Neil S. Buckholtz

Monoamine oxidase inhibition in brain and liver produced by β-carbolines: structure-activity relationships and substrate specificity

Abstract The ability of a series of β-carboline compounds to inhibit monoamine oxidase (MAO) was studied in homogenates of mouse brain and liver. The in vitro structure-activity study with tryptamine as substrate showed the following to affect the degree of MAO inhibition: methyl group at carbon-1; methoxy or hydroxy group at carbon-6 or 7; ring saturation. In general, there was greater in vitro inhibition of brain MAO than of liver MAO. 6-Methoxy-1,2,3,4-tetrahydro-β-carboline produced greater MAO inhibition with serotonin as the substrate than with β-phenylethylamine both in vitro and in vivo , suggesting that it is a relatively selective inhibitor of MAO type A.

Neurobiology of tetrahydro-β-carbolines

Abstract Tetrahydro-β-carbolines (THβCs) are tricyclic compounds structurally related to the indoleamines. Recent studies have reported the in vitro and in vivo formation of these compounds in brain and other tissues. This information and other data indicating that THβCs interact relatively specifically with various aspects of the serotonergic neurotransmitter system has suggested that THβCs may serve as endogenous neuromodulators of neurotransmitters. Evidence for the formation of these compounds as well as their neurochemical, neuroendocrinological, and behavioral effects is described in this review.

Serotonin2 agonist administration down-regulates rat brain serotonin2 receptors

Daily administration of D-lysergic acid diethylamide (LSD) was previously shown to decrease serotonin2 (5-HT2) receptor binding in rat brain. Recently, 4-substituted derivatives of 1-(2,5-dimethoxyphenyl)-2-aminopropane, the substitution being with either iodine (DOI) or bromine (DOB), have been suggested to be relatively selective 5-HT2 agonists. These compounds share common behavioral and neurophysiological effects with LSD, suggested to be 5-HT2 receptor mediated, and the purpose of the present study was to determine whether they also affect 5-HT2 receptor binding after systemic administration in a similar way to LSD. Administration of DOI (1.0 mg/kg) or DOB (0.5 mg/kg) for 7 days resulted in a decrease in 5-HT2 binding, as evaluated with [3H]ketanserin, similar to the decrease after LSD. In a further evaluation of the parallelism of LSD and 5-HT2 agonists, it was found that 24 hr after one administration of a low dose of LSD (130 ug/kg) or DOI (1.0 mg/kg), there was no change in binding, but there was a decrease 24 hr after a high dose (LSD, 650 micrograms/kg; DOI, 7.0 mg/kg). Four hours after the high dose of LSD or DOI there was also a decrease in 5-HT2 binding. Thus, results have shown that 5-HT2 agonists are capable of down-regulating 5-HT2 receptors and that LSD acts in a parallel fashion. This study has also demonstrated that 5-HT2 receptors can be modified within hours after drug administration.

Inhibition by β-carbolines of monoamine uptake into a synaptosomal preparation: Structure-activity relationships

The potency of a series of β-carboline compounds to inhibit 3H-serotonin (3H-5-HT) uptake into a synaptosomal suspension from mouse brain was studied. The in vitro structure-activity study showed the tetrahydro-β-carbolines to be the most potent inhibitors compared to unsaturated β-carbolines. In vitro inhibition of 3H-norepinephrine (3H-NE) and 3H-dopamine (3H-DA) uptake was determined for some tetrahydro-β-carbolines, and the degree of inhibition of uptake of these amines was less than that for 3H-5-HT (EC50s being 5–13 times those for 3H-5-HT). The tetrahydro-β-carbolines were also found to effectively inhibit 3H-5-HT but not 3H-NE or 3H-DA uptake when they were administered intra-peritoneally. These results suggest that the behavioral effects of the tetrahydro-β-carbolines which have been reported previously may be due to a relatively selective involvement of the serotonergic neurotransmitter system.

Daily LSD administration selectively decreases serotonin2 receptor binding in rat brain

The effect of ten daily injections of saline or d-lysergic acid diethylamide (LSD) (260 micrograms/kg i.p.) on serotonin1 (5-hydroxytryptamine1, 5-HT1) and 5-HT2 receptor binding was determined in brain membranes from rats killed 24 h after the last injection. [3H]LSD (3.0 nM) was used with either 30.0 nM 5-HT or 70.0 nM cinanserin to estimate 5-HT1 and 5-HT2 receptors, respectively. LSD administration decreased 5-HT2 binding in cortex, striatum, hippocampus, and diencephalon/midbrain without altering 5-HT1 or total specific binding.

Combined inhibition of serotonin uptake and oxidative deamination attenuates audiogenic seizures in DBA/2J mice ☆

We previously reported that 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THBC) attenuated audiogenic seizures (AGS) in 21-day-old DBA/2J mice and also inhibited brain monoamine oxidase-A (MAO-A) and serotonin (5-hydroxytryptamine, 5-HT) uptake leading to increased brain 5-HT concentration. In this study, the sensitivity of AGS to 5-HT manipulation was evaluated by utilizing drug combinations which paralleled the actions of 6-MeO-THBC and which also have been associated with the production of a serotonergic motor syndrome in rats. Combination of a specific 5-HT uptake inhibitor (fluoxetine or citalopram) with the MAO-A inhibitor clorgyline inhibited AGS more effectively than the individual drugs but combination with the MAO-B inhibitor deprenyl did not. Combined administration of clorgyline plus deprenyl also suppressed AGS. Inhibition of AGS by tryptophan was potentiated by combination with either of the mixed MAO inhibitors nialamide or tranylcypromine. The effects of these drugs individually and in combination on brain MAO-A and MAO-B activity and 5-HT uptake were also determined ex vivo and were consistent with expected mechanisms of action. These results suggest, first of all, that the inhibition of AGS produced by 6-MeO-THBC is a consequence of its combined MAO-A and 5-HT uptake inhibition properties. Secondly, the similarity of results of pharmacological manipulations of the 5-HT system which produce the rat motor syndrome and which inhibit AGS in the mouse suggests that AGS in 21-day-old DBA/2J mice may be a useful system for assessing functional consequences of these serotonergic manipulations.

6-Methoxy-1, 2, 3, 4-tetrahydro-β-carboline effects on retinal serotonin

Abstract The compound 6-Methoxy-1, 2, 3, 4-tetrahydro-β-carboline (6-MeO-THβC) belonging to the group of hallucinogenic β-carbolines has been found to alter various aspects of serotonergic transmission in the brain. The action of 6-MeO-THβC on the newly identified 5-HT system of bovine retina was studied in vitro . The drug inhibited the high affinity uptake of [3H] 5-HT in a competitive manner and had no evident effect on the uptake of dopamine or GABA. In addition the compound increased the potassium evoked release of 5-HT from retina. Endogenous β-carbolines have been proposed to be involved in the modulation of serotonergic activity in the brain and a similar action is proposed in the retina. In addition these findings raise the possibility of retinal 5-HT system as a site of action for β-carbolines and similar hallucinogenic drugs.

Anticonvulsant effects of 6-methoxy-1,2,3,4-tetrahydro-β-carboline on audiogenic and electroconvulsive seizures in mice

6-Methoxy-1, 2, 3, 4, -tetrahydro-beta-carboline (6-MeO-THBC) was tested for anticonvulsant properties against audiogenic seizures in DBA/2J and primed C57BL/6J mice (i.e., mice given a prior auditory exposure) and aginast electroconvulsive seizures in DBA/2J mice. 6-MeO-THBC (100 mg/kg) was found to attenuate both types of behavioral seizures 2 hr after injection as compared to saline controls. In addition, 6-MeO-THBC increased whole brain serotonin and decreased whole brain 5-hydroxyindoleacetic acid 2 hr after injection. These results support previous reports which suggest a serotonergic involvement in behavioral seizures.

Effects of caffeine and L-phenylisopropyladenosine on locomotor activity of mice ☆

C57BL/6J and DBA/2J mice were used to determine if possible differences in the behavioral response to caffeine might be related to differences in A1 adenosine receptors. Caffeine stimulated locomotor activity of both strains, but the dose-response relationship and time course of drug action differed according to strain. Although their response to caffeine differed, the strains did not differ in response to the A1 adenosine agonist L-phenylisopropyladenosine (PIA) nor in the binding of the A1 agonist [3H]N6-cyclohexyladenosine (CHA) in various brain regions. Thus, the behavioral differences in response to caffeine could not be accounted for by differences in adenosine binding. Of alternative mechanisms, strain differences in A2 receptors appear to be the most promising.

Brain serotonin and 5-hydroxyindoleacetic acid concentrations and serotonin synthesis following tetrahydro-β-carboline administration in mice

SummaryThe effects of various tetrahydro-β-carboline (THβC) compounds on serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations and serotonin synthesis in mouse brain were investigated. 6-Methoxy-THβC (6-MeO-THβC) (100 mg/kg i.p.) significantly increased 5-HT concentrations from 15 min until 4 h after administration, and decreased 5-HIAA concentrations from 15 min until 10 h. The lowest dose of 6-MeO-THβC which produced a significant 5-HT increase was 25 mg/kg whereas 12.5 mg/kg produced a significant decrease of 5-HIAA. Comparisons among the THβCs in their ability to decrease 5-HIAA concentrations showed the following order of potency: THβC>6-MeO-THβC>6-MeO-tetrahydroharman>6-OH-THβC, In order to determine the mechanism for the increase in 5-HT and decrease in 5-HIAA produced by 6-MeO-THβC, its effect was compared to that of reference drugs known to inhibit monoamine oxidase (e. g. clorgyline, deprenyl, pargyline) and synaptosomal 5-HT uptake (chlorimipramine, fluoxetine). These comparisons suggested that the increased 5-HT concentration produced by 6-MeO-THβC was primarily the result of its monoamine oxidase-A inhibiting properties, whereas the decrease in 5-HIAA was the result of a combination of monoamine oxidase-A inhibition and 5-HT uptake inhibition.Serotonin synthesis in mouse brain following treatment with various tetrahydro-β-carboline compounds and other drugs affecting serotonin metabolism was determined using the so-called in vivo tryptophan hydroxylase method. This involved administering either saline or test drug followed at various times later by an injection of the decarboxylase inhibitor RO4-4602 (800 mg/kg), killing the mouse 30 min later, and measuring 5-hydroxytryptophan (5-HTP) in brain. 6-MeO-THβC at 100 mg/kg decreased the formation of 5-HTP with respect to saline controls from 1 to 4 h after administration, and this decrease was shown to be dose-dependent as measured at 2 h. The other THβCs including THβC, 6-OH-THβC and 6-MeO-tetrahydroharman also decreased 5-HTP formation at 2 h. In order to determine the mechanism for this decreased 5-HTP formation produced by the THβCs, their effects were compared to those of reference drugs known to inhibit monoamine oxidase-A (clorgyline, Lilly 51641), to inhibit synaptosomal serotonin uptake (fluoxetine, chlorimipramine), or to be a serotonin receptor agonist (quipazine). These comparisons suggested that the decrease in 5-HTP formation produced by the THβCs was most likely the result of a combination of monoamine oxidase inhibition plus serotonin uptake inhibition with the latter effect somewhat more potent.