Accommodation of mouse DRG growth cones to electrically induced collapse: Kinetic analysis of calcium transients and set‐point theory

Abstract

Electrical stimulation causes growth cones of mouse dorsal root ganglion neurons to collapse. During chronic stimulation, however, growth cones resume motility. In addition, these growth cones are now resistant to the collapsing effects of subsequent stimulation, a process we term accommodation. We compared the kinetics of electrically induced Ca²⁺ transients in naive and accommodated growth cones in order to determine whether the accommodation process results from a change in the Ca²⁺ transient, or a change in the Ca²⁺ sensitivity of the growth cones. Three kinetics were determined: (1) the initial increase to peak Ca²⁺ levels produced by 10 Hz stimulation; (2) recovery from peak Ca²⁺ levels during stimulus trains lasting 15 min; and (3) clearing of Ca²⁺ from growth cones after terminating the stimulus. These kinetics were analyzed using single exponential fits to changes in fura‐2 fluorescence ratios. The electrically evoked increase in Ca²⁺ was significantly slower in accommodated growth cones (τ = 6.0 s) compared to naive growth cones (τ = 1.4 s). Desptie the slower increase of [Ca²⁺]_i in accommodated growth cones, peak [Ca²⁺]_i was similar to that reached in naive growth cones, and the steady‐state Ca²⁺ level was significantly elevated after chronic stimulation. Thus, accommodated growth cones maintained outgrowth at [Ca²⁺]_i that caused collapse initially. Time course experiments show that accommodation is a slow process (t_1/2 = about 3 h). Accommodation did not induce measurable changes in the rates of Ca²⁺ homeostasis during or after stimulus trains. The kinetics of Ca²⁺ recovery during (τ = 90 s) and after 15 min of stimulation (τ = 8.5 s) was not significantly different in accommodated versus naive growth cones. Rates of ⁴⁵Ca²⁺ efflux were also similar in both types of growth cones. These results suggest two regulatory processes contributing to growth cone motility during chronic stimulation: (1) recovery of [Ca²⁺]_i to levels permissive to neurite outgrowth, and (2) an increase in the range of optimal [Ca²⁺]_i for growth cone motility. These adaptive responses of mammalian growth cones to chronic stimulation could be involved in the modulation of CNS development by electrical activity of neurons. © 1993 John Wiley & Sons, Inc.^†