Maria Gabriela DE BRITO SANCHEZ
Max-Planck-Institut für Verhaltensphysiologie,
82319 Seewiesen/Starnberg, FRG.
In female moths of Bombyx mori, two olfactory receptor cells are found in each antennal sensilium trichodeum. They seem to be maximally activated by benzoic acid and a terpene alcohol (2,6-dimethyl-5-hepten-2-ol) so that they have been characterized as "benzoic-acid" and "terpene-cell" respectively (Priesner 1979). The latter cell type also responds to linalool (3,7-dimethyl-1,6-octadien-3-ol). The only known natural stimulus capable of activating the benzoic-acid cell is the meconium (intestinal waste products of the pupa, excreted by adult moth); however a behavioral response to meconium is not known (Heinbockel & Kaissling 1990). On the other hand. fluttering in females is elicited by linalool but not by benzoic acid (Priesner 1979). Studies on the structure -activity relationships of antenna! benzoicacid receptor cells ot the female of Bombyx mori were undertaken to characterize their electrophysiological response and provide a basis for inferences on the mechanisms of interaction with receptor sites. The benzoic-acid receptor cell may be considered as a specialist (Schneider 1969) since it responds maximally to a key substance. the benzoic acid, and has identical side spectra for less effective compounds.
Based on single-sensillum recordings, our studies demonstrate that the benzoic-acid cell responds to a halogen substitute of benzoic acid (mfluor benzoic acid) even more than to benzoic acid itself. Similar effects were observed for stimulation with m-fluor benzaldehyde: the response to this compound was greater than that to benzaldehyde. Using different halogen substitutes (F, Cl, Br, D, we also showed that, in both compounds, the effectiveness decreased with increasing atomic size of the substitute. The position of the halogen (ortho, meta, pare) also affected the cell response: this was greater with the halogen in mete position.
These results reveal that a critical feature of the stimulus molecule is the inductive effect generated by the halogen substitutes. Such an effect, which consists in the attraction of ~c electrons to the halogen, is greatest in the mete position. For benzoic acid, this results in a greater degree of dissociation, i. e. in a greater degree of acidity. Clearly, such an effect will be strongest for the most electronegative substitute, F. Increasing the atomic size of the halogen substitute could also affect the recognition of the molecule by the receptor cell due to steric effects. The fact that both m-fluor benzoic acid and m-fluor benzaldehyde elicit a greater response than benzoic acid and benzaidehyde; respectively, may be due to their increased volatility.
Stimulation with halogen substitutes of benzene generated an inhibition of the electrical response during stimulation, visible as a hyperpolarization and absence of nerve impulses, and was followed by a depolarization and an increase in the firing rate of action potentials. The inhibition and the subsequent activation were stronger for benzene with halogen substitutes of increasing size. This fact is contrary to the observed dependence of the cell response on the atomic size of the halogen substituted benzoic acid and benzaldehyde. The effects of Br and 1substituted benzene are similar to the effects of general anaesthetics (Stange and Kaissling 1995).
ReferencesThis work was also presented at the 24th Gottingen Neurobiology Conference 1996.