Multiple inhibitory pathways shape odor-evoked responses in lobster olfactory projection neurons

This study characterizes odor-evoked responses of the glomerular output neurons of the spiny lobster olfactory lobe, and implicates previously identified γ-aminobutyric acid (GABA)- and histamine-mediated inhibitory pathways in shaping these responses. Odor-evoked responses were more complex than electrically evoked responses, with up to three distinct components: a brief, short-latency (fast) depolarization, a longer-duration, longer-latency (slow) depolarization, and a slow hyperpolarization. Seventy-seven percent of all responses contained the hyperpolarization, while only 31% and 23% contained the fast and slow depolarizations, respectively. The broader tuning of the hyperpolarization relative to the other two components suggests that the hyperpolarization mediates lateral inhibitory interactions across olfactory glomeruli. Perfusing the brain with the GABA-receptor antagonist picrotoxin increased the amplitude of the hyperpolarization, while the histamine-receptor antagonist cimetidine decreased the hyperpolarization in some instances but increased it in others. Pharmacological enhancement or suppression of the hyperpolarization could mask or unmask, respectively, the slow depolarization. Both antagonists could also cause the appearance of the fast depolarization when it was not apparent prior to treatment. We conclude that GABA- and histamine-mediated inhibition contributes to the overall pattern of the response of projection neurons to odors by regulating the relative strength of these three distinct types of input.