EXTRASYNAPTIC LOCALIZATION OF TAURINE-LIKE IMMUNOREACTIVITY IN THE LAMPREY SPINAL-CORD

Taurine is an endogenous amino acid that can occur in nerve terminals in the central nervous system and that can produce inhibitory neuronal responses. It is unclear, however, whether this amino acid can functi on as a synaptic transmitter. To examine the distribution of taurine a t high anatomical resolution in a vertebrate, light and electron micro scopic immunocytochemical postembedding techniques were applied to the lamprey spinal cord (Ichtyomyzon unicuspis and Lampetra fluviatilis), which contains many large, unmyelinated axons. The most intense immun olabeling occurred in a population of liquor-contacting cells (tanycyt es), located around the central canal, which extended processes to the dorsal, lateral, and ventral margins of the spinal cord. In addition, a proportion of the taurine-immunoreactive cells contained gamma-amin obutyric acid (GABA)-like immunoreactivity. A moderate level of taurin e immunoreactivity was also present in ependymal cells, located around the central canal, as well as in astrocytes throughout all regions of the spinal cord. At the ultrastructural level, the taurine immunoreac tivity showed an even distribution in the cytoplasm of the labeled cel ls. In contrast to the glial labeling neuronal cell bodies and axons e xhibited very low levels of taurine labeling, which were similar to th e level of background labeling. The synaptic vesicle clusters within t he axons did not show any clear accumulation of taurine immunoreactivi ty. These results suggest that taurine may have metabolic roles in the lamprey spinal cord, and, as in other systems, it may take part in os moregulation. However, the lack of immunolabeling in presynaptic eleme nts is not consistent with a role of taurine as a synaptic transmitter . (C) 1994 Wiley-Liss, Inc.