Changes in brain met-enkephalin concentrations with peripheral CCK injections in Zucker rats

Divergent profile between hypothalamic and plasmatic aminopeptidase activities in WKY and SHR. Influence of beta-adrenergic blockade

AIMS

Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) differ in their renin-angiotensin system function and sympathetic tone. The metabolism of angiotensins and vasopressin depends on the action of certain aminopeptidases whose activity may be influenced by the autonomic nervous system. Their regulation may differ between WKY and SHR in hypothalamus and plasma according to the sympathetic tone. We analyzed aminopeptidases responsible for the hydrolysis of certain angiotensins, vasopressin, cholecystokinin or enkephalins in hypothalamus and plasma of WKY and SHR in untreated controls rats and under beta-adrenoceptor blockade. Systolic blood pressure, food intake, water intake and diuresis were measured as parameters modulated by the autonomic nervous system and the above mentioned peptides.

MAIN METHODS

Glutamyl-, aspartyl-, cystinyl- and alanyl-aminopeptidase activities were analyzed fluorimetrically in plasma and hypothalamus of control and propranolol-treated (100mg/kg/day administered in drinking water for 1month) WKY and SHR, using arylamide derivatives as substrates.

KEY FINDINGS

An opposite response of aminopeptidases to propranolol treatment between plasma and hypothalamus was observed in either WKY and SHR. Furthermore, the behavior of aminopeptidases was inversed between WKY and SHR either in hypothalamus and plasma: while the activity increased in hypothalamus and decreased in plasma of WKY, it decreased in hypothalamus and increased in plasma of SHR.

SIGNIFICANCE

These results revealed an inverse response of aminopeptidases between hypothalamus and plasma and also an opposite behavior of these enzymes between WKY and SHR in hypothalamus and plasma. These observations support the involvement of the sympathetic system in the modulation of aminopeptidase activities.

Common effects of fat, ethanol, and nicotine on enkephalin in discrete areas of the brain

Fat, ethanol, and nicotine share a number of properties, including their ability to reinforce behavior and produce overconsumption. To test whether these substances act similarly on the same neuronal populations in specific brain areas mediating these behaviors, we administered the substances short-term, using the same methods and within the same experiment, and measured their effects, in areas of the hypothalamus (HYPO), amygdala (AMYG), and nucleus accumbens (NAc), on mRNA levels of the opioid peptide, enkephalin (ENK), using in situ hybridization and on c-Fos immunoreactivity (ir) to indicate neuronal activity, using immunofluorescence histochemistry. In addition, we examined for comparison another reinforcing substance, sucrose, and also took measurements of stress-related behaviors and circulating corticosterone (CORT) and triglycerides (TG), to determine if they contribute to these substances' behavioral and physiological effects. Adult Sprague-Dawley rats were gavaged three times daily over 5 days with 3.5 mL of water, Intralipid (20% v/v), ethanol (12% v/v), nicotine (0.01% w/v) or sucrose (22% w/v) (approximately 7 kcal/dose), and tail vein blood was collected for measurements of circulating CORT and TG. On day five, animals were sacrificed, brains removed, and the HYPO, AMYG, and NAc processed for single- or double-labeling of ENK mRNA and c-Fos-ir. Fat, ethanol, and nicotine, but not sucrose, increased the single- and double-labeling of ENK and c-Fos-ir in precisely the same brain areas, the middle parvocellular but not lateral area of the paraventricular nucleus, central but not basolateral nucleus of the AMYG, and core but not shell of the NAc. While having little effect on stress-related behaviors or CORT levels, fat, ethanol, and nicotine all increased circulating levels of TG. These findings suggest that the overconsumption of these three substances and their potential for abuse are mediated by the same populations of ENK-expressing neurons in specific nuclei of the hypothalamus and limbic system.

Increased enkephalin in brain of rats prone to overconsuming a fat-rich diet

Recent studies have shown that the opioid enkephalin (ENK), acting in part through the hypothalamic paraventricular nucleus (PVN), can stimulate consumption of a high-fat diet. The objective of the present study was to examine sub-populations of Sprague-Dawley rats naturally prone to overconsuming a high-fat diet and determine whether endogenous ENK, in different brain regions, is altered in these animals and possibly contributes to their behavioral phenotype. An animal model, involving a measure of initial high-fat diet intake during a few days of access that predicts long-term intake, was designed to classify rats at normal weight that are either high-fat consumers (HFC), which ingest 35% more calories of the high-fat than low-fat chow diet, or controls, which consume similar calories of these two diets. Immediately after their initial access to the diet, the HFC compared to control rats exhibited significantly greater expression of ENK mRNA, in the PVN, nucleus accumbens and central nucleus of the amygdala, but not the arcuate nucleus or basolateral amygdala. This site-specific increase in ENK persisted even when the HFC rats were maintained on a chow diet, suggesting that it reflects an inherent characteristic that can be expressed independently of the diet. It was also accompanied by a greater responsiveness of the HFC rats to the stimulatory effect of a PVN-injected, ENK analogue, D-ala2-met-enkephalinamide, compared to saline on consumption of the high-fat diet. Thus, normal-weight rats predicted to overconsume a fat-rich diet exhibit disturbances in endogenous ENK expression and functioning that may contribute to their long-term, behavioral phenotype.

Developmental changes in opioid peptides and their receptors in Cpe(fat)/Cpe(fat) mice lacking peptide processing enzyme carboxypeptidase E

Carboxypeptidase E (CPE) is involved in the biosynthesis of a number of neuropeptides including opioid peptides. A point mutation in this gene results in a loss of enzyme activity, decrease in mature neuroendocrine peptides, and development of late onset obesity as seen in Cpe(fat)/Cpe(fat) mice. In this study, we examined the processing of peptides derived from prodynorphin and proenkephalin in various brain regions of these mice during development. At 6 to 8 weeks, an age prior to the onset of obesity, levels of dynorphin peptides are decreased in all brain regions, whereas levels of ir-Met-enkephalin are differentially altered. There is an accumulation of C-terminally extended forms of all three opioid peptides in Cpe(fat)/Cpe(fat) mice, consistent with a lack of CPE activity. Thus, it appears that there is no direct correlation between the level of mature opioid peptides and the development of obesity in these mice. Since altered levels of peptides can influence the opioid receptor system, we examined the functional activity of mu and kappa opioid receptors using [35S]guanosine-5'-O-(gamma-thio)-triphosphate binding assays. We find no differences in kappa receptor activity in Cpe(fat)/Cpe(fat) compared with control littermate mice. In contrast, the mu receptor activity is differentially altered in select regions of Cpe(fat)/Cpe(fat) mice in response to a mu-specific ligand. Taken together, these results suggest that the lack of CPE activity leads to alterations in the level of opioid peptides during development and that changes in peptide levels differentially affect opioid receptor activity in vivo.

ARC POMC mRNA and PVN alpha-MSH are lower in obese relative to lean zucker rats

Effects of obesity on gene expression for opioid peptides and neuropeptide-Y (NPY) in the arcuate nucleus (ARC), and on opioid peptides and alpha-melanocyte stimulating hormone (alpha-MSH) in the paraventricular nucleus (PVN) were examined in obese Zucker rats (18 weeks old). Obese Zucker rats are insulin-resistant, diabetic and hyperleptinemic as indicated by high serum glucose, insulin and leptin levels. ARC proOpiomelanocortin (POMC) mRNA levels were significantly lower in the obese relative to lean Zucker rats and ARC proNeuropeptide Y (proNPY) mRNA levels were higher (P<0.05). There were no differences in proDynorphin and proEnkephalin mRNA levels in the ARC (0.05). Obese Zucker rats had lower alpha-MSH and dynorphin A(1-17) peptide levels in the paraventricular nucleus (PVN) (P<0.05), but did not have lower PVN beta-endorphin peptide levels (0.05). The decrease in POMC in the ARC and decrease in alpha-MSH in the PVN seen in the obese Zucker rat in the present study suggest that reduced activity of the melanocortin system in the ARC to PVN pathway may contribute to the related hyperphagia. Reduced activity of the melanocortin system in the ARC to PVN pathway may be due to a disturbance of leptin signaling coupling to POMC.

Opioid receptor blockade promotes weight loss and improves the display of sexual behaviors in obese Zucker female rats

Obese Zucker female rats are hyperphagic, overweight, infertile, and hyporesponsive to the inductive effects of ovarian steroid hormones on sexual behaviors. It has been postulated that endogenous opioid activity may contribute to their obesity and reproductive dysfunction. To test this hypothesis, ovariectomized, adult obese Zucker rats were treated with the opioid receptor antagonist, naltrexone, or saline prior to measurement of steroid-induced sexual behaviors, food intake, and body weight. In estradiol benzoate (EB)-treated rats, naltrexone injection increased the display of sexual receptivity (lordosis quotient, LQ: saline, 11+/-10%; 5 mg/kg naltrexone, 54+/-15%, p < 0.05) and also elicited proceptivity (PRO), which was never observed after saline injection. In EB plus progesterone-treated animals, naltrexone administration enhanced both sexual receptivity and proceptivity (LQ: saline, 17+/-10%; 5 mg/kg naltrexone, 96+/-3%; p < 0.05; PRO: saline, 3.0+/-2.4 bouts/min; 5 mg/kg naltrexone, 45.3+/-12 bouts/min; p < 0.01). Naltrexone injection also decreased 24-h food intake (saline, 24.2+/-0.7 g; 5 mg/kg naltrexone, 17.6+/-1.2 g; p < 0.05) and weight change (saline, +7.3+/-0.8 g; 5 mg/kg naltrexone, -4.5+/-1.4 g, p < 0.01). Morphine treatment blocked these effects of naltrexone on sexual behaviors, food intake, and body weight. These data suggest that endogenous opioids contribute to hyperphagia, obesity, and behavioral hyporesponsiveness to ovarian steroid hormones in obese Zucker rats.

Long-term effects of CCK-agonist and -antagonist on food intake and body weight in Zucker lean and obese rats

The present study evaluates long-term effects of the CCK-agonist caerulein and the CCK-A antagonist loxiglumide in obese and lean Zucker rats. Caerulein and loxiglumide altered food intake neither in obese nor in lean rats. By as yet unknown mechanisms, however, weight increase was accelerated by loxiglumide and reduced by caerulein in obese and lean rats. Caerulein increased pancreatic weight only in lean but not in obese rats. Thus, obese rats show a resistance of pancreatic CCK-A receptors. The failure of CCK-agonist and -antagonist to alter food intake suggests that this CCK-resistance is not responsible for obesity in the genetically altered rats.

Evaluation of chronic opioid receptor antagonist effects upon weight and intake measures in lean and obese Zucker rats

Body weight and food intake are significantly reduced in rats during development of dietary obesity following chronic central administration of mu (beta-funaltrexamine, BFNA), mu1 (naloxonazine), kappa1 (nor-binaltorphamine, NBNI), delta1 ([D-Ala2,Leu5,Cys6]-enkephalin, DALCE) and delta2 (naltrindole isothiocyanate, NTII) opioid receptor subtype antagonists. In contrast, rats made obese by maintainance on a 'cafeteria' diet failed to display weight loss following chronic mu1 receptor antagonism. To test the hypothesis that chronic administration of opioid antagonists are less effective in controlling intake and weight in obese animals, the present study assessed whether chronic, central administration of either BFNA (20 micrograms), naloxonazine (50 micrograms), NBNI (20 micrograms), DALCE (40 micrograms) or NTII (20 micrograms) altered weight and intake in lean and obese Zucker rats over seven days. Body weight was reduced following chronic mu (lean: 42 g; obese: 49 g), mu1 (lean: 71 g; obese: 38 g), kappa1 (lean: 30 g; obese 14 g), delta1 (lean: 43 g; obese: 22 g) or delta2 (lean: 37.5 g; obese: 36 g) antagonism. Overall food intake was reduced following chronic mu (lean: 8.8 g; obese: 16.1 g), mu1 (lean: 12.6 g; obese: 17.0 g), kappa1 (lean: 6.5 g; obese 7.0 g), delta1 (lean: 9.7 g; obese: 11.1 g) or delta2 (lean: 9.4 g; obese: 14.3 g) antagonism. Therefore, both lean and obese Zucker rats display weight loss and reduced intake following chronic central administration of opioid receptor subtype antagonists.

The lateral hypothalamic area revisited: ingestive behavior

This article discusses the role of the lateral hypothalamic area (LHA) in feeding and drinking and draws on data obtained from lesion and stimulation studies and neurochemical and electrophysiological manipulations of the area. The LHA is involved in catecholaminergic and serotonergic feeding systems and plays a role in circadian feeding, sex differences in feeding and spontaneous activity. This article discusses the LHA regarding dietary self-selection, responses to high-protein diets, amino acid imbalances, liquid and cafeteria diets, placentophagia, "stress eating," finickiness, diet texture, consistency and taste, aversion learning, olfaction and the effects of post-operative period manipulations by hormonal and other means. Glucose-sensitive neurons have been identified in the LHA and their manipulation by insulin and 2-deoxy-D-glucose is discussed. The effects on feeding of numerous transmitters, hormones and appetite depressants are described, as is the role of the LHA in salivation, lacrimation, gastric motility and secretion, and sensorimotor deficits. The LHA is also illuminated as regards temperature and feeding, circumventricular organs and thirst and electrolyte dynamics. A discussion of its role in the ischymetric hypothesis as an integrative Gestalt concept concludes the review.

Central and peripheral control of postprandial pyloric motility by endogenous opiates and cholecystokinin in dogs

The role of endogenous opiates and cholecystokinin (CCK) in the control of postprandial pyloric myoelectric activity was investigated in conscious dogs with chronically implanted intraparietal electrodes at the gastroduodenal junction. Meals consisted of either 20 g/kg of canned food (standard meal) or the same food supplemented with 0.5 mL/kg of arachis oil (fat meal). During the 6 hours after standard and fat meals, the number of pyloric spike bursts, 2-4 seconds in duration, was 61.8 +/- 15.8 and 49.9 +/- 12.7/15 minutes, respectively. Administered 15 minutes before a fat meal, naloxone (50 micrograms/kg IV) decreased the number of spike bursts by 31.4%, whereas methyl-levallorphan, a peripheral opiate antagonist, increased postprandial spike activity by 22.2% when administered IV (0.5 mg/kg) and decreased it when administered intracerobroventricularly at a dose of 10 micrograms/kg. These two antagonists administered in the same conditions before a standard meal had no effect on the postprandial spike activity. A 1-hour infusion of cholecystokinin octapeptide (CCK-8), 500 ng.kg-1.h-1 IV and 50 ng.kg-1.h-1 intracerebroventricularly, performed 1 hour after a standard meal induced a 19.6% and 15.8% decrease in the number of pyloric spike bursts, respectively. Both naloxone IV (50 micrograms/kg) and methyl-levallorphan intracerebroventricularly (10 micrograms/kg) administered before the infusion of CCK-8 reinforced this pyloric inhibition, which was antagonized by methyl-levallorphan IV (0.5 mg/kg). The CCK antagonist asperlicin, 200 micrograms/kg IV and 20 micrograms/kg intracerebroventricularly, administered before a fat meal increased pyloric spike bursts by 22.0% and 31.5%, respectively. These results indicate that after a fat meal, endogenous opiates exert a peripheral inhibitory and central stimulatory control of pyloric motility; they suggest the involvement of both peripheral and central release of CCK.

Stimulation of kappa opiate receptors in intestinal wall affects stress-induced increase of plasma cortisol in dogs

In dogs, an acoustic stress (A.S.) produced by hearing of intense music (less than or equal to 90 dB) through earpieces for 1 h induced a 520% maximal rise in plasma cortisol 15-30 min after the beginning of stress. Oral administration of the specific kappa agonists, U-50488 (0.1 mg/kg) and PD 117302 (0.05 mg/kg), 30 min before the A.S. session reduced significantly (P less than 0.01) by 71.2% and 80.9% the maximal increase of plasma cortisol but did not affect the increase observed after intracerebroventricular administration of ovineCRF (100 ng/kg). These effects which are not reproduced by intravenous administration of the drugs at similar doses, were blocked by previous treatment with MR 2266 (0.1 mg/kg) or local anesthesia and vagotomy, suggesting that kappa opioid agonists inhibit the stress-induced activation of the hypothalamo-pituitary-adrenocortical (HPA) system by acting selectively on specific receptors located in the wall of the proximal gut.

Multiple brain sites sensitive to feeding stimulation by opioid agonists: a cannula-mapping study

Evidence suggests that brain opioid receptors of the mu, delta and kappa subtypes may be involved in the control of feeding behavior. However, limited information is available regarding the specific anatomical location of these feeding relevant opioid receptors. To address this problem, we microinjected three opioid agonists, morphine, (D-Ala2)-Met-enkephalinamide (DALA) or MR 2034, into one of 15 different brain areas and measured the subsequent feeding responses of satiated rats. Morphine (25 nmol) and DALA (6.8 nmol) both elicited strong feeding responses from the same five brain areas, namely, the paraventricular, dorsomedial and lateral hypothalamus, as well as from sites within the septum and amygdala. No other brain sites yielded significant responses to these opioid receptor agonists. In contrast to this anatomically specific pattern of effects, the opioid agonist MR 2034 (8.6 nmol) produced a feeding response which was generally smaller in magnitude and had little anatomical specificity. These findings suggest that opioid receptor systems for stimulating feeding exist in multiple discrete brain areas. Of the regions tested, specific sites within the hypothalamus, septum and amygdala are distinguished as being most sensitive to feeding stimulation by morphine and DALA.

Peripheral antagonistic action of trimebutine and kappa opioid substances on acoustic stress-induced gastric motor inhibition in dogs

The effects of intracerebroventricular (i.c.v.), intravenous (i.v.) and oral (p.o.) administration of trimebutine on the gastric motor inhibition induced by acoustic stress were investigated in fasted dogs fitted with strain-gauge transducers on the antrum and proximal jejunum. Started 40-50 min after the last migrating motor complex, a 1 h acoustic stress delayed by 111% the occurrence of the next gastric migrating motor complex without affecting the jejunal motor pattern. This inhibition of gastric migrating motor complex induced by acoustic stress was abolished by previous p.o. administration of trimebutine (1 mg/kg) but not by its i.v. (0.1 mg/kg) or i.c.v. (0.01 mg/kg) injection. The trimebutine blockade of gastric motor alterations induced by acoustic stress was suppressed after previous i.v. treatment with MR 2266 (0.3 mg/kg) but was unaffected by naloxone (0.3 mg/kg). Furthermore oral administration of U-50488H (10 micrograms/kg) and ethylketocyclazocine (10 micrograms/kg) respectively abolished and reduced the acoustic stress-induced delay of the occurrence of the gastric migrating motor complex. We concluded that trimebutine is able to antagonize the gastric motor disturbances induced in dogs by acoustic stress, probably by acting selectively on peripheral kappa receptors located in the wall of the proximal gut and directly stimulated from a mucosal site.

Decreased [3H]-naloxone binding and elevated dynorphin-A(1-8) content in Zucker rat brain

We have previously reported that female obese Zucker rats are hypersensitive to painful stimuli and are resistant to the analgesic effects of morphine. In continuation we hypothesized that these phenomena are possibly the result of diminished population of opioid receptors, or an overabundance of dynorphin interfering with morphine analgesia. We now report that female obese Zucker rats have decreased concentrations of mu opioid receptors in whole brain and elevated levels of Dynorphin A(1-8) (DYN) in a brain area known to be associated with responses to nociceptive stimuli.

Immunohistochemical distribution of cholecystokinin, dynorphin A and Met-enkephalin neurons in sheep hypothalamus

The distribution of cholecystokinin (CCK)-, Met-enkephalin (M-ENK)- and dynorphin (DYN)-like immunoreactive perikarya were examined in the sheep hypothalamus using the peroxidase-anti-peroxidase technique. CCK- and DYN-containing neurons were found primarily in the suprachiasmatic nucleus (SCH) and supraoptic nucleus (SO). No CCK- or DYN-containing neurons were found in the paraventricular nucleus (PVN). M-ENK-containing neurons were found mainly in the PVN of the hypothalamus. In addition, M-ENK neurons were found in the dorsomedial (DMH), lateral (LH), anterior (AH) and periventricular hypothalamic areas. The distribution of these neuropeptides may provide a basis for understanding differences in responsiveness to centrally administered peptides.

Circadian rhythm of feeding induced changes in hypothalamic Met-enkephalin concentrations

Accumulating evidence suggests that opioid peptides play an important role in the hunger component of the control of food intake. The enkephalins, one of the opioid peptide families, stimulate feeding when injected into specific hypothalamic areas and endogenous concentrations change with the fed/fasted condition of rats and sheep and with phase of circadian cycle. To demonstrate a possible circadian rhythm in feeding-induced changes in Met-enkephalin (MEK), 54 male rats initially weighing 255 +/- 3 g were adapted to a 12-hr fast during the light (light-fasted) or dark (dark-fasted) phase of the circadian cycle, then sacrificed before (non-fed) or after (fed) being allowed to eat a meal. In non-fed compared with fed rats, MEK concentrations were higher in the paraventricular nucleus (PVN, 170 vs. 109 pg/mg tissue, p less than 0.05) and ventromedial hypothalamus (VMH, 209 vs. 161 pg/mg tissue, p less than 0.05) in the dark (light-fasted) but not light (dark-fasted), even though rats ate a larger meal in the light (8.6 vs. 5.0 g, p less than 0.01). In rats fed the same amount of food in the light (dark-fasted) as ad lib fed rats in the dark (light-fasted), MEK concentrations did not differ in the PVN or VMH, suggesting that circadian rhythm is more important than meal size. Rats gavaged with an amount of milk equal in calories to dark ad lib-fed rats (light-fasted) had MEK concentrations not different from light-fasted non-fed rats (216 vs. 209 pg/mg tissue, NS) suggesting that feeding behavior, pregastric stimuli and/or form of diet is important for influencing MEK concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)