Tolerance and sensitization to the biphasic effects of low doses of morphine in the hamster

The development of behavioral sensitization induced by a single morphine exposure in adult zebrafish (Danio rerio)

BACKGROUND

Accumulating evidence suggest that behavioral sensitization is involved in the process of drug addiction. Zebrafish are sensitive to a variety of addictive drugs and are thus suitable for the study of behavioral sensitization. However, in contrast to mature rodent models of behavioral sensitization, how this phenomenon manifests in aquatic organisms, especially zebrafish, is largely unknown. In this study, we developed a morphine-induced behavioral sensitization adult zebrafish model and performed a preliminary investigation of the underlying mechanisms.

METHODS

Behavioral sensitization was established in zebrafish by observing their behavior after treatment and challenge with morphine. The effect of morphine was evaluated by a behavioral locomotor test. Different doses of morphine and withdrawal times were used to evaluate the establishment of the behavioral sensitization model.

RESULTS

Hyperlocomotion was induced after administration of morphine in adult zebrafish. After withdrawing the drug for a period, challenge with low-dose morphine evoked behavioral sensitization in zebrafish acutely pre-treated with morphine. Low-dose morphine failed to induce behavioral sensitization in zebrafish if the withdrawal time was less than 5 days or more than 7 days. Morphine induced behavioral sensitization in zebrafish may involve dopaminergic, glutamatergic and opioid systems.

CONCLUSION

A single low-dose of morphine could induce behavioral sensitization in zebrafish acutely pre-treated with morphine, and this phenomenon was highly correlated with drug dose and withdrawal time. These findings suggest that zebrafish is a suitable model for the study of behavioral sensitization.

Wheel running reduces high-fat diet intake, preference and mu-opioid agonist stimulated intake

The ranges of mechanisms by which exercise affects energy balance remain unclear. One potential mechanism may be that exercise reduces intake and preference for highly palatable, energy dense fatty foods. The current study used a rodent wheel running model to determine whether and how physical activity affects HF diet intake/preference and reward signaling. Experiment 1 examined whether wheel running affected the ability of intracerebroventricular (ICV) μ opioid receptor agonist D-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO) to increase HF diet intake. Experiment 2 examined the effects of wheel running on the intake of and preference for a previously preferred HF diet. We also assessed the effects of wheel running and diet choice on mesolimbic dopaminergic and opioidergic gene expression. Experiment 1 revealed that wheel running decreased the ability of ICV DAMGO administration to stimulate HF diet intake. Experiment 2 showed that wheel running suppressed weight gain and reduced intake and preference for a previously preferred HF diet. Furthermore, the mesolimbic gene expression profile of wheel running rats was different from that of their sedentary paired-fed controls but similar to that of sedentary rats with large HF diet consumption. These data suggest that alterations in preference for palatable, energy dense foods play a role in the effects of exercise on energy homeostasis. The gene expression results also suggest that the hedonic effects of exercise may substitute for food reward to limit food intake and suppress weight gain.

Androgen dependence in hamsters: overdose, tolerance, and potential opioidergic mechanisms

Anabolic steroids are drugs of abuse. However, the potential for steroid reward and addiction remains largely unexplored. This study used i.c.v. testosterone self-administration and controlled infusions of testosterone or vehicle in hamsters to explore central mechanisms of androgen overdose. Forty-two hamsters used nose-pokes to self-administer 1 microg/microl testosterone i.c.v. 4 h/day in an operant chamber. During 1-56 days of androgen self-administration, 10 (24%) hamsters died. Deaths correlated with peak daily intake of testosterone. Of the hamsters that self-administered a peak intake of <20 microg/day, there was 100% survival (10/10). Survival decreased to 86% (19/22) when daily testosterone intake peaked at 20-60 microg/day. Only 30% (three of 10) survived when daily testosterone intake exceeded 60 microg/day. Deaths are not due to volume or vehicle because i.c.v. infusions of 80 mul vehicle had no effect. Testosterone overdose resembles opiate intoxication. When male hamsters received infusions of 40 microg testosterone, locomotion (25.1+/-18.8 grid-crossings/10 min), respiration (72.7+/-5.4 breaths/min) and body temperature (33.5+/-0.4 degrees C) were significantly reduced, compared with males receiving vehicle infusions (186.1+/-8.1 crossings/10 min, 117.6+/-1.0 breaths/min, 35.9+/-0.1 degrees C, P<0.05). However, males developed tolerance to continued daily testosterone infusion. After 15 days, locomotion (170.2+/-6.3 crossings), respiration (118.4+/-1.3 breaths/min), and body temperature (35.3+/-0.3 degrees C) in testosterone-infused males were equivalent to that in vehicle controls (P>0.05). The depressive effects of testosterone infusion are blocked by the opioid antagonist, naltrexone. With naltrexone pre-treatment (10 mg/kg s.c.), locomotion (183.7+/-1.8 crossings/10 min), respiration (116.9+/-0.3 breaths/min), and body temperature (36.1+/-0.4 degrees C) during testosterone infusion were equivalent to vehicle controls. Likewise, naltrexone prevents the reinforcing effects of i.c.v. testosterone self-administration. These results indicate that testosterone at high doses causes central autonomic depression, which may be a factor in deaths during self-administration. As well, the depressive effects of large quantities of testosterone may be mediated, at least in part, by an opioidergic mechanism.

Naloxone suppression and morphine enhancement of voluntary wheel-running activity in rats

This study examines the effects of the opioid receptor antagonist, naloxone, and the agonist, morphine, on voluntary wheel-running activity (WR) in rats. Male Sprague-Dawley rats were given 1-h access to a running wheel under non-deprived conditions. Naloxone injections (1.0, 0.5, or 0.25 mg/kg, ip), administered immediately before access to running wheels, dose-dependently suppressed WR. In another experiment, subjects were given 6-h access to running wheels under nondeprived conditions for 5 consecutive days. Morphine injections (2.0 mg/kg, sc) were found to increase WR after an initial suppression. These data demonstrate that naloxone inhibits WR, while morphine both suppresses and enhances WR depending on time and dose. These are in agreement with data on other behaviors that indicate that endogenous opioid systems play a major role in the mediation of motivational behaviors.

The effect of ginseng extract on locomotor sensitization and conditioned place preference induced by methamphetamine and cocaine in mice

Repeated i.p. injections of 2 mg/kg methamphetamine (MA) or 20 mg/kg cocaine at 48-h intervals induced reverse tolerance to their ambulation-enhancing effects (behavioral sensitization). Furthermore, the reappearance of the sensitized state was observed at the time of readministration of MA or cocaine even after a 30-day discontinuation of drug administration. A concomitant injection of ginseng extract (GE), 200 mg/kg, i.p., suppressed the development of reverse tolerance and the reappearance of sensitization to MA and cocaine. Conditioned place preference to MA (1, 2, and 4 mg/kg, i.p.) and cocaine (1, 4, 10, and 20 mg/kg, i.p.), was completely blocked by GE, 200 mg/kg, i.p. combined treatment with MA of cocaine. Meanwhile, spontaneous motor activity and place preference were not affected by GE alone. These results provide evidence that GE may be useful clinically for the prevention of adverse actions of MA and cocaine.

A comparison of the effects of individual organic solvents and their mixture on brain stimulation reward

In spite of the prevalence of solvent abuse, there are only a few experimental investigations on the addictive potential of household organic solvents. In the present study we attempted to investigate the influence of glue thinner, a very popular glue used by glue-sniffing children, and the four organic solvents that compose this thinner (toluene, mixture of petroleum hydrocarbons, ethyl acetate, methylene chloride) on self-stimulation of the lateral hypothalamus (ICSS) in rats. Glue thinner, toluene, a mixture of petroleum hydrocarbons, and methylene chloride had a biphasic effect on ICSS, increasing frequency of self-stimulation at lower concentrations and decreasing it at higher concentrations. Ethyl acetate decreased frequency of self-stimulation at all concentrations. In contrast to classically abused drugs, solvents increased the threshold current of self-stimulation. The differences between concentration-response curves of ICSS for glue thinner and solvents permit the proposal that the mixture of solvents can be more dangerous than the individual components in potential for inducing solvent abuse.

Context-specific sensitization to naloxone-precipitated withdrawal in hamsters: effect of pimozide

Three experiments were conducted to investigate the development of context-specific sensitization to naloxone-precipitated withdrawal in hamsters. In Experiment 1, animals in group M/S were given morphine (15 mg/kg) injections in a distinctive environment and saline in the home cage. Animals in group S/M were given saline in the distinctive environment and morphine in the home cage, and animals in group S/S were given saline in both environments. All groups were challenged subsequently with naloxone (0.4 mg/kg) in the distinctive environment and then observed for signs of opiate withdrawal. The results showed that group M/S gave more naloxone-precipitated withdrawal signs than each of the other groups, which did not differ from one another. Experiment 2 was designed to test the effect of pimozide on context-specific sensitization to naloxone-precipitated withdrawal. The design was similar to that of Experiment 1 but group P/M/S, which received an injection of pimozide (0.5 mg/kg) 4 h prior to morphine, was added. The results indicated that context-specific sensitization developed as in Experiment 1, except among animals treated with pimozide. Experiment 3 was designed to determine whether pimozide interferes with the development or the expression of context-specific sensitization. Six groups, differing in the frequency and timing of the pimozide injection, were employed. The results indicated that pimozide interfered with context-specific sensitization, whenever it was given. It is concluded that pimozide interferes with the expression of context-specific sensitization, although a separate effect on the development of sensitization is not ruled out.

Conditioned activation induced by ethanol: role in sensitization and conditioned place preference

Previous studies of ethanol-induced activation and place preference conditioning have shown that repeated exposure to ethanol produces sensitization to ethanol's locomotor activating effect in mice. This experiment was designed to determine whether the behavioral sensitization to ethanol that occurs during place preference conditioning is due to development of a Pavlovian conditioned activity response. Mice (DBA/2J) in the experimental group (BEFORE) received four pairings of a distinctive floor stimulus with ethanol (2 g/kg, IP); a different floor stimulus was paired with saline (counterbalanced). Mice in two control groups were exposed equally to each floor stimulus and were handled and injected as often as experimental mice. One control group (AFTER) always received ethanol in the home cage 1 h after exposure to the floor stimulus, while the other control group (NO-DRUG) never received ethanol during conditioning. BEFORE group mice showed a significant conditioned place preference, whereas control mice did not. Activity tests after saline or ethanol indicated higher activity levels in BEFORE mice compared to control mice, regardless of floor stimulus. Moreover, BEFORE mice were more active on their CS+ floor than on their CS- floor during saline tests; activity was equally elevated on both floors during ethanol tests. These results support the hypothesis that sensitization to ethanol's activating effect is mediated by Pavlovian conditioning. Further, they suggest that place conditioning established-associative control by two kinds of stimuli; the specific tactile cues serving as CS+ and CS- and the general environmental cues common to both CS+ and CS- trials.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acute and chronic morphine on food intake in rats: modulation by oxytocin and vasopressin

The effects of acute and chronic morphine administration and the interaction with oxytocin and vasopressin on food intake response were investigated at various intervals during a 24-h schedule in rats. Acute morphine (5 mg/kg, IP) produced a generalized hyperphagic effect in both light (0-6 h) and dark (6-24 h) phases, the most marked effects being at 0-1 h, 1-3 h and 6-24 h. Chronic morphine (7 days) in an escalating dose schedule (5-35 mg/kg/day) produced (a) an enhancement of the hyperphagic effect in the light phase and (b) an attenuation of the food intake response during the dark phase. Neither oxytocin nor vasopressin had any significant influence on food intake, per se, after either acute or chronic administrations. However, both OXY and AVP reduced the hyperphagic response to acute morphine throughout the 24-h observation period. Further, on chronic administration, both neurohypophyseal peptides blocked the enhancements of morphine-induced hyperphagia (reverse tolerance) during light phase, whereas only vasopressin was effective in attenuating the reduction of hyperphagia (tolerance) during dark phase. These results are discussed in light of complex opiate-oxytocin/vasopressin interactions in the regulation of food intake.

Effects of cholecystokinin on morphine-elicited hyperactivity in hamsters

The effects of the octapeptide cholecystokinin (CCK) on hamster locomotor activity were investigated in three experiments. In Experiment 1, the effect of CCK (25, 50, 75 micrograms/kg) on morphine (2.5 mg/kg)-elicited hyperactivity was studied. Results indicated that CCK antagonized morphine-elicited hyperactivity and that CCK alone elicited hypoactivity. There were no effects of dose of CCK. In Experiment 2, the effects of intraperitoneal (IP) and subcutaneous (SC) routes of administration of CCK (25 micrograms/kg) on locomotor activity were studied. Compared to saline controls, CCK induced hypoactivity that was of greater magnitude and of longer duration when administered IP than SC. Experiment 3 was designed to replicate the route of administration effect observed in Experiment 2 and to determine whether sensitization to CCK-induced hypoactivity develops over the course of a few injections. Results indicated that CCK-induced hypoactivity was greater after IP than SC administration but that sensitization was not detectable. It is concluded that CCK antagonizes morphine-elicited hyperactivity in the hamster by acting, in part, independently of morphine to produce opposite behavioral effects.

Acute morphine dependence in the hamster

Two experiments investigated naloxone-precipitated withdrawal following a brief course of morphine administration in hamsters. In Experiment 1, observable withdrawal symptoms (e.g., wet-dog shakes) were elicited by two doses of naloxone (0.4 and 1.0 mg/kg) following four and eight daily injections of morphine (15 mg/kg), a regimen that replicated previous studies in our laboratory using a locomotor activity paradigm. At the lower dose of naloxone, the frequency of withdrawal signs was greater after eight than after four morphine injections. In Experiment 2, observable withdrawal symptoms were elicited by the same two doses of naloxone, 70 min after a single morphine injection. These results suggest that acute dependence in the hamster, as in other species, beings to develop with the first morphine exposure.

Tolerance and sensitization to the heart-rate effects of morphine

The effect of daily exposure to one of several doses of morphine (0, 2.0, 4.0 and 8.0 mg/kg IV) on heart rate was assessed in restrained (R) and unrestrained (U) rats. Initially, morphine produced a biphasic heart-rate response; bradycardia followed by tachycardia. Tolerance to the bradycardic effect was established in the 4 and 8 mg/kg U groups and in the 2 and 4 mg/kg U groups. Sensitization developed to the tachycardic effect in the 2 and 4 mg/kg U groups but not in the 8 mg/kg U group or any of the R groups. After several exposures to morphine, mean preinfusion heart rate increased in the 4 and 8 mg/kg dose groups but not in the 0 and 2 mg/kg dose groups. These results are generally consistent with the other data suggesting that tolerance develops only to the depressant effects of morphine, and either no change or sensitization develops to its stimulant effects. The development of higher preinfusion heart rates in the higher dose groups may represent a learned anticipatory response.

Tolerance to antinociceptive effects of morphine without tolerance to its effects on schedule-controlled behavior

The development of tolerance to behavioral effects of morphine was investigated in rats that responded on a two-lever, multiple-trial, multiple differential-reinforcement-of-low-rate fixed-ratio (mult DRL FR) schedule of food presentation. Stable performances were maintained when sessions were conducted just twice per week. The effects of cumulative doses of morphine (1.0-8.0 mg/kg) or chlordiazepoxide (CDP; 4.0-32.0 mg/kg) were evaluated once per week; saline injections were given in the intervening sessions. The effects of saline and morphine on nociception were also evaluated in hot-plate tests conducted on the same subjects 15 min after selected operant sessions. Initially, morphine produced dose-related decreases in response rates and reinforcement rates in the DRL and FR components as well as significant increases in hot-plate response latencies. Following weekly administration of morphine (1.0-8.0 mg/kg) for 10 weeks, there was little or no tolerance to its effects on operant behavior. In contrast, complete tolerance developed to the antinociceptive effects of morphine. These results suggest that tolerance to various behavioral effects of morphine may be dissociated, and that the loss of reinforcement may be insufficient by itself to produce tolerance to effects of morphine on operant behavior. Additionally, whereas CDP initially produced only dose-related decreases in DRL and FR response rates, following weekly morphine the smaller doses of CDP (4.0-16.0 mg/kg) produced increases in response rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholecystokinin antagonizes morphine induced hypoactivity and hyperactivity in hamsters

Three experimental replications were used to test the effects of three doses (25, 50 or 75 micrograms/kg) of cholecystokinin octapeptide (CCK-8) on morphine induced changes in activity. For each dose of CCK-8, running wheel activity of golden Syrian hamsters was monitored for three hours following a series of two injections. The first injection consisted of either saline or CCK-8, the second of either saline or morphine sulfate (15 mg/kg). Thus, in each replication four groups were created: Group SAL/SAL (n = 8) received two saline injections, Group CCK/SAL (n = 8) an injection of CCK-8 followed by an injection of saline, Group SAL/MS (n = 8) an injection of saline followed by an injection of morphine and Group CCK/MS (n = 8) an injection of CCK-8 followed by an injection of morphine. Results indicated that a 25 micrograms/kg dose of CCK-8 blocked the hypoactivity elicited by morphine 40-60 min after opiate injection, whereas a 75 micrograms/kg dose of CCK-8 blocked the hyperactivity elicited by morphine 80-100 min after opiate injection. These findings are consistent with previous reports that CCK-8 antagonizes the effects of opiate agonists on a variety of behaviors and is supportive of the hypothesis that endogenous CCK-8 may antagonize endogenous opioid peptides in the control of behavior.

Evidence for an underlying opponent process during morphine elicited hyperactivity in the hamster

Two experiments investigated the effects of naloxone on morphine elicited hyperactivity in the hamster. In Experiment 1, naloxone (0.4 mg/kg) administered two hours after morphine (15 mg/kg) produced sedation in animals running at high rates under the influence of morphine. Saline control animals running at comparable rates were unaffected by naloxone. In Experiment 2, naloxone administered two hours after morphine converted morphine elicited hyperactivity into sedation. These results are discussed in terms of a modified dual-action hypothesis which holds that morphine elicited hyperactivity masks an underlying opponent process.

Effects of naloxone on morphine induced sedation and hyperactivity in the hamster

Three experiments investigated the effects of naloxone on morphine elicited changes in hamster locomotor activity. In Experiment 1, a prior subcutaneous injection of naloxone (0.4 mg/kg) converted morphine (15 mg/kg) elicited hypoactivity into hyperactivity: Compared with saline controls, naloxone pretreated animals were hyperactive following a subcutaneous injection of morphine. Experiment 2 investigated the effects of four doses of naloxone (0, 0.04, 0.1, 0.4 mg/kg) on morphine elicited hyperactivity. Results indicated that naloxone reversal of morphine elicited hyperactivity is directly related to dose of naloxone. In Experiment 3, naloxone (0.4 mg/kg) was administered one and two hours after a morphine injection. Compared with saline controls, morphine treated animals were hypoactive for approximately 40 minutes after each of the naloxone injections. Results are discussed in terms of a modified dual-action hypothesis.

Dissociation of tolerance to the hypothermic and tachycardic effects of ethanol

Tolerance to the cardioacceleratory and hypothermic effects of ethanol was studied in unanesthetized, freely-moving rats surgically implanted with EKG electrodes and biotelemetric temperature sensors. Different groups received 0.0, 1,0 or 2.0 g ethanol/kg body weight in injections given every other day for a total of nine injections. Heart rate and body temperature were recorded for 1 hr before and 2 hr after each injection. Ethanol initially induced a monophasic dose-related cardioacceleration (80 bpm) and hypothermia (1.0 degrees C) that persisted throughout the 2-hr sample period. Tolerance developed to the hypothermic, but not to the tachycardic effect of ethanol. Assuming that tolerance depends on level of impairment in specific neuronal pathways, this outcome suggests that these two effects of ethanol are not mediated through a common autonomic mechanism (e.g., vasomotor depression) and/or that tolerance to the hypothermic effect is due to alterations in pathways unique to the thermoregulatory system. Overall, the finding is consistent with those of studies showing development of tolerance to depressant, but not to excitatory drug effects.

Morphine tolerance and sensitization in the hamster

The effects of morphine on hamster locomotor activity were studied. Repeated administration of morphine in doses from 5 to 40 mg/kg produced systematic changes in morphine's biphasic time effect pattern: morphine's sedative effects decreased (tolerance) while morphine's excitatory effects increased (sensitization). These results extend findings of behavioral tolerance and sensitization in the hamster to a range of higher doses than those used previously.

Evidence for the activation of the endogenous opiate system in hamsters infected with human blood flukes, Schistosoma mansoni

Nociceptive thresholds were investigated in golden hamsters infected with the human blood fluke, Schistosoma mansoni. Increases in thermal thresholds suggestive of analgesia were evident by 20-25 days of infection. These increased further during a 40-42 day period. The altered responses were suppressed by the opioid antagonist naloxone. Non-invasive inhibition of the activity of the pineal gland by exposure to light also reduced nocturnal analgesia in schistosome infected animals. Naloxone antagonism and pineal inhibition of morphine- induced analgesia was obtained similarly in control, uninfected animals. Taken together, these findings suggest strongly that infection with S. mansoni results in a chronic activation of the endogenous opiate system.

Locomotor activity and opiate effects in male and female hamsters

Locomotor activity of golden Syrian hamsters was investigated in three experiments. In Experiment 1, running wheel activity of male and female hamsters was compared under the following conditions: no injection, saline injection, morphine injection (15 mg/kg) and naltrexone injection (1 mg/kg). During two hour test sessions, females maintained high levels of activity, whereas males slowed down considerably during the second hour. The difference between males and females was evident under all conditions except following morphine which produced a biphasic time-effect pattern in both sexes. Naltrexone, however, had no detectable effects on either males or females. Experiment 2 investigated the effects of four doses of naltrexone (0.3, 1, 3, 10 mg/kg) on hamster locomotion. Results indicated that none of the doses tested had an effect. Experiment 3 demonstrated that a 1 mg/kg dose of naltrexone antagonizes two of morphine's (15 mg/kg) effects on activity. First, naltrexone partially blocked morphine elicited sedation. Second, naltrexone blocked the increase in activity that characterizes recovery from morphine.