The nociceptive response of stressed and lithium-treated rats is differently modulated by different flavors

Pleasant and unpleasant flavors and odors can modulate pain perception, and the efficacy of sweet flavors in reducing pain seems to be related to its hedonic value. Chronic variate stress paradigm is a model of depression, and is suggested to induce anhedonia. We observed previously that lithium may prevent behavioral and neurochemical alterations induced by chronic stress; so we hypothesized that chronically stressed animals may present different nociceptive response to pleasant and unpleasant tastes that could be prevented by lithium treatment. Adult male Wistar rats were divided into four groups, control and stressed, treated or not with lithium. A Chronic Variate Stress paradigm was used, and lithium was added to the chow. After 40 days of treatment, the tail flick latency of the animals was evaluated, and rats were immediately placed in a box with access to a 5% acetic acid solution (acid flavor). After 5 min, tail flick latency was measured again. On the following day, animals were submitted to the same procedure, with the substitution of acetic acid by condensed sweet milk (sweet flavor). The stressed group was the only group who did not present analgesia after sweet taste exposition. All groups, except the control group, presented increased tail flick latency after exposition to the acid flavor. These results indicate that pleasant and unpleasant flavors present different relevance for the induction of antinociception in stressed animals, and the absence of sweet flavor-induced analgesia may represent an anhedonic effect of the chronic variate stress paradigm. On the other hand, perception of different flavors may be more prominent in animals treated with lithium.