Evidence for glutamate as a neurotransmitter in spinothalamic tract terminals in the posterior region of owl monkeys.

Previous studies have suggested that glutamate is a neurotransmitter in ascending somatosensory pathways to the thalamus. The present study examined with quantitative immunohistochemical methods the presence of glutamate in spinothalamic tract terminals of owl monkeys (Aotus trivirgatus). Such terminals in the posterior region, in which a nucleus was recently identified as a specific pain and temperature relay in macaques and humans, were labeled by anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, injected into the spinal dorsal horn. Glutamate-like immunoreactivity was demonstrated with a postembedding immunogold procedure using a well-characterized glutamate antiserum. Quantitative analysis of the immunogold labeling demonstrated that the spinothalamic tract terminals contained more than twice the tissue average of glutamate-like immunoreactivity. Enrichment of glutamate-like immunoreactivity was also found in terminals of presumed cortical origin. Presynaptic dendrites, cell bodies and non-vesicle-containing dendrites displayed low levels of glutamate-like immunoreactivity. A strong positive correlation (r = 0.69; P < 0.0001) was found between the density of synaptic vesicles and the density of gold particles in spinothalamic tract terminals, in contrast to a weak negative relationship (r = -0.28; P = 0.089) present in GABAergic presynaptic dendrites. These data provide strong evidence that the gold labeling in the spinothalamic tract terminals represents transmitter labeling, implying that glutamate is a neurotransmitter for ascending nociceptive and thermoreceptive information in primates.