The temporal organization of masculine
sexual behavior in rats is highly stereotyped;
involving a sequence of mounts, intromissions
and ejaculations. Sexual behavior has been
described in exogamic and genetically
manipulated rodent species. In this work, we
compare the male sexual behavior of outbred
Sprague-Dawley (SD) to those of rats inbred for
high (HY)- and low (LY)- spontaneous yawning
frequency. In the first experiment, the
percentage of inexperienced rats' ejaculatory
behavior is significantly lower in the HY and LY
respect to Sprague-Dawley rats. The latency to
ejaculate for inexperienced HY was shorter than
the LY and SD rats. In the second experiment, we
examined the differences between inbred sublines
and Sprague-Dawley rats once the subjects had
become sexually experienced after four
copulatory sessions. HY rats still have slower
proportion of ejaculators respect to LY and SD
rats. Additionally, postejaculatory latencies
were longer for HY rats, with longer
intercopulatory intervals and higher number of
copulatory bouts that delayed ejaculation. Both
sublines show lower copulatory efficiency
respect to SD rats. In conclusion, both sublines
show alterations in the temporal organization of
sexual motor pattern that are due at least
partially to strong inbreeding process to select
them.
1. Introduction
The use of selected strains, lines and
sublines of rats that differ in their
spontaneous frequency of behavioral traits is a
powerful tool for studying genetic influences on
normal and altered behavior [1]. For
this reason, inbred groups of rats are useful
models to explore the inter- play between genes
and behavior. Male rat sexual behavior consists
of a highly stereotyped series of motor
patterns, of which timely sequential
organization is essential for reaching
ejaculation [2]. The masculine cop-
ulatory behavior of the rat consists of a series
of mounts, intromissions and ejaculations
[2]. Mounts are characterized by an
approach to the es- trous female rear
accompanied by forepaw palpations of the flanks
and pelvic thrusting. The intromission
behavioral pattern is similar to that of a
mount, but finishes abruptly with a quick
dismount after penile vagi- nal insertion.
Ejaculation is behaviorally characterized as a
deep pelvic thrust lasting several seconds
during which animals maintain an immo- bile
position, followed by a rapid dismount. During a
copulatory event, the male rat performs various
mounts and intromissions that lead to
ejaculation within approximately 10 to 15 min.
The number and timing of mounts and
intromissions that occur prior to ejaculation
are impor- tant measures associated with
pregnancy success [3,4]. The time to
dis- play the first mount and intromission after
introducing a receptive female to the test arena
are designated as mount and intromission la-
tencies and denote the motivational aspect of
copulation [2,4].
Furthermore, sexual experience results in
highly efficient male rats with ejaculation
latencies shorter than 15 min [4].
Additionally, the observa- tion of copulatory
behavior is particularly useful to analyze
temporal or- ganization patterns (for a review
see [5]).
Interestingly, some rat strains differ in
their expression of mounting, intromission or
ejaculation frequencies and timing to ejaculate,
suggest- ing that genetic background modulates
sexual behavior [6]. For exam- ple, it
has been described that male Lewis rats have
lower copulatory efficiency and frequently fail
to ejaculate, independently of the female
partner strain Lewis, Fischer (F344) or
Sprague&endash;Dawley (SD) [7]. Addi-
tionally, Flinders subline of male rats showed
lower proportion of ejac- ulators between
resistant as compared to those that are
sensitive to cholinergic drugs [8].
Similar findings were found in the Roman low-
avoidance (RLA) Wistar line male rats with a
decreased sexual perfor- mance respect to Roman
high-avoidance (RHA) [9].
In our laboratory, we selectively inbred two
sublines of Sprague&endash; Dawley rats: the
high-yawning (HY) and low-yawning (LY) sublines,
with a mean spontaneous frequency of 20 and 2
yawns/h, respectively [10]. We have
demonstrated that among rats in the HY subline,
yawning and penile erection are correlated; with
50% of the yawns occurring prior to, or
following penile erection in a 3 min period
[11]. Additionally, HY subline exhibited
a disorganized grooming sequence after fur wet-
ting or when exposed to a novel environment,
with more grooming bouts due to a higher
incidence of pauses [12,13].
Furthermore, dams of the HY subline also
displayed deficient maternal care associated
with a disorganized maternal behavior pattern,
which is characterized by shorter latencies to
retrieve the first pup and the litter, and
higher in- cidence of atypical retrievings from
the belly or the legs and re- retrievings when
the dams take the pup outside the nest
[14].
The aims of the present experiments were to
analyze the organiza- tion of the copulatory
behavior of sexually inexperienced males from
the HY and LY sublines. In a second series of
experiments, we evaluated the copulatory
behavior of HY and LY males with previous sexual
expe- rience (four copulatory sessions). For
comparison, a group of outbred SD rats was
included in both experiments. We hypothesized
that the HY and LY males would display deficient
sexual performance, relative to the outbred male
SD rats.
Discussion
In the rat, it has been demonstrated that
male sexual performance in adulthood is strongly
dependent on maternal care during the first days
of life and that stress responses in adulthood
are dependent on the ma- ternal care given by
the dams. Thus, the high-licking/grooming (LG)
dams produce offspring that are resistant to
stress and influence their sexual performance by
shorter inter-intromission intervals in these
males with respect to low-LG dams [30].
Therefore, the male sexual def- icit shown in
the present experiments in HY rats could be due
to the im- paired maternal care given by the
dams [14]. In fact, preliminary
experiments revealed that HY males fostered by
the LY or SD dams displayed improved sexual
performance (data not shown); suggesting an
interaction of genetic and epigenetic mechanisms
in the deficit in male sexual behavior
particularly in the HY rats.
Until now, only some selected sublines of
rats have been evaluated for their sexual
performance. For example, male Flinders rats
that are re- sistant to cholinergic agonist-
induced increase in central temperature showed
poor sexual performance. Similar results were
obtained with high-thermal responders to
systemic injection of serotonergic 1a sub- type
agonists, which also showed poor sexual
performance because of longer ejaculation and
post-ejaculatory latencies relative to lower
ther- mal responders [8,22]. Another
recent study revealed that the RLA male rats had
poor sexual performance relative to RHA male
rats [9].
Interestingly, the ejaculation latency is
one of the main parameters that differs across
different strains or lines/sublines of rats. For
example, male Lewis rats have longer ejaculatory
latencies than male Fischer or SD rats
[7]. Similarly, HY rats showed longer
ejaculatory and postejaculatory latencies
suggesting that inbreeding alters the mecha-
nisms that control them.
Another interesting relationship arises from
general activity and sexual performance, thus,
hyposexual non-copulating males showed
hyperactivity in the open-field arena, while
males with adequate sexual performance
consistently present lower ambulation scores
than their counterparts [31].
Remarkably, HY rats show higher ambulation
scores associated with a higher proportion of
non-copulating males [13,25, and present
results] as compared to LY subjects,
therefore, hyperactivity and sexual display can
be expressions of altered stress responses in
the HY subline of rats.
Sachs & Barfield [29] suggested
that the inter-copulatory intervals and the
number of copulatory bouts are essential
variables to analyze the internal organization
of copulation because they determine the time to
reach the ejaculation threshold [29]. It
has also been demon- strated that female
partners need various intromissions separated
with shorter intervals followed by an
ejaculation in order to be maxi- mally
fertilized [3]. This last evidence might
explain why HY rats are less effective at
inseminating HY female partners and why HY dams
have fewer pups [14,24].
The neurobiological bases underlying the
slower rate of copulation of HY rats are
unknown, but dopamine (DA) may be proposed as a
pu- tative candidate. DA serves primarily a
facilitator role because of its high
concentrations in the medial preoptic area
[23,32,33] and in the ventral striatum
when the subject intromitted or ejaculate
[34]. Additionally, dopaminergic drugs
administered systemically or in the medial
preoptic area shorten the intromission and
ejaculation latencies, as well as the number of
intromissions and of sexual bouts before
ejaculation; sug- gesting a primary role of this
neurotransmitter in male sexual motiva- tion
[4,35]. In line with this, a selective
D1 agonist, SKF-38393, increased the time spent
in a goal compartment with an estrous female and
copulatory display, supporting that dopamine
enhances sexual mo- tivation by stimulating D1
receptors [36,37]. We have demonstrated
a lower concentration of D1 dopamine receptors
in the ventral striatum of HY rats as compared
to LY rats [38]. Additionally, systemic
adminis- tration of SKF 38393 produced more
grooming in the HY than in the LY subline
[39] suggesting that dopaminergic
mechanisms may differ between sublines.
Interestingly, when the males of both
sublines were allowed to ex- perience seven
copulatory sessions, 79% of LY rats ejaculated,
while only 53% males of the HY achieved
ejaculation (data not shown). That is, even
after prior sexual experience, HY males still
showed poor per- formance and did not acquire
sexual experience efficiently.
In conclusion, HY rats are a suitable
model to analyze the possible causes of the
stereotyped motor sequence alterations due to
the in- breeding process. HY males showed a
disorganized sequence because the number of
copulatory bouts is higher and lower
intromission ratio that delay ejaculatory
threshold respect to that observed in SD rats.
Ac- quisition of sexual experience is lower in
HY and in a less degree in the LY subline, which
could be due to neurobiological changes caused
by in- breeding over greater than 80
generations. Therefore, HY are useful be- cause
the proportion of non-copulators and sluggish
subjects is one order of magnitude higher than
SD rats, making it easier to analyze the
mechanisms underlying this behavior.
