Viruses have proved instrumental to elucidating neuronal connectivity relationships in a variety of organisms. Recent advances in genetic technologies have facilitated analysis of neurons directly connected to a defined starter population. These advances have also made viral transneuronal mapping available to the broader neuroscience community, where one-step rabies virus mapping has become routine. This method is commonly used to identify inputs onto defined cell populations, to demonstrate the quantitative proportion of inputs coming from specific brain regions, or to compare input patterns between two or more cell populations. Furthermore, the number of inputs labeled is often assumed to reflect the number of synaptic connections, and these viruses are commonly believed to label strong synapses more efficiently than weak synapses. While these maps are often interpreted to provide a quantitative estimate of the synaptic landscape onto starter cell populations, in fact very little is known about how transneuronal transmission takes place. We do not know how these viruses transmit between neurons, if they display biases in the cell types labeled, or even if transmission is synapse-specific. In this review, we discuss the experimental evidence against or in support of key concepts in viral tracing, focusing mostly on the use of one-step rabies input mapping and related methods. Does spread of these viruses occur specifically through synaptic connections, preferentially through synapses, or non-specifically? How efficient is viral transneuronal transmission, and is this efficiency equal in all cell types? And lastly, to what extent does viral labeling reflect functional connectivity?

病毒已被证明有助于阐明各种生物体中的神经元连接关系。遗传技术的最新进展促进了对直接连接到确定的起始种群的神经元的分析。这些进展也使病毒跨神经元图谱可用于更广泛的神经科学界，其中一步狂犬病病毒图谱已成为常规。该方法通常用于识别已定义细胞群的输入，证明来自特定大脑区域的输入的定量比例，或比较两个或多个细胞群之间的输入模式。此外，通常假设标记的输入数量反映了突触连接的数量，并且这些病毒通常被认为比弱突触更有效地标记强突触。虽然这些图通常被解释为提供对起始细胞群突触景观的定量估计，但实际上对跨神经元传递是如何发生的知之甚少。我们不知道这些病毒如何在神经元之间传播，它们是否在标记的细胞类型中表现出偏差，或者即使传播是突触特异性的。在这篇综述中，我们讨论了反对或支持病毒追踪关键概念的实验证据，主要集中在使用一步狂犬病输入映射和相关方法。这些病毒的传播是通过突触连接特异性发生的，优先通过突触发生，还是非特异性发生？病毒跨神经元传播的效率如何，这种效率在所有细胞类型中都相同吗？最后，