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We compared the behavioral and caudate-putamen extracellular dopamine responses following intravenous (3.6 mg/kg) and subcutaneous (8 mg/kg) amphetamine administration using 2-min microdialysate sampling intervals, and doses of the drug selected to achieve comparable maximal brain concentrations. Following intravenous amphetamine, dopamine peaked within the first 2 min, then declined with a first-order decay rate of 0.018+/-0.007 min(-1). Following subcutaneous amphetamine, dopamine achieved maximum concentrations at 9 min and remained near peak levels for about 30 min before declining with a first-order decay rate of 0.019+/-0.008 min(-1). Maximal brain amphetamine levels and peak dopamine concentrations were equivalent following either route of drug administration. In contrast to the short latency to maximal extracellular dopamine, the onset of oral stereotypies was delayed until about 30 min following both routes of drug administration. Furthermore, in contrast to the behavioral response to amphetamine, apomorphine administration resulted in the rapid appearance of oral stereotypies within 5-10 min after drug administration. These results suggest that although caudate-putamen dopamine receptor activation may be a critical factor in the expression of focused oral stereotypies, other effects of amphetamine may interfere with the ability of animals to exhibit these behaviors.

To further delineate amphetamine-dopamine pharmacokinetic-pharmacodynamic relationships, we examined extracellular levels of dopamine and amphetamine in caudate-putamen after the s.c. administration of 8 mg/kg amphetamine. In a parallel group of animals, we also assessed caudate-putamen tissue levels of the drug. Extracellular concentrations of the transmitter and the drug exhibited similar temporal profiles, each achieving maximum concentrations within 30 min of drug administration. Tissue levels of amphetamine exhibited a similar, although slightly earlier time to maximum levels. The concentrations of amphetamine and dopamine in the extracellular fluid and amphetamine in tissue rapidly declined with similar rates of elimination. In contrast to the temporal profiles for both dopamine and amphetamine, stereotyped behaviors achieved maximum intensity at about 60 min. In addition, although transmitter and drug declined almost 10-fold from maximum values over the 4-hr interval after amphetamine administration, stereotyped behaviors persisted for at least 3 hr before abating. The results of these studies confirm our previous observation that the temporal profiles for stereotyped behaviors and extracellular dopamine are dissociated, and also extend this dissociation to extracellular amphetamine. In addition, although there was a close correspondence between dopamine and amphetamine within each experimental animal, individual animals exhibited a broad range of maximal dopamine responses, suggesting a differential responsiveness to amphetamine.

1. The cytochrome P450 isozymes involved in the deamination of amphetamine (AP) and benzphetamine (BZP) have been studied in liver microsomes from rabbit and rat using isozyme-specific inhibitors. 2. Metabolism of BZP in rat yielding phenylacetone and formaldehyde was moderately inhibited by testosterone and chloramphenicol. N-Debenzylation was thought to be P450-dependent, but all inhibitors except for a non-specific inhibitor, SKF-525A, failed to inhibit this reaction. 3. In rabbit, quinidine and testosterone were potent inhibitors of both BZP deamination and dealkylation. Deamination of AP in rabbit was extensively inhibited only with quinidine. 4. AP deamination with purified rabbit CYP2C3, which was previously identified as the major isozyme responsible for this metabolism, was extensively inhibited with quinidine, previously thought to be a specific inhibitor of CYP2D. 5. These results strongly support the notion that the CYP2C isozymes play a major role in the deamination of both AP and BZP, but not for N-debenzylation of BZP in rat. However, on the basis of different sensitivities toward inhibitors, multiple isozymes seem to be involved in BZP deaminations in both species.

Microdialysis in behaving animals was used to concomitantly characterize the dopamine and 5-HT responses in the caudate and the norepinephrine response in the hippocampus to the D- and L-isomers of amphetamine and methamphetamine. Doses of all four drugs which promoted similar stereotypy responses produced a D-amphetamine-like response profile of dopamine and dopamine metabolites, suggesting that all these drugs interact with dopamine systems to facilitate the release of transmitter. However, in contrast to the similar behavioral profiles, the magnitude of the dopamine responses diverged significantly. In addition, all four drugs increased extracellular norepinephrine and 5-HT, but the relative responses differed markedly from dopamine and from each other. The contrasting structure-activity relationships for these drugs likely reflect their differential potency at the various neuronal uptake transporters in promoting either transmitter release, and/or uptake blockade. In addition, the interaction of each drug at the vesicular transporters, as well as the availability of a cytoplasmic pool of transmitter likely also contribute to the neurotransmitter response. Because of the particularly divergent transmitter response profiles exhibited by L-methamphetamine, its behavioral and neurotransmitter effects were characterized over a more extended range of doses. Although the duration of the increase in extracellular dopamine was clearly proportional to dose, the dose-dependent increases in the magnitude of the dopamine response did not parallel the behavioral profiles. The results of these studies indicate that, while the dopamine, norepinephrine and 5-HT responses to these drugs probably contribute to the expression of stimulant-induced behaviors, simple relationships between the neurotransmitter responses and the behavioral profiles were not evident.

The metabolism of 1-amphetamine in rabbit liver is complex in that several routes may give rise to the same intermediate. In this study, the subsequent metabolism of the primary products of N- and C-oxidation were blocked by selecting appropriate incubation conditions. The resulting simplified system was used to investigate the enzymes involved in the two pathways. Phenobarbital treatment increased N- and C-hydroxylation, whereas 3-methylcholanthrene treatment had an inhibitory effect on both pathways. Inhibitors of cytochrome P-450 were either nonselective or were partially selective in inhibiting the two routes of amphetamine metabolism. Induction modulated the sensitivity toward the inhibitors of N- and C-oxidation.

1. Incubation of R(-), S(+) and RS(+/-) amphetamines with rabbit liver 9000 g supernatant indicated that R(-) was metabolized at a faster rate than S(+), but that racemic amphetamine was metabolized at the same rate as S(+) during one hour incubations. 2. N-Hydroxyamphetamine and 1-phenyl-2-propranol were the major compounds detected in both R(-) and S(+) amphetamine incubations. 3. Phenylacetone oxime was detected in significant quantities after 3 h incubations of R(-) amphetamine, but only in minor quantities from S(+). 4. A fall in the amount of N-hydroxyamphetamine present in R(-) amphetamine incubations after a 3 h period as compared to a 1 h incubation, paralleled by a rise in the amount of phenylacetone oxime during 3 h suggested that the oxime was derived as a secondary metabolics from N-hydroxyamphetamine. 5. R(-) and S(+) N-hydroxyamphetamines were both metabolized to phenylacetone oxime by rabbit liver 9000 g supernatant, but the R(-) enantiomer was converted at a faster rate than S(+).

1. The products of p-hydroxylation of amphetamine and phentermine by two different preparations of rat liver microsomes were identified and quantitatively determined. At low concentrations (muM) significant proportions of the substrates were metabolized to the p-hydroxy derivatives by an NADPH-dependent system. The enzyme system was inhibited by higher substrate concentrations (mM) and was not induced by either phenobarbital or 3-methylcholanthrene. 2. The properties of this in vitro system are consistent with reports on in vivo studies of this reaction.

The stereoselective formation of the major metabolites N-hydroxyamphetamine and 1-phenyl-2-propanol in incubations of amphetamine with 9,000 X g supernatant fractions of rabbit liver has been studied using deuterium labelling in conjunction with selected-ion-monitoring gas chromatography/mass spectrometry. Comparison of separate incubations of (R)-amphetamine with those of (S)-amphetamine-d3 and of (S)-amphetamine with those of (R)-amphetamine-d3 showed that larger amounts of the two metabolites were formed from the (R) enantiomer. When "pseudoracemic" mixture (R)-amphetamine/(S)-amphetamine-d3 or (R)-amphetamine-d3/(S)-amphetamine were incubated the two metabolites were preferentially formed from the (S) enantiomer. Studies on the disappearance of substrate during incubation gave results in agreement with the findings on stereoselective metabolite formation. It was concluded that (S)-amphetamine or one of its metabolites inhibits the metabolism of the (R) enantiomer.

Electroencephalographic (EEG) frequency analysis and evoked potentials (EPs) of 27 hyperactive children were examined to determine which, if any, of these electrophysiological parameters might be useful for the selection of those children likely to respond to stimulant medication. The children were treated with placebo, d-amphetamine, and l-amphetamine in double-blind fashion and in random order. EEGs for frequency analysis, visual EPs, and auditory EPs were obtained before and during treatment. Off medication, hyperactive responders to amphetamine had higher predominant beta frequency and shorter latencies of some EP waves than did nonresponders. Although the findings reported here would not select all responders, they would allow the exclusion of most nonresponders.