Effects of iontophoresis of noradrenaline and stimulation of the periaqueductal gray on single-unit activity in the rat superficial dorsal horn.

Recordings were made with a new form of low-noise carbon fibre microelectrode from 75 units in the superficial laminae of the lumbar dorsal horn of the anaesthetized rat. The response of each unit to adequate stimulation of its peripheral receptive field, to noradrenaline (NA) applied iontophoretically, and to electrical stimulation of the periaqueductal gray (PAG) was investigated. Only units that could be excited by iontophoresis of glutamate (10-100 nA) were analyzed. Recording sites in the spinal cord and stimulation sites in the brainstem were identified histologically at the end of each experiment. Forty-six units with low-threshold receptive fields and small spike amplitudes were found, mainly located in laminae II and III. Both stimulation of the PAG and NA iontophoresis excited the majority (32/46) of these units. The rest were unaffected. Eight high-threshold (HT) units were located in the region of lamina I. Twenty-one wide-dynamic-range (WDR) units were found mainly in deeper laminae. Both WDR and HT units were inhibited by NA and PAG stimulation. This inhibition affected both glutamate-evoked activity and responses to nociceptive stimuli. We suggest that the small LT units are inhibitory interneurones of the substantia gelatinosa (SG), which act on the WDR and HT units to produce nociceptive-specific inhibition. The inhibition can be modality-specific without necessarily being presynaptic because of the laminar arrangement of the dorsal horn. The PAG could thus exert its known antinociceptive effects at least partly via descending noradrenergic axons that excite these SG cells.