Perception of breath components by the tropical bont tick, <i>Amblyomma variegatum</i> Fabricius (Ixodidae): I. CO₂-excited and CO₂-inhibited receptors

Wall-pore olfactory sensilla located in the capsule of Haller's organ on the tarsus of <i>Amblyomma variegatum</i> ticks bear cells responding to vertebrate breath: one of these sensilla contains a CO₂-excited receptor and a second sensillum has a CO₂- inhibited receptor. Each of these antagonistic CO₂-receptors, which display typical phasic-tonic responses, monitors a different CO₂-concentration range. The CO₂-inhibited receptor is very sensitive to small concentration changes between 0 and ca. 0.2%, but variations of 0.01% around ambient (ca. 0.04%) induce the strongest frequency modulation of this receptor. An increase of just 0.001–0.002% (10–20 ppm) above a zero CO₂-level already inhibits this receptor. By contrast, the CO₂-excited receptor is not so sensitive to small CO₂ shifts around ambient, but best monitors changes in CO₂ concentrations above 0.1%. This receptor is characterized by a steep dose-response curve and a fast inactivation even at high CO₂-concentrations (>2%). In a wind-tunnel, <i>Amblyomma variegatum</i> is activated from the resting state and attracted by CO₂ concentrations of 0.04 to ca. 1%, which corresponds to the sensitivity range of its CO₂-receptors. The task of perceiving the whole concentration range to which this tick is attracted would thus appear to be divided between two receptors, one sensitive to small changes around ambient and the other sensitive to the higher concentrations normally encountered when approaching a vertebrate host.