A cyclic GMP analog decreases the currents underlying bursting activity in the Aplysia neuron R15

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Abstract
Bath application of 8-parachlorophenylthio-cyclic GMP (8-pcpt-cGMP) has been shown to increase the number of action potentials per burst in the Aplysia neuron R15. Here we report that 8-pcpt-cGMP can eventually inhibit R15's bursting activity and cause the cell to exhibit slow tonic spiking activity. This action is preceded by decreases in spike frequency and in the amplitude of the interburst hyperpolarization. The ionic mechanism underlying these various changes in burst pattern was investigated using voltage clamp. 8-pcpt-cGMP reduces steady-state inward current, and this effect is blocked by manganese but is unaffected by internal EGTA or elevation of extracellular potassium. Dopamine, which decreases the subthreshold calcium current in R15, diminishes the change in current induced by 8-pcpt-cGMP. 8-pcpt-cGMP also reduces the spike-evoked calcium-dependent inward and outward currents known to underlie burst generation. These data lead us to conclude that 8-pcpt-cGMP acts directly to reduce the steady-state subthreshold calcium current (ICa). This decrease in ICa may also indirectly reduce the calcium-dependent currents. It is suggested that 8-pcpt-cGMP produces its effects on bursting by decreasing both the inward and outward currents required for generating bursting behavior, and thus effectively dampens bursting until the cell is in a beating state. It is also shown that 8-pcpt-cGMP dramatically alters R15's responses to depolarizing and hyperpolarizing stimuli. Thus, this cyclic GMP analog alters not only the cell's intrinsic activity, but also its response to external inputs.