It is well established that receptors for
steroid sex hormones exist in various brain
regions of vertebrates (McEwen, Gerlach, Luine,
& Lieberburg, 1977) and that sex steroid
hormones influence catecholaminergic
processes in the brain (Heritage, Stumpf,
Sar, & Grant, 1980). The precise nature of
this influence is not clear. It has been
reported that the sensitivity of
catecholaminergic systems to exogenously applied
dopamine agonists like apomorphine (APO) is
increased after chronic estrogen treatment of
male rats (Hruska & Silbergeld, 1980). Other
authors, however, reported that the sensitivity
to APO is decreased after chronic estrogen
treatment (Euvrard, Oberlander, & Boissier,
1980). Thus far, interactions of androgens with
catecholaminergic systems have received little
attention. During studies on the low-APO-induced
yawning syndrome in rats (Nickolson &
Berendsen, 1980) it was noticed that male rats
reacted more strongly to low APO compared to
females. The present experiments were carried
out to study the possible involvement of sex
hormones in this phenomenon.
Randomly bred adult rats were used. Male rats
weighed 280-400 g, females weighed 200-340 g.
They were housed in black PVC cages (48 x 24 X
21 cm) under a controlled light-dark cycle
(lights on at 6:00 Am, lights off at 6:00 Pm).
They had access to standard pelleted food and
tap water ad lib. Experiments were done between
8:30 Am and 12:30 Pm.
Castration and ovariectomy were performed
under pentobarbital sodium anesthesia (40 mg/kg
ip). After surgery rats were allowed to recover
for at least 6 weeks. Female rats were checked
for complete ovariectomy by examination of
vaginal smears.
Unless stated otherwise, yawning was induced
by 0.08 mg/kg sc APO, freshly dissoived in 0.9%
NaCl in water containing 0.5 mg ascorbic acid
per milligram of APO. Ascorbic acid protects APO
from oxidation but does not affect the yawning
response (unpublished). Immediately after APO
injection rats were placed in perspex
observation cages (7.5 x 18 X 30 cm) and the
number of yawns was recorded for 20 min.
Experiments were carried out in blocks of five
animals. Resuits were analyzed statistically
with the use of the randomization test.
Intact or castrated male rats were treated sc
with either arachis oil (0.2 ml per rat per day)
or testosterone propionate (TEP) in
arachis oil (50 or 100 µg per rat per day)
or dihydrotestosterone (DHT, Stanolone)
in arachis oil (50 or 100 µg per rat per
day) during the 3 days preceding the yawning
experiment. After the experiment the castrated
rats were sacrificed and the weights of seminal
vesicles and ventral prostate glands were
determined. Intact and ovariectomized female
rats were treated with either arachis oil (0.2
ml per rat per day) or DHT in arachis oil (50 or
100 µg per rat per day) during the 3 days
preceding the yawning experiment.
Preliminary studies had shown that 0.08 mg/kg
APO induced yawning less effectively in female
rats than in male. To see whether this
difference was due to a difference in
sensitivity to APO or to a difference in
responsiveness, a dose-response study was
carried out. The dose-response curves for female
and for male rats, shown in Figs, lb and la,
respectively, indicate that the main sex
difference lies in the maximal magnitude of the
yawning response and not in the sensitivity to
APO. This renders it unlikely that the sex
difference in APO-induced yawning is due to a
difference in the metabolism of APO.
To see whether female sex hormones might be
responsible for the lower response to APO, the
effect of ovariectomy was studied. As can be
seen from Table 1, ovariectomy did not result
in an increase of APOinduced yawning. This
observation strongly suggests that female sex
hormones do not suppress APO-induced yawning,
although there is no absolute proof since extra
ovarian stores for female sex hormone like the
adrenals are not eliminated by ovariectomy. On
the other hand, removal of sex organs had a
profound effect on APO-induced yawning of male
rats. Castration resulted in a
considerable decrease of yawning when 0.08
mg/kg of APO was used. Higher doses of APO
failed to increase the yawning response (not
shown) which suggests an effect of castration on
responsiveness and not on the sensitivity to
APO.
Treatment of both intact and castrated
male rats with TEP resulted in an increase of
APO-induced yawning. To exclude the
possibility that the effect of TEP is caused by
estradiol formed from TEP the effects of DHT,
which cannot be metabolized to estradiol and is
therefore a more "pure" androgen, were studied.
Table 1 shows also that DHT treatment of
castrated male rats resulted in an increase of
APO-induced yawning, In intact male rats no
significant increase could be observed. Upon
examination of ventral prostate and seminal
vesicle weights of castrated rats a discrepancy
between the effects on APO-induced yawning and
on peripheral androgenic effects of TEP and DHT
became apparent. TEP, 50 and 100 µg,
resulted in 196 and 265% increases of ventral
prostate weight and 157 and 205% increases in
seminal vesicle weight, respectively, whereas
only 100 µg of TEP was effective in
increasing yawning. On the other hand, DHT, 50
and 100 µg, resulted in only 38 and 62%
increases of ventral prostate weight and 30 and
57% increases in seminal vesicle weight,
respectively, whereas both doses of DHT were
effective in increasing APO-induced yawning in
castrated rats. The present results thus confirm
previous findings that DHT is less potent than
TEP with regard to androgenic properties as
measured on ventral prostate and seminal vesicle
weight. With respect to yawning, however, both
steroids seem to be equally effective.
Further studies employing pharmacokinetically
similar steroids (e.g., testosterone vs
dihydrotestosterone or testosterone propionate
vs dihydrotestosterone propionate) are needed to
establish whether real differences exist between
the responsiveness to androgens of peripheral
sex organs and cerebral tissue.
Finally, the effect of DHT in intact and
ovariectomized female rats was studied. As in
castrated male rats, DHT in doses of 50 or 100
µg per rat per day increased APO-induced
yawning in both intact and ovariectomized
females, although the effect seemed to be
somewhat greater in ovariectomized rats (Table
1). To see whether the effects of DHT in
low-APO-induced yawning might be related to its
general anabolic rather than to its androgenic
character, a preliminary experiment was
performed with the anabolic steroid nandrolone
decanoate. Three-day treatment of female rats
with this steroid failed to increase
low-APO-induced yawning.
Taken together, the present results indicate
that low-APO-induced yawning is under the
influence of androgenic steroids. In contrast
to, e.g., the sensitization toward APO brought
about by des-tyrosine-gamma-endorphin the nature
of this influence seems to be permissive rather
than sensitizing. This
suggests that androgens do not directly affect
the receptor system which is challenged by low
doses of APO but facilitate the expression of
the yawning syndrome. In this respect
it is interesting to note that testosterone also
potentiates the effectiveness of
intraventricularly administered ACTH, in
inducing a syndrome which is characterized by
stretching and yawning.
Effects of testosterone in intact rats may be
explained by assuming that after subchronic
treatment with testosterone a stable, high level
of circulating testosterone is established, in
contrast to the level of circulating
testosterone in normal intact rats which shows a
diurnal variation which has its own
characteristics for each individual rat. This
may mean that in testosterone-treated intact
rats testosterone-dependent biological phenomena
show less variation and occur with an average
intensity higher than that in placebo-treated
intact rats.
In contrast to high-APO-induced behavioral
syndromes, estrogens seem to play only a minor
role in low-APOinduced yawning. In a preliminary
experiment TEP treatment had no effect on
high-APO-induced stereotyped gnawing in intact
female rats. Further studies are needed to
establish whether androgens also influence other
behavioral effects of low APO like depression of
open field activity and affect high-APO-induced
stereotypies.
