In Vivo Estimation of Imidazoline2 Binding Site Turnover

Ayahuasca: Psychological and Physiologic Effects, Pharmacology and Potential Uses in Addiction and Mental Illness

Background:

Ayahuasca, a traditional Amazonian decoction with psychoactive properties, is made from bark of the Banisteriopsis caapi vine (containing beta-carboline alkaloids) and leaves of the Psychotria viridis bush (supplying the hallucinogen N,N-dimethyltryptamine, DMT). Originally used by indigenous shamans for the purposes of spirit communi-cation, magical experiences, healing, and religious rituals across several South American countries, ayahuasca has been in-corporated into folk medicine and spiritual healing, and several Brazilian churches use it routinely to foster a spiritual experi-ence. More recently, it is being used in Europe and North America, not only for religious or healing reasons, but also for rec-reation.

Objective:

To review ayahuasca’s behavioral effects, possible adverse effects, proposed mechanisms of action and potential clinical uses in mental illness.

Method:

We searched Medline, in English, using the terms ayahuasca, dimethyltryptamine, Banisteriopsis caapi, and Psy-chotria viridis and reviewed the relevant publications.

Results:

The following aspects of ayahuasca are summarized: Political and legal factors; acute and chronic psychological ef-fects; electrophysiological studies and imaging; physiological effects; safety and adverse effects; pharmacology; potential psychiatric uses.

Conclusion:

Many years of shamanic wisdom have indicated potential therapeutic uses for ayahuasca, and several present day studies suggest that it may be useful for treating various psychiatric disorders and addictions. The side effect profile ap-pears to be relatively mild, but more detailed studies need to be done. Several prominent researchers believe that government regulations with regard to ayahuasca should be relaxed so that it could be provided more readily to recognized, credible re-searchers to conduct comprehensive clinical trials.

Autoradiographical distribution of imidazoline binding sites in monoamine oxidase A deficient mice

This study has used receptor autoradiography to characterize imidazoline binding sites (I-BS) in monoamine oxidase (MAO) A knockout and wild-type mice. A comparison between MAO-A and MAO-B, binding of the endogenous beta-carboline [(3)H]harmane, and I-BS, has been made using sections from brain and kidney. The loss of binding to MAO-A in the knockout animals was confirmed using the selective radioligand [(3)H]Ro41-1049, with labelling reduced to background levels. The binding of [(3)H]Ro19-6327 to MAO-B was unaffected, indicating no change in this isoform in response to the loss of MAO-A. A reduction in binding to the I(2)-BS, as labelled by both [(3)H]idazoxan and [(3)H]2-BFI (2-(2-benzofuranyl)-2-imidazoline), was seen in the MAO-A knockout animals in both brain and kidney sections, whereas binding to the I(1)-BS in kidney sections remained unchanged. The loss of I(2) binding was found to be regionally dependent and was positively correlated with the relative expression of MAO-A in specific regions in the wild-type animals. Using the MAO-A knockout mice it was also possible to demonstrate a non-MAO-A population of binding sites labelled by the putative I-BS endogenous ligand, harmane.

In vitro and ex vivo distribution of [3H]harmane, an endogenous beta-carboline, in rat brain

The endogenous beta-carboline, harmane, has been shown to bind to monoamine oxidase A (MAO-A) and a separate, high affinity, non-MAO site. Research in our laboratory has shown that harmane is an active component of clonidine-displacing substance (CDS), the proposed endogenous ligand for imidazoline binding sites (IBS). In the present study we have investigated the distribution of [3H]harmane in rat brain, and related the binding profile to the distribution of the MAO-A selective ligand [3H]Ro41-1049 and the I2BS ligand [3H]2-BFI. The in vivo distribution of [3H]harmane following intravenous administration was also investigated. Receptor autoradiography revealed a highly significant correlation for the distribution of [3H]harmane and [3H]Ro41-1049, and a significant correlation for [3H]harmane and the I2BS ligand [3H]2-BFI. The in vivo distribution of [3H]harmane suggests that the ligand accumulates in the adrenal gland and throughout the brain with the primary route of excretion occurring via the duodenum. In conclusion, these studies have shown that [3H]harmane labels a population of binding sites that reflect the distribution of MAO-A. Further evidence for a non-MAO, IBS [3H]harmane population has not been shown but the high level of expression of the MAO-A site is likely to have masked the much smaller population of I2BS.

Characterisation of imidazoline I2 binding sites in pig brain

The imidazoline I2 binding sites in the central nervous system have previously been described in several different species including rat, mouse, rabbit and frog. The present study has investigated the imidazoline I2 binding site, and its relationship to the monoamine oxidase isoforms, in pig whole brain and compared the results obtained with data from other species. Results from saturation binding studies revealed that the imidazoline I2-selective ligand, [3H]2BFI (2-(2-benzofuranyl)-2-imidazoline) labelled a single saturable population of sites with a KD=6.6 nM and Bmax=771.7 fmol/mg protein. The pharmacological characterisation of the sites was similar to that previously reported with a rank order of potency for the imidazoline I2 ligands of 2BFI>BU224>Idazoxan>BU226. Displacement by the imidazoline I1 ligands was low affinity and the monoamine oxidase inhibitors displaced with micromolar affinity. The majority of compounds displaced the binding in a monophasic manner, however, displacement by the putative endogenous ligand, harmane was biphasic. The relative populations of the two monoamine oxidase isoforms revealed a 10 fold greater expression of monoamine oxidase B relative to monoamine oxidase A. These data confirm the presence of imidazoline I2 binding sites in pig brain and show that their pharmacology is characteristic of that seen in other species. The proportion of monoamine oxidase A and B expressed in the pig brain is similar to that seen in the human brain therefore, given the association between imidazoline I2 binding sites and monoamine oxidase, the pig may provide a more useful model for human imidazoline I2 binding sites than other species such as the rat.