Conditioned increase in peripheral blood mononuclear cell (PBMC) number and corticosterone secretion in the rat

Femoral artery catheters were surgically implanted into male Lewis/N rats to allow blood sampling and drug infusion in the freely moving animal. After recovery, conditioned animals received four pairings of a peppermint odor, the conditioned stimulus (CS), and an infusion of 0.1 mg/kg nicotine bitartrate, an unconditioned stimulus (US) for an increase in the number of peripheral blood mononuclear cells (PBMC) and an increase in corticosterone concentration. When reexposed to the peppermint odor, conditioned animals showed a significant increase in PBMC number and corticosterone secretion when compared to saline and unpaired control groups and previously conditioned animals that were not reexposed to the CS. Increased PBMCs were found on the fifth unreinforced CS trial. Conditioned CORT responses were lost after the initial test trial. The data indicate that the distribution of immune cells can be influenced by learning processes and support the role of learning in the regulation of corticosterone secretion.