Effect of internal calcium concentration on calcium currents in rat sensory neurones

A biophysically detailed computational model of urinary bladder small DRG neuron soma

Bladder small DRG neurons, which are putative nociceptors pivotal to urinary bladder function, express more than a dozen different ionic membrane mechanisms: ion channels, pumps and exchangers. Small-conductance Ca2+-activated K+ (SKCa) channels which were earlier thought to be gated solely by intracellular Ca2+ concentration ([Ca]i) have recently been shown to exhibit inward rectification with respect to membrane potential. The effect of SKCa inward rectification on the excitability of these neurons is unknown. Furthermore, studies on the role of KCa channels in repetitive firing and their contributions to different types of afterhyperpolarization (AHP) in these neurons are lacking. In order to study these phenomena, we first constructed and validated a biophysically detailed single compartment model of bladder small DRG neuron soma constrained by physiological data. The model includes twenty-two major known membrane mechanisms along with intracellular Ca2+ dynamics comprising Ca2+ diffusion, cytoplasmic buffering, and endoplasmic reticulum (ER) and mitochondrial mechanisms. Using modelling studies, we show that inward rectification of SKCa is an important parameter regulating neuronal repetitive firing and that its absence reduces action potential (AP) firing frequency. We also show that SKCa is more potent in reducing AP spiking than the large-conductance KCa channel (BKCa) in these neurons. Moreover, BKCa was found to contribute to the fast AHP (fAHP) and SKCa to the medium-duration (mAHP) and slow AHP (sAHP). We also report that the slow inactivating A-type K+ channel (slow KA) current in these neurons is composed of 2 components: an initial fast inactivating (time constant ∼ 25-100 ms) and a slow inactivating (time constant ∼ 200-800 ms) current. We discuss the implications of our findings, and how our detailed model can help further our understanding of the role of C-fibre afferents in the physiology of urinary bladder as well as in certain disorders.

Activity-dependent regulation of T-type calcium channels by submembrane calcium ions

Voltage-gated Ca²⁺ channels are involved in numerous physiological functions and various mechanisms finely tune their activity, including the Ca²⁺ ion itself. This is well exemplified by the Ca²⁺-dependent inactivation of L-type Ca²⁺ channels, whose alteration contributes to the dramatic disease Timothy Syndrome. For T-type Ca²⁺ channels, a long-held view is that they are not regulated by intracellular Ca²⁺. Here we challenge this notion by using dedicated electrophysiological protocols on both native and expressed T-type Ca²⁺ channels. We demonstrate that a rise in submembrane Ca²⁺ induces a large decrease in T-type current amplitude due to a hyperpolarizing shift in the steady-state inactivation. Activation of most representative Ca²⁺-permeable ionotropic receptors similarly regulate T-type current properties. Altogether, our data clearly establish that Ca²⁺ entry exerts a feedback control on T-type channel activity, by modulating the channel availability, a mechanism that critically links cellular properties of T-type Ca²⁺ channels to their physiological roles.

DOI:http://dx.doi.org/10.7554/eLife.22331.001

Neurons, muscle cells and many other types of cells use electrical signals to exchange information and coordinate their behavior. Proteins known as calcium channels sit in the membrane that surrounds the cell and can generate electrical signals by allowing calcium ions to cross the membrane and enter the cell during electrical activities. Although calcium ions are needed to generate these electrical signals, and for many other processes in cells, if the levels of calcium ions inside cells become too high they can be harmful and cause disease.

Cells have a “feedback” mechanism that prevents calcium ion levels from becoming too high. This mechanism relies on the calcium ions that are already in the cell being able to close the calcium channels. This feedback mechanism has been extensively studied in two types of calcium channel, but it is not known whether a third group of channels – known as Cav3 channels – are also regulated in this way.

Cav3 channels are important in electrical signaling in neurons and have been linked with epilepsy, chronic pain and various other conditions in humans. Cazade et al. investigated whether calcium ions can regulate the activity of human Cav3 channels. The experiments show that these channels are indeed regulated by calcium ions, but using a distinct mechanism to other types of calcium channels. For the Cav3 channels, calcium ions alter the gating properties of the channels so that they are less easily activated . As a result, fewer Cav3 channels are “available” to provide calcium ions with a route into the cell.

The next steps following on from this work will be to identify the molecular mechanisms underlying this new feedback mechanism. Another challenge will be to find out what role this calcium ion-driven feedback plays in neurological disorders that are linked with altered Cav3 channel activity.

DOI:http://dx.doi.org/10.7554/eLife.22331.002

Off the beaten path: drug addiction and the pontine laterodorsal tegmentum

Drug addiction is a multileveled behavior controlled by interactions among many diverse neuronal groups involving several neurotransmitter systems. The involvement of brainstem-sourced, cholinergic neurotransmission in the development of addiction and in the persistent physiological processes that drive this maladaptive behavior has not been widely investigated. The major cholinergic input to neurons in the midbrain which are instrumental in assessment of reward and assignment of salience to stimuli, including drugs of abuse, sources from acetylcholine- (ACh-) containing pontine neurons of the laterodorsal tegmentum (LDT). Excitatory LDT input, likely cholinergic, is critical in allowing behaviorally relevant neuronal firing patterns within midbrain reward circuitry. Via this control, the LDT is positioned to be importantly involved in development of compulsive, addictive patterns of behavior. The goal of this review is to present the anatomical, physiological, and behavioral evidence suggesting a role of the LDT in the neurobiology underlying addiction to drugs of abuse. Although focus is directed on the evidence supporting a vital participation of the cholinergic neurons of the LDT, data indicating a contribution of noncholinergic LDT neurons to processes underlying addiction are also reviewed. While sparse, available information of actions of drugs of abuse on LDT cells and the output of these neurons as well as their influence on addiction-related behavior are also presented. Taken together, data from studies presented in this review strongly support the position that the LDT is a major player in the neurobiology of drug addiction. Accordingly, the LDT may serve as a future treatment target for efficacious pharmaceutical combat of drug addiction.

Activity-dependent changes in voltage-dependent calcium currents and transmitter release

Voltage-dependent Ca2+ channels are important in the regulation of neuronal structure and function, and as a result, they have received considerable attention. Recent studies have begun to characterize the diversity of their properties and the relationship of this diversity to their various cellular functions. In particular, Ca2+ channels play a prominent role in depolarization-secretion coupling, where the release of neurotransmitter is very sensitive to changes in voltage-dependent Ca2+ currents. An important feature of Ca2+ channels is their regulation by electrical activity. Depolarization can selectively modulate the properties of Ca2+ channel types, thus shaping the response of the neuron to future electrical activity. In this article, we examine the diversity of Ca2+ channels found in vertebrate and invertebrate neurons, and their short- and long-term regulation by membrane potential and Ca2+ influx. Additionally, we consider the extent to which this activity-dependent regulation of Ca2+ currents contributes to the development and plasticity of transmitter releasing properties. In the studies of long-term regulation, we focus on crustacean motoneurons where activity levels, Ca2+ channel properties, and transmitter releasing properties can be followed in identified neurons.

Early effects of PRL on ion conductances in CHO cells expressing PRL receptor

Chinese hamster ovary (CHO-K1) cells were stably transfected with prolactin (PRL) receptor cDNA. These cells (CHO-E32) expressed the long form of functional PRL receptor. Using microfluorimetric and patch-clamp techniques, we have investigated the effects of PRL on intracellular Ca2+ concentration ([Ca2+]i) and membrane ion conductances. Exposure of CHO-E32 cells to 5 nM PRL resulted in an increase in [Ca2+]i. Two types of response were observed: 1) a stimulation of Ca2+ entry and 2) an intracellular Ca2+ mobilization. As PRL inhibited voltage-activated Ca2+ current, the PRL-induced Ca2+ increase does not involve voltage-activated Ca2+ channels. PRL also increased a charybdotoxin-sensitive Ca(2+)-dependent K+ conductance. Simultaneous measurements showed that PRL hyperpolarized the membrane potential before increasing intracellular Ca2+ levels. In voltage clamp, hyperpolarizing voltage steps were associated with increased Ca2+ concentrations, whereas depolarizing voltage steps decreased [Ca2+]i. Cell-free patch-clamp experiments showed that PRL directly stimulates K+ channel activity. Our results suggest the existence of a regulatory complex involving a protein kinase tightly associated with the Ca(2+)-activated K+ channels and that PRL stimulates these channels by means of the activation of protein kinase. The resulting hyperpolarization stimulates Ca2+ entry, probably through voltage-insensitive nonspecific channels.

Inactivation of single Ca2+ channels in rat sensory neurons by extracellular Ca2+

1. Single Ca2+ channels conducting 20 mM Ba2+ from adult rat dorsal root ganglion cells were characterized using the two-electrode patch-clamp technique configuration. 2. Channels demonstrating specific characteristics of conductance, voltage dependence and dihydropyridine sensitivity were classified as high-threshold or L-type Ca2+ channels. 3. Mean single-channel current in 20 mM Ba2+ did not show inactivation, but inactivation occurred when using Ca2+ as a permeating ion. 4. Stimulus protocols were delivered alternately in the cell-attached and whole-cell electrode, while recording single-channel activity and total Ca2+ current simultaneously. 5. A mean single-channel Ba2+ current from a stimulated patch did not show inactivation. However, stimulation of a physiological whole-cell Ca2+ current induced a marked inactivation of mean single-channel Ba2+ current. 6. Complete Ca2+ current block by the addition of 200 microM Cd2+ in the external solution removed single-channel inactivation in patches stimulated through a whole-cell electrode.

Kindling-induced long-lasting enhancement of calcium current in hippocampal CA1 area of the rat: relation to calcium-dependent inactivation

Daily tetanization of the Schaffer collaterals (kindling) in the rat hippocampus induces a persistent epileptogenic focus in area CA1. Neurons were enzymatically isolated from the focal region one day or six weeks after seven class V generalized seizures had been evoked. Calcium currents were measured under voltage-clamp conditions in the whole-cell patch configuration. One day after kindling, as well as six weeks later, the amplitudes of a slow-inactivating (tau = 90 ms) and a non-inactivating calcium current component were, in comparison to controls, enhanced by 30 and 40%, respectively. This enhancement was therefore related to the kindled state of enhanced excitability. The enhancement of the calcium current was independent of the steady-state intracellular calcium concentration. Fast calcium-dependent inactivation was provoked with double-pulse protocols that conditioned the neuron with a defined calcium-influx in the first pulse. Despite the larger calcium current during the conditioning pulse, the relative calcium-dependent inactivation of the sustained current component was reduced in neurons from the kindled focus. Repetitive depolarizations, once every second, evoked a cumulative calcium-dependent inactivation. Nothwithstanding the larger calcium current, kindling also persistently reduced this slow inactivation of both transient and sustained high threshold calcium current. The reduction in calcium-dependent inactivation cannot be responsible for the increased current, but can certainly enhance the calcium influx during prolonged activation or seizures. The changes can be explained by assuming that additional calcium channels are recruited at a location that prevents calcium-dependent inactivation.

Separation of calcium currents in retinal ganglion cells from postnatal rat

A culture system of the postnatal rat retina was established to investigate Ca2+ currents and synaptic transmission in identified neurons. Methods are described that allowed us to select retinal ganglion neurons (RGNs) in short term cultures (up to 48 h in vitro) and in long-term cultures (3 to 21 days in vitro). The specific aim of the present study was to identify channel specific components in whole-cell Ca2+ currents of RGNs and to clarify the potential use of the lanthanide Gd3+ as a selective Ca2+ channel blocker. About one third of freshly dissociated RGNs generated both low voltage activated Ca2+ currents (ICa(LVA)) and high voltage activated Ca2+ currents (ICa(HVA)). The remaining 2/3 or RGNs in short term culture and most RGNs in long-term culture displayed only ICa(HVA). The latter comprised at least three different components that were functionally rather similar, but could be separated pharmacologically. A significant portion (about 40%) of ICa(HVA) was irreversibly blocked by the N channel antagonist omega-CgTx (5 microM). The L channel antagonist nifedipine (10 microM) eliminated about 25% of ICa(HVA). Thus, about 1/3 of the HVA Ca2+ or Ba2+ current remained unaffected by either omega-CgTx or nifedipine. omega-AgaTx (200 nM) completely failed to block HVA Ca2+ or Ba2+ currents in RGNs. Gd3+ exerted contrasting actions on LVA and HVA Ca2+ currents. While ICa(LVA) consistently increased in the presence of Gd3+ (0.32-3.2 microM), ICa(HVA) always decreased, especially when using higher concentrations of Gd3+ (10-32 microM). The blocking action of Gd3+ was not restricted to the omega-CgTx-sensitive HVA current component, but also concerned omega-CgTx- and nifedipine-resistant components. The decay of Ca2+ currents was accelerated in the presence of Gd3+. Even in RGNs lacking ICa(LVA), application of 3.2 microM Gd3+ significantly reduced the time constant of decay from an average of 64 ms to 36 ms (voltage steps from -90 to 0 mV; 10 mM [Ca2+]o; 26 degrees C). This is in contrast to what had to be expected if an N-type HVA current component was selectively suppressed by Gd3+.Gd3+ diminished glutamatergic spontaneous synaptic activity in retinal cultures tested during the 3rd week in vitro. Both frequency and amplitude were reduced. Occasionally, the application was followed by a rebound increase of EPSC frequency. A stimulatory effect during application of Gd3+ has never been observed. These experiments indicate that RGNs express at least 4 different types of Ca2+ currents, that resemble in some aspects T, N and L channel currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of antagonistic action of internal Ca2+ on serotonin-induced potentiation of Ca2+ currents in Helix neurones

The influence of internal Ca2+ ions has been investigated during intracellular perfusion of isolated neurones from pedal ganglia of Helix pomatia in which serotonin (5-HT) induces a cyclic-adenosine-monophosphate-(cAMP)-dependent enhancement of high-threshold Ca2+ current (ICa). Internal free Ca2+ ([Ca2+]i) was varied between 0.01 and 10 microM by addition of Ca(2+)-EGTA [ethylenebis(oxonitrilo)tetraacetate] buffer. Elevation of [Ca2+]i depressed the 5-HT effect. The dose/effect curve for the Ca2+ blockade had a biphasic character and could be described by the sum of two Langmuir's isotherms for tetramolecular binding with dissociation constants Kd1 = 0.063 microM and Kd2 = 1 microM. Addition of calmodulin (CM) antagonists (50 microM trifluoperazine or 50 microM chlorpromazine), phosphodiesterase (PDE) antagonists [100 microM isobutylmethylxanthine (IBMX) or 5 mM theophylline] and protein phosphatase antagonists [2 microM okadaic acid (OA)] in the perfusion solution caused "anticalcium" action and modified the Ca2+ binding isotherm. Using the effect of OA and IBMX, two components of the total Ca2+ inhibition were separated and evaluated. In the presence of one of these blockers tetramolecular curves with Kd1 = 0.04 microM and Kd2 = 0.69 microM were obtained describing the activation of the retained unblocked enzyme--PDE or calcineurin (CN) correspondingly. The sum of these isotherms gave a biphasic curve similar to that in control. Leupeptin (100 microM), a blocker of Ca(2+)-dependent proteases did not influence the amplitude of 5-HT effect, indicating that channel proteolysis is not involved in the depression. Our findings show that the molecular mechanism of Ca(2+)-induced suppression of the cAMP-dependent upregulation of Ca2+ channels is due to involvement of two Ca(2+)-CM-dependent enzymes: PDE reducing the cAMP level, and CN causing channel dephosphorylation. No other processes are involved in the investigated phenomenon at a Ca2+ concentration of less than or equal to 10 microM.

A cytoskeletal mechanism for Ca2+ channel metabolic dependence and inactivation by intracellular Ca2+

Many different types of voltage-dependent Ca2+ channels inactivate when intracellular ATP declines or intracellular Ca2+ rises. An inside-out, patch-clamp technique was applied to the Ca2+ channels of Lymnaea neurons to determine the mechanism(s) underlying these two phenomena. Although no evidence was found for a phosphorylation mechanism, agents that act on the cytoskeleton were found to alter Ca2+ channel activity. The cytoskeletal disrupters colchicine and cytochalasin B were found to speed Ca2+ channel decline in ATP, whereas the cytoskeletal stabilizers taxol and phalloidin were found to prolong Ca2+ channel activity without ATP. In addition, cytoskeletal stabilizers reduced Ca(2+)-dependent channel inactivation, suggesting that both channel metabolic dependence and Ca(2+)-dependent inactivation result from a cytoskeletal interaction.

Phencyclidine block of calcium current in isolated guinea-pig hippocampal neurones

1. Phencyclidine (PCP) block of Ca2+ channel current in enzymatically dissociated neurones from the CA1 region of the adult guinea-pig hippocampus was studied using whole-cell voltage clamp techniques. Ca2+ channel current was recorded with 3 mM-Ba2+ as the charge carrier. Na+ currents were blocked with tetrodotoxin and K+ currents were eliminated by using tetraethylammonium and N-methyl-D-glucamine as the predominant extracellular and intracellular cations, respectively. 2. Peak Ca2+ channel current evoked by depolarization from -80 to -10 mV was reduced in a use-dependent fashion by PCP. The apparent forward and reverse rate constants for block at the depolarized voltage were 10(6) s-1 M-1 and 11-14 s-1, respectively. These values were at least 60 times faster than the corresponding rates at the resting voltage. The steady-state block produced by PCP increased in a concentration-dependent fashion with an IC50 of 7 microM. Other dissociative anaesthetic drugs were substantially weaker inhibitors of the current (tiletamine > dizocilpine (MK-801) > ketamine). 3. The Ca2+ channel current recorded under identical conditions in rat dorsal root ganglion neurones was less sensitive to blockade by PCP (IC50, 90 microM). 4. PCP block of the hippocampal Ca2+ channel current occurred in a voltage-dependent fashion with the fractional block decreasing at positive membrane potentials. Analysis indicated that the PCP blocking site senses 56% of the transmembrane electric field. 5. Analysis of tail currents recorded at -80 mV demonstrated that PCP does not affect the voltage-dependent or time-dependent activation or deactivation of the Ca2+ channel current. 6. The rate and extent of inactivation of the Ca2+ channel current was maximal at -10 mV and diminished at more positive potentials. Experiments with Ba(2+)-free external solution demonstrated that inactivation of the Ca2+ channels is largely voltage-dependent and is not affected by Ba2+ influx. 7. PCP markedly increased the apparent extent of inactivation of the Ca2+ channel current during prolonged voltage steps. This increase in apparent inactivation was more pronounced at depolarized potentials. Inactivation at -10 mV proceeded in two exponential phases; PCP had little effect on the fast decay phase and caused a moderate speeding of the slow decay phase. Although block of the activated state evolved on the same time scale as inactivation, the apparent rate of inactivation was not increased in a concentration-dependent fashion by PCP indicating that the block does not occur by a conventional open channel mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium currents in rat motor nerve terminals

Ca2+ currents in response to an action potential were recorded extracellularly under non-voltage clamped conditions from rat motor nerve terminals. The Ca2+ current was blocked by Cd2+, Co2+, and Ni2+. A residual component that could not be blocked by inorganic cations was inhibited completely by tetrodotoxin (TTX). The Ca2+ current was also moderately sensitive to the N- and L-type Ca2+ channel-blocker omega-conotoxin but was insensitive to the L-type channel-specific dihydropyridines. When a fraction of the terminal K+ currents was blocked by 10 mM tetraethylammonium (TEA), the Ca2+ current duration decreased only slightly as stimulation frequency increased from 0.5 to 20 Hz. When K+ currents were blocked by TEA plus 3,4-diaminopyridine (250 microM) though, the Ca2+ current duration decreased from greater than 70 ms to 8-10 ms as stimulation frequency increased from 0.5 to 20 Hz. Recovery of the duration following 20-Hz stimulation occurred faster during subsequent stimulation at 0.5 Hz than at 2 Hz. ATP and ACh inhibit Ca2+ currents at stimulation frequencies ranging from 0.5 to 20 Hz; however, when the purinergic and cholinergic autoreceptors are blocked by theophylline (100 microM) and pirenzepine (3 microM), respectively, the frequency-induced decrease in current duration persisted. Thus, motor nerve terminal Ca2+ current duration is determined by stimulus repetition frequency; this appears to involve intracellular Ca2+ accumulation, although effects secondary to variability in the time course of changes in terminal membrane potentials cannot be ruled out.

Enhancement of calcium currents in rat hippocampal CA1 neurons induced by kindling epileptogenesis

Kindling of the Schaffer collaterals in the dorsal hippocampus of the rat induced an epileptogenic focus in area CA1. Pyramidal neurons were acutely isolated from this area in fully kindled rats one day after the last class five generalized seizure. Calcium currents were measured in these cells under the whole-cell patch voltage-clamp condition after blockade of sodium and potassium currents. Voltage-dependent calcium currents were activated by depolarizing voltage steps from different prepulse potentials. Calcium currents activated at 0 mV consisted of a sustained component and two voltage-dependent inactivating components. Current inactivation was fitted with two exponentials (time-constants of 13 and 72 ms) and a constant. When cells from kindled rats were compared with those from controls, the amplitudes of the slow-inactivating and the sustained component were significantly enhanced by 36% and 39%, respectively; the fast inactivating current showed only a small enhancement. Inactivation kinetics, time-to-peak and voltage dependency of activation and steady-state inactivation were unchanged. Shape and size of the analysed cells from kindled rats were not different from those in controls. We concluded that an increased specific calcium conductance of as yet unknown origin underlies the larger current. The magnitude of the observed changes is such that it will considerably increase calcium influx and consequently raise intracellular calcium concentration during tetanic stimulation and subsequent periods of paroxysmal activity. This increase will modulate calcium-dependent factors that regulate neuronal excitability and may lead to the enhanced excitability found in kindled tissue.

Bay K 8644 potentiates the anxiolytic effect of ethanol

The possible role of voltage-sensitive calcium channels (VSCCs) in the anxiolytic effect of ethanol was investigated using three different doses of ethanol (0.5, 1.0 and 2.0 g/kg) with calcium agonist Bay K 8644 (0.5 mg/kg) and calcium antagonist nifedipine (5 mg/kg) in rats. Ethanol produced an anxiolytic effect in a dose-dependent manner. The Bay K 8644-potentiated anxiolytic effect of ethanol, however, Bay K 8644 did not alter anxiety when used alone. Nifedipine itself showed an anxiolytic effect but did not change the ethanol-induced anxiolytic effect. This finding may lead to the consideration of the neurochemical mechanisms of the anxiolytic effects of ethanol and nifedipine as they vary from each other.

Kindling-induced epilepsy alters calcium currents in granule cells of rat hippocampal slices

Single electrode voltage-clamp recordings were obtained from dentate gyrus granule cells (GCs) in hippocampal slices of control and commissurally kindled rats. Two types of calcium currents, a transient and a sustained current, were studied in control and kindled neurons. The threshold of the transient calcium current was lowered in kindled GCs. The sustained calcium current was absent in kindled neurons but it could be restored by the intracellular administration of the calcium chelator EGTA. Our findings are consistent with the hypothesis that the loss of an intraneuronal calcium binding protein (Calbindin-D28K; CaBP) reduces the intraneuronal calcium buffering capacity in kindled neurons and results in the enhanced calcium-dependent inactivation of sustained calcium currents.

Neurons from neonatal hypertensive rats exhibit abnormal membrane properties in vitro

The in vitro membrane properties of neurons from superior cervical ganglia (SCG) of neonatal spontaneously hypertensive (SH), Wistar-Kyoto (WKY), and Sprague-Dawley (SD) rats were studied with microelectrodes. Neurons were obtained by enzymatic dissociation, plated, irradiated, and studied after 2-5 wk. Most SH neurons showed multiple action potentials in response to an intracellular long-duration depolarizing pulse (multiple firing), whereas most neurons from WKY or SD rats generated only one or two action potentials. Multiple firing was inhibited by low concentrations of cobalt (10(-5) M) but not by tetrodotoxin (TTX) (3 x 10(-6) M). Neither high calcium (5-10 x 10(-3) M) nor the Ca2+(-)channel opener BAY K 8644 (10(-6) M) could induce multiple firing in SD or WKY neurons. However, multiple firing was readily induced by apamin (10(-6) M) or tetraethylammonium chloride (5 x 10(-3) M) (Ca2+(-)activated K+(-)channels blockers), with cobalt and TTX sensitivities similar to native multiple-firing neurons. We conclude that 1) multiple firing is characteristic of neonate SH rats SCG neurons in vitro and depends on regenerative Ca2+ currents; 2) multiple firing in SH neurons results from a lack of activation of a Ca2+(-)activated K+ conductance and not from a lack of internal Ca2+ availability; and 3) multiple firing in SCG neurons mirrors a default in K+ conductance common to all cells in genetically hypertensive individuals.

Spontaneous rhythmic activity in the intermediolateral cell nucleus of the neonate rat thoracolumbar spinal cord in vitro

Intracellular recordings from the intermediolateral cell nucleus of the neonate rat thoracolumbar spinal cord slice preparation revealed a population of neurons which displayed three types of spontaneous rhythmic activity: burst firing, tonic beating and membrane oscillations. Most neurons displayed more than one of these types of activity. Neurons had mean resting potentials of -59 mV and input resistances ranging from 10 to 48 m omega. Spontaneous oscillations which were observed either independently or following hyperpolarization of neurons displaying tonic beating or bursting behaviour had a mean peak amplitude and frequency of approximately 14 mV and 1 Hz respectively. Oscillations were not obviously reversible as they were still apparent at potentials as negative as -120 to -140 mV. This suggests that the oscillations had a site of generation distant to the recording electrode. Neurons displaying tonic beating activity were characterized by low frequency firing activated at the peak of the depolarizing phase of the underlying oscillation and these neurons could be induced to exhibit burst behaviour by membrane depolarization. The frequency of firing in tonic beating neurons ranged from 0.1 to 8.8 Hz. Burst firing was characterized by: bursts of 3-17 action potentials; burst cycle frequency of approximately 1 Hz; an afterdepolarization potential mainly observed at the termination of a burst. Burst firing was abolished by cobalt and membrane hyperpolarization but not by barium, low calcium or tetraethylammonium chloride. The switch from tonic beating to burst firing may, in part, involve activation of a voltage- and calcium-dependent afterdepolarization potential. We conclude that a population of neurons in the lateral horn of the spinal cord are capable of rhythmic activity with underlying spontaneous pacemaker-like oscillations.

Calcium dependence of quantal release triggered by graded depolarization pulses to nerve terminals on crayfish and frog muscle

Quantal transmitter release was measured in small portions of neuromuscular junctions by means of a perfused macro-patch-clamp electrode. Release was elicited by graded current pulses through the recording electrode (excitation blocked by TTX). On increasing the stimulation current from a threshold amplitude, release rose steeply for several orders of magnitude and finally approached a saturation level of about 10 quanta/pulse. Reduction of the Ca concentration in the perfusate of the electrode, Cae, depressed the saturation level of release relatively little and had practically no effect on the threshold current amplitude, as long as the Ca concentration in the superfusion of the bath, Cab, remained high. When Cab was reduced too, the depression of release was more severe. The dependence of release on Cae was determined for a large range of Cae for saturating depolarization pulses. In crayfish, at 0 Cab, in double-logarithmic release-Cae plots the maximum slope was on average 3.9, and this slope dropped to on average 2.1 in 13.5 mM Cab. In frog, at 0 Cab, the respective double-logarithmic slope was 3.5, while in 1.8 mM Cab this slope declined dramatically, the rate of release decreasing on average only by a factor of 3.8 from 10 mM to 0.02 mM Cae. These results are interpreted by the assumption that the resting Ca concentration in the terminal, Cair, has strong influence on the rate of release due to depolarization pulses in low Cae, and that Cab has control on Cair in the terminal.

Activators and inactivators of calcium channels: effects in the central nervous system

The interactions of calcium antagonists or channel activators with the different classes of calcium channel are reviewed with particular emphasis on interactions with neuronal tissue; reasons for the failure of calcium antagonists to inhibit neurotransmitter release under normal circumstances are outlined. Calcium antagonists may be protective in several pathological situations and the possibilities of protection against ischaemic damage in the central nervous system are evaluated.

Inactivation properties of T-type calcium current in canine cardiac Purkinje cells

The kinetic behavior of T-type Ca2+ current (ICa-T) was studied in canine cardiac Purkinje cells using a single suction-pipette whole-cell voltage clamp method. ICa-T was studied without contamination of conventional L-type Ca2+ current (ICa-L). Ca2+, Sr2+, or Ba2+ were used as the charge carrier. During maintained depolarization ICa-T decayed rapidly, and under most conditions the decay showed a voltage-dependent single exponential time course that did not depend on the species of charge carrier. The development of inactivation did not depend on Ca2+, but the time course required more than a single exponential process. Just negative to the threshold voltage for activating ICa-T, inactivation slowly developed and there was a delay in its onset. The time course of recovery from inactivation was dependent on the protocol used to measure it. As the duration of an inactivating voltage step was increased, recovery slowed markedly and there was a delay in its onset. The time course of recovery could be fit as a biexponential. The fast and slow time constants of recovery were relatively constant, however, the relative amplitudes were dependent on the duration of the inactivating voltage step. Recovery was not dependent on Ca2+, and it was slower at a less negative voltage. These results suggest that the T-type Ca2+ channel in cardiac Purkinje cells follows a complex kinetic scheme dependent only on voltage. This behavior can be accounted for by incorporating into a Markovian model several inactivated and closed states.

Identification of ionic currents at presynaptic nerve endings of the lizard

1. Ionic currents associated with the invasion of an action potential into the motor nerve ending of the lizard, Anolis carolinensis, were measured with a focal extracellular electrode at several locations along the nerve ending. 2. These experimentally observed currents could be matched with computer simulations of action potential propagation into the nerve ending. They revealed that while Na+ channels are the major ionic current pathway in the heminode, K+ channels provide the major pathway in the terminal branches and boutons. 3. Calcium current in the presynaptic ending was unmasked by the application of tetraethylammonium (TEA). This current was blocked by: (a) cadmium, (b) omega-conotoxin GVIA and (c) nifedipine, but was unaffected by nickel at concentrations less than or equal to 100 microM. Nifedipine's action became more definitive when the duration of the action potential was greatly extended by pre-treatment with TEA. The effect of Bay K 8644 was inconsistent. 4. Transmitter release, as measured by postsynaptic current, had a pharmacological response profile similar to that of the Ca2+ current, with the exception that transmitter release was increased reliably and reversibly by Bay K 8644. 5. This pharmacological response profile is identical to that of the L type Ca2+ channel identified by Fox, Nowycky & Tsien (1987 alpha) in chick dorsal root ganglion neurones. We saw no evidence for more than a single type of Ca2+ channel in lizard motor nerve endings. 6. A calcium-activated K+ current IK(Ca) was revealed by application of 3,4-diaminopyridine (DAP), a delayed-rectifier K+ channel blocker. This K(Ca) current was blocked by TEA, charybdotoxin and by substitution of cobalt for extracellular calcium.

Intracellular Calcium and Control of Burst Generation in Neurons of Guinea-Pig Neocortex in Vitro

Response properties of neurons in brain slices of guinea pig parietal neocortex were examined following intracellular injection of the Ca2+ chelators, EGTA and BAPTA. Although chelator injection did not cause any consistent change in passive membrane properties, it did induce 81% of neurons encountered at all sub-pial depths to become 'bursters', in that just-threshold depolarizing current pulses triggered all-or-none bursts of 2 - 5 fast action potentials. Transition to 'burstiness' was associated with disappearance of an AHP and appearance of a DAP. Although chelator caused a slight increase in steady-state firing rate, marked accommodation persisted. Extracellular Co2+ or Mn2+ had an effect on steady-state firing rate similar to that of the intracellular chelators; however, exposure to these Ca2+ channel blockers also caused steady state depolarization, increased resting input resistance and time constant, and profound spike broadening. This treatment never induced transition to 'burstiness'. Chelator-injected neurons ceased to generate bursts when Ca2+ was replaced by Mn2+ in the Ringer's solution. During exposure to 10-6 M TTX and 20 mM TEA, 50 - 200 msec Ca2+ spikes followed brief depolarizing pulses. As chelator was injected into the cell, there was progressive prolongation of the Ca2+ plateaus, which was associated with slowing of the rate at which membrane resistance gradually recovered following the initial increase in conductance. These findings indicate that under normal conditions, activity-related increases in intracellular Ca2+ activate processes which prevent most neocortical neurons from being bursters. These processes probably include Ca2+-dependent K+ currents, and Ca2+-dependent Ca2+ channel inactivation.

Decay kinetics of calcium currents in rat sensory neurones: analysis at two internal free calcium concentrations

The patch-clamp technique in whole-cell configuration was used to investigate the kinetics of decay of calcium currents in rat sensory neurones. Whole-cell recording permitted control of the internal medium, particularly of the internal free calcium concentration, which was maintained at either 10(-9) M or 10(-6) M using a high concentration of Ca buffer. The inactivation decay of the total Ca current elicited above -10 mV was found to be faster at pCa 6 than at pCa 9. The total current contained three exponential components which were tentatively identified as the three types of Ca currents (IcaT, IcaN and IcaS). Kinetic analyses indicated that the control of the inactivation process by internal Ca results from an effect on both high-threshold Ca currents, IcaN and IcaS. The inactivation kinetics reported in the literature presents a large variability depending on the cell type. We propose that this variability may result from differences in the capacity of those cells to control their internal Ca.

Calcium current inactivation in insulin-secreting cells is mediated by calcium influx and membrane depolarization

Inactivation of voltage-dependent calcium currents was studied in single, dissociated insulin-secreting HIT cells voltage-clamped by the whole-cell patch-clamp method at room temperature. Na and K currents were suppressed by tetrodotoxin, tetraethylammonium, ATP, 4-aminopyridine and Cs. Ca currents activated in less than 10 ms by depolarizations beyond -50 mV from a holding potential of -100 mV and were identified, as in previous studies, by their sensitivity to divalent cation blockade and permeability to Ba as a charge carrier. Sustained depolarization revealed two kinetically distinct phases of inactivation: a rapid phase inactivated approximately 50% of the current in less than 100 ms while the remaining current was inactivated over the next 10-20 s. Rapid inactivation appeared to be due to Ca2+ influx since it was slowed markedly when Ba2+ was used as the current carrier, while the degree of inactivation increased and decreased with increasing depolarization in direct parallel with the U-shaped current-voltage relationship for inward Ca current. Slow inactivation appeared to be voltage-dependent since current could be inactivated (by approximately 20%) by 10 s long depolarizations to potentials below the threshold for activating Ca current, slow time constants of inactivation were voltage-dependent and slow inactivation persisted when Ca was replaced with Ba. Ca currents with low activation thresholds (in the -50 to -30 mV range) appeared to be preferentially inactivated by the rapid Ca-dependent mechanism. Recovery of slowly inactivated Ca current was very slow and currents inactivated by larger depolarizations required longer recovery time than those elicited by smaller depolarizations. Rapid and slow inactivation mechanisms may be important in understanding the fast spiking and slow plateau depolarizations seen in pancreatic B-cells exposed to stimulatory levels of glucose.

Two high voltage-activated calcium currents are present in isolation in adult rat spinal neurons

In neurons enzymatically isolated from adult rat dorsal root ganglia and used during the following 24 hours, the Ca2+ currents were investigated with the whole-cell patch-clamp technique. In contrast to the neonatal neurons, the salient feature of these adult neurons is the well separated (in the voltage-range) activation and inactivation properties of each recorded current. The low-threshold T-, the high-threshold inactivating N-, and the long-lasting L-currents have a threshold for activation at -60, -45 and -10 mV, and a 50% inactivation at -75, -45 and -5 mV respectively. The N and L currents were poorly affected by 100 microM Ni, a known blocker of T channels and completely blocked by 100 microM Cd2+. Frequently we could find neurons with only one type of current present. We conclude that adult sensory neurons are a better preparation for studying, in isolation, the physiological relevance of the three types of Ca2+ channels.

Diversity of calcium ion channels in cellular membranes

Successful introduction of techniques for separation of different ionic currents and recording of single channel activity has demonstrated the diversity of membrane structures responsible for generation of calcium signal during various forms of cellular activity. In excitable cells the electrically-operated calcium channels have been separated into two types functioning in different membrane potential ranges (low- and high-threshold ones). The low-threshold channels are ontogenetically primary and may play a role in regulation of cell development and differentiation. A similar function may also be characteristic of chemically-operated channels in some highly specialized cells (lymphocytes). The high-threshold channels in excitable cells generate an intracellular signal coupling membrane excitation and intracellular metabolic processes responsible for specific cellular reactions (among them retention of traces of previous activity in neurons--"learning"--being especially important). Chemically-operated N-methyl-D-aspartate-channels also participate in this function. The calcium signal can be potentiated by activation of calcium-operated channels in the membranes of intracellular structures, resulting in the liberation of calcium ions from the intracellular stores. Although different types of calcium channels have some common features in their structure which may indicate their genetic similarity, their specific properties make them well suited for participation in a wide range of cellular mechanisms.

Calcium currents in rat cerebellar Purkinje cells maintained in culture

Calcium permeabilities were examined in large cerebellar neurons maintained in culture, and morphologically identified as Purkinje cells. When cells were supplied with a Dulbecco Minimum Eagle's Medium with 10% horse serum added (5-10 days), somatic recordings revealed complex spikes and these were shown to be generated by Na and Ca components, the Na one being tetrodotoxin-sensitive. At the dendritic level, Ca currents were better resolved than at the soma. In dendrites, Ca entry was shown to occur through at least two distinct currents. The first was a low-threshold transient current (elicited above -60 mV from a holding potential of -80 mV) which was reduced by almost 30% by 50 microM cadmium. The second was a high-threshold current (above -20 mV) which gave rise to (1) a transient component exhibiting a steady-state inactivation and so requiring holding potentials at -80 mV, and (2) a sustained component. Both components were suppressed by 50 microns cadmium. We measured a total Ca current at the dendritic level reaching values of up to 1 nA. In another culture medium (Leibovitz medium) known to allow expression of three types of calcium currents in nodose cells we observed the development of the dendritic tree of Purkinje cells but with no simultaneous expression of the high-threshold Ca current.

Evidence of omega-conotoxin GV1A-sensitive Ca2+ channels in mammalian peripheral nerve terminals

The existence of omega-conotoxin GV1A (omega-CgTx)-sensitive, voltage-sensitive Ca2+ channels (VSCCs) in mammalian peripheral nerves was investigated in the guinea pig ileum myenteric plexus longitudinal smooth muscle preparation (GPI). omega-CgTx (0.01-1.0 microM) reduced the electrically stimulated GPI twitch height, failed to modify exogenously applied acetylcholine (ACh) contractions, and inhibited Ca2+-dependent KCl-stimulated ACh release as measured by chemiluminescence. The 1,4-dihydropyridine VSCC antagonist (-) 202-791 (0.1-1.0 microM) inhibited the GPI twitch height, reduced contractions to exogenous ACh, but failed to affect ACh release. In the rat aorta, a nerve free preparation, omega-CgTx failed to affect contractions to KCl which were inhibited by (-) 202-791 and potentiated by the VSCC agonist (+) 202-791. The results provide evidence of neuronal N type VSCCs in mammalian peripheral cholinergic nerve terminals.

Divalent cation dependent inactivation of the high-voltage-activated Ca-channel current in chick sensory neurons

We have used the whole-cell clamp technique to investigate inactivation of the omega-conotoxin sensitive high-voltage-activated Ca-channel current (HVA current [2]) carried either by Ca, Ba or Sr (2.5 mM) in chick sensory neurons. At a low internal EGTA concentration (0.1 mM), Ca-channel currents clearly inactivated irrespective of the species of divalent cation carrying the current. During 150 ms pulses, current inactivated to 0.57, 0.67 and 0.75 of the peak current in Ca, Ba and Sr solution, respectively. Time constants of inactivation (26 +/- 10 ms and 280 +/- 50 ms, mean +/- S.D., in Ba) were largely independent of the membrane potential. Double-pulse experiments showed that the amount of inactivation left by a pre-pulse was proportional to the amplitude of the current evoked by the pre-pulse. No inactivation was induced by an outward current elicited by a strong depolarization to +60 mV. With an internal EGTA concentration of 20 mM, the amount of inactivation was significantly smaller. In conclusion, the inactivation of the HVA Ca-channel currents during current flow depends mostly on the entry of divalent cations irrespective of their species.

Quantitative evaluation of the properties of a pyridazinyl GABA derivative (SR 95531) as a GABAA competitive antagonist. An electrophysiological approach

We have investigated the effects of an aryl-aminopyridazine derivative of GABA (SR 95531) on dose-response curves of GABA-induced depolarizations from dorsal root ganglion neurones recorded intracellularly. The reversible shift to the right of the dose-response curves in a parallel fashion and the dissociation constant (KB) value of 0.13 +/- 0.02 microM (n = 15) indicate that this compound is a potent competitive GABAA antagonist. The competitive nature of SR 95531-induced antagonism was confirmed by single channel analysis. In excised membrane patches from bovine chromaffin cells (outside out configuration), 0.2-0.5 microM SR 95531 did not alter the mean open time of GABA-activated channels and did not introduce further short closing gaps within bursts. Whole cell recordings from cultured nodose ganglion neurones indicated that SR 95531 (10 microM) did not modify significantly any of the 3 types of calcium currents already reported in sensory neurones. This result might be of importance for further studies of presynaptic GABA actions on transmitter release.

Voltage-sensitive calcium channels in normal and transformed 3T3 fibroblasts

Patch clamp recordings of whole-cell and single channel currents revealed the presence of two voltage-sensitive calcium channel types in the membrane of 3T3 fibroblasts. The two calcium channel types were identified by their unitary properties and pharmacological sensitivities. Both calcium channel types were present in all control 3T3 cells, but one type was selectively suppressed in 3T3 cells that had been transformed by activated c-H-ras, EJ-ras, v-fms, or polyoma middle T oncogenes. The presence of voltage-sensitive calcium channels in these nonexcitable cells and the control of their functional expression by transforming oncogenes raises questions about their role in the control of calcium-sensitive processes such as cell motility, cytoskeletal organization, and cell growth.

Regulation of calcium current by intracellular calcium in smooth muscle cells of rabbit portal vein

Effects of concentrations of intracellular calcium, [Ca2+]i, on the voltage-dependent Ca2+ current (ICa) recorded from dispersed single smooth muscle cells of the rabbit portal vein were studied, using a whole cell voltage clamp method combined with an intracellular perfusion technique. Outward currents were minimized by replacement of Cs+ -rich solution in the pipette and 20 mM tetraethylammonium in the bath. The ICa was evoked by command pulses of above -30 mV, and the maximum amplitude was obtained at about 0 mV. This ICa was dose dependently inhibited by increases in the [Ca2+]i above 30 nM. The Kd value of the [Ca2+]i required to inhibit the ICa was about 100 nM. The Ba2+ current was also inhibited by increases in the [Ca2+]i. Conversely, perfusion of Ba2+ into the cell up to 100 microM did not suppress the ICa. Changes in the [Ca2+]i did not modify the steady-state inactivation curve. The inhibition of the ICa evoked by the test pulse is most prominent when the preceding influx of Ca2+ during the conditioning pulse was large, as estimated using a double pulse protocol. This inhibition was proportionally reduced by increases in the concentration of the Ca2+ chelator, ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). Therefore, the Ca2+ -dependent inactivation of the Ca2+ channel may contribute toward regulating [Ca2+]i in smooth muscle cells of the rabbit portal vein.

Kinetic and pharmacological properties distinguishing three types of calcium currents in chick sensory neurones

1. Calcium currents in cultured dorsal root ganglion (d.r.g.) cells were studied with the whole-cell patch-clamp technique. Using experimental conditions that suppressed Na+ and K+ currents, and 3-10 mM-external Ca2+ or Ba2+, we distinguished three distinct types of calcium currents (L, T and N) on the basis of voltage-dependent kinetics and pharmacology. 2. Component L activates at relatively positive test potentials (t.p. greater than -10 mV) and shows little inactivation during a 200 ms depolarization. It is completely reprimed at a holding potential (h.p.) of -60 mV, and can be isolated by using a more depolarized h.p. (-40 mV) to inactivate the other two types of calcium currents. 3. Component T can be seen in isolation with weak test pulses. It begins activating at potentials more positive than -70 mV and inactivates quickly and completely during a maintained depolarization (time constant, tau approximately 20-50 ms). The current amplitude and the rate of decay increase with stronger depolarizations until both reach a maximum at approximately -40 mV. Inactivation is complete at h.p. greater than -60 mV and is progressively removed between -60 and -95 mV. 4. Component N activates at relatively strong depolarizations (t.p. greater than -20 mV) and decays with time constants ranging from 50 to 110 ms. Inactivation is removed over a very broad range of holding potentials (h.p. between -40 and -110 mV). 5. With 10 mM-EGTA in the pipette solution, substitution of Ba2+ for Ca2+ as the charge carrier does not alter the rates of activation or relaxation of any component. However, T-type channels are approximately equally permeable to Ca2+ and Ba2+, while L-type and N-type channels are both much more permeable to Ba2+. 6. Component N cannot be explained by current-dependent inactivation of L current resulting from recruitment of extra L-type channels at negative holding potentials: raising the external Ba2+ concentration to 110 mM greatly increases the amplitude of L current evoked from h.p. = -30 mV but produces little inactivation. 7. Cadmium ions (20-50 microM) virtually eliminate both N and L currents (greater than 90% block) but leave T relatively unaffected (less than 50% block). 200 microM-Cd2+ blocks all three components. 8. Nickel ions (100 microM) strongly reduce T current but leave N and L current little changed. 9. The dihydropyridine antagonist nifedipine (10 microM) inhibits L current (approximately 60% block) at a holding potential that inactivates half the L-type channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Single-channel recordings of three types of calcium channels in chick sensory neurones

1. T-, and L-type Ca2+ channels were studied in cell-attached patch recordings from the cell bodies of chick dorsal root ganglion neurones. All experiments were performed with isotonic BaCl2 (110 mM) in the recording pipette and with isotonic potassium aspartate in the bathing solution to zero the cell membrane potential. 2. L-type channels are distinguished by a unitary slope conductance of 25 pS, activation over the range of membrane potentials between 0 and +40 mV, little inactivation over the course of a 136 ms depolarization, and availability for opening even at depolarized holding potentials (h.p. greater than -40 mV). L channels show a predominant mode of gating (mode 1) characterized by brief openings (approximately 1 ms), occasionally interspersed with another pattern of gating characterized by much longer openings (mode 2). 3. The dihydropyridine (DHP) Ca2+ agonist Bay K 8644 promotes mode 2 activity and shifts the voltage dependence of L-type channel activation towards more negative potentials. It leaves the unitary current-voltage relation unchanged. 4. Nifedipine, a DHP Ca2+ antagonist, strongly inhibits L-type channel activity through an increase in the proportion of blank sweeps. 5. T-type Ca2+ channels are distinguished by a much smaller unitary slope conductance (8 pS) and by activation and inactivation over relatively negative ranges of potential. Inactivation is complete by the end of 136 ms pulses to test potentials beyond -20 mV. 6. N-type Ca2+ channels are distinguished by an intermediate unitary slope conductance (13 pS), and by activation over a range of potentials between those of T- and L-type channels. Inactivation of N-type channels takes place over an exceptionally broad range of holding potentials (-80 to -20 mV). 7. Cell-attached patch data on the voltage dependence of activation and inactivation of T- and N-type channels are in excellent agreement with results from whole-cell recordings (Fox, Nowycky & Tsien, 1987) if allowances are made for variations in external surface potential. 8. Patches containing one or two channels of a single type were used for analysis of gating kinetics. The predominant pattern of activity for each of the channel types is an exponential distribution of relatively brief (approximately 1 ms) openings, and a bi-exponential distribution of short and long closings. 9. Patches containing all possible combinations of channel types were observed. However, preliminary evidence suggests that channels are distributed unevenly over the cell body; clustering of N-type channels is particularly prominent.(ABSTRACT TRUNCATED AT 400 WORDS)

Opposing effects of noradrenaline on the two classes of voltage-dependent calcium channels of single vascular smooth muscle cells in short-term primary culture

The effects of noradrenaline (NA) were investigated on both fast and slow Ca2+ currents in isolated vascular cells from rat portal vein in short-term primary culture using the whole-cell patch-clamp technique. NA (1 microM) stimulated the fast Ca2+ current when the slow Ca2+ current was inhibited either by application of a dihydropyridine derivative or by increasing the cytoplasmic Ca2+ concentration. Noradrenaline and caffeine greatly reduced the slow Ca2+ current. As this inhibitory effect is suppressed after depletion of the intracellular Ca2+ store, we propose that reduction of the slow Ca2+ current by noradrenaline is mediated by a Ca2+ release from the sarcoplasmic reticulum.

Calcium channel currents and their inhibition by (-)-baclofen in rat sensory neurones: modulation by guanine nucleotides

1. The effect of intracellular application of the hydrolysis-resistant GTP and GDP analogues, guanosine 5'-O-3-thiotriphosphate (GTP-gamma-S), and guanosine 5'-O-2-thiodiphosphate (GDP-beta-S) has been examined on voltage-activated calcium-channel currents and the ability of the gamma-aminobutyric acid B agonist baclofen to inhibit them, in cultured rat dorsal root ganglion (d.r.g.) neurones. 2. Under control conditions, the calcium-channel current, recorded using the whole-cell patch technique with Ba2+ rather than Ca2+ as the permeant divalent cation, consists of an inactivating and a sustained current. In the presence of 500 microM-GTP-gamma-S included in the patch pipette, the calcium-channel current was activated more slowly and was largely non-inactivating during the 100 ms depolarization voltage step. The effects of GTP-gamma-S were abolished by pre-treatment of cells with pertussis toxin. 3. The calcium-channel current recorded in the presence of 500 microM-GDP-beta-S had a more marked transient component than the control calcium-channel current. The proportion of transient calcium-channel current in the presence of GDP-beta-S was not reduced in Na+-free medium. 4. No statistically significant effects of GTP-gamma-S and GDP-beta-S were observed on the calcium-activated potassium current IK(Ca), the transient outward potassium current activated in Ca2+-free medium, or on the inwardly rectifying current (Ih) activated by hyperpolarization. 5. GTP-gamma-S increased the ability of baclofen to inhibit calcium-channel currents, whereas this was decreased by GDP-beta-S and by pre-treatment of cells with pertussis toxin. The half-maximal effective dose (EC50) for baclofen was 2 microM in the presence of GTP-gamma-S, 15 microM for control and 50 microM in the presence of GDP-beta-S. Comparable results were obtained using a single concentration of the adenosine agonist 2-chloroadenosine (2-CA, 0.05 microM) to inhibit calcium-channel currents; its effect was significantly increased by GTP-gamma-S and reduced by GDP-beta-S. 6. The ability of baclofen to inhibit calcium-channel currents was not affected by 1 microM-forskolin or 50 microM-intracellular cyclic AMP. 7. It is concluded that calcium channels in d.r.g.s are associated with a nucleotide binding protein, and that this mediates the effect of baclofen and 2-CA on calcium-channel currents. The ability of GTP-gamma-S to inhibit the transient component of calcium-channel currents in the absence of agonist may represent a means of differentially regulating calcium-channel activity.

Intracellular effectors and modulators of GABA-A and GABA-B receptors: a commentary

The inhibitory neurotransmitter GABA activates two receptor subtypes that can be distinguished by their pharmacology. The GABA-A site is competitively antagonized by bicuculline and exclusively coupled to a chloride channel. The GABA-B receptor, for which baclofen is the only specific agonist, is resistant to bicuculline inhibition and, depending upon its localization, will activate K currents and/or inhibit Ca currents. Both electrophysiological and biochemical approaches have been applied to the study of each receptor. The membrane and intracellular components that to date have been implicated in GABA-B activation are discussed: G proteins, adenylate cyclase and intracellular calcium levels. This latter factor is also discussed with respect to GABA-A receptor action.

Characterization of calcium and sodium currents in porcine pars intermedia cells

The whole-cell configuration of the patch-clamp technique was applied to porcine pars intermedia cells. A tetrodotoxin-sensitive sodium current was recorded. Three types of calcium current were observed. Depolarizations from a holding potential of 100 mV elicited a transient current (ICaT), whereas depolarizations from a holding potential of -40 mV evoked a sustained current (ICaS). A third current (ICaN; N for neither) was activated by strong depolarization to +10 or +20 mV from holding potentials of - 100 mV. Increasing internal Ca2+ significantly reduced the amplitude of ICaS.