Dendro axonic neurotransmission. II. Morphological sites for the synthesis, binding and release of neurotransmitters in dopaminergic dendrites in the substantia nigra and cholinergic dendrites in the neostriatum

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Morphological evidence is presented indicating sites of synthesis, storage, and release of neurotransmitters in dendrites of dopaminergic cells of the substantia nigra and cholinergic cells of the neostriatum. Smooth endoplasmic reticulum can be identified in dopaminergic neurons touching the dendritic surface. The false transmitter for dopamine, 5-hydroxydopamine (5-OHDA), is localized to smooth endoplasmic reticulum or large vesicular structures which approach the dendritic surface. The dopamine synthesizing enzyme, tyrosine hydroxylase (TH), is localized to microtubules and smooth endoplasmic reticulum which approach the postsynaptic membrane. In the neostriatum, dopaminergic nerve endings make asymmetrical axospinous contacts. The postsynaptic spines often contain a few 'vesicles' near the postsynaptic thickenings. The surface and subsurface structures stain preferentially for choline acetyltransferase (CAT), the synthesizing enzyme for acetylcholine. It is hypothesized that neurotransmitters are released from dendrites as a general phenomenon in the CNS and that they can act upon axonal endings.