Changes in L1 and NCAM expression in the rat suprachiasmatic nucleus during growth and after orbital enucleation

Mammals possess a master circadian clock in the hypothalamic suprachiasmatic nucleus (SCN). In order to clarify the roles of the L1 adhesion molecule (L1) and neural cell adhesion molecule (NCAM), both members of the immunoglobulin superfamily, in the organization of the clock core, changes in the expression of these molecules in the SCN during the growth of rats were examined by immunohistochemistry. On postnatal day 7, L1 and NCAM were chiefly expressed in the region surrounding the SCN, but not in the SCN itself. In subsequent weeks, however, expression of both molecules shifted predominately to the SCN. This change seemed to parallel immunoreactivity increases in the SCN of synaptotagmin, a synapse marker, and of phosphotyrosine, a possible factor in the photic entrainment of the SCN clock. To further elucidate the roles of the L1 and NCAM adhesion molecules in the formation and maintenance of retinal neural projection into the SCN, the effects of orbital enucleation on their expression in the SCN were examined. L1 expression decreased on days 1 and 2 after the operation, in parallel with reductions in the tyrosine phosphorylation of several proteins, but recovered to the control level by the second week. In contrast, the expression of NCAM showed little change following orbital enucleation. These results suggest that L1 and NCAM are involved in the morphological organization of the SCN during the developmental stage, and that expression of L1 also contributes to the formation of the SCN network in a manner that is dependent on the retinal neural input to it.