Department of Cellular Animal Physiology, Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, Nijmegen, The Netherlands.
Synapse. 2011 Nov;65(11):1222-30. doi: 10.1002/syn.20962. Epub 2011 Jun 17.
On the basis of ultrastructural parameters, the concept was formulated that asymmetric Type I and symmetric Type II synapses are excitatory and inhibitory, respectively. This "functional Gray synapses concept" received strong support from the demonstration of the excitatory neurotransmitter glutamate in Type I synapses and of the inhibitory neurotransmitter γ-aminobutyric acid in Type II synapses, and is still frequently used in modern literature. However, morphological and functional evidence has accumulated that the concept is less tenable. Typical features of synapses like shape and size of presynaptic vesicles and synaptic cleft and presence of a postsynaptic density (PsD) do not always fit the postulated (excitatory/inhibitory) function of Gray's synapses. Furthermore, synapse function depends on postsynaptic receptors and associated signal transduction mechanisms rather than on presynaptic morphology and neurotransmitter type. Moreover, the notion that many synapses are difficult to classify as either asymmetric or symmetric has cast doubt on the assumption that the presence of a PsD is a sign of excitatory synaptic transmission. In view of the morphological similarities of the PsD in asymmetric synapses with membrane junctional structures such as the zonula adherens and the desmosome, asymmetric synapses may play a role as links between the postsynaptic and presynaptic membrane, thus ensuring long-term maintenance of interneuronal communication. Symmetric synapses, on the other hand, might be sites of transient communication as takes place during development, learning, memory formation, and pathogenesis of brain disorders. Confirmation of this idea might help to return the functional Gray synapse concept its central place in neuroscience.
基于超微结构参数，提出了不对称的Ⅰ型和对称的Ⅱ型突触分别为兴奋性和抑制性突触的概念。这种“功能性Gray 突触概念”得到了在Ⅰ型突触中谷氨酸作为兴奋性神经递质和在Ⅱ型突触中γ-氨基丁酸作为抑制性神经递质的证明的有力支持，并且仍然经常在现代文献中使用。然而，形态学和功能证据的积累表明，这个概念的可靠性较差。突触的典型特征，如突触前囊泡和突触间隙的形状和大小以及存在突触后密度（PsD），并不总是符合 Gray 突触假定的（兴奋性/抑制性）功能。此外，突触功能取决于突触后受体和相关的信号转导机制，而不是突触前形态和神经递质类型。此外，许多突触难以归类为不对称或对称的观点，使得人们对 PsD 的存在是兴奋性突触传递的标志这一假设产生了怀疑。鉴于不对称突触中 PsD 与膜连接结构（如粘着带和桥粒）之间的形态相似性，不对称突触可能作为突触后和突触前膜之间的连接物发挥作用，从而确保神经元间通讯的长期维持。另一方面，对称突触可能是在发育、学习、记忆形成和脑疾病发病机制过程中发生的短暂通讯的部位。这一观点的证实可能有助于使功能性 Gray 突触概念在神经科学中占据核心地位。
