Growth cone collapsing factors induce growth cone collapse or repulsive growth cone turning by interacting with membrane receptors that induce alterations in the growth cone cytoskeleton. A common change induced by collapsing factors in the cytoskeleton of the peripheral domain, the thin lamellopodial area of growth cones, is a decline in the number of radially aligned F-actin bundles that form the core of filopodia. The present study examined whether ML-7, a myosin light chain kinase inhibitor, serotonin, a neurotransmitter and TPA, an activator of protein kinase C, which induce growth cone collapse of Helisoma growth cones, depolymerized or debundled F-actin. We report that these collapsing factors had different effects. ML-7 induced F-actin reorganization consistent with debundling whereas serotonin and TPA predominately depolymerized and possibly debundled F-actin. Additionally, these collapsing factors induced the formation of a dense actin-ring around the central domain, the thicker proximal area of growth cones [Zhou and Cohan, 2001: J. Cell Biol. 153:1071-1083]. The formation of the actin-ring occurred subsequent to the loss of actin bundles. The ML-7-induced actin-ring was found to inhibit microtubule extension into the P-domain. Thus, ML-7, serotonin, and TPA induce growth cone collapse associated with a decline in radially aligned F-actin bundles through at least two mechanisms involving debundling of actin filaments and/or actin depolymerization.
The present experiments addressed the questions of how electrical stimulation influenced the magnitude, time course, and regional levels of free intracellular calcium of different identified neurons. The calcium concentration in the growth cones, neurites and cell bodies of Helisoma buccal neurons B4 and B19 was measured while somata were electrically stimulated via an intracellular electrode. The findings showed that calcium levels in B4 and B19 increased monotonically with increasing stimulation frequency. However, the range of calcium levels evoked by electrical stimulation differed significantly for each type of neuron. The greater increase in calcium concentration in B4 was correlated with its longer duration action potential compared to B19. The increase in calcium concentration was much smaller in the cell bodies than in the growth cones and neurites. Extending the duration of the B19 action potential produced a sixfold increase in the change in calcium concentration at 2 Hz stimulation. Under conditions where the electrical stimulation produced a calcium concentration of < 160 nM, the elevated level of free intracellular calcium remained constant. When calcium concentration increased above 200 nM in both identified neurons, an initial peak concentration was followed by a decline to a lower concentration suggesting increased calcium buffering occurring above 200 nM. By correlating the calcium concentration data herein with growth data from a previous study, we suggest that specific calcium levels that influence neurite outgrowth may differ widely between neurons.
The control of free intracellular calcium concentration ([Ca2+]i) is necessary for cell survival because of the ubiquitous and essential role this second messenger plays in regulating numerous intracellular processes. Calcium regulation in neurons is especially vigorous because of the large calcium influx that occurs through voltage-gated channels during membrane depolarization. In this study we examined changes in ionic currents that can limit calcium influx into neurons during electrical activity. We found that the [Ca2+]i in electrically stimulated Helisoma B4 neurons initially increased to a peak and then relaxed to lower concentrations in tandem with a decline in the action potential peak voltage. The decline in [Ca2+]i and the peak action potential voltage in this sodium and calcium driven neuron was found to be a dual manifestation of I(Na) and I(Ca) inactivation. I(Na) and I(Ca) both displayed voltage dependent inactivation. Additionally, I(Na) and I(Ca) progressively inactivated at [Ca2+]i above 200 nM, concentrations readily attained in electrically stimulated B4 neurons. Calcium and voltage dependent I(Na) and I(Ca) inactivation were found to reduce calcium influx during continuous electrical stimulation by decreasing both the magnitude of I(Ca) that could be activated and the percent of the available I(Ca) that would be activated due to the diminished peak action potential voltage. Calculations based on data herein suggest that the voltage and calcium dependent I(Na) and I(Ca) inactivation that occurs during continuous electrical stimulation dramatically reduces calcium influx in this sodium and calcium driven neuron and thus limits the increase in [Ca2+]i.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.