Oxytocin (5, 20 and 100_ng) injected
unilaterally into the bed nucleus of the stria
terminalis (BNST) of male rats stereotaxically
implanted with a microinjection cannula coupled
to a microdialysis probe, induces penile
erection and yawning that occur concomitantly
with a dose-dependent increase in the
extracellular concentration of glutamic acid,
dopamine and its main metabolite
3,4-dihydroxyphenilacetic acid (DOPAC), and
nitrites (NO2-) in the dialysate obtained from
the BNST by intracerebral microdialysis.
The responses induced by oxytocin (100_ng)
were all abolished by the oxytocin receptor
antagonist d(CH2)5Tyr(Me)2-Orn8-vasotocin
(1__g), and reduced by CNQX (1__g), a
competitive antagonist of the AMPA receptors,
both given into the BNST 25_min before
oxytocin.
In contrast, (+) MK-801 (1__g), a
non-competitive antagonist of NMDA receptors,
and SCH 23390 (1__g), a selective dopamine D1
receptor antagonist, reduced penile erection and
yawning, but not glutamic acid and dopamine
increases in the BNST dialysate induced by
oxytocin.
Immunohistochemistry revealed
oxytocin-labelled neuronal structures in close
proximity to tyrosine hydroxylase-labelled
neurons or nitric oxide synthase-labelled cell
bodies surrounded by intense vesicular glutamate
transporter1-stained synapses in BNST sections
where oxytocin injections induce the above
responses. Together, these findings show that
oxytocin injected into the BNST induces penile
erection and yawning by activating not only the
glutamatergic (and nitrergic) but also the
dopaminergic neurotransmission, leading in turn
to the activation of neural pathways mediating
penile erection and yawning.
Résumé
L'ocytocine (5, 20 et 100 ng)
injectée unilatéralement dans le
noyau de la stria terminalis (BNST) de rats
mâles implantés
stéréotaxiquement avec une canule
de microinjection couplée à une
sonde de microdialyse induit une érection
du pénis et des bâillements
concomitants, parallèlement à
l'augmentation de la concentration
extracellulaire en acide glutamique, en dopamine
et en son métabolite principal, l'acide
3,4-dihydroxyphénilacétique
(DOPAC) et en nitrites (NO2-) dans le dialysat
obtenu à partir du BNST par microdialyse
intracérébrale et cela
proportionnellement de façon doses
dépendantes.
Les réponses induites par l'ocytocine
(100ng) ont toutes été
supprimées par l'antagoniste du
récepteur de l'ocytocine d (CH2) 5Tyr
(Me) 2-Orn8-vasotocine (1µg) et
réduites par CNQX (1µg), un
antagoniste compétitif de l'AMPA
récepteurs, tous deux administrés
dans le BNST 25 minutes avant l'ocytocine.
En revanche, le (+) MK-801 (1 _g),
antagoniste non compétitif des
récepteurs NMDA, et le SCH 23390 (1 _g),
antagoniste sélectif des
récepteurs D1 de la dopamine,
réduisent l'érection du
pénis et les bâillements, mais non
l'augmentation de l'acide glutamique et de la
dopamine dans le dialysat de BNST induite par
l'ocytocine.
L'immunohistochimie a
révélé des structures
neuronales marquées à l'ocytocine
proches des neurones marqués à la
tyrosine hydroxylase ou à des corps
cellulaires marqués à l'oxyde
nitrique synthase entourés par des
intenses vésicules du transporteur
glutamate1 vésiculaire dans les sections
de BNST où les injections d'oxytocine
induisent les réponses ci-dessus.
Ensemble, ces résultats montrent que
l'ocytocine injectée dans la BNST induit
l'érection du pénis et les
bâillements en activant non seulement la
neurotransmission glutamatergique (et
nitrergique) mais également
dopaminergique, entraînant à son
tour l'activation des voies neurales induisant
l'érection du pénis et les
bâillements.
1. Introduction
The bed nucleus of the stria terminalis
(BNST) is a limbic forebrain structure that
includes many distinct nuclei broadly organized
into an anterior-posterior sequence and that is
also divided horizontally in a dorsal and
ventral part by the anterior commissure
[1&endash;4]. Extensive anatomical
studies of its connectivity suggest that BNST is
a relay center within neurocircuits coordinating
the activity of autonomic, neuroendocrine, and
somatic motor systems into fully organized
physiological functions and behaviors through
the action of numerous neurotransmitters and
neuropeptides present in this structure, which
include GABA, glutamic acid, noradrenaline,
dopamine, serotonin, nitric oxide (NO),
corticotropin-releasing factor (CRF) and
oxytocin [5&endash;8].
Among functions and behaviors in which the
BNST plays a role, are also erectile function
and sexual behavior. Accordingly, different
studies have shown that i) an increase in the
expression of c-fos (a putative marker of
neuronal activity) occurs in the BNST during
sexual behavior [9&endash;11], ii)
electrical stimulation of the BNST induces
penile erection in unanaesthetized and, to a
lesser extent, anesthetized rats [12],
and iii) radiofrequency or chemical excitotoxic
lesions produce a marked decrease in noncontact
erections [13, 14], which are observed
in sexually potent male rats exposed to an
inaccessible receptive female
[15&endash;17].
However, the above BNST lesions were found
able to exert only minor effects on reflexive
erections (induced by local stimulation of the
penis) and in some aspects of copulatory
behavior [13, 14, 18].
Recently oxytocin, which plays a main role
in socio-sexual behaviors (see [19] and
references therein), including erectile function
and sexual behavior (see
[20&endash;23]), was found able to
induce penile erection and yawning when injected
into the BNST of male rats, apparently by
increasing glutamatergic neurotransmission, with
dopamine and NO being involved in both
behavioral responses at BNST sites yet to be
identified [24]. Oxytocin induces penile
erection and yawning also when injected in the
paraventricular nucleus of the hypothalamus
(PVN)(from which oxytocinergic neurons
projecting to the BNST originate), the
hippocampus [25&endash;29], the ventral
tegmental area (VTA) [30, 31] and the
posterior nucleus of the amygdala of male rats
[27].
These oxytocin responses are mediated by the
stimulation of oxytocin receptors, since both
were abolished by the prior injection of d
(CH2)5Tyr(Me)2-Orn8-vasotocin, a potent oxytocin
receptor antagonist [32], into the above
brain areas, as found in the BNST [24].
Interestingly, the above brain areas are all
connected with the BNST. Accordingly, i) the
BNST receives a dense glutamatergic innervation
from the ventral hippocampus and the amygdala
(see [33&endash;37]), ii) these
glutamatergic synapses in the BNST are rich in
dopaminergic receptors (mainly of the D2 type),
which are activated by dopamine released from a
neuronal projection originating in the VTA (see
[35] and references therein) and iii),
the BNST contains the cell bodies of
glutamatergic neurons projecting back to the
VTA, the hypothalamus and its nuclei (i.e.,
PVN), the medial preoptic area (MPOA), the
ventral subiculum of the hippocampus and the
amygdala (see [33&endash;38]), all areas
well known to play a role in penile erection and
yawning (see [24, 27&endash;31,
39&endash;42]).
This raises the possibility that the BNST
participates in the complex central neural
circuit, which includes all the above brain
areas, earlier hypothesized to control both the
motivational anticipatory and consummatory
aspects of erectile function and sexual behavior
and also yawning, at least in part (see
[22&endash;24, 40]).
In order to provide further evidence
supporting the above hypothesis, we studied
first the effect of increasing doses of oxytocin
injected into the BNST not only on penile
erection and yawning but also on the
extracellular concentrations of glutamic acid,
dopamine and its main metabolite 3,
4-dihydroxyphenylacetic acid (DOPAC) and on the
concentration of nitrites (NO2_) (an index of NO
production) (see [31, 43, 44]), in the
dialysate obtained from the BNST by means of
intracerebral microdialysis. As oxytocin was
found able to increase both extracellular
dopamine and glutamic acid concentrations in the
BNST dialysate, we studied then whether the
changes in the extracellular concentration of
the above neurotransmitters induced by oxytocin
in the BNST dialysate were antagonized by
d(CH2)5-Tyr(Me)2-Orn8-vasotocin, which blocks
oxytocin receptors, by (+)MK-801 and CNQX, which
block NMDA and AMPA receptors, respectively, and
by SCH 23390, which blocks dopamine
D1-receptors, all given into the BNST at a dose
that antagonizes oxytocin-induced penile
erection and yawning.
Finally, the relationship between oxytocin
axons/fibers, dopamine, glutamic acid and NO was
studied in parallel within the BNST sections
where oxytocin injections induce penile erection
and yawning, by using oxytocin, tyrosine
hydroxylase (TH), NO synthase and vesicular
glutamate transporter (V-GLUT1)
immunohistochemistry (IHC).
In conclusion, the results of this
study show that oxytocin injected into the BNST
induces penile erection and yawning by acting on
oxytocin receptors whose stimulation leads to
the release of glutamic acid and dopamine in the
BNST. While it is confirmed that glutamic acid
released by oxytocin activates glutamatergic
neurons containing neuronal NO synthase possibly
projecting to brain areas involved in the
control of these behavioral responses (see
[23]), it is still unclear whether
dopamine released by oxytocin activates the same
neurons activated by glutamic acid or other
neurons that control penile erection and
yawning, mainly through the activation of
dopamine D1 receptors.
Although further experiments are necessary
to verify this possibility, these results
provide further evidence for a role of the BNST
in the complex circuitry controlling yawning and
both the consummatory (penile erection and
sexual performance) and the anticipatory phases
of sexual behavior (sexual arousal and sexual
motivation) through dopaminergic, glutamatergic
and oxytocinergic pathways that reciprocally
interconnect many of the brain areas recalled
above (see [23])
