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In mice, the drug N,N-dipropyltryptamine (DPT) produces head twitches via agonist binding at serotonin 5-HT2A receptors, and this behavioral effect may be
analogous to drug-induced hallucinogenic effects in humans. By pairing environmental stimuli in the form of a conditioning box with daily injections of DPT, the
box eventually became a conditioned stimulus and elicited a conditioned behavioral response. Significant numbers of head twitches occurred following saline
administration and placement in the conditioning box after daily conditioning trials with DPT, but not after daily conditioning trials with saline or after daily
administration of DPT in the home cage. We thus present the first example of conditioned behavioral effects with a hallucinogenic drug.
Tryptmaines are a class of drugs widespread throughout nature and known for their psychotropic effects. Natural tryptamines include serotonin, a neurotransmitter
in the brain, and psilocybin – the compound found in “magic mushrooms.”
N,N-dipropyltryptamine (DPT), a synthetic tryptamine that binds to serotonin (5-HT) receptors in the brain, has been known to elicit hallucinogenic effects in
humans, and is a drug of abuse.
In rodent studies, hallucinogens have been found to elicit head twitch behavior via their actions at 5-HT2A receptors. In humans, potency to induce hallucinogenic
effects for a given drug correlates with its binding affinity at 5-HT2A receptors. Thus, the observance of head twitch behavior in mice may be related
pharmacologically to hallucinogenic effects in humans.
In Pavlovian conditioning, if a stimulus that evokes a response is repeatedly paired with a neutral stimulus, in time, the neutral stimulus becomes a
conditioned stimulus and can produce a conditioned response, even in the absence of the unconditioned stimulus. In this case, the neutral stimulus is
the environment provided by the conditioning box, the unconditioned stimulus is the DPT injection, and the response is the occurrence of head twitches.
Preliminary Data
• 3.0 mg/kg DPT elicits the maximal number of head twitches in mice.
• Pretreatment with 0.01 mg/kg M100907, a selective 5-HT2A antagonist, attenuates this effect.
• At doses above 3.0 mg/kg, the number of head twitches observed decreases due to the saturation of 5-HT2A receptors and DPT binding to other receptors
that may inhibit head twitch behavior.
Twenty-four male Swiss Webster mice from Charles River Labs were separated into the following groups (n=6):
DPT PAIRED: For four days (Mon. – Thur.), each mouse was injected with either 3.0 or 10.0 mg/kg DPT and immediately placed in a conditioning box containing
black and white vertically-striped walls and a white flooring perforated with small holes for 60 minutes. The first fifteen minutes of these conditioning periods were
filmed to record the number of head twitches that occurred. On the fifth, eighth, and twelfth day, each mouse was tested for a conditioned drug response by
being placed in the same box for fifteen minutes following an injection of saline.
SALINE PAIRED: This group was treated identically to the DPT PAIRED mice, except that no DPT was ever administered. Instead, all mice received saline
prior to being placed in the conditioning box. On the fifth day, each mouse was tested for a conditioned drug response by being placed in the same box for
fifteen minutes following an injection of saline.
DPT UNPAIRED: This group was treated identically to the DPT PAIRED mice, except that they were injected with 3.0 mg/kg DPT and then returned to their
home cage on the four conditioning days. On the fifth day, each mouse was tested for a conditioned drug response in a conditioning box for fifteen minutes
following saline administration.
Mice were injected intraperitoneally and always conditioned in the same box. The volume of each injection was equal to the mass of the mouse, in grams,
divided by 100. After use by each mouse, every box was cleaned using a 5% acetic acid solution.
In contrast to the previous studies conducted over 10 minutes in a home cage environment, 3.0 and 10.0 mg/kg DPT elicited the same number of head twitches
in the conditioning boxes over a 15 minute observation period. Furthermore, the number of head twitches recorded for 3.0 and 10.0 mg/kg DPT was roughly
constant over the four days of drug pairings.
On the fifth day, administration of saline and placement in the conditioning box elicited a robust head twitch response in mice previously conditioned with 3.0 or
10.0 mg/kg DPT for 4 days, but not in mice treated with saline or DPT at home.
The conditioned head twitch response observed in mice previously treated with 3.0 mg/kg DPT decreased over time to levels not significantly different from
saline controls. Extinction trials with the 10.0 mg/kg DPT group are ongoing.
• Daily injections of 3.0 or 10.0 mg/kg DPT elicited head twitch behavior in the mouse that did not increase or decrease in magnitude over a four-day period.
• In contrast to preliminary studies conducted in a home cage environment, 3.0 and 10.0 mg/kg DPT elicited similar instances of head twitch behavior in the
conditioning box.
• Following four conditioning trials with DPT, the conditioning box acquired the capacity to elicit hallucinogen-like head twitch behavior.
• This effect was drug- and context-specific, as neither saline-paired nor DPT-unpaired mice exhibited head twitch behavior when placed into the conditioning
box on the test day.
• As previously observed with DPT-elicited twitches in the home cage environment, the conditioned effects of 3.0 mg/kg were greater than those of 10.0 mg/kg.
• The conditioned effects of 3.0 mg/kg DPT may be relatively long lasting.
• The serotonin 5-HT2A antagonist M100907, and the 5-HT1A antagonist WAY-100635 have been found to attenuate DPT-elicited head-twitch behavior.
• More experiments will be conducted to see if an injection of these antagonists will also attenuate conditioned head twitches, or if antagonist injections during
drug pairings will block acquisition of conditioned effects.
• The persistence of the conditioned effect will be better assessed by using multiple groups of mice in order to dissociate time since last pairing from extinction testing.
This research supported by:
• Howard Hughes Medical Institute Grant 52003727 (WX)
• Emory University Interdisciplinary Science Program for Integrating Research into Education (INSPIRE) Program (WX)
• Scholarly Inquiry and Research at Emory (SIRE) (WX)
• Yerkes National Primate Research Center (WEF)
• College on Problems of Drug Dependence (WEF)
• National Institute on Drug Abuse Grant DA020645 (WEF)
• DPT and M100907 were generous gifts of Kenner C. Rice, Ph.D., Laboratory of Medicinal Chemistry, NIH/NIDDK
1. Gonzalez-Maeso J, et al. (2003) Transcriptome fingerprints distinguish hallucinogenic and nonhallucinogenic 5-hydroxytryptamine 2A receptor agonist effects
in mouse somatosensory cortex. J Neurosci 23:8836-43
2. Sadzot B, et al. (1989) Hallucinogenic drug interactions at human brain 5-HT2 receptors: Implications for treating LSD-induced hallucinogenesis.
Psychopharmacology 98:495-499
Ivan Pavlov is famous for his experiment using "classical" conditioning. He noticed that his dogs would salivate when they were about to be fed. So, he began
ringing a bell when he set out for for his dogs. After several pairings of providing food with this "neutral stimulus," he could simply ring the bell and his dogs would
salivate without the original stimulus of food.
In this experiment, we paired a drug stimulus, which was an injection of N,N-Dipropyltryptamine (DPT), with a specific environment, which was a conditioning
box with black-and-white striped walls and a perforated floor. The normal response to DPT is the release of the neurotransmitter serotonin in the brain. This
causes head twitches in mice that can be observed and recorded. After four daily pairings, the mice were observed to twitch a fair amount above normal
head-twitch levels with only an injection of saline. In contrast, mice that had been paired with saline or unpaired with DPT in the same boxes did not show
head-twitch behaviour.
Intraperitoneal injection for the mouse, behavioral observation of head twitches, use of program JWatcher for scoring tapes
conditioned response, paired stimulus, neutral stimulus, unpaired stimulus, serotonin agonist, mouse, serotonin antagonist, head twitch behaviour,
N,N-Dipropyltryptamine, DPT
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