Imaging inhibitory synaptic potentials using voltage sensitive dyes

Abstract

Studies of the spatio-temporal distribution of inhibitory postsynaptic potentials (IPSPs) in a neuron have been limited by the spatial information that can be obtained by electrode recordings. We describe a method that overcomes these limitations by imaging IPSPs with voltage-sensitive dyes. CA1 hippocampal pyramidal neurons from brain slices were loaded with the voltage-sensitive dye JPW-1114 from a somatic patch electrode in whole-cell configuration. After removal of the patch electrode, we found that neurons recover their physiological intracellular chloride concentration. Using an improved voltage-imaging technique, dendritic GABAergic IPSPs as small as 1 mV could be resolved optically from multiple sites with spatial averaging. We analyzed the sensitivity of the technique, in relation to its spatial resolution. We monitored the origin and the spread of IPSPs originating in different areas of the apical dendrite and reconstructed their spatial distribution. We achieved a clear discrimination of IPSPs from the dendrites and from the axon. This study indicates that voltage imaging is a uniquely suited approach for the investigation of several fundamental aspects of inhibitory synaptic transmission that require spatial information.

Figure 1

Optical measurements of GABA-mediated synaptic potentials (A) (Top) Fluorescence image of a CA1 hippocampal pyramidal neuron (left). Dendritic region in recording position (right). (Upper trace) Electrical somatic recording (black) of a depolarizing evoked synaptic potential during dye loading with 40 mM Cl⁻ internal solution (first patch). (Middle trace) Hyperpolarizing synaptic potential evoked by the same stimulus and recorded optically as spatial average from all dendrites after electrode removal (no-patch). (Bottom trace) Superimposed electrical somatic (black) and optical dendritic (gray) recordings of the synaptic potential after repatch with an electrode containing 40 mM Cl⁻ (second patch). Synaptic potentials are averages of nine trials. (B) (Left/top) Electrical (black) and optical (gray) dendritic synaptic potential recordings during loading (40 mM Cl⁻) before and after addition of 20 μM bicuculline. (Left/bottom) optical dendritic recordings 30 min after patch termination and bicuculline washout before (upper trace) and after (lower trace) reapplication of bicuculline. (Right/top) Synaptic potential peak amplitude (mean ± SD; n = 5 cells) for electrical (white) and optical (gray) signals during loading before and after addition of 20 μM bicuculline. (Right/bottom) Optical synaptic potential signal peak amplitude (mean ± SD; n = 5 cells) after electrode removal before and after addition of bicuculline. Effects statistically significant (p < 0.01, paired t-test).

Figure 2

Optically recorded Cl⁻-mediated synaptic potentials are independent of the Cl⁻ concentration in the patch pipette used for dye loading. (A) (Left) Fluorescence image of a CA1 hippocampal pyramidal neuron: intracellular solution for dye loading contained 40 mM Cl⁻. (Right) Dendritic region in recording position (∼125 μm × 125 μm, apical dendrite). (B) (Left) Cl⁻-mediated synaptic potential in response to low-intensity (s1) stimulation recorded optically (average of nine trials) after patch termination and after cell recovery obtained as the spatial average from all dendritic branches. (Right) Action potential signal evoked by high-intensity (s2) stimulation and recorded from the same area as the IPSP signal. (C) (Left) Synaptic currents under voltage-clamp obtained at different holding potentials during second recording with 40 mM Cl⁻ internal solution. (Right) Optical recording of membrane potential signals under current-clamp conditions after s2 and s1 stimulations (second patch-electrode recording). (Upper traces) Action potential signals evoked at different baseline potentials. Gray trace is the action potential signal recorded before the patch-pipette was attached; estimated V_rest in B = −72 mV. (Lower trace) Response to s1 stimulations evoked at a baseline membrane potential of −84 mV. (D–F) Same as A–C for a cell dialyzed from an electrode with 5 mM Cl⁻ intracellular solution; estimated V_rest in E = −61 mV. (G) Estimates of [Cl⁻]_i plotted for neurons dialyzed from patch electrode containing 40 mM Cl⁻ (n = 7) and 5 mM Cl⁻ (n = 6). The values obtained after recovery period after patch-electrode removal are included. (H) Summary result (mean ± SD) for the data shown in G; The [Cl⁻]_i after recovery period for two groups did not differ significantly (p = 0.38, two-populations t-test).

Figure 3

Spatial resolution and sensitivity. (A) Fluorescence image of a neuron (left) and dendritic region in recording position (middle); regions 1–5 are ∼1.56 μm × 3.12 μm (2 pixels). (Right) Electrical somatic recording (black trace) and optical dendritic recordings (gray) of an IPSP ∼8 mV in amplitude from regions 1–5. Signals are averages of nine trials. (B) (Left) Image of a neuron (top) and apical dendrite in recording position (bottom); ∼75 μm long region of interest outlined. Right: electrical somatic recordings (top) and optical recording from the region of interest (bottom) of IPSPs ([Cl⁻]_i∼5 mM) in response to three stimulus intensities (4 μA, 5 μA, and 10 μA). Signals are averages of 16 trials. Four μA stimulation fails to evoke an IPSP. Stimulations of 5 μA and 10 μA evoke IPSPs of ∼800 μV and ∼2 mV in amplitude respectively. (C) Optical recording of the ∼800 μV IPSP in B from the ∼6.5 μm long regions of interest 1–4. (D) (Left) Image of a neuron (top) and an apical dendrite in recording position (bottom); one region of interest indicated. (Right) Optical recordings from the region of interest; (top traces) four single trials recorded at 500 Hz at 6% of the laser power; (middle trace) a single trial recorded at 2 kHz at 25% of the laser power; (bottom traces) the average of the four trials at 500 Hz and the trial at 2 kHz resampled at 500 Hz.

Figure 4

Spatial distribution of IPSPs along the apical dendrite. (A) (Top) Image of a neuron with an area of ∼300 μm × 300 μm projected onto the CCD camera outlined; position of a distal (dist) and a proximal (prox) stimulating electrode shown schematically. (Bottom) Optical recordings of IPSPs evoked by dist (black) or prox (red) stimulation. Signals are averages of nine trials from locations 1–3. Frame sequence indicated below. (B) Sequence of frames between the two arrows in A showing a color-coded display of the spatio-temporal pattern of IPSP initiation and spread after distal and proximal stimulation. See Movie S1. (C) (Left) Images of three additional neurons analyzed. (Right) Spatial distribution of the IPSPs in the dendritic tree shown by color-coded representation of signal peak amplitude. The IPSP peak corresponds to red. (D) (Top left) Schematic drawing of a CA1 hippocampal pyramidal neuron with dist and prox stimulating electrodes. The location of two separate areas (SLM and SR) indicated by blue and red rectangles, respectively. (Top right) Schematic drawing of interneurons excited by dist and prox electrodes based on anatomical information. (Bottom) Mean ± SE of the ratio of peak signals associated with dist and prox stimulation from six cells. Values from SLM and SR regions are significantly different (p < 0.001, two-sample t-test).

Figure 5

IPSPs recorded simultaneously in the axon and in the basal dendrite. (A) Image of a part of a CA1 pyramidal neuron in the stratum oriens with drawing of basal dendrites (gray) and axon (black). Part of a neuron projected onto a subset of pixels of the CCD camera for voltage-imaging (bottom image). Three recording locations are labeled 1–3. (B) (Left traces) Optical signals associated with an evoked action potential (top) and an IPSP (bottom) from locations 1–3 shown in C; location 1: dendritic signal; location 2: mixed signal; location 3: axonal signal. (Right traces) Optical signals from location 1 (dendrite, gray trace) and location 3 (axon, black trace) normalized in amplitude and superimposed. Action potential signal is present in the axon and in the dendrite. IPSP signal is absent in the axon (see also Movie S2). (C and E and D and F) Same sequence of measurements as in A and B from two additional neurons; IPSP observed both in the dendrite and in the axon. Action potential and IPSP signals were averages of 4 and 16 trials, respectively.