Glutamate-triggered events inducing corticostriatal long-term depression

Acute and chronic effects by nicotine on striatal neurotransmission and synaptic plasticity in the female rat brain

Introduction

Tobacco use is in part a gendered activity, yet neurobiological studies outlining the effect by nicotine on the female brain are scarce. The aim of this study was to outline acute and sub-chronic effects by nicotine on the female rat brain, with special emphasis on neurotransmission and synaptic plasticity in the dorsolateral striatum (DLS), a key brain region with respect to the formation of habits.

Methods

In vivo microdialysis and ex vivo electrophysiology were performed in nicotine naïve female Wistar rats, and following sub-chronic nicotine exposure (0.36 mg/kg free base, 15 injections). Locomotor behavior was assessed at the first and last drug-exposure.

Results

Acute exposure to nicotine ex vivo depresses excitatory neurotransmission by reducing the probability of transmitter release. Bath applied nicotine furthermore facilitated long-term synaptic depression induced by high frequency stimulation (HFS-LTD). The cannabinoid 1 receptor (CB1R) agonist WIN55,212-2 produced a robust synaptic depression of evoked potentials, and HFS-LTD was blocked by the CB1R antagonist AM251, suggesting that HFS-LTD in the female rat DLS is endocannabinoid mediated. Sub-chronic exposure to nicotine in vivo produced behavioral sensitization and electrophysiological recordings performed after 2-8 days abstinence revealed a sustained depression of evoked population spike amplitudes in the DLS, with no concomitant change in paired pulse ratio. Rats receiving sub-chronic nicotine exposure further demonstrated an increased neurophysiological responsiveness to nicotine with respect to both dopaminergic- and glutamatergic signaling. However, a tolerance towards the plasticity facilitating property of bath applied nicotine was developed during sub-chronic nicotine exposure in vivo. In addition, the dopamine D2 receptor agonist quinpirole selectively facilitate HFS-LTD in slices from nicotine naïve rats, suggesting that the tolerance may be associated with changes in dopaminergic signaling.

Conclusion

Nicotine produces acute and sustained effects on striatal neurotransmission and synaptic plasticity in the female rat brain, which may contribute to the establishment of persistent nicotine taking habits.

Pomegranate peel extracts effects to reduce mono sodium glutamate toxic effects on chicken embryos: Morphological studies

Background

Monosodium glutamate (MSG) is a flavoring agent added to various foods. This experimental study investigated MSG effects on chicken embryos morphology and the possible ameliorative effects of pomegranate peel extracts (PPE) at different incubation periods.

Methods

Seven hundred and twenty fertilized chicken eggs were used and divided into six groups: control, PPE, MSG, PPE + MSG, preventive (PPE–MSG) and therapeutic (MSG–PPE) groups. Fertile chicken eggs were injected with MSG (0.1 ml) and/or PPE (0.3 ml) twice before incubation at days 0, 1. Embryos were extracted at days 7, 10, 12, 14 and 16. Effects of MSG and/ or PPE on embryo development during different incubation periods were studied.

Results

MSG injected into embryos led to congenital anomalies that appeared mainly in MSG and MSG + PPE groups. These anomalies included growth retardation, absent eye, abdominal swelling and hernia. Mortality rate was the highest in MSG, then in MSG + PPE and MSG–PPE groups. PPE treatment reduced MSG toxic effects and these results were better in MSG–PPE and PPE–MSG groups than MSG + PPE group.

Conclusions

MSG injection affected chicken embryonic development causing growth retardation and decline in total body length, break length, and total body weight in all the treated groups. These harmful actions can be ameliorated with PPE treatment depending on embryo age.

Presynaptic AMPA Receptors in Health and Disease

AMPA receptors (AMPARs) are ionotropic glutamate receptors that play a major role in excitatory neurotransmission. AMPARs are located at both presynaptic and postsynaptic plasma membranes. A huge number of studies investigated the role of postsynaptic AMPARs in the normal and abnormal functioning of the mammalian central nervous system (CNS). These studies highlighted that changes in the functional properties or abundance of postsynaptic AMPARs are major mechanisms underlying synaptic plasticity phenomena, providing molecular explanations for the processes of learning and memory. Conversely, the role of AMPARs at presynaptic terminals is as yet poorly clarified. Accruing evidence demonstrates that presynaptic AMPARs can modulate the release of various neurotransmitters. Recent studies also suggest that presynaptic AMPARs may possess double ionotropic-metabotropic features and that they are involved in the local regulation of actin dynamics in both dendritic and axonal compartments. In addition, evidence suggests a key role of presynaptic AMPARs in axonal pathology, in regulation of pain transmission and in the physiology of the auditory system. Thus, it appears that presynaptic AMPARs play an important modulatory role in nerve terminal activity, making them attractive as novel pharmacological targets for a variety of pathological conditions.

Cholinergic modulation of striatal microcircuits

The purpose of this review is to bridge the gap between earlier literature on striatal cholinergic interneurons and mechanisms of microcircuit interaction demonstrated with the use of newly available tools. It is well known that the main source of the high level of acetylcholine in the striatum, compared to other brain regions, is the cholinergic interneurons. These interneurons provide an extensive local innervation that suggests they may be a key modulator of striatal microcircuits. Supporting this idea requires the consideration of functional properties of these interneurons, their influence on medium spiny neurons, other interneurons, and interactions with other synaptic regulators. Here, we underline the effects of intrastriatal and extrastriatal afferents onto cholinergic interneurons and discuss the activation of pre‐ and postsynaptic muscarinic and nicotinic receptors that participate in the modulation of intrastriatal neuronal interactions. We further address recent findings about corelease of other transmitters in cholinergic interneurons and actions of these interneurons in striosome and matrix compartments. In addition, we summarize recent evidence on acetylcholine‐mediated striatal synaptic plasticity and propose roles for cholinergic interneurons in normal striatal physiology. A short examination of their role in neurological disorders such as Parkinson's, Huntington's, and Tourette's pathologies and dystonia is also included.

Cocaine Place Conditioning Strengthens Location-Specific Hippocampal Coupling to the Nucleus Accumbens

Conditioned place preference (CPP) is a widely used model of addiction-related behavior whose underlying mechanisms are not understood. In this study, we used dual site silicon probe recordings in freely moving mice to examine interactions between the hippocampus and nucleus accumbens in cocaine CPP. We found that CPP was associated with recruitment of D2-positive nucleus accumbens medium spiny neurons to fire in the cocaine-paired location, and this recruitment was driven predominantly by selective strengthening of coupling with hippocampal place cells that encode the cocaine-paired location. These findings provide in vivo evidence suggesting that the synaptic potentiation in the accumbens caused by repeated cocaine administration preferentially affects inputs that were active at the time of drug exposure. This provides a potential physiological mechanism by which drug use becomes associated with specific environmental contexts.

Quantitative Analyses of the Projection of Individual Neurons from the Midline Thalamic Nuclei to the Striosome and Matrix Compartments of the Rat Striatum

A fundamental organizing principle of the striatum is the striosome/matrix system that is defined by inputs/outputs and neurochemical markers. The thalamostriatal projection is highly heterogeneous originating in many subnuclei of the thalamus including the midline (ML) and intralaminar (IL) nuclei. We examined the dendritic morphology and axonal trajectory of 15 ML and 11 IL neurons by single-neuron labeling with viral vectors in combination with mu-opioid receptor immunostaining in rat brains. Dendritic and axonal morphology defined ML neurons as type II cells consisting of at least two subclasses according to the presence or absence of striatal axon collaterals. In the striatum, ML neurons preferentially innervated striosomes, whereas parafascicular neurons preferentially innervated the matrix. Almost all single thalamostriatal neurons favoring striosome or matrix compartments also innervated the cerebral cortical areas that supplied cortical input to the same striatal compartment. We thus revealed that thalamostriatal projections are highly organized 1) by the similarity in morphological characteristics and 2) their preference for the striatal compartments and cortical areas. These findings demonstrate that the functional properties of striatal compartments are influenced by both their cortical and thalamic afferents presumably with a different time latency and support selective dynamics for the striosome and matrix compartments.

Genetics of tardive dyskinesia: Promising leads and ways forward

Tardive dyskinesia (TD) is a potentially irreversible and often debilitating movement disorder secondary to chronic use of dopamine receptor blocking medications. Genetic factors have been implicated in the etiology of TD. We therefore have reviewed the most promising genes associated with TD, including DRD2, DRD3, VMAT2, HSPG2, HTR2A, HTR2C, and SOD2. In addition, we present evidence supporting a role for these genes from preclinical models of TD. The current understanding of the etiogenesis of TD is discussed in the light of the recent approvals of valbenazine and deutetrabenazine, VMAT2 inhibitors, for treating TD.

L-DOPA Oppositely Regulates Synaptic Strength and Spine Morphology in D1 and D2 Striatal Projection Neurons in Dyskinesia

Dopamine depletion in Parkinson's disease (PD) produces dendritic spine loss in striatal medium spiny neurons (MSNs) and increases their excitability. However, the synaptic changes that occur in MSNs in PD, in particular those induced by chronic L-3,4-dihydroxyphenylalanine (L-DOPA) treatment, are still poorly understood. We exposed BAC-transgenic D1-tomato and D2-eGFP mice to PD and dyskinesia model paradigms, enabling cell type-specific assessment of changes in synaptic physiology and morphology. The distinct fluorescence markers allowed us to identify D1 and D2 MSNs for analysis using intracellular sharp electrode recordings, electron microscopy, and 3D reconstructions with single-cell Lucifer Yellow injections. Dopamine depletion induced spine pruning in both types of MSNs, affecting mushroom and thin spines equally. Dopamine depletion also increased firing rate in both D1- and D2-MSNs, but reduced evoked-EPSP amplitude selectively in D2-MSNs. L-DOPA treatment that produced dyskinesia differentially affected synaptic properties in D1- and D2-MSNs. In D1-MSNs, spine density remained reduced but the remaining spines were enlarged, with bigger heads and larger postsynaptic densities. These morphological changes were accompanied by facilitation of action potential firing triggered by synaptic inputs. In contrast, although L-DOPA restored the number of spines in D2-MSNs, it resulted in shortened postsynaptic densities. These changes in D2-MSNs correlated with a decrease in synaptic transmission. Our findings indicate that L-DOPA-induced dyskinesia is associated with abnormal spine morphology, modified synaptic transmission, and altered EPSP-spike coupling, with distinct effects in D1- and D2-MSNs.

Review: Modulation of striatal neuron activity by cyclic nucleotide signaling and phosphodiesterase inhibition

The cyclic nucleotides cAMP and cGMP are common signaling molecules synthesized in neurons following the activation of adenylyl or guanylyl cyclase. In the striatum, the synthesis and degradation of cAMP and cGMP is highly regulated as these second messengers have potent effects on the activity of striatonigral and striatopallidal neurons. This review will summarize the literature on cyclic nucleotide signaling in the striatum with a particular focus on the impact of cAMP and cGMP on the membrane excitability of striatal medium-sized spiny output neurons (MSNs). The effects of non-selective and selective phosphodiesterase (PDE) inhibitors on membrane activity and synaptic plasticity of MSNs will also be reviewed. Lastly, this review will discuss the implications of the effects PDE modulation on electrophysiological activity of striatal MSNs as it relates to the treatment of neurological disorders such as Huntington's and Parkinson's disease.

Changing the tune: plasticity and adaptation of retrograde signals

Retrograde signaling is a fundamental means by which neurons communicate. The acceptance of this statement has required a revision of how we view transmission and storage of information at the synapse. Although there is a substantial body of literature on the diverse molecules that serve as retrograde signals, less is known about how retrograde signal capacity can be modified. Is retrograde signaling plastic? How does this plasticity manifest? Are there behavioral correlates that may bias a neuron towards 'changing its tune', retrogradely speaking, of course? Here, we review recent findings that retrograde signaling is a highly labile process that adds additional layers of complexity that must be untangled to understand information processing in the nervous system.

Amygdala-prefrontal pathways and the dopamine system affect nociceptive responses in the prefrontal cortex

Background

We previously demonstrated nociceptive discharges to be evoked by mechanical noxious stimulation in the prefrontal cortex (PFC). The nociceptive responses recorded in the PFC are conceivably involved in the affective rather than the sensory-discriminative dimension of pain. The PFC receives dense projection from the limbic system. Monosynaptic projections from the basolateral nucleus of the amygdala (BLA) to the PFC are known to produce long-lasting synaptic plasticity. We examined effects of high frequency stimulation (HFS) delivered to the BLA on nociceptive responses in the rat PFC.

Results

HFS induced long lasting suppression (LLS) of the specific high threshold responses of nociceptive neurons in the PFC. Microinjection of N-methyl-D-aspartic acid (NMDA) receptor antagonists (2-amino-5-phosphonovaleric acid (APV), dizocilpine (MK-801)) and also metabotropic glutamate receptor (mGluR) group antagonists (α-methyl-4-carboxyphenylglycine (MCPG), and 2-[(1S,2S)-2-carboxycyclopropyl]-3-(9H-xanthen-9-yl)-D-alanine (LY341495)), prevented the induction of LLS of nociceptive responses. We also examined modulatory effects of dopamine (DA) on the LLS of nociceptive responses. With depletion of DA in response to 6-hydroxydopamine (6-OHDA) injection into the ipsilateral forebrain bundle, LLS of nociceptive responses was decreased, while nociceptive responses were normally evoked. Antagonists of DA receptor subtypes D2 (sulpiride) and D4 (3-{[4-(4-chlorophenyl) piperazin-1-yl] methyl}-1H-pyrrolo [2, 3-b] pyridine (L-745,870)), microinjected into the PFC, inhibited LLS of nociceptive responses.

Conclusions

Our results indicate that BLA-PFC pathways inhibited PFC nociceptive cell activities and that the DA system modifies the BLA-PFC regulatory function.

Down-regulated striatal gene expression for synaptic plasticity-associated proteins in addiction and relapse vulnerable animals

Reducing the likelihood of relapse represents one of the greatest obstacles in the successful treatment of cocaine addiction. Dysregulation of the synaptic plasticity processes long-term potentiation (LTP) and long-term depression (LTD) is thought to be associated with protracted relapse risk. To improve our understanding of the molecular mechanisms contributing to relapse vulnerability we trained rats (n=52) to self-administer cocaine and phenotyped animals as relapse-vulnerable or relapse-resilient using procedures adapted from Deroche-Gamonet et al. (Science 2004, 305, 1014-1017). Gene expression analysis, targeted at synaptic plasticity-related genes, revealed significant transcript down-regulation in the ventral and dorsal striatum of relapse-vulnerable animals compared to relapse-resilient controls. This included reduced expression of genes encoding proteins implicated in the dendritic translation of synaptic plasticity-related transcripts, the dynamic regulation and trafficking of ionotropic glutamate receptors important for LTP and LTD, along with neuronal surface receptors that initiate downstream signalling pathways associated with synaptic plasticity. Together, our data are consistent with recent reports of an inability to evoke LTD in the striatum of addiction-vulnerable rats. To our knowledge, this is the first study to demonstrate down-regulated synaptic plasticity-associated gene expression not only in the ventral striatum, where the majority of addiction-related synaptic plasticity studies have been conducted, but also in the dorsal striatum of animals categorized as relapse-vulnerable. As these neural correlates were elucidated using an approach incorporating individual behavioural differences, they potentially provide more relevant insight into addiction and assist the development of novel pharmacotherapies to treat relapse.

Nitric oxide donors enhance the frequency dependence of dopamine release in nucleus accumbens

Dopamine (DA) neurotransmission in the nucleus accumbens (NAc) is critically involved in normal as well as maladaptive motivated behaviors including drug addiction. Whether the striatal neuromodulator nitric oxide (NO) influences DA release in NAc is unknown. We investigated whether exogenous NO modulates DA transmission in NAc core and how this interaction varies depending on the frequency of presynaptic activation. We detected DA with cyclic voltammetry at carbon-fiber microelectrodes in mouse NAc in slices following stimuli spanning a full range of DA neuron firing frequencies (1-100 Hz). NO donors 3-morpholinosydnonimine hydrochloride (SIN-1) or z-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1-ium-1,2-diolate (PAPA/NONOate) enhanced DA release with increasing stimulus frequency. This NO-mediated enhancement of frequency sensitivity of DA release was not prevented by inhibition of soluble guanylyl cyclase (sGC), DA transporters, or large conductance Ca(2+)-activated K(+) channels, and did not require glutamatergic or GABAergic input. However, experiments to identify whether frequency-dependent NO effects were mediated via changes in powerful acetylcholine-DA interactions revealed multiple components to NO modulation of DA release. In the presence of a nicotinic receptor antagonist (dihydro-β-erythroidine), NO donors increased DA release in a frequency-independent manner. These data suggest that NO in the NAc can modulate DA release through multiple GC-independent neuronal mechanisms whose net outcome varies depending on the activity in DA neurons and accumbal cholinergic interneurons. In the presence of accumbal acetylcholine, NO promotes the sensitivity of DA release to presynaptic activation, but with reduced acetylcholine input, NO will promote DA release in an activity-independent manner through a direct action on dopaminergic terminals.

Nitric oxide signaling via cGMP-stimulated phosphodiesterase in striatal neurons

Nitric oxide (NO) acts in the nervous system to activate guanylyl cyclase and increase cGMP. One target for cGMP appears to be the cGMP-stimulated phosphodiesterase (PDE2A), which is widely expressed in the brain and provides a molecular mechanism for NO to regulate cAMP levels. We have found that PDE2A is highly expressed in the medium spiny neurons of the striatum, which project to the pallidum and substantia nigra. These cells express dopamine-stimulated adenylyl cyclase, and we have found that increases in cAMP in these neurons, produced by activation of the D1-type dopamine receptor, are dramatically enhanced by the general phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and the PDE2A-selective inhibitor erythro-p-(2-hydroxyl-3-nonyl)adenine (EHNA). These results indicate that PDE2A plays a major role in regulating dopamine-stimulated cAMP production in striatal neurons. EHNA also enhances NO-induced increases in striatal cGMP. In addition, dopamine appears to act via another receptor, activated by the agonist SKF83959, to increase striatal cGMP in a NO-dependent manner. Together, these observations indicate that striatal NO producing interneurons can act via the PDE2A in the medium spiny neurons to regulate the cAMP response to dopamine stimulation.

Nitric oxide-soluble guanylyl cyclase signaling regulates corticostriatal transmission and short-term synaptic plasticity of striatal projection neurons recorded in vivo

Striatal medium-sized spiny neurons (MSNs) contain the highest levels of soluble guanylyl cyclase (sGC) in the brain. Striatal sGC signaling is activated by nitric oxide (NO) and other neuromodulators. MSNs also express cGMP-dependent protein kinase and other components of the cGMP signaling system which are critically involved in integrating corticostriatal transmission and regulating synaptic plasticity in striatal networks. However, the influence of tonic and phasic activation of this signaling pathway on striatal MSN activity is poorly understood. The present study examined the impact of systemic administration of the selective sGC inhibitor [1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one] (ODQ) on spike activity evoked using low and high frequency electrical stimulation of the frontal cortex. MSN activity was monitored using single-unit extracellular recordings in urethane-anesthetized rats. ODQ administration significantly decreased spike activity evoked by low frequency cortical stimulation in a stimulus intensity- and time-dependent manner. Additionally, ODQ administered along with the neuronal NO synthase inhibitor 7-nitroindazole (7-NI) potently decreased the incidence of excitatory responses observed during high-frequency train stimulation of the contralateral frontal cortex. The short-term depression of cortically-evoked spike activity induced by train stimulation was enhanced following pretreatment with ODQ in MSNs exhibiting an excitatory response during cortical train stimulation. Unexpectedly, this effect of ODQ was reversed in animals receiving co-administration of ODQ and 7-NI. 7-NI/ODQ co-administration also reversed measures of short-term depression observed in MSNs exhibiting an inhibitory response during cortical train stimulation. These observations extend previous studies showing that tonic and phasic NO-sGC signaling modulates the responsiveness of MSNs to corticostriatal input. Moreover, phasic activation of NO signaling is likely to regulate short-term changes in corticostriatal synaptic plasticity via complex mechanisms involving both sGC-cGMP-dependent and independent pathways.

A kinetic model of dopamine- and calcium-dependent striatal synaptic plasticity

Corticostriatal synapse plasticity of medium spiny neurons is regulated by glutamate input from the cortex and dopamine input from the substantia nigra. While cortical stimulation alone results in long-term depression (LTD), the combination with dopamine switches LTD to long-term potentiation (LTP), which is known as dopamine-dependent plasticity. LTP is also induced by cortical stimulation in magnesium-free solution, which leads to massive calcium influx through NMDA-type receptors and is regarded as calcium-dependent plasticity. Signaling cascades in the corticostriatal spines are currently under investigation. However, because of the existence of multiple excitatory and inhibitory pathways with loops, the mechanisms regulating the two types of plasticity remain poorly understood. A signaling pathway model of spines that express D1-type dopamine receptors was constructed to analyze the dynamic mechanisms of dopamine- and calcium-dependent plasticity. The model incorporated all major signaling molecules, including dopamine- and cyclic AMP-regulated phosphoprotein with a molecular weight of 32 kDa (DARPP32), as well as AMPA receptor trafficking in the post-synaptic membrane. Simulations with dopamine and calcium inputs reproduced dopamine- and calcium-dependent plasticity. Further in silico experiments revealed that the positive feedback loop consisted of protein kinase A (PKA), protein phosphatase 2A (PP2A), and the phosphorylation site at threonine 75 of DARPP-32 (Thr75) served as the major switch for inducing LTD and LTP. Calcium input modulated this loop through the PP2B (phosphatase 2B)-CK1 (casein kinase 1)-Cdk5 (cyclin-dependent kinase 5)-Thr75 pathway and PP2A, whereas calcium and dopamine input activated the loop via PKA activation by cyclic AMP (cAMP). The positive feedback loop displayed robust bi-stable responses following changes in the reaction parameters. Increased basal dopamine levels disrupted this dopamine-dependent plasticity. The present model elucidated the mechanisms involved in bidirectional regulation of corticostriatal synapses and will allow for further exploration into causes and therapies for dysfunctions such as drug addiction.

Recent brain imaging and neurophysiological studies suggest that the striatum, the start of the basal ganglia circuit, plays a major role in value-based decision making and behavioral disorders such as drug addiction. The plasticity of synaptic input from the cerebral cortex to output neurons of the striatum, which are medium spiny neurons, depends on interactions between glutamate input from the cortex and dopaminergic input from the midbrain. It also links sensory and cognitive states in the cortex with reward-oriented action outputs. The mechanisms involved in molecular cascades that transmit glutamate and dopamine inputs to changes in postsynaptic glutamate receptors are very complex and it is difficult to intuitively understand the mechanism. Therefore, a biochemical network model was constructed, and computer simulations were performed. The model reproduced dopamine-dependent and calcium-dependent forms of long-term depression (LTD) and potentiation (LTP) of corticostriatal synapses. Further in silico experiments revealed that a positive feedback loop formed by proteins, the protein specifically expressed in the striatum, served as the major switch for inducing LTD and LTP. This model could allow us to understand dynamic constraints in reward-dependent learning, as well as causes and therapies of dopamine-related disorders such as drug addiction.

Priming of rotational behavior by a dopamine receptor agonist in Hemiparkinsonian rats: movement-dependent induction

Repetitive stimulation of dopamine receptors located in the basal ganglia may lead to the manifestation of sensitized, abnormal, motor responses in dopamine-denervated rats. In order to study the role of motor behavior execution on the expression of these altered motor responses, we evaluated how "priming", a phenomenon displaying neurochemical and behavioral features peculiar to a sensitized abnormal motor response in dopamine-denervated rats, depends on actual movement performance. To this end, unilaterally 6-hydroxydopamine-lesioned rats received apomorphine (0.2 mg/kg s.c.), being either allowed to move or immobilized (1 h) before, concomitantly to, or after its administration, respectively. Three days after apomorphine, the dopamine D(1) receptor agonist 1-Phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF 38393, 3 mg/kg s.c.) was administered to all animals. Rats that had performed rotational behavior following apomorphine administration displayed robust contraversive rotational behavior in response to SKF 38393, whereas rats that had been immobilized concomitantly to, but neither before nor after apomorphine, did not. To clarify whether stress, which may be increased by immobilization, mediated the results observed, additional rats received apomorphine paired with immobilization plus the corticosterone-synthesis inhibitor metyrapone (100 mg/kg i.p.), or apomorphine paired with a tail stressor, being not immobilized. Metyrapone did not affect the capacity of immobilization to prevent priming and tail stressor imposition did not affect priming magnitude, suggesting that stress has minimal or no effect on the results observed. This study demonstrates how movement performance following initial dopaminergic stimulation governs the occurrence of a sensitized, abnormal, motor response to a subsequent dopaminergic challenge in dopamine-denervated rats.

Feed-forward excitation of striatal neuron activity by frontal cortical activation of nitric oxide signaling in vivo

The gaseous neurotransmitter nitric oxide plays an important role in the modulation of corticostriatal synaptic transmission. This study examined the impact of frontal cortex stimulation on striatal nitric oxide efflux and neuron activity in urethane-anesthetized rats using amperometric microsensor and single-unit extracellular recordings, respectively. Systemic administration of the neuronal nitric oxide synthase inhibitor 7-nitroindazole decreased spontaneous spike activity without affecting activity evoked by single-pulse stimulation of the ipsilateral cortex. Train (30 Hz) stimulation of the contralateral frontal cortex transiently increased nitric oxide efflux in a robust and reproducible manner. Evoked nitric oxide efflux was attenuated by systemic administration of 7-nitroindazole and the non-selective nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester. Train stimulation of the contralateral cortex, in a manner identical to that used to evoke nitric oxide efflux, had variable effects on spike activity assessed during the train stimulation trial, but induced a short-term depression of cortically evoked activity in the first post-train stimulation trial. Interestingly, 7-nitroindazole potently decreased cortically evoked activity recorded during the train stimulation trial. Moreover, the short-term depression of spike activity induced by train stimulation was enhanced following pretreatment with 7-nitroindazole and attenuated after systemic administration of the dopamine D2 receptor antagonist eticlopride. These results demonstrate that robust activation of frontal cortical afferents in the intact animal activates a powerful nitric oxide-mediated feed-forward excitation which partially offsets concurrent D2 receptor-mediated short-term inhibitory influences on striatal neuron activity. Thus, nitric oxide signaling is likely to play an important role in the integration of corticostriatal sensorimotor information in striatal networks.

Dopamine D1 receptors and group I metabotropic glutamate receptors contribute to the induction of long-term potentiation in the nucleus accumbens

Long-term changes in the efficacy of glutamatergic synaptic transmission in the striatal complex are proposed to underlie motor learning and neuroadaptations leading to addiction. Dopamine and glutamate play key roles in the induction of long-term potentiation (LTP) and long-term depression (LTD) in the dorsal striatum, but their contribution to synaptic plasticity in the ventral striatum (nucleus accumbens, NAc) has been less extensively studied. We have examined the role of dopamine, glutamate and GABA in the induction of LTP in mouse brain slices containing the NAc. High-frequency stimulation of glutamatergic inputs elicited LTP of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in the core region of the NAc. GABA did not seem to participate in LTP induction because LTP was not altered in the presence of either a GABA(A)- (bicuculline) or a GABA(B)- (CGP 55845) receptor antagonist. However, the dopamine D1 receptor antagonist SCH 23390, but not the dopamine D2 receptor antagonist sulpiride, impaired LTP. The dopamine reuptake blocker nomifensine also inhibited LTP induction. We found that group I metabotropic glutamate receptors (mGluRs) contribute to LTP induction because the mGluR1 antagonist LY 367385, or the mGluR5 antagonist MPEP, blocked LTP induction. Furthermore, the glutamate reuptake blocker DL-TBOA also impaired LTP. The present results demonstrate that dopamine and glutamate play critical roles in the mechanisms of induction of LTP in the NAc through the activation of dopamine D1 receptors and group I mGluRs. However, LTP is negatively regulated when endogenous levels of dopamine or glutamate are elevated.

Frontal cortical afferents facilitate striatal nitric oxide transmission in vivo via a NMDA receptor and neuronal NOS-dependent mechanism

Striatal nitric oxide (NO) signaling plays a critical role in modulating neural processing and motor behavior. Nitrergic interneurons receive synaptic inputs from corticostriatal neurons and are activated via ionotropic glutamate receptor stimulation. However, the afferent regulation of NO signaling is poorly characterized. The role of frontal cortical afferents in regulating NO transmission was assessed in anesthetized rats using amperometric microsensor measurements of NO efflux and local field potential recordings. Low frequency (3 Hz) electrical stimulation of the ipsilateral cortex did not consistently evoke detectable changes in striatal NO efflux. In contrast, train stimulation (30 Hz) of frontal cortical afferents facilitated NO efflux in a stimulus intensity-dependent manner. Nitric oxide efflux evoked by train stimulation was transient, reproducible over time, and attenuated by systemic administration of either the NMDA receptor antagonist MK-801 or the neuronal NO synthase inhibitors 7-nitroindazole and NG-propyl-L-arginine. The interaction between NO efflux evoked via train stimulation and local striatal neuron activity was assessed using dual microsensor and local field potential recordings carried out concurrently in the contralateral and ipsilateral striatum, respectively. Systemic administration of the non-specific NO synthase inhibitor methylene blue attenuated both evoked NO efflux and the peak oscillation frequency (within the delta band) of local field potentials recorded immediately after train stimulation. Taken together, these observations indicate that feed-forward activation of neuronal NO signaling by phasic activation of frontal cortical afferents facilitates the synchronization of glutamate driven oscillations in striatal neurons. Thus, NO signaling may act to amplify coherent corticostriatal transmission and synchronize striatal output.

Spiking neurons, dopamine, and plasticity: timing is everything, but concentration also matters

While both dopamine (DA) fluctuations and spike-timing-dependent plasticity (STDP) are known to influence long-term corticostriatal plasticity, little attention has been devoted to the interaction between these two fundamental mechanisms. Here, a theoretical framework is proposed to account for experimental results specifying the role of presynaptic activation, postsynaptic activation, and concentrations of extracellular DA in synaptic plasticity. Our starting point was an explicitly-implemented multiplicative rule linking STDP to Michaelis-Menton equations that models the dynamics of extracellular DA fluctuations. This rule captures a wide range of results on conditions leading to long-term potentiation and depression in simulations that manipulate the frequency of induced corticostriatal stimulation and DA release. A well-documented biphasic function relating DA concentrations to synaptic plasticity emerges naturally from simulations involving a multiplicative rule linking DA and neural activity. This biphasic function is found consistently across different neural coding schemes employed (voltage-based vs. spike-based models). By comparison, an additive rule fails to capture these results. The proposed framework is the first to generate testable predictions on the dual influence of DA concentrations and STDP on long-term plasticity, suggesting a way in which the biphasic influence of DA concentrations can modulate the direction and magnitude of change induced by STDP, and raising the possibility that DA concentrations may inverse the LTP/LTD components of the STDP rule.

Dopamine D2 receptor-dependent modulation of striatal NO synthase activity

RATIONALE

Striatal nitric oxide (NO)-producing interneurons receive synaptic contacts from midbrain dopamine (DA) neurons and are regulated by phasic DA transmission. Classic antipsychotic drugs elevate neuronal NO synthase (NOS) expression in the rat striatum. Given that NO signaling potently modulates the membrane excitability of striatal projection neurons, it is plausible that up-regulation of NOS activity after DA D2 receptor blockade contributes to the therapeutic efficacy and/or motor side effects associated with antipsychotic drugs.

OBJECTIVES

This study assessed the impact of DA D(2) receptor activation on striatal NOS activity in vivo. Characterization of the dopaminergic regulation of striatal NO signaling will be relevant for understanding the mechanism(s) of action of antipsychotic drugs.

MATERIALS AND METHODS

Striatal NO efflux, evoked via electrical stimulation of the substantia nigra (SN) or systemic administration of the DA D(1) receptor agonist SKF 81297, was assessed in anesthetized rats using an NO-selective amperometric microsensor.

RESULTS

The facilitatory effect of SN stimulation on striatal NO efflux was attenuated by systemic administration of the DA D(2) receptor agonist quinpirole. Conversely, administration of the DA D(2) receptor antagonist eticlopride augmented evoked NO efflux. NO efflux induced by systemic administration of SKF 81297 was attenuated by quinpirole and restored by co-administration of quinpirole and eticlopride. The facilitatory effect of SKF 81297 on NO efflux was also significantly attenuated after pretreatment with the non-specific NOS inhibitor methylene blue.

CONCLUSIONS

Activation of NO synthesis by phasic DA transmission is down-regulated via a DA D2 receptor-dependent mechanism. DA D(2) receptor activation opposes DA D(1) receptor activation of NO synthesis at a site postsynaptic to the DA terminal. Further studies examining NO-DA dynamics may have potential to reveal novel therapeutic strategies to treat various brain disorders.

Difference in organization of corticostriatal and thalamostriatal synapses between patch and matrix compartments of rat neostriatum

The neostriatum, which possesses a mosaic organization consisting of patch and matrix compartments, receives glutamatergic excitatory afferents from the cerebral cortex and thalamus. Differences in the synaptic organization of these striatopetal afferents between the patch and matrix compartments were examined in the rat using confocal laser scanning and electron microscopes. Thalamostriatal terminals immunopositive for vesicular glutamate transporter (VGluT) 2 were less dense in the patch than in the matrix compartment, although the density of VGluT1-immunopositive corticostriatal terminals was almost evenly distributed in both the compartments. Quantitative analysis of ultrastructural images revealed that 84% of VGluT2-positive synapses in the patch compartment were formed with dendritic spines, whereas 70% in the matrix compartment were made with dendritic shafts. By contrast, VGluT1-positive terminals display a similar preference for specific synaptic targets in both compartments: about 80% made synapses with dendritic spines. In addition, VGluT2-positive axospinous synapses in the patch compartment were larger than the VGluT1-positive axospinous synapses in both compartments. As axospinous synapses are generally found in neuronal connections showing high synaptic plasticity, the present findings suggest that the thalamostriatal connection requires higher synaptic plasticity in the patch compartment than in the matrix compartment.

Alterations of rat corticostriatal synaptic plasticity after chronic ethanol exposure and withdrawal

BACKGROUND

The purpose of this study was to investigate the effects of chronic ethanol exposure (CEE) and withdrawal on corticostriatal plasticity in rats.

METHODS

We established an animal model of alcoholism using the method of Turchan et al. (1999). A synaptic model of long-term memory (long-term depression, LTD) was used as an index and the striatum, which is related to habit learning, was selected as a target region in the present study. The effects of CEE and withdrawal on the LTD were studied in striatal slices of ethanol-dependent rats using the extracellular recording method.

RESULTS

A stable LTD can be induced after high-frequency stimulation (HFS) in the slices of control rats. Chronic ethanol exposure and withdrawal suppressed the induction of corticostriatal LTD to different extents, with the strongest suppressive effects on LTD occurring in the slices of rats exposed to ethanol for 10 days and in those withdrawn from ethanol for 1 day. Notably, 3 days of withdrawal resulted in the shift of corticostriatal synaptic plasticity from LTD to long-term potentiation, and the peak latencies of the population spikes were obviously shortened compared with those of control rats. After 7 days of withdrawal, ethanol's effects tended to disappear.

CONCLUSIONS

These results suggest that the alterations of corticostriatal synaptic plasticity produced by CEE and withdrawal may play a prominent role in alcohol abuse and alcoholism.

NADPH-diaphorase histochemical changes in the hippocampus, cerebellum and striatum are correlated with different modalities of exercise and watermaze performances

Nitric oxide is involved in memory and motor learning. We investigated possible influences of exercise on spatial memory and NADPH-diaphorase (NADPH-d) histochemical activity in the hippocampus, striatum and cerebellum. Fifteen albino Swiss mice between the 22nd and 55th post-natal days were exercised in the following modalities: voluntary (V), acrobatic (A), acrobatic/voluntary (AV) and forced (F) and compared to inactive group (I). After the exercise period, all subjects were tested in the water maze for 3 days. Animal brains were processed for NADPH-d histochemistry. Densitometry of the neuropil of the hippocampus, striatum and cerebellum and morphometric analysis of NADPHd+ type I neurons of the striatum were done. Exercise groups presented higher levels of NADPH-d activity in the molecular and polymorphic layers of dentate gyrus and lacunosum molecular layer of CA1. The A group presented higher NADPH-d activity in the cerebellar granular layer than all other groups. Branching points and dendritic segment densities of NADPH-d type I neurons were higher in V, A and AV than in F and I groups. Exercise groups revealed best performances on water maze tests. Thus, different modalities of exercise increases in different proportions for the nitrergic activity in the hippocampus, striatum and cerebellum, and these changes seem to be beneficial to spatial memory.

Phasic dopaminergic transmission increases NO efflux in the rat dorsal striatum via a neuronal NOS and a dopamine D(1/5) receptor-dependent mechanism

Dysfunctional neurotransmission within striatal networks is believed to underlie the pathophysiology of several neurological and psychiatric disorders. Nitric oxide (NO)-producing interneurons have been shown to play a critical role in modulating striatal synaptic transmission. These interneurons receive synaptic contacts from midbrain dopamine (DA) neurons and may be regulated by DA receptor activation. In the current study, striatal NO efflux was measured in anesthetized male rats using an NO-selective electrochemical microsensor and the role of DA in modulating NO synthase (NOS) activity was assessed during electrical or chemical (bicuculline) stimulation of the substantia nigra (SN). Electrical stimuli were patterned to approximate the natural single spike or burst firing activity of midbrain DA neurons. Electrical stimulation of the SN at low frequencies induced modest increases in striatal NO efflux. In contrast, train stimulation of the SN robustly increased NO efflux in a stimulus intensity-dependent manner. NO efflux evoked by SN stimulation was similar in chloral hydrate- and urethane-anesthetized rats. The facilitatory effect of train stimulation on striatal NO efflux was transient and attenuated by systemic administration of the neuronal NOS inhibitor 7-nitroindazole and the nonselective NOS inhibitor methylene blue. Moreover, the increase in NO efflux observed during chemical and train stimulation of the SN was attenuated following systemic administration of the DA D(1/5) receptor antagonist SCH 23390. SCH 23390 also blocked NO efflux induced by systemic administration of the D(1/5) agonist SKF 81297. These results indicate that neuronal NOS is activated in vivo by nigrostriatal DA cell burst firing via a DA D(1/5)-like receptor-dependent mechanism.

The contribution of NMDA receptors in the dorsolateral striatum to egocentric response learning

The present study examined the effects of the N-methyl-D-aspartate (NMDA) competitive antagonist, 2-amino-5-phosphonopentanoic acid (AP-5), injected into the dorsolateral striatum on the acquisition and reversal learning of a response discrimination. Male Long-Evans rats were tested across 2 consecutive days in a modified cross-maze. An infusion of either saline or AP-5 (5 or 25 nM) occurred 5 min prior to testing. In acquisition rats learned to turn left or right. In reversal learning rats learned to turn in the opposite direction. An AP-5 infusion at 25 nmol, but not 5 nmol, impaired response acquisition. Neither AP-5 dose impaired response reversal learning. The results suggest that NMDA receptors in the dorsolateral striatum are critical for the initial learning of an egocentric response discrimination.

Induction and expression mechanisms of postsynaptic NMDA receptor-independent homosynaptic long-term depression

The induction of long-term depression (LTD) can be divided into two main forms, one dependent upon activation of postsynaptic NMDAR, and another independent of postsynaptic NMDAR. Non-postsynaptic NMDAR-LTD (non-NMDAR-LTD) occurs in many regions of the brain, and encompasses a wide variety of induction and expression mechanisms. In this article, the induction and expression mechanisms of such LTD in over 10 brain regions are described, with a number of common mechanisms compared across a large range of types of LTD. The article describes the involvement of different presynaptic or postsynaptic receptors in the induction of non-NMDAR-LTD, especially metabotropic glutamate receptors, cannabinoid receptors and dopamine receptors. An increase in presynaptic or postsynaptic intracellular Ca concentration is a key event in induction, commonly followed by activation of certain kinases, especially PKC, p38 MAPK and ERK. Expression mechanisms are either presynaptic via a reduction in release probability, or postsynaptic involving a decrease in AMPAR via phosphorylation of a glutamate receptor subunit, especially GluR2, followed by clathrin-mediated endocytosis. Retrograde signalling from postsynaptic to presynaptic occurs when induction is postsynaptic and expression is presynaptic.

Context-dependent modulation by D₁ receptors: Differential effects in hippocampus and striatum

Place-specific firing by hippocampal and striatal neurons was recorded simultaneously following injection of a D(1) receptor antagonist (SCH23390) and during spatial working memory task performance. SCH23390-induced changes in unit responses were observed during light and dark test conditions. Although hippocampal place field locations were altered by the contextual change, the reliability and specificity of place fields was disrupted only by combining D(1) antagonism and a change in context. Striatal place field locations were reorganized after either contextual change or D(1) antagonism, without altering place field reliability and specificity. Disrupted velocity encoding by place cells in both regions was induced by darkness, whereas greater stability in acceleration encoding followed removal of D(1) receptor activity. Dopamine may differentially regulate hippocampal context learning and striatum-based predictive codes.

Preservation of the direct and indirect pathways in an in vitro preparation of the mouse basal ganglia

We have developed a slice preparation of the mouse basal ganglia which contains portions of the striatum, external pallidum, subthalamic nucleus and substantia nigra and the neocortex. This basal ganglia slice is unique in preserving functional direct and indirect connections between the striatum and the substantia nigra as well as interconnectivity between the globus pallidus and the subthalamic nucleus. We used fiber tract tracing studies and electrophysiological recordings to demonstrate the full functionality of these pathways. Deposits of 1,1'-dioctadecyl-3,3,3',3'-tetra-methylindocarbocyamine perchlorate in the different basal ganglia resulted in labeled fibers in each of their target nuclei. Confirming these results, electrical stimulation of the different nuclei elicited whole-cell recorded postsynaptic potentials in their target neurons with an appropriate pharmacological profile. Electrical and glutamate activation of the striatum evoked bursts of glutamatergic and GABAergic activities in whole-cell recorded nigral neurons indicating that the direct and indirect pathways are operative in this slice. It also showed that the responses evoked are not due to fibers en passant but to the activation of striatal cell bodies. These findings provide the first direct evidence for a preserved basal ganglia circuitry in vitro and make the basal ganglia slice a suitable preparation for analyzing the activity of the direct and indirect pathways in physiological and pathological conditions.

Nucleus accumbens nitric oxide immunoreactive interneurons receive nitric oxide and ventral subicular afferents in rats

The nitric oxide generating neurons of the nucleus accumbens exert a powerful influence over striatal function, in addition, these nitrergic inputs are in a position to regulate the dopaminergic and glutamatergic inputs on striatal projection neurons. It was the aim of this study to establish the source of the glutamatergic drive to nitric oxide synthase interneurons of the nucleus accumbens. The nucleus accumbens nitric oxide-generating neurons receive asymmetrical, excitatory, presumably glutamatergic inputs. Possible sources of these inputs could be the limbic and cortical regions known to project to this area. To identify sources of the excitatory inputs to the nitric oxide synthase-containing interneurons of the nucleus accumbens in the rat we first examined the ultrastructural morphology of asymmetrical synaptic specializations contacting nitric oxide synthase-immunohistochemically labeled interneurons in the nucleus accumbens. Neurons were selected from different regions of the nucleus accumbens, drawn using camera lucida, processed for electron microscopic analysis, and the boutons contacting nitric oxide synthase-labeled dendrites were photographed and correlated to the drawings. Using vesicle size as the criterion the source was predicted to be either the prefrontal cortex or the ventral subiculum of the hippocampus. To examine this prediction, a further study used anterograde tracing from both the prefrontal cortex and the ventral subiculum, and nitric oxide synthase immunohistochemistry with correlated light and electron microscopy. Based on appositions by anterogradely labeled fibers, selected nitric oxide synthase-labeled neurons within the nucleus accumbens, were examined with electron microscopic analysis. With this technique we confirmed the prediction that subicular afferent boutons make synaptic contact with nitric oxide synthase interneurons, and demonstrated anatomically that nitric oxide synthase boutons make synaptic contact with the dendritic arbors of nitric oxide synthase interneurons. We suggest that the subicular input may excite the nitric oxide synthase neurons synaptically, while the nitric oxide synthase-nitric oxide synthase interactions underlie a nitric oxide signaling network which propagates hippocampal information, and expands the hippocampus's influence on 'gating' information flow across the nucleus accumbens.

Temporal sequence learning, prediction, and control: a review of different models and their relation to biological mechanisms

In this review, we compare methods for temporal sequence learning (TSL) across the disciplines machine-control, classical conditioning, neuronal models for TSL as well as spike-timing-dependent plasticity (STDP). This review introduces the most influential models and focuses on two questions: To what degree are reward-based (e.g., TD learning) and correlation-based (Hebbian) learning related? and How do the different models correspond to possibly underlying biological mechanisms of synaptic plasticity? We first compare the different models in an open-loop condition, where behavioral feedback does not alter the learning. Here we observe that reward-based and correlation-based learning are indeed very similar. Machine control is then used to introduce the problem of closed-loop control (e.g., actor-critic architectures). Here the problem of evaluative (rewards) versus nonevaluative (correlations) feedback from the environment will be discussed, showing that both learning approaches are fundamentally different in the closed-loop condition. In trying to answer the second question, we compare neuronal versions of the different learning architectures to the anatomy of the involved brain structures (basal-ganglia, thalamus, and cortex) and the molecular biophysics of glutamatergic and dopaminergic synapses. Finally, we discuss the different algorithms used to model STDP and compare them to reward-based learning rules. Certain similarities are found in spite of the strongly different timescales. Here we focus on the biophysics of the different calcium-release mechanisms known to be involved in STDP.

Complex response to afferent excitatory bursts by nucleus accumbens medium spiny projection neurons

The nucleus accumbens (NAc) of the ventral striatum is involved in attention, motivation, movement, learning, reward, and addiction. GABAergic medium spiny projection neurons that make up approximately 90% of the neuronal population are commonly driven by convergent bursts of afferent excitation. We monitored spiny projection neurons in mouse striatal slices while applying stimulus trains to mimic bursts of excitation. A stimulus train evoked a simple, short-lived postsynaptic response from CA1 hippocampal pyramidal neurons, but the train evoked a complex series of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) from the NAc spiny projection neurons. As is commonly seen with projection neurons, the EPSC amplitudes initially displayed facilitation followed by depression, and that pattern was sensitive to the extracellular calcium concentration. In addition, there were two other novel observations. The spiny projection neurons responded to the stimulus train with a prolonged depolarization that was accompanied by a posttrain increase of spontaneous glutamatergic synaptic activity. Blocking AMPA/kainate glutamate receptors strongly inhibited the evoked EPSP/EPSCs, the posttrain spontaneous synaptic activity, and the prolonged depolarization. A potassium channel inhibitor increased and extended the prolonged postsynaptic depolarization, causing a long-lasting depolarized plateau potential. Our results indicate that burst-like activity along ventral striatal afferents is extended in time by additional spontaneous glutamate release that is integrated by the postsynaptic spiny projection neurons into a prolonged depolarization. The results suggest that the posttrain quantal glutamate release helps to blend and maintain multiple afferent inputs. That convergent excitation is further integrated by the postsynaptic neuron into a prolonged depolarization that may contribute to the depolarized "up state" observed in vivo.

Repeated electroconvulsive stimulation impairs long-term depression in the neostriatum

BACKGROUND

Increasing and consistent findings of structural and functional abnormalities in patients with mood disorders demonstrate a clear involvement of the neostriatum. Therefore, the beneficial effect of electroconvulsive stimulation (ECS) treatment of acute state of mood disorder may relate to changes in striatal synaptic plasticity.

METHODS

We studied the effect of ECS treatment on the reported long-term depression (LTD) of synaptic excitatory afferents to striatal medium spiny neurons. Stimulation of the white matter between the cortex and the striatum elicited excitatory postsynaptic potentials (EPSPs) in medium spiny neurons in rat corticostriatal slices while recording using the whole-cell patch-clamp technique.

RESULTS

The EPSPs evoked in striatal neurons undergo LTD with repeated stimulation trains, and LTD of this pathway is impaired in rats after ECS treatment for 1 week, similar to what is reported in chronic lithium treatment. Electroconvulsive stimulation did not affect intrinsic membrane properties or the occurrence of spontaneous EPSCs. Dose-dependent inhibition of the EPSPs by a nonselective agonist of metabotropic glutamate receptor did not change in rats after ECS treatment.

CONCLUSIONS

Our data suggest that the effectiveness of electroconvulsive therapy in mood disorders may be a consequence of LTD impairment of excitatory cortical afferents to striatal projecting neurons.

Possible role of intramembrane receptor-receptor interactions in memory and learning via formation of long-lived heteromeric complexes: focus on motor learning in the basal ganglia

Learning in neuronal networks occurs by instructions to the neurons to change their synaptic weights (i.e., efficacies). According to the present model a molecular mechanism that can contribute to change synaptic weights may be represented by multiple interactions between membrane receptors forming aggregates (receptor mosaics) via oligomerization at both pre- and post-synaptic level. These assemblies of receptors together with inter alia single receptors, adapter proteins, G-proteins and ion channels form the membrane bound part of a complex three-dimensional (3D) molecular circuit, the cytoplasmic part of which consists especially of protein kinases, protein phosphatases and phosphoproteins. It is suggested that this molecular circuit has the capability to learn and store information. Thus, engram formation will depend on the resetting of 3D molecular circuits via the formation of new receptor mosaics capable of addressing the transduction of the chemical messages impinging on the cell membrane to certain sets of G-proteins. Short-term memory occurs by a transient stabilization of the receptor mosaics producing the appropriate change in the synaptic weight. Engram consolidation (long-term memory) may involve intracellular signals that translocate to the nucleus to cause the activation of immediate early genes and subsequent formation of postulated adapter proteins which stabilize the receptor mosaics with the formation of long-lived heteromeric receptor complexes. The receptor mosaic hypothesis of the engram formation has been formulated in agreement with the Hebbian rule and gives a novel molecular basis for it by postulating that the pre-synaptic activity change in transmitter and modulator release reorganizes the receptor mosaics at post-synaptic level and subsequently at pre-synaptic level with the formation of novel 3D molecular circuits leading to a different integration of chemical signals impinging on pre- and post-synaptic membranes hence leading to a new value of the synaptic weight. Engram retrieval is brought about by the scanning of the target networks by the highly divergent arousal systems. Hence, a continuous reverberating process occurs both at the level of the neural networks as well as at the level of the 3D molecular circuits within each neuron of the network until the appropriate tuning of the synaptic weights is obtained and, subsequently, the reappearance of the engram occurs. Learning and memory in the basal ganglia is discussed in the frame of the present hypothesis. It is proposed that formation of long-term memories (consolidated receptor mosaics) in the plasma membranes of the striosomal GABA neurons may play a major role in the motivational learning of motor skills of relevance for survival. In conclusion, long-lived heteromeric receptor complexes of high order may be crucial for learning, memory and retrieval processes, where extensive reciprocal feedback loops give rise to coherent synchronized neural activity (binding) essential for a sophisticated information handling by the central nervous system.

Morphine induction of c-fos expression in the rat forebrain through glutamatergic mechanisms: role of non-n-methyl-D-aspartate receptors

Acute injection of morphine induces expression of the immediate-early genes c-Fos and JunB in several forebrain regions of the rat, in part through an N-methyl-D-aspartate (NMDA) receptor-dependent mechanism. Because membrane depolarization through (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors is believed to be necessary for full activation of NMDA receptors, we determined the role of AMPA receptors in morphine-induced c-Fos expression. Rats were given the AMPA receptor antagonist GYKI-52466 (12.9 mg/kg, i.p.) 15 min before morphine (10 mg/kg, s.c.), or the AMPA receptor enhancer CX516 (30 mg/kg, i.p.) 5 min after morphine. The c-Fos response was attenuated by the antagonist and augmented by the enhancer. Using double immunocytochemistry, we found that morphine induced c-Fos in neurons containing the GluR2/3, but not the GluR1 and rarely the GluR4, subunits of the AMPA receptor. Double immunocytochemistry for mu opioid receptor and c-Fos showed that c-Fos expression was mainly absent in the patch compartment of the striatum, which is enriched in mu opioid receptors. The glutamatergic synapse often contains metabotropic receptors as well as ionotropic receptors. Type I metabotropic glutamate receptors are coupled to activation of protein kinase C, which has also been shown to mediate the immediate-early gene response to morphine. To determine if activation of metabotropic glutamate receptors is involved in rapid effects of morphine on the brain, rats were given the type I metabotropic glutamate receptor antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 0.2 mg/kg, i.p.) or vehicle 30 min before morphine treatment. Pretreatment with AIDA completely blocked morphine-induced c-Fos expression in the caudate-putamen.Taken together, these results demonstrate involvement of both AMPA and type I metabotropic glutamate receptors in the acute effects of morphine on the forebrain, supporting an important role for glutamatergic neurotransmission mediated by non-NMDA glutamate receptors in morphine's actions.

Glutamate receptor functions in sensory relay in the thalamus

It is known that glutamate is a major excitatory transmitter of sensory and cortical afferents to the thalamus. These actions are mediated via several distinct receptors with postsynaptic excitatory effects predominantly mediated by ionotropic receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate varieties (NMDA). However, there are also other kinds of glutamate receptor present in the thalamus, notably the metabotropic and kainate types, and these may have more complex or subtle roles in sensory transmission. This paper describes recent electrophysiological experiments done in vitro and in vivo which aim to determine how the metabotropic and kainate receptor types can influence transmission through the sensory thalamic relay. A particular focus will be how such mechanisms might operate under physiological conditions.

Regulation of striatal dopamine neurotransmission by nitric oxide: effector pathways and signaling mechanisms

An important role for the reactive gas nitric oxide (NO) in regulating striatal dopaminergic neurotransmission was identified shortly after initial observations indicated that this unorthodox neurotransmitter mediates many of the influences of glutamatergic neurotransmission in the cerebellum, cortex, and hippocampus. While the precise actions of NO on striatal presynaptic and postsynaptic elements remain to be fully characterized, the recent application of sophisticated anatomical, neurochemical, and electrophysiological approaches to the study of nitrergic signaling has revealed that NO exerts a powerful influence both on tonic extracellular dopamine (DA) levels and phasic DA neuron spike activity via the modulation of intrinsic striatal mechanisms and striatonigral feedback loops. Although the nature of the NO-mediated modulatory influence on DA neurotransmission was initially clouded by seemingly conflicting neurochemical observations, a growing body of literature and understanding of the diverse signaling mechanisms and effector pathways utilized by NO indicates that NO exerts a primary facilitatory influence over tonic and phasic dopaminergic neurotransmission under physiological conditions. A review of neurochemical and electrophysiological studies examining the influence of endogenous and exogenous NO on DA neurotransmission indicates that NO signaling exerts multiple effects on local striatal circuits and projection neurons involved in regulating basal ganglia output and nigrostriatal DA neuron activity. In addition to summarizing these influences, the current review focuses on the mechanisms utilized by striatal NO signaling pathways involved in modulating DA transmission at the level of the terminal and cell body and attempts to integrate these observations into a functional model of NO-dependent regulation of basal ganglia systems.

Dopamine-dependent plasticity of corticostriatal synapses

Knowledge of the effect of dopamine on corticostriatal synaptic plasticity has advanced rapidly over the last 5 years. We consider this new knowledge in relation to three factors proposed earlier to describe the rules for synaptic plasticity in the corticostriatal pathway. These factors are a phasic increase in dopamine release, presynaptic activity and postsynaptic depolarisation. A function is proposed which relates the amount of dopamine release in the striatum to the modulation of corticostriatal synaptic efficacy. It is argued that this function, and the experimental data from which it arises, are compatible with existing models which associate the reward-related firing of dopamine neurons with changes in corticostriatal synaptic efficacy.

Properties of LTD and LTP of retinocollicular synaptic transmission in the developing rat superior colliculus

The developing retinocollicular pathway undergoes synaptic refinement in order to form the precise retinotopic pattern seen in adults. To study the mechanisms which underlie refinement, we investigated long-term changes in retinocollicular transmission in rats aged P0-P25. Field potentials (FPs) in the superior colliculus (SC) were evoked by stimulation of optic tract fibers in an in vitro isolated brainstem preparation. High intensity stimulation induced long-term depression (LTD) in the SC after both low (1000 stimuli at 1 Hz) and higher (1000 stimuli at 50 Hz) frequency stimulation. The induction of LTD was independent of activation of NMDA and GABA(A) receptors, because D-APV (100 microM) and bicuculline (10 microM) did not block LTD. Induction of LTD was dependent upon activation of L-type Ca(2+) channels as 10 microM nitrendipine, an L-type Ca(2+) channel blocker, significantly decreased the magnitude of LTD. LTD was down-regulated during development. LTD magnitude was greatest in rats aged P0-P9 and significantly less in rats aged P10-P25. Long-term potentiation (LTP) was induced by low intensity stimulation and only after high frequency tetanus (1000 stimuli at 50 Hz). LTP was NMDA receptor dependent because d-APV (100 microM) completely abolished it. LTP induction was also blocked by the L-type Ca2+ channel blocker nitrendipine. The magnitude of LTP first increased with age, being significantly greater at P7-P13 than at P0-3 and then decreased at P23-25. In summary, both LTD and LTP are present during retinocollicular pathway refinement, but have different transmitter and ionic mechanisms and time courses of expression.

Impact of antipsychotic drug administration on the expression of nuclear receptors in the neocortex and striatum of the rat brain

We have recently shown that the expression of the nerve growth factor-inducible gene B (NGFI-B, or Nur77), a transcription factor belonging to the large ligand-activated nuclear receptor family, is modulated by antipsychotic drugs in the rat forebrain. In the present work, we have investigated the impact of antipsychotic drugs on a series of transcription factors also belonging to the nuclear receptor family. The receptors investigated include retinoid X receptor (RXR), thyroid hormone receptor (TR), retinoic acid receptor (RAR), RAR-related orphan receptor (RZR) and Rev-erb receptor isoforms in addition to the NGFI-B transcript. We have used in situ hybridization to monitor their mRNA levels after acute and chronic antipsychotic drug administration. RZRbeta and NGFI-B mRNA levels are down-regulated after chronic haloperidol or clozapine treatment in the primary somatosensory cortex. The TRbeta1 isoform mainly expressed in the cingulate cortex is modulated only after chronic clozapine treatment, whereas TRalpha isoform mRNAs are modulated by both antipsychotics in the cingulate cortex and nucleus accumbens shell; two brain areas associated with limbic functions. The RXRgamma1 isoform, mostly expressed in the dorsolateral portion of the striatum is modestly affected by antipsychotics. Modulation of the expression of transcription factors belonging to the ligand-activated nuclear receptor family by antipsychotics represents an additional molecular event in the mechanism of action of these drugs. We suggest that modification of the pattern of transcription factor expression may play a role in long-term cellular responses to these drugs.

Selective involvement of mGlu1 receptors in corticostriatal LTD

Although metabotropic glutamate receptors (mGluRs) have been proposed to play a role in corticostriatal long-term depression (LTD), the specific receptor subtype required for this form of synaptic plasticity has not been characterized yet. Thus, we utilized a corticostriatal brain slice preparation and intracellular recordings from striatal spiny neurons to address this issue. We observed that both AIDA (100 microM) and LY 367385 (30 microM), two blockers of mGluR1s, were able to fully prevent the induction of this form of synaptic plasticity, whereas MPEP (30 microM), a selective antagonist of the mGluR5 subtype, did not significantly affect the amplitude and time-course of corticostriatal LTD. Both AIDA and LY 367385 were ineffective on LTD when applied after its induction. The critical role of mGluR1s in the formation of corticostriatal LTD was confirmed in experiments performed on mice lacking mGluR1s. In these mice, in fact, a significant reduction of the LTD amplitude was observed in comparison to the normal LTD measured in their wild-type counterparts. We found that neither acute pharmacological blockade of mGluR1s nor the genetic disruption of these receptors affected the presynaptic modulation of corticostriatal excitatory postsynapic potentials (EPSPs) exerted by DCG-IV and L-SOP, selective agonists of group II and III mGluRs, respectively. Our data show that the induction of corticostriatal LTD requires the activation of mGluR1 but not mGluR5. mGluR1-mediated control of this form of synaptic plasticity may play a role in the modulatory effect exerted by mGluRs in the basal ganglia-related motor activity.