Estrous Cycle In Female Rats Research Articles

Latent inhibition is affected by phase of estrous cycle in female rats

Estrogen has been shown to have a strong modulatory influence on several types of cognition in both women and female rodents. Latent inhibition is a task in which pre-exposure to a neutral stimulus, such as a tone, later impedes the association of that stimulus with a particular consequence, such as a shock. Previous work from our lab demonstrates that high levels of estradiol (E2) administered to ovariectomized (OVX) female rats abolishes latent inhibition when compared to female rats with low levels of E2 or male rats. To determine if this E2-induced impairment also occurs with the natural variations of ovarian hormones during the estrous cycle, this behavior was investigated in cycling female rats. In addition, pre-pubertal male and female rats were also tested in this paradigm to determine if the previously described sex differences are activational or organizational in nature. In a latent inhibition paradigm using a tone and a shock, adult rats were conditioned during different points of the estrous cycle. Rats conditioned during proestrus, a period of high E2 levels, exhibited attenuated latent inhibition when compared to rats conditioned during estrus or metestrus, periods associated with low levels of E2. Moreover, this effect is not seen until puberty indicating it is dependent on the surge of hormones at puberty. This study confirms recent findings that high E2 interferes with latent inhibition and is the first to show this is based in the activational actions of hormones.

Working and reference memory across the estrous cycle of rat: A long-term study in gonadally intact females

The results of many studies conducted over the past two decades suggested a role of estrogen on mammal's ability to learn and remember. In the present paper, we analyzed the influence that the endogenous fluctuation of estrogen, naturally present across the different phases of estrous cycle of female rats, can exert over the performance of tasks utilized to assess memory. In particular, we analyzed the performances in an eight arms radial maze task, dependent upon working memory, and in a water maze (WM) task, dependent upon spatial reference memory. The water maze is aversively motivated by the desire to escape onto a safe platform, whereas the radial arm maze (RAM) is motivated by food reward. The difference in reinforcement may affect the speed of learning, the strategy adopted and the necessity for accurate navigation. Therefore, coherent results obtained through the two different tasks can be due to mnemonic factors. The study was conducted during a long period of time, 14 months, utilizing gonadally intact females, without pharmacological and surgical treatments. In order to evaluate the post-acquisition phase we first trained the animals to reach the criterion in performing tasks, and then we submitted them to experimental phase. Our results show that estrogen can have an effect on memory processes, and that this effect may be different in relation to different kinds of memory. In fact, in our study, estrogen selectively improved working memory, but not reference memory, during post-acquisition performance of a RAM task with four baited and four un-baited arms. Moreover, WM performances showed that estrogen have a negative effect on spatial reference memory.

Differential Activation of the Periaqueductal Gray by Mild Anxiogenic Stress at Different Stages of the Estrous Cycle in Female Rats

The effect of acute exposure to mild anxiogenic stress on cutaneous nociceptive threshold was investigated in female Wistar rats at different stages of the estrous cycle. Baseline tail flick latencies did not change significantly during the cycle. However after brief exposure to vibration stress (4 Hz for 5 min), rats in late diestrus, but not at other cycle stages, developed a hyperalgesia (decrease in tail flick latency). Animals in late diestrus revealed a more than fivefold increase in the density of Fos-like immunoreactive nuclei in the dorsolateral, lateral, and ventrolateral columns in the caudal half of the periaqueductal gray matter (PAG). There was no change in the density of Fos-like immunoreactive nuclei in the PAG in rats in estrus and early diestrus, although rats in proestrus showed a smaller (50%) but significant increase. Rats undergoing withdrawal from a progesterone dosing regimen (5 mg/kg i.p. twice daily for 6 days) designed to mimic the fall in progesterone that occurs naturally during late diestrus, exhibited a stress-induced hyperalgesia that was similar to animals in late diestrus and a significant increase in Fos-positive cells in the PAG. We suggest that falling levels of progesterone during late diestrus may be a predisposing factor for the development of stress-induced hyperalgesia, which is linked to differential activation of descending pain control circuits in the PAG. Similar changes in women, when progesterone levels fall during the late luteal phase of the menstrual cycle, may contribute to the development of premenstrual symptoms that include increased anxiety and hyperalgesia.

Effects of Nanpao, a Kampo Medicine, on the Decline in Estrous Cyclicity with Advancing Age in Female Rats, as Measured by Vaginal Impedance Methods

The present study was designed to evaluate the time-dependent effects of Nanpao, a kampo medicine, on age-related changes in the estrous cycle of female rats, and to investigate the utility of measuring electrical impedance in the vagina (EIV) for studying transitional changes in the estrous cycle. Rats were allocated to 3 groups: control, Nanpao 30 mg/kg/day, and 100 mg/kg/day groups. EIV measurements and cytology samples were taken for 14 days at the age of 6 months before the initial treatment. After the start of the treatment, these data were collected at about monthly intervals until the age of 10 months in the same manner. Observations at the ages of 7 (weeks 2-3 of dosing) and 8 months (weeks 6-7 of dosing) showed that loss of a regular estrous cycle in the 100 mg/kg/day group was inhibited as compared to the control group. Moreover, at the ages of 9 (weeks 11-12 of dosing) and 10 months (weeks 17-18 of dosing), these effects were identified not only in the 100 mg/kg/day group, but also in the 30 mg/kg/day group. Since vaginal cytology and EIV gave almost concordant results as indicators of estrous cyclicity, we concluded that the measurement of EIV was capable of detecting time-dependent changes in the estrous cycle as well as observations of vaginal smears. A short period of Nanpao administration inhibited loss of regular estrous cycles, and the EIV method is a worthwhile approach to a more precise study of estrous cyclicity in rats exhibiting abnormal estrous cycles.

Estrogen receptor alpha and beta specific agonists regulate expression of synaptic proteins in rat hippocampus

Changes in hippocampal CA1 dendritic spine density and synaptic number across the estrous cycle in female rats correlate with increased hippocampal-dependent cognitive performance in a manner that is dependent on estrogen receptors (ERs). Two isoforms of the estrogen receptor, α and β are present in the rat hippocampus and distinct effects on cognitive behavior have been described for each receptor. The present study generated a profile of synaptic proteins altered by administration of estradiol benzoate, the ERα selective agonist PPT (1,3,5-tris (4-hydroxyphenyl)-4-propyl-1H-pyrazole) and the ERβ selective agonist DPN (2,3-bis (4-hydroxyphenyl) propionitrile) alone and in combination in comparison to vehicle in the CA1 region of the dorsal hippocampus. In the stratum radiatum, estradiol, DPN, and PPT increased PSD-95 and AMPA-type glutamate receptor subunit GluR1. Only DPN administration regulated expression of AMPA receptor subunits GluR2 and GluR3, increasing and decreasing levels respectively. DPN also increased GluR2 expression in the other lamina of the CA1. These results support previous reports that estradiol and isoform specific agonists differentially activate ERα and ERβ to regulate protein expression. The distinct effects of DPN and PPT administration on synaptic proteins suggest that the desired therapeutic outcome of estrogen may be accomplished by using specific estrogen receptor agonists. Moreover, the effects of estradiol treatment on PSD-95 expression are consistent with a growing body of evidence that this postsynaptic protein is a key marker of estrogen action related to spine synapse formation.

Hippocampal and striatal dependent navigation: Sex differences are limited to acquisition

Estrogen has been demonstrated to enhance the use of hippocampal-based place learning while reducing the use of striatal-based motor-response strategy (Korol, D.L., Malin, E.L., Borden, K.A., Busby, R.A., & Couper-Leo, J. (2004). Shifts in preferred learning strategy across the estrous cycle in female rats. Horm. Behav. 45, 330–338). Previous research has focused on task acquisition and the switch from a place to motor-response navigation with training. The current paradigm allowed an examination of the interplay between these two systems by having well-trained animals switch strategies “on demand.” Female and male Sprague–Dawley rats were taught a motor-response task on a plus maze. The rats were then introduced to a place task and taught to switch, by cue, from the motor-response to place strategy. Finally, the rats were trained to continuously alternate between place and motor-responses strategies. The maze configuration allowed for an analysis of cooperative choices (both strategies result in the same goal arm), competitive choices (both strategies result in different goal arms), and single strategy choices (can only use the motor-response strategy). The results indicate that sex and estrogen-related effects on navigation strategy are limited to the initial stages of learning a task. The role of sex and estrogen is diminished once the task is well learned, and presumably, the relative involvement of the hippocampal and striatal systems is established.

Role of naloxone as an exogenous opioid receptor antagonist in spatial learning and memory of female rats during the estrous cycle

Opioid neurotransmitters play a modulatory role in learning and memory processing. Their levels fluctuate throughout the reproductive cycle. The purpose of this research described herein was to study the effects of an exogenous opioid receptor antagonist, naloxone, on spatial learning and memory during the estrous cycle in female rats. Proestrus and estrus female rats were trained in a Y-maze. After a 4-hour delay, spatial recognition memory was assessed. The rats were administered naloxone (2 mg/kg) or saline before training, after training or before the retention test. The administration of naloxone to the estrus and proestrus rats before and after training had no significant effects on their preference for the novel arm of the Y-maze. Injection of naloxone to estrus rats before the retention test enhanced their preference for the novel arm of the Y-maze, whereas, in the case of the proestrus rats, naloxone decreased their preference for the novel arm. Therefore, it can be concluded that naloxone enhances the retrieval of spatial recognition memory in the estrus phase rat, but that it impairs retrieval in the proestrus phase rat. This finding indicates that there is an interaction between ovarian hormone levels and opioids in cognitive function, so that naloxone prevents the facilitatory role of ovarian hormones in the retrieval of spatial memory.

Fluctuation of latent inhibition along the estrous cycle in the rat: Modeling the cyclicity of symptoms in schizophrenic women?

Latent inhibition (LI) is a cross-species selective attention phenomenon manifested as poorer conditioning of stimuli that had been experienced as irrelevant prior to conditioning. Disruption of LI by pro-psychotic agents such as amphetamine and its restoration by antipsychotic drugs (APDs) is a well-established model of psychotic symptoms of schizophrenia. There is evidence that in schizophrenic women symptom severity and treatment response fluctuate along the menstrual cycle. Here we tested whether hormonal fluctuation along the estrous cycle in female rats (as determined indirectly via the cellular composition of the vaginal smears) would modulate the expression of LI and its response to APDs. The results showed that LI was seen if rats were in estrus during pre-exposure stage and in metestrus during the conditioning stage of the LI procedure (estrus-metestrus) but not along the remaining sequential phases of the cycle (metestrus-diestrus, diestrus-proestrus and proestrus-estrus). Additionally, the efficacy of typical and atypical APDs, haloperidol and clozapine, respectively, in restoring LI depended on estrous condition. Only LI disruption in proestrus-estrus exhibited sensitivity to both APDs, whereas LI disruption in the other two phases was alleviated by clozapine but not haloperidol. Our results show for the first time that both the expression of LI and its sensitivity to APDs are modulated along the estrous cycle, consistent with fluctuations in psychotic symptoms and response to APDs seen along women's menstrual cycle. Importantly, the results indicate that although both low and high levels of hormones may give rise to psychotic-like behavior as manifested in LI loss, the pro-psychotic state associated with low hormonal level is more severe due to reduced sensitivity to typical APDs. The latter constellation may mimic states of increased vulnerability to psychosis coupled with reduced treatment response documented in schizophrenic women during periods associated with low levels of hormones.

Disrupted female reproductive physiology following neonatal exposure to phytoestrogens or estrogen specific ligands is associated with decreased GnRH activation and kisspeptin fiber density in the hypothalamus

It is well established that estrogen administration during neonatal development can advance pubertal onset and prevent the maintenance of regular estrous cycles in female rats. This treatment paradigm also eliminates the preovulatory rise of gonadotropin releasing hormone (GnRH). It remains unclear, however, through which of the two primary forms of the estrogen receptor (ERα or ERβ) this effect is mediated. It is also unclear whether endocrine disrupting compounds (EDCs) can produce similar effects. Here we compared the effect of neonatal exposure to estradiol benzoate (EB), the ERα specific agonist 1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT), the ERβ specific agonist diarylpropionitrile (DPN) and the naturally occurring EDCs genistein (GEN) and equol (EQ) on pubertal onset, estrous cyclicity, GnRH activation, and kisspeptin content in the anteroventral periventricular (AVPV) and arcuate (ARC) nuclei. Vaginal opening was significantly advanced by EB and GEN. By 10 weeks post-puberty, irregular estrous cycles were observed in all groups except the control group. GnRH activation, as measured by the percentage of immunopositive GnRH neurons that were also immunopositive for Fos, was significantly lower in all treatment groups except the DPN group compared to the control group. GnRH activation was absent in the PPT group. These data suggest that neonatal exposure to EDCs can suppress GnRH activity in adulthood, and that ERα plays a pivotal role in this process. Kisspeptins (KISS) have recently been characterized to be potent stimulators of GnRH secretion. Therefore we quantified the density of KISS immunolabeled fibers in the AVPV and ARC. In the AVPV, KISS fiber density was significantly lower in the EB and GEN groups compared to the control group but only in the EB and PPT groups in the ARC. The data suggest that decreased stimulation of GnRH neurons by KISS could be a mechanism by which EDCs can impair female reproductive function.

Neuronal somatic volume of posteroventral medial amygdala cells from males and across the estrous cycle of female rats

The posteroventral medial amygdala (MePV) is a brain area where gonadal hormones have neurotrophic effects in rats. The aim of the present study was to estimate the MePV neuronal somatic volume from males and diestrus, proestrus and estrus female Wistar rats (n=5 in each group) in an attempt to identify a possible sexual dimorphism in this parameter. The effect of laterality was also evaluated. The brains of adult animals were sectioned (1 microm), stained with 1% toluidine blue and serial-section reconstructions of each neuronal cell body were obtained. Images from both left and right MePV were studied and the somatic volume was estimated using the Cavalieri method in combination with the point counting technique. Results were compared according to sex and phase of the estrous cycle using a two-way ANOVA for repeated measures followed by the least significance difference test. Mean neuronal somatic volume showed a statistical difference among groups and the post hoc comparisons revealed that males present higher values than females in proestrus and estrus (p<0.05). On the other hand, neither a laterality effect (p=0.6) nor an interaction between groups and laterality (p=0.4) were found. Our results indicate that cell body volume in the MePV is distinct when comparing males to females in the different phases of the estrous cycle. Through dynamic changes modulated by sex steroids, it is likely that this morphological plasticity within the MePV may be affecting the functioning of local neurons and their integrated roles in neural circuits relevant for neuroendocrine control and reproductive behaviors.

Expression of brain prolactin releasing peptide (PrRP) changes in the estrous cycle of female rats

Prolactin releasing peptide (PrRP) is a neuropeptide with 31 or 20 amino acid residues and regarded as a potent and specific stimulator of pituitary prolactin. PrRP immunoreactive (PrRP-ir) neurons and mRNA are found in medulla oblongata and hypothalamus and the fibers containing PrRP are widely distributed in rat brains. Therefore, it is postulated that PrRP might act as a neurohormone or a neurotransmitter as well as a neuromodulator in the brain. In the present study, we probed the expression of brain PrRP in the estrous cycle of female rats and the relationship between brain PrRP and GnRH. Female rats were divided into four groups: the diestrus, the proestrus, the estrus and the metaestrus, which were identified by the vaginal cytological examination. Immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescent double labeling histochemistry combining confocal laser scanning microscope (CLSM) were used. The results showed that PrRP immunoreactive neurons in nucleus of solitary tract (NTS) and ventrolateral reticular nucleus (VLRN) in the proestrus were less than those in the diestrus, the estrus and the metaestrus. Similarly, the relative optical density of PrRP-ir fibers of the bed nucleus of stria terminalis (BST) in the proestrus was decreased compared with those in other three groups. However, the brain PrRPmRNA level was higher in the proestrus and estrus than those in the metaestrus and diestrus. We also observed the co-localization of GPR10-immunoreactive (GPR10-ir) and GnRH-immunoreactive (GnRH-ir) neurons in hypothalamic medial preoptic area (MPO). The present results provide morphological evidences that PrRP in the female rat brains might participate in the regulation of the rat estrous cycle at least in a direct way.

EFFECT OF XYLAZINE AND YOHIMBINE ON THE PHASIC PAIN DURING THE ESTROUS CYCLE IN THE RAT

*Summary The aim of the present study was to investigate the effect of α2-adrenergic agonist (xylazine) and antagonist (yohimbine) on phasic pain during estrous cycle in female rats. Adult female rats weighing 180220 g were kept under controlled temperature (21-24°C) and light/dark conditions (light on at 6:00 a.m. and light off at 6:00 p.m.). Animals were divided into four groups: 1) control group which received 0.3 ml of normal saline by intraperitoneal (IP) route; 2) IP experimental group which received 0.3 ml xylazine 3, 4.5 and 6 mg/kg and yohimbine 1, 2 and 4 mg/kg by IP route; 3) sham group which received 2 µl of artificial cerebrospinal fluid by intra cerebral ventricle (ICV) route and 4) ICV experimental which received 2 µl xylazine 10 and 20 µg/rat and yohimbine 5 and 10 µg/rat by ICV route. Cannulae were implanted into the left lateral ventricle using stereotaxic method. Pain sensitivity was measured by tail flick test, which was performed before injection, 15 and 30 min after injection in all groups. Xylazine decreased pain sensitivity significantly (P<0.05) during the estrous cycle; while higher analgesia was observed in the proestrus phase for IP and ICV routes. Yohimbine increased pain sensitivity significantly (P<0.05) during the estrous cycle; while higher hyperalgesia was observed in the metestrus phase for IP and ICV route groups. There was interaction (P<0.05) between endogenous steroids and the α2-adrenergic system in the modulation of phasic pain sensitivity.

The variation of nitric oxide synthase positive neurons in the medial amygdaloid nuclei during the estrous cycle in female rats

Estrogen receptor-β mRNA expression and estrogen positive neurons were found in the medial amygdaloid nuclei (ME), and neuronal nitric oxide synthase (nNOS) positive neurons were found in the amygdala in rats. We hypothesized that the expression of nNOS in the amygdala is associated with the estrogen level and exhibits periodicity during estrous cycle in female rats. 40 rats were divided into preoestrus, estrus, metoestrus, diestrus and control groups. The nNOS positive neurons were found in most amygdala regions. The ME contained a large number of densely stained and medium-sized (20~35μM) pyramidal cells. No nNOS positive structures were found in the control. The nNOS stain was stronger in the cytoplasm of neurons in preoestrus and metoestrus groups than in estrus and diestrus groups during the estrus cycle. The nNOS positive prominences were shorter in the ME than in other nuclei of the amgdala during whole estrous cycle. Most nNOS positive neurons were found in preoestrus groups (22.03±2.99), and some nNOS positive neurons were found in metoestrus groups (21.73±3.17), and few nNOS positive neurons were found in estrus groups (7.33±1.42) and diestrus groups (7.10±1.81). The number of nNOS positive neurons in preoestrus was higher than in estrus (p<0.01) and diestrus (p<0.01), and the number of nNOS positive neurons in metoestrus was higher than in estrus (p<0.01) and diestrus (p<0.01). The results indicate that the distribution of nNOS positive neurons in the ME varies with the estrogen level during whole estrus cycle. The estrogen level upregulates the amount of nNOS positive neurons in the ME. Interaction of estrogen and nitric oxide is involved in the neuroendocrine and sexual activity modulated partly by the amygdala.

Relation of sex and estrous phase to deficits in prepulse inhibition of the startle response induced by ecstasy (MDMA)

Sensorimotor gating is the ability of a weak sensory event to inhibit the motor response to an intense stimulus. Drugs that act as serotonin releasers, such as MDMA (3,4-methylenedioxymethamphetamine), impair sensorimotor gating, which is measured as a prepulse inhibition (PPI) of the acoustic startle response. The first objective of the present study was to compare the effect of different doses of MDMA on PPI and the acoustic startle response (ASR) in male and female Wistar rats. The second objective was to examine the effect of MDMA on PPI across the estrous cycle in female rats. MDMA was administered in doses of 2.5, 5 and 10 mg/kg s.c. 15 min before the start of the experiment. The controls received saline in equivalent volumes. MDMA dose-dependently decreased PPI in both the male and female rats and produced higher levels of ASR in the male rats compared to the females. In addition, we found that female rats in the diestrous and metestrous phases are more sensitive to MDMA and showed higher deficits in PPI than female rats in the proestrous and estrous phases. Our result showed that female rats in the proestrous and estrous phases were less sensitive to the disruption of PPI by MDMA.

Suppressive effect of neonatal treatment with a phytoestrogen, coumestrol, on lordosis and estrous cycle in female rats

The neural control systems for the ovulatory cycle and lordosis behavior are sexually differentiated by estrogen during the perinatal period in rats. In the present study, the effects of a single neonatal injection with the phytoestrogen, coumestrol, on female reproductive functions were investigated. Female rats were injected subcutaneously with 1 or 3 mg coumestrol (CM1, CM3), 1 mg genistein (GS1), 1 mg estradiol (E 2), or oil at day 5 after birth (birth day = day 1) and an estrous cycle check and lordosis behavior test were performed. As a result, vaginal opening was advanced in CM1-, CM3- or E 2-treated females. A vaginal smear check indicated that oil- or GS1-treated females showed a constant 4- or 5-day estrous cycle, whereas CM1-, CM3- or E 2-treated rats showed a persistent or prolonged estrus. Ovariectomy was performed in all females at 60 days of age. The ovary weights in the CM1-, CM3- or E 2-treated groups were lower than those in the oil- and GS1-treated groups and no corpora lutea were found in any rats of these three groups, except for two E 2-treated rats. Behavioral tests were carried out after implantation of E 2-tubes. All rats in the CM1-, GS1-treated groups showed a high lordosis quotient (LQ), being comparable to that in the oil-treated females. On the other hand, LQs in the CM3, E 2 or male groups were lower than that in the control female group. These results suggest that a single neonatal injection of 3 mg coumestrol was effective in suppressing the functions of ovulation-inducing mechanisms and the induction of lordosis, but 1 mg coumestrol was effective in only the estrous cycle of female rats.

Shifts in preferred learning strategy across the estrous cycle in female rats

The current status of the effects of ovarian steroids on learning and memory remains somewhat unclear, despite a large undertaking to evaluate these effects. What is emerging from this literature is that estrogen, and perhaps progesterone, influences learning and memory, but does so in a task-dependent manner. Previously, we have shown that ovariectomized rats given acute treatments of estrogen acquire allocentric or “place” tasks more easily than do rats deprived of estrogen, but acquire egocentric or “response” learning tasks more slowly than do those deprived of hormone, suggesting that estrogen treatment may bias the strategy a rat is able to use to solve tasks. To determine if natural fluctuations in ovarian hormones influence cognitive strategy, we tested whether strategy use fluctuated across the estrous cycle in reproductively intact female rats. We found that in two tasks in which rats freely choose the strategy used to solve the task, rats were more likely to use place strategies at proestrous, that is, when ovarian steroids are high. Conversely, estrous rats were biased toward response strategies. The data suggest that natural fluctuations in ovarian steroids may bias the neural system used and thus the cognitive strategies chosen during learning and memory.

Impaired hypothalamic regulation of reproductive function when exposed to neurotoxic compounds and melatonin

Diurnal rhythms of the contents of biogenic amines in the hypothalamus are considered to play an important role in regulation of many functions such as motor activity, feeding behaviour, sleep-awake cycle formation as well as hormonal and immune statuses. Mechanisms of regulation of reproduction that participate in a circadian signal transmission from the suprachiasmatic nuclei (SCN) of the hypothalamus to hypothalamic structures responsible for gonadoliberin (GnRH) synthesis and secretion (the preoptic area - PA, and the medial eminence - ME) are at present being intensively studied. It has been shown that the aging caused disturbances of estrous cycles in female rats are accompanied by a disappearance of diurnal rhythms of activity of monoaminergic and opioid hypothalamic systems playing a key role in regulation of GnRH synthesis and secretion.&#x0D; The experiments carried out have revealed diurnal rhythms of the contents of the studied biogenic amines in the PA, ME and SCN of the hypothalamus. The rhythms are due to the amines increasing in the morning hours when compared to their evening and night levels. The neurotoxic compounds used disturbed the rhythms of the amines. Chronic inhalation of toluene caused a disappearance of the dynamics of all the studied neurotransmitters that was typical of the control group. Meanwhile, a single administration of 1,2-dimethylhydrazine resulted in either a complete disappearance of the diurnal rhythms of the biogenic amines contents found in control, or their phase shifts. Severe disturbances of the GnRH content and of the ROS diurnal dynamics were also observed in the studied hypothalamic areas.&#x0D; The literature allowed supposition that melatonin possessing entraining and antioxidant properties can be used as a protector of disturbances of central mechanisms of regulation of reproduction. However, our experiments have shown that exogenous melatonin does not only eliminate the neurotoxic compounds caused disturbances of diurnal rhythms of the studied indices, but it itself causes deep changes in their dynamics. The effect of melatonin has been found to result in a significant decrease of the catecholamines contents in all the studied hypothalamic structures, particularly of the norepinephrine content, that is accompanied by a profound decrease of the GnRH content and by a disturbance of the diurnal dynamics of reactive oxygen species (ROS) in the PA and ME. The effect of 1,2-dimethylhydrazine on the studied indices was less pronounced when compared to that of melatonin, whereas an injection of the xenobiotic on the background of the pineal gland hormone administration resulted in a predominant effect of the latter.&#x0D; Both the data obtained and the literature review allow to offer a hypothesis for a possible mechanism of involvement of diurnal rhythms of biogenic amines in central regulation of reproduction. It is known that synthesis and secretion of endogenous opioid peptides in the mediobasal hypothalamus undergo diurnal oscillations with minimum in the morning and maximum in the evening and/or at night. The revealed diurnal rhythms of the biogenic amines contents had opposite oscillations. This fact allows a supposition that these rhythms are due to the opioid system activity changing. Both decreased endogenous opioids contents and simultaneously increased morning biogenic amines levels in proestrus can create a neurotransmitter background that is necessary for GnRH surge formation.&#x0D; An evaluation of diurnal rhythms of the biogenic amines contents, GnRH and ROS in the studied hypothalamic structures can be used for an assessment of disturbances of central mechanisms of regulation of reproduction that are caused by other neurotoxic compounds or developed in some diseases and in aging when the functional activity of the pineal gland is altered. The data obtained testify to possible negative side effects of melatonin on hypothalamic regulation of reproduction in therapeutic use of the hormone. The effects of melatonin realized via its receptors are known to highly depend on amplitude of its nocturnal surge and on duration of its increased blood level during a day. Therefore administration of melatonin on the background of its normal synthesis can lead to a number of negative effects due to its increased blood level when compared to its physiological level. Elucidation of the exogenous melatonin caused disturbance of the studied diurnal rhythms of central regulation of reproduction is of great practical use in development of prophylactics helping to eliminate or decrease an effect of damaging factors in unfavourable environmental conditions.

Functional expression of the dopamine-activated K(+) current in lactotrophs during the estrous cycle in female rats: correlation with prolactin secretory responses.

It is well established that hypothalamic dopamine (DA) is the major physiologic regulator of prolactin (PRL) secretion, exerting a tonic inhibition throughout most of the estrous cycle. A dramatic drop in the amount of DA perfusing the anterior pituitary occurs in the afternoon of proestrus and is critical for the production of the surge of PRL that occurs at that time. In my laboratory, we have identified and characterized a DA-activated K(+) channel (K(DA)) in lactotrophs derived from proestrous rats that underlies DA-induced membrane hyperpolarization of lactotrophs. We have also demonstrated that this hyperpolarization plays a critical role in both the inhibition of PRL release from proestrous cells and the PRL secretory rebound that occurs following DA withdrawal. We now report that the ability of DA to activate the K(DA) channel and elicit hyperpolarization in primary lactotrophs changes dramatically during the estrous cycle. Lactotrophs isolated from cycling female rats were studied using whole-cell voltage clamp. DA (1 microM) elicited a robust membrane K(+) current in the majority of proestrous lactotrophs (86%; 24.0 +/- 2.9 pA). By contrast, DA activated a considerably smaller membrane current (3.3 pA) in very few lactotrophs isolated from rats on either diestrus or estrus (8 and 0%, respectively). Using a perifusion system to examine temporal patterns of PRL release, we found that following application and withdrawal of DA, proestrous cells produced a robust secretory rebound, but diestrous and estrous cells did not. However, DA inhibited PRL release to the same extent regardless the stage of the cycle from which the cells were derived. These data are consistent with the presence of multiple DA effectors in lactotrophs and demonstrate that their relative importance shifts dramatically with changes in the endocrine status of the animal. We propose that the DA-activated K(+) channel (K(DA)) is a critical effector governing the unique secretory profile of PRL observed in proestrous animals.

Pineal melatonin synthesis and release are not altered throughout the estrous cycle in female rats.

Melatonin times reproduction with seasons in many photoperiodic mammalian species. Whether sexual hormones reflect on melatonin synthesis is still debated. The aim of this work was to study, using a large panel of technical approaches, whether the daily profile of pineal melatonin synthesis and release varies with the estrous cycle in the female rat. The mRNA levels and enzyme activities of the melatonin synthesizing enzymes, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase were similar at the four stages of the rat estrous cycle. The endogenous release of melatonin, followed by transpineal microdialysis during six consecutive days in cycling female rats, displayed no significant variation during this interval. Taken together, the present results demonstrate that there is no regular fluctuation in the pineal metabolism leading to melatonin synthesis and release throughout the estrous cycle in female rats.

The hormonal responses of lipoprotein lipase activity and lipolysis in adipose tissue differ depending on the stage of the estrous cycle in female rats.

This study was designed to elucidate whether there were differences in the hormonal responses of the parameters involving triacylglycerol (TG) deposition and mobilization in adipose tissue among the stages of the estrous cycle in female rats. Adipose tissue was obtained from the parametrial region in female rats at each stage of the estrous cycle. Lipoprotein lipase (LPL) activity in the extracts of acetone/ether powders of the tissues was measured as a parameter for TG deposition. Norepinephrine-stimulated lipolysis in isolated fat cells was measured as a parameter for TG mobilization. LPL activity changed periodically during the estrous cycle; the activity level was highest at diestrus, began to decrease at proestrus, reached a minimum at estrus, began to increase again at metestrus-1, and increased further at metestrus-2. At diestrus and proestrus, LPL activity was increased with an increase in plasma insulin levels, suggesting that plasma insulin was the predominant up-regulator of LPL. But at estrus, metestrus-1 and metestrus-2, LPL activity remained low even when plasma insulin levels were high, indicating that it was not up-regulated by plasma insulin. Norepinephrine-stimulated lipolysis in fat cells was high at estrus and metestrus-1 and low at diestrus. The hormonal responses of LPL activity and lipolysis in adipose tissue differed depending on the stage of the estrous cycle.