Metabotropic γ-aminobutyric acid (GABAB) receptors modulate feeding behavior in the calcisponge Leucandra aspera

Here, we report the presence of the g-aminobutyric acid (GABA)-ergic system in the calcisponge
Leucandra aspera and examine the cellular localization of the components of this system, including
GABA-like receptors using immunofluorescence and confocal microscopy. Furthermore, we demonstrate
for the first time that GABA plays a functional role as a messenger in regulating sponge-feeding
behavior. We found that both GABAB R1 and R2 subunits are present in the choanocytes of sponges as
well as in the eso- and endopinacocytes. The functional role of GABA in the feeding behavior of this
sponge was tested. The involvement of GABA receptors in the endocytic processes in L. aspera was
demonstrated with dextran conjugated to Texas Red as a marker for material ingestion and by treating
isolated sponge cells with a GABAB receptor agonist and an antagonist. The amount of dextran that
was ingested increased in dissociated sponge cells when the GABAB receptor agonist baclofen was
used, and this stimulatory effect was prevented by treatment with the GABAB receptor antagonist
phaclofen. The baclofen effect on uptake was blocked by treatment with pertussis toxin, thus indicating
a role for G proteins in modulating feeding behavior in L. aspera. Moreover, we found evidence that
GABA receptors are involved in the consumption of dissolved organic matter by sponge cells. These
findings suggest that GABA receptors and their functional role are highly conservative traits in the
animal kingdom prenervous system evolution.