![]() (E) Tactile paw withdrawal responses of rats investigated in A, C and D were measured throughout the duration of the experiment (days 14 – 120). (D) Sham or SNL surgery did not alter the last completed ratio (break point) on the progressive ratio task (n=12–16). (C) Naïve, sham, and SNL rats all exhibited similar numbers of lever presses during each session of the progressive ratio task (n=12–16). (B) Graphical representation demonstrating differences in the number of lever presses required to earn a reward on the two progressive ratio schedules (PR1 and PR2). (A) No significant difference in the number of lever presses during the FR1 acquisition phase was detected in SNL, sham, or naïve rats (n=12–16). Naïve, sham-operated and SNL-operated groups of rats were trained to lever press for sucrose pellets according to schedules outlined in the time-line (E). Adaptations that allow normal reward responding to food regardless of chronic pain may be of evolutionary benefit to promote survival. Thus, although acute ongoing inflammatory pain may transiently reduce reward motivation, we did not detect influences of chronic neuropathic pain on hedonic or motivational responses to food rewards. However, rats with inflammation showed decrements in lever pressing and break points on days 1 and 2 after complete Freund adjuvant injection that normalized by day 4, consistent with transient ongoing pain. Assessment of response acquisition and break points under the progressive ratio schedule revealed no differences between sham and spinal nerve ligation rats for up to 120 days after injury. To assess possible motivational deficits during acute and chronic pain, we used fixed- and progressive-ratio response paradigms of sucrose pellet presentation in rats with transient inflammatory or chronic neuropathic pain. Spinal nerve ligation rats did not differ from controls in either "liking" or "disliking" reactions to intraoral sucrose or quinine, respectively, at postsurgery day 21, suggesting no differences in perceived hedonic value of sweet or bitter tastants. Hedonic response was measured by implantation of intraoral catheters to allow passive delivery of liquid solutions, and "liking/disliking" responses were scored according to a facial reactivity scale. We independently evaluated hedonic qualities of sweet or bitter tastants and motivation to seek food reward in rats with experimental neuropathic pain induced by L5/6 spinal nerve ligation. Rewards influence responses to acute painful stimuli, but the relationship of chronic pain to hedonic or motivational aspects of reward is not well understood.
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