Developmentally regulated expression of calcitonin gene-related peptide in the superior olive

During postnatal development, the expression of calcitonin gene-related peptide (CGRP) and its mRNA was investigated in the superior olivary complex of the hamster in order to better understand its role in the development of the efferent olivocochlear (OC) pathway. Although both the peptide and its mRNA were expressed at birth in a few periolivary cells, neither CGRP mRNA nor any immunoreactivity could be detected in the lateral superior olive until after postnatal day (P) 5. By P9, CGRP expression had significantly increased and was mostly contained within the lateral superior olive. Between P7 and P18, there appears to be a transient increase in the transcript expression both in periolivary regions and in the lateral superior olive. Notably, both peptide and mRNA expression decreased precipitously throughout the superior olive after P18. In comparison, although both the facial and trigeminal motor nuclei had significant CGRP expression at birth, the facial motor nucleus demonstrated a decrease in the level of CGRP expression between P1 and P6, while the trigeminal motor nucleus reached a maximal level of expression around P18. If CGRP expression is related to synaptogenesis in OC neurons, as has been suggested for certain motor neurons, then we predict that the ephemeral increases in transcript expression in OC neurons are related to synaptogenetic mechanisms in the cochlear periphery. Importantly, the time course for CGRP expression in lateral OC neurons indicates that their OC terminals in the cochlear periphery may not begin forming synapses until near the end of the 1st postnatal week.