Phase shifts to light are altered by antagonists to neuropeptide receptors

Abstract

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 dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

“…VIP and GRP) we know very little about the functional consequences of their corelease. Only a few studies have examined the circadian effects of the interaction of multiple signals in the SCN (Albers et al, 1991; Peters et al, 1994; Chan et al, 2016). Similarly, GABA is likely released both synaptically and non-synaptically in the SCN.…”

Circuit development in the master clock network of mammals

“…Like VIP neurons, GRP neurons in the mature SCN are retino‐recipient (Drouyer et al., ; Fernandez, Chang, Hattar & Chen, ; Karatsoreos, Yan, LeSauter & Silver, ; Lokshin, LeSauter & Silver, ; Tanaka et al, ) and display light‐induced increases in firing rate, immediate early gene expression, and neuropeptide release (Earnest, DiGiorgio & Olschowka, ; Francl et al., ; Gamble, Kudo, Colwell & McMahon, ; Guillaumond et al., ; Lesauter, Silver, Cloues & Witkovsky, ; Romijn, Sluiter, Pool, Wortel & Buijs, ). In addition, potential interactions between GRP and VIP are illustrated by their co‐expression in a subset of SCN neurons (Okamura et al, ; Romijn, Sluiter, Wortel, Van Uum & Buijs, ) and synergistic responses to GRP and VIP co‐administration (Albers Liou, Stopa, & Zoeller, 1991; Albers et al, ; Chan et al, ). Furthermore, BB2 signalling can synchronize SCN neurons in the absence of VIP signalling (Brown et al, ; Maywood et al., ).…”

Functional Peptidomics: Stimulus- and Time-of-Day-Specific Peptide Release in the Mammalian Circadian Clock

“…Although a number of known, physiologically relevant neuropeptides were released in response to optic nerve stimulation, two anticipated neuropeptides, VIP and GRP, were not detected. VIP and GRP are established intrinsic SCN peptides with roles processing photic signals from the eye to the SCN clock 30, 52 . Several considerations may have contributed to this result.…”

“…A required role for VIP has been proposed in timing-specific, light-induced gene expression 55 . Recent studies suggest that GRP and VIP may play redundant and intermediate roles in light-induced phase shifting 30 . Thus, neither may contribute in the initial, more acute physiological response to a phase-shifting light stimulus.…”

“…Gastrin-releasing peptide (GRP), vasoactive intestinal polypeptide (VIP), and little SAAS are expressed within retinorecipient SCN neurons and are involved in SCN feed-forward relay of phase shifting signals 29–31 . The roles of these peptides in intercellular SCN communication have been investigated using a combination of both genetic knockout and exogenous in vitro chemical stimulation, 27,30–34 however confirmatory peptide release from the SCN downstream of light signal innervation has not been demonstrated.…”