Dopamine neurons projecting to the medial prefrontal cortex possess release-modulating autoreceptors

The ability of dopamine (DA) agonists and antagonists to modulate the K+-evoked overflow of radioactivity from superfused slices of prefrontal cortex of the rat, preincubated with [3H]DA in the presence of 1 microM desipramine, was examined. Apomorphine and the putative autoreceptor-selective DA agonist EMD 23 448 inhibited the K+-evoked overflow of radioactivity, while the DA antagonist sulpiride enhanced the evoked overflow in a dose-dependent and stereoselective manner. The latter effect was partially reversed by EMD 23 448. More than 95% of the radioactivity retained by the slices chromatographed with DA, while deaminated metabolites represented the majority of both the basal efflux (84% metabolites, 4-5% DA) and evoked overflow (84% metabolites, 14% DA) of radioactivity. These findings indicate that mesoprefrontal DA neurons possess release-modulating nerve terminal autoreceptors. Previous studies have shown that these neurons lack synthesis-modulating autoreceptors. Thus, autoreceptors on prefrontal DA terminals appear to be coupled to regulation of the release but not the synthesis of DA.     

Presynaptic long -term potentiation of neurotransmission in the pathway from auditory cortex to medial geniculate body

Objective To study the long- term potentiation （LTP） of synaptic transmission in the corticothalamic pathway to medial geniculate body （MGB） and mechanism underlying this synaptic potentiation. Methods After stimulating auditory thalamic radiation, excitatory postsynaptic currents （EPSCs） were recording at MGB neurons using whole- cell patch clamp technique in AC - MGB slices of postnatal day 14 - 20 rats. A brief 10Hz conditioning stimulation was used to study whether long - term potentiation （LTP） of synaptic transmission could be induced in MGB neurons.     

Presynaptic regulation of extracellular dopamine levels in the medial prefrontal cortex and striatum during tyrosine depletion

Rationale

Available neurochemical probes that lower brain dopamine (DA) levels in man are limited by their tolerability and efficacy. For instance, the acute lowering of brain tyrosine is well tolerated, but only modestly lowers brain DA levels. Modification of tyrosine depletion to robustly lower DA levels would provide a superior research probe.

Objectives

The objective of this study was to determine whether the subthreshold stimulation of presynaptic DA receptors would potentiate tyrosine depletion-induced effects on extracellular DA levels in the medial prefrontal cortex (MPFC) and striatum of the rat.

Methods

We administered quinpirole, a predominantly DA type 2 (D2R) receptor agonist, into the MPFC and striatum by reverse dialysis. A tyrosine- and phenylalanine-free neutral amino acid mixture [NAA(?)] IP was used to lower brain tyrosine levels. DA levels in the microdialysate were measured by HPLC with electrochemical detection.

Results

Quinpirole dose-dependently lowered DA levels in MPFC as well as in the striatum. NAA(?) alone transiently lowered DA levels (80 % baseline) in the striatum, but had no effect in MPFC. The co-administration of NAA(?) and a subthreshold concentration of quinpirole (6.25 nM) lowered DA levels (50 % baseline) in both the MPFC and striatum. This effect was blocked by the mixed D2R/D3R antagonist haloperidol at IP doses that on their own did not affect DA levels (10.0 nmol/kg in the MPFC and 0.10 nmol/kg in the striatum).

Conclusions

Pharmacological stimulation of inhibitory D2R receptors during tyrosine depletion markedly lowers the extracellular DA levels in the MPFC and striatum. The data suggest that combining tyrosine depletion with a low dose of a DA agonist should robustly lower brain regional DA levels in man.     

Stimulation of alpha2-adrenoceptors suppresses excitatory synaptic transmission in the medial prefrontal cortex of rat.

Stimulation of alpha2-, especially alpha2A-adrenoceptor (AR), in the prefrontal cortex (PFC) produces a beneficial effect on cognitive functions such as working memory. Alpha2-adrenergic agonists like clonidine and guanfacine have been used experimentally and clinically for treatment of psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and schizophrenia. However, the neurophysiological actions of alpha2-ARs in the PFC are poorly understood. In the present study, we recorded field excitatory post-synaptic potential (fEPSP) and evoked excitatory post-synaptic current (eEPSC) in the medial prefrontal cortex (mPFC) of rats, using in vivo field-potential recording and in vitro whole-cell patch-clamp recording techniques, and examined the effects of the alpha2-AR agonist clonidine and the selective alpha2A-AR agonist guanfacine on fEPSP and eEPSC. Systemic or intra-mPFC application of clonidine or guanfacine significantly reduced fEPSP in the mPFC, either in anesthetized or freely moving rats. Consistently, bath-application of guanfacine suppressed eEPSC in layer V/VI pyramidal neurons, and this effect was blocked by the alpha2-AR antagonist yohimbine or the Gi inhibitor NF023. Moreover, treatment with guanfacine had no effect on paired-pulse facilitation (PPF) of fEPSP and eEPSC. The present study provides the first electrophysiological evidence that stimulation of alpha2A-AR inhibits excitatory synaptic transmission in the mPFC through a post-synaptic mechanism.     

Mirtazapine-induced corelease of dopamine and noradrenaline from noradrenergic neurons in the medial prefrontal and occipital cortex

The novel antidepressant mirtazapine has been shown to increase extracellular noradrenaline and dopamine in the medial prefrontal cortex. Our previous studies indicate that extracellular dopamine in the cerebral cortex originates largely from noradrenergic terminals, such release being controlled by alpha(2)-adrenoceptors. Because mirtazapine inhibits alpha(2)-adrenoceptors, the possibility that it might corelease dopamine and noradrenaline was investigated. By means of microdialysis, the effect of mirtazapine on extracellular dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and noradrenaline in the medial prefrontal cortex, densely innervated by dopaminergic and noradrenergic neurons, and in the occipital cortex, receiving equal noradrenergic but scarce dopaminergic projections, was compared. Basal extracellular concentration of noradrenaline was similar in both cortices, while dopamine in the occipital cortex was only about 50% lower than in the medial prefrontal cortex, reflecting noradrenergic rather than dopaminergic projections. The intraperitoneal (i.p.) administration of mirtazapine (5 and 10 mg/kg) increased extracellular dopamine, DOPAC and noradrenaline to approximately the same extent in both cortices, an effect totally suppressed by the alpha(2)-adrenoceptors agonist clonidine (0.15 mg/kg, i.p.). To exclude the possibility that mirtazapine-induced increase in dopamine might result from reduced dopamine removal from extracellular space, noradrenaline and dopamine uptake mechanisms were blocked by perfusing 100 microM desipramine into either cortex. The combined i.p. administration of mirtazapine (5 mg/kg) and the local perfusion of desipramine produced an additional increase in extracellular dopamine, DOPAC and noradrenaline in the medial prefrontal cortex and occipital cortex compared with the increase produced by either drug given alone. The results suggest that mirtazapine by inhibiting alpha(2)-adrenoceptors produces a corelease of noradrenaline and dopamine from noradrenergic terminals in the cerebral cortex.     

Interaction between P2Y and NMDA receptors in layer V pyramidal neurons of the rat prefrontal cortex

In the first part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex were evoked by electrical stimulation of layer I. Recordings by intracellular sharp microelectrodes showed that EPSPs were concentration-dependently facilitated by the P2 receptor antagonistic ATP analogue 2-methylthio ATP (2-MeSATP), while ATP itself depressed the synaptic potentials. The inhibitory effect of ATP turned into facilitation in the presence of the adenosine A(1) receptor antagonist DPCPX. The 2-MeSATP-induced potentiation of EPSP amplitudes were prevented by the P2 receptor antagonists PPADS and Suramin. The EPSP was almost abolished by coapplication of the NMDA receptor antagonist AP-5 and the AMPA/kainate receptor antagonist CNQX. After blockade of the NMDA receptor-mediated part of the EPSP by AP-5, the stimulatory effect of 2-MeSATP disappeared. When NMDA or AMPA were pressure-applied onto pyramidal cells, only the NMDA-induced depolarization was potentiated by 2-MeSATP. In the second part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ATP, 2-MeSATP, UDP and UTP potentiated the response to NMDA, while ADP-beta-S was inactive. PPADS antagonized the effect of ATP. Synaptic isolation of pyramidal neurons by a Ca(2+)-free medium or tetrodotoxin did not alter the effect of ATP which, however, was markedly depressed when GTP in the micropipette was replaced by GDP-beta-S. These observations suggest that in layer V pyramidal neurons of the prefrontal cortex postsynaptically localized P2Y receptors interact with NMDA receptor-channels.     

Blockade of glutamate transporters leads to potentiation of NMDA receptor current in layer V pyramidal neurons of the rat prefrontal cortex via group II metabotropic glutamate receptor activation

Membrane currents of layer V pyramidal cells in slices of the rat prefrontal cortex (PFC) were recorded with the patch-clamp technique. In an Mg(2+)-free superfusion medium l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), a preferential blocker of astrocytic glutamate transporters, caused inward current due to the activation of NMDA receptors. The blockade of conducted action potentials by tetrodotoxin did not interfere with this effect. ATP was inactive when given alone and potentiated the NMDA-induced current in an Mg(2+)-containing but not Mg(2+)-free superfusion medium. Agonists of group I ((S)-3,5-dihydroxyphenylglycine; DHPG) and II ((1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0]hexane-4,6-dicarboxylic acid; LY 379268) metabotropic glutamate receptors (mGluRs) also potentiated responses to NMDA, whereas the group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) did not affect them. In contrast to ATP, PDC evoked inward current in the absence but not in the presence of external Mg(2+), when given alone, and facilitated the NMDA effect Mg(2+)-independently. The PDC-induced facilitation of NMDA responses was blocked by group II ((2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid; LY 341495), but not group I ((RS)-1-aminoindan-1,5-dicarboxylic acid; AIDA) or III (alpha-methyl-3-methyl-4-phosphonophenylglycine; UBP 1112) mGluR antagonists. In conclusion, the blockade of astrocytic glutamate uptake by PDC may lead to a stimulation of group II mGluRs, while the triggering of exocytotic glutamate release from astrocytes by ATP may cause activation of group I mGluRs, both situated postsynaptically at layer V PFC pyramidal cells. Either group of mGluRs may interact with NMDA receptors in a positive manner.     

Emerging role for the medial prefrontal cortex in alcohol-seeking behaviors

The medial prefrontal cortex (mPFC) plays an important role in high-order executive processes and sends highly organized projections to sub-cortical regions controlling mood, motivation and impulsivity. Recent preclinical and clinical studies have demonstrated alcohol-induced effects on the activity and composition of the PFC which are implicated in associative learning processes and may disrupt executive control over impulsivity, leading to an inability to self-limit alcohol intake. Animal studies have begun to dissect the role of the mPFC circuitry in alcohol-seeking behavior and withdrawal, and have identified a key role for projections to sub-cortical sites including the extended amygdala and the nucleus accumbens (NAc). Importantly, these studies have highlighted that alcohol can have contrasting effects on the mPFC compared to other addictive substances and also produce differential effects on the structure and activity of the mPFC following short-term versus long-term consumption. Because of these differences, how the mPFC influences the initial aspects of alcohol-seeking behavior and how we can better understand the long-term effects of alcohol use on the activity and connectivity of the mPFC need to be considered. Given the lack of preclinical data from long-term drinking models, an increased focus should be directed towards identifying how long-term alcohol use changes the mPFC, in order to provide new insights into the mechanisms underlying the transition to dependence.     

Neuroactive steroid pregnenolone sulphate inhibits long-term potentiation via activation of alpha2-adrenoreceptors at excitatory synapses in rat medial prefrontal cortex

Pregnenolone sulphate (PREGS) is one of the most important neuroactive steroids. Previous study showed that PREGS enhanced long-term potentiation (LTP) via activation of post-synaptic NMDA receptors at excitatory synapses in the hippocampus. The present paper studied the effect of PREGS on LTP at excitatory synapses in the pyramidal cells of layers V-VI of the medial prefrontal cortex (mPFC) using whole-cell patch-clamp in slices and made a comparison with that in the hippocampus. We also studied the mechanism of the effect of PREGS in the mPFC. We found that PREGS inhibited induction of LTP in the mPFC and had no influence on NMDA currents, which was different from its effect in the hippocampus. Moreover, the effect of PREGS on LTP in the mPFC was cancelled by alpha2-adrenoreceptor antagonist, alpha2A-adrenoreceptor antagonist, Gi protein inhibitor, adenylate cyclase inhibitor and protein kinase A inhibitor. These results suggest that PREGS inhibits LTP via activation of the alpha2-adrenoreceptor-Gi protein-adenylate cyclase-protein kinase A signalling pathway in the mPFC.     

Asenapine restores cognitive flexibility in rats with medial prefrontal cortex lesions

RATIONALE: Cognitive inflexibility in schizophrenia is treatment-resistant and predictive of poor outcome. This study examined the effect of asenapine, a novel psychopharmacologic agent being developed for schizophrenia and bipolar disorder, on cognitive dysfunction in the rat. OBJECTIVES: The objective of this paper was to establish whether asenapine has a beneficial effect on the performance of rats with ibotenic acid-induced lesion of the medial prefrontal cortex (mPFC) in an intradimensional/extradimensional (ID/ED) test of cognitive flexibility. METHODS: The effect of subcutaneously administered asenapine (0.75, 7.5, 75 mug/kg) on ID/ED performance of controls or mPFC-lesioned rats was examined using a within-subjects, repeated-measures design. In a second experiment, lesioned and control rats were tested with or without asenapine in a modified version of the task, with multiple set-shifts, before brains were processed for Fos-immunoreactivity in the mPFC. RESULTS: The mPFC lesion-induced deficit in the ID/ED task was stable with repeated testing over more than two months. Asenapine (75 mug/kg s.c., p < 0.05) completely restored the performance of lesioned rats. Experiment 2 replicated both lesion and asenapine effects and demonstrated that it is possible to measure set-shifting multiple times within a test session. Asenapine (75 mug/kg s.c.) was associated with differential activation of the neurons in the anterior mPFC of lesioned animals, but was without effect in controls. CONCLUSION: Asenapine can ameliorate mPFC lesion-induced impairment in attentional set-shifting, and is associated with a greater activation of the spared neurons in the anterior mPFC. These data suggest that asenapine may benefit impaired cognitive flexibility in disorders such as schizophrenia.     

Role of ventral medial prefrontal cortex in incubation of cocaine craving

Cue-induced drug-seeking in rodents progressively increases after withdrawal from cocaine, suggesting that cue-induced cocaine craving incubates over time. Here, we explored the role of the medial prefrontal cortex (mPFC, a brain area previously implicated in cue-induced cocaine seeking) in this incubation. We trained rats to self-administer cocaine for 10days (6h/day, infusions were paired with a tone-light cue), and then assessed after 1 or 30 withdrawal days the effect of exposure to cocaine cues on lever presses in extinction tests. We found that cue-induced cocaine-seeking in the extinction tests was higher after 30 withdrawal days than after 1day. The time-dependent increases in extinction responding were associated with large (ventral mPFC) or modest (dorsal mPFC) increases in ERK phosphorylation (a measure of ERK activity and an index of neuronal activation). After 30 withdrawal days, ventral but not dorsal injections of muscimol+baclofen (GABAa+GABAb receptor agonists that inhibit neuronal activity) decreased extinction responding. After 1 withdrawal day, ventral but not dorsal mPFC injections of bicuculline+saclofen (GABAa+GABAb receptor antagonists that increase neuronal activity) strongly increased extinction responding. Finally, muscimol+baclofen had minimal effect on extinction responding after 1day, and in cocaine-experienced rats, ventral mPFC injections of muscimol+baclofen or bicuculline+saclofen had no effect on lever presses for an oral sucrose solution. The present results indicate that ventral mPFC neuronal activity plays an important role in the incubation of cocaine craving.     

The medial prefrontal cortex mediates 3-methoxytyramine-induced behavioural changes in rat

L-3,4-Dihydroxyphenylalanine (L-DOPA) remains a common treatment for Parkinson's disease; however, side effects (i.e., dyskinesia and hallucinations) also remain problematic. We recently reported that the dopamine metabolite 3-methoxytyramine causes stereotypy in rats via dopamine receptors, raising the possibility that 3-methoxytyramine is involved in the adverse side effects of chronic L-DOPA treatment. Thus, the present study examined the sites of 3-methoxytyramine action in the rat brain. After intracerebroventricular administration of 3-methoxytyramine, significantly more neurones expressed c-Fos in mesocortico-limbic dopamine areas including frontal cortex, medial prefrontal cortex, parietal cortex, piriform cortex, the nucleus accumbens shell, and ventral tegmental area. 3-Methoxytyramine injection into the medial prefrontal cortex specifically resulted in behavioural changes characteristic of those elicited by the more general intracerebroventricular injection of 3-methoxytyramine. This suggests that the medial prefrontal cortex mediates the 3-methoxytyramine-induced behavioural changes and that a reduction of its action there may alleviate the adverse effects of chronic L-DOPA treatment.     

Delta 9-tetrahydrocannabinol enhances presynaptic dopamine efflux in medial prefrontal cortex

Acute administration of 1.0-2.0 mg/kg delta 9-tetrahydrocannabinol (delta 9-THC) increased presynaptic dopamine (DA) efflux in the medial prefrontal cortex of rats, as measured by intracerebral microdialysis in awake, behaving rats. These data are congruent with suggestions that (1) marijuana's euphorigenic effects and abuse potential may be related to augmentation of presynaptic DA mechanisms, and (2) the medial prefrontal cortex may be an important site of action for drugs of abuse in general and for delta 9-THC in particular.     

Neuropharmacological assessment of cocaine self-administration into the medial prefrontal cortex

Neuronal systems involved in the initiation of reinforcement following the response-contingent delivery of cocaine into the medial prefrontal cortex were investigated. Dose-effect analyses demonstrated that different concentrations of cocaine result in distinguishable patterns of self-administration which could be empirically determined by measuring the relative frequency distribution of the interinfusion intervals. The substitution of equimolar d-amphetamine or lidocaine resulted in rates and patterns of responding similar to vehicle or a low dose of cocaine, suggesting that reinforcement occurs from actions on specific receptors rather than through a local anesthetic neuronal blockade or through properties of a general psychomotor stimulant. The co-infusion of equimolar concentrations of sulpiride attenuated intake and produced patterns of responding similar to those seen after decreasing the cocaine dose consistent with an excitatory role for D2 dopaminergic receptors in these processes. Sulpiride and cocaine may act at separate sites since the decreased intake was not reversed by increasing the concentration of cocaine. D1 dopaminergic, muscarinic-cholinergic and beta-noradrenergic receptor antagonists either did not modulate drug-intake or had minimal effects. Cocaine reinforcement may result in part from an activation of D2 receptors initiating neuronal activity in pathways or circuits mediating reinforcement processes.     

Activation of pyramidal cells in rat medial prefrontal cortex projecting to ventral tegmental area by a 5-HT1A receptor agonist.

5-HT(1A) receptor agonists increase the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) and DA release in medial prefrontal cortex (mPFC). The mPFC is enriched in 5-HT(1A) receptors and projects to the VTA, where mesocortical dopaminergic neurons originate. We examined whether 5-HT(1A) receptor activation can modulate the activity of mPFC pyramidal neurons projecting to VTA. These were identified by antidromic stimulation from the VTA and were recorded extracellularly in anesthetized rats. The selective 5-HT(1A) agonist BAY x 3,702 (10-80 microg/kg i.v.) increased the firing rate in 14/19 neurons (283 +/- 79%) and reduced the activity of 5/19 neurons (22 +/- 11%), resulting in an overall 2.2-fold increase of the firing rate. Both effects were blocked by the selective 5-HT(1A) antagonist WAY-100635. These results suggest that the increase in dopaminergic activity produced by 5-HT(1A) receptor activation can be driven by an increase in the activity of projection neurons in mPFC.     

Olanzapine and clozapine but not haloperidol reverse subchronic phencyclidine-induced functional hyperactivity of N-methyl-D-aspartate receptors in pyramidal cells of the rat medial prefrontal cortex

In the present study, we have investigated and compared the ability of olanzapine, clozapine and haloperidol to modulate phencyclidine (PCP)-induced effect in pyramidal cells of the medial prefrontal cortex (mPFC) of rats using the techniques of intracellular recording and voltage-clamp. Subchronic treatment of rats with PCP (2 mg/kg, b.i.d., 7 days, 48-60 h withdrawal) produced: (1) a depolarized resting membrane potential, a decrease of slow after hyperpolarization (sAHP) and spike frequency adaptation, (2) a shift of the concentration response curve of N-methyl-D-aspartate (NMDA), but not (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), to the left, (3) a decrease of the paired pulse facilitation (PPF) with an increase of excitatory postsynaptic current variance (EPSC variance), and (4) a reduction of the blockade of NMDA response by in vitro application of PCP. Repeated treatment with either olanzapine or clozapine, but not haloperidol, completely prevented the aforementioned subchronic PCP-induced effects. The present results indicate that the atypical antipsychotic drugs (APDs) clozapine and olanzapine share a common property in preventing subchronic PCP-induced functional hyperactivity of NMDA receptors.     

Electrophysiological effects of phencyclidine in the medial prefrontal cortex of the rat

The effects of local applications of phencyclidine (PCP) and dopamine (DA) on neurons of the medial prefrontal cortex were investigated using single unit recording techniques. The activity of the majority of cells in the deeper layers of the medial prefrontal cortex was depressed by both phencyclidine and DA, whereas increases, as well as decreases, in the firing rates were observed in cells located in the superficial cortical layers. The stereospecificity of the responses of deeper cells to phencyclidine was demonstrated using the enantiomers of 1-(-1-phenylcyclohexyl)-3-methylpiperidine (PCMP). Phencyclidine was found to be 1.5 times more potent than (+) PCMP and 3 times more potent than (-) PCMP. Finally, the DA receptor antagonist fluphenazine, blocked the phencyclidine-elicited depressions of unit activity in the deep prefrontal cortex. Taken together, the data indicate that the DA-like effects of phencyclidine on neurons of the medial prefrontal cortex are mediated by DA receptors and provide pharmacological support for the idea that psychomotor stimulant drugs have specific actions on targets of the ventral tegmental area (A10) dopamine system.