Ultrastructural immunocytochemical localization of the N -methyl- d -aspartate receptor and tyrosine hydroxylase in the shell of the rat nucleus accumbens
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Abstract
The N-methyl-D-aspartate (NMDA)-type glutamate receptors in the shell region of the
nucleus accumbens (ACB) have been implicated in the modulation of dopamine release
and in amphetamine-induced neurotoxicity. We used electron microscopic immunocyto-chemistry
to determine the anatomical sites for NMDA-mediated effects of glutamate and for their
potential interactions with dopaminergic afferents identified by the presence of tyrosine
hydroxylase (TH) in this region of the rat brain. Immunogold and immunoperoxidase
methods were used to localize antisera against the R1 subunit of the NMDA receptor
(NMDAR1) alone or combined with TH. In single labeling experiments, approximately
half of the NMDAR1-like immunoreactivity (NMDAR1-LI) was localized to extrasynaptic
plasma membranes of neuronal processes, many (92 out of 215) of which were dendrites,
and only 33 out of 215 were unmyelinated axons or terminals. Surprisingly, the neuronal
labeling of NMDAR1 was almost equaled by that seen in astrocytic processes (88 out
of 215). Dual labeling for TH and NMDAR1 was rarely observed and was only seen in
axons. However, in favorable planes of section, NMDAR1 was noted along intervaricose
segments of axons in which TH was more readily seen in the varicosity. This differential
intra-axonal distribution suggests an underestimation of dual labeling in single coronal
sections through unmyelinated axons and terminals. The TH-immunoreactive terminals
were more often seen apposed to NMDA-immunoreactive astrocytic processes and dendrites.
These results provide the first ultrastructural evidence for presynaptic modulation
of dopamine release by NMDA receptors in the shell of the nucleus accumbens. They
also indicate that NMDA receptors modulate postsynaptic neurons receiving input from
the dopaminergic afferents and suggest a previously unsuspected functional association
involving glial NMDA receptors and dopaminergic afferents in this brain region.