CAG-FLEX-GCaMP6s AAV (Serotype 9)

The suprachiasmatic nucleus (SCN) synchronizes circadian rhythms of behavior and physiology to the external light cycle. As the neuropeptide vasoactive intestinal polypeptide (VIP) is important for circadian light responses, researchers tested the hypothesis that rhythmic VIP-producing SCN neurons mediate circadian light responses in male and female mice. Using in vivo fiber photometry over multiple days, they found daily rhythms in spontaneous calcium events in SCN VIP neurons that peaked during subjective daytime and were disrupted by constant light. Light-evoked calcium responses peaked at subjective dusk and were more robust during subjective night. Using novel VIP sensor cells, researchers found that activity patterns in SCN VIP neurons were tightly coupled to spontaneous and NMDA-evoked VIP release. Finally, hyperpolarization of VIP neurons in vivo attenuated light-induced circadian changes in locomotion. These findings suggest that SCN VIP neurons exhibit circadian rhythms in both spontaneous and light-responsive activity and are essential for normal resetting of daily rhythms by ambient light.

To determine the circadian regulation of SCN VIP neuron activity over multiple days, researchers used long-term in vivo fiber photometry to record hourly spontaneous calcium activity in the SCN of freely moving mice (Figure 1a-d). Cre-dependent AAVs encoding the fluorescent calcium sensor GCaMP6s (AAV9-CAG-Flex-GCaMP6s) or a control fluorophore (AAV9-CAGFlex-EGFP) were injected into the SCN of VIP-IRES-Cre knock-in mice. Researchers found that daily rhythms in intracellular calcium levels and event frequency persisted for the number of days recorded, peaking at daily minimums in locomotor activity during light/dark (LD) cycles or constant darkness (DD) (Figure 1e-j). The amplitude of the rhythm was greatly disrupted in constant light (LL) (Figure 1i, j). These results suggest that SCN VIP neurons exhibit in vivo daily rhythms in spontaneous calcium activity.

Figure 1. SCN VIP neurons exhibit daily rhythms in spontaneous calcium activity in vivo. (Jones J R, et al., 2018)