Altered host behaviour and brain serotonergic activity caused by acanthocephalans: evidence for specificity

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Abstract
Manipulative parasites can alter the phenotype of intermediate hosts in various ways. However, it is unclear whether such changes are just by-products of infection or adaptive and enhance transmission to the final host. Here, we show that the alteration of serotonergic activity is functionally linked to the alteration of specific behaviour in the amphipodGammarus pulexinfected with acanthocephalan parasites.Pomphorhynchus laevisand, to a lesser extent,Pomphorhynchus tereticollisaltered phototactism, but not geotactism, inG. pulex, whereas the reverse was true forPolymorphus minutus. Serotonin (5-hydroxytryptamine, 5-HT) injected to uninfectedG. pulexmimicked the altered phototactism, but had no effect on geotactism. PhotophilicG. pulexinfected withP. laevisorP. tereticollisshowed a 40% increase in brain 5-HT immunoreactivity compared to photophobic, uninfected individuals. In contrast, brain 5-HT immunoreactivity did not differ betweenP. minutus-infected and uninfectedG. pulex. Finally, brain 5-HT immunoreactivity differed significantly amongP. tereticollis-infected individuals in accordance with their degree of manipulation. Our results demonstrate that altered 5-HT activity is not the mere consequence of infection by acanthocephalans but is specifically linked to the disruption of host photophobic behaviour, whereas the alteration of other behaviours such as geotactism may rely on distinct physiological routes.

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