Hypothalamic neuropeptides, orexins A and B, differently inhibit nociceptive behavior. This difference is possibly due to a distinction between orexins A and B in modulating synaptic transmission in spinal substantia gelatinosa (SG) neurons that play a pivotal role in regulating nociceptive transmission. Although we previously reported a modulatory action of orexin B on synaptic transmission in adult rat SG neurons, it has not been fully examined how the transmission is affected by orexin A. The present study examined the effects of orexin A on spontaneous excitatory and inhibitory transmission in SG neurons of adult rat spinal cord slices by using the whole-cell patch-clamp technique. Like orexin B, orexin A produced an inward current at -70 mV and/or increased the frequency of spontaneous excitatory postsynaptic current without changing its amplitude. Half-maximal effective concentration values for their effects were 0.0045 and 0.030 mu M, respectively; the former value was four-fold smaller than that of orexin B while the latter value was comparable to that of orexin B. Orexin A enhanced not only glycinergic but also GABAergic transmission, although only glycinergic transmission was facilitated by orexin B in the majority of neurons tested. Orexin A activities were inhibited by an orexin-1 receptor antagonist (SB334867) but not an orexin-2 receptor antagonist (JNJ10397049), as different from orexin B whose activation was depressed by JNJ10397049 but not SB334867. These results indicate that orexin A has a different action from orexin B in SG neurons in efficacy for inward current production and in GABAergic transmission enhancement, possibly owing to orexin-1 but not orexin-2 receptor activation. This difference could contribute to at least a part of the distinction between orexins A and B in antinociceptive effects. (C) 2018 Elsevier Inc. All rights reserved.