The stretching and yawning syndrome (SYS) is
a complex behaviour specifically induced by the
central administration of ACTH-MSH peptides.
Recently we have found that ACTH activity in rat
hypothalamus is represented by a heterogeneous
population of peptides rather than by a unique
peptide.
During an attempt to identify the
hypothalamic forms of ACTH capable of inducing
SYS, we discovered a peptide which did not show
ACTH-like bioactivity and immunoreactivity, but
induced yawning and penile erection in rats.
Amino acid and sequence analysis showed the
peptide to be oxytocin. Since experimental
evidence indicates that oxytocin acts as a
neuropeptide in the central nervous system, we
further characterized the oxytocin effect on
yawning and penile erection. Male Sprague Dawley
rats (250-300 g) were used.
A stainless steel guide cannula was
implanted chronically into a lateral ventricle 3
days before the experiments (coordinates: 1 mm
anterior to bregma, 1.5 mm lateral to midline
and 2 mm ventral to dura, Pellegrino and
Cushman, 1971). Oxytocin was dissolved in saline
and injected intraventricularly (i.c.v.) in a
volume of 10µ l.
As shown in table 1, yawning and penile
erection were induced by a dose ox oxytocin as
low as 5 ng in 60% of the animals treated. The
most effective doses were 20 and 60 ng. The 120
and 1200 ng doses were ineffective in agreement
with previous studies.
Moreover, the oxytocin effect was prevented
by the antimuscarinic agent atropine (10 mg/kg
i.p.), but not by methylatropine (10 mg/kg i.p.)
or the opiate antagonist naloxone (1 mg/kg
i.p.).
It is not yet known whether or not the
oxytocin-induced and the ACTH-MSH
peptide-induced yawning and penile erection are
mediated by similar mechanisms (i.e. ACTH-MSH
peptides released by oxytocin in some brain area
or vice versa) and why the high doses of
oxytocin were ineffective. However, two
important features distinguish the
oxytocin-induced yawning and penile erection
from those induced by ACTH-MSH peptides: (1) the
doses of oxytocin that induce yawning and penile
erection (5-60 pmol) are much lower than those
of ACTH-MSH peptides that are active at doses of
1-2 nmol; and (2) oxytocin-induced yawning and
penile erection begin 8-10 min after i.c.v.
injection of the peptide, while a lag of 25-30
min is always observed after i.c.v. injection of
ACTH-MSH peptides before the start of SYS and
sexual behaviour (Bertolini and Gessa,
1981).
Although further experiments are necessary
to clarify the mechanism and the site(s) of
action of oxytocin, the present results suggest
that oxytocin plays a crucial role in the
expression of yawning and penile erection in
rats and provide further evidence that oxytocin
acts as a neuropeptide in the central nervous
system.
