Cortical and ventral tegmental systems exert opposing influences on self-stimulation from the prefrontal cortex

Animals with a schizophrenia-like phenotype are differentially sensitive to the motivational effects of cannabinoid agonists in conditioned place preference

Cannabis is the most consumed illicit drug worldwide, but among patients with a diagnosis of schizophrenia, this consumption is higher suggesting that they are differentially sensitive to cannabis. We chose to study this problematic using a neurodevelopmental model of schizophrenia: neonatal ventral hippocampus lesions (NVHL). In a first study, we compared the locomotor response to novelty, a mild stress and two doses of amphetamine (0.75 and 1.5 mg/kg) in sham and NVHL rats at post-natal day 35 (PD35) or 56 (PD56). In a second study, we investigated the valence of the motivational effect of Delta-9-tetrahydrocannabinnol (THC, 0.5 mg/kg, i.p.) and the cannabinoid receptor agonist, WIN55,212-2 (WIN, 1 mg/kg, i.p.), using the conditioned place preference paradigm; we used a biased procedure that comprised 12 days of testing with 3 paired-conditioning. The effects of this dose of WIN were also measured on locomotor activity. Results confirmed that the adult NVHL animals displayed a stronger locomotor response to the two doses of amphetamine, but not to novelty and a mild stress. In adult NVHL, but not sham animals, WIN stimulated locomotor activity and produced a conditioned place aversion. At the dose tested, THC tended to produce an aversion in adult sham but not NVHL animals. Taken together these findings show that adult animals with a schizophrenia-like phenotype are differentially sensitive to the motivational effect of cannabinoids.

Stress in adolescence and drugs of abuse in rodent models: role of dopamine, CRF, and HPA axis

RATIONALE

Research on adolescence and drug abuse increased substantially in the past decade. However, drug-addiction-related behaviors following stressful experiences during adolescence are less studied. We focus on rodent models of adolescent stress cross-sensitization to drugs of abuse.

OBJECTIVES

Review the ontogeny of behavior, dopamine, corticotropin-releasing factor (CRF), and the hypothalamic-pituitary-adrenal (HPA) axis in adolescent rodents. We evaluate evidence that stressful experiences during adolescence engender hypersensitivity to drugs of abuse and offer potential neural mechanisms.

RESULTS AND CONCLUSIONS

Much evidence suggests that final maturation of behavior, dopamine systems, and HPA axis occurs during adolescence. Stress during adolescence increases amphetamine- and ethanol-stimulated locomotion, preference, and self-administration under many conditions. The influence of adolescent stress on subsequent cocaine- and nicotine-stimulated locomotion and preference is less clear. The type of adolescent stress, temporal interval between stress and testing, species, sex, and the drug tested are key methodological determinants for successful cross-sensitization procedures. The sensitization of the mesolimbic dopamine system is proposed to underlie stress cross-sensitization to drugs of abuse in both adolescents and adults through modulation by CRF. Reduced levels of mesocortical dopamine appear to be a unique consequence of social stress during adolescence. Adolescent stress may reduce the final maturation of cortical dopamine through D2 dopamine receptor regulation of dopamine synthesis or glucocorticoid-facilitated pruning of cortical dopamine fibers. Certain rodent models of adolescent adversity are useful for determining neural mechanisms underlying the cross-sensitization to drugs of abuse.

The mesocortical dopamine projection to anterior cingulate cortex plays no role in guiding effort-related decisions

Both mesolimbic dopamine (DA) and the anterior cingulate cortex (ACC) have been implicated in enabling animals to expend effort to obtain greater reward. To investigate the role of the DA pathway to ACC in working for reward, the authors tested rats on a cost-benefit T-maze paradigm in which they could either climb a barrier to obtain large reward in 1 arm (high reward [HR]) or select the low-effort alternative containing less reward (low reward [LR]). Surprisingly, ACC DA depletions had no effect on choice performance. Manipulations of barrier and reward sizes demonstrated that lesioned rats were as sensitive to the costs and benefits of the alternatives as controls. These results imply that the DA projection to ACC is not involved in guiding effort-related decisions.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

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

Effects of cocaine on dopamine in subregions of the rat prefrontal cortex and their efferents to subterritories of the nucleus accumbens

The present study sought to investigate the contributions of the ventral prelimbic/infralimbic cortices and shell subterritory of the nucleus accumbens as well as the dorsal prelimbic/anterior cingulate cortices and core subregion of the nucleus accumbens to the acute systemic effects of cocaine (20 mg/kg i.p.) on both locomotor activity and simultaneous dialysate dopamine levels using a dual-probe microdialysis design. Basal dopamine levels were significantly higher in the ventral medial prefrontal cortex compared with the dorsal medial prefrontal cortex and higher concentrations of dopamine were also observed in the core of the nucleus accumbens compared with its shell counterpart. Cocaine produced a significant decrease in dopamine levels in both the ventral and dorsal medial prefrontal cortices. In contrast, cocaine significantly increased dialysate dopamine in the shell of the nucleus accumbens, whereas only a slight increase in dopamine was observed in the core subregion of the nucleus accumbens. A significant negative relationship between dopamine levels in the ventral and dorsal medial prefrontal cortices and dialysate dopamine concentrations in the shell and core of the nucleus accumbens was observed. Finally, in both the ventral and dorsal medial prefrontal cortices, the magnitude of the locomotor response to cocaine was inversely related to dialysate dopamine levels. In contrast, the magnitude of the locomotor response to cocaine became progressively larger as dopamine levels increased in the shell of the nucleus accumbens. These results show a dissociation in the pattern of dopamine release in subterritories of both the medial prefrontal cortex and nucleus accumbens in response to the acute systemic administration of cocaine.

Effect of adrenalectomy on cocaine facilitation of medial prefrontal cortex self-stimulation

Adrenalectomy (ADX) is known to block the acquisition of intravenous cocaine self-administration. A previous study therefore examined whether ADX decreases sensitivity of the 'brain reward system' in general, or its response to cocaine in particular, by measuring thresholds for intracranial self-stimulation with and without concurrent cocaine administration. ADX had no effect on thresholds for lateral hypothalamic self-stimulation (LHSS) and did not alter the cocaine dose-response curve for lowering the LHSS threshold. This result suggested that ADX does not affect sensitivity of the brain reward system. However, medial prefrontal cortex (MPFC) appears to be an important site in the mediation of cocaine reinforcing effects, and MPFC self-stimulation (MPFCSS) is mediated by a neural substrate that is largely independent of that which mediates LHSS. The present study therefore assessed whether ADX diminishes cocaine facilitation of MPFCSS. It was found that the threshold-lowering effect of cocaine (5.0, 10.0 and 20.0 mg/kg, i.p. ) did not differ between ADX rats maintained on 0.7% saline, ADX rats maintained on corticosterone (50 microg/ml) in 0.7% saline, and sham-operated controls. However, there was a trend toward desensitization of MPFCSS, itself, following ADX in the group that did not receive corticosterone supplementation. Based on this observation, and the similar responses of MPFCSS and cocaine self-administration to noncontingent priming stimulation, stress, and NMDA receptor antagonism, it is speculated that acquisition of MPFCSS and cocaine self-administration may be dependent upon a common sensitization process that is regulated by corticosterone.

Dissociable effects of anterior and posterior cingulate cortex lesions on the acquisition of a conditional visual discrimination: facilitation of early learning vs. impairment of late learning

Two experiments investigated the effects of quinolinic acid induced lesions of the anterior and posterior cingulate cortices on the acquisition and performance of a conditional visual discrimination (CVD) task, in which rats were required to learn a rule of the type: "If lights are flashing FAST, press the right lever; if SLOW press left". In Experiment 1, animals with lesions of the anterior cingulate cortex (ANT group) demonstrated a significant enhancement in learning during the early stages of task acquisition. Conversely, animals with lesions of the posterior cingulate cortex (POS group) were impaired in learning during the later stages of acquisition. There were no significant differences between the ANT and POS groups on the performance of the task when either variable inter-trial intervals or reduced stimulus durations were imposed. In Experiment 2, the specificity of the lesion effects for processes operative during the early and late stages of learning was tested. Animals were trained to a criterion of 70% correct choices on two consecutive sessions prior to lesioning, and subsequently allowed to continue to acquire the task to the mean asymptotic performance level of 85% correct choices on two consecutive sessions. Animals of the POS group were impaired in learning during this later stage of task acquisition, thus replicating the pattern of results obtained in Experiment 1. The animals in Experiment 2 were then tested following a 30-day retention interval and during extinction (removal of sucrose from the magazine). The extinction test revealed an impairment in the ability of animals in the ANT group to omit lever responses in the absence of reinforcement. These results indicate that the anterior and posterior cingulate cortices are functionally dissociable, and suggest that they may form part of complementary, but competing, learning and memory systems.

Dorsal lesions of the prefrontal cortex: effects on alcohol consumption and subcortical monoaminergic systems

Male Wistar rats were subjected to either bilateral aspiration lesions of the dorsal regions of the prefrontal cortex (PFC) or sham lesions and placed on a 6-week, modified sucrose-fading procedure. At the time of sacrifice, the size of the lesion, both in anterior-posterior and medial-lateral dimensions, was measured. Following sacrifice, levels of dopamine (DA), serotonin (5-HT), norepinephrine (NE), and their metabolites were measured in the midbrain (raphe) and nucleus accumbens (NA). Lesioned animals had reductions in 5-HT in the NA, and DA and NE in the raphe. The lesioned group drank more of a solution of 5% alcohol than controls early in the sucrose fading, and less during the later stages. In the lesioned group, the size of the left- and right-hemisphere lesions predicted 5-HIAA levels in the NA, and 5-HT and 5-HIAA levels in the raphe. A laterality effect was noted, such that the size of left-hemisphere lesions were positively associated with raphe 5-HT and 5-HIAA levels, and negatively associated with 5-HT levels in the NA, while right-hemisphere lesions showed the opposite relationships. In addition, the width of the left-hemisphere lesion predicted some measures of alcohol intake. These results suggest that, in the rat, the dorsal PFC is involved in the regulation of monoamines in subcortical regions known to be important in the regulation of reinforced behaviors, and that this regulation differs between hemispheres and shows a laterality effect. In addition, the dorsal PFC appears to have a subtle involvement in the regulation of alcohol intake.

Chronic morphine fails to enhance the reward value of prefrontal cortex self-stimulation

Dopamine (DA) plays an important role in the rewarding effects of drugs of abuse and intracranial self-stimulation (ICSS). We previously reported that ICSS derived from the prefrontal cortex appears insensitive to the reward-enhancing effects of amphetamine, a drug that increases DA release and reward at other ICSS sites. In the present study, rats with prefrontal electrodes were tested to see if morphine (7.5 or 10.0 mg/kg, IP) given once per day for 10 days enhanced prefrontal reward as assessed with the curve-shift method. Morphine initially produced sedation; however, after 3-4 days response rates increased sharply while frequency thresholds were unaffected. These results demonstrate that morphine does not enhance prefrontal ICSS reward and provide further evidence that prefrontal brain stimulation reward does not display the same characteristics as other ICSS sites.

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

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

Behaviourally derived estimates of excitability in striatal and medial prefrontal cortical self-stimulation sites

The refractory periods of the substrate underlying brain-stimulation reward were investigated in three rats with moveable electrodes implanted in the rostral caudate-putamen and the medial prefrontal cortex. Acquisition of caudate-putamen self-stimulation occurred within the first session, while self-stimulation for medial prefrontal cortex was observed only after three sessions of caudate-putamen stimulation. The currents required for self-stimulation ranged from 300 to 800 microA (0.1 ms pulse duration) across animals; the maximum response rates averaged roughly 40 bar presses per minute for both structures. Refractory period estimates were obtained from ten caudate-putamen and four medial prefrontal cortex sites. The time course of recovery had the following profile: the curves began to rise at 0.65 ms and 0.95 ms for caudate-putamen and medial prefrontal cortex stimulation, respectively, thereafter increasing to approach an asymptote at 6.00 ms for the caudate-putamen and 6.25 ms for the medial prefrontal cortex. The mean effectiveness value corresponding to the asymptotic portion of the curves was 73% for the caudate-putamen and 69% for the medial prefrontal cortex. Like other forebrain structures, the behaviourally derived refractory periods underlying caudate-putamen and medial prefrontal cortex stimulation, at least at these particular sites, are significantly longer than those observed in most medial forebrain bundle areas, both beginning and ending later. One interpretation for the similarity in their refractory period profiles and the apparent facilitating effect of caudate-putamen stimulation on acquisition of medial prefrontal cortex self-stimulation is that these two regions form part of the same reward substrate.

Haloperidol attenuates conditioned place preferences produced by electrical stimulation of the medial prefrontal cortex

A Conditioned Place Preference test procedure [Ettenberg and Duvauchelle (13)] was used to investigate the effects of dopamine antagonist challenge on the rewarding properties of medial prefrontal cortex (MPFC) electrical stimulation. Rats exhibited strong preferences for the side of a two-compartment test apparatus in which they experienced sessions of experimenter-administered 0.5-s trains of MPFC sine-wave 60-Hz stimulation. Pretreatment with the neuroleptic dopamine antagonist drug, haloperidol (0.0, 0.15, or 0.3 mg/kg IP), resulted in a dose-dependent reduction in the magnitude of observed place preferences. Preference tests were conducted 24 hours after drug-conditioning trials and, hence, were not subject to motoric or other nonspecific actions of the neuroleptic treatments. In a control experiment, haloperidol did not block the place aversions produced by dorsomedial tegmental stimulation. Animals can, therefore, recall place-associations formed in the presence of haloperidol, a result which challenges "state-dependent learning" explanations of the drug's actions. Together, these results are consistent with the view that dopamine neurotransmission is involved in the rewarding consequences of electrical stimulation in the medial prefrontal cortex.

Amphetamine affects the extinction of self-stimulation differently in prefrontal cortex and posterior hypothalamus of rats

The effects of amphetamine on the extinction of intracranial self-stimulation (ICSS) and on postextinction ICSS performance were examined in rats implanted with electrodes either in medial prefrontal cortex (mPFC) or in the posterior hypothalamus-ventral tegmental area (PH-VTA). Lever-pressing for ICSS was allowed to stabilize in daily 15-minute sessions before each animal was exposed to 5 minutes of extinction (responding without reward). Animals were administered either 0.25 mg/kg d-amphetamine or saline before baseline, extinction and postextinction sessions. After amphetamine treatment, the number of lever presses during extinction was higher in mPFC animals and lower in PH-VTA animals compared with saline-treated controls. Rates did not change immediately after extinction but, one day later, rates had increased in all saline-treated animals (both PH-VTA and mPFC animals) and had decreased in all amphetamine-treated animals. These findings demonstrated that the effects of amphetamine on the extinction of ICSS were different in cortical and hypothalamic sites, possibly because of regional differences in stimulus-evoked reinforcement and inhibitory processes.

Differences in sensitivity to neuroleptic blockade: medial forebrain bundle versus frontal cortex self-stimulation

The effects of systemic injections of the dopamine receptor antagonist, cis-flupenthixol were tested on intracranial self-stimulation at electrode sites in the medial forebrain bundle and the medial prefrontal cortex. Changes in the reward effectiveness of the brain stimulation were assessed using a curve-shift paradigm. Low to moderate doses of cis-flupenthixol (0.05, 0.1 and 0.15 mg/kg) consistently produced larger upward shifts in the rate-frequency function for medial forebrain bundle than for medial prefrontal self-stimulation. At the highest doses of cis-flupenthixol (0.15 and 0.2 mg/kg), some of the medial forebrain bundle rats failed to respond, whereas all medial prefrontal rats responded at these doses. These results demonstrate that medial forebrain bundle self-stimulation is much more dependent on dopamine systems than is prefrontal cortex self-stimulation.

Controllability of prestimulation of the medial prefrontal cortex determines the facilitation of self-stimulation and kindled seizures

Electrical stimulation of the medial prefrontal cortex (MPC) was administered according to the triadic design typically used to demonstrate learned helplessness. Three groups received either controllable, uncontrollable or no stimulation during the pretreatment phase. The effects of this pretreatment on the acquisition of self-stimulation at the same electrode site were investigated in the second phase of the experiment. Relative to unstimulated controls, both controllable and uncontrollable prestimulation facilitated the acquisition of self-stimulation and produced higher self-stimulation rates. In addition, compared with controllable stimulation, pretreatment with uncontrollable stimulation produced a greater facilitation in self-stimulation rate. The unambiguous demonstration of a behavioural facilitation produced by pretreatment with uncontrollable stimulation is, effectively, the inverse of the typical learned helplessness finding. It was also found, in the second phase of the experiment, that 6 of the 7 rats previously exposed to uncontrollable stimulation developed full class 5 seizures. No behavioural evidence of kindling was seen in any of the other rats or during the prestimulation procedure. These data are interpreted in terms of kindling and stress effects both proximal and distal to the site of stimulation.

Multiple reward systems and the prefrontal cortex

Electrical stimulation of the major divisions of the prefrontal cortex, the mediodorsal and sulcal areas, can serve as a reinforcing stimulus. Studies of self-stimulation of the prefrontal cortex have produced behavioral, anatomical and pharmacological evidence that the substrate of these rewarding effects can be dissociated from that subserving self-stimulation of ventral diencephalic sites such as the lateral hypothalamus. Other studies indicate that within the prefrontal cortex itself, self-stimulation of the medial and sulcal divisions can be attributed to dissociable processes. These observations suggest the existence of multiple, largely autonomous prefrontal subsystems involved in reinforcement. This raises the question of the functional significance of such systems, and of their organization. An approach to this problem is to consider the relationship between the behavioral functions of the prefrontal divisions and the characteristics of stimulation-induced reward obtained at each site. Studies of the effects of restricted prefrontal lesions indicate that the medial and sulcal divisions can be dissociated according to their involvement in the control of distinct types of sensory and motor events. Further experiments indicate that damage to each division causes selective deficits in the learning of stimulus-reinforcer and response-reinforcer relations, depending in part on the nature of the reinforcing event. Conditioning experiments further show that the rewarding effects produced by stimulation of these areas are preferentially associated to sensory events which correspond to the functional specialization of each division. These data are interpreted to suggest that different rewarding events and/or different attributes of rewarding stimuli are processed by distinct systems which are reflected by the organization of dissociable self-stimulation pathways.

Prefrontal cortex lesions attenuate substantia nigra self-stimulation: a reward summation analysis

A curve-shift paradigm was used to assess the effects of lesions of the prefrontal cortex on self-stimulation from electrode sites in the substantia nigra. Combined lesions of the medial and sulcal cortical regions severely attenuated substantia nigra self-stimulation. These results are discussed in the context of the frontal cortex and the substantia nigra as belonging to a reinforcement system that is largely independent of the medial forebrain bundle system.

Unilateral and bilateral lesions of the anteromedial cortex increase perseverative head movements of the rat

Lesions of the anteromedial cortex were made in rats trained to orient their heads to 9 positions located laterally and centrally in upper, middle and lower regions of space. The water-deprived subjects were tested in sessions of 20 trials in each of which all positions were baited once with a small amount of water. Training continued until no position was consistently missed and the number of returns to positions already selected on that trial (perseverative responses) was low and stable. In Expt. 1, rats with unilateral anteromedial lesions tested with 3 days recovery did not miss any position or significantly change the order in which correct positions were selected but they made more perseverative responses to all positions. Bilateral anteromedial lesions moderately increased misses but markedly increased perseverations. Visual cortex lesions comparable in size produced no changes. In Expt. 2, unilateral anteromedial lesions tested with 7 days' recovery did not increase misses and only slightly increased perseverations. In Expt. 3, unilateral anteromedial lesions tested within 1 day increased perseverations more than lesions tested with 6 days' recovery. Neither group increased misses or changed selection order. The perseverations appeared on a trial after 6 of the 9 correct selections and were directed to both recently and remotely selected positions. Lesions of the anteromedial cortex appear to produce a deficit in the suppressive control of head orienting.

The role of corticocortical projections in self-stimulation of the prelimbic and sulcal prefrontal cortex in rats

Four experiments were performed to assess the nature of the contribution of the corticocortical projections between the prelimbic and sulcal divisions of the rat prefrontal cortex to self-stimulation (SS) of these sites. The first experiment showed that transection of these projections by parasagittal knife cuts or bilateral electrolytic lesions of the prelimbic cortex had no effect on SS of the sulcal cortex. The second experiment demonstrated that SS of the prelimbic cortex could be obtained after transection of the corticocortical projection path. The third experiment demonstrated that the deficit in prelimbic SS, seen to follow such bilateral transections, is a function of the amount of exposure to the stimulation given to the animals after the lesion. The fourth experiment showed that the stimulation-dependent process underlying the acquisition of prelimbic and sulcal SS could be dissociated by the knife cuts. The discussion focused on the implications of these findings for an account of prefrontal self-stimulation behavior.