Input-specific learning rules at excitatory synapses onto hippocampal parvalbumin-expressing interneurons

Abstract

Hippocampal parvalbumin-expressing interneurons (PV INs) provide fast and reliable GABAergic signalling to principal cells and orchestrate hippocampal ensemble activities. Precise coordination of principal cell activity by PV INs relies in part on the efficacy of excitatory afferents that recruit them in the hippocampal network. Feed-forward (FF) inputs in particular from Schaffer collaterals influence spike timing precision in CA1 principal cells whereas local feedback (FB) inputs may contribute to pacemaker activities. Although PV INs have been shown to undergo activity-dependent long term plasticity, how both inputs are modulated during principal cell firing is unknown. Here we show that FF and FB synapses onto PV INs are endowed with distinct postsynaptic glutamate receptors which set opposing long-term plasticity rules. Inward-rectifying AMPA receptors (AMPARs) expressed at both FF and FB inputs mediate a form of anti-Hebbian long term potentiation (LTP), relying on coincident membrane hyperpolarization and synaptic activation. In contrast, FF inputs are largely devoid of NMDA receptors (NMDARs) which are more abundant at FB afferents and confer on them an additional form of LTP with Hebbian properties. Both forms of LTP are expressed with no apparent change in presynaptic function. The specific endowment of FF and FB inputs with distinct coincidence detectors allow them to be differentially tuned upon high frequency afferent activity. Thus, high frequency (>20 Hz) stimulation specifically potentiates FB, but not FF afferents. We propose that these differential, input-specific learning rules may allow PV INs to adapt to changes in hippocampal activity while preserving their precisely timed, clockwork operation.

Figure 1. Postsynaptic receptor content at feed-forward vs. feedback excitatory inputs onto PV interneurons (INs)

A, infrared differential interference contrast (left) and fluorescence (right) video images of a CA1 GFP-expressing interneuron in an acute hippocampal slice from Pvexponent::RCE mouse. Scale, 20 μm. B, evoked EPSCs recorded at +60 mV or −60 mV for a pyramidal neuron (Pyr) and PV IN (PV) in the presence of 100 μm APV. C and D, current–voltage relation (C) and rectification index (D) of AMPAR-mediated EPSCs recorded from pyramidal neurons (n= 4, grey) or PV INs (n= 7) upon stimulation of either FF (white) or FB (black) afferents. E, time course of Naspm-induced partial block of EPSCs evoked in PV INs upon FF (white) or FB (black) afferent stimulation (n= 10). F, normalized EPSC amplitude upon 15–25 min of Naspm application. Mean data from 2 pyramidal neurons and 10 PV INs. G, left: 15 consecutive evoked EPSCs (grey) and mean currents (black) recorded at +60 mV or −60 mV for a pyramidal neuron (Pyr) and PV IN (PV). G, right: NMDA/AMPA ratio for EPSCs recorded from PV INs upon stimulation of FF (white) or FB (black) afferents (n= 12). H, left: NMDAR-mediated EPSCs recorded in the presence of 10 μm NBQX, before (grey) and after (black) application of Ro25-6981. H, right, normalized amplitude of the NMDA EPSC upon application of Ro25-6981 (n= 5). *P < 0.05; n.s., non-significant.

Figure 2. Distinct plasticity rules at FF vs. FB excitatory synapses onto PV INs

A–D, after a 5 min. control recording, afferent-specific stimulation (400 pulses at 5 Hz, arrowhead) was paired with either postsynaptic hyperpolarization to –90 mV (anti-Hebbian induction, A, B) or depolarization to 0 mV (Hebbian induction, C, D). Left, averaged EPSCs are shown for individual recordings before (grey) and after (black) pairing. Right, summary data from all experiments. Anti-Hebbian plasticity was observed at both FF (A, n= 10) and FB synapses (B, n= 9). However, Hebbian plasticity was induced only at FB (D, n= 8) but not FF (C, n= 13) synapses onto PV INs.

Figure 3. Anti-Hebbian and Hebbian LTP in PV INs require activation of GluA2-lacking AMPARs and NMDARs, respectively

A, application of Naspm (100 μm) immediately prior to pairing (arrowhead) prevented anti-Hebbian LTP at FB excitatory afferents (n= 6). Normalized EPSC amplitudes are shown for control (grey circles) and paired (black) pathways. No significant difference in the recovery of EPSCs from Naspm blockade was observed between control and paired pathways. B, C, application of 100 μm APV prevented the induction of Hebbian LTP at FB afferents (B, n= 11) without affecting induction of anti-Hebbian LTP at FF afferents (C, n= 14). D, summary data of all experiments showing the mean amplitude of EPSCs normalized to control upon Hebbian (0 mV) or anti-Hebbian (−90 mV) pairing in control conditions or in the presence of either Naspm or APV for FF (white) or FB (black) afferents. Changes in EPSC amplitude upon pairing are compared in the presence of antagonists vs. control conditions. *P < 0.05; n.s., non-significant.

Figure 4. Presynaptic function is unaffected upon Hebbian and anti-Hebbian LTP in PV INs

A, averaged EPSCs evoked by paired pulses (50 ms apart) before (grey) and after (black) pairing-induced LTP. Top, anti-Hebbian LTP of FF inputs. Bottom, Hebbian pairing of FB inputs. Right, the second EPSC (EPSC2) is shown before and after pairing, after normalization to the peak amplitude of the first (EPSC1 scaled). No change in PPR was detectable in either condition. B, normalized PPR for control pathway or potentiated pathway. 5 Hz afferent stimulation was paired with hyperpolarization to −90 mV (FF inputs, white circles, n= 15; FB inputs, black circles, n= 14) or depolarization to 0 mV (FB inputs; black circles, n= 10). C, left: A1 receptor antagonist DPCPX (100 nM) increased EPSC amplitude in PV INs by about 40% (n= 10). C, right: compared effect of Naspm on evoked EPSCs in control conditions (grey circles, n= 17) or after potentiation by DPCPX (n= 10, black circles). EPSC amplitudes were normalized to mean values prior to Naspm application. The time course of Naspm-induced blockade of EPSCs (fitted to a single exponential) was faster in PV INs recorded in the presence of DPCPX. *P < 0.05. D, summary data of the decay time constant (tau) of EPSC blockade by Naspm in either condition. E, F, similar experiment in which the effect of Naspm was monitored on EPSCs evoked from two independent FF pathways (paired, black circles; control, white circles). Arrowhead indicates pairing. EPSC amplitudes were normalized to values prior to Naspm application. No significant difference in the decay of EPSC amplitude was detected between the two pathways (n= 10).

Figure 5. Postsynaptic Hebbian and anti-Hebbian LTP in PV INs

A, EPSCs evoked by focal uncaging of MNI-Glutamate onto proximal dendrites of PV INs in st. radiatum. Average of 30 uEPSCs normalized in amplitude and compared to 200 averaged mEPSCs recorded from the same cell showing fast onset kinetics despite slower decay of uEPSCs as compared to mEPSCs. B, right: uEPSCs evoked at various positions along a PV IN apical dendrite in st. radiatum. 10 individual uEPSCs (grey) are shown superimposed with averaged current (black) at each position. B, left, graph showing peak amplitude and decay time constant (decay tau) as a function of distance from the spot of maximal amplitude and minimal decay for the cell shown in right panel. Note the sharp decrease in amplitude and increase in decay kinetics of uEPSCs when light spot is moved away from optimal position. C, pairing consisting of 100 uEPSCs at 5 Hz with membrane hyperpolarization to –90 mV (arrowhead) leads to a persistent potentiation of uEPSC amplitude (white circles, n= 8 cells) as compared to control uEPSCs (grey circles, n= 3 cells). D, averaged uEPSCs from one such experiment before (n= 30) and 10–20 min after (n= 40) pairing. E, similar experiment on uEPSCs evoked by glutamate uncaging onto PV IN dendrites in st. oriens. Pairing of 100 uEPSCs at 5 Hz with membrane voltage clamped at 0 mV (arrowhead) leads to a potentiation of uEPSCs evoked at distal (>50 μm from soma close to alveus, black circles, n= 7 cells) but not proximal dendritic sites (20–40 μm from soma, black triangles, n= 6 cells) as compared to control (grey circles, n= 3 cells). F, averaged uEPSCs from one experiment with distally evoked uEPSCs before (n= 30) and 10–15 min after (n= 40) pairing.

Figure 6. Frequency-dependent plasticity at FF vs. FB inputs onto PV INs

A–D, long term plasticity induced by repetitive afferent stimulation (arrowheads) to either FF (white) or FB (black) inputs onto PV INs. 900 pulses were delivered at 1 (A), 5 (B) or 20 (C) Hz. In D, 100 pulses were delivered 5 times at 100 Hz while recording PV INs in current-clamp mode. Lower graphs represent averages of EPSC amplitudes recorded in voltage-clamp mode at –60 mV, normalized to control prior to conditioning stimulus; n= 5–7 cells for each frequency and each pathway. Upper graphs show sample recordings during conditioning stimulus at each frequency. Insets: detail of the first 150 ms. Note that 100 Hz stimulation only was sometimes sufficient to depolarize PV INs above firing threshold (action potentials are clipped due to filtering). E, 100 Hz stimulation was delivered to FB inputs in the presence of 100 μm APV to block NMDARs; n= 7 cells. F, 100 Hz stimulation was delivered to FF inputs while holding the postsynaptic cell to –90 mV in voltage-clamp (VC) mode to allow for activation of Ca²⁺-permeable AMPARs; n= 6 cells. G, summary of all experiments with afferent stimulations delivered at various frequencies (1, 5, 20 or 100 Hz) at FF (white) or FB (black) inputs onto PV INs, or FF inputs onto pyramidal neurons (grey). Each data point represents the mean ± SEM of normalized EPSC amplitudes measured between 10 and 20 min after conditioning stimulus. Note that beyond 20 Hz stimulation, FF inputs onto PV INs show no LTP, in contrast to FB inputs and FF inputs onto pyramidal cells. ^#P < 0.05 FF inputs onto pyramidal cells compared to FF and FB inputs onto PV INs. *P < 0.05 FF compared to FB inputs onto PV INs.