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Several studies have shown that guanine-based purines exert biological effects on the central nervous system (CNS), possibly through membrane receptors, but at the present there are not reports related to the identification of such specific receptor(s). We have identified the first guanosine G protein-coupled receptor GPR23, also known as LPA4 receptor, involved in the modulation of guanosine-mediated antiproliferative effects in human glioma cell line (U87). We report that the silencing of GPR23 reduces significantly the antiproliferative effects of guanosine, while stably transfected cell clones over-expressing GPR23 increase sensitivity to guanosine. [3H] Guanosine radioligand binding assay reveals that [3H]-Guanosine binding to membrane fractions is greatly enhanced by GPR23 overexpression, and inhibited by GPR23 silencing. Furthermore, in [35S] GTPγS binding assay experiments, Guanosine causes a functional G protein coupled receptor activation in U87GPR23 overexp! ressing cells with an EC50= 8,067 nM. The binding site for [3H]-guanosine is highly specific and both lysophosphatidic acid (LPA) and guanine agonists are 10 times less effective than guanosine in displacing 50 nM [3H]-guanosine binding. In order to correlate the effects of guanosine in the CNS to a putative activation of GPR23, we performed, in different brain areas, the following investigations: by PCR, the expression levels of GPR23; by [3H]-Guanosine radioligand binding assay, the binding properties of Guanosine; by [35S] GTPγS binding assay, the receptor activation properties in response to Guanosine. Among the examined brain areas, the cerebral cortex shows the highest GPR23 expression levels which correlate with the highest Bmax values for [3H]-Guanosine as compared to other brain regions, with the following rank order: cerebral cortex>hippocampus>striatum>spinal cord. [35S] GTPγS binding assay experiments confirm an activation of a G protein-cou! pled receptor in response to guanosine in the cerebral cortex (EC50 10 0 nM). Although these observations do not exclude a possible involvement of other unidentified receptors, they can suggest an involvement of GPR23 in the functional response of cerebral cortex to Guanosine. Overall, together these data suggest that GPR23 may act as a functional membrane receptor for Guanosine.

Several studies have shown that guanine-based purines exert biological effects on the central nervous system, possibly through membrane receptor. In a parallel work, we have identified the first guanosine G protein-coupled receptor GPR23, known as LPA4 receptor, involved in the modulation of guanosine-mediated antiproliferative effects in human glioma cell lines. Here, we performed in different brain areas the following studies: by PCR, the expression levels of GPR23; by [3H]-Guanosine radioligand binding assay, the binding properties of GPR23; by [35S] GTPγS binding assay, the receptor activation properties of guanosine. Among the examined areas, the cerebral cortex showed the highest GPR23 expression levels and affinity binding site for [3H]-Guanosine (KD 143.8 nM and Bmax 3713 nM) as compared to other brain regions with the following rank order: cerebral cortex>hippocampus>striatum>spinal cord. T! he activation of a G protein-coupled receptor in response to guanosine showed in the cerebral cortex an EC50 92 nM. The binding site for [3H]-guanosine was highly specific and both lysophosphatidic acid and guanine agonists were 10 times less effective than guanosine in displacing 50 nM [3H]-guanosine binding. Overall these data demonstrate the existence of different levels of GPR23 mRNAs and of guanosine binding in the brain areas examined. Nevertheless at this stage of study the guanosine binding observed could include, in addition to GPR23, other unidentified receptors.

Aim: The aim of present work was to test the hypothesis that Cx37, Cx39, Cx40, Cx43 and Cx45 expression could be regulated in adult regenerating skeletal muscle in response to injury promoting activation of satellite cells involved in myofibers repair and regeneration. Methods: Using in situ hybridization and immunohistochemistry procedures we examined the spatial and temporal expression pattern of above listed connexins in the regenerating gastrocnemious muscle following a mechanical injury. Results: Cx43 and Cx45 mRNA were up-regulated very early, by 3 hour following muscle injury, and were localised in satellite cells, M-cadherin positive cells, distributed around the area of lesion. Three days after lesion a large number of Cx43 and Cx45 mRNA labelled cells were found inside the area of lesion where they become myoblasts, myogenin positive, and participate to muscle regeneration. By contrast, Cx39, not expressed in control muscle, appears in myogenin positive cells 3 days following muscle lesion and its expression was restricted to the area of lesion. Cx39 mRNA was mainly localised in myoblast myogenin positive cells forming clusters or rows of closely apposed cell nuclei committed to form myotubes. Cx40 mRNA labelled cells were observed within 24 hours from injury in the endothelial cells of blood vessels around the area of lesion. After 3 days these cells were localised inside the area of lesion where they co-express myogenin and start to form myotubes. By contrast, Cx37 mRNA expression appeared by 24 hrs from lesion and never was colocalised in myogenin positive cells, but it was involved in new blood vessels formation. All transcripts examined reached a peak of expression between 7-9 days from injury and then progressively declined, being undetectable at 4-5 weeks from injury. Conclusion: Taken together these results support the suggestion that, in regenerating skeletal muscle several connexins may be differentially involved in communication of myogenic committed cells during the process of cell proliferation, aggregation and fusion to form new myotubes.

Although gap junctions have been observed in GABAergic interneurons of several brain regions, this chapter focuses on the distribution and functions of gap junctions and connexins in inhibitory interneurons of the cerebral cortex and hippocampus. Evidence for interconnections mediated by electrical synapses is reported for at least eight cerebral cortex interneuron types, classified on the basis of morphology, electrophysiology and molecular markers. The main differences in the organization of these interneuronal networks are summarized in terms of homologous and heterologous electrical coupling and mutual chemical inhibition. The role of connexin36 (Cx36) in forming neuronal electrical synapses in interneurons is reviewed by analyzing data derived by techniques such as immunoelectron microscopy, in situ hybridization, immunohistochemistry, reporter gene expression in transgenic mice, and functional and behavioral studies in knockout mice. Possible functional roles of Cx36-based gap junctions in GABAergic interneuronal networks are examined, from the molecular and cellular level to generation of oscillatory activities and synaptic plasticity.

Adenine-based purines play a pivotal role in the control of gastrointestinal motility in rodents. Recently, guanine-based purines have been also shown to exert extracellular effects in the central nervous system raising the possibility of the existence of distinct receptors for guanine-based purines. Thus, it seems likely to speculate that also guanine-based purines may play a role in the modulation of the intestinal contractility. Spontaneous and neurally-evoked mechanical activity was recorded in vitro as changes in isometric tension in circular muscle strips from mouse distal colon. Guanosine up to 3 mM or guanine up to 1 mM failed to affect the spontaneous mechanical activity, but reduced the amplitude of the electrical field stimulation (EFS)-induced cholinergic contractions, without affecting the early nitrergic relaxation. Both compounds failed to affect the direct contractile responses evoked by carbachol. No desensitization of the response was observed. Guanine-based purine effects were not altered by theophylline, P1 purinoceptor antagonist, by PPADS or suramin, P2 purinoceptor antagonists, by ODQ, guanilyl cyclase inhibitor, or by DDA, adenylyl cyclase inhibitor. Nucleoside uptake inhibitors, dipyridamole or 6-[(4-Nitrobenzyl)thio]-9-β-D-ribofuranosylpurine (NBTI), antagonized the inhibitory effects induced by guanosine without interfering with guanine. On the contrary, adenine, a competitive inhibitor of nucleobase uptake, antagonized guanine-induced effects. In conclusion, our data indicate that guanosine and guanine are able to modulate negatively the excitatory cholinergic neurotransmission in the circular muscle layer of mouse colon. Guanine-based purines appear to interfere with prejunctional acethylcoline release. Their effects are dependent by their cellular uptake, and independent by adenine-based purine receptors.

Calcium sensing receptor (CaSR), isolated for the first time from bovine and human parathyroid, is a G-protein-coupled receptors that has been involved in diverse physiological functions. At present a complete in vivo work on the identification of CaSR mRNA-expressing cells in the adult brain lacks and this investigation was undertaken in order to acquire more information on cell type expressing CaSR mRNA in the rat brain and to analyse for the first time its expression in different experimental models of brain injury. The expression of CaSR mRNAs was found mainly in scattered cells throughout almost all the brain regions. A double labeling analysis showed a colocalization of CaSR mRNA expression in neurons and oligodendrocytes, whereas it was not found expressed both in the microglia and in astrocytes. One week after kainate-induced seizure CaSR was found in the injured CA3 region of the hippocampus and very interestingly it was found up-regulated in the neurons of CA1-CA2 and dentate gyrus. Similarly, 1 week following ibotenic acid injection in the hippocampus, CaSR mRNA expression was increased in oligodendrocytes both in the lesioned area and in the contralateral CA1-CA3 pyramidal cell layers and dentate gyrus. One week after needle-induced mechanical lesion an increase of labeled cells expressing CaSR mRNA was observed along the needle track. In conclusion, the present results contribute to extend available data on cell type-expressing CaSR in normal and injured brain and could spur to understand the role of CaSR in repairing processes of brain injury.

In the adult skeletal muscle, various kinds of trauma promote proliferation of satellite cells that differentiate into myoblasts forming new myofibers or to repair the damaged one. The aim of present work was to perform a comparative spatial and temporal analysis of connexin (Cx) 37, Cx39, Cx40, Cx43, and Cx45 expression in the adult regenerating skeletal muscle in response to crush injury. Within 24 h from injury, Cx37 expression was upregulated in the endothelial cells of blood vessels, and, 5 days after injury, Cx37-expressing cells were found inside the area of lesion and formed clusters generating new blood vessels with endothelial cells expressing Cx37. Three days after injury, Cx39 mRNA was selectively expressed in myogenin-positive cells, forming rows of closely apposed cell nuclei fusing in myotubes. Cx40 mRNA-labeled cells were observed within 24 h from injury in the endothelium of blood vessels, and, 5 days after lesion, Cx40-labeled cells were found inside the area of lesion-forming rows of myogenin-positive, closely apposed cells coexpressing Cx39. Within 24 h from lesion, both Cx43 and Cx45 mRNAs were upregulated in individual cells, and some of them were positive for M-cadherin. Three days after injury, a large number of both Cx43 and Cx45 mRNA-labeled and myogenin-positive cells were found inside the area of lesion. Taken together, these results show that at least four Cxs, out of five expressed in regenerating skeletal muscle, can be differentially involved in communication of myogenic cells during the process of cell proliferation, aggregation, and fusion to form new myotubes or to repair damaged myofibers.

In order to understand the role of metabotropic glutamate receptors (mGluRs) in the brain, it is important to know how the mGluRs are differentially expressed among the different cell types. At present, the cellular expression of mGluR3 and mGluR5 has been mostly studied in terms of proteins with observations suggesting the expression of both mGluR3 and mGluR5 in neuronal and in glial cells. In order to verify the brain cell type-expressing mGluR3 and mGluR5 mRNAs, both in normal and injured brain, we performed a double labeling analysis, by in situ hybridization for mGluR3 or mGluR5 mRNA and immunohistochemistry for specific cellular markers. This approach allowed us to find mGluR3 mRNA expressed in neurons (NeuN-positive cells), and in glial cells, such as astrocytes (GFAP-positive cells) and oligodendrocytes (CNPase-positive cells). The same analysis showed that only NeuN-positive cells express mGluR5 mRNA. The time course of mGluR3 mRNA expression in two models of hippocampal formation lesion, kainate-induced seizures or ibotenic acid injection, showed an increased expression of mGluR3 in the area of lesion. This effect appears 1 week after the injury and was localized in GFAP- and CNPase-positive cells. In contrast, mGluR5 was not found expressed in the area of lesion. The present results contribute to extend available data on cell type-expressing mGluR3 and mGluR5 in normal and injured brain and could be relevant to understand the mechanisms that drive neuron–glial cells interaction both in normal and repairing processes.