Opiates inhibit the discharges of fine afferent units from inflamed knee joint of the cat.

The spontaneous discharges in 15 out of 19 small-diameter afferent units from inflamed knee joints of anaesthetized cats were significantly inhibited by one or several opiates (morphine in the dose range 1.0-5.0 mg/kg; gly-ol 0.5-5.0 mg/kg; U50488 1.0-10.0 mg/kg; ethylketocyclazacine 0.5-4.0 mg/kg administered by close arterial injection into the joint). In the majority of cases a subsequent injection of naloxone (1 mg/kg i.a.) significantly reversed this effect. These data provide an electrophysiological demonstration that opiates may act on opiate receptors located at peripheral sites of primary afferent fibres and hence exert a peripheral 'analgesic' effect.