ROLE OF PRIMARY AFFERENT NERVES IN ALLODYNIA CAUSED BY DIABETIC NEUROPATHY IN RATS G. M. KHAN, a S.-R. CHEN a and H.-L. PAN a;bà a Department of Anesthesiology, Penn State University College of Medicine, Hershey, PA 17033, USA b Department of Neuroscience and Anatomy, Penn State University College of Medicine, Hershey, PA 17033, USA AbstractöBoth myelinated and unmyelinated a¡erents are implicated in transmitting diabetic neuropathic pain. Although unmyelinated a¡erents are generally considered to play a signi¢cant role in diabetic neuropathic pain, patho- logical changes in diabetic neuropathy occur mostly in myelinated A-¢bers. In the present study, we ¢rst examined the role of capsaicin-sensitive C-¢bers in the development of allodynia induced by diabetic neuropathy. We then studied the functional changes of a¡erent nerves pertinent to diabetic neuropathic pain. Diabetes was induced in rats by i.p. streptozotocin. To deplete capsaicin-sensitive C-¢bers, rats were treated with i.p. resiniferatoxin (300 Wg/kg). Mechanical and thermal sensitivities were measured using von Frey ¢laments and a radiant heat stimulus. Single-unit activity of a¡erents was recorded from the tibial nerve. Tactile allodynia, but not thermal hyperalgesia, developed in diabetic rats. Resiniferatoxin treatment did not alter signi¢cantly the degree and time course of allodynia. Post-treatment with resin- iferatoxin also failed to attenuate allodynia in diabetic rats. The electrophysiological recordings revealed ectopic dis- charges and a higher spontaneous activity mainly in AN- and AL-¢ber a¡erents in diabetic rats regardless of resiniferatoxin treatment. Furthermore, these a¡erent ¢bers had a lower threshold for activation and augmented responses to mechanical stimuli. Thus, our study suggests that capsaicin-sensitive C-¢ber a¡erents are not required in the development of allodynia in this rat model of diabetes. Our electrophysiological data provide substantial new evidence that the abnormal sensory input from AN- and AL-¢ber a¡erents may play an important role in diabetic neuropathic pain. ß 2002 IBRO. Published by Elsevier Science Ltd. All rights reserved. Key words: diabetic neuropathy, resinoferatoxin, nociceptors, ectopic activity, myelinated a¡erents, hypersensitivity. Diabetic neuropathy is one of the most common late complications of diabetes mellitus and is frequently pain- ful, with the pain involving predominantly the distal extremities (Archer et al., 1983). Pain associated with diabetic neuropathy can occur either spontaneously or as a result of exposure to only mildly painful stimuli (hyperalgesia) or to stimuli not normally perceived as painful (allodynia) (Brown and Asbury, 1984; Clark and Lee, 1995). The underlying mechanisms of persistent pain in diabetic patients remain poorly understood. Dia- betic neuropathic pain is widely considered to be caused by peripheral neuropathy. Both myelinated and unmye- linated a¡erents are implicated in mediating diabetic neu- ropathic pain. It is not clear which ¢ber type is more important for development of neuropathic pain in dia- betes. It has been suggested that diabetic neuropathic pain results from hyperactivity of damaged small diam- eter C-¢bers (Burchiel et al., 1985; Chen and Levine, 2001). In this regard, electrophysiological studies have shown a signi¢cantly higher incidence of spontaneous discharges in comparison to normal nerves, and this spontaneous hyperactivity occurs almost exclusively in potentially nociceptive C-¢bers (Burchiel et al., 1985; Ahlgren et al., 1992; Chen and Levine, 2001). However, there is no substantial functional evidence to support the role of C-¢ber a¡erents in diabetic neuropathic pain. Resiniferatoxin (RTX), an ultrapotent analog of capsai- cin, has been used to study the action of nociceptive C-¢ber a¡erents (Szallasi et al., 1989; Ossipov et al., 1999; Szallasi et al., 1999). It has been well documented that systemic RTX produces long-lasting desensitization of unmyelinated nociceptive C-¢ber a¡erents (Szallasi and Blumberg, 1999). RTX has been utilized to de¢ne the role of capsaicin-sensitive C-¢ber a¡erents in neuro- pathic pain caused by traumatic nerve injury (Ossipov et al., 1999). In the present study, RTX was used as a pharmacological tool to determine the role of capsai- cin-sensitive C-¢ber a¡erents in the development of allo- dynia in diabetic rats. In addition to the C-¢ber a¡erents, various ultrastruc- tural damages to the myelinated ¢bers in diabetic neu- ropathy have been reported in both patients and animals (Yagihashi et al., 1990; Ochodnicka et al., 1995; Weis et al., 1995; Mizisin et al., 1998). Thus, myelinated a¡erent nerves also may play an important role in diabetic neuro- pathic pain. However, functional changes of myelinated a¡erent nerves in diabetic neuropathic pain have not been investigated systemically. In the present study, 291 *Correspondence to: H.-L. Pan, Department of Anesthesiology, H187, Penn State University College of Medicine, 500 University Drive, Hershey, PA 17033-0850, USA. Tel.: +1-717-531-8433; fax: +1-717-531-5434. E-mail address: hpan@psu.edu (H.-L. Pan). Abbreviations: RTX, resiniferatoxin; STZ, streptozotocin. NSC 5796 16-8-02 www.neuroscience-ibro.com Neuroscience Vol. 114, No. 2, pp. 291^299, 2002 ß 2002 IBRO. Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain PII:S0306-4522(02)00372-X 0306-4522 / 02 $22.00+0.00