Allosteric modulation of GABAA receptors in acutely dissociated neurons of the suprachiasmatic nucleus

Estrogen-mediated coupling via gap junctions in the suprachiasmatic nucleus

The circadian clock orchestrates many physiological and behavioural rhythms in mammals with 24-h periodicity, through a hierarchical organisation, with the central clock located in the suprachiasmatic nucleus (SCN) in the hypothalamus. The circuits of the SCN generate circadian rhythms with precision, relying on intrinsic coupling mechanisms, for example, neurotransmitters like arginine vasopressin (AVP), vasoactive intestinal peptide (VIP), neuronal gamma-aminobutyric acid (GABA) signalling and astrocytes connected by gap junctions composed of connexins (Cx). In female rodents, the presence of estrogen receptors (ERs) in the dorsal SCN suggests an influence of estrogen (E2) on the circuit timekeeping that could regulate circadian rhythm and coupling. To investigate this, we used SCN explants together with hypothalamic neurons and astrocytes. First, we showed that E2 stabilised the circadian amplitude in the SCN when rAVPs (receptor-associated vasopressin peptides) were inhibited. However, the phase delay induced by VIPAC2 (VIP receptors) inhibition remained unaffected by E2. We then showed that E2 exerted its effects in the SCN via ERβ (estrogen receptor beta), resulting in increased expression of Cx36 and Cx43. Notably, specific inhibition of both connexins resulted in a significant reduction in circadian amplitude within the SCN. Remarkably, E2 restored the period with inhibited Cx36 but not with Cx43 inhibition. This implies that the network between astrocytes and neurons, responsible for coupling in the SCN, can be reinforced through E2. In conclusion, these findings provide new insights into how E2 regulates circadian rhythms ex vivo in an ERβ-dependent manner, underscoring its crucial role in fortifying the SCN's rhythm.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

Direction of action of presynaptic GABAA receptors is highly dependent on the level of receptor activation

Many synapses, including parallel fiber synapses in the cerebellum, express presynaptic GABA_A receptors. However, reports of the functional consequences of presynaptic GABA_A receptor activation are variable across synapses, from inhibition to enhancement of transmitter release. We find that presynaptic GABA_A receptor function is bidirectional at parallel fiber synapses depending on GABA concentration and modulation of GABA_A receptors in mice. Activation of GABA_A receptors by low GABA concentrations enhances glutamate release, whereas activation of receptors by higher GABA concentrations inhibits release. Furthermore, blocking GABA_B receptors reduces GABA_A receptor currents and shifts presynaptic responses toward greater enhancement of release across a wide range of GABA concentrations. Conversely, enhancing GABA_A receptor currents with ethanol or neurosteroids shifts responses toward greater inhibition of release. The ability of presynaptic GABA_A receptors to enhance or inhibit transmitter release at the same synapse depending on activity level provides a new mechanism for fine control of synaptic transmission by GABA and may explain conflicting reports of presynaptic GABA_A receptor function across synapses. NEW & NOTEWORTHY GABA_A receptors are widely expressed at presynaptic terminals in the central nervous system. However, previous reports have produced conflicting results on the function of these receptors at different synapses. We show that presynaptic GABA_A receptor function is strongly dependent on the level of receptor activation. Low levels of receptor activation enhance transmitter release, whereas higher levels of activation inhibit release at the same synapses. This provides a novel mechanism by which presynaptic GABA_A receptors fine-tune synaptic transmission.

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.

Collective timekeeping among cells of the master circadian clock

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the master circadian clock that coordinates daily rhythms in behavior and physiology in mammals. Like other hypothalamic nuclei, the SCN displays an impressive array of distinct cell types characterized by differences in neurotransmitter and neuropeptide expression. Individual SCN neurons and glia are able to display self-sustained circadian rhythms in cellular function that are regulated at the molecular level by a 24h transcriptional-translational feedback loop. Remarkably, SCN cells are able to harmonize with one another to sustain coherent rhythms at the tissue level. Mechanisms of cellular communication in the SCN network are not completely understood, but recent progress has provided insight into the functional roles of several SCN signaling factors. This review discusses SCN organization, how intercellular communication is critical for maintaining network function, and the signaling mechanisms that play a role in this process. Despite recent progress, our understanding of SCN circuitry and coupling is far from complete. Further work is needed to map SCN circuitry fully and define the signaling mechanisms that allow for collective timekeeping in the SCN network.

Diurnal pituitary-adrenal activity during schedule-induced polydipsia of water and ethanol in cynomolgus monkeys (Macaca fascicularis)

RATIONALE

Intermittent delivery of an important commodity (e.g., food pellets) generates excessive behaviors as an adjunct to the schedule of reinforcement (adjunctive behaviors) that are hypothesized to be due to conflict between engaging and escaping a situation where reinforcement is delivered, but at suboptimal rates.

OBJECTIVES

This study characterized the endocrine correlates during schedule-induced polydipsia of water and ethanol using a longitudinal approach in non-human primates.

METHODS

Plasma adrenocorticotropic hormone (ACTH) and cortisol were measured in samples from awake cynomolgus monkeys (Macaca fascicularis, 11 adult males) obtained at the onset, mid-day, and offset of their 12-h light cycle. The monkeys were induced to drink water and ethanol (4 % w/v, in water) using a fixed time (FT) 300-s interval schedule of pellet delivery. The induction fluid changed every 30 sessions in the following order: water, 0.5 g/kg ethanol, 1.0 g/kg ethanol, and 1.5 g/kg ethanol. Following induction, ethanol and water were concurrently available for 22 h/day.

RESULTS

The FT 300-s schedule gradually increased ACTH, but not cortisol, during water induction to a plateau sustained throughout ethanol induction in every monkey. Upon termination of the schedule, ACTH decreased to baseline and cortisol below baseline. Diurnal ACTH and cortisol were unrelated to the dose of ethanol, but ACTH rhythm flattened at 0.5 g/kg/day and remained flattened.

CONCLUSIONS

The coincidence of elevated ACTH with the initial experience of drinking to intoxication may have altered the mechanisms involved in the transition to heavy drinking.

Ethanol modulates mammalian circadian clock phase resetting through extrasynaptic GABA receptor activation

Ethanol modulates the actions of multiple neurotransmitter systems, including GABA. However, its enhancing effects on GABA signaling typically are seen only at high concentrations. In contrast, although GABA is a prominent neurotransmitter in the circadian clock of the suprachiasmatic nucleus (SCN), we see ethanol modulation of clock phase resetting at low concentrations (<50 mM). A possible explanation is that ethanol enhances GABAergic signaling in the SCN through activating GABA(A) receptors that contain the delta subunit (GABA(Adelta) receptors), which are sensitive to low ethanol concentrations. Therefore, we investigated whether ethanol acts on GABA(Adelta) receptors in the SCN. Here we show that acute application of the GABA(Adelta) receptor antagonist, RO15-4513, to mouse hypothalamic slices containing the SCN prevents ethanol inhibition of nighttime glutamate-induced (photic-like) phase delays of the circadian clock. Diazepam, which enhances activity of GABA(A) receptors containing the gamma subunit (GABA(Agamma) receptors), does not modulate these phase shifts. Moreover, we find that RO15-4513 prevents ethanol enhancement of daytime serotonergic (non-photic) phase advances of the circadian clock. Furthermore, diazepam phase-advances the SCN circadian clock when applied alone in the daytime, while ethanol has no effect by itself at that time. These data support the hypothesis that ethanol acts on GABA(Adelta) receptors in the SCN to modulate photic and non-photic circadian clock phase resetting. They also reveal distinct modulatory roles of different GABA(A) receptor subtypes in circadian clock phase regulation.

Effects of muscarinic receptor stimulation on Ca2+ transient, cAMP production and pacemaker frequency of rabbit sinoatrial node cells

We investigated the contribution of the intracellular calcium (Ca_i²⁺) transient to acetylcholine (ACh)-mediated reduction of pacemaker frequency and cAMP content in rabbit sinoatrial nodal (SAN) cells. Action potentials (whole cell perforated patch clamp) and Ca_i²⁺ transients (Indo-1 fluorescence) were recorded from single isolated rabbit SAN cells, whereas intracellular cAMP content was measured in SAN cell suspensions using a cAMP assay (LANCE^®). Our data show that the Ca_i²⁺ transient, like the hyperpolarization-activated “funny current” (I_f) and the ACh-sensitive potassium current (I_K,ACh), is an important determinant of ACh-mediated pacemaker slowing. When I_f and I_K,ACh were both inhibited, by cesium (2 mM) and tertiapin (100 nM), respectively, 1 μM ACh was still able to reduce pacemaker frequency by 72%. In these I_f and I_K,ACh-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Ca_i²⁺ transient decay (r² = 0.98) and slow diastolic Ca_i²⁺ rise (r² = 0.73). Inhibition of the Ca_i²⁺ transient by ryanodine (3 μM) or BAPTA-AM (5 μM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Ca_i²⁺ transient and reduced the sarcoplasmic reticulum (SR) Ca²⁺ content, all in a concentration-dependent fashion. At 1 μM ACh, the spontaneous activity and Ca_i²⁺ transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10 μM) and I_K,ACh was inhibited by tertiapin (100 nM). Also, inhibition of the Ca_i²⁺ transient by ryanodine (3 μM) or BAPTA-AM (25 μM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Ca_i²⁺ affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Ca_i²⁺ transient via a cAMP-dependent signaling pathway. Inhibition of the Ca_i²⁺ transient contributes to pacemaker slowing and inhibits Ca_i²⁺-stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Ca_i²⁺ transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells.

Phenobarbital blockade of the preovulatory luteinizing hormone surge: association with phase-advanced circadian clock and altered suprachiasmatic nucleus Period1 gene expression

The suprachiasmatic nucleus (SCN) controls the timing of the preovulatory luteinizing hormone (LH) surge in laboratory rodents. Barbiturate administration during a critical period on proestrus delays the surge and prolongs the estrous cycle 1 day. Because a nonphotic timing signal (zeitgeber) during the critical period that phase advances activity rhythms can also induce the latter effect, we hypothesized that barbiturates delay the LH surge by phase-advancing its circadian timing signal beyond the critical period. In experiment 1, locomotor rhythms and estrous cycles were monitored in hamsters for 2-3 wk preinjection and postinjection of vehicle or phenobarbital and after transfer to darkness at zeitgeber time (ZT) 6 on proestrus. Phenobarbital delayed estrous cycles in five of seven hamsters, which exhibited phase shifts that averaged twofold greater than those exhibited by vehicle controls or phenobarbital-injected hamsters with normal cycles. Experiment 2 used a similar protocol, but injections were at ZT 5, and blood samples for LH determination were collected from 1200 to 1800 on proestrus and the next day via jugular cannulae inserted the day before proestrus. Phenobarbital delayed the LH surge 1 day in all six hamsters, but it occurred at an earlier circadian time, supporting the above hypothesis. Experiment 3 investigated whether phenobarbital, like other nonphotic zeitgebers, suppresses SCN Period1 and Period2 transcription. Two hours postinjection, phenobarbital decreased SCN expression of only Period1 mRNA, as determined by in situ hybridization. These results suggest that phenobarbital advances the SCN pacemaker, governing activity rhythms and hormone release in part by decreasing its Period1 gene expression.

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.

Structure and pharmacology of swelling-sensitive chloride channels, I(Cl,swell)

Since several years, the interest for chloride channels and more particularly for the enigmatic swelling-activated chloride channel (I(Cl,swell)) is increasing. Despite its well-characterized electrophysiological properties, the I(Cl,swell) structure and pharmacology are not totally elucidated. These channels are involved in a variety of cell functions, such as cardiac rhythm, cell proliferation and differentiation, cell volume regulation and cell death through apoptosis. This review will consider different aspects regarding structure, electrophysiological properties, pharmacology, modulation and functions of these swelling-activated chloride channels.

Mechanosensitivity of GIRK Channels Is Mediated by Protein Kinase C-dependent Channel-Phosphatidylinositol 4,5-Bisphosphate Interaction

Gprotein-activated inwardly rectifying K+ channel (GIRK or Kir3) currents are inhibited by mechanical stretch of the cell membrane, but the underlying mechanisms are not understood. In Xenopus oocytes heterologously expressing GIRK channels, membrane stretch induced by 50% reduction of osmotic pressure caused a prompt reduction of GIRK1/4, GIRK1, and GIRK4 currents by 16.6-42.6%. Comparable GIRK current reduction was produced by protein kinase C (PKC) activation (phorbol 12-myristate 13-acetate). The mechanosensitivity of the GIRK4 current was abolished by pretreatment with PKC inhibitors (staurosporine or calphostin C). Neither hypo-osmotic challenge nor PKC activation affected IRK1 currents. GIRK4 chimera (GIRK4-IRK1-(Lys207-Leu245)) and single point mutant (GIRK4(I229L)), in which the phosphatidylinositol 4,5-bisphosphate (PIP2) binding domain or residue was replaced by the corresponding region of IRK1 to strengthen the channel-PIP2 interaction, showed no mechanosensitivity and minimal PKC sensitivity. IRK1 gained mechanosensitivity and PKC sensitivity by reverse double point mutation of the PIP2 binding domain (L222I/R213Q). Overexpression of Gbetagamma, which is known to strengthen the channel-PIP2 interaction, attenuated the mechanosensitivity of GIRK4 channels. In oocytes expressing a pleckstrin homology domain of PLC-delta tagged with green fluorescent protein, hypo-osmotic challenge or PKC activation caused a translocation of the fluorescence signal from the cell membrane to the cytosol, reflecting PIP2 hydrolysis. The translocation was prevented by pretreatment with PKC inhibitors. Involvement of PKC activation in the mechanosensitivity of muscarinic K+ channels was confirmed in native rabbit atrial myocytes. These results suggest that the mechanosensitivity of GIRK channels is mediated primarily by channel-PIP2 interaction, with PKC playing an important role in modulating the interaction probably through PIP2 hydrolysis.

Circadian rhythm in intracellular Cl(-) activity of acutely dissociated neurons of suprachiasmatic nucleus

A link between the circadian rhythm and the function of Cl(-)-permeable gamma-aminobutyric acid (GABA) type A (GABA(A)) receptors on suprachiasmatic nucleus (SCN) neurons was studied by measuring intracellular activity of Cl(-) (aCl) at different times during a circadian cycle in SCN neurons acutely dissociated from rat brains. To measure aCl, the voltage-clamp mode of the gramicidin-perforated patch-clamp technique was used, and reversal potential of GABA-induced currents (E(GABA)) was converted to aCl. Measured aCl was significantly higher at around noon (20.1 +/- 1.4 mM) than at three other time zones of a circadian cycle (means ranging from 11.6 to 14.3 mM). Chord conductance of GABA-induced currents showed no circadian changes, indicating a lack of circadian changes in the number or single-channel conductance of GABA(A) receptors. These results suggest that aCl participates in modulating GABA(A) receptor functions on SCN neurons during the circadian rhythm.

Oestrogen receptor-alpha-immunoreactive neurones project to the suprachiasmatic nucleus of the female Syrian hamster

Ovarian steroid hormones regulate circadian period and phase, but classical receptors for these hormones are absent in the circadian pacemaker localized in the suprachiasmatic nucleus of the hypothalamus (SCN). In order to determine whether effects of oestrogen may be exerted through steroid-binding systems afferent to the SCN we have performed double label immunocytochemistry for oestrogen receptor-alpha(ER-alpha) and the retrograde tracer cholera toxin B subunit (CtB) after its application to the SCN. Most of the areas that contain ER-alpha-immunoreactive (ERalpha-ir) cells also contained cells afferent to the SCN. The percentage of neurones afferent to the SCN which show ERalpha-immunoreactivity varies between areas. As many as one-third of the neurones afferent to the SCN in some parts of the preoptic area and the corticomedial amygdala are ERalpha-ir. Very few of the afferent neurones from the septum and the central grey are ERalpha-ir, whereas an intermediate proportion of afferents from the bed nucleus of the stria terminalis and the arcuate nucleus are ERalpha-ir. Our retrograde tracing results were compared with results of anterograde tracing from some of the sites containing SCN afferents. Using a combined retrograde and anterograde tracing technique we tested the possibility that single ERalpha-ir neurones afferent to the SCN could receive reciprocal innervation by SCN efferents. Although we found SCN input to some SCN afferent neurones, we found no evidence of reciprocity between single ERalpha-ir cells and the SCN. Our results indicate the existence of oestrogen binding systems afferent to the SCN. These neuroanatomical pathways may mediate effects of gonadal steroid hormones on circadian rhythms.

Zinc and flunitrazepam modulation of GABA-mediated currents in rat suprachiasmatic neurons

The suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for generating circadian rhythms in mammals, and GABA is the predominant neurotransmitter in the SCN. Properties of gamma-aminobutyric acid-A (GABAA) responses in SCN neurons were examined in acutely prepared hypothalamic slices from 3- to 8-wk-old rats with the use of whole cell voltage-clamp techniques. Zn2+ reduced the amplitude of GABAA-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) in a concentration-dependent manner ranging from a reduction of control amplitude to 88% at 10 microM to 27% at 1,000 microM. Zn2+ reduced IPSC amplitude to a similar degree in the presence of tetrodotoxin and also significantly reduced the amplitude of currents evoked by application of exogenous GABA (100 microM, pressure applied). Zn2+ increased the frequency of IPSCs at lower concentrations and decreased it at higher ones. Flunitrazepam (100 nM) usually failed to potentiate the amplitude of sIPSCs, but prolonged sIPSC kinetics. Two exponential components were normally resolved in the sIPSC decay constants, and flunitrazepam significantly increased those two components. Thus flunitrazepam increased the duration of sIPSCs and potentiated the amplitude of currents evoked by pressure application of GABA. Zn2+ and benzodiazepine each modulated the effect of GABA in nearly all cells, suggesting that most SCN neurons have a similar GABAA receptor subunit composition in this respect. Zn2+ also affected sIPSC frequency, which suggests that Zn2+ increased neuronal firing rate at lower concentrations. These results begin to define the cellular roles that these GABAA receptor modulators might play in circadian regulation.

Development of GABAA receptors on medial septum/diagonal band (MS/DB) neurons after postnatal ethanol exposure

The impact of 'binge-like' ethanol exposure on postnatal days (PD) 4-9 was examined on development of gamma-aminobutyric acid type A receptors (GABAAR) during the first month of life in the rat. Whole-cell patch-clamp recordings in acutely isolated medial septum/diagonal band (MS/DB) neurons were used to define effects of rapidly applied ethanol and other allosteric modulators on bicuculline-sensitive GABA currents. Three age groups were examined including 'pups' (PD 4-10), 'juveniles' (PD 11-16) and 'young adults' (PD 25-35). In untreated neurons, maximum responses to GABA and the apparent GABA EC50 increased approximately 2-fold during the first month of life. Potentiation of GABA responses by pentobarbital, midazolam, and loreclezole all increased with age, while Zn2+ inhibition declined. Initial inhibition by ethanol switched to potentiation of GABA responses during this time. In vivo, binge-like ethanol treatment (4.5 g kg-1 day-1 divided into two doses, 2 h apart on PD 4-9) reduced both the GABA maximal response and GABA EC50 measured on PD 11-16. These measures returned to control levels by PD 25-35. After binge-like postnatal ethanol exposure, age-dependent loss of Zn2+ inhibition of GABA responses was increased, while potentiating actions of in vitro ethanol were blocked. GABAAR modulation by other drugs was unaffected. These data suggest that early postnatal ethanol exposure disrupts the expected developmental pattern of GABAAR function in MS/DB neurons, an action that could contribute to neurobehavioral deficits associated with the fetal alcohol syndrome. Whether these changes are due to cellular damage, delayed gene expression or post-translational modification needs to be determined.

Regulation of uterine smooth muscle function during gestation

The uterus is unique among smooth muscular organs in that, during pregnancy, it undergoes profound, largely reversible, changes orchestrated by the ovarian hormones. These changes facilitate uterine adaptation to the stretch induced by the growing fetus such that a state of myometrial contractile quiescence can be maintained. This quiescent state usually is maintained until fetal development is sufficient for extrauterine life, at which point unknown mechanisms precipitate conversion to a highly contractile state. Throughout pregnancy, signaling mechanisms for myometrial contractility are altered--first to promote quiescence and then again to promote contractions. The mechanisms responsible for these changes are only partially understood. This review attempts to summarize salient features of many of the changes in uterine contractile signaling and the current state of ongoing investigations of their mechanisms. We have also highlighted some newer information and concepts from nonuterine tissues, which we believe may provide insight into the control of uterine smooth muscle function. Some detail has been omitted, and can be found in the many excellent reviews cited. We hope that this discussion may stimulate the interests of other investigators. The diverse areas of inquiry offer hope that this decade will lead to a fuller understanding of myometrial function and the development of vastly improved approaches for the control of preterm labor.

Pacemaking in the heart: the interplay of ionic currents

1. There is still a degree of controversy about which currents drive pacemaking in the sinoatrial node or sinus venous. Early attempts to identify a single 'pacemaker current' in these tissues, based on voltage-clamp data, were largely unsuccessful, prompting the search for other mechanisms that may contribute to rhythmic activity. 2. Whole-cell patch-clamp recording from single cells isolated from the sinus venosus of the toad has shown that a voltage-dependent sodium current may play a role in pacemaking. This current has a transient component that contributes to the action potential upstroke and an inactivation-resistant component that contributes to the diastolic depolarization. The relative importance of this current in pacemaking is still controversial. 3. The development of computer models of pacemaking has contributed greatly to our understanding of how ionic currents can interact to produce rhythmic activity. Results are presented from one such model, 'Oxsoft Heart', to illustrate the different contributions of Ir and INa and to highlight the concept that pacemaking is driven by the integrated activity of many processes, rather than by any one current in particular. 4. Present models of pacemaking fail to accurately reproduce biological observations for certain situations. It is becoming clear that many processes contribute to pacemaking and have yet to be fully incorporated into models. Recent results regarding the role of intracellular calcium buffering and release and their implications, are discussed in this context. 5. The control of pacemaking by neurotransmitters is discussed. The limitations of single cell models in reproducing many of the complex responses to nerve stimulation of multicellular tissue, such as postinhibitory rebound, are discussed and possible improvements to models are suggested.

Characterization of the electrogenic Na+ -K+ pump in bipolar cells isolated from carp retina

The electrogenic Na+ -K+ pump current (Ip) in carp bipolar cells was investigated under voltage-clamp conditions. The Ip was activated in a concentration-dependent manner by adding external K+ (Ko+) and was completely suppressed with 10(-4) M ouabain (EC50=1.23 mM; Hill coefficient=1.36). The Ip was suppressed in a concentration-dependent manner by ouabain (IC50=1.90 mM; Hill coefficient=0.93). The Ip did not show a distinct voltage dependency either with or without Na(o)+. A large outward shift of the holding current was observed by completely removing Na(o)+. In the presence of Na(o)+, a steady Ip was observed even in the absence of internal Na+ (Na(i)+). These results suggest that continuous Na+ influxes exist across the membrane. When external and internal Na+ was removed, a transient Ip was observed (half decay time (t1/2) was 5.0+/-0.6 s), thus indicating that the transient Ip was activated by the residual Na(i)+. In the absence of Na(o)+, the transient Ip was also observed with lower than 8 mM Na(i)+. The t1/2 depended on Na(i)+. However, a steady Ip was observed with 10 mM Na(i)+ or more. The functional properties of the Ip are discussed.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.