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The Characterisation of an Adrenergic Signalling System Involved in the Encystment of the Ocular Pathogen Acanthamoeba spp.The aim of this study was to identify and characterise the receptor system involved in controlling encystment in Acanthamoeba using specific agonists and antagonists and to examine whether endogenous stores of catecholamines are produced by the organism. Acanthamoeba trophozoites suspended in axenic growth medium were exposed to adrenoceptor agonists and antagonists to determine which compounds promoted or prevented encystment. Secondly, trophozoites were cultured in medium containing a catecholamine synthesis inhibitor to investigate the effect this had on natural encystment. Non-specific adrenoceptor agonists including epinephrine, isoprotenerol and the selective β1 adrenoceptor agonist dobutamine were found to cause >90% encystment of Acanthamoeba trophozoites compared to <30% with the controls. The selective β1 antagonist metoprolol was able to inhibit epinephrine mediated encystment by >55%. Cultures of Acanthamoeba with the catecholamine synthesis inhibitor α-methyl-p-tyrosine significantly reduced the level of amoebic encystment compared to controls. In conclusion Acanthamoeba appear to contain a functional adrenergic receptor system of unknown structure which is involved in initiating the encystment process that can be activated and blocked by β1 agonists and antagonists respectively. Furthermore the presence of this receptor system in Acanthamoeba indicates that topical β adrenoceptor blockers may be effective adjunct therapy by reducing the transformation of trophozoites into the highly resistant cyst stage. This article is protected by copyright. All rights reserved.