Metabolic effects of blocking tone conditioning on the rat auditory system.

The Kamin blocking phenomenon occurs when behavioral expression of conditioning to a novel stimulus fails in the presence of a previously conditioned stimulus (CS). Neural metabolic effects of a tone conditioned as an excitor were compared to the effects of the same physical tone when excitatory conditioning was blocked by previous conditioning with a light. We examined the metabolic activity of the auditory system to test the hypothesis that auditory processing of a tone CS changes during blocking. Quantitative histochemistry of cytochrome oxidase (C.O.), the final mitochondrial enzyme for oxidative metabolism, was used to evaluate cumulative changes in the metabolic capacity of the auditory system resulting from blocking. Rats (Long-Evans) in the Blocking group received pairings of a light CS with a mild footshock unconditioned stimulus (US) during Phase 1 training. Rats in the Control group received random presentations of the same stimuli during Phase 1. Both groups then received the same Phase 2 training consisting of simultaneous tone and light presentations paired with footshock. The Control group exhibited significant suppression of drinking to tone alone presentations after training, whereas the Blocking group did not. Metabolic mapping results demonstrated that blocking effects were localized to auditory regions receiving direct US somatosensory projections. Significantly greater C.O. activity in the inferior colliculus and the dorsal cochlear nucleus was found for the Blocking group relative to the Control group. Input cell layers of secondary auditory cortex also demonstrated a group difference, in that layers II/III and IV had lower levels of C.O. activity in the Blocking group. These specific changes in C.O. activity linked to behavioral training demonstrated that the blocking phenomenon produced distinct neural metabolic changes in CS processing in the auditory system localized to regions with CS-US interactions.