Nitroglycerin-induced
penile erection and yawning in male rats:
mechanism of action in the brain
MR Melis, R Stancampiano, C Lai, A
Argiolas
Bernard B. Brodie Department
of Neuroscience, Cagliari, ltaly
Introduction : Penile erection and
yawning are two different behavioral patterns
that often occur concomitantly in physiological
and experimental conditions. Among substances
that induce these behavioral responses the best
known are dopamine agonists, adrenocorticotropin
(ACTH) and related peptides and
N-methyl-D-aspartic acid (NMDA). Several lines
of evidence suggest that the paraventricular
nucleus of the hypothalamus (PVN) plays a key
role in the expression of these behavioral
responses.
Accordingly, dopamine agonists, oxytocin and
NMDA induce penile erection and yawning when
injected in this hypothalamic nucleus. The
ability of nitric oxide (NO) synthase inhibitors
such as N-nitro-L-arginine methyl ester and
N-monomethyl-L-arginine when injected in the PVN
to prevent penile erection and yawning induced
not only by apomorphine and oxytocin, but also
by NMDA, raises the possibility that the novel
discovered neurotransmitter/ neuromodulator NO
plays a key role in the control ofthese
behavioral responses at level ofthe PVN. In
agreement with this hypothesis, we show here
that nitroglycerin, a drug that contains NO used
in humans in the treatment of angina pectoris
for its potent vasodilator effect induces penile
erection and yawning when injected into a
lateral ventricle (ICV) or into the PVN of male
rats. The effect of methylene blue or
hemoglobin, that antagonize NO activity,
d(CH,),5Tyr(Me)2 -Orn'-vasotocin, that blocks
oxytocinergic receptors and haloperidol, a
classical dopamine receptor antagonist, on
nitroglycerin responses is also reported.
[...]
Discussion : The present results show
that the ICV administration of nitroglycerin,
an organic nitrate well known for its potent and
rapid vasodilator effect, clinically used for
relieving anginal pain and as a hypotensive
agent, induces penile erection and yawning in
male rats.
One of the brain areas in which the drug
acts to induce these behavioral responses is the
PVN, which is involved in the control of these
responses induced by other substances, such as
NMDA, dopaminergic agonists and oxytocin. The
mechanism by means of which nitroglycerin
induces penile erection and yawning by acting in
the PVN is unknown and only some speculation is
possible at prescrit.
One possibility is that nitroglycerin acts
after being internalized in target neurons where
is biotransformed in S-nitroso-thiol compounds
that produce NO thereby activating enzymes such
as soluble guanylate cyclase, as already
suggested for explaining its vasodilator
properties in vascular beds. Accordingly, ICV
methylene blue, an inhibitor of guanylate
cyclase in several tissues, prevents
nitroglycerin responses.
However, the involvement of guanylate
cyclase in these nitroglycerininduced behavioral
responses must be considered with caution,
because both methylene blue, despite its ability
to inhibit guanylate cyclase in several tissues,
and hernoglobin, which would act as a NO
scavenger are ineffective when injected in the
PVN. Although the inability of hemoglobin to
prevent nitroglycerin responses when injected
directly in the PVN can be explained by the
inability of this compound to cross cellular
membranes these findings raise the possibility
that nitroglycerin induces penile erection and
yawning in this area by a mechanism not related
to the stimulation of guanylate cyclase, that
is, by a cyclic guanosine
3',5'-monophosphate-indepenilent mechanism.
Accordingly, if one assumes that
nitroglycerin responses are mediated by NO
produced in nitroglycerin target neurons, NO
might interact with numerous other enzymes that,
like guanylate cyclase, bind metal ions such as
iron, as described for instance in fibroblasts.
Whatever mechanism nitroglycerin-derived NO
activates in the PVN to induce penile erection
and yawning, the inability of hemoglobin to
prevent nitroglycerin responses suggests that NO
is not released out from the neurons where it is
formed but is acting intracellularly. This
implies a local second messengerrole of NO in
the PVN. This hypothesis iscomplicated by the
finding that ICV methylene blue prevents
nitroglycerin responses, which favours a
neurotransmitter role of NO in distant brain
areas from the PVN rather than a local second
messenger role.
A possible unifying explanation for such
discrepancy is that NO activates those neurons
where it is fonned, releasing in turn a
neurotransmitter(s) that activates a methylene
blue-sensitive guanylate cyclase in brain areas
distant from the PVN. Although only the
identification of the site(s) where methylene
blue acts to prevent nitroglycerin-induced
responses will clarify this point, this
hypothesis is supported by the ability of the
potent oxytocin receptor antagonist
d(CH,)_5Tyr(Me )2 -Orn'-vasotocin, given ICV but
not in the PVN, to prevent nitroglycerin-induced
penile erection and yawning. In tact, this
finding suggests that nitroglycerin induces
these behavioral responses by activating central
oxytocinergic transmission, as already suggested
for apomorphine-, oxytocin- and NMDA-induced
penile erection and yawning. The finding raises
also the possibility that nitroglycerin-derived
NO is produced in the cell bodies of
oxytocinergic neurons originating in the PVN and
projecting to extrahypothalamic brain areas,
and provides further support to the hypothesis
that PVN NO is involved in the control of penile
erection and yawning.
Indeed, a) NO-synthase inhibitors prevent
apomorphine-, oxytocin- and NMDA-induced penile
erection and yawning by acting in the PVN; b)
the PVN is one of the richest brain areas
containing NO-synthase; and C) NO-synthase is
localized in PVN oxytocinergic neurons.
Conversely, the inability of the dopamine
receptor antagomist haloperidol to prevent
nitroglycerin responses is in line with the
hypothesis that dopamine acts in the PVN to
induce penile erec- tion and yawning by
activating oxytocinergic transmission, by acting
therefore at sites located before oxytocin. The
finding rules out also the possibility that
nitroglycerin-induced penile erection and
yawning are mediated by the activation of
central dopaminergic transmission.
The interpretation given above of the
present results is based on two main
assumptions. The first is the existence of
central oxytocinergic pathways originating in
the PVN that are involved in the control of
penile erection and yawning, whose activity can
be modulated by several neurotransmitters,
including oxytocin itself. However the
possibility that nitroglycerin induces these
behavioral responses by activating oxytocinergic
transmission by a mechanism different from that
postulated above cannot be rule out by the
present results. Parallel or alternative
neuronal pathways might be involved as well. For
instance it was recently shown that the addition
of the NO precursor L-arginine or a N - synthase
inhibitor to the perfusion medium of a
microdialysis probe modulates in an opposite
manner dopamine and serotonin release in the
medial preoptic area. This area is very close to
the PVN and, most important, contains
NO-synthase and plays a key role in the control
of male sexual behavior, including penile
erection and seminal emission.
The second assumption, that needs further
experimental verification, is that nitroglycerin
is acting in the central nervous system
exclusively as a NO donor, as found in vascular
smooth muscles, including the corpus cavernosum,
since several reports on the use of
nitroglycerin as an effective topical ointment
for the treatment of erectile impotence have
been published. Our results show that
nitroglycerin can act also in the central
nervous system to induce penile erection. This
provides further evidence for a physiological
role of NO in the control of this primary male
sexual function not only in the corpus
cavernosum, but also in the central nervous
system.
