This rodent study (2014) investigated the role of the 5-HT2A, 5-HT2C, and mGlu2 receptors in DMT and another tryptamine. The authors conclude that 5-HT2A receptors likely play a major role in the effects of the substances, but that the other two types of receptors likely also play a modulatory role.
Abstract
“Rationale: Serotonin 5-HT2A and 5-HT2C receptors are thought to be the primary pharmacological mechanisms for serotonin-mediated hallucinogenic drugs, but recently there has been interest in metabotropic glutamate (mGluR2) receptors as contributors to the mechanism of hallucinogens.
Objective: The present study assesses the role of these 5-HT and glutamate receptors as molecular targets for two tryptamine hallucinogens, N,N-dimethyltryptamine (DMT) and N,N-diisopropyltryptamine (DiPT).
Methods: Drug discrimination, head twitch and radioligand binding assays were used. A 5-HT2AR inverse agonist (MDL100907), 5-HT2CR antagonist (SB242084) and mGluR2/3 agonist (LY379268) were tested for their ability to attenuate the discriminative stimulus effects of DMT and DiPT; an mGluR2/3 antagonist (LY341495) was tested for potentiation. MDL100907 was used to attenuate head twitches induced by DMT and DiPT. Radioligand binding studies and inosital-1-phosphate (IP-1) accumulation were performed at the 5-HT2CR for DiPT.
Results: MDL100907 fully blocked the discriminative stimulus effects of DMT, but only partially blocked DiPT. SB242084 partially attenuated the discriminative stimulus effects of DiPT, but produced minimal attenuation of DMT’s effects. LY379268 produced potent, but only partial blockade of the discriminative stimulus effects of DMT. LY341495 facilitated DMT- and DiPT-like effects. Both compounds elicited head twitches (DiPT>DMT) which were blocked by MDL1000907. DiPT was a low potency full agonist at 5-HT2CR in vitro.
Conclusions: The 5-HT2AR likely plays a major role in mediating the effects of both compounds. 5-HT2C and mGluR2 receptors likely modulate the discriminative stimulus effects of both compounds to some degree.”
Authors: Theresa M. Carbonaro, Amy J. Eshleman, Michael J. Forster, Kejun Cheng, Kenner C. Rice & Michael B. Gatch
Summary
Methods: A 5-HT2AR inverse agonist, 5-HT2CR antagonist, mGluR2/3 agonist and an mGluR2/3 antagonist were used to study the effects of DMT and DiPT on the brain.
Results: MDL100907 blocked the discriminative stimulus effects of DMT, but only partially blocked DiPT. LY379268 produced potent, but only partial blockade of DMT’s effects.
Introduction
DMT and DiPT are structurally similar hallucinogens, but produce somewhat different effects. DMT causes brief, episodic visual hallucinations, while DiPT produces auditory effects.
DMT, DOM, LSD and MDMA all fully substitute in DOM-trained rats, but DiPT fully substitutes in LSD-trained rats but DMT only produces 65% DAR in DiPT-trained rats. This may indicate that these compounds have different mechanisms of action.
The precise mechanism of action for hallucinogens is still unknown, but 5-HT2A, 5-HT2C, and 5-HT1A receptors may play important roles. DiPT may also bind to 5-HT2C and 5-HT1A receptors, but this has not yet been tested.
mGlu2/3 receptor agonists suppress glutamate release, whereas mGlu2/3 receptor antagonists increase the amount of glutamate in the synapse, creating a potentiation of hallucinogenic effects. mGlu2 receptors may co-localize with 5-HT2A receptors to form heteroreceptor complexes.
The goal of this study was to identify receptor mechanisms that account for differences in the behavioral effects produced by DMT and DiPT. The 5-HT2A and mGlu2 receptors played an important role in the discriminative stimulus of both DMT and DiPT.
Male Sprague – Dawley rats and male C57BL/6 mice were used in this study. They were housed individually, were maintained on a 12:12 light/dark cycle, and had ad libitum food and water access.
Drug discrimination procedures
16 rats were trained to discriminate N,N-dimethyltryptamine (DMT) from saline using a two-lever choice methodology. They received food when responding on the injection-appropriate lever and were tested between pairs of identical training sessions.
Rats received an intraperitoneal injection of either saline or the appropriate training drug, and were placed in an experimental chamber. They received 60 training sessions in total, and were selected for use in experiments after achieving 85% injection-appropriate responding.
Rats trained to discriminate DMT and DiPT from saline were tested with MDL100907, SB242084, LY341495 and LY379268. A repeated-measures design was used and a maximum of 20 min was allowed between test sessions.
Head Twitch Response
Groups of at least 7 mice were tested with a vehicle control, a positive hallucinogen control and doses of either DMT or DiPT. They were given the 5-HT2A inverse agonist or vehicle 10 min prior to injection of the test compound, vehicle or positive control.
[125I]DOI Binding
The binding of 5-HT2C receptors to cDNA was tested in human embryonic kidney cells expressing the human 5-HT2C receptor. The cells were washed with phosphate-buffered saline, scraped into 2 ml of phosphate-buffered saline, and frozen at 20°C until needed.
The binding assay included test compound, serotonin or buffer, cell homogenate, [125I]DOI, and buffer in a final volume of 250 l. The radioactivity was determined by liquid scintillation counting.
Inosital-1-Phosphate Accumulation
Activation of 5-HT2C receptors was tested by measuring the accumulation of inosital monophosphate (IP-1) in cell lysates. The maximal effect of serotonin was determined in each assay.
DMT, DiPT and DOI were dissolved in 0.9% saline, LY379268, LY341495 and SB242084 were dissolved in deionized water with sodium hydroxide, and MDL100907 was dissolved in saline and HCl and administered i.p. in a volume of 1 ml/kg.
Data analysis
Rats were tested for drug discrimination by responding to a lever with the appropriate drug or vehicle. The percentage of drug-appropriate responding and response rate were plotted as a function of dose of test compound.
Potencies of test compounds were calculated by fitting straight lines to the individual dose – response data for each compound by means of TableCurve 2D (Jandel Scientific, San Rafael, CA). The effects of vehicle, DOI, DMT and DiPT alone were analyzed by one-way analysis of variance.
Radioligand Binding and Functional Assay
5-HT2A receptor inverse agonist MDL100907 reduced DiPT-lever responding to 14.4 -14.3% at 0.1 mg/kg and 28.9 -18.4 and 33.2 -17.3% at 0.3 and 1.0 mg/kg, respectively.
SB242084 reduced drug-lever appropriate responding in DMT-trained rats to 63.2 – 17.2% at 2.5 mg/kg, but did not alter response rates in DiPT-trained rats.
LY379268, a mGlu2/3 receptor agonist, reduced DMT-appropriate responding to a plateau of approximately 60% drug-appropriate responding. It also decreased response rates and suppressed responding in 4 of 7 rats following 2.5 mg/kg.
LY341495 produced a dose-dependent leftward shift in both the DMT and DiPT dose-effect curves, and did not alter response rates in the DMT-trained rats. However, in the DiPT-trained rats, LY341495 did produce a dose-dependent decrease in response rates.
Head Twitch Assay
Figure 5 shows that DOI produced the most head twitches, followed by DMT and DiPT. The higher dose of MDL100907 reduced head twitches, but there was no difference in body weight of any of the treatment groups.
Discussion
The present study assessed the effects of DiPT on 5-HT2A, 5-HT2C and mGlu2/3 receptors in the brain and found that DiPT has a different effect on these receptors than DMT.
5-HT2A receptors
The 5-HT2A receptor inverse agonist MDL100907 completely blocked the discriminative stimulus effects of DMT, but did not completely block those of DiPT. Further, higher doses of MDL100907 were required to attenuate DiPT-appropriate responding.
MDL100907 attenuated head twitches produced by both DMT and DiPT, suggesting that 5-HT2A receptors are important in mediating the behavioral effects of both DMT and DiPT. However, the difference in the ability of MDL100907 to block the discriminative stimulus effects of DMT and DiPT may be due to dose differences.
5-HT2C receptors
A 5-HT2C receptor antagonist, SB242084, produced a U-shaped dose-effect curve when tested in combination with both DMT and DiPT, but was unable to fully block the discriminative stimulus effects of either drug.
mGlu2/3 receptors
In the present study, LY379268, an mGluR2/3 agonist, had no effect in DiPT-trained rats, but produced a small, but statistically non-significant attenuation of the discriminative stimulus effects of DMT and LSD.
The group II metabotropic glutamate receptors (mGluR2/3) have different roles in hallucinogen-induced head twitch response, with mGluR2 having the largest contribution to the effects of LSD and the least to those of DiPT.
The discriminative stimulus effects of DMT and DiPT are mediated through similar mechanisms, but the degree to which each of these mechanisms contribute appear to be different. 5-HT2A receptors were found to be the most important for the discriminative stimulus effects of DMT, whereas 5-HT2C and mGlu2 receptors may play modulatory roles.
Fig. 1.
MDL100907, a 5-HT2A receptor inverse agonist, increased the rate of responding to DMT and DiPT, compared to the saline control.
Fig. 2.
The 5-HT2C receptor antagonist (SB242084) inhibited the discriminative stimulus effects of DMT and DiPT. The rate of responding was also affected by the dose of the drug.
Fig. 3.
LY379268, an mGlu2/3 receptor agonist, inhibits the discriminative stimulus effects of DMT and DiPT in DMT/DiPT-trained rats.
Fig. 4.
LY341495, an mGlu2/3 receptor antagonist, facilitates the effects of DMT and DiPT in DMT/DiPT-trained rats. The effects are shown as a function of dose and as a function of rate of responding.
Fig. 5.
MDL100907, a 5-HT2A receptor inverse agonist, reduced head twitches in C57Bl/6 mice when used in combination with hallucinogens.