Commentary on the evolution of transmitters, receptors and ion channels in invertebrates

Management of Achatina fulica (Bowdich, 1822) (Pulmonata: Achatinidae) in lettuce (Lactuca sativa L.)

ABSTRACT: The giant African snail Achatina fulica was introduced in Brazil and since then has become an important pest, because of its resistance to abiotic conditions, hermaphroditism, polyphagia, and absence of natural predators. This study aims to evaluate the control of A. fulica in lettuce, in Alagoas, Brazil. Bioassays for the determination of lethal dose and lethal time to adults of A. fulica and the egg mortality were performed in the laboratory by applying commercial synthetic products, commercial and non-commercial alcoholic botanical extracts on mollusk adults. Additionally, the protein concentration, lipase activity and enzyme acetylcholinesterase (AChE), and butyrylcholinesterase (BuChE), in the stomach, intestine, nervous ganglion and liver were determined. The alcoholic extract of Capsicum frutescens caused higher mortality of A. fulica, and the alcoholic extract of C. frutescens and Piper tuberculatum oil can prevent the hatching of A. fulica. The lipase activity was present and in greater quantities in tissues, stomach, intestine, liver and ganglia of A. fulica, before and after exposure of the alcoholic extract of C. frutescens. The enzymatic activity of BuChE was present in the ganglia and liver of A. fulica, prior to exposure of the alcoholic extract of C. frutescens. The enzymatic activity of AChE was present only in the ganglion and absent in liver of A. fulica, prior to exposure of the alcoholic extract of C. frutescens. The concentration of 10% of the alcoholic extract of C. frutescens caused 84% mortality of adult A. fulica in lettuce in field conditions.

Mechanisms of carbacholine and GABA action on resting membrane potential and Na+/K+-ATPase of Lumbricus terrestris body wall muscles

This work was aimed to identify the action of several ion channel and pump inhibitors as well as nicotinic, GABAergic, purinergic and serotoninergic drugs on the resting membrane potential (RMP) and assess the role of cholinergic and GABAergic sensitivity in earthworm muscle electrogenesis. The nicotinic agonists acetylcholine (ACh), carbacholine (CCh) and nicotine depolarize the RMP at concentrations of 5 μM and higher. The nicotinic antagonists (+)tubocurarine, α-bungarotoxin, muscarinic antagonists atropine and hexamethonium do not remove or prevent the CCh-induced depolarization. Verapamil, tetrodotoxin, removal of Cl(-) and Ca(2+) from the solution also cannot prevent the depolarization by CCh. In a Na(+)-free medium, however, CCh lost this depolarization ability and this indicates that the drug opens the sodium permeable pathway. Serotonin, glutamate, glycine, adenosine triphosphate (ATP) and cis-4-aminocrotonic acid (GABA(C) receptor antagonist) had no effect on the RMP. On the other hand, isoguvacin, γ-aminobutyric acid (GABA) and baclofen (GABA(B) receptor agonist) hyperpolarized the RMP. Ouabain, bicucullin (GABA(A) antagonist) and phaclofen (GABA(B) antagonist), as well as the removal of Cl(-), suppressed the effect of GABA and baclofen. CCh did not enhance the depolarization generated by ouabain but, on the other hand, hindered the hyperpolarizing activity of baclofen both in the absence and presence of atropine and (+)tubocurarine. The long-term application of CCh depolarizes the RMP primarily by inhibiting the Na(+)/K(+)-ATPase. The muscle membrane also contains A and B type GABA binding sites, the activation of which increases the RMP at the expense of increasing the action of ouabain- and Cl(-) -sensitive electrogenic pumps.

Monoamines in the pedal plexus of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata)

In molluscs, the number of peripheral neurons far exceeds those found in the central nervous system. Although previous studies on the morphology of the peripheral nervous system exist, details of its organization remain unknown. Moreover, the foot of the terrestrial species has been studied less than that of the aquatic species. As this knowledge is essential for our experimental model, the pulmonate gastropod Megalobulimus oblongus, the aim of the present study was to investigate monoamines in the pedal plexus of this snail using two procedures: glyoxylic acid histofluorescence to identify monoaminergic structures, and the unlabeled antibody peroxidase anti-peroxidase method using antiserum to detect the serotonergic component of the plexus. Adult land snails weighing 48-80 g, obtained from the counties of Barra do Ribeiro and Charqueadas (RS, Brazil), were utilized. Monoaminergic fibers were detected throughout the pedal musculature. Blue fluorescence (catecholamines, probably dopamine) was observed in nerve branches, pedal and subepithelial plexuses, and in the pedal muscle cells. Yellow fluorescence (serotonin) was only observed in thick nerves and in muscle cells. However, when immunohistochemical methods were used, serotonergic fibers were detected in the pedal nerve branches, the pedal and subepithelial plexuses, the basal and lateral zones of the ventral integument epithelial cells, in the pedal ganglion neurons and beneath the ventral epithelium. These findings suggest catecholaminergic and serotonergic involvement in locomotion and modulation of both the pedal ganglion interneurons and sensory information. Knowledge of monoaminergic distribution in this snail s foot is important for understanding the pharmacological control of reflexive responses and locomotive behavior.

Activating properties of cocaine and cocaethylene in a behavioral preparation of Drosophila melanogaster

The use of Drosophila as a model to study the behavioral consequences of stimulant drugs was analyzed in an active preparation of decapitated Drosophila. Application of cocaine and cocaethylene to discrete nerve cord cells regulating motor programs of behavior produced striking patterns of behavioral activity in a concentration-related manner. In general, intense circling behavior and significant wing buzzing activity were distinguishable behavioral markers in flies treated with mM concentrations of cocaine or cocaethylene. The significant changes in motor behavior induced by stimulant drugs in decapitated flies were not reproduced by the application of apomorphine, a direct dopamine (DA) agonist, or octopamine, a naturally occurring transmitter in arthropods. Because both cocaine and cocaethylene interfere with DA reuptake in mammals, we characterized the role of DA receptors mediating increased stereotypy and motor behavior in flies. Coadministration of SCH-23390, a specific D1 receptor antagonist, significantly attenuated the behavior-activating properties of cocaine and cocaethylene in this active experimental preparation. Therefore, the receptor protein mediating the behavioral responses to stimulant drugs in Drosophila is pharmacologically similar to the mammalian D1 subtype. In rats, cocaine- and cocaethylene-induced behavioral activity is complex, with increasing evidence that the D1 receptor interacts significantly with N-methyl-D-aspartate (NMDA) receptor pathways to produce an altered behavioral phenotype. To further characterize additional receptor subtypes targeted by the actions of cocaine and cocaethylene, we pretreated flies with MK-801 and dextromethorphan. Both of these drugs are potent, selective noncompetitive NMDA receptor antagonists. Interestingly, MK-801 and dextromethorphan profoundly reduced the behavior-activating properties of cocaine and cocaethylene in Drosophila. Therefore, as in rats, the NMDA (and D1) receptor pathways in this arthropod represent obligatory targets for the behavioral effects of stimulant drugs.

Two novel types of L-glutamate receptors with affinities for NMDA and L-cysteine in the olfactory organ of the Caribbean spiny lobster Panulirus argus

A subset of olfactory receptor neurons of the Caribbean spiny lobster Panulirus argus possesses receptors for L-glutamate that can mediate both excitatory and inhibitory responses (P.C. Daniel, M.F. Burgess, C.D. Derby, Responses of olfactory receptor neurons in the spiny lobster to binary mixtures are predictable using a non-competitive model that incorporates excitatory and inhibitory transduction pathways, J. Comp. Physiol. A 178 (1992) 523-536). In this study, we have used biochemical and electrophysiological techniques to understand the role of these receptors in olfactory transduction, and to compare these olfactory glutamate receptors with peripheral and central L-glutamate receptors in other animals. Using a radioligand-binding assay with a membrane-rich preparation from the dendrites of olfactory receptor neurons, we have identified two types of binding sites for L-glutamate. Both sites showed rapid, reversible, and saturable association with radiolabeled L-glutamate, and their Kd values (1 nM and 3 microM) are effective in physiological studies of glutamate-sensitive olfactory neurons, suggesting these binding sites are receptors involved in olfactory transduction. Both sites were completely inhibited by high concentrations of NMDA and L-cysteine, and only partially inhibited by other L-glutamate analogs and odorants. Electrophysiological recordings from L-glutamate-best olfactory receptor neurons showed that NMDA and L-cysteine are both partial agonists and antagonists of glutamate receptors. Together, these results suggest the olfactory L-glutamate receptors of spiny lobsters are novel types of L-glutamate receptors that are functionally important in mediating olfactory responses.

Ionotropic GABA receptor from lobster olfactory projection neurons

This study reports an ionotropic GABA (gamma-aminobutyric acid) receptor in projection neurons acutely dissociated from the olfactory lobe of the brain of the spiny lobster and analyzed by whole cell and cell-free patch-clamp recording. GABA evokes a macroscopic current in the cells that is linear from -100 to + 100 mV, reverses at the imposed chloride equilibrium potential, has a permeability sequence of Cl- > acetate > bicarbonate > phosphate > propionate and SCN- > Br- > I- > Cl- > F-, and is reversibly blocked by the Cl channel blocker picrotoxin but not tert-butylbicyclophosphorothionate (TBPS). The current is bicuculline insensitive and activated by muscimol, isoguvacine, cis-4-aminocrotonic acid (CACA), and trans-aminocrotonic acid (TACA), as well as by the GABA(C)-receptor antagonists 4,5,6,7-tetrahydroisoxazolo [5,4,-c]pyridin-3-ol (THIP), 3-amino-1-propanesulfonic acid (3-APS), and imidazole-4-acetic acid (I-4AA), but not the GABA(B)-receptor agonists baclofen and 3-aminopropylphosphonic acid (3-APA). Agonist potency for the receptor is TACA > muscimol > GABA > I-4AA > isoguvacine > 3-APS > CACA > THIP. Unitary chloride currents in cell-free, outside-out patches from the cells share enough of these pharmacological properties to indicate that the channel underlies the macroscopic current. The receptor mediates an inhibitory current in the cells in vivo. The receptor is similar, if not identical, to one from neurons cultured from the thoracic ganglia of the clawed lobster. The more extensive pharmacological characterization of the receptor reported here indicates that this lobster CNS receptor is pharmacologically distinct from previously characterized ionotropic GABA receptors.

Dopaminergic modulation of spontaneous activity in the brain of the crayfishCherax destructor(Decapoda, Crustacea)

The role of dopamine in the crayfish (Cherax destructor) brain was studied in an isolated head preparation by means of extracellular recordings from unidentified neurons located in the circumoesophageal connectives. Perfusion of dopamine and the dopamine receptor agonist (±)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide into the brain induced excitatory as well as inhibitory modulation of spontaneous activity. The physiological effects of both drugs were reversible and showed concentration dependency in the concentration range 10−7to 10−3 M. Two vertebrate-derived dopamine receptor antagonists, chlorpromazine and fluphenazine, reversibly blocked the action of dopamine (10−5 M). The threshold for antagonistic blockade by chlorpromazine and fluphenazine occurred at a relatively low concentration (10−7 M) and was concentration dependent. These data collectively suggest that dopamine plays a physiological role in the crayfish brain, stimulating putative dopamine receptor(s) to alter neuronal activity.

Cloning and functional characterization of a novel dopamine receptor from Drosophila melanogaster

A cDNA clone is described that encodes a novel G-protein-coupled dopamine receptor (DopR99B) expressed in Drosophila heads. The DopR99B receptor maps to 99B3-5, close to the position of the octopamine/tyramine receptor gene at 99A10-B1, suggesting that the two may be related through a gene duplication. Agonist stimulation of DopR99B receptors expressed in Xenopus oocytes increased intracellular Ca2+ levels monitored as changes in an endogenous inward Ca2+-dependent chloride current. In addition to initiating this intracellular Ca2+ signal, stimulation of DopR99B increased cAMP levels. The rank order of potency of agonists in stimulating the chloride current is: dopamine > norepinephrine > epinephrine > tyramine. Octopamine and 5-hydroxytryptamine are not active (< 100 microM). This pharmacological profile plus the second-messenger coupling pattern suggest that the DopR99B receptor is a D1-like dopamine receptor. However, the hydrophobic core region of the DopR99B receptor shows almost equal amino acid sequence identity (40-48%) with vertebrate serotonergic, alpha 1- and beta-adrenergic, and D1-like and D2-like dopaminergic receptors. Thus, this Drosophila receptor defines a novel structural class of dopamine receptors. Because DopR99B is the second dopamine receptor cloned from Drosophila, this work establishes dopamine receptor diversity in a system amenable to genetic dissection.

Cholinergic inhibition of muscle fibres isolated from Schistosoma mansoni (Trematoda:Digenea)

Cholinergic compounds inhibit FMRFamide-induced contractions in dispersed muscle fibres isolated from adult Schistosoma mansoni. Acetylcholine (ACh) was the most effective cholinergic agonist tested with an EC50 < 100 nM. Less effective were propionylcholine and arecoline with EC50 < 1 microM and butyrylcholine and carbachol with EC50 < 10 microM. Choline, muscarine, pilocarpine, nicotine, DMPP (1,1-dimethylphenylpiperazine) and levamisole were all ineffective. Amongst tested antagonists, d-tubocurarine (100 microM), mecamylamine (1 mM), scopolamine (1 mM) and quinuclidinyl benzilate (10 microM) were all ineffective. Bicuculline, picrotoxin and strychnine were also ineffective. However alpha-bungarotoxin, at 100 nM, was able to block the inhibitory ACh effect. From these data it appears that the cholinergic receptor on the schistosome muscle fibres may be of the nicotinic type, but that its pharmacology is different from that of nicotinic receptors of vertebrates as well as of nematodes or a variety of other invertebrates.

Immunocytochemical localization of glutamate-like immunoreactivity within the nervous system of the cestode Mesocestoides corti and the trematode Fasciola hepatica

The localization and distribution of glutamate-like immunoreactivity (IR) in the nervous system of both the cestode Mesocestoides corti and the trematode Fasciola hepatica has been determined by an indirect immunofluorescent technique, in conjunction with confocal scanning laser microscopy (CSLM). Immunostaining was widespread in the central (CNS) and peripheral (PNS) nervous systems of both species examined. In the CNS, IR was evident in nerve cells and fibres in the cerebral ganglia, the cerebral commissure and the dorsal, ventral and longitudinal nerve cords. In the peripheral nervous system (PNS) of M. corti, IR was apparent in nerve plexuses associated with the subtegmental musculature and the musculature associated with the anteriorly positioned suckers. In F. hepatica, IR was evident in the innervation of both the oral and the ventral suckers. In the reproductive system of F. hepatica, glutamate-IR was observed around the ootype/Mehlis' gland complex.

Neuromuscular physiology and pharmacology of parasitic flatworms

The trematode and cestode flatworms include numerous parasitic forms of major medical and economic importance. A better knowledge of the neuromuscular physiology of these animals could lead to development of new control measures against these parasites. Since these animals are near the stem from which all other animals have evolved, better knowledge of these animals could also yield valuable information about the early evolution of nerve and muscle systems in the animal kingdom. This review focuses on what is known about the characteristics of the somatic muscle in these animals. The anatomy of the muscles is described along with a review of current information about their electrophysiology, including descriptions of the ion channels present. Also included is a summary of recently acquired data concerning the nature of serotonin, peptide, acetylcholine and glutamate receptors on the membranes of the muscles.

The effect of L-glutamate on the afferent resting activity in the cephalopod statocyst

The effects of bath application of L-glutamate and of excitatory amino acid agonists and antagonists on the resting activity of afferent crista fibers were studied in isolated preparations of the statocyst of the cuttlefish, Sepia officinalis. L-Glutamate (threshold 10(-5) M) and its agonists quisqualate and kainate (thresholds 10(-6) M) increased the resting activity in a dose-dependent manner. Glutamine (threshold 10(-5) M) was also excitatory, while D-glutamate had no effect. Also, no obvious excitatory effects were seen for NMDA and L-aspartate, nor was any antagonistic effect seen for the selective NMDA-receptor antagonist D-2-amino-5-phosphonovaleric acid (D-AP-5). The spider toxin Argiotoxin636 (threshold 10(-11) M), 2-amino-4-phosphonobutyric acid (AP-4), glutamic acid diethyl ester (GDEE), gamma-D-glutamylaminomethyl-sulfonic acid (GAMS), and kynurenic acid decreased the resting activity and effectively blocked or reversed the effect of L-glutamate and its non-NMDA agonists. Preliminary experiments with statocysts from the squid Sepioteuthis lessoniana and the octopod Octopus bimaculoides gave comparable results. All data show that in cephalopod statocysts L-glutamate, via non-NMDA receptors, has an excitatory effect on the activity of afferent fibers, an effect consistent with its possible function as a hair cell transmitter.

Serotonin and neuropeptide immunoreactivities in the intramolluscan stages of three marine trematode parasites

Using an indirect immunofluorescence technique interfaced with confocal scanning laser microscopy, whole-mount preparations of three genera of marine trematode larvae, Cryptocotyle lingua, Cercaria emasculans and Himasthla leptosoma, were screened for 5-hydroxytryptamine (5-HT) and selected neuropeptide immunoreactivities (IRs). IRs for pancreatic polypeptide (PP), peptide YY (PYY) and FMRFamide were found in the central nervous systems of the three species of cercariae, immunostaining the paired ganglia and central commissure and the longitudinal nerve cords, with slight differences in both distribution and intensity of IRs being observed for the different antisera used. PP, PYY and FMRFamide IRs were evident in both central and peripheral components of the nervous system in the rediae of C. lingua. 5-HT IR was confined to the peripheral nervous systems of the cercariae of C. emasculans and the rediae of C. lingua, appearing in the form of a network of immunoreactive fibres and associated large cell bodies. A moderate substance P IR was observed in the nervous system of the cercariae of C. lingua. The patterns of immunostaining described were compared with those obtained using antiserum directed to the C-terminal decapeptide amide of neuropeptide F (NPF), a native parasitic peptide from the cestode Moniezia expansa. Results demonstrated that serotoninergic and peptidergic components were present in the nervous systems of all of the trematode larvae studied and that some, if not all, of the IR for PP, PYY and FMRFamide was due to the presence of a trematode NPF homologue.

Hidden Markov Models of the G-protein-coupled receptor family

Hidden Markov Model techniques are used to derive a new model of the G-protein-coupled receptor family. The transition and emission parameters of the model are adjusted using a training set comprising 142 sequences. The resulting model is shown to perform well on a number of tasks, including multiple alignments, discrimination, large data base searches, classification, and fragment detection. General analytical results on the expectation and standard deviation of the likelihood of random sequences are also presented.

GABA receptor molecules of insects

Receptors for 4-aminobutyric acid (GABA) have been identified in both central and peripheral nervous systems of several invertebrate phyla. To date, much of the information derived from physiological and biochemical studies on insect GABA receptors relates to GABA-gated chloride channels that show some similarities with vertebrate GABAA receptors. Like their vertebrate central nervous system (CNS) counterparts, agonist activation of such insect GABA receptors leads to a rapid, picrotoxin-sensitive increase in chloride ion conductance across the cell membrane. In insects, responses to GABA can be modulated by certain benzodiazepines and barbiturates. However, recent studies have detected a number of striking pharmacological differences between GABA-gated chloride channels of insects and vertebrates. Receptor binding, electrophysiological and 36Cl- flux assays have indicated that many insect receptors of this type are insensitive to the vertebrate GABAA antagonists bicuculline and pitrazepin. Benzodiazepine binding sites coupled to insect GABA receptors display a pharmacological profile distinct from that of corresponding sites in vertebrate CNS. Receptor binding studies have also demonstrated differences between convulsant binding sites of insect and vertebrate receptors. Insect GABA receptor molecules are important target sites for several chemically-distinct classes of insecticidally-active molecules. By characterizing these pharmacological properties in detail, it may prove possible to exploit differences between vertebrate and insect GABA receptors in the rational design of novel, more selective pest control agents. The recent application of the powerful techniques of molecular biology has revealed a diversity of vertebrate GABAA receptor subunits and their respective isoforms that can assemble in vivo to form a multiplicity of receptor subtypes. Molecular cloning of insect GABA receptor subunits will not only enhance our understanding of invertebrate neurotransmitter receptor diversity but will also permit the precise identification of the sites of action of pest control agents.

Adenylate cyclase from sea urchin eggs is positively and negatively regulated by D-1 and D-2 dopamine receptors

The adenylate cyclase present in membranes prepared from sea urchin eggs is sensitive to dopamine stimulation. The receptor sites coupled to sea urchin adenylate cyclase were characterized by means of specific agonists and antagonists. The D-1 dopamine agonist SKF-38393 was able to stimulate enzyme activity, while the two D-1 dopamine antagonists, SCH-23390 and SKF-83566, suppressed the stimulatory effect of dopamine. In addition, the D-2 dopamine agonists, PPHT and metergoline, brought about a dose-dependent inhibition of dopamine-stimulated adenylate cyclase activity. These data show that: (i) in sea urchin eggs adenylate cyclase is regulated by dopamine receptors; (ii) these receptors share characteristics with D-1 and D-2 dopamine receptors present in the mammalian brain.

Sequence alignment of the G-protein coupled receptor superfamily

The multitude of G-protein coupled receptor (GPR) superfamily cDNAs recently isolated has exceeded the number of receptor subtypes anticipated by pharmacological studies. Analysis of the sequence similarities and unique features of the members of this family is valuable for designing strategies to isolate related cDNAs, for developing hypotheses concerning substrate-ligand and receptor-effector interactions, and for understanding the evolution of these genes. We have compiled and aligned the 74 unique amino acid sequences published to date and review the present understanding of the structural motifs contributing to ligand binding and G-protein coupling.

Evolutionary aspects of transmitter molecules, their receptors and channels

Classical transmitters are present in all phyla that have been studied; however, our detailed understanding of the process of neurotransmission in these phyla is patchy and has centred on those neurotransmitter receptor mechanisms which are amenable to study with the tools available at the time, for example, high-affinity ligands, tissues with high density of receptor protein, suitable electrophysiological recording systems. Studies also clearly show that many neurones exhibit co-localization of classical transmitters and neuropeptides. However, the physiological implications of this co-localization have yet to be elucidated in the vast majority of examples. The application of molecular biological techniques to the study of neurotransmitter receptors (to date mainly in vertebrates) is contributing to our understanding of the evolution of these proteins. Striking similarities in the structure of ligand-gated receptors have been revealed. Thus, although ligand-gated receptors differ markedly in terms of the endogenous ligands they recognize and the ion channels that they gate, the structural similarities suggest a strong evolutionary relationship. Pharmacological differences also exist between receptors that recognize the same neurotransmitter but in different phyla, and this may also be exploited to further the understanding of structure-function relationships for receptors. Thus, for instance, some invertebrate GABA receptors are similar to mammalian GABAA receptors but lack a modulatory site operated by benzodiazepines. Knowledge of the structure and subunit composition of these receptors and comparison with those that have already been elucidated for the mammalian nervous system might indicate the functional importance of certain amino acid residues or receptor subunits. These differences could also be exploited in the development of new agents to control agrochemical pests and parasites of medical importance. The study of the pharmacology of receptor proteins for neurotransmitters in invertebrates, together with the application of biochemical and molecular biological techniques to elucidate the structure of these molecules, is now gathering momentum. For certain receptors, e.g. the nicotinic receptor, we can expect to have fundamental information on the function of this receptor at the molecular level in both invertebrates and vertebrates in the near future.

The evolution of hormonal signalling systems

1. A comparative analysis was made of chemosignalling systems responsible for the action of hormones, hormone-like substances, pheromones, etc. in vertebrates--multicellular invertebrates--unicellular eukaryotes. Many common features revealed in structural-functional organization of the above systems give evidence of their evolutionary conservatism. 2. It was shown that some molecular components as well as signal transduction mechanisms similar to those of higher eukaryote hormonal signalling systems are present in such early organisms as bacteria. This allowed a suggestion that the roots of chemosignalling systems are likely to be found in prokaryotes. 3. The evolution of hormonal signalling systems is discussed in terms of current theories of the origin of eukaryotic cell, its organelles and components. A hypothesis is put forward about endosymbiotic genesis of these signal transduction systems in eukaryotes. 4. A possible evolutionary scenario of the formation of hormonocompetent systems is proposed with hormone-sensitive adenylate cyclase complex taken as an example.

Role of regulatory peptides in parasitic platyhelminths and their vertebrate hosts: possible novel factors in host-parasite interactions

The study of regulatory peptides has its origins in the classical work of Bayliss & Starling (1902). Their pioneering work on the presence of a factor in intestinal extracts which, when injected into the bloodstream of experimental animals, elicited pancreatic secretion, led to the genesis of the concept of the hormone, i.e. a chemical messenger which is released from one part of the body in response to a stimulus to travel in the bloodstream to a distant target tissue where it would elicit a physiological response appropriate to the original stimulus. In keeping with accepted scientific tradition, this concept had its critics. Pavlov, who had been studying secretory stimulation from a different perspective, concluded from his work on salivation in dogs, that this was mediated via neural pathways. With hindsight, and the benefits of knowledge obtained from nearly a century of scientific research, we now know that these pioneers were in actual fact studying different aspects of the same process and that both theories were complementary. In fact, it is becoming increasingly difficult to ascribe secretory control to either circulating or neuronal factors as both appear to be intimately involved in regulation.

The effect of various neurotransmitters and some of their agonists and antagonists on the crayfish abdominal positioning system

1. Crayfish abdominal nerve cords were perfused with selected transmitters or their agonists or antagonists. Motor activity underlying abdominal positioning behavior was monitored. 2. All the neurotransmitters except glycine had a measurable effect on this system. 3. Acetylcholine and its agonists were slightly stimulatory. Both muscarinic and nicotinic receptors were indicated. 4. GABA was weakly inhibitory. Picrotoxin was strongly stimulatory, perhaps as a result of its known ability to block GABA and inhibitory acetylcholine receptors. 5. Histamine was strongly inhibitory. Both H1 and H2 receptors were indicated. 6. Glutamate was found to be slightly inhibitory while its agonist, NMDA, showed no effect. 7. Finally, L-Dopa was stimulatory, but only at a high concentration.

The serotoninergic, cholinergic and peptidergic components of the nervous system in the monogenean parasite, Diclidophora merlangi: a cytochemical study

Confocal scanning laser microscopy has been employed with immunocytochemical techniques to map the distribution of serotoninergic and peptidergic components in the nervous system of the monogenean gill-parasite, Diclidophora merlangi; results are compared with the distribution of cholinergic components, following histochemical staining for cholinesterase activity. While all three neurochemical elements are present in the central and peripheral nervous systems, the cholinergic and peptidergic systems dominate the CNS, whereas the PNS has a majority of serotoninergic nerve fibres. The cholinergic and peptidergic neuronal pathways overlap extensively in staining patterns, suggesting possible co-localization of acetylcholine and neuropeptides. Within the peptidergic nervous system, immunoreactivity to the pancreatic polypeptide family of peptides and FMRFamide were the most prevalent. Gastrin/cholecystokinin (CCK)-, neuropeptide Y-, substance P-, neurokinin A- and eledoisin-like immunoreactivities have been demonstrated for the first time in a monogenean parasite. The gastrin/CCK- and tachykinin-like immunoreactivities had an apparently restricted distribution in the worm.

Cloning, localization, and permanent expression of a Drosophila octopamine receptor

A cDNA for a member of the G protein-coupled receptor family was isolated from Drosophila using a probe derived from a human beta 2-adrenergic receptor cDNA. This Drosophila receptor gene is localized at 99A10-B1 on the right arm of chromosome 3 and is preferentially expressed in Drosophila heads. The insect octopamine receptor has been permanently expressed in mammalian cells, where it mediates the attenuation of adenylate cyclase activity and exhibits a pharmacological profile consistent with an octopamine type 1 receptor. Sequence and pharmacological comparisons indicate that the octopamine receptor is unique but closely related to mammalian adrenergic receptors, perhaps as an evolutionary precursor.

A cytochemical study of the serotoninergic, cholinergic and peptidergic components of the reproductive system in the monogenean parasite, Diclidophora merlangi

The reproductive system of the monogenean gill parasite, Diclidophora merlangi, was examined for the presence of cholinergic, serotoninergic and peptidergic innervation using cytochemical and immunocytochemical techniques. Cholinesterase activity and 5-hydroxytryptamine immunoreactivity (5-HT-IR) were confined to neural elements of the male reproductive system, being evident in the innervation of the cirrus, whereas only 5-HT was present in nerves and somata of the elongate seminal vesicle. Peptidergic innervation was localised to both the male and female reproductive systems of the worm. Within the female reproductive apparatus pancreatic polypeptide, peptide tyrosine tyrosine, neuropeptide Y, substance P, neurokinin A, eledoisin, FMRFamide and gastrin/cholecystokinin immunoreactive fibres and somata were observed in the oviduct, vitelline reservoir and ovovitelline duct. Intense peptide immunoreactivity was identified in fibres in the wall of the ootype and in a surrounding population (greater than 100) of somata that were situated beyond Mehlis' gland cells and all of which were connected to the ootype wall by fine cytoplasmic connectives. The strategic location of this peptidergic cell population infers its involvement in the egg-forming sequence in this platyhelminth parasite.

GABA receptors on the somatic muscle cells of the parasitic nematode, Ascaris suum: stereoselectivity indicates similarity to a GABAA-type agonist recognition site

1. The gamma-aminobutyric acid (GABA) receptors on the somatic muscle cells of Ascaris, which mediate muscle cell hyperpolarization and relaxation, have been characterized by use of intracellular recording techniques. 2. These receptors are like mammalian GABAA-receptors in that the response is mediated by an increase conductance to chloride ions. The GABAA-mimetic, muscimol, has a relative potency of 0.40 +/- 0.02 (n = 3) compared to GABA. 3. The stereoselectivity of the GABA receptor on Ascaris is identical to that for the mammalian GABAA-receptor, as determined from the relative potency of three pairs of enantiomers of structural analogues of GABA. 4. The most potent agonist is (S)-(+)-dihydromuscimol which is 7.53 +/- 0.98 (n = 5) times more potent than GABA. 5. The Ascaris GABA receptor is not significantly blocked, at concentrations below 100 microM by the potent, competitive GABAA-receptor antagonist, SR95531. 6. The Ascaris GABA receptor does not recognise agents that are known to block the GABA gated chloride channel in mammalian preparations such as t-butylbicyclophosphorothionate (TBPS, 10 microM, n = 2) or the insecticide dieldrin (100 microM, n = 3). 7. GABAergic responses in Ascaris are not potentiated by pentobarbitone (100 microM, n = 3) or flurazepam (100 microM, n = 3). 8. The potencies of various GABA-mimetics in the Ascaris preparation have been compared with their potency at displacing GABAA-receptor binding in mammalian brain. Excluding the sulphonic acid derivatives of GABA, the correlation coefficient (r) between the potencies of compounds in the two systems is 0.74 (P less than 0.01). The significance of this correlation is discussed. 9. The pharmacology of the Ascaris GABA receptor is discussed in relation to other invertebrate systems and the mammalian subclassification of GABA receptors.