Figure 1: Mmmm, cough syrup.
(Disclaimer: cough syrup is *not* a treatment for depression.)
(Source: @aerial_m)
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Remember that thick purple cough syrup you took as a child? Maybe
you still take it when you’re
so sick you feel like you’re
hacking up a lung. One of the ingredients to suppress cough in some syrups is
dextromethorphan (DM). It has been in use for approximately 50 yrs and has a
low side effect profile. In addition to other effects, DM may act similarly to
ketamine, a dissociative anesthetic that blocks NMDA receptors and was recently
found to have fast-acting antidepressant properties. Because most current
antidepressants take weeks to provide symptom relief, both drugs are good candidates
for improved depression treatments. However, DM has fewer side effects than
ketamine. Anna Scandinaro is an undergraduate student at West Virginia
University in the lab of Dr. Rae Matsumoto, who is now Dean of the
College of Pharmacy at Touro University in California. Anna and her co-mentor, Linda
Nguyen, a MD/PhD student in Dr. Matsumoto’s lab are investigating a modified DM
molecule in the treatment of depression.
DM activates the sigma-1 receptor (σ1R), a receptor in the brain for which the function has not yet been fully determined. However, the Matsumoto lab has shown that DM can produce anti-depressant effects through σ1R binding (PLoS, open access). DM can also block NMDA receptors, the target of ketamine, which are excitatory and involved in learning and memory. As mentioned in another post, the two problems with current antidepressants are that they take several weeks to provide symptom relief and many people do not respond to them. DM and ketamine may address both issues because of the quick action and because they work by completely different mechanisms than current antidepressants.
However, DM has pharmacokinetic properties (how the body breaks it
down) that are not sufficient for use as it is. Having read the directions on a
bottle of cough medicine, you may know that it has a short half-life of only
2-3 hours (half of the time before you take another dose). This means the body
metabolizes it too quickly to be of use as an antidepressant because it
prevents sufficient levels from reaching the brain. Anna hypothesized that a modified
version of DM, d6-D,) that is metabolized more slowly would provide
antidepressant effects. She also hypothesized that the antidepressant action
would be mediated by σ1Rs,
similar to unmodified DM. Her study also examined whether the modified molecule
interacts with the original receptors and proteins the same way that DM does.
For the non-pharmacologists out there, you might be
wondering why metabolism matters in this context. DM enters the body and passes
from the stomach to the liver right away. There, it is changed (metabolized) into
dextrorphan, which produces problematic side effects. Adding a less common form
of hydrogen atom, deuterium, to the DM molecule may slow the breakdown of DM
into dextrorphan allowing it to stay active longer and minimizing the side
effects that would limit its use as a medication. Quinidine, a blocker of the
P4502D6 liver enzyme that breaks down DM into dextrorphan, could also slow the
metabolism.
Anna
used three doses of the modified DM molecule, d6-DM (injected into the abdomen),
to see which one would provide the best antidepressant effect. She used the
forced swim test to assess its antidepressant properties. In this test, mice
are placed into a deep (to them) beaker of water. The animals will try to get
out of the water and then give up and just float accepting that they cannot
escape. Administering an antidepressant prior to the test increases the
trying-to-get-out period and with d6-DM the dose at which this effect peaked
was 30 mg/kg.
Mouse locomotion was also examined using the open
field test, described here,
to make sure the increased activity in the water was truly due to an
antidepressant effect rather than just a higher level of activity in general.
They did find a significant increase in activity in mice treated with d6-DM
compared with saline-treated control mice. However, there was no correlation
between mobility in the swim test and stimulant activity in the open field.
Next, Anna treated groups of mice with either saline, d6-DM at
two doses, or d6-DM at the two doses plus quinidine, the liver enzyme inhibitor
that would further slow the metabolism of d6-DM (Table 1). When d6-DM and
quinidine were combined, there was a dose-dependent, but not a significant
difference in immobility in the forced swim test. However, the way the
experiment was designed required the statistical tests to be run on many groups
of mice, which lowers the ability to detect a true statistical difference
(called a Type 1 error). Kudos to Anna and Linda for reporting their stats
based on experimental design! Linda did say that if they ran a separate
statistical analysis on just the quinidine + d6-DM groups and compared them to
the control, the quinidine + d6-DM at the 30 mg/kg dose would be significantly
different. Also important, although further slowing the metabolism of d6-DM
with quinidine did not affect the antidepressant-like effects of d6-DM, it
significantly reduced the stimulant effects in the open field. This reinforces
the idea that the antidepressant effects of d6-DM are not dependent on its
stimulant effects.
Table 1: Experimental Groups
Control
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d6-DM alone
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d6-DM + quinidine
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Saline
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d6-DM, low dose
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quinidine + d6-DM, low dose
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Saline + quinidine
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d6-DM, higher dose
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quinidine + d6-DM, higher dose
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In order to determine if the antidepressant effect was mediated
by activation of σ1Rs,
Anna used the σ1R
antagonists BD1063 and BD1047 to block d6-DM from binding to the receptor. She performed the forced swim test again but did not find a
significant difference with either drug. This means the antidepressant effect
of d6-DM is not due to σ1R
binding. Linda thinks the σ1R
may still play a role, albeit a small one, as there appears to be partial reduction
of the antidepressant effects of d6-DM. It is possible that d6-DM may be using
other mechanisms of action (such as NMDA receptor binding), and that both these
mechanisms (and others not discussed here) are contributing to the
antidepressant effects of d6-DM observed in the forced swim test such that no
one antagonist can significantly inhibit the effects alone.
In future studies, the Matsumoto lab will continue to study how
DM could be modified to provide antidepressant effects and identify the
mechanisms that convey the therapeutic effects.
*Disclaimer: research on dextromethorphan as an antidepressant
is still in the early stages. Please do not try to use cough syrup as a
treatment for depression. If you are depressed, please see a medical doctor for
diagnosis and treatment options.
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