Ca2+ channel currents in rat sensory neurones: interaction between guanine nucleotides, cyclic AMP and Ca2+ channel ligands

Nitrergic modulation of ion channel function in regulating neuronal excitability

Nitric oxide (NO) signaling in the brain provides a wide range of functional properties in response to neuronal activity. NO exerts its effects through different signaling pathways, namely, through the canonical soluble guanylyl cyclase-mediated cGMP production route and via post-translational protein modifications. The latter pathways comprise cysteine S-nitrosylation and 3-nitrotyrosination of distinct tyrosine residues. Many ion channels are targeted by one or more of these signaling routes, which leads to their functional regulation under physiological conditions or facilities their dysfunction leading to channelopathies in many pathologies. The resulting alterations in ion channel function changes neuronal excitability, synaptic transmission, and action potential propagation. Transient and activity-dependent NO production mediates reversible ion channel modifications via cGMP and S-nitrosylation signaling, whereas more pronounced and longer-term NO production during conditions of elevated oxidative stress leads to increasingly cumulative and irreversible protein 3-nitrotyrosination. The complexity of this regulation and vast variety of target ion channels and their associated functional alterations presents a challenging task in assessing and understanding the role of NO signaling in physiology and disease.

Depolarization induces nociceptor sensitization by CaV1.2-mediated PKA-II activation

Depolarization drives neuronal plasticity. However, whether depolarization drives sensitization of peripheral nociceptive neurons remains elusive. By high-content screening (HCS) microscopy, we revealed that depolarization of cultured sensory neurons rapidly activates protein kinase A type II (PKA-II) in nociceptors by calcium influx through Ca_V1.2 channels. This effect was modulated by calpains but insensitive to inhibitors of cAMP formation, including opioids. In turn, PKA-II phosphorylated Ser1928 in the distal C terminus of Ca_V1.2, thereby increasing channel gating, whereas dephosphorylation of Ser1928 involved the phosphatase calcineurin. Patch-clamp and behavioral experiments confirmed that depolarization leads to calcium- and PKA-dependent sensitization of calcium currents ex vivo and local peripheral hyperalgesia in the skin in vivo. Our data suggest a local activity-driven feed-forward mechanism that selectively translates strong depolarization into further activity and thereby facilitates hypersensitivity of nociceptor terminals by a mechanism inaccessible to opioids.

Evidence for regulatory diversity and auto-regulation at the TAC1 locus in sensory neurones

The neuropeptide substance-P (SP) is expressed from the TAC1 gene in sensory neurones where it acts as a key modulator of neurogenic inflammation. The promoter of TAC1 (TAC1prom) plays a central role in the regulation of the TAC1 gene but requires the presence of a second regulatory element; ECR2, to support TAC1 expression in sensory neurones and to respond appropriately to signalling pathways such as MAPkinases and noxious induction by capsaicin. We examined whether the effect of capsaicin on ECR2-TAC1prom activity in larger diameter neurones was cell autonomous or non- cell autonomous. We demonstrate that TRPV1 is not expressed in all the same cells as SP following capsaicin induction suggesting the presence of a non-cell autonomous mechanism for TAC1 up-regulation following capsaicin induction. In addition, we demonstrate that induction of SP and ECR1-TAC1prom activity in these larger diameter neurones can be induced by potassium depolarisation suggesting that, in addition to capsaicin induction, transgene activity may be modulated by voltage gated calcium channels. Furthermore, we show that NK1 is expressed in all SP- expressing cells after capsaicin induction and that an agonist of NK1 can activate both SP and the transgene in larger diameter neurones. These observations suggest the presence of an autocrine loop that controls the expression of the TAC1 promoter in sensory neurones. In contrast, induction of the TAC1 promoter by LPS was not dependent on ECR2 and did not occur in large diameter neurones. These studies demonstrate the diversity of mechanisms modulating the activity of the TAC1 promoter and provide novel directions for the development of new anti-inflammatory therapies.

Ca2+ transient evoked by chemical stimulation is enhanced by PGE2 in vagal sensory neurons: role of cAMP/PKA signaling pathway

The effect of prostaglandin E(2) (PGE(2)) on chemical stimulation-evoked calcium (Ca(2+)) transient was investigated in isolated vagal sensory neurons of the rat using fura-2-based ratiometric Ca(2+) imaging. Application of capsaicin (3 x 10(-8) to 10(-7) M; 15 s) caused a rapid surge of intracellular Ca(2+) concentration in small- and medium-size neurons; the response was reproducible when >10 min elapsed between two challenges and was absent in nominally Ca(2+)-free solution. After pretreatment with PGE(2) (3 x 10(-7) M; 5 min), the peak of this capsaicin-evoked Ca(2+) transient was increased by almost fourfold, and its duration was also prolonged. This augmented response to capsaicin induced by PGE(2) gradually declined but remained higher than control after 15-min washout. Similarly, PGE(2) pretreatment also markedly enhanced the Ca(2+) transients induced by other chemical stimulants to C neurons, such as phenylbiguanide (PBG), adenosine 5'-triphosphate (ATP), and KCl. The Ca(2+) transients evoked by PBG, ATP, and KCl were potentiated after the pretreatment with PGE(2) to 242, 204, and 163% of their control, respectively. This potentiating effect of PGE(2) could be mimicked by forskolin (10(-6) M; 5 min), an activator of adenylyl cyclase, and 8-(4-chlorophenylthio)adenosine-3'-5'-cyclic monophosphate (CPT-cAMP; 3 x 10(-6) M, 10 min), a membrane-permeable cAMP analogue. Furthermore, the potentiating effects of PGE(2), forskolin, and CPT-cAMP were abolished by N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H89; 10(-5) M; 15-20 min), a protein kinase A (PKA) inhibitor. In summary, these results show that PGE(2) reversibly potentiates the chemical stimuli-evoked Ca(2+) transients in cultured rat vagal sensory neurons, and this potentiating effect is mediated through the cyclic AMP/PKA transduction cascade.

Modulation of cardiac Ca(V)1.2 channels by dihydropyridine and phosphatase inhibitor requires Ser-1142 in the domain III pore loop

Dihydropyridine-sensitive, voltage-activated calcium channels respond to membrane depolarization with two distinct modes of activity: short bursts of very short openings (mode 1) or repetitive openings of much longer duration (mode 2). Here we show that both the dihydropyridine, BayK8644 (BayK), and the inhibitor of SerThr protein phosphatases, okadaic acid, have identical effects on the gating of the recombinant cardiac calcium channel, Ca(V)1.2 (alpha(1)C). Each produced identical mode 2 gating in cell-attached patches, and each prevented rundown of channel activity when the membrane patch was excised into ATP-free solutions. These effects required Ser or Thr at position 1142 in the domain III pore loop between transmembrane segments S5 and S6, where dihydropyridines bind to the channel. Mutation of Ser-1142 to Ala or Cys produced channels with very low activity that could not be modulated by either BayK or okadaic acid. A molecular model of Ca(V)1.2 indicates that Ser-1142 is unlikely to be phosphorylated, and thus we conclude that BayK binding stabilizes mode 2 gating allosterically by either protecting a phospho Ser/Thr on the alpha(1)C subunit or mimicking phosphorylation at that site.

Prostaglandin E2-induced sensitization of bradykinin-evoked responses in rat dorsal root ganglion neurons is mediated by cAMP-dependent protein kinase A

Primary cultures of neonatal rat dorsal root ganglion (DRG) neurons were used to examine the mechanisms underlying both the direct activation and the sensitization of sensory neurons by prostanoids. Prostaglandin E2 (PGE2) elevated cytosolic calcium concentration ([Ca2+]i) in a subpopulation of small (< 19 microm) diameter, capsaicin-sensitive DRG neurons. PGE2 also stimulated substance P (SP) release from DRG cultures. In contrast to bradykinin, PGE2 did not stimulate phosphoinositidase C (PIC) and the PGE2-evoked increase in [Ca2+]i was dependent on extracellular calcium. Pre-treatment with PGE2 potentiated bradykinin-evoked increases in [Ca2+]i in small diameter neurons and increased the number of cells that responded to low concentrations of bradykinin. A similar effect was seen with prostaglandin I2 (PGI2) but not prostaglandin F2alpha (PGF2alpha). PGE2 pretreatment also potentiated bradykinin-evoked release of SP, inducing a leftward shift in the bradykinin concentration-response curve and an increase in the maximum response. PGE2 stimulated adenylyl cyclase activity in DRG cultures, at concentrations and times consistent with those required to observe both the direct and sensitizing effects of the prostanoid on [Ca2+]i responses. Furthermore, the direct and sensitizing effects of PGE2, on both [Ca2+]i responses and SP release, were mimicked by the membrane permeant cAMP analogue dibutyryl cAMP and inhibited by H89, an inhibitor of cAMP-dependent protein kinase A (PKA). These observations are consistent with the hypothesis that both direct activation and sensitization of sensory neurons by prostanoids, such as PGE2, are mediated by PKA-dependent phosphorylation mechanisms.

Ion channel modulation as the basis for neuroprotective action of MS-153

MS-153, (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline, is a new neuroprotective drug. Recent data in the literature suggest that it inhibits glutamate accumulation occurring during ischemia and the translocation of protein kinase C gamma (PKC gamma). The present study was undertaken to prove the hypothesis that MS-153 blocks neuroreceptors and ion channels involved in glutamate accumulation. Neurons isolated from rat dorsal root ganglia and frontal cortex were used for recording channel currents by the whole-cell patch clamp technique. The effects of bath-applied MS-153 were examined on tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels and high voltage-gated calcium channels of dorsal root ganglion neurons, and channels activated by glutamate, N-methyl-D-aspartate (NMDA), kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxarole propionic acid (AMPA), gamma-aminobutyric acid (GABA) and acetylcholine (ACh) in cortical neurons. MS-153 at a concentration of 300 microM had no effect on either tetrodotoxin-sensitive or tetrodotoxin-resistant sodium channels. High voltage-gated calcium channels were either suppressed or not affected by 1-300 microM MS-153. The variable blocking effect of MS-153 was due to the variable activity of intracellular components in individual neurons, especially that of PKC, whose translocation is known to be inhibited by MS-153. When 100 nM phorbol 12-myristate-13-acetate (PMA) was applied to neurons, MS-153 suppressed the calcium channel current more frequently. Calphostin C (0.5 microM), a specific PKC inhibitor, applied intracellularly via recording patch pipette, completely abolished MS-153 suppression of the calcium channel current. Currents induced by glutamate, NMDA, kainate, AMPA, GABA or ACh were not affected by MS-153 at 300 microM. It was concluded that MS-153 inhibited high voltage-gated calcium channels through interactions with PKC, thereby preventing massive release of glutamate from nerve terminals in ischemic conditions.

G-proteins are involved in riluzole inhibition of high voltage-activated calcium channels in rat dorsal root ganglion neurons

Effects of riluzole on high voltage-activated (HVA) calcium channels of rat dorsal root ganglion neurons were studied using the whole-cell patch-clamp technique. Riluzole at 30 microM inhibited the HVA currents. The onset and offset of riluzole inhibitory effect were slow usually taking more than 3 min. Riluzole inhibition of the HVA currents was abolished and partially reduced by addition of 500 microM GDP-beta-S and 1 mM N-ethylmaleimide, respectively, to the pipette solution. Pre-treatment with pertussis toxin or application of depolarizing pre-pulses did not affect riluzole's inhibitory effect on the HVA currents. Riluzole inhibition of the HVA currents was also blocked by internal application of 50 microg/ml protein kinase A inhibitory peptide. It was concluded that pertussis toxin-insensitive G-proteins and protein kinase A may be involved in riluzole inhibition of the HVA currents.

Facilitation of Ca2+ current in excitable cells

Voltage-dependent Ca2+ channels are one of the main routes for the entry of Ca2+ into excitable cells. These channels are unique in cell-signalling terms in that they can transduce an electrical signal (membrane depolarization) via Ca2+ entry into a chemical signal, by virtue of the diverse range of intracellular Ca(2+)-dependent enzymes and processes. In a variety of cell types, currents through voltage-dependent Ca2+ channels can be increased in amplitude by a number of means. Although the term facilitation was originally defined as an increase of Ca2+ current resulting from one or a train of prepulses to depolarizing voltages, there is a great deal of overlap between facilitation by this means and enhancement by other routes, such as phosphorylation.

Modulation of calcium currents by G-proteins and adenosine receptors in myenteric neurones cultured from adult guinea-pig small intestine

1. Whole-cell patch clamp methods were used to analyse voltage-dependent calcium currents in cultured myenteric neurones enzymatically isolated from adult guinea-pig small intestine. 2. Activation of G-proteins by intracellular administration of GTP-gamma-S (100-200 microM in pipette) decreased the amplitude of high voltage activated Ca2+ current (ICa) by more than 50%. Residual ICa was activated more slowly and was non-inactivating during 500 ms test pulses when GTP-gamma-S was included in the pipette solution. 3. Inclusion of 500 microM GDP-beta-S in the patch pipettes increased the amplitude of ICa by over 30% without altering the voltage-dependency. 4. Extracellular application of 2-chloroadenosine suppressed ICa dose-dependently by reducing both transient and sustained components of the current. 5. Pretreatment of the neurones with cholera toxin or forskolin did not alter the actions of GTP-gamma-S or GDP-beta-S or 2-chloroadenosine. 6. The results suggest that high threshold calcium channels in myenteric neurones are influenced by G-proteins and that the inhibitory action of 2-chloroadenosine on ICa involves G-protein coupling of the adenosine receptors to the Ca2+ channel.

Voltage-dependent potentiation of neuronal L-type calcium channels due to state-dependent phosphorylation

Modulation of Ca2+ channels during repetitive activity in excitable cells can have an important role in altering cellular function. In mammalian parasympathetic and dorsal root ganglion neurons, L-type Ca2+ channels are potentiated by single depolarizing prepulses or trains of short high-frequency depolarizing pulses. This type of potentiation takes place regardless of whether Ca2+ or Ba2+ is the charge carrier and requires phosphorylation by a adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase. The magnitude of facilitation was correlated with frequency of conditioning trains, was enhanced by 8-bromoadenosine 3',5'-cyclic monophosphate or the Sp diastereomer of adenosine 3',5'-cyclic monophosphothioate (cAMPS), and reduced by Rp-cAMPS or a peptide inhibitor of cAMP-dependent protein kinase. The N-type Ca2+ channels exhibited the opposite response to these agents. We propose that the potentiation of L-type Ca2+ channel currents in neurons is due to state-dependent phosphorylation by cAMP-dependent protein kinase (Sculptoreanu, A., T. Scheuer, and W. A. Catterall. Nature Lond. 364: 240-243, 1993; Sculptoreanu, A., E. Rotman, M. Takahashi, T. Scheuer, and W. A. Catterall. Proc. Natl. Acad. Sci. USA 90: 10135-10139, 1993.). Thus state-dependent phosphorylation in neurons may be a mechanism for the regulation of various functions including transmitter release.

Multiple second messenger routes enhance two high-voltage-activated calcium currents in molluscan neuroendocrine cells

Two types of high-voltage-activated calcium currents were identified in whole-cell voltage-clamp recordings of the neuroendocrine caudodorsal cells, which control egg-laying in the freshwater snail Lymnaea stagnalis. The currents were: (i) a rapidly inactivating high-voltage-activated current, with an activation threshold of -40 mV and maximal amplitude at +10 mV; and (ii) a slowly inactivating high-voltage-activated current, with a threshold of -10 mV and a peak at +30 mV. Both currents were reduced by nifedipine and verapamil, but not by omega-conotoxin GVIA, suggesting that they belong to the L-type family of calcium currents. The voltage-dependence of inactivation of the rapidly inactivating high-voltage-activated current was bell-shaped. Time-constants of inactivation ranged from 10 to 25 ms. Steady-state inactivation was characterized by a potential of half maximal inactivation of -21.7 +/- 3.4 mV and a slope factor of 8.1 +/- 1.7 mV. The voltage-dependence of inactivation of the slowly inactivating high-voltage-activated current was S-shaped. Time-constants of inactivation increased with depolarization up to a maximum of 300 ms. The steady-state inactivation parameters were a potential of half maximal inactivation of +6.8 +/- 2.2 mV and a slope factor of 6.0 +/- 1.1 mV. The membrane-permeable analog of cAMP, 8-chlorophenylthio-cyclic AMP, predominantly increased the slowly inactivating high-voltage-activated current, and shifted its voltage-dependence of activation and inactivation 10 mV to the left. The rapidly inactivating high-voltage-activated current was slightly increased by 8-chlorophenylthio-cyclic AMP. 8-Bromo-cyclic GMP and the phorbol ester, 12-O-tetradecanoyl-13-phorbol acetate, had qualitatively similar effects. Both agents enhanced the rapidly inactivating current and, to a lesser degree, the slowly inactivating current, without affecting their voltage-dependence. The cyclic AMP-dependent protein kinase inhibitor, Walsh inhibitor peptide, antagonized the stimulating effect of 8-chlorophenylthio-cyclic AMP. The broad-spectrum protein kinase inhibitor 1-(5-isoquino-linylsulfonyl)-2-methyl-piperazine (H-7) strongly attenuated the effects of 8-chlorophenylthio-cyclic AMP, 8-bromo-cyclic GMP and 12-O-tetradecanoyl-13-phorbol acetate, suggesting that all treatments increase both types of high-voltage-activated calcium currents through phosphorylation of the channel-complex.

Age-associated changes in beta-adrenergic modulation on rat cardiac excitation-contraction coupling

Previous studies have demonstrated that the ability of beta-adrenergic receptor (beta AR) stimulation to increase cardiac contractility declines with aging. In the present study, the control mechanisms of excitation-contraction (EC) coupling, including calcium current (ICa), cytosolic Ca2+ (Cai2+) transient and contraction in response to beta AR stimulation were investigated in ventricular myocytes isolated from rat hearts of a broad age range (2, 6-8, and 24 mo). While the baseline contractile performance and the Cai2+ transient did not differ markedly among cells from hearts of all age groups, the responses of the Cai2+ transient and contraction to beta-adrenergic stimulation by norepinephrine (NE) diminished with aging: the threshold concentration and the ED50 increased in rank order with aging; the maximum responses of contraction and Cai2+ transient decreased with aging. Furthermore, the efficacy of beta AR stimulation to increase ICa was significantly reduced with aging, and the diminished responses of the contraction and Cai2+ transient amplitudes to NE were proportional to the reductions in the ICa response. These findings suggest that the observed age-associated reduction in beta AR modulation of the cardiac contraction is, in part at least, due to a deficit in modulation of Cai2+, particularly the activity of L-type calcium channels.

Dual modulation of the L-type calcium current of rat osteoblastic cells by parathyroid hormone: opposite effects of protein kinase C and cyclic nucleotides

Using whole-cell voltage-clamp recording of rat osteoblastic cells, we show that PTH-(1-34), known to stimulate protein kinase C (PKC) and adenylate cyclase, has a dual effect on the L-type calcium current. It induces a long-lasting increase and a superimposed reversible decrease, which can be separated by repeating hormone applications. The stimulatory effect is the only effect induced by the (3-34) fragment, able to stimulate PKC but unable to stimulate adenylate cyclase. The L current is stimulated by an active phorbol ester and is reduced by permeable analogues of cyclic AMP. Thus, the effect of PTH-(1-34) can be explained by the opposite effects of PKC and cyclic AMP. Dibutyryl cyclic GMP reduces the L current even more potently than dibutyryl cyclic AMP. The above modulations are all voltage-insensitive. These results led us to reinvestigate the effects of some vitamin D3 metabolites known to stimulate PKC and/or guanylate cyclase, and previously reported to affect the voltage-sensitivity of the L current. We only detected voltage-insensitive effects.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Potentiating and depressant effects of metabotropic glutamate receptor agonists on high-voltage-activated calcium currents in cultured retinal ganglion neurons from postnatal mice

This study was aimed at clarifying the role of metabotropic glutamate receptors (mGluRs) in the regulation of intracellular Ca2+ concentration ([Ca2+]i in postnatal mouse retinal ganglion neurons (RGNs). RGNs were maintained for 1-2 weeks in vitro by adding brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF) to the culture medium. In order to select these cells for electrophysiological measurements, RGNs were vitally labelled with an antibody against Thy-1.2. Voltage-activated Ca2+ currents [ICa(V)] were recorded with patch electrodes in the whole-cell configuration. It was found that racemic +/--1-amino-cyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) or its active enantiomer 1S,3R-ACPD rapidly and reversibly either enhanced or depressed ICa(V). Quisqualate (QA), L-2-amino-4-phosphonobutyrate (L-AP4) and the endogenous transmitter glutamate induced similar effects when ionotropic glutamate receptors were blocked with D-2-amino-5-phosphonovalerate (D-APV) and 6,7-dinitroquinoxaline-2,3-dione (DNQX). omega-Conotoxin GVIA (omega-CgTx GVIA), but not nifedipine prevented modulation of ICa(V) by mGluR agonists. The depression of ICa(V) by t-ACPD was irreversible when cells were dialysed with guanosine-5'-O-(3-thiotriphosphate) (GTP[gamma-S]). Ratio measurements of fura-2 fluorescence in Thy-1+ cells showed that neither t-ACPD, QA nor L-AP4 affected [Ca2+]i by liberation of Ca2+ from intracellular stores. Our results suggest that cultured RGNs express mGluRs. These receptors cannot induce Ca2+ release from intracellular stores but regulate [Ca2+]i by a fast and reversible, G-protein-mediated action on a subpopulation of voltage-activated Ca2+ channels.

Epileptic focus induced by intrahippocampal cholera toxin in rat: time course and properties in vivo and in vitro

A small dose (0.5-1.0 micrograms) of cholera toxin injected into rat hippocampus induced an epileptic focus which discharged intermittently for 7-10 days. Epileptic discharges lasting from 70 ms to 2 min were recorded in vivo through implanted electrodes. The longer bursts could generalize to the neocortex, and occasionally caused motor seizures. The epileptic bursts reached a maximum 3-4 days after injection, and then declined to occasional brief interictal discharges by 9 days. Postmortem histology revealed no evidence of gross pathology or neuronal loss. Hippocampal slices prepared from rats < 8 days after injection of cholera toxin, and maintained in vitro, generated brief spontaneous and evoked epileptic bursts, usually lasting < 1 s. Spontaneous bursts always started in subregion CA3c, and propagated through the pyramidal layer at a mean of 0.18 m/s. Intracellular recordings from CA3 pyramidal layer cells always revealed simultaneous paroxysmal depolarization shifts during epileptic bursts. Epileptic activity, both in vivo and in vitro, required the whole toxin molecule. Injections of either the B subunit or the vehicle solution were not epileptogenic. Therefore binding of the toxin to neuronal membranes, which is mediated by the B subunit, was not sufficient for the epileptogenic effects of cholera toxin. This suggested that the activation of Gs which requires the whole molecule, was necessary. Gs activation is known to stimulate cyclic AMP production, but forskolin, which directly stimulates adenyl cyclase, failed to produce epileptic activity, even though it depressed action potential accommodation and afterhyperpolarizations (AHPs). While further work is required to resolve the basic mechanisms of cholera toxin induced epileptic foci, we propose that they require the activation of Gs, which can enhance Ca2+ currents and modify excitatory synaptic transmission directly. Cyclic AMP induced changes in these properties cannot be excluded. However, cyclic AMP induced reductions in action potential accommodation and AHPs, which are found in cholera toxin foci, may contribute to, but are not sufficient for, epileptogenesis. Cholera toxin differs from the commonly used epileptic agents in that its main action is on G proteins and second messenger systems, rather than on synaptic transmission directly. Furthermore it has a prolonged time course, and does not cause gross pathology. These features combine to make it a distinctive model for epilepsy and neuronal synchronization.

Epileptic focus induced in rat by intrahippocampal cholera toxin: neuronal properties in vitro

Injecting 0.5-1.0 microgram of cholera toxin into rat hippocampus induces a chronic epileptic focus which generates interictal discharges and brief epileptic seizures intermittently over the following seven to 10 days. Here we examined the electrophysiological properties of hippocampal slices prepared from these rats three to four days after injection, at the height of the epileptic syndrome. These slices generated epileptic discharges in response to electrical stimulation of afferent pathways. In many cases epileptic discharges occurred spontaneously in the CA3 subregion; these usually lasted < 200 ms, but they could last < 0.6 s. Intracellular recordings from pyramidal layer cells revealed depolarization shifts synchronous with the epileptic field potentials. These depolarization shifts had slow onsets compared with those induced by blocking inhibition with bicuculline (depolarizations started a mean of 57 ms before, and reached 5.2 mV by, the onset of the cholera toxin epileptic field potential, compared with 12 ms and 3.6 mV respectively for 70 microM bicuculline methiodide). Extracellular unit recordings showed that the slow predepolarization seen in the cholera toxin focus was associated with an acceleration of the firing of other pyramidal layer neurons. The epileptic activity in this model cannot be attributed to the loss of synaptic inhibition, because inhibitory postsynaptic potentials could be evoked when the synchronous bursts were blocked by increasing [Ca2+]o from 2 to 8 mM. Observations of monosynaptic inhibitory postsynaptic currents isolated by application of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione, 50 microM DL-2-amino-5-phosphonovaleric acid and 100-200 microM 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid showed a small effect of the toxin only on the time course of the inhibitory postsynaptic current. On the other hand, there were significant changes in the intrinsic properties of individual neurons. The membrane potentials of cells in the cholera toxin focus did not differ from those in slices from rats injected with vehicle solution, but their input resistances were significantly increased. Unlike the other cellular changes in this model, the increase in input resistance was not seen in slices exposed acutely to 1 micrograms/ml cholera toxin for 30 min, suggesting there may be morphological changes in the chronic focus. Action potential accommodation and the slow afterhyperpolarization were depressed in both acute and chronic epileptic tissue, indicating impairments of Ca(2+)- and/or voltage-dependent K+ currents, and we conclude that these provide the most likely basis for cholera toxin epileptogenesis.

2,3-butanedione monoxime blockade of Ca2+ currents in adult rat sympathetic neurons does not involve 'chemical phosphatase' activity

Effects of the 'chemical phosphatase' 2,3-butanedione monoxide (BDM) on voltage-activated Ca2+ currents of adult rat superior cervical ganglion neurons were investigated using the whole-cell patch-clamp technique. BDM produced a rapid (< 10 s), reversible and dose-dependent (IC50 = 18.3 mM) inhibition of Ca2+ currents. The action of BDM was not prevented by 1 mM 8-(4-chlorophenylthio)-cAMP or 50 microM isoproterenol. H-7, a non-specific protein kinase inhibitor at 200 microM, did not prevent the rapid recovery from BDM-induced inhibition. Our results suggest that BDM inhibition of Ca2+ currents in rat sympathetic neurons does not require a 'chemical phosphatase' activity.

Synergistic regulation of vasoactive intestinal polypeptide expression by cyclic AMP and calcium in newborn but not adult rat sensory neurons in culture

There is a spontaneous induction of vasoactive intestinal polypeptide (VIP) expression in adult rat dorsal root ganglion sensory neurons when grown in culture. The mechanism of this induction may be the same as that responsible for the increased VIP expression in sensory neurons following peripheral axotomy in vivo. This study investigates the effects of depolarization and cyclic AMP on VIP expression (measured by radioimmunoassay) in cultures of newborn and adult rat sensory neurons. Unlike adult neurons, newborn rat sensory neurons, supported in culture with nerve growth factor, did not spontaneously express VIP. However, potassium-evoked depolarization and drugs that increase intracellular cyclic AMP concentrations (forskolin, 8-bromo cyclic AMP, isobutylmethylxanthine) interacted synergistically to stimulate high levels of VIP expression in newborn rat neurons. The contribution of depolarization to this effect could be mimicked by the L-type calcium channel agonist Bay K 8644 and blocked by the channel antagonist nifedipine, implying the involvement of calcium influx through L-type channels. While depolarization and forskolin individually had small effects on VIP content of adult rat sensory neuron cultures, there was no synergism of the kind seen in newborn rat cultures. Immunostaining showed that VIP was localized within approximately 30% of both newborn and adult rat sensory neurons. Thus, a subpopulation of newborn rat sensory neurons exhibit marked plasticity of VIP expression in an integrated response to activation of cyclic AMP- and calcium-dependent signalling pathways. This property is no longer present in mature neurons, however, where VIP expression is regulated by as yet undetermined factors.

The effect of phosphatase inhibitors and agents increasing cyclic-AMP-dependent phosphorylation on calcium channel currents in cultured rat dorsal root ganglion neurones: interaction with the effect of G protein activation

Ca2+ channel currents have been recorded in cultured rat dorsal root ganglion neurones. The amplitude of IBa(GTP gamma S), recorded in the presence of GTP[ gamma S] (200 microM) in the patch pipette solution, is enhanced by external application of forskolin (10 microM), and there is an increase in the proportion of the rapidly activating component of the current. When forskolin (1 microM) is present in the bathing solution at the start of recording, or when 8-bromocyclic AMP (100 microM) is present in the patch pipette solution, the amplitude and rate of activation of IBa(GTP gamma S) are also increased compared to control IBa(GTP gamma S). The effect is mimicked by internal application of a 5 microM solution of a phosphopeptide fragment of inhibitor 1 (I1 PP), which inhibits phosphatase 1. The enhancement of IBa(GTP gamma S) caused by I1PP is not additive with that due to forskolin. Furthermore, the enhancement due to I1PP is reversibly lost when the holding potential is shifted from -80 mV to -30 mV, as was the enhancement due to forskolin and 8-bromocyclic AMP. I1PP also produced a less marked stimulation of the control Ca2+ channel current in the absence of G protein activation. The results suggest that phosphorylation regulates the interaction between calcium channels and G proteins in these neurones, and that phosphatase 1 is tonically active to dephosphorylate the relevant protein(s).

Novel suppression of an L-type calcium channel in neurones of murine dorsal root ganglia by 2,3-butanedione monoxime

1. Voltage-activated currents through calcium channels in primary cultures of murine dorsal root ganglion cells (DRG) were studied with the whole-cell and cell-attached patch recording techniques. 2. The chemical phosphatase 2,3-butanedione monoxime (BDM) reversibly reduced the amplitude of L-type calcium current (ICa) in a dose-dependent manner; at a concentration of 20 mM, BDM caused a 47% suppression of ICa. 3. Application of 10 mM-8-bromo-cyclic AMP or 50 microM-isoprenaline onto DRG treated with BDM completely restored ICa to the pre-BDM level. 4. In striking contrast, bath application of Bay K 8644 (0.5-5 microM) had no effect on the BDM-suppressed ICa. As expected, Bay K 8644 alone caused a two- to threefold increase of the maximal ICa and shifted its I-V relationship to the left. Interestingly, if a cell was first exposed to Bay K 8644 further treatment with 20 mM-BDM resulted in 100% suppression of ICa. This suggests that Bay K 8644 changes the conformation of the calcium channel to one which is more sensitive or more accessible to the action of the phosphatase. 5. Pre-treatment of DRG with an activator of protein kinase C, 12-O-tetradecanoyl-phorbol-13-acetate, did not antagonize BDM's effect on ICa. 6. The depressant action of BDM on ICa was distinct from that of nifedipine in that it did not exhibit use dependence. 7. When single calcium channel currents were recorded in cell-attached patches (barium as the charge carrier), bath application of BDM reduced the percentage of time that the channel spent in the open state. 8. Superfusion with 8-bromo-cyclic AMP restored the ensemble macroscopic 'ICa' to the pre-BDM amplitude. This was due to a dramatic enhancement of the frequency of channel openings. 9. We suggest that BDM acts through the cytoplasm to alter cyclic AMP-dependent protein kinase modulation of neuronal L-type calcium channels. The brief, high-frequency openings which 8-bromo-cyclic AMP activates in the presence of BDM may reflect a rapid phosphorylation-dephosphorylation sequence which controls channel gating.

Voltage-dependent regulation of L-type cardiac Ca channels by isoproterenol

The beta-adrenergic cascade is important for the regulation of voltage-dependent Ca channels by phosphorylation. Here we report that isoproterenol (ISO) profoundly alters the voltage-dependent properties of L-type Ca channels studied in rat ventricular cells. ISO (1 microM) shifted both threshold and maximal activation of Ba current (IBa) towards more negative potentials (approx. 10 mV). An equivalent shift was observed in the steady-state voltage-dependent inactivation curve. As a consequence, the potentiation induced by ISO on IBa was greater for weak depolarizations and from negative holding potentials (Vh). We have excluded that the contribution of minor uncompensated series resistances, the activation of Cl currents or changes in junction potential during the experiments account for these effects. In addition, ISO had a dual effect on IBa decay depending on the voltage step (acceleration below, slowing above -10 mV). In conclusion, it is postulated that the voltage dependence of the potentiating effects of ISO on Ca channels activity may ensure a selective regulation among heart tissues with different membrane resting potentials.

Modulation of vertebrate neuronal calcium channels by transmitters

A large number of neurotransmitters have now been shown to reduce the amplitude and slow the activation kinetics of whole cell HVA ICa in a great diversity of neurons. These transmitters include L-glutamate (AMPA/kainate, metabotropic and NMDA receptors), GABA (via GABAB receptors, NA (via alpha 2 receptors), 5-HT, NA (via alpha 2 receptors), DA and several peptides. Both whole-cell and single-channel studies have demonstrated that the N-channel is the most common channel type to be blocked by transmitters, although an inhibition of the L-type channel has also occasionally been reported. The suppression of the N-type Ca current was commonly shown to be voltage-dependent, with a relief at large positive voltages. Strong evidence has been put forward showing that the transmitter action is mediated by a G-protein, with GDP-beta-S blocking transmitter action, and GTP-gamma-S directly inhibiting the Ca channel. Moreover, pertussis toxin blocked the transmitter action in most neurons, and following such block, injection of the G-protein Go restored transmitter action. A direct link between the G-protein and the Ca channel has been widely theorized to mediate the action of transmitters on certain neurons. There is also some evidence that certain transmitters in specific neurons mediate calcium channel inhibition through a 2nd messenger, perhaps protein kinase C. Transmitters have also been found, although uncommonly, to inhibit HVA L-type and LVA T-type channels. In addition, an enhancement of both HVA and LVA Ca currents by transmitters has been demonstrated, and substantial evidence exists for mediation of this action by cAMP.