Liliana M. Cancela

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.

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

Abstract: 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 μg/0.5 μL) 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 ip). 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 ip 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 amphetamines 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.

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.

Chronic stress attenuation of α2-adrenoceptor reactivity is reversed by naltrexone

Abstract Low doses of clonidine (50–100 μg/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 α 2 -adrenoceptors involved in clonidine-induced sedation. In addition, a regulatory mechanism of endogenous opioids seems to participate on α 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.

A glutamate-dopamine interaction in the persistent enhanced response to amphetamine in nucleus accumbens core but not shell following a single restraint stress.

The administration of psychostimulant drugs or stress can elicit a sensitized response to the stimulating and reinforcing properties of the drug. We previously demonstrated that a single restraint stress session enhanced d-amphetamine (d-AMPH)-induced locomotion the day after the stress session, which lasted up to 8 days. The present experiments were designed to identify the contribution of major dopamine (DA) brain areas in the short- and long-lasting enhancement of d-AMPH-induced locomotion following a single stress, and to test the involvement of N-methyl-D-aspartate (NMDA) receptors in that phenomena. To achieve our goal, 24 h and 8 days after a 2-h restraint stress session either with or without a NMDA receptor blockade, we measured locomotor activity and DA overflow in nucleus accumbens (NAcc) core and shell and caudate putamen (CPu) following a d-AMPH injection (0.5 mg/kg i.p.). The stimulant effect of d-AMPH on DA overflow was enhanced in all nuclei at 24 h after a single stress, while at 8 days the enhanced responsiveness was maintained only in the NAcc core. When the rats were administered with MK-801 (0.1 mg/kg i.p.) 30 min before restraint stress, the d-AMPH-induced enhancement on locomotor activity and DA neurotransmission was prevented in all studied brain areas at both times. These findings show that a glutamate–dopamine link is underlying the short- and long- term d-AMPH-induced enhancement on DA and locomotor activity following stress. The persistent glutamate-dependent DA enhancement in NAcc core highlights the relevance of this region in the long-term proactive effects of stress on vulnerability to drug abuse.

Effect of acute and chronic stress restraint on amphetamine-associated place preference: involvement of dopamine D1 and D2 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 μg/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 D1 and D2 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.

Seven-day variable-stress regime alters cortical β-adrenoceptor binding and immunologic responses: Reversal by imipramine

Rats were submitted daily to a variable stressor for 1 week with or without concurrent imipramine (IMI) administration. One day after the last injection or stressful event, binding of cortical beta-adrenoceptors was determined in all experimental groups. Another group of chronically stressed animals with or without concurrent IMI administration were sacrificed 24 h following the last stress or injection treatment, and several immunologic parameters were evaluated. Chronically stressed rats showed an enhanced number of cortical beta-adrenergic sites without changes in their affinity. This effect was not present following concurrent administration with the antidepressant. In addition, a decreased percentage of T lymphocytes and a reduced delayed-type hypersensitivity reaction was also observed in stressed animals. Both responses were no longer evident when stressed rats were previously administered IMI. A possible link between behavioral, neurochemical, and immunologic alterations due to the stress regime is discussed.

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.

Opioid involvement in the adaptive change of 5-HT1 receptors induced by chronic restraint.

Rats immobilized for 2 h daily for 7 days showed an increased behavioral response (forepaw treading and hind-limb abduction) to 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) 24 h after the last stress session. An injection of naloxone before each stress session fully antagonized the increased behavioral reactivity to 5-MeODMT. Treatment with morphine or beta-endorphin associated with each immobilization session for 3 days produced a response to 5-MeODMT higher than that of animals subjected to immobilization only. Chronic immobilization for 7 days did not affect the shaking behavior induced by 5-hydroxytryptophan (5-HTP) 24 h after the last restraint session. These findings suggest that chronic stress may induce a selective adaptive change of the 5-HT1 site and activate an opioid mechanism that is most likely to be involved in the development of this adaptive change.