Effects of naloxone and its quaternary form on fluid consumption in rats

Gluten exorphin B5 stimulates prolactin secretion through opioid receptors located outside the blood-brain barrier

Gluten exorphin B5 (GE-B5) is a food-derived opioid peptide identified in digests of wheat gluten. We have recently shown that GE-B5 stimulates prolactin (PRL) secretion in rats; this effect is abolished by preadministration of the opioid receptor antagonist naloxone. However, since the structure of naloxone allows it to cross the blood-brain barrier (BBB) and antagonize opioid effects centrally as well as peripherally, it could not established, on the basis of those data, if GE-B5-induced PRL release is exerted through sites located inside or outside the BBB. In this study, we sought to determine the site of action of GE-B5 on PRL secretion, by pretreating male rats with naloxone methobromide (NMB), an opioid antagonist that does not cross the BBB. Four groups of rats were given the following treatments: 1) intravenous vehicle; 2) intravenous GE-B5 (3 mg kg(-1) body weight); 3) intraperitoneal NMB (5 mg kg(-1) body weight), followed by vehicle; 4) NMB, followed by GE-B5. Blood samples for PRL were taken at intervals for 40 minutes after vehicle or GE-B5 administration. GE-B5 stimulated PRL secretion; the effect was statistically significant at time 20. NMB preadministration completely abolished PRL response. Our experiment indicates that GE-B5 stimulates PRL secretion through opioid receptors located outside the BBB. Since opioid peptides do not exert their effect on PRL secretion directly, but via a reduced dopaminergic tone, our data suggest that GE-B5 can modify brain neurotransmitter release without crossing the BBB.

Effects of naloxone on neurohypophyseal peptide release by hypertonic stimulation in chicks

The effects of opioid peptides on the osmotic release of neurohypophyseal hormones, arginine vasotocin (AVT) and mesotocin (MT), were determined in 2-day-old chicks. Experiment 1 examined the effect of a variety of doses of naloxone, an opioid antagonist, on chicks administered isotonic or hypertonic solution. Plasma osmolality in chicks administered hypertonic solution was significantly higher than that in groups administered isotonic solution. None of the doses of naloxone affected plasma osmolality in response to isotonic and hypertonic solution. Plasma levels of AVT increased in hypertonic solution and this response was further enhanced by naloxone injection as the doses increased. The hypertonic solution alone did not affect plasma levels of MT, but additional treatment with naloxone slightly increased plasma levels of MT. Experiment 2 examined the effect of DAMGO ([d-Ala(2), N-Me-Phe(4),Gly-ol]-enkepha lin), a specific mu receptor agonist. Relatively high plasma osmolality caused by hypertonic solution was not affected by additional treatment with DAMGO. Plasma levels of AVT in response to hypertonic solution and to additional treatment with naloxone were reduced by higher doses of DAMGO. Experiment 3 examined the effect of naloxone on chicks administered different concentrations of NaCl. Administration of hypertonic solution resulted in an increase in plasma osmolality and plasma levels of AVT. Naloxone administration enhanced the increase in plasma AVT levels in response to hypertonic solution. Experiment 4 examined the effect of naloxone on different kinds of hypertonic solution, 0.15 M NaCl, 1.5 M NaCl, 2.55 M urea, and 1.95 M sucrose. The increases in plasma osmolality resulting from the administration of the urea and sucrose solutions were the same as those in the chicks injected with 1.5 M NaCl. In sucrose-treated chicks, plasma levels of AVT increased in chicks administered naloxone but not in chicks injected with normal saline. In contrast, no significant changes in plasma levels of AVT were observed in urea treatment with or without naloxone. In Experiments 3 and 4, plasma levels of MT after administration of hypertonic solutions did not change. However, naloxone administration enhanced plasma levels of MT in osmotically stimulated chicks. The results of the present study suggest that opioid peptides attenuate the increase in plasma AVT and MT in hypertonic states.

EEG evidence that morphine and an enkephalin analog cross the blood-brain barrier

The ability of naltrexone but not methyl naltrexone to cross the blood-brain barrier (BBB) was used to provide a different approach for the demonstration that opiates can enter the brain. Cortical electroencephalographic (EEG) measurements were made in rats receiving peripheral (IP) injections of naltrexone or methyl naltrexone and morphine or an enkephalin analog [Tyr-D-Ala-Gly-MePhe-Met(O)-ol]. Naltrexone significantly blocked the EEG effects of morphine and the enkephalin analog, but methyl naltrexone failed to do so. The results provide biological evidence that an opiate peptide can cross the BBB to affect the activity of the brain.

Naloxone: lack of effect of a very low dose on fluid intake and activity in rats

Opiate antagonists, such as naloxone, have been employed to indicate the possible involvement of endogenous opioids in a variety of behaviours including the pathogenesis of schizophrenia. This paper describes two experiments which were performed to determine the effects of naloxone on fluid intake and activity in rats. In experiment 1, the administration of 1mg/kg naloxone significantly (p<0.001) reduced water intake. 10mg/kg naloxone considerably reduced water intake, although this result was not significant. This influence was transient, since water intake was restored to control levels at the end of the 4 hours test period, and not dose related. A low dose of naloxone 0.01mg/kg produced no effect. In experiment 2, doses of 1 and 10mg/kg did not influence locomotor activity, rearing or grooming in the open field. These results suggest that naloxone may exert a primary antidipsogenic action that does not depend upon any suppression of concomitant activity. Furthermore, mechanisms controlling water intake and activity appear to be dissociated. A speculative role for the endogenous opioid system is discussed.

Naloxone eliminates passive avoidance retention deficits produced by pretest exposure to novelty in rats

Pretest exposure to novelty or injections of beta-endorphin can enhance passive avoidance (PA) retention (e.g., Izquierdo & McGaugh, 1985). Enhanced retention may result from a "state-dependent" match between the CNS state during test and the novelty-induced beta-endorphin state that is obtained during training in a novel apparatus. Our Experiment 1 suggests that, unlike PA, Pavlovian fear conditioning in a conditioned lick suppression (CLS) paradigm may be beta-endorphin "state-independent." Rats were given one tone-shock pairing in a novel environment. Baseline lick rates and CLS tested 48 h later in a familiar environment were not affected by pretest exposure to novelty and/or injections of 3.33 mg/kg naloxone HCl. In Experiment 2, the same rats were PA trained/tested in a new apparatus. Saline or naloxone injections and various exposure (novel, familiar, none) conditions preceded (1h) the 24-h retention test. Pretest exposure to novelty reduced retention and naloxone eliminated that deficit. In Experiment 3, naive rats given pretest exposure to novelty also showed a PA retention deficit. The results of Experiments 2 and 3 may complement rather than contradict previous findings. Pretest induction of a beta-endorphin state by novelty may either enhance state-dependent retrieval of a "weak" memory trace or make a "strong/well consolidated" training memory more vulnerable to retroactive interference from "new learning" during the pretest exposure period.

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.

Naloxone decreases ethanol consumption within a free choice paradigm in rats

The effect of subcutaneous naloxone administration on the consumption of a weak ethanol solution in rats on the three consecutive days (testing days) was investigated using a behavioral paradigm which includes a first forced ethanol exposure (conditioning day) followed by a two-bottle ethanol/water choice procedure. Besides reducing fluid intake, naloxone treatment prior to forced ethanol exposure interferes with the acquisition of ethanol preference. Post-conditioning naloxone administration fails to affect ethanol preference. Administration of naloxone prior to the first testing session induces a reduction on total fluid intake, at the day of treatment; a decrease on ethanol preference throughout the three consecutive testing days is also observed with the higher dose of the antagonist (5 mg/kg). An involvement of endogenous opioids in ethanol consumption is suggested through the modulation of alcohol reinforcement or the affective quality of the gustatory cue.

Central and peripheral injection of quaternary antagonist, SR58002C, reduces drinking

Earlier research has demonstrated that opioid involvement in drinking is primarily mediated centrally [8,10]. Drinking is attenuated by the centrally and peripherally active opioid antagonists naloxone and naltrexone in doses as low as 1.0 mg/kg, but not by the quaternary forms of these antagonists [4]. Peripherally administered quaternary forms of these antagonists fail to suppress drinking in doses as high as 10 mg/kg. We generated dose-response curves for centrally and peripherally administered SR58002C, a "newer" quaternary opioid antagonist purported to have high peripheral selectivity, on drinking in 23.5 hr water deprived rats. SR58002C was administered both intracerebroventricularly (ICV, 0, 10, 40 and 80 micrograms/rat) and intraperitoneally (IP, 0, 10, 40 and 80 mg/kg). Doses of SR58002C above 10 mg/kg IP or 10 micrograms ICV significantly reduced drinking in comparison to controls. However, SR58002C appears to be less potent than quaternary naltrexone in suppressing drinking after ICV administration.

Social conflict analgesia: studies on naloxone antagonism and morphine cross-tolerance in male DBA/2 mice

It has recently been reported that male mice exhibit pronounced analgesia in response to attack from aggressive conspecifics. Although several studies indicate that this reaction can be blocked by opiate antagonist pretreatment, unequivocal evidence of opioid involvement is very much more limited. In the present study, the phenomenon of conflict analgesia has been studied in male DBA/2 intruder mice following exposure to a criterion level of attack from aggressive BKW residents. Our findings indicate that this analgesia is blocked and reversed by naloxone unaltered by methyl naloxone, except at high doses (75 mg/kg) and fully cross-tolerant with morphine. This profile confirms and extends earlier findings with B6AF1 mice, indicating that the opioid mediation of this biologically-relevant form of environmental analgesia is not strain specific.

Intracerebroventricular drug administration in pigeons

For many procedures used in behavioral pharmacology, the intracerebroventricular (ICV) route of drug administration is infrequently used due, in part, to the lack of a reliable technique for determining cannula patency in vivo. This study describes an in vivo technique for assessing ICV cannula patency in pigeons. The technique was applied in an experiment designed to evaluate several drugs, which are presumed to differ in the extent to which they enter the central nervous system, for their rate-suppressing effects in pigeons trained to peck a key on a fixed-ratio 20 schedule of food reinforcement. The opioid agonist morphine and antagonist quaternary naltrexone were 100 and 280 times more potent, respectively, in suppressing responding when administered ICV, as compared to systemic administration. Tertiary naltrexone was approximately equipotent as an antagonist of morphine's rate-suppressing effects when administered ICV or systemically. Quaternary naltrexone did not antagonize morphine by either route of administration. The utility of this in vivo cannula verification technique is discussed, as well as the limitations of comparisons between systemically-administered tertiary and quaternary derivatives.

Opiate antagonism reduces placentophagia and pup cleaning by parturient rats

Since endogenous opiate mechanisms are activated during parturition, the present study examined in rats the effects of opiate antagonism on maternal care during and shortly after parturition. Endogenous opiate mechanisms were blocked in late pregnant rats by (1) naltrexone pellet implants (Experiment 1); (2) acute naloxone injections of 10 mg/kg (Experiment 2) or 0.1 mg/kg (Experiment 7); or (3) induction of opiate tolerance (Experiment 3). All methods resulted in a significant decrease in placentophagia and/or in cleaning pups of umbilical cords and birth fluids (Experiment 6). Other aspects of maternal care appeared relatively unaffected and 24 hr pup survival rats were lowered only by induction of morphine tolerance (probably via its effects on the young). In nonpregnant females, naloxone produced a small but significant decrease in placentophagia (Experiment 4) whereas morphine-tolerant nonpregnant females consumed placentas as readily as controls (Experiment 5). Thus the inhibition of placentophagia produced by opiate antagonism may be specific to conditions associated with parturition. These findings suggest that endogenous opiates support placenta eating and pup cleaning during and immediately after birth. Mediation may be via opiate effects on ingestive behavior, and/or via a reduction in the stress of parturition which otherwise can interfere with the female's ability to perform these tasks.

Sugar, opioids and binge eating

There is evidence that endogenous opiates are involved in the control of feeding in experimental animals. Several types of experimental obesity are associated with increased opiate production and/or increased numbers and sensitivity of opiate receptors. Research with experimental animals suggests that nutrients, particularly sugar, have an effect on feeding behavior that is mediated by opiates. For instance, the obesity-producing effect of a palatable diet in rodents is blocked by opiate antagonists. Stress induced feeding in rodents leads to preferential sucrose ingestion and is blocked by opiate antagonists and beta-endorphin. The effect of nutrients on the endogenous opiate system of humans is less clear. Clinical experience suggest that carbohydrates (sugar in particular) play a role in binge eating and obesity. Many binge eaters preferentially eat sweets during a binge. Many obese individuals consume more than half of their total daily calories as carbohydrates. Sweet snacking is a frequent behavior at times of stress. Recent evidence suggests that sugar can lead to increased beta-endorphin production in obese subjects.

The use of quaternary narcotic antagonists in opiate research

Quaternary ammonium derivatives of narcotic antagonists are commonly used in determining sites of action of opiates in the central nervous system and the periphery because it is widely assumed that they do not readily cross the blood-brain barrier, in contrast to their relatively non-polar tertiary counterparts. However, these compounds possess several unique pharmacological properties which have not been taken into consideration in the design of numerous investigations. This article reviews the current state of knowledge concerning the pharmacology of the quaternary narcotic antagonists, examines their use in physiological and behavioral studies of action of opiates, and proposes guidelines for the design of experiments involving these compounds.

Plasma immunoreactive beta-endorphin response to glucose ingestion in human obesity

Following the oral administration of 100 g of glucose, morbidly-obese subjects and non-obese controls demonstrated increased levels of plasma immunoreactive beta-endorphin. There was a slow rise in plasma immunoreactive beta-endorphin in the non-obese controls throughout the 3-h observation period. The obese subjects demonstrated a delayed and significantly greater rise of plasma immunoreactive beta-endorphin, when compared to the controls. These findings may have implications for further research in human obesity.

Dissociation of prey killing and prey eating by naloxone in the rat

Mouse killing rats matched for killing latency and prey eating were injected (IP) with 0.5, 2.0, or 5.0 mg/kg naloxone or 0.9% saline. Naloxone did not significantly inhibit prey killing or alter prey killing latency at any dose but did reduce prey eating by 50% at the two higher doses. The dissociation of prey killing and prey eating by naloxone is consistent with other evidence that these two behaviors are separate components of predation in rats.

Intracerebroventricular injection of ostrich beta-endorphin to satiated pigeons induces hyperphagia but not hyperdipsia

Satiated pigeons received intracerebroventricular injections of either ostrich (0.06, 0.3 and 1.5 nmoles) or human (0.06 and 1.5 nmoles) beta-endorphin, or a control solution, and their consumption of food and water was monitored during the half hour after these treatments. At each administered dose, ostrich beta-endorphin enhanced food without altering water intake. By contrast, human beta-endorphin did not induce any reliable alteration either of feeding or drinking. Together with previous studies performed in pigeons with opiate antagonists, these results suggest that the feeding system of pigeons is modulated by an endorphinergic mechanism. By contrast, no evidence exists so far that such a mechanism operates for controlling drinking, as appears to be the case in mammals.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.