Localization of somatostatin mRNAs in the brain and pancreas of rainbow trout (Oncorhynchus mykiss)

Rainbow trout possess three distinct mRNAs, each encoding a separate precursor: PPSS I, which contains a 14-amino acid sequence at its C-terminus (somatostatin-14) that is highly conserved among vertebrates, as well as two others, PPSS II′ and PPSS II″, both containing [Tyr⁷, Gly¹⁰]-somatostatin-14 at their C-terminus. In this study, we used RNA template-specific PCR and in situ hybridization to determine the distribution and cellular localization of PPSS mRNAs in the brain and Brockmann body of rainbow trout. PPSS I, PPSS II′ and PPSS II″ were expressed in the Brockmann body and pituitary; the expression of PPSS mRNAs in the brain was region specific. PPSS I mRNA was expressed in the Brockmann body predominantly by cells other than those that expressed PPSS IIs; however, there were several instances where PPSS I and PPSS IIs were co-expressed within the same cell. Of the PPSS II-expressing cells, many were observed to express both PPSS II′ and PPSS II″ mRNA; however, some cells expressed only PPSS II′ mRNA, while other cells expressed only PPSS II″ mRNA. In the brain, PPSS I mRNA was expressed in the optic tectum (OT) and in many hypothalamic nuclei, including the nucleus rotundus (NR), nucleus anterioris hypothalami (NAH), nucleus anterior tuberis (NAT), nucleus lateral tuberis (NLT), as well as in the pituitary (adenohypophysis). PPSS II″ mRNA was present in the same regions as PPSS I mRNA; however, PPSS II′ mRNA was present primarily in OT, NAT, NLT and adenohypohysis. These results indicate that PPSS mRNAs are expressed differently by different cells, suggesting that cell-specific mechanisms are involved with the control of PPSS expression and that particular biological responses may be associated with a specific SS isoform.