A Possible Role of the Non-GAT1 GABA Transporters in Transfer of GABA From GABAergic to Glutamatergic Neurons in Mouse Cerebellar Neuronal Cultures

Unveiling the crucial role of betaine: modulation of GABA homeostasis via SLC6A1 transporter (GAT1)

Betaine is an endogenous osmolyte that exhibits therapeutic potential by mitigating various neurological disorders. However, the underlying cellular and molecular mechanisms responsible for its neuroprotective effects remain puzzling.In this study, we describe a possible mechanism behind the positive impact of betaine in preserving neurons from excitotoxicity. Here we demonstrate that betaine at low concentration modulates the GABA uptake by GAT1 (slc6a1), the predominant GABA transporter in the central nervous system. This modulation occurs through the temporal inhibition of the transporter, wherein prolonged occupancy by betaine impedes the swift transition of the transporter to the inward conformation. Importantly, the modulatory effect of betaine on GAT1 is reversible, as the blocking of GAT1 disappears with increased extracellular GABA. Using electrophysiology, mass spectroscopy, radiolabelled cellular assay, and molecular dynamics simulation we demonstrate that betaine has a dual role in GAT1: at mM concentration acts as a slow substrate, and at µM as a temporal blocker of GABA, when it is below its K0.5. Given this unique modulatory characteristic and lack of any harmful side effects, betaine emerges as a promising neuromodulator of the inhibitory pathways improving GABA homeostasis via GAT1, thereby conferring neuroprotection against excitotoxicity.

Synaptic distribution of individually labeled mitral cells in the external plexiform layer of the mouse olfactory bulb

Mitral cells are the major projection neurons of the olfactory bulb. They receive olfactory inputs, regulate information, and project their axons to the olfactory cortex. To understand output regulation of mitral cells better, we established a method to visualize individual projection neurons and quantitatively examined their synaptic distribution. Individual mitral cells were labeled by viral injection, reconstructed three dimensionally with light microscopy, and serial sectioned for electron microscopy. Synaptic distributions were analyzed in electron microscopically reconstructed cell bodies, two regions of secondary dendrites (near the somata and ∼200 μm from the somata), and primary dendrites. The ratio of presynaptic sites (60%) and reciprocal synapses (60% presynaptic and 80% postsynaptic sites) were similar in each region. Characteristically, primary dendrite synapses were distributed mainly within the inner half of the external plexiform layer (EPL). For comparison, tufted cells were also examined, and the synaptic distribution in two secondary dendrite regions, which corresponded with mitral cells, was analyzed. The results showed that the ratio of reciprocal synapses (80% presynaptic and 90% postsynaptic sites) was greater than in mitral cells. The distribution of symmetrical synapses was also analyzed with synaptic and neuronal markers, such as parvalbumin, vesicular gamma-aminobutyric acid transporter, and gephyrin. Parvalbumin-expressing neurons tended to form synapses on secondary dendrites near the somata and were more uniformly distributed on primary dendrites of mitral cells. These results indicate that local mitral cell synaptic circuits are formed in accordance with their functional roles and restricted to the inner half of the EPL. J. Comp. Neurol. 525:1633-1648, 2017. © 2016 Wiley Periodicals, Inc.

Neuronal gamma-aminobutyric acid (GABA) type A receptors undergo cognate ligand chaperoning in the endoplasmic reticulum by endogenous GABA

GABA_A receptors mediate fast inhibitory neurotransmission in the brain. Dysfunction of these receptors is associated with various psychiatric/neurological disorders and drugs targeting this receptor are widely used therapeutic agents. Both the efficacy and plasticity of GABA_A receptor-mediated neurotransmission depends on the number of surface GABA_A receptors. An understudied aspect of receptor cell surface expression is the post-translational regulation of receptor biogenesis within the endoplasmic reticulum (ER). We have previously shown that exogenous GABA can act as a ligand chaperone of recombinant GABA_A receptors in the early secretory pathway leading us to now investigate whether endogenous GABA facilitates the biogenesis of GABA_A receptors in primary cerebral cortical cultures. In immunofluorescence labeling experiments, we have determined that neurons expressing surface GABA_A receptors contain both GABA and its degradative enzyme GABA transaminase (GABA-T). Treatment of neurons with GABA-T inhibitors, a treatment known to increase intracellular GABA levels, decreases the interaction of the receptor with the ER quality control protein calnexin, concomittantly increasing receptor forward-trafficking and plasma membrane insertion. The effect of GABA-T inhibition on the receptor/calnexin interaction is not due to the activation of surface GABA_A or GABA_B receptors. Consistent with our hypothesis that GABA acts as a cognate ligand chaperone in the ER, immunogold-labeling of rodent brain slices reveals the presence of GABA within the rough ER. The density of this labeling is similar to that present in mitochondria, the organelle in which GABA is degraded. Lastly, the effect of GABA-T inhibition on the receptor/calnexin interaction was prevented by pretreatment with a GABA transporter inhibitor. Together, these data indicate that endogenous GABA acts in the rough ER as a cognate ligand chaperone to facilitate the biogenesis of neuronal GABA_A receptors.

Effects of hyperammonemia on brain energy metabolism: controversial findings in vivo and in vitro

The literature related to the effects of elevated plasma ammonia levels on brain energy metabolism is abundant, but heterogeneous in terms of the conclusions. Thus, some studies claim that ammonia has a direct, inhibitory effect on energy metabolism whereas others find no such correlation. In this review, we discuss both recent and older literature related to this controversial topic. We find that it has been consistently reported that hepatic encephalopathy and concomitant hyperammonemia lead to reduced cerebral oxygen consumption. However, this may not be directly linked to an effect of ammonia but related to the fact that hepatic encephalopathy is always associated with reduced brain activity, a condition clearly characterized by a decreased CMRO2. Whether this may be related to changes in GABAergic function remains to be elucidated.