The principal features and mechanisms of dopamine modulation in the prefrontal cortex

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.

Attenuation of morphine withdrawal signs by a D1 receptor agonist in the locus coeruleus of rats

In the present study, the effects of intra-locus coeruleus injection of a dopamine D(1) receptor agonist (SKF38393) on naloxone-induced withdrawal signs of morphine-dependent rats were examined. Twenty different withdrawal signs were assessed. The total withdrawal score was calculated and used as an index of withdrawal intensity for comparison. The D(1) agonist and antagonist were injected 15 and 30 min prior to expression of naloxone-induced withdrawal signs, respectively. SKF38393 (2 and 4 microg/site) decreased while SCH23390 (a D(1) antagonist) had no effect on the total withdrawal score. On the other hand, SCH23390 (25 ng/site) reversed the SKF38393 effect. It may be concluded that activation of dopamine D(1) receptors in the locus coeruleus attenuates naloxone-induced withdrawal.

Overexpression of dopamine receptor genes and their products in the postnatal rat brain following maternal n-3 fatty acid dietary deficiency

A combination of PCR-Select cDNA subtraction and gene array hybridization was used to identify differentially expressed genomic markers in brains of rats fed for 3 weeks in utero and 2 weeks after birth on an n-3 polyunsaturated fatty acid (PUFA)-deficient diet supplied to dams. Total RNA was isolated, switch mechanism at 5'-end of the RNA transcripts (SMART) applied and used for PCR-Select subtraction of PUFA-deficient and adequately-fed control preparations. Subtracted and amplified ds-cDNA end-products were fragmented, terminally labeled with biotin-ddUTP and hybridized with a RN-U34A gene array. A 10-fold increase in potential genes with log2(Tester/Driver) = 1.4 was found compared with traditional gene array technology when the same chip was tested using non-subtracted targets. Reverse transcription-real-time relative PCR confirmed 30% of the transcripts. Among the validated transcripts, D1 and D2 receptors for dopamine (DA), were most prominent among a number of over-expressed neurotransmitter receptors and retinoic acid receptor (RXR alpha-2 and alpha-1). Immunohistochemical staining of brain sections from 2-week-old pups revealed a substantial enrichment of the D2 receptor in discrete regions of the mesolimbic and mesocortical pathways as well as in a large number of brain areas from the n-3 PUFA-deficient pups. Punches of the same areas run on western blots showed similar results. The overwhelming expression of D1 and D2 receptors may be attributed to a behavioral hypersensitivity caused by the possible impairment of DA production during brain development, which may have implications in certain disorders of the nervous system.

Dopaminergic Modulation of Short-Term Synaptic Plasticity in Fast-Spiking Interneurons of Primate Dorsolateral Prefrontal Cortex

Dopaminergic regulation of primate dorsolateral prefrontal cortex (PFC) activity is essential for cognitive functions such as working memory. However, the cellular mechanisms of dopamine neuromodulation in PFC are not well understood. We have studied the effects of dopamine receptor activation during persistent stimulation of excitatory inputs onto fast-spiking GABAergic interneurons in monkey PFC. Stimulation at 20 Hz induced short-term excitatory postsynaptic potential (EPSP) depression. The D1 receptor agonist SKF81297 (5 μM) significantly reduced the amplitude of the first EPSP but not of subsequent responses in EPSP trains, which still displayed significant depression. Dopamine (DA; 10 μM) effects were similar to those of SKF81297 and were abolished by the D1 antagonist SCH23390 (5 μM), indicating a D1 receptor-mediated effect. DA did not alter miniature excitatory postsynaptic currents, suggesting that its effects were activity dependent and presynaptic action potential dependent. In contrast to previous findings in pyramidal neurons, in fast-spiking cells, contribution of N-methyl-d-aspartate receptors to EPSPs at subthreshold potentials was not significant and fast-spiking cell depolarization decreased EPSP duration. In addition, DA had no significant effects on temporal summation. The selective decrease in the amplitude of the first EPSP in trains delivered every 10 s suggests that in fast-spiking neurons, DA reduces the amplitude of EPSPs evoked at low frequency but not of EPSPs evoked by repetitive stimulation. DA may therefore improve detection of EPSP bursts above background synaptic activity. EPSP bursts displaying short-term depression may transmit spike-timing-dependent temporal codes contained in presynaptic spike trains. Thus DA neuromodulation may increase the signal-to-noise ratio at fast-spiking cell inputs.

Cumulative effect of norepinephrine and dopamine carrier blockade on extracellular dopamine increase in the nucleus accumbens shell, bed nucleus of stria terminalis and prefrontal cortex

We investigated, by microdialysis in various brain areas, the possibility that dopamine could be captured by the norepinephrine transporter when the dopamine transporter is pharmacologically blocked. Administration of reboxetine, a selective blocker of the norepinephrine transporter, 20 min after the administration of GBR 12909, a selective blocker of the dopamine transporter, produced an increase of dopamine output in the nucleus accumbes shell (+408% above basal) greater than that obtained by GBR 12909 alone (+308% above basal). On the contrary, reboxetine did not increase further the dopamine output produced by GBR 12909 in the nucleus accumbens core or in the dorsal caudate, areas lacking a consistent noradrenergic innervations. A cumulative effect of dopamine and norepinephrine transporter blockade on the output of dopamine in dialysates was also observed in the bed nucleus of stria terminalis and in the prefrontal cortex. This study shows that dopamine extracellular concentration can be elevated by norepinephrine transporter blockade, even in areas where the dopamine transporter is predominant, when the latter is pharmacologically blocked. This phenomenon may have relevance in psychostimulant dependence as well as in antidepressant pharmacology.

Delta(9)-THC administered into the medial prefrontal cortex disrupts the spatial working memory

RATIONALE

Delta(9)-Tetrahydrocannabinol (Delta(9)-THC) disrupts working memory. The prefrontal cortex (PFC) is involved in the processing of working memory, and its medial portion (mPFC) is part of a brain reward circuit as constituted by the mesocorticolimbic dopaminergic system.

OBJECTIVE

This study examined the involvement of the mPFC in the effects of Delta(9)-THC on spatial working memory.

METHODS

Ten male Wistar rats well-trained in a radial arm maze and with bilateral cannula implanted in the mPFC received Delta(9)-THC intra-cortically (Delta(9)-THC IC) at doses of 0 (VEH), 32, 100 or 180 microg, 5 min before a 5-s or a 1-h delayed task in order to measure a short- or long-term spatial working memory, respectively. By contrast, 11 other animals received Delta(9)-THC intraperitoneally (Delta(9)-THC IP) at doses of 0 (VEH), 0.32, 1 or 1.8 mg/kg, 30 min before a 5-s or a 1-h delayed task. Additionally, after a 15-day washout, the effect of an IP or IC pre-exposure of Delta(9)-THC was examined by repeating both dose-effect curves in a crossover order for the routes of administration.

RESULTS

Delta(9)-THC IP produced significantly larger number of errors at doses of 0.32 or 1 mg/kg as compared to VEH in the 1-h post-delay performance. Delta(9)-THC 100 microg IC also produced significantly larger number of errors as compared to VEH and also to the other doses (32 or 180 microg) IC in the 1-h post-delay performance. Previous exposure to Delta(9)-THC IP or IC did not significantly affect the disruptive effect of this cannabinoid.

CONCLUSIONS

Delta(9)-THC administered directly in the mPFC impaired 1-h delayed task in the radial arm maze in a manner similar to that observed for its systemic administration, suggesting that the mPFC is involved in the disruptive effects of Delta(9)-THC on spatial working memory.

Tolerance to the procholinergic action of the D1 receptor full agonist dihydrexidine

RATIONALE

It has been well established that dopamine D1 receptor agonists increase acetylcholine (ACh) release in the brain, an effect that has been suggested to contribute to their procognitive properties.

OBJECTIVES

We sought to examine whether the ACh-releasing action of the D1 receptor full agonist dihydrexidine HCl (DHX) is altered after repeated administration.

RESULTS

DHX dose-dependently (3, 9, 18, and 36 mg/kg, i.p.) increased ACh efflux in the hippocampus of freely moving rats through D1 receptor stimulation, as the D1 receptor antagonist SCH 23390 (0.3 mg/kg, i.p.) abolished the effect of DHX (18 mg/kg, i.p.). Daily injections of DHX (18 mg/kg, i.p.) over 14 days did not significantly affect basal hippocampal ACh concentrations, but they significantly curtailed the stimulatory action of a challenge injection of DHX (18 mg/kg, i.p.) on ACh efflux.

CONCLUSIONS

Tolerance to the procholinergic action of DHX develops with repeated administration.

COMT genotype predicts longitudinal cognitive decline and psychosis in 22q11.2 deletion syndrome

Although schizophrenia is strongly hereditary, there are limited data regarding biological risk factors and pathophysiological processes. In this longitudinal study of adolescents with 22q11.2 deletion syndrome, we identified the catechol-O-methyltransferase low-activity allele (COMT(L)) as a risk factor for decline in prefrontal cortical volume and cognition, as well as for the consequent development of psychotic symptoms during adolescence. The 22q11.2 deletion syndrome is a promising model for identifying biomarkers related to the development of schizophrenia.

Reduced startle gating after D1 blockade: effects of concurrent D2 blockade

BACKGROUND

Prefrontal D1 systems have been implicated in the regulation of working memory and in the pathophysiology of schizophrenia. D1 hypofunction might contribute to reduced sensorimotor gating in schizophrenia patients since D1 activity in the medial prefrontal cortex (MPFC) regulates prepulse inhibition of startle (PPI) in animal models. We studied the neurochemical basis for the D1 regulation of PPI in rats.

METHODS

PPI to weak (1-5 dB over background) prepulses was measured after systemic or intra-MPFC administration of the D1 antagonist, SCH 23390, in rats pretreated systemically with the D2 antagonist, haloperidol (vehicle or 0.1 mg/kg).

RESULTS

After vehicle pretreatment, systemic and intra-MPFC SCH 23390 disrupted PPI produced by weak prepulses. This effect was not significantly opposed by pretreatment with haloperidol (0.1 mg/kg). In contrast, the PPI-disruptive effects of the DA agonist amphetamine were significantly opposed by this dose of haloperidol.

CONCLUSIONS

D1 blockade reduces PPI, but this effect does not appear to be mediated entirely via increased dopamine transmission at D2 receptors.

An integrated approach to classifying neuronal phenotypes

Characterizing the functional phenotypes of neurons is essential for understanding how genotypes can be related to the neural basis of behaviour. Traditional classifications of neurons by single features (such as morphology or firing behaviour) are increasingly inadequate for reflecting functional phenotypes, as they do not integrate functions across different neuronal types. Here, we describe a set of rules for identifying and predicting functional phenotypes that combine morphology, intrinsic ion channel species and their distributions in dendrites, and functional properties. This more comprehensive neuronal classification should be an improvement on traditional classifications for relating genotype to functional phenotype.

Presynaptic D1 dopamine receptors in primate prefrontal cortex: target-specific expression in the glutamatergic synapse

Dopaminergic modulation of glutamate neurotransmission in prefrontal cortex (PFC) microcircuits is commonly perceived as a basis for cognitive operations. Yet it appears that although the control of recurrent excitation between deep-layer prefrontal pyramids may involve presynaptic and postsynaptic D1 receptor (D1R) mechanisms, pyramid-to-interneuron communication will engage a postsynaptic D1R component. The substrate underlying such target-specific neuromodulatory patterns was investigated in the infragranular PFC with immunoelectron microscopy for D1R and parvalbumin, a marker for fast-spiking interneurons. In addition to their proverbial postsynaptic expression, gold-labeled D1Rs were distinctly distributed on perisynaptic/extrasynaptic membranes and the axoplasm of 13% of excitatory-like, presumably glutamatergic varicosities. Most importantly, presynaptic D1Rs were highly specific with regard to the cellular compartment and neurochemical identity of the postsynaptic neuron, being present in spine-targeting varicosities but distinctly absent from those synapsing with parvalbumin profiles often coexpressing D1Rs. We define therein an axonal D1 heteroreceptor component, apparently mediating volume neurotransmission, yet strategically positioned to convey target cell-specific modulation of the glutamatergic drive. We also indicate that presynaptic D1R mechanisms may indeed be associated with recurrent excitation in prefrontal microcircuits, consistent with physiological evidence for a role of these receptors in modulating the persistent activity-profile of neurons essential for working memory.

Modulation of neocortical interneurons: extrinsic influences and exercises in self-control

Neocortical GABAergic interneurons are a highly heterogeneous cell population that forms complex functional networks and has key roles in information processing within the cerebral cortex. Mechanisms that control the output of these cells are therefore crucial in regulating excitability within the neocortex during normal and pathophysiological activities. In addition to subtype-specific modulation of GABAergic cells by neurotransmitters released by afferents from subcortical nuclei, interneurons belonging to different classes are controlled by distinct self-modulatory mechanisms, each unique and powerful. In this article, we review the diverse responses of neocortical interneurons to extrinsic and intrinsic neuromodulators. We discuss how specificity of responses might differentially influence inhibition in somatodendritic compartments of pyramidal neurons and affect the balance of activities in neocortical circuits.

Prenatal disruption of neocortical development alters prefrontal cortical neuron responses to dopamine in adult rats

A growing body of evidence suggests that structural changes in the cortex may disrupt dopaminergic transmission in circuits involving the prefrontal cortex (PFC) and may contribute to the etiology of schizophrenia. In this study, we utilize a rodent model of neonatal disruption of cortical development using prenatal administration of the mitotoxin methylazoxymethanol acetate (MAM). Using intracellular recordings in vivo, we compare the physiology of prefrontal cortical neurons and their responses to topical administration of dopamine (DA) in intact animals and adult rats treated prenatally with MAM. Topical administration of DA hyperpolarized the membrane potential (MP) and decreased the firing rate of neurons recorded in deep layers of the PFC in intact animals. Furthermore, electrical stimulation of the VTA evoked fast onset epsps or long-lasting depolarizations in PFC neurons. In comparison, PFC neurons recorded in MAM-treated animals had significantly faster baseline firing rates. Moreover, topical administration of DA did not affect the MP or firing rate of the neurons in MAM-treated animals. However, MAM-treated animals exhibited an increase in the percentage of neurons responding with long-lasting depolarizations to stimulation of the VTA. The results of this study indicate that PFC neurons in the MAM-treated rats are not responsive to DA administered superficially, while at the same time exhibit greater responsiveness to VTA stimulation. These results are consistent with a rewiring of the corticolimbic system in response to neurodevelopmental insults.

Region-specific changes in prefrontal function with age: a review of PET and fMRI studies on working and episodic memory

Several neuroimaging studies of cognitive ageing have found that age-related deficits in working memory (WM) and episodic memory abilities are related to changes in prefrontal cortex (PFC) function. Reviews of these neuroimaging studies have generally concluded that with age there is a reduction in the hemispheric specialization of cognitive function in the frontal lobes that may either be due to dedifferentiation of function, deficits in function and/or functional reorganization and compensation. Moreover, previous reviews have considered the PFC as homogeneous in function and have not taken into account the possibility that region specific changes in PFC function may occur with age. In the current review we performed a qualitative meta-analytic review of all the functional magnetic resonance imaging ageing studies and positron emission tomography ageing studies of WM and episodic memory that report PFC activation, to determine if any region-specific changes occur. The results indicated that in normal ageing distinct PFC regions exhibit different patterns of functional change, suggesting that age-related changes in PFC function are not homogeneous in nature. Specifically, we hypothesize that normal ageing is related to the differentiation of cortical function in a bilateral ventral PFC and deficits in function in right dorsal and anterior PFC. As a result of these changes, functional compensation in left dorsal and anterior PFC may occur. We hope that future studies will be conducted to either confirm or counter these hypotheses.

Mechanisms underlying differential D1 versus D2 dopamine receptor regulation of inhibition in prefrontal cortex

Typically, D1 and D2 dopamine (DA) receptors exert opposing actions on intracellular signaling molecules and often have disparate physiological effects; however, the factors determining preferential activation of D1 versus D2 signaling are not clear. Here, in vitro patch-clamp recordings show that DA concentration is a critical determinant of D1 versus D2 signaling in prefrontal cortex (PFC). Low DA concentrations (<500 nm) enhance IPSCs via D1 receptors, protein kinase A, and cAMP. Higher DA concentrations (>1 microm) decrease IPSCs via the following cascade: D2-->G(i)-->platelet-derived growth factor receptor--> increase phospholipase C--> increase IP3--> increase Ca2+--> decrease dopamine and cAMP-regulated phosphoprotein-32--> increase protein phosphatase 1/2A--> decrease GABA(A). Blockade of any molecule in the D2-linked pathway reveals a D1-mediated increase in IPSCs, suggesting that D1 effects are occluded at higher DA concentrations by this D2-mediated pathway. Thus, DA concentration, by acting through separate signaling cascades, may determine the relative amount of cortical inhibition and thereby differentially regulate the tuning of cortical networks.

Electrospray ionization mass and tandem mass spectra of a series of N-pyrazolylmethyl and N-triazolylmethyl N-phenylpiperazines: new dopaminergic ligands with potential antipsychotic properties

Recently, two analogous series of N-pyrazolylmethyl and N-triazolylmethyl N-phenylpiperazines have been prepared and found to be potential antipsychotic drugs acting as new selective ligands of the dopamine D2 receptor. Herein we report a systematic study of their high-resolution electrospray ionization mass and tandem mass spectra in which the main dissociation routes of their protonated molecules are determined and rationalized. The ESI-MS/MS data is very characteristic for both series allowing straightforward isomeric differentiation. A single and dominant fragment ion for the pyrazole series and four major fragment ions for the triazole series are useful for selective reaction MS monitoring of these potential drugs in biological fluids.

Effects of caffeine on prospective duration judgements of various intervals depend on task difficulty

The effects of caffeine on prospective duration judgements were investigated in two double-blind placebo-controlled experiments. After taking either 200 mg of caffeine or a placebo, participants performed a task that demanded considerable attention, driving a car in a simulator (Experiment 1) or a task that demanded relatively little attention, watching a videotaped scene from a driven car (Experiment 2). Each participant made duration judgements of three target intervals: 15 s, 60 s and 300 s. Actively driving participants in the caffeine condition judged it as shorter than did those in the placebo condition. Caffeine had no effect on duration judgements following passive viewing. When people must perform a relatively difficult task, caffeine causes participants to allocate relatively more of their attentional resources to the task and relatively less to duration timing. Although caffeine may increase the pacemaker rate of an internal clock (via dopamine D(1) agonism), when external events are attention-demanding, caffeine mainly influences the relative allocation of attention to external events or to time (via dopamine D(2) agonism) in cerebral areas subserving the executive control of attention.

Differential modulation of anterior cingulate cortical activity by afferents from ventral tegmental area and mediodorsal thalamus

A distinct increase in cell firing activity is reported in prefrontal cortex during working memory tasks. The afferents that modulate this activity are not yet identified. Using in vivo intracellular recording and labelling of prefrontal cortical pyramidal neurons in anaesthetized rats, we systematically evaluated the influences of afferent projections arising from the ventral tegmental area (VTA) and mediodorsal thalamus (MD) by phasic electrical stimulation with a range of stimulus frequencies. Both VTA- and MD-responsive pyramidal neurons exhibited extensive intracortical axon arborization. Neither single shocks to the VTA at 0.2 Hz, nor low frequency trains of stimuli at 1-4 Hz (< 5 Hz) interrupted the periodicity of membrane bistability in bistable pyramidal neurons. However, high-frequency VTA-train stimulation (10-50 Hz) interrupted the bistability, and produced sustained membrane depolarizations accompanied by intense tonic firing in a frequency-dependent manner. Electrical stimulation of MD (10-50 Hz) did not produce sustained activity in the same PFC neurons. Thus, the sustained activity induced by high-frequency VTA trains is input selective. This effect of VTA-train stimulation was attenuated by systemic injection of the D1 receptor antagonist, SCH 23390, and blocked by acute dopamine (DA) depletion produced via alpha-methyl-para-tyrosine pre-treatment, suggesting that sustained cortical activity is mediated by DA. Chemical stimulation of VTA via intra-VTA infusion of NMDA induced sustained activity similar to VTA-train stimulation. Thus, while both VTA- and MD-responsive pyramidal neurons exhibited extensive intracortical axon arborization, VTA synapses (as opposed to MD synapses) may be critically positioned in the dendritic arborizations of anterior cingulate cortical pyramidal neurons, which may allow their modulation of sustained activity in prefrontal bistable neurons.

Dopamine D1-like receptor modulates layer- and frequency-specific short-term synaptic plasticity in rat prefrontal cortical neurons

The mesocortical dopamine (DA) input to the prefrontal cortex (PFC) is crucial for processing short-term working memory (STWM) to guide forthcoming behavior. Short-term plasticity-like post-tetanic potentiation (PTP, < 3 min) and short-term potentiation (STP, < 10 min) may underlie STWM. Using whole-cell voltage-clamp recordings, mixed glutamatergic excitatory postsynaptic currents (EPSCs) evoked by layer III or layer V stimulation (0.5 or 0.067 Hz) were recorded from layer V pyramidal neurons. With 0.5 Hz basal stimulation of layer III, brief tetani (2 x 50 Hz) induced a homosynaptic PTP (decayed: approximately 1 min). The D1-like antagonist SCH23390 (1 microm) increased the PTP amplitude and decay time without inducing changes to the tetanic response. The tetani may evoke endogenous DA release, which activates a presynaptic D1-like receptor to inhibit glutamate release to modulate PTP. With a slower (0.067 Hz) basal stimulation, the same tetani induced STP (lasting approximately 4 min, but only at 2x intensity only) that was insignificantly suppressed by SCH23390. With stimulation of layer-V-->V inputs at 0.5 Hz, layer V tetani yielded inconsisitent responses. However, at 0.067 Hz, tetani at double the intensity resulted in an STP (lasting approximately 6 min), but a long-term depression after SCH23390 application. Endogenous DA released by tetanic stimulation can interact with a D1-like receptor to induce STP in layer V-->V synapses that receive slower (0.067 Hz) frequency inputs, but suppresses PTP at layer III-->V synapses that receive higher (0.5 Hz) frequency inputs. This D1-like modulation of layer- and frequency-specific synaptic responses in the PFC may contribute to STWM processing.

Unmanageable motivation in addiction: a pathology in prefrontal-accumbens glutamate transmission

Prime diagnostic criteria for drug addiction include uncontrollable urges to obtain drugs and reduced behavioral responding for natural rewards. Cellular adaptations in the glutamate projection from the prefrontal cortex (PFC) to the nucleus accumbens have been discovered in rats withdrawn from cocaine that may underlie these cardinal features of addiction. A hypothesis is articulated that altered G protein signaling in the PFC focuses behavior on drug-associated stimuli, while dysregulated PFC-accumbens synaptic glutamate transmission underlies the unmanageable motivation to seek drugs.

Midbrain dopamine and prefrontal function in humans: interaction and modulation by COMT genotype

Using multimodal neuroimaging in humans, we demonstrate specific interactions between prefrontal activity and midbrain dopaminergic synthesis. A common V(108/158)M substitution in the gene for catecholamine-O-methyltransferase (COMT), an important enzyme regulating prefrontal dopamine turnover, predicted reduced dopamine synthesis in midbrain and qualitatively affected the interaction with prefrontal cortex. These data implicate a dopaminergic tuning mechanism in prefrontal cortex and suggest a systems-level mechanism for cognitive and neuropsychiatric associations with COMT.

A neural circuit basis for spatial working memory

The maintenance of a mental image in memory over a time scale of seconds is mediated by the persistent discharges of neurons in a distributed brain network. The representation of the spatial location of a remembered visual stimulus has been studied most extensively and provides the best-understood model of how mnemonic information is encoded in the brain. Neural correlates of spatial working memory are manifested in multiple brain areas, including the prefrontal and parietal association cortices. Spatial working memory ability is severely compromised in schizophrenia, a condition that has been linked to prefrontal cortical malfunction. Recent computational modeling work, in interplay with physiological studies of behaving monkeys, has begun to identify microcircuit properties and neural dynamics that are sufficient to generate memory-related persistent activity in a recurrent network of excitatory and inhibitory neurons during spatial working memory. This review summarizes recent results and discusses issues of current debate. It is argued that understanding collective neural dynamics in a recurrent microcircuit provides a key step in bridging the gap between network memory function and its underlying cellular mechanisms. Progress in this direction will shed fundamental insights into the neural basis of spatial working memory impairment associated with mental disorders.

Dopamine neurons mediate a fast excitatory signal via their glutamatergic synapses

Dopamine neurons are thought to convey a fast, incentive salience signal, faster than can be mediated by dopamine. A resolution of this paradox may be that midbrain dopamine neurons exert fast excitatory actions. Using transgenic mice with fluorescent dopamine neurons, in which the axonal projections of the neurons are visible, we made horizontal brain slices encompassing the mesoaccumbens dopamine projection. Focal extracellular stimulation of dopamine neurons in the ventral tegmental area evoked dopamine release and early monosynaptic and late polysynaptic excitatory responses in postsynaptic nucleus accumbens neurons. Local superfusion of the ventral tegmental area with glutamate, which should activate dopamine neurons selectively, produced an increase in excitatory synaptic events. Local superfusion of the ventral tegmental area with the D2 agonist quinpirole, which should increase the threshold for dopamine neuron activation, inhibited the early response. So dopamine neurons make glutamatergic synaptic connections to accumbens neurons. We propose that dopamine neuron glutamatergic transmission may be the initial component of the incentive salience signal.