Prior stress and behaviorally conditioned histamine release.

Male guinea pigs were either handled ('stressed') or not disturbed ('non-stressed') for four weeks prior to conditioning with a classical discrimination conditioning design. Animals were sensitized to bovine serum albumin (BSA) and four weeks later presented with either an odor (the CS+) paired with BSA or a second odor (the CS-) paired with saline. These pairs were presented in a random order for ten trials. Weekly blood samples were assayed for histamine and cortisol levels. Following the conditioning trials, animals were subjected to extinction trials during which the CS+ odor was presented but not paired with the BSA. The animals handled prior to the conditioning procedures learned the association between the odor and the BSA as indicated by increased histamine levels when exposed to the conditioned odor alone. The non-handled group did not learn. Additionally, the cortisol levels were significantly higher for the handled group vs. the non-handled group when the CS+ was presented during extinction. The role of stress in both learning and immunomodulation is discussed.