EFFECT OF A CONDITIONED AVERSIVE STIMULUS ON THE IMMUNE-RESPONSE IN 3STRAINS OF RATS

In the present study we investigated the effect of a brief exposure (1 5 s) to a conditioned aversive stimulus (CS) on the proliferative resp onse of spleen and peripheral blood lymphocytes (PBL) in Lewis, Fische r 344 and Sprague-Dawley rats. Plasma levels of ACTH and corticosteron e were also measured. For conditioning, rats were exposed to 10 presen tations of a 5 s duration foot-shock (1.6 mA) preceded by a 15 s tone. Seven days later, animals were exposed to the auditory signal without electric shock. Significant differences were found in both the kineti cs and the magnitude of altered mitogenic responsiveness of PBL betwee n the different strains of rats. Enhancement of PBL responsiveness to mitogens was observed in Fischer and Sprague-Dawley rats immediately a fter exposure to the CS. A significant decrease in the response of PBL to mitogens was found in Lewis and Sprague-Dawley rats 10 min after e xposure to the CS. The PBL response of Sprague-Dawley and Fischer rats returned to baseline at 30 min, but not in Lewis rats. Proliferative activity of spleen lymphocytes in response to the CS was suppressed fr om baseline in all rat strains, but the timing and degree of suppressi on differed. Fischer rats had the largest percentage of suppression. T he earliest suppression of spleen mitogenic function after exposure to the CS was in Fischer rats, while the Lewis rats had the latest onset of suppression, with the Sprague-Dawley rats being intermediate. Plas ma levels of ACTH and corticosterone peaked at 10 min in all strains o f rats. The magnitude of hormonal elevation differed in the different rat strains, suggesting that corticosterone may not have a variable im munomodulatory role in each strain. These data suggest that a brief ps ychological stressor results in activation of the HPA axis and is asso ciated with strain-dependent alterations of lymphocyte responsiveness to non-specific mitogens. The short-term exposure to a CS which produc es different parameters of lymphocyte functional modulation, provides a useful tool to study the mechanisms of stressor-induced immune alter ation.