Functionality of spines and filopodia-like processes located on dendrites of midbrain dopaminergic neurons

FARE Award Winner

Authors

• TA Hage
• Y Sun
• ZM Khaliq

Abstract

Dendritic spines are small protuberances proposed to play a role in compartmentalization of chemical and electrical signaling at synapses. In midbrain dopamine neurons, anatomical evidence for the presence of dendritic spines has been mixed with some studies observing the presence of spine-like processes while others concluding that dopamine neurons are largely aspiny. Furthermore, functional characterization of spines has not yet been established. Here, we report that dendritic spines of highly variable length were present on dopamine neurons visualized in live slices as well as in Golgi-stained tissue and tissue prepared by fast transcardial perfusion fixation. To test the functionality of dendritic spines, we used a combination of Ca2+ imaging, two-photon glutamate uncaging, fluorescence recovery after photobleaching (FRAP) measurements and electrophysiology. Local electrical stimulation resulted in spine-restricted Ca2+ signals, consistent with the presence of functional synapses located on spine heads. FRAP experiments showed that the time-constant of fluorescence indicator diffusion into the spine head slowed with neck length, consistent with stronger chemical compartmentalization in long-necked spines. Lastly, we used simultaneous two-photon glutamate uncaging to activate individual spines. We found no relationship between the amplitude of uncaging-evoked Ca2+ signals and spine length. However, we found an inverse correlation between the amplitude of the uncaging-evoked EPSP (uEPSP) and spine length, with longer spines resulting in substantially smaller uEPSPs. These results are an important initial step toward understanding the function of spiny synapses in synaptically-driven excitability in dopamine neurons.

Scientific Focus Area: Neuroscience

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