Localization of a sodium-dependent high-affinity serotonin transporter and recruitment of exogenous serotonin by the cestodeHymenolepis diminuta: an autoradiographic and immunohistochemical study

Abstract

Uptake of serotonin by tissues of intact and hemitransected Hymenolepis diminuta was studied by autoradiography and immunohistochemistry. Hemitransected worms were incubated in balanced saline containing 10 µM [ 3 H]serotonin and washed extensively. The density of silver grains over the serotonin-immunoreactive longitudinal nerve cords and commissural rings, male reproductive system, tissues surrounding the genital pouch, and deep longitudinal muscles was significantly greater than that over the parenchyma. The presence of serotonin in spermatozoa suggested a role for this amine in spermatozoon activity. In contrast, uptake of 10 µM [ 3 H]serotonin in sodium-free saline was significantly reduced compared with that in balanced saline in all tissues examined except the parenchyma. Analysis of the data revealed that the sodium-dependent high-affinity serotonin transport system is localized primarily in the serotonergic-like neurons of H. diminuta, which suggests possible recycling of neuronally released serotonin. Following incubation of intact worms in vitro for 12 h in 5 µM [ 3 H]serotonin, the density of silver grains was significantly higher over the serotonin-immunoreactive nerves, elements of the male reproductive system, tissues surrounding the genital pouch, and deep longitudinal muscles than over the parenchyma. These results demonstrate recruitment of exogenous serotonin by intact H. diminuta and suggest sequestration and concentration by the serotonin-immunoreactive neurons via the sodium-dependent high-affinity transporter. These data further suggest that although H. diminuta can synthesize serotonin, it may obtain serotonin from the host. Nonetheless, the amount of serotonin recruited by H. diminuta from the host in vivo compared with that which they synthesize is not known.Résumé : L'absorption de sérotonine dans les tissus de cestodes intacts et de cestodes Hymenolepis diminuta coupés en deux sur le sens de la longueur, a été suivie par autoradiographie et par immunohistochimie. Les vers coupés en deux ont été gardés en incubation dans une solution saline équilibrée contenant 10 µM d'[ 3 H]sérotonine, puis lessivés abondamment. La densité des grains d'argent le long des cordons nerveux longitudinaux immunoréactifs à la sérotonine et dans les anneaux des commissures, dans le système reproducteur mâle, dans les tissus entourant le sac génital et dans les muscles longitudinaux profonds était significativement plus élevée que sur le parenchyme. La présence de sérotonine dans les spermatozoïdes semble indiquer que cette amine a un rôle à jouer dans l'activité des spermatozoïdes. En revanche, l'absorption de 10 µM [ 3 H]sérotonine chez les vers gardés en solution saline en l'absence de sodium plutôt qu'en solution équilibrée a été significativement plus faible dans tous les tissus examinés sauf le parenchyme. L'analyse des données indique que le système de transport de la sérotonine qui exige la présence de sodium est localisé surtout dans les neurones de type sérotonergique d'H. diminuta, ce qui dénote peu...

Classical transmitters and their receptors in flatworms

“…Aside from the recruitment of host 5HT, the flatworm SERT is likely to play an important role in the recycling and inactivation of the amine. Autoradiographic studies of 3 H-5HT transport in Hymenolepis diminuta identified a concentrative, sodium-dependent transport system located within the CNS and musculature, as well as non-neuronal sites, including male reproductive structures and the tissues surrounding the genital pouch (Osloobi & Webb, 1999). This internal transport system is believed to mediate the re-uptake of neuronally released 5HT and may also sequester excess hostderived amine that would otherwise accumulate to toxic levels.…”

Serotonin Signalling in Flatworms: An Immunocytochemical Localisation of 5-HT7 Type of Serotonin Receptors in Opisthorchis felineus and Hymenolepis diminuta

“…Thus, the serotonin synthesis pathway was identified. The key enzyme for serotonin synthesis-the 5-hydroxytryptophan hydroxylase-was detected [25,37,[47][48][49]. It was shown that H. diminuta are capable not only of synthesising but actively absorbing the serotonin from the host, having a specialised transport system [38,132].…”

“…At the same time, it has been found that parasitic flatworms can synthesise serotonin using the serotonin synthesis enzyme, tryptophan hydroxylase, detected in various representatives of flatworm, expressing in both the parasitic and free-living stages of flatworm development. Thus, serotonin transporters and the synthesis pathway were identified in S. mansoni [43][44][45][46] and in H. diminuta [37,[47][48][49].…”