Abstract
Prior research has shown that an aversive event can produce either an increase (hyperalgesia) or a decrease in pain reactivity (hypoalgesia). Researchers have extensively examined both the behavioral and neurobiological basis of environmentally induced hypoalgesia. This research has shown that even a brief shock to a rat's tail can produce a strong decrease in responding to thermal stimulation. This hypoalgesia persists for at least 10 min and often appears to be opioid mediated because it is reduced by administration of an opiate antagonist. Much less is known about environmentally induced hyperalgesia. In fact, there has been considerable controversy as to whether hyperalgesia can be reliably observed in the laboratory. The first two experiments reported here show how a strong hyperalgesia can be obtained by measuring reactivity to shock. I found that the same mild shock procedure that yields hypoalgesia on the tail-flick test, produces hyperreactivity as measured by shock-induced vocalization, shock-elicited vocalization threshold, and shock-elicited tail movement threshold. In Experiment 3 I show that this effect represents a true change in responsiveness and not simply an artifact caused by nonspecific alterations in the amount of current and/or voltage that subjects experience. In Experiment 4 I assessed whether shock-induced hyperreactivity reflected either a conditioned or an unconditioned phenomenon. The result supported the latter hypothesis. In Experiment 5 I examined whether exposure to radiant heat can also produce hyperalgesia. I found that this stressor also produced a slight increase in reactivity to shock. However, the radiant heat had no effect on tail-flick latencies. In the final experiment I explored the role of endogenous opioids in shock-induced hyperreactivity. Mild shock produced heightened reactivity on the vocalization threshold test irrespective of whether subjects were injected with the opiate antagonist, naltrexone. Implications of the above results are discussed.
Illich, Paul Anthony (1993). Environmentally induced increases in pain reactivity. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1475410.