Influence of repeated cocaine exposure on the endocrine and behavioral responses to stress in rats

Influence of the single or combined administration of cocaine and testosterone in autonomic and neuroendocrine responses to acute restraint stress

Abuse of cocaine and androgenic-anabolic steroids (AASs) has become a serious public health problem. Despite reports of an increase in the incidence of simultaneous abuse of these substances, potential toxic interactions between cocaine and AASs are poorly known. In the present study, we investigated the effects of either single or combined administration of testosterone and cocaine for one or 10 consecutive days on autonomic (arterial pressure, heart rate and tail cutaneous temperature) and neuroendocrine (plasma corticosterone) responses induced by acute restraint stress in rats. Combined administration of testosterone and cocaine for 10 days reduced the increase in heart rate and plasma corticosterone level, as well as the fall in tail skin temperature induced by restraint stress. Furthermore, repeated administration of cocaine inhibited the increase in arterial pressure observed during restraint, and this effect was not affected by coadministration of testosterone. Ten-day combined administration of testosterone and cocaine increased basal values of arterial pressure. Moreover, chronic administration of testosterone induced rest bradycardia and elevated basal level of plasma corticosterone. One-day single or combined administration of the drugs did not affect any parameter investigated. In conclusion, the present study demonstrated that combined administration of testosterone and cocaine changed the autonomic and neuroendocrine responses to acute restraint stress. These findings suggest that interaction between AASs and cocaine may affect the ability to cope with stressful events.

Blockade of the serotonin 5-HT2A receptor suppresses cue-evoked reinstatement of cocaine-seeking behavior in a rat self-administration model

The serotonin 5-HT2A receptor (5-HT-sub(2A)R) may play a role in reinstatement of drug-seeking. This study investigated the ability of a selective 5-HT-sub(2A)R antagonist to suppress reinstatement evoked by exposure to cues conditioned to cocaine self-administration. Cocaine self-administration (0.75 mg/kg/0.1 mL/6 s infusion; FR 4) was trained in naïve, free-fed rats to allow interpretation of results independent from changes related to food deprivation stress. Pretreatment with the selective 5-HT-sub(2A)R antagonist M100907 (volinanserin) failed to reduce rates of operant responding for cocaine infusions. On the other hand, M100907 (0.001-0.8 mg/kg ip) significantly suppressed the cue-induced reinstatement of cocaine-seeking behavior following extinction; effective M100907 doses did not alter operant responding for cues previously associated with sucrose self-administration. Importantly, a greater magnitude of active lever presses on the initial extinction session (high extinction responders) predicted the maximal susceptibility to M100907-induced suppression of cue-evoked reinstatement. The findings indicate that blockade of the 5-HT-sub(2A)R attenuates the incentive-motivational effects of cocaine-paired cues, particularly in high extinction responders, and suggests that M100907 may afford a therapeutic advance in suppression of cue-evoked craving and/or relapse.

Cocaine-induced breakdown of the blood-brain barrier and neurotoxicity

Role of cocaine in influencing blood-brain barrier (BBB) function is still unknown. Available evidences suggest that cocaine administration results in acute hyperthermia and alterations in brain serotonin metabolism. Since hyperthermia is capable to induce the breakdown of the BBB either directly or through altered serotonin metabolism, a possibility exists that cocaine may induce neurotoxicity by causing BBB disruption. This hypothesis is discussed in this review largely based on our own laboratory investigations. Our observations in rats demonstrate that cocaine depending on the dose and routes of administration induces profound hyperthermia, increased plasma and brain serotonin levels leading to BBB breakdown and brain edema formation. Furthermore, cocaine was able to enhance cellular stress as seen by upregulation of heat shock protein (HSP 72 kD) expression and resulted in marked neuronal and glial cell damages at the time of the BBB dysfunction. Taken together, these observations are the first to suggest that cocaine-induced BBB disruption is instrumental in precipitating brain pathology. The possible mechanisms of cocaine-induced BBB breakdown and neurotoxicity are discussed.

Hormonal, neurochemical, and behavioral response to a forced swim test in adolescent rats throughout cocaine withdrawal

The use of cocaine in adults has been linked to depression and/or anxiety. Several studies have shown an association between cocaine-primed craving and depressive symptoms. In animal models, the forced swim test (FST) is frequently used for screening depressive-like behavior. This study aimed to verify the presence of depression-like symptoms in adolescent rats after chronic cocaine exposure by analyzing behavior in a FST. The subsequent alterations in neurotransmitters and hypothalamus-pituitary-adrenal axis activity induced by this test were also analyzed. Both male and female adolescent Wistar rats were submitted to a chronic "binge" pattern of administration of cocaine hydrochloride, and subjects were tested in a forced swim test 2 days after cocaine's last administration. At the end of the behavioral test, trunk blood was collected for quantification of corticosterone plasma levels, and hypothalamus, prefrontal cortex, amygdala, and hippocampus were dissected for neurochemical determinations. No significant differences were found in the behavior on the FST of both males and females after withdrawal from chronic cocaine administration. Nevertheless, plasma levels of corticosterone were increased in cocaine-treated males, although not significantly (P= 0.065). In females cocaine failed to affect corticosterone levels. Of interest, neurochemical analyses showed that dopamine turnover was decreased in amygdala in cocaine-treated males (not significantly, P= 0.055). No significant differences were found on neurotransmitter levels in the other brain regions analyzed. Withdrawal from chronic cocaine administration during adolescence did not have a significant effect on stress-induced behavioral alterations, although the neurochemical response to the stressful situation provided by FTS seemed to be affected.

Daily cocaine self-administration under long-access conditions augments restraint-induced increases in plasma corticosterone and impairs glucocorticoid receptor-mediated negative feedback in rats

Cocaine addiction appears to be associated with a drug-induced dysregulation of stressor responsiveness that may contribute to further cocaine use. The present study examined alterations in stressor-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis in rats provided daily access to cocaine for self-administration (SA) under long-access conditions (1.0 mg/kg/infusion; 6 hx14 days). Cocaine self-administering rats displayed reduced basal plasma corticosterone (CORT) levels but showed an augmented restraint-induced percent increase response from baseline compared to saline self-administering controls when measured 24 days after SA testing. This augmented CORT response may have been attributable to impaired glucocorticoid receptor (GR)-mediated feedback regulation of HPA function, since cocaine self-administering rats were also less susceptible to dexamethasone (0.01 mg/kg, i.p.) suppression of plasma CORT levels. GR protein expression measured using Western blot analysis was significantly reduced in the dorsomedial hypothalamus (including the paraventricular nucleus [PVN]) but not in the pituitary gland, ventromedial hypothalamus, dorsal hippocampus, ventral subiculum, medial prefrontal cortex or amygdala in cocaine self-administering rats. Surprisingly, basal corticotropin-releasing hormone (CRH) mRNA or post-restraint increases in CRH mRNA measured at a single (90 min) time-point in the PVN using in situ hybridization did not differ between groups. The findings suggest that cocaine use produces persistent changes in individual responsiveness to stressors that may contribute to the addiction process.

Restraint-induced corticosterone secretion and hypothalamic CRH mRNA expression are augmented during acute withdrawal from chronic cocaine administration

Stress responses during cocaine withdrawal likely contribute to drug relapse and may be intensified as a consequence of prior cocaine use. The present study examined changes in stressor-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis during acute withdrawal from chronic cocaine administration. Adult male Sprague-Dawley rats received daily administration of cocaine (30 mg/kg, i.p.) or saline for 14 days. Twenty-four hours after the last injection, rats in each group were sacrificed under stress-free conditions or following 30 min of immobilization. Plasma corticosterone (CORT) was measured in trunk-blood using radioimmunoassay, corticotropin-releasing hormone (CRH) mRNA levels in the paraventricular nucleus (PVN) of the hypothalamus were measured using in situ hybridization and glucocorticoid receptor (GR) protein expression in the pituitary gland and dissected brain regions was measured using Western blot analysis. Basal CRH mRNA in the PVN was unaltered as a result of prior cocaine administration. However, a significant increase in CRH mRNA was observed 90 min following the termination of restraint in cocaine withdrawn, but not saline-treated, rats. Basal CORT was also unaffected by prior cocaine administration, but the CORT response measured immediately after restraint was significantly augmented in cocaine-withdrawn rats. Differences in GR protein expression in number of regions implicated in negative feedback regulation of HPA function, including the hypothalamus, were not observed. These findings indicate that the HPA response to stressors is intensified during early withdrawal from cocaine administration and may be independent of changes in GR-mediated negative feedback.

Participants receiving dehydroepiandrosterone during treatment for cocaine dependence show high rates of cocaine use in a placebo-controlled pilot study

Twenty-three cocaine-dependent participants were randomly assigned to receive either dehydroepiandrosterone (DHEA; n = 11; 100 mg/day) or placebo (n = 12) in the context of 12 weeks of thrice weekly cognitive-behavioral group counseling. Outcomes were retention, urine drug screening, cocaine craving, adverse experiences, and medication compliance. DHEA-treated participants averaged 45.8 (SD = 28.8) days in treatment, compared with 70.7 (SD = 20.6) days for placebo, r(21) = -2.4, p =.03, and provided 26.8% (SD = 29.3) of urine samples free of cocaine metabolite compared with 70.6% (SD = 39.9) for the placebo condition, r(21) = -3.0, p =.01. No differences were detected between conditions for cocaine craving or adverse experiences. High levels of medication compliance were documented. Results argue against using high doses of DHEA as a pharmacotherapy for cocaine dependence.

Prolonged effects of repeated cocaine on medial prefrontal cortex dopamine response to cocaine and a stressful predatory odor challenge in rats

The present study examined the effects of seven daily saline (1 ml/kg, i.p.) or cocaine injections (15 mg/kg, i.p.) on extracellular dopamine levels in the medial prefrontal cortex (mPFC) after challenge with cocaine or stressful predatory odor presentation given 1 week (early withdrawal) or 3 weeks later (late withdrawal). Cocaine challenge at early withdrawal produced an increase in dopamine levels that was temporally shifted so that maximal levels of dopamine were significantly higher and attained 20 min earlier in the cocaine-pretreated group (maximal levels of saline controls=378% increase, cocaine=494% increase above baseline). Cocaine challenge at late withdrawal produced a similar effect on the temporal shift of maximal dopamine levels, with a significantly higher maximal percent increase of dopamine in cocaine-pretreated rats (saline-pretreated=420% increase, cocaine-pretreated=515% increase). Challenge with TMT, a predatory odor from fox that produces a stress response in rats, produced a maximal 75-200% increase in basal dopamine levels in both groups at both early and late withdrawal times. As with cocaine challenge, daily cocaine produced a leftward shift in the time at which maximal dopamine levels were attained in response to TMT. Cocaine-pretreated animals demonstrated maximal dopamine levels 40-80 min after TMT removal, while saline-pretreated rats showed maximal levels 100-140 min after TMT removal. These results suggest that there are long-term changes in the mPFC dopamine response to subsequent challenge with cocaine as well as a stressful predatory odor. The altered response of mPFC dopamine after repeated daily cocaine may impact relapse to drug-seeking or drug-taking behavior.

Stress-induced suppression of the immune system after withdrawal from chronic cocaine

Recent evidence suggests that withdrawal from cocaine shares similarities to the stress response. Here, we examine whether withdrawal from chronic cocaine produces immune system alterations and whether the hypothalamic-pituitary-adrenal axis is involved. Sprague-Dawley male rats received cocaine (10 mg/kg i.p., b.i.d.) or saline, followed by 2 h, 1, 2, 4, 6, and 14 days of withdrawal. Proliferation responses of peripheral blood lymphocytes to concanavalin A were significantly suppressed at the 2-h, 1- and 2-day time points, and persisted for up to 6 days during withdrawal from chronic cocaine. Flow cytometric analysis revealed no significant differences in the immunophenotype of blood lymphocytic populations of T cells, B cells, or monocytes at 2 or 6 days of withdrawal from cocaine. Consistent with the suppression in cellular immunity observed in the in vitro response, the in vivo delayed-type hypersensitivity response was also significantly decreased in cocaine withdrawing animals. Plasma corticosterone levels were significantly elevated 2 and 24 h after cessation of cocaine but returned to basal values by 2 days of withdrawal. The suppressive effects of cocaine withdrawal were no longer observed in either adrenalectomized animals or those treated with the glucocorticoid receptor antagonist mifepristone (RU486), when administered during the first 2 days of withdrawal. These data argue that repeated exposure to cocaine followed by withdrawal leads to an activation of the neuroendocrine stress response, which alters cellular immunity during the initial withdrawal phase and may contribute to an increased susceptibility to infection.

Alterations in hypothalamic-pituitary-adrenal axis activity and in levels of proopiomelanocortin and corticotropin-releasing hormone-receptor 1 mRNAs in the pituitary and hypothalamus of the rat during chronic 'binge' cocaine and withdrawal

Tolerance to the stimulatory effects of cocaine on the hypothalamic-pituitary-adrenal (HPA) axis develops after chronic 'binge' cocaine exposure in the rat. This blunting of HPA axis activity in response to cocaine is associated with a cocaine-induced reduction of corticotropin-releasing hormone (CRH) mRNA level in the hypothalamus. There is limited information about the effects of withdrawal from chronic cocaine on HPA activity. The present studies were undertaken to determine levels of the HPA hormones adrenocorticotropic hormone (ACTH) and corticosterone across 10 days of withdrawal following chronic 'binge' pattern cocaine administration (3 x 15 mg/kg/day at hourly intervals) for 14 days. Male Fischer rats showed a significantly attenuated HPA axis response to chronic 'binge' pattern cocaine administration 30 min after the last injection on the 14th day, as measured by both plasma ACTH and corticosterone levels at the nadir time point. Twenty-four hours following the final administration of 'binge' cocaine (the 1st day of withdrawal), a significant elevation of plasma ACTH levels and a modest, but significant, elevation of plasma corticosterone levels were found at the nadir time point. This acute withdrawal-related activation of the hormones of the HPA axis was no longer found on the 10th day of withdrawal. In the anterior pituitary, levels of both proopiomelanocortin (POMC) and CRH-receptor 1 (R1) mRNAs were significantly higher than saline controls on the 14th day of chronic 'binge' cocaine and were at control levels on the 4th day of withdrawal. In the neurointermediate lobe of the pituitary, a sustained reduction in POMC mRNA levels was observed on the 3rd, 7th and 14th day of chronic 'binge' cocaine, but POMC mRNA was at control levels by the 4th day of withdrawal. In the hypothalamus, POMC mRNA levels showed a transient decrease on the 1st day of 'binge' cocaine with no change during chronic 'binge' cocaine or its withdrawal. CRH mRNA levels in the hypothalamus were not different from saline controls on the 1st and 4th days of withdrawal. Taken together, the present results show that after development of adaptation or tolerance to chronic 'binge' cocaine there is an increase in HPA activity during acute cocaine withdrawal. In addition to being associated with CRH input from the hypothalamus, the activation of the HPA axis by cocaine withdrawal may be, at least in part, due to the increased POMC and/or CRH-R1 gene expression observed in the anterior pituitary after chronic 'binge' cocaine.

Social stress does not alter the expression of sensitization to cocaine

The effects of chronic social stress on behavioral sensitization to cocaine were investigated in the Syrian hamster. Adolescent animals received either 15 mg/kg i.p. of cocaine or saline twice per day for 7 consecutive days. Two weeks following the last injection (young adulthood), they were given a challenge dose of 5 mg/kg i.p. of cocaine and scored for locomotion. Motor activity was significantly greater in cocaine-treated animals, demonstrating sensitization to this psychostimulant. Following the results of the first study, another group of adolescent animals was exposed to either a novel clean cage (control) or an aggressive resident male hamster (social stress) for 15 min following an injection of cocaine (20 mg/kg i.p. once daily) or saline for 7 consecutive days. The groups were as follows: Social Stress/Cocaine (SSC), No Social Stress/Cocaine (NSSC), Social Stress/Saline (SSS) and No Social Stress/Saline (NSSS). Two weeks following the last injection (Day 21), all animals were given a challenge dose of cocaine (5 mg/kg i.p.) and were rescored for locomotion. At that time, the suppressive effect of stress on locomotion was no longer detectable, as the expression of sensitization was observed in the NSSC but not in the SSC group. These results suggest that chronic social stress administered during adolescence does not cross-sensitize with cocaine in young adult hamsters.

Adolescent cocaine and injection stress effects on the estrous cycle

Chronic cocaine exposure during critical periods of development induces short- and long-term effects. During the pubertal period, the hypothalamic-pituitary-gonadal (HPG) axis undergoes many dynamic changes. The present study investigated whether chronic periadolescent cocaine alters reproductive maturity in the rat. Sixty female Long-Evans hooded rats were randomly assigned to one of three conditions (20 mg cocaine/kg/day, saline injected and uninjected), for dosing from postnatal day 21 (P21) through P60. Several indicators of reproductive maturation and functioning were assessed during and following treatment. Cocaine exposure had no effect on the onset of puberty or on the date of first ovulation. The number of proestrus-estrus transitions was significantly lower in cocaine-exposed females compared to uninjected females, but not compared to saline-injected controls. This reduction was observed during exposure to cocaine, as well as after the cessation of injections. During the dosing period, cocaine-exposed rats also exhibited a greater number of cycles that had no clear P-E transition than did UN subjects; this effect disappeared once injections stopped. These alterations suggest immediate, and possibly persisting, alterations in the control of ovulation after chronic cocaine exposure throughout adolescence. Interestingly, during the injection period, the saline-injected females had a significantly greater number of diestrus days compared to uninjected and cocaine-injected animals, as well as a lower proportion of regular 4- and 5-day cycles. These differences disappeared once injections stopped. These results suggest a stress-induced irregularity of the estrous cycle, possibly attenuated by cocaine and recoverable after exposure. The present findings indicate that the HPG axis is susceptible to short-term, and possibly to long-term, alterations induced by cocaine exposure throughout the adolescent period.

Tolerance and sensitization to the behavioral effects of cocaine in rats: relationship to benzodiazepine receptors

Tolerance and sensitization to the behavioral effects of cocaine were investigated in rats responding under a fixed-consecutive-number eight schedule of food reinforcement. The development of tolerance or sensitization was induced by delivering the drug either immediately before or after each behavioral session during chronic administration. Chronic cocaine administered before each session resulted in tolerance, as indicated by the shift to the right in the cocaine dose response curve. This tolerance was more likely to develop in the presence of an external discriminative stimulus. On the other hand, when cocaine was delivered after each session, the injections did not disrupt responding and sensitization or increased sensitivity rather than tolerance developed. This sensitization was more likely to occur when the external discriminative stimulus was not present. These data suggest that either tolerance or sensitization to the behavioral effects of cocaine can occur following the same number of chronic injections, with the effect dependent on the context under which the drug is delivered. Significant differences in benzodiazepine receptor binding measured autoradiographically using [3H]flumazenil were observed between rats that received cocaine before or after each session, suggesting that the development of tolerance and sensitization may be mediated through changes in benzodiazepine receptors in discrete brain regions.