Connectivity and Plasticity of Inhibitory Circuits in the Mouse Anterior Piriform Cortex

小鼠前梨狀皮層的抑制性神經環路的連接及可塑性

Student thesis: Doctoral Thesis

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Award date28 Aug 2020

Abstract

The anterior piriform cortex (APC) is the largest area of olfactory cortex and serves as a primary role for olfactory sensory coding, such as odor identity and discrimination. It is also engaged in sensory related decision-making because of its intensive interaction with other brain regions (e.g. prefrontal cortex, entorhinal cortex). APC is only two synapses downstream from olfactory bulb (OB). Therefore, neural circuits in APC are regulated by ongoing sensory and learning activity via changes in upstream OB activity. Furthermore, neural activities in APC can provide feedback projection to OB. Hence, APC serves as a primary mediator for olfactory sensory learning and decision-making. 

APC, a three-layered olfactory cortex, have principal glutamatergic neurons that are extensively modulated by GABAergic interneurons. Principal neurons in APC are mainly located in L2 (Layer 2) and classified into two major groups: semilunar (SL) and superficial pyramidal (SP) cells, which can be distinguished by their input and output connectivity, and neurophysiological features. GABAergic interneurons mainly contain SST (somatostatin)- and PV (parvalbumin)- interneurons, which are mainly located in L3 (Layer 3). SST- and PV- interneurons have differential electrophysiological properties, targeting at different parts of neurons, therefore, playing different roles in in inhibitory circuits of SL vs SP cells. Although it is well-known that SL and SP cells receive distinctive excitatory activity, their inhibitory circuits were largely unexplored. I hypothesize that inhibitory circuits of SL and SP cells are differentially wired up and exhibit distinctively inhibitory plasticity. 

In order to uncover the inhibitory plasticity of SL vs SP cells in APC, I employed electrophysiology, optogenetics, immunostaining and genetic mice (PV- and SST- cre mice) to evaluate the role of PV and SST interneurons in regulation of SL vs SP cells. There are two kinds of inhibition: feedforward and feedback inhibition. Feedforward inhibition is largely mediated by L1 interneurons whereas feedback inhibition by L3 interneurons. Therefore, I employed the patterned illumination of L1, L123 or L3 to selectively activate the different layers of inhibitory interneurons to clarify how PV and SST interneurons differentially mediate the SL vs SP cells. 

First, I explored the basal inhibitory wiring of SL vs SP cells. Patch-clamp recordings showed that SL cells exhibited higher frequency but equivalent amplitudes of sIPSC compared with SP cells. Patterned stimulation of Chrimson+ axons in different layers revealed that stronger L1 inhibition in SL cells whereas stronger L3 inhibition in SP cells. The patterned stimulation of CheTA-expressing PV-positive interneurons exhibited the same target- and layer- specific connection. However, the patterned stimulation of CheTA-expressing SST-positive interneurons showed that both SL and SP cells received stronger inhibition from L1 than L3, and the SST inhibition in strength was approximately half of PV inhibition. Therefore, our data exhibited the first demonstration of layer-specific connectivity of GABAergic interneurons with L2 principal neurons (SL and SP cells), which was mainly contributed by PV interneurons.

Next, I performed the activity deprivation by naris occlusion. Because most of olfactory information travels ipsilaterally from OB to APC, ipsilateral naris occlusion dampened the neural activity in occluded APC. Patch-clamp recordings showed that activity deprivation did not alter the amplitude or frequency of sIPSC in SL cells, but significantly increased sIPSC amplitude in SP cells. Immunofluorescent staining showed that activity deprivation significantly reduced the protein expression of GAD67 and PV but not SST. Activity deprivation upregulated both EPSC and IPSC in SP cells, resulting in that the overall excitatory-inhibitory ratio was preserved. Patterned illumination of L1, L123 or L3 exhibited that activity deprivation only increased L3 PV inhibition in SP cells. And it did not alter SST inhibition in both SL and SP cells. Our data suggested that 1) L2 principal neurons in APC exhibited layer (L3)- and target (SP)- specific inhibitory plasticity after activity deprivation, and 2) homeostatic rules of E-I Balance were maintained after activity deprivation. 

As animals forage for food using olfactory sensory input, I hypothesized that SL and SP cells are differentially regulated during starvation. After long term (48 h) of food deprivation and attempting to new environment adaptation, our data uncovered that 1) food foraging and attempting to new environment adaptation selectively decreased L3 inhibition only in SP cells, and 2) food foraging decreased L3 but not L1 PV interneuron output in SP cells. Moreover, food foraging and attempting to new environment adaptation did not alter the E-I balance in both SL and SP cells. These results indicated that 1) L2 principal neurons in APC exhibited layer (L3)- and target (SP)- specific inhibitory plasticity in food foraging and attempting to new environment adaptation, and 2) homeostatic rules of E-I Balance were maintained in food foraging and attempting to new environment adaptation.

Overall, the inhibitory wiring of SL vs SP cells was differentially set up by SST and PV interneurons: PV → SL: L1 > L3; PV → SP: L1 < L3. SST → both SL and SP: L1 > L3. A physiological consequence of this layer- and target-specific connectivity is that activation of L1 PV neurons can selectively suppress SL activity, while activation of L3 PV neurons can selectively suppress SP activity, gating the output of APC. I believe that the layer (L3)- and target (SP)- specific inhibitory plasticity of L2 principal neurons is physiologically significant for olfactory information processing and learning.

    Research areas

  • SL cells, SP cells, APC, PV interneurons, SST interneurons, activity dependent, inhibitory plasticity