Norharman inhibition of [3H]diazepam binding in mouse brain

Exposure to beta-carbolines norharman and harman

The aromatic beta-carbolines norharman and harman have been implicated in a number of human diseases including Parkinson's disease, tremor, addiction and cancer. It has been shown that these compounds are normal body constituents formed endogenously but external sources have been identified. Here, we summarise literature data on levels of norharman and harman in fried meat and fish, meat extracts, alcoholic drinks, and coffee brews. Other sources include edible and medicinal plants but tobacco smoke has been identified as a major source. Exposure levels from these different dietary sources are estimated to a maximum of 4 microg norharman per kg body weight (bw) per day and 1 microg harman per kg bw per day. Exposure via tobacco smoke depends on smoking habits and type of cigarettes but can be estimated to 1.1 microg/kg bw for norharman and 0.6 microg/kg bw for harman per package of cigarettes smoked. Studies on toxicokinetics indicate that inhalative exposure leads to a rapid increase in plasma levels and high bioavailability of norharman and harman. Oral bioavailability is lower but there are indications that sublingual absorption may increase dietary uptake of beta-carbolines. Endogenous formation can be estimated to be 50-100 ng/kg bw per day for norharman and about 20 ng/kg bw per day for harman but these rates may increase with high intake of precursors. Biomarker studies on plasma levels of beta-carbolines reported on elevated levels of norharman, harman or both in diseased patients, alcoholics and following tobacco smoking or consumption of beta-carboline-containing food. Cigarette smoking has been identified as major influence but dietary exposure may contribute to exposure.

The impact of smoking and drinking on plasma levels of norharman

The hypothesized role of the beta-carboline norharman in processes of drug dependence forms the basis for several studies on plasma levels of norharman among substance-using populations, particularly among alcoholics and smokers. However, it is not clear whether norharman is implicated in processes of dependence to both substances, or only to tobacco smoke. In the present study plasma concentrations of norharman were measured among four groups of participants regarding heavy smokers who do or do not drink alcohol excessively and nonsmokers who do or do not drink alcohol excessively. All measurements were conducted on three different days with an interval of 2 months in between and at three times during the day to account for possible circadian or seasonal variations. Results showed that elevated plasma levels of norharman appear only in heavy smokers regardless of their drinking profile. The norharman plasma levels of nonsmoking excessive drinkers showed a similar pattern to that of the control group. The findings indicate that elevated plasma levels of norharman are due to heavy smoking and not to excessive drinking.

Occurrence and partition of the beta-carboline norharman in rat organs

The beta-carboline norharman was determined in plasma, brain, liver, kidney, spleen, heart and lung of the rat using HPLC with fluorescence detection. In order to improve the speed and sensitivity of this assay an earlier published sample clean-up extraction procedure and HPLC method were adjusted. Norharman was found to be present in plasma as well as in all organs tested, concentrations in organs being about 80 times higher than those in plasma. Intraperitoneal injections of 2 and 100 mg/kg norharman showed that the partition of norharman between organs and plasma is about 3. Only the highest dose was found to have behavioural effects, viz. alerting reactions, a decrease in motor and exploratory activity, sedation, loss of righting reflex and after 30 min complete muscle relaxation, but no catatonia was observed. Norharman was found to be metabolized by the liver with a half live of about 20 min, whereas all other organs tested did not show any norharman clearing capacity. The results suggest that norharman is not likely the cause of psychosis, but a natural sedative and by- or coproduct of a more primary biochemical derangement.

Inverse agonist properties of the THBC discriminative stimulus: asymmetrical generalization with FG 7142

Male rats were trained to discriminate the stimulus properties of the beta-carbolines 1,2,3,4-tetrahydro-beta-carboline (THBC) (15.0 mg/kg) or FG 7142 (5.0 mg/kg) from vehicle in a two-lever, food-motivated operant task. Consistent with the serotonergic properties of THBC, administration of the 5HT1B agonists TFMPP and mCPP to THBC-trained rats resulted in THBC-appropriate responding. Norharmane, a beta-carboline metabolite of THBC, also mimicked the THBC discriminative stimulus. In contrast, the benzodiazepine receptor partial inverse agonist FG 7142, the anxiogenic/convulsant pentylenetetrazole (PTZ), two physiological stressors and the alpha 2 adrenergic antagonists yohimbine and idazoxan failed to produce THBC-appropriate responding. In the FG 7142-trained rats, THBC and norharmane dose-dependently mimicked the FG 7142 discriminative stimulus. This generalization was not based upon the serotonergic properties of THBC and norharmane since administration of the serotonin agonist mCPP to FG 7142-trained rats failed to produce FG 7142-appropriate responding. The ability of THBC to substitute for the FG 7142 discriminative stimulus was antagonized by the benzodiazepine receptor mixed agonist/antagonist CGS 9896 and the benzodiazepine receptor antagonist RO 15-1788, indicating that THBC produces an inverse agonist stimulus in FG 7142-trained rats. These results suggest that THBC produces a discriminative stimulus which consists of both serotonergic and inverse agonist components.

Ro 15-1788 suppresses the development of kindling through the benzodiazepine receptor

beta-Carboline (norharman) has been shown to produce kindled seizures when given systemically for long periods of time. The expression of the kindled seizure activity can be blocked by ligands of the benzodiazepine receptor suggesting the receptor as a site of vulnerability in the kindling mechanism. The present data show that Ro 15-1788, a receptor antagonist, suppresses the development of kindled seizures as demonstrated by the delayed appearance of each behavioral stage and the decreased severity of symptoms within each stage. Animals treated with Ro 15-1788 still expressed lower behavioral stages when Ro 15-1788 was eliminated from the trials indicating that this compound suppresses the very process of kindling itself and not just the expression of the kindled seizures. Ro 15-1788 given with norharman causes an increase in Bmax of [3H]flunitrazepam binding to the benzodiazepine receptor in cortex. It is not known if this increase is instrumental in lowering the kindling rate.

Endogenously formed norharman (beta-carboline) in platelet rich plasma obtained from porphyric rats

Porphyria was induced in adult male Wistar rats starved for 24 hr by SC injection of 400 mg/kg allylisopropylacetamide (AIA). The presence of porphyria was shown by measuring excretion of delta-aminolevulinic acid (delta-ALA) and porphobilinogen (PBG) into the urine during 24 hr after AIA administration. Plasma levels of glycine, serine and of a number of other amino acids were decreased in porphyric rats as compared to controls. Intraperitoneal injection of 2 mmol/kg serine 24 hr after AIA administration was used as an animal model for an acute psychosis, by measuring catalepsy scores 30 min after serine injection. The concentration of 5 different beta-carbolines in platelet rich plasma (PRP) was measured using an HPLC-fluorometric method. An increase in the concentration of norharman (NH) in PRP, ranging from 0.57 nmoles/l in control rats to 1.88 nmoles/l in serine treated porphyric rats was found. The catalepsy duration was exponentially correlated with the NH concentrations in PRP. It is concluded that an elevated conversion of serine into glycine via serine hydroxymethyltransferase (SHMT) may be responsible for the enhanced NH biosynthesis.

Can drug effects on anxiety and convulsions be separated?

The effects of benzodiazepines, barbiturates, a series of novel putative anxiolytic compounds and anxiogenic compounds are reviewed in animal tests of anxiety and on experimentally-induced seizures. It is clear from the data that drug effects on anxiety and convulsions are not always in the same direction; certain compounds are apparently both anxiolytic and proconvulsant, others are anxiogenic and anticonvulsant, others have varied effects depending on the test situation. It is suggested that this work necessitates considerable revision of our traditional concepts of an "anticonvulsant." The extent to which drug-induced anxiety is correlated with weak epileptiform activity in the brain is discussed. Finally, the Discussion considers a number of possible mechanisms that could underlie the separation of drug effects on anxiety and convulsions that is observed.

Beta-carboline kindling of the benzodiazepine receptor

Beta-Carboline (norharman), an antagonist of the benzodiazepine receptor, given daily at a dose of 20 mg/kg (i.p.), produces kindled seizures. The mechanism of this epileptogenesis is unknown; however, other ligands of the benzodiazepine receptor, i.e. diazepam, R015-1788 and CL218-872 block the expression of kindled seizures in a dose-dependent manner. The results suggest that beta-carboline kindling is mediated through the benzodiazepine receptor.

Inhibition of benzodiazepine and GABA receptor binding by amino-gamma-carbolines and other amino acid pyrolysate mutagens

The effect of several pyrolysate mutagens on the benzodiazepine and GABA receptors was investigated. Of amino-gamma-carbolines, Trp-P-1 antagonized the suppressive effect of diazepam on the pentylenetetrazol-induced convulsions and death, whereas Trp-P-2 by itself precipitated seizures and death in male mice. Both Trp-P-1 and Trp-P-2 inhibited the specific binding of [3H]diazepam and [3H]muscimol in rat brain membranes mainly by increasing Kd, indicating that these gamma-carbolines bind on benzodiazepine and GABA receptors. IC50S of Trp-P-1 and Trp-P-2 on specific [3H]flunitrazepam binding were not changed by addition of GABA. The Hill coefficient of Trp-P-1 for displacing [3H]diazepam binding was about unity whereas that of Trp-P-2 was less than unity. These results suggest that Trp-P-1 and Trp-P-2 act as active antagonists or inverse agonists at benzodiazepine receptors. The convulsant effect of the gamma-carbolines may be mediated by an action on the central benzodiazepine receptors; however, the role of the effect on GABA receptors is not clear.

Interaction of convulsive ligands with benzodiazepine receptors

The gamma-aminobutyric acid (GABA)-benzodiazepine receptor complex, which is composed of distinct proteins embedded in the neuronal plasma membrane, is important for several effects of benzodiazepines, including protection afforded against convulsions. During structural modification of ethyl beta-carboline-3-carboxylate an agent was discovered which has high affinity for brain benzodiazepine receptors but which is a potent convulsant. Also in contrast to benzodiazepines, this type of benzodiazepine receptor ligand favors benzodiazepine receptors in the non-GABA-stimulated conformation, which may explain the convulsive properties.