Glutamate induces the rapid formation of spine head protrusions in hippocampal slice cultures

Abstract

Synaptic plasticity at neuronal connections has been well characterized functionally by using electrophysiological approaches, but the structural basis for this phenomenon remains controversial. We have studied the dynamic interactions between presynaptic and postsynaptic structures labeled with FM 4-64 and a membranetargeted GFP, respectively, in hippocampal slices. Under conditions of reduced neuronal activity (1 M tetrodotoxin), we observed extension of glutamate receptor-dependent processes from dendritic spines of CA1 pyramidal cells to presynaptic boutons. The formation of these spine head protrusions is blocked by ␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor antagonists and by agents that reduce the release of glutamate from presynaptic terminals. Moreover, spine head protrusions form in response to exogenously applied glutamate, with clear directionality toward the glutamate electrode. Our results suggest that spontaneously released glutamate is sufficient to activate nearby spines, which can then lead to the growth of new postsynaptic processes connecting to a presynaptic site. Spines thus can compare their recent history with that of neighboring synapses and modify local connectivity accordingly.T he majority of excitatory connections in the hippocampus are made at dendritic spines, which are small protrusions (Ϸ1 m long) that extend from dendrites (1, 2). Dendritic spines can undergo significant morphological change over a timescale of seconds (3), and this motility is both actin-dependent and responsive to synaptic activity (4-7). Spine motility plays a major role in synaptogenesis (8-10) but its physiological relevance in mature spines is unknown. It has been suggested that spine motility influences the compartmentalization of electrical and calcium signals, yet in CA1 pyramidal cells the resistance of the spine neck is insufficient for electrical filtering (11), and spine dynamics will have no effect on calcium signaling because calcium diffusion through the neck is negligible (12). Recent work from our laboratory also indicates a relationship between spine motility and the ability of proteins tethered to the inner leaflet of the membrane to diffuse (13). We have further explored possible physiological roles of spine motility by investigating dynamic interactions between presynaptic and postsynaptic sites and the regulation of these dynamics by glutamate. Our results indicate that the motility of mature spines can lead to changes in synaptic connectivity. We hypothesize that when a spine has not been recently activated by glutamate released by its associated bouton, it can respond to glutamate released by a neighboring synapse. Spines thus can compare their individual recent history to the level of activity of neighboring synapses and modify hippocampal microcircuitry accordingly. MethodsTransgenic Mice. Variegated mice were generated by using standard techniques. A construct was generated where the cDNA for EGFP was fused to the membrane-anchoring domain (first 41 aa) of a...

Differential Modulation of Drug-Induced Structural and Functional Plasticity of Dendritic Spines

“…Although dendritic spine plasticity and synaptic AMPA receptor plasticity have been shown to be highly correlated in a number of studies (McKinney et al, 1999;Matsuzaki et al, 2004;Richards et al, 2005;Okamoto et al, 2009), other studies have found that changes in spine morphology and synaptic AMPA receptors can be differentially mediated (Sdrulla and Linden, 2007;Wang et al, 2007). Until the present study, it has not been known whether these two cellular processes can be differentially mediated in drug-induced plasticity of excitatory synapses.…”

Cooperative Astrocyte and Dendritic Spine Dynamics at Hippocampal Excitatory Synapses

“…For example, glutamate stabilizes spines when spontaneously released from presynaptic terminals (McKinney et al, 1999) and causes the formation of small protrusions from the spine head when focally applied (Richards et al, 2005). Glutamate signaling through AMPA receptors is known to stabilize the actin cytoskeleton and spine motility (Fischer et al, 2000).…”

Paralemmin-1, a Modulator of Filopodia Induction Is Required for Spine Maturation

“…Changes in neuronal activity have been proposed to influence protrusion size and dynamics (Dunaevsky et al, 1999;Fischer et al, 2000;Nimchinsky et al, 2002;Richards et al, 2005; Zuo et al, 2005a,b). The rapid translocation of paralemmin-1 to the plasma membrane upon stimulation of neuronal activity prompted us to examine whether paralemmin-1 modulates activity-driven changes in dendritic protrusions.…”

“…Neuronal activity modulates protrusion formation, which in turn fine tunes synaptic strength and plasticity (Dunaevsky et al, 1999;Fischer et al, 2000;Nimchinsky et al, 2002;Richards et al, 2005;Zuo et al, 2005a,b). This process is thought to be mediated by the recruitment of proteins that alter membrane and cytoskeletal dynamics.…”