Pyramidal Neurons In Motor Cortex Research Articles

Increased glutamatergic synaptic transmission during development in layer II/III mouse motor cortex pyramidal neurons.

Postnatal maturation of the motor cortex is vital to developing a variety of functions, including the capacity for motor learning. The first postnatal weeks involve many neuronal and synaptic changes, which differ by region and layer, likely due to different functions and needs during development. Motor cortex layer II/III is critical to receiving and integrating inputs from somatosensory cortex and generating attentional signals that are important in motor learning and planning. Here, we examined the neuronal and synaptic changes occurring in layer II/III pyramidal neurons of the mouse motor cortex from the neonatal (postnatal day 10) to young adult (postnatal day 30) period, using a combination of electrophysiology and biochemical measures of glutamatergic receptor subunits. There are several changes between p10 and p30 in these neurons, including increased dendritic branching, neuronal excitability, glutamatergic synapse number and synaptic transmission. These changes are critical to ongoing plasticity and capacity for motor learning during development. Understanding these changes will help inform future studies examining the impact of early-life injury and experiences on motor learning and development capacity.

Changes in electrophysiological properties of pyramidal neuron in motor cortex during the postnatal early development of mice

To investigate the electrophysiological properties of pyramidal neurons in mouse motor cortex during the early postnatal development. Thirty-six mice were randomly divided into postnatal 1-, 2-, 3-Week and 1-, 2-,3-Month groups (n=6). Membrane properties, action potentials (AP) and spontaneous excitatory postsynaptic currents (sEPSCs) of motor cortex pyramidal neurons were recorded to evaluate the changes in the intrinsic electrophysilogical characteristics by using whole cell patch clamp. Pyramidal neurons and interneurons were distinguished according to the AP firing patterns. Comparing with interneurons, pyramidal neurons exhibited regular spiking (RS) with smaller frequency. During the period of postnatal 1 Week-3 Months, some of the intrinsic membrane properties of motor cortex pyramidal neurons changed. Compared to the 1-Week mice, the resting membrane potential (RMP) of 2-Week decreased significantly (P＜0.01), and the membrane input resistance (Rin) of 1-Month got a hyperpolarization (P＜0.01), and they showed no significant change in the next period, while the membrane capacitance (Cm) showed no significant changes during the whole postnatal development. The AP dynamic properties changed significantly during this period. Compared to the 1-Week mice, the absolute value of the AP threshold and the AP amplitude of the 3-Week increased significantly (P＜0.01), while the spike half width of the 2-Week decreased substantially (P＜0.05), and they showed no significant change in the next period. The sEPSCs frequency and amplitude of 1- Month increased significantly compared to the 1-Week mice(P＜0.01), while during the period of next 1 Month-3 Months, the amplitude and frequency showed no significant change. These results suggest that the motor cortex pyramidal neurons have time-specific eletrophysilogical properties during the postnatal development. The electrophysiological properties can be used as a functional index to detect the degree of neurons maturity, and as a marker to distinguish the pyramidal neurons and interneurons.

Oxidative Stress as a Potential Mechanism Underlying Membrane Hyperexcitability in Neurodegenerative Diseases.

Neurodegenerative diseases are characterized by gradually progressive, selective loss of anatomically or physiologically related neuronal systems that produce brain damage from which there is no recovery. Despite the differences in clinical manifestations and neuronal vulnerability, the pathological processes appear to be similar, suggesting common neurodegenerative pathways. It is well known that oxidative stress and the production of reactive oxygen radicals plays a key role in neuronal cell damage. It has been proposed that this stress, among other mechanisms, could contribute to neuronal degeneration and might be one of the factors triggering the development of these pathologies. Another common feature in most neurodegenerative diseases is neuron hyperexcitability, an aberrant electrical activity. This review, focusing mainly on primary motor cortex pyramidal neurons, critically evaluates the idea that oxidative stress and inflammation may be involved in neurodegeneration via their capacity to increase membrane excitability.

Age-Dependent Vulnerability to Oxidative Stress of Postnatal Rat Pyramidal Motor Cortex Neurons.

Oxidative stress is one of the main proposed mechanisms involved in neuronal degeneration. To evaluate the consequences of oxidative stress on motor cortex pyramidal neurons during postnatal development, rats were classified into three groups: Newborn (P2–P7); infantile (P11–P15); and young adult (P20–P40). Oxidative stress was induced by 10 µM of cumene hydroperoxide (CH) application. In newborn rats, using the whole cell patch-clamp technique in brain slices, no significant modifications in membrane excitability were found. In infantile rats, the input resistance increased and rheobase decreased due to the blockage of GABAergic tonic conductance. Lipid peroxidation induced by CH resulted in a noticeable increase in protein-bound 4-hidroxynonenal in homogenates in only infantile and young adult rat slices. Interestingly, homogenates of newborn rat brain slices showed the highest capacity to respond to oxidative stress by dramatically increasing their glutathione and free thiol content. This increase correlated with a time-dependent increase in the glutathione reductase activity, suggesting a greater buffering capacity of newborn rats to resist oxidative stress. Furthermore, pre-treatment of the slices with glutathione monoethyl ester acted as a neuroprotector in pyramidal neurons of infantile rats. We conclude that during maturation, the vulnerability to oxidative stress in rat motor neurons increases with age.

Fear conditioning and extinction induce opposing changes in dendritic spine remodeling and somatic activity of layer 5 pyramidal neurons in the mouse motor cortex

Multiple brain regions including the amygdala and prefrontal cortex are crucial for modulating fear conditioning and extinction. The primary motor cortex is known to participate in the planning, control, and execution of voluntary movements. Whether and how the primary motor cortex is involved in modulating freezing responses related to fear conditioning and extinction remains unclear. Here we show that inactivation of the mouse primary motor cortex impairs both the acquisition and extinction of freezing responses induced by auditory-cued fear conditioning. Fear conditioning significantly increases the elimination of dendritic spines on apical dendrites of layer 5 pyramidal neurons in the motor cortex. These eliminated spines are further apart from each other than expected from random distribution along dendrites. On the other hand, fear extinction causes the formation of new spines that are located near the site of spines eliminated previously after fear conditioning. We further show that fear conditioning decreases and fear extinction increases somatic activities of layer 5 pyramidal neurons in the motor cortex respectively. Taken together, these findings indicate fear conditioning and extinction induce opposing changes in synaptic connections and somatic activities of layer 5 pyramidal neurons in the primary motor cortex, a cortical region important for the acquisition and extinction of auditory-cued conditioned freezing responses.

Preferential stabilization of newly formed dendritic spines in motor cortex during manual skill learning predicts performance gains, but not memory endurance.

Previous findings that skill learning is associated with the formation and preferential stabilization of new dendritic spines in cortex have raised the possibility that this preferential stabilization is a mechanism for lasting skill memory. We investigated this possibility in adult mice using in vivo two-photon imaging to monitor spine dynamics on superficial apical dendrites of layer V pyramidal neurons in motor cortex during manual skill learning. Spine formation increased over the first 3 days of training on a skilled reaching task, followed by increased spine elimination. A greater proportion of spines formed during the first 3 training days were lost if training stopped after 3, compared with 15 days. However, performance gains achieved in 3 training days persisted, indicating that preferential new spine stabilization was non-essential for skill retention. Consistent with a role in ongoing skill refinement, the persistence of spines formed early in training strongly predicted performance improvements. Finally, while we observed no net spine density change on superficial dendrites, the density of spines on deeper apical branches of the same neuronal population was increased regardless of training duration, suggestive of a potential role in the retention of the initial skill memory. Together, these results indicate dendritic subpopulation-dependent variation in spine structural responses to skill learning, which potentially reflect distinct contributions to the refinement and retention of newly acquired motor skills.

Oxidative stress induced by cumene hydroperoxide produces synaptic depression and transient hyperexcitability in rat primary motor cortex neurons.

Pyramidal neurons of the motor cortex are selectively degenerated in Amyotrophic Lateral Sclerosis (ALS). The mechanisms underlying neuronal death in ALS are not well established. In the absence of useful biomarkers, the early increased neuronal excitability seems to be the unique characteristic of ALS. Lipid peroxidation caused by oxidative stress has been postulated as one of the possible mechanisms involved in degeneration motor cortex pyramidal neurons. This paper examines the effect of lipid peroxidation on layer V pyramidal neurons induced by cumene hydroperoxide (CH) in brain slices from wild type rats. CH induces a synaptic depression of pyramidal neurons in a time dependent manner, already observable on GABAergic synaptic transmission after 5min application of the drug. Altogether, our whole-cell patch-clamp recording data suggest that the functional changes induced by CH upon pyramidal neurons are due to pre- and postsynaptic mechanisms. CH did not alter mEPSCs or mIPSCs, but decreased the frequency, amplitude, and decay rate of spontaneous EPSCs and IPSCs. These effects may be explained by a presynaptic mechanism causing a decrease in action potential-dependent neurotransmitter release. Additionally, CH induced a postsynaptic inward current that underlies a membrane depolarization. Depressing the input flow from the inhibitory premotor interneurons causes a transient hyperexcitability (higher resistance and lower rheobase) in pyramidal neurons of the motor cortex by presumably altering a tonic inhibitory current. These findings, which resemble relevant cortical pathophysiology of ALS, point to oxidative stress, presumably by lipid peroxidation, as an important contributor to the causes underlying this disease.

Intracortical Microstimulation (ICMS) Activates Motor Cortex Layer 5 Pyramidal Neurons Mainly Transsynaptically.

Intracortical microstimulation (ICMS) is a technique used for a number of purposes including the derivation of cortical movement representations (motor maps). Its application can activate the output layer 5 of motor cortex and can result in the elicitation of body movements depending upon the stimulus parameters used. The extent to which pyramidal tract projection neurons of the motor cortex are activated transsynaptically or directly by ICMS remains an open question. Given this uncertainty in the mode of activation, we used a preparation that combined patch clamp whole-cell recordings from single layer 5 pyramidal neurons and extracellular ICMS in slices of motor cortex as well as a standard invivo mapping technique to ask how ICMS activated motor cortex pyramidal neurons. We measured changes in synaptic spike threshold and spiking rate to ICMS invitro and movement threshold invivo in the presence or absence of specific pharmacological blockers of glutamatergic (AMPA, NMDA and Kainate) receptors and GABAA receptors. With major excitatory and inhibitory synaptic transmission blocked (with DNQX, APV and bicuculline methiodide), we observed a significant increase in the ICMS current intensity required to elicit a movement invivo as well as to the first spike and an 85% reduction in spiking responses invitro. Subsets of neurons were still responsive after the synaptic block, especially at higher current intensities, suggesting a modest direct activation. Taken together our data indicate a mainly synaptic mode of activation to ICMS in layer 5 of rat motor cortex.

Behavioral state differentially regulates input sensitivity and firing rates of motor cortex pyramidal neurons

The primary motor cortex (M1) plays a prominent role in the initiation and control of voluntary movements [1], but the cellular mechanisms regulating motor output are not well understood. Due to its direct link with behavior, M1 is an ideal platform to study how brain state and behavior are related to single neuron dynamics. Here we combined in vivo patch-clamp recordings and computational modeling to characterize the effects of behavioral state on the membrane potential dynamics and integrative properties of layer 5B pyramidal cells in the awake motor cortex. Furthermore, we injected a range of somatic EPSC-like current pulses during quiet wakefulness and movement to investigate how behavioral state affects input-output transformations in layer 5B excitatory neurons. We found that during quiet wakefulness L5B cells (n = 24) display moderate firing rates (5.6+-3.5 Hz) and depolarized membrane potential (-52.9+-4.4 mV). In superficial and deep cortical layers of M1, we show that the membrane potential during quiet wakefulness is characterized by slow fluctuations in the delta-band range (2-4 Hz), resulting in a large Vm standard deviation (3.5+-1.0 mV) that was similar in magnitude to that observed in L2/3 of the barrel cortex of awake mice [2]. By modeling the neuron as a single compartment leaky integrate and fire unit, we estimated that the sum of the average excitatory and inhibitory synaptic conductances were similar in magnitude to the leak conductance ( = 2.2 nS, 3.0 nS, Gl = 5.5 nS) and consistent with values measured in primary visual cortex of awake mice [3]. We classified L5B cells (the main output neurons in M1) into two functional different sub-populations, which either suppressed (3.4+-3.5 Hz, L5Bsupp, n = 10) or enhanced (12.7 +- 5.6 Hz, L5Benh, n = 14) their firing rates during movement. During movement, we observed a global suppression of slow Vm fluctuations resulting in a decrease of the Vm standard deviation in L5Bsupp cells (2.4 +-0.3 mV, p = 0.002). By injecting small somatic EPSC waveforms, we demonstrated that the decrease in Vm variance reduced the input sensitivity of L5Bsupp cells during movement. In contrast, L5Benh cells displayed a net depolarization (to -49.1+-4.2 mV, n = 14, p = 0.0006) due to a moderate increase in average excitatory conductance (ΔGe = +0.7 nS) and enhanced Vm fluctuations in the high frequency band (12-50 Hz, Figure 1B). Based on integrate-and-fire simulations, we estimate that during movement excitatory inputs to L5Benh cells increased by +30%, with enhanced fine time-scale correlations (pairwise correlation coefficient of the inputs is ~1% larger). These changes increased the input sensitivity of L5Benh neurons. Thus we find that behavioural state differentially regulates the membrane potential dynamics and integrative properties of discrete subpopulations of output neurons in M1. Our findings suggest that sensorimotor information is preferentially routed through distinct subpopulations of excitatory neurons during motor behavior. During movement motor output from M1 is controlled by the interplay between a global suppression of slow Vm fluctuations and selective increase in the rate and synchrony of excitatory synaptic inputs.

Effect of temperature on spiking patterns of neocortical layer 2/3 and layer 6 pyramidal neurons

The spiking patterns of neocortical pyramidal neurons are shaped by the conductances in their apical dendrites. We have previously shown that the spiking patterns of layer 5 pyramidal neurons change with temperature, probably because temperature modulates the electrical coupling between somatic and dendritic compartments. Here we determine whether temperature has similar effects on the spiking patterns of layer 2/3 and layer 6 pyramidal neurons in acute slices of mouse primary motor cortex. In both cell types, decreasing temperature led to more irregular spiking patterns. Our results indicate that a decrease in spiking regularity with decreasing temperature, probably mediated by increased electrical coupling between soma and dendrites, is common to all pyramidal neurons in motor cortex.

Reduced GABAergic Inhibition Explains Cortical Hyperexcitability in the Wobbler Mouse Model of ALS

Amyotrophic lateral sclerosis (ALS) is a progressive degenerative disease of the central nervous system. Symptomatic and presymptomatic ALS patients demonstrate cortical hyperexcitability, which raises the possibility that alterations in inhibitory gamma-aminobutyric acid (GABA)ergic system could underlie this dysfunction. Here, we studied the GABAergic system in cortex using patch-clamp recordings in the wobbler mouse, a model of ALS. In layer 5 pyramidal neurons of motor cortex, the frequency of GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents was reduced by 72% in wobbler mice. Also, miniature inhibitory postsynaptic currents recorded under blockade of action potentials were decreased by 64%. Tonic inhibition mediated by extrasynaptic GABA(A) receptors was reduced by 87%. In agreement, we found a decreased density of parvalbumin- and somatostatin-positive inhibitory interneurons and reduced vesicular GABA transporter immunoreactivity in the neuropil. Finally, we observed an increased input resistance and excitability of wobbler excitatory neurons, which could be explained by lack of GABA(A) receptor-mediated influences. In conclusion, we demonstrate decreases in GABAergic inhibition, which might explain the cortical hyperexcitability in wobbler mice.

Pigmented creatine deposits in Amyotrophic Lateral Sclerosis central nervous system tissues identified by synchrotron Fourier Transform Infrared microspectroscopy and X-ray fluorescence spectromicroscopy

Amyotrophic Lateral Sclerosis (ALS) is an untreatable, neurodegenerative disease of motor neurons characterized by progressive muscle atrophy, limb paralysis, dysarthria, dysphagia, dyspnae and finally death. Large motor neurons in ventral horns of spinal cord and motor nuclei in brainstem, large pyramidal neurons of motor cortex and/or large myelinated axons of corticospinal tracts are affected. In recent synchrotron Fourier Transform Infrared microspectroscopy (sFTIR) studies of ALS CNS autopsy tissue, we discovered a small deposit of crystalline creatine, which has a crucial role in energy metabolism. We have now examined unfixed, snap frozen, post-autopsy tissue sections of motor cortex, brain stem, spinal cord, hippocampus and substantia nigra from six ALS and three non-degenerated cases with FTIR and micro-X-ray fluorescence (XRF). Heterogeneous pigmented deposits were discovered in spinal cord, brain stem and motor neuron cortex of two ALS cases. The FTIR signature of creatine has been identified in these deposits and in numerous large, non-pigmented deposits in four of the ALS cases. Comparable pigmentation and creatine deposits were not found in controls or in ALS hippocampus and substantia nigra. Ca, K, Fe, Cu and Zn, as determined by XRF, were not correlated with the pigmented deposits; however, there was a higher incidence of hot spots (Ca, Zn, Fe and Cu) in the ALS cases. The identity of the pigmented deposits remains unknown, although the absence of Fe argues against both erythrocytes and neuromelanin. We conclude that elevated creatine deposits may be indicators of dysfunctional oxidative processes in some ALS cases.

Partial reversal of Rett Syndrome-like symptoms in MeCP2 mutant mice

Rett Syndrome (RTT) is a severe form of X-linked mental retardation caused by mutations in the gene coding for methyl CpG-binding protein 2 (MECP2). Mice deficient in MeCP2 have a range of physiological and neurological abnormalities that mimic the human syndrome. Here we show that systemic treatment of MeCP2 mutant mice with an active peptide fragment of Insulin-like Growth Factor 1 (IGF-1) extends the life span of the mice, improves locomotor function, ameliorates breathing patterns, and reduces irregularity in heart rate. In addition, treatment with IGF-1 peptide increases brain weight of the mutant mice. Multiple measurements support the hypothesis that RTT results from a deficit in synaptic maturation in the brain: MeCP2 mutant mice have sparse dendritic spines and reduced PSD-95 in motor cortex pyramidal neurons, reduced synaptic amplitude in the same neurons, and protracted cortical plasticity in vivo. Treatment with IGF-1 peptide partially restores spine density and synaptic amplitude, increases PSD-95, and stabilizes cortical plasticity to wild-type levels. Our results thus strongly suggest IGF-1 as a candidate for pharmacological treatment of RTT and potentially of other CNS disorders caused by delayed synapse maturation.

Neuronal plasticity induced by self-stimulation rewarding experience in rats — a study on alteration in dendritic branching in pyramidal neurons of hippocampus and motor cortex

Self-stimulation rewarding experience promoted structural changes in pyramidal neurons of the CA3 region of the hippocampus and the Vth layer of the motor cortex in adult male Wistar rats. Self-stimulation experience was allowed for 1 h daily for a duration of 10 days through bipolar electrodes placed bilaterally in lateral hypothalamus and substantia nigra — ventral tegmental area. At the end of 10 days, rats were sacrificed, and rapid Golgi examination of the CA3 hippocampal and layer V pyramidal neurons of the motor cortex was made for a grand total of 1600 neurons from 80 rats divided into 4 groups. The neurons of the self-stimulation experienced (SS) group revealed a significant (ANOVA, F-test) increase in dendritic branching in the perisomatic domains. Such changes were not observed in neurons of sham control (SH), experimenter administered stimulation (EA) and normal control (NC) groups. SS animals also showed a significant increase in the thickness of lacunosum and radiatum laminae of CA3 neurons of the hippocampus. Our results reveal that both limbic and neocortical neurons undergo changes in dendritic branching patterns due to self-stimulation rewarding experience. It is tempting to hypothesize that neuronal plasticity is the result of motivation and learning experienced by rats which underwent self-stimulation.