PACAP and glutamate are co-stored in the retinohypothalamic tract.

The retinohypothalamic tract (RHT) relays photic information from the eyes to the suprachiasmatic nucleus (SCN). Activation of this pathway plays a role in adjusting circadian timing to the light/dark environment. Two transmitters, glutamate and pituitary adenylate cyclase activating polypeptide (PACAP) having phase shifting capacity during the night and day, respectively, are located in the RHT. Using double staining immunohistochemistry at the light and electron microscopic level, we showed that PACAP was co-stored with glutamate in a subset of retinal ganglion cells and in nerve terminals in the retino-recipient area of the SCN. These findings provide an anatomical basis for the recent demonstration of the interaction between these two transmitters on the SCN phase response at night.