Bank M, Schacher S
Center for Neurobiology and Behavior, Columbia University, College of Physicians and Surgeons, New York, New York 10032.
J Neurosci. 1992 Aug;12(8):2960-72. doi: 10.1523/JNEUROSCI.12-08-02960.1992.
Sensory cells of Aplysia form chemical synapses with the motor cell L7 in culture. Under certain conditions, sensory cells will also form electrical connections with each other. Sites of chemical synaptic interaction between the sensory cells and L7 are located at varicosities along sensory cell processes that overlie the main axons of L7, since these structures have been shown ultrastructurally to contain active zones. Previous studies have suggested that the distribution of sensory cell varicosities can be restricted to exclusive regions of the motor cell by the presence of other sensory cells. We wished to investigate (1) how this segregated pattern is generated over time and (2) whether electrical coupling between sensory cells has an effect on this segregated pattern. Using fluorescent dye injection and low-light video microscopy, we visualized the distribution of varicosities for each of two sensory cells growing on L7. In cases in which sensory cells are not electrically coupled, the varicosities from these two cells are spatially segregated on the target after 4 d in culture but not after 2 d in culture. Examination of the varicosity distribution of the same sensory cells on the second and third day of growth indicated both an increased rate in the elimination of varicosities from previously occupied areas and a restriction of varicosity formation in new areas of the target when a second sensory cell is present. For sensory cells that are electrically coupled, varicosities from these cells were not spatially segregated on the target even after 4 d in culture. These observations in vitro suggest that segregation of synaptic inputs by Aplysia sensory cells, which show little spontaneous activity of action potentials, can emerge over time via a process that includes both the elimination of existing sensory varicosities and the restriction of new varicosity formation. Our results also suggest that electrical connections between presynaptic cells can disrupt the segregation of their varicosities on a target, resulting in significant changes in the developing connectivity.
在培养条件下，海兔的感觉细胞与运动神经元L7形成化学突触。在某些条件下，感觉细胞之间也会形成电连接。感觉细胞与L7之间化学突触相互作用的位点位于感觉细胞突起的膨体处，这些突起覆盖在L7的主要轴突上，因为超微结构研究表明这些结构含有活性区。先前的研究表明，其他感觉细胞的存在可使感觉细胞膨体的分布局限于运动神经元的特定区域。我们希望研究：（1）这种分离模式是如何随时间产生的；（2）感觉细胞之间的电耦合是否会对这种分离模式产生影响。我们采用荧光染料注射和弱光视频显微镜技术，观察了在L7上生长的两个感觉细胞中每个细胞膨体的分布情况。在感觉细胞没有电耦合的情况下，培养4天后，这两个细胞的膨体在靶细胞上发生空间分离，但培养2天后没有分离。对生长第二天和第三天的同一感觉细胞的膨体分布进行检查发现，当存在第二个感觉细胞时，先前占据区域的膨体消除速率增加，且靶细胞新区域的膨体形成受到限制。对于电耦合的感觉细胞，即使培养4天后，这些细胞的膨体在靶细胞上也没有发生空间分离。这些体外观察结果表明，几乎没有动作电位自发活动的海兔感觉细胞的突触输入分离，可通过一个包括消除现有感觉膨体和限制新膨体形成的过程随时间出现。我们的结果还表明，突触前细胞之间的电连接可破坏其膨体在靶细胞上的分离，导致发育中的连接性发生显著变化。
