Until now we have highlighted the importance of the HPC and the mechanisms underlying memory formation mainly focusing on glutamatergic transmission. However, the HPC is made of two major neuronal types: Glutamatergic and GABAergic cells. Gamma amino butyric acid (GABA) mediates inhibitory neurotransmission, key to balance excitatory transmission and synchronize neuronal circuits (Soghomonian and Martin, 1998). GABA allows the generation of oscillatory activities and the emergence of cell assemblies that underlie cognitive processes such as learning and memory (Holtmaat and Caroni, 2016).
In contrast to excitatory synapses that occur on dendritic spines on post-synaptic cells (Bourne and Harris, 2008), GABAergic synapses in the hippocampus are formed on dendritic shafts or directly on the cell body and on axon initial segments. At the ultrastructural level, GABAergic synapses are symmetric synapses (type II) in contrast to asymmetric excitatory synapses. In the hippocampus, GABA binds two main types of post-synaptic receptors: GABAA, GABAB; GABAA, receptors are inotropic while GABAB are metabotropic (Bormann, 2000). Postsynaptic GABAA receptors interact with the scaffolding protein Gephyrin (Prior et al., 1992; Luscher and Keller, 2004; Fritschy et al., 2008) that regulates receptor clustering and function (Fritschy et al., 2008).
5.1 Inhibitory plasticity is regulated by neuronal activity
As for glutamatergic synapses, LTP and LTD occur as well at inhibitory synapses, inhibitory LTP (iLTP) and inhibitory LTD (iLTD) (Petrini et al., 2014). Together with synaptic plasticity, we have as well structural plasticity occurring at GABAergic synapses. GABAA
and Gly receptors directly interact with Gephyrin, the main scaffolding protein at inhibitory synapses (the homologous of post-synaptic density, PSD, at excitatory synapses) (Prior et al., 1992; Luscher and Keller, 2004; Fritschy et al., 2008). Gephyrin is a protein highly regulated by phosphorylation which can trigger its clustering at post-synaptic sites and eventually regulate GABAA and Gly receptors (Tyagarajan and Fritschy, 2014). Gephyrin interacts with many different molecules but most of their functions are still unclear. In 2015, Flores et al. reported that Gephyrin phosphorylation induced by increased levels of neuronal activity mediates GABAergic synapses plasticity in hippocampal principal neurons (Flores et al., 2015). NMDAR-dependent form of LTP activity in CA1 pyramidal cells promoted Gephyrin clustering at inhibitory synapses confirmed by correlative electron microscopy images and presented an increase in spontaneous GABAergic activity. This
form of GABAergic plasticity is mediated by CamKII Gephyrin phosphorylation at a specific serine (S305). Together, these results suggest that GABAergic synapses are plastic and undergo morphological and physiological changes in response to neuronal activity (Figure 8). However, there is no direct evidence showing whether iLTP or iLTD regulate memory.
Figure 8. Structural plasticity of GABAergic synapses, adapted from Flores and Mendez (2014).
Neuronal activity triggers structural plasticity of Gephyrin-containing inhibitory synapses. Neuronal activity can induce changes in the size of pre-existing synapses (left panel) or trigger their elimination (shaded grey, right panel). New Gephyrin clusters are formed at different dendritic locations in response to increased level of neuronal activity (right panel)(Flores and Mendez, 2014).
Although GABA has an inhibitory effect at postsynaptic sites in the adult brain, during early development, GABA has a depolarizing effect (Ben-Ari, 2002). Although GABA release is the main source of inhibition via chloride (Cl-) ion channels (GABAAr), it is necessary to realize that the Cl- concentration at inhibitory synapses may have different outcomes on cell activity. This further differentiation consists in hyperpolarizing and shunting inhibition. Importantly, GABA may drive differences in postsynaptic currents according to the synaptic reversal potential of GABAA receptors (Alger and Nicoll, 1979;
Andersen et al., 1980). Indeed, inhibition will be hyperpolarizing if the synaptic reversal potential is below the resting potential. Conversely, a shunting effect is observed when the synaptic reversal potential is between the resting potential and the threshold for AP firing.
At the circuit level, interaction between excitatory and inhibitory cells could be described according to 3 basic modes of functional interaction: Feedforward, feedback and disinhibition (Figure 9).
Figure 9. Basic local circuits in the hippocampus.
A. In the feedforward inhibition, axon collaterals from excitatory afferent fibers (input, black harrow head) contact local interneurons (IN, black ovals) inhibiting principal granule cells (GC, grey circles). B. In the feedback inhibition, axon collaterals from local principal granule cells (GC, grey circles with black borders) contact local interneurons (IN, black ovals), inhibiting principal cell activity following the generation of an output. C. In the disinhibition, some interneuron subtypes (IN, black ovals) contact other interneurons (IN, grey ovals). This pattern of connectivity can serve to impart spatiotemporally coordinated patterns of excitation and inhibition in the local circuit leading to rhythm generation.
5.2 Hippocampal cells diversity and the role of INs cells types
In the HPC, GABA is released by an extremely heterogeneous population of cells collectively known as interneurons (INs) (Freund and Buzsaki, 1996; Klausberger and Somogyi, 2008). Classification of GABAergic cells has been performed according to several criteria, including electrophysiological properties, molecular expression profile, morphology and connectivity pattern. Hippocampal interneurons innervate local principal excitatory cells and other INs. Using anatomical localization, morphology (i.e. their preferential axonal target of principal cells) and chemical identity (Lawrence, 2008) two main groups of INs can be defined (although not exhaustive):
1. A first class of interneurons we find in the HF are the basket cells which express parvalbumin (PV), cholecystokinin (CCK) or Vasoactive intestinal polypeptide (VIP).
Basket cells have their body in the stratum pyramidale (CA) or in the granule cells layer (DG) and have aspiny dendrites that spread in all the layers. Basket cells’ axon exclusively forms synaptic contacts in the soma and proximal dendrites regions of target neurons.
Notably, a single basket cell can receive inputs from more than 2000 different excitatory pyramidal cells whereas each pyramidal cell make only a single synapse with a basket cell (Per Andersen, 2007). Conversely, a single basket cells may contact more than a thousand pyramidal cells (Per Andersen, 2007). Basically, basket cells can be classified as fast-spiking (FS) and non-fast-spiking (NFS) cells (Freund and Katona, 2007). PV-expressing basket cells are known to be FS cells while CCK-PV-expressing cells are described as NFS cells. They have major influence on action potential (AP) generation and synchronization of principal cells. Within the same category, we also find the chandelier
A. Feedforward inhibition B. Feedback inhibition C. Disinhibition
excitatory contact
cells, also called axo-axonic (Figure 10-11). Chandelier cells are in the pyramidal (and granular) cell layer and their dendrites spread in all the different layers. Their axons are found mainly in the pyramidal cell layer where they contact the initial axon segment of pyramidal cells. One single chandelier cell can contact approximately 1’200 principal cells.
2. A second class of interneurons we find in the HF are the oriens lacunosum-moleculare associated cells (O-LM cells) and are found in the stratum oriens and mainly express somatostatin (SST) peptide (Figure 10-11). These types of interneurons have their dendritic arborization confined in the stratum lacunosum-moleculare, while send their axons via the stratum oriens to terminate in the stratum lacunosum-moleculare where they make synapses on distal apical dendrites of principal neurons creating a sort of disynaptic feedback.
This simplistic classification of INs does not highlight the huge diversity that has been used to date to describe them. According to Klausberger and Somogyi, there are at least 21 different INs subtype in the CA1 (Klausberger and Somogyi, 2008). These criteria include: the firing patterns, the distinct subcellular domains innervating principal cells, their molecular expression profile… (Freund and Buzsaki, 1996; Per Andersen, 2007;
Klausberger and Somogyi, 2008).
This huge diversity give INs the ability to support and control hippocampal oscillatory activities such as theta, gamma and ripples, known to underlie cognitive processes such as spatial representation and mnemonic function (Figure 11). Not only, as we will see in the following chapter INs can regulate synaptic plasticity (chapter 5.2.2) and information processing and memory (chapter 5.2.3).
Dendrite-targeting INs Perisomatic-targeting INs
Figure 10. Rodent’s dentate gyrus: morphological classification of interneurons, from Per Anderson et al.
(2007).
Granule cells are drawn in the background. Major inputs and their layer targets are indicated on the right.
Black circles represent the location of major interneurons’ cell bodies, while tick lined indicate the laminar distribution and orientation of the dendritic arborization.
Figure 11. Hippocampal connectivity supporting oscillatory activity, from Klausberger and Somogyi 2008.
On the left, spatiotemporal interaction between hippocampal principal cells (P) and interneurons during network oscillations (PV- expressing basket, axo-axonic, bistratified, O-LM, and three classes of CCK- expressing interneurons). On the right, histograms showing firing probability with different temporal patterns during theta, ripples and gamma oscillations according to the domains of pyramidal cells contacted by the different interneuron types.
5.3 Role of INs diversity in spatial information processing and memory Since INs can differentially modulate principal cells excitatory activity, it is important to dissect how different INs mediate information processing and learning. In 2012, Royer et al. showed that soma or dendrites targeting INs differently control the activity of hippocampal place cells (Royer et al., 2012). Optogenetic silencing of SST- or PV-INs during mice spatial exploration increased CA1 place cells firing rates without changing their preferred place field. PV-INs affected the coupling of place cell firing to hippocampal oscillatory activity to the theta phase while SST-INs inhibition increased burst firing of place cells. More recently, Grienberger et al. (Grienberger et al., 2017) have used in vivo recordings of neuronal activity and computational models to show that inhibition plays a key role in the refinement of CA1 place cells firing during spatial navigation. Indeed, local synaptic inhibition can increase the temporal and rate coding in CA1 place cells by suppressing other excitatory input acting as noise in the coding for spatial location. These two studies suggest that GABA regulates synaptic activity flow in hippocampal circuits mediating spatial information encoding.
An example of how INs types differentially modulated learning has been reported by
the firing of strongly active (disinhibited via CB1 receptors) and weakly active or inactive (still in-hibited by CCK interneurons) pyramidal cells, supporting the implementation of sparse coding in cell assemblies. The sum of PV- and CCK-expressing basket cell activity, together with axo-axonic cell firing, is maximal when pyramidal cell firing is minimal during theta oscillations.
The different spike timing of CCK- and PV-expressing interneurons is likely to be generated by synaptic inputs from distinct sources, thus demonstrating the cooperation of temporal and spatial organization.
In addition, the dendrites of pyramidal cells are also innervated by GABAergic neurogliaform cells, which provide slow GABAAreceptor–mediated (31,32) and also GABABreceptor–mediated in-hibition (33,34). Neurogliaform cells (type 11) innervate the apical dendritic tuft of CA1 py-ramidal cells co-aligned with the entorhinal input, whereas a related cell type, the Ivy cell (type 6), innervates more proximal pyramidal cell den-drites aligned with the CA3 input (Fig. 1). The spatially complementary axonal termination of Ivy and neurogliaform cells is mirrored by dis-tinct spike timing in vivo (35,36). Ivy cells ex-pressing nitric oxide synthase and neuropeptide Y, but neither PV nor CCK, represent the most numerous class of interneuron described so far.
They evoke slow GABAergic inhibition in pyram-idal cells, and through neuropeptide Y signaling
they are likely to modulate glutamate release from terminals of CA3 pyramidal cells, which, in contrast to perforant path terminals, express a high level of Y2 receptor (37). Ivy cells, together with neurogliaform cells, are a major source of nitric oxide, probably released by their extra-ordinarily dense axons. They modulate pre-and postsynaptic excitability at slower time scales and more diffusely than do other inter-neurons providing homeostasis to the network.
How the different firing patterns of distinct GABAergic neurons are generated remains large-ly unknown. For example, since the discovery of axo-axonic cells in 1983 (38), only one gluta-matergic input from CA1 pyramidal cells has been published (39); all other excitatory and inhibitory inputs remain inferential predictions.
Potential candidates for governing the activa-tion of interneurons include differential gluta-matergic and subcortical innervation (40,41), selective GABAergic and electrical coupling between interneurons, cell type–specific mod-ulatory regulation (42), cell type–specific ex-pression of distinct receptors and channels (43–46), or differential input from interneu-rons (Fig. 1, types 19 to 21), which apparently innervate exclusively other interneurons (47,48).
Little is known about the activity of the latter cell types in vivo. Interestingly, the only subcellular pyramidal cell domain that receives GABAergic input from a single source is the axon-initial
seg-ment, which highlights the unique place of axo-axonic cells in the cortex of mammals.
The Coordination of Network States Across Cortical Areas Is Supported by GABAergic Projection Neurons
Many distributed areas of the cerebral cortex participate in each cognitive process. Coordina-tion is supported by shared subcortical pathways and by inareal pyramidal cell projections ter-minating on both pyramidal cells and local GABAergic interneurons. In addition, GABAergic corticocortical connections are also present [e.g., (49)], including those in the temporal lobe (50).
Some neurons (Fig. 1, type 16) project to neighboring hippocampal subfields (51) and/or to the medial septum (type 18) (52), a key structure regulating network states. Recording and labeling GABAergic neurons in vivo revealed a variety of GABAergic projection neurons (50). Hippo-camposeptal neurons (type 18) also send thick, myelinated axons to the subiculum and other retrohippocampal areas; other GABAergic cells (types 15 and 17) project only to retrohippocampal areas, parallel with glutamatergic CA1 pyramidal cells. Because these projection cells fire rhyth-mically during sharp wave-associated ripple and gamma oscillations, they contribute to tempo-ral organization across the septohippocampal-subicular circuit. In addition, other GABAergic projection neurons (Fig. 1, type 12) emit long-P
CCK cells Axo-axonic cell Bistratified cell
Subcortical
Fig. 2.Spatiotemporal interaction between pyramidal cells and several classes of interneuron during network oscillations, shown as a schematic summary of the main synaptic connections of pyramidal cells (P), PV-expressing basket, axo-axonic, bistratified, O-LM, and three classes of CCK-expressing interneurons. The firing probability histograms show that
interneurons innervating different domains of pyramidal cells fire with distinct temporal patterns during theta and ripple oscillations, and their spike timing is coupled to field gamma oscillations to differing degrees. The same somatic and dendritic domains receive differentially timed input from several types of GABAergic interneuron (18,19,23,30). ACh, acetylcholine.
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Wolff and Gründemann (Wolff et al., 2014). By using optogenetic stimulations and in vivo recordings, they reported that two principal INs in the basolateral amygdala (BLA), the PV- and SST-expressing INs, bidirectionally modulate learning of auditory fear conditioning in mice via two separate mechanisms of disinhibition. In particular, during auditory cue, SST INs inhibition via activation of PV-INs disinhibited dendrites of principal neurons and promotes cue-shock association through increased auditory responses. On the other hand, shocks presentation resulted in both PV- and SST- INs inhibition and enhanced principal cells responses. This study reveal how different inhibitory circuits convey specific and distinct information to principal cells and that their precise interaction mediates learning.
Another study from Lovett-Barron et al. reported that contextual fear learning is regulated by the activation of CA1 SST-INs affecting principal cells activity (Lovett-Barron et al., 2014). By using in vivo imaging together with pharmacogenetics and optogenetics, they revealed that SST-INs mediate dendritic filtering of aversive stimuli information (US) in principal pyramidal CA1. Indeed, silencing of SST-INs during US presentation disrupted fear learning by activating more CA1 pyramidal cells.
Chen et al. (2015) have shown that motor learning induces subtype-specific plasticity in inhibitory circuits (Chen et al., 2015). They revealed by using in vivo imaging in mice, that motor learning induced spine reorganization in principal cortical cells coincides with a reduction in SST+ INs mediated inhibition in dendrites of principal cells and an increase in PV-INs mediated contacts on perisomatic regions of the same cells. Moreover, bidirectionally optogenetic manipulations of SST-INs altered principal cells spines and affected motor learning. This study suggests that dendritic SST-INs inhibition of principal neurons regulate learning also in the cortex.
In addition to the role of INs during memory encoding, a recent study from Dejean et al.
suggest that a specific population of INs, PV-INs in the dmPFC control memory recall and the expression of learned behavioral response (Dejean et al., 2016). The formation of functional ensemble of neurons in the dmPFC seems to underlie expression of conditioned fear in mice by the development of 4Hz oscillations. Optogenetic inhibition of PV-INs at different phases of the 4Hz oscillations can bidirectionally control fear responses in mice.
These results reveal that specific INs mediates, via phase-specific disinhibition mechanisms, the formation of cell assemblies involved in fear behavior.
Cichon and Gan (2015) showed that synaptic plasticity is induced by branch-specific dendritic Ca2+ spikes in pyramidal neurons of the mouse motor cortex (Cichon and Gan, 2015). In their study the revealed that dendritic Ca2+ spikes on pyramidal neurons of the
cortex induced by different motor learning tasks, resulted in potentiation of dendritic spines. Different motor tasks activate different branches and allow learning. However, under conditions of SST-INs inactivation, different motor tasks resulted in activation of the same dendritic branches. These findings reveal that, by controlling the generation of Ca2+
spikes on specific dendritic-branches, SST-INs play a major role in long-term plasticity required for discriminative learning.