NEUROSYSTEMS The role of the neuropeptides PACAP and VIP in the photic regulation of gene expression in the suprachiasmatic nucleus Joanna M. Dragich,* Dawn H. Loh, Louisa M. Wang, Andrew M. Vosko, Takashi Kudo, Takahiro J. Nakamura, Irene H. Odom, Sei Tateyama, Arkady Hagopian, James A. Waschek and Christopher S. Colwell Department of Psychiatry and Biobehavioral Sciences, University of California – Los Angeles, 760 Westwood Plaza, Los Angeles, CA 90024 1759, USA Keywords: circadian rhythms, Fos, Period1, pituitary adenylate cyclase-activating polypeptide, SCN, vasoactive intestinal peptide Abstract Previously, we have shown that mice deficient in either vasoactive intestinal peptide (VIP) or pituitary adenylate cyclase-activating polypeptide (PACAP) exhibit specific deficits in the behavioral response of their circadian system to light. In this study, we investigated how the photic regulation of the molecular clock within the suprachiasmatic nucleus (SCN) is altered by the loss of these closely-related peptides. During the subjective night, the magnitude of the light-induction of FOS and phosphorylated mitogen- activated protein kinase (p-MAPK) immunoreactive cells within the SCN was significantly reduced in both VIP- and PACAP-deficient mice when compared with wild-type mice. The photic induction of the clock gene Period1 (Per1) in the SCN was reduced in the VIP- but not in the PACAP-deficient mice. Baselines levels of FOS, p-MAPK or Per1 in the night were not altered by the loss of these peptides. In contrast, during the subjective day, light exposure increased the levels of FOS, p-MAPK and Per1 in the SCN of VIP- deficient mice, but not in the other genotypes. During this phase, baseline levels of these markers were reduced in the VIP-deficient mice compared with untreated controls. Finally, the loss of either neuropeptide reduced the magnitude of the light-evoked increase in Per1 levels in the adrenals in the subjective night without any change in baseline levels. In summary, our results indicate that both VIP and PACAP regulate the responsiveness of cells within the SCN to the effects of light. Furthermore, VIP, but not PACAP, is required for the appropriate temporal gating of light-induced gene expression within the SCN. Introduction In mammals, the part of the nervous system responsible for most circadian behavior can be localized to a pair of structures in the hypothalamus known as the suprachiasmatic nucleus (SCN). Neurons within the SCN generate robust, synchronized rhythms in the transcription, translation and degradation of key ‘clock genes’ in an autoregulatory loop that has an endogenous periodicity of approxi- mately 24 h (Reppert & Weaver, 2001; Ko & Takahashi, 2006). Functionally, the molecular clockwork must be synchronized to the external environment. The dominant environmental cue responsible for this synchronization or entrainment is the daily cycle of light and dark. The question of how photic cues from the environment regulate this molecular feedback loop in the SCN is one of the critical issues in the field of circadian rhythms. The SCN receives photic information directly through a monosyn- aptic projection from the retina known as the retinohypothalamic tract (RHT). The neuropeptide pituitary adenylate cyclase-activating poly- peptide (PACAP) and vasoactive intestinal peptide (VIP) are two closely related members of the secretin family (reviewed in Vaudry et al., 2000) that are both expressed in this sensory circuit. PACAP has emerged as a likely transmitter at the RHT ⁄ SCN synaptic connection (Hannibal et al., 2000, 2002). Many of the retinal recipient neurons within the ventral SCN express the neuropeptide VIP. Thus, the VIP- expressing cells in the ventral SCN directly receive photic information from the RHT. These retino-recipient cells must then convey this environmental information to the rest of the SCN. Receptors sensitive to these peptides (PAC 1 R and VPAC 2 R) are highly expressed in the SCN (Sheward et al., 1995; Cagampang et al., 1998a,b; Kalamatianos et al., 2004; Kallo et al., 2004). Furthermore, the circadian system of mice deficient in PAC 1 R (Hannibal et al., 2001, 2008), VPAC 2 R (Harmar et al., 2002), PACAP (Kawaguchi et al., 2003; Beaule ´ et al., 2009) and VIP each exhibited altered behavioral responses to light. In our own work, we have found that PACAP- and VIP-deficient mice exhibit a loss in the magnitude of light-induced phase advances and delays (Colwell et al., 2003, 2004). In order to better understand the role of these peptides in entrainment, we investigated the impact of the loss of VIP and PACAP on light-induced cellular events within the SCN and its peripheral target, the adrenal gland. We tested the Correspondence: Dr C. S. Colwell, as above. E-mail: ccolwell@mednet.ucla.edu *Present address: Department of Neurology, Columbia University, 650 West 168th Street, New York City, NY 10032, USA Received 2 June 2009, revised 18 November 2009, accepted 23 December 2009 European Journal of Neuroscience, Vol. 31, pp. 864–875, 2010 doi:10.1111/j.1460-9568.2010.07119.x ª The Authors (2010). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience