Morphine depresses dorsal horn neuron responses to controlled noxious and non-noxious cutaneous stimulation

Dorsal horn neurons of unanesthetized, decerebrated, low spinal cats were excited by controlled noxious and non-noxious natural stimulation as well as by intense transcutaneous electrical stimulation. Intravenous morphine (0.3--3.0 mg/kg) depressed the spontaneous activity, the electrically evoked discharge and the response to noxious cutaneous heat (greater than 45 degree C) of nociceptive dorsal horn neurons. In those nociceptive neurons receiving convergent non-noxious inputs, morphine also depressed responses to non-noxious cutaneous air-puff stimulation. The above morphine effects were all reserved by 0.3 mg/kg of naloxone i.v. In neurons, which were purely non-nociceptive, morphine had little or no effect on either spontaneous activity or evoked responses to non-noxious stimuli. These findings suggest that: 1) morphine has a spinal site of action in which noxious as well as non-noxious inputs are decreased; and 2) there is a group of purely non-nociceptive dorsal horn neurons which are not influenced by the spinal actions of morphine.