Crus cerebri lesions abolish amphetamine-induced ascorbate release in the rat neostriatum

Substantia nigra pars reticulata lesion induces preconvulsive behavior and changes in glutamate receptor gene expression in the rat brain

The substantia nigra pars reticulata (SNpr) has been proposed to play an important role in the control of the propagation and/or the generation of epileptic seizures. Earlier studies have shown differential effects of the lesion of the SNpr on seizure genesis that demonstrated a regional difference in the anterior and posterior parts of the SNpr in preconvulsive behavior induced by unilateral reticulata injection of dopamine (DA). This study was aimed to investigate some of the underlying mechanisms of the preconvulsive behavior elicited by unilateral SNpr DA injection by the study of changes in the gene expression of glutamate receptor subunits (GluR1, GluR2 and NMDAR1) and of changes in animal behavior following coinfusion of DA and a DA D1 antagonist SCH 23390 into the SNpr. Unilateral injection of exogenous DA into the anterior region of the SNpr induced rapid and short lasting preconvulsive behavior up to wet dog shakes stage and a significant reduction of gene expression for GluR1, GluR2 and NMDAR1 subunits in rat hippocampal subfields including CA1 through CA4 and dentate gyrus (DG) at 1 day after nigral DA injection. The effect was long lasting and persisted for at least 3 weeks. Both preconvulsive behavior and downregulation of glutamate receptor subunit genes were completely blocked by simultaneous coinfusion of DA and SCH 23390. The results suggest, for the first time, that DA D1 receptor in the SNpr may mediate the nigral-involved seizure development. Glutamate desensitization, and/or selective early neuronal damage might be responsible for the downregulation of glutamate receptor subunits by transient preconvulsive activity.

gamma-Aminobutyric acid infusion in substantia nigra pars reticulata in rats inhibits ascorbate release in ipsilateral striatum

A relatively high level of extracellular ascorbate in the striatum, which is known to modulate impulse flow in striatal neurons, originates primarily from glutamate-containing corticostriatal afferents. Increasing evidence suggests that ascorbate release from these fibers is regulated by a multisynaptic loop that includes gamma-aminobutyric acid (GABA) mechanisms in the substantia nigra. To assess the role that nigral GABA plays in striatal ascorbate release, extracellular ascorbate was monitored voltammetrically in the striatum during infusions of GABA into the substantia nigra pars reticulata (SNr) of awake, unrestrained rats. Compared to vehicle infusions, intranigral GABA lowered striatal ascorbate by >50%. In contrast, intranigral application of picrotoxin, a GABA antagonist, had the opposite effect. Neither GABA nor picrotoxin altered striatal 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine metabolite. Collectively, these results indicate that intranigral GABA exerts a tonic inhibitory influence on ascorbate release in the striatum.

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)

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Unilateral neostriatal kainate, but not 6-OHDA, lesions block dopamine agonist-induced ascorbate release in the neostriatum of freely moving rats

Unilateral kainate lesions of the neostriatum and 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle were used to assess the role of neostriatal and ascending dopaminergic neurons, respectively, on dopamine-agonist induced release of neostriatal ascorbate as measured voltammetrically in freely moving rats. Electrochemically modified, carbon-fiber electrodes recorded the effects of direct (a combination of 10 mg/kg SKF-38393 and 1.0 mg/kg quinpirole) as well as indirect (2.5 mg/kg D-amphetamine or 20.0 mg/kg GBR-12909) dopamine agonists. Relative to controls, kainate, but not 6-OHDA, lesions abolished the ability of both direct and indirect dopamine agonists to induce neostriatal ascorbate release. These results suggest that unlike dopaminergic afferents, neostriatal output pathways play a critical role in the modulation of neostriatal ascorbate levels.

Dopamine-, NMDA- and sigma-receptor antagonists exert differential effects on basal and amphetamine-induced changes in neostriatal ascorbate and DOPAC in awake, behaving rats

Amphetamine and other dopamine agonists elevate the extracellular level of neostriatal ascorbate, which has been shown to modulate neuronal function. To assess the receptor mechanisms underlying neostriatal ascorbate release, drug-induced changes in both basal and amphetamine-induced ascorbate release were monitored voltammetrically in the neostriatum of freely moving rats. A variety of dopamine receptor antagonists decreased basal ascorbate and reversed the increase induced by 2.5 mg/kg D-amphetamine. Thus, compared to vehicle treatment, administration of classical (haloperidol) and atypical (clozapine) neuroleptics or selective D1 (SCH-23390) and D2 (sulpiride) antagonists completely reversed the amphetamine-induced rise in ascorbate and also lowered basal levels by 20-40%. These same effects occurred following injection of dizocilpine (MK-801), a non-competitive NMDA antagonist, whereas BMY-14802, a sigma ligand, reversed the amphetamine-induced rise without altering basal levels. Simultaneous measurements of extracellular DOPAC, a major dopamine metabolite, revealed that haloperidol, clozapine, sulpiride and BMY-14802 elevated basal levels and reversed the amphetamine-induced decline. Dizocilpine also increased basal DOPAC but failed to alter the DOPAC response to amphetamine, whereas both basal and amphetamine-induced changes in DOPAC were unaffected by SCH-23390. A combination of subthreshold doses of SCH-23390 and sulpiride, however, reversed both the amphetamine-induced release of ascorbate and the corresponding decline in DOPAC. Collectively, these results suggest that whereas dopamine, sigma, and NMDA receptors modulate neostriatal ascorbate release, they exert an opposing influence on extracellular DOPAC. All drugs attenuated at least some components of the amphetamine behavioral response, suggesting a role for multiple mechanisms in the behavioral effects of this drug.

Effect of direct and indirect dopamine agonists on brain extracellular ascorbate levels in the striatum and nucleus accumbens of awake rats

Systemic administration of direct and indirect dopamine agonists resulted in increased extracellular ascorbic acid levels in the striatum and, to a lesser degree, in the nucleus accumbens as measured by in vivo voltammetry. Intraperitoneal d-amphetamine sulfate (5mg/kg) increased ascorbate concentrations in striatal extracellular fluid. Amphetamine also increased extracellular ascorbate levels in the nucleus accumbens although more gradually and to a lesser extent. Intraperitoneal phenethylamine hydrochloride (20 mg/kg) following pargyline hydrochloride pretreatment (20 mg/kg) increased extracellular ascorbate levels in the striatum significantly above the small increase seen in the nucleus accumbens. The direct acting dopamine agonists Ly-141865 and Ly-163502 when given i.p. at 1 mg/kg, resulted in increased extracellular ascorbate concentrations in both brain areas, again with a significantly greater effect in the striatum. These results indicate that brain extracellular ascorbate levels can be modulated by dopaminergic neuro-transmission and that this modulation is quantitatively different in different dopamine-containing brain structures.

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.

Repeated administration of high doses of amphetamine increases release of ascorbic acid in caudate but not nucleus accumbens

Linear sweep voltammetry with carbon paste electrodes was used to monitor extracellular ascorbic acid (AA) in the caudate nucleus and nucleus accumbens of behaving rats. Amphetamine (2 or 5 mg/kg) was administered 4, 6 and 8 days after surgery. In general the amphetamine-induced increase in AA was greater in the caudate than in the nucleus accumbens. Furthermore, in the nucleus accumbens the amphetamine-induced increase in AA was very similar on all test days, but in the caudate the increase in AA produced by 5 mg/kg amphetamine was progressively larger on each test day. Thus AA seems to be regulated differently in the caudate and nucleus accumbens.

Rapid changes in striatal ascorbate in response to tail-pinch monitored by constant potential voltammetry

The first peak in the voltammogram recorded with linear sweep and a carbon paste electrode implanted in the rat striatum is due to the oxidation of ascorbic acid. When the potential is held at a level slightly positive to this peak a current is recorded which is abolished by the microinjection of ascorbic acid oxidase in the vicinity of the electrode; this suggests that it is due to the oxidation of ascorbate. This current shows the same diurnal variation as the size of the ascorbate peak and its rise and fall coincides with the onset and offset of motor activity. A tail-pinch applied through a paper clip causes an immediate rise in the ascorbate current which begins to fall as soon as the paper clip is removed. Measurement of the ascorbate current at constant potential provides a technique for monitoring rapid changes in extracellular brain ascorbate in response to physiological stimuli.

d-amphetamine and active behavior--induced changes of regional brain ascorbic acid levels in the rat

In male Wistar rats, shuttle-box avoidance test (5 daily sessions of 50 min) significantly decreased (P less than 0.001) by 30% ascorbic acid (AsAc) level in hypothalamus (HYP), while it left it unmodified in striatum (ST) and in the remaining brain (RB), as opposed to untested rats; AsAc level in HYP showed a significant (P less than 0.001) trend to recovery 24 hours after the end of the test. No correlation was found between the rate of conditioned avoidance responses (CAR) and AsAc levels in the above brain regions. d-Amphetamine (d-A), given s.c. at the dose of 1.0 mg/kg/day for 5 consecutive days, significantly increased AsAc levels by 26% in HYP (P less than 0.005), by 14% in ST (P less than 0.05), and by 33% in RB (P less than 0.001), as opposed to untreated rats; return of AsAc toward baseline levels was negligible 24 hours after d-A withdrawal. The above d-A treatment schedule significantly increased (P 0.05) the rate of CAR in the shuttle-box test. AsAc levels resulted also significantly higher (P less than 0.001) in HYP (+81%) and in RB (+46%) at the end of the test; the AsAc level increase was negligible in ST. Twenty-four hours later, AsAc levels were still significantly higher in RB, but returned toward baseline level in HYP. Again, no correlation was found between CAR rate and regional brain AsAc levels. These results demonstrate that d-A is able to increase regional brain AsAc levels both in control and in tested rats. Since it is well known that systemic administration of AsAc antagonizes some d-A behavioral effects, it is suggested, as a working hypothesis, that the d-A CNS stimulating effect may cause a recruitment of endogenous AsAc, whose task could be a negative neuromodulatory action on the overall effect of d-A on neuronal activity.

The deafferentation syndrome in the rat: effects of intraventricular apomorphine

A deafferentation syndrome, produced in rats by dorsal root ganglionectomies, is expressed as scratching of partially deafferented limb areas and/or biting of anesthetic limb areas. This self-mutilation may be objective evidence of dysesthesias, thus serving as an experimental model to study chronic dysesthesias and/or pain from deafferentation in man. This study included behavioral observations of the syndrome and the effects of intraventricular apomorphine, a dopamine agonist, on its expression. Thirty-eight female Sprague-Dawley rats underwent unilateral C5-T2 dorsal root ganglionectomies followed immediately by stereotactically guided cannulation of the right lateral ventricle in 20 of the rats. For 2 weeks continuously via an osmotic minipump, 10 rats received apomorphine (5 micrograms/h) and 10 others received L-ascorbate (the vehicle). Rats with ganglionectomies only, as well as those receiving L-ascorbate, demonstrated early onset, more severe and later onset, less severe biting groups (P less than 0.05 Mann-Whitney U). Animals receiving apomorphine exhibited low autotomy scores irrespective of time of bite onset. Among the control groups, but not the experimental animals, the earlier the onset of biting, the more severe was the autotomy. The rats receiving vehicle via the minipump had earlier bite onsets than the rats in the ganglionectomy only group. This may indicate that the presence of the minipump is a stress which can accelerate the onset of biting. Intraventricular apomorphine can affect the deafferentation syndrome in the rat; it seems to decrease the level of autotomy and disrupt the relationship of bite onset with degree of biting.

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

Simultaneous recordings of single-unit activity and oxidation current were obtained bilaterally from the neostriatum of rats in response to a unilateral infusion of D-amphetamine (2.0 microliters of 10 micrograms/microliter) into the substantia nigra. Whereas neuronal activity increases ipsilaterally and decreases contralaterally, the electrochemical signals, which reflect extracellular ascorbate, increase bilaterally. In each case, changes in unit activity precede the change in ascorbate release. Systemic administration of haloperidol (0.2 mg/kg) blocks the increase in oxidation current bilaterally, but reverses the neuronal activity response only on the ipsilateral side. These results lend further support to the view that intranigral amphetamine facilitates neostriatal ascorbate release, but this effect is not correlated with changes in single-unit activity.