Noribogaine Generalization to the Ibogaine Stimulus: Correlation with Noribogaine Concentration in Rat Brain C
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Noribogaine Generalization to the Ibogaine Stimulus: Correlation with Noribogaine Concentration in Rat Brain C. Zubaran, M.D., M. Shoaib, Ph.D., I.P. Stolerman, Ph.D., J. Pablo, M.S., and D.C. Mash, Ph.D. The discriminative stimulus effects of ibogaine and administration of ibogaine and noribogaine. At the ED50 noribogaine in rats have been examined in relation to their doses for the discriminative effect, the estimated concentrations in blood plasma and brain regions and to concentrations of noribogaine in plasma, cerebral cortex, receptor systems through which they have been proposed to and striatum were similar regardless of whether ibogaine or act. Rats were trained to discriminate ibogaine (10 mg/kg noribogaine was administered. The findings suggest that IP), the NMDA antagonist dizocilpine (0.08 mg/kg IP) or the metabolite noribogaine may be devoid of NMDA the k-opioid agonist U50,488 (5 mg/kg IP) from vehicle in a antagonist and k-opioid agonist discriminative effects and standard two-lever operant conditioning procedure with a that it may play a major role in mediating the tandem VI-FR schedule of food reinforcement. Only rats discriminative stimulus effect of ibogaine. trained on ibogaine generalized to noribogaine, which was [Neuropsychopharmacology 21:119–126, 1999] approximately twice as potent as the parent compound. © 1999 American College of Neuropsychopharmacology. Noribogaine was detected in plasma and brain after Published by Elsevier Science Inc. KEY WORDS: Ibogaine; Noribogaine; Dizocilpine; k-opioids; claims have attracted the attention of scientists, and Drug discrimination; Rats there have been more than 60 scientific publications ad- dressing several issues related to ibogaine and drug de- Ibogaine is a naturally occurring psychoactive alkaloid pendence. derived from the roots of the African shrub Tabernanthe Animal studies and uncontrolled observations in hu- iboga. Extracts of T. iboga are used for religious purposes mans indicate that ibogaine can reduce withdrawal in African cults. In the period from 1985 onward, sev- symptoms and self-administration of some drugs (Cap- eral United States patents have claimed efficacy for pendijk and Dzoljic 1993; Sheppard 1994). Interestingly, ibogaine in the treatment of a wide variety of drug- in certain behavioral and neurochemical investigations, dependence syndromes (e.g., Lotsof 1985, 1992). These ibogaine has been characterized to have long-term ef- fects persisting for at least 19 hours after its administra- tion (Maisonneuve et al. 1991). In individuals given 5 From the Section of Behavioural Pharmacology (CZ, MS, IPS), Institute of Psychiatry, London, UK; Instituto de Biociências, (CZ), mg of ibogaine, this drug could not be detected after 4 Universidade Federal do Rio Grande do Sul Brazil, Rio Grande, Bra- hours using thin-layer chromatography (Ley et al. zil, SA; and the Department of Neurology (JP, DCM), University of 1996). Because the presence of ibogaine in plasma can- Miami Medical School, Miami, Florida, USA. Address correspondence to: Professor I.P. Stolerman, Section of not explain its protracted actions, the possibility that an Behavioural Pharmacology, Institute of Psychiatry, De Crespigny ibogaine metabolite persists for a longer period than the Park, London SE5 8AF, England. Tel: 144-171-919-3370; Fax: 144- parent compound has been considered. Using quantita- 171-740-5305. Received September 16, 1998; revised December 2, 1998; accepted tive gas chromatography/mass spectrometry, it was January 4, 1998. possible to detect the ibogaine metabolite 12-hydroxyi- NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 1 © 1999 American College of Neuropsychopharmacology Published by Elsevier Science Inc. 0893-133X/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0893-133X(99)00003-2 120 C. Zubaran et al. NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 1 bogamine (noribogaine) in plasma (Mash et al. 1995a); discrimination experiments. Ibogaine and noribogaine ap- while more than 90% of the ibogaine absorbed was pear to interact with several molecular targets (Deecher et eliminated 24 hours after its administration, the concen- al. 1992; Mash et al. 1995a, 1995b; Staley et al. 1996) but al- tration of the metabolite was still appreciable at this though their chemical structures are similar, the parent time. Ligand-binding studies suggest that noribogaine, compound and its metabolite display different pharmaco- like ibogaine itself, may interact with diverse types of logical and binding profiles. Noribogaine is several times receptor (Mash et al. 1995a; Staley et al. 1996). The pri- less potent than the parent compound as an inhibitor of mary metabolite noribogaine is produced through dem- [3H]dizocilpine binding (Mash et al. 1995b), but it is more ethylation of ibogaine (Mash et al. 1995a). Ibogaine is li- potent than ibogaine at both m- and k-opioid receptors pophilic and concentrated in fat, and might be and, unlike ibogaine, it is potent at the d-opioid receptor converted to noribogaine after slow release from fat tis- (Pearl et al. 1995). Additionally, noribogaine binds to the sue (Hough et al. 1996). Sequestration of ibogaine into serotonin transporter with 10-fold greater affinity than lipophilic compartments in the brain may result in low ibogaine and competitively inhibits [3H]dizocilpine and drug concentrations in the extracellular fluid and the [3H]TCP (1-[1-(2-thienyl)-cyclohexyl]piperidine) binding more polar nature of noribogaine suggests that it may to ion channels of NMDA receptors (Mash et al. 1995a; achieve relatively higher concentrations than the parent Popik et al. 1994; Sweetnam et al. 1995). compound (Staley et al. 1996). Evidence suggests that the effects of the metabolite Schechter and Gordon (1993) showed that rats can be noribogaine may be an important element in the stimu- trained to discriminate the interoceptive stimuli pro- lus properties of ibogaine. Rats trained to discriminate duced by ibogaine using drug discrimination proce- ibogaine partially generalized to noribogaine (Helsley dures similar to those employed for many other drugs. et al. 1997). In this study, we have attempted to deter- In such studies, ibogaine has been found to partially mine the role of the metabolite noribogaine in the dis- cross-generalize with several drugs acting directly or criminative stimulus effect of ibogaine. Thus we exam- indirectly as serotonergic agonists. Thus ibogaine gen- ined the relationship between tissue concentrations of eralized partially to fenfluramine, 1-(m-trifluorometh- ibogaine and noribogaine and the behavioral response ylphenyl)piperazine (TFMPP), lysergic acid diethylamide in rats of the same sex, strain, and age. We have also in- (LSD), 2,5-dimethoxy-4-methyl-amphetamine (DOM), vestigated whether rats trained to discriminate either and quipazine (Schechter and Gordon 1993; Helsley et the k-opioid agonist U50,488 or the uncompetitive al. 1998a). Conversely, in rats trained to discriminate NMDA antagonist dizocilpine will generalize to nori- fenfluramine, there was no generalization to ibogaine bogaine. (Schechter 1997) whereas in rats trained on LSD or DOM, partial generalization was seen (Palumbo and Winter 1992). In rats trained to discriminate ibogaine, partial gen- MATERIALS AND METHODS eralizations to drugs of other classes have been ob- Animals served. These drug classes include certain opioids and sigma agonists and N-methyl-D-aspartate (NMDA) an- Male hooded rats (Harlan Olac, Bicester) initially tagonists. Thus, several mixed agonist-antagonist opioids, weighing 210–280 g were housed individually in rooms such as (6)-pentazocine, nalorphine and diprenorphine, maintained at z218C, with a regular light-dark cycle (light and sigma agonists such as 1,3-di(2-tolyl)guanidine from 7:30 A.M. to 7:30 P.M.). The rats were fed restricted have elicited partial generalization in rats (Helsley et al. amounts of food to maintain their weights at z80% of 1998b). On the other hand, the selective k-opioid ago- normal, as determined from growth curves for rats with nists U50,488 and bremazocine and subtype-selective unrestricted access to food. The final weights of animals sigma agonists have not generalized (Helsley et al. in the experiment were 300–360 g. Water was available 1998b). Mice trained in a T-maze to discriminate the un- in the living cages at all times. competitive NMDA antagonist dizocilpine (but not the glycine antagonist R-(1)-3-amino-1-hydroxypyrrolid-2- Apparatus one) have also been reported to generalize to ibogaine (Popik et al. 1995; Witkin et al. 1995); however, the ef- Standard experimental chambers (Campden Instru- fect in dizocilpine-trained mice was only partial (67%) ments) were contained in sound-insulated, ventilated and it occurred with a very large (100 mg/kg) dose of enclosures. The chambers were fitted with two retract- ibogaine. It has also been found that uncompetitive able levers separated by a recess into which 45 mg pel- NMDA antagonists such as phencyclidine and dizo- lets of food could be delivered. White noise was present cilpine do not cross-generalize with ibogaine in either at all times to mask external sounds. The Arachnid soft- rats or monkeys (Helsley et al. 1998b; Jones et al. 1998). ware system running under RISC OS was used to con- The studies reviewed above indicate that ibogaine has trol the experiments and to record data (CeNes Ltd., complex and incompletely characterized effects in drug Cambridge, UK). NEUROPSYCHOPHARMACOLOGY 1999–VOL. 21, NO. 1 Noribogaine Generalization to the Ibogaine Stimulus 121 centage data were subjected to arc-sine transformation. Drug Discrimination The Dunnett t-test was used for multiple comparisons The procedure for establishing drug discrimination has with vehicle control data. The total number of re- been described in detail (Pratt et al. 1983; Stolerman et sponses was used as an index of response rate. All anal- al. 1984). Three groups of rats were used (n 5 5–6). One yses were performed using Unistat 4.5 software (Uni- group of rats was trained on ibogaine (10 mg/kg IP), stat, London, UK). another group was trained on dizocilpine (0.08 mg/kg IP), and a third group was trained on U50,488 (5 mg/kg SC).