Analysis of FMRF-amide effects of Aplysia bursting neurons

Abstract

The peptide L-phenylalanyl-L-methionyl-L-arginyl-L-phenylalaninamide (FMRF-amide) was pressure-applied onto the somata of bursting neurons L4 and L6 in the Aplysia abdominal ganglion. FMRF-amide causes a biphasic response, first depolarizing and then hyperpolarizing the neuron. In voltage-clamp experiments, FMRF-amide induces an inward current that begings 100-200 msec after applying the peptide and peaks in 2-10 sec. This is followed by an outward current that begins with a latency of 2-5 sec and peaks in 15-65 sec. The entire response lasts 1-5 min. Experiments were done to separate the two currents induced by FMRF-amide on the basis of ion selectivity and kinetics and to determine their I(V) relationships. The currents were studied using a method to quickly measure I(V) curves. The inward current is caused by a conductance increase and has a reversal potential of approximately +18 mV. This current depends on the concentration of extracellular Na ions but not Ca, Cl, or K ions and is insensitive to tetrodotoxin, hexamethonium, and curare. The outward current is caused by a conductance increase and has a reversal potential of approximately -61 mV, which is similar to the reversal potential of the fast, transient K current (I(A)) in the same cells. This current is sensitive to changes in the external K ion concentration but not to changes in Cl, Ca, or Na concentration. The outward current is partially blocked by 1 mM 4-aminopyridine but not TEA or curare. Neither current is significantly voltage dependent within the range from -70 to -40 mV. These data indicate that, in these neurons, FMRF-amide activates two separate types of ion channels. The likelihood that FMRF-amide acts as a transmitter or hormone in Aplysia is strengthened by these observations. The ionic mechanisms characterized here can lead to potent modulation of bursting pacemaker neurons by an endogenous peptide.