Cervicothalamic tract terminals are enriched in glutamate-like immunoreactivity: an electron microscopic double-labeling study in the cat

Abstract

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.