ANATOMICAL DISTRIBUTION OF SOMATOSTATIN RECEPTORS IN THE BRAIN-STEM OF THE HUMAN FETUS

The distribution of somatostatin binding sites was studied in the pens and medulla oblongata of three human fetuses (gestional ages 26, 28 a nd 30 weeks). The study was carried out by in vitro quantitative autor adiography using either [I-125-Tyr(0),D-Trp(8)]somatostatin-14 or [I-1 25-Tyr(11)]somatostatin-14 as radioligands. Somatostatin binding sites were observed in a number of nuclei subserving sensory, motor or inte grative functions within the pens and medulla. In addition, discrete t racts also contained significant amounts of binding sites. Among struc tures involved in sensory processes, a high density of binding sites ( 40-60 fmol/mg wet tissue) was measured in the dorsal cochlear nucleus and in the nucleus tractus spinalis trigemini caudalis. Moderate to hi gh levels of binding sites (30-40 fmol/mg wet tissue) were detected in the other sensory cranial nerve nuclei. A moderate density of sites ( 15-30 fmol/mg wet tissue) was measured in most motor nuclei, the highe st concentrations being observed in the dorsal motor nucleus of the va gus nerve, the facial nucleus, the hypoglossal nucleus and the nucleus ambiguus. The griseum pontis and the nucleus corporis pontobulbaris c ontained very high (> 60 fmol/mg wet tissue) and high concentrations o f somatostatin binding sites, respectively, while the other relay nucl ei contained low to moderate levels of binding. In monoaminergic nucle i, very high and moderate to high concentrations of somatostatin bindi ng sites were measured in the nucleus locus coeruleus and in its dorsa l subnucleus, respectively. Moderate densities of sites were detected in the ventral subnucleus of the nucleus locus coeruleus and in the di fferent parts of the raphe. In the white matter, low levels of binding were measured in the inferior cerebellar peduncle, the lateral and me dian lemnisci and the tractus solitarius. Conversely, moderate to high concentrations of somatostatin binding sites were measured in the med ian and superior cerebellar peduncles. The pyramis contained a very hi gh density of recognition sites. A marked heterogeneity in the density of binding sites was observed within a few structures, particularly i n the medial accessory olivary nucleus and the medial longitudinal fas ciculus. Selective ligands were used to determine the pharmacological profile of the [Tyr(11)]somatostatin-14 binding sites in various brain stem regions. In the dorsal cochlear nucleus and the pyramis, all soma tostatin binding sites belonged to the SSA subtype. Conversely, in the lateral paragigantocellular nucleus, all somatostatin binding sites b elonged to the SSB subtype. The other regions studied contained variou s proportions of SSA and SSB subtypes. In conclusion, the present stud y shows that high concentrations of somatostatin receptors are present in many regions of the human fetus brainstem. These data support the concept that somatostatin could be involved in the maturation of brain structures. Copyright (C) 1996 IBRO. Published by Elsevier Science Lt d.