Effects of neonatal testosterone treatment on pacing behaviors and development of a conditioned place preference

Hormone-dependent sexual responses of female mice in response to manual genital stimulation

Although copulatory behavior is thought to have a strong innate basis in mice, there is also clear evidence that sexual experience shapes its expression. Reinforcement of behavior through rewarding genital tactile stimulation is a primary candidate mechanism for this modification. In rats, manual tactile clitoral stimulation is rewarding only when it is temporally distributed, which is hypothesized to result from an innate preference for species-typical copulatory patterning. Here we test this hypothesis using mice, which have a temporal copulatory pattern which is distinctly less temporally distributed than that of rats. Female mice received manual clitoral stimulation which was either temporally continuous every second, or stimulation which was temporally distributed, occurring every 5 s, This pattern of stimulation was paired with environmental cues in a conditioned place preference apparatus to assess reward. Neural activation in response to this stimulation was evaluated by measuring FOS immunoreactivity. Results indicated that both temporal patterns of clitoral stimulation were rewarding, but that continuous stimulation better reproduced brain activation associated with sexual reward. Furthermore, continuous, but not distributed stimulation elicited a lordosis response in some females, and this response increased within and across days. Sexual reward, neural activation and lordosis resulting from tactile genital stimulation were eliminated by ovariectomy and restored with combined 17β-estradiol and progesterone treatment but not 17β-estradiol treatment alone. These observations are consistent with the hypothesis that sexual reward resulting from species-typical genital tactile stimulation has a permissive effect on copulatory behavior of female mice.

Perinatal testosterone exposure and maternal care effects on the female rat's development and sexual behaviour

Natural variations in maternal care have profound influences on offspring behaviour, brain activity and hormone release. Measuring the amount of time that a rat dam spends licking/grooming (LG) her pups during their first week of life allows for characterisation of distinctive Low, Mid and High LG phenotypes. We have previously found that female offspring of High LG mothers are less sexually receptive, less motivated to mate and show a later onset of puberty relative to Low LG offspring. Given that High LG females are exposed to greater levels of testosterone in utero, we hypothesise that differences in sexual behaviour between High and Low LG female offspring are driven in part by differences in prenatal hormone exposure. To test this hypothesis, pregnant dams pre-characterised as Low, Mid, or High LG mothers were implanted with testosterone or placebo on gestational day (GD) 16. Offspring body weight and anogenital index were assessed at GD 21 and in adulthood. Age of vaginal opening and oestrous cyclicity were assessed to determine the timing of pubertal onset. Testosterone exposure removed the difference between LG phenotypes in pubertal onset by delaying vaginal opening and the appearance of first pro-oestrus. In adulthood, sexual behaviour in a paced mating chamber after sham surgery or ovariectomy with steroid-replacement was examined. Our findings show that Low, Mid and High LG female offspring are differentially affected by perinatal testosterone exposure, and that this exposure removes the precocial pubertal onset of Low LG offspring and increases the sexual proceptivity and receptivity of High LG offspring. These results suggest that maternal programming of the female reproductive system may be mediated, in part, through differences in perinatal testosterone exposure, instead of solely through maternal behaviour.

Conditioned place preference for mating is preserved in rats with pelvic nerve transection

Female rats exhibit a conditioned place preference (CPP) for a context paired with mating. The present experiment tested the hypothesis that the activation of the pelvic nerve mediates the reinforcing effects of mating for female rats. Rats underwent bilateral pelvic nerve or sham transection and then received paced mating, nonpaced mating, or the control treatment during a CPP procedure. Pelvic nerve transection did not affect the CPP for paced or nonpaced mating. In tests of paced mating behavior, contact-return latencies following intromissions were significantly shorter in rats with pelvic nerve transection than they were in rats with sham transections. These results show that the pathway conveying the reinforcing effects of mating stimulation does not depend on the integrity of the pelvic nerve, but that activation of the pelvic nerve contributes to the display of paced mating behavior.

Infusions of bicuculline to the ventral tegmental area attenuates sexual, exploratory, and anti-anxiety behavior of proestrous rats

Actions of 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), in the midbrain ventral tegmental area (VTA) modulate sexual receptivity of female rats. Actions of 3alpha,5alpha-THP at GABAergic substrates in the VTA are known to modulate consummatory aspects of sexual behavior among rodents, such as lordosis. However, the extent to which GABA(A) receptors in the VTA are important for appetitive (exploratory, anti-anxiety, social) aspects of sexual receptivity is not well-understood. Proestrous rats were bilaterally-infused with saline or bicuculline (100 ng), a GABA(A) receptor antagonist, to the VTA or missed control sites. Rats were assessed for exploratory/anti-anxiety (open field/elevated plus maze), social (social interaction), and sexual (paced-mating) behavior. Compared to saline or missed site controls, intra-VTA bicuculline significantly reduced the number of central entries in an open field, time spent on the open arms of an elevated plus maze, frequency and intensity of lordosis, anti-aggression towards a male, pacing of sexual contacts, and 3alpha,5alpha-THP concentrations in midbrain and hippocampus. Bicuculline-infused rats also displayed less affiliation with a novel conspecific, fewer sexual solicitations, and had lower 3alpha,5alpha-THP concentrations in diencephalon and cortex, albeit these were not significant differences. Thus, actions at GABA(A) receptors in the midbrain VTA are essential for appetitive and consummatory aspects of sexual receptivity among rats.

The anabolic steroids testosterone propionate and nandrolone, but not 17alpha-methyltestosterone, induce conditioned place preference in adult mice

Anabolic androgenic steroids (AAS) are often misused by adolescents and athletes. Their effects vary according to chemical structure and metabolism, route of administration, and AAS regimen. In this study, adult C57Bl/6 male mice were systemically exposed to testosterone propionate (TP), nandrolone or 17alpha-methyltestosterone (17alpha-meT), type I, type II and type III AAS, respectively, in order to determine the hedonic or aversive properties of each drug. For this purpose, the conditioned place preference (CPP) test was employed at three different AAS doses (0.075, 0.75 and 7.5 mg/kg). Other behavioral domains monitored were light-dark transitions (side changes) and general activity. TP shifted place preference at all doses tested, and nandrolone shifted place preference at 0.75 and 7.5 mg/kg, but not at 0.075 mg/kg, the lower dose tested. Conversely, mice receiving 17alpha-meT did not show alteration in the preference score. The lower dose of nandrolone did modify exploratory-based anxiety showing a decrease in light-dark transitions if compared to vehicle-treated animals, while mice treated with TP or 17alpha-meT were not affected. Our data suggest that when studying hedonic and rewarding properties of synthetic androgens, distinction has to be made based on type of AAS and metabolism.

Artificial vaginocervical stimulation induces a conditioned place preference in female rats

Female rats express a conditioned place preference (CPP) for a context paired with mating. During a mating encounter, the female rat is exposed to several different types of stimuli, including, but not limited to, vaginocervical stimulation and social contact. The present experiment tested the hypothesis that two components of the mating interaction, vaginocervical stimulation or social contact, each induce a CPP in female rats. During conditioning rats received nonpaced mating, artificial vaginocervical stimulation, social interaction or a control treatment. Rats expressed a CPP for the context paired with nonpaced mating or artificial vaginocervical stimulation whereas social interaction and the control treatment did not induce a CPP. The present findings highlight the important role that vaginocervical stimulation plays in the reinforcing effects of mating in female rats.

Increasing 3alpha,5alpha-THP following inhibition of neurosteroid biosynthesis in the ventral tegmental area reinstates anti-anxiety, social, and sexual behavior of naturally receptive rats

The progesterone metabolite and neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), has actions in the midbrain ventral tegmental area (VTA) to modulate lordosis, but its effects on other reproductively relevant behaviors are not well understood. Effects on exploration, anxiety, and social behavior resulting from inhibition of 3alpha,5alpha-THP formation, as well as 3alpha,5alpha-THP enhancement, were investigated in the midbrain VTA. Naturally sexually receptive, female rats (n=8-10/group) received infusions aimed at the midbrain VTA of vehicle, PK11195 (an inhibitor of neurosteroidogenesis), and/or indomethacin (an inhibitor of 3alpha,5alpha-THP formation from prohormones), and were subsequently infused with vehicle or FGIN 1-27 (a neurosteroidogenesis enhancer). The rats were then assessed in a behavioral battery that examined exploration (open field), anxiety (elevated plus maze), social (social interaction), and sexual (paced mating) behavior. Inhibition of 3alpha,5alpha-THP formation decreased exploratory, anti-anxiety, social, and sexual behavior, as well as midbrain 3alpha,5alpha-THP levels. Infusions of FGIN 1-27 following 3alpha,5alpha-THP inhibition restored these behaviors and midbrain 3alpha,5alpha-THP levels to those commensurate with control rats that had not been administered inhibitors. These findings suggest that 3alpha,5alpha-THP formation in the midbrain VTA may influence appetitive, as well as consummatory, aspects of mating behavior.

Exploratory, anti-anxiety, social, and sexual behaviors of rats in behavioral estrus is attenuated with inhibition of 3alpha,5alpha-THP formation in the midbrain ventral tegmental area

The progesterone (P(4)) metabolite and neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) acts in the midbrain ventral tegmental area (VTA) to modulate lordosis of female rats. 3alpha,5alpha-THP also mediates exploratory, affective, and social behaviors; whether actions of 3alpha,5alpha-THP in the VTA mediate these behaviors is of interest. To elucidate the role of the VTA in mediating exploratory, affective, and social behaviors, the present study examined effects of inhibiting 3alpha,5alpha-THP formation in the VTA. Rats received intra-VTA infusions of either PK11195 (400ng/mul, which inhibits de novo 3alpha,5alpha-THP production), indomethacin (10mug/mul, which blocks metabolism of P(4) to 3alpha,5alpha-THP), PK11195 and indomethacin together, or beta-cyclodextrin vehicle and tested on a battery of anxiety (open field and elevated plus maze), social (partner preference and social interaction), and sexual (paced mating) tasks. Compared to rats infused with vehicle to the VTA, rats infused with inhibitor(s) demonstrated significant reductions in central entries in the open field, time on open arms of an elevated plus maze, time spent interacting with a conspecific, initiation and intensity of lordosis, sexual solicitations, and midbrain 3alpha,5alpha-THP levels. These findings suggest that actions of 3alpha,5alpha-THP in the VTA are important for mediating aspects of exploration, anxiety, and social behavior related to mating.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Infusions of 3alpha,5alpha-THP to the VTA enhance exploratory, anti-anxiety, social, and sexual behavior and increase levels of 3alpha,5alpha-THP in midbrain, hippocampus, diencephalon, and cortex of female rats

17beta-Estradiol (E2) and progesterone (P4) influence the onset and duration of sexual behavior and are also associated with changes in behaviors that may contribute to mating, such as exploration, anxiety, and social behaviors (socio-sexual behaviors). In the midbrain ventral tegmental area (VTA), the P4 metabolite, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), modulates lordosis of E2-primed rodents; 3alpha,5alpha-THP can also influence anxiety and social behaviors. To examine if 3alpha,5alpha-THP in the VTA mediates socio-sexual behaviors, we infused 3alpha,5alpha-THP to the VTA of diestrous and proestrous rats. As expected, proestrous, compared to diestrous, rats showed more exploratory (open field), anxiolytic (elevated plus maze), pro-social (partner preference, social interaction), and sexual (paced mating) behavior and had increased E2, P4, dihydroprogesterone (DHP), and 3alpha,5alpha-THP in serum, midbrain, hippocampus, diencephalon, and cortex. Infusions of 3alpha,5alpha-THP to the VTA, but not control sites, such as the substantia nigra (SN) or central grey (CG), of diestrous rats produced behavioral and endocrine effects akin to that of proestrous rats and increased DHP and 3alpha,5alpha-THP levels in midbrain, hippocampus, and diencephalon. Levels of DHP and 3alpha,5alpha-THP, but neither E2 nor P4 concentrations, in midbrain, hippocampus, diencephalon, and/or cortex were positively correlated with socio-sexual behaviors. Thus, 3alpha,5alpha-THP infusions to the VTA, but not SN or CG, can enhance socio-sexual behaviors and increase levels in midbrain, hippocampus, and diencephalon.

Engaging in paced mating, but neither exploratory, anti-anxiety, nor social behavior, increases 5alpha-reduced progestin concentrations in midbrain, hippocampus, striatum, and cortex

Sequential actions of 17beta-estradiol (E(2)) and progesterone (P(4)) in the hypothalamus and the P(4) metabolite, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), in the midbrain ventral tegmental area (VTA) respectively mediate the initiation and intensity of lordosis of female rats and may also modulate anxiety and social behaviors, through actions in these, and/or other brain regions. Biosynthesis of E(2), P(4), and 3alpha,5alpha-THP can also occur in brain, independent of peripheral gland secretion, in response to environmental/behavioral stimuli. The extent to which engaging in tasks related to reproductive behaviors and/or mating increased E(2) or progestin concentrations in brain was investigated. In Experiment 1, proestrous rats were randomly assigned to be tested in individual tasks, including the open field, elevated plus maze, partner preference, social interaction, or no test control, in conjunction with paced mating or no mating. Engaging in paced mating, but not other behaviors, significantly increased dihydroprogesterone (DHP) and 3alpha,5alpha-THP levels in midbrain, hippocampus, striatum, and cortex. In Experiment 2, proestrous rats were tested in the combinations of the above tasks (open field and elevated plus maze, partner preference, and social interaction) with or without paced mating. As in Experiment 1, only engaging in paced mating increased DHP and 3alpha,5alpha-THP concentrations in midbrain, hippocampus, striatum, and cortex. Thus, paced mating enhances concentrations of 5alpha-reduced progestins in brain areas associated with reproduction (midbrain), as well as exploration/anxiety (hippocampus and striatum) and social behavior (cortex).

Number of children and risk of Parkinson's disease

We investigated the association between number of children and Parkinson's disease (PD) in two independent studies. In a case-control study, we identified all subjects who developed PD in Olmsted County, MN, from 1976 through 1995, and matched them individually by age (+/-1 year) and sex to population controls (193 cases and 193 controls). The replication study was a population-based cohort study of 6,341 subjects from Rotterdam, the Netherlands (2,610 men). In the Olmsted County study, men who fathered at least one child had an increased risk of PD (unadjusted OR, 2.7; 95% CI, 1.2-6.1; P = 0.02), and the risk increased with increasing number of children. The findings in women were not significant. In the Rotterdam Study, the risk of PD increased significantly with increasing number of children in men (test for linear trend, unadjusted; P = 0.04), but not in women. The findings from both studies remained consistent in direction but reduced in magnitude of the association, and lost significance after simultaneous adjustment for education, cigarette smoking, alcohol consumption, and coffee consumption. The independent replication in two distinct populations and using different epidemiologic study designs may suggest a link between the number of children and PD restricted to men.

Female rats exhibit a conditioned place preference for nonpaced mating

Paced, but not nonpaced, mating behavior is reported to induce a conditioned place preference (CPP) in female rats. Contrary to these previous findings, Experiment 1 showed that female rats that received 15 intromissions from a single male rat during each of five conditioning sessions exhibited a CPP for the compartment associated with mating when the intromissions were delivered via a paced or nonpaced paradigm. Experiment 2 demonstrated that nonpaced mating induced a CPP when a single male delivered the 15 intromissions but not when the male was replaced following ejaculation and a new male allowed to complete the requisite number of intromissions. These findings invite reevaluation of the reinforcing aspects of mating behavior in female rats.

Sexual experience in female rodents: cellular mechanisms and functional consequences

The neurobiology of female sexual behavior has largely focused on mechanisms of hormone action on nerve cells and how these effects translate into the display of copulatory motor patterns. Of equal importance, though less studied, are some of the consequences of engaging in sexual behavior, including the rewarding properties of sexual interactions and how sexual experience alters copulatory efficiency. This review summarizes the effects of sexual experience on reward processes and copulation in female Syrian hamsters. Neural correlates of these sexual interactions include long-term cellular changes in dopamine transmission and postsynaptic signaling pathways related to neuronal plasticity (e.g., dendritic spine formation). Taken together, these studies suggest that sexual experience enhances the reinforcing properties of sexual behavior, which has the coincident outcome of increasing copulatory efficiency in a way that can increase reproductive success.

Prenatal blockade of androgen receptors reduces the number of intromissions needed to induce conditioned place preference after paced mating in female rats

Estrous female rats pace their coital contacts to control the rate of cervical/vaginal stimulation. Females that receive at least 10 paced intromissions develop a reward state, evaluated by conditioned place preference (CPP), whereas females that do not pace their coital contacts do not develop CPP. We asked if the blockade of androgen receptors could modify the number of intromissions needed during paced mating to develop CPP. Pregnant females received daily injections of the androgen receptor antagonist flutamide from day 12 of pregnancy until pups were born. When adults, females exposed prenatally to flutamide or vehicle were ovariectomized and hormonally primed to evaluate if paced and non-paced mating induced CPP. The prenatal flutamide treatment did not affect the capacity of females to develop CPP to a systemic morphine injection. Flutamide-treated females that paced their sexual contacts developed CPP with fewer intromissions than control females. No effect on conditioning was observed when females were not allowed to pace their sexual contacts. The results suggest the existence of a threshold of cervical/vaginal stimulation that correlates with the induction of a reward state and that this threshold can be reduced by prenatal blockade of androgen receptors.

Sexual behavior of female rats in a multiple-partner preference test

In this study, sexually experienced female rats were tested in a multiple-partner preference test (MPPT) in which they were allowed to pace their sexual contacts with four sexually active males. Four cylinders, with a small hole through which only the female could move freely from one cylinder to another, were assembled forming in the center an empty compartment. An intact female was placed in the central compartment and a sexually active male in each cylinder. Female sexual behavior was analyzed throughout the estrus cycle in four consecutive days. Each daily test lasted 15 min. The percentage of exits after intromission or ejaculation was significantly higher than the percentage of exits after each mount. The female spent significantly longer time with one of the males. We designated this male as the preferred male (PM). Although in each of the 4 days studied, females spent significantly longer time with the PM, however, the male selected was not the same throughout the estrus cycle. The number of entries into the compartment of the PM was significantly higher and increased around proestrus. Compared to previous studies, pacing behavior was notably lower in the conditions of the MPPT. No significant differences were observed during the estrous cycle concerning the other parameters recorded. The present results suggest that the MPPT could be a good model to study partner preference in the female rat.

Sexual experience and conditioned place preference in male rats

We have previously shown that sexual behavior induces a reward state, as evaluated by conditioned place preference (CPP), only when males or females are able to control the rate of sexual interaction. In the present experiment, we evaluated if male rats that are repeatedly tested in a situation in which they are not able to control the sexual interaction eventually develop CPP. Three groups of sexually naïve male rats were used. One group never mated. A second group was tested once a week for 10 consecutive weeks in a chamber in which they controlled the rate of the sexual interaction. The third group was mated for the same number of weeks in a chamber in which the female, but not the male, controlled mating. The three groups were then tested for CPP. Only the group able to control the sexual interaction developed CPP. The group that had no control over the rate of the sexual interaction did not develop CPP even after 10 tests in which they consistently displayed sexual behavior. These results suggest that an estrous female and/or sexual behavior are powerful incentives that maintain mating even if the rewarding properties of the incentive are reduced.

Sex dimorphisms in the neuroprotective effects of estrogen in an animal model of Parkinson's disease

The incidence of certain neurological disorders, including Parkinson's disease, appears to be more prevalent in men. Studies involving estrogen treatment of ovariectomised rodents attribute this largely to the neuroprotective effects of estrogen. However, a neuroprotective role for physiological levels of circulating hormones in males and females is less clear. Using the 6-hydroxydopamine (6-OHDA) model of Parkinson's disease to lesion the nigrostriatal dopaminergic (NSDA) pathway, we have shown that in females, endogenously produced estrogen is neuroprotective, whereas in males, gonadal factors increase striatal 6-OHDA toxicity. Intriguingly, estrogen, but not dihydrotestosterone, a nonaromatizable androgen, reversed the effects of orchidectomy on lesion size, raising the novel the hypothesis that enhanced male susceptibility may be attributable to the effects of endogenous testosterone only after its aromatization to estrogen. Thus, estrogen appears to exert opposite effects in the NSDA in males and females, being neuroprotective in females, but not in males, where it may even exacerbate neurodegenerative responses, with important implications for the clinical potential of estrogen-related compounds as neuroprotective agents. Preliminary experiments support the hypothesis that sex differences in the adult NSDA may result from the organisational actions of gonadal steroids during the critical neonatal period for the masculinization of the brain. Further studies are needed to determine whether this early organisation of a sexually differentiated neural circuitry may contribute to the emergence of neurodegenerative conditions such as Parkinson's disease.

Progestins and place preference conditioning after paced mating

When female rats pace their coital interaction, a reward state evaluated by conditioned place preference is induced. Progesterone (P) is essential for the expression of proceptive behavior and for the induction of CPP. However, the functional significance of ring A reduction of P for the induction of this state during estrous is unsettled. In the present study, we evaluated whether ring A-reduced metabolites of P are involved in the reward state induced when the females are allowed to pace their sexual contacts. Ovariectomized (ovx) female rats treated with estradiol benzoate (EB, 5 microg) and P (13 microg), Megestrol acetate (MA; 13 microg ), 5 alpha-pregnan-20 dione (5 alphaDHP; 3 microg), or 5 beta-pregnan-3 alpha-ol-20-one (5 beta,3 alpha-Pgl; 3 microg) were used. Progestins were dissolved in propylene glycol and intravenously (iv) injected through an indwelling jugular catheter before females were tested for pacing behavior. After 15 intromissions or one ejaculation, females were gently placed in the nonpreferred compartment of a CPP box. Paced mating in all groups treated with progestins induced a clear change of preference. The administration of progestins alone did not induce CPP. These results suggest that P and ring A-reduced metabolites facilitate the reward state following pacing.