Synergism Between Stress Responses Induced by Biting Flies and Predator Odours

Abstract

AbstractWe have previously shown that endogenous opioid systems (i.e. endorphins, enkephalins) are involved in the mediation of the behavioural and physiological effects of biting‐fly exposure. Opioid mediated reductions in pain sensitivity, or, more appropriately, nociceptive sensitivity (latency of a foot‐lifting response to a 50°C thermal surface), are evident in laboratory mice, Mus musculus domesticus, exposed to biting flies. Similar opioid‐mediated reductions in pain sensitivity (opioid analgesia) are also seen after exposure to a variety of other natural stressors such as the threat of predation. Here, we demonstrate that brief (30‐min) exposure of male mice to stable flies, Stomoxys calcitrans, significantly and synergistically augments either the concurrent or subsequent (60 min after fly exposure) analgesic effects induced by exposure to a predator (cat odour). These results demonstrate that the analgesic, and probably other opioid mediated behavioural and physiological stress responses induced by exposure to a relatively low number of biting flies, are markedly increased by the presence of another natural aversive stimulus. In addition, they show that biting‐fly exposure significantly exacerbates the effects of a subsequent stressful stimulus. These findings raise a possible mechanism whereby exposure to a low number of biting flies, which by themselves may not appear to have a great impact, can have marked behavioural and physiological consequences.