Electrophysiological properties of sodium current subtypes in small cells from adult rat dorsal root ganglia

Axotomy- and autotomy-induced changes in Ca2+ and K+ channel currents of rat dorsal root ganglion neurons

Sciatic nerve section (axotomy) increases the excitability of rat dorsal root ganglion (DRG) neurons. The changes in Ca2+ currents, K+ currents, Ca2+ sensitive K+ current, and hyperpolarization-activated cation current (I(H)) that may be associated with this effect were examined by whole cell recording. Axotomy affected the same conductances in all types of DRG neuron. In general, the largest changes were seen in "small" cells and the smallest changes were seen in "large" cells. High-voltage-activated Ca2+ channel current (HVA-I(Ba)) was reduced by axotomy. Although currents recorded in axotomized neurons exhibited increased inactivation, this did not account for all of the reduction in HVA-I(Ba). Activation kinetics were unchanged, and experiments with nifedipine and/or omega-conotoxin GVIA showed that there was no change in the percentage contribution of L-type, N-type, or "other" HVA-I(Ba) to the total current after axotomy. T-type (low-voltage-activated) I(Ba) was not affected by axotomy. Ca2+ sensitive K+ conductance (g(K,Ca)) appeared to be reduced, but when voltage protocols were adjusted to elicit similar amounts of Ca2+ influx into control and axotomized cells, I(K,Ca)(s) were unchanged. After axotomy, Cd2+ insensitive, steady-state K+ channel current, which primarily comprised delayed rectifier K+ current (I(K)), was reduced by about 60% in small, medium, and large cells. These data suggest that axotomy-induced increases in excitability are associated with decreases in I(K) and/or decreases in g(K,Ca) that are secondary to decreased Ca2+ influx. Because I(H) was reduced by axotomy, changes in this current do not contribute to increased excitability. The amplitude and inactivation of I(Ba) in all cell types was changed more profoundly in animals that exhibited self-mutilatory behavior (autotomy). The onset of this behavior corresponded with significant reduction in I(Ba) of large neurons. This finding supports the hypothesis that autotomy, that may be related to human neuropathic pain, is associated with changes in the properties of large myelinated sensory neurons.

Involvement of Na+ channels in pain pathways

Voltage-dependent Na+ channels in sensory nerves contribute to the control of membrane excitability and underlie action potential generation. Na+ channel subtypes exhibit a neurone-specific and developmentally regulated pattern of expression, and changes in both channel expression and function are caused by disease. Recent evidence implicates specific roles for Na+ channel subtypes Na(v)1.3 and Na(v)1.8 in pain states that are associated with nerve injury and inflammation, respectively. Insight into the role of Na(v)1.8 in pain pathways has been gained by the generation of a null mutant. Although drugs discriminate poorly between subtypes, the molecular diversity of channels and subtype-specific modulation might provide opportunities to target pain pathways selectively.

Block of neuronal tetrodotoxin-resistant Na+ currents by stereoisomers of piperidine local anesthetics

Tetrodotoxin (TTX)-sensitive Na(+) channels in the peripheral nervous system are the major targets for local anesthetics. In the peripheral nociceptive system, a Na(+) channel subtype resistant to TTX and with distinct electrophysiological properties seems to be of importance for impulse generation and conduction. A current through TTX-resistant Na(+) channels displays slower activation and inactivation kinetics and has an increased activation threshold compared with TTX-sensitive Na(+) currents and may have different pharmacological properties. We studied the effects of stereoisomers of piperidine local anesthetics on neuronal TTX-resistant Na(+) currents recorded with the whole-cell configuration of the patch clamp method in enzymatically dissociated dorsal root ganglion neurons of adult rats. Stereoisomers of mepivacaine, ropivacaine, and bupivacaine reversibly inhibited TTX-resistant Na(+) currents in a concentration and use-dependent manner. All drugs accelerated time course of inactivation. Half-maximal blocking concentrations were determined from concentration-inhibition relationships. Potencies for tonic and for use-dependent block increased with rising lipid solubilities of the drugs. Stereoselective action was not observed. We conclude that block of TTX-resistant Na(+) currents may lead to blockade of TTX-resistant action potentials in nociceptive fibers and consequently may be responsible for pain suppression during local anesthesia.

Sodium channels and pain therapy

Researchers have characterized changes in the nervous system that occur in response to tissue injury in order to identify possible targets for novel therapeutic interventions for the treatment of pain. That blockers of voltage-gated sodium channels (VGSCs) are clinically effective for the treatment of pain associated with certain types of tissue injury suggests that these channels constitute such a target. Furthermore, there are changes in biophysical properties, expression, and/or distribution of VGSCs in subpopulations of primary afferent and central nervous system neurons in response to injury that are consistent with a role for VGSCs in the generation and maintenance of pain. Injury-induced changes in four unique VGSCs have been described. However, each of these channels appears to contribute to pain associated with different forms of injury in different ways.

Antihyperalgesia effect of BmK IT2, a depressant insect-selective scorpion toxin in rat by peripheral administration

The study was undertaken to assess the antihyperalgesia effect of BmK IT2, a sodium channel-specific ligand purified from the venom of Chinese scorpion Buthus martensi Karsch in rat by peripheral injection. The peripheral inflammation of rat was induced by carrageenan resulted in hyperalgesia to heat stimulus. The heat hyperalgesia was measured by paw withdrawal latency (PWL). PWL was increased to 272 +/- 18%, 217 +/- 19% and 186 +/- 16% of the control by application of 10 microl BmK IT2 at the concentration of 0.1, 0. 01 and 0.001 mg/ml in inflammatory rats, respectively. In contrast, PWL was enhanced to about 177 +/- 16%, 141 +/- 15% and 133 +/- 15% of the control at the same applied concentrations of BmK IT2 in normal rats. The results thus suggest that BmK IT2 can produce peripheral antihyperalgesia and antinociception, which might be attributed a pathway of modulating the sodium channels on nociceptor.

Ketamine blockage of both tetrodotoxin (TTX)-sensitive and TTX-resistant sodium channels of rat dorsal root ganglion neurons

Ketamine, a general anesthetic, has been reported to block sodium channels. Two types of Na(+) channels, tetrodotoxin (TTX)-sensitive (TTX-s) and TTX-resistant (TTX-r), are expressed in dorsal root ganglion (DRG) neurons. The present study was to investigate the effects of ketamine on both types, particularly on TTX-r channels, using whole-cell patch-clamp recordings in dissociated rat DRG neurons. In addition to confirming ketamine-induced blockage of TTX-s Na(+) current, we showed for the first time that ketamine blocked TTX-r Na(+) channels on small DRG neurons in dose-dependent and use-dependent manner. Half-maximal inhibitory concentration (IC(50)) was 866.2 microM for TTX-r Na(+) channels. TTX-r Na(+) channels were more sensitive to ketamine in inactivated state (IC(50) = 314.8 microM) than in resting state (IC(50) = 866.2 microM). IC(50) was 146.7 microM for TTX-s Na(+) current. Activation and inactivation properties of both TTX-s and TTX-r Na(+) channels were affected by ketamine. Since TTX-r Na(+) channels were preferentially expressed in small DRG neurons known as nociceptors, blockage of TTX-r Na(+) channels by ketamine may result in reducing nociceptive signals conducting to the spinal cord. Moreover, both TTX-r and TTX-s Na(+) channels would be non-selectively blocked by ketamine at high concentration, suggesting that the high dose of ketamine might produce an action of local anesthesia.

Low dose of tetrodotoxin reduces neuropathic pain behaviors in an animal model

We hypothesize that the accumulation of tetrodotoxin (TTX) sensitive sodium channels in injured dorsal root ganglion (DRG) neurons plays a critically important role in the generation of ectopic discharges and mechanical allodynia after peripheral nerve injury. Using the segmental spinal nerve (L5) ligation model of neuropathic pain, this hypothesis was tested by examining the effect of TTX on the mechanical sensitivity of the affected hind paw. Various concentrations of TTX were applied topically to the L5 DRG by using chronically implanted polyethylene tubing. The data showed that application of TTX at low doses (12.5-50 nM), which are far less than those needed for blocking action potential conduction, produced a significant elevation of mechanical threshold in the paw for foot withdrawals, a sign of reduced allodynic behaviors. The data suggest that TTX-sensitive subtypes of sodium channels play an important role in maintaining allodynic behaviors in an animal model of neuropathic pain.

Diversity of expression of the sensory neuron-specific TTX-resistant voltage-gated sodium ion channels SNS and SNS2

The differential distribution of two tetrodotoxin resistant (TTXr) voltage-gated sodium channels SNS (PN3) and SNS2 (NaN) in rat primary sensory neurons has been investigated. Both channels are sensory neuron specific with SNS2 restricted entirely to those small dorsal root ganglion (DRG) cells with unmyelinated axons (C-fibers). SNS, in contrast, is expressed both in small C-fiber DRG cells and in 10% of cells with myelinated axons (A-fibers). All SNS expressing A-fiber cells are Trk-A positive and many express the vanilloid-like receptor VRL1. About half of C-fiber DRG neurons express either SNS or SNS2, and in most, the channels are colocalized. SNS and SNS2 are found both in NGF-responsive and GDNF-responsive C-fibers and many of these cells also express the capsaicin receptor VR1. A very small proportion of small DRG cells express either only SNS or only SNS2. At least four different classes of A- and C-fiber DRG neurons exist, therefore, with respect to expression of these sodium channels.

Nitric oxide is an autocrine regulator of Na(+) currents in axotomized C-type DRG neurons

In this study, we examined whether nitric oxide synthase (NOS) is upregulated in small dorsal root ganglion (DRG) neurons after axotomy and, if so, whether the upregulation of NOS modulates Na(+) currents in these cells. We identified axotomized C-type DRG neurons using a fluorescent label, hydroxystilbamine methanesulfonate and found that sciatic nerve transection upregulates NOS activity in 60% of these neurons. Fast-inactivating tetrodotoxin-sensitive (TTX-S) Na(+) ("fast") current and slowly inactivating tetrodotoxin-resistant (TTX-R) Na(+) ("slow") current were present in control noninjured neurons with current densities of 1.08 +/- 0. 09 nA/pF and 1.03 +/- 0.10 nA/pF, respectively (means +/- SE). In some control neurons, a persistent TTX-R Na(+) current was observed with current amplitude as much as approximately 50% of the TTX-S Na(+) current amplitude and 100% of the TTX-R Na(+) current amplitude. Seven to 10 days after axotomy, current density of the fast and slow Na(+) currents was reduced to 0.58 +/- 0.05 nA/pF (P < 0.01) and 0.2 +/- 0.05 nA/pF (P < 0.001), respectively. Persistent TTX-R Na(+) current was not observed in axotomized neurons. Nitric oxide (NO) produced by the upregulation of NOS can block Na(+) currents. To examine the role of NOS upregulation on the reduction of the three types of Na(+) currents in axotomized neurons, axotomized DRG neurons were incubated with 1 mM N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor. The current density of fast and slow Na(+) channels in these neurons increased to 0.82 +/- 0.08 nA/pF (P < 0.01) and 0.34 +/- 0.04 nA/pF (P < 0.05), respectively. However, we did not observe any persistent TTX-R current in axotomized neurons incubated with L-NAME. These results demonstrate that endogenous NO/NO-related species block both fast and slow Na(+) current in DRG neurons and suggest that NO functions as an autocrine regulator of Na(+) currents in injured DRG neurons.

Two tetrodotoxin-resistant sodium channels in human dorsal root ganglion neurons

Two tetrodotoxin-resistant (TTX-R) voltage-gated sodium channels, SNS and NaN, are preferentially expressed in small dorsal root ganglia (DRG) and trigeminal ganglia neurons, most of which are nociceptive, of rat and mouse. We report here the sequence of NaN from human DRG, and demonstrate the presence of two TTX-R currents in human DRG neurons. One current has physiological properties similar to those reported for SNS, while the other displays hyperpolarized voltage-dependence and persistent kinetics; a similar TTX-R current was recently identified in DRG neurons of sns-null mouse. Thus SNS and NaN channels appear to produce different currents in human DRG neurons.

Sodium channels, excitability of primary sensory neurons, and the molecular basis of pain

Following nerve injury, primary sensory neurons (dorsal root ganglion [DRG] neurons, trigeminal neurons) exhibit a variety of electrophysiological abnormalities, including increased baseline sensitivity and/or hyperexcitability, which can lead to abnormal burst activity that underlies pain, but the molecular basis for these changes has not been fully understood. Over the past several years, it has become clear that nearly a dozen distinct sodium channels are encoded by different genes and that at least six of these (including at least three distinct DRG- and trigeminal neuron-specific sodium channels) are expressed in primary sensory neurons. The deployment of different types of sodium channels in different types of DRG neurons endows them with different physiological properties. Dramatic changes in sodium channel expression, including downregulation of the SNS/PN3 and NaN sodium channel genes and upregulation of previously silent type III sodium channel gene, occur in DRG neurons following axonal transection. These changes in sodium channel gene expression are accompanied by a reduction in tetrodotoxin (TTX)-resistant sodium currents and by the emergence of a TTX-sensitive sodium current which recovers from inactivation (reprimes) four times more rapidly than the channels in normal DRG neurons. These changes in sodium channel expression poise DRG neurons to fire spontaneously or at inappropriately high frequencies. Changes in sodium channel gene expression also occur in experimental models of inflammatory pain. These observations indicate that abnormal sodium channel expression can contribute to the molecular pathophysiology of pain. They further suggest that selective blockade of particular subtypes of sodium channels may provide new, pharmacological approaches to treatment of disease involving hyperexcitability of primary sensory neurons.

The molecular pathophysiology of pain: abnormal expression of sodium channel genes and its contributions to hyperexcitability of primary sensory neurons

Although hyperexcitability and/or increased baseline sensitivity of primary sensory neurons following nerve injury can lead to abnormal burst activity associated with pain, the molecular mechanisms that contribute to it are not fully understood. Early studies demonstrated that, following axonal injury, neurons can display changes in excitability suggesting increased sodium channel expression. Consistent with this, abnormal accumulations of sodium channels have been observed at the tips of injured axons. But we now know that nearly a dozen distinct sodium channels are encoded by different genes, raising the question, what types of sodium channels underlie hyperexcitability of primary sensory neurons following injury? My laboratory has used molecular, electrophysiological, and pharmacological techniques to answer this question. Our studies have demonstrated that multiple sodium channels, with distinct physiological properties, are expressed within small dorsal root ganglion (DRG) neurons, which include nociceptive cells. Several DRG and trigeminal neuron-specific sodium channels have now been cloned and sequenced. There is a dramatic change in sodium channel expression in DRG neurons, with down-regulation of the SNS/PN3 and NaN sodium channel genes and up-regulation of previously silent Type III sodium channel gene, following injury to the axons of these cells. These changes in sodium channel gene expression can produce electrophysiological changes in DRG neurons which poise them to fire spontaneously or at inappropriate high frequencies. We have also observed changes in sodium channel gene expression in experimental models of inflammatory pain. The dynamic nature of sodium channel gene expression in DRG neurons, and the changes which occur in sodium channel and sodium current expression in these cells following axonal injury and in inflammatory pain models, suggest that abnormal expression of sodium channels contributes to the molecular pathophysiology of pain.

Building blocks of pain: the regulation of key molecules in spinal sensory neurones during development and following peripheral axotomy

The pathways, synapses and molecules involved in pain processing in the newborn are not only required to trigger repair and recuperation but are also involved in the process of forming a mature nervous system. Sensory neurons in the dorsal root ganglion and dorsal horn express a phenomenal array of molecules which contribute to their structural and functional characteristics and many of these are developmentally regulated both pre- and postnatally. In order to understand nociceptive signalling and pain in the neonate we need a clear picture of that regulation. This review concentrates on the changing expression of selected key molecules, receptors and channels in the embryo, neonate and adult, which both characterise the sensory neuron and contribute to its response to painful stimuli in normal and pathological conditions.

Ions in the fire: recent ion-channel research and approaches to pain therapy

Ion channels form a diverse and sophisticated collection of membrane-bound proteins. They are influenced by many endogenous compounds and physiological stimuli and modulate neuronal activity. It is thus not surprising that they provide attractive targets for the design of novel therapeutics. In this article, recent ion channel research and its relevance to modulation of sensory transmission is assessed. In pain research, specific blockade or activation of ion channels has long been considered a desired route for identification of analgesics. Historically, this has proven difficult to attain due to the incidence of side-effects associated with most ion-channel modulators. The recent discovery of several novel classes of ion channels, each of which has a specific distribution and role in sensory processing and nociception, has provided a plethora of targets for pharmaceutical intervention with the promise of an improved therapeutic index.

A comparison of the potential role of the tetrodotoxin-insensitive sodium channels, PN3/SNS and NaN/SNS2, in rat models of chronic pain

Alterations in sodium channel expression and function have been suggested as a key molecular event underlying the abnormal processing of pain after peripheral nerve or tissue injury. Although the relative contribution of individual sodium channel subtypes to this process is unclear, the biophysical properties of the tetrodotoxin-resistant current, mediated, at least in part, by the sodium channel PN3 (SNS), suggests that it may play a specialized, pathophysiological role in the sustained, repetitive firing of the peripheral neuron after injury. Moreover, this hypothesis is supported by evidence demonstrating that selective "knock-down" of PN3 protein in the dorsal root ganglion with specific antisense oligodeoxynucleotides prevents hyperalgesia and allodynia caused by either chronic nerve or tissue injury. In contrast, knock-down of NaN/SNS2 protein, a sodium channel that may be a second possible candidate for the tetrodotoxin-resistant current, appears to have no effect on nerve injury-induced behavioral responses. These data suggest that relief from chronic inflammatory or neuropathic pain might be achieved by selective blockade or inhibition of PN3 expression. In light of the restricted distribution of PN3 to sensory neurons, such an approach might offer effective pain relief without a significant side-effect liability.

Sodium channels and pain

Although it is well established that hyperexcitability and/or increased baseline sensitivity of primary sensory neurons can lead to abnormal burst activity associated with pain, the underlying molecular mechanisms are not fully understood. Early studies demonstrated that, after injury to their axons, neurons can display changes in excitability, suggesting increased sodium channel expression, and, in fact, abnormal sodium channel accumulation has been observed at the tips of injured axons. We have used an ensemble of molecular, electrophysiological, and pharmacological techniques to ask: what types of sodium channels underlie hyperexcitability of primary sensory neurons after injury? Our studies demonstrate that multiple sodium channels, with distinct electrophysiological properties, are encoded by distinct mRNAs within small dorsal root ganglion (DRG) neurons, which include nociceptive cells. Moreover, several DRG neuron-specific sodium channels now have been cloned and sequenced. After injury to the axons of DRG neurons, there is a dramatic change in sodium channel expression in these cells, with down-regulation of some sodium channel genes and up-regulation of another, previously silent sodium channel gene. This plasticity in sodium channel gene expression is accompanied by electrophysiological changes that poise these cells to fire spontaneously or at inappropriate high frequencies. Changes in sodium channel gene expression also are observed in experimental models of inflammatory pain. Thus, sodium channel expression in DRG neurons is dynamic, changing significantly after injury. Sodium channels within primary sensory neurons may play an important role in the pathophysiology of pain.

Tetrodotoxin-resistant Na+ currents and inflammatory hyperalgesia

Several mechanisms have been identified that may underlie inflammation-induced sensitization of high-threshold primary afferent neurons, including the modulation of voltage- and Ca2+-dependent ion channels and ion channels responsible for the production of generator potentials. One such mechanism that has recently received a lot of attention is the modulation of a tetrodotoxin (TTX)-resistant voltage-gated Na+ current. Evidence supporting a role for TTX-resistant Na+ currents in the sensitization of primary afferent neurons and inflammatory hyperalgesia is reviewed. Such evidence is derived from studies on the distribution of TTX-resistant Na+ currents among primary afferent neurons and other tissues of the body that suggest that these currents are expressed only in a subpopulation of primary afferent neurons that are likely to be involved in nociception. Data from studies on the biophysical properties of these currents suggest that they are ideally suited to mediate the repetitive discharge associated with prolonged membrane depolarizations. Data from studies on the effects of inflammatory mediators and antinociceptive agents on TTX-resistant Na+ currents suggest that modulation of these currents is an underlying mechanism of primary afferent neuron sensitization. In addition, the second-messenger pathways underlying inflammatory mediator-induced modulation of these currents appear to underlie inflammatory mediator-induced hyperalgesia. Finally, recent antisense studies have also yielded data supporting a role for TTX-resistant Na+ currents in inflammatory hyperalgesia. Although data from these studies are compelling, data presented at the Neurobiology of Pain colloquium raised a number of interesting questions regarding the role of TTX-resistant Na+ currents in inflammatory hyperalgesia; implications of three of these questions are discussed.

Pain

Advances in our understanding of the activation of peripheral damage-sensing neurons (nociceptors) over the past year have been complemented by electrophysiological and imaging studies of central nervous system pain-related centres. The manipulation of gene expression in a reversible and cell type specific way combined with imaging and electrophysiological studies holds promise for helping us to identify the spatial and molecular substrates of pain perception with increasing precision and gives hope for improved analgesic therapies.

The tetrodotoxin-resistant sodium channel SNS has a specialized function in pain pathways

Many damage-sensing neurons express tetrodotoxin (TTX)-resistant voltage-gated sodium channels. Here we examined the role of the sensory-neuron-specific (SNS) TTX-resistant sodium channel alpha subunit in nociception and pain by constructing sns-null mutant mice. These mice expressed only TTX-sensitive sodium currents on step depolarizations from normal resting potentials, showing that all slow TTX-resistant currents are encoded by the sns gene. Null mutants were viable, fertile and apparently normal, although lowered thresholds of electrical activation of C-fibers and increased current densities of TTX-sensitive channels demonstrated compensatory upregulation of TTX-sensitive currents in sensory neurons. Behavioral studies demonstrated a pronounced analgesia to noxious mechanical stimuli, small deficits in noxious thermoreception and delayed development of inflammatory hyperalgesia. These data show that SNS is involved in pain pathways and suggest that blockade of SNS expression or function may produce analgesia without side effects.

Differential role of GDNF and NGF in the maintenance of two TTX-resistant sodium channels in adult DRG neurons

Following sciatic nerve transection, the electrophysiological properties of small dorsal root ganglion (DRG) neurons are markedly altered, with attenuation of TTX-R sodium currents and the appearance of rapidly repriming TTX-S currents. The reduction in TTX-R currents has been attributed to a down-regulation of sodium channels SNS/PN3 and NaN. While infusion of exogenous NGF to the transected nerve restores SNS/PN3 transcripts to near-normal levels in small DRG neurons, TTX-R sodium currents are only partially rescued. Binding of the isolectin IB4 distinguishes two subpopulations of small DRG neurons: IB4+ neurons, which express receptors for the GDNF family of neurotrophins, and IB4- neurons that predominantly express TrkA. We show here that SNS/PN3 is expressed in approximately one-half of both IB4+ and IB4- DRG neurons, while NaN is preferentially expressed in IB4+ neurons. Whole-cell patch-clamp studies demonstrate that TTX-R sodium currents in IB4+ neurons have a more hyperpolarized voltage-dependence of activation and inactivation than do IB4- neurons, suggesting different electrophysiological properties for SNS/PN3 and NaN. We confirm that NGF restores SNS/PN3 mRNA levels in DRG neurons in vitro and demonstrate that the trk antagonist K252a blocks this rescue. The down-regulation of NaN mRNA is, nevertheless, not rescued by NGF-treatment in either IB4+ or IB4- neurons and NGF-treatment in vitro does not significantly increase the peak amplitude of the TTX-R current in small DRG neurons. In contrast, GDNF-treatment causes a twofold increase in the peak amplitude of TTX-R sodium currents and restores both SNS/PN3 and NaN mRNA to near-normal levels in IB4+ neurons. These observations provide a mechanism for the partial restoration of TTX-R sodium currents by NGF in axotomized DRG neurons, and demonstrate that the neurotrophins NGF and GDNF differentially regulate sodium channels SNS/PN3 and NaN.

cAMP-dependent phosphorylation of the tetrodotoxin-resistant voltage-dependent sodium channel SNS

1. Protein kinase A (PKA) modulation of tetrodotoxin-resistant (TTX-r) voltage-gated sodium channels may underly the hyperalgesic responses of mammalian sensory neurones. We have therefore examined PKA phosphorylation of the cloned alpha-subunit of the rat sensory neurone-specific TTX-r channel SNS. Phosphorylation of SNS was compared with that of a mutant channel, SNS(SA), in which all five PKA consensus sites (RXXS) within the intracellular I-II loop had been eliminated by site-directed mutagenesis (serine to alanine). 2. In vitro PKA phosphorylation and tryptic peptide mapping of SNS and mutant SNS(SA) I-II loops expressed as glutathione-S-transferase (GST) fusion proteins confirmed that the five mutated serines were the major PKA substrates within the SNS I-II loop. 3. SNS and SNS(SA) channels were transiently expressed in COS-7 cells and their electrophysiological properties compared. In wild-type SNS channels, forskolin and 8-bromo cAMP produced effects consistent with PKA phosphorylation. Mutant SNS(SA) currents, however, were not significantly affected by either agent. Thus, elimination of the I-II loop PKA consensus sites caused a marked reduction in PKA modulation of wild-type channels. 4. Under control conditions, the voltage dependence of activation of SNS(SA) current was shifted to depolarized potentials compared with SNS. This was associated with a slowing of SNS(SA) current inactivation at hyperpolarized potentials and suggested a tonic PKA phosphorylation of wild-type channels under basal conditions.5. We conclude that the major substrates involved in functional PKA modulation of the SNS channel are located within the intracellular I-II loop.