Rat tooth pulp projections to spinal trigeminal subnucleus caudalis are glutamate-like immunoreactive

It has been shown that glutamate-like immunoreactive axon terminals are present within the spinal trigeminal nucleus, including subnucleus caudalis. The morphology of many of these terminations is consistent with their identification as primary afferents. To establish whether primary afferent projections to subnucleus caudalis are glutamate-like immunoreactive, we injected an anterograde tract tracer into rat incisor tooth pulp, histochemically visualized this tracer within subnucleus caudalis, and then used an immunocytochemical technique to label glutamate-like immunoreactive profiles within these same sections. The anterograde tract tracer used, the B subunit of cholera toxin conjugated to horseradish peroxidase (B-HRP), is transported transganglionically and can be used to localize tooth pulp projection fibers in the spinal trigeminal nucleus. A majority of B-HRP projection fibers from rat lower incisors terminated ipsilaterally in axon terminals in the dorsal region of subnucleus caudalis. Labeled axon terminals were both scallop-shaped and smooth in profile. Small numbers of fibers containing B-HRP extended into laminae I-III caudally and were present in both the border zone between laminae IV and V and the most lateral region of lamina V rostrally. Approximately 75% of the B-HRP-labeled projection fibers were glutamate-like immunoreactive, providing evidence that the excitatory amino acid glutamate functions as a neurotransmitter in a subpopulation of these fibers. Terminals reactive for both B-HRP and glutamate-like immunoreactivity contained small, spherical round vesicles, formed asymmetric synapses, and participated in axoaxonic and axodendritic synaptic junctions. These results support the hypothesis that glutamate may be a transmitter of A delta and C fibers involved in relaying nociceptive information from the tooth pulp.