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

Infusions of ascorbic acid into the medial preoptic area facilitate appetitive sexual behavior in the female rat

Ascorbic acid (AA), also known as Vitamin C, enhances dopamine (DA) transmission in mesolimbic and nigrostriatal terminals and augments DA-mediated behaviors. It is not yet known whether AA has a similar influence in other DA terminals, in particular terminals of the incertohypothalamic system that modulate the function of the medial preoptic area (mPOA). In female rats, DA in the mPOA plays a critical role in the generation of appetitive sexual responses, notably solicitations, hops, and darts, and we have shown previously that the role of DA in this region on female sexual behavior changes depending on the hormonal profile of the female. Since AA has often been used as a vehicle control in the examination of rat sexual behavior, the present study examined the effect of infusions of AA to the mPOA of sexual experienced ovariectomized rats under two hormonal conditions: partially-primed with estradiol benzoate (EB) alone or fully-primed with EB and progesterone. Relative to saline baselines, females under both hormonal conditions displayed a significant increase in appetitive sexual behaviors following infusions of AA. No difference in lordosis behavior was observed following AA infusions relative to saline baselines. We suggest that the mechanism by which AA infusions to the mPOA increase appetitive sexual behaviors in female rats may be through dose-dependent DA receptor interactions, possibly through both presynaptic release mechanisms and postsynaptic DA D1-related messenger systems.

Hypoxia depletes ascorbate in the cat carotid body

Vitamin C or L-ascorbate is an effective endogenous reducing agent influencing the functions that are involved with the redox mechanism. Detection of oxygen tension changes by the carotid body chemoreceptors is one such function. In this study we investigated the hypothesis that the level of ascorbate, if present, could change in the carotid body as a result of ascorbate's interaction in the chemosensing process, which would be reflected in the production of the ascorbyl radical. We addressed this issue by examining changes of the ascorbyl radical, using the in vitro electron spin resonance spectroscopy (ESR), in the carotid bodies dissected from pentobarbitone anesthetized cats exposed in vivo to three eucapnic, breathing gas mixtures of different O2 concentration: normoxic, 21% O2; hypoxic, 7% O2; and hypoxic-reoxygenated, 7% O2 followed by 100% O2. Each group consisted of five cats, yielding five pairs of carotid bodies for the ESR signal recording. We found that the intensity of ESR signals, measured as peak-to-peak amplitude, was diminished by 39 and 43% in the hypoxic and hypoxic-reoxygenated groups, respectively, compared with that in normoxia. The study shows that ascorbate was present in the normoxic carotid body and was depleted by hypoxia. We conclude that ascorbate is part of free radical mechanisms operative in the carotid body in hypoxia.

Ascorbic acid and atypical antipsychotic drugs: modulation of amineptine-induced behavior in mice

To provide a detailed characterization of individual kinds of behavior produced by ascorbic acid in combination with typical (haloperidol) or atypical (clozapine, sulpiride and remoxipride) antipsychotic drugs, the 'open-field' test was selected. Amineptine, an indirect dopamine agonist, was used as an explicit model of dopaminergic activity. Results showed that amineptine (5-10-20 mg/kg i.p.), dose-dependently, increased ambulation and rearing. Ascorbic acid (62.5-125-250 mg/kg i.p.) markedly inhibited the behavior of mice as well as the amineptine-induced hyperactivity. A combination of each typical or atypical antipsychotic drug (except clozapine 2.5 mg/kg i.p.) with amineptine (20 mg/kg i.p.) induced a significant increase in ambulation and rearing over that seen with the antipsychotic drugs alone. The combination of antipsychotic drugs with ascorbic acid 250 mg/kg i.p. led to a decrease in open-field parameters when compared with controls. In conclusion, these data provide further in vivo support for the effect of ascorbic acid on dopaminergic system and demonstrate that the antidopaminergic effects of both typical and atypical antipsychotic drugs may be enhanced with concurrent administration of ascorbic acid.

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.

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.

Vitamin C in the treatment of schizophrenia

This report concerns a 37-year-old chronic schizophrenic patient who derived substantial benefit from the addition of ascorbic acid (vitamin C) to his neuroleptic treatment. The case highlights the potential usefulness of vitamin C as an adjunctive agent in the treatment of chronic schizophrenia.