Seiji Komagata

Enduring critical period plasticity visualized by transcranial flavoprotein imaging in mouse primary visual cortex

Experience-dependent plasticity in the visual cortex was investigated using transcranial flavoprotein fluorescence imaging in mice anesthetized with urethane. On- and off-responses in the primary visual cortex were elicited by visual stimuli. Fluorescence responses and field potentials elicited by grating patterns decreased similarly as contrasts of visual stimuli were reduced. Fluorescence responses also decreased as spatial frequency of grating stimuli increased. Compared with intrinsic signal imaging in the same mice, fluorescence imaging showed faster responses with ∼10 times larger signal changes. Retinotopic maps in the primary visual cortex and area LM were constructed using fluorescence imaging. After monocular deprivation (MD) of 4 d starting from postnatal day 28 (P28), deprived eye responses were suppressed compared with nondeprived eye responses in the binocular zone but not in the monocular zone. Imaging faithfully recapitulated a critical period for plasticity with maximal effects of MD observed around P28 and not in adulthood even under urethane anesthesia. Visual responses were compared before and after MD in the same mice, in which the skull was covered with clear acrylic dental resin. Deprived eye responses decreased after MD, whereas nondeprived eye responses increased. Effects of MD during a critical period were tested 2 weeks after reopening of the deprived eye. Significant ocular dominance plasticity was observed in responses elicited by moving grating patterns, but no long-lasting effect was found in visual responses elicited by light-emitting diode light stimuli. The present results indicate that transcranial flavoprotein fluorescence imaging is a powerful tool for investigating experience-dependent plasticity in the mouse visual cortex.

Visual Map Shifts based on Whisker-Guided Cues in the Young Mouse Visual Cortex

Mice navigate nearby space using their vision and whiskers, and young mice learn to integrate these heterogeneous inputs in perceptual space. We found that cortical responses were depressed in the primary visual cortex of young mice after wearing a monocular prism. This depression was uniformly observed in the primary visual cortex and was eliminated by whisker trimming or lesions in the posterior parietal cortex. Compensatory visual map shifts of responses elicited via the eye that had worn the prism were also observed. As a result, cortical responses elicited via each eye were clearly separated when a visual stimulus was placed in front of the mice. A comparison of response areas before and after prism wearing indicated that the map shifts were produced by depression with spatial eccentricity. Visual map shifts based on whisker-guided cues may serve as a model for investigating the cellular and molecular mechanisms underlying higher sensory integration in the mammalian brain.

Initial Phase of Neuropathic Pain within a Few Hours after Nerve Injury in Mice

We tested a hypothesis that the spinal plasticity induced within a few hours after nerve injury may produce changes in cortical activities and an initial phase of neuropathic pain. Somatosensory cortical responses elicited by vibratory stimulation were visualized by transcranial flavoprotein fluorescence imaging in mice. These responses were reduced immediately after cutting the sensory nerves. However, the remaining cortical responses mediated by nearby nerves were potentiated within a few hours after nerve cutting. Nerve injury induces neuropathic pain. In the present study, mice exhibited tactile allodynia 1–2 weeks after nerve injury. Lesioning of the ipsilateral dorsal column, mediating tactile cortical responses, abolished somatic cortical responses to tactile stimuli. However, nontactile cortical responses appeared in response to the same tactile stimuli within a few hours after nerve injury, indicating that tactile allodynia was acutely initiated. We investigated the trigger mechanisms underlying the cortical changes. Endogenous glial cell line-derived neurotrophic factor (GDNF), found in the Meissner corpuscles, induced basal firing ∼0.1 Hz or less in its Aβ tactile afferents, and disruption of the basal firing triggered the potentiation of nontactile cortical responses. Application of 10 nm LY341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid], a specific antagonist of group II metabotropic glutamate receptors (mGluRs), on to the surface of the spinal cord also induced the potentiation of nontactile cortical responses. Together, it is suggested that low-frequency afferent firing produced by GDNF in touch-sensitive nerve fibers continuously activated spinal group II mGluRs and that failure of this activation triggered tactile allodynia.

Restoration of Contralateral Representation in the Mouse Somatosensory Cortex after Crossing Nerve Transfer

Avulsion of spinal nerve roots in the brachial plexus (BP) can be repaired by crossing nerve transfer via a nerve graft to connect injured nerve ends to the BP contralateral to the lesioned side. Sensory recovery in these patients suggests that the contralateral primary somatosensory cortex (S1) is activated by afferent inputs that bypassed to the contralateral BP. To confirm this hypothesis, the present study visualized cortical activity after crossing nerve transfer in mice through the use of transcranial flavoprotein fluorescence imaging. In naïve mice, vibratory stimuli applied to the forepaw elicited localized fluorescence responses in the S1 contralateral to the stimulated side, with almost no activity in the ipsilateral S1. Four weeks after crossing nerve transfer, forepaw stimulation in the injured and repaired side resulted in cortical responses only in the S1 ipsilateral to the stimulated side. At eight weeks after crossing nerve transfer, forepaw stimulation resulted in S1 cortical responses of both hemispheres. These cortical responses were abolished by cutting the nerve graft used for repair. Exposure of the ipsilateral S1 to blue laser light suppressed cortical responses in the ipsilateral S1, as well as in the contralateral S1, suggesting that ipsilateral responses propagated to the contralateral S1 via cortico-cortical pathways. Direct high-frequency stimulation of the ipsilateral S1 in combination with forepaw stimulation acutely induced S1 bilateral cortical representation of the forepaw area in naïve mice. Cortical responses in the contralateral S1 after crossing nerve transfer were reduced in cortex-restricted heterotypic GluN1 (NMDAR1) knockout mice. Functional bilateral cortical representation was not clearly observed in genetically manipulated mice with impaired cortico-cortical pathways between S1 of both hemispheres. Taken together, these findings strongly suggest that activity-dependent potentiation of cortico-cortical pathways has a critical role for sensory recovery in patients after crossing nerve transfer.

Spinal mechanisms underlying potentiation of hindpaw responses observed after transient hindpaw ischemia in mice

Transient ischemia produces postischemic tingling sensation. Ischemia also produces nerve conduction block that may modulate spinal neural circuits. In the present study, reduced mechanical thresholds for hindpaw-withdrawal reflex were found in mice after transient hindpaw ischemia, which was produced by a high pressure applied around the hindpaw for 30 min. The reduction in the threshold was blocked by spinal application of LY354740, a specific agonist of group II metabotropic glutamate receptors. Neural activities in the spinal cord and the primary somatosensory cortex (S1) were investigated using activity-dependent changes in endogenous fluorescence derived from mitochondrial flavoproteins. Ischemic treatment induced potentiation of the ipsilateral spinal and contralateral S1 responses to hindpaw stimulation. Both types of potentiation were blocked by spinal application of LY354740. The contralateral S1 responses, abolished by lesioning the ipsilateral dorsal column, reappeared after ischemic treatment, indicating that postischemic tingling sensation reflects a sensory modality shift from tactile sensation to nociception in the spinal cord. Changes in neural responses were investigated during ischemic treatment in the contralateral spinal cord and the ipsilateral S1. Potentiation already appeared during ischemic treatment for 30 min. The present findings suggest that the postischemic potentiation shares spinal mechanisms, at least in part, with neuropathic pain.

Nociceptive cortical responses during capsaicin-induced tactile allodynia in mice with spinal dorsal column lesioning

We investigated nociceptive cortical responses using transcranial flavoprotein fluorescence imaging in anesthetized mice with capsaicin-induced allodynia. Tactile stimuli applied to the hindpaw produced fluorescence increases in the contralateral somatosensory cortex of naïve mice. Lesioning of the ipsilateral dorsal column in the spinal cord abolished most of the cortical responses. However, the responses to the same tactile stimuli appeared again after capsaicin was injected into the hindpaw. The capsaicin treatment reduced the thresholds of the hindpaw withdrawal responses. These findings strongly suggest that the responses to tactile stimuli in the lesioned mice after capsaicin injection represented nociceptive cortical responses.

Neuropathic pain is triggered by disruption of GDNF-induced firing at very low frequencies in peripheral sensory nerves

Brachial plexus (BP) injury is sometimes repaired by nerve crossing for bypassing the injured sites. Successful functional recovery after such operation suggests the presence of some plasticity after nerve crossing. To test this hypothesis, we investigated somatosensory cortical responses after nerve crossing in mice using transcranial flavoprotein fluorescence imaging. Vibratory stimuli applied to the left forepaw elicited bilateral cortical responses. Photo-inactivation of the left cortex suppressed the left and right responses, indicating involvement of callosal fibers for producing the right cortical responses. The right cortical responses after the nerve crossing were reduced in cortex-specific, heterotypic NR1 knockout mice, indicating that experience-dependent plasticity in inter-hemispheric pathways has an important role for functional recovery after nerve crossing in patients with BP injury.

Imaging of somatosensory cortical responses elicited by neuropathic pain in mice

S2-9-1-2 Imaging of somatosensory cortical responses elicited by neuropathic pain in mice Katsuei Shibuki 1 , Seiji Komagata 1, Shanlin Chen 1,2, Akiko Suzuki 3, Haruyoshi Yamashita 1,2, Ryuichi Hishida 1, Takeyasu Maeda 3, Minoru Shibata 2 1 Dept. Neurophysiol., Brain Res. Inst., Niigata Univ., Niigata 2 Dept. Plastic Surgery, Sch. Med., Niigata Univ., Niigata 3 Dept. Oral Bio. Sci., Sch. Dent., Niigata Univ., Niigata

Crossing nerve transfer in the brachial plexus produces bilateral somatosensory cortical representation in mice

Neocortical GABAergic interneurons are roughly classified into three subgroups and distinguished by chemical markers, such as parvalbumin (PV), somatostatin (SS) and the others. PV-expressing neurons, fast-spiking neurons, are a major component of GABAergic interneurons in the neocortex and have been implicated in higher order functions, such as learning and memory, by generating gamma frequency oscillations. We previously generated BAC transgenic mice expressing dendritic membrane-targeted GFP selectively in PV-expressing neurons, and succeeded in visualizing the somata and dendrites in a Golgi stain-like fashion. By combining the immunofluorescence labeling of GABAergic terminals with the antibody to vesicular GABA transporter, we revealed that GABAergic terminals preferentially apposed to the proximal dendrites and somata. It is, however, unclear which type of GABAergic interneurons innervates the proximal dendrites and somata of PV-expressing neurons. In the present study, we visualize the axon terminals of PVor SS-expressing neurons by immunofluorescence staining in the transgenic mice, observe the close appositions to PV-expressing neurons under confocal laser-scanning microscope, and analyze the synaptic inputs quantitatively. These experiments would provide new insights into the local circuits composed by neocortical interneurons.

Nerve crossing produces bilateral somatosensory cortical representation in mice

Brachial plexus (BP) injury is sometimes repaired by nerve crossing for bypassing the injured sites. Successful functional recovery after such operation suggests the presence of some plasticity after nerve crossing. To test this hypothesis, we investigated somatosensory cortical responses after nerve crossing in mice using transcranial flavoprotein fluorescence imaging. Vibratory stimuli applied to the left forepaw elicited bilateral cortical responses. Photo-inactivation of the left cortex suppressed the left and right responses, indicating involvement of callosal fibers for producing the right cortical responses. The right cortical responses after the nerve crossing were reduced in cortex-specific, heterotypic NR1 knockout mice, indicating that experience-dependent plasticity in inter-hemispheric pathways has an important role for functional recovery after nerve crossing in patients with BP injury.