Research report Dorsal horn neuron response patterns to graded heat stimuli in the rat Jasenka Borzan a , Stacey C. LaGraize a , Doyle L. Hawkins b , Yuan B. Peng a, * a Department of Psychology, P.O. Box 19528, University of Texas at Arlington, 501 S. Nedderman Drive, Arlington, TX 76019-0528, USA b Department of Mathematics, University of Texas at Arlington, Arlington, TX 76019, USA Accepted 15 March 2005 Available online 26 April 2005 Abstract Sensory input from various receptors in the periphery first becomes integrated in the spinal cord dorsal horn. The response of the spinal cord dorsal horn neurons to mechanical stimuli are classified as low threshold, high threshold, and wide dynamic range neurons. However, the response pattern of deep dorsal horn cells to heat has not been well described. In this study, the response of the spinal cord dorsal horn neurons to graded heat stimuli were characterized in 147 neurons in rats by extracellular single cell recording. After a differentiable cell was identified, the Peltier heat stimulator was applied to the receptive field and the base temperature was set at 30 -C. The heat stimulus was delivered for 10 s from 37 – 51 -C in 2 -C increments, with an inter-stimulus interval of 30 s. Out of the 147 neurons, five statistically distinguishable response patterns were identified by latent class cluster analysis. It is concluded that variation of temperature may account for the observed results and indicate functionally different subsets of heat-responsive cells in the deep dorsal horn. D 2005 Elsevier B.V. All rights reserved. Theme: Sensory systems Topic: Pain modulation: anatomy and physiology Keywords: Dorsal horn neuron; Spinal cord; Primary afferent; Synapse 1. Introduction In the rat, the response of the spinal cord dorsal horn neurons are typically classified as low threshold (LT), high threshold (HT), and wide dynamic range (WDR) neurons according to their response to mechanical stimuli [9,10,36]. Spinal cord dorsal horn neurons have been studied extensively for their response to thermal stimuli, ranging from noxious cold, cool, warm, to noxious heat [4,6 – 8,13,16,22,24,27,30,33,34,37,39–41]. Typically, dorsal horn neurons respond proportionally to graded heat stimuli. An inhibition of background discharge as well as to noxious mechanical stimulation was demonstrated in dorsal horn neurons in laminae III–V when warm temperatures (32– 42 -C) were applied [26]. However, there is little informa- tion about the response patterns of dorsal horn neurons to heat stimuli, particularly in deeper laminae. Although much is known about the properties of primary afferent warm and cool fibers in various mammalian species, much less is known about central mechanisms of thermoreception [1]. Spinothalamic tract (STT) neurons in lamina I of the dorsal horn in cat and monkey can be classified according to their response patterns to a range of mechanical and thermal stimuli. More specifically, in addition to the distinction between cold and warm cells, Craig and colleagues also differentiate between two other types of lamina I projection neurons: nociceptive specific (respond solely to noxious mechanical and thermal stimulation) and HPC cells (respond to noxious heat, cold, and pinch) [15]. Centrally, dorsal horn neurons responsive to thermal stimulation are generally thought to be located only in laminae I and II of the spinal cord although they have been reported in deeper laminae as well [21]. Consequently, there is a lack of classification pertaining to the properties of neurons in the 0006-8993/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2005.03.030 * Corresponding author. Fax: +1 817 272 2364. E-mail address: ypeng@uta.edu (Y.B. Peng). Brain Research 1045 (2005) 72 – 79 www.elsevier.com/locate/brainres