Hyperprolactinemia preferentially inhibits erectile function in adrenalectomized male rats

Patterns of prolactin secretion

Prolactin is pleotropic in nature affecting multiple tissues throughout the body. As a consequence of the broad range of functions, regulation of anterior pituitary prolactin secretion is complex and atypical as compared to other pituitary hormones. Many studies have provided insight into the complex hypothalamic-pituitary networks controlling prolactin secretion patterns in different species using a range of techniques. Here, we review prolactin secretion in both males and females; and consider the different patterns of prolactin secretion across the reproductive cycle in representative female mammals with short versus long luteal phases and in seasonal breeders. Additionally, we highlight changes in the pattern of secretion during pregnancy and lactation, and discuss the wide range of adaptive functions that prolactin may have in these important physiological states.

Prolactin Receptor Isoforms as the Basis of Tissue-Specific Action of Prolactin in the Norm and Pathology

The review describes functional and structural features of different isoforms of prolactin receptor, mechanisms of signaling pathway activation, and molecular messengers involved in the transmission and termination of signal from the prolactin receptor isoforms. Changes in the ratio between prolactin receptor isoforms, key mediators of prolactin signal transduction and termination in various organs and tissues, are analyzed. Special attention is given to the role of molecular mediators and the ratio between the isoforms in normal physiological functions and pathologies. Approaches for therapeutic correction of prolactin signaling impairments are discussed.

Effect of hyperprolactinemia on PRL-receptor expression and activation of Stat and Mapk cell signaling in the prostate of long-term sexually-active rats

The abnormal elevation of serum PRL, referred to as hyperprolactinemia (HyperPRL), produces alterations in several reproductive parameters of male rats such as penile erection or decreased tendency to reach ejaculation. Additionally, this situation produces a significant modification of prostate histology, as observed in the epithelial structure and alveolar area, which could reach a level of hyperplasia in the long-term. In this tissue, HyperPRL produces an increase in expression of PRL receptors and activation of the Stat3 signaling pathway that is correlated with the evolution of prostate pathologies. However, the impact of HyperPRL in long-term sexually active male rats is unknown. In this work, using constantly copulating Wistar male rats with induced HyperPRL, we analyzed the level of serum PRL, the effect on prostate PRL receptors, and activation of pStat3, pStat5 and Mapk signaling pathways. Two procedures to induce HyperPRL were employed, comprising daily IP administration or adenohypophysis transplant, and although neither affected the execution of sexual behavior, the serum PRL profile following successive ejaculations was affected. Messenger RNA expression of the short and long isoforms of the PRL receptor at the ventral prostate was affected in different ways depending on the procedure to induce HyperPRL. The ventral prostate did not show any modification in terms of activation of the pStat5 signaling pathway in subjects with daily administration of PRL, although this was significantly increased in ADH transplanted subjects in the second and fourth consecutive ejaculation. A similar profile was found for the pStat3 pathway which additionally showed a significant increase in the third and fourth ejaculation of daily-injected subjects. The Mapk signaling pathway did not show any modifications in subjects with daily administration of PRL, but showed a significant increase in the second and third ejaculations of subjects with ADH transplants. Thus, although sexual behavior was not modified, HyperPRL modified the expression of PRL receptors and the activation of signal pathways in the prostate tissue. Hence, it is probable that prostatic alterations precede the sexual behavioral deficits observed in subjects with HyperPRL.

Copulation is reactivated by bromocriptine in male rats after reaching sexual satiety with a same sexual mate

Male sexual satiety has been associated with a decrease in dopamine levels. Spontaneous recovery of copulatory behavior begins at least 72 h after sexual satiety is reached or in the condition in which a sexually-satiated male is exposed to a new receptive female distinct from the one with which sexual satiety was reached. The aim of the present study was to explore whether dopaminergic activation by bromocriptine (BrCr) can reactivate copulatory behavior with the same sexual mate immediately after sexual satiety is reached. Male rats were divided into three groups exposed to one of the following three conditions: 1) administration of 2 mg/kgs.c. of BrCr and exposure to the same female with whom sexual satiety was previously reached; 2) administration of 0.3 mLs.c. of the vehicle solution with exposure to the same female with whom sexual satiety was reached; and, 3) exposure to a new receptive female after sexual satiety was reached. Results showed that BrCr significantly reactivated copulatory capability in sexually-satiated males with the same receptive female. In contrast, no males in the vehicle group ejaculated with the same female after reaching sexual exhaustion. Copulation was reactivated by BrCr in a way similar to that observed in untreated males exposed to a new receptive female (i.e., the Coolidge effect). The reversal of sexual satiety in the males treated with BrCr could be explained by its action on D2 family receptors, which promotes a reactivation of sexual motivation at a level sufficient to allow renewed copulation with the same female mate.

Growth hormone, prolactin, and sexuality

GH and PRL, although not considered as 'classi cal' sexual hormones, could play a role in the endocrine control of sexual function both in men and women. Physiologically, PRL seems to be involved in the central control of sexual behavior and activity, by modulating mainly the effects of dopaminergic and serotoninergic systems on sexual function. Indeed, circulating PRL levels increase after orgasm and may potentially play a role in the acute regulation of further sexual arousal following orgasm both in men and women. On the other hand, either short-term or long-term PRL in crease can modulate central nervous system areas involved in the control of sexual function and, peripherally, can directly influence mechanisms of penile erection in men, and presently only as an hypothesis, mechanisms related to the sexual response of genitalia in women. Furthermore, chronic hyperprolactinemia is classically associated with hypogonadotropic hypogonadism and sexual dysfunction in both sexes. Successful treatment of chronic hyperprolactinemia generally restores normal sexual function both in men and women although this effect is not only related to relapse of gonadal function. Hypoprolactinemia is recently recognised as a possible risk factor of arteriogenic erectile dysfunction while a possible role on female sexual function is not known. The physiological role of GH on sexual function is not fully elucidated. GH is an important regulator of hypothalamus-pituitary-gonadal axis and seems to participate in the regulation of the sexual response of genitalia in men, and potentially also in women. Sexual function in men and women with GH deficiency (GHD) and GH excess, particularly in acromegaly, is scantily studied and GH- or IGF-I-dependent effects are difficult to quantify. Nevertheless, a decrease of desire and arousability both in men and women, together with an impairment of erectile function in men, have been described both in patients with GHD and acromegaly, although it is not clear whether they are dependent directly on the hormone defect or excess or they are consequence of the hypogonadism or the different clinical complications or the physical disfigurement and psychological imbalance, which are associated with the diseases, and are potentially affecting sexual function. Data on beneficial effects of GH replacement therapy and specific surgical or pharmacological approach for acromegaly are far to be fully elucidated although restoring normal GH/IGF-I levels have been associated to improvement of sexual function.

The aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) reduces disinhibitory behavior in intact adult male rats treated with a high dose of testosterone

Anabolic androgenic steroids and high testosterone doses have been reported to induce impulsive behavior in man and behavioral disinhibition in rats. The purpose of the present study was to investigate whether aromatization of testosterone to estradiol is of importance for the behavioral disinhibiting effect of a high testosterone dose in adult male rats. Testosterone administered via five testosterone-filled silastic capsules implanted subcutaneously (s.c.) to non-castrated, group-housed rats for six days induced behavioral disinhibition in a modified Vogel's drinking conflict model and yielded supraphysiological serum levels of testosterone and increased accessory sex organ weights. Moreover, concurrent administration of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD; 60 mg/kg/day s.c.) decreased behavioral disinhibition in testosterone-treated rats (without affecting accessory sex organ weights) while behavior was not significantly affected in sham-treated animals. Since some reports indicate that ATD, in addition to inhibit aromatase, also may affect the binding of testosterone to the androgen receptor, the effect of the non-steroidal androgen receptor antagonist flutamide was investigated. Flutamide treatment did not affect disinhibited behavior in testosterone-treated rats. However, in sham-treated animals, flutamide (50mg/kg/day) produced behavioral disinhibition. These results suggest that estradiol is of importance in the mechanisms underlying behavioral disinhibition in non-castrated rats treated with a high testosterone dose. Speculatively, aromatization may be involved in pro-impulsive effects of high testosterone doses in humans.

Signal transducers and activators of transcription 1 and 3 in prostate: Effect of sexual activity

The signal transducers and activators of transcription (Stat) are effector molecules downstream of cytokine receptors. Ligand occupancy of these receptors results in the tyrosine phosphorylation, dimerization and nuclear translocation of the Stat family of transcription factors and by these means regulate gene expression. Prolactin receptors as members of the cytokine-hematopoietin receptor superfamily, are linked to Stat activation. Sexual stimulation leads to an increase in prolactin secretion that might be involved in long-term changes in the protein repertoire associated to prostate hyperplasia. In order to gain insight into this phenomenon, we analyzed the tyrosine phosphorylation and DNA binding activity of two members of the Stat family in the prostate of sexual experienced rats after different number of ejaculations. A significant increase in Stat-1 and Stat-3 tyrosine phosphorylation was found after three ejaculations. Concomitantly an increase in Stat-1 and Stat-3 DNA-binding activity is detected after two and three ejaculation series. These results, favor the notion that ejaculation-induced prolactin secretion activates its prostate receptors resulting in Stat-1 and Stat-3 nuclear translocation, event likely to be associated to the so-called benign prostate hyperplasia.

Prostate response to prolactin in sexually active male rats

BACKGROUND

The prostate is a key gland in the sexual physiology of male mammals. Its sensitivity to steroid hormones is widely known, but its response to prolactin is still poorly known. Previous studies have shown a correlation between sexual behaviour, prolactin release and prostate physiology. Thus, here we used the sexual behaviour of male rats as a model for studying this correlation. Hence, we developed experimental paradigms to determine the influence of prolactin on sexual behaviour and prostate organization of male rats.

METHODS

In addition to sexual behaviour recordings, we developed the ELISA procedure to quantify the serum level of prolactin, and the hematoxilin-eosin technique for analysis of the histological organization of the prostate. Also, different experimental manipulations were carried out; they included pituitary grafts, and haloperidol and ovine prolactin treatments. Data were analyzed with a One way ANOVA followed by post hoc Dunnet test if required.

RESULTS

Data showed that male prolactin has a basal level with two peaks at the light-dark-light transitions. Consecutive ejaculations increased serum prolactin after the first ejaculation, which reached the highest level after the second, and started to decrease after the third ejaculation. These normal levels of prolactin did not induce any change at the prostate tissue. However, treatments for constant elevations of serum prolactin decreased sexual potency and increased the weight of the gland, the alveoli area and the epithelial cell height. Treatments for transient elevation of serum prolactin did not affect the sexual behaviour of males, but triggered these significant effects mainly at the ventral prostate.

CONCLUSION

The prostate is a sexual gland that responds to prolactin. Mating-induced prolactin release is required during sexual encounters to activate the epithelial cells in the gland. Here we saw a precise mechanism controlling the release of prolactin during ejaculations that avoid the detrimental effects produced by constant levels. However, we showed that minor elevations of prolactin which do not affect the sexual behaviour of males, produced significant changes at the prostate epithelium that could account for triggering the development of hyperplasia or cancer. Thus, it is suggested that minute elevations of serum prolactin in healthy subjects are at the etiology of prostate abnormal growth.

Prolactinergic and dopaminergic mechanisms underlying sexual arousal and orgasm in humans

Dopaminergic mechanisms play a major role in modulating sexual behavior in humans and animals. Animal data demonstrate important interactions between the dopaminergic and prolactinergic system. As recently demonstrated, dopamine agonists have facilitatory properties for penile erection but may also enhance sexual drive and orgasmic quality. In contrast, chronic elevations of prolactin inhibit appetitive as well as consummatory parameters of sexual behavior. Recent human studies show a marked increase in prolactin after orgasm in males and females. Concerning the biological relevance of acute prolactin alterations after orgasm, prolactin might serve as a neuroendocrine reproductive reflex for peripheral reproductive organs. Alternatively, prolactin may feedback to dopaminergic neurons in the central nervous system and thereby modulate sexual drive and satiation. Here, we provide a brief overview of the physiology of dopamine and prolactin in regulating sexual behavior. In addition, recent experimental and clinical evidence for a postulated feedback mechanism for prolactin and its implications for orgasmic disorders are discussed.

Disinhibitory behavior and GABA(A) receptor function in serotonin-depleted adult male rats are reduced by gonadectomy

Impulsive and aggressive behaviors in, e.g., personality or substance abuse disorders in man and corresponding behaviors in rats may involve serotonin (5-HT), gamma-amino-butyric acid(A)/benzodiazepine receptor complexes (GABA(A)/BDZ-RC) and steroid hormones, e.g., testosterone. Here, we studied the effect of gonadectomy on disinhibitory behavior in individually housed 5-HT-depleted rats and on GABA(A)/BDZ-RC function in vitro, in corticohippocampal synaptoneurosomes prepared from the brain of these animals. 5-HT depletion by intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT)-induced disinhibitory behavior in a shock-induced behavioral inhibition model (punished conflict model) 14 days after operation. Gonadectomy in connection with the 5-HT depletion reduced the disinhibitory behavior and testosterone substitution prevented this effect. Shock threshold and drinking motivation were not affected by gonadectomy and/or 5-HT depletion. The relative epididymides weight was increased in 5-HT-depleted as compared to sham-operated rats. However, the serum concentrations of testosterone and the relative testes weights were not different in 5-HT-depleted rats as compared to controls. GABA-induced (30, 100, 300 microM) 36Cl(-)-uptake into synaptoneurosomes was lower in 5,7-DHT+gonadectomized rats compared to the control group. This effect was reversed by substitution with testosterone. These results demonstrate that gonadectomy reduces disinhibitory behavior in 5-HT-depleted rats and that GABA(A)/BDZ-RC may be involved in this effect.

The "low-dose" concept and the paradoxical effects of prolactin on grooming and sexual behavior

The effects of prolactin on animal behavior include the stimulation of novelty-induced grooming in rats. This effect has been demonstrated in hyperprolactinaemic animals bearing pituitary homografts under the kidney capsule or after intracerebroventricular (i.c.v.) administration of prolactin. Since plasma prolactin levels in hyperprolactinaemic rats are similar to those of animals injected with low doses of rat prolactin, we studied the effects of this hormone injected subcutaneously (s.c.) in a dose range of 5-50 microg/kg. Novelty-induced grooming was enhanced only in rats injected with 5 or 10 microg/kg rat prolactin, whereas no effect was observed after the s.c. injection of the higher dose. The sexual behavior of male rats is also affected by prolactin. Male rats with normal mating activity showed enhanced sexual behavior when injected s.c. with rat prolactin (5, 10 or 50 microg/kg). In animals with poor sexual performance or in impotent rats, prolactin (5 or 10 microg/kg, but not 50 microg/kg) restored the full pattern of sexual behavior. An increased lordosis quotient was also observed in ovariectomized rats treated with prolactin 5 or 10 microg/kg. These results suggest that, besides the duration of hyperprolactinaemia, the effective level of plasma prolactin is important for the expression of the behavioral effects of this hormone.

Neuroendocrine correlates of separation stress in the Siberian dwarf hamster (Phodopus sungorus)

Siberian dwarf hamsters form monogamous male-female pair bonds. Disruption of the pair bond results in increases in body mass and behavioral alterations similar to profiles seen in human atypical depression. We examined behavioral and neuroendocrine correlates associated with separation of the male from his mate. Animals were paired (n = 28 pairs) for 3 weeks, then 15 pairs were separated and 13 pairs remained as controls. Behaviors of the males were observed in a novel environment following 3 weeks of pairing and 4 weeks of separation. The 5-min behavioral test monitored exploratory, territorial behavior, and locomotor activity. Separated males showed a significant increase in body mass (p < 0.01). paralleled by an increase in food consumption (p < 0.01). Separated males had decreased seminal vesicle mass (p < 0.05) and testicular mass (p < 0.05). Behavioral analysis revealed that separated males showed no significant differences in grooming, scent-marking, alert on 2 feet, or escape behavior when compared to paired males. Separated males did show significant increases in inactivity (p < 0.05). Plasma cortisol levels were significantly increased in separated animals (p < 0.05), but there were no significant effects on testosterone. Resting levels of plasma norepinephrine and epinephrine were less in separated males, but this was significant only for norepinephrine (p < 0.05). In conclusion, separation stress was accompanied by increased hypothalamic-pituitary-adrenal axis function and decreased peripheral sympathetic nervous system activity and decreased reproductive profiles.

Hormonal regulation of skeletal muscle hypertrophy in rats: the testosterone to cortisol ratio

This study determined the influence that the catabolic hormone, corticosterone (C), and the anabolic hormone, testosterone (T), had in regulating skeletal muscle hypertrophy using the rat hind limb ablation model. Specifically, the ratio of T:C (TCR) was manipulated via hormone implants and injections and concentrations measured to evaluate the relative contribution of each hormone to skeletal muscle protein balance. Skeletal muscle growth was measured 16 days after gastrocnemius muscle ablation. Elevations in plasma concentrations of C (via daily C injections, 50 mg kg*kg(-1) body mass) resulted in TCR of 0.007 that was less than the control group TCR of 0.249. In this C-injected group, whole body and skeletal muscle atrophy was elicited-this being greater in the fast-twitch plantaris muscle than in the slow-twitch soleus muscle. The overloaded leg resisted the C-induced atrophy. Castration of animals (TCR 0.024) resulted in less whole body and skeletal muscle growth. However, elevations in plasma concentrations of T (two groups, with TCR of 1.35 and 1.64) did not result in significantly greater muscle growth. Furthermore, T was also ineffective in antagonizing the C-induced atrophy in a group that received both T implants and C injections. This group had a TCR of 0.175 that was similar to the control group ratio of 0.249 that received no manipulations. We concluded that glucocorticoids were able to induce pronounced atrophy, but at the same time overloaded muscles were able to over-ride the glucocorticoid signal. Plasma concentrations of C were a better predictor of muscle growth/atrophy than T and/or the TCR. In addition, it is suggested that the volume of contractile activity of the muscle is perhaps an important determinant of C-induced atrophy, because less atrophy occurs in the more active slow twitch muscles.

Lack of an inhibitory effect of hyperprolactinemia on androgen-dependent marking

An experiment was performed to determine if hyperprolactinemia (chronically elevated serum prolactin levels), which inhibits testosterone-activated male sexual activity, also affects other androgen-dependent behaviors. Thus defecation and urine marking in response to a novel environment were examined in sham-operated and pituitary-grafted (hyperprolactinemic) male rats that had been castrated or castrated and given subcutaneous testosterone implants. Both castration and pituitary grafting significantly inhibited defecation, with the inhibitory effects of hyperprolactinemia being most pronounced in the castrated non-testosterone-treated animals. In contrast, castration significantly reduced the amount of urine marking observed, but pituitary grafting was without effect on this behavior. Thus, although hyperprolactinemia may inhibit sexual activity through an antagonism of the activational effects of testosterone, these results suggest that this effect is specific to sexual behavior and does not involve a more generalized inhibition of the effects of testosterone on androgen-dependent behaviors.