Choline acetyltransferase expression during a putative developmental waiting period.

The relationship between the cholinergic expression, morphological development, and target cell innervation of olivocochlear (OC) efferent neurons was investigated in the postnatal hamster. Similar to what was found in previous studies, tracer injections into the contralateral cochlea labeled cells bodies retrogradely in periolivary regions and labeled cell bodies only rarely in the lateral superior olive (LSO). Few morphological differences were found among cell bodies labeled between postnatal day 1 (P1) and P30. Tracer injections into the crossed OC bundles within the brainstem anterogradely labeled terminals below the inner hair cells of the cochlea prior to P5 and labeled terminals below outer hair cells after P5, consistent with a period of transient innervation, as hypothesized previously. Within the superior olive, choline acetyltransferase (ChAT) was expressed differentially. In periolivary regions, ChAT was expressed as early as P0. ChAT-immunoreactive cell bodies in periolivary regions were similar morphologically to retrogradely labeled OC neurons. In contrast, within the LSO, ChAT was not expressed until after P2. Consistent with a medical OC projection to the cochlea at early postnatal ages, ChAT immunoreactivity was detected below inner hair cells as early as P2 but was not detected below outer hair cells until after P6. Our results suggest that medial OC neurons not only provide transient connections to inner hair cells but also may express ChAT when they are below inner hair cells. Furthermore, these results raise the possibility that OC neurons may be capable of acetylcholine synthesis and release prior to or simultaneous with their innervation of the cochlea.

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.

Postnatal calcitonin gene-related peptide in the superior olivary complex.

The purpose of this study was to investigate quantitatively the distribution of calcitonin gene-related peptide (CGRP)-positive neurons within the superior olivary complex (SOC) at various postnatal ages. In the lateral superior olive (LSO), most if not all, CGRP-positive cells correspond to the cholinergic portion of the lateral olivocochlear system which innervates the cochlea. Brains from 1-day old (P1) through juvenile (P30) hamsters were used. At all ages, CGRP-positive neurons were seen throughout the various nuclei of the SOC. There was a dramatic shift in the distribution of CGRP-positive neurons from being predominantly periolivary to being predominantly confined within the LSO. The number of CGRP-positive neurons clearly increased as a function of increasing age. At the earliest postnatal ages, the LSO contained few if any immunostained cells, whereas at later ages the LSO contained the majority (greater than 70%) of the immunostained cells. Assuming that these CGRP-positive cells within the LSO correspond to olivocochlear neurons, these data suggest that in hamsters the lateral olivocochlear system may be immature at birth up until the second postnatal week.