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

Abstract

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.

Excitatory amino acid transporters tonically restrain nTS synaptic and neuronal activity to modulate cardiorespiratory function

“…Rats were placed prone in a stereotaxic apparatus (Kopf Instruments, Tujunga, CA) and a partial occipital craniotomy was performed to expose the brainstem as previously described (Mueller and Hasser 2006;Clark et al 2011;Ostrowski et al 2014). Following completion of surgical procedures, anesthesia was gradually converted from isoflurane to inactin (100 and 20 mg/kg iv supplements as required).…”

“…Rats were anesthetized with isoflurane (5%, induction; 2-2.5% maintenance, in 100% O 2 ) as previously described (Mueller and Hasser 2006;Clark et al 2011). Core temperature was monitored and maintained at ϳ38°C (Tele-Thermometer, Simpson Electric, Lac du Flambeau, WI).…”

“…Immunohistochemistry was performed as previously described (Kline et al 2010;Clark et al 2011;King et al 2012). Briefly, isoflurane anesthetized rats were transcardially perfused with heparinized, oxygenated Dulbecco's modified Eagle's medium (125 ml, pH 7.4; Sigma, St. Louis, MO) followed by 4% paraformaldehyde (500 ml, pH 7.4; Sigma).…”

“…The glial EAAT 1 and 2 have been identified in the nTS (Chounlamountry and Kessler 2011). Reducing EAAT glutamate uptake potentiates the slow excitation observed following repeated afferent stimulation of nTS neurons (Huda et al 2013;Zhao et al 2015).…”

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

“…In a recent ultrastructural study (Chounlamountry and Kessler 2011), we investigated the relationships between astrocytes and synapses in a sensory nucleus located in the dorsal medulla, the nucleus of the solitary tract (NST) which receives visceral primary afferent fibers. A large proportion of NST synapses including those made by visceral primary afferents use glutamate as their neurotransmitter (for review see Baude et al 2009).…”