Localization Of Peripheral Benzodiazepine Receptors Research Articles

Synthesis and Biological Evaluation of Substituted [18F]Imidazo[1,2-a]pyridines and [18F]Pyrazolo[1,5-a]pyrimidines for the Study of the Peripheral Benzodiazepine Receptor Using Positron Emission Tomography

The fluoroethoxy and fluoropropoxy substituted 2-(6-chloro-2-phenyl)imidazo[1,2- a]pyridin-3-yl)- N, N-diethylacetamides 8 (PBR102) and 12 (PBR111) and 2-phenyl-5,7-dimethylpyrazolo[1,5- a]pyrimidin-3-yl)- N, N-diethylacetamides 15 (PBR099) and 18 (PBR146) were synthesized and found to have high in vitro affinity and selectivity for the peripheral benzodiazepine receptors (PBRs) when compared with the central benzodiazepine receptors (CBRs). The corresponding radiolabeled compounds [ (18)F] 8 [ (18)F] 12, [ (18)F] 15, and [ (18)F] 18 were prepared from their p-toluenesulfonyl precursors in 50-85% radiochemical yield. In biodistribution studies in rats, the distribution of radioactivity of the [ (18)F]PBR compounds paralleled the known localization of PBRs. In the olfactory bulbs, where the uptake of radioactivity was higher than in the rest of the brain, PK11195 and Ro 5-4864 were able to significantly inhibit [ (18)F] 12, while little or no pharmacological action of these established PBR drugs were observed on the uptake of [ (18)F] 8, [ (18)F] 15, and [ (18)F] 18 compared to control animals. Hence, [ (18)F] 12 appeared to be the best candidate for evaluation as an imaging agent for PBR expression in neurodegenerative disorders.

Specific ligands of the peripheral benzodiazepine receptor induce apoptosis and cell cycle arrest in human esophageal cancer cells.

Esophageal cancer is the most markedly increasing tumor entity in Western countries. Due to very poor 5-year-survival, new therapeutic approaches are mandatory. Peripheral benzodiazepine receptors (PBR) have been implicated in growth control of various tumor models, but they have not been studied yet in esophageal cancer. We used esophageal cancer cell lines and primary cell cultures of human esophageal cancers and evaluated (i) expression and localization of PBR; (ii) PBR-ligand-induced inhibition of cell growth; (iii) induction of apoptosis; and (iv) alterations in cell cycle. Expression of PBR was detected both in cell lines and in primary cell cultures of human esophageal cancers. PBR was localized in the mitochondria. The PBR-specific ligands FGIN-1-27 and PK 11195, but not the centrally acting benzodiazepine clonazepam or the indolacetamide FGIN-1-52, neither of which displaying any affinity to the PBR, inhibited cell proliferation. FGIN-1-27 and PK 11195, but not clonazepam, potently induced apoptosis. FGIN-1-27 was shown to sequentially decrease the mitochondrial membrane potential, then to activate caspase-3 and finally to cause DNA fragmentation. In addition, PBR-specific ligands induced cell cycle arrest in the G1/G0 phase. Our data qualify PBR-specific ligands as innovative proapoptotic and antiproliferative substances. They might prove suitable for the treatment of esophageal cancer.

Cellular and subcellular localization of peripheral benzodiazepine receptors after trimethyltin neurotoxicity.

The peripheral benzodiazepine receptor (PBR) is currently used as a marker of inflammation and gliosis following brain injury. Previous reports suggest that elevated PBR levels in injured brain tissue are specific to activated microglia and infiltrating macrophages. We have produced hippocampal lesions using the neurotoxicant trimethyltin (TMT) to examine the cellular and subcellular nature of the PBR response. Degenerating, argyrophilic pyramidal neurons were observed in the hippocampus at 2 and 14 days after TMT exposure. Reactive microglia were also evident at both times with a maximal response observed at 14 days, subsiding by 6 weeks. Astrocytosis was observed at 14 days and 6 weeks, but not 2 days, after TMT administration, suggesting that the onset of the astroglia response is delayed, but more persistent, compared with microgliosis. Morphological evidence from [3H]PK11195 microautoradiography and PBR immunohistochemistry indicates that both astrocytes and microglia are capable of expressing high levels of PBR after injury. This was confirmed by double labeling of either Griffonia simplicifolia isolectin B4, a microglial-specific marker, or glial fibrillary acidic protein, an astrocyte-specific protein with PBR fluorescence immunohistochemistry. These results demonstrate that PBR expression is increased after brain injury in both activated microglia and astrocytes. Our findings also provide the first evidence for in situ nuclear localization of PBR in glial cells.

Temporal changes in glial fibrillary acidic protein messenger RNA and [ 3H]PK11195 binding in relation to imidazoline-I 2-receptor and α2-adrenoceptor binding in the hippocampus following transient global forebrain ischaemia in the rat

Immunohistochemical studies have demonstrated that following global forebrain ischaemia the selective neuronal loss that occurs in the CA1 pyramidal cell layer of the hippocampus is accompanied by a reactive astrocytosis, characterized by increases in glial fibrillary acidic protein, and activation of microglia. In this study the spatial changes in glial fibrillary acidic protein messenger RNA levels in the hippocampus have been mapped four, eight, 12, 16 and 20 days following 10 min of global forebrain ischaemia in the rat and related to changes in [ 3H]PK11195 binding to peripheral benzodiazepine receptors, a putative marker of activated microglia. Recent studies have suggested that the imidazoline-I 2-receptor, one of a class of non-adrenergic receptors related to, but structurally and functionally distinct from α 2-adrenoceptors, may have a functional role in controlling the expression of glial fibrillary acidic protein. To explore this possibility further we have also mapped changes in imidazoline-I 2-receptor and α 2-adrenoceptor binding sites. Following transient ischaemia there was a marked, biphasic increase in glial fibrillary acidic protein messenger RNA levels throughout the vulnerable CA1 region of the hippocampus, peaking four days after ischaemia and then increasing gradually during the remainder of the study period. There was also a sustained increase in [ 3H]PK11195 binding, however, in contrast to the initial increase in glial fibrillary acidic protein messenger RNA levels that peaked four days after ischaemia the density of [ 3H]PK11195 binding increased rapidly in all strata of the CA1 region over the first eight days and then increased more slowly throughout days 12 to 20. Despite the marked increase in glial fibrillary acidic protein messenger RNA levels there was no concomitant alteration in imidazoline-I 2-receptor binding sites detected using the specific radioligand, [ 3H]2-(2-benzofuranyl)-2-imidazoline, although α 2-adrenoceptor binding was decreased at eight days after ischaemia and did not recover. The time course and biphasic nature of the changes in the astrocytic marker, glial fibrillary acidic protein messenger RNA, in the hippocampus following ischaemia may reflect different functions of glial fibrillary acidic protein-reactive astrocytes in the post-ischaemic period. Differences in temporal expression of glial fibrillary acidic protein messenger RNA and [ 3H]PK11195 binding support the proposed localization of peripheral benzodiazepine receptors on activated microglia, as distinct from reactive astrocytes. There was no evidence in the present study that imidazoline-I 2-receptors are functionally linked to glial fibrillary acidic protein expression as the reactive astrocytosis in the hippocampus following ischaemia was not associated with changes in imidazoline-I 2-receptor binding site density.