Activation of presynaptic GABA_A receptors increases spontaneous glutamate release onto noradrenergic neurons of the rat locus coeruleus

In order to further explore how GABA can modulate the excitability of noradrenergic neurons of the locus coeruleus (LC), we investigated the presence of GABA_A receptors on glutamatergic nerve terminals and the functional consequences of their activation. We used mechanically dissociated immature rat LC neurons with adherent nerve terminals and patch-clamp recordings of spontaneous excitatory postsynaptic currents. Activation of presynaptic GABA_A receptors by muscimol facilitated spontaneous glutamate release by activating tetrodotoxin-sensitive Na⁺ channels and high-threshold Ca²⁺ channels. Bumetanide (10 μM), a potent blocker of Na ⁺-K⁺-Cl^- cotransporter, diminished the muscimol-induced facilitatory action of glutamate release. Our results indicate that the Na⁺-K⁺-Cl^- cotransporter accumulates Cl^- inside the nerve terminals so that activation of presynaptic GABA_A receptors causes depolarization. This GABA_A- receptor-mediated modulation of spontaneous glutamatergic transmission is another mechanism by which GABA and its analogues can regulate the excitability and activity of noradrenergic neurons in the LC.