The
octadecaneuropeptide ODN inhibits
apomorphine-induced yawning in
rats
Juana Garcia de Mateos-Verchere,
Jérôme Leprince, Marie-Christine
Tonon, Hubert Vaudry, Jean Costentin
Unité de
Neuropsychopharmacologie, CNRS UPRESA 6036,
Faculté de Médecine et Pharmacie
de Rouen, 76803 Saint-Etienne du Rouvray, Rouen,
France
Introduction : The term endozepine
designates a family of regulatory neuropeptides
that have been isolated from rat brain extracts
on the basis of their ability to displace
benzodiazepines from their binding sites. All
endozepines characterized so far derive from
diazepam-binding inhibitor (DBI), an 86-amino
acid polypeptide which can generate, through
proteolytic cleavage, several biologically
active fragments including the
triakontatetraneuropeptide DBI-(17-50) (TTN) and
the octadecaneuropeptide 13131-(33-50) (ODN).
Pharmacological studies have shown that ODN
interacts predominantly with central-type
benzodiazepine receptors while TTN is a
selective ligand for peripheral-type (i.e.,
mitochondrial) benzodiazepine receptors.
Endozepines may also activate a membrane
receptor positively coupled to phospholipase C
through a pertussis toxin sensitive G-protein.
Endozepines are widely distributed in the
central nervous system. In particular, high
concentrations of endozepines have been found in
brain areas sending or receiving dopaminergic
projections, such as the cortex, the amygdala,
the hypothalamus, the preoptic area, the
substantia nigra and the striatum. Concurrently,
it has been shown that agonists and antagonists
of the GABAa benzodiazepine receptor complex
modify dopamine release in the striaturn.
However, the effect of endozepines on
dopaminergic neurotransrmssion bas never been
investigated.
In order to explore possible interactions
between endozepines and the dopaminergic
systems, we have here investigated the effect of
ODN on apomorphine-induced yawning. Yawning
may be influenced by a variety of neuroamines
and neuropeptides (for review see, Argiolas
and Melis, 1998). In rat, the reference
direct dopamine receptor agonist apomorphine
modulates yawning in a biphasic manner: at low
doses apomorphine increases dose-dependently
yawning and penile erection whereas, at high
doses, apomorphine reduces yawning frequency .
The induction of yawning evoked by low doses of
apomorphine has been ascribed to the activation
of dopamine autoreceptors and the subsequent
decrease of the tonic dopaminergic transmission.
Conversely, the disappearance of yawning
provoked by high doses of apomorphine can be
accounted- for by direct activation of
postsynaptic dopamine D2 receptors which mimicks
the effect of the dopaminergic tone. The present
report provides evidence that the endozepine
ODN is a potent modulator of apomorphine-induced
yawning. [...]
Discussion : Apomorphine is a
reference direct agonist of dopamine receptors
which stimulates all types of identified
dopamine receptors, ie, D1, D2, D3, D4, D5.
However, since apomorphine exhibits a higher
affinity for autoreceptors which are associated
with either the somatodentritic region or the
axon terminals of dopamine neurons, low doses of
apomorphine cause primarily a decrease in
dopamine release.
In rat, the apomorphine-induced
depression of the tonic dopaminergic
transmission is associated with the occurrence
of a yawning behavior. It is assumed that
the decrease in the dopaminergic tone evoked by
apomorphine causes a direct or indirect
stimulation of cholinergic neurons and thereby
activates muscarinic receptors leading to the
appearance of yawning. As a matter of fact,
pilocarpine elicits yawning behavior in rats,
whereas the cholinesterase inhibitor
physostigmine potentiates the
apomorphine-induced yawning response by
preventing degradation of acetylcholine whose
release is triggered by suppression of the
dopaminergic tone. At higher doses than those
stimulating dopamine autoreceptors, apomorphine
also stimulates post-synaptic dopamine receptors
and thus restores the dopaminergic tone. As a
result, high doses of apomorphine repress the
cholinergic tone, so that the yawning response
disappears.
The present study bas demonstrated that
i.c.v. administration of ODN causes a
dose-dependent inhibition of apomorphine-induced
yawning. According to the functional pattern
aforementioned, ODN might have inhibited yawning
elicited by low doses of apomorphine, by
increasing dopaminergic transmission, and or by
inhibiting cholinergic transmission. Previous
reports have shown that ODN acts as an inverse
agonist of central-type benzodiazepine receptors
). Since benzodiazepines are known to depress
the activity of discrete populations of
dopaminergic neurons , it was conceivable that
ODN, as an inverse agonist of central-type
benzodiazepine receptors, might have activated
dopaminergic neurons involved in yawning.
However, our data did not support this
hypothesis, inasmuch as the effect of ODN on
apomorphine-induced yawning was not reversed by
diazepam. This observation indicates that
the effect of ODN cannot be ascribed to its
inverse agonistic activity on central-type
benzodiazepine receptors. Consistent with this
finding, it has been recently shown that the
stimulatory effect of ODN on cultured rat
astrocytes is mediated through activation of
non-benzodiazepinic receptor. In addition, it
appeared that the inhibitory effect of ODN on
yawning cannot be accounted for by a direct
action of the peptide on dopaminergic neurones
since ODN also inhibited the yawning response
evoked by pilocarpine. These observations
indicate that ODN likely operates downstream the
cholinergic synapse. The reappearance of yawning
induced by ODN in rats treated with 200
µg/kg apomorphine was associated with an
inhibition of the stereotyped sniffing. When the
dose of apomorphine was increased to 400
µg/kg, the effect of 100 ng ODN on the
sniffing score was not impaired while the
yawning was completely abolished. This
observation is consistent with a previous report
which showed that yawning and sniffing are two
mutually exclusive behaviors.
We have previously shown that the anxiogenic
effect of ODN may require cleavage of the
molecule to generate a biologically active
fragment. In the present study, we have found
that the C-terminal octapeptide fragment of ODN,
at a dose of 100 ng, mimicked the inhibitory
effect of ODN on apomorphine-induced yawning. In
addition, we have noticed that the effect of ODN
on yawning only culminated after a time lag of
60 min whereas the octapeptide was more rapidly
effective. These observations suggest that the
action of ODN on the yawning behavior might also
involve the formation of a biologically active
proteolytic fragment. The fact that i.c.v.
administration of ODN increases anxiety in rat
raised the question as to whether the inhibitory
effect of the peptide could bc related to its
anxiogenic activity. The present data have shown
that diazepam does not alter the effect of ODN
on yawning, whereas the anxiogenic responses
evoked by ODN are antagonized by diazepam
indicating that the effect of ODN on
apomorphine-induced yawning is not mediated by
central-type benzodiazepine receptors and cannot
be ascribed to the anxiogenic activity of the
peptide. Whether the recently described ODN
receptor coupled to phospholipase C is involved
in the effect of ODN on yawning deserves further
investigations.
Conclusion
ODN and its C-terminal octapeptide appear
to inhibit apomorphine-induced yawning. The
effect of ODN cannot be accounted for by its
inverse agonistic activity on the
GABAa-benzodiazepine receptor complex and
appears to take place downstream the
dopaminergic and cholinergic synapses involved
in yawning.
