Atomoxetine modulates spontaneous and sensory-evoked discharge of locus coeruleus noradrenergic neurons

Atomoxetine (ATM) is a potent norepinephrine (NE) uptake inhibitor and increases both NE and dopamine synaptic levels in prefrontal cortex, where it is thought to exert its beneficial effects on attention and impulsivity. At the behavioral level, ATM has been shown to cause improvements on the measures of executive functions, such as response inhibition, working memory and attentional set shifting across different species. However, the exact mechanism of action for ATM's effects on cognition is still not clear. One possible target for the cognitive enhancing effects of ATM is the noradrenergic locus coeruleus (LC), the only source of NE to key forebrain areas such as cerebral cortex and hippocampus. Although it is known that ATM increases NE availability overall by blocking reuptake of NE, the effects of this agent on impulse activity of LC neurons have not been reported. Here, the effect of ATM (0.1-1 mg/kg, ip) on NE-LC neurons was investigated by recording extracellular activity of LC neurons in isoflurane-anesthetized rats. ATM caused a significant decrease of the tonic activity of LC single-units, although leaving intact the sensory-evoked excitatory component of LC phasic response. Moreover, the magnitude of the inhibitory component of LC response to paw stimulation was increased after 1 mg/kg of ATM and its duration was prolonged at 0.3 mg/kg. Together, these effects of ATM produced an increase in the phasic-to-tonic ratio of LC phasic response to sensory stimulation. ATM also modulated the average sensory-evoked local field potential (LFP) and spike-field coherence in LC depending on the dose tested. The lower dose (0.1 mg/kg) significantly decreased early positive and negative components of the sensory-evoked LFP response. Higher doses (0.3-1 mg/kg) initially increased and then decreased the amplitude of components of the evoked fields, whereas the spike-field coherence was enhanced by 1 mg/kg ATM across frequency bands. Finally, coherence between LC fields and EEG signals was generally increased by 1 mg/kg ATM, whereas 0.1 and 0.3 mg/kg respectively decreased and increased coherence values in specific frequency bands. Taken together these results suggest that ATM effects on LC neuronal activity are dose-dependent, with different doses affecting different aspects of LC firing. This modulation of activity of LC-NE neurons may play a role in the cognitive effects of ATM. This article is part of a Special Issue entitled 'Cognitive Enhancers'.