Effects of morphine and indomethacin on single unit responses in the ventrobasal (VB) thalamus in adjuvant arthritic rats

Abstract

Rats rendered polyarthritic by injection of Mycobacterium butyricum into the tail were used as a model of chronic pain. (1) The activities of 154 neurones in the VB thalamus have been recorded in adjuvant arthritic rats anaesthetized with urethane (1200mg/kg i.p.). The nature of neurones which had receptive fields located in inflamed areas of hind paw (115/154 in 48 rats) were studied. 16 neurones were activated only by non-noxious stimuli (nN neurone) such as brushing or tapping and displayed the classical properties of leminiscal responses. 92 neurones were activated only by noxious stimuli (N neurone) such as tibial joint movement induced by electrical stimulation (1 Hz, 5msec, 0.5-8mA and train 30) to inflamed areas of hind paw. A relatively large number of N neurones (28/92) displayed spontaneous paroxysmal discharge. 8 neurones responded to both noxious and non-noxious stimuli (NnN neurone). All these neurones displayed long lasting afterdischarge (2-12 times of duration of stimulation). 12 N neurones were stimulated with the intensity of 0.5 to 8mA. In 8/12 neurones, these discharge rates increased linearly with intensity, although the rates of the remaining 4/12 reached a plateau at an intensity of 4 to 6mA. (2) The anti-nociceptive action of morphine and indomethacin on evoked neuronal responses of the 62 VB thalamus N neurones were investigated. Morphine lmg/kg, i.v., indomethacin lmg/kg i.v. and indomethacin 3ug/rat i.vent., produced marked depressive actions on the VB neuronal responses to joint movement induced by electrical stimulation. Spontaneous activity was not significantly modified by these drugs.