Download orexin a (hypocretin-1) application at the medial preoptic area

OREXIN A (HYPOCRETIN-1)
APPLICATION AT THE MEDIAL
PREOPTIC AREA POTENTIATES
MALE SEXUAL BEHAVIOR IN RATS
Neuroscience 116 (2003) 921–923
K. K. GULIA, H. N. MALLICK* AND V. M. KUMAR
Abstract—The medial preoptic area plays an important role in the
regulation of male sexual behavior in rats, and this area receives
orexinergic inputs. The role of orexinergic inputs in the medial
preoptic area in sexual behavior has not been studied, though they
have been shown to play a role in some other physiological
functions. In this study, the changes in male sexual behavior in
rats were studied after local injection of orexin A (Hypocretin-1) at
the medial preoptic area. The results of the study showed that
orexin A application at the medial preoptic area increased sexual
arousal as well as the copulatory performance. Sexual arousal is
one of the physiological stimuli, which influences wakefulness. It is
possible that the earlier reports showing increased wakefulness
(Espana et al., 2001; Thakkar et al., 2001), on application of orexin
A at the medial preoptic area/basal forebrain, has a contribution
from sexual arousal.
Orexins, also known as hypocretins, are the recently discovered
neuropeptides, which are synthesized exclusively by some
neurons located in the lateral hypothalamic area (de Lecea et
al., 1998; Sakurai et al., 1998). Therefore, it was initially
proposed that orexins participate in the control of feeding
behavior (Sakurai et al., 1998). However, a widespread
distribution of the axons of orexinergic neurons throughout the
brain suggests that they may have a role in autonomic
functions, energy homeostasis, sleepwakefulness,
neuroendocrine release and reproduction.
EXPERIMENTAL PROCEDURES
The study was conducted on ten adult male Wistar rats, weighing 180–
250 g. The animals were kept in controlled temperature 25_2 °C) under
14:10h light-dark schedule (light on at 6:00 A.M.). Food and water were
provided ad libitum. The rats were screened for sexual behavior using
standard criteria (Mittimohan et al., 1989; Mallick et al., 1996) and those
having a sex drive score above four were chosen for the study. Under
sodium pentobarbital anesthesia (40 mg/kg body wt, i.p.), 26 gauge
bilateral stainless steel guide cannulae with indwelling styli were
implanted in the brain, 1.5 mm above the mPOA, at the co-ordinates 7.8
mm anterior, 0.75 mm lateral and 1.5 mm below the interaural zero, as
per DeGroot atlas (DeGroot, 1959). The whole assembly was firmly
fixed to the skull with four implanted anchoring screws and dental
cement. Seven days after the implantation of the cannulae, the rats
were tested for their copulatory activity on three occasions, at an interval
of three days between the tests. Rats showing consistency in sexual
behavior as shown by analysis of variance were only used for further
study. Mean of these 3 days recordings were taken as control readings
for comparison with post-treatment values. OX-A (O-6012, Sigma
Chemical Co., St. Louis, MO, USA) at the dose of 0.3 nmol in 200 nl of
0.9% NaCl was bilaterally injected at the mPOA by a slow injector at the
rate of 100 nl/min. The injector cannula was left in place for 1 min
following injection. The cannula was then replaced with the stylus. The
injection was given only once in any one brain site in each of the
animals. The scoring was taken after 10 min of administration of the
drug. Sex behavior scoring was performed under dim illumination in a
wooden box (45_30_30 cm) with a sliding glass front, during the dark
phase of the light/dark cycle (20.0–23.0 h). Bilaterally ovariectomized
females of the same strain, primed with 25 _g of estradiol benzoate and
one mg of progesterone, were used as receptive partners. The male rat
was introduced into the test arena, 5 min prior to the introduction of the
female. Recording was initiated at the entry of the female into the box. A
computer program was used to record the latencies of pursuit, mount,
intromission and ejaculation, frequencies of pursuit, mount and
intromission, mean inter-intromission and post-ejaculatory intervals, and
sex drive score. The latencies and frequencies of the events (pursuit,
mount, intromission and ejaculation) were registered by manually
pressing the assigned keys. The computer did timing operation, using its
internal clock.
At the end of the experiment, the brain sites and spread of injection
were verified histologically by injecting 200 nl of 2% ferric chloride
through the implanted guide cannulae and then perfusing the brain with
10% formal-saline containing 3% potassium ferrocyanide. Injection sites
were bilaterally confirmed to be at the mPOA in seven rats.
RESULTS AND DISCUSSION
Seven rats in which OX-A was bilaterally injected at the mPOA
were grouped together for statistical analysis. The bilateral
microinjection of OX-A (0.3 nmol) in the mPOA (n_7) produced
significant decrease in latencies to mount, intromission and
ejaculation (Table 1). There was signifi- cant decrease in postejaculatory interval and mean interintromission interval (P_0.05)
and increase in pursuit, mount, intromission frequencies
(P_0.05) and sex drive scores (P_0.05).
The OX-A treated rats were moving around in the chamber
during the post-ejaculatory interval. During this period, they
were not indulging in active pursuit of the female. Rats generally
go into a phase of inactivity, lying prostrate for some time after
ejaculation (Sachs and Meisel, 1988). No change in the sex
behavior was observed in one rat in which OX-A was injected
above anterior commissure. There was an increase in sex drive
score from 7.88–8.28 in one, 14.5–24.27 in the other, where
injections were given unilaterally. They also showed a decrease
in ejaculation latency, post-ejaculatory interval and mean interintromission interval.