Sensitization after an Injury Increases Survival

Researchers at the University of Texas Medical School released the first piece of concrete evidence that nociceptive sensitization results in an adaptive advantage. Using squid (Doryteuthis pealeii)as their test organism, the scientists demonstrated that both behavioral and neurological changes resulting from noxious stimuli, like a minor injury, increase the likelihood of survival for animals (1).

In the experiment, squid were divided into four categories, injured (I), uninjured (U), injured while treated with anesthetic (IA) and uninjured while treated with anesthetic (UA). An injured squid received a small injury to one of its arms that produced no observable side effects after six hours (1). The injury did cause hyper-sensitization in the squid, which significantly lowered the thresholds for various electrical and mechanical stimuli (1, 2).

Anesthesia treatment in injured squid prevented this nociceptive sensitization response, while anesthesia treatment in uninjured squid caused no significant effects (1, 2). To test the effects of nociceptive sensitization on fitness, scientists then introduced a natural predator of squid, the black sea bass (Centropristis striata), to groups of squid from all categories (1).

In all cases, the bass targeted injured squids, whether they had been treated with anesthetic or not, over uninjured squids, even starting the chase for their prey from longer distances. Despite the lack of observable effects due to the injury, uninjured squids had much higher survival rates than injured squid (1).

However, injured squids that had not been exposed to anesthetic evaded capture more often than injured squids treated with anesthetic. Researchers observed that injured squids with the nociceptive sensitization response initiated secondary defenses faster than their injured, anesthetized group counterparts. As a result of nociceptive sensitization, the injured, non-anesthetized squid ran away from the bass at shorter “flight initiation” distances and began a series of behavioral changes of fluctuating body patterns followed by an “escape jet” and ink-releasing protean defense faster (1).

The results provide proof that nociceptive sensitization benefit the injured organism by counterbalancing some of the negative effects of the injury (1).

Although several differences exist in nociceptive responses between invertebrates like squid and mammals, this study contrasts previous clinical belief that sensitization is harmful or “maladaptive”(1, 2).  Since scientists do not believe that squid and other invertebrates feel pain as humans do, the findings provide interesting parallels and new methods of approaching responses to noxious stimuli in mammals (1, 2).

The implications of the study also hint at potential mechanisms for the development of hypersensitivity following injury (1).

References:

1. R. J. Crook, K. Dickson, R. T. Hanlon, E. T. Walters. Nociceptive sensitization reduces predation risk. Current Biology.24, 1-5 (May 2014).

2. R. J. Crook, R. T. Hanlon, E. T. Walters. Squid have nociceptors that display widespread long-term sensitization and spontaneous activity after bodily injury. The Journal of Neuroscience.33 10021-10026 (June 2013)