Suppression of oscillatory activity in crustacean pyloric neurons: implication of GABAergic inputs

Generation of rhythmic pyloric motor output in the crustacean stomatogastric ganglion results from synaptic connections and cellular properties of a 14-cell network of pyloric neurons. These cellular properties are under the influences of modulatory inputs, which act, for the most part, in an activating mode, i.e., they enhance the bursting properties of the pyloric neurons and/or their ability to express their regenerative properties. Here we attempt to demonstrate that the pyloric motor output is also under the control of suppressive afferent inputs that are able to stop the pyloric rhythm in a long-lasting manner. Immunohistochemistry, using GABA antibodies, indicates that GABAergic-like fibers are present in both the stomatogastric ganglion and its afferent nerve. Bath-applied GABA suppresses spontaneous pyloric rhythmic activity. This is due to an inability of the pyloric pacemakers to express their bursting properties. The suppressive effect of GABA is blocked by picrotoxin and mimicked by muscimol. Isolating the pyloric neurons from all descending spiking influences with tetrodotoxin demonstrates that exogenously applied GABA acts directly on the pyloric neurons. To confirm the existence of a physiological suppressive system for the pyloric motor pattern, we show that the stimulation of an afferent nerve, known to contain GABA-like fibers, also causes the cessation of rhythmic activity and the inability of the pyloric neurons to express their bursting properties.