Autoradiographical evaluation of [3H]glycine uptake in rat forebrain: cellular localization in the hippocampus

D-serine as a neuromodulator: regional and developmental localizations in rat brain glia resemble NMDA receptors

D-Serine is localized in mammalian brain to a discrete population of glial cells near NMDA receptors, suggesting that D-serine is an endogenous agonist of the receptor-associated glycine site. To explore this possibility, we have compared the immunohistochemical localizations of D-serine, glycine, and NMDA receptors in rat brain. In the telencephalon, D-serine is concentrated in protoplasmic astrocytes, which are abundant in neuropil in close vicinity to NMDA receptor 2A/B subunits. Ultrastructural examination of the CA1 region of hippocampus reveals D-serine in the cytosolic matrix of astrocytes that ensheath neurons and blood vessels, whereas NR2A/B is concentrated in dendritic spines. By contrast, glycine immunoreactivity in telencephalon is the lowest in brain. During postnatal week 2, D-serine levels in cerebellum are comparable to those in adult cerebral cortex but fall to undetectable levels by day 26. During week 2, we observe parallel ontogeny of D-serine in Bergmann glia and NR2A/B in Purkinje cells, suggesting a role for astrocytic D-serine in NMDA receptor-mediated synaptogenesis. D-Serine in the radial processes of Bergmann glia is also well positioned to regulate NMDA receptor-dependent granule cell migration. In the inner granule layer, D-serine is found transiently in protoplasmic astrocytes surrounding glomeruli, where it could regulate development of the mossy fiber/granule cell synapse. D-Serine seems to be the endogenous ligand of glycine sites in the telencephalon and developing cerebellum, whereas glycine predominates in the adult cerebellum, olfactory bulb, and hindbrain.

Cellular expression of glycine transporter 2 messenger RNA exclusively in rat hindbrain and spinal cord

High-affinity transporters mediate the removal of released neurotransmitters from synapses, thereby terminating their synaptic action. A novel glycine transporter has recently been cloned from a rat brain complementary DNA library. In this study we examined, by means of in situ hybridization with 35S-labelled oligodeoxynucleotide probes, the distribution of messenger RNAs encoding glycine transporter 2 in the rat CNS. Moreover, adjacent series of sections were labelled with [3H]strychnine to reveal the regional distribution of strychnine-sensitive glycine receptors. A very discrete pattern of distribution of the transcripts was found exclusively at the level of the brainstem/cerebellum and spinal cord. In the cerebellum, Golgi cells in the granule cell layer as well as a subpopulation of neurons in the interposed nuclei were consistently labelled. In the brainstem, where the bulk of the labelling was concentrated, several nuclei showed a high level of transcript expression, including the superior olivary complex, nucleus of the trapezoid body and the ventral nucleus of the lateral lemniscus. In the spinal cord, many neurons throughout all layers were labelled, including putative Renshaw cells and a few large neurons at the border of layers 7 and 9. No labelled cells were detected at the levels of the fore- and midbrain. The distribution of glycine transporter 2 messenger RNA-containing cell bodies was very different to that of other glycine transporter messenger RNAs (glycine transporter 1a and glycine transporter 1b), but similar to that of known glycine-immunoreactive neurons and correlated very well with that of strychnine-sensitive glycine receptors in most CNS regions except cerebellum. Our results show that glycine transporter 2 (but not glycine transporter 1) in the brainstem, spinal cord and cerebellum is probably involved in the reuptake of glycine from synapses containing classical strychnine-sensitive glycine receptors. Our findings also suggest that glycine acts as a neurotransmitter in cerebellar Golgi neurons. Whether the synaptic concentration of glycine, as co-agonist at NMDA receptors, is regulated (if at all) by transaminase activity or by a glycine transporter (GLYT1a?) distinct from that described here is not yet known.

Neurotransmitter transporters

In the past year, our knowledge of neurotransmitter transporters has increased significantly. Recently, new members of two families of plasma membrane uptake carriers have been cloned, and the stoichiometries, physiological function and mechanisms of modulation of some of these transporters are now better understood. These developments highlight the possible role of neurotransmitter transporters in disease states, in the development of the nervous system, and as targets for therapeutic drugs.