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18 mars 2004
Nitric oxide is involved in the ACTH-induced behavioral syndrome
R Poggioli, A Bennelli, ER Arletti, E Cavazzuti, A. Bertolini
Department of biomedical sciences, University Modena, Italy


Melanocortins (MSH/ACTH neuropeptides) induce in mammals one of the most complex and peculiar behavioral syndromes. Within a few minutes of injection into the cerebrospinal fluid or into specific brain areasbut not after peripheral administration they cause intense grooming and scratching that lasts about 1 h.
With a somewhat longer latency (10-20 min, depending on the site of intracranial injection), animals start to stretch and to yawn in the way they usually do when they awake from physiological sleep. These two components of the melanocortin-induced behavioral picture are usually associated and constitute a "syndrome within the syndrome," the so-called stretching and yawning syndrome (SYS), that lasts about 5-6 h (7).
In males, the behavioral picture is further complicated by the occurrence of spontaneous penile erections-independent of genital grooming or of the presence or absence of females-accompanied by pelvic thrusting as during copulation and usually terminating with ejaculation. Furthermore, melanocortins potently inhibit food intake, acting as satiety-inducing rather than as hunger-reducing agents, because they shorten the latency to stop eating without influencing the latency to start eating. Such effect, too, is observed only after intracranial administration.
Finally, ACTH or MSH and other ACTH fragments devoid of adrenocortical activity restore learning ability in hypophysectomized animals and dramatically influence performance of intact animaIs in a number of behavioral paradigms, improve shorttenn memory processes, and increase motivation and attention. These effects are obtained not only after injection into the cerebrospinal fluid, but also after peripherai administration. All these effects are adrenal independent.
Structure-activity studies have shown that these behavioral effects of melanocortins are produced by the melanocyte-stimulating cote of the molecule. An adequate activity of brain cholinergic neurons is required for the display of the syndrome, which is in fact prevented by the ICV injection of hemicholinium-3. The melanocortin-induced behavioral syndrome is also antagonized by the blockade of L- and N-type Ca++ channels; these data suggest that the influx of Ca into target neurons is a step of key importance in the occurrence of melanocortin-induced behavioral signs. Finally, quite recent data have shown that blockade of the NMDA receptor complex prevents the occurrence of excessive grooming, stretching, yawning, and penile erections following ICV administration of ACTH(1-24), suggesting an involvement of excitatory amino acids in the melanocortin-induced bebavioral syndrome.
Following activation of their respective receptors, both acetylcholine and excitatory amino acids stimulate guanylate cyclase, with consequent cyclic GMP accumulation in the brain, a process that requires Ca++.
NO is the endogenous stimulator of the soluble guanylate cyclase, and NO synthase activity is dependent on the free Ca++ concentration. Many experimental data suggest that, in the brain, NO may play a role in the effects of excitatory amino acids and of other neurotransmitters whose actions are associted with increases in cyclic GMP. All theese data prompted us to investigate the effect of NO synthase inhibition on the most typical behavioral symptoms induced in the male rat by the ICV injection of ACTH. [...]
Discussion : The observation that the melanocortin-induced behavioral syndrome is prevented by the functional blockade of brain cholinergic activity, by L- and N-type Ca++ channel blockade, and by blockade of the NMDA receptor complex (, and, on the other hand, the notion that NO synthase-whose activity is Ca++ dependent-plays an important role in the mechanism of action of excitatory amino acids and of acetylcholine, could suggest that NO synthesis is involved in the complex behavioral syndrome induced by the ICV injection of melanocortins.
Our present results indicate that inhibition of NO synthase indeed prevents the most typical signs of the melanocortin-induced behavioral picture (excessive grooming, stretching, yawning, penile erections), although in a different manner. So, excessive grooming and stretching are prevented by the systemic (IP) injection of NAME (which inhibits spontaneous grooming as well), but not by the ICV injection. On the other hand, yawning and penile erections are prevented after either ICV or IP administration of NAME.
Because dramatic changes in cardiovascular parameters, with marked hypertension, are induced by the peripheral, but not the ICV injection of NAME, we could not exclude the possibility that the effect of NAME on ACTH-induced stretching and grooming, observed after IP but not after ICV administration of the NO synthase inhibitor, is not specifically related to a role of NO in these ACTH-induced symptoms, but rather reflects a nonspecific consequence of increased blood pressure. So we studied the possible influence of a completely different hypertension inducing drug on the ACTH-induced behavioral syndrome. For this supplementary investigation we used the ganglion-stimulating agent dimethylphenylpiperazinium (DMPP) at the IP dose of 2 mg/kg, which in preliminary experiments in rats of the same strain caused an impressive (+41 % the basal mean arterial pressure value) and sustained (more than 90 min) hypertensive response. As shown in Fig. 3, DMPP had no influence on ACTH-induced yawning, stretching, and penile erections, but significantly reduced excessive grooming, F(2, 29) = 3.52, p < 0.05. This indicates that only grooming, and only in part, is inhibited by NAME, probably as a consequence of its hypertensive effect.
Thus, our present data demonstrate that NO plays a role in the ACTH-induced behavioral syndrome, at different levels and to a different degree depending on the different symptoms. This is a further confirmation of the repeatedly observed fact that melanocortin peptides induce their many behavioral effects through different mechanisms and acting at different sites.
Thus, for example, naloxone both mimies and potentiates ACTH-induced SYS and penile erections, but it antagonizes ACTH-induced grooming); SYS and penile erections are not prevented by dopaminergic: antagonists, whereas grooming is significantly attenuated. Clonidine inhibits ACTH-induced SYS and excessive grooming but not penile erections, and low doses of yohimbine increase the occurrence of ACTH-induced SYS and penile erection but not of grooming. Serotonin and serotoninergic agonists suppress melanocortin-induced excessive grooming and penile erections, but do not affect SYS.
Brain acetylcholine is essential for the display of the whole ACTH-induced behavioral syndrome, because the ICV injection of hemicholinium-3 completely abolishes stretching, yawning, grooming, and penile erections induced in adult rats by the ICV injection of ACTH(1 -24) but different acetylcholine receptors are involved for the different symptoms: the M2 antagonist, AF-DX 116, inhibits stretching, yawning, and grooming but not penile erections at low doses, whereas at higher doses it inhibits penile erections but not stretching, yawning, and grooming.
The nicotinic antagonist mecamylamine reduces excessive grooming and the number of penile erections, but not stretching and yawning. As far as the site of action is concerned, lesions of the amygdala, the mammillary bodies, and the dorsal and ventral hippocampus enhance melanocortin-induced SYS, but not grooming, which is in fact inhibited, and not enhanced, by partial lesioning of the hippocampal complex. Lesioning of the preoptic area eliminates the capacity of melanocortins to induce penile erections and ejaculation, but not to elicit SYS. The hypothalamic areas lining the third ventricle (particularly the anteroventral quadrant) are of critical importance for SYS and penile erections, whereas the periaqueductal gray is essential for groomin.
Even the structural requirements are different for the different melanocortin-induced behavioral symptoms: the core sequence for inducing the SYS is that of ACTH(4-12) the activity for excessive grooming resides within the 4-13 sequence, the 4-7 sequence being required but representing only part of the active core, the Cterminal elongation being needed for full expression of grooming. However, whereas ACTH(4-7) and ACTH(4-13) are active, ACTH(1 - 10) and ACTH(4- 10) are inactive. Finally, the minimum sequence for the induction of penile erection is the 4-10.
The data obtained in the present study indicate that the induction of yawning and penile erection involves brain nitrergic pathways, whereas the display of stretching and grooming involves peripheral nitrergic pathways.
-Poggioli R, Valerja Vergoni A, et al. Influence of clonidine on the ACTH-induced behavioral syndrome, European J. Pharmacol. 1984;101:299-301
-Poggioli R et al Nitric oxide is involved in the ACTH-induced behavioral syndrome Peptides 1995;16(7)1263-1268
-Poggioli R, Benelli A, Arletti R, et al. Old rats are unresponsive to the behavioral effects of adrenocorticotropin. Euro J Pharmacol 1994;271:253-257