The stereospecific effect of naloxone on rat dorsal horn neurones; Inhibition in superficial laminae and excitation in deeper laminae

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Abstract
The effect of systemic naloxone on the activity evoked by C-fiber stimulation in dorsal horn neurons of the rat spinal cord was investigated. Recordings were made in unanesthetized, decerebrate spinalized rats. Units (15) were recorded from laminae 4 and 5 of the dorsal horn; 11 of these units were excited by naloxone (0.2-1.0 mg/kg). The onset of this excitation was after 20 s-5 min and recovery to control levels occurred within 15-40 min. Of 17 units recorded in substantia gelatinosa of the dorsal horn, 13 were inhibited by naloxone. The latency of onset of this inhibition was short (2-10 s) and the effect persisted for 5-10 min. The effects were largely restricted to C-fiber evoked activity although sometimes A.delta. responses were similarly altered. Neurons stimulated at A.beta.-fiber threshold, or whose sole afferent input were A.beta.-fibers, were unaffected by the naloxone. The stereoisomer of naloxone, (+)naloxone which is inactive in opiate receptor binding tests, failed to produce the same changes found with (-)naloxone in 17 units. These results show a differential effect of naloxone on neurons in the dorsal horn which respond to C-fiber input. Units in the substantia gelatinosa are inhibited while units in deeper laminae are excited by naloxone. These effects may be mediated by the blockade of endogenous opioids in the spinal cord.