Theoretical review. Progesterone: inhibition of rodent sexual behavior

Tibolone induces lordosis behavior, but not concurrent or sequential inhibition, in Sprague Dawley rats

Activation of progesterone receptor (PR) facilitates lordosis 40 hr after estradiol treatment, but induces concurrent inhibition (CI) when given with estradiol, or sequential inhibition (SI) when given subsequent to the faciliatory time interval. Tibolone (TBL) is a broad spectrum gonadal steroid agonist that facilitates lordosis when given after estradiol and in place of progesterone (P). The present experiment examined whether it can also induce CI or SI of lordosis behavior in rats as a means of determining its dominant receptor mechanism of action. Subcutaneous (SC) injections of estradiol benzoate (EB), TBL, or P were varied in time to examine whether P induced CI in females pre-treated with TBL or EB, or whether P or TBL induced CI when injected prior to EB (Experiment 1); whether P or TBL induced SI after EB treatment (Experiment 2); and whether P induced SI after TBL treatment (Experiment 3). In Experiment 1, P injected 1 h before EB induced CI after a second P administration 40 h later. However, the same treatment of P to females primed with TBL did not induce CI. In Experiment 2, injections of P or TBL 40 h after EB or TBL induced lordosis within 4 h (facilitation test); however, a second injection of P, 24 h later, induced significant lordosis in rat pretreated with TBL, but not in rats pretreated with P (inhibition test). In Experiment 3, P injected 40 hs after different doses of TBL induced intense lordosis behavior (facilitation test); however, a second dose of P injected 64 h later induced SI, but not in females primed with the highest dose of TBL (inhibition test). Unlike P, TBL did not induce CI or SI. This suggests that TBL likely induces its facilitation of lordosis by an action that is independent of PR.

Towards a neurobiology of female aggression

Although many people think of aggression as a negative or undesirable emotion, it is a normal part of many species' repertoire of social behaviors. Purposeful and controlled aggression can be adaptive in that it warns other individuals of perceived breaches in social contracts with the goal of dispersing conflict before it escalates into violence. Aggression becomes maladaptive, however, when it escalates inappropriately or impulsively into violence. Despite ample data demonstrating that impulsive aggression and violence occurs in both men and women, aggression has historically been considered a uniquely masculine trait. As a result, the vast majority of studies attempting to model social aggression in animals, particularly those aimed at understanding the neural underpinnings of aggression, have been conducted in male rodents. In this review, we summarize the state of the literature on the neurobiology of social aggression in female rodents, including social context, hormonal regulation and neural sites of aggression regulation. Our goal is to put historical research in the context of new research, emphasizing studies using ecologically valid methods and modern sophisticated techniques. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity'.

Uptake of (3)H labeled synthetic progestin by rat brain and pituitary. A radioautography study

The distribution of cells concentrating R 5020, a potent synthetic progestin, in the brain and pituitary of estrogen-primed ovariectomized rat was investigated by radioautography. In the brain, radioactivity was found concentrated in nuclei of neurons in the preoptic region (nucleus (n.) preopticus pars suprachiasmatica, n. preopticus periventricularis, n. preopticus medialis) and in the medio basal hypothalamus (n. ventromedialis and n. arcuatus). In the anterior pituitary, a significant number of labeled cells was observed.

Sexual experience affects reproductive behavior and preoptic androgen receptors in male mice

Reproductive behavior in male rodents is made up of anticipatory and consummatory elements which are regulated in the brain by sensory systems, reward circuits and hormone signaling. Gonadal steroids play a key role in the regulation of male sexual behavior via steroid receptors in the hypothalamus and preoptic area. Typical patterns of male reproductive behavior have been characterized, however these are not fixed but are modulated by adult experience. We assessed the effects of repeated sexual experience on male reproductive behavior of C57BL/6 mice; including measures of olfactory investigation of females, mounting, intromission and ejaculation. The effects of sexual experience on the number of cells expressing either androgen receptor (AR) or estrogen receptor alpha (ERα) in the primary brain nuclei regulating male sexual behavior was also measured. Sexually experienced male mice engaged in less sniffing of females before initiating sexual behavior and exhibited shorter latencies to mount and intromit, increased frequency of intromission, and increased duration of intromission relative to mounting. No changes in numbers of ERα-positive cells were observed, however sexually experienced males had increased numbers of AR-positive cells in the medial preoptic area (MPOA); the primary regulatory nucleus for male sexual behavior. These results indicate that sexual experience results in a qualitative change in male reproductive behavior in mice that is associated with increased testosterone sensitivity in the MPOA and that this nucleus may play a key integrative role in mediating the effects of sexual experience on male behavior.

Progesterone signaling mechanisms in brain and behavior

Steroid hormone, progesterone, modulates neuroendocrine functions in the central nervous system resulting in alterations in physiology and behavior. These neuronal effects are mediated primarily by intracellular progestin receptors (PRs) in the steroid-sensitive neurons, resulting in transcription-dependent genomic actions (classical mechanism). In addition to progesterone, intracellular PRs can also be activated in a "ligand-independent" manner by neurotransmitters, peptide growth factors, cyclic nucleotides, and neurosteroids. Recent studies indicate that rapid, non-classical progesterone actions involving cytoplasmic kinase signaling and/or extranuclear PRs can result in both transcription-independent and transcription-dependent actions. Cross-talk between extranuclear and classical intracellular signaling pathways promotes progesterone-dependent behavior in mammals. This review focuses on the mechanisms by which progesterone-initiated signaling mechanisms converge with PRs in the brain to modulate reproductive behavior in female rodents.

Activation of progestin receptors in female reproductive behavior: Interactions with neurotransmitters

The steroid hormone, progesterone (P), modulates neuroendocrine functions in the central nervous system resulting in alterations in physiology and reproductive behavior in female mammals. A wide body of evidence indicates that these neural effects of P are predominantly mediated via their intracellular progestin receptors (PRs) functioning as "ligand-dependent" transcription factors in the steroid-sensitive neurons regulating genes and genomic networks. In addition to P, intracellular PRs can be activated by neurotransmitters, growth factors and cyclic nucleotides in a ligand-independent manner via crosstalk and convergence of pathways. Furthermore, recent studies indicate that rapid signaling events associated with membrane PRs and/or extra-nuclear, cytoplasmic PRs converge with classical PR activated pathways in neuroendocrine regulation of female reproductive behavior. The molecular mechanisms, by which multiple signaling pathways converge on PRs to modulate PR-dependent female reproductive behavior, are discussed in this review.

Non-intromissive mating stimuli are sufficient to enhance sexual behaviors in ovariectomized female rats

When ovariectomized/adrenalectomized female rats, injected with subthreshold doses of estradiol are given copulatory stimulation by a male rat at half hour intervals, the level of lordosis gradually increases over the course of a few hours. We tested the hypothesis that paracopulatory behaviors (behaviors that occur repetitively prior to and between mounts), also generally considered to be heavily dependent on progesterone, are enhanced by this stimulation as well. We have reported previously that the enhancement of copulatory behavior is dependent to a large extent on intromissive stimulation by the male. In the present study, mating stimulation induced high levels of paracopulatory behaviors, as well as lordosis. Surprisingly, though, and in contrast to previous findings, this increase was seen not only in rats receiving intromissive stimulation, but in those receiving non-intromissive stimulation as well. Furthermore, intromissive stimulation induced high levels of rejection behavior. In a subsequent experiment, experimenter-induced, mechanical stimulation increased only rejection behaviors, not copulatory behavior. The results collectively demonstrate that, under the conditions used in these experiments, non-intromissive stimulation is sufficient for inducing both copulatory and paracopulatory behaviors in estradiol-primed rats. However, under the conditions used in these studies, intromissive stimulation increases rejection behaviors.

Neuroendocrine regulation of feminine sexual behavior: lessons from rodent models and thoughts about humans

Much has been learned concerning the neuroendocrine processes and cellular mechanisms by which steroid hormones influence reproductive behaviors in rodents and other animals. In this review, a short discussion of hormones and feminine sexual behavior in some rodent species is followed by an outline of the main principles that have been learned from these studies. Examples are given of the importance of considering the timing of hormone treatments, dosage of hormone, use of a specific hormone, particular class of hormones, or form of hormone, interactions between hormones, route of administration, peripheral factors that influence hormonal response, and the possible mechanisms of action by which hormones and other factors influence sexual behaviors. Although cellular studies in humans are presently impossible to perform, mechanistic studies in rodents may provide clues about the neuroendocrine mechanisms by which hormones act and interact in the brain to influence behavior in all species, including humans.

Sexual behavior in ewes and other domestic ruminants

Similarities as well as differences across species in the control of sexual behavior are helping to fully understand the subtle relations between physiology and eco-ethological constraints and how the brain integrates such information. We will illustrate this with sexual behavior in domestic ruminants and especially ewes. Females of these species like humans, but unlike rodents, have a long luteal phase. A prolonged exposure to progesterone (Pg) before the preovulatory estradiol rise is necessary for estrous behavior to be displayed. Estradiol action and receptor localization is very similar to that observed in other species. But not too surprisingly, the role of Pg is rather different with a priming effect not observed in rodents. However, as in rodents, Pg also has an inhibitory effect, is necessary for the display of proceptivity and is responsible for the timing of the different periovulatory events. These steroids act on the central nervous system in similar areas across mammalian species to regulate estrous behavior. Steroid fluctuations during the estrous cycle cause changes in catecholaminergic activity in the hypothalamus. Interestingly, these neurotransmitters seem to have very similar effects in ewes and rats as illustrated by the norepinephrine rise after male-female interactions observed in both species. Similar comparisons can be made regarding the action of some neuropeptides, including oxytocin and GnRH, and more integrative processes like sexual differentiation and modulation of reproduction by social interactions. Data on sheep, goats and cows will be compared with those of rodents.

The role of insulin-like growth factor-I and growth factor-associated signal transduction pathways in estradiol and progesterone facilitation of female reproductive behaviors

We are examining the role of insulin-like growth factor-I (IGF-I) and downstream signal transduction pathways associated with growth factors (e.g., mitogen-activated protein kinase, MAPK) in estradiol and progesterone facilitation of female reproductive behavior in rats. Brain IGF-I receptor activity is required for the long-term, priming actions of estradiol on the female reproductive axis. Infusions of an IGF-I receptor antagonist during estradiol priming blocks induction of hypothalamic alpha(1B)-adrenergic receptors and luteinizing hormone surges, and attenuates lordosis behavior. Infusion of MAPK and phosphatidylinositol-3-kinase inhibitors inhibitors during estradiol priming completely blocks hormone-facilitated lordosis. Because progestin receptors (PRs) can be phosphorylated and activated by MAPKs, growth factor signaling pathways may also participate in progesterone facilitation of reproductive behaviors. Infusion of a MAPK inhibitor in estradiol-primed rats blocks progestin facilitation and sequential inhibition of lordosis and proceptive behaviors. Interference with MAPK signaling also inhibits behavioral responses to cGMP and a delta-opioid agonist, both of which can activate MAPK in some cells. Thus MAPK is involved in the facilitation of lordosis and proceptive behaviors, perhaps by phosphorylation of hypothalamic PRs.

Progesterone receptor activation signals behavioral transitions across the reproductive cycle of the female rabbit

The female rabbit is an exceptional experimental model to define mechanisms by which progesterone (P) controls the expression of reproductive behaviors. In the rabbit, the rise in P levels during pregnancy inhibits estrous scent marking ("chinning"), stimulates the excavation of a nest burrow ("digging"), and primes behaviors later used for nest construction. The pre-parturient fall of P triggers the construction of a straw nest ("straw carrying") that is lined with hair that she pulls from her own body ("hair pulling"). These behaviors can be replicated in ovariectomized (ovx) females given a schedule of estradiol (E) and P that mimics hormone levels during pregnancy (E from days 0 to 4, E + P from days 5 to 17, E from days 18 to 27). We administered PR antagonists RU486 or CDB(VA)2914 to ovx female rabbits during either the initial (days 5-11) or late (days 12-17) phases of P treatment, to determine the role of PR activation in coordinating the expression of these behaviors. Both antiprogestins attenuated the P-mediated decline in chinning and increase in digging when administered during days 5-11. When given across days 12-17, both antiprogestins triggered an early decline in digging, the onset of nest building in some Ss, and the reinstatement of chinning. These results point to a central role of PR activation for establishing and maintaining the behavioral phenotype of pregnancy, and for the behavioral transition from pregnancy to estrus.

Ovarian steroids influence cell proliferation in the dentate gyrus of the adult female rat in a dose- and time-dependent manner

In previous work, we have demonstrated that cell proliferation in the adult hippocampal formation is regulated by estrogen under both natural and experimental conditions. To determine the extent to which this regulation is affected by the dose or schedule of hormone treatment, or progesterone administration, we examined the impact of different acute and chronic ovarian hormone replacement regimens on cell production using the S-phase marker bromodeoxyuridine. Additionally, we investigated the long-term impact of surgical ovarian hormone depletion on the capacity of estrogen to stimulate cell proliferation and the production of new cells that express either TuJ1 (a marker of neuronal phenotype) or glial fibrillary acidic protein (GFAP; a marker of astroglial phenotype). Acute treatment with a moderate, but not a low or a high, dose of estrogen rapidly increased cell proliferation in ovariectomized (OVX) animals, an effect that was reversed by the administration of progesterone. In contrast, OVX animals that were chronically replaced with either estrogen alone (continuous or cyclic) or estrogen plus progesterone (cyclic) did not exhibit an estrogen-induced increase in cell proliferation 3 weeks following the onset of hormone replacement. In animals that were subjected to a prolonged absence of ovarian hormones, acute treatment with the moderate dose of estrogen failed to stimulate cell proliferation, and a decrease in the number of new cells expressing a neuronal phenotype was evident. Collectively, these results indicate that a prolonged reduction in ovarian hormones results in 1) a diminished responsiveness to estrogen over time in this system and 2) a decrease in neuron production that is unlikely to be reversible by standard regimens of hormone replacement.

The 26S proteasome participates in the sequential inhibition of estrous behavior induced by progesterone in rats

Estrous behavior induced by progesterone (P) treatment of estradiol-primed rats is followed by a period in which females do not respond behaviorally to a second administration of P [sequential inhibition (SI)]. SI is thought to involve P-dependent down-regulation of hypothalamic P receptor (PR) content. This study tested the hypothesis that the 26S proteasome participates in the regulation of SI and brain PR content in female rats. Ovariectomized, estrogen-primed (estradiol benzoate, 2 microg s.c.) adult rats were injected with P (1 mg s.c.) alone or P with the proteasome inhibitors Z-Ile-Glu (OBu(1))-Ala-Leu-H (PSI, 300 microg/100 g s.c.) or N alpha-tosyl-lysyl chloromethyl ketone (TLCK, 200 microg i.p.) administered 48 h after estradiol priming. Sexual behavior was assessed in all animals 4 h later. These two agents inhibit 26S proteasome-mediated protein degradation by different mechanisms. To explore SI, the animals received a second P injection 24 h after the first, and a second sexual behavior test was performed 4 h later. After this test, brains were excised, and proteins were extracted from the preoptic area and the hypothalamus and processed for semiquantitative immunoblotting. In the first sexual behavior test (facilitation test), all animals treated with estradiol + P exhibited intense lordosis behavior. In the second sexual behavior test (inhibition test), both lordosis and proceptivity were significantly reduced in response to the second administration of P (SI). The magnitude of SI was significantly attenuated by the administration of either PSI or TLCK concurrently with the first P injection. The first P injection reduced PR content in the hypothalamus but not in the preoptic area. In contrast, PSI and TLCK significantly increased PR content in both structures. Our results suggest that PR degradation by the 26S proteasome participates in the expression of P-induced SI in female rats.

Progestin receptors: neuronal integrators of hormonal and environmental stimulation

Although it originally was believed that neuronal steroid hormone receptors require binding to cognate ligand for activation, more recent evidence suggests that the receptors can be activated indirectly by other compounds, such as neurotransmitters and growth factors, acting through their own membrane receptors and specific intracellular signaling pathways. For example, as is the case with facilitation of sexual behavior by progesterone, facilitation of sexual behavior by D(1)/D(5) dopamine receptor agonists is blocked by disruption of progestin receptors. Therefore, some dopamine agonists facilitate sexual behavior at least in part by a progestin receptor-dependent mechanism, as does progesterone. This "ligand-independent activation" of neuronal progestin receptors is not limited to dopamine agonists; a variety of other compounds, as well as mating stimulation, facilitate sexual receptivity by a progestin receptor-dependent process. Steroid hormone receptors also can be regulated by afferent input in another way. Various neurotransmitters upregulate or downregulate steroid hormone receptors in some neurons. This, in turn, presumably confers greater or decreased sensitivity to the particular factors that can activate the particular steroid receptor in those particular neurons. Therefore, steroid hormones are but one class of factors that can regulate and activate steroid hormone receptors. Some additional factors that activate steroid hormone receptors have been identified, as have some factors that can regulate concentrations of receptors. Relatively little is known at this time about the range of neurotransmitters, humoral factors, and intracellular signaling pathways that are involved.

Tracing the Evolution of Brain and Behavior Using Two Related Species of Whiptail Lizards: Cnemidophorus uniparens and Cnemidophorus inornatus

Cnemidophorus whiptail lizards offer a unique opportunity to study behavioral and neural evolution because unlike most genera, ancestral and descendant species are still extant, and comparisons between species provide a window into correlated changes in biological organization through speciation. This review focuses on the all-female or parthenogenetic species Cnemidophorus uniparens (descendant species), which evolved through several hybridization events involving the sexually reproducing species Cnemidophorus inornatus (ancestral species). Data compiled over more than 2 decades include behavioral, endocrine, and neural differences between these two related species of whiptail lizards. For example, unlike females of the ancestral species, individuals of the descendant species display male-like mounting behavior (pseudocopulatory behavior) after ovulation. Pseudocopulatory behavior in the parthenogen is triggered by the progesterone surge after ovulation, and the behavioral capacity to respond to progesterone appears to be an ancestral trait that was inherited from C. inornatus males through the hybridization events. Interestingly, the regulation of sex steroid hormone receptor mRNA in brain areas critical for the expression of sociosexual behaviors differs between females of the two species and suggests that evolutionary changes in the regulation of gene expression could be a proximate mechanism that underlies the evolution of a novel social behavior in the parthenogen. Finally, because the sexual species is diploid, whereas the parthenogen is triploid, differences between the species could directly assess the effect of ploidy. The behavioral and neuroendocrinological data are pertinent for considering this possibility.

Differential effects of testosterone and progesterone on the activation and retention of courtship behavior in sexual and parthenogenetic whiptail lizards

Both testosterone (T) and progesterone (P) facilitate the expression of male-typical sexual behavior in a variety of animals, including rodents and lizards. In two species of whiptail lizards, Cnemidophorus inornatus and C. uniparens, both hormones elicit the full repertoire of courtship behavior. However, the relative efficacy of the two hormones is unknown. In Experiments 1 and 2 we assessed differences in capacity of exogenous T and P to induce male-typical courtship behavior in gonadectomized whiptail lizards. In both species, individuals implanted with T showed more frequent courtship behavior relative to those implanted with P or cholesterol. In Experiments 3 and 4 we examined whether T and P differentially affected the retention of courtship behavior following implant removal. In both species, individuals implanted with T showed more courtship behavior following implant removal than those previously given P. In these experiments, implants were removed at a time when individuals in both groups were behaviorally similar; therefore, the differences in behavior following implant removal were not due to differences in the amount of courtship experience. Taken together, the hormone that was more effective at activating courtship behavior was also more effective at maintaining courtship behavior following implant removal. In summary, though both T and P can elicit identical sexual behaviors in both whiptail species, T has a greater and more lasting effect on courtship behavior and possibly on the neural circuits underlying courtship behavior.

Endocrinological basis of sex differences in cocaine-induced behavioral responses

Currently, 1.8 million Americans use cocaine, 30% of whom are females. Sex differences in the pattern of cocaine abuse may reside in neuroendocrinological modulations that affect the use of and/or dependence on cocaine. This review discusses sex differences in cocaine-induced behavioral and molecular alterations in the central nervous system, with emphasis on the role of endocrine responses in the neuronal modulations of this drug. Mechanisms and data supporting the role of the hypothalamic-gonadal axis in the modulation of cocaine-induced behavioral and molecular alterations are also provided.

Ovarian hormone replacement affects cocaine-induced behaviors in ovariectomized female rats

To determine whether cocaine-induced behavioral alterations are modulated by ovarian hormones, ovariectomized rats were randomly assigned to one of two drug treatment conditions: "binge" cocaine (three 15-mg/kg intraperitoneal (ip) injections, 1 h apart) or saline administration; and four hormone pretreatment sub-groups: vehicle control, estrogen, progesterone, or estrogen+progesterone. Cocaine-treated animals displayed more locomotor activity than saline-treated animals and locomotor activity was higher after the third injection than after the first two injections. When analyzed according to hormone group, the administration of estrogen+progestrone suppressed cocaine-induced locomotion after the first injection; this effect was significant when compared to estrogen-pretreated animals. While in each condition cocaine-treated animals displayed significantly higher stereotypic activity than saline-treated animals, in the estrogen+progesterone replacement group, there was more activity after the second injection of cocaine than after the first. Interestingly, animals in the estrogen+progesterone group had significantly lower plasma levels of the cocaine metabolite, benzoylecgonine, than animals in the progesterone or estrogen groups. These results extend our earlier findings in the intact female rat, which suggest an interaction between the endocrine environment, cocaine metabolism, and cocaine-induced behaviors. These effects may underlie reported sex and estrous cycle differences in cocaine-induced behavioral activity.

Protein synthesis in the medial preoptic area is important for the mating-induced decrease in estrus duration in hamsters

Sexual receptivity in female hamsters potentially lasts for about 16 h. However, vaginal cervical stimulation (VCS) from a male during mating eventually reduces receptivity and can shorten the duration of behavioral estrus. The process by which this change in response to the male takes place is unknown. Recently, detection of the Fos protein has indicated that the medial preoptic area (POA) is one of the brain regions particularly responsive to VCS. Additionally, the POA may have an inhibitory effect on sexual receptivity. To determine if protein synthesis in the POA is required to initiate the VCS-induced decrease in estrus duration, a protein synthesis inhibitor (anisomycin, 0.50 microg) or a control substance (cholesterol) was applied bilaterally to the POA of steroid-primed ovariectomized female hamsters. Females were tested with a sexually active male at five time points following the initial test for sexual receptivity (hour 1, 2, 6, 12, and 24). Half of the females tested were allowed to receive VCS from a male, while half were fitted with vaginal masks to prevent penile intromission. Each group receiving VCS showed a significant decrease in lordosis duration evident between hour 2 and hour 6, except the group which received anisomycin in the POA. In this respect the POA anisomycin group was similar to animals which did not receive VCS. Hamsters with vaginal masks and the anisomycin/POA animals allowed to receive VCS exhibited their first decrease in lordosis duration between hour 6 and hour 12. These results indicate that protein synthesis is important for VCS-induced decrease in estrus duration in the POA.

Progesterone as a neurosteroid: synthesis and actions in rat glial cells

The central nervous system (CNS) and the peripheral nervous system (PNS) are targets for steroid hormones where they regulate important neuronal functions. Some steroid hormones are synthesized within the nervous system, either de novo from cholesterol, or by the metabolism of precursors originating from the circulation, and they were termed 'neurosteroids'. The sex steroid progesterone can also be considered as a neurosteroid since its synthesis was demonstrated in rat glial cell cultures of the CNS (oligodendrocytes and astrocytes) and of the PNS (Schwann cells). Both types of glial cells express steroid hormone receptors, ER, GR and PR. As in target tissue, e.g. the uterus, PR is estrogen-inducible in brain glial cell cultures. In the PNS, similar PR-induction could not be seen in pure Schwann cells derived from sciatic nerves. However, a significant PR-induction by estradiol was demonstrated in Schwann cells cocultured with dorsal root ganglia (DRG), and we will present evidence that neuronal signal(s) are required for this estrogen-mediated PR-induction. Progesterone has multiple effects on glial cells, it influences growth, differentiation and increases the expression of myelin-specific proteins in oligodendrocytes, and potentiates the formation of new myelin sheaths by Schwann cells in vivo. Progesterone and progesterone analogues also promotes myelination of DRG-Neurites in tissue culture, strongly suggesting a role for this neurosteroid in myelinating processes in the CNS and in the PNS.

Reversal of progesterone-induced sequential inhibition by progesterone metabolites

Previous reports have shown that intrabrain administration of progesterone (P) ring A-reduced metabolites into the medial preoptic area (MPOA) and ventromedial hypothalamus (VMH) induces facilitation of female sexual behavior in ovariectomized (ovx) rats pretreated with estrogen. Present studies were designed to explore the possibility that ring-A reduced progesterone metabolites might play a role in controlling the duration of estrous behavior. To this aim ovariectomized (ovx) Sprague Dawley rats implanted with guide cannulae directed towards the VMH or the MPOA were submitted to a systemic hormonal treatment to provoke P-induced sequential inhibition (estradiol benzoate (EB) at time O + P at 44 h + P at 68 h). The second dose of P was administered simultaneously with the i.c. implantation of one of the following P metabolites: 3 beta-hydroxy-5 beta-pregnan-20-one (5 beta,3 alpha P), 3 alpha-hydroxy-5 beta-pregnan-20-one (5 beta,3 alpha P) or 3 beta-hydroxy-5 beta- pregnan-20-one (5 alpha,3 beta P) into the MPOA or VMH. Lordosis behavior was evaluated by the lordosis quotient (LQ = number of lordosis/10 male mount x 100) and by the percentage of responding subjects. Results show that 5 beta,3 beta P implanted into the VMH or MPOA counteracted the sequential inhibitory effect induced by systemic administration of P.5 alpha,3 beta P was also able to counteract sequential inhibition, but with less potency and only in the VMFI. Results showed that P-induced sequential inhibition can be counteracted by intrabrain administration of ring-A reduced progestins in both the VMH and MPOA. Data are discussed in terms of a putative physiological role of naturally occurring P metabolites in P-mediated female sexual behavior expression.

Lordosis-inhibiting effect of progesterone in female rats with lesions in septum, preoptic area, or dorsal raphe nucleus

Radiofrequency lesions in the septum (SL), the preoptic area (POAL), or the dorsal raphe nucleus (DRL) were made in ovariectomized rats. In a control group of 16 females, ovariectomy, but no brain surgery, was performed. All animals except half of the control rats received injections of 5 mg progesterone (P) 1 h prior to the injection of 5 micrograms/kg b.w. of estradiol benzoate (EB). Instead of 5 mg P, oil was administered to half of the controls. Forty-four hours after EB, all females received 0.5 mg P. A sexual behavior test was performed 4 h after the last injection of P. The result was that oil-treated control rats showed high lordosis quotient (LQ) and soliciting behavior. In contrast, low scores of LQ and no soliciting behavior were observed in all of the 5 mg P-treated rats, even if the SL, POAL, or DRL was made. These results suggest that the septum, the preoptic area, and the dorsal raphe nucleus are not essential for the female sexual behavior-inhibiting mechanisms of progesterone.

Inhibitory effect of progesterone on sexual receptivity in female rats: a temporal relationship to estrogen administration

The inhibitory effect of progesterone (P) injected at various times on female sexual behavior was investigated in estradiol benzoate (EB) treated ovariectomized rats. Four behavioral tests were carried out at two-week intervals. All females received 5 micrograms/kg b.w. EB and 0.5 mg P 44 hr after the EB. In the P-control group, an additional 5 mg P was administered at the same time as the injection of EB in four tests. Instead of P, oil was given concurrently with EB in the Oil control group. In the experimental groups, female rats were treated with 5 mg P from 1 to 40 hr before (PB group) or after (PA group) the EB-injection. A sexual behavioral test was started 4 hr after 0.5 mg P. The results show that low levels of lordosis and soliciting behavior were observed in the P group, compared to the Oil-control group. In the PB groups, lordosis quotient (LQ) was low when P was given from 1 to 24 hr before EB. Moreover, animals in which P was given 27-40 hr before EB showed lower LQ than Oil-control animals, but higher LQ than rats in the P-control group. In the PA groups, when P was administered from 1 to 24 hr after EB, low levels of lordosis response were observed, whereas animals which received P 27-40 hr after EB showed a high score of LQ, being comparable to that in the Oil control. These results suggest that the period of 24 hr before and after EB injection is a critical period for inhibitory action of P on female sexual behavior in female rats.

Convergent pathways for steroid hormone- and neurotransmitter-induced rat sexual behavior

Estrogen and progesterone modulate gene expression in rodents by activation of intracellular receptors in the hypothalamus, which regulate neuronal networks that control female sexual behavior. However, the neurotransmitter dopamine has been shown to activate certain steroid receptors in a ligand-independent manner. A dopamine receptor stimulant and a D1 receptor agonist, but not a D2 receptor agonist, mimicked the effects of progesterone in facilitating sexual behavior in female rats. The facilitory effect of the neurotransmitter was blocked by progesterone receptor antagonists, a D1 receptor antagonist, or antisense oligonucleotides to the progesterone receptor. The results suggest that in rodents neurotransmitters may regulate in vivo gene expression and behavior by means of cross-talk with steroid receptors in the brain.

Benextramine, a putative neuropeptide Y receptor antagonist, attenuates the termination of receptivity

Sexual behavior in female rats is dependent on gonadal steroids. In ovariectomized rats, progesterone treatment typically exerts a biphasic effect on copulatory behavior induced by prior treatment with estradiol. Thus, there is an initial augmentation of the facilitative effect of estradiol occurring 4-10 h after progesterone. This is followed by a profound inhibitory effect, essentially terminating receptivity. We hypothesized that the receptivity terminating effect of progesterone could be due to increased synthesis and release of neuropeptide Y in relevant brain regions. Rats were tested for female sexual behavior 4 h after progesterone (52 h postestradiol). Immediately following this test, benextramine was administered (0, 3, or 15 mg/kg, IP). Subsequently, behavioral tests were administered 24, 48, 72, and 96 h postbenextramine. Benextramine treatment attenuated the inhibitory effects of progesterone on receptivity (lordosis quotients and percent of responding animals) without affecting either proceptive or rejection behaviors. These data suggest that blockade of NPY (and alpha-adrenergic) receptors is associated with selective enhancements of specific components of sexual behavior in female rats. Specifically, blockade of NPY receptors by benextramine is associated with continued receptivity.

A review and reevaluation of the role of serotonin in the modulation of lordosis behavior in the female rat

The role of serotonin (5-HT) in the modulation of sexual receptivity (lordosis) in the female rat is reviewed and reevaluated. The effects on lordosis of drug treatments that decrease or increase the activity and availability of central 5-HT are first discussed, and this is followed by an evaluation of the effects of drugs that act directly at 5-HT receptors. In order to shed light on the physiological significance of effects of serotonergic drugs on lordosis, there is also a review of what is known of changes in levels of serotonergic activity and densities of 5-HT receptors in the female rat brain that take place through the estrous cycle and in response to administration of behaviorally effective doses of gonadal steroids. Serotonin has generally been thought to have a tonic, inhibitory effect on lordosis. However, it is concluded that 5-HT can either inhibit or facilitate lordosis depending on which subtypes of central 5-HT receptors become activated. Because of a lack of consistent or compelling evidence of effects of ovarian hormones on serotonergic activity or 5-HT receptors in critical areas of the brain, it is stated that there is at present no basis to conclude that the effects of pharmacological manipulations of serotonergic activity on lordosis reflect an important, physiological role of 5-HT in the modulation of lordosis behavior in the female rat.

Ethopharmacological studies on the effects of antihormones on rodent agonistic behavior with especial emphasis on progesterone

The effects of a range of antiandrogens and antiestrogens on conflict behaviors in laboratory rats and mice are reassessed in the light of recent studies applying ethophamacological analyses (recording the full spectrum of behaviors) to such investigations. It is argued that any antihostility properties of the antiandrogen cyproterone acetate are largely a consequence of indirect actions on odor communication, whereas antiestrogens (e.g., tamoxifen and CI 680) seem to have more fundamental motivational effects in addition to communicatory actions. A detailed example of the approach is provided in which progesterone (which can be antiandrogenic) is given to rats paired in different ways. The type of pairing has a very substantial effect on the actions seen after treatment, and the ethopharmacological approach generates a better picture of antihormone effect than traditional psychopharmacological tests.

Effects of pretreatment with adrenocorticotropin on endocrine and behavioral responses of bulls to sexual activity

Peripheral concentrations of cortisol, growth hormone and testosterone were determined in two experiments which examined the endocrine and behavioral responses of sexually mature Angus bulls to an estrous female (Experiment 1) and to female exposure 5 hours following an adrenocorticotropin (ACTH) injection (Experiment 2). Sexual activity of bulls in Experiment 1 significantly increased levels of cortisol when compared with concentrations before exposure to a female. Administration of ACTH in Experiment 2 consistently elevated levels of cortisol by 30-fold (P<0.01) when compared with pre-ACTH concentrations. This heightened level of cortisol persisted throughout the period of exposure to an estrous cow, although a gradual decline in cortisol concentrations occurred over time (P<0.05). In Experiment 1, growth hormone profiles tended to increase in response to sexual activity (P<0.10), whereas in Experiment 2, growth hormone increased in response to ACTH administration (P<0.01) and to female exposure (P<0.01). Concentrations of testosterone were unaffected (P>0.10) by mating activity in Experiment 1. In Experiment 2, acute suppression (P<0.01) in testosterone concentrations 5 hours after ACTH administration coincided with the exposure period to the estrous female. Frequencies of mounting behavious (penis extension, mounting, intromission and ejaculation) exhibited by ACTH-treated bulls were significantly lower compared with the frequencies two days earlier. Exogenous ACTH administration suppressed reproductive behaviors of bulls and altered secretion of cortisol, growth hormone and testosterone. Furthermore, these data provide evidence that specific mating behaviors of the bull can be influenced by circulating steroids.

Sustained influence of previous estradiol or testosterone treatments on sexual behaviors of female pigs

Two studies were conducted to determine the consequences of extended treatment with estradiol or testosterone on sexual behavior in postpubertal, female pigs. After ovariectomy, either steroid was administered for 6 weeks at dosages sufficient to maintain serum concentrations similar to those observed in mature male pigs. Behavioral evaluations were initiated 2 months after the last steroid treatment. These treatments reduced receptivity (immobile stance when placed with a mature male) and proceptivity (preference to remain near a mature male) in association with an increase in aggressive behavior. In females treated previously with both estradiol and progesterone, sexual behaviors 2 months later were similar to those of control females. When evaluations were repeated 5 months after extended estradiol treatment had ceased, receptivity and proceptivity had returned to that of control pigs and aggressive behavior had diminished greatly. Interpretation of these changes in behavior is that extended periods of estradiol or testosterone treatment sustain activational influences for a considerable amount of time after treatments cease and progesterone antagonizes estradiol's effect on these behaviors. In a companion study, pubertal and post-pubertal females were similar for receptivity but pubertal females spent less time near a mature male. This difference in proceptivity likely reflects a maturational change associated with sexual development in female pigs. Collectively, these observations in postpubertal, female pigs document that prolonged estrogen treatment will activate aggressive behaviors in association with reduced proceptivity and receptivity. Because these behavioral changes are reversible by 5 months after cessation of treatment, they are not the result of sexual differentiation.

Down-regulation of progestin receptors in guinea pig brain: new findings using an immunocytochemical technique

Progesterone injection in estradiol-primed, ovariectomized guinea pigs results in down-regulation of hypothalamic progestin receptors determined by in vitro binding assays. In order to determine if progesterone also decreases immunostaining of progestin receptors and if progestin receptors are down-regulated preferentially in particular neuroanatomical areas, ovariectomized guinea pigs were injected with doses of estradiol benzoate (10 micrograms at 42 h before progesterone injection) and progesterone (500 micrograms at 4, 12, or 24 h before perfusion) that reliably induce the expression of lordosis and subsequent behavioral refractoriness to progesterone. Progestin receptor-immunoreactive cells were counted in sections from discrete parts of the preoptic area and hypothalamus. As expected, estradiol dramatically increased cell nuclear, and, to a lesser extent, cytoplasmic, immunostaining in defined regions of the preoptic area and hypothalamus. By 12 h after progesterone injection, the number of progestin receptor-immunoreactive cells had decreased in some areas, but not others. The rostral and caudal aspects of the ventrolateral hypothalamus were particularly responsive showing a substantial decrease in progestin receptor-immunoreactivity by 12 h after injection. No decreases in the progestin receptor-immunoreactive cell number were observed in any of the preoptic regions examined, although obvious decreases in immunostaining intensity were seen. The results of these immunocytochemical experiments extend earlier findings from in vitro progestin binding experiments and demonstrate that as with progestin binding, progestin receptor-immunoreactivity decreases when progesterone is injected in a behavioral desensitization procedure. Furthermore, they point to the ventrolateral hypothalamus as one site in which the down-regulation of progestin receptors may be particularly responsive to progesterone.

Facilitation of sexual receptivity by hypothalamic and midbrain implants of progesterone in female hamsters

In the first experiment, ovariectomized female hamsters were stereotaxically implanted with bilateral guide cannulae aimed at the medial preoptic area (POA), ventromedial hypothalamus (VMH), or ventral tegmentum (VTA). The following week these females were injected SC with 10 micrograms estradiol benzoate (EB) and then had 27-gauge cannulae containing crystalline progesterone inserted through the guide tubes. Sexual receptivity was observed in 3 of 11 animals with VMH implants of progesterone, in 2 of 10 with VTA progesterone, but in none with POA implants. In the second experiment, the amount of intracranial progesterone was increased by mechanically expelling a 1.5 micrograms progesterone pellet from the tip of each cannula insert. This treatment facilitated receptivity in 10 of 20 hamsters with VTA implants and in 9 of 32 VMH-implanted animals. This induction of receptivity required approximately 2 hr. Progesterone pellets in the POA, mammillary region, and lateral mesencephalon were generally ineffective. In hamsters, progesterone into either the VMH or the VTA is sufficient to facilitate receptivity, although neither site is highly sensitive to progesterone. These results differ from those in recent studies in rats and this difference may reflect important species differences in the control of lordosis.

Lordosis inhibiting effects of endogenous progesterone in the male rat primed with estrogen

The aim of this study was to investigate the mechanisms involved in the inhibitory action of progesterone on estrogen-induced facilitatory effects of estradiol benzoate on lordosis behavior in the male rat. Intact adult male rats were given 1) 25 micrograms estradiol benzoate (EB) and 100 micrograms progesterone (P) at an interval of 42 hr. EB injected animals served as controls 2) EB followed by 3 doses of 400 micrograms dexamethasone (DEXA) and P as above. EB + DEXA injected animals served as controls. Testing for lordosis behavior was performed by 50 +/- hr after EB injection. A significant decrease in the number of the males displaying lordosis in response to the mounts of stimulus males resulted from P injection following EB treatment as compared to EB controls. DEXA treatment significantly reduced the number of EB animals showing lordosis responses but completely prevented the inhibitory effects of exogenous P to occur. Blood P values appeared to be significantly lower in EB + DEXA males than in their EB counterparts. The results provide evidence that endogenous P is involved in the display of lordosis behavior by EB-treated intact males. They mainly suggest that the effects of exogenous P on estrogen-induced lordosis behavior in the intact male rat result from sequential inhibitory mechanisms involving exposure of the animals to the successive action of endogenous and exogenous P.

Immunohistochemical evidence of the presence of estrogen and progesterone receptors in the same neurons of the guinea pig hypothalamus and preoptic area

Estrogen and progesterone interact in the regulation of various brain functions including mechanisms controlling gonadotropin secretion and female sexual behavior. Various methods have been used to map the regions where these hormones act and where specific receptors can be detected. However, it remains unknown if both steroids act on the same neuron or if there are neural populations responding to either one or the other of these hormones To answer this question, we used various immunocytochemical procedures to detect estrogen and progesterone receptors (ER-IR and PR-IR) on the same histological section, taking advantage of the fact that anti-ER monoclonal antibodies were raised from rats whereas anti-PR monoclonal antibodies were raised from mice. Initial experiments showed that the number of cells displaying ER and PR immunoreactivity changed with hormonal treatment. Prolonged treatment of ovariectomized guinea pigs with high doses of estradiol benzoate (EB) (10 or 15 micrograms/day for 4 or 6 days) increased the number of PR-positive cells in the preoptic area and the hypothalamus. Inversely, with this estrogen priming regimen the number of ER-positive cells decreased, as did the immunostaining intensity in their nuclei. With a lower dose of estrogen administered for a shorter period (2-microgram dose of EB for 2 days) the two receptor populations could be easily observed. Under these hormonal conditions, the sequential procedure revealed that the vast majority of cells containing ER-IR in the preoptic area (nucleus preopticus periventricularis, medialis) and the mediobasal hypothalamus (nucleus periventricularis, arcuatus, ventrolateral part of nucleus ventromedialis and premamillaris) showed PR immunoreactivity. This was true regardless of the order in which dual immunocytochemical staining was performed. There were, however, a few ER-positive/PR-negative cells in these regions. However, only ER-positive neurons were found in the n. interstitialis stria terminalis, the n. amygdaloideus medialis and the n. supraopticus. Thus, in guinea pigs receiving moderate doses of estrogen, all PR-positive cells contain ER-IR whereas there is a population of ER-positive cells which are devoid of PR-IR.

Plasma levels of dihydrotestosterone in the cycling rat: implications for the regulation of lordosis behavior

Exogenously administered dihydrotestosterone (DHT) is a potent inhibitor of sexual behavior in ovariectomized, estrogen-primed rats and endogenous DHT has been implicated as an inhibitor of sexual behavior in cycling rats. To determine the temporal relationship between DHT levels and the expression of sexual behavior, females were tested at midnight of each day of the 4-day estrous cycle for lordosis response to male mounts and subsequently bled. Plasma DHT levels were highest (177 +/- 10 pg/ml) at proestrus (P), when lordosis was fully expressed, fell to significantly lower levels (136 +/- 8, p less than 0.025) at estrus (E), and rose at diestrus I and diestrus II (DII) (152 +/- 8 and 154 +/- 10 respectively). Other female rats were bled from a jugular cannula at 4-hr intervals between midnight of DII and midnight of E. Plasma DHT was elevated from 1200 of P to 0400 of E and fell rapidly through 0800 of E. This elevation of endogenous DHT coincides with the expression of lordosis behavior, and the magnitude of this peak is many times lower than the concentrations reached by the minimal exogenous dose that inhibits lordosis behavior. This suggests that cyclic changes of DHT in the peripheral circulation do not inhibit lordosis behavior during the estrous cycle.

Hormonal control of courtship and copulatory behavior in male Cnemidophorus inornatus, a direct sexual ancestor of a unisexual, parthenogenetic lizard

The hormonal control of courtship and copulatory behavior in male Cnemidophorus inornatus, one of the gonochoristic ancestral species of the all-female, parthenogenetic C. uniparens, was determined. Significantly fewer castrated males courted females than intact and sham-castrated control males. Silastic capsules containing dihydrotestosterone or testosterone reinstated courtship and copulatory behavior in a significant number of castrates. While significantly greater numbers of castrates treated with androgens resumed courtship, some of the castrates receiving progesterone implants also resumed intense courtship and copulatory behavior. Exogenous progesterone also maintained courtship behaviors in a significant number of intact males at a time when control males ceased to court. The implications of these findings for our understanding of the evolution of hormone-brain-behavior relationships and sex steroid hormone mechanisms of action are discussed.

Estrogen induction of sexual behavior in female rats and synthesis of polyadenylated messenger RNA in the ventromedial nucleus of the hypothalamus

To test the hypothesis that estrogen stimulates sexual behavior by inducing transcription of polyadenylated messenger RNA, we studied the effects of cordycepin, an adenosine analog that disrupts polyadenylation, on the lordotic responses of ovariectomized female rats made sexually receptive with systemic injections of estradiol benzoate (EB) and progesterone (P). Cordycepin inhibited lordosis when infused into the ventromedial nucleus of the hypothalamus within an hour before the females received EB; its effectiveness varied linearly with dose. It did not significantly alter sexual behavior when infused into the medial preoptic area. A dose of cordycepin that decreased lordosis when infused 1 h before injection of 0.5 microgram EB did not affect the behavior when infused 1 h before injection of 500 micrograms P. Cordycepin does not suppress behavior by blocking estrogen uptake since it did not alter estrogen accumulation by hypothalamic cell nuclei. Cordycepin inhibits ribosomal RNA (rRNA) synthesis as well as polyadenylation. While this probably contributes to cordycepin's inhibitory effects on lordosis, it cannot fully account for them since a cytidine analog that inhibits rRNA synthesis without inhibiting polyadenylation did not mimic cordycepin's behavioral effects. Cordycepin may suppress synthesis of P receptors; however, this could not fully account for its behavioral effects since cordycepin also inhibited lordosis when the P receptor was bypassed by substituting methysergide for P. As assessed by protein synthesis autoradiography, cordycepin's effects are highly localized. The data support the notion that estrogen facilitates female sexual behavior by altering gene expression in the brain.

Anxiolytic effects of progesterone are sexually dimorphic

We evaluated the effects of progesterone administration to estrogen-primed rats on licking behavior during a lick suppression test. This test assesses the behavior of water-deprived rats to lick a drinking tube for water reinforcement that is paired to electric shock. Chlordiazepoxide (a commonly used tranquilizer) is very effective in increasing licking during this test. Normal females and neonatally castrated males displayed an increase in shock-punished responses in tests conducted after hormone injections. Normal males and neonatally androgenized females were unaffected. Our results suggest that ovarian steroids, and particularly progesterone, can modulate anxiety in females and feminized males. In addition, the anxiolytic effect of progesterone appears to be mediated by a mechanism different from that of Chlordiazepoxide.

Evolution of mechanisms controlling mating behavior

The proximate mechanisms underlying mating behavior in naturally occurring species can be fundamentally different from those in more commonly studied laboratory and domesticated forms. In naturally occurring species, reproductive strategies are much more diverse, and mechanisms controlling behavior are correspondingly diverse. A variety of hormonal, environmental, and social cues can be used to activate mating behavior. Which cues are used by particular species depends on differences in environmental and physiological constraints imposed by particular reproductive strategies. Study of this diversity of mechanisms promises to identify specific selective forces that have shaped their evolution. This evolutionary perspective leads to widely applicable generalizations and provides a useful context within which to conceptualize differences between species, populations, and individuals.

Progesterone distribution in the brain of the PMSG treated female rat

The uptake of progesterone in the intact female rat brain was studied in 25, immature, female rats which were given an injection of 4 IU PMSG on the 25th day of life. The rats were killed 53-55 h after their first ovulation. Amounts of progesterone were measured in seven brain areas, peripheral fat and muscle tissue and blood plasma. The analyses were done by radioimmunoassay after extraction with ethanol or diethylether. The highest mean concentration was found in the cerebral cortex, followed by the hypothalamus, the hippocampus, the striatum, the midbrain, the cerebellum and the medulla oblongata (mean +/- SE pg/mg 29 +/- 2.0, 27 +/- 5.2, 19 +/- 2.4, 15 +/- 1.8, 14 +/- 2.0, 12 +/- 1.7 and 11 +/- 1.9). The concentration of progesterone in the cerebral cortex was significantly higher than in all other areas except the hypothalamus (p less than 0.001). The cerebral cortex, the hypothalamus and the hippocampus had progesterone levels that were significantly higher than in peripheral fat tissue (p less than 0.01). The plasma progesterone level correlated positively with that in the cerebral cortex (r = 0.62, p less than 0.01). Some earlier studies have shown high accumulation in the hypothalamus, but low in the cerebral cortex. One reason for the different results might be the nonphysiological models used in the earlier studies.

Pubertal changes in the medio-basal hypothalamic area after neonatal suprachiasmatic nucleus lesions in the rat

In order to get more insight into the mechanism by which the onset of puberty is controlled, a developmental study on the displacement of catalase- and dopamine-containing cells in the hypothalamic region was done in rats which received a neonatal lesion of the suprachiasmatic nucleus. The displacement of cells is delayed after these lesions. However, the time lost at the beginning of the displacement is made up at the end of the migration.

Progesterone decreases the concentration of hypothalamic and anterior pituitary estrogen receptors in ovariectomized rats

In a study of cellular mechanisms of progesterone's antiestrogenic action on behavior and neuroendocrine responses, we investigated the influence of progesterone on the concentration of estrogen receptors in the hypothalamus-preoptic area (HP), anterior pituitary gland (AP), and uterus of chronically estradiol-treated ovariectomized rats. Ovariectomized (OVX) rats were implanted s.c. with 15 mm silastic capsules of estradiol. One week later, they were injected with progesterone or oil vehicle and killed 6 h or 24 h later. Confirming previous reports, progesterone caused a decrease in the concentration of uterine cytosol and nuclear estrogen receptors at both times. Less consistent results were obtained in HP and AP; a decrease in the concentration of HP cytosol estrogen receptors was detected at 6 h, as was a small decrease in the concentration of HP nuclear estrogen receptors at 24 h. More consistent results were seen when a low priming dose of estradiol was used. Although progesterone was without effect on the concentration of nuclear estrogen receptors in HP and AP at 6 h, cytosol receptor levels were depressed by 25% in HP and 14% in AP. At 24 h after progesterone injection, nuclear estrogen receptor levels were decreased in all tissues, while cytosol estrogen receptor levels remained depressed. A study of the time course of progesterone's suppression of cytosol estrogen receptor concentration revealed that the effect is transient, occurring by 6 h after progesterone injection, but returning to baseline by 48 h after injection. Scatchard analysis confirmed that the decreased concentration of cytosol binding in HP was due to a decrease in the concentration of binding sites. As with nearly all of progesterone's neuroendocrine effects, the suppression of estrogen receptor levels requires estrogen priming. HP and AP cytosol from progesterone-treated rats did not seem to contain an estrogen receptor-regulatory factor as do uterine cell nuclei; loss of binding sites at 37 degrees C was no faster in cytosol from progesterone-treated rats. These results demonstrate that, under some conditions, progesterone decreases HP and AP estrogen receptor concentrations. Unlike progesterone's action in the uterus, the primary effect in the brain and pituitary gland seems to be on the cytosol receptor.

Progesterone in the ventromedial hypothalamus of ovariectomized, estrogen-primed rats inhibits subsequent facilitation of estrous behavior by systemic progesterone

The putative neural target sites of progesterone's (P) inhibitory influence on estrous behavior were re-examined utilizing intracranial hormone implants. Subjects were estrogen-primed ovariectomized, Long-Evans rats, and all were outfitted with permanent indwelling guide cannulae aimed for the ventromedial hypothalamus (VMH), preoptic area (POA), or midbrain. In a series of 4 weekly testing paradigms, the ability of blank or P-filled implants to both facilitate estrous responsiveness and to interfere with an animal's ability to exhibit estrous behavior following a systemic injection of P 24 hours later was assessed. P-filled implants placed bilaterally into the VMH inhibited the subsequent facilitation of estrous behavior by systemic P administration. Neither P-filled implants in other brain regions nor blank implants lowered into the VMH had this same effect. Seven of the females that exhibited progesterone-induced behavioral refractoriness 24 h after P stimulation of the VMH exhibited facilitation of estrous responsiveness in behavioral tests given 4.5 h after cannulae placement suggesting that progesterone may exert its facilitative and inhibitory actions on estrous responsiveness at the same neuroanatomical locus.

Behavioral effects of progestin in the brain

In this article we review research on the role of progestins in the regulation of estrous responsiveness in female rats. Estrous responsiveness normally results from a synergistic action of estradiol (E2) and progesterone (P). E2 primes the system but normally does not result in estrous behavior. The full expression of estrous responsiveness results from the action of P on the E2-primed system. It has been demonstrated with implants of dilute E2 (1 part E2: 250 parts cholesterol) that the site of E2 priming is the ventromedial nucleus of the hypothalamus (VMN). In females primed with systemically administered E2, P also acts on the VMN to facilitate full estrous responsiveness. It has been shown in addition that estrous responsiveness results from sequential application of E2 and P to the VMN but not to other areas of the brain. The VMN is also the site at which P produces sequential inhibition of estrous responsiveness. The time course of P action in facilitating full estrous responsiveness is about two hours, regardless of whether the hormone is administered intracerebrally or intravenously. The duration of estrous responsiveness is directly correlated with the length of time P is in contact with brain tissue. Experiments with the protein synthesis inhibitor anisomycin are consistent with the view that P acts in the VMN by way of a protein synthetic mechanism to facilitate estrous behavior; however, other mechanisms must be considered as alternatives. Finally, we address the question of whether estrogenic priming depends upon induction of progestin receptors in the VMN. Results indicate that estrogenic priming of estrous responsiveness may occur without concomitant induction of progestin receptors.