Mating-induced
analgesia is dependent of copulatory male
pattern in high- and low- yawning male
rats
Gómora-Arrati P, Cortes C,
Trujillo A,
Encarnación-Sánchez JL,
Galicia-Aguas YL, González-Flores
O,
Eguibar JR.
Abstract
Mating behavior in rodents can modulate pain
sensations in both sexes. In males, the
execution of mounts, intromissions, and
ejaculations induced a progressive increase in
their vocalization thresholds induced by tail
shocks and other types of noxious stimuli. We
selectively inbred two sublines from
Sprague-Dawley (SD) rats that differed in their
spontaneous yawning frequency. The high-yawning
(HY) subline had a mean of 20 yawns/h and a
different pattern of sexual behavior
characterized by longer interintromission
intervals and more sexual bouts that delayed
ejaculation. The low-yawning (LY) subline and SD
rats yawned as a mean 2 and 1 yawns/h,
respectively. So, we determine mating-induced
analgesia in HY, LY, and SD male rats by
measuring vocalization thresholds in response to
noxious electric tail shocks. Our results showed
that the magnitude of mating-induced analgesia
was lower in HY and LY rats with respect to SD
rats. When the rats performed different
components of male sexual pattern, both sublines
exhibited a significantly lower increase in
their vocalization thresholds with respect to SD
rats-being sublines less responsive regarding
mating-induced analgesia. Pain modulation
mechanisms depend on responses to stress, so the
low levels of analgesia obtained in the yawning
sublines may be due either to differences in
their response to stress in other paradigms, or
to atypical performance of male sexual behavior
during mating, an event which as a stressful
event in rats. Therefore, the yawning sublines
are a suitable model for analyzing how a
different temporal pattern in the display of
male sexual behavior affects analgesia
mechanisms. Our results concur with Wistar rats
with different endophenotypes that could apply
to humans as well
Résumé
Le comportement d'accouplement chez les
rongeurs peut moduler les sensations de douleur
dans les deux sexes. Chez les mâles,
l'exécution d'intromissions et
d'éjaculations induit une augmentation
progressive de leurs seuils de vocalisation
induits par des chocs de la queue et d'autres
types de stimuli nocifs. Les auteurs ont
consanguinisé sélectivement deux
sous-lignées de rats Sprague-Dawley (SD)
qui différaient par la fréquence
de leurs bâillements spontanés. La
sous-lignée à bâillements
élevés (HY) avait une moyenne de
20 bâillements/h et un modèle
différent de comportement sexuel
caractérisé par des intervalles
plus longs entre les intromissions et un plus
grand nombre de rapports sexuels qui retardaient
l'éjaculation. La sous-lignée LY
(low yawning) et les rats SD ont
bâillé en moyenne 2 et 1
bâillement/h, respectivement. Ils ont
déterminé l'analgésie
induite par l'accouplement chez les rats
mâles HY, LY et SD en mesurant les seuils
de vocalisation en réponse à des
chocs électriques nocifs de la queue. Les
résultats ont montré que l'ampleur
de l'analgésie induite par l'accouplement
était plus faible chez les rats HY et LY
par rapport aux rats SD. Lorsque les rats ont
exécuté différentes
composantes du schéma sexuel masculin,
les deux sous-lignées ont
présenté une augmentation
significativement plus faible de leurs seuils de
vocalisation par rapport aux rats SD,
étant des sous-lignées moins
sensibles à l'analgésie induite
par l'accouplement. Les mécanismes de
modulation de la douleur dépendent des
réponses au stress, ainsi les faibles
niveaux d'analgésie obtenus dans les
sous-lignées de bâillements+
peuvent être dus soit à des
différences dans leur réponse au
stress dans d'autres paradigmes, soit à
une performance atypique du comportement sexuel
mâle pendant l'accouplement, un
événement qui est un
événement stressant chez les rats.
Par conséquent, les sous-lignes de
bâillement sont un modèle
approprié pour analyser comment un
schéma temporel différent dans
l'affichage du comportement sexuel mâle
affecte les mécanismes
d'analgésie. Ces résultats
concordent avec ceux des rats Wistar dont les
différents endophénotypes
pourraient s'appliquer également aux
humains.
Male sexual behavior differs among mammalian
species and can induce different endocrine,
behavioral, and adaptive changes including
alterations in pain sensation [9, 14, 20,
44]. For example, male rats have vocalized
less frequently in response to suprathreshold
electrical stimuli applied to the tail after
ejaculation [20]. Additionally,
paw-shake behavior in response to a constant
electric shock increased after ejacu- lation in
the golden hamster [9]. Comparable
results were obtained from rats in the hot-plate
test: specifically, paw-lick latency increased
after ejaculation [14].
Gonza _lez-Mariscal et al., [20]
conducted a detailed study in which they
observed an increase in vocalization thresholds
during the different components of sexual
behavior in male rats, i.e., after the display
of mounts, intromission, or ejaculation.
Remarkably, the authors showed that when two
copulatory series occurred sequentially, the
vocalization threshold was higher during the
second, indicating that mating-induced analgesia
could have an additive effect. Specifically, the
maximum analgesic effect occurred 20-min after
ejaculation and reached steady values even after
longer periods (60-min). Importantly, if the
subject's movement was restricted, causing a
stress response, this also had addi- tive
effects with respect to mating-induced analgesia
[20]. The above results suggest that
genital stimulation received by male rats during
the execution of mounts, intromissions,
ejaculations, and the subsequent ano-genital
grooming intensify sexual arousal as measured by
the number of penile erections, and gradually
increases the threshold for pain sensation
([20, 27]; Sachs, 2007).
Using different rat sublines, it is possible
to analyze analgesia levels that differ
according to male sexual behavior patterns. In
the present study, we used two selectively
inbred sublines from Sprague-Dawley (SD) rats
that differed in terms of the frequency of
spontaneous yawning. Specifically, the
high-yawning (HY) subline had a mean of 20
yawns/h, and the low-yawning (LY) subline had
just 2 yawns/h [8, 46]. HY male rats
also had more spontaneous penile erections, and
50% of these erections were temporally
associated with a yawn within a time window of
just 3 min [13, 23]. Sexually
experienced HY rats exhibit a copulatory pattern
that has a different organization compared with
sexually experienced SD rats. Specifically,
these rats engage in longer inter-intromission
intervals and more copulatory bouts that delay
ejac- ulation [11]. Additionally, the
proportion of HY rats that were non-copulators
was higher in HY compared with LY and SD rats
[38]. In contrast, LY rats had an
intermediate sexual pattern with respect to
sexually experienced HY and SD rats.
Based on these findings, the aim of this
study was to evaluate copulation-induced
analgesia in HY, LY, and SD rats. As an
indicator of pain modulation in a socio-sexual
context, we measured vocalization thresholds in
response to electric shocks applied to the tail
during the execution of different components of
male sexual behavior in freely moving rats.
4. Discussion
In this study, we measured mating-induced
analgesia according to threshold changes in tail
shock-induced audible vocalizations in both
yawning sublines and compared them with respect
to SD rats. Importantly, analgesia levels in the
SD strain were like those already obtained in
Wistar rats, which indicates that analgesia
mechanisms had similar spinal cord and
supraspinal physiological mechanisms between
both albino strains, bred from Norwegian rats
(Rattus norvegicus; [20, 27]). The
degree of VTTS changes were significantly lower
in the HY and LY sublines may be explained, at
least in part, by the differing display of
different components of male sexual behavior
between the sublines among the SD rats
[11].
Specifically, we corroborate that HY rats
have longer interintromission intervals that
delay ejaculation, so the amount of afferent
information during mating differs between HY
subline with respect to SD rats (see Table 2).
The performance of male sexual behavior in LY
rats is between that of HY and SD rats, and it
is important that our experimental paradigm
allowed us to measure the VTTS without
disrupting the characteristic organization of
sexual performance in either subline, as
previously reported [11].
The different temporal patterns of male
sexual behavior exhibited by the HY subline
likely produced unique inflows of genital
stimulation travelling through pelvic,
hypogastric, and even vagal afferents, which are
involved in the analgesia induced by mating
[20, 27]. The longer interintromission
intervals during copulation in the HY rats
induced an interruption of the ongoing genital
stimulation, including the activation of spinal
or supraspinal analgesic mechanisms. This may
also have occurred in lower magnitude in the LY
male rats. In fact, the amount of afferent
inflow depended on the temporal organization of
mounts, intromissions or ejaculation; and
successive intromissions led to increase pain
thresholds as shown in male rats with different
endophenoty- pes, called sluggish, intermediate
and rapid ejaculators in Wistar rats
[1]. It is well established that pain
sensations travel through peripheral A_ and C
fibers that arrive at the dorsal horn of the
spinal cord [3, 5, 31]. They then travel
through the dorsal column pathway and the
spino-thalamic tracts to reach upper brain
centers. There here they are processed and then
activate descending fibers from the dorsal raphe
nuclei, or from the locus coeruleus, inducing
analgesia at spinal cord level [15, 22,
49]. Inhibitory mechanisms can modulate
inhibitory spinal interneurons, which then
modulate primary afferents or second-order
interneurons at the level of the lumbosacral
spinal cord [3,5, 31].
In fact, during copulatory analgesia,
different excitatory and inhibitory amino acids
are released at lumbo-sacral levels, and release
opioid and oxytocin peptides that can modulate
pain at primary afferents or
interneurons [19, 32, 33, 39]. Our
results indicate that yawning sublines did not
experience significant analgesia after the
performance of five mounts probably because
mounts do not produce enough afferent in-
formation from the penis. However, after five
intromissions produced a characteristic pattern
of information inflow from the penis, scrotum
and perigenital area that induced a maximum
amount of analgesia with a long-time course (see
Table 1), HY rats reached lower percentage
changes in VTTS due to the characteristic longer
inter-intromission intervals that significantly
delayed ejaculation (see Table 2; [11]).
In Wistar rats with a rapid phenotype it has
been shown that shorter inter-intromission
intervals induce higher levels of analgesia and
that the opposite happens with sluggish
phenotype [1]. After ejaculation, the
afferent inflow reaches its maximum, and the
amount of analgesia is also the highest, and
concomitantly the differences among SD and
yawning sublines diminish (see Fig. 4C). These
effects are not due to age differences among the
subjects, an important factor in pain and
analgesic mechanisms [16].
In conclusion, the temporal organization
pattern of male sexual display impacts the
mating-induced analgesia of both yawning
sublines, as well as in Wistar rats with
different mating endophenotypes [1, 11].
In the case of yawning sublines, we previously
demonstrated that they differ in the maternal
behavior display with respect to SD dams
[45]. It has also been demonstrated that
maternal licking changes penile sensitivity, and
that low level of genital licking induces lowers
dendritic density in the bulbospongiousus
motoneurons [29]. We have already
demonstrated that HY male rats exhibited more
yawning and penile erections induced by D
2dopaminergic agonists or by the central
administration of oxytocin with respect to SD
male rats [10, 13]. Additional
experiments are needed to determine the role of
maternal care in the display of penile erection
under different ex-copula circumstances such as
non-contact penile erection on both yawning
sublines.
Another important aspect is that
mating-induced analgesia in female rats
[18]; in fact, it is possible to induce
analgesia through vaginal or rectal stimulation
with a decrease in the vibrissa or
leg-withdrawal re- flexes elicited by pinching
the base of the ear or the foot, respectively,
with fine mouse-tooth forceps [28].
Preliminary data showed that both sublines had
lower tail-flick latencies with respect to SD
female rats [41] supporting that
inbreeding process as well as the different
susceptibility to stress on yawning sublines
modify their analgesic responses as we
demonstrated with our results on mating-induced
analgesia particularly after the execution of 5
intromissions (see Fig. 4B and Table
1[4]).
Additionally, we recently showed that
atosiban, an oxytocin receptor antagonist, can
reduce analgesia induced by mounts and
intromissions, thus supporting the analgesic
role of oxytocin [17]. In this context,
it is relevant that our results, obtained with
intracerebroventricular (i.c.v.) injections of
different doses of oxytocin, induced higher
frequencies of yawning and penile erections in
HY rats with respect to LY and SD rats
[10]. These effects are mediated by
activity in the paraventricular nucleus of the
hypothalamus [2, 8].
Furthermore, it has been demonstrated that
mating induces the release of oxytocin in the
PVN, and this peptide it is known to promote
social bonding and produce anxiolysis
([48]: [35]), that could be
responsible, at least in part, for different
amounts of mating-induced analgesia.
It is well established, that oxytocin is
known to fine-tune general and social
anxiety-related behaviors, supporting the link
between anxiety disorders and imbalanced in the
oxytocinergic mechanisms [36]. HY rats
were found to ambulate more in the open field
arena and had fewer fecal boluses compared to
LY, supporting [the hypothesis?] that HY
are less emotionally reactive than LY subline
male rats [34]. Further, recent
unpublished data indicate that HY rats are
resilient to stressful conditions because they
spent more time in the open arms of the elevated
plus maze test compared with the LY subline
[12]. As oxytocin peptide plays a
central role in pair bonding, it is possible
that the differences in pain thresholds observed
between both sublines are due to different
levels of arousal and concomitant emotional
display during copulation, essential components
of mating behavior [42, 47], and can
even induce fear-avoidance behavior (Brauer et
al., 2014). These factors had additive effects
with the display of sexual behavior, which
clearly differed be- tween sublines with respect
to the SD male rats in our study. This is an
important consideration because pain thresholds
are correlated with anxiety, as correspondingly
demonstrated in humans [37]. Indeed,
copulation appears to be a stressful activity
that produces stress-induced analgesia in rats
(Buttler, 2009). Thus, the yawning sublines are
suitable models for analyzing these stress
effects with respect to mating interactions,
which can modulate pain mechanisms (Neuman 2008;
[47]).
A further important consideration regarding
our experimental design is the possibility that
through social cues the observer rats change
their behavior due to the experience of the
experimental rats because it has been
demonstrated that fear responses can be
transmitted in a conditioned paradigm [6,
25, 26, 40]. Because we used
counter-balancing and random assignation of both
sublines and the SD rats to the control and
experimental groups, we expected potential
social communication to be equal along all
experimental sessions. However, further
experiments are needed to determine role of the
observer rat in mating-induced analgesia on the
yawning sublines.
In conclusion, we demonstrated that both
yawning sublines had lower levels of
mating-induced analgesia with respect to SD
rats. These effects are probably due to
differences in the regulation of nociceptive
mechanisms at spinal or supraspinal levels that
modulate pain afferents. These sublines may
represent a useful model of altered sexual
performance and associated physiological
consequences such as analgesia.
To the best of our knowledge, this is the
first report to examine differences in
mating-induced analgesia in animals with
different phenotypic backgrounds.
