Learning-dependent plasticity in the antennal lobe improves discrimination and recognition of odors in the honeybee

Early life imidacloprid and copper exposure affects the gut microbiome, metabolism, and learning ability of honey bees (Apis mellifera)

The pesticide imidacloprid and the heavy metal copper provide some degree of protection to plants, while at the same time causing varying degrees of damage to bees. However, few studies have investigated the negative effects of imidacloprid and copper exposure on newly emerged bees (young bees), especially when both are present in a mix. In this study, young bees were exposed to sterile sucrose solutions containing imidacloprid (10 μg/L, 100 μg/L), copper (10 mg/L, 50 mg/L), or a mix of both (10 μg/L + 10 mg/L) for 5 days to assess their gut system and behavior, with survival and dietary consumption recorded over 21 days. We found that imidacloprid and copper reduced honeybee survival, dietary intake, and learning ability, decreased gut microbiota diversity, and caused metabolic disruptions. Notably, the mix of imidacloprid and copper had a synergistic negative effect. Correlation analyses revealed that the honeybee gut microbiota influences bee immunity and behavior by regulating metabolic pathways related to ascorbate, tryptophan, and carbohydrates. Our results demonstrate that imidacloprid and copper, either alone or in a mix, alter young bee health through a complex mechanism of toxicity. These findings highlight imidacloprid and copper's negative effects on young honeybees, offering insights for future pesticide and heavy metal impact research.

Plasticity in inhibitory networks improves pattern separation in early olfactory processing

Distinguishing between nectar and non-nectar odors is challenging for animals due to shared compounds and varying ratios in complex mixtures. Changes in nectar production throughout the day and over the animal's lifetime add to the complexity. The honeybee olfactory system, containing fewer than 1000 principal neurons in the early olfactory relay, the antennal lobe (AL), must learn to associate diverse volatile blends with rewards. Previous studies identified plasticity in the AL circuits, but its role in odor learning remains poorly understood. Using a biophysical computational model, tuned by in vivo electrophysiological data, and live imaging of the honeybee's AL, we explored the neural mechanisms of plasticity in the AL. Our findings revealed that when trained with a set of rewarded and unrewarded odors, the AL inhibitory network suppresses responses to shared chemical compounds while enhancing responses to distinct compounds. This results in improved pattern separation and a more concise neural code. Our calcium imaging data support these predictions. Analysis of a graph convolutional neural network performing an odor categorization task revealed a similar mechanism for contrast enhancement. Our study provides insights into how inhibitory plasticity in the early olfactory network reshapes the coding for efficient learning of complex odors.

De novo variants in CDKL1 and CDKL2 are associated with neurodevelopmental symptoms

The CDKL (cyclin-dependent kinase-like) family consists of five members in humans, CDKL1-5, that encode serine-threonine kinases. The only member that has been associated with a Mendelian disorder is CDKL5, and variants in CDKL5 cause developmental and epileptic encephalopathy type 2 (DEE2). Here, we study four de novo variants in CDKL2 identified in five individuals, including three unrelated probands and monozygotic twins. These individuals present with overlapping symptoms, including global developmental delay, intellectual disability, childhood-onset epilepsy, dyspraxia, and speech deficits. We also identified two individuals with de novo missense variants in CDKL1 in the published Deciphering Developmental Disorders (DDD) and GeneDx cohorts with developmental disorders. Drosophila has a single ortholog of CDKL1-5, CG7236 (Cdkl). Cdkl is expressed in sensory neurons that project to specific regions of the brain that control sensory inputs. Cdkl loss causes semi-lethality, climbing defects, heat-induced seizures, hearing loss, and reduced lifespan. These phenotypes can be rescued by expression of the human reference CDKL1, CDKL2, or CDKL5, showing that the functions of these genes are conserved. In contrast, the CDKL1 and CDKL2 variants do not fully rescue the observed phenotypes, and overexpression of the variant proteins leads to phenotypes that are similar to Cdkl loss. Co-expression of CDKL1 or CDKL2 variants with CDKL1, CDKL2, or CDKL5 references in the mutant background suppresses the rescue ability of the reference genes. Our results suggest that the variants act as dominant negative alleles and are causative of neurological symptoms in these individuals.

Sexual dimorphism and morphological integration in the orchid bee brain

Sex-specific behaviours are common across animals and often associated with sexual dimorphism in the nervous system. Using micro-CT scanning we standardized sex-specific brain atlases and tested for sexual dimorphism in the brain of the orchid bee Euglossa dilemma, a species with marked sex differences in social behaviour, mating strategies and foraging. Males show greater investment in all primary visual processing neuropils and are uniquely integrated with the central complex, evidenced by a strong positive covariation. This suggests that males invest more on locomotor control, flight stability and sky-compass navigation which may have evolved in response to sex-specific behaviours, like courtship display. In contrast, females have larger mushroom bodies that strongly and positively covary with the optic lobes and have increased volume of the Kenyon cell cluster, implying greater capabilities for visual associative memory. We speculate this is an adaptation to social and nest-building behaviours, and reliance on learning visual landmarks required for central place foraging. Our study provides the first record of sexually dimorphic morphological integration in the brain of an insect, an approach that revealed sex-specific brain traits that lack an apparent morphological signal. These subtle differences provide further evidence for the causal link between brain architecture and behaviour.

Experience-dependent tuning of the olfactory system

Insects rely on their sense of smell to navigate complex environments and make decisions regarding food and reproduction. However, in natural settings, the odors that convey this information may come mixed with environmental odors that can obscure their perception. Therefore, recognizing the presence of informative odors involves generalization and discrimination processes, which can be facilitated when there is a high contrast between stimuli, or the internal representation of the odors of interest outcompetes that of concurrent ones. The first two layers of the olfactory system, which involve the detection of odorants by olfactory receptor neurons and their encoding by the first postsynaptic partners in the antennal lobe, are critical for achieving such optimal representation. In this review, we summarize evidence indicating that experience-dependent changes adjust these two levels of the olfactory system. These changes are discussed in the context of the advantages they provide for detection of informative odors.

Gain modulation and odor concentration invariance in early olfactory networks

The broad receptive field of the olfactory receptors constitutes the basis of a combinatorial code that allows animals to detect and discriminate many more odorants than the actual number of receptor types that they express. One drawback is that high odor concentrations recruit lower affinity receptors which can lead to the perception of qualitatively different odors. Here we addressed the contribution that signal-processing in the antennal lobe makes to reduce concentration dependence in odor representation. By means of calcium imaging and pharmacological approach we describe the contribution that GABA receptors play in terms of the amplitude and temporal profiles of the signals that convey odor information from the antennal lobes to higher brain centers. We found that GABA reduces the amplitude of odor elicited signals and the number of glomeruli that are recruited in an odor-concentration-dependent manner. Blocking GABA receptors decreases the correlation among glomerular activity patterns elicited by different concentrations of the same odor. In addition, we built a realistic mathematical model of the antennal lobe that was used to test the viability of the proposed mechanisms and to evaluate the processing properties of the AL network under conditions that cannot be achieved in physiology experiments. Interestingly, even though based on a rather simple topology and cell interactions solely mediated by GABAergic lateral inhibitions, the AL model reproduced key features of the AL response upon different odor concentrations and provides plausible solutions for concentration invariant recognition of odors by artificial sensors.

Chronic larval exposure to thiacloprid impairs honeybee antennal selectivity, learning and memory performances

The use of agricultural neonicotinoid insecticides has sub-lethal chronic effects on bees that are more prevalent than acute toxicity. Among these insecticides, thiacloprid, a commonly used compound with low toxicity, has attracted significant attention due to its potential impact on the olfactory and learning abilities of honeybees. The effect of sub-lethal larval exposure to thiacloprid on the antennal activity of adult honeybees (Apis mellifera L.) is not yet fully understood. To address this knowledge gap, laboratory-based experiments were conducted in which honeybee larvae were administered thiacloprid (0.5 mg/L and 1.0 mg/L). Using electroantennography (EAG), the impacts of thiacloprid exposure on the antennal selectivity to common floral volatiles were evaluated. Additionally, the effects of sub-lethal exposure on odor-related learning and memory were also assessed. The results of this study reveal, for the first time, that sub-lethal larval exposure to thiacloprid decreased honeybee antenna EAG responses to floral scents, leading to increased olfactory selectivity in the high-dose (1.0 mg/L) group compared to the control group (0 mg/L vs. 1.0 mg/L: p = 0.042). The results also suggest that thiacloprid negatively affected odor-associated paired learning acquisition, as well as medium-term (1 h) (0 mg/L vs. 1.0 mg/L: p = 0.019) and long-term memory (24 h) (0 mg/L vs. 1.0 mg/L: p = 0.037) in adult honeybees. EAG amplitudes were dramatically reduced following R-linalool paired olfactory training (0 mg/L vs. 1.0 mg/L: p = 0.001; 0 mg/L vs. 0.5 mg/L: p = 0.027), while antennal activities only differed significantly in the control between paired and unpaired groups. Our results indicated that exposure to sub-lethal concentrations of thiacloprid may affect olfactory perception and learning and memory behaviors in honeybees. These findings have important implications for the safe use of agrochemicals in the environment.

Honey bees can store and retrieve independent memory traces after complex experiences that combine appetitive and aversive associations

Real-world experiences do often mix appetitive and aversive events. Understanding the ability of animals to extract, store and use this information is an important issue in neurobiology. We used honey bees as model organism to study learning and memory after a differential conditioning that combines appetitive and aversive training trials. First of all, we describe an aversive conditioning paradigm that constitutes a clear opposite of the well known appetitive olfactory conditioning of the proboscis extension response. A neutral odour is presented paired with the bitter substance quinine. Aversive memory is evidenced later as an odour-specific impairment in appetitive conditioning. Then we tested the effect of mixing appetitive and aversive conditioning trials distributed along the same training session. Differential conditioning protocols like this were used before to study the ability to discriminate odours, however they were not focused on whether appetitive and aversive memories are formed. We found that after a differential conditioning, honey bees establish independent appetitive and aversive memories that do not interfere with each other during acquisition or storage. Finally, we moved the question forward to retrieval and memory expression to evaluate what happens when appetitive and the aversive learned odours are mixed during test. Interestingly, opposite memories compete in a way that they do not cancel each other out. Honey bees showed the ability to switch from expressing appetitive to aversive memory depending on their satiation level.