Modulation of the lobster pyloric rhythm by the peptide proctolin

The modulation of the pyloric network of the stomatogastric ganglion (STG) of the lobster Panulirus interruptus by the neuropeptide proctolin is described. First, the effects of proctolin on the pyloric motor patterns were characterized in terms of frequency and phase relations. Pyloric cycle frequency and lateral pyloric (LP) neuron activity increased and ventricular dilator (VD) neuron activity decreased with increasing concentrations (10(-9)-10(-6) M) of applied proctolin. Next, the effects of proctolin on the individual neurons that constitute the pyloric network were determined. Identified neurons were isolated from chemical and electrical presynaptic inputs by using pharmacological agents (Marder and Eisen, 1984a) and/or photoinactivation following Lucifer yellow injection (Miller and Selverston, 1979). Proctolin increased the amplitude and frequency of bursts produced by isolated pacemaker anterior burster (AB) neurons. Isolated LP and pyloric (PY) neurons responded to proctolin with increases in activity only when they were at or above threshold. All other pyloric neurons were unaffected. To determine how the direct effects of proctolin on isolated neurons resulted in the observed changes in frequency and phase relations in the motor pattern of the intact pyloric circuit seen in proctolin, individual neurons were deleted from the circuit. A comparison of proctolin's effects on isolated neurons with those on the intact network shows that the synaptic connectivity among neurons directly affected by proctolin and those unaffected by it shapes the network's response to proctolin.