Inhibitory and excitatory networks balance cell coupling in the suprachiasmatic nucleus: A modeling approach
Introduction
The way organisms anticipate the timing of daytime and nighttime, referred to as circadian rhythms, is essential for good health and optimal timing of metabolic processes and behavior (Kondratova and Kondratov, 2012, McClung, 2007, Xu et al., 2005). Circadian rhythms may be disrupted in otherwise healthy individuals by a variety of perturbations such as jet lag, social jet lag, rotating shift work and seasonal changes (Sack et al., 2007). Psychiatric disorders such as schizophrenia (Boivin, 2000, Wulff et al., 2012) and neurodegenerative disorders such as Alzheimer׳s disease (Satlin et al., 1995, Wu and Swaab, 2007) are also characterized by loss of circadian rhythms. Those afflicted with circadian disruption suffer from sleep loss and erratic wake times (Buysse et al., 2005). Reduced cognitive performance has also been exhibited by employees whose jobs require shift rotation or flight crew members with over eight hours of jet lag per week (Cho et al., 2000, Rouch et al., 2005, Viitasalo et al., 2014). Maladies such as obesity, diabetes, and heart attacks have also been correlated to human social jet lag (Roenneberg et al., 2012) and knockout mice lacking circadian rhythmicity (Shi et al., 2013, Turek et al., 2005). Therefore, circadian disruption represents a serious public health concern.
In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus generates these rhythms and responds to cues such as light and feeding (Klein et al., 1991). Mice with their SCN surgically ablated have their circadian rhythms abolished, while the SCN confers host rhythmic behavior to transplant recipients (Sujino et al., 2003). Remarkably, a coherent signal is produced by the SCN despite it being composed of 20,000 heterogeneous neural oscillators (Herzog et al., 2004, Webb et al., 2009, Welsh et al., 1995). In order for these cells to develop a consensus circadian rhythm, they form networks to exchange information about their individual and collective oscillatory patterns (Reppert and Weaver, 2002). Robust rhythms are necessary for ensuring that regular sleep and other behavioral schedules are followed even when external cues are absent; however networks must also be flexible so that organisms may adjust their schedule to seasonal changes or time shifts (Herzog, 2007, Meijer et al., 2010, Pfeuty et al., 2012).
The neurotransmitter vasoactive intestinal peptide (VIP) has been shown to be essential for synchronizing SCN neurons (Aton et al., 2005). VIP secretion follows a circadian pattern with peaks during the subjective day (Shinohara et al., 1995). Mice lacking genes for VIP or its receptor, VPAC2, have highly disrupted circadian rhythms (Bechtold et al., 2008, Maywood et al., 2006), while doses of VIP have entrained or phase shifted SCN tissue oscillations in vitro (Reed et al., 2001, Watanabe et al., 2000). Furthermore, when the dorsolateral SCN shell, which lacks VIP secreting cells, is separated from the ventromedial SCN core, which contains many VIP secreting cells, the core remains synchronized while rhythms are not observed in the shell (Belenky et al., 2008, Yamaguchi et al., 2003). VPAC2 activation, in conjunction with cytosolic calcium oscillations, stimulate Per1 and Per2 gene expression through the cAMP response element-binding protein (CREB) signaling cascade (Nielsen et al., 2002, Tischkau et al., 2003). Therefore, VIP is likely paramount among neurotransmitters for connecting SCN cells into a functional network.
Meanwhile, GABA is the principal inhibitory neurotransmitter in the brain and is also pervasive throughout the SCN where its functionality is controversial (Castel and Morris, 2000, Moore and Speh, 1993). GABA has been reported to synchronize (Liu and Reppert, 2000) or desynchronize (Freeman et al., 2013) SCN neurons. A potential link between VIP and GABA is also strongly suggested by the increase in GABA secretion from SCN neurons administered exogenous VIP (Itri and Colwell, 2003). One study has shown that while GABA opposes VIP-mediated synchrony during steady-state conditions, it facilitates resynchronization from antiphase conditions induced by long days (Evans et al., 2013). Although GABA, unlike VIP, has no known mechanism for directly influencing the molecular core clock of SCN neurons (Aton et al., 2006), it does open GABAA chloride ion channels (Itri et al., 2004) and its concentration oscillates throughout the day in phase with VIP. Whether GABA networks are excitatory or inhibitory has been shown experimentally (Wagner et al., 1997) and in models (Vasalou et al., 2011) to depend on the intracellular chloride concentration, but in the SCN core GABA networks seem to only be inhibitory (Albus et al., 2005). The aim of our modeling study was therefore to explore new ways of how GABA-induced inhibitory post-synaptic currents (IPSCs) could modulate network properties in conjunction with VIP signaling.
To discover new relationships between VIP and GABA networks in the SCN, we performed computational analyses with a modified version of a heterogeneous multicellular model previously developed by our group (Vasalou et al., 2011). Heterogeneous cell populations were of particular interest because of the wide phenotypic behavior that exists, even within classically defined regions such as the core or the shell. Our network population consisted of neurons that in the absence of neurotransmitter signaling exhibited either sustained, damped, or no oscillations (Vasalou and Henson, 2010). Neurons with different intrinsic properties have different requirements for entrainment (Abraham et al., 2010), and so a network of heterogeneous oscillators would ideally include local differences in neurotransmitter concentrations that are responsive to receptor cell feedback (Fig. 1). This responsiveness is similar to what was observed by Itri and Colwell when they found that GABA was released by neurons in the presence of high levels of exogenous VIP (Itri and Colwell, 2003). When entraining to periods different from their intrinsic periods, we expected weak oscillators to require excitatory positive feedback networks for generating robust rhythms. Meanwhile we expected inhibitory negative feedback networks to weaken strong oscillators, having the effect of increasing their range of entrainment (Abraham et al., 2010). So while VIP is ultimately necessary for rhythmic coupling, we hypothesized that too strong of an excitatory signal could push a cell into circadian disruption whereas a subtler influence would more effectively shift the neuron to the consensus periodicity. Therefore, we expected that a system where the strongest oscillators also released the highest levels of inhibitory GABA (thus slowing VIP and GABA release from cells in its local network) would achieve this neuroexcitatory balance locally and confer improved synchronization globally.
Our model was particularly useful for testing these hypotheses because it included not only the effect of VIP on the core molecular clock (Leloup and Goldbeter, 2003) but also an electrophysiological component that captured the effect of GABA on the resting membrane potential and firing rate (Vasalou and Henson, 2010). Since the two neurotransmitters influenced different components of the model, their combined action would be indirectly rather than directly antagonistic for a coupled network of heterogeneous SCN neurons. Our modeling studies were therefore designed to determine if GABA signaling could counterbalance high concentrations of secreted VIP to improve network performance. We used two scenarios to test this hypothesis, one where the network synchronized in the dark and another where the network resynchronized to an imposed light shift.
Section snippets
GABA and VIP coordinate to change network properties
By modulating the maximum rates of release of VIP and GABA (vVIP and vGABA, respectively), the model predicted that VIP and GABA had differential roles in determining network properties. The heat maps in Fig. 2 show the effects of these modulations on network synchrony, mean peak firing rate, amplitude, and period, with the values of vVIP and vGABA centered on previously published values (Vasalou et al., 2011). We also reported variability of these values across five simulations performed with
Discussion
A core tenet of this study was that proper functioning of the SCN required a balance between excitatory and inhibitory networks. This hypothesis was derived from the importance of excitatory VIP to intercellular coupling (Aton et. al., 2005) and the overall prevalence of inhibitory GABA in the SCN (Belenky et. al., 2008). Our multicellular model predicted that the SCN network had an optimal peak firing rate for synchronization that could be achieved by an appropriate balance between the VIP and
Author contributions
NJK participated in simulation design, performed the simulations, analyzed the data, produced the images, and wrote the paper. SRT and MAH participated in simulation design, data analysis, and writing the paper.
Conflicts of interest
The authors declare no conflict of interest in the publishing of this work.
Acknowledgments
This research was funded by National Institutions of Health grants GM078993 and
1R01GM096873-01.
References (68)
- et al.
A GABAergic mechanism is necessary for coupling dissociable ventral and dorsal regional oscillators within the circadian clock
Curr. Biol.
(2005) - et al.
A Gq-Ca 2+axis controls circuit-level encoding of circadian time in the suprachiasmatic nucleus
Neuron
(2013) - et al.
Dynamic interactions mediated by nonredundant signaling mechanisms couple circadian clock neurons
Neuron
(2013) - et al.
GABA networks destabilize genetic oscillations in the circadian pacemaker
Neuron
(2013) - et al.
GABA synchronizes clock cells within the suprachiasmatic circadian clock
Neuron
(2000) - et al.
Synchronization and maintenance of timekeeping in suprachiasmatic circadian clock cells by neuropeptidergic signaling
Curr. Biol.
(2006) - et al.
GABA is the principal neurotransmitter of the circadian system
Neurosci. Lett.
(1993) - et al.
The circadian visual system, 2005
Brain Res. Rev.
(2006) - et al.
Social jetlag and obesity
Curr. Biol.
(2012) - et al.
Circadian locomotor activity and core-body temperature rhythms in Alzheimer׳s disease
Neurobiol. Aging
(1995)
Circadian disruption leads to insulin resistance and obesity
Curr. Biol.
Suprachiasmatic nucleus grafts restore circadian behavioral rhythms of genetically arrhythmic mice
Curr. Biol.
Ca2+/cAMP response element-binding protein (CREB)-dependent activation of Per1 is required for light-induced signaling in the suprachiasmatic nucleus circadian clock
J. Biol. Chem.
A multicellular model for differential regulation of circadian signals in the core and shell regions of the suprachiasmatic nucleus
J. Theor. Biol.
Multicellular model for intercellular synchronization in circadian neural networks
Biophys. J.
In vitro entrainment of the circadian rhythm of vasopressin-releasing cells in suprachiasmatic nucleus by vasoactive intestinal polypeptide
Brain Res.
Individual neurons dissociated from rat suprachiasmatic nucleus express independently phased circadian firing rhythms
Neuron
Disturbance and strategies for reactivation of the circadian rhythm system in aging and Alzheimer׳s disease
Sleep Med.
Highly sensitive luciferase reporter assay using a potent destabilization sequence of calpain 3
J. Biotechnol.
Coupling governs entrainment range of circadian clocks
Mol. Syst. Biol.
Intracellular Calcium as a Clock Output from SCN Neurons. Mechanisms of Circadian Systems in Animals and Their Clinical Relevance
A neuropeptide speeds circadian entrainment by reducing intercellular synchrony
Proc. Natl. Acad. Sci.
Timing of neuropeptide coupling determines synchrony and entrainment in the mammalian circadian clock
PLoS Comput. Biol.
GABA and Gi/o differentially control circadian rhythms and synchrony in clock neurons
Proc. Natl. Acad. Sci.
Vasoactive intestinal polypeptide mediates circadian rhythmicity and synchrony in mammalian clock neurons
Nat. Neurosci.
Metabolic rhythm abnormalities in mice lacking VIP-VPAC2 signaling
Am. J. Physiol.-Regulatory, Integr. Comp. Physiol.
Heterogeneous expression of γ-aminobutyric acid and γ-aminobutyric acid-associated receptors and transporters in the rat suprachiasmatic nucleus
J. Comp. Neurol.
Influence of sleep-wake and circadian rhythm disturbances in psychiatric disorders
J. Psychiatry Neurosci.
Circadian patterns of sleep, sleepiness, and performance in older and younger adults
Sleep
Morphological heterogeneity of the GABAergic network in the suprachiasmatic nucleus, the brain׳s circadian pacemaker
J. Anat.
Chronic jet lag produces cognitive deficits
J. Neurosci.
Distinct roles for GABA across multiple timescales in mammalian circadian timekeeping
Proc. Natl. Acad. Sci.
GABAA receptor activation suppresses Period 1 mRNA and Period 2 mRNA in the suprachiasmatic nucleus during the mid-subjective day
Eur. J. Neurosci.
Cited by (11)
The functional changes of the circadian system organization in aging
2019, Ageing Research ReviewsCitation Excerpt :Although the role of inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the SCN neuronal synchronization is still unclear, it is hypothesized that VIP works with GABA to promote synchrony of heterogeneous oscillators in the SCN. In this model, VIP produced by VIP-secreting cells in the SCN works as an essential driver for synchronization while GABA generated from both VIP-secreting and non-VIP-secreting cells serves as a negative feedback mechanism to balance cell coupling (Kingsbury et al., 2016). Similar to the instability of isolated individual cellular oscillators in the SCN, single Drosophila central clock neurons are weak oscillators that need to be in an intact circadian circuit to generate robust circadian rhythms (Sabado et al., 2017).
A systems theoretic approach to analysis and control of mammalian circadian dynamics
2016, Chemical Engineering Research and DesignCitation Excerpt :Neurotransmitters, or pharmaceuticals which modulate neurotransmitter expression, offer another possible path to control of the circadian oscillator. In particular, the neurotransmitters VIP and GABA have attracted attention, due to their preeminent role in synchronizing the SCN (An et al., 2013; Kingsbury et al., 2016). However, these neurotransmitters are also implicated in a variety of non-circadian functions, and may not be sufficiently precise to target solely the clock.
Expression of the vesicular GABA transporter within neuromedin S<sup>+</sup> neurons sustains behavioral circadian rhythms
2023, Proceedings of the National Academy of Sciences of the United States of AmericaModelling the functional roles of synaptic and extra-synaptic γ-aminobutyric acid receptor dynamics in circadian timekeeping
2021, Journal of the Royal Society InterfaceAstrocytic Modulation of Neuronal Activity in the Suprachiasmatic Nucleus: Insights from Mathematical Modeling
2020, Journal of Biological Rhythms