Spontaneous bursting induced by convulsant agents in an identified insect neurone.

1. Using an in vitro preparation, intracellular recordings were made from the cell body of the "fast" coxal depressor motoneurone (Df) from the cockroach, Periplaneta americana. 2. This cell was normally quiescent in the absence of injected current but could respond to applied depolarizing current with one or more plateau potentials: regenerative depolarizations which can drive production of axonal impulses. However, when the convulsant agents picrotoxin (PTX; 10(-5) M) or pentylenetetrazole (PeTZ; 25 mM) were applied Df exhibited spontaneous bursting behaviour. 3. The bursting activity induced by the two agents was qualitatively different. The bursting pattern induced by PTX was irregular and occurred only after a delay of 20-40 min; the transformation to bursting activity occurred without any consistent changes in membrane potential and effective membrane resistance (5.9 +/- 0.4 M omega; n = 25; mean +/- SEM) remained unchanged (P > 0.1). PTX did not induce spontaneous bursting or cause a significant change in plateau potential threshold or membrane potential in isolated somata. Therefore, it appeared to exert its actions or more distant regions of the neurone, probably by enhancing the effects of excitatory synaptic input. 4. In contrast, PeTZ rapidly (30 sec-2 min) induced regular bursting activity. This agent could also produce spontaneous bursting activity in isolated somata; thus it exerted its actions primarily upon intrinsic membrane properties, although it also enhanced the effects of electrically driven synaptic stimulation of the non-isolated Df somata. 5. Experiments with PTX also revealed that bursting could be recorded in cells in which it was not possible to evoke plateau potentials by electrically depolarizing the soma. This suggests that other regions of the cell possess the machinery necessary to produce burst-like behaviour.