Effects of systemic or intracerebroventricular naloxone injection on basal and 2-deoxy-D-glucose-induced ingestive behavior

In order to examine the role and site of action of opiates on both hunger and thirst and food and water intake in rats after short term (3 hr.) food deprivation alone or in combination with 2DG-induced glucoprivic stress, naloxone was given to rats in either the jugular vein or the lateral ventricle. Both basal and 2DG-induced food and water intake were reduced by naloxone injected either peripherally or centrally. Latencies to eat and drink were used as measures of hunger and thirst respectively. Only central injection of naloxone significantly reduced 2DG-induced but not basal hunger. These results suggest a central site of action of naloxone on both food and water intake even if some peripheral effects cannot be totally ruled out. Our findings indicate central nervous system opiate receptor involvement in the hunger response to 2DG-induced glucoprivation. In all other treatment conditions, decreases in food intake cannot be related to reduction of hunger but may be due to potentiation of satiation during opiate receptor blockade.     

Proopiomelanocortin (POMC) products in the central regulation of sympathetic and cardiovascular dynamics: studies on melanocortin and opioid interactions

The proopiomelanocortin (POMC)-derived peptides are important regulators in a number of central nervous system pathways especially as they relate to food intake as well as metabolic and autonomic responses. In this study, we investigated the sympathetic nervous and cardiovascular responses to intracerebroventricular (i.c.v.) administration of alpha melanocyte stimulating hormone (alphaMSH), beta-endorphin (beta-END) and adrenal corticotrophic hormone (ACTH) alone or in the presence of a melanocortin antagonist, or an opioid antagonist, in normal animals. The i.c.v. administration of alphaMSH and ACTH resulted in a significant increase in the lumbar sympathetic nerve activity (LSNA) that was accompanied by an increase in mean arterial pressure (MAP). On the other hand i.c.v. administration of beta-END decreased the LSNA and MAP. The pretreatment of animals with the melanocortin-4 (MC-4) receptor antagonist, agouti protein, significantly antagonized the response to alphaMSH and also, paradoxically, not only antagonized the response to beta-END but converted its inhibitory responses on both the LSNA and MAP to a sympathetic activated and pressor response. Pretreatment with the opioid antagonist, naloxone, significantly antagonized the sympathetic nervous and cardiovascular response to beta-END. It partially but significantly antagonized the MAP response to alphaMSH, but the sympathetic response was unaffected. Neither agouti protein nor naloxone altered the sympathetic nervous and cardiovascular response to ACTH. From these studies, we conclude that i.c.v. administration of alphaMSH and ACTH increases the LSNA and cardiovascular dynamics, whereas beta-END decreases it. And, the MC-4 receptor antagonist reverses the endorphin response and the opioid antagonist attenuates the alphaMSH response suggesting possible receptor or central neural pathway interactions between MC-4 and the opioid receptor mediated effects.     

MORPHINE INDUCED CALCIUM DEPLETION IN DISCRETE REGIONS OF RAT BRAIN1

The in vivo administration of a single dose of morphine produces a decrease of tissue calcium in the rat brain. This decrease is observed to be linear, dose-dependent, time-dependent and to occur to an equal degree in 8 discrete brain regions. This effect of morphine is blocked by naloxone and exhibits a high degree of sterospecificity. The reserpine induced decrease of brain calcium was not antagonized by naloxone. Differentiation of this response using reserpine and naloxone indicates the possibility of calcium pools in the central nervous system. The results are discussed in terms of a specific effect of opiate drugs and the role of calcium in opiatereceptor interactions.     

The interference of naloxone hydrochloride in the teratogenic activity of opiates

Diamorphine hydrochloride, methadone hydrochloride, and the synthetic enkephalin analogue FK 33-824 are potent teratogens for the central nervous system in mouse embryos. They induce the "neurotropic syndrome of malformations," which is restricted to the central nervous system if administered during the critical period of neural tube closure. Pretreatment with corresponding equimolecular doses of the antagonist naloxone hydrochloride applied 30 minutes before treatment with the opiate agonists abolishes the major severe malformations, i.e., exencephaly, craniorachischisis, and brachyury, and reduces the number of cases of kinking of the spinal cord. Dilation of the fourth brain ventricle remains unaffected. It is suggested that the mechanism of interference in the teratogenicity of the opiates by naloxone hydrochloride reported here is based on competition for opiate receptors. In general, these observations are regarded as evidence that the pharmacological affinity of opiate agonists to receptors in the central nervous system is responsible for the malformations caused by them in this system.     

Central versus peripheral opioid regulation of ingestive behavior in the domestic fowl

1. Three experiments were conducted to determine whether opioid regulation of ingestive behavior in the domestic fowl is mediated at sites within the central nervous system (CNS) or peripheral tissues. 2. Food and water intake were significantly decreased by the intramuscular (im) injection of naloxone hydrochloride (NHCl) and naloxone methobromide, which have a high and low ability, respectively, to cross the blood-brain barrier. 3. Water, but not food, intake was significantly decreased by the intracerebroventricular (ICV) injection of NHCl. However, water intake was not affected by the im injection of doses which were effective when given ICV. 4. These results suggest that in the domestic fowl there is a peripheral component to opioid regulation of food intake, while opioid regulation of water intake seems to be mediated at peripheral sites and within the CNS.     

Endogenous opiates modulate release of growth hormone in response to electroacupuncture

The effect of electroacupuncture on serum growth hormone levels was investigated in 5 normal subjects and in 10 patients with chronic musculoskeletal pain. Serum growth hormone did not change in the normal subjects but there was an approximate 5-fold increase in the chronic pain subjects. This effect was partially inhibited by prior administration of the opiate antagonist naloxone, suggesting that the rise in growth hormone was mediated via release of central nervous system opioids.     

Opioid agonist/antagonist effect of naloxone in modulating rabbit jejunum contractility in vitro.

Opioid peptides are the most effective drugs in controlling pain; their action is elicited by binding to specific membrane receptors. The gastrointestinal tract represents, after the nervous system, the site in which the opioid receptors are expressed at high levels. The opioid agonist morphine has a significant inhibitory effect on intestinal motility, this action is blocked by naloxone an opioid antagonist mainly active at mu and kappa receptors. In this study the presence of mu opioid receptor on rabbit jejunum was investigated by western blot. The effects of beta-endorphin, the endogenous opioid peptide with the highest affinity to the mu opioid receptor and those of naloxone on spontaneous rabbit jejunum contractions were evaluated. Beta-endorphin (10(-6) M) showed a relaxant effect on jejunum contractility while naloxone showed a dual effect inducing an increase of spontaneous contractility at low concentrations (10(-6) M, 10(-7) M, 10(-8) M) and a decrease when high concentrations (10(-3) M, 10(-4) M, 10(-5) M) were utilized. The obtained results demonstrate that mu opioid receptor is expressed in rabbit jejunum and suggest that this receptor may be involved in mediating the effects of both opioid agonist and antagonist on jejunum contractions.     

Opioids and neuropeptides: mechanisms in circulatory shock

Endogenous opioid systems are activated in stressful situations such as circulatory shock. The opiate antagonist naloxone improves cardiovascular function in several models of shock caused by endotoxemia, hypovolemia, anaphylaxis, and spinal trauma. The ergotropic neuropeptide, thyrotropin-releasing hormone, in supraphysiological doses, also improves cardiovascular function in these shock models, but this effect does not result from action at the opiate receptor. For both these agents a central nervous system (CNS) site of action has been partially characterized. A variety of neuropeptides, including the opioids, seem capable of modulating autonomic function through their CNS actions. In addition, they may play a role in peripheral integration and transmission of autonomic nervous activity by actions at the ganglia and/or at nerve endings. Some neuropeptides also have direct autacoid effects on cells, including those of the microvasculature. This raises new questions concerning possible peripheral functions of neuropeptides during circulatory shock, and the nature of their interactions with other potential shock mediators such as monokines and arachidonic acid derivatives.     

Possible role of cyclic AMP and dopamine in morphine tolerance and physical dependence.

In chronically morphinized rats undergoing naloxone induced withdrawal the cerebellar Cyclic 3′, 5′ adenosine monophosphate (Cyclic AMP) was significantly higher than the controls. The cerebellar dopamine (DA) and norepinephrine (NE) were decreased, elevated or unchanged depending on the duration of morphine treatment. The corpus striatal DA levels during withdrawal were markedly elevated and the striatal cyclic AMP levels were unchanged. The NE levels in the striatal tissue were either elevated or unchanged depending upon the duration of morphine administration. In sharp contrast to the chronically morphinized rats undergoing naloxone induced withdrawal, the rats made morphine dependent over a period of eight weeks showed quite moderate changes in the striatal and cerebellar cyclic AMP and DA levels. Thus alterations in the DA and the cyclic AMP levels in the central nervous system (CNS) may play an important role in the naloxone induced stereotyped morphine withdrawal behavior.     

Opiate receptor blockade in man reduces 2-deoxy-d-glucose-induced food intake but not hunger,thirst, and hypothermia

Opioid peptides may act as neuromodulators in the central nervous system to conserve energy stores and water in mammals. To examine this hypothesis in man, the effect of opiate receptor blockade with naloxone on the hunger, thirst, and hypothermic response to 2-deoxy-D-glucose-induced glucoprivic stress was assessed. Opiate receptor blockade decreased stress-induced food intake but did not reduce marked increases in hunger produced by glucoprivation. Naloxone infusions did not change the hypercortisolemic, polydipsic, hypothermic, and thermogenic response to 2-deoxy-D-glucose. While these results do not suggest a major role for a β-endorphin modulation of stress-induced hunger, hypothermia and water conservation, the reduction of food intake could be due to augmented satiety, perhaps associated with retardation of gastric emptying during opiate receptor blockade.     

Orexin-A suppresses the pulsatile secretion of luteinizing hormone via beta-endorphin

Orexins, the novel hypothalamic neuropeptides that stimulate feeding behavior, have been shown to suppress the pulsatile secretion of LH in ovariectomized rats. However, the mechanism of this action is still not clear. We examined the effect of naloxone, a specific opioid antagonist, on the suppression of the pulsatile secretion of LH by orexins to determine whether beta-endorphin is involved in this suppressive effect. We administered orexins intracerebroventricularly and injected naloxone intravenously in ovariectomized rats, and we measured the serum LH concentration to analyze the pulsatile secretion. Administration of orexin-A significantly reduced the mean LH concentration and the pulse frequency, but coadministration of naloxone significantly restored the mean LH concentration and the pulse frequency. Administration of orexin-B also significantly reduced the mean LH concentration and the pulse frequency, and coadministration of naloxone did not restore them. These results indicate that orexin-A, but not orexin-B, suppresses GnRH secretion via beta-endorphin.     

Suggestive evidence for receptors for morphine and methionine-enkephalin on normal human blood T lymphocytes.

This study reports the in vitro influence of morphine, dextromoramide, levomoramide, and methionine-enkephalin upon normal human T blood lymphocytes by using the active and total rosette tests. Morphine and dextromoramide inhibited the percentage of active T rosettes. This effect was completely reversed in the presence of naloxone, their specific antagonist. The specificity was further demonstrated by the absence of the effect of levomoramide, the inactive enantiomere, upon the rosette system. Methionine-enkephalin increased the percentage of active T rosettes. This effect was specifically inhibited by naloxone. These observations suggest that normal human blood T lymphocytes bear surface receptor-like structures for morphine, dextromoramide, and methionine-enkephalin. Such findings may provide a link between the central nervous system and the immune system.     

Opiate receptor blockade and diurnal pituitary and adrenal hormone levels

Peripheral pituitary hormone levels exhibit circadian variations though the mechanism of these changes is unknown. In order to investigate the possible role of endogenous opiates in such changes we have studied the influence of opiate receptor blockade with naloxone (6.8 mg) on pituitary hormones in the morning and again in the evening in six normal male volunteers. Basal ACTH, cortisol, aldosterone and prolactin were higher in the morning than in the evening. Following naloxone at 0700h both ACTH and cortisol rose indicating a tonic inhibition of ACTH by endogenous opiates at that time. At 2230h cortisol rose following naloxone but ACTH did not, suggesting that endogenous opiates do not play an important role in the diurnal rhythm of this hormone and consistent with the suggestion that endogenous opiates can effect cortisol levels independently of their action on ACTH. Neither aldosterone nor prolactin were influenced by naloxone. In contrast TSH was unaffected by naloxone in the morning but fell in the evening (mean + SE decrement over 120 min -0.6 +/- 0.3 mU/l as compared with the control +0.6 +/- 0.4 mU/l; p less than 0.01). Thus, endogenous opiates probably tonically stimulates TSH levels in the evening when TSH may increase and possibly play a role in the circadian rhythm of TSH.     

Binding sites of opiates and endogenous opioids in the oocytes of the toad Bufo viridis

The binding of labelled naloxone, morphine and (D-Ala2,D-Leu5)enkephalin (DADL) to oocyte membranes of the toad Bufo viridis was investigated. The opiate antagonist naloxone binds to the membranes much more effectively than morphine or DADL. The binding of [3H]naloxone is reversible and saturating. The bound [3H]naloxone is readily replaced by unlabelled naloxone or bremazocine (kappa-agonist), far less effectively by morphine (mu-agonist) and SKF 10.047 (sigma-agonist) and is not practically replaced by DADL (delta-agonist), beta-endorphin (epsilon-agonist) and other neuropeptides. Analysis of experimental results in Scatchard plots revealed two types of binding sites with a high (Kd = 15 nM) and low (Kd = 10(3) nM) affinity for naloxone. The number of sites responsible for the binding of naloxone possessing a high affinity is 16 pmol-/mg of oocyte homogenate protein, i.e., 20-50 times as great as in the toad or rat brain. Trypsin and p-chloromercurybenzoate decrease the binding of [3H]naloxone. The oocyte extract is capable of replacing the membrane-bound [3H]naloxone, on the one hand, and of inhibiting the smooth muscle contracture of the rabbit vas deferens, on the other. This inhibition is reversed by naloxone and can also be induced by bremazocine, but not by morphine, DADL and SKF 10.047. In all probability oocytes contain compounds that are similar to opiate kappa-agonists. It may also be possible that these compounds mediate their effects via specific receptors and are involved in the control over maturation of oocytes and early development of toad eggs.     

Suppression of appetitive behavior in the rat by naloxone: lack of effect of prior morphine dependence

Naloxone (0.3–10 mg/kg) produced a dose-related suppression of eating and drinking in rats that had been deprived of food for 48 hr or water for 24 hr. The suppression of water intake by naloxone was unaltered in rats that had been physically dependent upon morphine one week earlier and which were tolerant to the analgesic effect of morphine at the time naloxone was tested. These results confirm the ability of naloxone to suppress appetitive behavior in the rat but do not resolve the issue of whether or not this effect of naloxone is the consequence of an interaction with an endogenous opioid system.     

Modulation of luteinizing hormone and follicle-stimulating hormone in circulation by interactions between endogenous opioids and oestradiol during the peripubertal period of heifers.

The aim of this study was to determine whether the decline in oestradiol inhibition of circulating luteinizing hormone (LH) and follicle-stimulating hormone (FSH) during the peripubertal period of heifers is associated with a change in opioid modulation of LH and FSH secretion. Opioid inhibition of LH secretion was determined by response to administration of the opioid antagonist naloxone. Prepubertal heifers (403 days old) were left as intact controls, ovariectomized or ovariectomized and chronically administered oestradiol. Control heifers were used to determine time of puberty. Three weeks after ovariectomy, four doses of naloxone (0.13-0.75 mg kg-1 body weight) or saline were administered to heifers in the treatment groups in a latin square design (one dose per day). Blood samples were collected at intervals of 10 min for 2 h before and 2 h after administration of naloxone. This procedure was repeated four times at intervals of 3 weeks during the time intact control heifers were attaining puberty. All doses of naloxone induced a similar increase in concentration of serum LH within a bleeding period. During the initial bleeding period (before puberty in control heifers), administration of naloxone induced an increase in LH concentration, but the response was greater for heifers in the ovariectomized and oestradiol treated than in the ovariectomized group. At the end of the study when control heifers had attained puberty (high concentrations of progesterone indicated corpus luteum function), only heifers in the ovariectomized and oestradiol treated group responded to naloxone. Opioid inhibition of LH appeared to decline in heifers during the time control heifers were attaining puberty. Heifers in the ovariectomized group responded to naloxone at the time of administration with an increase in FSH, but FSH did not respond to naloxone at any other time. Administration of naloxone did not alter secretion of FSH in ovariectomized heifers. These results suggest that opioid neuropeptides and oestradiol are involved in regulating circulating concentrations of LH and possibly FSH during the peripubertal period. Opioid inhibition of gonadotrophin secretion appeared to decline during the peripubertal period but was still present in ovariectomized heifers treated with oestradiol after the time when age-matched control heifers had attained puberty. We conclude that opioid inhibition is important in regulating LH and FSH in circulation in heifers during the peripubertal period. However, opioids continue to be involved in regulation of circulating concentrations of LH after puberty.     

Effects of a 60 Hz magnetic field on central cholinergic systems of the rat

We studied the effects of an acute (45 min) exposure to a 60 Hz magnetic field on sodium-dependent, high-affinity choline uptake in the brain of the rat. Decreases in uptake were observed in the frontal cortex and hippocampus after the animals were exposed to a magnetic field at flux densities ? 0.75 mT. These effects of the magnetic field were blocked by pretreating the animals with the narcotic antagonist naltrexone, but not by the peripheral opioid antagonist, naloxone methiodide. These data indicate that the magnetic-field-induced decreases in high-affinity choline uptake in the rat brain were mediated by endogenous opioids in the central nervous systems. © 1993 Wiley-Liss, Inc.     

A study of the actions of naloxone and morphine on gastric acid secretion and gastric mucosal damage in the rat

There exists a considerable controversy in the literature with regard to the effect of either opiate receptor blockade or that of morphine in different gastric and intestinal ulcer models in the rat. We performed experiments to evaluate the effects of naloxone and morphine on gastric acid secretion and gastric mucosal damage in different experimental models of gastric mucosal injury, namely in indomethacin-, HCl (0.6N)- and ethanol (96%)-models. We found that: 1) 10 mg/kg naloxone ip given twice, effectively protected gastric mucosa against indomethacin (30 mg/kg ip) and against the acid-dependent injury caused by 0.6 N HCl (1 mL ig), but not against the non acid-dependent injury caused by 96% ethanol (1 mL ig); 2) morphine (10 + 10 mg/kg ip) increased ulcers in the HCl-model, but had no effect in the two other models; 3) this ulcer-aggravating effect of morphine in the HCl-model was blocked by pretreatment of 2 mg/kg ip naloxone; and 4) both naloxone (5 + 5 and 10 + 10 mg/kg ip) significantly decreased gastric acid secretion in 1-h pylorus ligated rats. We conclude that: 1) naloxone dose-dependently protects against the indomethacin- and HCl-, but not against the ethanol-induced gastric mucosal damage; 2) morphine aggravates the HCl-induced ulcerogenesis; and 3) both opiod receptor agonist and antagonist decrease gastric acid secretion.     

Effects of (D-ala2, met5)-enkephalinamide and naloxone on ACTH and corticosterone secretion

An intravenous administration of (D-ala2, met5)-enkephalinamide (DALA) caused a significant elevation of plasma ACTH and corticosterone at 10 to 20 min after injection in unanesthetized freely moving rats. An intraperitoneal administration of cyproheptadine tended to reduce plasma ACTH and corticosterone levels at 60 min after injection, but it did not attenuate the DALA-induced ACTH and corticosterone elevation. A large dose of naloxone (1-10 mg/kg body weight) caused a significant elevation in plasma corticosterone, but naloxone at 10 mg/kg body weight reduced the basal ACTH level and DALA-induced ACTH elevation. When both DALA and naloxone were injected, the steroidogenic effect was attenuated. Neither DALA nor naloxone affected the basal ACTH release and CRF-induced ACTH stimulation in rat anterior pituitary cell cultures. These results suggest that DALA acts at the extra-hypophyseal level to stimulate ACTH and corticosterone and that the naloxone stimulatory effect on steroidogenesis acts on the adrenal gland or is mediated by stimulating corticosterone stimulating factors other than ACTH.     

Analeptic and antianaleptic effects of naloxone and naltrexone in rabbits.

Naloxone (5 mg/kg), but not naltrexone, shortened the duration of anaesthesia in rabbits pretreated with pentobarbital. This analeptic effect was blocked by atropine, but not by methylatropine; it thus appears that a central cholinergic mechanism is involved. In contrast, smaller doses of both naloxone and naltrexone attenuated the arousal property of thyrotropin releasing hormone (TRH). Naloxone, but not naltrexone, also antagonized the analeptic property of d-amphetamine. In conscious animals naloxone potentiated, whereas naltrexone attenuated, the excitatory effects of TRH and d-amphetamine.