MEDULLARY ON-CELL ACTIVITY DURING TAIL-FLICK INHIBITION PRODUCED BY HETEROTOPIC NOXIOUS-STIMULATION

Reflex responses and neuronal excitation elicited by noxious stimuli a pplied to a given body site can be inhibited by application of noxious stimulation to another, even distant body region. Such heterotopic no xious stimulation (HNS) has been proposed to act via 'diffuse noxious inhibitory controls' (DNIC) which involve supraspinal components. The so-called on-cells of the rostral ventromedial medulla (RVM) in rats a re thought to facilitate nociceptive transmission. Experimental manipu lations that inhibit on-cells also inhibit withdrawal reflexes and noc iceptive thalamic responses. In the present study on-cell activity was recorded in relation to the tail flick (TF) elicited by noxious heat applied to the tail both before and during either immersion of a paw i n water above 56 degrees C or application of strong pinch to various b ody regions. Such HNS elicited strong activation of on-cells, followed by depression even when I-INS continued. When this depression was int ense, tail-heating failed to elicit vigorous on-cell firing, and TF wa s retarded or abolished. These results are compatible with the hypothe sis that antinociception elicited by HNS involves depression of on-cel l firing and hence lack of facilitation of nociceptive transmission.