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mise à jour du
25 mars 2004
Europ J Neurosci
1996; 8; 2056-2063
Dopamine agonists increase nitric oxide
production in the paraventricular nucleus
of the hypothalamus:
correlation with penile erection and yawning
MR Melis, S Succu, A Argiolas
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.