Arginine–vasopressin-expressing neurons in the murine suprachiasmatic nucleus exhibit a circadian rhythm in network coherence in vivo

Abstract

The suprachiasmatic nucleus (SCN) is composed of functionally distinct subpopulations of GABAergic 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, and which aspects result from neuronal interaction within a network. Here, we utilize in vivo miniaturized microscopy to measure fluorescent GCaMP-reported calcium dynamics in arginine vasopressin (AVP)-expressing neurons in the intact SCN of awake, behaving mice. We report that SCN AVP neurons exhibit periodic, slow calcium waves which we demonstrate, using in vivo electrical recordings, likely reflect burst firing. Further, we observe substantial heterogeneity of function in that AVP neurons exhibit unstable rhythms, and relatively weak rhythmicity at the population level. Network analysis reveals that correlated cellular behavior, or coherence, among neuron pairs also exhibited stochastic rhythms with about 33% of pairs rhythmic at any time. Unlike single-cell variables, coherence exhibited a strong rhythm at the population level with time of maximal coherence among AVP neuronal pairs at CT/ZT 6 and 9, coinciding with the timing of maximal neuronal activity for the SCN as a whole. These results demonstrate robust circadian variation in the coordination between stochastically rhythmic neurons and that interactions between AVP neurons in the SCN may be more influential than single-cell activity in the regulation of circadian rhythms. Furthermore, they demonstrate that cells in this circuit, like those in many other circuits, exhibit profound heterogenicity of function over time and space.

Immediate responses to ambient light in vivo reveal distinct subpopulations of suprachiasmatic VIP neurons

“…FP is relatively easy to perform and robust, while single-cell imaging from the SCN has become feasible only recently. 50 , 51 Acquiring high-quality data is challenging because the SCN is a small (400 μm ML, 700 μm AP) and deep (5.5mm) target that suffers from high levels of motion artifacts. Performing both analyses, as we do here, allows comparison of the results to achieve a better understanding.…”

Hypertension: Causes and Consequences of Circadian Rhythms in Blood Pressure

“…237 Such findings suggest steady-state autoregulation of CBF is effective in the SCN, similar to many other brain regions. 183,237 Considering changes in Ca 2+ dynamics, neural activity, and transcription that occur in the SCN during a circadian cycle, [232][233][234]240 significant changes in local CBF might be predicted to occur due to neurovascular coupling or other mechanisms. Defining to what extent local CBF or BBB function potentially change over a 24-hour cycle could be revealing with potential implications for the mechanistic basis by which the SCN functions.…”

“…Electrical lesions of the SCN eliminate the circadian rhythmicity in BP. 106,230 Despite its importance as the central circadian clock in the mammalian brain, [231][232][233][234] little is known regarding regulation of blood flow to the SCN. The SCN is located within the anterior hypothalamus above the optic chiasma on each side of the third ventricle.…”

Discrete photoentrainment of mammalian central clock is regulated by bi-stable dynamic network in the suprachiasmatic nucleus

“…This in vivo setup allows us to track single neurons across different ZT time points longitudinally (Figure 1C). Furthermore, two-photon system also allow us to image across several hundred microns in dorsoventral axis (Figure 1D) compared to single photon system 71 . Here, each recording session consisted of six trials of a 30-second dark baseline followed by a 90-second light exposure (488 nm, 1.76 μW/mm2) to the eyes.…”