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

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 gamma-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 i 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 micrograms 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.