Evidence for vesicular storage of glutamate in primary afferent terminals.

The correlation between the density of gold particles signalling fixed glutamate and the density of synaptic vesicles was examined in primary afferent terminals in the rat dorsal horn. Statistically significant positive correlations between these parameters were found in lamina I and III-V primary afferent terminals, identified through axonal transport of choleragenoid-horseradish peroxidase conjugate, and in lamina II dense sinusoid axon terminals of presumed thin calibre primary afferent origin. Profiles containing pleomorphic synaptic vesicles displayed negative or statistically insignificant correlations between the density of synaptic vesicles and that of gold particles. These observations indicate that primary afferent terminals store glutamate in synaptic vesicles and provide further evidence for a role of glutamate as a primary afferent neurotransmitter.