Time-of-day differences in dopamine clearance in the rat medial prefrontal cortex and nucleus accumbens

Circadian rhythms influence cocaine-seeking behavior in rats, and this behavior may be mediated by variability in the rate of extracellular dopamine clearance across the day:night cycle. We used rotating disk electrode voltammetry to examine dopamine clearance and inhibition of clearance by cocaine in the rat medial prefrontal cortex (mPFC) and nucleus accumbens (NAc). Rats were housed under light:dark conditions (LD, 12 h:12 h) or in constant darkness (DD), the latter given just prior to the day of sacrifice. Tissue was collected at 4-h intervals under LD and DD conditions. Under LD, dopamine clearance in both brain regions was greatest at 4h after lights on. Under DD, there was a blunted but still rhythmic pattern of dopamine clearance across the 24-h cycle. Cocaine-induced inhibition of dopamine clearance in the mPFC was not different across the day:night cycle in rats under LD. Paradoxically, under DD, dopamine clearance in the mPFC was enhanced by cocaine at ZT16, 4 h into the subjective night, and only minimally inhibited at other times. In the NAc, cocaine inhibition of dopamine clearance was lowest at ZT4 under LD, and did not vary under DD. We conclude that dopamine clearance varies both in a diurnal and possibly in a circadian manner in the mPFC, and in a diurnal manner in the NAc. These results indicate that light itself may be used to manipulate molecules implicated in drug addiction.     

Cholinergic modulation of basal and amphetamine-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens

Behavioral evidence suggests that muscarinic/cholinergic inhibition of brain dopaminergic activity may be a useful principle for developing novel antipsychotic drugs (APDs). Thus, oxotremorine, a muscarinic agonist, attenuates amphetamine-induced locomotor activity in rodents, an effect also produced by a wide variety of proven APDs, whereas scopolamine, a muscarinic antagonist, has the opposite effect. Since atypical APDs such as clozapine, olanzapine, risperidone, ziprasidone and quetiapine, increase brain acetylcholine as well as dopamine (DA) release in a region-specific manner, their effects on cholinergic and dopaminergic neurotransmission may also contribute to various actions of these drugs. Oxotremorine (0.5-1.5 mg/kg) dose-dependently and preferentially increased DA release in rat medial prefrontal cortex (mPFC), compared to the nucleus accumbens (NAC). However, S-(-)-scopolamine (0.5-1.5 mg/kg) produced similar increases in DA release in the mPFC, but the effect was much less than that of oxotremorine. Whereas a dose of S-(-)-scopolamine of 0.5 mg/kg comparably increased DA release in the mPFC and NAC, 1.5 mg/kg had no effect on DA release in the NAC. Oxotremorine-M (0.5 mg/kg), a M(1/4)-preferring agonist, also increased DA release in the mPFC, but not the NAC, an effect completely abolished by telenzepine (3 mg/kg), a M(1/4)-preferring antagonist, which by itself had no effect on DA release in either region. Oxotremorine (0.5, but not 1.5, mg/kg) attenuated amphetamine (1 mg/kg)-induced DA release in the NAC, whereas S-(-)-scopolamine did not. Oxotremorine (1.5 mg/kg) and S-(-)-scopolamine (0.5 mg/kg) modestly but significantly potentiated amphetamine (1 mg/kg)-induced DA release in the mPFC. These results suggest that stimulation of muscarinic receptors, in particular M(1/4), as indicated by the effect of oxotremorine-M and telenzepine, may preferentially increase cortical DA release and inhibit amphetamine-induced DA release in the NAC.     

Persistent elevations in dopamine and its metabolites in the nucleus accumbens after mild subchronic stress in rats with ibotenic acid lesions of the medial prefrontal cortex

This study assessed the possible influence of the medial prefrontal cortex (MPFC) on the response of subcortical dopamine (DA) systems to subchronic, mild stress. DA and its metabolites as well as noradrenaline were assayed in the nucleus accumbens and corpus striatum, 1 and 7 days after one week of daily intraperitoneal saline injections (Stress) or no handling (No stress), in rats with sham (Sham) or ibotenic acid (IA) lesions of the MPFC. One day after the last saline injection the level of dihydroxyphenylacetic acid (DOPAC) was elevated in the nucleus accumbens of IA/Stress rats in comparison to the Sham/No stress, Sham/Stress, and IA/No Stress groups. Levels of mesolimbic DA, DOPAC and homovanillic acid were still elevated 7 days after the last injection in IA/Stress animals in comparison to all other groups. There were no other significant differences between the groups. The data suggest that in rats with MPFC impairment, mild subchronic stress can induce alterations in mesolimbic DA activity that persist beyond the duration of the stress.     

Injections of N-methyl-D-aspartate into the ventral hippocampus increase extracellular dopamine in the ventral tegmental area and nucleus accumbens

The nucleus accumbens septi receives inputs from dopaminergic neurons of the ventral tegmental area (VTA) and glutamatergic neurons of the ventral subiculum (VS). The convergence of these inputs in the NAS is important for the normal expression of exploratory locomotion; stimulation of the VS by injection of the glutamate receptor agonist N-methyl-D-aspartate (NMDA) causes dopamine-dependent increases in locomotion. In the present study, in vivo microdialysis in conjunction with high-performance liquid chromatography and electrochemical detection (HPLC-EC) was used to estimate changes in extracellular dopamine in the VTA and NAS in response to intra-VS injections of NMDA (0.074, 0.28, 0.74 microg). NMDA injections caused dose-dependent elevations in extracellular dopamine in each region. Each dose of NMDA clearly increased extracellular dopamine in the NAS, whereas only the two higher doses increased dopamine significantly in the VTA. The highest dose of NMDA elevated extracellular dopamine to approximately 180% of baseline in each region. Whereas elevations in NAS dopamine might be induced by impulse-independent local mechanisms, elevations of dopamine in the VTA are presumed to reflect increased somatodendritic release associated with increased impulse flow through dopamine neurons. Thus, the present study suggests that the modulation of dopaminergic neurotransmission by the ventral subiculum results from a trans-synaptic activation of dopamine cell bodies in the VTA.     

Chemical stimulation of the ventral hippocampus elevates nucleus accumbens dopamine by activating dopaminergic neurons of the ventral tegmental area.

Dual-probe microdialysis (with HPLC and electrochemical detection) in freely moving rats and single-unit recording in anesthetized rats were used to study the extent to which impulse flow through the ventral tegmental area (VTA) contributes to elevations in nucleus accumbens (NAS) dopamine (DA) evoked by stimulation of the ventral subiculum (VS). During perfusion of artificial extracellular fluid into the VTA, injections of 0.74 microgram of the excitatory amino acid NMDA into the VS elevated accumbens DA to >150% of basal values. During intra-VTA perfusion of either 1 microM tetrodotoxin (which blocks impulse flow) or 1 mM kynurenic acid (which blocks excitatory glutamate receptors), injections of NMDA into the VS failed to elevate accumbens DA. Thus, increased impulse flow through VTA DA neurons, mediated by excitatory glutamate inputs to this region, appears critical for VS stimulation to elevate NAS DA. Increased impulse flow through VTA DA neurons was confirmed using single-unit recording in anesthetized rats. Intra-VS NMDA injections increased the firing rates of 45% (14 of 31), decreased the firing rates of 13% (4 of 31), and had no effect on 42% (13 of 31) of VTA DA neurons. Increases in firing rates were evident within 15 min of NMDA injections, a time at which VS NMDA injections elevate accumbens DA in awake animals. The results of the present experiments identify the VTA as a critical site through which outputs from the VS modulate NAS dopaminergic neurotransmission.     

Protein kinase C inhibition differentially affects 3,4-methylenedioxymethamphetamine-induced dopamine release in the striatum and prefrontal cortex of the rat

The acute administration of 3,4-methylenedioxymethamphetamine (MDMA) elevates extracellular concentrations of dopamine (DA) and serotonin (5-HT) in the rat striatum and medial prefrontal cortex (mPFC). The release of DA induced by MDMA is thought to involve both transporter and impulse-mediated processes. Furthermore, the impulse-dependent release of DA in the striatum elicited by MDMA appears to involve 5-HT2 receptor activation. Since 5-HT2 receptors are known to utilize protein kinase C (PKC) for intracellular signaling, we examined the effects of modulators of PKC activity on DA release stimulated by MDMA. Reverse dialysis of the PKC inhibitors bisindolylmaleimide I (BIM; 30 microM) or chelerythrine chloride (100 microM) through a microdialysis probe significantly attenuated the MDMA (10 mg/kg, i.p.)-induced increase in the extracellular concentration of DA in the striatum. In contrast, BIM did not significantly alter the increase in the extracellular concentration of DA in the striatum elicited by amphetamine (5 mg/kg, i.p.). Reverse dialysis of a PKC activator, phorbol 12,13-dibutyrate (PDBu) (0.5 microM), through the microdialysis probe into the striatum, significantly increased MDMA-induced DA release. In contrast to the inhibitory effects of the PKC inhibitors on MDMA-induced DA release in the striatum, intracortical infusion of BIM enhanced MDMA-induced release of DA in the mPFC. These data suggest that PKC-mediated signaling pathways differentially modulate MDMA-induced DA release from mesocorticolimbic and nigrostriatal neurons.     

Dendritic morphology changes in neurons from the ventral hippocampus,amygdala and nucleus accumbens in rats with neonatal lesions into the prefrontal cortex

Neonatal prefrontal cortex (nPFC) lesions in rats could be a potential animal model to study the early neurodevelopmental abnormalities associated with the behavioral and morphological brain changes observed in schizophrenia. Morphological alterations in pyramidal neurons from the ventral hippocampus (VH) have been observed in post‐mortem schizophrenic brains, mainly because of decreased dendritic arbor and spine density. We assessed the effects of nPFC‐lesions on the dendritic morphology of neurons from the VH, basolateral‐amygdala (BLA) and the nucleus accumbens (NAcc) in rats. nPFC lesions were made on postnatal day 7 (PD7), after dendritic morphology was studied by the Golgi‐Cox stain procedure followed by Sholl analysis at PD35 (prepubertal) and PD60 (adult) ages. We also evaluated the effects of PFC‐lesions on locomotor activity caused by a novel environment. Adult animals with nPFC lesions showed a decreased spine density in pyramidal neurons from the VH and in medium spiny cells from the NAcc. An increased locomotion was observed in a novel environment for adult animals with a PFC‐lesion. Our results indicate that PFC‐lesions alter the neuronal dendrite morphology of the NAcc and the VH, suggesting a disconnection between these limbic structures. The locomotion paradigms suggest that dopaminergic transmission is altered in the PFC lesion model. This could help to understand the consequences of an earlier PFC dysfunction in schizophrenia. To evaluate possible dendritic changes in neonatal prefrontal cortex lesions in schizophrenia‐related regions including nucleus accumbens, ventral hippocampus and basolateral amygdala, we used the Golgi‐Cox stain samples at PD35 and PD70. Our results suggest that neonatal prefrontal cortex damage alters dendritic parameters in limbic regions, and this has potential implications for schizophrenia. Synapse 69:314–325, 2015 . © 2015 Wiley Periodicals, Inc.     

Differential tonic influence of lateral habenula on prefrontal cortex and nucleus accumbens dopamine release

Conditions of increased cognitive or emotional demand activate dopamine release in a regionally selective manner. Whereas the brief millisecond response of dopamine neurons to salient stimuli suggests that dopamine's influence on behaviour may be limited to signalling certain cues, the prolonged availability of dopamine in regions such as the prefrontal cortex and nucleus accumbens is consistent with the well described role of dopamine in maintaining motivation states, associative learning and working memory. The behaviourally elicited terminal release of dopamine is generally attributed to increased excitatory drive on dopamine neurons. Our findings here, however, indicate that this increase may involve active removal of a tonic inhibitory control on dopamine neurons exerted by the lateral habenula (LHb). Inhibition of LHb in behaving animals transiently increased dopamine release in the prefrontal cortex, nucleus accumbens and dorsolateral striatum. The inhibitory influence was more pronounced in the nucleus accumbens and striatum than in the prefrontal cortex. This pattern of regional dopamine activation after LHb inhibition mimicked conditions of reward availability but not increased cognitive demand. Electrical or chemical stimulation of LHb produced minimal reduction of extracellular dopamine, suggesting that in an awake brain the inhibition associated with tonic LHb activity represents a near-maximal influence on dopamine neurotransmission. These data indicate that LHb may be critical for functional differences in dopamine neurons by preferentially modulating dopamine neurons that project to the nucleus accumbens over those neurons that primarily project to the prefrontal cortex.     

Extracellular dopamine in the rat ventral tegmental area and nucleus accumbens following ventral tegmental infusion of cocaine

Rats were implanted with dual dialysis probes, one in the ventral tegmental area, and another one ipsilateral in the nucleus accumbens. Infusion of cocaine (10, 100, 1000 mM) into the ventral tegmental area gradually increased extracellular dopamine to 164, 329 and 991% of baseline in the ventral tegmental area, but reduced dopamine to 76, 47 and 38% of baseline in the nucleus acumbens. These results are consistent with cocaine-induced feedback regulation of dopamine cell activity involving somatodendritec impulse-regulating dopamine D₂ autoreceptors.     

Reduction of dopamine utilization in the prefrontal cortex but not in the nucleus accumbens after selective destruction of noradrenergic fibers innervating the ventral tegmental area in the rat

The present study was made to determine the role of the noradrenergic (NA) neurons which innervate the ventral tegmental area (VTA) in the regulation of VTA dopaminergic (DA) neurons projecting to the prefrontal cortex and the nucleus accumbens. For this purpose, a 6-hydroxydopamine lesion was made in benzotropine pretreated rats medially just above the decussatio of the pedunculus cerebellaris superior in order to specifically destroy the NA fibers innervating the VTA without affecting those projecting to the prefrontal cortex. Seven days later the ratio of DOPAC and DA levels was estimated in the prefrontal cortex and the nucleus accumbens and used as an index of DA utilization. In the lesioned rats the DOPAC/DA ratio was significantly decreased in the prefrontal cortex but not in the nucleus accumbens. These results suggest that the NA neurons which innervate the VTA exert a specific tonic excitatory influence on the mesocortico-prefrontal DA neurons.     

Differential release of dopamine in the nucleus accumbens evoked by low-versus high-frequency medial prefrontal cortex stimulation

The medial prefrontal cortex (mPFC) coordinates goal-directed behaviors, which may be mediated through mPFC regulation of dopamine release in the nucleus accumbens (NAc). Furthermore, frequency-specific oscillatory activity between the frontal cortex and downstream structures may facilitate inter-region communication. Although high-frequency (e.g., 60 Hz) mPFC stimulation is known to increase basal dopamine levels in the NAc, little is known about how phasic dopamine release is affected by mPFC stimulation. Understanding the frequency-specific control of phasic dopamine release by mPFC stimulation could elucidate mechanisms by which the mPFC modulates other regions. It could also inform optimization of deep brain stimulation for treatment of neurological disorders.

Dendritic morphology of neurons in prefrontal cortex and ventral hippocampus of rats with neonatal amygdala lesion

Neonatal basolateral amygdala (nBLA) lesions in rats have been widely used as a neurodevelopmental model that mimics schizophrenia-like behaviors. Recently, we reported that nBLA lesions result in significant decreases in the dendritic spine number of layer 3 prefrontal cortex (PFC) pyramidal cells and medium spiny neurons of the nucleus accumbens (NAcc), which all changes after puberty. At present, we aimed to evaluate the effect of this lesion in pyramidal neurons of CA1 of the ventral hippocampus (VH) and layer 5 of the PFC. In order to assess the effects of nBLA lesions on the dendritic morphology of the PFC and VH neurons, we carried out nBLA lesions in rats on postnatal day (PD) 7, and then we studied the dendritic morphology of these two limbic subregions at prepubertal (PD35) and postpubertal (PD60) ages. Dendritic characteristics were measured by Golgi-Cox procedure followed by Sholl analysis. We also evaluated the effects of nBLA lesions on the prepulse inhibition (PPI) and acoustic startle responses. The nBLA lesion induced a significant increase in dendritic length of layer 5 pyramidal neurons of the PFC at both ages, with a decrease in the dendritic spines density after puberty. The spine density of CA1 VH pyramidal neurons showed significant decreases at both ages. PPI was decreased in adulthood in the animals with an nBLA lesion. These results show that an nBLA lesion alters the dendritic morphology at the level of the PFC and VH in distinct ways before puberty, suggesting a disconnection between these limbic structures at an early age, and increasing our understanding of the implications of the VH in early amygdala dysfunction in schizophrenia.     

Differential effect of cocaine on extracellular dopamine levels in rat medial prefrontal cortex and nucleus accumbens: comparison to amphetamine

The technique of in vivo microdialysis was used to measure the extracellular levels of dopamine in the nucleus accumbens septi and medial prefrontal cortex of chloral-hydrate-anaesthetized rats following systemic administration of cocaine and amphetamine. Intravenous injection of cocaine increased the extracellular levels of dopamine in the medial prefrontal cortex and the nucleus accumbens septi in a dose-dependent manner. However, the magnitude of increase was significantly greater in nucleus accumbens than in medial prefrontal cortex. In comparison to cocaine, amphetamine increased the extracellular levels of dopamine in the nucleus accumbens and medial prefrontal cortex to the same degree. Based on the relatively small increase of extracellular dopamine levels in medial prefrontal cortex by cocaine, it is postulated that dopaminergic innervation of other structures besides medial prefrontal cortex may be involved in maintenance of cocaine self-administration.     

Glutamate-dopamine in vivo interaction in the prefrontal cortex modulates the release of dopamine and acetylcholine in the nucleus accumbens of the awake rat

Summary. In the context of the in vivo neurochemical literature this article reviews some recent microdialysis studies in our laboratory of glutamate-dopamine interaction in the prefrontal cortex (PFC) and its possible role in modulating dopamine and acetylcholine release in the nucleus accumbens. A functional glutamate-dopamine interaction in PFC has been reported by microdialysis studies showing that both stimulation and blockade of prefrontal glutamate receptors produce changes of basal and/or stimulated release of dopamine in this area of the brain. These studies suggest that dopamine function in PFC is modulated locally by prefrontal glutamatergic inputs and that glutamate and dopamine can act simultaneously to regulate GABA release in PFC. In particular, dopamine can enhance the increases of extracellular GABA produced by the stimulation of prefrontal glutamate receptors. We report also that the stimulation of prefrontal D2 and mGluI receptors and their interaction changes the release of dopamine and acetylcholine in the nucleus accumbens.     

Ontogeny of altered dendritic morphology in the rat prefrontal cortex, hippocampus, and nucleus accumbens following Cesarean delivery and birth anoxia

We used a delayed Cesarean birth model and the Golgi-Cox staining method to investigate the effects of perinatal anoxia on prefrontal cortex (PFC) and hippocampal (CA1) pyramidal neurons as well as nucleus accumbens (NAcc) medium spiny neurons. Dendritic morphology in these regions was studied on postnatal days (P) 2, 7, 14, 21, 35, and 70 in male Sprague-Dawley rats born either vaginally (VAG) or by Cesarean section either with (C + anoxia) or without (C-only) anoxia. The most striking birth group differences seen were at the level of dendritic spine densities on P35. During this postnatal period the dendritic spine density of PFC neurons was significantly lower in C + anoxia and C-only animals than in VAG controls; however, by P70 PFC spine densities in all birth groups were comparable. In contrast, hippocampal spine densities on P35 were comparably greater in C + anoxia animals than in VAG controls, whereas in C-only animals spine densities were lower than controls; here again, by P70 all groups had comparable hippocampal spine densities. In NAcc greater spine densities were seen on medium spiny neurons of C + anoxia animals on P35. These findings provide evidence that perinatal insult in the form of Cesarean birth with or without anoxia alters the dendritic development of PFC and hippocampal pyramidal neurons and to some extent also of NAcc medium spiny neurons. They also suggest that perinatal anoxia can alter the neuronal development of key structures thought to be affected in such late-onset dopamine-related disorders as schizophrenia and Attention Deficit Hyperactivity Disorder (ADHD).     

NMDA receptors in nucleus accumbens modulate stress-induced dopamine release in nucleus accumbens and ventral tegmental area

Converging evidence suggests that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated locally by an excitatory amino acid (EAA)-containing input possibly originating in medial prefrontal cortex (PFC). In the present study, we examined the effects of intra-NAcc administration of EAA receptor antagonists on stress-induced increases of NAcc DA levels and of dendritically released DA in the ventral tegmental area (VTA). Local injection of the NMDA receptor antagonist—AP-5 (0.05, 0.5, and 5.0 nmoles)—dose-dependently potentiated increases in NAcc DA levels elicited by 15 min of restraint stress. In contrast, local application of equivalent doses of the kainate/AMPA receptor antagonist—DNQX—failed to alter the NAcc DA stress response reliably. In a separate experiment, we found that intra-NAcc injection of AP-5 also potentiated stress-induced increases in VTA DA levels. These results indicate that EAAs acting at NMDA receptors in NAcc can modulate stress-induced DA release in this region. Our data indicate, however, that this action exerts an inhibitory influence on the NAcc DA stress response, suggesting that the relevant population of NMDA receptors are not located on NAcc DA terminals. The fact that intra-NAcc AP-5 injections also potentiated the DA stress response in VTA suggests instead an action mediated by NMDA receptors located on NAcc neurons that feedback, directly or indirectly, to cell bodies of the mesocorticolimbic DA system. Synapse 26:225–234, 1997. © 1997 Wiley-Liss Inc.     

Comparing phasic dopamine dynamics in the striatum,nucleus accumbens,amygdala, and medial prefrontal cortex

Midbrain dopaminergic neurons project to and modulate multiple highly interconnected modules of the basal ganglia, limbic system, and frontal cortex. Dopamine regulates behaviors associated with action selection in the striatum, reward in the nucleus accumbens (NAc), emotional processing in the amygdala, and executive functioning in the medial prefrontal cortex (mPFC). The multifunctionality of dopamine likely occurs at the individual synapses, with varied levels of phasic dopamine release acting on different receptor populations. This study aimed to characterize specific aspects of stimulation‐evoked phasic dopamine transmission, beyond simple dopamine release, using in vivo fixed potential amperometry with carbon fiber recording microelectrodes positioned in either the dorsal striatum, NAc, amygdala, or mPFC of anesthetized mice. To summarize results, the present study found that the striatum and NAc had increased stimulation‐evoked phasic dopamine release, faster dopamine uptake (leading to restricted dopamine diffusion), weaker autoreceptor functioning, greater supply levels of available dopamine, and increased dopaminergic responses to DAT blockade compared to the amygdala and mPFC. Overall, these findings indicate that phasic dopamine may have different modes of communication between striatal and corticolimbic regions, with the first being profuse in concentration, rapid, and synaptically confined and the second being more limited in concentration but longer lasting and spatially dispersed. An improved understanding of regional differences in dopamine transmission can lead to more efficient treatments for disorders related to dopamine dysfunction.     

Ethanol induced differences in medial prefrontal cortex dopamine asymmetry and in nucleus accumbens dopamine metabolism in left- and right-turning rats

Ethanol (0.5 g/kg i.p.) 15 min prior to sacrifice increased homovanillic acid (HVA) levels in the left medial prefrontal cortex (mPFC) of left-turning rats and in the right mPFC of right-turning rats. In the nucleus accumbens (NAS), ethanol decreased dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and HVA levels in rats that exhibited low levels of locomotor activity but not in rats that exhibited high levels of locomotor activity. This laboratory has previously shown that rats exhibiting differences in turning and locomotor activity behavior display different preferences for ethanol. The present results suggest that ethanol-induced differences in mPFC and NAS DA activity may be related to individual differences in the susceptibility to abuse ethanol.