Biogenic amines synthesized from decarboxylation of amino acid are classified into indoleamine (serotonin), catecholamines (dopamine, noradrenaline and adrenaline), -aminogmonoamines (octopamine and tyramine), imidazole (histamine) and amino acid ( butyric acid (GABA)). Noradrenaline has been shown to induce metamorphosis without settlement in the larvae of the Eastern and the Pacific oysters. Dopamine and serotonin agonists and antagonists have been shown to inhibit larval settlement in barnacle B. amphitrite. Adrenoreceptor agonists in solution and adsorbed on a surfaces have been shown to inhibit barnacle Balanus improvisus larval settlement. Adrenoreceptor antagonists have also been shown to inhibit larval settlement in B. amphitrite (Cirripedia), Bugula neritina (Bryozoa) and Hydroides elegans (Polychaeta). Thus the concept of interfering with the surface exploration and attachment mechanisms involved in larval settlement is being explored as the most promising and environmentally benign antifouling option .
The involvement of adrenoreceptor signaling in invertebrate larval settlement and metamorphosis pathways is well demonstrated by these studies. Majority of these studies have focused on screening such pharmacological agents to investigate their ability to inhibit larval settlement. Little information is thus available about the effects of adrenoreceptor agonists or antagonists at a cellular level, or effects of such compounds on the larval ecdysis and metamorphosis. As the neurotransmitters and neuromodulators are potent even at extremely low concentrations from nano-moles to pico-moles, the use of such compounds as biocides deployed in leachable marine paints may not be permitted by various environmental protection agencies of the world. The mechanism through which such compounds manifest their effects on target species is yet to be fully understood and the effects these compounds may have on non-target species is an area still largely unexplored. Thus it may not be wise to deploy such agents in antifouling coatings if are to avoid the environmental devastation similar to that caused by TBT.
Thus the focus of this dissertation is to understand the cellular and signal transduction mechanisms mediated by adrenoreceptor agonist noradrenaline and the effect it has on the larval ecdysis and metamorphosis. A novel strategy of covalently conjugating adrenoreceptor agonists to polymer surface is also investigated. Such a strategy would eliminate the need of a biocide leaching out of the paint. For this study the catecholamine noradrenaline was chosen as a candidate adrenoreceptor agonist. Noradrenaline (NA) has been demonstrated to inhibit oyster larval settlement while inducing metamorphosis via α1-adrenoreceptor activation. Although NA prevented oyster larval settlement, it induced metamorphosis and was not toxic to the larvae.. Effects of noradrenaline have been studied extensively by Lacoste et al from a molluscan immunological perspective. Noradrenaline has been shown to be the principle circulating catecholamine in the hemolymph of C. gigas and is up-regulated when oysters are under environmental stress. NA has also been shown to induce apoptosis in the hemocytes of C. gigas through the β-adrenergic receptor signal transduction pathway . As adrenoreceptor agonists have been shown to inhibit larval settlement in B. improvisus, it was hypothesized that NA would also be able inhibit larval settlement in B. amphitrite while promoting metamorphosis.