Modulatory effects of hypocretin-1/orexin-A with glutamate and gamma-aminobutyric acid on freshly isolated pyramidal neurons from the rat prefrontal cortex.

It is widely known that hypocretins are essential for the regulation of wakefulness. Our recent reports have found that hypocretin-1 shows a direct postsynaptic excitatory effect on rat prefrontal cortex (PFC) pyramidal neurons. It remains unclear whether hypocretin-1 may interact with two classical neurotransmitter systems, glutamate and gamma-aminobutyric acid (GABA) in rat PFC. For this reason, we here investigated the modulatory actions of hypocretin-1 with these two transmitters on freshly isolated PFC pyramidal neurons using whole-cell patch-clamp recordings. We found that coadministration of hypocretin-1 and glutamate showed a synergistic effect on the recorded cells, and hypocretin-1 could excite the neurons even if GABA was present. Thus, our data suggest that there may be hypocretin-glutamate and hypocretin-GABA interactions in the PFC.

Signaling pathways of hypocretin-1 actions on pyramidal neurons in the rat prefrontal cortex.

We have investigated the direct excitatory effects of hypocretin-1 on acutely isolated prefrontal cortical pyramidal neurons and explored the signaling mechanisms of these actions. Puff application of hypocretin-1 caused an excitation in the recorded neurons. These effects of hypocretin-1 were abolished by a phospholipase C inhibitor D609, demonstrating that phospholipase C mediates the actions of hypocretin-1. A specific protein kinase C inhibitor, bisindolylmaleimide II, blocked the excitatory actions of hypocretin-1, suggesting that protein kinase C plays a key role. Finally, protein kinase A inhibitor applied intracellularly did not affect the responses. These results indicate that hypocretin-1 excites prefrontal neurons by activation of phospholipase C and protein kinase C pathways, but not protein kinase A.