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Pharmacological Reports
2006, 58, 680–691
ISSN 1734-1140
Copyright © 2006
by Institute of Pharmacology
Polish Academy of Sciences
Vinpocetine and piracetam exert antinociceptive
effect in visceral pain model in mice
Omar M.E. Abdel Salam
Department of Pharmacology, National Research Centre, Tahrir St., Dokki, Cairo, Egypt
Correspondence: Omar M.E. Abdel Salam,
e-mail: [email protected]
Abstract:
The effect of vinpocetine or piracetam on thermal and visceral pain was studied in mice. In the hot plate test, vinpocetine (0.9 and
1.8 mg/kg), but not piracetam, produced a reduction in nociceptive response. Vinpocetine (0.45–1.8 mg/kg, ip) or piracetam
(75–300 mg/kg, ip) caused dose-dependent inhibition of the abdominal constrictions evoked by ip injection of acetic acid. The effect
of vinpocetine or piracetam was markedly potentiated by co-administration of propranolol, guanethidine, atropine, naloxone,
yohimbine or prazosin. The marked potentiation of antinociception occurred upon a co-administration of vinpocetine and baclofen
(5 or 10 mg/kg). In contrast, piracetam antagonized antinociception caused by the low (5 mg/kg), but not the high (10 mg/kg) dose of
baclofen. The antinociception caused by vinpocetine was reduced by sulpiride; while that of piracetam was enhanced by haloperidol
or sulpiride. Either vinpocetine or piracetam enhanced antinociception caused by imipramine. The antinociceptive effects of
vinpocetine or piracetam were blocked by prior administration of theophylline. Low doses of either vinpocetine or piracetam
reduced immobility time in the Porsolt’s forced-swimming test. This study indicates that vinpocetine and piracetam possess visceral
antinociceptive properties. This effect depends on activation of adenosine receptors. Piracetam in addition inhibits GABA-mediated
antinociception.
Key words:
vinpocetine, piracetam, thermal pain, visceral pain, mice
Introduction
Piracetam, the first of the pyrrolidone derivatives
(2-oxopyrrolidine), is a widely used drug in the treatment of cerebral stroke [20], dementia or cognitive
impairment in the elderly [41], and myoclonus epilepsy [10]. The drug reverses hippocampal membrane
alterations in Alzheimer’s disease [8]. Vinpocetine,
a synthetic derivative of the alkaloid vincamine having antianoxic, antiischemic and neuroprotective
properties, is frequently used in the therapy of cerebral disorders of vascular origin [11]. Although both
drugs belong to distinct chemical classes, they share
the name nootropic, the term introduced by Giurgea
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[16], to indicate this category of drugs that enhance
memory, facilitate learning and protect memory processes against conditions which tend to disrupt them.
Despite the fact that piracetam and vinpocetine
have been used in clinical practice for around three
decades, their precise mechanism of action within the
brain remains largely unknown. Enhancement of oxidative glycolysis, anticholinergic effects, positive effects on the cerebral circulation, effects on membrane
physics, and also reduction of platelet aggregation
and improvement in erythrocyte function are likely to
account for a cognition-enhancing effect and a neuroprotective effect of piracetam in stroke [36]. On the
other hand, vinpocetine is a potent vasodilator at the
cerebral vascular bed, increasing cerebral blood flow,
Antinociception by vinpocetine and piracetam
Omar M.R. Abdel Salam
that increases the regional cerebral glucose uptake
and, to a certain extent, glucose metabolism in the
so-called peri-stroke region as well as in the relatively
intact brain tissue [40]. Vinpocetine has been found to
interfere with various stages of the ischemic cascade:
adenosine triphosphate (ATP) depletion, activation of
voltage-sensitive Na+- and Ca++-channels, glutamate
and free radicals release [17].
Apart from their cognitive enhancing properties,
both drugs are likely to possess other important pharmacological properties. Gastric protective effects
have been described for vinpocetine [28], whereas piracetam has been suggested to exert anti-inflammatory
properties [27]. Prolonged consumption of piracetam
resulted in a 3-fold decrease in beta-endorphin concentration in plasma of rats [39], but its ip administration (100 mg/kg) inhibited gamma aminobutyric acid
(GABAergic) antinociception caused by baclofen
[13]. Nefiracetam, a new member of the piracetam
group of cognition enhancers, reversed the thermal hyperalgesia observed in nerve-injured mice and the thermal
hyperalgesia observed in streptozotocin-induced diabetic
mice [33].
Vinpocetine is capable of blocking NaV1.8 sodium
channel activity, which suggests a potential additional
utility of the drug in various sensory abnormalities
arising from abnormal peripheral nerve activity [44].
The above data suggest that these nootropic drugs are
likely to modulate nociception and might prove useful
in certain types of pain. In the light of the above and
since the nootropic agents vinpocetine and piracetam
are widely prescribed for the pharmacotherapy of
memory and neurodegenerative disorders, it, therefore, looked pertinent to investigate and compare the
pharmacological effects of piracetam and vinpocetine
in two experimental pain models in mice: the acetic
acid-induced writhing response, a model of visceral
inflammatory pain (chemonociception), and the tailflick test, a model of supraspinal analgesia. Moreover,
the effect of the two drugs on locomotor activity and
their potential antidepressant effect were evaluated.
Materials and Methods
Animals
Male albino Swiss mice 20–22 g of body weight were
used. Standard laboratory food and water were pro-
vided ad libitum. All experimental procedures were
conducted in accordance with the guide for the care
and use of laboratory animals and were reviewed by
the Local Animal Care and Use Committee. The
doses of vinpocetine or piracetam used in the study
were based upon the human dose after conversion to
that of rat [31].
Hot-plate assay
The hot-plate test was performed by using an electronically controlled hot plate (Ugo Basile, Italy)
heated to 52 ± 0.1°C). The cut-off time was 30 s.
Groups of mice (n = 6/group) were given vinpocetine
(0.45, 0.9 or 1.8 mg/kg, sc, 0.2 ml, n = 6/group) or piracetam (75, 150 or 300 mg/kg, sc, 0.2 ml) or saline
(control), 30 min prior to testing. The experimenter
was blind to the dose. Latency to lick a hind paw or
jump out of the apparatus was recorded for the control
and drug-treated groups.
Abdominal constriction assay
Separate groups of 6 mice each were given vinpocetine (0.45, 0.9 or 1.8 mg/kg, ip, 0.2 ml), piracetam (75,
150 or 300 mg/kg, ip, 0.2 ml), indomethacin
(20 mg/kg, ip, 0.2 ml) or saline (0.2 ml, ip, control).
Unless otherwise indicated in the text, drugs or saline
were administered 30 min prior to an ip injection of
0.2 ml of 0.6% acetic acid [22]. Each mouse was then
placed in an individual clear plastic observational
chamber, and the total number of writhes experienced
by each mouse was counted for 30 min after acetic
acid administration by an observer who was not aware
of the animals’ treatment. Afterwards, animals were
sacrificed by CO2 exposure.
Further experiments were designed in an attempt to
elucidate the mechanisms by which vinpocetine or piracetam exerts its anti-nociceptive effect. The dose of
1.8 mg/kg of vinpocetine or 300 mg/kg of piracetam
was then selected to be used in the subsequent experiments.
Thus, we examined the effects of co-administration
of the a-1 adrenoceptor antagonists prazosin (2 mg/kg,
ip) and doxazosin (4 or 16 mg/kg, ip), a-2 adrenoceptor antagonist yohimbine (4 mg/kg, ip), b-adrenoceptor antagonist, propranolol (2 mg/kg, ip), muscarinic
acetylcholine receptor antagonist atropine (2 mg/kg
ip), non-selective opioid receptor antagonist naloxone
(5 mg/kg ip), GABA agonist baclofen (5 or 10 mg/kg)
Pharmacological Reports, 2006, 58, 680–691
681
and ATP-gated potassium channel blocker glibenclamide (5 mg/kg, ip) on antinociception caused by vinpocetine or piracetam. Drugs were administered
30 min prior to the abdominal constriction assay.
The effect of adrenergic neuron blockade with
guanethidine (16 mg/kg, ip) co-administered with either vinpocetine or piracetam was also investigated.
The abdominal constriction assay was carried out
30 min after the administration of vinpocetine or piracetam.
In addition, the effect of the non-selective adenosine receptor antagonist theophylline (20 mg/kg ip) or
the nonsteroidal anti-inflammatory drug indomethacin (20 mg/kg, ip) was examined. Theophylline or
indomethacin was administered 30 min prior to vinpocetine or piracetam (60 min prior to the abdominal
constriction assay).
In further experiments, the effect of coadministration of the centrally acting dopamine D2
receptor antagonists, sulpiride (10 or 20 mg/kg, ip)
and haloperidol (0.5, 1.5 or 3 mg/kg, ip) or D2 receptor agonist bromocryptine (1.5 or 3 mg/kg, ip) on vinpocetine (1.8 mg/kg, ip) or piracetam (300 mg/kg, ip)
antinociception was examined. Lastly, the effect of
vinpocetine or piracetam on antinociception caused
by the tricyclic agent imipramine was studied.
Rota-rod test
Motor performance in mice was measured as the latency to fall from an accelerating rota-rod located
over plates connected to an automatic counter (Ugo
Basile, Varese, Italy). Mice were trained to remain on
a rotating rod for 2 min as the rod rotated toward the
animal. After the 2-min training period, the mice were
administered vehicle (saline) or drugs (sc) and 30 min
later were placed on the rotating rod as it accelerated
from 4 to 40 rpm over 5 min and the time they could
remain on the accelerating rod was recorded [25]. The
cut-off time was 600 s. The time was measured from
the start of the acceleration period. The test was repeated 2 h after vehicle or drug injection. Six animals
were used per dose and for the controls.
Porsolt’s forced-swimming test
In this test, a commonly used tool for screening of potential antidepressants, the floating time, which is
considered as the measure of despair, is decreased by
the administration of antidepressant drugs [32].
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Pharmacological Reports, 2006, 58, 680–691
Briefly, each mouse was placed individually in a glass
cylinder (diameter 12 cm, height 24 cm) filled with
water to a height of 12 cm. Water temperature was
maintained at 24–25°C. The animal was forced to
swim for 6 min and the duration of immobility was
measured. The mouse was considered as immobile
when it stopped struggling and moved only to remain
floating in the water, keeping its head above water
[32].
In two separate sets of experiments, the floating
time was recorded after treatment with different doses
of vinpocetine (0.45–1.8 mg/kg, sc) or piracetam
(37.5–300 mg/kg, sc) and compared to that of imipramine (15 mg/kg, sc). The control group received
saline. In the third set of experiments, the possible
modulation of the antidepressant effect of imipramine
(15 mg/kg, sc) by either vinpocetine (1.8 mg/kg, sc)
or piracetam (300 mg/kg, sc) or the possible effect of
combined vinpocetine (1.8 mg/kg, sc) and piracetam
(300 mg/kg, sc) administration was investigated.
Drugs
Vinpocetine (Vinporal, Amrya. Pharm. Ind., Cairo,
ARE), piracetam (Nootropil, Chemical Industries Development; CID, Cairo, ARE), guanethidine, propranolol hydrochloride, yohimbine hydrochloride, naloxone hydrochloride (Sigma, St. Louis, USA), imipramine hydrochloride, bromocryptine (Novartis
Pharma, Cairo, ARE), amitriptyline hydrochloride,
haloperidol, indomethacin (Kahira Pharm & Chem.
IND Co., Cairo, ARE), glibenclamide (Hoechst Orient, Cairo, ARE), atropine sulfate, baclofen (Misr
Pharm Co., Cairo, ARE), domperidone (JanssenCilag, Switzerland), clozapine (APEX Pharma, Cairo,
ARE) were used. Analytical-grade glacial acetic acid
(Sigma, St. Louis, USA) was diluted with pyrogenfree saline to provide a 0.6% solution for ip injection.
All drugs were dissolved in isotonic (0.9% NaCl) saline solution immediately before use.
Statistical analyses
Data are expressed as the mean ± SE. Differences between vehicle control and treatment groups were
tested using one- and two-way ANOVA followed by
Duncan’s multiple comparison test. When there were
only two groups, a two-tailed Student’s t-test was
used. A probability value less than 0.05 was considered statistically significant.
Antinociception by vinpocetine and piracetam
Omar M.R. Abdel Salam
Results
Effect of vinpocetine or piracetam on thermal
nociception
Hot-plate latency (s)
The mean reaction time in the hot plate test was significantly delayed at 30 min after the administration
of vinpocetine at doses of 0.9 mg/kg or 1.8 mg/kg,
compared with basal (pre-drug) values, denoting decreased nociception (p < 0.05, one way ANOVA). Piracetam given at doses of 75–300 mg/kg failed to affect the nociceptive response (Fig. 1).
Fig. 2. Effect of different doses of vinpocetine (0.45, 0.9 or 1.8 mg/kg),
piracetam (75, 150 or 300 mg/kg), indomethacin (20 mg/kg) or saline
(control) on abdominal constrictions caused by injection of dilute
acetic acid in mice. Drugs or saline (control) were administered
30 min prior to testing. Data are expressed as the mean ± SE and
percent inhibition (%) compared to the control animals. * p < 0.05 .
control, n = 6
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mice in a dose-dependent manner. The degree of inhibition was 42.3, 53, 63.9% for vinpocetine at 0.45, 0.9
or 1.8 mg/kg (Fig. 2A) and 48.1, 51.3, 76.8% for piracetam at 75, 150 or 300 mg/kg, respectively (Fig. 2B).
In comparison, the non-steroidal anti-inflammatory drug
indomethacin (20 mg/kg) decreased the number of abdominal constrictions by 54.9%.
Fig. 1. Effect of different doses of vinpocetine (0.45, 0.9 or 1.8 mg/kg)
or piracetam (75, 150 or 300 mg/kg) in the hot-plate test of thermal
pain. Basal and 30-min latencies were determined for each treatment group (saline, vinpocetine or piracetam). The first column represents the basal (pre-drug) latencies and the second column represents the 30 min- (post-drug) values in each treatment group. Saline
or drugs were administered 30 min prior to testing. Data are expressed as the mean ± SE. Hot-plate latencies comparisons were
made between baseline paw withdrawal latencies (pre-drug values)
as compared with the respective post-drug values. * p < 0.05 . basal (pre-drug value), n = 6
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Effect of vinpocetine or piracetam on visceral
nociception
Either vinpocetine (0.45, 0.9 or 1.8 mg/kg) or piracetam (75, 150 or 300 mg/kg) inhibited the number
of writhes induced by ip acetic acid administration in
Investigation of the mechanism of visceral
antinociception by vinpocetine or piracetam
Figures 3–5 show the effect of the a-2 adrenoceptor
antagonist yohimbine (4 mg/kg) and a-1 adrenoceptor
antagonists prazosin (2 mg/kg) and doxazosin
(16 mg/kg) on the nociceptive responses. The administration of prazosin or doxazosin caused a marked reduction in the number of abdominal constrictions.
The administration of yohimbine or prazosin at the
above-mentioned doses significantly enhanced the
antinociceptive action of vinpocetine or piracetam in
the abdominal constriction assay. When administered
at 16 mg/kg, doxazosin given alone or combined with
either vinpocetine or piracetam markedly inhibited
the abdominal constrictions by 94.8–90.2% (Fig. 4).
To delineate whether there is any synergism between
Pharmacological Reports, 2006, 58, 680–691
683
Fig. 5. Effect of doxazosin (4 mg/kg) on antinociception caused by
vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg) in the abdominal
constriction assay. Drugs or saline (control) were administered
30 min prior to testing. Data are expressed as the mean ± SE, * p < 0.05
compared to control group. The plus sign (+) indicates significant
change from the other treatment groups, n = 6
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Fig. 3. Effect of yohimbine (4 mg/kg, ) or prazosin (2 mg/kg, ) on
antinociception caused by vinpocetine (1.8 mg/kg, ) or piracetam
(300 mg/kg, ) in the abdominal constriction assay. Drugs or saline
(control) were administered 30 min prior to testing. Data are expressed as the mean ± SE and percent inhibition (%) compared to
the control animals. * p < 0.05 . control and between different
groups as shown in the figure. The plus sign (+) indicates significant
difference from the yohimbine-treated group, n = 6
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Fig. 6. Effect of atropine (2 mg/kg) or propranolol (2 mg/kg) on
antinociception caused by vinpocetine (1.8 mg/kg) or piracetam
(300 mg/kg) in the abdominal constriction assay. Drugs or saline
(control) were administered 30 min prior to testing. Data are expressed as the mean ± SE and percent inhibition (%) compared to
the control animals. * p < 0.05 compared to control and between different groups as shown in the figure. The plus sign (+) indicates significant change from the atropine-treated group. The (#) sign indicates significant difference from the vinpocetine + atropine or vinpocetine + propranolol-treated groups, n = 6
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Fig. 4. Effect of doxazosin (16 mg/kg) on antinociception caused by
vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg) in the abdominal
constriction assay. Drugs or saline (control) were administered
30 min prior to testing. Data are expressed as the mean ± SE, * p < 0.05
compared to control group, n = 6
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Pharmacological Reports, 2006, 58, 680–691
Antinociception by vinpocetine and piracetam
Fig. 7. Effect of guanethidine
(16 mg/kg) on visceral antinociception
caused by vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg). Drugs
or saline (control) were administered 30 min prior to testing. Data
are expressed as the mean ± SE and percent inhibition (%) compared to the control animals. * p < 0.05 compared to control and between different groups as shown in the figure. The plus sign (+) indicates significant change from the guanethidine-treated group, n = 6
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doxazosin and either nootropic, doxazosin was administered at a lower dose of 4 mg/kg. Figure 5 shows
that doxazosin at the dose of 4 mg/kg was able to enhance the piracetam-induced antinociception.
The effect of vinpocetine was markedly potentiated
by co-administration of the beta-adrenergic antagonist
propranolol or muscarinic receptor antagonist atropine, with 99.5% inhibition of the abdominal constriction response. Propranolol or atropine also enhanced
the piracetam effect (Fig. 6). Antinociception caused
by vinpocetine or piracetam was, in addition, enhanced by co-administration of the adrenergic neuron
blocking agent guanethidine. Guanethidine by itself
elicited a significant reduction in the number of abdominal constrictions as compared to the control
group (Fig. 7).
Potentiation of antinociception caused by vinpocetine or piracetam was also observed when the nonspecific opioid receptor antagonist naloxone was administered along with the nootropics. Naloxone administered alone increased the writhing response (Fig.
8A). The potassium channel blocker gibenclamide administered ip 30 min prior to testing inhibited
abodominal constrictions but did not affect antino-
Glibenclamide + Piracetam
Glibenclamide 5 mg/kg
Glibenclamide + Vinpocetine
N
N
Omar M.R. Abdel Salam
Fig. 8. (A) Effect of naloxone (5 mg/kg) on antinociception caused by
vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg) in the abdominal
constriction assay. * p < 0.05 . control. The plus sign (+) indicates
significant change from the naloxone-treated group. The (#) sign indicates significant difference from the vinpocetine + naloxone or piracetam + naloxone-treated groups. (B) Effect of gibenclamide
(5 mg/kg) on visceral antinociception caused by vinpocetine
(1.8 mg/kg) or piracetam (300 mg/kg). * p < 0.05 compared to control. The plus sign (+) indicates significant change from the
vinpocetine-treated group. Drugs or saline (control) were administered 30 min prior to testing. Data are expressed as the mean ± SE
and percent inhibition (%) compared to the control animals, n = 6
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ciception by vinpocetine or piracetam (Fig. 8B). The
non-steroidal anti-inflammatory drug indomethacin
given 30 min prior to vinpocetine or piracetam increased the antinociceptive response (Fig. 9). The vinpocetine- or piracetam-induced antinociception was,
however, inhibited by pretreatment with the nonselective adenosine receptor antagonists theophylline
administered at the dose of 20 mg/kg (Fig. 9).
The antinociception induced by the nootropic drugs
vinpocetine and piracetam was also examined in the
presence of the dopamine D2 receptor antagonists,
sulpiride and haloperidol or the dopamine D2 receptor
agonist bromocryptine. As shown in Figure 10, the
antinociception caused by vinpocetine was reduced
by sulpiride; while that of piracetam was enhanced by
sulpiride. The administration of either vinpocetine or
piracetam with haloperidol at 1.5 or 3 mg/kg, almost
abolished the writhing response. However, haloperidol administered at 1.5 or 3 mg/kg by itself inhibited
the writhing response by 93.1 and 98.4%, respecPharmacological Reports, 2006, 58, 680–691
685
Fig. 9. Effect of theophylline
(20 mg/kg) or indomethacin (20 mg/kg)
on antinociception caused by vinpocetine (1.8 mg/kg) or piracetam
(300 mg/kg). Theophylline or indomethacin was administered ip
30 min prior to vinpocetine or piracetam (60 min prior to the abdominal
constriction assay). Data are expressed as the mean ± SE and percent
inhibition (%) compared to the control animals. * p < 0.05 compared to
control and between different groups as shown in the figure, n = 6
Fig. 10. Effect of sulpiride (10 or 20 mg/kg) and haloperidol (1.5 or
3 mg/kg) on antinociception caused by vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg). Drugs or saline (control) were administered
30 min prior to testing. Data are expressed as the mean ± SE and percent inhibition (%) compared to the control animals. * p < 0.05 compared to control. The plus sign (+) indicates significant change from
the sulpiride (20 mg/kg)-treated group. The (#) sign indicates significant difference from the sulpiride (10 mg/kg)-treated group, n = 6
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Pharmacological Reports, 2006, 58, 680–691
Fig. 11. Effect of haloperidol (0.5 mg/kg) on antinociception caused
by vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg). Drugs or saline
(control) were administered 30 min prior to testing. Data are expressed as the mean ± SE and percent inhibition (%) compared to
the control animals. * p < 0.05 compared to control. The plus sign (+)
or the sign (#) indicates significant change from the other treatment
groups, n = 6
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Fig. 12. Effect of bromocryptine (1.5 or 3 mg/kg) on visceral antinociception caused by vinpocetine (1.8 mg/kg) or piracetam
(300 mg/kg). Drugs or saline (control) were administered 30 min
prior to testing. Data are expressed as the mean ± SE and percent inhibition (%) compared to the control animals. * p 0.05 compared to
control. The plus sign (+) indicates significant change from the piracetam or bromocryptine (1.5 mg/kg)-treated groups, n = 6
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Antinociception by vinpocetine and piracetam
Omar M.R. Abdel Salam
Effect of vinpocetine or piracetam in rota-rod test
Vinpocetine (0.45, 0.9 or 1.8 mg/kg) or piracetam (75,
150 or 300 mg/kg) did not produce any significant
changes on the rota-rod performances of the mice.
Both controls and either vinpocetine- or piracetamtreated mice remained on accelerating rotating rod
during the acceleration period (5 min) and for 5 min
thereafter (data not shown).
Effect of vinpocetine or piracetam in Porsolt’s
forced-swimming test
Effect of baclofen (5 mg/kg) on antinociception caused
by vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg). * p < 0.05 compared to control. The plus sign (+) indicates significant change from
the piracetam + baclofen (5 mg/kg)-treated group. The (#) sign indicates significant difference from the vinpocetine-treated group, n =
6. (B) Effect of imipramine (15 mg/kg) alone or co-administered with
either vinpocetine (1.8 mg/kg) or piracetam (300 mg/kg) or vinpocetine (1.8 mg/kg) + piracetam (300 mg/kg) on visceral nociception. * p
< 0.05 compared to control. The plus (+) sign indicates significant
difference from the vinpocetine + piracetam-treated group, n = 6.
Drugs or saline (control) were administered 30 min prior to testing.
Data are expressed as the mean ± SE and percent inhibition (%)
compared to the control animals, n = 6
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tively, vs. the saline-treated control group (Fig. 10).
Thus, the effect of a lower dose of haloperidol
(0.5 mg/kg, ip) was examined. Figure 11 shows that
the piracetam effect was enhanced by a lower dose
(0.5 mg/kg) of the dopamine D2 receptor antagonist
haloperidol. On the other hand, the administration of
the D2 receptor agonist bromocryptine (1.5 or 3 mg/kg)
reduced the number of abdominal constrictions in
a dose-dependent manner by 30.1 and 61.3%, respectively (Fig. 12). A marked and significant antinociception was still observed when either nootropic
was co-administered with bromocryptine (Fig. 12).
Figure 13A shows that marked potentiation of
antinociception occurred upon a co-administration of
vinpocetine and GABA agonist baclofen (5 or 10 mg/kg).
In contrast, piracetam antagonized antinociception
caused by the low (5 mg/kg), but not the high
(10 mg/kg) dose of baclofen (Fig. 13A).
When we examined the effect of vinpocetine or piracetam on the imipramine-induced inhibition of the
writhing response, we observed that either nootropic enhanced antinociception caused by imipramine (Fig. 13B).
Duration of immobility (s)
Fig. 13. (A)
The floating time, was reduced by 19.9 and 20.8% after treatment with low doses of vinpocetine
(0.45 mg/kg) or piracetam (37.5 mg/kg), respectively,
but was markedly reduced by 63.4% after treatment
with imipramine (15 mg/kg) (Fig. 14A, B). The coadministration of either vinpocetine (1.8 mg/kg) or pi-
Fig. 14. Porsolt’s forced-swimming test. The graph shows three sets
of experiments. In parts A & B the effect of different doses of
vinpocetine (0.45–3.6 mg/kg) or piracetam (37.5–300 mg/kg) were
investigated and compared to that of imipramine (15 mg/kg) or
saline. * p < 0.05 compared to saline control. The plus (+) sign
indicates significant difference from the imipramine-treated group.
The (#) sign indicates significant difference from the vinpocetine
(0.45 mg/kg)-treated group. In part C the effect of combined
administration of either vinpocetine (1.8 mg/kg) or piracetam
(300 mg/kg) with imipramine (15 mg/kg) or the effect of vinpocetine
(1.8 mg/kg) combined with piracetam (300 mg/kg) was studied.
* p < 0.05 compared to saline control. The plus (+) sign indicates
significant difference from the vinpocetine + piracetam-treated
group. Data are expressed as the mean ± SE and percent inhibition
(%) compared to the control animals, n = 6
Pharmacological Reports, 2006, 58, 680–691
687
racetam (300 mg/kg) with imipramine (15 mg/kg) had
no significant effect on the antidepressant effect of
imipramine (Fig. 14C). Meanwhile, a decrease in immobility time by 31.8% was noted in mice treated
with a combination of vinpocetine (1.8 mg/kg) and piracetam (300 mg/kg), suggesting an additive antidepressant effect of either nootropic (Fig. 14C).
Discussion
The present study provides evidence that the
nootropic drugs vinpocetine and piracetam exert
pain-modulating effects. In the hot-plate test of thermal pain, vinpocetine showed antinociceptive effect.
Vinpocetine is capable of blocking NaV1.8 sodium
channel activity, which is likely to be responsible for
the analgesic properties of the drug [44]. In this model
of supraspinal analgesia, piracetam failed to alter nociception. In another study, nefiracetam, a new member of the piracetam group of cognition enhancers, reversed the thermal hyperalgesia in mouse models of
neuropathic pain [33]. It should be noted, however,
that members of the 2-oxopyrrolidine group though
structurally close to piracetam differ markedly in their
pharmacological properties and clinical effects.
On the other hand, either vinpocetine or piracetam
clearly inhibited the abdominal constrictions induced
in mice by injection of dilute acetic acid into the peritoneal cavity, a model of visceral inflammatory pain,
brought about by the release of prostaglandins [2].
Vinpocetine and piracetam did not impair motor performance in the rota-rod test, thus ruling out the confounding influence of a possible sedative effect. The
inhibition of adrenergic, cholinergic and opioidergic
systems appears to facilitate vinpocetine- or piracetam-induced antinociception.
The induction of pain behaviors in animals relies
upon a stimulus applied to a nociceptive neuron and
the activation of a pain pathway and reflexes. Several
drugs induce analgesia or antinociception by interfering with the neuronal pathways involved in the receipt
and transmission of nociceptive information from the
periphery to higher centers in the central nervous system. In this context, the spinal cholinergic system and
muscarinic receptors are important for modulation of
nociception. Spinal muscarinic agonists, such as carbachol and the cholinesterase inhibitor neostigmine,
688
Pharmacological Reports, 2006, 58, 680–691
induce a potent analgesia in the rat; spinal M1 and/or
M3 receptor subtypes [26] likely mediate these effects. Activation of spinal muscarinic receptors inhibits dorsal horn neurons through inhibition of the glutamatergic synaptic input [23] and potentiation of synaptic GABA release in the spinal cord [43]. Also,
M1-muscarinic agonists increased the pain threshold
in the mouse acetic acid writhing test and in rat paw
pressure test [1]. In contrast, atropine, a cholinergic
muscarinic antagonist, has been reported to produce
analgesia when administered at low doses of 1–100 µg/kg,
while hyperalgesia was observed with 5 mg/kg of the
agent. The analgesia caused by very low doses of atropine was attributed to amplification of cholinergic
transmission by a selective blockade of presynaptic
muscarinic autoreceptors [15]. In the present study,
atropine administered ip at 2 mg/kg increased the
number of writhes in the abdominal constriction assay, yet amplified the antinociceptive action of vinpocetine or piracetam.
The sympathetic nervous system has also been implicated in the modulation of nociceptive transmission. Certain pain conditions involve the sympathetic
nervous system, e.g., visceral pain due to abdominal
and pelvic cancers; ischemic pain from peripheral
vascular disease, arterial spasm or frostbite and others. Sympathetcomy or intravenous regional sympathetic block with guanethidine can be carried out to
reduce pain [35]. Afferent nociceptive fibers from the
viscera accompany sympathetic nerves to enter the
spinal cord at the dorsal horn. Transmission of pain
impulses through the spinal cord can be inhibited by
descending reflexes that originate in the noradrenergic neurons of the mesencephalic periductal gray matter, and the pontine locus ceruleus, thereby altering
ascending transmission mediating visceral sensation
[4]. Behavioral, neurophysiological and clinical evidence shows that most forms of gastrointestinal pain
are mediated by activity in visceral afferent fibers
running in sympathetic nerves [5]. Sympathetic block
interrupts these pathways and also the efferent
viscero-visceral reflexes so that ischemia and spasm
are relieved [9]. Voltage-gated sodium channels
(Nav1.7) have been found predominantly in sensory
and sympathetic neurons [42]. Sympathectomy produced in rats by administering 6-hydroxydopamine
reduced spontaneous activity of sacral spinal cord
neurons and attenuated visceral nociceptive responses
due to colorectal distension [19]. Duarte et al. [7] observed a significant inhibition of writhing in the acetic
Antinociception by vinpocetine and piracetam
Omar M.R. Abdel Salam
acid abdominal constriction assay with the adrenergic
neuron blocker guanethidine (27% at 30 mg/kg, sc).
In the present study, a 30% reduction in the nociceptive response was observed with 16 mg/kg of ip
guanethidine. The latter is taken up into adrenergic
neurons, where it binds to the storage vesicles and
prevents release of the neurotransmitter in response to
a neuronal impulse, which results in generalized decrease in sympathetic tone. In the present study,
guanethidine potentiated antinociception when coadministered with either nootropic. A decrease in
writhing response was also observed after a nonselective beta-adrenoceptor antagonist propranolol.
The latter enhanced antinociception elicited by piracetam, whereas its administration with vinpocetine
almost abolished the writhing response. Based upon
these results, it might be suggested that a decrease in
sympathetic tone facilitates antinociception caused by
vinpocetine or piracetam.
Descending brainstem systems influence nociceptive stimuli spinally while other systems modulate nociceptive input supraspinally. The administration of
a-2 adrenoceptor agonists produces anti-nociception
in rodents by inhibiting synaptic transmission in the
spinal cord dorsal horn and there is an evidence of descending noradrenergic system, the stimulation of
which results in the activation of spinal a-2-adrenergic
receptors and antinociception [37]. Visceral nociception induced by ip administered 2% solution of acetic
acid was reduced dose-dependently by prazosin, an
a-1-adrenoceptor antagonist and clonidine, an a-2adrenoceptor agonist [21]. Results of the present
study indicated that visceral nociception was markedly inhibited by the administration of a-1 antagonists. The administration of a-1 or a-2 antagonists
amplified the vinpocetine or piracetam antinociception. Piracetam exerts GABAB antagonistic effect
[13]. There is evidence to suggest that spinally projecting noradrenergic A7 neurons in the dorsolateral
pontine tegmentum may be under tonic inhibitory
control by GABA neurons [29]. Removal of inhibitory GABAB mechanisms by piracetam might thus facilitate a descending noradrenergic pathway so as to
alter nociception. This hypothesis, however, cannot
explain the potentiating effect of a-1 or a-2 antagonists on vinpocetine antinociception.
Unexpected also was the finding that the visceral
antinociceptive effects of vinpocetine or piracetam
were enhanced by co-administration the opioid receptor antagonist naloxone. The latter increased the no-
ciceptive behavior in the abdominal constriction assay
when administered alone and was thus expected to reduce antinociception by the two nootropic drugs.
Voltage-gated potassium channels can also contribute
to the sensitization of primary afferents observed in
gastrointestinal pain states [6]. The possible involvement of ATP-gated potassium channels in the mediation of antinociception induced by vinpocetine or piracetam is ruled out, in view of the inability of glibenclamide, an ATP-sensitive potassium channels
blocker to reduce their antinociceptive effect.
Adenosine is an inhibitory neuromodulator that can
increase nociceptive thresholds in response to noxious
stimulation [34] and blockade of adenosine receptors
by theophylline, a non-selective adenosine receptor
antagonist at A1 and A2 was shown to induce hyperalgesia [30]. The antinociceptive effects of vinpocetine
or piracetam were reversed by prior administration of
the adenosine receptor blocker theophylline. Therefore, it is suggested that an adenosine sensitive
mechanism is likely to be responsible for the visceral
antinociceptive properties of both nootropic drugs.
Baclofen, a prototypical agonist of GABAB receptors, alters nociception at the level of the spinal cord
by acting on GABAB receptors located on primary afferent terminals and is known to produce analgesia in
man and animals [18]. In the present work, baclofen
markedly inhibited the nociceptive response in the abdominal constriction assay. The drug potentiated
antinociception elicited by vinpocetine. In contrast,
piracetam antagonized antinociception caused by the
low dose (5 mg/kg), though not the high dose
(10 mg/kg) of baclofen. The prevention of baclofen
antinociception is in accordance with other studies.
Piracetam (and also aniracetam) antagonized GABAergic antinociception caused by baclofen [13].
Dopamine D2 receptors are involved in nociceptive
and analgesic mechanisms [12, 38] and there is evidence to suggest that dopamine may acts tonically in
the cortex to inhibit nociception [3]. In the present
study, haloperidol itself dose-dependently inhibited
the nociceptive behavior. In contrast, sulpiride, a highly
selective D2-dopamine receptor antagonist, increased
the nociceptive response. It is likely that different
pharmacological receptor binding profiles of the two
antagonists might be associated with different effects
on visceral nociception. Haloperidol is a partially selective dopamine D2 receptor antagonist [24]. The involvement of other dopamine receptors as well is
likely to account for the antinociceptive effect of ha-
Pharmacological Reports, 2006, 58, 680–691
689
loperidol seen in the present study. Haloperidol at
0.5 mg/kg potentiated antinociception caused by piracetam. On the other hand, sulpiride reduced antinociception caused by vinpocetine, but enhanced that of
piracetam. Blockade of D2 receptors thus potentiates
piracetam antinociception, but might reduce vinpocetine antinociception. These results suggest alteration
of dopaminergic neurotransmission by modifying the
D2-receptors which can alter antinociception induced
by the two nootropic drugs.
Antidepressant drugs such as clomipramine or imipramine can also modulate visceral nociception [14].
Data from the present study indicate that vinpocetine
or piracetam are likely to enhance visceral antinociception caused by imipramine. Interestingly, when
examined in the forced-swimming test in mice,
a commonly used tool for screening of potential antidepressants [32], low doses of vinpocetine and also
piracetam displayed antidepressant-like activity.
In summary, vinpocetine, but not piracetam reduced thermal nociception. The two nootropics, however, displayed marked antinociceptive effect on visceral pain caused by ip injection of acetic acid in
mice. This effect depended on the activation of adenosine receptors. Cholinergic muscarinic blockade with
atropine, adrenorecptor blockade or opioid antagonism with naloxone potentiated vinpocetine or
piracetam-induced antinociception. An interaction at
the level of dopamine D2 receptors by the two
nootropic agents is also suggested.
Acknowledgment:
The author acknowledges Wael Zedan and Mohamed Abd-Elaziz
for their technical assistance in the experiments.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
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Received:
April 6, 2006; in revised form August 22, 2006.
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