Response characteristics of neurons in the medial component of the medial geniculate nucleus during Pavlovian differential fear conditioning in rabbits.

Recent evidence suggests that the amygdaloid central nucleus (ACE) may contribute significantly to Pavlovian fear-conditioned bradycardic responses during the presentation of conditioned emotional stimuli. Because the medial component of the medial geniculate nucleus (MGm) is a major source of input to the region of the ACE, the extracellular single-unit responses of MGm neurons were examined during Pavlovian differentially conditioned bradycardic responding in rabbits. Conditioning involved pairing one tone (CS+) with paraorbital shock and presenting another tone (CS-) in the absence of shock. Two general classes of MGm neurons were identified based on their conditioned-response characteristics. Both groups responded differentially to the CSs. One group responded with greater increases in activity and at a shorter latency to the CS+ compared with the CS-, whereas the other group responded with greater increases in activity and at a shorter latency to the CS- compared with the CS+. Recordings from MGm neurons in naive rabbits prior to conditioning provided evidence that the acoustic stimuli used subsequently as the CS+ and CS- did not evoke differential responses. These results suggest that the MGm along with the ACE may be forebrain components of a neural circuit involved in the acquisition and/or expression of Pavlovian fear-conditioned bradycardic responses.