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
