Metabotropic glutamate 2 receptors modulate synaptic inputs and calcium signals in striatal cholinergic interneurons

The Biology and Pathobiology of Glutamatergic, Cholinergic, and Dopaminergic Signaling in the Aging Brain

The elderly population is growing worldwide, with important health and socioeconomic implications. Clinical and experimental studies on aging have uncovered numerous changes in the brain, such as decreased neurogenesis, increased synaptic defects, greater metabolic stress, and enhanced inflammation. These changes are associated with cognitive decline and neurobehavioral deficits. Although aging is not a disease, it is a significant risk factor for functional worsening, affective impairment, disease exaggeration, dementia, and general disease susceptibility. Conversely, life events related to mental stress and trauma can also lead to accelerated age-associated disorders and dementia. Here, we review human studies and studies on mice and rats, such as those modeling human neurodegenerative diseases, that have helped elucidate (1) the dynamics and mechanisms underlying the biological and pathological aging of the main projecting systems in the brain (glutamatergic, cholinergic, and dopaminergic) and (2) the effect of defective glutamatergic, cholinergic, and dopaminergic projection on disabilities associated with aging and neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Detailed knowledge of the mechanisms of age-related diseases can be an important element in the development of effective ways of treatment. In this context, we briefly analyze which adverse changes associated with neurodegenerative diseases in the cholinergic, glutaminergic and dopaminergic systems could be targeted by therapeutic strategies developed as a result of our better understanding of these damaging mechanisms.

Potential Therapeutic Application for Nicotinic Receptor Drugs in Movement Disorders

Emerging studies indicate that striatal cholinergic interneurons play an important role in synaptic plasticity and motor control under normal physiological conditions, while their disruption may lead to movement disorders. Here we discuss the involvement of the cholinergic system in motor dysfunction, with a focus on the role of the nicotinic cholinergic system in Parkinson's disease and drug-induced dyskinesias. Evidence for a role for the striatal nicotinic cholinergic system stems from studies showing that administration of nicotine or nicotinic receptor drugs protects against nigrostriatal degeneration and decreases L-dopa-induced dyskinesias. In addition, nicotinic receptor drugs may ameliorate tardive dyskinesia, Tourette's syndrome and ataxia, although further study is required to understand their full potential in the treatment of these disorders. A role for the striatal muscarinic cholinergic system in movement disorders stems from studies showing that muscarinic receptor drugs acutely improve Parkinson's disease motor symptoms, and may reduce dyskinesias and dystonia. Selective stimulation or lesioning of striatal cholinergic interneurons suggests they are primary players in this regulation, although multiple central nervous systems appear to be involved.

IMPLICATIONS

Accumulating data from preclinical studies and clinical trials suggest that drugs targeting CNS cholinergic systems may be useful for symptomatic treatment of movement disorders. Nicotinic cholinergic drugs, including nicotine and selective nAChR receptor agonists, reduce L-dopa-induced dyskinesias, as well as antipsychotic-induced tardive dyskinesia, and may be useful in Tourette's syndrome and ataxia. Subtype selective muscarinic cholinergic drugs may also provide effective therapies for Parkinson's disease, dyskinesias and dystonia. Continued studies/trials will help address this important issue.

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.

A new outlook on cholinergic interneurons in Parkinson's disease and L-DOPA-induced dyskinesia

Traditionally, dopamine (DA) and acetylcholine (ACh) striatal systems were considered antagonistic and imbalances or aberrant signaling between these neurotransmitter systems could be detrimental to basal ganglia activity and pursuant motor function, such as in Parkinson's disease (PD) and L-DOPA-induced dyskinesia (LID). Herein, we discuss the involvement of cholinergic interneurons (ChIs) in striatally-mediated movement in a healthy, parkinsonian, and dyskinetic state. ChIs integrate numerous neurotransmitter signals using intrinsic glutamate, serotonin, and DA receptors and convey the appropriate transmission onto nearby muscarinic and nicotinic ACh receptors to produce movement. In PD, severe DA depletion causes abnormal rises in ChI activity which promote striatal signaling to attenuate normal movement. When treating PD with L-DOPA, hyperkinetic side effects, or LID, develop due to increased striatal DA; however, the role of ChIs and ACh transmission, until recently has been unclear. Fortunately, new technology and pharmacological agents have facilitated understanding of ChI function and ACh signaling in the context of LID, thus offering new opportunities to modify existing and discover future therapeutic strategies in movement disorders.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

Spontaneous Synaptic Activation of Muscarinic Receptors by Striatal Cholinergic Neuron Firing

Cholinergic interneurons (CHIs) play a major role in motor and learning functions of the striatum. As acetylcholine does not directly evoke postsynaptic events at most striatal synapses, it remains unclear how postsynaptic cholinergic receptors encode the firing patterns of CHIs in the striatum. To examine the dynamics of acetylcholine release, we used optogenetics and paired recordings from CHIs and medium spiny neurons (MSNs) virally overexpressing G-protein-activated inwardly rectifying potassium (GIRK) channels. Due to the efficient coupling between endogenous muscarinic receptors and GIRK channels, we found that firing of individual CHIs resulted in monosynaptic spontaneous inhibitory post-synaptic currents (IPSCs) in MSNs. Paired CHI-MSN recordings revealed that the high probability of acetylcholine release at these synapses allowed muscarinic receptors to faithfully encode physiological activity patterns from individual CHIs without failure. These results indicate that muscarinic receptors in striatal output neurons reliably decode CHI firing.

mGlu2 Receptor Agonism, but Not Positive Allosteric Modulation, Elicits Rapid Tolerance towards Their Primary Efficacy on Sleep Measures in Rats

G-protein-coupled receptor (GPCR) agonists are known to induce both cellular adaptations resulting in tolerance to therapeutic effects and withdrawal symptoms upon treatment discontinuation. Glutamate neurotransmission is an integral part of sleep-wake mechanisms, which processes have translational relevance for central activity and target engagement. Here, we investigated the efficacy and tolerance potential of the metabotropic glutamate receptors (mGluR2/3) agonist LY354740 versus mGluR2 positive allosteric modulator (PAM) JNJ-42153605 on sleep-wake organisation in rats. In vitro, the selectivity and potency of JNJ-42153605 were characterized. In vivo, effects on sleep measures were investigated in rats after once daily oral repeated treatment for 7 days, withdrawal and consecutive re-administration of LY354740 (1–10 mg/kg) and JNJ-42153605 (3–30 mg/kg). JNJ-42153605 showed high affinity, potency and selectivity at mGluR2. Binding site analyses and knowledge-based docking confirmed the specificity of JNJ-42153605 at the mGluR2 allosteric binding site. Acute LY354740 and JNJ-42153605 dose-dependently decreased rapid eye movement (REM) sleep time and prolonged its onset latency. Sub chronic effects of LY354740 on REM sleep measures disappeared from day 3 onwards, whereas those of JNJ-42153605 were maintained after repeated exposure. LY354740 attenuated REM sleep homeostatic recovery, while this was preserved after JNJ-42153605 administration. JNJ-42153605 enhanced sleep continuity and efficiency, suggesting its potential as an add-on medication for impaired sleep quality during early stages of treatment. Abrupt cessation of JNJ-42153605 did not induce withdrawal phenomena and sleep disturbances, while the initial drug effect was fully reinstated after re-administration. Collectively, long-term treatment with JNJ-42153605 did not induce tolerance phenomena to its primary functional effects on sleep measures, nor adverse effects at withdrawal, while it promoted homeostatic recovery sleep. From the translational perspective, the present rodent findings suggest that mGluR2 positive allosteric modulation has therapeutic potential based on its superior long term efficacy over agonists in psychiatric disorders, particularly of those commonly occurring with REM sleep overdrive.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Proneurogenic Group II mGluR antagonist improves learning and reduces anxiety in Alzheimer Aβ oligomer mouse

Proneurogenic compounds have recently shown promise in some mouse models of Alzheimer's pathology. Antagonists at Group II metabotropic glutamate receptors (Group II mGluR: mGlu2, mGlu3) are reported to stimulate neurogenesis. Agonists at those receptors trigger γ-secretase-inhibitor-sensitive biogenesis of Aβ42 peptides from isolated synaptic terminals, which is selectively suppressed by antagonist pretreatment. We have assessed the therapeutic potential of chronic pharmacological inhibition of Group II mGluR in Dutch APP (Alzheimer's amyloid precursor protein E693Q) transgenic mice that accumulate Dutch amyloid-β (Aβ) oligomers but never develop Aβ plaques. BCI-838 is a clinically well-tolerated, orally bioavailable, investigational prodrug that delivers to the brain BCI-632, the active Group II mGluR antagonist metabolite. Dutch Aβ-oligomer-forming APP transgenic mice (APP E693Q) were dosed with BCI-838 for 3 months. Chronic treatment with BCI-838 was associated with reversal of transgene-related amnestic behavior, reduction in anxiety, reduction in levels of brain Aβ monomers and oligomers, and stimulation of hippocampal neurogenesis. Group II mGluR inhibition may offer a unique package of relevant properties as an Alzheimer's disease therapeutic or prophylactic by providing both attenuation of neuropathology and stimulation of repair.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

The role of inhibition in oscillatory wave dynamics in the cortex

Cortical oscillations arise during behavioral and mental tasks, and all temporal oscillations have particular spatial patterns. Studying the mechanisms that generate and modulate the spatiotemporal characteristics of oscillations is important for understanding neural information processing and the signs and symptoms of dynamical diseases of the brain. Nevertheless, it remains unclear how GABAergic inhibition modulates these oscillation dynamics. Using voltage-sensitive dye imaging, pharmacological methods, and tangentially cut occipital neocortical brain slices (including layers 3-5) of Sprague-Dawley rat, we found that GABAa disinhibition with bicuculline can progressively simplify oscillation dynamics in the presence of carbachol in a concentration-dependent manner. Additionally, GABAb disinhibition can further simplify oscillation dynamics after GABAa receptors are blocked. Both GABAa and GABAb disinhibition increase the synchronization of the neural network. Theta frequency (5-15-Hz) oscillations are reliably generated by using a combination of GABAa and GABAb antagonists alone. These theta oscillations have basic spatiotemporal patterns similar to those generated by carbachol/bicuculline. These results are illustrative of how GABAergic inhibition increases the complexity of patterns of activity and contributes to the regulation of the cortex.

Group II metabotropic glutamate receptors in the striatum of non-human primates: dysregulation following chronic cocaine self-administration

A growing body of evidence has demonstrated a role for group II metabotropic glutamate receptors (mGluRs) in the reinforcing effects of cocaine. These receptors are important given their location in limbic-related areas, and their ability to control the release of glutamate and other neurotransmitters. They are also potential targets for novel pharmacotherapies for cocaine addiction. The present study investigated the impact of chronic cocaine self-administration (9.0mg/kg/session for 100 sessions, 900 mg/kg total intake) on the densities of group II mGluRs, as assessed with in vitro receptor autoradiography, in the striatum of adult male rhesus monkeys. Binding of [(3)H]LY341495 to group II mGluRs in control animals was heterogeneous, with a medial to lateral gradient in binding density. Significant elevations in the density of group II mGluRs following chronic cocaine self-administration were measured in the dorsal, central and ventral portions of the caudate nucleus (P<0.05), compared to controls. No differences in receptor density were observed between the groups in either the putamen or nucleus accumbens. These data demonstrate that group II mGluRs in the dorsal striatum are more sensitive to the effects of chronic cocaine exposure than those in the ventral striatum. Cocaine-induced dysregulation of the glutamate system, and its consequent impact on plasticity and synaptic remodeling, will likely be an important consideration in the development of novel pharmacotherapies for cocaine addiction.

Metabotropic glutamate receptors modulate glutamatergic and GABAergic synaptic transmission in the central nucleus of the inferior colliculus

Fast glutamatergic and GABAergic transmission in the central nucleus of the inferior colliculus (ICC), a major auditory midbrain structure, is mediated respectively by alpha-amino-3-hydroxy-5-methylisoxazole-4 propionic acid (AMPA) and gamma-aminobutyric acid (GABA)(A) receptors. In this study, we used whole-cell patch clamp recordings in brain slices to investigate the effects of activation of metabotropic glutamate receptors (mGluRs) on synaptic responses mediated by AMPA and GABA(A) receptors in ICC neurons of young rats. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) mediated respectively by AMPA and GABA(A) receptors were elicited by stimulation of the lateral lemniscus, the major afferent pathway to the ICC. The agonists for groups I and II mGluRs, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD), and for group III mGluRs, L-2-amino-3-hydroxypropanoic acid 3-phosphate (L-SOP), did not affect intrinsic membrane properties of the ICC neurons. The agonist for group II mGluRs, (1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0] hexane-4,6-dicarboxylic acid (LY379268), significantly reduced the AMPA receptor-mediated EPSCs and GABA(A) receptor-mediated IPSCs. The effects were reversed by the group II mGluR antagonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495). The agonists for groups I and III, (RS)-3,5-dihydroxyphenylglycine (DHPG) and L-SOP, respectively, did not affect AMPA or GABA(A) receptor-mediated responses. The reduction of the synaptic responses by LY379268 was accompanied by a substantial increase in a ratio of the second to the first AMPA receptor-mediated EPSCs and GABA(A) receptor-mediated IPSCs to paired-pulse stimulation. The results suggest that group II mGluRs regulate both fast glutamatergic and GABAergic synaptic transmission in the ICC, probably through a presynaptic mechanism due to reduction of transmitter release.

Neurotransmitter roles in synaptic modulation, plasticity and learning in the dorsal striatum

The dorsal striatum is a large forebrain region involved in action initiation, timing, control, learning and memory. Learning and remembering skilled movement sequences requires the dorsal striatum, and striatal subregions participate in both goal-directed (action-outcome) and habitual (stimulus-response) learning. Modulation of synaptic transmission plays a large part in controlling input to as well as the output from striatal medium spiny projection neurons (MSNs). Synapses in this brain region are subject to short-term modulation, including allosteric alterations in ion channel function and prominent presynaptic inhibition. Two forms of long-term synaptic plasticity have also been observed in striatum, long-term potentiation (LTP) and long-term depression (LTD). LTP at glutamatergic synapses onto MSNs involves activation of NMDA-type glutamate receptors and D1 dopamine or A2A adenosine receptors. Expression of LTP appears to involve postsynaptic mechanisms. LTD at glutamatergic synapses involves retrograde endocannabinoid signaling stimulated by activation of metabotropic glutamate receptors (mGluRs) and D2 dopamine receptors. While postsynaptic mechanisms participate in LTD induction, maintained expression involves presynaptic mechanisms. A similar form of LTD has also been observed at GABAergic synapses onto MSNs. Studies have just begun to examine the roles of synaptic plasticity in striatal-based learning. Findings to date indicate that molecules implicated in induction of plasticity participate in these forms of learning. Neurotransmitter receptors involved in LTP induction are necessary for proper skill and goal-directed instrumental learning. Interestingly, receptors involved in LTP and LTD at glutamatergic synapses onto MSNs of the "indirect pathway" appear to have important roles in habit learning. More work is needed to reveal if and when synaptic plasticity occurs during learning and if so what molecules and cellular processes, both short- and long-term, contribute to this plasticity.

Effect of the metabotropic glutamate antagonist MPEP on striatal expression of the Homer family proteins in levodopa-treated hemiparkinsonian rats

RATIONALE

Striatal glutamatergic hyperactivity through the metabotropic receptors and their intracellular signaling pathways is considered critical in the development of levodopa-induced dyskinesias in Parkinson's disease and in experimental parkinsonism.

OBJECTIVE

We investigated whether the administration of the metabotropic glutamate antagonist, MPEP, modifies striatal expression of Homer family proteins which are involved in the intracellular mechanisms mediated by these receptors.

MATERIALS AND METHODS

Sprague-Dawley rats were unilaterally lesioned in the nigrostriatal pathway with 6-hydroxydopamine (8 microg) and treated with: levodopa (12 mg/kg, i.p.) plus vehicle (n=10) divided in two daily injections; levodopa plus MPEP (1.5 and 3 mg/kg, i.p.; n=6-13) divided in two daily injections; or saline (n=7) for 10 consecutive days. Axial, limb, and orolingual dyskinesias were evaluated. Striatal expression of tyrosine hydroxylase (TH), Homer 1a, 1b/c, and deltaFosB were measured by Western Blot.

RESULTS

Animals treated with levodopa showed an increase of dyskinesia score (p<0.01) that was attenuated by the administration of MPEP (p<0.01). In the ipsilateral side of the lesion, striatal TH expression was decreased (p<0.01). No significant differences in striatal Homer 1a or b/c expression were observed between the groups of treatment. Striatal deltaFosB expression increased in the animals treated with levodopa (p<0.05) being attenuated after MPEP administration (p<0.05). MPEP effect was not paralleled by any modification of striatal Homer proteins expression.

CONCLUSIONS

These results suggest that Homer protein family is not causally involved in the development of dyskinetic movements induced by levodopa treatment in this animal model of parkinsonism.

Synergistic roles of GABAA receptors and SK channels in regulating thalamocortical oscillations

Rhythmic oscillations throughout the cortex are observed during physiological and pathological states of the brain. The thalamus generates sleep spindle oscillations and spike-wave discharges characteristic of absence epilepsy. Much has been learned regarding the mechanisms underlying these oscillations from in vitro brain slice preparations. One widely used model to understand the epileptiform oscillations underlying absence epilepsy involves application of bicuculline methiodide (BMI) to brain slices containing the thalamus. BMI is a well-known GABAA receptor blocker that has previously been discovered to also block small-conductance, calcium-activated potassium (SK) channels. Here we report that the robust epileptiform oscillations observed during BMI application rely synergistically on both GABAA receptor and SK channel antagonism. Neither application of picrotoxin, a selective GABAA receptor antagonist, nor application of apamin, a selective SK channel antagonist, alone yielded the highly synchronized, long-lasting oscillations comparable to those observed during BMI application. However, partial blockade of SK channels by subnanomolar concentrations of apamin combined with picrotoxin sufficiently replicated BMI oscillations. We found that, at the cellular level, apamin enhanced the intrinsic excitability of reticular nucleus (RT) neurons but had no effect on relay neurons. This work suggests that regulation of RT excitability by SK channels can influence the excitability of thalamocortical networks and may illuminate possible pharmacological treatments for absence epilepsy. Finally, our results suggest that changes in the intrinsic properties of individual neurons and changes at the circuit level can robustly modulate these oscillations.

Direct enhancement of presynaptic calcium influx in presynaptic facilitation at Aplysia sensorimotor synapses

Regulation of synaptic transmission by modulation of the calcium influx that triggers transmitter release underlies different forms of synaptic plasticity, and thus could contribute to learning. In the mollusk Aplysia, the neuromodulator serotonin (5-HT) increases evoked transmitter release from sensory neurons and thereby contributes to dishabituation and sensitization of defensive reflexes. We combined electrophysiological recording with fluorescence measurements of intracellular calcium in sensory neuron synapses in culture to test whether direct up-modulation by 5-HT of calcium influx triggered by single action potentials contributes to facilitation of transmitter release. We observe increases in a previously undescribed calcium influx that are strongly correlated with increases in the amplitude of the evoked postsynaptic potentials and which cannot be accounted for by action potential prolongation. Our results suggest that direct modulation of a presynaptic calcium conductance that controls neurotransmitter release contributes to the presynaptic facilitation that underlies a simple form of learning.

Enhanced sensitivity to group II mGlu receptor activation at corticostriatal synapses in mice lacking the familial parkinsonism-linked genes PINK1 or Parkin

An altered glutamatergic input at corticostriatal synapses has been shown in experimental models of Parkinson's disease (PD). In the present work, we analyzed the membrane and synaptic responses of striatal neurons to metabotropic glutamate (mGlu) receptor activation in two different mouse models of inherited PD, linked to mutations in PINK1 or Parkin genes. Both in PINK1 and Parkin knockout ((-/-)) mice, activation of group I mGlu receptors by 3,5-DHPG caused a membrane depolarization coupled to an increase in firing frequency in striatal cholinergic interneurons that was comparable to the response observed in the respective wild-type (WT) interneurons. The sensitivity to group II and III mGlu receptors was tested on cortically-evoked excitatory postsynaptic potentials (EPSPs) recorded from medium spiny neurons (MSNs). Both LY379268 and L-AP4, agonists for group II and III, respectively, had no effect on intrinsic membrane properties, but dose-dependently reduced the amplitude of corticostriatal EPSPs. However, both in PINK1(-/-) and Parkin(-/-) mice, LY379268, but not L-AP4, exhibited a greater potency as compared to WT in depressing EPSP amplitude. Accordingly, the dose-response curve for the response to LY379268 in both knockout mice was shifted leftward. Moreover, consistent with a presynaptic site of action, both LY379268 and L-AP4 increased the paired-pulse ratio either in PINK1(-/-) and Parkin(-/-) or in WT mice. Acute pretreatment with L-dopa did not rescue the enhanced sensitivity to LY379268. Together, these results suggest that the selective increase in sensitivity of striatal group II mGlu receptors represents an adaptive change in mice in which an altered dopamine metabolism has been documented.

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlus) are a family of G-protein-coupled receptors activated by the neurotransmitter glutamate. Molecular cloning has revealed eight different subtypes (mGlu1-8) with distinct molecular and pharmacological properties. Multiplicity in this receptor family is further generated through alternative splicing. mGlus activate a multitude of signalling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of neurotransmitter release. In this review, we summarize anatomical findings (from our work and that of other laboratories) describing their distribution in the central nervous system. Recent evidence regarding the localization of these receptors in peripheral tissues will also be examined. The distinct regional, cellular and subcellular distribution of mGlus in the brain will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.

Altered seizure susceptibility in mice lacking the Ca(v)2.3 E-type Ca2+ channel

PURPOSE

Recently the Ca(v)2.3 (E/R-type) voltage-gated calcium channel (VGCC) has turned out to be not only a potential target for different antiepileptic drugs (e.g., lamotrigine, topiramate) but also a crucial component in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and epileptiform activity in CA1 neurons. The aim of our study was to perform an electroencephalographic analysis, seizure-susceptibility testing, and histomorphologic characterization of Ca(v)2.3-/- mice to unravel the functional relevance of Ca(v)2.3 in ictogenesis.

METHODS

Generalized and brain-specific Ca(v)2.3 knockout animals were analyzed for spontaneous epileptiform discharges by using both electrocorticographic and deep intracerebral recordings. In addition, convulsive seizure activity was induced by systemic administration of either 4-aminopyridine (4-AP; 10 mg/kg, i.p.) or pentylenetetrazol (PTZ; 80 mg/kg, s.c.) to reveal possible alterations in seizure susceptibility. Besides histomorphologic analysis, expression studies of other voltage-gated Ca2+ channels in Ca(v)2.3-/- brains were carried out by using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

Both electrocorticographic and deep intrahippocampal recordings exhibited no spontaneous epileptiform discharges indicative of convulsive or nonconvulsive seizure activity during long-term observation. Gross histology and expression levels of other voltage-gated Ca2+ channels remained unchanged in various brain regions. Surprisingly, PTZ-induced seizure susceptibility was dramatically reduced in Ca(v)2.3-deficient mice, whereas 4-AP sensitivity remained unchanged.

CONCLUSIONS

Ca(v)2.3 ablation results in seizure resistance, strongly supporting recent findings in CA1 neurons that Ca(v)2.3 triggers epileptiform activity in specialized neurons via plateau potentials and afterdepolarizations. We provide novel insight into the functional involvement of Ca(v)2.3 in ictogenesis and seizure susceptibility on the whole-animal level.

Striatal metabotropic glutamate receptors as a target for pharmacotherapy in Parkinson’s disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by the loss of dopamine (DA)-containing neurons in the substantia nigra pars compacta (SNc). The symptoms are resting tremor, slowness of movement, rigidity and postural instability. Evidence that an imbalance between dopaminergic and cholinergic transmission takes place within the striatum led to the utilization of DA precursors, DA receptor agonists and anticholinergic drugs in the symptomatic therapy of PD. However, upon disease progression the therapy becomes less effective and debilitating effects such as dyskinesias and motor fluctuations appear. Hence, the need for the development of alternative therapeutic strategies has emerged. Several observations in different experimental models of PD suggest that blockade of excitatory amino acid transmission exerts antiparkinsonian effects. In particular, recent studies have focused on metabotropic glutamate receptors (mGluRs). Drugs acting on group I and II mGluRs have indeed been proven useful in ameliorating the parkinsonian symptoms in animal models of PD and therefore might represent promising therapeutic targets. This beneficial effect could be due to the reduction of both glutamatergic and cholinergic transmission. A novel target for drugs acting on mGluRs in PD therapy might be represented by striatal cholinergic interneurons. Indeed, the activation of mGluR2, highly expressed on this cell type, is able to reduce calcium-dependent plateau potentials by interfering with somato-dendritic N-type calcium channel activity, in turn reducing ACh release in the striatum. Similarly, the blockade of both group I mGluR subtypes reduces cholinergic interneuron excitability, and decreases striatal ACh release. Thus, targeting mGluRs located onto cholinergic interneurons might result in a beneficial pharmacological effect in the parkinsonian state.

An influence of ligands of metabotropic glutamate receptor subtypes on parkinsonian-like symptoms and the striatopallidal pathway in rats

Several data indicate that inhibition of glutamatergic transmission may be important to alleviate of parkinsonian symptoms. Therefore, the aim of the present paper is to review recent studies on the search for putative antiparkinsonian-like effects of mGluR ligands and their brain targets. In order to inhibit glutamatergic transmission, the group I mGluRs (mGluR1 and mGluR5) were blocked, and group II (mGluR2/3) or III (mGluR4/7/8) mGluRs were activated. Systemic or intrastriatal administration of group I mGluR antagonists (mGluR5 - MPEP, MTEP; mGluR1 - AIDA) was found to inhibit parkinsonian-like symptoms (catalepsy, muscle rigidity) in rats. MPEP administered systemically and mGluR1 antagonists (AIDA, CPCCOEt, LY367385) injected intrastriatally reversed also the haloperidol-increased proenkephalin (PENK) mRNA expression in the striatopallidal pathway. Similarly, ACPT-1, a group III mGluR agonist, administered into the striatum, globus pallidus or substantia nigra inhibited the catalepsy. Intrastriatal injection of this compound reduced the striatal PENK expression induced by haloperidol. In contrast, a group II mGluR agonist (2R,4R-APDC) administered intrastriatally reduced neither PENK expression nor the above-mentioned parkinsonian-like symptoms. Moreover, a mixed mGluR8 agonist/AMPA antagonist, (R,S)-3,4-DCPG, administered systemically evoked catalepsy and enhanced both the catalepsy and PENK expression induced by haloperidol. The results reviewed in this article seem to indicate that group I mGluR antagonists or some agonists of group III may possess antiparkinsonian properties, and point at the striatopallidal pathway as a potential target of therapeutic intervention.

Group-II metabotropic glutamate receptors negatively modulate NMDA transmission at striatal cholinergic terminals: Role of P/Q-type high voltage activated Ca++ channels and endogenous dopamine

Striatal cholinergic nerve terminals express functional group-II metabotropic (mGlu) and NMDA glutamate receptors. To investigate whether these receptors interact to regulate ACh release, LY354740 (a group-II mGlu receptor agonist) and NMDA were co-applied in striatal synaptosomes and slices. LY354740 prevented the NMDA-evoked [3H]-choline release from synaptosomes and ACh release from slices. In synaptosomes, this modulation was prevented by omega-agatoxin IVA, suggesting that it was mediated by P/Q-type high voltage activated Ca++ channels. In slices, LY341495 (a group-II mGlu receptor antagonist) enhanced the NMDA-induced ACh release, suggesting that group-II mGlu receptor activation by endogenous glutamate inhibits NMDA transmission. Co-immunoprecipitation studies excluded direct group-II mGlu-NMDA receptor interactions. Finally, group-II mGlu negative modulation of NMDA transmission was abolished in dopamine-depleted synaptosomes and slices, suggesting that it relied on endogenous dopamine. We conclude that group-II mGlu receptors attenuate NMDA inputs at striatal cholinergic terminals via Ca++ channel modulation and dopamine-sensitive pathways.

Metabotropic glutamate receptor 2 modulates excitatory synaptic transmission in the rat globus pallidus

While group II metabotropic glutamate receptors (mGluRs) are known to be expressed in the rat globus pallidus (GP), their functions remain poorly understood. We used standard patch clamping technique in GP slices to determine the effect of group II mGluR activation on excitatory transmission in this region. Activation of group II mGluRs with the group-selective agonist DCG-IV or APDC reduced the amplitude of the evoked excitatory postsynaptic currents (EPSCs) and significantly increased the paired pulse ratio suggesting a presynaptic site of action. This was further supported by double-labeling electron microscopy data showing that group II mGluRs (mGluR2 and 3) immunoreactivity is localized in glutamatergic pre-terminal axons and terminals in the GP. Furthermore, we found that LY 487379, an mGluR2-specific allosteric modulator, significantly potentiated the inhibitory effect of DCG-IV on the excitatory transmission in the GP. Co-incubation with 30 microM LY 487379 increased the potency of DCG-IV about 10-fold in the GP. We were thus able to pharmacologically isolate the mGluR2-mediated function in the rat GP using an mGluR2-specific allosteric modulator. Therefore, our findings do not only shed light on the functions of group II mGluRs in the GP, they also illustrate the therapeutic potential of mGluR-targeting allosteric modulators in neurological disorders such as Parkinson's disease.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

Metabotropic glutamate receptors in the basal ganglia motor circuit

In recent years there have been tremendous advances in our understanding of the circuitry of the basal ganglia and our ability to predict the behavioural effects of specific cellular changes in this circuit on voluntary movement. These advances, combined with a new understanding of the rich distribution and diverse physiological roles of metabotropic glutamate receptors in the basal ganglia, indicate that these receptors might have a key role in motor control and raise the exciting possibility that they might provide therapeutic targets for the treatment of Parkinson's disease and related disorders.

Thalamic regulation of striatal acetylcholine efflux is both direct and indirect and qualitatively altered in the dopamine-depleted striatum

Striatal cholinergic interneurons play a pivotal role in the integrative sensorimotor functions of the basal ganglia. The major excitatory input to these interneurons arises from glutamatergic neurons of the parafascicular nucleus of the thalamus (Pf). Thalamic regulation of cholinergic interneurons, however, may also include an indirect inhibitory component mediated by the axon collaterals of GABAergic medium spiny neurons that are also innervated by Pf. The present study examined thalamic regulation of striatal cholinergic interneurons by employing dual probe in vivo microdialysis in freely moving animals to determine the effect of pharmacological manipulation of Pf on acetylcholine (ACh) efflux in intact and dopamine-lesioned striata. In intact animals, reverse dialysis application of the GABA(A) antagonist bicuculline (50 microM) into Pf, likely disinhibiting Pf neurons, significantly decreased striatal ACh efflux. When striatal GABA(A) receptors were blocked by simultaneous reverse dialysis application of bicuculline (10 microM), however, the same manipulation significantly increased ACh efflux. Qualitatively similar results were obtained in experiments employing a higher concentration of bicuculline (200 microM). Application of the GABA agonist muscimol (500 microM) into Pf, likely inhibiting Pf neurons, decreased ACh efflux only when the experiment was conducted under blockade of striatal GABA(A) receptors. These data are consistent with the existence of an indirect, inhibitory, GABA(A) receptor-mediated component of ACh regulation that is most clearly manifested when Pf is disinhibited and with the existence of a direct excitatory component of ACh regulation, evident when Pf is inhibited. Manipulation of Pf using very high concentrations of drug (500 microM bicuculline, 2 mM muscimol), however, yielded data consistent only with direct excitatory thalamic regulation. In contrast to results obtained in intact animals, in animals with prior (3 weeks) unilateral lesion of the dopaminergic nigrostriatal pathway, bicuculline application (50 muM) in Pf significantly increased striatal ACh efflux, irrespective of simultaneous blockade of striatal GABA(A) receptors. The results of experiments in which muscimol (500 microM) was applied in Pf were similar to those obtained in intact animals, however. Baseline ACh efflux was not significantly elevated in dopamine-lesioned animals. These results indicate a qualitative alteration in the effectiveness of an inhibitory component of the thalamic regulation of ACh efflux in the dopamine depleted striatum, evident during increased thalamostriatal input. Such altered regulation of striatal ACh output is likely to have profound consequences for integrative function in the parkinsonian basal ganglia.

Suppression of potassium channels elicits calcium-dependent plateau potentials in suprachiasmatic neurons of the rat

By using whole-cell recordings in acute and organotypic hypothalamic slices, we found that following K+ channel blockade, sustained plateau potentials can be elicited by current injection in suprachiasmatic neurons. In an attempt to determine the ionic basis of these potentials, ion-substitution experiments were carried out. It appeared that to generate plateau potentials, calcium influx was required. Plateau potentials were also present when extracellular calcium was replaced by barium, but were independent upon an increase in the intracellular free calcium concentration. Substitution of extracellular sodium by the impermeant cation N-methyl-D-glucamine indicated that sodium influx could also contribute to plateau potentials. To gain some information on the pharmacological profile of the Ca++ channels responsible for plateau potentials, selective blocker of various types of Ca++ channel were tested. Plateau potentials were unaffected by isradipine, an L-type Ca++ channel blocker. However, they were slightly reduced by omega-conotoxin GVIA and omega-agatoxin TK, blockers of N-type and P/Q-type Ca++ channels, respectively. These data suggest that R-type Ca++ channels probably play a major role in the genesis of plateau potentials. We speculate that neurotransmitters/neuromodulators capable of reducing or suppressing potassium conductance(s) may elicit a Ca++-dependent plateau potential in suprachiasmatic neurons, thus promoting sustained firing activity and neuropeptide release.

Modulatory action of metabotropic glutamate receptor (mGluR) 5 on mGluR1 function in striatal cholinergic interneurons

Within basal ganglia, group I metabotropic glutamate receptor subtypes (mGluR1 and 5) frequently co-localize in the same neuron. However, little is known about how these receptors functionally interact. We addressed this issue by means of electrophysiological recordings of striatal cholinergic interneurons, a neuronal subtype that co-express both group I mGluRs. The group I non-selective agonist 3,5-DHPG induced a membrane depolarization/inward current that was prevented by co-application of LY 367385, a selective mGluR1 antagonist, and SIB 1757 or MPEP, blockers of mGluR5 subtype. The reversal potential for the response to 3,5-DHPG was close to the equilibrium potential for potassium channels. Repeated bath or focal applications of 3,5-DHPG induced a progressive decline in the amplitude of the membrane depolarization, suggesting that group I mGluRs undergo receptor desensitization. Interestingly, in the presence of the mGluR5 blocker, SIB 1757, this event was not observed, whereas it occurred in LY 367385. PKC blockers chelerythrine and calphostin C mimicked the inhibitory effect of SIB 1757. In a subset of interneurons, in MPEP or SIB 1757, 3,5-DHPG induced a 0.5-1 Hz oscillatory response, that was prevented by L-type Ca2+ channel blockers, and by the tyrosine kinase inhibitors genistein and lavendustin. Together, these data suggest that mGluR5 modulates mGluR1 activity to shape cell excitability.

Coordinate high-frequency pattern of stimulation and calcium levels control the induction of LTP in striatal cholinergic interneurons

Current evidence appoints a central role to cholinergic interneurons in modulating striatal function. Recently, a long-term potentiation (LTP) of synaptic transmission has been reported to occur in these neurons. The relationship between the pattern of cortico/thalamostriatal fibers stimulation, the consequent changes in the intracellular calcium concentration ([Ca2+]i), and the induction of synaptic plasticity was investigated in striatal cholinergic interneurons from a rat corticostriatal slice preparation by means of combined electrophysiological intracellular recordings and microfluorometric techniques. Different protocols of stimulation were considered, varying both the frequency and the duration of the train of stimuli. High-frequency stimulation (HFS) (three trains at 100 Hz for 3 sec, 20-sec interval) induced a rise in [Ca2+]i, exceeding by fivefold the resting level, and caused a LTP of synaptic transmission. Tetanic stimulation delivered at lower frequencies (5-30 Hz) failed to induce long-term changes of synaptic efficacy. The observed elevation in [Ca2+]i during HFS was primarily mediated by L-type high-voltage activated (HVA)-Ca2+ channels, as it was fully prevented by nifedipine. Conversely, blockade of NMDA and AMPA glutamate receptor did not affect either LTP or the magnitude of the [Ca2+]i rise. Interestingly, the pharmacological analysis of the post-tetanic depolarizing postsynaptic potential (DPSP) revealed that LTP was attributable, to a large extent, to the potentiation of the GABA(A)-mediated component. In conclusion, the expression of LTP in striatal cholinergic interneurons is a selective response to a precise stimulation pattern of induction requiring a critical rise in [Ca2+]i.

Immunolocalization of CB1 receptor in rat striatal neurons: a confocal microscopy study

Several lines of evidence indicate that cannabinoids, among other functions, are involved in motor control. Although cannabinoid receptors (CB(1)) mRNA has been observed in medium-sized spiny neurons of the striatum, a description of the precise localization of CB(1) at a protein level among striatal cells is still lacking. Therefore, we performed immunohistochemical studies with light and confocal microscopy to identify neuronal subpopulations that express CB(1) and to assess the distribution of the receptor within these neurons. In our single label light microscopy study, CB(1) was observed in most medium-sized neurons of the caudate-putamen. However, CB(1) was also present in large-sized neurons scattered throughout the striatum. Our dual-label study showed that 89.3% of projection neurons in matrix contain CB(1), and that 56.4% of projection neurons in patch are labeled for CB(1). To investigate the presence of CB(1) among the different subclasses of striatal interneurons we performed a double-labeling study matching CB(1) and each of the striatal interneuron markers, namely, choline acetyl-transferase, parvalbumin, calretinin, and nitric oxide synthase. Our double-label study showed that most parvalbumin immunoreactive interneurons (86.5%), more than one-third (39.2%) of cholinergic interneurons, and about one-third (30.4%) of the NOS-positive neurons are labeled for CB(1). Calretinin-immunolabeled neurons were devoid of CB(1).

The corticostriatal input to giant aspiny interneurons in the rat: a candidate pathway for synchronising the response to reward-related cues

Tonically active neurons (TANs) in the mammalian striatum show a pause in their ongoing firing activity in response to an auditory cue that is paired with a reward. This response to reward-related cues develops through learning and becomes expressed synchronously by TANs located throughout the striatum. The pause response is abolished by inactivating the thalamic inputs to the striatum but a short-latency excitatory response to reward-related cues remains, which may originate in the cortex. We investigated the cortical inputs to striatal neurons to determine the electrophysiological properties of their cortical projections. We made in vivo intracellular recordings from 14 giant aspiny interneurons (which correspond to the TANs) and from a control group of spiny projection neurons (n=18) in urethane-anaesthetised rats. All giant aspiny interneurons were tonically active (firing rate: 3.0+/-1.5 Hz) and displayed small-amplitude subthreshold fluctuations in membrane potential. These fluctuations in membrane potential were correlated with the cortical electroencephalogram (EEG). Test stimulation of the contralateral cortex induced postsynaptic potentials (PSPs) in giant aspiny interneurons. These PSPs were significantly shorter in latency (5.1+/-1.6 ms) than those measured in spiny projection neurons (9.3+/-2.8 ms; p<0.01), whereas the latencies of ipsilaterally evoked PSPs did not differ. Taken together, these observations suggest that giant aspiny interneurons are under the significant influence of spontaneous excitatory inputs and receive specialised input from either faster conducting or less branching cortical fibres than spiny projection neurons. These inputs may be involved in the synchronised convergence of reward-related cues from spatially distinct cortical areas onto giant aspiny interneurons.

Proton magnetic resonance spectroscopic imaging of the brain in childhood autism

BACKGROUND

Autism is a developmental disorder of unknown neurologic basis. Based on prior work, we used proton magnetic resonance spectroscopic imaging ((1)H- MRSI) to investigate brain structures, including cingulate and caudate, that we hypothesized would reveal metabolic abnormalities in subjects with autism.

METHODS

In 22 children with autism, 5 to 16 years old, and 20 age-matched healthy control subjects, (1)H-MRSI assessed levels of N-acetyl compounds (NAA), choline compounds (Cho), and creatine plus phosphocreatine (Cr) at 272 msec echo-time and 1.5 T.

RESULTS

In subjects with autism compared with control subjects, Cho was 27.2% lower in left inferior anterior cingulate and 19.1% higher in the head of the right caudate nucleus; Cr was 21.1% higher in the head of the right caudate nucleus, but lower in the body of the left caudate nucleus (17.9%) and right occipital cortex (16.6%).

CONCLUSIONS

Results are consistent with altered membrane metabolism, altered energetic metabolism, or both in the left anterior cingulate gyrus, both caudate nuclei, and right occipital cortex in subjects with autism compared with control subjects.

Protective role of group-II metabotropic glutamate receptors against nigro-striatal degeneration induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

To examine how mGlu2/3 metabotropic glutamate receptors affect nigro-striatal degeneration, we used the agonist, LY379268, and the antagonist, LY341495, in mice challenged with the nigro-striatal toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In control mice, high doses of MPTP (20 mg/kg, i.p., injected four times with 2 h of interval) induced a nearly total degeneration of the nigro-striatal pathway, as shown by measurements of striatal dopamine (DA) levels and by immunohistochemical analysis of tyrosine hydroxylase, high affinity dopamine transporter, and glial fibrillary acidic protein in the corpus striatum and substantia nigra. Lower cumulative doses of MPTP (30 mg/kg, i.p., injected only once) produced a partial lesion of the nigro-striatal pathway (about 50% reduction of striatal DA content). Systemic injection of LY379268 (1 mg/kg, i.p., 30 min prior to each injection of MPTP) partially reduced the extent of nigro-striatal degeneration induced by high doses of MPTP. Similar results were obtained by continuously delivering LY379268 (1 mg/kg/d for 7 d) by means of a subcutaneous osmotic minipump. The protective effect of LY379268 was antagonized by LY341495 (also delivered by the osmotic minipump). In mice challenged with the lower cumulative dose of MPTP, injection of LY379268 did not produce a significant neuroprotective effect. In contrast, the lesion was amplified by the antagonist, LY341495. Neither LY379268 nor LY341495 influenced the central bioavailability and the local half-life of MPTP, as shown by measurements of the toxin and its active metabolite, MPP(+), in the striatum. We conclude that mGlu2/3 receptors play a protective role against MPTP toxicity, and that the efficacy of the agonist, LY379268, critically depends on the extent of the nigro-striatal lesion.

Seizure, neurotransmitter release, and gene expression are closely related in the striatum of 4-aminopyridine-treated rats

The present experiments aimed to compare the length of seizure activity with the time-related increase of transmitter release and the induction of c-fos gene expression in the striatum of the rat. Anesthetized Wistar rats were intraperitoneally treated with 7 mg/kg 4-aminopyridine, and the transmitter levels in the striatum were measured by means of in vivo microdialysis, 30, 60, 90, 120, and 150 min following the treatment. Striatal and neocortical electric activity was monitored with depth and surface electrodes, respectively. The expression level of the c-fos gene was estimated by counting the striatal c-fos-immunostained cell nuclei at the time intervals of the microdialysis. 4-aminopyridine elicited high-frequency seizure discharges in the EEG and significantly increased glutamate, aspartate, GABA, serotonin, noradrenaline, and dopamine levels in the extracellular dialysates. The number of c-fos-stained cell nuclei in the striatum displayed a prolonged increase, showing significantly elevated numbers throughout the experiment. The increase of c-fos expression in time correlated best with the increase of glutamate release, which was also significantly elevated at every sampling time. The GABA release, culminating at 60 min after the seizure onset, correlated best with the cessation of the electrographic seizure. Aspartate, norepinephrine, serotonin, and dopamine displayed transient but significant elevations. We conclude that glutamate plays the essential role (most probably through ionotropic and metabotropic receptors) in the extracellular signaling, which eventually leads to intracellular cascades and c-fos gene expression in the striatum during convulsions.

Plasticity of glutamatergic control of striatal acetylcholine release in experimental parkinsonism: opposite changes at group-II metabotropic and NMDA receptors

To investigate whether adaptive changes of glutamatergic transmission underlie dysfunction of the cholinergic system in experimental parkinsonism, the effects of group-II metabotropic glutamate and NMDA receptor ligands on acetylcholine release was studied in striatal slices and synaptosomes obtained from naive rats, 6-hydroxydopamine hemi-lesioned rats and 6-hydroxydopamine hemi-lesioned rats chronically treated with levodopa (L-DOPA) plus benserazide (non-dyskinetic). Group-II metabotropic glutamate receptor agonists LY354740, DCG-IV and L-CCG-I inhibited the electrically-evoked endogenous acetylcholine release from slices, while NMDA facilitated it. LY354740 also inhibited K+-evoked acetylcholine release from synaptosomes. LY354740-induced inhibition was prevented by the group-II metabotropic glutamate receptor antagonist LY341495. In hemi-parkinsonian rats, sensitivity towards LY354740 was reduced while that to NMDA was enhanced in the lesioned (denervated) compared with unlesioned striatum. Moreover, dizocilpine inhibited acetylcholine release in the lesioned compared with unlesioned striatum. Chronic treatment with L-DOPA normalized sensitivity towards glutamatergic agonists. We conclude that striatal dopamine denervation results in plastic changes at group-II metabotropic glutamate and NMDA receptors that may shift glutamatergic control of acetylcholine release towards facilitation. From a clinical perspective, L-DOPA and NMDA antagonists appear effective in counteracting overactivity of striatal cholinergic interneurones associated with Parkinson's disease.

Behavioural learning-induced increase in spontaneous GABAA-dependent synaptic activity in rat striatal cholinergic interneurons

Cholinergic striatal interneurons play a crucial role in cognitive aspects of context-dependent motor behaviours. They are considered to correspond to the tonically active neurons (TANs) of the primate striatum, which phasically decrease their discharge at the presentation of reward-related sensory stimuli. The origin of this response is still poorly understood. Therefore, in the present paper, we have investigated whether synaptic changes establish in cholinergic interneurons from young rats that have learned a rewarded, externally cued sensorimotor task. Corticostriatal slices were prepared from both control and trained rats. No significant change in intrinsic membrane properties and evoked synaptic activity was observed in cholinergic interneurons, nor the responsiveness to exogenously applied dopaminergic and glutamatergic agonists was modified. Conversely, an increased occurrence of spontaneous bicuculline-sensitive depolarizing postsynaptic potentials (sDPSP) was recorded. The frequency of the GABAA-mediated sDPSP was increased in comparison to not-conditioned rats. Overall, these results suggest that after learning a rewarded sensorimotor paradigm an increased GABA influence develops on cholinergic interneurons. The origin of this effect might be searched in collaterals of GABAergic output spiny neurons as well as in GABAergic striatal interneurons impinging onto cholinergic interneurons. This intrastriatal mechanism might be involved in the phasic suppression of discharge of TANs at the presentation of reward-related sensory stimuli.