Morphine-sensitive and morphine-insensitive actions of C-fibre input on the rat spinal cord.

A dose-dependent differential action of morphine on the hamstring flexor withdrawal reflex recorded in flexor alpha-motoneurones has been found in the decerebrate-spinal rat. At low doses (0.5 mg/kg) morphine pretreatment reduces the prolonged heterosynaptic facilitation of the reflex that follows brief (20 s) C-primary afferent fibre conditioning stimuli, without modifying the activation of the motoneurones by C-fibre input or noxious peripheral stimuli. Moderate doses (5 mg/kg) of morphine are required to suppress the C-fibre-induced excitability increase once it is established by the conditioning stimulus buy only very high doses (20 mg/kg) reduce the direct reflex activation of the flexor motoneurones by either C-fibre input or noxious stimuli. The relevance of these findings to the antinociceptive and analgesic actions of morphine are discussed.