Ethological analyses of the effects of naloxone and the opiate antagonist ICI 154, 129 on social interactions in male house mice

The influences of the mu blocker naloxone and the novel opioid delta receptor antagonist ICI, 154, 129 on videotaped encounters between individually-housed, male Swiss mice and anosmic male ‘standard opponents’ were assessed using a variety of ethological analyses. The effects of drugs were studied on individual elements and on the times allocated by subjects to broad categories of behaviour. Neither of the drugs significantly altered times allocated to broad categories of behaviour. Both doses of both compounds significantly increased the incidences of some ‘fearful’/defensive postures. A more detailed analysis considered the effects of the drugs on the sequences of postures used in the resident's behaviour. This involved the generation of ‘dendrograms’ which provided support for the view that both naloxone and ICI 154, 129 altered the associations between behavioural elements seen in saline controls (especially at higher doses) and that the effects of these antagonists were qualitatively different.     

Cadmium effects on hypothalamic-pituitary-testicular axis in male rats

This study analyzes cadmium effects at the hypothalamic-pituitary-testicular axis. Male rats were given cadmium during puberty or adulthood. Cadmium exposure through puberty increased norepinephrine content in all hypothalamic areas studied, but not in the median eminence. Metal exposure increased serotonin turnover in median eminence and the anterior hypothalamus, while decreased it in mediobasal hypothalamus. Also, decreased plasma levels of testosterone were found. Cadmium exposure during adulthood increased norepinephrine content in posterior hypothalamus and decreased the neuro-transmitter content in anterior and mediobasal hypothalamus. Decreased circulating levels of luteinizing hormone (LH) and testosterone and increased plasma follicle stimulating hormone (FSH) levels were also observed. Cadmium accumulated in all analyzed tissues. Various parameters showed age-dependent changes. These data suggest that cadmium globally effects hypothalamic-pituitary-testicular axis function by acting at the three levels analyzed and that an interaction between cadmium exposure and age emerge.     

In vivo characterization of the opioid antagonist nalmefene in mice

AimsThe current study assessed the in vivo antagonist properties of nalmefene using procedures previously used to characterize the opioid antagonists naloxone, naltrexone, 6β-naltrexol and nalbuphine.Main methodsICR mice were used to generate antagonist dose–response curves with intraperitoneal (i.p.) nalmefene against fixed A₉₀ doses of morphine in models of morphine-stimulated hyperlocomotion and antinociception. Additional dose–response curves for antagonist precipitated opioid withdrawal were run in mice treated acutely (100 mg/kg, s.c., ? 4 h) or chronically (75 mg pellet, s.c., ? 72 h) with morphine. Comparisons were made between antagonist potency and degree of precipitated withdrawal.Key findingsNalmefene produced dose- and time-related antagonism of morphine-induced increases in locomotor activity with a calculated ID₅₀ (and 95% confidence interval) of 0.014 (0.007–0.027) mg/kg. Nalmefene produced rapid reversal of morphine-induced locomotor activity (5.1 min for 50% reduction in morphine effect). A 0.32 mg/kg dose of nalmefene produced blockade of morphine-induced antinociception in the 55 °C tail-flick test that lasted approximately 2 h. Nalmefene was able to potently precipitate withdrawal in mice treated acutely or chronically with morphine.SignificanceThese results demonstrate that nalmefene is similar to naloxone and naltrexone with respect to its in vivo pharmacology in mice. Specifically, nalmefene produces potent antagonism of morphine agonist effects while precipitating severe withdrawal. The compound has a slower onset and longer duration of action compared to naloxone and naltrexone. The data allows for a more complete preclinical comparison of nalmefene against other opioid antagonists including the putative opioid neutral antagonist 6β-naltrexol.     

Testosterone does not influence opiate binding sites in the male rat brain

It has been reported previously that castration produces testosterone-reversible increases in the density of 3H-naltrexone binding sites in the male rat brain. Unfortunately, we were unable to replicate these observations in a comprehensive series of studies. Specifically, we found that castration failed to produce changes in the Kd or Bmax of opiate binding sites in whole male rat brain, or in the hypothalamus, utilizing 3H-dihydromorphine (a mu receptor ligand), 3H-D-alanine, D-leucine enkephalin (delta) or 3H-naltrexone (ubiquitous). Furthermore, we found that the relative proportion of mu and delta binding sites in brain was unchanged by castration. The reasons for the discrepancy between the present results and those previously reported are unclear, but it appears that the provocative hypothesis that testosterone influences opioid receptors in brain must be carefully reevaluated.     

Role of adrenals on the 24-hour periodicity in hypothalamo-pituitary axis in male rat

Studies were carried out to investigate the influence of adrenalectomy on the 24 hour periodicity in the hypothalamo-pituitary axis in adult male rat. Adrenal gland ablation resulted in the shift in the occurrence of peak content of hypothalamic LHRH and attenuation in the responsiveness of the pituitary to synthetic LHRH. Following adrenalectomy, the circadian serum LH peak was attenuated and serum FSH showed a 24 hour periodicity. In conclusion, the results suggest the possible role of adrenal gland in the maintenance of 24 hour periodicity of hormones in the hypothalamo-hypophyseal axis in male rat.     

Unexpected beta adrenergic effects of the antitumor agent, cyclocytidine, on rat salivary glands.

In addition to its potent antileukemic properties, cyclocytidine has a sialogogue action that depends on stimulation of beta adrenergic ereceptors of salivary glands. Furthermore, when chronically administered (for 3 days), cyclocytidine caused enlargement of parotid and submaxillary glands and heart that resembled the hypertrophy caused by chronic isoproterenol administration. The salivas evoked by cyclocytidine also closely resembled those evoked by isoproterenol, and were extremely viscous, and high in K+, (121 plus or minus 5.6, for submaxillary, and 42 plus or minus 2.9, for parotid), low in flow rate (0.007 mg/min times mg) and parotid saliva contained high concentrations of amylase (805 plus or minus 33 mg/mg gland). Cyclocytidine also caused marked emptying of parotid gland amylase. The cyclocytidine-induced salivary flow and gland emptying of amylase were prevented for 90 min when propranolol (but not dibenzyline or atropine) was administered prior to injection of the cyclocytidine. In addition, when the superior cervical ganglion was acutely removed, administration of cyclocytidine elicited salivary flow from the denervated as well as the innervated glands. These findings suggest that cyclocytidine does not affect salivary glands through indirect central or ganglionic actions. Cyclocytidine action does not exclusively involve beta receptors, since even in the presence of propranolol, secretory flow was evident after 90 min but when dibenzyline was given with the propranolol, complete blockade of cyclocytidine-stimulated saliva was effected. The dominant effect is, however, a beta adrenergic one. The undesirable side effects of cyclocytidine (parotid pain, postural hypotension, and cardiac hypertrophy) probably stem chiefly from its beta adrenergic properties and might be eliminated (or at least modified) by administration of propranolol with the cyclocytidine.     

Social aggressiveness of female and subordinate male crickets is released by opiate receptor antagonist

1. In the cricket Gryllus bimaculatus, effects of opiate receptor antagonist naloxone, 9 or 30 microg per animal, on aggressive behavior were investigated. 2. Naloxone had no significant impact on aggression of isolated and dominant males. In contrast, the drug caused a dramatic release of social aggression in female and subordinate male crickets. 3. The results suggest that activity of the opioid system contributes to suppress aggression in subordinate males, as well as in females, during social contacts.     

Effect of stress on the activity of the hypothalamic-pituitary-gonadal axis: peripheral and central mechanisms.

This article reviews the mechanisms believed to mediate stress-induced inhibition of reproductive functions and the anatomical sites at which these effects take place. Particular emphasis is placed on the potential modulating role of hormones or neurotransmitters released during stress. At the level of the gonads, adrenal corticoids, pro-opiomelanocortin (POMC)-like peptides, and corticotropin-releasing factor (CRF) are reported to interfere with the stimulatory action of gonadotropins on sex steroid-producing cells. Increased circulating corticosteroid levels may also decrease pituitary responsiveness to GnRH. There is, however, increasing evidence that these mechanisms are primarily involved in mediating the effects of prolonged stress, but not those of an acute stimulus. In contrast, a variety of hormones or neurotransmitters, including CRF, POMC peptides, and biogenic amines act within the brain to mediate the inhibitory influence of both acute and prolonged stresses on reproductive function.     

Unexpected male choosiness for mates in a spider

Sexual selection theory traditionally considers choosiness for mates to be negatively related to intra-sexual competition. Males were classically considered to be the competing, but not the choosy, sex. However, evidence of male choosiness is now accumulating. Male choosiness is expected to increase with an individual's competitive ability, and to decrease as intra-sexual competition increases. However, such predictions have never been tested in field conditions. Here, we explore male mate choice in a spider by studying size-assortative pairing in two natural sites that strongly differ in the level of male-male competition. Unexpectedly, our results demonstrate that mate choice shifts from opportunism to high selectivity as competition between males increases. Males experiencing weak competition did not exhibit size-related mating preferences. By contrast, when competition was intense we found strong size-assortative pairing due to male choice: while larger, more competitive males preferentially paired with larger, more fecund females, smaller males chose smaller females. Thus, we show that mating preferences of males vary with their competitive ability. The distinct preferences exhibited by males of different sizes seem to be an adaptive response to the lower reproductive opportunities arising from increased competition between males.     

Effect of oral nalmefene, an opiate-receptor antagonist, on meal-stimulated gastric acid secretion and serum gastrin concentration in man

We evaluated whether nalmefene, an orally administered opiate-receptor antagonist, would inhibit gastric acid secretion in response to a meal in healthy humans. On separate days either 50 mg nalmefene or a placebo tablet was administered by mouth 90 min before a blenderized steak meal was infused into the stomach through a nasogastric tube. Compared to placebo, nalmefene inhibited meal-stimulated acid secretion in each of 6 subjects studied (P less than 0.05). During the second and third hours after the meal, nalmefene inhibited mean acid secretion by 16%. Nalmefene also resulted in significantly higher meal-stimulated serum gastrin concentrations than placebo (P less than 0.05) even though intragastric pH was kept constant at 5.0 in both experiments. These studies indicate that an orally administered opiate-receptor antagonist can inhibit gastric acid secretion in response to a meal in humans, yet increase meal-stimulated serum gastrin concentrations.     

Effect of the opiate antagonist naloxone on body temperature in rats.

The specific opiate antagonist naloxone was used to assess the hypothesis that an endogenous opioid plays a significant role in temperature regulation in the rat. A very slight hypothermic effect was observed at a naloxone dose sufficient to block the opiate receptors. Even under conditions of cold stress, the magnitude of the effect was so small as to lend little support to the hypothesis.     

Alteration of the hypothalamic-pituitary-gonadal axis in estrogen- and androgen-treated adult male leopard frog, <Emphasis Type="Italic">Rana pipiens</Emphasis>

Background  

Gonadal steroids, in particular 5 alpha-dihydrotestosterone (DHT) and 17 beta-estradiol (E2), have been shown to feed back on the hypothalamic-pituitary-gonadal (HPG) axis of the ranid frog. However, questions still remain on how DHT and E2 impact two of the less-studied components of the ranid HPG axis, the hypothalamus and the gonad, and if the feedback effects are consistently negative. Thus, the goal of the study was to examine the effects of DHT and E2 upon the HPG axis of the gonadally-intact, sexually mature male leopard frogs, Rana pipiens.     

Kisspeptin and the regulation of the hypothalamic-pituitary-gonadal axis in the rhesus monkey (Macaca mulatta)

The present article reviews recent studies of monkeys and, in some cases, humans that have been conducted to examine the role of kisspeptin-GPR54 signaling in the regulation of the hypothalamic-pituitary-gonadal axis in higher primates. This area of peptide biology was initiated in 2003 by the discovery that loss of function mutations of GPR54 in man were associated with hypogonadotropic hypogonadism and absent or delayed puberty. Puberty in the monkey, an experimental model commonly used to study this fundamental developmental stage, is first described. This is followed by a review of the role of kisspeptin in the regulation of the postnatal ontogeny of GnRH pulsatility. The roles of kisspeptin in GnRH pulse generation and in the feedback loops governing gonadotropin secretion in primates are then discussed. A brief section on kisspeptin-GPR54 signaling at the pituitary and gonadal levels is also included. The review concludes with a discussion of the phenomenon of GPR54 downregulation by continuous exposure to kisspeptin and its therapeutic implications.     

Effects of TGF-betas and a specific antagonist on apoptosis of immature rat male germ cells in vitro

Massive apoptosis of pubertal male germ cells is important for the development of functional spermatogenesis in the adult testis. Although the trigger(s) for male germ cell loss at puberty remain undefined, we have hypothesized that transforming growth factor-betas (TGF-βs) play an active role. Here we demonstrate that the three mammalian TGF-β isoforms, TGF-β1, TGF-β2 and TGF-β3, induce distinct apoptosis of pubertal spermatogonia and spermatocytes in a dose-dependent manner. Induction of male germ cell death by activation of caspase-3 was most pronounced with TGF-β2 compared to TGF-β1 and TGF-β3. Furthermore, we found colocalization of activated caspase-3 with apoptotic protease-activating factor-1 (Apaf-1) in apoptotic germ cells, thus indicating the importance of the intrinsic mitochondrial pathway in TGF-β-induced apoptosis. The specificity of the TGF-β effects was proven by addition of recombinant latency-associated peptide against TGF-β1 (rLAP-TGF-β1) which completely abolished TGF-β1-induced and TGF-β3-induced germ cell apoptosis. Although TGF-β2-triggered germ cell death also was significantly reduced by rLAP-TGF-β1, inhibition was not maximal. Our results suggest that the three TGF-β isoforms induce apoptosis of pubertal male germ cells via the mitochondrial pathway in vitro and are thus likely candidates involved in the excessive first wave of apoptosis of male germ cells during puberty. Lutz Konrad and Marcel Munir Keilani contributed equally to this work.     

Estrogen-like effects of 7,12-dimethylbenz(a)anthracene on the female rat hypothalamo-pituitary axis

We have recently demonstrated that 7,12-dimethylbenz(a)anthracene (DMBA), a potent inducer of mammary tumors in rodents, can in vitro decrease the number of membrane dopamine D2 receptors and stimulate prolactin (PRL) release, by direct estrogen-like actions on anterior pituitary. In the present study, we tested the ability of DMBA to mimic the in vivo estradiol (17 beta E2) effects on pituitary D2 receptors and on PRL as well as LH release. We have found that DMBA, like 17 beta E2, when injected to ovariectomized rats, induced a decrease in the number of anterior pituitary D2 receptors, a release of PRL and exerted a biphasic (acute negative and longer term positive) action on LH secretion. We thus examined the ability of DMBA to interact with 17 beta E2 receptors in the hypothalamo-pituitary axis: DMBA binds to the pituitary cytosolic estrogen receptors with an affinity 0.001% that of 17 beta E2. Finally [3H]DMBA binds to hypothalamus-containing brain sections. This binding was displaced partially by RU 2858 a pure estrogen agonist and totally by tamoxifen, a purported estrogen antagonist. No competition for [3H]DMBA binding was observed with an androgen (RU 1881) or a glucocorticoid (RU 26988) agonist. From these data, it may be concluded that DMBA can act as a partial estrogen in pituitary and hypothalamic tissues.