Effect of morphine on far-field somatosensory evoked potentials in the rat.

Far-field somatosensory evoked potentials were obtained by electrical stimulation of the volar surface of the forepaw and were recorded from the skull overlying the contralateral somatosensory area of the cerebral cortex. Three distinct peaks were discernable in the somatosensory evoked potentials prior to the first cortical component. Effects of different doses of morphine (0.5, 1.0, 2.5 and 5.0 mg/kg body weight), administered intravenously, were studied. Morphine selectively reduced the amplitude of component III in a dose-dependent manner. The maximum effect was obtained at a dose of 2.5 mg/kg. This effect was reversed within 2 min by intravenous administration of naloxone (0.25 mg/kg). The amplitudes of peaks I and II and the latencies of components I, II and III were not significantly affected by morphine. Infusion of morphine (25 micrograms/kg) into the thalamus showed effects on the far-field somatosensory evoked potentials similar to the maximal effect obtained on intravenous infusion. Therefore, the action of morphine on somatosensory evoked potentials could be attributed to its effect on the thalamic neurones.