Dopamine in a molluscan nervous system: synthesis and fluorescence histochemistry

The distribution and evolutionary dynamics of dopaminergic neurons in molluscs

Dopamine is one of the most versatile neurotransmitters in invertebrates. It's distribution and plethora of functions is likely coupled to feeding ecology, especially in Euthyneura (the largest clade of molluscs), which presents the broadest spectrum of environmental adaptations. Still, the analyses of dopamine-mediated signaling were dominated by studies of grazers. Here, we characterize the distribution of dopaminergic neurons in representatives of two distinct ecological groups: the sea angel - obligate predatory pelagic mollusc Clione limacina (Pteropoda, Gymnosomata) and its prey - the sea devil Limacina helicina (Pteropoda, Thecosomata) as well as the plankton eater Melibe leonina (Nudipleura, Nudibranchia). By using tyrosine hydroxylase-immunoreactivity (TH-ir) as a reporter, we showed that the dopaminergic system is moderately conservative among euthyneurans. Across all studied species, small numbers of dopaminergic neurons in the central ganglia contrast to significant diversification of TH-ir neurons in the peripheral nervous system, primarily representing sensory-like cells, which predominantly concentrated in the chemotactic areas and projecting afferent axons to the central nervous system. Combined with α-tubulin immunoreactivity, this study illuminates the unprecedented complexity of peripheral neural systems in gastropod molluscs, with lineage-specific diversification of sensory and modulatory functions.

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.

Localization of tyrosine hydroxylase-like immunoreactivity in the nervous systems of Biomphalaria glabrata and Biomphalaria alexandrina, intermediate hosts for schistosomiasis

Planorbid snails of the genus Biomphalaria are major intermediate hosts for the digenetic trematode parasite Schistosoma mansoni. Evidence suggests that levels of the neurotransmitter dopamine (DA) are reduced during the course of S. mansoni multiplication and transformation within the snail. This investigation used immunohistochemical methods to localize tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of catecholamines, in the nervous system of Biomphalaria. The two species examined, Biomphalaria glabrata and Biomphalaria alexandrina, are the major intermediate hosts for S. mansoni in sub-Saharan Africa, where more than 90% of global cases of human intestinal schistosomiasis occur. TH-like immunoreactive (THli) neurons were distributed throughout the central nervous system (CNS) and labeled fibers were present in all commissures, connectives, and nerves. Some asymmetries were observed, including a large distinctive neuron (LPeD1) in the pedal ganglion described previously in several pulmonates. The majority of TH-like immunoreactive neurons were detected in the peripheral nervous system (PNS), especially in lip and foot regions of the anterior integument. Independent observations supporting the dopaminergic phenotype of THli neurons included 1) block of LPeD1 synaptic signaling by the D2/3 antagonist sulpiride, and 2) the similar localization of aqueous aldehyde (FaGlu)-induced fluorescence. The distribution of THli neurons indicates that, as in other gastropods, dopamine functions as a sensory neurotransmitter and in the regulation of feeding and reproductive behaviors in Biomphalaria. It is hypothesized that infection could stimulate transmitter release from dopaminergic sensory neurons and that dopaminergic signaling could contribute to modifications of both host and parasite behavior.

Peptidergic modulation of patterned motor activity in identified neurons of Helisoma

The neuroactive peptides SCP(B) (small cardioactive peptide B) and FMRFamide (Phe-Met-Arg-Phe-NH(2)), both originally isolated from molluscs, have potent modulatory effects upon the production of patterned motor activity in identified neurons (e.g., B5 and B19) in the buccal ganglia of the snail Helisoma. Such patterned motor activity has previously been shown to underlie feeding behavior. Micromolar concentrations of SCP(B) initiate patterned motor activity in quiescent ganglia and increase the rate of activity in ganglia that are spontaneously active. Micromolar concentrations of FMRFamide inhibit patterned motor activity in Helisoma buccal ganglia, and 10 muM FMRFamide completely suppresses such activity. In addition, there are both anti-SCP(B)-and anti-FMRFamide-immunoreactive neurons in Helisoma buccal ganglia. Our results suggest that peptides may play a prominent role in the regulation of feeding behavior in Helisoma.

Serotonin regulates electrical coupling via modulation of extrajunctional conductance: H-current

Synaptic strength can be highly variable from animal to animal within a species or over time within an individual. The process of synaptic plasticity induced by neuromodulatory agents might be unpredictable when the underlying circuits subject to modulation are themselves inherently variable. Serotonin (5-hydroxytryptomine; 5HT) and serotonergic signaling pathways are important regulators of animal behavior and are pharmacological targets in a wide range of neurological disorders. We have examined the effect of 5HT on electrical synapses possessing variable coupling strengths. While 5HT decreased electrical coupling at synapses with weak electrical connectivity, synapses with strong electrical coupling were less affected by 5HT treatment, as follows from the equations used for calculating coupling coefficients. The fact that the modulatory effect of 5HT on electrical connections was negatively correlated with the strength of electrical coupling suggests that the degree of electrical coupling within a neural network impacts subsequent neuromodulation of those synapses. Biophysical studies indicated that these effects were primarily due to 5HT-induced modulation of membrane currents that indirectly affect junctional coupling at synaptic contacts. In support of these experimental analyses, we created a simple model of coupled neurons to demonstrate that modulation of electrical coupling could be due solely to 5HT effects on H-channel conductance. Therefore, variability in the strength of electrical coupling in neural circuits can determine the pharmacological effect of this neuromodulatory agent.

Dopamine stimulates snail albumen gland glycoprotein secretion through the activation of a D1-like receptor

The catecholamine dopamine is present in both the central nervous system and in the peripheral tissues of molluscs, where it is involved in regulating reproduction. Application of exogenous dopamine to the isolated albumen gland of the freshwater pulmonate snail Helisoma duryi (Wetherby) induces the secretion (release) of perivitelline fluid. The major protein component of the perivitelline fluid of Helisoma duryi is a native 288 kDa glycoprotein that is secreted around individual eggs and serves as an important source of nutrients for the developing embryos. The secretion of glycoprotein by the albumen gland is a highly regulated event that must be coordinated with the arrival of the fertilized ovum at the carrefour (the region where the eggs receive albumen gland secretory products). In order to elucidate the intracellular signalling pathway(s) mediating dopamine-induced glycoprotein secretion, albumen gland cAMP production and glycoprotein secretion were measured in the presence/absence of selected dopamine receptor agonists and antagonists. Dopamine D1-selective agonists dihydrexidine, 6,7-ADTN and SKF81297 stimulated cAMP production and glycoprotein secretion from isolated albumen glands whereas D1-selective antagonists SCH23390 and SKF83566 suppressed dopamine-stimulated cAMP production. Dopamine D2-selective agonists and antagonists generally had no effect on cAMP production or protein secretion. Based on the effects of these compounds, a pharmacological profile was obtained that strongly suggests the presence of a dopamine D1-like receptor in the albumen gland of Helisoma duryi. In addition, secretion of albumen gland glycoprotein was not inhibited by protein kinase A inhibitors, suggesting that dopamine-stimulated protein secretion might occur through a protein kinase A-independent pathway.

Dopaminergic neurons in the brain and dopaminergic innervation of the albumen gland in mated and virgin helisoma duryi (mollusca: pulmonata)

BACKGROUND

Dopamine was shown to stimulate the perivitelline fluid secretion by the albumen gland. Even though the albumen gland has been shown to contain catecholaminergic fibers and its innervation has been studied, the type of catecholamines, distribution of fibers and the precise source of this neural innervation has not yet been deduced. This study was designed to address these issues and examine the correlation between dopamine concentration and the sexual status of snails.

RESULTS

Dopaminergic neurons were found in all ganglia except the pleural and right parietal, and their axons in all ganglia and major nerves of the brain. In the albumen gland dopaminergic axons formed a nerve tract in the central region, and a uniform net in other areas. Neuronal cell bodies were present in the vicinity of the axons. Dopamine was a major catecholamine in the brain and the albumen gland. No significant difference in dopamine quantity was found when the brain and the albumen gland of randomly mating, virgin and first time mated snails were compared.

CONCLUSIONS

Our results represent the first detailed studies regarding the catecholamine innervation and quantitation of neurotransmitters in the albumen gland. In this study we localized catecholaminergic neurons and axons in the albumen gland and the brain, identified these neurons and axons as dopaminergic, reported monoamines present in the albumen gland and the brain, and compared the dopamine content in the brain and the albumen gland of randomly mating, virgin and first time mated snails.

Feeding stimulants activate an identified dopaminergic interneuron that induces the feeding motor program in Helisoma

The neurotransmitter dopamine is shown to play a fundamental role in the generation of the feeding motor pattern and resultant feeding behavior in Helisoma. Application of exogenous dopamine triggered the fictive feeding motor pattern in the isolated CNS and triggered feeding movements in semi-intact preparations. Application of feeding stimulants to the oral cavity excited the putatively dopaminergic buccal interneuron N1a, and depolarization of interneuron N1a triggered the production of the fictive feeding motor pattern. The ability of dopamine superfusion and of interneuron N1a stimulation to activate the fictive feeding motor pattern was blocked by the dopamine antagonist sulpiride. The phase of the fictive feeding motor pattern was reset by brief hyperpolarization of interneuron N1a, demonstrating that interneuron N1a is an integral component of the buccal central pattern generator (CPG). During spontaneous fictive feeding patterns, prolonged hyperpolarizations of interneuron N1a inhibited the production of patterned activity. Exogenous dopamine maintained the fictive feeding motor pattern in the absence of interneuron N1a activity. Interneuron N1a was labeled by the formaldehyde-glutaraldehyde histochemical technique, which is indicative of the presence of dopamine in mollusks. These data suggest that interneuron N1a is an endogenous source of the neuromodulator dopamine, intrinsic to the buccal CPG, and that interneuron N1a has a prominent role in the sensory-motor integration triggering the consummatory response.

In vivo neuropharmacological and in vitro laser ablation techniques as tools in the analysis of neuronal circuits underlying behavior in a molluscan model system

1. This paper reviews the selective lesioning techniques employed to elucidate the role of the neurotransmitters dopamine and serotonin and single, identified interneurons in the feeding system of the pond snail Lymnaea stagnalis. 2. The pathway lesioning work reviewed in this paper showed that dopamine is necessary for the feeding response to occur and serotonin has a mainly modulatory role in the feeding system of Lymnaea. 3. The photoinactivation results reviewed here assist in the elucidation of the different roles that different types of interneurons play in the initiation and modulation of patterned neuronal activity underlying feeding.

Behavioural and biochemical changes in the feeding system of Lymnaea induced by the dopamine and serotonin neurotoxins 6-hydroxydopamine and 5,6-dihydroxytryptamine

The neurotoxins 5,6-dihydroxytryptamine (5,6-DHT) and 6-hydroxydopamine (6-OH-DA) were used to examine the role of monoamines in the feeding system of the snail Lymnaea stagnalis . Biting responses to sucrose were monitored up to 25 days after injection with drugs. Cerebral and buccal ganglia and cerebro-buccal connectives from the same groups of snails were examined for changes in serotonin and dopamine levels by high performance liquid chromatography and the glyoxylic acid histo-fluorescence technique. Twelve to eighteen days after injection with 5,6-DHT only 57% of the snails responded to sucrose with biting movements, compared with 98% of controls. Those that did respond had a longer latency to the first bite and the bites were of shorter duration and occurred at a lower rate when compared with controls. This was accompanied by a 39% drop in 5-HT levels in the cerebro-buccal commissure and nerves and a loss of fluorescence in the axons of the paired cerebral giant cells, the main serotonergic neurons involved in feeding. Earlier behavioural effects of injecting 5,6-DHT at 20 min and 3-4 h after injection could not be explained by specific changes in 5-HT levels. Recovery of both behavioural response and serotonin levels occurred between 22-25 days after injection. Injection of 6-OH-DA also inhibited feeding responses but the effects were quicker (1-3 days) and more dramatic, with only 40% of snails showing any biting response to food compared with a 98% response in controls. Reduction in dopamine (DA) levels of 40%, together with a loss of DA fluorescence in nerve fibres accompanied the reduction of behavioural responsiveness. An early (3-4 h) effect of 6-OH-DA injection could not be correlated with a specific reduction in DA levels. Behavioural responses and DA levels returned to normal by 4-7 days after injection. Both neurotoxins inhibited uptake of their target monoamines and this appeared to be the main mechanism for depleting 5-HT and DA. Early effects of neurotoxin injection probably directly inhibited monoamine uptake, whereas long-term inhibition was a secondary effect because of degeneration of nerve fibres. Neither neurotoxin caused release of monoamines. Results from both neurotoxins suggest that monoamines play an important role in the initiation and maintainance of the feeding response, the consummatory phase of feeding. Neither neurotoxin prevented the occurrence of the initial appetitive phase of feeding when snails orientate towards food in the experimental chamber.

Dopaminergic neuron B20 generates rhythmic neuronal activity in the feeding motor circuitry of Aplysia

We have identified a buccal neuron (B20) that exhibits dopamine-like histofluorescence and that can drive a rhythmic motor program of the feeding motor circuitry of Aplysia. The cell fires vigorously during episodes of patterned buccal activity that occur spontaneously, or during buccal programs elicited by stimulation of identified cerebral command-like neurons for feeding motor programs. Preventing B20 from firing, or firing B20 at inappropriate times, can modify the program driven by the cerebral feeding command-like neuron CBI-2. When B20 is activated by means of constant depolarizing current it discharges in phasic bursts, and evokes a sustained coordinated rhythmic buccal motor program. The program incorporates numerous buccal and cerebral neurons associated with aspects of feeding responses. The B20-driven program can be reversibly blocked by the dopamine-antagonist ergonovine, suggesting that dopamine may be causally involved in the generation of the program. Although firing of B20 evokes phasic activity in cerebral command-like neurons, the presence of the cerebral ganglion is not necessary for B20 to drive the program. The data are consistent with the notion that dopaminergic neuron B20 is an element within the central pattern generator for motor programs associated with feeding.

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.

Catecholamine neurons in Aplysia: improved light-microscopic resolution and ultrastructural study using paraformaldehyde and glutaraldehyde (FaGlu) cytochemistry

We have modified the formaldehyde-glutaraldehyde (FaGlu) histofluorescence method of Furness, Costa, and Blessing (1977a) and Furness, Costa, and Wilson (1977b) to examine wholemounts and sections of both juvenile and adult ganglia as well as peripheral tissues of Aplysia californica. FaGlu fluorescence is the result of a reaction between formaldehyde and tissue catecholamines to produce water-insoluble (fixed) fluorophores. In serially sectioned cerebral ganglia, 70-80 positive neurons were observed (many in clusters of 10-20 cells), many more than were found using the glyoxylic acid technique. Catecholamine-containing varicosities were densely packed in localized portions of the neuropil of all central ganglia. Exclusive localization in the neuropil of presumed dopamine release sites is similar to that previously found for the neuropeptide SCP but differs from the widespread ramification of varicose neurites containing 5-HT, FMRFamide, and ELH. The FaGlu technique also enabled us to study the ultrastructure of catecholamine-containing neurons. In contrast to the larger vesicles found in serotonergic and histaminergic neurons, these dopaminergic neurons contain 70 nm dense-cored vesicles.

Distribution of monoamines within the central nervous system of the juvenile pulmonate snail, Achatina fulica

The distributions of serotonin and catecholamines were examined within the central ganglia of juvenile Achatina through the histological localization of serotonin-like immunoreactivity and of glyoxylic acid-induced fluorescence. Somata containing these amines were widely distributed throughout all central ganglia except the two pleural ganglia and the left parietal ganglion. Most catecholaminergic neurons were very small (5-10 microns in diameter) and located in clusters in the cerebral and pedal ganglia, although a few, somewhat larger catecholaminergic neurons were also scattered throughout other locations. Catecholamines also appeared to be heavily concentrated in certain neuropilar regions of the central ganglia. Serotonergic neurons were generally much larger than the catecholaminergic neurons, and some of these somata reached relatively large sizes (up to 50-70 microns in diameter). The majority of serotonergic cells were located in the pedal ganglia but major populations were also located in the paired cerebral, the right parietal and the visceral ganglia. Several of the serotonergic cells could be reliably recognized as distinct individuals which appear to be identical to those described in previous studies. Among the previously identified cells which appear to contain serotonin are v-RCDN ad v-LCDN (the right and left metacerebral giant cells) of the cerebral ganglia, d-LPeLN of the left pedal ganglion, and TAN, TAN-2, and TAN-3 of the right parietal ganglion. Comparisons are drawn with general distribution patterns of monoamines and with identified monoaminergic cells and cell populations found in other gastropod species.

Localization of catecholamines in the buccal ganglia of Aplysia californica

The distribution of catecholamines and serotonin in the buccal ganglia and buccal nerves of Aplysia californica was examined using glyoxylic acid-induced histofluorescence. Three identifiable, medium-sized cell bodies, two paired and one unpaired, and 5 smaller cells fluoresced blue-green indicating the presence of catecholamines. Numerous fluorescent axons were observed in the neuropil and peripheral nerves, including a network of catecholaminergic processes emerging from the esophageal nerve and surrounding the base of the esophagus. The presence of catecholaminergic cells and processes in the buccal system suggests that these transmitters are used in the control of feeding behavior and digestion.

Dopamine and serotonin inhibition of neurite elongation of different identified neurons

This study demonstrates that a second classical neurotransmitter, dopamine, can act to suppress regenerative neurite outgrowth. Single identified neurons were dissected from two central ganglia of the snail Helisoma, and growth cone motility was studied as neurites regenerated in cell culture. Both dopamine and serotonin inhibited growth cone motility and elongation of neurites. Outgrowth inhibition ranged from sustained arrest to a similar but transient response. The effects of dopamine and serotonin are neuron-selective. Specific neurons affected by dopamine and serotonin represent distinct sets. One neuron was found that responds to both agents. The implications of neurotransmitter regulation of the dynamics of neuronal morphology are discussed.

Distribution of monoamines in the central nervous system of the nudibranch gastropod, Hermissenda crassicornis

The distribution of monoamines in the central ganglia of the nudibranch gastropod Hermissenda crassicornis was examined through the histological localization of both glyoxylic acid-induced fluorescence and serotonin-like immunoreactivity. Glyoxylic acid histochemistry revealed several clusters of catecholamine-containing cells which were located principally in the cerebropleural ganglia. One large unpaired catecholamine-containing cell was also located in the right pedal ganglion. Glyoxylic acid histochemistry and immunohistochemistry together revealed several serotonin-containing cells. The most prominent of these was a bilateral pair of cells (the metacerebral giants or MCG's) with somata located in the anterior lobes of the cerebropleural ganglia and each with a single large axon running through the ipsilateral cerebrobuccal connective to the buccal ganglia. Apart from the MCG's and a few smaller ones in the cerebropleural ganglia, most other serotonergic cells were located in the pedal ganglia. Among the serotonergic cells identified in the pedal ganglia was a single unpaired giant cell (LP1) located only on the left side. The neurites of LP1 projected through the cerebropleural ganglia to the contralateral pedal ganglion. Similarities in the distribution of monoamines in different gastropod species are discussed.

Age-dependent changes in fluorescent neurons in the brain of Notoplana acticola, a polyclad flatworm

The formaldehyde-glutaraldehyde-sucrose (FGS) method for in situ localization of catecholamines has been applied to the nervous system of the marine polyclad flatworm Notoplana acticola. This histochemical fluorescence technique revealed the presence of a small population of fluorescent cells within the brain. The number and positions of these neurons were constant in animals of the same size, but varied with the size of the worm. The brains of small animals (8 mm in length) were found to contain 20 fluorescent cells, whereas the largest animals studied (30 mm in length) were found to have 28 such cells. Various intermediate cell numbers were found in animals between these two sizes. The origin of the newly added fluorescent cells is uncertain. Peripheral fluorescence was found in association with the tentacular ocelli (eyespots) and interneurons within the ventral submuscular nerve plexus. The fluorescent spectrum from these cells measured in situ had a lambda max of 526 nm. Treatment with HCl shifts this peak to 530 nm. L-dopamine fluoresces with a similar peak emission before HCl treatment (525.5 nm) and shifts to the appropriate longer wavelength (530 nm) following acidification. This strongly suggests that the fluorescent substance in the neurons is dopaminergic in nature.

Monoamine-containing neurons in the Aplysia brain

The localization of monoamine-containing neurons in the CNS of Aplysia depilans has been studied by fluorescent histochemistry (the glyoxylic acid condensation method) and microspectrofluorimetry. Yellow fluorescent nerve cells and fibers show the emission maximum at 515-520 nm which corresponds to that of serotonin fluorophore in a model system. Green fluorescent nerve cells have the emission maximum at 485 nm which corresponds to that of catecholamine. Central catecholamine-containing neurons were found in cerebral, buccal, pedal and unpaired abdominal ganglia. The majority of them were revealed in cerebral ganglia (about 40). Serotonin-containing neurons are abundant in cerebral and pedal ganglia. More than 30 serotonin-containing nerve cells were localized in cerebral ganglia. In the right pedal ganglion approximately 100 neurons were revealed; in the left one about 150. In the abdominal ganglion all nerve cells of this chemical type (except one) are located in the right hemiganglion. The results are summarized in corresponding schemes.

Localization and synthesis of monoamines in regions of Limax CNS controlling feeding behavior

Localization and synthesis of dopamine and serotonin in the cerebral and buccal ganglia of Limax maximus were studied. A combination of fluorescence histochemistry, immunocytochemistry, and microchemical analysis showed that both amines were localized to particular cell groups and fiber tracts within and between the two sets of ganglia. Since these ganglia control feeding behavior, which is readily modified by associative learning, these studies have direct bearing on analysis of both motor control and learning mechanisms.

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.