Comparative study of acetylcholine and 5-hydroxytryptamine receptors on single snail neurons

Diphasic postsynaptic potential: a chemical synapse capable of mediating conjoint excitation and inhibition

Two identified interneurons in each buccal ganglion of Aplysia can mediate conjoined excitation and inhibition to a single follower cell. A single presynaptic action potential in one of these interneurons produces a diphasic, depolarizing-hyperpolarizing synaptic potential apparently as a result of a single transmitter acting on two types of postsynaptic receptors in the follower cell. These receptors produce synaptic potentials with differing reversal potentials, ionic conductances, time courses, rates of decrement with repetition, pharmacological properties, and functional consequences. The excitatory receptor controls a sodium conductance, the inhibitory receptor controls a chloride conductance. Both components of the synaptic potentials can be produced by iontophoretic application of acetylcholine on the cell body of the follower cell, and each component is differentially sensitive to different cholinergic blocking agents.

Invertebrate preparations and their contribution to neurobiology in the second half of the 20th century

This review summarized the contribution to neurobiology achieved through the use of invertebrate preparations in the second half of the 20th century. This fascinating period was preceded by pioneers who explored a wide variety of invertebrate phyla and developed various preparations appropriate for electrophysiological studies. Their work advanced general knowledge about neuronal properties (dendritic, somatic, and axonal excitability; pre- and postsynaptic mechanisms). The study of invertebrates made it possible to identify cell bodies in different ganglia, and monitor their operation in the course of behavior. In the 1970s, the details of central neural circuits in worms, molluscs, insects, and crustaceans were characterized for the first time and well before equivalent findings were made in vertebrate preparations. The concept and nature of a central pattern generator (CPG) have been studied in detail, and the stomatogastric nervous system (STNS) is a fine example, having led to many major developments since it was first examined. The final part of the review is a discussion of recent neuroethological studies that have addressed simple cognitive functions and confirmed the utility of invertebrate models. After presenting our invertebrate "mice," the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, our conclusion, based on arguments very different from those used fifty years ago, is that invertebrate models are still essential for acquiring insight into the complexity of the brain.

Autoradiographic localization and characterization of [125I]lysergic acid diethylamide binding to serotonin receptors in Aplysia

The sensitive serotonergic radioligand 2-[125I]lysergic acid diethylamide was used to study the distribution and pharmacological binding properties of serotonin receptors in Aplysia californica. The high specific activity of this radioligand allowed us to develop a methodology for the investigation of receptor binding properties and receptor distribution in a single ganglion. [125I]Lysergic acid diethylamide labels a population of high-affinity serotonergic sites (Kd = 0.41 nM) in Aplysia ganglia whose regional distribution matches that expected from previous electrophysiological and immunohistochemical studies. The properties of [125I]lysergic acid diethylamide binding sites in Aplysia are in general agreement with previous studies on [3H]lysergic acid diethylamide binding in this system but these sites differ from the serotonergic receptor subtypes described in the mammalian brain. Guanine nucleotides were shown to modulate agonist but not antagonist affinity for the [125I]lysergic acid diethylamide binding site in Aplysia, suggesting that this site is coupled to a G-protein. Images of serotonin receptor distribution in the Aplysia nervous system were obtained from autoradiograms of [125I]lysergic acid diethylamide binding. Serotonin receptors in ganglia tissue sections are located primarily within the neuropil. In addition, a subset of neuronal soma are specifically labeled by [125I]lysergic acid diethylamide. These studies indicate that [125I]lysergic acid diethylamide binds to sites in the Aplysia nervous system which display a regional distribution, pharmacological binding properties and evidence of coupling to a G-protein consistent with labeling of a subset of functional serotonin receptors. In addition, the techniques used in this investigation provide a general approach for rapidly characterizing the pharmacological properties and anatomical distribution of receptor binding sites in single invertebrate ganglia. Individual neurons containing these receptor subtypes can be identified by these methods and correlated with physiological responses in the same cell.

A new potent channel blocker: effects on glutamate responses at the crayfish neuromuscular junction

The glutamate blocking action of 5-methyl-1-phenyl-2-(3-piperidinopropylamino)-hexane-1-ol (MLV-5860) was studied at the crayfish neuromuscular junction using electrophysiological techniques. The opener muscle of the dactyl in the first leg of the crayfish was used to examine the action of the drug on the glutamate response. MLV-5860 reduced the amplitude of repetitively-induced glutamate potentials in a dose-dependent manner at the crayfish neuromuscular junction and this reduction was time- and activity-dependent. The minimum effective concentration of MLV-5860 to reduce the glutamate response was estimated to be lower than 50 nM, and therefore MLV-5860 is the most powerful glutamate blocker known at the crayfish neuromuscular junction. Pretreatment of the muscle fiber with concanavalin A did not affect the action of MLV-5860. MLV-5860 reduced the amplitude of excitatory junctional potentials (EJPs) and increased the decay rate of extracellularly-recorded EJPs in a dose-dependent manner. Quisqualate responses were also reduced by this drug but the conductance increase of the muscle membrane induced by GABA was not affected. MLV-5860 did not cause a significant change in the input resistance of the opener muscle fiber at concentrations less than 10 microM. The action of the drug is possibly explained in part by the open channel block of the glutamate-activated ion channels. The forward rate constant for channel blockade was estimated from the difference between the decay rate constants of extracellular EJPs in the absence and presence of the drug and the estimated value was 6.5 +/- 1.4 X 10(7) M-1 s-1.

Cholinergic activation of the electrocorticogram: role of the substantia innominata and effects of atropine and quinuclidinyl benzilate

Systemic injection of quinuclidinyl benzilate partially abolished low voltage fast activity (LVFA) in the neocortex of waking rats, resulting in the appearance of large irregular slow waves during Type 2 behaviors (e.g. immobility, sniffing without head movement, face washing). These slow waves did not occur during Type 1 behavior (e.g. walking, head movement). Atropine sulfate produced a similar effect but it was less potent by a factor of about 12. Injection of kainic acid into the substantia innominata: (a) destroyed local cells which contain acetylcholinesterase (AChE) and reduced AChE staining in the ipsilateral neocortex; and (b) produced large slow waves in the ipsilateral neocortex during Type 2 behavior but not during Type 1 behavior. These slow waves were abolished by systemic injection of pilocarpine. Kainic acid injection into the thalamus produced extensive local cell loss but failed to produce slow waves in the neocortex. The data suggest that the LVFA which is normally present in the neocortex during waking Type 2 behavior is dependent on a cholinergic input to the neocortex from the substantia innominata. The relevance of these findings to Alzheimer's disease is discussed.

The effects of avermectin and drugs related to acetylcholine and 4-aminobutyric acid on neurotransmission in Ascaris suum

We have examined some aspects of the neuropharmacology of the nematode Ascaris suum using a divided chamber and selective stimulation technique to localize the sites of action of drugs. These techniques enabled us to investigate separately excitatory neuromuscular transmission, inhibitory neuromuscular transmission, and transmission from interneurons to excitatory motorneurons. We find that a curare-sensitive mechanism is involved in the excitation of the excitatory motorneuron via interneurons. The anthelmintic avermectin Bla (AVM) also blocks interneuronal stimulation of excitatory motorneurons. This action of AVM can be reversed by picrotoxin. AVM has no effect on excitatory neuromuscular transmission. Two GABAergic agonists in nematodes, muscimol and piperazine, mimic the effects of AVM when applied ventrally. This suggests that the action of AVM is related to a GABAergic mechanism. Ventral inhibitory neuromuscular transmission is also blocked by AVM, but this action is not reversed by picrotoxin. Thus AVM has two distinct sites of action in A. suum.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Block of glutamate-activated synaptic channels by curare and gallamine

Excitatory junctional currents (e.j.cs) and glutamate-activated currents have been examined in voltage-clamped locust muscle fibres exposed to curare or gallamine. Although these drugs have little action on channel kinetics at the resting potential, there is an increasingly pronounced effect at hyperpolarized levels. In the presence of curare (5-100 microM), fibres held at hyperpolarized potentials showed e.j.cs with an initial 'fast component' followed by a 'slow tail'. In many fibres, hyperpolarization beyond -50 mV decreased the amplitude of the peak synaptic current; the decay time constant of the fast component was decreased by hyperpolarization while the time constant of the slow component was increased. Iontophoretic application of brief pulses of glutamate also produced two-component glutamate currents in these conditions. Gallamine was considerably more effective than curare, markedly altering the decay time and amplitude of the e.j.c. and the glutamate current at 1-5 microM. Its effects appeared qualitatively similar to those of curare. The observations are consistent with the idea that curare and gallamine produce a transient block of glutamate-activated synaptic channels.

Aspartate: distinct receptors on Aplysia neurons

Aplysia neurons have specific aspartate receptors that are distinct from those to glutamate. In some cells, asparate selectively increases the membrane permeability to chloride, giving rise to a hyperpolarization, while on other cells it increases the permeability to sodium, causing a depolarization. There are also specific receptors for L-glutamate which mediate sodium, chloride, or potassium conductance increases, and another class of receptors activated by both glutamate and aspartate.

Ionic mechanism of 5-hydroxytryptamine induced hyperpolarization and inhibitory junctional potential in body wall muscle cells of Hirudo medicinalis

Five-hydroxytryptamine (5-HT) causes a hyperpolarization and increased conductance of the leech body wall muscle cell membrane. If 5-HT is applied in the absence of the Cl minus ion, the response appears as a depolarization, whereas if 5-HT is applied in the absence of the K+ ion, the response is a hyperpolarization. In both cases, the conductance of the muscle cell membrane is increased. Stimulation of the peripheral nerve to the body wall muscle produces a complex junctional potential in muscle cells. Exposing the muscle to d-tubocurarine (d-TC) eliminates the excitatory component (EJP) of the complex potential. The inhibitory potential (IJP) that remains has an equilibrium potential at approximately 65 mV. Furthermore, this IJP appears as a depolarization when the nerve is stimulated in the presence of d-TC and low CL minus, whereas this is not the case if the nerve is stimulated in the presence of d-TC and low K+. The drugs BOL-148 and cyproheptadine block the IJP's in the body wall muscle. These data are interpreted as indicating that 5-HT acts on leech body wall muscle cells by increasing the conductance to the Cl minus ion and that the IJP's caused by nerve stimulation are probably the result of 5-HT release at nerve terminals. As a final point, it has been shown that the inhibition by 5-HT of the spontaneous EJP's that occur on the leech body wall muscle results from an inhibition of central neurons and not from any direct effect on the muscle cell or on peripheral synapses.

Effect of Dendroaspis neurotoxins on synaptic transmission in the spinal cord of the frog

Six neurotoxins from Dendroaspis venom, and α -bungarotoxin, were tested on a cholinergic synaptic pathway in the isolated spinal cord of the frog. Four dendrotoxins, which block neuromuscular transmission, also blocked the cholinergic pathway in the spinal cord. In both cases the block is slowly reversible and not due to presynaptic action. The dendrotoxins appear to act by blocking acetylcholine receptors and are therefore potentially useful for the localization and isolation of acetylcholine receptors in the central nervous system of vertebrates.

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.

Analysis of Mauthner cell responses to iontophoretically delivered pulses of GABA, glycine and L-glutamate

1. The intracellularly recorded responses of goldfish Mauthner neurones to iontophoretically applied pulses of amino acids have been analysed: their time courses have been compared with each other, and with those predicted from diffusion theory.2. The rise time of the response to GABA is slower than that to glycine or L-glutamate. The response curves of the latter substances were very similar, and unlike that of GABA were markedly affected by increasing the distance of pipette-tip from the membrane. The results suggest that the time course of the responses to glycine and L-glutamate are determined mainly by free diffusion in the brain tissue (at least within about 200 mum of the cell), while that to GABA must be rate-limited by other factors, e.g. drug-receptor activation time.3. The possibility that the responses are influenced by some desensitizing process was investigated by applying a second (test) drug pulse during the response to a prior conditioning one. In the case of glycine and of L-glutamate there was no attenuation of the response to a second pulse at any time. With GABA, however, the second response was reduced during the period of the conditioning response; the reduction was progressively less marked the later the test pulse occurred. A similar effect with GABA was seen when glycine was used as the test pulse. The responses to long-maintained drug pulses also indicated that for GABA, but not for glycine or glutamate, there seems to be some desensitizing process present.4. Calculated time courses of responses to brief pulses of glycine and of L-glutamate (based upon diffusion theory) differed somewhat from the observed curves, largely during the falling phase. However, when the calculations were based upon second-order reactions (two molecules of drug per receptor) the diffusion model gave results very like the observed ones.5. Possible implications of these results for the role these three amino acids may have as neuro-transmitters are mentioned.

Inhibitory and excitatory effects of dopamine on Aplysia neurones

1. Electrophoretic application of dopamine (DA) on Aplysia neurones elicits both excitatory and inhibitory effects, which in many cases are observed in the same neurone, and often result in a biphasic response.2. The DA receptors are localized predominantly on the axons. Desensitization, which occurs after repeated injections or with bath application of DA, is more marked for excitatory responses.3. Tubocurarine and strychnine block the DA excitatory responses without affecting the inhibitory ones, which can be selectively blocked by ergot derivatives. It is concluded that the excitatory and inhibitory effects are mediated by two distinct receptors.4. The two DA receptors can be pharmacologically separated from the three ACh receptors described in the same nervous system.5. In some neurones the dopamine inhibitory responses can be inverted by artificial hyperpolarization of the membrane at the potassium equilibrium potential, E(K), indicating that dopamine causes a selective increase in potassium permeability.6. In other neurones the reversal potential of dopamine inhibitory responses is at a more depolarized level than E(K), but can be brought to E(K) by pharmacological agents known to block the receptors mediating the excitatory effects of DA.7. In still other neurones, the hyperpolarization induced by DA cannot be inverted in normal conditions, but a reversal can be induced by ouabain or by the substitution of external sodium by lithium. These results are discussed in terms of an hypothesis in which dopamine increases the potassium permeability of a limited region of the axonal membrane.8. It is concluded that a selective increase in potassium permeability probably accounts for all dopamine inhibitory effects in the neurones studied.

Acetylcholine receptors: topographic distribution and pharmacological properties of two receptor types on a single molluscan neurone

1. The iontophoretic application of acetylcholine (ACh) on to identified neurones in the buccal ganglion of the mollusc Navanax produced a biphasic or monophasic membrane potential change which was a function of the current intensity and site of ACh application.2. Low iontophoretic currents, 200 msec in duration, applied to the somatic surface facing the neuropile, caused a monophasic potential change of 6-10 sec duration, which had a reversal potential of about - 50 mV, varied with changes in the [Cl](o) of the bathing medium, and was not blocked by the cholinolytics tested.3. ACh applied more distal to the soma, in the neuropile, produced a 1-3 sec monophasic response whose reversal potential was more positive than - 30 mV, varied in amplitude with changes in the [Na](o) of the medium, and was blocked by cholinolytics such as tubocurarine, hexamethonium and atropine.4. With larger iontophoretic currents a biphasic response could be obtained, depolarization followed by hyperpolarization, which represented a superposition of the above monophasic potentials.5. The cholinomimetics propionylcholine and butyrylcholine caused a biphasic response like that to ACh. Carbamylcholine and tetramethylammonium also produced a biphasic response but with a more prominent Cl component than that to ACh. Acetyl-beta-methylcholine, oxytremorine and pilocarpine only produced a response comparable to the chloride phase of the ACh response.6. Anticholinesterases prolonged both phases of the ACh response.7. It was concluded that each of the identified neurones possess two types of cholinoceptive sites, which are pharmacologically distinct, produced different changes in membrane permeability and are distributed differently over the axo-somatic membrane complex.

Acetylcholine: possible neuromuscular transmitter in Crustacea

The tonic flexor muscles of the crayfish abdomen respond with a large depolarizing potential to acetylcholine iontophoresed onto a neuromuscular Junction, but not to glutamate. Excitatory junctional potentials are abolished by d-tubocurarine and enhanced by a cholinesterase inhibitor. The membrane is depolarized and the junctional potentials are desensitized by excess acetylcholine. Thus acetylcholine is thought to be the neuromuscular transmitter.

Serotonin: two different inhibitory actions on snail neurons

Serotonin (5-hydroxytryptamine) inhibits snail neurons through two different mechanisms. Whereas on some cells it increases selectively the membrane permeability to chloride ions thus giving rise to a net influx of this ion, on other neurons it increases the permeability to potassium ions causing a net potassium efflux. The serotonin receptors involved in these two inhibitions are different; they also differ from the receptors involved in the excitatory action of serotonin previously described in snail neurons.

On the conformations of hallucinogenic molecules and their correlation

There are only a few possible conformations of D-lysergic acid diethylamide and hallucinogenic derivatives of tryptamine and phenylethylamine. Of these possible conformations there is a high structural correlation among the probable conformations of active hallucinogenic molecules and between these conformations and the known conformations of several central nervous system transmitter molecules.