Response properties of higher level neurons in the central olfactory pathway of the crayfish

Abstract

We have examined the electrical activity of interneurons within the higher levels of the crayfish olfactory system. In unstimulated isolated crayfish head preparations, local protocerebral interneurons (LPI) of the hemiellipsoid bodies generate periodic, low-frequency membrane depolarizations. The most reasonable explanation for these baseline fluctuations, which were exhibited by all of the LPIs examined and which were reversibly abolished by either tetrodotoxin or low-calcium saline solution, is that they reflect periodic synaptic drive from the axon terminals of olfactory projection neurons. One-third of tested LPIs generated impulses in response to the odor stimuli we applied to the antennules. Those cells that did respond exhibited a brief excitatory postsynaptic potential and one or two action potentials, even during prolonged odor pulses. Many of the responding neurons also exhibited a delayed impulse burst 1 or 2 s following the stimulus pulse. Most of the responding cells recovered their sensitivity to odors very slowly, exhibiting disadaptation periods of several minutes. The apparent refractory nature of individual LPIs to olfactory stimulation is attributed in part to a hypothesized selectivity of connections between projection neurons and protocerebral targets and in part to the electrical isolation of the recording electrode from many regions of the extensive LPI dendritic tree.