Cortical dendritic spine heads are not electrically isolated by the spine neck from membrane potential signals in parent dendrites

Abstract

The evidence for an important hypothesis that cortical spine morphology might participate in modifying synaptic efficacy that underlies plasticity and possibly learning and memory mechanisms is inconclusive. Both theory and experiments suggest that the transfer of excitatory postsynaptic potential signals from spines to parent dendrites depends on the spine neck morphology and resistance. Furthermore, modeling of signal transfer in the opposite direction predicts that synapses on spine heads are not electrically isolated from voltages in the parent dendrite. In sharp contrast to this theoretical prediction, one of a very few measurements of electrical signals from spines reported that slow hyperpolarizing membrane potential changes are attenuated considerably by the spine neck as they spread from dendrites to synapses on spine heads. This result challenges our understanding of the electrical behavior of spines at a fundamental level. To re-examine the specific question of the transfer of dendritic signals to synapses of spines, we took advantage of a high-sensitivity Vm-imaging technique and carried out optical measurements of electrical signals from 4 groups of spines with different neck length and simultaneously from parent dendrites. The results show that spine neck does not filter membrane potential signals as they spread from the dendrites into the spine heads.

Keywords: cerebral cortex; cortical spines; optical recording; plasticity; voltage-sensitive dyes.

Figure 1.

Spine model. (A) Schematic diagram of a spine attached to a dendrite; morphology and dimensions correspond to an actual spine; V(dend): membrane potential of the dendrite at the base of the spine; V(head): membrane potential of the spine head. (B) The simplest conceptual model (equivalent electrical circuit) of a spine attached to a dendrite; R(neck): axial resistance of the spine neck; R(head): transmembrane resistance of the spine head. (C) Multicompartmental model of a spine attached to a dendrite with all membrane resistances and distributed capacitance preserved. (D) Simulation results obtained with each of the 3 sets of biophysical parameters described in the text for spine neck length of 1 μm, given a 100 mV spike in the dendrite starting from a resting potential of −66 mV. Spine neck and head diameters were 0.2 and 1 μm, respectively. Traces from different simulations are superimposed; note that there is no discernible difference between V(dend) and V(head) in all cases.

Figure 2.

Spatial resolution and spine dimensions. (A) Fluorescence image of a dendritic spine in recording position obtained with low-resolution CCD for voltage imaging. (B) Low-magnification fluorescence image of a dendrite with spines reconstructed from a z-stack of high-resolution confocal images. The white rectangle outlines the region shown in panels A and C. (C) Confocal image of the same spines at high magnification.

Figure 3.

Sensitivity and spatial resolution. (A) Voltage-sensitive dye fluorescence image of a mushroom spine with a long neck in recording position obtained with CCD for voltage imaging. Single-trial recordings of bAP-related signals from spine head (location 1) and from an analogous region without spine (location 2) are shown on the right. (B) Similar sensitivity and spatial resolution of recording bAP signals from a stubby spine head separated from the dendrite by a very short spine neck; temporal average of 4 trials.

Figure 4.

Temporal resolution. Recordings of bAP signals from a dendritic spine (shown in upper micrograph) using frame rates from 1 to 20 kHz. Recordings from spine head (red) and parent dendrite (blue) are shown. Frame rates lower than 5 kHz did not allow accurate reconstruction of the bAP signal. Superimposed signals from the spine head and from the dendrite (right traces) scaled to the same height were identical.

Figure 5.

Steady-state pulse signal amplitude in spine head and parent dendrite. A typical measurement of a voltage-sensitive dye signal (ΔF/F) from a spine head (red) and from the dendrite at the base of the spine (blue) as indicated in the upper fluorescent image of a spiny dendrite in recording position. Optical signals are related to a long lasting hyperpolarizing membrane potential transient generated under voltage-clamp from a somatic patch-electrode (bottom trace).

Figure 6.

Steady-state signal amplitude as a function of spine neck length. Scatterplot of the amplitude of optical signals recorded from the spine head and parent dendrite from 4 groups of spines with different neck length. Optical signals were related to a long-lasting hyperpolarizing membrane potential transient. Mean ± SD shown for individual groups (left) and for all spines (right).

Figure 7.

Backpropagating AP signal amplitude normalized to long-lasting hyperpolarizing transient. (A) Confocal fluorescence image (upper) and low-resolution image (lower) of a spine in recording position. Recording regions: spine head, red; dendrite, blue. (B) Optical signals related to bAPs and hyperpolarizing calibration pulses recorded simultaneously from identical locations: spine head, red; parent dendrites, blue. Left traces: unprocessed signals. Right traces: bAP spine signal normalized to long-lasting hyperpolarizing transients.

Figure 8.

Backpropagating AP signal amplitude as a function of spine neck length. Scatterplot of the amplitude of optical signals related to bAPs recorded from the spine head and parent dendrite from 4 groups of spines with different neck length. Raw values (left) and normalized spine signals (right) are shown (mean ± SD).

Figure 9.

Amplitude and waveform comparison of bAP signals in spines and dendrites. (A) Superimposed scaled bAP signals from spine heads (solid line) and dendrites (dashed line) on expanded time scale; representative measurements from 2 spines. (B) Spine head/dendrite ratios for bAP signal amplitude and time course (FWHH) recorded in 4 groups of spines with different neck length.