Expression of cocaine sensitization: regulation by the medial prefrontal cortex

Effects of non-contingent cocaine on 3alpha-androstanediol. I. Disruption of male sexual behavior

One of the hallmarks of drug abuse is a reduction in the salience of, and motivation for, natural rewards, such as mating. The effects of psychostimulants on male sexual interest and performance are conflicting; use of psychostimulants can produce increases in risky sexual behaviors but have detrimental effects on sexual ability. We hypothesize that these conflicting effects on sexual behavior are due to interactions between cocaine and androgens, such as testosterone and its neuroactive metabolite, 3α-androstanediol (3α-diol). Male rats were administered saline or cocaine (5, 10, or 20mg/kg, i.p.). Motor behavior was observed in the first 30min following drug-administration, and then sexual responding was assessed for 15min. Levels of androgens (testosterone, 3ɑ-diol, and testosterone's aromatized metabolite, estradiol) were measured in circulation and brain regions (frontal cortex, hippocampus, hypothalamus/striatum (hypo/str), and midbrain). Cocaine had no effect on measures of sexual interest (i.e. anogenital investigation). However, cocaine had substantial effects on consummatory sexual behaviors, such as the latency to mount/intromit and the number of sexual contacts. Frontal cortex and hypo/str 3α-diol levels were strongly correlated with consummatory behaviors in saline administered rats; however, this relationship was disrupted by cocaine at all dosages, concomitant with impaired sexual behaviors. Additionally, there was a shift in metabolism at low dosages of cocaine to push testosterone metabolism in the midbrain towards 3α-diol. On the contrary, moderate and high dosages of cocaine shifted testosterone metabolism towards estradiol. These data demonstrate that the association between cortical and hypo/str 3α-diol levels and sexual behavior of male rats is disrupted by non-contingent cocaine and that there may be dose-dependent effects of acute cocaine on androgen metabolism.

Pacing stereotypies in laboratory rhesus macaques: Implications for animal welfare and the validity of neuroscientific findings

Stereotypic behaviours are commonly observed in captive animals and are usually interpreted as a sign of poor welfare. Stereotypies have also been linked with brain abnormalities. However, stereotypies are a heterogeneous class of behaviours and mounting evidence indicates that different stereotypies can have different causes, and can be linked to different affective states. As a consequence, the implications of a specific stereotypy in a specific species cannot be safely inferred from evidence on other stereotypies or species. Here we review what is known about pacing behaviour in laboratory rhesus macaques, a common stereotypy in this species. Our review highlights the current lack of understanding of the causal factors underlying pacing behaviour. According to current knowledge, the welfare of pacing macaques could be either better, worse or equivalent to that of non-pacing individuals. It is also unclear whether pacing results from brain abnormalities. Since rhesus macaques are widely used as a model of healthy humans in neuroscience research, determining if pacing behaviour reflects an abnormal brain and/or poor welfare is urgent.

Glycogen synthase kinase-3β inhibition in the medial prefrontal cortex mediates paradoxical amphetamine action in a mouse model of ADHD

Psychostimulants show therapeutic efficacy in the treatment of attention-deficit hyperactivity disorder (ADHD). It is generally assumed that they ameliorate ADHD symptoms via interfering with monoaminergic signaling. We combined behavioral pharmacology, neurochemistry and molecular analyses to identify mechanisms underlying the paradoxical calming effect of amphetamine in low trait anxiety behavior (LAB) mice, a novel multigenetic animal model of ADHD. Amphetamine (1 mg/kg) and methylphenidate (10 mg/kg) elicited similar dopamine and norepinephrine release in the medial prefrontal cortex (mPFC) and in the striatum of LAB mice. In contrast, amphetamine decreased, while methylphenidate increased locomotor activity. This argues against changes in dopamine and/or norepinephrine release as mediators of amphetamine paradoxical effects. Instead, the calming activity of amphetamine corresponded to the inhibition of glycogen synthase kinase 3β (GSK3β) activity, specifically in the mPFC. Accordingly, not only systemic administration of the GSK3β inhibitor TDZD-8 (20 mg/kg), but also local microinjections of TDZD-8 and amphetamine into the mPFC, but not into the striatum, decreased locomotor activity in LAB mice. Amphetamine effects seem to depend on NMDA receptor signaling, since pre- or co-treatment with MK-801 (0.3 mg/kg) abolished the effects of amphetamine (1 mg/kg) on the locomotion and on the phosphorylation of GSK3β at the level of the mPFC. Taken together, the paradoxical calming effect of amphetamine in hyperactive LAB mice concurs with a decreased GSK3β activity in the mPFC. This effect appears to be independent of dopamine or norepinephrine release, but contingent on NMDA receptor signaling.

Role of medial prefrontal and orbitofrontal monoamine transporters and receptors in performance in an adjusting delay discounting procedure

Performance in an adjusting delay discounting procedure is predictive of drug abuse vulnerability; however, the shared underlying specific prefrontal neural systems linking delay discounting and increased addiction-like behaviors are unclear. Rats received direct infusions of methylphenidate (MPH; 6.25, 25.0, or 100μg), amphetamine (AMPH; 0.25, 1.0, or 4.0μg), or atomoxetine (ATO; 1.0, 4.0, or 16.0μg) into either medial prefrontal cortex (mPFC) or orbitofrontal cortex (OFC) immediately prior to performance in an adjusting delay task. These drugs were examined because they are efficacious in treating impulse control disorders. Because dopamine (DA) and serotonin (5-HT) receptors are implicated in impulsive behavior, separate groups of rats received microinfusions of the DA receptor-selective drugs SKF 81297 (0.1 or 0.4µg), SCH 23390 (0.25 or 1.0µg), quinpirole (1.25 or 5.0µg), and eticlopride (0.25 or 1.0µg), or received microinfusions of the 5-HT receptor-selective drugs 8-OH-DPAT (0.025 or 0.1μg), WAY 100635 (0.01 or 0.04μg), DOI (2.5 or 10.0μg), and ketanserin (0.1 or 0.4μg). Impulsive choice was not altered significantly by MPH, AMPH, or ATO into either mPFC or OFC, indicating that neither of these prefrontal regions alone may mediate the systemic effect of ADHD medications on impulsive choice. However, quinpriole (1.25μg) and eticlopride infused into mPFC increased impulsive choice, whereas 8-OH-DPAT infused into OFC decreased impulsive choice. These latter results demonstrate that blockade of DA D2 receptors in mPFC or activation of 5-HT1A receptors in OFC increases impulsive choice in the adjusting delay procedure.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

Modification of cocaine-induced behavioral and neurochemical effects by serotonin1A receptor agonist/antagonist in mice

Administration of cocaine causes a locomotor stimulant effect and increases extracellular levels of serotonin (5-HT) and dopamine (DA) in the brains of rodents. Previous studies show that 5-HT1A receptor agonist and antagonist modify the cocaine-induced behavioral and neurochemical effects in the rats. However, the role of the 5-HT system on the effects of cocaine has not been studied in the prefrontal cortex. The present study examined in ddY-strain male mice the effects of the 5-HT1A receptor agonist osemozotan and the receptor antagonist WAY100635 on cocaine-induced locomotor stimulant effect and increases in extracellular levels of 5-HT and DA in the prefrontal cortex. The cocaine-induced locomotor stimulant effect was attenuated by osemozotan and enhanced by WAY100635. The cocaine-induced increase in extracellular levels of 5-HT was attenuated by osemozotan, and enhanced by WAY100635. The cocaine-induced increase in extracellular levels of DA was enhanced by osemozotan, but not affected by WAY100635. These results suggest that the prefrontal 5-HT system plays a pivotal role in the locomotor stimulant effect of cocaine in mice.

Administration of SCH 23390 into the medial prefrontal cortex blocks the expression of MDMA-induced behavioral sensitization in rats: an effect mediated by 5-HT2C receptor stimulation and not by D1 receptor blockade

Akin to what has been reported for cocaine, systemic administration of the dopamine D1 receptor antagonist, SCH 23390 ((R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride), blocks the expression but not the induction of 3,4-methylenedioxymethamphetamine (MDMA)-induced behavioral sensitization. Since the medial prefrontal cortex (mPFC) appears to regulate the expression of sensitization to cocaine, this study examined whether microinjection of SCH 23390 into the mPFC would alter the expression of MDMA sensitization. Saline or MDMA was administered for 5 consecutive days. After 12 days of withdrawal, rats received a bilateral intra-mPFC microinjection of SCH 23390 or saline followed by an intraperitoneal (i.p.) challenge dose of MDMA. While SCH 23390 enhanced locomotion in MDMA-naïve rats, it completely suppressed the expression of sensitization in MDMA-pretreated animals. Since, SCH 23390 has a fairly good affinity for 5-HT(2C) receptors, we went further to study the role of mPFC D1 and 5-HT(2C) receptors in this, apparently, paradoxical effect shown by SCH 23390. Thus, the microinjection of both SKF 81297 (R-(+)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide) and MK 212 (6-chloro-2-(1-piperazinyl)pyrazine hydrochloride), a D1 and 5-HT(2C) receptor agonist, respectively, blocked MDMA sensitization. By contrast, the 5-HT(2C) receptor antagonist, RS 102221 (8-[5-(2,4-dimethoxy-5-(4-trifluoromethylphenylsulfonamido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4,5]decane-2,4-dione hydrochloride), had no effect in MDMA-naïve or MDMA-sensitized animals, but reversed the effects of SCH 23390 in MDMA-pretreated rats. These results demonstrate that suppression of MDMA-induced sensitization by SCH 23390 is mediated by 5-HT(2C) receptor stimulation in the mPFC and not by the blockade of mPFC D1 receptors. Furthermore, these data indicate that stimulation of 5-HT(2C) receptors by SCH 23390 is not a minor issue and should be considered when interpreting future data.

Netrin receptor deficient mice exhibit functional reorganization of dopaminergic systems and do not sensitize to amphetamine

Netrins are guidance cues that play a fundamental role in organizing the developing brain. The netrin receptor, DCC (deleted in colorectal cancer), is highly expressed by dopaminergic (DA) neurons. DCC may therefore participate in the organization of DA circuitry during development and also influence DA function in the adult. Here we show that adult dcc heterozygous mice exhibit a blunted behavioral response to the indirect DA agonist amphetamine and do not develop sensitization to its effects when treated repeatedly. These behavioral alterations are associated with profound changes in DA function. In the medial prefrontal cortex, dcc heterozygotes exhibit increased tyrosine hydroxylase (TH) protein levels and dramatic increases in basal concentrations of DA and DA metabolites. In contrast, in the nucleus accumbens, dcc heterozygotes show no changes in either TH or DA levels, but exhibit decreased concentrations of DA metabolites, suggesting reduced DA activity. In addition, dcc heterozygous mice exhibit a small, but significant reduction in total number of TH-positive neurons in midbrain DA cell body regions. These results demonstrate for the first time that alterations in dcc expression lead to selective changes in DA function and, in turn, to differences in DA-related behaviors in adulthood. These findings raise the possibility that changes in dcc function early in life are implicated in the development of DA dysregulation observed in certain psychiatric disorders, such as schizophrenia, or following chronic use of drugs of abuse.

Ibotenic acid lesions of the medial prefrontal cortex block the development and expression of 3,4-methylenedioxymethamphetamine-induced behavioral sensitization in rats

There is ample evidence that plastic changes in the nervous system require the excitatory amino acid transmission. This appears to be also the case for psychostimulant-induced behavioral sensitization. More specifically the glutamatergic input from the medial prefrontal cortex (mPFC) to the VTA and the NAc appears to be involved in behavioral sensitization processes. However, dissociations regarding the role of the mPFC with respect to the development and expression of sensitization, as well as with respect to the psychostimulant being studied (amphetamine versus cocaine) appear to exist. The present study examined the role of the dorsal mPFC in the development and expression of 3,4-methylenedioxymethamphetamine (MDMA)-induced sensitization. Bilateral ibotenic acid or sham lesions of the dorsal mPFC were performed 7 days prior to or 4 days after a context-dependent sensitization-inducing regimen of MDMA (15 mg/kg i.p.) or saline. Rats were then challenged with MDMA (5 mg/kg i.p.) after 12 days of withdrawal. Ibotenic acid lesions did not affect the activating effects of MDMA, but prevented the development and expression of MDMA sensitization. Thus, the distance traveled during the development phase of sensitization increased in sham-lesioned rats but not in ibotenic-lesioned animals. Similarly, sham-lesioned rats showed a sensitized response when challenged with MDMA after the withdrawal period, an effect not observed in ibotenic-lesioned animals. These data reinforce the view that the dorsal mPFC is involved in psychostimulant sensitization and more specifically they indicate that the dorsal mPFC plays a key role in the development and expression of MDMA-induced behavioral sensitization.

Time-dependent effects of repeated cocaine administration on dopamine transmission in the medial prefrontal cortex

The medial prefrontal cortex (mPFC) has been implicated in the development of behavioral sensitization, which is the progressive enhancement of locomotor activity that occurs with repeated administration of psychostimulants. Previous data suggest that mPFC dopamine (DA) transmission may be attenuated in cocaine-sensitized animals, but the onset and duration of this effect have not been investigated. After recovery from stereotaxic surgeries, animals were given four daily injections of saline (1 ml/kg, i.p.) or cocaine (15 mg/kg, i.p.) and were subsequently challenged with saline or cocaine after 1, 7 or 30 d of withdrawal, on which days in vivo microdialysis of the mPFC was conducted simultaneously with monitoring of locomotor activity. Compared to acutely administered controls, the results in cocaine-pretreated animals were as follows: 1d of withdrawal was associated with a significant attenuation in cocaine-induced locomotion and mPFC DA overflow; after 7d, behavioral sensitization was accompanied by a significant attenuation in cocaine-induced elevations in mPFC DA levels; 30 d of withdrawal led to the expression of sensitized behaviors paralleled by an augmentation in cocaine-induced mPFC DA. These data suggest that repeated cocaine produces temporally distinct behavioral effects associated with alterations in mPFC DA responsiveness to cocaine that may be involved in the development of behavioral sensitization.

Effects of cocaine-induced behavioural sensitization on GABA transmission within rat medial prefrontal cortex

Studies support the involvement of mPFC dopaminergic and glutamatergic systems in the development of cocaine sensitization. GABA is known to modulate dopamine and glutamatergic systems in the mPFC. In addition, recent reports have suggested that cocaine sensitization might be associated with a decrease in GABAB receptor responsiveness in the mPFC. Hence, in vivo microdialysis of the mPFC was performed to examine the effects on extracellular GABA levels within the mPFC of a cocaine challenge subsequent to repeated cocaine administration. Male Sprague-Dawley rats were given four daily injections of saline (1.0 mL/kg, i.p.) or cocaine (15 mg/kg, i.p.) and challenged with the same dose of saline or cocaine 1, 7 or 28 days later. Acute cocaine produced a motor-stimulant response that was significantly augmented in repeated cocaine animals at all withdrawal time points. Moreover, sensitized animals exhibited a significant increase in extracellular GABA levels after 1 and 7 days but not 28 days following repeated cocaine exposure. These data suggest that cocaine-induced sensitization is associated with a transient increase in mPFC GABA transmission.

Studies on the role of dopamine D1 receptors in the development and expression of MDMA-induced behavioral sensitization in rats

RATIONALE

There is a large body of evidence indicating that the mesoaccumbens dopamine pathway is critically involved in the expression of behavioral sensitization to amphetamine and cocaine, but its role in the development of sensitization to psychostimulants is not that sound. Very few studies, however, have examined the role of dopamine transmission in 3,4-methylenedioxymethamphetamine (MDMA)-induced sensitization.

OBJECTIVES

The effects of the D1 receptor antagonist SCH 23390 on the development and expression of MDMA-induced behavioral sensitization were investigated in rats.

METHODS

During the development phase of sensitization, SCH 23390 was administered 15 min before every administration of MDMA. After 12 days of withdrawal, a MDMA challenge dose was given and locomotor activity was measured. In separate experiments, 15 min before the challenge injection of MDMA, SCH 23390 was administered either systemically or directly into the core of the nucleus accumbens (NAc) of MDMA-pretreated rats.

RESULTS

SCH 23390 did not prevent the development of MDMA-induced behavioral sensitization but completely blocked the expression when given before the challenge dose of MDMA. The same results were obtained when SCH 23390 was locally applied into the core of the NAc.

CONCLUSIONS

The present data suggest that D1 receptor stimulation is not critical for the development of long-term MDMA sensitization, in agreement with what has been reported for cocaine. By contrast, expression of sensitization depends on the activation of D1 receptors located in the NAc core.

Prolonged effects of repeated cocaine on medial prefrontal cortex dopamine response to cocaine and a stressful predatory odor challenge in rats

The present study examined the effects of seven daily saline (1 ml/kg, i.p.) or cocaine injections (15 mg/kg, i.p.) on extracellular dopamine levels in the medial prefrontal cortex (mPFC) after challenge with cocaine or stressful predatory odor presentation given 1 week (early withdrawal) or 3 weeks later (late withdrawal). Cocaine challenge at early withdrawal produced an increase in dopamine levels that was temporally shifted so that maximal levels of dopamine were significantly higher and attained 20 min earlier in the cocaine-pretreated group (maximal levels of saline controls=378% increase, cocaine=494% increase above baseline). Cocaine challenge at late withdrawal produced a similar effect on the temporal shift of maximal dopamine levels, with a significantly higher maximal percent increase of dopamine in cocaine-pretreated rats (saline-pretreated=420% increase, cocaine-pretreated=515% increase). Challenge with TMT, a predatory odor from fox that produces a stress response in rats, produced a maximal 75-200% increase in basal dopamine levels in both groups at both early and late withdrawal times. As with cocaine challenge, daily cocaine produced a leftward shift in the time at which maximal dopamine levels were attained in response to TMT. Cocaine-pretreated animals demonstrated maximal dopamine levels 40-80 min after TMT removal, while saline-pretreated rats showed maximal levels 100-140 min after TMT removal. These results suggest that there are long-term changes in the mPFC dopamine response to subsequent challenge with cocaine as well as a stressful predatory odor. The altered response of mPFC dopamine after repeated daily cocaine may impact relapse to drug-seeking or drug-taking behavior.

Activation of dopamine D1 receptors in the medial prefrontal cortex produces bidirectional effects on cocaine-induced locomotor activity in rats: effects of repeated stress

We examined the effects of repeated stress and D1 receptor activation in the medial prefrontal cortex (mPFC) on acute-cocaine-induced locomotor activity in rats. Male rats were given 7 days of either handling (Controls) or a variety of stressors. After 8-17 days' withdrawal, rats received an intra-mPFC microinjection of the full D1 agonist, SKF 81297: 0, 0.03, 0.1 or 0.3 microg/side followed by an i.p. saline or cocaine injection (15 mg/kg, i.p.). The target sites were either the dorsal or ventral mPFC. We also divided rats into either high or low responders based on their locomotor response to an acute cocaine injection. In the dorsal PFC, low responder Control and Stress groups demonstrated an augmentation of cocaine-induced increases in activity after SKF 81297, compared with vehicle, microinjection. In contrast, high responder rats demonstrated a suppression of cocaine-induced increases in activity after intra-mPFC SFK 81297 infusion, with an apparent 10 times higher sensitivity in the Stress group. In the ventral PFC, low responder Controls showed no changes after SKF 81297 infusion, while the Stress group showed an increase in cocaine-induced activity in response to SKF 81297. In high responders given SFK 81297 into the ventral mPFC, cocaine-induced activity was suppressed in Controls, while stress pretreatment rendered animals resistant to SKF 81297 effects. These results indicate that D1 receptor activation effects in the mPFC are bidirectional depending on whether rats have a high or low locomotor response to cocaine. Further, daily stress alters the sensitivity of the mPFC to SKF 81297, which is dependent on whether the dorsal or ventral mPFC is targeted.

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.

What we know and what we need to know about the role of endogenous CCK in psychostimulant sensitization

The unique distribution of CCK and its receptors and its co-localization with dopamine makes it ideally situated to pay a role in dopamine-mediated reward and psychostimulant sensitization. A number of studies support the hypothesis that CCK acting through the CCK 1 and CCK 2 receptors is an endogenous modulator of dopamine neurotransmission. Behavioral studies with CCK antagonists and CCK 1 receptor mutant rats support a role for endogenous CCK in behavioral sensitization to psychostimulants. CCK microdialysis studies in the nucleus accumbens (NAC) have demonstrated that extracellular CCK is increased in the NAC by psychostimulants, providing neurochemical evidence that CCK could be involved in the behavioral response to psychostimulants. A model for how CCK may be acting in multiple brain regions to foster sensitization is presented and the gaps in our knowledge about the role of CCK in psychostimulant sensitization are described.

Regulation of medial prefrontal cortex dopamine by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptors

In the medial prefrontal cortex, repeated cocaine produces tolerance of the extracellular dopamine response to subsequent cocaine injection. These studies characterized the influence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptors on the medial prefrontal cortex dopamine response to acute cocaine, amphetamine and potassium chloride as a first step to assess whether these receptor subtypes may be candidates for mediating dopamine tolerance after repeated cocaine. Local infusion of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) produced an approximate 40% increase in dopamine levels in the medial prefrontal cortex, while a 30 microM dose did not alter basal levels infused over a 3-h period. Thus, 30 microM CNQX was chosen for the remaining experiments, and was infused for 1 h prior to and during all in vivo treatments. Local medial prefrontal cortex infusion of the 30 microM dose blocked the small increase in dopamine levels elicited by systemic saline injection (maximum of 26%), as well as the much larger increase in response to acute cocaine injection (maximum of 340%). Local infusion of D-amphetamine (3 and 30 microM) through the probe increased dopamine to 300 and 600% of basal levels, respectively. Co-infusion of CNQX partially blocked the response for the first 40 min, but dopamine levels recovered by 60 min later. Local infusion of 100 mM potassium chloride elicited a 600% increase in dopamine levels, which was attenuated approximately 50% by CNQX co-infusion. Potassium-stimulated release of dopamine was also measured in vitro in medial prefrontal cortical and striatal tissue. By 30 s after potassium addition, dopamine levels increased to 800% above baseline in the medial prefrontal cortex, and this increase was blocked by the presence of 30 microM CNQX. In contrast, potassium-stimulated dopamine release in striatal tissue was approximately 250% above basal levels, with no effect of CNQX on dopamine release. Locomotor behavior collected during dialysis experiments demonstrated that increased activity induced by local infusion of potassium chloride was severely attenuated by co-infusion of 30 microM CNQX, while no effects of this drug were found for cocaine-elicited behavior. These results suggest a potent influence of glutamate via alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptors on extracellular dopamine in the medial prefrontal cortex, and these receptors may regulate dopamine release through a presynaptic mechanism. The findings may help elucidate the role of medial prefrontal cortex dopamine-glutamate interactions in drug abuse and stress- and drug-precipitated psychosis.

Adaptation to repeated cocaine administration in rats

Quantitative electroencephalogram (EEG) studies in cocaine-dependent human patients show deficits in slow-wave brain activity, reflected in diminished EEG power in the delta and theta frequency bands. In the present study, electrophysiological measures were monitored in 10 nonanesthetized, adult male Sprague-Dawley rats via bipolar, epidural electrodes implanted over the somatosensory cortex. Control electrocorticograms (ECoG) were recorded twice within a two-week interval to establish a baseline. Rats were subsequently injected daily with cocaine HCl at 15 mg/kg, i.p., for two weeks. The ECoG was recorded during a 1-h session one day after the last injection. Total concentrations of dopamine (DA) and its metabolites were assayed in caudate nucleus (CN) and frontal cortex (FC) using HPLC/EC. Compared with controls, marked increases in DA concentrations were observed in both regions. The DA turnover decreased significantly. The power spectra, obtained by use of a fast Fourier transformation, revealed a significant decrease in slow-wave delta frequency bands following repeated exposure to cocaine. These data are consistent with reported findings in humans that repeated exposures to cocaine result in a decrease in slow-wave brain activity. Further studies are necessary to establish whether regional alterations in blood flow and metabolic activity may underlie such observations.

D-amphetamine-induced behavioral sensitization: effect of lesioning dopaminergic terminals in the medial prefrontal cortex, the amygdala and the entorhinal cortex

The behavioral sensitization produced by the repeated administration of D-amphetamine is known to involve dopaminergic neurons in the mesoaccumbens pathway. Induction of this process is dependent on action of the drug in the ventral tegmental area while its expression involves action in the nucleus accumbens. We studied here the putative involvement of dopaminergic projections other than the mesoaccumbens in this phenomenon. We examined the influence of dopaminergic lesion of the medial prefrontal cortex, the amygdala and the entorhinal cortex in the behavioral sensitization produced by repeated injections of amphetamine either peripherally or directly into the ventral tegmental area of the brain. The repeated administration of amphetamine induced a behavioral sensitization, with the ventral tegmental area a critical site for induction of the process. This sensitization to amphetamine cross-reacted with morphine and was still observed 2 weeks after cessation of the treatment. Bilateral 6-hydroxydopamine lesion of dopaminergic terminals in either the medial prefrontal cortex or the amygdala, but not in the entorhinal cortex, prevented the development of behavioral sensitization to amphetamine and the cross-sensitization with morphine, whether the amphetamine pretreatment was administered peripherally or directly into the ventral tegmental area. In conclusion, these results indicated that behavioral sensitization to amphetamine, which involves dopaminergic neurons of the ventral tegmental area, is also dependent on dopaminergic neurotransmission of the medial prefrontal cortex and amygdala but not of the entorhinal cortex.

Cocaine sensitization and craving: differing roles for dopamine and glutamate in the nucleus accumbens

The repeated administration of cocaine produces enduring neuroadaptations that are associated with enhanced behavioral responsiveness to cocaine administration and lead to cocaine addiction and the manifestation of paranoid psychosis. This review describes the effect of chronic cocaine administration on dopamine and glutamate transmission in the nucleus accumbens, and discusses the relevance of these changes in the initiation of drug craving and relapse to drug abuse. Recent findings suggest that glutamate transmission in the nucleus accumbens is a dominant precipitator of relapse to drug-seeking activity, whereas both dopamine and glutamate transmission are important for sustaining drug-taking behavior.

The effects of acute or repeated cocaine administration on nerve terminal glutamate within the rat mesolimbic system

Cocaine administration alters glutamate function within several brain regions. Using quantitative electron microscopic immunocytochemistry, the present study investigated the effect of repeated intermittent cocaine (resulting in behavioral sensitization) or acute cocaine administration on the density of glutamate immunogold labeling within nerve terminals. Rats were treated daily with saline or cocaine for 7 days. Following a 14-day withdrawal animals were challenged with saline or cocaine. On the challenge day, most (75%) animals that received cocaine repeatedly showed a heightened locomotor response to cocaine compared to the first day of cocaine administration, and were considered behaviorally sensitized.Three days after the challenge, glutamate immunogold labeling was quantified in nerve terminals making asymmetrical synaptic contacts within the core and shell of the nucleus accumbens, ventral tegmental area and medial prefrontal cortex. There was a decrease in such labeling in the nucleus accumbens in the group receiving acute cocaine. Locomotor activity was positively correlated with glutamate immunolabeling within nerve terminals in the nucleus accumbens core only for the cocaine-sensitized group. Nerve terminal glutamate immunolabeling in the nucleus accumbens core, but not the shell, was increased in the non-sensitized compared to the cocaine-sensitized group. In the ventral tegmental area, glutamate immunolabeling was significantly higher in the cocaine-sensitized compared to the acute cocaine group. In the prefrontal cortex, there were no significant differences in glutamate immunogold labeling between treatment groups. This study indicates that acute cocaine administration significantly decreases nerve terminal glutamate immunoreactivity in the nucleus accumbens. We suggest that sensitization results in differential changes in the nucleus accumbens core versus the shell, and may alter presynaptic mechanisms regulating glutamate release or re-uptake in the core.

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

The dopaminergic pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) is well known to be involved in the reinforcing properties of many drugs of abuse. The medial prefrontal cortex (mPFC) has been shown to exhibit significant influence over activity in this pathway, and has also been implicated in drug abuse. The present experiment investigated the ability of D1 activity in the mPFC to influence accumbal dopamine levels. NAcc dopamine (DA) was monitored before, immediately after, and 24 h following mPFC infusion of a D1 agonist (SKF 38393), D1 antagonist (SCH 23390), or a vehicle solution. Immediately following infusion of dopaminergic agents or vehicle, no significant changes in accumbal DA were observed. However, 24 h following infusion of the antagonist but not the agonist, significant elevations of accumbal DA were observed. Since elevated NAcc DA was only observed 24 h after treatment, these results provide evidence that long-term neural adaptations can be induced by transient neuropharmacological treatment.

Characterization of the role of medial prefrontal cortex dopamine receptors in cocaine-induced locomotor activity

Medial prefrontal cortex (mPFC) dopamine (DA) modulates the motor-stimulant response to cocaine. The present study examined the specific mPFC DA receptor subtypes that mediate this behavioral response. Intra-mPFC injection of the DA D2-like receptor agonist quinpirole blocked cocaine-induced motor activity, an effect that was prevented by coadministration of the D2 receptor antagonist sulpiride. Intra-mPFC injection of the selective D4 receptor agonist PD 168,077 or the selective D1 receptor agonist SKF 81297 did not alter the motor-stimulant response to cocaine. Finally, it was found that an intermediate dose of quinpirole, which only attenuated cocaine-induced motor activity, was not altered by SKF 81297 coadministration, suggesting a lack of synergy between mPFC D1 and D2 receptors. These results suggest that D2 receptor mechanisms in the mPFC are at least partly responsible for mediating the acute motor-stimulant effects of cocaine.

Behavioral sensitization following exposure to low doses of trimethylolpropane phosphate

Behavioral sensitization is commonly studied within the context of drugs known to directly increase activity in the brain's dopamine system, particularly drugs of abuse. However, the present research suggests such behavioral changes can also be observed following exposure to other compounds that indirectly affect the dopamine system. One such compound is trimethylolpropane phosphate (TMPP), a bridged organophosphate that can be produced by the partial pyrolysis of certain synthetic lubricants used on military ships and aircraft. Although TMPP is a potent convulsant, it has been demonstrated that treatment with doses below seizure threshold results in long-term behavioral sensitization. The effect has been demonstrated with a number of neurobehavioral endpoints, particularly those assessing appetitive responding. More specifically, sensitization has been observed in acquisition of schedule-induced polydipsia (SIP), appetitive reinforcer approach sensitization (ARAS) and social interaction as measured in neonatal ultrasonic vocalizations, juvenile play and adult conspecific approach. Overall, the rats demonstrated a heightened appetitive response pattern. More specifically, TMPP reliably reduced the number of SIP sessions necessary to induce asymptotic drinking level and increased the time spent investigating (sniffing) a food reinforcer as measured in the ARAS task. Specific effects of TMPP on social interaction were an increase in ultrasonic vocalizations when the neonate was isolated from the dam and littermates and an increase in both measures of juvenile play (pins and dorsal contacts). A complex set of interactions emerged for the measures of adult social investigation where the drug effect was modulated by such factors as sex and neutral vs. stress-inducing experiences coincident with the drug treatment. In contrast to the above results, no behavioral changes were recorded for measures in the elevated plus maze and open field exploration. These results suggest that TMPP produces neurophysiological changes that persist much longer than the pharmacological effect of the compound, particularly in the neural correlates for appetitive behavior.

GABA(B) receptors: altered coupling to G-proteins in rats sensitized to amphetamine

Modified dopamine and glutamate neurotransmission in discrete brain regions is implicated in stimulant-induced behavioral sensitization. Release of both neurotransmitters is influenced by GABA(B) metabotropic receptors for the principal inhibitory neurotransmitter GABA. Accordingly, GABA(B) receptors were examined in rats sensitized to amphetamine by measuring receptor density and coupling to G-proteins indicated as [(3)H]baclofen binding and baclofen-mediated [(35)S]GTP gamma S binding. Repeated treatment with (+)-amphetamine (5mg/kg per day, i.p., for five days) sensitized the rats to amphetamine challenge (1mg/kg) at 14 days, but not one day, later. GABA(B) receptor density was not altered at either time. Baclofen-mediated [(35)S]GTP gamma S binding, however, was selectively augmented in the prefrontal cortex and attenuated in the nucleus accumbens at 14 days, but not one day, after amphetamine treatment. Changes in GABA(B) receptor coupling to G-proteins in rats sensitized to amphetamine, but not in similarly treated but unsensitized rats, lead us to suggest that altered GABA(B) receptor functioning may contribute to the expression of amphetamine-induced behavioral sensitization.

Expression of sensitization to amphetamine and dynamics of dopamine neurotransmission in different laminae of the rat medial prefrontal cortex

The present study investigated the effect of acute and repeated administrations of amphetamine (AMPH) on dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the two main cytoarchitectonic subterritories of the medial prefrontal cortex (mPFC) (anterior cingulate and dorsocaudal prelimbic cortices vs ventral prelimbic and rostral infralimbic cortices). Both the acute locomotor effects of AMPH and the expression of behavioral sensitization following its repeated administration were also simultaneously assessed. The repeated, intermittent administration of AMPH over five consecutive days led to a significant sensitized locomotor response to a subsequent challenge that occurred following a 48-h withdrawal period. Basal dialysate DA levels were higher in the ventral mPFC compared with its dorsal counterpart in naive animals, that is prior to the acute administration of AMPH. However, the inverse relationship was observed in animals that had developed sensitization: basal dialysate DA levels were significantly lower in the ventral mPFC compared with the dorsal mPFC. In naïve animals, AMPH produced a significant decrease in DA levels in both the ventral and dorsal subregions of the mPFC. However, the inverse relationship was observed in animals that had developed sensitization: dialysate DA levels in response to AMPH remained significantly decreased in the dorsal mPFC, whereas DA went back to baseline levels in the ventral mPFC. Given that a critical concentration of DA is required for normal function of the mPFC, our results suggest that AMPH-induced changes in DA levels in different subregions of the mPFC are critical for both the acute effects of the drug and the expression of behavioral sensitization to its repeated administration by producing either less or more selectivity or sharpening of stimuli to cortico-cortical dendrites and subcortical synaptic afferents to the pyramidal cells located in the dorso-ventral axis of the mPFC.

Characterization of extracellular dopamine clearance in the medial prefrontal cortex: role of monoamine uptake and monoamine oxidase inhibition

In vitro rotating disk electrode (RDE) voltammetry and in vivo microdialysis were used to characterize dopamine clearance in the rat medial prefrontal cortex (mPFC). RDE studies indicate that inhibition by cocaine, specific inhibitors of the dopamine transporter (DAT) and norepinephrine transporter (NET), and low Na(+) produced a 50-70% decrease in the velocity of dopamine clearance. Addition of the monoamine (MAO) inhibitors, l-deprenyl, clorgyline, pargyline, or in vivo nialamide produced 30-50% inhibition. Combined effects of uptake inhibitors with l-deprenyl on dopamine clearance were additive (up to 99% inhibition), suggesting that at least two mechanisms may contribute to dopamine clearance. Dopamine measured extracellularly 5 min after exogenous dopamine addition to incubation mixtures revealed that most conditions of DAT/NET inhibition did not produce elevated dopamine levels above controls. Inhibition of MAO produced elevated dopamine levels only after long-term, but not short-term, incubation in vitro. Short-term incubation of l-deprenyl combined with DAT and NET uptake inhibitors increased dopamine above control levels, consistent with more than one mechanism of dopamine clearance. Local infusion of pargyline (100 or 300 microm) into the mPFC or striatum via microdialysis produced more pronounced and immediate increases in mPFC dopamine levels compared with striatum. Furthermore, dopamine elevation in the mPFC was not accompanied by a decrease in the dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, as found in the striatum. These findings may have revealed a unique mechanism of mPFC dopamine clearance and therefore contribute to the understanding of multiple behaviors that involve mPFC dopamine transmission, such as schizophrenia, drug abuse, and working memory function.

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.

The adolescent brain and age-related behavioral manifestations

To successfully negotiate the developmental transition between youth and adulthood, adolescents must maneuver this often stressful period while acquiring skills necessary for independence. Certain behavioral features, including age-related increases in social behavior and risk-taking/novelty-seeking, are common among adolescents of diverse mammalian species and may aid in this process. Reduced positive incentive values from stimuli may lead adolescents to pursue new appetitive reinforcers through drug use and other risk-taking behaviors, with their relative insensitivity to drugs supporting comparatively greater per occasion use. Pubertal increases in gonadal hormones are a hallmark of adolescence, although there is little evidence for a simple association of these hormones with behavioral change during adolescence. Prominent developmental transformations are seen in prefrontal cortex and limbic brain regions of adolescents across a variety of species, alterations that include an apparent shift in the balance between mesocortical and mesolimbic dopamine systems. Developmental changes in these stressor-sensitive regions, which are critical for attributing incentive salience to drugs and other stimuli, likely contribute to the unique characteristics of adolescence.

Amphetamine withdrawal alters bistable states and cellular coupling in rat prefrontal cortex and nucleus accumbens neurons recorded in vivo

Repeated amphetamine administration is known to produce changes in corticoaccumbens function that persist beyond termination of drug administration. We have found previously that long-term alteration in dopamine systems leads to changes in gap junction communication, expressed as dye coupling, between striatal neurons. In this study, the cellular bases of amphetamine-induced changes were examined using in vivo intracellular recordings and dye injection in ventral prefrontal-accumbens system neurons of control and amphetamine-treated rats. Rats that had been withdrawn from repeated amphetamine displayed a significant increase in the incidence of dye coupling in the prefrontal cortex and nucleus accumbens, which persisted for up to 28 d after withdrawal. The increased coupling was limited to projection neurons in both prefrontal cortical and accumbens brain regions, as identified by their axonal trajectory or the absence of interneuron-selective immunocytochemical markers. These changes occurred with no substantial loss of tyrosine hydroxylase-immunoreactive terminals in these cortical and accumbens regions, ruling out dopamine degeneration as a precipitating factor. Previous studies showed that nitric oxide plays a role in the regulation of coupling; however, amphetamine-withdrawn rats had fewer numbers of neurons and processes that stained for nitric oxide synthase immunoreactivity. In amphetamine-treated rats, a higher proportion of cortical cells fired in bursts, and a larger proportion of accumbens and prefrontal cortical neurons exhibited bistable membrane oscillations. By increasing corticoaccumbens transmission, amphetamine withdrawal may lead to neuronal synchronization via gap junctions. Furthermore, this adaptation to amphetamine treatment persists long after the drug is withdrawn.

A single exposure to amphetamine is sufficient to induce long-term behavioral, neuroendocrine, and neurochemical sensitization in rats

Repeated treatment with psychostimulant drugs causes long-lasting behavioral sensitization and associated neuroadaptations. Although sensitization induced by a single psychostimulant exposure has also been reported, information on the behavioral and neurochemical consequences of a single psychostimulant exposure is sparse. Therefore, to evaluate whether behavioral sensitization evoked by single and repeated psychostimulant pretreatment regimens represent the same neurobiological phenomenon, the time-dependent expression of behavioral, neurochemical, and neuroendocrine sensitization after a single exposure to amphetamine was investigated in rats. A single exposure to amphetamine (5 mg/kg, i.p.) caused context-independent sensitization of the locomotor effects of amphetamine, which intensified over time. Thus, sensitization to amphetamine was marginal at 3 d after treatment and more evident after 1 week, whereas 3 weeks after treatment, profound sensitization, as well as cross-sensitization, to cocaine was observed. Amphetamine pretreatment caused an increase in the electrically evoked release of [(3)H]dopamine from nucleus accumbens, caudate putamen, and medial prefrontal cortex slices and of [(14)C]acetylcholine from accumbens and caudate slices. The hyperreactivity of dopaminergic nerve terminals appeared to parallel the development of locomotor sensitization, i.e., whereas hyperreactivity of accumbens dopaminergic terminals increased between 3 d and 3 weeks after treatment, the hyperreactivity of medial prefrontal dopaminergic terminals decreased. Pre-exposure to amphetamine also sensitized the hypothalamus-pituitary-adrenal axis response to amphetamine at 1 and 3 weeks, but not at 3 d after treatment. Because these data closely resemble those reported previously for repeated amphetamine pretreatment, it is concluded that a single exposure to amphetamine is sufficient to induce long-term behavioral, neurochemical, and neuroendocrine sensitization in rats.

Dopamine depletion in the medial prefrontal cortex induces sensitized-like behavioral and neurochemical responses to cocaine

It has been postulated that behavioral sensitization to cocaine is associated with an attenuation of cocaine-induced dopamine (DA) transmission in the medial prefrontal cortex (mPFC). Hence, experiments were designed to examine the effects of chemically-induced cortical DA depletion on the acute behavioral and neurochemical responses to cocaine. One week following two bilateral 6-hydroxydopamine (6-OHDA) injections into the mPFC, animals received injections of cocaine (7.5, 15 or 30 mg/kg, i.p.) or saline (1 ml/kg, i.p.) in a randomized fashion with a minimum 3 day intertrial interval. Cocaine produced a dose-dependent increase in motor activity which was significantly enhanced in animals depleted (mean of 76%) of dopamine in the mPFC. Likewise, 6-OHDA lesions of the mPFC produced a significant enhancement of cocaine-induced DA transmission in the nucleus accumbens (NAC) as estimated by in vivo microdialysis. These data indicate a permissive involvement of cortical DA in mediating behavioral and neurochemical responses to cocaine, as well as confirm the ability of the mPFC to influence subcortical structures in response to an acute injection of cocaine. Collectively, the present findings suggest that alterations in cortical DA transmission may be a neural substrate mediating the development of sensitization to cocaine, and thus, may contribute to the addictive properties of cocaine.