Intensity of odorant stimulation affects mode of Ca2+ dynamics in rat olfactory receptor neurons

We investigated the relation between the intensity of odorant stimulation and the mode of spatiotemporal Ca2+ dynamics in Fluo-4-loaded rat olfactory receptor neurons (ORNs) using a confocal laser scanning microscope. We found that relatively smaller Ca2+ transients remained confined to the knob while larger ones spread to the soma with latency. Prolonged odor exposure ensured the spread of Ca2+ transients from the knob to the soma. Upon exposing ORNs to progressively increasing concentrations of odor, the Ca2+ transients that were confined to the knob at lower concentrations extended to the soma at higher concentrations. Stimulation with progressively increasing concentrations of forskolin plus IBMX yielded identical results. Partial inhibition of adenylyl cyclase by MDL12330A changed the odor response extending to the soma to a response confined to the knob. Blocking of L-type Ca2+ channels by nifedipine reduced the magnitude of the response extending to the soma but had no effect on the response confined to the knob. It is thus suggested that Ca2+ transients confined to the knob represent weak stimulation, and, speculatively, such responses either constitute inhibitory responses or indicate weak excitatory responses that fail to outstand the spontaneous electrical noise of ORNs.