Multiple Types of GABAA Receptors Mediate Inhibition in Brainstem Parasympathetic Cardiac Neurons Running Head: Diversity of GABA Receptors in Cardiac Vagal Neurons

Recent work suggests neurons can have different types of GABAA receptors that mediate phasic inhibitory post-synaptic currents (IPSCs) and tonic currents. This study examines the diversity of GABAergic synaptic currents in parasympathetic cardioinhibitory neurons that receive rhythmic bursts of GABAergic neurotransmission. Focal application of gabazine (25 μM) to cardiac vagal neurons in-vitro did not change the frequency of firing in spontaneously active neurons or the resting membrane potential, however picrotoxin (100 μM) significantly depolarized cardiac vagal neurons and increased their firing. Similarly, gabazine (25 μM) selectively blocked GABAergic IPSCs but did not change holding current in cardiac vagal neurons, whereas picrotoxin (100 μM) not only blocked GABAergic IPSCs but also rapidly decreased the tonic current. Since the tonic current could be due to activation of GABA receptors by ambient GABA, or alternatively, spontaneous opening of constitutively active GABA channels, an antagonist for the GAT-1 GABA transporter, NO-711 (10 μM), was applied to distinguish between these possibilities. NO-711 did not significantly alter the holding current in these neurons. The benzodiazepine flunitrazepam (1 μM) significantly increased the tonic current and GABAergic IPSC decay time, surprisingly, however, in the presence of gabazine flunitrazepam failed to elicit any change. These results suggest cardiac vagal neurons possess gabazine-sensitive GABAA receptors that mediate phasic synaptic currents, a gabazineinsensitive but picrotoxin-sensitive extrasynaptic tonic current that when blocked depolarizes and increases the firing rate of cardiac vagal neurons, and benzodiazepines recruit a third type of GABAA receptor which is sensitive to gabazine and augments the extrasynaptic tonic current. keywords: GABA, bradycardia; parasympathetic; ambiguus Page 2 of 26