Behavioral, adrenal, and sympathetic responses to long-term administration of an oral corticotropin-releasing hormone receptor antagonist in a primate stress paradigm.
CRH is a main regulator of the stress response. This neuropeptide and its specific receptors, CRHR-1 and CRHR-2, are disseminated throughout the central nervous system. There is a significant interspecies difference in the distribution of CRHR within the central nervous system. CRH-R1 antagonists may attenuate stress-related behavior in rats without compromising adrenal function, but few studies have addressed the same question in higher mammals. Antalarmin (AA) is a specific CRHR-1 antagonist suitable for oral administration. Social separation is a potent stressor for rhesus monkeys. Therefore, we sought to investigate the hormonal responses to chronic administration of AA using a primate stress model. Eight preadolescent (4-6 kg) male rhesus monkeys received AA (20 mg/kg.d) or placebo (PBO) orally. All animals were on a regular day/light cycle and were fed with standard monkey chow daily. The study (114 d) was comprised of the following consecutive phases: adaptation, baseline, separation (stress), recovery, and cross-over. During social separation, solid panels separated the individuals. Cerebrospinal fluid (CSF) and femoral venous blood samples were obtained once a week on the fourth day of separation under ketamine anesthesia. Serum samples were also obtained 1 and 2 h after separation. CSF samples were assayed for CRH, AA, norepinephrine (NE) and epinephrine (EPI). Plasma was assayed for ACTH, cortisol, NE, and EPI. AA was detected in the plasma of each monkey while they were taking the active drug and in none of the animals on PBO. Among the behaviors assessed, environmental exploration, a behavior inhibited by stress, was increased during AA administration. However, AA at this dose did not affect other anxiety-related behavioral end points, including self-directed behavior, vocalization, or locomotion. We also observed that: 1) ACTH decreased between adaptation and baseline, indicating that the animals had adjusted to the novel environment; 2) ACTH and cortisol increased significantly after social separation, indicating that social separation was an adequate model for acute stress; 3) NE and EPI increased significantly during acute stress in the AA and PBO groups (P < 0.005, NE; P < 0.001, EPI); 4) after chronic stress, by d 4 of separation, ACTH levels were no longer significantly different from baseline, and NE and EPI remained slightly elevated when compared with baseline (P < 0.05, NE; P < 0.01, EPI); and 5) all the animals remained healthy and gained the expected weight during the study. In summary, oral chronic administration of a specific CRH-R1 antagonist to rhesus monkeys does not blunt the sympathoadrenal response to stress while increasing environmental exploration, a behavior that is normally suppressed during stressful events. Taken together, these findings suggest that CRHR-1 antagonists may be a valid treatment for stress-related disorders.