Cultured olfactory interneurons from Limax maximus: Optical and electrophysiological studies of transmitter-evoked responses

1. The olfactory processing network in the procerebral (PC) lobe of the terrestrial mollusk Limax maximus exhibits a coherent oscillation of local field potential that is modulated by odor input. To understand the cellular basis of this oscillation, we developed a cell culture preparation of isolated PC neurons and studied the responses of isolated cells to stimulation with neurotransmitters known to be present in the PC lobe. 2. The distribution of PC soma diameters suggests at least two different populations of neurons. Approximately 95% of isolated cells had soma diameters of 7-8 μm, with the remaining cells having larger diameters (10-15 μm). 3. Extracellular measurements of action potentials and optical measurements of intracellular calcium concentrations in fura-2-loaded cells were made. Serotonin and dopamine excited PC neurons and promoted transitions from steady to bursty activity. Both amines elicited increases in intracellular calcium, presumably concomitant with the increase in action-potential frequency. 4. Glutamate suppressed action-potential firing and reduced intracellular calcium. This effect was seen most clearly when glutamate was applied to cells excited by high potassium medium. Quisqualate is an effective glutamate agonist in this system, whereas kainate is not. 5. Combined with anatomic and biochemical data and with studies of the effects of these neurotransmitters on the oscillating local field potential of the intact PC network, the data from isolated PC neurons are consistent with the hypothesis that dopamine and serotonin modulate network dynamics, whereas glutamate is involved in generating the basic oscillation of local field potential in the PC. 6. The optical studies of fura-2-loaded cells showed that several treatments that increase the rate of action-potential production lead to elevations in intracellular calcium. Optical studies of intracellular calcium may be useful for multisite measurements of activity in the intact, oscillating PC lobe network.