Effects of Peripheral Blockade of Hyperpolarised Activated Cyclic Nucleotide Gated (HCN) Channels on Inflammatory and Neuropathic Pain Behaviours and Changes in HCN expression in DRG Neurones After Chronic Hindlimb Inflammation

Trevor Smith, Laiche Djouhri. University of Liverpool, Liverpool, United Kingdom.

Chronic inflammatory and neuropathic pain is characterised in humans by spontaneous/ongoing pain (stimulus-independent), increased pain in response to normally painful stimuli (hyperalgesia), and pain in response to normally non-painful stimuli (allodynia). This hypersensitivity is partly due to sensitization of primary afferent dorsal root ganglion (DRG) neurons. Abnormal (ectopic) spontaneous activity (SA) in these neurons has been implicated in chronic pain development and maintenance. Changes in HCN channels (HCN1-4), which mediate the hyperpolarisation-activated (Ih) current, could contribute to this abnormal SA. We investigated: a) the effects of peripheral blockade of HCN activity with the Ih selective blocker, ZD7288, on chronic pain behaviours and b) to determine immunohistochemically whether HCN expression is altered in DRG neurons in an animal model of inflammatory pain. We used two widely used rat models of chronic pain. These were produced under Isoflurane anaesthesia and were: 1) an inflammatory pain model that involved injection of 100µl of Complete Freund’s adjuvant (CFA) into the left hindlimb of male Wistar rats (200-350g, n=9) and 2) a modified spinal nerve ligation model which involved transection of the L5 spinal nerve plus loose ligation of the L4 spinal nerve with neuroinflammation-inducing chromic gut (n=11). Three behavioural signs representing different aspects of chronic pain were studied. These were: spontaneous foot lifting (SFL), a sign of spontaneous pain in animals, mechanical allodynia (MA) inferred by a decrease in paw withdrawal threshold to pressure, and heat hyperalgesia (HH) inferred by a decrease in paw withdrawal latency to a noxious heat stimulus. 3 days after CFA injection and 7 days after spinal nerve injury, rats showed clear signs of spontaneous pain (increased SFL duration), MA and HH. Intraplantar administration of ZD7288 (100 µM) reduced SFL and MA, but not HH, in both animal models. Immunoreactivity of HCN3, but not HCN1 or 2, was increased in small putative nociceptive DRG neurons 7 days after hindlimb inflammation. These results suggest that HCN channels may contribute to hyperexcitability of DRG neurons and hypersensitivity associated with tissue inflammation and nerve injury.