Functional identification of synaptic plasticity in the rat auditory cortex

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In this study, we investigated the physiological roles of synaptic plasticity in the rat auditory cortex. We studied long-term depression (LTD). The synaptic inputs to pyramidal neurons from the medial geniculate body exhibited LTD, which was sensitive to an antagonist of NMDA receptors. In contrast, LTD was sensitive to an antagonist of metabotropic glutamate receptors in the synapses between cortical pyramidal neurons. We further investigated sequence-dependent properties of long-term potentiation (LTP). The sequence-dependence was observed on the scale of seconds, so that LTP was observed only in the synapses stimulated earlier. The sequence-dependence of LTP was abolished in the presence of antagonists of muscarinic acetylcholine receptors. The importance of endogenous cholinergic was also shown in the sound-sequence discrimination ability of rats using behavioral analyses. These results suggest that the sequence-dependent LTP may have a role in temporal information processing in the auditory cortex. Finally, we investigated the properties between the auditory cortex and higher cortices. Primary sensory cortices can be activated by memory recall or imagery via feedback projections from higher cortices. We studied the properties of the cortical boundary between the auditory cortex and area 18a in rat cerebral slices. Activities in the auditory cortex were usually not elicited by the stimulation of area 18a. However, the whole auditory cortex was invaded by polysynaptic feedback activities triggered in area 18a, after area 18a and the auditory cortex were alternately stimulated several times. Our findings indicate that different cortical areas are quickly bound with feedback activities depending on previous activities.