High opioid doses inhibit whereas low doses enhance neuritogenesis in PC12 cells

Exposure to opiates affects brain development, cell growth as well as in vitro cell differentiation [33,34]. Perinatal treatment with morphine has been reported to impair neuronal plasticity after neonatal lesion with 5,7-dihydroxytryptamine (5,7-DHT) [8]. This study has investigated the use of mu, delta and kappa opioid receptor ligands to examine the selective receptor mediated inhibition of PC12 neurite formation. Morphine and D-Ala2,D-Leu5-enkephalin (DADLE) had a comparable inhibitory potency with a maximal effect at 1 mM concentration, while both naltrexone and naltrindole antagonized their effect at only 10 nM. D-Ala2-MePhe4,Gly-ol5-enkephalin (DAMGO) showed only a transient inhibitory effect. The administration of 10 nM guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) prevented morphine inhibition. It is suggested that opiate inhibition of neuritogenesis may be mediated by a receptor with delta-like characteristics coupled to G proteins. On the other hand, the activation of this receptor with morphine at a very low concentration (1 pM) actually enhanced nerve growth factor (NGF) neurite promoting activity.

Lack of effect of gamma-hydroxybutyrate on mu, delta and kappa opioid receptor binding

gamma-Hydroxybutyrate (GHB) and morphine induce a number of similar effects. Moreover, the effects they elicit can be reversed by the opiate antagonist naloxone (NX), suggesting that GHB may produce at least some of its central effects by acting as an opiate agonist. The present study considered this possibility by examining the effect of GHB on mu, delta, and kappa-opioid receptor binding in concentrations of 1 nM-0.1 mM. GHB was inactive in each instance, at every dose examined. GHB is consequently not a direct opiate receptor agonist. It is also unlikely to be an indirect (enkephalin or dynorphin release-stimulating) agonist. The mechanism of action involved whereby NX can reverse the effects of GHB must therefore not involve opioid mechanisms; at least not directly.

Unsignaled morphine delivery does not disrupt the development of associative morphine tolerance in the rat

When morphine administration is paired with a distinctive context, tolerance to morphine's analgesic effects comes readily under the associative control of the drug-paired context. These associative tolerance effects are eliminated when a relatively short (i.e., 6 h) interdose interval (IDI) is used for conditioning. Contemporary models of learned tolerance explain the absence of learning at short IDIs by positing that residual morphine effects from a recent drug exposure disrupt the formation of drug-context associations. The present studies examined the impact of unsignaled morphine injections given 6 h prior to drug-context pairings on the development of associative tolerance. Analgesia was measured by the tail-flick method, and tolerance levels were assessed by dose-response curve methodology. Morphine preexposure had no detectable influence on the acquisition of associative tolerance when rats were tested immediately after conditioning, after a 30-day rest interval, or after a 30-day period of daily saline injections in their home-cage environment. These data suggest disruption of associative tolerance effects at short IDIs is not attributable to residual effects of morphine from the immediately preceding trial.

The effects of cocaine and nandrolone co-administration on aggression in male rats

1. Cocaine and anabolic-androgenic steroids are among the more commonly abused substances in selected populations. These agents, when used alone or in combination, have been reported to cause aggressive tendencies in both laboratory-based animal models and in human clinical situations. This project, using a resident-intruder paradigm, examined the effects of co-administration of cocaine and a typical anabolic-androgenic steroid, nandrolone decanoate, on the development of aggression in male Sprague-Dawley rats. 2. Dose response studies demonstrated that low dose cocaine (1 mg/kg) produced more aggression in a greater percentage of animals than for either the controls or groups receiving higher doses (up to 20 mg/kg). Initially, high intermittent doses of nandrolone (20 mg twice weekly) produced more aggression; however, low daily doses of nandrolone (2 mg) produced greater levels of aggression following 4 weeks of treatment. 3. Optimal doses of cocaine and nandrolone, when administered together, resulted in aggression scores that were not significantly different from controls or either drug singly. However, a greater percentage of animals receiving both drugs exhibited aggression than did rats receiving either drug alone. 4. These results support the interpretation that the drugs interact to produce unique effects in the development of aggression. However, the complexity and extent of the interactions is great and remains to be fully elucidated.

Detailed investigations of 5-HT3 compounds in a drug discrimination model

Serotonin type-3 (5-HT3) receptors modulate both dopamine (DA) release and locomotor stimulation induced by cocaine, yet appear to be ineffective at blocking its stimulus and reinforcing effects. To more thoroughly characterize a potential modulatory role of 5-HT3 receptors in the stimulus effects of cocaine, rats (n = 8/group) were trained to discriminate cocaine (10 mg/kg, IP) or the 5-HT3 agonist 1-(meta-chlorophenyl)-biguanide (mCPBG: 15 mg/kg, IP) from saline using a standard drug discrimination task. In rats trained to discriminate cocaine, mCPBG (2.5-20 mg/kg) produced, at best, a partial substitution while mCPBG (10 mg/kg) did not alter the cocaine dose-response relationship. The 5-HT3 antagonists MDL 72222 (10 mg/kg) and ondansetron (1.25-16 mg/kg) did not attenuate the cocaine cue. In rats trained to discriminate mCPBG from saline, the 5-HT precursor l-5-hydroxytryptophan (12.5-50 mg/kg) dose-dependently substituted for mCPBG, whereas the 5-HT3 antagonist zacopride (0.1-10 mg/kg) partially antagonized the mCPBG cue, demonstrating that mCPBG produces distinct discriminable effects that appear to be mediated by 5-HT, possibly 5-HT3, receptors. However, cocaine (5-20 mg/kg) did not substitute in mCPBG-trained rats. Overall, these data support previous findings to suggest that 5-HT3 receptors play little role in mediating the discriminative stimulus effects of cocaine and suggest that the neurochemical mechanisms and/or sites of action important for the generation of the discriminative stimulus vs. locomotor stimulatory effects of cocaine may be dissociable.

Effect of isolation on morphine-induced running and changes in body temperature

1. The influence of isolation of three durations 8, 15 and 30 days has been examined in mice on the effects of morphine on rectal temperature and on locomotor activity. Isolated mice were compared to non isolated mice with the same age. 2. Morphine (20 mg/kg ip) induced in mice an early hypothermia followed by a late hyperthermia. The hypothermic effect was significantly reduced following isolation, but the duration of isolation (8, 15, 30 days) had no influence. Isolation did not modify the hyperthermic effect of morphine. 3. Morphine (40 mg/kg ip) induced in mice an increase in locomotor activity called "running". The running activity was significantly increased following isolation. The duration of isolation (8, 15, 30 days) did not seem to influence this effect. 4. These results show that isolation does not modify in the same way every effects of morphine, they suggest that isolation alters the mechanism involved in the running activity and in the hypothermic effect. The nature of these mechanisms is discussed.

Serotonergic lesions alter cocaine-induced locomotor behavior and stress-activation of the mesocorticolimbic dopamine system

The aim of this study was to examine the effects of serotonergic lesions to the dorsal raphe on midbrain dopaminergic systems. 5,7-Dihydroxytryptamine lesions of the dorsal raphe resulted in a substantial loss of serotonin in the medial prefrontal cortex (about 75%) and the nucleus accumbens (about 50%), while no change in DA levels or DA metabolism were noted in either region at 12 days postlesion. A transient basal locomotor activation was noted in the lesioned animals compared to the sham controls 7 to 12 days after the lesions. The locomotor response to an acute dose of cocaine was also enhanced in 5,7-dihydroxytryptamine lesioned rats, however, no change in the time course or magnitude of the behavioral locomotor response to repeated cocaine administration was observed. Restraint for 30 min increased DA metabolism in both the NAS and mPFC of sham rats, as expected. However, in 5,7-dihydroxytryptamine lesioned rats, restraint stress enhanced the usual stress-induced increase in DA metabolism by about 50 and 150% in the medial prefrontal cortex and nucleus accumbens, respectively. Our results indicate the 5,7-dihydroxytryptamine lesions of the dorsal raphe lower serotonin in both the mPFC and NAS leading to an enhanced responsiveness of the DA projections in both regions. This effect may be explained by a loss of sensitivity of DA receptors in 5,7-dihydroxytryptamine denervated rats. This interpretation implies that the stimulated, but not basal, release of DA in the mPFC and NAS is dependent on serotonin tone.

Repeated intravenous cocaine administration: locomotor activity and dopamine D2/D3 receptors

The dopamine D3 receptor has been implicated as a possible mediator in the reinforcement or abuse of psychostimulants such as cocaine. The present studies examined the effects of repeated (14 day) intravenous cocaine administration (saline vehicle, 0.5, 1.0 and 3.0 mg/kg) on locomotor activity and dopamine D2 and D3 receptor density in the rat striatum and nucleus accumbens. Male Sprague-Dawley rats (n = 40) were implanted with an intravenous access port and allowed to recover for 2 days. An additional group of naive rats was included to control for surgical/injection stress (n = 10). Following 2 days of habituation trials, total, peripheral and central activity (photocell interruptions) data were collected during alternate daily 60-minute test sessions. Repeated cocaine treatment resulted in a significant dose-dependent increase in striatal D3 receptors which was predicted by daily 60-minute central locomotor activity. Conversely, D3 receptors in the nucleus accumbens exhibited a significant dose-dependent reduction which was predicted by the initial 5 minutes of central locomotor activity observed on peak sensitization days (days 6, 8 and 10). Sensitization to the locomotor stimulatory effects of cocaine was dose-dependent, with the time to peak sensitization day following the rank order of 0.5 > 1.0 > 3.0 mg/kg. The density of D2 receptors in the striatum and nucleus accumbens was unchanged by cocaine administration. These data suggest striatal and nucleus accumbens D3 receptor involvement in the expression of cocaine-induced behavioral sensitization. Thus, the D3 receptors in the striatum and nucleus accumbens may be differentially involved in the locomotor stimulation (striatal D3) and reinforcing aspects (nucleus accumbens D3) of repeated cocaine administration.

Opiates, opioids and addiction

Increasing our knowledge about the major addictive diseases, opiate and cocaine addiction and alcoholism, is of great importance from a public, as well as personal health perspective. Each disease is associated with profound and negative impacts on physical and mental health and also each has devastating social and economic consequences. Each of these addictive diseases has been associated with major infectious diseases including AIDS, hepatitis B, C, D and G, either through parenteral or sexual transmission. Since 1967, we have addressed the research question related to our early hypotheses on the development of an addiction, that atypical responsitivity to stress and stressors may play a central role in the acquisition and persistence of, and relapse to, drug abuse. We have been conducting studies both in humans and in animal models focused on the role of disruption of the stress responsive hypothalamic-pituitary-adrenal axis in opiate addiction, cocaine dependency and alcoholism. We also have conducted studies of the role of the endogenous opioid system in modulation of this axis, as well as the interaction of the endogenous opioid system with the dopaminergic system and other neurotransmitter and neuropeptides related to the reinforcing effects of drugs of abuse.

Orally delivered methadone as a reinforcer for rhesus monkeys: the relationship between drug concentration and choice

The relative reinforcing effects of orally delivered methadone were studied in five male rhesus monkeys. Drug deliveries were available under either a fixed-ratio (FR) or a fixed-interval (FI) schedule. Three concentrations of methadone, low (0.05 mg/ml), intermediate (0.2 mg/ml), and high (0.8 mg/ml) were delivered in 0.65 ml volumes. In the first experiment, monkeys were presented with a choice paradigm. Under independent FR schedules responding led to a delivery of either a methadone solution or the water vehicle. For each concentration, deliveries of a methadone solution maintained higher response rates than did deliveries of water. In the second experiment, methadone concentrations were tested in pairs in the following sequence: high vs. low, high vs. intermediate, intermediate vs. low, high vs. intermediate (retest), and high vs. low (retest). The retest of the last two pairs was designed to counterbalance the test sequence, so that order effects, if they existed, could be detected. Regardless of the schedule, the higher concentration of the methadone pair maintained a greater response rate than did the lower concentration. The present results are consistent with the generalization based on other studies that over a broad range of concentrations and across pharmacological classes, reinforcement schedules, and routes of administration, reinforcing effects increase with increases in drug concentration.

Exposure to perinatal morphine promotes developmental changes in rat striatum

This study shows that perinatal exposure to morphine promotes developmental changes (up to 8 months of life) in the striatum by up-regulating concentrations of substance P and met-enkephalin with changes of prometenkephalin A mRNA expression at the day of birth only. Dopamine metabolism (up to 60 days) is also increased as suggested by the reduced concentrations of dopamine and increased content of 3,4-dihydroxyphenylacetic acid. Tyrosine hydroxylase mRNA expression is selectively reduced only in the substantia nigra by perinatal morphine. Serotonin content is reduced only during the early postnatal days and is unaffected thereafter. Supplementation of naltrexone to morphine-exposed rats prevents monoaminergic and neuropeptidergic changes in the striatum, which directly implicates opioid receptors in the developmental changes caused by morphine. The data suggest that perinatal morphine may inhibit met-enkephalin release, causing accumulation of the peptide without corresponding changes in specific mRNA. Dopamine release may also be increased as indicated by a higher metabolism and consequent reduction of tyrosine hydroxylase mRNA expression in the substantia nigra.

Acute and repeated exposure to caffeine: effects on reinstatement of extinguished cocaine-taking behavior in rats

Previously, we demonstrated that caffeine dose-dependently reinstated extinguished cocaine-taking behavior in rats. In the present study, we determined whether this effect of caffeine would extinguish with repeated exposures. Rats were first trained to self-administered cocaine intravenously. Once reliable self-administration was obtained, the pumps that delivered cocaine were turned off and the lever-pressing behavior was extinguished. Every 4 days the rats were given an injection of caffeine (20.0 mg/kg) and its ability to reinstate responding was measured. Some rats received each of four exposures to caffeine in the previously cocaine-associated environment. Other rats received the first three exposures to caffeine in the home cage and the last exposure to caffeine in the previously cocaine-associated environment. The results indicated that although caffeine was an effective cue for reinstatement of extinguished cocaine taking, the effect was reduced when repeated exposures occurred in the test environment. In addition, when 4 drug-free days were interspersed between self-administration and reinstatement testing, the caffeine effect was greater than when testing was conducted 1 day following the last self-administration session. Thus, extended withdrawal increases the priming effects of caffeine. The results are discussed in terms of the effectiveness of cue exposure as an adjunct to current therapies for cocaine abuse.

Ibogaine fails to interrupt the expression of a previously established one-trial morphine place preference

1. Ibogaine, a proposed anti-addictive agent, has been found to interfere with the acquisition of a weak morphine-induced place preference. The present series of experiments determined if ibogaine would interfere with the expression of a previously established morphine (5 mg/kg) place preference. 2. A single injection of 40 mg/kg of ibogaine 24 h, 12 h or 4 h prior to the preference test (Experiment 1) or 80 mg/kg of ibogaine 24 hr prior to the preference test (Experiment 3) did not interfere with the expression of a morphine conditioned place preference. 3. Furthermore, two injections of 40 mg/kg of ibogaine 48 h and 24 h or 24 h and 4 h prior to testing (Experiment 2) did not interfere with the expression of a morphine place preference. 4. Ibogaine appears to be incapable of attenuating the expression of a previously established one-trial morphine place preference.

Dextrorphan effects on cocaine and brainstem perturbation

Dextrorphan is a noncompetitive blocker of N-methyl-D-aspartate (NMDA) receptors. Since NMDA blockers are known to reduce the locomotor stimulatory and toxic effects of cocaine, it was speculated that dextrorphan would attenuate cocaine-induced behavioral excitatory motor activity associated with and without mechanical perturbation of the brainstem.

TECHNIQUE

Motor activity was recorded following dextrorphan and/or cocaine challenge in 25 SHR rats. Ten were naive subjects. Mini-osmotic pumps delivering cocaine (2.5 mg/0.49 ul/hr) were placed in 15 subjects, and infusion was halted after the third infusion day. On the fifth day either a dextrorphan (25 mg/kg, subcutaneous) or a dextrorphan and cocaine (40 mg/kg, intraperitoneal) challenge was done. Ten rats had bipolar electrode implants in the bilateral brainstem. Five were treated with DC current lesions in each of 12 days over a 3-week period. The effects of brainstem lesions on escape behavior were also evaluated in those five subjects.

RESULTS

In the naive subjects, dextrorphan reduced motor activity (P = .0001), whereas combined cocaine and dextrorphan increased motor activity (P = 0.04). In lesioned subjects, dextrorphan decreased motor activity (P = 0.0001). In electrode implant subjects, combined dextrorphan and cocaine challenge decreased the motor activity (P = 0.04). Hyperactivity in the electrode implant group was greater than in the lesioned subjects. Midbrain electrolytic lesions attenuated escape behavior. A variety of behaviors were produced by brainstem lesions.

CONCLUSIONS

Dextrorphan and brainstem lesions reduced motor hyperactivity and escape behavior. In electrode implant subjects dextrorphan counteracted the expected cocaine excitatory motor effects. Dextrorphan did not activate nor facilitate seizures.

Rewarding properties of some drugs studied by place preference conditioning

Ethanol, morphine, cocaine and amphetamine were examined in place conditioning. After determination of initial preferences, animals were conditioned with ethanol (1 g/kg), morphine (5 mg/kg), cocaine (5 mg/kg) and amphetamine (5 mg/kg) alone or with combinations of these drugs plus naloxone (1 mg/kg). Naloxone prevented the ability of all drugs used to produce a place preference. The reinforcing properties of ethanol and morphine were reduced by sodium nitroprusside at a dose equal to 1/10 of LD50 given before preference testing. Molsidomine (1/10 LD50 and 1/20 LD50) altered the expression of the conditioned place preference produced by ethanol but not by morphine. Results of the present study suggest the involvement of endogenous opioids and probably of nitric oxide in the rewarding actions of drugs of abuse.

Pregnancy enhances cocaine-induced stimulation of uterine contractions in the chronically instrumented rat

OBJECTIVE

Our purpose was to test whether cocaine stimulates uterine activity in nonpregnant and pregnant rats.

STUDY DESIGN

The carotid artery and jugular vein were chronically catheterized, and a microballoon probe was inserted into the uterine cavity of 15 pregnant and 14 nonpregnant female rats. Conscious animals received a bolus dose of either cocaine or saline solution intravenously. Cardiovascular and uterine contractile responses were studied.

RESULTS

Cocaine (2.5 mg/kg) induced a marked increase in uterine activity and arterial blood pressure in both pregnant and nonpregnant animals without producing systemic toxicity. The maximum change in uterine contractions was greater in the pregnant group than in the nonpregnant group, and blood pressure responses were transient in both.

CONCLUSION

This study is the first demonstration that cocaine stimulates the rat uterus in vivo, with a greater increase in contractions in pregnant compared with nonpregnant animals. These differences are not related to the hemodynamic response or pharmacokinetic profile of cocaine.

The stimulatory effects of opioids and their possible role in the development of tolerance

Opioids have stimulatory as well as the traditional inhibitory effects on neurotransmission, but the underlying mechanisms are poorly understood. Here, Darren Smart and David Lambert review the stimulatory effects of opioids on second messengers, including inositol (1,4,5)-trisphosphate (IP3), protein kinase C (PKC), Ca2+, and cAMP, and propose that these coordinated changes at the cellular level underlie the facilitatory effects of opioids on neurotransmission. The evidence for a possible role for these stimulatory effects, particularly the activation of PKC by opioids, in the development of tolerance is also discussed.

Intrasynaptosomal free calcium levels in rat forebrain synaptosomes: modulation by sigma (sigma) receptor ligands

The sigma receptor ligands (+) and (-)pentazocine and BD1008 (1-100 microM) were added to rat forebrain synaptosomes. Their effects on intrasynaptosomal free calcium ([Ca2+(+)]i) levels under basal conditions and after depolarisation with high potassium buffer (45 mM KCl), veratridine (25 microM) and 4-aminopyridine (4-AP, 1 mM) were determined. The sigma ligands elicited significant, concentration-dependent decreases in basal [Ca2+]i levels with an order of potency (-)pentazocine > (+)pentazocine = BD1008. The sigma ligands (at the maximum effective concentrations) also significantly inhibited the rise in [Ca2+]i levels produced by depolarisation with KCl, veratridine and 4-AP. The effect of (+) and (-)pentazocine (100 microM) to inhibit the depolarisation-dependent increase in [Ca2+]i levels was greater when veratridine and 4-aminopyridine were used to depolarise the synaptosomes than with KCl, whereas the effect of BD1008 (100 microM) was approximately equipotent using all three depolarising agents. However, BD1008 was more potent to inhibit the KCl-induced rise in [Ca2+]i compared to (+) and (-)pentazocine. The data demonstrate for the first time that sigma ligands decrease [Ca2+]i levels in rat forebrain synaptosomes and this suggests a possible mechanism for the changes to neuronal protein phosphorylation and neurotransmitter release previously observed with sigma ligands.

Spinally-mediated behavioural responses evoked by intrathecal high-dose morphine: possible involvement of substance P in the mouse spinal cord

Intrathecal (i.t.) administration of morphine in the spinal subarachnoid space of mice produced a severe hindlimb scratching followed by biting and licking. The onset of the scratching behaviour was observed 60-70 s after i.t. injection of morphine (60 and 90 nmol), and had a duration of 3-4 min. The morphine-induced behaviour was increased additively by i.t. co-administration of substance P (SP). This characteristic behavioural response was inhibited dose-dependently by i.t. co-administration of the tachykinin NK-1 receptor antagonists, sendide and CP-96,345. Significant antagonistic effects of SP (1-7), a putative antagonist for NK-1 receptors and [D-Phe7, D-His9]SP (6-11), a selective antagonist for SP receptors, were observed against the morphine-induced behaviour. Pretreatment with i.t. SP antiserum and i.t. capsaicin resulted in reduction of the response to morphine. I.t. administration of somatostatin (SOM) antiserum, cysteamine, a relatively selective depletor of SOM and cyclo-SOM, a SOM receptor antagonist, produced no inhibitory effect on the morphine-induced behaviour. These results demonstrate that a spinal system of neurones containing SP may be involved in elicitation of the behavioural episode following i.t. injection of morphine in mice.

Acute pharmacological blockade of corticosterone secretion reverses food restriction-induced sensitization of the locomotor response to cocaine

Several data indicate that a blockade of stress-induced corticosterone secretion prevents the development of the stress-induced sensitization of the behavioral effects of drugs of abuse. In this report we investigated if an acute blockade of corticosterone secretion could reverse stress-induced sensitization once it is already established. Food restriction (90% of initial body weight) was used as stressor. Corticosterone secretion was blocked by the corticosterone synthesis inhibitor metyrapone (100 mg/kg). After 8 days of food restriction, animals received an injection of metyrapone and 3 h later they were tested either for the locomotor response to cocaine or for the corticosterone secretion in response to stress (restraint, 30 min). Neither metyrapone nor food restriction had any effect on the locomotor response to a saline injection. In contrast, food-restricted animals, compared to ad libitum-fed controls, showed a higher locomotor response to cocaine and higher corticosterone levels. Treatment with metyrapone totally abolished these effects. Food-restricted animals, receiving a single injection of metyrapone, did not differ from ad libitum-fed controls for both locomotor response to cocaine and corticosterone secretion. Metyrapone treatment also similarly reduced the response to cocaine and corticosterone secretion in ad libitum-fed controls. In conclusion, this study provides further evidence that the enhancement in drug effects produced by stress depends on an increase in corticosterone levels. Since stress-induced sensitization is considered one of the conditions predisposing to drug abuse, the present results might have implications for the treatment of addiction.

Relevance of phosphorylation state to opioid responsiveness in opiate naive and tolerant/dependent tissue

This laboratory previously reported that the mu-selective opiate receptor agonist, sufentanil, produces a naloxone-reversible, concentration-dependent facilitation or inhibition of the stimulated formation of cAMP in the myenteric plexus. Chronic in vivo exposure to morphine results not only in the loss of inhibitory opioid responsiveness but in the reversal of inhibition to enhancement. The present study demonstrates, in tolerant/dependent as well as opiate naive tissue, that the state of phosphorylation is a critical determinant of the balance between positive and negative opioid modulation of stimulated cAMP formation. In vitro treatment of chronic morphine-treated preparations with inhibitors of protein kinases, abolishes the previously observed reversal of opioid inhibition to enhancement and restores sufentanil inhibitory responsiveness. The established kinase-type selectivity profile of the inhibitors employed suggests the involvement of protein kinase C (PKC) in the tolerant-associated reversal from opioid inhibition to enhancement of cAMP formation. Conversely, treatment of opiate naive tissue with the protein phosphatase inhibitor okadaic acid or a phorbol ester activator of protein kinase C, phorbol 12-myristate 13-acetate (PMA), not only attenuates sufentanil inhibition of evoked cAMP formation but reverses it to a facilitation (as occurs following chronic in vivo morphine exposure). This effect of PMA is abolished by the PKC-selective inhibitor chelerythrine. Moreover, the longitudinal muscle myenteric plexus content of PKC alpha and PKC beta is substantially elevated following chronic morphine treatment. These results underscore the relevance of opioid bimodality to the manifestation of tolerance/dependence and suggest that augmented phosphorylation (mediated at least in part via PKC) is a critical determinant of some of the sequelae of chronic morphine exposure.

Morphine- and cocaine-induced conditioned place preference: effects of quinpirole and preclamol

The role of dopamine in opioid reward is unresolved. Furthermore, the issue is somewhat unclear regarding cocaine and the place preference paradigm. In the present study we investigated whether the drugs activating dopamine autoreceptors affect cocaine- and morphine-induced place preference in rats. Neither the dopamine D2/D3 receptor agonist, quinpirole (0.05 mg/kg, SC), nor the partial dopamine autoreceptor agonist, preclamol (2 or 8 mg/kg, SC), induced place conditioning by itself. Quinpirole had no significant influence on the place preference induced either by morphine (3 mg/kg, SC) or cocaine (5 mg/kg, IP). Preclamol, when given at the dose of 8 mg/kg SC, significantly attenuated the effect of cocaine but failed to modify the effect of morphine. Our results suggest that the rewarding properties of morphine involve DA-independent mechanisms whereas in the cocaine-induced reward the role of brain DA is critical. Furthermore, as regards place conditioning, we propose that the activation of DA autoreceptors is not sufficient to reliably modify the rewarding effect of cocaine.

Acute and chronic effects of morphine in pigeons responding under a progressive-ratio schedule of food delivery

Although progressive-ratio schedules have often been used by behavioral pharmacologists to index the relative reinforcing effects of drugs of abuse, they have been ignored in the study of tolerance to opioids. The present study examined tolerance to morphine in pigeons responding under a progressive-ratio 5 schedule of food delivery. Acute administrations of morphine produced general dose-dependent reductions in response rates and breaking points. Dose-response curves for both measures shifted rightward substantially (roughly fivefold) following chronic (daily) exposure to morphine, indicating that tolerance developed to the drug's effects.

Cocaine self-administration increased by compounding discriminative stimuli

Presenting independently established discriminative stimuli in compound can substantially increase response rates under food and shock-avoidance schedules. To determine whether this effect extends to drug self-administration, rats were trained to press a lever to receive cocaine intravenously. A tone and a light were independently established as discriminative stimuli for cocaine self-administration, then presented in combination in a stimulus-compounding test. Compared to tone and light alone, the tone-plus-light compound stimulus increased responding approximately three-fold when cocaine was withheld during testing, and it increased drug intake approximately two-fold when cocaine was made available during testing. Compounding did not increase responding after training in a truly random control condition where tone and light were presented uncorrelated with the availability of cocaine. The results obtained with this animal model of drug abuse define conditions under which combinations of environmental stimuli might substantially increase human drug use.

Intravenous heroin and ethanol self-administration by alcohol-preferring AA and alcohol-avoiding ANA rats

The alcohol-preferring AA rats have previously been shown to drink more solution containing the opioid etonitazene than the alcohol-avoiding ANA rats. The present experiments were initiated to see whether the line difference in opioid and alcohol intake would persist if an intravenous (i.v.) route of self-administration is used. Following establishment of stable heroin responding (0.03 mg/kg per infusion), AA and ANA rats were first subjected to three within-session dose-response determinations during which they were allowed to respond for ascending heroin doses (0.0075, 0.015, 0.03, and 0.06 mg/kg per infusion) and then to one progressive-ratio schedule session. AA rats obtained more heroin infusions than ANAs during the first acquisition sessions but there were no significant differences between the lines either in their baseline heroin responding, across the ascending within-session doses, or on the progressive ratio probe. When, after additional heroin baseline sessions, ethanol (1.0 mg/kg per infusion) was substituted for heroin, AA rats initially increased their responding and showed stable rates for responding across ascending ethanol doses (2.0 and 4.0 mg/kg), whereas ANAs declined below their heroin baseline. These findings give evidence for only an initial line difference in i.v. opiate self-administration but for a sustained difference in i.v. ethanol self-administration, thus suggesting that the differential alcohol drinking of the AA and ANA rats is dependent at least partly on non-oral factors.