Selective effects of morphine on the nociceptive responses of thalamic neurones in the rat.

1. The effects of intravenous morphine on the firing of single sensory neurones in the thalamus of the anaesthetized rat are described. 2. Low doses (0.38 to 2.00 mg/kg) of morphine depressed the excitation of nucleus lateralis neurones produced by natural noxious stimuli but were without effect on spontaneous activity. The excitation of some of these neurones by iontophoretically applied acetylcholine or glutamate was also depressed by morphine. These effects were reversed by intravenous (0.25 to 1.17 mg/kg) but not iontophoretically applied (50 to 200 nA) naloxone. 3. Similar or higher doses of morphine (0.58 to 5.07 mg/kg) did not prevent the excitation of ventrobasal thalamic neurones by non-noxious stimuli. 4. The possibility of a direct action of intravenous morphine in the thalamus is discussed.