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

A single neuronal activity was recorded extracellularly in the ventrobasal (VB) nucleus of the thalamus in adjuvant arthritic rats under urethane anesthesia (1200 mg/kg, i.p.). The effects of morphine and indomethacin on the evoked responses elicited by noxious stimuli (transcutaneous electrical stimulation and tibial nerve electrical stimulation) and/or non-noxious stimuli were investigated. Intravenous administration of morphine and indomethacin depressed the evoked responses elicited by either transcutaneous electrical stimulation or tibial nerve electrical stimulation without affecting any background activities. By contrast, the responses of all neurones investigated responding to non-noxious stimuli were never depressed by the intravenous administration of morphine and indomethacin. Morphine showed the same depressant effects on the evoked responses activated by both noxious stimuli, but the depressant effects due to indomethacin on the evoked discharges were more sensitively produced by transcutaneous electrical stimulation than tibial nerve electrical stimulation. Depressant effects of morphine were restored by intravenous naloxone administration, but not observed in case of indomethacin. These results suggest that an analgesic mechanism of morphine and indomethacin may reside in the neo-spinothalamic projection system of adjuvant arthritic rats and that the site of action of indomethacin may also reside in a peripheral site. However, the mode of the central action of indomethacin was different from that of morphine.