Conditioned rewarding stimulus associated with cocaine self-administration reverses the depression of catecholamine brain systems following cocaine withdrawal in rats.

Craving phenomena are related to induction of substance-seeking behaviour by stimuli associated with the availability of the drug. We investigated the changes in monoamine metabolism in regions of the brains of rats that, following a period of training of cocaine self-administration, were either killed 2 h after the last session or underwent extinction trials, during which cocaine was withdrawn. During the training, acoustic and visual stimuli announced the availability of cocaine. After 10 d of daily extinction trials, rats were re-introduced into the cage, and a signal associated with cocaine availability was applied to half of the animals. The rats were immediately killed and concentrations of dopamine and serotonin and their metabolites in various brain areas, and the concentration of noradrenaline and MHPG in the brainstem were assessed to calculate their metabolism rate indices. In rats self-administering cocaine, the levels of metabolites of all three amines were depressed, indicating a depression of the activity of monoaminergic systems. In the period of extinction, the dopamine levels in the nucleus accumbens and striatum and the level of the noradrenaline metabolite, MHPG, in the brainstem were reduced, suggesting a long-lasting disturbance of the catecholaminergic system, while serotonin levels and metabolism returned to normal values. The presence of the signal associated with previous cocaine availability, which invariably caused the reinstatement of cocaine-seeking behaviour annulled the changes observed in the group receiving no stimulus, bringing the concentration values of dopamine, and dopamine and noradrenaline metabolites to yoked-saline control rats. The results suggest that the stabilized self-administration of cocaine depresses the activity of all biogenic amine systems, and the changes in serotonin system are reversible, in contrast to those observed in catecholaminergic systems, which show the signs of a long-lasting impairment. The stimulus associated with cocaine availability activates the catecholaminergic system in animals after extinction procedure.

Effects of CP 154,526, a CRF1 receptor antagonist, on behavioral responses to cocaine in rats.

We examined the influence of CP 154,526, a selective antagonist of corticotropin-releasing factor (CRF)1 receptors, in the locomotor, sensitizing, discriminative stimulus and rewarding effects of cocaine, as well as on the cocaine-induced reinstatement of cocaine-seeking behavior in male Wistar rats. CP 154,526 in doses of 5, 10 and 20 mg/kg, which did not affect basal locomotor activity, dose-dependently reduced the hyperactivation evoked by cocaine. To assess the effects of CP 154,526 on the expression of cocaine sensitization, the rats were injected with either saline or cocaine (10 mg/kg) for 5 days, and were then challenged with cocaine (10 mg/kg) after pretreatment with saline or CP 154,526 on day 5 of withdrawal. The cocaine-induced hyperactivity in sensitized rats was reduced by CP 154,526 (10 and 20 mg/kg). In rats trained to discriminate cocaine (10 mg/kg) from saline, pretreatment with CP 154,526 (5-20 mg/kg) did not affect the cocaine (1.25-10 mg/kg)-induced discriminative stimulus effects. In a self-administration model, the rats were trained to self-administer cocaine (0.5 mg/kg/infusion) in the FR 5 schedule of reinforcement. Administration of CP 154,526 (10-20 mg/kg) did not alter the rewarding effects of cocaine, assessed as the number of active-lever presses and infusions; however, following a 10-day extinction phase, CP 154,526 (5-20 mg/kg) significantly decreased in a dose-dependent manner the cocaine (10 mg/kg) priming-induced reinstatement of cocaine-seeking behavior. The present study implies that CRF1 receptors control the expression of cocaine hyperactivation and sensitization as well as the cocaine-induced relapse behavior, but do not play any role in cocaine discrimination and self-administration. These findings may suggest that CRF1 receptor antagonists should be considered as possible medications in the treatment of cocaine addiction.

Effects of 5-HT1B receptor ligands microinjected into the ventral tegmental area on the locomotor and sensitizating effects of cocaine in rats.

The present study was aimed at finding out whether 5-HT(1B) receptors located in the ventral tegmental area (VTA) played a role in the locomotor hyperactivity induced by a single dose of cocaine and in the sensitization evoked by repeated exposure to the psychostimulant in rats. Male Wistar rats, implanted bilaterally with cannulae in the VTA, were microinjected with GR 55562 (an antagonist of 5-HT(1B) receptors) or CP 93129 (an agonist of 5-HT(1B) receptors). GR 55562 (0.3-3 microg/side) did not affect locomotor hyperactivity response to a single dose of cocaine (10 mg/kg). CP 93129 in a dose of 1 microg/side (but not lower), which stimulated basal locomotor activity, enhanced the cocaine-induced locomotor hyperactivity. The rats that were treated repeatedly (for 5 days) with cocaine (10 mg/kg) and then challenged with cocaine (10 mg/kg) after 5-day withdrawal period (day 10 of the experiment) showed significantly higher locomotor hyperactivity compared to the effect observed in saline-pretreated and cocaine-challenged rats. GR 55562 (a dose of 3 microg/side, but not lower), administered for 5 days into the VTA just prior to daily cocaine, attenuated cocaine sensitization. When injected for 5 days into the VTA, CP 93129 (0.03-0.1 microg/side) enhanced the development of cocaine sensitization. The enhancing effect of CP 93129 (0.1 microg/side) was blocked by GR 55562 (1 microg/side). To examine the effects of GR 55562 and CP 93129 on the expression of cocaine sensitization, the 5-HT(1B) receptor ligands were given acutely before the challenge dose of cocaine administered on day 10. No change in cocaine sensitization was observed after intra-VTA microinjections of GR 55562 (0.3-3 microg/side) or CP 93129 (0.1-1 microg/side). Our findings suggest that 5-HT(1B) receptors located in the VTA play a permissive role in the development of cocaine sensitization, but are not involved in the locomotor hyperactivity induced by a single dose of cocaine or in the expression of the sensitization to the psychostimulant.

Effect of thyrotropin-releasing hormone on the locomotor and sensitizing effects of cocaine in rats.

The present study was designed to find out whether single and repeated treatment with thyrotropin-releasing hormone (TRH) changed the cocaine-evoked hyperactivation or sensitization, and whether cross-sensitization occurred between TRH and cocaine. Like cocaine (10 mg/kg), TRH (10 mg/kg) increased the basal activation of rats; however, when given in combination with cocaine (10 mg/kg), TRH (5-10 mg/kg) did not change the locomotor effect of cocaine. On day 10, cocaine challenge of rats treated repeatedly with the psychostimulant (days 1-5) significantly enhanced locomotor hyperactivity compared to the effect of acute cocaine injection in saline-treated (days 1-5) animals. When co-administered with cocaine for 5 days during the development of sensitization, TRH (10 mg/kg) enhanced the effect of the challenge dose of cocaine (10 mg/kg) following a 5-day withdrawal. Given acutely with cocaine on day 10 to cocaine-treated animals, TRH (5-10 mg/kg) did not change the expression of cocaine sensitization. Significant enhancement of the locomotor response to TRH (10 mg/kg) challenge was observed in animals treated repeatedly with TRH. The response to TRH (5-10 mg/kg) was stronger in repeated cocaine-treated rats than in saline-injected ones; similarly, the response to cocaine (10 mg/kg) was more potent in TRH-treated animals compared to saline-injected ones (cross-sensitization). In conclusion, our results indicate that exposure to TRH induces sensitization to its locomotor hyperactivity effect. They also show that TRH enhances the development of cocaine sensitization, but affects neither the expression phase of the phenomenon nor the locomotor hyperactivity induced by a single dose of cocaine. Moreover, cross-sensitization between cocaine and TRH has also been demonstrated. These findings also may provide an insight into the relationship between TRH and cocaine in humans exposed to the psychostimulant.

Effects of 5-HT1B receptor ligands microinjected into the ventral tegmental area on cocaine discrimination in rats.

Some recent data indicate a significant interaction between serotonin (5-hydroxytryptamine; 5-HT) and dopamine in mesolimbic brain structures (e.g. the ventral tegmental area) which modulate the behavioral effects of cocaine in rats. The present study investigated the role of 5-HT(1B) receptors in the ventral tegmental area in the discriminative stimulus effects of cocaine in rats. Male Wistar rats were trained to discriminate cocaine (10 mg/kg, intraperitoneally (i.p.)) from saline (i.p.) in a two-choice, water-reinforced fixed-ratio 20 procedure. After reaching the cocaine-saline discrimination criterion, the rats were stereotaxically implanted with bilateral cannulae in the ventral tegmental area and were then microinjected with selective 5-HT(1B) receptor ligands. In substitution studies, microinjections of the 5-HT(1B) receptor antagonist, 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR 55562; 0.1-1 microg/side), did not evoke cocaine-lever responding, whereas the 5-HT(1B) receptor agonist, 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one (CP 93129; 0.3-1 microg/side), induced partial substitution for cocaine. Intra-tegmental microinjections with the 5-HT(1B) receptor antagonist, GR 55562 (0.1-1 microg/side), before cocaine (5 mg/kg), which alone produced 98% cocaine-lever responses, decreased in a dose-dependent manner the discriminative stimulus effects of the psychostimulant. On the other hand, combination tests using a fixed dose of CP 93129 (0.3 or 1 microg/side), given into the ventral tegmental area prior to low systemic doses of cocaine (1.25-2.5 mg/kg), increased cocaine discrimination. These results seem to indicate that tegmental 5-HT(1B) receptors are necessary to express the discriminative stimulus effects of cocaine.

Blocking impact of clozapine on cocaine locomotor and sensitizing effects in rats.

In the present study, we attempted to determine the effects of an atypical antipsychotic drug clozapine on the locomotor activation as well as sensitization to cocaine in male Wistar rats. When given acutely to rats, cocaine (10 mg/kg, ip) increased 4-fold the animals' locomotor activity. Repeated administration (1-5 days) of cocaine (10 mg/kg, ip) to rats significantly enhanced on day 10 the locomotor activation induced by its challenge dose given after 5-day withdrawal (sensitization). When given in combination with acute cocaine, clozapine (10 mg/kg, but not 2.5-5 mg/kg) attenuated the locomotor effects of the psychostimulant. In animals pretreated with clozapine (5-10 mg/kg, but not 2.5 mg/kg) before each of the 5 daily cocaine injections, a significant dose-dependent reduction of the locomotor response of animals to the challenge dose of cocaine (10 mg/kg) was observed on day 10. A decrease in that response was also found in animals treated repeatedly with cocaine (days 1-5) and challenged with the psychostimulant combined with clozapine (10 mg/kg, but not 2.5-5 mg/kg) on day 10. The obtained results indicate the ability of clozapine to reduce both acute and sensitizing locomotor responses to cocaine. These findings seem to be in line with recent clinical reports showing that clozapine may be useful in the treatment of cocaine abuse, even in schizophrenic patients.

Effects of 5-HT(1B) receptor ligands microinjected into the accumbal shell or core on the sensitization to cocaine in rats.

The present study was designed to find out whether 5-HT(1B) receptors located in subareas of the nucleus accumbens played a role in cocaine sensitization in rats, and whether pharmacological activation of these receptors could modify this drug effect. Male Wistar rats implanted bilaterally with cannulae into the accumbens shell or core were microinjected with GR 55562 (an antagonist of 5-HT(1B) receptors) or CP 93129 (an agonist of 5-HT(1B) receptors). The rats, which were repeatedly (for 5 days) administered with cocaine (10 mg/kg) and then challenged with cocaine (10 mg/kg) after 5-day withdrawal period, showed significantly higher locomotor hyperactivity in comparison with the effect observed in saline-pretreated and cocaine challenged rats. GR 55562 (0.1-10 microg/side), administered for 5 days into the accumbens shell, but not into the core, prior to cocaine dose-dependently attenuated cocaine sensitization. When injected for 5 days into either the accumbens shell or core before cocaine, CP 93129 (0.1-10 microg/side) had no effect on the development of cocaine sensitization. To examine the effects of GR 55562 and CP 93129 on the expression of cocaine sensitization, the drugs were given acutely before a challenge dose of cocaine (10 mg/kg) on day 10. No change in cocaine sensitization was observed after injection of GR 55562 (0.1-10 microg/side) to the either accumbens subregion or after injection of CP 93129 (0.1-10 microg/side) into the core. However, an intra-accumbens shell injection of CP 93129 (10-30 microg/side) increased sensitization to cocaine, and this enhancement was attenuated after local injection of GR 55562 (1 microg/side). Our findings indicate that behavioral sensitization to cocaine may be modulated by 5-HT(1B) receptor ligands in the accumbens shell, but not into the core. They suggest that the inhibition of 5-HT(1B) receptors in the accumbens shell attenuates the development, whereas their pharmacological activation enhances the expression of cocaine sensitization.

Effects of serotonin (5-HT)(1B) receptor ligands, microinjected into accumbens subregions, on cocaine discrimination in rats.

Recent data indicate a significant input of serotonin (5-HT) on mesoaccumbens dopamine-dependent behavioral effects of cocaine in rats. The present study investigated the role of 5-HT(1B) receptors in nucleus accumbens subregions (the shell and the core) and the effect of stimulation of those receptors in the discriminative stimulus effects of cocaine in rats. Male Wistar rats were trained to discriminate cocaine (10 mg/kg, i.p.) from saline (i.p.) in a two-choice, water-reinforced fixed-ratio 20 procedure. After reaching the cocaine-saline discrimination criterion, rats were stereotaxically implanted with bilateral cannulae in the accumbens shell or core, and then were microinjected with selective 5-HT(1B) receptor ligands. In substitution studies, microinjections of the 5-HT(1B) receptor antagonist GR 55562 (0.1-10 microg/side) or the 5-HT(1B) receptor agonist CP 93129 (0.1-10 microg/side) into accumbens subregions did not evoke cocaine-lever responding. Pretreatment with the 5-HT(1B) receptor antagonist GR 55562 (0.1-10 microg/side) in the accumbens shell or core failed to modulate the discriminative stimulus effects of cocaine (5 mg/kg). Combination tests using a fixed dose of CP 93129 (1-10 microg/side) into the accumbens shell prior to cocaine administration (0.6-5.0 mg/kg) did not affect cocaine discrimination. CP 93129 (1 microg/side, but not 0.1 microg/side) microinjected in the accumbens core, and low doses of systemic cocaine (0.6-2.5 mg/kg) produced a leftward shift in the cocaine dose-response curve and a decrease in its ED(50) value. GR 55562 (1 microg/side) significantly attenuated the enhancement of cocaine discrimination evoked by a combination of CP 93129 (1 microg/side) and cocaine (1.25 mg/kg or 2.5 mg/kg). These results seem to exclude a major role for the accumbens shell and core 5-HT(1B) receptors in controlling the discriminative stimulus effects of cocaine. However, they do suggest that the stimulation of 5-HT(1B) receptors in the accumbens core, but not in the shell, enhances cocaine discrimination in rats.

Role of dopamine D3 receptors in controlling the expression of cocaine sensitization in rats.

It is established that dopamine (DA) is an important brain mediator of the behavioral (i.e. sensitizing) effects of cocaine in rodents. Among DA receptors, recent findings point to engagement of DA D3 receptors in cocaine addictive actions. In the present study, we attempted to determine the role of DA D3 receptors in the expression phase of sensitization to cocaine in rats, using the selective ligands 7-OH-PIPAT (an agonist) and nafadotride (an antagonist) of these receptors. Repeated administration (1-5 days) of cocaine (10 mg/kg, ip) to male Wistar rats significantly enhanced the locomotor activation induced by its challenge dose given after 5-day withdrawal (on day 10). 7-OH-PIPAT (1 mg/kg, but not 0.01-0.1 mg/kg, sc) administered together with a challenge dose of cocaine significantly decreased the response to cocaine in rats treated repeatedly with cocaine. On the other hand, the expression of cocaine sensitization was increased when that drug was combined with nafadotride (0.4 mg/kg, ip) on day 10. The results indicate a role of DA D3 receptors in controlling the expression of cocaine sensitization in rats, and may suggest an importance of DA D3 receptor agonists in the therapy of cocaine abuse.

Effect of intra-tegmental microinjections of 5-HT1B receptor ligands on the amphetamine-induced locomotor hyperactivity in rats.

Previous research demonstrated that the mesoaccumbens dopamine (DA) pathway played a critical role in the behavioral effects of amphetamine in rodents. Nonetheless, recent findings have also indicated involvement of 5-hydroxytryptamine (5-HT, serotonin) transmission in these effects. In the present study, we investigated the role of 5-HT1B receptors located in the ventral tegmental area (VTA) in the amphetamine-induced locomotor hyperactivity in rats. Male Wistar rats, implanted bilaterally with cannulae in the VTA were infused with saline (0.2 microl/side), GR 55562 (an antagonist of 5-HT1B receptors; 0.1-1 microg/side) or CP 93129 (an agonist of 5-HT1B receptors; 0.003-0.03 microg/side) immediately prior to the injection of saline (1 ml/kg, ip) or amphetamine (0.5 mg/kg, ip). The monitoring of locomotor activity in photobeam chambers began at once and proceeded for 60 min. Neither GR 55562 nor CP 93129 affected basal locomotor activity. Pretreatment with GR 55562 (0.1-1 microg/side) did not affect the locomotor hyperactivity evoked by amphetamine. On the other hand, microinjections of CP 93129 (0.01-0.03 microg/side) enhanced the amphetamine-induced hyperlocomotor activity. GR 55562 (1 microg/side) markedly reduced the enhancing effects of CP 93129 (0.01 microg/side) on the amphetamine-induced hyperactivity. These findings indicate that 5-HT1B receptors located in the VTA do not play a major role in the hyperlocomotion elicited by amphetamine, whereas their activation may modulate the behavioral response to the psychostimulant.