Involvement of prefrontal dopamine neurones in behavioural blockade induced by controllable vs uncontrollable negative events in rats

Periadolescent stress exposure exerts long-term effects on adult stress responding and expression of prefrontal dopamine receptors in male and female rats

Recent research has demonstrated that experiential/environmental factors in early life can program the adult stress response in rats, and this is manifest as altered hypothalamic-pituitary-adrenocortical activity and behavior in response to a stressor. Very little work has been devoted to investigating whether the environment during adolescence plays a similar role in modulating ongoing developmental processes and how this might affect adult stress responding. Periadolescent predator odor (PPO) exposure was used here as a naturalistic model of repeated psychological stress. Behavioral and endocrine responses to PPO changed across the exposure period, and behavioral alterations persisted into adulthood. While adolescent rats showed pronounced avoidance responses upon initial PPO exposure, hyperactivity increased across the exposure period, especially in females. Corticosterone (cort) responses to stressor exposure also changed in females, with higher physiological baseline levels observed at the end of the exposure period. In adulthood, relative to rats who had received a control manipulation during adolescence, PPO-exposed rats were more fearful in a novel open field and displayed altered responses to a predator odor stress test in adulthood. Moreover, lower levels of the D2 dopamine (DA) receptor were measured in prefrontal (infralimbic and dorsopeduncular) cortices of PPO-exposed rats. These findings suggest that the adolescent period may represent a sensitive period during which developmental programming of the stress response occurs.

Animal models with potential applications for screening compounds for the treatment of obsessive-compulsive disorder

The availability of an animal model for obsessive-compulsive disorder (OCD) is necessary for the development of novel pharmacological treatments. To be useful, the model must be predictive of clinical performance, possess characteristic criteria and distinguish anti-OCD from antidepressant compounds. Due to the lack of OCD models useful for drug discovery, all compounds currently used for OCD were developed first as antidepressants. In this article, we discuss the relative merits of: stereotypic behaviours (canine acral lick, feather picking, amphetamine- and 5-HT-induced stereotypy); adjunctive and displacement behaviours (schedule-induced polydipsia, wheel running, resident-intruder grooming); anxiolytic tests (separation and shock-induced ultrasonic vocalisation and marble burying); and depression tests (inescapable shock-induced escape and immobility in forced swim) as potential OCD models. We conclude that adjunctive and displacement behaviours, and in particular schedule-induced polydipsia, may prove to be the best models for compulsive behaviour in animals that can be used for the discovery of novel anti-OCD agents.

Cognitive deficits caused by late gestational disruption of neurogenesis in rats: a preclinical model of schizophrenia

Late gestational disruption of neurogenesis in rats has been shown to induce behavioral abnormalities thought to mimic aspects of positive and negative symptoms of schizophrenia. Furthermore, it has been shown that the morphological changes produced by the perturbation are relevant to schizophrenia with reduced thickness of the hippocampus, thalamus, and cortical regions. In addition to the positive and negative symptoms, schizophrenia is associated with deficits in a wide variety of cognitive domains. In the present studies, we assessed whether the cognitive deficits are modeled by disruption of neurogenesis late during gestation (gestational day 17) in the rat. In the battery of tests utilized, we describe that rats in which neurogenesis was disrupted have deficits in a reversal-learning paradigm of the Morris water maze and in object recognition, and that they exhibit perseveration in the Porsolt forced swimming test. Additionally, we found deficient associative learning in an acquisition of an active avoidance paradigm and deficits in latent inhibition. No deficits were observed in the reference memory version of the Morris water maze and in a non-match-to position experiment, showing that the deficits are limited to certain aspects of cognition.

Limbic and motor circuitry underlying footshock-induced reinstatement of cocaine-seeking behavior

The role of limbic, cortical, and striatal circuitry in a footshock reinstatement model of relapse to cocaine seeking was evaluated. Transient inhibition of the central extended amygdala [CEA; including the central nucleus of the amygdala (CN), ventral bed nucleus of the stria terminalis (BNSTv), and nucleus accumbens shell (NAshell)], ventral tegmental area (VTA), and motor circuitry [including the dorsal prefrontal cortex (PFCd), nucleus accumbens core (NAcore), and ventral pallidum (VP)] blocked the ability of footshock stress to reinstate lever pressing previously associated with cocaine delivery. However, inhibition of the basolateral amygdala, mediodorsal nucleus of the thalamus, or the ventral prefrontal cortex had no effect on drug-seeking behavior. These data suggest that footshock stress activates limbic circuitry of the CEA that, via the VTA, activates motor output circuitry responsible for producing lever press responding. Consistent with this notion, the D1/D2 dopamine receptor antagonist fluphenazine blocked footshock-induced reinstatement when infused into the PFCd. Further, inhibition of the NAshell blocked a footshock-induced increase in dopamine within the PFC and concomitantly blocked reinstatement responding. Also supporting the idea of a CEA-VTA-motor circuit in stress-induced reinstatement of cocaine seeking, inactivation of the PFCd was shown to block stress-induced glutamate release within the NAcore while concurrently inhibiting reinstatement responding. Taken together, these data suggest that footshock activates limbic circuitry in the CEA, which in turn activates a VTA dopamine projection to the PFCd. The rise in dopamine within the PFCd initiates reinstatement via a glutamatergic projection to the NAcore.

Monoamine metabolism changes following the mouse forced swimming test but not the tail suspension test

Microdialysis, binding and behavioural studies have shown that the dopaminergic system plays a role in antidepressant treatment. It has been suggested that stress may provoke a modification in dopamine (DA) release in different brain areas and that the forced swimming test (FST), in its own accord as a stressor, may be responsible for this modification. Naive male Swiss mice, receiving saline solution, were used in two animal models of depression, the FST and the tail suspension test (TST). In order to understand the locomotor aspect of each test, groups of mice were studied for effects on locomotor activity. Following each test, mice were killed by cervical dislocation, brains were removed and concentrations of amines in the whole brain were analysed by high-performance liquid chromatography. DA concentration increased from 5 min of the FST, dihydroxyphénylacetate (DOPAC), from 20 min of FST and serotonin, from 8 min of FST. No modification of noradrenaline was observed during the FST and no modification of the neurotransmitter concentrations was observed during the TST. Following an FST of 2-min duration, a hypolocomotor effect was observed in the subsequent actimeter test. The same effect was observed after a TST of 8 min and onwards. This study confirms the fact that although these two tests are used to study depression, they involve different neuronal mechanisms.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Antidepressant-like effects in various mice strains in the forced swimming test

RATIONALE

Strain differences in mice have been reported in response to drugs in the mouse forced swimming test (FST), even if few antidepressants were examined.

OBJECTIVES

The aim of the present study was to investigate the influence of genetic factors, using five antidepressants (imipramine, desipramine, citalopram, paroxetine and bupropion) in the mouse FST, in outbred strains (Swiss, NMRI) and inbred strains (DBA/2, C57BL/6J Rj). Moreover, whole brain levels of dopamine (DA), noradrenaline (NA), serotonin (5-HT) in vehicle treated animals, which were or were not subjected to the FST, were measured by HPLC analysis in an attempt to explain behavioural differences.

METHODS

For each antidepressant, a dose range (1-16 mg/kg) was tested in the locomotor apparatus and only non-psychostimulant doses were then tested in the FST in order to detect antidepressant-like activity.

RESULTS

No baseline differences among Swiss, NMRI, DBA/2 and C57BL/6J Rj strains were observed in our experiments, allowing the comparison of different antidepressants in each strain. Imipramine (16 mg/kg), desipramine, citalopram (4-16 mg/kg) and paroxetine (8 and 16 mg/kg) treatment decreased the immobility time in the Swiss strain and the size of the effect reached more than 20% for each of these antidepressants. C57BL/6J Rj was the only strain sensitive to bupropion (2 and 4 mg/kg). In the NMRI strain, only paroxetine treatment decreased the immobility time (16 mg/kg).

CONCLUSION

Our study showed that drug sensitivity is genotype dependent. FST results have shown that Swiss mice are the most sensitive strain to detect 5-HT and/or NA treatment. The use of DBA/2 inbred mice may be limited, as an absence of antidepressant-like response was observed in the FST. The lack of sensitivity to antidepressant treatment in DBA/2 strains could be due to high DA, NA and 5-HT whole brain concentrations.

The Vogel conflict test: procedural aspects, gamma-aminobutyric acid, glutamate and monoamines

A multitude of mechanisms are involved in the control of emotion and in the response to stress. These incorporate mediators/targets as diverse as gamma-aminobutyric acid (GABA), excitatory amino acids, monoamines, hormones, neurotrophins and various neuropeptides. Behavioural models are indispensable for characterization of the neuronal substrates underlying their implication in the etiology of anxiety, and of their potential therapeutic pertinence to its management. Of considerable significance in this regard are conflict paradigms in which the influence of drugs upon conditioned (trained) behaviours is examined. For example, the Vogel conflict test, which was introduced some 30 years ago, measures the ability of drugs to release the drinking behaviour of water-deprived rats exposed to a mild aversive stimulus ("punishment"). This model, of which numerous procedural variants are discussed herein, has been widely used in the evaluation of potential anxiolytic agents. In particular, it has been exploited in the characterization of drugs interacting with GABAergic, glutamatergic and monoaminergic networks, the actions of which in the Vogel conflict test are summarized in this article. More recently, the effects of drugs acting at neuropeptide receptors have been examined with this model. It is concluded that the Vogel conflict test is of considerable utility for rapid exploration of the actions of anxiolytic (and anxiogenic) drugs. Indeed, in view of its clinical relevance, broader exploitation of the Vogel conflict test in the identification of novel classes of anxiolytic agents, and in the determination of their mechanisms of action, would prove instructive.

Extracellular dopamine in the rat prefrontal cortex during reward-, punishment- and novelty-associated behaviour. Effects of diazepam

Variations of extracellular dopamine (DA(ext)) levels in prefrontal cortex were assessed by in vivo microdialysis. In rats trained in an operant fixed interval (FI(30s)) schedule of food delivery, acute exposure to contingent foot shocks resulted in a suppression of responding that was reversed by diazepam (4 mg/kg, ip). No changes in cortical DA(ext) levels occurred during this period in both control and treated rats. By contrast, in control rats, cortical DA(ext) levels increased (+25-40%) during the nonpunished component of the operant session, and during noncontingent food delivery (+25%). Control rats placed into an unfamiliar brightly lit openfield exhibited a marked increase in cortical DA(ext) levels (+100%). This effect occurred neither in rats given diazepam at a dose (2 mg/kg) which stimulated motor activity, nor during a second exposure to the openfield. In conclusion, a benzodiazepine-sensitive activation of mesoprefrontal DA neurones is induced by exposure to novel stressful surroundings and by food availability and consumption. The fact that cortical DA(ext) levels remained unchanged in rats that exerted complete control upon negative stimuli indicates that an activation of the mesoprefrontal DA system is not required for punishment-induced behavioural blockade.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

FMRI visualization of brain activity during a monetary incentive delay task

Comparative studies have implicated striatal and mesial forebrain circuitry in the generation of autonomic, endocrine, and behavioral responses for incentives. Using blood oxygen level-dependent functional magnetic resonance imaging, we sought to visualize functional activation of these regions in 12 normal volunteers as they anticipated and responded for monetary incentives. Both individual and group analyses of time-series data revealed significant activation of striatal and mesial forebrain structures (including insula, caudate, putamen, and mesial prefrontal cortex) during trials involving both monetary rewards and punishments. In addition to these areas, during trials involving punishment, group analysis revealed activation foci in the anterior cingulate and thalamus. These results corroborate comparative studies which implicate striatal and mesial forebrain circuitry in the elaboration of incentive-driven behavior. This report also introduces a new paradigm for probing the functional integrity of this circuitry in humans.

Animal models of acute drug-induced akathisia - a review

Akathisia is a complex neurobehavioural side effect of neuroleptics and some other drugs which is characterised by subjective report and objective manifestations of restlessness. Its pathophysiology is poorly understood and there are many limitations to its investigation in humans. This paper reviews the various attempts that have been made in modelling acute akathisia in animals. Homologous as well as isomorphic models have been attempted, but most models are partial as they reproduce either the subjective or the objective features of the syndrome. None of the available models has been fully validated. Neuroleptic-induced defecation in the rat, even though constrained by a lack of symptom similarity and thereby face validity, has been most studied as a model of subjective akathisia. Rat models of restlessness, in particular those involving the use of serotonergic drugs or lesions of the ventral tegmentum or medial prefrontal cortex, are interesting partial models that should be further investigated. Neuroleptic-induced akathisia is observed in primates and has been modelled in dogs, and these should be studied further for their validation. It is also necessary to consider the subtypes of akathisia in the attempts to develop these models.

Engagement in a non-escape (displacement) behavior elicits a selective and lateralized suppression of frontal cortical dopaminergic utilization in stress

Although the preferential activation of the prefrontal cortical (PFC) dopaminergic system is generally observed in stress, limited exceptions to this have been observed. Certain non-escape behaviors have been demonstrated to attenuate physiological indices of stress (e.g., coping or displacement responses). One well-characterized non-escape behavior observed in stress is chewing, or gnawing, of inedible objects. Engagement in this behavior attenuates stress-related activation of the hypothalamopituitary-adrenal axis, in a variety of species. We examined the degree to which engagement in this non-escape behavior modulates stressor-induced activation of the PFC dopamine (DA) system. Rats and mice were exposed to a brightly lit novel environment (novelty stress) in the presence or absence of inedible objects. Following novelty exposure, various dopaminergic terminal fields were collected and dopamine and its major catabolite, DOPAC, were measured using HPLC with electrochemical detection. DOPAC/DA ratios were calculated as an index of DA utilization. In some cases serotonin (5-HT) and its major catabolite, 5-HIAA, were also measured. In animals that did not chew, novelty exposure elicited significant increases in DOPAC/DA levels within PFC, nucleus accumbens (shell and core subdivisions), and striatum (relative to quiet-controls). DOPAC/DA responses were greater in the right PFC than in the left PFC. Animals that chewed displayed significantly lower DOPAC/DA responses in PFC, but not other dopaminergic terminal fields. This effect of chewing was always observed in the right PFC and less consistently in the left PFC. Chewing did not alter novelty-induced increases in PFC 5-HIAA/5-HT responses. Thus, engagement in this non-escape behavior elicits a neuroanatomically and neurochemically specific attenuation of the PFC DA response in stress. Given the pivotal role of the PFC in certain cognitive and affective processes, behavioral regulation of PFC DA utilization may modulate cognitive and/or affective function in stress.

Dissociations in dopamine release in medial prefrontal cortex and ventral striatum during the acquisition and extinction of classical aversive conditioning in the rat

Dual perfusion in vivo brain microdialysis was used to monitor extracellular levels of dopamine in the medial prefrontal cortex and ventral striatum during the acquisition and extinction of a classical aversive conditioning paradigm in rats. The main finding was a dissociation in the pattern of release in the two brain areas. The first stimulus-footshock pairing elicited large increases in cortical dopamine over baseline levels that were much greater than the increases elicited by different stimuli of equivalent salience that were unpaired with footshock. In contrast, dopamine levels in ventral striatum were unchanged under these conditions. Over the next two pairings, there was a decline in the cortical response and an increase in the response in ventral striatum. The first presentation of the aversive conditioned stimulus in a separate context elicited the largest response in ventral striatum. Post-conditioning, the cortical response to the conditioned stimulus was smaller than that elicited by the initial stimulus-footshock pairing and was equivalent in magnitude to that elicited by stimuli unpaired with footshock. Over the final two conditioned stimuli presentations, in the absence of the footshock reinforcer (extinction), responses declined in both brain areas. Simultaneous monitoring of behaviour indicated that the neurochemical events were accompanied by effective aversive learning, as indexed by conditioned freezing responses. The data are discussed in terms of the hypothesis that medial prefrontal cortex is especially engaged during novel circumstances which may, potentially, require new learning, whilst ventral striatal dopamine more closely follows the expression of conditioned responding during learning and extinction.

The nature of interactions involving prefrontal and striatal dopamine systems

A number of converging lines of evidence from work in rodents suggest that dopamine (DA) function in the prefrontal cortex (PFC) and striatal terminal fields may be linked, possibly in an 'inverse' manner, whereby a change in prefrontal dopamine transmission in one direction occasions an opposite change in dopamine function in striatal territories. The present article considers the possible functional importance of this concept in the light of recent neuroanatomical data and new data from our own laboratory indicating that, at the neurochemical level, the basic finding of an inverse relationship between dopamine function in prefrontal and striatal regions also holds good in the non-human primate. The main conclusion is that the simple idea of an inverse relationship between prefrontal and striatal dopamine systems emphasizing presynaptic release mechanisms is unlikely to underlie, solely, the full repertoire of functional interactions. Whilst there is evidence consistent with dynamic interactions between prefrontal and striatal dopamine release under some circumstances, specifically, during the early phases of aversive learning, a complete account of possible interactions between prefrontal and striatal dopamine systems requires consideration of additional factors. Such factors include: (1) the precise nature of the psychological function investigated, (2) the possibility of acute, localized changes in striatal postsynaptic function secondary to changes in presynaptic function and (3) the possibility of manipulations of prefrontal cortex leading to adaptive changes in striatal function, at a diffuse, neural systems level.

Rapid sampling of extracellular dopamine in the rat prefrontal cortex during food consumption, handling and exposure to novelty

We report the effects of physiological stimuli on extracellular dopamine (DA) in the medial prefrontal cortex (PFC) of the rat determined on-line in dialysates obtained every 5.5 min. The detection limit for DA was 0.03-0.1 pg/5 microl injection using a conventional HPLC set-up. Basal levels in PFC were at the detection limit, therefore 3 microM nomifensine was included in the Ringer perfusion fluid, producing readily detectable DA levels of 0.9 pg/injection. Perfusion with 3 microM TTX for 30 min decreased DA within 11 min to 10% of control. The routine use of rapid sampling of extracellular DA was applied to study cortical DA release in relation to behaviour. Exposure to a novel environment for 5.5 min led to an increase to 135%. Presentation of a food pellet to food-deprived rats resulted in a rapid increase to 150% within 5.5 min, which lasted 30-40 min, which is 10-20 min more than the time spent eating. Handling the rat for 5.5 min increased DA in PFC within 5.5 min to 160% and in 11 min to 190% of control followed by a 25-min period of a 50% increase, probably reflecting increased arousal. The results suggest that emotional arousal is a common denominator of increased cortical DA release and that responses are graded depending on the intensity of the stimulus.

Deficient sensorimotor gating after 6-hydroxydopamine lesion of the rat medial prefrontal cortex is reversed by haloperidol

The present study sought to test the hypothesis that dopamine in the prefrontal cortex exerts an inhibitory influence on subcortical dopamsine systems and that depletion of prefrontal dopamine may affect behaviour via an increase in dopamine release in the basal ganglia. We used prepulse inhibition of the acoustic startle response, i.e. the inhibition of the acoustic startle response by a preceding non-startling stimulus, as the behavioural test, because this phenomenon of sensorimotor gating is modified in opposite directions by dopamine in the prefrontal cortex and in the basal ganglia. Rats were tested for prepulse inhibition before and after injections of the neurotoxin 6-hydroxydopamine into the medial prefrontal cortex. We attempted to differentiate the contributions of prefrontal dopamine and noradrenaline by pretreating the animals with desipramine (6-OHDAMI rats) or bupropion (6-OHDABUP rats), selective inhibitors of noradrenaline and dopamine reuptake respectively. 6-Hydroxydopamine lesion reduced prefrontal dopamine by 90% and noradrenaline by 80% in 6-OHDADMI rats, while prefrontal dopamine was reduced by 54% and noradrenaline by 95% in 6-OHDABUP rats. The ability of an acoustic prepulse (75 dB, 10 kHz) to inhibit the response to a startle pulse (100 dB noise burst) was maintained in sham-lesioned rats and in 6-OHDABUP rats. However, there was a marked reduction of prepulse inhibition (by 26%) in the 6-OHDADMI rats. Systemic administration of the dopamine antagonist haloperidol (0.05 mg/kg), which did not affect prepulse inhibition in sham-lesioned and in 6-OHDABUP rats, antagonized the lesion-induced deficit in prepulse inhibition in 6-OHDADMI rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dopamine depletions in the medial prefrontal cortex on active avoidance and escape in the rat

Dopamine systems have been implicated in the performance of avoidance behavior, and the dopaminergic innervation of medial prefrontal cortex is known to be responsive to stressful stimuli. In the present investigation, injections of 6-hydroxydopamine were used to produce moderate depletions of dopamine in the medial prefrontal cortex of rats trained to perform an active avoidance/escape task. In this task, 0.5 mA shock was presented for 5 s every 30 s, and the rat could escape shock presentation, or avoid the shock for 30 s, by pressing a lever. Depletion of dopamine in the medial prefrontal cortex did not affect total number of responses, and did not impair avoidance responding (i.e. responding when the shock was off), and in fact dopamine-depleted animals tended to make slightly more avoidance responses than control animals. Prefrontal dopamine depletions did result in a significant decrease in the number of escape responses (i.e. responding to terminate shock when the shock was on). Moreover, dopamine depletions significantly decreased response efficiency, which is an index of the reduction of shock time produced per lever pressing response. Previous work has indicated that dopamine antagonists and accumbens dopamine depletions have dramatic effects on avoidance behavior; thus, the present results indicate that prefrontal cortex dopamine depletions do not mimic the effects of interference with subcortical dopamine systems. The selective effects of dopamine depletions on escape behavior in the present study suggest that rats with medial prefrontal dopamine depletions have an impairment in the ability to respond appropriately to the direct presentation of footshock.(ABSTRACT TRUNCATED AT 250 WORDS)

Does learned helplessness induction by haloperidol involve serotonin mediation?

Learned helplessness (LH) is a behavioral depression following inescapable stress. Helpless behavior was induced in naive rats by the dopamine D2 receptor blocker haloperidol (HDL) in a dose-dependent manner, with the greatest effects seen at 20 mg/kg (IP). Rats were tested 24 h after injection. Haloperidol (IP) increased release of serotonin (5-HT) in medial prefrontal cortex (MPC) as measured by in vivo microdialysis. Perfusion of HDL through the probe in MPC caused increased cortical 5-HT release, as did perfusion of both dopamine and the dopamine agonist apomorphine. Our previous work found that increased 5-HT release in MPC correlates with the development of LH. The present work suggests that increased DA release in MPC, known to occur with both inescapable stress and with HDL, may play a necessary but not sufficient role in the development of LH. Also, this suggests that increased DA activity in MPC leads to increased 5-HT release in MPC and to subsequent behavioral depression.

Effects of dopamine depletions in the medial prefrontal cortex on DRL performance and motor activity in the rat

Two experiments assessed the behavioral effects of dopamine depletions in the medial prefrontal cortex that were produced by injection of the neurotoxic agent 6-hydroxydopamine. In the first experiment, rats were trained to respond on a differential reinforcement of low rates of responding-30 second (DRL30) schedule. On this schedule, rats were only reinforced if they withheld responding for 30 s. Rats with prefrontal dopamine depletions were found to be impaired in the DRL task. These animals responded more than controls, received fewer reinforcers, and were less efficient in their responses. Moreover, an analysis of interresponse times (IRTs) revealed that rats with medial prefrontal dopamine depletions made more responses with short-duration IRTs, and fewer responses with long-duration IRTs. In the second experiment, rats were tested on open field activity, amphetamine-induced locomotor activity and stereotypy. No increase in spontaneous locomotor activity was found following surgery; however, increases in amphetamine-induced locomotor activity and stereotypy were observed. These results are consistent with hypothesized role for the prefrontal cortex in behavioral inhibition, and indicates that prefrontal cortical dopamine is an integral part of the system.

6-Hydroxydopamine lesions of the medial prefrontal cortex of rats do not affect dopamine metabolism in the basal ganglia at short and long postsurgical intervals

Dopamine (DA) in the medial prefrontal cortex (mPFC) has been implicated in the regulation of subcortical DA function. To further characterize the potential interaction between cortical and subcortical DA systems, the short- and long-term neurochemical consequences of 6-hydroxydopamine (6-OHDA) lesions of the mPFC of rats were investigated in the mPFC and in its subcortical target structures. 4 to 5, 10 to 12 and 32 to 36 days after infusion of 6-OHDA, DA was depleted to a larger extent than noradrenaline and serotonin. No lesion-induced changes of DA and its metabolites were detected in subcortical structures. These results show that prefrontal 6-OHDA lesions produce immediate and long lasting depletions of prefrontal monoamines, especially of DA, without increasing basal DA metabolism in the striatum and nucleus accumbens.

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)

Lateralized changes in prefrontal cortical dopamine activity induced by controllable and uncontrollable stress in the rat

Exposure to stressors that are not controlled results in a variety of changes in behavior and in brain chemistry. Among these is the activation of dopamine-containing neuronal systems projecting to the medial prefrontal cortex (PFC), to a lesser extent the nucleus accumbens (NAC) and, in a few studies, the striatum. Previous data have shown that stressor evoked PFC activation is asymmetrical. The present experiments were designed to assess the effects of controlled and uncontrolled stressors on these DA systems using the procedures of the learned helplessness (LH) model. In a first experiment, 80 trials of either a controllable (ESC) or identical uncontrollable footshock stressor (YOK) caused an activation, as indicated by increased metabolite concentrations of DA in the PFC, NAC and striatum. In the PFC, YOK caused a bilateral DA depletion, relative to ESC and control animals, and a right > left increase in DOPAC/DA which was not seen in ESC animals. These findings suggested a preferential effect of YOK in the right PFC. A second experiment used rats that had been grouped according to their turning behavior, YOK right-biased rats showed an increase in DOPAC on the right side of the PFC and YOK left-biased rats displayed a similar increase on the left side in response to a brief (5 min) controllable footshock stressor. Since right-turning rats had been shown to be more sensitive to a LH behavioral phenomenon, the data suggested that right PFC activation is responsible for the greater LH sensitivity. A final experiment evaluated the neurochemical and behavioral responses to a prolonged footshock stressor 24 h after uncontrolled footshock. Right-biased YOK animals displayed depressed footshock escape behavior and a right > left depletion in PFC DA and HVA. Across-groups footshock escape performance was correlated with DA and HVA concentrations on the right but not on the left side of the PFC. Thus a disturbance of right PFC DA utilization was again associated with compromised coping behavior. The data suggest that the inability to control a stressor causes a lateralized alteration of PFC DA and this results in a disruption of the ability to respond to a new stressor. These findings indicate that the two sides of the PFC are differentially specialized for responding to a stressor.

Effects of postnatal stress on dopamine mesolimbic system responses to aversive experiences in adult life

The effects of postnatal stress on mesolimbic dopamine (DA) functioning in 90-day-old mice were investigated. Postnatal stress consisted of 15 min daily exposure to clean bedding (CB) in the absence of the mother for the first two weeks of life. Controls were daily exposed to home cage bedding (HCB) in the absence of the mother. A single brief (5-10 min) exposure to restraint produced a clear-cut increase in DA metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT)) in the nucleus accumbens septi (NAS) of adult HCB but not CB mice. Moreover, when tested in an elevated plus maze, CB mice showed more exploration and reduced fearfulness in comparison with HCB mice. Taken together, these results indicate reduced emotional reactivity in adult mice repeatedly stressed during postnatal development. Moreover, HCB mice but not CB mice showed altered behavioral responsiveness to apomorphine following repeated restraint stress (10 daily 120 min) in adult life, although no difference in the behavioral response to either a low or a high dose of apomorphine was observed in adult unstressed mice of the CB and HCB groups. These results indicate that the effects of early experiences on brain DA functioning may not be evident in basal conditions and be revealed only under environmental pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of the slow acquisition of medical and sulcal prefrontal cortex self-stimulation: an individual differences approach

Stimulation-naive rats were tested for motor activity during noncontingent electrical stimulation of the medial prefrontal cortex (MPC) or sulcal prefrontal cortex (SPC). Defecation during stimulation was also measured. The rats were then tested using a conditioned taste aversion paradigm for aversion to a novel flavor (0.1% saccharin) paired with stimulation. Finally, the rats were trained to acquire self-stimulation over 26 days of training. Large individual differences were seen in motor activity, defecation, and conditioned taste aversion to initial stimulation and in the subsequent speed of self-stimulation acquisition. In the MPC-stimulated group, acquisition speed was positively correlated with motor activity to initial stimulation and negatively correlated with defecation to this stimulation. In the SPC-stimulated group, the same correlations were evident, but only when rats suffering seizures prior to self-stimulation acquisition were excluded from the analysis. Such preacquisition seizures, which were only found in the SPC-stimulated group, retarded self-stimulation acquisition. In most rats, MPC or SPC stimulation failed to condition a taste aversion to saccharin. These results suggest that the slow acquisition of MPC and SPC self-stimulation may be partly related to the motor suppressive, aversive, and convulsive properties of initial stimulation.

Contrasting effects of stress on medial and sulcal prefrontal cortex self-stimulation

Male Wistar rats were subjected to either 25 controllable or uncontrollable footshocks and then tested for changes in fixed-interval 5-second (FI-5) self-stimulation of the medial prefrontal cortex (MPC), sulcal prefrontal cortex (SPC) or nucleus accumbens (NAS). Controllable footshock caused a moderate facilitation of MPC self-stimulation (30% above baseline rates) but inhibited SPC self-stimulation (32% below baseline rates). Uncontrollable footshock had no effect on MPC self-stimulation but inhibited SPC self-stimulation (52% below baseline rates). An inhibition of SPC self-stimulation was also evident 24 hours following controllable or uncontrollable footshock. NAS self-stimulation was unaffected by footshock. Changes in locomotor activity were not consistently related to changes in self-stimulation following footshock. These results are discussed in terms of the different effects of mild stress on the release of reward-relevant neurotransmitters in the MPC, SPC and NAS. The possible role of stress-induced hypoalgesia in determining the stress-induced facilitation of MPC self-stimulation is also discussed.

The acquisition of self-stimulation of the medical prefrontal cortex following exposure to escapable or inescapable footshock

The effect of acute stress on the acquisition of an instrumental action reinforced by electrical stimulation of the medial prefrontal cortex (MPC) was investigated by exposing rats to either escapable, inescapable or no footshock prior to daily self-stimulation training sessions. Treatment with inescapable footshock did not affect the number of sessions required for acquisition of MPC self-stimulation but did increase the rate of responding over acquisition sessions compared with the no-shock group. When the treatment footshock was escapable, however, both a facilitation in acquisition, as indexed by a reduction in the number of sessions to criterion, and an increase in the rate of MPC self-stimulation was found. These data were interpreted as offering evidence for the operation of a dopaminergic mechanism in the acquisition of MPC self-stimulation. Further, they indicate, contrary to the reported effects of footshock on self-stimulation of other brain areas, that exposure to acute stress has a facilitatory effect on the rate of self stimulation of the MPC.