Input transformation by dendritic spines of pyramidal neurons

Abstract

In the mammalian brain, most inputs received by a neuron are formed on the dendritic tree. In the neocortex, the dendrites of pyramidal neurons are covered by thousands of tiny protrusions known as dendritic spines, which are the major recipient sites for excitatory synaptic information in the brain. Their peculiar morphology, with a small head connected to the dendritic shaft by a slender neck, has inspired decades of theoretical and more recently experimental work in an attempt to understand how excitatory synaptic inputs are processed, stored and integrated in pyramidal neurons. Advances in electrophysiological, optical and genetic tools are now enabling us to unravel the biophysical and molecular mechanisms controlling spine function in health and disease. Here I highlight relevant findings, challenges and hypotheses on spine function, with an emphasis on the electrical properties of spines and on how these affect the storage and integration of excitatory synaptic inputs in pyramidal neurons. In an attempt to make sense of the published data, I propose that the raison d'etre for dendritic spines lies in their ability to undergo activity-dependent structural and molecular changes that can modify synaptic strength, and hence alter the gain of the linearly integrated sub-threshold depolarizations in pyramidal neuron dendrites before the generation of a dendritic spike.

Keywords: biophysical processes; dendritic computation; input-output transformation; plasticity; spine neck; synaptic integration; synaptic transmission; two-photon uncaging.

Figure 1

Dendritic spines are tiny protrusions that cover the dendrites of pyramidal neurons and are the sites at which excitatory connections are made. (A) Confocal scanning image of a representative dendrite, covered with dendritic spines, of a layer 5 pyramidal neuron from a thy1-YFP-H transgenic mouse expressing the yellow fluorescent protein. (B) Synaptic contacts occur at spines. Reconstruction of electron micrographs taken from serial sections of dendritic segments from neocortical pyramidal neurons. Note the distribution of postsynaptic contacts (PSD, red; excitatory asymmetric contacts). Only a few percent of dendritic protrusions are devoid of synaptic contacts (blue). Note that the shaft lacks excitatory synaptic contacts. Scale bar = 2 μm (Modified with permission from Arellano et al., 2007b). (C) Simplified circuit diagram of a passive dendritic spine. C_m(h), capacitance of the spine head membrane; C_m(N), capacitance of the spine neck membrane; C_m(d), dendritic membrane capacitance; R_m(h), membrane resistance of the spine head; R_m(N), membrane resistance of the spine neck; R_m(d), membrane resistance of the dendrite; E_h, reversal potential at the spine head; E_syn, synaptic reversal potential; E_d, reversal potential at the dendrite; R_N, neck resistance; R_d dendritic resistance; G_rest, spine's conductance at rest; G_syn, spine's synaptic conductance; G_N, spine neck's conductance.

Figure 2

Schematic representing excitatory synaptic transmission and the sources of Ca²⁺ accumulations at the spine head in pyramidal neurons. Left, drawing showing how presynaptically released glutamate activates glutamate (AMPA, NMDA and mGluR) receptors leading to spine head depolarization. Right, spine depolarization will generate Ca²⁺ transients at the spine by removing the magnesium block from NMDA receptors, triggering the activation of voltage-sensitive Ca²⁺ channels (VSCCs) and the generation of second messengers like IP₃ (for details see text).

Figure 3

Effect of the spine neck on spine uncaging potentials. (A) Examples of two-photon glutamate uncaging potentials in spines with different neck lengths. Red dots indicate the site of uncaging, and traces correspond to averages of 10 uncaging potentials from each spine. (B) Three neighboring spines with different neck lengths. Note the different uncaging potentials generated at the soma of the neuron. (C) Plot of the uncaging potential peak amplitude versus neck length. Line is the linear regression of the data with a weighted fit. Standard errors are provided for each point (Figure taken with permission from Araya et al., 2006b).

Figure 4

Measurements of the ratio of spine-to-dendrite voltage amplitude, to estimate R_N using the voltage divider equation. Harnett et al. (2012) estimated R_N by combining two-photon Ca²⁺ imaging and glutamate uncaging with dual dendritic patch-clamp current injection and voltage recording from hippocampal CA1 pyramidal neurons in acute slices. (A) Uncaging potential [light blue trace in (A)] was produced by uncaging onto a single spine and recording in the dendrite (V rec. V_out) while measuring spine head Ca²⁺ responses (Sp-Ca²⁺ response, light green trace) mediated exclusively by voltage-sensitive Ca²⁺ channels (VSCC) (see Harnett et al., for details). Next, current injection (I_inj.) into the dendrite was performed to depolarize the spine to a level that triggers Ca²⁺ responses in the spine head (dark green traces) similar to the ones produced by glutamate uncaging (V_in in voltage divider equation. Also see comparison of V_in and V_out spine head Ca²⁺ responses). Assuming a lack of voltage attenuation from the dendrite to the spine, the uncaging potential and the voltage generated by I_inj provide a good estimate of the spine head potentials (V_in). Two-photon image of a dendrite patched with two patch electrodes, and the voltage and calcium traces were taken from Harnett et al. (2012). (B) The amount of electrical compartmentalization produced by the spine can be measured as the amplitude ratio (AR) of the voltage at the spine head when an input impinges on the spine (EPSP_spine), to the voltage at the dendrite when the synapse impinges on the spine (EPSP_dend(sp)). (C) Calculation of R_N was obtained by the equation depicted in (B). Modified with permission from Harnett et al. (2012). See Harnett et al. (2012) for details.

Figure 5

Summation of excitatory uncaging potentials on spines and dendritic shafts. (A1) Left, drawing of a dendrite from a layer V pyramidal cell showing the protocol for testing summation in spines and shafts. Red dots indicate the sites of uncaging in spines or shafts. Middle, two-photon images showing the uncaging locations in spines or shafts (red dots). Right, voltage responses were recorded with a patch electrode in current-clamp configuration. Two-photon uncaging of glutamate was performed first at each spine or shaft location (1 or 2) and then in either both spines together or in both shaft locations (1 + 2). Summation in spines: Red trace corresponds to an average of 10 depolarizations caused by uncaging over the two spines, and black traces correspond to the expected algebraic (linear) sum of the individual events of each spine. Summation in shafts: Data are presented as for spines. Note how the average uncaging response when spines are activated is close to the expected value. However, when inputs impinge on shaft locations, the integration is sublinear (Image modified from Araya et al., 2006a). (A2) Summary of results from Araya et al. (2006a). Data are presented as averages ± s.e.m. (B) Data taken with permission from Losonczy and Magee (2006). (B1) Two-photon image stack from a CA1 pyramidal neuron. Inset, red circles indicate the site of uncaging in spines—up to 20 spines in this example. (B2) Two-photon uncaging potentials evoked at a 0.1 ms interval, ranging from 2 to 20 activated spines. (B3) Input/output plot for the experiment. Note how inputs onto spines integrate linearly before additional inputs generate a dendritic spike.

Figure 6

(A) Spatially clustered inputs: Excitatory inputs directed to clustered spines add linearly (A1, compare the expected algebraic (linear) sum (Exp., black trace) of the individual events of each spine with the observed response after simultaneous activation of all spines (Obs., gray trace)) before the generation of a dendritic spike (A5, dotted line indicates the threshold for triggering a dendritic spike). In contrast, excitatory inputs directed to clustered shaft locations will shunt each other (A3 for 2 inputs, and A4 for 3 inputs). Note that more shunting is expected if more clustered inputs are directed to the shaft (compare Exp. (black trace) vs. Obs. (yellow trace) in A3,A4). A5, Plot of the observed vs. expected amplitude (mV) for uncaging events in spines (black) or shafts (yellow) along the dendrite of layer 5 pyramidal neurons. Plasticity (for simplicity I only focus here on neck plasticity as reported in Araya et al., 2014): Spine-STDP will generate a significant change in the neck length (and probably a conductance change) of the stimulated spine (A2, red spine) with a concomitant change in synaptic weight (red trace, gray box shows the amplitude change from control). Single spine-STDP will increase the input/output gain (A5, red trace and arrow indicating the change in gain from the control (black) trace) of the neuron without affecting the linear integration of subthreshold excitatory inputs. (B) Spatially distributed inputs: Distributed excitatory inputs directed to spine (red dots) or shaft locations (yellow dots) will integrate linearly (B1, compare Exp. Vs. Obs.) by preventing large variations in the input impedance of the dendrite, thus avoiding shunting interactions that would otherwise be expected if clustered inputs are directed to the dendritic shaft (A3,A4). B3 same as A5 but with distributed inputs onto spine and shaft locations. (C) Representation of the summation of excitatory inputs directed to spines of a pyramidal neuron. The simultaneous synaptic activation of a few distributed (C1) or clustered (C2) spines (red) would trigger a voltage response that matches the arithmetic linear sum of each spine's voltage contribution. If tens of spines are activated simultaneously within the same branch (C3), then a supralinear response, or dendritic spike, will be generated.