The Journal of Neurophysiology Vol. 86 No. 1 July 2001, pp. 49-58
Copyright ©2001 by the American Physiological Society

GABA_A and 5-HT₃ Receptors Are Involved in Dorsal Root Reflexes: Possible Role in Periaqueductal Gray Descending Inhibition

 ¹Pain and Neurosensory Mechanisms Branch, National Institute of Dental and Craniofacial Research and  ²Center for Scientific Review, National Institutes of Health, Bethesda, Maryland 20892; and  ³Department of Anatomy and Neurosciences, University of Texas Medical Branch, Galveston, Texas 77555-1069

Peng, Yuan Bo, Jing Wu, William D. Willis, and Daniel R. Kenshalo. GABA_A and 5-HT₃ Receptors Are Involved in Dorsal Root Reflexes: Possible Role in Periaqueductal Gray Descending Inhibition. J. Neurophysiol. 86: 49-58, 2001. The dorsal root reflex (DRR) is a measure of the central excitability of presynaptic inhibitory circuits in the spinal cord. Activation of the periaqueductal gray (PAG), a center for descending inhibition of spinal cord nociceptive transmission, induces release of variety of neurotransmitters in the spinal cord, including GABA and serotonin (5-HT). GABA has been shown to be involved in generation of DRRs. In this study, pharmacological agents that influence DRRs and their possible mechanisms were investigated. DRRs were recorded in anesthetized rats from filaments teased from the cut central stump of the left L₄ or L₅ dorsal root, using a monopolar recording electrode. Stimulating electrodes were placed either on the left sciatic nerve or transcutaneously in the left foot. Animals were paralyzed and maintained by artificial ventilation. Drugs were applied topically to the spinal cord. A total of 64 units were recorded in 34 Sprague-Dawley rats. Peripheral receptive fields were found for nine of these units. In these units, DRRs were evoked by brush, pressure, and pinch stimuli. Nine units were tested for an effect of electrical stimulation in the periaqueductal gray on the DRRs. In eight cases, DRR responses were enhanced following PAG stimulation. The background activity was 4.2 ± 1.9 spikes/s (mean ± SE; range: 0-97.7; n = 57). The responses to agents applied to the spinal cord were (in spikes/s): artificial cerebrospinal fluid, 7.1 ± 3.6 (range: 0-86.9; n = 25); 0.1 mM GABA, 16.8 ± 8.7 (range: 0-191.0; n = 22); 1.0 mM GABA, 116.0 ± 26.5 (range: 0.05-1001.2; n = 50); and 1.0 mM phenylbiguanide (PBG), 68.1 ± 25.3 (range: 0-1,073.0; n = 49). Bicuculline (0.5 mM, n = 27) and ondansetron (1.0 mM, n = 10) blocked the GABA and PBG effects, respectively (P < 0.05). Significant cross blockade was also observed. It is concluded that GABA_A receptors are likely to play a key role in the generation of DRRs, but that 5-HT₃ receptors may also contribute. DRRs can be modulated by supraspinal mechanisms through descending systems.