Figure 1. At birth, CB1 is prominently expressed in axon tracts in hindbrain. A–A”, Immunolocalization of CB1 in sagittal hindbrain sections spanning cerebellar vermis and paravermis at P0 shows robust expression in developing axons, including long-range axon tracts in the brainstem and in the fibers cruising through the inner regions of the developing cerebellar cortex. Immediately relevant for the developing pontocerebellar system, CB1 immunolocalization is seen in mcp and scp [mcp (A”) and scp (A’), highlighted by green arrowheads], and in axons filling pWM (A–A”, green arrowheads). CB1 expression in icp (A”, green dotted outline) is very weak or absent. B–B”, Sagittal sections showing localization of CB1 mRNA in the hindbrain at E18 from the Allen Brain Atlas Developmental Mouse database (2008 Allen Institute for Brain Science. Allen Developing Mouse Brain Atlas, available from http://developingmouse.brain-map.org/experiment/show/100024806). Notably for pontocerebellar development, CB1 mRNA expression is prominent both in the Pn (B, maroon arrowhead) and the IO (B, maroon arrowhead), among many other brainstem nuclei which project axons to the cerebellum (marked by maroon triangles and anatomic abbreviations in summary diagrams in panels C–C”). In the cerebellum, CB1 mRNA is most prominent in deeper layers (B, maroon arrowhead). CB1 mRNA positive region spans developing IGL and pWM, marked in summary diagram in C. CB1 mRNA expression is also prominent in vestibulocerebellar nucleus (VeCb; B”, maroon arrowhead). In addition, restricted regions of the iEGL (B–B”, maroon arrow) show thin layer of CB1-positive cells. C–C”, Traced outlines of sections shown in A, summarizing perinatal distribution of CB1 expression relative to anatomic landmarks. Green shapes mark CB1 immunolocalization in axon tracts as shown in A, red triangles mark brain regions with robust CB1 mRNA expression as shown in B. Lines in coronal insets represent mediolateral plane of sectioning. Anatomical locations are adapted from Franklin and Paxinos (2008) and from Paxinos et al. (2007). bsc, brachium of superior colliculus; CAT, nucleus of the central acoustic tract; CB, cerebellum; CnF, cuneiform nucleus; cp, cerebral peduncle; csc, commissure of superior colliculus; Cu, cuneate nucleus; DC, dorsal cochlear nucleus; DTg, dorsal tegmental nucleus; fr, fasciculus retroflexus; Gi, gigantocellular reticular nucleus; GiV, gigantocellular reticular nucleus, ventral part; g, gracile fasciculus; Gr, gracile nucleus; icp, inferior cerebellar peduncle; IC, inferior colliculus; IO, inferior olivary nucleus; IP, interpeduncular nucleus; IRt, intermediate reticular nucleus; ll, lateral lemniscus; LRt, lateral reticular nucleus; mcp, middle cerebellar peduncle; mcp, middle cerebellar peduncle; MiTg, microcellular tegmental nucleus; mlf, medial longitudinal fasciculus; MPB, medial parabrachial nucleus; MVe, medial vestibular nucleus, magnocellular part; PDTg, posterodorsal tegmental nucleus; PMn, paramedian reticular nucleus; Pn, pontine nucleus; PnC, pontine reticular nucleus, caudal part; PrCnF, precuneiform area; py, pyramidal tract; RLi, rostral linear nucleus (midbrain); RR, retrobulbar nucleus; Rt, reticular nucleus; RtTg, reticulotegmental nucleus of the pons; SC, superior colliculus; scp, superior cerebellar peduncle; Sol, solitary nucleus; sp5, spinal trigeminal tract; Tz, nucleus of the trapezoid body; tz, trapezoid body; VeCb, vestibulocerebellar nucleus; vsc, ventral spinocerebellar tract; VTg, ventral tegmental nucleus; X, nucleus X, dorsal nucleus of the vagus nerve.