Shock-induced hyperalgesia: IV. Generality.

Abstract

Brief-moderate shock (3, 0.75 s, 1.0 mA) has opposite effects on different measures of pain, inducing antinociception on the tail-flick test while lowering vocalization thresholds to shock and heat (hyperalgesia) and enhancing fear conditioned by a gridshock unconditioned stimulus (US). This study examined the generality of shock-induced hyperalgesia under a range of conditions and explored parallels to sensitized startle. Reduced vocalization thresholds to shock and antinociception emerged at a similar shock intensity. Severe shocks (3, 25 s, 1.0 mA or 3, 2 s, 3.0 mA) lowered vocalization threshold to shock but increased vocalization and motor thresholds to heat and undermined fear conditioned by a gridshock or a startling tone US. All shock schedules facilitated startle, but only brief-moderate shock inflated fear conditioning. The findings suggest that brief-moderate shock enhances the affective impact of aversive stimuli, whereas severe shocks attenuate pain.

Brief exposure to a mild stressor enhances morphine-conditioned place preference in male rats

“…The present results shed light on the mechanisms that underlie findings of shock-induced sensitization of pain reactivity, startle, and learning reported in previous studies (Boulis and Davis 1989;Davis 1989;Illich et al 1995;King et al 1996;Crown et al 2000;Meagher et al 2001). One possible mechanism was that exposure to an aversive event has an effect that is limited to stimuli with the same valence.…”

“…However it is possible that unconditioned fear or anxiety mediates the effects of moderate shock. Although we (Meagher et al 2001) and others (Davis 1989) have reported that shock enhances startle in a new context, a finding indicative of a general sensitization of stimulus processing, this does not necessarily implicate anxiety. Indeed, none of the common unconditioned fear/ anxiety tests examined (freezing, elevated plus maze, shock-probe burying, the social interaction test) has yielded evidence of anxiety-like behavior (Sieve et al 1999;Ferguson et al unpublished observations).…”

“…Using startle-eliciting auditory stimuli, we found that prior shock exposure enhanced startle magnitude and the degree to which these stimuli could support fear conditioning (Meagher et al 2001). The present study extends this work by testing whether prior shock exposure also enhances the processing of a non-aversive, appetitive stimulus-morphine administration.…”

“…Elsewhere, we have examined whether this enhancement is limited to cutaneous stimuli (Meagher et al 2001). Using startle-eliciting auditory stimuli, we found that prior shock exposure enhanced startle magnitude and the degree to which these stimuli could support fear conditioning (Meagher et al 2001).…”

“…Exposure to moderate levels of shock [3, 0.75 s, 1 mA, 20 s interstimulus interval (ISI)] can enhance reactivity to subsequent aversive stimuli in rats (Illich et al 1995;King et al 1996;McLemore et al 1999;Meagher et al 2001). This sensitization enhances pain reactivity (hyperalgesia), lowering vocalization latencies to both gradually incremented shock and radiant heat applied to the tail (Illich et al 1995).…”