Identification and pharmacological characterization of the histamine H3 receptor in cultured rat astrocytes.

Recently we reported that cultured rat cortical astrocytes express histamine H3 receptor that is functionally coupled to Gi/o proteins and participates to the stimulatory effect of histamine. Due to the lack of data on the distribution of histamine H3 receptors on glial cells we further investigated their presence in cultured astrocytes from different brain regions. Real-time PCR was performed to examine the expression of native histamine H3 receptor in cultured rat astrocytes from cortex,cerebellum, hippocampus and striatum.Double-antigen immunofluorescence staining and[3H]N-α-methylhistamine([3H]NαMH) binding studies were utilized to specifically identify and characterize receptor binding sites in astrocytes. Histamine H3 receptor mRNA was detected in rat astrocytes from all the regions under investigation with the highest levels in striatal astrocytes followed by hippocampal astrocytes and approximately equal levels in cerebellar and cortical astrocytes.Double-antigen immunofluorescence confirmed the presence of histamine H3 receptors on the membrane of all examined astroglial populations.[3H]NαMH bound with high affinity and specificity to an apparently single class of saturable sites on cortical astrocytic membranes(KD¼4.5570.46 nM; Bmax¼5.6370.21 fmol/mg protein)and competition assays with selective agonists and antagonists were consistent with labeling of histamine H3 receptor(range of pKi values 7.50–8.87). Our study confirmed the ability of cultured astrocytes from different rat brain regions to express histamine H3 receptors.The observed diverse distribution of the receptors within various astrocytic populations possibly mirrors their heterogeneity in the brain and indicates their active involvement in histamine-mediated effects.