The effects of the acetic acid “pain” test on feeding, swimming, and respiratory responses of rainbow trout (Oncorhynchus mykiss)

The acetic acid test is commonly used in conscious mammals to determine nociceptive thresholds and efficacy of analgesics. The purpose of the present study was to examine the response of rainbow trout that were not anaesthetised during the acetic acid test. Rainbow trout (n = 31) were trained to eat in response to a light cue and assigned to one of four treatments: (i) control (handled only), (ii) saline subcutaneous (SC) injection in the lip region, (iii) 2% acetic acid (AA) SC injection in the lip region, and (iv) 5% AA SC injection in the lip region. Swimming behaviour and respiratory frequency were monitored prior to, and 5, 30, 60, 90, 120, 240, and 360 min after treatment. Nine of the 16 fish from both acetic acid treated groups lost equilibrium for 1.7 ± 0.6 min before returning to an upright position swimming normally in the current. The respiratory frequency of all fish significantly increased by 69% (P < 0.001) after treatment; the saline and control group returned to their resting levels after 120 min while the acetic acid groups were 12% higher than resting levels 120 min after treatment until the experiment was terminated at 360 min. Food was presented 15 min after treatment and every fish ate immediately. Compared with two previous studies that used anaesthetised rainbow trout, the acetic acid test in the current study negatively affected the swimming behavioural response for a much shorter duration and did not affect the feeding behavioural response. However, results for respiratory frequency were comparable to those of anaesthetised rainbow trout in the other work.