ACTH and
alpha-MSH induced grooming, stretching, yawning
and penile erection in male rats: site of action
in the brain and role of melanocortin
receptors
Introduction : Adrenocorticotropin
(ACTH) and aalpha-melanocyte-stimulating homone
(a-MSH) induce a variety of behavioural
responses when injected into the central nervous
system. The best known of these responses are
the so-called stretching-yawning
syndrome, penile erection and ejaculation,
grooming, changes in feeding, pain perception,
inflammation, learning and memory and
sociosexual behaviours. The mechanism by means
of' which ACTH, a-MSH and related peptides
induce these responses, is still little
understood. Indeed, it proved very difficult to
find ACTH and/or a-MSH binding sites in the
brain, although some of these responses were
discovered as early as the 1960s, and although
it was discovered in the 1980s that ACTH and
a-MSH derive from the common 31 kD protein
precursor, pro-opiomelanocorticotropin, that
proopionielanocorticotropin-containing neurons
were present in the brain, and that ACTH
receptors in adrenal tissue stimulate adenylate
cyclase.
Cloning studies led to the characterization
of five subtypes of melanocortin (MC) receptors,
named MC1 MC2 MC3 M4 and MC5, which are
differently distributed in peripheral and brain
tissues. The melanocortin MC1 receptor is the
a-MSH receptor prescrit in melanocytes involved
in pigmentation and in immune cells
(macrophages), while the melanocortin MC2
receptor is the ACTH receptor present in the
adrenal tissues. Of the remaming receptors, the
melanocortin MC3 and MC4 receptors are found
almost exclusively in the central nervous
system, especially in the hypothalamus while the
melanocortin MC5 receptor seems involved mainly
in the control of exocrine glands. The selective
expression of these melanocortin receptors in
cultured transfected cells allowed the
characterization of ACTH-MSH analogues with
different affinities for, and that act as
agonists or antagonists on the above receptor
subtypes. These analogues led to the
identification of central melanocortin MC4
receptors as those mediating the inhibitory
effect of ACTH and a-MSH ou feeding behaviour,
in fine with other studies showing that the
melanocortin receptors are involved in the
agouti obesity syndrome. There is also evidence
that the melanocortin MC4 receptor mediates
grooming, yawning and stretching, but not
penile erection induced by a-MSH. Indeed HS014,
a selective antagonist of melanocortin MC,
receptors, prevents grooming, stretching,
yawning but not penile erection induced by a-MSH
injected into the lateral ventricles.
This study was aimed at identifying brain
areas that respond to the injection
ol'ACTH(1-24) and a-MSH with grooming,
stretching yawning and penile erection. The
effect of HS014 injected into an identified
brain area responsive to ACTH (1 -24) or a-MSH,
on grooming, stretching, yawning and penile
erection induced by the above peptides is also
reported.
[...]
Discussion : The present study shows
that ACTH(1-24) and a-MSH induce stretching,
yawning and penile erection in male rats when
injected into the hypothalamic periventricular
region surrounding, the third ventricle. In
particular, the two peptides were found active
when injected into the
paraventricular nucleus, the dorsomedial
nucleus, the anterior hypothalamic area, the
ventromedial nucleus and surrounding areas, and
inactive when injected into the preoptic area,
the caudate nucleus or the CA 1 field of the
hippocampus. This confirms and extends previous
results showing that ACTH(1 -24) induces
grooming and yawning when injected in the
para\entricular nucleus and surrounding areas.
Indeed, to our knowledge this is the first
report showing that ACTH(1-24) and a-MSH induce
penile erection when injected into the
hypothalamic periventricular area. The
behavioural responses induced by ACTH(1-24) and
a-MSH injected into the hypothalamic
periventricular region are indistinguishable
from those induced by the two peptides given
intracerebroventricularly. In this regard, it is
pertinent to recall that the ACTH(1-24)- and
a-MSH-induced stretching, yawning and penile
erection are peculiar in several aspects:
-firstly, they start usually 25-35 min after
the peptide injection into the lateral
ventricles,
-secondly they are usually long lasting.
This also occurs when ACTH(1-24) and a-MSH are
injected into the hypothalamic periventricular
region.
Indeed, irrespective of the injection site,
stretching, yawning and penile erection started
usually 15-30 min after treatment, while
grooming started in 5-7 min after treatment. The
above pecularities distinguish the ACTH and
a-MSH responses from those similar induced by
other agents. For instance, also oxytocin,
dopamine receptor agonists and
N-methyl-D-aspartic acid (NMDA) induce yawning
and penile erection when injected into the
paraventricular nucleus of the hypothalamus,
however these responses start usually in 5-7 min
afiter treatment and last for 45-60 min. Perhaps
most important, oxytocin. dopamine receptor
agonists and NMDA act to induce the above
behavioural responses exclusively in the
paraventricular nucleus, while ACTH and a-MSH
act pratically in all the periventricular region
of the hypothalamus.
The extension of the area in which
ACTH(1-24) and a-MSH induce the above responses
might also explain why bilateral electrolytic
lesions of the paraventricular nucleus prevent
yawning and penile erection induced by oxytocin
and dopamine agonists, but not by ACTH(1-24)
given intracerebroventricularly. Indeed,
although the paraventricular nucleus is a site
where ACTH(1-24) given into the lateral
ventricles induces the behavioural syndrome, the
peptide can still induce the behavioural
responses by acting in other hypothalamic
periventricular sites in animals with bilateral
lesions of the paraventricular nucleus, unlike
oxytocin and dopamine receptor agonists. The
same interpretation might also be used to
explain the inability of w-conotoxin injected
into the paraventricular nucleus to prevent
ACTH(1-24)-induced stretching, yawning and
penile erection, despite the ability of the
peptide toxin to prevent oxytocin- and dopamine
receptor agonist-induced responses. Together,
the above resuils are in line with earlier
hypotheses that ACTH(1 -24) and (a-MSH induce
grooming, stretching, yawning and penile
erection by acting w ith a mechanism(s)
different from that of oxytocin, dopamine
agonists and NMDA. Accordingly, the behavioural
syndrome induced by ACTH(1-24) is not prevented
by oxytocin receptor antagonists, which prevent
oxytocin-, dopamine agonist- and NMDA-induced
yawning and penile erection.
The present study shows also that HSO14, a
selective antagonist of the melancortin MC4
receptor, prevents grooming, stretching and
yawning, but not penile erection induced by
ACTH(1-24) and a-MSH when injected into the same
hypothalamic periventricular area. These results
confirm and extend previous studies showing that
a-MSH given into the lateral ventricles induces
groorning, stretching and yawning, but not
penile crection by acting through melanocortin
MC4 receptors. In line with this hypothesis,
firstly, the hypothalamus contains high
concentration of melanocortin MC4 receptors,
secondly, melanocortin receptors recognise both
ACTH(1-24) and a-MSH, and thirdly, ACTH(1-24) is
slightly more effective than a-MSH, as found in
melanocortin MC4 receptor transfecte cells in
vitro. Since the prevention hy HSO 14 of
ACTH(1-24) and a-MSH-induced grooming,
stretching and yawning is complete, the
involvement of other melanocortin receptors in
these reponses, i.e., melanocortin MC3
receptors, which hare also present in the
hypothalamus, seems unlikely at least in the
hypothalamic periventricular region. Similarly,
the failure of HS014 to prevent penile erection
suggests that melanocortin MC4 receptors are not
involved in this sexual reponse, at least in the
hypothalamic periventricular region.
In view of the high concentration of
melanocortin MC3 receptors in the hypothalamus,
the finding raises the possibility that penile
erection induced by ACTH(1-24) and a-MSH is
mediated by melanocortin MC3 receptors. However,
this is unlikely since HS014 blocks also
melanocortin MC3 receptors in vitro, although
less effectively than melanocortin MC4
receptors.
Therefore, if one assumes that penile
erection is mediated by melanocortin MC3
receptors, some prevention of this response by
HS014 would have been found, at least with the
highest dose tested. More selective agonists or
antagonists of these receptors are necessary to
verify such a possibility. Conversely, a role of
other melanocortin receptors MC1 and MC5
receptors) cannot be completely ruled out, since
also these two receptors are found in the
central nervous system, although not so abundant
as the melanocortin MC3 and MC4 receptors.
The moleculair mechanisins by means ofwhich
the stimulation of melanocortinn receptors by
ACTH(1-24) and a-MSH in the hypothalamic
periventricular region induce the above
behavioural is unknown at present. Several
neurotransmitters and neuromodulators are
involved in the above responses induced by
ACTH-MSH peptides. Among these are Ca2+ and K+
ions, nitric oxyde and acetylcholine.
Indeed ACTH- induced stretching, yawning and
penile erection are prevented by Ca2+ channel
blockers and by K+ channel openers, by nitric
oxide synthase inhibitors [32] and by
muscarinic receptor antagonists that cross the
blood-brain barrier. As in these studies
ACTH(1-24) was usually given into the lateral
ventricles, it is unknown at prescrit if the
above results apply also to this study, in which
ACTH(1-24) and a-MSH are given directly into the
hypothalamic periventricular region. As recalled
above, w-conotoxin, a selective blocker of
neuronal Ca2+ channels of the N-type, prevents
ACTH(1-24) induced stretching, yawning, and
penile erection when given into the lateral
ventricles but not when given into the
paraventricular nucleus. Further complications
derive from the fact that nitric oxide synthase
inhibitors and muscarinic receptor antagonists
were given systematically to prevent ACTH(1-24)
responses. This makes it impossible to ascertain
whether the prevention of the ACTH(1-24)
response is due to the blockade of nitric oxide
synthase and/or muscarinic receptors in the
hypothalamic periventricular region or ni other
brain areas. Nevertheless, it is noteworthy that
melanocortin receptors have been found coupled
to adenylate cyclase-cAMP- or to
phosphatidyl-inositol-Ca2+ mediated signalling
systems not only in the periphery, but also in
the central nervous systern.
Unfortunately, the discovery that
hypothalainic melanocortin MC4 receptors
apparently mediate stretching and yawning does
not provide any explanation for the delayed
appearance recalled to above of these two
responses after ACTH(1-24) and a-MSH, nor does
it clarify if the delay is related somehow to
the extension of the hypothalainic area in which
ACTH(1-24) and a-MSH induce their effect. Since
penile erection also occurs after a delay
similar to that of stretching and yawning, while
groorning does not, it is likely that the
stimulation of different melanocortin receptors
by the above peptides activates several
distinct, still undiscovered mechanisms in the
hypothalainic periventricular area that lead to
the above behavioural responses.
In conclusion, the present results show
that ACTH(1-24) and a-MSH induce grooming,
stretching, yawning and penile erection when
injected into the hypothalamic periventricular
region. Apparently grooming, stretching and
yawning are probably mediated by melanocortin
MC4 receptors, while penile erection is
not.
