Electrophysiological responses of the chemoreceptor neurones in the antennal taste sensilla to plant alkaloids and glucosides in a granivorous ground beetle

The electrophysiological response of chemoreceptor neurones from the antennal chaetoid taste sensilla of the omnivorous ground beetle Pterostichus oblongopunctatus to several plant alkaloids and glucosides is investigated. A quinine‐sensitive neurone responding to quinine and quinine hydrochloride is found, most probably related to the granivorous feeding habit of P. oblongopunctatus. The response to quinine hydrochloride is concentration‐dependent at 0.001–50 mm , with the response threshold at 0.01 mm and a maximum rate of firing of 67 spikes/s at 50 mm . The stimulatory effect of caffeine is very weak, where the firing rate increases by only 1.4 spikes/s at a concentration of 10 mm compared with that evoked by a control stimulus. In addition, both quinine and quinine hydrochloride strongly inhibit spike production by the salt‐ and pH‐sensitive neurones when presented in mixtures with 10 mm NaCl. Several tested plant secondary compounds (i.e. salicin, sinigrin, caffeine and nicotine), which have only little or no effect on the firing rate of the quinine‐sensitive neurone, greatly reduce the responses of the salt‐ and pH‐sensitive neurones. The results of the present study suggest that the antennal taste sensilla of P. oblongopunctatus may detect plant defensive compounds both through the activation of a quinine‐sensitive neurone and via peripheral inhibition of other chemoreceptor neurones of the taste sensillum.     

Antennal sugar sensitivity in the click beetle Agriotes obscurus

The occurrence of salt‐, sugar‐sensitive neurones and a mechanoreceptor neurone in the antennal hair‐like gustatory sensilla of the click beetle Agriotes obscurus L. (Coleoptera, Elateridae) is demonstrated using the electrophysiological sensillum tip‐recording technique. The stimulating effect of 13 water soluble sugars at 100 mm is tested on the neurones of these sensilla. Sucrose and fructose are the two most stimulating sugars for the sugar‐sensitive neurone, evoking almost 30 spikes s^?1 at 100 mm . The stimulating effect of arabinose, glucose, mannose, maltose and raffinose is three‐ to five‐fold lower, in the range 5.9–9.6 spikes s^?1. The remaining six sugars, xylose, galactose, rhamnose, cellobiose, trehalose and lactose, have very low (<1 spikes s^?1) or no ability to stimulate the sugar‐sensitive neurone. Concentration/response curves of the sugar‐sensitive neurone to sucrose, fructose and glucose at 0.01–100 mm overlap to a large extent in hibernating, cold reactivated and reproductively‐active beetles. A remarkable 9–50% decrease in the number of spikes evoked by 100 mm fructose and 10–100 mm sucrose occurs, however, in reproductively‐active beetles in June compared with beetles at the beginning of hibernation in October. These findings show that A. obscurus is capable of sensing a wide range sugars via their antennal gustatory sensilla.     

Sensitivity of antennal gustatory receptor neurones to aphid honeydew sugars in the carabid Anchomenus dorsalis

Using electrophysiology, the stimulating effect of 13 sugars and three sugar alcohols (each at a concentration of 100 mm ) to antennal gustatory receptor neurones (GRNs) is tested in the carabid beetle Anchomenus dorsalis (Pontoppidan, 1763) (Coleoptera, Carabidae). Maltose, sucrose, glucose and raffinose are the most stimulating sugars for the sugar‐sensitive neurone (SuN), evoking 6.7–18.6 spikes s^?1 in fed insects, whereas the others had little or no effect. The firing rate of the antennal GRNs is not affected by any of the tested sugar alcohols, dulcitol, inositol and sorbitol. Additionally, concentration/response curves for sucrose and maltose are obtained in the range 0.01–100 mm . The responses of beetles starved for 96 h to this range of sucrose are two‐ to three‐fold higher compared with those of fed beetles. The presence of a terminal α‐glucose unit is an important feature of the molecular structure determining the stimulating properties of the two disaccharides, maltose and sucrose, as well as glucose. The other monosaccharide unit of the molecule is also of great importance in determining the stimulating properties of various disaccharides. The sensitivity of the SuN to the four most prevalent aphid honeydew sugars suggests that A. dorsalis uses these chemicals as sensory cues when searching for aphids as prey.