Unilateral, intranigral infusions of amphetamine produce differential, bilateral changes in unit activity and extracellular levels of ascorbate in the neostriatum of the rat

Endothelial nitric oxide synthase activity involves in the protective effect of ascorbic acid against penicillin-induced epileptiform activity

Ascorbic acid and nitric oxide are known to play important roles in epilepsy. The aim of present study was to identify the involvement of nitric oxide (NO) in the anticonvulsant effects of ascorbic acid on penicillin-induced epileptiform activity in rats. Intracortical injection of penicillin (500, International Units (IU)) into the left sensorimotor cortex induced epileptiform activity within 2-5 min. Thirty minutes after penicillin injection, nitric oxide synthase (NOS) inhibitor, N(G)-nitro-l-arginine methyl ester (l-NAME, 100mg/kg), neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI, 40 mg/kg), NO substrate, l-arginine (500 mg/kg) were administered with the most effective dose of ascorbic acid (100 mg/kg) intraperitoneally (i.p.). The administration of l-arginine significantly decreased the frequency of epileptiform activity while administration of l-NAME did not influence the mean frequency of epileptiform activity. Injection of 7-NI decreased the mean frequency of epileptiform activity but did not influence amplitude. Ascorbic acid decreased both the mean frequency and amplitude of penicillin-induced epileptiform activity in rats. The application of l-NAME partially and temporarily reversed the anticonvulsant effects of ascorbic acid. The results support the hypothesis of neuro-protective role for NO and ascorbic acid. The protective effect of ascorbic acid against epileptiform activity was partially and temporarily reversed by nonspecific nitric oxide synthase inhibitor l-NAME, but not selective neuronal nitric oxide synthase inhibitor 7-NI, indicating that ascorbic acid needs endothelial-NOS/NO route to decrease penicillin-induced epileptiform activity.

Adrenergic neural connections between the bilateral supraoptic nuclei of the rat hypothalamus

Our previous study has demonstrated that unilateral microinjection of norepinephrine (NE) into the right supraoptic nucleus (SON) of anesthetized hydrated rats elicited dose-dependent decreases in the urine outflow rate. This was antagonized by pretreatment with phenoxybenzamine (an alpha-antagonist) and timolol (a beta-antagonist) in the same SON. In the present study, we examined the effects of NE, microinjected into the right, left and bilateral SON, on the urine outflow rate in order to investigate neural connections between the bilateral SON. NE administered by those three routes dose-dependently decreased the urine outflow rate. The order for the antidiuretic potency was as follows: the effect elicited by the intrabilateral-SON microinjection > the intra-left-SON microinjection = the intra-right-SON microinjection. The antidiuresis of NE microinjected into the right SON was inhibited by an electrolytic left-SON lesion and by pretreatment with phenoxybenzamine (20 nmol) and timolol (100 nmol), but not by atropine (300 nmol) in the left SON. These findings suggest adrenergic neural connections from the right to left SON, contributing to the regulation of urine production. Furthermore, there is a possibility that stimulation of endogenously-released NE in the bilateral SON is amplified through these neurons and elicits more potent effects than those produced in either the right or left nucleus.

Cortical ablation and drug-induced changes in striatal ascorbic acid oxidation and behavior in the rat

Rats whose frontoparietal cortex had been bilaterally ablated were allowed 21 days for recovery and then treated with apomorphine (APO), 1 mg/kg SC or scopolamine (SCOP), 0.6 mg/kg SC. Soon after a behavioral test, dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), ascorbic acid (AA), and dehydroascorbic acid (DHAA) levels were determined by HPLC/EC in striatal synaptosomes (left side) and whole striatum (right side). SCOP behavioural effects were attenuated by cortical ablation, while those of APO were affected to a lesser extent. In the striatum of unoperated and sham-operated rats DHAA contents and DHAA/AA ratio resulted increased after drugs administration. No change in AA oxidation was observed in the striatum of ablated rats. In the synaptosomes of unoperated and sham-operated rats both drugs led to a decrease in DHAA contents and DHAA/AA ratio. In unoperated and sham-operated rats APO and SCOP caused a decrease of the DOPAC/DA ratio in the whole striatum and striatal synaptosomes. In ablated rats APO caused a decrease of DOPAC/DA ratio in the whole striatum and synaptosomes, while SCOP effects on DA turnover resulted attenuated in the whole striatum and abolished in synaptosomes. We conclude that drug-induced AA oxidation is likely to occur in the extracellular space and requires intact corticostriatal glutamatergic pathways. The latter may play an enabling role in SCOP behavioral effects.

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)

The effects of haloperidol, scopolamine, and MK-801 on amphetamine-induced increases in ascorbic and uric acid as determined by voltammetry in vivo

Amphetamine (which enhances dopaminergic, cholinergic, and glutamatergic activity) increases release of ascorbic acid (AA) and uric acid (UA) in the caudate nucleus. In this study, linear sweep voltammetry with carbon past electrodes was used to investigate the effects of haloperidol (a DA receptor blocker), scopolamine (a muscarinic receptor blocker), and MK-801 (an NMDA receptor blocker) alone and in combination on amphetamine-induced increases in AA and UA in the caudate nucleus. Both scopolamine (0.5 mg/kg, IP) and MK-801 (0.5 mg/kg, IP) significantly reduced amphetamine-induced increases in AA. Also, scopolamine did not affect MK-801-induced reductions of amphetamine-induced increases in AA. Unexpectedly, a subthreshold dose of haloperidol (0.1 mg/kg, IP) potentiated the ability of scopolamine to block amphetamine-induced increases in AA. Therefore, the data suggest that acetylcholine release and subsequent binding to cholinergic receptors in the caudate, are components of amphetamine-induced increases in AA. In addition, scopolamine modulated haloperidol-induced reductions of amphetamine-induced increases in release of UA. Thus, our data demonstrate that cholinergic and dopaminergic systems may interact to control release of UA.

Ascorbic acid in the brain

Ascorbic acid is highly concentrated in the central nervous system. Measurement of the extracellular concentration of ascorbate in animals, mainly by the technique of voltammetry in vivo, has demonstrated fluctuation in release from neuropil, both spontaneously and in response to physical stimulation of the animal and to certain drugs. Although in the adrenal medulla ascorbate is co-released with catecholamines, release of ascorbate from brain cells is associated principally with the activity of glutamatergic neurones, mainly by glutamate-ascorbate heteroexchange across cell membranes of neurones or glia. This phenomenon is discussed in relation to a possible role of ascorbate as a neuromodulator or neuroprotective agent in the brain.

Effects of cortical ablation on apomorphine- and scopolamine-induced changes in dopamine turnover and ascorbic acid catabolism in the rat striatum

Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), ascorbic acid and dehydroascorbic acid (DHAA) were measured by HPLC in the striatum of rats whose fronto-parietal cortex had been unilaterally ablated after a single injection of apomorphine (1 mg/kg s.c.), scopolamine (0.6 mg/kg s.c.) or L-glutamate (500 mg/kg i.p.). Unilateral cortical ablation decreased striatal levels of glutamate in both striata ipsilateral (35%) and contralateral (17-25%) to the lesion. Apomorphine and scopolamine significantly increased (+94 and +122%, respectively) the DHAA/ascorbic acid ratio in the striata ipsilateral to the lesion in unoperated and sham-operated rats (+72 and +34%, respectively), but both drugs failed to increase it in ablated rats. L-Glutamate significantly increased the DHAA/ascorbic acid ratio in unoperated (+53%) and ablated rats (+37%). The increase in sham-operated rats (+34%) did not reach statistical significance. Apomorphine and scopolamine significantly decreased the DOPAC/DA ratio in the striata ipsilateral to the lesion of unoperated, sham-operated and ablated rats. The decrease in the DOPAC/DA ratio induced by apomorphine and scopolamine was greater in ablated rats than in sham-operated rats. L-Glutamate induced only minor changes in striatal DA and DOPAC levels. We conclude that the apomorphine- and scopolamine-induced increase in ascorbic acid oxidation in the striatum requires intact cortico-striatal glutamatergic pathways. Cortical ablation potentiates the apomorphine- and scopolamine-induced inhibition of striatal DA turnover.

The effects of cortical ablation on d-amphetamine-induced changes in striatal dopamine turnover and ascorbic acid catabolism in the rat

Dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), ascorbic acid (AA) and dehydroascorbic acid (DHAA) levels were determined by HPLC in the striatal synaptosomal fraction and in the whole striatum of rats, whose fronto-parietal cortex had been bilaterally ablated, after a single injection of d-amphetamine (2.0 mg/kg i.p.). d-Amphetamine significantly increased the DHAA/AA ratio in unoperated and sham-operated rats, but failed to increase it in ablated rats, as compared to pertinent saline-treated groups. In the synaptosomal fraction, d-amphetamine significantly decreased the DHAA/AA ratio in unoperated, sham-operated and ablated rats. d-Amphetamine significantly decreased the DOPAC/DA ratio in the whole striatum and significantly increased it in the striatal synaptosomal fraction in all experimental groups. Cortical ablation greatly increased d-amphetamine-induced motor hyperactivity. We conclude that the d-amphetamine-induced increase in AA striatal oxidation requires integrity of the cortico-striatal glutamatergic pathways. Further, AA oxidation occurs in the extracellular space. The cortico-striatal glutamatergic pathways exert an inhibitory modulation on d-amphetamine behavioral effects.

Regional distribution of ascorbate and 3,4-dihydroxyphenylacetic acid (DOPAC) in rat striatum

In vivo voltammetry was used to study the regional distribution of extracellular ascorbate (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine metabolite, in the striatum of the rat. An electrochemically-modified carbon-fiber electrode, which provides distinct oxidation curves for each of these substances, was lowered in 1-mm increments through one of four striatal regions selected to sample the entire extent of this structure, including the nucleus accumbens. In anteromedial striatum, the level of AA was highest in the most dorsal and ventral aspects and lowest in the middle, whereas DOPAC levels generally showed the opposite pattern. This inverse relationship between AA and DOPAC was not evident in either lateral or posterior areas. To the extent that AA and DOPAC are released from different axon terminals, as mounting evidence suggests, regional differences in the extracellular concentration of these compounds may reflect the different and, in some cases, reciprocal distributions of two neuronal populations.

Stimulation of both D1 and D2 dopamine receptors increases behavioral activation and ascorbate release in the neostriatum of freely moving rats

Electrochemically modified carbon-fiber electrodes were used to assess the effects of indirect (amphetamine and GBR-12909) as well as direct D1 (SKF-38393) and D2 (quinpirole) dopamine agonists on extracellular ascorbate in the neostriatum of awake, behaving rats. Relative to controls, 2.5 mg/kg d-amphetamine and 20.0 mg/kg GBR-12909 produced marked behavioral activation concomitant with a significant increase in ascorbate. Comparable effects were observed following the combined administration of 10.0 mg/kg SKF-38393 and 1.0 mg/kg quinpirole, but not after either of these drugs alone. Thus, behavioral activation and release of neostriatal ascorbate were closely related to the concurrent stimulation of both D1 and D2 dopamine receptors.

Decrease of Na(+)-Ca2+ exchange activity by ascorbate in rat brain membrane vesicles

Na(+)-dependent Ca2+ uptake in rat brain microsomal membrane vesicles was inhibited by preincubating the vesicles with ascorbic acid at 0.1-10 mM. The inhibitory effect of ascorbate was blocked by simultaneous addition of ascorbate oxidase. The decrease in activity was not reversed upon removing the ascorbate. The kinetic study showed that the treatment with ascorbate decreased Bmax without a change in Km for Ca2+. The inhibitory effect by ascorbate was also observed in membrane vesicles derived from osmotically shocked synaptosomes and in reconstituted membrane vesicles. The effect by ascorbate was specific: it did not affect either ATP-dependent Ca2+ uptake in the presence of o-phenanthroline, an inhibitor of lipid peroxidation, or Na(+)-dependent glutamate uptake in the membrane vesicles. The activity of Na(+)-Ca2+ exchange was also decreased by isoascorbic acid, but not by ascorbate 2-sulfate at 1 mM. The treatment with glutathione or 2-mercaptoethanol did not affect the Na(+)-Ca2+ exchange activity, while 1 mM dithiothreitol caused the inhibition which was completely blocked by o-phenanthroline. The effect of ascorbate on Na(+)-dependent Ca2+ uptake was observed even under the conditions which suppress peroxidation of membrane phospholipids.

Trauma-induced increase of extracellular ascorbate in rat cerebral cortex

Extracellular (EC) ascorbate concentrations were measured in microdialysates from the cerebral cortex in rats subjected to cortical compression-contusion trauma. The trauma induced a transient, dramatic increase in EC ascorbate compared to the basal level before the insult and compared to control animals. The data support the presence of a releasable intracellular pool of ascorbate in the neocortex. The possibility that ascorbate may influence traumatic brain damage by its proposed neuromodulatory property and/or by its ability to induce lipid peroxidation is considered.

Corticostriatal and thalamic regulation of amphetamine-induced ascorbate release in the neostriatum

Lesions of cerebral cortex and ventromedial nucleus (VM) of the thalamus were made in rats to investigate the contribution of these structures to amphetamine (AMPH)-induced ascorbate (AA) release in the neostriatum as measured by in vivo voltammetry. Following a recovery period of at least one week, rats were anesthetized, and electrochemically modified, carbon-fiber electrodes were lowered into the neostriatum. Compared to data obtained from sham-operated and unoperated controls, bilateral aspiration lesions of cerebral cortex significantly lowered both the basal level of AA and the amount of AA released by AMPH in the neostriatum. Similar results were obtained after bilateral, but not unilateral electrolytic lesions of the VM thalamus. Collectively, these results suggest that the corticostriatal pathway and the VM thalamic nuclei participate in the regulation of basal and AMPH-induced AA release in the neostriatum.

Blockade of both D1- and D2-dopamine receptors inhibits amphetamine-induced ascorbate release in the neostriatum

In vivo recordings with electrochemically modified microvoltammetric electrodes revealed that several neuroleptic drugs, including haloperidol, clozapine, and thioridazine, blocked the rise in extracellular ascorbate produced by amphetamine in the neostriatum of urethane-anesthetized rats. This effect was also observed in animals that received a combined injection of Sch-23390 and sulpiride, but not when either of these drugs were administered alone or in combination with the 5-HT2 blocker, ritanserin. These results indicate that a combined blockade of D1- and D2-dopamine receptors blocks amphetamine-induced ascorbate release.

Increased extracellular levels of ascorbate in the striatum after middle cerebral artery occlusion in the rat monitored by intracerebral microdialysis

Extracellular (EC) ascorbate concentrations were measured in microdialysates from the striatum bilaterally in rats subjected to unilateral middle cerebral artery occlusion (MCAO). The focal cerebral ischemia induced a dramatic increase in ascorbate on the ipsilateral (operated) side while the levels remained at the preocclusion level in the striatum of the contralateral (control) hemisphere. The possibility that ascorbate may aggravate ischemic neuronal damage by its proposed neuromodulatory properties and/or by its ability to induce lipid peroxidation is discussed.

Concurrent on-line analysis of striatal ascorbate, dopamine and dihydroxyphenylacetic acid concentrations by in vivo voltammetry

A new method for on-line analysis of in vivo differential normal pulse voltammograms is reported. They are fitted by least squares to an expression describing the contribution of different electroactive species and the baseline. Its validity for resolving ascorbate (AA), dopamine (DA) and DOPAC signals is evidenced by both in vitro testing and the changes recorded in the striatum of anesthetized rats following drug treatments having well-known effects on DA release and metabolism. Thus, pargyline and amphetamine treatments respectively increased and decreased DA and DOPAC, whereas they both were increased by haloperidol. AA levels followed those of DA except when haloperidol was given.

Striatal grafts in rats with unilateral neostriatal lesions--II. In vivo monitoring of GABA release in globus pallidus and substantia nigra

GABA release was recorded in vivo by push-pull perfusion from the globus pallidus and substantia nigra of control rats, rats with unilateral ibotenic acid lesions of the neostriatum, and rats with embryonic striatal tissue grafts implanted in the lesioned striatum. The lesions reduced baseline levels of GABA release to 5% of control levels in the globus pallidus and to 13% of control levels in the substantia nigra pars reticulata. GABA release was substantially restored in both the globus pallidus and substantia nigra of the grafted rats, to 34 and 60%, respectively. Peripheral injection of the dopaminergic stimulant methamphetamine induced a short (lasting approximately 20 min) 4-5 fold increase in GABA release in the intact globus pallidus and a longer (lasting longer than 80 min) increase in the substantia nigra. The stimulatory effect of methamphetamine on GABA release was completely abolished in both sites by the strial lesions, suggesting that the effect was mediated via a direct or indirect dopaminergic action on striatal output neurons. The grafts reinstated methamphetamine-induced stimulation of GABA release in striatal output targets to a level (as a proportion of baseline) that was similar to that seen in the control rats. The results support the view that activation of the dopaminergic inputs to the striatum is functionally excitatory on the major striatal output projections to the globus pallidus and substantia nigra pars reticulata. The results also support the hypothesis that striatal grafts have the capacity to become functionally incorporated by reciprocal graft-host connections into the neural circuitry of the host brain.