Silent Synapses in Developing Rat Nucleus Tractus Solitarii Have AMPA Receptors

Abstract

NMDA-only synapses, called silent synapses, are thought to be the initial step in synapse formation in several systems. However, the underlying mechanism and the role in circuit construction are still a matter of dispute. Using combined morphological and electrophysiological approaches, we searched for silent synapses at the level of the nucleus tractus solitarii (NTS), a brainstem structure that is a gateway for many visceral sensory afferent fibers. Silent synapses were detected at birth by using electrophysiological recordings and minimal stimulation protocols. However, anatomical experiments indicated that nearly all, if not all, NTS synapses had AMPA receptors. Based on EPSC fluctuation measurements and differential blockade by low-affinity competitive and noncompetitive glutamate antagonists, we then demonstrated that NTS silent synapses were better explained by glutamate spillover from neighboring fibers and/or slow dynamic of fusion pore opening. Glutamate spillover at immature NTS synapses may favor crosstalk between active synapses during development when glutamate transporters are weakly expressed and contribute to synaptic processing as well as autonomic circuit formation.

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

“…It stains a single band of *400 kDa on Western blots of rat brain tissue extract (manufacturer's technical information). In addition, we have previously reported that this antibody generates a punctate pattern of labeling that associates with various markers of either glutamatergic or gabaergic/glycinergic synapses on rat brainstem sections (Balland et al 2008;Dufour et al 2010).…”

Glutamatergic neurotransmission in the nucleus tractus solitarii: Structural and functional characteristics

“…Conversion of these silent synapses into functional ones occurs by AMPA receptors insertion via an LTP-like mechanism. Electrophysiological recordings performed in the NTS of rat pups indicate a high occurrence of pure NMDA responses at birth (Balland et al, 2008). However, these responses are unlikely to originate from AMPA-lacking synaptic contacts but rather result from glutamate spillover.…”

“…Some information regarding the location of the GluR2 subunit is available from work carried out using confocal microscopy and antigen-retrieval methods that unmask synaptic receptors (see Fritschy et al, 1998). These studies (Lachamp et al, 2003;Balland et al, 2006Balland et al, , 2008 show that GluR2 is abundantly and widely expressed in the NTS. More importantly, they indicate that GluR2-containing AMPA receptor are mainly associated with glutamatergic terminals and localized close to presynaptic active zones, presumably in PSDs.…”

“…Several studies report the presence of an APV-sensible component in synaptic responses elicited by electrical stimulation of the solitary tract (Aylwin et al, 1997;Doyle and Andresen, 2001;Bonham and Chen, 2002;Jin et al, 2003). However, these data should be interpreted with caution since electrical stimulation may evoke NMDA currents that result from the activation of extrasynaptic receptors by glutamate spillover (see Balland et al, 2008). In line with this interpretation, it should be noted that monosynaptic responses elicited by solitary tract stimulation on horizontal brainstem slices often lack an NMDA component, thereby suggesting that at least some NTS glutamatergic synapses are devoid of NMDA receptors (Doyle and Andresen, 2001;Doyle et al, 2002;Jin et al, 2003).…”

Putative roles of neuropeptides in vagal afferent signaling

“…Anatomical studies indicate that most if not all NTS neurons express non-NMDA receptors [54]. Although, gastric distension or duodenal nutrient stimuli activate NTS neurons that express the AMPA receptors, GLUR2/3 [55], pharmacological studies have failed to demonstrate a role for non-NMDA receptors in satiation.…”