Conditioned suppression of humoral immunity in the rat.

Rats were conditioned by pairing consumption of a novel sodium saccharin drinking solution with the effects of an ip injection of 75 mg/kg cyclophosphamide, an immunosuppressive drug. Five and ten days after conditioning, an experimental group of conditioned animals (Group CS) was reexposed to the saccharin drinking solution. Control animals (Group CSo) were conditioned but were not reexposed to saccharin. On Day 10, 15, or 25 after conditioning, animals were injected ip with sheep erythrocytes (SRBC), and independent subgroups were sampled for hemagglutinating antibody titer 4, 6, or 8 days later. There was a significant effect of sample time (antibody titers 4 days after immunization were lower than values observed 6 and 8 days after immunization) and a significant effect of treatment; conditioned animals reexposed to the CS had an attenuated antibody response. There were no significant differences between Group CSo and a group of placebo-treated animals, but conditioned animals reexposed to the CS had lower antibody titers than placebo-treated animals 4, 6, and 8 days after antigenic stimulation. These differences are more pervasive than those previously reported and suggest that reexposure to a CS may have long-lasting effects. More generally, these data provide further documentation of conditioned immunopharmacologic effects and the impact of behavioral factors in modifying immunologic reactivity.