Opioid involvement in parasite-induced behavioural modifications: evidence from hamsters infected with Schistosoma mansoni

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Abstract
Although it is well established that parasites can modify the behaviour of their hosts, relatively little is known about the neurochemical correlates of such effects. In the present study we examined the relationships between parasite infection, endogenous opioid system, and host behaviour. Male golden hamsters (Mesocricetus auratus) infected with the blood fluke Schistosoma mansoni for 25 days (prepatent) displayed a significant enhancement of their locomotory activity levels, whereas patent animals infected for 40 days displayed a significant decrease in their activity. The schistosome-induced increase in activity was blocked by peripheral administration of the specific antagonist toward δ-opiate receptors, ICI 154,129, while the decrease in activity was blocked by the prototypic μ-opiate antagonist naloxone. In a parallel manner, the δ antagonist blocked the increased locomotor activity induced by a low dose (5.0 mg/kg) of the prototypic opiate agonist morphine, while the μ antagonist blocked the reduction in activity induced by a higher dose (10 mg/kg) of morphine. These observations, along with previously described opioid-mediated analgesic responses in parasitized hamsters, suggest that schistosome infection can lead to a differential augmentation of endogenous opioid activity. These findings suggest that parasite-induced modifications of host behaviour may, in part, arise from alterations in opioid modulation of behaviour.