The Journal of Neuroscience, March 1, 1998, 18(5):1923-1933
Cellular Sites for Activation of 
-Opioid Receptors in the Rat
Nucleus Accumbens Shell: Relationship with
Met5-Enkephalin
Adena L.
Svingos,
Cathy L.
Clarke, and
Virginia M.
Pickel
Department of Neurology and Neuroscience, Division of Neurobiology,
Cornell University Medical College, New York, New York, 10021
The shell compartment of the nucleus accumbens (AcbSh) is
prominently involved in the rewarding aspects of 
-opioid receptor (DOR) agonists, including one of its putative endogenous ligands, Met5-enkephalin (Enk). We examined the
ultrastructural immunocytochemical localization of an antipeptide DOR
antiserum and an antibody against Enk to determine the major cellular
sites for DOR activation and the spatial relationship between DOR and
Enk in this region. Sixty percent of DOR-immunoreactive profiles were
axon terminals and small unmyelinated axons, whereas the remainder were
mainly dendrites and dendritic spines. In axons and terminals, DOR
labeling was distributed along plasma and vesicular membranes.
DOR-containing terminals were mainly large and primarily formed
symmetric synapses or occasionally asymmetric synapses. DOR
immunoreactivity also was associated with terminals that were small and
formed punctate symmetric or nonrecognizable synapses. Dual
immunoperoxidase and immunogold labeling showed that 35% of
DOR-labeled axons apposed other terminals that contained Enk. In
addition, 25% of the DOR-labeled terminals contained Enk. Thirty-five
percent of DOR labeling was observed within dendrites and dendritic
spines. DOR-labeled spines showed intense immunoreactivity within
asymmetric postsynaptic junctions, which were formed by terminals that
lacked Enk immunoreactivity. DOR-labeled spines, however, were apposed
to Enk-containing terminals in 13% of all associations between dually
labeled profiles. These results provide ultrastructural evidence that
activation of DOR in the AcbSh is primarily involved in modulating the
presynaptic release of mainly inhibitory, but also excitatory,
neurotransmitters. These data also suggest that DOR plays a role in
determining the postsynaptic response to excitatory afferents.
Key words:
enkephalin; reinforcement; ultrastructure; electron
microscopy; nucleus accumbens; opiate
Copyright © 1998 Society for Neuroscience 0270-6474/98/1851923-11$05.00/0
