Structural determinants underlying the high efficacy of synaptic transmission and plasticity at synaptic boutons in layer 4 of the adult rat 'barrel cortex'

Excitatory layer 4 (L4) neurons in the ‘barrel field’ of the rat somatosensory cortex represent an important component in thalamocortical information processing. However, no detailed information exists concerning the quantitative geometry of synaptic boutons terminating on these neurons. Thus, L4 synaptic boutons were investigated using serial ultrathin sections and subsequent quantitative 3D reconstructions. In particular, parameters representing structural correlates of synaptic transmission and plasticity such as the number, size and distribution of pre- and postsynaptic densities forming the active zone (AZ) and of the three functionally defined pools of synaptic vesicles were analyzed. L4 synaptic boutons varied substantially in shape and size; the majority had a single, but large AZ with opposing pre- and postsynaptic densities that matched perfectly in size and position. More than a third of the examined boutons showed perforations of the postsynaptic density. Synaptic boutons contained on average a total pool of 561 ± 108 vesicles, with ~5 % constituting the putative readily releasable, ~23 % the recycling, and the remainder the reserve pool. These pools are comparably larger than other characterized central synapses. Synaptic complexes were surrounded by a dense network of fine astrocytic processes that reached as far as the synaptic cleft, thus regulating the temporal and spatial glutamate concentration, and thereby shaping the unitary EPSP amplitude. In summary, the geometry and size of AZs, the comparably large readily releasable and recycling pools, together with the tight astrocytic ensheathment, may explain and contribute to the high release probability, efficacy and modulation of synaptic transmission at excitatory L4 synaptic boutons. Moreover, the structural variability as indicated by the geometry of L4 synaptic boutons, the presence of mitochondria and the size and shape of the AZs strongly suggest that synaptic reliability, strength and plasticity is governed and modulated individually at excitatory L4 synaptic boutons

Structural and functional determinants of synaptic transmission and plasticity at layer 4 synapses in the neocortex

Synapses are the contact sites between neurons in the central nervous system and represent the key elements for synaptic transmission and plasticity within a given connection. In the present study input synapses terminating on morphologically and physiologically identified spiny neurons in layer 4 of the “barrel field” of the adult rat somatosensory cortex were investigated. These neurons play a key role in cortical information processing since they are the main recipients of sensory signals from the rat whisker pad. The knowledge about structural and functional properties of intra- and translaminar synaptic connections of layer 4 neurons of the somatosensory cortex has steadily increased. In contrast, relatively little is known about the structure of cortical synapses terminating on layer 4 excitatory neurons, in particular their quantitative geometry. Synaptic connections between L4 excitatory neurons, namely between spiny stellate neurons, are one of the most powerful connections in the neocortex as indicated by a low coefficient of variation and a very low failure rate. To understand these functional characteristics a detailed morphological description of those parameters underlying synaptic transmission and plasticity is an essential pre-requisite. Therefore, structural key elements of synapses such as the size of synaptic boutons, the number and size of active zones, the structural correlate of transmitter release sites, and the size and organization of the pool of synaptic vesicles were obtained from 3D reconstructions based on serial ultrathin sections and digital electron microscopic images through the dendritic domain of layer 4 excitatory neurons. Furthermore, using Freeze Fracture Replica preparations combined with postimmunogold-labelling the abundance, density, and co-localization of AMPA- and NMDA-receptor subunits at these synapses were analyzed and quantified. Input synapses terminating on dendrites of excitatory layer 4 spiny neurons are relatively small cortical synapse that varied substantially in size and shape. The majority of synapses contained a large, but a single active zone with a perforated and/or nonperforated appearance. Most synaptic vesicles were found within a 150 nm perimeter of the active zone suggesting a relatively large readily releasable and recycling pool. Layer 4 input synapses were ensheathed by a network of astrocytes. Astrocytic processes were seen to reach the active zones as far as the synaptic cleft. The presence of fine glial processes at active zones is therefore likely to prevent glutamate spillover and as a consequence synaptic cross-talk. Glutamatergic receptors from the AMPA- and NMDA-type are present at shaft and spine post-synaptic densities. However, on shaft postsynaptic densities the NMDA-receptors outnumber the AMPA-receptors significantly, resulting in a dominance of the NMDA-receptor component at these synapses. Spine postsynaptic densities are significantly more occupied by AMPA- than NMDA-receptors suggesting that the fast excitatory synaptic transmission at spine postsynaptic densities is mainly mediated by AMPA-receptors rather than by the NMDA-receptor. The results of the present study clearly demonstrated that the structural geometry and the composition of subelements together with the density and distribution of glutamate receptors of the AMPA- and NMDA-type underlie the unique functional characteristics of layer 4 synapses. Clinical and experimental evidence suggest a dysregulation of NMDA-receptor function and glutamatergic pathways in the pathophysiology of schizophrenia. A mouse model for schizophrenia which only expresses 12% of the NR1-subunit of the NMDA-receptor was behaviorally characterized and pharmacologically treated with respect to the negative symptoms of the disorder that are mostly treatment-resistant

Structural and functional determinants of synaptic transmission and plasticity at layer 4 synapses in the neocortex

Synapses are the contact sites between neurons in the central nervous system and represent the key elements for synaptic transmission and plasticity within a given connection. In the present study input synapses terminating on morphologically and physiologically identified spiny neurons in layer 4 of the “barrel field” of the adult rat somatosensory cortex were investigated. These neurons play a key role in cortical information processing since they are the main recipients of sensory signals from the rat whisker pad. The knowledge about structural and functional properties of intra- and translaminar synaptic connections of layer 4 neurons of the somatosensory cortex has steadily increased. In contrast, relatively little is known about the structure of cortical synapses terminating on layer 4 excitatory neurons, in particular their quantitative geometry. Synaptic connections between L4 excitatory neurons, namely between spiny stellate neurons, are one of the most powerful connections in the neocortex as indicated by a low coefficient of variation and a very low failure rate. To understand these functional characteristics a detailed morphological description of those parameters underlying synaptic transmission and plasticity is an essential pre-requisite. Therefore, structural key elements of synapses such as the size of synaptic boutons, the number and size of active zones, the structural correlate of transmitter release sites, and the size and organization of the pool of synaptic vesicles were obtained from 3D reconstructions based on serial ultrathin sections and digital electron microscopic images through the dendritic domain of layer 4 excitatory neurons. Furthermore, using Freeze Fracture Replica preparations combined with postimmunogold-labelling the abundance, density, and co-localization of AMPA- and NMDA-receptor subunits at these synapses were analyzed and quantified. Input synapses terminating on dendrites of excitatory layer 4 spiny neurons are relatively small cortical synapse that varied substantially in size and shape. The majority of synapses contained a large, but a single active zone with a perforated and/or nonperforated appearance. Most synaptic vesicles were found within a 150 nm perimeter of the active zone suggesting a relatively large readily releasable and recycling pool. Layer 4 input synapses were ensheathed by a network of astrocytes. Astrocytic processes were seen to reach the active zones as far as the synaptic cleft. The presence of fine glial processes at active zones is therefore likely to prevent glutamate spillover and as a consequence synaptic cross-talk. Glutamatergic receptors from the AMPA- and NMDA-type are present at shaft and spine post-synaptic densities. However, on shaft postsynaptic densities the NMDA-receptors outnumber the AMPA-receptors significantly, resulting in a dominance of the NMDA-receptor component at these synapses. Spine postsynaptic densities are significantly more occupied by AMPA- than NMDA-receptors suggesting that the fast excitatory synaptic transmission at spine postsynaptic densities is mainly mediated by AMPA-receptors rather than by the NMDA-receptor. The results of the present study clearly demonstrated that the structural geometry and the composition of subelements together with the density and distribution of glutamate receptors of the AMPA- and NMDA-type underlie the unique functional characteristics of layer 4 synapses. Clinical and experimental evidence suggest a dysregulation of NMDA-receptor function and glutamatergic pathways in the pathophysiology of schizophrenia. A mouse model for schizophrenia which only expresses 12% of the NR1-subunit of the NMDA-receptor was behaviorally characterized and pharmacologically treated with respect to the negative symptoms of the disorder that are mostly treatment-resistant

Gamma synchrony: Towards a translational biomarker for the treatment-resistant symptoms of schizophrenia

The lack of efficacy for antipsychotics with respect to negative symptoms and cognitive deficits is a significant obstacle for the treatment of schizophrenia. Developing new drugs to target these symptoms requires appropriate neural biomarkers that can be investigated in model organisms, be used to track treatment response, and provide insight into pathophysiological disease mechanisms. A growing body of evidence indicates that neural oscillations in the gamma frequency range (30–80 Hz) are disturbed in schizophrenia. Gamma synchrony has been shown to mediate a host of sensory and cognitive functions, including perceptual encoding, selective attention, salience, and working memory – neurocognitive processes that are dysfunctional in schizophrenia and largely refractory to treatment. This review summarizes the current state of clinical literature with respect to gamma-band responses (GBRs) in schizophrenia, focusing on resting and auditory paradigms. Next, preclinical studies of schizophrenia that have investigated gamma-band activity are reviewed to gain insight into neural mechanisms associated with these deficits. We conclude that abnormalities in gamma synchrony are ubiquitous in schizophrenia and likely reflect an elevation in baseline cortical gamma synchrony (‘noise’) coupled with reduced stimulus-evoked GBRs (‘signal’). Such a model likely reflects hippocampal and cortical dysfunction, as well as reduced glutamatergic signaling with downstream GABAergic deficits, but is probably less influenced by dopaminergic abnormalities implicated in schizophrenia. Finally, we propose that analogous signal-to-noise deficits in the flow of cortical information in preclinical models are useful targets for the development of new drugs that target the treatment-resistant symptoms of schizophrenia