Effects of noradrenaline on frequency tuning of auditory cortex neurons during wakefulness and slow-wave sleep

This study shows the effects of noradrenaline (NA) on receptive fields of a uditory cortex neurons in awake animals; it is the first one to describe th e effects of NA on neurons in sensory cortex, in different natural states o f vigilance. The frequency receptive field of 250 auditory cortex neurons w as determined before, during and after ionophoretic application of NA while recording the state of vigilance of unanaesthetized guinea-pigs, When NA s ignificantly changed the spontaneous activity (85 out of 250 cells), the do minant effect was a decrease (61 out of 85 cells, 72%). When NA significant ly changed the evoked activity (107 out of 250 cells), the dominant effect was also a decrease (84 out of 107 cells, 78%). During and after NA applica tion, the signal-to-noise ratio (S/N, i.e. evoked/spontaneous activity) was unchanged, but the selectivity for pure-tone frequencies was enhanced. Whe n the effects occurring in wakefulness and in slow-wave sleep (SWS) were co mpared, it appeared that the predominantly inhibitory effect of NA on spont aneous and evoked activity was present in both states. The SIN ratio was un changed and the selectivity was increased in both states. However, during S WS, the percentage of cells inhibited by NA was lower, and the effects on t he frequency selectivity were smaller than in wakefulness. In contrast, GAB A produced similar inhibitory effects on spontaneous and on evoked activity during wakefulness and SWS. Comparisons with previous data obtained using the same protocol in urethane anaesthetized animals (Manunta & Edeline, 199 7) indicate that the effects of NA were qualitatively the same. Based on th ese results, we suggest that any hypothesis concerning the role of NA in co rtical plasticity should take into account the fact that the predominantly inhibitory effects of NA lead to decrease the size of the receptive field.