Ultrastructural aspects of the coeruleo-spinal projection

Few studies have focussed on the ultrastructure of the coeruleo-spinal projection. In rat the projections from the area of the locus coeruleus (LC) and subcoeruleus (SC) to lumbar motoneuronal cell groups exhibited two different types of terminals: E-type terminals, containing many very small vesicles and S-type terminals, containing many spherical vesicles and an occasional dense-cored vesicle. These findings are in agreement with data indicating the existence of a noradrenergic (NA) and a non-NA projection from the area of the LC and SC to the spinal cord. A study on dopamine-beta-hydroxylase (D beta H)-immunoreactive terminals in lumbar motoneuronal cell groups showed that they contained several granular vesicles, which were not found in the E- and S-type terminals. Only a few immunoreactive terminals exhibited a synaptic specialization in a single, thin section. A low incidence of synaptic junctions was also found for the E-type terminals, but not for the S-type. Based on this and other data, it is suggested that the E-type terminal is NA, while the S-type may contain a non-NA transmitter, possibly acetylcholine. A low incidence of synaptic junctions in single, thin sections may indicate the presence of non-synaptic NA terminals, but direct evidence from serial-section analysis is not available. In the superficial dorsal horn, terminals derived from the area of the LC and SC were identified at the ultrastructural level in two studies, one using the anterograde degeneration technique in opossum, the other (presented in this chapter) using WGA-HRP anterograde tracing in rat. It was found in both studies that most of the labeled structures were small axons (mostly unmyelinated), while few terminals were labeled. They contained mostly spherical vesicles and, according to the degeneration study, a variable number of dense-cored vesicles. The labeled terminals appeared to make regular synaptic contacts mostly with small dendrites and occasionally with spines. They were not present in glomeruli or engaged in presynaptic arrangements. A study on NA terminals showed similar results, although large granular vesicles were not observed and fewer synapses were seen. On the few data available at present it is concluded that in the spinal superficial dorsal horn, most terminals derived from the area of the LC and SC are NA and establish conventional synapses. However, a non-NA component cannot be excluded.