Dopamine and serotonin receptors mediating contractions of the snail, Helix pomatia, salivary duct

Dopamine as a Multifunctional Neurotransmitter in Gastropod Molluscs: An Evolutionary Hypothesis

AbstractThe catecholamine 3,4-dihydroxyphenethylamine, or dopamine, acts as a neurotransmitter across a broad phylogenetic spectrum. Functions attributed to dopamine in the mammalian brain include regulation of motor circuits, valuation of sensory stimuli, and mediation of reward or reinforcement signals. Considerable evidence also supports a neurotransmitter role for dopamine in gastropod molluscs, and there is growing appreciation for its potential common functions across phylogeny. This article reviews evidence for dopamine's transmitter role in the nervous systems of gastropods. The functional properties of identified dopaminergic neurons in well-characterized neural circuits suggest a hypothetical incremental sequence by which dopamine accumulated its diverse roles. The successive acquisition of dopamine functions is proposed in the context of gastropod feeding behavior: (1) sensation of potential nutrients, (2) activation of motor circuits, (3) selection of motor patterns from multifunctional circuits, (4) valuation of sensory stimuli with reference to internal state, (5) association of motor programs with their outcomes, and (6) coincidence detection between sensory stimuli and their consequences. At each stage of this sequence, it is proposed that existing functions of dopaminergic neurons favored their recruitment to fulfill additional information processing demands. Common functions of dopamine in other intensively studied groups, ranging from mammals and insects to nematodes, suggest an ancient origin for this progression.

Recent advances in pharmacological research on the management of irritable bowel syndrome

Irritable bowel syndrome (IBS), a common gastrointestinal (GI) disorder, is associated with various factors, including lifestyle, infection, stress, intestinal flora, and related diseases. The pharmacotherapeutic stimulation of receptors and downstream signaling pathways is effective in reducing IBS symptoms; however, it is still associated with adverse effects. Various receptors related to GI motility and visceral hypersensitivity should be considered to enhance the benefit/risk ratio of IBS treatments. This review discusses recent pharmacological advances in IBS management. Several receptors related to GI motility and abdominal pain are investigated in various angles. 5-Hydroxytryptamine (5-HT) is an important neurotransmitter that activates the colonic mucosal 5-HT₄ receptor without causing severe cardiovascular adverse effects. The clinical potential of ramosetron for diarrhea-predominant IBS has been suggested because of a lower risk of ischemic colitis than conventional 5-HT₃ receptor antagonists. Toll-like receptors (TLRs), especially TLR2 and TLR4, show a significant effect on the post-infection symptoms and lipopolysaccharide-mediated regulation of GI motility. Histamine is a well-known nitrogenous compound that regulates inflammatory responses and visceral hypersensitivity. Histamine 1 receptor-mediated sensitization of the transient receptor potential vanilloid 1 is associated with IBS. Pharmacological approaches based on these signaling pathways could be useful in the development of novel IBS treatments.

Endocannabinoid-mediated potentiation of nonnociceptive synapses contributes to behavioral sensitization

Endocannabinoids, such as 2-arachidonoyl glycerol (2-AG) and anandamide, can elicit long-term depression of both excitatory and inhibitory synapses. This latter effect will result in disinhibition and would therefore be expected to produce an increase in neural circuit output. However, there have been no examples directly linking endocannabinoid-mediated disinhibition to a change in a functional neurobehavioral circuit. The present study uses the well-characterized central nervous system of the medicinal leech, Hirudo verbana, to examine the functional/behavioral relevance of endocannabinoid modulation of an identified afferent synapse. Bath application of 2-AG potentiates synaptic transmission by pressure-sensitive sensory neurons (P cells) as well as the magnitude of the defensive shortening reflex elicited by P-cell stimulation. This potentiation requires activation of TRPV-like channels. Endocannabinoid/TRPV signaling was found to produce sensitization of the shortening reflex elicited by either direct stimulation of nearby nociceptive afferents (N cells) or noxious stimulation applied to skin several segments away. In both cases, heterosynaptic potentiation of P-cell synapses was observed in parallel with an increase in the magnitude of elicited shortening and both synaptic and behavioral effects were blocked by pharmacological inhibition of 2-AG synthesis or TRPV-like channel activation. Serotonin (5-HT) is known to play a critical role in sensitization in Hirudo and other animals, and the 5-HT2 receptor antagonist ritanserin also blocked behavioral sensitization and the accompanying synaptic potentiation. These findings suggest a novel, endocannabinoid-mediated contribution to behavioral sensitization that may interact with known 5-HT-dependent modulatory processes. NEW & NOTEWORTHY There is considerable interest in the analgesic potential of cannabinoids. However, there is evidence that the cannabinoid system can have both pro- and antinociceptive effects. This study examines how an endogenous cannabinoid transmitter can potentiate nonnociceptive synapses and enhance their capacity to elicit a nocifensive behavioral response.

Responses of Withdrawal Interneurons to Serotonin Applications in Naïve and Learned Snails Are Different

Long-term changes in membrane potential after associative training were described previously in identified premotor interneurons for withdrawal of the terrestrial snail Helix. Serotonin was shown to be a major transmitter involved in triggering the long-term changes in mollusks. In the present study we compared the changes in electrophysiological characteristics of identifiable premotor interneurons for withdrawal in response to bath applications of serotonin (5-HT) or serotonin precursor 5-hydroxytryptophan (5-HTP) in preparations from naïve, neurotoxin-injected or associatively trained snails. It was found that 5-HT or 5-HTP applications caused a significant decrease of membrane potential in premotor interneurons of naïve snails, associatively trained snails and snails with impaired serotonergic system by injection of a selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) 1 week before the experiments. Applications of 5-HT or 5-HTP did not cause significant changes in the action potential (AP) threshold potential of these neurons in naïve snails. Conversely, applications of 5-HT or 5-HTP to the premotor interneurons of previously trained or 5,7-DHT-injected snails caused a significant increase in the firing threshold potential in spite of a depolarizing shift of the resting membrane potential. Results demonstrate that responsiveness of premotor interneurons to extracellularly applied 5-HT or 5-HTP changes for days after the associative training or serotonin depletion. Similarity of the effects in trained and 5,7-DHT-injected animals may be due to massive release of serotonin elicited by 5,7-DHT injection. Our results suggest that serotonin release due to aversive conditionining or elicited by the neurotoxin administration triggers similar changes in resting membrane potential and AP threshold in response to bath applications of 5-HT or its precursor 5-HTP.

Modulation of defensive reflex conditioning in snails by serotonin

Highlights Daily injection of serotonin before a training session accelerated defensive reflex conditioning in snails.Daily injection of 5-hydroxytryptophan before a training session in snails with a deficiency of serotonin induced by the "neurotoxic" analog of serotonin 5,7-dihydroxytryptamine, restored the ability of snails to learn.After injection of the "neurotoxic" analogs of serotonin 5,6- and 5,7-dihydroxytryptamine as well as serotonin, depolarization of the membrane and decrease of the threshold potential of premotor interneurons was observed. We studied the role of serotonin in the mechanisms of learning in terrestrial snails. To produce a serotonin deficit, the "neurotoxic" analogs of serotonin, 5,6- or 5,7-dihydroxytryptamine (5,6/5,7-DHT) were used. Injection of 5,6/5,7-DHT was found to disrupt defensive reflex conditioning. Within 2 weeks of neurotoxin application, the ability to learn had recovered. Daily injection of serotonin before a training session accelerated defensive reflex conditioning and daily injections of 5-HTP in snails with a deficiency of serotonin induced by 5,7-DHT restored the snail's ability to learn. We discovered that injections of the neurotoxins 5,6/5,7-DHT as well as serotonin, caused a decrease in the resting and threshold potentials of the premotor interneurons LPa3 and RPa3.

Minocycline inhibits D-amphetamine-elicited action potential bursts in a central snail neuron

Minocycline is a second-generation tetracycline that has been reported to have powerful neuroprotective properties. In our previous studies, we found that d-amphetamine (AMPH) elicited action potential bursts in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac. This study sought to determine the effects of minocycline on the AMPH-elicited action potential pattern changes in the central snail neuron, using the two-electrode voltage clamping method. Extracellular application of AMPH at 300 μM elicited action potential bursts in the RP4 neuron. Minocycline dose-dependently (300-900 μM) inhibited the action potential bursts elicited by AMPH. The inhibitory effects of minocycline on AMPH-elicited action potential bursts were restored by forskolin (50 μM), an adenylate cyclase activator, and by dibutyryl cAMP (N(6),2'-O-Dibutyryladenosine 3',5'-cyclic monophosphate; 1mM), a membrane-permeable cAMP analog. Co-administration of forskolin (50 μM) plus tetraethylammonium chloride (TEA; 5mM) or co-administration of TEA (5mM) plus dibutyryl cAMP (1mM) also elicited action potential bursts, which were prevented and inhibited by minocycline. In addition, minocycline prevented and inhibited forskolin (100 μM)-elicited action potential bursts. Notably, TEA (50mM)-elicited action potential bursts in the RP4 neuron were not affected by minocycline. Minocycline did not affect steady-state outward currents of the RP4 neuron. However, minocycline did decrease the AMPH-elicited steady-state current changes. Similarly, minocycline decreased the effects of forskolin-elicited steady-state current changes. Pretreatment with H89 (N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride; 10 μM), a protein kinase A inhibitor, inhibited AMPH-elicited action potential bursts and decreased AMPH-elicited steady-state current changes. These results suggest that the cAMP-protein kinase A signaling pathway and the steady-state current are involved in the inhibitory effects of minocycline upon AMPH-elicited action potential bursts.

Long-term depression of nociceptive synapses by non-nociceptive afferent activity: role of endocannabinoids, Ca²+, and calcineurin

Activity in non-nociceptive afferents is known to produce long-lasting decreases in nociceptive signaling, often referred to as gate control, but the cellular mechanisms mediating this form of neuroplasticity are poorly understood. In the leech, activation of non-nociceptive touch (T) mechanosensory neurons induces a heterosynaptic depression of nociceptive (N) synapses that is endocannabinoid-dependent. This heterosynaptic, endocannabinoid-dependent long-term depression (ecLTD) is observed where the T- and N-cells converge on a common postsynaptic target, in this case the motor neuron that innervates the longitudinal muscles (L-cells) that contributes to a defensive withdrawal reflex. Depression in the nociceptive synapse required both presynaptic and postsynaptic increases in intracellular Ca²⁺. Activation of the Ca²⁺-sensitive protein phosphatase calcineurin was also required, but only in the presynaptic neuron. Heterosynaptic ecLTD was unaffected by antagonists for NMDA or metabotropic glutamate receptors, but was blocked by the 5-HT₂ receptor antagonist ritanserin. Depression was also blocked by the CB1 receptor antagonist rimonabant, but this is thought to represent an effect on a TRPV-like receptor. This heterosynaptic, endocannabinoid-dependent modulation of nociceptive synapses represents a novel mechanism for regulating how injury-inducing or painful stimuli are transmitted to the rest of the central nervous system.

Report on the 12th symposium on invertebrate neurobiology held 31 August-4 September 2011 at the Balaton Limnological Research Institute of the Hungarian Academy of Sciences, Tihany, Hungary

In August 2011, the 12th international symposium of ISIN was held by Lake Balaton in Tihany, Hungary. This convivial and stimulating meeting provided a forum for discussion of a range of invertebrate organisms in neuroscience research. Here the main topics covered at the meeting are reviewed.

Properties of cannabinoid-dependent long-term depression in the leech

Previously, a cannabinoid-dependent form of long-term depression (LTD) was discovered at the polysynaptic connection between the touch mechanosensory neuron and the S interneuron (Li and Burrell in J Comp Physiol A 195:831-841, 2009). In the present study, the physiological properties of this cannabinoid-dependent LTD were examined. Increases in intracellular calcium in the S interneuron are necessary for this form of LTD in this circuit. Calcium signals contributing to cannabinoid-dependent LTD are mediated by voltage-dependent calcium channel and release of calcium from intracellular stores. Inositol triphosphate receptors, but not ryanodine receptors, appear to mediate this store-released calcium signal. Cannabinoid-dependent LTD also requires activation of metabotropic serotonin receptors, possibly a serotonin type 2-like receptor. Finally, this form of LTD involves the stimulation of nitric oxide synthase and a decrease in cyclic adenosine monophosphate signaling, both of which appeared to be downstream of cannabinoid receptor activation. Based on these findings, the cellular signaling mechanisms of cannabinoid-dependent LTD in the leech are remarkably similar to vertebrate forms of cannabinoid-dependent synaptic plasticity.

Peptidergic modulation of serotonin and nerve elicited responses of the salivary duct muscle in the snail, Helix pomatia

In the present study, the ability of a range of endogenous neuropeptides to modulate neuromuscular transmission was examined in the salivary duct neuromuscular preparation of the terrestrial snail, Helix pomatia. Immunohistochemical and physiological techniques were used to localize the neuropeptides (GSPYFVamide, CARP, FMRFamide and APGWamide) and to investigate whether contractions elicited by the stimulation of the salivary nerve or by exogenously applied 5-HT are subject to peptidergic modulation. All of the neuropeptides studied decreased the tonus by a direct action on the muscle fibers in a concentration dependent manner in a range of 10(-9) to 10(-6)M. Neuropeptides distinctly affected the 5-HT evoked contraction or relaxation and GSPYFVa and APGWa decreased also the amplitude of contractions elicited by the stimulation of the salivary nerve. All four neuropeptides facilitated the relaxation phase providing further evidence for the postsynaptic action of neuropeptides. Low Ca(2+)/high Mg(2+) saline abolished the nerve-elicited contractions, however the denervated muscle retained the ability to contract due to the mobilization of the Ca(2+) from intracellular stores. It was concluded, that peptides belonging to different peptide families exerted their effects through pre- and postsynaptic mechanisms. The modulatory effect of neuropeptides can be assigned to the partial co-localization of 5-HT and neuropeptides in the nerves innervating muscles of the salivary duct, as it was demonstrated by double-labeling immunohistochemistry. A double origin of the 5-HTergic innervation was demonstrated, including efferents originating from both the cerebral and visceral ganglia.

Localization of biogenic amines in the foregut of Aplysia californica: catecholaminergic and serotonergic innervation

This study examined the catecholaminergic and serotonergic innervation of the foregut of Aplysia californica, a model system in which the control of feeding behaviors can be investigated at the cellular level. Similar numbers (15-25) of serotonin-like-immunoreactive (5HTli) and tyrosine hydroxylase-like-immunoreactive (THli) fibers were present in each (bilateral) esophageal nerve (En), the major source of pregastric neural innervation in this system. The majority of En 5HTli and THli fibers originated from the anterior branch (En(2)), which innervates the pharynx and the anterior esophagus. Fewer fibers were present in the posterior branch (En(1)), which innervates the majority of the esophagus and the crop. Backfills of the two En branches toward the central nervous system (CNS) labeled a single, centrifugally projecting serotonergic fiber, originating from the metacerebral cell (MCC). The MCC fiber projected only to En(2). No central THli neurons were found to project to the En. Surveys of the pharynx and esophagus revealed major differences between their patterns of catecholaminergic (CA) and serotonergic innervation. Whereas THli fibers and cell bodies were distributed throughout the foregut, 5HTli fibers were present in restricted plexi, and no 5HTli somata were detected. Double-labeling experiments in the periphery revealed THli neurons projecting toward the buccal ganglion via En(2). Other afferents received dense perisomatic serotonergic innervation. Finally, qualitative and quantitative differences were observed between the buccal motor programs (BMPs) produced by stimulation of the two En branches. These observations increase our understanding of aminergic contributions to the pregastric regulation of Aplysia feeding behaviors.

(±)3,4-Methylenedioxyamphetamine elicits action potential bursts in a central snail neuron

The effects of (+/-)3,4-methylenedioxyamphetamine (MDA) were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials. Extracellular or intracellular application of MDA elicited action potential bursts of the central RP4 neuron. The action potential bursts elicited by MDA were not blocked when neurons were immersed in high-Mg2+ solution, Ca2+-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, sulpiride or methiothepin. Notably, the induction of action potential bursts was blocked by pretreatment with protein kinase C (PKC) inhibitors, chelerythrine and Ro 31-8220, but not by protein kinase A (PKA) inhibitors, KT-5720 and H89, nor by the phospholipase C (PLC) inhibitor, U73122. PKC activators, i.e., phorbol 12,13-dibutyrate (PDBu) and 1-oleoyl-2-acety-sn-glycerol (OAG; a membrane-permeant DAG analog), facilitate the induction of action potential bursts elicited by MDA. Voltage-clamp studies revealed that MDA decreased the delayed rectifying K+ current (I(KD)) of the RP4 neuron. Further, although Ro 31-8220 did not affect the I(KD), Ro 31-8220 decreased the inhibitory effect of MDA on the I(KD). These results suggest that the generation of action potential bursts elicited by MDA was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic, dopaminergic or serotoninergic receptors of the excitable membrane. Instead, the MDA-elicited action potential bursts are closely related to PKC activity and the inhibitory effects on the I(KD).

Electrical properties and cell-to-cell communication of the salivary gland cells of the snail, Helix pomatia

The aim of the present study was to assess the cellular mechanism of secretion in the salivary gland of the snail, Helix pomatia, using electrophysiological, electron microscopic and immunohistochemical techniques. A homogeneously distributed membrane potential (-56.6 +/- 9.8 mV) was determined mainly by a K+ -electrochemical gradient and partly by the contribution of the electrogenic Na+ -pump and Cl- conductance. Low resistance electrical coupling sites were identified physiologically. Transmission electron microscopy and innexin 2 antibody revealed the presence of gap-junction-like membrane structures between gland cells. It is suggested that gap-junctions are sites of electrotonic intercellular communication, which integrate the gland cells into a synchronized functional unit in the acinus. Stimulation of the salivary nerve elicited secretory potentials (depolarization) which could be mimicked by local application of acetylcholine, dopamine or serotonin. In voltage-clamp experiments four major conductances were identified: a delayed rectifier (IK), a transient (IA) and a Ca2+ -activated outward K+ current (IK(Ca)) and Ca2+ -inward currents (ICa). It is suggested that one or more of these conductances may give rise to a stimulus activated secretory potential leading to excitation-secretion coupling and subsequent the release of the mucus from the gland cells.

Cocaine elicits action potential bursts in a central snail neuron: The role of delayed rectifying K+ current

The effects of cocaine were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials and bath application of cocaine (0.3-1 mM) reversibly elicited action potential bursts of the central RP4 neuron in a concentration-dependent manner. The action potential bursts were not blocked when neurons were immersed in high-Mg(2+)solution, Ca(2+)-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, or sulpiride. Similarly, the action potential bursts were not abolished by pretreatment with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester or anisomycin. Injection of hyperpolarizing current at an intensity of greater than 2 nA effectively suppressed the cocaine-elicited action potential bursts and no postsynaptic potentials were observed under these conditions. These results suggest that the generation of action potential bursts elicited by cocaine was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic or dopaminergic receptors of the excitable membrane, or (3) the cAMP second messengers and new protein synthesis of the RP4 neuron. Notably, the induction of action potential bursts was blocked by pretreatment with 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione. Voltage-clamp studies conducted on the RP4 neuron revealed that cocaine at 0.3 mM decreased (1) the Ca(2+) current, (2) the delayed rectifying K(+) current, (3) the fast-inactivating K(+) current and (4) the Ca(2+)-activated K(+) current, but had no remarkable effects on the Na(+) current. Perfusion with Ca(2+)-free solution, which may abolish the Ca(2+) current and Ca(2+)-activated K(+) current, did not cause any bursts of action potentials in control RP4 neurons. Application of 4-aminopyridine, an inhibitor of fast-inactivating K(+) current, and paxilline, an inhibitor of Ca(2+)-activated K(+) current, failed to elicit action potential bursts, whereas tetraethylammonium chloride, a blocker of Ca(2+)-activated K(+) current and delayed rectifying K(+) current, and tacrine, an inhibitor of delayed rectifying K(+) current, successfully elicited action potential bursts. Further, while 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione did not affect the delayed rectifying K(+) current of the RP4 neuron, 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione decreased the inhibitory effect of cocaine on the delayed rectifying K(+) current. It is concluded that cocaine elicits action potential bursts in the central snail RP4 neuron and that the effect is closely related to the inhibitory effects on the delayed rectifying K(+) current.

Biospectra Analysis: Model Proteome Characterizations for Linking Molecular Structure and Biological Response

Establishing quantitative relationships between molecular structure and broad biological effects has been a long-standing goal in drug discovery. Evaluation of the capacity of molecules to modulate protein functions is a prerequisite for understanding the relationship between molecular structure and in vivo biological response. A particular challenge in these investigations is to derive quantitative measurements of a molecule's functional activity pattern across different proteins. Herein we describe an operationally simple probabilistic structure-activity relationship (SAR) approach, termed biospectra analysis, for identifying agonist and antagonist effect profiles of medicinal agents by using pattern similarity between biological activity spectra (biospectra) of molecules as the determinant. Accordingly, in vitro binding data (percent inhibition values of molecules determined at single high drug concentration in a battery of assays representing a cross section of the proteome) are useful for identifying functional effect profile similarity between medicinal agents. To illustrate this finding, the relationship between biospectra similarity of 24 molecules, identified by hierarchical clustering of a 1567 molecule dataset as being most closely aligned with the neurotransmitter dopamine, and their agonist or antagonist properties was probed. Distinguishing the results described in this study from those obtained with affinity-based methods, the observed association between biospectra and biological response profile similarity remains intact even upon removal of putative drug targets from the dataset (four dopaminergic [D1/D2/D3/D4] and two adrenergic [alpha1 and alpha2] receptors). These findings indicate that biospectra analysis provides an unbiased new tool for forecasting structure-response relationships and for translating broad biological effect information into chemical structure design.

Dopaminergic modulation of neurosecretory cells in the crayfish

The main aims of this paper are (a) to locate possible dopaminergic neurons in the eyestalk with anti-tyrosine hydroxylase antibodies, (b) to search for the presence of dopamine (DA) in the nervous structures of the eyestalk, (c) to explore its release, and (d) to test the effect of DA on neurosecretory cells in the eyestalk. Experiments were performed in adult crayfishes Procambarus clarkii, in isolated optic peduncle. Immunocytochemistry was made with the antibody against its precursor synthesizing enzyme tyrosine-hydroxylase. The content and release studies of DA were made using high performance liquid chromatography (HPLC). Extracellular and intracellular recordings were conducted with conventional recording techniques. A large number (approximately 2000) of immunopositive somata of different sizes and shapes were identified in various regions of the eyestalk. The majority of somata are of the smallest size (5-25 microm diameter). DA content in the eyestalk was 5.6 +/- 0.1 pmol per structure; the greatest content is in the MT (over 60%). A basal level release of DA was observed. Incubation of eyestalks in solution containing a high K+ concentration increased the DA release (79%). Two effects of DA on the excitability of X-organ neurons were observed; an excitatory effect on neurons of approximately 25 microm somata diameter and another inhibitory effect in the group of approximately 35-microm somata diameter neurons. The excitation occurs with a depolarization and decrement of membrane conductance in the cell soma while the inhibition occurs with a hyperpolarization and increment of membrane conductance in soma. We concluded the following: (1) Dopamine is present in each optic ganglia of the crayfish eyestalk. (2) There is a basal release of DA from the isolated eyestalk. (3) DA release is enhanced threefold by eyestalk incubation in 40 mM [K+] solution. (4) DA selectively excites a population of neurons with low-speed conduction axons, and small somata in the X-organ-sinus gland system, while inhibiting another population characterized by higher axonal conduction speed and large somata. (5) These observations support a role for DA as a neurotransmitter or neuromodulator in the X-organ neurons of the crayfish eyestalk.

Functional morphology of the salivary gland of the snail, Helix pomatia: a histochemical and immunocytochemical study

Functional morphology of Helix pomatia salivary gland cells was studied at light microscopic level by using different histochemical methods. Three cell types could be demonstrated in the salivary gland: mucocytes, granular and vacuolated cells. The distribution and the number of the different cell types were different in active and inactive snails. In active feeding animals, dilatated interlobular salivary ducts were observed, which were never present in inactive ones. In active animals an additional cell type, the cystic cell could also be observed. Periodic acid Schiff staining revealed both mucuos and serous elements in the salivary gland. Furthermore, hematoxyline-eosin staining indicated the occurrence of a cell layer with high mitotic activity in the acini. Applying immunohistochemical methods with monoclonal mouse anti-human Ki-67 clone, B56 and polyclonal rabbit anti-human Ki-67 antibodies, we also were able to demonstrate the occurrence of dividing cells in the salivary gland. Analysis of 1-2 microm semi-thin Araldite sections stained with toluidine-blue showed that the saliva can be released, in addition to possible exocytosis, by the lysis of cystic cells. Using an apoptosis kit, we could also establish that this process was due to rather an apoptotic than a necrotic mechanism. In the salivary gland of active snails, where an intensive salivation takes place, significantly more apoptotic cells occurred, if compared to that of inactive animals. It is suggested that programmed cell death may also be involved in the saliva release.

Effects of procaine on a central neuron of the snail, Achatina fulica Ferussac

Effects of procaine on a central neuron (RP1) of the giant African snail (Achatina fulica Ferussac) were studied pharmacologically. The RP1 neuron showed spontaneous firing of action potential. Extra-cellular application of procaine (10 mM) reversibly elicited bursts of potential. The bursts of potential elicited by procaine were not blocked after administration of (1) prazosin, propranolol, atropine, d-tubocurarine, (2) calcium-free solution, (3) ryanodine (4) pretreatment with KT-5720 or chelerythrine. The bursts of potential elicited by procaine were blocked by adding U73122 (10 microM) and the bursts of potential were decreased if physiological sodium ion was replaced with lithium ion or incubated with either neomycin (3.5 mM) or high magnesium solution (30 mM). Preatment with U73122 (10 microM) blocked the initiation of bursts of potential. Ruthenium red (100 microM) or caffeine (10 mM) facilitated the procaine-elicited bursts of potential. It is concluded that procaine reversibly elicits bursts of potential in the central snail neuron. This effect was not directly related to (1) the extra-cellular calcium ion fluxes, (2) the ryanodine sensitive calcium channels in the neuron, or (3) the PKC or PKA related messenger systems. The procaine-elicited bursts of potential were associated with the phospholipase activity and the calcium mobilization in the neuron.

Dopamine stimulates snail albumen gland glycoprotein secretion through the activation of a D1-like receptor

The catecholamine dopamine is present in both the central nervous system and in the peripheral tissues of molluscs, where it is involved in regulating reproduction. Application of exogenous dopamine to the isolated albumen gland of the freshwater pulmonate snail Helisoma duryi (Wetherby) induces the secretion (release) of perivitelline fluid. The major protein component of the perivitelline fluid of Helisoma duryi is a native 288 kDa glycoprotein that is secreted around individual eggs and serves as an important source of nutrients for the developing embryos. The secretion of glycoprotein by the albumen gland is a highly regulated event that must be coordinated with the arrival of the fertilized ovum at the carrefour (the region where the eggs receive albumen gland secretory products). In order to elucidate the intracellular signalling pathway(s) mediating dopamine-induced glycoprotein secretion, albumen gland cAMP production and glycoprotein secretion were measured in the presence/absence of selected dopamine receptor agonists and antagonists. Dopamine D1-selective agonists dihydrexidine, 6,7-ADTN and SKF81297 stimulated cAMP production and glycoprotein secretion from isolated albumen glands whereas D1-selective antagonists SCH23390 and SKF83566 suppressed dopamine-stimulated cAMP production. Dopamine D2-selective agonists and antagonists generally had no effect on cAMP production or protein secretion. Based on the effects of these compounds, a pharmacological profile was obtained that strongly suggests the presence of a dopamine D1-like receptor in the albumen gland of Helisoma duryi. In addition, secretion of albumen gland glycoprotein was not inhibited by protein kinase A inhibitors, suggesting that dopamine-stimulated protein secretion might occur through a protein kinase A-independent pathway.

Serotonergic regulation of the central heart auricles of Sepia officinalis L. (Mollusca, Cephalopoda)

In pharmacological bioassays on isolated isotonically suspended auricles of Sepia officinalis, the regulatory action of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) on these autonomous contractile compartments was demonstrated. 5-HT induced concentration-dependent positive effects on frequency and tone, whereas the concentrations/response curve for the amplitude showed a biphasic course. All applied antagonists inhibited mainly the effect of 5-HT on frequency and amplitude. The chronotropic effects of 5-HT were blocked mainly by the 5-HT(1,2) antagonist methiothepin (pA(2)=8.01), the 5-HT(1a) antagonist NAN-190 (pA(2)-) and in lesser extent by the 5-HT(1,2) antagonist mianserin (pA(2)=6.81). In the presence of each antagonist applied the 5-HT action on amplitude was transformed into a positive inotropic effect with the maximum under influence of NAN-190 and the 5-HT(2,1c)-antagonist ketanserin. The auricular tone was also influenced by the antagonists and in combination with methiothepin it turned into strong negative tonotropic effect. In addition to the pharmacological bioassays, the presence of 5-HT in nerve endings within the auricle wall was demonstrated by immunohistochemical and fluorescence microscopic findings. Altogether the findings presented here confirm that 5-HT evokes excitatory effects on the autonomous contractile auricle of S. officinalis and acts obviously over different receptors, whereby a 5-HT(1)- and a 5-HT(2)-like seem to be involved.