Thalamic mast cell activity is associated with sign-tracking behavior in rats.

Mast cells are resident immune cells in the thalamus that can degranulate and release hundreds of signaling molecules (i.e., monoamines, growth factors, and cytokines) both basally and in response to environmental stimuli. Interestingly, mast cell numbers in the brain show immense individual variation in both rodents and humans. We used a Pavlovian conditioned approach (PCA) procedure to examine whether mast cells are associated with individual variation in the attribution of incentive-motivational value to reward-related cues. During the PCA procedure, a lever response-independently predicts the delivery of a food pellet into a magazine, and over training sessions three conditioned responses (CRs) develop: sign-tracking (lever-directed CRs), goal-tracking (magazine-directed CRs), and an intermediate response (both CRs). In Experiment 1, we measured thalamic mast cell number/activation using toluidine blue and demonstrated that sign-trackers have increased degranulated (activated) but not granulated (inactive) mast cells. In Experiment 2, we infused the mast cell inhibitor, cromolyn (200µg/rat; i.c.v.), immediately before five daily PCA training sessions and demonstrated that mast cell inhibition selectively impairs the acquisition of sign-tracking behavior. Taken together, these results demonstrate that thalamic mast cells contribute to the attribution of incentive-motivational value to reward-related cues and suggest that mast cell inhibition may be a novel target for addiction treatment.