Localization of high affinity [3H]glycine transport sites in the cerebellar cortex

Compartmentalized Input-Output Organization of Lugaro Cells in the Cerebellar Cortex

Purkinje cells (PCs) are principal cerebellar neurons, and several classes of interneurons modulate their activity. Lugaro cells (LCs) are one such inhibitory interneuron with distinctive cytology and location, but still most enigmatic among cerebellar neurons. Here we serendipitously produced a novel transgenic mouse line, where a half of Yellow Cameleon (YC)(+) cells in the cerebellar cortex were judged to be LCs, and YC(+) LCs were estimated to constitute one-third of the total LC populations. Neurochemically, two-thirds of YC(+) LCs were dually GABAergic/glycinergic, with the rest being GABAergic. Beneath the PC layer, they extended a sheet of somatodendritic meshwork interconnected with neighboring LCs by adherens junctions, and received various inputs from climbing fibers, mossy fibers, granule cell axons, recurrent PC axons, Golgi cell axons, LC axons, and serotonergic fibers. Intriguingly, somatodendritic elements of individual LCs preferentially extended within a given cerebellar compartment defined by aldolase C expression. In turn, YC(+) LCs projected a dense lattice of ascending and transverse axons to the molecular layer, and innervated molecular layer interneurons (basket and stellate cells) and Golgi cells, but not PCs. Of note, ascending axons profusely innervated individual targets within a cerebellar compartment, while transverse axons ran across several compartments and innervated targets sparsely. This unique circuit configuration highlights that LCs integrate various excitatory, inhibitory, and modulatory inputs coming to the belonging cerebellar compartment and that, as an interneuron-selective interneuron, LCs can effectively disinhibit cerebellar cortical activities in a compartment-dependent manner through inhibition of inhibitory interneurons selectively targeting PCs and granule cells.

Active release of glycine or D-serine saturates the glycine site of NMDA receptors at the cerebellar mossy fibre to granule cell synapse

The current and calcium influx generated by NMDA receptors depend on the concentration of the coagonist glycine, or its analogue d-serine, in the synaptic cleft. If there is no release of glycine, the ionic stoichiometry of the glial GlyT1 glycine transporters expressed near NMDA receptors in the brain should be able to lower the extracellular glycine concentration to below the EC50 for coactivation of NMDA receptors. We examined whether changing the glycine or d-serine concentration in the superfusion solution altered the NMDA receptor mediated component of the synaptic current at the rat cerebellar mossy fibre to granule cell synapse. Adding up to 100 microM glycine or d-serine had no effect, implying that the glycine site is saturated. Using the competitive glycine site antagonist 7-chlorokynurenate, and plausible values for the kinetic parameters of NMDA receptors, we estimate that during activation of the mossy fibres the concentration of glycine or d-serine in the synaptic cleft is at least 4.6 microM or 1.5 microM, respectively, requiring active release of glycine or d-serine.

IPSC kinetics at identified GABAergic and mixed GABAergic and glycinergic synapses onto cerebellar Golgi cells

In the rat cerebellum, Golgi cells receive serotonin-evoked inputs from Lugaro cells (L-IPSCs), in addition to spontaneous inhibitory inputs (S-IPSCs). In the present study, we analyze the pharmacology of these IPSCs and show that S-IPSCs are purely GABAergic events occurring at basket and stellate cell synapses, whereas L-IPSCs are mediated by GABA and glycine. Corelease of the two transmitters at Lugaro cell synapses is suggested by the fact that both GABA(A) and glycine receptors open during individual L-IPSCs. Double immunocytochemical stainings demonstrate that GABAergic and glycinergic markers are coexpressed in Lugaro cell axonal varicosities, together with the mixed vesicular inhibitory amino acid transporter. Lugaro cell varicosities are found apposed to glycine receptor (GlyR) clusters that are localized on Golgi cell dendrites and participate in postsynaptic complexes containing GABA(A) receptors (GABA(A)Rs) and the anchoring protein gephyrin. GABA(A)R and GlyR/gephyrin appear to form segregated clusters within individual postsynaptic loci. Basket and stellate cell varicosities do not face GlyR clusters. For the first time the characteristics of GABA and glycine cotransmission are compared with those of GABAergic transmission at identified inhibitory synapses converging onto the same postsynaptic neuron. The ratio of the decay times of L-IPSCs and of S-IPSCs is a constant value among Golgi cells. This indicates that, despite a high cell-to-cell variability of the overall IPSC decay kinetics, postsynaptic Golgi cells coregulate the kinetics of their two main inhibitory inputs. The glycinergic component of L-IPSCs is responsible for their slower decay, suggesting that glycinergic transmission plays a role in tuning the IPSC kinetics in neuronal networks.

Morphological and neurochemical differentiation of large granular layer interneurons in the adult rat cerebellum

The granular layer of the cerebellar cortex consists of densely packed neuronal cells, classified into granule cells and large interneurons. In this study, we provide a comparative survey of large granular layer interneurons in the adult rat cerebellum based on both morphological and neurochemical criteria. To this end, double immunofluorescence histochemistry was performed by combining antibodies against the cytoplasmic antigen Rat-303, calretinin, the metabotropic glutamate receptor mGluR2 and somatostatin. Based on Rat-303/calretinin double immunohistochemistry, three distinct populations of large granular layer interneurons could be discerned: cells immunopositive for Rat-303, calretinin or both. Rat-303 or calretinin single-labeled cells represented Golgi cells and unipolar brush cells, respectively. Rat-303/calretinin double-labeled cells located just underneath the Purkinje cell layer represented Lugaro cells. Morphometrical analysis distinguished two populations of Rat-303-positive Golgi cells according to their location: vermis versus hemisphere. Immunostaining for the metabotropic glutamate receptor mGluR2 combined with Rat-303 or calretinin revealed that the majority of Golgi cells (about 90%) appeared to be mGluR2 positive. Lugaro cells were mGluR2 negative. In addition, a limited population of large polymorphous interneurons in the depth of the granular layer with morphological features resembling Golgi cells also displayed Rat-303/calretinin immunoreactivity and were mGluR2 negative. Double immunohistochemistry for Rat-303 and somatostatin revealed three populations of labeled cells in the depth of the granular layer. Besides double-labeled Golgi cells, Rat-303 or somatostatin single-labeled cells were present. Based on mGluR2/somatostatin and calretinin/somatostatin double immunostainings, Rat-303 single-labeled cells were found to correspond to Rat-303/calretinin-positive, mGluR2-negative Golgi-like cells, while the identity of somatostatin single-labeled cells remained unclear. The data presented in this article elaborate previous reports on the morphological and neurochemical differentiation of large interneurons in the rat cerebellar granular layer. In addition, they indicate that the current classification of these cells into Golgi cells, Lugaro cells and unipolar brush cells does not describe the observed neurochemical heterogeneity.

Cerebellum as a target for toxic substances

The Purkinje cells and the granule cells are the most important targets in cerebellum for toxic substances. The Purkinje cells are among the largest neuron in the brain and are very sensitive to ischaemia, bilirubin, ethanol and diphenylhydantoin. The granule cells are small and seem to be sensitive to loss of intracellular glutathione. Granule cells are sensitive to methyl halides, thiophene, methyl mercury, 2-chloropropionic acid and trichlorfon. The Purkinje cells appear in the rat brain on pre-natal day 14-16, whereas the granule cells appear post-natally. Both cells are sensitive to excitotoxic chemicals and also to an effect on DNA or its repair mechanisms.

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.

Metabotropic glutamate receptors mGluR2 and mGluR5 are expressed in two non-overlapping populations of Golgi cells in the rat cerebellum

The metabotropic glutamate receptor subtypes mGluR2 and mGluR5, which are thought to be coupled respectively to the inhibitory cyclic adenosine monophosphate (cAMP) cascade and the phosphatidylinositol hydrolysis/Ca2+ cascade, are known to be expressed on Golgi cells in the granular layer of the rat cerebellar cortex. In the present immunohistochemical study with a monoclonal antibody against mGluR2 and a polyclonal antibody for mGluR5, we examined whether or not mGluR2- and mGluR5-like immunoreactivities were both present in single Golgi cells in the rat cerebellar cortex. In double immunofluorescence histochemistry, no Golgi cells showed mGluR2- and mGluR5-like immunoreactivities simultaneously. Of the total number of Golgi cells immunoreactive for mGluR2 or mGluR5, about 90% were mGluR2-like immunoreactive, and about 10% were mGluR5-like immunoreactive. Golgi cells with mGluR2-like immunoreactivity were distributed evenly in the granular layer of all the cerebellar regions, while those with mGluR5-like immunoreactivity were distributed more frequently in the I, II, VII-X lobules of the vermis and the copula pyramidis of the hemisphere than in other cerebellar regions. The results indicate that Golgi cells containing mGluR2 are segregated from those possessing mGluR5. These two populations of Golgi cells, each equipped with a different metabolic glutamate receptor coupled to a different intracellular signal transduction system, may play different roles in the glutamatergic neuronal circuits in the cerebellar cortex.

Glycine and GABAA receptor-mediated chloride fluxes in synaptoneurosomes from different parts of the rat brain

Strychnine-sensitive, inhibitory glycine receptors have not until lately been considered to play a significant role in neurotransmission in mammalian forebrain regions. In order to investigate the role of glycine as a neurotransmitter in brain we have measured glycine induced chloride fluxes in different adult rat forebrain areas using synaptoneurosomes and a chloride-sensitive fluorescent indicator. The results have been compared to those obtained with GABA. The synaptoneurosomes from every brain area investigated responded to both glycine and GABA with chloride fluxes in a picrotoxin sensitive manner. The effect of glycine was inhibited by strychnine, which had no effect on the GABA-induced Cl-flux. Bicuculline inhibited the effect of GABA, but had no effect on the glycine-induced Cl-flux. Addition of GABA did not affect the response to glycine and vice versa. The endogenous content of glycine and GABA in the synaptoneurosome preparations was about the same and synaptoneurosomes from every brain area investigated released both glycine and GABA upon depolarisation with KCl. The depolarisation induced release of both GABA and glycine was partly Ca(2+)-dependent and partly Ca(2+)-independent. These results indicate that glycine can induce inhibitory Cl- fluxes distinct from GABA induced fluxes in every investigated brain area and that glycine can be released upon depolarisation.

Whole-cell and single-channel currents activated by GABA and glycine in granule cells of the rat cerebellum

1. Patch-clamp methods have been used to characterize GABA-and glycine-activated channels and spontaneous synaptic currents in granule cells in thin cerebellar slices from 7- to 20-day-old rats. 2. All granule cells responded to 10 microM GABA, while approximately 60% responded to 100 microM glycine. With repeated against application, whole-cell responses to GABA, but not those to glycine, declined over a period of minutes unless the pipette solution contained Mg-ATP. 3. Whole-cell concentration-response curves gave EC50 values at 45.2 and 99.6 microM and Hill slopes of 0.94 and 2.6 for GABA and glycine, respectively. At saturating concentrations, currents evoked by GABA were fivefold larger than those evoked by glycine. 4. Whole-cell current-voltage (I-V) relationships of GABA- and glycine-activated currents reversed close to the predicted Cl- equilibrium potential. Partial replacement of intracellular Cl- with F- shifted the GABA reversal potential to a more negative value. 'Instantaneous' I-V relationships produced by ionophoretic application of GABA were linear, while 'steady-state' I-V relationships produced by ramp changes in potential showed outward rectification. For glycine, 'steady-state' I-V plots were linear. 5. Responses to GABA were blocked by the GABAA receptor antagonists bicuculline (15 microM), SR-95531 (10 microM) and picrotoxinin (100 microM) while responses to glycine were selectively blocked by strychnine (200 nM), indicating the presence of two separate receptor types. 6. In outside-out membrane patches, GABA opened channels with conductances of 16 and 28 pS. The proportion of openings to each of the conductances varied between patches, possibly indicating the activation of two distinct channel types. Glycine-activated single-channel currents had conductances of 32, 55 and 104 pS. Single-channel I-V relationships were linear. 7. Spontaneous synaptic currents with a rapid rise time and biexponential decay were present in more than half of the cells examined. These currents were eliminated by bicuculline (15 microM) or SR-95331 (10 microM) and were greatly reduced in frequency by tetrodotoxin (TTX; 300 nM), suggesting that they were mediated by GABA and arose from spontaneous activity in Golgi interneurones. In granule cells where this spontaneous synaptic activity was apparent, glycine and low concentrations of GABA increased the frequency of the synaptic currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional distribution and developmental variation of the glycine transporters GLYT1 and GLYT2 in the rat CNS

The high-affinity glycine transporter in neurons and glial cells is the primary means of inactivating synaptic glycine. Previous molecular cloning studies have indicated heterogeneity of glycine transporters in the CNS. Here the distribution of glycine transporter GLYT1 and GLYT2 transcripts and proteins in different regions and developmental stages of the rat brain were analysed by Northern, Western and in situ hybridization techniques. Sequence-specific riboprobes and two specific antibodies raised against fusion proteins were used, containing either 76 or 193 amino acids of the C or N terminus of the GLYT1 and GLYT2 transporters respectively. High levels of GLYT1 transcripts were found in the spinal cord, brainstem and cerebellum, and moderate levels in forebrain regions such as the cortex or hippocampus. GLYT2 transcripts are restricted to the spinal cord, brainstem and cerebellum. The onset of both GLYT1 and GLYT2 expression in the brainstem occurred in late fetal life, and full expression of these proteins was observed before weaning. There was a stepwise increase in the levels of mRNA and protein for these two transporters, reaching a maximum by the second postnatal week, followed by a slight decrease until adult values were reached by the fourth postnatal week. These data reveal interesting parallelism between the distribution of different glycine transporters and glycine receptor subunits, and suggest discrete roles for distinct glycine transporters.

Glycinergic synaptic currents in Golgi cells of the rat cerebellum

Recordings were made from Golgi cells in slices from rat cerebellar cortex using whole-cell and outside-out configurations of the patch-clamp technique. Exogenous glycine and gamma-aminobutyric acid (GABA) both activated chloride currents, which could be differentially blocked by strychnine and SR95531, respectively. Inhibitory synaptic currents occurred spontaneously in all Golgi cells. Some were blocked by strychnine while the others were blocked by SR95531. The single channel events occurring during the decay of these two types of inhibitory postsynaptic currents had different amplitudes, which matched the main conductance states of the channels gated by glycine and GABA in outside-out patches. It was concluded that Golgi cells receive both glycinergic and GABAergic synaptic inputs.

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.

Pyridoxal kinase immunoreactivity in rabbit brain

Murine polyclonal antibody against purified bovine brain pyridoxal kinase (EC 2.7.1.35) was generated and showed cross-reactivity with rabbit brain pyridoxal kinase. This antibody was used to immunohistochemically examine the distribution of pyridoxal kinase in the rabbit brain. The cytoplasm of neuronal cells and neuroglial cells in the cerebral cortex, hippocampal region, brain nuclei and cerebellar cortex showed positive staining with various degrees of intensity. The neuronal cells and surrounding fibers in some brain nuclei, such as the area tegmentalis ventralis or the substantia nigra, showed intense staining. The neuronal cells of the hippocampal region showed somewhat weak reactivity, but some with intense reactivity were found sparsely distributed and positive staining fiber networks of a very low density were also observed.

Potentiation of N-methyl-D-aspartate-evoked elevation of intracellular Ca2+ concentrations by exogenous glycine in cerebellar granule cells

The effect of glycine on the intracellular free Ca2+ concentration ([Ca2+])i response to N-methyl-D-aspartate (NMDA) was examined in small groups of cerebellar granule cells loaded with fura 2. NMDA alone evoked a long-lasting monophasic [Ca2+]i plateau, which was abolished by removal of extracellular Ca2+, or addition of the NMDA channel antagonist dizocilpine or the glycine site antagonist 5,7-dichlorokynurenic acid, virtually unaffected by the L-type Ca2+ channel antagonist (-)-PN 202 791, and greatly, though variably, potentiated by addition of glycine. In the presence of glycine the response to NMDA was clearly biphasic. However, there was no consistent relationship between the magnitudes of the peak and plateau phases of the response, and their temporal relationship was also highly variable. The potentiation seen with exogenous glycine was highly dependent on plating density, which may be the result of higher levels of endogenous glycine in more dense cultures. Our results provide an explanation of the inconsistent findings previously reported by different groups on the potentiation of the [Ca2+]i response to NMDA by exogenous glycine.

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

The cellular elements responsible for the uptake of [3H]glycine into rat hippocampal slices were investigated. The diffuse laminar distribution of labelling observed under control conditions was greatly reduced seven days after intrahippocampal injection of a neurotoxic dose of quinolinic acid, suggesting a neuronal localization. Glycine was also taken up into glial cells, since dense clusters of silver grains were present on small sized cells throughout the hippocampus which were apparently increased in number after the lesion. The pattern of [3H]glycine uptake into rat cerebral cortex and cerebellar slices was also consistent with both neuronal and glial localization. These glycine transport sites may be strategically located to control excitatory neurotransmission mediated by the N-methyl-D-aspartate sub-type of glutamate receptors.

Cholinergic innervation of the human cerebellum

Cholinergic innervation of the human cerebellum was investigated immunocytochemically by using a polyclonal rabbit antiserum against choline acetyltransferase. Immunoreactive structures were found throughout the cerebellar cortex but were localized predominantly in the vermis, flocculus, and tonsilla. These included 1) a population of Golgi cells in the granular layer; 2) a subpopulation of mossy fibers and glomerular rosettes; 3) thin, varicose fibers closely associated with the Purkinje cell layer and the molecular layer; and 4) a relatively dense network of fibers and terminals contributing to the glomerular formations in the granular layer. In the cerebellar nuclei, some cells stained positively for choline acetyltransferase, and a terminal field pattern could be detected with a distinct but sparse network of varicose fibers. Acetylcholine appears to be a primary transmitter in the vestibulocerebellar pathways at several levels, which may account for the potent effects of muscarinic antagonists in diminishing vestibular vertigo in humans.

Colocalization of neurotransmitters in the deep cerebellar nuclei

An abundance of glycine and glycine receptor immunoreactivities was found in all three parts of the deep cerebellar nuclei. Glycine immunoreactivity was restricted to small neurons throughout most of the deep cerebellar nuclei except for a few large positive neurons in the ventral part of the fastigial nuclei. In addition, glycine immunoreactivity was found in boutons outlining somata of large glycine negative neurons. Complementary to the glycine positive boutons was an intense glycine receptor immunoreactivity on large deep cerebellar nuclei neurons. Comparisons of immunoreactivities for glycine, GABA and aspartate in consecutive one micron sections revealed that many small neurons colocalized glycine and GABA, while some large neurons in the fastigal region colocalized glycine and aspartate. Ultrastructural investigations revealed glycine receptors on postsynaptic sites of dendrites and somata. Most boutons, which were presynaptic to glycine receptor sites, were filled with small flattened vesicles; however, a small percentage of boutons had round clear or dense core vesicles. Frequently, each bouton apposed multiple active zones on the dendrite or soma. One of these active zones was positive for glycine receptor and another was negative. This study supports: (1) glycine as a neurotransmitter in deep cerebellar nuclei, and (2) glycine and GABA colocalization in the same cell and bouton, but releasing to different receptor sites on the target neuron. Furthermore, the coexistence of glycine with GABA in the same deep cerebellar neuron may play an important role in controlling the conset and duration of signal transmission.

gamma-Aminobutyric acid and glycine modulate each other's release through heterocarriers sited on the releasing axon terminals of rat CNS

The ability of gamma-aminobutyric acid (GABA) and glycine (Gly) to modulate each other's release was studied in synaptosomes from rat spinal cord, cerebellum, cerebral cortex, or hippocampus, prelabeled with [3H]GABA or [3H]Gly and exposed in superfusion to Gly or to GABA, respectively. GABA increased the spontaneous outflow of [3H]Gly (EC50, 20.8 microM) from spinal cord synaptosomes. Neither muscimol nor (-)-baclofen, up to 300 microM, mimicked the effect of GABA, which was not antagonized by either bicuculline or picrotoxin. However, the effect of GABA was counteracted by the GABA uptake inhibitors nipecotic acid and N-(4,4-diphenyl-3-butenyl)nipecotic acid. Moreover, the GABA-induced [3H]Gly release was Na+ dependent and disappeared when the medium contained 23 mM Na+. The effect of GABA was Ca2+ independent and tetrodotoxin insensitive. Conversely, Gly enhanced the outflow of [3H]GABA from rat spinal cord synaptosomes (EC50, 100.9 microM). This effect was insensitive to both strychnine and 7-chlorokynurenic acid, antagonists at Gly receptors, but it was strongly Na+ dependent. Also, the Gly-evoked [3H]GABA release was Ca2+ independent and tetrodotoxin insensitive. GABA increased the outflow of [3H]Gly (EC50, 11.1 microM) from cerebellar synaptosomes; the effect was not mimicked by either muscimol or (-)-baclofen nor was it prevented by bicuculline or picrotoxin. The GABA effect was, however, blocked by GABA uptake inhibitors and was Na+ dependent. Gly increased [3H]GABA release from cerebellar synaptosomes (EC50, 110.7 microM) in a strychnine- and 7-chlorokynurenic acid-insensitive manner. This effect was Na+ dependent. The effects of GABA on [3H]Gly release seen in spinal cord and cerebellum could be reproduced also with cerebrocortical synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning, expression, and localization of a rat brain high-affinity glycine transporter

A cDNA clone encoding a glycine transporter has been isolated from rat brain by a combined PCR and plaque-hybridization strategy. mRNA synthesized from this clone (designated GLYT1) directs the expression of sodium- and chloride-dependent, high-affinity uptake of [3H]glycine by Xenopus oocytes. [3H]Glycine transport mediated by clone GLYT1 is blocked by sarcosine but is not blocked by methyl-aminoisobutyric acid or L-alanine, a substrate specificity similar to that described for a previously identified glycine-uptake system called system Gly. In situ hybridization reveals that GLYT1 is prominently expressed in the cervical spinal cord and brainstem, two regions of the central nervous system where glycine is a putative neurotransmitter. GLYT1 is also strongly expressed in the cerebellum and olfactory bulb and is expressed at lower levels in other brain regions. The open reading frame of the GLYT1 cDNA predicts a protein containing 633 amino acids with a molecular mass of approximately 70 kDA. The primary structure and hydropathicity profile of GLYT1 protein reveal that this protein is a member of the sodium- and chloride-dependent superfamily of transporters that utilize neurotransmitters and related substances as substrates.

The ontogeny of the uptake systems for glutamate, GABA, and glycine in synaptic vesicles isolated from rat brain

The ontogeny of the uptake of glutamate, GABA and glycine into synaptic vesicles isolated from rat brain has been investigated. The vesicular uptake of the three amino acids increased with developmental age in parallel with synaptogenesis, indicating a functional role of uptake of the amino acids by synaptic vesicles in the nerve terminals. Uptake of the amino acids by plasma membrane particles (synaptosomes) in brain homogenate showed a somewhat different developmental profile. The uptake of glutamate increased markedly with developmental time, while the uptake of GABA showed only a slight increase. Uptake of glycine by plasma membrane particles was very low and therefore not registered. The observed developmental increase in uptake of glycine by synaptic vesicles isolated from brain, supports previous reports indicating that glycine can be taken up by vesicles from non-glycine terminals.

Immunocytochemical evidence for the involvement of glycine in sensory centers of the rat brain

Glycine-like immunoreactivity was localized to a number of sites in the rat brain which are involved in processing sensory information. In the auditory and vestibular systems, glycine immunoreactivity was seen in dorsal and ventral cochlear nuclei, superior olive, trapezoid body, medial and lateral vestibular nuclei, and inferior colliculus. Staining in the visual system was seen in retina, dorsal lateral geniculate nucleus, and superior colliculus. The olfactory system exhibited staining in the olfactory bulb and accessory olfactory formation. Somatosensory centers with glycine immunoreactivity included the dorsal column nuclei, spinal trigeminal nucleus, principal sensory nucleus of V, reticular formation, and periaqueductal gray. Glycine-immunoreactive neurons were also seen in cerebellar cortex, deep cerebellar nuclei, hippocampus, cerebral cortex, and striatum. The distribution of staining indicates that glycine plays a major role in sensory centers with actions at both strychnine-sensitive and strychnine-insensitive receptors.

Uptake of glycine, GABA and glutamate by synaptic vesicles isolated from different regions of rat CNS

Synaptic vesicle fractions have been isolated from cerebral cortex, subcortical telencephalon, whole brain and spinal cord by density gradient centrifugation. The Mg2+ ATP-dependent vesicular uptake and the Na(+)-dependent synaptosomal uptake of glycine, GABA and L-glutamate has been compared in the different regions. All these regions contain GABA as inhibitory neurotransmitter, whereas glycine only plays a dominant role as such in the spinal cord. The ratio between GABA and glycine uptake in the different vesicle fractions was similar, and the ratios differed greatly from the ratios in the synaptosomal uptake. In contrast, the ratio between glutamate and GABA uptake in vesicles from different regions differed, and these ratios corresponded to the ratios in the synaptosomal uptake. These results indicate that glycine is taken up into synaptic vesicles from non-glycine terminals, and we suggest that GABA and glycine can be taken up into the same vesicle population.

Inhibition of gamma-aminobutyrate and glycine uptake into synaptic vesicles

The substrate specificity of vesicular GABA and glycine uptake was studied in vesicle fractions from brain and spinal cord, respectively. Glycine, beta-alanine and gamma-vinyl-GABA were competitive inhibitors of the GABA uptake were competitive inhibitors of the GABA uptake by synaptic vesicles in brain. Likewise GABA and beta-alanine turned out to be competitive inhibitors of vesicular uptake of glycine in spinal cord. The apparent K1 values were in the same range as the respective Km values for the transport systems. Accumulation of different amino acids were examined, and the structurally related amino acids GABA, beta-alanine and glycine were all taken up by both vesicle fractions. In the present study, we suggest that there are similarities in the vesicular transporters for GABA and glycine, and the two amino acids are probably taken up into the same vesicle population. The key factor in differentiating between GABA and glycine as transmitters in the terminals could be the synthesis and the high-affinity synaptosomal uptake.

Localization of glycine receptor alpha 1 subunit mRNA-containing neurons in the rat brain: an analysis using in situ hybridization histochemistry

The localization of glycine receptors in the rat brain was examined by means of in situ hybridization histochemistry using an oligonucleotide probe to the sequence of the alpha 1 subunit. Strongly- or moderately-labeled neurons were found in the cranial nuclei, sensory nuclei such as the spinal trigeminal nucleus, principal trigeminal nucleus, gracile and cuneate nuclei, dorsal and ventral cochlear nuclei, superior olivary nucleus, medial and lateral trapezoid nuclei, lateral lemniscus and vestibular nuclei, red nucleus, parabrachial area, cerebellar nuclei, dorsal tegmental nucleus, reticular formation and parafascicular nucleus. This study thus demonstrated the localization of neurons which are regulated by glycine via strychnine-sensitive glycine receptors in the rat brain.

Dual-component NMDA receptor currents at a single central synapse

Present thinking about the way that the NMDA (N-methyl-D-aspartate) class of glutamate receptor operates at central synapses relies mainly on information obtained from single-channel and whole-cell recordings from cultured neurons stimulated by exogenous NMDA receptor agonists. The mechanisms that operate in the postsynaptic membrane of a normal neuron following release of the natural transmitter are far less clear. An important problem is that most normal neurons receive many excitatory synapses (10(3)-10(5) per cell) and these synapses are located on slender dendritic elements far away from the somatic recording site, making the study of discrete synaptic events difficult. Typically, when populations of synapses are activated, NMDA receptor-mediated synaptic potentials appear as slowly rising, long-lasting waves superimposed on faster, non-NMDA-receptor potentials. Although believed to be critical for NMDA receptor function, this slow time-course would not be predicted from single-channel kinetics and its origin remains puzzling. We have now analysed the events occurring at the level of a single excitatory synapse using a simple, small, neuron--the cerebellar granule cell--which has an unusually simple glutamatergic input. By applying high-resolution whole-cell recording techniques to these cells in situ, we were able to study the nature of elementary NMDA receptor-mediated synaptic currents. Contrary to expectations, the prominent currents are fast but are followed by slow ones. Both types of current are strongly voltage-dependent but differ subtly in this respect. Furthermore, the currents are absent unless glycine is provided.

A subtype of cerebellar Golgi cells may be cholinergic

In cerebellar sections of the feline brain processed for choline acetyltransferase-like immunoreactivity, a population of distinctly stained cells was discovered in the granular layer of the cortex in both vermis and hemispheres. Their position and morphology qualify them as Golgi cells, but their density indicated that they comprise less than 5% of all cerebellar Golgi cells. Varicose immunoreactive fiber nets in all cortical layers also contribute to seemingly widespread cholinergic systems in the cerebellar cortex.

The expression of excitatory amino acid binding sites during neuritogenesis in the developing rat cerebellum

The present study has examined excitatory amino acid transmitter binding sites as measured autoradiographically in cryostat sections prepared from developing rat cerebella during the period of granule cell neuritogenesis. The external germinal layer (EGL) and molecular layer (ML), which during development contain granule cells at early stages of axon growth, contained only low levels of NMDA-displaceable L-[3H]glutamate binding sites. Similarly, [3H]glycine binding to the NMDA receptor linked binding site was not enriched in the EGL. Radioligand binding to the NMDA receptor was always greater in the granular layer (GL) than in the ML. The developmental increases in NMDA-displaceable L-[3H]glutamate and in [3H]glycine binding to the GL were similar but NMDA displaceable L-[3H]glutamate binding density increased before [3H]glycine binding sites. Glycine increased NMDA-displaceable L-[3H]glutamate binding only in the adult cerebellum. These results suggest that NMDA stimulation of neuritogenesis in granule cell cultures may reflect stimulation of dendritogenesis in the developing glomerulus rather than a stimulation of axon growth in the EGL. Also, NMDA receptors may be present in an immature form during cerebellar development and have different properties to the adult receptor. Binding sites for [3H]kainate and [3H]AMPA were present in both the GL and ML and increased during development. At all times the amount of binding sites for [3H]kainate were highest in the GL whereas those for [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate were highest in the ML.

Uptake and release of glycine in cerebellar granule cells and astrocytes in primary culture: potassium-stimulated release from granule cells is calcium-dependent

The properties of [3H]glycine uptake and release were studied with cerebellar granule cells, 7-9 days in vitro, (DIV) and astrocytes, 14-15 DIV, in primary cultures. The uptake of glycine in both cell types consisted of a saturable high-affinity transport and nonsaturable diffusion. The transport constant (Km) and maximal velocity (V) were significantly higher in granule cells than in astrocytes. Uptake was strictly Na+-dependent and also markedly diminished in low-Cl medium. The specificity of the uptake was similar in both cell types. The spontaneous release of glycine from granule cells and astrocytes was fast. Homoexchange with extracellularly added glycine in granule cells suggests that the efflux is at least partly mediated via membrane transport sites in these cells. Kainate stimulated the release more effectively in neurons than in glial cells, the effect apparently being mediated by specific kainate-sensitive receptors in both cell types. The release was enhanced by veratridine and by depolarization of cell membranes by high K (50 mM) in both neurons and astrocytes. The potassium-stimulated release was partially Ca-dependent in neurons but Ca-independent in glial cells. The results suggest a functional role for glycine in both cerebellar astrocytes and glutamatergic granule cells.

What confers specificity on glycine for its receptor site?

1. The structural requirements for activation of the glycine receptor were studied in isolated ventromedial hypothalamic neurones of rats by use of a 'concentration-clamp' technique under single-electrode voltage-clamp conditions. 2. alpha-Amino acids (L-alpha-alanine, and D-alpha-alanine, and L-serine), and glycine-methylester, glycine-ethylester and beta-amino acids (beta-alanine and taurine) produced a transient inward Cl- current, which was similar to that induced by glycine. 3. The responses to individual alpha- and beta-amino acids were selectively antagonized by strychnine, but were not affected by bicuculline, picrotoxin or the taurine antagonist, TAG (6-aminomethyl-3-methyl-4H,1,2,4-benzothiadiazine-1,1-dioxide hydrochloride), suggesting that alpha- and beta-amino acids activate the same glycine receptor. 4. beta-Amino acids were slightly more potent than the alpha-amino acids in causing cross-desensitization of the glycine response. 5. From the results of the structure-activity analysis of the optical isomers of alpha-alanine, serine and cysteine, a tentative structure of the glycine receptor is proposed.

Quantitative electron microscopic immunocytochemistry of neuroactive amino acids

Amino acids are of crucial importance in brain function, not only as metabolic intermediates and building blocks of proteins, but also as mediators of interneuronal communication. This dual role of the amino acids distinguishes them from other neurotransmitter candidates, and implies that they are unlikely to be restricted to neurons using them as transmitters. This calls for a quantitative approach when attempts are made to analyse the distribution of transmitter amino acids by means of immunocytochemistry. The present review deals with recent methodological developments that have made it possible to utilize specific antisera to explore the cellular and subcellular distribution of neuroactive amino acids in a quantitative manner.

Modulation of glutamate receptors by phencyclidine and glycine in the rat cerebellum: cGMP increase in vivo

In rats receiving N-methyl-D-aspartate (NMDA) intraventricularly or intracisternally the cerebellar cyclic guanosine monophosphate (cGMP) content increases in a dose-related manner. This response was used to study phencyclidine (PCP) and glycine interactions with the glutamate receptor subtype stimulated by NMDA. The increase of cGMP elicited by NMDA was inhibited by PCP and potentiated by glycine. Moreover, 2-amino-5-phosphonovalerate (APV) abolished the NMDA response. Since the increase in cerebellar cGMP induced by kainate, a synthetic agonist of another glutamate receptor subtype, was not modified by APV, the specificity of its action on NMDA response was confirmed. The increase of cerebellar cGMP content elicited by glycine was inhibited by PCP and APV but not by strychnine. Binding studies failed to demonstrate an apparent competitive interaction between PCP, glycine and NMDA. This suggests that the observed interaction is not of the isosteric type. The present results provide evidence that glycine, in vivo, acting at strychnine-insensitive recognition sites modulates allosterically in a positive manner the function of NMDA-sensitive glutamate receptors.

Somatostatin expression in the cerebellar cortex during postnatal development. An immunohistochemical study in the rat

The distribution of somatostatin-immunoreactive (SOM-IR) elements in the cerebellar cortex of the rat has been studied at different stages of postnatal development (from birth to day 30) and in adult animals using immunohistochemistry. The results showed that in vermis of new born animals there are three main groups of SOM-IR structures within the cortex which subsequently spread along the Purkinje cell layer. In addition, both in the vermis and in the lateral lobes, numerous more evenly distributed SOM-positive cells and fibers could be seen. SOM-IR Golgi cells, Purkinje cells and climbing fibers could then be recognized during the subsequent developmental stages. In the vermal zone, SOM-IR Purkinje cells formed patches, which seemed to be part of a sagittal columnar or band-like organization. This was most obvious between days 5 and 21 of postnatal development. Subsequently there was a reduction in the number of immunoreactive Purkinje cells but a patchy disposition remained. In addition high numbers of SOM-IR Purkinje and Golgi cells and also climbing fibers were identified in the flocculus and paraflocculus at all stages of development studied, and they were also seen in the adult rats in these regions. In the lateral lobes expression of SOM-like immunoreactivity (LI) decreased and almost completely disappeared in adult animals. The present results demonstrate that a SOM or a SOM-LI peptide can be transiently detected in many Purkinje and Golgi cells in the cerebellar cortex, suggesting a role in events related to developmental processes. However, in some regions and structures SOM-LI can be seen also in adult animals.

Colocalization of glycine-like and GABA-like immunoreactivities in Golgi cell terminals in the rat cerebellum: a postembedding light and electron microscopic study

Consecutive sections of rat cerebella were incubated with antisera raised against glycine or gamma-aminobutyric acid (GABA) conjugated to protein by glutaraldehyde. The sections were subsequently processed according to the peroxidase-antiperoxidase technique (semithin sections) or treated with a secondary antibody coupled to colloidal gold particles (ultrathin sections). Corroborating previous light microscopic observations based on pre-embedding immunocytochemistry, a major proportion (about 70%) of the Golgi cell bodies showed immunoreactivity for both glycine and GABA. Analyses of semithin sections further suggested that the two immunoreactivities were colocalized in the same glomeruli and even in the same Golgi cell terminals. This was confirmed by electron microscopy. Quantification of the immunogold labelling for glycine (which is assumed to play metabolic roles in addition to its presumed role as a transmitter) showed that the net gold particle density was an order of magnitude higher over Golgi cell terminals than over the other constituents of the cerebellar glomeruli (mossy fibre terminals and granule cell dendrites). The total particle density over the latter was only slightly higher than the background level (over empty resin), suggesting that the concentration of 'metabolic' glycine is generally low compared to the concentration of glycine in Golgi cells. The stellate and basket cell terminals (which similarly to the Golgi cells are thought to release GABA as transmitter) were immunoreactive for GABA, but (with very few exceptions) virtually unlabelled for glycine, suggesting that our results were not confounded by any crossreactivity of the glycine antiserum with fixed GABA. Direct evidence that the sera reacted selectively with fixed glycine or GABA under the conditions used was obtained by incubating the tissue sections together with test sections containing a series of different amino acid-glutaraldehyde-brain macromolecule conjugates. Adsorption tests with soluble amino acid-glutaraldehyde complexes similarly suggested that the double-labelling of the Golgi terminals indeed reflected a colocalization of glycine and GABA. The results show that two 'classical' transmitters, both being inhibitory and acting on Cl- channels, may coexist in the same nerve terminals.

Stimulus-coupled release of amino acids from cerebellar granule cells in culture

Cerebellar cultures greatly enriched in excitatory granule neurons were depolarized by exposure to either elevated K+ or veratrine. Stimulation of the release of not only Glu, but also of certain amino acids, including Gly, Ala and Ser, was observed. The effect was specific, as depolarization did not induce the release of all the estimated amino acids or of lactate dehydrogenase. In comparison with the characteristics of the evoked release of Glu, those of the responsive neutral amino acids were similar in terms of Ca2+-dependence, but differences were also noted. Thus, upon stimulation, the relative rise was smaller than for Glu and the degree of depolarization causing maximal release was lower. The questions of whether stimulus-coupled release of the non-transmitter amino acids from granule cells may play a neuromodulatory role in the cerebellum is discussed.

Localization of glycine receptors in the rat central nervous system: an immunocytochemical analysis using monoclonal antibody

The localization of glycine receptors was immunocytochemically examined in the rat brain using a monoclonal antibody against the affinity-purified glycine receptor. Glycine receptors were concentrated in the lower brainstem, whereas no immunoreactivity was observed in the diencephalon and forebrain except in a few diencephalic nuclei. The highest density of receptors was found in the cranial motor nuclei, reticular formation, parabrachial area, dorsal and ventral cochlear nuclei, and dorsal and ventral tegmental nuclei. Differences were observed in the distribution of immunoreactive elements in the various brain regions. In the cerebellar cortex, the immunoreactivity was exclusively seen along the dendrites of the Purkinje cells. On the other hand, glycine receptors were detected on the cellular membrane of the soma of the cochlear nuclei, trigeminal motor nucleus, parabrachial area, lateral reticular nucleus, dorsal nucleus of the lateral lemniscus, cerebellar nuclei, trigeminal spinal nucleus, anterior horn and reticular formation. In other regions, the receptors were evenly distributed throughout the neuropil.

Neurotransmitters of the cerebellar glomeruli: uptake and release of labeled gamma-aminobutyric acid, glycine, serotonin and choline in a purified glomerulus fraction and in granular layer slices

We have studied some properties of the uptake and release of labeled gamma-aminobutyric acid (GABA), glycine, serotonin and choline in a purified fraction of glomeruli and in slices of the granular layer of the rat cerebellum. The uptake of both GABA and glycine into the glomerulus particles was dependent on the presence of Na+ in the medium. In contrast, the uptake of both serotonin and choline was Na+-independent. In slices of the granular layer also a slight Na+-dependence was observed for both serotonin and choline uptake; imipramine and hemicholinium partially inhibited the uptake of serotonin and choline, respectively. Choline uptake into the glomerulus particles showed two components, with apparent Km values of 16.8 and 102 microM. GABA release was stimulated by K+-depolarization about 100% (peak stimulation) and this value was reduced to 50% when Ca2+ was omitted. The release of glycine was stimulated more rapidly and notably than GABA (200%) and this stimulation was completely abolished in the absence of Ca2+. Serotonin release from the glomerulus particles was only slightly stimulated by depolarization, but this stimulation was strictly Ca2+-dependent. In slices of the granular layer, this stimulation was considerably larger (about 40%) and it was also almost totally dependent on Ca2+. In contrast, after loading with labeled choline the release of radioactivity from both the glomerulus particles and the cerebellar slices was not stimulated at all by K+-depolarization, either in the presence or in the absence of Ca2+. Most of the radioactivity released spontaneously corresponded to choline, and only a small proportion (8-14%) to acetylcholine. From the results of the release experiments and taking into account the pertinent data from the literature, it is concluded that GABA and glycine are probably the transmitters of different populations of Golgi axon terminals, whereas serotonin might be the transmitter of at least a certain population of the mossy fiber giant terminals, in the rat cerebellar glomeruli. In contrast, acetylcholine does not seem to have any transmitter role in the synaptic structures of the glomeruli.

Glycine immunoreactivity localized in the cochlear nucleus and superior olivary complex

Polyclonal antibodies were made in rabbits against glycine conjugated to bovine serum albumin with glutaraldehyde and were used for immunocytochemical studies in the cochlear nucleus and superior olivary nucleus of the guinea-pig. Antibodies selective for glycine were prepared by affinity chromatography. By dot-blot analysis this preparation showed a strong recognition of glycine conjugates and relatively little recognition of conjugates of most other amino acids tested. However, there was a significant reaction with conjugates of alanine and beta-alanine, and this cross-reaction could not be removed by affinity chromatography without eliminating the preparation's recognition of glycine. The affinity-purified preparation showed only a weak recognition of conjugates of gamma-aminobutyrate (GABA) which was detectable at high concentrations of primary antibody. Immunocytochemical studies showed several intensely staining cell bodies in the cochlear nucleus and superior olivary complex. Most immunoreactive cell bodies in the cochlear nucleus were in the dorsal cochlear nucleus, being present in both the superficial and deep layers. Scattered immunoreactive cells were present in the ventral cochlear nucleus. Intense staining of cell bodies was seen in the medial nucleus of the trapezoid body, and these cells appear to correspond to the principal cells of that nucleus. Punctate labelling, suggestive of immunoreactive presynaptic terminals, was also apparent, particularly in the ventral cochlear nucleus and lateral superior olive. In the ventral cochlear nucleus, immunoreactive puncta were found around unlabeled cell bodies, at times nearly covering the perimeter of the cell. A population of glycine-immunoreactive cell bodies in the superficial dorsal cochlear nucleus also labeled with anti-GABA antibodies as determined through double-labeling studies. However, glycine-positive cells in the deep dorsal cochlear nucleus were not labeled with anti-GABA antibodies, and some populations of GABA-positive cells in the superficial layers were not labeled with anti-glycine antibodies. In the hippocampus intense staining of cell bodies and puncta was seen with anti-GABA antibodies while essentially no staining was seen with anti-glycine antibodies. These results suggest that anti-glycine antibodies can be useful for immunocytochemical identification of glycinergic neurons. From this study several populations of putative glycinergic neurons are identified in the auditory nuclei of the brain stem using these antibodies. Some populations of GABA-containing neurons also contain high levels of glycine or a related molecule.

gamma-Aminobutyric acid-containing terminals can be apposed to glycine receptors at central synapses

The distributions of terminals containing gamma-aminobutyric acid (GABA) and of endings apposed to glycine receptors were investigated cytochemically in the ventral horn of the rat spinal cord. For this purpose, a polyclonal antibody raised to recognize glutamic acid decarboxylase (GAD), a synthetic enzyme for GABA, and three monoclonal antibodies (mAb's) directed against the glycine receptor were used. Double immunofluorescence showed that, surprisingly, GAD-positive terminals are closely associated in this system with glycine receptors at all the investigated cells, most of which were spinal motoneurons. Furthermore, double labeling was performed with immunoenzymatic recognition of GAD and indirect marking of mAb's with colloidal gold. With this combined approach, it was found, at the electron microscopic level, that all GAD-positive terminals are in direct apposition with glycine receptors while, on the other hand, not all glycine receptors are in front of GABA-containing boutons. This result is not due to a cross-reactivity of mAb's with GABA receptors as shown by using as a control synapses known to use GABA as a neurotransmitter in the cerebellar cortex. Indeed, no glycine receptor immunoreactivity was detected on Purkinje cells facing basket axon terminals. However, Purkinje neurons can express glycine receptor immunoreactivity at other synaptic contacts. Assuming that the presence of postsynaptic receptors for glycine indicates that this amino acid is used for neurotransmission at a given synapse, our results strongly support the notion that GABA and glycine, two classical inhibitory transmitters, coexist at some central connections. However, such is not always the case; in the cerebellum, Golgi terminals impinging on the dendrites of granule cells are either GAD-positive or face glycine receptors, in a well-segregated manner.

Light microscopic autoradiographic localisation of [3H]glycine and [3H]strychnine binding sites in rat brain

Receptor autoradiography has been employed to determine the distribution of strychnine-insensitive glycine binding sites in rat brain using [3H]glycine as a ligand. The location was significantly different from and more widespread than glycine sensitive [3H]strychnine binding sites. Highest binding densities were observed in hippocampus, cortex, subiculum and amygdala followed by striatum, cerebellum and olfactory areas. Characterisation of the binding indicated that it was saturable, of high affinity, stereoselective and displaced by structurally related amino acids. The results support the existence of two glycine receptor subtypes: strychnine-sensitive and strychnine-insensitive.

Sites of D-[3H]aspartate accumulation in mouse cerebellar slices

Slices of mouse cerebellar vermis, cut in the parasagittal plane, were incubated for various times (up to 3 h) in the presence of 1 microM D-[3H]aspartate, a non-metabolized substrate for the glutamate/aspartate carrier in brain tissue. Light microscopic autoradiography indicated that in regions away from the cut edges of the slices the amino acid accumulated in glia and granule cells. Relatively few grains were seen over Purkinje, Golgi, stellate and basket cells or over white matter. Grain counts over the granule cell layers in the middle parts of the slices indicated that after short (15 min) exposures to the labelled substrate, non-granule cell areas (which included glia) contained, on average, slightly more grains than granule cells but with longer exposures (1.5 and 3 h) the relative grain density over granule cells became much higher, possibly because glial uptake prevents D-[3H]aspartate gaining access to neuronal sites in adequate amounts during short incubations and/or because the longer incubations allow time for retrograde migration of the label from parallel fibre terminals to occur. The demonstration of selective uptake of D-[3H]aspartate into granule cells contrasts with previous autoradiographic results (possible reasons for which are discussed) and supports the notion that L-glutamate is the transmitter of granule cells. The results also have a bearing on the importance of the metabolic compartmentation of glutamate in relation to its proposed transmitter role.

Concentration of free amino acids in primary cultures of neurones and astrocytes

The cellular distribution of free amino acids was estimated in primary cultures (14 days in vitro) composed principally of cerebellar interneurones or cerebellar and forebrain astrocytes. In cultured neural cells, the overall concentration of amino acids resembled that found in brain at the corresponding age in vivo. In the two neural cell types, there were marked differences in the distribution of amino acids, in particular, those associated with the metabolic compartmentation of glutamate. In neuronal cell cultures, the concentrations of glutamate, aspartate, and gamma-aminobutyric acid were, respectively, about three, four, and seven times greater than in astrocytes. By contrast, the amount of glutamine was approximately 65% greater in astroglial cell cultures than in interneurone cultures. An unexpected finding was a very high concentration of glycine in astrocytes derived from 8-day-old cerebellum, but the concentrations of both serine and glycine were greater in nerve cell cultures than in forebrain astrocytes. The essential amino acids threonine, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, lysine, and arginine were all present in the growth medium, and small cellular changes in the contents of some of these amino acids may relate to differences in their influx and efflux during culturing and washing procedures. The present results, together with our previous findings, provide further support for the model assigning the "small" compartment of glutamate to glial cells and the "large" compartment to neurones, and also underline the metabolic interaction between these two cell types in the brain.

Release of endogenous and accumulated exogenous amino acids from slices of normal and climbing fibre-deprived rat cerebellar slices

Efflux of various amino acids from slices of rat cerebellar hemispheres was determined under resting or depolarizing conditions. It was increased under high K+(50 mM) as compared to low K+ (5 mM) conditions by 1258 pmol/mg protein for aspartate, 478 for gamma-aminobutyric acid (GABA), 44,693 for glutamate, and 615 for glycine. These were significantly higher than the corresponding values obtained under low-Ca2+ (0.1 mM), high-Mg2+ (12 mM) conditions, whereas for 11 other amino acids the K+-induced efflux was similar under normal and low-Ca2+ concentrations. The K+-induced efflux of exogenously accumulated L-[3H]aspartate, D-[3H]aspartate, and L-[3H]glutamate was higher by factors of 2, 5.8, and 6.3, respectively, under normal Ca2+ conditions, as compared with low-Ca2+, high-Mg2+ conditions. After climbing fibre degeneration induced by destruction of the inferior olive with 3-acetylpyridine, release of endogenous aspartate and exogenous L-[3H]glutamate and D-[3H]aspartate was significantly reduced, by 26%, 38%, and 27%, respectively. These results support the hypothesis that climbing fibres may use aspartate or a related compound as a neurotransmitter. In rat cerebellar tissue, L-[3H]glutamate and L-[3H]aspartate differ in several aspects: (1) L-[3H]glutamate uptake was 4 times higher than that of L-[3H]aspartate; (2) fractional rate constant of K+- evoked release of L-[3H]aspartate was 7% X 2.5 min-1, and of L-[3H]glutamate 36% X 2.5 min-1; and (3) specific activity of L-[3H]glutamate in the eluate collected during K+ stimulation was 3.5 times the value in the tissue, whereas for L-[3H]aspartate, specific activities in the eluate and tissue were similar.