Motor Training Promotes Both Synaptic and Intrinsic Plasticity of Layer II/III Pyramidal Neurons in the Primary Motor Cortex

Cerebral Cortex, August 2016;26: 3494–3507 doi:10.1093/cercor/bhw134 Advance Access Publication Date: 18 May 2016 Original Article ORIGINAL ARTICLE Motor Training Promotes Both Synaptic and Intrinsic Plasticity of Layer II/III Pyramidal Neurons in the Primary Motor Cortex Hiroyuki Kida1, Yasumasa Tsuda1, Nana Ito1, Yui Yamamoto2, Yuji Owada2, Yoshinori Kamiya3, and Dai Mitsushima1 1 Department of Physiology, 2Department of Organ Anatomy, Yamaguchi University Graduate School of Medicine, 1-1-1 Minami-Kogushi, Ube, Yamaguchi 755-8505, Japan and 3Uonuma Institute of Community Medicine, Niigata University Medical and Dental Hospital, 4132 Urasa, Minami-uonuma, Niigata 949-7302, Japan Address correspondence to Dai Mitsushima, Department of Physiology, Yamaguchi University Graduate School of Medicine, 1-1-1 Minami-Kogushi, Ube, 755-8505, Japan. Email: Abstract Motor skill training induces structural plasticity at dendritic spines in the primary motor cortex (M1). To further analyze both synaptic and intrinsic plasticity in the layer II/III area of M1, we subjected rats to a rotor rod test and then prepared acute brain slices. Motor skill consistently improved within 2 days of training. Voltage clamp analysis showed significantly higher α-amino3-hydroxy-5-methyl-4-isoxazolepropionic acid/N-methyl--aspartate (AMPA/NMDA) ratios and miniature EPSC amplitudes in 1-day trained rats compared with untrained rats, suggesting increased postsynaptic AMPA receptors in the early phase of motor learning. Compared with untrained controls, 2-days trained rats showed significantly higher miniature EPSC amplitude and frequency. Paired-pulse analysis further demonstrated lower rates in 2-days trained rats, suggesting increased presynaptic glutamate release during the late phase of learning. One-day trained rats showed decreased miniature IPSC frequency and increased paired-pulse analysis of evoked IPSC, suggesting a transient decrease in presynaptic γ-aminobutyric acid (GABA) release. Moreover, current clamp analysis revealed lower resting membrane potential, higher spike threshold, and deeper afterhyperpolarization in 1-day trained rats—while 2-days trained rats showed higher membrane potential, suggesting dynamic changes in intrinsic properties. Our present results indicate dynamic changes in glutamatergic, GABAergic, and intrinsic plasticity in M1 layer II/III neurons after the motor training. Key words: AMPA receptor, GABA, glutamic acid, motor learning Introduction The primary motor cortex (M1) is considered a central region required for skilled voluntary movements. During voluntary movements, neurons of the M1 vigorously discharge to encode various parameters, such as force (Evarts 1968), direction (Georgopoulos et al. 1982), and speed of spontaneous movements (Moran and Schwartz 1999). M1 neurons form a glutamatergic/γ-aminobutyric acidergic (GABAergic) neural circuit (Kaneko 2013), and synaptic transmission efficacy can be altered through motor experience. For example, forelimb motor training reportedly strengthens horizontal connections in M1 layer II/III (Rioult-Pedotti et al. 1998, 2000). In vivo imaging studies further demonstrated structural remodeling of dendritic spines of the M1 pyramidal neurons after skilled motor tasks, suggesting the learning-dependent © The Author 2016. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact Motor Training-Dependent Neocortical Plasticity plasticity of the M1 (Xu et al. 2009; Yang et al. 2009; Yu and Zuo 2011; Ma et al. 2016). Dendritic spines express glutamate receptors on their postsynaptic surface (Shi et al. 1999; Takumi et al. 1999). Activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)type glutamate receptors induces fast excitatory transmission enabling memory and enhancement of task-related behavior, and N-methyl--aspartate (NMDA) receptor activation is implicated in maintenance of spatial memory and associative learning (Riedel et al. 2003). Studies using virus-mediated in vivo gene delivery with in vitro patch-clamp recordings have also demonstrated that AMPA receptor delivery into the synapses is involved in synaptic strengthening (Malinow and Malenka 2002; Takahashi et al. 2003). Recently, we further showed that AMPA receptor delivery into CA1 synapses is required for episodic memory (Mitsushima et al. 2011). Since the LTP-like plasticity of M1 contributes to motor skill retention (Rioult-Pedotti et al. 2000; Cantarero et al. 2013), we hypothesized that an increase of postsynaptic AMPA receptors leads to synaptic strengthening among layer II/III neurons. Most research regarding plasticity focuses on excitatory synapses, but GABAergic inhibitory synapses are also strengthened by frequent presynaptic fiber stimulation (Caillard et al. 1999; Kurotani et al. 2008). Experience-dependent plasticity of GABAergic synapses was first reported in the hippocampus (Cui et al. 2008). We further demonstrated that contextual memory requires both AMPA and GABAA receptor-mediated postsynaptic plasticity, forming a wide diversity of excitatory/inhibitory inputs at hippocampal CA1 synapses (Mitsushima et al. 2013). However, motor learning rapidly reduces local GABA concentration in the human sensorimotor cortex (Floyer-Lea et al. 2006), suggesting that skilled motor learning may reduce GABAergic inhibitory transmission in the M1. In the present study, we further examined motor training-dependent plasticity at GABAergic synapses, as well as the balance of excitatory/inhibitory synaptic inputs (Liu 2004). Neuronal excitability after learning is impacted by synaptic plasticity, but also by long-term changes in neuronal properties, such as membrane potential, spike threshold, and afterhyperpolarization (Daoudal and Debanne 2003; Saar and Barkai 2003). In the hippocampus, operant conditioning training changes the intrinsic properties of CA1 neurons, without affecting the resting membrane potential or resistance (Moyer et al. 1996; Saar et al. 1998). Olfactory training reduces afterhyperpolarization to enhance excitability in the piriform cortex, suggesting long-term plasticity of intrinsic excitability (Saar and Barkai 2003). However, it is completely unknown whether motor training changes neuronal properties in the M1. Here we show that an accelerated rotor rod task promoted dynamic changes in the glutamatergic, GABAergic, and intrinsic plasticity in M1 layer II/III neurons. These findings provide functional evidence for the structural remodeling of dendritic spines after the motor skills training. Kida et al. | 3495 performed according to the Guidelines for Animal Experimentation of Yamaguchi University Sc (...truncated)