Neural substrates for tone-conditioned bradycardia demonstrated with 2-deoxyglucose. II. Auditory cortex plasticity.

The 2-deoxyglucose (2-DG) method was used to map the metabolic activity of the auditory cortex (AC) during and after conditioning. Using separate groups of animals, the effects of both paired and unpaired presentations of a 4-5 kHz FM tone (CS) and midbrain reticular stimulation (US) were compared for acquisition, extinction and sensitization training. Rats with cardiac deceleration conditioned to the FM tone showed a pattern of AC metabolic activity distinctly different from that seen in control animals. The tonotopic pattern of 2-DG labeling consisted of two contiguous spindle-shaped bands corresponding to the location of neurons with best frequency response in the 4-5 kHz band width. Reticular stimulation alone or combined with the tones produced a widespread increase of 2-DG uptake. At least two types of modulatory effects appeared to interact with the tonotopic pattern. The first involved a selective enhancement of evoked activity in the AC region of convergence of CS-US effects. This effect may be related to learning because it was restricted to the conditioning group. The second effect involved a general increase in background uptake of 2-DG in AC. This effect may be related to reticular sensitization because it was common to all groups subjected to reticular stimulation. The present findings are the first anatomical demonstration of the modulatory effects of auditory learning on AC metabolic activity.