We studied the involvement of zinc in the sexual behavioral response of male rats. The study design employed a rat model to predict the human sexual response to elemental zinc supplementation. Rats were used because they are very social and copulate under a variety of circumstances, regardless of the presence of a human experimenter. They are practical (small and easy to handle) and certain tissues and neuroendocrine systems are strikingly similar to humans.[13
Parameters evaluated in this study are determinants of some important aspects of the male sexual competence. Libido index, intromission and mount latencies are indicators of libido, arousability and motivation. Copulatory efficiency and inter-copulatory intervals are indicators of sexual vigor, while penile thrusting is an indicator of penile erection.[8
] Our results showed that the libido index was significantly reduced in the 10 mg/day zinc saulphate treated group. In the same group, majority of the animals (62%) failed to complete their sexual behavioral cycle within the observed period. Though these unsuccessful rats showed the initial steps of the cycle such as licking, physical contacts with the females and few mounts and intromissions, they were inactive at later stages. Therefore a significant reduction of the libido index, number of intromissions, mounts, and ejaculations were observed in the high dose of zinc treated group compared to controls.
The group treated with the lower concentration of zinc (1 mg/day) did not show an alteration in any of the observed parameters. However, supplementation with a dose of 5 mg/day per rat caused substantial prolonged ejaculatory latency and increased in number of penile thrusting. The other parameters studied remained unchanged indicating uninterrupted libido, sex vigor and performance. Majority of male rats (75 %) showed the prominent actions of sexual behaviour (mount, intromission and penile thrusting) and did not ejaculate within the 15-minute observation period.
Though higher doses of zinc reduce libido, supplementation with a medium dose (5 mg/day) has some beneficial effect on the sexual competence of adult male rats. The major significant effects of this dose of zinc are prolongation of ejaculatory latency without disturbing sexual arousability, motivation, penile erection and sex vigor. Also, the partner preference index of the 5 mg/day group was positive and comparable to the controls. A positive partner preference index is indicative of unchanged sexual interest of males.[16
] These results confirmed that libido and sexual interest are not affected by zinc supplementation with a 5 mg/day dose. However, mild reduction in percentage of intromission was observed in this group and it is postulated that this may be situational rather than an effect of supplemented zinc. This is based on our observation where mild rejection by the females at the initial phase of the behavior led some males to refrain from sexual activity.
Reduction of the libido index was the major disadvantage that we observed with zinc supplementation. Substances that affect libido usually act centrally and may reduce desire by causing sedation or hormonal disturbances.[17
] The role of elevated levels of PRL in serum as an inhibitor of sexual drive and gonadal function is well established.[18
] This reduction of sex drive may occur through the modification of activity of dopaminergic neurons in the CNS that are regarded as controlling sexual motivation and function.[19
] Our study demonstrated a significant increase of serum PRL level (2.9 to 7.22 ng/dl) within two weeks of supplementation of zinc (5 mg/day). This is a possible explanation for the reduced libido with increasing doses of zinc observed in this study.
In a study with human subjects, Kruger et al
. have reported that acute changes in the normal physiological levels of PRL led to a significantly prolonged ejaculatory latency, but minor reductions of sexual drive and function.[19
] Although zinc induced elevation of PRL was not an acute effect, findings similar to the human study (prolongation of ejaculation and mild reduction of libido index with medium dose of zinc) were observed in this study. However, the prolongation of ejaculatory latency may not be merely due to effects of elevated PRL because elevated PRL levels are known to be associated with the negative aspects of sexual activities (decreased sexual desire and frequency of sexual intercourse).[20
Although we observed an elevation of serum PRL levels with supplementation of zinc, in most studies on humans, the relationship between these two parameters is inverse.[21
] However, there are no published data on rats to support our finding.
In male rats, main olfactory epithelium (MOE) exerts an important role in regulating sexual behavior. Intranasal irrigation with zinc sulphate has been reported to destroy the MOE and completely abolish the sex behavior.[6
] In this study supplementation of zinc was done using a feeding tube and precautions were taken to avoid contacting nasal area. Hence the possibility of reducing sexual performance due to MOE disturbance is ruled out. Some humans experience gastrointestinal irritation with supplementation of zinc.[23
] If the same is applicable to animals it may be another possible explanation for the reduction of libido index with elevated doses of zinc. One drawback of our study is that we did not compare the weight of animals before and after treatment.
The prolongation of ejaculatory latency may be beneficial when present with unimpaired arousability, penile erection and sex vigor. The ejaculatory latency can also be prolonged due to some disorders in the neuroendocrine or reproductive system.[24
] But the duration of zinc supplementation in our study was only two weeks which is not long enough to have an impact on the neuroendocrine or reproductive system.
The number of intromissions in the 5 mg/day group was comparable with controls. On the other hand number of penile thrusting was significantly increased with the same dose. These are indications of unimpaired erectile function due to supplemented zinc. Our observations also confirmed the lack of aberrant sexual behavior which is indicative of uninhibited penile tactile sensations.[15
] Sexual behavior may also be changed with motor weaknesses and incoordination.[12
] Supplementation with zinc did not impair muscle strength or coordination when tested with bar and bridge tests, indicating that prolongation of ejaculation was not mediated through these mechanisms.
In the present study zinc caused an elevation of T. This showed an increase from 2.39 to 8.21 ng/dl after two weeks of zinc treatment. This elevated T level may have contributed to the increase in number of penile thrusting (from 26.5 to 52.8) observed. Supplementation with 459 μmol/day of zinc for three months, in marginally zinc deficient healthy elderly men, has been shown to increase the levels of serum T from 8.3 to 16 ng/dl.[25
] Laboratory experiments indicate that the nitric oxide erectile pathway is T dependent.[26
] Many studies using animal models have confirmed that T is important in modulating the central and peripheral regulation of erectile dysfunction. T deprivation has a negative impact on the structure of penile tissues and erectile nerves.[27
] Thus, elevated T levels subsequent to zinc supplementation may increase the sexual competence via rigid and sustained erection. This may promote greater tactile stimulation of the penis due to increased contact with vagina.[25