GABA-mediated synaptic inhibition of projection neurons in the antennal lobes of the sphinx moth, Manduca sexta

1. Responses of neurons in the antennal lobe (AL) of the moth Manduca sexta to stimulation of the ipsilateral antenna by odors consist of excitatory and inhibitory synaptic potentials (Fig. 2A). Stimulation of primary afferent fibers by electrical shock of the antennal nerve causes a characteristic IPSP-EPSP synaptic response in AL projection neurons (Fig. 2B). 2. The IPSP in projection neurons reverses below the resting potential (Fig. 3), is sensitive to changes in external (Fig. 4) and internal (Fig. 5) chloride concentration, and thus is apparently mediated by an increase in chloride conductance. 3. The IPSP is reversibly blocked by 100 μM picrotoxin (Fig. 6) or bicuculline (Fig. 7). 4. Many AL neurons respond to application of GABA with a strong hyperpolarization and an inhibition of spontaneous spiking activity (Fig. 8). GABA responses are associated with an increase in neuronal input conductance (Fig. 9) and a reversal potential below the resting potential (Fig. 11). 5. Application of GABA blocks inhibitory synaptic inputs (Fig. 12 A) and reduces or blocks excitatory inputs (Fig. 12B). EPSPs can be protected from depression by application of GABA (Fig. 12B). 6. Muscimol, a GABA analog that mimics GABA responses at GABA_A receptors but not at GABA_B receptors in the vertebrate CNS, inhibits many AL neurons in the moth (Fig. 13).