Differential processing of laser stimuli by Aδ and C fibres in major depression

Clinical studies have revealed that up to 92% of major depressed patients report pain complaints such as back or abdominal pain. Furthermore, patients suffering from depression exhibit increased superficial pain thresholds and decreased ischemic (deep) pain thresholds during experimental pain testing in comparison to healthy controls. Here, we aimed to investigate a putative role of Aδ- and C-fibre activation in altered pain perception in the disease. Laser-evoked potentials (LEPs) of 27 unmedicated depressed patients and 27 matched controls were recorded. Aδ and C fibres were activated separately. Amplitudes and latencies of N2 and P2 peaks of Aδ- (Aδ-LEP) and C-fibre- (C-LEP) related LEPs were evaluated. Depressed patients showed significantly decreased Aδ-LEP amplitudes (N2 peak: P=0.019; P2 peak: P=0.024) and delayed C-LEP latencies (P2 peak: P=0.0495; N2 peak: P=0.0556). In contrast, C-LEP amplitudes and Aδ-LEP latencies were unaffected. Our results might be suggestive of the differential impact of physiological changes on pain processing in depression. Thus, Aδ-LEP might reflect the physiological correlate of the augmented superficial pain thresholds during depression. On the contrary, the C-fibre component mediates the facets of pain processing, outlasting the stimulation period, and has been shown to be exaggerated in chronic pain states. Therefore, the functional over-representation of the C-fibre component found in our study might be a possible link between depression and associated pain complaints.