Neuroscience Letters 419 (2007) 147–152 Superficial dorsal horn neurons with double spike activity in the rat Gerardo Rojas-Piloni a , Anthony H. Dickenson c , Miguel Cond´ es-Lara a,b,∗ a Departamento de Neurobiolog´ ıa del Desarrollo y Neurofisiolog´ ıa, Instituto de Neurobiolog´ ıa, Universidad Nacional Aut´ onoma de M´ exico, Campus Juriquilla, Quer´ etaro 76230, Mexico b Instituto Nacional de Psiquiatr´ ıa “Ram ´ on de la Fuente”, M´ exico, D.F., Mexico c Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, UK Received 22 March 2007; received in revised form 13 April 2007; accepted 14 April 2007 Abstract Superficial dorsal horn neurons promote the transfer of nociceptive information from the periphery to supraspinal structures. The membrane and discharge properties of spinal cord neurons can alter the reliability of peripheral signals. In this paper, we analyze the location and response properties of a particular class of dorsal horn neurons that exhibits double spike discharge with a very short interspike interval (2.01 ± 0.11 ms). These neurons receive nociceptive C-fiber input and are located in laminae I–II. Double spikes are generated spontaneously or by depolarizing current injection (interval of 2.37 ± 0.22). Cells presenting double spike (interval 2.28 ± 0.11) increased the firing rate by electrical noxious stimulation, as well as, in the first minutes after carrageenan injection into their receptive field. Carrageenan is a polysaccharide soluble in water and it is used for producing an experimental model of semi-chronic pain. In the present study carrageenan also produces an increase in the interval between double spikes and then, reduced their occurrence after 5–10 min. The results suggest that double spikes are due to intrinsic membrane properties and that their frequency is related to C-fiber nociceptive activity. The present work shows evidence that double spikes in superficial spinal cord neurones are related to the nociceptive stimulation, and they are possibly part of an acute pain-control mechanism. © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Double spike; Nociception; Spinal cord; C-fiber input; Dorsal horn Superficial laminae of the spinal dorsal horn play an impor- tant role in the processing and relay of nociceptive information. Intrinsic neurons of the superficial dorsal horn modulate and integrate afferent and efferent nociceptive information before it is transferred to supraspinal structures [6]. Several in vivo and in vitro studies on the electrophysiological properties of dorsal horn neurons [13,18,19,21,25] have revealed the importance of understanding the intrinsic properties of dorsal horn cells in the processing and modulation of nociceptive information. It could be assumed that sensory systems convey peripheral afferent signals to the central nervous system with a high degree of fidelity but transmission is complicated by the fact that the membrane receptors involved can also mediate changes such as wind-up that alter the relation between input and output [8]. So, ∗ Corresponding author at: Departamento de Neurofisiolog´ ıa, Instituto de Neurobiolog´ ıa, Universidad Nacional Aut´ onoma de M´ exico, Campus UNAM- Juriquilla, Quer´ etaro, Mexico. Tel.: +52 442 238 1042; fax: +52 442 238 1042. E-mail address: condes@servidor.unam.mx (M. Cond´ es-Lara). the intrinsic properties of sensory or relay neurons will have an influence on sensory signaling [2,23], and their membrane and discharge properties will determine, at least in part, the input–output relationship of neurons that is important for the performance of the neural circuits [1]. The purpose of the present study was to provide experimen- tal evidence of the localization and response properties of a particular class of superficial dorsal horn neurons that exhibits evoked and spontaneous double spikes with very short inter- spike intervals (e.g. 2 ms). These neurons with double spikes receive synaptic input from C-fiber nociceptors. Double spike activities have been implicated in sensory abnormalities asso- ciated with nerve injuries and disease [3,7,24]. Our study will further the understanding of the electrophysiological properties of double spike dorsal horn neurons and their role in nocicep- tive transmission at the dorsal horn level. In order to prove the double spike participation in nociceptive processes we used the electrical stimulation of the receptive field and the subcutaneous carrageenan injection into the receptive field. Carrageenan has been used elsewhere [11,15] as a semi-chronic model of pain. 0304-3940/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2007.04.032