Is there a role for autophagy in pain processing?

Chronic pain is a debilitating condition affecting life quality and has dramatic economic impact on national health systems. However, our ability to treat this condition is currently limited and there is an urgent need for more effective treatments. This goal can be achieved through a better understanding of the neurobiology of pain and its mechanisms. We focussed our research on autophagy, a physiological mechanism contributing to protein and organelles degradation, cellular remodelling and survival. Because of its critical homeostatic role, any imbalance in the autophagic flux can impact basal functions leading to cellular dysfunction. Both insufficient and excessive autophagy have therefore differently implicated in several human pathologies such as neurodegenerative diseases and cancer. Changes occurring during spinal processing of sensory information consequent to a peripheral lesion are important for chronic pain conditions. We have recently shown that autophagy is impaired in the spinal cord following peripheral nerve injury and suggested a potential role for this degradative pathway in spinal pain processing (Berliocchi et al., 2011; Berliocchi et al., 2012). The formation of the autophagic marker LC3-II, together with the accumulation of autophagy substrates like p62 detected by western blot in the spinal dorsal horn following spinal cord ligation (SNL; Kim and Chung, 1992), indicates that autophagy is impaired in this experimental model of neuropathic pain. The increased expression of these autophagic markers is restricted to the spinal cord side ipsilateral to the ligation in SNL mice, correlates with the up-regulation of the calcium channel subunit α2δ-1, and is not present in mice that underwent sham surgery. Immunofluorescence and confocal microscopy analysis suggest that these changes in the expression of the autophagic markers occur within specific cell populations in the spinal dorsal horn. Seven days after surgery, increased p62 immunoreactivity is detectable in the most superficial laminae of the spinal dorsal horn on the injury side of SNL mice and is less evident in mice that underwent sham surgery, thus confirming the western blot data. Double immunostainings with some main cellular markers, indicate the presence of p62 mainly in NeuN-positive cell bodies, occasionally in GFAP-positive processes, but not in Iba1-positive cells, thus suggesting a predominant expression in the neuronal compartment. Analogous changes were observed also in the formalin test, a model of acute inflammatory pain characterised by a peripheral and a central component. Finally, we investigated the consequences of autophagy impairment on pain behaviour. Cloroquine is known to increase lysosomal pH disrupting the normal autophagic flux and resulting in autophagosome accumulation. When injected intrathecally in naïve mice, cloroquine (CQ, 100μM) was able to induce a spinal accumulation of LC3-II and p62 indicative of a block in autophagosome clearance. This was not associated to cell death, but induced an increased mechanical sensitivity suggesting a participation of the autophagic flux into spinal mechanisms of central sensitization. Altogether, our data indicate that spinal autophagy is impaired in pain states and suggest that this event may be playing a relevant role in pain processing, thus providing a potential novel target for pain control.