Action and uptake of neurotransmitters in CNS tissue culture

Extremely Low Frequency Magnetic Fields Modulate Bicuculline-lnduced-Convulsion in Rats

The effect of extremely low frequency (ELF, 60 Hz) magnetic fields (MFs) on convulsions was investigated in rats. We determined the onset and duration of convulsions induced by bicuculline alone or by co-exposure to MFs and bicuculline. In addition, we measured the GABA concentrations in the rat brains using HPLC-ECD. MFs strengthened the convulsion induced by bicuculline (0.3, 1, and 3 microg, i.c.v.), with a shortening of the onset time, but lengthening of the duration time. Co-exposure to MFs and bicuculline decreased the GABA levels in the cortex, hippocampus and hypothalamus, whereas MFs alone reduced the level of GABA only in the hippocampus. These results suggest that the exposure to MFs may modulate bicuculline-induced convulsions due to GABA neurotransmissions in rat brains.

Baclofen analgesia: involvement of the GABAergic system

The effect of baclofen, a GABA(B) agonist, has been studied in the hot plate test in mice, to analyze the possible involvement of the GABAergic system in baclofen analgesia. Baclofen (1-3 mg kg(-1) intraperitoneal (i.p.)) was found to elicit a dose-dependent antinociceptive effect. The antinociceptive effect of baclofen cannot be due to motor incoordination or sedation as the doses of baclofen which produce analgesia did not induce these effects during the rota-rod test. The antinociceptive effect of baclofen was reversed by 2-hydroxysaclofen, a GABA(B) antagonist by both systemic (3 mg kg(-1)) and intra cisterna magna (intracisternal (i.c.)) (0.3 mg kg(-1)) administration. The antagonist dose administered via i.c. produced a complete blockade and was 10-fold lower than the dose employed in i.p. administration. The data suggest that the antinociceptive effect of baclofen is GABA(B) receptor-mediated and reveal a central location of the analgesic effect of baclofen.

Pharmacokinetic-pharmacodynamic modeling of the antinociceptive effect of baclofen in mice

The aim of this study was the development of a pharmacokinetic-pharmacodynamic (PK/PD) model of the antinociceptive effect of baclofen in mice. We studied the dose response curve of the analgesic action of baclofen in mice by hot plate test. Baclofen produced a dose dependent antinociceptive effect with doses between 1-3 mg/kg administered intraperitoneally (i.p.) (ED50: 1.94 mg/kg of racemate) and this effect fits to a linear pharmacodynamic model. Blood and brain concentrations of (-)3H-baclofen were determined by Thin-Layer Chromatography (TLC) and counted in the scintillation-counter. The PK/PD models were analyzed with the PC-TOPFIT V.2.0 and the tests for distinguishing between models were several adjustment parameters as Akaike information criterion (AIC), Imbimbo criterion (Ip), standard deviation (SD) and the correlation coefficient (r2). Accordingly with these adjustment parameters, a 2 compartment open model was selected where plasma is the central compartment and brain is in the peripheral compartment. In this model, the effect is linked to the peripheral compartment. When the antinociceptive effect of baclofen was plotted against blood concentration, the resulting curve exhibited an anticlockwise hysteresis loop, but on the other hand, when the antinociceptive effect was plotted against the brain concentration, the hysteresis was collapsed. These results confirmed the selected model in our study, as the best adjustment was shown when the pharmacological response was linked to the peripheral compartment.

The influences of extremely low frequency magnetic fields on clonidine-induced sleep in 2-day-old chicks

1. It has been shown that magnetic fields (MFs) affect a variety of biological effects in animal brains. There have been few experiments on the effects of MFs on sleep. Therefore, we investigated whether extremely low frequency (ELF) MFs affect the sleep induced by clonidine, a central alpha(2)-adrenoceptor agonist. Clonidine produced dose-related increase of the sleeping time and dose-related decrease of the onset time in 2-day-old chicks. 2. Exposure of chicks to MFs (5, 10, 20 G; for 3, 6, 9, 12 h) significantly increased the clonidine-induced sleep time as a direct function of intensity and duration of MF application. Clonidine reduced noradrenaline or tyrosine in the brain, an effect which was not further changed in animals exposed to MF. 3. To determine whether the gamma amino butyric acid A (GABA(A))/benzodiazepine (BZD) receptor system is involved in the decrease in clonidine-induced sleep caused by activation of central alpha(2)-adrenergic systems, we examined exposure of chicks to the effects of the BZD receptor antagonist flumazenil (0.5 mg kg(-1), i.p.) and GABA(A) antagonist bicuculline (0.1 mg kg-1, i.p.) on clonidine-induced sleep. Bicuculline and flumazenil inhibited the increase of clonidine-induced sleep time by MFs. Clonidine or MFs did not change GABA levels in the brain. 4. These results suggest that MFs can increase clonidine-induced sleep via a change of GABA(A) and BZD receptor system irrespective of the concentration of GABA or noradrenaline in the brain of 2-day-old chicks.

GABA(A) but not GABA(B) receptor stimulation induces antianxiety profile in rats

The effect of GABA receptor agonists and antagonists on anxiety behavior in rats in the elevated-plus-maze has been investigated. The increase in two parameters of %open arm entries (%OAE) and %time spent in the open arms (%OAT) and decrease in the %time spent in closed arm (%CAT) was considered as antianxiety effects. Intracerebroventricular (i.c.v.) injection of different doses of the GABA(A) receptor agonist muscimol (0.25, 0.5, and 1 microg/rat) increased %OAE and %OAT and decreased %CAT in rats dose-dependently. The higher response was obtained with 1 microg/rat of the drug. Neither icv (0.05, 0.1, and 0.2 microg/rat) nor intraperitoneal (i.p.) (1, 2, and 4 mg/kg) injection of the GABA(B) receptor agonist baclofen altered %OAE, %OAT, and %CAT. However, the GABA(B) receptor antagonist CGP35348 (5, 10, and 30 microg/rat i.c.v.) increased %OAE and %OAT and decreased %CAT in the animals. The response induced by injection of muscimol (0.5 microg/rat i.c.v.) or administration of CGP35348 (10 microg/rat i.c.v.) was reduced by i.c.v. (1, 2, and 4 microg/rat) or i.p. (0.25, 0.5, and 0.75 mg/kg) injection of the GABA(A) receptor antagonist bicuculline, except the effect of CGP35348 on %CAT which was not significantly altered by i.p. administration of bicuculline. Ip but not i.c.v. administration of bicuculline by itself reduced both %OAE and %OAT but did not alter %CAT. None of the drugs altered the locomotor activity of the animals. The current findings support our hypothesis that the anxiolytic effects of GABA(B) antagonist are mediated by autoreceptor blockade-induced release of endogenous GABA, which in turn activates postsynaptic GABA(A) receptors.

Pharmacological and functional characterization of astrocytic GABA transport: a short review

GABA neurotransmission is terminated by high affinity transport mediated by a number of carriers on neurons and astrocytes. So far four different carriers have been cloned and their cellular distribution has been partly worked out. It is generally believed that GAT-1 (mouse homologue GAT1) is the quantitatively most important of the transporters and it is primarily present on GABAergic neurons but also to some extent on astrocytes. The pharmacological properties of neuronal and astrocytic GABA uptake have been studied extensively and recently the GABA analogue N-methyl-Exo-THPO has been reported to act as a selective and potent (IC50 28 microM) astroglial GABA transport inhibitor with a 15-fold selectivity. It has moreover been reported to act as an anticonvulsant in animal models of epilepsy. This may underline the functional importance of astrocytic GABA uptake in relation to seizure activity.

Effects of GABAergic system on naloxone-induced jumping in morphine-dependent mice

The effect of GABA (gamma-aminobutyric acid system) receptor agonists and antagonists on naloxone-induced jumping in morphine-dependent mice was examined. Intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) injection of different doses of the GABA(B) receptor agonist, baclofen (2.5, 5 and 10 mg/kg), reduced naloxone-induced jumping in morphine-dependent mice. The i.p. administration of the GABA(B) receptor antagonist, CGP35348 (P-[3-aminopropyl]-p-diethoxymethyl-phosphinic acid), but not the i.c.v. injection of the drug, increased naloxone-induced jumping. The antagonist also decreased the baclofen response. Administration of the GABA(A) receptor agonist, muscimol, but not the GABA(A) receptor antagonists bicuculline and picrotoxin, decreased the naloxone response in morphine-dependent animals. It is concluded that both GABA(A) and GABA(B) receptor subtypes may have an inhibitory influence on naloxone-induced withdrawal jumping in mice.

Effects of CCK antagonists on GABA mechanism-induced antinociception in the formalin test

In this work, the influences of CCK receptor antagonists on antinociception induced by the GABA receptor agonist, baclofen, and the GABA uptake inhibitor, THPO, in the formalin test have been studied. GABA-B agonist baclofen (0.75, 1.25 and 2.5 mg/kg), THPO, a GABA uptake inhibitor (1 and 2 mg/kg) and morphine (1.5, 3 and 6 mg/kg) induced antinociception in both phases of the formalin test in mice. The selective CCK receptor antagonists, L-365,260, MK-329 (0.05, 0.125 and 0.25 mg/kg) and non-selective CCK receptor antagonist, proglumide (2.5, 5, 10 and 20 mg/kg) induced antinociception only in high doses. The CCK receptor antagonists potentiated baclofen (0.75, 1.25 and 2.5 mg/kg) or THPO (1 and 2 mg/kg) responses. It may be concluded that the CCK receptor mechanism may interact with GABA-function in its antinociceptive effect.

Interactions between antinociception induced by cholecystokinin antagonists and GABA agonists in the tail-flick test

In this work, the influences of cholecystokinin receptor antagonists L-365,260, MK-329 and proglumide on antinociception induced by baclofen and GABA uptake inhibitor 4,5,6,7-tetrahydroisoxazolo [4,5-c]pyridin-3-ol (THPO) in the tail flick test has been studied. Baclofen and THPO induced antinociception in the tail flick test. Morphine, and the CCK receptor antagonists, MK-329, L-365,260 and proglumide also induced antinociception. The CCK receptor antagonists potentiated antinociceptive response induced by both baclofen and THPO. It may be concluded that cholecystokinin receptor mechanism(s) may interact with antinociception induced by GABA receptor mechanism(s).

Effects of GABAergic drugs on physostigmine-induced improvement in memory acquisition of passive avoidance learning in mice

1. The effect of gamma-aminobutyric acid (GABA) receptor agonists and antagonists on acquisition of a step-down passive avoidance learning in mice was measured in the presence and absence of physostigmine. 2. Intraperitoneal injection of different doses of the anticholinesterase drug physostigmine (0.1-0.3 mg/kg) increased acquisition in mice dose dependently. The maximum response was obtained with 0.3 mg/kg of the drug. Higher doses of the drug impaired acquisition of the learned response. To show the effect of the GABAergic system on acquisition, GABAA receptor agonists and antagonists were challenged against 0.2 mg/kg of physostigmine. 3. Administration of the GABAA receptor agonist muscimol but not the GABAB receptor agonist baclofen decreased the acquisition of the learned task. However, the improvement induced by physostigmine (0.2 mg/kg) was decreased by both muscimol and baclofen. A combination of both agonists caused a higher inhibitory effect on the physostigmine response. 4. Pretreatment of animals with the higher doses of GABAA receptor antagonists bicuculline and picrotoxin but not the GABAB receptor antagonist phaclofen impaired learning. Both the GABAA and GABAB receptor antagonists reduced the learning improvement induced by physostigmine. The inhibitory effects of the GABAA and GABAB receptor antagonists are lost when combined together. 5. Bicuculline, picrotoxin or phaclofen increased the impairment of learning induced by muscimol, whereas a combination of either of the antagonists with baclofen did not alter the learning. The GABAA antagonists reduced the inhibitory effect of muscimol, whereas a higher dose of phaclofen increased the inhibition of the physostigmine response induced by muscimol and baclofen on physostigmine-induced learning improvement. 6. Phaclofen decreased but a higher dose of bicuculline increased the baclofen-induced inhibition of physostigmine effect. 7. It is concluded that both GABAA and GABAB activation inhibit improvement of acquisition induced by physostigmine.

Evidence for the existence of galanin receptors on cultured astrocytes of rat CNS: colocalization with cholinergic receptors

The cellular localization of binding sites for [125I]galanin was studied in explant cultures of rat neocortex, cerebellum, locus coeruleus and spinal cord by means by of autoradiography. Binding sites for the peptide were observed on a great number of astrocytes in all CNS regions studied. In addition to astrocytes, many neurones were intensely labelled by [125I]galanin. Binding of [125I]galanin (10(-8) M) to both astrocytes and neurones was markedly reduced or inhibited by the unlabelled peptide at high concentration (10(-6) M), suggesting 'specific' binding of the radioligand. Evidence for the colocalization of galanin and cholinergic receptors on astrocytes was provided by combined autoradiographic and immunohistochemical studies. Many astrocytes were labelled by [125I]galanin and immunostained with antibodies to either muscarinic or nicotinic receptors. Electrophysiological studies revealed that addition of galanin (10(-9) to 10(-7) M) to the bathing fluid caused a dose-dependent hyperpolarization of the majority of astrocytes studied. When galanin (10(-8) M) and the cholinergic agonists muscarine and nicotine (10(-6) M) were tested on the same astrocyte, all three compounds induced a hyperpolarization, suggesting a colocalization of functional galanin and cholinergic receptors on the glial membrane.

The rapid L- and D-aspartate uptake in cultured astrocytes

Astrocytes in primary culture possess a rapid L-aspartate saturable transport system (K(m) = 93 microM; V(max) = 81 nmol/min/mg protein), which shows certain stereospecificity since V(max) was 36% lower for D-aspartate uptake. These are values obtained at short incubation time (15 seconds), to obtain approximate initial rate conditions. Metabolic energy inhibitors, rotenone and iodoacetate very potently inhibited the L- and D-aspartate uptake processes, indicating that the transport process is an active one. However, the accumulation of L-aspartate was "enhanced" by inhibitors of L-aspartate metabolism, such as the aspartate aminotransferase inhibitor, aminooxyacetate and L-methionine sulfoximine, an inhibitor of glutamine synthetase, whereas D-aspartate (a non-metabolizable analog of L-aspartate) uptake was not affected. The accumulated levels of L-aspartate in the presence of aminooxyacetate were similar to the levels of D-aspartate. These effects of L-aspartate metabolic inhibitors, suggest that due to metabolism of the the L-aspartate, short incubation time (eg., 15 seconds) is necessary to measure the initial rate of L-aspartate uptake, in order to obtain the "true" kinetic parameters.

Autoradiographic studies on the uptake of 3H-dopamine by neurons and astrocytes in explant and primary cultures of rat CNS: effects of uptake inhibitors

The cellular localization of the uptake of 3H-dopamine was studies in explant and primary cultures from various regions of rat central nervous system by means of autoradiography. In explant cultures of substantia nigra, 3H-dopamine was taken up by cell bodies and processes of many neurons. In cultures from striatum, cerebellum and spinal cord, neuronal cell bodies were not labelled, whereas outgrowing nerve fibres revealed intense uptake of the monoamine. Uptake of 3H-dopamine by neurons was Na(+)- and temperature-dependent, suggesting an active uptake mechanism. In explant cultures, astrocytes did not accumulate 3H-dopamine, whereas in primary cultures, which were prepared from the same regions of rat central nervous system as the explant cultures, astrocytes also revealed uptake of this monoamine. The intensity of labelling was dependent on the incubation time. Little uptake of 3H-dopamine was observed after an incubation time of 5 min and only after 10-15 min did the astrocytes show moderate labelling. Uptake of 3H-dopamine by astrocytes was not Na(+)- and temperature-dependent, indicating that glial cells do not possess an active uptake mechanism for this monoamine. This is consistent with biochemical investigations by other laboratories, demonstrating that astrocytes accumulate 3H-dopamine by a facilitated diffusion system. Addition of the uptake inhibitors nomifensine or GBR 12909 to explant cultures markedly reduced or inhibited uptake of 3H-dopamine by neurons at a concentration of 10(-6) M. In contrast, accumulation of 3H-dopamine by astrocytes in primary cultures was only slightly affected by nomifensine at 10(-6) M. At the highest concentration used (10(-5) M), nomifensine also blocked the uptake of 3H-dopamine by astrocytes. Our finding that GBR 12909 almost completely inhibited the uptake of 3H-dopamine by astrocytes already at 10(-6) M suggests that this compound is a more potent inhibitor of the glial uptake of dopamine than nomifensine.

Binding and competitive inhibition of amine uptake at postsynaptic neurones (transport-P) by tricyclic antidepressants

1. We have provided evidence for a novel amine uptake process for which prazosin is a substrate in postsynaptic neurones, characterized by a paradoxical increase in accumulation of the radioligand when the concentration of the unlabelled drug is increased above 10(-7) M. This increase is due to activation of a proton-dependent, vacuolar type-ATPase-linked uptake process which is blocked by desipramine but is resistant to reserpine. We have now examined the effects of tricyclic antidepressants on this uptake system in a cell line derived from hypothalamic peptidergic neurones, known to be innervated by noradrenergic nerve terminals in vivo. 2. [3H]-imipramine bound to the cells and was displaced by unlabelled imipramine, desipramine, amitriptyline and nortriptyline. The data fitted a single binding site model. This is the first demonstration of antidepressant binding sites in postsynaptic neurones. 3. There was no increase in the binding of [3H]-imipramine at high concentrations of unlabelled imipramine, suggesting that antidepressants inhibit uptake but are not themselves accumulated by peptidergic gonadotrophin releasing hormone neurones. 4. Accumulation of prazosin was competitively inhibited by antidepressants. Tertiary amines were slightly more potent than secondary amines and the presence of a nitrogen atom in the heterocyclic ring enhanced blocking activity. 5. The affinities of the antidepressants for the uptake process are within the range of plasma concentrations that are observed during therapeutic use of these compounds. Since it is likely that this uptake process has a physiological function, its inhibition by antidepressants may provide a new avenue for investigating the mechanism of action of these compounds.

Functional properties of the uptake of amines in immortalised peptidergic neurones (transport-P)

1. Most neurotransmitters are inactivated by uptake into presynaptic nerve terminals and into glial cells. We recently provided evidence for uptake of amines in postsynaptic neurones. Uptake was evident at nanomolar concentrations of prazosin, but at concentrations of unlabelled prazosin greater than 10(-7) M, there was a further activation of uptake, manifested by a paradoxical increase in accumulation of the radioligand. We have now studied further characteristics of amine uptake in immortalised gonadotrophin-releasing hormone (GnRH) neurones. Control cells included SK-N-SH neuroblastoma cells (which possess presynaptic type amine transporters) and non-neuronal (COS-7) cells. 2. [3H]-prazosin bound to intact GnRH cells and was displaced by unlabelled prazosin in concentrations of 10(-9) to 10(-7) M. However, at higher concentrations of unlabelled prazosin, there was an increase in apparent [3H]-prazosin binding, as we had previously described. This paradoxical increase in accumulation of the radioligand was abolished by desipramine. 3. Desipramine had no effect on the association of prazosin with COS-7 cells. There was no paradoxical increase in accumulation of [3H]-prazosin in COS-7 cells, indicating that this effect requires the presence of a desipramine-blockable uptake process. 4. The increase in binding of the radioligand that was observed in the GnRH cells is not a general property of neuronal transporters; in SK-N-SH cells, there was no increase in accumulation of (-)-[3H]-noradrenaline in the presence of concentrations of unlabelled (-)-noradrenaline greater than 10(-7) M. 5. The uptake of prazosin and the increase in accumulation of [3H]-prazosin were abolished in the cold, indicating that this is an active, energy-requiring process. 6. Desipramine-sensitive uptake of prazosin was demonstrable in the GnRH cells in the absence of sodium. Further, the Na+/K(+)-ATPase inhibitor, vanadate, abolished noradrenaline uptake in SK-N-SH cells but had no effect on prazosin uptake in GnRH cells. Thus, the uptake of prazosin does not derive its energy from the sodium pump. 7. Prazosin uptake was inhibited by the V-ATPase inhibitor bafilomycin A1, the H+/Na+ ionophore, monensin and the organic base, chloroquine, indicating that uptake derives its energy from a proton pump. In contrast to other proton-dependent amine transporters, the uptake of prazosin was unaffected by reserpine. 8. Increasing extracellular pH did not increase the uptake of prazosin into GnRH cells, indicating that it is unlikely to be due to non-specific diffusion and concentration of a lysosomotropic drug into intracellular acidic particles. 9. The uptake of prazosin was unaffected by steroid hormones. 10. In COS-7 cells transfected with alpha 1-adrenoceptor cDNA, [3H]-prazosin was displaced by unlabelled prazosin without causing an increase in binding of the radioligand. This indicated that the increase in accumulation of the radioligand is unlikely to be due simply to some function of alpha 1-adrenoceptors. 11. Thus, peptidergic neurones possess an uptake process with properties that are distinguishable from known amine transporters.

Effects of GABAergic drugs on physostigmine-induced yawning in rats

In the present work the effects of GABA agonists and antagonists on yawning induced by physostigmine have been studied. Intraperitoneally (IP) injection of physostigmine (0.05-0.3 mg/kg) induced dose-related yawning in rats. The maximum yawning response was observed with 0.2 mg/kg of the drug. The GABA agonists muscimol (1-4 mg/kg) and baclofen (0.125-1 mg/kg) decreased yawning induced by physostigmine (0.2 mg/kg) dose dependently. Combination of both GABA agonists elicited greater inhibition of yawning. The GABA-A antagonists bicuculline or picrotoxin but not the GABA-B antagonist phaclofen reduced the inhibitory response induced by muscimol, whereas phaclofen but not bicuculline or picrotoxin reduced baclofen's inhibitory effect. Administration of bicuculline, picrotoxin or phaclofen also decreased the yawning induced by physostigmine. However, when the GABA-A and GABA-B antagonists were employed in combination, the inhibitory responses of both drugs were lost. It is concluded that GABA-A and/or GABA-B receptor stimulation may inhibit physostigmine-induced yawning.

Autoradiographic studies on the uptake of 3H-noradrenaline and 3H-serotonin by neurones and astrocytes in explant and primary cultures of rat CNS: effects of antidepressants

Autoradiographic studies were made on the uptake of 3H-noradrenaline and 3H-serotonin in explant cultures and primary astrocyte cultures from various regions of rat central nervous system (cortex, cerebellum, locus coeruleus, nucleus raphé, spinal cord). In explant cultures from locus coeruleus and nucleus raphé cell bodies and processes of many neurones revealed intense labelling by 3H-noradrenaline and 3H-serotonin, respectively. In cultures from cortex, cerebellum and spinal cord the cell bodies of neurones did not show labelling by the monoamines but many nerve fibres in the outgrowth zone had taken up 3H-noradrenaline and 3H-serotonin. Astrocytes in explant cultures did not take up 3H-noradrenaline and 3H-serotonin whereas astrocytes in primary cultures showed heavy uptake of both monoamines. In contrast, amino acid transmitters such as 3H-GABA and 3H-glutamate were accumulated by astrocytes in explant as well as in primary cultures. Uptake of both 3H-noradrenaline and 3H-serotonin by neurones and astrocytes was considerably reduced or inhibited in Na(+)-free incubation medium or at low temperature, suggesting an active uptake mechanism. Addition of the antidepressants maprotiline and (+)oxaprotiline inhibited the uptake of 3H-noradrenaline by neuronal cell bodies and fibres in explant cultures and by astrocytes in primary cultures. The uptake of 3H-serotonin by neurones and astrocytes was blocked by citalopram and paroxetine. Our studies demonstrate that astrocytes in primary cultures are able to actively take up 3H-noradrenaline and 3H-serotonin whereas there was no uptake of monoamines into astrocytes in explant cultures, suggesting that there is a difference between astrocytes in different culture systems (explant cultures vs primary cultures) with respect to the uptake of monoamine transmitters.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of adenosine receptors in a model of cultured neurons from rat forebrain

The neuromodulator adenosine is acting through specific receptors coupled to adenylate cyclase via G-proteins. The expression of both adenosine receptors A1 and A2 as well as forskolin binding sites was investigated by radioligand binding techniques in 8-day-old neurons isolated from fetal rat forebrain and cultured in chemically-defined medium. Adenosine A1 receptors were specifically labeled with [3H]chloro-N6-cyclopentyladenosine (CCPA), whereas [3H]CGS 21680 was used for the analysis of A2 receptors. Cultured neurons exhibited high affinity binding sites for CCPA (Bmax = 160 fmol/mg protein; Kd = 2.9 nM), and for CGS 21680 (Bmax = 14 fmol/mg protein; Kd = 1.7 nM). These data correlate well with those obtained in crude membranes isolated from the newborn rat forebrain. The incubation of culture membranes in the additional presence of guanylyl-5'-imidodiphosphate (Gpp(NH)p, a GTP analogue) led to significantly increased Kd-values, suggesting the association of adenosine receptors with G-proteins. Finally, cultured neurons also bound specifically [3H]forskolin with characteristics close to those found in the newborn brain, indicating that cultured neurons appear as an appropriate model for studying the neuromodulatory properties of adenosine.

Cell-type specific expression of Na+, K(+)-ATPase catalytic subunits in cultured neurons and glia: evidence for polarized distribution in neurons

Na+,K(+)-ATPase (the sodium pump) is a family of proteins consisting of catalytic (alpha) and glycoprotein (beta) subunit isoforms which are differentially expressed in excitable tissue. To gain insight into the cell-type distribution of sodium pump protein, we determined the expression pattern of fetal rat telencephalic cultures, of telencephalic cultures depleted of neurons, and of pure astrocyte cultures. Isoform-specific antibodies were used for immunoblotting and immunohistochemistry, with supplemental [3H]ouabain binding to assess levels of functional alpha 2/alpha 3 protein. The results show that neurons of mixed telencephalic cultures uniquely express alpha 3 and high levels of alpha 1. The marked similarity in the distribution of microtubule-associated protein-2 and alpha 1 immunocytochemical staining strongly suggests that alpha 1 subunits are enriched in dendrites. Further, highly correlative growth cone-associated protein-43 and alpha 3 staining is consistent with a preferential expression of alpha 3 subunits in axons, which are also characterized by low levels of alpha 1 and no alpha 2 immunoreactivity. Process-bearing glia are intimately associated with neuronal aggregates and express high levels of both alpha 1 and alpha 2 protein, as well as GFAP. Interestingly, polygonal, flat glia not within neuronal aggregates are weakly immunopositive only for alpha 1 and GFAP. Pure astrocytic cultures possess appreciable alpha 1 protein and GFAP, but lack both alpha 2 and alpha 3 immunoreactivity. As predicted by the immunohistochemical findings, [3H]ouabain binding was low in pure astrocytic cultures, and much higher in the neuron-enriched mixed cultures. These observations confirm that neurons express all three catalytic isoforms of the sodium pump. They also suggest that specific alpha-isoforms may be polarized to targeted membrane regions of neurons. Further, glia intimately associated with neurons express alpha 2, bind significant amounts of [3H]ouabain, and possess much higher levels of alpha 1 and GFAP compared to glia not near neurons. Thus, neurons may regulate glial sodium pump expression.

Autoradiographic localization of binding sites for arginine vasopressin and atrial natriuretic peptide on astrocytes and neurons of cultured rat central nervous system

The cellular localization of binding sites for [125I]arginine vasopressin and [125I]atrial natriuretic peptide was studied in explant cultures of rat spinal cord, brain stem and cerebellum by means of autoradiography. In brain stem cultures, especially in the nucleus of the solitary tract, a great number of neurons revealed binding sites for both peptides. In spinal cord cultures, many neurons of various sizes were labelled by [125I]arginine vasopressin, whereas only a small number of cells showed binding sites for [125I]atrial natriuretic peptide. Neurons in cerebellar cultures revealed little or no binding for the peptides. In addition to neurons, binding sites for [125I]arginine vasopressin and [125I]atrial natriuretic peptide were also observed on glial cells. Simultaneous staining of the cultures with glial fibrillary acidic protein has shown that the labelled cells were glial fibrillary acidic protein-positive and could therefore be identified as astrocytes. Labelling of the cells by [125I]arginine vasopressin and [125I]atrial natriuretic peptide was more intense in spinal cord and brain stem cultures than in cultures of cerebellum, providing evidence for a heterogeneity of astrocytes in different regions of the central nervous system. Binding of both [125I]arginine vasopressin and [125I]atrial natriuretic peptide to neurons and astrocytes could be competed by the unlabelled peptides, suggesting specific binding of the radioligands. Our autoradiographic studies provide good evidence that in addition to neurons, astrocytes also express receptors for arginine vasopressin and atrial natriuretic peptide.

Adrenergic receptor regulation of amino acid neurotransmitter uptake in astrocytes

Using culture techniques it has been demonstrated that astroglia possess uptake carriers for amino acid neurotransmitters and enzyme systems for inactivation of several neurotransmitters. They express membrane receptors functionally coupled to second messenger systems and they can regulate the extracellular ionic milieu including a clearing of K+ from the extracellular space. With these specific functional characteristics and their strategic anatomy the cells might influence the passage of information between neurons.

Autoradiographic evidence for endothelin receptors on astrocytes in cultures of rat cerebellum, brainstem and spinal cord

The cellular localization of binding sites for 125I-endothelin-1 and -3 (ET-1, ET-3) was studied in explant cultures of rat cerebellum, brain stem and spinal cord by means of autoradiography. The majority of astrocytes in these cultures expressed binding sites for both ET-1 and ET-3. There was a difference in the intensity of labelling between glial cells in the same culture. Some cells revealed intense radioactivity whereas neighbouring astrocytes were only slightly labelled. Besides glial cells, cerebellar neurones presumably Purkinje cells and granule cells as well as medium-sized and large neurones in brain stem and spinal cord cultures showed binding sites for both peptides. Our results provide strong evidence for the existence of ET receptors on astrocytes.

Light and electron microscopic localization of GABAA-receptors on cultured cerebellar granule cells and astrocytes using immunohistochemical techniques

GABAA-receptors were localized in explant cultures of rat cerebellum and in dissociated primary cultures of rat cerebellar granule cells and rat cerebellar astrocytes using the monoclonal antibody bd-17 directed against the beta-subunit of the GABAA/benzodiazepine/chloride channel complex. At the light microscope level specific staining of GABAA-receptors was localized in various types of neurones in explant cultures of rat cerebellum using the indirect peroxidase-antiperoxidase (PAP) technique, whereas no specific staining was found in astrocytes. At the electron microscope level labeling of GABAA-receptors was observed in the plasma membrane of both the cell bodies and processes in dissociated primary cultures of cerebellar granule cells using an indirect preembedding immunogold staining technique which in contrast to the classical PAP technique allows quantitative estimations to be performed. Quantification of the labeling intensity revealed a higher concentration of GABAA-receptors per microns plasma membrane in the cell bodies than in the processes. In discrete areas an extremely high density of the GABAA-receptors was observed. No specific labeling of GABAA-receptors was observed in dissociated primary cultures of cerebellar astrocytes.

Action of baclofen, GABA and antagonists on the membrane potential of cultured astrocytes of rat spinal cord

The action of gamma-aminobutyric acid A (GABAA) and B (GABAB)-agonists has been studied on the membrane potential of astrocytes in explant cultures of rat spinal cord by means of intracellular microelectrode recordings. The GABAB-agonists (-)-baclofen and CGP 27 492 (3-aminopropyl phosphonous acid; 10(-6) to 10(-4) M) caused a hyperpolarization of the majority of astrocytes studied. On approximately 25% of the cells, the compounds had no effect. The hyperpolarization by baclofen (10(-4) M) was reversibly antagonized by the GABAB-antagonist 5-hydroxysaclofen (10(-4) M). GABA and the GABAA-agonist muscimol (10(-4) and 10(-3) M) depolarized approximately two thirds of the glial cells tested, whereas the remaining third remained unaffected. The GABAA-antagonist bicuculline (10(-4) and 10(-3) M) only reduced the depolarization by GABA (10(-4) M) but did not completely block it. On half of the cells tested, the depolarization by GABA was not affected by bicuculline, suggesting that the glial GABAA-receptor is different from the neuronal GABAA-receptor. Our electrophysiological investigations together with recent autoradiographic binding studies strongly suggest the existence of GABAB-receptors on astrocytes whereas there is less evidence for GABAA-sites on these cells.

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.

Comparative marker analysis of the ependymocytes of the subcommissural organ in four different mammalian species

The subcommissural organ (SCO), classified as one of the circumventricular organs, is composed mainly of modified ependymal cells, attributable to a glial lineage. Nevertheless, in the rat, these cells do not possess glial markers such as glial fibrillary acidic protein (GFAP), protein S100, or the enzyme glutamine synthetase (GS). They receive a synaptic 5-HT input and show pharmacological properties for uptake of GABA resembling the uptake mechanism of neurons. In this study, we examine the phenotype of several mammalian SCO (cat, mouse, rabbit) and compare them with the corresponding features of the rat SCO. In all these species, the SCO ependymocytes possess vimentin as an intermediate filament, but never express GFAP or neurofilament proteins. They do not contain GS as do glial cells involved in GABA metabolism, and when they contain protein S100 (rabbit, mouse), its rate is low in comparison to classical glial or ependymal cells. Thus, these ependymocytes display characteristics that differentiate them from other types of glial cells (astrocytes, epithelial ependymocytes and tanycytes). Striking interspecies differences in the capacity of SCO-ependymocytes for uptake of GABA might be related to their innervation and suggest a species-dependent plasticity in their function.

Autoradiographic localization of binding sites for vasoactive intestinal peptide and angiotensin II on neurons and astrocytes of cultured rat central nervous system

The cellular localization of binding sites for [125I] vasoactive intestinal polypeptide and [3H]angiotensin II was studied in explant cultures of rat spinal cord, brain stem, cerebellum and cortex by means of autoradiography. In spinal cord cultures, interneurons of the dorsal horn and motoneurons of the ventral horn were labelled by [125I]vasoactive intestinal polypeptide and [3H]angiotensin II. In many brain stem cultures, groups of large neurons revealed intense binding of both peptides. In contrast, only few medium-sized cerebellar neurons, probably interneurons, showed binding sites for [125I]vasoactive intestinal polypeptide and [3H]angiotensin II. Furthermore, the intensity of labelling of cerebellar neurons was usually weaker than that of neurons of the brain stem and spinal cord. Many neurons in cultures of neocortex were also labelled by [125I]vasoactive intestinal polypeptide, whereas little binding was found with [3H]angiotensin II. In addition to neurons, binding sites for both peptides were also observed on astrocytes. Labelling of these cells was more intense in spinal cord and brain stem cultures than in cultures of cerebellum and cortex, suggesting that only a certain type or a certain population of astrocytes possesses receptors for vasoactive intestinal polypeptide and angiotensin II, or that glial cells in different parts of the CNS have different physiological and pharmacological properties.

GABAA-antagonists and baclofen analgesia

1. Intraperitoneal (i.p.) injection of different doses of baclofen (5, 7.5 and 10 mg/kg) induced analgesia in tail-flick test. The effect was dose-dependent. 2. The antinociception induced by baclofen (10 mg/kg, i.p.) was decreased in animals pretreated with bicuculline (1.5 mg/kg, i.p., 30 min), but not with naloxone (1.5 mg/kg, i.p., 30 min). 3. In picrotoxin (1 mg/kg, i.p., 15 min) pretreated mice, baclofen (5 mg/kg, i.p.) showed a significant analgesic effect. 4. Morphine (6 mg/kg, s.c.) induced analgesia which was antagonized by naloxone pretreatment (1.5 mg/kg, i.p.), while bicuculline or picrotoxin did not alter the morphine response. 5. These data suggest that a part of analgesic effect of baclofen may be mediated through GABAA receptor sites, and differs from that of morphine.

Oligodendroglial and astroglial heterogeneity in mouse primary central nervous system culture as demonstrated by differences in GABA and D-aspartate transport and immunocytochemistry

Using simultaneous autoradiography and immunofluorescence we have investigated the functional heterogeneity amongst oligodendrocytes and astrocytes in primary mouse central nervous system (CNS) culture as expressed by differences in their ability to accumulate gamma-[3H]aminobutyric acid [( 3H]GABA) and D-[3H]aspartate. We have used a range of specific antibodies that identify oligodendrocytes and astrocytes, from precursor to fully mature cells, to address the question of whether all neuroglial cells are capable of expressing this function. Our results showing that A2B5-, 03-, and galactocerebroside-positive cells became heavily labelled with these two neuroactive amino acids, whereas cells expressing the myelin proteins 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and myelin basic protein (MBP) did not, demonstrate that this capacity is already present in oligodendrocytes at early developmental stages but may not extend to fully mature cells. Astrocytes in culture exhibited a large degree of variability with respect to their ability to transport GABA and D-aspartate. When grown in either serum-containing or serum-free hormone supplemented culture medium two morphologically distinct of glial fibrillary acidic protein (GFAP)-positive astrocyte were identified, process-bearing and epithelioid. Process-bearing cells became heavily labelled with the amino acids under both growth conditions, whereas, data showed that although epithelioid astrocytes were not, or only lightly, labelled with either amino acid in serum-containing cultures, when grown in serum-free culture medium they became more heavily labelled. Thus the expression, in culture, by epithelioid astrocytes, of one of the functions attributed to these cells is largely dependent on growth conditions.

Simultaneous immunofluorescence and autoradiography: a useful technique for investigating neurotransmitter uptake by neurons and glia in primary central nervous system culture

Previous studies on the localization of radiolabelled neurotransmitters in cultured cells of neural origin have relied on the comparison of cell morphology, as determined by immunocytochemistry, with the patterns of labelling on autoradiograms. We present here a method combining simultaneously autoradiography, following the uptake of tritium-labelled amino acid transmitters, with indirect immunofluorescence using antibodies against both surface and intracellular antigens. Using a fixative containing only a low concentration of glutaraldehyde (4% paraformaldehyde, 0.1% glutaraldehyde), a similar retention of gamma-[3H]aminobutyric acid (GABA) and D-[3H]aspartate was achieved as with the higher concentrations commonly used, with the advantage that the autofluorescence associated with glutaraldehyde fixed tissue was eliminated, and the immunoreactivity of the antigens to be localized was not destroyed. Using this method GABA and D-aspartate accumulating cells, in dissociated mouse central nervous system (CNS) cultures, could be reliably identified as oligodendrocytes, and some multiprocessed astrocytes, by anti-galactocerebroside (GC) and anti-glial fibrillary acidic protein (GFAP) immunofluorescence respectively. GABA-accumulating neuron-specific enolase (NSE) positive neurons could be clearly identified but no D-aspartate accumulating neurons were found. This technique should have a wide application in the investigation of whether selective transport mechanisms coexist with antigens characteristic of a certain cell type or sub-type.

GABA immunoreactive neurons in rat visual cortex

An antiserum to gamma-aminobutyric acid (GABA) was used in a light and electron microscopic immunocytochemical study to determine the morphology and distribution of GABA-containing neurons in the rat visual cortex and to ascertain whether all classes of nonpyramidal neurons in this cortex are GABAergic. The visual cortex used for light microscopy was prepared in such a way that the antibody penetrated completely through tissue sections, and in these sections large numbers of GABA immunoreactive neurons were apparent. The labeled neurons could be identified as being either multipolar, bitufted, bipolar, or horizontal neurons. In layers II through VIa, GABA immunostained cells were distributed uniformly and accounted for approximately 15% of all neurons, but in layer I all neurons appeared to be immunostained. Electron microscopy of GABA immunostained visual cortex prepared to ensure good fine structural preservation confirmed the presence in layers II through VIa of numerous immunoreactive bipolar neurons, both small and large varieties, as well as multipolar and bitufted neurons. Additionally, electron microscopy reveals that astrocytes are frequently GABA immunoreactive. From a correlated light and electron microscopic evaluation of neurons in GABA immunostained visual cortex, it was possible to confirm which kinds of neurons are GABAergic and what proportion of the neuronal population they represent. Thus, from an analysis of some 950 neurons, it was found that pyramidal neurons were never immunoreactive and that except for 20% of the bipolar cell population, all examples of other types of nonpyramidal neurons encountered in this material were GABA immunoreactive.

Development of transmitter-releasing capacity in neuron-enriched tissue cultures

Dissociated cell cultures derived from whole brains of foetal rats (17 days of gestation) were maintained for periods of up to 21 days in vitro for the purpose of studying the transmitter-releasing properties of the dopaminergic neuronal cells and glial cells. In the neuron-enriched cultures, after 3 days in vitro, [3H]dopamine was released in response to depolarizing stimuli. Both the potassium and veratrine-evoked release of dopamine was Ca2+ dependent. Veratrine-evoked release was reduced in the presence of the calcium channel blocker verapamil and was tetrodotoxin sensitive. Glial cultures, after 7 days in vitro, did not respond to any depolarizing stimuli, although they displayed a significant ability to take up [3H]dopamine. Comparison between static incubations and perfused cultures showed no difference in the patterns of release resulting from veratrine stimulation. Tyrosine hydroxylase activity increased progressively in neuron-enriched cultures but was not detectable in glial cultures. These results show that neuron-enriched cultures respond to depolarizing stimuli in a manner similar to excised adult basal ganglia tissue, with the appearance of functional ionic channels after 3 days in vitro.

Astroglial response in the excitotoxically lesioned neostriatum and its projection areas in the rat

The anatomical distribution of the astrocytic glial reactions, following ibotenic acid-induced neuronal degeneration of the neostriatum in the rat, has been studied immunohistochemically using an antibody directed against the astrocytic marker, glial fibrillary acidic protein. The acute astroglial response to the excitotoxic lesion, determined 7 days post lesion, was compared with a sham-operated group and a chronic group that had received the excitotoxic lesion 6 months prior to histological evaluation. Total doses of 16-20 micrograms ibotenic acid injected unilaterally into the head of the neostriatum caused not only a marked neuronal cell loss but was also accompanied by a large increase in the number and size (about 5 times) of glial fibrillary acidic protein-stained astrocytes throughout the neostriatum by 7 days after lesion. Reactive astrocytes were also observed in the major neostriatal projection areas, the globus pallidus and the substantia nigra pars reticulata, at 7 days post lesion, although no neuronal cell loss could be detected in these regions using regular Cresyl Violet staining. Previous studies of lesions identical to the ones used here have shown that globus pallidus and substantia nigra are deafferented as a result of the neostriatal neuronal degeneration. The reactive astrocytes in the striatal projection areas had a 3-5 times larger size than control astrocytes from the same anatomical region. In animals that received a larger dose of ibotenic acid into the neostriatum (25 micrograms), neuronal cell loss was also observed in the neocortex and reactive glial fibrillary acidic-stained astrocytes were found in the entire neocortex of the injected hemisphere. In the chronic group, 6 months after the excitotoxic lesion, the astroglial response was clearly diminished or absent in the major neostriatal projection areas, but was still present within the lesioned neostriatum. The results suggest that focal neuronal destruction can result in widespread astrocytic glial reactions which follow the anatomical connectivity of the lesioned area. This may have implications for the understanding of the multifocal distribution of glial reactions seen in patients with striatal degeneration as a result of Huntington's disease.

Effects of arachidonic acid on glutamate and gamma-aminobutyric acid uptake in primary cultures of rat cerebral cortical astrocytes and neurons

The effects of arachidonic acid on glutamate and gamma-aminobutyric acid (GABA) uptake were studied in primary cultures of astrocytes and neurons prepared from rat cerebral cortex. The uptake rates of glutamate and GABA in astrocytic cultures were 10.4 nmol/mg protein/min and 0.125 nmol/mg protein/min, respectively. The uptake rates of glutamate and GABA in neuronal cultures were 3.37 nmol/mg protein/min and 1.53 nmol/mg protein/min. Arachidonic acid inhibited glutamate uptake in both astrocytes and neurons. The inhibitory effect was observed within 10 min of incubation with arachidonic acid and reached approximately 80% within 120 min in both types of culture. The arachidonic acid effect was not only time-dependent, but also dose-related. Arachidonic acid, at concentrations of 0.015 and 0.03 mumol/mg protein, significantly inhibited glutamate uptake in neurons, whereas 20 times higher concentrations were required for astrocytes. The effects of arachidonic acid were not as deleterious on GABA uptake as on glutamate uptake in both astrocytes and neurons. In astrocytes, GABA uptake was not affected by any of the doses of arachidonic acid studied (0.015-0.6 mumol/mg protein). In neuronal cultures, GABA uptake was inhibited, but not to the same degree observed with glutamate uptake. Lower doses of arachidonic acid (0.03 and 0.015 mumol/mg protein) did not affect neuronal GABA uptake. Other polyunsaturated fatty acids, such as docosahexaenoic acid, affected amino acid uptake in a manner similar to arachidonic acid in both astrocytes and neurons. However, saturated fatty acids, such as palmitic acid, exerted no such effect. The significance of the arachidonic acid-induced inhibition of neurotransmitter uptake in cultured brain cells in various pathological states is discussed.

Mutual inhibition kinetic analysis of gamma-aminobutyric acid, taurine, and beta-alanine high-affinity transport into neurons and astrocytes: evidence for similarity between the taurine and beta-alanine carriers in both cell types

The transport kinetics of gamma-aminobutyric acid (GABA), taurine, and beta-alanine in addition to the mutual inhibition patterns of these compounds were investigated in cultures of neurons and astrocytes derived from mouse cerebral cortex. A high-affinity uptake system for each amino acid was demonstrated both in neurons (Km GABA = 24.9 +/- 1.7 microM; Km Tau = 20.0 +/- 3.3 microM; Km beta-Ala = 73.0 +/- 3.6 microM) and astrocytes (Km GABA = 31.4 +/- 2.9 microM, Km Tau = 24.7 +/- 1.3 microM; Km beta-Ala = 70.8 +/- 3.6 microM). The maximal uptake rates (Vmax) determined were such that, in neurons, Vmax GABA greater than Vmax beta-Ala = Vmax Tau, whereas in astrocytes, Vmax beta-Ala greater than Vmax Tau = Vmax GABA. Taurine was found to inhibit beta-alanine uptake into neurons and astrocytes in a competitive manner, with Ki values of 217 microM in neurons and 24 microM in astrocytes. beta-Alanine was shown to inhibit taurine uptake in neurons and astrocytes, also in a competitive manner, with Ki values of 72 microM in neurons and 71 microM in astrocytes. However, beta-alanine was found to be a weak noncompetitive inhibitor of neuronal and astrocytic GABA uptake, whereas in reverse experiments, GABA displayed weak noncompetitive inhibition of neuronal and astrocytic uptake of beta-alanine. Likewise, taurine was a weak noncompetitive inhibitor of GABA uptake in neurons and similarly, GABA was a weak noncompetitive inhibitor of taurine uptake into neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective uptake of neuroactive amino acids by both oligodendrocytes and astrocytes in primary dissociated culture: a possible role for oligodendrocytes in neurotransmitter metabolism

CNS glia may be involved in the modulation of neuronal excitability through their capacity to accumulate and metabolize neuroactive amino acids. To investigate the possible role of oligodendrocytes in amino acid neurotransmitter metabolism, we have used light microscopic autoradiography, following the uptake of 3H-labelled amino acids by dissociated cultures of neonatal mouse brain, characterized immunocytochemically using cell-type specific markers. Oligodendrocytes, recognized by their characteristic galactocerebroside membrane staining, rapidly accumulated [3H] gamma-aminobutyric acid (GABA), becoming intensely labelled over cell body and processes after short incubations. In contrast, oligodendrocytes became only lightly labelled with [3H]L-glutamate and aspartate, which preferentially labelled astrocytes. [3H]D-aspartate, a non-metabolized analogue of L-glutamate, was avidly accumulated by oligodendrocytes, labelling cell bodies and processes after short incubations, to a similar extent as GABA. Thus, oligodendrocytes possess a transport mechanism for these excitatory amino acids, but rapidly metabolize them and release the metabolites. Not only the GC-positive cells but also the GC-negative undifferentiated oligodendrocyte precursors accumulated both GABA and D-aspartate, suggesting that this may be a function expressed early in the differentiation of oligodendrocytes. Net uptake of [3H] beta-alanine and [3H]glycine by oligodendrocytes was not observed under any conditions tested. A small number of oligodendrocytes were labelled with [3H]taurine after longer incubations. The uptake of certain neuroactive amino acids is thus a property shared by astrocytes and oligodendrocytes, the latter acting in a protective fashion around neuronal perikarya and axons.

Binding sites for [3H]dopamine and dopamine-antagonists on cultured astrocytes of rat striatum and spinal cord: an autoradiographic study

The cellular localization of binding sites for [3H]dopamine, and dopamine-antagonists (D1 and D2) was studied in organotypic cultures of rat striatum and spinal cord by means of autoradiography. In both types of cultures, many astrocytes were labelled by [3H]dopamine, the D1-antagonist [3H]cis-flupenthixol and the D2-antagonists [3H]domperidone and [3H]spiperone (10(-9) to 10(-8) M). Addition of unlabelled dopamine and antagonists at high concentrations (10(-6) to 10(-4) M) inhibited or markedly reduced binding of the radioligands indicating 'specific' binding of the compounds. Our autoradiographic studies are consistent with biochemical investigations by other authors, suggesting that astrocytes possess receptors for dopamine.

Amino acid and monoamine transport in primary astroglial cultures from defined brain regions

The uptake of L-[3H]glutamate, L-[3H]aspartate, gamma-[3H]aminobutyric acid (GABA), [3H]dopamine, DL-[3H]norepinephrine and [3H]5-hydroxytryptamine (5-HT) was studied in astrocytes cultured from the cerebral cortex, striatum and brain stem of newborn rat and grown for 2 weeks in primary cultures. The astrocytes exhibited a high-affinity L-glutamate uptake with Km values ranging from 11 to 110 microM. Vmax values were 4.5 in cerebral cortex, 39.1 in striatum, and 0.4 in brain stem, nmol per mg cell protein per min. There was a less prominent high-affinity uptake of L-aspartate with Km values from 88 to 187 microM. Vmax values were 7.4 in cerebral cortex, 37.1 in striatum, and 3.1 in brain stem, nmol per mg cell protein per min. The high-affinity GABA uptake exhibited Km values ranging from 5 to 17 microM and Vmax values were 0.01 for cerebral cortex, 0.04 for striatum, and 0.1 for brain stem, nmol per mg cell protein per min. No high-affinity, high-capacity uptake was found for the monoamines. The results demonstrate a heterogeneity among the astroglial cells cultivated from the different brain regions concerning the uptake capacity of amino acid neurotransmitters. Furthermore, amino acid transmitters and monoamines are taken up by the cells in different ways.

Effects of inhibitors of Na+,K+-ATPase on the membrane potentials and neurotransmitter efflux in rat brain slices

The potassium potential EK, of rat brain slices was estimated by determining the uptake of 86Rb+. The ERb was the same for slices prepared from five rostral brain regions, the average value being 66.4 mV. The ERb values in the presence of 20 microM ouabain were only slightly lower than the resting values; increasing concentrations of ouabain above 20 microM resulted in a graded depolarization in all five brain regions. High concentrations (1 mM) of two other inhibitors of Na+,K+-ATPase, dihydro-ouabain and strophanthidin, produced no more depolarization than did 20 microM ouabain. Competitive binding studies indicated that the differential effects were due to the relative binding to brain slices. Erythrosin B, an inhibitor of Na+,K+-ATPase, had no measurable effect on ERb. Intermediate concentrations of the Na+/H+ ionophore monensin slightly hyperpolarized striatal slices, whereas the same monensin concentrations plus 20 microM ouabain, 1 mM strophanthidin or 70 microM erythrosin B resulted in marked depolarization. Measurement of the membrane potential via uptake of methyltriphenylphosphonium cation indicated that ERb was indeed a valid estimation of the membrane potential. EK was measured directly by monitoring 42K+ uptake in striatal slices and was found to be essentially identical to ERb. Uptake of 22Na+ was consistent with the values for ERb or EK. Several conditions that resulted in little or no measurable depolarization of striatal slices did induce efflux of exogenously loaded GABA and dopamine; these conditions included 20 microM ouabain, 1 mM dihydro-ouabain or strophanthidin, and 70 microM erythrosin B. Neurotransmitter efflux in the absence of general cell depolarization was not accompanied by altered rates of respiration or decreased ATP levels.

Transport of monoamine and amino acid neurotransmitters by primary astroglial cultures

The transport of [3H]L-glutamate, [3H]L-aspartate, [3H] gamma-aminobutyric acid [( 3H]GABA), [3H]dopamine, [3H]norepinephrine and [3H]5-hydroxytryptamine (3H-5-HT) was measured in primary astroglial cultures from newborn rat cerebral hemispheres. There was a high-affinity uptake with a Km of 69.0 microM for L-glutamate, 12.3 microM for L-aspartate and 3.1 microM for GABA. The uptake showed properties of high capacity with a Vmax of 17.0 nmol X mg prot-1 X min-1 for L-glutamate, 1.1 nmol X mg prot-1 X min-1 for L-aspartate and 0.04 nmol X mg prot-1 X min-1 for GABA. No high-affinity high capacity transport system was found for the monoamines studied. Autoradiographic examination demonstrated a heavy deposit of grains suggesting a prominent accumulation of [3H]L-glutamate and [3H]L-aspartate in the astroglial-like cells of the cultures, while the [3H]GABA accumulation was less intense. On the other hand, there was only a weak accumulation of grains after incubating the cultures with [3H]dopamine, [3H]norepinephrine or [3H]5-HT. Thus, astroglial cells in culture accumulate amino acid neurotransmitters and monoamines in different ways with a high-affinity high-capacity uptake of glutamate, aspartate and GABA and a diffusion-uptake of dopamine, norepinephrine and 5-HT.

Selective culture of neurons from rat cerebral cortex: morphological characterization, glutamate uptake and related enzymes during maturation in various culture media

Dissociated cerebral cortex of fetal rat was grown in a serum-free, chemically defined medium (CDM) (containing insulin, progesterone, estradiol, transferrin, putrescine, selenium and 15 mM KCl) and compared with cultures grown in a medium containing 20% fetal calf serum (SCM). Neurons survived well using either medium, but in the serum-free medium the cellular population was exclusively neuronal (at 96%), while glial cells began to proliferate after one week in the SCM. The various cellular morphologies are described in the present report and the presence of immunological markers characteristic of neurons was investigated. Autoradiographic experiments have been performed after incubation with various putative neurotransmitters and we have shown the presence of a strikingly high proportion of glutamatergic neurons in these cultures. Glutamate high affinity uptake was also greatly increased in neuronal cultures maintained in a CDM compared to a SCM, especially in young cultures. The development of different enzymes involved in the metabolism of glutamate was also studied; the specific activity of glutaminase increased in culture and was found to be higher in a CDM than in a SCM, while the inverse was true for glutamine synthetase. The relative proportion of both enzymes in neurons compared to glial cells was opposite, as neuronal cultures had higher levels of glutaminase and glial cultures were enriched in glutamine synthetase activity. It seems that the proportion of glutamate-neurons increases when cultured in a CDM compared to a SCM and we suggest that this culture procedure may provide a purely neuronal population enriched in mature glutamatergic neurons. It may thus be useful for future in vitro studies on glutamate and GABA metabolism in neurons.

Uptake of [3H]GABA by oligodendrocytes in dissociated brain cell culture: a combined autoradiographic and immunocytochemical study

Uptake of [3H]GABA by dissociated mixed cell cultures of fetal mouse brain was studied using light microscopic autoradiography. Major cell types in the cultures were identified and quantified by immunocytochemical localization of reliable cell type-specific antigenic markers. In 12 days in vitro (DIV) cultures [3H]GABA uptake was predominantly into neurons and oligodendrocytes, whilst at 28 DIV the only surface cells labeled were oligodendrocytes. This was confirmed by complement-dependent antibody-mediated cytotoxicity against galactocerebroside-positive oligodendrocytes. There was a moderate labeling of almost all flat cells, the majority of which were glial fibrillary acidic protein (GFAP)-positive astrocytes. Heavily labeled astrocytes were only occasionally observed. Oligodendrocytes accumulated [3H]GABA more rapidly than astrocytes but slower than neurons. Oligodendroglial labeling was predominantly over the cell body, whereas neuronal labeling was more uniformly distributed over cell body and processes. The uptake was inhibited by diaminobutyric acid (DABA) and nipecotic acid, but not by beta-alanine, and thus had similar characteristics to neuronal rather than astroglial uptake. Oligodendrocytes did not accumulate [3H]beta-alanine, which labeled only astrocytes. Oligodendroglial [3H]GABA uptake was Na+-dependent and sensitive to ouabain, but was only slightly enhanced by aminooxyacetic acid (AOAA), whereas astroglial uptake was not sensitive to ouabain but was markedly enhanced by AOAA. The results indicate that oligodendrocytes, in addition to astrocytes, may also be involved in the modification of neuronal function by the uptake and inactivation of neuroactive substances.

Autoradiographic localization of binding sites for [3H]histamine and H1- and H2-antagonists on cultured neurones and glial cells

By means of autoradiography we have studied the cellular localization of binding of [3H]histamine and H1- and H2-antagonists in explant cultures of rat cerebellum, brain stem and spinal cord. In brain stem and spinal cord cultures, a relatively great number of neurones revealed binding sites for [3H]histamine and to a lesser extent also for the H1-antagonist [3H]pyrilamine and for the H2-antagonist [3H]tiotidine. In contrast, only a small number of labelled neurones was found in cerebellar cultures. The intensity of labelling was usually much stronger for [3H]histamine than for its antagonists, suggesting that binding sites for histamine might reflect both H1- and H2-receptors. Glial cells also showed binding sites for [3H]histamine and the H1- and H2-antagonists, the number of labelled astrocytes by these radioligands was, however, smaller than that observed with [3H]noradrenaline and alpha- and beta-adrenergic antagonists. It is suggested that in addition to alpha- and beta-adrenoceptors, glial cells also possess receptors for histamine.

Glutamate and kainate receptors induced by rat brain messenger RNA in Xenopus oocytes

Xenopus laevis oocytes injected with poly(A)+ mRNA extracted from rat brain became sensitive to serotonin, glutamate, kainate, acetylcholine and gamma-aminobutyrate. Application of these substances to mRNA-injected oocytes elicited membrane currents. The glutamate- and acetylcholine-induced currents usually showed oscillations, while the kainate current was smooth. The current oscillations during glutamate application reversed direction at about the chloride equilibrium potential (-24 mV), but the reversal potential for the kainate current was close to 0 mV. The current-voltage relation for the glutamate-induced current oscillations showed strong rectification at hyperpolarized potentials, while that for the kainate current was nearly linear. In some oocytes, glutamate elicited smooth membrane currents, with oscillations either absent, or appearing after a delay. The reversal potential of this component was close to 0 mV, and was clearly different from that of the oscillatory component. The appearance of glutamate and kainate sensitivity in the oocyte membrane is due to the translation of the foreign messenger RNA, and not to activation of the oocytes' own genome, because oocytes still become sensitive when transcription is prevented by enucleation or by treatment with actinomycin D. It appears that mRNA from rat brain contains translationally active messengers which code for various neurotransmitter receptors. When this mRNA is injected into Xenopus oocytes, the messengers are translated and receptors are inserted into the oocyte membrane, where they form functionally active receptor-channel complexes.

Enzyme activities of monoamine oxidase, catechol-O-methyltransferase and gamma-aminobutyric acid transaminase in primary astroglial cultures and adult rat brain from different brain regions

The activities of monoamine oxidase (MAO), catechol-O-methyltransferase (COMT) and gamma-aminobutyric acid transaminase (GABA-T) were measured in primary cultures from newborn rat cultivated from 6 different brain regions. These primary cultures contained mostly astroglial cells, evaluated by the presence of the glial fibrillary acidic protein (GFAp, alpha-albumin) and the S-100 protein. The enzyme activities in the corresponding brain area from adult rat were also quantified. MAO activities were on the same level in 14-day old cultures and in adult rat brain homogenates, with significantly lower values in brain stem as compared to the other brain regions examined. COMT activities were on a higher level in the cultures than in adult rat brain homogenates. Astroglial cells from hippocampus were found to have the highest and those from brain stem the lowest COMT-activities. GABA-T activities were lower in the cultures than in adult rat homogenates. No significant differences were seen in the various astroglial cultures. Accumulation of [3H]dopamine and [3H]gamma-aminobutyric acid (GABA) visualized by autoradiography showed only a slight uptake of dopamine in comparison with the uptake of GABA. It is concluded that astroglial cells in culture have enzymatic properties similar to those of astroglial cells in different brain regions of adult rat brain. Studies are in progress to evaluate if the regional heterogeneity observed among cultivated astroglial cells is affected by in vivo differentiation until cultivation and/or time in culture.