Morphologically heterogeneous met-enkephalin terminals form synapses with tyrosine hydroxylase-containing dendrites in the rat nucleus locus coeruleus.
J. Comp. Neurol.. 1995-12-18; 363(3): 423-438
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1. J Comp Neurol. 1995 Dec 18;363(3):423-38.
Morphologically heterogeneous met-enkephalin terminals form synapses with
tyrosine hydroxylase-containing dendrites in the rat nucleus locus coeruleus.
Van Bockstaele EJ(1), Branchereau P, Pickel VM.
(1)Department of Neurology and Neuroscience, Cornell University Medical College,
New York 10021, USA.
Physiological and anatomical studies have suggested that the endogenous opioid
peptide, methionine-enkephalin (ENK), may directly modulate noradrenergic
neurons. Additionally, chronic opiate administration has been shown to increase
the levels of a number of G-proteins and phosphoproteins including the
catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). We combined
immunogold-silver localization of tyrosine hydroxylase and immunoperoxidase
labeling for ENK in single sections through the nucleus locus coeruleus (LC) in
the rostral pons to determine potential substrates for the divergent actions of
this opioid peptide. Light microscopic analysis of ENK immunoreactivity in the LC
area indicated that ENK fibers are dense and highly varicose. In coronal
sections, ENK-immunoreactive processes were punctate and appeared to envelop
LC-cell bodies. More rostrally, in the region of catecholamine-immunoreactive
extranuclear dendrites, ENK-immunoreactive varicose processes were interdigitated
with TH-labeled processes. Electron microscopy of this rostral region revealed
that ENK-immunoreactive axon terminals contained small clear as well as large
dense core vesicles. The large dense core vesicles (1-10/terminal) were
consistently the most immunoreactive and were identified toward the periphery of
the axon terminal distal to the active zone of the synapse. Unlabeled axon
terminals and glial processes were the most commonly observed elements located
adjacent to the plasmalemma of axons containing the labeled dense core vesicles.
Axon terminals containing ENK immunoreactivity varied in size (0.3 micron to 2.0
microns) as well as formation of synaptic specializations (i.e., asymmetric
versus symmetric). The ENK-labeled terminals formed synapses with dendrites with
and without detectable TH immunoreactivity. These results provide the first
direct ultrastructural evidence that morphologically heterogeneous terminals
containing ENK immunoreactivity form synapses with catecholamine dendrites within
the LC. The formation of asymmetric and symmetric synaptic specializations
suggests that the opioid peptide, ENK, may be colocalized with other
neurotransmitters. Furthermore, the distribution of ENK immunoreactivity in axon
terminals apposed to other unlabeled afferents or astrocytic processes suggests
that actions of ENK may also include presynaptic modulation of other transmitters
and/or effects on astrocytes.
PMID: 8847409 [Indexed for MEDLINE]