Rapid use‐dependent down‐regulation of γ‐aminobutyric acid type A receptors in rat mesencephalic trigeminal neurons

Rundown is ubiquitously seen in response to repetitive activation of receptor or ion channels as a use‐dependent down‐regulation through various mechanisms. In contrast to AMPA receptors, γ‐aminobutyric acid type A receptor (GABA_AR) are believed to display no rapid use‐dependent down‐regulation. We report here a rapid use‐dependent down‐regulation of GABA_AR in primary sensory neurons of rat mesencephalic trigeminal nucleus (MTN), which express synaptic GABA_ARs in addition to extrasynaptic ones, unlike other primary sensory neurons. When muscimol was repetitively puff‐applied to an MTN neuron every 2 min before, during, and after the muscimol bath application for 5 min, both the GABA_A responses obtained under both current‐ and voltage‐clamp conditions were almost completely depressed during the bath application. However, the former and latter GABA_A responses recovered to 26% ± 7% and 36% ± 7% of their control amplitudes, respectively, 15 min after washout of the bath‐applied muscimol. By contrast, when examined in the presence of chelerythrine, a protein kinase C (PKC) inhibitor, together with a stringent chelation of intracellular Ca²⁺, the puff responses were almost completely recovered, whereas those were recovered to 40–60% of the control by either chelerythrine or EGTA alone. A phosphatidylinositol 3‐kinase inhibitor (PI3K), wortmannin, which blocks various signal transductions, including vesicular trafficking, significantly enhanced the rundown of the puff responses examined every 2 min. These findings indicate that the rundown of GABA_A response in MTN neurons is mediated by the use‐dependent down‐regulation of GABA_AR, which is reversed by PKC inhibition together with intracellular Ca²⁺ chelation, while being facilitated by PI3K inhibition. © 2009 Wiley‐Liss, Inc.