Hormonal responses to different sexually related conditions in male rats

Plasma levels of corticosterone (C) and testosterone (T) increase after sexual activity in males of several species. However, the physiological significance of these increases has not been elucidated. In the present study, hormonal response to different conditions linked to sexual activity was assessed. In the first experiment, plasma levels of C and T were assessed both in sexually experienced and naive male rats after the following conditions: (A) control group, without sexual stimulation; (B) males exposed to ovariectomized females; (C) males exposed to intact, non-receptive females; (D) males exposed to receptive females with the vagina obstructed, to avoid intromission; (E) males exposed to receptive females: but separated by a grid that prevents physical contact; (F) males exposed to receptive females during 30 min. In a second experiment, experienced male rats were allowed to repeatedly copulate until reaching the criteria for sexual exhaustion, and 24 h later, they were allowed to copulate. Once sexually related conditions ended, males were killed and their blood was obtained. C and T plasma levels were assessed by HPLC with ultraviolet (UV) detection. Results indicate that T did not increase significantly in naive male in any sexual condition, while in the experienced males, significant increases were observed with the mere presence of a receptive female and also after ejaculation. These increases were significantly larger in experienced males. On the other hand, C also increased in all sexual conditions, both in experienced and naive rats; however, the increase observed was larger in experienced males. Regarding sexual satiety, both C and T increased after copulating ad libitum to satiety. T increased almost three-fold compared to control, while C increased two-fold. No significant changes were observed in either one of the steroids 24 h after sexual exhaustion, even though males remained with a receptive female during an hour. These results show that sexual experience has an important influence on the hormonal response to sexual activity. C rises could be directly related to sexual arousal involved in the different sexual conditions, while T rises seem to have a direct relationship with both the motivation and execution aspects of masculine sexual behavior.     

Positive correlation between the body weight of anestrous goats and their response to the male effect with sexually active bucks

In the present study, we analyzed the results of two years of response to the male effect in seasonally anestrous goats to investigate whether the activation of female reproductive activity by the male effect is related to the body weight of the females. Seventy-nine adult female Mexican mixed breed goats were used. The anestrous females were exposed during 15 days to sexually active males, and were classified into three categories according to their mean body weight +/-SD (42 +/- 9 kg) (Light: < or = 33 kg, n = 19; Medium: 34-50 kg, n = 46; Heavy: > or = 51 kg, n = 14). More than 98% of the goats from the Medium and Heavy groups showed at least one estrus behavior within the first 15 days following the introduction of the bucks, versus only 63% of the females from the Light group (P < 0.01). The interval between the introduction of the males and the onset of estrus behavior was longer in the females of the Light and Medium groups (4.2 +/- 0.8 and 3.3 +/- 0.3 days) than in the females of the Heavy group (2.0 +/- 0.2 days; P < 0.03). Also, body weight was negatively correlated with latency to first estrus (Spearman r = -0.57; P < 0.001). These results are in agreement with the hypothesis that the ability of anestrous goats to respond to the male effect is positively influenced by their body weight.     

Presence of glycoconjugates in prolactin granules of male rats

Summary Prolactin granules in the anterior pituitary glands of male rats contain densely stained materials at the periphery of the matrix. These occur in both small spherical and large polymorphic types of granules. The presence of densely stained materials around secretory granules may be a useful criterion for identification of prolactin cells since the dense structure was observed in 95% of these cells after conventional staining by uranyl acetate and lead citrate. The localization of glycoconjugates in the prolactin granules was examined by applying concanavalin A (Con A) on the ultrathin sections. HRP-Con A or ferritinconjugated Con A bound specifically to the densely stained materials in the peripheral region of the prolactin granule matrix, indicating that this densely stained matrix contains glycoconjugates; the significance thereof is discussed with reference to the concentration and packaging of secretory product.     

Serotonergic control of prolactin release in male rats.

To further clarify the relationship between the central serotonergic system and the control of prolactin secretion, we studied the effect of dorsal raphe' lesions, electrical stimulation of the midbrain raphe' nucleus and treatment with parachlorophenylalanine (PCPA) on prolactin secretion. Radio frequency destruction of serotonergic cell bodies in the midbrain dorsal raphe' nucleus or PCPA decreased forebrain serotonin (5HT) and 5-hydroxyindoleacetic acid (5HIAA) concentration and prolactin secretion. Electrical stimulation of the raphe' increased forebrain serotonin turnover and prolactin secretion. These observations indicate that serotonergic neurons located in the raphe' nuclei may be involved in regulating prolactin secretion in male rats.     

Plasma prolactin levels in maturing intact and cryptorchid male rats: development of stress response.

In order to determine the possible role of the seminiferous tubules in the regulation of prolactin secretion during sexual development, male rats were rendered cryptorchid at 22 days of age, and thereafter different groups of animals were decapitated at 8-10 day intervals between Day 32 and Day 70. Cryptorchid rats showed destruction of the germinal epithelium accompanied by increased plasma FSH and, to a much lesser extent, increased plasma LH titers. Nevertheless, plasma prolactin levels were similar to those of intact controls throughout the entire period studied. Plasma prolactin titers in intact controls remained uniformly low from Day 20 to Day 70, contrasting with previous reports in which increasing prolactin levels have been observed during sexual development. To determine the reason for this apparent discrepancy, a longitudinal experiment was conducted in which intact and cryptorchid male rats were bled every 10 days from Day 30 to Day 70, following a 3-min period of exposure to ether fumes. The prolactin response to this stress increased markedly with age. A similar pattern of prolactin was observed in a cross-sectional study in which different groups of intact animals were bled following a 3-min period of ether exposure, at ages ranging from 20 to 70 daysmthe results indicate that unlike FSH secretion, prolactin secretion is not controlled by the seminiferous tubules. In addition, they suggest that the pattern of increasing plasma prolactin previously described in the developing male rat is at least in part caused by an age-dependent increase in responsiveness of prolactin to stress.     

Prolactin response to stress in neonatally estrogenized male rats

Male Wistar rats were injected either 500 micrograms of estradiol benzoate or olive oil on their first day of life. Blood samples were obtained from the adult by decapitation, by decapitation after 15 min of restraint, by decapitation 10 min after a 5 min period of ether exposure or by jugular venipuncture after 60 s of ether exposure. Prolactin (PRL) plasma levels were measured by RIA. The PRL levels in samples obtained by decapitation were similar to control and estrogenized groups. A similar response to restraint was also found in both groups. Sixty s of exposure to ether stress stimulates PRL secretion only in the estrogenized males, this effect being blocked by treatment with Normifensine (5 mg/kg two hours prior to blood sampling). These results suggested that estrogenized male rats show greater sensitivity to acute ether stress than the controls, and that changes in the dopaminergic system could be involved in this response.     

Differential responsiveness of LH and prolactin to p-tyramine in male and female rats

The effect of p-tyramine, a natural amine which is found in the rat brain in trace amounts, was evaluated for its capacity to influence LH and prolactin secretion in male and female rats under different hormonal conditions. p-Tyramine (40 mg/kg ip) was ineffective in modifying LH levels in either female or male rats which had been gonadectomized for 2 days, but if the animals were injected with 12.5 micrograms of estradiol benzoate (EB) on the day of castration, p-tyramine was able to release LH in female but not in male rats. To evaluate whether early androgenization of brain structures which control LH secretion was involved in the sexual difference observed, p-tyramine was tested in female androgenized rats (200 micrograms of testosterone propionate on the day of birth), and in male rats castrated at birth. The trace amine was ineffective in altering LH levels in both experimental models, even if rats were pretreated with EB as control females. On the other hand, p-tyramine inhibited prolactin secretion in male rats pretreated with EB, and not in similarly treated female rats. The present results suggest that p-tyramine may be involved not only in prolactin regulation as it has been previously shown, but also in LH control, and that the hormonal response to this amine is sexually differentiated in the rat.     

Ovariectomized Sprague-Dawley and Long-Evans rats release prolactin differently in response to estrogen

Mature female Sprague-Dawley (SD) and Long-Evans (LE) rats were ovariectomized (OVX), fitted with indwelling atrial catheters and given a single sc injection of either 25 or 100 μg polyestradiol phosphate (PEP); seven days later blood samples were withdrawn at two hour intervals from 1100 to 2100 hours to detect the presence of an afternoon surge of prolactin (PRL). Other groups of OVX rats of both strains also treated with PEP and catheterized as above were sampled before and at 2, 5, 10 and 30 min after iv administration of 1 μg synthetic thyrotropin releasing hormone (TRH). Pituitary (AP) and uterine weights were determined following sacrifice one day after TRH treatment. Separate groups of OVX rats of both strains treated with 100 μg PEP were decapitated 7 days later and each AP was removed and homogenized. The AP homogenates and plasma samples were assayed for PRL by radioimmunoassay. Rats of both strains had afternoon PRL surges and in both strains the magnitude and/or duration of the surges were enhanced by the higher dose of PEP. However, within each PEP dose LE rats released significantly more PRL during the surge than did SD rats. Rats of both strains also released PRL in response to TRH and this response was enhanced in both strains by the higher of the two doses of PEP. However, there were no differences between the strains at 25 μg PEP and at 100 μg PEP SD rats released significantly more PRL to TRH than did LE rats. Pituitary weight and PRL concentration were not different between the strains at either dose of PEP but LE rats had significantly heavier uteri at both doses of PEP compared to SD rats. These data not only show that strain differences exist in estrogen-induced or mediated PRL release in the rat but also indicate that the differences are not uniform. This latter observation suggests that the estrogen-induced mechanisms examined in this study are for the most part independent of each other.     

The prostate response to prolactin modulation in adult castrated rats subjected to testosterone replacement

Despite the androgenic dependence, other hormones, growth factors, and cytokines are necessary to support prostatic growth and maintain the glandular structure; among them, prolactin is a non-steroidal hormone secreted mainly by the pituitary gland. However, extra-pituitary expression of prolactin, such as in the prostate, has also been demonstrated, highlighting the paracrine and autocrine actions of prolactin within the prostate. Here, we investigated whether prolactin modulation alters ventral prostate (VP) morphophysiology in adult castrated rats. Sprague Dawley rats were castrated and after 21 days, divided into ten experimental groups (n?=?6/group): castrated control: castrated animals that did not receive treatment; castrated+testosterone: castrated animals that received T (4 mg/kg/day); castrated+PRL (PRL): castrated animals receiving prolactin (0.3 mg/kg/day); castrated+T+PRL: castrated animals that received a combination of testosterone and prolactin; and castrated+bromocriptine (BR): castrated animals that received bromocriptine (0.4 mg/kg/day). The control group included intact animals. The animals were treated for 3 or 10 consecutive days. At the end of experimental period, the animals were euthanized, and the blood and VP lobes were collected and analyzed by different methods. The main findings were that the administration of prolactin to castrated rats did not exert anabolic effects on the VP. Although we observed activation of downstream prolactin signaling after prolactin administration, this was not enough to overcome the prostatic androgen deficiency. Likewise, there was no additional glandular involution in the castrated group treated with bromocriptine. We concluded that despite stimulating the downstream signaling pathway, exogenous prolactin does not act on VP in the absence or presence of high levels of testosterone.     

Morphine stimulates prolactin release in normal but not in castrated male rats

Morphine (200 micrograms/rat) was injected intraventricularly (i.v.t.) into normal and into long-term castrated (4 weeks) adult male rats. Animals were killed 10, 20, 40 and 60 min after treatment. In normal animals, the treatment with morphine resulted in a significant increase of serum prolactin concentrations at all time intervals considered. However, the i.v.t. injection of 200 micrograms morphine/rat into castrated rats did not exert any significant effect on prolactin release at any time interval considered. When morphine (200 micrograms/rat) was administered i.v.t. together with the specific opioid receptor blocker naloxone (7.5 or 15 micrograms/rat) the stimulatory effect of morphine on prolactin release was diminished at 10 min, and totally blocked at 20 min. Naloxone given alone did not influence serum prolactin concentrations. The results suggest that the presence of endogenous androgens is essential to permit the stimulatory effect of morphine on prolactin release.     

Pituitary LH response to LHRH in male rats following pentobarbital anesthesia

Plasma LH concentrations were determined in unanesthetized or pentobarbital (PB) anesthetized male rats following several doses of LHRH administered into the lateral ventricle of jugular vein. Regardless of the route of injection of LHRH, plasma LH concentrations were similar whether animals received PB anesthesia or not. No evidence was found that PB enhanced or diminished the response of the pituitary to LHRH in male rats.     

Effect of corticosterone on the prolactin response to psychological and physical stress in rats

Both corticosterone and prolactin (PRL) levels increase in response to stress. In these studies we examined the effect of corticosterone on the PRL response to both physical (footshock) and psychological (novel environment) stress. Three groups of rats were used: sham adrenalectomized (SHAM), adrenalectomized (ADX), and adrenalectomized with corticosterone replacement (ADX+CORT). The corticosterone-treated animals received 80 ug corticosterone/ml drinking water. Blood samples were drawn via an indwelling cannula and PRL values determined using radioimmunoassay. ADX rats showed a consistently greater PRL response to being placed on a platform above water (novel environment) or when receiving intermittant footshock than did ADX+CORT rats. The PRL response of the latter group was similar to that of the SHAM animals. These findings indicate that corticosterone levels of an animal can significantly attenuate the magnitude of the PRL response to both physical and psychological stress. These findings further emphasize that the PRL response to stress is dependent not only upon the immediate action of the stressor, but also the prior stress history of the animal.     

Effects of centrally injected amylin on sexually behavior of male rats.

Amylin (AMY) is a peptide of pancreatic origin principally involved in the carbohydrate metabolism, but that may interfere with central and peripheral dopamine (DA) pathways. The peptide, injected intracerebroventricularly (ICV) at the dose of 2.5 microg/rat, induced a decrease of copulatory activity in good copulators (GCO) and sluggish (SLU) male rats. The dose of 0.1 microg/rat did not affect significantly the sexual behavior of GCO rats, whereas AMY 0.5 microg/rat increased only the latency and reduced the frequency of ejaculation. At the dose of 2.5 microg/rat AMY antagonized the activation of sexual behavior induced by the DA receptor agonist, apomorphine administered subcutaneously (SC) at the dose of 100 microg/kg. Moreover, this inhibitory effect was blocked by the calcitonin gene-related peptide and AMY receptor antagonist, CGRP (8-37) fragment (injected ICV at the dose of 1 microg/rat). These data suggest that AMY may exert inhibitory effects on male sexual behavior in rats, probably interfering with central DA neurotransmission and with CGRP receptors.     

Effects of serotonergic agents on plasma prolactin levels in pyridoxine-deficient adult male rats

Plasma concentration of prolactin was significantly reduced in pyridoxine-deficient as compared to control (pyridoxine-supplemented) adult male rats. Administration of pyridoxine to deficient rats resulted in a significant increase in plasma prolactin. The reduction in plasma prolactin in pyridoxine-deficient rats corresponded with the significantly reduced hypothalamic contents of pyridoxal phosphate and serotonin in pyridoxine-deficient rats. Plasma prolactin concentrations were also measured in response to serotonergic agents in both groups of rats. The administration of the 5HT_1A agonist (8-hydroxy 2-n-dipropylamino tetralin) resulted in a significant increase in plasma prolactin and that of the specific 5HT_1A antagonist spiroxatrine had the opposite effect. The results suggest that the hypothalamic serotonergic regulation of prolactin release is impaired in pyridoxine deficiency.     

Plasma prolactin levels in parental male rats: effects of increased pup stimuli

The effects of pup exposure on plasma prolactin (PRL) levels were measured in unrestrained, steriod-primed, orchidectomized parental rats. Castrated adult male rats were treated with estradiol and progesterone for 3 weeks and then exposed to rat young until parental behavior was induced. Blood samples were then collected from parental males in both the presence and the absence of pups. Samples were obtained through the use of indwelling cannulas connected via polyethylene tubing to syringes located outside each subject's cage. Despite the intense pup stimulation provided by exposure to nine rat pups, acute rises in plasma PRL levels were not observed in these unrestrained parental male rats. Parental male rats, therefore, do not appear to have the capacity to secrete prolactin in response to acute exposure to young.     

Effects of LHRH on copulatory behavior and locomotor activity in sexually inexperienced male rats

To determine whether luteinizing hormone-releasing hormone (LHRH) contributes to the sexual behavior and locomotor activity of sexually inexperienced male rats, subcutaneous injections of LHRH (500 ng) were given to males. The males showed significant facilitation of a few aspects of sexual behavior 2h after LHRH injection, compared with saline-injected controls. However, the locomotor activity of males injected with LHRH showed no significant change. These results may indicate that LHRH mechanisms play a direct role in the normal regulation of sexual arousal of male behavior in rats.