Reduced sexual motivation of diabetic female rats: Restoration with insulin.

The aim of this study was to evaluate female rat sexual motivation in a model of diabetes mellitus type 1. Severe hyperglycemia was induced in ovariectomized Wistar rats by injecting streptozotocin [STZ, 100 mg/kg, i.p.]. Ten days later, females received estradiol benzoate (10 µg/rat, s.c.) plus progesterone (3 mg/rat, s.c.). A group of STZ-treated animals was administered with insulin (2-4 U) every 12 h for 10 days, which normalized glucose levels. In the partner preference (PP) and sexual incentive motivation (SIM) tests, control females spent more time close to a sexually experienced male (SE) than with a castrated male (CM). STZ-treated females stayed the same amount of time with both stimuli, that is, they lost their sexual preference. We also evaluated the sense of smell using two behavioral tests, one related to sexual odors (SO) and another one to food odors (FO). In the SO test, control females spent more time sniffing the sawdust coming from cages that contained SE males; hyperglycemic females remained the same amount of time sniffing the sawdust of both stimuli: SE and CM. In the FO test, no differences were found between control and STZ-treated groups. Insulin treatment reverted the changes observed in hyperglycemic females in the PP, SIM and SO tests. These data suggest that severe hyperglycemia decreases sexual motivation and that insulin recovers such diminution.

Sex, stress, and mood disorders: at the intersection of adrenal and gonadal hormones.

The risk for neuropsychiatric illnesses has a strong sex bias, and for major depressive disorder (MDD), females show a more than 2-fold greater risk compared to males. Such mood disorders are commonly associated with a dysregulation of the hypothalamo-pituitary-adrenal (HPA) axis. Thus, sex differences in the incidence of MDD may be related with the levels of gonadal steroid hormone in adulthood or during early development as well as with the sex differences in HPA axis function. In rodents, organizational and activational effects of gonadal steroid hormones have been described for the regulation of HPA axis function and, if consistent with humans, this may underlie the increased risk of mood disorders in women. Other developmental factors, such as prenatal stress and prenatal overexposure to glucocorticoids can also impact behaviors and neuroendocrine responses to stress in adulthood and these effects are also reported to occur with sex differences. Similarly, in humans, the clinical benefits of antidepressants are associated with the normalization of the dysregulated HPA axis, and genetic polymorphisms have been found in some genes involved in controlling the stress response. This review examines some potential factors contributing to the sex difference in the risk of affective disorders with a focus on adrenal and gonadal hormones as potential modulators. Genetic and environmental factors that contribute to individual risk for affective disorders are also described. Ultimately, future treatment strategies for depression should consider all of these biological elements in their design.

Reduction in the latency of action of antidepressants by 17 beta-estradiol in the forced swimming test.

RATIONALE: Antidepressants (ADs) are slow to produce their therapeutic effect. This long latency promotes the development of new strategies to short their onset of action. Previous reports indicated that 17beta-estradiol (E(2)) promotes the antidepressant-like activity of fluoxetine (FLX) and desipramine (DMI) in the forced swimming test (FST). OBJECTIVE: The aim of the present work was to analyze if E(2) reduces the antidepressant-like onset of action of venlafaxine (VLX), FLX, and DMI. MATERIALS AND METHODS: Independent groups of ovariectomized female Wistar rats were tested in the FST and in the open field after chronic (1 to 14 days) treatment with VLX (20 mg/kg/day), FLX (1.25 mg/kg/day), or DMI (1.25 mg/kg/day) alone or in combination with a single injection of E(2) (2.5 microg/rat sc, 8 h before FST). RESULTS: VLX, FLX, or DMI by themselves at these doses did not induce changes in the FST at short intervals after their injection (from 1 to 7 days). The addition of E(2) promoted the antidepressant-like effect of VLX and DMI as early as day 1. Such action was also evident after 3, for FLX, and 14 days for both FLX and DMI, but not for VLX. The behavioral actions of these ADs combined with E(2) were not accompanied by increases in general activity in the open-field test. CONCLUSION: E(2) clearly reduced the latency to the onset of action for these ADs in the FST. These results represent an interesting therapeutic strategy for the treatment of depression in perimenopausal women.

Aging increases the susceptibility to develop anhedonia in male rats.

The objective of this study was to establish the effect of aging on the development of anhedonia, a core feature of depression. Young and old male Wistar rats (of around 3-5 and 12-15 months, respectively) were exposed to a chronic variable stress (CVS) schedule for 3 weeks. CVS produced anhedonia, indicated by a reduction in the intake of a sucrose solution (1%), in 8 out of 23 (35%) young rats and in 19 out of 26 (73%) old rats, implying that old animals are more susceptible to stress and develop anhedonia more readily than young animals. Young and old anhedonic rats showed a similar temporal course in the reduction of sucrose consumption, reaching the anhedonic state after 2 weeks of CVS exposure. Compared with young animals, old rats had lower basal serum testosterone and estradiol levels. The systemic levels of corticosterone did not vary between both age groups. No significant pathological condition was detected in old animals. It is suggested that the higher susceptibility to develop anhedonia in male rats could be associated to neuroendocrine changes consequent to aging.

Neuropeptide and steroid hormone mediators of neuroendocrine regulation.

To maintain the health and well-being of all mammals, numerous aspects of physiology are controlled by neuroendocrine mechanisms. These mechanisms ultimately enable communication between neurones and glands throughout the body and are centrally mediated by neuropeptides and/or steroid hormones. A recent session at the International Workshop in Neuroendocrinology highlighted the essential roles of some of these neuropeptide and steroid hormone mediators in the neuroendocrine regulation of stress-, reproduction- and behaviour-related processes. Accordingly, the present review highlights topics presented in this session, including the role of the neuropeptides corticotrophin-releasing factor and gonadotrophin-releasing hormone in stress and reproductive physiology, respectively. Additionally, it details an important role for gonadal sex steroids in the development of behavioural sex preference.

c-Fos expression related to sexual satiety in the male rat forebrain.

The long term inhibition of masculine sexual behavior after repeated ejaculations is known as sexual satiety. To investigate the brain areas that may regulate sexual satiety, c-Fos expression was studied in different groups of sexually experienced male rats: controls not allowed to copulate, males allowed two or four ejaculations and animals allowed to reach sexual satiety. Interestingly, males that ejaculated two or four times had similar c-Fos densities in all the evaluated brain regions, except for the suprachiasmatic nucleus. Similarly, sexually satiated males had analogous c-Fos densities in all the evaluated brain areas independently of the number of ejaculations required to reach satiety. Sexual activity (evidenced in males that ejaculated two or four times) increased c-Fos levels in the anteromedial bed nucleus of the stria terminalis, claustrum, entorhinal cortex, medial preoptic area, nucleus accumbens core, suprachiasmatic nucleus and supraoptic nucleus; however, sexual satiety did not modify c-Fos expression in these regions. Sexually satiated males had increased c-Fos densities in the ventrolateral septum and the anterodorsal and posteroventral medial amygdala, compared with animals allowed to copulate but that did not reach sexual satiety, and decreased c-Fos density in the piriform cortex. These results suggest that the network that underlies sexual satiety is different from that which regulates copulation.

Facilitating antidepressant-like actions of estrogens are mediated by 5-HT1A and estrogen receptors in the rat forced swimming test.

Previous studies have shown that 17beta-estradiol (E2) induces antidepressant-like actions per se and potentiates those produced by fluoxetine (FLX) in the forced swimming test (FST). The aim of the present work was to explore the participation of serotonin 1A receptors (5-HT1A) and estrogen receptors (ERs) in the antidepressant-like actions of E2, FLX or their combination in the FST. Although all antidepressants reduce behavioral immobility, antidepressants that modulate serotonergic neurotransmission increase swimming behavior whereas those that modulate the catecholaminergic neurotransmission increase climbing behavior. Thus, using this animal model, it is possible to infer which neurotransmitter system is modulating the action of an antidepressant compound. Ovariectomized female Wistar rats were used in all experiments. In the first experiment, an effective dose of E2 (10 microg/rat, -48 h) was combined with several doses (0.5, 1.0 and 2 mg/kg) of RU 58668 (a pure ER antagonist) 48 h previous to the FST. The second experiment evaluated the action of (1 mg/kg, -48 h or -23, -5 and -1 h) WAY 100635 (5-HT1A receptor antagonist) on the antidepressant-like action of FLX (10 mg/kg, -23, -5 and -1 h). In the third experiment, the effect of RU 58668 (2 mg/kg, -48) or WAY 100635 (1 mg/kg, -48 h) on the antidepressant-like action of the combination of a sub-optimal dose of E2 (2.5 microg/rat, -48 h) plus a non-effective dose of FLX (2.5 mg/kg, -23,-5 and -1 h) was evaluated. The results showed that RU 58668, the antagonist to the ER, canceled the antidepressant-like action of E2 in a dose-dependent manner. The antagonist to the 5-HT1A receptor blocked the antidepressant action of FLX only when administered simultaneously with FLX, i.e. -23, -5 and -1 h before the FST. Finally, the administration of both RU 58668, and WAY100635 canceled the antidepressant-like action of the combination of E2/FLX. These results imply that both 5-HT1A receptors and ERs participate in the facilitating actions of E2 on the antidepressant-like action of FLX in the FST.

Comparative analysis of immunoreactive cells for androgen receptors and oestrogen receptor alpha in copulating and non-copulating male rats.

In some species, including gerbils, guinea pigs, mice, rams and rats, some apparently normal males fail to mate. These kinds of animals have been named 'noncopulating (NC)'. The cause of this behavioural deficit is unknown. The present study aimed to determine whether NC male rats have alterations in the amount of androgen (AR) and oestrogen receptor alpha (ERalpha) in a neuronal circuit important for the control of male sexual behaviour; the vomeronasal projection pathway. We evaluated the number of AR and ERalpha immunoreactive (AR-IR and ERalpha-IR) cells in the accessory olfactory bulb (AOB), the bed nucleus of the stria terminalis (BNST), the anterior-dorsal medial amygdala (MeAD), the posterior dorsal amygdala (MePD) and the medial preoptic area (MPOA). The results demonstrate that the number of AR-IR cells in NC males was significantly higher compared to copulating (C) males in the MePD, but no significant differences were found in any of the other structures analysed. ERalpha-IR cells were more abundant in NC than in C males in the MeAD and the MePD. However, in the MPOA the number of ERalpha-IR cells was significantly reduced in NC males. No significant differences were found in the AOB or in the BNST. A similar pattern of results was observed when regions within these structures that are activated by Fos expression, on mating or exposure to sexually relevant cues were analysed. The differences in the number of AR and ER in particular brain areas could be associated with alterations in sexual behaviour as well as partner and olfactory preference for receptive females seen in NC male rats.

Rutin antinociception involves opioidergic mechanism and descending modulation of ventrolateral periaqueductal grey matter in rats.

BACKGROUND: Rutin is a bioflavonoid found in fruits, vegetables and plants used in traditional medicine to alleviate pain. However, rutin's scientific evidence for the modulation of pain and its mechanism of action is lacking. It is well known that the periaqueductal grey matter (PAG) contains opioidergic neural circuits involved in the modulation of descending nociception. The aim of this study was to investigate if antinociceptive activity of rutin is modulated by the PAG circuitry involving participation of opioid receptors. METHODS: The experimental design included groups of rats receiving rutin systemically (30-1000 mg/kg) or microinjected into the vlPAG (8-32 nmol/4 µL) alone or in the presence of an opioid antagonist, naltrexone (5 mg/kg, i.p. or 26 nmol/4 µL, respectively). Nociception was assessed using the formalin test and compared versus the reference drugs, tramadol and morphine. RESULTS: Systemic or intra-vlPAG administration of rutin significantly decreased both phases of the formalin test. Antinociceptive responses of the reference drugs were prevented by naltrexone, whereas the antinociceptive effect of rutin was inhibited by this antagonist mainly in the phase II of the formalin test. CONCLUSIONS: Our results provide evidence that rutin produces antinociceptive effects involving central modulation of the vlPAG descending circuit partly mediated by an opioidergic mechanism.

Influence of inflammatory nociception on the anxiolytic-like effect of diazepam and buspirone in rats.

RATIONALE: The effect of anxiety on nociception has been evaluated but not that of nociception on anxiety. OBJECTIVE: The study was conducted to analyse the influence of nociception on basal levels of anxiety-like behaviour and on the action of anxiolytic drugs. METHODS: Nociception was induced by an intra-articular injection of uric acid at 3.75 or 7.5%. Experimental anxiety was determined in the rat burying behaviour and the elevated plus maze tests. To separate specific anxiety-related drug actions, a spontaneous ambulatory test was included. The anxiolytics, buspirone (2.5 and 5.0 mg/kg, i.p.) and diazepam (0.5, 1.0 and 2.0 mg/kg, i.p.), were used. RESULTS: In the nociception test, the pain-induced functional impairment rat model, uric acid at 3.75 and 7.5% had an effect of around 35 and 75%, respectively. Uric acid (UA) at the lower dose (3.75%) lacked an effect on burying behaviour but significantly increased the time spent and number of entries to the open arms; the higher UA dose (7.5%) produced a significant increase in the time spent and number of entries to the open arms and a statistically significant reduction in cumulative burying. Diazepam and buspirone produced a clear dose-dependent reduction in cumulative burying. In the plus maze, diazepam also induced an increase in the time spent and number of entries to the open arms. In the burying behaviour test, rats with a mild level of nociception (uric acid at 7.5%) were insensitive to the anxiolytic-like effect of these anxiolytic drugs. In the plus maze test, the anxiolytic-like effect of diazepam (1.0 mg/kg) was blocked under both levels of nociception. CONCLUSIONS: These data demonstrate that nociception modifies the response to anxiolytic drugs. The role of factors with anxiogenic properties produced during inflammation, which may modify diazepam and buspirone effects, is discussed.

Experimental anxiety in the black and white model in cycling, pregnant and lactating rats.

This study demonstrates changes in experimental anxiety assessed in the black and white paradigm during various reproductive states of female rats. Low levels of experimental anxiety were observed during late proestrus and on day 17 of gestation, stages related to high progesterone (P) levels. In estrus, metestrus, diestrus and on day 21 of gestation, stages characterized by low P concentrations, high levels of experimental anxiety, similar to those exhibited by ovariectomized females, were found. No changes in experimental anxiety were observed on day 8 of lactation compared to ovariectomized females. These data are discussed from the standpoint of the putative anxiolytic-like effect of progestins.

Unraveling the modulatory actions of serotonin on male rat sexual responses.

Animal studies and clinical investigations reveal that serotonin plays a central role in the control of the ejaculatory threshold. The chronic use of selective serotonin reuptake inhibitors (SSRIs) frequently results in sexual dysfunction, inviting to analyze the modulatory actions of serotonin on male sexual function in depth. Even though the main effect of serotonin on male sexual responses is inhibitory, this neuromodulator also mediates brief important stimulatory actions. Serotonin (5-HT) can activate two intracellular signaling pathways: a lower-threshold facilitatory pathway, and a higher-threshold inhibitory pathway, leading to biphasic effects. We propose that these divergent actions are related to the stimulation or inhibition of glutamatergic and GABAergic interneurons. Experimental evidence suggests that low 5-HT concentrations produce stimulatory actions on male ejaculatory aspects that might be mediated by the blockade of the GABAergic neurotransmission in the MPOA and spinal cord, which in turn releases a tonic inhibition that allows other neurotransmitters such as glutamate, noradrenaline, oxytocin and dopamine to initiate a sequence of molecular events resulting in the expression of ejaculation. Similar serotonin actions, mediated via interneurons, have been proposed for the regulation of other processes and occur in many central nervous system areas, indicating that it is not an isolated phenomenon.

Sex differences in androgen receptors of the human mamillary bodies are related to endocrine status rather than to sexual orientation or transsexuality.

In a previous study we found androgen receptor (AR) sex differences in several regions throughout the human hypothalamus. Generally, men had stronger nuclear AR immunoreactivity (AR-ir) than women. The strongest nuclear labeling was found in the caudal hypothalamus in the mamillary body complex (MBC), which is known to be involved in aspects of cognition and sexual behavior. The present study was carried out to investigate whether the sex difference in AR-ir of the MBC is related to sexual orientation or gender identity (i.e. the feeling of being male or female) or to circulating levels of androgens, as nuclear AR-ir is known to be up-regulated by androgens. Therefore, we studied the MBC in postmortem brain material from the following groups: young heterosexual men, young homosexual men, aged heterosexual castrated and noncastrated men, castrated and noncastrated transsexuals, young heterosexual women, and a young virilized woman. Nuclear AR-ir did not differ significantly between heterosexual and homosexual men, but was significantly stronger than that in women. A female-like pattern of AR-ir (i.e. no to weak nuclear staining) was observed in 26- to 53-yr-old castrated male-to-female transsexuals and in old castrated and noncastrated men, 67--87 yr of age. In analogy with animal studies showing strong activational effects of androgens on nuclear AR-ir, the present data suggest that nuclear AR-ir in the human MBC is dependent on the presence or absence of circulating levels of androgen. The group data were, moreover, supported by the fact that a male-like AR-ir (i.e. intense nuclear AR-ir) was found in a 36-yr-old bisexual noncastrated male-to-female transsexual and in a heterosexual virilized woman, 46 yr of age, with high levels of circulating testosterone. In conclusion, the sexually dimorphic AR-ir in the MBC seemed to be clearly related to circulating levels of androgens and not to sexual orientation or gender identity. The functional implications of these alterations are discussed in relation to reproduction, cognition, and neuroprotection.

Sex differences in the distribution of androgen receptors in the human hypothalamus.

The present study reports for the first time the distribution of androgen receptor immunoreactivity (AR-ir) in the human hypothalamus of ten human subjects (five men and five women) ranging in age between 20 years and 39 years using the antibody PG21. Prolonged postmortem delay (72:00 hours) or fixation time (100 days) did not influence the AR-ir. In men, intense nuclear AR-ir was found in neurons of the horizontal limb of the diagonal band of Broca, in neurons of the lateromamillary nucleus (LMN), and in the medial mamillary nucleus (MMN). An intermediate nuclear staining was found in the diagonal band of Broca, sexually dimorphic nucleus of the preoptic area, paraventricular nucleus, suprachiasmatic nucleus, ventromedial nucleus, and infundibular nucleus, whereas weaker labeling was found in the bed nucleus of the stria terminalis, medial preoptic area, dorsal and ventral zones of the periventricular nucleus, supraoptic nucleus, and nucleus basalis of Meynert. In most brain areas, women revealed less staining than men. In the LMN and the MMN, a strong sex difference was found. Cytoplasmic labeling was observed in neurons of both sexes, although women showed a higher variability in the intensity of such staining. However, no sex differences in AR-ir were observed in the bed nucleus of the stria terminalis, the nucleus basalis of Meynert, or the islands of Calleja. Species differences and similarities of the AR-ir distribution are discussed. The present results suggest the participation of androgens in the regulation of various hypothalamic processes that are sexually dimorphic.

Changes in burying behavior during the estrous cycle: effect of estrogen and progesterone.

This study demonstrates changes in cumulative burying behavior during various phases of the rat estrous cycle. Low levels of burying behavior were observed during late proestrus. Similar burying behavior levels among females tested in early proestrus, metestrus or diestrus and ovariectomized females were found. In ovariectomized females, treatment with estrogen (E) produced a slight, non-significant reduction in burying behavior. In contrast, progesterone (P) treatment resulted in a dose-dependent, statistically significant decrease in time spent burying. The combined treatment of E and P produced no effect. These data are discussed from the standpoint of the putative anxiolytic effect of P.

Sexual differentiation modifies the allopregnanolone anxiolytic actions in rats.

The administration of progesterone (0.0, 1.0 and 2.0 mg/rat, s.c.) and allopregnanolone (5 alpha, 3 alpha dihydroprogesterone) (0.0, 0.5 and 1.0 mg/rat, s.c.) to both, males and females, produced a similar reduction in burying behavior. Only allopregnanolone showed a gender-dependent effect on burying behavior latency. Allopregnanolone actions were established in five groups of animals according to their neonatal hormonal manipulation: intact males and females, neonatally-testosterone propionate-treated female rats (TP, 30 and 120 micrograms/rat, s.c. at day 5) and neonatally (4-12 h after delivery) castrated males. Males and females showed a reduction in anxiety after treatment with allopregnanolone. Both neonatally-androgenized-females and -castrated males were completely insensitive to allopregnanolone anxiolytic action tested in both burying behavior and plus-maze paradigm. The virilizing action of neonatally administered TP was demonstrated by dose-dependent delayed vaginal opening, a persistent estrus in their vaginal smears and the presence of polifollicular ovaries. Results are discussed on the bases of the differences and similarities between males, females, androgenized females and neonatally castrated males to anxiolytic steroids and the underlying possible processes.

The differential effect of the anxiolytic agent 8-OH-DPAT during lactation is independent of pup withdrawal and maternal behavior.

Injection of the serotonergic agonist, 8-hydroxy-2-(di-n-propylamino-tetralin (8-OH-DPAT) (0.5 mg/kg ip) produced a clear anxiolytic-like effect (as measured in the burying behavior test), after parturition, which remains until day 6 of lactation. Thereafter 8-OH-DPAT completely lacked action. In order to analyze whether lactation prevented the action of 8-OH-DPAT, dams were separated from their pups for five consecutive days. The blockade of the anxiolytic effect of 8-OH-DPAT does not disappear by isolation of the mothers from their offspring or from neighboring pups. Finally, to investigate the possible role of maternal behavior in the blockade of the anxiolytic effect of 8-OH-DPAT a third experiment was made in which ovariectomized females were rendered maternal by the sensitization procedure. These females respond normally to the antianxiety actions of 8-OH-DPAT. Results suggest that a long-term clue triggered by lactation, possibly related to prolactin secretion, interferes with the anxiolytic effect of 8-OH-DPAT.

Blockade of the anxiolytic-like action of ipsapirone and buspirone, but not that of 8-OH-DPAT, by adrenalectomy in male rats.

The effect of the 5-HT1A agonists ipsapirone (5 mg/kg), buspirone (5 mg/kg) and 8-OH-DPAT (0.5 mg/kg) on experimental anxiety was examined in sham-operated, adrenalectomized and adrenally demedullated male rats. The animal model of anxiety used was the defensive burying test. At the doses selected, all 5-HT1A compounds produced an anxiolytic-like action by reducing the burying behavior in both sham-operated and demedullated rats. However, in adrenalectomized subjects, while 8-OH-DPAT still reduced burying behavior, ipsapirone and buspirone lost their action. Data suggest that adrenocortical secretions play a role in the anxiolytic-like actions of buspirone and ipsapirone, but not in those of 8-OH-DPAT. Buspirone and ipsapirone also produced a reduction in burying behavior latency in sham-operated animals that was not observed in adrenalectomized or adrenally demedullated rats. These data suggest that adrenaline may be participating in the action of these compounds on the burying behavior latency. Present findings support possible direct relationships between the stimulation of 5-HT1A receptors and adrenal secretions.

Diazepam and progesterone produce sexually dimorphic actions on the rat EEG: role of the neonatal sexual differentiation process.

This study was designed to analyse if the effect of diazepam or progesterone on the wakeful rat EEG vary according to sex and neonatal sexual differentiation. Diazepam or progesterone was administered to males, females, neonatally castrated males and neonatally virilised females. Comparisons of drug actions were made versus baseline values. Both compounds produced clear sexual dimorphic responses, males being more sensitive than females. In normal males and neonatally virilised females diazepam produced a clear increase in the absolute power of fast frequencies. In normal females and neonatally castrated males this anxiolytic only produced a moderate increase in the absolute power of the beta 2 band. Regarding interparietal correlation, diazepam produced an increase in males in the alpha, beta 1 and beta 2 bands, while in females this drug caused a reduction in the same bands. Neonatally castrated males or virilised females showed an intermediate pattern. These data suggest that sexual dimorphism in diazepam action depends upon neonatal sexual differentiation. Progesterone, in contrast with diazepam, produced effects on the EEG that were also sexually dimorphic but independent of the sexual differentiation process. This steroid in males induced an increase in the absolute power of the fast bands of the EEG (alpha, beta 1 and beta 2) accompanied by an increased interparietal correlation of the alpha band. In females, progesterone only reduced the interparietal correlation in the fast theta and alpha bands. Data show similarities between diazepam and progesterone effects on the EEG that are discussed in the light of the anxiolytic-like and hypnotic properties of these compounds. A similar mechanism of action for both substances, involving the GABA(A) receptor, is sustained.

Interaction of desipramine with steroid hormones on experimental anxiety.

The present study analyzes if estradiol benzoate and/or progesterone interact with desmethylimipramine (DMI) to diminish experimental anxiety. The animal model of anxiety used was the conditioned defensive burying test. Dose response curves for DMI (0.625, 1.25 and 2.5 mg/kg, every 24 h, during 21 days), estradiol benzoate (0.5, 1.0, 2.0 and 4.0 micrograms/rat, 48 h) and progesterone (0.5, 1.0 and 2.0 mg/rat, -4 h) were made in ovariectomized rats. DMI per se decreased dose dependently the cumulative burying time, an effect considered as anxiolytic-like. Progesterone produced a decrease in burying at the highest dose, while estradiol benzoate had no effect on defensive burying. Both, progesterone (0.5 mg/rat) and estradiol benzoate (4.0 micrograms/rat) were able to decrease the cumulative burying behavior when injected with a subthreshold dose of DMI (1.25 mg/kg). In addition, the effect of DMI (1.25 mg/kg) plus the combination of estradiol benzoate and progesterone, sequentially administered (48 h and 4 h before the tests, respectively), also produced a synergistic decrease in burying behavior. In general, the treatments produced no changes in burying behavior latency, neither in spontaneous ambulation or in nociception. It is concluded that DMI synergizes its anxiolytic-like effect when administered with estradiol alone or in combination with progesterone. Present data provide experimental evidence suggesting an interaction between hormones and antidepressants. Results are discussed on the basis of the interaction between steroids and serotonergic or GABAergic receptors.