Effects Of Dopamine On Olfactory Learning Of Bactrocera Dorsalis

Bactrocera dorsalis(Hendel) is a worldwide quarantine pest.It is common to use methyl eugenol(ME),a male attractant,to reduce the population of male mixing with pesticides.With the widely use of ME,the control effect is decreasing.Previous studies have shown that adults of B.dorsalis can combine ME with a saturated salt solution for aversive olfactory learning,and confirmed that aversive olfactory learning is an important factor leading to the decline of ME attraction activity.Learning and memory are two inseparable processes of neural activity that alter the insect’s preferences,while dopamine(DA)is ubiquitous in insects and regulates learning and memory behavior.We explored the effect of dopamine on the olfactory learning and memory of B.dorsalis.Firstly,we optimised of unconditional stimuli in aversive olfactory learning conditoning of B.dorsalis and used the classical olfactory conditioning to identify the aversive olfactory learning ability of ME combined with various concentration of N,N-Diethyl-3-methyl benzoyl amide(DEET)and saturated sodium chloride solution.Then,the effects of DA antagonists on the olfactory learning and memory of Bactrocera dorsalis were studied by behavioral and pharmacological,and then the neurobiological mechanism of DA on learning and memory of B.dorsalis was illuminated,which could provide a theoretical basis for developing insect behavioral regulators to improve the trapping effect of ME and provide a reference for the development of biological pesticides targeting on DA receptors.The main findings are as follows:(1)Optimization of unconditional stimuli in aversive olfactory learning conditioning of B.dorsalis.Combined with ME and different concentrations(0.2%,0.4%,0.6% and 0.8%)of DEET and saturated sodium chloride solution,the aversive olfactory conditioning of B.dorsalis was carried out 5 times,and the learning performance was strengthened with the increase of the concentration of aversive substances.The proportion of proboscis extension reflex(PER)decreased from 100.00% to 35.00%.However,when the concentration of the aversive substance reached 0.8%,the learning performance of B.dorsalis was reduced,and the proportion of PER decreased from 100.00% to 43.33%.At the same time,through the bioassay of learning effects after conditioning,we found that the lured proportion of the paired group was significantly lower than the untrained group and the unpaired group.Feeding B.dorsalis with different concentrations(0.2%,0.4%,0.6% and 0.8%)of DEET and saturated sodium chloride solution showed that the intake of saturated sodium chloride solution was significantly less than that of all DEET groups.The results showed that B.dorsalis can combine the ME with DEET for aversive learning,and DEET can be used as unconditioned stimuli olfactory learning memory for B.dorsalis.When the DEET concentration is 0.6%,the olfactory learning performance of B.dorsalis is the strongest.(2)Determination of memory retention time of B.dorsalis after aversive olfactory conditioning.When the concentration of 0.6% DEET was used as aversive substance,the aversive olfactory conditioning was carried out on B.dorsalis.At the end of conditioing,the PER was measured at 10 min,1 h,3 h,6 h,12 h and 24 h.We found that the memory of B.dorsalis showed a decay of the proportion of PER with the increase of time,but there was a significant difference between before and after conditioning.The proportion of PER to ME were 35.00%,55.00%,51.67%,55.00%,76.67%,76.67% and 61.67%.Ours results showed that the memory time can be persisted for at least 24 hours after aversive olfactory conditioning,and the formation of long-term memory can provide necessary conditions for subsequent behavioral determination and pharmacological studies.(3)The effect of DA on the olfactory learning of B.dorsalis.Before the aversive olfactory learning conditioning of combined ME with 0.6% DEET,the DA antagonist Fluphenazine was injected into B.dorsalis,and the proportion of PER of the 30 min group after the corresponding training times was significantly higher than that of the 60 min group.The proportion of PER of the 50 μM and 500 μM treatment groups were 46.67% and 67.74%,respectively,which were significantly higher than the control group(28.13%).These results showed that DA was involved in the regulation of aversive olfactory learning of B.dorsalis,and there was a time and concentration effect.However,before the appetitive olfactory conditioning of orange essential oil and sucrose solution,DA antagonists were injected into B.dorsalis,and it was found that DA was not involved in the regulation of the olfactory learning of Bactrocera dorsalis.(4)The effect of DA on the learning and memory of the olfactory learning of B.dorsalis.DA antagonists Fluphenazine and Chlorpromazine were injected into B.dorsalis 24 hours after the end of aversive olfactory learning conditioning by combined ME with 0.6% DEET.The proportion of PER of Fluphenazine group and Chlorpromazine group were 66.67% and 63.33%,respectively.It was higher than the control group(36.67%),which indicated that DA was involved in the regulation of the aversive olfactory learning and memory retention of B.dorsalis;after 24 hours of the end of the appetitive olfactory learning conditioning of orange essential oil and sucrose solution injected DA antagonists Fluphenazine and Chlorpromazine.Fluphenazine group and Chlorpromazine group were 13.33% and 16.67%,respectively,and there were significant differences between the treatment group and the control group(53.33%),indicating that DA also participates in the regulation of appetitive olfactory learning and memory retention.