Pituitary Adenylate Cyclase-Activating Peptide (PACAP) in the Retinohypothalamic Tract: A Potential Daytime Regulator of the Biological Clock

Abstract

The retinohypothalamic tract (RHT) relays photic information from the eyes to the suprachiasmatic nucleus (SCN). Activation of this pathway by light plays a role in adjusting circadian timing via a glutamatergic pathway at night. Here we report a new signaling pathway by which the RHT may regulate circadian timing in the daytime as well. We used dual immunocytochemistry for pituitary adenylate cyclase-activating peptide (PACAP) and the in vivo tracer cholera toxin subunit B and observed intense PACAP-immunoreactivity (PACAP-IR) in retinal afferents in the rat SCN as well as in the intergeniculate leaflet (IGL) of the thalamus. This PACAP-IR in the SCN as well as in the IGL was nearly lost after bilateral eye enucleation. PACAP afferents originated from small ganglion cells distributed throughout the retina. The phase of circadian rhythm measured as SCN neuronal activity in vitro was significantly advanced (3.5 +/- 0.4 hr) by application of 1 x 10(-6) M PACAP-38 during the subjective day [circadian time (CT)-6] but not at night (CT14 and CT19). The phase-shifting effect is channeled to the clock via a PACAP-R1 receptor, because mRNA from this receptor was demonstrated in the ventral SCN by in situ hybridization. Furthermore, vasoactive intestinal peptide was nearly 1000-fold less potent in stimulating a phase advance at CT6. The signaling mechanism was through a cAMP-dependent pathway, which could be blocked by a specific cAMP antagonist, Rp-cAMPS. Thus, in addition to its role in nocturnal regulation by glutamatergic neurotransmission, the RHT may adjust the biological clock by a PACAP/cAMP-dependent mechanism during the daytime.

Mass spectrometry-based discovery of circadian peptides

“…These releasate profiles may not be complete, because neuropeptides mediating photic input or intra-SCN synchronization, such as pituitary adenylate cyclase-activating peptide (27,28), vasoactive intestinal polypeptide (29,30), and gastrin-releasing peptide (31, 32), are not observed or, in the case of gastrinreleasing peptide, are not present in amounts sufficient to produce peak intensities that exceed our significance criterion. Release of these peptides may be dependent on specific stimulation parameters such as frequency, intensity, or time of day.…”

Mice lacking the PACAP type I receptor have impaired photic entrainment and negative masking

“…We have previously shown that the neuropeptide pituitary adenylate cyclase activating peptide (PACAP) is costored with glutamate in the RHT (13) and that the two neurotransmitters are mediators of photic adjustment of the circadian system in a rather complex interplay (9). After the demonstration of PACAP in the RHT, (12) a number of studies have delineated the role of PACAP in light entrainment, which primarily seems to be exerted via the PACAPspecific PACAP receptor type 1 (PAC1) receptor (3,4,11,14,17,23). In vitro, PACAP has been shown to modulate glutamatergic signaling in a time-and concentration-dependent manner targeting the core clock genes Per1 and Per2 via cyclic AMP/PKA and the inositotriphospate/phospholipase C signaling pathway (reviewed in Ref.…”

Sildenafil accelerates reentrainment of circadian rhythms after advancing light schedules

“…The cAMP/PKA pathway has traditionally been involved in nonphotic resetting in the daytime domain (20,21). Application of cAMP analogs in vitro induces prominent phase advances in the subjective day but not in the subjective night (22).…”