Bernard B. Brodie Department
of Neuroscience, University of Cagliari,
Italy
Introduction : Penile erection and
yawning are two different behavioural patterns
that often occur concomitantly in physiological
and experimental conditions. Although the
importance of penile erection in reproduction
does not need to be stressed, the response can
also be observed in other contexts, such as
manipulation of the genitalia, erotic fantasies
and sleep in humans. Depending on the context in
which penile erection occurs, different neural
and/or humoral mechanisms may participate in its
regulation. The physiological significance of
yawning is less clear: the act alone or
associated with stretching is considered an
ancestral vestige, surviving through evolution,
that subserves the purpose of arousal, although
its role is not entirely certain.
Among substances that induce both these
responses the best known are dopamine receptor
agonists, oxytocin, adrenocorticotropin and
related peptides, N methyl-D-aspartic acid and
serotonin receptor agonists that act mainly on
the serotonin, receptor subtype. Interestingly,
the paraventricular nucleus of the hypothalamus
(PVN) seems to play a key role in these
behavioural responses when they are induced by
the dopamine receptor agonists,
N-methyl-D-aspartic acid and oxytocin, but not
when they are induced by adrenocorticotropin or
serotonin receptor agonists.
Indeed, the former, but not the latter
compounds induce penile erection and yawning
when injected in the PVN. These responses are
apparently mediated by the activation of
oxytocinergic transmission. since they are
prevented by the central administration of
oxytocin receptor antagonists and by
electrolytic lesions of the PVN that induce an
almost complete depletion of oxytocin content in
extra-hypothalamic brain areas and spinal cord.
However, since PVN lesions do not prevent penile
erection and yawning induced by
adrenocorticotropin or penile reflexes and
erections in copula ), additional neuronal
pathways must be involved in the control of
these behavioural responses.
Recently we found that the putative
neurotransmitter/neuromodulator nitric oxide is
involved in penile erection and yawning induced
by apomorphine, oxytocin, N-methyl-D-aspartic
acid and serotonin agonists. In particular, the
abovecompounds (except serotonin agonists) seem
to induce these behavioural responses by
activating NO synthase in the PVN, which in turn
leads to the activation of central oxytocinergic
transmission.
Accordingly, NO synthase inhibitors prevent
apomorphine-, N-methylaspartic acid- and
oxytocin-induced penile erection and yawning
when injected in the PVN, while NO donors
injected into the PVN induce penile erection and
yawning that are indistinguishable from those
induced by the NO synthase activators and are
prevented by the injection of oxytocin receptor
antagonist d(CH2),5Tyr(Me)-Orn8-vasotocin into
the lateral ventricles.
In order to provide further support for a
role of paraventricular NO in penile erection
and yawning induced by apomorphine, we studied
the effect of a low dose of apomorphine and
other dopamine receptor agonists on the
production of NO in the PVN in vivo. This was
achieved by measuring the concentration of the
reaction products of newly formed NO with O2,
NO2- and NO3-, which provides an indirect but
reliable indicator of NO production in vivo, in
the dialysate collected from a vertical
microdialysis probe implanted in the PVN.
[...]
Discussion : The present results show
that a dose of apomorphine that induced penile
erection and yawning increased basal NO2- and,
to a lesser extent, NO3- concentration in the
PVN dialysate of male rats. A similar increase
of NO2- concentration was found with LY 171555,
a selective D2 receptor agonist that, like
apomorphine, induced penile erection and
yawning, but not with SKF 38393, a D1 receptor
agonist that was unable to induce these
behavioural responses.
These results are in line with the
hypothesis that dopamine receptor agonists
induce penile erection and yawning by activating
D2 receptors in the PVN and that the stimulation
of these receptors leads to the activation of NO
synthase in this hypothalamic nucleus.
Accordingly, L-sulpiride, a dopamine receptor
antagonist that acts mainly on D2 receptors,
prevented not only penile erection and yawning
but also the NO2- concentration increase induced
by apomorphine in the PVN dialysate.
The results also confirm previous studies
showing that SCH 23390, a D1 receptor
antagonist, was able to prevent penile erection
and yawning induced by D2 receptor agonists.
Hence, by showing that SCH 23390, unlike
haloperidol or L-sulpiride, is unable to prevent
the increase in NO2- concentration induced by
apomorphine in the PVN dialysate, despite its
efficacy in preventing penile erection and
yawning, the present results suggest that D1
receptors control these behavioural responses by
a mechanism either not involving paraventricular
NO synthase or acting in brain areas different
from the PVN.
As far as the possibility that dopamine
agonists stimulate NO synthase in the PVN is
concerned, it is noteworthy that this
hypothalamic nucleus is one of the brain areas
richest in NO synthaseand that the enzyme is
present in the cell bodies of oxytocinergic
neurons.
Furthermore. the increased concentrations of
NO2- and NO3- induced in the PVN dialysate by
these agents would reflect almost exclusively an
increased conversion of L-arginine to NO that is
in turn oxidized mainly to NO2 and, to a lesser
extent. NO2-, as found in other biological
fluids not containing blood cells.
In agreement with the above hypothesis,
NG-nitro-L-arginine methyl ester, a potent
inhibitor of NO synthase given i.c.v. prevented
both apomorphine-induced increase of NO2-
concentration and penile erection and yawning.
Conversely, the apomorphine-induced increase in
NO2- concentration was also prevented by
haemoglobin, a potent NO scavenger.
However, this interpretation is complicated
by the inability of haemoglobin to prevent
apomorphine-induced penile erection and yawning.
Such apparent discrepancy can be explained by
assuming that NO formed by apomorphine
stimulation of dopamine receptors in the PVN
acts as an intracellular messenger in the
neurons in which is formed rather than an
extracellular transmitter. In fact, due to its
higgh molecular weight haemoglobin is unable to
cross cellular membranes and would therefore be
expected to scavenge extracellular rather than
intracellular NO-although this does not rule out
the possibility that NO released from the cells
in which it is produced acts as an extracellular
transmitter and is involved for instance in
other hypothalamic responses induced by
apomorphine.
The present results show also that
d(CH2)5Tyr(Me)-Om 8-vasotocin, a potent oxytocin
receptor antagonist, has no effect on the
apomorphine-induced increase of NO2-
concentration, despite its ability to prevent
penile erection and yawning. This apparent
discrepancy may be explained by the inability of
Oxytocin receptor antagonists to prevent
apomorphine-induced penile erection and yawning
when injected in the PVN. In fact, if one
assumes that apomorphine stimulates PVN D2
receptors to activate PVN NO synthase and
oxytocinergic neurons that in turn release
oxytocin in brain areas distant front the PVN to
induce penile erection and yawning, the oxytocin
receptor antagonist would be expected to act at
the sites where oxytocin is released, i.e. after
NO formation, to prevent these responses. The
mechanism by which NO formed by dopamine
receptor stimulation leads to the activation of
PVN oxytocinergic neurons to mediate penile
erection and yawning is unknown. One possibility
is that NO increases the firing rate of
oxytocinergic neurons and/or oxytocin synthesis.
Indeed, the dose of apomorphine used in this
study was shown to increase oxytocin
concentration in the hippocampus as weil as in
blood of male rats.
An similar interpretation can also explain
the ability of methylene blue to prevent penile
erection and yawning but not the NO2
concentration increase induced by apomorphine.
Indeed, methylene blue prevents
apomorphine-induced penile erection and yawning
when given i.c.v. but not in the PVN. Thus it is
likely that methylene blue, like oxytocin
receptor antagonists, acts at sites distant from
the PVN to prevent the behavioural responses of
apomorphine. Since methylene blue is an
inhibitor of guanylate cyclase, one of the best
known targets of NO, its inability to prevent
the apomorphine-induced increase in NO2-
concentration provides further evidence that NO
acts in the PVN to control penile erection and
yawning by a mechanism not related to the
activation of guanylate cyclase. Accordingly,
the injection of a stable cyclic guanosine 3',5'
monophosphate analogue (e.g. 8-bromo-cyclic
,auanosine 3',5' monophosphate) into the PVN was
not able to induce penile erection or yawning.
The above results do not, however, rule out the
possibility that guanylate cyclase is involved
in the control of these bebavioural responses in
sites distant froin the PVN.
In conclusion, the present study shows that
dopamine agonists, at doses that induce penile
erection and yawning by activating oxytocinergic
transmission, increase the concentration of NO2-
and N03- in the PVN dialysate through the
stimulation of D2 receptors. Since NO2- and NO3-
concentration in extracellular fluids is a
reliable index of the activity of NO synthase,
the results provide further evidence that NO
plays an important role in the control of these
behavioural responses at the level of this
hypothalamic nucleus.
