Effect of serotonin and acetylcholine on neurons in the central nervous system of snails

Differences in the distribution of neurons distinguished by their responses to serotonin and acetylcholine were found in the central nervous system ofHelix pomatia. When applied to the body of the neuron acetylcholine hyperpolarizes the cell more often than it depolarizes it, but depolarization predominates in some regions, e.g., on the dorsal surface of the visceral ganglion. In most cases serotonin stimulates activity and induces depolarization or the appearance of pacemaker oscillations of membrane potential. The oscillogenic effect of serotonin is characteristic, in particular, of white (peptidergic) neurons and the depolarization effect is characteristic of other neurons, including the paired giant metacerebral neurons which contain serotonin in their cytoplasm. Both effects failed to appear in sodium-free solution. A group of neurons in which hyperpolarization was observed in response to serotonin application was found in the visceral ganglion of hibernating snails. The same cells in active snails were stimulated by serotonin. A giant neuron with two variously located cholinergic structures is present on the ventral surface of the ganglion among this group of cells: acetylcholine hyperpolarized it when applied to the cell body but depolarized it when applied to the axon.     

The effect of plastic restructuring of the local chemoexcitable zone in the somatic membrane of the PPa4 neuron of Helix lucorum on the excitability of neighboring chemo- and electrically excited zones

Rhythmic application of acetylcholine or serotonin to the local zone of somatic membrane was used to study the effect of extinction of RPa4 neuron depolarization in Helix lucorum on the excitability of adjacent chemo- and electroexcitable zone. It has been found that the extinction of response to iontophoretic application of acetylcholine to one somatic zone decreases the sensitivity of serotonin and cholinoreceptors in adjacent zones, as well as the excitability of electroexcitable membrane. The effect on the excitability of adjacent zones does not depend on the type of receptors activated rhythmically, as the extinction of RPa4 response to the repeated application of serotonin also reduces the sensitivity of adjacent cholinoreceptor zones. A cause of this effect may lie in modification of chemoreceptors and ionic channels, by intracellular regulatory systems that become activated by repeated stimulation.     

Role of calcium ions in processes of serotonin-induced modulation of neuronal response to acetylcholone application in Helix pomatia

The role of calcium ions in modulating serotonin action on acetylcholine (ACh) response in nonidentified and identified (LPa3 and RPa3) neurons ofHelix pomatia was investigated using voltage-clamping at the neuronal membrane. Exposure for 1 min to serotonin prior to ACh application reduced response to ACh in neuron LPa3 and raised it in RPa3. The same two patterns of modulating ACh-induced response were produced by extracellular application of theophylline and dibutyryl c-AMP. Injecting calcium ions into neuron LPa3 led to reinforcement of ACh-induced current in the presence of serotonin, thus changing the pattern of serotonin-induced modulation of ACh response in this unit. In neuron RPa3, the same process enhanced the serotonin-induced modulating effect on ACh response but without changing the pattern of modulation, while injected EDTA produced the reverse effects. Increased intracellular concentration of calcium ions brought about a reduction in the degree of serotonin-induced modulation of ACh response in neuron RPa3. Possible reasons are discussed for changes in serotonin-induced bimodal modulation of ACh response in test neurons produced by altering the extracellular concentration of calcium ions.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 5, pp. 666–671, September–October, 1988.     

Ultrastructural changes in the autonomic interneuronal synapse during activation in the presence of acetylcholinesterase suppression

Structural-functional reconstructions of the frog autonomic interneuronal synapsis have been studied at its activation with endogenic acetylcholine under conditions of acetylcholinesterase suppression. The investigation has been performed with preparations of isolated sympathetic trunk of Rana temporaria treated with armine (5 X 10(-6) M) and subjected to electrostimulation (5 imp/sec) up to a complete block of the synaptic transmission. Certain structural changes are revealed in the axo-somatic synapses, demonstrating an increased adhesive properties of the membranes, ("hatch-like" membranes, numerous submembranous aggregates, aggregates of the intercellular cleft and neuronal-glial contacts). In the terminals changed according to the "light type", with poorly manifested changes, light synaptic vesicles loose their spheric form, their diameter decreases. In the boutons with more intensive changes, the vesicles gradually change into the mass of cluster-floccular material. In the boutons with intensively manifested disorders in the ultrastructure, a complete destruction of the light vesicles is observed. The great part of the ganglionic neuron bodies changes according the "dark type". In their neuroplasm a great amount of subsuperficial cisterns of the endoplasmic reticulum and formation of powerful fasciculi of microfilaments are noted to appear.     

Serotonin inhibition of synaptic transmission: Galpha(0) decreases the abundance of UNC-13 at release sites

We show that serotonin inhibits synaptic transmission at C. elegans neuromuscular junctions, and we describe a signaling pathway that mediates this effect. Release of acetylcholine from motor neurons was assayed by measuring the sensitivity of intact animals to the acetylcholinesterase inhibitor aldicarb. By this assay, exogenous serotonin inhibited acetylcholine release, whereas serotonin antagonists stimulated release. The effects of serotonin on synaptic transmission were mediated by GOA-1 (a Galpha0 subunit) and DGK-1 (a diacylglycerol [DAG] kinase), both of which act in the ventral cord motor neurons. Mutants lacking goa-1 G(alpha)0 accumulated abnormally high levels of the DAG-binding protein UNC-13 at motor neuron nerve terminals, suggesting that serotonin inhibits synaptic transmission by decreasing the abundance of UNC-13 at release sites.     

The characteristics of the effect of mediator substances on the dynamic oxygen tension in the gastric mucosa]

The studies on rats were carried out to determine dynamics of pO2 in the mucous membrane of the stomach under the effect of acetylcholine, norepinephrine, serotonin, histamine, prostaglandin E2 and ATP. As to the changes in pO2 the mediator substances were arranged as follows (from more intensive effect to less pronounced one): serotonin, acetylcholine, prostaglandin E2, norepinephrine, histamine and ATP. As to the duration of the action--PGE2 acetylcholine, serotonin, norepinephrine, ATP and histamine. Under the joint action of the mediator substances (serotonin, norepinephrine, histamine) with acetylcholine the effects of domination and modulation of acetylcholine effect are found.     

Effect of serotonin and acetylcholine on the electrical activity of isolated cortex in the rabbit

Neuronal isolation of the rabbit's cerebral hemisphere shifts the EEG spectrum in direction of slower processes. Application of acetylcholine on the cortex elicits EEG activation and appearance of the theta-rhythm. Initially serotonin application is accompanied by the appearance of the theta-rhythm periods; in the process of subsequent administration of the drug these periods are gradually substituted by slow delta-waves. Combined application of serotonin and acetylcholine on isolated cortex elicits bursts of high-amplitude activity, abruptly substituted by "silence" phases. In contrast to the intact cortex where serotonin elicited prolonged and rhythmic alternation on EEG of phases of high-amplitude activity and of "silence" periods, in the isolated cortex the bursts of activity of about 1 min duration appeared only after application of the acetylcholine to serotonin-saturated cortex. Repeated phases of activation were either absent or were of short duration and extinguished rapidly.     

Action of local iontophoretic application of acetylcholine and serotonin to neurons of the rabbit superior cervical ganglion

The action of ionotophoretic application of acetylcholine and serotonin (5-hydroxytryptamine) on neurons of the isolated rabbit superior cervical ganglion was investigated by intracellular recording. The soma of neurons in the ganglion was shown to have no muscarinic receptors and to have only nicotinic receptors scattered irregularly over the whole surface of the neuron soma membrane. Acetylcholine has an excitatory action on presynaptic endings. In about half of the neurons of the ganglion the soma was shown to possess serotonin receptors.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 5, pp. 519–524, September–October, 1978.     

Synaptic and Extrasynaptic Secretion of Serotonin

Serotonin is a major modulator of behavior in vertebrates and invertebrates and deficiencies in the serotonergic system account for several behavioral disorders in humans.The small numbers of serotonergic central neurons of vertebrates and invertebrates produce their effects by use of two modes of secretion: from synaptic terminals, acting locally in hard wired circuits, and from extrasynaptic axonal and somatodendritic release sites in the absence of postsynaptic targets, producing paracrine effects.In this paper, we review the evidence of synaptic and extrasynaptic release of serotonin and the mechanisms underlying each secretion mode by combining evidence from vertebrates and invertebrates. Particular emphasis is given to somatic secretion of serotonin by central neurons.Most of the mechanisms of serotonin release have been elucidated in cultured synapses made by Retzius neurons from the central nervous system of the leech. Serotonin release from synaptic terminals occurs from clear and dense core vesicles at active zones upon depolarization. In general, synaptic serotonin release is similar to release of acetylcholine in the neuromuscular junction.The soma of Retzius neurons releases serotonin from clusters of dense core vesicles in the absence of active zones. This type of secretion is dependent of the stimulation frequency, on L-type calcium channel activation and on calcium-induced calcium release.The characteristics of somatic secretion of serotonin in Retzius neurons are similar to those of somatic secretion of dopamine and peptides by other neuron types. In general, somatic secretion by neurons is different from transmitter release from clear vesicles at synapses and similar to secretion by excitable endocrine cells.     

Cation-pi interactions in ligand recognition by serotonergic (5-HT3A) and nicotinic acetylcholine receptors: the anomalous binding properties of nicotine

A series of tryptophan analogues has been introduced into the binding site regions of two ion channels, the ligand-gated nicotinic acetylcholine and serotonin 5-HT(3A) receptors, using unnatural amino acid mutagenesis and heterologous expression in Xenopus oocytes. A cation-pi interaction between serotonin and Trp183 of the serotonin channel 5-HT(3A)R is identified for the first time, precisely locating the ligand-binding site of this receptor. The energetic contribution of the observed cation-pi interaction between a tryptophan and the primary ammonium ion of serotonin is estimated to be approximately 4 kcal/mol, while the comparable interaction with the quaternary ammonium of acetylcholine is approximately 2 kcal/mol. The binding mode of nicotine to the nicotinic receptor of mouse muscle is examined by the same technique and found to differ significantly from that of the natural agonist, acetylcholine.     

Modulation of swimming behavior in the medicinal leech

Expression of swimming in the medicinal leech (Hirudo medicinalis) is modulated by serotonin, a naturally occurring neurohormone. Exogenous application of serotonin engenders spontaneous swimming activity in nerve-cord preparations. We examined whether this activity is due to enhanced participation of swim motor neurons (MNs) in generating the swimming rhythm. We found that depolarizing current injections into MNs during fictive swimming are more effective in shifting cycle phase in nerve cords following serotonin exposure. In such preparations, the dynamics of membrane potential excursions following current injection into neuronal somata are substantially altered. We observed: 1) a delayed outward rectification (relaxation) during depolarizing current injection, most marked in inhibitory MNs; and 2) in excitor MNs, an enhancement of postinhibitory rebound (PIR) and afterhyperpolarizing potentials (AHPs) following hyperpolarizing and depolarizing current pulses, respectively. In contrast, we found little alteration in MN properties in leech nerve cords depleted of amines. We propose that enhanced expression of swimming activity in leeches exposed to elevated serotonin is due, partly, to enhancement of relaxation, PIR and AHP in MNs. We believe that as a consequence of alterations in cellular properties and synaptic interactions (subsequent paper) by serotonin, MNs are reconfigured to more effectively participate in generating and expressing the leech swimming rhythm.Abbreviations AHP Afterhyperpolarizing potential - DCC Discontinuous current clamp - DE Dorsal excitor motor neuron - DI Dorsal inhibitor motor neuron - IPSP Inhibitory postsynaptic potential - MN Motor neuron - PIR Postinhibitory rebound - VE Ventral excitor motor neuron - VI Ventral inhibitor motor neuron     

Microiontophoretic study of the effect of biogenic amines on unit activity in the rabbit visual cortex

The effect of acetylcholine, noradrenalin, and serotonin on spontaneous activity of visual cortical neurons and on their activity evoked by flashes, recorded extracellularly, was studied by microiontophoresis in unanesthetized rabbits. The ability of visual cortical neurons to respond to light does not correlate with their sensitivity to acetylcholine. This substance, which changes the spontaneous firing rate of many of the neurons tested, was less effective against their evoked activity. Noradrenalin had a powerful depressant action on both spontaneous and evoked activity of most neurons studied. Serotonin acted in different ways on the spontaneous and evoked activity of some neurons tested. It is postulated that acetylcholine mediates reticulo-cortical inputs, noradrenalin is a true inhibitory mediator in the cerebral cortex, and serotonin has a presynaptic action by preventing the liberation of natural mediators.     

Acetylcholine and histamine are transmitter candidates in identifiable mechanosensitive neurons of the spider Cupiennius salei: an immunocytochemical study

Histochemical and indirect immunocytochemical techniques were used to search for neuroactive substances and transmitter candidates in identified sensory neurons of two types of cuticular mechanoreceptors in the spider Cupiennius salei Keys.: (1) in lyriform slit-sense organ VS-3 (comprising 7-8 cuticular slits each innervated by 2 bipolar neurons), and (2) in tactile hairs (each supplied by 3 bipolar sensory cells). All neurons are mechanosensitive. A polyclonal antibody against choline acetyltransferase (ChAT) strongly labeled all cell bodies and afferent fibers of both mechanoreceptor types. Western blot analysis using the same antibody against samples of spider sensory hypodermis and against samples from the central nervous system demonstrated a clear band at 65 kDa, corresponding to the molecular mass of ChAT in insects. Moreover, staining for acetylcholine esterase (AChE) revealed AChE activity in one neuron of each mechanoreceptor type. Incubation with a polyclonal antibody against histamine clearly labeled one neuron in each set of sensilla, whereas activity in the remaining one or two cells was near background. All mechanoreceptor preparations treated with a polyclonal antiserum against serotonin tested negative, whereas sections through the central nervous system of the same spiders were clearly labeled for serotonin. The presence of ChAT-like immunoreactivity and AChE implicates acetylcholine as a transmitter candidate in the two mechanoreceptive organs. We assume that histamine serves as a mechanosensory co-transmitter in the central nervous system and may also act at peripheral synapses that exist in these sensilla. Received: 15 July 1996 / Accepted: 26 August 1996     

Acute effects of pulsed microwaves and 3-nitropropionic acid on neuronal ultrastructure in the rat caudate-putamen

Ultrastructure of the medium sized "spiny" neuron in rat dorsal-lateral caudate-putamen was assessed after administration of 3-nitropropionic acid (3-NP) and exposure to pulsed microwaves. Sprague-Dawley male rats were given two daily intraperitoneal doses of 0 or 10 mg/kg 3-NP and 1.5 h after each dose were exposed to microwave radiation at a whole body averaged specific absorption rate (SAR) of 0 (sham exposure), 0.6, or 6 W/kg for 30 min. Microwave exposure consisted of 1.25 GHz radiation delivered as 5.9 micros pulses with repetition frequency 10 Hz. Tissue samples taken 2-3 h after the second sham or microwave exposure showed no injury with light microscope methods. Blinded qualitative assessment of ultrastructure of randomly selected neurons from the same samples did reveal differences. Subsequent detailed, quantitative measurements showed that, when followed by sham exposure, administration of 3-NP significantly increased endoplasmic reticulum (ER) intracisternal width, ER area density, and nuclear envelope thickness. Microwave exposure at 6 W/kg alone also significantly increased these measures. Exposure of 3-NP treated animals at 6 W/kg significantly increased effects of 3-NP on ultrastructure. Although exposure at 0.6 W/kg alone did not affect ultrastructure measures, exposure of 3-NP treated animals at 0.6 W/kg reduced the effects of 3-NP. We concluded that 3-NP changed neuronal ultrastructure and that the microwave exposures used here changed neuronal ultrastructure in ways that depended on microwave SAR and neuron metabolic status. The apparent cancellation of 3-NP induced changes by exposure to pulsed microwaves at 0.6 W/kg indicated the possibility that such exposure can protect against the effects of mitochondrial toxins on the nervous system.     

L-NAME enhances pulmonary vasoconstriction without inhibiting EDRF-dependent vasodilation.

The purpose of the present study was to determine the influence of NG-nitro-L-arginine methyl ester (L-NAME) on pulmonary vascular responses to endothelium-dependent relaxing factor- (EDRF) dependent and EDRF-independent substances in the pulmonary vascular bed of the anesthetized cat. Because pulmonary blood flow and left atrial pressure were kept constant, changes in lobar arterial pressure directly reflect changes in pulmonary vascular resistance. When pulmonary vasomotor tone was actively increased by intralobar infusion of U-46619, intralobar bolus injections of acetylcholine, bradykinin, serotonin, and 5-carboxyamidotryptamine (a serotonin1A receptor agonist) decreased lobar arterial pressure in a dose-related manner. The pulmonary vasodilator response to serotonin, but not to 5-carboxyamidotryptamine, acetylcholine, and bradykinin, was significantly decreased by L-NAME (100 mg/kg i.v.). Administration of ritanserin (0.5 mg/kg i.v.), but not L-arginine (1 g/kg i.v. with 60 mg.kg-1 x min-1 i.v. infusion), reversed the inhibitory effects of L-NAME on the pulmonary vasodilator response to serotonin and abolished the enhanced pulmonary vasoconstrictor response to (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminoproprane hydrochloride (a serotonin2 receptor agonist) after L-NAME administration. In conclusion, the present experiments suggest that L-NAME inhibits the pulmonary vasodilator response to serotonin by increasing the sensitivity of serotonin2 receptor-mediated vasoconstriction and not by inhibiting EDRF formation. Because the pulmonary vasodilator responses to bolus administration of acetylcholine and bradykinin were not inhibited by L-NAME, these data suggest that L-NAME does not appear to be an adequate probe to study the role of endogenous EDRF in the adult feline pulmonary vascular bed in vivo.     

Interactive effects of serotonin and acetylcholine on neurite elongation

Serotonin (5-HT) inhibits elongation of neurites of specific identified neurons. Here we report a novel, growth-enabling action of another neurotransmitter, acetylcholine (ACh). When applied simultaneously with serotonin, ACh prevents the inhibition of Helisoma neuron B19 neurite elongation that would occur in response to application of 5-HT alone. We also report that ACh prevents the rise in growth cone Ca2+ that would occur in response to application of 5-HT alone and that ACh blocks the electrical excitatory effect of 5-HT on neuron B19. These results support the hypothesis that growth cone motility and neurite elongation can be regulated by voltage-gated Ca2+ fluxes and suggest that the dynamics of neurite morphology may be complexly regulated by an array of neurotransmitters, as is functional electrical activity.     

Voltage-dependent actions of endogenous and exogenous serotonin on identified neurones

1. Impulse activity in an identified serotonin-containing neuron (GSN) produces a slow excitatory synaptic response in another identified neuron, the A neuron. An axon process can be traced close to the follower neuron perikaryon after Lucifer Yellow injection of the GSN perikaryon. 2. The synaptic response is markedly voltage-sensitive being increased at depolarized potentials and almost abolished and not inverted at potentials in excess of about -55mV. 3. Serotonin locally applied produces a similar response. 4. The response to serotonin does not involve a change in conductance to either sodium or chloride ions, but calcium ions do appear to be important either because of their influence on potassium ion permeability or in a direct transfer of charge across the membrane. 5. Another follower neuron exhibits a complex GSN-induced synaptic response comprising a slow potential similar to that seen in the A neuron and also a fast, probably sodium dependent, potential. 6. In addition to producing a weak direct excitation of the A neuron, GSN-activation can also partly reverse accommodation and also prolong the duration of the impulse in the A neuron. 7. Exogenously applied serotonin produces a similar voltage-dependent inward current response in the GSN as seen in the A neuron. It is suggested that the receptors mediating the response on the GSNs may normally be involved in feedback regulation. 8. Cyproheptadine (reversibly), methergoline, mianserin and propranolol (all irreversibly) antagonised the response in the GSN. These agents probably all have action on the ionic mechanism underlying the serotonin response.     

Serotonin in the developing stomatogastric system of the lobster,Homarus americanus

We studied the development of the serotonergic modulation of the stomatogastric nervous system of the lobster, Homarus americanus. Although the stomatogastric ganglion (STG) is present early in embryonic development, serotonin immunoreactivity is not visible in the STG until the second larval stage. However, incubation of the STG with exogenous serotonin showed that a serotonin transporter is present in embryonic and early larval stages. Serotonin uptake was blocked by paroxetine and 0% Na⁺ saline. The presence of a serotonin transporter in the embryonic STG suggests that hormonally liberated serotonin could be taken up by the STG, and potentially released as a “borrowed transmitter”. Consistent with a potential hormonal role, serotonin is found in the pericardial organs, a major neurosecretory structure, by midembryonic development. The rhythmic motor patterns produced by embryonic and larval STGs were decreased in frequency by serotonin. Lateral Pyloric (LP) neuron‐evoked excitatory junctional potentials (EJPs) in the embryos and the first larval stage (LI) were larger, slower, and more variable than those in the adult. The amplitude of adult LP neuron‐evoked EJPs was increased more than twofold in serotonin, but in embryos and LI preparations this effect was negligible. In embryos and LI preparations, serotonin increased the occurrence of muscle fiber action potentials and altered the EJP wave‐form. These data demonstrate that serotonin receptors are present in the stomatogastric nervous system early in development, and suggest that the role of serotonin changes from modulation of muscle fiber excitability early in development to enhancement of neurally evoked EJPs in the adult. © 2002 Wiley Periodicals, Inc. J Neurobiol 54: 380–392, 2003     

Serotonergic Regulation of Acetylcholine Release in Rat Frontal Cortex

Abstract: The extent to which serotonin regulates the activity of cortically projecting cholinergic neurons was studied using in vivo microdialysis to monitor interstitial concentrations of acetylcholine in the frontal cortex of freely moving rats. Systemic administration of the serotonin release-inducing agent fenfluramine (3 or 10 mg/kg, i.p.) increased acetylcholine release by 110–130%. The fenfluramine-induced increase in acetylcholine release was significantly attenuated by pretreatment with the selective serotonin uptake inhibitor fluoxetine (10 mg/kg, i.p.). Pretreatment with the selective dopamine D₁ receptor antagonist SCH-23390 (0.3 mg/kg, s.c.) failed to prevent the fenfluramine-induced increase in acetylcholine release. In contrast, the serotonin 5-HT_2A receptor antagonist ketanserin (5 mg/kg, i.p.) blocked fenfluramine-induced increases in acetylcholine release. In contrast to previous studies that have concluded that serotonin has inhibitory actions on cortical acetylcholine release, the present results indicate that fenfluramine increases cortical acetylcholine release in vivo by its ability to enhance serotonin transmission and that serotonin produces these effects at least in part via actions at serotonin 5-HT_2A receptors.