Cell-Type Specificity of Callosally Evoked Excitation and Feedforward Inhibition in the Prefrontal Cortex

Abstract

Excitation and inhibition are highly specific in the cortex, with distinct synaptic connections made onto subtypes of projection neurons. The functional consequences of this selective connectivity depend on both synaptic strength and the intrinsic properties of targeted neurons but remain poorly understood. Here, we examine responses to callosal inputs at cortico-cortical (CC) and cortico-thalamic (CT) neurons in layer 5 of mouse prelimbic prefrontal cortex (PFC). We find callosally evoked excitation and feedforward inhibition are much stronger at CT neurons compared to neighboring CC neurons. Elevated inhibition at CT neurons reflects biased synaptic inputs from parvalbumin and somatostatin positive interneurons. The intrinsic properties of postsynaptic targets equalize excitatory and inhibitory response amplitudes but selectively accelerate decays at CT neurons. Feedforward inhibition further reduces response amplitude and balances action potential firing across these projection neurons. Our findings highlight the synaptic and cellular mechanisms regulating callosal recruitment of layer 5 microcircuits in PFC.

Keywords: circuits; excitation; feedforward; h-current; inhibition; interneuron; optogenetics; prefrontal cortex; pyramidal neuron; synaptic transmission.

Figure 1. Callosal Inputs Evoke Stronger Excitation and Inhibition at L5 CT Neurons

(A) Left, injections of AAV-ChR2 into the contralateral PFC (cPFC) to label callosal inputs, and CTB into the cPFC or ipsilateral mediodorsal thalamus (MD) to label CC and CT neurons. Right, injection sites, with dashed regions indicating prelimbic (PL), infralimbic (IL), and MD (scale bar, 500 μm). (Representative images, n = 3 mice.) (B) Confocal images showing the distributions of cPFC axons (left), CC neurons (middle), and CT neurons (right) across multiple layers of PL PFC (scale bar, 100 μm). Dashed white box indicates the band in layer 5 (L5) from which electrophysiological recordings were made. Far right, confocal images of CC neurons (white) CT neurons (red) and intermingled cPFC axons (green) (scale bar, 25 μm). (C) Morphological reconstructions made from 2-photon images of CC neurons (black) and CT neurons (red) (scale bar, 50 μm). (D) Average monosynaptic cPFC-evoked EPSCs at pairs of CC (black) and CT (red) neurons recorded in the presence of TTX and 4-AP (n = 7 pairs, 4 mice). Arrow indicates light pulse (4-ms duration). (E) Summary of EPSC amplitudes at pairs of CC and CT neurons (left) and CT/CC amplitude ratio with y axis on log2 scale (right). (F) Average cPFC-evoked EPSCs recorded at −70 mV and IPSCs recorded at +10 mV in CC (black) and CT (red) neurons, recorded across a range of light durations (1–4 ms) in the absence of TTX and 4-AP (n = 13 pairs, 8 mice). Traces are averages taken across all cells at each LED pulse duration. (G) Summary of amplitudes of EPSCs (left) and IPSCs (right) recorded at pairs of CC and CT neurons in response to 4 ms light stimulation. See also Figure S1. Values are mean ± SEM (E, left and G) or geometric mean ± CI (E, right), *p < 0.05.

Figure 2. Both PV⁺ and SOM⁺ Interneurons Are Engaged by Callosal Inputs

(A) Distributions of PV⁺ (green) and SOM⁺ (purple) interneurons across multiple layers of prelimbic PFC, in virally injected PV-Cre and SOM-Cre mice (scale bars, 100 μm). (Representative images, n = 3 mice each for PV- and SOM-Cre.) (B) Left, schematic of conditional rabies virus (RV) tracing, with helper AAVs injected on day 0 and RV on day 14 in the ipsilateral PFC (iPFC), followed by imaging on day 22 in the iPFC and cPFC. Right, representative anatomy for PV-Cre (top) and SOM-Cre (bottom) mice (n = 3 mice each for PV- and SOM-Cre), showing TVA⁺ interneurons in the iPFC (red), starter interneurons in the iPFC (yellow), and connected presynaptic neurons in the iPFC and cPFC (green) (scale bar, 100 μm). (C) Average cPFC-evoked EPSCs at pairs of CC neurons (black) and neighboring PV⁺ interneurons (green) (n = 9 pairs, 4 mice) or SOM⁺ interneurons (purple) (n = 9 pairs, 4 mice). Arrow indicates light pulse (4-ms duration). (D) Summary of interneuron (INT)/CC amplitude ratio from recordings in (C), with y axis on log10 scale. (E) cPFC-evoked EPSPs and action potentials (APs) at PV⁺ (green) and SOM⁺ (purple) interneurons, recorded across a range of light durations (n = 9 pairs, 5 mice for PV⁺, n = 7 pairs, 4 mice for SOM⁺). APs have been truncated to highlight subthreshold responses. Arrows indicate light pulse. Insets show average cPFC-evoked EPSCs at adjacent CC neurons (black). Responses shown at a range of light durations (1–4 ms), evoking cPFC input of increasing magnitude. Darker lines represent longer pulse durations. (F) Summary of AP probability at PV⁺ and SOM⁺ interneurons as a function of EPSC amplitude at adjacent CC neurons. Data are shown across increasing light durations (1–4 ms), with each cell contributing 4 data points. See also Figure S2. Values are geometric mean ± CI (D) or mean ± SEM (F), *p < 0.05.

Figure 3. PV⁺ and SOM⁺ Interneurons Preferentially Target CT Neurons

(A) Injections used to conditionally express ChR2 in interneurons of PV-Cre or SOM-Cre mice, while also retrogradely labeling CC and CT neurons. (B) Average IPSCs evoked by either PV⁺ interneurons (left) or SOM⁺ interneurons (right) at pairs of CC neurons (black) and CT (red) neurons, recorded in the presence of TTX and 4-AP (n = 7 pairs, 7 mice for PV⁺, n = 13 pairs, 8 mice for SOM⁺). Arrows indicate light pulse (4-ms duration). (C) Summary of amplitudes of IPSCs evoked by PV⁺ (left) and SOM⁺ (middle) interneurons at pairs of CC and CT neurons. Right, summary of CT/CC amplitude ratios, with y axis on log2 scale. See also Figure S3. Values are mean ± SEM (C, left, middle) or geometric mean ± CI (C, right), *p < 0.05.

Figure 4. Callosally Evoked EPSPs Are Similar in Amplitude but Faster at CT Neurons

(A) Intrinsic physiological responses of CC (black) and CT (red) neurons held at −75 mV to depolarizing and hyperpolarizing current steps. Light red trace for CT neurons shows response to larger current injections, yielding a similar number of APs. (B) Average response to −50 pA current step at CC and CT neurons, where solid line is average, shaded region is SEM, and dotted line is peak-scaled CT response (n = 10 CC neurons, n = 11 CT neurons) (n = 5 mice total). (C) Summary of input resistance (R_in) (left) and voltage sag during hyperpolarization (sag ratio) (middle) at −75 mV and resting membrane potential (V_rest) (right) for neurons recorded in (B). (D) Average cPFC-evoked EPSPs at pairs of CC neurons held at −75 mV (black) and CT neurons held at both −75 (red) and −65 mV (blue) (n = 8 pairs, 6 mice). Arrow indicates light pulse (4-ms duration). (E) Summary of EPSP amplitudes (left) and decays (right) at pairs of CC and CT neurons, where CT neurons were recorded at both −75 (red) and −65 mV (blue). See also Figure S4. Values are mean ± SEM, *p < 0.05.

Figure 5. Isolated EPSPs and IPSPs Are Also Much Faster at CT Neurons

(A) Average cPFC-evoked EPSPs at pairs of CC (black) and CT (red) neurons held at −75 mV, with inhibition blocked by gabazine (GZ) (n = 7 pairs, 6 mice). Arrow indicates light pulse. (B) Left, average PV⁺-evoked IPSPs at pairs of CC and CT neurons held at −55 mV, with excitation blocked by NBQX and CPP (n = 7 pairs, 7 mice). Right, similar for SOM⁺-evoked IPSPs held at −55 mV (n = 8 pairs, 7 mice). (C) Summary of CT/CC amplitude (left) and decay (right) ratios for EPSPs and IPSPs, with y axis on log2 scale. (D) Summary of the input resistance (R_in) of CC and CT neurons (from Figure 4C) held at multiple membrane potentials. (E) Left, summary of EPSP decay versus input resistance from data recorded in (A), pooling data from recordings at −65, −75, and −85 mV (see also Figures S5A and S5B). Right, summary of IPSP decay versus input resistance from data recorded in (B), pooling data at −55, −65, and −85 mV (empty circles = PV⁺ IPSP; closed circles = SOM⁺ IPSP) (see also Figures S5C–S5F). Solid lines indicate linear fits to each dataset. See also Figure S5. Values are geometric mean ± CI (C) or mean ± SEM (D), *p < 0.05.

Figure 6. H-Current Influences Subthreshold Synaptic Responses at CT Neurons

(A) Intrinsic physiological responses of CC (black) and CT (red) neurons, held at −75 mV, to depolarizing and hyperpolarizing current steps, with h-current blocked by ZD-7288. (B) Average response of CT neurons to −50 pA current step in the absence (red) and presence (orange) of ZD-7288, where dotted line is peak-scaled control (data without ZD-7288 taken from Figure 4B). (C) Summary of input resistance (R_in) (left), voltage sag during hyperpolarization (sag ratio) (middle) at −75 mV, and resting membrane potential (Vrest) (right), in the presence of ZD-7288 (n = 10 CC neurons, n = 9 CT neurons) (n = 5 mice total). Light blue bars indicate values in control conditions (from Figure 4). (D) Left, average cPFC-evoked EPSPs at pairs of CC (black) and CT (red) neurons held at −75 mV in the presence of ZD-7288, where dotted line is peak-scaled version of CC response (n = 7 pairs, 6 mice). Middle, similar for PV⁺-evoked IPSPs for neurons held at −55 mV (n = 7 pairs, 7 mice). Right, similar for SOM⁺-evoked IPSPs for neurons held at 55 mV (n = 7 pairs, 7 mice). Arrows indicates light pulse. (E) Summary of CT/CC amplitude (left) and decay (right) ratios, with y axis on log2 scale. Light blue bars indicate average values in control conditions (from Figure 5). Values are mean ± SEM (C) or geometric mean ± CI (E), *p < 0.05.

Figure 7. Targeted Inhibition Dampens EPSPs and Suppresses AP Firing

(A) Average conductance-evoked EPSPs recorded in dynamic clamp at CC neurons held at −75 mV (left) (n = 10 neurons) and CT neurons held at either −75 mV (middle) (n = 11 neurons) or −65 mV (right) (n = 6 of the 11 neurons recorded at −75 mV) (n = 6 mice total). Traces show response to either excitation alone (E) or excitation and inhibition (E + I). Inset traces show injected conductances, where dark traces are excitation and light traces are inhibition. Arrows indicate conductance onset. (B) Summary of ([E + I] / E) amplitude (left) and decay (right) ratios, with y axis on log2 scale. (C) Suprathreshold responses recorded in dynamic clamp, where APs have been truncated to highlight subthreshold responses. Inset traces show injected conductances with 3× scale factor. (D) Summary of number of APs evoked at CC (n = 8 neurons) and CT (n = 8 neurons) neurons in response to different scale factors of excitation alone (left) or excitation and inhibition (right) (n = 4 mice total). (E) Summary of inhibition of firing, calculated as the difference (Δ) in APs evoked by excitation alone (E) or excitation and inhibition (E + I) at the 10× scale factor. See also Figures S6 and S7. Values are geometric mean ± CI (B) or mean ± SEM (D and E), *p < 0.05.