Spatiotemporal heterogeneity in the electrical activity of suprachiasmatic nuclei neurons and their response to photoperiod.

The coordinated activity of thousands of cellular oscillators in the suprachiasmatic nuclei (SCN) temporally regulates mammalian physiology to anticipate daily environmental changes across the seasons. The phasing of clock gene expression varies according to anatomical location in the SCN and is thought to encode photoperiodic information. However, it is unclear whether similar variations in phase occur in the electrical activity of SCN neurons, a measure of both intraSCN signaling and clock output. To address this, we recorded single-unit and multiunit activity (SUA/MUA) from dorsal and ventral subregions of the middle level of the rostrocaudal axis of the SCN in coronal brain slices prepared from mice housed under different photoperiods. We demonstrate that under a symmetrical (12 h light:12 h dark) photoperiod, cells in the dorsal SCN are less tightly synchronized than those in the ventral SCN. Comparison of recordings made from mice under short (8 h light:16 h dark) or long (16 h light:8 h dark) photoperiods shows that the phase distribution of ventral, but not dorsal, SCN neurons expands with increasing day length. Conversely, the duration that individual neurons are active increases in dorsal, but not ventral, SCN under long days. These data indicate that in the ventral SCN photoperiod is encoded at the network level, while this coding occurs at the level of individual cells in the dorsal SCN.