Journal
BRAIN STRUCTURE & FUNCTION
Volume 226, Issue 4, Pages 1031-1052Publisher
SPRINGER HEIDELBERG
DOI: 10.1007/s00429-021-02224-9
Keywords
Somatostatin; Lampreys; Hypothalamus; Differential expression; Spinal cord; Neuropeptides
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Funding
- FEDER/Ministerio de Ciencia, Innovacion y Universidades-Agencia Estatal de Investigacion [BFU-2017-87079-P]
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The three somatostatin (SST) genes (SSTa, SSTb, and SSTc) in lampreys were found to be expressed in specific neuronal populations in the hypothalamus, with clear differential expression in other regions of the brain. The study provides important new insights into the evolution and distribution of the somatostatinergic system in vertebrates.
The identification of three somatostatin (SST) genes (SSTa, SSTb, and SSTc) in lampreys (Tostivint et al. Gen Comp Endocrinol 237:89-97 , 2016) prompted us to study their expression in the brain and spinal cord of the sea lamprey by in situ hybridization. These three genes were only expressed in equivalent neuronal populations in the hypothalamus. In other regions, SST transcripts showed clear differential expression. In the telencephalon, SSTc-positive cells were observed in the medial pallium, ventral part of the lateral pallium, striatum, subhippocampal lobe, and preoptic region. In the diencephalon, SSTa-positive cells were observed in the thalamus and SSTc-positive cells in the prethalamus, posterior tubercle, pretectal area, and nucleus of the medial longitudinal fascicle. In the midbrain, SSTc-positive cells were observed in the torus semicircularis, lateral reticular area, and perioculomotor tegmentum. Different SSTa- and SSTc-positive populations were observed in the isthmus. SSTc neurons were also observed in the rostral octavolateralis area and caudal rhombencephalon. In the spinal cord, SSTa was expressed in cerebrospinal-fluid-contacting (CSF-c) neurons and SSTc in non-CSF-c interneurons. Comparison with previous immunohistochemical studies using anti-SST-14 antibodies strongly suggests that SST-14-like neurons correspond with the SSTa populations. Thus, the SSTc populations were not reported previously in immunohistochemical studies. Cluster-based analyses and alignments of mature peptides suggested that SSTa is an ortholog of SST1 and that SSTb is closely related to SST2 and SST6. These results provide important new insights into the evolution of the somatostatinergic system in vertebrates.
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