Noradrenergic neurons in the locus coeruleus referred to as locus coeruleus neurons, provide the major supply of norepinephrine to the forebrain and play important roles in behavior through regulation of wakefulness and arousal. In a previous study using brain slice preparations, we reported that locus coeruleus neurons are subject to tonic inhibition mediated by gamma-aminobutyric acid B receptors (GABA(B)Rs) and that the extent of tonic inhibition varies with ambient GABA levels. Since ambient GABA in the locus coeruleus was reported to fluctuate during the sleep-wakefulness cycle, here we tested whether GABA(B)R-mediated tonic inhibition of locus coeruleus neurons could be a mechanism underlying changes in brain arousal. We first demonstrated that GABA(B)R-mediated tonic inhibition of locus coeruleus neurons also exists in vivo by showing that local infusion of CGP35348, a GABA(B)R antagonist, into the locus coeruleus increased the firing rate of locus coeruleus neurons in anesthetized rats. We then showed that this manipulation accelerated the behavioral emergence of rats from deep anesthesia induced by isoflurane. Together, these observations show that GABA(B)R-mediated tonic inhibition of locus coeruleus neurons occurs in vivo and support the idea that this effect may be important in regulating the functional state of the brain.
