Dendritic spine geometry is critical for AMPA receptor expression in hippocampal CA1 pyramidal neurons

Abstract

Dendritic spines serve as preferential sites of excitatory synaptic connections and are pleomorphic. To address the structure-function relationship of the dendritic spines, we used two-photon uncaging of glutamate to allow mapping of functional glutamate receptors at the level of the single synapse. Our analyses of the spines of CA1 pyramidal neurons reveal that AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptors are abundant (up to 150/ spine) in mushroom spines but sparsely distributed in thin spines and filopodia. The latter may be serving as the structural substrates of the silent synapses that have been proposed to play roles in development and plasticity of synaptic transmission. Our data indicate that distribution of functional AMPA receptors is tightly correlated with spine geometry and that receptor activity is independently regulated at the level of single spines.Most excitatory synaptic transmission in the mammalian central nervous system relies on glutamate as a neurotransmitter, and postsynaptic glutamate receptors are central in both the acquisition and maintenance of memory 1, 2 . Substantial biochemical evidence indicates that glutamate receptors in postsynaptic densities (PSDs) are regulated by various protein machineries that link the receptors to the cytoskeleton 3,4,5 and that control the insertion [6][7][8][9][10] and phosphorylation of the receptors 11,12 . Individual dendritic spines have been thought to act as functional compartments of glutamate receptor expression, given that they are physically and thus metabolically separated from the body of the dendrite by the narrow spine neck [13][14][15][16] . Indeed, the Hebbian principle of learning as well as most theories of neuronal networks assume that the strength of synaptic connections is subject to independent control 17 . Moreover, spine geometry has been proposed to be a key determinant of synaptic function and memory in the brain 13, 14, 18-22 . Correspondence to: Haruo Kasai. These various hypotheses have not been tested experimentally, however, because it is not possible to systematically investigate postsynaptic glutamate sensitivities at the level of the individual spine by classical electrophysiological approaches 16,23 . Also, the number of functional AMPA-sensitive glutamate receptors in individual spines has not been estimated directly. Two-photon excitation of caged-glutamate compounds may overcome these difficulties 24 as a result of the inherent three-dimensional resolution of neurotransmitter application associated with this technique. However, caged-glutamate compounds with a cross-section for two-photon absorption, a rate of photolysis, and a stability in aqueous solution sufficient for such studies have not previously been described [25][26][27] . NIH Public AccessWe developed a caged-glutamate compound and microscopic system for two-photon excitation that allowed systematic investigation of functional glutamate receptors at the level of the individual synapse. Our experiment...

BDNF enhances dendritic Ca2+ signals evoked by coincident EPSPs and back-propagating action potentials in CA1 pyramidal neurons

“…These larger spines with wider necks are thought to represent spines that acquired AMPA receptors immediately after the induction of long-term potentiation (Matsuzaki et al, 2001;Kasai et al, 2003), and thus enhance spine-dendrite coupling leading to widespread dendritic Ca 2+ signaling during excitatory synaptic transmission (Noguchi et al, 2005). Together with the morphological effects of BDNF on presynaptic terminals (Tyler et al, 2002b) and postsynaptic spine growth and form Pozzo-Miller, 2001, 2003;Alonso et al, 2004), we propose that the larger Ca 2+ signals in spiny dendrites during coincident pre-and postsynaptic activation represent a physiological consequence of the structural BDNF actions at hippocampal synapses.…”

Automated analysis of spine dynamics on live CA1 pyramidal cells

“…Twophoton microscopy (2PM), which uses infrared light to locally excite fluorescence, is especially suited to image fine neuronal structures deeply embedded in intact tissue. To study the dynamics of excitatory synaptic connections between neurons, dendritic spines are often used as a proxy: the size of a dendritic spine is correlated with the strength of the synapse impinging on it (Matsuzaki et al, 2001), and the density of spines on the dendrite is altered in many mental disorders . The tiny volume of dendritic spines is below the resolution limit of light microscopy and therefore not easy to measure or to track over time.…”

Novel approach to real-time flash photolysis and confocal [Ca2+] imaging

“…Our method offers both advantages and disadvantages compared with these approaches. As mentioned, 2P photolysis has been used by Niggli's group in studies with caged Ca 2+ in heart cells (Lipp and Niggli, 1998;DelPrincipe et al, 1999b;Lindegger and Niggli, 2005) and by several other groups in different types of tissue (Denk, 1994;Wang et al, 2000;Matsuzaki et al, 2001;Soeller et al, 2003). The main advantage of 2P photolysis is the potentially superb spatial resolution it allows-photolysis is confined to an ellipsoidal volume extending ∼700 nm along the optical axis of the microscope and ∼300 nm in the plane of focus.…”