The mechanism of action of the neuropeptide galanin in sensory pathways with special reference to nociception

The neuropeptide galanin is up-regulated after nerve injury in areas involved in nociception, and enhances the analgesic effects of morphine. Galanin binds to G-protein coupled receptors (GPCRs) GAL1, GAL2,GAL3, and galanin acting via GAL1 and GAL2 is linked with pain modulation. GAL1 and GAL2 are present in small to medium-sized primary sensory neurons of the DRG, whereas GAL2 is present in microglia in the dorsal horn. Microglia provide a cellular response to damaged nerves, however prolonged microglial activation can lead to chronic pain states. While little is known about the effect of galanin on microglial activation, galanin has been shown to inhibit cytokine release from microglia. Therefore the actions of galanin on Ca2+ channels and [Ca2+]i in DRG neurons and microglia were investigated. Dissociated DRG neurons from Sprague-Dawley (SD) rats were used acutely for patch clamp recordings or cultured overnight in supplemented media before Ca2+ imaging using Fura-2. KCl-induced Ca2+ transients partially due to voltage-gated Ca2+ channels (VGCC) were inhibited by galanin acting via GAL2 in small nociceptive DRG neurons. In addition, galanin repeatedly inhibits both high voltage activated (HVA) Ca2+currents and Ca2+ transients, in a concentration-dependant manner. Galanin inhibited both N and P/Q-type VGCCs in DRG neurons, which differentiates galanin from analgesic agents that inhibit transmitter release only via actions on N-type channels. In DRG neurons [Ca2+]i responses to depolarisation are not only mediated by VGCCs, but also via Ca2+ induced Ca2+ release (CICR) and store operated Ca2+ entry (SOCE). While galanin was able to inhibit VGCC, the results also suggested that inhibition of CICR and/or SOCE may be involved, but further investigation is required. The effect of galanin on microglial activity was investigated by dissociating microglia from neonatal SD rats kept in mixed glial culture for 10-14 days before isolation and Ca2+ imaging using Fura-2. Results from Ca2+ imaging recordings indicate that galanin can modulate both the lipopolysaccharide (LPS) and adenosine triphosphate (ATP) responses in microglia. Galanin was shown to augment the short-term effects of ATP on microglial activity. Indeed ATP-induced activation of microglia can lead to symptoms of neuropathic pain, suggesting a pro-inflammatory role for galanin in the ATP signalling pathway. Therefore, GAL2 receptor antagonists may have the potential to prevent chronic pain states from developing. While galanin enhances ATP actions in the short-term, it did not change the response to LPS exposure in the short term. However, galanin did appear to enhance the effects of LPS signalling pathways after longer exposure durations. This study has provided the basis for future investigation into the effect of galanin on microglial activity. It has also provided information that may inform therapeutic strategies to reduce neuropathic pain states related to prolonged microglial activation. Together with the dual action of galanin on DRG neurons, this suggests galanin receptor mimetics have the potential to be developed as analgesics for both acute and chronic neuropathic pain.