Glutamatergic mechanisms of comorbidity between acute stress and cocaine self-administration.

There is substantial comorbidity between stress disorders and substance use disorders (SUDs), and acute stress augments the locomotor stimulant effect of cocaine in animal models. Here we endeavor to understand the neural underpinnings of comorbid stress disorders and drug use by determining whether the glutamatergic neuroadaptations that characterize cocaine self-administration are induced by acute stress. Rats were exposed to acute (2 h) immobilization stress, and 3 weeks later the nucleus accumbens core was examined for changes in glutamate transport, glutamate-mediated synaptic currents and dendritic spine morphology. We also determined whether acute stress potentiated the acquisition of cocaine self-administration. Acute stress produced an enduring reduction in glutamate transport and potentiated excitatory synapses on medium spiny neurons. Acute stress also augmented the acquisition of cocaine self-administration. Importantly, by restoring glutamate transport in the accumbens core with ceftriaxone the capacity of acute stress to augment the acquisition of cocaine self-administration was abolished. Similarly, ceftriaxone treatment prevented stress-induced potentiation of cocaine-induced locomotor activity. However, ceftriaxone did not reverse stress-induced synaptic potentiation, indicating that this effect of stress exposure did not underpin the increased acquisition of cocaine self-administration. Reversing acute stress-induced vulnerability to self-administer cocaine by normalizing glutamate transport poses a novel treatment possibility for reducing comorbid SUDs in stress disorders.

Cross-sensitization between cocaine and acute restraint stress is associated with sensitized dopamine but not glutamate release in the nucleus accumbens.

Repeated administration of psychostimulant drugs or stress can elicit a sensitized response to the stimulating and reinforcing properties of the drug. Here we explore the mechanisms in the nucleus accumbens (NAc) whereby an acute restraint stress augments the acute locomotor response to cocaine. This was accomplished by a combination of behavioral pharmacology, microdialysis measures of extracellular dopamine and glutamate, and Western blotting for GluR1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor (AMPAR). A single exposure to restraint stress 3 weeks before testing revealed that enduring locomotor sensitization to cocaine was paralleled by an increase in extracellular dopamine in the core, but not the shell subcompartment, of the NAc. Wistar rats pre-exposed to acute stress showed increased basal levels of glutamate in the core, but the increase in glutamate by acute cocaine was blunted. The alterations in extracellular glutamate seem to be relevant, as blocking AMPAR by 6-cyano-7-nitroquinoxaline-2,3-dione microinjection into the core prevented both the behavioral cross-sensitization and the augmented increase in cocaine-induced extracellular dopamine. Further implicating glutamate, the locomotor response to AMPAR stimulation in the core was potentiated, but not in the shell of pre-stressed animals, and this was accompanied by an increase in NAc GluR1 surface expression. This study provides evidence that the long-term expression of restraint stress-induced behavioral cross-sensitization to cocaine recapitulates some mechanisms thought to underpin the sensitization induced by daily cocaine administration, and shows that long-term neurobiological changes induced in the NAc by acute stress are consequential in the expression of cross-sensitization to cocaine.

Adrenocorticotropic hormone influences the development of adaptive changes in dopamine autoreceptors induced by chronic administration of desipramine.

The influence of adrenocorticotropic hormone (ACTH) in the adaptive changes on central dopamine (DA) autoreceptors following chronic administration of desipramine (DMI) has been examined in rats. Dopamine had an inhibitory effect on basal and K(+)-induced release of [3H]DA from slices of striatum and n. accumbens of rats treated chronically (10 days) with ACTH (50 IU/kg, s.c.), DMI (10 mg/kg, i.p.) or the combination of ACTH and DMI. In slices of n. accumbens, but not in slices of striatum of rats exposed to the combined treatment of ACTH and DMI, a significant decrease in the inhibitory effect of exogenous DA on stimulated release of [3H]DA was observed. Chronic administration of ACTH or DMI alone had no effect. The effect of the combined treatment with both agents, on the reactivity of these DA receptors was evaluated by means of apomorphine-induced hypoactivity. The administration of ACTH and DMI (5 mg/kg, i.p.) reduced the hypoactivity induced by apomorphine, as compared to hypoactivity in rats treated with ACTH or DMI alone. Experiments with ACTH4-10 revealed that the peptide modified biochemical and behavioural parameters of dopaminergic function, which may implicate a direct action of the peptide on the brain, rather than on the release of adrenal hormones. These findings suggest that ACTH accelerates the onset of DMI-induced adaptive changes on dopamine in the mesolimbic area. However, because the effect of ACTH4-10 on release of adrenocortical hormone was not investigated, the possibility cannot be disregarded that the effect of the peptide was secondary to an enhancement of release of adrenal hormone.

Effect of naloxone and amphetamine on acquisition and memory consolidation of active avoidance responses in rats.

Pretraining IP injection of naloxone (0.3 mg/kg) or amphetamine (2 mg/kg) enhanced performance during acquisition, but did not improve retention of active avoidance responses in rats. Naloxone (0.1 or 3 mg/kg) had no effect on acquisition or on retention. The combination of naloxone (0.3 mg/kg) plus amphetamine (2 mg/kg) did not produce the facilitation observed when each of the two drugs was administered alone. Pretreatment with the higher dose of naloxone (3 mg/kg) blocked the facilitative effect of amphetamine on acquisition. Post-training administration of naloxone (0.3 mg/kg) or amphetamine (2 mg/kg) improved retention. Naloxone (0.1 or 3 mg/kg) had no effect. When naloxone and amphetamine were combined, at respective doses of 0.3 mg/kg and 2 mg/kg, the improvement did not occur, i.e., the higher dose of naloxone prevented the facilitative effect of amphetamine. In addition, an ineffective dose of amphetamine (0.5 mg/kg), given either pre- or post-training together with the lower dose of naloxone (0.1 mg/kg), produced a significant enhancement of acquisition or consolidation, respectively. The results are consistent with the possibility that naloxone might exert its facilitative action on acquisition and memory consolidation through the release of catecholaminergic systems from inhibitory influences of opioids.

A single exposure to restraint stress induces behavioral and neurochemical sensitization to stimulating effects of amphetamine: involvement of NMDA receptors.

Evidence indicates that repeated exposure to stressful events sensitizes the motor and addictive effects of drugs of abuse in rats. Regarding a single exposure to one restraint stress, previous findings have shown that it is sufficient to induce behavioral sensitization to stimulating and reinforcing properties of abuse drugs (e.g., amphetamine and morphine), as measured by locomotor activity and conditioned place preference, respectively. It is well known that enhanced dopaminergic neurotransmission in the nucleus accumbens and striatum plays a critical role in the development and/or expression of repeated stress-induced or drug-induced sensitization. In addition, involvement of NMDA receptors has been implicated in its development. However, whether sensitization induced by a single restraint stress exposure represents the same neurobiologic phenomenon is unknown. We studied the following issues: (a) influence of a single restraint exposure on the stimulating effects of amphetamine on dopamine release by microdialysis from striatum and (b) involvement of glutamatergic pathways, specifically those innervating striatum, on stress-induced sensitization to amphetamine, by administering MK-801 ip (0.1 mg/kg) or intrastriatally (1 microg/0.5 microL) previous to an acute restraint stress. For microdialysis studies (a) or intrastriatal administration of MK-801 (b), Wistar rats (250-330 g) were implanted stereotactically under anesthesia with a guide cannula in the striatum. After 2 days, animals were immobilized for 2 hours in a Plexiglas device. Control animals remained in their home cages. The following day we evaluated the stimulating effect of amphetamine on (a) dopamine release from striatum or (b) locomotor activity. In studies (a), dialysis probes were inserted into the guide cannula, and baseline dopamine levels were collected for 2 hours before a challenge of amphetamine (1.5 mg/kg i.p.). Dialysates were then collected by 3 hours. Amphetamine challenge induced a significantly higher increase in dopamine release and locomotor activity in animals previously subjected to one restraint stress exposure, relative to that observed in the no-restraint stress group. MK-801 administered i.p. or intrastriatally blocked the restraint stress-induced sensitization to amphetamine. First, our results point out that a single restraint stress exposure is a pertinent stimulus to induce sensitization of amphetamine's stimulating effects on dopaminergic neurotransmission in the striatum. Secondly, NMDA-glutamatergic receptors, specifically those placed in the striatum, are implicated in the development of stress restraint-induced sensitization.

A dopaminergic mechanism is involved in the 'anxiogenic-like' response induced by chronic amphetamine treatment: a behavioral and neurochemical study.

The purpose of this study was to examine the influence of chronic d-amphetamine (AMPH) treatment (2 mg/kg i.p., for 9 consecutive days) on behavioral and neurochemical responses to a subsequent exposure - 4 days after the last AMPH injection--to the elevated plus-maze (EPM), as well as to determine the involvement of a dopaminergic mechanism in that influence. Results showed that chronic AMPH treatment induced an 'anxiogenic-like' response when animals were evaluated in the EPM test. Pretreatment with either haloperidol (HAL, 1 mg/kg i.p., 20 min prior to each injection) or SCH-23390 (0.1 mg/kg i.p., 10 min prior to each injection) completely abolished the chronic AMPH-induced 'anxiogenic-like' effect displayed in the EPM test. However, sulpiride pretreatment (60 mg/kg i.p., 10 min prior to each AMPH injection) did not modify such effect. In addition, rats treated with AMPH and subsequently exposed to the EPM, showed a decrease in the maximal GABA-stimulated chloride uptake in cortical microsacs. HAL pretreatment restored the maximal chloride uptake induced by chronic AMPH. Altogether, these results suggest that: (1) previous exposure to chronic AMPH treatment induces an increased emotional response following a conflict situation, (2) dopamine D(1) receptors are mainly involved in chronic AMPH-induced changes in the behavior displayed in EPM test, and (3) an interaction between GABAergic and dopaminergic mechanisms may be implicated in neurochemical and behavioral changes induced by chronic AMPH treatment.

Reduced apomorphine-induced sedation following chronic stress.

In agreement with previous reports, low doses of apomorphine (10-50 micrograms/kg, 5 min before test) produced a dose-dependent decrease in motor activity of control rats. This effect was markedly attenuated 24 h after a prolonged immobilization stress (2 h daily during 7 days). A single stress session had no effect on motor activity. These behavioral observations suggest a reduced sensitivity of presynaptic dopamine receptors after chronic stress.

Effect of acute and chronic stress restraint on amphetamine-associated place preference: involvement of dopamine D(1) and D(2) receptors.

The purpose of this study was to determine the D-amphetamine (1.0, 1. 5 and 2 mg/kg i.p.)-induced place preference in rats pre-exposed to acute or chronic restraint stress, using the conditioned place preference model. We also studied the involvement of opioid and dopamine mechanisms in the acute restraint stress-induced increase of D-amphetamine-induced place preference. A single restraint session (2 h) but not chronic restraint (2 h/day for 7 days) leading to adaptation to the stressor, enhanced the D-amphetamine-induced place preference. This enhancing effect was prevented by haloperidol administration (0.4 mg/kg i.p.), (+/-)-sulpiride (60 mg/kg i.p.) or R(+)-SCH-23390 hydrochloride (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrochloride, 30 microg/kg i.p.) 10-20 min prior to the acute restraint session. However, naltrexone pretreatment (1 or 2 mg/kg i.p.) failed to prevent the acute restraint-induced enhancement of D-amphetamine-induced place preference. These results suggest that: (1) the enhancement of D-amphetamine-induced place preference occurred after a single restraint stress but not following chronic restraint stress, (2) the stimulation of both dopamine D(1) and D(2) receptors is necessary for the development of single restraint stress-induced enhancement of D-amphetamine-induced place preference and (3) apparently, an opioid system is not involved in this acute restraint-induced effect.

Anxiolytic-like effect induced by chronic stress is reversed by naloxone pretreatment.

The present study assesses the influence of different restraint schedules on behavioral parameters determined by a conflict test, namely the light-dark transitions (LDT) as well as the opiate modulation on the behavioral consequences induced by chronic restraint. Finally, another group of animals that received naloxone (NAL) and/or chronic stress was either exposed to a single foot shock session or administered a single dose of the beta-carboline FG 7142 (N'-methyl-beta-carboline-3-carboxamide) immediately prior to the LDT test. We observed that a single restraint session (2 h) induced a decrease of LDT and time spent in the lit box, while chronic restraint (2 h per day for up to 7 days) induced a significant increase in both parameters. However, this increasing effect was blocked by a NAL administration (2 mg/kg IP) prior to each of the seven restraint events. A single foot shock or FG administration produced a clear anxiogenic response, an effect that was absent in animals previously submitted to chronic stress. In addition, NAL pretreatment abolished the chronic stress-induced attenuating effect on the behavioral suppression induced after either foot shock or FG administration. Therefore, these findings demonstrate that a previous history of chronic stress, leading to adaptation, induced an anxiolytic-like effect, and attenuated the behavioral suppression produced by acute stressors. There seems to be an endogenous opiate mechanism involved in the behavioral influence induced by chronic stress.

Effect of different restraint schedules on the immobility in the forced swim test: modulation by an opiate mechanism.

The present research was conducted to evaluate the influence of different stress schedules on behaviors displayed during both phases of the forced swim test (FST). In addition, the involvement of an opiate mechanism in the behavioral consequences of chronic restraint was investigated. Exposure to a single, but not to chronic, restraint event induced an increase in the immobility score obtained during the 10-min initial swimming exposure (initial test) of the FST. Animals submitted to a previous regime of repeated restraint showed a significant increase in immobility during the 5-min second swimming exposure (retest period) of this behavioral task. However, naloxone (NAL) administered before each of the seven restraint events, blocked the higher immobility observed in chronically stressed rats during the retest period suggesting the involvement of an opiate mechanism. Results concerning the effect of chronic stress on the behavior displayed during the FST were discussed with reference to previous reports which have proposed that immobility performed during the retest period of the FST represents an efficient adaptive response in this inescapable aversive experience.

Gangliosides attenuate stress-induced changes on body weight, motor activity and on the behavioral response to 5-methoxy-N,N-dimethyltryptamine.

The major goal of this study was to evaluate the influence of gangliosides (GANG) treatment on the onset of adaptive changes and the sequelae induced by stress exposure. With this purpose, the behavioral response to 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 5 mg/kg, IP) and motor activity were evaluated in rats previously submitted either to a single restraint session (2 h) or to a daily restraint event for 3 consecutive days, combined or not to GANG administration (30 mg/kg IP). GANG was always injected 2 h before stress exposure. In addition, differences in body weights were recorded throughout the experiments. A similar behavioral response after 5-MeODMT was observed between saline (SAL) and GANG unstressed rats. Exposure to one or three restraint sessions did not modify the behavioral response to 5-MeODMT, whereas the association of GANG and stress during 3 consecutive days enhanced forepaw treading and hindlimb abduction. SAL-treated animals submitted to a single or to three stressful stimuli showed reduced locomotion and rearing. The combination of GANG and stress for 3 days, but not after a unique association, reversed the decrease on motor activity induced by the aversive experience. The decrease of body weights produced by one or three stress sessions was recovered only in animals treated with GANG and stress for 3 days. These findings suggest that GANG may accelerate the onset of adaptive changes on 5-HT1 sites and attenuate certain sequelae induced by previous stress experience.

Chronic stress-induced changes in locus coeruleus neuronal activity.

Locus coeruleus (LC) activity was assessed in rats exposed to either acute or chronic stress. After one immobilization session, the number of spontaneously active LC neurons dramatically decreased. On the other hand, repeated restraint sessions enhanced noradrenergic (NA) transmission and the inhibitory effect of clonidine (CLON) was greater on these cells than in those of controls. These results bear on the adaptive changes in the NA system following acute or chronic stress.

Opioid influence on some aspects of stereotyped behavior induced by repeated amphetamine treatment.

Rats were administered repeated IP injections of dl-amphetamine (AMPH) according to a chronic escalating dose schedule (three doses per 24 hr, for four days, two days or one day). Animals treated for four days exhibited a diminished oral stereotypy in response to a challenge of 12 mg/kg AMPH or 2 mg/kg SC apomorphine (APO), 72 hr after withdrawal. Pretreatment with 2 mg/kg IP naloxone (NAL) during the period of chronic AMPH administration prevented the reduction in oral stereotypy induced by AMPH or APO. No differences were detected among the mean of stereotypy scores from the different treatments in response to a challenge dose of 6 mg/kg AMPH. Neurochemical data showed that NAL pretreatment reversed the depletion of striatal dopamine content induced by chronic AMPH. When repeated injections of AMPH were given only one day, the diminished stereotypy response to AMPH or APO was not observed. Animals treated simultaneously with 1 mg/kg IP morphine or 5 micrograms/kg IP beta-endorphin and repeated AMPH injections for one day, showed a reduced stereotyped response to AMPH or APO. These results suggest that opioid peptides are involved in the mechanisms underlying the decrease in oral behaviors following AMPH treatment.

Chronic stress attenuation of alpha 2-adrenoceptor reactivity is reversed by naltrexone.

Low doses of clonidine (50-100 micrograms/kg IP) evoke a clear dose-dependent hypoactivity response. Seven daily immobilization sessions prevented the motor activity decrease induced by clonidine. On the contrary, a single stress failed to modify clonidine response. Pretreatment with naltrexone (2 mg/kg IP) fully antagonized the attenuating effect induced by chronic stress on clonidine sedative action. These evidences suggest that chronic but not acute stress reduces the reactivity of alpha 2-adrenoceptors involved in clonidine-induced sedation. In addition, a regulatory mechanism of endogenous opioids seems to participate on alpha 2-adrenoceptors adaptative changes.

D1 and D2 dopamine and opiate receptors are involved in the restraint stress-induced sensitization to the psychostimulant effects of amphetamine.

The time course of the restraint stress-induced sensitization to the stimulant effects of amphetamine (AMPH, 0.5 mg/kg IP) on locomotor activity was investigated for up to 8 days. In a series of separate experiments, the involvement of opioid and dopaminergic mechanisms in the development of acute restraint stress-induced behavioral sensitization were characterized. Both a single restraint session (2 h) and chronic restraint (2 h per day for 7 days) similarly potentiated the effects of AMPH on motor activity. This behavioral sensitization was prevented by the administration of naltrexone (2 mg/kg IP), haloperidol (1 mg/kg IP), sulpiride (60 mg/kg IP) or SCH23390 (0.5 mg/kg IP) 10-20 min prior to restraint. These results indicate that 1) the development of sensitization to amphetamine-induced effects on motor activity does not depend on the length of exposure to stress (acute or chronic). 2) the stimulation of both D1 and D2 dopaminergic receptors is necessary for the development of the restraint stress-induced sensitization to AMPH and 3) and opioid system is also implicated in this sensitization process.

Chronic amphetamine facilitates immunosuppression in response to a novel aversive stimulus: reversal by haloperidol pretreatment.

The effect of chronic d-amphetamine sulfate (AMPH) treatment (nine daily injections, 2 mg/kg i.p.) on subsequent foot shock stress-induced immunological response was investigated. In addition, the potential role of a dopaminergic (DA) mechanism in the development of chronic AMPH-induced changes in stress-influenced immune responses was characterized. Exposure to foot shock stress decreased the percentage of T-lymphocytes, and reduced the delayed-type hypersensitivity reaction (DTH) in chronically AMPH-pretreated rats relative to vehicle-treated controls. Both of those stress-induced immunosuppressive responses were no longer evident when AMPH-pretreated rats were injected with haloperidol (HAL, 1 mg/kg i.p.) 30 min prior to each daily AMPH injection. The present findings are indicative of a modulatory role for dopamine in the facilitating process induced by AMPH on stress-induced immunosuppressive effects.

Opiate agonist-induced changes in behavioral sensitivity to clonidine are observed in perinatally malnourished rats exposed to chronic stress.

Sensitivity of alpha2-adrenoceptors following repeated immobilization sessions plus morphine (MOR) or beta-endorphin (BETA) was assayed by examining clonidine (CLO)-induced hypoactivity in adult malnourished rats at perinatal age. As previously described, chronic restraint did not attenuate the hypoactivity elicited by CLO in malnourished rats, although chronic restraint did have such an effect on motor activity in control animals. MOR and BETA administration prior to each restraint session induced subsensitivity of alpha2-adrenoceptors in malnourished rats as determined by a blunted response to clonidine challenge. An injection of naloxone (NAL) prior to BETA before each stress session fully antagonized the subsensitivity to clonidine observed in malnourished animals. A possible deficiency in the functional role of the opiate system in the process of adaptation to chronic stress in perinatal malnourished rats is suggested.

Lack of adaptive changes in 5-HT sites in perinatally undernourished rats after chronic stress: opioid influence.

The reactivity of 5-HT receptors following repeated immobilization sessions or after immobilization plus morphine was measured through 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) or 8-hydroxy-2-(dipropyl-amino)tetralin (8-OH-DPAT)-induced serotonergic syndrome in adult rats undernourished at perinatal age. Repeated stress enhanced the scores of forepaw treading and hindlimb abduction elicited by 5-MeODMT in control animals. In a similar way, forepaw treading induced by 8-OH-DPAT was enhanced in chronically stressed control rats. These results indicate the development of supersensitivity in 5-HT1 receptors. Conversely, this effect was not observed in undernourished animals. Morphine injections before each stress session instaured the increased reactivity to 5-HT1 sites in malnourished animals. An injection of naloxone prior to morphine before each stress session fully antagonized the increased behavioral reactivity to 5-MeODMT observed in deprived animals. A possible deficiency in the functional role of the opiate system involved in the process of adaptation to chronic stress in early undernourished rats is suggested.

Behavioral responses to ethanol in rats perinatally exposed to low lead levels.

Wistar rats were exposed to 220 ppm of lead (Pb) in the drinking water from conception to the end of the nursing period (postnatal day 25). Maternal blood Pb levels at this time were 25 microg/dl. Male offspring were tested at the age of 35 or 70 days. We studied the anxiolytic response to 0.5-2.0 g/kg ethanol in an elevated plus maze test and preference for increasing ethanol solutions (2%, 4%, and 6%, v/v) in a free-choice paradigm; we also determined basal blood levels of corticosterone. Results demonstrated that, at 35 days of age, experimental rats were hypersensitive to the anxiolytic effect of ethanol and showed greater voluntary intake of this drug. In addition, 35-day-old Pb-treated rats exhibited higher basal levels of corticosterone as compared with those of controls. These differences disappeared at 70 days. Our findings are discussed in terms of either Pb-induced alterations in the development of the CNS or higher levels of corticosterone in experimental animals. Possible Pb-ethanol effects interactions are also considered.

Amphetamine and stress responses in developmentally lead-exposed rats.

In this study, pregnant Wistar dams were exposed to 220 ppm of lead (Pb) in drinking water during gestation and lactation. The response to the locomotor-stimulating effects of 0.5 mg/kg of amphetamine (AMPH) was evaluated in 35-day-old male offspring. The results demonstrated that developmental Pb exposure induced no differences in the response to the drug, although an increase in locomotor activity induced by a single saline (SAL) injection was observed selectively in the Pb-exposed group. Considering evidence that suggests a relationship between increased locomotor activity and stress response, a time course analysis of corticosterone (CS) secretion and locomotor performance was carried out. Higher basal levels of CS and elevated stress-induced secretion of this hormone in response to the injection were observed in Pb-exposed rats compared to controls, a pattern that showed a time-related increase in locomotor activity. Habituation to SAL injections prior to testing restored both CS secretion and locomotor response to SAL to levels comparable to controls and did not modify AMPH locomotor response measured in these new experimental conditions. Additionally, we demonstrated that these behavioral/hormonal disruptions were no longer detectable later in adulthood. Collectively, these data suggest that the stimulant-locomotor effect of AMPH in Pb-exposed rats is independent of the arousal of the animal at the time of its administration. They also support a unique profile of integrated behavioral and hormonal hyperresponsiveness in 35-day-old low-level Pb-exposed rats evidenced as hyperlocomotion and altered secretion of CS in response to an environmental manipulation, an effect that was no longer present later in life.