PRESENCE OF SOMATOSTATIN SST(2) RECEPTORS IN THE DEVELOPING RAT AUDITORY-SYSTEM

A transient expression of somatostatin mRNA as well as of the peptide itself has been described in the developing mammalian auditory brainst em. However, little is known about the presence, and the spatial and t emporal pattern of somatostatin (SRIF) receptor subtypes in this syste m. Therefore, we investigated the distribution of SRIF receptor bindin g sites labeled with the radioligands [I-125]LTT-SRIF-28, [I-125]Tyr(3 )-octreotide, and [I-125]CGP 23996 (in buffers containing either Mg2or Na+ ions) within the developing auditory brainstem of the rat. In a ddition, we performed in situ hybridization with a P-33-labeled oligop robe, specific for somatostatin sst, receptor mRNA. We observed a tran sient expression of SRIF receptors, labeled with [I-125]LTT-SRIF-28, [ I-125]Tyr(3)-octreotide, and [I-125]CGP 23996 (only in the presence of Mg2+ ions), in all principal auditory nuclei during neonatal developm ent. In the adult rats, however, only the inferior colliculus displaye d significant SRIF receptor binding. A very similar spatiotemporal lab eling pattern was found for sst(2) receptor mRNA. Our in situ hybridiz ation data, together with those on ligand binding, suggest a predomina ntly transient expression of sst(2) receptors in the auditory system. Since sst(2) sites (and possibly sst(3) and sst(5)) as well as SRIF it self appear to be co-expressed during a period when synapse maturation occurs, we suggest that sst(2) receptors are involved in this process of the developing auditory system.