Early reports
on the stretching and yawning syndrome (SYS)
induced in rats by intracisternal injections of
ACTH and MSH were soon followed by others which
described that intraventricular or intracerebral
administration of ACTH or MSH also produced
signs of sexual excitement (penile erection (for
the sake of brevity we will use the expression
penile erection to describe a behavioral pattern
which, in the rat, may also include genital
grooming, pelvic thrusts or ejaculation and
licking of the genitalia in the male, lordotic
posture in the female), not only in rodents but
also in other mammals. Different pharmacological
evidence suggesting an association between
yawning and sexual excitement began with work by
Baraldi and Benassi-Benelli and Mogilnicka
and Klimek demonstrating that systemic
administration of apomorphine (APO) and other
dopamine (DA) agonists elicits both yawning and
penile erections in the rat.
Systematic observation of spontaneous and
pharmacologically induced yawning and penile
erections in a line of Sprague-Dawley rats
selectively bred in our laboratory to establish
a high incidence of yawning has led us to
suggest that yawning and penile erection are
correlated even in the normal spontaneously
behaving animal. As reasonable parallelism
exists between the elicitation or inhibition of
yawning and penile erections by drugs acting
through DA pathways, we further suggest that
dopaminergic mechanisms operate in the
regulation or modulation of these two behavioral
patterns. [...]
Discussion
A strict correlation between spontaneous
yawning and penile erections was certainly
difficult to establish in Wistar rats, because
of the low frequency of these behavioral
patterns. In that strain of rats spontaneous
yawning has, according to different authors
average frequencies within a range from 0,1 to 2
yawns/hr. For penis erection, an occurrence of
30% and an average frequency of 0,3 erections/hr
were reported by Bertolini et al, from
experiments in which the animals were observed
during two hours. Their data show a yawning
occurrence of the order of 60%, with an average
frequency of 1.5 yawns/hr. The progress made in
our laboratory, in the selective breeding of a
'frequent yawning' line of Sprague-Dawley rats
has allowed us to demonstrate that penile
erection frequency increased linearly from
around 0.4 erections/hr in the group of
nonyawning rats, to 2 erections/hr in the group
that had an average spontaneous frequency within
a range from 26 to 30 yawns/hr. This correlation
suggests that some common underlying factors may
participate in the regulation of these two
behavioral patterns. For Bertolini et al, when
both behavioral patterns occur under
physiological conditions, they might be elicited
by an endogenous peptide closely related to CRF,
and containing the peptide sequence required for
eliciting stretching, yawning and sexual
stimulation.
The very similar dose-effect curves obtained
by us for yawning and penile erection,
when elicited with two DA agonists (APO and
bromocriptine) and their equivalent depression
in frequency by the selective presynaptic DA
antagonist, metoclopramide strongly suggest that
penile erection and yawning are both under
dopaminergic control. In relation to
yawning, which is also elicited in rats by
cholinomimetic drugs, physostigmine or
pilocarpine, a hypothetical model of its central
control mechanism has been proposed, which
includes a dopaminergic inhibitory-cholinergic
excitatory link. Low doses of APO or other
dopaminergic agonists, by activating presynaptic
DA autoreceptors would inhibit DA release and
thus disinhibit the cholinergic neurones
exciting yawning. Wood et al have proposed that
septalhippocampal cholinergic neurones are
involved in the specific SYS elicited by
intraventricular injection of MSH or ACTH.
It is therefore tempting to suggest, as
Yamada and
Furukawa implicitly do, that the
dopaminergiccholinergic link in yawning might
involve the DA pathway ascending from the A1O
mesencephalic cell group to the limbic region
and the septal-hippocampal cholinergic neurones.
It seems interesting to recall that Passouant et
al had observed yawning in cats during the
post-discharge period following electrical
stimulation of the hippocampus.
With respect to genital function, consistent
results from MacLean and coworkers (as reviewed
by MacLean showed that electrical stimulation of
several sites in the limbic areas, leading to
the buildup of high-voltage potentials or after
discharges in the hippocampus, are generally
accompanied by penile erection. In later
experiments done with Kinnard, MacLean found
that depositing a-MSH or ACTH in solid form in
the septo-preoptic region of the squirrel monkey
resulted in recurrent episodes of stretching,
yawning, scratching of the body and full penile
erections which, in about three hours, could
reach a frequency of twice per minute.
The reviewed evidence points strongly to the
existence of some common regulating or
modulating elements interrelating, under certain
physiological or experimental circumstances two
behavioral patterns as different as yawning and
penile erection. The complex neural circuitry
interconnecting the different structures of the
limbic brain may offer neuroanatomical sites for
this interaction. Common modulating influences
seem also important. Hormones such as
testosterone, or a particular polypeptide
sequence contained in a-MSH and ACTH facilitate
both yawning and penile erection. Tonic changes
in neurotransmitter influences, as exerted by
the ascending mesolimbic pathway or other DA
pathways, may inhibit or disinhibit these
behavioral patterns.
The most recent report by Serra
et al that hypophysectomy in rats prevents
yawning and penile erections induced by
apomorphine is quite a strong argument in favor
of their hypothesis that both behavioral effects
are mediated by pituitary hormones. They suggest
that activation of DA autoreceptors in the
hypothalamo-hypophyseal DA neurones by low doses
of apomorphine might remove an inhibitory
control on MSH relcase, the hormone reaching
target areas in the brain by retrograde portal
flow. This hypothesis appears very attractive
because it offers a unifying explanation of the
elicitation of yawning and penile erections by
so different substances as the common
polypeptide sequence contained in a-MSH and
ACTH, and DA agonists.
