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[Display omitted] • Acquisition of motor skills in the rotor rod task varies among individual animals. • Specific properties of M1 layer V neurons correlate with individual performance. • Individual motor performance correlates synaptic plasticity of M1 layer V neurons. • Rapid spine plasticity of M1 layer V neurons correlates with individual performance. • Training-induced ACh increases in M1 correlate with individual motor performance. Layer V neurons in primary motor cortex (M1) are required for motor skill learning. We analyzed training-induced plasticity using a whole-cell slice patch-clamp technique with a rotor rod task, and found that training induces diverse changes in intrinsic properties and synaptic plasticity in M1 layer V neurons. Although the causal relationship between specific cellular changes and motor performance is unclear, by linking individual motor performance to cellular/synaptic functions, we identified several cellular and synaptic parameters that represent acquired motor skills. With respect to cellular properties, motor performance was positively correlated with resting membrane potential and fast afterhyperpolarization, but not with the membrane resistance, capacitance, or threshold. With respect to synaptic function, the performance was positively correlated with AMPA receptor-mediated postsynaptic currents, but not with GABA A receptor-mediated postsynaptic currents. With respect to live imaging analysis in Thy1-YFP mice, we further demonstrated a cross-correlation between motor performance, spine head volume, and self-entropy per spine. In the present study, we identified several changes in M1 layer V pyramidal neurons after motor training that represent acquired motor skills. Furthermore, training increased extracellular acetylcholine levels known to promote synaptic plasticity, which is correlated with individual motor performance. These results suggest that systematic control of specific intracellular parameters and enhancement of synaptic plasticity in M1 layer V neurons may be useful for improving motor skills. [ABSTRACT FROM AUTHOR]

Synaptic heterogeneity is a hallmark of nervous systems that enables complex and adaptable communication in neural circuits. To understand circuit function, it is thus critical to determine the factors that contribute to the functional diversity of synapses. We investigated the contributions of voltage-gated calcium channel (VGCC) abundance, spatial organization, and subunit composition to synapse diversity among and between synapses formed by two closely related Drosophila glutamatergic motor neurons with distinct neurotransmitter release probabilities (Pr). Surprisingly, VGCC levels are highly predictive of heterogeneous Pr among individual synapses of either low- or high-Pr inputs, but not between inputs. We find that the same number of VGCCs are more densely organized at high-Pr synapses, consistent with tighter VGCC-synaptic vesicle coupling. We generated endogenously tagged lines to investigate VGCC subunits in vivo and found that the α2δ–3 subunit Straightjacket along with the CAST/ELKS active zone (AZ) protein Bruchpilot, both key regulators of VGCCs, are less abundant at high-Pr inputs, yet positively correlate with Pr among synapses formed by either input. Consistently, both Straightjacket and Bruchpilot levels are dynamically increased across AZs of both inputs when neurotransmitter release is potentiated to maintain stable communication following glutamate receptor inhibition. Together, these findings suggest a model in which VGCC and AZ protein abundance intersects with input-specific spatial and molecular organization to shape the functional diversity of synapses.

Pronounced differences in neurotransmitter release from a given presynaptic neuron, depending on the synaptic target, are among the most intriguing features of cortical networks. Hippocampal pyramidal cells (PCs) release glutamate with low probability to somatostatin expressing oriens- lacunosum- moleculare (O- LM) interneurons (INs), and the postsynaptic responses show robust short- term facilitation, whereas the release from the same presynaptic axons onto fast- spiking INs (FSINs) is -10- fold higher and the excitatory postsynaptic currents (EPSCs) display depression. The mechanisms underlying these vastly different synaptic behaviors have not been conclusively identified. Here, we applied a combined functional, pharmacological, and modeling approach to address whether the main difference lies in the action potential- evoked fusion or else in upstream priming processes of synaptic vesicles (SVs). A sequential two- step SV priming model was fitted to the peak amplitudes of unitary EPSCs recorded in response to complex trains of presynaptic stimuli in acute hippocampal slices of adult mice. At PC-FSIN connections, the fusion probability (Pfusion) of well- primed SVs is 0.6, and 44% of docked SVs are in a fusion- competent state. At PC-O- LM synapses, Pfusion is only 40% lower (0.36), whereas the fraction of well- primed SVs is 6.5- fold smaller. Pharmacological enhancement of fusion by 4- AP and priming by PDBU was recaptured by the model with a selective increase of Pfusion and the fraction of well- primed SVs, respectively. Our results demonstrate that the low fidelity of transmission at PC-O- LM synapses can be explained by a low occupancy of the release sites by well- primed SVs. [ABSTRACT FROM AUTHOR]

To determine the critical timing for learning and the associated synaptic plasticity, we analyzed developmental changes in learning together with training-induced plasticity. Rats were subjected to an inhibitory avoidance (IA) task prior to weaning. While IA training did not alter latency at postnatal day (PN) 16, there was a significant increase in latency from PN 17, indicating a critical day for IA learning between PN 16 and 17. One hour after training, acute hippocampal slices were prepared for whole-cell patch clamp analysis following the retrieval test. In the presence of tetrodotoxin (0.5 µM), miniature excitatory postsynaptic currents (mEPSCs) and inhibitory postsynaptic currents (mIPSCs) were sequentially recorded from the same CA1 neuron. Although no changes in the amplitude of mEPSCs or mIPSCs were observed at PN 16 and 21, significant increases in both excitatory and inhibitory currents were observed at PN 23, suggesting a specific critical day for training-induced plasticity between PN 21 and 23. Training also increased the diversity of postsynaptic currents at PN 23 but not at PN 16 and 21, demonstrating a critical day for training-induced increase in the information entropy of CA1 neurons. Finally, we analyzed the plasticity at entorhinal cortex layer III (ECIII)-CA1 or CA3-CA1 synapses for each individual rat. At either ECIII-CA1 or CA3-CA1 synapses, a significant correlation between mean α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/N-methyl-D-aspartic acid (AMPA/NMDA) ratio and learning outcomes emerged at PN 23 at both synapses, demonstrating a critical timing for the direct link between AMPA receptor-mediated synaptic plasticity and learning efficacy. Here, we identified multiple critical periods with respect to training-induced synaptic plasticity and delineated developmental trajectories of learning mechanisms at hippocampal CA1 synapses. [ABSTRACT FROM AUTHOR]

Layer V neurons in the primary motor cortex (M1) are important for motor skill learning. Since pretreatment of either CNQX or APV in rat M1 layer V impaired rotor rod learning, we analysed training‐induced synaptic plasticity by whole‐cell patch‐clamp technique in acute brain slices. Rats trained for 1 day showed a decrease in small inhibitory postsynaptic current (mIPSC) frequency and an increase in the paired‐pulse ratio of evoked IPSCs, suggesting a transient decrease in presynaptic GABA release in the early phase. Rats trained for 2 days showed an increase in miniature excitatory postsynaptic current (mEPSC) amplitudes/frequency and elevated AMPA/NMDA ratios, suggesting a long‐term strengthening of AMPA receptor‐mediated excitatory synapses. Importantly, rotor rod performance in trained rats was correlated with the mean mEPSC amplitude and the frequency obtained from that animal. In current‐clamp analysis, 1‐day‐trained rats transiently decreased the current‐induced firing rate, while 2‐day‐trained rats returned to pre‐training levels, suggesting dynamic changes in intrinsic properties. Furthermore, western blot analysis of layer V detected decreased phosphorylation of Ser408–409 in GABAA receptor β3 subunits in 1‐day‐trained rats, and increased phosphorylation of Ser831 in AMPA receptor GluA1 subunits in 2‐day‐trained rats. Finally, live‐imaging analysis of Thy1‐YFP transgenic mice showed that the training rapidly recruited a substantial number of spines for long‐term plasticity in M1 layer V neurons. Taken together, these results indicate that motor training induces complex and diverse plasticity in M1 layer V pyramidal neurons. Key points: Here we examined motor training‐induced synaptic and intrinsic plasticity of layer V pyramidal neurons in the primary motor cortex.The training reduced presynaptic GABA release in the early phase, but strengthened AMPA receptor‐mediated excitatory synapses in the later phase: acquired motor performance after training correlated with the strength of excitatory synapses rather than inhibitory synapses.As to the intrinsic property, the training transiently decreased the firing rate in the early phase, but returned to pre‐training levels in the later phase.Western blot analysis detected decreased phosphorylation of Ser408‐409 in GABAA receptor β3 subunits in the acute phase, and increased phosphorylation of Ser831 in AMPA receptor GluA1 subunits in the later phase.Live‐imaging analysis of Thy1‐YFP transgenic mice showed rapid and long‐term spine plasticity in M1 layer V neurons, suggesting training‐induced increases in self‐entropy per spine. [ABSTRACT FROM AUTHOR]

Multiple synaptic adhesion proteins are thought to collectively define the properties of specific synapses and thereby shape the architectures of neural circuits. Growing evidence supports a molecular model in which a set of central hub proteins interacts with a vast number of other proteins to organize multifarious synaptic adhesion pathways. However, several fundamental open questions remain, partly owing to drawbacks in current approaches and interpretations. In this opinion, we provide an overview of synaptic adhesion pathways, underscoring open questions to be addressed in future work, and highlighting approaches for advancing understanding of synaptic adhesion processes. Synaptic cell adhesion molecules (CAMs) are critical components that determine the properties of neural circuits. Defining canonical functions of synaptic CAMs has been challenging. Several important questions remain incompletely explored, which partly reflects a lack of standardized approaches in the field. We highlight some of the key emerging questions and provide potential avenues for addressing them. [ABSTRACT FROM AUTHOR]

Decades of literature indicate that the AMPA-type glutamate receptor is among the fastest acting of all neurotransmitter receptors. These receptors are located at excitatory synapses, and conventional wisdomsays that they activate in hundreds of microseconds, deactivate in milliseconds due to their low affinity for glutamate and also desensitize profoundly. These properties circumscribe AMPA receptor activation in both space and time. However, accumulating evidence shows that AMPA receptors can also activate with slow, indefatigable responses. They do so through interactions with auxiliary subunits that are able promote a switch to a high open probability, high-conductance 'superactive' mode. In this review, we show that any assumption that this phenomenon is limited to heterologous expression is false and rather that slow AMPA currents have been widely and repeatedly observed throughout the nervous system. Hallmarks of the superactive mode are a lack of desensitization, resistance to competitive antagonists and a current decay that outlives free glutamate by hundreds of milliseconds. Because the switch to the superactive mode is triggered by activation, AMPA receptors can generate accumulating 'pedestal' currents in response to repetitive stimulation, constituting a postsynaptic mechanism for short-term potentiation in the range 5-100 Hz. Further, slow AMPA currents span 'cognitive' time intervals in the 100 ms range (theta rhythms), of particular interest for hippocampal function, where slow AMPA currents are widely expressed in a synapse-specific manner. Here, we outline the implications that slow AMPA receptors have for excitatory synaptic transmission and computation in the nervous system. [ABSTRACT FROM AUTHOR]

Comprehending how the brain functions requires an understanding of the dynamics of neuronal assemblies. Previous work used a mean-field reduction method to determine the collective dynamics of a large heterogeneous network of uniformly and globally coupled theta neurons, which are a canonical formulation of Type I neurons. However, in modeling neuronal networks, it is unreasonable to assume that the coupling strength between every pair of neurons is identical. The goal in the present work is to analytically examine the collective macroscopic behavior of a network of theta neurons that is more realistic in that it includes heterogeneity in the coupling strength as well as in neuronal excitability. We consider the occurrence of dynamical structures that give rise to complicated dynamics via bifurcations of macroscopic collective quantities, concentrating on two biophysically relevant cases: (1) predominantly excitable neurons with mostly excitatory connections, and (2) predominantly spiking neurons with inhibitory connections. We find that increasing the synaptic diversity moves these dynamical structures to distant extremes of parameter space, leaving simple collective equilibrium states in the physiologically relevant region. We also study the node vs. focus nature of stable macroscopic equilibrium solutions and discuss our results in the context of recent literature.

Comprehending how the brain functions requires an understanding of the dynamics of neuronal assemblies. Previous work used a mean-field reduction method to determine the collective dynamics of a large heterogeneous network of uniformly and globally coupled theta neurons, which are a canonical formulation of Type I neurons. However, in modeling neuronal networks, it is unreasonable to assume that the coupling strength between every pair of neurons is identical. The goal in the present work is to analytically examine the collective macroscopic behavior of a network of theta neurons that is more realistic in that it includes heterogeneity in the coupling strength as well as in neuronal excitability. We consider the occurrence of dynamical structures that give rise to complicated dynamics via bifurcations of macroscopic collective quantities, concentrating on two biophysically relevant cases: (1) predominantly excitable neurons with mostly excitatory connections, and (2) predominantly spiking neurons with inhibitory connections. We find that increasing the synaptic diversity moves these dynamical structures to distant extremes of parameter space, leaving simple collective equilibrium states in the physiologically relevant region. We also study the node vs. focus nature of stable macroscopic equilibrium solutions and discuss our results in the context of recent literature. [ABSTRACT FROM AUTHOR]

A stunning example of synaptic diversity is the postsynaptic target cell-type-dependent difference in synaptic efficacy in cortical networks. Here, we show that CA1 pyramidal cell (PC) to fast spiking interneuron (FSIN) connections have 10-fold larger release probability (P v) than those on oriens lacunosum-moleculare (O-LM) interneurons. Freeze-fracture immunolabeling revealed that different nano-topologies and coupling distances between Ca2+ channels and release sites (RSs) are not responsible for the distinct P v. Although [Ca2+] transients are 40% larger in FSINs innervating boutons, when [Ca2+] entry is matched in the two bouton populations, EPSCs in O-LM cells are still 7-fold smaller. However, application of a phorbol ester analog resulted in a ∼2.5-fold larger augmentation at PC – O-LM compared to PC – FSIN synapses, suggesting incomplete docking or priming of vesicles. Similar densities of docked vesicles rule out distinct RS occupancies and demonstrate that incompletely primed, but docked, vesicles limit the output of PC – O-LM synapses. [Display omitted] • Probability of release from pyramidal cells differs depending on the target cell type • Replica immunolabeling reveals similar vesicle - Ca2+ channel coupling distances • At low P v synapses, PDBU is more effective in increasing P v than elevating [Ca2+] • Similar docked vesicle densities suggest distinct priming states underlie different P v Aldahabi et al. demonstrate that the target cell-type-dependent difference in release probability from hippocampal pyramidal cell axons is not the consequence of distinct coupling distances or nano-topologies of docked vesicles and Ca2+ channels. Instead, their data reveal robust differences in the priming state of docked vesicles. [ABSTRACT FROM AUTHOR]

Synapses of the mammalian CNS are diverse in size, structure, molecular composition, and function. Synapses in their myriad variations are fundamental to neural circuit development, homeostasis, plasticity, and memory storage. Unfortunately, quantitative analysis and mapping of the brain's heterogeneous synapse populations has been limited by the lack of adequate single-synapse measurement methods. Electron microscopy (EM) is the definitive means to recognize and measure individual synaptic contacts, but EM has only limited abilities to measure the molecular composition of synapses. This report describes conjugate array tomography (AT), a volumetric imaging method that integrates immunofluorescence and EM imaging modalities in voxel-conjugate fashion. We illustrate the use of conjugate AT to advance the proteometric measurement of EM-validated single-synapse analysis in a study of mouse cortex. [ABSTRACT FROM AUTHOR]