Circadian rhythms of mammalian physiology and behavior are coordinated by the suprachiasmatic nucleus (SCN) in the hypothalamus. Within SCN neurons, various aspects of cell physiology exhibit circadian oscillations, including circadian clock gene expression, levels of intracellular Ca2+ ([Ca2+]i), and neuronal firing rate. [Ca2+]i oscillates in SCN neurons even in the absence of neuronal firing. To determine the causal relationship between circadian clock gene expression and [Ca2+]i rhythms in the SCN, as well as the SCN neuronal network dependence of [Ca2+]i rhythms, we introduced GCaMP3, a genetically encoded fluorescent Ca2+ indicator, into SCN neurons from PER2::LUC knock-in reporter mice. Then, PER2 and [Ca2+]i were imaged in SCN dispersed and organotypic slice cultures. In dispersed cells, PER2 and [Ca2+]i both exhibited cell autonomous circadian rhythms, but [Ca2+]i rhythms were typically weaker than PER2 rhythms. This result matches the predictions of a detailed mathematical model in which clock gene rhythms drive [Ca2+]i rhythms. As predicted by the model, PER2 and [Ca2+]i rhythms were both stronger in SCN slices than in dispersed cells and were weakened by blocking neuronal firing in slices but not in dispersed cells. The phase relationship between [Ca2+]i and PER2 rhythms was more variable in cells within slices than in dispersed cells. Both PER2 and [Ca2+]i rhythms were abolished in SCN cells deficient in the essential clock gene Bmal1. These results suggest that the circadian rhythm of [Ca2+]i in SCN neurons is cell autonomous and dependent on clock gene rhythms, but reinforced and modulated by a synchronized SCN neuronal network.

Heterodimers of CLOCK and BMAL1 are the major transcriptional activators of the mammalian circadian clock. Because the paralog NPAS2 can substitute for CLOCK in the suprachiasmatic nucleus (SCN), the master circadian pacemaker, CLOCK-deficient mice maintain circadian rhythms in behavior and in tissues in vivo. However, when isolated from the SCN, CLOCK-deficient peripheral tissues are reportedly arrhythmic, suggesting a fundamental difference in circadian clock function between SCN and peripheral tissues. Surprisingly, however, using luminometry and single-cell bioluminescence imaging of PER2 expression, we now find that CLOCK-deficient dispersed SCN neurons and peripheral cells exhibit similarly stable, autonomous circadian rhythms in vitro. In CLOCK-deficient fibroblasts, knockdown of Npas2 leads to arrhythmicity, suggesting that NPAS2 can compensate for loss of CLOCK in peripheral cells as well as in SCN. Our data overturn the notion of an SCN-specific role for NPAS2 in the molecular circadian clock, and instead indicate that, at the cellular level, the core loops of SCN neuron and peripheral cell circadian clocks are fundamentally similar.

Like neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in the brain, single fibroblasts can function as independent oscillators. In the SCN, synaptic and paracrine signaling among cells creates a robust, synchronized circadian oscillation, whereas there is no evidence for such integration in fibroblast cultures. However, interactions among single-cell fibroblast oscillators cannot be completely excluded, because fibroblasts were not isolated in previous work. In this study, we tested the autonomy of fibroblasts as single-cell circadian oscillators in high and low density culture, by single-cell imaging of cells from PER2::LUC circadian reporter mice. We found greatly reduced PER2::LUC rhythmicity in low density cultures, which could result from lack of either constitutive or rhythmic paracrine signals from neighboring fibroblasts. To discriminate between these two possibilities, we mixed PER2::LUC wild type (WT) cells with non-luminescent, non-rhythmic Bmal1−/− cells, so that density of rhythmic cells was low but overall cell density remained high. In this condition, WT cells showed clear rhythmicity similar to high density cultures. We also mixed PER2::LUC WT cells with non-luminescent, long period Cry2−/− cells. In this condition, WT cells showed a period no different from cells cultured with rhythmic WT cells or non-rhythmic Bmal1−/− cells. In previous work, we found that low K+ suppresses fibroblast rhythmicity, and we and others have found that either low K+ or low Ca2+ suppresses SCN rhythmicity. Therefore, we attempted to rescue rhythmicity of low density fibroblasts with high K+ (21 mM), high Ca2+ (3.6 mM), or conditioned medium. Conditioned medium from high density fibroblast cultures rescued rhythmicity of low density cultures, whereas high K+ or Ca2+ medium did not consistently rescue rhythmicity. These data suggest that fibroblasts require paracrine signals from adjacent cells for normal expression of rhythmicity, but that these signals do not have to be rhythmic, and that rhythmic signals from other cells do not affect the intrinsic periods of fibroblasts.

Objective To compare pain control (opioid consumption and postsurgical pain scores) in head and neck (H&N) free flap reconstruction patients who undergo traditional means of postoperative analgesia including use of opioids versus a novel protocol that includes ketamine and gabapentin. Methods Single‐institution retrospective cohort study. Results Eighty‐six patients who underwent H&N free flap reconstruction from 2015 to 2018 were included. Forty‐three patients were in the control cohort treated with opioids only, and 43 patients were in the treatment group. There was a statistically significant decrease in opioid consumption in each of the first 5 postoperative days ranging from 80% to 83% in the treatment group. The daily pain scores were significantly lower in the treatment group in the first 2 postoperative days. At the 1‐month postoperative visit, there was no significant difference in pain scores between the groups; however, by the 2‐month visit, the treatment group reported significantly lower pain scores than the control group (P = 0.001). No adverse outcomes of ketamine or gabapentin were experienced. Conclusion Ketamine and gabapentin are safe and effective analgesics in H&N free flap surgery that significantly decrease opioid use in the acute postoperative setting and may improve pain control. Level of Evidence 3a Laryngoscope, 130:1686–1691, 2020