Day–night variations in zinc sensitivity of GABAA receptor-channels in rat suprachiasmatic nucleus

Modelling the functional roles of synaptic and extra-synaptic γ-aminobutyric acid receptor dynamics in circadian timekeeping

In the suprachiasmatic nucleus (SCN), γ-aminobutyric acid (GABA) is a primary neurotransmitter. GABA can signal through two types of GABAA receptor subunits, often referred to as synaptic GABAA (gamma subunit) and extra-synaptic GABAA (delta subunit). To test the functional roles of these distinct GABAA in regulating circadian rhythms, we developed a multicellular SCN model where we could separately compare the effects of manipulating GABA neurotransmitter or receptor dynamics. Our model predicted that blocking GABA signalling modestly increased synchrony among circadian cells, consistent with published SCN pharmacology. Conversely, the model predicted that lowering GABAA receptor density reduced firing rate, circadian cell fraction, amplitude and synchrony among individual neurons. When we tested these predictions, we found that the knockdown of delta GABAA reduced the amplitude and synchrony of clock gene expression among cells in SCN explants. The model further predicted that increasing gamma GABAA densities could enhance synchrony, as opposed to increasing delta GABAA densities. Overall, our model reveals how blocking GABAA receptors can modestly increase synchrony, while increasing the relative density of gamma over delta subunits can dramatically increase synchrony. We hypothesize that increased gamma GABAA density in the winter could underlie the tighter phase relationships among SCN cells.

Examination of Zinc in the Circadian System

Zinc is a trace element that is essential for a large number of biological and biochemical processes in the body. In the nervous system zinc is packaged into synaptic vesicles by the ZnT3 transporter, and synaptic release of zinc can influence the activity of postsynaptic cells, either directly through its own cognate receptors, or indirectly by modulating activation of receptors for other neurotransmitters. Here, we explore the anatomical and functional aspects of zinc in the circadian system. Melanopsin-containing retinal ganglion cells in the mouse retina were found to colocalize ZnT3, indicating that they can release zinc at their synaptic targets. While the master circadian clock in the hamster suprachiasmatic nucleus (SCN) was found to contain, at best, sparse zincergic input, the intergeniculate leaflet (IGL) of hamsters and mice were found to have prominent zincergic input. Levels of zinc in these areas were not affected by time of day. Additionally, IGL zinc staining persisted following enucleation, indicating other prominent sources of zinc instead of, or in addition to, the retina. Neither enhancement nor chelation of free zinc at either the SCN or IGL altered circadian responses to phase-shifting light in hamsters. Finally, entrainment, free-running, and circadian responses to light were explored in mice lacking the ZnT3 gene. In every aspect explored, the ZnT3 knockout mice were not significantly different from their wildtype counterparts. These findings highlight the presence of zinc in areas critical for circadian functioning but have yet to identify a role for zinc in these areas.

Circadian ATP Release in Organotypic Cultures of the Rat Suprachiasmatic Nucleus Is Dependent on P2X7 and P2Y Receptors

The circadian rhythms in physiological and behavioral functions are driven by a pacemaker located in the suprachiasmatic nucleus (SCN). The rhythms continue in constant darkness and depend on cell-cell communication between neurons and glia. The SCN astrocytes generate also a circadian rhythm in extracellular adenosine 5′-triphosphate (ATP) accumulation, but molecular mechanisms that regulate ATP release are poorly understood. Here, we tested the hypothesis that ATP is released via the plasma membrane purinergic P2X7 receptors (P2X7Rs) and P2Y receptors (P2YRs) which have been previously shown to be expressed in the SCN tissue at transcriptional level. We have investigated this hypothesis using SCN organotypic cultures, primary cultures of SCN astrocytes, ATP bioluminescent assays, immunohistochemistry, patch-clamping, and calcium imaging. We found that extracellular ATP accumulation in organotypic cultures followed a circadian rhythm, with a peak between 24:00 and 04:00 h, and the trough at ~12:00 h. ATP rhythm was inhibited by application of AZ10606120, A438079, and BBG, specific blockers of P2X7R, and potentiated by GW791343, a positive allosteric modulator of this receptor. Double-immunohistochemical staining revealed high expression of the P2X7R protein in astrocytes of SCN slices. PPADS, a non-specific P2 antagonist, and MRS2179, specific P2Y1R antagonist, also abolished ATP rhythm, whereas the specific P2X4R blocker 5-BDBD was not effective. The pannexin-1 hemichannel blocker carbenoxolone displayed a partial inhibitory effect. The P2Y1R agonist MRS2365, and the P2Y2R agonist MRS2768 potentiated ATP release in organotypic cultures and increase intracellular Ca²⁺ level in cultured astrocytes. Thus, SCN utilizes multiple purinergic receptor systems and pannexin-1 hemichannels to release ATP.

Temporal Regulation of GABAA Receptor Subunit Expression: Role in Synaptic and Extrasynaptic Communication in the Suprachiasmatic Nucleus

Recent molecular studies suggest that the expression levels of δ and γ2 GABA_A receptor (GABA_AR) subunits regulate the balance between synaptic and extrasynaptic GABA neurotransmission in multiple brain regions. We investigated the expression of GABA_Aδ and GABA_Aγ2 and the functional significance of a change in balance between these subunits in a robust local GABA network contained within the suprachiasmatic nucleus of the hypothalamus (SCN). Muscimol, which can activate both synaptic and extrasynaptic GABA_ARs, injected into the SCN during the day phase advanced the circadian pacemaker, whereas injection of the extrasynaptic GABA_A superagonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) had no effect on circadian phase. In contrast, injection of either THIP or muscimol during the night was sufficient to block the phase shifting effects of light. Gene expression analysis of the whole SCN revealed different temporal patterns in GABA_Aδ and GABA_Aγ2 mRNA expression. When examined across all subregions of the SCN, quantitative immunohistochemical analysis found no significant variations in GABA_Aδ protein immunoreactivity (IR) but did find significant variations in GABA_Aγ2 protein-IR in hamsters housed in either LD cycles or in constant darkness. Remarkably, significant interactions in the ratio of GABA_Aδ:GABA_Aγ2 subunits between lighting condition and circadian phase occurred only within one highly discrete anatomical area of the SCN; a region that functions as the input for lighting information from the retina. Taken together, these data support the hypothesis that the balance between synaptic and extrasynaptic GABA_ARs determines the functional response to GABA, and that this balance is differentially regulated in a region-specific manner.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

Potentiation of inhibitory synaptic transmission by extracellular ATP in rat suprachiasmatic nuclei

The hypothalamic suprachiasmatic nuclei (SCN), the circadian master clock in mammals, releases ATP in a rhythm, but the role of extracellular ATP in the SCN is still unknown. In this study, we examined the expression and function of ATP-gated P2X receptors (P2XRs) in the SCN neurons of slices isolated from the brain of 16- to 20-day-old rats. Quantitative RT-PCR showed that the SCN contains mRNA for P2X 1-7 receptors and several G-protein-coupled P2Y receptors. Among the P2XR subunits, the P2X2 > P2X7 > P2X4 mRNAs were the most abundant. Whole-cell patch-clamp recordings from SCN neurons revealed that extracellular ATP application increased the frequency of spontaneous GABAergic IPSCs without changes in their amplitudes. The effect of ATP appears to be mediated by presynaptic P2X2Rs because ATPγS and 2MeS-ATP mimics, while the P2XR antagonist PPADS blocks, the observed enhancement of the frequency of GABA currents. There were significant differences between two SCN regions in that the effect of ATP was higher in the ventrolateral subdivision, which is densely innervated from outside the SCN. Little evidence was found for the presence of P2XR channels in somata of SCN neurons as P2X2R immunoreactivity colocalized with synapsin and ATP-induced current was observed in only 7% of cells. In fura-2 AM-loaded slices, BzATP as well as ADP stimulated intracellular Ca(2+) increase, indicating that the SCN cells express functional P2X7 and P2Y receptors. Our data suggest that ATP activates presynaptic P2X2Rs to regulate inhibitory synaptic transmission within the SCN and that this effect varies between regions.

The GABAergic network in the suprachiasmatic nucleus as a key regulator of the biological clock: does it change during senescence?

GABA is the main neurotransmitter of the hypothalamic suprachiasmatic nucleus (SCN) and plays a key role in the function of this master circadian pacemaker. Despite the evidence that disturbances of biological rhythms are common during aging, little is known about the GABAergic network in the SCN of the aging brain. We here provide a brief overview of the GABAergic structures and the role of GABA in the SCN. We also review some age-related changes of the GABAergic system occurring in the brain outside the SCN. Finally, we present preliminary data on the GABAergic system within the SCN comparing young and aging mice. In particular, our study on age-related changes in the SCN focused on the daily expression of the alpha3 subunit of the GABA(A) receptor and on the density of GABAergic axon terminals. Interestingly, our preliminary findings point to alterations of the GABAergic network in the biological clock during senescence.

Molecular, pharmacological and functional properties of GABA(A) receptors in anterior pituitary cells

Anterior pituitary cells express gamma-aminobutyric acid (GABA)-A receptor-channels, but their structure, distribution within the secretory cell types, and nature of action have not been clarified. Here we addressed these questions using cultured anterior pituitary cells from postpubertal female rats and immortalized alphaT3-1 and GH(3) cells. Our results show that mRNAs for all GABA(A) receptor subunits are expressed in pituitary cells and that alpha1/beta1 subunit proteins are present in all secretory cells. In voltage-clamped gramicidin-perforated cells, GABA induced dose-dependent increases in current amplitude that were inhibited by bicuculline and picrotoxin and facilitated by diazepam and zolpidem in a concentration-dependent manner. In intact cells, GABA and the GABA(A) receptor agonist muscimol caused a rapid and transient increase in intracellular calcium, whereas the GABA(B) receptor agonist baclofen was ineffective, suggesting that chloride-mediated depolarization activates voltage-gated calcium channels. Consistent with this finding, RT-PCR analysis indicated high expression of NKCC1, but not KCC2 cation/chloride transporter mRNAs in pituitary cells. Furthermore, the GABA(A) channel reversal potential for chloride ions was positive to the baseline membrane potential in most cells and the activation of ion channels by GABA resulted in depolarization of cells and modulation of spontaneous electrical activity. These results indicate that secretory pituitary cells express functional GABA(A) receptor-channels that are depolarizing.

Circadian Rhythmicity in AVP Secretion and GABAergic Synaptic Transmission in the Rat Suprachiasmatic Nucleus

A variety of physiological and behavioral functions exhibit circadian changes and these circadian rhythms are driven by oscillatory expression of clock genes in the suprachiasmatic nuclei (SCN). It is still unknown how this molecular clockwork is controlled by extracellular neurohormones and neurotransmitters and which membrane receptors undergo circadian modulation. Circadian rhythm can be measured as a secretion of arginine vasopressin (AVP) in organotypic SCN culture for several weeks. Melatonin applied directly to the SCN late in the day induces a phase advance, when applied late at night or at the beginning of the day melatonin causes a phase delay. The time window for phase advance corresponds with the highest level of melatonin receptors in the SCN but the mechanism of melatonin-induced phase delay is unknown. The principal neurotransmitter on SCN synapses is gamma-aminobutyric acid (GABA), which acts at postsynaptic GABA(A) receptors. Spontaneous release of GABA from presynaptic nerve terminals, recorded as miniature inhibitory postsynaptic currents in the presence of TTX, does not change, but zinc sensitivity of exogenous GABA-induced currents varies during the day and night, possibly due to changes in subunit composition of GABA(A) receptors. We conclude that there is daily variation in the postsynaptic, but not presynaptic, function in the SCN.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

The neurobiology of zinc in health and disease

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'.

Distribution of zincergic neurons in the mouse forebrain

Synaptically released zinc is thought to play an important role in neuronal signaling by modulating excitatory and inhibitory receptors and intracellular signaling proteins. Consequently, neurons that release zinc have been implicated in synaptic plasticity underlying learning and memory as well as neuropathological processes such as epilepsy, stroke, and Alzheimer's disease. To characterize the distribution of these neurons, investigators have relied on a technique that involves the retrograde transport of zinc-selenium crystals from axonal boutons to the cell bodies of origin. However, one major problem with this method is that labeling of cell bodies is obscured by high levels of staining in synaptic boutons, particularly within forebrain structures where this staining is most intense. Here, we used a modification of the retrograde labeling method that eliminates terminal staining for zinc, thereby enabling a clear and comprehensive description of these neurons. Zincergic neurons were found in all cerebral cortical regions and were arranged in a distinct laminar pattern, restricted to layers 2/3, 5, and 6 with no labeling in layer 4. In the hippocampus, labeling was present in CA1, CA3, and the dentate gyrus but not in CA2. Labeled cell bodies were also observed in most amygdaloid nuclei, anterior olfactory nuclei, claustrum, tenia tecta, endopiriform region, lateral ventricle, lateral septum, zona incerta, superior colliculus, and periaqueductal gray. Moreover, retrograde labeling was also noted in the dorsomedial and lateral hypothalamus, regions that previously were thought to be devoid of neurons with a zincergic phenotype. Collectively these data show that zincergic neurons comprise a large population of neurons in the murine forebrain and will provide an anatomical framework for understanding the functional importance of these neurons in the mammalian brain.

GABAergic synapses of the suprachiasmatic nucleus exhibit a diurnal rhythm of short-term synaptic plasticity

We examined synaptic plasticity at intrasuprachiasmatic nucleus (SCN) gamma-aminobutyric acid (GABA)ergic synapses by measuring the paired-pulse ratio between pairs of evoked inhibitory postsynaptic currents (IPSCs). Interstimulus intervals were chosen to represent the range of spontaneous action potential firing frequencies found in SCN neurons. A majority of synapses studied during the day exhibited paired-pulse depression (PPD), whereas a majority of synapses studied during the night showed no PPD. Two types of PPD were found. Type 1 PPD expresses the greatest inhibition at shorter interstimulus intervals, is predominant in the early morning and is likely to be a result of vesicle depletion. Type 2 showed the greatest inhibition at interstimulus intervals between 175 and 225 ms, is found throughout the day yet rarely at night and is likely to be a result of a Ca(2+)-dependent mechanism that is independent of pertussis toxin-sensitive G-proteins. Thus, multiple mechanisms of synaptic plasticity modulate intra-SCN communication throughout the diurnal cycle.