PDE4 control on cAMP/PKA compartmentation revealed by biosensor imaging in neurons.

Electrophysiological recordings (using the slow-AHP potassium current) together with novel biosensor imaging methods (with AKAR and Epac sensors) were used in preparations of rodent brain slices to record PKA activation in real time and in individual neurons. The experiments revealed the propagation of the PKA signal from the membrane to the cytosol and eventually to the nucleus. The experiments show how the geometry of the neurons combined with phosphodiesterase activities (mostly rolipram-sensitive PDE4) contributes to a functional compartmentation of the cAMP in subcellular domains.

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.

Alpha2-adrenoceptor activation increases a cationic conductance and spontaneous GABAergic synaptic activity in dopaminergic neurones of the rat substantia nigra.

Noradrenaline (NA) plays an important role in compensating for the loss in dopaminergic (DA) function following lesions of the DA neurones of the substantia nigra (SN). Alpha2-adrenoceptors are largely expressed in these neurones, but the cellular response to their activation is unknown. Whole-cell patch-clamp recordings were made from DA neurones of rat SN. At a holding potential of -60 mV, bath application of NA (50 microM) induced an inward current (-20.3+/-10.0 pA) in 50% of the recorded neurones. This effect was mimicked by UK-14304 (50 microM), a specific alpha2-adrenoceptor agonist, whereas alpha1-adrenoceptor and beta-adrenoceptor agonists failed to induce a response. Surprisingly, alpha2-adrenoceptor antagonists (idazoxan, RX-811059, SKF-86466 and yohimbine) also induced an inward current that could occlude the one induced by UK-14304, suggesting that they may act as alpha2-adrenoceptor agonists. The inward current results from an increase in cationic conductance identical to the one previously described in these neurones, as neurotensin (1 microM), known to activate it, occluded the inward current induced by UK-14304. In addition, GABAergic miniature inhibitory postsynaptic current frequency was increased by activation of presynaptic alpha2-adrenoceptors. We conclude that the effects of NA on alpha2-adrenoceptors can contribute to the previously described composite action of NA on DA neurone firing and can be pharmacologically differentiated from the effect of NA on DA and neighbouring neurones known to be mediated through alpha1-adrenoceptors.

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.

Serotonin- and dopamine-sensitive adenylate cyclase in molluscan nervous system. Biochemical and electrophysiological analysis of the pharmacological properties and the GTP-dependence.

Helix aspersa neuronal cell membranes contain distinct serotonin (5-HT) and dopamine (DA) sensitive adenylate cyclases. We have taken advantage of the fact that in this system, both in vitro (enzymatic assays) and in vivo (electrophysiological measurements) experiments can be used to explore the GTP dependence and the pharmacological properties of this neurotransmitter-sensitive enzyme system. The first property was studied using non-hydrolysable GTP analogs (guanosine 5'-O-(3-thio-triphosphate) or GTP gamma S, and guanosine 5'-imido diphosphate or Gpp(NH)p). In vitro, these two components stimulate the enzyme activity but with different potencies (Kapparent = 10(-8) to 5 X 10(-8) M for GTP gamma S, and 10(-5) M for Gpp(NH)p). Intracellular injections of GTP gamma S, but not of Gpp(NH)p, produced an electrophysiological response similar to the one elicited by 5-HT and DA. These results imply that, even in the presence of the high endogenous GTP concentration normally present in the cell (10(-3) M), GTP gamma S may bind to the GTP-binding protein. Such an interpretation is consistent with the in vitro competition experiments between GTP and GTP gamma S for adenylate cyclase activation. The pharmacology of 5-HT and DA receptors involved in adenylate cyclase stimulation and electrophysiological responses was studied. Serotoninergic antagonists and neuroleptics inhibited the 5-HT-sensitive adenylate cyclase in a stereospecific manner. However, their inhibition was not simply competitive. Our results suggest that they irreversibly bind a component localized on the cytoplasmic side of the membrane. Unexpectedly, the DA receptor coupled with adenylate cyclase was insensitive to any of the several antagonists tested.

Phosphodiesterase type 2 and the homeostasis of cyclic GMP in living thalamic neurons.

The ubiquitous second messenger cyclic GMP (cGMP) is synthesized by soluble guanylate cyclases in response to nitric oxide (NO) and degraded by phosphodiesterases (PDE). We studied the homeostasis of cGMP in living thalamic neurons by using the genetically encoded fluorescence resonance energy transfer sensor Cygnet, expressed in brain slices through viral gene transfer. Natriuretic peptides had no effect on cGMP. Basal cGMP levels decreased upon inhibition of NO synthases or soluble guanylate cyclases and increased when PDEs were inhibited. Single cell RT-PCR analysis showed that thalamic neurons express PDE1, PDE2, PDE9, and PDE10. Basal cGMP levels were increased by the PDE2 inhibitors erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) and BAY60-7550 but were unaffected by PDE1 or PDE10 inhibitors. We conclude that PDE2 regulates the basal cGMP concentration in thalamic neurons. In addition, in the presence of 3-isobutyl-1-methylxanthine (IBMX), cGMP still decreased after application of a NO donor. Probenecid, a blocker of cGMP transporters, had no effect on this decrease, leaving PDE9 as a possible candidate for decreasing cGMP concentration. Basal cGMP level is poised at an intermediate level from which it can be up or down-regulated according to the cyclase and PDE activities.

The pharmacological profile of the acetylcholine response of a crustacean muscle.

A pharmacological analysis was made of the depolarizing acetylcholine (ACh) response found on the gastric mill I muscles of the crabs Cancer pagurus, Cancer irroratus and Cancer borealis. Acetylcholine, carbamylcholine, trimethylammonium, nicotine, and dimethyl-4-phenyl-piperazinium were effective in producing contractures and depolarizations in these muscles. No response to decamethonium, suberyldicholine, acetyl-beta-methylcholine, carbamyl-beta-methylcholine, pilocarpine and oxotremorine could be detected. High concentrations of muscarinic agonists (10(-4) to 10(-3) M) potentiated and prolonged the ACh iontophoretic response. When the acetylcholinesterase activity was inhibited with neostigmine, or when the response was elicited with carbamylcholine, muscarinic agonists partially inhibited the response. ACh responses were most effectively blocked by vertebrate nicotinic ganglionic antagonists, including dihydro-beta-erythroidine, pempidine, and mecamylamine. alpha-Bungarotoxin was without effect on the ACh response.

The pharmacological properties of some crustacean neuronal acetylcholine, gamma-aminobutyric acid, and L-glutamate responses.

1. A study was performed of the L-glutamate, gamma-aminobutyric acid (GABA), and acetylcholine (ACh) responses of cells in the stomatogastric ganglion of the crab, Cancer pagurus. 2. Ionophoretic or pressure application of L-glutamate revealed three classes of responses: a K+-dependent inhibition which reversed at 15-20 mV more negative than the resting potential; a Cl- dependent inhibitory response which was at equilibrium at the resting potential; and a depolarizing response. 3. Ionophoretic or pressure applications of GABA likewise produced three kinds of responses: an increase in K+ conductance, an increase in Cl- conductance, and a depolarizing response. 4. Picrotoxin (10(-6)-10(-5) M) was effective in blocking both the glutamate inhibitory responses. 10(-4) M-picrotoxin, which was necessary to produce a 50% block of the GABA-K+-dependent response, had no effect on the GABA-Cl- response. 5. beta-Guanidinopropionic acid (beta-GP) was found to be an agonist for the GABA-K+ response, but was ineffective in mimicking or blocking the GABA-Cl- response. 6. ACh applications produced large depolarizing responses with a pharmacological profile similar to that of the nicotinic ganglionic response in vertebrates. 7. The muscarinic agonist, acetyl-beta-methyl choline (MeCh), produced depolarizations which decreased in amplitude as the membrane was hyperpolarized from -40 to -100 mV. Pilocarpine and oxotremorine produced changes in the endogenous activity of ganglionic neurones. 8. Implications of these results for the identification of synaptic transmitters in the somatogastric ganglion are discussed.

Neurotensin inhibition of the hyperpolarization-activated cation current (Ih) in the rat substantia nigra pars compacta implicates the protein kinase C pathway.

1. Whole-cell patch-clamp recording was performed from principal neurones of the substantia nigra pars compacta (SNc). In 66% of these neurones, neurotensin (NT) induced, at -60 mV, an inward current associated with an increase in conductance. 2. Principal neurones displayed, in response to hyperpolarizing voltage steps, the voltage-dependent inward cationic current, Ih. This current activated at potentials more negative than -65 mV and reached a maximum at -106 +/- 4 mV, with a half-activation potential of -86 +/- 3 mV. Its estimated reversal potential was -43 +/- 7 mV and its activation curve was fitted with two exponentials. 3. In 41% of neurones showing the inward current, NT (0.5 microM) also reversibly reduced the amplitude of Ih. The diminution was 48.5 +/- 12% when voltage steps were made from -60 to -95 mV. The decrease in Ih resulted from a reduction in the maximal current with no change in the voltage dependence of activation. 4. Forskolin (10 microM), an activator of adenylate cyclase, increased Ih by shifting its activation range to more positive potentials, but it did not alter the NT inhibition of Ih. 5. The effect of NT was blocked by staurosporine (0.5 microM) and by PKC-(19-31) (0.5 microM), a specific protein kinase C (PKC) inhibitor, but was unaffected by Walsh's peptide (100 microM), a specific inhibitor of protein kinase A. The reduction of Ih was mimicked by 1-oleoyl-2-acetyl-sn-glycerol (0.5-10 microM), an analogue of diacylglycerol, an endogenous PKC activator. 6. These results suggest that the inhibition of Ih by NT involves a phosphorylation mechanism that implies activation of PKC.

Ionic mechanisms and receptor properties underlying the responses of molluscan neurones to 5-hydroxytryptamine.

1. Molluscan neurones have been found to show six different types of response (three excitatory and three inhibitory) to the iontophoretic application of 5-hydroxytryptamine (5-HT). The pharmacological properties of the receptors and the ionic mechanisms associated with these responses have been analysed.2. Four of the responses to 5-HT (named A, A', B and C) are consequent upon an increase in membrane conductance whereas the other two (named alpha and beta) are caused by a decrease in membrane conductance.3. The A-response to 5-HT consists of a ;fast' depolarization due to an increase mainly in Na(+)-conductance; the A'-response is a ;slow' depolarization also associated with a Na(+)-conductance increase. Receptors mediating the A- and A'-depolarizations have different pharmacological properties and may exist side by side on the same neurone.4. Both the B- and C-responses are inhibitory. The B-response is a ;slow' hyperpolarization due to an increase in K(+)-conductance, the C-response is a fast hyperpolarization associated with an increase in Cl(-)-conductance.5. The alpha-response to 5-HT is a depolarization which becomes reduced in amplitude with cell hyperpolarization and reverses at -75 mV; it is caused by a decrease in K(+)-conductance. The beta-response is an hyperpolarization which increases in amplitude with cell hyperpolarization and reverses at -20/-30 mV. It results from a decrease in conductance to both Na(+) and K(+) ions.6. The receptors involved in the 5-HT responses associated with a conductance increase may be recognized by the action of specific antagonists: 7-methyltryptamine blocks only the A-receptors, 5-methoxygramine only the B-receptors and neostigmine only the C-receptors. Curare blocks the A- and C-receptors and bufotenine, the A-, A'- and B-receptors. No specific antagonists have yet been found for the 5-HT responses caused by a conductance decrease.7. The significance of the multiplicity of receptors is discussed. Their functional significance at synapses is analysed in the following paper.

On the transmitter function of 5-hydroxytryptamine at excitatory and inhibitory monosynaptic junctions.

1. Two symmetrical giant neurones located in the cerebral ganglion of Aplysia californica contain 4-6 p-mole 5-hydroxytryptamine (5-HT) and are able to synthesize it (Weinreich, McCaman, McCaman & Vaughn, 1973; Eisenstadt, Goldman, Kandel, Koike, Koester & Schwartz, 1973). Stimulation of each of these neurones evokes excitatory and inhibitory potentials in various cells of the ipsilateral buccal ganglion. In nine buccal neurones it evokes excitatory potentials, in other three, ;classical' inhibitory potentials and in one neurone an ;atypical' inhibitory potential.2. The connexion between the giant cerebral neurone and the cells receiving either an excitatory or a ;classical' inhibitory input from it are monosynaptic. TEA injection into the cerebral giant neurone, which prolongs the presynaptic spike, causes a gradual increase of both the excitatory and the inhibitory potentials. On the other hand, high Ca(2+) media, which block polysynaptic pathways, do not suppress these synaptic potentials.3. The iontophoretic application of 5-HT to the buccal neurones receiving excitatory input from the giant cerebral neurones evokes depolarizations showing the pharmacological properties of both A- and A'-responses to 5-HT (see preceding paper). Antagonists which block only the A-receptors (curare, 7-methyltryptamine, LSD 25) block partially the synaptic depolarizing potentials. Bufotenine, which blocks both the A- and A'-receptors, completely blocks the excitatory potentials. Thus, the post-synaptic membrane of these buccal neurones appears to be endowed with both A- and A'-receptors to 5-HT.4. The ;classical' inhibitory potentials elicited in three buccal neurones are hyperpolarizations which reverse at - 80 mV and are due to an increase in K(+)-conductance. The iontophoretic application of 5-HT to these post-synaptic neurones evokes hyperpolarizing B-responses which are also generated by an increase in K(+)-conductance. Antagonists which block the B-responses (bufotenine, methoxygramine) also block the inhibitory potentials.5. The ;atypical' inhibitory potential evoked in one buccal neurone consists in an hyperpolarization which increases in amplitude with cell hyperpolarization. Iontophoretic application of 5-HT to this buccal cell evokes an hyperpolarizing beta-response which also increases in amplitude with cell polarization and results from a decrease in both Na(+)- and K(+)- conductances. The monosynaptic character of the ;atypical' inhibitory potential is not yet fully proven.6. It can be concluded that the excitatory and inhibitory synaptic effects evoked in the buccal neurones by the stimulation of the 5-HT-containing-giant cerebral neurones are very likely mediated by 5-HT.

Effect of catecholamines on the hyperpolarization-activated cationic Ih and the inwardly rectifying potassium I(Kir) currents in the rat substantia nigra pars compacta.

Whole-cell ruptured-patch and perforated-patch recordings were used in principal neurons of the rat substantia nigra pars compacta (SNc) to study the effect of catecholamines both on the hyperpolarization-activated cationic (Ih) and the inwardly rectifying potassium (I(Kir)) currents. In internal potassium, a 2 min bath application of noradrenaline (NA; 50 microM) or dopamine (DA; 50 microM) both inhibited Ih and induced an outward current associated with an increase in I(Kir) conductance. These two effects recovered poorly after wash-out. Protein kinase A (PKA), protein kinase C (PKC) and phosphatases 1 and 2A inhibitors did not modify the NA and DA effects on the amplitude of Ih and I(Kir) currents. They also had no effect on the recovery of the catecholamine responses. In perforated-patch experiments, NA and DA also induced an inhibition of Ih and revealed an outward current associated with an increase in conductance. However, both effects recovered in less than 5 min following the wash-out. These results indicate that neither PKA, PKC, nor phosphatases 1 or 2A were required in the NA and DA modulation of these two currents and that an intracellular factor, that could be either washed-out or inversely up-regulated in the ruptured-patch configuration, was implicated in the recovery of both effects. In the presence of external barium (300 microM) or internal caesium which both blocked the outward current and the increase in conductance, neither NA nor DA affected Ih, suggesting that the effect on Ih observed is secondary to the activation of the I(Kir) channels. Increasing chloride conductance of the cell by activation of GABA(A) receptors also induced an inhibition of Ih. All together these results suggest that the NA or DA induced inhibition of Ih could result from an occlusion of Ih by a space-clamp effect.

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)

Release of endogenous serotonin from two identified serotonin-containing neurones and the physiological role of serotonin re-uptake.

1. The amounts of endogenous serotonin (5-HT) released into the medium by the cerebro-buccal ganglionic ring of Aplysia californica incubated in artificial sea water (ASW) were measured. The rate of spontaneous 5-HT release varied between 0.4 and 1.2 p-mole per hour, which is less than 1% of the total 5-HT present in this preparation.2. Direct stimulation of the ordinarily silent 5-HT-containing giant cerebral neurones resulted in a 80-100% increase of the 5-HT released, but only when the 5-HT uptake was blocked by chlorimipramine (1-10 muM).3. High K(+) media (50 mM) also caused a significant increase in the amount of 5-HT released from the preparation provided that chlorimipramine (1-10 muM) was present in the incubation fluid.4. Co(2+) ions (10-30 mM) added to the incubating medium blocked the spontaneous leak of endogenous 5-HT as well as the release, in the presence of chlorimipramine, evoked either by stimulation of the 5-HT-giant cerebral neurones or high K(+)-media.5. In the presence of chlorimipramine or desmethylimipramine, the duration and/or the amplitude of the excitatory or the inhibitory synaptic potentials evoked in the buccal neurones by the stimulation of the 5-HT giant cerebral neurones were markedly enhanced.6. These results strongly support the idea that 5-HT is the synaptic transmitter released at the excitatory and inhibitory junctions established by the 5-HT giant cerebral neurones in the ipsilateral buccal ganglia. In addition, they underline the role of amine re-uptake in the physiological inactivation of 5-HT as a transmitter.

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 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.

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.

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.