the doors of reception: functionally selective receptor mosaics and the plasticity-inducing...

7/30/2019 The Doors of Reception: Functionally selective receptor mosaics and the plasticity-inducing psychedelics that bind

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The Doors of Reception:

Functionally selective receptor mosaics

and the plasticity-inducing psychedelics that bind them.

Evan Martin

[email protected]

Spring 2011

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ABSTRACT

The past decade has seen many exciting new developments in neurobiology. Three particularly

paradigm-rattling revelations reviewed in this report include; first, the elucidations of 'functional

receptivity,' which expand multi-fold the elegant complexity of receptor function by showing that, not

one, but rather, myriad unique cascades of intracellular signals leading to discrete profiles of gene

activation can result from multiple receptor conformations, as opposed to merely an active or inactive

state. Second: epigenetic and state-dependent neuroplasticity which suggests monumental therapeutic

potential and challenges the status quos of biological reductionism and pharmaceutical industry. And

last: receptor heteromerization wherein metabotropic receptors belonging to separate families form

complexes engaging in functional, allosteric co-modulation and neurotransmitter signal integration,

humbling the current level of neurological comprehension while presenting the potential for vastly

improved pharmacological interventions. The author makes use of psychedelics as a vector connecting

these exciting areas noting how they have played, and will continue to play, an indispensable role in

their exploration.

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INTRODUCTION

Psychedelics*

are commonly described as 'mind expanding' drugs. Perhaps it wouldn't be untrue

either to identify them as science expanding drugs: work with them in the 1950's and 60's helped reveal

the significance of the serotonin system, develop novel concepts in cognitive neuroscience, illuminate

new realms of psychology, and facilitate the development of many pivotal psychopharmacological

medicines. Now, after three decades of scientists being politically barred access to these remarkable

substances, the psychedelics have re-entered the labs and are again helping us discover and delineate

new and extraordinarily profound insights into the functions and dysfunctions of the nervous system.

Just as psychedelics might allow reality appear to users of them as though viewed through a fractal

lens, wherein a universe can be seen in a grain of sand, so has the depth and detail of the dynamic

complexity of neural architecture expanded by several orders of magnitude over the past decade

through the application of psychedelics as biochemical research probes. When fifty years ago

psychedelics were helping us identify discrete neurotransmitters and correlate them with aspects of

cognition and the major psychiatric disorders, we are now, with the help of psychedelics, learning that

single receptors can initiate myriad discreet intracellular signal cascades directed by conformational

adjustments to various ligands, each inducing a unique set of protein activations and gene translation

outcomes relating to neural metabolism and synaptic organization. Psychedelics are also helping

elucidate the fascinating revelation that metabotropic G protein-coupled receptors (GPCRs) can form

mosaic-like complexes with other categorically distinct GPCRs, essentially forming super-receptors

that are capable of receiving and integrating input from 2, 3 and possibly more distinct ligands,

allosterically modulating each other and further complexifying the conformation directed intracellular

*It is common to refer these materials in the scientific literature as hallucinogens, psychotomimetics or simply 'drugs of abuse' and while all these terms are

valid in various circumstances, the designation psychedelic is the most socially accepted and descriptively accurate term in most contexts. However, I

often find hallucinogen to be a more sonorous, easier word, so I will use it in most circumstances, but have taken the liberty to alternate terms dependingon context.

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signaling pathways. These discoveries allow for the development of medicinal therapies far more

specific and with far less side effects than previously possible.

In this review I will briefly describe some of these ground-shattering breakthroughs,

spearheaded by work with psychedelic probes and models, and discuss their far-reaching implications.

FUNCTIONAL SELECTIVITY

G protein-coupled receptors (GPCRs) are characterized by their seven transmembrane -helices

and are a highly conserved family of membrane receptors, found in nearly everything from plants to

protozoa (Gonzalez-Maeso & Sealfon 2009a). They are among the largest family of vertebrate

receptors comprising 1 to 3% of the mammalian genome and 1 to 5% of the total cell proteins

(Gonzalez-Maeso & Sealfon 2009a). The GPCRs are involved in countless different physiological

responses from vision to olfaction. Their malfunctions can lead to pathologies ranging from diabetes

and asthma to immunological and neurological disorders and their importance in medicine is attested to

by the estimates that 30 to 40% of currently available pharmaceutical drugs target the GPCR's (Albizu

2010).

Traditionally, metabotropic events initiated by a specific type of GPCR were thought to be the

same for all varieties of agonists that bound to it, merely varying in different degrees of efficiency.

Over the past two decades, however, it became increasingly clear that different agonists bearing similar

binding affinities for the same sites, could trigger distinctly separate signaling cascades within the cell,

based upon subtle differences in the conformation taken on by the receptor in response to the different

ligands. This phenomenon has been referred to as agonist-trafficking of receptor signaling, biased

agonism, conformational selection, functional selectivity, among other terms (Simmons 2005).

One of the most illustrative cases of functional selectivity has been the example of the 5-HT2A

receptor agonist hallucinogens such as lysergic acid diethylamide (LSD), mescaline and psilocin.

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When these compounds bind to 5-HT2A receptors (Vollenweider 1998) they elicit profound changes in

cognition, perception, mood and behavior, however, when molecules with similar pharmacological

activity, such as lisuride, ergotamine or serotonin itself, bind to the same site, no such changes in

behavior or consciousness arise (Gonzalez-Maeso & Sealfon 2009a). This intimated that

hallucinogenic 5-HT2A receptor agonists induce intracellular responses different from those of the non-

hallucinogenic agonists. The behaviorally distinct effects of the hallucinogenic and non-hallucinogenic

agonists also serves a unique utility in experimental systems exploring this phenomenon of diverging

cellular events in vivo.

In response to extracellular cues, signal transduction cascades eventually regulate gene

expression, so in 2003, Gonzalez-Maeso et al. assayed mouse somatosensory cortex cells for

differential levels of mRNA transcripts in response to receptor activation by various hallucinogenic and

non-hallucinogenic 5-HT2A receptor agonists in vitro and in vivo. Although gene expression assays had

been conducted before on GPCRs to monitor response, this was the first reported time it had been done

to differentiate various agonist-driven responses of the same receptor (Gonzalez-Maeso et al. 2003).

They were able to identify 23 transcripts specifically dependent on 5-HT2A receptor activation, and

each agonist tested showed its own unique and reproducible profile of varying levels of these

transcribed genes; its own transcriptome fingerprint. Even more notably, they found that there were

certain gene expressions that were induced by the hallucinogenic agonists but not by the non-

hallucinogenic ones (Gonzalez-Maeso et al. 2003). These particular genes mainly belong to the family

of early growth response (EGR) elements which are primarily known to be involved with neural

growth and plasticity (Leah & Wilce 2002). Two gene transcriptions, egr-1 and egr-2, were induced by

all six hallucinogens tested (DOI, DOM, DOB, mescaline, LSD and psilocin, representing 4

structurally diverse classes of 5-HT2A receptor agonists) with no induction at all by the various non-

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hallucinogen 5-HT2A receptor agonists (Gonzalez-Maeso et al. 2007). The hallucinogen-specific

transcript for the protein Egr-2 showed particularly robust expression.

Further investigations by Gonzalez-Maeso et at. elucidated details concerning the signaling

cascades that lead to the gene transcriptions. They found that the psychotropic effects of the

hallucinogens depended on the same Gq/11 protein activated pathway that the non-hallucinogenic 5-HT2A

receptor agonists did, however, the hallucinogens also co-activated a G i/0protein-initiated cascade

simultaneously, which was responsible for the unique hallucinogen-specific pattern of gene induction

(Gonzalez-Maeso et al. 2007). This data contributed mightily to the interpretation that GPCRs can

adopt multiple, functional conformations directed by the binding of different ligands and commencing

distinct cellular responses.

The fact that receptors can exhibit such a variety of precise and functional pathways in response

to varying agonists, producing a specific set of gene transcriptions for every different agonist tested, is

interesting to contemplate. Might this imply that single receptor types may have evolved to bind to

numerous endogenous or exogenous ligands? Some, perhaps, existing in trace amounts, sequestered

into sparse vesicles or unassuming astrocytes, hidden from our blunt instruments or only synthesized in

rare circumstances? We rightly assumed that the tightest binding endogenous ligand was the receptor's

main key to it's lock, but perhaps we should take a closer look at the cellular events and gene

transcriptions that take place when less obvious endogenous molecules bind a receptor.

PLASTICITY

It is unlikely the expression of the psychedelic-specific genes are directly responsible for the

immediate physiological and behavioral effects of the psychedelic drugs, which can begin before the

genes are even induced (Gonzalez-Maeso et al. 2007). The mechanisms underlying those more salient

aspects of psychedelic experience remain a mystery. However, the gene expressions are directly

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involved in neuroplastic effects that might correlate with the extremely long term efficacy of

psychedelic experiences in therapeutic contexts for relieving psychiatric symptoms such as depression,

anxiety, OCD, PTSD, chronic pain, addiction and other maladaptive behaviors (Multidisciplinary

Association for Psychedelic Studies 2011) and likewise for the lingering emotional, behavioral and

perceptual side effects that can be elicited by taking hallucinogens in a threatening or non-conducive

setting. Hallucinogens have been shown to stimulate long-term potentiation (LTP) and long-term

depression (LTD) of various receptors (Vollenweider 2010) and induce transient increases in dendritic

spine size of cortical neurons (Jones et al. 2009) as well as rapid synapse formation (Li et al. 2010). 5-

HT2A agonists, particularly the Hallucinogens have also been shown to preferentially activate proteins

involved in the regulation of microtubule structure and function such as kalirin-7, PAK (Jones et al.

2009) and -arrestin-2 (Schmid et al. 2008) and others (Woerkom 1990). Indeed, tryptamines such as

serotonin and melatonin interact robustly with cytoskeletal elements such as actin, tubulin and

micotubule-associated proteins (MAPs) and play apical roles in differentiation and cell morphology;

measures of which are correlated with neuropathologies such as schizophrenia and depression

(Woerkom 1990; Azmitia 2001; Massimiliano, et al. 2003; Bianchi 2005; Bellon et al. 2007; Gardiner

2011).

Synaptic plasticity and neural mitosis are burgeoning areas of research and medicine. Neural

degeneration and matter-loss in specific brain areas have been correlated with many psychiatric

diseases. The monoamine deficiency hypothesis has been the basis of the pharmacological treatment of

depression for over five decades, yet the reuptake inhibitor class of antidepressants are not effective in

all patients, and when clinical efficacy does occur, it requires 2 to 5 weeks to begin, even though

monoamine concentrations in the synaptic cleft increase within hours of administration (Vidal et al.

2011). Drugs that increase monoamine levels through other mechanisms have shown to be ineffective

in the treatment of depression (Vidal et al. 2011), further suggesting that a simple deficiency in the

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monoaminergic neurotransmitters, while being a symptom, is likely not the primary cause of depressive

disorders.

Untreated depressed subjects show decreased hippocampal volume (Martinowich et al. 2007)

and it has been established that the antidepressants enhance neural proliferation, particularly in the

hippocampus (Martinowich et al. 2007; Vidal et al. 2011). The time required for the differentiation and

maturation of new nerve cells correlates with the lag time seen in clinical response to antidepressants

(Vidal et al. 2011). Brain-derived neurotrophic factor (BDNF) exerts a potent trophic effect on

serotonin and norepinephrine neurons and displays increased expression by the chronic use of

antidepressant medications and decreased BDNF levels are found in subjects with untreated major

depression (Baudry et al. 2005; Vidal 2011). The increased levels of BDNF are also correlated with

the onset of the therapeutic effects of antidepressants (Baudry et al. 2005) and direct infusions of

BDNF into the midbrain has shown to have antidepressant effects in animal models (Martinowich et al.

2007). Data such as this and that which is to follow in this report has led to what has been called the

neuroplastic or neurotrophin hypothesis of depression (Baudry et al. 2005; Martinowich et al. 2007).

Different antidepressants enhanceBdnfexpression using different combinations of promoters,

of whichBdnfhas at least seven (Martinowich et al. 2007), which indicates possibilities for more

guided and specific pharmacological regulations of the gene. Also leading to increased complexity and

confusion while simultaneously presenting better therapeutic possibilities, BDNF is known to exist in

two forms: a precursor, proBDNF, and its cleaved form, mature BDNF (mBDNF). mBDNF is what is

responsible for long-term potentiation (LTP) while proBDNF actually facilitates long-term depression

(LTD) (Martinowich et al. 2007). Each form has it's own receptors and while the balance of the two

seems to help regulate synaptic plasticity, it is not clear whether an overabundance of proBDNF, a

deficiency mBDNF, or both are involved with the etiology of depression, however increasing evidence

points toward toward the conversion process being very significant. For example, knocking out the

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genes involved in the BDNF conversion pathway or the pro & mBDNF receptors does not directly

cause symptoms of depression, but it does render antidepressants ineffective, indicating that they elicit

their effects by activating the BDNF conversion pathway (Martinowich et al. 2007) .

Another intriguing result from the investigations of the catalytic process involved in this

conversion involves the protein p11. Like BDNF, decreased expression levels of p11 have been

correlated with depression and the efficacy of antidepressants. Svenningsson et al. attribute this effect

to p11's ability to upregulate and localize 5-HT1B receptors (Svenningsson et al. 2006), however,

Martinowich et al. pointed out that p11 is a dramatic enhancer of tPA, the enzyme that converts

proBDNF to mBDNF (Martinowich et al. 2007), which would also offer an explanation for its

antidepressant properties.

A number of other neurotrophins are also being investigated for their apparent roles in cortical

and midbrain plasticity such as fibroblast growth factor (FGF), insulin-like growth factor (IGF-1),

nerve growth factor (NGF) and vasoendothelial growth factor (VEGF) (Krystal et al. 2009).

As mentioned previously, the transcriptome fingerprints of cellular responses to 5-HT2A receptor

agonists revealed transcriptions unique to compounds with hallucinogenic activity. The products of

these transcriptions mainly belong to the class of early growth response proteins (EGRs), which are

transcription factors critically involved in glial proliferation (Mayeret al2009) and neural plasticity

(Knapska & Kaczmarek 2004), and are being investigated for their roles in the circadian regulation of

the pineal gland (Man & Carter 2008), long-term memory formation (Davis et al. 2003) and various

other cognitive processes (DeSteno et al. 2008). Their genes are a subclass of the immediate early

genes (IEGs) which are the first gene targets activated by intracellular signal pathways. Because they

are inducible transcription factors they continue the signaling cascade to further downstream genes,

acting perhaps as both messengers and gateways to later gene responses (Beckmann & Wilce 1996).

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There are four principle EGR proteins; Egr-1, Egr-2, Egr-3 and Egr-4. Out of the four,

transcripts for Egr-1 and Egr-2 were found to be induced only by hallucinogenic 5-HT2A receptor

agonists and not by non-hallucinogenic 5-HT2A receptoragonists, with Egr-2 showing the strongest

response (a four-fold increase) from every hallucinogen (Gonzalez-Maeso & Sealfon 2009b). For this

reason I will review some of the data on Egr-2, although most research thus far has been conducted on

Egr-1 and Egr-3.

Egr-2 controls genes that are required for the onset and completion of Schwann cell myelination

in the peripheral nervous system and appears to be involved in a process that regulates myelination and

demyelination programs as well as the maintenance of the myelinated state (Decker, et al. 2006).

Mutations affecting Egr-2 result in various human hereditary peripheral neuropathies, including

congenital hypomyelinating neuropathy and Charcot-Marie-Tooth disease (Warneret al. 1998) which is

one of the most common inherited neurological disorders (Pareyson et al. 2000).

The Egr-2 transcription factor is the only Egr protein necessary for life. Deletion of the gene is

lethal in mice, due to peripheral nerve myelination failure (Beckmann and Wilce 1997). Egr-2 is also

required for the segmentation of the hindbrain, which arises from Hox genes under Egr-2's control.

EGRs are also necessary for astrocyte proliferation (Mayeret al. 2009). Astrocytes, which

outnumber neurons five-fold, are now widely acknowledged for their potentially complex

computational roles in the brain's circuitry. They can share nearly all the same channels and receptors

as neurons, including such pharmacological darlings as the 5-HT2A, mGlu2/3 and dopamine D2 receptors

(Xu & Pandey 2000; Aronica et al. 2000; Khan et al. 2001, respectively). The involvement of

astrocytes and other glia in the neuropatholgy of schizophrenia (Moises et al. 2002) and mood

disorders has been attracting increasing investigation. Reductions in glial number and density have

been found in the fronto-limbic brain regions of subjects with depression and bipolar disorder

(Rajkowska & Miguel-Hidalgo 2007), so gliogenesis could prove to be an important therapeutic target.

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Genes encoding at least half a dozen growth factors have been identified as targets of the EGR

transcription factors in astrocytes (Mayeret al. 2009).

Less is known about Egr-2 than Egr-1 and Egr-3 (DeSteno & Schmauss 2008), yet it is apparent

that Egr-2 bears some unique qualities distinct from the rest of the family. For example, it is the only

EGR that exists in the cytoplasm as well as the nucleus, and in the nucleus it remains for substantially

longer the others (Perez-Cadaha et al. 2011). Egr-2 can be induced by BDNF like the other EGRs but

also uniquely by NGF, and it has specific involvement in mossy fiber sprouting (Ludwig et al. 2011).

If activated in a tumor cell such as a glioma, however, Egr-2 reverses it's role as a growth facilitator and

instead induces apoptosis (Unoki 2003), linking this psychedelically-induced gene to tumor

suppression.

Another unique property of Egr-2 is that is has a different pattern of expression in and across the

cerebral cortex than the others. For instance, there is a strong basal expression of Egr-1, -3 and -4 in

layers II and VII of the cortex, whereas Egr-2 occurs mostly in layers II and III (Leah & Wilce 2002).

Interestingly, hallucinogens induced Egr-2 in layer V, but not in layers II and III (Gonzalez-Maeso et al.

2007). Layer III is also the cortical layer where schizophrenic patients show significant diminished

dendritic spine density compared to other layers and compared to controls (Glantz & Lewis 2000),

possibly mirroring a neuroplastic neglect of that layer by hallucinogens, which could potentially be

another correlation between hallucinogen pharmacology and schizophrenia.

An association between mutations of the EGR genes and schizophrenia was reported in Japan as

well as postmortem studies that revealed EGR transcripts were down-regulated in the prefrontal cortex

of schizophrenics, but not bipolar, patients. (Yamada et al. 2007) A similar link between EGR

mutations and schizophrenia was later reported in Korea (Kim et al. 2010) whereas no such correlation

was found in a study of the Chinese population (Liu et al. 2010).

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Differential expressions of individual members of the EGR family were induced during

different cognitive processes in mice and it was reported that Egr-2, but not Egr-1 nor Egr-3, was

induced by cognitive tasks associated with attention and attentional shifts. The level of Egr-2

expression was proportional to the magnitude of attentional demand. In tasks involving spatial

working memory, the reverse was true; Erg-1 and Egr-3 were expressed but not Egr-2 (DeSteno &

Schmauss 2008). In other tests, Egr-1 was shown to be essential for the formation of long-term

memories (Davis et al. 2003) and Egr-3 was shown to have a pivotal role in adaptation to novelty and

stress (Gallitano-Mendel et al. 2007). Correlating these neuroplastic elements with various cognitive

domains may reveal some interesting and useful things about our brains. Being able to selectively

strengthen the circuits involved with attention may turn out be a more effective treatment for childhood

ADD than amphetamines, and transcription factors associated with long term memory are enticing for

many reasons.

Plasticity is highly activity-dependent phenomenon. In mouse models, environments enriched

with toys, colors, wheels and ramps lead to marked increases in plasticity and production of BDNF and

NGF. Even in aged mice, a five-fold increase in neurogenesis was observed, actually reversing deficits

imparted by impoverished youth conditions (Baudry 2005). The hypothesis that depressive disorders

involve impaired activity-dependent plasticity and that antidepressants work by ameliorating this

deficit is supported by the interesting findings that depressed individuals exhibit impaired visual system

plasticity in response to visual stimuli and also that healthy, non-depressed individuals chronically

administered sertraline (Zoloft) developed increased visual stimulus-dependent plasticity (Krystal et al.

2009). It was also shown that ocular dominance plasticity was was enhanced in rats administered

fluoxetine (Prozac) (Krystal et al. 2009). These non-mood-related plasticity effects correlated with

depression and antideprssants may point towards a more causal mechanism than a mere transmitter

imbalance.

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Considering the activity-dependence of plastic outcomes, it follows that patients taking

antidepressants would experience different results based upon which behaviors and outlooks were

reinforced by the environment. Indeed, the case was demonstrated that cognitive-behavioral therapy in

combination with antidepressant medication was significantly more effective than either treatment

alone (Kelleret al. 2000).

D-cycloserine (DCS) is a partial agonist at the glycine coagonist site of NMDA receptors,

having the effect of stabilizing NMDA receptor activity against fluxing glycine levels. DCA has been

observed to increase neuroplasticity, yet it has negligible psychoactive effects when compared to

antidepressants and related psychiatric medications. Because of the known role of NMDA-dependent

amygdalar functions in Pavlovian fear conditioning, consolidation, and extinction, DCA was tested and

found to effectively promote the extinction of fear conditioning in animals, a neuroplastic process

involving associational learning and adaptation to novelty (Krystal et al. 2009). The first studies of

DCS in humans was for acrophobia and patients who used DCS in combination with virtual reality

exposure therapy benefited more than those who received placebo (Davis et al. 2005). The results were

replicated and trials using DCS and exposure therapy have also begun for patients with social anxiety,

panic disorder, obsessive compulsive disorder (Krystal et al. 2009), cocaine addiction (Paolone &

Stewart 2009) and schizophrenia (Goffet al. 2008).

The results so far from DCS studies are mostly positive, but with a wide range of efficacy from

insignificant to highly significant. DCS may also have potentially moderate CNS side effects.

However, a key facet of these trials was that the patients who benefited from the DCS / exposure

therapy combination were found during follow up to have retained their cognitive-behavioral

improvements after discontinuation of the treatment. Many current psychopharmacological therapies

require continual and often indefinite administration because of the high risk of relapse There is also

the concern that such long-term pharmacological symptom maintenance simply aggravates the

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underlying psychiatric condition into an increasingly chronic state. In either case, a more curative,

permanent treatment is immeasurably more desirable by a patient yet, sadly, would be neglected by the

profit-driven pharmaceutical industry.

As neuroplasticity-based therapies emerge and begin their refinement, the two novel and central

attributes they entail, that of qualitative activity-dependence and potentially permanent curative

capability, parallel precisely the two key aspects of psychedelic therapy that have, over the past six

decades, been a conceptual and ideological challenge for conventional medical science to address and

antithetical to its economic landscape.

The long-standing dominant model of biological medicine has been based on the premise that

medications acting on biochemical features produce their efficacy on psychiatric conditions in a

bottom-up (body to mind) manner, and that this trend would continue indefinitely, simply becoming

ever more specific and efficacious as science advances forward. Psychedelics have long existed as a

thorn in the side of this kind of biological reductionism and behavioral materialism because the effects

of identical doses of the same drug can produce extremely varied psychological and somatic responses

between subjects or even in the same subject during different administrations.

It was learned early on that that the variable effects of psychedelics were not random but were

shaped entirely by two factors (three if one includes the obvious factor of dose): the external

environment in which the experience takes place as well as the internal bio-psychological milieu such

as personality, anxiety, fears, intentions and state of physical health. The influential Harvard

psychologist Dr. Timothy Leary coined the terminology 'set and setting' to describe these internal and

external factors that shape a psychedelic experience. While set and setting can influence the

experience of many psychoactive drugs ranging from alcohol to amphetamines, in the case of

psychedelic drugs it seems to dictate nearly the entirety of the effects, wherein stripped away of the

environmental stimuli and subjective mental dynamics, such as in an anesthetized subject, there would

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be no externally observable effects such as excitation, sedation nor alterations of heart rate or

metabolism. This type of action has led many to concur with the psychiatrist Dr. Stanislav Grof's

description of psychedelics as non-specific amplifiers of consciousness.

This priority of consciousness as a factor in the action of psychedelics did not quite fit into the

materialistic paradigm of Western medicine where drugs have a specific given effect. Psychedelics are

simply not something one's doctor would recommend to take two and call me in the morning. Their

ability to heal, or aggravate, deep psychological wounds is utterly activity-dependent, just as

neuroplastic adaptations are. Indeed, it is increasingly apparent that neuroplastic brain rewiring are

exactly what the psychedelics provoke.

Those currently working on developing neuroplastic therapies have been confronted with the

challenge of how to develop and obtain FDA approval for therapeutic strategies that entail

pharmacological medications conjoined with cognitive-behavioral elements (Krystal et al. 2009).

Luckily for them, this very same problem has been worked on diligently for the past 10 years by those

pursuing clinical approval for the therapeutic applications of psychedelic medicines in conjunction with

psychotherapy by developing and carrying out dozens of protocols and clinical trials, some of which

are nearly ready for phase III (Multidisciplinary Association of Psychedelic Studies 2011).

The other critical attribute of psychedelic therapy and, likely, other neuroplastic therapies yet to

be developed, is that the desired effects are very long-lasting (Griffiths, et al. 2008; Doblin 1998),

potentially even permanent. While the benefits of not having to indefinitely take side-effect-laden

medications are obviously desirably in every way for patients, they pose a challenge for the medical

establishment's current economic structure. Pharmaceutical companies can not make money from pills

that only need to be taken once or a dozen times so they have no incentive to fund the multi-million-

dollar clinical trials required to bring them to market. At this time, psychedelics are still somewhat

controversial so large-scale corporate funding and philanthropy has not yet lent support to the venture

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and most governments wouldn't let public funds anywhere near such an endeavor, which is why the

expensive clinical trials for psychedelic medicines thus far have been paid for entirely by the private

sector through the fund-raising and outreach efforts of non-profits such as The Multidisciplinary

Association for Psychedelic Studies (MAPS). However, it is feasible that other kinds of non-

psychedelic neuroplastic treatments could gather funds for development from more conventional

sources of capital since they would not bear the unfortunate stigma that psychedelics drugs do.

Deciphering the biological mechanisms of hallucinogenic drugs holds very high potential for

the discovery of non-psychoactive neuroplastic agents targeted at a wide range of psychiatric illnesses.

LSD has been found to downregulate 5-HT2Areceptors in the frontal cortex (Gresch et al. 2005) which

may help to explain the antidepressant effects of hallucinogens considering that frontal cortex 5-HT2A

receptor density was found to be increased in untreated depressed patients and that antidepressants also

reduce prefrontal 5-HT2A receptor density (Vollenweider 2010). Fronto-limbic 5-HT2Areceptor density

upregulation has also been correlated with anxiety, the ability to cope with stress and responses to tonic

pain (Vollenweider 2010); all conditions that have been successfully treated with hallucinogens (Grob

2010).

Dissociative anesthetics such as ketamine also share some of the same subjective effects as the

classical psychedelics. Ketamine has been getting much recent attention for it's unique antidepressant

properties. While being too psychoactive for efficient regular use in treating depression, a single

administration of ketamine has immediate and pronounced antidepressants effects that last a relatively

long period of approximately two to three weeks, which is likely due to a very quickly initiating

neuroplastic effect (Li et al. 2010) [2012 Update: Confirmed (Kavalali 2012 & Schmidt 2012).], albeit

a more transient one than the classical psychedelics can induce. Unlike the 5-HT2A agonists, ketamine

is an antagonist of the NMDA receptor, however, both 5-HT2A agonism by hallucinogens and NMDA

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antagonism by ketamine result in activation of prefrontal-limbic glutamatergic circuits, which have

been demonstrated to be required for the hallucinogenic effects (Vollenweider 2010). Neuroimaging

studies also reveal that psilocybin and ketamine activate very similar prefrontal cortical areas

(Vollenweider 2010), further indicating common modes of action. Patients with depression have been

observed to have reduced prefrontal glutamatergic activity (Vollenweider 2010) and not only do 5-HT2A

agonist hallucinogens and ketamine increase glutamate levels in the prefrontal-limbic areas, they also

increase BDNF levels in the same areas, which might play a role in the antidepressant effects of these

drugs that persist long after the psychotropic effects have ceased (Vollenweider 2010).

While hallucinogens have shown efficacy in treating a vastly wide range of psychopathologies,

one illness they seem to be contraindicated for, at least in modern medical contexts, is schizophrenia.

Although most hallucinogens tend to predominantly induce visual phenomena while auditory

hallucinations are more common in schizophrenia, the similarities between hallucinogenic drug effects

and psychosis has been long appreciated. (It has been reported that some novel synthetic 5-HT2A

agonist hallucinogens do tend to produce more to auditory distortions than visual ones. Indeed, the

spectra of qualitative variations in perceptual effects among the hallucinogens may prove very useful in

future neurocognitive and neuropathological investigations.) Based upon the stimulant-model of

psychosis, the first wave of antipsychotics were the D2 receptor blockers. Later, atypical antipsychotics

came into favor which also blocked D2 receptors but blocked 5-HT2A with a much greater affinity,

indicating 5-HT2A receptor function abnormality in the etiology of schizophrenia. This hypothesis was

later supported when it was found that the hallucinogens also acted on 5-HT 2A receptors and by the fact

that the atypical antipsychotics partially block the effects of hallucinogens (Gonzales-Maeso & Sealfon

2009b).

Later, the NMDA antagonist phencyclidine (PCP) was shown to mimic the effects of

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schizophrenia even more accurately than D2 stimulants or LSD-like drugs, implicating glutamate

hypofunction in schizophrenia, however, glutamate receptor agonists were scant and no such

medications were developed until recently, where successful trials with an mGlu2/3 receptor agonist in

treating schizophrenia have caused quite a stir in the field (Patil et al. 2007). Congruent with these

pharmacological findings, levels of 5-HT2A receptors in the cortices of untreated schizophrenic subjects

were found to be significantly higher and mGlu2 receptors significantly lower than controls (Gonzales-

Maeso et al. 2008).

It has been discovered that not only do the atypical antipsychotics block the effects of 5-HT2A

receptor agonist hallucinogens, they also block the effects of NMDA antagonist dissociative

anesthetics. Further, the new class of mGlu2 receptor agonists currently being looked at for

antipsychosis applications were also found to abolish the effects of 5-HT2A receptor agonist

hallucinogens as well as the NMDA antagonist dissociative anesthetics (Gonzales-Maeso & Sealfon

2009b). The evidence that 5-HT2A receptors and mGlu2 receptors exhibit multiple forms of cross-talk

has been observed and investigated for some time and now the reason has become quite clear: these

two different types GPCRs form a functional dimer on the postsynaptic side of cortical pyramidal

neurons (Gonzales-Maeso et al. 2008).

RECEPTOR COMPLEXES

The 5-HT2A-mGlu2 heterodimer has unique properties beyond either receptor individually. It

exhibits both positive and negative ligand-dependent cooperative binding through allosteric

interactions, with mGlu2 receptor activation increasing the affinity of the 5-HT2A receptor agonists

while, in contrast, 5-HT2A receptor activation decreases the affinity of mGlu2 receptor agonists

(Gonzales-Maeso et al. 2008), in what appears to be regulatory integration of serotonin and glutamate

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neurotransmission into specific patterns of cellular response.

There exist unique G protein pathways associated with the dimer that are not activated by either

receptor alone. For example, the 5-HT2A receptor hallucinogen-specific pathway involving the Gi/0

signaling cascade leading to the induction ofegr-2 requires the dimerization with the mGlu2 receptor

and the psychotropic effects of hallucinogens are abolished by the elimination of the mGlu2 receptor

(Moreno et al. 2011). This and a wealth of other data support the finding that it is the 5-HT2A-mGlu2

heterodimer that is the prime target of hallucinogens, atypical antipsychotics andthe new class of

metabotropic glutamatereceptor agonist antipsychotics (Gonzales-Maeso & Sealfon 2009b).

There still remains the challenge of how to tie in the dopamine hypothesis of schizophrenia with

the emerging serotonin-glutamate hypothesis. The activation of the 5-HT2A-mGlu2 heterodimer with

hallucinogens or via NMDA receptor antagonism does have downstream effects on dopaminergic

activity in the nucleus accumbens where D2 receptor antagonists appear to have their antipsychotic

effect. The nucleus accumbens is involved in emotions and motivation which might explain why the

D2 receptor antagonist typical antipsychotics are effective against negative symptoms such as social

withdrawal and less so with the positive ones such as delusions. The prefrontal cortex circuits

activated by hallucinogens also innervate dopaminergic pathways of the ventral tegmental area and lead

to increased concentration of dopamine in the striatum, which has been correlated with feelings of

euphoria induced by hallucinogens, but blocking D2 receptors with haloperidol only decreases

psilocybin-induced euphoria by about 30% (Vollenweider 2010), implying another more salient

mechanism involving dopamine in schizophrenia.

The existence of GPCR receptor dimers such as the T1R1/T1R3 taste receptor have been

recognized for over a decade now, but in the past few years the number of homodimers, heterodimers,

heterotrimers, heterotetramers and even mosaics of recepter oligomer complexes involving dozens of

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discreet receptor types has been pushed up to scores of identified or hypothesized combinations and the

number is increasing rapidly (Kniazeffet al. 2011). Current speculations are that, given the large

number of GPCR genes and their ability to form combinations, there may be tens or even hundreds of

thousands of unique receptor heteromers in the brain and nervous system. (Albizu et al. 2010)

Dopamine D2 receptors, for example, appear to exist in dimeric and trimeric combinations with

with D1, D3, CB1, mGluR5 and A2a receptors, giving rise to complexity previously unimagined. It has

even been found that D2 partial agonists surprisingly behave as D2 antagonists at the D2-D3 heterodimer

(Fuxe et al. 2009)! Investigating the the D2 containing heteromers and receptor mosaics along the

circuits implicated in schizophrenia could reveal important aspects of receptor dysfunction and new

targets for pharmacological intervention. Indeed, countless diseases can be approached anew with this

understanding. One example of a novel pharmacological approach to be taken is to develop bivalent

ligands: two ligands attached by amino acid or perhaps alkyl links, that bind both individual

components of the heterodimer (Albizu et al. 2010).

CONCLUSION

While the scope of complexity revealed by such leaps in neuroscientific comprehension as the

existence of ligand-directed signal trafficking, epigenetic neural plasticity and receptor

heteromerization may appear daunting, the implications for medicinal and therapeutic opportunities are

staggering. Given that psychedelics play such a crucial role in exploring and expanding the

neuroscientific landscape, one may take pause and wonder if the field of neuro-cognitive science would

not be several years more advanced than it is now if these compounds were never wrenched from hands

of scientists and kept out of the labs for nearly 3 decades. Perhaps our society, groggily waking from

the slumber of a dark age of authoritarian fundamentalism, was not yet ready nor capable of fathoming

the magnitude of mysteries such keys to the doors of perception would reveal, but now, faced with the

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