Effects of acetylcholine and atropine on plasticity of central auditory neurons caused by conditioning in bats

In the big brown bat (Eptesicus fuscus), conditioning with acoustic stimuli followed by electric leg-stimulation causes shifts in frequency-tuning cur ves and best frequencies (hereafter BF shifts) of collicular and cortical n eurons, i.e., reorganization of the cochleotopic (frequency) maps in the in ferior colliculus (IC) and auditory cortex (AC). The collicular BF shift re covers 180 min after the conditioning, but the cortical BF shift lasts long er than 26 h. The collicular BF shift is not caused by conditioning, as the AC is inactivated during conditioning. Therefore it has been concluded tha t the collicular BF shift is caused by the corticofugal auditory system. Th e collicular and cortical BF shifts both are not caused by conditioning as the somatosensory cortex is inactivated during conditioning. Therefore it h as been hypothesized that the cortical BF shift is mostly caused by both th e subcortical (e.g., collicular) BF shift and the activity of nonauditory s ystems such as the somatosensory cortex excited by an unconditioned leg-sti mulation and the cholinergic basal forebrain. The main aims of our present studies are to examine whether acetylcholine (ACh) applied to the AC augmen ts the collicular and cortical BF shifts caused by the conditioning and whe ther atropine applied to the AC abolishes the cortical BF shift but not the collicular BF shift, as expected from the preceding hypothesis. In the awa ke bat, we made the following findings. ACh applied to the AC augments not only the cortical BF shift but also the collicular BF shift through the cor ticofugal system. Atropine applied to the AC reduces the collicular BF shif t and abolishes the cortical BF shift which otherwise would be caused. ACh applied to the IC significantly augments the collicular BF shift but affect s the cortical BF shift only slightly. ACh makes the cortical BF shift long -lasting beyond 4 h, but it cannot make the collicular BF shift long-lastin g beyond 3 h. Atropine applied to the IC abolishes the collicular BF shift. It reduces the cortical BF shift but does not abolish it. Our findings fav or the hypothesis that the BF shifts evoked by the corticofugal system, and an increased ACh level in the AC evoked by the basal forebrain are both ne cessary to evoke a long-lasting cortical BF shift.