Enhanced sensitivity to naltrexone is associated with an up-regulation in GABA receptor function

Relative potency of the opioid antagonists naloxone and 6-alpha-naloxol to precipitate withdrawal from acute morphine dependence varies with time post-antagonist

The current study compared the potency of naloxone versus 6-alpha-naloxol to precipitate opioid withdrawal under varying conditions of morphine pretreatment history using suppression of operant responding for food reward as the index of withdrawal. Male Wistar rats trained to respond on a lever for food reward received pretreatment with either Vehicle (Morphine-Naïve), a single subcutaneous (SC) injection of 5.6 mg/kg morphine (Single Morphine), or two morphine injections at 24 h intervals (Repeat Morphine), with varying doses of naloxone or 6-alpha-naloxol injected SC 4 h post-morphine and 5 min prior to the 30 min test session. When responding over the entire 30 min operant session was examined, naloxone was only 5-fold more potent than 6-alpha-naloxol in suppressing operant responding under Morphine Naïve conditions, but this increased to a 65-fold potency difference after Single or Repeat Morphine pretreatment. Examination of the relative potency of these antagonists in the Early Phase of operant testing (5-15 min post-antagonist) revealed an even greater 100-fold potency difference between naloxone and 6-alpha-naloxol, but in the Late Phase of testing (25-35 min post-antagonist), this had declined to a 9-fold potency difference, comparable to the relative potency of naloxone to 6-alpha-naloxol under Morphine-Naïve conditions. The results confirm a differential potency of naloxone to its reduced conjugate 6-alpha-naloxol in vivo, and extend the observation of this phenomenon to an acute (single) pretreatment with a low dose of morphine and an additional sign of opioid withdrawal to those previously used. However, the results also indicate that delay in onset of action of 6-alpha-naloxol at opioid receptors in the central nervous system may contribute significantly to its reduced potency relative to naloxone under certain morphine pretreatment conditions.

Repeated experience with naloxone facilitates acute morphine withdrawal: potential role for conditioning processes in acute opioid dependence

Single injections with morphine can induce a state of acute opioid dependence in humans and animals, typically measured as precipitated withdrawal when an antagonist such as naloxone is administered 4-24 h after morphine. Repeated treatment with morphine at 24-h intervals can result in a progressive shift in potency of naloxone to produce such acute withdrawal signs, including suppression of operant responding for food reward. The current study characterized fully both morphine and naloxone dose-effect functions in an effort to establish the relative contributions of repeated morphine vs. repeated naloxone (Nal) experience to these potency shifts. Rats trained on an FR15 schedule for food received four vehicle or morphine injections (0.56-5.6 mg/kg sc), spaced 24 h apart. Four hours after each morphine pretreatment (Repeat Nal), or 4 h after the fourth and final morphine pretreatment only (Single Nal), a cumulative dose-effect function for naloxone-induced suppression of responding was determined. Vehicle-pretreated (Morphine Naive) rats showed little change in the naloxone dose effect function even after four cumulative dose-effect determinations. By contrast, a progressive increase in naloxone potency was observed following successive pretreatments with morphine under Repeat Nal conditions, and the magnitude of naloxone potency shift was morphine dose dependent. At a morphine dose of 5.6 mg/kg, repeated naloxone experience in the presence of morphine was not an absolute requirement to produce an increase in naloxone potency across days, but repeated naloxone could potentiate the magnitude of the observed shift, indicating both experience-independent and experience-dependent processes at work. At lower doses of morphine (1.0 and 3.3 mg/kg) no shift in naloxone potency was observed across days of morphine treatment in the absence of repeated naloxone experience (Single Nal conditions), indicating an increasing contribution of naloxone experience-dependent processes as dose of morphine was decreased. It is argued that these experience-dependent processes in the progressive shift of naloxone potency observed in the current study may reflect an important role of conditioning in the early development of opioid dependence.

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.

Regulation of opioid receptors by opioid antagonists: implications for rapid opioid detoxification

Opioid receptor antagonists have long been used in the diagnosis of opioid dependence and in the treatment of both opioid overdose and addiction. More recently they have been used in rapid opioid detoxification, a technique which has generated much ethical and scientific controversy. Because of this, the present review aims to integrate and summarize the current state of knowledge on adaptational changes to opioid systems as a result of antagonist administration. It is generally accepted that chronic treatment with an opioid antagonist results in opioid receptor upregulation. However, the mechanism(s) underlying this resultant opioid supersensitivity remain unresolved. In addition, there is not yet consensus regarding whether changes in opioid receptor number are directly responsible for the functional changes observed after chronic opioid antagonist treatment. Moreover, changes in opioid receptor number and sensitivity to opioid agonists and antagonists after chronic opioid antagonist treatment are dependent on dosing regimes as well as the kinetic properties of the antagonist itself. The role of these variables is appraised critically given the implication that an opioid antagonist can enhance functional responses. For example, such responses are an important consideration in the use of opioids because of possible adverse outcomes, such as overdose, after cessation of administration. Based on the literature discussed in this review it is concluded that caution is essential in the use of opioid antagonists for rapid opioid detoxification.

Effect of Leu-enkephalin on the memory reactivation produced by blockade of the benzodiazepine/GABA-chloride ionophore receptor complex

This investigation sought to characterize the interaction between GABA/benzodiazepine and opioid systems in memory retrieval deficit induced by detaining an animal in the training apparatus after acquisition. Mice pretreated with saline or Leu-enkephalin (0.2 mg/kg) were trained in one-trial passive avoidance test with following detention. Pre-testing administration of bicuculline (1 mg/kg), picrotoxin (1 mg/kg), or flumazenil (10 mg/kg) produced the memory-enhancing effect in the saline-pretreated mice. Pretraining treatment with Leu-enkephalin blocked the reactivation of memory produced by bicuculline and picrotoxin, but not flumazenil. The present investigation suggest that both benzodiazepine/GABA and opioid systems are important modulators of memory retrieval and that a specific interaction between these systems is responsible for the observed recovery of impaired memory trace.

Long-term sensitization to the behavioral effects of naltrexone is associated with regionally specific changes in the number of mu and delta opioid receptors in rat brain

Enhanced sensitivity to some of the behavioral effects of the opioid antagonist naltrexone (NTX) develops following once-weekly injections of cumulative doses of the drug. Rats treated with this regimen of NTX injections show enhanced sensitivity to the operant response rate decreasing effects of NTX and NTX-induced salivation. The enhanced sensitivity is long-lasting and appears to be produced through conditioning processes. We have conducted saturation binding assays to assess possible changes in the number and affinity of mu and delta opioid receptors in cortical, midbrain and hindbrain membrane preparations from Long-Evans rats treated once weekly for 8 weeks with cumulative doses of the drug (1, 3, 10, 30 and 100 mg/kg). 3H-DAMGO (0.5-21 nM) and 3H-pCl-DPDPE (0.04-4 nM) were used to characterize mu and delta receptors, respectively. NTX treatment had no effect on 3H-DAMGO binding in cortex, but decreased binding in midbrain and increased binding in hindbrain relative to saline-treated controls. Saturation analyses revealed that these differences reflected changes in the number, but not the affinity of mu receptors. NTX treatment also increased the amount of 3H-pCl-DPDPE bound to delta receptors in midbrain and hindbrain, but not in cortex. Again, these changes were due to changes in the number of receptors. Thus, chronic NTX differentially affects the number of mu and delta opioid receptors in various brain regions.

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.

Endogenous opiates: 1992

This paper is the fifteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1992 involving the behavioral, non-analgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Pharmacological specificity of enhanced sensitivity to naltrexone in rats

Rats treated weekly with cumulative doses (1-100 mg/kg, IP) of naltrexone develop an enhanced sensitivity to the operant response-rate decreasing effect of naltrexone. In the present experiment the pharmacological specificity of that enhanced sensitivity was determined by testing a variety of drugs for cross-sensitivity to naltrexone. Cross-sensitivity was evaluated with two procedures. In one, dose-effect functions were determined for each of the test compounds before and after the development of enhanced sensitivity to naltrexone in a single group of rats. In the second procedure, one group of rats was made sensitive to naltrexone, while a second was not. Test compounds were then evaluated in both groups. For both procedures, a shift to the left in the dose-effect functions similar to naltrexone was considered evidence of cross-sensitivity. Of the opioid antagonists tested, only naloxone showed clear cross-sensitivity to naltrexone, although MR 2266 and diprenorphine also showed evidence of cross-sensitivity. The opioid antagonist quadazocine did not show cross-sensitivity to naltrexone on the day of testing, although some evidence of cross-sensitivity was evident 24 h later. In addition, the dose-effect function for d-amphetamine was significantly changed following naltrexone treatment. No evidence of cross-sensitivity was observed for the optical isomer of naloxone, d-naloxone, or for naloxone's quaternary derivative, naloxone methiodide. None of the opioid agonists or agonist-antagonists tested showed cross-sensitivity to naltrexone (i.e. morphine, U-50, 488H, ethylketocyclazocine, N-allylnormetazocine and pentazocine). The non-opioid drugs chlordiazepoxide and phencyclidine also failed to show evidence of cross-sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)