Photic and non-photic interactions in the functioning of the circadian clock
The biological clock relies on the integration of both photic and non-photic information in order to synchronize and entrain to the environment. In this thesis I examine the interaction between light and NPY, a known mediator of many non-photic stimuli. Previous studies have shown that both photic and non-photic stimuli interact in vivo and in vitro. However, the precise neural pathways and cellular mechanisms utilized in such a complex interaction are as yet unknown. In my first set of experiments I have shown that microinjections of NPY directly onto the suprachiasmatic nucleus (SCN) in vivo, block the phase advancing effects of light and attenuate phase delays. Furthermore, I found that a specific NPY Y1/Y5 receptor agonist was able to inhibit photic phase shifts in a similar manner to that observed with NPY. In the following series of studies I utilized a newly developed NPY Y5 receptor antagonist. It was found that injection of the Y5 receptor antagonist during light exposure prevented NPY from inhibiting light induced phase shifts during the night. Also, the administration of a specific Y1 receptor antagonist had no effect on the ability of NPY to influence the resetting of light during the early and late night. It was previously shown that phase shifts to NPY were mediated via GABAergic interneurons. However, I found that NPY did not inhibit photic phase shifts via a similar GABA dependent mechanism, as the presence of bicuculline, a GABAA antagonist did not prevent NPY from altering phase shifts to light during the night. Finally, to determine the possible site of interaction between NPY and light a time course study was designed. It was found that microinjections of NPY up to 60 minutes post light exposure were able to attenuate photic phase advances. This was also the case when animals were presented with a novel running wheel 60 minutes after an advancing light pulse. Overall my work has shown that NPY interacts with light during the early and late night. Furthermore, NPY utilizes the Y5 receptor subtype and a neural pathway independent of GABAA activation. Finally the present data indicates that the possible site of interaction between NPY and light lies downstream from receptor binding sites within SCN cells.