Abstract
Previous studies have revealed that the stimulation of the rostral ventromedial medulla (RVM), including raphe magnus inhibits nociceptive transmission in the spinal doral horn and multiple neurotransmitters besides 5-HT are shown to be involved in the raphe-spinal system (RSS). To elucidate the inhibitory mechanisms of the RSS, we analyzed IPSCs of the substantia gelatinosa neurons (SG) in the adult rat spinal cord elicited by glutamate injection and electrical stimulation of the RVM (RVM-GI and RVM-ES) using in vivo patch-clamp technique. In the voltage-clamp mode (VH, 0 mV), the RVM-GI and RVM-ES facilitated both of frequency and amplitude of IPSCs in the SG neurons. The RVM-ES evoked IPSCs were considered to be monosynaptic and conduction velocities of the evoked IPSCs were calculated as over 3 m/sec (3.1-20.7 m/s), suggesting that the IPSCs were mediated by myelinated fibers. Furthermore, the evoked IPSCs had either long or short decay time courses and were blocked by bicuculline and strychnine, respectively, but not by 5-HT antagonists. In the current-clamp mode, action potentials elicited by noxious mechanical stimuli applied to the receptive field of the ipsilateral hindlimb were suppressed by the RVM-ES in 63% of the neurons tested (10/16). These findings suggest that the RSS includes the direct GABAergic and glycinergic descending inputs to the SG. And these non-serotonergic descending inhibitory systems may exert distinct modulatory effects on the excitatory noxious transmission in the spinal dorsal horn. [Jpn J Physiol 55 Suppl:S144 (2005)]