The orienting response of Lake Michigan mottled sculpin is mediated by canal neuromasts

Lake Michigan mottled sculpin, Cottus bairdi, exhibit a naturally occurring and unconditioned orienting response that can be triggered by both live pr ey and chemically inert vibrating spheres, even in blinded animals. CoCl2-i nduced reductions of the orienting response demonstrate that the lateral li ne is required for this behavior in the absence of non-mechanosensory cues (such as vision), but shed no light on the relative contributions of superf icial and canal neuromasts to this behavior, To determine the relative role s of these two subsystems, we measured the frequency with which mottled scu lpin oriented towards a small vibrating sphere before and after two treatme nts: (i) immersion of fish in a solution of gentamicin, an aminoglycoside a ntibiotic that damages hair cells in canal, but not superficial, neuromasts ; and (ii) scraping the skin of the fish, which damages the superficial, bu t not the canal, neuromasts. To ensure that both superficial and canal neur omasts were adequately stimulated, we tested at different vibration frequen cies (10 and 50 Hz) near or at the best frequency for each type of neuromas t, At both test frequencies, response rates before treatment were greater t han 70 % and were significantly greater than 'spontaneous' response frequen cies measured in the absence of sphere vibration. Response rates fell to sp ontaneous levels after 1 day of gentamicin treatment and did not return to pre-treatment levels for 10-15 days. In contrast, response rates stayed app roximately the same after superficial neuromasts had been damaged by skin a brasion. Scanning electron microscopy confirmed hair cell damage (loss of a pical cilia) in canal, but not superficial, neuromasts of gentamicin-treate d animals after as little as 24 h of treatment. The sensory epithelium of c anal neuromasts gradually returned to normal, following a time course simil ar to behavioral loss and recovery of the orienting response, whereas that of superficial neuromasts appeared normal throughout the entire period. Thi s study shows that the orienting response of the mottled sculpin is mediate d by canal neuromasts.