Deciphering Ibogaine’s Matrix Pharmacology: Multiple Transporter Modulation at Serotonin Synapses

Ibogaine and its main metabolite noribogaine inhibit the vesicular monoamine transporter 2 (VMAT2) with submicromolar potency, as shown in cell-based assays and two-photon microscopy of mouse brain synaptic vesicle clusters. Noribogaine also induces partial serotonin release from synaptic vesicles and binds VMAT2 at a distinct site from the established inhibitor dihydrotetrabenazine. These compounds additionally inhibit plasma membrane monoamine transporters, prominently the serotonin transporter (SERT), and a novel target, organic cation transporter 2 (OCT2). Several iboga analogs display dual inhibition of VMAT2 and SERT with comparable potencies, termed "Synaptic Reuptake Inhibitors" (SynRIs). This profile explains why ibogaine and noribogaine do not induce catalepsy, unlike other VMAT2 inhibitors, and illustrates the complex "matrix pharmacology" of iboga compounds.

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Abstract

) that has a unique therapeutic potential across multiple indications, including opioid dependence, substance use disorders, depression, anxiety, posttraumatic stress disorder (PTSD), and traumatic brain injury (TBI). We systematically examined the effects of ibogaine, its main metabolite noribogaine, and a series of iboga analogs at monoamine neurotransmitter transporters, some of which have been linked to the therapeutic effects of these substances. We report that ibogaine and noribogaine inhibit the transport function of the vesicular monoamine transporter 2 (VMAT2) with submicromolar potency in cell-based fluorometry assays and at individual synaptic vesicle clusters in mouse brain as demonstrated via two-photon microscopy. Examining the uptake and release of radiolabeled serotonin in isolated brain synaptic vesicles, we confirmed that ibogaine and noribogaine act as inhibitors of VMAT2 and showed that noribogaine induces partial serotonin release from synaptic vesicles. Noribogaine does not compete with the binding of dihydrotetrabenazine, an established VMAT2 inhibitor, indicating that noribogaine engages the VMAT2 transporter at a distinct binding site or conformational state. The iboga compounds also inhibit the plasma membrane monoamine transporters (MATs), prominently including the serotonin transporter (SERT), and a novel iboga target, the organic cation transporter 2 (OCT2). SERT transport inhibition was demonstrated in serotonin axons and somata in mouse brain slices and in rat brain synaptosomes, where ibogaine and its analogs did not act as substrate-type serotonin releasers. Noribogaine, oxa-noribogaine, and several analogs displayed dual inhibition of VMAT2 and SERT with comparable potencies, a relatively uncommon activity in the pharmacopoeia of monoamine transporter inhibitors, and were hence labeled as "Synaptic Reuptake Inhibitors" ("SynRIs"). The SynRI profile provides an explanatory model for previously reported neurochemical effects of ibogaine in rodents. Together, the updated profile of the monoamine transporter modulation offers insight into the grand complexity of the iboga pharmacology, which we termed "matrix pharmacology". The matrix pharmacology hypothesis is outlined and used to conceptualize why ibogaine and noribogaine do not induce catalepsy, as demonstrated in our study, in contrast to other VMAT2 inhibitors.