Excitatory effects of GABA on procerebrum neurons in a slug.

Classical neurotransmitters, such as glutamate and γ-aminobutyric acid (GABA), often have different actions on invertebrate neurons from those reported for vertebrate neurons. In the terrestrial mollusk Limax, glutamate was found to function as an inhibitory transmitter in the procerebrum (PC), but it has not yet been clarified how GABA acts in the PC. We thus examined what effects GABA exerts on PC neurons in the present study. For this purpose, we first applied GABA to isolated PC preparations and recorded postsynaptic currents and potentials in PC neurons. The GABA application reduced the amplitude of inhibitory postsynaptic currents and depolarization-induced outward currents recorded in nonbursting neurons and increased the number of spontaneous spikes of nonbursting neurons. However, direct GABA-induced currents were not observed in either bursting or nonbursting neurons. These results suggest a potential direct effect of GABA on outward currents resulting in enhanced excitability of PC neurons. Next, we measured the change in [Ca(2+)](i) in cultured PC neurons by application of GABA. The GABA application increased spontaneous Ca(2+) events in cultured neurons. These Ca(2+) events were ascribable to the influx of extracellular Ca(2+). We then confirmed the presence of GABA and GABA receptors in the PC. The GABA-like immunoreactivity was observed in the neuropil layers of the PC, and the mRNAs for both GABA(A) and GABA(B) receptors were expressed in the PC. In particular, GABA(B) receptor mRNA, rather than GABA(A), was found to be more abundantly expressed in the PC. These results suggest that GABA functions as an excitatory modulator for PC neurons via mainly GABA(B) receptors.