Department of Pharmacology, Nihon University School of Dentistry, 1-8-13 Kanda-Surugadai, Tokyo 101-8310, Japan.
Neuroscience. 2012 Mar 29;206:201-11. doi: 10.1016/j.neuroscience.2012.01.014. Epub 2012 Jan 18.
The gustatory cortex (GC), a part of the insular cortex (IC), receives gustatory inputs from the parvicellular part of the ventroposteromedial thalamic nucleus (VPMpc). Transcallosal projections from the contralateral GC modulate neural responses to gustatory stimulation. However, the spatiotemporal dynamics of the amplitude and area of excitation induced by contralateral GC stimulation remain unclear. First, we demonstrated the distribution patterns of neurons projecting to the GC by injecting the anterograde tracer, biotinylated dextranamine (BDA), and retrograde tracer, Fluorogold (FG), into the unilateral putative GC throughout the layers in five male Sprague-Dawley and two vesicular GABA transporter-Venus rats. FG-labeled pyramidal neurons were found in the contralateral GC and ipsilateral VPMpc. The contralateral GC and ipsilateral VPMpc received BDA-positive fibers, suggesting that the GCs of both hemispheres are reciprocally connected. Second, the spatiotemporal profiles of neural responses evoked by five train pulses of electrical stimulation (50 Hz) were quantified by in vivo optical imaging with a voltage-sensitive dye in male Sprague-Dawley rats (n=56). Stimulation of the ipsilateral VPMpc evoked potent GC activation that was followed by propagation to the surrounding IC; this propagation was similar to that following ipsilateral GC stimulation. Contralateral stimulation of the somatosensory area I, dorsal IC, and ventral IC evoked excitation in the ipsilateral each corresponding area, suggesting that transcallosal fibers are symmetrically connected. Contralateral GC stimulation induced a similar spatial profile of excitation as ipsilateral GC stimulation; however, the latency was longer (~20 ms), and the excitation was frequently followed by a GABA(B) receptor antagonist-sensitive inhibitory signal. Excitation by ipsilateral GC stimulation was potentiated by simultaneous contralateral GC stimulation, especially in cases where the amplitude of the response to ipsilateral stimulation was small. These results suggest that the transcallosal projection may support the detection of gustatory inputs by potentiating weak gustatory signals in the GC.
味觉皮层（GC）是脑岛皮层（IC）的一部分，接收来自腹后内侧核（VPMpc）小细胞部的味觉输入。来自对侧 GC 的胼胝体投射调节对味觉刺激的神经反应。然而，对侧 GC 刺激引起的兴奋幅度和面积的时空动力学尚不清楚。首先，我们通过将顺行示踪剂生物素化葡聚糖胺（BDA）和逆行示踪剂荧光金（FG）注射到单侧假定 GC 的整个层中，在五只雄性 Sprague-Dawley 和两只囊泡 GABA 转运体-荧光素酶大鼠中证明了投射到 GC 的神经元的分布模式。在对侧 GC 和同侧 VPMpc 中发现了 FG 标记的锥体神经元。对侧 GC 和同侧 VPMpc 接收 BDA 阳性纤维，表明两个半球的 GC 是相互连接的。其次，我们通过在雄性 Sprague-Dawley 大鼠（n=56）中使用电压敏感染料进行活体光学成像，量化了由 5 个电刺激（50 Hz）的脉冲串引起的神经反应的时空分布。同侧 VPMpc 的刺激引发了 GC 的强烈激活，随后传播到周围的 IC；这种传播类似于同侧 GC 刺激后的传播。同侧躯体感觉区 I、背侧 IC 和腹侧 IC 的对侧刺激在同侧相应区域引起兴奋，表明胼胝体纤维是对称连接的。对侧 GC 刺激引起的兴奋空间分布与同侧 GC 刺激相似；然而，潜伏期较长（~20 ms），兴奋后常伴有 GABA(B)受体拮抗剂敏感的抑制信号。同侧 GC 刺激的兴奋被同时的对侧 GC 刺激增强，特别是在对侧刺激的反应幅度较小的情况下。这些结果表明，胼胝体投射可能通过增强 GC 中弱味觉信号来支持味觉输入的检测。
