Oxidative stress and age-related olfactory memory impairment in the honeybee Apis mellifera.

Abstract

is one of the most impor-tant insect species to study olfactorylearning and memory because it canbe conditioned to respond with feed-ing movements of the mouthparts (pro-boscis)to avariety offloral odors by usingProboscis Extension Reflex (PER) condi-tioning. Honeybee workers switch fromin-hive (nursing) to outside-hive (forag-ing) tasks depending on their age, colonydemand, and outside conditions. Workersuse their sophisticated cognitive abilitiesin both foraging, and task performancewithin the colony (Menzel and Giurfa,2001; Frasnelli et al., 2014). The honey-bee olfactory system possesses olfactorysensory neurons inside the cuticle-coveredsensillae along the antennae, which areequivalenttoolfactoryepitheliawithinthenasal cavity in vertebrates. From antennathe information is carried via axons ofolfactory sensory neurons directly to theantennal lobe (AL) that is equivalent tothe olfactory bulb (OB) in vertebrates.This information is processed in the ALthen relayed by projection neurons tothe mushroom bodies (MB), which con-tribute to memory consolidation asso-ciated with long-term potentiation andsynaptic organization (Hourcade et al.,2010).Theexhibitingsynapticplasticityinthe MB is similar to that of mammalianhippocampal synaptic plasticity (Bliss andCollingridge, 1993).The neuronal synaptic plasticity canbe regulated by important signalingmolecules called as “biogenic amines”that modulate synaptic morphology,number of synapses, and receptors,influencing animal behaviors includ-ing complex behaviors such as learningand memory formation in both ver-tebrates and invertebrates. Prominentexamples of biogenic amines includeepinephrine, norepinephrine, dopamine,serotonin, octopamine, and tyramine.Norepinephrine and epinephrine arepreferentially synthesized by vertebrates;whereas octopamine and tyramine arepreferentially synthesized by invertebrates.Biogenic amines exert their activity byinteracting with specific G-protein cou-pled receptors, causing changes in thelevels of intracellular second messengers(Scheiner et al., 2006). In vertebrates,both the OB and cortex receive heavyinputs from cholinergic, noradrenergic,and serotonergic modulatory systems,exerting profound effects on both odorprocessing and odor memory by actingon both inhibitory local interneurons γ-amino butyric acid (GABA) and outputneurons in both regions (Fletcher andChen, 2010). The primary sensory affer-ents from the olfactory neuroepitheliumto OB can be modulated by a presynap-tic inhibition-mediated by GABA anddopamine released from bulbar interneu-rons. Increased levels of octopamine inthe AL mediate an important role in areinforcementpathwayinvolvingolfactorylearning and memory in