Subsets of GABAergic neurons are able to maintain high frequency discharge patterns, which requires efficient replenishment of the releasable pool of GABA. Although glutamine is considered a preferred precursor of GABA, the identity of transporters involved in glutamine uptake by GABAergic neurons remains elusive. Molecular analyses revealed that SAT1 (Slc38a1) features system A characteristics with a preferential affinity for glutamine, and that SAT1 mRNA expression is associated with GABAergic neurons. By generating specific antibodies against SAT1 we show that this glutamine carrier is particularly enriched in GABAergic neurons. Cellular SAT1 distribution resembles that of GAD67, an essential GABA synthesis enzyme, suggesting that SAT1 can be involved in translocating glutamine into GABAergic neurons to facilitate inhibitory neurotransmitter generation.

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The reorganizations of the overall intrinsic glutamatergic and gamma-aminobutyric acid (GABA)-ergic hippocampal networks as well as the time course of these reorganizations during development of pilocarpine-induced temporal lobe epilepsy were studied with in situ hybridization and immunohistochemistry experiments for the vesicular glutamate transporter 1 (VGLUT1) and the vesicular GABA transporter (VGAT). These transporters are particularly interesting as specific markers for glutamatergic and GABAergic neurons, respectively, whose expression levels could reflect the demand for synaptic transmission and their average activity. We report that 1) concomitantly with the loss of some subpopulations of VGAT-containing neurons, there was an up-regulation of VGAT synthesis in all remaining GABA neurons as early as 1 week after pilocarpine injection. This enhanced synthesis is characterized by marked increases in the relative amount of VGAT mRNAs in interneurons associated with increased intensity of axon terminal labeling for VGAT in all hippocampal layers. 2) There was a striking loss of mossy cells during the latent period, demonstrated by a long-term decrease of VGLUT1 mRNA-containing hilar neurons and associated loss of VGLUT1-containing terminals in the dentate gyrus inner molecular layer. 3) There were aberrant VGLUT1-containing terminals at the chronic stage resulting from axonal sprouting of granule and pyramidal cells. This is illustrated by a recovery of VGLUT1 immunoreactivity in the inner molecular layer and an increased VGLUT1 immunolabeling in the CA1-CA3 dendritic layers. These data indicate that an increased activity of remaining GABAergic interneurons occurs during the latent period, in parallel with the loss of vulnerable glutamatergic and GABAergic neurons preceding the reorganization of glutamatergic networks.

The sense of smell is conveyed by the olfactory sensory neurons of the olfactory mucosa. Uniquely for sensory systems, the olfactory neurons directly face the external environment and are thus vulnerable to infections and changes in the airway surface liquid, but the surface liquid production and maintenance is not well understood. Here we show in rats and mice that Bowman's glands secrete the mucin MUC5AC. Aquaporin-5 was present at the apical face of the olfactory epithelium, completing a water transport pathway to the surface of the epithelium. Immunogold electron microscopy analysis revealed an intricate network of fine Aquaporin-1-positive fibroblast processes that surround Bowman's glands, whereas deeper blood vessels were unlabeled for Aquaporin-1. Our results show how the olfactory mucosa might be protected against infections and dehydration generally and how neuronal function is protected against ion concentration changes in the airway surface liquid by rapid replacement of water losses through the aquaporin pathways.

Aquaporin-4 (AQP4) is a water channel found at high concentrations around blood vessels in the brain and is organized into elaborate assemblies called square arrays. The natural functions of AQP4 and the square arrays remain unknown, but under pathophysiological conditions, AQP4 has been shown to influence brain edema, synapse function, and cellular migration. AQP4 was recently found to have six isoforms, where AQP4a (also known as M1), AQP4c (also known as M23), and AQP4e are functional water transport channels. Furthermore, by two-dimensional blue native polyacrylamide gel electrophoresis (BN-PAGE) analysis of the internal composition of square arrays, three distinct isoforms were visualized. Here we combine these advances in technique with mutational analysis to test a series of current hypotheses about AQP4 functional structure. We find that the square array destabilizing N-terminus of AQP4a is partly functional through the C13 and C17 amino acids, and not through R8 and R9. We find a discrepancy between our data and the proposed tetramer-tetramer binding site based on the in vitro AQP4 two-dimensional crystal structure. On the other hand, we find that isoforms AQP4a and AQP4e, while not being able to form square arrays alone, are able to interact with AQP4c and be incorporated into higher-order structures. Our results with the novel BN-PAGE analysis technique point toward a model in which the presence of accessory isoforms (AQP4a and AQP4e) regulates the square array assembly process of the main isoform, AQP4c.