Trifluoperazine and calcium antagonists accelerate cholinergic desensitization in Aplysia neurons

Tyrosine Phosphorylation Determines Afterdischarge Initiation by Regulating an Ionotropic Cholinergic Receptor

Changes to neuronal activity often involve a rapid and precise transition from low to high excitability. In the marine snail, Aplysia, the bag cell neurons control reproduction by undergoing an afterdischarge, which begins with synaptic input releasing acetylcholine to open an ionotropic cholinergic receptor. Gating of this receptor causes depolarization and a shift from silence to continuous action potential firing, leading to the neuroendocrine secretion of egg-laying hormone and ovulation. At the onset of the afterdischarge, there is a rise in intracellular Ca²⁺, followed by both protein kinase C (PKC) activation and tyrosine dephosphorylation. To determine whether these signals influence the acetylcholine ionotropic receptor, we examined the bag cell neuron cholinergic response both in culture and isolated clusters using whole-cell and/or sharp-electrode electrophysiology. The acetylcholine-induced current was not altered by increasing intracellular Ca²⁺ via voltage-gated Ca²⁺ channels, clamping intracellular Ca²⁺ with exogenous Ca²⁺ buffers, or activating PKC with phorbol esters. However, lowering phosphotyrosine levels by inhibiting tyrosine kinases both reduced the cholinergic current and prevented acetylcholine from triggering action potentials or afterdischarge-like bursts. In other systems, acetylcholine receptors are often modulated by multiple signals, but bag cell neurons appear to be more restrictive in this regard. Prior work finds that, as the afterdischarge proceeds, tyrosine dephosphorylation leads to biophysical alterations that promote persistent firing. Because this firing is subsequent to the cholinergic input, inhibiting the acetylcholine receptor may represent a means of properly orchestrating synaptically induced changes in excitability.

Effects of concanavalin A on desensitization kinetics of GABA responses in Achatina fulica neurons

The effects of the lectin concanavalin A (Con A), on the kinetics of desensitization of the responses of voltage clamped Achatina fulica LP5 neuron to microperfused acetylcholine (ACh) and GABA were compared. Both ACh and GABA elicited increases in chloride conductance which decayed biphasically during prolonged applications of these agonists; an initial rapid decay was followed by a later slow decay. Con A (5 micrograms/ml) accelerated both the fast and the slow decays of responses to ACh. Con A (5 micrograms/ml) also accelerated the fast decay of responses to GABA, but the slow decay was unaffected, even by 20 micrograms/ml or more of the lectin. It is suggested that, at least in the case of GABA receptor, the fast and slow decays involve distinct desensitization kinetics. The effects of Con A on the desensitization of the ACh and GABA responses were reversed by D-mannose, a competitive and specific inhibitor of Con A binding to membrane sugar residues. These results provide further evidence that receptor desensitization can be influenced by perturbing the sugar moieties associated with the subunits comprising these signalling macromolecules. The carbohydrate residues may play an important role in regulating desensitization of transmitter receptors.

Interactions of anticholinesterases with Achatina fulica acetylcholine responses and electrogenic sodium pump

The dose-dependent effects of the anticholinesterases, neostigmine and mycotoxin territrem-B, were determined on: (i) Cl(-)-responses of voltage clamped Achatina fulica neurons to microperfused acetylcholine; (ii) the 4 K(+)-induced outward currents evoked by an electrogenic sodium pump in the same neuron; and (iii) acetylcholinesterase activity of Achatina fulica ganglionic homogenates. Both compounds at low doses potentiated the peak acetylcholine responses. However, they had different effects at higher (> 1 microM) doses in that neostigmine now antagonized acetylcholine responses, while territrem-B still produced a maximal potentiation. At all doses neostigmine produced a dose-dependent inhibition of acetylcholinesterase activity. The cholinolytic effect of high doses of neostigmine was associated with the inhibition of 4 K(+)-induced current in the same neuron, while territrem-B neither altered the K(+)-induced current nor antagonized acetylcholine responses. The cholinolytic effect of neostigmine was completely antagonized by the inhibition of electrogenic sodium pump by ouabain or by perfusion with K(+)-free solution. These results suggest that neostigmine at high concentrations inhibits the electrogenic sodium pump and that the cholinolytic effect of high doses of neostigmine is secondary to this action. Territrem-B, on the other hand, had no effect on the electrogenic sodium pump and had no effect on the neuronal membrane properties other than to inhibit acetylcholinesterase. Thus, territrem-B may be a useful tool for studying the interaction between acetylcholinesterase and acetylcholine receptors.

Methoxyverapamil reduction of nicotine-induced catecholamine release involves inhibition of nicotinic acetylcholine receptor currents

The mechanism by which the putative Ca2+ channel blocker methoxyverapamil (D600) inhibits nicotine-induced catecholamine release was investigated in bovine adrenal chromaffin cells and in neurons from paravertebral sympathetic ganglia of chick embryos. We found D600 to prevent catecholamine release evoked by 30 s applications of nicotine with a significantly higher potency than the release induced either by 30 s K+ depolarizations or by electrical field stimulation of sympathetic neurons. Like the use-dependent action of D600 upon Ca2+ channels, the magnitude of inhibition of the K(+)-evoked secretion depended on the duration of stimulation (10 s to 5 min). Data on catecholamine release were supplemented by patch-clamp recordings. We found whole-cell currents in chromaffin cells evoked by (extrapolated) 0.5 s applications of nicotine to be significantly more sensitive to D600 than Ca2+ currents induced by a 0.5 s depolarization from -80 to 0 mV. In both instances, the potency of D600 depended on the duration of the (nicotinic and depolarizing) stimuli. Our data suggest that D600 inhibits nicotine-induced catecholamine release by reducing nicotinic acetylcholine receptor currents rather than voltage-gated Ca2+ currents. Hence, in chromaffin cells as well as in sympathetic neuronal preparations, D600 does not appear to be a suitable tool to investigate the part voltage-activated Ca2+ currents play in cellular events induced by nicotine.

Interaction of concanavalin A and wheat germ agglutinin with Helix acetylcholine receptors

The effects of lectins concanavalin A (Con A) and wheat germ agglutinin (WGA), were studied on acetylcholine (ACh) responses of physically isolated internally dialyzed Helix aspersa neurons using the concentration clamp method and on the binding of [3H]alpha-bungarotoxin to the cluster of neurons. Con A and WGA have different simple sugar specificity and produced different actions on ACh-evoked Cl conductance responses, which were antagonized by Con A (5 micrograms/ml) but were not altered by WGA. Con A depressed ACh responses when applied extracellularly while it had no effect on ACh responses of the same neuron when added to the intracellular solution, thus indicating that Con A specific glycoproteins are exposed on the surface of the neuron. The studies of the effect of Con A on the properties of the ACh binding site (receptor) have demonstrated, that (1) the onset of desensitization of ACh responses of the dialyzed neurons, determined from the decay of ACh-current from peak to plateau in the continued presence of agonist and best fitted by a double exponential function, was accelerated by Con A; (2) Con A depressed the maximal ACh induced current in a dose response relationship and altered the Hill coefficients; (3) Con A depressed the binding of [3H]alpha-bungarotoxin to the cluster of neurons. These results indicate that Con A receptors on the surface of the neuronal membrane play a regulatory role in the ACh-receptor system and suggest that binding of lectin molecules to their receptors leads to inhibition of binding of ACh to ACh-receptors and to acceleration of the kinetics of desensitization of ACh receptors. All the effects of Con A, that is, on the peak amplitude, desensitization, dose-response relationship of ACh induced current and binding of [3H]alpha-bungarotoxin, could be recovered by D-mannose, a competitive inhibitor of Con A binding to its receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of trifluoperazine on synaptically evoked potentials and membrane properties of CA1 pyramidal neurons of rat hippocampus in situ and in vitro

The effects of trifluoperazine (TFP), a phenothiazine antipsychotic, on hippocampal activity were studied in the CA1 subfield, both in situ and in slices. In the extracellular studies in situ and in vitro, both somatic population spikes and dendritic excitatory postsynaptic potentials (EPSP) fields were depressed reversibly by TFP, applied by microiontophoresis or in the bath (50-100 microM). Similar effects were also seen during iontophoretic applications of sphingosine in situ. Like TFP (at micromolar concentrations) sphingosine is a dual Ca2+/calmodulin-dependent kinase and protein kinase C (PKC) inhibitor. In intracellular recordings from slices, 50-100 microM TFP induced a slow depolarization and a decrease in input resistance (RN), probably through a gamma-aminobutyric acid (GABA)-mediated increase in Cl- conductance (GCl). TFP also reduced the slow afterhyperpolarization (AHP) as well as electrically evoked inhibitory postsynaptic potentials (IPSPs), but EPSPs were augmented in both amplitude and duration. When CA1 neurons were voltage clamped, TFP elicited a corresponding inward current (consistent with depolarization), increased the leak conductance, and enhanced excitatory synaptic currents; whereas inhibitory synaptic currents and high-threshold Ca2+ currents were reduced. In conclusion, these effects of TFP--which cannot be readily explained by its potent antidopamine action--are in keeping with other evidence that both Ca2+/calmodulin-dependent kinase and PKC can modulate GCl-conductance and high-threshold Ca(2+)-conductance, as well as inhibitory and excitatory postsynaptic currents.

Penicillin-induced triphasic modulation of GABAA receptor-operated chloride current in frog sensory neuron

Effects of penicillin-G (PCN) on GABA-evoked Cl- current (IGABA) were investigated in freshly dissociated frog sensory neurons by the use of the concentration-clamp technique combined with the suction-pipette method. Under conditions where the internal and external solutions allowed only Cl- permeability, PCN elicited triphasic modulation on IGABA, consisting of two modes of blockade on IGABA and a following rebound (rebound-like transient IGABA). Simultaneously applied PCN and GABA depressed IGABA immediately (phasic blockade), with the depressed IGABA slightly recovering in amplitude to achieve a stable level of blockade (tonic blockade). When a solution containing a mixture or PCN and GABA was quickly replaced by one containing GABA alone, a rebound-like transient Cl- current (IR) was evoked. Each component of the PCN actions on IGABA was PCN- and GABA-concentration-dependent. The reversal potential for each component of the PCN actions on IGABA was close to the chloride equilibrium potential (ECl) calculated using the Nernst equation. The current-voltage (I-V) relations for both the phasic and tonic blockade revealed inward rectification, while I-V curves for the control IGABA and the IR were outwardly rectified. The degree of IGABA-desensitization and the amplitude of the IR correlated well. The data suggest that partial removal of the GABAA receptor-desensitization may result in generation of the IR.

Mechanisms of the synergistic interactions between organic calcium channel antagonists and various neuromuscular blocking agents

The effects of Mn2+, neomycin and four organic Ca2(+)-channel antagonists (OCA): nicardipine, nifedipine, diltiazem and verapamil on the neuromuscular blocking activities of tubocurarine, succinylcholine (SCh), decamethonium and neomycin were studied in isolated mouse phrenic nerve-diaphragm preparations. The effective concentration of SCh for 50% inhibition (IC50) of single indirect twitch responses were reduced markedly by more than 3-fold when the preparations were pretreated with OCA at 10 microM; the latter alone did not appreciably affect the indirect twitch response or the amplitude of miniature endplate potentials. The neuromuscular blocking effect of decamethonium was also enhanced synergistically by OCA to a similar extent. On the other hand, under the comparable condition. the combined uses of OCA plus tubocurarine or neomycin, neomycin plus tubocurarine or SCh, and Mn2+ plus tubocurarine, SCh or neomycin all resulted in insignificant potentiation. These results suggest that OCA have a specific effect to enhance the agonist effect of depolarizing agents on nicotinic acetylcholine receptors. Nicardipine at 2 microM non-competitively inhibited depolarizations of endplates elicited by SCh and decamethonium and abolished them completely at 10 microM nicardipine. The IC50's in inhibiting endplate potentials and miniature endplate potentials by SCh and decamethonium were also reduced 2 to 3.5-fold by nicardipine. It is inferred that OCA are endowed with a unique capability to allosterically affect the postsynaptic nicotinic acetylcholine receptor, promoting its desensitization liability, hence synergistic interaction with depolarizing agents. Presynaptic effects of OCA are probably not involved.

Influences of trypsin and collagenase on acetylcholine responses of physically isolated single neurons of Aplysia californica

1. The influences of enzyme treatments (trypsin and collagenase) on responses to perfused acetylcholine were examined on physically isolated single Aplysia neurons, using the voltage-clamp, internal perfusion, and rapid external perfusion technique. 2. During treatment with trypsin (0.025 to 0.1%) for 10 to 30 min at room temperature (22 to 25 degrees C), the peak amplitude of the Na current induced by acetylcholine increased in a time- and dose-dependent manner, and the decay in the continued presence of acetylcholine was slowed. This effect of trypsin treatment was irreversible after washing for 60 min without enzyme. 3. Edrophonium, a cholinesterase inhibitor, has previously been shown to augment the Na acetylcholine response in this preparation by inhibition of acetylcholinesterase. After treatment of the neuron with trypsin, the augmentation after edrophonium was abolished. Furthermore, in the presence of edrophonium, trypsin also failed to increase the response. The dose-response curve for acetylcholine after treatment of trypsin was similar to that in the presence of edrophonium. These results suggest that the modification of the current response by trypsin is a result of removal of cholinesterase activity from the membrane. 4. In contrast to the effects of trypsin, collagenase (0.03 to 0.1%) for 10 to 60 min did not change the current amplitude of the acetylcholine response. However, collagenase treatment did alter the kinetics of the acetylcholine response in a dose-dependent manner, in that the rate of decay was accelerated. A similar acceleration was seen in the acetylcholine responses on other neurons which were due to Cl or K currents, suggesting that the effect was independent on the type of channel. This effect of collagenase was reversible after 30 to 60 min of washing of the neuron. 5. In the presence of edrophonium or after the treatment with trypsin, collagenase still accelerated the current kinetics of the acetylcholine response, indicating that cholinesterase activity is not related to this effect. Furthermore, heated collagenase (presumably inactivated) had a similar action, suggesting that the enzymatic activity of collagenase is not related to the modification of the response. 6. These results suggest that Aplysia acetylcholinesterase is sensitive to trypsin but not to collagenase. However, the preparation of a collagenase used in these studies contains some factor which alters the response to acetylcholine, but this effect is reversible and unrelated to enzymatic activity.

Desensitization kinetics of a K+ acetylcholine response in Aplysia

We have studied the process of acetylcholine receptor desensitization on Aplysia medial pleural neurons under voltage clamp conditions. Acetylcholine, applied by microperfusion, elicits a biphasic response on these neurons, a rapid component which reverses polarity at about -60 mV and is Cl-dependent, and a slower component which reverses at about -85 mV and is K-dependent. Both components show desensitization, and the present study focuses on the K-dependent component, which could be isolated by maintaining membrane potential at the Cl equilibrium potential or by blocking the Cl component pharmacologically. K-dependent acetylcholine responses on these neurons varied in regard to time to peak of response and rate of desensitization. While the rising phase of the response was always fitted by a single exponential process, times to peak were divided somewhat arbitrarily into three broad groups of fast (less than 3 s), medium (3-6 s) and slow (greater than 6 s). Desensitization of fast responses was best described by two exponential processes plus a constant, medium responses by a double exponential, and slow responses by single exponential plus a constant. The apparent dissociation constant of acetylcholine was 17.3 +/- 1.6 microM. The best fit of responses for a given cell remained constant over a range of acetylcholine doses, but the kinetics of both fast and slow components accelerated with dose and depolarization. The fast component of desensitization was very temperature dependent. In neurons where it was present it was abolished by cooling, while in neurons with no fast component at room temperature it would appear with warming. The time constant of the fast component varied inversely with temperature. The time constant of the slow component was maximal at 22-24 degrees C, and fell on either side of this temperature. These results suggest that receptor desensitization for acetylcholine K responses is, like Na-dependent responses, composed of two independent processes. When responses to the acetylcholine agonists, carbachol and arecoline, were compared to those of acetylcholine on fast-type neurons, the times to peak varied in the order acetylcholine less than carbachol less than arecoline. The carbachol response was best fitted by two exponential functions, while arecoline was best fitted by a single exponential plus a constant.

Desensitization kinetics of a chloride acetylcholine response in Aplysia

The kinetics of desensitization of acetylcholine-evoked Cl conductance increased response of Aplysia RC neurons of the abdominal ganglion were studied under voltage-clamp conditions for comparison with results of similar studies on acetylcholine Na and K responses. The response evoked by acetylcholine on RC neurons was an outward current at resting potential (about -45 mV) that reversed at about -65 mV and was blocked by D-tubocurarine and strychnine but not hexamethonium and was not activated by arecoline. From the current-voltage relation this response can be ascribed to a pure conductance increase to Cl. The apparent KD was 40.6 microM. Upon prolonged exposure to acetylcholine the response peaked within 200-400 ms, and then decayed to a plateau current in the continued presence of the agonist. The peak and plateau currents reversed at the same potential, indicating that there had not been significant redistribution of Cl. The current decay in every cell was best fit by a double exponential function plus a constant, and the average time constants were tau fast = 1.8 +/- 0.2 s and tau slow = 16.2 +/- 1.0 s. Both components were slowed by cooling. While tau fast did not change with dose, tau slow increased with dose. Both components accelerated with hyperpolarization and upon application of trifluoperazine (2 microM). These results are consistent with the interpretation that desensitization of the acetylcholine Cl response is composed of two independent processes. This conclusion is the same as that derived from studies of the acetylcholine Na and K responses, and is in general consistent with desensitization being a property of a common acetylcholine receptor, and independent of the ionic selectivity of the associated channel. There are, however, significant differences in voltage, temperature and trifluoperazine dependence of the two components of the three ionic responses which may reflect influence of the different ion channels and/or transduction mechanisms.

Verapamil, neuromuscular transmission and the nicotinic receptor

The effect of verapamil on neuromuscular transmission was studied in the frog by analysing ionophoretic endplate current (iEPC) trains and the growth and decay phases of miniature endplate currents (mepcs). In addition, single channel data on the interaction of verapamil with the nicotinic acetylcholine receptor were obtained from cultured embryonic chick skeletal muscle cells. Verapamil caused both open and closed channel blockade in the iEPC trains. Mepc amplitude was decreased at low micromolar concentrations, and at higher concentrations there was also accelerated mepc decay indicating open channel blockade. The latter effect could not be explained by a sequential channel occlusion mechanism. Analysis of the mepc rising phase showed that low micromolar concentrations of the drug decreased the pool of receptors which could be activated. Single channel data confirmed the specific interaction of verapamil with the nicotinic receptor, showing closed channel blockade at low concentrations, and at higher levels the shortening of open channel lifetime. It is suggested that both forms of blockade may involve desensitization processes.

Modulation of acetylcholine receptor desensitization by forskolin is independent of cAMP

Biochemical and electrophysiological studies suggest that adenosine 3',5'-monophosphate (cAMP)-dependent phosphorylation of the nicotinic acetylcholine receptor channel is functionally significant because it modifies the receptor's rate of desensitization to acetylcholine. In studies that support this conclusion researchers have used forskolin to stimulate cAMP-dependent phosphorylation in intact muscle. It is now shown that although forskolin facilitated desensitization in voltage-clamped rat muscle, this effect was not correlated with the abilities of forskolin and forskolin analogs to activate adenylate cyclase or phosphorylate the receptor. Furthermore, elevation of intracellular cAMP or addition of the catalytic subunit of A-kinase failed to alter desensitization. Therefore, in intact skeletal muscle, cAMP-dependent phosphorylation does not modulate desensitization.

Antagonists of GABA responses, studied using internally perfused frog dorsal root ganglion neurons

Responses of frog dorsal root ganglion neurons to GABA were studied under conditions of internal perfusion. Conductances to Na, Ca and K were pharmacologically blocked, C1 concentrations were maintained equal on both sides of the membrane and a small holding potential was used. Under these conditions GABA-induced C1 currents could be studied in isolation without shifts in EC1 occurring after GABA application. GABA currents were blocked by a variety of agents. The blockade by bicuculline and Zn was competitive, while that to penicillin was competitive at low concentrations (6 x 10(-5) M) and non-competitive at high concentrations (3 x 10(-4) M). Picrotoxin was non-competitive at all concentrations studied. The time course of the GABA-induced currents was changed in the presence of antagonists, including those that were competitive. These actions appear to be due to a change in the rates of receptor desensitization rather than shifts in EC1. Pretreatment with antagonists increased the degree of inhibition only for picrotoxin as compared to simultaneous application of GABA plus antagonist. The voltage dependence of the GABA response was altered by penicillin but not by other antagonists. GABA responses on frog dorsal root ganglion cell were also depressed by a variety of other metal ions (Cd, Ni, Cu, Co, Mn) and other drugs (strychnine, curare, 4-acetamide, 4'-isothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt, 4,4'-diisothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt trihydrate, bemegride and folic acid). In this preparation bicuculline and the heavy metal ions appear to block at or very near to the agonist binding site, while penicillin probably blocks the ion channel. The non-competitive action of picrotoxin appears not to be channel blockade, but to be due to a slowly equilibrating action at a site different from either the agonist binding site or the channel.

Multiple mechanisms of antagonism of gamma-aminobutyric acid (GABA) responses

Gamma-aminobutyric acid (GABA) is one of the most important neurotransmitters in the brain. In an effort to understand the operation of the GABA receptor-ionophore complex, the antagonism of GABA responses by four substances was studied in bullfrog dorsal root ganglion cells by concentration-clamp and internal-perfusion techniques. Two antagonists (bicuculline and Zn2+) were competitive; two (picrotoxin and penicillin) were noncompetitive. However, significant changes in the kinetics of activation and inactivation were produced by the antagonists, including those that were competitive. The causes of these changes may be important clues to the structure and operation of the GABA receptor-ionophore complex.

Modulation of an acetylcholine receptor responsiveness by filipin and chlorpromazine studied in neurons of Aplysia californica

The responsiveness of Aplysia acetylcholine receptors (AChR) was studied using a polyene antibiotic, filipin, which specifically complexes cholesterol, and another compound, chlorpromazine (CPZ), which inserts at the proteolipidic interface. Both substances enhanced the evoked postsynaptic responses or responses to iontophoretic application of carbachol only on the H-type receptor (opening a Cl-permeability), whereas at the same concentrations filipin was without effect on the D-type receptor (opening a cationic permeability) while CPZ depressed the D-type response. The facilitation observed specifically for the H-type receptor was similar to that previously described after acetylcholinesterase (AChE) inhibition or when low concentrations of detergents were applied to this preparation. No additive effect was obtained after the addition of chlorpromazine following a maximal potentiation obtained with an anticholinesterase agent. Since at Aplysia central neurons, AChE is a membranal protein, we propose that the facilitation of H-type responses is attributable to the removal of a modulatory action of AChE on AChR. Filipin or chlorpromazine might disrupt the interaction between AChR and AChE.

Augmentation of succinylcholine-induced neuromuscular blockade by calcium channel antagonists

The block of endplate currents (EPCs) by succinylcholine, in frog cutaneous pectoris muscles, is significantly potentiated by certain organic calcium channel antagonists, at concentrations at which the calcium channel antagonists have no effect on EPCs themselves. Succinylcholine alone blocked the current in a dose-dependent manner with an IC50 = 2.7 microM in voltage-clamped muscle fibers. The ability of succinylcholine to block endplate currents was potentiated by nicardipine (3.5- and 31-fold by 1 and 10 microM, respectively), by bepridil (4.3-fold by 1 microM), and by verapamil (7.7-fold by 10 microM). The dihydropyridine nifedipine (10 microM), did not potentiate the succinylcholine effect. Since the calcium channel antagonists do not affect endplate currents directly at these concentrations, a channel blocking or receptor antagonistic effect of these drugs is not indicated. We therefore suggest that the enhancement of block by succinylcholine may be due to the increased desensitization of the receptor. Furthermore, this enhancement of desensitization may explain the effects of these calcium channel antagonists seen in whole animal studies, where the contractures caused by acetylcholine and succinylcholine, applied systemically, are blocked.

Multiple interactions of anticholinesterases with Aplysia acetylcholine responses

The effects of the carbamate anticholinesterases neostigmine and pyridostigmine on the kinetics of desensitization of responses of isolated, voltage-clamped Aplysia neurons to microperfused acetylcholine (ACh) was examined. The peak ACh-induced current was potentiated at low carbamate doses and antagonized at higher doses (greater than 10(-5) M); neostigmine was more potent than pyridostigmine in producing both effects. These effects suggest two mechanisms of action of these compounds: (a) inhibition of acetylcholinesterase at low doses, which increases the effective ACh dose, and (b) direct antagonism of the response at higher concentrations, which is associated with a slowing of both the activation and desensitization of the ACh response. These compounds may therefore have direct actions on the excitatory ACh receptor in Aplysia neurons which are similar to the effects of these drugs at the vertebrate endplate.

Cholinergic suppression: a postsynaptic mechanism of long-term associative learning

Food avoidance learning in the mollusc Pleurobranchaea entails reduction in the responsiveness of key brain interneurons in the feeding neural circuitry, the paracerebral feeding command interneurons (PCNs), to the neurotransmitter acetylcholine (AcCho). Food stimuli applied to the oral veil of an untrained animal depolarize the PCNs and induce the feeding motor program (FMP). Atropine (a muscarinic cholinergic antagonist) reversibly blocks the food-induced depolarization of the PCNs, implicating AcCho as the neurotransmitter mediating food detection. AcCho applied directly to PCN somata depolarizes them, indicating that the PCN soma membrane contains AcCho receptors and induces the FMP in the isolated central nervous system preparation. The AcCho response of the PCNs is mediated by muscarinic-like receptors, since comparable depolarization is induced by muscarinic agonists (acetyl-beta-methylcholine, oxotremorine, pilocarpine), but not nicotine, and blocked by muscarinic antagonists (atropine, trifluoperazine). The nicotinic antagonist hexamethonium, however, blocked the AcCho response in four of six cases. When specimens are trained to suppress feeding behavior using a conventional food-avoidance learning paradigm (conditionally paired food and shock), AcCho applied to PCNs in the same concentration as in untrained animals causes little or no depolarization and does not initiate the FMP. Increasing the concentration of AcCho 10-100 times, however, induces weak PCN depolarization in trained specimens, indicating that learning diminishes but does not fully abolish AcCho responsiveness of the PCNs. This study proposes a cellular mechanism of long-term associative learning--namely, postsynaptic modulation of neurotransmitter responsiveness in central neurons that could apply also to mammalian species.