Surface-accessible GABA supports tonic and quantal synaptic transmission

Abstract

Exocytosis is commonly viewed as the only secretory process able to account for quantal forms of fast synaptic transmission. However, the demonstrated variability and composite properties of miniature postsynaptic signals are not easily explained by all-or-none exocytotic discharge of transmitter in solution from inside vesicles. Recent studies of endocrine secretion have shown that hormone release does not coincide with exocytosis due to its trapping in the core matrix of the granule. Thus, we tested whether the synaptic transmitter GABA could also be held in a matrix before being released. Using confocal microscopy and flow cytometry of embryonic rat hippocampal neurons, we found a GABA immunoreaction at the surface of live cell bodies and growth cones that coincided spatially and quantitatively with the binding of tetanus toxin fragment C (TTFC). TTFC binds predominantly at membrane sites containing the trisialoglycosphingolipid GT1b. Using flow cytometry, GT1b-containing liposomes preincubated in 100 nM GABA exhibited the same relationship between GABA and TTFC surface binding as found on neurons and growth cones. Embryonic neurons differentiated in culture expressed initially a tonic, and after 3-5 days, transient, postsynaptic signals mediated by GABA acting at GABA(A) receptor/Cl(-) channels. A stream of saline applied to the neuronal surface rapidly and reversibly suppressed both tonic and transient signals. A brief application of the GABAmimetic isoguvacine immediately transformed both tonic and transient GABAergic signals into tonic and transient isoguvacinergic signals. These results and those in the literature are consistent with an immediately releasable compartment of transmitter accessible from the presynaptic surface.

GABA Expression Dominates Neuronal Lineage Progression in the Embryonic Rat Neocortex and Facilitates Neurite Outgrowth via GABAAAutoreceptor/Cl−Channels

“…4 B), suggesting that these elements represented sheared-off processes or growth cones from these cells. These data are supported by previous observations, which showed that TnTx labels the surface of embryonic cortical neurons quite uniformly (Maric et al, 2000b) and that TnTx ϩ GABA ϩ growth cones are readily detected in short-term cultures of embryonic neurons (Vautrin et al, 2000). To test GABA release, TnTx/ A2B5-labeled neocortical dissociates were treated for 15 min with increasing [K ϩ ] o to depolarize cells or ionomycin to elevate intracellular Ca 2ϩ .…”

“…The similarities in the kinetics of baseline and GABA-activated channels strongly suggest that (1) GABA rather than other substances mediates the baseline C l Ϫ channel activity and (2) submicromolar to micromolar concentrations in solution mimic the intensity of the autocrine GABAergic signal. Superf usion eliminated the baseline current, demonstrating that it was mediated by GABA equilibrating in a surface-accessible compartment, which is consistent with results in embryonic hippocampal neurons (Valeyev et al, 1993(Valeyev et al, , 1998Vautrin et al, 2000). Channel activity during superf usion most likely reflected autocrine signaling at the neuronal surface apposing the coverslip, which would be less accessible to superf usion and similar to the interstitial space.…”

Enrichment of presynaptic and postsynaptic markers by size‐based gating analysis of synaptosome preparations from rat and human cortex

“…Recent work suggests that flow cytometry analysis of synaptosomes may prove a valuable tool for studies of presynaptic vesicle traffic and release (24,25). In addition, because lysis or damage to synaptosomes results in a loss of particles within the large gate (7), size gating can also be used as a direct toxicity indicator in synaptosome studies.…”

Dynamic Equilibrium of Neurotransmitter Transporters: Not Just for Reuptake Anymore

“…During the last decade, it has been recognized that there is also a tonic form of inhibition like that seen after vigabatrin treatment, but occurring spontaneously (i.e., without drug treatment). This tonic inhibition is due to an ambient [GABA] that is sufficiently high to continuously activate GABA A receptors on some neurons (Bai et al 2001;Brickley et al 1996Brickley et al , 2001Liu et al 2000;Nusser and Mody 2002;Otis et al 1991;Rossi and Hamann 1998;Stell and Mody 2002;Vautrin et al 2000;Wall and Usowicz 1997;Yeung et al 2003). The magnitude of this tonic inhibition can be surprisingly high with the total charge transfer greatly exceeding that due to phasic GABA release (Brickley et al 1996;Nusser and Mody 2002;Wall and Usowicz 1997).…”