Ultrastructural Localization of the Vesicular Monoamine Transporter-2 in Midbrain Dopaminergic Neurons: Potential Sites for Somatodendritic Storage and Release of Dopamine

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Abstract
Midbrain dopaminergic neurons are known to release dopamine from somata and/or dendrites located in the substantia nigra (SN) and the ventral tegmental area (VTA). There is considerable controversy, however, about the subcellular sites for somatodendritic dopamine storage in these regions. In the present study, we used dual-labeling electron microscopic immunocytochemistry to localize the vesicular monoamine transporter-2 (VMAT2), a novel marker for sites of intracellular monoamine storage, within identified dopaminergic (tyrosine hydroxylase-containing) neurons in the rat SN and VTA. In dopaminergic perikarya, immunogold labeling for VMAT2 was localized to the Golgi apparatus, tubulovesicles that resembled smooth endoplasmic reticulum (SER), and the limiting membranes of multivesicular bodies. In dopaminergic dendrites, VMAT2 was extensively localized to tubulovesicles that resembled saccules of SER, and less frequently localized to isolated small synaptic vesicles (SSVs) or large dense-core vesicles (DCVs). In rare cases, VMAT2-immunoreactive SSVs were clustered within the cytoplasm of an SN or a VTA dendrite. Dopaminergic dendrites in the VTA contained a significantly higher number of immunogold particles for VMAT2 per unit area than those in the SN. Together, these observations support the proposal that dopamine is stored in and may be released from dendritic SSVs and DCVs, but suggest that the SER is the major site of dopamine storage within midbrain dopaminergic neurons. In addition, they provide new evidence that dopaminergic dendrites in the VTA may have greater potential for reserpine-sensitive storage and release of dopamine than those in the SN.