Sensitized Fos expression in subterritories of the rat medial prefrontal cortex and nucleus accumbens following amphetamine sensitization as revealed by stereology

Total Number Is Important: Using the Disector Method in Design-Based Stereology to Understand the Structure of the Rodent Brain

The advantages of using design-based stereology in the collection of quantitative data, have been highlighted, in numerous publications, since the description of the disector method by Sterio (1984). This review article discusses the importance of total number derived with the disector method, as a key variable that must continue to be used to understand the rodent brain and that such data can be used to develop quantitative networks of the brain. The review article will highlight the huge impact total number has had on our understanding of the rodent brain and it will suggest that neuroscientists need to be aware of the increasing number of studies where density, not total number, is the quantitative measure used. It will emphasize that density can result in data that is misleading, most often in an unknown direction, and that we run the risk of this type of data being accepted into the collective neuroscience knowledge database. It will also suggest that design-based stereology using the disector method, can be used alongside recent developments in electron microscopy, such as serial block-face scanning electron microscopy (SEM), to obtain total number data very efficiently at the ultrastructural level. Throughout the article total number is discussed as a key parameter in understanding the micro-networks of the rodent brain as they can be represented as both anatomical and quantitative networks.

Low-anxiety rats are more sensitive to amphetamine in comparison to high-anxiety rats

This study utilised the two injection protocol of sensitisation (TIPS) and the conditioned place preference test to validate and extend previous findings on the effects of amphetamine on positive reinforcement-related 50 kHz ultrasonic vocalisation (USV) in rats. We also examined changes in the expression of c-Fos and the NMDA receptor 2B (GluN2B) subunit, markers of neuronal activity and plasticity, in brain regions of rats in response to TIPS. We used low anxiety-responsive (LR) and high anxiety-responsive (HR) rats, which are known to exhibit different fear-conditioned response strengths, different susceptibilities to amphetamine in the TIPS procedure and different amphetamine-dependent 50 kHz USV responses. The LR rats, compared to the HR rats, not only vocalised much more intensely but also spent significantly more time in the amphetamine-paired compartment. After the second dose of amphetamine, the LR rats exhibited more c-Fos and GluN2B activation in layers II and III of the M1/M2 motor cortex area and prefrontal cortex (PRE, PRL, IL) and also presented with more GluN2B activation in the basal amygdala. These data reveal that HR and LR rats exhibit different levels of reactivity in the cortical-limbic pathway, which controls reward-related motivational processes. These findings contribute to the general hypothesis that heterogeneity in emotional processes is one of the causes of sensitisation to amphetamine and drug addiction.

Effects of withdrawal from repeated amphetamine exposure in peri-puberty on neuroplasticity-related genes in mice

Although extensive evidence demonstrates that repeated administration of amphetamine (AMPH) induces behavioral and neurochemical sensitization, the influence of the developmental timing of AMPH administration is unknown. This is an important issue to address because it could help clarify the influence of early drug exposure on neuronal plasticity and the involvement of dopaminergic sensitization in the etiopathology of neuropsychiatric disorders. Thus, we decided to investigate the molecular alterations induced by the administration of AMPH during adolescence, when repeated exposure to the psychostimulant may interfere with developmental neuroplasticity. We investigated the expression of the neurotrophin brain-derived neurotrophic factor (BDNF) and of two inducible-early genes (arc and cfos) that bridge neuronal activity with long-lasting functional alterations. We found that peri-pubertal treatment with AMPH induces long-lasting changes in the expression of bdnf and of activity-regulated genes in the hippocampus and in the prefrontal/frontal cortex, and leads to alterations of their short-term modulation in response to a subsequent acute AMPH challenge. These data suggest that AMPH exposure in peri-puberty may negatively affect the maturation of brain structures, such as the prefrontal cortex, which facilitate the development of dopamine sensitization and may contribute to dopamine-dependent behavioral dysfunctions and molecular alterations in adulthood.

Sensitized activation of Fos and brain-derived neurotrophic factor in the medial prefrontal cortex and ventral tegmental area accompanies behavioral sensitization to amphetamine

Behavioral sensitization, or augmented locomotor response to successive drug exposures, results from neuroadaptive changes contributing to addiction. Both the medial prefrontal cortex (mPFC) and ventral tegmental area (VTA) influence behavioral sensitization and display increased immediate-early gene and BDNF expression after psychostimulant administration. Here we investigate whether mPFC neurons innervating the VTA exhibit altered Fos or BDNF expression during long-term sensitization to amphetamine. Male Sprague-Dawley rats underwent unilateral intra-VTA infusion of the retrograde tracer Fluorogold (FG), followed by 5 daily injections of either amphetamine (2.5 mg/kg, i.p.) or saline vehicle. Four weeks later, rats were challenged with amphetamine (1.0 mg/kg, i.p.) or saline (1.0 mL/kg, i.p.). Repeated amphetamine treatment produced locomotor sensitization upon drug challenge. Two hours later, rats were euthanized, and mPFC sections were double-immunolabeled for either Fos-FG or Fos-BDNF. Tissue from the VTA was also double-immunolabeled for Fos-BDNF. Amphetamine challenge increased Fos and BDNF expression in the mPFC regardless of prior drug experience, and further augmented mPFC BDNF expression in sensitized rats. Similarly, more Fos-FG and Fos-BDNF double-labeling was observed in the mPFC of sensitized rats compared to drug-naïve rats after amphetamine challenge. Repeated amphetamine treatment also increased VTA BDNF, while both acute and repeated amphetamine treatment increased Fos and Fos-BDNF co-labeling, an effect enhanced in sensitized rats. These findings point to a role of cortico-tegmental BDNF in long-term amphetamine sensitization.

Reward expectation alters learning and memory: the impact of the amygdala on appetitive-driven behaviors

The capacity to seek and obtain rewards is essential for survival. Pavlovian conditioning is one mechanism by which organisms develop predictions about rewards and such anticipatory or expectancy states enable successful behavioral adaptations to environmental demands. Reward expectancies have both affective/motivational and discriminative properties that allow for the modulation of instrumental goal-directed behavior. Recent data provide evidence that different cognitive strategies (cue-outcome associations) and neural systems (amygdala) are used when subjects are trained under conditions that allow Pavlovian-induced reward expectancies to guide instrumental behavioral choices. Furthermore, it has been demonstrated that impairments typically observed in a number of brain-damaged models are alleviated or eliminated by embedding unique reward expectancies into learning/memory tasks. These results suggest that Pavlovian-induced reward expectancies can change both behavioral and brain processes.

Expression of amphetamine sensitization is associated with recruitment of a reactive neuronal population in the nucleus accumbens core

RATIONALE

Repeated exposure to psychostimulant drugs causes a long-lasting increase in the psychomotor and reinforcing effects of these drugs and an array of neuroadaptations. One such alteration is a hypersensitivity of striatal activity such that a low dose of amphetamine in sensitized animals produces dorsal striatal activation patterns similar to acute treatment with a high dose of amphetamine.

OBJECTIVES

To extend previous findings of striatal hypersensitivity with behavioral observations and with cellular activity in the nucleus accumbens and prefrontal cortex in sensitized animals.

MATERIALS AND METHODS

Rats treated acutely with 0, 1, 2.5, or 5 mg/kg i.p. amphetamine and sensitized rats challenged with 1 mg/kg i.p. amphetamine were scored for stereotypy, rearing, and grooming, and locomotor activity recorded. c-fos positive nuclei were quantified in the nucleus accumbens and prefrontal cortex after expression of sensitization with 1 mg/kg i.p. amphetamine.

RESULTS

Intense stereotypy was seen in animals treated acutely with 5 mg/kg amphetamine, but not in the sensitized group treated with 1 mg/kg amphetamine. The c-fos response to amphetamine in the accumbens core was augmented in amphetamine-pretreated animals with a shift in the distribution of optical density, while no effect of sensitization was seen in the nucleus accumbens shell or prefrontal cortex.

CONCLUSIONS

A lack of stereotypy in the sensitized group indicates a dissociation of behavioral responses to amphetamine and striatal immediate-early gene activation patterns. The increase in c-fos positive nuclei and shift in the distribution of optical density observed in the nucleus accumbens core suggests recruitment of a new population of neurons during expression of sensitization.

Repeated amphetamine administration induces Fos in prefrontal cortical neurons that project to the lateral hypothalamus but not the nucleus accumbens or basolateral amygdala

RATIONALE

The development of sensitization to amphetamine (AMPH) is dependent on increases in excitatory outflow from the medial prefrontal cortex (mPFC) to subcortical centers. These projections are clearly important for the progressive enhancement of the behavioral response during drug administration that persists through withdrawal.

OBJECTIVES

The objective of this study was to identify the mPFC subcortical pathway(s) activated by a sensitizing regimen of AMPH.

MATERIALS AND METHODS

Using retrograde labeling techniques, Fos activation was evaluated in the predominant projection pathways of the mPFC of sensitized rats after a challenge injection of AMPH.

RESULTS

There was a significant increase in Fos-immunoreactive cells in the mPFC, nucleus accumbens (NAc), basolateral amygdala (BLA), and lateral hypothalamus (LH) of rats treated repeatedly with AMPH when compared to vehicle-treated controls. The mPFC pyramidal neurons that project to the LH but not the NAc or BLA show a significant induction of Fos after repeated AMPH treatment. In addition, we found a dramatic increase in Fos-activated orexin neurons.

CONCLUSIONS

The LH, a region implicated in natural and drug reward processes, may play a role in the development and persistence of sensitization to repeated AMPH through its connections with the mPFC and possibly through its orexin neurons.

Regional adaptations in PSD-95, NGFI-A and secretogranin gene transcripts related to vulnerability to behavioral sensitization to amphetamine in the Roman rat strains

Genetically selected for high or low two-way active avoidance, Roman high-avoidance (RHA) and Roman low-avoidance (RLA) rats differ in their central dopaminergic activity, sensation/novelty- and substance-seeking profiles. These animals are, therefore, well suited to identify anatomical and neurochemical concomitants of behavioral sensitization, a phenomenon linked to addictive liability. We submitted inbred RHA (RHA-I), inbred RLA (RLA-I) and Sprague-Dawley-OFA (SD-OFA) rats to a sensitization regimen with amphetamine and studied the behavioral response to an amphetamine challenge after a 2-week withdrawal period. The expression patterns of nerve growth factor inducible clone A (NGFI-A), secretogranin, post-synaptic density protein of 95 Kd (PSD-95), prodynorphin and proenkephalin mRNA were also analyzed using in situ hybridization, after the challenge with amphetamine. RHA-I rats showed stronger sensitization than SD-OFA rats. RLA-I rats did not show sensitization but were hyper-reactive to amphetamine. Expression of behavioral sensitization in RHA-I rats activated secretogranin and PSD-95 mRNA in the nucleus accumbens core. On the other hand, high induction of NGFI-A mRNA in the central amygdala was observed in RLA-I rats when they experienced amphetamine for the first time in the challenge. Our results reveal that 1) the acute locomotor response to amphetamine does not predict vulnerability to behavioral sensitization and 2) differences in vulnerability to sensitization may involve distinctive cellular adaptations at particular brain locations which may be related to addictive vulnerability.

Differential laminar effects of amphetamine on prefrontal parvalbumin interneurons

The increase in excitatory outflow from the medial prefrontal cortex is critical to the development of sensitization to amphetamine. There is evidence that psychostimulant-induced changes in dopamine-GABA interactions are key to understanding the behaviorally sensitized response. The objective of this study was to characterize the effects of different amphetamine paradigms on the Fos activation of GABAergic interneurons that contain parvalbumin in the medial prefrontal cortex. Although a sensitizing, repeated regimen of amphetamine induced Fos in all cortical layers, only layer V parvalbumin-immunolabeled cells were activated in the infralimbic and prelimbic cortices. Repeated amphetamine treatment was also associated with a loss of parvalbumin immunoreactivity in layer V, but only in the prelimbic cortex. An acute amphetamine injection to naive rats was associated with an increase in Fos, but in parvalbumin-positive neurons of the prelimbic cortex, where it was preferentially induced in layer III. These data indicate that distinct substrates mediate the response to repeated or acute amphetamine treatment. They also suggest that a sensitizing amphetamine regimen directs medial prefrontal cortex (mPFC) outflow, via changes in inhibitory neuron activation, toward subcortical centers important in reward.

A scale-free systems theory of motivation and addiction

Scale-free organizations, characterized by uneven distributions of linkages between nodal elements, describe the structure and function of many life-based complex systems developing under evolutionary pressures. We explore motivated behavior as a scale-free map toward a comprehensive translational theory of addiction. Motivational and behavioral repertoires are reframed as link and nodal element sets, respectively, comprising a scale-free structure. These sets are generated by semi-independent information-processing streams within cortical-striatal circuits that cooperatively provide decision-making and sequential processing functions necessary for traversing maps of motivational links connecting behavioral nodes. Dopamine modulation of cortical-striatal plasticity serves a central-hierarchical mechanism for survival-adaptive sculpting and development of motivational-behavioral repertoires by guiding a scale-free design. Drug-induced dopamine activity promotes drug taking as a highly connected behavioral hub at the expense of natural-adaptive motivational links and behavioral nodes. Conceptualizing addiction as pathological alteration of scale-free motivational-behavioral repertoires unifies neurobiological, neurocomputational and behavioral research while addressing addiction vulnerability in adolescence and psychiatric illness. This model may inform integrative research in defining more effective prevention and treatment strategies for addiction.

Changes in Proenkephalin mRNA expression in forebrain areas after amphetamine-induced behavioural sensitization

Acute and repeated psychostimulant administration induces a long-lasting enhanced behavioural response to a subsequent drug challenge, known as behavioural sensitization. This phenomenon involves persistent neurophysiological adaptations, which may lead to drug addiction. Brain dopaminergic pathways have been implicated as the main neurobiological substrates of behavioural sensitization, although other neurotransmitters and neuromodulators may also participate. In order to investigate a possible involvement of opioid systems in amphetamine (AMPH) behavioural sensitization, we studied the AMPH-induced changes in Proenkephalin (Pro-Enk) mRNA expression in forebrain areas in both drug-naïve and AMPH-sensitized rats. Male Sprague-Dawley rats were sensitized to AMPH by means of a single AMPH (1 mg/kg s.c.) injection and the same dose was injected 7 days later to assess the expression of sensitization. Pro-Enk mRNA levels were evaluated by in situ hybridization in coronal brain sections. AMPH injection induced an increase in Pro-Enk mRNA expression in the nucleus accumbens and the medial-posterior caudate-putamen in drug-naïve rats. Challenge with AMPH to rats injected 1 week earlier with AMPH induced motor sensitization and increased and decreased Pro-Enk mRNA expression in the prefrontal cortex and the anterior medial caudate-putamen, respectively. Our results suggest that alterations in cortical and striatal enkephalinergic systems could contribute to the expression of AMPH behavioural sensitization.

The psychogenetically selected Roman high- and low-avoidance rat lines: A model to study the individual vulnerability to drug addiction

The Roman high- (RHA) and low-avoidance (RLA) rat lines were selected for, respectively, rapid vs poor acquisition of two-way active avoidance in the shuttle-box. Here, we review experimental evidence indicating that, compared with their RLA counterparts, RHA rats display a robust sensation/novelty seeking profile, a marked preference and intake of natural or drug rewards, and more pronounced behavioral and neurochemical responses to the acute administration of morphine and psychostimulants. Moreover, we show that (i) the repeated administration of morphine and cocaine elicits behavioral sensitization in RHA, but not RLA, rats, (ii) in sensitized RHA rats, acute morphine and cocaine cause a larger increment in dopamine output in the core, and an attenuated dopaminergic response in the shell of the nucleus accumbens, as compared with RHA rats repeatedly treated with saline, and (iii) such neurochemical changes are not observed in the mesoaccumbens dopaminergic system of the sensitization-resistant RLA line. Behavioral sensitization plays a key role in several cardinal features of addiction, including drug craving, compulsive drug seeking and propensity to relapse following detoxification. Comparative studies in the Roman lines may therefore represent a valid approach to evaluate the contribution of the genotype on the neural substrates of drug sensitization and addiction.

Long-term functional consequences of quinolinic acid striatal lesions and their alteration following an addition of a globus pallidus lesion assessed using pharmacological magnetic resonance imaging

The present study tested the hypothesis that lesion to the rat globus pallidus (GP) can "normalize" the functioning of the basal ganglia-thalamocortical circuits in striatal-lesioned rats by assessing the functional connectivity of these regions using functional magnetic resonance imaging (fMRI). Changes in brain activation following systemic administration of amphetamine were assessed in (1) rats sustaining a unilateral lesion to the striatum, (2) rats sustaining a combined striatal and pallidal lesion, and (3) control rats. Striatal-lesioned rats showed attenuated cortical activation following amphetamine administration and lower correlations between the responses to amphetamine in different brain regions compared to control rats. Although the addition of an excitotoxic GP lesion failed to prevent striatal lesion-induced attenuation of cortical activation by amphetamine, it was effective in "normalizing" the correlations between the responses to amphetamine in the different areas. These results suggest that, although the GP lesion is ineffective in correcting the global changes in activity caused by the striatal lesion, it may have the capacity to partially restore alterations in functional connectivity resulting from the striatal lesion. These results are further discussed in view of our previous demonstration that lesions to the GP can reverse several behavioral deficits produced by a striatal lesion.

Selective disruption of nucleus accumbens gating mechanisms in rats behaviorally sensitized to methamphetamine

Behavioral sensitization of psychostimulant-induced locomotor activity in rats has been proposed as a model of addiction and is accompanied by neuroadaptations in the nucleus accumbens and related circuits. Here, we used in vivo intracellular recordings to examine electrophysiological properties of accumbens neurons from animals that did or did not exhibit behavioral sensitization after repeated methamphetamine (5.0 mg/kg; 5 d). Although spontaneous activity of accumbens neurons was virtually unchanged, multiple synaptic interactions controlling membrane potential states were disrupted in sensitized animals. For example, stimulation of the ventral tegmental area attenuated accumbens responses to prefrontal cortex activation in nonsensitized and saline-treated animals, but not in sensitized animals. Acute methamphetamine (0.5 mg/kg) abolished accumbens up and down states in nonsensitized and saline-treated animals, suggesting a disruption of normal information processing in this area. However, acute methamphetamine failed to affect this pattern in accumbens neurons from sensitized animals. These results suggest that both acute and repeated methamphetamine administration can disrupt synaptic interactions in the nucleus accumbens; however, the nature of these alterations depends critically on the extent of behavioral sensitization. It is speculated that the response to acute methamphetamine in nonsensitized and saline-treated animals may be functionally adaptive, whereas the neuroadaptations observed in sensitized animals may be maladaptive and detrimental to accumbens information processing.

Differential activation of dopamine release in the nucleus accumbens core and shell after acute or repeated amphetamine injections: A comparative study in the Roman high- and low-avoidance rat lines

The selectively bred Roman high- and low-avoidance rats differ in emotionality and responsiveness to the motor effects of acute and repeated psychostimulant administration. These lines also show drastic differences in the neurochemical responses of their mesolimbic dopamine systems to addictive drugs. The nucleus accumbens is critically involved in the locomotor activation produced by psychostimulants and in the augmentation of this effect observed upon repeated drug administration (i.e. behavioral sensitization), although there is not a general consensus as to whether the nucleus accumbens-core or the nucleus accumbens-shell is preferentially involved in such alterations. This study was designed to evaluate the effects of acute amphetamine (0.20 mg/kg, s.c.) on dopamine output in the nucleus accumbens-shell and nucleus accumbens-core of the Roman lines under basal conditions (i.e. naïve rats) and after the repeated administration of amphetamine (1 mg/kg, s.c. x 10 days) or saline. We show that (1) in naïve rats, amphetamine caused a larger increment in dopamine output in the nucleus accumbens-shell vs the nucleus accumbens-core only in the Roman high-avoidance line; (2) repeated amphetamine elicits behavioral sensitization in Roman high-avoidance, but not Roman low-avoidance, rats; (3) in sensitized Roman high-avoidance rats, amphetamine provokes a larger increment in dopamine output in the nucleus accumbens-core, and an attenuated dopaminergic response in the nucleus accumbens-shell, as compared with Roman high-avoidance rats repeatedly treated with saline; and (4) such neurochemical changes are not observed in the mesoaccumbens dopaminergic system of the sensitization-resistant Roman low-avoidance line. We propose that (1) Roman high-avoidance and Roman low-avoidance rats differ in the vulnerability to develop psychostimulant sensitization, (2) the nucleus accumbens-core and nucleus accumbens-shell subserve distinct functional roles in this phenomenon, and (3) comparative studies in the Roman lines may provide insight into the influence of neural substrates and genetic background on the individual vulnerability to addiction.

The induction of behavioural sensitization is associated with cocaine-induced structural plasticity in the core (but not shell) of the nucleus accumbens

Repeated exposure to cocaine increases the density of dendritic spines on medium spiny neurons in the nucleus accumbens (Acb) and pyramidal cells in the medial prefrontal cortex (mPFC). To determine if this is associated with the development of psychomotor sensitization, rats were given daily i.p. injections of 15 mg/kg of cocaine (or saline) for 8 days, either in their home cage (which failed to induce significant psychomotor sensitization) or in a distinct and relatively novel test cage (which induced robust psychomotor sensitization). Their brains were obtained 2 weeks after the last injection and processed for Golgi-Cox staining. In the Acb core (AcbC) cocaine treatment increased spine density only in the group that developed psychomotor sensitization (i.e. in the Novel but not Home group), and there was a significant positive correlation between the degree of psychomotor sensitization and spine density. In the Acb shell (AcbS) cocaine increased spine density to the same extent in both groups; i.e. independent of psychomotor sensitization. In the mPFC cocaine increased spine density in both groups, but to a significantly greater extent in the Novel group. Furthermore, when rats were treated at Home with a higher dose of cocaine (30 mg/kg), cocaine now induced psychomotor sensitization in this context, and also increased spine density in the AcbC. Thus, the context in which cocaine is experienced influences its ability to reorganize patterns of synaptic connectivity in the Acb and mPFC, and the induction of psychomotor sensitization is associated with structural plasticity in the AcbC and mPFC, but not the AcbS.

In vivo gene delivery of urokinase-type plasminogen activator with regulatable lentivirus induces behavioural changes in chronic cocaine administration

Serine proteases play a key function in extracellular processes affecting central nervous system plasticity. Recently, the role of extracellular proteolytic processes in regulating synaptic structure and function has been described. However, to date direct evidence linking extracellular serine protease activity with drug-related behavioural changes has not been documented. Importantly, in a screening for genes induced after drug treatment we found that urokinase plasminogen-type activator (uPA) was strongly regulated by cocaine in several protocols of drug administration. Cocaine-induced up-regulation could be verified on microarray analysis under several protocols of drug administration, then further fully confirmed by means of qRT-PCR. As a result, we chose to investigate further uPA function in the mesolimbic dopaminergic pathway, a major target area of cocaine and drugs of misuse. Our approach was based on the characterization of cocaine-induced behavioural changes following lentiviral vector delivery of a doxycycline-regulated uPA expression cassette (or of its mutated form), into specific rat brain areas (the hippocampus, the nucleus accumbens and the ventral tegmental area). We show that doxycycline-dependent over-expression of uPA in these regions yields a 10- to 12.3-fold increase in locomotor activity after cocaine administration. These behavioural effects were completely abolished when the active site of the protease was point-mutated and used as a dominant negative. The physiological relevance of these drastic behavioural changes is discussed.

A progressive ratio schedule of self-stimulation testing in rats reveals profound augmentation of d-amphetamine reward by food restriction but no effect of a "sensitizing" regimen of d-amphetamine

RATIONALE

Prior research indicates that psychostimulant-induced sensitization is not expressed in lateral hypothalamic electrical self-stimulation (LHSS)-based measures of drug reward, although the augmenting effect of chronic food restriction is. Neuroadaptations within the brain dopamine system have been identified in both psychostimulant-sensitized and food-restricted animals. Consequently, a variant of the LHSS paradigm in which responding is particularly sensitive to changes in dopaminergic tone may be best suited to detect and compare effects of chronic d-amphetamine and food restriction. Instrumental responding on a progressive ratio (PR) schedule is more sensitive to dopaminergic manipulations than is responding on a continuous reinforcement (CRF) schedule, but has not previously been used to examine chronic psychostimulant and food restriction effects on LHSS-based measures of drug reward.

OBJECTIVE

The first aim of this study was to determine whether a regimen of d-amphetamine treatment, that produces locomotor sensitization (5 mg/kg per day x5 days), increases the reward-potentiating effect of d-amphetamine in a PR LHSS protocol. The second aim, was to determine whether chronic food restriction produces a marked increase in the reward-potentiating effect of d-amphetamine in the PR LHSS protocol and, if so, whether it is reversible in parallel with body weight recovery when free feeding is restored.

METHOD

Reward-potentiating effects of a challenge dose of d-amphetamine (0.25 mg/kg, IP) were measured in terms of the break point of LHSS responding on a PR schedule of reinforcement, in ad libitum fed and food-restricted rats.

RESULTS

A regimen of d-amphetamine treatment that produced locomotor sensitization did not increase the break point for LHSS in the presence or absence of d-amphetamine. Chronic food restriction produced a marked increase in the break point-increasing effect of d-amphetamine (3-fold), which returned to baseline in parallel with body weight recovery over a 4-week period of restored free-feeding.

CONCLUSIONS

A locomotor-sensitizing regimen of d-amphetamine treatment does not increase the rewarding effect of LH electrical stimulation or the reward-potentiating effect of d-amphetamine in a PR LHSS protocol. The augmenting effect of chronic food restriction on drug reward is mechanistically and functionally different from psychostimulant sensitization and may be controlled by signals associated with adipose depletion and repletion.

The use of stereological counting methods to assess immediate early gene immunoreactivity

The issue of whether profile and stereological counting methods are interchangeably accurate when assessing immediate early gene expression still needs to be resolved. To compare these two counting techniques, we quantified the expression of c-fos in the nucleus accumbens core and shell, and in the lateral septum as a control structure, of rats treated with neuroleptics. With the profile counting method, which relies on selective placement of a counting grid within a structure, we evaluated the density of c-fos labeled cells within a box of fixed dimension. With stereology, which applies random and systematic sampling methods, we used the optical fractionator method and counted the absolute number of c-fos labeled cells within the contours of each structure examined. Our results showed that the substantial increase in c-fos expression in the shell and core induced by haloperidol treatment was detected by both stereological and profile counting methods; in contrast, the weaker effect of clozapine on c-fos expression was detected differentially by the two methods. Whereas the profile counting method reported a reduction of c-fos in the core by clozapine, and an increase in c-fos in the lateral septum, these effects were not replicated using stereology. These findings suggest that stereological and profile counting methods do not always produce equivalent results. This may be particularly relevant when a measured effect is relatively small, and it is not distributed homogeneously within a structure. In this respect, the random and systematic sampling methods of stereology may yield more accurate and unbiased results than the profile counting method, and therefore may be preferred for a more accurate and thorough investigation of a treatment effect on immediate early gene expression in a specific brain region.

The medial prefrontal cortex in the rat: evidence for a dorso-ventral distinction based upon functional and anatomical characteristics

The prefrontal cortex in rats can be distinguished anatomically from other frontal cortical areas both in terms of cytoarchitectonic characteristics and neural connectivity, and it can be further subdivided into subterritories on the basis of such criteria. Functionally, the prefrontal cortex of rats has been implicated in working memory, attention, response initiation and management of autonomic control and emotion. In humans, dysfunction of prefrontal cortical areas with which the medial prefrontal cortex of the rat is most likely comparable is related to psychopathology including schizophrenia, sociopathy, obsessive-compulsive disorder, depression, and drug abuse. Recent literature points to the relevance of conducting a functional analysis of prefrontal subregions and supports the idea that the area of the medial prefrontal cortex in rats is characterized by its own functional heterogeneity, which may be related to neuroanatomical and neurochemical dissociations. The present review covers recent findings with the intent of correlating these distinct functional differences in the dorso-ventral axis of the rat medial prefrontal cortex with anatomical and neurochemical patterns.

Evidence of increased dopamine receptor signaling in food-restricted rats

It is well established that chronic food restriction enhances sensitivity to the rewarding and motor-activating effects of abused drugs. However, neuroadaptations underlying these behavioral effects have not been characterized. The purpose of the present study was to explore the possibility that food restriction produces increased dopamine (DA) receptor function that is evident in behavior, signal transduction, and immediate early gene expression. In the first two experiments, rats received intracerebroventricular (i.c.v.) injections of the D1 DA receptor agonist SKF-82958, and the D2/3 DA receptor agonist quinpirole. Both agonists produced greater motor-activating effects in food-restricted than ad libitum-fed rats. In addition, Fos-immunostaining induced by SKF-82958 in caudate-putamen (CPu) and nucleus accumbens (Nac) was greater in food-restricted than ad libitum-fed rats, as was staining induced by quinpirole in globus pallidus and ventral pallidum. In the next two experiments, neuronal membranes prepared from CPu and Nac were exposed to SKF-82958 and quinpirole. Despite the documented involvement of cyclic AMP (cAMP) signaling in D1 DA receptor-mediated c-fos induction, stimulation of adenylyl cyclase (AC) activity by SKF-82958 in CPu and Nac did not differ between groups. Food restriction did, however, decrease AC stimulation by the direct enzyme stimulant, forskolin, but not NaF or MnCl(2), suggesting a shift in AC expression to a less catalytically efficient isoform. Finally, food restriction increased quinpirole-stimulated [(35)S]guanosine triphosphate-gammaS binding in CPu, suggesting that increased functional coupling between D2 DA receptors and G(i) may account for the augmented behavioral and pallidal c-Fos responses to quinpirole. Results of this study support the hypothesis that food restriction leads to neuroadaptations at the level of postsynaptic D1 and D2 receptor-bearing cells which, in turn, mediate augmented behavioral and transcriptional responses to DA. The signaling pathways mediating these augmented responses remain to be fully elucidated.