Prolonged, naloxone-reversible inhibition of the flexion reflex in the cat

A long-lasting inhibition of the flexion reflex was produced by prolonged electrical stimulation of a peripheral nerve with high intensity and low frequency pulses in decerebrate and spinal cats. The flexion reflex in single active motor axons was recorded from filaments of the L7, Sl or S2 ventral roots. The reflex was elicited either by electrical stimulation of a cutaneous or mixed hind limb nerve or by natural forms of stimulation applied to the foot. A late flexion reflex discharge could be elicited by electrical stimuli that activated Aδ and C afferent fibres. Conditioning stimulation of the common peroneal or tibial nerve at a suprathreshold intensity for C fibers at a rate of 2 Hz for 15 or 30 min produced an inhibition of the flexion reflex late discharges which outlasted the conditioning stimuli. Maximum inhibition on average was to 40.1% and 42.7% of control reflex value in decerebrate and spinal cats, respectively. In decerebrate cats, the duration of inhibition varied from less than 10 min to over 1 h beyond the termination of the conditioning stimuli, depending on the unit (mean = 37.1 ± 13.2 min). However, inhibition lasted over 20 min for all units tested in spinal animals (mean = 38.3 ± 5.4 min). This long-lasting inhibition of the flexion reflex was reversed completely by systemic injection of naloxone hydrochloride (0.05 mg/kg). The long-lasting inhibition of the flexion reflex produced by peripheral nerve stimulation is discussed in relation to peripheral nerve stimulation produced analgesia.