Effect of combined feeding stimuli on ingestion by the nematodeTrichostrongylus colubriformis

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Abstract
Ingestion by the nematodeTrichostrongylus colubriformis was stimulated by serotonin, histamine, and dopamine. Immune serum inhibited feeding by both sexes while octopamine depressed feeding by only male nematodes. However, the combined influence of these excitatory and inhibitory stimuli is unknown. Feeding in vitro by both sexes of the nematodeT. colubriformis was similar in millimolar ratios of histamine and dopamine or histamine and serotonin, but not dopamine and serotonin. Feeding by nematodes that was stimulated by histamine, dopamine, and serotonin was decreased by subsequent exposure to octopamine. Conversely, elevated concentrations of histamine, dopamine, and serotonin decreased the inhibitory action of octopamine on helminth ingestion in vitro. Doses of immune serum also decreased the stimulated feeding of nematodes that was induced by histamine, dopamine or serotonin. However, the addition of elevated concentrations of these biogenic amines to immune serum reversed the inhibitory effect. The feeding activity of nematodes may represent a summation of the inhibitory and excitatory stimuli that occur in the chemical environment of the helminth.