Localization and expression of GABA transporters in the suprachiasmatic nucleus.

GABA is a principal neurotransmitter in the suprachiasmatic hypothalamic nucleus (SCN), the master circadian clock. Despite the importance of GABA and GABA uptake for functioning of the circadian pacemaker, the localization and expression of GABA transporters (GATs) in the SCN has not been investigated. The present studies used Western blot analysis, immunohistochemistry and electron microscopy to demonstrate the presence of GABA transporter 1 (GAT1) and GAT3 in the SCN. By using light microscopy, GAT1 and GAT3 were co-localized throughout the SCN, but were not expressed in the perikarya of arginine vasopressin- or vasoactive intestinal peptide-immunoreactive (-ir) neurons of adult rats, nor in the neuronal processes labelled with the neurofilament heavy chain. Using electron microscopy, GAT1- and GAT3-ir was found in glial processes surrounding unlabelled neuronal perikarya, axons, dendrites, and enveloped symmetric and asymmetric axo-dendritic synapses. Glial fibrillary acidic protein-ir astrocytes grown in cell culture were immunopositive for GAT1 and GAT3 and both GATs could be observed in the same glial cell. These data demonstrate that synapses in the SCN function as 'tripartite' synapses consisting of presynaptic axon terminals, postsynaptic membranes and astrocytes that contain GABA transporters. This model suggests that astrocytes expressing both GATs may regulate the extracellular GABA, and thereby modulate the activity of neuronal networks in the SCN.

Simultaneous electrophysiological recording and calcium imaging of suprachiasmatic nucleus neurons.

Simultaneous electrophysiological and fluorescent imaging recording methods were used to study the role of changes of membrane potential or current in regulating the intracellular calcium concentration. Changing environmental conditions, such as the light-dark cycle, can modify neuronal and neural network activity and the expression of a family of circadian clock genes within the suprachiasmatic nucleus (SCN), the location of the master circadian clock in the mammalian brain. Excitatory synaptic transmission leads to an increase in the postsynaptic Ca(2+) concentration that is believed to activate the signaling pathways that shifts the rhythmic expression of circadian clock genes. Hypothalamic slices containing the SCN were patch clamped using microelectrodes filled with an internal solution containing the calcium indicator bis-fura-2. After a seal was formed between the microelectrode and the SCN neuronal membrane, the membrane was ruptured using gentle suction and the calcium probe diffused into the neuron filling both the soma and dendrites. Quantitative ratiometric measurements of the intracellular calcium concentration were recorded simultaneously with membrane potential or current. Using these methods it is possible to study the role of changes of the intracellular calcium concentration produced by synaptic activity and action potential firing of individual neurons. In this presentation we demonstrate the methods to simultaneously record electrophysiological activity along with intracellular calcium from individual SCN neurons maintained in brain slices.

Causes and consequences of hyperexcitation in central clock neurons.

Hyperexcited states, including depolarization block and depolarized low amplitude membrane oscillations (DLAMOs), have been observed in neurons of the suprachiasmatic nuclei (SCN), the site of the central mammalian circadian (~24-hour) clock. The causes and consequences of this hyperexcitation have not yet been determined. Here, we explore how individual ionic currents contribute to these hyperexcited states, and how hyperexcitation can then influence molecular circadian timekeeping within SCN neurons. We developed a mathematical model of the electrical activity of SCN neurons, and experimentally verified its prediction that DLAMOs depend on post-synaptic L-type calcium current. The model predicts that hyperexcited states cause high intracellular calcium concentrations, which could trigger transcription of clock genes. The model also predicts that circadian control of certain ionic currents can induce hyperexcited states. Putting it all together into an integrative model, we show how membrane potential and calcium concentration provide a fast feedback that can enhance rhythmicity of the intracellular circadian clock. This work puts forward a novel role for electrical activity in circadian timekeeping, and suggests that hyperexcited states provide a general mechanism for linking membrane electrical dynamics to transcription activation in the nucleus.

Vasoactive intestinal polypeptide requires parallel changes in adenylate cyclase and phospholipase C to entrain circadian rhythms to a predictable phase.

Circadian oscillations in the suprachiasmatic nucleus (SCN) depend on transcriptional repression by Period (PER)1 and PER2 proteins within single cells and on vasoactive intestinal polypeptide (VIP) signaling between cells. Because VIP is released by SCN neurons in a circadian pattern, and, after photic stimulation, it has been suggested to play a role in the synchronization to environmental light cycles. It is not known, however, if or how VIP entrains circadian gene expression or behavior. Here, we tested candidate signaling pathways required for VIP-mediated entrainment of SCN rhythms. We found that single applications of VIP reset PER2 rhythms in a time- and dose-dependent manner that differed from light. Unlike VIP-mediated signaling in other cell types, simultaneous antagonism of adenylate cyclase and phospholipase C activities was required to block the VIP-induced phase shifts of SCN rhythms. Consistent with this, VIP rapidly increased intracellular cAMP in most SCN neurons. Critically, daily VIP treatment entrained PER2 rhythms to a predicted phase angle within several days, depending on the concentration of VIP and the interval between VIP applications. We conclude that VIP entrains circadian timing among SCN neurons through rapid and parallel changes in adenylate cyclase and phospholipase C activities.

Neuropeptide-mediated calcium signaling in the suprachiasmatic nucleus network.

Neuroactive peptides and the intracellular calcium concentration ([Ca(2+) ](i) ) play important roles in light-induced modulation of gene expression in the suprachiasmatic nucleus (SCN) neurons that ultimately control behavioral rhythms. Vasoactive intestinal peptide (VIP) and arginine vasopressin (AVP) are expressed rhythmically within populations of SCN neurons. Pituitary adenylate cyclase-activating peptide (PACAP) is released from retinohypothalamic tract (RHT) terminals synapsing on SCN neurons. Nociceptin/orphanin FQ (OFQ) receptors are functionally expressed in the SCN. We examined the role of several neuropeptides on Ca(2+) signaling, simultaneously imaging multiple neurons within the SCN neural network. VIP reduced the [Ca(2+) ](i) in populations of SCN neurons during the day, but had little effect at night. Stimulation of the RHT at frequencies that simulate light input signaling evoked transient [Ca(2+) ](i) elevations that were not altered by VIP. AVP elevated the [Ca(2+) ](i) during both the day and night, PACAP produced variable responses, and OFQ induced a reduction in the [Ca(2+) ](i) similar to VIP. During the day, VIP lowered the [Ca(2+) ](i) to near nighttime levels, while AVP elevated [Ca(2+) ](i) during both the day and night, suggesting that the VIP effects on [Ca(2+) ](i) were dependent, and the AVP effects independent of the action potential firing activity state of the neuron. We hypothesize that VIP and AVP regulate, at least in part, Ca(2+) homeostasis in SCN neurons and may be a major point of regulation for SCN neuronal synchronization.

GABAergic signaling induces divergent neuronal Ca2+ responses in the suprachiasmatic nucleus network.

Intercellular communication between gamma-aminobutyric acid (GABA)ergic suprachiasmatic nucleus (SCN) neurons facilitates light-induced phase changes and synchronization of individual neural oscillators within the SCN network. We used ratiometric Ca(2+) imaging techniques to record changes in the intracellular calcium concentration ([Ca(2+)](i)) to study the role of GABA in interneuronal communication and the response of the SCN neuronal network to optic nerve stimulations that mimic entraining light signals. Stimulation of the retinohypothalamic tract (RHT) evoked divergent Ca(2+) responses in neurons that varied regionally within the SCN with a pattern that correlated with those evoked by pharmacological GABA applications. GABA(A) and GABA(B) receptor agonists and antagonists were used to evaluate components of the GABA-induced changes in [Ca(2+)](i). Application of the GABA(A) receptor antagonist gabazine induced changes in baseline [Ca(2+)](i) in a direction opposite to that evoked by GABA, and similarly altered the RHT stimulation-induced Ca(2+) response. GABA application induced Ca(2+) responses varied in time and region within the SCN network. The NKCC1 cotransporter blocker, bumetanide, and L-type calcium channel blocker, nimodipine, attenuated the GABA-induced rise of [Ca(2+)](i). These results suggest that physiological GABA induces opposing effects on [Ca(2+)](i) based on the chloride equilibrium potential, and may play an important role in neuronal Ca(2+) balance, synchronization and modulation of light input signaling in the SCN network.

Postinhibitory rebound spikes are modulated by the history of membrane hyperpolarization in the SCN.

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates biological circadian time thereby directly impacting numerous physiological processes. The SCN is composed almost exclusively of gamma-aminobutyric acid (GABA)ergic neurons, many of which synapse with other GABAergic cells in the SCN to exert an inhibitory influence on their postsynaptic targets for most, if not all, phases of the circadian cycle. The overwhelmingly GABAergic nature of the SCN, along with its internal connectivity properties, provide a strong model to examine how inhibitory neurotransmission generates output signals. In the present work we show that hyperpolarizations that range from 5 to 1000 ms elicit rebound spikes in 63% of all SCN neurons tested in voltage-clamp in the SCN of adult rats and hamsters. In current-clamp recordings, hyperpolarizations led to rebound spike formation in all cells; however, low-amplitude or short-duration current injections failed to consistently activate rebound spikes. Increasing the duration of hyperpolarization from 5 to 1000 ms is strongly and positively correlated with enhanced spike probability. Additionally, the magnitude of hyperpolarization exerts a strong influence on both the amplitude of the spike, as revealed by voltage-clamp recordings, and the latency to peak current obtained in either voltage- or current-clamp mode. Our results suggest that SCN neurons may use rebound spikes as one means of producing output signals from a largely interconnected network of GABAergic neurons.

Calcium response to retinohypothalamic tract synaptic transmission in suprachiasmatic nucleus neurons.

Glutamate released from retinohypothalamic tract (RHT) synapses with suprachiasmatic nucleus (SCN) neurons induces phase changes in the circadian clock presumably by using Ca2+ as a second messenger. We used electrophysiological and Ca2+ imaging techniques to simultaneously record changes in the membrane potential and intracellular calcium concentration ([Ca2+]i) in SCN neurons after stimulation of the RHT at physiologically relevant frequencies. Stimulation of the RHT sufficient to generate an EPSP did not produce detectable changes in [Ca2+]i, whereas EPSP-induced action potentials evoked an increase in [Ca2+]i, suggesting that the change in postsynaptic somatic [Ca2+]i produced by synaptically activated glutamate receptors was the result of membrane depolarization activating voltage-dependent Ca2+ channels. The magnitude of the Ca2+ response was dependent on the RHT stimulation frequency and duration, and on the SCN neuron action potential frequency. Membrane depolarization-induced changes in [Ca2+]i were larger and decayed more quickly in the dendrites than in the soma and were attenuated by nimodipine, suggesting a compartmentalization of Ca2+ signaling and a contribution of L-type Ca2+ channels. RHT stimulation at frequencies that mimicked the output of light-sensitive retinal ganglion cells (RGCs) evoked [Ca2+]i transients in SCN neurons via membrane depolarization and activation of voltage-dependent Ca2+ channels. These data suggest that for Ca2+ to induce phase advances or delays, light-induced signaling from RGCs must augment the underlying oscillatory somatic [Ca2+]i by evoking postsynaptic action potentials in SCN neurons during a period of slow spontaneous firing such as occurs during nighttime.

Mutations of presenilin genes in dilated cardiomyopathy and heart failure.

Two common disorders of the elderly are heart failure and Alzheimer disease (AD). Heart failure usually results from dilated cardiomyopathy (DCM). DCM of unknown cause in families has recently been shown to result from genetic disease, highlighting newly discovered disease mechanisms. AD is the most frequent neurodegenerative disease of older Americans. Familial AD is caused most commonly by presenilin 1 (PSEN1) or presenilin 2 (PSEN2) mutations, a discovery that has greatly advanced the field. The presenilins are also expressed in the heart and are critical to cardiac development. We hypothesized that mutations in presenilins may also be associated with DCM and that their discovery could provide new insight into the pathogenesis of DCM and heart failure. A total of 315 index patients with DCM were evaluated for sequence variation in PSEN1 and PSEN2. Families positive for mutations underwent additional clinical, genetic, and functional studies. A novel PSEN1 missense mutation (Asp333Gly) was identified in one family, and a single PSEN2 missense mutation (Ser130Leu) was found in two other families. Both mutations segregated with DCM and heart failure. The PSEN1 mutation was associated with complete penetrance and progressive disease that resulted in the necessity of cardiac transplantation or in death. The PSEN2 mutation showed partial penetrance, milder disease, and a more favorable prognosis. Calcium signaling was altered in cultured skin fibroblasts from PSEN1 and PSEN2 mutation carriers. These data indicate that PSEN1 and PSEN2 mutations are associated with DCM and heart failure and implicate novel mechanisms of myocardial disease.

Retrograde suppression of GABAergic currents in a subset of SCN neurons.

Many postsynaptic neurons release a retrograde transmitter that modulates presynaptic neurotransmitter release. In the suprachiasmatic nucleus (SCN), retrograde signaling is suggested by the presence of dendritic dense-core vesicles. Whole-cell voltage-clamp recordings were made from rat SCN neurons to determine whether a retrograde messenger could modulate the activity of afferent gamma-aminobutyric acid (GABA)ergic inputs. The frequency and amplitude of spontaneous GABAergic currents was significantly reduced in a subpopulation of SCN neurons (eight out of 13) following a postsynaptic depolarization. Similarly, a postsynaptic depolarization significantly reduced the amplitude of evoked GABAergic currents during both day and night recordings. A postsynaptic depolarizing pulse eliminated paired-pulse inhibition of GABAergic currents consistent with a presynaptic mechanism. Muscimol-activated currents were not altered by postsynaptic depolarization, demonstrating that the activity of GABA(A) receptors was not altered. Depolarization-induced inhibition of the GABAergic currents was not observed when a Ca2+ chelator was included in the microelectrode. Postsynaptic depolarization significantly increased the Ca2+ concentration in both the soma and dendrites. The dendritic Ca2+ levels increased faster, to a higher concentration and decayed faster than in the soma. The depolarization-induced inhibition of the evoked GABAergic current was blocked by the G-protein uncoupling agent N-ethylmaleimide, suggesting that the retrograde messenger acts on a pertussis toxin-sensitive G-protein-coupled receptor. Because the majority of SCN neurons receive GABAergic input from neighboring cells, these results describe a retrograde signaling mechanism by which SCN neurons can modulate GABAergic synaptic input.

Presynaptic GABA(B) receptors regulate retinohypothalamic tract synaptic transmission by inhibiting voltage-gated Ca2+ channels.

Presynaptic GABA(B) receptor activation inhibits glutamate release from retinohypothalamic tract (RHT) terminals in the suprachiasmatic nucleus (SCN). Voltage-clamp whole cell recordings from rat SCN neurons and optical recordings of Ca2+-sensitive fluorescent probes within RHT terminals were used to examine GABA(B)-receptor modulation of RHT transmission. Baclofen inhibited evoked excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner equally during the day and night. Blockers of N-, P/Q-, T-, and R-type voltage-dependent Ca2+ channels, but not L-type, reduced the EPSC amplitude by 66, 36, 32, and 18% of control, respectively. Joint application of multiple Ca2+ channel blockers inhibited the EPSCs less than that predicted, consistent with a model in which multiple Ca2+ channels overlap in the regulation of transmitter release. Presynaptic inhibition of EPSCs by baclofen was occluded by omega-conotoxin GVIA (< or = 72%), mibefradil (< or = 52%), and omega-agatoxin TK (< or = 15%), but not by SNX-482 or nimodipine. Baclofen reduced both evoked presynaptic Ca2+ influx and resting Ca2+ concentration in RHT terminals. Tertiapin did not alter the evoked EPSC and baclofen-induced inhibition, indicating that baclofen does not inhibit glutamate release by activation of Kir3 channels. Neither Ba2+ nor high extracellular K+ modified the baclofen-induced inhibition. 4-Aminopyridine (4-AP) significantly increased the EPSC amplitude and the charge transfer, and dramatically reduced the baclofen effect. These data indicate that baclofen inhibits glutamate release from RHT terminals by blocking N-, T-, and P/Q-type Ca2+ channels, and possibly by activation of 4-AP-sensitive K+ channels, but not by inhibition of R- and L-type Ca2+ channels or by Kir3 channel activation.