Synergistic elevation of brain 3,4-dihydroxyphenylacetic acid concentration by methylphenidate and spiperone in control but not reserpine-pretreated rats

Exogenous tyrosine potentiates the methylphenidate-induced increase in extracellular dopamine in the nucleus accumbens: a microdialysis study

Previous work has shown that the synthesis and release of dopamine may, under certain conditions, be influenced by an increase in the availability of its amino acid precursor, tyrosine. To examine whether exogenous tyrosine could potentiate the methylphenidate-induced increase in extracellular dopamine, male rats were implanted with microdialysis probes aimed at the right nucleus accumbens. Samples were collected from awake animals beginning 22 h after surgery. A repeated measures design was used involving the continuous collection of 20-min samples for a 4-h period once a day for 3 consecutive days. On a given day, an animal was infused with methylphenidate, tyrosine, or methylphenidate plus tyrosine. Periods of infusion with the active compounds were preceded and followed by baseline conditions, and treatments were counterbalanced to control for possible order effects. Methylphenidate plus tyrosine significantly increased extracellular levels of dopamine in comparison to drug alone. This effect was long-lasting, persisting into the post-treatment sampling period and peaking 40 min after the peak induced by methylphenidate alone. Tyrosine alone induced a small but significant increase in extracellular dopamine in the absence of any treatment to accelerate the firing of dopamine cells. These findings may have implications for the treatment of attention deficit hyperactivity disorder.

Evidence for presynaptic antagonism by amantadine of indirectly acting central stimulants

In mice, amantadine pretreatment (150 mg/kg, but not 10 mg/kg, 2 h prior to testing) markedly inhibited the locomotor stimulation produced by submaximal doses of d-amphetamine, amfonelic acid, methylphenidate, caffeine, memantin, phencyclidine, and cocaine. A 50-mg/kg dose was ineffective in blocking the effects of caffeine and memantin, but blocked the responses to the other five stimulants. Amantadine did not modify the locomotor stimulant effect of apomorphine in reserpinized mice. These results indicate that amantadine acts as a presynaptic antagonist to the above seven stimulants. Even the highest dose of amantadine did not modify the hyperactivity induced in mice by morphine and levorphanol. This result is consistent with evidence showing opiate actions at postsynaptic striatal neurons, sites where presumably amantadine is unable to exert an antagonist effect. Amantadine did not modify the central depressant effects of ethyl alcohol and pentobarbital. Amantadine could be of value as a pharmacological tool in understanding the mode of action of central stimulants, and in the management of stimulant abuse. The present data do not support the currently held view that the antiparkinsonism effect of amantadine results from its ability to potentiate the central effects of dopamine.

Differential effects of d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate on the rate of dopamine synthesis in terminals of nigrostriatal and mesolimbic neurons and on the efflux of dopamine metabolites into cerebroventricular perfusates of rats

The in vivo effects of four psychomotor stimulants (d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate) were determined on: 1) the rate of dopamine (DA) synthesis, as measured by the accumulation of dihydroxyphenylalanine (DOPA) after aromatic L-amino acid decarboxylase inhibition, in the striatum (terminals of nigrostriatal neurons) and in the nucleus accumbens and olfactory tubercle (terminals of mesolimbic neurons) and 2) the efflux of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) into cerebroventricular perfusates of conscious, freely-moving rats. d-Amphetamine and beta-phenylethylamine produced biphasic responses with lower doses of each drug increasing both the efflux of DOPAC and the rate of DA synthesis in the striatum. Higher doses of each drug either had no effect or actually decreased the efflux of DOPAC and also decreased the rate of DA synthesis in the striatum. On the other hand, cocaine and methylphenidate only decreased the efflux of DOPAC and the rate of DA synthesis in the striatum. The effects of the drugs on the rate of DA synthesis in the nucleus accumbens and olfactory tubercle were similar to, but less pronounced than those seen in the striatum. These results are consistent with the following suggestions: 1) low doses of d-amphetamine and beta-phenylethylamine facilitate the neuronal release of DA while higher doses of both drugs facilitate release and inhibit neuronal reuptake of the amine, and 2) cocaine and methylphenidate preferentially block the neuronal reuptake of DA.

The effect of phencyclidine on dopamine synthesis and metabolic in rat striatum

Previous behavioral and neurochemical studies indicate that phencyclidine (PCP), a potent psychotomimetic agent, interacts with central dopaminergic systems. We have examined the effects of PCP on the rate of accumulation of 3,4-dihydroxyphenylalanine (DOPA) after the inhibition of L-aromatic amino acid decarboxylase and on the levels of dopamine (DA) metabolites: 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in rat striatum. PCP, in doses from 2.5 to 50 mg/kg, decreased the rate of striatal DOPA accumulation. PCP did not antagonized the increase in the rate of striatal DOPA formation caused by haloperidol, reserpine or gamma-butyrolactone (GBL). When given alone, PCP decreased striatal levels of DOPAC and HVA, while it greatly potentiated the haloperidol-induced rise in striatal levels of these two metabolites. PCP is considerably less effective than d-amphetamine in promoting the release of 3H-DA from preloaded striatal slices in vitro. Our results are consistent with the interpretation that PCP potentiates the synaptic effects of endogenous DA. Its mechanism of action appears to be closely related to that of a category of drugs known as non-amphetamine stimulants, which, among others, includes methylphenidate, amfonelic acid and cocaine.