Naloxone as a GABA antagonist: evidence from iontophoretic, receptor binding and convulsant studies

Susceptibility to Pentylenetetrazole-Induced Seizures in Mice with Distinct Activity of the Endogenous Opioid System

Currently, pharmacotherapy provides successful seizure control in around 70% of patients with epilepsy; however, around 30% of cases are still resistant to available treatment. Therefore, effective anti-epileptic therapy still remains a challenge. In our study, we utilized two mouse lines selected for low (LA) and high (HA) endogenous opioid system activity to investigate the relationship between down- or upregulation of the opioid system and susceptibility to seizures. Pentylenetetrazole (PTZ) is a compound commonly used for kindling of generalized tonic-clonic convulsions in animal models. Our experiments revealed that in the LA mice, PTZ produced seizures of greater intensity and shorter latency than in HA mice. This observation suggests that proper opioid system tone is crucial for preventing the onset of generalized tonic-clonic seizures. Moreover, a combination of an opioid receptor antagonist-naloxone-and a GABA receptor agonist-diazepam (DZP)-facilitates a significant DZP-sparing effect. This is particularly important for the pharmacotherapy of neurological patients, since benzodiazepines display high addiction risk. In conclusion, our study shows a meaningful, protective role of the endogenous opioid system in the prevention of epileptic seizures and that disturbances in that balance may facilitate seizure occurrence.

Comparison of the effects of fentanyls and other μ opioid receptor agonists on the electrical activity of respiratory muscles in the rat

Introduction: Deaths due to overdose of fentanyls result primarily from depression of respiration. These potent opioids can also produce muscle rigidity in the diaphragm and the chest muscles, a phenomenon known as Wooden Chest Syndrome, which further limits ventilation. Methods: We have compared the depression of ventilation by fentanyl and morphine by directly measuring their ability to induce muscle rigidity using EMG recording from diaphragm and external and internal intercostal muscles, in the rat working heart-brainstem preparation. Results: At equipotent bradypnea-inducing concentrations fentanyl produced a greater increase in expiratory EMG amplitude than morphine in all three muscles examined. In order to understand whether this effect of fentanyl was a unique property of the phenylpiperidine chemical structure, or due to fentanyl's high agonist intrinsic efficacy or its lipophilicity, we compared a variety of agonists with different properties at concentrations that were equipotent at producing bradypnea. We compared carfentanil and alfentanil (phenylpiperidines with relatively high efficacy and high to medium lipophilicity, respectively), norbuprenorphine (orvinolmorphinan with high efficacy and lipophilicity) and levorphanol (morphinan with relatively low efficacy and high lipophilicity). Discussion: We observed that, agonists with higher intrinsic efficacy were more likely to increase expiratory EMG amplitude (i.e., produce chest rigidity) than agonists with lower efficacy. Whereas lipophilicity and chemical structure did not appear to correlate with the ability to induce chest rigidity.

Lesson of the month 1: Sodium valproate-induced encephalopathy

A 69-year-old man developed reduced consciousness of sudden onset. Examination and parameters were normal, except for a Glasgow Coma Scale (GCS) score of six. Brain imaging and blood tests were also normal, except for high plasma ammonia. His past medical history included epilepsy, hypertension and colitis. He was taking multiple antiepileptic medications, including sodium valproate, with no recent dose alterations. Medical intervention led to the sodium valproate being stopped and naloxone being administered. The patient's level of responsiveness and ammonia levels gradually improved. The patient was also being treated with ciprofloxacin for a urinary tract infection and a newly developed syndrome of inappropriate antidiuretic hormone secretion treated with demeclocycline. There is an association between long-term sodium valproate use and low carnitine levels, especially in the setting of polypharmacy. This in turn precipitates hyperammonaemia and encephalopathy. This case highlights the importance of an adequate drug history and the awareness of serious but uncommon adverse effects.

Lack of Specific Involvement of (+)-Naloxone and (+)-Naltrexone on the Reinforcing and Neurochemical Effects of Cocaine and Opioids

Effective medications for drug abuse remain a largely unmet goal in biomedical science. Recently, the (+)-enantiomers of naloxone and naltrexone, TLR4 antagonists, have been reported to attenuate preclinical indicators of both opioid and stimulant abuse. To further examine the potential of these compounds as drug-abuse treatments, we extended the previous assessments to include a wider range of doses and procedures. We report the assessment of (+)-naloxone and (+)-naltrexone on the acute dopaminergic effects of cocaine and heroin determined by in vivo microdialysis, on the reinforcing effects of cocaine and the opioid agonist, remifentanil, tested under intravenous self-administration procedures, as well as the subjective effects of cocaine determined by discriminative-stimulus effects in rats. Pretreatments with (+)-naloxone or (+)-naltrexone did not attenuate, and under certain conditions enhanced the stimulation of dopamine levels produced by cocaine or heroin in the nucleus accumbens shell. Furthermore, although an attenuation of either cocaine or remifentanil self-administration was obtained at the highest doses of (+)-naloxone and (+)-naltrexone, those doses also attenuated rates of food-maintained behaviors, indicating a lack of selectivity of TLR4 antagonist effects for behaviors reinforced with drug injections. Drug-discrimination studies failed to demonstrate a significant interaction of (+)-naloxone with subjective effects of cocaine. The present studies demonstrate that under a wide range of doses and experimental conditions, the TLR4 antagonists, (+)-naloxone and (+)-naltrexone, did not specifically block neurochemical or behavioral abuse-related effects of cocaine or opioid agonists.

Using [(11)C]Ro15 4513 PET to characterise GABA-benzodiazepine receptors in opiate addiction: Similarities and differences with alcoholism

The importance of the GABA-benzodiazepine receptor complex and its subtypes are increasingly recognised in addiction. Using the α1/α5 benzodiazepine receptor PET radioligand [¹¹C]Ro15 4513, we previously showed reduced binding in the nucleus accumbens and hippocampus in abstinent alcohol dependence. We proposed that reduced [¹¹C]Ro15 4513 binding in the nucleus accumbens was a marker of addiction whilst the reduction in hippocampus and positive relationship with memory was a consequence of chronic alcohol abuse. To examine this further we assessed [¹¹C]Ro15 4513 binding in another addiction, opiate dependence, and used spectral analysis to estimate contributions of α1 and α5 subtypes to [¹¹C]Ro15 4513 binding in opiate and previously acquired alcohol-dependent groups. Opiate substitute maintained opiate-dependent men (n = 12) underwent an [¹¹C]Ro15 4513 PET scan and compared with matched healthy controls (n = 13). We found a significant reduction in [¹¹C]Ro15 4513 binding in the nucleus accumbens in the opiate-dependent compared with the healthy control group. There was no relationship between [¹¹C]Ro15 4513 binding in the hippocampus with memory. We found that reduced [¹¹C]Ro15 4513 binding was associated with reduced α5 but not α1 subtypes in the opiate-dependent group. This was also seen in an alcohol-dependent group where an association between memory performance and [¹¹C]Ro15 4513 binding was primarily driven by α5 and not α1 subtype. We suggest that reduced α5 levels in the nucleus accumbens are associated with addiction since we have now shown this in dependence to two pharmacologically different substances, alcohol and opiates.

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Lower [¹¹C]Ro15 4513 binding is evident in the nucleus accumbens of opiate addicts.

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This appears primarily due to lower levels of α5 subtype of the GABA-A receptor.

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Lower [¹¹C]Ro15 4513 binding is similarly found in alcoholism.

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Lower levels of α5 GABA-A receptor in nucleus accumbens may underpin addiction.

Effects of naloxone and flumazenil on antinociceptive action of acetaminophen in rats

BACKGROUND

Studies of acetaminophen suggest that multiple nociceptive pathways are involved in the drug's analgesic action.

OBJECTIVE

The purpose of this study was to determine whether naloxone and flumazenil were able to modify or antagonize the antinociceptive effect of acetaminophen in rats.

METHODS

Adult albino Wistar rats were used in the study and randomly allocated to 1 of 4 groups. The acetaminophen group (A group) was administered IP saline and then 300 mg/kg IP acetaminophen 5 minutes thereafter. The acetaminophen + naloxone group (AN group) was pretreated with 1 mg/kg IP naloxone, followed by 300 mg/kg IP acetaminophen 5 minutes later. The acetaminophen + flumazenil group (AF group) was pretreated with 1 mg/kg IP flumazenil, followed by 300 mg/kg IP acetaminophen 5 minutes later. The control group received 2.5 mL IP saline, followed by an additional 2.5 mL IP injection of saline 5 minutes later. The paw-withdrawal latency period of the rats was assessed by an investigator blinded to treatment using the hot-plate test at 30, 45, 60, and 90 minutes after administration of acetaminophen.

RESULTS

Thirty-two rats were evenly randomized by envelope method into 4 groups of 8 rats each. Baseline values for the A, AN, AF, and control groups were not significantly different (9.1 [2.3], 10.5 [2.7], 9.8 [3.0], and 8.9 [1.4] sec, respectively). In the AF group, flumazenil appeared to antagonize the analgesic effect exerted by the acetaminophen in the hot-plate test (30 min, 10.3 [3.7] sec; 45 min, 11.7 [5.1] sec; 60 min, 12.1 [5.1] sec; and 90 min, 12.2 [4.9] sec) and values were not significantly different from those obtained in the control group (30 min, 9.8 [2.2] sec; 45 min, 9.0 [1.6] sec; 60 min, 9.2 [1.6] sec; and 90 min, 8.5 [2.0] sec). In the AN group, naloxone did not significantly affect the values observed in the hot-plate test (30 min, 18.0 [4.5] sec; 45 min, 21.5 [7.8] sec; 60 min, 20.5 [5.9] sec; and 90 min, 22.3 [7.4] sec) and values at all time points were not significantly different from those obtained in the A group (30 min, 17.8 [7.6] sec; 45 min, 20.9 [6.9] sec; 60 min, 21.5 [7.3] sec; and 90 min, 23.8 [8.6] sec). All postbaseline values in the A and AN groups were significantly increased versus baseline and versus the control group values (all, P < 0.05). All postbaseline values in the A group were significantly greater than those in the AF group (all, P < 0.05).

CONCLUSION

Flumazenil antagonized the analgesic effect exerted by acetaminophen, while naloxone had no significant effect on acetaminophen's antinociceptive action in this pain model in rats.

Polydrug abuse: a review of opioid and benzodiazepine combination use

This paper reviews studies examining the pharmacological interactions and epidemiology of the combined use of opioids and benzodiazepines (BZDs). A search of English language publications from 1970 to 2012 was conducted using PubMed and PsycINFO(®). Our search found approximately 200 articles appropriate for inclusion in this paper. While numerous reports indicate that the co-abuse of opioids and BZDs is ubiquitous around the world, the reasons for the co-abuse of these medications are not entirely clear. Though the possibility remains that opioid abusers are using BZDs therapeutically to self-medicate anxiety, mania or insomnia, the data reviewed in this paper suggest that BZD use is primarily recreational. For example, co-users report seeking BZD prescriptions for the purpose of enhancing opioid intoxication or "high," and use doses that exceed the therapeutic range. Since there are few clinical studies investigating the pharmacological interaction and abuse liability of their combined use, this hypothesis has not been extensively evaluated in clinical settings. As such, our analysis encourages further systematic investigation of BZD abuse among opioid abusers. The co-abuse of BZDs and opioids is substantial and has negative consequences for general health, overdose lethality, and treatment outcome. Physicians should address this important and underappreciated problem with more cautious prescribing practices, and increased vigilance for abusive patterns of use.

The role of mu-opioid receptor signaling in the dorsolateral periaqueductal gray on conditional and unconditional responding to threatening and aversive stimuli

Here we examined how mu-opioid receptor signaling in the periaqueductal gray (PAG) mediates conditional and unconditional responses to aversive stimuli. The mu-opioid agonist morphine (MOR) and/or the partially mu-selective antagonist naltrexone (NAL) were infused into dorsolateral PAG (dlPAG) during a fear conditioning task, in which rats were trained to fear an auditory conditional stimulus (CS) by pairing it with a unilateral eyelid shock unconditional stimulus (US). During drug-free test sessions, the CS elicited movement suppression responses (indicative of freezing) from trained rats that had not recently encountered the US. In trained rats that had recently encountered the US, the CS elicited flight behavior characterized by turning in the direction away from the eyelid where US delivery was anticipated. Infusions of MOR (30 nmol/side) into dlPAG prior to the test session did not impair CS-evoked movement suppression, but did impair CS-evoked turning behaviors. MOR infusions also reduced baseline motor movement, but US-evoked reflex movements remained largely intact. NAL was infused at two dosages, denoted 1x (26 nmol/side) and 10x (260 nmol/side). Infusions of NAL into dlPAG did not affect CS- or US-evoked behavioral responses at the 1x dosage, but impaired CS-evoked movement suppression at the 10x dosage, both in the presence and absence of MOR. When rats were co-infused with MOR and NAL, MOR-induced effects were not reversed by either dosage of NAL, and some measures of MOR-induced movement suppression were enhanced by NAL at the 1x dosage. Based on these findings, we conclude that mu-opioid receptors in dlPAG may selectively regulate descending supraspinal motor pathways that drive active movement behaviors, and that interactions between MOR and NAL in dlPAG may be more complex than simple competition for binding at the mu receptor.

Benefit effect of naloxone in benzodiazepines intoxication: Findings of a preliminary study

Background: Naloxone, as a low-priced and available drug, may be useful in improvement of signs and symptoms of benzodiazepines intoxication. The aim of this study was assessment of its effect on benzodiazepines poisoning. Methods: In this clinical-trial study, patients with typical signs and symptoms of benzodiazepines poisoning, who were referred to a poisoning center in Tehran in 2008, were selected. After recording of patients’ characteristics, supportive treatment was initiated and patients were randomly assigned to the case group with intravenous (IV) injection of two 0.4 mg naloxone ampules or to the control group. Their signs and symptoms were evaluated again 0.5 hour later. Each of diazepam, clonazepam and alperazolam drug group had 30 patients and lorazepam drug group had 26 patients, half of which patients in each drug group received naloxone. Results: Most of the participants were female and the mean age was 28 years. There were no significant differences between case and control groups in age, sex, time of drug consumption, tablet counts, signs and symptoms and level of consciousness at the admission time in each drug types. After naloxone injection in case groups, all signs and symptoms significantly improved in all drug types in comparison to control groups except nystagmus. In addition, level of consciousness significantly improved in case groups in all drug types except lorazepam. Conclusion: Findings of the study showed that naloxone is effective in management of benzodiazepines poisoning. However, future clinical trials with greater sample size are recommened.

Chronic valproic acid intoxication: reversal by naloxone

A 76-year-old woman being treated with sodium valproate for bipolar depression presented with a 4 day history of acute confusion and tremulousness. She had apnoeic episodes, reduced conscious level and generalised myoclonic movements. Her plasma valproate concentration was 848 micromol/l (normal 300-600 micromol/l). Administration of naloxone 0.8 mg led to rapid clinical improvement. Naloxone may be useful in reversing the features of chronic valproate toxicity.

Fatal valproate overdose in a newborn baby

Valproate is a widely used drug in the treatment of epilepsy in children and adults. However, it is not safe for patients under two years of age, especially during the newborn period. This study presents a case of fatal valproate overdose in a 26-day-old female newborn, who is the youngest patient in the literature.

Opioid antagonism of cannabinoid effects: differences between marijuana smokers and nonmarijuana smokers

In non-human animals, opioid antagonists block the reinforcing and discriminative-stimulus effects of Delta(9)-tetrahydrocannabinol (THC), while in human marijuana smokers, naltrexone (50 mg) enhances the reinforcing and subjective effects of THC. The objective of this study was to test a lower, more opioid-selective dose of naltrexone (12 mg) in combination with THC. The influence of marijuana-use history and sex was also investigated. Naltrexone (0, 12 mg) was administered 30 min before oral THC (0-40 mg) or methadone (0-10 mg) capsules, and subjective effects, task performance, pupillary diameter, and cardiovascular parameters were assessed in marijuana smoking (Study 1; n=22) and in nonmarijuana smoking (Study 2; n=21) men and women. The results show that in marijuana smokers, low-dose naltrexone blunted the intoxicating effects of a low THC dose (20 mg), while increasing ratings of anxiety at a higher THC dose (40 mg). In nonmarijuana smokers, low-dose naltrexone shifted THC's effects in the opposite direction, enhancing the intoxicating effects of a low THC dose (2.5 mg) and decreasing anxiety ratings following a high dose of THC (10 mg). There were no sex differences in these interactions, although among nonmarijuana smokers, men were more sensitive to the effects of THC alone than women. To conclude, a low, opioid-selective dose of naltrexone blunted THC intoxication in marijuana smokers, while in nonmarijuana smokers, naltrexone enhanced THC intoxication. These data demonstrate that the interaction between opioid antagonists and cannabinoid agonists varies as a function of marijuana use history.

gamma-Hydroxybutyrate/sodium oxybate: neurobiology, and impact on sleep and wakefulness

gamma-Hydroxybutyrate (GHB) is an endogenous short chain fatty acid and a, mostly oral, pharmacological compound that has been utilised in a variety of ways. Endogenously, GHB is synthesised locally within the CNS, mostly from its parent compound GABA. Sodium oxybate is the sodium salt of GHB and is used for the exogenous oral administration of GHB. It is likely that supraphysiological concentrations of GHB from exogenous administration produce qualitatively different neuronal actions than those produced by endogenous GHB concentrations. Evidence suggests a role for GHB as a neuromodulator/neurotransmitter. Under endogenous conditions and concentrations, and depending on the cell group affected, GHB may increase or decrease neuronal activity by inhibiting the release of neurotransmitters that are co-localised with GHB. After exogenous administration, most of the observed behavioural effects appear to be mediated via the activity of GHB at GABA(B) receptors, as long as the concentration is sufficient to elicit binding, which does not happen at endogenous concentrations. Endogenous and exogenous GHB is rapidly and completely converted into CO(2) and H(2)O through the tricarboxylic acid cycle (Krebs cycle). Sodium oxybate has been observed to modulate sleep in nonclinical study participants, and sleep and wakefulness in clinical populations, including groups with insomnia, fibromyalgia and narcolepsy. In narcolepsy, sodium oxybate has shown dose-related effects on various properties of sleep, including increases in slow-wave sleep duration and delta power, and a reduced number of night-time awakenings. Furthermore, multiple measures of daytime sleepiness and cataplexy demonstrated consistent short- and long-term improvement in response to night-time sodium oxybate therapy. The most common reported adverse events include dose-related headache, nausea, dizziness and somnolence.

Windup in dorsal horn neurons is modulated by endogenous spinal mu-opioid mechanisms

The mu-opioid receptor (MOR) plays a critical role in morphine analgesia and nociceptive transmission. However, the physiological roles for endogenous MOR mechanisms in modulating spinal nociceptive transmission, and particularly in the enhanced excitability of spinal nociceptive neurons after repeated noxious inputs, are less well understood. Using a MOR gene knock-out (-/-) approach and an MOR-preferring antagonist, we investigated the roles of endogenous MOR mechanisms in processing of acute noxious input and in neuronal sensitization during windup-inducing stimuli in wide dynamic range (WDR) neurons. Extracellular single-unit activity of WDR neurons was recorded in isoflurane-anesthetized MOR(-/-) and wild-type C57BL/6 mice. There were no significant differences between the genotypes in the responses of deep WDR cells to acute mechanical stimuli, graded electrical stimuli, and noxious chemical stimuli applied to the receptive field. Intracutaneous electrical stimulation at 1.0 Hz produced similar levels of windup in both genotypes. In contrast, 0.2 Hz stimulation induced significantly higher levels of windup in MOR(-/-) mice compared with the wild-type group. In wild-type mice, spinal superfusion with naloxone hydrochloride (10 mM, 30 microl) significantly enhanced windup to 0.2 Hz stimulation in both deep and superficial WDR cells. A trend toward facilitation of windup was also observed during 1.0 Hz stimulation after naloxone treatment. These results suggest that endogenous MOR mechanisms are not essential in the processing of acute noxious mechanical and electrical stimuli by WDR neurons. However, MORs may play an important role in endogenous inhibitory mechanisms that regulate the development of spinal neuronal sensitization.

Naltrexone plus benzodiazepine aids abstinence in opioid-dependent patients

Naltrexone (NTX) is widely used to prevent relapse of opioid-dependent patients but its association with insomnia and "hyperexcitability" can result in treatment withdrawal. We evaluated whether NTX combined with the benzodiazepine prazepam was more effective than NTX in keeping patients opioid-free. We determined the relapse rate over 6 months in 56 opioid-dependent subjects, divided into 4 equal groups. All groups received psychological support and underwent urine tests for drug metabolites twice weekly. Group 1 did not receive pharmacological treatment (controls). Group 2 received NTX alone (one 50-mg tablet daily); group 3 received NTX (one 50-mg tablet daily) plus placebo (one tablet twice daily); and group 4 received NTX (one 50-mg tablet daily) plus prazepam (one 10-mg tablet twice daily). Ten patients of group 1 relapsed within 3 months, one after 6 months and three remained opioid-free. Six patients of group 2 relapsed within three months, two after 6 months, and six remained opioid-free. Seven patients of group 3 relapsed three months, one after 6 months and six patients remained opioid-free. In group 4, one patient relapsed within 3 months and one patient after 6 months; 12 patients of this group remained opioid-free. At urine tests, a significantly higher percent patients of group 4 remained free of Delta(9)-tetrahydrocannabinol versus patients of groups 2 and 3. In conclusion, many patients remained opioid-free on NTX alone or combined with prazepam, with a significant advantage for the NTX plus prazepam group.

Possible mechanism involved in the anticonvulsant action of butorphanol in mice

The study was designed to examine the effect of butorphanol, a classical opioid on convulsive behaviour using maximal electroshock (MES) test. An attempt was also made to investigate the role of possible receptor mechanisms involved. MES seizures were induced in mice via transauricular electrodes (60 mA, 0.2 s). Seizure severity was assessed by the duration of tonic hindlimb extensor phase and mortality due to convulsions. Intraperitoneal administration of butorphanol produced a dose-dependent (0.25-2 mg/kg) protection against hindlimb extensor phase. The anticonvulsant effect of butorphanol was antagonized by all the three opioid receptor antagonists (i.e., naloxone [mu], MR2266 [kappa], and naltrindole [delta], respectively). Coadministration of gamma-aminobutyric acid (GABA)-ergic drugs (diazepam, GABA, muscimol, and baclofen) and N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (MK801), with butorphanol augmented the anticonvulsant action of the latter drug. In contrast, flumazenil, a central benzodiazepine (BZD) receptor antagonist, reversed the facilitatory effect of diazepam on the anti-MES effect of butorphanol. Similarly, delta-aminovaleric acid (DAVA), a GABA(B) receptor antagonist, antagonized the facilitatory effect of baclofen, a GABA(B) agonist on anti-MES action of butorphanol. These BZD-GABAergic antagonists, flumazenil or DAVA, per se also counteracted the anti-MES effect of butorphanol given alone. These data exemplify the benefits of using the MES test, which is sensitive to opioidergic compounds and distinguished convulsive behavioural changes associated with GABAergic and NMDAergic effects. Taken together, the results implicate a role for multitude of neurotransmitter systems, i.e., opioid (mu, kappa, delta), NMDA channel, BZD-GABA(A) chloride channel complex, and GABA(B) receptors in the anti-MES action of butorphanol.

GABA(A) receptor activation at medullary sympathetic neurons contributes to postexercise hypotension

A single bout of exercise results in a postexercise hypotension (PEH) that is accompanied by a reduced baroreflex function. Based on the role of rostral ventrolateral medulla (RVLM) neurons in controlling sympathetic nerve activity (SNA) and blood pressure, the role of gamma-aminobutyric acid (GABA) in controlling RVLM neuronal activity, and the reduced baroreflex-SNA relationship during PEH, we determined whether: 1) RVLM neuronal activity is decreased during PEH, 2) GABA(A)-receptor mechanisms mediate the decrease, and 3) baroreflex control of RVLM activity is reduced. Spontaneously hypertensive rats (SHR) were subjected to 40 min of treadmill or sham exercise (Sham PEH). PEH lasted 10 h in conscious and anesthetized SHR, indicating that the anesthetics did not affect the expression of PEH. Extracellular RVLM neuronal activity having a cardiac and sympathetic rhythm, lumbar SNA, and blood pressure were recorded at rest and during baroreflex function curves. Resting RVLM neuronal activity was lower and was increased to a greater extent by GABA(A)-receptor antagonism in PEH versus Sham PEH (P < 0.05). Baroreflex control of RVLM neuronal activity operated with a reduced gain (P < 0.05). Thus increased GABA signaling at RVLM neurons may contribute to PEH.

Use of naloxone in valproic acid overdose: case report and review

We present a case of a mixed ingestion of valproic acid, gabapentin, mexilitine, and ethanol with central nervous system depression that was reversed by naloxone. This report represents the fourth case demonstrating the antidotal efficacy of naloxone in reversing central nervous system depression associated with acute valproic acid overdose. Increasing clinical experience will more fully elucidate indications for, and optimal dosing of, naloxone in valproic acid toxic states.

GHB: a new and novel drug of abuse

There has been increasing attention in the United States to problems of abuse of gamma-hydroxybutyrate (GHB), with some evidence for problems in other parts of the world as well. In vitro and animal research show that, while GHB shares some properties with abused central nervous system depressant drugs, it has unique aspects of its pharmacology as well, including actions at a specific neural receptor which probably mediates many of its effects. Abuse potential assessment of GHB using standard animal models has not yielded a picture of a highly abusable substance, but little human testing has yet been done. Very little systematic data exist on tolerance and dependence with GHB, but both have been seen in human users. Quantitative data on the prevalence of GHB abuse is incomplete, but various qualitative measures indicate that a mini-epidemic of abuse began in the late 1980s and continues to the present. GHB is often included with the group of 'club drugs', and can be used as an intoxicant. It also has been used as a growth promoter and sleep aid and has been implicated in cases of 'date rape', usually in combination with alcohol. Undoubtedly the easy availability of GHB and some of its precursors has contributed to its popularity. Recent changes in the control status of GHB in the US may reduce its availability with as yet unknown consequences for the scope of the public health problem. Drug abuse experts need to familiarize themselves with GHB as possibly representing a new type of drug abuse problem with some unique properties.

Tonic opioid inhibition of the subiculo-accumbens pathway

There is evidence to suggest that medium spiny neurons (MSNs) in the nucleus accumbens (NAS) should be sensitive to opiate compounds. However, neuronal responses in the NAS evoked by fimbria stimulation (F-D) are insensitive to systemically or iontophoretically administered morphine. The hypothesis of this study was that fimbria-evoked NAS responses may fail to demonstrate sensitivity to morphine because they are under tonic opioid inhibition and can't be further inhibited by opiates. If correct, then pharmacological inhibition of opioid actions on these NAS neuronal responses should result in an increase of response to fimbria stimulation. The effects of systemic and iontophoretic administrations of naloxone on NAS responses evoked by fimbria stimulation were observed. Systemically and locally administered naloxone selectively increased the excitability of accumbens single-unit responses to fimbria stimulation. Conversely, systemic or iontophoretic administration of morphine was without effect on the same types of NAS responses. These observations are consistent with the hypothesis that a tonic opioid inhibition may regulate this pathway. In contrast, naloxone and morphine effect other NAS circuit responses differently than F-D NAS responses. In some cases naloxone and morphine tests have been conducted on different evoked responses from the same neuron. Those results have shown that different responses from the same cell may be differentially affected. Consequently, opioid modulation of activity in the NAS is probably pathway-specific rather than neuron-specific.

Anxiolytic-like effects of meprobamate. Interactions with an opiate antagonist in Swiss and BALB/c mice

Naloxone has previously been shown to block the effects of benzodiazepines in the Swiss but not in the BALB/c strain. We have also reported that naloxone potentiates subeffective doses of benzodiazepines in Swiss mice. In the present studies we first determined whether naloxone could block anxiolytic-like effects of meprobamate in Swiss and BALB/c mice. Then we evaluated if subeffective doses of meprobamate could be potentiated in Swiss as well as in BALB/c mice. The elevated plus-maze test and the light/dark choice procedure were used. The lowest dose of meprobamate with anxiolytic-like effects was 60 mg/kg in the BALB/c mice. This dose was effective in both the plus-maze and in the light/dark choice procedure. In Swiss mice the same dose was effective in the plus-maze, whereas 120 mg/kg was required in the light/dark choice procedure. When an effective dose of meprobamate was combined with naloxone, 10 mg/kg, no blockade of anxiolytic-like effects was obtained in any strain in any procedure. To the contrary, when a subeffective dose of meprobamate was combined with naloxone, 10 mg/kg, an anxiolytic-like effect was obtained in both strains in both procedures. The present series of experiment shows that the ability of naloxone to block anxiolytic-like drug effects do not apply to meprobamate. However, the naloxone-induced potentiation of subeffective doses previously observed after treatment with benzodiazepines or buspirone was present also after treatment with meprobamate. Moreover, although blockade of anxiolytic-like drug effects with naloxone has not been observed in BALB/c mice, potentiation was as evident in that strain as in the Swiss. This suggests that the mechanisms behind naloxone's blockade of anxiolytic-like effects are independent from those behind its potentiation of such effects.

Naloxone reverses disinhibitory/aggressive behavior in 5,7-DHT-lesioned rats; involvement of GABA(A) receptor blockade?

Effective medical treatment for impulsive aggression and several impulse control disorders is needed. Disinhibited, impulsive behavior of e.g. murderers, arsonists, suicidal patients, and patients suffering from antisocial personality or substance abuse disorders has been associated with signs of a deficiency in brain serotonin (5-HT) systems. Depletion of brain 5-HT consistently produces disinhibition and aggression also in experimental animals. The present series of experiments using a modified Vogel's conflict test indicates that the disinhibitory behavior of 5-HT-lesioned rats can be reversed by the commonly used opiate receptor antagonist naloxone at doses (0.1-5.0 mg/kg, s.c.) that do not significantly affect behavior in sham-lesioned controls. Moreover, this effect of naloxone, which resembles that previously observed after administration of negative modulators of gamma-aminobutyric acidA (GABA(A))/benzodiazepine receptor complexes, was reversed by a low inert dose (2.0 mg/kg, i.p.) of amobarbital. Furthermore, both naloxone (5.0 mg/kg, s.c.) and Ro 15-4513 (1.0 mg/kg, p.o.; a partial inverse agonist at benzodiazepine receptors) significantly decreased the number of attacks and the time spent in aggressive acts in 5,7-DHT-lesioned male residents. These results taken together with previous behavioral and neurochemical data suggest that the behavioral effects of naloxone observed here may involve an antagonistic action at brain gamma-aminobutyric acidA (GABA(A))/benzodiazepine receptor complexes. Thus, naloxone, its stable analogue naltrexone or other weak negative modulators of brain GABA(A)/benzodiazepine receptor complexes may represent a new pharmacological principle for the treatment of impulse control disorders.

The effects of naltrexone maintenance on the response to yohimbine in healthy volunteers

BACKGROUND

Preclinical research suggests that opiate antagonists may alter stress responsiveness. This study describes the effect of pretreatment with the opioid antagonist naltrexone on the response to a noradrenergic stressor, the alpha-2-receptor-antagonist, yohimbine, in healthy subjects. The current study was designed to compare the change in responses to yohimbine after 2 weeks of treatment with naltrexone to the response after at least 2 weeks of treatment with placebo.

METHODS

After a week of placebo naltrexone treatment, ten subjects were randomized into a double-blind cross-over to placebo or active naltrexone (50 mg p.o. daily) on weeks 2 to 4, and the converse condition for weeks 5 to 7. Subjects received challenges in a random, fixed sequence with placebo and active yohimbine (i.v., 0.2 mg/kg) on weeks 1, 4, and 7. The active-active combination generally had the strongest drug effects.

RESULTS

There were statistically significant (p < .05) interactions of naltrexone condition X yohimbine condition for subject ratings of "nervous," "not liking the drug effect," "talkative," and "urge to urinate," and a trend (p < .10) for cortisol levels.

CONCLUSIONS

The results suggest that clinically used naltrexone doses alter sensitivity to yohimbine.

Different effect of high doses of naloxone on spinal reflexes in normal subjects and chronic paraplegic patients

There is still controversy over the effects of naloxone on spinal reflexes in view of the fact that both facilitatory and inhibitory activities have been observed. Dosage, supraspinal influences and interactions with different opiate receptors may account for the different findings. We investigated the effect of placebo (saline) and high doses of naloxone (1.66 mg/kg) on the monosynaptic (H reflex) and nociceptive polysynaptic reflex (RIII reflex) in five normal subjects and three chronic paraplegic subjects. Following the administration of naloxone, there were no changes in the RIII reflex threshold in either group. By contrast, there was a marked facilitation of the H reflex amplitude in the normal subjects, but not in the spinal cord-injured subjects after treatment with naloxone. Saline induced no changes in the RIII reflex threshold or the H reflex amplitude in either of the two groups. Our data suggest that under normal conditions the opiatergic modulation of the nociceptive reflex is not functionally active whereas the tonic inhibitory modulation of the monosynaptic reflex is mediated by descending pathways.

Stereoselective mu- and delta-opioid receptor-related antinociception and binding with (+)-thebaine

In vivo and in vitro binding studies with natural thebaine and its enantiomer, (+)-thebaine were conducted to elucidate further their interactions with the opioid system. (-)-Thebaine a key intermediate in the biosynthesis of morphine in the poppy plant (Papaver somniferum) and in mammalian tissue, was poorly effective antinociceptively in mice at doses to 30 mg/kg. Its principal behavioral manifestation was lethal convulsions. Naltrindole, at doses of 1 and 10 mg/kg did not block either the convulsions or lethal effects, suggesting that the delta-opioid receptor system was not involved in this action. Surprisingly, the dextrorotatory isomer exhibited significant antinociceptive activity in the tail-flick [ED50 = 8.9 (3.4-22.1) mg/kg], hot-plate [ED50 = 22.9 (10.9-48.1) mg/kg] and phenylquinone [ED50) = 1.9 (1.6-9.5) mg/kg] assays. Studies with opioid receptor-subtype antagonists, beta-funaltrexamine, nor-binaltorphimine and naltrindole, indicated that antinociception was associated with mu- and delta-opioid receptors. Results of displacement experiments supported the in vivo data. Significant competition for [3H]diprenorphine binding with both isomers for cloned mu- and delta-opioid receptors was observed. However, (-)-thebaine was more effective at the delta-opioid receptor (Ki = 1.02+/-0.01 microM) whereas (+)-thebaine was more effective at the mu-opioid receptor ( Ki = 2.75+/-0.01 microM). Opioid-induced antinociception associated with unnatural thebaine raises the possibility of additional mu- and delta-opioid receptor sites.

No interactive effects of naltrexone and benzodiazepines on pain during phobic fear

Animal research has shown that anxiety may inhibit pain through the release of endogenous opioids. On the other hand, anxiety is often believed to exacerbate pain in clinical situations, and anxiety reduction has been shown to attenuate the affective component of pain. In the present study phobic anxiety was induced by confronting forty-eight spider phobic subjects with a spider, after which they received two mildly painful electrical stimuli at two different current levels. The benzodiazepine alprazolam (1 mg) was administered to investigate the influence on pain of a reduction in anxiety, while the role of endogenous opioids was studied by administering the opioid antagonist naltrexone (50 mg). Alprazolam resulted in lower anxiety and pain ratings during pain stimulation, supporting the idea that (presumably pain-related) anxiety may increase the experience of pain. Naltrexone did not influence pain and anxiety ratings, nor was there a significant interaction between the two pharmacological manipulations. These findings confirm previous evidence that phobic fear does not necessarily induce an endogenous opioid-mediated analgesia.

Naloxone inhibits gastric emptying in the rat

Naloxone is generally considered to be a pure antagonist, but it may produce several behavioral effects, such as hyperalgesia or stimulation of respiration. We studied the effect of naloxone on gastric emptying and gastrointestinal transit in rats. Six to eight Wistar rats (200-250 g) were used for each experiment. Either saline or naloxone (0.01-10 mg/kg) was injected intraperitoneally at 0 min. At 30 min, radiolabeled saline or milk 1 mL was infused into the stomach. At 60 min, gastric emptying and gastrointestinal transit were calculated by measuring the radioactivity in the gastrointestinal tract. Naloxone significantly inhibited gastric emptying of saline (P = 0.002) and of milk (P < 0.05), but not the gastrointestinal transit of either (P > 0.05). Gastric emptying of saline showed a significant peak (P < 0.05) in the dose-response curve at 0.7 mg/kg. Therefore, naloxone significantly inhibits gastric emptying of saline and milk, but not the gastrointestinal transit of either.

IMPLICATIONS

Although naloxone is generally considered to be a pure opioid receptor antagonist, it delays gastric emptying of saline or milk, as does morphine in the rat. However, it is uncertain from our results whether naloxone inhibited gastric emptying by antagonizing the effects of endogenous opioids.

The role of subtypes of the opioid receptor in the anxiolytic action of chlordiazepoxide

Previous studies have shown that the opiate antagonist naloxone blocks the anxiolytic-like effects of benzodiazepines in several models of anxiety, including the elevated plus-maze. Although naloxone preferentially binds to the mu opioid receptor, its selectivity is rather low. The opioid receptor subtype important for anxiolytic-like actions of benzodiazepines in the plus-maze remains, therefore, unknown. In the present experiments, the ability of antagonists selective for subtypes of the opioid receptor to block the anxiolytic-like effects of chlordiazepoxide in the elevated plus-maze was evaluated in Swiss mice. Chlordiazepoxide, 5 mg/kg, increased the proportion as well as the number of open arms entries without modifying closed arms entries. Lower doses of the benzodiazepine were ineffective. The mu receptor antagonist beta-funaltrexamine, 10 and 20 mg/kg, the delta antagonist naltrindole, 10 mg/kg, and the kappa antagonist nor-binaltorphimine, 2.5 and 5 mg/kg, were then combined with chlordiazepoxide, 5 mg/kg. beta-funaltrexamine, 10 mg/kg, reduced the effects of the benzodiazepine while the dose of 20 mg/kg completely blocked the effects. Nor-binaltorphimine was ineffective at a dose of 2.5 mg/kg, but completely inhibited the actions of chlordiazepoxide when the dose was 5 mg/kg. Naltrindole was ineffective. None of the antagonists affected plus-maze behavior when administered alone. It was concluded that the mu and kappa receptors are important for the anxiolytic-like actions of chlordiazepoxide in the elevated plus maze.

Effect of naloxone on behavioral changes induced by subchronic administration of ethanol in rats

Endogenous opioid peptides appear to be involved in acute behavioral effects induced by single doses of ethanol. However, its role in repeated ethanol exposure has not been studied. In the present study ethanol was given to rats at the doses of 2 and 4 g/kg by a stomach gauge for 15 days, and its effects on spontaneous motility, open-field activity, and active avoidance behavior recorded on the 3rd, the 6th and the 15th days. Then the effect of naloxone (0.5 and 2 mg/kg by intraperitoneal route) was tested against a challenge ethanol dose, administrated by oral route, on the 16th day. Control animals received tap water and saline instead of ethanol or naloxone, respectively. Both doses of ethanol induced a decrease in spontaneous motility that was antagonized by naloxone. Open-field ambulations were increased by the high dose of ethanol, low-dose lacking effect; naloxone did not modify these ethanol effects. The low dose of ethanol shortened latency time in shuttlebox, the high dose causing escape and freezing responses; none of these effects were modified by naloxone. Therefore, endogenous opioid peptides appear to play a limited role in the chronic effects of ethanol in rats; particularly its effects in tests inducing an increase in the level of anxiety were resistant to naloxone.

GABAergic function in detoxified heroin addicts: relationship to anxiety disorders

The function of the GABAergic system was examined in 20 subjects with heroin dependence and abuse, 2 months after detoxification, and in 10 healthy volunteers, by measuring the growth hormone (GH) response to a challenge with the GABA B receptor agonist baclofen. Ten heroin addicts had comorbid anxiety disorder (Group A), while the other ten had heroin addiction uncomplicated by Axis I and II psychopathologies (Group B). GH responses to baclofen stimulation of Group A patients were significantly blunted, while those of Group B subjects did not differ from responses of healthy volunteers. Our data show that the function of the GABAergic system is impaired only in heroin addicts with comorbid anxiety disorders (anxious cluster), suggesting that the GABA system is not persistently influenced by prolonged exposure to opioid receptor stimulation.

Endogenous opioid peptides acting at mu-opioid receptors in the dorsal horn contribute to midbrain modulation of spinal nociceptive neurons

Activation of neurons in the midbrain periaqueductal gray (PAG) inhibits spinal dorsal horn neurons and produces behavioral antinociception in animals and analgesia in humans. Although dorsal horn regions modulated by PAG activation contain all three opioid receptor classes (mu, delta, and kappa), as well as enkephalinergic interneurons and terminal fields, descending opioid-mediated inhibition of dorsal horn neurons has not been demonstrated. We examined the contribution of dorsal horn mu-opioid receptors to the PAG-elicited descending modulation of nociceptive transmission. Single-unit extracellular recordings were made from rat sacral dorsal horn neurons activated by noxious heating of the tail. Microinjections of bicuculline (BIC) in the ventrolateral PAG led to a 60-80% decrease in the neuronal responses to heat. At the same time, the responses of the same neurons to iontophoretically applied NMDA or kainic acid were not consistently inhibited. The inhibition of heat-evoked responses by PAG BIC was reversed by iontophoretic application of the selective mu-opioid receptor antagonists, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). A similar effect was produced by naloxone; however, naloxone had an excitatory influence on dorsal horn neurons in the absence of PAG-evoked descending inhibition. This is the first demonstration that endogenous opioids acting via spinal mu-opioid receptors contribute to brain stem control of nociceptive spinal dorsal horn neurons. The inhibition appears to result in part from presynaptic inhibition of afferents to dorsal horn neurons.

The use of the plasma concentration-effect relationship as a tool for the study of the mechanism of action of naloxone effects on mood and endocrine function

The relationship between naloxone-plasma concentrations and their effects on mood and endocrine function was studied. Ten healthy volunteers received 1.0 mg/kg i.v. naloxone or placebo following a randomized double-blind design. Effects on mood, determined by a visual analogue scale and luteinizing hormone (LH) and naloxone-plasma concentrations were measured at selected times. Naloxone induced significant effects on confusion, bewilderment and indifference, and an increment in LH levels. The timecourse of the responses on confusion and bewilderment was similar to that of naloxone-plasma concentration, suggesting that these effects are directly related to the action of naloxone on its receptors. Responses for indifference and LH, however, exhibited a delayed onset. This delay could be due to an indirect action, i.e. to the participation of additional physiological mechanisms in a cascade-like manner. The results show that analysis of the concentration-effect relationship can be a useful tool for understanding naloxone effects on mood and endocrine function.

Effects of naltrexone and cross-tolerance to morphine in a learned helplessness paradigm

Opiates have been implicated in learned helplessness (LH), a phenomenon known to be related to opiate stress-induced analgesia (SIA). In the present study, we investigated the role of opiates in the induction of LH and SIA under different conditions. Adult female Wistar rats were trained either by receiving 60 inescapable 1-mA footshocks (IS group, N = 114) or by confinement in the shock box (control or NS group, N = 92). The pain threshold of some of the animals was immediately evaluated in a tail-flick test while the rest were used 24 h later in a shuttle box experiment to examine their escape performance. The opiate antagonist naltrexone (0 or 8 mg/kg, ip) and the previous induction of cross-tolerance to morphine by the chronic administration of morphine (0 or 10 mg/kg, sc, for 13 days) were used to identify opiate involvement. Analysis of variance revealed that only animals in the IS group demonstrated antinociception and an escape deficit, both of which were resistant to the procedures applied before the training session. However, the escape deficit could be reversed if the treatments were given before the test session. We conclude that, under our conditions, induction of the LH deficit in escape performance is not opiate-mediated although its expression is opiate-modulated.

The gamma-hydroxybutyrate signalling system in brain: organization and functional implications

gamma-Hydroxybutyrate is a metabolite of GABA which is synthesized and accumulated by neurons in brain. This substance is present in micromolar quantities in all brain regions investigated as well as in several peripheral organs. Neuronal depolarization releases gamma-hydroxybutyrate into the extracellular space in a Ca(2+)-dependent manner. Gamma-hydroxybutyrate high-affinity receptors are present only in neurons, with a restricted specific distribution in the hippocampus, cortex and dopaminergic structures of rat brain (the striatum in general, olfactory bulbs and tubercles, frontal cortex, dopaminergic nuclei A9, A10 and A12). Stimulation of these receptors with low amounts of gamma-hydroxybutyrate induces in general hyperpolarizations in dopaminergic structures with a reduction of dopamine release. However, in the hippocampus and the frontal cortex, it seems that gamma-hydroxybutyrate induces depolarization with an accumulation of cGMP and an increase in inositol phosphate turnover. Some of the electrophysiological effects of GHB are blocked by NCS-382, a gamma-hydroxybutyrate receptor antagonist while some others are strongly attenuated by GABAB receptors antagonists. Gamma-hydroxybutyrate penetrates freely into the brain when administered intravenously or intraperitoneally. This is a unique situation for a molecule with signalling properties in the brain. Thus, the gamma-hydroxybutyrate concentration in brain easily can be increased more than 100 times. Under these conditions, gamma-hydroxybutyrate receptors are saturated and probably desensitized and down-regulated. It is unlikely that GABAB receptors could be stimulated directly by GHB. Most probably, GABA is released in part under the control of GHB receptors in specific pathways expressing GABAB receptors. Alternatively, GABAB receptors might be specifically stimulated by the GABA formed via the metabolism of gamma-hydroxybutyrate in brain. In animals and man, these GHBergic and GABAergic potentiations induce dopaminergic hyperactivity (which follows the first phase of dopaminergic terminal hyperpolarization), a strong sedation with anaesthesia and some EEG changes with epileptic spikes. It is presumed that, under pathological conditions (hepatic failure, alcoholic intoxication, succinic semialdehyde dehydrogenase defects), the rate of GHB synthesis or degradation in the peripheral organ is modified and induces increased GHB levels which could interfere with the normal brain mechanisms. This pathological status could benefit from treatments with gamma-hydroxybutyric and/or GABAB receptors antagonists. Nevertheless, the regulating properties of the endogenous gamma-hydroxybutyrate system on the dopaminergic pathways are a cause for the recent interest in synthetic ligands acting specifically at gamma-hydroxybutyrate receptors and devoid of any role as metabolic precursor of GABA in brain.

Naltrexone, naltrindole, and CTOP block cocaine-induced sensitization to seizures and death

I.c.v. injection for 9 days of either naltexone (NTX) (5, 10, 20, 40 micrograms/rat) or a selective mu peptide (CTOP) (0.125, 0.25, 0.5, 1, 3, 6 micrograms/rat) or delta (naltrindole) (NLT) (5, 10, 20 micrograms/rat) subtype opioid receptor antagonist affected sensitization to cocaine (COC) (50 mg/kg, i.p.) administered 10 min after. NTX (5 and 40 micrograms/rat), NLT (10 and 20 micrograms/rat), and the peptide CTOP (0.25-0.5 microgram/rat) attenuated seizure parameters (percent of animals showing seizures, mean score and latency) in a day-related manner. The DD50 (days to reach 50% of death) value for COC was 2.69, whereas it was 9.67 and 7.27 for NTX 5 and 40 micrograms/rat, 8.59 for NLT (10 micrograms/rat), and 6.11, 5.95, and 4.30 for CTOP (0.25, 0.5, and 1 microgram/rat respectively). These findings suggest a concurrent involvement of mu- and delta-opioid receptor subtype in COC-induced sensitization to toxic effects.

Naloxone reverses the inhibitory effect of gamma-hydroxybutyrate on central DA release in vivo in awake animals: a microdialysis study

gamma-Hydroxybutyrate (GHB) is a 4-carbon anesthetic that acts primarily by inhibiting presynaptic dopamine (DA) release in vivo. A number of studies have reported a reversal of many of the central effects of GHB by the allegedly pure opiate antagonist naloxone (NX) but its mechanism of action is unclear. In vivo microdialysis performed in the present preliminary study disclosed a significant inhibitory effect of GHB (500 mg/kg) on striatal DA release which was completely reversed by a low dose of NX (0.8 mg/kg). The results indicate that NX likely inhibits many of the central effects produced by GHB primarily through its reversal of the GHB induced inhibition of central DA release.

Medial septal injection of naloxone elevates acetylcholine release in the hippocampus and induces behavioral seizures in rats

The effects of injections of naloxone, a universal opioid receptor antagonist, into the medial septal nucleus on hippocampal acetylcholine (ACh) release and behavior were investigated in freely moving rats by means of the microdialysis method. The injection of naloxone (2, 10 and 20 micrograms) produced a marked increase in hippocampal ACh release in a dose-dependent manner. These effects of naloxone were reversed by the post-injection of [D-Ala2, N-Me-Phe4, Gly-ol]-enkephalin (DAGO; 10 micrograms), an opioid mu receptor agonist. Furthermore, basal release of hippocampal ACh was significantly reduced by the injection of DAGO alone. It was also found that rats given an injection of naloxone showed an increase in motor activity and occasionally exhibited behavioral seizures. These effects of naloxone were also reversed by the post-injection of DAGO. The present results suggest that endogenous opioids ionically inhibit the activity of septo-hippocampal cholinergic neurons via mediation of mu opioid receptors in the medial septal nucleus. They also suggest that endogenous opioids modulate the incidence of seizures, at least in part, through opioid mu receptors in the medial septal nucleus.

Chronotoxicity of sodium valproate in pregnant mouse and embryo

The influence of dosing time on the embryotoxicity of sodium valproate (valproic acid, VPA) was investigated in ICR mice under a light-dark (12:12) cycle. A significant circadian rhythm was shown for VPA-induced embryotoxicity, with the highest value at 1700 and the lowest at 0100. A similar pattern of rhythm was also shown for VPA-induced toxicity in pregnant and nonpregnant mice. No significant dosing time-dependent difference between injection at 1700 and 0100 was demonstrated for VPA concentrations in the embryo, plasma and brain. The rhythm in the embryotoxicity seems to be related to the rhythm in the sensitivity of the embryo and dam to the drug. Embryotoxicity and VPA concentrations were significantly higher on gestational day 13 than day 7. The pharmacokinetics of VPA contribute, at least partly, to the gestational stage-dependent embryotoxicity of VPA. The timing of drug administration (i.e., gestational stage and circadian phase) appears to be essential for studies on the embryotoxicity of VPA in mice.

Effects of GABAergic drugs on the recovery of reflex potentials after spinal cord ischemia in cats

Effects of GABAergic drugs on the recovery of reflex potentials after spinal cord ischemia were examined in anesthethized spinal cats. Monosynaptic reflex (MSR) and polysynaptic reflex (PSR) potentials, elicited by electrical stimulation of the tibial nerve in spinal cats, were recorded from the lumbo-sacral ventral root. The spinal reflex potentials were immediately depressed by occlusion of the thoracic aorta and the bilateral mammary arteries for 10 min. The potentials recovered gradually to the control level within 90 min after reperfusion. Pretreatment with bicuculline (0.3 mg/kg, i.v.), a GABA antagonist, or semicarbazide (200 mg/kg, i.v.), an inhibitor of GABA synthesis, accelerated the recovery of PSR potentials after the removal of the arterial occlusion. In contrast, pretreatment with aminooxyacetic acid (10 mg/kg, i.v.), an inhibitor of GABA degradation, retarded the recovery of PSR potentials, and this effect was overcome by the addition of the opioid antagonist naloxone (10 mg/kg, i.v.). These results suggest that the GABAergic system retards the recovery of PSR potentials after a brief spinal cord ischemia, which can be antagonized by naloxone.

Naloxone blocks the antianxiety but not the motor effects of benzodiazepines and pentobarbital: experimental studies and literature review

The role of opioid systems in the anticonflict effect of chlordiazepoxide, diazepam and pentobarbital was evaluated with a modified Vogel procedure. First, morphine, ineffective by itself, was combined with subeffective or marginally effective doses of the benzodiazepines in order to detect possible potentiation. However, the combined treatment reduced licking in the Vogel procedure as well as in a licking test where no shock was administered. Several doses of the benzodiazepines and pentobarbital were then administered in combination with several doses of the opiate antagonist naloxone. A dose-dependent inhibition of anticonflict effect was obtained. In an additional experiment, it was shown that naloxone blocked the effects of diazepam in the elevated plus-maze procedure. Motor deficiencies, as evaluated with a rotarod test, produced by the benzodiazepines and pentobarbital could not be antagonized by naloxone. It is concluded that opioids are important for the anticonflict but not for the motor effects of these drugs. An analysis of published studies concerning the interaction of opioids and benzodiazepines in several procedures supposed to reflect anxiolytic effects shows that the inhibition obtained with naloxone is reliable and not procedure specific. The mechanisms by which opiate antagonists produce this inhibition of anticonflict activity are not known. It is tentatively suggested that opioid activation associated with stress may be a necessary component of anxiolysis.

The influence of various experimental conditions on the expression of naloxone-induced withdrawal symptoms in mice

1. Physical dependence was induced in mice by repeated injections of increasing doses of morphine for either 4 or 7 days. 2. Withdrawal symptoms induced by naloxone (1 mg/kg, i.p.) given 3 hr postmorphine were more severe in mice treated for 7 than for 4 days. 3. In mice that developed a similar degree of dependence, various doses of naloxone (0.1-10 mg/kg) given 3 hr postmorphine produced withdrawal symptoms of different intensities. 4. Withdrawal jumping was maximal at naloxone (1 mg/kg) but declined with further increases in the dose of the antagonist. 5. "Wet dog" shakes progressively increased with increasing doses of naloxone (0.1-10 mg/kg). 6. Variation in the temporal time interval between the last dose of morphine and that of naloxone (1 mg/kg) influenced the intensity of withdrawal symptoms. 7. In male and female mice that developed a similar degree of dependence, no major differences were observed in the severity of withdrawal symptoms-induced by naloxone (1 mg/kg) given 3 hr postmorphine.

GABA receptor-linked chloride channels and the behavioral effects of naltrexone in rats

The present study was conducted to determine whether the effects of naltrexone on schedule-controlled behavior in rats were mediated, at least in part, by the GABAergic system. Because the enhanced sensitivity that has been shown to occur following naltrexone treatment might alter the effects of the treatment compounds, a variety of compounds interacting with the GABA system were tested in both sensitized and nonsensitized animals. Of all the compounds tested in this manner, only the dose-effect function for the GABA agonist muscimol was altered by the naltrexone treatment, with the higher doses of muscimol producing response-rate decreasing effects only in naltrexone-sensitized rats. In the naltrexone-treated animals, these same GABA agonists and antagonists were used as pretreatments prior to the determination of the naltrexone dose-effect function. Although shifts in the naltrexone dose-effect function were observed, the effects were not consistent either within or across receptor class. In contrast, the chloride-channel antagonist picrotoxin clearly shifted the naltrexone dose-effect function in sensitized animals to the left, while the chloride-channel facilitator pentobarbital shifted the function to the right. These results indicate that the effects of naltrexone are at least partially mediated by an action at the GABA-linked chloride channel, rather than directly at the GABA receptor.