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Spinal substantia gelatinosa (SG) is a site of action of administered and endogenous opioid agonists and is an important element in the system of antinociception. However, little is known about the types of neurons serving as specific postsynaptic targets for opioid action within the SG. To study the spinal mechanisms of opioidergic analgesia, the authors compared the action of μ-opioid agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) on SG neurons with different intrinsic firing properties.Whole cell patch clamp recordings from spinal cord slices of Wistar rats were used to study the sensitivity of SG neurons to DAMGO.Three groups of neurons with distinct distributions in SG were classified: tonic-, adapting-, and delayed-firing neurons. DAMGO at 1 μm concentration selectively hyperpolarized all tonic-firing neurons tested, whereas none of the adapting- or delayed-firing neurons were affected. The effect of DAMGO on tonic-firing neurons was due to activation of G protein–coupled inward-rectifier K+ conductance, which could be blocked by 500 μm Ba2+ and 500 μm Cs+ but increased by 50 μm baclofen. As a functional consequence of DAMGO action, a majority of tonic-firing neurons changed their pattern of intrinsic firing from tonic to adapting.It is suggested that tonic-firing neurons, presumably functioning as excitatory interneurons, are primary postsynaptic targets for administered and endogenous opioid agonists in spinal SG. Functional transition of cells in this group from tonic to adapting firing mode may represent an important mechanism facilitating opioidergic analgesia.