Timing isn’t everything: opposing roles for perisomatic inhibition
Authors: Adam P. Caccavano, Chris J. McBain
Journal: Neuron (2021)

Perisomatic inhibition from parvalbumin-containing (PV) interneurons is critical for timing, but the role of cholecystokinin-containing (CCK) interneurons remains obscure. Utilizing a novel mouse model, Dudok et al. demonstrate fundamentally distinct behavioral roles for these neuronal subpopulations during behavior. Perisomatic inhibition from parvalbumin-containing (PV) interneurons is critical for timing, but the role of cholecystokinin-containing (CCK) interneurons remains obscure. Utilizing a novel mouse model, Dudok et al. demonstrate fundamentally distinct behavioral roles for these neuronal subpopulations during behavior. Inhibition from the diverse GABAergic neuronal population finely tunes excitatory principal cell activity through selective innervation along the somatodendritic axis. Perhaps more so than any other source, perisomatic inhibition directly onto soma and proximal dendrites has an outsized role in controlling neuronal activity. Within the hippocampus, perisomatic inhibition largely arises from parvalbumin-containing (PV) and cholecystokinin-containing (CCK) basket cells (BCs), named for the dense mesh of axonal contacts surrounding their post-synaptic targets (Pelkey et al., 2017Pelkey K.A. Chittajallu R. Craig M.T. Tricoire L. Wester J.C. McBain C.J. Hippocampal GABAergic Inhibitory Interneurons.Physiol. Rev. 2017; 97: 1619-1747Crossref PubMed Scopus (283) Google Scholar). Extensive research has established the unique characteristics of fast-spiking PV cells. With sub-millisecond precision, PV cells are exquisitely tuned to provide reliable feedforward and feedback control, with an essential role in theta oscillations and sharp wave ripples (SWRs), thought to support memory encoding and consolidation (Hu et al., 2014Hu H. Gan J. Jonas P. Interneurons. Fast-spiking, parvalbumin+ GABAergic interneurons: from cellular design to microcircuit function.Science. 2014; 345: 1255263Crossref PubMed Scopus (537) Google Scholar). In contrast, CCK cells exhibit regular-spiking patterns and asynchronous transmitter release (Hefft and Jonas, 2005Hefft S. Jonas P. Asynchronous GABA release generates long-lasting inhibition at a hippocampal interneuron-principal neuron synapse.Nat. Neurosci. 2005; 8: 1319-1328Crossref PubMed Scopus (405) Google Scholar), a baffling feature for efficient information storage, as this would be predicted to result in the loss of critical temporal information. The compelling study by Dudok et al., 2021Dudok B. Klein P.M. Hwaun E. Lee B.R. Yao Z. Fong O. Bowler J.C. Terada S. Sparks F.T. Szabo G.G. et al.Alternating sources of perisomatic inhibition during behavior.Neuron. 2021; 109 (this issue): 997-1012Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar finally suggests an intriguing potential function for this asynchronous release. In an innovative set of experiments, the authors observed opposing activity patterns between perisomatic-targeting PV and CCK cells. The novel observation that CCK cells are more active during disorganized periods of activity between running behavior and SWR replay suggests they may serve to orchestrate an important "idle state" of the hippocampus. The wealth and dearth of knowledge concerning PV and CCK cells, respectively, can at least partially be attributed to the availability of genetic tools. With a unique expression profile of the protein parvalbumin, as well as the easily distinguishable fast-spiking pattern, PV cells have been a relatively accessible target for research. In contrast, CCK cells have long resisted investigation. Cholecystokinin is expressed by several neuronal populations, and the regular-spiking pattern is not clearly distinct. The authors overcame these obstacles by building upon recent transcriptomic observations that the Sncg gene (encoding gamma synuclein) preferentially labels CCK interneuron subtypes (Gouwens et al., 2020Gouwens N.W. Sorensen S.A. Baftizadeh F. Budzillo A. Lee B.R. Jarsky T. Alfiler L. Baker K. Barkan E. Berry K. et al.Integrated Morphoelectric and Transcriptomic Classification of Cortical GABAergic Cells.Cell. 2020; 183: 935-953.e19Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar), prompting the development of a novel Sncg transgenic mouse. In transfecting with conditional viral reporters, the authors observed Scng mice labeled CCK interneurons with greater specificity than other markers and greater sensitivity than combinatorial approaches that only label CCK subpopulations. Sncg cells co-expressed CCK and the cannabinoid receptor CB1R and exhibited basket cell morphology, with axons restricted to the pyramidal cell (PC) layer. An exhaustive Patch-seq analysis in acute slice confirmed morphological, transcriptomic, and electrophysiological profiles consistent with CCK BCs. Within the CCK population, the additional markers VGluT3 (vesicular glutamate transporter 3) and VIP (vasoactive intestinal peptide) are generally considered to comprise nonoverlapping CCK subpopulations (Somogyi et al., 2004Somogyi J. Baude A. Omori Y. Shimizu H. El Mestikawy S. Fukaya M. Shigemoto R. Watanabe M. Somogyi P. GABAergic basket cells expressing cholecystokinin contain vesicular glutamate transporter type 3 (VGLUT3) in their synaptic terminals in hippocampus and isocortex of the rat.Eur. J. Neurosci. 2004; 19: 552-569Crossref PubMed Scopus (149) Google Scholar). Notably, Sncg cells included those co-expressing VGluT3, a minority co-expressing VIP, and some expressing neither additional marker. Optogenetic activation of Sncg cells resulted in inhibitory synaptic responses in PCs consistent with CCK interneurons, with adapting and asynchronous presynaptic release, fast rise kinetics, and depolarization-induced suppression of inhibition (DSI), a CB1R-dependent retrograde signaling unique to CCK cells. The ability to specifically label the CCK subpopulation represents a significant advancement to study in vivo CCK cell function. Prior approaches have relied on laborious juxtacellular recordings and post hoc identification, necessitating modest sample sizes (Klausberger et al., 2005Klausberger T. Marton L.F. O'Neill J. Huck J.H. Dalezios Y. Fuentealba P. Suen W.Y. Papp E. Kaneko T. Watanabe M. et al.Complementary roles of cholecystokinin- and parvalbumin-expressing GABAergic neurons in hippocampal network oscillations.J. Neurosci. 2005; 25: 9782-9793Crossref PubMed Scopus (305) Google Scholar). With this novel genetic tool, the authors were able to perform dual in vivo calcium imaging of PV and Sncg/CCK populations and made an exciting discovery: PV and Sncg/CCK cells display opposing activity profiles at the behavioral timescale of seconds. During running behavior, PV cells are highly active while Sncg/CCK cells are suppressed, an activity pattern that inverts during immobile periods characterized by disorganized activity, in which PV cell activity is reduced while Sncg/CCK cells are heavily recruited. While this behavioral modulation of PV cells is consistent with several studies, no such observation has previously been possible for the whole CCK BC population. At the shorter timescale of milliseconds, prior work has established that PV and CCK BCs fire at unique phases of theta oscillations (Klausberger et al., 2005Klausberger T. Marton L.F. O'Neill J. Huck J.H. Dalezios Y. Fuentealba P. Suen W.Y. Papp E. Kaneko T. Watanabe M. et al.Complementary roles of cholecystokinin- and parvalbumin-expressing GABAergic neurons in hippocampal network oscillations.J. Neurosci. 2005; 25: 9782-9793Crossref PubMed Scopus (305) Google Scholar), likely subserving different inhibitory control. However, longer-term activity patterns for CCK cells have not been quantified, with CCK spiking activity observed during both activity-associated theta oscillations and immobility-associated SWRs (Klausberger et al., 2005Klausberger T. Marton L.F. O'Neill J. Huck J.H. Dalezios Y. Fuentealba P. Suen W.Y. Papp E. Kaneko T. Watanabe M. et al.Complementary roles of cholecystokinin- and parvalbumin-expressing GABAergic neurons in hippocampal network oscillations.J. Neurosci. 2005; 25: 9782-9793Crossref PubMed Scopus (305) Google Scholar). Due to the difficulty in specifically targeting the CCK cell population, there are scant studies of behavioral modulation of CCK cells. Recent evidence has suggested that subpopulations of CCK cells are supportive of memory processes and modulated by behavior. A subset of CCK/VGluT3 cells play an important role in memory, with developmental disruption of their connectivity impairing spatial learning and memory and reducing the power of theta oscillations during exploratory behavior (Del Pino et al., 2017Del Pino I. Brotons-Mas J.R. Marques-Smith A. Marighetto A. Frick A. Marín O. Rico B. Abnormal wiring of CCK+ basket cells disrupts spatial information coding.Nat. Neurosci. 2017; 20: 784-792Crossref PubMed Scopus (39) Google Scholar). Additionally, a subset of CCK/VIP cells were found to be negatively modulated by velocity (Turi et al., 2019Turi G.F. Li W.K. Chavlis S. Pandi I. O'Hare J. Priestley J.B. Grosmark A.D. Liao Z. Ladow M. Zhang J.F. et al.Vasoactive Intestinal Polypeptide-Expressing Interneurons in the Hippocampus Support Goal-Oriented Spatial Learning.Neuron. 2019; 101: 1150-1165.e8Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar). The present study extends these findings to the larger CCK BC population, demonstrating that these cells are strongly modulated by running behavior. Perhaps most striking of all, the authors found evidence for direct functional connections from PV to Sncg/CCK cells. The reliably anti-correlated activity of PV and CCK cells suggests direct reciprocal inhibitory control. Anatomical connections exist between PV and CCK cells, but paired recordings have suggested that only CCK to PV innervation, and not the inverse, has functional relevance (Karson et al., 2009Karson M.A. Tang A.H. Milner T.A. Alger B.E. Synaptic cross talk between perisomatic-targeting interneuron classes expressing cholecystokinin and parvalbumin in hippocampus.J Neurosci. 2009; 29: 4140-4154Crossref PubMed Scopus (92) Google Scholar). In contrast, the authors observed direct PV inhibitory control of Sncg/CCK cells with optogenetics/patch clamp electrophysiology in acute slice and calcium imaging in vivo, demonstrating that activation of PV cells is sufficient to suppress the increase in Sncg/CCK activity during immobility. Having previously eluded investigators, this surprising finding suggests that PV cells contribute to disinhibitory microcircuit motifs in addition to their well-characterized roles in feedforward and feedback inhibition of PCs. It will require further study to determine if this functional connection is selective to Sncg cells or particular CCK subpopulations. The inverse activity patterns of PV and CCK cells suggest they subserve entirely distinct functions. PV cell activity was highly correlated with the activity of the network at large, as well as ensembles of place cells encoding the animal's position. This observation is consistent with the prominent role PV cells play in organizing synchronous network oscillations underlying memory encoding and consolidation. Increased cellular activity during theta oscillations and SWRs is consistent with a highly interconnected cell with a prominent role in feedback and feedforward inhibition. In contrast, the activity profile of CCK cells suggests they instead contribute to broad network silencing during disorganized periods of network activity. The mechanism underlying this hand-off in inhibitory tone warrants further examination. It is unclear if the reliable and sustained increase in Sncg/CCK activity during immobility is principally due to PV cell disinhibition and if disinhibition alone could cause a sustained increase over seconds. The authors addressed an important possibility that increased Sncg/CCK activity during disorganized periods could be driven by increased excitatory drive. Calcium responses were recorded from the principal CA1 inputs, including lateral and medial entorhinal cortex (LEC and MEC, respectively) and hippocampal areas CA2 and CA3. During transitions from running to immobility, when Sncg/CCK cells underwent a dramatic increase in activity, most glutamatergic axons showed no change. However, a small fraction of excitatory fibers from LEC and CA2 were recruited, indicating the behavioral increase in Sncg/CCK cell activity may have both excitatory and disinhibitory underpinnings. The contributions of long-range inhibitory and neuromodulatory input to CCK cells also require further study. Additionally, future studies will hopefully shed light on the makeup of this Sncg/CCK cell population and determine if similar motifs are present in dendritic-targeting CCK cells. But even considering these outstanding questions, Dudok et al., 2021Dudok B. Klein P.M. Hwaun E. Lee B.R. Yao Z. Fong O. Bowler J.C. Terada S. Sparks F.T. Szabo G.G. et al.Alternating sources of perisomatic inhibition during behavior.Neuron. 2021; 109 (this issue): 997-1012Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar provide a compelling description of CCK function in vivo, a notoriously challenging cell to study. The Sncg mouse represents a significant advance for the scientific community in understanding this under-explored neuronal population. The exciting observation that PV and CCK activities are strongly anti-correlated on the behavioral timescale of seconds has eluded prior studies and suggests fundamentally distinct in vivo function. The surprising disinhibitory role of PV cells provides a mechanism for this behavioral switch of CCK cells, and the observation that CCK cells are highly active during disorganized periods of activity proposes a function for the curious feature of asynchronous release. A major outstanding question in contemporary neuroscience is why the brain displays such vast neuronal diversity. For what purpose has the brain developed over twenty distinct inhibitory subpopulations in the CA1 region alone? The present study offers intriguing hints. PV and SOM (somatostatin) interneurons, deriving from the medial ganglionic eminence (MGE), are considered to represent the lion's share of control over feedback and feedforward circuits. Their cellular physiology is tuned such that they are well positioned to function as gatekeepers of activity. In contrast, cells derived from the caudal ganglionic eminence (CGE) do not appear to play this role. CGE-derived VIP (vasoactive intestinal peptide) cells have a higher level of control, functioning to disinhibit circuits, predominantly through SOM innervation. CGE-derived neurogliaform cells are known to provide blanket inhibition via volume transmission. CCK cells may play a similar role in providing an idling basal inhibitory tone. Notably, Dudok et al., 2021Dudok B. Klein P.M. Hwaun E. Lee B.R. Yao Z. Fong O. Bowler J.C. Terada S. Sparks F.T. Szabo G.G. et al.Alternating sources of perisomatic inhibition during behavior.Neuron. 2021; 109 (this issue): 997-1012Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar were able to reduce the level of CCK-mediated inhibition through optogenetics, suggesting this idling inhibitory tone may function as a rheostat. Moreover, these findings are supportive of the suggested dichotomy of perisomatic inhibition, with a timing-related role for PV cells and a mood-related role for CCK cells (Freund and Katona, 2007Freund T.F. Katona I. Perisomatic inhibition.Neuron. 2007; 56: 33-42Abstract Full Text Full Text PDF PubMed Scopus (458) Google Scholar). While significantly more effort is required to elucidate the in vivo function of all neuronal subpopulations, the present study represents a substantial step forward and provides further evidence for a fundamentally distinct role for MGE- and CGE-derived interneurons. Alternating sources of perisomatic inhibition during behaviorDudok et al.NeuronFebruary 1, 2021In BriefDudok et al. show a complementary perisomatic inhibitory system consisting of CCK- and PV-expressing interneurons that are active at distinct times in an alternating fashion during spontaneous behaviors. Due to PV to CCK inhibition, CCK basket cells are the most active when PV and pyramidal cells are relatively silent. Full-Text PDF Open Archive