Modulation of brain Na+ channels by a G-protein-coupled pathway

Whole-cell plasticity in cocaine withdrawal: reduced sodium currents in nucleus accumbens neurons

The nucleus accumbens is a forebrain region that mediates cocaine self-administration and withdrawal effects in animal models of cocaine dependence. Considerable evidence suggests an important role of dopamine D1 receptors in these effects. Using a combination of current-clamp recordings in brain slices and whole-cell patch-clamp recordings from freshly dissociated neurons, we found that nucleus accumbens neurons are less excitable in cocaine withdrawn rats because of a novel form of plasticity: reduced whole-cell sodium currents. Three days after discontinuation of repeated cocaine injections, nucleus accumbens neurons recorded in brain slices were less responsive to depolarizing current injections, had higher action potential thresholds, and had lower spike amplitudes. Freshly dissociated nucleus accumbens neurons from cocaine-pretreated rats exhibited diminished sodium current density and a depolarizing shift in the voltage-dependence of sodium channel activation. These effects appear to be related to enhanced basal phosphorylation of sodium channels because of increased transmission through the dopamine D1 receptor/cAMP-dependent protein kinase pathway. The effects of repeated cocaine administration were not mimicked by repeated injections of the local anesthetic lidocaine and were not observed in neurons within the motor cortex, indicating that they did not result from local anesthetic actions of cocaine. Because nucleus accumbens neurons are normally recruited to coordinate response patterns of movement and affect, the decreased excitability during cocaine withdrawal may be related to symptoms such as anergia, anhedonia, and depression.

Persistent sodium currents through brain sodium channels induced by G protein betagamma subunits

Persistent Na+ currents are thought to be important for integration of neuronal responses. Here, we show that betagamma subunits of G proteins can induce persistent Na+ currents. Coexpression of G beta2gamma3, G beta1gamma3, or G beta5gamma3, but not G beta1gamma1 subunits with rat brain type IIA Na+ channel alpha subunits in tsA-201 cells greatly enhances a component of Na+ current with a normal voltage dependence of activation but with dramatically slowed and incomplete inactivation and with steady-state inactivation shifted +37 mV. Synthetic peptides containing the proposed G betagamma-binding motif, Gln-X-X-Glu-Arg, from either adenylyl cyclase 2 or the Na+ channel alpha subunit C-terminal domain reversed the effect of G beta2gamma3 subunits. These results are consistent with direct binding of G betagamma subunits to the C-terminal domain of the Na+ channel, stabilizing a gating mode responsible for slowed and persistent Na+ current. Modulation of Na+ channel gating by G betagamma subunits is expected to have profound effects on neuronal excitability.

The Ca2+-dependent binding of calmodulin to an N-terminal motif of the heterotrimeric G protein beta subunit

Ca2+ ion concentration changes are critical events in signal transduction. The Ca2+-dependent interactions of calmodulin (CaM) with its target proteins play an essential role in a variety of cellular functions. In this study, we investigated the interactions of G protein betagamma subunits with CaM. We found that CaM binds to known betagamma subunits and these interactions are Ca2+-dependent. The CaM-binding domain in Gbetagamma subunits is identified as Gbeta residues 40-63. Peptides derived from the Gbeta protein not only produce a Ca2+-dependent gel mobility shifting of CaM but also inhibit the CaM-mediated activation of CaM kinase II. Specific amino acid residues critical for the binding of Gbetagamma to CaM were also identified. We then investigated the potential function of these interactions and showed that binding of CaM to Gbetagamma inhibits the pertussis toxin-catalyzed ADP-ribosylation of Galphao subunits, presumably by inhibiting heterotrimer formation. Furthermore, we demonstrated that interaction with CaM has little effect on the activation of phospholipase C-beta2 by Gbetagamma subunits, supporting the notion that different domains of Gbetagamma are responsible for the interactions of different effectors. These findings shed light on the molecular basis for the interactions of Gbetagamma with Ca2+-CaM and point to the potential physiological significance of these interactions in cellular functions.

Chronic cocaine intoxication alters hippocampal sodium channel function

Repeated daily administration of subconvulsive doses of cocaine results in the appearance and increase in convulsive responsiveness to the drug and its lethal effects. The mechanisms involved in this increased susceptibility to cocaine-induced seizure are yet unknown. In this study, we used whole cell patch-clamp recording techniques to examine the functional changes in voltage-dependent Na+ channels produced by subconvulsive doses of cocaine (45 mg/kg per day, i.p.) in rat hippocampal CA1 pyramidal neurons. Intact animals were injected with cocaine for 5-6 days. Acutely dissociated hippocampal neurons were then recorded in vitro. Our results show that an augmentation of peak Na+ currents and a shift in depolarizing direction of the steady-state inactivation were present in neurons from drug-treated rats. These changes, by making a larger proportion of Na+ channels available for opening, could increase the excitability of CA1 neurons and may contribute to the increase in convulsive responsiveness to cocaine.

Corticosteroid effects on sodium and calcium currents in acutely dissociated rat CA1 hippocampal neurons

Consequences of corticosteroid receptor activation on voltage-dependent Na+ conductances were studied in acutely dissociated CA1 hippocampal neurons. This preparation was selected because of the compact electrotonic properties of dissociated neurons, allowing reliable voltage-clamp of the large and fast Na+ currents. The Na+ currents were studied in (i) neurons of adrenalectomized animals (no steroid receptors occupied), (ii) neurons from tissue of adrenalectomized rats treated in vitro with corticosterone and the glucocorticoid receptor antagonist RU38486 (selectively occupying the mineralocorticoid receptor), (iii) corticosterone-treated neurons of adrenalectomized animals (occupying both the mineralocorticoid and glucocorticoid receptors) and (iv) neurons of sham-operated animals. Activation and steady-state inactivation properties of the Na+ current recorded in neurons of adrenalectomized animals were slightly shifted (3-5 mV) to hyperpolarized potentials as compared to the Na+ currents from neurons of the other experimental groups. Furthermore, the removal from inactivation of the Na+ current in the group of neurons of adrenalectomized animals was relatively slow. Although small, these effects could influence neuronal properties like action potential generation and accommodation. Under the present experimental conditions, no apparent differences were seen between cells with predominant mineralocorticoid receptor activation and cells where both mineralocorticoid and glucocorticoid receptors were occupied. In contrast to Na+ currents, voltage-dependent Ca2+ currents displayed no steroid-dependent shifts in voltage-dependent properties. However, Ca2+ current amplitudes were increased by approximately 160% in CA1 neurons of adrenalectomized animals as compared to Ca2+ currents from neurons of the other experimental groups. We conclude that corticosteroid receptor activation affects various properties of voltage-dependent Na+ and Ca2+ conductances in CA1 neurons, indicating that the steroid receptors are involved in the modulation of neuronal excitability in these cells.

Modulation of the human cardiac sodium channel alpha-subunit by cAMP-dependent protein kinase and the responsible sequence domain

1. In order to investigate the modulation of human hH1 sodium channel alpha-subunits by cAMP-dependent protein kinase (PKA), the channel was expressed in oocytes of Xenopus laevis. 2. Cytosolic injection of cAMP, as well as of SP-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (SP-cAMPS, the S-diastereoisomeric configuration of the compound with respect to the phosphorus atom), resulted in a marked and significant increase in peak sodium current (INa,p). Cytosolic injections of RP-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (RP-cAMPS; a compound inhibitory to PKA) had no effect on peak current. 3. Kinetic parameters of steady-state activation, inactivation and recovery from inactivation were unchanged following stimulation of PKA activity, but a 42 +/- 5% (mean +/- S.E.M.) increase in maximal sodium conductance (delta gmax) could account for the observed increase in INa,p. 4. A set of chimerical sodium channels made from portions of the human cardiac hH1 alpha-subunit and the rat skeletal muscle SkM1 alpha-subunit (which is not affected by PKA stimulation) was generated. These were used to localize the structural determinant in the hH1 sequence responsible for PKA modulation of hH1. From our data we conclude that the effects of PKA on hH1 are conferred by the large cytosolic loop interconnecting transmembrane domains I and II, which is not conserved among sodium channel subtypes.

Spatial distribution of synaptically activated sodium concentration changes in cerebellar Purkinje neurons

The spatial distribution of Na(+)-dependent events in guinea pig Purkinje cells was studied with a combination of high-speed imaging and simultaneous intracellular recording. Individual Purkinje cells in sagittal cerebellar slices were loaded with either fura-2 or the Na+ indicator sodium binding benzofuran isophthalate (SBFI) with sharp electrodes or patch electrodes on the soma or dendrites. [Na+]i changes were detected in response to climbing fiber and parallel fiber stimulation. These changes were located both at the anatomically expected sites of synaptic contact in the dendrites and in the somatic region. The variation in time course of these [Na+]i changes in different locations implies that Na+ enters at the synapse and diffuses rapidly to locations of lower initial [Na+]i. The synaptically activated somatic [Na+]i changes probably reflect Na+ entry through voltage-sensitive Na+ channels because they were detected only when regenerative potentials were recorded in the soma. [Na+]i changes in response to antidromically or intrasomatically evoked Na+ action potentials also were confined to the cell body. These observations are in agreement with other evidence that Na+ spikes are generated in the somatic region of the Purkinje neuron and spread passively into the dendrites. Plateau potentials, evoked by depolarizing pulses to the soma or dendrites, caused [Na+]i changes only in the soma, indicating that the noninactivating Na+ channels contributing to this potential also were concentrated in this region. The climbing fiber-activated [Na+]i changes were blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that these changes were not due to direct stimulation of the Purkinje neuron or activation of metabotropic receptors. Direct depolarization of the soma or dendrites never caused dendritic [Na+]i increases, suggesting that the climbing fiber-activated [Na+]i changes in the dendrites are due to Na+ entry through ligand-gated channels. A climbing fiber-like regenerative potential could be recorded in the soma after anode break stimulation, parallel fiber activation, or depolarizing pulses to the soma. The [Na+]i changes evoked by all of these potentials were confined to the cell body region of the Purkinje cell. [Ca2+]i changes in the dendrites evoked by the anode break potential were small relative to climbing fiber-activated changes, suggesting that a Ca2+ spike was not evoked by this response. The anode break and directly responses were blocked by tetrodotoxin. These results suggest that the somatically recorded climbing fiber response is predominantly a Na(+)-dependent event, consisting of a few fast action potentials and a slower regenerative response activating the same channels as the Na+ plateau potential.

The equine periodic paralysis Na+ channel mutation alters molecular transitions between the open and inactivated states

1. The Na+ channel mutation associated with equine hyperkalaemic periodic paralysis (HPP) affects a highly conserved phenylalanine residue in an unexplored region of the alpha-subunit. This mutation was introduced into the rat skeletal muscle Na+ channel gene at the corresponding location (i.e. F1412L) for functional expression and characterization in Xenopus oocytes. 2. In comparison with wild-type (WT) channels, equine HPP channels showed clear evidence for disruption of inactivation: increased time-to-peak current, slowed rates of whole-cell current decay, significant increases in sustained current, rightward shifts in the steady-state inactivation curve by 9.5 mV, a 6-fold acceleration in the rate of recovery from inactivation at -80 mV, decreased number of blank single-channel sweeps, repetitive opening of single channels throughout depolarizing steps, increased open probability per sweep, and an increased mean open time. 3. The observed disruption of inactivation in HPP occurred without measurable changes in steady-state activation and first latency kinetics of channel opening. 4. Kinetic modelling demonstrates that the equine HPP phenotype can be simulated by altering the rate constants for transitions entering and leaving the inactivated states resulting from an energetic destabilization of the inactivated state. 5. These results suggest that the highly conserved cytoplasmic end of the third transmembrane segment (S3) in the fourth internal repeat domain (domain IV) plays a critical role in Na+ channel inactivation.

Protein kinase A activation increases sodium current magnitude in the electric organ of Sternopygus

The inactivation kinetics of the Na+ current of the weakly electric fish Sternopygus are modified by treatment with androgens. To determine whether phosphorylation could play a role in this effect, we examined whether activation of protein kinase A by 8 bromo cyclic AMP (8 Br cAMP) altered voltage-dependent properties of the current. Using a two-electrode voltage-clamp procedure, we found no effect of 8 Br cAMP on inactivation kinetics or other voltage-dependent properties of the Na+ current of the electrocytes. However, treatment with 8 Br cAMP did produce a dose-dependent increase in the Na+ current compared with saline controls: 17.6% at 100 microM, 42.4% at 1 mM, and 43.1% at 5 mM. This effect was blocked by 30 microM H89, a PKA inhibitor, indicating that the observed effect was attributable to 8 Br cAMP activation of PKA. We conclude that androgen-induced changes in Na+ current inactivation are not mediated by PKA and suggest that PKA-mediated increases in Na+ current underlie increases in the amplitude of the electric organ discharge observed in social interactions or with changes in water conductance.

Protein kinase A activation increases sodium current magnitude in the electric organ of Sternopygus

The inactivation kinetics of the Na+ current of the weakly electric fish Sternopygus are modified by treatment with androgens. To determine whether phosphorylation could play a role in this effect, we examined whether activation of protein kinase A by 8 bromo cyclic AMP (8 Br cAMP) altered voltage-dependent properties of the current. Using a two-electrode voltage-clamp procedure, we found no effect of 8 Br cAMP on inactivation kinetics or other voltage-dependent properties of the Na+ current of the electrocytes. However, treatment with 8 Br cAMP did produce a dose-dependent increase in the Na+ current compared with saline controls: 17.6% at 100 microM, 42.4% at 1 mM, and 43.1% at 5 mM. This effect was blocked by 30 microM H89, a PKA inhibitor, indicating that the observed effect was attributable to 8 Br cAMP activation of PKA. We conclude that androgen-induced changes in Na+ current inactivation are not mediated by PKA and suggest that PKA-mediated increases in Na+ current underlie increases in the amplitude of the electric organ discharge observed in social interactions or with changes in water conductance.

G-protein regulation of ion channels

Even though the vast majority of ion channels are regulated by voltage, extracellular ligands, phosphorylation, intracellular ions, or a combination of these influences, probably only a handful of ion channels are regulated by direct interaction with activated G proteins. Although results from electrophysiological studies of some channels are consistent with the hypothesis of regulation via direct physical interactions with G proteins, strong biochemical evidence for such interactions is still lacking. In most cases, such evidence has been difficult to obtain because ion channels are present at very low abundances in cell membranes, or because the molecular identity of the channel is unknown. The recent cloning of members of the inwardly rectifying K+ channel and voltage-gated Ca2+ channel families should facilitate the rigorous study of the putative interactions between G proteins and ion channels.