Effects of discrimination learning on the rat amygdala dopamine release: a microdialysis study

Sex differences in sensitivity to dopamine receptor manipulations of risk-based decision making in rats

Risky decision making involves the ability to weigh risks and rewards associated with different options to make adaptive choices. Previous work has established a necessary role for the basolateral amygdala (BLA) in mediating effective decision making under risk of punishment, but the mechanisms by which the BLA mediates this process are less clear. Because this form of decision making is profoundly sensitive to dopaminergic (DA) manipulations, we hypothesized that DA receptors in the BLA may be involved in risk-taking behavior. To test this hypothesis, male and female Long-Evans rats were trained in a decision-making task in which rats chose between a small, safe food reward and a larger food reward that was associated with a variable risk of footshock punishment. Once behavioral stability emerged, rats received intra-BLA infusions of ligands targeting distinct dopamine receptor subtypes prior to behavioral testing. Intra-BLA infusions of the dopamine D2 receptor (D2R) agonist quinpirole decreased risk taking in females at all doses, and this reduction in risk taking was accompanied by an increase in sensitivity to punishment. In males, decreased risk taking was only observed at the highest dose of quinpirole. In contrast, intra-BLA manipulations of dopamine D1 or D3 receptors (D1R and D3R, respectively) had no effect on risk taking. Considered together, these data suggest that differential D2R sensitivity in the BLA may contribute to the well-established sex differences in risk taking. Neither D1Rs nor D3Rs, however, appear to contribute to risky decision making in either sex.

The amygdala mediates the facilitating influence of emotions on memory through multiple interacting mechanisms

Emotionally arousing experiences are better remembered than neutral ones, highlighting that memory consolidation differentially promotes retention of experiences depending on their survival value. This paper reviews evidence indicating that the basolateral amygdala (BLA) mediates the facilitating influence of emotions on memory through multiple mechanisms. Emotionally arousing events, in part by triggering the release of stress hormones, cause a long-lasting enhancement in the firing rate and synchrony of BLA neurons. BLA oscillations, particularly gamma, play an important role in synchronizing the activity of BLA neurons. In addition, BLA synapses are endowed with a unique property, an elevated post-synaptic expression of NMDA receptors. As a result, the synchronized gamma-related recruitment of BLA neurons facilitates synaptic plasticity at other inputs converging on the same target neurons. Given that emotional experiences are spontaneously remembered during wake and sleep, and that REM sleep is favorable to the consolidation of emotional memories, we propose a synthesis for the various lines of evidence mentioned above: gamma-related synchronized firing of BLA cells potentiates synapses between cortical neurons that were recruited during an emotional experience, either by tagging these cells for subsequent reactivation or by enhancing the effects of reactivation itself.

Projection-Target-Defined Effects of Orexin and Dynorphin on VTA Dopamine Neurons

Circuit-specific signaling of ventral tegmental area (VTA) dopamine neurons drives different aspects of motivated behavior, but the neuromodulatory control of these circuits is unclear. We tested the actions of co-expressed lateral hypothalamic peptides, orexin A (oxA) and dynorphin (dyn), on projection-target-defined dopamine neurons in mice. We determined that VTA dopamine neurons that project to the nucleus accumbens lateral shell (lAcbSh), medial shell (mAcbSh), and basolateral amygdala (BLA) are largely non-overlapping cell populations with different electrophysiological properties. Moreover, the neuromodulatory effects of oxA and dyn on these three projections differed. OxA selectively increased firing in lAcbSh- and mAcbSh-projecting dopamine neurons. Dyn decreased firing in the majority of mAcbSh- and BLA-projecting dopamine neurons but reduced firing only in a small fraction of those that project to the lAcbSh. In conclusion, the oxA-dyn input to the VTA may drive reward-seeking behavior by tuning dopaminergic output in a projection-target-dependent manner.

Basolateral amygdala and stress-induced hyperexcitability affect motivated behaviors and addiction

The amygdala integrates and processes incoming information pertinent to reward and to emotions such as fear and anxiety that promote survival by warning of potential danger. Basolateral amygdala (BLA) communicates bi-directionally with brain regions affecting cognition, motivation and stress responses including prefrontal cortex, hippocampus, nucleus accumbens and hindbrain regions that trigger norepinephrine-mediated stress responses. Disruption of intrinsic amygdala and BLA regulatory neurocircuits is often caused by dysfunctional neuroplasticity frequently due to molecular alterations in local GABAergic circuits and principal glutamatergic output neurons. Changes in local regulation of BLA excitability underlie behavioral disturbances characteristic of disorders including post-traumatic stress syndrome (PTSD), autism, attention-deficit hyperactivity disorder (ADHD) and stress-induced relapse to drug use. In this Review, we discuss molecular mechanisms and neural circuits that regulate physiological and stress-induced dysfunction of BLA/amygdala and its principal output neurons. We consider effects of stress on motivated behaviors that depend on BLA; these include drug taking and drug seeking, with emphasis on nicotine-dependent behaviors. Throughout, we take a translational approach by integrating decades of addiction research on animal models and human trials. We show that changes in BLA function identified in animal addiction models illuminate human brain imaging and behavioral studies by more precisely delineating BLA mechanisms. In summary, BLA is required to promote responding for natural reward and respond to second-order drug-conditioned cues; reinstate cue-dependent drug seeking; express stress-enhanced reacquisition of nicotine intake; and drive anxiety and fear. Converging evidence indicates that chronic stress causes BLA principal output neurons to become hyperexcitable.

Input-specific contributions to valence processing in the amygdala

Reward and punishment are often thought of as opposing processes: rewards and the environmental cues that predict them elicit approach and consummatory behaviors, while punishments drive aversion and avoidance behaviors. This framework suggests that there may be segregated brain circuits for these valenced behaviors. The basolateral amygdala (BLA) is one brain region that contributes to both types of motivated behavior. Individual neurons in the BLA can favor positive over negative valence, or vice versa, but these neurons are intermingled, showing no anatomical segregation. The amygdala receives inputs from many brain areas and current theories posit that encoding of positive versus negative valence by BLA neurons is determined by the wiring of each neuron. Specifically, many projections from other brain areas that respond to positive and negative valence stimuli and predictive cues project strongly to the BLA and likely contribute to valence processing within the BLA. Here we review three of these areas, the basal forebrain, the dorsal raphe nucleus and the ventral tegmental area, and discuss how these may promote encoding of positive and negative valence within the BLA.

Modulation of risk/reward decision making by dopaminergic transmission within the basolateral amygdala

RATIONALE

Dopamine (DA) transmission within cortico-limbic-striatal circuitry is integral in modulating decisions involving reward uncertainty. The basolateral amygdala (BLA) also plays a role in these processes, yet how DA transmission within this nucleus regulates cost/benefit decision making is unknown.

OBJECTIVES

We investigated the contribution of DA transmission within the BLA to risk/reward decision making assessed with a probabilistic discounting task.

METHODS

Rats were well-trained to choose between a small/certain reward and a large/risky reward, with the probability of obtaining the larger reward decreasing (100-12.5 %) or increasing (12.5-100 %) over a session. We examined the effects of antagonizing BLA D1 (SCH 23390, 0.1-1 μg) or D2 (eticlopride, 0.1-1 μg) receptors, as well as intra-BLA infusions of agonists for D1 (SKF 81297, 0.1-1 μg) and D2 (quinpirole, 1-10 μg) receptors. We also assessed how DA receptor stimulation may induce differential effects related to baseline levels of risky choice.

RESULTS

BLA D1 receptor antagonism reduced risky choice by decreasing reward sensitivity, whereas D2 antagonism did not affect overall choice patterns. Stimulation of BLA D1 receptors optimized decision making in a baseline-dependent manner: in risk-averse rats, infusions of a lower dose of SKF81297 increased risky choice when reward probabilities were high (50 %), whereas in risk-prone rats, this drug reduced risky choice when probabilities were low (12.5 %). Quinpirole reduced risky choice in risk-prone rats, enhancing lose-shift behavior.

CONCLUSIONS

These data highlight previously uncharacterized roles for BLA DA D1 and D2 receptors in biasing choice during risk/reward decision making through mediation of reward/negative feedback sensitivity.

18-Methoxycoronaridine blocks context-induced reinstatement following cocaine self-administration in rats

Numerous studies utilizing drug self-administration have shown the importance of conditioned cues in maintaining and reinstating addictive behaviors. However, most used simple cues that fail to replicate the complexity of cues present in human craving and addiction. We have recently shown that music can induce behavioral and neurochemical changes in rats following classical conditioning with psychostimulants. However, such effects have yet to be characterized utilizing operant self-administration procedures, particularly with regard to craving and relapse. The goal of the present study was to validate the effectiveness of music as a contextual conditioned stimulus using cocaine in an operant reinstatement model of relapse. Rats were trained to lever press for cocaine with a musical cue, and were subsequently tested during reinstatement sessions to determine how musical conditioning affected drug seeking behavior. Additionally, in vivo microdialysis was used to determine basolateral amygdala involvement during reinstatement. Lastly, tests were conducted to determine whether the putative anti-addictive agent 18-methoxycoronaridine (18-MC) could attenuate cue-induced drug seeking behavior. Our results show that music-conditioned animals exhibited increased drug seeking behaviors when compared to controls during reinstatement test sessions. Furthermore, music-conditioned subjects exhibited increased extracellular dopamine in the basolateral amygdala during reinstatement sessions. Perhaps most importantly, 18-MC blocked musical cue-induced reinstatement. Thus,music can be a powerful contextual conditioned cue in rats, capable of inducing changes in both brain neurochemistry and drug seeking behavior during abstinence. The fact that 18-MC blocked cue-induced reinstatement suggests that α3β4 nicotinic receptors may be involved in the mechanism of craving, and that 18-MC may help prevent relapse to drug addiction in humans.

Cerebral antioxidant enzyme increase associated with learning deficit in type 2 diabetes rats

In this study, we examined alterations in the enzymatic antioxidant defenses associated with learning deficits induced by type 2 diabetes, and studied the effects of the peroxisome proliferator-activated receptor γ agonist pioglitazone on these learning deficits. Learning ability was assessed by visual discrimination tasks in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, as a model of spontaneous type 2 diabetes. Levels of the antioxidant enzymes glutathione peroxidase (GPx), Cu(2+)-Zn(2+) superoxide dismutase (CuZn-SOD) and manganese SOD were measured in the cortex, hippocampus and striatum. Half the rats received oral pioglitazone (20mg/kg/day) from the early stage of diabetes (22 weeks old) to 27 weeks old. OLETF rats showed learning deficits compared with control, Long-Evans Tokushima Otsuka (LETO) rats. GPx levels in the cortex and hippocampus were increased in OLETF rats compared with LETO rats, with an inverse correlation between GPx in the hippocampus and learning score. CuZn-SOD levels were also increased in the hippocampus in OLETF rats. Pioglitazone reduced blood glucose and increased serum adiponectin levels, but had no effect on learning tasks or antioxidant enzymes, except for CuZn-SOD. These results suggest that an oxidative imbalance reflected by increased brain antioxidant enzymes plays an important role in the development of learning deficits in type 2 diabetes. Early pioglitazone administration partly ameliorated diabetic symptoms, but was unable to completely recover cerebral oxidative imbalance and functions. These results suggest that diabetes-induced brain impairment, which results in learning deficits, may have occurred before the appearance of the symptoms of overt diabetes.

Dopamine D1 receptors within the basolateral amygdala mediate heroin-induced conditioned immunomodulation

This study investigates the role of basolateral amygdala (BLA) dopamine in heroin-induced conditioned immunomodulation. Animals underwent conditioning in which heroin administration was repeatedly paired with placement into a conditioning chamber. Six days after the final conditioning session animals were returned to the chamber and received intra-BLA microinfusions of dopamine, D(1) or D(2), antagonist. Antagonism of D(1), but not D(2), receptors within the BLA blocked the suppressive effect of heroin-associated environmental stimuli on iNOS, TNF-α and IL-1β. This study is the first to demonstrate that the expression of heroin's conditioned effects on proinflammatory mediators require dopamine D(1) receptors within the BLA.

How conditioned stimuli acquire the ability to activate VTA dopamine cells: a proposed neurobiological component of reward-related learning

The ability to learn about conditioned stimuli (CS) associated with rewards is a crucial adaptive mechanism. Activity in the mesocorticolimbic dopamine (DA) system, as well as in the ventral tegmental area (VTA), is correlated with responding to and learning about CSs. The mechanism by which VTA neurons become activated by signals associated with conditioned stimuli is not fully understood. Our model suggests that NMDA receptor stimulation in the VTA allows originally weak glutamate signals carrying information about environmental stimuli, coincident with strong excitation correlated with primary rewards, to be strengthened and thereby acquire the ability to activate VTA neurons in themselves, producing approach. Furthermore, once synaptic strengthening occurs, the model suggests that NMDA receptor stimulation in VTA is not necessary for the expression of reward-related learning. In this review we survey evidence that VTA cells respond to cues associated with primary rewards, that this responding is acquired, and that the VTA possesses the attributes to function as a site of integration of signals of primary and conditioned stimuli.

Intranasal administration of progesterone increases dopaminergic activity in amygdala and neostriatum of male rats

We evaluated the effects of intranasal administration of progesterone (PROG) on the activity of dopaminergic neurons in the brain of anesthetized rats by means of microdialysis. Male Wistar rats were implanted with guide cannulae in the basolateral amygdala and neostriatum. Three to 5 days later, they were anesthetized with urethane, and dialysis probes were inserted. After a stabilization period of 2 h, four 30-min samples were collected. Thereafter, the treatment (0.5, 1.0 or 2.0 mg/kg of PROG dissolved in a viscous castor oil mixture, or vehicle) was applied into the nose in a volume of 10 microl (5 microl in each nostril). In other animals, an s.c. injection of PROG (1.0, 2.0 or 4.0 mg/kg) or vehicle was given. Samples of both application ways were collected at 30-min interval for 4 h after the treatment and immediately analyzed with high performance liquid chromatography and electrochemical detection. Intranasal administration of 2 mg/kg of PROG led to an immediate (within 30 min after the treatment) significant increase in the basolateral amygdala dopamine levels. In the neostriatum, the 2 mg/kg dose led to a delayed significant increase in dopamine. S.c. administration of 4 mg/kg of PROG was followed by a delayed significant increase in dopamine, both, in the basolateral amygdala and neostriatum, but smaller in magnitude in comparison to the intranasal treatment. This is the first study to demonstrate dopamine-enhancing effects of PROG, not only in the neostriatum, but also in the basolateral amygdala. Our results indicate that the intranasal route of administration of PROG is a more efficacious way for targeting the brain than the s.c. route.

Specific timing of taurine supplementation affects learning ability in mice

The effects of taurine supplementation on visual discrimination in mice were examined. Taurine, 2-aminoethane-sulphonic acid, found in high concentrations in the central nervous system of mammals and in human milk, has been shown to be essential for development. Male mice were divided into four groups according to taurine supplementation periods. 1) Lifelong: taurine (400 mg/kg/day) was dissolved in distilled water and provided as drinking water. In the prenatal period, taurine was given via the mother. After weaning mice were administered taurine in drinking water. 2) Pre-weaning: mice were exposed to taurine prior to weaning, 3) Post-weaning: mice were exposed to taurine after weaning. 4) CONTROL: no supplementation of taurine. It was shown that the Lifelong group required a longer period of time to acquire visual discrimination than the CONTROL group. Conversely, in the Post-weaning group, mice learned the task faster than CONTROLs. Visual discrimination learning time in the Pre-weaning group showed no significant difference compared with that in the CONTROL group. From these results, we suggest that the perinatal to early postnatal period is a "sensitive period" where taurine supplementation can result in retardation of learning in later life. At the same time, taurine supplementation after weaning improved visual discrimination learning. Thus, timing of taurine supplementation affected learning.

Instrumental learning, but not performance, requires dopamine D1-receptor activation in the amygdala

Substantial experimental evidence exists suggesting a critical role for dopamine in reinforcer-related processes, such as learning and drug addiction. Dopamine receptors, and in particular D1 receptors, are widely considered as modulators of synaptic plasticity. The amygdala contains both dopamine terminals and dopamine D1 receptors and is intimately involved in motivation and learning. However, little is known about the involvement of D1 receptor activation in two subnuclei of the mammalian amygdala, the central nucleus and basolateral complex in instrumental learning. Following recovery from surgery and preliminary training, rats with bilateral indwelling cannulae aimed at the central nucleus or basolateral complex were trained to lever-press for sucrose pellets over 12 sessions. Infusion of the selective D1 antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (0.3 nmol and 3.0 nmol) prior to the first five training sessions dose-dependently impaired instrumental learning when compared with vehicle-infused controls. All rats were then exposed to five sessions drug-free; lever-pressing quickly reached equal levels across groups. A drug infusion prior to an 11th session revealed no effect on performance. Control experiments indicated that basic motivational processes and general motor responses were intact, such as spontaneous feeding and locomotor activity. These results show an essential role for D1-receptor activation in both the central nucleus and basolateral complex on the acquisition of lever pressing for sucrose pellets in rats, but not the performance of the behavior once conditioned. We propose that instrumental learning is dependent on plasticity in the central nucleus and basolateral complex amygdala, and that D1 receptor activation participates in transcriptional processes that underlie this plasticity.

Hippocalcin-deficient mice display a defect in cAMP response element-binding protein activation associated with impaired spatial and associative memory

Hippocalcin is a member of the neuronal calcium sensor (NCS) protein family that is highly expressed in hippocampal pyramidal cells and moderately expressed in the neurons of cerebral cortex, cerebellum and striatum. Here we examined the physiological roles of hippocalcin using targeted gene disruption. Hippocalcin-deficient (-/-) mice displayed no obvious structural abnormalities in the brain including hippocampal formation at the light microscopic level. Deletion of hippocalcin did not result in up-regulation of the hippocalcin-related proteins; neural visinin-like Ca(2+)-binding proteins (NVP) 1, 2, and 3. The synaptic excitability of hippocampal CA1 neurons appeared to be normal, as estimated by the shape of field excitatory postsynaptic potentials elicited by single- and paired-pulse stimuli, and by tetanic stimulation. However, N-methyl-d-aspartate stimulation- and depolarization-induced phosphorylation of cAMP-response element-binding protein (CREB) was significantly attenuated in -/- hippocampal neurons, suggesting an impairment in an activity-dependent gene expression cascade. In the Morris water maze test, the performance of -/- mice was comparable to that of wild-type littermates except in the probe test, where -/- mice crossed the previous location of the platform significantly less often than +/+ mice. Hippocalcin-deficient mice were also impaired on a discrimination learning task in which they needed to respond to a lamp illuminated on the left or right side to obtain food reinforcement. No abnormalities were observed in motor activity, anxiety behavior, or fear learning. These results suggest that hippocalcin plays a crucial role in the Ca(2+)-signaling pathway that underlies long-lasting neural plasticity and that leads to spatial and associative memory.

Extracellular level of basolateral amygdalar dopamine responding to reversal of appetitive-conditioned discrimination in young and old rats

Young and old rats, aged 3 and 24 months old, respectively, were conditioned to press a lever under lamp-on conditions for reward acquisition and lamp-off for no reward using a variable interval reinforcement schedule that averaged 15 s; i.e., the minimal requirement was 4 responses/min. Over a 30-day period, young and old groups showed increased response to lamp-on from 22 to 35/min and from 10 to 23/min, respectively, and shortened response to lamp-off after initial training. Response to lamp-on as a percentage of total response to lamp-on and -off (the discrimination ratio) was over 80%. For the next 30 days, reversal learning was imposed to reinforce discrimination of the lamp-off state. Young rats showed a steadily increasing discrimination ratio from 40% to 70%, and old rats from 30% to 60%. In comparison with the initial training, young rats showed a total response increase from 50% to 60%, while old rats showed an approximately 5% decrease without loss of reward-obtaining efficiency. In vivo microdialysis during reversal revealed that young rats had higher dopamine transmission in the basolateral amygdala than old rats. The dopamine level was positively correlated with the number of responses to state of reward in young rats and negatively with the numbers of both NRF and RF responses to lamp-on and -off states in old rats. These results suggest that in reversal discrimination, basolateral amygdalar dopamine efflux correlates with the manner of age-related conditioned response rather than the ability to learn.

Drug-induced neurobehavioral plasticity: the role of environmental context

Repeated administrations of addictive drugs produce long-lasting changes in brain and behavior. However, drug-induced neurobehavioral plasticity is not a mere function of the neuropharmacological actions of drugs, but the result of complex drug-environment interactions. In the present review we summarize results obtained in a series of studies using an animal model of drug-environment interaction, showing that environmental context and past drug history interact to modulate the effects of amphetamine, cocaine and morphine on behavior, gene expression and structural plasticity. These findings may help shed some light on the conditions necessary for addictive drugs to enduringly alter brain and behavior.

Neonatal thyroxine treatment: changes in the number of corticotropin-releasing-factor (CRF) and neuropeptide Y (NPY) containing neurons and density of tyrosine hydroxylase positive fibers (TH) in the amygdala correlate with anxiety-related behavior of wistar rats

Neonatal hyperthyroidism induces persisting alterations in the adult brain, e.g. in spatial learning and hippocampal morphology. In the present study, the relationship between anxiety-related behavior and amygdala morphology was investigated in the adult rat after transient neonatal hyperthyroidism (daily s.c. injections of 7.5 microg L-thyroxine in 0.5 ml 0.9% NaCl solution from postnatal day p1 to p12). The behavioral tests used to study anxiety-related behavior were the motility test, elevated plus-maze and fear-sensitized acoustic startle response. In the amygdala, the number of neurons containing the anxiogenic peptide corticotropin releasing factor (CRF-ir and CRF mRNA) and anxiolytic neuropeptide Y (NPY-ir), the total number of neurons and the density of tyrosine hydroxylase immunoreactive (TH-ir) fibers were quantified. Thyroxine-treated pups presented an accelerated development including opening of eyes and snout elongation as typical signs of hyperthyroidism. Thyroxine-treated adult animals displayed a reduced anxiety in the motility box and elevated plus maze, a reduction in the number of CRF-ir neurons in the central nucleus of the amygdala, as well as an increase in the number of NPY-ir neurons and density of TH-ir fibers in nuclei of the basolateral complex of the amygdala. Moreover, there was a reduction in the total number of neurons in all nuclei of the basolateral complex (despite the higher number of NPY-ir neurons), but not central nucleus of the amygdala. The number of CRF-ir neurons in the central nucleus correlated positively with anxiety-related behavior, and the number of NPY-ir neurons and the density of TH-ir fibers in the basolateral complex correlated inversely with anxiety-related behavior. The findings suggested a shift toward an anxiolytic rather than anxiogenic distribution of peptidergic neurons and fibers in the amygdala at adult age following transient neonatal hyperthyroidism.

Role of the basolateral amygdala in memory consolidation

Typically, emotionally charged events are better remembered than neutral ones. This paper reviews data indicating that the amygdala is responsible for this facilitation of memory by emotional arousal. Pharmacological and behavioral studies have shown that the release of adrenal stress hormones facilitates memory consolidation. The available evidence suggests that this effect depends on a central action of stress hormones involving the release of the neuromodulators noradrenaline (NA) and acetylcholine in the basolateral complex of the amygdala (BLA). Indeed, BLA lesions block the memory modulating effects of stress hormones. Moreover, microdialysis studies have revealed that BLA concentrations of NA and acetylcholine are transiently (2h) elevated following emotionally arousing learning episodes. Last, post-learning intra-BLA injections of beta-adrenergic or muscarinic receptor antagonists reduce retention. These results have led to the hypothesis that NA and acetylcholine increase the activity of BLA neurons in the hours after the learning episode. In turn, the BLA would facilitate synaptic plasticity in other brain structures, believed to constitute the storage sites for different types of memory. Consistent with this, post-learning treatments that reduce or enhance the excitability of BLA neurons respectively decrease or improve long-term retention on various emotionally charged learning tasks. However, a number of issues remain unresolved. Chief among them is how the BLA facilitates synaptic plasticity elsewhere in the brain. The present review concludes with a consideration of this issue based on recent advances in our understanding of the BLA. Among other possibilities, it is suggested that rhythmic BLA activity at the theta frequency during arousal as well as the uniform conduction times of BLA axons to distributed rhinal sites may promote plasticity in co-active structures of the temporal lobe.

Influence of postweaning social isolation in the rat on brain development, conditioned behavior, and neurotransmission

There is substantial evidence that early life events influence brain development and subsequent adult behavior and play an important role in the causation of certain psychiatric disorders including schizophrenia and depression. The underlying mechanism of the effects of these early environmental factors is still not understood. It is a challenge to attempt to model early environmental factors in animals to gain understanding of the basic mechanisms that underlie the long-term effects. This paper reviews the effects of rearing rats from weaning in social isolation and reports some recent results indicating hippocampal dysfunction. Isolation rearing in rats from weaning produces a range of persistent behavioral changes in the young adult, including hyperactivity in response to novelty and amphetamine and altered responses to conditioning. These are associated with alterations in the central aminergic neurotransmitter functions in the mesolimbic areas and other brain regions. Isolation-reared rats have enhanced presynaptic dopamine (DA) and 5-HT function in the nucleus accumbens (NAC) associated with decreased presynaptic 5-HT function in the frontal cortex and hippocampus. Isolation-reared rats have reduced presynaptic noradrenergic function in the hippocampus, but have enhanced presynaptic DA function in the amygdala. These neurochemical imbalances may contribute to the exaggerated response of the isolated rat to a novel stimulus or to stimuli predictive of danger, and isolation-induced behavioral changes. These changes have neuroanatomical correlates, changes which seem to parallel to a certain degree those seen in human schizophrenia. A greater understanding of the processes that underlie these changes should improve our knowledge of how environmental events may alter brain development and function, and play a role in the development of neuropsychiatric disorders.

Dopamine modulates the response of the human amygdala: a study in Parkinson's disease

In addition to classic motor signs and symptoms, Parkinson's disease (PD) is characterized by neuropsychological and emotional deficits, including a blunted emotional response. In the present study, we explored both the neural basis of abnormal emotional behavior in PD and the physiological effects of dopaminergic therapy on the response of the amygdala, a central structure in emotion processing. PD patients and matched normal controls (NCs) were studied with blood oxygenation level-dependent functional magnetic resonance imaging during a paradigm that involved perceptual processing of fearful stimuli. PD patients were studied twice, once during a relatively hypodopaminergic state (i.e., > or =12 hr after their last dose of dopamimetic treatment) and again during a dopamine-replete state. The imaging data revealed a robust bilateral amygdala response in NCs that was absent in PD patients during the hypodopaminergic state. Dopamine repletion partially restored this response in PD patients. Our results demonstrate an abnormal amygdala response in PD that may underlie the emotional deficits accompanying the disease. Furthermore, consistent with findings in experimental animal paradigms, our results provide in vivo evidence of the role of dopamine in modulating the response of the amygdala to sensory information in human subjects.

Blockade of sensitisation-induced facilitation of appetitive conditioning by post-session intra-amygdala nafadotride

Prior D-amphetamine experience has been reported to enhance appetitive Pavlovian conditioning. The present study assessed the involvement of the mesoamygdaloid dopamine projection in this effect. Bilateral post-session intra-amygdala infusions of the D3 dopamine receptor antagonist, L-nafadotride, or vehicle were given during acquisition of a Pavlovian association in sensitised and unsensitised rats. During these sessions, subjects received presentations of a stimulus (CS(+)) paired with 10% sucrose availability. A second stimulus (CS(-)) was also presented but never paired with sucrose. Sensitised animals infused post-session with vehicle acquired a Pavlovian conditioned approach response during CS(+) presentations more rapidly than controls, as we have shown previously. However, post-session intra-amygdala L-nafadotride selectively retarded conditioned responding to the CS(+) in both groups of animals, abolishing the difference between sensitised and unsensitised rats. These results, therefore, extend the evidence for the involvement of the mesoamygdaloid dopamine projection in Pavlovian conditioning, and the facilitation of associative learning following sensitisation.

Isolation rearing-induced facilitation of Pavlovian learning: abolition by postsession intra-amygdala nafadotride

It has been shown previously in this laboratory that rats reared in social isolation acquire a Pavlovian-conditioned approach task much more rapidly than their respective controls. This study assessed the involvement specifically of the mesoamygdaloid dopamine pathway in this facilitated learning of isolates. Thus, animals were required to associate arbitrary stimuli with a pulsed light stimulus (unconditioned stimulus, US). The US, while without biological significance, was nevertheless capable of eliciting an intrinsic and sustained alerting response. Procedures ensured that the arbitrary stimuli (tone or clicker) did not elicit a response in the first instance, and were presented either paired (CS+) or unpaired (CS-) with the US. Isolates and socially reared controls received intra-amygdala infusions of the D3 dopamine receptor antagonist, L-nafadotride, or vehicle immediately following the end of each training session. The conditioned response increased over sessions in both groups of vehicle-infused rats during presentations of the CS+ stimulus, but not CS-, and isolates acquired this association more rapidly than controls. However, acquisition of this association was abolished by postsession intra-amygdala L-nafadotride. Responding to the US was largely unaffected by drug or rearing conditions. Hence, these data provide strong evidence for the specific involvement of the mesoamygdaloid dopamine projection in the facilitation of associative learning by isolation rearing.

Functional aspects of dopamine metabolism in the putative prefrontal cortex analogue and striatum of pigeons (Columba livia)

Dopamine (DA) in mammalian associative structures, such as the prefrontal cortex (PFC), plays a prominent role in learning and memory processes, and its homeostasis differs from that of DA in the striatum, a sensorimotor region. The neostriatum caudolaterale (NCL) of birds resembles the mammalian PFC according to connectional, electrophysiological, and behavioral data. In the present study, DA regulation in the associative NCL and the striatal lobus parolfactorius (LPO) of pigeons was compared to uncover possible differences corresponding to those between mammalian PFC and striatum. Extracellular levels of DA and its metabolites (homovanillic acid [HVA], dihydroxyphenylacetic acid [DOPAC]) and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were investigated by in vivo microdialysis of urethane-anesthetized pigeons under basal conditions and after systemic administration of D-amphetamine. DA was reliably determined only in LPO dialysates, and DA metabolite levels were significantly higher in LPO than in NCL. The HVA/DOPAC ratio, indicating extracellular lifetime of DA, was more than twice as high in NCL than in LPO dialysates. After amphetamine, DA increased in LPO while still being undetectable in NCL, and DA metabolites decreased in both regions. 5-HIAA slightly decreased in NCL dialysates. Amphetamine effects were delayed in NCL compared with the striatum. In conclusion, effects of amphetamine on the pigeon's ascending monoamine systems resemble those found in mammals, suggesting similar regulatory properties. The neurochemical differences between NCL and LPO parallel those between associative regions, such as PFC and dorsal striatum in mammals. They may reflect weaker regulation of extracellular DA, favoring DAergic volume transmission, in associative than striatal forebrain regions.

Cellular mechanisms of infralimbic and prelimbic prefrontal cortical inhibition and dopaminergic modulation of basolateral amygdala neurons in vivo

The basolateral amygdala (BLA) is believed to be involved in schizophrenia, depression, and other disorders that display affective components. The neuronal activity of the BLA, and BLA-mediated affective behaviors, are driven by sensory stimuli transmitted in part from sensory association cortical regions. These same behaviors may be regulated by prefrontal cortical (PFC) inputs to the BLA. However, it is unclear how two sets of glutamatergic inputs to the BLA can impose opposing actions on BLA-mediated behaviors; specifically, it is unclear how PFC inputs exert inhibitory actions over BLA projection neurons. Dopamine (DA) receptor activation enhances BLA-mediated behaviors. Although we have demonstrated that DA suppresses medial PFC inputs to the BLA and enhances sensory cortical inputs, the precise cellular mechanisms for its actions are unknown. In this study we use in vivo intracellular recordings to determine the means by which glutamatergic inputs from the PFC inhibit BLA projection neurons, contrast that with glutamatergic inputs from the association sensory cortex (Te3) that drive BLA projection neurons, and examine the effects of DA receptor activation on neuronal excitability, spontaneous postsynaptic potentials (PSPs), and PFC-evoked PSPs. We found that PFC stimulation inhibits BLA projection neurons by three mechanisms: chloride-mediated hyperpolarization, a persistent decrease in neuronal input resistance, and shunting of PSPs; all effects are possibly attributable to recruitment of inhibitory interneurons. DA receptor activation enhanced neuronal input resistance by a postsynaptic mechanism (via DA D2 receptors), suppressed spontaneously occurring and PFC-evoked PSPs (via DA D1 receptors), and enhanced Te3-evoked PSPs.

Elevation of prefrontal acetylcholine is related to the extinction of learned behavior in rats

To investigate whether changes occur in acetylcholine (ACh) levels in the rat medial prefrontal cortex (mPFC) during initial lever-press extinction training, in vivo microdialysis was used to measure mPFC ACh. Elevated ACh was found during this training period. Furthermore, this elevation significantly correlated with the number of responses found in the re-training session the next day, but not with that in the initial training. These results suggest that the mPFC ACh elevation during the initial training period enhances the progress of lever-press extinction across sessions.

Dopamine attenuates prefrontal cortical suppression of sensory inputs to the basolateral amygdala of rats

The basolateral complex of the amygdala (BLA) plays a significant role in affective behavior that is likely regulated by afferents from the medial prefrontal cortex (mPFC). Studies suggest that dopamine (DA) is a necessary component for production of appropriate affective responses. In this study, prefrontal cortical and sensory cortical [temporal area 3 (Te3)] inputs to the BLA and their modulation by DA receptor activation was examined using in vivo single-unit extracellular recordings. We found that Te3 inputs are more capable of driving BLA projection neuron firing, whereas mPFC inputs potently elicited firing from BLA interneurons. Moreover, mPFC stimulation before Te3 stimulation attenuated the probability of Te3-evoked spikes in BLA projection neurons, possibly via activation of inhibitory interneurons. DA receptor activation by apomorphine attenuated mPFC inputs, while augmenting Te3 inputs. Additionally, DA receptor activation suppressed mPFC-induced inhibition of Te3-evoked spikes. Thus, the mPFC may attenuate sensory-driven amygdala-mediated affective responses via recruitment of BLA inhibitory interneurons that suppress sensory cortical inputs. In situations of enhanced DA levels in the BLA, such as during stress and after amphetamine administration, mPFC regulation of BLA will be dampened, leading to a disinhibition of sensory-driven affective responses.

Memory-related acetylcholine efflux from rat prefrontal cortex and hippocampus: a microdialysis study

To investigate the relationship between the prefrontal and hippocampal acetylcholine (ACh) systems and working memory, an in vivo microdialysis study was conducted. A group of rats was trained to perform a working memory task, delayed alternation, in an operant chamber for food reinforcement. The rats had to choose one of two response levers in an alternative manner in each trial, with a certain interval between trials. They had to remember which lever they chose in the previous trial without the assistance of external cues. Another group was trained to perform a reference memory task, cued alternation, in which the behavioral sequence was identical, but an external cue was provided. After stable behavior was established, a dialysis probe was implanted into the prefrontal cortex or the hippocampus of each rat. The extracellular concentration of ACh in the dialysates from the prefrontal cortex increased during performance of the delayed alternation task, while the hippocampal ACh showed a more distinct increase during performance of the cued alternation task. These results suggest that the prefrontal ACh is mainly related to working memory, whereas the hippocampal ACh is mainly related to reference memory.

Environmental novelty differentially affects c-fos mRNA expression induced by amphetamine or cocaine in subregions of the bed nucleus of the stria terminalis and amygdala

The environmental context in which amphetamine or cocaine are administered modulates both their acute psychomotor activating effects and their ability to induce sensitization. Here we report that environmental context differentially affects patterns of amphetamine- and cocaine-induced c-fos mRNA expression in the bed nucleus of the stria terminalis (BST) and amygdala of male rats. In the medial amygdala and medial posterior BST, exposure to novelty resulted in a marked increase in c-fos mRNA. Amphetamine given at home did not induce c-fos mRNA, and when given in the novel environment, did not increase levels beyond that observed for novelty alone. In the basolateral and lateral amygdala, amphetamine or cocaine at home or exposure to novelty induced c-fos mRNA. When amphetamine or cocaine was given in a novel environment the c-fos mRNA response was significantly enhanced. In the central nucleus of the amygdala (CEA) and oval subnucleus of the BST (BSTov), amphetamine administration at home produced a robust increase in c-fos mRNA expression, whereas exposure to novelty had little effect. In contrast to other brain regions examined, the c-fos mRNA response to amphetamine in a novel versus home environment was significantly smaller. In both "home" and "novel" amphetamine groups, c-fos mRNA in the BSTov and CEA was predominantly expressed in enkephalin-containing cells; coexpression with corticotropin-releasing hormone was rare. These data suggest that the context in which psychostimulants are given powerfully and differentially alters the response of limbic structures that have been functionally implicated in drug reinforcement and emotional behaviors.

Role for dopamine in the behavioral functions of the prefrontal corticostriatal system: implications for mental disorders and psychotropic drug action

We have discussed the role of dopamine in modulating the interactions between cortical and striatal regions that are involved in behavioral regulation. The evidence reviewed seems to suggest that dopamine acts, overall, to promote stimulus-induced responding for conditioned or reward-related stimuli by integrative actions at multiple forebrain sites. It is thus not surprising that dopaminergic dysfunction has been implicated in a number of neuropsychiatric disorders that involve abnormal cognitive and affective function. Future studies aimed at pinpointing the precise anatomical sites of action and molecular mechanisms involved in dopaminergic transmission within the corticolimbic circuit are critical for trying to disentangle the cellular mechanisms by which dopamine exerts its actions. Moreover, the afferent control of dopamine neurons from brainstem and forebrain sites need to be fully explored in order to begin to understand what mechanisms are involved in regulating the dopaminergic response to stimuli with incentive value. Finally, the post-synaptic consequences of prolonged and supranormal dopaminergic activation need to be investigated in order to understand what persistent neuroadaptations result from chronic activation of this neuromodulatory system (e.g. in drug addiction). Answers to these sorts of questions will undoubtedly provide important insights into the nature of dopaminergic function in the animal and human brain.

Increased dopamine release in the human amygdala during performance of cognitive tasks

Accumulating data support a critical involvement of dopamine in the modulation of neuronal activity related to cognitive processing. The amygdala is a major target of midbrain dopaminergic neurons and is implicated in learning and memory processes, particularly those involving associations between novel stimuli and reward. We used intracerebral microdialysis to directly sample extracellular dopamine in the human amygdala during the performance of cognitive tasks. The initial transition from rest to either a working memory or a reading task was accompanied by significant increases in extracellular dopamine concentration of similar magnitude. During a sustained word paired-associates learning protocol, increase in dopamine release in the amygdala related to learning performance. These data provide evidence for sustained activation of the human mesolimbic dopaminergic system during performance of cognitive tasks.

Modulation of basolateral amygdala neuronal firing and afferent drive by dopamine receptor activation in vivo

The basolateral amygdala (BLA) is implicated in responding to affective stimuli. Dopamine (DA) is released in the BLA during numerous conditions; however, the neurophysiological effects of DA in the BLA have not been examined in depth. In this study, the effects of DA receptor manipulation on spontaneous and afferent-driven neuronal firing were examined using in vivo extracellular single-unit recordings in parallel with systemic and iontophoretic drug application, and stimulation of the substantia nigra/ventral tegmental area in the rat. The effects of DA receptor activation in the BLA were found to depend on the characteristics of the BLA neuron examined, causing an increase in the firing rate of putative interneurons and a decrease in the firing of identified projection neurons. Additionally, DA receptor activation attenuated short-latency spikes evoked by electrical stimulation of prefrontal cortical and mediodorsal thalamic inputs to the BLA while potentiating the responses evoked by electrical stimulation of sensory association cortex. DA receptor activation can thus attenuate BLA projection neuron firing via two mechanisms: (1) by a direct inhibition, and (2) by indirect actions mediated via activation of BLA interneurons. This is hypothesized to lead to a global filtration of weaker inputs. Moreover, DA potentiates sensory inputs and attenuates medial prefrontal cortex inputs to the BLA. Conditions in which DA is released in the BLA, such as during the presentation of an affective stimulus, will lead to a potentiation of the strongest sensory input and a dampening of cortical inhibition over the BLA, thus augmenting the response to affective sensory stimuli.

Effects of delta 9-tetrahydrocannabinol and diazepam on feeding behavior in mice

The present study examined effects of diazepam (DZP) alone or in combination with delta 9-tetrahydrocannabinol (THC) on feeding behavior as well as body weight in male ddY strain mice at 5 weeks of age. Because we saw no hyperphagic effect of DZP with or without THC in mice, we explored the hyperphagia elicitable by DZP. THC [2 (THC2) or 4 (THC4) mg/kg/day s.c.] was given daily for 7 days. For the last day the mice were starved and injected i.p. with DZP (2 mg/kg) 10 min prior to a food or maze test. Controls received vehicle injections. Feeding behavior was measured after giving food for 2 hr. THC4 significantly reduced body weight gain. DZP, with or without THC, induced hyperphagia. THC4 alone also induced hyperphagia that was not significantly affected by DZP. Time taken to find food was extended by DZP and further with THC. Both DZP and THC can therefore interact on food ingestion but synergize on food seeking in mice through different mechanisms.

Enhanced dopamine efflux in the amygdala by a predictive, but not a non-predictive, stimulus: facilitation by prior repeated D-amphetamine

Extracellular levels of dopamine within the amygdala were monitored using in vivo microdialysis during performance of an appetitive Pavlovian conditioning task in sensitized rats and unsensitized controls. Animals received exposure either to D-amphetamine or to vehicle for seven consecutive days (2 mg/kg/day, i.p.) in the home cage. Training began following a further seven injection-free days. Animals were exposed to two session types: during conditioning sessions, a stimulus (tone or light) immediately preceded sucrose pellet delivery. During control sessions, the alternative stimulus was also presented, but not in temporal proximity to an otherwise identical schedule of pellet delivery. There was a total of three alternating presentations of each session type during training. Sensitization enhanced Pavlovian conditioned approach behaviour to the stimulus predictive of imminent pellet delivery, and was without effect upon approach behaviours either to the food pellets themselves or to the control stimulus. Extracellular levels of dopamine within the amygdala were assessed during the fourth conditioning and control sessions. Mesoamygdaloid dopamine efflux increased significantly during the conditioning test session, but not during the control session, and this dopaminergic response was more marked in rats with prior repeated D-amphetamine experience. Hence, these results add to evidence suggesting a role for amygdaloid dopamine in appetitive Pavlovian conditioning, and in the facilitation of associative learning following prior experience of D-amphetamine.

Dopaminergic innervation of the amygdala is highly responsive to stress

The amygdala has been implicated in the neuronal sequelae of stress, although little is known about the neurochemical mechanisms underlying amygdala transmission. In vivo microdialysis was employed to measure extracellular levels of dopamine in the basolateral nucleus of the amygdala in awake rats. Once it was established that impulse-dependent release of dopamine could be measured reliably in the amygdala, the effect of stress, induced by mild handling, on amygdala dopamine release was compared with that in three other dopamine-innervated regions, the medial prefrontal cortex, nucleus accumbens, and caudate nucleus. The magnitude of increase in dopamine in response to the handling stimulus was significantly greater in the amygdala than in the nucleus accumbens and prefrontal cortex. This increase was maximal during the application of stress and diminished after the cessation of stress. In contrast, the increases in extracellular dopamine levels in other regions, in particular the nucleus accumbens, were prolonged, reaching maximal values after the cessation of stress. These results suggest that dopaminergic innervation of the amygdala may be more responsive to stress than that of other dopamine-innervated regions of the limbic system, including the prefrontal cortex, and implicate amygdalar dopamine in normal and pathophysiological processes subserving an organism's response to stress.

Dopamine and acetylcholine elevation on lever-press acquisition in rat prefrontal cortex

To determine whether the rat medial prefrontal cortex (PFC) is involved in acquiring operant learning, we observed changes in extracellular concentration of dopamine (DA) and acetylcholine (ACh) in the rat medial PFC during lever-press acquisition (acquisition group) or retrieval (retention group) using in vivo microdialysis. We found that DA or ACh elevation related to acquisition occurred. DA elevation was observed in the acquisition group only. These results indicate that the medical PFC is related to acquisition, and suggest that interaction between DA and ACh may be involved in learning acquisition.

Isolation rearing enhances acquisition in a conditioned inhibition paradigm

Isolation rearing from weaning has been reported to enhance excitatory conditioning. The present study employed a conditioned inhibition procedure to examine whether this result was attributable to locomotor hyperactivity. Rats were raised from 21 days old in isolation or in groups of five. In Phase 1, presentation of stimulus A+ was followed immediately by sucrose availability (excitatory conditioning). In Phase 2, sucrose was again presented after A+ alone, but not after a compound presentation of A+ with a second stimulus, B-. Thus, B is believed to acquire conditioned inhibitory properties, countering the excitatory impact of A, and reducing responding specifically to this stimulus compound. Isolates showed enhanced excitatory conditioning in Phase 1. Furthermore, acquisition of conditioned inhibition in Phase 2 was also facilitated by isolation rearing. In Phase 3, B- was paired with a period of sucrose availability. Isolation rearing initially retarded responding to B-, confirming that this stimulus possessed a greater degree of behavioural inhibition in these animals. Later in training, isolates showed enhanced excitatory conditioning to B-, as shown previously to A+ in Stage 1. These results suggest that isolation rearing enhances the acquisition of appetitive Pavlovian associations, independently of locomotor hyperactivity.

Chronic cerebral hypoperfusion disrupts discriminative behavior in acquired-learning rats

We describe an 'acquired-learning' rat model that was used to investigate the effects of chronic cerebral hypoperfusion on the maintenance of previously acquired discriminative behavior using the discriminating learning task. Male Wistar rats, aged 11 weeks, were trained to discriminate between lamp-on and lamp-off states under an operant-type learning procedure. After 30 sessions, we selected 'acquired-learning' rats with an average discrimination ratio higher than 75% recorded during the last three sessions. Chronic hypoperfusion was then induced by permanent ligation of both common carotid arteries under pentobarbital anesthesia. The rats were tested after surgery over a period of 12 weeks and brain tissue was analyzed for muscarinic acetylcholine receptor (mACh-R) binding. Cerebral hypoperfusion resulted in a significant reduction in the discrimination ratio throughout the observation period, compared with sham-operated rats. However, chronic hypoperfusion would not affect on motor function. The maximum number of mACh-R examined 12 weeks after the operation was significantly reduced in the frontal cortex and hippocampus in the hypoperfusion group. Impaired discrimination learning was associated with a reduction in mACh-R. Our findings suggest that chronic cerebral hypoperfusion in acquired-learning rats is a useful model for investigating the pathophysiology of dementia and that cortical and/or hippocampal cholinergic systems contributes to learning impairment, at least, in our learning task.

Dopamine release in the amygdaloid complex of the rat, studied by brain microdialysis

The dopaminergic projection from the ventral tegmental area to the amygdaloid complex may be modulatory on the processes of associative learning in the latter region. We measured dopamine in four different amygdaloid subfields in the rat, using brain microdialysis. Extracellular levels of dopamine in two sites in the lateral nucleus were not consistently measurable, even after treatment with amphetamine. However, basal dopamine levels were measurable in more medial locations (basolateral and central nuclei), with higher concentrations in the caudal than in the rostral probe placement, and were increased around 3-fold by systemic amphetamine. Similarly, dopamine levels in caudal-medial amygdala were increased by local potassium stimulation and by mild footshock in a calcium-dependent manner, indicating a neurotransmitter origin.

Isolation rearing enhances the rate of acquisition of a discriminative approach task but does not affect the efficacy of a conditioned reward

Male Lister hooded rats were reared from weaning either alone (isolation reared) or in groups of five (socially reared controls). Experiments began at 18-weeks postweaning. Subjects were trained to associate an arbitrary stimulus with 10% sucrose reward. Trials (VT30sec) consisted of a 5-s light stimulus (houselight off, wall lights on), followed by a 5-s period of access to the sucrose reward. Alcove approach between trials delayed the next trial by the duration of approach plus 3 s. Activity not associated directly with approach behaviour was also recorded. Isolation rearing enhanced the rate of acquisition of the discriminative approach response. Acquisition of both conditioned approach behaviour during trials and conditioned avoidance between trials was more rapid in isolates. In general, effects were most evident early in training, with asymptotic performance least affected. Horizontal and vertical activity extraneous to approach behaviour was enhanced in isolates during the first training session and increased further relative to social controls after several training sessions. Subsequently, two novel levers were presented: a response on one lever resulted in a 0.5-s presentation of the conditioned stimulus (CS) (probability 0.5), whereas the second lever had no programmed consequences. Sucrose reward was not available at any time. Both groups of animals showed a preference for the CS-associated lever, although rates of response by isolates were higher than social controls on both levers. The proportion of responses emitted upon the active lever, by comparison with the total number of responses recorded upon both levers, was, however, unaffected by isolation rearing.

6-OHDA lesions to the central amygdala abolish angiotensins facilitation of object recognition in rats

1. We have previously reported that the dopaminergic projection from A10 ventral tegmental neurons and A9 neurons of substantia nigra to the central amygdala (CA) is, in part, responsible for the facilitatory effect of angiotensin II (AII) and its 3-7 fragment [AII(3-7)] on the retrieval of information in memory that is motivated affectively. 2. In this study, the influence of both angiotensins, given intracerebroventricularly at the dose of 1 nmol each in rats lesioned with 6-OHDA to CA, on recognition memory, was tested. 3. AII and its 3-7 fragment significantly improved object recognition in sham-operated groups of rats. Bilateral 6-OHDA lesions to CA totally abolished the facilitatory effect of both angiotensins on object recognition. As insignificant increase of spontaneous locomotor activity in rats lesioned to CA did not interfere with the cognitive effect of AII and AII(3-7). 4. These results suggest that the dopaminergic projection at the CA takes part in the facilitatory effect of angiotensins on recognition memory.

Substance P facilitates memory in goldfish in an appetitively motivated learning task

The aim of this study was to investigate if the neuropeptide substance P (SP) can improve learning and memory in goldfish (Carassius auratus). Four groups of fish were trained for seven days to find food in one out of two compartments until discrimination was achieved. On the last training day, they were injected (intra-abdominal) with haloperidol or vehicle before the training, and with SP or vehicle immediately after the training session. Each group of fish received either: (1) vehicle+vehicle (n = 18); (2) vehicle + SP, (n = 20, SP 50 mg/kg); (3) haloperidol+ vehicle (n = 15, haloperidol 2 mg/kg); or (4) haloperidol+ SP (n = 14, haloperidol 2 mg/kg, SP 50 mg/kg). Twenty-four hours later, the time spent to find the food was recorded. Reversal training was done for four consecutive days after this post-injection test and the time spent to find the food was recorded again. The results indicate that only the group treated with vehicle + SP needed more time to reach reversal training than control fish (Mann-Whitney U-test, P = 0.0009). It is suggested that SP can enhance memory in fish and that this effect may have a dopaminergic mediation in discrimination learning task.

Effects of discrimination learning on the rat striatal dopaminergic activity: a microdialysis study

The prefrontal cortex (PFC) has anatomical and functional relationships with the striatum. In a previous study we showed that dopamine (DA) turnover in the PFC of rats is enhanced during the performance of a discrimination task. In the present study, we used an in vivo microdialysis method to examine whether DAergic activity in the striatum could also be altered by the discrimination task. The results showed a substantial and sustained suppression of DAergic activity during and after the discriminative behaviour. The fact that the discriminative performance induced opposite changes in DAergic activity in the striatum and the PFC is consistent with the results of biochemical studies, suggesting that the suppressed DA turnover in the striatum may be induced by the enhanced DAergic activity in the PFC during the discrimination task.

Brain microdialysis and its application for the study of animal behaviour

Microdialysis is a sampling method that is used to determine the extracellular concentration of neurotransmitters in the brain. The method can be applied to conscious and unrestrained animals and is very suitable for the study of the chemistry of endogenous behaviour. This article reviews the contribution that microdialysis made to our understanding of the chemistry of behaviour. Methodological and practical considerations such as the implantation time and the use of guide cannulas are reviewed. The question whether neurotransmitters and related metabolites in dialysates reflect true synaptic release is critically discussed. There is much evidence that dopamine, noradrenaline, acetylcholine and serotonin in dialysates are related to neurotransmission, but there is serious doubt whether this is the case with amino acid transmitters such as GABA, glutamate and aspartate. Until now far over 100 papers appeared that used microdialysis in behavioural studies. Behavioural activation, the sleep-awake cycle and diurnal rhythms were subject of several of these studies. Various workers have described neurochemical changes in the brain that are related to feeding. Other studies were concerned with sexual behaviour and the sexual cycle in females. Parturition, maternal behaviour and offspring recognition have been studied in a series of microdialysis studies carried out in sheep. An overview is given of the microdialysis studies that were carried out to understand the biochemistry of stress. In this respect dopamine and noradrenaline have received much attention. A great number of microdialysis studies dealt with the role of dopamine in self-stimulation, reward and aversive emotions. It is concluded that microdialysis is at presently the most versatile and practical method to study the chemistry of behaviour and it is to be expected that it will soon be a routine methodology in behavioural research. Finally, perspectives and possible future developments of the methods are discussed.

Septo-hippocampal cholinergic system under the discrimination learning task in the rat: a microdialysis study with the dual-probe approach

To investigate regulation of the septo-hippocampal cholinergic system by dopaminergic inputs to the septum in rats which performed a discrimination learning task, an in vivo microdialysis method with the dual-probe approach was used. Rats were trained to discriminate between lamp-on and -off states under an operant-type learning procedure. After stable discriminative behavior was established, dialysis probes were implanted into the hippocampus and the lateral septum area of each rat. The concentration of dopamine (DA) in the septum rapidly increased within 20 min after the beginning of a learning session. However, another group of rats trained on a similar but non-discriminative task showed no such increase. The concentration of acetylcholine (ACh) in the hippocampus was significantly enhanced during the learning session and rapidly returned to the basal value after the session, but showed a delayed and diminished increase in the non-discrimination group. These results suggest that DAergic inputs to the septum may be involved in control of the septo-hippocampal cholinergic system which is of importance for discrimination learning behavior.

Sensitization of cocaine-stimulated increase in extracellular levels of corticotropin-releasing factor from the rat amygdala after repeated administration as determined by intracranial microdialysis

Using intracranial microdialysis, the effect of repeated cocaine (30 mg/kg i.p.) versus saline administration for 10 consecutive days upon basal and stimulated release of corticotropin-releasing factor (CRF) was examined in the central amygdaloid nucleus (CeA) of anesthetized rats. No significant differences in basal CRF levels between daily cocaine and saline treated groups were found. However, after cocaine challenge (10 mg/kg i.p.) the increase in CRF overflow was significantly greater in cocaine- as opposed to saline-pretreated rats (266 +/- 55.4% versus 149 +/- 8.5% of basal levels). Local administration of 4-aminopyridine produced a significant increase in CRF efflux (195 +/- 58.5%) in daily cocaine-treated rats with only a weak response in the control group (127 +/- 30.9%). These data demonstrate that repeated administration of cocaine enhances cocaine-induced release of CRF in the rat CeA. The sensitization of CRF release may play a significant role in psychostimulant-induced sensitization phenomena.