Journal of Neuroscience, Vol 12, 204-221, Copyright © 1992 by Society for Neuroscience
Cervicothalamic tract terminals are enriched in glutamate-like immunoreactivity: an electron microscopic double-labeling study in the cat
J Broman and OP Ottersen
Department of Cell Biology, Faculty of Health Sciences, University of Linkoping, Sweden.
The distribution of glutamate-like immunoreactivity (Glu-LI) in the
thalamic ventral posterolateral nucleus (VPL) of cats was studied with the
EM immunogold technique in order to identify nerve terminal populations
that may use glutamate as a neurotransmitter. The investigation was focused
on cervicothalamic tract (CTT) terminals, which were labeled by WGA-HRP
transported anterogradely from injection sites in the lateral cervical
nucleus (LCN). The amount of Glu-LI in different profiles was evaluated
quantitatively by counting the number of gold particles and then
calculating the areal density of gold particles over different profile
types. The highest density of gold particles was found over terminals with
morphologic characteristics of terminals of cortical origin (RS terminals),
a finding that further supports the glutamatergic nature of these terminals
suggested by previous studies. Enrichment of Glu-LI was also found in CTT
terminals and in non-peroxidase-labeled terminals with the same morphologic
characteristics as CTT terminals (RL terminals). The labeling density over
these terminals was about twice the average tissue density of gold
particles. The labeling density over large VPL neuronal cell bodies was on
average 127%, and that over vesicle-containing dendritic appendages and
truncs (presynaptic dendrites) about 80%, of the average tissue density of
gold particles. Immunogold labeling with antiserum against glutamine (Gln)
indicated low levels of Gln-like immunoreactivity in CTT terminals and a
high Glu:Gln ratio as compared to astrocytes and the average Glu:Gln ratio
in the VPL. The present findings provide further support for a transmitter
role of glutamate in terminals of ascending somatosensory afferents to the
VPL, including the CTT. Taken together with previous findings of an
enrichment of Glu-LI in terminals of the spinocervical tract (Broman et
al., 1990), our results suggest that synaptic transmission in the
spinocervicothalamic pathway is dependent on the release of glutamate both
at the levels of the LCN and the VPL.