Classical transmitters are present in all phyla that have been studied; however, our detailed understanding of the process of neurotransmission in these phyla is patchy and has centred on those neurotransmitter receptor mechanisms which are amenable to study with the tools available at the time, for example, high-affinity ligands, tissues with high density of receptor protein, suitable electrophysio-logical recording systems. Studies also clearly show that many neurones exhibit co-localization of classical transmitters and neuropeptides. However, the physiological implications of this co-localization have yet to be elucidated in the vast majority of examples.

The application of molecular biological techniques to the study of neurotransmitter receptors (to date mainly in vertebrates) is contributing to our understanding of the evolution of these proteins. Striking similarities in the structure of ligand-gated receptors have been revealed. Thus, although ligand-gated receptors differ markedly in terms of the endogenous ligands they recognize and the ion channels that they gate, the structural similarities suggest a strong evolutionary relationship. Pharmacological differences also exist between receptors that recognize the same neurotransmitter but in different phyla, and this may also be exploited to further the understanding of structure-function relationships for receptors. Thus, for instance, some invertebrate GABA receptors are similar to mammalian GABAa receptors but lack a modulatory site operated by benzodiazepines. Knowledge of the structure and subunit composition of these receptors and comparison with those that have already been elucidated for the mammalian nervous system might indicate the functional importance of certain amino acid residues or receptor subunits. These differences could also be exploited in the development of new agents to control agrochemical pests and parasites of medical importance.

The study of the pharmacology of receptor proteins for neurotransmitters in invertebrates, together with the application of biochemical and molecular biological techniques to elucidate the structure of these molecules, is now gathering momentum. For certain receptors, e.g. the nicotinic receptor, we can expect to have fundamental information on the function of this receptor at the molecular level in both invertebrates and vertebrates in the near future.

This paper provides an overview of research on the nervous system of parasitic platyhelminths. We have emphasized studies concerned with the physiological, pharmacological and biochemical nature of the major small molecule neurotransmitters of these parasites. We have attempted to provide a critical review of the work by focusing on important unresolved issues. Finally, we have focused on some recent work in our laboratory, using patch-clamp recording techniques and quantitative fluorescence cytometry, as an example of newer methods that will hopefully resolve some of the unanswered questions concerning the nervous system of these parasites.

The organization of Ascaris motoneurones and nervous system is summarized. There is an anterior nerve ring and associated ganglia, main dorsal and ventral nerve cords which run longitudinally, and a small set of posterior ganglia. Cell bodies of motoneurones are found in the ventral nerve cord and occur in 5 repeating ‘segments’; each contains 11 motoneurones. Seven morphological types of excitatory or inhibitory motoneurone are recognized.

Each Ascaris somatic muscle cell is composed of the contractile spindle; the bag region, containing the nucleus; the arm; and the syncytial region, the location of neuromuscular junctions. The resting membrane potential of muscle is approximately — 30 mV and shows regular depolarizing, Ca-dependent ‘spike potentials’ superimposed on smaller Na+- and Ca2+-dependent ‘slow waves’ and even slower ‘modulation waves’. The membrane shows high Cl- permeability. Adjacent cells are electrically coupled so that electrical activity in the cells is synchronized. Acetylcholine (ACh) and γ-aminobutyric acid (GABA) affect the electrical activity. Bath-applied ACh increases membrane cation conductance, depolarizes the cells, alters the frequency and amplitude of spike potentials and produces contraction. Bath-applied GABA increases Cl- conductance, decreases spike activity and causes hyperpolarization and muscle relaxation.

The extra-synaptic ACh receptors on the bag region of Ascaris muscle can be regarded as a separate subtype of nicotinic receptor. ACh and anthelmintic agonists (pyrantel, morantel, levamisole) produce a dose-dependent increase in cation conductance and membrane depolarization which is blocked by tubocurarine, mecamylamine but not by hexamethonium. The potency, of GABA agonists, with the exception of sulphonic acid derivatives, correlates with the vertebrate GABAa receptor. The potency of antagonists does not. Thus, bicuculline, securinine, pitrazepine, SR95531 and RU5135 are potent vertebrate GABAa antagonists but have little effect on GABA receptors. The potency order of the arylaminopyridazine GABA antagonists: SR95103, SR95132, SR42666, SR95133, SR95531, SR42627 and SR42640 at the Ascaris GABA receptors contrasts with that at vertebrate GABAa receptors. It has been suggested that the receptor is referred to as a GABAn receptor.

Patch-clamp studies show that ACh activates a non-selective cation channel which has a main conductance of 40–50pS and apparent mean open time of 1·3 ms; a smaller channel of 20–30 pS with a similar open-time is also activated. Pyrantel and levamisole also produce openings with similar conductances and open-times. GABA activates a Cl- channel with a main state conductance of 22 pS and an apparent mean open duration of 32 ms; conductance states of 10 and 15 pS are also seen. Piperazine similarly activates this channel but the mean open-time is shorter (14 ms). Ivermectin in high doses, is an antagonist which reduces the GABA channel conductance and Popen; it does, however, open ‘small’ Cl- channels when applied to the outside surface of membrane. These channels have a conductance of 9–15 pS and very long open times (> 100 mS). 5-HT does not have a direct effect on membrane potential or conductance but acts on cAMP levels and glycogen metabolism. Dopamine, octopamine and AF1 may act as neurotransmitters or neuromodulators.

Many medically important diseases of man are caused by blood-sucking arthropods which serve as vectors for a wide range of viral, bacterial, protozoal and nematode infections (Table 1). Furthermore, serious economic losses are caused by the numerous arthropod parasites which infect domesticated animals (for examples, see Table 2). Among these the ixodid hard ticks are particularly important and it has been estimated that the global cost of hard tick infections is $7000 million per annum (F.A.O., 1984). Not surprisingly, there have been strenuous efforts to control infections caused by arthropods.

At the cellular and molecular levels, the small and simpler nervous systems of invertebrates do not differ fundamentally from the larger more complex ones of vertebrates. It seems therefore that the special properties of the human brain arise more from the fact that it has ten trillion cellular components than from any unusual properties of the components themselves. By studying invertebrates we can gain insight into what basic functions are performed by the cells and molecules of the nervous system and this will contribute to a more fundamental understanding of what goes on in a system in which the same functions are performed by uncountable numbers of neurones. Invertebrate studies are also important for an entirely different reason; they are interesting and important in their own right. Moreover, a relatively small number of invertebrates are pests; either parasites, vectors of serious parasitic diseases or pests of our agricultural production. It is no accident that most of the methods that are used to control such organisms act on their nervous system. That is because the nervous system is a complex chemical machine which works through a great variety of chemical interactions between a wide diversity of receptors and ligands. Many currently used control methods work because they disrupt these interactions. For this reason I would imagine that the new generation of compounds developed to control invertebrates will depend for their activity on interactions with the nervous system. Since most of the chemical effectors (transmitters, modulators and hormones) in the nervous system are peptides, a number of these newly developed approaches will depend upon a fundamental knowledge of peptidergic systems in parasites. This essay is about peptidergic systems and indicates how we might exploit their vulner-ability to interference.

The neuropeptide story began in 1928 with the description by Ernst Scharrer of gland-like nerve cells in the hypothalamus of the minnow, Phoxinus laevis. Because these nerve cells were overwhelmingly specialized for secretory activity, overshadowing other neuronal properties, Scharrer termed them ‘neurosecretory neurons’. What was even more remarkable about the cells was that their products were released into the bloodstream to act as hormones, specifically neurohormones. Neurosecretory cells were identified largely on morphological grounds. That is, they could be stained with special techniques, such as chrome-haematoxylin and paraldehyde-fuchsin, although the techniques are far from specific, staining non-neurosecretory cells as well. However, the basis for the ‘special’ neurosecretory techniques is the demonstration of sulphur-containing proteins – so they are indicative of peptide-producing neurones. An alternative characteristic of neurosecretory cells is the presence of large (> 100 nm), dense-cored vesicles at the electron microscope level; these are the so-called elementary granules of neurosecretion, or ENGs. However, implicit in the concept of neurosecretion is that the prime function of the neurosecretory cell is in endocrine regulation, exerting a hormone-like control over some aspect of the organism's metabolism, by controlling endocrine glands and other effector organs. To satisfy this criterion, evidence had to be obtained of cycles of secretory activity within the cell that could be correlated with a change in the physiological condition of the organism.

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

Most of the successful anti-nematode drugs currently available affect the nematode locomotory system. Their success is due to their interactions with molecules associated with the main neuro-transmitters of the motor nervous system, acetylcholine and GABA. These drugs tend to have a relatively broad spectrum of action, affecting a wide variety of nematodes, presumably because nematode motor nervous systems are conservative in their use of these transmitters.

Although structurally simple, the nervous systems of parasitic helminths are biochemically complex. As well as the classical transmitters, the nervous systems of helminths contain a range of regulatory peptides and so far some 29 different peptides have been described. The only group of compounds thought to be transmitters in free-living organisms which have not yet been shown to be transmitters in helminths are the purines. There is evidence in helminths, as in vertebrates, for the co-localization, in the same cell, of classical transmitters and peptidergic molecules and for the non-neuronal occurrence of some regulatory peptides. Co-localization would effectively increase the number of transmitter types, since compounds could be released in different combinations. How such a selective release might be achieved is unknown. Different regulatory peptides are localized in different parts of the helminth nervous system, but it is quite probable that more than one peptide can occur in the same cell. Whether the relative proportions of the different peptides change under different physiological or developmental conditions is not known. The classical transmitters similarly show a differential distribution. It is presumed that the regulatory peptides in helminths, as in other organisms, are synthesized by post-translational modification of a high molecular weight precursor protein and that the peptides are not recycled but are removed from their site of action by specific peptidases. The latter are located near the receptor sites and may be membrane-bound. In vertebrates the precursor molecules usually contain several different peptides with different biological functions and processing of the precursor often shows tissue-specific variations.