Initiation and propagation of calcium-dependent action potentials in a coupled network of olfactory interneurons

Coherent oscillatory electrical activity and apical-basal wave propagation have been described previously in the procerebral (PC) lobe, an olfactory c enter of the terrestrial slug Limax maximus. In this study, we investigate the physiological basis of oscillatory activity and wave propagation in the PC lobe. Calcium green dextran was locally deposited in the PC lobe; this led to cellular uptake and transport of dye by bursting and nonbursting neu rons of the PC lobe. The change of intracellular calcium concentration was measured at several different positions in neurites of individual bursting neurons in the PC lobe with a two-photon laser-scanning microscope. Fluores cence measurements were also made from neurons intracellularly injected wit h calcium green-1. Two different morphological classes of bursting neurons were found, varicose (VB) and smooth (SB). Our results from concurrent opti cal and intracellular recordings suggest that Ca2+ is the major carrier for the inward current during action potentials of bursting neurons. Intracell ular recordings from bursting neurons with nystatin perforated-patch electr odes made while simultaneously recording the local field potential (LFP) wi th extracellular electrodes indicate that the burster spikes are precisely phase-locked to the periodic LFP events. By referencing successive calcium measurements to the common LFP signal, we could therefore accurately determ ine the relative timing of calcium transients at different points along a n eurite. Measuring the relation of temporal to spatial differences allowed u s to estimate the velocity of action potential propagation, which was 4.3 /- 0.2 (SE) mm/s in VBs, and 1.3 +/- 0.2 mm/s in SB.