Phase differences in electrical discharge rhythms between neuronal populations of the left and right suprachiasmatic...

The circadian pacemaker of the suprachiasmatic nuclei is a complex multioscillator system, which controls circadian and seasonal rhythmicity. A number of clock genes have been identified that play a key role in the generation of circadian rhythms. These clock genes are expressed in a circadian manner as has been shown in mice, rats and hamsters. The time at which their expression reaches peak values differs among the several genes. Expression profiles for a specific gene may also differ among subdivisions of the suprachiasmatic nuclei. It has been shown that mPer1 peaks slightly out of phase in the left and right suprachiasmatic nuclei and that the rhythm in c-fos expression is significantly different between the dorsomedial and ventrolateral regions. In the special case that the animal shows splitting of its locomotor activity pattern, mPer1 in the left and right suprachiasmatic nuclei appeared to oscillate in antiphase. Whether the molecular organization within the suprachiasmatic nuclei plays a role in seasonal rhythmicity, allowing animals to track daylength and become reproductive at the proper phase of the annual cycle, receives increasing interest (). The differences in peak expression times that exist between different genes, and the spatial differences in peak time for single genes, are suggestive of a genetic mechanism underlying the multioscillator structure. It is unknown, however, whether phase differences that are observed at the molecular level exist at the level of electrical activity rhythms in the suprachiasmatic nuclei in order to become potentially functional. In this study we investigated the presence of phase differences in neuronal discharge rhythms in the suprachiasmatic nuclei of the rat. To this purpose we combined simultaneous electrophysiological recordings of neuronal populations in the left and right suprachiasmatic nuclei with a detailed analysis of the phase relationship between them. The results demonstrate that neuronal subpopulations of the suprachiasmatic nuclei show phase differences both in their peak and half-maximum times of up to 4 h. We propose that these phase differences may play a role in the plasticity of the circadian timing system.