Effects Of Opiate Antagonists Research Articles

Opiate receptor blockade on human granulosa cells inhibits VEGF release

The objectives of this study were to determine whether the main opioid receptor (OPRM1) is present on human granulosa cells and if exogenous opiates and their antagonists can influence granulosa cell vascular endothelial growth factor (VEGF) production via OPRM1. Granulosa cells were isolated from women undergoing oocyte retrieval for IVF. Complementary to the primary cells, experiments were conducted using COV434, a well-characterized human granulosa cell line. Identification and localization of opiate receptor subtypes was carried out using Western blot and flow cytometry. The effect of opiate antagonist on granulosa cell VEGF secretion was assessed by enzyme-linked immunosorbent assay. For the first time, the presence of OPRM1 on human granulosa cells is reported. Blocking of opiate signalling using naloxone, a specific OPRM1 antagonist, significantly reduced granulosa cell-derived VEGF levels in both COV434 and granulosa-luteal cells (P < 0.01). The presence of opiate receptors and opiate signalling in granulosa cells suggest a possible role in VEGF production. Targeting this signalling pathway could prove promising as a new clinical option in the prevention and treatment of ovarian hyperstimulation syndrome.

Naloxone Successfully Counters Life-Threatening Toxicity of Benzodiazepine in a Patient in Methadone-Maintenance Treatment

Naloxone Successfully Counters Life-Threatening Toxicity of Benzodiazepine in a Patient in Methadone-Maintenance Treatment

Hepatic Methionine-enkephalin may interfere with response to antiviral therapy in chronic hepatitis C

The immunoreactivity of methionine-enkephalin (Met-enkephalin), one of the endogenous opioid peptides, is expressed in the liver in different types of chronic diseases. This finding suggests that the liver may be a source of endogenous opioids in disease state. In-vitro studies have shown that morphine, which exerts its effect by binding to opioid receptors, enhances hepatitis C virus replication and that it interferes with the antiviral effects of interferon. Thus, it was hypothesized that liver-derived endogenous opioids, which also bind to opioid receptors, may interfere with the response to antiviral therapy with interferon. To correlate the expression of Met-enkephalin immunoreactivity (MEIR) in the liver of patients with chronic hepatitis C with their response to treatment with pegylated interferon and ribavirin. We sought to investigate the expression of the immunoreactivities of Met-enkephalin and of the δ-opioid receptor 1 (DOR1), to which Met-enkephalin binds, preferentially, in liver samples from 23 patients with chronic hepatitis C who had undergone antiviral therapy. Twelve patients obtained a sustained virological response, and 11 patients were relapsers after or nonresponders to treatment. Among the 12 patients with sustained virological response, one patient (8.3%) expressed MEIR and another one expressed DOR1 immunoreactivity (8.3%), whereas none of the patients expressed both immunoreactivities. Among the group of nonresponders/relapsers, one patient expressed MEIR (9%), two patients expressed DOR1 immunoreactivity (18.2%), and seven patients expressed both (63.6%). The difference between responders and nonresponders in the expression of both immunoreactivities was significantly different (P<0.001). Our data are in favor of the hypothesis that enhanced expression of MEIR and DOR1 immunoreactivity correlates with poor response to antiviral therapy that includes interferon. Thus, this study provides a rationale to study the effect of opiate antagonists in combination with antiviral therapy with interferon and ribavirin in patients who express MEIR in the liver and who have not responded to or who have relapsed after treatment with those antiviral medications.

Pharmacodynamic effects of opiate receptor antagonism: An agonist challenge study

Background Pharmacodynamic effects of opiate antagonists and agonists such as naltrexone (NTX) and fentanyl (FE) have been documented in separate studies and various populations. However, no data are available for a direct comparison in the same group of subjects. Methods NTX 50mg QD and a matched placebo were compared in a randomized double-blind crossover study conducted in a research hospital setting. NTX was administered for 3 days to 16 healthy males. ACTH, cortisol, FSH, LH, PRL and pupillary diameter (PD) were measured on Day 1. On Day 3, blockade of agonist-induced effects was tested by use of an IV bolus of FE. Results On Day 1, NTX induced an increase in LH, ACTH and cortisol (p<0.01 each; 90%CI ratio[1.29–1.91],[1.42–2.40],[1.26–2.06]). No effect was observed on FSH, PRL and PD. On Day 3, NTX was associated with a blockade of the following FE effects: increase in ACTH (p<0.0001,[0.15–0.59]), cortisol (p=0.01,[0.38–0.81]) and PRL (p<0.0001,[0.12–0.22]) and reduction in PD (p<0.0001,[2.47–3.32]). NTX did not influence pain perception during the cold pressor test performed after FE. Conclusion Blockade of FE-induced changes in PRL and PD were the most significant treatment effects observed. In contrast to FE-induced decreases in ACTH/cortisol reported in stressed or surgical patients, an increase was observed in healthy volunteers. This effect was blocked by NTX. Clinical Pharmacology & Therapeutics (2005) 77, P28–P28; doi: 10.1016/j.clpt.2004.11.110

The delta opioid receptor 1 is expressed by proliferating bile ductules in rats with cholestasis: Implications for the study of liver regeneration and malignant transformation of biliary epithelium

In sharp contrast with the normal adult liver, the fetal human and rat livers and the liver of rats with cholestasis secondary to bile duct resection express the preproenkephalin mRNA, which codes for the endogenous opioid peptide Met-enkephalin. Furthermore, Met-enkephalin immunoreactivity is detected in hepatocytes and in proliferating bile ductules in the cholestatic rat liver. These data suggest that in cholestasis endogenous opioids may have a local effect in the cholestatic liver. As endogenous opioids exert their effect by binding to opioid receptors, the presence of opioid receptors in the cholestatic livers would support the hypothesis that Met-enkephalin plays a role in situ. Preliminary data presented in this manuscript reveals the expression of the delta opioid receptor in the liver of rats with cholestasis. This finding suggests that there is a scenario in which Met-enkephalin can bind to opioid receptors in the liver in cholestasis to exert a local effect. In vivo studies in this model of cholestasis with the use of opioid agonist and antagonist will shed light on the possible role of opioidergic regulation of liver regeneration. Studies on the effect of opiate antagonists on the evolution of cholestasis in this animal model may provide insight into the mechanisms of liver regeneration. In addition, as some conditions associated with cholestasis and bile ductular proliferation can be complicated by malignancy, the expression of the delta opioid receptor in malignant tumors of the biliary tree merits research.

Evidence for Functional Release of Endogenous Opioids in the Locus Ceruleus during Stress Termination

Endogenous opioids target noradrenergic locus ceruleus (LC) neurons and potently inhibit LC activity. Nonetheless, it has been difficult to demonstrate functional regulation of the LC-noradrenergic system by endogenous opioids because of the lack of effect of opiate antagonists. The present findings provide evidence that endogenous opioids regulate LC neuronal activity during the termination of a stressor. LC neuronal discharge was recorded from halothane-anesthetized rats before, during, and after hypotensive stress elicited by intravenous nitroprusside infusion. In naive rats, mean arterial blood pressure was temporally correlated with LC activity such that hypotension was associated with increased LC discharge and a return to the normotensive state was associated with a decrease in LC discharge below pre-stress values. After microinfusion of an antagonist of the stress neuropeptide corticotropin-releasing factor (CRF) into the LC, the increase in LC discharge associated with hypotension was prevented, whereas LC inhibition associated with termination of the challenge occurred at an earlier time and was of a greater magnitude. In contrast, microinfusion of naloxone into the LC completely abolished LC inhibition associated with termination of the stressor. Naloxone microinfusion did not prevent LC inhibition associated with hypertension produced by intravenous vasopressin administration, suggesting that endogenous opioids may be selectively engaged during the termination of hypotensive stress. These results provide evidence for a functional release of endogenous opioids within the LC. This action of endogenous opioids may serve to counterbalance excitatory effects of CRF on the LC-norepinephrine system, thereby limiting its activation by stress.

Effect of opiate antagonists on stress-induced oesophageal hypersensitivity to distension

Effect of opiate antagonists on stress-induced oesophageal hypersensitivity to distension

S.27.01 - Finding susceptibility genes for schizophrenia

A recent neuropathological study has reported decreased levels of dynorphin A immunoreactivity in striato-pallidal fibers in the brain of a patient with severe Gilles de la Tourette's syndrome (TS). This observation, taken with the neuroanatomic distribution of dynorphin and its broad range of motor and behavioral effects, has led to speculation concerning its role in the pathobiology of TS. We report on the presence of elevated concentrations of dynorphin A [1–8] in the CSF of 7 TS patients, aged 20 to 45 years. The increase in CSF dynorphin was found to be associated with the severity of the obsessive compulsive symptoms but not with tic severity in these patients. Although CSF studies lack the precision necessary to address questions of selective involvement of neuronal systems in specific CNS locations, these findings suggest that endogenous opioids are involved in the pathobiology of TS and related disorders.Tourette's syndrome (TS) is a chronic neuropsychiatric disorder of childhood onset that is characterized by multiple motor and phonic tics that wax and wane in severity and an array of behavioral problems including some forms of obsessive compulsive disorder (OCD) (1). Once thought to be a rare condition, the prevalence of TS is now estimated to be on case per 1, 000 boys and one case per 10, 000 girls, and milder variants of the syndrome are likely to occur in a sizeable percentage of the population (2). Although the etiology of TS remains unknown, the vertical transmission of TS within families follows a pattern consistent with an autosomal dominant form of inheritance (3, 4). Neurobiologic and pharmacological data have implicated central monoaminergic and neuropeptidergic systems in the pathophysiology of TS, and basal ganglia structures remain the prime candidates as the neuroanatomical origin for TS and related conditions (1).Endogenous opioids, including dynorphin and met-enkephalin are concentrated in structures of the basal ganglia (5), are known to interact with central dopaminergic neurons (6, 7), and may play an important role in the control of motor functions (8). Post-mortem brain studies have directly implicated opioids in the pathophysiology of Parkinson's disease (9), Huntington's disease (10), and most recently in TS (11). The neuropathological study of Haber et al. (11) reported decreased levels of dynorphin A [1–17] immunoreactivity in striatal fibers projecting to the globus pallidus in the brain of a patient with severe TS. This observation, taken with the neuroanatomic distribution of dynorphin and its broad range of motor and behavioral effects, has led to speculation concerning its role in the pathobiology of TS. Additional evidence supporting the role of endogenous opioids comes from reports of dramatic, but poorly documented, effects of opiate antagonists in the treatment of TS patients (12, 13).

Opiate antagonists inhibit feeding induced by 8-OH-DPAT: possible mediation in the nucleus accumbens

Previous work has shown that the 5-hydroxytryptamine (5-HT) 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) elicits a variety of behaviours, including feeding in rats. These effects are accompanied by reduced 5-HT neurotransmission resulting from activation of somatodendritic 5-HT receptors located in the midbrain raphe nuclei. Dopamine antagonists injected either peripherally or into the nucleus accumbens reverse 8-OH-DPAT-induced feeding. Thus a facilitation of dopamine activity, secondary to reduced 5-HT activity, may be involved in mediating 8-OH-DPAT-induced feeding. Opiate antagonists have been shown previously to reduce several dopamine-dependent behaviours including feeding induced by dopaminergic drugs, tail pinch and electrical brain stimulation. Therefore experiments were conducted to assess the effects of opiate antagonists on feeding induced by peripheral, and raphe injection of 8-OH-DPAT in free-feeding rats. Following SC injection naloxone (0.1–10 mg/kg) dose-dependently reduced the feeding response induced by 100 μg/kg 8-OH-DPAT (SC). The lowest effective dose of naloxone was 1 mg/kg. This dose of naloxone also suppressed feeding induced by 8-OH-DPAT injected into either the dorsal (1 μg) or median (0.5 μg) raphe. Microinjecting 2 μg naloxone together with 8-OH-DPAT into either of these sites failed to prevent the increased feeding. These results indicate that the effects of naloxone are mediated at sites distal to the raphe nuclei. One possible site may be the nucleus accumbens, since methyl-naltrexone (0.3, 1 or 3 μg) injected into this site blocked the feeding responses to intra-raphe 8-OH-DPAT. Examination of the effects of naloxone and methyl-naltrexone in food-deprived rats ruled out intrinsic anorectic actions of these drugs as an explanation for the antagonism of 8-OH-DPAT-induced feeding. The results may indicate that reduced 5-HT function increases opiate activity in the nucleus accumbens. Alternatively opiate antagonists may prevent the indirect facilitation of dopamine activity resulting from a reduction in 5-HT neurotransmission. Opiate and dopamine systems in the nucleus accumbens play a major role in reward processes. Since opiate and dopamine antagonists reverse 8-OH-DPAT-induced feeding the possible role of reward mechanisms in mediating the effect of 8-OH-DPAT is discussed.

Electroacupuncture analgesia in rats: naltrexone antagonism is dependent on previous exposure

Inhibition of pain by low frequency electroacupuncture (EA) has been thought to be mediated by endogenous opioids. We reported in a previous paper, however, that naloxone (NAL) and naltrexone (NTX) either potentiated or had no effect on analgesia in EA-naive rats, independent of origin (American or Chinese), sex, weight, geographic location (the U.S.A. or China), or numerous variations of experimental methodology. In the present study, we hypothesized that the number of exposures to EA treatment may be responsible for the discrepant effect of opiate antagonists. We found, as previously demonstrated, analgesia in EA-naive rats was potentiated by NTX. After two pre-exposures to EA, however, NTX antagonized analgesia. These results indicate that, in rats: (1) pre-exposure is a key variable for opiate antagonists to produce antagonism or potentiation of analgesia; (2) environmental cues paired with the initial analgesic manipulation may be responsible for converting analgesia from non-opioid to opioid, as already demonstrated with classically conditioned and learned helplessness induced analgesia; and (3) EA analgesia in rats is a multidimensional manipulation which can be influenced by subtle environmental changes.

Interaction between opiate subtype and alpha-2 adrenergic agonists in suppression of noxiously evoked activity of WDR neurons in the spinal dorsal horn.

Several studies have demonstrated synergistic antinociception following low-dose administration of morphine and alpha-2 adrenergic agonists at the spinal level. This study was carried out in order to identify the opiate subtypes that are likely to be involved in such synergistic suppression of noxiously evoked activity of wide-dynamic-range (WDR) neurons in the dorsal horn of the spinal cord. We also examined the effect of opiate antagonists and alpha-2 adrenergic antagonists on the suppression produced by opiate or alpha-2 adrenergic agonists. Extracellular activity of single WDR neurons in the spinal dorsal horn, which was evoked by a radiant heat stimulus (51 degrees C), was recorded in decerebrate, spinally transected cats. Agonists were administered spinally and antagonists intravenously. In the synergism study, ineffective doses of the moderately selective mu agonist morphine (25 micrograms), the delta agonist DADL (20 micrograms), and the selective delta agonist DPDPE (30 micrograms), when combined with an ineffective dose of the alpha-2 adrenergic agonist clonidine (5 micrograms) produced significant synergistic suppression of noxiously evoked WDR neuronal activity. However, the ineffective or slightly effective dose of the selective mu agonist DAGO (1 or 1.5 micrograms, respectively) did not show any synergistic action with clonidine. Furthermore, the synergism between morphine and clonidine was reversed by the selective delta antagonist ICI 174,864. We interpret these results to indicate that opiates interact at spinal delta receptors to produce a synergistic suppression of evoked WDR neuronal activity in the presence of spinal clonidine. An alternative explanation is that ICI 174,864 may interact in some way with alpha-adrenergic systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereoselectivity and subtype of the opiate receptor involved in stress-induced hypertension

Naloxone enantiomorphs were given intracerebroventricularly (i.c.v.) to rats socially deprived for a brief period of time (7–14 days) in order to ascertain the mediation of central opiate receptors in the reversal effect of opiate antagonists on the high systolic blood pressure induced by this type of stress. While the active enantiomorph ((−)-naloxone, 20 nmol per rat) lowered the elevated blood pressure, the (+)-enantiomorph (which shows a 10 000-fold lower affinity for opiate receptors) had no effect. Additionally, the antihypertensive effect induced by i.c.v. administration of an antagonist of the μ-opiate receptor (β-funaltrexamine, 20 nmol per rat), but not of the δ-opiate receptor (ICI 174, 864, 15 nmol per rat) pointed to the involvement of μ-opiate receptors as the endogenous component of the hypertensive response of rats to stress.

Opponent process theory of motivation: Neurobiological evidence from studies of opiate dependence

One hypothetical model for a mechanism of drug dependence involves the development of an adaptive process that is initiated to counter the acute effects of the drug. This adaptive process persists after the drug has been cleared from the brain, leaving an opposing reaction unopposed (abstinence signs). From a motivational perspective a particularly attractive hypothesis has been that of opponent process theory (32). Here many reinforcers elicit positive affective and hedonic processes that are opposed by negative affective and hedonic processes. Thus the intense pleasure of the opiate drug "rush" or "high" would be opposed by aversive withdrawal symptoms. The present paper presents neurobiological evidence to support the opponent process concept and suggests neural circuitry that may be involved. The region of the nucleus accumbens in the forebrain of the rat has been shown to be a particularly sensitive substrate not only for the acute reinforcing properties of opiate drugs, but also for the response disruptive effects of opiate antagonists in opiate dependent rats. This region also appears to be particularly sensitive to the aversive stimulus effects of opiate antagonists using a place aversion measure in dependent rats. These results suggest that the region of the nucleus accumbens and its neural circuitry may be an important neural substrate for both the positive and negative motivational aspects of drug dependence.

Effects of opiate antagonists and their quaternary analogues on nucleus accumbens self-stimulation

Naloxone and naltrexone were compared with their quaternary analogues naloxone methobromide and naltrexone methobromide for efficacy in suppressing intracranial self-stimulation behavior. These quaternary analogues effectively block opiate receptors in the periphery, but since they do not readily cross the blood-brain barrier they have little effect on central receptors. Rats with electrodes in the nucleus accumbens were trained to self-stimulate in daily 60-min sessions. Naloxone (0.2, 2.0 and 20 mg/kg) and naltrexone (20 mg/kg) potently suppressed self-stimulation behavior. In contrast, neither naloxone methobromide (0.2 and 20 mg/kg) nor naltrexone methobromide (20mg/kg) had any significant effects on this behavior. These results suggest that blockade of peripheral opiate receptors alone is insufficient to suppress self-stimulation, and therefore support the idea that opiate antagonists suppress self-stimulation by blockade of central receptors that mediate reinforcement.

γ‐Hydroxybutyrate Stimulation of the Formation of Cyclic GMP and Inositol Phosphates in Rat Hippocampal Slices

The presence of gamma-hydroxybutyrate (GHB) (300-600 microM) in the incubation medium of rat hippocampal slices led to an increase of intracellular cyclic GMP and inositol phosphates. This phenomenon is dependent on the time and the dose of GHB used and might be the result of the stimulation of GHB receptor sites which are abundant in rat hippocampus. The increase of cyclic GMP and inositol phosphates is blocked by some anticonvulsants and opiate antagonists. These results seems to indicate that, like many substances inducing epileptic phenomena, GHB provokes neuronal depolarization in hippocampus which is accompanied by formation of cyclic GMP and inositol phosphates. The effect of opiate antagonists can be explained by the possible implication of an opiate synapse which mediates GHB effects in rat hippocampus.

Paradoxical and subtype‐specific effects of opiate antagonists on the expression of opioid receptors in rat brain cultures

Cultures of aggregating fetal rat brain cells express mu, delta, and kappa opioid receptors. The potent and long-lasting opioid antagonist naltrexone was used to investigate whether different regulatory mechanisms are involved in the expression of the three receptor subtypes. In cultures treated for seven days, naltrexone increased dose-dependently the binding of 3H-diprenorphine to the three receptor subtypes, with the mu sites being affected at a lower concentration than the other two; A Scatchard analysis indicated that this receptor up-regulation was obtained by an increase in the BMax, with no significant change in the affinity of the ligand to the receptors. In contrast to these effects in cultures treated for 7 days, it was surprising to find that a 48 hr treatment with naltrexone had an apparent converse and subtype-specific influence; the antagonist decreased significantly the binding of 3H-diprenorphine to both mu and delta receptors but had no effect on kappa sites. Two other opioid antagonists, naloxone and levallorphan, had a similar effect. Further analysis of naltrexone's mode of action was obtained by studying its effect on the adenylate cyclase activity. Of several inducers of this enzyme, the beta-adrenergic inducer isoproterenol gave the highest increase in cyclic AMP. Naltrexone had no significant effect on the basal adenylate cyclase activity but it altered the pattern of cyclic AMP formation in isoproterenol-stimulated cultures. Overall, the results indicate that in addition to its classic antagonistic activity, naltrexone exhibits in fetal brain aggregates some properties associated with opiate agonists.

Relationship between behavioral and nociceptive changes in attacked mice: effects of opiate antagonists

The relationship between analgesia and behavior during and after an aggressive encounter was investigated in saline- and opiate antagonist-treated DBA mice. A low number of bites induced an analgesia that was reversed by beta-chlornaltrexamine but not by naloxone, and that correlated positively with increased displays of defensive upright and immobility upon contact with the opponent. Extended attacks induced a naloxone-sensitive analgesia that was linked to a delayed occurrence of "panic" escape behavior. In the post-conflict phase, the degree of immobility and analgesia correlated positively in attacked mice. Naltrexone prevented this analgesia and lowered immobility. Endogenous opioids released during social conflict may induce analgesia and immobility in DBA mice.

Naloxone, but not Tyr-MIF-1, reduces volitional ethanol drinking in rats: Correlation with degree of spontaneous preference

The possible relationship between the actions of ethanol and opiates led us to examine the effect of opiate antagonists on ethanol intake in rats with a free choice of water. Naloxone (NAL) significantly reduced intake of ethanol. This effect was much greater in “high-preferring” (ethanol/total fluid intake> 60%) than in “low-preffering” (ethanol/total fluid intake <30%) rats. Furthermore, a correlation was found between the degree of spontaneous preference (ethanol/total fluid intake ratio) and the reduction of ethanol drinking by NAL. Sensitivity to NAL increased with increased preference for ethanol. Neither Try-MIF-1 (Tyr-Pro-Leu-Gly-NH 2) nor MIF-1 (Pro-Leu-Gly-NH 2) caused a significant modification of ethanol intake. This study shows that NAL can reduce volitional ethanol intake in rats and provides further evidence that Tyr-MIF-1 does not always act like NAL.

Effect of opiate antagonists on middle cerebral artery occlusion infarct in the rat.

The authors examined the effect of the opiate antagonists naloxone and thyrotropin-releasing hormone (TRH) on neurological outcome and the size of areas of cerebral infarction in a rat model of focal cerebral ischemia. The middle cerebral artery (MCA) was permanently occluded in 66 adult Sprague-Dawley rats. The rats were randomly divided into three groups. In 20 Group I rats, TRH in normal saline was administered initially as a 2-mg/kg bolus followed by continuous infusion of 2 mg/kg/hr for 4 hours. In 20 Group II rats, naloxone in normal saline was administered initially as a 2-mg/kg bolus followed by continuous infusion of 2-mg/kg/hr for 4 hours. In 26 Group III rats, physiological saline was administered as an initial 0.5-cc bolus followed by continuous infusion of 0.5 cc/hr for 4 hours. All solutions were given in volumes of 0.5 cc for the bolus and 0.5 cc/hr for continuous infusion, and all infusions were begun within 10 minutes of MCA occlusion. Twenty-four hours after treatment, the rats underwent a careful neurological examination and were then sacrificed immediately. The size of areas of cerebral infarction was evaluated using 2,3,5-triphenyltetrazolium chloride staining techniques. The neurological grade of the rats correlated with the size of infarcted areas among all grades, irrespective of treatment (p less than 0.01). Neither naloxone nor TRH improved neurological function or reduced the size of infarction compared to saline-treated control rats. Treatment with TRH caused a significant increase in mean arterial blood pressure during infusion, but naloxone had no effect. These results suggest that neither TRH nor naloxone are effective in the treatment of acute focal cerebral ischemia.

Opiate antagonists and rewarding brain stimulation

This review examines the literature on the effects of opiate antagonists on brain stimulation (ICSS) reward. Antagonists should have predictable effects if endogenous opioids modulate ICSS. Naloxone is the antagonist most often used, and it has produced inconsistent results in some ICSS paradigms. When schedules of intermittent reinforcement are used, however, naloxone reliably reduces the rate of responding. It reverses the effects of opiate agonists on ICSS behavior, and it also attenuates the effects of psychomotor stimulants, such as amphetamine. The results produced by naloxone are consistent with a modulatory effect of endogenous opioid systems on reward, and suggest that the opiate and dopamine systems together exert significant control over ICSS. Further research is needed to characterize better the actions of the antagonists on ICSS behavior, and productive research directions are proposed. Data obtained in future studies might suggest how the endogenous opioid systems modulate both natural and brain stimulation reward.