The excitatory amino acid carrier 1 (EAAC1) in the rat nucleus of the solitary tract: subcellular localization suggests no major role in glutamate clearance.

The excitatory amino acid carrier 1 (EAAC1) is a sodium-dependent glutamate transporter widely found in the mammalian brain and mainly localized in the somatodendritic compartment of neurons. The present study was performed to determine whether EAAC1 is present in the rat nucleus of the solitary tract (NST, a sensory brainstem nucleus involved in visceroception) and to document its subcellular localization. Using fluorescent immunolabeling, peroxidase immunostaining and quantitative immunogold labeling, we showed that both intracellular and plasma membrane-associated pools of EAAC1 transporters existed in dendrites of NST neurons. Although plasma membrane-associated transporters were more concentrated in the vicinity of synapses, no labeling was found at the axon-dendrite interface, suggesting that EAAC1 was not (or barely) expressed in this portion of dendritic membrane. Using computer simulation, we next showed that the ability of EAAC1 to efficiently take up synaptically released glutamate was very low outside the axon-dendrite interface. These data suggest that EAAC1 transporters present on NST dendrites may play a minor role if any in glutamate clearance.

Remodeling of glial coverage of glutamatergic synapses in the rat nucleus tractus solitarii after ozone inhalation.

Besides the well-described inflammatory and dysfunction effects on the respiratory tract, accumulating evidence indicates that ozone (O3 ) exposure also affects central nervous system functions. However, the mechanisms through which O3 exerts toxic effects on the brain remain poorly understood. We previously showed that O3 exposure caused a neuronal activation in regions of the rat nucleus tractus solitarii (NTS) overlapping terminal fields of vagal lung afferents. Knowing that O3 exposure can impact astrocytic protein expression, we decided to investigate whether it may induce astroglial cellular alterations in the NTS. Using electron microscopy and immunoblot techniques, we showed that in O3 -exposed animals, the astrocytic coverage of NTS glutamatergic synapses was 19% increased while the astrocyte volume fraction and membrane density were not modified. Moreover, the expression of glial fibrillary acidic protein and S100ß, which are known to be increased in reactive astroglia, did not change. These results indicate that O3 inhalation induces a glial plasticity that is restricted to the peri-synaptic coverage without overall astroglial activation. Taken together, these findings, along with our previous observations, support the conclusion that O3 -induced pulmonary inflammation results in a specific activation of vagal lung afferents rather than non-specific overall brain alterations mediated by blood-borne agents. Exposure to ozone, a major atmospheric pollutant, induces an increase in the glial coverage of neurons that is restricted to peri-synaptic compartments. This observation does not support the view that the ozone-induced neuronal disorders are related to non-specific overall brain alterations. It rather argues for a specific activation of the vagus nerve in response to pulmonary inflammation.

The ultrastructure of perisynaptic glia in the nucleus tractus solitarii of the adult rat: Comparison between single synapses and multisynaptic arrangements.

Astrocytes are now considered as essential partners of neurons. In particular, they play important roles in glutamatergic transmission, including transmitter inactivation by uptake. Here, we investigated the organization of astroglia in the Nucleus Tractus Solitarii (NTS), a sensory nucleus located in the caudal medulla. Special attention was given to perisynaptic astroglial processes. Investigations were performed at the light and electron microscope levels, using immunodetection of glial glutamate transporters, stereological methods, and serial reconstruction. In the NTS, the main glutamate transporter expressed by astrocytes was GLT1. The volume fraction of astrocyte processes and the density of astrocyte membranes reached 15% and 2.8 µm(2) µm(-3) , respectively. In spite of the relative abundance of astrocyte processes, we found that NTS glutamatergic synapses were not entirely surrounded by glia. Measurements were performed on 43 reconstructed asymmetric junctions which were either single synapses (n = 22) or synapses involved in multisynaptic arrangements (n = 21). Single synapses had 58% of their perimeter contacted by astrocyte processes on average. In multisynaptic arrangement, glial coverage was restricted to the outer part of synaptic diameters and amounted to 50% of this outer part on average. Incomplete glial coverage of NTS synapses may allow glutamate to diffuse out of the synaptic cleft and to activate extrasynaptic receptors as well as receptors from neighboring synapses. Especially, in multisynaptic arrangements, the lack of intervening glia may favor functional coupling between individual contacts.