Marks Carolyn A, Cheng Kai, Cummings Diana M, Belluscio Leonardo
Developmental Neural Plasticity Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Neurosci. 2006 Nov 1;26(44):11257-66. doi: 10.1523/JNEUROSCI.2805-06.2006.
In mammals, each olfactory bulb contains two mirror-symmetric glomerular maps. Isofunctional glomeruli within each bulb are specifically linked through a set of reciprocal intrabulbar projections (IBPs) to form an intrabulbar map. We injected neural tracers into the glomerular layer on one side of the bulb and examined the resulting projection on the opposite side. In adult mice, the size of the projection tuft is directly proportional to the size of the injected region. Using this ratio as a measure of IBP maturity, we find an immature 5:1 projection to injection ratio at 1 week of age that gradually refines to a mature 1:1 by 7 weeks. Moreover, whereas the glomerular map is able to form despite the elimination of odorant-induced activity, the intrabulbar map shows clear activity dependence for its precise formation. Here we show through experiments with both naris-occluded and anosmic mice that odorant-induced activity is not required to establish IBPs but is crucial for projection refinement. In contrast, increased glomerular activation through exposure to distinct odorants during map development can accelerate the refinement of projections associated with the activated glomeruli. These findings illustrate a clear role for odorant-induced activity in shaping the internal circuitry of the bulb. Interestingly, activity deprivation can alter the organization of both the developing and the mature map to the same degree, demonstrating that intrabulbar map plasticity is maintained into adulthood with no discernible critical period.
在哺乳动物中，每个嗅球包含两个镜像对称的肾小球图谱。每个嗅球内功能相同的肾小球通过一组相互的球内投射（IBP）特异性连接，形成一个球内图谱。我们将神经示踪剂注入嗅球一侧的肾小球层，并检查另一侧产生的投射。在成年小鼠中，投射簇的大小与注射区域的大小成正比。以这个比例作为IBP成熟度的指标，我们发现1周龄时投射与注射的比例为不成熟的5:1，到7周时逐渐细化为成熟的1:1。此外，尽管消除了气味诱导的活动，肾小球图谱仍能形成，但球内图谱的精确形成显示出明显的活动依赖性。在这里，我们通过对鼻孔堵塞和嗅觉缺失小鼠的实验表明，气味诱导的活动对于建立IBP不是必需的，但对于投射细化至关重要。相反，在图谱发育过程中通过暴露于不同气味剂增加肾小球激活，可以加速与激活的肾小球相关的投射细化。这些发现说明了气味诱导的活动在塑造嗅球内部回路中的明确作用。有趣的是，活动剥夺可以在相同程度上改变发育中和成熟图谱的组织，表明球内图谱可塑性在成年期得以维持，没有明显的关键期。
