Electrophysiological effects of ghrelin on pedunculopontine tegmental neurons in rats: An in vitro study

Ghrelin is a potent stimulant for growth hormone (GH) secretion and feeding. Recent studies further show a critical role of ghrelin in the regulation of sleep-wakefulness. Pedunculopontine tegmental nucleus (PPT), which regulates waking and rapid eye movement (REM) sleep, expresses GH secretagogue receptors (GHS-Rs). Thus, the present study was carried out to examine electrophysiological effects of ghrelin on PPT neurons using rat brainstem slices, and to determine the ionic mechanism involved. Whole cell recording revealed that ghrelin depolarizes PPT neurons dose-dependently in normal artificial cerebrospinal fluid (ACSF). The depolarization persisted in tetrodotoxin-containing ACSF, although action potentials did not occur. Application of [d-Lys3]-GHRP-6, a selective antagonist for GHS-Rs, almost blocked the ghrelin-induced depolarization. Furthermore, the ghrelin-induced depolarization was reduced in high K+ ACSF or low Na+ ACSF, and abolished in high K+–low Na+ ACSF or in a combination of low Na+ ACSF and recordings with Cs+-containing pipettes. An inhibitor of Na+/Ca2+ exchanger had no effect on the depolarization. Most of the PPT neurons recorded were characterized by an A-current or both the A-current and a low threshold Ca2+ spike, and they were predominantly cholinergic as revealed by nicotinamide adenine dinucleotide phosphate-diaphorase staining. These results suggest that ghrelin depolarizes PPT neurons postsynaptically and dose-dependently via GHS-Rs, and that the ionic mechanisms underlying the ghrelin-induced depolarization include a decrease of K+ conductance and an increase of non-selective cationic conductance. The results also support the notion that ghrelin plays a role in the regulation of sleep-wakefulness.