in the
paraventricular nucleus of the
hypothalamus
of male
rats: correlation with
penile
erection and yawning
MR Melis, S Succu, U Iannucci, A
Argiolas
Bernard B. Brodie Department ofNeuroscience,
Cagliari, Italy
Introduction : Penile erection and
yawning are two different behavioral patterns
that often occur concomitantly in physiological
and experimental conditions. While the
importance of penile erection in reproduction
does not need to be stressed, yawning alone
or associated with stretching can be considered
an ancestral vestige surviving through evolution
that subserves the purpose of arousal. Among
substances that induce both behavioral
responses, the neurohypophyseal peptide,
oxytocin, centrally administered, is certainly
one of the most potent. This effect is mediated
by the stimulation of specific uterinetype
oxytocinergic receptors, possibly located in the
paraventricular nucleus of the hypothalamus
(PVN) being induced by oxytocin-related
peptides and prevented by selective oxytocin
receptor antagonists, given centrally with a
potency that was parallel with the potency of
these compounds in stimulating and blocking
oxytocinergic receptors, respectively. These
oxytocin responses are apparently mediated by
the stimulation of oxytocinergic neurons
originating in the PVN and projecting to
extrahypothalamic brain areas. These neurons
seem to mediate penile erection and yawning
induced not only by oxytocin but also by
dopamine receptor agonists and
N-methyl-D-aspartic acid.
We recently found that oxytocin,
apomorphine, Nmethyl-D-aspartic acid, but not
serotonin, receptor agonists, induce penile
erection and yawning by activating NO synthase
in the PVN, which in turn leads to the
activation
of central oxytocinergic transmission. This
suggests that the recently discovered
neurotransmitter/neuromodulator, nitric oxide
(NO) is involved in the control of penile
erection and yawning induced by these
substances. In agreement with this hypothesis,
NO synthase has been identified in the cell
bodies of paraventricular oxytocinergic neurons
and NO donors injected into the PVN induce
penile erection and yawning indistinguishable
from those induced by apomorphine, oxytocin,
N-methyl-D-aspartic acid which, however, are
prevented by the oxytocin receptor antagonist
d(CH,-)5Tyr(Me)-Orns-vasotocin injected into the
lateral ventricles (i.c.v.). In order to provide
further support for a role of paraventricular NO
in penile erection and yawning induced by
oxytocin, we studied the effect of a dose of
oxytocin and related peptides that induce penile
erection and yawning on the concentration of the
reaction products of newly formed NO with O2,
NO2 and NO in the dialysate collected from a
vertical microdialysis probe implanted in the
PVN of male rats. Indeed, the concentration of
NO, and NO- in the dialysate represents an
indirect but reliable indicator of NO production
in vivo. [...]
Discussion : The present results show
that a dose of oxytocin that induces penile
erection and yawning increased basal NO, and,
but to a lesser extent, NO concentration in the
PVN dialysate of male rats. NO2 concentration
was also increased by the oxytocin-related
peptides [Thr 4 Gly'loxytocin and
oxytocin(1-8) that, like oxytocin, induce penile
erection and yawning, but not by oxytocin(I-6)
and oxytocin(7-9), which were unable to induce
these behavioral responses. These findings are
in line with the hypothesis that oxytocin
induces penile erection and yawning by
activating its own receptors in the PVN and
that the stimulation of these receptors leads to
the activation of paraventricular NO synthase.
Accordingly, d(CH2)5Tyr(Me)-Orn8-vasotocin, a
selective oxytocin receptor antagonist,
prevented not only penile erection and yawning
but also the NO2 concentration increase induced
by oxytocin in the PVN dialysate. As to the
mechanism by means of which oxytocin activates
NO synthase in the PVN, one posisibility is that
oxytocin increases Ca2+ influx in the cell
bodies of oxytocinergic neurons mediating penile
erection and yawning, since these neurons
contain NO synthase, which is Ca2+
calmodulin-dependent, and Ca2+ ions mediate the
oxytocin effect on penile erection and
yawning.
Furthermore, the increase of NO2 and NO3
concentration in the PVN dialysate induced by
oxytocin and related peptides found in this
study, like that induced by dopamine agonists,
which induce penile erection and yawning by
increasing oxytocinergic transmission, would
reflect almost exclusively an increased
conversion of L-arginine to NO that is, in turn,
oxidized mainly to NO2 and, te, a lesser extent,
to NO3 , as found in other biological fluids not
containing blood cells. In agreement with the
above hypothesis, this study shows that L-NAME,
a potent inhibitor of NO synthase given i.c.v.,
prevented oxytocin-induced increase of NO,
concentration, penile erection and yawning.
Conversely, the oxytocin-induced increase of NO2
concentration was also prevented by hemoglobin,
a potent NO scavenger.
However, the hypothesis that an increase of
NO production by NO synthase is correlated to an
increase of these behavioral responses is
complicated by the inability of hemoglobin to
prevent oxytocin-induced penile erection and
yawning, despite its ability to prevent the
increase of NO, concentration in the PVN.
Nevertheless, NO, is measured in the
extracellular dialysate, while NO is formed by
NO synthase intracellularly. A possible
explanation for this contradiction is that NO
formed by oxytocin stimulation of oxytocinergic
receptors in the PVN acts as an intracellular
messenger, rather than an extracellular
transmitter, in those neurons, inducing these
behavioral responses. In fact hemoglobin would
be expected to scavenge only extracellular and
not intracellular NO, because it is unable to
cross cellular membranes because of its high
molecular weight. However, this does not rule
out the possibility that NO released from those
cells by which it is produced, acts as an
extracellular transmitter and is involved, for
instance, in other hypothalamic effects of
oxytocin.
The present results show also that the
dopamine receptor antagonist, haloperidol, is
unable to prevent oxytocin effects on NO2
concentration, penile erection and yawning,
despite its ability to prevent these responses
when induced by dopamine agonists. This is in
agreement with previous studies showing that
dopamine acts before oxytocin in the PVN for the
induction of these behavioral responses. In
fact, if one assumes that dopamine agonists and
oxytocin increase PVN NO synthase in the same
oxytocinergic neurons mediating penile erection
and yawning, by stimulating specific
dopaminergic and oxytocinergic receptors,
respectively, haloperidol would prevent dopamine
agonist- but not oxytocininduced responses.
As to the mechanism by means of which NO
produced in the PVN by oxytocin leads to the
activation of oxytocinergic neurons mediating
penile erection and yawnîng, the inability
of methylene blue, an inhibitor of guanylate
cyclase one of the best known targets of NO in
preventing the increase of NO2 concentration
induced by oxytocin as well as penile erection
and yawning when injected in the PVN provides
further support to the hypothesis that NO acts
in the PVN to control these behavioral responses
by a mechanism not related to the activation of
guanylate cyclase. Accordingly, the injection of
a stable cyclic guanosine 3':5'-mono-phosphate
analog (e.g., 8-bromo-cyclic guanosine 3': 5'-
mono- phosphate) in the PVN, unlike that of
nitric oxide donors, was found to be unable to
induce penile erection and yawning.
Nevertheless, since methylene blue given i,c.v.
prevents these behavioral responses induced by
oxytocin, the possibility that guanylate cyclase
is involved in the control of penile erection
and yawning in sites distant from the PVN cannot
be ruled out.
In conclusion, the present results
suggest that oxytocin and oxytocin-related
peptides at doses that induce penile erection
and yawning, increase the concentration of NO2
and NO3 in the PVN dialysate. Since NO2 and NO3
concentration in extracellular fluids is a
reliable index of the activity of NO synthase,
these results provide further evidence that NO
plays an important role in the control of these
behavioral responses ut the level of this
hypothalamic nucleus.
