Noradrenalin enhances the activity of cochlear nucleus neurons in the rat

The cochlear nucleus of rats is heavily innervated by noradrenergic fibres from the locus coeruleus. The physiological meaning of this innervation is poorly understood. Therefore, iontophoretically applied noradrenalin was tested on single neurons of the cochlear nucleus in urethane-anaesthetized rats. Iontophoresis of noradrenalin had a dual effect. During application noradrenalin led to moderate inhibition of tone-evoked activity in 37% of the tested neurons. In contrast, approximately 20-30 s after the onset of iontophoresis a long-lasting increase in discharge activity was found in most neurons. Data from iontophoresis of the alpha1-receptor agonist phenylephrine and the alpha2-receptor agonist clonidine suggest that the fast moderate inhibition is mediated by alpha2-receptors while the pronounced long-lasting elevated neuronal firing is mediated by alpha1-receptors. However, these data do not exclude the possibility that part of the response to noradrenalin is also mediated by beta-receptors. Electrical stimulation of the locus coeruleus resulted in an increase in discharge activity comparable with iontophoresis of noradrenalin or phenylephrine. Thus, activation of the locus coeruleus predominantly increases spontaneous and tone-evoked neuronal firing in the cochlear nucleus of the rat. This alpha-receptor-mediated enhanced discharge activity may serve to increase the sensitivity of acoustic processing mechanisms or to lower the threshold for short-latency acoustic reflexes.