Programmed cell death of an identified motoneuron examined in vivo: electrophysiological and morphological correlates

A paucity of information exists about the electrophysiological and anatomical correlates of neurons committed to die in vivo. Thus, we examined how an identified neuron, motoneuron MN-12, dies during development in the intact moth Manduca sexta. The developmental programmed cell death of this motoneuron was examined because of its defined commitment point of death. In addition, its ability to be unambiguously identified between animals and its accessibility to recording and dye injection facilitated our examination. MN-12 becomes committed to die approximately 28-30 h after adult emergence. At this time, MN-12 can no longer be saved by manipulations of steroid hormone levels, protein synthesis, or removal of descending inputs. Our initial prediction was that within a few hours after the commitment point, MN-12 would begin showing a gradual loss of central arbors and alterations in membrane properties. Contrary to our expectations, we found the MN-12 motoneuron to exhibit a stable central morphology and electrophysiological profile for approximately 12 h, followed by a rapid dismantling that occurred within a 1- to 2-h period. Several hours prior to the commitment point, the target muscle of MN-12 was no longer viable; yet, this did not affect the death of MN-12 or cause retraction of its motor terminals. We conclude that the delayed (12-h) onset of rapid cell death is not preceded by a slow accumulation of damages to the neuronal membrane (e.g., ion channels or cytoskeletal components) as both the electrical activity and morphology of MN-12 remained measurably unaltered during this 12-h lag.