Failure of naloxone to affect initial and acquired tolerance to ethanol in rats

Selective mu- and kappa-opioid receptor antagonists administered into the nucleus accumbens interfere with rapid tolerance to ethanol in rats

BACKGROUND

Previous findings have shown that intra-accumbens injection of naltrexone, a non-selective opioid antagonist, blocks the acquisition of rapid tolerance to ethanol in rats. This study investigates the effects of intra-accumbens injection of the selective mu-, delta-, and kappa-opioid antagonists, respectively, naloxonazine, naltrindole, and nor-binaltorphimine, on rapid tolerance to ethanol.

METHODS

Male Wistar rats with guide cannulae directed to the shell or the core portions of the nucleus accumbens received a microinjection of naloxonazine (2-4 microg), naltrindole (2-4 microg), nor-binaltorphimine (2.5-5 microg), or vehicle. After 5 min, each group was divided in two groups that received ethanol (2.7 g/kg i.p.) or saline. Rats were then tested for motor coordination on the tilting plane apparatus. Twenty four hours later, all rats received a challenge dose of ethanol (2.7 g/kg i.p.) and were tested on the tilt plane again.

RESULTS

Repeated injections of ethanol caused a reduction in motor impairment suggesting the development of tolerance. However, rats injected with 4 microg naloxonazine into either core or shell portions of the nucleus accumbens did not exhibit tolerance when challenged with ethanol on day 2. Rats treated with 5 microg nor-binaltorphimine into accumbens core plus intraperitoneal saline on day 1 showed reduced motor impairment when challenged with ethanol on day 2, suggesting cross-tolerance to ethanol.

CONCLUSIONS

Taken together, our results suggests that mu-opioid receptors in both shell and core portions of the nucleus accumbens, and possibly kappa-opioid in the core, participate in the modulation of rapid tolerance to ethanol.

Systemic and intra-accumbens microinjections of naltrexone interfere with tolerance to ethanol in rats

RATIONALE

Evidence suggests a role for the opioid system in the control of ethanol reinforcement and drinking. Previous findings have shown that naltrexone, an opioid antagonist that decreases ethanol consumption in humans and experimental animals, reduces the acquisition of acute ethanol tolerance in rats. However, there are few data regarding the role of the opioid system in the acquisition of ethanol tolerance, particularly in brain areas involved in the rewarding actions of ethanol.

OBJECTIVES

This study investigates the effects of systemic and of intra-accumbens injections of naltrexone on the development of rapid tolerance to ethanol.

METHODS

Wistar rats received intraperitoneal injections of naltrexone (0.1-3.0 mg/kg) or microinjections into the core or shell portions of the nucleus accumbens (5-20 microg) before ethanol (2.7 g/kg i.p.). The animals were tested for motor coordination on the tilting plane apparatus. Tolerance was assessed 24 h later by administering the same dose of ethanol to all animals and retesting them on the tilting plane.

RESULTS

The second injection of ethanol resulted in less motor incoordination on Day 2, suggesting the development of rapid tolerance. Pretreatment with naltrexone, either i.p. (0.3 and 0.6 mg/kg) or intra-accumbens (5-20 microg), on Day 1, blocked the development of rapid tolerance to the motor-incoordinating effects of ethanol on Day 2 without affecting the motor performance of the animals on Day 1.

CONCLUSIONS

The results suggest that the opioid system may be involved in the development of ethanol tolerance, and that the nucleus accumbens may play a role in this phenomenon.

Narcotic antagonism of seizures induced by a dopamine-derived tetrahydroisoquinoline alkaloid

This paper describes experiments designed to evaluate whether the narcotic antagonist naloxone significantly interferes with seizures induced by tetrahydroisoquinolines (TIQs). In these experiments we found that naloxone significantly reduced seizure scores induced by intra-cranially infusing mice with 50 micrograms of the dopamine-derived tetrahydroisoquinoline (TIQ) alkaloid, 6,7-dihydroxy TIQ. These findings support an opioid involvement in the actions of TIQs and may lead to further understanding of opioid-mediated novel excitatory receptors.

Ethanol-induced hypothermia in rats: possible involvement of opiate kappa receptors

The effect of the opiate antagonists naloxone and MR2266 on ethanol-induced hypothermia and changes in Ca2+-stimulated Mg2+-ATPase activity in brain regions were investigated in the present study. Administration of different doses of ethanol (0.5-2 g/kg, IP) produced a dose-dependent hypothermia. Ca2+/Mg2+-ATPase activity in the hypothalamus was stimulated at 30 min and 2 hr after ethanol (2 g/kg, IP) treatment. In cortex, enzyme activity was inhibited by ethanol at 30 min with no change seen at 2 hr. Naloxone (7.5 mg/kg, SC) at a dose which did not affect body temperature or enzyme activity, partially inhibited ethanol-induced hypothermia and enzyme activity at the earliest time (30 min) but not at 2 hours. The opiate Kappa antagonist MR2266 (5 mg/kg, SC), however, significantly protected against ethanol hypothermia and enzyme activation measured at 30-120 min. This evidence suggests that ethanol-induced hypothermia and subsequent activation changes of Ca2+/Mg2+-ATPase in the hypothalamus may be regulated by opiate Kappa receptors, and that Ca2+ ions play an important role in mediating the effects of ethanol.

Evidence from behavioural and in vitro receptor binding studies that the enkephalinergic system does not mediate acute ethanol effects

The behavioural disturbances produced by acute exposure to ethanol have been related to changes in function of the opioid systems in the CNS. However, evidence in the literature is conflicting. The present report concerns the possible role of the enkephalinergic system in the mediation of acute ethanol effects. We used rats to study the ability of a selective opioid delta receptor antagonist (ICI 154129) to prevent the effect of ethanol on pain sensitivity, body temperature, sensorimotor performance and level of consciousness. Furthermore, in vitro receptor binding was measured to investigate whether or not ethanol, within a non-lethal concentration range, would change the binding parameters of the delta receptor ligand [3H][D-ala2, D-Leu5]enkephalin. ICI 154129 did not significantly influence the effects of ethanol in the behavioural experiments. Ethanol did not significantly change the binding parameters whether saturation or competition was measured in the receptor binding experiments. Thus, there was no evidence that the enkephalinergic system mediated the acute ethanol effects.

Naloxone does not reverse ethanol analgesia in man

The effects of intravenously administered ethanol and morphine on pain threshold, reaction time, motor skills and short-term memory were investigated, and the ability of naloxone to reverse any changes was studied. Morphine (loading dose 0.2 mg/kg with an infusion of 0.004 mg/kg per min) and ethanol (loading dose 0.75 ml/kg with an infusion of 0.0025 ml/kg per min) produced a similar increase in pain threshold of 6.3 (s.e.m. = 1.5, n = 8) pain units and 7.7 (s.e.m. = 1.9, n = 8) pain units, respectively. Naloxone 0.015 mg/kg produced a significant reduction in pain threshold in the morphine group, but not in the ethanol group, and there was a significant difference between the groups following naloxone (P less than 0.05, t-test, 7 d.f.). Ethanol produced a significantly greater deterioration in motor skills than did morphine (P less than 0.05, t-test, 7 d.f.) and performance in both groups was improved following naloxone (P less than 0.05, t-test, 7 d.f.). There was no significant change in the other modalities studied. It is concluded that the reversal of ethanol effects by naloxone is probably due to a non-specific analeptic action rather than blockade of opioid receptors.

Effects of naloxone and buprenorphine on intravenous acetaldehyde self-injection in rats

Rats can be induced to self-inject acetaldehyde under an appropriate operant conditioning schedule. The narcotic antagonist naloxone (30 mg/kg) is shown to produce a decrease in schedule-induced acetaldehyde self-injection, but was without effect on both barpress responding and spontaneous activity in rats tested individually for fine, gross and total activity. On the other hand buprenorphine (0.3 and 3 mg/kg), the mixed agonist-antagonist derived from the opium alkaloid thebaine, also produced a significant decrease in acetaldehyde self-injection. However, a significant effect of buprenorphine on barpressing in otherwise drug naive rats indicated that this finding should not be dissociated from a possible involvement of buprenorphine on motor responding. While the findings are consistent with the hypothesis of opiate involvement in acetaldehyde self-administration, caution must be exercised when drawing conclusions about the participation of endogenous opiates in acetaldehyde-mediated behavior.

Attenuation of ethanol-induced conditioned taste aversion by naloxazone: behavioral evidence for an opiate receptor-mediated morphine-ethanol interaction

When rats are presented with a novel saccharin solution and immediately injected with either morphine or ethanol, they subsequently develop a conditioned taste aversion (CTA) to the saccharin solution which reflects the aversive component of the conditioning drug. The present study provides evidence which suggests that both morphine-induced and ethanol-induced CTAs can be blocked by the specific high-affinity binding opiate antagonist, naloxazone.

The effect of naloxone on intragastric ethanol self-administration

The acute effect of 1.25 and 2.50 mg/kg naloxone was tested in a group of male Wistar rats readily self-administering 10% w/v ethanol intragastrically following 12 days of forced ethanol intoxication. Compared to saline pretreatment, naloxone did not alter 24 hr intakes of food, water or ethanol. However, both does strongly and significantly inhibited lever pressing for ethanol during 2 hr following pretreatment. The results indicate that naloxones's inhibition of ethanol intake does have a transient postabsorptive component, although this component is unlikely to be specific to ethanol.

Symmetrical effect of pre-exposure between alcohol and morphine on conditioned taste aversion

It has previously been reported that pre-exposure to a psychoactive drug can block the conditioned taste aversion associated with that drug. This study was an attempt to investigate alcohol-morphine interactions using this pre-exposure paradigm. After two weeks of adaptation to a schedule of daily 30-minute access to water, rats were pre-exposed to morphine, ethanol, or the respective vehicle control every second day for three days before (Days 1, 3, 5) and after the first conditioning day (Days 8, 10, 12). On conditioning days (Days 7, 14), animals were first presented with a saccharin solution for 30 minutes following which animals that were pre-exposed to morphine were injected with ethanol while those pre-exposed to ethanol were administered with morphine. Saccharin was again presented on three more occasions (Days 21, 28, 35) without drug injection. Using the percent change in saccharin consumed from the first presentation as a measure of aversion, it was found that pre-exposure to morphine blocked ethanol conditioned taste aversion. Similarly, animals pre-exposed to ethanol showed less aversion to the saccharin paired with morphine. This is the first demonstration of a symmetrical relationship between alcohol and the opiates.

Naloxone and alcohol intoxication in the dog

1 The effects of naloxone upon ethanol-induced coma have been investigated in dogs. In a double blind study, 15 mongrel dogs received ethanol i.v. (4 g/kg) followed by a single dose of naloxone (12 micrograms/kg). 2 Naloxone failed to affect either the duration of respiratory arrest or the time to recovery of motor coordination. In similar animals treated with the narcotic analgesic, fentanyl, naloxone induced a dramatic and complete reversal of the narcotic effects within 30 seconds. 3 It is concluded that, if naloxone has any effect in alcoholic coma, it is not comparable with its dramatic action in narcotic coma.

Decrease in ethanol consumption by naloxone in naive and dependent rats

Acute effects of the opiate antagonist, naloxone, on alcohol intake have been examined and compared in naive and behaviorally dependent rats. In naive rats the aversion to an 8% alcohol solution exhibited in a 30 min presentation was selectively augmented by an IP administration of naloxone (1 mg/kg) 30 min before a morning drinking session. In other rats, behavioral dependence was established by 15 days of IG administration of intoxicating doses of alcohol. This dependence was exhibited by a sustained preference for ethanol for 6 days. Naloxone (1 mg/kg) abolishes the acquired preference for ethanol tested during an 8 hour day time presentation. These effects of naloxone on alcohol intake in ethanol naive and dependent rats are interpreted in relation to a general non-specific action of naloxone on preferred or aversive flavoured solutions.

Does ethanol stimulate brain opiate receptors? Studies on receptor binding and naloxone inhibition of ethanol-induced effects

The ability of ethanol to stimulate opiate receptors was investigated. Administration of ethanol 2-3.5 g/kg in rats induced analgesia as tested by the tail flick method, reduced body temperature, impaired sensorimotor performance and induced "sleep". Naloxone HCI (0.5-10 mg/kg) did not significantly attenuate these ethanol-induced behavioural changes. [3H]Etorphine binding to opiate receptors in membrane preparations from rat and mouse brains was unchanged by ethanol concentrations of 0.05-3% (v/v). It was concluded that the ethanol effects studied were not mediated by naloxone-sensitive opiate receptors.