Early development of cerebellar afferent systems that contain corticotropin‐releasing factor

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Abstract
Corticotropin‐releasing factor (CRF) and CRF binding sites have been described in the cerebellum of several species, including the North American opossum (Didelphis marsupialis virginiana), the species used in the present study. Iontophoretic application of this peptide in the adult cerebellum enhances the spontaneous and amino acid‐induced firing rate of Purkinje cells and overcomes the GABA‐induced suppression of Purkinje cell activity. The present account provides immunohistochemical evidence for the localization of CRF in the North American opossum within developing axons and their growth cones prior to the formation of the Purkinje cell and granule cell layers. CRF mRNA is present on postnatal day (PD) 1 within the internal migratory stream of the ventral lateral medulla, which contains migrating olivary neurons, and within the ventral medulla in the region, where inferior olivary neurons first aggregate to form the inferior olivary complex. The olivary complex can first be identified on PD2 and is well defined by PD3. CRF‐immunoreactive axons are evident within the cerebellar primordium on PD4 and penetrate the nascent Purkinje cell layer between PD14 and PD26. By PD26, CRF‐immunoreactive puncta are organized within the Purkinje cell layer as parasagittal bands. Thus, olivary neurons express CRF mRNA prior to the time that the first CRF‐labeled axons are present in the cerebellar anlage (PD4), suggesting that olivary axons are among the first to reach the developing cerebellum. Coincident (PD1–3) with the early transcription of CRF mRNA in the inferior olive, cells in the medullary reticular formation (PD1) and locus coeruleus (PD2) also transcribe CRF mRNA. These brainstem sites also could provide CRF‐immunoreactive axons to the developing cerebellum; however, based on the results of this study and correlative data reported in the literature, we propose that the primary source of early‐arriving CRF fibers is the inferior olivary complex. The early arrival of CRF‐containing axons in the cerebellum prior to synaptogenesis and migration of both granule cells and Purkinje cells suggests a role for this peptide in target recognition and synaptic organization.