Microinjections of the Hypocretin Antagonist Almorexant Vs. Gabaergic Agonist Zolpidem in Basal Forebrain Show Differential Effects on Cortical Adenosine Levels in Freely-Moving Rats

Abstract

Hypocretin (Hcrt-1 and Hcrt-2) peptides are well-known to regulate sleep and alertness and send projections to the basal forebrain (BF), an area critical for promoting wakefulness. The BF contains a heterogeneous mix of neurons that send diverse projections important for cortical arousal. Almorexant (ALM) is a dual Hcrt receptor antagonist that reversibly blocks signaling of both Hcrt receptors, whereas Zolpidem (ZOL) is a benzodiazepine receptor agonist affecting Cl- channels. Previous studies have shown that oral delivery of ALM elicits somnolence without cataplexy and, in rat, decreases active wake and increases the time spent in non-rapid eye movement (NREM) and (REM) sleep with differential effects on various neurotransmitter systems. To date, no studies have reported the effects of central microinjections of ALM or ZOL and its effect on behavior or transmitter levels in brain. We used in vivo microdialysis and HPLC analysis to examine cortical adenosine (ADO) levels following BF microinjections of ZOL (0.6 µg/0.2 µl), ALM (1.0 µg/0.2 µl), or VEH (aCSF) combined with behavioral analyses. Preliminary analyses revealed a significant main effect of drug on ADO levels. Post-hoc comparisons showed that ALM microinjected in to the BF (n=3 rats; * p<0.05) caused a significant increase in cortical ADO that lasted up to 6 h post microinjection compared to VEH control (n=3). Conversely, administration of ZOL (n=3) to the BF significantly decreased cortical ADO levels (# p<0.05) compared to VEH and ALM. These results provide novel evidence for dynamic neurochemical changes underlying Hcrt modulation of sleep-wakefulness.