Mating-induced analgesia is dependent of copulatory male pattern in high- and low- yawning male rats.

Mating behavior in rodents can modulate pain sensations in both sexes. In males, the execution of mounts, intromissions, and ejaculations induced a progressive increase in their vocalization thresholds induced by tail shocks and other types of noxious stimuli. We selectively inbred two sublines from Sprague-Dawley (SD) rats that differed in their spontaneous yawning frequency. The high-yawning (HY) subline had a mean of 20 yawns/h and a different pattern of sexual behavior characterized by longer interintromission intervals and more sexual bouts that delayed ejaculation. The low-yawning (LY) subline and SD rats yawned as a mean 2 and 1 yawns/h, respectively. So, we determine mating-induced analgesia in HY, LY, and SD male rats by measuring vocalization thresholds in response to noxious electric tail shocks. Our results showed that the magnitude of mating-induced analgesia was lower in HY and LY rats with respect to SD rats. When the rats performed different components of male sexual pattern, both sublines exhibited a significantly lower increase in their vocalization thresholds with respect to SD rats-being sublines less responsive regarding mating-induced analgesia. Pain modulation mechanisms depend on responses to stress, so the low levels of analgesia obtained in the yawning sublines may be due either to differences in their response to stress in other paradigms, or to atypical performance of male sexual behavior during mating, an event which as a stressful event in rats. Therefore, the yawning sublines are a suitable model for analyzing how a different temporal pattern in the display of male sexual behavior affects analgesia mechanisms. Our results concur with Wistar rats with different endophenotypes that could apply to humans as well.

Threshold for copulation-induced analgesia varies according to the ejaculatory endophenotypes in rats.

Analgesia may be modulated by multiple internal and external factors. In prior studies, copulatory-induced analgesia was demonstrated using the vocalization threshold to tail shock (VTTS) in male and female rats. Three ejaculatory endophenotypes have been characterized in male Wistar rats based upon their ejaculation latency (EL). Since intromissions and ejaculations produce analgesia, and these copulatory patterns are performed with different frequency depending on the male's ejaculatory endophenotype, we hypothesized that copulation-induced analgesia would vary in relation to these endophenotypes. In the present study, we used three groups according to the EL (medians): rapid ejaculators (236 s; n = 21), intermediate ejaculators (663.2 s; n = 20) and sluggish ejaculators (1582.2 s; n = 8). Our aim was to evaluate whether copulation-induced analgesia is related to the ejaculatory endophenotypes during two consecutive ejaculatory series (EJS). In the first EJS, the VTTS of the rapid ejaculators was significantly higher than that of intermediate and sluggish rats. At the onset of the second EJS, the VTTS of the rapid and intermediate ejaculators was significantly higher than that of the sluggish rats. No differences in VTTS were observed during the first or second post-ejaculatory intervals among the three groups. These findings provide evidence that the more intromissions that occurred per unit time, the higher was the level of analgesia.

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.

Increasing the antinociceptive effect of ingested glycinamide in female rats by increasing its palatability.

BACKGROUND: These studies were undertaken to investigate whether the ingestion of glycinamide, a precursor of glycine, made more palatable by mixing with a chocolate suspension, improves antinociception in rats. METHODS: Two nociception threshold models were employed: the tail-flick latency and vocalization to tail shock, in restricted and freely-moving rats. Glycinamide in a highly palatable commercial chocolate aqueous suspension was provided for ad-lib ingestion after 24 hours of water deprivation. Antinociception threshold testing was performed before and 10, 30, 60, 90, 120, 180, and 240 minutes after the ingestion of the chocolate-glycinamide mixture. RESULTS: Ingestion of the glycinamide-in-chocolate suspension induced antinociception based on the tail shock vocalization and tail-flick latency tests. Ingestion of the glycinamide-in-chocolate suspension induced an 80% elevation in the antinociceptive threshold that persisted for 4 hours. CONCLUSIONS: Rats readily ingest the glycine precursor, glycinamide, in an aqueous chocolate mixture, which induces potent and prolonged antinociception.

Protein kinase inhibitors infused intraventricularly or into the ventromedial hypothalamus block short latency facilitation of lordosis by oestradiol.

An injection of unesterified oestradiol (E2 ) facilitates receptive behaviour in E2 benzoate (EB)-primed, ovariectomised female rats when it is administered i.c.v. or systemically. The present study tested the hypothesis that inhibitors of protein kinase A (PKA), protein kinase G (PKG) or the Src/mitogen-activated protein kinase (MAPK) complex interfere with E2 facilitation of receptive behaviour. In Experiment 1, lordosis induced by i.c.v. infusion of E2 was significantly reduced by i.c.v. administration of Rp-cAMPS, a PKA inhibitor, KT5823, a PKG inhibitor, and PP2 and PD98059, Src and MAPK inhibitors, respectively, between 30 and 240 minutes after infusion. In Experiment 2, we determined whether the ventromedial hypothalamus (VMH) is one of the neural sites at which those intracellular pathways participate in lordosis behaviour induced by E2 . Administration of each of the four protein kinase inhibitors into the VMH blocked facilitation of lordosis induced by infusion of E2 also into the VMH. These data support the hypothesis that activation of several protein kinase pathways is involved in the facilitation of lordosis by E2 in EB-primed rats.

Copulation-induced antinociception in female rats is blocked by atosiban, an oxytocin receptor antagonist.

AIMS: We hypothesized that copulation-induced temporary anti-nociception in female rats is mediated by the activation of central and/or peripheral oxytocin receptors. To test this hypothesis, we assessed the effects of intraperitoneal (ip), intrathecal (it), and intra-cerebroventricular (icv) administration of an oxytocin receptor antagonist (atosiban), on copulation-induced temporary anti-nociception in estrous rats. MAIN METHODS: The treatment groups were ovariectomized rats pre-treated subcutaneously (sc) with 10 µg of estradiol benzoate (EB) followed 24 h later by an sc injection of 5 µg EB, and 4 h later, by an sc injection of 2 mg progesterone (P4). Rats were then administered saline vehicle (ip, it, or icv: control groups) or atosiban (500 µg/kg ip; 500 ng it; or 500 ng icv: experimental groups). Thirty minutes after drug or saline administration, their sexual behavior was tested by pairing with a sexually-experienced male rat. Brief pulse trains of 50 Hz, 300 ms duration, supra-threshold tail electrical shocks (STS) were delivered before and during copulatory activity i.e., while the female was receiving mounts, intromissions, or ejaculations, and we recorded whether vocalization occurred in response to each STS. KEY FINDINGS: Replicating our previous findings, the vocalization response to STS in control rats was significantly attenuated during intromissions and ejaculations, compared to their baseline (pre-mating) response, indicative of anti-nociception. By contrast, rats pre-treated with atosiban (each route of administration) failed to show an attenuation of the vocalization response to shock. SIGNIFICANCE: These findings provide evidence that the temporary anti-nociceptive effect of copulation in female rats is mediated by copulation-induced release of endogenous oxytocin in brain, spinal cord and periphery.

Sex hormone levels and expression of their receptors in lactating and lactating pregnant rats.

Parturient rats show a postpartum estrus, a period of sexual receptivity that occurs from 6 to 15 h after the birth of a litter, which allows the mother to gestate a second litter while simultaneously nursing the first one (lactating and pregnant). The present study investigated hormone levels and the expression pattern of estrogen receptor α, and ß, progesterone receptor isoforms and SRC1 in the hypothalamus and the preoptic area of lactating as well as in lactating-pregnant rats. In the latter, estradiol levels were 3-fold higher than those observed in lactating rats on day 14, meanwhile progesterone levels did not change in any condition. There were higher levels of prolactin in both lactating and lactating-pregnant rats on day 7 and decreased on the following days. In the hypothalamus of the lactating rat, the content of ERα increased during lactation meanwhile that of ERß decreased 50% on day 10. The content of both estrogen receptor subtypes in the hypothalamus increased 3-fold on day 21 in lactating-pregnant rats. In the preoptic area, the content of ERα was higher in lactating-pregnant rats on days 14 and 21 while the content of progesterone receptor isoforms was lower as compared with those found in lactating animals on days 7 and 10. The content of SRC1 increased 2-fold in the preoptic area only in lactating rats at day 14 and 21. These findings suggest that lactating- pregnant animals should exhibit differential neuroendocrine and molecular characteristics as compared to lactating animals.

Estrogen receptor α and ß are involved in the activation of lordosis behavior in estradiol-primed rats.

The present study was designed to assess the participation of estrogen receptors alpha (ERα) and beta (ERß) in the short-term facilitation of lordosis behavior in ovariectomized (ovx), estradiol (E2) primed rats. In experiment 1, dose response curves for PPT and DPN (ERα and ERß agonists, respectively) facilitation of lordosis behavior (lordosis quotient and lordosis score) were established by infusing these agonists into the right lateral ventricle (icv) in female rats injected 40h previously with 5µg of E2 benzoate. PPT doses of 0.08 and 0.4ng produced high lordosis quotients starting at 30min and continuing at 120 and 240min post-injection. DPN induced high levels of lordosis behavior at all times tested. However, the intensity of lordosis induced by both agonists was weak. In experiment 2, we tested the involvement of each ER in facilitation of lordosis by icv infusion of MPP (ERα-selective antagonist) or PHTPP (ERß-selective antagonist) prior to infusion of 2ng of free E2. Icv infusion of either MPP or PHTPP 30min before free E2 significantly depressed E2 facilitation of lordosis. The results suggest that both forms of ER are involved in the short-latency facilitation of lordosis behavior in E2-primed rats.

The importance of the chemical structure of pregnanes in the concurrent inhibition of estrous behavior in the female rat.

An investigation of aspects ranging from behavior to molecular electronic structure and physicochemical properties was performed to explore the role of 5α-pregnanedione (5α-DHP), 5ß-pregnanedione (5ß-DHP) and their precursor progesterone (P) on the concurrent inhibition of the sexual lordosis response in female rats. The concurrent inhibition of lordosis behavior occurs when ovariectomized rodents are primed simultaneously with estradiol (E2) and P. Thus, a second administration of P 40h later fails to induce the expected sexual response that takes place when E2 and P are administered sequentially 40h apart. In this study, it is hypothesized that the modulation of the sexual behavior display depends to some extent on the molecular structure and associated physicochemical properties of steroid hormones such as P and its metabolites. Therefore, these molecules must be studied chemically and structurally to explain their role in sexual behavior, including the concurrent inhibition effect. Analysis of the electronic structure and physicochemical properties demonstrated striking differences in the A-ring region of P, 5α-DHP and 5ß-DHP, particularly in atomic charges, dipole moment (DM) and electrostatic potentials. Similarly, the structural differences between the trans (5α-DHP) and cis (5ß-DHP) configurations were remarkable. 5α-DHP most significantly promoted the concurrent inhibition of the lordosis behavior, followed by P and 5ß-DHP. These data indicate that variations in pregnane structure are related to the extent of the concurrent inhibition effect and also suggest that P may act as a prehormone in certain functions of the central nervous system.

Sexual receptivity facilitated by unesterified estradiol: Dependence on estrogen and progestin receptors and priming dose of estradiol benzoate.

In some conditions, female sexual behavior in ovariectomized rats can be induced by continuous exposure of estradiol (E2) alone or by a single injection of a high dose of the long-lasting, esterified estradiol benzoate (EB). However, there are inconsistencies in the literature on the role of estrogens during priming or in the facilitation on female sexual behavior in EB-primed rats, as well as the cellular mechanisms involved. Either subcutaneous (sc) or intracerebral (icv) administration of some doses of free unesterified E2, induced lordosis in EB-primed rats. Either sc or icv injection of E2, immediately prior to testing, induced high levels of sexual receptivity when the female rats were primed with an EB sc injection of 2 µg EB. The roles of progesterone receptor (PR) and estrogen receptor on lordosis induced by sc or icv administration of E2 were explored. Tamoxifen or RU486 administrated sc or icv; each reduced lordosis induced by E2. Similarly, antisense oligonucleotides directed at PR-B or total PR (PR-A + PR-B) administrated icv immediately before EB injection inhibited lordosis induced by daily injections of EB. These results suggest that lordosis facilitated by free E2 is dependent on priming dose of EB. Furthermore both ERs and PRs are involved in this action of E2.

Lordosis facilitation by leptin in ovariectomized, estrogen-primed rats requires simultaneous or sequential activation of several protein kinase pathways.

The present study tested the hypothesis that the Janus kinase 2, Src tyrosine kinases, and mitogen-activated protein kinase interact to regulate lordosis behavior induced by leptin in ovariectomized, estrogen-primed rats. The role of protein kinase A and protein kinase C in lordosis facilitation by leptin was also assessed. In experiment 1, the intracerebroventricular administration of leptin to ovariectomized, estradiol-primed rats significantly stimulated lordosis behavior at 1, 2 and 4 h post-injection tests. In experiment 2, the Janus kinase 2 inhibitor AG490, the Src tyrosine kinase inhibitor PP2 and the mitogen-activated protein kinase inhibitor PD98059 were administered into the right lateral ventricle before leptin. The lordosis quotient and the lordosis score induced by leptin were significantly decreased by each of these kinase inhibitors. In experiment 3, we examined the effects of RpcAMPS and bisindolylmaleimide, protein kinase A and protein kinase C inhibitors on the lordosis elicited by leptin administration. Lordosis behavior induced by leptin was significantly decreased by both the protein kinase A and protein kinase C inhibitors at 1 h post-leptin injection. The results confirm that multiple intracellular pathways participate in the expression of lordosis behavior in estrogen-primed rats elicited by leptin.

Bimodal binding and free energy of the progesterone receptor in the induction of female sexual receptivity by progesterone and synthetic progestins.

Synthetic progestins (SPs) are used for regulation of fertility, contraception and hormone replacement therapy. The acetylated medroxyprogesterone (MPA), megestrol (MGA) and chlormadinone (CLA) are related to progesterone (P). Other SPs are 19-nortestosterone derivatives such as: norethisterone (NET), norethynodrel (NED) or the 13-ethyl gonane, levonorgestrel (LNG). We studied MPA, NET, NED and LNG in a dose-response manner to induce sexual receptivity in rats. Results showed that MPA, NET and NED act as partial agonists, with similar or lower potency than P. However, LNG is a full agonist. Additionally, the molecules of MPA, MGA, CLA, NET, NED, LNG, and P, were submitted to computer calculations at ab initio quantum mechanics theory, to obtain their electronic structure and molecular properties. The aim was to correlate their behavioral effect with their physicochemical properties. In addition, the crystals of P, NET and LNG bound to the progesterone receptor (PR) were studied. The PR crystallizes as a dimer forming two monomers (mA and mB), in which Gln725 interacts in either of two possible ways with the C3-carbonyl pharmacophore of progestins. P binds differentially to both PR monomers, while NET binds exclusively as mA and LNG binds only as mB in both monomers with no difference. Energetically, binding of LNG and P to mB, is more favorable than that of NET and P to mA. Consequently, this bimodal mechanism increases the action possibilities of SPs on biological systems. Interestingly, progestin potency depends mostly on local molecular structure and electronic features, prevailing over total molecular properties.

Src kinase signaling mediates estrous behavior induced by 5ß-reduced progestins, GnRH, prostaglandin E2 and vaginocervical stimulation in estrogen-primed rats.

The progesterone receptor (PR) is a dual function protein that acts in the nucleus as a transcriptional factor and at the cytoplasm as a scaffold for the Src-MAPK signaling pathway. Several agents lacking affinity for the PR, such as 5ß-reduced progestins, GnRH or prostaglandin E(2) (PGE(2)) facilitate estrous behavior in ovariectomized (ovx), estrogen-primed rats yet their action is blocked by the antiprogestin RU486. We hypothesize that these agents act by using the PR-Src-mitogen activated protein kinase alternative pathway. To test this hypothesis we used PP2, a specific inhibitor of the Src kinase family. Intraventricular infusion of 30 µg of PP2, 30 min before behavioral testing, significantly attenuated estrous behaviors induced in estradiol benzoate (E(2)B)-primed rats by 5ß-dihydroprogesterone (5ß-DHP), 5ß-pregnan-3ß-ol-20-one (5ß,3ß-Pgl), GnRH, PGE(2) and by manual flank/vaginocervical stimulation. These results suggest that the Src signaling system, by activating mitogen-activated protein kinases, participates in the facilitation of estrous behavior in E(2)B-primed rats induced by agents lacking affinity for the PR.

Expression pattern of Tau in the rat brain during pregnancy and the beginning of lactation.

Pregnancy involves changes in brain function that implicate a re-organization in neuronal cytoskeleton. We analyzed the content of the microtubule associated protein Tau (65kDa isoform) and its phosphorylated form (PhosphoTau) in several rat brain regions throughout pregnancy and on day 2 of lactation by Western blot. In hypothalamus the content of Tau increased on days 2 and 18 of gestation compared with days 14, 21 and in lactation. PhosphoTau content increased throughout pregnancy. In preoptic area Tau content did not show significant changes throughout pregnancy or lactation, however, the content of PhosphoTau presented a decrease on day 21 of gestation. In hippocampus Tau content decreased on day 14 until day 21 compared with day 2 of gestation, however, in lactation day 2 the content of Tau increased meanwhile PhosphoTau content progressively increased throughout pregnancy. In frontal cortex Tau content decreased on day 21 of gestation compared with days 2, 14 and 18, with an increase in lactation, whereas PhosphoTau did not show significant changes. In cerebellum Tau protein decreased on days 14, 18 and 21 of pregnancy with an increase in lactation. PhosphoTau content increased throughout pregnancy and on day 2 of lactation. PhosphoTau/Tau ratio changes in each brain area along pregnancy and in lactation. Our data suggest that Tau expression and its phosphorylation pattern change in a tissue-dependent manner throughout pregnancy and the beginning of lactation in the rat brain.

The nitric oxide pathway participates in lordosis behavior induced by central administration of leptin.

Intracerebroventricular (icv) administration of leptin facilitates lordosis behavior in ad libitum-fed, estrogen-primed rats. The cellular mechanism involved in this response is unknown. The present study tested the hypothesis that the nitric oxide-guanylyl cyclase, cGMP-dependent protein kinase (PKG) pathway is involved in the facilitation of lordosis behavior induced by the central administration of leptin. We tested the importance of the nitric oxide/cGMP pathway for lordosis stimulation by either icv infusion of a nitric oxide synthase inhibitor (L-NAME) or a nitric oxide-dependent, soluble guanylyl cyclase inhibitor (ODQ) 30 min before leptin administration (1 µg). This dose of leptin reliably induced lordosis behavior in ovariectomized estradiol benzoate treated rats. The lordosis induced by leptin at 1 and 2h after infusion was significantly reduced by the previous injection of either L-NAME or by ODQ. Intracerebroventricular infusion of the PKG inhibitor (KT5823) 30 min before leptin infusion, also significantly inhibited the lordosis behavior induced by leptin at 1 and 2h after hormone administration. These data support the hypothesis that the nitric oxide/cGMP/PKG pathway is involved in the facilitation of lordosis by leptin in estrogen-primed female rats.

Effects of RU486 in the expression of progesterone receptor isoforms in the hypothalamus and the preoptic area of the rat during postpartum estrus.

In several mammalian species females undergo postpartum estrus, a brief period of ovulation and sexual receptivity that in rats usually occurs during the first 24h following parturition. The maximal lordotic expression occurs at 12h after the initiation of parturition and depends on intracellular progesterone receptor (PR). We studied the regulation of PR expression by its antagonist, RU486 in the hypothalamus and the preoptic area of the rat during postpartum estrus by Western blot. Adult female rats were treated with RU486 (1.25 and 5mg) 3h after parturition, and Western blot was performed to assess the expression of PR-A and PR-B at 12h postpartum. RU486 (1.25 and 5mg) reduced the expression of PR-A (63% and 95%) and that of PR-B (75% and 99%), respectively in the preoptic area whereas it had no effects in the hypothalamus. These results suggest a differential regulation of PR expression in the rat brain during postpartum estrus.

Progesterone receptor isoforms differentially regulate the expression of tryptophan and tyrosine hydroxylase and glutamic acid decarboxylase in the rat hypothalamus.

Progesterone exerts a variety of actions in the brain through the interaction with its receptors (PR) which have two isoforms with different function and regulation: PR-A and PR-B. Progesterone may modulate neurotransmission by regulating the expression of neurotransmitters synthesizing enzymes or their receptors in several brain regions. The role of PR isoforms in this modulation is unknown. We explored the role of PR isoforms in the regulation of tryptophan (TPH) and tyrosine (TH) hydroxylase, and glutamic acid decarboxylase (GAD) expression in the hypothalamus of ovariectomized rats. Two weeks after ovariectomy, animals were subcutaneously injected with 5 µg of estradiol benzoate (EB), and 40 h later, progesterone (P) was intracerebroventricularly (ICV) injected. Each animal received two ICV injections of 1 µg/µl (4 nmol) of PR-B and total PR (PR-A+PR-B) sense or antisense (As) oligonucleotides (ODNs). First injection was made immediately before sc EB injection, and 24h later animals received the second one. Twenty-four hours after P administration, rats were euthanized and brains removed to measure the expression of PR-A and PR-B, TPH, TH and GAD by Western blot. We observed that sense ODNs modified neither PR isoforms nor enzymes expression in the hypothalamus, whereas PR A+B antisense (PR A+B As) clearly decreased the expression of both PR isoforms in this region. ICV administration of PR-B As only decreased PR-B isoform expression with no significant effects on PR-A expression. A differential protein expression of TPH, TH and GAD was observed after PR isoforms antisense administration. PR-B As administration decreased the expression of TPH (65% with respect to control). In contrast, PR A+B As and PR-B As administration increased (51.6% and 34.4%, respectively) TH expression. The administration of PR A+B As and PR-B As diminished GAD expression (33.4% and 41.6%, respectively). Our findings indicate that PR isoforms play a differential role in the regulation of the content of TPH, TH and GAD in the rat hypothalamus.

Role of progesterone receptors during postpartum estrus in rats.

We studied the role of progesterone receptor (PR) in the display of female sexual behavior during postpartum estrus in rats. Adult female rats were treated with the PR antagonist, RU486 (1.25 and 5 mg), 3 h after parturition and sexual behavior was evaluated throughout the first postpartum day. Estradiol and progesterone serum levels changed during the first 24 h postpartum. The highest estradiol and progesterone levels were found at 9 and 12 h postpartum, respectively. The predominant PR isoform in the hypothalamus and the preoptic area was PR-A during postpartum day. The content of PR-A increased at 6 h postpartum in the hypothalamus and the preoptic area, and decreased in both regions at 9 h. PR-B content only increased in the preoptic area at 12 h postpartum. The highest display of lordotic and proceptive behaviors were found at 12 h postpartum. The treatment with 1.25 and 5 mg of RU486 respectively reduced lordosis by 61% and 92% at 12 h postpartum. These results suggest that PR is essential in the display of postpartum estrus in rats.

Leptin facilitates lordosis behavior through GnRH-1 and progestin receptors in estrogen-primed rats.

Dose response curves for leptin facilitation of estrous behavior (lordosis and proceptivity) were made by infusing the peptide into the lateral ventricle (icv) of ovariectomized (ovx), ad libitum-fed rats injected 40h previously with 5µg of estradiol benzoate. Leptin doses of 1 and 3µg produced significant lordosis quotient at 60min post-injection, with maximal lordosis being displayed at 120min. Yet the intensity of lordosis was weak, and a high incidence of rejection behaviors was found. Moreover, leptin did not induce significant proceptive behaviors at any dose. The leptin doses of 1 and 3µg were selected for determining whether antide, a GnRH-1 receptor antagonist, or the progestin receptor antagonist RU486 could modify the lordosis response to leptin. Icv injection of either antide or RU486 1h before leptin significantly depressed leptin facilitation of lordosis. The results suggest that leptin stimulates lordosis by releasing GnRH, which in turn activates GnRH-1 and progestin receptors. The physiological role of leptin in the control of estrous behavior remains to be determined.

Changes in the content of GFAP in the rat brain during pregnancy and the beginning of lactation.

Several changes in brain function, including learning and memory, have been reported during pregnancy but the molecular mechanisms involved in these changes are unknown. Due to the fundamental role of glial cells in brain activity, we analyzed the content of glial fibrillary acidic protein (GFAP) in the hippocampus, frontal cortex, preoptic area, hypothalamus and cerebellum of the rat on days 2, 14, 18, and 21 of pregnancy and on day 2 of lactation by Western blot. A differential expression pattern of GFAP was found in the brain during pregnancy and the beginning of lactation. GFAP content was increased in the hippocampus throughout pregnancy, whereas a decrease was observed in cerebellum. GFAP content was increased in the frontal cortex and hypothalamus on days 14 and 18, respectively, with a decrease in the following days of pregnancy in both regions. In preoptic area a decrease in GFAP content was observed on day 14 with an increase on days 18 and 21. In the frontal cortex and cerebellum, GFAP content was increased on day 2 of lactation, while it was maintained as on day 21 of pregnancy in the other regions. Our data suggest a differential expression pattern of GFAP in the rat brain during pregnancy and the beginning of lactation that should be associated with changes in brain function during these reproductive stages.