Balanced feedforward inhibition and dominant recurrent inhibition in olfactory cortex

Throughout the brain, the recruitment of feedforward and recurrent inhibition shapes neural responses. However, disentangling the relative contributions of these often-overlapping cortical circuits is challenging. The piriform cortex provides an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition are anatomically segregated in layer (L) 1 and L2/3 respectively. Here we use a combination of optical and electrical activation of interneurons to profile the inhibitory input received by three classes of principal excitatory neuron in the anterior piriform cortex. In all classes, we find that L1 interneurons provide weaker inhibition than L2/3 interneurons. Nonetheless, feedforward inhibitory strength covaries with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L2/3 evokes strong inhibition that dominates recurrent excitation in all classes. Finally, we find that the relative contributions of feedforward and recurrent pathways differ between principal neuron classes. Specifically, L2 neurons receive more reliable afferent drive and less overall inhibition than L3 neurons. Alternatively, L3 neurons receive substantially more intracortical inhibition. These three features—balanced afferent drive, dominant recurrent inhibition, and differential recruitment by afferent vs. intracortical circuits, dependent on cell class—suggest mechanisms for olfactory processing that may extend to other sensory cortices.The recruitment of inhibition is an essential feature of cortical processing. Feedforward and recurrent inhibitory circuits have been implicated in controlling the timing, strength, and tuning of cortical responses (for review, see ref. 1). In sensory cortices, including the olfactory cortex, neural responses to sensory stimuli depend on the relative balance of inhibition with respect to excitation in both feedforward and recurrent pathways (2–7). Moreover, numerous theoretical studies have suggested that balanced cortical networks underlie the selectivity, sparseness, and correlations of cortical activity (8–15). These studies highlight the importance of quantifying the relationship between excitation and inhibition in cortical networks. However, isolating the contributions of feedforward vs. recurrently evoked inhibition to cortical responses is difficult because these circuits are often coactive and frequently share interneurons (16–18). The piriform cortex is an ideal system to address this issue because the interneurons responsible for feedforward and recurrent inhibition differ by class and laminar location and, thus, are differentially recruited by afferent and intracortical excitation (19–22).The piriform cortex is a trilaminar cortex responsible for processing olfactory stimuli. Principal excitatory neurons are found in layer (L) 2/3 and send dendrites to L1, where they receive odor-related excitation directly from the olfactory bulb via the lateral olfactory tract (LOT) (23). LOT afferents also drive horizontal and neurogliaform inhibitory interneurons within L1, yielding feedforward inhibition of principal neurons (24). Within the cortex, principal neurons send axon collaterals throughout L2/3 and to an intracortical fiber tract in L1b (25, 26). Intracortical excitation recruits a number of interneuron classes within L2/3 that, in turn, provide recurrent inhibition to principal neurons (20, 24, 27, 28). Stimulation of the LOT evokes short- latency feedforward inhibition that targets principal neuron dendrites, as well as long-latency, recurrent inhibition that is somatic (24, 28, 29). These findings are consistent with the different laminar locations of inhibitory interneurons mediating feedforward and recurrent inhibition respectively.Previous studies have shown that electrical stimulation of the LOT as well as odors recruit mixed feedforward and recurrent inhibition in vivo (3, 4, 30, 31). However, in vivo and in vitro studies focusing on different principal neuron classes have led to conflicting reports of the relative contributions of feedforward and feedback inhibition during afferent odor processing (3, 24, 27, 28, 32). Furthermore, because of the disynaptic nature of inhibition, estimates of feedforward or recurrent inhibitory strength depend on the quality of afferent and intracortical excitatory recruitment, which varies with the different stimulation protocols used in each study. Here, we resolve these discrepancies by comparing feedforward and recurrent inhibition evoked by direct optical activation of interneurons that express channelrhodopsin (ChR2) (33), as well as electrical stimulation of excitatory pathways in all three classes of principal neuron in the anterior piriform cortex (APC).In the APC, principal excitatory neuron classes differ in laminar location and in the proportion of afferent vs. intracortical excitatory input received (34–37). Within L2, semilunar cells (SLCs) receive predominantly afferent excitation, whereas superficial pyramidal cells (sPCs) receive weaker afferent and stronger intracortical excitatory drive. In L3, deep pyramidal cells (dPCs) receive minimal afferent, but substantial intracortical excitation. Given these differences in excitatory drive, we hypothesized that inhibition mediated by feedforward and recurrent inhibitory circuits also differs between principal neuron classes. In this study, we find that principal neuron classes are weakly inhibited by L1 interneurons that mediate feedforward inhibition, compared with L2/3 interneurons that provide strong recurrent inhibition. As predicted, feedforward inhibitory strength varies in a manner consistent with the amount of afferent excitation received by each class of principal neuron. In contrast, intracortical stimulation of L3 evokes strong recurrent inhibition that dominates excitation in all classes. Moreover, excitatory and inhibitory profiles differ between SLCs, sPCs, and dPCs. Taken together, our results demonstrate that inhibitory circuits in the piriform cortex provide both balanced feedforward inhibition and dominant recurrent inhibition, as well as segregate principal excitatory neuron classes during cortical processing.