Phase shifts to light are altered by antagonists to neuropeptide receptors

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is a heterogeneous structure. Two key populations of cells that receive retinal input and are believed to participate in circadian responses to light are cells that contain vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP). VIP acts primarily through the VPAC2 receptor, while GRP works primarily through the BB2 receptor. Both VIP and GRP phase shift the circadian clock in a manner similar to light when applied to the SCN, both in vivo and in vitro, indicating that they are sufficient to elicit photic-like phase shifts. However, it is not known if they are necessary signals for light to elicit phase shifts. Here we test the hypothesis that GRP and VIP are necessary signaling components for the photic phase shifting of the hamster circadian clock by examining two antagonists for each of these neuropeptides. The BB2 antagonist PD176252 had no effect on light-induced delays on its own, while the BB2 antagonist RC-3095 had the unexpected effect of significantly potentiating both phase delays and advances. Neither of the VIP antagonists ([d-p-Cl-Phe6, Leu17]-VIP, or PG99-465) altered phase shifting responses to light on their own. When the BB2 antagonist PD176252 and the VPAC2 antagonist PG99-465 were delivered together to the SCN, phase delays were significantly attenuated. These results indicate that photic phase shifting requires participation of either VIP or GRP; phase shifts to light are only impaired when signalling in both pathways are inhibited. Additionally, the unexpected potentiation of light-induced phase shifts by RC-3095 should be investigated further for potential chronobiotic applications.

The serotonergic anxiolytic buspirone attenuates circadian responses to light

Serotonergic drugs modify circadian responses to light, with agonists attenuating and some partial agonists or antagonists potentiating photic phase shifts. The anxiolytic buspirone is a 5-HT1A receptor partial agonist. Given that buspirone is used therapeutically to manage generalised anxiety disorder, it would be useful to understand if and how this drug may modify circadian responses to light, not only to help manage side effects, but also to examine its potential use as a chronobiotic. Here we examined behavioral and molecular responses to phase-shifting light in mice and hamsters treated with buspirone. Phase advances to late subjective night light pulses in hamsters and wildtype mice were significantly attenuated by buspirone. 5-HT1A receptor knockout mice exhibited potentiated photic phase shifts when pretreated with buspirone. In wildtype mice, the attenuated phase shifts were accompanied by increased cFos expression in the suprachiasmatic nucleus, whereas potentiated phase shifts in knockouts were accompanied by increased phosphorylation of extracellular signal-regulated kinase (ERK) and cyclic AMP response element-binding protein (CREB), and decreased cFos expression. Attenuated photic phase shifts in buspirone-treated hamsters were accompanied by decreased phosphorylation of ERK and CREB. Chronic buspirone treatment decreased the amplitude of wheel-running rhythms, lengthened the duration of the active phase and advanced the phase angle of entrainment. Buspirone administration at midday produced non-photic phase advances in wildtype but not 5-HT1A receptor knockout mice. These findings suggest that buspirone affected the circadian system in a manner similar to the 5-HT1A/7 agonist (±)-8-Hydroxy-2-dipropylaminotetralin hydrobromide, primarily through the 5-HT1A receptor, and suggest that therapeutic use of buspirone to manage anxiety may impact circadian function.