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

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 amphipod Gammarus pulex infected with acanthocephalan parasites. Pomphorhynchus laevis and, to a lesser extent, Pomphorhynchus tereticollis altered phototactism, but not geotactism, in G. pulex, whereas the reverse was true for Polymorphus minutus. Serotonin (5-hydroxytryptamine, 5-HT) injected to uninfected G. pulex mimicked the altered phototactism, but had no effect on geotactism. Photophilic G. pulex infected with P. laevis or P. tereticollis showed a 40% increase in brain 5-HT immunoreactivity compared to photophobic, uninfected individuals. In contrast, brain 5-HT immunoreactivity did not differ between P. minutus-infected and uninfected G. pulex. Finally, brain 5-HT immunoreactivity differed significantly among P. 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.

Figure 1

Phototactic behaviour in acanthocephalan-infected and uninfected (site specific) G. pulex. (a) Samples collected in the river Ouche; (b) samples collected in the river Be`ze. Figures show median phototactism scores, interquartile range, sample size and significance as determined by Wilcoxon–Mann–Whitney two-sample test (P. laevis), Kruskal–Wallis one-way test (P. tereticollis and P. minutus) and 2×2 significance as determined by post hoc tests, ^***p≤0.0001.

Figure 2

(a) Phototactic behaviour of uninfected and non-injected G. pulex (C) and those injected with 1 μl Ringer solution (R), octopamine (5 μg μl⁻¹) (O) or 5-HT (5 μg μl⁻¹) (S). (b) Geotactic behaviour of non-injected G. pulex (C) and those injected with 1 μl Ringer solution (R) or 5-HT (5 μg μl⁻¹) (S). Figures show median phototactism/geotactism scores, associated interquartile range, sample size and significance as determined by Kruskal–Wallis one-way test and 2×2 significance as determined by post hoc tests, ^***p≤0.0001.

Figure 3

Images show 5-HT immunoreactivity (yellow) within the brains of (a) uninfected (arrow shows position of TGN cell body) and (b) P. laevis-, (c) P. tereticollis- and (d) P. minutus-infected individuals. No differences, gross or fine, in brain anatomy from infected and uninfected individuals were observed. Bar shows 100 μm.

Figure 4

Optical densitometric measures of 5-HT immunoreactivity within the tritocerebral region of G. pulex infected with P. laevis, P. tereticollis and P. minutus, shown as relative to uninfected controls (dotted line). Only the brains of those individuals showing extreme alteration of normal phototactic behaviour were examined here. Figures show standardized means against site-specific controls, associated 95% CI, sample size and significance as determined by ANOVA. No effect of experimental block was found (p=0.05 for all).

Figure 5

Optical densitometric measures of 5-HT immunoreactivity within the tritocerebral region of G. pulex infected with P. tereticollis showing altered and unaltered phototactic behaviour. Figure shows mean values, associated 95% CI, sample size and significance as determined by ANOVA.