Asymmetric uptake of 2-deoxy--[C]glucose in the dorsal cochlear nucleus during Pavlovian conditioning in the rabbit

Uptake of 2-deoxy--[¹⁴C]glucose was measured during Pavlovian conditioning of the rabbit's nictitating membrane response by both qualitative autoradiography and by quantitative measurement of radioactivity in samples of brain tissue. Conditioning was accomplished by pairing a tone stimulus delivered to both ears with an air-puff stimulus delivered to the right eye. Infusion of 2-deoxy--[¹⁴C]glucose during the first day of conditioning when there was no evidence of acquisition or during the 7th day of conditioning when animals demonstrated 68% conditioned responses resulted in a significantly greater uptake of radioactivity by the caudal portions of the left as compared with the right dorsal cochlear nucleus. Similar changes were not observed in other auditory and non-auditory nuclei. Rabbits that had acquired conditioned responses across 6 days of training and were then exposed only to the tone-conditioned stimulus on the 7th day of testing exhibited 69% conditioned responses but no asymmetry in the uptake of 2-deoxy--[¹⁴C]glucose. Control animals receiving unpaired presentations of tone and air puff or no stimulation did not acquire conditioned responses and did not demonstrate asymmetric uptake of radioactivity in the dorsal cochlear nucleus. These results indicate that the asymmetric uptake of radioactivity by the dorsal cochlear nucleus did not result from the effects of stimulation per se or the prior occurrence of learning but was due to the explicit pairing of the tone stimulus with the asymmetric delivery of the air puff. It would appear that the caudal dorsal cochlear nucleus not only serves as a signal transducer for auditory stimuli but also receives input from other sensory systems thus allowing it to both recognize when an auditory stimulus is followed by a biologically significant event and to transmit such information to other brain regions that are, in turn, responsible for learning.