The Journal of Neurophysiology Vol. 81 No. 5 May 1999, pp. 2226-2233
Copyright ©1999 by the American Physiological Society

Receptor Subtype Mediating the Adrenergic Sensitivity of Pain Behavior and Ectopic Discharges in Neuropathic Lewis Rats

 ¹Marine Biomedical Institute;  ²Department of Anatomy and Neurosciences;  ³Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston, Texas 77555-1069

Lee, Doo Hyun, Xianzeng Liu, Hyun Taek Kim, Kyungsoon Chung, and Jin Mo Chung. Receptor Subtype Mediating the Adrenergic Sensitivity of Pain Behavior and Ectopic Discharges in Neuropathic Lewis Rats. J. Neurophysiol. 81: 2226-2233, 1999.Receptor subtype mediating the adrenergic sensitivity of pain behavior and ectopic discharges in neuropathic Lewis rats. We attempted to identify the subtype of -adrenergic receptor (-AR) that is responsible for the sympathetic (adrenergic) dependency of neuropathic pain in the segmental spinal injury (SSI) model in the Lewis strain of rat. This model was chosen because our previous study showed that pain behaviors in this condition are particularly sensitive to systemic injection of phentolamine (PTL), a general -AR blocker. We examined the effects of specific ₁- and ₂-AR blockers on 1) behavioral signs of mechanical allodynia, 2) ectopic discharges recorded in the in vivo condition, and 3) ectopic discharges recorded in an in vitro setup. One week after tight ligation of the L5 and L6 spinal nerves, mechanical thresholds of the paw for foot withdrawals were drastically lowered; we interpreted this change as a sign of mechanical allodynia. Signs of mechanical allodynia were significantly relieved by a systemic injection of PTL (a mixed ₁- and ₂-AR antagonist) or terazosin (TRZ, an ₁-AR antagonist) but not by various ₂-AR antagonists (idazoxan, rauwolscine, or yohimbine), suggesting that the ₁-AR is in part the mediator of the signs of mechanical allodynia. Ongoing ectopic discharges were recorded from injured afferents in fascicles of the L5 dorsal root of the neuropathic rat with an in vivo recording setup. Ongoing discharge rate was significantly reduced after intraperitoneal injection of PTL or TRZ but not by idazoxan. In addition, by using an in vitro recording setup, spontaneous activity was recorded from teased dorsal root fibers in a segment in which the spinal nerve was previously ligated. Application of epinephrine to the perfusion bath enhanced ongoing discharges. This evoked activity was blocked by pretreatment with TRZ but not with idazoxan. This study demonstrated that both behavioral signs of mechanical allodynia and ectopic discharges of injured afferents in the Lewis neuropathic rat are in part mediated by mechanisms involving ₁-ARs. These results suggest that the sympathetic dependency of neuropathic pain in the Lewis strain of the rat is mediated by the ₁ subtype of AR.