The Journal of Neurophysiology Vol. 80 No. 5 November 1998, pp. 2718-2726
Copyright ©1998 The American Physiological Society

Na⁺-Dependent Neuritic Spikes Initiate Ca²⁺-Dependent Somatic Plateau Action Potentials in Insect Dorsal Paired Median Neurons

Corine Amat¹, Bruno Lapied¹, Andrew S. French², and Bernard Hue¹

¹ Laboratoire de Neurophysiologie Récepteurs et Canaux Ioniques Membranaires, Université d'Angers, F-49045 Angers Cedex, France; and ² Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada

Amat, Corine, Bruno Lapied, Andrew S. French, and Bernard Hue. Na⁺-dependent neuritic spikes initiate Ca²⁺-dependent somatic plateau action potentials in insect dorsal paired median neurons J. Neurophysiol. 80: 2718-2726, 1998. The origin of plateau action potentials was studied in short-term cultures of dorsal paired median (DPM) neurons dissociated from the terminal abdominal ganglion of the cockroach, Periplaneta americana. Spontaneous plateau action potentials were recorded by intracellular microelectrodes in cell bodies that had neurite stumps. These action potentials featured a fast initial depolarization followed by a plateau. However, only fast spikes of short duration were observed when the cell was hyperpolarized from the resting membrane potential. These two different components of the action potentials could be separated by applying depolarizing current pulses from a hyperpolarized holding potential. Application of 200 nM tetrodotoxin (TTX) abolished both fast and slow phases, but depolarization to the original resting potential by steady current injection triggered slow monophasic action potentials that could be blocked by 3 mM CoCl₂. In contrast, DPM neurons without neurites were not spontaneously active. In these cells, calcium-dependent slow monophasic action potentials were only recorded immediately after impalement or with current pulse stimulation. Immunocytochemical observations showed that dorsal unpaired median (DUM) neuron cell bodies, which are known to exhibit spontaneous sodium-dependent action potentials, reacted with an antibody directed against a synthetic peptide corresponding to the SP19 segment of voltage-activated sodium channels. In contrast, the antibody did not stain DPM neuron cell bodies but gave intense, patchy staining only in the neurite. Whole cell patch-clamp experiments performed on isolated DPM neuron cell bodies without a neurite revealed the presence of an inward current that did not inactivate completly within the duration of the test pulse. This current was insensitive to both 100 nM TTX and sodium-free saline. It was defined as a high-voltage-activated calcium current according to its high threshold of activation (30 mV) and its sensitivity to 1 mM CdCl₂ and 100 nM -conotoxin GVIA. Our findings demonstrate that spontaneous sodium-dependent spikes arising from the neurite are required to initiate slow somatic calcium-dependent action potentials in DPM neurons.

This article has been cited by other articles:

				D. Wicher, S. Messutat, C. Lavialle, and B. Lapied A New Regulation of Non-capacitative Calcium Entry in Insect Pacemaker Neurosecretory Neurons: INVOLVEMENT OF ARACHIDONIC ACID, NO-GUANYLYL CYCLASE/cGMP, AND cAMP J. Biol. Chem., November 26, 2004; 279(48): 50410 - 50419. [Abstract] [Full Text] [PDF]

				D. Wicher Peptidergic Modulation of an Insect Na+ Current: Role of Protein Kinase A and Protein Kinase C J Neurophysiol, January 1, 2001; 85(1): 374 - 383. [Abstract] [Full Text] [PDF]

				F Grolleau and B Lapied Dorsal unpaired median neurones in the insect central nervous system: towards a better understanding of the ionic mechanisms underlying spontaneous electrical activity J. Exp. Biol., January 6, 2000; 203(11): 1633 - 1648. [Abstract] [PDF]