Dopamine D1/5 receptor-mediated long-term potentiation of intrinsic excitability in rat prefrontal cortical neurons: Ca2+-dependent intracellular signaling

Rewiring of Prelimbic Inputs to the Nucleus Accumbens Core Underlies Cocaine-Induced Behavioral Sensitization

BACKGROUND

Unbalanced activity of medium spiny neurons (MSNs) of the direct and indirect pathways mediates reward-related behaviors induced by addictive drugs. Prelimbic (PL) input to MSNs in the nucleus accumbens core (NAcC) plays a key role in cocaine-induced early locomotor sensitization (LS). However, the adaptive plastic changes at PL-to-NAcC synapses underlying early LS remain unclear.

METHODS

Using transgenic mice and retrograde tracing, we identified NAcC-projecting pyramidal neurons (PNs) in the PL cortex based on the expression of dopamine receptor types (D1R or D2R). To examine cocaine-induced alterations in PL-to-NAcC synapses, we measured excitatory postsynaptic current amplitudes evoked by optostimulation of PL afferents to MSNs. Riluzole was chosen to test the effects of PL excitability on cocaine-induced changes of PL-to-NAcC synapses.

RESULTS

NAcC-projecting PNs were segregated into D1R- and D2R-expressing PNs (D1- and D2-PNs, respectively), and their excitability was opposingly regulated by respective dopamine agonists. Both D1- and D2-PNs exhibited balanced innervation of direct MSNs and indirect MSNs in naïve animals. Repeated cocaine injections resulted in biased synaptic strength toward direct MSNs through presynaptic mechanisms in both D1- and D2-PNs, although D2R activation reduced the D2-PN excitability. Under group 1 metabotropic glutamate receptors coactivation, however, D2R activation enhanced the D2-PN excitability. The cocaine-induced rewiring accompanied LS, and both rewiring and LS were precluded by PL infusion of riluzole, which reduced the intrinsic excitability of PL neurons.

CONCLUSIONS

These findings indicate that cocaine-induced rewiring of PL-to-NAcC synapses correlates well with early behavioral sensitization and that rewiring and LS can be prevented by riluzole-induced reduction of excitability of PL neurons.

Long-term memory, synaptic plasticity and dopamine in rodent medial prefrontal cortex: Role in executive functions

Mnemonic functions, supporting rodent behavior in complex tasks, include both long-term and (short-term) working memory components. While working memory is thought to rely on persistent activity states in an active neural network, long-term memory and synaptic plasticity contribute to the formation of the underlying synaptic structure, determining the range of possible states. Whereas, the implication of working memory in executive functions, mediated by the prefrontal cortex (PFC) in primates and rodents, has been extensively studied, the contribution of long-term memory component to these tasks received little attention. This review summarizes available experimental data and theoretical work concerning cellular mechanisms of synaptic plasticity in the medial region of rodent PFC and the link between plasticity, memory and behavior in PFC-dependent tasks. A special attention is devoted to unique properties of dopaminergic modulation of prefrontal synaptic plasticity and its contribution to executive functions.

Dopamine activates astrocytes in prefrontal cortex via α1-adrenergic receptors

The prefrontal cortex (PFC) is a hub for cognitive control, and dopamine profoundly influences its functions. In other brain regions, astrocytes sense diverse neurotransmitters and neuromodulators and, in turn, orchestrate regulation of neuroactive substances. However, basic physiology of PFC astrocytes, including which neuromodulatory signals they respond to and how they contribute to PFC function, is unclear. Here, we characterize divergent signaling signatures in mouse astrocytes of the PFC and primary sensory cortex, which show differential responsiveness to locomotion. We find that PFC astrocytes express receptors for dopamine but are unresponsive through the Gs/Gi-cAMP pathway. Instead, fast calcium signals in PFC astrocytes are time locked to dopamine release and are mediated by α1-adrenergic receptors both ex vivo and in vivo. Further, we describe dopamine-triggered regulation of extracellular ATP at PFC astrocyte territories. Thus, we identify astrocytes as active players in dopaminergic signaling in the PFC, contributing to PFC function though neuromodulator receptor crosstalk.

Pittolo et al. demonstrate that the neuromodulator dopamine targets astrocytes, a type of brain cell, via receptors specific to another neuromodulator—norepinephrine. This study provides groundwork on how dopamine affects non-neuronal brain cells and suggests that crosstalk between neuromodulatory pathways occurs in vivo, with possible clinical implications.

Dopaminergic regulation of vestibulo-cerebellar circuits through unipolar brush cells

While multiple monoamines modulate cerebellar output, the mechanistic details of dopaminergic signaling in the cerebellum remain poorly understood. We show that dopamine type 1 receptors (Drd1) are expressed in unipolar brush cells (UBCs) of the mouse cerebellar vermis. Drd1 activation increases UBC firing rate and post-synaptic NMDAR -mediated currents. Using anatomical tracing and in situ hybridization, we test three hypotheses about the source of cerebellar dopamine. We exclude midbrain dopaminergic nuclei and tyrosine hydroxylase-positive Purkinje (Pkj) cells as potential sources, supporting the possibility of dopaminergic co-release from locus coeruleus (LC) axons. Using an optical dopamine sensor GRABDA2h, electrical stimulation, and optogenetic activation of LC fibers in the acute slice, we find evidence for monoamine release onto Drd1-expressing UBCs. Altogether, we propose that the LC regulates cerebellar cortex activity by co-releasing dopamine onto UBCs to modulate their response to cerebellar inputs. Pkj cells directly inhibit these Drd1-positive UBCs, forming a dopamine-sensitive recurrent vestibulo-cerebellar circuit.

Conventional measures of intrinsic excitability are poor estimators of neuronal activity under realistic synaptic inputs

Activity-dependent regulation of intrinsic excitability has been shown to greatly contribute to the overall plasticity of neuronal circuits. Such neuroadaptations are commonly investigated in patch clamp experiments using current step stimulation and the resulting input-output functions are analyzed to quantify alterations in intrinsic excitability. However, it is rarely addressed, how such changes translate to the function of neurons when they operate under natural synaptic inputs. Still, it is reasonable to expect that a strong correlation and near proportional relationship exist between static firing responses and those evoked by synaptic drive. We challenge this view by performing a high-yield electrophysiological analysis of cultured mouse hippocampal neurons using both standard protocols and simulated synaptic inputs via dynamic clamp. We find that under these conditions the neurons exhibit vastly different firing responses with surprisingly weak correlation between static and dynamic firing intensities. These contrasting responses are regulated by two intrinsic K-currents mediated by Kv1 and Kir channels, respectively. Pharmacological manipulation of the K-currents produces differential regulation of the firing output of neurons. Static firing responses are greatly increased in stuttering type neurons under blocking their Kv1 channels, while the synaptic responses of the same neurons are less affected. Pharmacological blocking of Kir-channels in delayed firing type neurons, on the other hand, exhibit the opposite effects. Our subsequent computational model simulations confirm the findings in the electrophysiological experiments and also show that adaptive changes in the kinetic properties of such currents can even produce paradoxical regulation of the firing output.

Ketamine Rapidly Enhances Glutamate-Evoked Dendritic Spinogenesis in Medial Prefrontal Cortex Through Dopaminergic Mechanisms

BACKGROUND

Ketamine elicits rapid onset antidepressant effects in patients with clinical depression through mechanisms hypothesized to involve the genesis of neocortical dendritic spines and synapses. Yet, the observed changes in dendritic spine morphology usually emerge well after ketamine clearance, raising questions about the link between rapid behavioral effects of ketamine and plasticity.

METHODS

Here, we used two-photon glutamate uncaging/imaging to focally induce spinogenesis in the medial prefrontal cortex, directly interrogating baseline and ketamine-associated plasticity of deep layer pyramidal neurons in C57BL/6 mice. We combined pharmacological, genetic, optogenetic, and chemogenetic manipulations to interrogate dopaminergic mechanisms underlying ketamine-induced rapid enhancement in evoked plasticity and associated behavioral changes.

RESULTS

We found that ketamine rapidly enhances glutamate-evoked spinogenesis in the medial prefrontal cortex, with timing that matches the onset of its behavioral efficacy and precedes changes in dendritic spine density. Ketamine increases evoked cortical spinogenesis through dopamine Drd1 receptor (Drd1) activation that requires dopamine release, compensating blunted plasticity in a learned helplessness paradigm. The enhancement in evoked spinogenesis after Drd1 activation or ketamine treatment depends on postsynaptic protein kinase A activity. Furthermore, ketamine's behavioral effects are blocked by chemogenetic inhibition of dopamine release and mimicked by activating presynaptic dopaminergic terminals or postsynaptic Gα_s-coupled cascades in the medial prefrontal cortex.

CONCLUSIONS

Our findings highlight dopaminergic mediation of rapid enhancement in activity-dependent dendritic spinogenesis and behavioral effects induced by ketamine.

Modulation of dopamine D1 receptors via histamine H3 receptors is a novel therapeutic target for Huntington's disease

Early Huntington's disease (HD) include over-activation of dopamine D₁ receptors (D₁R), producing an imbalance in dopaminergic neurotransmission and cell death. To reduce D₁R over-activation, we present a strategy based on targeting complexes of D₁R and histamine H₃ receptors (H₃R). Using an HD mouse striatal cell model and HD mouse organotypic brain slices we found that D₁R-induced cell death signaling and neuronal degeneration, are mitigated by an H₃R antagonist. We demonstrate that the D₁R-H₃R heteromer is expressed in HD mice at early but not late stages of HD, correlating with HD progression. In accordance, we found this target expressed in human control subjects and low-grade HD patients. Finally, treatment of HD mice with an H₃R antagonist prevented cognitive and motor learning deficits and the loss of heteromer expression. Taken together, our results indicate that D₁R - H₃R heteromers play a pivotal role in dopamine signaling and represent novel targets for treating HD.

Dopamine Receptor Activation Modulates the Integrity of the Perisynaptic Extracellular Matrix at Excitatory Synapses

In the brain, Hebbian-type and homeostatic forms of plasticity are affected by neuromodulators like dopamine (DA). Modifications of the perisynaptic extracellular matrix (ECM), which control the functions and mobility of synaptic receptors as well as the diffusion of transmitters and neuromodulators in the extracellular space, are crucial for the manifestation of plasticity. Mechanistic links between synaptic activation and ECM modifications are largely unknown. Here, we report that neuromodulation via D1-type DA receptors can induce targeted ECM proteolysis specifically at excitatory synapses of rat cortical neurons via proteases ADAMTS-4 and -5. We showed that receptor activation induces increased proteolysis of brevican (BC) and aggrecan, two major constituents of the adult ECM both in vivo and in vitro. ADAMTS immunoreactivity was detected near synapses, and shRNA-mediated knockdown reduced BC cleavage. We have outlined a molecular scenario of how synaptic activity and neuromodulation are linked to ECM rearrangements via increased cAMP levels, NMDA receptor activation, and intracellular calcium signaling.

A Schizophrenia-Related Deletion Leads to KCNQ2-Dependent Abnormal Dopaminergic Modulation of Prefrontal Cortical Interneuron Activity

Altered prefrontal cortex function is implicated in schizophrenia (SCZ) pathophysiology and could arise from imbalance between excitation and inhibition (E/I) in local circuits. It remains unclear whether and how such imbalances relate to genetic etiologies. We used a mouse model of the SCZ-predisposing 22q11.2 deletion (Df(16)A+/- mice) to evaluate how this genetic lesion affects the excitability of layer V prefrontal pyramidal neurons and its modulation by dopamine (DA). Df(16)A+/- mice have normal balance between E/I at baseline but are unable to maintain it upon dopaminergic challenge. Specifically, in wild-type mice, D1 receptor (D1R) activation enhances excitability of layer V prefrontal pyramidal neurons and D2 receptor (D2R) activation reduces it. Whereas the excitatory effect upon D1R activation is enhanced in Df(16)A+/- mice, the inhibitory effect upon D2R activation is reduced. The latter is partly due to the inability of mutant mice to activate GABAergic parvalbumin (PV)+ interneurons through D2Rs. We further demonstrate that reduced KCNQ2 channel function in PV+ interneurons in Df(16)A+/- mice renders them less capable of inhibiting pyramidal neurons upon D2 modulation. Thus, DA modulation of PV+ interneurons and control of E/I are altered in Df(16)A+/- mice with a higher excitation and lower inhibition during dopaminergic modulation.

The intersection of stress and reward: BNST modulation of aversive and appetitive states

The bed nucleus of the stria terminalis (BNST) is widely acknowledged as a brain structure that regulates stress and anxiety states, as well as aversive and appetitive behaviours. The diverse roles of the BNST are afforded by its highly modular organisation, neurochemical heterogeneity, and complex intrinsic and extrinsic circuitry. There has been growing interest in the BNST in relation to psychopathologies such as anxiety and addiction. Although research on the human BNST is still in its infancy, there have been extensive preclinical studies examining the molecular signature and hodology of the BNST and their involvement in stress and reward seeking behaviour. This review examines the neurochemical phenotype and connectivity of the BNST, as well as electrophysiological correlates of plasticity in the BNST mediated by stress and/or drugs of abuse.

Epigenetic Effects Induced by Methamphetamine and Methamphetamine-Dependent Oxidative Stress

Methamphetamine is a widely abused drug, which possesses neurotoxic activity and powerful addictive effects. Understanding methamphetamine toxicity is key beyond the field of drug abuse since it allows getting an insight into the molecular mechanisms which operate in a variety of neuropsychiatric disorders. In fact, key alterations produced by methamphetamine involve dopamine neurotransmission in a way, which is reminiscent of spontaneous neurodegeneration and psychiatric schizophrenia. Thus, understanding the molecular mechanisms operated by methamphetamine represents a wide window to understand both the addicted brain and a variety of neuropsychiatric disorders. This overlapping, which is already present when looking at the molecular and cellular events promoted immediately after methamphetamine intake, becomes impressive when plastic changes induced in the brain of methamphetamine-addicted patients are considered. Thus, the present manuscript is an attempt to encompass all the molecular events starting at the presynaptic dopamine terminals to reach the nucleus of postsynaptic neurons to explain how specific neurotransmitters and signaling cascades produce persistent genetic modifications, which shift neuronal phenotype and induce behavioral alterations. A special emphasis is posed on disclosing those early and delayed molecular events, which translate an altered neurotransmitter function into epigenetic events, which are derived from the translation of postsynaptic noncanonical signaling into altered gene regulation. All epigenetic effects are considered in light of their persistent changes induced in the postsynaptic neurons including sensitization and desensitization, priming, and shift of neuronal phenotype.

Differential Expression of Dopamine D5 Receptors across Neuronal Subtypes in Macaque Frontal Eye Field

Dopamine signaling in the prefrontal cortex (PFC) is important for cognitive functions, yet very little is known about the expression of the D5 class of dopamine receptors (D5Rs) in this region. To address this, we co-stained for D5Rs, pyramidal neurons (neurogranin+), putative long-range projection pyramidal neurons (SMI-32+), and several classes of inhibitory interneuron (parvalbumin+, calbindin+, calretinin+, somatostatin+) within the frontal eye field (FEF): an area within the PFC involved in the control of visual spatial attention. We then quantified the co-expression of D5Rs with markers of different cell types across different layers of the FEF. We show that: (1) D5Rs are more prevalent on pyramidal neurons than on inhibitory interneurons. (2) D5Rs are disproportionately expressed on putative long-range projecting pyramidal neurons. The disproportionately high expression of D5Rs on long-range projecting pyramidals, compared to interneurons, was particularly pronounced in layers II-III. Together these results indicate that the engagement of D5R-dependent mechanisms in the FEF varies depending on cell type and cortical layer, and suggests that non-locally projecting neurons contribute disproportionately to functions involving the D5R subtype.

The Dopamine D5 Receptor Is Involved in Working Memory

Pharmacological studies indicate that dopamine D₁-like receptors (D₁ and D₅) are critically involved in cognitive function. However, the lack of pharmacological ligands selective for either the D₁ or D₅ receptors has made it difficult to determine the unique contributions of the D₁-like family members. To circumvent these pharmacological limitations, we used D₅ receptor homozygous (-/-) and heterozygous (+/-) knockout mice, to identify the specific role of this receptor in higher order cognitive functions. We identified a novel role for D₅ receptors in the regulation of spatial working memory and temporal order memory function. The D₅ mutant mice acquired a discrete paired-trial variable-delay T-maze task at normal rates. However, both [Formula: see text] and [Formula: see text] mice exhibited impaired performance compared to [Formula: see text] littermates when a higher burden on working memory faculties was imposed. In a temporal order object recognition task, [Formula: see text] exhibited significant memory deficits. No D₅-dependent differences in locomotor functions and interest in exploring objects were evident. Molecular biomarkers of dopaminergic functions within the prefrontal cortex (PFC) revealed a selective gene-dose effect on Akt phosphorylation at Ser473 with increased levels in [Formula: see text] knockout mice. A trend toward reduced levels in CaMKKbeta brain-specific band (64 kDa) in [Formula: see text] compared to [Formula: see text] was also evident. These findings highlight a previously unidentified role for D₅ receptors in working memory function and associated molecular signatures within the PFC.

M1 muscarinic activation induces long-lasting increase in intrinsic excitability of striatal projection neurons

The dorsolateral striatum is critically involved in movement control and motor learning. Striatal function is regulated by a variety of neuromodulators including acetylcholine. Previous studies have shown that cholinergic activation excites striatal principal projection neurons, medium spiny neurons (MSNs), and this action is mediated by muscarinic acetylcholine subtype 1 receptors (M₁) through modulating multiple potassium channels. In the present study, we used electrophysiology techniques in conjunction with optogenetic and pharmacological tools to determine the long-term effects of striatal cholinergic activation on MSN intrinsic excitability. A transient increase in acetylcholine release in the striatum by optogenetic stimulation resulted in a long-lasting increase in excitability of MSNs, which was associated with hyperpolarizing shift of action potential threshold and decrease in afterhyperpolarization (AHP) amplitude, leading to an increase in probability of EPSP-action potential coupling. The M₁ selective antagonist VU0255035 prevented, while the M₁ selective positive allosteric modulator (PAM) VU0453595 potentiated the cholinergic activation-induced persistent increase in MSN intrinsic excitability, suggesting that M₁ receptors are critically involved in the induction of this long-lasting response. This M₁ receptor-dependent long-lasting change in MSN intrinsic excitability could have significant impact on striatal processing and might provide a novel mechanism underlying cholinergic regulation of the striatum-dependent motor learning and cognitive function. Consistent with this, behavioral studies indicate that potentiation of M₁ receptor signaling by VU0453595 enhanced performance of mice in cue-dependent water-based T-maze, a dorsolateral striatum-dependent learning task.

Modulation of Synaptic Plasticity in the Cortex Needs to Understand All the Players

The prefrontal cortex (PFC) is involved in cognitive tasks such as working memory, decision making, risk assessment and regulation of attention. These functions performed by the PFC are supposed to rely on rhythmic electrical activity generated by neuronal network oscillations determined by a precise balance between excitation and inhibition balance (E/I balance) resulting from the coordinated activities of recurrent excitation and feedback and feedforward inhibition. Functional alterations in PFC functions have been associated with cognitive deficits in several pathologies such as major depression, anxiety and schizophrenia. These pathological situations are correlated with alterations of different neurotransmitter systems (i.e., serotonin (5-HT), dopamine (DA), acetylcholine…) that result in alterations of the E/I balance. The aim of this review article is to cover the basic aspects of the regulation of the E/I balance as well as to highlight the importance of the complementarity role of several neurotransmitters in the modulation of the plasticity of excitatory and inhibitory synapses. We illustrate our purpose by recent findings that demonstrate that 5-HT and DA cooperate to regulate the plasticity of excitatory and inhibitory synapses targeting layer 5 pyramidal neurons (L5PyNs) of the PFC and to fine tune the E/I balance. Using a method based on the decomposition of the synaptic conductance into its excitatory and inhibitory components, we show that concomitant activation of D1-like receptors (D1Rs) and 5-HT_1ARs, through a modulation of NMDA receptors, favors long term potentiation (LTP) of both excitation and inhibition and consequently does not modify the E/I balance. We also demonstrate that activation of D2-receptors requires functional 5-HT_1ARs to shift the E-I balance towards more inhibition and to favor long term depression (LTD) of excitatory synapses through the activation of glycogen synthase kinase 3β (GSK3β). This cooperation between different neurotransmitters is particularly relevant in view of pathological situations in which alterations of one neurotransmitter system will also have consequences on the regulation of synaptic efficacy by other neurotransmitters. This opens up new perspectives in the development of therapeutic strategies for the pharmacological treatment of neuronal disorders.

Influence of SKF38393 on changes of gene profile in rat prefrontal cortex during chronic paradoxical sleep deprivation

Chronic paradoxical sleep deprivation (CSD) can induce dramatic physiological and neurofunctional changes in rats, including decreased body weight, reduced learning and memory, and declined locomotor function. SKF38393, a dopamine D1 receptor agonist, can reverse the above damages. However, the mechanism of CSD syndrome and reversal role of SKF38393 remains largely unexplained. To preliminarily elucidate the mechanism of the neural dysfunction caused by CSD, in the present study we use gene chips to examine the expression profile of more than 28,000 transcripts in the prefrontal cortex (PFC). Rats were sleep deprived by modified multi-platform method for 3 weeks. Totally 59 transcripts showed differential expressions in CSD group in contrast to controls; they included transcripts coding for caffeine metabolism, circadian rhythm, drug metabolism and some amino acid metabolism pathway. Among the 59 transcripts, 39 increased their expression and 20 decreased. Two transcripts can be specifically reversed with SKF38393, one of them is Homer1, which is related to 20 functional classifications and coding for Glutamatergic synapse pathway. Our findings in the present study indicate that long-term sleep deprivation may trigger the changes of some certain functions and pathways in the PFC, and lead to the dysfunction of this advanced neuron, and the activation of D1 receptor by SKF38393 might ameliorate these changes via modulation of some transcripts such as Homer1, which is involved in the Ca(2+) pathway and MAPK pathway related to Glutamatergic synapse pathway.

Suppression of outward K(+) currents by activating dopamine D1 receptors in rat retinal ganglion cells through PKA and CaMKII signaling pathways

Dopamine plays an important role in regulating neuronal functions in the central nervous system by activating the specific G-protein coupled receptors. Both D1 and D2 dopamine receptors are extensively distributed in the retinal neurons. In the present study, we investigated the effects of D1 receptor signaling on outward K(+) currents in acutely isolated rat retinal ganglion cells (RGCs) by patch-clamp techniques. Extracellular application of SKF81297 (10 μM), a specific D1 receptor agonist, significantly and reversibly suppressed outward K(+) currents of the cells, which was reversed by SCH23390 (10 μM), a selective D1 receptor antagonist. We further showed that SKF81297 mainly suppressed the glybenclamide (Gb)- and 4-aminopyridine (4-AP)-sensitive K(+) current components, but did not show effect on the tetraethylammonium (TEA)-sensitive one. Both protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways were likely involved in the SKF81297-induced suppression of the K(+) currents since either Rp-cAMP (10 μM), a cAMP/PKA signaling inhibitor, or KN-93 (10 μM), a specific CaMKII inhibitor, eliminated the SKF81297 effect. In contrast, neither protein kinase C (PKC) nor mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling pathway seemed likely to be involved because both the PKC inhibitor bisindolylmaleimide IV (Bis IV) (10 μM) and the MAPK/ERK1/2 inhibitor U0126 (10 μM) did not block the SKF81297-induced suppression of the K(+) currents. These results suggest that activation of D1 receptors suppresses the Gb- and 4-AP-sensitive K(+) current components in rat RGCs through the intracellular PKA and CaMKII signaling pathways, thus modulating the RGC excitability.

Dopamine modulation of transient receptor potential vanilloid type 1 (TRPV1) receptor in dorsal root ganglia neurons

The transient receptor potential vanilloid type 1 (TRPV1) receptor plays a key role in the modulation of nociceptor excitability. To address whether dopamine can modulate the activity of TRPV1 channels in nociceptive neurons, the effects of dopamine and dopamine receptor agonists were tested on the capsaicin-activated current recorded from acutely dissociated small diameter (<27 μm) dorsal root ganglia (DRG) neurons. Dopamine or SKF 81297 (an agonist at D1/D5 receptors), caused inhibition of both inward and outward currents by ∼60% and ∼48%, respectively. The effect of SKF 81297 was reversed by SCH 23390 (an antagonist at D1/D5 receptors), confirming that it was mediated by activation of D1/D5 dopamine receptors. In contrast, quinpirole (an agonist at D2 receptors) had no significant effect on the capsaicin-activated current. Inhibition of the capsaicin-activated current by SKF 81297 was mediated by G protein coupled receptors (GPCRs), and highly dependent on external calcium. The inhibitory effect of SKF 81297 on the capsaicin-activated current was not affected when the protein kinase A (PKA) activity was blocked with H89, or when the protein kinase C (PKC) activity was blocked with bisindolylmaleimide II (BIM). In contrast, when the calcium-calmodulin-dependent protein kinase II (CaMKII) was blocked with KN-93, the inhibitory effect of SKF 81297 on the capsaicin-activated current was greatly reduced, suggesting that activation of D1/D5 dopamine receptors may be preferentially linked to CaMKII activity. We suggest that modulation of TRPV1 channels by dopamine in nociceptive neurons may represent a way for dopamine to modulate incoming noxious stimuli.

Catecholaminergic based therapies for functional recovery after TBI

Among the many pathophysiologic consequences of traumatic brain injury are changes in catecholamines, including dopamine, epinephrine, and norepinephrine. In the context of TBI, dopamine is the one most extensively studied, though some research exploring epinephrine and norepinephrine have also been published. The purpose of this review is to summarize the evidence surrounding use of drugs that target the catecholaminergic system on pathophysiological and functional outcomes of TBI using published evidence from pre-clinical and clinical brain injury studies. Evidence of the effects of specific drugs that target catecholamines as agonists or antagonists will be discussed. Taken together, available evidence suggests that therapies targeting the catecholaminergic system may attenuate functional deficits after TBI. Notably, it is fairly common for TBI patients to be treated with catecholamine agonists for either physiological symptoms of TBI (e.g. altered cerebral perfusion pressures) or a co-occuring condition (e.g. shock), or cognitive symptoms (e.g. attentional and arousal deficits). Previous clinical trials are limited by methodological limitations, failure to replicate findings, challenges translating therapies to clinical practice, the complexity or lack of specificity of catecholamine receptors, as well as potentially counfounding effects of personal and genetic factors. Overall, there is a need for additional research evidence, along with a need for systematic dissemination of important study details and results as outlined in the common data elements published by the National Institute of Neurological Diseases and Stroke. Ultimately, a better understanding of catecholamines in the context of TBI may lead to therapeutic advancements. This article is part of a Special Issue entitled SI:Brain injury and recovery.

Alcohol-induced alterations in dopamine modulation of prefrontal activity

Long-term alcohol use leads to persistent cognitive deficits that may be associated with maladaptive changes in the neurocircuitry that mediates executive functions. Impairments caused by these changes can persist well into abstinence and have a negative impact on quality of life and job performance, and can increase the probability of relapse. Many of the changes that affect cognitive function appear to involve dysregulation of the mesocortical dopamine system. This includes changes in dopamine release and alterations in dopamine receptor expression and function in the medial prefrontal cortex (PFC). This review summarizes the cellular effects of acute and chronic ethanol exposure on dopamine release and dopamine receptor function in the PFC with the goal of providing greater understanding of the effects of alcohol-use disorders on the dopamine system and how this relates to deficits in the executive function of the PFC.

Dopaminergic modulation of synaptic plasticity in rat prefrontal neurons

The prefrontal cortex (PFC) is thought to store the traces for a type of long-term memory - the abstract memory that determines the temporal structure of behavior often termed a "rule" or "strategy". Long-term synaptic plasticity might serve as an underlying cellular mechanism for this type of memory. We therefore studied the induction of synaptic plasticity in rat PFC neurons, maintained in vitro, with special emphasis on the functionally important neuromodulator dopamine. First, the induction of long-term potentiation (LTP) was facilitated in the presence of tonic/background dopamine in the bath, and the dose-dependency of this background dopamine followed an "inverted-U" function, where too high or too low dopamine levels could not facilitate LTP. Second, the induction of long-term depression (LTD) by low-frequency stimuli appeared to be independent of background dopamine, but required endogenous, phasically-released dopamine during the stimuli. Blockade of dopamine receptors during the stimuli and exaggeration of the effect of this endogenously-released dopamine by inhibition of dopamine transporter activity both blocked LTD. Thus, LTD induction also followed an inverted-U function in its dopamine-dependency. We conclude that PFC synaptic plasticity is powerfully modulated by dopamine through inverted-U-shaped dose-dependency.

Effects of chronic REM sleep restriction on D1 receptor and related signal pathways in rat prefrontal cortex

The prefrontal cortex (PFC) mediates cognitive function that is sensitive to disruption by sleep loss, and molecular mechanisms regulating neural dysfunction induced by chronic sleep restriction (CSR), particularly in the PFC, have yet to be completely understood. The aim of the present study was to investigate the effect of chronic REM sleep restriction (REM-CSR) on the D₁ receptor (D₁R) and key molecules in D₁R' signal pathways in PFC. We employed the modified multiple platform method to create the REM-CSR rat model. The ultrastructure of PFC was observed by electron microscopy. HPLC was performed to measure the DA level in PFC. The expressions of genes and proteins of related molecules were assayed by real-time PCR and Western blot, respectively. The general state and morphology of PFC in rats were changed by CSR, and DA level and the expression of D₁R in PFC were markedly decreased (P < 0.01, P < 0.05); the expression of phosphor-PKAcα was significantly lowered in CSR rats (P < 0.05). The present results suggested that the alteration of neuropathology and D₁R expression in PFC may be associated with CSR induced cognitive dysfunction, and the PKA pathway of D₁R may play an important role in the impairment of advanced neural function.

Effect of dopaminergic D1 receptors on plasticity is dependent of serotoninergic 5-HT1A receptors in L5-pyramidal neurons of the prefrontal cortex

Major depression and schizophrenia are associated with dysfunctions of serotoninergic and dopaminergic systems mainly in the prefrontal cortex (PFC). Both serotonin and dopamine are known to modulate synaptic plasticity. 5-HT1A receptors (5-HT1ARs) and dopaminergic type D1 receptors are highly represented on dendritic spines of layer 5 pyramidal neurons (L5PyNs) in PFC. How these receptors interact to tune plasticity is poorly understood. Here we show that D1-like receptors (D1Rs) activation requires functional 5HT1ARs to facilitate LTP induction at the expense of LTD. Using 129/Sv and 5-HT1AR-KO mice, we recorded post-synaptic currents evoked by electrical stimulation in layer 2/3 after activation or inhibition of D1Rs. High frequency stimulation resulted in the induction of LTP, LTD or no plasticity. The D1 agonist markedly enhanced the NMDA current in 129/Sv mice and the percentage of L5PyNs displaying LTP was enhanced whereas LTD was reduced. In 5-HT1AR-KO mice, the D1 agonist failed to increase the NMDA current and orientated the plasticity towards L5PyNs displaying LTD, thus revealing a prominent role of 5-HT1ARs in dopamine-induced modulation of plasticity. Our data suggest that in pathological situation where 5-HT1ARs expression varies, dopaminergic treatment used for its ability to increase LTP could turn to be less and less effective.

Impaired neuronal operation through aberrant intrinsic plasticity in epilepsy

OBJECTIVE

Patients with temporal lobe epilepsy often display cognitive comorbidity with recurrent seizures. However, the cellular mechanisms underlying the impairment of neuronal information processing remain poorly understood in temporal lobe epilepsy. Within the hippocampal formation neuronal networks undergo major reorganization, including the sprouting of mossy fibers in the dentate gyrus; they establish aberrant recurrent synapses between dentate granule cells and operate via postsynaptic kainate receptors. In this report, we tested the hypothesis that this aberrant local circuit alters information processing of perforant path inputs constituting the major excitatory afferent pathway from entorhinal cortex to dentate granule cells.

METHODS

Experiments were performed in dentate granule cells from control rats and rats with temporal lobe epilepsy induced by pilocarpine hydrochloride treatment. Neurons were recorded in patch clamp in whole cell configuration in hippocampal slices.

RESULTS

Our present data revealed that an aberrant readout of synaptic inputs by kainate receptors triggered a long-lasting impairment of the perforant path input-output operation in epileptic dentate granule cells. We demonstrated that this is due to the aberrant activity-dependent potentiation of the persistent sodium current altering intrinsic firing properties of dentate granule cells.

INTERPRETATION

We propose that this aberrant activity-dependent intrinsic plasticity, which lastingly impairs the information processing of cortical inputs in dentate gyrus, may participate in hippocampal-related cognitive deficits, such as those reported in patients with epilepsy.

Chronic cocaine disrupts mesocortical learning mechanisms

The addictive power of drugs of abuse such as cocaine comes from their ability to hijack natural reward and plasticity mechanisms mediated by dopamine signaling in the brain. Reward learning involves burst firing of midbrain dopamine neurons in response to rewards and cues predictive of reward. The resulting release of dopamine in terminal regions is thought to act as a teaching signaling to areas such as the prefrontal cortex and striatum. In this review, we posit that a pool of extrasynaptic dopaminergic D1-like receptors activated in response to dopamine neuron burst firing serve to enable synaptic plasticity in the prefrontal cortex in response to rewards and their cues. We propose that disruptions in these mechanisms following chronic cocaine use contribute to addiction pathology, in part due to the unique architecture of the mesocortical pathway. By blocking dopamine reuptake in the cortex, cocaine elevates dopamine signaling at these extrasynaptic receptors, prolonging D1-receptor activation and the subsequent activation of intracellular signaling cascades, and thus inducing long-lasting maladaptive plasticity. These cellular adaptations may account for many of the changes in cortical function observed in drug addicts, including an enduring vulnerability to relapse. Therefore, understanding and targeting these neuroadaptations may provide cognitive benefits and help prevent relapse in human drug addicts.

Cell-attached single-channel recordings in intact prefrontal cortex pyramidal neurons reveal compartmentalized D1/D5 receptor modulation of the persistent sodium current

The persistent Na⁺ current (I_Nap) is believed to be an important target of dopamine modulation in prefrontal cortex (PFC) neurons. While past studies have tested the effects of dopamine on I_Nap, the results have been contradictory largely because of difficulties in measuring I_Nap using somatic whole-cell recordings. To circumvent these confounds we used the cell-attached patch-clamp technique to record single Na⁺ channels from the soma, proximal dendrite (PD) or proximal axon (PA) of intact prefrontal layer V pyramidal neurons. Under baseline conditions, numerous well resolved Na⁺ channel openings were recorded that exhibited an extrapolated reversal potential of 73 mV, a slope conductance of 14–19 pS and were blocked by tetrodotoxin (TTX). While similar in most respects, the propensity to exhibit prolonged bursts lasting >40 ms was many fold greater in the axon than the soma or dendrite. Bath application of the D1/D5 receptor agonist SKF81297 shifted the ensemble current activation curve leftward and increased the number of late events recorded from the PD but not the soma or PA. However, the greatest effect was on prolonged bursting where the D1/D5 receptor agonist increased their occurrence 3 fold in the PD and nearly 7 fold in the soma, but not at all in the PA. As a result, D1/D5 receptor activation equalized the probability of prolonged burst occurrence across the proximal axosomatodendritic region. Therefore, D1/D5 receptor modulation appears to be targeted mainly to Na⁺ channels in the PD/soma and not the PA. By circumventing the pitfalls of previous attempts to study the D1/D5 receptor modulation of I_Nap, we demonstrate conclusively that D1/D5 receptor activation can increase the I_Nap generated proximally, however questions still remain as to how D1/D5 receptor modulates Na⁺ currents in the more distal initial segment where most of the I_Nap is normally generated.

Dopamine D2 receptor desensitization by dopamine or corticotropin releasing factor in ventral tegmental area neurons is associated with increased glutamate release

Neurons of the ventral tegmental area (VTA) are the source of dopaminergic (DAergic) input to important brain regions related to addiction. Prolonged exposure of these VTA neurons to moderate concentrations of dopamine (DA) causes a time-dependent decrease in DA-induced inhibition, a complex desensitization called DA inhibition reversal (DIR). DIR is mediated by conventional protein kinase C (cPKC) through concurrent stimulation of D2 and D1-like DA receptors, or by D2 stimulation concurrent with activation of some Gq-linked receptors. Corticotropin releasing factor (CRF) acts via Gq, and can modulate glutamater neurotransmission in the VTA. In the present study, we used brain slice electrophysiology to characterize the interaction of DA, glutamate antagonists, and CRF agonists in the induction and maintenance of DIR in the VTA. Glutamate receptor antagonists blocked induction but not maintenance of DIR. Putative blockers of neurotransmitter release and store-operated calcium channels blocked and reversed DIR. CRF and the CRF agonist urocortin reversed inhibition produced by the D2 agonist quinpirole, consistent with our earlier work indicating that Gq activation reverses quinpirole-mediated inhibition. In whole cell recordings, the combination of urocortin and quinpirole, but not either agent alone, increased spontaneous excitatory postsynaptic currents (sEPSCs) in VTA neurons. Likewise, the combination of a D1-like receptor agonist and quinpirole, but not either agent alone, increased sEPSCs in VTA neurons. In summary, desensitization of D2 receptors induced by dopamine or CRF on DAergic VTA neurons is associated with increased glutamatergic signaling in the VTA.

Prefrontal deficits in a murine model overexpressing the down syndrome candidate gene dyrk1a

The gene Dyrk1a is the mammalian ortholog of Drosophila minibrain. Dyrk1a localizes in the Down syndrome (DS) critical region of chromosome 21q22.2 and is a major candidate for the behavioral and neuronal abnormalities associated with DS. PFC malfunctions are a common denominator in several neuropsychiatric diseases, including DS, but the contribution of DYRK1A in PFC dysfunctions, in particular the synaptic basis for impairments of executive functions reported in DS patients, remains obscure. We quantified synaptic plasticity, biochemical synaptic markers, and dendritic morphology of deep layer pyramidal PFC neurons in adult mBACtgDyrk1a transgenic mice that overexpress Dyrk1a under the control of its own regulatory sequences. We found that overexpression of Dyrk1a largely increased the number of spines on oblique dendrites of pyramidal neurons, as evidenced by augmented spine density, higher PSD95 protein levels, and larger miniature EPSCs. The dendritic alterations were associated with anomalous NMDAR-mediated long-term potentiation and accompanied by a marked reduction in the pCaMKII/CaMKII ratio in mBACtgDyrk1a mice. Retrograde endocannabinoid-mediated long-term depression (eCB-LTD) was ablated in mBACtgDyrk1a mice. Administration of green tea extracts containing epigallocatechin 3-gallate, a potent DYRK1A inhibitor, to adult mBACtgDyrk1a mice normalized long-term potentiation and spine anomalies but not eCB-LTD. However, inhibition of the eCB deactivating enzyme monoacylglycerol lipase normalized eCB-LTD in mBACtgDyrk1a mice. These data shed light on previously undisclosed participation of DYRK1A in adult PFC dendritic structures and synaptic plasticity. Furthermore, they suggest its involvement in DS-related endophenotypes and identify new potential therapeutic strategies.

Dopaminergic control of long-term depression/long-term potentiation threshold in prefrontal cortex

Long-term memory in the prefrontal cortex is a necessary component of adaptive executive control and is strongly modulated by dopamine. However, the functional significance of this dopaminergic modulation remains elusive. In vitro experimental results on dopamine-dependent shaping of prefrontal long-term plasticity often appear inconsistent and, altogether, draw a complicated picture. It is also generally difficult to relate these findings to in vivo observations given strong differences between the two experimental conditions. This study presents a unified view of the functional role of dopamine in the prefrontal cortex by framing it within the Bienenstock-Cooper-Munro theory of cortical plasticity. We investigate dopaminergic modulation of long-term plasticity through a multicompartment Hodgkin-Huxley model of a prefrontal pyramidal neuron. Long-term synaptic plasticity in the model is governed by a calcium- and dopamine-dependent learning rule, in which dopamine exerts its action via D1 and D2 dopamine receptors in a concentration-dependent manner. Our results support a novel function of dopamine in the prefrontal cortex, namely that it controls the synaptic modification threshold between long-term depression and potentiation in pyramidal neurons. The proposed theoretical framework explains a wide range of experimental results and provides a link between in vitro and in vivo studies of dopaminergic plasticity modulation. It also suggests that dopamine may constitute a new player in metaplastic and homeostatic processes in the prefrontal cortex.

Protein kinase A regulates the long-term potentiation of intrinsic excitability in neonatal trigeminal motoneurons

Although much is known about neuronal plasticity in the mammalian hippocampus and other cortical neurons, the subcellular mechanisms underlying plasticity at the level of motor pools are less well characterized. Protein kinase A (PKA) activation plays an essential role in long-term potentiation of intrinsic excitability (LTP-IE) in layer V (LV) visual cortical neurons and may be involved in other systems as well. Trigeminal motoneurons (TMNs) participate in rhythmical motor behaviors, such as suckling, chewing, and swallowing. Using the whole-cell patch clamp method and various kinase inhibitors and activators, we investigated the mechanism of LTP-IE in neonatal rat TMNs. Ca(2+) depletion using ACSF with 0mM Ca(2+) or the Ca(2+) chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) blocked the long-lasting increase in intrinsic excitability in TMNs, showing that intracellular Ca(2+) during the induction protocol is necessary for the induction of LTP-IE. We next used specific inhibitors of PKA, protein kinase C, and calcium/calmodulin-dependent protein kinase II during the induction protocol. Only the PKA inhibitor H-89 blocked the increase in the firing rate induced by the induction protocol. In addition, forskolin, which activates PKA, induced a long-lasting increase in excitability that resembled the excitability produced by the induction protocol. Thus, we conclude that LTP-IE in TMNs is calcium-dependent, and PKA is the primary regulator of this process.

Contribution of dopamine d1/5 receptor modulation of post-spike/burst afterhyperpolarization to enhance neuronal excitability of layer v pyramidal neurons in prepubertal rat prefrontal cortex

Dopamine (DA) receptors in the prefrontal cortex (PFC) modulate both synaptic and intrinsic plasticity that may contribute to cognitive processing. However, the ionic basis underlying DA actions to enhance neuronal plasticity in PFC remains ill-defined. Using whole-cell patch-clamp recordings in layer V-VI pyramidal cells in prepubertal rat PFC, we showed that DA, via activation of D1/5, but not D2/3/4, receptors suppress a Ca(2+)-dependent, apamin-sensitive K(+) channel that mediates post-spike/burst afterhyperpolarization (AHP) to enhance neuronal excitability of PFC neurons. This inhibition is not dependent on HCN channels. The D1/5 receptor activation also enhanced an afterdepolarizing potential (ADP) that follows the AHP. Additional single-spike analyses revealed that DA or D1/5 receptor activation suppressed the apamin-sensitive post-spike mAHP, further contributing to the increase in evoked spike firing to enhance the neuronal excitability. Taken together, the D1/5 receptor modulates intrinsic mechanisms that amplify a long depolarizing input to sustain spike firing outputs in pyramidal PFC neurons.

D1 receptor modulation of action potential firing in a subpopulation of layer 5 pyramidal neurons in the prefrontal cortex

Dopamine modulation in the prefrontal cortex is important for cognitive processing and disrupted in diverse neuropsychiatric diseases. Activation of D1 receptors is thought to enable working memory by enhancing the firing properties of pyramidal neurons. However, these receptors are only sparsely expressed in the prefrontal cortex, and how they impact individual neurons remains unknown. Here we study D1 receptor modulation of layer 5 pyramidal neurons in acute slices of the mouse prefrontal cortex. Using whole-cell recordings and two-photon microscopy, we show that neurons expressing D1 receptors have unique morphological and physiological properties. We then demonstrate that activation of these receptors selectively enhances the firing of these neurons by signaling via the protein kinase A pathway. This finding of robust D1 receptor modulation in only a subpopulation of neurons has important implications for cognitive function and disease.

Dopaminergic modulation of synaptic transmission in cortex and striatum

Among the many neuromodulators used by the mammalian brain to regulate circuit function and plasticity, dopamine (DA) stands out as one of the most behaviorally powerful. Perturbations of DA signaling are implicated in the pathogenesis or exploited in the treatment of many neuropsychiatric diseases, including Parkinson's disease (PD), addiction, schizophrenia, obsessive compulsive disorder, and Tourette's syndrome. Although the precise mechanisms employed by DA to exert its control over behavior are not fully understood, DA is known to regulate many electrical and biochemical aspects of neuronal function including excitability, synaptic transmission, integration and plasticity, protein trafficking, and gene transcription. In this Review, we discuss the actions of DA on ionic and synaptic signaling in neurons of the prefrontal cortex and striatum, brain areas in which dopaminergic dysfunction is thought to be central to disease.

Generation and characterization of hD5 and C-terminal Mutant hD(5m) transgenic rats

Dopamine D1-like receptors play important roles in many brain activities such as cognition and emotion. We have generated human hD5 and mutant human hD5 (hD(5m)) transgenic rats. The C-terminal juxtamembrane domain of mutant hD5 was identical to that of hD5 pseudogenes. The transgenes were driven by the CAMKII promoter that led the expression mainly in the cerebral cortex and hippocampus. We have used different dopamine receptor agonists to compare the pharmacological profiles of the human hD5 and hD(5m) receptors. The results showed that they exhibited distinct pharmacological properties. Our results of pharmacological studies indicated that the C-terminal of D5 receptor could play important roles in agonist binding affinity. Hippocampal long-term potentiation (LTP) evoked by tetanic stimulation was significantly reduced in both transgenic rats. In addition, we found that the overexpression of dopamine hD5 and hD(5m) receptors in the rat brain resulted in memory impairments. Interestingly, an atypical D1-like receptor agonist, SKF83959, could induce anxiety in hD(5m) receptor transgenic rats but had no effect on the anxiety-like behavior in D5 receptor transgenic and wild-type rats.

Amphetamine modulation of long-term potentiation in the prefrontal cortex: dose dependency, monoaminergic contributions, and paradoxical rescue in hyperdopaminergic mutant

Amphetamine can improve cognition in healthy subjects and patients with schizophrenia, attention-deficit hyperactivity disorder, and other neuropsychiatric diseases; higher doses, however, can impair cognitive function, especially those mediated by the prefrontal cortex. We investigated how amphetamine affects prefrontal cortex long-term potentiation (LTP), a cellular correlate of learning and memory, in normal and hyperdopaminergic mice lacking the dopamine transporter. Acute amphetamine treatment in wild-type mice produced a biphasic dose-response modulation of LTP, with a low dose enhancing LTP and a high dose impairing it. Amphetamine-induced LTP enhancement was prevented by pharmacological blockade of D(1) - (but not D(2)-) class dopamine receptors, by blockade of β-adrenergic receptors, or by inhibition of cAMP-PKA signaling. In contrast, amphetamine-induced LTP impairment was prevented by inhibition of post-synaptic protein phosphatase-1, a downstream target of PKA signaling, or by blockade of either D(1) - or D(2)-class dopamine, but not noradrenergic, receptors. Thus, amphetamine biphasically modulates LTP via cAMP-PKA signaling orchestrated mainly through dopamine receptors. Unexpectedly, amphetamine restored the loss of LTP in dopamine transporter-knockout mice primarily by activation of the noradrenergic system. Our results mirror the biphasic effectiveness of amphetamine in humans and provide new mechanistic insights into its effects on cognition under normal and hyperdopaminergic conditions.

Dopamine modulates synaptic plasticity in dendrites of rat and human dentate granule cells

The mechanisms underlying memory formation in the hippocampal network remain a major unanswered aspect of neuroscience. Although high-frequency activity appears essential for plasticity, salience for memory formation is also provided by activity in ventral tegmental area (VTA) dopamine projections. Here, we report that activation of dopamine D1 receptors in dentate granule cells (DGCs) can preferentially increase dendritic excitability to both high-frequency afferent activity and high-frequency trains of backpropagating action potentials. Using whole-cell patch clamp recordings, calcium imaging, and neuropeptide Y to inhibit postsynaptic calcium influx, we found that activation of dendritic voltage-dependent calcium channels (VDCCs) is essential for dopamine-induced long-term potentiation (LTP), both in rat and human dentate gyrus (DG). Moreover, we demonstrate previously unreported spike-timing-dependent plasticity in the human hippocampus. These results suggest that when dopamine is released in the dentate gyrus with concurrent high-frequency activity there is an increased probability that synapses will be strengthened and reward-associated spatial memories will be formed.

Confocal Analysis of Cholinergic and Dopaminergic Inputs onto Pyramidal Cells in the Prefrontal Cortex of Rodents

Cholinergic and dopaminergic projections to the rat medial prefrontal cortex (mPFC) are both involved in cognitive functions including attention. These neuronal systems modulate mPFC neuronal activity mainly through diffuse transmission. In order to better understand the anatomical level of influence of these systems, confocal microscopy with triple-fluorescent immunolabeling was used in three subregions of the mPFC of rats and Drd1a-tdTomato/Drd2-EGFP transgenic mice. The zone of interaction was defined as a reciprocal microproximity between dopaminergic and cholinergic axonal segments as well as pyramidal neurons. The density of varicosities, along these segments was considered as a possible activity-dependant morphological feature. The percentage of cholinergic and dopaminergic fibers in microproximity ranged from 12 to 40% depending on the layer and mPFC subregion. The cholinergic system appeared to have more influence on dopaminergic fibers since a larger proportion of the dopaminergic fibers were within microproximity to cholinergic fibers. The density of both cholinergic and dopaminergic varicosities was significantly elevated within microproximities. The main results indicate that the cholinergic and dopaminergic systems converge on pyramidal cells in mPFC particularly in the layer V. In transgenic mice 93% of the pyramidal cells expressed the transgenic marker for Drd2 expression, but only 22% expressed the maker for Drd1ar expression. Data presented here suggest that the modulation of mPFC by dopaminergic fibers would be mostly inhibitory and localized at the output level whereas the cholinergic modulation would be exerted at the input and output level both through direct interaction with pyramidal cells and dopaminergic fibers.

Roles of fragile X mental retardation protein in dopaminergic stimulation-induced synapse-associated protein synthesis and subsequent alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) receptor internalization

Fragile X syndrome, the most common form of inherited mental retardation, is caused by the absence of the RNA-binding protein fragile X mental retardation protein (FMRP). FMRP regulates local protein synthesis in dendritic spines. Dopamine (DA) is involved in the modulation of synaptic plasticity. Activation of DA receptors can regulate higher brain functions in a protein synthesis-dependent manner. Our recent study has shown that FMRP acts as a key messenger for DA modulation in forebrain neurons. Here, we demonstrate that FMRP is critical for DA D1 receptor-mediated synthesis of synapse-associated protein 90/PSD-95-associated protein 3 (SAPAP3) in the prefrontal cortex (PFC). DA D1 receptor stimulation induced dynamic changes of FMRP phosphorylation. The changes in FMRP phosphorylation temporally correspond with the expression of SAPAP3 after D1 receptor stimulation. Protein phosphatase 2A, ribosomal protein S6 kinase, and mammalian target of rapamycin are the key signaling molecules for FMRP linking DA D1 receptors to SAPAP3. Knockdown of SAPAP3 did not affect surface expression of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) GluR1 receptors induced by D1 receptor activation but impaired their subsequent internalization in cultured PFC neurons; the subsequent internalization of GluR1 was also impaired in Fmr1 knock-out PFC neurons, suggesting that FMRP may be involved in subsequent internalization of GluR1 through regulating the abundance of SAPAP3 after DA D1 receptor stimulation. Our study thus provides further insights into FMRP involvement in DA modulation and may help to reveal the molecular mechanisms underlying impaired learning and memory in fragile X syndrome.

Localization of dopamine- and cAMP-regulated phosphoprotein-32 and inhibitor-1 in area 9 of Macaca mulatta prefrontal cortex

The actions of dopamine D1 family receptors (D1R) depend upon a signal transduction cascade that modulates the phosphorylation state of important effector proteins, such as glutamate receptors and ion channels. This is accomplished both through activation of protein kinase A (PKA) and the inhibition of protein phosphatase-1 (PP1). Inhibition of PP1 occurs through PKA-mediated phosphorylation of dopamine- and cAMP-regulated phosphoprotein 32 kDa (DARPP-32) or the related protein inhibitor-1 (I-1), and the availability of DARPP-32 is essential to the functional outcome of D1R activation in the basal ganglia. While D1R activation is critical for prefrontal cortex (PFC) function, especially working memory, the functional role played by DARPP-32 or I-1 is less clear. In order to examine this more thoroughly, we have utilized immunoelectron microscopy to quantitatively determine the localization of DARPP-32 and I-1 in the neuropil of the rhesus monkey PFC. Both were distributed widely in the different components of the neuropil, but were enriched in dendritic shafts. I-1 label was more frequently identified in axon terminals than was DARPP-32, and DARPP-32 label was more frequently identified in glia than was I-1. We also quantified the extent to which these proteins were found in dendritic spines. DARPP-32 and I-1 were present in small subpopulations of dendritic spines, (4.4% and 7.7% and respectively), which were substantially smaller than observed for D1R in our previous studies (20%). Double-label experiments did not find evidence for colocalization of D1R and DARPP-32 or I-1 in spines or terminals. Thus, at the least, not all prefrontal spines which contain D1R also contain I-1 or DARPP-32, suggesting important differences in D1R signaling in the PFC compared to the striatum.

Characterization of intrinsic properties of cingulate pyramidal neurons in adult mice after nerve injury

The anterior cingulate cortex (ACC) is important for cognitive and sensory functions including memory and chronic pain. Glutamatergic excitatory synaptic transmission undergo long-term potentiation in ACC pyramidal cells after peripheral injury. Less information is available for the possible long-term changes in neuronal action potentials or intrinsic properties. In the present study, we characterized cingulate pyramidal cells in the layer II/III of the ACC in adult mice. We then examined possible long-term changes in intrinsic properties of the ACC pyramidal cells after peripheral nerve injury. In the control mice, we found that there are three major types of pyramidal cells according to their action potential firing pattern: (i) regular spiking (RS) cells (24.7%), intrinsic bursting (IB) cells (30.9%), and intermediate (IM) cells (44.4%). In a state of neuropathic pain, the population distribution (RS: 21.3%; IB: 31.2%; IM: 47.5%) and the single action potential properties of these three groups were indistinguishable from those in control mice. However, for repetitive action potentials, IM cells from neuropathic pain animals showed higher initial firing frequency with no change for the properties of RS and IB neurons from neuropathic pain mice. The present results provide the first evidence that, in addition to synaptic potentiation reported previously, peripheral nerve injury produces long-term plastic changes in the action potentials of cingulate pyramidal neurons in a cell type-specific manner.

Protracted withdrawal from alcohol and drugs of abuse impairs long-term potentiation of intrinsic excitability in the juxtacapsular bed nucleus of the stria terminalis

The juxtacapsular bed nucleus of the stria terminalis (jcBNST) is activated in response to basolateral amygdala (BLA) inputs through the stria terminalis and projects back to the anterior BLA and to the central nucleus of the amygdala. Here we show a form of long-term potentiation of the intrinsic excitability (LTP-IE) of jcBNST neurons in response to high-frequency stimulation of the stria terminalis. This LTP-IE, which was characterized by a decrease in the firing threshold and increased temporal fidelity of firing, was impaired during protracted withdrawal from self-administration of alcohol, cocaine, and heroin. Such impairment was graded and was more pronounced in rats that self-administered amounts of the drugs sufficient to maintain dependence. Dysregulation of the corticotropin-releasing factor (CRF) system has been implicated in manifestation of protracted withdrawal from dependent drug use. Administration of the selective corticotropin-releasing factor receptor 1 (CRF(1)) antagonist R121919 [2,5-dimethyl-3-(6-dimethyl-4-methylpyridin-3-yl)-7-dipropylamino-pyrazolo[1,5-a]pyrimidine)], but not of the CRF(2) antagonist astressin(2)-B, normalized jcBNST LTP-IE in animals with a history of alcohol dependence; repeated, but not acute, administration of CRF itself produced a decreased jcBNST LTP-IE. Thus, changes in the intrinsic properties of jcBNST neurons mediated by chronic activation of the CRF system may contribute to the persistent emotional dysregulation associated with protracted withdrawal.

Multiple forms of activity-dependent intrinsic plasticity in layer V cortical neurones in vivo

Synaptic plasticity is classically considered as the neuronal substrate for learning and memory. However, activity-dependent changes in neuronal intrinsic excitability have been reported in several learning-related brain regions, suggesting that intrinsic plasticity could also participate to information storage. Compared to synaptic plasticity, there has been little exploration of the properties of induction and expression of intrinsic plasticity in an intact brain. Here, by the means of in vivo intracellular recordings in the rat we have examined how the intrinsic excitability of layer V motor cortex pyramidal neurones is altered following brief periods of repeated firing. Changes in membrane excitability were assessed by modifications in the discharge frequency versus injected current (F-I) curves. Most (approximately 64%) conditioned neurones exhibited a long-lasting intrinsic plasticity, which was expressed either by selective changes in the current threshold or in the slope of the F-I curve, or by concomitant changes in both parameters. These modifications in the neuronal input-output relationship led to a global increase or decrease in intrinsic excitability. Passive electrical membrane properties were unaffected by the intracellular conditioning, indicating that intrinsic plasticity resulted from modifications of voltage-gated ion channels. These results demonstrate that neocortical pyramidal neurones can express in vivo a bidirectional use-dependent intrinsic plasticity, modifying their sensitivity to weak inputs and/or the gain of their input-output function. These multiple forms of experience-dependent intrinsic changes, which expand the computational abilities of individual neurones, could shape new network dynamics and thus might participate in the formation of mnemonic motor engrams.

Genetic dissection of the role of catechol-O-methyltransferase in cognition and stress reactivity in mice

The COMT (catechol-O-methyltransferase) gene has been linked to a spectrum of human phenotypes, including cognition, anxiety, pain sensitivity and psychosis. Doubts about its clinical impact exist, however, because of the complexity of human COMT polymorphism and clinical variability. We generated transgenic mice overexpressing a human COMT-Val polymorphism (Val-tg), and compared them with mice containing a null COMT mutation. Increased COMT enzyme activity in Val-tg mice resulted in disrupted attentional set-shifting abilities, and impaired working and recognition memory, but blunted stress responses and pain sensitivity. Conversely, COMT disruption improved working memory, but increased stress responses and pain sensitivity. Amphetamine ameliorated recognition memory deficits in COMT-Val-tg mice but disrupted it in wild types, illustrating COMT modulation of the inverted-U relationship between cognition and dopamine. COMT-Val-tg mice showed increased prefrontal cortex (PFC) calcium/calmodulin-dependent protein kinase II (CaMKII) levels, whereas COMT deficiency decreased PFC CaMKII but increased PFC CaMKKbeta and CaMKIV levels, suggesting the involvement of PFC CaMK pathways in COMT-regulated cognitive function and adaptive stress responses. Our data indicate a critical role for the COMT gene in an apparent evolutionary trade-off between cognitive and affective functions.

Dopamine Increases the Gain of the Input-Output Response of Rat Prefrontal Pyramidal Neurons

Dopaminergic modulation of prefrontal cortical activity is known to affect cognitive functions like working memory. Little consensus on the role of dopamine modulation has been achieved, however, in part because quantities directly relating to the neuronal substrate of working memory are difficult to measure. Here we show that dopamine increases the gain of the frequency-current relationship of layer 5 pyramidal neurons in vitro in response to noisy input currents. The gain increase could be attributed to a reduction of the slow afterhyperpolarization by dopamine. Dopamine also increases neuronal excitability by shifting the input-output functions to lower inputs. The modulation of these response properties is mainly mediated by D1 receptors. Integrate-and-fire neurons were fitted to the experimentally recorded input-output functions and recurrently connected in a model network. The gain increase induced by dopamine application facilitated and stabilized persistent activity in this network. The results support the hypothesis that catecholamines increase the neuronal gain and suggest that dopamine improves working memory via gain modulation.