Roles of light and serotonin in the regulation of gastrin‐releasing peptide and arginine vasopressin output in the hamster SCN circadian clock

Abstract

Daily timing of the mammalian circadian clock of the suprachiasmatic nucleus (SCN) is regulated by photic input from the retina via the retinohypothalamic tract. This signaling is mediated by glutamate which activates SCN retinorecipient units communicating to pacemaker cells in part through the release of gastrin-releasing peptide (GRP). Efferent signaling from the SCN involves another SCN-containing peptide, arginine vasopressin (AVP). It is notable that little is known concerning the mechanisms regulating these peptides, as literature on in vivo peptide release in the SCN is sparse. Here, microdialysis-radioimmunoassay procedures were used to characterize mechanisms controlling GRP and AVP release in the hamster SCN. In animals housed under a 14hr:10hr 24hr LD cycle both peptides exhibited daily fluctuations of release, with levels increasing during the morning to peak around midday. Under constant darkness, this pattern persisted for AVP, but rhythmicity was altered for GRP, characterized by a broad plateau throughout the subjective night and early subjective day. Neuronal release of the peptides was confirmed by their suppression with reverse-microdialysis perfusion of calcium blockers and stimulation with depolarizing agents. Reverse-microdialysis perfusion with the 5-HT 1A,7 agonist, 8-OH-DPAT, during the day significantly suppressed GRP but had little effect on AVP. Also, perfusion with the glutamate agonist NMDA, or exposure to light at night, increased GRP but did not affect AVP. These analyses reveal distinct daily rhythms of SCN peptidergic activity, with GRP but not AVP release attenuated by serotonergic activation that inhibits photic phase-resetting, and activated by glutamatergic and photic stimulation that mediate this phase-resetting. Keywordssuprachiasmatic; microdialysis; neuropeptide; serotonin; glutamate; photic The master circadian clock in mammals is located within the suprachiasmatic nucleus (SCN) of the anterior hypothalamus (Klein et al., 1991;Moore, 1983;Rusak and Zucker 1979). Clock timing is synchronized to the 24 hr light-dark cycle directly by photic signaling relayed from the retina to the SCN via the retinohypothalamic tract (RHT; Johnson et al., 1988;Moore and Lenn 1972;Pickard, 1982), and indirectly from the intergeniculate leaflet via the geniculohypothalamic tract (GHT; Card and Moore, 1982;Johnson et al., 1989). The SCN also receives nonphotic entraining input from the GHT and the midbrain raphe nuclei (Albers and Ferris, 1984;Biello et al., 1994;Marchant et al., 1997;Meyer-Bernstein and Morin, 1996). The mechanism by which this diverse input is processed by the SCN to produce a stably entrained rhythm involves interplay among these systems, including three populations of peptidergic neurons indigenous to the SCN. These include cells producing The SCN photic signaling cascade is initiated by glutamate release from RHT terminals located primarily in the ventral SCN core region. This is confirmed by observations that glutamate in the SCN induces time-dependent phase-s...

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

“…In addition, administration of VIP within the slice significantly increases GABA A -induced IPSCs, an effect that can be blocked by a VPAC 2 receptor antagonist. Consistent with the hypothesis that synaptically released VIP induces the rhythm in GABA A -induced IPSCs is the finding that VIP is synaptically released in the SCN during the middle of the light period in hamsters entrained to a LD cycle (Francl et al, 2010a, 2010b). Further support for this hypothesis comes from the finding that VIP neurons in the SCN have a higher repetitive firing rate during the second half of the light period than at night in slices prepared from mice previously entrained to a LD cycle (Hermanstyne et al, 2016).…”

“…Light reduces VIP mRNA and protein and increases GRP mRNA and protein in rodents housed in LD cycles (Takahashi et al, 1989; Albers et al, 1990; Zoeller et al, 1992; Okamoto et al, 1991; Duncan et al, 1995), while in constant lighting conditions no rhythms in the levels of VIP or GRP mRNA and protein are observed (Takahashi et al, 1989; Zoeller et al, 1992; Shinohara et al, 1993). Lighting conditions also influence SCN release of VIP and GRP (Francl et al, 2010a, 2010b). Neurons in the ventral SCN exhibit increased levels of Per1 and Per2 mRNA and protein in response to phase shifting pulses of light.…”

“…Induction of high levels of GABA A -mediated currents requires VIP/PHI because they are absent in VIP/PHI-deficient mice. It will be important to determine whether the persistent rhythmicity in GABA A mediated currents seen in constant darkness represents a damped rhythm like that seen for GAD 65 mRNA, because rhythms in extracellular levels of VIP in the SCN rapidly damp out in constant darkness, at least in hamsters (Francl et al, 2010a, 2010b). …”

“…Although VIP acting on VPAC2 receptors is of substantial importance in neuropeptide coupling of clock cells, GRP and AVP may also play a role (Brown et al, 2005; Maywood et al, 2011). It will be interesting to determine how VIP mediates this cellular coupling, because only a small percentage of VIP containing cells display rhythmicity in culture, and rhythms of VIP mRNA, protein, release, and receptor number in the SCN are absent in intact animals under free-running conditions (Francl et al, 2010a, 2010b; Zoeller et al, 1992; Shinohara et al, 1993; Ajpru et al, 2002). …”

Role of Na+/Ca2+exchanger in Ca2+homeostasis in rat suprachiasmatic nucleus neurons

“…Indeed, high-K ϩ -induced, Ca 2ϩ -dependent somatodendritic release of dense-core vesicles has been previously demonstrated in the rat SCN (Castel et al 1996). Furthermore, experiments with reverse-microdialysis perfusion also show high-K ϩ -induced, Ca 2ϩ -dependent release of VIP, GRP, and AVP from the hamster SCN (Francl et al 2010a(Francl et al , 2010b. How and to what extent the NCX may participate in the regulation of neuropeptide release will await further investigations.…”