The pharmacology of molluscan neurons

Serotonergic regulation of the buccal (feeding) rhythm of the pond snail, Lymnaea stagnalis. An immunocytochemical, biochemical and pharmacological approach

Hatching is an important phase of the development of pulmonate gastropods followed by the adult-like extracapsular foraging life. Right before hatching the juveniles start to display a rhythmic radula movement, executed by the buccal complex, consisting of the buccal musculature (mass) and a pair of the buccal ganglia. In order to have a detailed insight into this process, we investigated the serotonergic regulation of the buccal (feeding) rhythm in 100% stage embryos of the pond snail, Lymnaea stagnalis, applying quantitative immunohistochemistry combined with the pharmacological manipulation of the serotonin (5-HT) synthesis, by either stimulating (by the 5-HT precursor 5-hydroxytryptophan, 5-HTP) or inhibiting (by the 5-HT synthesis blocker para-chlorophenylalanine, pCPA) it. Corresponding to the direction of the drug effect, significant changes of the fluorescence intensity could be detected both in the cerebral ganglia and the buccal complex. HPLC-MS assay demonstrated that 5-HTP increased meanwhile pCPA decreased the 5-HT content both of the central ganglia and the buccal complex. As to the feeding activity, 5-HTP induced only a slight (20%) increase, whereas the pCPA resulted in a 20% decrease of the radula protrusion frequency. Inhibition of 5-HT re-uptake by clomipramine reduced the frequency by 75%. The results prove the role of both central and peripheral 5-HTergic processes in the regulation of feeding activity. Application of specific receptor agonists and antagonists revealed that activation of a 5-HT1-like receptor depressed the feeding activity, meanwhile activation of a 5-HT6,7-like receptor enhanced it. Saturation binding plot of [3H]-5-HT to receptor and binding experiments performed on membrane pellets prepared from the buccal mass indicated the presence of a 5-HT6-like receptor positively coupled to cAMP. The results suggest that 5-HT influences the buccal (feeding) rhythmic activity in two ways: an inhibitory action is probably exerted via 5-HT1-like receptors, while an excitatory action is realized through 5-HT_6,7-like receptors.

Excitatory effects of GABA on procerebrum neurons in a slug

Classical neurotransmitters, such as glutamate and γ-aminobutyric acid (GABA), often have different actions on invertebrate neurons from those reported for vertebrate neurons. In the terrestrial mollusk Limax, glutamate was found to function as an inhibitory transmitter in the procerebrum (PC), but it has not yet been clarified how GABA acts in the PC. We thus examined what effects GABA exerts on PC neurons in the present study. For this purpose, we first applied GABA to isolated PC preparations and recorded postsynaptic currents and potentials in PC neurons. The GABA application reduced the amplitude of inhibitory postsynaptic currents and depolarization-induced outward currents recorded in nonbursting neurons and increased the number of spontaneous spikes of nonbursting neurons. However, direct GABA-induced currents were not observed in either bursting or nonbursting neurons. These results suggest a potential direct effect of GABA on outward currents resulting in enhanced excitability of PC neurons. Next, we measured the change in [Ca2+]i in cultured PC neurons by application of GABA. The GABA application increased spontaneous Ca2+ events in cultured neurons. These Ca2+ events were ascribable to the influx of extracellular Ca2+. We then confirmed the presence of GABA and GABA receptors in the PC. The GABA-like immunoreactivity was observed in the neuropil layers of the PC, and the mRNAs for both GABAA and GABAB receptors were expressed in the PC. In particular, GABAB receptor mRNA, rather than GABAA, was found to be more abundantly expressed in the PC. These results suggest that GABA functions as an excitatory modulator for PC neurons via mainly GABAB receptors.

Distribution of serotonin-, Gamma-Aminobutyric Acid- and substance P-like immunoreactivity in the central and peripheral nervous system of Mytilus galloprovincialis

The distribution of serotonin-, GABA- and substance P-like immunoreactivity has been studied in the cerebral and visceral ganglia and in some peripheral tissues of Mytilus galloprovincialis (Moleusca, Bivalvia). Cerebral ganglia contain a developed serotonin-immunoreactive neuronal subpopulation and numerous GABA-immunoreactive neurons, whereas neurons positive for substance P are sparse. In peripheral tissues innervated by the cerebral ganglia (labial palps and oesophagus) only serotonin-immunoreactive nerve fibers were found. In the visceral ganglia, serotonin- and GABA-immunoreactive neurons are far less numerous than in the cerebral ganglia, whereas several neurons positive for substance P are scattered in all cortical zones. Serotonin-immunoreactive plexuses innervate the posterior adductor muscle and the gill filaments which contain also a developed nerve network positive for substance P. The distribution pattern of the immunoreactive elements in the ganglia and in peripheral territories indicates that GABA should exert only a central action, whereas serotonin and a substance P-like peptide are involved both in central and peripheral neurotransmission.

In hemocytes from Mytilus galloprovincialis Lmk., treatment with corticotropin or growth factors conditions catecholamine release

The cells in charge of the innate immune response in the sea mussel Mytilus galloprovincialis Lmk. are the hemocytes, which have the capacity to release catecholamines when subjected to stressing conditions. Hemocytes were kept in culture before stimulation. That is, their behaviour was not studied immediately after extraction from the mollusc, as happens in most studies. This avoids the interference and variability caused by the conditions in which mussels may be when collected. This work describes the great variability found in the pattern of catecholamine release when the hemocytes are stimulated with either corticotropins or growth factors. Dopamine, adrenaline and noradrenaline release differs with each of the inducers assayed, with stimulation time and with the season of hemocyte collection. One of the results presented is particularly remarkable; such is the great amount of adrenaline and noradrenaline released to the medium when the hemocytes obtained in summer are stimulated with transforming growth factor-beta1 (TGF-beta1) for 60 min.

Serotonin stimulates [Ca2+]i elevation in ciliary ectodermal cells of echinoplutei through a serotonin receptor cell network in the blastocoel

A full-length serotonin receptor mRNA from the 5Hthpr gene was sequenced from larvae of the sea urchin, Hemicentrotus pulcherrimus.The DNA sequence was most similar to 5HT-1A of the sea urchin Strongylocentrotus purpuratus found by The Sea Urchin Genome Project,and the protein sequence predicted the presence of seven transmembrane domains. Immunohistochemistry with anti-5HThpr antibodies indicated that the protein was expressed on blastocoelar cells that comprised the major blastocoelar network (serotonin receptor cell network). These network cells inserted their processes into the ectoderm in various regions, including the ciliary band region. Serotonin injected into the blastocoel stimulated a transient elevation of cytoplasmic Ca2+ concentration([Ca2+]i) in the ectoderm, as detected by Oregon-Green dextran, injected earlier in development. The calcium transient propagated as a wave at about 175 μm s–1, but was not detectable in the serotonin receptor-positive cell network. In larvae treated with p-chlorophenylalanine, a potent and irreversible serotonin synthesis inhibitor, serotonin application did not stimulate[Ca2+]i, the serotonin receptor cell network did not develop properly, and the swimming behavior of the larvae was abnormal. However, formation of a different nervous system comprising synaptotagmin-possessed neurites was not affected by p-chlorophenylalanine treatment. These results imply that serotonin secreted from the apical ganglion into the blastocoel stimulates the elevation of [Ca2+]i in the larval ectodermal cells through the serotonin receptor cell network.

Cholinergic sensory inputs to command neurons in edible snail

We studied cholinergic component of visceral sensory input to defensive behavior command neurons in edible snail. Nicotinic receptor antagonist tubocurarine and muscarinic receptor antagonist atropine reversibly decreased the amplitude of the total excitatory postsynaptic potential induced by electrostimulation of the peripheral region in the mechanosensory receptor field of command neurons on the surface of internal organs. Our results indicate that acetylcholine is involved in sensory signal transduction from the visceral sac to command neurons of snail parietal ganglia. The subsynaptic membrane of visceral synaptic input contains nicotinic and muscarinic receptors.

Neuroanatomical and immunocytochemical studies of the head retractor muscle innervation in the pond snail, Lymnaea stagnalis L

Axonal tracing and immunocytochemical techniques were used to study the innervation of the head retractor muscle (HRM) in the pond snail Lymnaea stagnalis L. Fibers of both the superior and inferior cervical nerves which innervate the HRM form endings that comply with the structure of chemical synapses. The somata of neurons with axons in these nerves are located in all except the buccal ganglia of the central nervous system, and this seems to be a special feature of the HRM motor system. By staining the filamentous actin with Oregon-green conjugated phalloidin, we demonstrated that the HRM has a multiterminal innervation and one muscle fiber can contain several synaptic endings which appear to be both morphologically and physiologically different. The morphological diversity of synaptic vesicles suggests a multiplicity of neurotransmitters acting on these nerve-muscle junctions. Immunocytochemical evidence was found for a strong serotonergic and FMRFamidergic innervation of muscle fibers through axons of the inferior cervical nerve. The thin fibers of the inferior cervical nerve possess immunoreactivity to glutamate, gamma-aminobutyric acid (GABA) and choline-acetyltransferase, and form sparser innervation patterns in the muscle. Our results indicate that several neurotransmitters are present in the nerves innervating the Lymnaea HRM and may therefore participate in the control of this muscle. The possible behavioral significance of such different neurotransmitter sets involved in the regulation of contractions is discussed.

In vitro effects of LPS, IL-2, PDGF and CRF on haemocytes of Mytilus galloprovincialis Lmk

The cells in charge of the innate immune response in the marine mussel Mytilus galloprovincialis Lmk. are the haemocytes. These cells respond in different ways to agents such as lipopolysaccharide (LPS), interleukin-2 (IL-2), platelet-derived growth factor (PDGF) and corticotropin releasing factor (CRF). After stimulation of the haemocytes, the expression of molecules reactive with monoclonal antibodies raised to the alpha chain of the IL-2 receptor, present in their membrane, differed depending on the agent used. The same happened with regard to the levels of dopamine, adrenaline and noradrenaline released to the medium by the haemocytes. It should also be noted that no catecholamine release was detected and the level of expression of IL-2Ralpha showed no significant variation in cultured cells that had not been treated with inducers. These facts would indicate that most haemocytes were in the same starting condition at the moment that the stimulation was performed. Therefore, cultured haemocytes can be a highly reliable model in the study of the innate immune system.

Noradrenaline modulates oyster hemocyte phagocytosis via a beta-adrenergic receptor-cAMP signaling pathway

Catecholamines (CA) regulate several physiological processes in molluscs. Experiments have been conducted to determine the effects of noradrenaline (NA), the principal CA circulating in bivalve hemolymph, on oyster hemocytephagocytosis. Results show that NA had a dose-dependent inhibitory effect on phagocytosis at physiological concentrations of 0.1 microM and above. The beta-adrenoceptor agonist isoproterenol mimicked the inhibitory effects of NA on phagocytosis, whereas the alpha-adrenoceptor agonist phenylephrine had no significant effect. Furthermore, the beta-adrenoceptor antagonist propanolol, but not the alpha-adrenoceptor antagonist prazosin, prevented the inhibition of phagocytosis by NA. The type IV phosphodiesterase inhibitor rolipram acted synergistically with a suboptimal concentration of isoproterenol to inhibit phagocytosis, and the protein kinase A inhibitor H-89, but not the protein kinase C inhibitor calphostin C, attenuated the effect of isoproterenol. These results show that NA can modulate oyster hemocyte phagocytosis via a beta-adrenergic receptor/cAMP/protein kinase A signaling pathway.

5-Hydroxytryptamine stimulates net Ca2+ flux in the ventricular muscle of a mollusc (Busycon canaliculatum) during cardioexcitation

Noninvasive, self-referencing calcium (Ca2+) electrodes were used to study the mechanisms by which 5-hydroxytryptamine (5-HT) affects net Ca2+ flux across the sarcolemma of myocytes from ventricular trabeculae (from a marine gastropod, Busycon canaliculatum). Treatment of isolated trabeculae with 5-HT causes a net Ca2+ efflux, which is 30% blocked by verapamil. These findings suggest that the efflux is in part the result of a previous Ca2+ influx through L-type Ca2+ channels and is due to a rapid Ca2+ extrusion mechanism inherent to the sarcolemma of these myocytes. 5-HT-induced net Ca2+ efflux is also reduced by about 40% by treatment with a sodium (Na+)-free, lithium (Li+)-substituted saline, which shuts down the Na-Ca exchanger during Ca2+ extrusion. Cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum (SR) Ca2+ ATPase, almost completely abolishes the 5-HT-induced net Ca2+ efflux, suggesting that the SR rather than the extracellular pool is the primary Ca2+ reservoir serving 5-HT-induced excitation.

Noradrenaline modulates hemocyte reactive oxygen species production via beta-adrenergic receptors in the oyster Crassostrea gigas

Catecholamines (CA) are known to be present in the microenvironment of molluscan immunocytes. In the present study, experiments were conducted to determine the effects of noradrenaline (NA), the principal CA circulating in bivalve hemolymph, on the luminol-dependent chemiluminescence (CL) of oyster Crassostrea gigas hemocytes. Results show that NA had a dose-dependent inhibitory effect on the CL-response at the physiological concentration of 0.1 microM and above. The alpha-adrenoceptor agonist phenylephrine had no significant effect on the CL-response whereas the beta-adrenoceptor agonist isoproterenol mimicked the inhibitory effects of NA on the CL-response. The beta-adrenoceptor antagonist propanolol, but not the alpha-adrenoceptor antagonist prazosin, prevented the negative effects of NA on the CL-response. Taken together, these results show that beta-adrenergic receptors are present at the surface of oyster hemocytes and allow NA to down-regulate the CL-response.

Stress-induced catecholamine changes in the hemolymph of the oyster Crassostrea gigas

The stress response is a series of coordinated physiological reactions increasing an organism's capacity to maintain homeostasis in the presence of threatening agents. This fundamental process is known to involve hormonal signaling to rapidly modulate key physiological functions in vertebrates, but data are lacking concerning neuroendocrine responses to stress in invertebrates. The present study examined circulating catecholamine (CA) responses to stress in oysters. Mechanical disturbances (consisting of shaking the animals) and temperature or salinity variations were applied to the animals because these three types of stressors are commonly encountered by oysters in aquaculture or in their natural habitat. Results show that both circulating noradrenaline (NA) and dopamine (DA) concentrations increased in response to stress. The catecholaminergic response to acute mechanical stressors was rapid (less than 5 min), transient (a return to basal CA levels was observed after 60-90 min), and reflected both the intensity and duration of the perturbation. In contrast, responses to temperature and salinity variations were long lasting (up to 72 h). CA concentrations varied from 1.61 +/- 0.30 ng NA/ml and 0.41 +/- 0.05 ng DA/ml to maximal values of 22.07 +/- 0.97 ng NA/ml and 2.24 +/- 0.19 ng DA/ml. Such CA concentrations are known to induce physiological responses in bivalves, suggesting that stress-induced NA and DA changes exert a regulatory function in oysters.

Structure and function of invertebrate 5-HT receptors: a review

Over the last decade, knowledge of invertebrate serotonin receptors has expanded greatly. The first 5-HT receptor from Drosophila was cloned 10 years ago, and subsequently, 11 additional receptor genes have been cloned from Drosophila, molluscs (Lymnaea and Aplysia) and nematodes (Caenorhabditis and Ascaris). Information has also accumulated from physiological and biochemical studies that have used vertebrate serotonergic ligands to characterize endogenous invertebrate receptors. Although the endogenous receptors are often classified according to mammalian-based categories, in many cases the pharmacological properties of vertebrate and invertebrate receptors differ significantly and the actual identity of the latter is questionable. By providing information on the gene structure and amino acid sequence, molecular cloning studies offer a more definitive way to identify and classify invertebrate 5-HT receptors. This review summarizes information on the pharmacological and transductional properties of cloned invertebrate 5-HT receptors, and considers recent studies of endogenous receptors in the light of this new data.

Evidence for a form of adrenergic response to stress in the mollusc Crassostrea gigas

Catecholamines and pro-opiomelanocortin (POMC)-derived peptides, some of the central regulators of the stress-response systems of vertebrates, are also present in invertebrates. However, studies are needed to determine how these hormones participate in the organisation of neuroendocrine stress-response axes in invertebrates. Our present work provides evidence for the presence of an adrenergic stress-response system in the oyster Crassostrea gigas. Noradrenaline and dopamine are released into the circulation in response to stress. Storage and release of these hormones take place in neurosecretory cells presenting morphological and biochemical similarities with vertebrate chromaffin cells. Both in vivo and in vitro experiments showed that applications of the neurotransmitters acetylcholine or carbachol caused no significant release of noradrenaline or dopamine. Moreover, the nicotinic antagonists hexamethonium and α -bungarotoxin and the muscarinic antagonist atropine caused no significant inhibition of catecholamine release in stressed oysters. Adrenocorticotropic hormone (ACTH) induced a significant release of noradrenaline, but the release of dopamine in response to ACTH was not significant. These results suggest that, unlike that of vertebrates, the adrenergic stress-response system of oysters is not under the control of acetylcholine and that other factors, such as the neuropeptide ACTH, might control this system.

Modulation of oscillatory neural activities by cholinergic activation of interneurons in the olfactory center of a terrestrial slug

The neurons in the procerebrum (PC) of the terrestrial slug Limax marginatus show regular oscillation of their membrane potential, and the oscillation has been implicated in olfactory processing. The neural mechanisms for the generation and modulation of the oscillation have been poorly understood. In the present work, we examined the ionic conductances evoked by acetylcholine (ACh) in the PC neurons and the effects of ACh application on the population activities of intrinsic and extrinsic neurons. The PC neurons are categorized into bursting neurons, which are putative local inhibitory neurons, and nonbursting neurons, which likely mediate the input and output of information in the PC. Bath application of ACh augmented the local field potential oscillation in the PC. Perforated patch recording from single PC neurons revealed that ACh has direct excitatory effects on bursting neurons, while it suppresses the activity of nonbursting neurons, possibly via augmented inhibitory synaptic input from bursting neurons. The correlation between the membrane potential of bursting neurons and the frequency of oscillation suggests that bursting neurons are the main determinant of the oscillation frequency. Application of ACh also resulted in a reduction of the oscillation amplitude in the olfactory nerve, suggesting that the frequency modulation in the oscillatory network could change the activities in the follower neurons.

Amine and amino acid transmitters in the eye of the mollusc Bulla gouldiana: an immunocytochemical study

We identified putative transmitters of the photoreceptors and circadian pacemaker neurons and found candidates for efferent control in the eye of the marine mollusc Bulla gouldiana. Established antisera against octopamine, dopamine, serotonin, histamine, glutamate, gamma-aminobutyric acid (GABA), and taurine were used, and central ganglia were processed in parallel to evaluate general staining quality. Photoreceptors and circadian pacemaker cells both expressed immunoreactivity for glutamate and taurine. The eye and its sheath were devoid of GABA-like immunoreactive material, and none of the antisera directed against biogenic amines labelled cells or processes in the nervous tissue of the eye. However, dopamine and octopamine antisera stained large spherical granules (diameter 2-3 microm) contained in granular cells that are located in the connective tissue encapsulating the eye and the optic nerve. The serotonin antiserum revealed a sparse distribution of varicose axon fibers in the optic nerve and eye sheath. No histamine-immunoreactive processes were revealed in the eye. The functional significance of these findings for the molluscan eye and its circadian clock is discussed.

Topographic organization of serotonergic and dopaminergic neurons in the cerebral ganglia and their peripheral projection patterns in the head areas of the snail Helix pomatia

The distribution of monoaminergic neurons within the cerebral ganglia was investigated in the pulmonate snail Helix pomatia. Simultaneous serotonin and tyrosine hydroxylase double immunostaining revealed that the immunoreactive cell groups are concentrated in a putative monoaminergic center on the ventral surface of the cerebral ganglia. Simultaneous cobalt (Co)- and nickel (Ni)-lysine backfills of cerebral nerves were combined with 5, 6-dihydroxytryptamine pigment-labelling of serotonergic neurons, or with fluorescence immunocytochemistry of dopaminergic neurons. This showed that the serotonergic and dopaminergic cell groups can be divided into smaller subgroups on the basis of their axonal projections into different cerebral nerves. These subgroups show a topographic organization within the serotonergic and dopaminergic neuronal clusters. In the serotonergic system, the different regions of the head are represented in a rostrocaudal direction, whereas a caudorostral organization is characteristic for the dopaminergic system. No serotonin- or dopamine-immunoreative cell bodies but numerous fibers were observed in the head areas, indicating that these are innervated by cerebral monoaminergic neurons and show different innervation patterns. Serotonin-immunoreactive fibers mostly innervate muscle fibers, whereas dopamine-immunoreactive processes do not innervate effector cells, but terminate within the nerve branches of the head areas. On the basis of their innervation pattern, we suggest that dopaminergic neurons may take part in en route modulation of sensory afferent and efferent processes in an as yet unknown manner. The serotonergic neurons, on the other hand, may play a direct role in the modulation of muscle function.

Properties of cholinergic responses in isolated parapodial muscle fibers of Aplysia

The parapodial neuromuscular junction in the marine snail Aplysia brasiliana is a model synapse for the investigation of neural modulation. The parapodial muscle fibers are innervated by cholinergic motoneurons and by serotonergic modulatory cells. The physiological properties of voltage-gated currents of the muscle membranes and the effects of serotonin on these currents have been published previously. However, the pharmacological properties of the cholinergic receptors have not been investigated. Acetylcholine (ACh) applied exogenously to dissociated muscle fibers produces a response with a reversal potential of about -52 mV; the resting membrane potential of the average muscle fiber is approximately -56 mV. ACh induces variable responses (depolarizations or hyperpolarizations) in individual cells, but the transmitter never causes a depolarization adequate to produce muscle contraction. We demonstrate that the ACh response is the result of the activation of two distinct receptors. One receptor is linked to a chloride channel and induces a hyperpolarization with a reversal potential near -70 mV. This receptor is activated selectively by suberyldicholine and by nicotine and is antagonized by curare but not by hexamethonium. The second response, presumably caused by increased conductance to mixed cations, results in muscle fiber depolarization with a reversal potential near -35 mV and does induce muscle contraction. This receptor is activated by methylcarbamylcholine and selectively blocked by hexamethonium; atypically, this receptor is not activated by nicotine nor by carbachol. The depolarizing, cation-selective receptors likely are associated with identified excitatory cholinergic motoneurons the activity of which typically results in muscle contractions because the reversal potential for this ACh response is more depolarized than the activation threshold for voltage-gated calcium channels in these fibers. The hyperpolarizing, chloride-selective receptors may be associated with inhibitory motoneurons; such motoneurons have yet to be identified, but their presence is inferred because of the occurrence of spontaneous inhibitory junctional potentials recording from muscle fibers in situ. Muscle fiber responses to exogenously applied ACh reflect the relative contribution of each receptor type in each muscle fiber.

Alternative splicing and genomic organization of the L5-67 gene of Aplysia californica

The L5-67 gene was first identified on the basis of its high expression level in the LUQ neurons, a group of four giant cells located in the left upper quadrant of the abdominal ganglion of Aplysia californica. Its mRNA and peptides were later shown to be present in these cells, as well as in about 100 other smaller neurons in the CNS. L5-67 propeptide and/or mature peptides are also present in peripheral organs, particularly in the kidney, which is the target of most LUQ processes. Using RT-PCR, we show the presence of an alternatively spliced L5-67 transcript arising from the exclusion of the fourth exon from the mature mRNA. This alternative splicing event occurs specifically in the kidney, although we could not identify the cells in which it takes place. Translation of this transcript generates a 52 amino acid (aa) propeptide in which the first N-terminal 45 aa are identical to the original L5-67 propeptide. The last seven C-terminal aa are unrelated to the previously characterized L5-67 peptides due to a change in the open reading frame.

Cholinergic interneurons in the feeding system of the pond snail Lymnaea stagnalis . III. Pharmacological dissection of the feeding rhythm

The feeding activity of the pond snail Lymnaea stagnalis was stimulated by depolarization of a modulatory interneuron (SO) or of a N1 pattern-generating interneuron. The cholinergic antagonists phenyltrim ethylammonium (PTMA), methylxylocholine (MeXCh), hexamethonium (HMT) and atropine (ATR) were applied at 0.5 mM in the bath and their effects on the rhythmic feeding pattern were monitored. Each of the antagonists slowed or blocked the feeding rhythm. The block was due to interference in the pattern generating network, not to disturbance of modulatory inputs. The experimental results favour a model in which the alternation of protraction (N l) and retraction (N2) phases occurs by recurrent inhibition. The results would be more difficult to explain on the reciprocal inhibition model. When all the N1 output was blocked, the N1 neurons fired rhythmic bursts endogenously.

Inhibition of the Met-enkephalin-induced K+ current in B-cluster neurons of Aplysia by nitric oxide donor

The effects of sodium nitroprusside (SNP), a nitric oxide (NO) donor, on a methionine-enkephalin (Met-E)-induced K+ current recorded from B-cluster neurons in Aplysia cerebral ganglion were investigated with voltage-clamp and pressure ejection techniques. Bath-applied SNP (10-25 microM) reduced the Met-E-induced K+ current in the neurons without affecting the resting membrane conductance and holding current. The inhibitory effects of SNP were reversible. Pretreatment with methylene blue (10 microM), a non-specific inhibitor of guanylate cyclase, and hemoglobin (50 microM), a NO scavenger, decreased the SNP-induced inhibition of the Met-E-induced current. Intracellular injection of 1 mM guanosine 3',5'-cyclic monophosphate (cGMP) or bath-applied 3-isobutyl-1-methylxanthine (IBMX; 50 microM), a nonspecific phosphodiesterase inhibitor, inhibited the Met-E-induced current. Furthermore, 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 1 microM), a more specific inhibitor of NO-stimulated guanylate cyclase, decreased the SNP-induced inhibition of the Met-E-induced current. These results suggest that SNP induces suppression of the Met-E-induced K+ current recorded from B-cluster neurons of Aplysia cerebral ganglion via stimulation of cGMP formation.

Functional characterisation of a 5-HT2 receptor cDNA cloned from Lymnaea stagnalis

A G-protein-coupled receptor (5-HT2Lym) resembling members of the 5-HT2 receptor subfamily was cloned from the mollusc Lymnaea stagnalis. Serotonin induces a concentration-dependent increase in intracellular inositol phospates in HEK293 cells expressing this receptor (EC50 = 114 nM). 5-HT2Lym differs from mammalian 5-HT2 receptors by the presence of a large amino-terminal region. This large domain appears to preclude an adequate level of expression of 5-HT2Lym in HEK293. Therefore, we constructed a cDNA encoding an amino-terminally truncated receptor (delta N-5-HT2Lym) that appeared to be much better expressed in HEK293 cells. delta N-5-HT2Lym-expressing cells exhibit a serotonin-induced stimulation of phosphatidylinositol bisphosphate hydrolysis (EC50 = 11.4 nM) and a high-affinity binding of the 5-HT2-selective antagonist [3H]mesulergine (Kd = 4 nM). Inhibition of this binding by several 5-HT2 antagonists and agonists revealed a pharmacological profile most closely resembling those of 5HT2Dro, 5-HT2B and 5-HT2C.

Antagonism of glutamate receptors by a chromatographic fraction from the exudate of the sea anemone Phyllactis flosculifera

In the search for new glutamate antagonists it seems promising to characterize the effects of venom from invertebrates that prey mainly on crustaceans. In this work, the exudate of the sea anemone Phyllactis flosculifera was used as a source of this type of compound. The action of chromatographic fraction D from P. flosculifera was tested upon microion-tophoretically evoked glutamate responses in intracellular recordings from central neurons of the land snail Zachrysia guanensis. Bath application of fraction D (2-8 mg/ml, n = 13) diminished both the excitatory and the inhibitory components of glutamate agonists in Z. guanensis neurons; this action was dose-dependent and partially reversible. Fraction D actions were also tested in the multiunit spontaneous and mechanically evoked responses of the glutamatergic junction between hair cells and afferent neurons of the axolotl Ambystoma tigrinum. Pressure ejection of fraction D in concentrations ranging from 0.5 to 2 mg/ml (n = 9) decreased the spontaneous and mechanically evoked activity of semicircular canal afferent neurons and the responses evoked by kainic acid and alpha-amino-3-hydroxy-5-methylisoxasole-4-propionic acid. This action was also dose-dependent and partially reversible. These results indicate that fraction D acts as a glutamate receptor antagonist in snail and amphibian neurons. Further studies are required to characterize the active compounds responsible for this action and its specificity upon the subtypes of glutamate receptors.

Neurite outgrowth is enhanced by conditioning factor(s) released from central ganglia of Aplysia californica

Single neurons of Aplysia californica were cultured on control and conditioned medium-treated cover slips (CM, derived from central ganglia of Aplysia) and fed with L-15 medium plus 20% hemolymph. After 5 h 36.9 +/- 3.3% of all neurons plated on CM-treated cover slips sprouted, whereas only 22.9 +/- 2.7% sprouted in the control group. After 24 h the average neurite length in the presence of putative conditioning factor(s) was 621 +/- 42 microns versus 411 +/- 20 microns in control, but the number of primary processes was the same (4.3 +/- 0.5 versus 4.2 +/- 0.9).

Opioid peptides in the nervous system of Aplysia: a combined biochemical, immunocytochemical, and electrophysiological study

1. We have used biochemical, immunocytochemical, and electrophysiological techniques to evaluate the role of opioid peptides in the central nervous system of the marine mollusc, Aplysia californica. 2. Binding studies using 3H-D-Ala2, met-enkephalinamide (3H-DAMA) showed a single class of high-affinity binding sites with a Kd of 1.3 nM and a binding density of 45 pmol/g. 3. HPLC extracts of ganglia revealed multiple peaks with immunoreactivity for either leu (LEU-IR)- or met-enkephalin (MET-IR), but the amounts were not uniformly distributed in all ganglia. 4. LEU-IR and MET-IR neurons were demonstrated immunocytochemically in all ganglia, but MET-IR neurons were more frequent and were concentrated in pedal and pleural ganglia. While absorption control studies abolished MET-IR, LEU-IR was only partially abolished in the neuropil. 5. In electrophysiological studies, both depolarizing and hyperpolarizing responses were found to D-Ala2-leu-enkephalin (DALEU) and D-Ala2-met enkephalin (DAMET) on some and different neurons. 6. HPLC fractions from regions with retention times corresponding to authentic leu- or met-enkephalin showed physiologic responses similar to those of DALEU and DAMET, respectively. 7. These studies suggest that a variety of endogeneous opioid peptides play physiologically important roles in the nervous system of Aplysia, including but not necessarily limited to leu- and met-enkephalin.

Ultrastructural, biochemical and electrophysiological changes induced by 5,6-dihydroxytryptamine in the CNS of the snail Helix pomatia L

The serotonin neurotoxin, 5,6-dihydroxytryptamine (5,6-DHT), was injected into the body cavity of snails. Changes induced in the central nervous system (CNS) by the neurotoxin were studied by morphological, electrophysiological and biochemical techniques for up to 90 days following injection. The neurotoxin induced a variety of ultrastructural alterations during the early phase (1st to 6th days) following treatment. On day 6 after treatment, membranous structures first appeared in the synaptic-like areas and apparently migrated to cell bodies where they were detected by day 14. Their number increased with time. Neurotoxin-induced structural alterations were found in neuronal processes and cell bodies of the serotonergic metacerebral giant cells injected intracellularly with horseradish peroxidase and in serotonin immunoreactive axons. These findings suggest that the toxin-induced alterations are rather selective for the serotonin-containing neuronal elements. The neurotoxin decreased the concentration of 5-HT in and [3H]5-HT uptake into cerebral and pedal ganglia, with a maximum effect between the 3rd and 5th day following drug administration. 5-HT levels and 5-HT uptake returned to normal by 19-21 days after treatment. The concentration of dopamine and of [3H]DA uptake capacity were reduced between 3-5 days after injection of 5,6-DHT by 6-7 days following treatment. The transmission from identified serotonergic synapses to targets was reduced beyond day 5 after 5,6-DHT administration. By 15 days after treatment, synaptic transmission between the metacerebral giant cell (MGC) and buccal followers was blocked. Transmission recovered by day 21 after 5,6-DHT. Comparison of the time-course of functional and structural recovery indicates that while functional recovery takes place within 21 days after treatment, certain structural alterations, e.g. the membranous structures and dense particles, remain in the nerve fibres and cell bodies. These may serve as specific intracellular markers of the serotonin-containing neuronal elements long after functional recovery from the effect of 5,6-DHT.

Release of neurite outgrowth promoting factors by Helisoma central ganglia depends on neural activity

Identified buccal neurons B5 and B19 from the mollusc, Helisoma trivolvis, were plated into cell culture in order to assay for neurite outgrowth promoting factors released from central ring ganglia. The release and attachment of neurite promoting factors to the substratum of poly-lysine coated dishes could be inhibited by blocking spontaneous bioelectric activity in central ring ganglia used to condition the medium and dishes. Bioelectric activity within neurons in central ring ganglia was assayed by intracellular recording and found to be inhibited by exposure to the sodium channel blocker, tetrodotoxin (TTX; 2 x 10(-5) M), or CoCl2 (10 mM). Neither of these agents appeared to be toxic over a three day period since activity within neurons in central ring ganglia was restored following superfusion with saline. To examine the effect of blocking neural activity on the ability of central ring ganglia to release neurite outgrowth promoting factors, we compared the percentage of neurons that extended processes under 5 different conditions: (1) dishes containing conditioned medium and substrate attached growth factors (Super SAM); (2) dishes with substrate attached growth factors only and defined medium (SAM); (3) dishes containing substrate attached growth factors prepared in the presence of TTX; or (4) CoCl2; and (5) dishes containing unconditioned defined medium. The percentage of neurons extending processes under the 5 conditions were: (1) 71% (n = 32); (2) 51% (n = 33); (3) 14% (n = 37); (4) 15% (n = 47); (5) 0% (n = 40), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine-immunoreactive neurones in the central nervous system of the pond snail Lymnaea stagnalis

The distribution of dopamine and dopamine-immunoreactive neurones was studied in the central nervous system of the snail Lymnaea stagnalis. The results from immunocytochemical labelling were compared with those from the application of the glyoxylic acid fluorescence method and 6-hydroxydopamine-induced pigment labelling. Comparisons were also made between the number of dopamine immunoreactive neurones and the dopamine content of the ganglia, measured by high-performance liquid chromatography. Dopamine immunocytochemistry proved to be superior to the other two histochemical techniques in terms of specificity and sensitivity. The 6-hydroxydopamine-induced pigment labelling failed to prove a useful tool for the in vivo identification of all dopamine-containing neurones. The distribution and number of dopamine-immunoreactive neurones and levels of biochemically measured dopamine in specific ganglia showed a close correspondence. By using the results of the dopamine immunocytochemistry and glyoxylic acid technique, a detailed map of dopamine-containing neurones was constructed. Dopamine-containing inter- and intra-ganglionic axon tracts were also demonstrated. The mapping of dopamine-containing neurones will facilitate further neurophysiological analysis of dopaminergic neural mechanisms in Lymnaea.

Serotonin-immunoreactive varicosities in the cell body region and neural sheath of the snail, Helix pomatia, ganglia: an electron microscopic immunocytochemical study

The distribution and connections of serotonin-immunoreactive fibers in the cell body region and neural sheath of the central ganglia of the snail, Helix pomatia, have been examined. The cell body region of the ganglia is supplied by an extremely dense network of varicose serotonin-immunoreactive fibers which surround neuronal perikarya in the ganglia. Immunoreactive processes also run to the neural sheath of both the ganglia and the peripheral nerve roots, forming a dense network. Electron microscopy revealed five different connections of serotonin-immunoreactive varicosities, according to their target: (i) non-specialized contacts with neuronal perikarya; (ii) non-specialized contacts with axon processes on the surface of the peripheral nerve roots; (iii) non-specialized neuromuscular connections with smooth muscle fibers in the neural sheath; (iv) varicosities engulfed by glial processes in both the cell body region and neural sheath; (v) varicosities embedded in the connective tissue elements of the sheath either partly or completely free of glial processes. In all cases of appositions no membrane specializations could be observed on either site of the contacts. These observations provide morphological evidence for non-synaptic regulatory actions of serotonin-containing neurons in Helix central nervous system: (i) modulation of the activity of neuronal perikarya; (ii) involvement in neuromuscular regulation; (iii) neurohormonal modulation of peripheral processes by release through the neural sheath.

Indications for a hormonal function of dopamine in the central nervous system of the snail Lymnaea stagnalis

In the present paper we collected evidence for the occurrence of D2-like dopamine receptors on the cell bodies of the neuroendocrine growth hormone-producing cells (GHCs) in the central nervous system (CNS) of the snail Lymnaea stagnalis. Measurements of the membrane potential of GHCs in situ as well as isolated GHCs revealed that stimulation of these dopamine receptors results in a hyperpolarization. Although immunohistochemical analysis of the CNS of L. stagnalis clearly revealed the occurrence of dopamine containing cells and nerve fibers, no projections of dopamine immunopositive fibers to the GHC cell bodies could be observed. By using HPLC with electrochemical detection we found that the blood concentration of dopamine in L. stagnalis is in the range of concentrations hyperpolarizing GHCs in vitro (0.1-10 microM). On the basis of these findings it is proposed that dopamine is involved in hormonal communication in the CNS of L. stagnalis.

A comparison of four techniques for mapping the distribution of serotonin and serotonin-containing neurons in fixed and living ganglia of the snail, Lymnaea

The distribution of serotonin and serotonin-containing neurons was studied in the ganglia of the CNS of the snail Lymnaea stagnalis. Results of the application of three different labelling techniques on wholemount preparations were compared with each other and with the serotonin content of the ganglia, measured by high-performance liquid chromatography. Serotonin immunocytochemistry resulted in the highest number of labelled neurons, but the more recently developed in vivo method of 5,6- or 5,7-dihydroxytryptamine-induced pigmentation also proved to be a reliable technique for the visualization of serotonin-containing cell bodies. In comparison with these two techniques, the glyoxylic acid fluorescence method appeared to be less sensitive. The distribution and number of serotonin-containing neurons and biochemically measured serotonin in specific ganglia showed a close correlation. By combining the results of the three labelling techniques, a detailed map of serotonin-containing neurons was constructed, and this was compared with maps of identified neurons prepared from earlier electrophysiological studies. Previously described serotonergic neurons were consistently found, as well as several new serotonin-containing cell types in the cerebral, visceral and parietal ganglia. A network of serotonin-containing inter- and intraganglionic axon tracts, and thin serotonergic fibres in the perineurium were also demonstrated. This in vivo and in vitro identification of serotonin-containing neurons will facilitate further neurophysiological analysis of serotonergic neural mechanisms in Lymnaea.

Neurochemical and immunocytochemical studies of serotonin in the Hermissenda central nervous system

In the preceding paper we showed that serotonin mimics the effects of associative conditioning on Type B photoreceptors of Hermissenda. Here we show that serotonin is present in the Hermissenda central nervous system, and that it is released in a calcium-dependent manner. A large number of serotonergic cell bodies are present in the cerebropleural ganglia (CPG). Cell bodies in the CPG are located in three small, symmetrical clusters. One cluster (2-3 cells) located at the anterior end of the CPG contains a large cell that projects to the buccal ganglion. A second cluster (2-3 cells) is located posterior to the first and near the midline of the nervous system. A third cluster (1-2 cells) was located slightly more posteriorly than the second. Type B photoreceptors and the S/E optic ganglion cell project to a region of the CPG neuropil which is heavily innervated by serotonin immunoreactive fine processes. Since serotonin mimicked the effects of associative conditioning on Type B cells, and is present in an appropriate region of the CPG neuropil, serotonin may mediate some effects of associative conditioning on Type B cells. Results supporting this hypothesis are presented in an accompanying paper.

Effects of hypothalamic releasing hormones and biogenic amines on identified neurones in the circumoesophageal ganglia of the water snail (Planorbis corneus)

1. The effect of locally applied releasing hormones, thyrotropin-releasing hormone (TRH) and luteinizing-hormone-releasing hormone (LHRH) and the putative neurotransmitters, acetylcholine (ACh) and dopamine (DA), on the neuronal excitability of identified invertebrate giant dopaminergic neurone (GDN) and serotoninergic neurone (5-HT) (Planorbis corneus) were investigated by intracellular recording in vitro. 2. The membrane potential of GDN was of the order of -60 to -70 mV. The microiontophoretically applied substances produced membrane depolarization as well as spike activation. Their order of efficacy was as follows: TRH greater than ACh greater than DA greater than LHRH. 3. The effects of the tested TRH, ACh, LHRH and DA on serotoninergic neurones were less pronounced. 4. During ACh depolarization the membrane resistance of GDN was found to be strongly reduced, whereas TRH produced only a small reduction in membrane resistance. 5. Dihydro-beta-erythroidin (DHE) added to the bath solution reversibly blocked ACh depolarization without influencing TRH depolarization. Concentrations of atropine sulfate required to block the ACh depolarization were higher by at least 100 order of magnitude. 6. These effects are discussed in relation to the immunoreactive TRH detected earlier in the central nervous system of invertebrates and vertebrates. The results are consistent with the postulate that TRH acts as a neuromodulator and/or neurotransmitter on invertebrate and vertebrate neurones.

Humoral factors released during trauma of Aplysia body wall. II. Effects of possible mediators

1. Preliminary, general chemical characteristics of substances in artificial sea water (ASW) washed through stimulated body wall (SBW) and in hemolymph taken from noxiously stimulated animals (SHL) were consistent with those of classical neurotransmitters, amino acids, and small- to medium-sized peptides. 2. 5-Hydroxytryptamine (5HT) and acetylcholine (ACh), unlike SBW and SHL, caused relaxation when perfused into isolated body wall. FMRFamide produced a biphasic response--brief contraction followed by prolonged relaxation. 3. Small cardioactive peptide (SCPB) caused body wall contractions similar to those produced by SBW and SHL, except that SCPB contractions displayed more desensitization and were completely blocked by 30 mM CoCl2. SCPB and SBW contractions were synergistic. 4. Dopamine caused persistent body wall contractions similar to those of SBW and SHL. Dopamine contractions were reduced but not blocked by 30 mM CoCl2. Unlike SBW activity, dopamine activity was reduced by alkalinization. 5. Glutamate and taurine produced strong but usually short-lasting body wall contractions. Adenosine, octopamine, arginine vasotocin, and cholecystokinin (CCK-8) caused weak or variable contractions. Met-enkephalin and somatostatin caused no obvious body wall responses. 6. When superfused over the fully sheathed abdominal ganglion, FMRFamide, met-enkephalin, glutamate, aspartate, and taurine reduced the magnitude of the gill-withdrawal reflex elicited by siphon nerve stimulation. 7. Taken together with earlier results, these data suggest a preliminary framework for trauma signal pathways. It is proposed that stress hormones (perhaps including FMRFamide, SCPs, 5HT, and dopamine) are released into hemolymph from neuroendocrine cells. Effective amounts of active intracellular solutes such as amino acids may also be released by extensive cellular rupture. Various humoral signals produce slow effects that contribute to hemostasis, balling up, increased cardiac output, and reflex suppression.

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

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