Technical Abstract: P2X ion channels have been functionally characterised from a range of eukaryotes. Whilst these receptors can be broadly classified into fast and slow desensitising, the molecular mechanisms underlying current desensitisation are not fully understood. Here we describe the characterisation of a P2X channel from the cattle tick Boophilus microplus (BmP2X) displaying extremely slow current kinetics, little desensitisation during ATP application and marked run-down in current amplitude between sequential responses. ATP (EC50 67.1 µM) evoked concentration dependent currents at BmP2X which were antagonised by suramin (IC50 4.8 µM) and potentiated by the antiparasitic drug amitraz. Ivermectin did not potentiate BmP2X currents but the mutation M362L conferred ivermectin sensitivity. To investigate the mechanisms underlying slow desensitisation we generated intracellular domain chimeras between BmP2X and the rapidly desensitising HdP2X receptor from Hypsibius dujardini. Exchange of N- or C-termini between these fast and slow desensitising receptors altered the rate of current desensitisation towards that of the donor channel. Truncation of the BmP2X C-terminus identified the penultimate residue (R413) as important for slow desensitisation. Removal of positive charge at this position in the mutant R413A resulted in significantly faster desensitisation which was further accentuated by the negatively charged substitution R413D. R413A and R413D however still displayed current run-down to sequential ATP application. Mutation to a positive charge (R413K) reconstituted the wild-type phenotype. This study identifies a new paradigm by which positive charge in the C-terminal domain regulates P2X receptor desensitisation during ATP application and further demonstrates that run-down and desensitisation are governed by distinct mechanisms.