Lumbar intrathecal morphine alters activity of putative nociceptive modulatory neurons in rostral ventromedial medulla

Two physiologically and pharmacologically distinct classes of putative nociceptive modulatory neurons have been identified in the rostral ventral medulla (RVM) of the lightly anesthetized rat: on-cells and off-cells. We have previously shown that administration of morphine either systematically or by microinjection into the periaqueductal gray (PAG) produces an increase in the activity of all off-cells and a depression of the activity of all on-cells concomitant with inhibition of the tail flick reflex. We now demonstrate that morphine applied intrathecally has effects on RVM neurons that are indistinguishable from those of systemic or PAG administration. This may contribute to the known multiplicative effects of concurrent administration of opioids at spinal and supraspinal sites.