Francisco J. Camacho

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.

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.

Reward value of intromissions and morphine in male rats evaluated by conditioned place preference

The present experiment was designed to determine if intromissions alone could induce a reward state as evaluated by conditioned place preference (CPP). We also compared the rewarding properties of one ejaculation and a morphine injection. We evaluated if intromissions alone could induce CPP in males with one or three ejaculations as previous sexual experience. Different groups of males were allowed to display 5, 10, and 15 intromissions or an ejaculation with a sexually receptive female before being placed in the originally non-preferred compartment of a CPP cage. On alternate days they were placed in the preferred compartment. The groups that displayed 5 or 10 intromissions did not modify their original preference, regardless of whether they had experienced 1 or 3 ejaculations before the conditioning procedure. The groups that had experienced 1 ejaculations that were allowed to display 15 intromissions or one ejaculation modified their original preference indicating the induction of a reward state. These results suggest that male rats displaying sexual behavior require a minimum amount of stimulation, 15 intromissions or an ejaculation, in order for sex to be sufficiently rewarding to induce CPP. In a separate experiment we evaluated if a morphine injection (1mg/kg) has the same reward value that one ejaculation in male rats has. Two groups of sexually active males were used, in one group ejaculation was paired with the initially non-preferred compartment and morphine administration was paired with the initially preferred compartment. In the other group morphine injection was paired with the non-preferred compartment and ejaculation with the preferred compartment. None of the groups changed their originally preferred compartment suggesting that morphine and one ejaculation have the same reward value in male rats.

Hormonal and testing conditions for the induction of conditioned place preference by paced mating

The ability to control or pace the sexual interaction has important physiological and behavioral consequences for the female rat. Paced mating favors reproduction and induces a positive affective state as revealed by conditioned place preference (CPP). In the present experiment we evaluated: 1) If paced mating induces CPP in naturally cycling females; 2) If females developed a positive affective state if they paced the sexual interaction through a 1- or a 3-hole pacing chamber; 3) If females that mate with the same male without pacing the sexual interaction develop CPP. In the first experiment intact females were divided in 4 different groups; 2 paced the sexual interaction until receiving 1 or 3 ejaculations; the other 2 groups mated, without pacing the sexual interaction, until receiving 1 or 3 ejaculations. Only the group that paced the sexual interaction until receiving 3 ejaculations developed a positive affective state. In experiments 2 and 3 hormonally treated ovariectomized females were used. In experiment 2 females were allowed to pace the sexual interaction through a 1- or a 3-hole pacing chamber: A clear positive affective state was induced in both testing conditions. Finally, in experiment 3 females did not develop CPP for non-paced sex despite the fact that they mated with the same male in the conditioning sessions. These results demonstrate that the pattern of vaginocervical stimulation that the females received by engaging in approach and avoidance behaviors to pace the sexual interaction can induce a positive affective state in naturally cycling females. They also confirm the existence of a threshold of vaginocervical stimulation for paced mating to induce CPP in female rats.

Sexual activity increases the number of newborn cells in the accessory olfactory bulb of male rats

In rodents, sexual behavior depends on the adequate detection of sexually relevant stimuli. The olfactory bulb (OB) is a region of the adult mammalian brain undergoing constant cell renewal by continuous integration of new granular and periglomerular neurons in the accessory (AOB) and main (MOB) olfactory bulbs. The proliferation, migration, survival, maturation, and integration of these new cells to the OB depend on the stimulus that the subjects received. We have previously shown that 15 days after females control (paced) the sexual interaction an increase in the number of cells is observed in the AOB. No changes are observed in the number of cells when females are not allowed to control the sexual interaction. In the present study we investigated if in male rats sexual behavior increases the number of new cells in the OB. Male rats were divided in five groups: (1) males that did not receive any sexual stimulation, (2) males that were exposed to female odors, (3) males that mated for 1 h and could not pace their sexual interaction, (4) males that paced their sexual interaction and ejaculated one time and (5) males that paced their sexual interaction and ejaculated three times. All males received three injections of the DNA synthesis marker bromodeoxyuridine at 1h intervals, starting 1 h before the beginning of the behavioral test. Fifteen days later, males were sacrificed and the brains were processed to identify new cells and to evaluate if they differentiated into neurons. The number of newborn cells increased in the granular cell layer (GrCL; also known as the internal cell layer) of the AOB in males that ejaculated one or three times controlling (paced) the rate of the sexual interaction. Some of these new cells were identified as neurons. In contrast, no significant differences were found in the mitral cell layer (also known as the external cell layer) and glomerular cell layer (GlCL) of the AOB. In addition, no significant differences were found between groups in the MOB in any of the layers analyzed. Our results indicate that sexual behavior in male rats increases neurogenesis in the GrCL of the AOB when they control the rate of the sexual interaction.

Facilitation of ejaculation induced by 8-OH-DPAT does not produce conditioned place preference in male rats.

The serotonin 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) produces a drastic facilitation of ejaculation characterized by a significant reduction in the number of pre-ejaculatory intromissions and a shortening of ejaculation latency. In the present study, the authors evaluated whether this facilitation of ejaculation can induce a reward state assessed by conditioned place preference. Males treated with 0.1 or 1.0 mg/kg of 8-OH-DPAT showed a clear facilitation of ejaculation but did not develop conditioned place preference. These results clearly indicate that the pharmacological facilitation of ejaculation and the reduction of the number of intromissions does not necessarily make sex rewarding.

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.

Behavioral characterization of non-copulating male mice.

Non-copulating (NC) males are those animals that do not mate in spite of repeated testing with sexually receptive females. They have been observed in several species including rats and mice. The present experiment was designed to perform a detailed behavioral characterization of NC male mice. Thus, we evaluated their sexual incentive motivation for a sexually receptive female or a sexually active male, olfactory preference for volatile and non-volatile odors from females or males, and olfactory discrimination between female and male volatile odors and food related odors (milk versus vinegar). We compared the activity of the accessory olfactory system (AOS) in copulating (C) and NC males in response to estrous bedding using the induction of Fos-immunoreactivity (Fos-IR) as a measure of neuronal activation. We also determined if estradiol or dopamine treatment could induce sexual behavior in NC males. Finally, we compared the testis weight and the number of penile spines in C, NC, and gonadectomized males. In the sexual incentive motivation test C males spend significantly more time in the female incentive zone than in the male incentive zone. On the other hand, NC males spend the same amount of time in both incentive zones. In tests of olfactory preference, NC males spent less time investigating estrous odors than C males. As well, NC males discriminate urine from conspecifics but they spend less time smelling these odors than C males. In addition, no increase in Fos expression is observed in NC males when they are exposed to odors from estrous females. Our data also suggest that the deficits observed in NC males are not due to lower circulating levels of gonadal hormones, because estradiol supplementation does not induce sexual behavior in these animals, and their testis weight and the number of penile spines are normal. The results suggest that NC males are not sexually motivated by the receptive females and their odors.

The role of the dorsolateral tegmentum in the control of male sexual behavior: A reevaluation

The medial preoptic area/anterior hypothalamus (MPOA/AH) plays a key role in the control of male sexual behavior. Independently of the type, MPOA/AH lesions permanently eliminate male sexual behavior in the rat. The MPOA/AH projects among other structures to the dorsolateral tegmentum (DLT). Bilateral electrolytic lesions of the DLT or the unilateral electrolytic destruction of the MPOA/HA combined with a contralateral electrolytic lesion of the DLT eliminate male sexual behavior. In the present experiment, we evaluated if neurotoxic lesions of the DLT produce the same behavioral deficits as those observed after electrolytic lesions. This would allow us to evaluate if neurons of the DLT or the fibers passing through this area are important in the control of male sexual behavior. To this aim, sexually experience male rats were tested for socio-sexual behavior, partner preference and motor execution in order to determine if the possible behavioral changes could be attributed to alterations in sexual motivation or motor execution. One week after the bilateral DLT lesions the animals were evaluated in the same behavioral tests. The lesions were identified by glial fibrillary acidic protein (GFAP) and neuronal nuclear protein (Neu-N) immunohistochemistry. No significant consistent effects upon sexual behavior were observed in any of the groups, including the group with clear bilateral damage of the DLT. A reduction in the percentage of males displaying ejaculation in the first post-lesion test was observed for all groups injected with quinolinic acid. No effects upon partner preference or motor coordination were observed after the lesion in any of the groups. The lack of effect of DLT neurotoxic lesions upon mating suggests that neurons of this structure are not involved in the control of male sexual behavior.

Perinatal Inhibition of Aromatization Enhances the Reward Value of Sex

Paced mating reduces the aversive properties and increases the positive characteristics of mating, inducing a reward state. Pacing is able to induce conditioned place preference (CPP), whereas nonpaced mating does not. The authors hypothesized that the aversive properties of mating are caused by androgens from adjacent males or from the mother during fetal life. To test whether aromatization of androgens induces the aversive properties of mating, female rats were treated perinatally with 1,4,6-androstatriene-3, 17-dione (ATD) to inhibit aromatization. When adults, these females were ovariectomized and hormonally primed to evaluate CPP after paced and nonpaced mating. During paced mating, control females showed higher return latencies after ejaculation, whereas ATD-treated females did not show a similar increase. In CPP tests, both paced and nonpaced mating induced a reward state in ATD-treated females, whereas only paced mating induced a reward state in control females. These results show that the perinatal inhibition of aromatization enhances the rewarding properties of mating, suggesting that estradiol induced the aversive properties of mating and/or modified the perinatal organization of the neuronal pathways in females.