Infusions of naloxone into the medial preoptic area, ventromedial nucleus of the hypothalamus, and amygdala block conditioned place preference induced by paced mating behavior

Paced mating induces positive affect as revealed by conditioned place preference (CPP) in female rats. It has been suggested that endogenous opioids are involved in the generation of this positive affect since systemic administration of the opioid antagonist naloxone blocks mating-induced CPP. Several brain structures, including the medial preoptic area (mPOA), the ventromedial nucleus of the hypothalamus (VMH), the amygdala (Me), and the nucleus accumbens (Acb) have been implicated in the control of female sexual behavior. However, it is not known if these structures also participate in the positive affect produced by paced mating. To this end we determined the effects of intracranial administration of naloxone methiodide into the mPOA, VMH, Me and Acb on conditioned place preference induced by paced mating in female rats. Regardless of the site of infusion 5 μg of naloxone did not affect any of the sexual behavior parameters measured during copulation. When CPP was evaluated, the groups infused with naloxone into the mPOA, the VMH, and the Me before each conditioning session did not develop place preference. Only the group infused with naloxone in the Acb and the control groups did so. These results demonstrate that opioid receptors within the mPOA, VMH and Me are necessary for the rewarding aspects of paced mating. We suggest that the Me and VMH are important for the transmission of sensory information produced by copulation while the mPOA is the site where the positive affect is originated.     

Different doses of estradiol benzoate induce conditioned place preference after paced mating

The ovarian hormones estrogen and progesterone are required for the complete display of sexual behavior in female rats. Paced mating produces a reward state in intact cycling and ovariectomized (OVX), hormonally primed females as evaluated by the conditioned place preference (CPP) paradigm. Most of the studies that have evaluated CPP induced by paced mating in OVX females have used relatively high doses of estradiol benzoate (EB). In the present study we determined if different doses of EB, combined with progesterone (P), could induce CPP after paced mating. For this purpose OVX female rats were divided in five groups that received one of different doses of estradiol benzoate (5, 2.5, 1.25 or 0.625 μg estradiol + 0.5 mg of progesterone) before being allowed to pace the sexual interaction and conditioned in a CPP paradigm. We found that the lowest dose of EB used (0.625 μg) significantly reduced the lordosis quotient and the lordosis coefficient. Even though these females paced the sexual interaction, they didn't change its original preference, suggesting that sexual interaction did not induce a positive affective, reward state. Females allowed to pace the sexual interaction with higher doses of EB developed CPP after paced mating. These results indicate that a threshold of estradiol is required for paced mating to induce CPP.     

Only Self-Paced Mating Is Rewarding in Rats of Both Sexes

When rats are mated in a traditional mating chamber (with one male and one female) in which the male dictates the pace of the copulatory sequence, males develop a reward state as evaluated by conditioned place preference (CPP). In this mating situation no reward state is induced in females. However, when female rats are able to control (pace) the rate of sexual stimulation, thereby reducing the aversive consequences associated with mating, a clear CPP is observed. In the present study the CPP paradigm was used to determine whether if the reinforced state induced by coital interactions in male rats can be maintained when females pace the sexual interaction. Adult male and female rats were mated in one of two different conditions: (1) where subjects were able to pace their coital interactions or (2) where subjects were not able to pace their sexual contacts. The results showed that when males had control over the sexual interaction they developed a clear place preference while males that mated with females that paced their coital contacts did not develop CPP. Similarly, only females that were able to pace their sexual contacts developed place preference. These results suggest that coital interactions in males, as well as in females, can induce a reward state only when they are able to control the sexual interaction. Under seminatural conditions sexual behavior in rats is highly promiscuous, they mate in groups and repeatedly change partners in the middle of copulation. This behavioral sequence allows both, male and female to control the rate of sexual interaction, assuring the induction of a reward state outlasting the actual performance of coital responses.     

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.     

Extended paced mating tests induces conditioned place preference without affecting sexual arousal

One way to evaluate sexual arousal is by measuring approach behavior to sexual incentive stimuli. In our case we measure approach behavior to an originally non-preferred compartment which is associated with the physiological state induced by mating. This change of preference indicative of a positive affective (reward) state can be evaluated by conditioned place preference (CPP). We have shown that the CPP induced by paced mating is mediated by opioids. The administration of opioids also induces a reward state. The present study was designed to compare the rewarding properties of paced mating and a morphine injection. One group of females was allowed to pace the sexual interaction before being placed in the non-preferred compartment. In alternate sessions they received a morphine injection before being placed in the preferred compartment. In another group of females, the treatments were reversed. Only the females placed in the originally non-preferred compartment after paced mating changed their original preference, suggesting that paced mating induces a positive affective, reward, state of higher intensity than a morphine injection of 1 mg/kg. In a second experiment we determined if females allowed to pace the sexual interaction for 1 h would still developed CPP. No change in preference was observed in the females that mated for 1 h without pacing the sexual interaction. On the other hand, females that received between 10 and 15 paced intromissions as well as females that paced the sexual interaction for 1 h developed a clear CPP. The second experiment demonstrated that pacing is rewarding even in an extended mating session in which the females received around 25 intromissions and several ejaculations. These results further demonstrate the biological relevance associated with the ability of the female to space coital stimulation received during mating. This positive affective state will contribute to increase sexual arousal the next time a rat finds an appropriate mate.     

Activation of mu-opioid receptors inhibits lordosis behavior in estrogen and progesterone-primed female rats

The present study investigated the effect of highly selective mu-opioid receptor (OR) agonists on lordosis behavior in ovariectomized rats treated with 3 microg of estradiol benzoate followed 48 h later by 200 microg of progesterone. Ventricular infusion of the endogenous mu-OR agonists endomorphin-1 and -2 suppressed receptive behavior in a time- and dose-dependent fashion. At 6 microg, both endomorphin-1 and -2 inhibited lordosis behavior within 30 min. However, while the effect of endomorphin-1 lasted 60 min, endomorphin-2 inhibition lasted up to 120 min after infusion. Pretreatment with naloxone (5 mg/kg sc) was able to block both endomorphin-1 and endomorphin-2 effects on lordosis. Site-specific infusions of endomorphin-1 or endomorphin-2 into the medial preoptic area (mPOA), the ventromedial nucleus of the hypothalamus (VMH), or into the mesencephalic central gray did not affect receptivity. In contrast, infusion of 1 mug of either compound into the medial septum/horizontal diagonal band of Broca inhibited lordosis in a pattern very similar to that seen after intraventricular infusions. Infusion of the potent synthetic mu-OR agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (0.08 microg) into the VMH and mPOA inhibited lordosis behavior for at least 60 min after infusion. The nonspecific opioid receptor antagonist naloxone was able to facilitate lordosis in partially receptive female rats when infused into the mPOA but not when infused into the VMH. The behavioral effects of the agonists and antagonist used in this study suggest that the endogenous mu-opioid system modulates estrogen and progesterone-induced lordosis behavior.     

Female rats develop conditioned place preferences for sex at their preferred interval

Female rats engage in approach and avoidance behaviors directed toward the male to "pace" the rate of copulation. These pacing behaviors result in a pattern of vaginocervical stimulation that triggers a neuroendocrine reflex that is important for pregnancy to result from insemination. Each female rat has a preferred pacing interval, and females develop conditioned place preferences for paced sex versus nonpaced sex. Research from this laboratory has reported that extracellular dopamine concentrations in striatum and nucleus accumbens are greater in female rats that are engaging in paced sex compared with those engaging in nonpaced sex. Furthermore, females who have males removed at their preferred intervals during a copulatory bout show extracellular dopamine concentrations comparable to females engaging in paced sex. It is unclear, however, whether they would also develop a conditioned place preference for sex under such conditions. This experiment was designed to address this question. Female rats had six exposures each to a chamber in which they engaged in nonpaced sex and a chamber in which they engaged in paced or preferred pacing interval sex. Following conditioning trials, females were tested for a conditioned place preference. The findings indicate that female rats develop conditioned place preferences for paced sex and for sex in which the male is removed at her preferred interval. This suggests that sexual behavior is reinforcing to female rats when their preferred interval is achieved, whether or not they are actively controlling the rate of copulation.     

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.     

Dansyl-PQRamide, a possible neuropeptide FF receptor antagonist, induces conditioned place preference

Neuropeptide FF (NPFF) is an endogenous anti-opioid peptide. NPFF could potentiate the naloxone-precipitated morphine withdrawal syndromes in morphine-dependent rats, indicating the possible involvement of the endogenous NPFF system in opioid analgesia and dependence. The present study was performed to examine the effects of dansyl-PQRamide (dns-PQRa), a putative NPFF antagonist, on conditioned place preference (CPP), in addition, its interaction with the opioid system. Two CPP experiments were conducted. First, rats were treated with dns-PQRa (4-13 mg/kg, i.p.) and paired with the non-preferred compartment while the vehicle was paired with the preferred compartment. Second, similar to experiment 1 except naloxone (1 mg/kg, i.p.) was given 10 min prior to each dns-PQRa administration. The post-drug place preference was examined after 4 alternative pairings. Another group of animals after repetitive dns-PQRa treatments were analyzed for levels of neurotransmitters in discrete brain areas. Dns-PQRa (4-13 mg/kg, i.p.) induced a significant dose-dependent CPP. The dns-PQRa-induced CPP was completely blocked by pretreatment with 1 mg/kg i.p. naloxone, while naloxone alone did not induce any place aversion. The chronic dns-PQRa-treated (13 mg/kg, i.p., b.i.d.) rats caused a significant increase in 3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid in the olfactory tubercle compared to the vehicle-treated controls. There was also an increase in the turnover of serotonin in the olfactory tubercle, nucleus accumbens and medial prefrontal cortex. These results suggest that blockade of the NPFF system produces rewarding, possibly via an inhibition of the anti-opioid action of NPFF. These results also reveal a close relationship between NPFF, drug rewarding and the dopaminergic and serotoninergic neurons in the mesolimbic system.     

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

Two experiments assessed the effects of neonatal testosterone treatment on paced mating behavior and conditioned place preference in female rats. In both experiments, females received s.c. injections of 5.0 microg testosterone propionate or oil vehicle at three days postpartum. As adults, females were ovariectomized and given s.c. injections of 10 microg estradiol benzoate and 500 microg progesterone, 48 and 4 h before mating, respectively. In Experiment 1, TP- and Oil-treated females exhibited similar high levels of lordosis responsiveness, but TP-treated females showed increased intervals between mounts and between intromissions in paced and non-paced mating conditions compared to control females. The effect was particularly pronounced during paced mating, when contact return latencies were increased approximately 2-fold by TP treatment. TP-treated females showed exaggerated pacing behavior, showing significantly greater return latencies after intromissions than Oil-treated females. In Experiment 2, TP- and Oil-treated groups were tested in a conditioned place preference paradigm to determine if the behavioral changes observed in Experiment 1 were in part a result of changes in the perceived reward produced by paced mating. TP treated and control females developed equivalent preferences for places associated with paced but not non-paced mating, indicating that neonatal TP treatment at this dosage does not disrupt or enhance the conditioned place preference induced by paced mating. The results of the two experiments demonstrate that neonatal TP treatment alters the display of pacing behavior but not the reward state induced by paced mating, and suggest that TP affects neural substrates involved in performance of paced mating without effects on those controlling lordosis or place preference conditioning.     

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.     

Lesions of orexin neurons block conditioned place preference for sexual behavior in male rats

The hypothalamic neuropeptide orexin (hypocretin) mediates reward related to drugs of abuse and food intake. However, a role for orexin in sexual reward has yet to be investigated. Orexin neurons are activated by sexual behavior, but endogenous orexin does not appear to be essential for sexual performance and motivation in male rats. Therefore, the goal of the current study was to test the hypothesis that orexin is critically involved in processing of sexual reward in male rats. First, it was demonstrated following exposure to conditioned contextual cues associated with sexual behavior in a conditioned place preference paradigm that cFos expression is induced in orexin neurons, indicating activation of orexin neurons by cues predicting sexual reward. Next, orexin-cell specific lesions were utilized to determine the functional role of orexin in sexual reward processing. Hypothalami of adult male rats were infused with orexin-B-conjugated saporin, resulting in greater than 80% loss of orexin neurons in the perifornical-dorsomedial and lateral hypothalamus. Orexin lesions did not affect expression of sexual behavior, but prevented formation of conditioned place preference for a sexual behavior paired chamber. In contrast, intact sham-treated males or males with partial lesions developed a conditioned place preference for mating. Orexin lesioned males maintained the ability to form a conditioned place aversion to lithium chloride-induced visceral illness, indicating that orexin lesions did not disrupt associative contextual memory. Overall, these findings suggest that orexin is not essential for sexual performance or motivation, but is critical for reward processing and conditioned cue-induced seeking of sexual behavior.     

Yes, I am ready now: differential effects of paced versus unpaced mating on anxiety and central oxytocin release in female rats

Sexual activity and partner intimacy results in several positive consequences in the context of stress-coping, both in males and females, such as reduced state anxiety in male rats after successful mating. However, in female rats, mating is a rewarding experience only when the estrous female is able to control sexual interactions, i.e., under paced-mating conditions. Here, we demonstrate that sex-steroid priming required for female mating is anxiolytic; subsequent sexual activity under paced mating conditions did not disrupt this anxiolytic priming effect, whereas mating under unpaced conditions increased anxiety-related behavior. In primed females, the release of the neuropeptide oxytocin (OT) within the hypothalamic paraventricular nucleus was found to be elevated and to further increase during paced, but not unpaced mating. Central administration of an OT receptor antagonist partly prevented priming/mating-induced anxiolysis indicating the involvement of brain OT in the anxiolysis triggered by priming and/or sexual activity.These findings reveal that the positive consequences of mating in females are dependent on her ability to control sexual interactions, and that brain OT release is at least in part the underlying neurobiological correlate.     

Conditioned place preference induced by sexual interaction in female rats.

The present study was undertaken to establish whether the conditioned place preference paradigm can be utilized to investigate and elucidate the neuroendocrine basis of the appetitive elements of female sexual behavior. Females were exposed to a male with which copulation occurred in a distinctive compartment of the place preference apparatus and did not receive an incentive in the alternative compartment. After six pairings to each compartment a place preference test was conducted. Both estradiol benzoate and estradiol benzoate plus progesterone treated, ovariectomized females showed a preference for the compartment associated with sexual interaction. A second group of estradiol plus progesterone treated females was exposed to a male with which copulation occurred in one compartment of the place preference apparatus and to a sexually active, but caged, male in the other. The females tended to prefer the compartment paired with the caged male. After noncontingent intromissions, immediately preceding an additional test, the females showed a place preference for the compartment paired with sexual interaction. The presented observations indicate the potential use of the place preference procedure in studying opposing motivational processes associated with the unconditioned sequence of responses that characterize the species-specific pattern of sexual behavior.     

Distinct accumbal subareas are involved in place conditioning of amphetamine and cocaine

In considering the heterogeneous function of the nucleus accumbens (NAC), the present work evaluated the conditioned place preference (CPP) after local infusion of d-amphetamine (AMP; 10, 15 microg/side) or cocaine (COC; 50, 100 microg/side) into two subareas of NAC, core and shell. A regular two-compartment CPP apparatus was used to test the place conditioning effects after 6 pairings of drug in one compartment and 6 pairings of vehicle in the other one. Significant CPP was observed with either AMP infused in the core area or COC infused into the shell area. Neither AMP in shell nor COC in core significantly produced CPP. These results indicate important differences between two neural substrates within NAC for the rewarding effects of AMP and COC on the CPP task.     

The effects of mating stimulation on c-Fos immunoreactivity in the female hamster medial amygdala are region and context dependent

During mating in hamsters, both tactile and nontactile sensory stimulation experienced by the female affect sexual behavior and progestational neuroendocrine reflexes. To test the interactions of these types of mating stimulation, c-Fos immunohistochemistry measured brain cellular activity during sexual behavior under conditions that included combinations of tactile and nontactile mating stimulation. Test groups received: (1) mating stimulation from a male, females being either fully mated or mated while wearing a vaginal mask, or (2) experimenter applied manual vaginocervical stimulation (VCS)-with or without males present, or (3) handling similar to VCS but without insertions-with or without males present. Numbers of c-Fos immunoreactive cells were counted in specific subdivisions of the posterior medial amygdala (MeP) and ventromedial hypothalamus (VMH). The medial amygdala dorsal and ventral subdivisions responded differentially to components of mating stimulation. The posterodorsal Me (MePD) cellular activation was greatest during mating conditions that included VCS and/or males present. However, the posteroventral Me (MePV) was sensitive to male exposure and not to VCS. Also, MePV and VMH shell responses mirrored each other, both being primarily sensitive to male exposure. In separate tests, manual VCS induced pseudopregnancy, though the procedure was most effective with additional nontactile stimulation from males present. In summary, contextual cues provided by nontactile male stimulation enhance the effect of vaginocervical and other tactile stimulation on reproductive processes. Furthermore, c-Fos expression in the female hamster medial amygdala is region and context dependent.     

Maturation of social reward in adult male Syrian hamsters does not depend on organizational effects of pubertal testosterone

The rewarding value of female sexual stimuli develops across puberty, as sexually-naïve adult, but not prepubertal, male hamsters show a conditioned place preference (CPP) for both vaginal secretions and a receptive female. Similarly, only adults show an endogenous testosterone surge when they encounter vaginal secretions. Testosterone by itself can condition a place preference in male rodents. Therefore, Experiment 1 assessed whether the endogenous testosterone surge elicited by vaginal secretions is necessary to show a CPP. Both gonad-intact and gonadectomized, testosterone-treated adult males showed a CPP for vaginal secretions, indicating that the rewarding value of this social cue is independent of an endogenous testosterone surge. However, organizational effects of pubertal testosterone could be necessary for adolescent development of social reward, as pubertal testosterone organizes adult-typical expression of sexual behavior. To investigate this possibility, in Experiment 2, sexually-naïve prepubertal and adult male hamsters were gonadectomized and received testosterone-filled capsules four weeks later. Testing began after two weeks of testosterone replacement. Adult males showed a CPP for both vaginal secretions and a receptive female, whether or not they experienced pubertal testosterone. Thus, the acquisition of positive valence of sexual stimuli is not organized by pubertal testosterone. Taken together, the ability of female sexual stimuli to serve as an unconditioned reward to adult male hamsters is independent of the chemosensory-induced endogenous testosterone surge and also organizational effects of pubertal testosterone. Instead, sexual reward may be dependent either on activational effects of testosterone or gonadal hormone-independent mechanisms.     

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.     

Naloxone blocks conditioned place preference induced by substance P and [pGlu6]-SP(6-11).

The effect of prior treatment with the opioid receptor (opioceptor) antagonist naloxone on conditioned place preference produced by the neurotachykinin substance P (SP) and its C-terminal hexapeptide analog [pGlu6]-SP(6-11) (SPC) was investigated in rats. Place conditioning was assessed using a circular open field partitioned into four quadrants that were equally preferred by the rats prior to drug treatment. On three successive days, rats received an intraperitoneal (i.p.) injection of naloxone-HCl (1 mg/kg) or vehicle 15 min before an i.p. injection of either 37 nmol/kg SP, equimolar dosed SPC or corresponding diluent vehicle. After injection the rats were placed into their assigned treatment corral for 15 min. During the test for conditioned corral preference (CCP), when provided a choice between the four quadrants, rats injected with SP or SPC spent more time in the treatment corral compared to vehicle controls, indicative of a positive reinforcing action of these peptides. The pre-treatment with naloxone blocked the positive reinforcing effects of both SP and SPC; when injected alone, naloxone did not influence the preference behavior. Gross locomotor activity was affected by neither treatment. Thus, the positive reinforcing effects of SP and SPC may be mediated via interactions with the endogenous opioid system(s).     

Intracranial dialysis and microinfusion studies suggest that morphine may act in the ventromedial hypothalamus to inhibit female rat sexual behavior.

The present investigation examined the neural sites and mechanisms of opiate inhibition of female sexual behavior. Systemic administration of morphine (10 mg/kg) significantly reduced ovarian steroid-induced estrous behavior in female rats. This behavioral inhibition was prevented when the opiate receptor antagonist naloxone (5 mg/kg) was administered 30 min prior to morphine. Bilateral infusion of morphine directly into the ventromedial hypothalamus (VMH) also inhibited hormone-dependent estrous behavior for at least 2 hr. Furthermore, naloxone infusion into the VMH 20 min before behavior testing reduced the inhibitory effects of systemically administered morphine on lordosis. These results suggest that morphine may inhibit female sexual behavior by acting directly on the VMH, the primary site at which ovarian steroids facilitate this behavior. In a separate experiment we used in vivo brain microdialysis to test the hypothesis that morphine inhibits lordosis by interfering with norepinephrine (NE) neurotransmission in the VMH. In control rats, the onset of mating was associated with increased NE release in the VMH. Morphine-treated animals displayed neither behavioral estrus nor elevated NE release from the VMH when tested with stimulus males. These data are consistent with the hypothesis that morphine suppresses NE release in the VMH. Nevertheless, mechanisms other than or in addition to attenuation of hypothalamic NE release may contribute to the inhibitory effects of morphine on lordosis.