Monoamine-containing neurons in planaria

Planarian nociception: Lessons from a scrunching flatworm

In addition to being studied for their exceptional regeneration abilities, planarians (i.e., flatworms) have also been extensively used in the context of pharmacological experiments during the past century. Many researchers used planarians as a model system for the study of drug abuse because they display high similarities with the nervous system of vertebrates at cellular and molecular levels (e.g., neuronal morphology, neurotransmitter ligands, and receptor function). This research field recently led to the discovery of causal relationships between the expression of Transient Receptor Potential ion channels in planarians and their behavioral responses to noxious stimuli such as heat, cold or pharmacological analogs such as TRP agonists, among others. It has also been shown that some antinociceptive drugs modulate these behaviors. However, among the few authors that tried to implement a full behavior analysis, none reached a consensual use of the terms used to describe planarian gaits yet, nor did they establish a comprehensive description of a potential planarian nociceptive system. The aim of this review is therefore to aggregate the ancient and the most recent evidence for a true nociceptive behavior in planarians. It also highlights the convenience and relevance of this invertebrate model for nociceptive tests and suggests further lines of research. In regards to past pharmacological studies, this review finally discusses the opportunities given by the model to extensively screen for novel antinociceptive drugs.

Neurotransmitters of sleep and wakefulness in flatworms

STUDY OBJECTIVES

Sleep is a prominent behavioral and biochemical state observed in all animals studied, including platyhelminth flatworms. Investigations into the biochemical mechanisms associated with sleep-and wakefulness-are important for understanding how these states are regulated and how that regulation changed with the evolution of new types of animals. Unfortunately, beyond a handful of vertebrates, such studies on invertebrates are rare.

METHODS

We investigated the effect of seven neurotransmitters, and one pharmacological compound, that modulate either sleep or wakefulness in mammals, on flatworms (Girardia tigrina). Flatworms were exposed via ingestion and diffusion to four neurotransmitters that promote wakefulness in vertebrates (acetylcholine, dopamine, glutamate, histamine), and three that induce sleep (adenosine, GABA, serotonin) along with the H1 histamine receptor antagonist pyrilamine. Compounds were administered over concentrations spanning three to five orders of magnitude. Flatworms were then transferred to fresh water and video recorded for analysis.

RESULTS

Dopamine and histamine decreased the time spent inactive and increased distance traveled, consistent with their wake-promoting effect in vertebrates and fruit flies; pyrilamine increased restfulness and GABA showed a nonsignificant trend towards promoting restfulness in a dose-dependent manner, in agreement with their sleep-inducing effect in vertebrates, fruit flies, and Hydra. Similar to Hydra, acetylcholine, glutamate, and serotonin, but also adenosine, had no apparent effect on flatworm behavior.

CONCLUSIONS

These data demonstrate the potential of neurotransmitters to regulate sleep and wakefulness in flatworms and highlight the conserved action of some neurotransmitters across species.

Post-SSRI Sexual Dysfunction: A Bioelectric Mechanism?

Selective serotonin reuptake inhibitor (SSRI) drugs, targeting serotonin transport, are widely used. A puzzling and biomedically important phenomenon concerns the persistent sexual dysfunction following SSRI use seen in some patients. What could be the mechanism of a persistent physiological state brought on by a transient exposure to serotonin transport blockers? In this study, we briefly review the clinical facts concerning this side effect of serotonin reuptake inhibitors and suggest a possible mechanism. Bioelectric circuits (among neural or non-neural cells) could persistently maintain alterations of bioelectric cell properties (resting potential), resulting in long-term changes in electrophysiology and signaling. We present new data revealing this phenomenon in planarian flatworms, in which brief SSRI exposures induce long-lasting changes in resting potential profile. We also briefly review recent data linking neurotransmitter signaling to developmental bioelectrics. Further study of tissue bioelectric memory could enable the design of ionoceutical interventions to counteract side effects of SSRIs and similar drugs.

Predator odor produces anxiety-like behavioral phenotype in planarians that is counteracted by fluoxetine

Avoidant behavior is a characteristic feature post-traumatic stress disorder (PTSD) and is modeled in mammals with predator odor. Light avoidance is a hallmark behavioral reaction in planarians. We hypothesized that planarians exposed to frog extract would display enhanced light avoidance that is prevented by fluoxetine. Enhanced light avoidance (i.e., less time spent in light compartment of a dish split into light and dark sides) after a 30-min frog extract exposure (0.0001-0.01%) manifested 15 min post-exposure, persisted for at least 24 h, and was counteracted by fluoxetine (10 μM). These results suggest conservation of an anxiety-like behavioral phenotype.

Physiology and pharmacology of turbellarian neuromuscular systems

Our understanding of the neurobiology of the Platyhelminthes has come in large part from free-living turbellarians. In addition to providing considerable information about the capabilities of the rudimentary nervous system present in all members of the phylum, turbellarians have provided the most definitive information about the variety of ion channels present in the membranes of neurones and muscle cells, and about the physiology and pharmacology of those channels. Furthermore, preparations of single, viable muscle cells have provided some of the most conclusive evidence about the variety of transmitters present, and the types of response they evoke. Here, we review what is known about the physiology and pharmacology of the turbellarian neuromuscular system. Particular attention is given to the triclad flatworm Bdelloura Candida, the best studied species in this respect, but other species are included where relevant.

Behavioral characterization of serotonergic activation in the flatworm Planaria

Serotonin (5-hydroxytryptamine, 5-HT) receptors have been identified in Planaria, a model used for studying the pharmacology of behavioral phenomena. This study characterized the behavioral and locomotor effects of 5-HT, a 5-HT1A agonist, a 5-HT1B/2C agonist, and a 5-HT1A antagonist to examine the role of 5-HT receptor activation in this species. Planarians were video recorded individually in a clear plastic cube containing drug solution or vehicle. To quantify locomotor velocity (pLMV), planarians were placed individually into a dish containing drug solution or vehicle and the rate of gridline crossings was recorded. For the antagonist experiments, four conditions were studied: water alone, agonist alone, antagonist alone, and agonist plus antagonist. The decrease in pLMV induced by the5-HT1A agonist (8-OH-DPAT), and the 5-HT1B/2C agonist (mCPP), was antagonized by pretreatment with the 5-HT1A antagonist (WAY-100635) at a dose that had no effect of its own on pLMV. At a higher concentration of WAY-100635, further decreases in pLMV induced by 8-OH-DPAT were observed. Each agonist produced increased occurrences of 'C-like position' and 'screw-like hyperkinesia', 5-HT and mCPP produced 'writhing', and only mCPP produced a significant increase in duration of 'headswing' behavior. The results demonstrate that the 5-HT1A receptor identified in Planaria mediates behavioral responses to 5-HT receptor ligands, supporting the notion that planarians possess functional 5-HT receptors and might serve as a simple model for their study.

Neuropharmacology and behavior in planarians: translations to mammals

Planarians are the simplest animals to exhibit a body plan common to all vertebrates and many invertebrates, characterized by bilateral rather than radial symmetry, dorsal and ventral surfaces, and a rostrocaudal axis with a head and a tail, including specialized sense organs and an aggregate of nerve cells in the head. Neurons in planarian more closely resemble those of vertebrates than those of advanced invertebrates, exhibiting typical vertebrate features of multipolar shape, dendritic spines with synaptic boutons, a single axon, expression of vertebrate-like neural proteins, and relatively low spontaneously generated electrical activity. Here we report the most relevant contribution to the knowledge of the neuropharmacology of planarians, with particular reference to the behavioral consequences of the exposure to drugs acting on neural transmission. Neurochemical and histochemical data indicate the presence of several neurotransmitter-receptor systems in planarians. Moreover, a variety of experimental studies characterized specific behavioral patterns of these animals following the exposure to drugs acting on neural transmission. There is also evidence of the interactions between discrete neurotransmitter-receptor systems in modulating behavior in planarians. Finally, the model has proved efficacy for investigating the neurotoxicology of the dopamine neurons, and for the initial screening of the neuroprotective potential of drugs. In conclusion, these findings indicate that interactions between discrete neurotransmitter-receptor systems occur very early along phylogeny, although they may have evolved from very fundamental behaviors, such as motor activity in planarian, to more complex and integrated functions in vertebrates.

The kappa-opioid receptor antagonist nor-BNI inhibits cocaine and amphetamine, but not cannabinoid (WIN 52212-2), abstinence-induced withdrawal in planarians: an instance of 'pharmacologic congruence'

The broad applicability of receptor theory to diverse species, from invertebrates to mammals, provides evidence for the evolution in complexity of pharmacologic receptor diversification and of receptor-effector signal transduction mechanisms. However, pre-mammalian species have less receptor subtype differentiation, and thus, might share signal transduction pathways to a greater extent than do mammals, a phenomenon that we term 'pharmacologic congruence'. We have demonstrated previously that the lowest species considered to have a centralized nervous system, planarians, display both abstinence-induced and antagonist-precipitated withdrawal signs, indicative of the development of physical dependence. We report here: (1) amphetamine abstinence-induced withdrawal, and (2) the attenuation of cocaine and amphetamine, but not cannabinoid agonist (WIN 52212-2), abstinence-induced withdrawal by the opioid receptor antagonist naloxone and by the selective kappa-opioid receptor subtype antagonist nor-BNI (nor-Binaltorphimine), but not by the selective mu-opioid or the delta-opioid receptor subtype antagonists CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) and naltrindole. These results provide evidence that the withdrawal from cocaine and amphetamine, but not cannabinoids, in planarians is mediated through a common nor-BNI-sensitive (kappa-opioid receptor-like) pathway.

Flumazenil-sensitive dose-related physical dependence in planarians produced by two benzodiazepine and one non-benzodiazepine benzodiazepine-receptor agonists

Two benzodiazepine (midazolam and clorazepate) and one non-benzodiazepine (zolpidem) benzodiazepine-receptor agonists produced dose-related physical dependence, as evidenced by abstinence-induced decrease in planarian locomotor velocity (pLMV) when drug-exposed planarians were placed into drug-free water, but not when they were placed into drug-containing water (i.e., an abstinence-induced withdrawal, since the effect was only obtained in the removal of drug and not in the continued presence of drug). We have previously shown that the decrease in pLMV is associated with specific and transient withdrawal signs. In the present study, the selective benzodiazepine-receptor antagonist flumazenil significantly antagonized (P<0.05), by co-application, the ability of each agonist to produce the withdrawal. These results: (1) suggest that benzodiazepine-receptor agonists, for two different chemical categories, produce dose-related physical dependence manifested as abstinence-induced withdrawal in this simple and convenient model, and (2) in the absence of cloning or radioligand binding literature, suggest a possible specific interaction site (receptor?) for these compounds in planarians.

Subadditive withdrawal from cocaine/κ-opioid agonist combinations in Planaria

We have previously developed and extensively characterized a convenient and sensitive metric for the quantification of withdrawal responses using Planaria. Planaria are particularly valuable for these studies because of their permeable exteriors and their relevant neurotransmitter systems (e.g., dopaminergic, opioid, and serotonergic). In the present study, we used this metric and mathematically rigorous joint-action analysis to investigate poly-drug withdrawal from fixed-ratio cocaine/kappa-opioid agonist combinations. The D50 (concentration producing half-maximal effect) for cocaine and U-50,488H was 10.3 and 1.02 microg, respectively. The D50 for 19:1 or 1:19 combinations did not differ significantly (p>0.05) from expected additive values (11.6+/-3.0 vs. 9.9+/-1.4 and 1.1+/-0.2 vs. 1.5+/-0.1, respectively), but the 3:1, 1:1, and 1:3 ratios did (34.5+/-6.9 vs. 7.7+/-1.1; 55.1+/-10.0 vs. 5.7+/-0.7; and 40.8+/-8.9 vs. 3.3+/-0.4, respectively), indicating subadditive interaction at these ratios. The finding of subadditivity in this model suggests that abstinence-induced withdrawal from the combination is less intense than that predicted from the individual drug potencies. The concept that certain combinations of drugs leads to attenuated withdrawal might generalize to humans.

Measurement of glutamate and aspartate in Planaria

INTRODUCTION

The major excitatory neurotransmitters in the mammalian central nervous system are glutamate and aspartate. We developed a rapid and efficient method for the extraction and measurement of these amino acids in Planaria--a valuable model for mammalian processes because of their simple, centralized nervous system and similar neurotransmitter systems.

METHOD

The method utilized buffer extraction (perchloric acid containing 0.025% of L-cystine and Na2EDTA), simple derivatization, high-pressure liquid chromatography (HPLC), and fluorescence detection.

RESULTS

The mean+/-S.E.M. amounts of glutamate and aspartate were 322.6+/-43.6 and 188.6+/-27.6 pmol/mg-planarian, respectively.

DISCUSSION

The method provides the ability to investigate changes in glutamate and aspartate in response to drug administration or withdrawal.

Description and quantification of cocaine withdrawal signs in Planaria

Previous work provided indirect evidence that planarians undergo abstinence-induced withdrawal from cocaine. The present study's purpose was to determine if planarians display withdrawal signs and, if so, to quantify the behaviors. Planarians were soaked in cocaine then transferred to either the same cocaine concentration or cocaine-free water. Compared to the cocaine/cocaine group, the cocaine/water group displayed a significant number of atypical behaviors, providing direct evidence of a 'withdrawal phenomenon' in planarians.

Amphetamine-induced increase in planarian locomotor activity and block by UV light

The dopamine D2-receptor antagonist sulpiride decreases spontaneous locomotor velocity of planarians (pLMV) in an enantiomeric-selective and dose-dependent manner and is significantly attenuated by UV light (254 and 366 nm). We now report that amphetamine (10 microM) produced the opposite effect and was also reversed by UV light. These findings strengthen the hypothesis that the effects of dopaminergic ligands and UV light on pLMV relate to interaction with neurotransmitter transduction process(es).

Disruption of a drug-induced choice behavior by UV light

A considerable body of evidence suggests that UV light disrupts ligand binding in vitro. In vivo, UV light effects have been reported to disrupt simple behaviors such as spontaneous locomotor activity. However, there are no reports of UV light blocking a more complex drug-altered behavior. We now report that: (1) cocaine dose-relatedly reversed planarians' usual selection of dark over light (from 19.4+/-4.4% to 73.3+/-6.7%) (this effect was not due to an increase in locomotor activity, since cocaine only minimally increases locomotor activity at the highest dose tested); and (2) the cocaine-induced alteration of behavioral choice was attenuated significantly (P<0.05) by UV light (366 nm and 254 nm). The results demonstrate alteration of a relatively complex choice behavior by UV light.

Basiepidermal nervous system in Nemertoderma westbladi (Nemertodermatida): GYIRFamide immunoreactivity

The Nemertodermatida are a small group of microscopic marine worms. Recent molecular studies have demonstrated that they are likely to be the earliest extant bilaterian animals. What was the nervous system (NS) of a bilaterian ancestor like? In order to answer that question, the NS of Nemertoderma westbladi was investigated by means of indirect immunofluorescence technique and confocal scanning laser microscopy. The antibodies to a flatworm neuropeptide GYIRFamide were used in combination with anti-serotonin antibodies and phalloidin-TRITC staining. The immunostaining revealed an entirely basiepidermal NS. A ring lying outside the body wall musculature at the level of the statocyst forms the only centralisation, the "brain". No stomatogastric NS has been observed. The GYIRFamide immunoreactive part of the "brain" is formed of loosely packed nerve fibres with multiple small neurones and a few large ones. The peptidergic and aminergic patterns of the NS do not correspond to each other: the former is more developed on the ventral side, the latter is more pronounced on the dorsal side. A pair of GYIRFamide immunoreactive nerve cords innervates the ventral side of the animal, the mouth and the male genital opening. The nemertodermatids studied to-date display no common NS pattern. Possible synapomorphies of the Acoelomorpha are discussed. The study demonstrates that the nemertodermatid NS possesses a number of plesiomorphic features and appears more primitive than the NS in other worms, except the Xenoturbellida. The bilaterian ancestor supposedly possessed only a basiepidermal nerve net and had no centralised brain-like structures and no stomatogastric NS.

Serotonin in the nervous system of the head region of the land planarian Bipalium kewense

The presence and distribution of serotonin (5-hydroxytryptamine, or 5-HT) in the head region of the land planarian Bipalium kewense has been investigated by an indirect immunofluorescence technique combined with confocal scanning laser microscopy (CSLM), and also by immunogold labeling at ultrastructural level. Serotonin immunoreactivity (IR) was restricted to elements of the nervous system, such as the cerebral ganglion, and the peripheral nerve net. Most of 5-HT-immunoreactive neurons are at the periphery of the brain; they were identified as unipolar, bipolar, and multipolar neurons. The ultrastructural results using immunogold labeling confirm the location of 5-HT within electron-dense vesicles (50-120 nm in diameter), clustered both in the cell bodies and in their processes. The intense 5-HT-IR herein demonstrated for B. kewense adds new data to the poorly studied nervous system of land planarians.

kappa-Opioid withdrawal in Planaria

Many drug-abusers engage in poly-drug abuse, but there has been relatively little quantification of withdrawal from poly-drug use. Planarians are an advantageous model for these studies due to mammalian-relevant neurotransmitter systems (e.g. dopamine, opioid, and 5-HT). We recently developed a metric that quantified an acute cocaine withdrawal phenomenon in planarians. However, despite much indirect evidence, we lacked direct evidence of a receptor- or carrier-mediated effect. We now report dose-related, naloxone- and nor-binaltorphine-sensitive acute abstinence-induced withdrawal and naloxone-precipitated withdrawal from the kappa-opioid agonist U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl)-benzeneacetamide). The less active enantiomer [1R,2R]U-50,488 produced significantly less withdrawal and U-50,488H withdrawal was not due to pH or osmolarity. These data provide pharmacologic evidence of a kappa-opioid receptor-mediated withdrawal phenomenon and neuroadaptation to a pharmacologic stimulus (adaptations in transduction mechanisms) in this model.

When does a ganglion become a brain? Evolutionary origin of the central nervous system

A brain, a neural structure located in the head, differs from a ganglion by the following characteristics: (1) a brain subserves the entire body, not just restricted segments; (2) it has functionally specialized parts; (3) it is bilobar; (4) commissures and neurons form the surface with axons in the central core; (5) interneurons are more numerous than primary motor or primary sensory neurons; and (6) multisynaptic rather than monosynaptic circuits predominate. A "cephalic ganglion" does not exist in any living animal and probably never occurred even in extinct ancestral species. It also is not a developmental stage in the ontogenesis of any vertebrate. Amphioxus may represent an intermediate stage in the evolution of the vertebrate nervous system, but the anatomic relationship between the notochord and neural tube is more complex. The decussating interneuron of amphioxus, to mediate a primitive coiling reflex away from any stimulus, provides a phylogenetic explanation for the pattern of crossed long ascending and descending pathways in the subsequent evolution of the vertebrate central nervous system. The evolution of the vertebrate central nervous system may have begun with free-living flatworms (planaria) that evolved before the divergence of metazoans into invertebrate and chordate branches. The planarian is the simplest animal to develop a body plan of bilateral symmetry and axes of growth with gradients of genetic expression, enabling cephalization, dorsal and ventral surfaces, medial and lateral regions, and an aggregate of neural cells in the head that form a bilobed brain. Neurons of the planarian brain more closely resemble those of vertebrates than those of advanced invertebrates, exhibiting typical vertebrate features of multipolar shape, dendritic spines with synaptic boutons, a single axon, expression of vertebrate-like neural proteins, and relatively slow spontaneously generated electrical activity. The planarian is thus not only the first animal to possess a brain, but may be the ancestor of the vertebrate brain.

Cannabinoid-induced stimulation of motor activity in planaria through an opioid receptor-mediated mechanism

Planaria, the most primitive example of centralization and cephalization of the nervous system along phylogeny, shows specific stereotyped behavioral patterns following exposure to drugs acting on neural transmission. In this study, the authors investigated the effects of exposure to the synthetic cannabinoid receptor agonist WTN55212.2 on motor activity in planaria. WTN55212.2 produced dose-dependent stimulation of motor behavior. High doses of the drug caused stereotyped activities identical to those seen previously with opioid agonists. These effects were antagonized by coexposure to cannabinoid or opioid receptor antagonists. The results indicate that functional interactions between cannabinoid and opioid systems are highly conserved along phylogeny, at least at the behavioral level.

Quantitative assessment of dopamine D2 antagonist activity using invertebrate (Planaria) locomotion as a functional endpoint

INTRODUCTION

Dopaminergic ligands, including drugs of abuse, modulate the locomotor activity of planarians and induce characteristic abnormal patterns of motility at high doses. It has been presumed that the effect is related to dopamine receptors based on ligand specificity and effects on second messenger levels. However, to date, the measured changes have been mostly qualitative in nature and it is not completely clear that the effect is related to stereospecific receptor mechanisms.

METHODS

The present study addressed these issues by devising a convenient and sensitive metric (locomotor velocity, pLMV) and applied the method to test Planaria enantiomer-sensitivity to a dopamine D2-receptor antagonist.

RESULTS

pLMV was remarkably constant over the observation period and established a stable baseline against which to study and quantitate pharmacologic intervention. Further, S(-)-sulpiride at low doses (10(-10) to 10(-8) M) attenuated pLMV in a dose-dependent manner, but R(+)-sulpiride was only 1/25th as potent.

DISCUSSION

The new methodology thus provides a method for quantifying actions of D2 ligands in a simple in vivo system.

Reinforcing effects of methamphetamine in planarians

Reinforcing properties of dopamine agonist, methamphetamine, for planarians were examined with the conditioned place preference (CPP) procedure. The planarians showed preference for the environment associated with methamphetamine administration. This reinforcing effect was antagonized by pretreatment with non-selective dopamine antagonist, haloperidol. Both selective D1 antagonist SCH23390 and selective D2 antagonist sulpiride also blocked the reinforcing effect of methamphetamine. These results suggest that reinforcing effects of dopaminergic drugs can be traced back to invertebrates such as planarians.

Acetylcholine/dopamine interaction in planaria

Planaria represents the most primitive example of centralization and cephalization of nervous system. Previous reports indicate that planaria shows specific behavioral patterns, analogous to mammalian stereotypes, in response to drugs acting on acetylcholine or dopamine transmission. Here we further characterized these responses, and investigated the interactions between cholinergic and dopaminergic systems by means of behavioral methods. Exposure to cholinergic agonists physostigmine or nicotine produced hypokinesia with 'bridge-like' and 'walnut' positions, respectively. Blockade of muscarinic receptors by atropine produced 'screw-like' hyperkinesia. Exposure to dopamine agonists (nomifensine, apomorphine) produced marked hyperkinesia with 'screw-like' movements. Finally, exposure to dopamine antagonists produced immobility or 'bridge-like' position. Pre-exposure to physostigmine blocked the behavioral effects of nomifensine and reduced and markedly delayed the behavioral effects of apomorphine. Pre-exposure to apomorphine slightly reduced and delayed the behavioral changes by physostigmine. Finally, planaria exposed to atropine after either SCH23388 or sulpiride showed 'C-like' or 'screw-like' hyperkinesia, respectively. Thus, reduction of cholinergic transmission seems to play a pivotal role in determining hyperkinesia in planaria. Under these conditions, different patterns of hyperkinetic activities occur, according to the subpopulation of dopamine receptors stimulated by drugs. These findings suggest that interactions between cholinergic and dopaminergic systems occur very early in animal phylogeny.

Circadian rhythms of melatonin-synthesizing enzyme activities and melatonin levels in planarians

In most vertebrates and several insects, melatonin (N-acetyl-5-methoxytryptamine) is synthesized enzymatically from serotonin (5-hydroxytryptamine) by the sequential action of arylalkylamine N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT). In the freshwater planarian Dugesia japonica, which belongs to the most primitive metazoan phylum, activities of NAT and HIOMT, as well as melatonin, were found. The apparent Michaelis constants for substrates of NAT and HIOMT in the planarian were similar to those reported for the mammalian pineal gland and retina. When the planarians were maintained under a 12 h light:12 h dark cycle, the activities of NAT and HIOMT and melatonin levels exhibited a significant diurnal variation, peaking at the mid-dark time. In constant darkness, NAT activity and melatonin levels fluctuated with a circadian (about 24 h) rhythm. These data demonstrate that the planarian synthesizes melatonin through the same pathways as those in most vertebrates and several insects, and that its melatonin synthesis fluctuates in a circadian manner. Thus, it is strongly suggested that the planarian contains a circadian clock controlling melatonin synthesis.

Protective effect of talipexole on MPTP-treated planarian, a unique parkinsonian worm model

The planarian, a flatworm, has a high potential for regeneration, and dopamine plays a key role in its behavior. Planarians treated with MPTP underwent autolysis and individual death in a concentration-dependent manner. When the planarian body was cut into anterior, middle and posterior pieces, each piece subsequently regenerated and reorganized to form a new individual within approximately 10 days. The anterior piece was significantly more sensitive than the middle and posterior pieces to MPTP cytotoxicity. Concomitant treatment with talipexole, an anti-parkinsonian drug, inhibited MPTP-induced autolysis and individual death in a concentration-dependent manner. Pramipexole showed a similar protective effect. In addition, post-treatment with talipexole at 1 hr after MPTP completely inhibited MPTP-induced individual death. Although MPTP treatment caused 30% of the planarians to undergo autolysis and individual death within 12 hr, post-treatment with talipexole even at 12 hr completely rescued the remaining 70% of the planarians from death. These results suggest that the MPTP-treated planarian may be useful as a novel parkinsonian model in which talipexole has a protective effect even in the case of post-treatment.

The nervous system of Tricladida. I. Neuroanatomy of Procerodes littoralis (Maricola, Procerodidae): an immunocytochemical study

The organization of the nervous system of Procerodes littoralis (Tricladida, Maricola, Procerodidae) was studied by immunocytochemistry, using antibodies to authentic flatworm neuropeptide F (NPF) (Moniezia expansa). Compared to earlier investigations of the neuroanatomy of tricladid flatworms, the pattern of NPF immunoreactivity in Procerodes littoralis reveals differences in the following respects: 1. Shape and structure of the brain. 2. Number and composition of longitudinal nerve cords. 3. Shape of branches of, and transverse connections between, main ventral nerve cords. 4. Composition of the pharyngeal nervous system. The rich innervation by NPF immunoreactive (IR) fibres and cells of the subepithelial muscle layer, the pharynx musculature and the musculature of the male copulatory apparatus indicates a neurotransmitter or neuromodulatory influence on muscular activity.

The nervous system of Tricladida. II. Neuroanatomy of Dugesia tigrina (Paludicola, Dugesiidae): an immunocytochemical study

The nervous system (NS) of Dugesia tigrina has been studied by immunocytochemical double-staining, using the authentic flatworm neuropeptide, neuropeptide F (NPF), and serotonin (5-HT) on cryosections. This technique provides a precise morphological (descriptive) account of the NS. The results show that the central nervous system is shaped like a horseshoe. The brain is composed of two lateral lobes connected by three commissures, one antero-dorsal in front of the cerebral eyes and two, more ventral, behind the eyes. The pair of main nerve cords extend from the lateral lobes of the brain to the tail end of the worm. Cross sections reveal a very close contact between lateral branches from the main cords and the submuscular plexus. Thin cord-like lateral nerves are formed by longitudinal plexal fibres. No dorsal cords were observed. The patterns of immunoreactivity to NPF and 5-HT differ from each other in several respects. In the walls of gut diverticula only NPF immunoreactive (IR) cells and fibres were observed. Only NPF-immunoreactive cells occur in the parenchyma along dorso-ventral nerve fibres connecting the dorsal and ventral parts of the submuscular plexus. The number of 5-HT-immunoreactive cells associated with the main nerve cords (MCs) is greater than that of the NPF-immunoreactive cells, and the spongy structure of the MCs is more apparent following immunostaining for 5-HT. Thin 5-HT-immunoreactive fibres were observed in the subepithelial plexus, penetrating the basal lamina and innervating a rhabdite-free ventro-lateral sensory area along the body periphery. The correspondence between MCs in the lower flatworms (Catenulida and Macrostomida) and the Seriata (Tricladida and Proseriata) confirms the status of the MCs in flatworms as the most important and stable neuronal characteristic, and constitutes support for the hypothesized common origin of the MCs in flatworms.

The peptidergic nervous system of the triclad turbellarian, Bdelloura candida (Maricola, Bdellouridae): an immunocytochemical study using an antiserum raised to an endogenous neuropeptide, GYIRFamide

The organisation of the nervous system of Bdelloura candida (Tricladida, Maricola, and Bdellouridae) was studied by immunocytochemistry, by using an antiserum raised to the authentic B. candida FMRFamide-related peptide (FaRP), GYIRFamide. Immunostaining was intense and abundant throughout both the central and peripheral nervous systems, being localised to the brain, the longitudinal nerve cords and their transverse and lateral connections, the pharyngeal plexus, the extensive sub-epidermal and sub-muscular plexuses, and elements of the reproductive apparatus. Compared to an earlier anatomical investigation of this species, and also to the neuroanatomy of other triclad turbellarians, the pattern of GYIRFamide-immunoreactivity reveals differences in the following aspects: the shape and structure of the brain, the distribution of longitudinal nerve cords and their relationships with the peripheral nervous system, the structure and distribution of the lateral nerves and the transverse connectives between the longitudinal nerve cords, organisation of the pharyngeal nervous system, and innervation of the eyespots and epidermal sensory structures. Although this study focuses on a descriptive account of the neuroanatomy of Bdelloura candida, by using anti-GYIRFamide as a neuronal marker, the possible functions of the native peptide are also discussed. The quality and reproducibility of the immunostaining obtained during this work highlights the effectiveness of the GYIRFamide antiserum in the neuroanatomical study of flatworms, and also the suitability of B. candida as a model species in studies of the turbellarian nervous system.

gamma-Aminobutyric acid in the nervous system of a planarian

The amino acid gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in both vertebrates and invertebrates. Despite this, no reports of GABA in flatworms have to date been published. We have studied the presence of GABA in the planarian Dugesia tigrina with immunocytochemical methods and high-pressure liquid chromatography. Fibers showing GABA-like immunoreactivity (GABA-IR) were present in abundance in the longitudinal nerve cords and lateral nerves. GABA-IR was revealed in fibers forming commissures in the brain. The ventral part of the subepidermal plexus showed GABA-IR. No cell somata containing GABA-IR could be identified with certainty. The chromatographic analysis showed that the average GABA concentration in D. tigrina is 533.6 pmol/mg protein. This is substantially higher than the concentrations of dopamine (62.87 pmol/mg) and serotonin (233.20 pmol/mg). An enzyme assay confirmed the capacity for GABA-synthesis in D. tigrina. The results indicate that GABA-containing neurons appeared earlier in evolution than was previously thought and that GABA may serve an important role already in the flatworms.

3,4-Dihydroxyphenylethylamine, L-3,4-dihydroxyphenylalanine and 3,4,5-trihydroxyphenylalanine: oxidation and binding to membranes. A comparative study of a neurotransmitter, a precursor and a neurotransmitter candidate in primitive nervous systems

At neutral (7.0) and slightly basic (8.2) pH, L-3,4-dihydroxyphenylalanine (L-DOPA), 3,4,5-trihydroxyphenylalanine (5-OH-DOPA) and 3,4-dihydroxyphenylethylamine (dopamine) undergo autoxidation. The binding of radiolabeled oxidation products of L-DOPA, 5-OH-DOPA and dopamine to membrane proteins was compared by a filtration procedure. Membranes from tentacles of the sea anemone Metridium senile bind significantly more 5-OH-DOPA than L-DOPA and dopamine. Membranes from rat brain and brains from the three-spined stickleback Gasterosteus aculeatus, bind significantly more dopamine than L-DOPA and 5-OH-DOPA. Membranes from Metridium contain an o-diphenol O2: oxidoreductase (tyrosinase). In the absence of inhibitors, enzymatic oxidation causes a fiftyfold increase in binding of L-DOPA and a more than tenfold increase in binding of dopamine, whereas the binding of 5-OH-DOPA only is increased by 10%. It is concluded than 5-OH-DOPA more easily undergo autoxidation than L-DOPA and dopamine, but its quinone form is probably less reactive with membrane proteins. The suitability of tyrosinase-mediated biosynthesis of L-DOPA and 5-OH-DOPA versus tyrosine hydroxylase-mediated biosynthesis of L-DOPA and dopamine in primitive nervous systems and in the vertebrate CNS is discussed on the basis of the cytotoxic potential through irreversible binding to membrane proteins of oxidation products of the catechol compounds formed.

Immunohistochemical and electrochemical detection of serotonin in the nervous system of Fasciola hepatica, a parasitic flatworm

The head region of the trematode parasite Fasciola hepatica contains 3.47 +/- 0.42 pmol/mg wet wt. of serotonin as measured by high-performance liquid chromatography coupled to electrochemical detection. The head region includes the cerebral ganglia, the transverse commissure and associated nervous tissue that innervates the musculature of the oral sucker, pharynx and body wall. Tissue from the tail, which contains little nervous innervation, has approximately 20 times less serotonin (0.18 +/- 0.01 pmol/mg wet wt.). Immunohistochemistry was used to identify serotonin-like immunoreactive cells. Bipolar and multipolar cell bodies in the cerebral ganglia show serotonin-like immunoreactivity. Also evident are serotonin-like immunoreactive processes in the neuropile and in the transverse commissure that connects the ganglia, and immunoreactive peripheral bipolar cell bodies innervating the musculature of the pharynx and body wall. The cell bodies containing serotonin are organized in bilateral symmetry with homologous cell bodies and processes represented in each ganglion and on both sides of the pharynx.

Photoperiodic modulation of cephalic melatonin in planarians

Endogenous melatonin was detected by high-performance liquid chromatography (HPLC) with electrochemical detection (EC) in the head of the planarian Dugesia dorotocephala. The identity of this elution peak was further confirmed by radioimmunoassay. In groups of planarians adapted to either normal or reversed photoperiods, the melatonin levels were always higher in those heads collected in the dark period than in those collected in the light period. This indicates that primitive animals such as planarians have already evolved a melatonin-metabolizing system that is photically driven in a manner suggestive of the way melatonin synthesis is influenced by light and dark cycles in vertebrates.

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.

The brain of the planarian as the ancestor of the human brain

The planarian is the simplest living animal having a body plan of bilateral symmetry and cephalization. The brain of these free-living flatworms is a bilobed structure with a cortex of nerve cells and a core of nerve fibres including some that decussate to form commissures. Special sensory input from chemoreceptors, photoreceptor cells of primitive eyes, and tactile receptors are integrated to provide motor responses of the entire body, and local reflexes. Many morphological, electrophysiological, and pharmacological features of planarian neurons, as well as synaptic organization, are reminiscent of the vertebrate brain. Multipolar neurons and dendritic spines are rare in higher invertebrates, but are found in the planarian. Several neurotransmitter substances identified in the human brain also occur in the planarian nervous system. The planarian evolved before the divergence of the phylogenetic line leading to vertebrates. This simple worm therefore is suggested as a living example of the early evolution of the vertebrate brain. An extraordinary plasticity and regenerative capacity, and sensitivity to neurotoxins, provide unique opportunities for studying the reorganization of the nervous system after injury. Study of this simple organism may also contribute to a better understanding of the evolution of the human nervous system.

Effects of dopaminergic agents on monoamine levels and motor behaviour in planaria

1. Dopamine, serotonin and, in lesser amounts, norepinephrine were detected in Dugesia gonocephala using electrochemical detection coupled with liquid chromatography (LCED). 2. Treatment with L-dopa induced hyperkinesias, and a rise in dopamine, serotonin and norepinephrine content, whereas reserpine reduced motor activity and the concentrations of all three monoamines. 3. Haloperidol reduced motor activity and dopamine and norepinephrine levels. 4. Apomorphine induced hyperkinesias without altering monoamine levels.

Planarians as a model system for in vitro teratogenesis studies

Free-living flatworms such as planarians are inexpensive to culture, maintain, and use for toxicologic testing in the laboratory. A considerable number of basic studies by ourselves and others indicate that, in simplified miniature, they possess many features of biochemical and physiologic organization similar to higher animals such as mammals. These include a well-developed brain with a varied behavioral repertoire including complex maneuvers of prey capture and learning, with a number of the same neurotransmitters used in mammalian brain. They are sensitive to a variety of the same toxicants. Undifferentiated totipotent stem cells, i.e., "neoblasts," which are capable of mitosis and differentiation into any of the various specialized cell types, permit regeneration of complete planarians from fragments. They also provide new cells to replace those lost in the normal cellular turnover of nonregenerating planarians. Both regeneration of surgical fragments and aberrant remodeling of whole planarians model important features of embyrogenesis and are potentially useful for assaying teratogens. Results are described from studies in which various representative teratogenic toxicants were tested in these two different planarian paradigms. The potential of planarian cephalic regeneration for behavioral teratogenesis investigations is also indicated.