Endogenous hallucinogens as ligands of the trace amine receptors: A possible role in sensory perception

No explicit Methods section or systematic search strategy is reported in the extracted text; the paper is a theoretical synthesis and hypothesis article drawing on published pharmacology, genetics and behavioural studies. Wallach assembles evidence from a range of sources including in vitro receptor pharmacology (binding affinities and functional assays), animal behavioural models (for example the mouse head-twitch response), human antagonist studies with psychedelic agents, and genetic association studies implicating TAAR family members in psychiatric symptoms. The author compares receptor binding and functional data for several compounds (DMT, 5-MeO-DMT, DET, MDMA, DOI, psilocin, mescaline, AMT) across 5-HT subtypes and various TAARs, and discusses expression patterns of TAAR genes in brain regions relevant to sensory processing. Examples of quantitative data cited include IC50 or affinity values and relative cAMP responses in TAAR1 assays; however, the paper does not describe a formal meta-analytic or statistical method, nor does it present new experimental protocols or participant sampling. Where relevant, Wallach evaluates behavioural and clinical findings—mouse htr as a proxy for hallucinogenic potential and human antagonist studies using ketanserin and risperidone measured by the APZ scale—but notes that many of the cited human pharmacological antagonist studies predate the discovery of TAARs. The narrative therefore seeks to integrate disparate lines of evidence to motivate targeted experimental tests (for example receptor binding panels that include TAARs, use of hallucinogen rating scales, and trials with selective antagonists), rather than to provide pooled estimates from primary datasets.

The paper compiles several lines of evidence that the author interprets as supporting a TAAR–endogenous hallucinogen signalling hypothesis. Key empirical points reported in the text include: - TAAR biology and distribution: Nine TAAR genes have been observed in humans, and TAAR subtypes are expressed across CNS regions involved in sensory processing (prefrontal cortex, hippocampus, substantia nigra, amygdala, hippocampus and basal ganglia). A specific mutation in the TAAR6 gene (previously referred to as TRAR4) has been associated with delusional and hallucinatory symptoms in genetic studies, suggesting TAAR6 expression in sensory-processing areas. - Receptor pharmacology and functional assays: Several hallucinogens act as partial agonists at serotonin receptors (5-HT1A, 5-HT2A, 5-HT2C) but also show activity at TAAR subtypes. The paper cites affinity values indicating DMT has lower affinity for 5-HT2A (75 ± 16 nM) than 5-MeO-DMT (14 ± 1 nM), while 5-MeO-DMT shows greater affinity at 5-HT1A (6.5 ± 1.5 nM) compared with DMT (170 ± 35 nM). S(+)-alpha-methyltryptamine (AMT) is reported with a 5-HT2A affinity of 46 ± 6 nM and much weaker 5-HT1A activity (1900 ± 375 nM). Relative to DMT, 5-MeO-DMT produces a smaller cAMP response at TAAR1, interpreted as lower potency at that receptor. - Behavioural and clinical correlates: The head-twitch response (htr) in mice is described as mediated by 5-HT2A activity and often used to predict hallucinogenic potential. Human antagonist studies with psilocybin plus 5-HT2A antagonists (ketanserin, risperidone) reduced scores on the Swiss altered states of consciousness (APZ) scale, including visionary restructuring effects, but these trials were conducted before TAARs were known. The author notes that some compounds with strong 5-HT2A affinity (for example 5-MeO-DMT) lack prominent visual effects, while DMT is strongly visual despite lower 5-HT2A affinity, suggesting additional receptor mechanisms (candidate TAARs) mediate visual phenomena. MDMA is noted as a TAAR1 agonist in mice yet lacks significant visual restructuring (VR) activity in humans, indicating TAAR1 may not be the TAAR subtype responsible for visual effects. - Conceptual comparisons: Wallach uses examples such as endogenous morphine and codeine concentrations in CNS tissue (reported as 2–339 fmol/ml) to illustrate that low plasma levels do not preclude potent synaptic effects; by analogy, plasma measures of endogenous DMT may not reflect synaptic concentrations relevant to TAAR-mediated neurotransmission. The author argues that low doses of administered DMT may produce subjective effects primarily via 5-HT receptors because of relative receptor abundance, and only higher concentrations engage TAAR-mediated responses analogous to synaptic release of an endogenous transmitter.

Wallach interprets the assembled evidence to argue that endogenous hallucinogens are more plausibly neurotransmitters than hormones, acting in a compartmentalised and regulated manner at TAARs in brain regions that underlie sensory perception. In this model, ordinary waking perception is conceptualised as a tightly regulated psychedelic-like process; altered states of consciousness (dreaming, psychosis, near-death experiences) arise when release or regulation of these transmitters becomes decoupled from external sensory input. The author positions this TAAR-centric hypothesis relative to the serotonin literature by acknowledging that 5-HT2A activation accounts for many hallucinogen effects but is insufficient to explain qualitative differences between compounds—for example, why some 5-HT2A agonists produce strong visual phenomena while others produce mainly emotional effects. Wallach therefore suggests that visual phenomena may be mediated by one or more TAAR subtypes rather than 5-HT2A alone. He further proposes TAAR6 as a likely candidate for an endogenous hallucinogen receptor because of its CNS distribution and genetic associations with positive symptoms of schizophrenia. Limitations and uncertainties are acknowledged: the paper is hypothetical and synthesises indirect evidence rather than presenting direct demonstrations of endogenous hallucinogen–TAAR signalling. Legal restrictions on these compounds have limited experimental work, and many cited human pharmacology studies predate the discovery of TAARs so their antagonist profiles at TAARs are unknown. Wallach calls for targeted empirical work to resolve these questions: comprehensive receptor binding assays that include TAARs, evaluation of antagonist affinities at TAAR sites, application of standardised hallucinogen rating scales (APZ, HRS) to characterise subjective effects across compounds, and experimental manipulations such as pretreatment with antagonists (for example pindolol) to test receptor interactions. Finally, the discussion proposes a translational implication: TAAR antagonists might represent novel pharmacological approaches for psychotic disorders if TAAR signalling by endogenous hallucinogens contributes to positive psychotic symptoms. The author emphasises that not all TAARs will be involved in sensory experience and that different endogenous hallucinogens may target different TAAR subtypes, so further receptor-specific work is required.

Wallach concludes that the endogenous hallucinogens most likely act as true neurotransmitters within the CNS rather than as systemic hormones, with compartmentalised release and rapid clearance underpinning their physiological role in sensory perception. The theory predicts that when this regulatory system fails, altered states and psychotic phenomena may result. TAAR6 is highlighted as a plausible receptor candidate for one or more endogenous hallucinogens because of its limbic and frontal cortical distribution and genetic links to positive symptoms of schizophrenia. The paper ends by calling for more research into these compounds and their receptors, arguing that such work could yield novel treatments for psychological disorders and advance understanding of the neurochemistry of sensory experience.