Ligula intestinalis (Cestoda) infections of roach (Rutilas rutilus) (Cyprinidae): immunocytochemical investigations into the salmon- and chicken-II type gonadotrophin-releasing hormone (GnRH) systems in host brains

The tapeworm Ligula intestinalis interferes with the pituitary-gonadal axis of its fish host so that parasitised fish are unable to reproduce. To inve stigate how this effect may be brought about, antibodies against salmon- an d chicken-II type gonadotrophin-releasing hormones (sGnRH and cGnRH-II, res pectively) have been used in an immunocytochemical investigation of the bra in of roach, Rutilus rutilus. Brains were removed, without the pituitary gl and, from fish obtained from the wild during the period from February to Ap ril, prior to spawning. The sGnRH-positive neuronal system comprised cell b odies and fibres located in the forebrain and midbrain areas. The cGnRH-II- positive neuronal system comprised cell bodies, tracts and fibres mainly lo cated in the posterior hypothalamic lobes and medulla. Salmon-type GnRH-pos itive cell bodies were demonstrated in the midbrain tegmentum. In the foreb rain, the olfactory lobes contained only a few short fibres immunoreactive for sGnRH whilst the floor of the ventral telencephalon was highly reactive for sGnRH. Long tracts of cGnRH-II-positive fibres were observed to course laterally within the hypothalamic lobes, whilst cGnRH-II-positive fibres o riginating in the medulla formed a continuous channel coursing to the spina l chord. GnRH-positive cells were not observed in the preoptic area using a nti-sGnRH or anti-cGnRH-II antibodies. The presence of the tapeworm Ligula intestinalis had no discernible effect on the GnRH-positive cells or fibres . The interaction of L. intestinalis with the pituitary-gonadal system of t he fish host is discussed.