Intra-prefrontal cortex injections of SCH 23390 influence nucleus accumbens dopamine levels 24 h post-infusion

Concurrent LI-rTMS induces changes in c-Fos expression but not behavior during a progressive ratio task with adult ephrin-A2A5-/- mice

Changes within the dopaminergic system induced by repetitive transcranial magnetic stimulation (rTMS) may contribute to its therapeutic effects; however, dopamine-related behavioral effects of rTMS have not been widely investigated. We recently showed that ephrin-A2A5^-/- mice completed significantly fewer trials in a visual task than wildtype mice, and that concurrent low-intensity (LI-) rTMS during the task could partially rescue the abnormal behavior [Poh et al. 2018, eNeuro, vol. 5]. Here, we investigated whether the behavioral differences in ephrin-A2A5^-/- mice are due to abnormal motivation, primarily a dopamine-modulated behavior, and whether LI-rTMS would increase motivation. Ephrin-A2A5^-/- and wildtype mice underwent 14 daily sessions of progressive ratio (PR) tasks and received either sham or LI-rTMS during the first 10 min. Ephrin-A2A5^-/- mice responded more than wildtype comparisons, and LI-rTMS did not influence task performance for either strain. Therefore concurrent stimulation does not influence motivation in a PR task. However, ephrin-A2A5^-/- mice did have abnormal performance in the PR tasks after a change in the PR schedule which suggests perseverative behavior. We stained for c-Fos in the prelimbic area (PrL), ventral tegmental area and nucleus accumbens (NAc) core and shell to examine neuronal activity from the final PR session. Sham ephrin-A2A5^-/- mice had lower c-Fos expression in the PrL and NAc vs. wildtype mice. Ephrin-A2A5^-/- mice that received LI-rTMS showed c-Fos expression closer to wildtype levels in the NAc. Combined with high PR performance, ephrin-A2A5^-/- mice show an abnormal shift to habitual responding and LI-rTMS may attenuate this shift.

Relationship between impulsivity, prefrontal anticipatory activation, and striatal dopamine release during rewarded task performance

Impulsivity, and in particular the negative urgency aspect of this trait, is associated with poor inhibitory control when experiencing negative emotion. Individual differences in aspects of impulsivity have been correlated with striatal dopamine D2/D3 receptor availability and function. This multi-modal pilot study used both positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to evaluate dopaminergic and neural activity, respectively, using modified versions of the monetary incentive delay task. Twelve healthy female subjects underwent both scans and completed the NEO Personality Inventory Revised to assess Impulsiveness (IMP). We examined the relationship between nucleus accumbens (NAcc) dopaminergic incentive/reward release, measured as a change in D2/D3 binding potential between neutral and incentive/reward conditions with [(11)C]raclopride PET, and blood oxygen level-dependent (BOLD) activation elicited during the anticipation of rewards, measured with fMRI. Left NAcc incentive/reward dopaminergic release correlated with anticipatory reward activation within the medial prefrontal cortex (mPFC), left angular gyrus, mammillary bodies, and left superior frontal cortex. Activation in the mPFC negatively correlated with IMP and mediated the relationship between IMP and incentive/reward dopaminergic release in left NAcc. The mPFC, with a regulatory role in learning and valuation, may influence dopamine incentive/reward release.

Natural rewards, neuroplasticity, and non-drug addictions

There is a high degree of overlap between brain regions involved in processing natural rewards and drugs of abuse. "Non-drug" or "behavioral" addictions have become increasingly documented in the clinic, and pathologies include compulsive activities such as shopping, eating, exercising, sexual behavior, and gambling. Like drug addiction, non-drug addictions manifest in symptoms including craving, impaired control over the behavior, tolerance, withdrawal, and high rates of relapse. These alterations in behavior suggest that plasticity may be occurring in brain regions associated with drug addiction. In this review, I summarize data demonstrating that exposure to non-drug rewards can alter neural plasticity in regions of the brain that are affected by drugs of abuse. Research suggests that there are several similarities between neuroplasticity induced by natural and drug rewards and that, depending on the reward, repeated exposure to natural rewards might induce neuroplasticity that either promotes or counteracts addictive behavior.

Adolescent social defeat alters markers of adult dopaminergic function

Stressful experiences during adolescence can alter the trajectory of neural development and contribute to psychiatric disorders in adulthood. We previously demonstrated that adolescent male rats exposed to repeated social defeat stress show changes in mesocorticolimbic dopamine content both at baseline and in response to amphetamine when tested in adulthood. In the present study we examined whether markers of adult dopamine function are also compromised by adolescent experience of social defeat. Given that the dopamine transporter as well as dopamine D1 receptors act as regulators of psychostimulant action, are stress sensitive and undergo changes during adolescence, quantitative autoradiography was used to measure [(3)H]-GBR12935 binding to the dopamine transporter and [(3)H]-SCH23390 binding to dopamine D1 receptors, respectively. Our results indicate that social defeat during adolescence led to higher dopamine transporter binding in the infralimbic region of the medial prefrontal cortex and higher dopamine D1 receptor binding in the caudate putamen, while other brain regions analyzed were comparable to controls. Thus it appears that social defeat during adolescence causes specific changes to the adult dopamine system, which may contribute to behavioral alterations and increased drug seeking.

Behavioral, thermal and neurochemical effects of acute and chronic 3,4-methylenedioxymethamphetamine ("Ecstasy") self-administration

3,4-Methylenedioxymethamphetamine (MDMA) is a popular methamphetamine derivative associated with young adults and all-night dance parties. However, the enduring effects of MDMA at voluntary intake levels have not been extensively investigated. In this study, MDMA-influenced behaviors and core temperatures were assessed over the course of 20 daily MDMA self-administration sessions in rats. In vivo microdialysis techniques were used in a subsequent MDMA challenge test session to determine extracellular nucleus accumbens dopamine (NAcc DA) and serotonin (5-HT) levels in MDMA-experienced and naïve animals before and after a self-administered MDMA injection (3.0mg/kg, i.v.). During self-administration sessions, gradual and significant increases in MDMA intake and MDMA-stimulated locomotor activity were observed across sessions. Core temperature significantly decreased during initial MDMA sessions, but was unaltered by the last 10 sessions. In the MDMA challenge test, MDMA-naïve rats showed significantly higher NAcc 5-HT responses compared to MDMA-experienced rats, though MDMA experience did not affect the magnitude of NAcc DA response. The overall findings suggest that changes in MDMA-induced responses over the course of increasing levels of drug exposure may reflect the development of tolerance to a number of MDMA effects.

The antipsychotic potential of l-stepholidine--a naturally occurring dopamine receptor D1 agonist and D2 antagonist

RATIONALE

l-Stepholidine, a dopamine D(2) antagonist with D(1) agonist activity, should in theory control psychosis and treat cognitive symptoms by enhancing cortical dopamine transmission. Though several articles describe its impact on the dopamine system, it has not been systematically evaluated and compared to available antipsychotics.

MATERIALS AND METHODS

We examined its in vitro interaction with dopamine D(2) and D(1) receptors and compared its in vivo pharmacokinetic profile to haloperidol (typical) and clozapine (atypical) in animal models predictive of antipsychotic activity.

RESULTS

In vitro, l-stepholidine showed significant activity on dopamine receptors, and in vivo, l-stepholidine demonstrated a dose-dependent striatal receptor occupancy (RO) at D(1) and D(2) receptors (D(1) 9-77%, 0.3-30 mg/kg; D(2) 44-94%, 1-30 mg/kg), though it showed a rather rapid decline of D(2) occupancy related to its quick elimination. In tests of antipsychotic efficacy, it was effective in reducing amphetamine- and phencyclidine-induced locomotion as well as conditioned avoidance response, whereas catalepsy and prolactin elevation, the main side effects, appeared only at high D(2)RO (>80%). This preferential therapeutic profile was supported by a preferential immediate early gene (Fos) induction in the nucleus accumbens over dorsolateral striatum. We confirmed its D(1) agonism in vitro, and then using D(2) receptor, knockout mice showed that l-stepholidine shows D(1) agonism in the therapeutic dose range.

CONCLUSIONS

Thus, l-stepholidine shows efficacy like an "atypical" antipsychotic in traditional animal models predictive of antipsychotic activity and shows in vitro and in vivo D(1) agonism, and, if its rapid elimination does not limit its actions, it could provide a unique therapeutic approach to schizophrenia.

Prefrontal cortex-nucleus accumbens interaction: in vivo modulation by dopamine and glutamate in the prefrontal cortex

Previous experimental studies have shown that the prefrontal cortex (PFC) regulates the activity of the nucleus accumbens (NAc), and in particular the release of dopamine in this area of the brain. In the present report we review recent microinjections/microdialysis studies from our laboratory on the effects of stimulation/blockade of dopamine and glutamate receptors in the PFC that modulate dopamine, and also acetylcholine release in the NAc. Stimulation of prefrontal D2 dopamine receptors, but not group I mGlu glutamate receptors, reduces the release of dopamine and acetylcholine in the NAc and spontaneous motor activity. This inhibitory role of prefrontal D2 receptors is not changed by acute systemic injections of the NMDA antagonist phencyclidine. On the other hand, the blockade of NMDA receptors in the PFC increases the release of dopamine and acetylcholine in the NAc as well as motor activity which suggests that the hypofunction of prefrontal NMDA receptors is able to produce the neurochemical and behavioural changes associated with a dysfunction of the corticolimbic circuit. We suggest here that dopamine and glutamate receptors are, in part, segregated in specific cellular circuits in the PFC. Thus, the stimulation/blockade of these receptors would have a different net impact on PFC output projections to regulate dopamine and acetylcholine release in the NAc and in guided behaviour. Finally, it is speculated that environmental enrichment might produce plastic changes that modify the functional interaction between the PFC and the NAc in both physiological and pathological conditions.

Dopamine modulation of hippocampal-prefrontal cortical interaction drives memory-guided behavior

Information gleaned from learning and memory processes is essential in guiding behavior toward a specific goal. However, the neural mechanisms that determine how these processes are effectively utilized to guide goal-directed behavior are unknown. Here, we show that rats utilize retrospective and prospective memory and flexible switching between these 2 memory processes to guide behaviors to obtain rewards. We found that retrospective memory is mainly processed in the hippocampus (HPC) but that this retrospective information must be incorporated within the prefrontal cortex (PFC) to be used to switch to an anticipatory response strategy involving prospective memory. Furthermore, switching between memory processes is regulated by the mesocortical dopamine (DA) system. Thus, DA D1 and D2 receptor activation in the PFC differentially affects retrospective memory processing within the HPC via an indirect feedback pathway. In contrast, D1, but not D2, receptor activation is crucial for incorporation of HPC-based retrospective information into the PFC. However, once this takes place, D2 receptor activation is required for further processing of information to effect preparation of future actions. These results provide a unique perspective on the mechanism of memory-based goal-directed behavior.

Prefrontal cortex D1 modulation of the reinforcing properties of cocaine

The involvement of the dopaminergic pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) in the reinforcing properties of many drugs of abuse is well established. Though the prefrontal cortex (PFC) exhibits significant influence over activity in this pathway, its role in drug abuse is less defined. The present experiment investigated the impact of PFC D1 activity on cocaine self-administration (0.25, 0.75 mg/kg/inj) under progressive (PR) and fixed ratio (FR) schedules of reinforcement by assessing immediate and delayed effects of bilateral intra-PFC infusions of a D1 agonist (SKF 38393; 0.23 microg/side) and antagonist (SCH 23390; 0.25 microg/side). Immediately following infusion of dopaminergic agents or vehicle, no significant changes in self-administration occurred under any tested condition. However, 24 h after intra-PFC antagonist treatment, significantly lower PR breakpoints were observed for low (0.25 mg/kg), but not moderate (0.75 mg/kg) unit doses of self-administered cocaine. Locomotor activity levels during these assessments were unaffected by intra-PFC treatments. On an FR-3 schedule of reinforcement, the 0.25 cocaine unit dose elicited higher total cocaine intake and hyperlocomotor activation during a shorter session, but intra-PFC treatment had no significant effects on the number of reinforced responses or behavioral activity. The observation of decreased cocaine breakpoints after intra-PFC DA antagonist treatment reflects decrements in cocaine reinforcement efficacy. This finding corresponds temporally with previous work showing increased NAcc DA levels after similar treatment. Current findings demonstrate that transient changes in PFC DA neurotransmission can specifically influence reinforced behaviors without affecting overall behavioral activation.

Nucleus accumbens and medial prefrontal cortex dopaminergic response to self-administered cocaine in naive rats

Cocaine reinforcement is strongly associated with increased nucleus accumbens dopamine (NAcc DA). The involvement of medial prefrontal cortex (mPFC) DA in cocaine reward is less defined, but substantial evidence indicates that increased mPFC DA may suppress NAcc DA levels. Using in vivo microdialysis, NAcc or mPFC DA was determined in cocaine-naive rats after a self-administered cocaine injection (3.0 mg/kg). Extracellular levels of NAcc DA were dramatically enhanced 10 min post-cocaine injection, but dropped significantly at each subsequent assessment. mPFC DA also increased significantly, but to a lesser extent than observed in the NAcc. Findings of prominent DA increases in both the NAcc and mPFC terminals during the test session indicate that NAcc DA responses do not appear to be inhibited by increased mPFC DA during cocaine self-administration.

Cholinergic interneurons of the nucleus accumbens and dorsal striatum are activated by the self-administration of cocaine

The nucleus accumbens, a major component of the ventral striatum, and the dorsal striatum are primary targets of the mesolimbic dopamine pathway, which is a pathway that plays a critical role in reward and addiction. The shell compartment of the nucleus accumbens and the ventromedial striatum, in particular, receive extensive afferent projections from the ventral tegmental area, which is the major afferent source of the mesolimbic pathway [Prog Brain Res 99 (1993) 209; J Neurosci 7 (1987) 3915]. The present study focused on striatal cholinergic interneurons as potential key neurons involved in the neural basis of drug reinforcement. The main finding of this study is that cholinergic interneurons located in the shell compartment of the nucleus accumbens and the ventromedial striatum were activated, as measured by Fos labeling, following a 1 h session of the self-administration of cocaine in rats. A direct correlation existed between the percent of cholinergic interneurons that were activated and the amount of cocaine that was self-administered. The greatest amount of administered cocaine (approximately 10 mg/kg) resulted in the activation of approximately 80% of the cholinergic neurons. No such correlation existed in the group of animals that self-administered saline. In addition, activation was not found in the core compartment of the nucleus accumbens or the dorsolateral striatum, which receive extensive innervation from the substantia nigra and thus are more closely tied to the motor effects of the drug. In conclusion, cocaine-driven neuronal activation was specific to the shell compartment of the nucleus accumbens (R(2)=0.9365) and the ventromedial striatum (R(2)=0.9059). These findings demonstrate that cholinergic interneurons are involved in the initial stage of cocaine intake and that these neurons are located in areas of the nucleus accumbens and dorsal striatum that are more closely tied to the rewarding and hedonic effects rather than the motor effects of cocaine intake.