Further evidence for a strong depressive effect of low doses of morphine on VB thalamic neuronal responses (a study on arthritic rats)

The effects of various i.v. morphine doses (30, 100 and 1000 micrograms/kg) were studied upon unitary ventral basal (VB) neuronal responses elicited by joint stimulation in 24 Freund's adjuvant induced arthritic rats. The depressive effect of morphine was significantly dose-related and generally naloxone-reversible; however there were sometimes some difficulties to reverse morphine effect with the lowest dose of naloxone (10 micrograms/kg). The effect of morphine was not significantly different from that obtained in normal rats upon responses of specific nociceptive VB neurons. Although these results do not explain enhancement of morphine analgesia in arthritic rats by comparison with normal rats, they do confirm efficiency of low doses of morphine upon VB neuronal responses elicited by stimuli which induce nociceptive reaction in freely moving animals.