Opiates distinguish spinal excitation from inhibition evoked by noxious heat stimuli in the rat: relevance to theories of analgesia

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Abstract
1 Experiments were performed to test the hypothesis that a significant part of the action of opiates in reducing responses to noxious stimuli is a reduction in the release of neurotransmitter from primary afferent fibres. 2 The effects of locally and systemically administered opiates were examined on the excitatory and γ-aminobutyric acid (GABA)-mediated inhibitory responses of spinal dorsal horn neurones to noxious heat stimulation in the anaesthetized rat: the inhibitions are thought to involve the same C-fibre afferents as the excitation. 3 Microionophoretically administered morphine reduced the excitatory response in a small proportion of the cells, reduced the background firing in a larger proportion but was ineffective on the inhibition. 4 Intravenously injected morphine (0.5–6 mg kg−1) or etorphine (0.1–2 μg kg−1) invariably attenuated the excitation of dorsal horn neurones by noxious stimuli but had no effect on the inhibition. 5 It was concluded that the data do not support the hypothesis that the production of analgesia is due mainly to a reduction in the release of transmitter from primary afferent fibres.