Electrophysiological study on spinal antinociceptive interactions between adenosine and morphine in the dorsal horn of the rat.

The effects of intrathecal N6-cyclopentyladenosine (N6-CPA), an A1-receptor agonist, and morphine, a mu opioid-receptor agonist were studied on the electrically evoked responses of deep dorsal horn neurones in the rat. Both caused inhibitions, when applied alone, of the C-fibre evoked response and post-discharges. Enhanced inhibitory effects were observed when N6-CPA (5 micrograms) was given with morphine (1 microgram). Increasing the dose of morphine (5 micrograms) with the same dose of N6-CPA occluded the expected inhibitions. By contrast, the A delta-fibre evoked response, inhibited by morphine alone, was significantly facilitated by the combination. Parallel activation of purinergic and opioid inhibitory systems within the dorsal horn as well as complex interactions between them are likely.