Functional units of a compound nose: Aesthetasc sensilla house similar populations of olfactory receptor neurons on the crustacean antennule

The lateral flagellum of the antennule of the spiny lobster Panulirus argus houses more than 1,000 morphologically similar olfactory sensilla, called aesthetascs. By using a high-resolution activity labeling technique that de pends on entry of agmatine into olfactory receptor neurons (ORNs) through c ation channels during odor stimulation, we examined the distribution of dif ferent functional types of ORNs within and across mature aesthetascs. A sig nificant number of ORNs in mature aesthetascs are labeled with agmatine dur ing stimulation by single odorants, including adenosine-5'-monophosphate, a mmonium chloride, cysteine, glycine, proline, and taurine. The percentage o f ORNs per aesthetase that was agmatine labeled during odor stimulation ave raged 0.5-1.6% for single compounds and 4.6% for a 33-component mimic of oy ster tissue. For most antennules and antennular regions studied, the percen tage of agmatine-labeled ORNs by stimulation with single or complex odorant s was statistically homogeneous across most or all aesthetascs. The extent of heterogeneity among mature aesthetascs was correlated with their age: ex tensive heterogeneity was observed only in the distal part of the flagellum containing the oldest aesthetascs and their ORNs. Thus, it appears that ov er most of the length of the aesthetase-bearing region of the lateral flage llum, different and distinct functional types of aesthetascs do not exist. Rather, aesthetascs appear to be repetitive morphological and functional un its in olfactory coding. However, because odor sensitivity of ORNs can chan ge with the age of an aesthetase, some development-related functional heter ogeneity exists among aesthetascs. J, Comp. Neurol. 418:270-280, 2000. (C) 2000 Wiley-Liss, Inc.