The Anxiolytic Effect of the Neuropeptide Cycloprolylglycine
Is Mediated by AMPA and TrkB Receptors
Corresponding Member of the RAS T. A. Gudashevaa,
Cyclo-L-prolylglycine (CPG) was designed at the
Zakusov Research Institute of Pharmacology as a
topological analogue and presumable endogenous
peptide prototype of the classical nootropic piracetam
[1]. In 1996, CPG was indeed identified in the rat
brain as an endogenous compound at a concentration
of ∼10–6 M [2]. Recently, CPG was also found in
human blood plasma and cerebrospinal fluid at concentrations of ∼10–7 M [3]. It is assumed that CPG is
formed in the body from the tripeptide Gly-Pro-Glu,
an N-terminal metabolite of the insulin-like growth
factor [4].
In animal experiments, CPG exhibits pharmacological effects characteristic of piracetam, though at
doses 2–3 orders of magnitude lower. In particular,
the nootropic [1, 2], anxiolytic [5], neuroprotective
[4], analgesic [6], and antidepressant [7] activities of
this cyclodipeptide were detected. In 2016, we found
[8] that CPG increases AMPA currents in isolated rat
cerebellar Purkinje cells and, thus, exhibits the properties of a positive modulator of glutamate AMPA
receptors. It is known that piracetam and some other
racetams affect AMPA receptors and mediate the
deceleration of desensitization and deactivation of the
latter [9]. In addition, using neuronal cell cultures, it
was shown [10] that CPG increases the level of the
brain-derived neurotrophic factor (BDNF), which is
characteristic of positive AMPA receptor modulators
[11]. The stimulation of BDNF synthesis by CPG was
also observed in experiments in vivo on a rat model of
Alzheimer’s disease induced by intracerebral injection
of β-amyloid, where the neuroprotective effect of
CPG was accompanied by an increase in the BDNF
mRNA level and activation of the ERK signaling pathway [12].
These data suggest that the pharmacological effects
of CPG are determined by the stimulation of the
BDNF/TrkB signaling pathway, mediated by the activation of AMPA receptors. To test this hypothesis, in
this work we studied the effect of the AMPA receptor
blocker DNQX and the TrkB receptor blocker K252A
on the anxiolytic activity of CPG in the elevated plus
maze (EPM) test.
The study was performed on 80 Balb/C male mice
weighing 20–22 g.
CPG was dissolved in distilled water and injected
into the mice intraperitoneally (ip) at doses of
1.0 mg/kg (experiment with K252A) or 0.1 mg/kg
(experiment with DNQX) 15 min before the EPM test.
CPG doses of 1.0 mg/kg and 0.1 mg/kg were selected
on the basis of preliminary experiments as the most
active, respectively, under low scattered light (illumination of open arms, 10–15 lux) and bright illumination of
open arms (400–500 lux). Compound K252A was dissolved in 0.1% DMSO in saline and was injected ip at a
dose of 25 μg/kg [13] immediately before injecting
CPG. Compound DNQX was dissolved in 1% DMSO
in saline and injected ip at a dose of 10 mg/kg [14]
15 min before injecting CPG.
The AMPA receptor blocker DNQX at a dose of
10 mg/kg did not affect the behavior of mice in the
EPM test (Table 1). CPG in the EPM test variant with
a brightly illuminated arena had a statistically signifiBIOCHEMISTRY, BIOPHYSICS,
AND MOLECULAR BIOLOGY
a Zakusov Research Institute of Pharmacology,
Moscow, Russia
*e-mail: [email protected]
DOKLADY BIOCHEMISTRY AND BIOPHYSICS Vol. 493 2020
THE ANXIOLYTIC EFFECT OF THE NEUROPEPTIDE CYCLOPROLYLGLYCINE 191
cant effect, judging by the two most important parameters characterizing the anxiolytic activity, increasing
both the spent time and the percentage of time spent
in the open arms. DNQX injected together with CPG
prevented the manifestation of the anxiolytic activity
of CPG by both parameters (Table 1). Thus, AMPA
receptors are involved in the pharmacodynamic
mechanisms of the anxiolytic activity of CPG.
The TrkB blocker, compound K252A, in the EPM
test at a dose of 25 kg/kg did not change the time spent
in the open arms, the percentage of time spent in the
open arms, the number of visits to the open arms, and
the percentage of visits to open arms compared to the
control. CPG under conditions of a moderately illuminated arena showed anxiolytic activity by each
parameter. After the addition of K252A, the anxiolytic
activity of CPG was not detected (Table 2); therefore,
the BDNF/TrkB signaling pathway is involved in the
formation of the anxiolytic activity of CPG.
Thus, the results of this study confirmed the
involvement of the TrkB-dependent mechanisms
mediated by the activation of AMPA receptors in the
effect of CPG, which is typical for ampakines. The
data obtained allow us to characterize CPG as an
endogenous ampakine.
FUNDING
This work was supported by the Russian Foundation for
Basic Research (project no. 20-015-00102 “Identification
of the Role of Glutamatergic and BDNFergic Mechanisms
in the Pharmacological Effects of the Neuropeptide
Cycloprolylglycine”).
COMPLIANCE WITH ETHICAL STANDARDS
Conflict of interest. The authors declare that they have no
conflict of interest.
Statement on the welfare of animals. In experiments, we
complied with the requirements formulated in Order no.
199 of the Ministry of Health of the Russian Federation
“On Approving the Rules of Good Laboratory Practice”
and in the Decision of the EEC Council no. 81 “On
Approving the Rules of Good Laboratory Practice of the
Eurasian Economic Union in the Sphere of Medicines.” All
manipulations with animals were approved by the Bioethical Commission of the Zakusov Research Institute of Pharmacology.
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Table 1. AMPA receptor blocker DNQX completely eliminates the anxiolytic effects of CPG in EPM test
Data are represented as the mean value and the standard error of the mean. * p < 0.05 compared to the control, # p < 0.05 compared to
Table 2. Neurotrophin tyrosine kinase receptor blocker K252A completely eliminates the anxiolytic effects of CPG in EPM
Data are represented as the mean value and the standard error of the mean. * p < 0.05 compared to the control, # p < 0.05 compared to
DOKLADY BIOCHEMISTRY AND BIOPHYSICS Vol. 493 2020
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