Frequency-specific plasticity of single unit discharges in the rat medial geniculate body

The effects of conditioning (tone-footshock pairing) on single unit discharges in the magnocellular medial geniculate (mMG) were examined in rats submitted to pairing under ketamine. For all the semichronic animals, the conditioned suppression of lever pressing for food observed subsequently indicated that at the behavioral level, an acquisition takes place during the pairing. The activity of 37 single unit activities was recorded both during pairing and during determination of frequency-selectivity before and after pairing. The results showed that 54% (20/37) of the cells exhibited plasticity of their evoked response (increase or decrease) during pairing compared with the habituation or the sensitization phase. Moreover, 75% of these changes (15/20 cells) were specific for the frequency used as conditioned stimulus during conditioning. Modifications of background activity were observed during pairing or sensitization and were never correlated with modifications of the evoked responses. These results suggest that specific modifications of the frequency receptive field of the mMG cells occur during acquisition of classical conditioning.