Uncovering the Mechanisms of Action of Psychedelic Drugs

We know that psychedelic drugs modify the brain’s activity, leading to changes in mood, sensory perception, thought, and sense of self. How exactly they do that, however, is a question that still amazes scientists. One of them, Dr. Boris Heifets, assistant professor at Stanford School of Medicine, presented his recent research in a webinar hosted by Analytical Cannabis.

According to him, psychedelics drugs can be classified into three main groups: serotonergic classic hallucinogens (LSD and psilocybin, for example), dissociative anesthetics (ketamine and nitrous oxide), and entactogens (MDMA and mephedrone).

“Although psilocybin, ketamine and MDMA are different types of drugs, they all do share an important property: a rapid acting therapeutic effect for neuropsychiatric disorders,” Heifets said.

Ketamine, for instance, is already being used in patients with treatment-resistant depression; MDMA-assisted psychotherapy has proved to be an effective treatment for post-traumatic stress disorder (PTSD), while psilocybin can be used to reduce the symptoms of depression.

But how exactly do these drugs affect the brain?

Basic neuropharmacology of psilocybin, ketamine and MDMA

Classic hallucinogens, such as psilocybin, are agonists of the serotonin receptors and thus exert their pharmacological effects primarily through the serotonin system. MDMA is an agonist of serotonin receptors as well, however, it also inhibits serotonin, dopamine, and norepinephrine reuptake transporters (SERT, DAT, and NET, respectively), leading to an increase in the extracellular levels of these neurotransmitters.

Ketamine, on the other hand, is an antagonist of the NMDA glutamate receptor. The three drugs are thought to increase the release of glutamate using different pathways. High levels of glutamate, in turn, increase the release of the brain-derived neurotrophic factor (BDNF), promoting neuroplasticity, synaptogenesis, as well as spine and neurite growth.

Understanding the neuropharmacology of these compounds in important; as Heifets points out, “pharmacology alone is not enough to understand how these drugs work”. Indeed, the human brain is an incredibly complex system, and we still face major gaps in our knowledge regarding the mechanism of action of these substances.

Modelling psychedelics effects in mice

The profound subjective experiences induced by psychedelics seem uniquely human and raise doubts about the validity of animal models in psychedelic research. However, evidence shows that some neuropsychological effects elicited by psychedelics can be extrapolated from rodents to humans. Thus, although rodent behavioral models have several limitations, they can be used to make clear predictions that can be later tested in clinical experiments. This is exactly the approach Heifets and collaborators have been implementing.

“We have tremendous genetic access to mice and a lot of tools for manipulating activity of brain circuits”, he explained. “Our approach starts with modelling drug effects with mouse behavior and then using genetic tools to manipulate the activity of specific brain circuits. From there, we can identify synaptic and circuit mechanisms that underlie these drug effects. And finally, we can test that mechanism in humans.”

This strategy works as a cycle – “when we have mechanistic insight from human studies, we can go back and model those in mice,” Heifets explained. The cycle starts again. Thus, this type of integrated approach can not only help to refine our understanding on how clinical psychedelics-based therapy works but also improve the accuracy of preclinical models.

Modeling pro-social effects of MDMA

MDMA is a synthetic amphetamine derivative that has unique pro-social effects.

“In addition to producing feelings of euphoria – which is common in other stimulants – it’s fairly unique in that it seems to enhance trust, emotional openness, empathy and the bond between therapist and patient,” Heifets said of MDMA.

However, MDMA widespread adoption in psychotherapy can be limited by its potential for abuse and incompatibility with selective serotonin reuptake inhibitors. A deeper understanding of the pharmacology and neural dynamics underlying MDMA can help to support its use as psychotherapeutic. And Heifets’ lab is working toward this end.

“Wouldn’t it be great if we could somehow separate out the pro-social effects from the less desirable effects that might limit and MDMA broader use in people with PTSD?” Heifets said.

With this goal in mind, the team modeled MDMA’s pro-social and nonsocial rewarding effects in mice. They found that pro-social effects can be explained by MDMA acting at the serotonin transporter within the nucleus accumbens, while MDMA’s acute rewarding effects – most likely linked to its addictive potential – involve the dopaminergic system.

Although, these results are very interesting, Heifets knows that they are not enough to fully understand MDMA effects in the brain. That is why his lab has spent the last few years developing a new technique to map the neuronal activity in the whole brain.

“The beauty of this technique is that it is unbiased; it doesn’t assume any specific pharmacology or particular brain region. The only thing that we’re assuming is that neuronal activity is somehow related to the mechanism of action of the drug,” he explained.

Using this technique, the team mapped the neuronal activity evoked by MDMA and identified three areas that are selectively active when the drug is administered in a social context: the nucleus accumbens, the dorsal endopiriform nucleus, and the olfactory tubercle. Similarly, by mapping the brain activity when MDMA is given in a non-social setting, it is possible to identify which areas are involved in the psychostimulant effects of the drug.

This method resembles human neuroimaging studies. Yet animal models allow for further investigation on the causality of these results. As Heifers pinpointed, “in mice we have the tools to perturb the system and test the direct involvement of the drug on the resulting activity map.”

One of these tools is a genetic technique that allows for the capture and control (turning on/off) of the set of neurons that are active during a particular state. To test if these areas mediate MDMA’s pro-social effect, the team first captured the ensemble of cells that are activated by MDMA in a social context. Next, they turned off these cells in the areas of interest (nucleus accumbens, dorsal endopiriform nucleus or olfactory tubercle) and tested if MDMA still elicited a pro-social effect. Interestingly, “MDMA pro-social effects disappear when we turned off the nucleus accumbens or the dorsal endopiriform nucleus,” Heifets enthusiastically stated.

The next big step would be to test if stimulation of these particular areas induced pro-social effects in patients.

Ketamine and the opioid system

For some time, it was thought that ketamine’s fast antidepressant effect was mediated by inhibition of the glutamate NMDA receptor. However, other antagonists of this receptor lack antidepressant effects, suggesting that ketamine’s striking antidepressant action involves other pathways. Ketamine also inhibits opioids receptors – albeit, with significantly lower affinity – and a recent clinical study revealed that the opioid system is involved in its antidepressant effect. Following this result, Heifet’s lab tried to model this effect in mice.

“If we can understand how the system works in mice, that should give us some understanding of how ketamine interacts with opioid receptors in humans,” he explained. He and his team mapped the neuronal activity evoked by ketamine and compared it with the map obtained when ketamine was administered together with naltrexone (an opioid receptor antagonist).

“The difference between those two states (ketamine versus ketamine + naltrexone) should reveal a circuit linked to ketamine interaction with the opioid system,” Heifets remarked. The comparison of the two maps showed that the central amygdala – an area involved in fear and emotional processing – is involved in the ketamine-opioid interaction. Further, when Heifets’ team blocked opioid receptors specifically in that brain area, ketamine behavioral effects were abolished.

“This result leads us to believe that the central amygdala mediates the ketamine-opioid system interaction and is now something that we want to pursue in human trials,” Heifers concluded.

Setting-dependent effects of psilocybin

One interesting feature of psychedelics in general – and psilocybin in particular – is that the setting in which the drug is taken can influence its short- and long-term effects. Heifet’s team is also using mice to dissect the setting-dependent effects of psilocybin. For this experiment, mice were kept in two different settings, either their typical home cage or an enriched environment that included toys and exercise equipment. Mice in both settings were then injected with either psilocybin or saline. Finally, their brains were imaged and the neuronal activity maps under the two conditions were compared.

The study is still in progress, but preliminary results showed that psilocybin elicited both setting-independent and setting-dependent effects; some brain areas were activated/suppressed independently of the setting, while others were activated/suppressed depending on whether the mice were in a home cage or in an enriched environment.

“We’ve done something you cannot do with human imaging,” Heifets said. “We’ve counted the number of active cells in these hotspots.”

Overall, psilocybin setting-independent effects include an increase of active cells in the cortex and central nucleus of the amygdala as well as a decrease of active cells in the hypothalamus. On the other hand, psilocybin setting-dependent effects include an increase of active cells in the somatosensory area as well as a decrease of active cells in the piriform area and zona incerta.

“This is a proof of principle demonstration that we could potentially link setting-dependent brain physiology to behavioral outcomes, and that’s something we wish to model in in mice in the future,” Heifers concluded.

Conclusion

No doubt we still need to learn a lot about the multiple effects psychedelic drugs exert in our brains and how they can be used to treat mental health syndromes. Despite their limitations, animal models offer a broad range of advantages that can be leveraged to dissect the mechanism of action of these compounds and generate new hypothesis to test in humans. An integrative approach that links pharmacology, large scale neural dynamics and behavior in both preclinical and clinical studies is essential to tie up loose ends.

This article originally appeared in the Analytical Cannabis - April 2023 Digest.