No abstract

Theoreticians predict that animal 'personality' traits may be maladaptive if fixed throughout different contexts, so the present study aimed to test whether these traits are fixed or plastic. Rainbow trout (Onchorhyncus mykiss) were given emboldening or negative experiences in the forms of watching bold or shy individuals responding to novelty or winning or losing fights to examine whether prior experience affected boldness. Bold individuals that lost fights or watched shy demonstrators became more shy by increasing their latency to approach a novel object, whereas shy observers that watched bold demonstrators remained cautious and did not modify their responses to novelty. Shy winners became bolder and decreased their latency to approach a novel object, but shy losers also displayed this shift. In comparison, control groups showed no change in behaviour. Bold fishes given negative experiences reduced their boldness which may be an adaptive response; however, shy fishes may base their strategic decisions upon self-assessment of their relative competitive ability and increase their boldness in situations where getting to resources more quickly ensures they outcompete better competitors.

This study examined stimulus-response properties of somatosensory receptors on the head of rainbow trout, Oncorhynchus mykiss, using extracellular recording from single cells in the trigeminal ganglion. Of 121 receptors recorded from 39 fish, 17 were polymodal nociceptors, 22 were mechanothermal nociceptors, 18 were mechanochemical receptors, 33 were fast adapting mechanical receptors and 31 were slowly adapting mechanical receptors. Mechanical thresholds were higher in polymodal nociceptors than in either slowly adapting or fast adapting mechanical receptors, whereas thermal thresholds of mechanothermal nociceptors were higher than those of polymodal nociceptors. Polymodal nociceptors and mechanochemical receptors gave similar responses to topical applications of acid. All receptor types except mechanothermal nociceptors showed an increase in peak firing frequency with increased strength of mechanical stimulation, with evidence of response saturation at higher intensities. Mechanothermal, but not polymodal, nociceptors showed an increase in firing response to increased temperature. None out of 120 receptors tested gave any response to the temperature range +4 degrees C to -7 degrees C, indicating an absence of cold nociceptors. Attempts to evoke sensitization of receptors using chemical or heat stimuli were unsuccessful, with receptors showing either a return to control responses or irreversible damage. Comparisons are made between somatosensory receptors characterized here in a fish and those of higher vertebrates.

A potentially painful experience may modify normal behavioural responses. To gauge the importance of pain relative to predation or social status, we presented competing stimuli, a predator cue or an unfamiliar social group, to two groups of noxiously treated rainbow trout, Oncorhynchus mykiss. In the predator cue experiment, fish were classified as bold or shy. Noxiously stimulated fish did not show antipredator responses, suggesting that pain is the imperative. In the social status experiment, noxiously stimulated fish held individually and undisturbed showed an increase in respiration rate and plasma cortisol. As a comparison, we used the dominant or subordinate fish in a group as the noxiously stimulated fish. After the noxious treatment, we returned this test fish to a familiar or unfamiliar social group. Neither dominants nor subordinates showed a negative change in physiology compared to their controls. However, in a familiar group the dominant was much less aggressive, suggesting a behavioural impairment in response to noxious stimulation. In an unfamiliar group, no reduction of aggression was seen, suggesting that maintaining dominance status took priority over showing signs of pain. These findings may reflect an ability to prioritize motivational drivers in fish, and as such provides evidence for central processing of pain rather than merely showing a nociceptive reflex. In animal models of pain, exposure to a new circumstance or a potentially fear-inducing or stressful situation reduces pain reactivity (e.g.

Corneal receptors have not previously been identified in lower vertebrates. The present study describes the properties of trigeminal ganglion corneal receptors in a teleost fish, the rainbow trout (Oncoryhnchus mykiss). Out of 27 receptors, 7 were polymodal nociceptors, 6 were mechanothermal nociceptors, 2 were mechanochemical receptors and the largest group, 12, were only responsive to mechanical stimulation. No cold responsive receptors were found on the trout cornea. Mechanical and thermal thresholds were lower and receptive field diameters smaller than those of cutaneous trigeminal receptors in the trout, demonstrating greater sensitivity in the cornea. The lack of cold sensitive neurons may provide evidence for the evolution of cold nociceptors in vertebrates that is related to the transition from poikilothermy to homeothermy.