Dopamine has bidirectional effects on synaptic responses to cortical inputs in layer II of the lateral entorhinal cortex

T-type Ca2+ channels and inward rectifier K+ channels contribute to the orexin-induced facilitation of GABAergic transmission onto pyramidal neurons in the prefrontal cortex of juvenile mice

Orexin is a neuropeptide restrictedly synthesized in the hypothalamus, but extensively modulates the whole brain region activity including prefrontal cortex (PFC), and involved in the pathophysiology of psychiatric disorders. GABAergic interneurons in the mPFC are a promising pharmacological target for developing antidepressant therapies. Here, we examined the effects of the orexin on GABAergic transmission onto pyramidal neurons in the deep layers of the mPFC. We found that bath application of orexin dose-dependently increased the amplitude of evoked IPSCs (eIPSCs). Orexin increased the frequency but not the amplitude of miniature IPSCs (mIPSCs). Ca²⁺ influx through T-type voltage-gated Ca²⁺ channels is required for orexin-induced increases in GABA release. We also found orexin increases GABA release probability and the number of releasable vesicles. Orexin depolarizes somatostatin (Sst) interneurons without effects on the firing rate of action potentials (APs) of Sst interneurons. Orexin-induced depolarization of Sst interneurons is independent of extracellular Na⁺, Ca²⁺ and T-type Ca²⁺ channels, but requires inward rectifier K⁺ channels (Kirs). The present study suggests that orexin enhances GABAergic transmission onto mPFC pyramidal neurons through inhibiting Kirs on Sst interneurons, which further depolarizes interneurons leading to increase in Ca²⁺ influx via T-type Ca²⁺ channels. Our results may provide a cellular and molecular mechanism that helps explain the physiological functions of orexin in the brain.

Involvement of dopamine D2 -like receptors in the antiepileptogenic effects of deep brain stimulation during kindling in rats

AIMS

Deep brain electrical stimulation (DBS), as a potential therapy for drug resistive epileptic patients, has inhibitory action on epileptogenesis. In the present investigation, the role of dopamine D₂ -like receptors in the antiepileptogenic action of DBS was studied.

METHODS

Seizures were induced in adult rats by stimulating the perforant path in a semi-rapid kindling method. Five minutes after the last kindling stimulation, daily DBS was applied to the perforant path at the pattern of low frequency stimulation (LFS; 1 Hz; pulse duration: 0.1 ms; intensity: 50-150 μA; 4 trains of 200 pulses at 5 min intervals). Sulpiride (10 μg/1 μl, i.c.v.), a selective dopamine D₂ -like receptor antagonist, was administered prior to the daily LFS application.

RESULTS

Kindling stimulations increased cumulative daily behavioral seizure stages, daily afterdischarge duration (dADD), and population spike amplitude (PS) in dentate gyrus following perforant path stimulation, while applying LFS decreased the kindled seizures' parameters. In addition, kindling potentiated the early (at 10-50 ms inter-pulse interval) and late (at 150-1000 ms inter-pulse interval) paired-pulse inhibition and decreased the paired-pulse facilitation (at 70-100 ms inter-pulse interval). These effects were also inhibited by applying LFS. All inhibitory effects of LFS on kindling procedure were prevented by sulpiride administration.

CONCLUSION

These data may suggest that LFS exerts its preventive effect on kindling development, at least partly, through the receptors on which sulpiride acts which are mainly dopamine D₂ -like (including D₂ , D₃ , and D₄ ) receptors.

Activation of Dopamine D2 Receptors Alleviates Neuronal Hyperexcitability in the Lateral Entorhinal Cortex via Inhibition of HCN Current in a Rat Model of Chronic Inflammatory Pain

Functional changes in synaptic transmission from the lateral entorhinal cortex to the dentate gyrus (LEC-DG) are considered responsible for the chronification of pain. However, the underlying alterations in fan cells, which are the predominant neurons in the LEC that project to the DG, remain elusive. Here, we investigated possible mechanisms using a rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. We found a substantial increase in hyperpolarization-activated/cyclic nucleotide-gated currents (Ih), which led to the hyperexcitability of LEC fan cells of CFA slices. This phenomenon was attenuated in CFA slices by activating dopamine D2, but not D1, receptors. Chemogenetic activation of the ventral tegmental area -LEC projection had a D2 receptor-dependent analgesic effect. Intra-LEC microinjection of a D2 receptor agonist also suppressed CFA-induced behavioral hypersensitivity, and this effect was attenuated by pre-activation of the Ih. Our findings suggest that down-regulating the excitability of LEC fan cells through activation of the dopamine D2 receptor may be a strategy for treating chronic inflammatory pain.

Navigating for reward

An organism's survival can depend on its ability to recall and navigate to spatial locations associated with rewards, such as food or a home. Accumulating research has revealed that computations of reward and its prediction occur on multiple levels across a complex set of interacting brain regions, including those that support memory and navigation. However, how the brain coordinates the encoding, recall and use of reward information to guide navigation remains incompletely understood. In this Review, we propose that the brain's classical navigation centres - the hippocampus and the entorhinal cortex - are ideally suited to coordinate this larger network by representing both physical and mental space as a series of states. These states may be linked to reward via neuromodulatory inputs to the hippocampus-entorhinal cortex system. Hippocampal outputs can then broadcast sequences of states to the rest of the brain to store reward associations or to facilitate decision-making, potentially engaging additional value signals downstream. This proposal is supported by recent advances in both experimental and theoretical neuroscience. By discussing the neural systems traditionally tied to navigation and reward at their intersection, we aim to offer an integrated framework for understanding navigation to reward as a fundamental feature of many cognitive processes.

The Neurotransmitter Receptor Architecture of the Mouse Olfactory System

The uptake, transmission and processing of sensory olfactory information is modulated by inhibitory and excitatory receptors in the olfactory system. Previous studies have focused on the function of individual receptors in distinct brain areas, but the receptor architecture of the whole system remains unclear. Here, we analyzed the receptor profiles of the whole olfactory system of adult male mice. We examined the distribution patterns of glutamatergic (AMPA, kainate, mGlu_2/3, and NMDA), GABAergic (GABA_A, GABA_A(BZ), and GABA_B), dopaminergic (D_1/5) and noradrenergic (α₁ and α₂) neurotransmitter receptors by quantitative in vitro receptor autoradiography combined with an analysis of the cyto- and myelo-architecture. We observed that each subarea of the olfactory system is characterized by individual densities of distinct neurotransmitter receptor types, leading to a region- and layer-specific receptor profile. Thereby, the investigated receptors in the respective areas and strata showed a heterogeneous expression. Generally, we detected high densities of mGlu_2/3Rs, GABA_A(BZ)Rs and GABA_BRs. Noradrenergic receptors revealed a highly heterogenic distribution, while the dopaminergic receptor D_1/5 displayed low concentrations, except in the olfactory tubercle and the dorsal endopiriform nucleus. The similarities and dissimilarities of the area-specific multireceptor profiles were analyzed by a hierarchical cluster analysis. A three-cluster solution was found that divided the areas into the (1) olfactory relay stations (main and accessory olfactory bulb), (2) the olfactory cortex (anterior olfactory cortex, dorsal peduncular cortex, taenia tecta, piriform cortex, endopiriform nucleus, entorhinal cortex, orbitofrontal cortex) and the (3) olfactory tubercle, constituting its own cluster. The multimodal receptor-architectonic analysis of each component of the olfactory system provides new insights into its neurochemical organization and future possibilities for pharmaceutic targeting.

GPCR-Based Dopamine Sensors-A Detailed Guide to Inform Sensor Choice for In vivo Imaging

Understanding how dopamine (DA) encodes behavior depends on technologies that can reliably monitor DA release in freely-behaving animals. Recently, red and green genetically encoded sensors for DA (dLight, GRAB-DA) were developed and now provide the ability to track release dynamics at a subsecond resolution, with submicromolar affinity and high molecular specificity. Combined with rapid developments in in vivo imaging, these sensors have the potential to transform the field of DA sensing and DA-based drug discovery. When implementing these tools in the laboratory, it is important to consider there is not a 'one-size-fits-all' sensor. Sensor properties, most importantly their affinity and dynamic range, must be carefully chosen to match local DA levels. Molecular specificity, sensor kinetics, spectral properties, brightness, sensor scaffold and pharmacology can further influence sensor choice depending on the experimental question. In this review, we use DA as an example; we briefly summarize old and new techniques to monitor DA release, including DA biosensors. We then outline a map of DA heterogeneity across the brain and provide a guide for optimal sensor choice and implementation based on local DA levels and other experimental parameters. Altogether this review should act as a tool to guide DA sensor choice for end-users.

Dopamine Suppresses Synaptic Responses of Fan Cells in the Lateral Entorhinal Cortex to Olfactory Bulb Input in Mice

The lateral entorhinal cortex (LEC) is involved in odor discrimination, odor-associative multimodal memory, and neurological or neuropsychiatric disorders. It receives direct axonal projections from both olfactory bulb (OB) output neurons and midbrain dopaminergic neurons. However, the cellular targets in LEC receiving direct synaptic input from OB output neuron, the functional characteristics of these synapses, and whether or how dopamine (DA) modulates the OB-LEC pathway remain undetermined. We addressed these questions in the present study by combing optogenetic and electrophysiological approaches with four major findings: (1) selective activation of OB input elicited glutamate-mediated monosynaptic responses in all fan cells, the major output neurons in layer II of the LEC; (2) this excitatory synaptic transmission exhibited robust paired-pulse facilitation (PPF), a presynaptically derived short-term synaptic plasticity; (3) DA dramatically attenuated the strength of the OB input-fan cell synaptic transmission via activation of D1 receptors; and (4) DA altered the PPF of this transmission but neither intrinsic properties of postsynaptic neurons nor the kinetic profile of postsynaptic responses, suggesting that presynaptic mechanisms underlie the DA inhibitory actions. This study for the first time demonstrates the FCs in the LEC layer II as the postsynaptic target of direct OB input and characterizes DA modulation of the OB input-fan cell pathway. These findings set the foundation for future studies to examine the synaptic transmission from the OB output neuron axon terminals to other potential cell types in the LEC and to pinpoint the pathophysiological mechanisms underlying olfactory deficits associated with DA-relevant neurological and neuropsychiatric disorders.

Dopamine and memory dedifferentiation in aging

The dedifferentiation theory of aging proposes that a reduction in the specificity of neural representations causes declines in complex cognition as people get older, and may reflect a reduction in dopaminergic signaling. The present pharmacological fMRI study investigated episodic memory-related dedifferentiation in young and older adults, and its relation to dopaminergic function, using a randomized placebo-controlled double-blind crossover design with the agonist Bromocriptine (1.25mg) and the antagonist Sulpiride (400mg). We used multi-voxel pattern analysis to measure memory specificity: the degree to which distributed patterns of activity distinguishing two different task contexts during an encoding phase are reinstated during memory retrieval. As predicted, memory specificity was reduced in older adults in prefrontal cortex and in hippocampus, consistent with an impact of neural dedifferentiation on episodic memory representations. There was also a linear age-dependent dopaminergic modulation of memory specificity in hippocampus reflecting a relative boost to memory specificity on Bromocriptine in older adults whose memory was poorer at baseline, and a relative boost on Sulpiride in older better performers, compared to the young. This differed from generalized effects of both agents on task specificity in the encoding phase. The results demonstrate a link between aging, dopaminergic function and dedifferentiation in the hippocampus.

A behavioral defect of temporal association memory in mice that partly lack dopamine reuptake transporter

Temporal association memory, like working memory, is a type of episodic memory in which temporally discontinuous elements are associated. However, the mechanisms that govern this association remain incompletely understood. Here, we identify a crucial role of dopaminergic action in temporal association memory. We used hemizygote hyperdopaminergic mutant mice with reduced dopamine transporter (DAT) expression, referred to as DAT^+/− mice. We found that mice with this modest dopamine imbalance exhibited significantly impaired trace fear conditioning, which necessitates the association of temporally discontinuous elements, and intact delay auditory fear conditioning, which does not. Moreover, the DAT^+/− mice displayed substantial impairments in non-matching-to-place spatial working-memory tasks. Interestingly, these temporal association and working memory deficits could be mimicked by a low dose of the dopamine D2 receptor antagonist haloperidol. The shared phenotypes resulting from either the genetic reduction of DAT or the pharmacological inhibition of the D2 receptor collectively indicate that temporal association memory necessitates precise regulation of dopaminergic signaling. The particular defect in temporal association memory due to partial lack of DAT provides mechanistic insights on the understanding of cognitive impairments in multiple neurodevelopmental disorders.

Activation of Phosphatidylinositol-Linked Dopamine Receptors Induces a Facilitation of Glutamate-Mediated Synaptic Transmission in the Lateral Entorhinal Cortex

The lateral entorhinal cortex receives strong inputs from midbrain dopamine neurons that can modulate its sensory and mnemonic function. We have previously demonstrated that 1 µM dopamine facilitates synaptic transmission in layer II entorhinal cortex cells via activation of D1-like receptors, increased cAMP-PKA activity, and a resulting enhancement of AMPA-receptor mediated currents. The present study assessed the contribution of phosphatidylinositol (PI)-linked D1 receptors to the dopaminergic facilitation of transmission in layer II of the rat entorhinal cortex, and the involvement of phospholipase C activity and release of calcium from internal stores. Whole-cell patch-clamp recordings of glutamate-mediated evoked excitatory postsynaptic currents were obtained from pyramidal and fan cells. Activation of D1-like receptors using SKF38393, SKF83959, or 1 µM dopamine induced a reversible facilitation of EPSCs which was abolished by loading cells with either the phospholipase C inhibitor U-73122 or the Ca2+ chelator BAPTA. Neither the L-type voltage-gated Ca2+ channel blocker nifedipine, nor the L/N-type channel blocker cilnidipine, blocked the facilitation of synaptic currents. However, the facilitation was blocked by blocking Ca2+ release from internal stores via inositol 1,4,5-trisphosphate (InsP3) receptors or ryanodine receptors. Follow-up studies demonstrated that inhibiting CaMKII activity with KN-93 failed to block the facilitation, but that application of the protein kinase C inhibitor PKC(19-36) completely blocked the dopamine-induced facilitation. Overall, in addition to our previous report indicating a role for the cAMP-PKA pathway in dopamine-induced facilitation of synaptic transmission, we demonstrate here that the dopaminergic facilitation of synaptic responses in layer II entorhinal neurons also relies on a signaling cascade dependent on PI-linked D1 receptors, PLC, release of Ca2+ from internal stores, and PKC activation which is likely dependent upon both DAG and enhanced intracellular Ca2+. These signaling pathways may collaborate to enhance sensory and mnemonic function in the entorhinal cortex during tonic release of dopamine.

Patterns of Spontaneous Local Network Activity in Developing Cerebral Cortex: Relationship to Adult Cognitive Function

Detecting neurodevelopμental disorders of cognition at the earliest possible stages could assist in understanding them mechanistically and ultimately in treating them. Finding early physiological predictors that could be visualized with functional neuroimaging would represent an important advance in this regard. We hypothesized that one potential source of physiological predictors is the spontaneous local network activity prominent during specific periods in development. To test this we used calcium imaging in brain slices and analyzed variations in the frequency and intensity of this early activity in one area, the entorhinal cortex (EC), in order to correlate early activity with level of cognitive function later in life. We focused on EC because of its known role in different types of cognitive processes and because it is an area where spontaneous activity is prominent during early postnatal development in rodent models of cortical development. Using rat strains (Long-Evans, Wistar, Sprague-Dawley and Brattleboro) known to differ in cognitive performance in adulthood we asked whether neonatal animals exhibit corresponding strain-related differences in EC spontaneous activity. Our results show significant differences in this activity between strains: compared to a high cognitive-performing strain, we consistently found an increase in frequency and decrease in intensity in neonates from three lower performing strains. Activity was most different in one strain considered a model of schizophrenia-like psychopathology. While we cannot necessarily infer a causal relationship between early activity and adult cognition our findings suggest that the pattern of spontaneous activity in development could be an early predictor of a developmental trajectory advancing toward sub-optimal cognitive performance in adulthood. Our results further suggest that the strength of dopaminergic signaling, by setting the balance between excitation and inhibition, is a potential underlying mechanism that could explain the observed differences in early spontaneous activity patterns.

Stimulation of α1-adrenoceptors facilitates GABAergic transmission onto pyramidal neurons in the medial prefrontal cortex

Whereas activation of α1-adrenoceptors (α1-ARs) modulates glutamatergic transmission, the roles of α1-ARs in GABAergic transmission in the medial prefrontal cortex (mPFC) are elusive. Here, we examined the effects of the α1-AR agonist phenylephrine (Phe) on GABAergic transmission onto pyramidal neurons in the deep layers of the mPFC. We found that bath application of Phe dose-dependently increased the amplitude of evoked IPSCs (eIPSCs). Phe increased the frequency but not the amplitude of miniature IPSCs (mIPSCs). Ca(2+) influx through T-type voltage-gated calcium channels is required for Phe-induced increases in GABA release. Phe increases GABA release probability and the number of releasable vesicles. Phe depolarizes the fast-spiking (FS) interneurons without effects on the firing rate of action potentials (APs) of interneurons. Phe-induced depolarization is independent of extracellular Na(+), Ca(2+) and T-type calcium channels, but requires inward rectifier K(+) channels (Kirs). The present study demonstrates that Phe enhances GABAergic transmission onto mPFC pyramidal neurons through inhibiting interneurons Kirs, which further depolarizes interneurons leading to increase in Ca(2+) influx via T-type calcium channels. Our results may provide a cellular and molecular mechanism that helps explain α1-AR-induced PFC dysfunction.

Dopaminergic enhancement of excitatory synaptic transmission in layer II entorhinal neurons is dependent on D₁-like receptor-mediated signaling

The modulatory neurotransmitter dopamine induces concentration-dependent changes in synaptic transmission in the entorhinal cortex, in which high concentrations of dopamine suppress evoked excitatory postsynaptic potentials (EPSPs) and lower concentrations induce an acute synaptic facilitation. Whole-cell current-clamp recordings were used to investigate the dopaminergic facilitation of synaptic responses in layer II neurons of the rat lateral entorhinal cortex. A constant bath application of 1 μM dopamine resulted in a consistent facilitation of EPSPs evoked in layer II fan cells by layer I stimulation; the size of the facilitation was more variable in pyramidal neurons, and synaptic responses in a small group of multiform neurons were not modulated by dopamine. Isolated inhibitory synaptic responses were not affected by dopamine, and the facilitation of EPSPs was not associated with a change in paired-pulse facilitation ratio. Voltage-clamp recordings of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptor-mediated excitatory postsynaptic currents (EPSCs) were facilitated by dopamine, but N-methyl-D-aspartate receptor-mediated currents were not. Bath application of the dopamine D₁-like receptor blocker SCH23390 (50 μM), but not the D₂-like receptor blocker sulpiride (50 μM), prevented the facilitation, indicating that it is dependent upon D₁-like receptor activation. Dopamine D₁ receptors lead to activation of protein kinase A (PKA), and including the PKA inhibitor H-89 or KT 5720 in the recording pipette solution prevented the facilitation of EPSCs. PKA-dependent phosphorylation of inhibitor 1 or the dopamine- and cAMP-regulated protein phosphatase (DARPP-32) can lead to a facilitation of AMPA receptor responses by inhibiting the activity of protein phosphatase 1 (PP1) that reduces dephosphorylation of AMPA receptors, and we found here that inhibition of PP1 occluded the facilitatory effect of dopamine. The dopamine-induced facilitation of AMPA receptor-mediated synaptic responses in layer II neurons of the lateral entorhinal cortex is therefore likely mediated via a D₁ receptor-dependent increase in PKA activity and a resulting inhibition in PP1-dependent dephosphorylation of AMPA receptors.

Exposure to cues associated with palatable food reward results in a dopamine D₂ receptor-dependent suppression of evoked synaptic responses in the entorhinal cortex

BACKGROUND

The lateral entorhinal cortex receives inputs from ventral tegmental area dopamine neurons that are activated by exposure to food-related cues, and exogenously applied dopamine is known to modulate excitatory synaptic responses within the entorhinal cortex.

METHODS

The present study used in vivo synaptic field potential recording techniques to determine how exposure to cues associated with food reward modulates synaptic responses in the entorhinal cortex of the awake rat. Chronically implanted electrodes were used to monitor synaptic potentials in the entorhinal cortex evoked by stimulation of the piriform (olfactory) cortex, and to determine how synaptic responses are modulated by food-related cues.

RESULTS

The amplitudes of evoked synaptic responses were reduced during exposure to cues associated with delivery of chocolate, and during delivery of chocolate for consumption at unpredictable intervals. Reductions in synaptic responses were not well predicted by changes in behavioural mobility, and were not fully blocked by systemic injection of either the D₁-like receptor antagonist SCH23390, or the muscarinic receptor antagonist scopolamine. However, the reduction in synaptic responses was blocked by injection of the D₂-like receptor antagonist eticlopride.

CONCLUSIONS

Exposure to cues associated with palatable food results in a suppression of synaptic responses in olfactory inputs to the entorhinal cortex that is mediated in part by activation of dopamine D₂ receptors.

Dopaminergic modulation of GABAergic transmission in the entorhinal cortex: concerted roles of α1 adrenoreceptors, inward rectifier K⁺, and T-type Ca²⁺ channels

Whereas the entorhinal cortex (EC) receives profuse dopaminergic innervations from the midbrain, the effects of dopamine (DA) on γ-Aminobutyric acid (GABA)ergic interneurons in this brain region have not been determined. We probed the actions of DA on GABAA receptor-mediated synaptic transmission in the EC. Application of DA increased the frequency, not the amplitude, of spontaneous IPSCs (sIPSCs) and miniature IPSCs (mIPSCs) recorded from entorhinal principal neurons, but slightly reduced the amplitude of the evoked IPSCs. The effects of DA were unexpectedly found to be mediated by α1 adrenoreceptors, but not by DA receptors. DA endogenously released by the application of amphetamine also increased the frequency of sIPSCs. Ca(2+) influx via T-type Ca(2+) channels was required for DA-induced facilitation of sIPSCs and mIPSCs. DA depolarized and enhanced the firing frequency of action potentials of interneurons. DA-induced depolarization was independent of extracellular Na(+) and Ca(2+) and did not require the functions of hyperpolarization-activated (Ih) channels and T-type Ca(2+) channels. DA-generated currents showed a reversal potential close to the K(+) reversal potential and inward rectification, suggesting that DA inhibits the inward rectifier K(+) channels (Kirs). Our results demonstrate that DA facilitates GABA release by activating α1 adrenoreceptors to inhibit Kirs, which further depolarize interneurons resulting in secondary Ca(2+) influx via T-type Ca(+) channels.

A neuroanatomical model of prefrontal inhibitory modulation of memory retrieval

Memory of past experience is essential for guiding goal-related behavior. Being able to control accessibility of memory through modulation of retrieval enables humans to flexibly adapt to their environment. Understanding the specific neural pathways of how this control is achieved has largely eluded cognitive neuroscience. Accordingly, in the current paper I review literature that examines the overt control over retrieval in order to reduce accessibility. I first introduce three hypotheses of inhibition of retrieval. These hypotheses involve: (i) attending to other stimuli as a form of diversionary attention, (ii) inhibiting the specific individual neural representation of the memory, and (iii) inhibiting the hippocampus and retrieval process more generally to prevent reactivation of the representation. I then analyze literature taken from the White Bear Suppression, Directed Forgetting and Think/No-Think tasks to provide evidence for these hypotheses. Finally, a neuroanatomical model is developed to indicate three pathways from PFC to the hippocampal complex that support inhibition of memory retrieval. Describing these neural pathways increases our understanding of control over memory in general.

Adenosine modulates the excitability of layer II stellate neurons in entorhinal cortex through A1 receptors

Stellate neurons in layer II entorhinal cortex (EC) provide the main output from the EC to the hippocampus. It is believed that adenosine plays a crucial role in neuronal excitability and synaptic transmission in the CNS, however, the function of adenosine in the EC is still elusive. Here, the data reported showed that adenosine hyperpolarized stellate neurons in a concentration-dependent manner, accompanied by a decrease in firing frequency. This effect corresponded to the inhibition of the hyperpolarization-activated, cation nonselective (HCN) channels. Surprisingly, the adenosine-induced inhibition was blocked by 3 μM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) receptor antagonists, but not by 10 μM 3,7-dimethyl-1-propargylxanthine (DMPX), a selective A(2) receptor antagonists, indicating that activation of adenosine A(1) receptors were responsible for the direct inhibition. In addition, adenosine reduced the frequency but not the amplitude of miniature EPSCs and IPSCs, suggesting that the global depression of glutamatergic and GABAergic transmission is mediated by a decrease in glutamate and GABA release, respectively. Again the presynaptic site of action was mediated by adenosine A(1) receptors. Furthermore, inhibition of spontaneous glutamate and GABA release by adenosine A(1) receptor activation was mediated by voltage-dependent Ca(2+) channels and extracellular Ca(2+) . Therefore, these findings revealed direct and indirect mechanisms by which activation of adenosine A(1) receptors on the cell bodies of stellate neurons and on the presynaptic terminals could regulate the excitability of these neurons.

Relevance of norepinephrine-dopamine interactions in the treatment of major depressive disorder

Central dopaminergic and noradrenergic systems play essential roles in controlling several forebrain functions. Consequently, perturbations of these neurotransmissions may contribute to the pathophysiology of neuropsychiatric disorders. For many years, there was a focus on the serotonin (5‐HT) system because of the efficacy of selective serotonin reuptake inhibitors (SSRIs), the most prescribed antidepressants in the treatment of major depressive disorder (MDD). Given the interconnectivity within the monoaminergic network, any action on one system may reverberate in the other systems. Analysis of this network and its dysfunctions suggests that drugs with selective or multiple modes of action on dopamine (DA) and norepinephrine (NE) may have robust therapeutic effects. This review focuses on NE‐DA interactions as demonstrated in electrophysiological and neurochemical studies, as well as on the mechanisms of action of agents with either selective or dual actions on DA and NE. Understanding the mode of action of drugs targeting these catecholaminergic neurotransmitters can improve their utilization in monotherapy and in combination with other compounds particularly the SSRIs. The elucidation of such relationships can help design new treatment strategies for MDD, especially treatment‐resistant depression.

Activation of glycogen synthase kinase-3 beta is required for hyperdopamine and D2 receptor-mediated inhibition of synaptic NMDA receptor function in the rat prefrontal cortex

The interactions between dopamine and glutamate systems play an essential role in normal brain functions and neuropsychiatric disorders. The mechanism of NMDA receptor regulation through high concentrations of dopamine, however, remains unclear. Here, we show the signaling pathways involved in hyperdopaminergic regulation of NMDA receptor functions in the prefrontal cortex by incubating cortical slices with high concentration of dopamine or administering dopamine reuptake inhibitor 1-(2-[bis-(4-fluorophenyl)methoxy]ethyl)- 4-(3-phenylpropyl)piperazine (GBR12909) in vivo. We found that, under both conditions, the synaptic NMDA receptor-mediated currents were significantly attenuated by excessive dopamine stimulation through activation of D(2) receptors. Furthermore, high dose of dopamine failed to affect NMDA receptor-mediated currents after blockade of NR2B subunits but triggered a dynamin-dependent endocytosis of NMDA receptors. The high-dose dopamine/D(2) receptor-mediated suppression of NMDA receptors was involved in the increase of glycogen synthase kinase-3beta (GSK-3beta) activity, which in turn phosphorylates beta-catenin and disrupts beta-catenin-NR2B interaction, but was dependent on neither Gq11 nor PLC (phospholipase C). Moreover, the hyperdopamine induced by GBR12909 significantly decreased the expression of both surface and intracellular NR2B proteins, as well as NR2B mRNA levels, suggesting an inhibition of protein synthesis. These effects were, however, completely reversed by administration of either GSK-3beta inhibitor or D(2) receptor antagonist. These results therefore suggest that GSK-3beta is required for the hyperdopamine/D(2) receptor-mediated inhibition of NMDA receptors in the prefrontal neurons and these actions may underlie D(2) receptor-mediated psychostimulant effects and hyperdopamine-dependent behaviors in the brain.

Hyperdopaminergia and NMDA receptor hypofunction disrupt neural phase signaling

Neural phase signaling has gained attention as a putative coding mechanism through which the brain binds the activity of neurons across distributed brain areas to generate thoughts, percepts, and behaviors. Neural phase signaling has been shown to play a role in various cognitive processes, and it has been suggested that altered phase signaling may play a role in mediating the cognitive deficits observed across neuropsychiatric illness. Here, we investigated neural phase signaling in two mouse models of cognitive dysfunction: mice with genetically induced hyperdopaminergia [dopamine transporter knock-out (DAT-KO) mice] and mice with genetically induced NMDA receptor hypofunction [NMDA receptor subunit-1 knockdown (NR1-KD) mice]. Cognitive function in these mice was assessed using a radial-arm maze task, and local field potentials were recorded from dorsal hippocampus and prefrontal cortex as DAT-KO mice, NR1-KD mice, and their littermate controls engaged in behavioral exploration. Our results demonstrate that both DAT-KO and NR1-KD mice display deficits in spatial cognitive performance. Moreover, we show that persistent hyperdopaminergia alters interstructural phase signaling, whereas NMDA receptor hypofunction alters interstructural and intrastructural phase signaling. These results demonstrate that dopamine and NMDA receptor dependent glutamate signaling play a critical role in coordinating neural phase signaling, and encourage further studies to investigate the role that deficits in phase signaling play in mediating cognitive dysfunction.

Memory encoding and dopamine in the aging brain: a psychopharmacological neuroimaging study

Normal aging brings with it changes in dopaminergic and memory functions. However, little is known about how these 2 changes are related. In this study, we identify a link between dopamine, episodic memory networks, and aging, using pharmacological functional magnetic resonance imaging. Young and older adults received a D2-like agonist (Bromocriptine, 1.25 mg), a D2-like antagonist (Sulpiride, 400 mg), and Placebo, in a double-blind crossover procedure. We observed group differences, during memory encoding, in medial temporal, frontal, and striatal regions and moreover, these regions were differentially sensitive across groups to dopaminergic perturbation. These findings suggest that brain systems underlying memory show age-related changes and that dopaminergic function may be key in understanding these changes. That these changes have behavioral consequences was suggested by the observation that drug modulations were most pronounced in older subjects with poorer recognition memory. Our findings provide direct evidence linking ageing, memory, and dopaminergic change.

Dopaminergic suppression of synaptic transmission in the lateral entorhinal cortex

Dopaminergic projections to the superficial layers of the lateral entorhinal cortex can modulate the strength of olfactory inputs to the region. We have found that low concentrations of dopamine facilitate field EPSPs in the entorhinal cortex, and that higher concentrations of dopamine suppress synaptic responses. Here, we have used whole-cell current clamp recordings from layer II neurons to determine the mechanisms of the suppression. Dopamine (10 to 50 μM) hyperpolarized membrane potential and reversibly suppressed the amplitude of EPSPs evoked by layer I stimulation. Both AMPA- and NMDA-mediated components were suppressed, and paired-pulse facilitation was also enhanced indicating that the suppression is mediated largely by reduced glutamate release. Blockade of D₂-like receptors greatly reduced the suppression of EPSPs. Dopamine also lowered input resistance, and reduced the number of action potentials evoked by depolarizing current steps. The drop in input resistance was mediated by activation of D₁-like receptors, and was prevented by blocking K⁺ channels with TEA. The dopaminergic suppression of synaptic transmission is therefore mediated by a D₂ receptor-dependent reduction in transmitter release, and a D₁ receptor-dependent increase in a K⁺ conductance. This suppression of EPSPs may dampen the strength of sensory inputs during periods of elevated mesocortical dopamine activity.

Sustained administration of bupropion alters the neuronal activity of serotonin, norepinephrine but not dopamine neurons in the rat brain

Bupropion is widely used in the treatment of depression. There are, however, limited data on its long-term effects on monoaminergic neurons and therefore the mechanism of its delayed onset of action is at present not well understood. The present study was conducted to examine the effects of prolonged bupropion administration on the firing activity of dorsal raphe nucleus (DRN), locus coeruleus (LC), and ventral tegmental area (VTA) neurons. Spontaneously firing neurons were recorded extracellularly in rats anesthetized with chloral hydrate. Bupropion (30 mg/kg/day) was administered using subcutaneously implanted minipumps. In the DRN, the firing rate of serotonin (5-HT) neurons was significantly increased after 2, 7 and 14 days of administration. The suppressant effect of LSD was significantly diminished after the two-day regimen, indicating a desensitization of 5-HT1A autoreceptors. In the LC, the firing rate of norepinephrine (NE) neurons was significantly attenuated after a 2-day regimen, but recovered progressively over 14 days of administration. The suppressant effect of clonidine on NE neuronal firing was significantly attenuated in rats treated with bupropion for 14 days, indicating a desensitization of alpha2-adrenoceptors. In the VTA, neither 2 nor 14 days of bupropion administration altered the firing and burst activity of dopamine neurons. These results indicate that bupropion, unlike 5-HT reuptake inhibitors, promptly increased 5-HT neuronal activity, due to early desensitization of the 5-HT1A autoreceptor. The gradual recovery of neuronal firing of NE neurons, due to the desensitization of alpha2-adrenoceptors, in the presence of the sustained increase in 5-HT neuronal firing, may explain in part the delayed onset of action of bupropion in major depression.

Inhibiting dopamine reuptake blocks the induction of long-term potentiation and depression in the lateral entorhinal cortex of awake rats

Synaptic plasticity in olfactory inputs to the lateral entorhinal cortex may result in lasting changes in the processing of olfactory stimuli. Changes in dopaminergic tone can have strong effects on basal evoked synaptic responses in the superficial layers of the entorhinal cortex, and the current study investigated whether dopamine may modulate the induction of long-term potentiation (LTP) and depression (LTD) in piriform cortex inputs to layer II of the lateral entorhinal cortex in awake rats. Groups of animals were pretreated with either saline or the selective dopamine reuptake inhibitor GBR12909 prior to low or high frequency stimulation to induce LTD or LTP. In saline-treated groups, synaptic responses were potentiated to 122.4+/-6.4% of baseline levels following LTP induction, and were reduced to 84.5+/-4.9% following induction of LTD. Changes in synaptic responses were maintained for up to 60min and returned to baseline levels within 24h. In contrast, induction of both LTP and LTD was blocked in rats pretreated with GBR12909. Dopaminergic suppression of synaptic plasticity in the entorhinal cortex may serve to restrain activity-dependent plasticity during reward-relevant behavioral states or during processing of novel stimuli.

State-dependent modulation of amygdala inputs by dopamine-induced enhancement of sodium currents in layer V entorhinal cortex

Interaction between the entorhinal cortex (EC) and basolateral amygdala (BLA) may be a fundamental component in the consolidation of many forms of affective memory, such as inhibitory avoidance. Dopamine (DA) in the EC is necessary for, and may facilitate, this form of learning. This effect of DA on affective behaviors may be accomplished in part through modulation of amygdala inputs. Although it is known that DA can modulate neuronal activity in the EC, it is not known whether DA modulates inputs from the BLA. In this study, we used in vitro patch-clamp recordings and Ca2+ imaging of layer V neurons in the rat lateral EC to determine whether DA modulates the integration of inputs from the BLA and the mechanism for this modulation. We found that DA exerted actions that depended on the neuronal state. Near resting membrane potentials, DA suppressed integration of inputs, whereas at depolarized potentials, DA enhanced integration. DA enhanced the integration by a D2-mediated enhancement of Na+ currents, via phospholipase C. These experiments demonstrate that DA can exert actions in the EC that depend on the membrane voltage. This effect of DA may affect a wide range of inputs. Functionally, by enhancement of amygdala inputs that arrive in concert with other inputs, or during depolarized states, DA can facilitate the impact of affect on memory in a subset of conditions.

Effects of serotonin on the intrinsic membrane properties of layer II medial entorhinal cortex neurons

Although serotonin (5-HT) is an important neuromodulator in the superficial layers of the medial entorhinal cortex (mEC), there is some disagreement concerning its influences upon the membrane properties of neurons within this region. We performed whole cell recordings of mEC Layer II projection neurons in rat brain slices in order to characterize the intrinsic influences of 5-HT. In current clamp, 5-HT evoked a biphasic response consisting of a moderately short latency and large amplitude hyperpolarization followed by a slowly developing, long lasting, and small amplitude depolarization. Correspondingly, in voltage clamp, 5-HT evoked a robust outward followed by a smaller inward shift of holding current. The outward current evoked by 5-HT showed a consistent current/voltage (I/V) relationship across cells with inward rectification, and demonstrating a reversal potential that was systematically dependent upon the extracellular concentration of K(+), suggesting that it was predominantly carried by potassium ions. However, the inward current showed a less consistent I/V relationship across different cells, suggesting multiple independent ionic mechanisms. The outward current was mediated through activation of 5-HT(1A) receptors via a G-protein dependent mechanism while inward currents were evoked in a 5-HT(1A)-independent fashion. A significant proportion of the inward current was blocked by the I(h) inhibitor ZD7288 and appeared to be due to 5-HT modulation of I(h) as 5-HT shifted the activation curve of I(h) in a depolarizing fashion. Serotonin is thus likely to influence, in a composite fashion, the information processing of Layer II neurons in the mEC and thus, the passage of neocortical information via the perforant pathway to the hippocampus.