Studies of dopamine pharmacology in molluscs

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.

Monoamines, Insulin and the Roles They Play in Associative Learning in Pond Snails

Molluscan gastropods have long been used for studying the cellular and molecular mechanisms underlying learning and memory. One such gastropod, the pond snail Lymnaea stagnalis, exhibits long-term memory (LTM) following both classical and operant conditioning. Using Lymnaea, we have successfully elucidated cellular mechanisms of learning and memory utilizing an aversive classical conditioning procedure, conditioned taste aversion (CTA). Here, we present the behavioral changes following CTA training and show that the memory score depends on the duration of food deprivation. Then, we describe the relationship between the memory scores and the monoamine contents of the central nervous system (CNS). A comparison of learning capability in two different strains of Lymnaea, as well as the filial 1 (F1) cross from the two strains, presents how the memory scores are correlated in these populations with monoamine contents. Overall, when the memory scores are better, the monoamine contents of the CNS are lower. We also found that as the insulin content of the CNS decreases so does the monoamine contents which are correlated with higher memory scores. The present review deepens the relationship between monoamine and insulin contents with the memory score.

Relationship between the grades of a learned aversive-feeding response and the dopamine contents in Lymnaea

The pond snail Lymnaea learns conditioned taste aversion (CTA) and remembers not to respond to food substances that initially cause a feeding response. The possible relationship between how well snails learn to follow taste-aversion training and brain dopamine contents is not known. We examined this relationship and found the following: first, snails in the act of eating just before the commencement of CTA training were poor learners and had the highest dopamine contents in the brain; second, snails which had an ad libitum access to food, but were not eating just before training, were average learners and had lower dopamine contents; third, snails food-deprived for one day before training were the best learners and had significantly lower contents of dopamine compared to the previous two cohorts. There was a negative correlation between the CTA grades and the brain dopamine contents in these three cohorts. Fourth, snails food-deprived for five days before training were poor learners and had higher dopamine contents. Thus, severe hunger increased the dopamine content in the brain. Because dopamine functions as a reward transmitter, CTA in the severely deprived snails (i.e. the fourth cohort) was thought to be mitigated by a high dopamine content.

A dopa decarboxylase modulating the immune response of scallop Chlamys farreri

Background

Dopa decarboxylase (DDC) is a pyridoxal 5-phosphate (PLP)-dependent enzyme that catalyzes the decarboxylation of L-Dopa to dopamine, and involved in complex neuroendocrine-immune regulatory network. The function for DDC in the immunomodulation remains unclear in invertebrate.

Methodology

The full-length cDNA encoding DDC (designated CfDDC) was cloned from mollusc scallop Chlamys farreri. It contained an open reading frame encoding a polypeptide of 560 amino acids. The CfDDC mRNA transcripts could be detected in all the tested tissues, including the immune tissues haemocytes and hepatopancreas. After scallops were treated with LPS stimulation, the mRNA expression level of CfDDC in haemocytes increased significantly (5.5-fold, P<0.05) at 3 h and reached the peak at 12 h (9.8-fold, P<0.05), and then recovered to the baseline level. The recombinant protein of CfDDC (rCfDDC) was expressed in Escherichia coli BL21 (DE3)-Transetta, and 1 mg rCfDDC could catalyze the production of 1.651±0.22 ng dopamine within 1 h in vitro. When the haemocytes were incubated with rCfDDC-coated agarose beads, the haemocyte encapsulation to the beads was increased significantly from 70% at 6 h to 93% at 24 h in vitro in comparison with that in the control (23% at 6 h to 25% at 24 h), and the increased haemocyte encapsulation was repressed by the addition of rCfDDC antibody (which is acquired via immunization 6-week old rats with rCfDDC). After the injection of DDC inhibitor methyldopa, the ROS level in haemocytes of scallops was decreased significantly to 0.41-fold (P<0.05) of blank group at 12 h and 0.47-fold (P<0.05) at 24 h, respectively.

Conclusions

These results collectively suggested that CfDDC, as a homologue of DDC in scallop, modulated the immune responses such as haemocytes encapsulation as well as the ROS level through its catalytic activity, functioning as an indispensable immunomodulating enzyme in the neuroendocrine-immune regulatory network of mollusc.

An aplysia dopamine1-like receptor: molecular and functional characterization

In Aplysia, the neurotransmitter dopamine is involved in the regulation of various physiological processes and motor functions, like feeding behaviour, and in the siphon-gill withdrawal reflex. In this paper, we report the characterization of the first Aplysia D1-like dopamine receptor (Apdop1) mainly expressed in the CNS, heart and buccal mass. Following expression of the Apdop1 receptor in HEK293 cells, a higher level of cAMP was observed in the absence of the receptor ligand, showing that Apdop1 is constitutively active. This activity was blocked by the inverse agonist flupentixol. Application of dopamine (EC50 of 35 nm) or serotonin (EC50 of 36 microm) to Apdop1-transfected HEK293 cells further increased the level of cAMP, suggesting that the receptor is linked to the stimulatory Gs protein pathway. When expressed in cultured sensory neurons, Apdop1 immunoreactivity was observed in the cell body and neurites. Control sensory neurons responded to dopamine with a decrease in excitability mediated by a pertusis toxin-sensitive G protein. Expression of Apdop1 produced an increase in hyperpolarization in the absence of agonist and an increase in membrane excitability following stimulation by dopamine. In the presence of pertussis toxin to inhibit the Gi protein inhibitory pathway responsible for decrease in excitability mechanism, Stimulation of membrane excitability was observed. Apdop1 sensitivity to dopamine makes it a potential modulator of operant conditioning procedure.

Reinforcement in an in vitro analog of appetitive classical conditioning of feeding behavior in Aplysia: blockade by a dopamine antagonist

In a recently developed in vitro analog of appetitive classical conditioning of feeding in Aplysia, the unconditioned stimulus (US) was electrical stimulation of the esophageal nerve (En). This nerve is rich in dopamine (DA)-containing processes, which suggests that DA mediates reinforcement during appetitive conditioning. To test this possibility, methylergonovine was used to antagonize DA receptors. Methylergonovine (1 nM) blocked the pairing-specific increase in fictive feeding that is usually induced by in vitro classical conditioning. The present results and previous observation that methylergonovine also blocks the effects of contingent reinforcement in an in vitro analog of appetitive operant conditioning suggest that DA mediates reinforcement for appetitive associative conditioning of feeding in Aplysia.

Dopamine receptor subtype density as a function of age in Aplysia californica

The age-associated changes in dopamine subtype receptors were examined in Aplysia californica. The density of the subtype receptors D1, D2, D3 and D4 was examined in the ganglia from 4.5-, 6-, 8-, 9- and 12-month animals. Receptor analysis was performed by examining the binding of radiolabeled ligands to the individual subtypes. [3H]SCH23390 and [3H]Clozapine were used to analyze D1 and D4 specific binding. [3H]Quinpirole was used for determining D2 and D3 specific binding. Specific binding was found to be present for all four receptor subtypes. All receptor subtypes showed an increase in density from 4.5 to 6 months. From 6 to 8 months D2 and D3 decreased, while D1 and D4 increased. D4 showed the strongest increase. All four subtypes examined showed decreases from 8 to 12 months. ANOVA results indicated age was a significant factor in the subtype receptor density for all receptor types.

Modulation of fictive feeding by dopamine and serotonin in aplysia

The buccal ganglia of Aplysia contain a central pattern generator (CPG) that mediates rhythmic movements of the buccal apparatus during feeding. Activity in this CPG is believed to be regulated, in part, by extrinsic serotonergic inputs and by an intrinsic and extrinsic system of putative dopaminergic cells. The present study investigated the roles of dopamine (DA) and serotonin (5-HT) in regulating feeding movements of the buccal apparatus and properties of the underlying neural circuitry. Perfusing a semi-intact head preparation with DA (50 microM) or the metabolic precursor of catecholamines (L-3-4-dihydroxyphenylalanine, DOPA, 250 microM) induced feeding-like movements of the jaws and radula/odontophore. These DA-induced movements were similar to bites in intact animals. Perfusing with 5-HT (5 microM) also induced feeding-like movements, but the 5-HT-induced movements were similar to swallows. In preparations of isolated buccal ganglia, buccal motor programs (BMPs) that represented at least two different aspects of fictive feeding (i.e., ingestion and rejection) could be recorded. Bath application of DA (50 microM) increased the frequency of BMPs, in part, by increasing the number of ingestion-like BMPs. Bath application of 5-HT (5 microM) did not significantly increase the frequency of BMPs nor did it significantly increase the proportion of ingestion-like BMPs being expressed. Many of the cells and synaptic connections within the CPG appeared to be modulated by DA or 5-HT. For example, bath application of DA decreased the excitability of cells B4/5 and B34, which in turn may have contributed to the DA-induced increase in ingestion-like BMPs. In summary, bite-like movements were induced by DA in the semi-intact preparation, and neural correlates of these DA-induced effects were manifest as an increase in ingestion-like BMPs in the isolated ganglia. Swallow-like movements were induced by 5-HT in the semi-intact preparation. Neural correlates of these 5-HT-induced effects were not evident in isolated buccal ganglia, however.

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.

Distribution of catecholamines and of immunoreactivity to substances like vertebrate enzymes for the synthesis of catecholamines within the central nervous system of the snail, Lymnaea stagnalis

Catecholamines (CAs) were detected histochemically within over 185 cell bodies in the central nervous system (CNS) of juvenile and young adult Lymnaea. This distribution of CA-containing cells in all central ganglia except the pleural ganglia is more widespread than previously described but is consistent with other reports suggesting numerous roles for CAs within the nervous system. This study also describes the distribution of substances which are antigenically similar to four bovine enzymes for catecholamine synthesis, but the distribution patterns showed little or no overlap with each other or with CA. These results suggest the need for caution in the interpretation of such immunohistochemical studies.

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.

Characterization of pre- and postsynaptic dopamine receptors in Lymnaea

1. The effects of dopamine and several synthetic agonists and antagonists were studied using two identified neurons of the snail Lymnaea stagnalis. 2. In both the buccal-2 (B-2) neurons and the pedal giant (RPeD1) neuron dopamine elicited a hyperpolarizing response at least partly due to potassium efflux. RPeD1 is itself dopaminergic, implicating autoreceptors in its response to dopamine. 3. The following agents were tested: agonists--LY171555, pergolide, SKF38393, (-)-3-PPP, R(-)NPA and dopamine; antagonists--SCH23390, sulpiride, and metaclopramide. Dibutyryl cAMP was applied to determine whether the response is cAMP-mediated. 4. Results indicate that the pharmacological profiles of dopamine receptors on these neurons are inconsistent with those of either D-1, D-2 or autoreceptors in mammals.

Pharmacological evidence that alpha 1-adrenoceptors mediate metamorphosis of the Pacific oyster, Crassostrea gigas

Oyster larvae can be induced to metamorphose by exposure to the natural vertebrate adrenergic agonists, epinephrine and norepinephrine. The larval receptors mediating this induction were pharmacologically characterized by testing the ability of a variety of adrenergic agonists and selected structural analogs of epinephrine and norepinephrine to induce oyster metamorphosis, and by testing the ability of various adrenergic antagonists to block the induction of metamorphosis by epinephrine. Oyster metamorphosis can be induced by vertebrate adrenergic agonists with relative potencies: cirazoline greater than epinephrine greater than phenylephrine greater than or equal to norepinephrine greater than alpha-methylnorepinephrine greater than isoproterenol much greater than methoxamine = clonidine. Other structural analogs of epinephrine and norepinephrine, including dopamine and octopamine, were ineffective at inducing metamorphosis. Induction of metamorphosis by epinephrine can be blocked by vertebrate adrenergic antagonists with relative potencies: chlorpromazine greater than or equal to prazosin greater than phentolamine greater than WB4101 greater than propranolol greater than yohimbine greater than metoprolol. These data demonstrate that receptors similar to vertebrate-type alpha 1-adrenoceptors mediate oyster metamorphosis. This is the first evidence for alpha 1-adrenoceptors in molluscs, and provides an important clue to the control of the complex process of molluscan metamorphosis and to the evolution of vertebrate adrenergic receptors.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

Further pharmacological characterization of a D-2-like dopamine receptor on growth hormone producing cells in Lymnaea stagnalis

A preliminary study has revealed that a mammalian D-2-like dopamine (DA) receptor mediates hyperpolarization of the neuroendocrine growth hormone-producing cells (GHCs) in the snail Lymnaea stagnalis. An extensive pharmacological characterization of this receptor was performed in the present study. Several mammalian D-2 receptor agonists (e.g. aminotetralins) and antagonists (e.g.(-)-sulpiride) showed agonistic and antagonistic effects, respectively. However, some selective D-2 receptor agonists (e.g. N 0437) and antagonists (e.g. domperidone) failed to show agonistic or antagonistic effects, respectively. It is concluded that the dopamine receptor mediating hyperpolarization of the GHCs displays, besides some similarities, several differences from the mammalian D-2 receptor.

Modulation of dopamine receptors in the Tapes clam by dextroamphetamine and phenylethanolamine

The mechanism underlying the modulation, by dextroamphetamine and compounds related to phenylethanolamine, of responses to dopamine and serotonin has been studied in the isolated ventricle and aortic bulb of the clam Tapes watlingi. Dextroamphetamine and phenylethanolamine but not cocaine and benztropine have the ability to unmask inhibitory responses to both dopamine and serotonin in the ventricle. Chlordimeform but not clozapine attenuates the inhibitory response to both dextroamphetamine and phenylethanolamine in concentrations which have little or no effect on the inhibitory response to dopamine in the ventricle. Phenylethanolamine, dextroamphetamine, phenylpropylolamine and p-chloro-phenylethanolamine but not octopamine or noradrenaline attenuate the contractile responses to both dopamine and serotonin in preparations of the quiescent aortic bulb. These data show that there are specific receptors for phenylethanolamine in the Tapes heart capable of modulating responses to dopamine and serotonin, and suggests that this biogenic phenethylamine can act as an environmental and physiological factor which may determine how the mollusc heart responds to dopamine.

Serotonin- and dopamine-sensitive adenylate cyclase in molluscan nervous system. Biochemical and electrophysiological analysis of the pharmacological properties and the GTP-dependence

Helix aspersa neuronal cell membranes contain distinct serotonin (5-HT) and dopamine (DA) sensitive adenylate cyclases. We have taken advantage of the fact that in this system, both in vitro (enzymatic assays) and in vivo (electrophysiological measurements) experiments can be used to explore the GTP dependence and the pharmacological properties of this neurotransmitter-sensitive enzyme system. The first property was studied using non-hydrolysable GTP analogs (guanosine 5'-O-(3-thio-triphosphate) or GTP gamma S, and guanosine 5'-imido diphosphate or Gpp(NH)p). In vitro, these two components stimulate the enzyme activity but with different potencies (Kapparent = 10(-8) to 5 X 10(-8) M for GTP gamma S, and 10(-5) M for Gpp(NH)p). Intracellular injections of GTP gamma S, but not of Gpp(NH)p, produced an electrophysiological response similar to the one elicited by 5-HT and DA. These results imply that, even in the presence of the high endogenous GTP concentration normally present in the cell (10(-3) M), GTP gamma S may bind to the GTP-binding protein. Such an interpretation is consistent with the in vitro competition experiments between GTP and GTP gamma S for adenylate cyclase activation. The pharmacology of 5-HT and DA receptors involved in adenylate cyclase stimulation and electrophysiological responses was studied. Serotoninergic antagonists and neuroleptics inhibited the 5-HT-sensitive adenylate cyclase in a stereospecific manner. However, their inhibition was not simply competitive. Our results suggest that they irreversibly bind a component localized on the cytoplasmic side of the membrane. Unexpectedly, the DA receptor coupled with adenylate cyclase was insensitive to any of the several antagonists tested.

The binding of acidic amino acids to snail, Helix aspersa, periesophagic ring membranes reveals a single high-affinity glutamate/kainate site

The characterization of specific acidic amino acid binding sites to snail, Helix aspersa, ganglia membranes has been assayed using tritiated glutamate (L-[3H]Glu), aspartate (L-[3H]Asp), cysteine sulfinate (L-[3H]CSA) and kainate. At 2 degrees C, only L-[3H]Glu and [3H]kainate specific binding could be measured using a filtration procedure to separate bound from free ligand. The analysis of L-[3H]Glu specific binding reveals the presence of one class of high-affinity binding sites with Kd = 0.12 microM and Bmax = 30 pmol/mg protein. This L-[3H]Glu binding was specific, reversible and saturable. The order of potency of different substances, agonists or antagonists of the rat brain excitatory amino acid receptors, has been determined. Kainate was the best displacing agent, followed by ibotenate = L-Glu greater than L-alpha-aminoadipate (L-alpha-AA) greater than homocysteate (HCA). Using 10 nM [3H]kainate, a single class of binding site was detected. Its pharmacological properties indicate that it is likely identical to the L-[3H]Glu binding site. This L-Glu-kainate site possesses most of the properties expected for a specific receptor. However, whereas L-[3H]Glu binding could be detected on purified neuronal membranes, the major component of specifically bound L-[3H]Glu appeared to be located on the sheaths surrounding neuronal cell bodies. These findings suggest that Glu or another endogenous acidic amino acid may function as a transmitter at neuromuscular junctions in Helix periesophagic ring, acting at a receptor distinct from those on nerve cells.

Dopamine receptor stimulation induces a potassium dependent hyperpolarizing response in growth hormone producing neuroendocrine cells of the gastropod mollusc Lymnaea stagnalis

Of several putative transmitters used, dopamine was the only one which caused (at low concentrations) a hyperpolarizing response (H-response) in growth hormone producing cells (GHCs) of the freshwater snail Lymnaea stagnalis. Membrane resistance changes, and shifts in the reversal potential of this H-response in different K+-concentrations, indicate that the response is due to an increase in potassium conductance. The dopamine induced H-response is blocked by (-)-sulpiride, 4-aminopyridine, dibutyryl cAMP, 8CPT-cAMP, forskolin and IBMX. These data suggest that dopamine induces the H-response by stimulating a receptor resembling the mammalian D-2 receptor and that this effect of dopamine is mediated by a decrease in the formation of intracellular cAMP.

Dual effects of the snake venom polypeptide vipoxin on receptors for acetylcholine and biogenic amines in Aplysia neurons

Vipoxin, a 13,000-dalton polypeptide component of Russell's viper venom, has a dual pattern of effects on the responses of voltage-clamped Aplysia neurons to acetylcholine and biogenic amines. Application of low doses of vipoxin by pressure ejection reversibly antagonized all three types of ionic response to acetylcholine and carbachol. The blockade by vipoxin of acetylcholine responses was not prevented by eserine. The order of susceptibility of acetylcholine responses to blockade by vipoxin was Na+ greater than K+ greater than Cl-. Low doses of vipoxin also produced a reversible potentiation of excitatory responses to dopamine with a slower time course of onset and recovery. Inhibitory responses to dopamine (Cl-, K+) and both excitatory and inhibitory responses to histamine and 5-hydroxytryptamine were little affected by vipoxin. Higher doses of vipoxin directly evoked current responses which were always of the same ionic type as that evoked by acetylcholine or carbachol. Responses to cholinergic agonists and vipoxin were both blocked by cholinergic antagonists but not by antagonists of biogenic amine receptors, which reversibly antagonized the responses to amines on the same cell. These results suggest that vipoxin, which has no demonstrated actions on vertebrate acetylcholine receptors, acts as a partial agonist at all three types of acetylcholine receptor in Aplysia neurons. Our observations thus provide evidence for some degree of phylogenetic difference between vertebrate and molluscan acetylcholine receptors.

Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.

Opposing roles for D-1 and D-2 dopamine receptors in regulating the excitability of growth hormone-producing cells in the snail Lymnaea stagnalis

Dopamine hyperpolarizes growth hormone-producing cells (GHC) in the CNS of Lymnaea stagnalis. This effect of dopamine was mimicked by the D-2 receptor agonist LY 141865 and antagonized by the D-2 receptor antagonists (-)-sulpiride and YM 09151-2. SKF 38393, a selective D-1 receptor agonist, increased the excitability of the GHC. This effect was mimicked by intracellular injection of cyclic AMP and antagonized by the D-1 receptor antagonist SCH 23390. Dopamine (in the presence of (-)-sulpiride) also increased the excitability of the GHC. It is concluded that both a D-1 and a D-2 receptor regulate the electrical activity of the GHC in the CNS of Lymnaea stagnalis.