初級感覺皮質內興奮性神經元之可塑性誘導：大鼠觸鬚模型

Abstract

神經可塑性研究過去多著眼於少數神經的突觸間權重變化，如神經活動時間相依可塑性（STDP）研究等，本研究則試圖探討大群神經的功能可塑性。我們使用結構上有良好對應關係（topography）的大鼠觸鬚、大腦皮質桶狀組織系統（whisker barrel system）做為研究模型，引入光遺傳學技術做刺激操弄，並參照神經活動時間相依可塑性之實驗流程，以不同時間間隔重複呈現光刺激和觸鬚物理刺激。觸鬚神經首先根據其反應特性來定義主要觸鬚（principal whisker, PW）與鄰近觸鬚（adjacent whisker, AW），在進行光刺激和物理刺激配對時僅撥動其一（PW or AW）。我們在大鼠的桶狀皮質區域（barrel field）進行胞外電生理記錄，藉由比較配對前與配對後神經活動改變來瞭解是否誘發可塑性。另外，大部分桶狀皮質內的神經都有方向性偏好（direction selectivity），故於配對時只選擇單一固定方向撥動觸鬚，來瞭解觸鬚方向偏好是否也具可塑性。我們針對對光刺激和兩根觸鬚刺激皆有反應的神經進行分析（n = 168），將近一半（n = 82/168）的神經在配對前後的反應有顯著改變，其改變的方向性（上升或下降）與神經在配對前能被誘發反應的機率呈現負相關。我們也發現配對觸鬚刺激的方向性和神經本身方向選擇性的相關會顯著影響可塑性：當配對神經主要觸鬚並在其偏好角度移動時，神經誘發反應會下降，而當配對神經主要觸鬚並在其偏好角度相反方向移動時，神經誘發反應反而會會提升。另外出乎我們意料，刺激配對中兩刺激的時間間隔對神經可塑性的程度及改變方向性沒有任何影響。綜合以上結果，光遺傳學技術可以在大鼠完整觸鬚皮質區產生神經可塑性，神經誘發反應改變的方向，與神經本身的反應機率，以及配對觸鬚移動的方向性有關，但是不受兩配對刺激間的時間與順序影響。

Neuroplasticity, such as spike timing dependent plasticity (STDP), has been investigated primarily based on the change of synaptic weights among few neurons. In this research, we studied the functional plasticity in the topographically organized circuitry of rat whisker barrel system by manipulating activities in a large number of neurons. We hypothesized that though repetitive delivery of stimuli pairs, neural activity would be altered following the STDP rules. Furthermore, we expected neural function such as direction selectivity would also be different after our manipulation. We paired neural activities induced by stimulating a single whisker [principal whisker (PW) or adjacent whisker (AW)] with optogenetic stimulation (100 repeats) with different time delays between them. We recorded extracellularly from neurons in the cortical barrel corresponding to the principal whisker, and measured changes of neuronal activity before and after the parings. Directional selectivity for PW and AW deflections were measured separately by stimulating them in eight directions at 8 Hz before and after pairings. During the pairings, the chosen whisker (PW or AW) was deflected in a fixed direction in order to verify whether the paired direction was a determinant factor in neuroplasticity. Among the 39% of neuronal population (n = 168/430) responded to both optical and physical (PW and AW) stimulation, almost half of them (49%, n = 82/168) shown significant changes in their mean firing rate after the optical-physical pairing. The change of the firing rate is negatively correlated with the cell’s original firing probability. Moreover, the difference between the neuron’s original preferred direction and the paired direction has a strong effect on whether the neuron was upregulated or downregulated. In the PW-pairing condition, the mean response of PW was upregulated when the paired direction was opposite to the neuron’s preferred direction, but was downregulated when the paired direction was the same as the neuron’s preferred direction. To our surprise, the order and the duration between optical stimulation and physical stimulation has no effect on the amplitude and the direction of the change in firing rate. Overall, optical stimulation can induce neuroplasticity in vivo in intact barrel cortex. The change of the induced neuroplasticity by pairing physical and optical stimulations is related to the neuron’s original firing probability and the deflecting direction of the paired whisker, but is not affected by the order and the timing between the two paired stimuli.