METABOLIC EFFECTS OF BLOCKING TONE CONDITIONING ON THE RAT AUDITORY-SYSTEM

The Kamin blocking phenomenon occurs when behavioral expression of con ditioning to a novel stimulus fails in the presence of a previously co nditioned stimulus (CS). Neural metabolic effects of a tone conditione d as an exciter were compared to the effects of the same physical tone when excitatory conditioning was blocked by previous conditioning wit h a light. We examined the metabolic activity of the auditory system t o test the hypothesis that auditory processing of a tone CS changes du ring blocking. Quantitative histochemistry of cytochrome oxidase (C.O. ), the final mitochondrial enzyme for oxidative metabolism, was used t o evaluate cumulative changes in the metabolic capacity of the auditor y system resulting from blocking. Rats (Long-Evans) in the Blocking gr oup received pairings of a light CS with a mild footshock unconditione d stimulus (US) during Phase 1 training. Rats in the Control group rec eived random presentations of the same stimuli during Phase 1. Both gr oups then received the same Phase 2 training consisting of simultaneou s tone and light presentations paired with footshock. The Control grou p exhibited significant suppression of drinking to tone alone presenta tions after training, whereas the Blocking group did not. Metabolic ma pping results demonstrated that blocking effects were localized to aud itory regions receiving direct US somatosensory projections. Significa ntly greater C.O. activity in the inferior colliculus and the dorsal c ochlear nucleus was found for the Blocking group relative to the Contr ol group. Input cell layers of secondary auditory cortex also demonstr ated 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 blockin g phenomenon produced distinct neural metabolic changes in CS processi ng in the auditory system localized to regions with CS-US interactions . (C) 1997 Academic Press.