Histamine H3 and H4 receptors are expressed on distinct endocrine cell types in the rat fundic mucosa

The present study was designed to explore the expression and distribution of histamine H3 and H4 receptors within the rat gastric fundus by immunohistochemistry using our unique immunological probes. Groups of rats were either fed ad libitum or fasted for 24h before the sacrifice. Tissue samples from the fundic region of the stomach were removed, fixed and processed for immunohistochemistry, using our validated anti-H3 (349-358) and anti–H4 (374-390) antibodies. The results showed that cells immunoreactive for H3R and H4R are located in the fundic epithelium. The vast majority of H3R immunopositive cells occupied the lower half of the glands, and were absent in the superficial portion. Their number per gland was 5.38 ± 0.19 in freely fed rats and 3.21 ± 0.40 in fasted rats. Conversely, cells immunoreactive for H4R were dispersed throughout the glands. Their number per gland was 0.75 ± 0.03 in fed rats and 0.90 ± 0.05 in fasted rats. Double immunostaining revealed that the H3R is not colocalized with the H4R, while both H3R and H4R positive cells were immunoreactive for chromogranin A, suggesting that these immunopositive cells are endocrine cells. Because five different endocrine cell types have been identified in the rat fundic epithelium, we used double immunostaining to identify the cell types expressing H3R and H4R, respectively. Approximately 90% of cells positive for H3R were also immunoreactive for histidine decarboxylase, demonstrating that the H3R is located on ECL cells. Cells positive for H4R were immunoreactive for ghrelin, originating from A-like cells. These findings may imply that histamine, via a balanced interaction with H3R and H4R expressed on ECL and A-like cells respectively, regulates not only acid secretion and mucosal protection but could also have a role in feeding behaviour and growth hormone release.