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Departments of
¹ Integrative Physiology
² Orthopaedic Surgery, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan

To study the functional projection patterns of the primary afferents in the spinal cord, the postsynaptic responses of substantia gelatinosa (SG) neurones evoked by L5 dorsal root stimulation (DRS) were examined from the neurones located at L2 to S1 in horizontal slices of the adult rat spinal cord using a blind whole-cell patch-clamp technique. In the voltage-clamp mode, the L5 DRS evoked the A- and C-afferent-mediated excitatory postsynaptic currents (EPSCs) in more than 70% of the neurones tested at the L5 level. Both A- and C-afferent EPSCs were also recorded in more than 50% of the neurones at L4. At L3 and L6, the number of neurones receiving the C-afferent EPSCs (> 40%) was significantly greater than that of A-afferent EPSCs (< 20%). On the other hand, the A- and C-afferent-mediated inhibitory postsynaptic currents (IPSCs) elicited by L5 DRS were almost equally observed from L2 to S1. In the current-clamp mode, L5 DRS evoked A- and C-afferent-mediated EPSPs, some of which initiated action potentials (APs). Most of the A-afferent-mediated APs were limited at the L5 level, while C-afferent-mediated APs were observed at L5 and L4. As the L2 DRS-evoked APs in the L2 SG neurones were suppressed by L5 DRS, the widespread distribution of the inhibitory inputs was considered to be functional. These findings suggest that the excitatory projection of the C afferents to the SG neurones was thus spread more rostrocaudally than that of the A afferents, thereby contributing to more diffuse pain transmission. In addition, the widespread distribution of the inhibitory inputs may thus play a role as a lateral inhibitory network and thereby prevent the expansion of the excitatory inputs of noxious stimuli.

(Received 27 May 2004; accepted after revision 2 August 2004; first published online 5 August 2004)
Corresponding author T. Katafuchi: Department of Integrative Physiology, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan. Email: kataf{at}physiol.med.kyushu-u.ac.jp

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