mystery of yawning
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
 
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
 
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
http://www.baillement.com

 

 

 

 

 

 

 

 

 

 

haut de page

mise à jour du
22 mars 2012
Behav Brain Res
2012;230:355-364
Dopamine agonist-induced penile erection and yawning:
Differential role of D(2)-like receptor subtypes
and correlation with nitric oxide production
in the paraventricular nucleus of the hypothalamus
of male rats
Sanna F, Succu S, Melis MR, Argiolas A.
 
Bernard B. Brodie Department of Neuroscience and Centre of Excellence for the Neurobiology of Addictions, University of Cagliari, Italy.
 

Chat-logomini

 Tous les travaux de MR Melis & A Argiolas 
Tous les travaux de M Eguibar & G Holmgren
Abstract
 
The dopamine D(3) preferring agonist pramipexole (50ng) induced penile erection and yawning when injected into the paraventricular nucleus of the hypothalamus of male rats, like the mixed D(1)/D(2)-like agonist apomorphine (50ng), while the D(4) agonist PD 168,077 (100ng), induced penile erection only. These responses lasted for 45-60min and occurred with an increase of NO(2)- and NO(3)- concentrations in the dialysate obtained from the paraventricular nucleus by intracerebral microdialysis. Pramipexole and apomorphine responses were reduced by the D(2) preferring antagonist L-741,626 (5?g), but not by the D(3) preferring antagonist SB-277011A (10?g), or the D(4) preferring antagonist L-745,870 (5?g), injected into the PVN before the dopamine agonist. In contrast, PD 168,077 responses were reduced by L-745,870, but not by L-741,626 or SB-277011A. Pramipexole, apomorphine and PD 168,077 effects were also reduced by the nitric oxide synthase inhibitor S-methyl-l-thiocitrulline (20?g) and the N-type voltage-dependent Ca(2+) channels blocker ?-conotoxin (5ng), given into the paraventricular nucleus, and by the oxytocin antagonist d(CH(2))(5)Tyr(Me)(2)-Orn(8)-vasotocin (2?g), given intracerebroventricularly but not into the paraventricular nucleus before dopamine agonists. These results suggest that stimulation of D(2), but not D(3) or D(4) receptors, by pramipexole or apomorphine increases Ca(2+) influx in cell bodies of oxytocinergic neurons. This increases the production of nitric oxide, which activates oxytocinergic neurotransmission in extra-hypothalamic brain areas and spinal cord, leading to penile erection and yawning. However, the stimulation of D(4) receptors by PD 168,077 also increases Ca(2+) influx/nitric oxide production leading to penile erection, but not yawning.
 
paraventricular nucleus 
 
1. Introduction
 
Dopamine agonists induce penile erection and yawning in laboratory animals and humans, these responses being mediated by dopamine receptors of the D2 but not of the D1 family [1&endash;7]. The paraventricular nucleus of the hypothalamus (PVN) is a brain area where dopamine agonists act to induce penile erection and yawning. Accordingly, D2-like, but not D1-like agonists, induce these responses when injected into the PVN of male rats and D2-like, but not D1-like antagonists, impair these responses when given into the PVN before D2-like agonists [1,2,8&endash;10]. Apparently, dopamine agonists stimulate D2-like receptors located in the cell bodies of oxytocinergic neurons originating in the PVN and projecting to1. Introduction Dopamine agonists induce penile erection and yawning in laboratory animals and humans, these responses being mediated by dopamine receptors of the D2 but not of the D1 family [1&endash;7]. The paraventricular nucleus of the hypothalamus (PVN) is a brain area where dopamine agonists act to induce penile erection and yawning. Accordingly, D2-like, but not D1-like agonists, induce these responses when injected into the PVN of male rats and D2-like, but not D1-like antagonists, impair these responses when given into the PVN before D2-like agonists [1,2,8&endash;10]. Apparently, dopamine agonists stimulate D2-like receptors located in the cell bodies of oxytocinergic neurons originating in the PVN and projecting to extra-hypothalamic brain areas and to the spinal cord [2,8&endash;10]. Indeed, PVN oxytocinergic cell bodies are innervated by incertohypothalamic dopaminergic neurons originating in the A13&endash;A14 cell groups of Dahlstrom and Fuxe [11,12]. The stimulation of PVN D2-like receptors increases Ca2+ influx into the cell bodies of oxytocinergic neurons, leading to the activation of nitric oxide (NO) synthase. NO in turn activates oxytocinergic neurons to release oxytocin in the spinal cord and extra-hypothalamic brain areas, leading to penile erection and yawning [2,9,10]. Molecular biology studies allowed the characterization of at least three D2-like receptor subtypes, D2 (with the D2 short and D2 long splice variants), D3 and D4 [13&endash;15].
 
These receptor subtypes were cloned and inserted into cultured cells allowing the synthesis of selective agonists and antagonists. Binding studies with these molecules allowed the identification of dopamine receptor subtypes across the brain, and suggested their involvement in specific central functions and pathologies, from schizophrenia to autism [15]. Recently, a few D4 agonists were found capable of inducing penile erection when given systemically or into the PVN [16&endash;19]. This led to compare the effect of selective D2, D3 and D4 agonists and antagonists on penile erection and yawning with that of the D3-preferring agonist pramipexole and apomorphine given systemically and/or into the PVN. While some of these studies supported a main role for D3 receptors in penile erection and yawning induced by dopamine agonists (see [20&endash;22]), other studies supported a main role for D2 receptors (see [23,24]). However, at variance from [24] the above studies [21,23] failed to confirm a pro-erectile role of D4 receptors. In order to further characterize the role of PVN D2, D3 and D4 receptors, which have been recently found to co-exist in the cell bodies of PVN oxytocinergic neurons [25,26], and the mechanism by means of which pramipexole induces penile erection and yawning, the effects of pramipexole, apomorphine and the D4 preferring agonist PD 168,077 given into the PVN on penile erection, yawning and on the production of NO, measured by the concentration of NO2 - and NO3 - in the PVN dialysate obtained by intracerebral microdialysis, were compared. The effect of D2, D3 and D4 preferring receptor antagonists given into the PVN before dopamine agonists on these responses was also studied.
 
4. Discussion
 
The present results show for the first time that pramipexole, a D3 preferring receptor agonist [30&endash;32], when injected into the PVN of male rats at a dose that induces penile erection and yawning, increases the production of NO in the PVN as determined by the increase of the concentration of NO2 - and NO3 - in the paraventricular dialysate obtained by intracerebral microdialysis. These pramipexole responses were similar to those obtained with apomorphine, a classical mixed D1/D2-like receptor agonist, which also induces penile erection and yawning when injected into the PVN and increases NO production in the paraventricular dialysate [8,10,27]. Indeed, at the dose used (50 ng for both drugs), the efficacy of the two compounds (e.g., the maximal number of penile erections/rat and the maximal increase in NO production elicited in the first hour after treatment), did not differ significantly. PD 168,077, a selective D4 receptor agonist [33], also induces penile erection when injected into the PVN, which occurs concomitantly to an increase in NO production in the paraventricular dialysate, although less effectively than pramipexole and apomorphine. Indeed, at the dose of 100 ng, PD 168,077 increased the number of penile erection episodes and the concentration of NO2 - and NO3 - by about 60&endash;65% only of the values obtained with 50 ng of pramipexole and apomorphine [18,34]. However, at variance from pramipexole and apomorphine, this D4 receptor agonist, at least at the dose used in this study, was found unable to induce yawning, as if D4 receptors in the PVN were not involved in such response. Although it cannot be completely ruled out that different responses might have been obtained with higher doses of these dopamine agonists, this is unlikely since doses of pramipexole, apomorphine and PD 168,077 higher than 50 ng for pramipexole and apomorphine and 100 ng for PD 168,077 up to 200 ng were found unable to increase further the number of penile erection episodes when injected into the PVN [24].
 
The present results also show that the effects of pramipexole and apomorphine on penile erection, yawning and paraventricular NO production and of PD 168,077 on penile erection and paraventricular NO production when given directly into the PVN, are differentially influenced by dopamine preferring antagonists of the three dopamine receptor subtypes, D2, D3 or D4, given into the PVN before the dopamine agonists. Accordingly, penile erection, yawning and the increase of NO production in the paraventricular dialysate induced by pramipexole, were reduced almost completely by L-741,626, a D2 preferring receptor antagonist [35,36], but not by SB-277011A, a D3 preferring receptor antagonist [37,38] or L- 745,870, a D4 preferring receptor antagonist [39]. Similar results were obtained when penile erection, yawning and the increase of NO production in the paraventricular dialysate were induced by apomorphine, with the only difference that these responses (but not yawning) were found to be also decreased slightly (by about 25%) by L-745,870, despite the inability of this D4 receptor antagonist to reduce pramipexole responses. In contrast, penile erection and the increase in NO production induced by PD 168,077 were antagonized by L-745,870, which blocks D4 receptors, as expected [17,18,34], but were not reduced by L-741,626 or SB-277011A, which block D2 and D3 receptors, respectively. Together these findings are in line with the idea that penile erection, yawning and the increase of NO production induced by pramipexole and apomorphine, are mediated mainly by the stimulation of dopamine receptors of the D2 subtype, and that D3 and D4 receptors play only a minor role, if any, in the pro-erectile effect of these dopamine receptor agonists and in their ability to increase NO production in the PVN, as already discussed in detail for pramipexole-induced penile erection (see [24] and below). Briefly, it is unlikely that different results should have been obtained with higher doses of the dopamine antagonists used in this study. Accordingly, the doses of L-741,626, SB 277011A and L-745,870 used in this study were chosen because of the results of dose&endash;response curves showing that while 5 g of L-741,626 injected into the PVN (the same used in this study) reduced by more than 80% the increase of penile erection episodes induced by pramipexole given into the PVN, doses up to 5 g of SB-277011A (half the dose used in this study) and of FAUC 365 (another D3 preferring antagonist with a binding affinity for D3 receptors 20 times higher than that of SB-277011A [40]), and of L-745,870 up to 5 g, were completely ineffective against this dopamine agonist [24].
 
yawning
 
The present findings also confirm that the stimulation of D4 receptors in the PVN seems sufficient per se to induce penile erection, which also occurs concomitantly with an increase of NO production in the PVN. Nonetheless, in spite of these similarities with pramipexole and apomorphine in increasing penile erection episodes and NO production, this study shows that PD 168,077 does not increase yawning when injected into the PVN, at least at the dose used in this study. Although it cannot be ruled out that much higher doses of this compound may be found capable of triggering yawning when injected into the PVN, the finding is in line with other studies showing that PD 168,077 and other D4 agonists, such as ABT 724 and RO-10-5824, are unable to induce yawning when given systemically [20,21,23], as if D4 receptors, including those in oxytocinergic neurons in the PVN [25,26] were not involved in the facilitatory effect of D2-like dopamine agonists on this behavioral response. Whatever mechanism is responsible for the inability of PD 168,077 to induce yawning, the present results are in line with those of early studies showing that D2-like agonists injected into the PVN induce penile erection and yawning by acting on dopamine receptors located in the cell bodies of paraventricular oxytocinergic neurons [25,26] rich in NO synthase and projecting to the spinal cord and extra-hypothalamic brain areas (see [10,41] and references therein). The stimulation of these receptors causes an increase in Ca2+ influx into the cell bodies of these oxytocinergic neurons leading to the activation of NO synthase, a Ca2+&endash;calmodulin dependent enzyme, which increases NO production. NO in turn activates oxytocinergic neurons to release oxytocin in extra-hypothalamic brain areas controlling penile erection and yawning, apparently by a cGMP-independent mechanism [41,42]. In line with this hypothesis, the increase of penile erection and yawning and of NO production in the PVN dialysate induced by pramipexole, are strongly reduced by the prior injection into the PVN of S-methyl-l-thiocitrulline, which inhibits NO synthase [43] and by -conotoxin, a potent blocker of N-type-voltage dependent Ca2+ channels [44], while d(CH2)5Tyr(Me)2-Orn8-vasotocin, which blocks oxytocin receptors [45], reduces penile erection and yawning without affecting the increase in NO production when injected into the lateral ventricles but not into the PVN. These results are similar to those found with apomorphine and PD 168,077, as expected, since pramipexole, like apomorphine and PD 168,077, injected into the PVN acts on dopamine receptors localized in the cell bodies of oxytocinergic neurons [25,26] and not on oxytocin receptors to activate oxytocinergic neurons mediating penile erection and yawning, as discussed extensively in early studies [10,41]. Perhaps even more importantly for this work, G-protein-coupled D2-like receptors that may be linked to an increase in Ca2+ influx in cell preparations have been identified in cloned versions of D2-like receptors, mainly of the D4 subtype (see [16,46]). In this regard, it is noteworthy that PD 168,077 injected into the PVN, induces not only penile erection and increases NO production, but also increases c- Fos and extra-cellular signal regulated (ERK) expression in the PVN [47], effects that are all antagonized by D4 receptor antagonists [18,47]. The increased c-Fos expression induced by PD 168,077 in the PVN resembles the ability of pro-erectile doses of apomorphine to increase c-Fos content in discrete brain areas, including the PVN, when given systemically [48]. Unfortunately, it has still to be confirmed that the increased c-Fos expression induced by PD 168,077 occurs in PVN oxytocinergic neurons and if this increase is related to the pro-erectile effect of this D4 agonist [26].
 
Finally, this study provides further evidence for a differential role of D2, D3 and D4 receptors present in the PVN in mediating penile erection and yawning induced by dopamine D2-like receptor agonists. In particular, the major role of D2 vs. D3 receptor in penile erection and yawning induced by the D3 preferring agonist pramipexole suggested by this study, is in line with the recent study by Depoortère [23] and Sanna [24] and their coworkers, but is in contrast with those by Collins and coworkers [20,21], which supports a major role for D3 vs. D2 receptors. Briefly, while Depoortère [23] and Sanna [24] and their coworkers found that apomorphine- and pramipexole-induced penile erection and yawning are strongly reduced by L-741,626, but not by SB- 277011-A and other D3 antagonists, Collins and coworkers [20,21] found that pramipexole-induced penile erection and yawning are significantly reduced not only by L-741,626, but also by SB- 277011-A given at doses similar to that used by Depoortère [23] and Sanna [24] and their coworkers, and completely abolished by a very high doses of this compound, which is likely to act not only on D3 receptors but also on D2 receptors (see [23,24] and references therein). In spite of the above similarities and differences with the results of Sanna and coworkers [24] and this study, the studies by Collins [21] and by Depoortère [23] and coworkers failed to confirm the pro-erectile effect of D4 receptor agonists, such as PD 168,077 (used also in this study) and other D4 receptor agonists as well (e.g., ABT 724, A 412997, etc.) [16&endash;19,49,50]. The reasons of these discrepancies are unknown, although they may be due to methodological differences in the experimental protocols (i.e., several doses of each antagonist vs. a full dose&endash;response curve for each agonist [20&endash;22], against several doses of each antagonist vs. a single dose of agonist [23,24], different rat strains [23], different nutritional state [51], etc.).
 
In this regard, it must be emphasized that in the present study dopamine agonists and antagonists were given directly into the PVN, therefore the observed responses are the results of the selective action of these drugs on dopamine receptors in this specific brain area only. This is at variance from the studies of Collins [20,21] and Depoortère [23] and their coworkers, in which drug effects were studied only after systemic administration. This allows administered drugs to reach and act in every part of the central nervous system and in the periphery as well. Therefore, the observed behavioral effects are the results of the algebraic sum of the effects of these drugs at different brain and peripheral sites, since dopamine can influence penile erection and yawning by acting in different brain areas (see [41] and references therein). Moreover, the D2, D3 and D4 antagonists used in these studies have been identified for their ability to act on the different dopamine receptor subtypes mainly in in vitro experiments with cell membranes or cultured cells. It would be therefore considered not surprising that drugs with a similar profile in vitro, show different effects when administered in vivo and that these effects may be different when these drugs are given systemically or directly into specific brain areas, i.e., the PVN. Finally, although direct injection into the PVN eliminates eventual pharmacokinetic differences (ability to cross blood brain barrier, different metabolism rate, and so on), also in this case it is very difficult to prove that D2, D3 and D4 agonists and antagonists act on dopamine receptor subtypes with a profile identical to that found in in vitro experiments. Therefore, dopamine receptor agonists and antagonists with selectivities for the different dopamine receptor subtypes much higher than those of the drugs now available are necessary to define the exact contribution of PVN D2, D3 and D4 receptor subtypes in penile erection and yawning induced by D2-like dopamine agonists.
 
In conclusion, the present results show that pramipexole injected into the PVN of male rats at a dose that induces penile erection, induces also yawning and increases NO production in this hypothalamic nucleus with potency similar to that of apomorphine. PD 168,077 also injected into the PVN, induces penile erection and increases NO production, although with an efficacy lower than that of pramipexole and apomorphine as expected, but was unable to induce yawning. Since pramipexole responses, like those of apomorphine, were reduced almost completely by the prior injection into the PVN of dopamine receptor antagonists, which block preferentially D2 receptors, this suggests a main role of receptors of the D2 subtype in these responses, at least when induced by these dopamine agonists. However, the stimulation of D4 receptors in the PVN is sufficient per se to induce penile erection and to increase NO production. As pramipexole-induced penile erection and yawning are also reduced by the prior injection into the PVN of S-methyl-l-thiocitrulline, which inhibits NO synthase, of -conotoxin, which blocks N-type voltage dependent Ca2+ channels and of d(CH2)5Tyr(Me)2-Orn8- vasotocin, which blocks oxytocin receptors, these findings show that this drug also, like apomorphine, acts in the PVN to induce penile erection and yawning by increasing NO production, thereby activating central oxytocinergic neurotransmission.