Recent data indicate that 'classical' neurotransmitters can also act as co-transmitters. This notion has been strengthened by the demonstration that three vesicular glutamate transporters (vesicular glutamate transporter 1 (VGLUT1), VGLUT2 and VGLUT3) are present in central monoamine, acetylcholine and GABA neurons, as well as in primarily glutamatergic neurons. Thus, intriguing questions are raised about the morphological and functional organization of neuronal systems endowed with such a dual signalling capacity. In addition to glutamate co-release, vesicular synergy - a process leading to enhanced packaging of the 'primary' transmitter - is increasingly recognized as a major property of the glutamatergic co-phenotype. The behavioural relevance of this co-phenotype is presently the focus of considerable interest.

SUMMARY The natural response to itch sensation is to scratch, which relieves the itch through an unknown mechanism. Interaction between pain and itch has been frequently demonstrated, and the selectivity hypothesis of itch, based on data from electrophysiological and behavioral experiments, postulates the existence of primary pain afferents capable of repressing itch. Here, we demonstrate that deletion of vesicular glutamate transporter (VGLUT) 2 in a subpopulation of neurons partly overlapping with the vanilloid receptor (TRPV1) primary afferents resulted in a dramatic increase in itch behavior accompanied by a reduced responsiveness to thermal pain. The increased itch behavior was reduced by administration of antihistaminergic drugs and by genetic deletion of the gastrin-releasing peptide receptor, demonstrating a dependence on VGLUT2 to maintain normal levels of both histaminergic and nonhistaminergic itch. This study establishes that VGLUT2 is a major player in TRPV1 thermal nociception and also serves to regulate a normal itch response.

Glutamatergic excitatory neurotransmission is dependent on glutamate release from presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6 -8, which function as vesicular glutamate transporters (VGLUTs). Here, we show that VGLUT2 (Slc17a6) is required for life ex utero. Vglut2 null mutant mice die immediately after birth because of the absence of respiratory behavior. Investigations at embryonic stages revealed that neural circuits in the location of the pre-Bötzinger (PBC) inspiratory rhythm generator failed to become active. However, neurons with bursting pacemaker properties and anatomical integrity of the PBC area were preserved. Vesicles at asymmetric synapses were fewer and malformed in the Vglut2 null mutant hindbrain, probably causing the complete disruption of AMPA/kainate receptor-mediated synaptic activity in mutant PBC cells. The functional deficit results from an inability of PBC neurons to achieve synchronous activation. In contrast to respiratory rhythm generation, the locomotor central pattern generator of Vglut2 null mutant mice displayed normal rhythmic and coordinated activity, suggesting differences in their operating principles. Hence, the present study identifies VGLUT2-mediated signaling as an obligatory component of the developing respiratory rhythm generator.

The "One neuron-one neurotransmitter" concept has been challenged frequently during the last three decades, and the coexistence of neurotransmitters in individual neurons is now regarded as a common phenomenon. The functional significance of neurotransmitter coexistence is, however, less well understood. Several studies have shown that a subpopulation of dopamine (DA) neurons in the ventral tegmental area (VTA) expresses the vesicular glutamate transporter 2 (VGLUT2) and has been suggested to use glutamate as a cotransmitter. The VTA dopamine neurons project to limbic structures including the nucleus accumbens, and are involved in mediating the motivational and locomotor activating effects of psychostimulants. To determine the functional role of glutamate cotransmission by these neurons, we deleted VGLUT2 in DA neurons by using a conditional gene-targeting approach in mice. A DAT-Cre/Vglut2Lox mouse line (Vglut2 f/f;DAT-Cre mice) was produced and analyzed by in vivo amperometry as well as by several behavioral paradigms. Although basal motor function was normal in the Vglut2 f/f;DAT-Cre mice, their risk-taking behavior was altered. Interestingly, in both home-cage and novel environments, the gene targeted mice showed a greatly blunted locomotor response to the psychostimulant amphetamine, which acts via the midbrain DA system. Our results show that VGLUT2 expression in DA neurons is required for normal emotional reactivity as well as for psychostimulant-mediated behavioral activation.amphetamine | midbrain | neurotransmission | reward | striatum

Cyclin-dependent kinase inhibitors of the Cip͞Kip family play critical roles in regulating cell proliferation during embryogenesis. However, these proteins also influence cell differentiation by mechanisms that have remained unknown. Here we show that p57 Kip2 is expressed in postmitotic differentiating midbrain dopamine cells. Induction of p57 Kip2 expression depends on Nurr1, an orphan nuclear receptor that is essential for dopamine neuron development. Moreover, analyses of p57 Kip2 gene-targeted mice revealed that p57 Kip2 is required for the maturation of midbrain dopamine neuronal cells. Additional experiments in a dopaminergic cell line demonstrated that p57 Kip2 can promote maturation by a mechanism that does not require p57 Kip2 -mediated inhibition of cyclin-dependent kinases. Instead, evidence indicates that p57 Kip2 functions by a direct protein-protein interaction with Nurr1. Thus, in addition to its established function in control of proliferation, these results reveal a mechanism whereby p57 Kip2 influences postmitotic differentiation of dopamine neurons. C ell differentiation and withdrawal from the cell cycle are tightly coordinated processes during development. Thus, mechanisms essential for regulation of the cell cycle also influence processes of cell differentiation (1). An important mechanism for cell cycle control involves inhibition of cyclindependent kinases (CDKs) by CDK inhibitors (Ckis) (2). Members of the Cip͞Kip family of Ckis, consisting of p21 Cip1 , p27 Kip1 , and p57 Kip2, control cell cycle exit and cell differentiation in vivo (1, 3). However, only p57 Kip2 has been shown to play essential roles during embryogenesis for which other Ckis cannot compensate (4-6). With the exception of abnormal maturation of retina amacrine interneurons, central nervous system-related deficiencies have not been reported in p57-null mutant mice (6). Dopamine (DA)-producing cells are generated in the ventral floor of the embryonic midbrain (7,8). These cells are degenerating in Parkinson's disease and are therefore of major clinical interest (9-11). Early signaling by the secreted factors sonic hedgehog and fibroblast growth factor 8 contributes to patterning events and the establishment of a proliferating dopaminergic progenitor cell population expressing aldehyde dehydrogenase 1 (Aldh1a1) (12-14). As dividing progenitor cells stop proliferating, they begin to express the orphan nuclear receptor Nurr1 (NR4A2) (13). Nurr1 lacks a cavity for ligand binding as revealed from the recently solved x-ray crystal structure of the Nurr1 ligand-binding domain and is therefore defined as a ligand-independent member of the nuclear receptor family (15). Nurr1 has been shown to be essential for midbrain DA neuron development because Nurr1 knockout animals lack tyrosine hydroxylase (TH) and other dopaminergic characteristics (16-18). Nurr1 is required for sustained expression of DA cellspecific genes, normal cell migration, target area innervation, and cell survival (13,18,19). Nurr1 overexpression in stem cells h...

Recent studies have proposed that glutamate corelease by mesostriatal dopamine (DA) neurons regulates behavioral activation by psychostimulants. How and when glutamate release by DA neurons might play this role remains unclear. Considering evidence for early expression of the type 2 vesicular glutamate transporter in mesencephalic DA neurons, we hypothesized that this cophenotype is particularly important during development. Using a conditional gene knock-out approach to selectively disrupt the Vglut2 gene in mouse DA neurons, we obtained in vitro and in vivo evidence for reduced growth and survival of mesencephalic DA neurons, associated with a decrease in the density of DA innervation in the nucleus accumbens, reduced activity-dependent DA release, and impaired motor behavior. These findings provide strong evidence for a functional role of the glutamatergic cophenotype in the development of mesencephalic DA neurons, opening new perspectives into the pathophysiology of neurodegenerative disorders involving the mesostriatal DA system.

Themesostriataldopamine(DA)systemcontributestoseveralaspectsofresponsestorewardingsubstancesandisimplicatedinconditionssuch asdrugaddictionandeatingdisorders.AsubsetofDAneuronshasbeenshowntoexpressthetype2Vesicularglutamatetransporter(Vglut2)and may therefore corelease glutamate. In the present study, we analyzed mice with a conditional deletion of Vglut2 in DA neurons (Vglut2 f/f;DAT-Cre ) to address the functional significance of the glutamate-DA cophenotype for responses to cocaine and food reinforcement. Biochemical parameters of striatal DA function were also examined by using DA receptor autoradiography, immediate-early gene quantitative in situ hybridization after cocaine challenge, and DA-selective in vivo chronoamperometry. Mice in which Vglut2 expression had been abrogated in DA neurons displayed enhanced operant self-administration of both high-sucrose food and intravenous cocaine. Furthermore, cocaine seeking maintained by drug-paired cues was increased by 76%, showing that reward-dependent plasticity is perturbed in these mice. In addition, several lines of evidence suggest that adaptive changes occurred in both the ventral and dorsal striatum in the absence of VGLUT2: DA receptor binding was increased, and basal mRNA levels of the DA-induced early genes Nur77 and c-fos were elevated as after cocaine induction. Furthermore, in vivo challenge of the DA system by potassium-evoked depolarization revealed less DA release in both striatal areas. This study demonstrates that absence of VGLUT2 in DA neurons leads to perturbations of reward consumption as well as reward-associated memory, features of particular relevance for addictive-like behavior.

Nurr1 is a transcription factor that is expressed in the embryonic ventral midbrain and is critical for the development of dopamine (DA) neurons. It belongs to the conserved family of nuclear receptors but lacks an identified ligand and is therefore referred to as an orphan receptor. Recent structural studies have indicated that Nurr1 belongs to a class of ligand-independent nuclear receptors that are unable to bind cognate ligands. However, Nurr1 can promote signaling via its heterodimerization partner, the retinoid X receptor (RXR). RXR ligands can promote the survival of DA neurons via a process that depends on Nurr1-RXR heterodimers. In developing DA cells, Nurr1 is required for the expression of several genes important for DA synthesis and function. However, Nurr1 is probably also important for the maintenance of adult DA neurons and plays additional less-well-elucidated roles in other regions of the central nervous system and in peripheral tissues.