Low dose of morphine strongly depresses responses of specific nociceptive neurones in the ventrobasal complex of the rat

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Abstract
Effects of low i.v. doses of morphine (30, 100 and 1000 .mu.g/kg) upon unitary responses of 22 nociceptive and 5 non-nociceptive units recorded in the ventrobasal (VB) complex of the rat were analyzed. The responses of the non-noxious neurons were not depressed by morphine. For all these doses there was a decrease of the total number of spikes and of the maximal firing rate of the responses of the noxious neurons. The depressive effect was significantly dose-related (with linear semi-logarithmic dose-response curve) and naloxone-reversible. Similar effects were observed upon responses to pinches and noxious heat. The ED50 which was close to 90 .mu.g/kg for these thalamic responses to pinches is much lower than that evaluated for spinal dorsal horn responses under the same anesthetic conditions. The depressive effect of low doses observed for VB neurons seems to be mainly of supraspinal origin.