Orexin A (OX_A) and orexin B (OX_B) are neuropeptides produced in the lateral hypothalamus and show widespread distribution in the CNS. These neuropeptides play essential roles in sleep-wake control and metabolism regulation. Recently cardiovascular function is also reported. Orexin-1 receptor (OX₁R) and orexin-2 receptor (OX₂R) were identified and the former is reported to be selective for OX_A and the latter is nonselective for OX_A and OX_B. The nucleus tractus solitarius (NTS) expresses OX₁R and OX₂R. OX_B shows excitatory effects on excitatory synaptic transmission but the effects of OX_A is not examined. This report examined the effects of OX_A on excitatory synaptic transmission in the rat NTS neurons using a slice patch-clamp technique.
OX_A (1 micro M) increased the frequency of spontaneous EPSCs (sEPSCs) in with or without TCS-OX2-29 (10 micro M:OX₂R blocker). Prior application of L-NAME (100 micro M) blocked the effect of OX_A under the presence of TCS-OX2-29. Contrary, OX_A (1 micro M) decreased the amplitude of tractus solitarius (TS) evoked EPSCs (eEPSCs) in with or without TCS-OX2-29 (10 micro M). Prior application of L-NAME (100 micro M) did not block the effect of OX_A under the presence of TCS-OX2-29. The action of OX_A on both sEPSC and eEPSC seemed to be presynaptic.
These results suggest that the activation of OX₁R in the NTS facilitates spontaneous excitatory synaptic transmission through NO production and inhibits TS-evoked excitatory synaptic transmission through distinct mechanism other than NO production.