The effect of ovariectomy, estradiol and progesterone on opioid modulation of feeding

Pregnenolone rescues schizophrenia-like behavior in dopamine transporter knockout mice

Pregnenolone belongs to a class of endogenous neurosteroids in the central nervous system (CNS), which has been suggested to enhance cognitive functions through GABA(A) receptor signaling by its metabolites. It has been shown that the level of pregnenolone is altered in certain brain areas of schizophrenic patients, and clozapine enhances pregnenolone in the CNS in rats, suggesting that pregnenolone could be used to treat certain symptoms of schizophrenia. In addition, early phase proof-of-concept clinical trials have indicated that pregnenolone is effective in reducing the negative symptoms and cognitive deficits of schizophrenia patients. Here, we evaluate the actions of pregnenolone on a mouse model for schizophrenia, the dopamine transporter knockout mouse (DAT KO). DAT KO mice mirror certain symptoms evident in patients with schizophrenia, such as the psychomotor agitation, stereotypy, deficits of prepulse inhibition and cognitive impairments. Following acute treatment, pregnenolone was found to reduce the hyperlocomotion, stereotypic bouts and pre-pulse inhibition (PPI) deficits in DAT KO mice in a dose-dependent manner. At 60 mg/kg of pregnenolone, there were no significant differences in locomotor activities and stereotypy between wild-type and DAT KO mice. Similarly, acute treatment of 60 mg/kg of pregnenolone fully rescued PPI deficits of DAT KO mice. Following chronic treatment with pregnenolone at 60 mg/kg, the cognitive deficits of DAT KO mice were rescued in the paradigms of novel object recognition test and social transmission of food preference test. Pregnenolone thus holds promise as a therapeutic candidate in schizophrenia.

Brain distribution and behavioral effects of progesterone and pregnenolone after intranasal or intravenous administration

Neurosteroids hold great promise for the treatment of diseases of the central nervous system (CNS). We compared the uptake by 11 brain regions and appearance in blood of tritium-labeled pregnenolone and progesterone after intranasal and intravenous (IV) injection. Both neurosteroids appeared in blood and brain after either method of administration, but with important differences in uptake. Bioavailability based on appearance in arterial serum showed that about 23% and 14% of the intranasal administered doses of pregnenolone and progesterone, respectively, entered the blood. Brain levels were about two fold lower after intranasal administration for the two neurosteroids. With intranasal administration, brain levels of the two steroids did not vary over time (2-120 min), whereas brain levels were higher early (10 min or less) after i.v. administration. With i.v. administration, uptake by brain regions did not vary, whereas the olfactory bulb, hippocampus, and hypothalamus had high uptake rates after intranasal administration. Intranasal administration of prenenolone improved memory, whereas progesterone decreased anxiety, thus demonstrating that therapeutic levels of neurosteroids can be delivered to the brain by intranasal administration. The neurosteroids were rapidly degraded after i.v. or intranasal delivery, but pregnenolone was more resistant to degradation in the brain after intranasal administration and in serum after i.v. administration. These results show that either the i.v. or intranasal routes of administration can deliver neurosteroids to blood and brain, but that the two routes have significant differences with intranasal administration favoring some brain regions.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

Naltrexone does not prevent the weight gain and hyperphagia induced by the antipsychotic drug sulpiride in rats

Few pharmacological tools are currently available to counteract the excessive body weight gain often observed during prolonged administration of antipsychotic drugs. Most antipsychotic drugs block dopamine receptors, and both the brain dopaminergic and opioid systems appear to be involved in initiation and maintenance of feeding behavior, respectively. We evaluated whether the opioid antagonist naltrexone (NAL, 0.5-16 mg/kg/ip for 21 days) (a) affects body weight and food intake in gonadally-intact and drug-free female rats, (b) prevents obesity, hyperphagia, hyperprolactinemia and vaginal cycle disruption induced by long-term administration of the antipsychotic drug sulpiride (SUL, 20 mg/kg/ip for 21 days), or (c) reverses the acute hyperphagia induced by SUL (15 microg bilaterally), when directly applied in the perifornical lateral hypothalamus (PFLH). In drug-free rats, only NAL doses above 4 mg/kg, significantly decreased weight gain and food intake. Even though NAL (1 and 8 mg/kg) significantly attenuated SUL-induced hyperphagia and hyperprolactinemia, it did not reverse at any dose the weight gain and permanent diestrous induced by SUL. In addition, local NAL did not prevent the hyperphagia and polidypsia observed after acute intrahypothalamic SUL. Unexpectedly, the cumulative and 24 h food intake in SUL-treated rats was significantly increased by NAL. Collectively, these results do not support a role for endogenous opiates in the neural and endocrine mechanisms involved in weight gain during prolonged antipsychotic drug administration in rats.

Estradiol accelerates extinction of a conditioned taste aversion in female and male rats

Exogenous testosterone treatment prolongs extinction of conditioned taste aversions and estradiol treatment prevents testosterone from prolonging extinction in both gonadectomized males and females. Estradiol could require the presence of testosterone for its effect or its action alone could accelerate extinction. The first series of experiments were designed to test the hypothesis that estradiol accelerates extinction when it is given in the absence of testosterone. The results showed that estradiol accelerates extinction of conditioned taste aversions in the absence of testosterone in gonadectomized Sprague-Dawley females and Fischer 344 females and males. The second series of experiments were designed to determine whether estradiol and testosterone differ in the temporal requirements for their opposite effects on extinction. The results showed that estradiol can accelerate extinction when it is present before and during acquisition (from 8 days before until 3 days after acquisition) or when it is present before and during extinction (from 2 days after acquisition, which was 23 days before extinction, until extinction trials were terminated). This is in contrast to a previous finding that testosterone prolongs extinction only when it is present before and during extinction. The following two hypotheses were suggested to account for the temporal effects of estradiol on extinction of conditioned taste aversions: (1) the presence of estradiol during acquisition reduces the effectiveness of LiCl through its action on the opioid system, and the presence of estradiol during extinction activates a neural pathway, such as that associated with activity levels, that accelerates extinction of passive avoidance tasks in general or (2) the presence of estradiol before, not during, acquisition or extinction accelerates extinction because of its illness-inducing properties. Most of the evidence supports the second hypothesis.

Menstrual cycle and sex differences influence salt preference

Previous studies demonstrate that some women have a greater preference for palatable high-sugar and high-fat foods premenstrually. Because salt may also be considered palatable, it was of interest to discover whether salt preference also varies. To determine whether there are differences in salt preference across the menstrual cycle, 49 women and 31 men rated popcorn sprayed with five different concentrations (0M, 1M, 2M, 3M, 3M+) of salt solution. Between-subjects comparisons of women revealed that those tested in separate weeks of their menstrual cycle had differing preferences for salt. Specifically, women in their luteal week preferred unsalted popcorn (0M) significantly less than women tested during their ovulatory or follicular weeks. Women in the menstrual week also tended to find the saltiest popcorn (3M+) much less palatable than women tested in their follicular, luteal, or ovulatory weeks. When men's and women's preference ratings of the same stimuli were compared, an overall sex difference was found. Men liked the mildly salty popcorn (2M) more than women. There were no significant differences in perceived saltiness ratings between men and women or among women tested in different weeks of their cycle.

Interactions among aging, gender, and gonadectomy effects upon naloxone hypophagia in rats

The present study examined the dose-dependent (0.25-5 mg/kg) effects of systemic naloxone upon deprivation-induced intake and high-fat intake as functions of age (4, 8, 14, and 20 months), gender, and gonadectomy in rats. Significant increases in body weight were observed as functions of age and gonadectomy. Whereas aging significantly reduced basal deprivation-induced intake, it generally failed to alter basal high-fat intake. Whereas age, gender, and gonadectomy failed to alter the decreases in deprivation-induced intake following low (0.25-2.5 mg/kg) naloxone doses, sham males displayed significantly greater age-related and gender-related inhibition following the 5 mg/kg dose of naloxone. Young gonadectomized rats displayed significant increases in naloxone's inhibition of deprivation-induced intake as well. More dramatic changes occurred in naloxone's inhibition of high-fat intake. Naloxone's potency increased in sham female rats as a function of age, and decreased in sham males and ovariectomized females as a function of age. Whereas sham males and ovariectomized females were most sensitive to naloxone's inhibition of high-fat intake at young ages, sham females were most sensitive at older ages. These data indicate that effects of age, gender, and gonadectomy upon naloxone-induced hypophagia dissociate as a function of the type of intake. Because selective opioid antagonist studies demonstrate that deprivation-induced intake is mediated by the mu1 receptor and high-fat intake is mediated by kappa and mu2 receptors, it is postulated that the differential effects of aging, gender, and gonadectomy variables upon opioid mediation of the two forms of intake may reflect their interaction with different opioid receptor subtypes.

Diet and estrous cycle influence pain sensitivity in rats

Effects of estrous cycle and acute and chronic access to palatable fluids on tail-flick latency and opiate-induced analgesia were assessed in 124 female Long Evans rats. Following three consistent cycles, rats were water deprived for 8 h and then given ad lib access to 20 ml of either water, a 32% sucrose solution, or corn oil for 5 h. Nociceptive testing was conducted immediately preceding and 30, 60, and 90 min following an SC injection of morphine sulfate (7.5 mg/kg). Diestrus rats had prolonged premorphine tail-flick latencies compared to rats in proestrus. Rats that consumed corn oil had longer tail-flick latencies preceding and 30 min following morphine injections than rats that drank water or the sucrose solution. Rats were retested after they had ad lib access to the same fluid for 3 weeks. No estrous cycle differences were noted following chronic consumption. Rats with chronic access to sucrose showed increased baseline pain sensitivity and increased morphine-induced analgesia at 30, 60, and 90 min postinjection. These data support the notion that palatable fluid consumption attenuates estrous cycle-dependent differences in pain sensitivity.

Hormonal milieu affects tailflick latency in female rats and may be attenuated by access to sucrose

The steroid hormones estrogen (E) and progesterone (P) are known to modify pain sensitivity; however, the relative role of each of these hormones in this process is not well understood. To systematically investigate the effects of E and P on nociception, pain sensitivity was assessed under several hormone conditions. Tailflick (TF) latencies were measured every other day in 10 cycling female rats and 10 female rats during luteal functioning (pseudopregnancy). Thirty ovariectomized (OVX) rats were tested for TF latency following administration of 10 micrograms estradiol benzoate (EB) and either 0.0, 0.5, or 1.0 mg of P. Significant differences in TF latency were seen across days of the estrous cycle but not during luteal functioning. Tailflick latencies during luteal functioning were elevated relative to latencies in normally cycling animals. Among OVX rats, those administered EB and P (1.0 mg) displayed significant reductions in TF latency compared to vehicle controls. As a separate line of research indicated that consumption of highly palatable foods modified pain sensitivity, whether chronic sucrose consumption might overide the influence of hormones on nociception was examined. Ovariectomized rats given EB and P (0.0, 0.5, or 1.0 mg) were allowed chronic exposure to a 32% sucrose solution. Our preliminary findings suggest that chronic sucrose consumption attenuates hormonally induced differences in nociception.

Pineal involvement in the alimentary behavior and taste preferences in the rat

The purpose of this experiment was to explore the effects of pinealectomy or sham pinealectomy on circadian rhythms of taste preferences, food and fluid intake and body weight gain in rats. We compared the body weight gain, the amounts of food eaten and that of deionized water, total fluid, salt, sour, sweet and bitter near-threshold solutions consumed by rats before all surgical manipulations and after pinealectomy or sham pinealectomy. The results showed that the pineal gland does not exert a major influence on circadian organization of taste preferences, drinking, feeding and body weight modifications in the rat. The failure of pinealectomy to modify light/dark rhythms of taste preferences and other related alimentary behaviors may be explained by the fact that pinealectomy does not completely eliminate circulating melatonin.

Naloxone disrupts the expression but not the acquisition by male rats of a conditioned place preference response for an oestrous female

Two experiments were conducted to investigate the possible role of endogenous opioid peptides in the regulation of masculine sexual reward. In experiment 1 sexually experienced male rats, which had recently been castrated or left gonadally intact, were allowed to mate with an oestrous female in an initially "non-preferred" chamber of a test apparatus. On alternate days these males were placed alone in the initially "preferred" chamber of the same apparatus. After eight such conditioning sessions both intact and castrated males had acquired a conditioned place preference (CPP) for the initially "non-preferred" chamber whereas control males, which were never given access to an oestrous female, showed no evidence of a significant shift in their preference for either chamber. Administration of the opioid receptor antagonist, naloxone (1 or 5 mg/kg, SC) prior to each conditioning session had no significant influence on the acquisition of a CPP for an oestrous female. By contrast, in experiment 2 naloxone treatment significantly attenuated the expression of a previously established CPP for an oestrous female in both gonadally intact and castrated males. The results suggest that opioid components of neural reward circuits are normally activated in the male rat by conditioned incentive stimuli, but not by the primary rewarding stimuli associated with access to and mating with an oestrous female.

Ullrich-Turner syndrome and anorexia nervosa

We report on a 6-year-old girl with Ullrich-Turner syndrome and anorexia nervosa. The diagnosis was made at 6 years and she became anorectic at 14 years. She had been treated with low doses of estrogen just before the onset of anorexia. In spite of remarkable decrease in food intake, her body weight was in the normal range compared to standard weight. Rohrer indices were also normal, probably due to abnormal habitus in individuals with the syndrome. The pathogenetic relationship between this disorder and the hormone treatment in the onset of anorexia nervosa is discussed.

Experimental manipulations of eating: advances in animal models for studying anorectic agents

The material set out in this text has been designed to show the wide range of procedures which have the capacity to modify eating behavior--to produce hyper- or hypophagia, to alter the profile of eating patterns, or to adjust dietary preferences and selection. Accordingly, in investigating anorectic drugs it seems necessary to observe the effects of drug actions in a variety of experimental models. This strategy will provide a more complete description of the effect of a drug, will throw light on the mechanism of action, and will provide a more realistic base for predicting the effects of drugs in man.

The effect of naloxone on nocturnal food intake in female and male rats

Previous studies have shown that estradiol and progesterone can alter the response of female rats to naloxone. For example, ovariectomized rats receiving estradiol were found to be less sensitive to the anorexic effect of naloxone than ovariectomized rats receiving oil (vehicle) or progesterone. In the present paper, we evaluated the effect of naloxone on nocturnal food intake in female rats during each stage of the estrous cycle to determine whether changing levels of gonadal hormones in intact female rats would affect their response to naloxone. To evaluate the role testosterone might play in modulating the male rat's feeding response to naloxone we studied the effect of peripherally administered naloxone (0.1, 1.0 and 10 mg/kg) on nocturnal food intake of intact, castrate and castrate + testosterone propionate male rats. During late metestrus, diestrus and proestrus, female rats decreased nocturnal food intake following the administration of naloxone (1.0 and 10 mg/kg) SC (p less than 0.05). During estrus, female rats failed to decrease food intake following any of the doses of naloxone administered. The male rat's response to naloxone does not appear to be altered by the presence or absence of testosterone. Thus, the level of estradiol and progesterone at different stages of the estrous cycle may affect the female rat's response to the satiety effect of naloxone.

Endorphins and food intake: kappa opioid receptor agonists and hyperphagia

Evidence from studies which utilise either opiate receptor agonists and antagonists strongly indicate a role for endorphinergic mechanisms in the control of feeding responses. Two means by which these compounds may exert an effect on feeding can be singled-out. Firstly, emerging evidence suggests that the process of achieving satiety (terminating a meal, or choice of a commodity) may be accelerated following treatments with opiate receptor antagonists. Secondly, the preference for highly palatable solutions (sweet solutions have received most attention) in two-bottle tests is blocked after injection of opiate receptor antagonists. This finding has been interpreted in terms of the abolition of the reward or incentive quality associated with the particularly attractive flavour. These two mechanisms of action may represent two aspects of a single, fundamental process. Following an introduction to rat urination model of in vivo kappa agonist activity, the consistent effect of several kappa agonists (including the highly selective U-50,488H) to stimulate food consumption is described. Recognising that members of the dynorphin group of endogenous opioid peptides are kappa receptor ligands, some with a high degree of selectivity, and the evidence the dynorphins and neo-endorphins produce hyperphagia in rats is particularly interesting. Such lines of evidence lead to the hypothesis that peptides of the dynorphin group may act endogenously to promote the expression of normal feeding behaviour.

The effect of opiates and naloxone on food intake in virgin and lactating rats

Lactation provides an excellent model of non-obese hyperphagia. There is accumulating evidence that endogenous opioids play a role in the modulation of the hormonal changes that occur during lactation. Because endogenous opioids appear also to play a role in the regulation of feeding, we studied the effects of the opiate agonist, butorphanol tartrate, and an opiate antagonist, naloxone, on food intake in virgin female rats and in rats during early, mid and late lactation and during post-weaning. It has been reported that female rats are less sensitive to the suppressant effects of nalmefene, an opioid antagonist, than male rats. Therefore, we also examined the effect of naloxone, an opioid antagonist, on spontaneous nocturnal feeding and 24 hour food deprivation-induced food intake in virgin female rats. We found that female rats were relatively insensitive to the food suppressant effects of naloxone following 24 hour food deprivation, while male rats tested under similar conditions had a decreased intake in response to naloxone. Despite the marked hyperphagia that occurred during lactation, there were minimal alterations in the response to opiate agonists and antagonists during this time period. Our data suggest that endogenous opioids may not play a pivotal role in the hyperphagia of lactation.

Endogenous opiates: 1984

This paper is the seventh in an annual series of reviews of research involving the endogenous opiate peptides, each installment being restricted to work published during the previous year. As in the past three years, the review this year is limited to non-analgesic and behavioral studies of the opiate peptides. The specific topics this year include: stress, tolerance and dependence, consummatory responses, gastric and renal activity, alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, seizures and neurological disorders, activity, and miscellaneous other topics.

Opioids and consummatory behavior

Since the second decade of this century it has been known that opiates can influence ingestive behaviors. Generally, opioid agents enhance feeding and opioid antagonists decrease feeding. The present paper reviews the responsiveness of different animal species to opiates in relation to ingestive behaviors, the opioid receptors involved in such consummatory behaviors, the site of action of opioid modulation of feeding, the role of glucose in opioid induced feeding, and endocrine effects on opioid feeding systems. We emphasize the finding that more than one opioid receptor is involved in the modulation of feeding. A large body of evidence indicates a major role for the dynorphin/alpha-neo-endorphin kappa opioid receptor as one of the receptors involved in feeding modulation. Opioids appear to exert their effect predominantly within the central nervous system, though peripheral effects on taste and gastrointestinal function may play a role in opioid-induced feeding. Although opioid blockade acutely blocks food intake, chronic administration of opiate antagonists to humans and laboratory animals has not proven to be an effective means of decreasing body weight. Chronic opiate administration decreases body weight and autosensitization of beta-endorphin increases body weight. Thus, although it is clear that opioids can effect food intake, it is not clear what effect chronic administration of opioids has no food intake or body weight.

The pineal gland and opiate-induced feeding

Opiate systems in the brain are thought to play a major, though not exclusive, role in the regulation of intake. The rough correspondence of feeding and pineal activity rhythms in the rat offers the possibility that the pineal may also modulate ingestive behavior. In these studies we measured the possibility that pineal manipulations would influence feeding responses to opiate agonists and the antagonist naloxone. Male rats received one of four treatments (or a corresponding control treatment): pinealectomy, removal of the superior cervical ganglia (SCG), transection of the optic nerves or chronic melatonin treatment (1 mg/kg daily). Pinealectomy and melatonin treatment reduced intake during the first half of the dark period, and removal of the SCG reduced intake during the second half of the light period. The most striking effect was seen after optic nerve transection, which reduced nocturnal and increased diurnal intake. Pinealectomy, but no other manipulation, caused a slight decrease in sensitivity to the inhibitory effects of naloxone on intake. None of the treatments affected daytime feeding responses to morphine, ketocyclazocine, or butorphanol. These results suggest that the pineal gland has a minimal role in modulating the opioid regulation of food intake.