Up-regulation in expression of vesicular glutamate transporter 3 in substantia nigra but not in striatum of 6-hydroxydopamine-lesioned rats

Fast neurotransmitter identity of MCH neurons: Do contents depend on context?

Hypothalamic melanin-concentrating hormone (MCH) neurons participate in many fundamental neuroendocrine processes. While some of their effects can be attributed to MCH itself, others appear to depend on co-released neurotransmitters. Historically, the subject of fast neurotransmitter co-release from MCH neurons has been contentious, with data to support MCH neurons releasing GABA, glutamate, both, and neither. Rather than assuming a position in that debate, this review considers the evidence for all sides and presents an alternative explanation: neurochemical identity, including classical neurotransmitter content, is subject to change. With an emphasis on the variability of experimental details, we posit that MCH neurons may release GABA and/or glutamate at different points according to environmental and contextual factors. Through the lens of the MCH system, we offer evidence that the field of neuroendocrinology would benefit from a more nuanced and dynamic interpretation of neurotransmitter identity.

Genetic deletion of vesicular glutamate transporter in dopamine neurons increases vulnerability to MPTP-induced neurotoxicity in mice

A subset of midbrain dopamine (DA) neurons express vesicular glutamate transporter 2 (VgluT2), which facilitates synaptic vesicle loading of glutamate. Recent studies indicate that such expression can modulate DA-dependent reward behaviors, but little is known about functional consequences of DA neuron VgluT2 expression in neurodegenerative diseases like Parkinson's disease (PD). Here, we report that selective deletion of VgluT2 in DA neurons in conditional VgluT2-KO (VgluT2-cKO) mice abolished glutamate release from DA neurons, reduced their expression of brain-derived neurotrophic factor (BDNF) and tyrosine receptor kinase B (TrkB), and exacerbated the pathological effects of exposure to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Furthermore, viral rescue of VgluT2 expression in DA neurons of VglutT2-cKO mice restored BDNF/TrkB expression and attenuated MPTP-induced DA neuron loss and locomotor impairment. Together, these findings indicate that VgluT2 expression in DA neurons is neuroprotective. Genetic or environmental factors causing reduced expression or function of VgluT2 in DA neurons may place some individuals at increased risk for DA neuron degeneration. Therefore, maintaining physiological expression and function of VgluT2 in DA neurons may represent a valid molecular target for the development of preventive therapeutic interventions for PD.

High-frequency stimulation of the subthalamic nucleus modifies the expression of vesicular glutamate transporters in basal ganglia in a rat model of Parkinson's disease

Background

It has been suggested that glutamatergic system hyperactivity may be related to the pathogenesis of Parkinson’s disease (PD). Vesicular glutamate transporters (VGLUT1-3) import glutamate into synaptic vesicles and are key anatomical and functional markers of glutamatergic excitatory transmission. Both VGLUT1 and VGLUT2 have been identified as definitive markers of glutamatergic neurons, but VGLUT 3 is also expressed by non glutamatergic neurons. VGLUT1 and VGLUT2 are thought to be expressed in a complementary manner in the cortex and the thalamus (VL/VM), in glutamatergic neurons involved in different physiological functions. Chronic high-frequency stimulation (HFS) of the subthalamic nucleus (STN) is the neurosurgical therapy of choice for the management of motor deficits in patients with advanced PD. STN-HFS is highly effective, but its mechanisms of action remain unclear. This study examines the effect of STN-HFS on VGLUT1-3 expression in different brain nuclei involved in motor circuits, namely the basal ganglia (BG) network, in normal and 6-hydroxydopamine (6-OHDA) lesioned rats.

Results

Here we report that: 1) Dopamine(DA)-depletion did not affect VGLUT1 and VGLUT3 expression but significantly decreased that of VGLUT2 in almost all BG structures studied; 2) STN-HFS did not change VGLUT1-3 expression in the different brain areas of normal rats while, on the contrary, it systematically induced a significant increase of their expression in DA-depleted rats and 3) STN-HFS reversed the decrease in VGLUT2 expression induced by the DA-depletion.

Conclusions

These results show for the first time a comparative analysis of changes of expression for the three VGLUTs induced by STN-HFS in the BG network of normal and hemiparkinsonian rats. They provide evidence for the involvement of VGLUT2 in the modulation of BG cicuits and in particular that of thalamostriatal and thalamocortical pathways suggesting their key role in its therapeutic effects for alleviating PD motor symptoms.

Multisite intracerebral microdialysis to study the mechanism of L-DOPA induced dopamine and serotonin release in the parkinsonian brain

L-DOPA is currently one of the best medications for Parkinson's disease. It was assumed for several years that its benefits and side effects were related to the enhancement of dopamine release in the dopamine-depleted striatum. The use of intracerebral microdialysis combined with a pharmacological approach has led to the discovery that serotonergic neurons are responsible for dopamine release induced by L-DOPA. The subsequent use of multisite microdialysis has further revealed that L-DOPA-stimulated dopamine release is widespread and related to the serotonergic innervation. The present Review emphasizes the functional impact of extrastriatal release of dopamine induced by L-DOPA in both the therapeutic and side effects of L-DOPA.

The metabotropic glutamate receptor antagonist 2-methyl-6-(phenylethynyl) pyridine decreases striatal VGlut2 expression in association with an attenuation of L-DOPA-induced dyskinesias

The striatal glutamatergic hyperactivity is considered critical in the development of levodopa-induced dyskinesias (LID) in Parkinson's disease (PD). Pharmacological antagonism of the metabotropic glutamate receptors (mGluRs), in particular, the subtype mGluR5, can inhibit the expression of dyskinesia in both rodent and nonhuman primate models of PD. However, the exact mechanisms underlying the mGluR5 antagonism effects are not completely known. The vesicular glutamate transporters (VGluts) are localized in the synaptic vesicles of the striatal glutamatergic axonal terminals. The effects of mGluR5 antagonism modulating VGlut1 and VGlut2, as selective markers for the corticostriatal and thalamostriatal pathways, respectively, are still unknown. We investigated the effects of the mGluR5 antagonist, 2-methyl-6-(phenylethynyl) pyridine (MPEP) on the striatal expression of VGlut1 and VGlut2 in levodopa-treated hemiparkinsonian rats. Male Sprague-Dawley rats received a unilateral 6-hydroxydopamine (6-OHDA) administration in the nigrostriatal pathway. Rats were treated with: (a) levodopa (12 mg/kg/day with benserazide 15 mg/kg, ip) + vehicle; (b) MPEP (1.5 mg/kg/day, ip) + vehicle; (c) levodopa + MPEP, or (d) saline for 10 days. Levodopa treatment induced dyskinesias and did not modify the striatal expression of either VGlut1 or VGlut2. The administration of MPEP significantly attenuated LID and decreased the levels of VGlut2, but not the VGlut1, in the striatum ipsilateral to the lesion (P < 0.05). Our results suggest that the effects of MPEP on LID might be mediated by a modulating effect on VGlut 2 expression.

A sensitizing D-amphetamine dose regimen induces long-lasting spinophilin and VGLUT1 protein upregulation in the rat diencephalon

Numerous studies in this lab and others have reported psychostimulant-induced alterations in both synaptic protein expression and synaptic density in striatum and prefrontal cortex. Recently we have shown that chronic D-amphetamine (D-AMPH) administration in rats increased synaptic protein expression in striatum and limbic brain regions including hippocampus, amygdala, septum, and paraventricular nucleus of the thalamus (PVT). Potential synaptic changes in thalamic nuclei are interesting since the thalamus serves as a gateway to cerebral cortex and a nodal point for basal ganglia influences. Therefore we sought to examine drug-induced differences in synaptic protein expression throughout the diencephalon. Rats received an escalating (1-8 mg/kg) dosing regimen of D-AMPH for five weeks and were euthanized 28 days later. Radioimmunocytochemistry (RICC) revealed significant upregulation of both spinophilin and the vesicular glutamate transporter, VGLUT1, in PVT, mediodorsal (MD), and ventromedial (VM) thalamic nuclei as well as in lateral hypothalamus (LH) and habenula. Strong positive correlations were observed between VGLUT1 and spinophilin expression in PVT, medial habenula, MD, VM and LH of D-AMPH-treated rats. No significant D-AMPH effect was seen in sensorimotor cortices for either protein. Additionally, no significant differences in the general vesicular protein synaptophysin were observed for any brain region. These findings add to evidence suggesting that long-lasting stimulant-induced synaptic alterations are widespread but not ubiquitous. Moreover, they suggest that D-AMPH-induced synaptic changes may occur preferentially in excitatory synapses.

Anatomical and functional characterization of neuropil in the gracile fasciculus

A fundamental organizational principle of the central nervous system is that gray matter is the province of neuronal somata, white matter their processes. However, the rat and primate dorsal columns (archetypal spinal "white matter" tracts) are actually of intermediate character, insofar as they contain a surprisingly prominent neuropil of unknown function. Here I report on the morphology, inputs, projections, and functional properties of these neurons. Small fusiform and larger lentiform neurons are most abundant in the gracile fasciculus of the cervical and lumbar enlargements and are absent from the cuneate fasciculus and corticospinal tract. Many have dendrites that run along the dorsal pia, and, although in transverse sections these neurons appear isolated from the gray matter, they are also connected to area X by varicose and sometimes loosely fasciculated dendrites. These neurons receive neurochemically diverse, compartmentalized synaptic inputs (primary afferent, intrinsic and descending), half express the substance P receptor, and some project supraspinally. Unlike substantia gelatinosa neurons, they do not express protein kinase C gamma. Functionally, they have small receptive fields, which are somatotopically appropriate with respect to their anterior-posterior position along the neuraxis. They respond to innocuous and/or noxious mechanical stimulation of the distal extremities, and some are prone to central sensitization or "windup." Morphologically, neurochemically, and functionally, therefore, these cells most closely resemble neurons in laminae III-VI in the dorsal horn. The proximity of their dorsal dendrites to the pia mater may also reflect an ability to integrate internal (e.g., changes in cerebrospinal fluid compostition) and external (e.g., somatic) stimuli.

Coexpression of the mu-opioid receptor splice variant MOR1C and the vesicular glutamate transporter 2 (VGLUT2) in rat central nervous system

It has been reported that mu-opioid agonists depress glutamate release in some neurons but the specific receptor subtype mediating this effect is unclear. The purpose of the present study was to examine whether a particular mu-opioid receptor (MOR) splice-variant, MOR(1C), is expressed in rat central nervous system (CNS) by terminals expressing the vesicular glutamate transporter2 (VGLUT2), a marker of glutamatergic neurons. Several MOR splice variants have been identified in mice and MOR(1C) appears mainly to be localized to fibers and terminals, from which most neurotransmitter release would be expected. In addition, VGLUT2 has been found in the CNS and antibodies to it are reliable markers for glutamatergic terminals. Using fluorescence immunohistochemistry and confocal microscopy to examine spatial relationships between MOR(1C) and VGLUT2, we found that MOR(1C) and VGLUT2 puncta were widely distributed throughout the rat CNS; moreover, many regions contained terminals that expressed both. Thus, it appears that coexpression of MOR(1C) and VGLUT2 is common in the rat CNS. We hypothesize that activation of MOR(1C) by mu-opioid agonists at some glutamatergic terminals may be a mechanism by which glutamate release is inhibited.

Synaptic vesicle protein trafficking at the glutamate synapse

Expression of the integral and associated proteins of synaptic vesicles is subject to regulation over time, by region, and in response to activity. The process by which changes in protein levels and isoforms result in different properties of neurotransmitter release involves protein trafficking to the synaptic vesicle. How newly synthesized proteins are incorporated into synaptic vesicles at the presynaptic bouton is poorly understood. During synaptogenesis, synaptic vesicle proteins sort through the secretory pathway and are transported down the axon in precursor vesicles that undergo maturation to form synaptic vesicles. Changes in protein content of synaptic vesicles could involve the formation of new vesicles that either mix with the previous complement of vesicles or replace them, presumably by their degradation or inactivation. Alternatively, new proteins could individually incorporate into existing synaptic vesicles, changing their functional properties. Glutamatergic vesicles likely express many of the same integral membrane proteins and share certain common mechanisms of biogenesis, recycling, and degradation with other synaptic vesicles. However, glutamatergic vesicles are defined by their ability to package glutamate for release, a property conferred by the expression of a vesicular glutamate transporter (VGLUT). VGLUTs are subject to regional, developmental, and activity-dependent changes in expression. In addition, VGLUT isoforms differ in their trafficking, which may target them to different pathways during biogenesis or after recycling, which may in turn sort them to different vesicle pools. Emerging data indicate that differences in the association of VGLUTs and other synaptic vesicle proteins with endocytic adaptors may influence their trafficking. These observations indicate that independent regulation of synaptic vesicle protein trafficking has the potential to influence synaptic vesicle protein composition, the maintenance of synaptic vesicle pools, and the release of glutamate in response to changing physiological requirements.