Introduction : Penile erection and
yawning are two distinct behavioral patterns
that often occur concomitantly under certain
physiological and experimental conditions.
Penile erection is a primary sexual response in
mammals, including humans, its achievement being
essential for the success of reproduction. As to
the physiological significance of yawning. it is
pertinent to recall that yawning is considered
to be an ancestral vestige surviving throughout
evolution that subserves the purpose of arousal.
Among substances that induce both penile
erection and yawning, dopamine receptor agonists
(i.e. apomorphine), oxytocin,
adrenocorticotropin (ACTH) and
N-methylD-aspartic acid (NMDA) are certainly the
most widely known.
However, it was recently shown that penile
erection and yawning can also be induced in male
rats by drugs that modify serotonin (5-HT)
transmission, in particular 5-HT receptor
agonists that act on the 5-HT, receptor subtype,
such as 1-(3-chlorophenyl)-piperazine (m-CPP)
and N-(3-trifluoromethylphenyl)-piperazine
(TFMPP)
Accordingly, 5-HT1c receptor agonist-induced
penile erection and yawning are prevented by
5-HT antagonists with a potency that is parallel
to their potency in blocking the 5-HT1c receptor
subtype. As to the mechanism(s) responsible for
the induction of these behavioral responses by
5-HT1c receptor agonists, very little
information is available at present. Briefly,
these compounds seem to act by mechanisms
different from those that are activated by
dopaminergic agonists and oxytocin. Indeed m-CPP
or TFMPP induced penile erection and yawning are
prevented neither by the dopamine receptor
antagonist haloperidol nor by the oxytocin
receptor antagonist
d(CH2)5Tyr(Me)2-Orn8-vasotocin, nor iliese
5-HT1c receptor agonists induce penile erection
and yawning when injecied in the paraventricular
nucleus of the hypothalamus (PVN), unlike
dopamine agonists or oxytocin. We report here
that m-CPP- and TFMPP induced penile erection
and yawning, are prevented by the
intracerebroventricular (i.c.v.) administration
of Ng-nitro-L-arginine methyl ester (NAME) and
Ng-monomethyl-L-arginine (NMMA), inhibitors of
nitric oxide (NO) synthase, the enzyme
responsible for the formation of NO, a novel
discovered neutrotransmitter - neuromodulator in
the peripheral and central nervous system. Since
guanylate cyclase is one of the major targets of
NO, the effect of guanylate cyclase inhibitors
and NO scavengers on m-CPP- and TFMPP-induced
penile erection and yawning is also reported.
[...]
Discussion :
The present results show that penile
erection and yawning induced by m-CPP and TFMPP,
two 5-HT1c receptor agonists, are prevented by
NAME and NMMA given i.c.v. at doses insufficient
to act peripherally with a potency that is
parallel to their potency in inhibiting
NO-synthase, the calmodulin-dependent
iron-containing enzyme that forms the novel
discovered neurotransmitter/neuromodulator NO in
peripheral and central neural tissues from
L-arginine.
The finding suggests that endogenous NO is
involved in the expression of these behavioral
responses induced by 5-HT1c receptor agonists.
In agreement with this hypothesis, the
inhibitory effect of NAME on m-CPP- and
TFMPP-induced penile erection and yawning is
prevented by the simultaneous excess
administration of the precursor of NO
L-arginine, which per se did not alter m-CPP and
TFMPP responses. Together these findings are in
line with previous studies showing that central
NO is involved in the control of spontaneous or
drug-induced penile erection and yawning.
Accordingly,
NO-synthase inhibitors prevent these
behavioural responses by the dopaminergic
agonist apomorphine, oxytocin and NMDA when
injected in the PVN, that contains one of the
highest concentrations of NO-synthase in
brain
nitroglycerin, a NO donor well known for its
vasodilator and antihypertensive action, induces
penile erection and yawning when administered
centrally or in the PVN. Despite these findings,
which support a key role of paraventricular NO
in the control of penile erection and yawning,
and the fact that the PVN receives a dense
serotoninergic projection from the medullary
raphe nuclei, the inability of NAME when
injected directly into the PVN to prevent
in-CPP-or TFMPP-induced penile erection yawning
suggests that paraventricular NO is not involved
in the 5-HT1c receptor agonist-induced
responses.
The finding also rules out the possibility
that m-CPP or TFMPP induces penile erection and
yawning by activating oxytocinergic
transmission, possibly by increasing NO synthase
activily in paraventricular oxytocinergic
neurons projecting to extrahypothalamic brain
areas.
This is in agreement with previous findings
showing thal m-CPP and TFMPP injected in the PVN
falled to induce penile erecction and yawning,
and that their effect was not prevented by a
potent oxytocin antagonist given i.c.v., unlike
apomorphine. NMDA, nitroglycerin and oxytocin
itself. Il is then more likely that 5-HT1c
receptor agonists induce and NO-synthase
inhibitors prevent these responses by acting at
sites located downstream of processes involving
oxytocin in some yet unidentified brain area. In
this regard, it is pertinent to recall thal a
serotoninergic pathway originating in the
nucleus paragigantocelluris of the ventral
medullaa sending projections to the lumbar
spinal cord in the region of the spinal nucleus
of the bulbo cavernuosus inhibits penile
erection has been recently identifie. Perhaps
more intriguing, NO-synthase was recently
localized in neurons of the ventral medulla,
including the nucleus paragigantocellularis as
well as of the spinal cord. Since electolytic
lesions of the nucleus paragigantocellularis
facilitate penile reflexes and copulatory
behavior in male rats and drugs that enhance
5-HT transmission, especially 5HT2 receptor
agonists, impair these sexual responses, it is
tempting to speculate that NO-synthase
inhibitors prevent m-CPP and TFMPP-induced
penile erection by inhibiting NO synthase in the
medulla and/or the spinal cord. This is in
agreement with the possibility that penile
erection by 5-HT1c receptor agonists is
rnediated by the stimulalion of post-synaptic
5-HT1c receptors in the ventral medulla and/or
spinal nucleus of the bulbocavernosus.
As to the molecular mechanisins by means of
which NO, formed by 5-HT1c receptor stimulation,
induces penile erection and yawning, one
possibility is that m-CPP and TFMPP activate the
so-called NO-cyclicguanosine 3': 5'
monophosphate (cGMP) pathway. In agreement with
this hypotliesis m-CPP- and TFMPP-induced penile
erection and yawning are prevenied by inhibitors
of guanylate cyclase, a major target of NO, such
as methyleneblue and LY 83583. Since NO-synthase
containing neurons are often just opposed to
guanylate cyclase-containing largels, the
stimulation of 5-HT1c receptors would lead to
the activation of NO synthase, thereby producing
NO which would in turn act as an intercellular
messager activating guanylate cyclase in target
cells mediating penile erection and yawning.
However, this interpretation is complicated by
the inability of the potent NO scavenger
hemoglobin to preventm-CPP or TFMPP responses.
In fact, this high molecular weight substance
would bind NO exclusively in the extraccellular
space, being unable to cross cellular membranes,
precluding it from reacaching larget cells,
thereby preventing, the activation of guanylate
cyclase and in turn the behavioral responses.
One unilving explanation for such discrepancy
might be that NO acts as an intracellular
messenger activating, in turn those
neuronsni which is formed, thereby releasing a
neurotransmitter(s) that activates a methylene
blue- or LY 83583-sensitive guanylate cyclase in
areas involved in the control of these
bahavioral responses. If the latter hypothesis
were correct, NO would act on a larget different
from guanylate cyclase, that is by a c-GMP-
independent mechanism. Indeed, NO might interact
with numerous other enzymes that, like guanylate
cyclase, bind metal ions such as iron, as
described for instance in fibroblasts.
Accordingly, NO donors were found to be able to
activate the ADP-ribosylation, catalyzed by
cellular ADP-ribosyl-transferases, of several
brain proteins including
glyceraldehyde-3'-phosphate dehydrogenase and
guanosine triphosphate (GTP) binding proteins.
In addition to the points discussed above, other
arguments against a direct activation by NO of
guanylate cyclase after m-CPP or TFMPP come from
the possibility of methylene blue to inhibit
directly NO-synthase and of LY 83583 Io inhibit
indirectly guanylate cyclase, for instance by
generating superoxide anion radicals that act as
NO scavengers.
Although further studies are necessary to
identify the brain site(s) where NO-synthase
inhibitors act to prevent m-CPP- and
TFMPP-induced penile erection and yawning and to
clarify the role of guanylate cyclase, the
present results provide further evidence for a
key role of central NO in the control of these
behavioral responses.
