Induction of selective plasticity in the frequency tuning of auditory cortex and auditory thalamus neurons by locus coeruleus stimulation.

Neurons in primary sensory cortices display selective receptive field plasticity in behavioral situations ranging from classical conditioning to attentional tasks, and it is generally assumed that neuromodulators promote this plasticity. Studies have shown that pairing a pure-tone and a stimulation of the nucleus basalis magnocellularis mimics the selective receptive field facilitations described after classical conditioning. Here, we evaluated the consequences of repeated pairings between a particular sound frequency and a phasic stimulation of locus coeruleus (LC) on the frequency tuning of auditory thalamus and auditory cortex neurons. Selective alterations for the paired frequency were observed for more than 30% of the cells recorded both in cortex and in thalamus. There were as much selective increases as selective decreases at the cortical level, whereas selective increases were prevailing at the thalamic level. Selective changes usually persisted 15 min after pairing in cortex; they dissipated in thalamus, and so did the general increases in both structures. In animals with stimulation sites outside the LC, pairing induced either general changes or no effect. These results indicate that the selective plasticity induced in the frequency tuning of auditory cortex neurons by LC stimulation is bidirectional, thereby suggesting that noradrenergic activation can contribute to the different forms of plasticity observed after distinct behavioral paradigms.

Noradrenergic induction of selective plasticity in the frequency tuning of auditory cortex neurons.

Neuromodulators have long been viewed as permissive factors in experience-induced cortical plasticity, both during development and in adulthood. Experiments performed over the last two decades have reported the potency of acetylcholine to promote changes in functional properties of cortical cells in the auditory, visual, and somatosensory modality. In contrast, very few attempts were made with the monoaminergic systems. The present study evaluates how repeated presentation of brief pulses of noradrenaline (NA) concomitant with presentation of a particular tone frequency changes the frequency tuning curves of auditory cortex neurons determined at 20 dB above threshold. After 100 trials of NA-tone pairing, 28% of the cells (19/67) exhibited selective tuning modifications for the frequency paired with NA. All the selective effects were obtained when the paired frequency was within 1/4 of an octave from the initial best frequency. For these cells, selective decreases were prominent (15/19 cases), and these effects lasted > or =15 min after pairing. No selective effects were observed under various control conditions: tone alone (n = 10 cells), NA alone (n = 11 cells), pairing with ascorbic acid (n = 6 cells), or with GABA (n = 20 cells). Selective effects were observed when the NA-tone pairing was performed in the presence of propranolol (4/10 cells) but not when it was performed in the presence phentolamine (0/13 cells), suggesting that the effects were mediated by alpha receptors. These results indicate that brief increases in noradrenaline concentration can trigger selective modifications in the tuning curves of cortical neurons that, in most of the cases, go in opposite direction compared with those usually reported with acetylcholine.