Whole-cell recording coupled with biocytin injection revealed four types of interneurons intrinsic to the olfactory lobe (OL) of the spiny lobster Panulirus argus. Each type of neuron had a distinct pattern of arborization within the three anatomically defined regions of OL glomeruli (cap, subcap and base). Type I interneurons innervated all three regions, while types II, III and IV branched only in the cap, subcap and base, respectively. Type I interneurons responded to electrical stimulation of the antennular (olfactory) nerve with a burst of 1­20 action potentials and a 1­10 s depolarization. Type II (cap) interneurons responded to the same input with a burst of 1­3 action potentials followed by a shorter hyperpolarization. Type III (subcap) interneurons responded with a burst of 1­6 action potentials followed by a delayed, 0.5­4 s depolarization. Type IV (base) interneurons responded with a brief depolarization or a burst of 1­3 action potentials followed by a 1 s hyperpolarization. The regionalized arborization and the different response properties of the type II, III and IV interneurons strongly imply that lobster olfactory glomeruli contain functionally distinct regions, a feature that should be useful in understanding the multiple synaptic pathways involved in processing olfactory input.

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