Postsynaptic action of GABA in modulating sensory transmission in co‐cultures of rat carotid body via GABA_A receptors

Abstract

GABA is expressed in carotid body (CB) chemoreceptor type I cells and has previously been reported to modulate sensory transmission via presynaptic GABA B receptors. Because low doses of clinically important GABA A receptor (GABA A R) agonists, e.g. benzodiazepines, have been reported to depress afferent CB responses to hypoxia, we investigated the potential contribution of GABA A R in co-cultures of rat type I cells and sensory petrosal neurones (PNs). During gramicidin perforated-patch recordings (to preserve intracellular Cl − ), GABA and/or the GABA A agonist muscimol (50 μm) induced a bicuculline-sensitive membrane depolarization in isolated PNs. GABA-induced whole-cell currents reversed at ∼ −38 mV and had an EC 50 of ∼10 μm (Hill coefficient = ∼1) at −60 mV. During simultaneous PN and type I cell recordings at functional chemosensory units in co-culture, bicuculline reversibly potentiated the PN, but not type I cell, depolarizing response to hypoxia. Application of the CB excitatory neurotransmitter ATP (1 μm) over the soma of functional PN induced a spike discharge that was markedly suppressed during co-application with GABA (2 μm), even though GABA alone was excitatory. RT-PCR analysis detected expression of GABAergic markers including mRNA for α1, α2, β2, γ2S, γ2L and γ3 GABA A R subunits in petrosal ganglia extracts. Also, CB extracts contained mRNAs for GABA biosynthetic markers, i.e. glutamate decarboxylase (GAD) isoforms GAD 67A,E, and GABA transporter isoforms GAT 2,3 and BGT-1. In CB sections, sensory nerve endings apposed to type I cells were immunopositive for the GABA A R β subunit. These data suggest that GABA, released from the CB during hypoxia, inhibits sensory discharge postsynaptically via a shunting mechanism involving GABA A receptors.

Signal processing at mammalian carotid body chemoreceptors

“…The reversal potential for GABA-evoked current is ∼−38 mV. Interestingly, however, in these same petrosal neurons, excitatory responses elicited by ATP are markedly suppressed by GABA [54]. These data suggest that GABA plays an inhibitory postsynaptic role in the carotid body by shunting ATP-evoked excitatory currents in petrosal afferent terminals.…”

“…Recent immunohistochemical studies in tissue sections from rat carotid bodies demonstrate that GABA(A) receptors are localized to petrosal afferent terminals near type I cells. Also, several subunits of the GABA(A) receptor have been identified by RT-PCR in petrosal ganglia [54]. In the co-culture preparation of the carotid body, GABA depolarizes petrosal neurons and GABA responses are inihibited by bicuculline.…”

Carbon monoxide (CO) and hydrogen sulfide (H2S) in hypoxic sensing by the carotid body

“…AOAA is also known to increase GABA levels in tissues (Wallach, 1961) and disrupts mitochondrial function (Kauppinen et al, 1987). GABA inhibits carotid body sensory response to hypoxia (Zhang et al, 2009). It is also known that inhibition of mitochondrial function affects carotid body response to hypoxia (see Kumar and Prabhakar, 2012 for ref).…”

Prenatal Nicotine Exposure Increases GABA Signaling and Mucin Expression in Airway Epithelium

“…Currents were recorded from confluent cultured monkey BECs (7-10 days). The majority of electrophysiologic experiments involved the gramicidin perforated patch and whole-cell patch-clamp technique (6,23). The external Krebs solution bathing the BECs contained 130 mM NaCl, 3 mM KCl, 2.5 mM CaCl 2 , 1 mM MgCl 2 , 10 mM NaHCO 3 , 5 mM Hepes, and 10 mM glucose, pH 7.35-7.4.…”