Transient and prolonged facilitation of tone-evoked responses induced by basal forebrain stimulations in the rat auditory cortex.

We investigated the relationships between cortical arousal and cholinergic facilitation of evoked responses in the auditory cortex. The basal forebrain (BF) was stimulated unilaterally, while cluster recordings were obtained simultaneously from both auditory cortices in urethane-anesthetized rats. The global electroencephalogram (EEG; large frontoparietal derivation) and the local EEG (from the auditory cortex) were recorded. The BF was stimulated at two intensities, a lower one which did not desynchronize the EEG and a higher one which did. Twenty pairing trials were delivered, during which a tone was presented 50 ms after the end of the BF stimulation. At low intensity, the pairing procedure led to a transient increase in the ipsilateral tone-evoked responses. At high intensity, the pairing increased the ipsilateral evoked responses up to 15 min after pairing. Such effects were not observed for the contralateral recordings. Systemic atropine injection prevented the facilitations observed ipsilaterally. BF stimulations alone did not induce any increased evoked response either at low or at high intensity. These results show (1) that a tone, presented while the cortex is activated by cholinergic neurons of the BF, evokes enhanced cortical responses, and (2) that the duration of this facilitation is dependent on the stimulation intensity. These results are discussed in the context of neural mechanisms involved in general arousal and cortical plasticity.