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The objective of this study was to determine the effects of grapefruit juice and seville orange juice on dextromethorphan (DM) pharmacokinetics. Eleven healthy volunteers were studied over a 3-week period consisting of 5 study days each separated by a three-day washout. All subjects refrained from drinking caffeine containing beverages (coffee, soda, etc.) 8 h before orally taking DM (30 mg) with 200 ml water, 200 ml grapefruit juice, 200 ml water, 200 ml seville orange juice, and 200 ml water on Study Days 1 to 5. Aliquots of urine samples were assayed and analysed for DM, and the DM metabolites dextrorphan, 3-methoxymorphinan and 3-hydroxymorphinan using a validated HPLC method employing a phenyl column and a fluorescence detection. Results suggests that DM could provide some useful information on P-glycoprotein or related membrane efflux protein activity in the human gastro-intestinal tract. Bioavailability (F) of DM increased significantly with grapefruit and seville orange juice, but only returned to half the baseline value after three days of washout. This confirms that grapefruit and seville orange juice are long-lasting and perhaps irreversible inhibitors of gut CYP3A/P-glycoprotein. Grapefruit and seville orange juice appeared to have the same overall effect on DM pharmacokinetics. In addition, this paper presents a novel method of phenotyping for CYP2D6, CYP3A and P-glycoprotein using DM as a probe.

Buprenorphine is a partial opioid agonist available in France as an alternative to methadone in the treatment of opiate-dependent individuals. Twenty deaths have been reported in patients who have ingested buprenorphine in combination with benzodiazepines. Since buprenorphine and many benzodiazepines are CYP3A substrates, the effect of buprenorphine on CYP3A activity was examined in order to assess the likelihood of a pharmacokinetic interaction. The formation of 6beta-hydroxytestosterone was measured in dexamethasone-induced rat liver microsomes and in human liver microsomes under control conditions and in the presence of buprenorphine. Buprenorphine was found to be a weak inhibitor of CYP3A with a 50% decrease in enzyme activity occurring at a concentration of 118 microM (IC50) in human liver microsomes. IC50 was 0.3 microM for ketoconazole in the same system. Since the IC50 for buprenorphine is roughly 2000 times higher than typical plasma concentrations, this drug is unlikely to cause clinically significant inhibition of CYP3A in patients. Excessive CNS depression due to the combination of buprenorphine and benzodiazepines is most likely due to additive or synergistic pharmacologic effect unrelated to a pharmacokinetic interaction between the drugs.

The effect of various citrus juices and solutions of naringin on CYP3A activity in rat liver microsomes was compared by measuring the formation of 6 beta-hydroxytestosterone from testosterone. Control enzyme activity was reduced by more than 70% by grapefruit juice. Freshly-squeezed sour (Seville) orange juice containing 20% as much naringin was comparable to grapefruit juice in its ability to inhibit microsomal activity. An aqueous solution of naringin at the same concentration as in grapefruit juice produced only a small decrease in 6 beta-hydroxytestosterone formation. Dilution of grapefruit juice with a naringin solution reduced the inhibitory activity of the juice even though naringin concentration did not change. Naringenin did not form under the incubation conditions used indicating that it did not contribute to the inhibition produced by grapefruit juice. Finally, organic extracts of grapefruit juice possessed considerable inhibitory activity even though naringin was not extractable. These data suggest that grapefruit juice inhibits CYP3A activity in vitro and that neither naringin nor naringenin are primarily responsible for this effect. A compound present in both grapefruit juice and Seville orange juice and extractable into organic solvents appears to be responsible.

Rats were exposed for 10 minutes to one of several enclosures graded in novelty. In one experiment they were then simply sacrificed and plasma corticosterone determinations made in order to obtain an index of the relative stressfulness of these enclosures. In a second experiment the animals received haloperidol and were tested for catalepsy, 2 hours or two weeks following the novel experience. The most novel experience, exposure to a black box, resulted in the highest corticosterone levels and was the only one of our pre-treatments to induce significant enhancement of catalepsy as well as alteration of nucleus accumbens dopamine levels, 2 weeks --but not 2 hours--later. These findings indicate that brief exposure of adult animals to a psychological stressor can induce a long-term alteration in both behavioral and neurochemical responses to a drug and that this effect requires a minimum level of stress to get started and once triggered gets stronger with the passage of time.

The effects of the tricyclic antidepressant imipramine (20 mg/kg) and the β-adrenoceptor agonist clenbuterol (0.5 mg/kg) on the serum concentrations of tyrosine, tryptophan, glucose and insulin were compared 30 min after intraperitoneal injection. The drugs had nearly identical effects on serum tyrosine, which was reduced to 73% of control by both drugs, and on tryptophan, which was reduced to 72% by imipramine and to 66% by clenbuterol. In contrast, whereas clenbuterol raised serum glucose to 174% and insulin to 379% of control, imipramine had no significant effects on either glucose or insulin. The results clearly demonstrate that the effects of imipramine on blood amino acid levels are dissociable from effects on glucose and insulin. We conclude, therefore, that hypoaminoacidemia caused by imipramine is not mediated by stimulating insulin release.

Rats given bilateral lesions of the medial hypothalamus, using either direct or radio frequency current, and killed 2 hours later showed a significant elevation in striatal concentration of homovanillic acid (HVA), while striatal dopamine (DA) was unaltered. After unilateral damage the elevated HVA was seen only in the hemisphere ipsilateral to the lesion. In rats killed 2 days after such damage, the striatal HVA did not differ from controls. The elevation of HVA, suggesting an enhanced release of striatal DA, is associated with a resistance to the cataleptic action of the DA receptor blocking agent droperidol. The present findings suggest that medial hypothalamic lesions can increase neurotransmission within brain DA neurons, and that this neurochemical event may account for at least some of the short-term behavioral effects of these lesions. The relationship of these brain events to the long-term behavioral effects of the lesion remains an important issue for future research.

Spontaneously hypertensive rats (SHR) of both sexes and from two sources exhibited enhanced stereotyped behavior following amphetamine (AM) administration. Both the intensity and the duration of sniffing and licking were higher in SHR than in Wistar or Sprague Dawley controls. SHR exhibited longer catalepsy after haloperidol (HALO) than did controls. Rats made hypertensive by the Goldblatt one kidney method showed neither increased AM stereotypy nor longer HALO catalepsy. The increased effect of AM in SHR was not due to greater drug accumulation in the brain. These findings are discussed in terms of altered brain catecholamine metabolism in SHR, and are related to experiments of chronic stress and the sensitization of AM's effects.

A gas-liquid chromatographic method for the determination of methylhistamine in urine is described. Methylhistamine is reacted with 2,6-dinitro-4-trifluoromethylbenzene sulfonic acid and the derivative thus formed is quantitated with electron-capture detection. The twenty-four hour urinary excretion of methylhistamine in the male rat is about five-fold greater than that in the female rat. Amino-guanadine, a diamine oxidase inhibitor, causes a four-to five-fold increase in methylhistamine excretion in both male and female rats. Treatment with pargyline, a monoamine oxidase inhibitor, causes only a small increase in methylhistamine excretion in male and female rats thereby suggesting that oxidative deamination of methylhistamine is a relatively minor pathway.

The administration of a dose of 200 mg/kg of tyrosine (as either the free amino acid or the ethyl ester) increased the 24-hour excretion of p-hydroxyphenethyleneglycol (p-HPG) and p-hydroxyphenylethanol, metabolites of octopamine and tyramine, by 147 and 50%, respectively. One hour after this dose of tyrosine, brain levels of p-HPG and p-hydroxyphenylacetic acid (p-HPA), another metabolite of tyramine, were increased by 82 and 196%, respectively. Pretreatment with Ro4-4602, a peripheral decarboxylase inhibitor, reduced by 50% the tyrosine-induced increases in brain p-HPA levels, suggesting that tyramine was partially formed in the brain parenchyma. Tyrosine caused only slight, but non-significant increases in brain levels of catecholamine metabolites. These results suggest that tyrosine-induced increases in the production of tyramine and octopamine in brain may account for some of the effects of tyrosine, such as its antihypertensive and reported antidepressant properties.

The enzymatic activities of types A and B MAO are differently distributed among small, discrete regions of the hamster brain. The A/B ratios are highest in the septal region and medial hypothalamic areas and lowest in the cerebellum and the medial and lateral amygdala. The A/B ratios are at least 2 to 3-fold lower in the pineal and pituitary glands than in any brain region.

The characteristics of phenylethanolamine as both a competitive inhibitor and as a substrate for monoamine oxidase (MAO) were studied using rat brain and liver homogenates. Although phenylethanolamine, even at high concentrations (1 mM), produced minimal inhibition of MAO when serotonin (a substrate for type A MAO) was used as the substrate, it was a potent competitive inhibitor (Ki=11 μM) of the deamination of phenylethylamine (a substrate for type B MAO). When phenylethanolamine was used as a substrate, deprenyl, a selective inhibitor of type B MAO, was found to produce a single sigmoid inhibition curve at low concentrations of the inhibitor (pI50=7.5). These results indicate that phenylethanolamine is a specific substrate for type B MAO. Identification of the products formed under the assay conditions show that phenylethanolamine is converted to both mandelic acid and phenylethylene glycol by liver homogenates but only to the latter, neutral metabolite by brain homogenates.

Rabbit platelets were found to contain both types A and B MAO activities. The specific enzymatic activity of rabbit platelet MAO was higher for the substrate serotonin than for phenylethylamine. The Km's for rabbit platelet MAO indicated that the MAO-B enzyme was similar to human platelet MAO and that both MAO-A and MAO-B enzymes in the rabbit platelet are similar to the corresponding forms in the rabbit brain. The drugs clorgyline and deprenyl confirmed the existence of types A and B MAO in the platelet and furthermore indicated that the type A form accounted for approximately 90% of the total enzymatic activity. Amitriptyline at low (micromolar) concentrations selectively inhibited MAO-B activity in both rabbit platelets and brain.

Tricyclic antidepressant drugs were found to inhibit human platelet MAO. The I50 for the inhibition by amitriptyline was 4 × 10 −6 M, 1.6 × 10 −5 M, and 2 × 10 −4 M when phenylethylamine, tryptamine, and benzylamine were used as substrates. Amitriptyline exhibited noncompetitive inhibition with the substrates phenylethylamine and tryptamine but competitive inhibition with benzylamine. Solubilization and partial purification of platelet MAO did not alter the inhibitory effects of tricyclics. Treatment of the partially purified enzyme with the chaotropic agent sodium perchlorate produced only a slight increase in the inhibition constant for amitriptyline. Our findings suggest that selective inhibition of phenylethylamine oxidation may mediate the antidepressant actions of tricyclics. In addition, our studies provide some evidence for the existence of multiple catalytic sites of MAO activity in the human platelet.

Imipramine (IMI; 20 mg/kg) in rats decreased the plasma tyrosine concentration by 21% (90 min), whereas norepinephrine (NE; 1.25 mg/kg) raised it by 72% (40 min). Since NE raised plasma tyrosine by stimulating alpha-adrenoceptors, as shown by phenoxybenzamine (PB) completely abolishing this increase, an experiment was done to find out whether IMI lowered plasma tyrosine by blocking alpha-adrenoceptors. In contrast to PB, IMI pretreatment failed to alter the NE-induced elevation in plasma tyrosine, suggesting that at this dose IMI is not an effective alpha-adrenergic antagonist in vivo. Thus, IMI would not appear to reduce plasma tyrosine by blocking alpha-adrenoceptors. In a separate experiment, propranolol blocked the ability of IMI to lower plasma tyrosine. Propranolol also prevented a 17% elevation in brain tryptophan levels induced by IMI but did not alter the 29% decrease in plasma tryptophan. PB by itself decreased plasma tyrosine, but this decrease was not greater by additionally treating with IMI. Salbutamol (10 mg/kg), a beta 2 agonist, lowered plasma tyrosine to 76% and raised brain tryptophan to 143% of control. These results suggest that IMI decreases tyrosine concentrations in plasma and raises tryptophan in brain by stimulating beta-adrenoceptors.