Beta-adrenoceptors and antidepressants: possible 2-phenylethylamine mediation of chronic phenelzine effects.

Effects of chronic administration of antidepressant drugs on beta-adrenoceptor function were assessed. Tricyclics (imipramine 30 mg/kg/day, desipramine 5 and 10 mg/kg/day) and monoamine oxidase inhibitors [(+/-)-tranylcypromine 1 mg/kg/day, phenelzine 5 and 10 mg/kg/day] were administered to Male Sprague-Dawley rats (n = 8), via Alzet 2ML2 osmotic minipumps for 28 days. Pumps were implanted subcutaneously in the interscapular region and replaced after 14 days. On days 21 and 22 motor-suppressant actions of the beta-adrenoceptor agonist salbutamol (3 mg/kg intraperitoneally [IP]) were assessed as a measure of beta-adrenergic receptor sensitivity. On day 28 the animals were killed and their brains used for measurement of drug levels and monoamine oxidase activity. Liver tissue was used to measure the trace amine 2-phenylethylamine. Each drug induced a decrease in the response to salbutamol. With phenelzine the decreased response to salbutamol was not observed at the lower dose. Differences in monoamine oxidase inhibition following phenelzine did not correspond to differential effects on functional beta-adrenergic sensitivity. Levels of 2-phenylethylamine, an endogenous amine that is also a metabolite of phenelzine, were significantly higher in the 10-mg/kg/day phenelzine group. These data suggest that 2-phenylethylamine may be one mediator of the chronic actions of phenelzine on beta-adrenoceptors.

5HT3 receptor antagonists do not block nicotine induced hyperactivity in rats.

The effects of 5-HT3 receptor antagonists (ondansetron 0.1 mg kg-1 SC 30 min; bemesetron 0.03 mg kg-1 SC 45 min) on nicotine-induced increases in locomotor activity were measured in male Sprague-Dawley rats. Intermittent daily injections of nicotine (0.3-1.2 mg kg-1 SC 30 min) resulted in increased locomotor activity as measured by photocell counts. The effect of nicotine was not affected by administration of the 5-HT3 receptor antagonists at doses that are reported to block nicotine- and morphine-induced place-preference conditioning. Neither of the 5-HT3 receptor antagonists tested affected activity counts in vehicle treated animals. Nicotine-induced hyperactivity was blocked by the dopamine antagonist haloperidol (0.03 mg kg-1 SC 2 h) and by the nicotinic antagonist mecamylamine (1 mg kg-1 SC 1 min). The effects of a range of doses (0-1 mg kg-1) of the 5-HT3 receptor antagonists ondansetron, bemesetron, granisetron and tropisetron on hyperactivity induced by 0.6 mg kg-1 nicotine were then assessed. Only tropisetron at 1 mg kg-1 attenuated nicotine-induced hyperactivity. To demonstrate the efficacy of the present range of doses of the 5-HT3 receptor antagonists in this study, conditioned taste aversion experiments were conducted. Ondansetron (0.1 mg kg-1) failed to attenuate a conditioned taste aversion to saccharin induced by nicotine (0.6 mg kg-1), but did induce a reduction in saccharin preference in choice tests following three saccharin-ondansetron pairings.(ABSTRACT TRUNCATED AT 250 WORDS)

A rapid procedure for the analysis of phenylalanine in brain tissue utilizing electron-capture gas chromatography.

A rapid procedure has been developed for the analysis of phenylalanine in brain tissue. Perchloric acid extracts of brain tissue were made basic, and benzoyl chloride was added to derivatize the amine function. The aqueous layer was retained and made slightly acidic. To derivatize the carboxylic acid group, a solution of pentafluorobenzyl alcohol was added in the presence of the coupling agent dicyclohexylcarbodiimide and chloroform. After shaking for 15 min, the organic phase was retained and taken to dryness. The residue was taken up in toluene, washed, and an aliquot used for analysis on a gas chromatograph equipped with an automatic injector, a capillary column and an electron-capture detector. The procedure has been utilized for analysis of phenylalanine in brains of rats treated with vehicle or phenylalanine.

Chronic antidepressant drug treatment attenuates motor-suppressant effects of apomorphine without changing [3H]GBR 12935 binding.

The effects of chronic administration (28 days s.c. via osmotic minipumps) of the antidepressants phenelzine sulphate, desipramine hydrochloride and clomipramine hydrochloride (each at 10 mg/kg per day) on dopamine function have been measured in rats. Both phenelzine and desipramine attenuated the suppression of locomotor activity induced by apomorphine hydrochloride (0.05 mg/kg s.c. 15 min). Clomipramine did not affect the behavioural response to apomorphine. Analyses of brain tissue from these animals using the radioligand [3H]GBR 12935 revealed that there were no changes in dopamine uptake site density or affinity following the administration of phenelzine, desipramine or clomipramine. Analyses of brain monoamine oxidase activity and tricyclic levels were used to confirm the efficacy of the drug administration protocol. These data indicate that changes in dopamine uptake site density do not mediate antidepressant-induced changes in behavioural responses to apomorphine.

Monoamine oxidase inhibitors: effects on tryptophan concentrations in rat brain.

It has been suggested that inhibition of tryptophan (Trp) pyrrolase and a subsequent elevation of brain Trp may contribute to the actions of antidepressant drugs. In our laboratories, we have conducted a series of experiments measuring brain Trp levels in the rat after both acute and chronic administration of several monoamine oxidase (MAO) inhibitors. The drugs studied during the course of the long-term (28 day) experiments were phenelzine, N2-acetylphenelzine, tranylcypromine, 4-fluorotranylcypromine, 4-methoxytranylcypromine and (-)-deprenyl. High-pressure liquid chromatography with electrochemical detection was employed to measure Trp levels in brains of both MAO inhibitor- and vehicle-treated animals. No significant increases in brain Trp levels were observed as a consequence of MAO inhibitor treatment. Acute time-response (up to 24 h) and dose-response studies were conducted following the administration of phenelzine and tranylcypromine. Only after administration of high doses of these drugs was an elevation in brain Trp observed and the increase was relatively short-lived. These results suggest that elevation of brain Trp may be an important factor in the actions of MAO inhibitors only at high doses of these drugs.

Failure to detect amphetamine or 1-amino-3-phenylpropane in humans or rats receiving the MAO inhibitor tranylcypromine.

BACKGROUND: There have been conflicting reports in the literature about whether or not tranylcypromine is metabolized to amphetamine. In the current report, we investigated this possible route of metabolism in both rats and humans. Body fluid samples from patients and rats and brain, liver and heart samples from rats were analyzed for levels of amphetamine and 1-amino-3-phenylpropane, another potential product of cleavage of the cyclopropyl ring of tranylcypromine after administration of tranylcypromine. Extracted samples were reacted with pentofluorobenzenesulfonyl chloride and analyzed using electron-capture gas chromatography. RESULTS: Amphetamine or 1-amino-3-phenylpropane were not found in any of the samples, indicating that opening of the cyclopropyl ring of tranylcypromine is not a significant route of metabolism for this drug at usual doses. LIMITATIONS: The assay procedure did not permit analysis of 1-amino-2-phenylpropane (another possible product of cleavage of the cyclopropyl ring of tranylcypromine) or of N-methylamphetamine. CONCLUSIONS: These studies support the growing body of evidence indicating that opening of the cyclopropyl ring of tranylcypromine to form amphetamine, a drug of abuse, is not significant at usual doses of tranylcypromine.

Induction of functional down-regulation of beta-adrenoceptors in rats by 2-phenylethylamine.

The effects of chronic administration of antidepressant drugs and 2-phenylethylamine on beta-adrenoceptor function were assessed. Monoamine oxidase inhibitors [phenelzine sulfate, 5 or 10 mg kg-1 per day, and (-)-deprenyl HCl, 1 mg kg-1 per day] and 2-phenylethylamine HCl (10 mg kg-1 per day) were administered to male Sprague-Dawley rats via Alzet osmotic minipumps. On days 21 and 22, the motor-suppressant actions of the beta-adrenoceptor agonist salbutamol hemisulfate (3 mg kg-1 intraperitoneally after 15 min) were assessed as a measure of beta-adrenoceptor sensitivity. On day 28, the animals were killed, and their brains were used for the measurement of monoamine oxidase activity and concentrations of 2-phenylethylamine, an endogenous amine and a metabolite of phenelzine. Phenelzine sulfate at 10 mg kg-1 per day (but not 5 mg kg-1 per day) and the combination of (-)-deprenyl and 2-phenylethylamine resulted in a decrease in the response to salbutamol. These treatments also resulted in substantial increases in brain 2-phenylethylamine concentrations. The phenelzine treatments each resulted in an equivalent inhibition of brain monoamine oxidase activity. These results support the proposal that 2-phenylethylamine may, at least in part, mediate the effects of phenelzine on beta-adrenoceptor function.

Effects of two substituted hydrazine monoamine oxidase (MAO) inhibitors on neurotransmitter amines, gamma-aminobutyric acid, and alanine in rat brain.

Time- and dose-response analyses were undertaken to investigate the effects of the substituted hydrazine monoamine oxidase (MAO) inhibitors iproniazid and nialamide on the following: MAO-A and -B activity; levels of gamma-aminobutyric acid (GABA), alanine (ALA), and the neurotransmitter amines dopamine, noradrenaline, and 5-hydroxytryptamine (serotonin) and their acid metabolites; and the activity of GABA-transaminase and ALA-transaminase. The results showed that these drugs are relatively potent MAO inhibitors but, unlike the unsubstituted hydrazine MAO inhibitor phenelzine, they do not produce increased GABA and ALA levels in brain. These experiments suggest that a free hydrazine group is necessary for MAO inhibitors to also have marked effects on GABA and ALA.

Down-regulation of beta-adrenergic and dopaminergic receptors induced by 2-phenylethylamine.

1. The effects of chronic administration (28 days s.c. via Alzet osmotic minipumps) of 2-phenylethylamine.HCl (10 mg kg-1 per day) and/or (-)-deprenyl.HCl (1 mg kg-1 per day) on dopamine and noradrenaline receptor subtypes have been measured in rat brain. 3H-CGP 12177 was used to label beta-adrenoceptors; 3H-spiperone and 3H-SCH 23390 were used to label D2-like and D1-like receptors. 2. Total cortical beta-adrenoceptor density was reduced by (-)-deprenyl but not 2-phenylethylamine alone. Combined administration of 2-phenylethylamine and (-)-deprenyl resulted in a significantly larger decrease than (-)-deprenyl alone. Subtype density analysis by competition experiments with ICI 89406 revealed that the (-)-deprenyl effect in cortex was due to a decrease in beta 1-adrenoceptor density. The combination of 2-phenylethylamine and (-)-deprenyl resulted in a significant decrease in both cortical beta 1- and cortical beta 2-adrenoceptors. Cerebellar beta-adrenoceptor density was not altered by the present drug treatments. The Kd values for total beta-adrenoceptor densities and Ki values for beta-adrenoceptor subtype densities were not altered by drug treatment in either cortex or cerebellum. 3. Administration of 2-phenylethylamine and of (-)-deprenyl resulted in a decrease in the density of D1-like 3H-SCH 23390 but not D2-like 3H-spiperone binding to dopamine receptors in the striatum. The effects of combined 2-phenylethylamine and (-)-deprenyl treatment on 3H-SCH 23390 binding were additive. These drug treatments did not alter Kd values for these binding sites. 4. The down-regulation of catecholamine receptors following chronically increased availability of 2-phenylethylamine may be due to the catecholamine releasing or uptake blocking effects of this amine. These effects may also be attributable to a direct neuromodulatory action of 2-phenylethylamine on catecholamine receptors. 5. The parallels between effects of increased 2-phenylethylamine availability and effects of administration of MAO inhibitor antidepressants on catecholamine receptor systems indicate that this substrate for MAO may mediate some of the effects of MAO inhibitor antidepressants.

2-Phenylethylamine-induced changes in catecholamine receptor density: implications for antidepressant drug action.

It is now established that (1) concentrations of 2-phenylethylamine (PEA) are greatly increased in brain following administration of monoamine oxidase inhibitor (MAOI) antidepressants; (2) PEA is a metabolite of the MAOI antidepressant phenelzine; and (3) PEA may be a neuromodulator of catecholamine activity. On the basis of these observations, the effects of long term increases in brain PEA on catecholamine receptors have been assessed. Both PEA and antidepressants induced a reduction in the behavioural response to the beta 2 adrenoceptor agonist salbutamol. Radioligand binding measurements revealed that 28 day administration of PEA in combination with the type B MAOI (-)-deprenyl results in a decrease in the density of beta 1 adrenoceptors but not beta 2 adrenoceptors in rat cerebral cortex and cerebellum. (-)-Deprenyl alone also induced a significant decrease in beta 1-adrenoceptors but when PEA was added to this treatment there was a further decrease in beta 1-adrenoceptor density. Only changes in beta 1 adrenoceptor density were evident following 28 day administration of MAOI antidepressants. PEA also induced a decrease in the density of D1-like dopamine (DA) receptors in the rat striatum. MAOI antidepressants induced a decrease in the density of both D1-like and D2-like DA receptors. These data are discussed in terms of a possible role of PEA-catecholamine interactions in antidepressant drug action.

Effects of chronic antidepressant treatment on dopamine-related [3H]SCH 23390 and [3H]spiperone binding in the rat striatum.

1. The effects of chronic administration of antidepressants on dopamine-related [3H]SCH 23390 and [3H]spiperone binding to rat striatal membranes were assessed. 2. The monoamine oxidase inhibitors phenelzine (5 or 10 mg kg-1/day) and tranylcypromine (1 mg kg-1/day) and the tricyclic desipramine (10 mg kg-1/day) were administered for 28 days by constant subcutaneous infusion using Alzet (2ML4) osmotic minipumps. 3. These treatments did not alter Kd estimates for either [3H]SCH 23390 or [3H]spiperone binding sites. The monoamine oxidase inhibitors induced a decrease in the Bmax values for both [3H]SCH 23990 and [3H]spiperone binding sites. Desipramine induced a decrease in the Bmax value for [3H]SCH 23390 binding but had no effect on the Bmax value for [3H]spiperone binding.

Effects of chronic antidepressant treatment on beta-adrenoceptor subtype binding in the rat cerebral cortex and cerebellum.

The effects of desipramine, tranylcypromine, and phenelzine on beta-adrenoceptor subtype binding were measured in rat cerebral cortex and cerebellum. The drugs were administered to male Sprague Dawley rats for 28 d sc via 2ML4 Alzet osmotic minipumps (desipramine HCl 10, tranylcypromine HCl 1, phenelzine sulfate 5, 10 mg kg-1/d). Binding of [3H]CGP 12177 was measured to determine total beta-adrenoceptor density in each brain region. The relative densities of beta 1- and beta 2-adrenoceptors were directly calculated from competition analysis using ICI 89406 to displace [3H]CGP 12177. beta 1-Adrenoceptor density was decreased in cortical tissue following each antidepressant treatment, but no effects on beta 2-adrenoceptor density were evident. With cerebellar tissue, despite a higher density of beta 2-adrenoceptors in this area, no effects of antidepressants on beta 2-adrenoceptor density were evident. The present data confirm and extend previous reports of beta 1- but not beta 2-adrenoceptor downregulation in brain following chronic antidepressant drug treatment.

Beta-adrenergic effects on plasma and brain large neutral amino acids are unaltered by chronic administration of antidepressants.

Effects of isoproterenol (3 mg kg-1, i.p. for 60 min) and salbutamol (3, 10 mg kg-1, i.p. for 60 min) on large neutral amino acid concentrations in rat plasma and brain were assessed. Phenylalanine, leucine, isoleucine, and valine were measured by gas chromatography with electron-capture detection; tyrosine and tryptophan were measured by HPLC with electrochemical detection. These drugs induced increases in brain tryptophan, tyrosine, phenylalanine, and valine and decreases in plasma tryptophan, tyrosine, leucine, isoleucine, and valine. Effects of salbutamol (3 mg kg-1, i.p. for 60 min) were assessed following chronic administration of phenelzine sulfate and desipramine.HCl (each drug 10 mg kg-1 per day, s.c. via Alzet 2ML4 osmotic minipumps for 28 days). There were no effects of these antidepressants on basal levels of large neutral amino acids in brain and plasma. In both brain and plasma, salbutamol-induced changes in large neutral amino acids were unaffected by these antidepressants. The results indicate that beta-adrenoceptor-regulated availability of plasma and brain large neutral amino acids is unaffected by chronic administration of tricyclic or monoamine oxidase inhibitor antidepressants.

Phenylalanine in the caudate nucleus of dopamine depleted rats.

Dopamine depleting lesions of the substantia nigra result in a reduction of the striatal accumulation of 2-phenylethylamine following monoamine oxidase inhibition. It is established that this effect may not be due to a change in availability of aromatic L-amino acid decarboxylase in striatum. Nevertheless, the possibility remains that striatal concentrations of phenylalanine (the precursor of 2-phenylethylamine) may be altered by dopamine-depleting lesions. The present experiments assessed the effects of dopamine depletion induced by 6-OHDA (7 days following 8 micrograms/4 microliters unilateral substantia nigra injection) on striatal concentrations of phenylalanine, dopamine, 5-hydroxytryptamine and their metabolites. In addition, the effects of reserpine-induced (10 mg kg-1, 2h, sc) amine depletion on these striatal levels were also assessed. Under equivalent conditions reserpine is reported to increase striatal accumulation of 2-phenylethylamine. 6-OHDA induced a significant unilateral depletion of dopamine, DOPAC and HVA and increased 5-HIAA but had no significant effect on phenylalanine levels. Reserpine decreased dopamine and 5-hydroxytryptamine and increased DOPAC, HVA and 5-HIAA levels, no changes in phenylalanine were observed. This pattern of results was also observed when lesioned animals or reserpine-treated animals were pretreated with (-)-deprenyl (2 mg kg-1, 2 hr, sc), the treatment previously used to induce accumulation of 2-phenylethylamine. These data indicate that changes in 2-phenylethylamine previously observed under these conditions may not simply be secondary to a change in striatal phenylalanine concentrations.

Electron-capture gas chromatographic procedure for simultaneous determination of amphetamine and N-methylamphetamine.

An electron-capture gas chromatographic procedure for the simultaneous determination of amphetamine and N-methylamphetamine in biological samples is described. The method employs extraction with the ion-pairing reagent bis(2-ethylhexyl)phosphoric acid, and back-extraction with 0.5 M hydrochloric acid. The hydrochloric acid phase is basified, and the amphetamines and the internal standard benzylamine are derivatized with pentafluorobenzenesulfonyl chloride prior to analysis on a gas chromatograph equipped with a capillary column. Levels of amphetamine and N-methylamphetamine have been determined in the urine and liver of rats treated chronically with (-)-deprenyl.