Laboratory of
Pathophysiology, Center of Mental Health,
Russian Academy of Medical Sciences, Moscow,
Russia.
Abstract
Tripeptide Pro-Gly-Pro interacted with
dopamine receptors in vitro and reduced
behavioral manifestations of apomorphine-induced
hyperfunction of the dopamine system in
verticalization, stereotypy, and yawning tests.
Presumably, the behavioral effects of
Pro-Gly-Pro tripeptide were mediated through
post- and presynaptic D(2)and
D(3)receptors.
Glyprolines is a recently discovered family
of bioactive peptides consisting of glycine
(Gly) and proline (Pro). One of them,
Pro-Gly-Pro tripeptide, redu ces blood clotting
and inhibits ulcer formation in the sto mach
[8]; its preclinical studies have been
carried out. The effects of PGP on CNS activity
were de monstrated in animal experiments,
primarily the stress-protective effect. The drug
prevents increase in anxiety and suppression of
orientation and exploratory ac ti vity of
stressed rats [5]. Presumably, the
direct effects of glyprolines on the CNS
structures involved in the formation of response
to stress play the key role in these effects.
These characteristics are assumed to be
essential for the realization of other effects
of glyprolines, for example, their
antiulcerogenic effect on the gastric mucosa
under conditions of stress inducing ulcer
formation [8]. In addition, PGP
exhibited a neuroprotective effect in vitro on
nerve cell culture under conditions of oxidative
stress [6] and in vivo on hypoxic
animals [9]. Presumably, the effect of
PGP on hypoxic resistance is due to blood fl ow
stimulation and more rapid saturation of tissues
with oxygen as a result of glyproline
preinjection. Incorporation of glyproline in the
C-terminal structure of other bioactive peptides
improves the resistance of the resultant
molecule to proteases, stimulates and even
modulates the physiological effects of the
initial peptide, as exemplifi ed by such drugs
as Semax (Met-Glu-His-Phe-Pro-Gly-Pro) with
marked nootropic and neurotropic effects and
Selanc (Thr-Lys-Pro-Arg-Pro-Gly-Pro) anxiolytic.
Despite great variety of detected forms of
biological activities of glyprolines, the
neurochemical mechanism of their action received
little attention. We studied the interactions of
PGP with the dopamine system at the behavioral
and receptor levels.
RESULTS
The curves of 3H-spiperone exclusion from
sites of its specifi c binding to the membrane
fraction of striatal cells, where the density of
D2 dopamine receptors is maximum, with sulpiride
(selective ligand of these receptors) and PGP
are presented in Figure 1. The curve of
3H-spiperone exclusion with sulpiride reaches a
plateau without attaining 50% binding level.
This indicates the presence of receptors other
that D2 in the striatum binding spiperone. The
maximum exclusion of 3H-spiperone with PGP
reaches 90%, 50% exclusion being attained at
peptide concentration of 9 nM.
Hence, PGP presumably interacts not only
with D2, but also with some other spiperone
receptors and its affi nity for them is by an
order of magnitude lower than that of spiperone.
Spiperone is a nonselective ligand of 5-HT1 and
5-HT2 serotonin receptors, D2 dopamine receptors
(D2, D3, D4), and of ?1 adrenoceptors. In our
study under conditions optimal for 3H-spiperone
reactions with 5-HT2 receptors in the frontal
cortex (the brain site with the highest density
of these receptors [10]), PGP in
concentrations up to 100 ?M virtually did not
modify binding of 3H-spiperone. Independent
experiments also showed that tritium-labeled PGP
specifi cally bound to the basal nuclear
membranes, but not to other compartments of rat
forebrain [4].
The results suggest that PGP reacts with D2
dopamine receptors and adrenoreceptors. This is
confi rmed by the data on the capacity of PGP
and its fragments to reduce the tone of the
aorta, elevated by norepinephrine [2].
PGP in a dose of 10 mg/kg reduced behavioral
manifestations of drug-induced hyperactivation
of the dopamine system in vivo. This effect was
observed in the verticalization test refl ecting
the increase of dopaminergic transmission in the
cerebral mesolimbic structures induced by
injection of high doses of apomorphine (5 mg/kg;
Table 1). Apomorphine in a medium dose (0.75
mg/kg) stimulated dopaminergic transmission in
the nigrostriatal system, thus causing
stereotypy. Under these conditions PGP in a dose
of 10 mg/kg decreased the duration and number of
stereotypical movements in rats by 20%.
It is assumed that psychomimetics stimulate
motor and stereotypical activities of
experimental animals through postsynaptic D1 and
D2 dopamine receptors, while D3 receptor
stimulation inhibits locomotion [12,13].
On the other hand, it was shown that the yawning
phenomenon is primarily caused by stimulation of
presynaptic D3 receptors, while D2 autoreceptors
mediate its inhibition [11]. The PGP
reduced the number of yawns by 35% in rats
(Table 1), this indicating blocking of
presynaptic D3 receptors. Hence, our study
detected the PGP capacity to react with D2
dopamine receptors in vitro and block
apomorphine reactions with post- and presynaptic
dopamine receptors of D2 and D3 types in vivo.
Opposite effects me dia ted by these receptors
can be responsible, along with other factors,
for just 20-30% realization of behavioral
effects of PGP.
Very low efficiency of PGP in vivo can be
also due to its pharmacokinetics. It enters
unchanged into the circulation and crosses the
blood-brain barrier [3]. However, just
one thousandth of the injected peptide reaches
the brain and is gradually hydrolyzed with the
formation of GP as the main metabolite
[1]. The neurotropic effects of this
dipeptide on stressed animals are no less
pronounced than the effects of PGP [5].
GP is the main metabolite of Selanc, a synthetic
peptide anxiolytic [1]. We also observed
a depriming effect of Selanc in verticalization
test, but this peptide did not modify
3H-spiperone binding to D2 receptors in vitro
[7]. Summing up these data, we think
that not only PGP, but also its GP fragment are
characterized by dopamine-blocking effects.
It is assumed that the development of
positive symptoms in schizophrenia is associated
with hyperactivation of the dopamine system,
while blocking of dopamine receptors is the main
mechanism of pharmacological effect of
neuroleptics. PGP unites the characteristics of
a D2 receptor antagonist with the
neuroprotective [6,9] and nootropic
[5] effects, and hence, can be regarded
as a candidate antipsychotic drug effective for
therapy of positive symptoms and cognitive
disorders in schizophrenia and other
psychoses.
