EVOLUTION OF INHIBITORY NEUROTRANSMISSION: GABAERGIC SIGNALING IN A NUMERICALLY SIMPLER NERVOUS SYSTEM

Inhibitory neurotransmission is a fundamental functional property of nervous systems. However, the evolution of GABAergic signaling has largely been unexplored. Here, we provide a comprehensive analysis of the GABAergic system in the CNS of a widely studied molluscan model, the great pond snail (Lymnaea stagnalis).
Using cluster analysis and phylogenetics, we mapped the complete GABAergic toolkit across prebilaterian and bilaterian lineages. Our results revealed that GABA-B receptors are found even in animals without defined nerve cells, whereas GABA-A receptors are exclusive for cnidarians and bilaterians. Moreover, canonical GABA-A receptor subunits are conserved, while non-canonical subunits represent vertebrate-specific types, highlighting a deep evolutionary divergence in inhibitory receptor architecture. Comparative analyses also showed lineage-specific differences in GABA transporters, suggesting that mechanisms of synaptic clearance have evolved differently across lineages. Neurochemical assays confirmed the presence of GABA and its precursors within the CNS, and immunogold electron microscopy provided a priori ultrastructural evidence of GABA localization within synaptic varicosities of a gastropod CNS. The expression pattern of GABA-A and GABA-B receptor subunits in the CNS visualized by in situ hybridization, as well as electrophysiological recordings on identified single neurons, seems to confirm the postsynaptic effects of GABA. Ligand-binding assays showed no interaction between GABA and Lymnaea GABA-B receptor, indicating unique regulatory mechanisms which are absent in vertebrates.
Together, our findings contribute to our understanding how inhibitory signaling has diversified during evolution, showing that while the fundamental principles of GABAergic transmission are deeply conserved, lineage-specific modifications may shape circuit dynamics and, consequently, behavioral complexity.