Synapse-specific inhibitory control of hippocampal feedback inhibitory circuit

Abstract

Local circuit and long-range GABAergic projections provide powerful inhibitory control over the operation of hippocampal inhibitory circuits, yet little is known about the input- and target-specific organization of interacting inhibitory networks in relation to their specific functions. Using a combination of two-photon laser scanning photostimulation and whole-cell patch clamp recordings in mice hippocampal slices, we examined the properties of transmission at GABAergic synapses formed onto hippocampal CA1 stratum oriens - lacunosum moleculare (O-LM) interneurons by two major inhibitory inputs: local projection originating from stratum radiatum interneurons and septohippocampal GABAergic terminals. Optical mapping of local inhibitory inputs to O-LM interneurons revealed that vasoactive intestinal polypeptide- and calretinin-positive neurons, with anatomical properties typical of type III interneuron-specific interneurons, provided the major local source of inhibition to O-LM cells. Inhibitory postsynaptic currents evoked by minimal stimulation of this input exhibited small amplitude and significant paired-pulse and multiple-pulse depression during repetitive activity. Moreover, these synapses failed to show any form of long-term synaptic plasticity. In contrast, synapses formed by septohippocampal projection produced higher amplitude and persistent inhibition and exhibited long-term potentiation induced by theta-like activity. These results indicate the input and target-specific segregation in inhibitory control, exerted by two types of GABAergic projections and responsible for distinct dynamics of inhibition in O-LM interneurons. The two inputs are therefore likely to support the differential activity- and brain state-dependent recruitment of hippocampal feedback inhibitory circuits in vivo, crucial for dendritic disinhibition and computations in CA1 pyramidal cells.

Keywords: GABAergic circuits; interneuron-specific interneuron; medial septum; mouse; plasticity; synapse.

Figure 1

Laser-scanning photostimulation by glutamate uncaging of RAD INs. (A) Two-photon Dodt ISGC images of the hippocampal CA1 area showing the positioning of a puff pipette filled with MNI-Glu (A1) near the cell of interest (A2). Red circles with numbers (1–3) correspond to the areas of uncaging from which the recordings were obtained (B). (B) Postsynaptic responses evoked in the interneuron of interest (A2) by glutamate uncaging with different laser power (B1, uncaging area 1) and at different locations (B2, uncaging areas 1–3; laser power = 32 mW). Note that the puff application of MNI-Glu or uncaging alone (area 1; laser power = 32 mW) did not produce any postsynaptic responses (B3). Horizontal bars below the traces indicate the duration of uncaging pulses. (C) Contour plot of the spatial profile of IN photoexcitability with color coding for the slope of uncaging-evoked responses. A slope of 0.5 ± 0.04 mV/ms corresponds to the EPSP-spike sequence. The black circle indicates the cell body position. Note the high localization of action potential generation at the cell body level. (D) Spike probability as a function of laser power and pulse duration. Dotted lines correspond to individual cells and solid lines to the average spike probability for a given duration. Note that a minimal laser power of ∼30 mW was required to evoke a single spike using pulses of 113 ms. Successful photostimulation with shorter uncaging pulses (20–44 ms) required an increase in the laser power above 50 mW. (E) Neurolucida reconstruction of a RAD IN tested for photoexcitability and filled with biocytin. Soma and dendrites are shown in black, and axonal arborization is shown in red.

Figure 2

Optical targeting of local inhibitory input to O–LM INs. (A) Representative traces of IPSCs evoked in O–LM INs by local glutamate uncaging on PYR/RAD INs (uIPSCs; superimposition of three consecutive traces with an individual trace shown as an inset) in control and after application of bicuculline. The right panel shows summary data of uIPSC peak amplitude before and after bicuculline application (n = 6). Horizontal bars below the traces indicate the duration of uncaging pulses. (B) The distribution histograms of uIPSC peak amplitude (left), rise time (middle), and decay time constant (right) from all cells (n = 5). (C) Neurolucida reconstruction of monosynaptically connected RAD and O–LM INs filled with biocytin, showing anatomical features (bipolar orientation, extensive axonal arborization in the O/A) of an interneuron innervating an O–LM cell. Soma and dendrites of the presynaptic RAD IN are shown in green, and its axonal arborization is shown in blue. Soma and dendrites of the postsynaptic O–LM IN are shown in dark blue and its axonal arborization is shown in red. (D) Responses of the presynaptic RAD IN to current pulses (top) and a plot of the interspike interval as a function of pulse duration at different levels of membrane depolarization (bottom) demonstrating the irregular firing pattern of the RAD IN innervating the O–LM cell.

Figure 3

PYR/RAD INs positive for vasoactive intestinal polypeptide target O–LM INs. (A) Maximal projection of a two-photon z-stack acquired in the CA1 region of the hippocampus of a VIP-eGFP mouse, showing bipolarly oriented VIP-positive INs with a cell body located at the PYR/RAD border and a dense axonal arborization in the O/A. (B) Immunofluorescence images of neurons positive for calretinin (top) and VIP (middle) as well as their superimposition (bottom). Scale bar: 20 μm. (C) Sample traces of uIPSCs (six consecutive traces with an average trace shown in red; top) evoked in O/LM IN (D) by photostimulation of the bipolarly oriented VIP-positive cell (D, inset; scale bar: 20 μm) and the distribution histograms of uIPSC peak amplitude (left), rise time (middle), and decay time constant (right) from all VIP-positive cells (n = 4). (D) Neurolucida reconstruction of an O–LM IN from which the recording was obtained (C). Inset shows a VIP-positive IN, which was stimulated by uncaging. (E) Example of a firing pattern (top) of a VIP-positive IN and a summary plot of firing frequency as a function of pulse duration (bottom left; data from individual cells are shown in black and the group average is shown in red) and interspike interval histogram (bottom right) showing irregularity in IN firing. INs were depolarized to −40 mV. (F) Reconstruction of a bipolarly oriented VIP-positive cell, showing anatomical features of a putative type III ISI (soma and dendrites are shown in black and its axon is shown in red).

Figure 4

Input-specific properties of inhibitory synapses onto O–LM INs. (A) Schematic of the recording and stimulation configuration in hippocampal (A1) and septohippocampal (A2) slices. (B) Sample traces of IPSCs (10 consecutive traces with an average trace shown in black) evoked in O–LM INs by local minimal stimulation in PYR adjacent to st. radiatum (B1) and by minimal stimulation of the SH inhibitory projection (B2). (C) Graphs of normalized distribution histograms of IPSC peak amplitudes (C1), rise times (C2), and a relationship between the rise time and the IPSC peak amplitude (C3), with a line corresponding to a linear fit between the data points.

Figure 5

Distinct dynamics of inhibition at local and septohippocampal synapses. (A) Schematic of the recording and stimulation configuration in hippocampal (A1) and septohippocampal (A2) slices (top) and sample traces of eIPSCs (average of 10 sweeps; failures excluded) evoked by paired-pulse stimulation at local (A1) and SH (A2) synapses together with summary plots (bottom), showing the coefficient-of-variation analysis of paired-pulse depression of eIPSCs at two synapses. The inverse of the square of the coefficient of variation of the second response (${\text{CV}}_{\text{A}2}^{-2}$) was plotted versus the mean peak amplitude; data were normalized by the ${\text{CV}}_{\text{1}}^{\text{-2}}$ and the mean of the first response, respectively. Thick lines with open symbols correspond to the average values. Interpulse interval was 50 ms. (B,C) Short-term plasticity of eIPSCs at two synapses during repetitive stimulation at 10 Hz. (B), IPSCs evoked by the first (left) and last (right) five stimuli in a train of 20 stimuli (top), and a superimposition of the first and fifth (bottom left), first and 10th (bottom middle) and first and 20th (bottom right) responses during the train; the traces represent averages of three sweeps. (C) Time course of eIPSC amplitude during 10-Hz train. The curve at C1 corresponds to a single exponential fit to the data points, with τ = 0.16 s. Note that there is a significant depression at local synapses (n = 6) and no change during the train at SH synapses (n = 7). Error bars throughout represent SEM.

Figure 6

Input-specific expression of long-term plasticity at inhibitory synapses on O–LM INs. (A) Graphs of eIPSC amplitude versus time from representative INs. Average eIPSCs (average of 30 sweeps; failures included) in control (a) and 20 min after 10-Hz stimulation (b) are shown on top. (B) Normalized group data of eIPSC amplitude (normalized to the first 5 min of recordings) as a function of time. (C) Summary bar graphs of group data (C1, n = 5; C2, n = 5) for experiments illustrated in (B) showing changes in the eIPSC peak amplitude, the paired-pulse ratio (PPR), the coefficient of variation (CV) and the failure rate obtained 20 min after 10-Hz stimulation. Data are expressed as percentage of control parameters obtained before 10-Hz stimulation. *P < 0.05. Error bars throughout represent SEM.