Serotonergic enhancement of circadian responses to light: Role of the raphe and intergeniculate leaflet

Light serves as the primary stimulus that synchronizes the circadian clock in the suprachiasmatic nucleus (SCN) to the external day-night cycle. Appropriately timed light exposure can reset the phase of the circadian clock. Some serotonergic drugs that bind to the 5-HT1A receptor can enhance phase shifts to light. The mechanism by which this potentiation occurs is not well understood. In this study we examine where in the hamster brain one of these drugs, BMY7378, might be working. Systemic (5 mg/kg), intradorsal raphe and intra-median raphe (both 15.6 nmol in 0.5 μl), but not intra-SCN (7.8 nmol or 15.6 nmol in 0.5 μl) injections of BMY7378 significantly potentiated phase shifts to light. Potentiation of photic shifts persisted when serotonergic innervation of the SCN was lesioned with infusions of the serotonin neurotoxin 5,7-dihydroxytryptamine into the SCN. Light-induced c-Fos expression in the rostral and caudal intergeniculate leaflet (IGL) was attenuated with systemic BMY7378, suggesting that the IGL may be involved in this response. Both complete IGL lesions and depletion of serotonergic innervation of the IGL prevented systemic BMY7378 from potentiating photic phase shifts. Together these findings suggest that the mechanism by which BMY7378 enhances photic responses is by changing the activity of the raphe nuclei to influence how the IGL responds to light, which subsequently influences the SCN as one of its downstream targets. Identification of the network that underlies this potentiation could lead to the development of useful therapeutic interventions for treating sleep and circadian disorders