Role of amygdala astrocytes in different phases of contextual fear memory

Growing evidence indicates a critical role of astrocytes in learning and memory. However, little is known about the role of basolateral amygdala complex (BLA-C) astrocytes in contextual fear conditioning (CFC), a paradigm relevant to understand and generate treatments for fear- and anxiety-related disorders. To get insights on the involvement of BLA-C astrocytes in fear memory, fluorocitrate (FLC), a reversible astroglial metabolic inhibitor, was applied at critical moments of the memory processing in order to target the acquisition, consolidation, retrieval and reconsolidation process of the fear memory. Adult Wistar male rats were bilaterally cannulated in BLA-C. Ten days later they were infused with different doses of FLC (0.5 or 1 nmol/0.5 µl) or saline before or after CFC and before or after retrieval. FLC impaired fear memory expression when administered before and shortly after CFC, but not one hour later. Infusion of FLC prior and after retrieval did not affect the memory. Our findings suggest that BLA-C astrocytes are critically involved in the acquisition/early consolidation of fear memory but not in the retrieval and reconsolidation. Furthermore, the extinction process was presumably not affected (considering that peri-retrieval administration could also affect this process).

Resistance to fear memory destabilization triggers exaggerated emotional-like responses following memory reactivation

Fear memory reactivation does not always lead to memory destabilization-reconsolidation. For instance, fear memories formed following withdrawal from chronic ethanol consumption or a stressful event are less likely to become destabilized after reactivation, with the effect of recall of these memories on the affective state still requiring elucidation. Here, we investigated the negative emotional-like responses following fear memory reactivation in ethanol-withdrawn (ETOH) rats by focusing on the possible role played by destabilization. Our findings indicated that ETOH rats displayed an increased freezing in a novel context and an anxiogenic-like response in the elevated plus maze (EPM) following memory reactivation, whereas the behavior of CON animals was not affected. The destabilization blockade by pre-reactivation nimodipine (16 mg/kg, s.c) administration promoted in CON animals a similar behavior in the EPM and in a novel environment as that exhibited by ETOH rats after the reminder. Moreover, facilitating destabilization by pre-reactivation d-cycloserine (5 mg/kg, i.p) administration prevented the emotional-like disturbances observed in ETOH rats. Finally, using restraint stress, which is also an inductor of a fear memory resistant to destabilization, an increased fear response in an unconditioned environment and an anxiogenic-like state was also found after the presentation of the fear reminder in stressed rats. Our results suggest that, in the context of resistant fear memories, the occurrence of destabilization influences how animals respond to subsequent environmental challenges following reactivation.

Prior stress promotes the generalization of contextual fear memories: Involvement of the gabaergic signaling within the basolateral amygdala complex

Fear generalization occurs when a response, previously acquired with a threatening stimulus, is transferred to a similar one. However, it could be maladaptive when stimuli that do not represent a real threat are appraised as dangerous, which is a hallmark of several anxiety disorders. Stress exposure is a major risk factor for the occurrence of anxiety disorders and it is well established that it influences different phases of fear memory; nevertheless, its impact on the generalization of contextual fear memories has been less studied. In the present work, we have characterized the impact of acute restraint stress prior to contextual fear conditioning on the generalization of this fear memory, and the role of the GABAergic signaling within the basolateral amygdala complex (BLA) on the stress modulatory effects. We have found that a single stress exposure promoted the generalization of this memory trace to a different context that was well discriminated in unstressed conditioned animals. Moreover, this effect was dependent on the formation of a contextual associative memory and on the testing order (i.e., conditioning context first vs generalization context first). Furthermore, we observed that increasing GABA-A signaling by intra-BLA midazolam administration prior to the stressful session exposure prevented the generalization of fear memory, whereas intra-BLA administration of the GABA-A antagonist (Bicuculline), prior to fear conditioning, induced the generalization of fear memory in unstressed rats. We concluded that stress exposure, prior to contextual fear conditioning, promotes the generalization of fear memory and that the GABAergic transmission within the BLA has a critical role in this phenomenon.

The effect of Midazolam and Propranolol on fear memory reconsolidation in ethanol-withdrawn rats: influence of d-cycloserine

BACKGROUND

Withdrawal from chronic ethanol facilitates the formation of contextual fear memory and delays the onset to extinction, with its retrieval promoting an increase in ethanol consumption. Consequently, manipulations aimed to reduce these aversive memories, may be beneficial in the treatment of alcohol discontinuation symptoms. Related to this, pharmacological memory reconsolidation blockade has received greater attention due to its therapeutic potential.

METHODS

Here, we examined the effect of post-reactivation amnestic treatments such as Midazolam (MDZ, 3 mg/kg i.p) and Propranolol (PROP, 5 mg/kg i.p) on contextual fear memory reconsolidation in ethanol- withdrawn (ETOH) rats. Next, we examined whether the activation of N-methyl-D-aspartate (NMDA) receptors induced by d-cycloserine (DCS, 5 mg/kg i.p., a NMDA partial agonist) before memory reactivation can facilitate the disruptive effect of PROP and MDZ on fear memory in ETOH rats.

RESULTS

We observed a resistance to the disruptive effect of both MDZ and PROP following memory reactivation. Although intra-basolateral amygdala (BLA; 1.25 ug/side) and systemic PROP administration attenuated fear memory in DCS pre-treated ETOH rats, DCS/MDZ treatment did not affect memory in these animals. Finally, a decrease of both total and surface protein expression of the α1 GABAA receptor (GABAA-R) subunit in BLA was found in the ETOH rats.

CONCLUSIONS

Ethanol withdrawal facilitated the formation of fear memory resistant to labilization post-reactivation. DCS administration promoted the disruptive effect of PROP on memory reconsolidation in ETOH rats. The resistance to MDZ's disruptive effect on fear memory reconsolidation may be, at least in part, associated with changes in the GABAA-R composition induced by chronic ethanol administration/withdrawal.

Memory beyond expression

The idea that memories are not invariable after the consolidation process has led to new perspectives about several mnemonic processes. In this framework, we review our studies on the modulation of memory expression during reconsolidation. We propose that during both memory consolidation and reconsolidation, neuromodulators can determine the probability of the memory trace to guide behavior, i.e. they can either increase or decrease its behavioral expressibility without affecting the potential of persistent memories to be activated and become labile. Our hypothesis is based on the findings that positive modulation of memory expression during reconsolidation occurs even if memories are behaviorally unexpressed. This review discusses the original approach taken in the studies of the crab Neohelice (Chasmagnathus) granulata, which was then successfully applied to test the hypothesis in rodent fear memory. Data presented offers a new way of thinking about both weak trainings and experimental amnesia: memory retrieval can be dissociated from memory expression. Furthermore, the strategy presented here allowed us to show in human declarative memory that the periods in which long-term memory can be activated and become labile during reconsolidation exceeds the periods in which that memory is expressed, providing direct evidence that conscious access to memory is not needed for reconsolidation. Specific controls based on the constraints of reminders to trigger reconsolidation allow us to distinguish between obliterated and unexpressed but activated long-term memories after amnesic treatments, weak trainings and forgetting. In the hypothesis discussed, memory expressibility--the outcome of experience-dependent changes in the potential to behave--is considered as a flexible and modulable attribute of long-term memories. Expression seems to be just one of the possible fates of re-activated memories.

Activation of ERK2 in basolateral amygdala underlies the promoting influence of stress on fear memory and anxiety: influence of midazolam pretreatment

Exposure to emotionally arousing experiences elicits a robust and persistent memory and enhances anxiety. The amygdala complex plays a key role in stress-induced emotional processing and in the fear memory formation. It is well known that ERK activation in the amygdala is a prerequisite for fear memory consolidation. Moreover, stress elevates p-ERK2 levels in several areas of the brain stress circuitry. Therefore, given that the ERK1/2 cascade is activated following stress and that the role of this cascade is critical in the formation of fear memory, the present study investigated the potential involvement of p-ERK2 in amygdala subnuclei in the promoting influence of stress on fear memory formation and on anxiety-like behavior. A robust and persistent ERK2 activation was noted in the Basolateral amygdala (BLA), which was evident at 5min after restraint and lasted at least one day after the stressful experience. Midazolam, a short-acting benzodiazepine ligand, administered prior to stress prevented the increase in the p-ERK2 level in the BLA. Pretreatment with intra-BLA infusion of U0126 (MEK inhibitor), but not into the adjacent central nucleus of the amygdala, attenuated the stress-induced promoting influence on fear memory formation. Finally, U0126 intra-BLA infusion prevented the enhancement of anxiety-like behavior in stressed animals. These findings suggest that the selective ERK2 activation in BLA following stress exposure is an important mechanism for the occurrence of the promoting influence of stress on fear memory and on anxiety-like behavior.

Effect of kosher salt application on microbial profiles of poultry carcasses

The effect of conventionally applied kosher salt on the microbiological profile of posteviscerated chicken carcasses obtained from a local commercial processing facility was evaluated. The broiler carcasses were divided into treatments 1 through 8. Standard sampling methods were used to evaluate Salmonella prevalence, aerobic plate counts, coliforms, generic Escherichia coli, and psychrotroph counts. Results indicate significant reductions in microbial populations in all the salted groups compared with controls. Significant reductions (1.45, 2.31, 2.81, and 1.48 log cfu/mL of rinse) were obtained for aerobic plate count (APC), coliforms, generic E. coli, and psychrotroph counts, respectively, on prechill salt-treated carcasses compared with controls. Salt-treated carcasses sampled after chilling had lower microbial populations compared with control chilled samples with significant reductions in coliforms and generic E. coli (1.25 and 1.77 log, respectively). Salt-treated samples had lower counts on APC and psychrotrophs after 10 d of refrigerated storage compared with controls. Finally, drip loss of salt-treated carcasses was lower after 24 h compared with nontreated controls. Based on the results, it can be concluded that salting process is an effective contributor to microbial reductions during processing that needs further investigation as a possible intervention in commercial poultry processing settings.

Enhancing a declarative memory in humans: the effect of clonazepam on reconsolidation

A consolidated memory recalled by a specific reminder can become unstable (labile) and susceptible to facilitation or impairment for a discrete period of time. This labilization phase is followed by a process of stabilization called reconsolidation. The phenomenon has been shown in diverse types of memory, and different pharmacological agents have been used to disclose its presence. Several studies have revealed the relevance of the GABAergic system to this process. Consequently, our hypothesis is that the system is involved in the reconsolidation of declarative memory in humans. Thus, using our verbal learning task, we analyzed the effect of benzodiazepines on the re-stabilization of the declarative memory. On Day 1, volunteers learned an association between five cue- response-syllables. On Day 2, the verbal memory was labilized by a reminder presentation, and then a placebo capsule or 0.25 mg or 0.03 mg of clonazepam was administered to the subjects. The verbal memory was evaluated on Day 3. The volunteers who had received the 0.25 mg clonazepam along with the specific reminder on Day 2, exhibited memory improvement. In contrast, there was no effect when the drug was given without retrieval, when the memory was simply retrieved instead of being reactivated or when short-term memory testing was performed 4 h after reactivation. We discuss the GABAergic role in reconsolidation, which shows a collateral effect on other memories when the treatment is aimed at treating anxiety disorders. Further studies might elucidate the role of GABA in the reconsolidation process associated with dissimilar scenarios. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The influence of stress on fear memory processes

It is well recognized that stressful experiences promote robust emotional memories, which are well remembered. The amygdaloid complex, principally the basolateral complex (BLA), plays a pivotal role in fear memory and in the modulation of stress-induced emotional responses. A large number of reports have revealed that GABAergic interneurons provide a powerful inhibitory control of the activity of projecting glutamatergic neurons in the BLA. Indeed, a reduced GABAergic control in the BLA is essential for the stress-induced influence on the emergence of associative fear memory and on the generation of long-term potentiation (LTP) in BLA neurons. The extracellular signal-regulated kinase (ERK) subfamily of the mitogen-activated protein kinase (MAPK) signaling pathway in the BLA plays a central role in the consolidation process and synaptic plasticity. In support of the view that stress facilitates long-term fear memory, stressed animals exhibited a phospho-ERK2 (pERK2) increase in the BLA, suggesting the involvement of this mechanism in the promoting influence of threatening stimuli on the consolidation fear memory. Moreover, the occurrence of reactivation-induced lability is prevented when fear memory is encoded under intense stressful conditions since the memory trace remains immune to disruption after recall in previously stressed animals. Thus, the underlying mechanism in retrieval-induced instability seems not to be functional in memories formed under stress. All these findings are indicative that stress influences both the consolidation and reconsolidation fear memory processes. Thus, it seems reasonable to propose that the emotional state generated by an environmental challenge critically modulates the formation and maintenance of long-term fear memory.

ERK activation in the amygdala and hippocampus induced by fear conditioning in ethanol withdrawn rats: modulation by MK-801

The extracellular signal-regulated kinase (ERK) pathway, which can be activated by NMDA receptor stimulation, is involved in fear conditioning and drug addiction. We have previously shown that withdrawal from chronic ethanol administration facilitated the formation of contextual fear memory. In order to explore the neural substrates and the potential mechanism involved in this effect, we examined: 1) the ERK1/2 activation in the central (CeA) and basolateral (BLA) nuclei of the amygdala and in the dorsal hippocampus (dHip), 2) the effect of the NMDA receptor antagonist MK-801 on fear conditioning and ERK activation and 3) the effect of the infusion of U0126, a MEK inhibitor, into the BLA on fear memory formation in ethanol withdrawn rats. Rats made dependent via an ethanol-containing liquid diet were subjected to contextual fear conditioning on day 3 of ethanol withdrawal. High basal levels of p-ERK were found in CeA and dHip from ethanol withdrawn rats. ERK activation was significantly increased both in control (60min) and ethanol withdrawn rats (30 and 60min) in BLA after fear conditioning. Pre-training administration of MK-801, at a dose that had no effect on control rats, prevented the increase in ERK phosphorylation in BLA and attenuated the freezing response 24h later in ethanol withdrawn rats. Furthermore, the infusion of U0126 into the BLA, but not the CeA, before fear conditioning disrupted fear memory formation. These results suggest that the increased fear memory can be linked to changes in ERK phosphorylation, probably due to NMDA receptor activation in BLA in ethanol withdrawn rats.

Increased voluntary ethanol consumption and c-Fos expression in selected brain areas induced by fear memory retrieval in ethanol withdrawn rats

Withdrawal from chronic ethanol administration facilitated the formation of contextual fear memory. The effect of fear memory retrieval on subsequent ethanol consumption, by employing a two-bottle free-choice procedure with either water or ethanol (2-8% v/v), was investigated in ethanol withdrawn rats. The effect of fear memory extinction with or without d-cycloserine (DCS, 5mg/kgi.p.) on subsequent ethanol consumption was also evaluated. In addition, we examined c-Fos expression in different brain areas following the fear memory recall. The retrieval of such fear memory induced a significant increase in ethanol consumption in ethanol withdrawn but not in control animals. Regardless of DCS treatment, this increase was attenuated by extinction training. In ethanol withdrawn rats, context-dependent memory retrieval was accompanied by an increased c-Fos expression in the basolateral amygdala, ventrolateral periaqueductal gray, dentate gyrus and dorsomedial periaqueductal gray. Among these brain areas suggested to be implicated in the modulation of motivation and of emotional states, the basolateral amygdala has a crucial role in the emergence of negative affective state during ethanol withdrawal. These data suggest that retrieval of fear memory in ethanol withdrawn rats affected ethanol consumption and that amygdala may be involved in this effect.

Previous stress attenuates the susceptibility to Midazolam's disruptive effect on fear memory reconsolidation: influence of pre-reactivation D-cycloserine administration

It is well known that, under certain boundary conditions, the retrieval of a stable consolidated memory results into a labile one. During this unstable phase, memory can be vulnerable to interference by a number of pharmacological agents, including benzodiazepines. One of the goals of this study was to evaluate the vulnerability to midazolam (MDZ) after reactivation of recent and remote contextual fear memories in animals that experienced a stressful situation before learning. Animals were subjected to a restraint session and trained in a contextual fear paradigm the following day; consolidated memories were reactivated at different times after learning and different MDZ doses (1.5, 3.0 mg/kg) were administered to rats after reactivation. Our results show that MDZ did not affect memory reconsolidation in older-than-one-day memories of stressed animals, even after the administration of a higher MDZ dose and a longer reactivation session (5 min). In contrast, MDZ was effective in blocking reconsolidation at all memory ages in unstressed animals. In addition, the current research investigated whether activating NMDA sites before reactivation promotes the destabilization of resistant memories such as those of stressed animals. We tested the influence of pre-reactivation D-cycloserine (DCS), a partial NMDA agonist, on MDZ's effect on fear memory reconsolidation in stressed animals. Our findings indicate that DCS before reactivation promotes retrieval-induced lability in resistant memory traces, as MDZ-induced memory impairment in stressed rats became evident with pre-reactivation DCS but not after pre-reactivation sterile isotonic saline.

Involvement of dopaminergic mechanisms in the nucleus accumbens core and shell subregions in the expression of fear conditioning

The involvement of dopamine (DA) mechanisms in the nucleus accumbens (NAC) in fear conditioning has been proposed by many studies that have challenged the view that the NAC is solely involved in the modulation of appetitive processes. However, the role of the core and shell subregions of the NAC in aversive conditioning remains unclear. The present study examined DA release in these NAC subregions using microdialysis during the expression of fear memory. Guide cannulae were implanted in rats in the NAC core and shell. Five days later, the animals received 10 footshocks (0.6 mA, 1 s duration) in a distinctive cage A (same context). On the next day, dialysis probes were inserted through the guide cannulae into the NAC core and shell subregions, and the animals were behaviorally tested for fear behavior either in the same context (cage A) or in a novel context (cage B). Dialysates were collected every 5 min for 90 min and analyzed by high-performance liquid chromatography. The rats exhibited a significant fear response in cage A but not in cage B. Moreover, increased DA levels in both NAC subregions were observed 5-25 min after the beginning of the test when the animals were tested in the same context compared with accumbal DA levels from rats tested in the different context. These findings suggest that DA mechanisms in both the NAC core and shell may play an important role in the expression of contextual fear memory.

Influence of ethanol withdrawal on fear memory: Effect of D-cycloserine

Animals made dependent via an ethanol (ETOH) -containing liquid diet (6% v/v) for 14 days were subjected to a contextual fear conditioning paradigm 3 days after the last consumption day. After conditioning, rats were subjected to four extinction trials by exposing the animals to the conditioned context and their freezing was evaluated for each trial. Immediately after the first extinction trial, animals were injected with D-cycloserine (DCS) 5 mg/kg i.p., a dose that did not influence the extinction in control rats. Spontaneous recovery of learned fear was tested seven days after the last extinction trial. The following day, animals were subjected to a reacquisition or a reinstatement procedure and their freezing responses evaluated 24 h later. The present study shows that: 1. discontinuation from chronic ETOH administration facilitated the formation of a new fear memory concomitant with a marked resistance to being extinguished, 2. administration of DCS (5 mg/kg) facilitated the extinction process only in ETOH withdrawn rats, 3. both reinstatement and reacquisition procedures restored the increased freezing in ETOH withdrawn animals after extinction, 4. DCS administered immediately after the first extinction trial prevented the increase in freezing following both reacquisition and reinstatement. The enhanced sensitivity to the facilitatory effect of DCS in ETOH withdrawn animals may be mediated by adaptive changes in N-methyl-D-aspartate (NMDA) receptor provoked by ETOH dependence.

Fear state induced by ethanol withdrawal may be due to the sensitization of the neural substrates of aversion in the dPAG

The neural substrate underlying the aversive effects induced by ethanol abstinence is still unclear. One candidate for such effects is the dorsal periaqueductal gray (dPAG), a core structure of the brain aversion system. The main aim of this study is to examine the role of the dPAG as a possible locus of the aversive effects following abrupt alcohol withdrawal. To this end, rats were subjected to an oral ethanol self-administration procedure, in which animals were offered 6-8% (v/v) ethanol solution for a period of 21 days followed by an abrupt discontinuation of the treatment on the two subsequent days. Control animals received control dietary fluid for similar periods of time. The effects of ethanol withdrawal were examined in the elevated plus-maze (EPM) (Exp. I), on the prepulse inhibition of startle to loud sounds (Exp. II) and on the freezing and escape responses induced by electrical stimulation of the dPAG (Exp. III). In Experiment III, rats were implanted with an electrode aimed at the dPAG and the number and duration of ultrasonic vocalizations (USVs) were also recorded in the rats that received dPAG stimulation at freezing and escape thresholds. Data obtained showed that ethanol withdrawal elicited significant "anxiety-like" behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM. Startle reflex and prepulse inhibition remained unchanged in withdrawn animals. In addition, discontinuation from the chronic ethanol regimen caused a reduction in the stimulation thresholds for freezing and escape and in the number and duration of USVs. Together, these effects have been interpreted in the frame of a high fear state elicited by activation of the dPAG. These findings are indicative that ethanol withdrawal sensitizes the substrates of fear at the level of this midbrain structure.

Midazolam disrupts fear memory reconsolidation

The current research examines the influence of midazolam (MDZ) on memory reconsolidation using a contextual fear paradigm in rats, based on three context-shock training trials (0.7 mA, 3 s). First, we evaluate the effect of MDZ (1 mg/kg, i.p.) injected shortly after the training procedure. Second, we examined the influence of MDZ after a brief exposure (90 s) either in the training context (reactivation procedure) or in a neutral environment (no reactivation procedure) and one day later, freezing behavior was scored when rats were re-exposed to the training environment. Third, we investigate both the effect of MDZ administered at different times following reactivation on fear memory and the persistence of such effect 10 days after reactivation. Finally, we test whether the MDZ effect could be reverted by a single weak training trial (0.2 mA, 3 s) or by the presentation of the same unconditioned stimulus in the absence of the conditioned stimulus as a reminder which proves to induce significant freezing in rats not previously trained. Results show that MDZ interferes with the formation of a contextual fear memory only when administered after the reactivation procedure but not after the training procedure. This interference was effective up to 60 min after reactivation and not at a later time. No spontaneous recovery of freezing behavior was observed 11 days after MDZ injection which was not reverted by a weak training trial and by the unconditioned stimulus alone. All these data support the idea that stimulating GABA A receptor sites via MDZ selectively disrupts the reconsolidation process of a contextual fear memory.

Gabaergic modulation of the stress response in frontal cortex and amygdala

GABAergic neurotransmission is thought to play an important role in the modulation of the central response to stress. In the present study we evaluate the influence of a brief restraint exposure on GABA-stimulated chloride influx in diverse brain areas presumed to have a major role in the mediation of emotional behaviors following aversive stimulation. A reduced chloride uptake after stress exposure was only observed in frontal cortex and amygdala. Moreover, rats subjected to such stressor performed an anxiogenic behavior when exposed later to the elevated plus-maze. A comparable behavior in the elevated plus-maze was observed between animals that were allowed to chew during the restraint experience and those without any stressful manipulation, suggesting that chewing served as an efficient coping behavioral strategy during such threatening situations. In order to explore if chewing during the restraint experience could suppress the reduction in GABA-stimulated chloride uptake induced by this stressor, rats were allowed or not to chew during restraint and in both cases GABA-stimulated chloride influx was assayed in frontal cortex and amygdala. The finding of this experiment showed that restrained rats that have the possibility to chew exhibited a similar GABA-stimulated chloride uptake in cortical tissue to that shown by control, unstressed rats. Moreover, chewing in response to restraint attenuated the reduction of GABA-stimulated chloride uptake in amygdala, supporting the notion that chewing is an effective coping response to restraint. These experiments suggest that a reduced GABAergic inhibitory control in these areas could be implicated in the emotional sequelae generated by this uncontrollable stressor and that the suppression of this reduction seems to be associated with the occurrence of coping behavioral response to such fear-inducing stimulus.

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.

Chronic benzodiazepine administration facilitates the subsequent development of ethanol dependence

This study investigated the effect of chronic benzodiazepine (BZD) administration and its abrupt discontinuation on later subsequent ethanol consumption employing a free choice paradigm between water and increasing ethanol concentrations. In addition, we also studied the anxiolytic and reinforcing properties of ethanol assessed in the elevated plus maze (EPM) and in the conditioned place preference paradigm, respectively. Adult male Wistar rats were subjected to a chronic diazepam (DZM) treatment (2 mg/kg/day, i.p.) during 21 days. Twenty-four hours after that treatment and, in another experiment, 10 days after the last DZM injection, rats were subjected to an oral ethanol self-administration procedure (ethanol was increased in concentration (v/v) on 4 consecutive days as follows: 2%, 4%, 6%, 8% followed by an additional period of 8 days in which animals were offered a 10% (v/v) ethanol solution. Diazepam treated rats showed a higher ethanol intake and spontaneous signs of ethanol withdrawal when the access to ethanol was discontinued. These results were observed when ethanol was available at day 1 of withdrawal but not when DZM treated rats were initiated in the ethanol choice test 10 days after BDZ withdrawal. Furthermore, DZM treated rats exhibited an increased anxiolytic ethanol induced effect (1 g/kg, i.p.) in the EPM and a significant ethanol-induced conditioned place preference (1 g/kg, i.p.). These data suggest that early DZM treatment facilitates ethanol consumption and the development of ethanol dependence.

Electrical stimulation of the midbrain tectum enhances dopamine release in the frontal cortex

One widely used animal model of anxiety is the electrical stimulation of a given structure supposed to be involved in the neural circuitry underlying emotional behavior. Indeed, electrical stimulation of midbrain structures with substrates for the processing of fear-like responses, such as the dorsal periaqueductal gray matter (DPAG) or the inferior colliculus (IC), produces behavioral, sensorial and autonomic responses very similar to the defense reactions observed in environmental threatening situations. It has also been proposed that the required level of integration of all these components of the defense reaction needs an integrative process situated at higher brain level, as the prefrontal cortex. As a matter of fact, substantial cortical inputs to the midbrain tectum have already been found. In view of this evidence, it seems important to know whether animals stimulated in the midbrain tectum would present neurochemical changes in the prefrontal cortex. To this end, we examined the temporal course of the effects of the electrical stimulation of the DPAG and IC on the dopamine (DA) release in the prefrontal cortex. Electrical stimulation of these structures was performed at the alertness (control) and escape thresholds. Electrical stimulation of the inferior colliculus at the escape threshold produced a long-lasting increase in the levels of corticofrontal dopamine in relation to these measurements in the control group. No significant changes in extracellular DA release in this cortical area could be observed following DPAG electrical stimulation. These findings bring evidence for the involvement of dopamine of the frontal cortex in the setting up of adaptive responses to stressful situations generated at the inferior colliculus level.

Prior electrical stimulation of the inferior colliculus sensitizes rats to the stress of the elevated plus-maze test

Besides its primary function in the transmission of acoustic signals, the inferior colliculus (IC) is involved in conveying auditory information of aversive nature to higher cortical structures. In the field of anxiety research, one widely used animal model is the electrical stimulation of a given structure supposed to be involved in the neural circuitry underlying emotional behavior. Indeed, electrical stimulation of the inferior colliculus produces fear-like responses. Moreover, prior exposure to stressful events sensitizes rodents' responsivity to fearful stimuli when they are subsequently tested in the elevated plus-maze. Based on these evidence it seems to be important to know whether animals stimulated in the inferior colliculus would show a heightened behavioral responsivity to subsequent stressful events. To this end, we examined the temporal course of the effects of the electrical stimulation of this midbrain region (5, 10 and 15 min afterward) on the conventional and ethological measures of the behavior of rats exposed to the elevated plus-maze test. Prior electrical stimulation of the inferior colliculus produced an 'anxiogenic' profile in the elevated plus-maze, i.e. a significant reduction in the number of entries and time spent into the open arms. Still, previous electrical stimulation of the inferior colliculus caused a significant decrease in rearing, scanning, peeping out, head dipping and end-arm activity while increased immobility. All these changes occurred after 5 min of inferior colliculus electrical stimulation. Therefore, stimulation of the inferior colliculus causes a heightened responsivity to anxiogenic stimuli inherent to the elevated plus-maze test. These findings bring additional support to the proposed role of this midbrain structure in the elaboration of adaptive responses to stressful situations.

The involvement of an opiate mechanism in the sensitized behavioral deficit induced by early chronic variable stress: influence of desipramine

The influence of early chronic variable stress (CVS) associated with persistent desipramine (DMI) administration was examined on escape performance. Animals were exposed to CVS and 1 day later administered DMI (5 mg/kg, i.p. twice a day) or vehicle (VH) during six consecutive days. Escape performance was assessed over 24 h following inescapable shock (IS) exposure. Higher escape failures were observed in CVS shocked rats compared with unstressed shocked animals. DMI normalized escape failures in both groups. In order to investigate the role of an endogenous opiate mechanism presumably activated by CVS exposure in this behavioral deficit, rats were administered naltrexone (NAL, 2 mg/kg i.p.) or VH prior to each daily stressor of the CVS regime. NAL pretreatment blocked escape failures performed only by CVS shocked rats. In addition, animals were daily administered morphine (MOR, 10 mg/kg, i.p.) or VH during seven consecutive days and subsequently administered DMI. A significant increase in escape deficit in shocked rats was observed after chronic MOR but not following the associated treatment with MOR and DMI. These behavioral data suggest that early experience with a CVS facilitated the onset of escape deficit induced by a brief IS event, an effect that can be prevented by chronic DMI. Furthermore, this sensitized escape deficit response seems to be partially modulated by the previous activation of an opiate mechanism.

Benzodiazepine withdrawal facilitates the subsequent onset of escape failures and anhedonia: influence of different antidepressant drugs

The effect of benzodiazepine (BDZ) withdrawal on escape acquisition and on the behavioral response to two different reinforcing stimuli was investigated. In addition, the influence of antidepressant drugs (AD) differing in their mechanism of action on these behavioral outputs was also evaluated. Rats subjected to withdrawal from a chronic treatment with diazepam (DZM; 2 mg/kg per day, i.p.) during 21 days were subsequently exposed to a brief inescapable shock session (IS) and 48 h later to an active avoidance test. Only withdrawn animals exposed to the IS exhibited enhanced escape failures. In an additional experiment, withdrawn rats were repeatedly administered with vehicle (VEH), desipramine (DMI; 5 mg/kg, i.p.), fluoxetine (FLU; 5 mg/kg, i.p.) or phenelzine (PHEN; 5 mg/kg, i.p.) and subsequently exposed to IS and to active avoidance task. A significant reversal of escape deficit was only observed following DMI and PHEN but not after FLU. Furthermore, withdrawn rats showed a reduced preference for a sexually relevant olfactory cue, this reduced sensitivity was only normalized following DMI but not after the administration of FLU or PHEN. Finally, rats exposed to abrupt cessation of chronic BDZ administration did not exhibit preference for a context previously associated with amphetamine (AMP) under the conditioned place preference (CPP) procedure. All these findings are indicative that BDZ withdrawal facilitates the subsequent occurrence of behavioral changes-escape failures and reduced behavioral response to rewarding stimuli-suggested to parallel important symptoms of human depression. In addition, DMI seems to be much more effective in restoring such behavioral abnormalities as compared to a MAO inhibitor and to a inhibitor of 5-HT uptake.

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.

Metyrapone pretreatment prevents the behavioral and neurochemical sequelae induced by stress

In the present study, we examined the effect of metyrapone, an inhibitor of corticosterone (CS) synthesis, on the behavioral and neurochemical sequelae induced by a brief restraint session. A 15-min stress period induced an anxiogenic-like behavior on the elevated plus-maze (EPM), which was reversed with metyrapone (75 mg/kg i.p.) injected 3 h prior to the stress event. It was further demonstrated that metyrapone pretreatment normalized the decrease in maximal chloride uptake following GABA stimulation observed in brain cortex tissue obtained from animals exposed to both restraint and the EPM. In addition, plasma CS levels were assessed both after restraint and following EPM exposure. Furthermore, the administration of both CS (2.5 mg/kg s.c. at a dose that mimics CS levels induced by restraint) or dexamethasone (DEXA, 1.25 microg/kg s.c) resulted in an anxiogenic response in the EPM comparable to that induced by restraint. Taken together, all these evidence suggest that CS released in response to stress seems to be associated with functional changes at the GABAergic supramolecular complex which could underlie the enhanced anxiety observed following the exposure to an aversive experience.

Carbamazepine normalizes the altered behavioral and neurochemical response to stress in benzodiazepine-withdrawn rats

Rats chronically treated with diazepam (2 mg/kg per day, i.p.) for 21 days were tested 96 h after the last injection in both the forced swim test (inescapable stress) and in an active avoidance test (escapable stress). The influence of carbamazepine (7.5 mg/kg, i.p.) administered 25 min prior to each behavioral task was investigated. Withdrawn animals showed a reduced time spent in immobility in the forced swim test and an enhanced latency to escape in the active avoidance test. Both behavioral effects were normalized by a single carbamazepine administration. An additional experiment was performed to investigate the effect of a forced swim experience on cortical chloride uptake following GABA (gamma-aminobutyric acid) stimulation 96 h after diazepam withdrawal, and the influence of a single administration of carbamazepine on these effects. An increased chloride uptake was observed in vehicle-treated rats but not in diazepam-withdrawn animals following the swimming experience. Carbamazepine pretreatment enhanced chloride uptake after diazepam withdrawal but did not modify chloride flux in stressed or unstressed vehicle-treated rats. These results support the hypothesis that diazepam withdrawal affects the ability to develop adaptive responses to stress and that carbamazepine can normalize such an alteration.

Prior exposure to a brief restraint session facilitates the occurrence of fear in response to a conflict situation: behavioral and neurochemical correlates

The influence of two different stressors on the behavioral and neurochemical responses to a subsequent exposure to the elevated plus maze (EPM) was examined. Rats were submitted to either a 15-min restraint period or to a 15-min forced swimming test (FS) and one day later exposed to the EPM. Animals with early restraint exhibited a significant decrease in the percent time spent and in the number of entries on the open arms. In addition, restraint induced a reduction in the total number of entries. An identical behavior in the EPM was observed between unstressed rats and those exposed to a previous swimming experience. As a humoral index of stress, corticosterone (CS) secretion in response to each stressor was evaluated. A similar increase of CS release was observed following each aversive stimulus. Exposure to both restraint and EPM decreased the cortical chloride uptake following GABA stimulation. Similar values of chloride flux were obtained from animals submitted to either restraint but without subsequent exposure to the EPM, exposed only to the EPM, or without any manipulation (controls). These findings are discussed in terms of a facilitated behavioral and neurochemical response to a fearful situation following an early and brief restraint experience.

Altered behavioral and neurochemical response to stress in benzodiazepine-withdrawn rats

Rats chronically treated with diazepam (DZM) (2 mg/kg/day i.p.) for 21 days, were subsequently tested at 24, 48, 72 and 96 h after the last injection in the elevated plus-maze. When compared with their respective controls, withdrawn animals showed an anxiogenic response since they exhibited a significant decrease in the % of time spent in the open arms, and a reduction in open and total arms entries at 24 and 48 h following the last benzodiazepine administration. No other behavioral differences were observed in the remaining groups. Ninety-six hours following DZM withdrawal, when the anxiogenic response was no longer evident, withdrawn rats showed a significant decrease in immobility time when exposed to a forced-swim test (FST). GABA stimulation of chloride uptake in cortical tissue was measured in BDZ-withdrawn rats or vehicle-treated animals with or without exposure to the FST. An enhanced chloride uptake following GABA stimulation was observed in vehicle-treated rats following the swimming trial. However, similar values of chloride uptake were found among rats withdrawn from DZM at 96 h either exposed or not to the FST and vehicle-treated animals without prior stress exposure. These findings show that BDZ withdrawal alters the neurochemical and behavioral response to a subsequent stressful experience. These lines of evidence may indicate that BZD withdrawal reduced the ability to develop an adaptive response to stress.

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.

Play behavior and stress responsivity in periadolescent offspring exposed prenatally to cocaine

Play behavior and stress responsiveness were examined in offspring exposed gestationally to cocaine. The subjects were offspring of Sprague-Dawley rat dams given s.c. injections of 40 mg/kg/3 cc cocaine HC1 daily from gestational days 8-20 (C40), pair-fed dams injected daily with saline (PF), and untreated control dams (LC). Periadolescent (postnatal day (P) 30-36) male and female rats were assigned to either pretest Stress or No Stress conditions. Every other day Stress animals were exposed to a stressor (on P30--foot shock; P32--white noise; P34--forced swim; P36--foot shock), with each stressor being administered 4 h prior to a play session. Immobility during one of the stressors, foot shock, was used to assess stress responsiveness. Play sessions consisted of pairing each experimental animal with a same-sex, nonexperimentally manipulated conspecific for 7 min. The results indicated that periadolescent offspring exposed gestationally to cocaine differed from controls in their stress responsivity, as evidenced by a failure to show increased immobility during the final foot shock session. Also, while cocaine-exposed juveniles did not differ from controls in their own play behavior, these offspring elicited less play solicitation from conspecifics, as evidenced by an increased latency to be pounced, and decreased frequency and duration of being pounced. These findings parallel earlier evidence for altered stress responsiveness in adult cocaine-exposed rats and also suggest that prenatal exposure to cocaine results in altered social cues.

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.

Chronic variable stress enhances the stimulatory action of a low dose of morphine: reversal by desipramine

Adult male rats were exposed to a chronic variable stress treatment, an animal model of depression, with or without concurrent daily administration of desipramine. Animals given chronic and variable stress were submitted daily to a different stressor following an injection of either saline or desipramine (5 mg/kg, i.p.), whereas control animals were unmanipulated except for the injection process. One day after the last event of the chronic procedure, control and stressed animals were administered saline or morphine (0.75 or 1.5 mg/kg, i.p.) and their locomotion assessed for 90 min. In an additional experiment, 24 h after the last stressor, stressed and control rats were challenged with either saline or one of two higher doses (behaviorally suppressant) of morphine (5 and 10 mg/kg, i.p.). A significantly greater increase in locomotor activity following a low dose (1.5 mg/kg) of morphine was observed in chronically stressed rats as compared to control rats. This potentiated locomotor response to morphine in stressed rats was prevented by desipramine pretreatment. The chronic and variable stress treatment did not modify the sedative response to the high doses of morphine. These data support the suggestion that a chronic and variable stress procedure results in sensitization to the stimulant effect of opioid stimulation, and that pretreatment with the antidepressant agent desipramine blocks the development of this sensitization.

Chronic restraint attenuates the immunosuppressive response induced by novel aversive stimuli

The exposure to a novel aversive event, such as foot shock, induced a decrease in the percentage of T lymphocytes and a clear reduction in the delayed-type hypersensitivity reaction (DTH). This immunosuppressive response to an acute stressor was absent in rats that were previously exposed to a chronic immobilization stress regime (2 h daily during 7 consecutive days), but was still present in animals with prior exposure to only one or three restraint sessions. No stress effect was observed in other immunologic parameters, such as the percentage of B lymphocytes or the hemagglutinin titer, in any of the experimental treatments. The possible involvement of central adaptive mechanisms in the attenuation of the immunosuppressive response induced by an acute stress is discussed.

The behavioral response to stress is altered in adult rats exposed prenatally to cocaine

The behaviors displayed during and following acute stress exposure were investigated in rats prenatally exposed to cocaine. Offspring from Sprague-Dawley rat dams given SC injections of 40 mg/kg/3 cc cocaine HCl (C40) daily from gestational days 8-20, pair-fed dams injected daily with saline (PF), and untreated control dams (LC) were examined. Adult male rats 60 days old (P60) from the three prenatal groups were given either a 5-min forced swim test (FS), 10 min of intermittent foot shock exposure, or were not exposed to either acute stressor. Amount of immobility or inactivity during acute stress exposure was recorded. One day later, all animals were given a 5-min open field test. During both of the acute stressors, C40 offspring exhibited significantly less immobility than PF and LC control offspring. Whereas prior foot shock experience enhanced the amount of immobility shown by LC and PF offspring in the open field, similar immobility scores were observed between shocked and nonstressed C40 rats. In contrast, prior foot shock experience reduced locomotion in all prenatal treatment groups. These findings suggest that prenatal cocaine exposure results in altered behavioral reactivity in terms of the immediate and delayed response to stressful experiences.

Chronic variable stress or chronic morphine facilitates immobility in a forced swim test: reversal by naloxone

The behaviors displayed in a forced swim test were investigated in rats previously exposed to a chronic variable stress treatment or chronic administration of morphine. In addition, to further explore the participation of an endogenous opiate mechanism in these behavioral effects, naloxone was either administered during the chronic treatment (prior to each stress or morphine exposure) or immediately prior to the forced swim test. Animals were submitted daily to a different stressor for 1 week or injected with morphine (10 mg/kg, IP) for 6 days, whereas controls were unmanipulated except for the injection process. On the day following the last stressor, control and stressed animals were administered saline or naloxone (2 mg/kg, IP) 15 min prior to the forced swim test. Morphine treated animals were similarly tested on the third day following the last morphine injection. In a separate group of rats, naloxone (2 mg/kg, IP) was administered daily 10 min prior to each stressor of the chronic stress regime or each daily morphine injection. A significant increase in the time spent in immobility was observed in stressed animals as well as in rats chronically treated with morphine. In both groups, this potentiated immobility was attenuated by naloxone pretreatment prior to the forced swim test or when given before each daily stressor or morphine injection. In addition, the concurrent exposure to stress or morphine along with naloxone administration enhanced struggling in the first 5 min of the forced swim test.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Repeated exposure of rat pups to isolation attenuates isolation-induced ultrasonic vocalization rates: reversal with naltrexone

Young rat pups are dependent on the dam for their survival, thus isolation of the neonatal rat pup from the dam presents the young organism with a variety of stressors. The question examined in this study concerns the ability of the young rat pup to modify its response to isolation following repeated exposure to that isolation as well as the role played by endogenous opiates in this process. Following repeated isolations, pups were seen to decrease vocalization rates. Altering the context in an attempt to dishabituate animals failed to reverse the decreased vocalization rate. However, opiate receptor blockade attenuated this decrease when administered subsequent to the first isolation period but not prior to the last isolation period. These results suggest that the development of this attenuated response to isolation stress is opiate-mediated but that once established, its expression is not dependent on endogenous opiate release.

Gangliosides facilitate the recovery of behavioral response mediated by dopaminergic sites following their irreversible blockade

The effect of acute and chronic exogenous ganglioside (G) administration on the functional recovery of dopaminergic receptors following their blockade by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was investigated by means of apomorphine (APO)-induced stereotyped behavior. Animals previously treated with EEDQ exhibited a lack of behavioral response to APO 6 or 24 h later. A progressive behavioral recovery was already evident at day 3 and reached control values at day 7. G pretreatment accelerated the behavioral recovery after EEDQ administration, because a higher behavioral response to APO in these animals as compared with rats treated with EEDQ alone at the same time was observed. These findings indicate that G accelerates the functional recovery of dopaminergic sites following their irreversible blockade.

Fighting during shock exposure attenuates the reduction in the number of low-affinity GABAA sites in the cerebral cortex

Rats were submitted either singly or in pairs to a series of foot-shocks. Immediately following the stress event, animals were sacrificed and the binding of low-affinity GABAA sites assayed in the cerebral cortex. A reduced number of sites was observed in rats singly shocked. However, rats shocked in pairs and thus afforded the opportunity to fight during the shock did not present changes in the density of GABAA receptors as compared to unstressed animals. Our data may suggest that fighting during an aversive event could decrease the deleterious consequence of a highly stressful experience.

A fostering study of the effects of prenatal cocaine exposure: II. Offspring behavioral measures

The impact of rearing condition was assessed in Sprague-Dawley dams given 40 mg/kg cocaine (C40) or saline (LC control) subcutaneously (SC) from gestational days 8-20 and their offspring. Treated pups reared by their biological dams (LC/LC; C40/C40), treated pups reared by surrogate dams (FOS/LC; FOS/C40), and foster pups raised by treated dams (LC/FOS; C40/FOS) were examined. On postnatal day 7 (P7), pups received either 0 (unpaired) 2, 3, or 4 pairings of an odor and footshock and were tested for their aversion to this odor. Foster and LC pups, regardless of rearing condition, exhibited significant odor aversions following either 2, 3, or 4 training trials. In contrast, C40 pups reared by surrogate dams required 4 trials to acquire the aversion, and C40 pups reared by their own dams did not exhibit conditioning even after 4 trials. At P17, no differences were seen among the groups in the aversion formed to an auditory or an olfactory stimulus that was paired with footshock. At P60, shock-elicited aggression among pairs of siblings was examined. Regardless of prenatal exposure condition, offspring reared by dams given cocaine showed a decreased latency to the first aggressive contact, an effect that was evident without any alteration in shock sensitivity. Together these data suggest that being reared by a dam previously exposed to cocaine has an impact on offspring behavioral function apart from the effects of prenatal cocaine exposure per se. The implications of the data regarding the cognitive performance of pups exposed prenatally to cocaine are also discussed.

A fostering study of the effects of prenatal cocaine exposure: I. Maternal behaviors

The effect of rearing condition and prenatal exposure to cocaine on maternal behaviors was examined. Sprague-Dawley dams were given SC injections of 40 mg/kg/3cc cocaine HCl (C40) or saline (LC) daily from gestational days 8-20. Maternal behavior was assessed in treated dams rearing their biological pups (LC/LC; C40/C40), treated dams rearing untreated pups (LC/FOS; C40/FOS), and foster dams rearing treated pups (FOS/LC; FOS/C40). All dams were monitored for home cage behavior (time eating, drinking, and with pups) for 2 h during both the light and dark cycle on postnatal day 4 (P4), pup retrieval on P5-P9, and maternal aggression to a female intruder (latency to the first attack, number of attacks, boxing, pins, intruder time spent submissive and motionless) on P10. No differences were observed in nest behavior or in tests of pup retrieval among the six groups. Dams rearing their biological litter (LC/LC and C40/C40) were significantly quicker to initiate the first attack when compared to all other groups. This increased aggression was maintained throughout the test session in the C40/C40 dams who made significantly more intruder attacks than all other groups, with the intruder spending significantly more time in a submissive posture (lying on back). In contrast, LC/LC dams did not exhibit an increased number of attacks during the test, apparently responding to an increased freezing in their intruders with a reduction in aggressive behavior. Taken together these findings suggest that prior cocaine exposure results in alterations in maternal aggression that is evident when these dams rear their own pups.

ACTH accelerates the attenuation of alpha 2-adrenoceptors response in nucleus accumbens following chronic desipramine

The inclusion of clonidine (CLO) induced a dose-dependent reduction of K(+)-evoked [3H] dopamine ([3H]DA) release in slices from rat nucleus accumbens. This inhibition was clearly attenuated in animals previously administered desipramine daily (DMI, 10 mg/kg i.p.) during 21 days, but not in rats submitted to a persistent treatment with DMI during 10 days. However, the coadministration of adrenocorticotrophic hormone (ACTH, 50 IU/kg s.c.) and DMI (10 mg/kg i.p.) for 10 days provoked a clear decrease in the inhibition produced by alpha 2-adrenoceptor stimulation, while ACTH alone had no effect. These results may indicate that ACTH accelerates the onset of DMI-induced adaptive changes on central alpha 2-adrenoceptor in the mesolimbic area.

Effects of chronic variable stress and antidepressant drugs on behavioral inactivity during an uncontrollable stress: interaction between both treatments

Rats submitted to a chronic variable stress regime exhibited more inactivity during inescapable shock as compared with unstressed rats. Besides, chronic treatments with desipramine, imipramine, clomipramine, or phenelzine reduce the amount of inactivity exhibited during this aversive experience. Furthermore, the combination of both chronic treatments--stress and antidepressants--resulted in a potentiation of the antidepressant effect alone. This higher antidepressant efficacy may result from the interaction of the neural mechanisms triggered by chronic stress exposure and antidepressant drugs.

Chronic variable stress facilitates tumoral growth: reversal by imipramine administration

The present study was conducted to examine whether a chronic variable stress procedure (CVS)--an animal model of depression--facilitates tumor growth, and whether this effect can be modified by concurrent administration of the antidepressant imipramine (IMI). Unstressed rats, with or without previous administration of the immunosuppressive agent cyclosporine (CS), were inoculated with 5 x 10(6) cells of a sarcoma. Another group of rats was inoculated with tumoral cells and later exposed to the CVS procedure with or without concurrent administration of IMI (10 mg/kg, i.p.). An additional group of animals was treated with CS and later given daily injections of IMI (10 mg/kg, i.p.) without stress manipulation. A lack of tumoral development was observed in unstressed animals without previous injections of CS, whereas, prior injections of this immunosuppressive agent increased tumoral growth in unstressed animals. Exposure to the CVS procedure facilitated tumoral growth even in animals without CS injections. This effect was clearly attenuated when chronically stressed rats were concurrently given IMI. These findings support the notion that the development of a tumoral process is facilitated when a state of experimental depression is induced and that the reversal of such a state by antidepressant treatment results in the inhibition of tumor development.

Antidepressants reduce inactivity during both inescapable shock administration and shuttle-box testing

The occurrence of inactivity during inescapable shock is a good predictor of escape deficit because a positive correlation was obtained between the two factors. Chronic desipramine (DMI) administration prior to inescapable shock clearly reduced the inactivity during this aversive experience. Moreover, repeated DMI administration either before or after inescapable shock reversed the increase, in escape failures and in inactivity during shuttle-box testing produced by this inescapable shock. A single injection with DMI before either inescapable shock or shuttle-box test altered neither inactivity nor escape deficit. Clomipramine and phenelzine were also effective to reduce inactivity and escape failures. However, other drugs without antidepressant properties, such as haloperidol, amphetamine, and diazepam reduced neither inactivity nor escape deficit. These data suggest that an anti-inactivity effect is critically involved in the reversal of inescapable shock-induced escape deficit following chronic antidepressants. The involvement of antidepressant-induced changes in beta-adrenoceptors with this behavioral reversal is discussed.

Antidepressants reverse the inhibition of shock-induced aggression elicited by a prior inescapable shock

Animals were exposed to long-duration inescapable shock (IS) and six days later submitted in pairs to a foot-shock session in order to induce shock-elicited aggression (SIF). Shocked rats subsequently displayed a lower aggressive response as compared to unshocked animals. This reduction was prevented by repeated treatment with different antidepressant drugs administered either prior or following IS exposure. In addition, rats chronically administered with antidepressant drugs before the IS showed less inactivity during the application of the uncontrollable aversive event. These data indicate that persistent administration with these pharmacological compounds prevent the induction and impede the further expression of the reduced aggressive response induced by a previous IS.

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 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.

Shuttle-box deficits induced by chronic variable stress: reversal by imipramine administration

Escape performance in a shuttle-box task was evaluated in rats chronically exposed to a series of unpredictable stressors either during 14 or 7 consecutive days. Failure in escape responses was observed when animals were subjected to both regimes of variable aversive situations. The association between chronic exposure to unpredictable stressors with imipramine resulted in a significant reversal of escape deficits. Furthermore, animals submitted to repeated immobilization sessions during 7 days presented similar escape response to control rats. A possible involvement of beta-adrenergic sites on this behavioral response is discussed.

Chronic antidepressant counteracts the conditioned analgesia induced by a context previously associated with inescapable shock

Rats exposed to a context previously paired with an inescapable shock session showed an increase in the paw lick and escape latencies in the hotplate test. This analgesic response was clearly reversed following chronic antidepressant drug administration. These findings indicate that chronic antidepressant treatment can influence the biological processes involved in the inescapable shock-induced conditioned analgesia.