The receptor mGluR5 is a metabotropic glutamate receptor with messenger RNA abundantly present throughout cortex, hippocampus, and caudate/putamen that is also coupled to phosphatidyl inositide hydrolysis and calcium mobilization. In this study, the distribution of mGluR5 was examined in rat brain by immunocytochemistry. The antibody utilized is highly specific and does not cross react with the most closely related other metabotropic glutamate receptor, as determined by Western blot analysis of nonneuronal cells transfected with metabotropic receptor coding sequences. The receptor mGluR5 is widely expressed with the highest density in olfactory bulb, caudate/putamen, lateral septum, cortex, and hippocampus, as confirmed with both immunocytochemistry and Western blot analysis. Electron microscopic studies in hippocampus and cortex indicate that the labeling is mostly on membranes of dendritic spines and shafts. Light and electron microscopic evidence indicates that some mGluR5 immunoreactivity is located in presynaptic axon terminals, suggesting that mGluR5 may function as a presynaptic receptor.

The sequences of the metabotropic glutamate receptors (mGluRs) show little homology with other members of the G protein-coupled receptor family and exhibit several distinctive features, including a large N-terminal extracellular domain with 17 cysteines in conserved positions. Here we demonstrate that mGluR5, as well as other mGluRs, behave as species approximately twice as large as expected from their sequence, but reducing conditions cause a decrease to the predicted molecular mass. Co-immunoprecipitation experiments using wild type and epitope-tagged receptors demonstrate that this is due to specific, disulfide-dependent dimerization of the receptor. The intermolecular disulfide that mediates dimerization occurs in the extracellular domain, within about 17 kDa from the N terminus.Glutamate is the primary neurotransmitter for excitatory neurotransmission in the vertebrate central nervous system and as such is responsible for a broad range of physiological and pathophysiological roles. These include transmission in sensory pathways, higher brain functions such as learning and memory, and cytotoxicity and neuronal death.Two classes of receptors for glutamate are present on neural cells: the ionotropic glutamate receptors (iGluRs) 1 and the metabotropic glutamate receptors (mGluRs). The iGluRs are ligand-gated cation channels, and they mediate rapid synaptic transmission. The iGluRs include the N-methyl-D-aspartate, ␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and kainic acid families of receptors. At least eight mGluRs have been molecularly characterized, and these activate effectors via interactions with heterotrimeric G proteins (1). Thus, the mGluRs are important for neuromodulatory functions, although mGluRs clearly mediate transmission at the retinal photoreceptor-depolarizing bipolar cell synapse (2-4) and at certain thalamic sensory neurons (5).Although the mGluRs possess seven transmembrane domains, there are important differences between these receptors and other G protein-coupled receptors. There is no primary sequence similarity between the mGluRs and the rhodopsinlike receptors (4). The recently described Ca 2ϩ

Although T-type calcium channels were first described in sensory neurons, their function in sensory processing remains unclear. In isolated rat sensory neurons, we show that redox agents modulate T currents but not other voltage- and ligand-gated channels thought to mediate pain sensitivity. Similarly, redox agents modulate currents through Ca(v)3.2 recombinant channels. When injected into peripheral receptive fields, reducing agents, including the endogenous amino acid L-cysteine, induce thermal hyperalgesia. This hyperalgesia is blocked by the oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) and the T channel antagonist mibefradil. DTNB alone and in combination with mibefradil induces thermal analgesia. Likewise, L-cysteine induces mechanical DTNB-sensitive hyperalgesia in peripheral receptive fields. These data strongly suggest a role for T channels in peripheral nociception. Redox sites on T channels in peripheral nociceptors could be important targets for agents that modify pain perception.

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G-protein-coupled receptors are well known for converting an extracellular signal into an intracellular response. Here we showed that the metabotropic glutamate receptor 5 (mGlu5) plays a dynamic intracellular role in signal transduction. Activation of endogenously expressed mGlu5 on striatal nuclear membranes leads to rapid, sustained calcium (Ca 2؉ ) responses within the nucleoplasm that can be blocked by receptor-specific antagonists. Extracellular ligands such as glutamate and quisqualate reach nuclear receptors via both sodiumdependent transporters and cystine glutamate exchangers. Inhibition of either transport system blocks radiolabeled agonist uptake as well as agonist-induced nuclear Ca 2؉ changes. can enter the nucleoplasm via the nuclear pore complex or from the nuclear lumen, the presence of nuclear mGlu5 receptors appeared to amplify the latter process generating a faster nuclear response in heterologous cells. In isolated striatal nuclei, nuclear receptor activation results in the de novo appearance of phosphorylated CREB protein. Thus, activation of nuclear mGlu5 receptors initiates a signaling cascade that is known to alter gene transcription and regulate many paradigms of synaptic plasticity. These studies demonstrated that mGlu5 receptors play a dynamic role in signaling both on and off the plasma membrane.The structure and function of G-protein-coupled receptors have received intense scrutiny over the past decades. These studies point to a dynamic environment in which receptors are not static but rather move on and off the plasma membrane according to environmental stimuli, specific targeting information, protein-protein interactions, etc. In this model, intracellular receptors are considered transitional, i.e. receptors that are either ready to be inserted into the plasma membrane or that have just been sequestered from such a site. Emerging data, however, suggest that some intracellular receptors may have intracellular functions as well. For example, a number of G-protein-coupled receptors such as the apelin, angiotensin AT1 and ATII, bradykinin B2, and lysophosphatidate LP1 receptors have been localized within the nucleoplasm itself (1-3). In contrast, prostaglandin E 2 receptors have been found on the nuclear envelope together with their ligand-generating enzymes (4, 5). Similarly, endothelin receptors A and B are also localized to the perinuclear region of cardiac ventricular myocytes where they mediate nuclear Ca 2ϩ levels and activate nuclear protein kinases (6). Finally, we have shown that the metabotropic glutamate receptor, mGlu5, 1 can be expressed on nuclear membranes where it can couple with endogenous signaling components to induce changes in nuclear Ca 2ϩ (7). Because most studies investigating the properties of nuclear receptors have been performed in heterologous cell types with overexpressed receptors, the question arises as to whether such phenomena are physiologically relevant and, in the case of mGlu5, how a ligand such as glutamate has access to this receptor.Widely expre...

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CNS function depends on a capacity for plasticity during development, following injury, and in response to changing environmental conditions. Functional alterations in signal transduction pathways and in neurotransmitter receptor expression are possible mechanisms for the expression of such plasticity. In the present report, we demonstrate that exposure of astrocytes to specific growth factors alters both the functional activity and the protein levels of a specific glutamate receptor. Exposure of astrocytes to basic fibroblast growth factor, epidermal growth factor, or transforming growth factor-alpha produced marked increases in the ability of metabotropic glutamate receptor (mGluR) agonists to stimulate phosphoinositide hydrolysis. Using Western immunoblotting, we demonstrate that an increase in the levels of one of the phosphoinositide-coupled mGluR subtypes, mGluR5, accompanies the increased ability of mGluR agonists to stimulate phosphoinositide hydrolysis. In contrast, another phosphoinositide-coupled subtype of this receptor family, mGluR1 alpha, was not present at detectable levels in these cultures. The enhanced stimulation of phosphoinositide hydrolysis showed little sensitivity to pertussis toxin, and appeared to be selective to mGluR agonists, as there was not a similar increase in the ability of norepinephrine or carbachol to stimulate phosphoinositide hydrolysis. These findings demonstrate that expression of mGluRs in astrocytes is plastic, and indicate a novel pathway through which specific growth factors may selectively modulate neurotransmitter action.