Abstract
In the present study, our aim was to compare physiological and behavioural responses to different noxious stimuli to those of a standardized innocuous stimulus, to possibly identify aversive responses indicative of injury detection in a commercially important marine teleost fish, the Atlantic cod. Individual fish were administered with a noxious stimulus to the lip under short-term general anaesthesia (MS-222). The noxious treatments included injection of 0.1% or 2% acetic acid, 0.005% or 0.1% capsaicin, or piercing the lip with a commercial fishing hook. Counts of opercular beat rate (OBR) at 10, 30, 60, 90 and 120 min and observations of behaviour at 30 and 90 min post-treatment were compared with pre-treatment values and with control fish injected with physiological saline, an innocuous stimulus. Circulatory levels of physiological stress indicators were determined in all fish at 120 minutes post-treatment. All treatments evoked temporarily increased OBR that returned to pre-treatment levels at 60 minutes (saline, 0.005% capsaicin, hook), 90 minutes (0.1% acetic acid, 0.1% capsaicin), or 120 minutes (2% acetic acid), but with no significant differences from the control group at any time point. Fish treated with 0.1% and 2% acetic acid and 0.1% capsaicin displayed increased hovering close to the bottom of the aquaria and fish given 2% acetic acid and 0.1% capsaicin also displayed a reduced use of shelter. The only effect seen in hooked fish was brief episodes of lateral head shaking which were not seen pre-treatment or in the other groups, possibly reflecting a resiliency to tissue damage in the mouth area related to the tough nature of the Atlantic cod diet. There were no differences between groups in circulatory stress indicators two hours after treatment. This study provides novel data on behavioural indicators that could be used to assess potentially aversive events in Atlantic cod.
Highlights
The ability of animals to detect stimuli that can cause harm to their tissues is a universal feature termed nociception [1]
Post hoc tests for use of shelter using the Bonferroni correction showed that 2.0% Acetic acid significantly reduced percentage of time the sheltering behaviour was observed (1.6% 61.6 at 30 min and 0.060.0% at 90 min) compared with 0.1% Acetic acid (39.8616.7% at 30 min and 43.1620.1% at 90 min) (p = 0.033) and Saline (57.1620.2% at 30 and 90 min) (p = 0.021). 0.1% Capsaicin decreased the % of time the sheltering behaviour was observed (16.0612.5% at 30 min and 18.8613.1% at 90 min) compared with 0.1% Acetic acid (39.8616.7% at 30 min and 43.1620.1% at 90 min) (p = 0.020) and Saline (57.1620.2% at 30 and 90 min) (p = 0.013)
Post hoc results (Bonferroni correction, mean % of time6S.E.) associated with hovering on the bottom demonstrated that 2.0% Acetic Acid significantly increased the percentage of time (47.8614.0% at 30 min and 21.566.8% at 90 min) that cod were observed hovering on the bottom of the aquaria as compared with 0.005% Capsaicin (2.8 2.2% at 30 min and 10.867.0% at 90 min) (p = 0.010) and Saline (2.061.9% at 30 min and 0.360.2% at 90 min) (p = 0.001)
Summary
The ability of animals to detect stimuli that can cause harm to their tissues is a universal feature termed nociception [1] This sensory ability encompasses the neural processing of noxious stimuli and may include the induction of both physiological and behavioural responses but with the sensation of pain not necessarily being implied [2,3,4]. It has been argued that the neural processing of noxious stimuli may involve peripheral and central nociception but does not allow for pain perception, and that behavioural responses to noxious stimuli may represent an activation of nocifensive motor programs which does not involve conscious awareness [6,7]. Other studies have demonstrated nociception and suggested the existence of affective states and potentially the ability for pain perception in teleost fishes Studies on the rainbow trout (Oncorhynchus mykiss), for example, have demonstrated A-delta and C-fibre nociceptors [12,13,14,15] and shown that subcutaneous injections of acetic acid or bee venom to the lips caused enhanced opercular beat rate (OBR)
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