as well as gastric acid secretion.