Associative learning of plant odorants activating the same or different receptor neurones in the moth Heliothis virescens.

The importance of olfactory learning in host plant selection is well demonstrated in insects, including the heliothine moths. In the present study olfactory conditioning of the proboscis extension response was performed to determine the moths' ability to learn and discriminate three plant odorants: beta-ocimene and beta-myrcene (activating the same receptor neurone type), and racemic linalool (activating two different types). The conditioned stimulus (CS) was an air puff with each odorant blown into a constant air stream and over the antennae, and the unconditioned stimulus (US) was sucrose solution applied first to the antennal taste sensilla, then to the proboscis. Conditioning with increasing odorant concentrations induced increased learning performance. The concentration threshold for learning was 100 times lower for racemic linalool than for the two other odorants, a fact that can be correlated with a higher sensitivity of the moths' antennae to racemic linalool as shown in electroantennogram recordings. After correcting for the different odour sensitivities, the moths' ability to discriminate the odorants was studied. Differential conditioning experiments were carried out, in which moths had to distinguish between a rewarded (CS+) odorant and an explicitly unrewarded odorant (CS-), choosing odour concentrations giving the same learning rate in previous experiments. The best discrimination was found with beta-myrcene as the rewarded odorant and racemic linalool as the unrewarded. The opposite combination gave lower discrimination, indicating a higher salience for beta-myrcene than for racemic linalool. The moths could also discriminate between beta-ocimene and beta-myrcene, which was surprising, since they activate the same receptor neurone type. No difference in salience was found between these two odorants.

Representation of primary plant odorants in the antennal lobe of the moth Heliothis virescens using calcium imaging.

The primary olfactory centre, the antennal lobe of Heliothis virescens moths, contains 62 glomeruli which process plant odour information and four male-specific glomeruli which form the macroglomerular complex, involved in processing information about pheromone and interspecific signals. Using calcium imaging, we recorded the spatio-temporal activity pattern of the glomeruli in the anterior antennal lobe during stimulation with odorants produced by plants or insects. Each odorant elicited specific excitatory responses in one or a few glomeruli: the major pheromone component did so exclusively in the large glomerulus of the macroglomerular complex and the plant odours exclusively in the ordinary glomeruli. Eight glomeruli, with corresponding plant odour responses and positions, were identified within each sex. Glomeruli responded specifically to linalool, beta-ocimene/beta-myrcene or germacrene D/alpha-farnesene. Responses to two essential plant oils covered the response areas of their major constituents, as well as activating additional glomeruli. Stronger activation in the AL due to increased odour concentration was expressed as increased response strength within the odorant-specific glomeruli as well as recruitment of less sensitive glomeruli.