The in vitro uptake of biogenic amines by snail (Heliz pomatia) nervous tissue

Serotonergic and dopaminergic influence of the duration of embryogenesis and intracapsular locomotion of Lymnaea stagnalis L

The role of the dopaminergic and serotonergic system was studied during the embryonic development of the pond snail Lymnaea stagnalis, with special attention to the effect of dopamine and serotonin as well as their agonists and antagonists on the rotation of the veliger larvae, and to the effect of precursors and inhibitors of the synthetizing enzymes on the duration of the embryonic life. Serotonin, D-lysergic acid diethylamide and N,N-dimethyltryptamine increased at a concentration of 1 microM the rotation by 50%, 90% and 87% respectively, and among them D-Lysergic acid diethylamide was found to be the most potent agonist. Other serotonergic agonists and antagonists enhanced the frequency of the rotation (from 165% to 355%) at higher threshold concentrations in the following rank order: methysergid > tryptamine > 2,5-dimethoxy-4-iodoamphetamine > 5-carboxyamidotryptamine > bromo-lysergic acid diethylamide > 7-methyltryptamine. Application of 1-(2-methoxyphenyl) piperazine decreased the rotation by 76%. The reuptake inhibitor desipramine completely blocked the rotation and killed the embryos. Dopaminergic agonists accelerated the rotation by 62% to 233%, and their effect was ranged as follows: dopamine > apomorphine > m-tyramine approximately equal to p-tyramine. Chlorpromazine at 100 microM concentration killed the embryos. At a concentration of 100 microg/ml, tyrosine, the precursor of DA, slowed down the embryonic development by increasing the duration of the embryonic life from 8 to 10 days. Decarboxylase inhibitors, alpha-methyl-3,4-dihydroxyphenyl-alanine (25 microg/ml) and m-hydroxybenzylhydrazin (5 microg/ml), killed 50% of the embryos, meanwhile the rest hatched ten days later, compared to the control animals. The development was partially blocked by the serotonin precusor L-tryptophane (50 microg/ml). Trytophan hydroxylase blocker, p-chlorphenylalanine (50 microg/ml) resulted in a distortion of the body pattern of the embryos, and prevented the hatching of most (95%) of the animals.

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

The combination of high performance liquid chromatography, bioassay and immunocytochemistry was applied to study the regulation of the salivary duct muscle of the snail, Helix pomatia. The major function of the duct appears to be to propel the saliva toward the buccal cavity during feeding. It has been established that serotonin and dopamine applied exogenously mimic the effect on the duct exerted by the stimulation of the salivary nerve. Immunohistochemistry revealed the presence of serotonin, but not dopaminergic nerve elements in the nerve and along the duct surface. However, both serotonin (14.9-15.5 pmol/mg) and dopamine (0.38-0.58 pmol/mg), as well as the synthesizing enzymes (tyrozine hydroxylase 0.28 pmol/mg tissue/h and DOPA 0.32 nmol synthesized DA/mg tissue/h) could regularly be assayed in the salivary duct by high performance liquid chromatography. When released following the stimulation of the salivary nerve, both monoamines were shown to interact with distinct membrane receptors. Dopamine elicited a sustained increase of the muscle tone in concentration-dependent manner (K(d)=1.5 microM). Mammalian D(1) receptor antagonist flupenthixol and fluphenazine attenuated, whereas the D(1) receptor agonist SKF-38393 mimicked the effect elicited by exogenous dopamine. Serotonin had a double effect on the salivary duct: a relaxing and a contracting one with different K(d) values 76 nM and 2.4 microM, respectively. 5-HT(2) receptor antagonist ritanserin and ketanserin attenuated the serotonin-induced relaxation. In contrast 5-HT(3) antagonist metoclopramide and MDL2222 decreased and 5-HT(3) receptor agonist 1-(m-chlorophenyl)-biguanide mimicked the serotonin-induced contraction, suggesting that serotonin exerted its action on two different receptor subtypes. The release of radiolabeled serotonin and dopamine upon nerve stimulation was found to be Ca-dependent. Furthermore, the increase in serotonin concentration induced a decrease of the potency of dopamine to elicit sustained contraction. These results provide evidence for the transmitter role of serotonin and dopamine in salivary duct. It is concluded that receptors reveal a pharmacological profile related to vertebrate D(1), 5-HT(2) and 5-HT(3) receptor subtypes. Moreover, it was found that the process of conveying the saliva is modulated by an interaction of dopamine and serotonin.

Parturition in Anodonta cygnea induced by selective serotonin reuptake inhibitors (SSRIs)

The selective serotonin reuptake inhibitors (SSRIs) fluoxetine (Nodep) and fluvoxamine (Dumyrox) are drugs commonly prescribed for the treatment of depression in humans. They act to increase serotonin (5-hydroxytryptamine) neurotransmission by inhibiting reuptake transport proteins at synapses. A similar effect of SSRIs seems to occur naturally in one group of invertebrates, the molluscs. In this study the action of these drugs at different concentrations and under different conditions was tested in the freshwater mussel Anodonta cygnea. Fluoxetine was more potent than fluvoxamine, inducing an intense release of larvae (parturition) at a concentration of 1 × 10–6 M in the presence of light. The non-SSRI antidepressants trazodone (Triticum), mianserine (Tolvon), and L-5-hydroxytryptophan (Cincofarm), which are known to have different serotoninergic mechanisms, had a strong, no, and a weak effect, respectively, on larval parturiton in A. cygnea. These effects suggest that serotonin could be a normal and relevant mediator of larval parturition in A. cygnea. Other parallel visible signs due to incubation with SSRIs were an increase in the volume of the foot and gills through water uptake and stimulation of valve movements. These results indicate that incubation with SSRIs is potentially important in culturing the larvae of freshwater bivalves, since it facilitates control of the intensity and timing of larval parturition.

An octopaminergic system in the CNS of the snails, Lymnaea stagnalis and Helix pomatia

Octopamine (OA) levels in each ganglion of the terrestrial snail, Helix pomatia, and the pond snail, Lymnaea stagnalis, were measured by using the HPLC technique. In both species an inhomogeneous distribution of OA was found in the central nervous system. The buccal ganglia contained a concentration of OA (12.6 pmol mg^-1 and 18.8 pmol mg^-1) that was two to three times higher than the pedal (4.93 pmol mg^-1 and 9.2 pmol mg^-1) or cerebral (4.46 pmol mg^-1 and 4.9 pmol mg^-1) ganglia of Helix and Lymnaea, respectively, whereas no detectable amount of OA could be assayed in the visceroparietal complex. In Lymnaea ganglia, the OA uptake into the synaptosomal fraction had a high (K_m1 = 4.07 ± 0.51 μM, V_max1 = 0.56 ± 0.11 pmol mg^-1 per 20 min), and a low (K_m2 = 47.6 ± 5.2 μM, V_max2 = 4.2 ± 0.27 pmol mg^-1 per 20 min), affinity component. A specific and dissociable ³H-OA binding to the membrane pellet prepared from the CNS of both Helix and Lymnaea was demonstrated. The Scatchard analysis of the ligand binding data showed a one-binding site, representing a single receptor site. The K_d and B_max values were found to be 33.7 ± 5.95 nM and 1678 ± 179 fmol g^-1 tissue in Helix and 84.9 ± 17.4 nM and 3803 ± 515 fmol g^-1 tissue in Lymnaea preparation. The pharmacological properties of the putative molluscan OA receptor were characterized in both species and it was demonstrated that the receptor resembled the insect OA₂ rather than to the cloned Lymnaea OA receptor. Immunocytochemical labelling demonstrated the presence of OA-immunoreactive neurons and fibres in the buccal, cerebral and pedal ganglia in the central nervous system of both species investigated. Electrophysiological experiments also suggested that the Lymnaea brain possessed specific receptors for OA. Local application of OA onto the identified buccal B2 neuron evoked a hyperpolarization which could selectively be inhibited by the OAergic agents phentolamine, demethylchlordimeform and 2-chloro-4-methyl-2-(phenylimino)-imidazolidine. Among the dopamine antagonists, ergotamine reversibly inhibited the OA response, whereas sulpiride had no effect. Based on our findings, a neurotransmitter-modulator role of OA is suggested in the gastropod CNS.

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.

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.

Further characterisation of the dopamine-inhibitory receptor in Helix and evidence for a noradrenaline-preferring receptor

1. The cells in this study responded with a hyperpolarization to the following agents in this order of potency; dopamine greater than noradrenaline phenylephrine = octopamine. 2. 6,7 ADTN had a relative potency of 0.1 compared to dopamine. 5,6 ADTN did not inhibit the cells in this study. 3. The D1 receptor agonists SKF38393 and dihydroxynomifensine mimicked the effect of dopamine on these cells but were over 100 times less active, whereas the D2 selective agonists quinpirole and RU24213 were without effect. 4. Both the D1 antagonist SCH23390 and the D2 antagonist sulpiride antagonised the dopamine response with pA2 values of 6.1 and 6.7, respectively. 5. Five cells that responded to dopamine with a hyperpolarization were depolarized by noradrenaline. The order of potency of compounds at eliciting this depolarization, noradrenaline greater than phenylephrine greater than octopamine indicated that this response may be mediated by a noradrenaline-preferring receptor.

Uptake and release of [3H]-serotonin in photophores of the midshipman fish, Porichthys notatus

A kinetic analysis of [3H]-5-HT uptake in the photocytes of the photophores of Porichthys notatus revealed a high affinity (Km: 1.71 X 10(-7] and low affinity component (Km: 1.10 X 10(-5) M). The high affinity uptake was sodium- and potassium-dependent but largely insensitive to temperatures between 0 and 20 C. Ouabain (5 X 10(-3) M) and dinitrophenol (10(-3) M) reduced uptake significantly. DMI, imipramine and fluoxetine, in that order of potency, greatly inhibited [3H]-5-HT uptake. Noradrenaline and adrenaline reduced uptake in a non-competitive manner, while dopamine, tryptophan, 5-hydroxytryptophan and Cypridina luciferin had little or not effect on uptake. Non-facilitated luminescent responses to electrical stimulation were accompanied by release of [3H]-5-HT accumulated in the photocytes. Facilitatory luminescence excitation consistently failed to induce the release of [3H]-5-HT. Electrical and adrenaline (10(-5) M) stimulation of photophores after [3H]-5-HT release has occurred, failed to elicit any additional luminescent response. The photophores were responsive to KCN (10(-3) M) under these conditions. The results indicate that a specific carrier-mediated transport system is responsible for photocytic [3H]-5-HT uptake, and that release of photocytic [3H]-5-HT is stringently regulated and followed by inhibition of luminescence excitability.

The pharmacology of molluscan neurons

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

Studies of dopamine pharmacology in molluscs

Dopamine has been established as a putative neurotransmitter in several species of molluscs. Biochemical and neurophysiological studies of the cellular pharmacology of dopamine have revealed several properties of molluscan dopamine receptors. The biochemical synthesis and degredation of dopamine in molluscs follows the same pathways that have been described in mammals. Adenylate cyclase is present, and the receptor mediating CAMP production is blocked by neuroleptics and certain ergot alkaloids. Studies of this enzyme and of radioligand binding indicate that molluscan dopamine receptors and serotonin receptors share certain characteristics. Neurophysiological studies have shown that dopamine induces several forms of ionic conductance changes in molluscan neurons. The receptors mediating these conductance changes may be differentiated pharmacologically. Neuroleptics are antagonists at certain receptors and ergot alkaloids have been shown to be either partial agonists or antagonists. Present evidence indicates that molluscan and mammalian CNS dopamine receptors have some similarities. However, further biochemical and neurophysiological investigations will be necessary to fully characterize molluscan dopamine receptors.

Characterization of the dopamine stimulated adenylate cyclase in the pedal ganglia of Mytilus edulis: interactions with etorphine, beta-endorphin, DALA, and methionine enkephalin

The dopamine-stimulated adenylate cyclase activity was studied both in vivo and in vitro in the central nervous system of the bivalve mollusc Mytilus edulis. Dopamine, epinine, and apomorphine stimulated the enzyme system. Fluphenazine, haloperidol, chlorpromaxine, and to a lesser extent BOL inhibited the dopamine-stimulated adenylate cyclase. Etorphine, beta-endorphine, DALA, and methionine enkephalin depressed cyclic AMP levels. This phenomena was naloxone reversible. In addition, the opioids inhibited the stimulation of adenylate cyclase by dopamine. This phenomena was also naloxone reversible. The study demonstrates an interaction among dopamine, the opioids, and cyclic AMP.

In vitro experiments on the metabolism, uptake and release of 5-hydroxytryptamine in bovine retina

The accumulation, metabolism and potassium-induced release of 5-hydroxytryptamine (5-HT) in bovine retinae was studied. When isolated retina pieces were incubated at 37 degrees C in a medium containing [14C]5-HT, tissue:medium ratios of about 5 were obtained after only 10 min. Within this time the accumulated amine is hardly metabolized and even after a 60 min period, only approximately 12% of the amine is metabolized to form 5-hydroxyindoleacetic acid. The process responsible for this accumulation showed properties of an active transport system: it was temperature-sensitive, sodium-dependent and inhibited in particular by Lilly 110140, chlorimipramine and desimipramine. The uptake of [14C]5-HT was saturated with increasing 5-HT concentrations and could be accounted for by two saturable processes; a high-affinity system with a Km1 of 1.3 X 10(-7) M and Vmax1 of 0.65 pmol/mg/10 min, and a low affinity system with a Km2 of 1.2 X 10(-5) and Vmax2 of 29 pmol/mg/10 min. A rapid efflux of [14C]5-HT from tissue loaded with [14C]amine was observed when exposed to 70 mM KCl. This release was inhibited when the calcium content in the medium was replaced by sucrose. Cobalt ions added to the incubation medium also counteracted the effect of KCl, while chlorimipramine added to the medium enhanced the release of 5-HT caused by KCl. Bovine retinae also possess the enzymes necessary to produce 5-HT from tryptophan. These data generally support the idea that 5-HT is a transmitter in the retina, although as this study shows, the amine is present only in minute quantities.

The accumulation of DL-glutamate by the central nervous system of the snail Helix pomatia

Isolated snail ganglia are capable of maintaining their free amino acid levels steady for the first 60 min of incubation in physiological saline. Within this time the ganglia also possess an uptake mechanism for DL-glutamate which can be divided into sodium-sensitive and -insensitive components. The accumulation of DL-glutamate showed saturation kinetics typical of a carrier-mediated process. The Vmax value for the uptake is 1.5 x 10(-8) mole/g/min and the Km value 1.1 x 10(-4) M. The amino acid accumulation is quite specific towards L-dicarboxylic acids and insensitive to a number of metabolic inhibitors. It is unlikely to be due to a homoexchange phenomenon because the ganglia are capable of achieving a net uptake of glutamate and the efflux of DL-[3H]glutamate is not increased by the addition of non-radioactive L-glutamate to the incubation medium.

Radioautographic study of 5-hydroxytryptamine-containing nerve terminals in central ganglia of Planorbis corneus: comparison with other species and characteristics of the serotoninergic nerve terminal

Central ganglia from Planorbis corneus were incubated with [3H]5-hydroxytryptamine and [3H]-leucine. After fixation, sections were examined by light and electron microscopic autoradiography. With [3H]5-hydroxytryptamine there was a high level of uptake into a small number of axons and terminal processes in the neuropil. The terminal processes contained granular vesicles (diameter 50--120 nm), some of which possessed eccentrically placed cores, and agranular vesicles (mean diameter 60 nm). No membrane synaptic specializations were observed. With [3H]leucine there was a general distribution of radioactivity throughout the ganglia. The vesicular inclusions and non-specialized nature of the labelled terminal processes were very similar to presumed 5-hydroxytryptamine-containing terminals in other invertebrates and in the mammalian brain. It appears that several anatomical features of serotoninergic nerve terminals are common in all animals studied, although there is no specific characteristic which allows positive identification.

Effects of 5,7-dihydroxytryptamine on an identified 5-hydroxytryptamine-containing neurone in the central nervous sytem of the snail Helix pomatia

1. The effect of 5,7-dihydroxytryptamine (5,7-DHT) on an identified 5-hydroxytryptamine (5-HT)-containing neurone in the CNS of the snail was studied by histochemical, biochemical and electrophysiological methods. 2. Low concentrations of 5,7-DHT decreased the endogenous 5-HT content of the neurone without affecting the amino acids, while relatively large amounts of the drug proportionately lowered 5-HT and in addition slightly decreased the tryptophan and methionine content of the cell. 3. 5,7-DHT blocked the uptake of [3H]-5-HT into the neurone; the close analogue 5,6-DHT was more potent. 4. As well as slightly influencing the accumulation of [3H]-tryptophan by the neurone 5,7-DHT inhibited the metabolism of this amino acid to form 5-HT, probably by affecting the enzyme tryptophan-hydroxylase. 5. 5,7-DHT produced a postsynaptic blockade of transmission from the neurone by blocking the 5-HT receptors of the follower neurones. This effect appeared to be specific for 5-HT receptors. 6. The data support the idea that 5,7-DHT is neurotoxic for indoleamine-containing neurones.