Adenosine 3',5'-monophosphate in snail (Helix pomatia) nervous system: analysis of dopamine receptors

Effects of dopamine on snail neurones

The pharmacological features of dopamine receptors in identifiable giant neurone types of a snail (Achatina fulica Férussac) were studied. Under voltage clamp, two neurone types, LVMN (left ventral multiple spike neurone) and d-RPeAN (dorsal-right pedal anterior neurone), produced an inward current (Iin) in response to dopamine, (-)-noradrenaline and epinine, whereas v-LCDN (ventral-left cerebral distinct neurone) produced an outward current (Iout) in response to dopamine and epinine. Mammalian dopamine receptor agonists, fenoldopam (dopamine D1-like receptor agonist), (+/-)-SKF 38393 (1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8- diol) (D1-like), apomorphine (D2-like), (-)-quinpirole (D3 and D4) and methylergometrine showed slight or no effect. (+/-)-SKF 83566 ((+/-)-7-bromo-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zepine) (dopamine D1-like receptor antagonist) and (+)-UH 232 (cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin) (D3 and D2) non-competitively inhibited the Iin of LVMN and d-RPeAN, but (+/-)-sulpiride (D2-like) was without effect. In contrast, (+/-)-sulpiride competitively inhibited Iout of v-LCDN, (+)-UH 232 non-competitively inhibited Iout of v-LCDN but (+/-)-SKF 83566 was without effect. H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine) (non-selective protein kinase inhibitor) inhibited Iin of LVMN and d-RPeAN, but did not affect Iout of v-LCDN. Dopamine-induced Iin was Na(+)-dependent; Iout was K(+)-dependent. Ouabain did not affect these currents. We propose that the pharmacological features of Achatina neuronal dopamine receptors are not fully comparable to those of mammals, although intracellular signal transduction systems linked with dopamine receptors may similarly exist in different animal species.

DARPP-32 like protein in specific snail (Helix aspersa) neurones

Monoclonal antibodies to DARPP-32 recognise an antigen which is present in specific neurones in the snail (Helix aspersa). Consecutive sections 10-microns-thick processed for the localisation of DARPP-32 and tyrosine-hydroxylase immunoreactivity did not show a coexistence in any neuronal structures. DARPP-32 positive cells were, however, often morphologically closely associated with tyrosine-hydroxylase positive cells, implying a functional relationship consistent with the proposed role of DARPP-32. Immunochemical analysis of the DARPP-32 immunoreactive material in the snail nervous system shows that the substance has a molecular weight of 28 kDa and therefore different from the DARPP-32 protein found in the rat brain.

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.

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.

An analysis of the inhibitory responses of dopamine and octopamine on Helix central neurons

1. Electrophysiological recordings were made from identified neurons in the isolated suboesophageal ganglionic mass of Helix aspersa. Cells were voltage clamped at testing membrane potential. 2. Bath addition of 1 microM dibutyryl cAMP caused a time dependent enhancement of an evoked IPSP and the dopamine (DA) and octopamine (OA) induced outward currents obtained in these neurons. Forskolin, 0.1 microM, which enhances and MDL 12,330A, 0.12 microM, which depresses adenylate cyclase activity also modified these responses. 3. The DA and OA inhibitory responses were both shown to be potassium mediated events. They were preferentially antagonised by low micromolar concentrations of 4-aminopyridine. Two other potassium channel antagonists, tetraethylammonium and apamin had little effect on the DA and OA responses. 4. Cell sensitivity to DA and OA was greatly enhanced in calcium free/2 mM cobalt Ringer. The reversal potential of the DA response was shifted to a more negative value in calcium free Ringer. Sodium free Ringer was also found to alter the responses to DA or OA but those results were not consistent.

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.

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.

Activation by dopamine of patterned motor output from the buccal ganglia of Helisoma trivolvis

The buccal ganglia of the snail, Helisoma trivolvis, contain an intrinsic system of dopamine-containing neurons (Trimble, Barker, and Bullard, 1983). Dopamine, when bath applied to the isolated buccal ganglia, activates patterned motor output in a dose-dependent fashion. Haloperidol blocks the activating effect of dopamine, but the similar activation evoked by serotonin is not blocked by haloperidol. We suggest that there are two separate mechanisms for activating patterned motor output from the buccal ganglia. One is serotonergic, emanating from identified cerebral ganglion cells (Granzow and Kater, 1977), while the other is dopaminergic, involving neurons intrinsic to the buccal ganglia.

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.

Pre- and postsynaptic effects of dopamine antagonists on dopaminergic synaptic transmission in Helix pomatia

The rate of transmitter mobilization in identified dopaminergic synapses was decreased by the dopamine antagonists pimozide, chlorpromazine, haloperidol, cis-flupenthixol, curare, clozapine and high concentrations of ergometrine. The depolarizing postsynaptic potential was inhibited by pimozide, chlorpromazine, haloperidol, cis-flupenthixol, curare, clozapine, (+)-butaclamol and high concentrations of ergometrine. The hyperpolarizing synaptic potential was inhibited by naloxone, methysergide, (+)-butaclamol, haloperidol, 6-hydroxydopamine and low concentrations of ergometrine, while pimozide, cis-flupenthixol, trans-flupenthixol, curare, clozapine, promethazine, chlorpromazine and (-)-butaclamol had no clear effect. The presynaptic receptors involved in modulation of the mobilization rate showed similarities with the dopamine receptors mediating depolarizations. The dopamine antagonists changed dynamics of synaptic transmission.

Action of dextroamphetamine on dopamine sensitive cells in the snail brain

Presynaptic and postsynaptic actions of dextroamphetamine (DEX) were studied on dopamine (DA) sensitive neurons of the subesophageal ganglion of the garden snail Helix aspersa utilizing standard microelectrode techniques. Dextroamphetamine (5.5 X 10(-7)-10(-4)M) produced effects on DA-sensitive neurons similar to that caused by DA (5.5 X 10(-7)-10(-4)M). On cells excited by DA, surfused DEX (5.5 X 10(-7)M) caused an excitation that could be blocked by chlorpromazine (0.5-1 X 10(-6)M) or haloperidol (0.5-1 X 10(-6)M). Elevating the extracellular Mg2+ from 4 to 20 mM reduced the depolarization caused by DEX from 11 to 2.5 mV without affecting the response to DA. The response remaining is attributed to a direct response to DEX on DA receptors. Surfused DEX caused an inhibition of cells inhibited by DA. Both DA and DEX effects were selectively blocked by dihydroergotamine (0.5-1 X 10(-6)M). Elevating the [Mg2+] decreased the hyperpolarization caused by DEX from 11 to 3 mV without affecting the DA response. The effect of elevated magnesium in decreasing responses to surfused DEX suggests that the primary action of DEX is at the nerve terminal to cause DA release. Iontophoretic application of DEX caused minimal excitation or inhibition of DA neurons. This is attributed to the fact that DA receptors at the site of drug application are not associated with synaptic innervation. The response obtained with iontophoretically applied DEX suggest a weak direct action on DA 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.