Mechanisms mediating effects of nitric oxide on perifornical lateral hypothalamic neurons

The perifornical-lateral hypothalamic area (PF-LHA) is a major wake-promoting structure. It predominantly contains neurons that are active during behavioral and cortical activation. Nitric oxide (NO) is a gaseous neurotransmitter that has been implicated in the regulation of sleep. Recently we found that NO levels in the PF-LHA are higher during sustained waking and that NO exerts predominantly inhibitory effects, especially on PF-LHA neurons excited by tactile stimulation. The mechanisms underlying this NO-evoked inhibitory action on the PF-LHA neurons were assessed in the present study. We investigated the contributions of adenosinergic, GABAergic, and sGC-cGMP signaling mechanisms in mediating nitrergic influences on the PF-LHA neurons. The extracellular discharge activity of PF-LHA neurons was recorded in combination with microdialytic delivery of pharmacological agents adjacent to the recorded neurons in urethane-anesthetized rats. First, we quantified changes in the discharge activity of the PF-LHA neurons during the blockade of the adenosine A(1) receptor, GABA(A) receptor, and sGC-cGMP pathway. Then, we determined the efficacy of blocking adenosine A(1) receptor, GABA(A) receptor, and sGC signaling mechanisms in attenuating the inhibitory influences of 3,3-bis(aminoethyl)-1-hydroxy-1-oxo-1-triazene (a NO donor) (NOC-18), a NO donor, on the discharge activity of the PF-LHA neurons. We found that NOC-18-induced suppression in the discharge activity of PF-LHA neurons was significantly attenuated during the blockade of adenosine A(1) receptor-, GABA(A) receptor-, and sGC-cGMP-mediated signaling. These findings suggest that NO-evoked inhibition of PF-LHA neurons involves a complex mechanism including, but may not be limited to, adenosinergic, GABAergic and sGC-cGMP signaling pathways. The findings are consistent with a generalized sleep-promoting role of NO within the PF-LHA and, given the sleep-promoting roles of adenosinergic and GABAergic systems in this area, further suggest that this effect may be mediated through nitrergic interactions with other neurotransmitters and neuromodulators.