Histamine H(1) receptor (H1R) level varies under various pathological conditions, and these changes may be responsible for some pathogenesis, such as allergic rhinitis. Previously, we showed that H1R was heterologously down-regulated (through degradation of H1R) by prolonged stimulation with muscarinic M(3) receptor (M3R) in Chinese hamster ovary (CHO) cells stably expressing H1R and M3R. However, this cell was inadequate for studying the effects on H1R gene regulation, because the cell expresses H1R, which is under the control of the SV40 promoter. Therefore, in this study, we have investigated the possible role of M3R stimulation in the H1R gene transcription and H1R mRNA stability by using U373 astrocytoma cells that express endogenous H1R and transfected M3R. Stimulation of M3R significantly increased H1R promoter activity and H1R mRNA level without alteration in H1R mRNA stability. The H1R level was also up-regulated by M3R activation (150% of control by treatment with carbachol for 24 h). These M3R-mediated events were almost completely blocked by the protein kinase C (PKC) inhibitor, Ro 31-8220, suggesting the involvement of PKC. These results indicated that M3R was involved in the up-regulation of H1R by activating H1R gene transcription through a PKC-dependent process.