Expression pattern of calcitonin gene-related peptide in the superior colliculus during postnatal development: demonstration of its intrinsic nature and possible roles.

Calcitonin gene-related peptide (CGRP) is a widespread neuropeptide with multiple central and peripheral targets. In an analysis on the expression of this peptide throughout the rat brain during postnatal development, we observed a discrepancy between results obtained by immunohistochemistry and by in situ hybridization. In the superior colliculus (SC), only the immunohistochemical signal could be detected (Terrado et al. [1997] Neuroscience 80:951-970). Here we focus our attention on this structure because the temporal pattern of CGRP immunoreactivity observed in the SC suggested the participation of this peptide in the postnatal maturation of the SC. In the present study, we describe in detail the postnatal development of collicular CGRP-immunoreactive structures and their spatiotemporal relationship with cholinergic modules and definitively demonstrate the local expression of CGRP in the SC. CGRP-immunopositive axons and neurons were distributed within the most ventral part of superficial strata and in the intermediate strata of the SC, showing a peak in staining intensity and density at the end of the first postnatal week. At P14, CGRPergic terminal fibers are arranged in small, clearly defined patches in a complementary manner with respect to the cholinergic modules, which start forming at this stage. By using Western blot and RT-PCR analyses, and by means of injections of antisense oligonucleotides, both the presence of CGRP peptide in the SC and the local expression of alpha-CGRP transcripts in collicular neurons were demonstrated. A possible role of CGRP is discussed in the context of postnatal modular compartmentalization of collicular afferents.