A possible homologue of the suprachiasmatic nucleus in the hypothalamus of lampreys (Lampetra fluviatilis L.).

This study in lampreys (Lampetra fluviatilis L.) is concerned with the phylogenetic age of the suprachiasmatic nucleus of the hypothalamus which in mammals receives input from the retinohypothalamic tract and acts as circadian pacemaker. Tracing experiments with the fluorescent dye DiI demonstrated a retinohypothalamic projection in lampreys which terminates in a distinct area of the hypothalamus corresponding to the infrachiasmatic part of the nucleus of the postoptic commissure. Immunocytochemical investigations showed vasotocinergic neurons and a relatively high concentration of neuropeptide Y-, substance P- and serotonin-immunoreactive nerve fibers in this region. These hodological and immunocytochemical data suggest that the infrachiasmatic part of the nucleus of the postoptic commissure of lampreys might be homologous to the suprachiasmatic nucleus of other craniates.