ELECTRON-MICROSCOPIC EVIDENCE FOR COEXISTENCE OF LEUCINE(5)-ENKEPHALIN AND GAMMA-AMINOBUTYRIC-ACID IN A SUBPOPULATION OF AXON TERMINALS IN THE RAT LOCUS-COERULEUS REGION
Ej. Vanbockstaele et J. Chan, ELECTRON-MICROSCOPIC EVIDENCE FOR COEXISTENCE OF LEUCINE(5)-ENKEPHALIN AND GAMMA-AMINOBUTYRIC-ACID IN A SUBPOPULATION OF AXON TERMINALS IN THE RAT LOCUS-COERULEUS REGION, Brain research, 746(1-2), 1997, pp. 171-182
We recently described ultrastructural evidence for morphologically het
erogeneous axon terminals containing the endogenous opioid peptide, me
thionine(5)-enkephalin (ENK), that formed synapses with neurons contai
ning the catecholamine synthesizing enzyme, tyrosine hydroxylase, in t
he locus coeruleus (LC) of the rat brain. The morphological characteri
stics of these terminals suggested that ENK may be co-localized with e
ither an excitatory or inhibitory amino acid. To further test this hyp
othesis, we combined immunogold-silver localization of gamma-aminobuty
ric acid (GABA) and immunoperoxidase labeling for ENK in single sectio
ns through the LC, in the present study, to determine whether ENK and
GABA were contained within single axon terminals. Light microscopic an
alysis of ENK and GABA immunoreactivities in the LC indicated that bot
h transmitters were enriched in the dorsal pens. Although electron mic
roscopy revealed that ENK and GABA were located primarily in axon term
inals, some dendrites also contained immunolabeling for GABA. The dens
e core vesicles were consistently the most immunoreactive in ENK conta
ining axon terminals and were identified toward the periphery of the a
xon terminal distal to the synaptic specialization. Axon terminals con
taining either ENK or GABA immunoreactivities contained pleomorphic ve
sicles as well as large dense core vesicles, varied in size and formed
heterogeneous types of synaptic specializations (i.e. asymmetric vs.
symmetric). Approximately 38% (n = 76) of the axon terminals containin
g ENK immunoreactivity (n = 200) also contained GABA. Some axon termin
als containing peroxidase labeling for ENK (22%; n = 44) converged on
common targets with GABA-labeled axon terminals. Finally, a few ENK-la
beled axon terminals (14%; n = 28) formed asymmetric (excitatory-type)
synapses with dendrites containing gold-silver labeling for GABA. The
results, therefore, indicate that the opioid peptide, ENK, and the in
hibitory amino acid, GABA, may influence LC neurons by concerted actio
ns via (1) release from a common axon terminal, and (2) via separate s
ets of afferents converging on similar portions of the plasmalemma of
target neurons. Furthermore, these studies also suggest a cellular sub
strate for opioid inhibition of LC neurons via activation (i.e. asymme
tric synapses) of inhibitory GABAergic neurons. Future studies are req
uired to determine whether the receptive sites for ENK and GABA are lo
cated at similar sites on the plasma membranes of LC neurons pre- or p
ostsynaptically and whether there is differential release of either tr
ansmitter from single terminals in the LC.