Activation of Intracellular 5-HT2A Receptors Required for Psychedelic-Induced Neuroplasticity

A recently published landmark study titled “Psychedelics promote neuroplasticity through the activation of intracellular 5-HT2A receptors” has shed further light on the mechanisms through which psychedelics promote neuroplasticity.

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The 5-HT2A receptor is a type of serotonin receptor that is mainly found in the central nervous system (CNS). Activation of 5-HT2A receptors triggers a cascade of biochemical events that ultimately lead to changes in brain activity.

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In this paper, neuroplasticity refers to the growth of neuronal dendrites — specialized extension of neurons, or nerve cells, that receive and process information from other neurons or sensory receptors — in response to changes in neural activity. Dendrites are typically branched and have many small protrusions called dendritic spines, which serve as sites of communication with other neurons. They play a crucial role in the processing and integration of information in the nervous system.  

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Neuroplasticity can be induced by classic psychedelics — LSD, psilocybin, DMT, mescaline, and 5-MeO-DMT — as well as other psychedelic-like compounds such as ketamine. However, neuroplasticity induced by classic psychedelics and ketamine is achieved via different brain mechanisms.

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In the case of classic psychedelics, activation of 5-HT2A receptors produces psychedelic effects, which then leads to activation of downstream signaling pathways inside the neuron that are responsible for dendrite growth.

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The important study in question, conducted by Maxemiliano V. Vargas and colleagues working out of the Olson Lab at UC Davis, measured the neuroplasticity-promoting potential of a variety of compounds that activate 5-HT2A receptors, including serotonin and the psychedelic compounds DMT and 5-MeO-DMT. 

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Locating 5-HT2A Receptors Inside Neurons 

Some compounds that do not necessarily produce psychedelic effects can still promote neuroplasticity via the activation of 5-HT2A receptors. The authors of the study hypothesized that such non-psychedelic compounds do so by selectively activating 5-HT2A receptors that are situated in a particular location within the neuron. This phenomenon is known as location bias.

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Scientists typically think of receptors as being located on the surface of the cell membrane. However, receptors can also be located on internal membranes within the cell, such as the “Golgi body,” also known as the “Golgi apparatus” or “Golgi complex,” —  an “organelle” or small structure in a cell that is involved in the processing, sorting, and packaging of proteins and lipids.

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Using fluorescent tags to image 5-HT2A receptors, the researchers discovered a significant number of these receptors deep within the neurons, particularly in the Golgi body. This does not mean that the receptors are exclusively found inside the cells, but a significant portion of them was observed to be located there.

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For compounds to activate these intracellular receptors, however, they must first penetrate the neuron.

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The primary finding of this landmark paper is that the neuroplasticity effect is solely attributed to the activation of intracellular 5-HT2A receptors. Therefore, the ability of a 5-HT2A agonist compound to induce neuroplasticity relies on its capability to access these intracellular receptors.

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Testing for Neuroplasticity 

To determine the neuroplasticity-inducing effects of different compounds, the researchers administered several compounds based on serotonin and the psychedelics DMT and 5-MeO-DMT to isolated neurons in culture (in vitro) and to neurons in living mice (in vivo). They then assessed the effects that these compounds had on dendritic complexity — the number of dendrites, their length, their branching pattern, and the number and shape of dendritic spines, which are small protrusions on dendrites where synapses with other neurons are formed.

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Notably, DMT and 5-MeO-DMT are more lipophilic (fat soluble) than serotonin. This enabled the researchers to test the relationship between the lipophilicity of a compound and its ability to promote neuroplasticity. 

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Results revealed that the ability of a compound to promote neuroplasticity was dependent on how fat soluble it is. This is a particularly noteworthy finding, and here’s why; For a compound to enter a neuron, it first needs to diffuse freely through its fatty plasma membrane (the phospholipid bilayer), and only compounds that are lipophilic can accomplish this task efficiently. Non-lipophilic (polar) compounds, such as serotonin, are unable to do so effectively.

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This suggests that psychedelic compounds may be inducing neuroplasticity by activating intracellular 5-HT2A receptors as opposed to 5-HT2A receptors located on the cell membrane. However, this finding alone does not definitively prove that this is actually the case. So, further experimentation was in order… 

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Lipophilicity Required for Dendritic Growth 

To further test whether neuroplasticity is induced by activating intracellular 5-HT2A receptors, the researchers created a series of non-lipophilic (not fat soluble), highly polar, water soluble compounds unable to pass through the cell membrane. These highly polar compounds should fail to induce dendritic growth.

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The researchers observed that these compounds were still able to bind to and activate the 5-HT2A receptors on the surface of the cell, but failed to induce neuroplasticity. This finding strongly suggests that the ability of compounds to promote neuroplasticity is dependent on their ability to access intracellular 5-HT2A receptors.

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However, the researchers sought to confirm this hypothesis and rule out any possibility that the molecular changes that took place in the creation of these compounds was responsible for the lack of neuroplasticity.

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To do so, they used electroporation, a microbiology technique in which an electrical field is applied that temporarily increases the permeability of the cell membrane, allowing polar compounds to be introduced to the neuron. As expected, when applied to neurons subjected to electroporation, these polar compounds were able to reach intracellular 5-HT2A receptors and induce neuroplasticity, confirming the importance of intracellular 5-HT2A receptors in neuroplasticity induction.

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Activation of Cell Surface Receptors Not Necessary for Neuroplasticity

As we have already noted, 5-HT2A receptors are present not only on the inside of the neuron but also on the neuron’s outer plasma membrane. Hence, to definitively establish that the induction of neuroplasticity does not require the activation of both internal and surface 5-HT2A receptors, further experimentation was required.

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The researchers used ketanserin, a selective 5-HT2A antagonist that can easily pass through the plasma membrane and block both surface and internal 5-HT2A receptors. When DMT, a psychedelic compound that typically induces neuroplasticity, was applied in the presence of ketanserin, no neuroplasticity was observed.

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To determine whether DMT required only the intracellular 5-HT2A receptors to induce neuroplasticity, the researchers modified the ketanserin molecule to create a charged methylated ketanserin analogue that was unable to pass through the plasma membrane and, as a consequence, could only block surface receptors. In the presence of this analogue, DMT was still able to induce neuroplasticity, demonstrating that only intracellular 5-HT2A receptors are necessary for dendritic growth.

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When the researchers then subjected the neurons to electroporation to allow the charged ketanserin analogue to enter the cell where it could block intracellular 5-HT2A receptors, DMT was prevented from inducing neuroplasticity. This finding seems to have firmly established that activation of intracellular 5-HT2A receptors is responsible for the stimulation of neuroplasticity observed with psychedelics. 

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Intracellular Serotonin Induces Neuroplasticity

The findings outlined above raised important questions regarding the inability of serotonin to induce neuroplasticity.

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Up until this point, scientists thought that this had something to do with the way in which serotonin activated 5-HT2A receptors. However, the experiments we have discussed above suggest that the inability of serotonin to induce neuroplasticity may be due to its lack of lipophilicity and consequent inability to reach intracellular 5-HT2A receptors, rather than how it interacts with and activates the 5-HT2A receptor.

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To test this hypothesis, the researchers expressed a serotonin transporter protein (SERT) on the surface of neurons that can import serotonin from the outside of the neuron to the inside. When they did this, the authors found that if a water-soluble compound like serotonin is artificially introduced into the neurons, it too can induce neuroplasticity. 

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These experiments with serotonin were carried out on isolated neurons in vitro. As such, the researchers also sought to verify whether extracellular serotonin in a living brain could also induce neuroplasticity if it could enter the neurons.

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To test this, they employed specialized techniques to introduce the SERT transporter into the neurons of the prefrontal cortex of mice, and then raised extracellular serotonin levels in the brains of the mice using the serotonin-releasing drug, para-chloroamphetamine (PCA). The authors found that the SERT did transport this extracellular serotonin into the neurons which indeed did lead to dendritic growth.

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Moreover, the mice displayed an improvement in the “forced swim test,” a behavioral test that is commonly used in preclinical research to measure the antidepressant-like effects of drugs.

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Psychedelic-induced Neuroplasticity in a Nutshell

In summary, the authors have multiple lines of evidence indicating that psychedelics induce neuroplasticity by activating 5-HT2A receptors that are located inside neurons. They also showed that any compound that activates intracellular 5-HT2A receptors can induce neuroplasticity, regardless of whether it is psychedelic. 

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The study also indicates that the effects of psychedelics on neuroplasticity are related to their observed antidepressant effects. The outcome of the forced swim test in this study indicates that dendritic growth could be linked to an improvement in depression symptoms.

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This study is a significant step in understanding the cellular and molecular mechanisms of psychedelics in the brain, and it provides exciting opportunities for exploring novel treatments for psychiatric illnesses.

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If you are interested in getting deeper into the weeds of this fascinating paper, you can find a more in depth distillation at computational neurobiologist, pharmacologist, and chemist Andrew R. Gallimore’s wonderfully insightful substack. 

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