Arginine-Vasopressin Expressing Neurons in the Murine Suprachiasmatic Nucleus Exhibit a Circadian Rhythm in Network Coherence in vivo

Abstract

AbstractThe Suprachiasmatic Nucleus (SCN) is composed of functionally distinct sub-populations of GABAergic neurons such as vasoactive intestinal polypeptide (VIP)-, arginine vasopressin (AVP)-, gastrin releasing peptide (GRP)-, and neuromedin S (NMS)-expressing neurons which form a neural network responsible for synchronizing most physiological and behavioral circadian rhythms in mammals. To date, little is known regarding which aspects of SCN rhythmicity are generated by individual SCN neurons or neuronal sub-populations and which aspects result from neuronal interaction within a network. In this study, we address this question utilizing in vivo miniaturized microscopy to measure fluorescent GCaMP-mediated calcium dynamics in AVP neurons in the intact SCN of awake, behaving mice. This approach permits analysis of rhythms of single cells, populations, and correlational analysis among groups of AVP neurons in a field of view across the circadian and diurnal day and night. We report that AVP neurons in the murine SCN exhibit a periodic oscillatory increase in calcium of approximately 14 seconds across the day and night, in both constant darkness and under a 12:12 light-dark (LD) cycle. Using in vivo optogentically-targeted single unit activity recording, we demonstrated that these slow calcium waves are likely the result of burst-firing characteristic of AVP neurons previously reported for other brain regions. Rhythmicity analysis of several fluorescence measures suggests that individual AVP neurons exhibit unstable and stochastic rhythms, with approximately 30% of the neurons rhythmic during any given day across lighting conditions, and weak or absent rhythmicity at the population level. Network-level cross-correlational analysis revealed that coherence among neuron pairs also exhibited stochastic rhythms with about 25% of pairs rhythmic at any time. Notably, this analysis revealed a stronger rhythm at the population level than was observed in single cell analysis. The peak time of maximal coherence among AVP neuronal pairs occurs between CT/ZT 6 and 9, coinciding with the timing of maximal neuronal activity with the SCN as a whole. These results are the first to demonstrate robust circadian variation in the coordination between apparently weakly rhythmic or arrhythmic neurons suggesting that, for AVP neurons, interactions between neurons in the SCN are more influential than individual or single subpopulation activity in the regulation of mammalian circadian rhythms.