Disinhibitory Circuitry Gates Associative Synaptic Plasticity in Olfactory Cortex

Abstract

Inhibitory microcircuits play an essential role in regulating cortical responses to sensory stimuli. Interneurons that inhibit dendritic or somatic integration act as gatekeepers for neural activity, synaptic plasticity, and the formation of sensory representations. Conversely, interneurons that selectively inhibit other interneurons can open gates through disinhibition. In the anterior piriform cortex, relief of inhibition permits associative LTP of excitatory synapses between pyramidal neurons. However, the interneurons and circuits mediating disinhibition have not been elucidated. In this study, we use an optogenetic approach in mice of both sexes to identify the inhibitory interneurons and disinhibitory circuits that regulate LTP. We focused on three prominent interneuron classes: somatostatin (SST), parvalbumin (PV), and vasoactive intestinal polypeptide (VIP) interneurons. We find that LTP is gated by the inactivation SST or PV interneurons and by the activation of VIP interneurons. Further, VIP interneurons strongly inhibit putative SST cells during LTP induction but only weakly inhibit PV interneurons. Together, these findings suggest that VIP interneurons mediate a disinhibitory circuit that gates synaptic plasticity during the formation of olfactory representations.SIGNIFICANCE STATEMENT Inhibitory interneurons stabilize neural activity during sensory processing. However, inhibition must also be modulated to allow sensory experience shape neural responses. In olfactory cortex, inhibition regulates activity-dependent increases in excitatory synaptic strength that accompany odor learning. We identify two inhibitory interneuron classes that act as gatekeepers preventing excitatory enhancement. We demonstrate that driving a third class of interneurons inhibits the gatekeepers and opens the gate for excitatory enhancement. All three inhibitory neuron classes comprise disinhibitory microcircuit motifs found throughout the cortex. Our findings suggest that a common disinhibitory microcircuit promotes changes in synaptic strength during sensory processing and learning.

Keywords: circuit; cortex; inhibition; olfactory; plasticity.

Figure 1.

Associative LTP of intracortical synapses is gated by dendritic disinhibition. A1, Schematic of APC circuit and stimulation paradigm. GZ was focally applied to the dendrite region in L1b (tan oval). A2, LTP induction protocol: Strong (s) TBS of afferents (L1a) is paired with weak (w) single pulses at the intracortical pathway (L1b). B, Representative average traces pre (gray) and post induction in control (black) and with GZ(Tan). C1, Average EPSP amplitude (mV) pre (−5 to 0 min) and post induction (25-30 min) with inhibition intact (black) or with dendritic GZ (tan). C2, Average slope (mV/ms) of the rising phase of the EPSP pre and post induction. D, Normalized EPSP amplitude, and slope 30 min post induction in control (black) and with GZ (tan). E1, Time course of normalized EPSP amplitude pre and post induction (gray box, t = 0) in control (black) and with dendritic GZ (tan). E2, Time course of normalized EPSP slope. Colors as in E1.

Figure 2.

SST-IN inactivation promotes associative LTP. A1, Schematic, SST-INs express Arch. A2, SST-IN spike responses during TBS in control (black) and inactivated (green) conditions (*p < 0.05, WSR test). B, PN responses for a single TBS burst. Left, Inactivation of SST-INs enhanced PN depolarization (green vs black trace). Right, Depolarization during SST-IN inactivation (green) is reduced by APV (gray trace). C1, Average EPSP amplitude (mV) pre (−5 to 0 min) and post induction (25-30 min) with SST inactivation (magenta) or with SST inactivation plus bath application of the NMDAR antagonist, APV (black). C2, Average slope (mV/ms) of the rising phase of the EPSP pre and post induction. Colors as in B2. D, Normalized EPSP amplitude and slope following pairing with SST-IN inactivation (magenta circles). LTP is blocked by bath application APV (black). E, Representative average traces pre (gray) and post induction with SST-IN inactivation (magenta) or inactivation plus APV (Black). F1, Time course of normalized EPSP amplitude pre and post induction with inactivation of SST-INs (green box, t = 0) in control (magenta) and with bath application of APV (black). F2, Time course of normalized EPSP slope. Colors as in F1.

Figure 3.

PV-IN inactivation during induction promotes associative LTP. A1, Schematic, PV-INs express Arch. A2, PV-IN spike responses during TBS in control (black) and inactivated (green) conditions. B, Representative PN responses for a single TBS for control (black) versus PV-IN inactivation (green). C1, Average EPSP amplitude (mV) pre (−5 to 0 min) and post induction (25-30 min) with PV inactivation (blue) or with PV inactivation plus bath application of the NMDAR antagonist, APV (black). C2, Average slope (mV/ms) of the rising phase of the EPSP pre and post induction. Colors as in B2. D, Normalized EPSP amplitude and slope following pairing with PV-IN inactivation (blue circles) or with inactivation plus bath application APV (black). E, Representative average traces pre (gray) and post induction with PV-IN inactivation (blue) or inactivation plus APV (black). F1, Time course of normalized EPSP amplitude pre and post induction with inactivation of PV-INs (green box, t = 0) in control (blue) and with bath application of APV (black). F2, Time course of normalized EPSP slope. Colors as in F1.

Figure 4.

Inhibition by VIP-INs in piriform cortex. A1, VIP-INs express ChR2. Optically evoked IPSCs were recorded in putative(p) pSST-INs (magenta), pPV-INs (blue), and PNs (black). A2, IPSCs recorded in response to 10 light pulses (100 ms duration, 4 Hz, same neurons as in A1). B1, IPSC amplitudes were stronger in pSST-INs versus pPV-INs (**p = 0.0008) or PNs (**p = 0.001), ANOVA. B2, IPSC amplitude diminishes by the fifth pulse of theta stimulation (**p = 0.002, *p = 0.02). C1, IPSCs from VIP-INs delay pSST-IN spike responses during suprathreshold depolarization (4 overlaid traces) Left, Control. Right, Activation of VIP-INs (100 ms pulse, blue). Magenta trace represents VIP-IN-mediated IPSC during subthreshold depolarization. C2, Interspike interval (ISI) was significantly increased during optical activation of VIP-INs (blue circles, p = 0.016, paired t test, n = 11) compared with light off trials (black circles). D1, Spike responses in pSST-INs during TBS in control (magenta trace) and during pulsed light (blue trace). D2, pSST-IN FRs decreased during TBS on light trials (blue) versus control (black circles, p = 0.002, paired t test, n = 11).

Figure 5.

VIP-IN activation promotes associative LTP. A1, Circuit schematic, VIP-INs express ChR2 and were activated using theta pulsed light during L1a+L1b pairing. A2, VIP-IN responses during TBS without (black) and with light (blue). FRs increase during pairing with light (blue circles, p < 0.05, WSR, n = 7). B, Top, Activation of VIP-INs enhanced PN depolarization during TBS stimulation (blue vs black trace). Bottom, PN depolarization during VIP-IN activation (blue trace) is reduced by APV (gray trace). C1, Average EPSP amplitude (mV) pre (−5 to 0 min) and post induction (25-30 min) with VIP-IN activation (green) or with VIP-IN activation plus bath application of APV (black). C2, Average slope (mV/ms) of the rising phase of the EPSP pre and post induction. Colors as in C1. D, Normalized EPSP amplitude and slope following pairing with VIP-IN activation (green circles) or with activation plus APV (black). E, Representative average traces pre (gray) and post induction with VIP-IN activation (green) or activation plus APV (black). F1, Time course of normalized EPSP amplitude pre and post induction with activation of VIP-INs (blue box, t = 0) in control (green) and with APV (black). F2, Time course of normalized EPSP slope. Colors as in F1.