Antidepressant-Like Effects of the Corticotropin-Releasing Factor 1 Receptor Antagonist, SSR125543, and the Vasopressin 1b Receptor Antagonist, SSR149415, in a DRL-72 s Schedule in the Rat

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Abstract
The vasopressin 1b receptor antagonist, SSR149415, and the corticotropin-releasing factor 1 receptor antagonist, SSR125543, are orally active non-peptidic compounds with anxiolytic- and antidepressant-like activities in animal models. Presently, SSR149415 and SSR125543 were evaluated in a differential reinforcement of low-rate 72 s (DRL-72 s) schedule, a procedure known to respond differentially to antidepressants and anxiolytics. Male Wistar rats were trained to lever-press for food reinforcement, but only lever-presses occurring after a 72 s delay were reinforced; otherwise, presses were not rewarded, and the timer was reset to 0 s. The selective serotonin reuptake inhibitor, fluoxetine, and the benzodiazepine anxiolytic, diazepam, were tested in parallel. SSR149415 (10–30 mg/kg, i.p.) and SSR125543 (30 mg/kg, i.p.) increased the percentage of responses emitted in the inter-response time (IRT) bin (49–96 s), which resulted in a greater number of reinforced presses. Both compounds shifted the frequency distribution of responses toward longer IRT durations, with a preservation of the bell shape of the IRT distribution curve. Fluoxetine (10 mg/kg, i.p.) had an effect on DRL-72 s similar to that of SSR149415 and SSR125543. By contrast, diazepam increased the number of responses in IRT bin (0–12 s), and the IRT distribution curve was shifted toward shorter IRT durations and flattened. In summary, these results show that SSR149415 and SSR125543 displayed antidepressant-like activity in a DRL-72 s schedule in rat, confirming their therapeutic potential for the treatment of pathological states induced by chronic frustration such as depression.

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