Selective and Reversible Blocking of the Lateral Line in Freshwater Fish

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Abstract
Fish possess two separate systems for detection of low-level sound and water motions in the low-frequency range: the inner ear and the lateral line. The relative roles of these systems in normal fish behaviour is still not clear. There is, for instance, a lack of experimental evidence showing the involvement of the lateral line and the inner ear in detection of infrasound, in directional hearing in the near field, and in detection and attack of swimming prey below the surface. To provide a useful tool for such studies, we have developed a pharmacological method for selective and reversible blocking of the lateral line in the roach (Rutilus rutilus). By recording multi-unit activity from the lateral line nerve and microphonic potentials from the inner ear, we have shown that cobalt ions in the external water may completely block the mechanosensitivity of the lateral line without affecting the utricular microphonic activity. This inhibiting effect of Co2+ is antagonized by Ca2+, making the ratio between these ions the important blocking factor. For practical work, we recommend 12–24h exposure to 0.1 mmol 1−1 Co2+ at a Ca2+ concentration of less than 0.1 mmol 1−1. The fish showed no sign of general behavioural disorders even after 1 week in this solution, and the microphonic sensitivity of the inner ear was not reduced. The blocking effect of Co2+ was clearly reversible, and the recovery was dependent upon both the duration of the Co2+ exposure and the Ca2+ concentration of the recovery solution.