5-hydroxytryptamine (5-HT) is called serotonin, and is one of the major neurotransmitters in the central nervous system. It carries signals between nerve cells, and normally controls many physiological processes, such as sleep, mood, cognition, thermoregulation, motor, cardiovascular and respiratory functions. It also plays a major role in psychiatric disorders, such as depression, anxiety, schizophrenia and drug addiction. Based on structural, biochemical and pharmacological differences, it is possible to identify seven subtypes of serotonin receptors. In this article, we will take you through a recent paper by Adam Halberstadt and colleagues that uses genetic knock-out mice to demonstrate the role of the serotonin-2A receptor in the action of phenylalkylamine hallucinogens (Halberstadt 2013).
One of the best ways to study how hallucinogens work in the nervous system is through rodent studies. Mice and rats have long served as a research tool in all avenues of neuroscience. The advantage of using mice is their short life cycle, the lower costs and ethical standards compared to human studies, and their genetic similarity to humans. Mice share 90-95% of our genetic makeup, with over 99% of the genes having a homologue (a gene that is thought to share the same function) in the human DNA (Chinwalla et. al 2002). The shared genetic material is associated with another great advantage of using rodents – the ability to use so-called ‘genetic knock-outs’.
In genetic knock-outs, a single gene is removed or rendered nonfunctional. This technique allows researchers to compare the behaviour of a normal individual (so called wild-type) and a knock-out individual (the one with the deleted gene), and is therefore valuable in identifying the function of a gene. For example, knocking out the gene which encodes the serotonin receptor 2A allows scientists to investigate the role of this molecule. Additionally, behaviour in both wild-type and knock-out could be compared before and after administration of a substance. If the effect of the substance is seen in both the wild type and the knock-out, then it can be concluded that the given molecule plays no part in producing the effects of the drug. In contrast, if the behavioural differences after substance administration are eliminated in the knock-outs, then it means that the given molecule is necessary for the action of the drug.
One of the difficulties of studying psychedelics with mice is that they are unable to report about their experiences; hence scientists must find some externally measurable behavioural quantity to quantify the action of the drug. After the administration of a psychedelic substance, the most obvious observable change in the behaviour of rodents is a spontaneous increase of locomotor activity. Therefore, to evaluate the behavioural effects of hallucinogens in mice, a behavioural Pattern Monitor (BPM) was used. A BPM consists of a chamber with clear plexiglas walls with holes and an opaque floor. BPM detects distance travelled and rearing behaviour by using infrared photobeams placed on each wall. The distance travelled is used as a measure of the locomotor activity in these studies. Furthermore ‘holepoking’ is assessed as a quantitative proxy for exploratory behaviour.
In Halberstadt's study, the action of phenylalkylamine hallucinogens are investigated. Phenylalkylamine hallucinogens are substances with a chemical structure resembling amphetamine and methamphetamine, with the most well-known example being mescaline. The action of these so-called substituted amphetamines, however, does not resemble that of the unsubstituted amphetamine, which is a stimulant drug without psychedelic properties. The following hallucinogenic substances were used in the study:
2,5-dimethoxy-4-ethylamphetamine hydrochloride (DOET);
2,5-dimethoxy-4-propylamphetamine hydrochloride (DOPR);
2,4,5-trimethoxyamphetamine hydrochloride (TMA-2);
(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide (TCB-2);
And the nonhallucinogenic 2,5-dimethoxy-4-tert-butylamphetamine hydrochloride (DOTB)
Mescaline is naturally found in the many cacti such as peyote or San Pedro cactus and it is well known for its hallucinogenic effects. DOET, DOPR and TMA-2 are all substituted amphetamines and are considered as a “heavy duty psychedelics”. TCB-2 is one of the most potent phenylalkylamine hallucinogens, and is mostly used in scientific research to assess serotonin-2A receptor function.
The experiment had the following structure: mice were placed in the BPM chambers after the administration of the substance. The experiment in the BPM took 60 min. Distance was examined in 10- or 30- min time blocks, rearings and holepokes in 30-min time blocks.
So what were the results of this study? Firstly, mescaline, DOET and DOPR dose-dependently suppressed investigatory behaviour, which is probably related to the decreased cognitive flexibility during hallucinogenic intoxication in humans. Secondly, mescaline, DOET, DOPR and TCB-2 produced a dose-dependent effect on locomotion: whereas low doses acted similarly to saline at the beginning of the experiment and increased locomotion later on, high doses of these hallucinogens reduced locomotion at the beginning and increased it towards the end of the 60-min experiment. TMA-2 was a bit different, in that it caused hyperlocomotion at all times. TMA-2 has not been studied elsewhere; so this could be the result of using an inappropriate (too high) dosage.
The study further showed that both mescaline and TCB-2 induced an increase in locomotor activity, and this was blocked by the serotonin-2A antagonist (antagonists bind to that receptor and block its activity), demonstrating that this receptor is necessary to produce the action of these substances. To further support the role of the serotonin-2A receptor subtype in rodent behaviour, both mescaline and TCB-2 produced hyperlocomotion in wild-type mice, however there was no effect in knock-out mice. These results indicate that mescaline and TCB-2 produce their increased locomotor activity through activating the serotonin-2A receptor.
Similar effects on locomotor activity were shown with other phenylalkylamine hallucinogens such as DOI and DOM, in a previous study by the same researchers (Halberstadt et al., 2009). Therefore it can be hypothesised that hyperactivity is an effect common to all phenylalkylamine hallucinogens. The ability to induce behavioural changes in this experiment was congruent with hallucinogenic effects in humans (Shulgin and Dyer, 1975; Shulgin and Shulgin, 1991), with the most potent being DOPR and the least being mescaline ( DOPR > DOI > DOET > TMA-2 > mescaline).
Further, the results of the presented study also confirmed that even though DOTB has high affinity for the serotonin-2A receptor, it is not a true psychedelic (Shulgin and Dyer, 1975); in mice, it does not produce a hallucinogenic-induced increase in locomotor activity. TCB-2 being one of the most potent hallucinogen has not been shown to be as much perception-altering as other phenylalkylamine according to Erowid.org users. In the presented study, TCB-2- induced an increase in locomotor activity that was partially decreased by low dose and completely blocked by a high dose of serotonin-2A antagonist. Moreover, TCB-2 had no effect on investigatory behaviour in both wild-type and serotonin-2A knock-out mice, indicating that TCB-2 does not produce this behaviour through this receptor. It can be therefore concluded that TCB-2 acts mainly through activating serotonin-2A, but collaboration with other receptors is needed to produce investigatory behaviour.
It is important to note that locomotor hyperactivity in rodents is related to increased dopamine in the mesolimbic system, and does not represent a model of hallucinogenic effect in humans. Rather, many studies indicate that hyperlocomotion in rodents is a more complex and multifactorial behaviour, with dopamine being the final part of the motor pathway. Some researchers point out that the increased locomotor activity in mice could be comparable to psychotic symptoms in humans, namely the increased agitation and disorganised behaviour seen in acute psychotic phases (Bubenikova-Valesova et al, 2008). Agitated behaviour is normally associated with elevated dopamine in humans, and it has been demonstrated that psychedelics can increase the release of dopamine (Vollenweider et al, 1999). The problem gets more complicated, as previous research with other class of hallucinogens – indoleamines such as psilocin and 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) - showed rather reduced locomotor activity in mice – an effect contributed to an activation of the serotonin-1A receptor (Halberstadt et al., 2011a). It could be therefore deduced that altered locomotor activity in rodents after administration of hallucinogens represents rather complex functional interactions between serotonergic, dopaminergic and other neurotransmitter systems. To gain more insight into the mechanisms of hallucinogens, there are many other behavioural aspects that could be observed in mice - namely perseveration of behaviour (excessive grooming in rodents correlates to stereotypic behaviours in psychotic individuals), deficits in information processing (measurement of the sensory overload and ability to prevent it) or memory impairment (correlates to cognitive deficits seen in psychotic patients).
In conclusion, Halberstadt and colleagues demonstrated with the use of genetically modified mice that phenylalkylamines produce their locomotor effect through the serotonin-2A receptor – although one should bear in mind the difference between humans and animal models. For example, different dosage regimens, metabolisms or permeability of the blood-brain barrier could feasibly limit the validity of the rodent model. Despite these differences, preclinical research still has a firm place in the study of hallucinogens. The long-term aim is to be able to translate what we learn from rodent models into a better understanding of human mental disorders.
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