GABA as a Neurotransmitter in Gastropod Molluscs

Abstract

The neurotransmitter gamma-aminobutyric acid (GABA) is widely distributed in the mammalian central nervous system, where it acts as a major mediator of synaptic inhibition. GABA also serves as a neurotransmitter in a range of invertebrate phyla, including arthropods, echinoderms, annelids, nematodes, and platyhelminthes. This article reviews evidence supporting the neurotransmitter role of GABA in gastropod molluscs, with an emphasis on its presence in identified neurons and well-characterized neural circuits. The collective findings indicate that GABAergic signaling participates in the selection and specification of motor programs, as well as the bilateral coordination of motor circuits. While relatively few in number, GABAergic neurons can influence neural circuits via inhibitory, excitatory, and modulatory synaptic actions. GABA's colocalization with peptidergic and classical neurotransmitters can broaden its integrative capacity. The functional properties of GABAergic neurons in simpler gastropod systems may provide insight into the role of this neurotransmitter phenotype in more complex brains.

Keywords: BCI, buccal-cerebral interneuron; CBC, cerebral-buccal connective; CBI, cerebral-buccal interneuron; CNS, central nervous system; CPG, central pattern generator; Cr-Aint, cerebral A interneuron; DA, dopamine; EPSP, excitatory postsynaptic potential; FCAP, feeding circuit activating peptide; GABA, gamma-aminobutyric acid; GABAli, GABA-like immunoreactivity; IPSP, inhibitory postsynaptic potential; PKC, protein kinase C.

Figure 1.

GABA as a neurotransmitter in gastropod molluscs. (A₁, A₂) Initial demonstration of inhibitory (A₁) and excitatory (A₂) actions of GABA on gastropod neurons. Perfusion of GABA (1 mmol L⁻¹; white lines under recordings) while recording from neurons in the viscero-abdominal ganglia of Helix aspersa. Calibration bars: 2 s, 25 mV, from another panel in original paper, apply to both (A₁) and (A₂). Reprinted from Gerschenfeld and Lasansky, 1964. Int. J. Neuropharmacol. 3: 301–314, with permission from Elsevier. (B₁, B₂) Common properties of acetylcholine and GABA responses on cell R2 of Aplysia. (B₁) In control solution, [Cl⁻] 5 593 mmol L⁻¹, the reversal potential for both ACh and GABA was near 258 mV. (B₂) When the external Cl² concentration was reduced to 296 mmol L⁻¹, the reversal potential for both drugs was shifted to −40 mV. ACh and GABA were delivered from independent iontophoretic pipettes. Reprinted from Yarowsky and Carpenter, 1978. J. Neurophysiol. 41: 531–541, with permission from the American Physiological Society.

Figure 2.

GABA-like immunoreactive (GABAli) neurons in the subesophageal ganglia of Lymnaea stagnalis. (A) GABAli neurons in the visceral ganglion (V g.) and right parietal ganglion (R Pa g.). A single large cell (20–30 μm, arrowhead) was located at the posterior edge of the right parietal ganglion, and a group of three cells (arrow) was located at the posterior pole of the visceral ganglion. One of the visceral ganglion cells was significantly larger (40–50 μm, asterisk). Calibration bar = 50 μm. (B₁) Intracellular recording from the large putative VD1 neuron in the visceral ganglion disclosed rhythmic spiking activity. (B₂) Repetitive impulses were also recorded from cell RPD2. Calibration bars = 2 s, 10 mV. (C₁) Injection of the large visceral GABAli neuron with neurobiotin showed branching of its axon (arrow) and projections toward the right and left parietal ganglia. (C₂) Same field of view as (C₁), after processing for GABA-like immunoreactivity. (C₃) Overlay of (C₁) and (C₂). Merge of magenta and green appears white. Calibration bar = 50 μm, applies to (C₁–C₃). (D₁) Injection of RPD2 with neurobiotin labeled a projection toward the visceral ganglion (arrow). (D₂) Same field of view as (D₁) after processing for GABA-like immunoreactivity. (D₃) Merge of (D₁) and (D₂). Calibration bar = 20 μm, applies to (D₁–D₃).

Figure 3.

Identified GABAergic neurons. (A₁, A₂) Identified GABAergic neuron cerebral A interneuron (Cr-Aint) in the prey capture circuit of Clione limacia. (A₁) A single CR-Aint (arrow) was filled with neurobiotin and visualized with Texas Red-labeled avidin. (A₂) GABA-like immunoreactivity, same cerebral ganglion as (A₁). The cell body of Cr-Aint is indicated by the arrow. Scale bars = 200 μm. Reprinted from Norekian, 1999. J. Neurosci. 19: 1863–1875, with permission from the Society for Neuroscience. (B₁, B₂) Colocalization of THli and GABAli in neuron B20 in the buccal ganglion of Aplysia californica. (B₁) THli was observed in one neuron (arrow) on the rostral surface of each buccal hemiganglion (only left hemiganglion shown) near the buccal commissure (b c.). (B₂) GABAli was present in the same cell (arrow). Calibration bar = 40 μm, applies to both (B₁) and (B₂). Reprinted from Díaz-Ríos and Miller, 2002. J. Comp. Neurol. 445: 29–46, with permission from John Wiley.

Figure 4.

GABA modulates rapid dopaminergic signaling from B20 to the radula closer motor neuron B8. (A₁) Bath application of GABA (1 mmol L⁻¹) increased the amplitude of excitatory postsynaptic potentials (EPSPs) produced in B8 by impulses evoked in B20 (con, control). (A₂) GABA_B agonist baclofen (1 mmol L⁻¹) also augmented the B20-to-B8 EPSP. Reprinted from Díaz-Ríos and Miller, 2005. J. Neurophysiol. 93: 2142–2156, with permission from the American Physiological Society. (B₁, B₂) GABA and baclofen potentiate dopamine (DA) currents in B8. (B₁) Perfusion of GABA (100 μmol L⁻¹) augmented the inward current evoked by dopamine puffed from a micropipette onto the soma of B8. This potentiation was blocked by the GABA_B antagonist phaclofen (Phaclo, 100 mmol L⁻¹). (B₂) Perfusion of baclofen (100 μmol L⁻¹) also potentiated the inward current evoked by dopamine in B8. The effect of baclofen was also blocked by phaclofen. All experiments were conducted in the presence of tetrodotoxin (10 μmol L⁻¹) to suppress impulses and synaptic activity. Reprinted from Svensson et al., 2014. J. Neurophysiol. 112: 22–29, with permission from the American Physiological Society.