Dendritic spines shape up

For some time, the role of the spine neck in dendrites has been controversial. Kasai and colleagues go some way to resolving this debate and propose that the shape of spine necks is crucial for determining dendritic Ca²⁺ signalling, and therefore long-term potentiation.

The regulation of synaptic plasticity depends on increases in intracellular Ca²⁺ ([Ca²⁺]_i), mediated by glutamate receptors that are sensitive to NMDA (N-methyl-D-aspartate). Synaptic function and plasticity are thought to depend on the structure of synapses, and long-term plasticity seems to occur more efficiently in smaller spines. However, the relationship between spine structure, including spine neck structure, and NMDA receptor (NMDAR)-dependent Ca²⁺ signalling has, until now, not been established.

Kasai and colleagues used two-photon excitation to uncage and activate a glutamate compound to specifically apply glutamate to various types of dendritic spine in rat CA1 pyramidal neurons, thereby allowing selective induction of NMDAR-dependent long-term plasticity. They coupled this method with Ca²⁺ imaging to determine the relationship between the structure of spines in these neurons and NMDAR-mediated Ca²⁺ signalling.

First, the authors showed that the extent of NMDAR-mediated current in spines, and therefore, by implication, the expression of functional NMDARs, was positively correlated with the volume of spine heads. Next, they investigated the effects of spine neck structure on Ca²⁺ signalling. They showed that small spines tend to have narrow necks, which hinders the flow of Ca²⁺ into the dendritic shaft, and, consequently, there is a larger increase in [Ca²⁺]_i in the spine compartments relative to concentrations in spines with larger necks.

Although more work is needed to refine our understanding of spine structure, these findings provide a new link between spine structure and synaptic plasticity, and reveal that the spine head and neck have distinct roles in the regulation of NMDAR-mediated Ca²⁺ signalling. The volume of the spine head is important for the regulation of synaptic transmission, possibly through variations in expression levels of functional glutamate receptors. By contrast, the specific structure of the spine neck seems to directly influence synaptic Ca²⁺ signalling in dendrites, and is therefore a determining factor in the selective induction of synaptic plasticity.