GABA transporters regulate tonic and synaptic GABA_A receptor-mediated currents in the suprachiasmatic nucleus neurons

Abstract

GABA is a principal neurotransmitter in the hypothalamic suprachiasmatic nucleus (SCN) that contributes to intercellular communication between individual circadian oscillators within the SCN network and the stability and precision of the circadian rhythms. GABA transporters (GAT) regulate the extracellular GABA concentration and modulate GABA receptor (GABAR)-mediated currents. GABA transport inhibitors were applied to study how GABAR-mediated currents depend on the expression and function of GAT. Nipecotic acid inhibits GABA transport and induced an inward tonic current in concentration-dependent manner during whole cell patch-clamp recordings from SCN neurons. Application of either the selective GABA transporter 1 (GAT1) inhibitors NNC-711 or SKF-89976A, or the GABA transporter 3 (GAT3) inhibitor SNAP-5114, produced only small changes of the baseline current. Coapplication of GAT1 and GAT3 inhibitors induced a significant GABAR-mediated tonic current that was blocked by gabazine. GAT inhibitors decreased the amplitude and decay time constant and increased the rise time of spontaneous GABAR-mediated postsynaptic currents. However, inhibition of GAT did not alter the expression of either GAT1 or GAT3 in the hypothalamus. Thus GAT1 and GAT3 functionally complement each other to regulate the extracellular GABA concentration and GABAR-mediated synaptic and tonic currents in the SCN. Coapplication of SKF-89976A and SNAP-5114 (50 µM each) significantly reduced the circadian period of expression in the SCN by 1.4 h. Our studies demonstrate that GAT are important regulators of GABAR-mediated currents and the circadian clock in the SCN. In the suprachiasmatic nucleus (SCN), the GABA transporters GAT1 and GAT3 are expressed in astrocytes. Inhibition of these GABA transporters increased a tonic GABA current and reduced the circadian period of expression in SCN neurons. GAT1 and GAT3 showed functional cooperativity: inhibition of one GAT increased the activity but not the expression of the other. Our data demonstrate that GABA transporters are important regulators of GABA receptor-mediated currents and the circadian clock.

The circadian clock: a tale of genetic–electrical interplay and synaptic integration

“…The specific role of GABA neurotransmission in maintaining cellular synchrony remains controversial (see recent reviews by [23–25]). GABA acts on synaptic GABA A receptors to mediate fast “phasic” signaling between SCN neurons, and extrasynaptic GABA A receptor activation provides SCN cells with a “tonic” GABA A current [26]. As a network, activity signaling by the tonic GABA-GABA A receptor current is a strong candidate to regulate the coupling strength between individual SCN neuronal oscillators [27,28].…”

Astrocytic Modulation of Neuronal Activity in the Suprachiasmatic Nucleus: Insights from Mathematical Modeling

“…Our model predicted that the astrocytic modulation of SCN neurons depended on mutual contributions from multiple intercellular signals (VIP: Marpegan et al, 2009; GABA: Barca-Mayo et al, 2017; Moldavan et al, 2017; and glutamate: Brancaccio et al, 2017; Scofield, 2018) regulated not only by neurons but also by astrocytes. Our signaling-blockade simulations suggested that the astrocytic binding of neuronally released VIP is an essential mechanism for establishing bidirectional communication between the 2 cell types.…”

“…We modeled calcium dynamics and internal calcium pools (i.e., IP3 and ryanodine stores; Mesiti et al, 2015; Vasalou and Henson, 2010) and included VPAC2 receptors (Masmoudi-Kouki et al, 2007) for binding VIP released by SCN neurons (Marpegan et al, 2009). The astrocyte model also included GABA transporters, which potentially regulate GABA concentrations in synaptic and extrasynaptic regions (Moldavan et al, 2015, 2017) as well as glutamate transporters for glutamate uptake and release (Brancaccio et al, 2017, 2019). The astrocyte was treated as a GABAergic and glutamatergic cell (Brancaccio et al, 2017; Schousboe et al, 2013; Yoon et al, 2012) that produced glutamate and stored GABA with circadian variations.…”

“…Inhibition of GABA transporters expressed in SCN astrocytes resulted in an elevated tonic GABA A receptor–mediated current and affected the periodicity of Per1 expression in SCN neurons (Moldavan et al, 2017). Astrocytes also have been shown to synaptically communicate with and tune SCN neurons through glutamatergic transmission (Brancaccio et al, 2017; Scofield, 2018).…”

GABAergic‐astrocyte signaling: A refinement of inhibitory brain networks

“…GAT-3 is the most abundant in astrocyte processes facing synapses and neuronal bodies (Boddum et al, 2016). GABA uptake via GAT-1 and GAT-3 activity influences GABAAR-mediated inhibitory transmission (Moldavan, Cravetchi, & Allen, 2017;Song et al, 2013); but additionally, GAT-3 activation stimulates the release of ATP/ adenosine from hippocampal astrocytes, which contributes to downregulate excitatory transmission via activation of presynaptic adenosine receptors (A1Rs) (Boddum et al, 2016), and enhances inhibitory transmission via activation of postsynaptic A1Rs (Matos et al, 2018). This downregulation of excitatory transmission via GABAergic activation contributes to the hippocampal heterosynaptic depression phenomena (Boddum et al, 2016;Serrano, Haddjeri, Lacaille, & Robitaille, 2006).…”