Selective calcium imaging of olfactory interneurons in a land mollusk.

The procerebrum (PC) of the land slug Limax is the olfactory center involved in olfactory discrimination and learning. The PC contains two types of neurons, bursting (B) and nonbursting (NB) neurons. B neurons are local interneurons and show spontaneous bursting while NB neurons directly receive input from olfactory nerve fibers, and are possibly involved in odor coding. In order to observe activity of NB neurons, selective calcium imaging was made by retrograde staining of NB neurons with the calcium sensitive dye, rhod-2 AM, from the internal mass (IM) of the PC. The stained cells, presumably NB neurons, appeared to be mostly silent, but showed transient calcium elevation in response to electrical stimulus to the tentacle nerve. Stimulation of the tentacle with an odorant also evoked calcium events in some of the stained cells. These included one or multiple calcium events, and these events were synchronized with each other. These results revealed characteristics of the activity of NB neurons.