Figure 1.

Intrinsic bursting properties that emerge in upper lumbar ventromedial interneurons by removing [Ca²⁺]_o are related neither to small Ca²⁺-activated K⁺ conductance nor to intracellular Ca²⁺ processes. A1, Voltage traces showing a typical ventromedial interneuron switching its firing pattern from tonic spiking to bursting after a progressive depletion of the [Ca²⁺]_o. A2, Repetitive events recorded in the cell-attached configuration from the surface of a bursting ventromedial interneuron. A3, Slow afterhyperpolarizations after a spike (truncated) evoked by a 3 ms current pulse (bottom trace) before (black trace) and after removing [Ca²⁺]_o (gray trace). Each trace in A3 represents the average of three sweeps. The initial holding potential was −60 mV. B1–B3, Voltage traces from a ventromedial interneuron exhibiting a bursting behavior in [Ca²⁺]_o-free ACSF (B1) but not in 1.2 mm [Ca²⁺]_o (B2) even after bath application of 100 nm apamin (B3). B4, Voltage traces (average of 3 sweeps) showing slow afterhyperpolarizations that follow spikes (truncated) evoked by a 3 ms current pulse (bottom trace) before (black trace) and after 100 nm apamin (gray trace). The initial holding potential was −60 mV. C1, C2, Voltage traces of a neuron recorded by means of an electrode filled with BAPTA (5 mm) before (C1) and after the removal of the extracellular calcium (C2). The inset (3 sweeps averaged) in C1 shows the absence of slow afterhyperpolarizations after spikes (truncated) evoked by a 3 ms current pulse. The initial holding potential was −50 mV.