GABAergic and glycinergic IPSCs in ganglion cells of rat retinal slices.

GABAergic and glycinergic IPSCs were studied in identified retinal ganglion cells (RGCs) of light-adapted rat retinal slices, using whole-cell recording techniques. GABAergic IPSCs were blocked specifically by SR95531 (3 microM) and bicuculline (3 microM) and glycinergic IPSCs by strychnine (0.3 microM). From 37 RGCs studied, 25 showed exclusively GABAergic IPSCs, 6 presented only glycinergic IPSCs, and 6 showed both. This distribution may result from differences in amacrine cells input rather than from receptor heterogeneity, because both GABA and glycine elicited Cl--selective currents in all RGCs tested. TTX markedly reduced GABAergic IPSCs frequency, whereas glycinergic IPSCs were unaffected. Ca2+-free media, with or without high Mg2+, blocked TTX-resistant GABAergic and glycinergic IPSCs. These results suggest that GABAergic IPSCs in RGCs can be elicited either by Na+-dependent action potentials or by local Ca2+ influx in medium or large dendritic field GABAergic amacrine cells, whereas glycinergic IPSCs are generated by action potential-independent Ca2+ influx in narrow field glycinergic amacrine cells. Both types of IPSCs had fast rise times and biexponential decays, but glycinergic IPSC decay was significantly slower than that of GABAergic IPSCs. An elementary conductance of 54 pS for the glycine-gated channels was estimated from single-channel events, clearly detected in the falling phase of glycinergic IPSCs, and from responses to exogenous glycine.

Enhancement by muscarinic agonists of a high voltage-activated Ca2+ current via phosphorylation in a snail neuron.

1. In previous work we have shown that in the snail Helix aspersa neuron F1 carbamylcholine (CCh) and other muscarinic agonists enhance the inward current carried through high voltage-activated Ca2+ channels by Ba2+ (HVA-ICa). It was also found that cyclic nucleotides, inositol trisphosphate or arachidonic acid are not involved in this modulation. Moreover, despite the effect of CCh being blocked by intracellular injection of EGTA, neither protein kinase C nor Ca(2+)-calmodulin-dependent protein kinase II appeared to play a role. 2. In the present paper, the intracellular mechanism of this muscarinic modulation was investigated further by studying the effects of inhibitors of Ser-Thr protein phosphatases (PP) on both the HVA-ICa of neuron F1 and its enhancement by CCh. 3. Intracellular injections in the F1 neuron of either microcystin LR or okadaic acid, both inhibitors of PP1 and PP2A, mimic the action of CCh on the HVA-ICa and occlude the effects of CCh on this current. In contrast, cyclosporin A, an inhibitor of PP2B (calcineurin), affects neither the HVA Ca2+ current itself nor its modulation by CCh. 4. The efficacy of PP inhibitors was tested in F1 neurons in which serotonin (5-HT) induces an inward current involving intracellular increases in cAMP and a protein kinase A-dependent closing of K+ channels. We found that intracellular injection of either microcystin LR or okadaic acid mimicked the 5-HT-induced inward current and occluded the effect of further application of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic enhancement of inhibitory synaptic activity in rat cerebellar stellate and Purkinje cells.

1. Using the tight-seal whole-cell recording technique, we studied the effects of noradrenaline (NA) on the spontaneous inhibitory synaptic currents (IPSCs) of stellate and Purkinje cells in rat cerebellar slices. 2. In both types of cells, NA (10 microM) induced a marked increase in the frequency of the IPSCs. This effect was observed both in the absence and in the presence of TTX in the saline bathing the cerebellar slices. 3. The NA-induced increase in frequency of IPSCs and miniature IPSCs (mIPSCs) in the two cell types was mimicked by bath applications of isoprenaline (10 microM) and of the adenylyl cyclase activator forskolin (20 microM). Neither phenylephrine nor clonidine changed the frequency of IPSCs in stellate or Purkinje cells. 4. In stellate cells, the beta-agonists and forskolin had variable effects on the amplitudes of both IPSCs and mIPSCs. None of these compounds altered the amplitude of mIPSCs in Purkinje cells. 5. The responses to local applications of GABA to Purkinje cells were unchanged by bath applications of beta-adrenergic agonists or forskolin. A decrease in the response to GABA after treatment with these agents was observed in half the stellate cells examined. 6. We conclude that the major effect of NA on stellate and Purkinje cells is an increase in the frequency of occurrence of spontaneous inhibitory synaptic currents. This action is exerted through the activation of beta-adrenergic receptors and is probably mediated by an intracellular mechanism involving cAMP. The beta-adrenergic modulation of IPSC frequency takes place at the presynaptic level and may involve a change in the process of transmitter release.

Inhibitory synaptic currents in stellate cells of rat cerebellar slices.

1. In thin cerebellar slices of rats aged 14-21 days, voltage-gated currents, synaptic currents and GABA responses were studied with the tight-seal whole-cell recording technique from stellate cells (8-9 micrograms soma diameter) located in the outer two-thirds of the molecular layer. 2. In symmetrical Cl- conditions, stellate cells voltage-clamped at -60 mV showed spontaneous inhibitory postsynaptic currents (IPSCs). As were the GABAA responses of the same cells, the IPSCs were blocked by bicuculline. The frequency of occurrence of IPSCs ranged from 0.2 to 1.9 events per second (21 cells). The mean amplitude of the events ranged from 61 to 226 pA (mean +/- S.E.M.: 132 +/- 11; n = 21). 3. The temporal course of IPSCs was characterized by a rapid rise (mean +/- S.E.M. of the time to peak: 1.1 +/- 0.1 ms, n = 7) and a slow decay. The decay phase was described by a double exponential function with time constants of 8.7 +/- 0.6 ms, and 40.9 +/- 3.7 ms respectively (means +/- S.E.M.; n = 7). 4. A minor fraction (15 to 20%) of the spontaneous synaptic events recorded in control saline had a faster onset than that of the IPSCs and decayed with a rapid mono-exponential decay (time constant of 1.0-1.3 ms). These were excitatory postsynaptic currents (EPSCs) unaffected by bicuculline and blocked by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 5. Bath application of TTX (0.5-1 microM), which blocked voltage-gated Na+ currents in stellate cells, induced a variable decrease in the frequency of IPSCs (mean +/- S.E.M. of the frequency ratio in TTX over control: 0.47 +/- 0.09; n = 12). However, the toxin had no significant effect either on the mean amplitude or on the kinetics of the IPSCs. The mean amplitude of the miniature IPSCs was 141 +/- 13 pA (mean +/- S.E.M.; n = 22). 6. In TTX-containing solutions, the frequency of the IPSCs was unaffected when Ca2+ currents were eliminated either by removal of extracellular Ca2+ and addition of EGTA, or by addition of Cd2+. Miniature IPSCs of 200-300 pA were still observed. 7. In symmetrical Cl- conditions, local application of GABA to stellate cells induced an inward current and an increase in membrane noise. Responses to prolonged applications of GABA showed desensitization in both whole-cell mode and somatic outside-out patches. The chord conductance estimated from recording single GABA channel events in somatic outside-out patches was 28 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscarinic enhancement of the voltage-dependent calcium current in an identified snail neuron.

1. In the F1 neuron of the snail Helix aspersa bathed in a Ba2+ and 4-aminopyridine-containing saline, carbamylcholine (CCh) enhanced the inward current carried by Ba2+ through the voltage-dependent Ca2+ channels. 2. This effect of CCh on the F1 neuron was not affected by the nicotinic antagonists (+)-tubocurarine and hexamethonium, but it was mimicked by oxotremorine and blocked by both atropine and pirenzepine. 3. The intracellular injection of GTP gamma S (guanosine 5'-O-(3- thiotriphosphate] into the F1 neuron caused both a decrease in Ca2+ current and a blockade of the CCh-induced enhancement of the Ca2+ current. 4. Neither cyclic AMP, cyclic GMP nor arachidonic acid mimicked the effect of CCh on the Ca2+ current in the F1 neuron. In contrast, the intracellular injection of EGTA blocked the CCh-induced enhancement of the Ca2+ current thus suggesting that cytosolic Ca2+ is involved in the CCh-induced response. 5. We then investigated the possible role of inositol 1,4,5-trisphosphate (InsP3) and Ca(2+)-dependent protein kinases in the CCh-induced enhancement of the Ca2+ current. The intracellular injection of InsP3 in the F1 neuron elicited no consistent change in the Ca2+ current. Diacylglycerol analogues (OAG and DOG) decreased the Ca2+ current amplitude, i.e. an effect opposite to that produced by CCh. This effect of the diacylglycerol analogues resulted from the activation of protein kinase C (PKC) since it was blocked by staurosporine. In addition, staurosporine did not affect the CCh-induced increase in Ca2+ current. 6. The intracellular injection of either Ca(2+)-calmodulin-dependent protein kinase II (Ca(2+)-CaM-PK) or a peptide inhibitor of this enzyme into the F1 neuron affected neither the Ca2+ current nor its enhancement by CCh. 7. We conclude that the CCh-induced enhancement of the Ca2+ current in the snail F1 neuron involves the activation via muscarinic receptors of an intracellular transduction mechanism in which cytosolic Ca2+ plays a key role. However, InsP3, protein kinase C and Ca(2+)-CaM-PK do not appear to be directly involved in this CCh-induced response.

Serotonin Induces a Cyclic AMP-Mediated Outwardly Rectifying Slow K+ Current in a Single Identified Snail Neuron.

In the F2 neuron of the parietal ganglion of the snail Helix aspersa either bath or iontophoretic application of serotonin (5-HT) induces an outward current. This current has a long latency (10 - 60 s) and a slow time course, a 500 ms iontophoretic application of 5-HT evoking a response lasting 3 - 5 min. This slow outward current reverses at -80 mV, a value equal to EK. After doubling the extracellular K+ concentration the reversal potential of the 5-HT response is shifted by 19 mV, as predicted by the Nernst equation. The I-V curves reveal that the 5-HT-induced slow outward current is outwardly rectifying. This 5-HT response is blocked by intracellular Cs+ and tetraethylammonium (TEA+) and by extracellular TEA+ and Ba2+, but is not affected by the removal of extracellular Ca2+ or the intracellular injection of ethyleneglycol-bis-(beta-amino-ethylether)-N,N,N',N'-tetra-acetic acid (EGTA). These results indicate that the outwardly rectifying slow outward current induced by 5-HT in the F2 neuron is carried by K+ and is Ca2+-independent. In the single isolated F2 neuron, 5-HT induces a 2.5-fold stimulation of the adenylate cyclase activity. In addition, both the intracellular injection of 3',5'-adenosine monophosphate (cAMP) and the application of forskolin mimic the effect of 5-HT on the F2 neuron. The phosphodiesterase inhibitor isobutylmethylxanthine also induces a slow outward current and potentiates the 5-HT response. The intracellular injection of a synthetic 20-residue peptide inhibitor of the cAMP-dependent protein kinase blocks the slow outward K+ current induced by 5-HT. These results show that in the F2 neuron, 5-HT elicits a slow K+ current via the stimulation of adenylate cyclase, an increase in intracellular cAMP and the activation of the cAMP-dependent kinase which probably phosphorylates a population of outwardly rectifying K+ channels or some protein/s associated with these channels.

Intracellular mechanism of neurotransmitter-induced modulations of voltage-dependent Ca current in snail neurons.

This paper reviews our work on the modulation of voltage-dependent Ca currents in identified snail neurons. Ca currents of snail neurones are enhanced or decreased by neurotransmitters. Serotonin and acetylcholine enhance the Ca current of identified neurons, the effect of serotonin being mediated by cGMP and cGMP-dependent protein kinase. Cholecystokinin (CCK8) and dopamine both decrease the Ca current of identified neurons. The effect of CCK8 is irreversible and involves the activation of protein kinase C. The dopamine-induced decrease in Ca current is reversible and involves an alpha 40 subunit of a snail G protein immunologically and functionally related to alpha o of mammalian brain.

An alpha 40 subunit of a GTP-binding protein immunologically related to Go mediates a dopamine-induced decrease of Ca2+ current in snail neurons.

Dopamine induces a decrease in voltage-dependent Ca2+ current in identified neurons of the snail H. aspersa. This effect is blocked by intracellular injection of activated B. pertussis toxin and of an affinity-purified antibody against the alpha subunit of bovine Go protein. The dopamine effect is mimicked by intracellular injection of mammalian alpha o. In snail nervous tissue, pertussis toxin ADP-ribosylates a single protein band on SDS gels, and this band is recognized in immunoblots by the anti-alpha o antibody. We propose that this is a 40 kd alpha subunit of a molluscan G protein immunologically related to alpha o and that it mediates the effect of dopamine on Ca2+ currents in identified snail neurons.

Cholecystokinin induces a decrease in Ca2+ current in snail neurons that appears to be mediated by protein kinase C.

Three distinct classes of protein kinases have been shown to regulate Ca2+ current in excitable tissues. Cyclic AMP-dependent protein kinase mediates the action of noradrenaline on the Ca2+ current of cardiac muscle cells. Cyclic GMP-dependent protein kinase mediates the serotonin-induced modulation of the Ca2+ current in identified snail neurons. The Ca2+/diacylglycerol-dependent protein kinase (protein kinase C) has also been found to regulate Ca2+ currents of neurons. However, no neurotransmitter has yet been shown to regulate Ca2+ current through the activation of protein kinase C. We now report that cholecystokinin, a widely occurring neuropeptide which is present in molluscan neuron, modulates the Ca2+ current in identified neurons of the snail Helix aspersa, and that this effect appears to be mediated by protein kinase C. Specifically, sulphated cholecystokinin octapeptide 26-33 (CCK8), activators of protein kinase C, and intracellular injection of protein kinase C, all shorten the Ca2+-dependent action potential and decrease the amplitude of the Ca2+ current in these cells. All these effects are not reversible within the duration of the experiments. Moreover, intracellular injections of low concentrations of protein kinase C, which are ineffective by themselves, enhance the effectiveness of low concentrations of CCK8 on the Ca2+ current.

cGMP-dependent protein kinase enhances Ca2+ current and potentiates the serotonin-induced Ca2+ current increase in snail neurones.

Protein phosphorylation catalysed by cyclic AMP-dependent, Ca2+/calmodulin-dependent and Ca2+/diacylglycerol-dependent protein kinases is important both in the modulation of synaptic transmission and in the regulation of neuronal membrane permeability (for reviews see refs 5-7). However, there has previously been no evidence for the involvement of cyclic GMP-dependent protein kinase (cGMP-PK) in the regulation of neuronal function. Serotonin induces an increase of Ca2+ current in a group of identified ventral neurones of the snail Helix aspersa. This effect is probably mediated by cGMP because it is mimicked by the intracellular injection of cGMP or the application of zaprinast, an inhibitor of cGMP-dependent phosphodiesterase. We have now found that the effect of either serotonin or zaprinast on the Ca2+ current is potentiated by the intracellular injection of cGMP-PK. Moreover, the intracellular injection of activated cGMP-PK (cGMP-PK + 1 microM cGMP) greatly enhances the Ca2+ current of the identified ventral neurones seen in the absence of serotonin. These results indicate that cGMP-PK has a physiological role in the control of the membrane permeability of these neurones.

Serotonin and cyclic GMP both induce an increase of the calcium current in the same identified molluscan neurons.

Serotonin (5-HT) has previously been shown to evoke an increase in the duration of the Ca2+-dependent spike of molluscan neurons by decreasing the S current (Klein et al., 1982), a K+ current controlled by cAMP. However, in a group of identified ventral neurons of the snail Helix aspersa in which 5-HT (1-10 microM) also prolonged the duration of the Ca2+-dependent action potential, no 5-HT-induced depression of S current or of any other outward current was observed. Instead, 5-HT was found to evoke the prolongation of the somatic spike by inducing an increase in Ca2+ membrane conductance. This 5-HT-induced increase of Ca2+-current was mimicked neither by the intracellular injection of cAMP nor by the extracellular application of forskolin (20 microM). In contrast, it was mimicked by the intracellular injection of cGMP and by the extracellular application of 100 nM zaprinast, a cGMP-phosphodiesterase inhibitor. The extracellular application of phorbol ester TPA (100 nM), an activator of protein kinase C, was also found to increase the Ca2+ current in the identified snail ventral neurons, but this enhancing effect had a different time course from that induced by 5-HT. These results indicate that there is a second mechanism for prolonging the Ca2+ spike of molluscan neurons, consisting of an increase in Ca2+ current, in which cGMP may play a role as second messenger.

The neuropeptide FMRF-amide decreases both the Ca2+ conductance and a cyclic 3',5'-adenosine monophosphate-dependent K+ conductance in identified molluscan neurons.

The molluscan neuropeptide FMRF-amide (10 to 50 microM) decreases the duration of the Ca2+-dependent action potential recorded in the cell body of identified neurons of the snail Helix aspersa (cells D3 and E2). In these neurons, FMRF-amide evokes a decrease of the Ca2+ current resulting from a decrease in Ca2+ conductance. In another single neuron, cell E11, FMRF-amide, besides evoking a decrease of the Ca2+ conductance, induces a decrease of the S-current (Klein, M., J. S. Camardo, and E. R. Kandel (1982) Proc. Natl. Acad Sci. U. S. A. 79: 5713-5717), a K+ current controlled by cyclic AMP. However, in this E11 cell, FMRF-amide also evokes a decrease of the amplitude of the Ca2+ spike plateau. As discussed in the preceding paper (Paupardin-Tritsch, D., L. Colombaioni, P. Deterre, and H. M. Gerschenfeld (1985) J. Neurosci. 5: 2522-2532), it is suggested that these FRMF-amide-induced modulations of ionic conductances involved in the Ca2+-dependent spike recorded in these neuronal somata may intervene in processes of presynaptic inhibition and facilitation.

Two different mechanisms of calcium spike modulation by dopamine.

Dopamine (10 to 50 microM) modulates in two different ways the duration of the Ca2+-dependent action potential recorded in the cell body of identified neurons of the snail Helix aspersa. In some neurons (cells E13 and F1) dopamine increases the amplitude of their Ca2+-dependent spike plateau by decreasing the S-current (Klein, M., J.S. Camardo, and E. R. Kandel (1982) Proc. Natl. Acad. Sci. U.S.A. 79: 5713-5717), a K+ current controlled by cyclic AMP. In another neuron (cell D2), dopamine decreases the Ca2+-dependent plateau of the somatic action potential by evoking a decrease in Ca2+-current resulting from a decrease in Ca2+ conductance. Both modulatory effects could be observed in the same single neuron in which dopamine induces decreases of both the Ca2+ conductance and cyclic AMP-dependent K+ conductance. Nevertheless, in these cells (such as cell F5) dopamine only evokes a decrease of the amplitude of the Ca2+ spike plateau. Since the modulation of the duration of the Ca2+ action potential recorded in the neuronal soma has been shown to constitute a good model of events taking place at synaptic endings, it is suggested that these modulatory mechanisms evoked by dopamine may be involved in processes of presynaptic facilitation and inhibition.

Decrease of gap junction permeability induced by dopamine and cyclic adenosine 3':5'-monophosphate in horizontal cells of turtle retina.

The axon terminals of the H1 horizontal cells of the turtle retina are electrically coupled by extensive gap junctions. Dopamine (10 nM to 10 microM) induces a narrowing of the receptive field profile of the H1 horizontal cell axon terminals, increases the coupling resistance between them, and decreases the diffusion of the dye Lucifer Yellow in the network formed by the coupled axon terminals. These actions of dopamine involve the activation of D1 receptors located on the membrane of the H1 horizontal cell axon terminals proper. Increases of the intracellular cyclic AMP concentration induced by either stimulating the adenylate cyclase activity with forskolin or inhibiting the phosphodiesterase activity with isobutylmethylxanthine, theophylline, aminophylline, or compound RO 20-1724 elicit effects similar to those of dopamine on the receptive field profile of the H1 horizontal cell axon terminals, on their coupling resistance, and on the diffusion of Lucifer Yellow in the axon terminal network. It is concluded that dopamine decreases the permeability of the gap junctions between the axon terminals of the H1 horizontal cells of the turtle retina and that this action probably involves cyclic AMP as a second messenger.

cAMP-mediated decrease in K+ conductance evoked by serotonin and dopamine in the same neuron: a biochemical and physiological single-cell study.

The extracellular application of either serotonin or dopamine and the intracellular injection of cAMP all evoke in the same identified neurons of the snail Helix aspersa inward currents associated with a decrease in K+ conductance. The serotonin-, dopamine-, and cAMP-induced inward currents all show the same maximal amplitude. When the response to one transmitter is maximal, the response to the other is blocked. Using a single-cell microassay, we found that both serotonin and dopamine stimulate the adenylate cyclase [adenosine triphosphate pyrophosphate-lyase (cyclizing), EC 4.6.6.1] activity of the neurons giving the inward-current responses; on the other hand, the adenylate cyclase activity of a neuron that does not show the serotonin- and dopamine-induced currents was not stimulated by the transmitters. In contrast with the nonsummation of the maximal inward-current responses, the maximal stimulating effects of the transmitters on the enzyme activity are additive. The diterpene forskolin, which stimulates the adenylate cyclase activity of the single cells 9-fold, also evokes an inward current. We conclude that single snail neurons are endowed with independent serotonin and dopamine receptors linked to the adenylate cyclase. Activation of each of these receptors evokes a cAMP-mediated decrease in K+ conductance. The physiological interaction between the transmitters probably takes place at a late step in the chain of events leading from the increase in cAMP to the closing of the K+ channels.

gamma-Aminobutyric acid antagonists decrease junctional communication between L-horizontal cells of the retina.

The antagonists of gamma-aminobutyric acid, bicuculline and picrotoxin, were found to narrow the receptive field profile of the large field horizontal cell (L1HC) in the turtle retina when added to the perfusion medium in micromolar concentrations. The coupling resistance between neighboring L1HCs was increased by bicuculline or picrotoxin. Under control conditions, the dye Lucifer yellow injected into one L1HC diffused into a large number of neighboring L1HCs; bicuculline or picrotoxin greatly restricted dye passage between these same cells. We conclude that antagonists of gamma-aminobutyric acid decrease the conductance of gap junctions between L1HCs.

Horizontal cells of turtle retina: a neurotransmitter control of electrical junctions?

Recent experiments suggest that GABA and dopamine can modulate the permeability of the electrical junctions between horizontal cells of the turtle retina. The possible significance and functional advantages of a chemical control of the electrical junctions are briefly discussed. A similar mechanism could combine in a network of interacting elements the low energy cost and low intrinsic noise typical of the electrical synapses with the great functional plasticity typical of the chemical transmission.

Relationship between two voltage-dependent serotonin responses of molluscan neurones.

A serotonin (5-HT)-induced slow inward current was reanalyzed in identified snail neurones and found to result from a decrease in a voltage-dependent K+-conductance, sensitive to [Ca2+]0 changes. 5-HT evoked in the same neurones an increase in the spike plateau known to be associated to a K+-conductance decrease. Both 5-HT responses appear to reflect the same decrease in K+-conductance.

Involvement of small-field horizontal cells in feedback effects on green cones of turtle retina.

Light stimuli depolarize green cones of turtle retina through a circuit involving a feedback connection from luminosity horizontal cells (L-HC) to green cones. In turtle retina two types of L-HC have been distinguished: large-field L-HC and small-field L-HC. The spatial properties of the feedback depolarizations of green cones were compared with those of both large- and small-field L-HC. Green cones were found to be more effectively depolarized by relatively small spots of red light than by large red annuli. Moreover, red light stimulation of the periphery of the receptive field could reduce the depolarizing influence of central red stimuli. These spatial properties greatly differ from those of the large-field L-HC, whereas they strongly resemble those of the small-field L-HC. These results suggest that the small-field L-HC mediate the feedback action on green cones.

Role of cyclic AMP in a serotonin-evoked slow inward current in snail neurones.

One model of synaptic transmission suggests that transmitters modify postsynaptic permeability through the intermediary of cyclic AMP. Thus, serotonin (5-hydroxytryptamine) evokes in molluscan neurones a decrease in a voltage-dependent K+ conductance which in turn generates a slow inward current when studied in steady voltage-clamp conditions. The serotonin-induced increase of the plateau phase of the spike of an Aplysia sensory neurone can be mimicked by both intracellularly injected cyclic AMP and extracellularly applied phosphodiesterase inhibitors, suggesting that cyclic AMP mediates the effect. We have tested whether a similar mechanism could account for the serotonin slow inward current in identified snail neurones and have found that the intracellular injection of cyclic AMP, but not of cyclic GMP or 5'-AMP, evokes a slow inward current showing similar voltage dependence, inversion potential and ionic properties to the serotonin slow inward current. Phosphodiesterase inhibitors at low concentrations (1-20 microM) potentiate the serotonin slow inward current and at higher concentrations evoke by themselves an inward current, partially or totally occluding the serotonin and cyclic AMP currents. Finally, we have found that in homogenates of pooled identified snail neurones serotonin stimulates the adenylate cyclase, increasing its activity by 50-100%.