Vaginocervical stimulation induces Fos in glutamate neurons in the ventromedial hypothalamus: attenuation by estrogen and progesterone

Effects of chronic tamoxifen treatment in female rat sexual behaviour

The medial preoptic (MPN) and the ventromedial hypothalamic nuclei (VMN) modulate the estrogen receptor (ER)-dependent female sexual behavior, a response that is inhibited by tamoxifen (TAM), a modulator of the steroid receptor activation. With the objective to assess TAM action in the brain areas involved in the modulation sexual cues, an animal model on long-term TAM therapy to intact female rats, was used to mimic the 5-year prophylactic TAM therapy offered to women at higher risk of breast cancer. After three months treatment, female sexual behavior with a stud male rat was evaluated. Upon sacrifice, the brains were removed and the MPN and the ventrolateral division of the VMN were screened for the effects of TAM in the expression of ERα, ERβ and progesterone receptor. Results show that TAM inhibited the receptive component of the female sexual behavior. Even though TAM decreased estrogen and progesterone levels to values similar to the ones of estrous and diestrus rats, the biochemical data failed to demonstrate such possible causation for the behavioral response. In fact, TAM administration induced a constant low level of ovarian hormones that changed the pattern of ER and PR expression as well as receptor co-expression in the brain areas regulating the behavioral response, dissimilar to the ones seen in the cycle phases with the same low hormone levels. Nevertheless, present data suggests that by affecting ER- and/or PR-dependent mechanisms, TAM may modulate the hypothalamus, a region known to participate in several social behaviors.

Apelin-13 facilitates lordosis behavior following infusions to the ventromedial hypothalamus or preoptic area in ovariectomized, estrogen-primed rats

In normal hormonal conditions, increased neuronal activity in the ventromedial hypothalamus (VMH) induces lordosis whereas activation of the preoptic area (POA) exerts an opposite effect. In the present work, we explored the effect of bilateral infusion of different doses of the apelin-13 (0.37, 0.75, 1.5, and 15 μg) in both brain areas on the expression of lordosis behavior. Lordosis quotient and lordosis reflex score were performed at 30, 120, and 240 min. Weak lordosis was observed following the 0.37 μg dose of apelin-13 at 30 min in the VMH of EB-primed rats; however, the rest of the doses induced significant lordosis relative to the control group. At 120 min, all doses induced lordosis behavior, while at 240 min, the highest dose of 15 μg did not induce significant differences. Interestingly, only the 0.75 μg infusion of apelin in the POA induced significant lordosis at 120 and 240 min. These results indicate that apelin-13 acts preferably in HVM and slightly in POA to initiate lordosis behavior in estrogen-primed rats.

Glutamate in Male and Female Sexual Behavior: Receptors, Transporters, and Steroid Independence

The survival of animal species predicates on the success of sexual reproduction. Neurotransmitters play an integral role in the expression of these sexual behaviors in the brain. Here, we review the role of glutamate in sexual behavior in rodents and non-rodent species for both males and females. These encompass the release of glutamate and correlations with glutamate receptor expression during sexual behavior. We then present the effects of glutamate on sexual behavior, as well as the effects of antagonists and agonists on different glutamate transporters and receptors. Following that, we discuss the potential role of glutamate on steroid-independent sexual behavior. Finally, we demonstrate the interaction of glutamate with other neurotransmitters to impact sexual behavior. These sexual behavior studies are crucial in the development of novel treatments of sexual dysfunction and in furthering our understanding of the complexity of sexual diversity. In the past decade, we have witnessed the burgeoning of novel techniques to study and manipulate neuron activity, to decode molecular events at the single-cell level, and to analyze behavioral data. They pose exciting avenues to gain further insight into future sexual behavior research. Taken together, this work conveys the essential role of glutamate in sexual behavior.

Preference of kittens for scratchers

OBJECTIVES

The objective of this study was to determine kitten preferences towards different scratchers and the effects of catnip and cat odor on kitten scratching behaviors.

METHODS

Two-choice preference tests were conducted to compare scratchers and preferred scratchers with or without additives (ie, catnip, catnip oil, cat hair) in six studies. Kittens (n = 40, <8 weeks old) had access to two scratchers on the floor of a simulated living room for 20 mins and interactions were video-recorded. The time each kitten spent scratching each scratcher was compared.

RESULTS

In study 1, the S-shaped cardboard was preferred over a hemp post with a toy on top, and no difference was observed in the other pairs of scratchers compared. In study 2, the S-shaped cardboard was preferred by kittens compared with a raised scratcher covered with window screen or with carpet, and no differences were observed between the latter two scratchers. In study 3, the scratcher covered with window screen set upright was preferred over the same scratcher laid horizontally on the floor. The S-shaped cardboard was preferred over the scratcher covered with bubble wrap. In study 4, kittens preferred the S-shaped cardboard over the scratchers covered with window screen or short-fiber carpet but not over the scratcher covered with long-fiber carpet. In study 5, the S-shaped cardboard was strongly preferred over the long cardboard and rectangular cardboard but not over the boat-shaped cardboard. In study 6, dried catnip plant, catnip oil spray or the hair from other cats did not alter the scratching behavior exhibited by the kittens vs the control S-shaped cardboard alone.

CONCLUSIONS AND RELEVANCE

The S-shaped cardboard scratcher was a preferred scratcher for kittens. Catnip or odor of other cats did not alter this behavior in kittens. S-shaped cardboard may be a preferred scratching device for kittens.

Do rats have orgasms?

BACKGROUND

Although humans experience orgasms with a degree of statistical regularity, they remain among the most enigmatic of sexual responses; difficult to define and even more difficult to study empirically. The question of whether animals experience orgasms is hampered by similar lack of definition and the additional necessity of making inferences from behavioral responses.

METHOD

Here we define three behavioral criteria, based on dimensions of the subjective experience of human orgasms described by Mah and Binik, to infer orgasm-like responses (OLRs) in other species: 1) physiological criteria that include pelvic floor and anal muscle contractions that stimulate seminal emission and/or ejaculation in the male, or that stimulate uterine and cervical contractions in the female; 2) short-term behavioral changes that reflect immediate awareness of a pleasurable hedonic reward state during copulation; and 3) long-term behavioral changes that depend on the reward state induced by the OLR, including sexual satiety, the strengthening of patterns of sexual arousal and desire in subsequent copulations, and the generation of conditioned place and partner preferences for contextual and partner-related cues associated with the reward state. We then examine whether physiological and behavioral data from observations of male and female rats during copulation, and in sexually-conditioned place- and partner-preference paradigms, are consistent with these criteria.

RESULTS

Both male and female rats display behavioral patterns consistent with OLRs.

CONCLUSIONS

The ability to infer OLRs in rats offers new possibilities to study the phenomenon in neurobiological and molecular detail, and to provide both comparative and translational perspectives that would be useful for both basic and clinical research.

The role of orgasm in the development and shaping of partner preferences

BACKGROUND

The effect of orgasm on the development and shaping of partner preferences may involve a catalysis of the neurochemical mechanisms of bonding. Therefore, understanding such process is relevant for neuroscience and psychology.

METHODS

A systematic review was carried out using the terms Orgasm, Sexual Reward, Partner Preference, Pair Bonding, Brain, Learning, Sex, Copulation.

RESULTS

In humans, concentrations of arousing neurotransmitters and potential bonding neurotransmitters increase during orgasm in the cerebrospinal fluid and the bloodstream. Similarly, studies in animals indicate that those neurotransmitters (noradrenaline, oxytocin, prolactin) and others (e.g. dopamine, opioids, serotonin) modulate the appetitive and consummatory phases of sexual behavior and reward. This suggests a link between the experience of orgasm/sexual reward and the neurochemical mechanisms of pair bonding. Orgasm/reward functions as an unconditioned stimulus (UCS). Some areas in the nervous system function as UCS-detection centers, which become activated during orgasm. Partner-related cues function as conditioned stimuli (CS) and are processed in CS-detector centers.

CONCLUSIONS

Throughout the article, we discuss how UCS- and CS-detection centers must interact to facilitate memory consolidation and produce recognition and motivation during future social encounters.

Vaginocervical stimulation attenuates the sensitization of appetitive sexual behaviors by estradiol benzoate in the ovariectomized rat

The acute administration of estradiol benzoate (EB) to the ovariectomized (OVX) rat induces low levels of lordosis while sexually appetitive behaviors (e.g., hops, darts, solicitations) are absent, yet the repeated administration of EB results in a behavioral sensitization in which lordosis is potentiated and sexually appetitive behaviors are induced. We have shown that repeated copulation attenuates the sensitization of appetitive sexual behaviors. Here, we assessed which component of male stimulation during copulation is involved in the attenuation. On 8 occasions, sexually experienced OVX Long-Evans rats were treated with 10μgEB and 48h later assigned to one of six groups that differed in their experience on intermediates tests (2-7). One was given repeated access to a male (EB/Male), and another was placed in the copulation chamber alone (EB/Alone) on intermediate tests. Three groups were given one of three somatosensory stimuli by the experimenter: manual flank stimulation (FLS), clitoral stimulation (CLS), or vaginocervical stimulation (VCS). Finally, the control group was left undisturbed in the animal care facility (ACF). Sexual behaviors were measured on Tests 1 and 8. VCS received from the experimenter (VCS) or from the male during copulation (EB/Male) attenuated the magnitude of the sensitization of appetitive sexual behaviors compared with those that were not brought to the testing rooms (ACF), and the effect was most pronounced on sexual solicitations. These results suggest that VCS received during penile intromission inhibits the sensitization of sexually appetitive behaviors by repeated administration of EB. As such, repeated administration of EB may oppose those mechanisms that induce estrous termination, perhaps by sensitizing inhibitory processes within the ventromedial hypothalamus that typically prevent the display of sexual behaviors (i.e., by facilitating disinhibition).

RU486 facilitates or disrupts the sensitization of sexual behaviors by estradiol in the ovariectomized Long-Evans rat: Effect of timecourse

An acute injection of estradiol benzoate (EB) to the ovariectomized (OVX) rat activates low levels of lordosis, and subsequent progesterone (P) administration augments lordosis and recruits a complete pattern of sexual behavior including appetitive behaviors (e.g., hops/darts and solicitations). However, repeated injections of 5μg or 10μg EB (but not 2μg EB), administered every 4days to sexually-experienced OVX rats results in a behavioral sensitization, such that lordosis quotients (LQs) and appetitive behaviors progressively increase. We have shown that adrenal P does not play a critical role because behavioral sensitization to EB is not prevented by adrenalectomy. Here we tested whether P receptors play a role by examining the effect of chronic administration of the P receptor antagonist RU486 at a dose that reliably inhibits sexual behavior in fully primed OVX rats. Females were treated with EB (5 or 10μg), and 5mg RU486 dissolved in 0.4mL vehicle (VEH; 80% sesame oil, 15% benzyl benzoate, 5% benzyl alcohol) 48h and 5h prior to each of 7 tests, respectively, occurring at 4-day intervals in unilevel 4-hole pacing chambers. Control animals were treated with 2, 5, or 10μg EB+VEH. As expected, sensitization did not occur in females treated with 2μg EB+VEH, and those females received fewer intromissions and ejaculations than all other groups. RU486 did not prevent the sensitization of LQ, moderate and high lordosis magnitudes (LM2 and LM3) or appetitive sexual behaviors on early tests, and in fact potentiated appetitive behaviors, LQ, LM2 and LM3, consistent with its facilitative actions in females treated with EB-alone, as we and others have reported previously. However, despite the initial facilitation, blocking P receptors by chronic administration of RU486 inhibited the maintenance of behavioral sensitization to EB.

Behavioral defeminization by prenatal androgen treatment in rats can be overcome by sexual experience in adulthood

Exposure to testosterone during a critical period of prenatal development disrupts the normal display of sexual behaviors in adult ovariectomized (OVX) rats treated with estradiol benzoate (EB) followed by progesterone (P). The organizational hypothesis posits that prenatally androgenized females (PNAFs) are desensitized to EB. We tested this hypothesis by first treating PNAFs with varying doses of EB (2.5, 5, 10, 20μg) followed by P (500μg), and second by subjecting females to an established EB behavioral sensitization paradigm where females are first given sexual experience with EB (10μg) and P prior to repeated sexual behavior testing with EB alone. Long-Evans females were androgenized in utero by a s.c. injection of 500μg testosterone propionate or the oil control to pregnant dams on gestational day 18. Female offspring were OVX on postnatal day 80 and tested one week later in the unilevel 4-hole pacing chamber. Genital tissue was defeminized in PNAFs, and the lordosis quotient (LQ) and partial (i.e., hops/darts) and full solicitations were significantly lower, while defensive behaviors were higher, in PNAF females, relative to non-PNAF females regardless of the acute EB priming dose. However, repeated testing with EB alone (10μg), or EB and P eliminated the differences between groups on LQ and hops/darts, indicating that the behavioral deficit can be overcome by sexual experience. These results suggest that PNAFs are not desensitized to EB, and despite disruptions in sexual differentiation of anatomical structures, the deficiency in sexual behavior in response to acute EB and P can be experientially overcome. PNAFs appear, however, to have a chronic deficit in the expression of full solicitations.

Repeated administration of estradiol promotes mechanisms of sexual excitation and inhibition: Glutamate signaling in the ventromedial hypothalamus attenuates excitation

Repeated administration of 10 μg of estradiol benzoate (EB) every 4 days to the ovariectomized (OVX) rat induces a behavioral sensitization of sexual behaviors. Repeated copulation or the receipt of vaginocervical stimulation (VCS) attenuates the sensitization of appetitive sexual behaviors, suggesting that VCS acts in opposition to the mechanisms that induce the sensitization. It is known that VCS accelerates the onset of estrous termination (characterized by a decrease in appetitive sexual behaviors, and an increase in defensive behaviors prior to the decline in lordosis), and glutamate transmission in the ventromedial hypothalamus (VMH), particularly via AMPA receptor signaling, is an important regulator of this effect. Thus, the current studies examined whether mechanisms of estrous termination are involved in the attenuated sensitization to EB that occurs with repeated copulation. In the first study, OVX rats received infusions of AMPA to the VMH on tests 2-4, and sexual behavior was measured on tests 1 and 5. Appetitive sexual behaviors were lower in females that received AMPA infusions in place of copulation compared to saline, suggesting that AMPA receptor activation by VCS may be playing a role in the attenuation of sensitization. In the second study, females that were not given the opportunity to copulate on tests 2-4 fell out of behavioral estrus faster than those that did, suggesting that both excitatory and inhibitory mechanisms of sexual behavior become sensitized with repeated administration of EB. Together these findings extend our hypothesis that repeated episodes of heat sensitize the activation of sexual behaviors to increase the probability of eventual fertilization.

Enhanced male-evoked responses in the ventromedial hypothalamus of sexually receptive female mice

Social encounters often start with routine investigatory behaviors before developing into distinct outcomes, such as affiliative or aggressive actions. For example, a female mouse will initially engage in investigatory behavior with a male but will then show copulation or rejection, depending on her reproductive state. To promote adaptive social behavior, her brain must combine internal ovarian signals and external social stimuli, but little is known about how socially evoked neural activity is modulated across the reproductive cycle [1]. To investigate this, we performed single-unit recordings in the ventrolateral region of the ventromedial hypothalamus (VMHvl) in freely behaving, naturally cycling, female mice interacting with conspecifics of both genders. The VMHvl has been implicated in rodent sociosexual behavior [2, 3]: it has access to social sensory stimuli [4-8] and is involved in aggression and mating [9-11]. Furthermore, many VMHvl neurons express ovarian hormone receptors [12, 13], which play a central role in female sociosexual behavior [14-16]. We found that a large fraction of VMHvl neurons was activated in the presence of conspecifics with preference to male stimuli and that the activity of most VMHvl neurons was modulated throughout social interactions rather than in response to specific social events. Furthermore, neuronal responses to male, but not female, conspecifics in the VMHvl were enhanced during the sexually receptive state. Thus, male-evoked VMHvl responses are modulated by the reproductive state, and VMHvl neural activity could drive gender-specific and reproductive state-dependent sociosexual behavior.

Sensitization of sexual behaviors in ovariectomized Long-Evans rats is induced by a subthreshold dose of estradiol benzoate and attenuated by repeated copulation

Ovariectomy (OVX) abolishes the expression sexual behaviors in the rat, but they can be fully reinstated by sequential administration of estradiol benzoate (EB) followed by progesterone (P). When administered alone, 5 or 10 μg EB (but not 2 μg) acutely induce only low levels of lordosis, whereas repeated administration potentiates lordosis and induces sexually appetitive behaviors (e.g., hops, darts, solicitations, ear wiggles). The mechanisms mediating this behavioral sensitization are poorly understood, and it is not clear whether stimulation from the male during repeated copulation plays a role. OVX Long-Evans rats were given 4 sexual training sessions with EB (10 μg) and P (500 μg) 48 and 4h prior to testing, respectively, in a unilevel 4-hole pacing chamber followed by a 2-week hormone washout. Females were then treated with 2 μg or 10 μg EB 48 h prior to copulation on Tests 1 and 8. On Tests 2-7, a group of females was treated with 10 μg EB and allowed to copulate with a male (10 μg EB/Male, n = 16), or treated with 2 μg or 10 μg EB and placed in the chamber alone (2 μg EB/Alone, n = 6; 10 μg EB/Alone, n = 18). A negative control group was treated with the oil vehicle and placed in the chamber alone (Oil/Alone, n = 6) on Tests 2-7, but treated with 2 μg EB prior to copulatory Tests 1 and 8. All groups, except Oil, displayed behavioral sensitization to EB, suggesting that repeated administration EB is both necessary and sufficient to induce sensitization. Appetitive behaviors were attenuated in those that copulated on every session. Pacing was disrupted in all groups. Together these results suggest that EB activates excitatory mechanisms to promote the expression of sexual behaviors, which are potentiated across time under certain conditions. In contrast, copulatory stimulation attenuates behavioral sensitization to EB.

The role of oxytocin in male and female reproductive behavior

Oxytocin (OT) is a nonapeptide with an impressive variety of physiological functions. Among them, the 'prosocial' effects have been discussed in several recent reviews, but the direct effects on male and female sexual behavior did receive much less attention so far. As our contribution to honor the lifelong interest of Berend Olivier in the control mechanisms of sexual behavior, we decided to explore the role of OT in the present review. In the successive sections, some physiological mechanisms and the 'pair-bonding' effects of OT will be discussed, followed by sections about desire, female appetitive and copulatory behavior, including lordosis and orgasm. At the male side, the effects on erection and ejaculation are reviewed, followed by a section about 'premature ejaculation' and a possible role of OT in its treatment. In addition to OT, serotonin receives some attention as one of the main mechanisms controlling the effects of OT. In the succeeding sections, the importance of OT for 'the fruits of labor' is discussed, as it plays an important role in both maternal and paternal behavior. Finally, we pay attention to an intriguing brain area, the ventrolateral part of the ventromedial hypothalamic nucleus (VMHvl), apparently functioning in both sexual and aggressive behavior, which are at first view completely opposite behavioral systems.

Glutamate release in the ventromedial hypothalamus of the female rat during copulation: modulation by estradiol

Binding of glutamate or its ionotropic receptor agonists in the ventromedial hypothalamus (VMH) of female rats inhibits both appetitive and consummatory aspects of sexual behavior. Because vaginocervical stimulation activates glutamate neurons in the VMH, and administration of estradiol benzoate (EB) and progesterone (P) delays this effect, the present study examined the effects of hormonal priming on glutamate release within the VMH of female rats paired with sexually vigorous males. Ovariectomized, sexually experienced rats were implanted with guide cannula aimed at the ventrolateral VMH, through which microdialysis probes were inserted prior to testing. Females were assigned randomly to one of three hormone treatment conditions: EB+P, EB alone, or the oil vehicle. Testing was conducted over 5h, including a 120-min period of habituation to the testing chamber, a 60-min period of baseline sample collection, and a 120-min period during which a sexually vigorous male was introduced into the testing chamber. Dialysates were collected every 20min during the test and were analyzed for glutamate using HPLC. Females primed with oil had large and significant increases in glutamate release from baseline once the male was introduced to the chamber. Treatment with EB alone decreased glutamate release in response to male cues. Although treatment with EB+P did not differ significantly from EB alone, the degree of reduced glutamate release was less than with EB alone. These results indicate that priming with EB reduces glutamate transmission in the VMH in response to male cues. Taken together with our previous findings, estradiol blunts the activation of glutamate neurons in the VMH thus allowing female rats to copulate.

Temporal and concentration-dependent effects of oestradiol on neural pathways mediating sexual receptivity

The acceptance of oestradiol signalling through receptors found in the cell membrane, as well as, the nucleus, has provided for a re-examination of the timing and location of the actions of oestradiol on neural circuits mediating sexual receptivity (lordosis). Oestradiol membrane signalling involves the transactivation of metabotrophic glutamate receptors (mGluRs) that transduce steroid information through protein kinase C signalling cascades producing rapid activation of lordosis-regulating circuits. It has been known for some time that oestradiol initially produces an inhibition of the medial preoptic nucleus. We have demonstrated that underlying this inhibition is oestradiol acting in the arcuate nucleus to induce β-endorphin release, which inhibits the medial preoptic nucleus through a μ-opioid receptor mechanism. This transient inhibition is relieved by either subsequent progesterone treatment or longer exposure to higher doses of oestradiol to facilitate lordosis behaviour. We review recent findings about oestradiol membrane signalling inducing dendritic spine formation in the arcuate nucleus that is critical for oestradiol induction of sexual receptivity. Moreover, we discuss the evidence that, in addition to oestrogen receptor α, several other putative membrane oestrogen receptors facilitate lordosis behaviour through regulation of the arcuate nucleus. These include the GRP30 and the STX activated Gq-mER. Finally, we report on the importance of GABA acting at GABAB receptors for oestradiol membrane signalling that regulates lordosis circuit activation and sexual receptivity.

Sensitization of sexual behavior in ovariectomized rats by chronic estradiol treatment

The ovariectomized (OVX) rat treated with estradiol benzoate (EB) is used to elucidate neuroendocrine mechanisms of sexual behavior. Chronic behavioral and pharmacological manipulations can be confounded by rising baselines, since females are behaviorally more sensitive to repeated EB injections. The literature lacks a systematic examination of chronic effects of EB administered alone to the sexually experienced OVX rat. Long-Evans rats were repeatedly treated (8 tests) with s.c. injections of 2, 5, or 10 μg EB at different time intervals (4 or 8 days). Female sexual behaviors as well as receipt of mounts, intromissions and ejaculations from the male were observed in the unilevel 4-hole pacing chamber. The effects of adrenalectomy (ADX) and strain (Long-Evans vs. Wistar) were also assessed. Long-Evans OVX rats treated with 5 μg EB every 8 days showed persistently low levels of sexual behavior. Sensitization was most robust following 10 μg EB at 4-day intervals. Very few sexual behaviors were ever induced by 2 μg EB. ADX did not affect the development of behavioral sensitization by 10μg EB. Therefore, to achieve a low steady state of sexual behaviors in sexually experienced Long-Evans OVX rats 5μg of EB administered every 8days is optimal, whereas a persistently high level of sexual behaviors is induced with 10 μg EB administered every 4 days. OVX Wistar rats are behaviorally more sensitive to EB. Behavioral sensitization to EB may serve as a mechanism to optimize reproductive success.

Sex for fun: a synthesis of human and animal neurobiology

Sex is a fundamental pleasure, and crucial to the survival of our species. Though not many people would disagree with the proposition that sexual behaviour depends on the brain, the neuroscientific study of human sex is still relatively taboo and much remains to be discovered. On the contrary, excellent experimental animal models (mostly rat) are available that have uncovered major behavioural, neurochemical, and neuroanatomical characteristics of sexual behaviour. Restructuring sexual behaviour into broader terms reflecting behavioural states (wanting, liking, and inhibition) facilitates species comparison, revealing many similarities between animal and human sexual pleasure cycles, some of which can serve as potential avenues of new human sex research. In particular, behavioural and brain evidence clearly shows that motivational and consummatory phases are fundamentally distinct, and that genitally-induced sexual reward is a major factor in sexual learning mechanisms.

AMPA/kainate receptors in the ventromedial hypothalamus mediate the effects of glutamate on estrus termination in the rat

Infusions of glutamate or its selective receptor agonists to the VMH of ovariectomized (OVX) female rats primed with estradiol benzoate (EB) and progesterone (P) inhibit both appetitive and consummatory aspects of sexual behavior whereas selective glutamate receptor antagonists facilitate these measures in females primed with EB alone. Because vaginocervical stimulation (VCS) activates glutamate neurons in the VMH, and induces a faster termination of estros behavior, the present study examined the effects of the AMPA/kainate receptor antagonist DNQX on the induction of estrus termination by manual VCS. Ovx, sexually-experienced rats were primed with EB and P and subsequently received either 1 or 50 distributed VCSs, over the course of an hour, 12 h before a test with sexually vigorous males. Half of the females in each stimulus group received bilateral infusions of 1 μl/side of either DNQX (19.8 mmol/μl) or saline aimed at the VMH immediately prior to VCS or sham stimulation. Saline-infused females given VCS had lower lordosis quotients compared to females given sham stimulation. In contrast, females infused with DNQX prior to VCS displayed more appetitive behaviors and higher lordosis quotients and magnitudes compared to females infused with saline. These data indicate that activation of AMPA/kainate receptors in the VMH by increased glutamate transmission induced by VCS mediates estrus termination.

Maternal programming of reproductive function and behavior in the female rat

Parental investment can be used as a forecast for the environmental conditions in which offspring will develop to adulthood. In the rat, maternal behavior is transmitted to the next generation through epigenetic modifications such as methylation and histone acetylation, resulting in variations in estrogen receptor alpha expression. Natural variations in maternal care also influence the sexual strategy adult females will adopt later in life. Lower levels of maternal care are associated with early onset of puberty as well as increased motivation to mate and greater receptivity toward males during mating. Lower levels of maternal care are also correlated with greater activity of the hypothalamus-pituitary-gonadal axis, responsible for the expression of these behaviors. Contrary to the transition of maternal care, sexual behavior cannot simply be explained by maternal attention, since adoption studies changed the sexual phenotypes of offspring born to low caring mothers but not those from high caring dams. Indeed, mothers showing higher levels of licking/grooming have embryos that are exposed to high testosterone levels during development, and adoption studies suggest that this androgen exposure may protect their offspring from lower levels of maternal care. We propose that in the rat, maternal care and the in utero environment interact to influence the reproductive strategy female offspring display in adulthood and that this favors the species by allowing it to thrive under different environmental conditions.

Orphanin FQ in the mediobasal hypothalamus facilitates sexual receptivity through the deactivation of medial preoptic nucleus mu-opioid receptors

Sexual receptivity, lordosis, can be induced by sequential estradiol and progesterone or extended exposure to high levels of estradiol in the female rat. In both cases estradiol initially inhibits lordosis through activation of β-endorphin (β-END) neurons of the arcuate nucleus of the hypothalamus (ARH) that activate μ-opioid receptors (MOP) in the medial preoptic nucleus (MPN). Subsequent progesterone or extended estradiol exposure deactivates MPN MOP to facilitate lordosis. Opioid receptor-like receptor-1 (ORL-1) is expressed in ARH and ventromedial hypothalamus (VMH). Infusions of its endogenous ligand, orphanin FQ (OFQ/N, aka nociceptin), into VMH-ARH region facilitate lordosis. Whether OFQ/N acts in ARH and/or VMH and whether OFQ/N is necessary for steroid facilitation of lordosis are unclear. In Exp I, OFQ/N infusions in VMH and ARH that facilitated lordosis also deactivated MPN MOP indicating that OFQ/N facilitation of lordosis requires deactivation of ascending ARH-MPN projections by directly inhibiting ARH β-END neurons and/or through inhibition of excitatory VMH-ARH pathways to proopiomelanocortin neurons. It is unclear whether OFQ/N activates the VMH output motor pathways directly or via the deactivation of MPN MOP. In Exp II we tested whether ORL-1 activation is necessary for estradiol-only or estradiol+progesterone lordosis facilitation. Blocking ORL-1 with UFP-101 inhibited estradiol-only lordosis and MPN MOP deactivation but had no effect on estradiol+progesterone facilitation of lordosis and MOP deactivation. In conclusion, steroid facilitation of lordosis inhibits ARH β-END neurons to deactivate MPN MOP, but estradiol-only and estradiol+progesterone treatments appear to use different neurotransmitter systems to inhibit ARH-MPN signaling.

Variation in maternal care influences ventromedial hypothalamus activation in the rat

Natural variation in maternal care in the rat is an important source of individual differences in the female neuroendocrine system and sexual behaviours. Thus, females reared by low licking and grooming (LG) mothers are sexually more receptive to males, showing higher lordosis ratings, and are more motivated to mate compared to female offspring of high LG mothers. In the present study, we investigated the effect of natural variations in maternal care on the ventrolateral part of the ventromedial hypothalamus (VMHvl) cell population and on the reproductive success of the female rat. Immunohistochemistry for phosphorylated oestrogen-receptor (pER) α and progesterone receptor (PR) were used to study the VMHvl of female offspring of high and low LG mothers at pro-oestrus and dioestrus. A second experiment investigated sexual behaviour and the effect of mating on c-Fos expression in the VMHvl of pro-oestrus and ovariectomised high and how female offspring. Lastly, we investigated the maternal effect on the establishment of the progestational state. A greater number of VMHvl pERα immunoreactive cells was found in the pro-oestrous female offspring of low LG mothers and PR was most abundant at pro-oestrus compared to dioestrus in both high and low LG females. Interestingly it is the less receptive high females that show the greater c-Fos expression in the VMH after mating in the pro-oestrous group. The difference in c-Fos expression after mating disappeared when the two groups were ovariectomised and received steroid replacement. Finally, low LG female offspring reached pseudopregnancy more often when receiving only seven intromissions at a 5-min interval compared to high LG females. Lower levels of maternal care may favour the reproductive success of low LG offspring by increasing pERα and oestrogen-dependent lordosis behaviour and lowering c-Fos after mating, resulting in inhibition of termination of oestrus.

Context alters the ability of clitoral stimulation to induce a sexually-conditioned partner preference in the rat

We have shown previously that clitoral stimulation (CLS) of female rats induces significant conditioned place preference (CPP), indicating that it is rewarding. The present study asked whether CLS could induce a conditioned partner preference. In the first experiment, sexually naïve females received 10 alternating trials of CLS and No-CLS in the presence of a male rat behind a wire-mesh screen. For one group, CLS was made in the presence of the male scented with almond extract. On alternating trials, those females received sham CLS in the presence of an unscented male behind the screen. The order was reversed for the other group. After 5 trials in each condition, females were placed into an open field with two sexually vigorous males, one scented and the other unscented. Contrary to expectation, females displayed a preference for the male associated with sham CLS. The second experiment examined whether a partner preference could be conditioned by associating CLS with the almond odor alone. A new group of sexually naive females received the same CLS-odor, No-CLS-No Odor pairings as above, but with the odor presented on cotton gauze in the chamber. During the final open field test, those females selectively solicited the scented male. We conclude that CLS that induces CPP also induces conditioned partner preference. However, we propose that CLS in the presence of an inaccessible male created a sexual inhibitory state for female rats.

Clitoral stimulation modulates appetitive sexual behavior and facilitates reproduction in rats

In rats, sexual reward, appetitive sexual behaviors and reproduction are modulated by the amount and rate of vaginocervical stimulation. Here the effect of clitoral stimulation (CLS) on proceptivity was assessed. In Exp 1, ovariectomized, hormone-primed Wistar females formed three groups: G1 (1 CLS every second), G2 (1 CLS every 5s) and G3 (no CLS). Precopulatory CLS consisted of 5cycles of 1min of stimulation with the tip of a cotton swab connected to a vibrator device, followed by 1-2min of rest. CLS increased proceptive behavior in G1 compared to G2, but not compared to G3. In Exp 2, gonadally-intact rats in late proestrous received CLS prior to copulation. No differences in sexual behavior were detected between the groups, but CLS enhanced reproduction in females that received >9 intromissions. 28, 66 and 10% of females became pregnant in G1, G2, and G3, respectively. These data indicate that precopulatory CLS affects proceptive behaviors depending on the pattern and rhythm of stimulation in hormone-primed females. In virgin rats that have received sufficient vagino cervical stimulation CLS also increases fertility.