Electrophysiological studies on the effects of intrathecal morphine on nociceptive neurones in the rat dorsal horn.

We have studied the effects of intrathecal morphine on the responses of 38 dorsal horn neurones in the intact rat under halothane anaesthesia to A and C fibre electrical stimulation and to natural stimuli applied to their receptive fields. Morphine selectively reduced the C fibre and pinch evoked activity in a dose-dependent naloxone-reversible manner with an ED50 of 7 nmoles. The 'wind-up' of neurones to repetitive stimulation was little altered except with the highest doses (50-150 nmoles) tested. By contrast, the A fibre evoked responses of the neurones were only slightly reduced by morphine and both the tactile responses and receptive field size to innocuous stimuli enhanced for certain cells. The results are discussed in relation to the spinal actions of opiates and their clinical applications.