DUAL INHIBITORY PATHWAYS MEDIATED BY GABA AND HISTAMINERGIC INTERNEURONS IN THE LOBSTER OLFACTORY LOBE

Antisera to GABA and histamine (HA) label distinct populations of inte rneurons that innervate glomeruli in the olfactory lobe (OL) of the sp iny lobster. GABA-immunoreactive interneurons branch most heavily in t he cap of the glomeruli, while HA-immunoreactive interneurons branch m ostly in the glomerular subcap. Perfusing GABA or HA into the isolated brain increases the intensity of electrical stimulation of the antenn ular nerve necessary to elicit action potentials in OL projection neur ons. The GABA receptor antagonist picrotoxin (30-100 mu mol . l(-1)) a nd the HA receptor antagonist cimetidine (1-5 mmol . l(-1)) both reduc e the stimulus intensity needed to elicit action potentials. However, cimetidine also eliminates the hyperpolarizing phase of the evoked res ponse and reveals a delayed, prolonged excitation of up to 10 s, where as picrotoxin enhances the hyperpolarization and, at higher concentrat ions, transiently suppresses all phases of the evoked response. We con clude that GABA- and HA-ergic interneurons constitute two overlapping, yet functionally distinct inhibitory pathways in the OL, an organizat ional feature which may be fundamental to processing at this level of the olfactory pathway.