The GABAergic-like system in the Marine demosponge Chondrilla nucula

Gamma-amino butyric acid (GABA) is believed to be the principal inhibitory neurotransmitter
in the mammalian central nervous system, a function that has been extended to a
number of invertebrate systems. The presence of GABA in the marine demosponge Chondrilla
nucula was verified using immunofluorescence detection and high-pressure liquid chromatography.
A strong GABA-like immunoreactivity (IR) was found associated with choanocytes, exopinacocytes,
endopinacocytes lining inhalant, and exhalant canals, as well as in archaeocytes scattered in
the mesohyl. The capacity to synthesize GABA from glutamate and to transport it into the vesicles
was confirmed by the presence in C. nucula of glutamate decarboxylase (GAD) and vesicular
GABA transporters (vGATs), respectively. GAD-like and vGAT-like IR show the same distribution
as GABA-like IR. Supporting the similarity between sponge and mammalian proteins, bands with
an apparent molecular weight of about 65–67 kDa and 57 kDa were detected using antibodies
raised against mammalian GAD and vGAT, respectively. A functional metabotropic GABAB-like receptor
is also present in C. nucula. Indeed, both GABAB R1 and R2 isoforms were detected by immunoblot
and immunofluorescence. Also in this case, IR was found in choanocytes, exopinacocytes,
and endopinacocytes. The content of GABA in C. nucula amounts to 1225.75 6 79 pmol/mg proteins
and GABA is released into the medium when sponge cells are depolarized. In conclusion, this
study is the first indication of the existence of the GABA biosynthetic enzyme GAD and of the
GABA transporter vGAT in sponges, as well as the first demonstration that the neurotransmitter
GABA is released extracellularly