Augmentation of plasticity of the central auditory system by the basal forebrain and/or somatosensory cortex.

Auditory conditioning (associative learning) or focal electric stimulation of the primary auditory cortex (AC) evokes reorganization (plasticity) of the cochleotopic (frequency) map of the inferior colliculus (IC) as well as that of the AC. The reorganization results from shifts in the best frequencies (BFs) and frequency-tuning curves of single neurons. Since the importance of the cholinergic basal forebrain for cortical plasticity and the importance of the somatosensory cortex and the corticofugal auditory system for collicular and cortical plasticity have been demonstrated, Gao and Suga proposed a hypothesis that states that the AC and corticofugal system play an important role in evoking auditory collicular and cortical plasticity and that auditory and somatosensory signals from the cerebral cortex to the basal forebrain play an important role in augmenting collicular and cortical plasticity. To test their hypothesis, we studied whether the amount and the duration of plasticity of both collicular and cortical neurons evoked by electric stimulation of the AC or by acoustic stimulation were increased by electric stimulation of the basal forebrain and/or the somatosensory cortex. In adult big brown bats (Eptesicus fuscus), we made the following major findings. 1) Collicular and cortical plasticity evoked by electric stimulation of the AC is augmented by electric stimulation of the basal forebrain. The amount of augmentation is larger for cortical plasticity than for collicular plasticity. 2) Collicular and cortical plasticity evoked by AC stimulation is augmented by somatosensory cortical stimulation mimicking fear conditioning. The amount of augmentation is larger for cortical plasticity than for collicular plasticity. 3) Collicular and cortical plasticity evoked by both AC and basal forebrain stimulations is further augmented by somatosensory cortical stimulation. 4) A lesion of the basal forebrain tends to reduce collicular and cortical plasticity evoked by AC stimulation. The reduction is small and statistically insignificant for collicular plasticity but significant for cortical plasticity. 5) The lesion of the basal forebrain eliminates the augmentation of collicular and cortical plasticity that otherwise would be evoked by somatosensory cortical stimulation. 6) Collicular and cortical plasticity evoked by repetitive acoustic stimuli is augmented by basal forebrain and/or somatosensory cortical stimulation. However, the lesion of the basal forebrain eliminates the augmentation of collicular and cortical plasticity that otherwise would be evoked by somatosensory cortical stimulation. These findings support the hypothesis proposed by Gao and Suga.