Synaptic communication within the microcircuits of pyramidal neurons and basket cells in the mouse prefrontal cortex.

Basket cells are inhibitory interneurons in cortical structures with the potential to efficiently control the activity of their postsynaptic partners. Although their contribution to higher order cognitive functions associated with the medial prefrontal cortex (mPFC) relies on the characteristics of their synaptic connections, the way that they are embedded into local circuits is still not fully uncovered. Here, we determined the synaptic properties of excitatory and inhibitory connections between pyramidal neurons (PNs), cholecystokinin-containing basket cells (CCKBCs) and parvalbumin-containing basket cells (PVBCs) in the mouse mPFC.

Functionally linked amygdala and prefrontal cortical regions are innervated by both single and double projecting cholinergic neurons.

Cholinergic cells have been proposed to innervate simultaneously those cortical areas that are mutually interconnected with each other. To test this hypothesis, we investigated the cholinergic innervation of functionally linked amygdala and prefrontal cortical regions. First, using tracing experiments, we determined that cholinergic cells located in distinct basal forebrain (BF) areas projected to the different nuclei of the basolateral amygdala (BLA).

Fear learning and aversive stimuli differentially change excitatory synaptic transmission in perisomatic inhibitory cells of the basal amygdala.

Inhibitory circuits in the basal amygdala (BA) have been shown to play a crucial role in associative fear learning. How the excitatory synaptic inputs received by BA GABAergic interneurons are influenced by memory formation, a network parameter that may contribute to learning processes, is still largely unknown. Here, we investigated the features of excitatory synaptic transmission received by the three types of perisomatic inhibitory interneurons upon cue-dependent fear conditioning and aversive stimulus and tone presentations without association.

Structural Organization of Perisomatic Inhibition in the Mouse Medial Prefrontal Cortex.

Perisomatic inhibition profoundly controls neural function. However, the structural organization of inhibitory circuits giving rise to the perisomatic inhibition in the higher-order cortices is not completely known. Here, we performed a comprehensive analysis of those GABAergic cells in the medial prefrontal cortex (mPFC) that provide inputs onto the somata and proximal dendrites of pyramidal neurons.

CaMKIIα Promoter-Controlled Circuit Manipulations Target Both Pyramidal Cells and Inhibitory Interneurons in Cortical Networks.

A key assumption in studies of cortical functions is that excitatory principal neurons, but not inhibitory cells express calcium/calmodulin-dependent protein kinase II subunit α (CaMKIIα) resulting in a widespread use of CaMKIIα promoter-driven protein expression for principal cell manipulation and monitoring their activities. Using neuroanatomical and electrophysiological methods we demonstrate that in addition to pyramidal neurons, multiple types of cortical GABAegic cells are targeted by adeno-associated viral vectors (AAV) driven by the CaMKIIα promoter in both male and female mice. We tested the AAV5 and AAV9 serotype of viruses with either Channelrhodopsin 2 (ChR2)-mCherry or Archaerhodopsin-T-green fluorescent protein (GFP) constructs, with different dilutions.

Hippocampal sharp wave-ripples and the associated sequence replay emerge from structured synaptic interactions in a network model of area CA3.

Hippocampal place cells are activated sequentially as an animal explores its environment. These activity sequences are internally recreated ('replayed'), either in the same or reversed order, during bursts of activity (sharp wave-ripples [SWRs]) that occur in sleep and awake rest. SWR-associated replay is thought to be critical for the creation and maintenance of long-term memory.

Interneuron Types and Their Circuits in the Basolateral Amygdala.

The basolateral amygdala (BLA) is a cortical structure based on its cell types, connectivity features, and developmental characteristics. This part of the amygdala is considered to be the main entry site of processed and multisensory information delivered cortical and thalamic afferents. Although GABAergic inhibitory cells in the BLA comprise only 20% of the entire neuronal population, they provide essential control over proper network operation.

Total Number and Ratio of GABAergic Neuron Types in the Mouse Lateral and Basal Amygdala.

GABAergic neurons are key circuit elements in cortical networks. Despite growing evidence showing that inhibitory cells play a critical role in the lateral (LA) and basal (BA) amygdala functions, neither the number of GABAergic neurons nor the ratio of their distinct types has been determined in these amygdalar nuclei. Using unbiased stereology, we found that the ratio of GABAergic neurons in the BA (22%) is significantly higher than in the LA (16%) in both male and female mice.

Excitation of Diverse Classes of Cholecystokinin Interneurons in the Basal Amygdala Facilitates Fear Extinction.

There is growing evidence that interneurons (INs) orchestrate neural activity and plasticity in corticoamygdala circuits to regulate fear behaviors. However, defining the precise role of cholecystokinin-expressing INs (CCK INs) remains elusive due to the technical challenge of parsing this population from CCK-expressing principal neurons (CCK PNs). Here, we used an intersectional genetic strategy in CCK-Cre;Dlx5/6-Flpe double-transgenic mice to study the anatomical, molecular and electrophysiological properties of CCK INs in the basal amygdala (BA) and optogenetically manipulate these cells during fear extinction.

Vasoactive Intestinal Polypeptide-Immunoreactive Interneurons within Circuits of the Mouse Basolateral Amygdala.

In cortical structures, principal cell activity is tightly regulated by different GABAergic interneurons (INs). Among these INs are vasoactive intestinal polypeptide-expressing (VIP+) INs, which innervate preferentially other INs, providing a structural basis for temporal disinhibition of principal cells. However, relatively little is known about VIP+ INs in the amygdaloid basolateral complex (BLA).

Differential excitatory control of 2 parallel basket cell networks in amygdala microcircuits.

Information processing in neural networks depends on the connectivity among excitatory and inhibitory neurons. The presence of parallel, distinctly controlled local circuits within a cortical network may ensure an effective and dynamic regulation of microcircuit function. By applying a combination of optogenetics, electrophysiological recordings, and high resolution microscopic techniques, we uncovered the organizing principles of synaptic communication between principal neurons and basket cells in the basal nucleus of the amygdala.

Morphological and physiological properties of CCK/CB1R-expressing interneurons in the basal amygdala.

Principal neurons in cortical regions including the basal nucleus of the amygdala (BA) are innervated by several types of inhibitory cells, one of which expresses the neuropeptide cholecystokinin (CCK) and the type 1 cannabinoid receptor (CB1R). CCK/CB1R-expressing interneurons may have a profound impact on amygdalar function by controlling its output. However, very little is known about their properties, and therefore their role in circuit operation cannot be predicted.

Perisomatic GABAergic synapses of basket cells effectively control principal neuron activity in amygdala networks.

Efficient control of principal neuron firing by basket cells is critical for information processing in cortical microcircuits, however, the relative contribution of their perisomatic and dendritic synapses to spike inhibition is still unknown. Using in vitro electrophysiological paired recordings we reveal that in the mouse basal amygdala cholecystokinin- and parvalbumin-containing basket cells provide equally potent control of principal neuron spiking. We performed pharmacological manipulations, light and electron microscopic investigations to show that, although basket cells innervate the entire somato-denditic membrane surface of principal neurons, the spike controlling effect is achieved primarily via the minority of synapses targeting the perisomatic region.

Different output properties of perisomatic region-targeting interneurons in the basal amygdala.

The perisomatic region of principal neurons in cortical regions is innervated by three types of GABAergic interneuron, including parvalbumin-containing basket cells (PVBCs) and axo-axonic cells (AACs), as well as cholecystokinin and type 1 cannabinoid receptor-expressing basket cells (CCK/CB1BCs). These perisomatic inhibitory cell types can also be found in the basal nucleus of the amygdala, however, their output properties are largely unknown. Here, we performed whole-cell recordings in morphologically identified interneurons in slices prepared from transgenic mice, in which the GABAergic cells could be selectively targeted.

Synaptic Organization of Perisomatic GABAergic Inputs onto the Principal Cells of the Mouse Basolateral Amygdala.

Spike generation is most effectively controlled by inhibitory inputs that target the perisomatic region of neurons. Despite the critical importance of this functional domain, very little is known about the organization of the GABAergic inputs contacting the perisomatic region of principal cells (PCs) in the basolateral amygdala. Using immunocytochemistry combined with in vitro single-cell labeling we determined the number and sources of GABAergic inputs of PCs at light and electron microscopic levels in mice.

Tonic endocannabinoid-mediated modulation of GABA release is independent of the CB1 content of axon terminals.

The release of GABA from cholecystokinin-containing interneurons is modulated by type-1 cannabinoid receptors (CB1). Here we tested the hypothesis that the strength of CB1-mediated modulation of GABA release is related to the CB1 content of axon terminals. Basket cell boutons have on average 78% higher CB1 content than those of dendritic-layer-innervating (DLI) cells, a consequence of larger bouton surface and higher CB1 density.

Strategically positioned inhibitory synapses of axo-axonic cells potently control principal neuron spiking in the basolateral amygdala.

Axo-axonic cells (AACs) in cortical regions selectively innervate the axon initial segments (AISs) of principal cells (PCs), where the action potentials are generated. These GABAergic interneurons can alter the activity of PCs, but how the efficacy of spike control correlates with the number of output synapses remains unclear. Moreover, the relationship between the spatial distribution of GABAergic synapses and the action potential initiation site along the AISs is not well defined.

Mechanisms of sharp wave initiation and ripple generation.

Replay of neuronal activity during hippocampal sharp wave-ripples (SWRs) is essential in memory formation. To understand the mechanisms underlying the initiation of irregularly occurring SWRs and the generation of periodic ripples, we selectively manipulated different components of the CA3 network in mouse hippocampal slices. We recorded EPSCs and IPSCs to examine the buildup of neuronal activity preceding SWRs and analyzed the distribution of time intervals between subsequent SWR events.

Anatomically heterogeneous populations of CB1 cannabinoid receptor-expressing interneurons in the CA3 region of the hippocampus show homogeneous input-output characteristics.

A subpopulation of GABAergic cells in cortical structures expresses CB1 cannabinoid receptors (CB1 ) on their axon terminals. To understand the function of these interneurons in information processing, it is necessary to uncover how they are embedded into neuronal circuits. Therefore, the proportion of GABAergic terminals expressing CB1 and the morphological and electrophysiological properties of CB1 -immunoreactive interneurons should be revealed.

Presynaptic calcium channel inhibition underlies CB₁ cannabinoid receptor-mediated suppression of GABA release.

CB1 cannabinoid receptors (CB1) are located at axon terminals and effectively control synaptic communication and thereby circuit operation widespread in the CNS. Although it is partially uncovered how CB1 activation leads to the reduction of synaptic excitation, the mechanisms of the decrease of GABA release upon activation of these cannabinoid receptors remain elusive. To determine the mechanisms underlying the suppression of synaptic transmission by CB1 at GABAergic synapses, we recorded unitary IPSCs (uIPSCs) at cholecystokinin-expressing interneuron-pyramidal cell connections and imaged presynaptic [Ca(2+)] transients in mouse hippocampal slices.

Feedforward inhibition underlies the propagation of cholinergically induced gamma oscillations from hippocampal CA3 to CA1.

Gamma frequency (30-80 Hz) oscillations are implicated in memory processing. Such rhythmic activity can be generated intrinsically in the CA3 region of the hippocampus from where it can propagate to the CA1 area. To uncover the synaptic mechanisms underlying the intrahippocampal spread of gamma oscillations, we recorded local field potentials, as well as action potentials and synaptic currents in anatomically identified CA1 and CA3 neurons during carbachol-induced gamma oscillations in mouse hippocampal slices.

Input-output features of anatomically identified CA3 neurons during hippocampal sharp wave/ripple oscillation in vitro.

Hippocampal sharp waves and the associated ripple oscillations (SWRs) are implicated in memory processes. These network events emerge intrinsically in the CA3 network. To understand cellular interactions that generate SWRs, we detected first spiking activity followed by recording of synaptic currents in distinct types of anatomically identified CA3 neurons during SWRs that occurred spontaneously in mouse hippocampal slices.

Different input and output properties characterize parvalbumin-positive basket and Axo-axonic cells in the hippocampal CA3 subfield.

In the hippocampus, parvalbumin-expressing basket (BC) and axo-axonic cells (AAC) show different discharge patterns during distinct network states, but the cellular mechanisms underlying these differences are not well understood. Using whole-cell patch-clamp techniques, we investigated the single-cell properties and excitatory synaptic features of anatomically identified BCs and AACs in the CA3 region of mouse hippocampal slices. The results showed that BCs had lower threshold for action potential (AP) generation and lower input resistance, narrower AP and afterhyperpolarization than AACs.