Localization of a suprachiasmatic nucleus subregion regulating locomotor rhythmicity

The bilaterally symmetrical suprachiasmatic nuclei (SCN) of the hypothalamu s are the loci of the mammalian clock controlling circadian rhythms. Previo us studies suggested that all regions of the SCN are equipotential as circa dian rhythmicity is sustained after partial ablation, as long as similar to 25% of the nuclei are spared. In contrast to these results, we found that animals bearing partial lesions of the SCN that spared the subregion delimi ted by cells containing the calcium-binding protein calbindin-D-28K (CaBP), sustained circadian locomotor rhythms. Furthermore, there was a correlatio n between the strength of the rhythm and the number of spared CaBP cells. P artial lesions that destroyed this region but spared other compartments of the SCN resulted in loss of rhythmicity. The next study indicates that tran splants of half-SCN grafts that contain CaBP cells restore locomotor rhythm s in SCN-lesioned host animals, whereas transplants containing SCN tissue b ut lacking cells of this subnucleus fail to restore rhythmicity, Finally, t here was a correlation between the number of CaBP-positive cells in the gra ft and the strength of the restored rhythm. Taken together, the results ind icate that pacemakers in the region of the CaBP subnucleus are necessary an d sufficient for the control of locomotor rhythmicity and that the SCN is f unctionally heterogeneous.