A novel membrane sodium current induced by injection of cyclic nucleotides into gastropod neurones

cAMP, Ca2+, pHi, and NO Regulate H-like Cation Channels That Underlie Feeding and Locomotion in the Predatory Sea Slug Pleurobranchaea californica

A systems approach to regulation of neuronal excitation in the mollusc Pleurobranchaea has described novel interactions of cyclic AMP-gated cation current (INa,cAMP), Ca2+, pHi, and NO. INa,cAMP appears in many neurons of feeding and locomotor neuronal networks. It is likely one of the family of hyperpolarization-activated, cyclic-nucleotide-gated currents (h-current) of vertebrate and invertebrate pacemaker networks. There are two isoforms. Ca2+ regulates both voltage dependence and depolarization-sensitive inactivation in both isoforms. The Type 1 INa,cAMP of the feeding network is enhanced by intracellular acidification. A direct dependence of INa,cAMP on cAMP allows the current to be used as a reporter on cAMP concentrations in the cell, and from there to the intrinsic activities of the synthetic adenyl cyclase and the degradative phosphodiesterase. Type 2 INa,cAMP of the locomotor system is activated by serotonergic inputs, while Type 1 of the feeding network is thought to be regulated peptidergically. NO synthase activity is high in the CNS, where it differs from standard neuronal NO synthase in not being Ca2+ sensitive. NO acidifies pHi, potentiating Type 1, and may act to open proton channels. A cGMP pathway does not mediate NO effects as in other systems. Rather, nitrosylation likely mediates its actions. An integrated model of the action of cAMP, Ca2+, pHi, and NO in the feeding network postulates that NO regulates proton conductance to cause neuronal excitation in the cell body on the one hand, and relief of activity-induced hyperacidification in fine dendritic processes on the other.

Excitatory action of vasopressin in the brain of the rat: role of cAMP signaling

Brain vasopressin plays a role in behavioral and cognitive functions and in pathological conditions. Relevant examples are pair bonding, social recognition, fear responses, stress disorders, anxiety and depression. At the neuronal level, vasopressin exerts its effects by binding to V1a receptors. In the brainstem, vasopressin can excite facial motoneurons by generating a sustained inward current which is sodium-dependent, tetrodotoxin-insensitive and voltage-gated. This effect is independent of intracellular calcium mobilization and is unaffected by phospholipase Cβ (PLCβ) or protein kinase C (PKC) inhibitors. There are two major unsolved problems. (i) What is the intracellular signaling pathway activated by vasopressin? (ii) What is the exact nature of the vasopressin-sensitive cation channels? We performed recordings in brainstem slices. Facial motoneurons were voltage-clamped in the whole-cell configuration. We show that a major fraction, if not the totality, of the peptide effect was mediated by cAMP signaling and that the vasopressin-sensitive cation channels were directly gated by cAMP. These channels appear to exclude lithium, are suppressed by 2-aminoethoxydiphenylborane (2-APB) and flufenamic acid (FFA) but not by ruthenium red or amiloride. They are distinct from transient receptor channels and from cyclic nucleotide-regulated channels involved in visual and olfactory transduction. They present striking similarities with cation channels present in a variety of molluscan neurons. To our knowledge, the presence in mammalian neurons of channels having these properties has not been previously reported. Our data should contribute to a better knowledge of the neural mechanism of the central actions of vasopressin, and may be potentially significant in view of clinical applications.

Nitric oxide potentiates cAMP-gated cation current by intracellular acidification in feeding neurons of pleurobranchaea

A pH-sensitive cAMP-gated cation current (I(Na,cAMP)) is widely distributed in neurons of the feeding motor networks of gastropods. In the sea slug Pleurobranchaea this current is potentiated by nitric oxide (NO), which itself is produced by many feeding neurons. The action of NO is not dependent on either cGMP or cAMP signaling pathways. However, we found that NO potentiation of I(Na,cAMP) in the serotonergic metacerebral cells could be blocked by intracellular injection of MOPS buffer (pH 7.2). In neurons injected with the pH indicator BCECF, NO induced rapid intracellular acidification to several tenths of a pH unit. Intracellular pH has not previously been identified as a specific target of NO, but in this system NO modulation of I(Na,cAMP) via pH(i) may be an important regulator of the excitability of the feeding motor network.

Functional role of cyclic nucleotide-gated channels in rat medial vestibular nucleus neurons

Although cyclic nucleotide-gated (CNG) channels are expressed in numerous brain areas, little information is available on their functions in CNS neurons. The aim of the present study was to define the distribution of CNG channels in the rat medial vestibular nucleus (MVN) and their possible involvement in regulating MVN neuron (MVNn) excitability. The majority of MVNn expressed both CNG1 and CNG2 A subunits. In whole-cell current-clamp experiments carried out on brainstem slices containing the MVNn, the membrane-permeant analogues of cyclic nucleotides, 8-Br-cGMP and 8-Br-cAMP (1 mM), induced membrane depolarizations (8.9 +/- 0.8 and 9.2 +/- 1.0 mV, respectively) that were protein kinase independent. The cGMP-induced depolarization was associated with a significant decrease in the membrane input resistance. The effects of cGMP on membrane potential were almost completely abolished by the CNG channel blockers, Cd(2+) and L-cis-diltiazem, but they were unaffected by blockade of hyperpolarization-activated cyclic nucleotide-gated channels. In voltage-clamp experiments, 8-Br-cGMP induced non-inactivating inward currents (-22.2 +/- 3.9 pA) with an estimated reversal potential near 0 mV, which were markedly inhibited by reduction of extracellular Na(+) and Ca(2+) concentrations. Membrane depolarization induced by CNG channel activation increased the firing rate of MVNn without changing the action potential shape. Collectively, these findings provide novel evidence that CNG channels affect membrane potential and excitability of MVNn. Such action should have a significant impact on the function of these neurons in sensory-motor integration processes. More generally, it might represent a broad mechanism for regulating the excitability of different CNS neurons.

Persistent sodium current is a target for cAMP-induced neuronal plasticity in a state-setting modulatory interneuron

We have identified a TTX-resistant low-threshold persistent inward sodium current in the cerebral giant cells (CGCs) of Lymnaea, an important state-setting modulatory cell type of molluscan feeding networks. This current has slow voltage-dependent activation and de-activation kinetics, ultra-slow inactivation kinetics and fast de-inactivation kinetics. It activates at approximately -90 mV, peaks at approximately -30 mV, reverses at approximately +35 mV and does not show full voltage-dependent inactivation even at positive voltage steps. Lithium-sodium replacement experiments indicate that the persistent sodium current makes a significant contribution to the CGC membrane potential. Injection of cyclic adenosine monophosphate (cAMP) into the CGC cell body produces a large increase in the persistent sodium current that lasts for several hours. cAMP injection also leads to increased bursting, a significant decrease in the resistance and a significant depolarization of the soma membrane, indicating that cAMP-dependent mechanisms induce prolonged neuronal plasticity in the CGCs. Our observations provide the first link between cAMP-mediated modulation of a TTX-resistant persistent sodium current and prolonged neuronal plasticity in an identified modulatory cell type that plays an important role in behavioral state setting.

The multiple actions of GLP-1 on the process of glucose-stimulated insulin secretion

The physiological effects of glucagon-like peptide-1 (GLP-1) are of immense interest because of the potential clinical relevance of this peptide. Produced in intestinal L-cells through posttranslational processing of the proglucagon gene, GLP-1 is released from the gut in response to nutrient ingestion. Peripherally, GLP-1 is known to affect gut motility, inhibit gastric acid secretion, and inhibit glucagon secretion. In the central nervous system, GLP-1 induces satiety, leading to reduced weight gain. In the pancreas, GLP-1 is now known to induce expansion of insulin-secreting beta-cell mass, in addition to its most well-characterized effect: the augmentation of glucose-stimulated insulin secretion. GLP-1 is believed to enhance insulin secretion through mechanisms involving the regulation of ion channels (including ATP-sensitive K(+) channels, voltage-dependent Ca(2+) channels, voltage-dependent K(+) channels, and nonselective cation channels) and by the regulation of intracellular energy homeostasis and exocytosis. The present article will focus principally on the mechanisms proposed to underlie the glucose dependence of GLP-1's insulinotropic effect.

Activation of protein kinase C by oxytocin-related conopressin underlies pacemaker current in Lymnaea central neurons

The vasopressin/oxytocin-related neuropeptide Lys-conopressin activates two pacemaker currents in central neurons of the mollusk Lymnaea stagnalis. A high-voltage-activated current (I-HVA) is activated at potentials greater than -40 mV and resembles pacemaker currents found in many molluscan neurons. A low-voltage-activated current (I-LVA) activates throughout the range of -90 to 0 mV. Based on sequence homologies, Lymnaea conopressin receptors are thought to couple to Q-type G proteins and protein kinase C (PKC). Alternatively, agonist-induced pacemaker currents in molluscan neurons have traditionally been attributed to cAMP-dependent protein kinase (PKA) activation. Accordingly, this study aimed at resolving possible involvement of cAMP/PKA and diacylglycerol/PKC in the conopressin response. Injection of cAMP into anterior lobe neurons induced a slow inward current with a voltage dependence resembling that of I(LVA) (and not I(HVA)). However, lack of effect of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and the absence of cross-desensitization between cAMP and conopressin suggest that neither current is dependent on intracellular cAMP. The PKC-activating phorbol ester 12-O-tetradecanoylphorbol 13-acetate (but not inactive phorbol 12-myristate 13-acetate) mimicked activation of I(HVA), but not I(LVA), and occluded subsequent responses to conopressin. Activation of I(HVA) was blocked by general protein kinase inhibitors and the PKC-inhibitor GF-109203X. Modulation of the calcium buffering capacity of the pipette medium did not affect the conopressin response, suggesting that calcium dynamics are not of major importance. We conclude that conopressin activates the ion channels carrying I(LVA) and I(HVA) through different second-messenger cascades and that PKC-dependent phosphorylation underlies activation of I(HVA).

Low-threshold Na+ currents: a new family of receptor-operated inward currents in mammalian nerve cells

In the mammalian nervous system, various neurotransmitters can modulate cell excitability by inducing slow membrane potential changes. In the last decade, inhibition of potassium currents has been characterized as the primary mechanism by which neurones can undergo sustained depolarization. More recently (1990s), a new class of inward currents, which are voltage-dependent and mainly carried by sodium ions, has been found to be activated by various neurotransmitter receptors in mammalian central and peripheral neurones. Because the channels involved pass depolarizing current, are open at more negative membrane potentials than the resting potential, and are voltage-gated and persistent, these currents are capable of producing regenerative and maintained depolarizations and play an important role in neuronal signalling.

Vasopressin/oxytocin-related conopressin induces two separate pacemaker currents in an identified central neuron of Lymnaea stagnalis

The molluscan vasopressin/oxytocin analogue Lys-conopressin excites neurons in the anterior lobe of the right cerebral ganglion of the snail Lymnaea stagnalis. Persistent inward currents that underlie the excitatory response were studied with the use of voltage-ramp protocols in the identified neuron RCB1 and other anterior lobe neurons. Under whole cell voltage-clamp conditions, two types of conopressin-activated current could be distinguished on the basis of their voltage dependence: 1) a pacemaker-like current that was activated at potentials above -40 mV (high-voltage-activated current, I(HVA)) and 2) an inward current that was activated at all potentials between -90 and +10 mV (low-voltage-activated current, I(LVA)). Ion substitution experiments indicate that sodium is the main charge carrier for I(HVA) and I(LVA). Both currents are differentially affected by cadmium. I(HVA) and I(LVA) differ in dose dependence, with median effective concentration values of 7.7 x 10(-8) M and 2.2 x 10(-7) M, respectively. Vasopressin and oxytocin act as weak agonists for the conopressin responses. The kinetics of desensitization and washout of I(HVA) and I(LVA) are different. The HVA response shows little desensitization, whereas the LVA response desensitizes within minutes (time constant 80 +/- 28 s, mean +/- SD). The time constant of washout on removal of conopressin is 159 +/- 63 s for I(HVA) and 36 +/- 13 s for I(LVA). These results suggest that two distinct conopressin receptors are involved in the activation of both currents. The conopressin-activated currents induce or enhance a region of negative slope resistance in the steady-state current-voltage relation. They differ from a third persistent inward current that is carried by calcium and completely blocked by cadmium. The presumed functional roles of these currents, possibly including autoregulation, are discussed.

Cyclic AMP levels, adenylyl cyclase activity, and their stimulation by serotonin quantified in intact neurons

In molluscan central neurons that express cAMP-gated Na+ current (INa,cAMP), estimates of the cAMP binding affinity of the channels have suggested that effective native intracellular cAMP concentrations should be much higher than characteristic of most cells. Using neurons of the marine opisthobranch snail Pleurobranchaea californica, we applied theory and conventional voltage clamp techniques to use INa,cAMP to report basal levels of endogenous cAMP and adenylyl cyclase, and their stimulation by serotonin. Measurements were calibrated to iontophoretic cAMP injection currents to enable expression of the data in molar terms. In 30 neurons, serotonin stimulated on average a 23-fold increase in submembrane [cAMP], effected largely by an 18-fold increase in adenylyl cyclase activity. Serotonin stimulation of adenylyl cyclase and [cAMP] was inversely proportional to cells' resting adenylyl cyclase activity. Average cAMP concentration at the membrane rose from 3.6 to 27.6 microM, levels consistent with the expected cAMP dissociation constants of the INa,cAMP channels. These measures confirm the functional character of INa,cAMP in the context of high levels of native cAMP. Methods similar to those employed here might be used to establish critical characters of cyclic nucleotide metabolism in the many cells of invertebrates and vertebrates that are being found to express ion currents gated by direct binding of cyclic nucleotides.

Analysis of spontaneous electrical activity in cerebellar Purkinje cells acutely isolated from postnatal rats

Whole-cell patch recording techniques were used to analyze spontaneous electrical activity in cerebellar Purkinje cells acutely isolated from postnatal rats. Spontaneous activity was present in 65% of the cells examined, and it included simple and complex firing patterns which persisted under conditions that eliminated residual or reformed synaptic contacts. Under voltage clamp, both spontaneous and quiescent cells displayed similar voltage-dependent conductances. Inward current was carried by Na+ through tetrodotoxin (TTX)-sensitive channels and by Ca2+ through P-type and T-type Ca channels. P-type current was present in all cells examined. T-type current was found in <50%, and it did not correlate with spontaneous activity. We found no evidence of a transient (A-type) potassium current or hyperpolarization-activated cationic current in either spontaneous or quiescent cells. Spontaneous activity did correlate with a lower activation threshold of the Na current, resulting in substantial overlap of the activation and inactivation curves. TTX reduced the holding current of spontaneous cells clamped between -50 and -30 mV, consistent with the presence of a Na "window" current. We were unable, however, to measure a persistent component of the Na current using voltage steps, a result which may reflect the complex gating properties of Na channels. An Na window current could provide the driving force underlying spontaneous activity, as well as plateau potentials, in Purkinje cells.

Identification and characterization of a Ca(2+)-sensitive nonspecific cation channel underlying prolonged repetitive firing in Aplysia neurons

The afterdischarge of Aplysia bag cell neurons has served as a model system for the study of phosphorylation-mediated changes in neuronal excitability. The nature of the depolarization generating the afterdischarge, however, has remained unclear. We now have found that venom from Conus textile triggers a similar prolonged discharge, and we have identified a slow inward current and corresponding channel, the activation of which seems to contribute to the onset of the discharge. The slow inward current is voltage-dependent and Ca(2+)-sensitive, reverses at potentials slightly positive to O mV, exhibits a selectivity of K approximately equal to Na >> Tris > N-methyl-D-glucamine (NMDG), and is blocked by high concentrations of tetrodotoxin. Comparison of these features with those observed in channel recordings provides evidence that a Ca(2+)-sensitive, nonspecific cation channel is responsible for a slow inward current that regulates spontaneous repetitive firing and suggests that modulation of the cation channel underlies prolonged changes in neuronal response properties.

Identification and characterization of a Ca(2+)-sensitive nonspecific cation channel underlying prolonged repetitive firing in Aplysia neurons

The afterdischarge of Aplysia bag cell neurons has served as a model system for the study of phosphorylation-mediated changes in neuronal excitability. The nature of the depolarization generating the afterdischarge, however, has remained unclear. We now have found that venom from Conus textile triggers a similar prolonged discharge, and we have identified a slow inward current and corresponding channel, the activation of which seems to contribute to the onset of the discharge. The slow inward current is voltage-dependent and Ca(2+)-sensitive, reverses at potentials slightly positive to O mV, exhibits a selectivity of K approximately equal to Na >> Tris > N-methyl-D-glucamine (NMDG), and is blocked by high concentrations of tetrodotoxin. Comparison of these features with those observed in channel recordings provides evidence that a Ca(2+)-sensitive, nonspecific cation channel is responsible for a slow inward current that regulates spontaneous repetitive firing and suggests that modulation of the cation channel underlies prolonged changes in neuronal response properties.

Nitric oxide induces an increased Na+ conductance in identified neurons of Aplysia

The ionic mechanism of the effects of micropressure ejections of hydroxylamine (HOA) and sodium nitroprusside (SNP), nitric oxide (NO) generators, on the membrane of identified neurons (R9-R12) of Aplysia kurodai was investigated with conventional voltage-clamp, micropressure ejection, and ion-substitution techniques. Micropressure ejection of HOA and SNP onto the neurons caused a marked depolarization in the unclamped neurons. Clamping the same neurons at their resting potential level (-60 mV) and reejecting HOA and SNP with the same dose produced a slow inward current (Ii(HOA) and Ii(SNP), 3-7 nA in amplitude, 15-60 s in duration) associated with an increase in input membrane conductance. Bath-applied hemoglobin (50 microM), a nitric oxide scavenger, almost completely blocked Ii(HOA) and Ii(SNP), and 3-isobutyl-1-methylxanthine (IBMX, 50 microM) prolonged and enhanced both Ii(HOA) and Ii(SNP). An intracellular injection of cyclic guanosine 3',5'-monophosphate (cGMP) into the same neurons produced a slow inward current (Ii(cGMP)) which resembled the responses to HOA and SNP, and this current was enhanced in IBMX. Bath-applied methylene blue (10 microM), an inhibitor of guanylate cyclase, significantly reduced Ii(HOA) and Ii(SNP). The inward currents induced by HOA, SNP and cGMP were sensitive to changes in the external Na+ concentration. These results suggest that extracellular NO can induce a slow inward current associated with an increase in Na+ conductance, mediated by an increase in intracellular cGMP.

The neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) directly gates two ion channels in an identified Helix neurone

FMRFamide (i.e. Phe-Met-Arg-Phe-NH2) application to the C2 neurone of Helix caused a depolarizing response which consisted of a large, rapidly developing, and rapidly desensitizing inward current, underlain by a smaller, slower inward current which did not desensitize. Both currents were carried through sodium-selective channels which were insensitive to D-tubocurarine, and the to the fast sodium channel blockers tetrodotoxin (TTX) and lignocaine. Only the faster, desensitizing current could be blocked by amiloride. FMRFamide also activated two types of unitary inward currents with slightly differing amplitudes in outside-out patches taken from the C2 neurone, both through sodium-selective ion channels. Only the smaller unitary currents readily desensitized and were susceptible to block by amiloride, and they also activated more rapidly. Unitary currents of both types were recorded in outside-out patches in the absence of freely diffusible intracellular mediators, and were also activated when guanosine 5'-O-(2-thiodiphosphate) (GDP [beta-S]) was included in the recording pipette solution. This supports a tight receptor/channel coupling for both responses, with no involvement of GTP-binding proteins. Further, the very fast rate of activation of the smaller channels, which generally carry the major part of the FMRFamide-induced current, strongly indicates that these channels are ligand gated.

Activation of adenylate cyclase attenuates the hyperpolarization following single action potentials in brain noradrenergic neurons independently of protein kinase A

Afterhyperpolarizations that follow action potentials are a prominent mechanism for the control of neuronal excitability. Such afterhyperpolarizations in many neurons are modulated by a variety of second messenger systems. Here, we examined the regulation of afterhyperpolarizations in noradrenergic locus coeruleus neurons by the adenylate cyclase system. Although superfusion of the adenylate cyclase activator, forskolin, had no effect on hyperpolarizations following trains of action potentials, both forskolin and a membrane permeable analog of cyclic AMP, 8-bromo-cyclic AMP, attenuated the amplitude of afterhyperpolarizations which followed single action potentials of locus coeruleus neurons recorded intracellularly in brain slices. In contrast, superfusion of 1,9-dideoxyforskolin, the forskolin analog that does not activate adenylate cyclase, had no effect on these single action potential afterhyperpolarizations. Co-application of a protein kinase inhibitor (H8, KT5720, staurosporin or Rp-cAMPS) with either forskolin or 8-bromo-cyclic AMP failed to block the reduction of afterhyperpolarization amplitude, but blocked the cyclic AMP-dependent enhancement of opiate responses in the same locus coeruleus neurons. Furthermore, application of a membrane permeable analog of 5'-AMP, 8-bromo-5'-AMP, the cyclic AMP metabolite that does not activate a protein kinase, potently reduced the amplitudes of single action potential afterhyperpolarizations. The afterhyperpolarization amplitude was also reduced in locus coeruleus neurons taken from chronically morphine-treated rats, a treatment known to increase adenylate cyclase activity. These results indicate that elevation of intracellular cyclic AMP or 5'-AMP reduces the single action potential afterhyperpolarization in locus coeruleus neurons. This action may be mediated through a mechanism independent of protein kinase activation.

Patch-clamp recording of cAMP-induced membrane current noise in Helix pomatia neurons

Membrane current noise evoked by intracellular cAMP injection was studied in isolated Helix pomatia neurons with the patch-clamp technique. Fluctuation analysis was used to estimate the elementary current amplitude (i) and the single channel conductance. It was found that i decreased linearly with cell depolarization and the extrapolated reversal potential was approximately -12 mV. The calculated single-channel conductance was 0.9 +/- 0.14 pS, a value quite different from those obtained for cAMP-activated channels in Pleurobranchaea neurons.

A novel tetrodotoxin-insensitive, slow sodium current in striatal and hippocampal neurons

Slow inward currents modulate neuronal firing patterns and may generate depolarizing afterpotentials (DAPs). We report a novel, slow Na+ current (INaS) in striatal and hippocampal neurons that can generate DAPs. INaS activated at depolarizations greater than -40 mV, was tetrodotoxin insensitive, and activated and deactivated slowly over hundreds of milliseconds. INaS was dependent upon extracellular Na+, but was not affected by 0 mM extracellular Ca2+ or by Ca2+ channel blockers (Mn2+, Cd2+, or Co2+). A tetrodotoxin-insensitive, Na(+)-dependent plateau potential that was likely generated by INaS was shown to underlie DAPs during intracellular recordings from hippocampal CA1 pyramidal neurons. Membrane depolarizations and DAPs generated by INaS may contribute to alterations in neuronal firing and epileptiform bursting.

Co-regulation of cAMP-activated Na+ current by Ca2+ in neurones of the mollusc Pleurobranchaea

1. The cAMP-gated Na+ current (INa, cAMP) was studied in axotomized neurons of the pedal ganglion of the sea slug Pleurobranchaea. INa, cAMP responses were elicited by iontophoretic injection of cAMP and recorded in voltage clamp. 2. The current-voltage relation for INa, cAMP was flat between -90 and -50 mV, but declined steeply with depolarization from -50 to -30 mV. Depolarizing pulses also suppressed the INa, cAMP response, which recovered slowly over tens of seconds. 3. The inactivating effects of depolarization on the current were abolished both by blockade of Ca2+ current and intracellular injection of Ca2+ chelator. Thus, Ca2+ influx through voltage-dependent Ca2+ channels probably mediates inactivation of INa, cAMP within its normal physiological range of action. 4. Increasing intracellular cAMP levels antagonized the effects of Ca2+ influx on INa, cAMP. The mutual antagonism of the ions suggests that cAMP and Ca2+ act competitively in regulation of the INa, cAMP channel. 5. Measures of fractional inactivation of INa, cAMP provided evidence for the existence of an appreciable basal level of current, and hence cAMP, in the unstimulated neuron. Since INa, cAMP is a direct function of cAMP activity, measures of fractional inactivation permit quantification of cAMP levels in the living neuron. 6. Calcium inactivation of INa, cAMP completes a negative feedback loop that can contribute to endogenous burst activity. Over the burst cycle, depolarization and action potential activity driven by INa, cAMP would lead to Ca2+ influx, consequent inactivation of the inward current, and hyperpolarization. This mechanism of endogenous bursting resembles other in which the burst cycle has been found to be regulated by kinetics of Ca2+ influx and removal. However, INa, cAMP may vary in its Ca2+ sensitivity in different neurons and these variations may affect the functional expression of endogenous oscillatory activity.

Transduction mechanism for glutamate-induced potassium current in neurones of the mollusc Planorbarius corneus

1. The potassium currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. Glutamate responses were not modified by increasing intracellular cyclic nucleotide concentrations (treatment with 8-Br-cAMP, 8-Br-cGMP, forskolin and/or the phosphodiesterase inhibitor isobutylmethylxantine, IBMX), whereas inward-going currents induced by the nucleotides were observed. It follows that glutamate currents are independent of intracellular cyclic nucleotide control. 3. Protein kinase C activation with phorbol esters or oleoylacetylglycerol induced a slowly developing outward current and reduced glutamate response amplitude. Staurosporine itself did not affect the glutamate responses but completely prevented the effects of phorbol esters and oleoylacetylglycerol. This indicated that protein kinase C was not involved in the transduction mechanism for the potassium component of the glutamate response. 4. The possible involvement of inositol-1,4,5-trisphosphate seems to be improbable because the glutamate responses were independent of intracellular calcium concentration. Intracellular injection of calcium buffer BAPTA, failed to affect any of the glutamate currents, although it effectively blocked the after-hyperpolarization following directly evoked action potentials. 5. Nordihydroguaiaretic acid (NDGA) and indomethacin, inhibitors of the lipoxygenase and cyclo-oxygenase pathways of arachidonic acid metabolism, correspondingly, did not change the glutamate responses of these neurones. 6. The failure to demonstrate the involvement of any known secondary messenger systems in glutamate response transduction favours two assumptions: (1) the receptor-G protein complex controls the potassium channel directly; or (2) some still unknown transduction system is used.

Evidence that potassium channels mediate the effects of serotonin on the ocular circadian pacemaker of Aplysia

The eye of the marine mollusk Aplysia californica contains a photo-entrainable circadian pacemaker that drives an overt circadian rhythm of spontaneous compound action potentials in the optic nerve. Serotonin is known to influence the phase of this ocular rhythm. The aim of the present study was to evaluate whether potassium channels are involved in effects on the ocular circadian rhythm. Our experimental approach was to study the effect of the potassium channel antagonist barium on serotonin-induced phase shifts of this rhythm. The application of barium was found to block serotonin-induced phase shifts whereas barium alone did not cause significant phase shifts. The effects of barium were found to be dose dependent. In addition, barium blocked forskolin-induced phase advances but did not interfere with serotonin-induced increases in cAMP content. Finally, barium antagonized serotonin-induced suppression of compound action potential activity. These results are consistent with a model in which the application of serotonin phase shifts the ocular pacemaker by causing a membrane hyperpolarization which is mediated by a cAMP-dependent potassium conductance.

Cyclic AMP analog activates Na(+)-dependent inward currents in dissociated frog motoneurons

Effects of intracellular accumulation of 3',5'-cyclic adenosine monophosphate (cAMP) were studied using 3',5'-cyclic 8-bromoadenosine monophosphate (8-Br-cAMP) and forskolin on single motoneurons acutely dissociated from adult bullfrog spinal cord. 8-Br-cAMP (10(-3) M) and forskolin (1.5 x 10(-6) M) activated inward currents under K(+)-free conditions at a holding potential of -70 mV. The currents were dependent on extracellular Na+ concentration, and were never reversed within the range of membrane potentials tested (-130 to 30 mV). These results indicate that accumulation of intracellular cAMP induces Na(+)-dependent inward currents in frog motoneurons.

Activation of beta-adrenergic receptor induces Na(+)-dependent inward currents in acutely dissociated motoneurons of bullfrog spinal cord

Effects of adrenergic drugs on single motoneurons acutely dissociated from the lumbar enlargement of adult bullfrogs were examined. The dissociated large cells were identified as motoneurons by retrograde labeling with a fluorescent dye. Adrenaline caused membrane depolarization with a decrease in input resistance. Under whole-cell voltage clamp conditions at a holding potential of -70 mV, adrenergic drugs induced inward currents in a dose-dependent manner. Adrenaline was more potent than noradrenaline. Under K(+)-free conditions, adrenaline (10(-6)-10(-5) M) induced inward currents which were blocked by propranolol (10(-6) M) but not by phentolamine (10(-5) M). CoCl2 (1 mM) did not affect the currents. Substitution of choline+ in the recording solution for Na+ abolished the currents, but tetrodotoxin (TTX, 10(-6) M) had no effect on them. The adrenaline-induced currents exhibited a characteristic voltage-dependency: the conductance became large at hyperpolarized membrane potential (-150 to -30 mV) and approached zero at the depolarized membrane potential (greater than -30 mV), but was never reversed up to 30 mV, suggesting that the currents are different from non-specific cation currents. Substitution of isethionate- for Cl- in the recording solution had no effect on the voltage-dependency of the adrenaline-induced currents, whereas substitution of choline+ for Na+ apparently attenuated the voltage-dependency of the currents. These results indicate that adrenaline induces Na(+)-dependent inward currents through activation of beta-adrenergic receptors in bullfrog motoneurons.

Properties of cyclic AMP-dependent inward current in two identified neurons of the snail Lymnaea stagnalis

1. Intracellular iontophoresis of cyclic AMP induces a slow, inward sodium current in identified neurons B1 and RPD1 of Lymnaea stagnalis, which is TTX-insensitive and voltage independent. 2. High extracellular Ca2+ or Co2+ ions cause a reduction in the amplitude of the current, whereas low Ca2+ enhances it. 3. Agents known to increase intracellular cyclic AMP lead to depolarization and increased activity in B1, but RPD1 is unaffected. 4. The properties of cyclic AMP-stimulated inward current in neurons of L. stagnalis are compared with those of similar currents in other gastropod species.

Pacemaker activity of locus coeruleus neurons: whole-cell recordings in brain slices show dependence on cAMP and protein kinase A

Noradrenergic neurons of the rat locus coeruleus (LC) are endogenous pacemakers that exhibit slow, tonic firing even in the complete absence of synaptic inputs. In the present study a time-dependent decline in LC spontaneous firing activity was found on intracellular dialysis during whole-cell recording with low-resistance patch electrodes; this decline was accentuated by a specific inhibitor of cAMP-dependent protein kinase (PKI5-24). Conversely, the inclusion of cAMP, 8-Br-cAMP, or the catalytic subunit of cAMP-dependent protein kinase (PKAcat) in the patch pipettes dose-dependently increased firing rate; intracellular PKI5-24 blocked both 8-Br-cAMP and PKAcat-induced firing in LC neurons. These results indicate that endogenous cAMP, via a phosphorylation-dependent route, drives tonic pacemaker activity in LC neurons.

Modulation of calcium-activated non-specific cation currents by cyclic AMP-dependent phosphorylation in neurones of Helix

1. Currents through calcium-activated non-specific cation (CAN) channels were studied in the fast burster neurone of Helix aspersa and Helix pomatia. CAN currents were activated by reproducible intracellular injections of small quantities of Ca2+ utilizing a fast, quantitative pressure injection technique. 2. External application of forskolin (10-25 microM), an activator of adenylate cyclase, caused the endogenous bursting activity of the cells to be replaced by beating activity. These same concentrations of forskolin reduced CAN currents reversibly to about 50%. 3. External application of IBMX (3-isobutyl-1-methylxanthine, 100 microM), an inhibitor of phosphodiesterase, the enzyme which breaks down cyclic AMP, reduced CAN currents reversibly to about 40%. 4. External application of the membrane-permeable cyclic AMP analogues 8-bromo-cyclic AMP and dibutyryl-cyclic AMP (100 microM) caused almost complete block of the CAN current. A marked reduction in the CAN current was also observed following quantitative injections of cyclic AMP (internal concentrations up to 50 microM) directly into the cells from a second pressure injection pipette. 5. Similar results were obtained with quantitative injections of the catalytic subunit (C-subunit) of the cyclic AMP-dependent protein kinase (internal concentrations 10(-4) units of enzyme) directly into the cells from a second pressure injection pipette. 6. Injection of the non-hydrolysable GTP analogue, GTP-gamma-S (internal concentrations 100 microM), which stimulates G-proteins, produced a prolonged increase in CAN current amplitude by as much as 300%. 7. External application of serotonin (100-200 microM) caused a transition from bursting to beating activity of the neurones and mimicked cyclic AMP's effects on CAN currents. Two other neurotransmitters, dopamine and acetylcholine, were not significantly effective in reducing CAN currents. 8. Injection of a peptide inhibitor of cyclic AMP-dependent protein kinase suppressed serotonin's action on bursting and on CAN current. 9. Our results indicate that CAN currents in Helix burster neurones are modulated by cyclic AMP-dependent membrane phosphorylation. They suggest that the physiological transmitter that induces this second messenger action is serotonin. The dual control of CAN channels by two second messengers, namely Ca2+ and cyclic AMP, has important functional implications. While Ca2+ activates these channels which generate the pacemaker current in these neurones, cyclic AMP-dependent phosphorylation down-regulates them, thereby resulting in modulation of neuronal bursting activity.

The role of cGMP in the extinction of the reactions of identified neurons of the edible snail in response to acetylcholine

The possible role of cGMP in the regulation of the extinction of the reactions of the RPa4, RPa3, and LPa3 neurons of the edible snail in response to acetylcholine (ACh), applied rhythmically to the soma of the neuron by means of microiontophoresis, has been investigated. It was demonstrated that activators of guanylate cyclase which increased the level of cGMP in the cell, namely, sodium nitroprusside and sodium azide (5.10(-4)-10(-3) mole/liter), when applied intracellularly, intensify the extinction of inward transmembrane current and of depolarization of the membrane in response to ACh. The hypothesis of the participation of cGMP-dependent phosphorylation of membrane proteins in the regulation of the rate of development, depth, and duration of short-lived plasticity of the cholinoreceptors of the neuron is proposed.

Cyclic adenosine monophosphate (cAMP)-induced potentiation of synaptic responses in Helix neurons

1. The effect of intracellularly injected cAMP on the amplitude of excitatory postsynaptic potentials was studied using identified neurons of the snail Helix pomatia. 2. In 25% of the experiments, postsynaptic cAMP elevation caused a pronounced augmentation of the excitatory postsynaptic potential (EPSP) amplitude, lasting up to 15-30 min. 3. The results suggest that a cAMP increase in the postsynaptic neuron may be involved in the enhancement of synaptic efficiency.

Oxytocin-induced sodium current is mediated by cAMP-dependent protein phosphorylation in an identified snail neuron

The intracellular biochemical process underlying oxytocin-induced change of membrane properties was analyzed in an identified neuron of Achatina fulica Férussac, using pressure injection technique and pharmacological tools. Oxytocin dose-dependently enhanced the negative slope resistance (NSR) region on the current-voltage relation. The oxytocin-induced current was attenuated by a reduction of extracellular Na+ and not influenced by the addition of 100 microM tetrodotoxin (TTX) to the medium, suggesting that this current is predominantly due to the activation of TTX-resistant Na+ channels. In the Ca2(+)-free state, substituted by an equivalent amount of Co2+, the amplitude of oxytocin-induced current was somewhat reduced at the NSR region but it was not influenced at less than -60 mV. Application of 100 microM isobutylmethylxanthine, a phosphodiesterase inhibitor, augmented the oxytocin-induced current. Pressure injection of 10 mM adenosine 3',5'-cyclic monophosphate (cAMP) elicited a Na(+)-dependent inward current similar to the oxytocin response. The further role of cAMP linked with the oxytocin-induced current was investigated using two kinds of cAMP-dependent protein kinase inhibitors, isoquinolinesulfonamide (H-8) and protein kinase inhibitor (PKI). Extracellular application of H-8 or pressure injection of PKI, prior to oxytocin application, both blocked the oxytocin-induced current. Based on these results, oxytocin-elicited inward currents may mediate cAMP-dependent protein phosphorylation mainly by activation of Na+ channels.

Proctolin activates a slow, voltage-dependent sodium current in motoneurons of the lobster cardiac ganglion

The two-electrode voltage-clamp technique was used to study the ionic basis of the slow depolarization caused by the neuropeptide proctolin. At negative membrane potentials, proctolin caused a dose-dependent slow inward current. This current reversed and became outward at membrane potentials positive to 0 to +20 mV. Current-voltage curves also showed the response to be voltage-dependent, with a reversal potential at positive membrane potentials. The response was blocked in sodium-free solutions. Solutions with reduced sodium concentration caused a shift of the reversal potential in a manner expected for a response that is largely sodium-dependent. We conclude that proctolin causes an inward current by increasing a voltage-dependent cation conductance that is predominantly permeable to sodium.

Excitation of locus coeruleus neurons by vasoactive intestinal peptide: evidence for a G-protein-mediated inward current

Vasoactive intestinal polypeptide (VIP) caused a reversible increase in the firing rate of locus coeruleus (LC) neurons. Voltage-clamp at -60 mV revealed that VIP induced an inward current associated with a small increase in conductance. The inward current persisted in the presence of Co2+ (to block Ca2+ channels) or tetrodotoxin (to block fast voltage-dependent Na+ channels). Substitution (80%) of Na+ with choline or Tris reduced the VIP-elicited inward current by approximately 75%. Changing external K+ concentrations did not alter the effect of VIP. The inward current induced by VIP became irreversible after the intracellular administration of GTP gamma S, a hydrolysis-resistant analog of GTP which can cause a prolonged activation of G-proteins. The intracellular application of GDP beta S, which can interfere with G-protein activation, attenuated the effect of VIP. Pertussis toxin, an inactivator of certain G-proteins, did not block the effect of VIP. We conclude that VIP directly excites LC neurons by inducing a largely Na-dependent inward current. As this effect became irreversible in the presence of intracellular GTP gamma S, was attenuated by GDP beta S, and was not eliminated by pertussis toxin, mediation through a pertussis toxin-insensitive G-protein is suggested.

The cyclic GMP-induced inward current in neuron R14 of Aplysia californica: similarity to a FMRFamide-induced inward current

Injecting cGMP into Aplysia neuron R14 induced an inward current similar to one elicited by application of FMRFamide to the outside of that cell. In contrast, injection of cAMP into R14 caused a long-lasting outward current and conductance increase. Phosphodiesterase inhibitors increased the cGMP and FMRFamide-induced inward currents in R14. The cGMP-induced inward current is voltage dependent and is largely carried by Na+. It is also strongly and inversely dependent on both external [Ca2+] and [Cl-], although these ions are not significant current carriers. Changing external [K+] had no effect. Voltage and ion dependencies of the cGMP-induced inward current are similar to those of an inward current induced by FMRFamide. Thus cGMP may be a second messenger to FMRFamide in producing a slow inward current in R14. cGMP does not appear to be a second messenger to FMRFamide in most Aplysia neurons.

Dopamine-induced depolarizing responses associated with negative slope conductance in LB-cluster neurones of Aplysia

1. Current- and voltage-clamp methods were used to evaluate the intracellular and ionic mechanisms involved in dopamine-induced slow depolarizations recorded from neurones of the LB cluster in the abdominal ganglion of Aplysia kurodai. 2. In voltage-clamped cells, dopamine induced a slow inward current that, over the range studied (-40 to -110 mV), decreased in amplitude with hyperpolarization of the cell, but failed to invert when the cell was hyperpolarized beyond the reversal potential for K+,(E)K. 3. Bathing the ganglion in 3-isobutyl-1-methylxanthine (IBMX) caused a significant increase in the dopamine response. 4. Most of the responses to dopamine were markedly augmented in Ca2+-free media, but were depressed in Na+-free media. 5. An intracellular injection of cyclic adenosine 3',5'-monophosphate (cyclic AMP) into the same cell type produced an inward current which, like the response to dopamine, diminished in amplitude with hyperpolarization of the cell. 6. Like the dopamine response, the cyclic AMP response increased in the presence of IBMX, was enhanced in Ca2+-free media, was depressed in Na+-free media, and was unaffected by changes in external potassium. 7. In a few cells, although the cyclic AMP-induced responses disappeared in Na+-free media, the dopamine-induced slow inward current responses did not. However, these Na+-free resistant responses disappeared completely in Na+- and Ca2+-free media. 8. It was concluded that most of the dopamine-induced inward current responses were produced by an increase in permeability, mainly to Na+, triggered by a receptor-controlled increase in intracellular cyclic AMP.

Stimulation of sodium current by cyclic AMP is mediated through protein phosphorylation in Euhadra neurons

The protein kinase inhibitors, protein kinase inhibitor isolated from rabbit muscle and isoquinolinesulfonamide, abolished the inward Na current which was elicited by cAMP.

Enhancement of cyclic AMP-dependent sodium current by the convulsant drug pentylenetetrazol

The convulsant drug pentylenetetrazol (PTZ) causes paroxysmal depolarizing shifts (PDS) and bursting in molluscan neurons. PDS has been found to be accompanied by increased levels of cyclic AMP (cAMP) and supported by persistent Na+ current. In neurons of the snail Lymnaea stagnalis the blocker of cAMP degradation isobutylmethylxanthine (IBMX) mimicks PTZ action. Na+ dependence of PTZ-induced inward shift in holding current in voltage-clamped cells supports the potential Na+ current origin of PDS. Intracellular cAMP injection elicits a transient Na+ current whose amplitude and duration are enhanced by both PTZ and IBMX. PTZ may cause PDS partly through slowing cAMP degradation, thus enhancing the cAMP-dependent Na+ current. PDS-generated bursts cause partial inactivation of the Na+ current, which may contribute towards burst termination.

Parallel processing of short-term memory for sensitization in Aplysia

How is the short-term memory for a single form of learning distributed among the various elements of a neuronal circuit? To answer this question, we examined the short-term memory for sensitization, using the siphon component of the defensive gill- and siphon-withdrawal reflex. We found that the memory for short-term sensitization is represented by at least four sites of circuit modification, each involving a different type of plasticity. These include (1) presynaptic facilitation of the sensory neuron connections onto both interneurons and motorneurons; (2) presynaptic inhibition at the connections of the L30 inhibitory neurons onto the excitatory interneuron L29; (3) posttetanic potentiation of the excitatory connections made by L29 onto a specific subclass of siphon motorneurons, the LFS cells; and (4) an increase in the tonic firing rate of the LFS siphon motor neurons, resulting in neuromuscular facilitation. Each of the heterosynaptic changes seems to involve a common modulatory transmitter and to utilize a common second messenger system. Moreover, each of these sites seems capable of encoding a different component of the short-term memory. Facilitation of the connections of sensory neurons should contribute to the increase in amplitude of the response; the disinhibition of the L29 interneurons and the posttetanic potentiation at L29 synapses should contribute to an increase in the duration of the response; and the increase in tonic firing of the LFS subclass of siphon motor neurons seems capable of contributing both to an increase in response amplitude and to changes in response topography.

Edrophonium-induced inward membrane current in single neurons physically isolated from Aplysia californica

The action of edrophonium on Aplysia neurons was studied using a concentration clamp technique which combines internal perfusion and a rapid drug application. Edrophonium elicited a dose-dependent inward current in the concentration range 10(-6) to 10(-4) M. At higher concentrations (10(-3) and 10(-2) M), the amplitude of the current often decreased and there was a rapid decay of the current. At these high concentrations, the current increased immediately after washing the neuron with normal solution. These results suggest that edrophonium blocks the ion channel which it opens. Removal of Na+ from the external solution greatly reduced the current amplitude by more than 90%. Removal of Ca2+ also reduced the amplitude of the response; however an increase of Ca2+ did not augment the response. These results suggest that Ca2+ does not carry the current, but is necessary for generation of an Na+-dependent inward current. Edrophonium, 10(-2) M, which completely blocked the current it induced within 20 s, did not significantly affect the voltage-dependent Na+ current. Tetrodotoxin, 1 x 10(-6) M, did not affect the edrophonium response. Hexamethonium, 1 x 10(-4) M, did not change the response elicited by edrophonium, while it significantly reduced the ACh response mediated by Na+. In some neurons edrophonium elicited an inward current, but ACh induced an outward current. Therefore the Na+ channels opened by edrophonium appear to be distinct from both the voltage-gated and ACh receptor-operated Na+ channels.

Cyclic AMP-stimulated sodium current in identified feeding neurons of Lymnaea stagnalis

Iontophoretic injection of cAMP elicits a slow, transient inward current in identified buccal feeding motoneurons and in the giant cerebral interneuron of the snail, Lymnaea stagnalis. The current is voltage independent, and is abolished in the absence of extracellular Na+. Application of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) causes a marked increase in both amplitude and duration of cAMP-stimulated inward current. The amplitude of the current is reduced following prolonged application of depolarizing pulses to the cell. However, generation of high-frequency bursts of action potentials lasting up to 20 s has no significant effect on the amplitude of the cAMP-induced current measured subsequently. Bath application of the cAMP analogue 8-chlorophenylthio-cAMP or of IBMX leads to enhanced bursting activity in buccal motoneurons. It is suggested that cAMP sensitivity in feeding motoneurons provides a mechanism for adjusting the cells' responsiveness to rhythmic synaptic inputs during the generation of feeding motor output.

Calcium dependence of voltage sensitivity in adenosine 3',5'-cyclic phosphate-stimulated sodium current in Pleurobranchaea

1. Ionophoretic injection of cyclic AMP into a voltage-clamped molluscan neurone caused a transient slow inward current (Isi) whose amplitude was enhanced by depolarization. Na+-replaced salines abolished the current, placing it with cyclic AMP-stimulated Na+ currents of other gastropod species. 2. Isi amplitude was suppressed by extracellular Ca2+. The amplitude increased up to 4-fold at holding potentials of -50 mV in nominally Ca2+-free saline. Ion substitutions showed that Ca2+ suppressed Isi more effectively than Mg2+, Co2+, Cd2+, Mn2+, Ba2+ or Sr2+. 3. Voltage sensitivity of Isi was abolished by low-Ca2+ salines, by the Ca2+ current blocker Co2+ and by substitution of Ba2+ or Sr2+ as Ca2+ channel current carriers. In such salines Isi showed no appreciable change in amplitude at holding potentials between -70 and -25 mV. 4. Intracellular injection of the Ca2+ chelator EGTA both augmented the amplitude of the current and its duration. EGTA injection failed to suppress the Ca2+-dependent voltage sensitivity of Isi. Intracellular injection of concentrated 3-N-(morpholino) propanesulphonic acid (MOPS) pH buffer to inhibit secondary, Ca2+-dependent intracellular acidification also failed to suppress the voltage sensitivity, as did injections of a mixed EGTA and MOPS solution. 5. While the data indicate a requirement for extracellular Ca2+ in conferring voltage sensitivity, they do not support a role for an intracellular action. An extracellular binding site for Ca2+ could mediate the voltage sensitivity, either by local depolarization-dependent changes in extracellular Ca2+ concentration or through direct voltage-sensitive block of the Isi channel.

Depolarization induced by injection of cyclic nucleotides into frog taste cell

In order to identify the intracellular transmitter involved in the taste transduction process, cyclic nucleotides were iontophoretically injected into the frog taste cells while membrane potentials were recorded intracellularly. Injection of either cyclic GMP or cyclic AMP induced a depolarization response of about 5 mV in the taste cells, but injection of Cl- had no effect. The rate of a repolarization after the depolarization elicited by cyclic GMP was larger than that after cyclic AMP. The possible role of cyclic nucleotide in the taste transduction was discussed.

Differential effects of pentylenetetrazole on ion currents of Aplysia neurones

The effect of the convulsant drug pentylenetetrazole (PTZ) on separated membrane current components has been studied in identified voltage-clamped Aplysia neurones. External PTZ blocks the voltage-dependent Na+, Ca2+ currents and the delayed rectifier current (INa, ICa and IK,V, respectively). The amplitude of the Ca2+-activated K+ current (IK,Ca) is increased. The amplitude of the fast inactivating K+ current (IA) is transiently increased at low concentrations of PTZ but is depressed at higher concentrations or after long-lasting application of the drug. The effect of PTZ on leakage current (IL) seems to depend on the cell type. In some cells (R-15, L-7, LP-1) IL is decreased while it is increased in other cells (L-11, BL-1, BR-1). PTZ accelerates the inactivation of IK,V and IA and shifts the current-voltage relation of ICa to negative voltages by 5-8 mV. Pressure injection of PTZ into the neurone did not affect IK,V or IK,Ca. Thus PTZ seems to act on the outside of the plasma membrane. The effect of external PTZ on INa, ICa, IK,V and IL is also observed if the internal Ca2+ activity is buffered with EGTA suggesting that an increase in the internal Ca2+ activity is not involved. At -40 mV PTZ induces a tetrodotoxin-insensitive inward current carried by Na+ ions. PTZ transforms the beating pacemaker cell L-11 into a bursting pacemaker and the bursting pacemaker cell R-15 exhibits 'square-wave'-like oscillations of the membrane potential.

Direct ion channel gating: a new function for intracellular messengers

1. There is widespread belief that intracellular messengers [e.g., Ca2+, cyclic AMP, cyclic GMP, inositol-1,4,5-triphosphate (IP3)] assert their actions primarily through activation of protein kinases. 2. In studies of excitable cells protein kinase activation has been shown to alter membrane ionic conductance, presumably through phosphorylation of ion channels (see Levitan, 1985). However, recent reports from several laboratories indicate that intracellular messengers can also affect membrane ionic conductances directly without invoking protein kinase activation. 3. In this article we examine those examples of direct activation of ionic conductances by intracellular messengers which are supported by single-channel studies of isolated membrane patches. The list of cell types displaying this kind of response is growing and includes cells of neuronal as well as nonneuronal origin.

Compartmentalization of cyclic AMP elevation in neurons of Aplysia californica

We have measured by radioimmunoassay the amount of total, free, and bound forms of cyclic AMP (cAMP) within the abdominal ganglion and in five identified cell bodies of neurons from Aplysia californica. In the abdominal ganglion the unbound (free) cAMP levels comprised approximately 25-30% of the total cAMP content under the unstimulated condition, i.e., bathed in high-magnesium saline. Under pharmacological conditions that blocked endogenous phosphodiesterase and activated adenylate cyclase, ganglionic free cAMP levels were elevated more than fourfold, while bound cAMP levels more than doubled. Freeze-substitution techniques were employed to facilitate isolation of individual cell bodies either before or after pharmacological manipulation of cAMP levels. The basal, free cAMP content of cells R2, LP1, R15, L11, and L2-L6 was in the range of 10-40 pmol/mg of cell protein, which accounted for approximately one-half of the total cAMP content per cell body. Determinations of individual cell volumes indicated that the basal, free cAMP concentrations ranged from 1 to 6 microM. Under the same pharmacological conditions that elevated ganglionic cAMP in levels, no changes were measured in either the free or the bound forms of cAMP in isolated cell bodies. Our results indicate that the cAMP elevation was compartmentalized within the neuropilar region of the ganglion, most likely within the processes of the nerve cells. Previous results demonstrated that cAMP injections into the same Aplysia neurons studied here induced a cAMP-activated sodium current, INa (cAMP). In this report we discuss the possibility that pharmacological elevation of cAMP within neuronal processes may reach concentrations similar to those produced by cAMP injections into somata.

Modulation of potassium conductances by an endogenous neuropeptide in neurones of Aplysia californica

1. Macroscopic and single-channel currents were recorded from voltage-clamped neurones in the abdominal and pleural ganglia of Aplysia californica in order to investigate conductance changes elicited by application of the endogenous peptide FMRFamide (Phe-Met-Arg-Phe-NH2) and related neuropeptides to the cell surface. 2. The Ca-dependent K current, IK(Ca), when elicited at a constant voltage by intracellular injection of Ca2+, was insensitive to FMRFamide or its derivative YGG-FMRFamide (Tyr-Gly-Gly-Phe-Met-Arg-Phe-NH2). 3. Under steady voltage clamp, certain cells responded to a brief puff of FMRFamide or YGG-FMRFamide with a transient outward current lasting about 1 min. Unclamped cells responded with a corresponding hyperpolarization. These responses reversed at about -75 mV. Ion substitution indicated that the current is carried by K+. 4. FMRFamide and YGG-FMRFamide were equally effective in activating the outward current, whereas FMRF, met-enkephalin and leu-enkephalin were ineffective. 5. At voltages negative to -30 mV and, in the absence of extracellular Ca2+, also at more positive potentials, the FMRFamide-sensitive current showed no voltage dependence beyond that predicted from constant-field considerations. 6. The response to FMRFamide was relatively insensitive to extracellular tetraethylammonium (TEA, KD approximately 75 mM) and 4-aminopyridine (4-AP, KD approximately 6 mM). It was suppressed in Ba-containing solutions, but was unaffected by injection of the Ca chelating agent EGTA. The response was blocked by serotonin and other agents known to elevate intracellular adenosine 3',5'-phosphate (cyclic AMP) levels, and by direct injection of cyclic AMP into the cell. 7. In its pharmacological properties and lack of voltage dependence, the FMRFamide-activated current resembles the 'S' current, IK(S), a K current suppressed by application of serotonin in Aplysia neurones. 8. The similarity between the FMRFamide-sensitive current and the 'S' current was confirmed in cell-attached patch-clamp studies, in which activity of 'S' channels was found to be reduced by serotonin, and enhanced by FMRFamide. 9. Thus, FMRFamide may function in Aplysia to counteract the serotonergic modulation of 'S' channels, which has been proposed as a mechanism of presynaptic plasticity in this mollusc.

Excitation of locus coeruleus neurons by an adenosine 3',5'-cyclic monophosphate-activated inward current: extracellular and intracellular studies in rat brain slices

The firing rate of locus coeruleus (LC) neurons in rat brain slices was increased reversibly by agents that either elevate intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) or mimic its actions (e.g., forskolin, and activator of adenylate cyclase, 8-Br-cAMP, a membrane permeable analog of cAMP, and Ro20-1724, a preferential inhibitor of cAMP-phosphodiesterase). Intracellular recordings showed that 8-Br-cAMP and forskolin induce a depolarization of LC neurons, accompanied by a decrease in input resistance. The 8-Br-cAMP- and forskolin-elicited depolarization persisted in the presence of cobalt, a calcium channel blocker. Steady-state current-voltage curves revealed that in the voltage range of -50 to -120 mV, 8-Br-cAMP and forskolin induced an inward current, which did not reverse at the potassium equilibrium potential and could not be blocked by tetrodotoxin. Partial replacement of sodium with Tris or choline markedly reduced the depolarization elicited by 8-Br-cAMP. We conclude that 8-Br-cAMP and forskolin act through a common mechanism to increase the firing rate of locus coeruleus neurons by inducing a cAMP-activated inward current, carried out at least in part by sodium ions.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.