Department de farmacologica,
Universidad federal de Santa Catarina,
Florianoplois, Brazil
Abstract : The effect of systemic
administration of the cannabinoid antagonist SR
141716A
(N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide
) on penile erection and yawning induced
by apomorphine was investigated in rats.
SR 141716A (2 mg/kg, i.p.) administered 40
min before apomorphine (40 and 80 microg/kg,
s.c.) increased the number of penile erection
and yawning responses. The administration of
cannabinoid agonist Delta9-tetrahydrocannabinol
(1.25 mg/kg, i.p.) 15 min before apomorphine (40
and 80 microg/kg, s.c.) did not affect penile
erection, however it decreased yawning. The
present results provide additional evidence that
cannabinoid agonists interfere with dopaminergic
systems and that SR 141716A together with a
dopaminergic agonist could be useful to
potentiate dopaminergic activity.
Penile erection is a behavior of pivotal
importance in mammalian reproduction, the
physiology of which involves a complex cascade
of neuronal and neuroanatomical events. Yawning
is a reflex or stereotyped event exhibited by
all mammals and vertebrates. Although its
mechanisms and physiological role are not
entirely understood, it bas been used as a tool
to study physiological responses to and
mechanisms of action of some drugs and hormones.
Penile erection and yawning are behavioral
features that are under the control of several
neurotransmitters and neuropeptides:
acetylcholine, serotonin, dopamine, GABA,
oxitocine, adrenocorticotrophin, opioid peptides
and nitric oxide. Both behaviors can occur
concomitantly under some conditions, and among
them the administration of dopamine receptor
agonists is certainly one of the most effective.
Apomorphine, a nonselective dopamine agonist,
bas been extensively studied in animals,
primarily rodents, and is a prototypical
dopaminergic initiator of erectile pathways, as
well as yawning pathways in the brain.
Accordingly, low doses of apomorphine elicit
yawning and penile erection responses that may
be mediated by a prefèrential activation
of presynaptic dopamine receptor D2, which
result in inhibition of dopamine release and a
decrease of its synthesis.
Recent studies showed that administration of
cannabinoid agonists is able to inhibit both
behaviors induced by dopaminergic agonists. The
colocalization of cannabinoid and dopamine in
those brain areas which have a prominent role in
regulating psychomotor activities has stimulated
research in the field. Therefore, because of the
possibility of interaction between
endocannabinoids and dopaminergic systems for
the modulation of behavior, in the present study
we examined the action of the cannabinoid
antagonist SR 141716A (N-(piperidin- 1
-yl)-5-(4-chlorophenyl)-4-methyl- 1
H-pyrazole-3-carboxyamide) in penile erection
and yawning induced by apomorphine.
Male Wistar rats (250-300 g, 9-10weeks old)
were maintained in groups in the colony room of
the Department of Pharmacology under a
light/dark cycle of 12 h (lights on at 7:00
a.m.) with food and water ad libitum. On the day
of the experiment the animals were brought to
the laboratory and placed in individual wire
cages. During the 30 min period immediately
following the administration of apomorphine
(Sigma; 20, 40 or 80 µg/kg, s.c., diluted
in saline with 0.1% Tween-80, vehicle) or
control solution, the number of yawns and penile
erections was counted. Penile erection was
scored when the glans emerged from the prepucial
sheath, an action that was usually accompanied
by intense penile grooming and slight hip
movements.
Yawning was scored when the rat
sustained an open mouth for more than a second.
Control solution and Æ9-tetrahydrocannabinol
(Æ9-THC) (obtained from the National Institute
of Drug Abuse, Bethesda, MD, at 200 mg/ml in
alcohol; 1.25 mg/kg, i.p., prepared immediately
before use by evaporating the alcohol and
emulsifying the residue with Tween-80) were
administered 15 min before control solution or
apomorphine, and SR 141716A (SanofiSynthelabo,
Montpellier, France; 2.0 mg/kg, i.p., suspeniled
in 1 ml/kg saline with 0. 1 % Tween-80) was
injected 40 min before the first treatment. All
measurements were taken in the afternoon,
beginning at 2:00 p.m. The applied doses were
chosen from previous experiments. Control
solution was prepared with the corresponding
vehicle. AU substances were administered at 1
ml/kg.
It's shows the effects of different
pretreatments on apomorphine-induced penile
erection (top) and yawning (bottom). Two-way
ANOVA on the number of erections showed a
significant effect for pretreatment (F(2,75) =
12.74, P < 0.00001), a significant effect for
treatment (ffl,75) = 24.87, P < 0.000001) and
a significant interaction between these factors
(F(6, 75) = 3.99, P < 0.001). SR 141716A
potentiated the penile erection induced by 40
µg/kg apomorphine (P < 0.001,
NewmanKeuls test). Pretreatment with Æ9-THC
(1.25 mg/kg) did not inhibit the penile erection
induced by apomorphine (80 µg/ kg),
although it did show a tendency towards
inhibition (40 µg/kg, P < 0.07). Two-way
ANOVA on yawning responses showed a significant
effect for pretreatment (F(2,75) = 12.64, P <
0.00001) and treatment (ffl,75) = 17.07, P <
0.000001) and a significant interaction between
these factors (F(6,75) = 3.39, P < 0.005).
Pretreatment with Æ9-THC significantly decreased
the number of yawns induced by 40 and 80
µg/kg of apornorphine (P < 0.02 and P
< 0.04, respectively), while SR 141716A
potentiated the yawning induced by 80 µg/kg
apomorphine (P < 0.01).
The present results show
that cannabinoids affect penile erection and
yawning induced by apomorphine, a dopaminergic
agonist, confirming that they can modulate the
dopaminergic system. Such interaction has been
previously shown in brain regions related to
motor and emotional responses.
Although the present study did not
investigate the neuroanatomical basis of penile
erection and yawning, previous studies reported
the importance of the paraventricular nucleus of
the hypothalamus and dopamine to both behavioral
responses. Potentiation of penile erection
and yawning responses to apomorphine by the
cannabinoid receptor antagonist, SR 141716A (2
mg/kg), suggests an antagonism of cannabinoid
receptors, blocking endocannabinoid action and
thereby decreasing dopaminergic activation, an
action that remains to be clarified as a single
dose was used in the present study. The finding
that the stimulation of motor behavior elicited
by systemic administration of the D2-like
agonist quinpirole was increased by SR 141716A
supports this interaction and led these authors
to suggest that the endocannabinoid systern may
modulate D2 dopamine activation of psychomotor
activity. In line with this hypothesis it is
noteworthy that there is an interaction between
endocannabinoid and doparninergic systems in
hypothalamic structures related to hypothermia
and prolactin secretion. In addition, our
finding that Æ9-THC significantly reduced
yawning, although it did not fully reduce the
number of penile erections, strongly supports
the idea that cannabinoids might interact
with dopaminergic receptors.
Several studies have shown that cannabinoid
agonists can either increase doparninergic
activity or produce a dopaminergic
antagonistic-like effect. The antagonistic-like
action of cannabinoid agonists might best
explain the present findîngs since yawning
responses were significantly reduced by Æ9-THC
and no sign of stereotypy or hyperactivity was
observed in animals treated with the combination
of Æ9-THC with apomorphine. Moreover, additional
support for this contention comes from our
recent study examining the effects of
cannabinoid agonists on yawning induced by
pilocarpine, a cholinergic agonist, or
apomorphine. Yawning induced by low doses of
apomorphine was much more sensitive to
antagonism by both Æ8-THC and Æ9-THC than that
produced by pilocarpine. This antagonistic-like
action of cannabinoid agonists may involve D2
dopamine receptor mediation. Beltramo
et al. recently showed that both anandamide
transport inhibitor AM 404 and anandamide by
itself counteract two characteristic responses
mediated by activation of D2 family receptors,
that is, yawning induced by apomorphine in a
dose equivalent to the highest one employed in
our study (80 µg/kg s.c.) and
quinpiroleinduced stimulation of motor
behaviors.
The reduction of yawning by Æ9-THC
could be attributed to its known hypotensive
properties and/or motor suppressive effect. In
regard to the former effect, our study cannot
discard this possibility, but it is important to
remember that SR 1411716A which dues not cause
hemodynamic changes potentiated the penile
erection responses. Concerning the latter
effects of cannabinoid agonists, the locomotor
activity data from the literature provide
evidence against this possibility. That is, a
study using the radial arm maze task did not
observe differences in locomotor activity of
rats treated with the same dose used here, 1.25
mg/kg . Furthermore, experiments evaluating the
role of cannabinoids on movement found an
increase in ocomotor activity in rats injected
with either 1.5 or 2 mgfkg of Æ9-THC.
Taken together, the prescrit results show
that although SR 141716A has no overt effect
when administered alone, it enhances the
dopaminergic effects induced by low doses of
apomorphine, while the cannabinoid agonist,
Æ9-THC, decreases these responses. As was
mentioned before, there are many other systems
that are known to modulate yawning and penile
erection responses and certainly further studies
are necessary to clarify this proposed mediation
of these responses by cannabinoid and dopamine
interaction.
Considering the importance of dopamine to
motor and limbic functions and the number of
reports that dopamine agonists are effective in
various psychiatric conditions as well as
neurological disorders, the possibility of SR
141716A increasing dopaminergic
neurotransmission may be useful as a potential
strategy to ameliorate dysfunctions related to
this system.
Yawning
and penile erection induced by apomorphine is
potentited by cannabinoid recptors antagonist in
rats
Eur Neuropsychopharmacol 2002;12; s3;
S391
R.N. Takahashi. UFSC, Depto Farmacologia,
Florianopolis SC, Brazil
It is known that cannabinoid and dopamine
receptors are colocalised in brain areas that
have important role in regulating motor and
emotional responses. Yawning responses have been
observed during the SR 141716 precipitated
cannabis withdrawal in rats. Moreover, yawning
and penile erection constitute behavioral
patterns that occur concomitantly under the
effects of dopamine receptors agonists. The
present study was undertaken to examine the
influence exerted by an acute treatment with SR
1411716, a specific cannabinoid receptor
antagonist, on typical behavioral responses
induced by low doses of apomorphine, a dopamine
receptor agonist. Wistar male rats were injected
with apomorphine (20, 40 or 80 mg/kg, se) and
individually placed in wire cages. The number of
yawns and penile erections were counted for a
period of 30 min. Pretreatment with SR 141716 (2
mg/kg, ip) was given 40 min before apomorphine.
A separate two-way ANOVA of penile erection and
yawning data showed significant treatment
effects [F(3, 49)= 21.2, p<O.Ol and F(3,
49)=14,3, p<O.OOl, respectively], a
significant pretreatment effects [F(1,
49)=5.l8, p<0.027 and F( 1, 49)=4.12,
p<O.047, respectively], and a significant
interaction factors [F(3, 49)=2.80,
p<O.049 and F(3, 49)=3.84, p<0.0l5,
respectively]. Further comparisons indicated
that penile erection elicited by apomorphine (40
mg/kg) and yawning elicited by the dose of 80
mg/kg was significantly potentiated by SR 141716
pretreatment (Tukey LSD test, p<O.O5). These
results indicate that cannabinoid antagonists
facilitate dopaminergic stimulatory responses
and they also confirm the cross-talk between
cannabinoid and dopaminergic systems.
