GABAB Receptors Mediated Tonic Inhibition of Locus Coeruleus Neuron in Vivo and Its Role in Sleep-wakefulness Regulation in Rats

In mammalian brain, locus coeruleus (LC) provides most of norepinephrine (NE) for the whole brain. LC-NE system is involved in many brain behaviors, such as nociception, attention and wake-sleep cycle. We have previously shown a tonic inhibition of LC neurons mediated by GABAB receptor in acute brain slice, and that manipulating it by varying ambient GABA could effectively tune LC neuron firing rate. Furthermore, we predict that GABAB receptor-mediated tonic inhibition could play an important role in the wake-sleep cycle behavior. Here we tested whether GABAB receptor tonic inhibition of LC neurons also occurs in vivo. Since ambient GABA in LC is reported to be higher in sleep than in wakefulness, we tested the role of GABAB receptor tonic inhibition of LC neurons in hypnosis induced by isoflurane. To investigate the hypothesis, righting reflex behavior test were made in SD (Sprague-Dawley) rats (male, 8-10 weeks) which implanted a cannula for drug injection and electrodes for EEG (electroencephalography) and EMG (electromyography) recording under isoflurane anesthesia. Righting reflex occurs during the position out of upright, was used to define emergence from general anesthesia. Local infusion of CGP35348 but not the vehicle into LC significantly reduced recovery latency from hypnosis. We found that the latency of righting reflex behavior decreased after GABAB receptor antagonist injection. A custom-made probe consisting of microwires and a fine cannula was implanted into LC to allow recording of LC single- unit and local drug application in rats. After application of CGP35348, a neutral GABAB receptor antagonist, robustly increases firing rate of LC single-unit about 3.382 times higher (n = 6 units; P=0.01497, paired-t test); In contrast, vehicle infusion did not have an effect. Above all, we suggest that not only the firing rate of LC neurons is inhibited by GABAB receptor in vivo, but also the activity of GABAB receptor on LC neurons play an important role in sleep-wakefulness regulation.