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 29,987 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Mating in female rats induces an acute prolactin (PRL) release within 60 min and twice-daily surges of PRL throughout the first 10 days of pregnancy to maintain luteal function. Little is known about the brain mechanism whereby the vaginocervical stimulation is processed to induce PRL release. Our recent results revealed an increase in Fos expression in the arcuate nucleus (ARC) following mating in the intact estrous rat, suggesting that a neuronal network in the brain area may participate in conveying and integrating the genitosensory stimulation. To further investigate the phenotype of activated neurons in the ARC, the present study examined whether beta-endorphin (beta-END) and/or dopamine (DA) neurons are activated by mating, and if so, whether activation is involved in the mating-induced acute release of PRL and the establishment of the twice-daily surges of PRL. In experiment 1, proestrous rats receiving intromissions (mated group) from males or mounts without intromission (mounted group) were sacrificed along with rats taken directly from their home cage (control group) 60 min after the beginning of mating or mounting. Expression of Fos in beta-END neurons and expression of fos-related antigen (FRA) in DA neurons, which were labeled by tyrosine hydroxylase (TH) antibody in the ARC were examined by double-label immunocytochemistry. In experiment 2, proestrous females with indwelling atrial catheters were mated with males. Naloxone (10 microl/min, 2 mg/10 min), an opiate antagonist, or saline was infused before, during and after mating. Blood samples were collected during the mating session and also at several times 3 days after mating. The results showed that mating induced a significant increase in the percentage of beta-END/Fos colabeled neurons and a significant decrease in the number of beta-END cells in all subdivisions of the ARC. In contrast, neither the percentage of FRA/TH colabeled cells nor the number of TH cells was influenced by mating. Mating induced an acute increase in PRL release in saline-treated control animals within 30 min and a subsequent diurnal surge (18.00 h) and a nocturnal surge of PRL (2.00 h) 3 days after mating. Naloxone infusion during mating blocked the mating-induced acute PRL response and the diurnal surge of PRL 3 days after mating, but affected neither the nocturnal surge of PRL nor the incidence of pregnancy. These results demonstrate that (1) beta-END neurons but not DA neurons in the ARC are activated in response to mating in proestrous rats, and (2) the mating-induced activation of beta-END neurons may participate in the acute response of PRL release to mating and the memory mechanism for the establishment of the diurnal PRL surge, but not the nocturnal PRL surge in early pregnancy. These results lead to a conclusion that endogenous opioid peptides may be involved in the neuronal transmission of genitosensory stimulation to induce PRL secretion.
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PMID:Involvement of endogenous opioidergic neurons in modulation of prolactin secretion in response to mating in the female rat. 1094 Jul 35

We previously reported that voluntary intake of corn oil in the light box showed place preference in the conditioned place preference (CPP) test in mice. In the present study, we investigated the contribution of opioidergic systems to the corn oil-induced CPP in mice. Acquisition of the place preference by corn oil intake was blocked by i.p. injections of an opioid mu antagonist, naloxone (0.1 and 0.3 mg/kg), and delta antagonists, 7-benzylidenenaltrexone (0.5 mg/kg) and naltriben (0.5 mg/kg) 15 min before conditioning. The opioid kappa agonist U-50488H (1 and 3 mg/kg i.p.) also blocked corn oil-induced CPP. Naloxone (1 mg/kg, i.p.) and naltriben (0.5 mg/kg, i.p.) did not affect corn oil intake in the home cage. However, 7-benzylidenenaltrexone (0.5 mg/kg, i.p.) and U-50488H (1 mg/kg i.p.) decreased and increased the corn oil intake, respectively. These results suggested that the rewarding effects of corn oil in the CPP test are at least partially mediated via opioidergic systems through mu and delta receptors. Further, we showed that an opioid kappa agonist reduced the rewarding effects of corn oil in the CPP test in mice, although it increased corn oil intake.
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PMID:Opioidergic contribution to conditioned place preference induced by corn oil in mice. 1127 90

Evidence suggests that endogenous opioid peptides are implicated in the suckling-induced prolactin rise. We explored the role of the opioid system and the participation of ovarian hormones in the regulation of prolactin induced by the suckling stimulus at the end of pregnancy in rats with developed maternal behavior, and during lactation. Suckling for 24 h induced a significant increase in serum prolactin on day 19 of pregnancy, which was increased more than three times when naloxone (2 mg/kg s.c.) or mifepristone (2 mg/kg) was administered. The combination of naloxone and mifepristone did not increase serum prolactin more than either compound alone. Administration of tamoxifen (500 microg/kg orally) on days 14 and 15 of pregnancy completely abolished the effect of naloxone, indicating a role for estrogens in establishing this inhibitory role of opioids. To examine the participation of the opioid system during lactation, we used groups of rats on days 1, 3, 5, 12 and 19 postpartum either (i) isolated from the pups for 4 h, or (ii) isolated from the pups for 3.5 h and reunited with them and suckled for 30 min. Naloxone, given just before replacing the pups, prevented the increase in serum prolactin levels observed in the suckled group of rats but had no effect on the basal levels of the isolated rats. To examine whether the participation of the opioid system in the release of prolactin is dependent on the variation of progesterone levels, rats on day 20 of pregnancy were implanted with two cannulae containing progesterone (that blocked postpartum ovulation) or cholesterol, and cesarean surgery was performed on day 21. To maintain lactation, pups (1-3 days old) were replaced every 24 h, and 4 days after the cesarean eight pups were placed in the cage at 1800 h to maintain a strong suckling stimulus during the following 24 h. Naloxone administration significantly reduced serum prolactin levels in control (cholesterol) rats but progesterone implants prevented the inhibitory effect of naloxone and this effect was not modified by treatment with estrogen. These results indicate that the opioid system modulates suckling-induced prolactin secretion, passing from an inhibitory action before delivery to a stimulatory action during lactation. This regulatory shift seems to be dependent on the fall in progesterone concentration at the end of pregnancy and the subsequent increase after the postpartum ovulation and luteal phase.
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PMID:Regulation by endogenous opioids of suckling-induced prolactin secretion in pregnant and lactating rats: role of ovarian steroids. 1183 43

Many similarities exist between the overconsumption of food, which results in obesity, and drug addiction. The present study investigated the effects of anorectic drugs on responding maintained by high incentive, but nutritionally unnecessary, food reinforcers using an FI15(fixed-ratio 10:S) schedule of reinforcement, similar to that used in studies on the incentive properties of drugs of abuse. Rats were trained to respond on a lever to gain access to two high incentive foods--chocolate chip cookies and cheese. Under the FI15(FR10:S) schedule, every 10th response (fixed-ratio 10) delivered a tone and light conditioned stimulus. The first ratio completed 15 min after the start of the session produced the conditioned stimulus and opened a door to give access to a piece of cookie. After 5 min to consume the high incentive food, a second 15-min interval was started, terminating in access to a second reinforcer, cheese. Once trained, the rats were given free access to laboratory chow in the home cage. They continued to work for the high incentive foods for a period of over 1 year, showing a pattern of responding appropriate to an FI(fixed-ratio) schedule. Naloxone (1.0 mg/kg), fenfluramine (1 and 2 mg/kg), D-amphetamine (0.25 and 0.5 mg/kg), and rimonabant (3 mg/kg) significantly reduced responding, especially in the second interval. In contrast, complete removal of the high incentive food from the test procedure did not immediately reduce the rate of responding, tending to increase it in the second of the intervals. Apparently, the drugs did not reduce responding by reducing the experienced magnitude of the high incentive food, but more probably by reducing the animals' motivation.
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PMID:The effects of anorexic drugs on free-fed rats responding under a second-order FI15-min (FR10:S) schedule for high incentive foods. 1721 98


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