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 i n 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 triv irgatus). Such terminals in the posterior region, in which a nucleus w as recently identified as a specific pain and temperature relay in mac aques and humans, were labeled by anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, injected into the spi nal dorsal horn. Glutamate-like immunoreactivity was demonstrated with a postembedding immunogold procedure using a well-characterized gluta mate antiserum. Quantitative analysis of the immunogold labeling demon strated that the spinothalamic tract terminals contained more than twi ce the tissue average of glutamate-like immunoreactivity. Enrichment o f glutamate-like immunoreactivity was also found in terminals of presu med cortical origin. Presynaptic dendrites, cell bodies and non-vesicl e-containing dendrites diplayed low levels of glutamate-like immunorea ctivity. A strong positive correlation (r=0.69; P<0.0001) was found be tween the density of synaptic vesicles and the density of gold particl es in spinothalamic tract terminals, in contrast to a weak negative re lationship (r=-0.28; P=0.089) present in GABAergic presynaptic dendrit es. These data provide strong evidence that the gold labeling in the s pinothalamic tract terminals represents transmitter labeling, implying that glutamate is a neurotransmitter for ascending nociceptive and th ermoreceptive information in primates.