Temporal summation and a C-fibre reflex in the rat: effects of morphine on facilitatory and inhibitory mechanisms.

In intact rats, an inhibitory mechanism counteracts the increase in excitability of a flexor reflex, which is seen in spinal animals following temporal summation of C-fibre inputs; the Rostral Ventromedial Medulla is involved in this inhibitory mechanism. Electromyographic responses elicited by electrical stimulation of the sural nerve were recorded from the biceps femoris in four types of preparations, namely intact, sham-operated, Rostral Ventromedial Medulla-lesioned and decerebrate-spinal rats. The excitability of the C-fibre reflex was tested during and following high intensity homotopic electrical conditioning stimuli. Morphine (2 mg/kg) did not significantly change the basal test response but increased the excitability of the spinal cord during conditioning. This effect was triggered by the strength of inputs, involved the Rostral Ventromedial Medulla and was probably related to some forms of motor stimulation through dopaminergic transmission. While wind-up was not reduced, the inhibition related to Diffuse Noxious Inhibitory Controls, which occurred following the conditioning period, did. In spinal animals where inhibitory mechanisms disappear, the depressive effects of morphine were unmasked for both wind-up and post-conditioning facilitations. All effects of morphine were completely reversed by naloxone.