A new surgical approach to the ram suprachiasmatic nuclei

In mammals, lesions of the suprachiasmatic nuclei (SCN) cause alterations in several biological rhythms. Investigation of the role of the ovine SCN requires complete bilateral lesions of the nuclei. However, the elongated shape, location and horizontal orientation of this hypothalamic structure prohibit a classical stereotaxic vertical approach. We present here a new trans-sinusal surgical technique to reach the SCN for lesion or other studies, which avoids damage to hypothalamic or other structures. The surgical procedure allows direct viewing of the SCN, located dorsal to the optic chiasm on the rostro-ventral surface of the brain. The orientation of the access tunnel is roughly aligned with the nearly horizontal main axis of the nuclei. An example of the use of such a surgical procedure is described: the bilateral destruction of the SCN. Following surgery, the extent of lesions was assessed by histology, showing complete destruction of the SCN. In order to demonstrate the physiological effects of the lesions, melatonin secretion profiles were followed over 48 h periods and compared before and after surgery. Bilateral SCN lesions induced alterations in the rhythm of melatonin secretion: levels were no longer correlated with light/dark cycles.