Electrical Stimulation of Large Myelinated Afferents Inhibits Responses of Rat Spinal Dorsal Horn Neurons to Noxious and Innocuous Cutaneous Stimulation

The inhibitory effects of electrical stimulation of large myelinated afferents on_ spinal dorsal horn neurons were investigated. In rats anesthetized with thiamylal sodium,_ responses of 153 dorsal horn neurons to noxious heating or innocuous mechanical stimulation_ (light brushing) of the tail were recorded from the sacral and coccygeal levels of the spinal cord_ by extracellular microelectrodes. Of these neurons, 45 were low-threshold mechanoreceptive_ (LTM), 47 were nociceptive-specific (NS), and 61 were wide-dynamic-range (WDR) neurons_. In 22.2% of the LTM neurons, responses to innocuous mechanical stimulation were inhibited_ by electrical stimulation of large myelinated afferents applied to the ipsilateral hindlimb for_ 5 min. This conditioning stimulation (large myelinated afferent stimulation, LMAS) also inhibited noxious heat responses in 19.2% of the NS neurons. The inhibitory effect of LMAS_ on WDR neurons was nonselective in that both responses to light brushing and noxious heating were inhibited. Of the WDR neurons, 27.3 % were inhibited by LMAS. These results_ indicate that inhibition produced by LMAS is exerted on all 3 classes of spinal dorsal horn neu_ rons.