Parvalbumin Interneurons Are Differentially Connected to Principal Cells in Inhibitory Feedback Microcircuits along the Dorsoventral Axis of the Medial Entorhinal Cortex

Abstract

The medial entorhinal cortex (mEC) shows a high degree of spatial tuning, predominantly grid cell activity, which is reliant on robust, dynamic inhibition provided by local interneurons (INs). In fact, feedback inhibitory microcircuits involving fast-spiking parvalbumin (PV) basket cells (BCs) are believed to contribute dominantly to the emergence of grid cell firing in principal cells (PrCs). However, the strength of PV BC-mediated inhibition onto PrCs is not uniform in this region, but high in the dorsal and weak in the ventral mEC. This is in good correlation with divergent grid field sizes, but the underlying morphologic and physiological mechanisms remain unknown. In this study, we examined PV BCs in layer (L)2/3 of the mEC characterizing their intrinsic physiology, morphology and synaptic connectivity in the juvenile rat. We show that while intrinsic physiology and morphology are broadly similar over the dorsoventral axis, PV BCs form more connections onto local PrCs in the dorsal mEC, independent of target cell type. In turn, the major PrC subtypes, pyramidal cell (PC) and stellate cell (SC), form connections onto PV BCs with lower, but equal probability. These data thus identify inhibitory connectivity as source of the gradient of inhibition, plausibly explaining divergent grid field formation along this dorsoventral axis of the mEC.

Keywords: GABAergic interneurons; entorhinal cortex; feedback inhibition; microcircuit; morphology; synapse.

Figure 1.

Morphology and physiological signature of fast-spiking PV BCs in the dorsal and ventral mEC. A, Representative confocal images from the slices of the dorsal (left panel) and ventral mEC (right panel) immunostained for PV (in green pseudocolor) and counterstained by DAPI (grayscale). Higher-magnification images illustrate the superficial layers (L1–L3) of the mEC corresponding to the white rectangles in the overview images in the separate channels. B, C, Reconstructions of biocytin-filled fast-spiking PV BCs recorded in L2/3 of the dorsal (B) and ventral mEC (C). Soma and dendrites of the INs are depicted in black, axons in red; boundaries of the layers (L1–L3) are indicated by dotted lines. Insets on the right illustrate the PV immunoreactivity (in green) in the biocytin-filled somata of the INs (Bioc, grayscale). D, E, Voltage responses of the two visualized PV BCs to hyperpolarizing (−500 to −100 pA) and depolarizing current pulses (100 and 500 pA, 500 ms in duration, see inset in the middle). Note the fast-spiking non-accommodating AP discharge pattern in response to the strong depolarizing current pulse in both INs. F, G, Summary bar charts of the amplitude (F) and frequency of spontaneous EPSPs in dorsal (D, red bars) and ventral PV BCs (V, blue bars). Data from individual neurons are superimposed as open circles; numbers of recorded neurons are indicated in parenthesis under the bars.

Figure 2.

Neuroanatomical properties of PV BCs in the dorsal and ventral mEC. A, D, Summary bar charts of the length of dendrites (A) and axons (D) of dorsal (D, red bars) and ventral PV BCs (V, blue bars). Data from individual neurons is superimposed as open circles; numbers of analyzed neurons are indicated in parenthesis under the bars. B, E, Sholl analysis of the dendritic (B) and axonal arbors (E) of dorsal (in red) and ventral PV BCs (in blue). Sholl radius was set to 25 μm, and significance was tested using Fischer’s exact test; asterisks indicate significant differences at the level of p = 0.05. C, F, Cumulative heat maps of the spatial densities of dendritic (C) and axonal distributions (F) for dorsal (left) and ventral PV BCs (right). Individual INs viewed in the plane of the slices were aligned with their somata to the middle of the plots. The color code for the density (in arbitrary units) is on the left. One-dimensional density plots on the right and bottom illustrate the spatial integrals of the densities along the x- and y-axes, respectively, for dorsal (in red) and ventral INs (in blue). G, A confocal image of axon collaterals of intracellularly-filled PV BCs displaying varicosities in the dorsal (left) and ventral mEC (right). H, Summary bar chart of the density of varicosities along axon collaterals of PV BCs from the dorsal (D, red bars) and ventral mEC (V, blue bars). Data from individual neurons are superimposed as open circles; numbers of analyzed neurons are indicated in parenthesis under the bars. Statistical significance: *p < 0.05 and ***p < 0.001.

Figure 3.

Connectivity of recorded IN-PrC pairs shows greater coupling probability in the dorsal mEC. A, Morphologic reconstructions of a dorsal and a ventral synaptically-coupled PV BC-PrC pair. PV BC somata and dendrites are in black, axons in red; for PrCs only soma and dendrites are shown (in cyan); boundaries of the L1–L3 are indicated by dotted lines. Insets on the bottom illustrate the PV immunoreactivity (in green pseudocolor) in the biocytin-filled somata of the BCs (Bioc, grayscale). B, Representative traces illustrate presynaptic APs evoked in PV BCs (upper traces) and short latency unitary IPSCs in concurrently recorded synaptically coupled PrCs (lower traces, averaged trace in black, individual IPSCs are superimposed in gray) in the dorsal (left) and ventral mEC (right). C, Summary bar chart of the connectivity of PV BCs onto PrCs in the dorsal (D, red bars) and ventral mEC (V, blue bars). D, Summary box plot of the unitary IPSC amplitudes in the dorsal (D, red bars) and ventral mEC (V, blue bars). Individual amplitude data from the pairs are superimposed as open circles on the bars. Numbers of analyzed simultaneous BC-PrC recordings are indicated in parenthesis under the bars. Statistical significance: **p < 0.01.

Figure 4.

Reciprocal IN-PrC pairs in the mEC show higher excitation and lower inhibition than non-reciprocal pairs. A, Schemes and representative traces illustrating the observed connectivity patterns of BC-PrC pairs: a reciprocally connected pair (left panel), a unidirectionally connected pair (middle panel) displaying only a unitary EPSC in the BC (bottom left trace, in gray) and unidirectionally connected PrC-BC pair (right panel) displaying only unitary IPSC in the PrC (top right trace in cyan). The representative presynaptic APs and the evoked unitary synaptic responses (average of 10 traces) are illustrated side-by-side in the BCs (left traces, in gray) and PrCs (right traces, in cyan). B, Bar chart of the total number of connected PV BC and PrC pairs (reciprocal connections, in gray). C, Summary bar chart of the unitary IPSC and EPSC amplitudes for reciprocal and non-reciprocal connections. Individual peak amplitude data from the pairs are superimposed as open circles on the bars. D, Ratio of excitation and inhibition in non-reciprocal (NR, white bar) versus reciprocal pairs (R, gray bar). Statistical significance: *p < 0.05, **p < 0.01.

Figure 5.

Properties of synaptic coupling of SCs and PCs to PV BCs. A, Voltage responses of a PC (top) and a SC (bottom) to hyperpolarizing (−500 to −100 pA, 500 ms in duration) and a suprathreshold depolarizing current pulse (500 pA). B, Dendrogram illustrates the separation of two subpopulations of PrCs corresponding to PCs (in cyan) and SCs (in ochre) by cluster analysis. C, E, Morphologic reconstructions of a synaptically-coupled PV BC-PC (C) and a PV BC-SC pair (E). PV BC somata and dendrites are in black, axons in red; for the PrCs only soma and dendrites are shown (PC in cyan; SC in ochre); boundaries of L1–L3 are indicated by dotted lines. Insets on the bottom illustrate the PV immunoreactivity (in green pseudocolor) in the biocytin-filled IN somata (Bioc, grayscale). D, Electrophysiological data from a reciprocally connected BC-PC pair. Representative trace illustrates presynaptic APs evoked in a PV BC (top trace) and the short latency unitary IPSCs in the concurrently recorded PC (upper middle trace, averaged response in black, individual IPSCs are superimposed in gray). Similarly, presynaptic APs evoked in the PC (lower middle trace) were followed by short latency unitary EPSCs in the concurrently recorded IN (bottom trace). F, Electrophysiological data from a reciprocally connected BC-SC pair as illustrated in D. G, Summary bar chart of the connection probabilities between PV BCs and PCs and SCs in the mEC. H, I, Bar charts of the amplitudes (H) and paired pulse ratios (I) of unitary IPSCs and EPSCs between PV BCs and synaptically coupled PCs and SCs. Individual values from the pairs are superimposed as open circles on the bars.