mPFC Dopamine Research Articles

Glucocorticoid receptors in the prefrontal cortex regulate dopamine efflux to stress via descending glutamatergic feedback to the ventral tegmental area

Enhanced dopamine (DA) efflux in the medial prefrontal cortex (mPFC) is a well-documented response to acute stress. We have previously shown that glucocorticoid receptors in the mPFC regulate stress-evoked DA efflux but the underlying mechanism is unknown. DA neurons in the ventral tegmental area (VTA) receive excitatory input from and send reciprocal projections to the mPFC. We hypothesize that blockade of prefrontal glucocorticoid receptors can reduce activity of descending glutamatergic input to the VTA, thereby attenuating stress-evoked DA efflux in the mPFC. Using in vivo microdialysis, we demonstrate that acute tail-pinch stress leads to a significant increase in glutamate efflux in the VTA. Blockade of prefrontal glucocorticoid receptors with the selective antagonist CORT 108297 attenuates stress-evoked glutamate efflux in the VTA together with DA efflux in the mPFC. Furthermore, blockade of ionotrophic glutamate receptors in the VTA attenuates stress-evoked DA efflux in the mPFC. We also examine the possible role of glucocorticoid-induced synthesis and release of endocannabinoids acting presynaptically via cannabinoid CB1 receptors to inhibit GABA release onto prefrontal pyramidal cells, thus enhancing descending glutamatergic input to the VTA leading to an increase in mPFC DA efflux during stress. However, administration of the cannabinoid CB1 receptor antagonist into the mPFC does not attenuate stress-evoked DA efflux in the mPFC. Taken together, our data indicate that glucocorticoids act locally within the mPFC to modulate mesocortical DA efflux by potentiation of glutamatergic drive onto DA neurons in the VTA.

Pre-gestational stress alters stress-response of pubertal offspring rat in sexually dimorphic and hemispherically asymmetric manner

BackgroundThere is increasing evidence that maternal stress may have long-term effects on brain development in the offspring. In this study, we examined whether pre-gestational stress might affect offspring rats on the medial prefrontal cortical (mPFC) dopaminergic activity in response to acute stress in puberty and if so, whether such effects exhibited hemispheric asymmetry or sexual dimorphism.ResultsWe used behavioral tests to assess the model of chronic unpredictable stress (CUS). We found that the activity in the open field test and sucrose intake test were lower for maternal rats in the CUS group than those in the control group. Offspring rats in the CUS group floated more and swam or climbed less as compared to the offsprings in the control group in the forced swimming test. The floating time was longer and swimming or climbing time was shorter in the female offspring rats than those in the males. Serum corticosterone and corticotrophin-releasing hormone levels were significantly higher for CUS maternal rats and their offsprings than the respective controls. The ratio of dihydroxy-phenyl acetic acid (DOPAC) to dopamine (DA), DA transporter (DAT), norepinephrine transporter (NET) were lower in the mPFC of offspring rats in the CUS group than the control group. Levels of catechol-O-methyltransferase (COMT) in the left mPFC of female offspring rats and in the right mPFC of both female and male offspring rats were lower in the CUS group than those in the controls, but there was no difference in the left mPFC of male offspring between the CUS and control groups. DOPAC, the ratio of DOPAC to DA, NET and COMT were lower in the right mPFC than in the left mPFC of offspring rats in the CUS group. The ratio of DOPAC to DA in the right mPFC was lower in the female offspring rats than male offspring rats in the CUS group. The NET and COMT levels in both left and right mPFC were lower in the female offspring rats than those of the male offsprings in the CUS group.ConclusionOur data provide evidence that the effect of pre-gestational stress on the mPFC dopaminergic activity in response to acute stress exhibited hemispheric asymmetry and sexual dimorphism in the pubertal offspring rats.

Prefrontal Cortical Kappa-Opioid Receptor Modulation of Local Neurotransmission and Conditioned Place Aversion

Kappa-opioid receptors (KORs) are important for motivation and other medial prefrontal cortex (mPFC)-dependent behaviors. Although KORs are present in the mPFC, their role in regulating transmission in this brain region and their contribution to KOR-mediated aversion are not known. Using in vivo microdialysis in rats and mice, we demonstrate that intra-mPFC administration of the selective KOR agonist U69,593 decreased local dopamine (DA) overflow, while reverse dialysis of the KOR antagonist nor-Binaltorphimine (nor-BNI) enhanced mPFC DA overflow. Extracellular amino-acid levels were also affected by KORs, as U69,593 reduced glutamate and GABA levels driven by the glutamate reuptake blocker, l-trans-pyrrolidine-2,4-dicarboxylate. Whole-cell recordings from mPFC layer V pyramidal neurons revealed that U69,593 decreased the frequency, but not amplitude, of glutamatergic mini EPSPs. To determine whether KOR regulation of mPFC DA overflow was mediated by KOR on DA terminals, we utilized a Cre recombinase-driven mouse line lacking KOR in DA neurons. In these mice, basal DA release or uptake was unaltered relative to controls, but attenuation of mPFC DA overflow by local U69,593 was not observed, indicating KOR acts directly on mPFC DA terminals to locally inhibit DA levels. Conditioning procedures were then used to determine whether mPFC KOR signaling was necessary for KOR-mediated aversion. U69,593-mediated conditioned place aversion was blocked by intra-mPFC nor-BNI microinjection. These findings demonstrate that mPFC KORs negatively regulate DA and amino-acid neurotransmission, and are necessary for KOR-mediated aversion.

Reorganization of Circuits Underlying Cerebellar Modulation of Prefrontal Cortical Dopamine in Mouse Models of Autism Spectrum Disorder

Imaging, clinical, and pre-clinical studies have provided ample evidence for a cerebellar involvement in cognitive brain function including cognitive brain disorders, such as autism and schizophrenia. We previously reported that cerebellar activity modulates dopamine release in the mouse medial prefrontal cortex (mPFC) via two distinct pathways: (1) cerebellum to mPFC via dopaminergic projections from the ventral tegmental area (VTA) and (2) cerebellum to mPFC via glutamatergic projections from the mediodorsal and ventrolateral thalamus (ThN md and vl). The present study compared functional adaptations of cerebello-cortical circuitry following developmental cerebellar pathology in a mouse model of developmental loss of Purkinje cells (Lurcher) and a mouse model of fragile X syndrome (Fmr1 KO mice). Fixed potential amperometry was used to measure mPFC dopamine release in response to cerebellar electrical stimulation. Mutant mice of both strains showed an attenuation in cerebellar-evoked mPFC dopamine release compared to respective wildtype mice. This was accompanied by a functional reorganization of the VTA and thalamic pathways mediating cerebellar modulation of mPFC dopamine release. Inactivation of the VTA pathway by intra-VTA lidocaine or kynurenate infusions decreased dopamine release by 50 % in wildtype and 20-30 % in mutant mice of both strains. Intra-ThN vl infusions of either drug decreased dopamine release by 15 % in wildtype and 40 % in mutant mice of both strains, while dopamine release remained relatively unchanged following intra-ThN md drug infusions. These results indicate a shift in strength towards the thalamic vl projection, away from the VTA. Thus, cerebellar neuropathologies associated with autism spectrum disorders may cause a reduction in cerebellar modulation of mPFC dopamine release that is related to a reorganization of the mediating neuronal pathways.

Intravenous Ethanol Increases Extracellular Dopamine in the Medial Prefrontal Cortex of the Long–Evans Rat

Ethanol (EtOH) affects prefrontal cortex functional roles such as decision making, working memory, and behavioral control. Yet, the pharmacological effect of EtOH on dopamine, a neuromodulator in the medial prefrontal cortex (mPFC), is unclear. Past studies exploring this topic produced conflicting outcomes; however, a handful of factors (temporal resolution, method of drug administration, estrous cycle) possibly contributed to these discrepancies. We sought to mitigate these factors in order to elucidate EtOH's pharmacological effects on mPFC dopamine in Long-Evans rats. We administered experimental solutions via an intravenous (iv), handling-free route, monitored dopamine in the mPFC via microdialysis (10-minute samples), and used male rats to avoid estrous cycle/EtOH interactions. First, we rapidly (approximately 2.7 ml/min) or slowly (approximately 0.6 ml/min) administered 1.0 g/kg EtOH and saline infusions, showing that the experimental methods did not contribute to dopamine changes. Then, a cumulative dosing protocol was used to administer 0.25, 0.75, 1.50, and 2.25 g/kg iv EtOH doses to evaluate dose-response. Finally, we monitored dialysate EtOH levels during an oral EtOH self-administration session to compare the dialysate EtOH levels achieved during the pharmacological experiments to those seen during self-administration. IV administration of a rapid or slow 1.0 g/kg EtOH infusion resulted in similar significant 55 ± 9 and 63 ± 15% peak dialysate dopamine increases, respectively. The 0.25, 0.75, 1.50, and 2.25 g/kg EtOH doses produced a nonsignificant 17 ± 5% and significant 36 ± 15, 68 ± 19, and 86 ± 20% peak dialysate dopamine increases, respectively. Self-administration dialysate EtOH concentrations fell within the range of concentrations noted during the EtOH dose-response curve. These experiments show that, using experimental methods that minimize possibly confounding factors, acute iv EtOH increases extracellular dopamine in the mPFC in a dose-dependent manner, thereby clarifying EtOH's pharmacological effects on the mesocortical dopamine system.

Lesion of medial prefrontal dopamine terminals abolishes habituation of accumbens shell dopamine responsiveness to taste stimuli

Taste stimuli increase extracellular dopamine (DA) in the nucleus accumbens (NAc) and in the medial prefrontal cortex (mPFC). This effect shows single-trial habituation in NAc shell but not in core or in mPFC. Morphine sensitization abolishes habituation of DA responsiveness in NAc shell but induces it in mPFC. These observations support the hypothesis of an inhibitory influence of mPFC DA on NAc DA. To test this hypothesis, we used in vivo microdialysis to investigate the effect of mPFC 6-hydroxy-dopamine (6-OHDA) lesions on the NAc DA responsiveness to taste stimuli. 6-OHDA was infused bilaterally in the mPFC of rats implanted with guide cannulae. After 1 week, rats were implanted with an intraoral catheter, microdialysis probes were inserted into the guide cannulae, and dialysate DA was monitored in NAc shell/core after intraoral chocolate. 6-OHDA infusion reduced tissue DA in the mPFC by 75%. Tyrosine hydroxylase immunohistochemistry showed that lesions were confined to the mPFC. mPFC 6-OHDA lesion did not affect the NAc shell DA responsiveness to chocolate in naive rats but abolished habituation in rats pre-exposed to the taste. In the NAc core, mPFC lesion potentiated, delayed and prolonged the stimulatory DA response to taste but failed to affect DA in pre-exposed rats. Behavioural taste reactions and motor activity were not affected. The results indicate a top-down control of NAc DA by mPFC and a reciprocal relationship between DA transmission in these two areas. Moreover, habituation of DA responsiveness in the NAc shell is dependent upon an intact DA input to the mPFC.

Intact risk-based decision making in rats with prefrontal or accumbens dopamine depletion

The medial prefrontal cortex (mPFC) and the core region of the nucleus accumbens (AcbC) are key regions of a neural system that subserves risk-based decision making. Here, we examined whether dopamine (DA) signals conveyed to the mPFC and AcbC are critical for risk-based decision making. Rats with 6-hydroxydopamine or vehicle infusions into the mPFC or AcbC were examined in an instrumental task demanding probabilistic choice. In each session, probabilities of reward delivery after pressing one of two available levers were signaled in advance in forced trials followed by choice trials that assessed the animal's preference. The probabilities of reward delivery associated with the large/risky lever declined systematically across four consecutive blocks but were kept constant within four subsequent daily sessions of a particular block. Thus, in a given session, rats need to assess the current value associated with the large/risky versus small/certain lever and adapt their lever preference accordingly. Results demonstrate that the assessment of within-session reward probabilities and probability discounting across blocks were not altered in rats with mPFC and AcbC DA depletions, relative to sham controls. These findings suggest that the capacity to evaluate the magnitude and likelihood of rewards associated with alternative courses of action seems not to rely on intact DA transmission in the mPFC or AcbC.

Role of medial prefrontal cortex dopamine in age differences in response to amphetamine in rats: Locomotor activity after intra‐mPFC injections of dopaminergic ligands

Changes in medial prefrontal cortex (mPFC) dopamine receptor expression and in mPFC projections to the nucleus accumbens in adolescence suggest that there may be age differences in the regulation of drug-related behavior by the mPFC. The age-specific role of prelimbic D1 dopamine receptors on amphetamine-induced locomotor activity was investigated. In experiment 1, rats aged postnatal day 30 (P30), P45, and P75, corresponding to early and late adolescence and adulthood, were given an injection of D1 and D2 antagonists into the prelimbic mPFC before a systemic injection of 1.5 mg/kg of amphetamine and locomotor activity was recorded. In experiment 2, effects of intra-prelimbic injections of a D1 agonist and antagonist on locomotor activity produced by a lower dose (0.5 mg/kg) of amphetamine were investigated. D2 receptor antagonist did not alter amphetamine-induced activity, whereas the D1 receptor antagonist reduced activity produced by 1.5 mg/kg of amphetamine more in P30 than in P45 and P75 rats. In addition, D1 agonist enhanced the locomotor activating effects of 0.5 mg/kg of amphetamine in adolescent rats and decreased activity in adult rats. These results suggest that insufficient activation of mPFC D1 receptors may underlie the reduced activity at the low dose of amphetamine in early adolescent compared to adult rats.

Ginkgo bilobaleaf extract (EGb 761®) and its specific acylated flavonol constituents increase dopamine and acetylcholine levels in the rat medial prefrontal cortex: possible implications for the cognitive enhancing properties of EGb 761®

Experimental and clinical data suggest that the Ginkgo biloba standardized extract EGb 761® exerts beneficial effects in conditions which are associated with impaired cognitive function. However, the neurochemical correlates of these memory enhancing effects are not yet fully clarified. The aim of this study was to examine the effect of repeated oral administration of EGb 761® and some of its characteristic constituents on extracellular levels of dopamine (DA), noradrenaline (NA), serotonin (5-HT), acetylcholine (ACh) and the metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) in the medial prefrontal cortex (mPFC) of awake rats by use of in vivo microdialysis technique. Subacute (14 days, once daily), but not acute, oral treatment with EGb 761® (100 and 300 mg/kg) or the flavonoid fraction, which represents about 24% of the whole extract caused a significant and dose-dependent increase in extracellular DA levels in the mPFC. Repeated administration of EGb 761® also caused a modest but significant increase in the NA levels, whereas the concentrations of 5-HT and those of the metabolites DOPAC, HVA and 5-HIAA were not affected. The same treatment regimen was used in a subsequent study with the aim of investigating the effects of two Ginkgo-specific acylated flavonols, 3-O-(2''-O-(6'''-O-(p-hydroxy-trans-cinnamoyl)-β-D-glucosyl)-α-L-rhamnosyl)quercetin (Q-ag) and 3-O-(2''-O-(6'''-O-(p-hydroxy-trans-cinnamoyl)-β-D-glucosyl)-α-L-rhamnosyl)kaempferol (K-ag). Both compounds together represent about 4.5% of the whole extract. Repeated oral treatment with Q-ag (10 mg/kg) for 14 days caused a significant increase in extracellular DA levels of 159% and extracellular acetylcholine (ACh) levels of 151% compared to controls. Similarly, administration of K-ag (10 mg/kg) induced a significant rise of DA levels to 142% and ACh levels to 165% of controls, whereas treatment with isorhamnetin, an O-methylated aglycon component of EGb 761® flavonol glycosides had no effect. None of the tested flavonoids had a significant effect on extracellular DOPAC and HVA levels. The present findings provide evidence that the subacute treatment with EGb 761® and its flavonol constituents increases DA and ACh release in the rat mPFC, and suggest that the two Ginkgo-specific acylated flavonol glycosides Q-ag and K-ag are active constituents contributing to these effects. As seen for isorhamnetin, the effect on neurotransmitter levels seems not to be a general effect of flavonols but rather to be a specific action of acylated flavonol glycosides which are present in EGb 761®. The direct involvement of these two flavonol derivatives in the increase of dopaminergic and cholinergic neurotransmission in the prefrontal cortex may be one of the underlying mechanisms behind the reported effects of EGb 761® on the improvement of cognitive function.

Postnatal Aversive Experience Impairs Sensitivity to Natural Rewards and Increases Susceptibility to Negative Events in Adult Life

Evidence shows that maternal care and postnatal traumatic events can exert powerful effects on brain circuitry development but little is known about the impact of early postnatal experiences on processing of rewarding and aversive stimuli related to the medial prefrontal cortex (mpFC) function in adult life. In this study, the unstable maternal environment induced by repeated cross-fostering (RCF) impaired palatable food conditioned place preference and disrupted the natural preference for sweetened fluids in the saccharin preference test. By contrast, RCF increased sensitivity to conditioned place aversion (CPA) and enhanced immobility in the forced swimming test. Intracerebral microdialysis data showed that the RCF prevents mpFC dopamine (DA) outflow regardless of exposure to rewarding or aversive stimuli, whereas it induces a strong and sustained prefrontal norepinephrine (NE) release in response to different aversive experiences. Moreover, the selective mpFC NE depletion abolished CPA, thus indicating that prefrontal NE is required for motivational salience attribution to aversion-related stimuli. These findings demonstrate that an unstable maternal environment impairs the natural propensity to seek pleasurable sources of reward, enhances sensitivity to negative events in adult life, blunts prefrontal DA outflow, and modulates NE release in the reverse manner depending on the exposure to rewarding or aversive stimuli.

P.1.003 Brain-derived neurotrophic factor as a biomarker of clinical response in bipolar disorder: 16-week follow-up study

Simultaneous infusion of the selective 5-HT2A-receptor antagonist M100907, as well as asenapine, completely blocked the DOI-induced increase in dopamine and serotonin release and partially the increased noradrenaline release, indicating a similar mechanism by M100907 and asenapine. When given alone, infusion of M100907 or asenapine, in the concentrations used, did not affect monoamine release. Intracortical administration of the selective a2adrenoceptor agonist clonidine significantly decreased dopamine, noradrenaline and serotonin release in the mPFC (maximal effects; 61.0±8.0, 50.4±7.6, and 67.0±4.1% of baseline, respectively; p< 0.01–0.001). Simultaneous infusion of the a2-adrenoceptor antagonist idazoxan blocked the clonidine-induced reduction of dopamine and noradrenaline release. The effect of idazoxan on serotonin was less pronounced and not statistically significant. Simultaneous infusion of asenapine did not acutely block the clonidine-induced decrease in transmitter release. However, following termination of the clonidine infusion the transmitter release remained at a low level. During this interval, i.e. the following hour after the clonidine infusion, intracortical infusion of asenapine restored the catecholamine release to baseline level, indicating an a2-adrenoceptor blocking effect of asenapine which was somewhat delayed. When given alone, the infusion of idazoxan or asenapine, in the concentrations used, did not affect monoamine release. Our results propose that local application of asenapine in the mPFC indeed exhibits a pharmacological significant 5-HT2Aand, to a weaker extent, a2-receptor antagonistic activity. Whereas its 5-HT2A blocking property preferentially influenced the release of serotonin and dopamine but only to a lesser extent noradrenaline, blockage of a2-adrenoceptors preferentially influenced dopamine and noradrenaline release, albeit the effect was somewhat delayed. Thus, 5-HT2A-receptor antagonism and a2-adrenoceptor blockage induced by asenapine in the mPFC may contribute to enhance prefrontal monoamine release in vivo and, secondarily, its effect on positive and negative symptoms as well as pro-cognitive and antidepressant effects [1,2].

Dose-dependent effects of prefrontal dopamine on behavioral state in rats.

An inverted-U mechanism has been proposed to explain findings that both under- and overstimulation of dopamine (DA) receptors in medial prefrontal cortex (mPFC) result in working memory impairments. Dopamine release in the mPFC is also associated with the mediation of other behavioral processes such as affective regulation, reward, and nociception. The present studies investigated mPFC DA dose-dependent effects on the performance of tasks that tap these other processes, including delayed alternation in the T maze for working memory, a water maze task for escape, the elevated plus maze for anxiolytic/anxiogenic effects, place preference conditioning for reward effects, the tail flick test for nociception, and a measure of locomotor activity for general arousal. Injection of 5 μg of DA improved working memory, was anxiolytic in the plus maze, and increased pain sensitivity, but did not have any effects on water maze escape, place preference or locomotor activity. Doses of 10 μg and 20 μg impaired working memory and substantially decreased pain sensitivity, but did not affect plus maze behavior or locomotor activity. The 20-μg dose also enhanced water maze escape. Taken together, these findings suggest a profile of two distinct behavioral states induced by low or high levels of mPFC DA: a low level facilitates foraging functions such as working memory and exploratory behaviors, with increased pain sensitivity, while higher levels facilitate stress-related adaptations, such as escape from threat and reduced pain sensitivity. These findings are concordant with behavioral flexibility views of mPFC DA functions.

Dissociation of prefrontal cortex and nucleus accumbens dopaminergic systems in conditional learning in rats

There is converging evidence that the prefrontal and mesolimbic dopaminergic (DAergic) systems are involved in the performance of a variety of tasks that require the use of contextual, or task-setting, information to select an appropriate response from a number of candidate responses. Performance on tasks of this nature are impaired in schizophrenia and in rats exposed to psychotomimetics; impairments that are often attenuated by administration of dopamine (DA) antagonists. Rats were trained on either a complex instrumental discrimination task, that required the use of task-setting cues, or a simple discrimination task that did not. Following training, microdialysis probes were implanted unilaterally in either the medial prefrontal cortex (mPFC) or nucleus accumbens (NAc) and samples were collected in freely moving animals during a behavioural test session. In Experiment 1, we found no difference in levels of DA in the mPFC of rats while they were performing the two discrimination tasks. Rats that performed the complex task did, however, show significantly higher mPFC DA levels relative to rats in the simple discrimination condition following the end of the behavioural test session. In Experiment 2, rats performing the conditional discrimination showed lower levels of DA in the NAc compared to the simple discrimination group both during the test session and after it. These results provide direct evidence that conditional discrimination tasks engage frontal and mesolimbic DAergic systems and are consistent with the proposal that regulation of fronto-striatal DA is involved in aspects of cognitive control that are known to be impaired in individuals with schizophrenia.

Connecting the dots of the cerebro‐cerebellar role in cognitive function: Neuronal pathways for cerebellar modulation of dopamine release in the prefrontal cortex

Cerebellar involvement in autism, schizophrenia, and other cognitive disorders is typically associated with prefrontal cortical pathology. However, the underlying neuronal mechanisms are largely unknown. It has previously been shown in mice that stimulation of the dentate nucleus (DN) of the cerebellum evokes dopamine (DA) release in the medial prefrontal cortex (mPFC). Here, we investigated the neuronal circuitry by which the cerebellum modulates mPFC DA release. Fixed potential amperometry was used to determine the contribution of two candidate pathways by which the cerebellum may modulate mPFC DA release. In urethane anesthetized mice, DA release evoked by DN stimulation (50 Hz) was recorded in mPFC following local anesthetic lidocaine (0.02 μg) or ionotropic glutamate receptor antagonist kynurenate (0.5 μg) infusions into the mediodorsal or ventrolateral thalamic nucleus (ThN md; ThN vl), or the ventral tegmental area (VTA). Following intra-VTA lidocaine or kynurenate infusions, DA release was decreased by ∼50%. Following intra-ThN md and ThN vl infusions of either drug, DA release was decreased by ∼35% and 15%, respectively. Reductions in DA release following lidocaine or kynurenate infusions were not significantly different indicating that neuronal cells in the VTA and ThN were activated primarily if not entirely by glutamatergic inputs. The present study suggests that neuropathological changes in the cerebellum commonly observed in autism, schizophrenia, and other cognitive disorders could result in a loss of functionality of cerebellar-mPFC circuitry that is manifested as aberrant dopaminergic activity in the mPFC. Additionally, these results specifically implicate glutamate as a modulator of mPFC dopaminergic activity.

The Netrin Receptor DCC Is Required in the Pubertal Organization of Mesocortical Dopamine Circuitry

Netrins are guidance cues involved in neural connectivity. We have shown that the netrin-1 receptor DCC (deleted in colorectal cancer) is involved in the functional organization of the mesocorticolimbic dopamine (DA) system. Adult mice with a heterozygous loss-of-function mutation in dcc exhibit changes in indexes of DA function, including DA-related behaviors. These phenotypes are only observed after puberty, a critical period in the maturation of the mesocortical DA projection. Here, we examined whether dcc heterozygous mice exhibit structural changes in medial prefrontal cortex (mPFC) DA synaptic connectivity, before and after puberty. Stereological counts of tyrosine-hydroxylase (TH)-positive varicosities were increased in the cingulate 1 and prelimbic regions of the pregenual mPFC. dcc heterozygous mice also exhibited alterations in the size, complexity, and dendritic spine density of mPFC layer V pyramidal neuron basilar dendritic arbors. Remarkably, these presynaptic and postsynaptic partner phenotypes were not observed in juvenile mice, suggesting that DCC selectively influences the extensive branching and synaptic differentiation that occurs in the maturing mPFC DA circuit at puberty. Immunolabeling experiments in wild-type mice demonstrated that DCC is segregated to TH-positive fibers innervating the nucleus accumbens, with only scarce DCC labeling in mPFC TH-positive fibers. Netrin had an inverted target expression pattern. Thus, DCC-mediated netrin-1 signaling may influence the formation/maintenance of mesocorticolimbic DA topography. In support of this, we report that dcc heterozygous mice exhibit a twofold increase in the density of mPFC DCC/TH-positive varicosities. Our results implicate DCC-mediated netrin-1 signaling in the establishment of mPFC DA circuitry during puberty.

Role of dorsal medial prefrontal cortex dopamine D1-family receptors in relapse to high-fat food seeking induced by the anxiogenic drug yohimbine.

In humans, relapse to maladaptive eating habits during dieting is often provoked by stress. In rats, the anxiogenic drug yohimbine, which causes stress-like responses in both humans and nonhumans, reinstates food seeking in a relapse model. In this study, we examined the role of medial prefrontal cortex (mPFC) dopamine D1-family receptors, previously implicated in stress-induced reinstatement of drug seeking, in yohimbine-induced reinstatement of food seeking. We trained food-restricted rats to lever press for 35% high-fat pellets every other day (9-15 sessions, 3 h each); pellet delivery was accompanied by a discrete tone-light cue. We then extinguished operant responding for 10-16 days by removing the pellets. Subsequently, we examined the effect of yohimbine (2 mg/kg, i.p.) on reinstatement of food seeking and Fos (a neuronal activity marker) induction in mPFC. We then examined the effect of systemic injections of the D1-family receptor antagonist SCH23390 (10 μg/kg, s.c.) on yohimbine-induced reinstatement and Fos induction, and that of mPFC SCH23390 (0.5 and 1.0 μg/side) injections on this reinstatement. Yohimbine-induced reinstatement was associated with strong Fos induction in the dorsal mPFC and with weaker Fos induction in the ventral mPFC. Systemic SCH23390 injections blocked both yohimbine-induced reinstatement and mPFC Fos induction. Dorsal, but not ventral, mPFC injections of SCH23390 decreased yohimbine-induced reinstatement of food seeking. In addition, dorsal mPFC SCH23390 injections decreased pellet-priming-induced reinstatement, but had no effect on ongoing high-fat pellet self-administration or discrete-cue-induced reinstatement. Results indicate a critical role of dorsal mPFC dopamine D1-family receptors in stress-induced relapse to palatable food seeking, as well as relapse induced by acute re-exposure to food taste, texture, and smell.

Peri-pubertal emergence of UNC-5 homologue expression by dopamine neurons in rodents.

Puberty is a critical period in mesocorticolimbic dopamine (DA) system development, particularly for the medial prefrontal cortex (mPFC) projection which achieves maturity in early adulthood. The guidance cue netrin-1 organizes neuronal networks by attracting or repelling cellular processes through DCC (deleted in colorectal cancer) and UNC-5 homologue (UNC5H) receptors, respectively. We have shown that variations in netrin-1 receptor levels lead to selective reorganization of mPFC DA circuitry, and changes in DA-related behaviors, in transgenic mice and in rats. Significantly, these effects are only observed after puberty, suggesting that netrin-1 mediated effects on DA systems vary across development. Here we report on the normal expression of DCC and UNC5H in the ventral tegmental area (VTA) by DA neurons from embryonic life to adulthood, in both mice and rats. We show a dramatic and enduring pubertal change in the ratio of DCC:UNC5H receptors, reflecting a shift toward predominant UNC5H function. This shift in DCC:UNC5H ratio coincides with the pubertal emergence of UNC5H expression by VTA DA neurons. Although the distribution of DCC and UNC5H by VTA DA neurons changes during puberty, the pattern of netrin-1 immunoreactivity in these cells does not. Together, our findings suggest that DCC:UNC5H ratios in DA neurons at critical periods may have important consequences for the organization and function of mesocorticolimbic DA systems.

ORIGINAL RESEARCH—BASIC SCIENCE: Acute and Repeated Flibanserin Administration in Female Rats Modulates Monoamines Differentially Across Brain Areas: A Microdialysis Study

Hypoactive sexual desire disorder (HSDD) is defined as persistent lack of sexual fantasies or desire marked by distress. With a prevalence of 10% it is the most common form of female sexual dysfunction. Recently, the serotonin-1A (5-HT(1A)) receptor agonist and the serotonin-2A (5-HT(2A)) receptor antagonist flibanserin were shown to be safe and efficacious in premenopausal women suffering from HSDD in phase III clinical trials. The current study aims to assess the effect of flibanserin on neurotransmitters serotonin (5-HT), norepinephrine (NE), dopamine (DA), glutamate, and gamma-aminobutyric acid (GABA) in brain areas associated with sexual behavior. Flibanserin was administered to female Wistar rats (280-350 g). Microdialysis probes were stereotactically inserted into the mPFC, NAC, or MPOA, under isoflurane anesthesia. The extracellular levels of neurotransmitters were assessed in freely moving animals, 24 hours after the surgery. Dialysate levels of DA, NE, and serotonin from medial prefrontal cortex (mPFC), nucleus accumbens (NAC), and hypothalamic medial preoptic area (MPOA) from female rats. Acute flibanserin administration decreased 5-HT and increased NE levels in all tested areas. DA was increased in mPFC and MPOA, but not in the NAC. Basal levels of NE in mPFC and NAC and of DA in mPFC were increased upon repeated flibanserin administration, when compared to vehicle-treated animals. The basal levels of 5-HT were not altered by repeated flibanserin administration, but basal DA and NE levels were increased in the mPFC. Glutamate and GABA levels remained unchanged following either repeated or acute flibanserin treatment. Systemic administration of flibanserin to female rats differentially affects the monoamine systems of the brain. This may be the mechanistic underpinning of flibanserin's therapeutic efficacy in HSDD, as sexual behavior is controlled by an intricate interplay between stimulatory (catecholaminergic) and inhibitory (serotonergic) systems.

Dopamine release induced by atypical antipsychotics in prefrontal cortex requires 5-HT1A receptors but not 5-HT2A receptors

Atypical antipsychotic drugs (APDs) increase dopamine (DA) release in prefrontal cortex (PFC), an effect probably mediated by the direct or indirect activation of the 5-HT(1A) receptor (5-HT(1A)R). Given the very low in-vitro affinity of most APDs for 5-HT(1A)Rs and the large co-expression of 5-HT(1A)Rs and 5-HT(2A) receptors (5-HT(2A)Rs) in the PFC, this effect might result from the imbalance of 5-HT(1A)R and 5-HT(2A)R activation after blockade of these receptors by APDs, for which they show high affinity. Here we tested this hypothesis by examining the dependence of the APD-induced DA release in medial PFC (mPFC) on each receptor by using in-vivo microdialysis in wild-type (WT) and 5-HT(1A)R and 5-HT(2A)R knockout (KO) mice. Local APDs (clozapine, olanzapine, risperidone) administered by reverse dialysis induced a dose-dependent increase in mPFC DA output equally in WT and 5-HT(2A)R KO mice whereas the DA increase was absent in 5-HT(1A)R KO mice. To examine the relative contribution of both receptors to the clozapine-induced DA release in rat mPFC, we silenced G-protein-coupled receptors (GPCRs) in vivo with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) while 5-HT(1A)Rs or 5-HT(2A)/2CRs in the mPFC were selectively protected with the respective antagonists WAY-100635 or ritanserin. The inactivation of GPCRs while preserving ∼70% of 5-HT(2A)/(2C)Rs prevented the clozapine-induced DA rise in mPFC. In contrast, clozapine increased DA in mPFC of EEDQ-treated rats whose 5-HT(1A)Rs were protected (∼50% of control rats). These results indicate that (1) 5-HT(1A)Rs are necessary for the APDs-induced elevation in cortical DA transmission, and (2) this effect does not require 5-HT(2A)R blockade by APDs.

The novel neurotensin analog NT69L blocks phencyclidine (PCP)-induced increases in locomotor activity and PCP-induced increases in monoamine and amino acids levels in the medial prefrontal cortex

Schizophrenia is a life-long, severe, and disabling brain disorder that requires chronic pharmacotherapy. Because current antipsychotic drugs do not provide optimal therapy, we have been developing novel treatments that focus on receptors for the neuropeptide neurotensin (NT). NT69L, an analog of neurotensin(8-13), acts like an atypical antipsychotic drug in several dopamine-based animal models used to study schizophrenia. Another current animal model utilizes non-competitive antagonists of the NMDA/glutamate receptor, such as the psychotomimetic phencyclidine (PCP). In the present study, we investigated the effects of NT69L on PCP-induced behavioral and biochemical changes in the rat. The top of an activity chamber was modified to allow us to perform microdialysis in rat brain, while simultaneously recording the locomotor activity of a rat. PCP injection significantly increased activity as well as the extracellular concentration of norepinephrine (NE), 5-HT, dopamine (DA), and glutamate in the medial prefrontal cortex (mPFC). Pretreating with NT69L blocked the PCP-induced hyperactivity as well as the increase of DA, 5-HT, NE, and glutamate in mPFC. Interestingly and unexpectedly, NT69L markedly increased glycine levels, while PCP was without effect on glycine levels. Thus, NT69L showed antipsychotic-like effects in this glutamate-based animal model for studying schizophrenia. Previous work from our group suggests that NT69L also has antipsychotic-like effects in dopaminergic and serotonergic rodent models. Taken together, these data suggest that NT69L in particular and NT receptor agonists in general, will be useful as broad-spectrum antipsychotic drugs.