In vivo measurement of cell-type-specific synaptic connectivity and synaptic transmission in layer 2/3 mouse barrel cortex

Abstract

Intracellular recordings of membrane potential in vitro have defined fundamental properties of synaptic communication. Much less is known about the properties of synaptic connectivity and synaptic transmission in vivo. Here, we combined single-cell optogenetics with whole-cell recordings to investigate glutamatergic synaptic transmission in vivo from single identified excitatory neurons onto two genetically defined subtypes of inhibitory GABAergic neurons in layer 2/3 mouse barrel cortex. We found that parvalbumin-expressing (PV) GABAergic neurons received unitary glutamatergic synaptic input with higher probability than somatostatin-expressing (Sst) GABAergic neurons. Unitary excitatory postsynaptic potentials onto PV neurons were also faster and more reliable than inputs onto Sst neurons. Excitatory synapses targeting Sst neurons displayed strong short-term facilitation, while those targeting PV neurons showed little short-term dynamics. Our results largely agree with in vitro measurements. We therefore demonstrate the technical feasibility of assessing functional cell-type-specific synaptic connectivity in vivo, allowing future investigations into context-dependent modulation of synaptic transmission.

Figure 1

In Vivo Measurement of uEPSPs (A) On day 1, eGFP- and ChR2-encoding plasmid DNAs together with Alexa 488 dye are electroporated into a single excitatory neuron in L2/3 mouse barrel cortex. On day 2, juxtacellular recording of the ChR2-expressing excitatory neuron is carried out to assess optogenetic control of AP firing. Whole-cell (WC) recordings of nearby tdTomato-expressing neurons are then performed sequentially to measure synaptic potentials. Local field potential (LFP) is recorded simultaneously. (B) Example in vivo two-photon images of a single L2/3 excitatory neuron filled with Alexa 488 dye in a Sst-Cre × LSL-tdTomato mouse taken immediately after electroporation (above) and 24 hr later showing eGFP expression in soma and dendrites (below). (C) Juxtacellular recording of the AP firing response to a single 1 ms light pulse delivered at 1 Hz to the ChR2-expressing neuron in (B). LFP recording allowed identification of DOWN (gray) and UP states (white) (left). A single AP was elicited with precise timing by each light pulse during DOWN states (right). (D) Whole-cell recording of a synaptically connected neuron, Sst 1 in (B), with simultaneous LFP recording (left). Example single-trial uEPSPs and synaptic failures recorded during DOWN states (right). (E) Same as (D), but for an unconnected Sst neuron, Sst 2 in (B). See also Movie S1.

Figure 2

Precise Optogenetic Stimulation of Action Potential Firing in Single Excitatory Neurons In Vivo (A) Example single AP elicited by a single 1 ms light pulse recorded juxtacellularly in a L2/3 ChR2-expressing excitatory neuron. (B) Population peristimulus time histogram of light-evoked AP timing (left) and light-evoked AP probability, latency, and jitter (right) for single 1 ms light pulses delivered during the DOWN states. (C) Same cell as in (A), but for an optogenetic stimulus made of a 50 Hz train of five 1 ms light pulses. (D) Same analysis as in (B), but for an optogenetic stimulus made of a 50 Hz train of five 1 ms light pulses. (E) Light-evoked AP probability, latency, and jitter quantified at the beginning (black, t = 0 hr) and end (red, t = 4.5 hr) of the recording session for single 1 ms light pulses (above) and 50 Hz trains of five 1 ms light pulses (below). (F) Example APs elicited by a single 1 ms light pulse delivered at 1 Hz recorded juxtacellularly during UP and DOWN states. (G) Population peristimulus time histogram of light-evoked AP timing (left) and light-evoked AP probability, latency, and jitter (right) for 1 ms optogenetic stimuli occurring in DOWN (black) and UP states (gray). Data are represented as mean ± SD. Two-tail Wilcoxon signed-rank test assessed statistical significance. See also Figure S1.

Figure 3

Cell-Type-Specific Features of Excitatory Synaptic Transmission In Vivo (A) Example whole-cell recording of uEPSPs elicited in a PV neuron (red) and a Sst neuron (brown) during DOWN states by 1 ms light pulses. Single trial uEPSPs are shown above and average uEPSP below. The in vivo two-photon images show the whole-cell recording pipette (Alexa 488 dye, green), the recorded tdTomato-expressing neuron (yellow), and part of the presynaptic eGFP- and ChR2-expressing neuron (green). (B) Connectivity rate is higher from excitatory neurons onto PV neurons than onto Sst neurons. (C) Connectivity rate is uncorrelated with intersomatic distance for both Exc→PV (p = 0.56) (left) and Exc→Sst pairs (p = 0.89) (right) over the short range tested. (D) uEPSP grand average of all connected PV and Sst neurons, as well as that of all nonconnected (NC) neurons (gray) (left) and uEPSP amplitude distribution (right). The uEPSP amplitude for each cell was computed as the average across both failure and success trials. (E) uEPSP amplitude is anticorrelated with the failure rate for both Exc→PV and Exc→Sst synapses. (F) uEPSP coefficient of variation (CV) is larger for Sst neurons compared to PV neurons. (G) uEPSP 20%–80% rise time, full-width at half-maximum amplitude, and exponential decay time constant (Tau) are slower for Sst neurons compared to PV neurons. (H) Example whole-cell recording of uEPSPs elicited in a PV neuron during DOWN (below) and UP states (above) by 1 ms light pulses. Single trial uEPSPs are shown on the left and average uEPSPs on the right. (I) uEPSPs elicited in DOWN states on average have an amplitude similar to that of uEPSPs elicited in UP states for both PV and Sst neurons (left). One Sst and five PV neurons show a significant decrease in uEPSP amplitude in UP compared to DOWN states (black lines). Red line represents neuron in (H). (J) Baseline V_m at uEPSP onset is more depolarized in UP compared to DOWN states for both PV and Sst neurons (right). Data are represented as mean ± SD. χ² test assessed for statistical difference in connectivity rates. Linear regression tested distance dependence of connectivity. Two-tail Wilcoxon rank-sum test assessed the difference in uEPSP CV, rise time, half-width, and Tau decay. Two-tail Wilcoxon signed-rank test assessed the differences in uEPSP amplitude and baseline V_m between UP and DOWN states. Spearman’s ρ assessed the correlation between uEPSP amplitude and failure rate. See also Figures S2 and S3 and Tables S1 and S2.

Figure 4

In Vivo Short-Term Synaptic Dynamics (A) Reconstruction of connected pairs of L2/3 Exc→PV and Exc→Sst neurons. Dendrites of the presynaptic excitatory neurons are colored in green, axons in gray. Dendrites of postsynaptic PV and Sst neurons are colored in red and brown, respectively. Example whole-cell recording of uEPSPs elicited in the PV (red) and Sst (brown) neuron during DOWN states by a 50 Hz train of five 1 ms light pulses. Single trial uEPSPs are shown above and average uEPSPs below. (B) Grand average uEPSPs for all connected PV and Sst neurons evoked by 50 Hz train of optogenetic stimuli during DOWN states. (C) Population uEPSP amplitude ratios comparing the amplitude of each uEPSP in the train to the amplitude of the first uEPSP for PV and Sst neurons (left). Individual neuron uEPSP amplitude ratios for uEPSP2 and uEPSP5 (right). Exc→Sst synapses facilitate, whereas Exc→PV synapses show little short-term dynamics. (D) Population difference in baseline V_m of each uEPSP in the train relative to the baseline V_m of the first uEPSP for PV and Sst neurons (left). Differences across individual neurons in baseline V_m at onset of uEPSP2 and uEPSP5 (right). uEPSPs summate prominently in Sst neurons, but not in PV neurons. Data are represented as mean ± SEM. Two-tail Wilcoxon rank-sum test assessed statistical significance. See also Figure S4.