Mechanisms underlying odorant-induced and spontaneous calcium signals in olfactory receptor neurons of spiny lobsters, Panulirus argus
We determined if a newly developed antennule slice preparation allows studying chemosensory properties of spiny lobster olfactory receptor neurons under in situ conditions with Ca2+ imaging. We show that chemical stimuli reach the dendrites of olfactory receptor neurons but not their somata, and that odorant-induced Ca2+ signals in the somata are sufficiently stable over time to allow stimulation with a substantial number of odorants. Pharmacological manipulations served to elucidate the source of odorant-induced Ca2+ transients and spontaneous Ca2+ oscillations in the somata of olfactory receptor neurons. Both Ca2+ signals are primarily mediated by an influx of extracellular Ca2+ through voltage-activated Ca2+ channels that can be blocked by CoCl2 and the L-type Ca2+ channel blocker verapamil. Intracellular Ca2+ stores contribute little to odorant-induced Ca2+ transients and spontaneous Ca2+ oscillations. The odorant-induced Ca2+ transients as well as the spontaneous Ca2+ oscillations depend on action potentials mediated by Na+ channels that are largely TTX-insensitive but blocked by the local anesthetics tetracaine and lidocaine. Collectively, these results corroborate the conclusion that odorant-induced Ca2+ transients and spontaneous Ca2+ oscillations in the somata of olfactory receptor neurons closely reflect action potential activity associated with odorant-induced phasic-tonic responses and spontaneous bursting, respectively. Therefore, both types of Ca2+ signals represent experimentally accessible proxies of spiking.