Synapse-Specific Inhibitory Control of Hippocampal Feedback Inhibitory Circuit

Local circuit and long-range GABAergic projections provide powerful inhibitory control over the operation of hippocampal inhibitory circuits, yet little is known about the input- and target-specific organization of interacting inhibitory networks in relation to their specific functions. Using a combination of two-photon laser scanning photostimulation and whole-cell patch clamp recordings in mice hippocampal slices, we examined the properties of transmission at GABAergic synapses formed onto hippocampal CA1 stratum oriens – lacunosum moleculare (O–LM) interneurons by two major inhibitory inputs: local projection originating from stratum radiatum interneurons and septohippocampal GABAergic terminals. Optical mapping of local inhibitory inputs to O–LM interneurons revealed that vasoactive intestinal polypeptide- and calretinin-positive neurons, with anatomical properties typical of type III interneuron-specific interneurons, provided the major local source of inhibition to O–LM cells. Inhibitory postsynaptic currents evoked by minimal stimulation of this input exhibited small amplitude and significant paired-pulse and multiple-pulse depression during repetitive activity. Moreover, these synapses failed to show any form of long-term synaptic plasticity. In contrast, synapses formed by septohippocampal projection produced higher amplitude and persistent inhibition and exhibited long-term potentiation induced by theta-like activity. These results indicate the input and target-specific segregation in inhibitory control, exerted by two types of GABAergic projections and responsible for distinct dynamics of inhibition in O–LM interneurons. The two inputs are therefore likely to support the differential activity- and brain state-dependent recruitment of hippocampal feedback inhibitory circuits in vivo, crucial for dendritic disinhibition and computations in CA1 pyramidal cells

Properties and function of somatostatin-containing inhibitory interneurons in the somatosensory cortex of the mouse

GABAergic inhibitory interneurons play a pivotal role in balancing neuronal activity in the neocortex. They can be classified into different classes according to their variable morphological, electrophysiological, and neurochemical properties, including two major groups: parvalbumin-containing (PV+), fast-spiking (FS) cells and somatostatin-containing (SOM+) cells. Using transgenic mice, we identified two subgroups, distinct by all criteria, of SOM+ cells in the somatosensory (barrel) cortex of the mouse, one (called X94) in layer 4 and 5B, and the other one (X98) in deep layers (Ma et al., 2006). We found that X98 cells were calbindin-expressing (CB+), infragranular, layer 1--targeting Martinotti cells, and had a propensity to fire low-threshold calcium spikes, whereas X94 cells did not express CB, targeted mostly layer 4, discharged in stuttering pattern and with quasi fast-spiking properties. In the barrel cortex, it was previously shown that SOM+ cells mediate disynaptic inhibition in supragranular and infragranular layers. However, the roles of layer 4 SOM+ cells remain largely unknown. We used dual whole-cell recording to elucidate the synaptic circuits in layer 4 and the function of layer 4 SOM+ cells during cortical network activities. We found that layer 4 X94 SOM+ cells received strongly facilitating excitatory input and generated relatively slow rising inhibitory postsynaptic currents (IPSCs) compared to those evoked by FS cells. Strikingly, our data showed that SOM+ cells mediated strong synaptic inhibition of FS cells with connection probability greater than 90% in layer 4, but received very little reciprocal inhibition from FS cells, and no reciprocal inhibition from other SOM+ cells. Moreover, 100% of recorded SOM+-SOM+ cell pairs were electrically coupled with higher coupling ratio compared to that of electrically coupled FS cell pairs. In order to examine the functions of SOM+ cells, we applied 0 Mg2+ artificial cerebrospinal fluid (ACSF) to induce episodes of cortical network activity and observed that, during episodes of network activity, SOM+ cells fired robustly and synchronously, and produced strong inhibition of regular-spiking (RS) excitatory cells and inhibitory FS cells, especially the latter. Taken together, our data reveal that SOM+ cells in the barrel cortex can be sub-divided into different subtypes, and that layer 4 SOM+ cells exert a powerful inhibitory effect during high frequency network activity

GABA signaling in the thalamus

Inhibition of neuronal activity in networks of the mammalian central nervous system is essential for all fundamental brain functions, ranging from perception, to consciousness, to action. Both exacerbation and diminution of inhibition dramatically affect our behavioral capacities, indicating that, in the healthy brain, strength and dynamics of inhibition must be precisely balanced. Inhibitory functions are primarily accomplished by neurons releasing the neurotransmitter GABA. According to their wide variety of functions, GABAergic neurons show a tremendous diversity in morphological, biochemical and functional characteristics. The combination of these diverse properties allows the brain to generate interneurons acting as, for examples, filters, co-incidence detectors or contrast enhancers. GABAergic signaling in thalamus plays an essential role in controlling sensory information flow from the periphery to the cortical processing centers, and in generating sleep-related neuronal rhythms. Surprisingly, however, the diversity of GABAergic neurons is remarkably limited in thalamic networks. Both functions mentioned have been tightly associated with two homogeneous groups of GABAergic neurons arising within thalamic nuclei or within the nucleus reticularis, a shell of inhibitory nuclei surrounding the dorsal thalamus. The results arising from the present thesis challenge the view that the diversity of GABAergic signaling in thalamus is comparatively limited and proposes that, to fully understand GABAergic signaling in thalamus, at least two additional aspects have to be considered. First, it shows that GABAergic signaling arising from the nucleus reticularis can have a profound effect on the synthesis of second messenger compounds that are important in the control of neuronal rhythmicities and in the statedependent control of gene expression. Second, it demonstrates the functional relevance of a previously undescribed extrathalamic and extrareticular inhibitory pathway that arises within the anterior pretectal nuclei, indicating that the architecture of GABAergic signaling in thalamus has to be complemented by a conceptually novel, powerful afferent pathway. The first part investigates the modulation of cAMP synthesis by GABA in thalamocortical neurons through the activation of the Gi-coupled GABAB receptors. GABAB receptors can provide two different cAMP signals in the neurons. First, GABAB receptor activation depresses the level of cAMP inside thalamocortical neurons. However, a large and long cAMP signal is observed when GABAB receptors are activated concomitantly with b-adrenergic receptors, which are Gscoupled receptors. In the presence of GABAB receptor agonists, the moderate cAMP increase produced by b-adrenergic receptor activation is transformed into a large synthesis of cAMP. Remarkably, the activation of the GABAB receptors at the synapses between reticular neurons and thalamocortical neurons also potentiates the effects of b-adrenergic receptors. Thus, GABAB receptors modulate cAMP signals at synapses that are important for the regulation of the state of arousal. The second part provides the first electrophysiological description of synaptic connections between the anterior pretectum group and the thalamic higher-order nuclei. Electric stimulation in the anterior pretectum group evoked inhibitory postsynaptic responses (IPS) in the thalamocortical neurons of the higher-order nuclei. We showed that the IPS responses were mediated via the GABAA receptors activated through monosynaptic connections between the APT and the higher-order nuclei. Functionally, the anterior pretectum modulated the discharge properties of the thalamocortical neurons, suggesting an important role of this nucleus in the dialogue between the thalamus and the cortex

Focal Augmentation of Somatostatin Interneuron Function and Subsequent Circuit Effects in Developmentally Malformed, Epileptogenic Cortex

Drug-resistant epilepsy (DRE) is a common clinical sequela of developmental cortical malformations such as polymicrogyria. Unfortunately, much remains unknown about the aberrant GABA-mediated circuit alterations that underlie DRE\u27s onset and persistence in this context. To address this knowledge gap, we utilized the transcranial freeze lesion model in optogenetic mice lines (Somatostatin (SST)-Cre or Parvalbumin (PV)-Cre x floxed channelrhodopsin-2) to dissect features of the SST, PV, and pyramidal neuron microcircuit that are potentially associated with DRE. Investigations took place within developmental microgyria’s known pathological substrate, the adjoined and epileptogenic paramicrogyral region (PMR). As well, microcircuit relationships within the previously unexplored range of normal-appearing cortex beyond PMR’s terminus were also interrogated. We previously demonstrated SST interneuron output enhancement onto postsynaptic layer V pyramidal neurons of PMR. Dissertation studies elaborated on this SST-interneuron mediated effect through the utilization of ex vivo slice electrophysiology in conjunction with selective optical activation of either SST or PV interneurons. An ostensible mechanism was identified in the form of a novel structural schematic for SST interneurons of PMR whereby they exhibit wider reaching, within-layer arborization of axons within this pathological substrate. Also, within PMR, SST interneuron output was not enhanced onto postsynaptic layer V PV interneurons, indicating targeting specificity of the SST to pyramidal neuron effect. Moving beyond PMR, past its terminus, SST interneuron output onto layer V pyramidal cells was found to be equivalent to controls, indicating effect focality. Finally, a novel disinhibitory relationship was demonstrated beyond PMR’s terminus, wherein PV interneurons exhibited output enhancement onto postsynaptic layer V SST interneurons. This indicates a putative in vivo mechanism for the PMR-focality of the SST to pyramidal neuron output enhancement scheme. These novel discoveries will provide the field with more context as to the role SST and PV interneurons potentially play in the emergence and/or modulation of drug-resistant epilepsy in and outside the terminus of PMR

Steep, Spatially Graded Recruitment of Feedback Inhibition by Sparse Dentate Granule Cell Activity

The dentate gyrus of the hippocampus is thought to subserve important physiological functions, such as 'pattern separation'. In chronic temporal lobe epilepsy, the dentate gyrus constitutes a strong inhibitory gate for the propagation of seizure activity into the hippocampus proper. Both examples are thought to depend critically on a steep recruitment of feedback inhibition by active dentate granule cells. Here, I used two complementary experimental approaches to quantitatively investigate the recruitment of feedback inhibition in the dentate gyrus. I showed that the activity of approximately 4% of granule cells suffices to recruit maximal feedback inhibition within the local circuit. Furthermore, the inhibition elicited by a local population of granule cells is distributed non-uniformly over the extent of the granule cell layer. Locally and remotely activated inhibition differ in several key aspects, namely their amplitude, recruitment, latency and kinetic properties. Finally, I show that net feedback inhibition facilitates during repetitive stimulation. Taken together, these data provide the first quantitative functional description of a canonical feedback inhibitory microcircuit motif. They establish that sparse granule cell activity, within the range observed in-vivo, steeply recruits spatially and temporally graded feedback inhibition

Signalling properties at single synapses and within the interneuronal network in the CA1 region of the rodent hippocampus

Understanding how the complexity of connections among the neurons in the brain is established and modified in an experience- and activity-dependent way is a challenging task of Neuroscience. Although in the last decades many progresses have been made in characterising the basic mechanisms of synaptic transmission, a full comprehension of how information is transferred and processed by neurons has not been fully achieved. In the present study, theoretical tools and patch clamp experiments were used to further investigate synaptic transmission, focusing on quantal transmission at single synapses and on different types of signalling at the level of a particular interneuronal network in the CA1 area of the rodent hippocampus. The simultaneous release of more than one vesicle from an individual presynaptic active zone is a typical mechanism that can affect the strength and reliability of synaptic transmission. At many central synapses, however, release caused by a single presynaptic action potential is limited to one vesicle (univesicular release). The likelihood of multivesicular release at a particular synapse has been tied to release probability (Pr), and whether it can occur at Schaffer collateral\u2013CA1 synapses, at which Pr ranges widely, is controversial. In contrast with previous findings, proofs of multivesicular release at this synapse have been recently obtained at late developmental stages; however, in the case of newborn hippocampus, it is still difficult to find strong evidence in one direction or another. In order to address this point, in the first part of this study a simple and general stochastic model of synaptic release has been developed and analytically solved. The model solution gives analytical mathematical expressions relating basic quantal parameters with average values of quantities that can be measured experimentally. Comparison of these quantities with the experimental measures allows to determine the most probable values of the quantal parameters and to discriminate the univesicular from the multivesicular mode of glutamate release. The model has been validated with data previously collected at glutamatergic CA3-CA1 synapses in the hippocampus from newborn (P1-P5 old) rats. The results strongly support a multivesicular type of release process requiring a variable pool of immediately releasable vesicles. Moreover, computing quantities that are functions of the model parameters, the mean amplitude of the synaptic response to the release of a single vesicle (Q) was estimated to be 5-10 pA, in very good agreement with experimental findings. In addition, a multivesicular type of release was supported by various experimental evidences: a high variability of the amplitude of successes, with a coefficient of variation ranging from 0.12 to 0.73; an average potency ratio a2/a1 between the second and first response to a pair of stimuli bigger than 1; and changes in the potency of the synaptic response to the first stimulus when the release probability was modified by increasing or decreasing the extracellular calcium concentration. This work indicates that at glutamatergic CA3-CA1 synapses of the neonatal rat hippocampus a single action potential may induce the release of more than one vesicle from the same release site. In a more systemic approach to the analysis of communication between neurons, it is interesting to investigate more complex, network interactions. GABAergic interneurons constitute a heterogeneous group of cells which exert a powerful control on network excitability and are responsible for the oscillatory behaviour crucial for information processing in the brain. They have been differently classified according to their morphological, neurochemical and physiological characteristics. In the second part of this study, whole cell patch clamp recordings were used to further characterize, in transgenic mice expressing EGFP in a subpopulation of GABAergic interneurons containing somatostatin (GIN mice), the functional properties of EGFPpositive cells in stratum oriens of the CA1 region of the hippocampus, in slice cultures obtained from P8 old animals. These cells showed passive and active membrane properties similar to those found in stratum oriens interneurons projecting to stratum lacunosum-moleculare. Moreover, they exhibited different firing patterns which were maintained upon membrane depolarization: irregular (48%), regular (30%) and clustered (22%). Paired recordings from EGFP-positive cells often revealed electrical coupling (47% of the cases), which was abolished by carbenoxolone (200 mM). On average, the coupling coefficient was 0.21 \ub1 0.07. When electrical coupling was particularly strong it acted as a powerful low-pass filter, thus contributing to alter the output of individual cells. The dynamic interaction between cells with various firing patterns may differently control GABAergic signalling, leading, as suggested by simulation data, to a wide range of interneuronal communication. In additional paired recordings of a presynaptic EGFP positive interneuron and a postsynaptic principal cell, trains of action potentials in interneurons rarely evoked GABAergic postsynaptic currents (3/45 pairs) with small amplitude and slow kinetics, and that at 20 Hz exhibited short-term depression. In contrast, excitatory connections between principal cells and EGFP-positive interneurons were found more often (17/55 pairs) and exhibited a frequency and use-dependent facilitation, particularly in the gamma band. In conclusion, it appears that EGFP-positive interneurons in stratum oriens of GIN mice constitute a heterogeneous population of cells interconnected via electrical synapses, exhibiting particular features in their chemical and electrical synaptic signalling. Moreover, the dynamic interaction between these interneurons may differentially affect target cells and neuronal communication within the hippocampal network

Distance-dependent inhibition facilitates focality of gamma oscillations in the dentate gyrus

Gamma oscillations (30–150 Hz) in neuronal networks are associated with the processing and recall of information. We measured local field potentials in the dentate gyrus of freely moving mice and found that gamma activity occurs in bursts, which are highly heterogeneous in their spatial extensions, ranging from focal to global coherent events. Synaptic communication among perisomatic-inhibitory interneurons (PIIs) is thought to play an important role in the generation of hippocampal gamma patterns. However, how neuronal circuits can generate synchronous oscillations at different spatial scales is unknown. We analyzed paired recordings in dentate gyrus slices and show that synaptic signaling at interneuron-interneuron synapses is distance dependent. Synaptic strength declines whereas the duration of inhibitory signals increases with axonal distance among interconnected PIIs. Using neuronal network modeling, we show that distance-dependent inhibition generates multiple highly synchronous focal gamma bursts allowing the network to process complex inputs in parallel in flexibly organized neuronal centers

The thalamocortical symphony:How thalamus and cortex play together in schizophrenia and plasticity

The work presented in this thesis aimed at investigating the function and mechanism of corticothalamic-thalamocortical network in schizophrenia and experience-dependent plasticity, further discussed their possible connection.In Chapter 2, we examined the effects of low-dose ketamine on the corticothalamic circuit (CTC) system. Our findings reveal that ketamine induces abnormal spindle activity and gamma oscillations in the CTC system. Notably, ketamine also leads to a transition in thalamic neurons from burst-firing to tonic action potential mode, which may underlie deficits in spindle oscillations. Chapter 3 addresses sensory perception deficits in schizophrenia, emphasizing disruptions in beta and gamma frequency oscillations due to signal-to-noise ratio imbalances. Chapter 4 explores experience-dependent plasticity, highlighting the role of thalamic synaptic inhibition in ocular dominance plasticity and the influence of cortical feedback. Chapter 5 investigates the involvement of endocannabinoids, particularly CB1 receptors, in inhibitory synaptic maturation and ocular dominance plasticity within the primary visual cortex.The general discussion raises the possibility of a link between neural plasticity and schizophrenia, particularly during the transformative phase of adolescence when the brain undergoes significant changes. An abnormal balance between inhibition and excitation, influenced by GABAergic maturation deficits, connectivity disruptions, and altered perceptual information transfer, may contribute to the development of schizophrenia.This thesis offers valuable insights into the intricate mechanisms underlying schizophrenia, with a particular focus on the CTC circuit, NMDA receptors, and endocannabinoids in the context of neuronal plasticity and cognitive function

The thalamocortical symphony:How thalamus and cortex play together in schizophrenia and plasticity

The work presented in this thesis aimed at investigating the function and mechanism of corticothalamic-thalamocortical network in schizophrenia and experience-dependent plasticity, further discussed their possible connection.In Chapter 2, we examined the effects of low-dose ketamine on the corticothalamic circuit (CTC) system. Our findings reveal that ketamine induces abnormal spindle activity and gamma oscillations in the CTC system. Notably, ketamine also leads to a transition in thalamic neurons from burst-firing to tonic action potential mode, which may underlie deficits in spindle oscillations. Chapter 3 addresses sensory perception deficits in schizophrenia, emphasizing disruptions in beta and gamma frequency oscillations due to signal-to-noise ratio imbalances. Chapter 4 explores experience-dependent plasticity, highlighting the role of thalamic synaptic inhibition in ocular dominance plasticity and the influence of cortical feedback. Chapter 5 investigates the involvement of endocannabinoids, particularly CB1 receptors, in inhibitory synaptic maturation and ocular dominance plasticity within the primary visual cortex.The general discussion raises the possibility of a link between neural plasticity and schizophrenia, particularly during the transformative phase of adolescence when the brain undergoes significant changes. An abnormal balance between inhibition and excitation, influenced by GABAergic maturation deficits, connectivity disruptions, and altered perceptual information transfer, may contribute to the development of schizophrenia.This thesis offers valuable insights into the intricate mechanisms underlying schizophrenia, with a particular focus on the CTC circuit, NMDA receptors, and endocannabinoids in the context of neuronal plasticity and cognitive function