Journal - Journal of Neuroscience

Abstract :

Hypothalamic fibers containing the wake-promoting peptides,hypocretins (Hcrts) or orexins, provide a dense innervationto the medial septum–diagonal band of Broca (MSDB), asleep-associated brain region that has been suggested to showintense axonal degeneration in canine narcoleptics. The MSDB,via its cholinergic and GABAergic projections to the hippocampus,controls the hippocampal theta rhythm and associated learningand memory functions. Neurons of the MSDB express very highlevels of the Hcrt receptor 2, which is mutated in canine narcoleptics.In the present study, we investigated the electrophysiologicaleffects of Hcrt peptides on septohippocampal cholinergic neuronsthat were identified in living brain slices of the MSDB usinga selective fluorescent marker. Hcrt activation of septohippocampalcholinergic neurons was reversible, reproducible, and concentrationdependent and mediated via a direct postsynaptic mechanism.Both Hcrt1 and Hcrt2 activated septohippocampal cholinergicneurons with similar EC50 values. The Hcrt effect was dependenton external Na+, reduced by external Ba2+, and also reducedin recordings with CsCl-containing electrodes, suggesting adual underlying ionic mechanism that involved inhibition ofa K+ current, presumably an inward rectifier, and a Na+-dependentcomponent. The Na+ component was dependent on internal Ca2+,blocked by replacing external Na+ with Li+, and also blockedby bath-applied Ni2+ and KB-R7943, suggesting involvement ofthe Na+–Ca2+ exchanger. Using double-immunolabeling studiesat light and ultrastructural levels, we also provide definitiveevidence for a hypocretin innervation of cholinergic neurons.Thus Hcrt effects within the septum should increase hippocampalacetylcholine release and thereby promote hippocampal arousal.