Extrasynaptic localization of taurine‐like immunoreactivity in the lamprey spinal cord

Abstract

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 function as a synaptic transmitter. To examine the distribution of taurine at high anatomical resolution in a vertebrate, light and electron microscopic immunocytochemical postembedding techniques were applied to the lamprey spinal cord (Ichtyomyzon unicuspis and Lampetra fluviatilis), which contains many large, unmyelinated axons. The most intense immunolabeling occurred in a population of liquor-contacting cells (tanycytes), 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-aminobutyric acid (GABA)-like immunoreactivity. A moderate level of taurine 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 immunoreactivity showed an even distribution in the cytoplasm of the labeled cells. In contrast to the glial labeling, neuronal cell bodies and axons exhibited very low levels of taurine labeling, which were similar to the level of background labeling. The synaptic vesicle clusters within the axons did not show any clear accumulation of taurine immunoreactivity. These results suggest that taurine may have metabolic roles in the lamprey spinal cord, and, as in other systems, it may take part in osmoregulation. However, the lack of immunolabeling in presynaptic elements is not consistent with a role of taurine as a synaptic transmitter.

Synaptic Vesicular Localization and Exocytosis ofl-Aspartate in Excitatory Nerve Terminals: A Quantitative Immunogold Analysis in Rat Hippocampus

“…This supports the idea that changes in Asp-LI and Glu-LI reflect changes in synaptic release. The Tau results further add to the notion that Tau does not act as a transmitter at synapses in the CNS (Shupliakov et al, 1994).…”

Levels of Amino Acid Neurotransmitters during Neurogenesis and in Histotypic Cultures of Mouse Spinal Cord

“…The localization of taurine in presumptive inhibitory axon terminals provides anatomical support for the hypothesis that taurine may play an inhibitory role in the superficial dorsal horn of the spinal cord and may have a selective antinociceptive effect on chemically induced nociception [22]. On the other hand, its localization in astrocytes and endothelial cells within both the dorsal and ventral horns of the spinal cord relates to its involvement in osmoregulation [23].…”

Synaptic and nonsynaptic monoaminergic neuron systems in the lamprey spinal cord

“…The DA-ir LC cells can be reliably classified as neurons as they receive input synapses and establish output synapses with spinal neurons. Spinal LC neurons containing various neuroactive substances have previously been identified in different vertebrate species (e.g., Baumgarten, 1972;Vigh and Vigh-Teichman, 1973;Vigh et al, 1983;Christenson et al, 1991b;Rodriguez et al, 1992;Roberts et al, 1989;Shupliakov et al, 1994;Gonzalez et al, 1994;Reiner et al, 1994;Meek, 1994). The present study is, however, the first in which their connectivity with other cell types is established.…”

“…These LC cells were generally larger than the tanycytes, a non-neuronal cell population containing taurine (t in Fig. 4B; Shupliakov et al, 1994). …”

“…The central canal of the vertebrate spinal cord is lined by three types of LC cells, including ependymal cells, tanycytes, and neurons (e.g., Vigh and Vigh-Teichman, 1973;Vigh et al, 1983;Peters et al, 1991;Rodriguez et al, 1992;Shupliakov et al, 1994). The DA-ir LC cells can be reliably classified as neurons as they receive input synapses and establish output synapses with spinal neurons.…”