Substrates of command ability in a buccal neuron of Pleurobranchaea - I. Mechanisms of action potential broadening

1. Bilaterally paired ventral white cells (VWCs) in the buccal ganglion of Pleurobranchaea are putative command neurons which in part derive their ability to drive the neural network controlling the buccal mass from the progressive broadening of action potentials during repetitive firing (Fig. 1) (Gillette et al. 1978, 1980). We have conducted an investigation of the parameters and mechanisms of spike broadening using conventional intracellular recording methods, ion substitutions, intracellular injections, and pharmacological agents. In the course of these studies we found evidence for a role for [Ca⁺⁺]_i in modulating spike broadening. 2. During a current-driven train of broadening spikes, the overshoot amplitudes initially increase progressively and then decline. The rate of broadening is slowest during growth of the overshoot and maximal during overshoot decline (Fig. 2), suggesting that changes in overshoot amplitude contribute to spike broadening. However, a continuous decline in spike undershoot amplitudes throughout the train suggests a relation between the factors underlying progressive spike broadening and progressive decline of undershoot (Fig. 3). 3. Progressive spike broadening is dependent on the presence of external Ca⁺⁺ (Fig. 4), indicating that late Ca⁺⁺ current supports the prolonged depolarization of the broadened spike. In contrast, the progressive decay of the amplitudes of the K⁺-dependent undershoots is not dependent on external Ca⁺⁺ (Fig. 5). Tetraethylammonium ion (TEA) applied extracellularly or injected intracellularly causes extreme spike prolongation (Fig. 6), indicating the presence of the delayed K⁺ current known to inactivate with depolarization in other molluscan neurons (Aldrich et al. 1979a). Both by analogy with a previous study of spike broadening (Aldrich et al. 1979b) and direct ly, these data suggest that spike broadening during repetitive firing is largely due to a progressive decrease in a delayed K⁺ current, which thus permits spike prolongation by inward Ca⁺⁺ current. 4. While the major contribution to spike broadening appears to arise from decrement of the TEA-sensitive K⁺ current, internal Ca⁺⁺ levels appear to have a significant role in modulating the rate and extent of spike broadening. This role may be effected through a Ca⁺⁺-activated K⁺ current (I_K,Ca), and possibly by regulation of the Ca⁺⁺ conductance itself. I_K,Ca is demonstrable in the undershoots of single, unbroadened action potentials, whose waveforms and amplitudes are affected by agents and treatments known to suppress I_K,Ca (low Ca⁺⁺, Ba⁺⁺, Co⁺⁺, and injections of EGTA) or enhance it (high Ca⁺⁺, high Ca⁺⁺-buffer injection) (Figs. 7, 8, 9 and 10). 5. Intracellular injection of the Ca⁺⁺ chelator, EGTA, or replacement of external Ca⁺⁺ by Ba⁺⁺ enhances progressive spike broadening (Fig. 11). Conversely, intracellular injection of high Ca⁺⁺ (EGTA) buffers suppresses broadening (Figs. 12 and 13). These experiments suggest a possible role for intracellular Ca⁺⁺ regulation in modulating this form of functional neuronal plasticity (Fig. 14).