An electrophysiological study of 5-hydroxytryptamine receptors of neurones in the molluscan nervous system

Synaptic connexions of two symmetrically placed giant serotonin-containing neurones

1. Each giant serotonin cell in Helix pomatia makes synaptic connexions with three non-amine-containing neurones: the anterior, middle and posterior buccal cells.2. Individual e.p.s.p.s, of 500-600 msec duration, were observed in both left and right middle cells following each evoked giant serotonin cell action potential. They were facilitated with repetitive stimulation of the giant serotonin cells and summed to give rise to an action potential. The membrane resistance of the middle cells was reduced when the giant serotonin cells were stimulated to fire rapidly. Evidence is presented which suggests that the link between each giant serotonin cell and each middle cell is monosynaptic.3. Iontophoretically applied serotonin produced a depolarizing potential change in the middle cell perikaryon; the response rapidly desensitized on repetitive application.4. Morphine abolished reversibly the middle cell serotonin potential and antagonized transmission from the giant serotonin cells to the middle cells. Lowering the Na concentration of the medium reversibly diminished the size of the serotonin potential and the giant serotonin cell elicited e.p.s.p.s in the middle cells.5. Reserpine, which depletes serotonin in the giant serotonin cell, impaired transmission from these cells to the middle cells.6. The results suggest that serotonin is the synaptic transmitter released from the giant serotonin cells on to the middle cells and that this system is a suitable model for further analysis of the neuronal role of serotonin.

Attention deficit disorders: are we barking up the wrong tree?

Attention deficit disorder (AAD) and attention deficit/hyperactivity disorder (ADHD) are very frequent and protean developmental disorders without a definite biologic marker. This review proposes a framework to understand the enlarged spectrum of its manifestations based on current knowledge of the mechanisms underlying arousal and attention variations during sleep/wake cycle. The neuro-modulation's pivotal role in this process as well as in the fine tuning of synaptic architecture during development must be taken into account when trying to understand the marked fuzziness of the symptoms and the very high prevalence of reported co-morbidities. The series of related interactions includes a cyclic deactivation of the dorso-lateral portion of the prefrontal cortex (DLPFC) during sleep, suspending executive functions, co-occurring with rhythmic periods of decreased noradrenergic tonus. A protracted unbalance in modulation, with catecholaminergic relative deficiency, could explain less-than-optimum waking DLPFC activation and the most important manifestations of ADD. Beside the well documented dopaminergic effects of stimulant medication used in ADD and ADHD, a more important role must be assigned to noradrenaline (NA). At this light hyperactivity and impulsivity are less important dimensions. Rather, an attention deficit spectrum disorder should probably be regarded as a complication of a core defect in prefrontal cortex dependent inhibitory control, underlying inattention.

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.

Monoamine pharmacology of the lobster cardiac ganglion

Monoamine agonists and antagonists were applied to the lobster cardiac ganglion in an attempt to clarify the different actions of 5-hydroxytryptamine (5HT) and dopamine (DA) on this rhythmic pattern generator. Experiments were designed to determine whether the similar responses to 5HT and DA applied to the anterior region of the ganglion could be separated by pharmacological approaches, and whether the different responses to 5HT applied to the anterior and posterior regions of the ganglion could be attributed to mediation by different receptors. A small number of the 5HT agonists which were tested mimic the effects of 5HT, in that they increase the frequency of bursting and decrease burst duration when applied to the whole ganglion, but decrease burst frequency and increase burst duration when applied only to the posterior half. Other 5HT agonists decrease frequency and prolong bursts when applied to the whole ganglion. Of the DA agonists tested, none acts as DA itself does. Rather, they mimic the effects of 5HT applied to the posterior ganglion, by slowing bursting and prolonging bursts. The actions of agonists do not correspond in any clear way to the receptor specificities as defined in vertebrates. Most antagonists tested do not show similar specificities to their effects in vertebrates. In particular, most of the DA antagonists tested are more effective in blocking exogenous 5HT than DA. One monoamine agonist directly alters the properties of endogenous burst-organizing potentials (driver potentials) in the motorneurons of the ganglion.

Blockage of muscle and neuronal nicotinic acetylcholine receptors by fluoxetine (Prozac)

Fluoxetine (Prozac), a widely used antidepressant, is said to exert its medicinal effects almost exclusively by blocking the serotonin uptake systems. The present study shows that both muscle and neuronal nicotinic acetylcholine receptors are blocked, in a noncompetitive and voltage-dependent way, by fluoxetine, which also increases the rate of desensitization of the nicotinic receptors. Because these receptors are very widely distributed in the both central and peripheral nervous systems, the blocking action of fluoxetine on nicotinic receptors may play an important role in its antidepressant and other therapeutical effects. Our findings will help to understand the mode of action of fluoxetine, and they may also help to develop more specific medicinal drugs.

Ligand-gated ion channels opened by 5-HT in molluscan neurones

1. 5-Hydroxytryptamine (5-HT) activated a fast (70 ms to half maximum) and desensitizing inward current through non-selective channels conducting predominantly monovalent cations in neurons of Helix aspersa. 2. alpha-Methyl-5-HT was equipotent with 5-HT in activating this current, but the known selective agonists at vertebrate 5-HT3 receptors, 2-methyl-5-HT and arylbiguanides were ineffective (< 100 microM). 5-Methoxytryptamine which is inactive on vertebrate 5-HT3 receptors was a very weak agonist. 3. The responses were antagonized by the specific vertebrate 5-HT3 receptor blocker MDL-72222 (IC50 = 1 microM), but were only weakly affected by ondansetron (10 microM). The 5-HT2-type antagonist, ketanserin (< 5 microM) had no effect. The responses were also antagonized by the non-specific antagonists (+)-tubocurarine and strychnine. 4. Unitary currents through channels non-selective for monovalent cations, and with a conductance of 2pS, could be activated repeatedly by 5-HT or alpha-methyl-5-HT in outside-out patches from neurones exhibiting the fast 5-HT-activated current (I[5-HT]fast), even in the presence of 500 microM GDP-[beta S] in the recording pipette. This strongly supports direct-gating of these channels by 5-HT. The properties of these unitary currents resembled those of I[5-HT]fast. 5. The pharmacological properties of this molluscan 5-HT-operated, ligand-gated channel differed sufficiently from known vertebrate 5-HT3-type receptors to suggest that it represents a new class of 5-HT receptor.

Blockage of nicotinic acetylcholine receptors by 5-hydroxytryptamine

The action of 5-hydroxytryptamine (5HT) on nicotinic acetylcholine receptor (nAChR) channels was investigated in mouse myotubes, human cloned TE671/RD cells, and Xenopus laevis oocytes. The decay of the ACh-activated whole-cell currents was reversibly accelerated in the presence of 5HT (10(-5) to 10(-3) M), in a dose-dependent manner. 5HT also reduced the size and accelerated the decay of currents elicited by ACh in Xenopus oocytes injected with mRNA extracted from C2 myotubes or Torpedo electroplaques, or oocytes injected with cloned mouse muscle AChR subunit mRNAs. The effect of 5HT was promptly reversed after washout, or by depolarizing the oocyte beyond -10 mV. In patch-clamp recordings from myotubes, bath-application of 5HT did not exert an indirect influence on the ACh-activated channels within the patch membrane. In contrast, when the patch membrane was exposed to 5HT (10(-6) M), ACh unit responses appeared as bursts of short pulses. It is concluded that the regulation of ACh responses by 5HT results from a fast noncompetitive blocking action of nAChR-channels. These results show that ligand-gated channels, activated by their specific neurotransmitter, may be regulated by a different neurotransmitter through a direct action on the receptor molecule.

Tryptamine: a metabolite of tryptophan implicated in various neuropsychiatric disorders

Although early interest in the biomedical relevance of tryptamine has waned in recent years, it is clear from the above discussion that the study of tryptamine is worthy of serious consideration as a factor in neuropsychiatric disorders. The study of [3H]-tryptamine binding sites indicates an adaptive responsiveness characteristic of functional receptors. The question raised by Jones (1982d) on whether tryptamine is acting centrally as a neurotransmitter or a neuromodulator still remains mostly unanswered, although the evidence cited within this review strongly suggests a modulatory role for this neuroactive amine (see also Juorio and Paterson, 1990). The synthesis and degradative pathways of tryptamine, as well as the intricate neurochemical and behavioral consequences of altering these pathways, are now more fully understood. It is not yet clear what the role of tryptamine is under normal physiological [homeostatic] conditions, however, its role during pathological conditions such as mental and physical stress, hepatic dysfunction and other disorders of metabolism (i.e. electrolyte imbalance, increased precursor availability, enzyme induction or alterations in enzyme co-factor availability) may be quite subtle, perhaps accounting for various sequelae hitherto considered idiopathic. The evidence for a primary role for tryptamine in the etiology of mental or neurological diseases is still relatively poor, although the observations that endogenous concentrations of tryptamine are particularly susceptible to pharmacological as well as physiological manipulations serve to reinforce the proposition that this indoleamine is not simply a metabolic accident but rather a neuroactive compound in its own right. Finally, one might wonder what proportion of the data attributed to modifications of 5-HT metabolism might, in fact, involve unrecognized changes in the concentrations of other neuroactive metabolites of tryptophan such as tryptamine.

Vitamin E regulates acetylcholine receptor function of molluscan neurons

1. Intracellular recordings were made from identified LP11, RBc4, D1 and E4 neurons in perioesophageal ganglionic ring with buccal ganglia of the mollusc Helix pomatia. 2. The modulations of acetylcholine (ACh)-induced current by vitamin E in these neurons were investigated using two-microelectrode intracellular recording and voltage-clamp techniques. 3. ACh receptors function on LP11 and RBc4 neurons was strongly regulated by intracellular calcium ions. For these ACh receptors application of 10(-6) to 10(-4) M vitamin E and calcium influx both induced an enhancement of the ACh-induced chloride current. Application of 10(-5) to 5.10(-5) M arachidonic acid on the same identified LP11 and RBc4 neurons was shown to evoke a decrease of the ACh-induced chloride current. 4. The elevation of calcium levels into D1 and E4 neurons induced a faint decrease of ACh-induced chloride current, but vitamin E and arachidonic acid were ineffective. 5. The calmodulin inhibitor, chloropromazine (6.10(-5) M), strongly inhibited the enhancing effect of calcium influx on ACh-induced chloride current in LP11 and RBc4 neurons, but it had a weak influence on the effect of vitamin E. 6. The effect of vitamin E on surface distribution of functional ACh receptors in LP11 and RBc4 neurons was found. 7. Application of 10(-4) to 10(-6) M vitamin E (DL-alpha-tocopherol) triggered mechanisms, which after a 5 to 45-min period lead to appearance of functional ACh receptors on the parts of neuronal soma, which were further from the axon. 8. Arachidonic acid (vitamin F) evoked a disappearance of functional ACh receptors, which were activated by vitamin E.

Actions of the molluscan neuropeptide FMRF-amide on neurones in the suboesophageal ganglia of the snail Helix aspersa

The effects of the molluscan neuropeptide FMRF-amide were tested on several neurones in the suboesophageal ganglia of the snail Helix aspersa. Almost all neurones tested responded to the peptide, some being hyperpolarized (H response) and others depolarized (D response). The H response is due primarily to an inward potassium current and may be blocked in 20 microM 4-aminopyridine. The hyperpolarizing actions of FMRF-amide and dopamine may be separated by ergometrine which blocks the response to dopamine but not to FMRF-amide. The D response is due mainly to an inward sodium current but this is not blocked by d-tubocurarine, morphine or TTX. It appears to be mediated by a distinct receptor/ionophore as excitation by ACh and 5-HT are both antagonized by d-tubocurarine. The Leu2-substituted analogue FLRF-amide was found to produce similar H responses to FMRF-amide, but was much less potent at producing D responses. It did, however, produce cross-desensitization of the D response to FMRF-amide, suggesting that it does bind to the FMRF-amide receptor.

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)

5-Hydroxytryptamine receptors of visceral primary afferent neurones on rabbit nodose ganglia

1. The electrophysiological characteristics of 5-hydroxytryptamine (5-HT) receptors distributed on visceral primary afferent neurones (the nodose ganglion cells of the vagus) in rabbits were investigated with intracellular recording and voltage-clamp techniques.2. In response to 5-HT applied by superfusion (>/= 10 mum) or by ionophoresis (>/= 5 nA, 50 msec), the majority of type C neurones (mean axonal conduction velocity: 0.83+/-0.25 m/sec) showed a rapid depolarization of 20-30 mV in amplitude which was followed by a hyperpolarization of a few millivolts. Both the initial depolarization and afterhyperpolarization were associated with a reduction in membrane resistance.3. Type A neurones (mean axonal conduction velocity: 7.7+/-0.4 m/sec) did not show any significant alterations in membrane potential and resistance during or after application of 5-HT.4. The initial depolarization induced by 5-HT was abolished by Na(+)-free Krebs solution and showed a reduction of a few millivolts in K(+)-free or Ca(2+)-free Krebs solution. The response in normal Krebs solution was reversed at a membrane potential level of +7.3+/-1.1 mV.5. The afterhyperpolarization disappeared in Na(+)-free or Ca(2+)-free Krebs solution, while it was markedly enhanced in K(+)-free Krebs solution. The response in normal Krebs solution reversed at a membrane potential of -88.7+/-0.8 mV, and was abolished at membrane potentials more positive than -20 mV.6. Unlike 5-HT voltage responses, which were biphasic in the majority of neurones examined, 5-HT induced currents were usually monophasic when recorded at holding membrane levels ranging from -80 to +50 mV. The reversal potential of the inward current was +7.5+/-0.8 mV which was in good agreement with the reversal level for 5-HT-induced depolarizations. The reversal potentials for inward currents which were obtained at various concentrations of Na(+) or K(+) corresponded to the theoretical values calculated by the equivalent circuit equation.7. These results suggest that the initial depolarization induced by 5-HT is due mainly to simultaneous increases in Na(+) and K(+) conductances, while the afterhyperpolarization is brought about by an increase of K(+) conductance which is triggered by a voltage-dependent influx of Na(+) and Ca(2+).8. The mean value for the ;limiting slope' of conductance change vs. 5-HT concentration and the slope of 5-HT current vs. 5-HT concentration obtained by superfusion of 5-HT, were in good agreement, 1.84+/-0.26 and 1.88+/-0.31, respectively. On the other hand, the mean Hill coefficient obtained from the dose-response curves for the inward current induced by ionophoresis was 2.51+/-0.14.9. Tetrodotoxin (0.2 mum) blocked the soma action potential completely, but did not show any effect on 5-HT-induced responses.10. (+)-Lysergic acid diethylamide and methysergide (1-100 mum) had no depressant effect on the 5-HT-induced depolarization.11. (+)-Tubocurarine at low concentrations (1-5 mum) inhibited the 5-HT induced inward current competitively. The mode of its inhibitory action became noncompetitive at higher concentrations (10-20 mum).

Tryptamine: a neuromodulator or neurotransmitter in mammalian brain?

Tryptamine synthesized by decarboxylation of L-tryptophan occurs as an endogenous constituent of mammalian brain albeit at very low concentrations (low ng/g range). It is primarily metabolized by oxidative deamination by MAO and possesses an extremely rapid turnover and half-life. Subcellular localization appears to be in nerve terminals and it is releasable by electrical or potassium evoked depolarization. Neuropharmacological and electrophysiological data strongly suggest the existence of post-synaptic receptors for tryptamine independent of those for 5HT. There may exist a rostrally projecting neuronal tryptamine containing system arising from cell bodies in or close to the nucleus raphé medianus. The demonstration of specific receptors for tryptamine in the CNS strongly indicates a transmitter role, although a strong case can be made for a role as a modifier of central 5HT systems. The possibility also exists that 5HT and tryptamine may be mediators of functionally opposite neuronal pathways. Whatever the role of tryptamine in the CNS it is clear that it not simply present as an accident of metabolism or a "biological artefact." The indications are that it possesses important functions in central neurotransmission.

Chemoreceptors for serotonin (5-HT), acetylcholine (ACh), bradykinin (BK), histamine (H) and gamma-aminobutyric acid (GABA) on rabbit visceral afferent neurons

The somata of type 'C' neurons in rabbit nodose ganglion are endowed with receptor sites for 5-HT, BK, ACh, II and GABA. 5-HT and ACh application to type 'C' neurons in the nodose ganglion of rabbits produced a rapid depolarization associated with an increased membrane conductance, most likely to Na+ and K+. BK and H elicited slow depolarizations accompanied by a decreased membrane conductance probably to K+. GABA induced a rapid depolarization associated with an increased conductance to Cl-. In contrast, type 'A' neurons were insensitive to the four algesic agents but responded to GABA. d-Tubocurarine or picrotoxin at relatively low concentrations blocked ACh, 5-HT and GABA depolarizations without affecting membrane properties. Hexamethonium blocked ACh responses but not 5-HT responses. In addition, no desensitization occurred between the substances 5-HT, ACh or BK. The results suggest that the depolarizing effect of these agents on visceral neurons might be exerted via different receptors.

On the time course of the electrical response of salivary gland cells of Nauphoeta cinerea to ionophoretically applied dopamine

1. An attempt has been made to account for the latency and slow time course of the electrical responses of salivary gland cells of Nauphoeta cinerea (Olivier) to nerve stimulation and ionophoretically applied dopamine from a pipette placed as close as possible to the acinar surface. 2. The effects have been investigated on the time course of the ionophoretic response of changes in the distance of the pipette from the acinar surface and of the amount of dopamine ejected. 3. The observed changes were smaller than those predicted by models in which the rate limiting step was assumed to be diffusion or slow receptor kinetics. Indirect evidence suggests that the time course of the response is determined by processes subsequent to receptor activation.

Serotonin shifts the phase of the circadian rhythm from the Aplysia eye

A putative neurotransmitter, serotonin, may be used to transmit temporal information in the eye of Aplysia, because it can shift the phase of the circadian rhythm of spontaneous optic nerve impulses from the eye and the eye contains a significant quantity of serotonin. Serotonin acts either directly on the cell, or cells, containing the circadian pacemaker or on cells electronically coupled to the pacemaker cells.

Antipsychotic drugs and dopamine-mediated responses in Aplysia neurons

The effect of antipsychotic drugs was tested on responses to micro-electrophoretically applied dopamine, acetylcholine and 5-hydroxytryptamine in identified neurons of the marine gastropod Aplysia californica. Fluphenazine was able to depress the response to DA in concentration of 10 muM, with 100 muM DA-responses of many neurons were blocked completely. Thioridazine (10 and 100 muM) and haloperidol (50 muM) were also effective in depressing DA-responses, while the non-antipsychotic phenothiazines mepazine (10 and 100 muM) and promethazine (100 muM) had only a slight action on DA-receptors. ACh- and 5-HT-responses were slightly affected only by high concentrations after long lasting perfusion. The investigated drugs had no persistent or only an insignificant effect on resting membrane potential and amplitude of action potentials of the neurons. With haloperidol depolarizing afterpotentials leading to double discharges were observed in some neurons. In a few instances spontaneous EPSPs disappeared with the DA-response under the influence of anti-psychotic drugs. The results render a direct neurophysiological evidence for the blockade of DA-receptors by antipsychotic drugs in correspondence to their clinical efficacy and agree with data from clinical observations and obtained in neurochemical, behavioral and indirect neurophysiological experiments.

Regulation of neuronal properties by afferent connections. I. Functional changes in snail neurons following section of presysnaptic nerve fibers

The left cerebral connective of snail ganglia containing afferent inputs to a particular neuron was sectioned. One week after the operation synaptic potentials were not obtained when the connective was stimulated, and they had not reappeared after 60 days. The current-voltage relationship of denervated cells showed a rectification in the hyperpolarizing direction, whereas that of control cells had an ohmic behavior. Depolarization or conductance changes produced by ACh or carbachol showed a supersensitivity in denervated cells, evidenced by a shift to the left of the dose-response curve and an increase of the maximal responses obtained. The reversal potential of ACh shifted from--22 mV in the control to--8 mV in denervated cells. Depolarization produced by serotonin was similar in control and denervated cells.

Some effects of 5-hydroxytryptamine, dopamine and noradrenaline on neurones in the submucous plexus of guinea-pig small intestine

1. Responses to the iontophoretic application of 5-hydroxytryptamine (5-HT), dopamine and noradrenaline were examined in neurones of the submucous plexus of guinea-pig small intestine. 2. Every neurone was excited by 5-HT. 3. In a proportion of cells, dopamine or noradrenaline caused an increase in membrane potential. This response was only observed in cells which received in inhibitory innervation. The responses closely resembled inhibitory synaptic potentials evoked by transmural stimulation. 4. Both inhibitory synaptic potentials and inhibitory responses to dopamine and noradrenaline were blocked by methysergide. 5. It seems possible that these two catecholamines may interact with similar receptors to those activated by inhibitory transmitter.

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.

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.

Differential chemosensitivity of synaptic and extrasynaptic areas on the neuronal surface membrane in parasympathetic neurons of the frog, tested by microapplication of acetylcholine

1. The chemosensitivity of the surface of parasympathetic neurons in the interatrial septum of the heart of the frog has been explored by focal iontophoretic application of acetylcholine (ACh). 2. There is a specific highly chemosensitive area on the cell surface near synaptic boutons. ACh released close to visually identified boutons produced depolarizing responses which arise more rapidly and are larger than responses evoked at randomly chosen spots on the neuronal surface. 3. Individual synapses, or small numbers of them, can be desensitized by applied ACh without blocking synaptic transmission at nearby synapses on the same neuron. This shows that the method of ACh application resolves chemosensitivity in patches restricted to a few micrometres in diameter.

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.

Chemical and ultrastructural identification of 5-hydroxytryptamine in an identified neuron

The two largest cells in a typical ganglion of the leech (Hirudo medicinalis) nervous system are the colossal cells of Retzius. These cells show a positive chromaffin reaction, and it has been suggested that they contain 5-hydroxytryptamine (5-HT). In this study, the presence of 5-HT in the colossal cells was confirmed by microspectrofluorometry and by thin-layer chromatography and spectrofluorometry of extracts of individually dissected and pooled colossal cell bodies. A single colossal cell body was found to contain, on the average, 3.8 x 10(-10) g (6mM) 5-HT. Electron microscopy shows that the colossal cells are distinguished by the presence of 1000 A granules with irregular, electron-opaque cores. Since the granules are distributed in the same pattern as the 5-HT fluorescence, we have suggested that they contain 5-HT. Furthermore, a chromaffin reaction modified for the electron microscope provides evidence that 5-HT is present in the granule cores. These data can now serve as a basis for further studies on the metabolism, distribution, and function of 5-HT in these identified neurons.