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mise à jour du
11 septembre 2003
Europ J Pharmacol
1998; 357; 121-126
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.


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.