The roles of putative neurotransmitters and neuromodulators in annelids and related invertebrates

The velvet worm brain unveils homologies and evolutionary novelties across panarthropods

Background

The evolution of the brain and its major neuropils in Panarthropoda (comprising Arthropoda, Tardigrada and Onychophora) remains enigmatic. As one of the closest relatives of arthropods, onychophorans are regarded as indispensable for a broad understanding of the evolution of panarthropod organ systems, including the brain, whose anatomical and functional organisation is often used to gain insights into evolutionary relations. However, while numerous recent studies have clarified the organisation of many arthropod nervous systems, a detailed investigation of the onychophoran brain with current state-of-the-art approaches is lacking, and further inconsistencies in nomenclature and interpretation hamper its understanding. To clarify the origins and homology of cerebral structures across panarthropods, we analysed the brain architecture in the onychophoran Euperipatoides rowelli by combining X-ray micro-computed tomography, histology, immunohistochemistry, confocal microscopy, and three-dimensional reconstruction.

Results

Here, we use this detailed information to generate a consistent glossary for neuroanatomical studies of Onychophora. In addition, we report novel cerebral structures, provide novel details on previously known brain areas, and characterise further structures and neuropils in order to improve the reproducibility of neuroanatomical observations. Our findings support homology of mushroom bodies and central bodies in onychophorans and arthropods. Their antennal nerve cords and olfactory lobes most likely evolved independently. In contrast to previous reports, we found no evidence for second-order visual neuropils, or a frontal ganglion in the velvet worm brain.

Conclusion

We imaged the velvet worm nervous system at an unprecedented level of detail and compiled a comprehensive glossary of known and previously uncharacterised neuroanatomical structures to provide an in-depth characterisation of the onychophoran brain architecture. We expect that our data will improve the reproducibility and comparability of future neuroanatomical studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01196-w.

Unexpected diversity of bioluminescence in planktonic worms

The vast majority of pelagic bioluminescent organisms emit a blue light with emission maxima (λ_max ) ranging from 450 to 490 nm. Among the known outliers, the tomopterids (Annelida: Polychaeta) are usually described as yellow-emitters (λ_max  = 565-570 nm) for which bioluminescence functions as a specific recognition signal. Here, we report the first data regarding the colours emitted by four different tomopterid species, Tomopteris pacifica, T. carpenteri, T. septentrionalis and T. planktonis. Surprisingly, T. planktonis is a blue-emitter (λ_max  = 450 nm). Our pharmacological results on T. planktonis support cholinergic control, as recently demonstrated in the yellow-emitter, T. helgolandica. Moreover, as revealed by epifluorescence microscopy, the light seems to be produced in both species from the same yellow-pigmented parapodial glands. Despite these similarities, tomopterids express an unexpected diversity of bioluminescent colour patterns. This leads us to reassess the ecological value of bioluminescence within this group.

Physiological control of bioluminescence in a deep-sea planktonic worm, Tomopteris helgolandica Greeff, 1879

Tomopteris helgolandica Greeff 1879 (Tomopteridae) is a transparent holoplanktonic polychaete that can emit a bright light. In this work, we investigated the emission pattern and control of this deep-sea worm's luminescence. Potassium chloride depolarisation applied on anesthetised specimens triggered a maximal yellow light emission from specific parapodial sites, suggesting that a nervous control pathway was involved. A pharmacological screening revealed a sensitivity to carbachol, which was confirmed by a dose-light response associated with a change in the light emission pattern, where physiological carbachol concentrations induced flashes and higher concentrations induced glows. The light response induced by its hydrolysable agonist, acetylcholine, was significantly weaker but was facilitated by eserine pretreatment. In addition, a specific inhibitory effect of tubocurarine was observed on carbachol-induced emission. Lastly, KCl- and carbachol-induced light responses were significantly reduced when preparations were pre-incubated in Ca2+-free artificial sea water or in different calcium channel blockers (verapamil, diltiazem) and calmodulin inhibitor (trifluoperazine) solutions. All of these results strongly suggest that T. helgolandica produces its light flashes via activating nicotinic cholinergic receptors and a calcium-dependent intracellular mechanism involving L-type calcium channels.

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.

Correlative morphometric and electrochemical measurements of serotonin content in earthworm muscles

Distribution of serotonin (5-HT) content of nervous fibers in both the somatic and the visceral muscle of Eisenia fetida have been investigated using immunocytochemical staining and voltammetric measurements. The somatic muscles in the body wall are richer innervated with serotoninergic fibers than the visceral ones in the pharynx and gizzard. The relative density of immunopositive fibers in the circular muscle layer of the body wall was found to be 2.73% while in the prostomium it was 1.02%. In the case of the muscle in pharynx 1.12% and in gizzard 1.28% density values were found. Differential Pulse Voltammetric (DPV) measurements with carbon fiber electrodes in the above mentioned muscle layers gave 272.5 nA, 135.0 nA, 122.5 nA, 137.5 nA peak heights, respectively. In the statistical analysis T-test was used at a confidence level of 95% (p<0.05). DPV current peak (i(p)) values reflect clearly the 5-HT concentration differences. Significant correlation was found between the innervation density and the i(p) values recorded in different areas. The i(p) values recorded at different times in different locations are determined by instantaneous serotonin concentration of the living tissue. As far as we know this is the first report using in vivo voltammetry investigating serotonin content in earthworm, E. fetida.

Embryogenesis of the serotonergic system in the earthwormEisenia fetida (Annelida, Oligochaeta): Immunohistochemical and biochemical studies

Organization of the serotonergic system and changes of the serotonin (5-HT) content were studied during the embryogenesis of the earthworm Eisenia fetida, using immunocytochemistry and HPLC. A gradual emergence of 5-HT immunoreactive (IR) cells and their axon projections in the several ganglia of the central (CNS) and peripheral nervous system are described in the context of a staged time-scale of development. The first 5-HT-IR neurons appear in the subesophageal ganglion at an early embryonic stage (E2), followed by neurons in some rostrally located ventral ganglia. In the cerebral ganglion, 5-HT-IR cells can be detected only from stage E5. The number of labeled cells in each ganglion of the embryo increases until hatching, when it is still considerably lower than that observed in adults. This shows that the development of the 5-HTergic system is far from complete by the end of embryogenesis. Organization of 5-HT-IR innervation of the body wall starts by stages E3 to E4. In the stomatogastric nervous system the first 5-HT-IR fibers can be detected by stage E5. By stage E9 5-HT immunopositive neurons can be observed in both the stomatogastric ganglia and the enteric plexus. Both 5-HT levels and the numbers of the labeled cells show a significant increase before hatching, which indicate a functional maturation of the 5-HTergic system. Based on the early appearance of 5-HT, we suppose that it may play a regulatory role in both the gangliogenesis and the maturation of peripheral functions necessary during postembryonic life.

Neurochemical characterization of nervous elements innervating the body wall of earthworms (Lumbricus, Eisenia): immunohistochemical and pharmacological studies

The distribution and chemical neuroanatomy of nervous elements and certain pharmacological-physiological characteristics of the innervation of the body wall in earthworms are described. Solitary sensory bipolar cells can be found among the epithelial cells. These bipolar cells contain serotonin, tyrosine hydroxylase, histamine, gamma-amino-butyric acid (GABA), Eisenia tetradecapeptide, proctolin or rhodopsin in various combinations. In the body wall, the plexus sub-muscularis is composed of nerve fibres only, whereas the plexus sub-epithelialis and muscularis also contain solitary nerve cells. These cells display histamine, GABA or neuropeptide Y immunoreactivity. The fibres of the three plexuses are reactive to serotonin, histamine, Eisenia tetradecapeptide, proctolin, GABA and neuropeptide Y antibodies. FMRFamide-immunoreactive fibres of the plexus muscularis originate from the central nervous system, whereas axons containing the other studied molecules are derived from both peripheral and central structures. High pressure liquid chromatography assays have revealed serotonin, dopamine and histamine in the body wall. Contractions of the body wall musculature can be elicited with serotonin and FMRFamide. Serotonin-evoked contractions are suppressed by the application of GABA. Serotonin acts both directly on the muscle cell receptors and indirectly through initiating transmitter release from the nervous elements, whereas the FMRFamide-induced contractions seem to be mediated through the muscle cell receptors only. The pharmacological profiles of the serotonin and GABA receptors resemble those of the vertebrate 5-HT(3) and GABA(B) receptor types. Our findings indicate that both the sensory and efferent system of the annelid body wall operate by means of a variety of neuroactive compounds, suggesting a complex role of signalling systems in the regulation of this organ.

Reorganization of the GABAergic system following brain extirpation in the earthworm (Eisenia fetida, Annelida, Oligochaeta)

The reorganization of the GABAergic system was studied by means of immunohistochemistry after the symmetrical and asymmetrical (unilateral) extirpation of the brain of the annelid Eisenia fetida. GABA-immunoreactive neurons were first observed in the wound tissue on the 3rd postoperative day. Thereafter the number of labelled cells gradually increased, and by postoperative days 76-80 all GABA-immunoreactive cells (approx. 140 neurons) could be found in their final positions in the symmetrically regenerated brain. After asymmetrical brain extirpation, nearly all cells (70-75) could be detected in the regenerating hemisphere by postoperative days 50-56. In the early stages of the asymmetrical regeneration of the brain, more GABAergic cells were concentrated dorsally and laterally in the preganglion than during the symmetrical type of regeneration. In both types of regeneration, the immunoreactive neurons in the regenerated brain originated in part from undifferentiated neuroblasts situated in different parts of the body, and in part from dividing neurons localized mainly in the pharyngeal nerve plexus. Both exogenous GABA and picrotoxin, applied during the early stages (days 10- 12) of brain regeneration, inhibited the development of the wound tissue and the migration of the neuroblasts and the enteric neurons. At the same time, exogenous GABA application accelerated the proliferation of the pharyngeal neurons. No effect on the process of regeneration could be demonstrated when exogenous GABA and picrotoxin were given together.

Tissue distribution of PACAP27 and -38 in oligochaeta: PACAP27 is the predominant form in the nervous system of Lumbricus polyphemus

The levels of pituitary adenylate cyclase-activating polypeptide (PACAP)27 and -38 were measured in the nervous, intestinal, excretory, and reproductive systems of Lumbricus polyphemus by radioimmunoassay. Although both PACAP27 and -38 were significantly detectable in all of the examined tissues, the distribution of the peptides was very heterogeneous. Their highest concentrations were found in the cerebral ganglia and the ventral cord, followed by the alimentary tract and the nephridial system, respectively. Moreover, the reproductive system also contained a substantial amount of PACAP. The dominant form of the peptide discovered in the majority of tissues was PACAP27. Interestingly, about 10 times more PACAP27 than PACAP38 was found, with the latter representing only a fraction of PACAP-like immunoreactivity in the tissues of Lumbricus polyphemus.

Pharmacological identification of acetylcholine receptor subtypes in echinoderm smooth muscle (Sclerodactyla briareus)

Contractions of an echinoderm (sp. Sclerodactyla briareus) smooth muscle, the longitudinal muscle of the body wall (LMBW), were evoked by acetylcholine (ACh) and agonists: epibatidine, muscarine and nicotine (in order of force generation: ACh>muscarine=epibatidine>nicotine). ACh-induced contractions were blocked by atropine by 50%, and methoctramine, by 30%. ACh responses were also blocked by 25% by methyllycaconitine (MLA) but not by D-tubocurarine (dTC). Muscarine initiated large contractions that were completely blocked by atropine. To elucidate possible muscarinic ACh receptor (mAChR) subtypes, muscarinic agonists (oxotremorine, pilocarpine) and antagonists (methoctramine, pirenzepine) were tested. Oxotremorine, pilocarpine, and pirenzepine each enhanced resting tonus and potentiated ACh-induced contractions (order of potency: pilocarpine>oxotremorine=pirenzepine). Muscarine, oxotremorine or pirenzepine generated phasic, rhythmic contractions. Nicotine-induced contractions were almost completely blocked by dTC but were not altered by atropine. Large contractions evoked by epibatidine were potentiated by dTC whereas atropine had no effect on them. MLA blocked spontaneous rhythmicity. Cholinesterase inhibitors, neostigmine or physostigmine, caused marked potentiation of ACh-induced contractions and initiated rhythmic slow wave contractions in previously quiescent muscles. The present pharmacological evidence points to the co-existence of excitatory nicotinic ACh receptor (nAChRs) and mAChRs where nAChRs possibly modulate tone, and the mAChRs initiate and enhance rhythmicity.

Octopamine-containing neurons in the alimentary tract of the earthworm (Eisenia fetida)

Octopamine-containing nerve cells have been demonstrated in the enteric plexus of the earthworm (Eisenia fetida), applying immunocytochemistry and HPLC assay. A few octopamine-immunoreactive neurons occurred in the fore- and hindgut, whereas their number in the midgut was considerably greater. Octopamine levels detected by HPLC correlated with the distribution of octopamine-containing nerve cells. A regulatory role for these intrinsic octopaminergic neurons is suggested in the enteric plexus in the earthworm alimentary tract. This is the first report on the occurrence of octopamine-containing nerve cells in the peripheral nervous system of an invertebrate.

Co-localization of serotonin and FMRFamide-like immunoreactivities in the central nervous system of the earthworm, Eisenia fetida

In the present study co-localization of serotonin and FMRFamide-like immunoreactivities was studied in the central nervous system of the earthworm, Eisenia fetida. Alternate serial sections were stained with antisera raised against serotonin and FMRFamide, using the peroxidase-antiperoxidase method. The immunostained consecutive sections were compared by light-microscopy and on reconstructed images using NIH Image and Neurolucida programs. Although there was a large number of serotonin and FMRFamide-like immunoreactive neurons in the central nervous system of Eisenia, co-localization of immunostaining was relatively rare. In the cerebral ganglion no cells were found that stained for both antisera. In the subesophageal and ventral cord ganglia the neurons reacting for both antisera represented approximately 17-23% and 10-14% of the serotonin- and 7-11% and 9-17% of the FMRF-amide-like immunoreactive cells, respectively. It is suggested, that FMRFamide-related peptides act as neuromodulators and may also influence the activity of serotonergic neurons.

Ca2+ influx into leech neuropile glial cells mediated by nicotinic acetylcholine receptors

The effect of cholinergic agonists and antagonists on the intracellular free Ca2+ concentration ([Ca2+]i) of leech neuropile glial cells was investigated by use of iontophoretically injected fura-2. In neuropile glial cells, cholinergic agonists induced a marked increase in [Ca2+]i that was inhibited by d-tubocurarine, alpha-bungarotoxin, strychnine, and atropine. The efficacy of the various agonists and antagonists indicates that the [Ca2+]i increase is mediated by the nicotinic acetylcholine (ACh) receptors that have been characterized previously in these cells by using electrophysiological methods. In the presence of high agonist concentrations, [Ca2+]i partly recovered, suggesting that the ACh receptors desensitize. The [Ca2+]i increase induced by cholinergic agonists was abolished in Ca2(+)-free solution, which indicates that it is caused by Ca2+ influx from the external medium. The agonist-induced [Ca2+]i increase was partly preserved in Na(+)-free solution, whereas the agonist-induced membrane depolarization was strongly suppressed. The agonist-induced [Ca2+]i increase was also partly preserved in the presence of 5 mM Ni2+, which almost abolished the K(+)-induced [Ca2+]i increase mediated by voltage-dependent Ca2+ channels. It is concluded that at low agonist concentrations the [Ca2+]i increase in leech neuropile glial cells is mediated exclusively by the ion channels associated with the nicotinic ACh receptors. At high agonist concentrations, voltage-dependent [Ca2+]i increase in leech neuropile glial cells is mediated exclusively by the ion channels associated with the nicotinic ACh receptors. At high agonist concentrations, voltage-dependent Ca2+ channels activated by the concomitant membrane depolarization also contribute to the agonist-induced Ca2+ influx.

Physiological and pharmacological studies on annelid and nematode body wall muscle

1. This review covers the pharmacology and physiology of the body wall muscle systems of nematodes and annelids. 2. Both acetylcholine and gamma-aminobutyric acid (GABA) play important roles in the control of body wall muscle in both phyla. In annelids and nematodes, acetylcholine is the excitatory neuromuscular transmitter while GABA is the inhibitory neuromuscular transmitter. In addition, 5-hydroxytryptamine (5-HT) has a modulatory role at annelid body wall muscle but little if any effect on nematode body wall muscle. 3. The acetylcholine receptor of the body wall muscle can be classified as nicotinic-like in both phyla though the annelid receptor has not been analysed in detail. In nematodes, vertebrate ganglionic nicotinic agonists were the most effective of those so far examined while mecamylamine and benzoquinonium were the most effective antagonists. Both neuronal bungarotoxin and neosurugatoxin were potent antagonists of acetylcholine excitation at the nematode receptor. 4. The GABA receptor of the body wall muscle exhibits similarities with the vertebrate GABA-A receptor in both phyla. Picrotoxin is a very weak or inactive antagonist at leech and nematode GABA receptors, while bicuculline methiodide blocks leech GABA receptors but is inactive on nematode GABA receptors. Picrotoxin does block GABA responses of earthworm body wall muscle. All these GABA responses are chloride mediated. 5. Neuroactive peptides of the RFamide family occur in both phyla and FMRFamide has been identified in leeches. RFamides probably have an important role in heart regulation in leeches and in modulation of their body wall muscles. RFamides also modulate nematode body wall muscle activity with KNEFIRFamide raising muscle tone while SDPNFLRFamide relaxes the muscle. It is likely that this family and other neuroactive peptides play an important role in the physiology of body wall muscle throughout both phyla.

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.

Serotoninlike immunoreactivity in the central and peripheral nervous system of the scale worm Harmothoe imbricata (Polychaeta)

The distribution of serotoninergic neurons in the nervous system of the scale worm Harmothoe imbricata was visualized in the anterior half of the body by the peroxidase-antiperoxidase (PAP) immunohistochemical method with a specific antiserotonin antibody. Immunoreactive neuronal somata were localized in discrete ganglion cell masses of the dorsally situated cerebral ganglion and in segmental ganglia of the ventral nerve cord. They also make up the majority of neurons present in the parapodial ganglia. Large and small varicose fibers stained in the neuropile of all the above-mentioned ganglia but also in interganglionic connectives and segmental nerves. On the basis of soma size and location and of fiber distribution, the reactive neurons were identified as primarily interneuronal with a few motoneurons and presumptive afferent neurons. The presence of a motor component was substantiated by observations of several reactive varicose fibers spread over longitudinal muscle layers of the trunk. In addition, neurites of the subepidermal nerve plexus and enterochromaffinlike cells of the gut epithelium reacted with the serotonin antibody. It is concluded that serotoninergic pathways are ubiquitous elements in the organization of the central and peripheral nervous system of this polychaete. The significance of these findings in relation to other annelid groups and to the physiological role of serotonin is discussed.

The Retzius cells in the leech: a review of their properties and synaptic connections

1. The Retzius cells (RCs) project an axonal branch in each anterior, posterior and dorsal segmental root. 2. RCs are the only serotonin-containing neurons projecting to the periphery. 3. RCs are activated by mechano-sensory neurons, by serotonin-containing neurons and by two pairs of subesophageal neurons, Tr 1 and Tr 2. 4. RCs also receive an excitatory and an inhibitory input from sensilla. 5. These inputs could form two systems, one converging onto RCs of each ganglion and one distributing to other ganglia after processing by RCs. 6. RCs play a role in muscle tension, in mucous release and in swimming activity.

Purinergic modulation in the brain of the spiny lobster

Earlier studies identified purinergic chemoreceptors in the olfactory organ of the spiny lobster, Panulirus argus. In this study, electrophysiological experiments demonstrate that purinergic substances can modulate both the spontaneous activity and the evoked responses of neurons within the brain of this animal. Perfusion of the brain with 100 microM adenosine 5'-monophosphate (AMP) modulated the spontaneous activity of 71% of the brain interneurons that were monitored. AMP also modulated the electrically or chemically evoked activity of 25% of the monitored interneurons. The effects were dose-dependent (down to 1 microM) and reversible. The modulatory effects of adenosine were similar to those of AMP, and were antagonized by the adenosine receptor antagonist 1,3-dipropyl-8-p-sulfophenylxanthine. The modulation by AMP or adenosine was depressive in most but not all neurons, as is the case with purinergic effects in the brain of vertebrates. We believe this is the first demonstration of modulatory effects of purinergic substances in the nervous system of any invertebrate.

The action of cholinomimetic and cholinolytic agents, hemicholinium-3 and alpha- and beta-bungarotoxin on the body wall muscle of the earthworm, Lumbricus terrestris

Strips of muscle, approximately 12 segments in length, were prepared from the body wall of the earthworm, Lumbricus terrestris, from which the nerve cord and viscera had been removed. Contractions to electrical stimulation and acetylcholine agonists were recorded using an isometric transducer. A range of nicotinic and muscarinic agonists and antagonists were tested on this preparation and the results indicate that the acetylcholine receptor on this muscle cannot be classified as either nicotinic or muscarinic. Hemicholinium-3 abolished electrically induced muscle twitches at concentrations which had no effect on the acetylcholine response. Alpha-Bungarotoxin blocked the responses to both electrical stimulation and acetylcholine while beta-bungarotoxin blocked the contractions induced by electrical stimulation but potentiated the acetylcholine contraction.

Serotonin-immunoreactive neurons in the brain of the honeybee

The distribution of serotonin-immunoreactive neurons in the brain of the worker honey bee Apis mellifera was studied by means of immunocytochemical staining by using a well-characterized antibody to serotonin (5-HT). About 75 immunoreactive perikarya are grouped into clusters in the optic lobe and in the median and dorsal protocerebrum. Immunoreactive fibers were resolved in all areas of the brain. The optic lobe shows restricted layers of 5-HT-immunoreactive fibers in the lamina and medulla organized perpendicular to the retinotopic elements. Immunoreactive fibers in the lobula represent invasions of protocerebral giant wide-field neurons. The nonglomerular neuropil of the brain exhibits a meshwork of immunoreactive fibres invading glomerular neuropil of the mushroom bodies, central body complex, and antennal lobes. Mushroom body stalks and lobes contain immunoreactive fibers arranged perpendicular to the Kenyon cell fibers and matching subcompartments of these corpora pedunculata areas. The calyces are devoid of immunofluorescence. Serotonin-positive fibres in the central body complex are arranged in its subcompartments. No 5-HT immunoreactivity was found in the pons. Antennal glomeruli contain immunoreactive fibers restricted around the margin of the glomeruli. The selective mapping of 5-HT-immunoreactive neurons complements studies on the distribution of monoamine-containing neurons in the bee brain. Serotonin- and catecholamine-containing neurons often occur together in the same brain areas and subcompartments. The immunohistochemical approach in chemoneuroanatomy gives new evidence for a more complicated architecture of the brain than could be deduced from the classical neuroanatomical studies.

The possible site of action of 5-hydroxytryptamine, 6-hydroxytryptamine, tryptamine and dopamine on identified neurons in the central nervous system of the snail, Helix aspersa

Intracellular recordings were made from identified neurons in the right parietal ganglion of the snail, Helix aspersa. Cells F 4, 5 and 6 were excited by 5-hydroxytryptamine (5-HT) and inhibited by dopamine while cells in the F 30 area were inhibited by both compounds. Low doses of both tryptamine and 6-HT produced weak excitation of cells F 4, 5 and 6 while higher doses of both compounds inhibit the activity of these cells. In terms of the inhibitory responses, tryptamine and 6-HT are approximately equipotent but between 10 and 100 times less potent than dopamine. d-Tubocurarine reversibly antagonized the excitatory action of 5-HT on cells F 4, 5 and 6 and converted tryptamine and 6-HT excitation to inhibition. In the presence of the antagonist, ergometrine, the dopamine inhibitory response was almost completely abolished while the inhibitory responses to tryptamine and 6-HT were converted to weak excitation. All four agonists inhibited cells in the F 30 area with the following potency ratios: dopamine much greater than tryptamine/6-HT greater than 5-HT. Tubocurarine had no antagonist effects on these responses while ergometrine reduced or blocked all four, often irreversibly. In potassium-free Ringer the inhibitory responses to all four agonists were enhanced. It is concluded that on cells F 4, 5 and 6, low concentrations of tryptamine and 6-HT act on 5-HT receptors while higher concentrations of both agonists act on dopamine receptors. On cells in the F 30 area, 5-HT, 6-HT and tryptamine all act on a dopamine receptor.

Pharmacological evidence for the existence of presynaptic alpha-like receptors in Mytilus muscle

The effects of alpha-adrenoceptor agonists and antagonists on contractions of the ABRM of M. edulis were examined. Naphazoline (10(-11)-10(-7)M) as well as octopamine potentiated both contractions in response to ACh and to repetitive electrical stimulation with brief pulses, while clonidine (10(-12)-10(-6)M), imidazole (10(-4)-10(-3)M) and tolazoline (10(-6)-10(-4)M) potentiated only the contraction in response to repetitive electrical stimulation. Potentiating actions of octopamine and naphazoline on ACh contraction were blocked by tolazoline but were little affected by clonidine and imidazole. Potentiating after-effect of repetitive electrical stimulation on subsequent ACh-contraction was not affected by clonidine and imidazole but was blocked by tolazoline. These results suggest that in the ABRM there are probably at least two classes of alpha-like receptors for octopamine; one may be on the muscle fibres and the other may be on the excitatory nerve terminals. Activation of the latter class of receptors may result in enhancement of ACh release from the terminals.