The suprachiasmatic nucleus (SCN) receives information about ambient light levels through the retinohypothalamic tract. This information is used to reset the molecular clock of individual SCN neurons, leading to entrainment of overt animal behavior and physiology. Recently, a new class of intrinsically photosensitive retinal ganglion cells (ipRGC's) utilizing melanopsin as their photopigment was discovered and found to provide the majority of retinal input to the SCN. However, many questions remain about how these melanopsin ganglion cells communicate with individual SCN neurons. One complicating factor is that classic rod and cone pathways also provide input to the SCN, relayed by melansopin ganglion cells and conventional ganglion cell types. Due to difficulties in recording in vivo light-evoked responses in the SCN, little is known about the organization and function of these various input pathways and how they lead to resetting of the biological clock. To circumvent this issue, we have developed a novel in vitro SCN brain- slice preparation that maintains functional connectivity to both retinas, enabling patch-clamp recordings from visually identified SCN neurons that are capable of responding to light. This allows us to conduct experiments concerning light processing in the SCN with unprecedented control and knowledge of the cells we record from. One of the main advantages is our ability to target cells for recording in the dorsal shell or ventrolateral core of the SCN, two sub-regions thought to play different roles in processing retinal input. Utilizing this novel slice preparation, the goal of the proposed study is to delineate the functional organization of rod/cone and melansopin based input to the SCN using patch-clamp recording techniques in combination with pharmacology, and to understand how these various inputs shape light responses from individual SCN cells. In addition, the function of endogenous PACAP, a neuropeptide found in melanopsin ganglion cells, will be characterized during processing of visual information in the SCN. Because of the similarities between rat and human circadian systems, studying how the SCN processes retinal input in our novel slice preparation will provide valuable information about the cellular basis of circadian rhythms, photic entrainment, and sleep regulation in humans. This will hopefully lead to standards of work conditions and schedules for millions of shift workers where disruption of circadian rhythms and sleep deprivation is a major occupational hazard, and in the case of hospitals, a threat to public health. [unreadable] [unreadable] [unreadable]