Effects of morphine and indomethacin on evoked neuronal responses of ventrobasal thalamic neurones: site of action of analgesic drugs in non-adjuvant arthritic rats.

Single neuronal activity was recorded extracellularly in the ventrobasal (VB) nucleus of the thalamus in non-adjuvant arthritic rats under urethane (1200 mg/kg, i.p.) anesthesia. The effects of morphine and indomethacin on the evoked responses elicited by noxious stimuli such as transcutaneous electrical stimulation (TES) or tibial nerve electrical stimulation (TNES), or non-noxious stimulation such as repetitive brushing were examined. Intravenous administered morphine (0.1, 0.3 and 1 mg/kg) depressed the evoked responses elicited by either TES or TNES without affecting any background activities. In contrast, intravenous administered indomethacin (1, 3 and 10 mg/kg) depressed the evoked responses induced by TES, but failed to depress the evoked responses induced by TNES. At doses of 3 and 10 mg/kg, indomethacin slightly depressed the background activities of the nociceptive neurones. Depressant effects of morphine were restored by intravenous naloxone (0.5 mg/kg) administration, but not observed in case of indomethacin. The evoked responses induced by non-noxious stimulation failed to depress either indomethacin or morphine administration. These results suggest that the site of action of indomethacin in non-adjuvant arthritic rats is mainly in the periphery. In contrast, morphine produced an antinociceptic action due to the central mechanism.