Heterosexual experience prevents the development of conditioned same-sex partner preference in male rats

The impact of subchronic hypercaloric and restriction diets on sexual behavior, serum testosterone, and prostate histology in rats

The role of diet in health is crucial, with calorie intake playing a significant role. Hypercaloric diets (HD) often lead to adipose tissue accumulation and increased risk of chronic diseases, including reproductive impairments. By contrast, restriction diets (RD) help with weight loss, improve cardiovascular function, and ameliorate reproduction. Herein we sought to investigate the impact of subchronic HD and RD on body weight, sexual behavior, serum testosterone and prostate histology in rats. Hence, 10-week old male rats gained sexual experience during five trials with ovariectomized, hormone-primed females. Then at postnatal week PW15 the males were organized in three groups, depending on the feeding they received until PW18: HD, RD and standard diet (SD). During PW19-22 they were tested for sexual behavior, and at PW23 were euthanized for prostate histology (hematoxylin & eosin stain) and hormone analysis. Results indicated that HD males increased their body weight (16-23%) compared to SD and RD. Furthermore, HD males showed 65% less testosterone than RD males. The prostate of HD males revealed histological alterations, including a notable increase in epithelium height and other abnormal features, while no changes were observed in the performance of sexual behavior between HD and RD, although HD appeared to facilitate ejaculation when compared to SD. The histological features of RD males were comparable to SD males. Accordingly, we argue that subchronic modifications in calorie intake can alter body weight (in HD), serum testosterone levels (HD and RD in opposite directions), and prostate histology (in HD), while having no immediate effect on male sexual behavior.

The Sexual Motivation of Male Rats as a Tool in Animal Models of Human Health Disorders

Normal or dysfunctional sexual behavior seems to be an important indicator of health or disease. Many health disorders in male patients affect sexual activity by directly causing erectile dysfunction, affecting sexual motivation, or both. Clinical evidence indicates that many diseases strongly disrupt sexual motivation and sexual performance in patients with depression, addiction, diabetes mellitus and other metabolic disturbances with obesity and diet-related factors, kidney and liver failure, circadian rhythm disorders, sleep disturbances including obstructive sleep apnea syndrome, developmental and hormonal disorders, brain damages, cardiovascular diseases, and peripheral neuropathies. Preclinical studies of these conditions often require appropriate experimental paradigms, including animal models. Male sexual behavior and motivation have been intensively investigated over the last 80 years in animal rat model. Sexual motivation can be examined using such parameters as: anticipatory behavior and 50-kHz ultrasonic vocalizations reflecting the emotional state of rats, initiation of copulation, efficiency of copulation, or techniques of classical (pavlovian) and instrumental conditioning. In this review article, we analyze the behavioral parameters that describe the sexual motivation and sexual performance of male rats in the context of animal experimental models of human health disorders. Based on analysis of the parameters describing the heterogeneous and complex structure of sexual behavior in laboratory rodents, we propose an approach that is useful for delineating distinct mechanisms affecting sexual motivation and sexual performance in selected disease states and the efficacy of therapy in preclinical investigations.

Establishment of partner preference in male rats: Effect of prenatal letrozole and sexual experience

Repeated testing for masculine sexual behavior influences female sex preference in males. Males perinatally treated with aromatase inhibitors show male preference, but also copulate with the receptive female. Such copulation modifies sex preference most likely because of its rewarding properties. In this study, we intended to equal the incentive value of both stimuli -in the sex preference test- by using receptive females with vaginal occlusion. Vehicle and letrozole-treated (0.56 μg/kg, gestation days 10-21) males were repeatedly tested for sex preference at 40, 55, 70, 85 and 100 days of age. These ages were selected because males of 40 days are unable to copulate, while by 100 days of age almost all males show the complete repertoire of masculine sexual behavior. At 40 days of age, males of all groups fail to show sex preference and none of them was able to copulate. In controls of 100 days of age all males showed female-sex preference and all intromitted the female. A large proportion (44%) of vehicle-treated males that could not copulate the female showed male preference. Twenty to 30% of the prenatally letrozole treated males also had same-sex preference even if they could copulate; and most of them (67%) had a male preference when copulation was precluded. These data support the idea that copulation is crucial for developing a female preference in control animals. The results suggest that brain changes produced by estrogens along early development and stimuli coming from the partner are essential for shaping sex preference.

Brain activation associated to olfactory conditioned same-sex partner preference in male rats

Sexual preferences can be strongly modified by Pavlovian learning. For instance, olfactory conditioned same-sex partner preference can occur when a sexually naïve male cohabits with an scented male during repeated periods under the effects of enhanced D2-type activity. Preference is observed days later via social and sexual behaviors. Herein we explored brain activity related to learned same-sex preference (Fos-Immunoreactivity, IR) following exposure to a conditioned odor paired with same-sex preference. During conditioning trials males received either saline or the D2-type receptor agonist quinpirole (QNP) and cohabitated during 24 h with a stimulus male that bore almond scent on the back as conditioned stimulus. This was repeated every 4 days, for a total of three trials. In a drug-free final test we assessed socio/sexual partner preference between the scented male and a receptive female. The results indicated that QNP-conditioned males developed a same-sex preference observed via contact, time spent, olfactory investigations, and non-contact erections. By contrast, saline-conditioned and intact (non-exposed to conditioning) males expressed an unconditioned preference for the female. Four days later the males were exposed to almond scent and their brains were processed for Fos-IR. Results indicated that the QNP-conditioned group expressed more Fos-IR in the nucleus accumbens (AcbSh), medial preoptic area (MPA), piriform cortex (Pir) and ventromedial nucleus of the hypothalamus (VMH) as compared to saline-conditioned. Intact males expressed the lowest Fos-IR in AcbSh and VMH, but the highest in MPA and Pir. We discuss the role of these areas in the learning process of same-sex partner preferences and olfactory discrimination.