Glutamatergic systems in Hydrozoa (Cnidaria)
The origins and early diversification of intercellular signaling molecules in animals remain poorly understood because comparative data across basal metazoan lineages are limited. Cnidarians form the sister group to bilaterian animals, and characterizing their transmitter systems is critical to understanding how complex adaptations within integrative systems shape evolutionary trajectories.…
Cnidarians, which are the closest living relatives to bilaterian animals, have long been a mystery when it comes to their intercellular signaling molecules. To better understand how complex adaptations within integrative systems shape evolutionary trajectories, researchers have focused on characterizing their transmitter systems. Although glutamate is known to be a crucial transmitter in bilaterian animals, its role in cnidarians remains elusive, with limited information on its neuronal function and signaling.
To shed light on this topic, researchers studied eight hydrozoan species with diverse ecologies, including Aequorea victoria, Eutonina indicans, Clytia gregaria, Bougainvillia principis, Euphysa flammea, Polyorchis penicillatus, Aglantha digitalis, and Nanomia septata. By examining glutamate immunoreactivity (Glu IR), they identified and visualized distinct populations of glutamate-immunoreactive (Glu-ir) cells, which included nematocytes, neurons, and muscle cells.
The study found a broad diversity of Glu-ir nematocytes across all studied species, indicating that these cells play a significant role in glutamatergic signaling. However, glutamate-immunoreactive (Glu-ir) neural cells were only found in three species: Aequorea, Nanomia, and Aglantha. The morphology and localization of these neural cells were found to be species-specific, suggesting that different hydrozoan species have evolved unique ways to utilize glutamate in their signaling systems.
Interestingly, the research also revealed that some striated and smooth myoepithelial cells were either Glu-ir or GABA-ir (gamma-aminobutyric acid immunoreactive). This finding suggests that both glutamatergic and GABAergic systems were independently recruited more than three times as neurotransmitters across cnidarians. The researchers propose that these recruitments are fundamentally rooted in the bioenergetic demands of these organisms, highlighting the importance of understanding the origins and diversification of glutamatergic systems in Hydrozoa (Cnidaria).
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