Measuring honey bee feeding rhythms with the BeeBox, a platform for nectar foraging insects.

In honey bees, most studies of circadian rhythms involve a locomotion test performed in a small tube, a tunnel, or at the hive entrance. However, despite feeding playing an important role in honey bee health or fitness, no demonstration of circadian rhythm on feeding has been performed until recently. Here, we present the BeeBox, a new laboratory platform for bees based on the concept of the Skinner box, which dispenses discrete controlled amounts of food (sucrose syrup) following entrance into an artificial flower. We compared caged groups of bees in 12 h-12 h light/dark cycles, constant darkness and constant light and measured average hourly syrup consumption per living bee. Food intake was higher in constant light and lower in constant darkness; mortality increased in constant light. We observed rhythmic consumption with a period longer than 24 h; this is maintained in darkness without environmental cues, but is damped in the constant light condition. The BeeBox offers many new research perspectives and numerous potential applications in the study of nectar foraging animals.

Effect of Adult Male Diet on Fertilization and Hatching in an Insect.

As in other animals, diet is known to influence insect reproduction, and its impact has been intensively investigated in females. In our study, we examined the effects of various diets on male reproductive success in the moth Agrotis ipsilon, a pest of many crops. Our experiments showed an increase in the rates of fertilization and hatching when males fed with various sugars (sucrose, fructose, and glucose) supplemented with sodium. Such results provide valuable initial information on the nutritional ecology of male moths and could serve to the development of nutritional attractants for the management of crop pests.

Diet impacts the reproductive system's maturation in the male moth Agrotis ipsilon (Noctuidae, Lepidoptera).

In male moth Agrotis ipsilon, sexual maturation occurs between the third and the fifth day of adult life and is characterized by the development of the reproductive organs such as testes and accessory sex glands. Since sexual maturation requires considerable energy investment, we hypothesized that diet would be an essential regulatory factor in this developmental process. Indeed, the links between the male diet and reproductive physiology have not been described as in females. To test the previous hypothesis, we offered male moths diets corresponding to different flower nectars found in nature, and measured morphological and functional changes in the testes and accessory sex glands. In comparison to a diet composed of sucrose only, males fed with a diet composed of diverse sugars, including glucose, supplemented with sodium led to an earlier increase in the length and the protein content of accessory sex glands, as well as a reduction of the testicular volume accompanied by an acceleration of the sperm bundle transfer from the testes to the duplex. These results show that these specific diets accelerate the maturation of the reproductive system in male moth Agrotis ipsilon.

Impact of single and combined exposure to priority pollutants on gene expression and post-embryonic development in Drosophila melanogaster.

Many priority pollutants are concentrated in the environment due to human activity. Most are highly toxic to various organisms, including endocrine disruptors EDCs, aromatic polycyclic hydrocarbons PAHs, pesticides. While the effects of single and binary exposure have been widely explored, several pollutants can be simultaneously present at the same time in the environment, in in more or less polluted matrices. Effective pollution control requires the presence and sources of contamination to be identified. Previously we used Drosophila melanogaster to investigate metal pollution. Here, we re-used Drosophila to identify the biomarkers of pollution, and to determine if they can be used for specific types of pollution. Single and combined exposure of Bis(2-ethylhexyl) phthalate (DEHP), bisphenol A, nonylphenol, benzo(a)pyrene, and glyphosate was investigated. The impact of these pollutants on post-embryonic development and the expression pattern of 38 molecular targets were examined using qPCR. During single exposure, different profiles were observed at the molecular level. In complex mixtures, the expression profile resembled that of bisphenol A. In contrast, relatively specific gene expression profiles were obtained for the effects of each pollutant separately. While direct pollutant-gene profiling remains difficult in mixtures, molecular biology analyses enhance pollution monitoring, and should be incorporated in toxicological studies.

Pyrroline-5-carboxylate dehydrogenase is an essential enzyme for proline dehydrogenase function during dark-induced senescence in Arabidopsis thaliana.

During leaf senescence, nitrogen is remobilized and carbon backbones are replenished by amino acid catabolism, with many of the key reactions occurring in mitochondria. The intermediate Δ1 -pyrroline-5-carboxylate (P5C) is common to some catabolic pathways, thus linking the metabolism of several amino acids, including proline and arginine. Specifically, mitochondrial proline catabolism involves sequential action of proline dehydrogenase (ProDH) and P5C dehydrogenase (P5CDH) to produce P5C and then glutamate. Arginine catabolism produces urea and ornithine, the latter in the presence of α-ketoglutarate being converted by ornithine δ-aminotransferase (OAT) into P5C and glutamate. Metabolic changes during dark-induced leaf senescence (DIS) were studied in Arabidopsis thaliana leaves of Col-0 and in prodh1prodh2, p5cdh and oat mutants. Progression of DIS was followed by measuring chlorophyll and proline contents for 5 days. Metabolomic profiling of 116 compounds revealed similar profiles of Col-0 and oat metabolism, distinct from prodh1prodh2 and p5cdh metabolism. Metabolic dynamics were accelerated in p5cdh by 1 day. Notably, more P5C and proline accumulated in p5cdh than in prodh1prodh2. ProDH1 enzymatic activity and protein amount were significantly down-regulated in p5cdh mutant at Day 4 of DIS. Mitochondrial P5C levels appeared critical in determining the flow through interconnected amino acid remobilization pathways to sustain senescence.

Regulation of feeding dynamics by the circadian clock, light and sex in an adult nocturnal insect.

Animals invest crucial resources in foraging to support development, sustenance, and reproduction. Foraging and feeding behaviors are rhythmically expressed by most insects. Rhythmic behaviors are modified by exogenous factors like temperature and photoperiod, and internal factors such as the physiological status of the individual. However, the interactions between these factors and the circadian clock to pattern feeding behavior remains elusive. As Drosophila, a standard insect model, spends nearly all its life on food, we rather chose to focus on the adults of a non-model insect, Agrotis ipsilon, a nocturnal cosmopolitan crop pest moth having structured feeding activity. Our study aimed to explore the impact of environmental cues on directly measured feeding behavior rhythms. We took advantage of a new experimental set-up, mimicking an artificial flower, allowing us to specifically monitor feeding behavior in a naturalistic setting, e.g., the need to enter a flower to get food. We show that the frequency of flower visits is under the control of the circadian clock in males and females. Feeding behavior occurs only during the scotophase, informed by internal clock status and external photic input, and females start to visit flowers earlier than males. Shorter duration visits predominate as the night progresses. Importantly, food availability reorganizes the microstructure of feeding behavior, revealing its plasticity. Interestingly, males show a constant number of daily visits during the 5 days of adult life whereas females decrease visitations after the third day of adult life. Taken together, our results provide evidence that the rhythmicity of feeding behavior is sexually dimorphic and controlled by photoperiodic conditions through circadian clock-dependent and independent pathways. In addition, the use of the new experimental set-up provides future opportunities to examine the regulatory mechanisms of feeding behavior paving the way to investigate complex relationships between feeding, mating, and sleep-wake rhythms in insects.

Single and mixed exposure to cadmium and mercury in Drosophila melanogaster: Molecular responses and impact on post-embryonic development.

Heavy metals, like many other chemical elements, are naturally present in the environment; however, the concentrations of these metals in various environmental matrices have increased through their intensive use in many human activities (such as industry, mining and agriculture). Among the heavy metals, cadmium (Cd) and mercury (Hg) induce a wide variety of defects in animals. While the effects of these heavy metals have been widely documented, a single exposure paradigm is typically used. Few studies have focused on evaluating combined exposure to these metals. However, in the environment, animals are confronted with a plethora of substances simultaneously; thus, the presence and origin of such substances must be determined to reduce the sources of contamination. Using the model of the fruit fly Drosophila melanogaster, for which many tools are readily available, we investigated how different concentrations of Cd and Hg in single and combined exposures impact post-embryonic development. In parallel, we evaluated the extended expression pattern of 38 molecular targets used as potential biomarkers of exposure through qPCR. Our results showed that both metals caused developmental delays and mortality in dose-dependent responses. Both metals were able to deregulate genes involved in hormonal control, general stress, and oxidative stress. Importantly, we confirmed synergistic interactions between Cd and Hg. Our results indicate the importance of assessing several biomarkers and their kinetics in mixtures. Drosophila represents a useful model for monitoring the toxicity of substances in polluted environments.

Modulatory effects of pheromone on olfactory learning and memory in moths.

Pheromones are chemical communication signals known to elicit stereotyped behaviours and/or physiological processes in individuals of the same species, generally in relation to a specific function (e.g. mate finding in moths). However, recent research suggests that pheromones can modulate behaviours, which are not directly related to their usual function and thus potentially affect behavioural plasticity. To test this hypothesis, we studied the possible modulatory effects of pheromones on olfactory learning and memory in Agrotis ipsilon moths, which are well-established models to study sex-pheromones. To achieve this, sexually mature male moths were trained to associate an odour with either a reward (appetitive learning) or punishment (aversive learning) and olfactory memory was tested at medium- and long-term (1 h or 1.5 h, and 24 h). Our results show that male moths can learn to associate an odour with a sucrose reward, as well as a mild electric shock, and that olfactory memory persists over medium- and long-term range. Pheromones facilitated both appetitive and aversive olfactory learning: exposure to the conspecific sex-pheromone before conditioning enhanced appetitive but not aversive learning, while exposure to a sex-pheromone component of a heterospecific species (repellent) facilitated aversive but not appetitive learning. However, this effect was short-term, as medium- and long-term memory were not improved. Thus, in moths, pheromones can modulate olfactory learning and memory, indicating that they contribute to behavioural plasticity allowing optimization of the animal's behaviour under natural conditions. This might occur through an alteration of sensitization.

Effects of DEHP on the ecdysteroid pathway, sexual behavior and offspring of the moth Spodoptera littoralis.

Bis(2-ethylhexyl) phthalate (DEHP) is a widely produced plasticizer that is considered to act as an endocrine-disrupting chemical in vertebrates and invertebrates. Indeed, many studies have shown that DEHP alters hormonal levels, reproduction and behavior in vertebrates. Few studies have focused on the effects of DEHP on insects, although DEHP is found almost everywhere in their natural habitats, particularly in soils and plants. Here, we investigated the effects of DEHP on the sexual behavior and physiology of a pest insect, the noctuid moth Spodoptera littoralis. In this nocturnal species, olfaction is crucial for sexual behavior, and ecdysteroids at the antennal level have been shown to modulate sex pheromone detection by males. In the present study, larvae were fed food containing different DEHP concentrations, and DEHP concentrations were then measured in the adults (males and females). Hemolymphatic ecdysteroid concentrations, the antennal expression of genes involved in the ecdysteroid pathway (nuclear receptors EcR, USP, E75, and E78 and calmodulin) and sexual behavior were then investigated in adult males. The success and latency of mating as well as the hatching success were also studied in pairs consisting of one DEHP male and one uncontaminated female or one DEHP female and one uncontaminated male. We also studied the offspring produced from pairs involving contaminated females to test the transgenerational effect of DEHP. Our results showed the general downregulation of nuclear receptors and calmodulin gene expression associated with the higher concentrations of DEHP, suggesting peripheral olfactory disruption. We found some effects on male behavior but without an alteration of the mating rate. Effects on offspring mortality and developmental rates in the N + 1 generation were also found at the higher doses of DEHP. Taken together, the results of the study show for the first time that larval exposure to DEHP can induce delayed endocrine-disruptive effects in the adults of a terrestrial insect as well as effects on the next generation. To date, our study is also the first description of an impact of endocrine disrupter on olfaction in insects.

A plant volatile alters the perception of sex pheromone blend ratios in a moth.

Mate finding in most moths is based on male perception of a female-emitted pheromone whose species specificity resides in component chemistry and proportions. Components are individually detected by specialized olfactory receptor neurons (ORNs) projecting into the macroglomerular complex (MGC) of the male brain. We asked how robust ratio recognition is when challenged by a plant volatile background. To test this, we investigated the perception of the pheromone blend in Agrotis ipsilon, a moth species whose females produce a blend of Z7-dodecenyl acetate (Z7-12:Ac), Z9-tetradecenyl acetate (Z9-14:Ac), and Z11-hexadecenyl acetate in a 4:1:4 ratio optimally attractive for males. First, we recorded the responses of specialist ORNs for Z7 and Z9 and showed that heptanal, a flower volatile, activated Z7 but not Z9 neurons. Then, we recorded intracellularly the responses of MGC neurons to various ratios and showed that heptanal altered ratio responses of pheromone-sensitive neurons. Finally, we analyzed the behavior of males in a wind tunnel and showed that their innate preference for the 4:1:4 blend was shifted in the presence of heptanal. Pheromone ratio recognition may thus be altered by background odorants. Therefore, the olfactory environment might be a selective force for the evolution of pheromone communication systems.

Responsiveness to Sugar Solutions in the Moth Agrotis ipsilon: Parameters Affecting Proboscis Extension.

Adult moths need energy and nutrients for reproducing and obtain them mainly by consuming flower nectar (a solution of sugars and other compounds). Gustatory perception gives them information on the plants they feed on. Feeding and food perception are integrated in the proboscis extension response, which occurs when their antennae touch a sugar solution. We took advantage of this reflex to explore moth sugar responsiveness depending on different parameters (i.e., sex, age, satiety, site of presentation, and composition of the solution). We observed that starvation but not age induced higher response rates to sucrose. Presentation of sucrose solutions in a randomized order confirmed that repeated sugar stimulations did not affect the response rate; however, animals were sometimes sensitized to water, indicating sucrose presentation might induce non-associative plasticity. Leg stimulation was much less efficient than antennal stimulation to elicit a response. Quinine prevented and terminated sucrose-elicited proboscis extension. Males but not females responded slightly more to sucrose than to fructose. Animals of either sex rarely reacted to glucose, but curiously, mixtures in which half sucrose or fructose were replaced by glucose elicited the same response rate than sucrose or fructose alone. Fructose synergized the response when mixed with sucrose in male but not female moths. This is consistent with the fact that nectars consumed by moths in nature are mixtures of these three sugars, which suggests an adaptation to nectar perception.

Effects of bisphenol A on post-embryonic development of the cotton pest Spodoptera littoralis.

Endocrine-disrupting chemicals encompass a variety of chemicals that may interfere with the endocrine system and produce negative effects on organisms. Among them, bisphenol A is considered a major pollutant in numerous countries. The harmful effects of BPA on environmental and human health are intensely studied. However, the effects of BPA on terrestrial insects are still poorly investigated, despite that several plants can accumulate BPA in their tissues, leading to potential contamination of herbivorous insects. Here, we used the leafworm Spodoptera littoralis, a polyphagous species, to study BPA effects on post-embryonic development. We studied the effects of BPA ingestion at environmental doses (e.g., 0.01, 0.1, and 1 µg/g of BPA) and high doses (e.g., 25 µg/g) on larval weight and stage duration, pupal length and sex ratio. BPA effects were investigated in more detail during the last larval instar, a crucial period for preparing pupation and metamorphosis, which are under endocrine control. We monitored the haemolymph concentration of ecdysteroids, hormones controlling moult and metamorphosis, as well as the expression levels of several nuclear receptors involved in the ecdysteroid signalling pathway. Our integrative study showed that, upon exposure doses, BPA can induce various effects on the viability, developmental time, growth and sex ratio. These effects were correlated with a delay of the ecdysteroid peak during the last larval instar and a modification of expression of EcR, USP, E75AB, E75D and Br-c. We provide new evidence about the events that occur after BPA exposure in insect contaminated by food ingestion.

Thymol Affects Congruency Between Olfactory and Gustatory Stimuli in Bees.

Honey bees learn to associate sugars with odorants in controlled laboratory conditions and during foraging. The memory of these associations can be impaired after exposure to contaminants such as pesticides. The sub-lethal effects of acaricides such as 5-methyl-2-(propan-2-yl)-phenol (thymol) introduced into colonies to control varroa mites are of particular concern to beekeeping, due to detrimental effects of some acaricides on bees. Here we assess whether various odorant/sugar pairs are identically memorized in a differential appetitive olfactory conditioning experiment and whether this learning is affected by thymol exposure. Responses to odorants in retrieval tests varied according to the sugar they were paired with, a property called congruency. Interestingly, congruency was altered by pre-exposure to some thymol concentrations during retrieval tests, although electroantennography recordings showed it left odorant detection intact. This highlights the importance of taking into account subtle effects such as odor/sugar congruency in the study of the effect of pesticides on non-target insects, in addition to the simpler question of memory impairment.

Effects of DEHP on post-embryonic development, nuclear receptor expression, metabolite and ecdysteroid concentrations of the moth Spodoptera littoralis.

Di (2-ethylhexyl) phthalate (DEHP) is recognized in vertebrates as an Endocrine Disrupting Chemical (EDC). DEHP can alter steroid hormones production, development, reproduction and behavior in vertebrates. Only few studies investigated DEHP effects on insects. However, some recent studies on aquatic insects showed that DEHP could also act as an EDC by interfering with the signaling pathways of ecdysteroids, the main hormones involved in the control of insect post-embryonic development and physiology. The aim of the study was to investigate (1) the fate of DEHP within a terrestrial insect species by exposing larvae to food containing a wide range of DEHP concentrations and (2) the effects of this chemical on their post-embryonic development and metamorphosis, by using a multi-level approach. DEHP was shown to be present both in larvae and resulting stages, with higher concentrations in chrysalises and adults than in larvae. DEHP concentrations also decreased at the end of the last larval instar, suggesting the metabolic transformation or excretion of this chemical during this time. Only the two highest DEHP doses induced higher insect mortality, whereas low and intermediate concentrations increased larval food consumption without affecting body weight. Metabolic profiles showed that in control insects, the last three days before metamorphosis correspond to a metabolic transition, but with time-dependent changes in treated insects. Interestingly, DEHP treatments also alter both hemolymphatic ecdysteroid titers and expression levels of ecdysteroid response genes. These results confirm that DEHP can alter insect post-embryonic development and metamorphosis, by interfering with ecdysteroid pathways.

Exposure to Conspecific and Heterospecific Sex-Pheromones Modulates Gustatory Habituation in the Moth Agrotis ipsilon.

In several insects, sex-pheromones are essential for reproduction and reproductive isolation. Pheromones generally elicit stereotyped behaviors. In moths, these are attraction to conspecific sex-pheromone sources and deterrence for heterospecific sex-pheromone. Contrasting with these innate behaviors, some results in social insects point toward effects of non-sex-pheromones on perception and learning. We report the effects of sex-pheromone pre-exposure on gustatory perception and habituation (a non-associative learning) in male Agrotis ipsilon moths, a non-social insect. We also studied the effect of Z5-decenyl acetate (Z5), a compound of the sex-pheromone of the related species Agrotis segetum. We hypothesized that conspecific sex-pheromone and Z5 would have opposite effects. Pre-exposure to either the conspecific sex-pheromone or Z5 lasted 15 min and was done either immediately or 24 h before the experiments, using their solvent alone (hexane) as control. In a sucrose responsiveness assay, pre-exposure to the conspecific sex-pheromone had no effect on the dose-response curve at either delays. By contrast, Z5 slightly improved sucrose responsiveness 15 min but not 24 h after pre-exposure. Interestingly, the conspecific sex-pheromone and Z5 had time-dependent effects on gustatory habituation: pre-exposing the moths with Z5 hindered learning after immediate but not 24-h pre-exposure, whereas pre-exposure to the conspecific sex-pheromone hindered learning at 24-h but not immediate pre-exposure. They did not have opposite effects. This is the first time a sex-pheromone is reported to affect learning in a non-social insect. The difference in modulation between conspecific sex-pheromone and Z5 suggests that con- and hetero-specific sex-pheromones act on plasticity through different cerebral pathways.

Dopamine Receptor Activation Is Required for GABAergic Spike Timing-Dependent Plasticity in Response to Complex Spike Pairing in the Ventral Tegmental Area.

One of the most influential synaptic learning rules explored in the past decades is activity dependent spike-timing-dependent plasticity (STDP). In STDP, synapses are either potentiated or depressed based on the order of pre- and postsynaptic neuronal activation within narrow, milliseconds-long, time intervals. STDP is subject to neuromodulation by dopamine (DA), a potent neurotransmitter that significantly impacts synaptic plasticity and reward-related behavioral learning. Previously, we demonstrated that GABAergic synapses onto ventral tegmental area (VTA) DA neurons are able to express STDP (Kodangattil et al., 2013), however it is still unclear whether DA modulates inhibitory STDP in the VTA. Here, we used whole-cell recordings in rat midbrain slices to investigate whether DA D1-like and/or D2-like receptor (D1R/D2R) activation is required for induction of STDP in response to a complex pattern of spiking. We found that VTA but not Substantia nigra pars compact (SNc) DA neurons exhibit long-term depression (LTDGABA) in response to a combination of positive (pre-post) and negative (post-pre) timing of spiking (a complex STDP protocol). Blockade of either D1Rs or D2Rs prevented the induction of LTDGABA while activation of D1Rs did not affect the plasticity in response to this complex STDP protocol in VTA DA neurons.Our data suggest that this DA-dependent GABAergic STDP is selectively expressed at GABAergic synapses onto VTA DA neurons which could be targeted by drugs of abuse to mediate drug-induced modulation of DA signaling within the VTA, as well as in VTA-projection areas, thereby affecting reward-related learning and drug-associated memories.

Morphine-induced synaptic plasticity in the VTA is reversed by HDAC inhibition.

Dopamine (DA) dysfunction originating from the ventral tegmental area (VTA) occurs as a result of synaptic abnormalities following consumption of drugs of abuse and underlies behavioral plasticity associated with drug abuse. Drugs of abuse can cause changes in gene expression through epigenetic mechanisms in the brain that underlie some of the lasting neuroplasticity and behavior associated with addiction. Here we investigated the function of histone acetylation and histone deacetylase (HDAC)2 in the VTA in recovery of morphine-induced synaptic modifications following a single in vivo exposure to morphine. Using a combination of immunohistochemistry, Western blot, and whole cell patch-clamp recording in rat midbrain slices, we show that morphine increased HDAC2 activity in VTA DA neurons and reduced histone H3 acetylation at lysine 9 (Ac-H3K9) in the VTA 24 h after the injection. Morphine-induced synaptic changes at glutamatergic synapses involved endocannabinoid signaling to reduce GABAergic synaptic strength onto VTA DA neurons. Both plasticities were recovered by in vitro incubation of midbrain slices with a class I-specific HDAC inhibitor (HDACi), CI-994, through an increase in acetylation of histone H3K9. Interestingly, HDACi incubation also increased levels of Ac-H3K9 and triggered GABAergic and glutamatergic plasticities in DA neurons of saline-treated rats. Our results suggest that acute morphine-induced changes in VTA DA activity and synaptic transmission engage HDAC2 activity locally in the VTA to maintain synaptic modifications through histone hypoacetylation.

Behavioral and metabolic effects of sublethal doses of two insecticides, chlorpyrifos and methomyl, in the Egyptian cotton leafworm, Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae).

Insecticides have long been used as the main method in limiting agricultural pests, but their widespread use has resulted in environmental pollution, development of resistances, and biodiversity reduction. The effects of insecticides at low residual doses on both the targeted crop pest species and beneficial insects have become a major concern. In particular, these low doses can induce unexpected positive (hormetic) effects on pest insects, such as surges in population growth exceeding what would have been observed without pesticide application. Methomyl and chlorpyrifos are two insecticides commonly used to control the population levels of the cotton leafworm Spodoptera littoralis, a major pest moth. The aim of the present study was to examine the effects of sublethal doses of these two pesticides, known to present a residual activity and persistence in the environment, on the moth physiology. Using a metabolomic approach, we showed that sublethal doses of methomyl and chlorpyrifos have a systemic effect on the treated insects. We also demonstrated a behavioral disruption of S. littoralis larvae exposed to sublethal doses of methomyl, whereas no effects were observed for the same doses of chlorpyrifos. Interestingly, we highlighted that sublethal doses of both pesticides did not induce a change in acetylcholinesterase activity in head of exposed larvae.

Spike timing-dependent plasticity at GABAergic synapses in the ventral tegmental area.

Persistent changes in excitatory and inhibitory synaptic strengths to the ventral tegmental area (VTA) dopamine (DA) neurons in response to addictive drugs may underlie the transition from casual to compulsive drug use. While an enormous amount of work has been done in the area of glutamatergic plasticity of the VTA, little is known regarding the learning rules governing GABAergic plasticity in the VTA. Spike timing-dependent plasticity, STDP, has attracted considerable attention primarily due to its potential roles in processing and storage of information in the brain and there is emerging evidence for the existence of STDP at inhibitory synapses. We therefore used whole-cell recordings in rat midbrain slices to investigate whether near-coincident pre- and postsynaptic firing induces a lasting change in synaptic efficacy of VTA GABAergic synapses. We found that a Hebbian form of STDP including long-term potentiation (LTP) and long-term depression (LTD) can be induced at GABAergic synapses onto VTA DA neurons and relies on the precise temporal order of pre- and postsynaptic spiking. Importantly, GABAergic STDP is heterosynaptic (NMDA receptor dependent): triggered by correlated activities of the presynaptic glutamatergic input and postsynaptic DA cells. GABAergic STDP is postsynaptic and has an associative component since pre- or postsynaptic spiking per se did not induce STDP. STDP of GABAergic synapses in the VTA provides physiologically relevant forms of inhibitory plasticity that may underlie natural reinforcement of reward-related behaviours. Moreover, this form of inhibitory plasticity may mediate some of the reinforcing, aversive and addictive properties of drugs of abuse.

A-kinase anchoring protein-calcineurin signaling in long-term depression of GABAergic synapses.

The postsynaptic scaffolding A-kinase anchoring protein 79/150 (AKAP79/150) signaling complex regulates excitatory synaptic transmission and strength through tethering protein kinase A (PKA), PKC, and calcineurin (CaN) to the postsynaptic densities of neurons (Sanderson and Dell'Acqua, 2011), but its role in inhibitory synaptic transmission and plasticity is unknown. Using immunofluorescence and whole-cell patch-clamp recording in rat midbrain slices, we show that activation of postsynaptic D(2)-like family of dopamine (DA) receptor in the ventral tegmental area (VTA) induces long-term depression (LTD) of GABAergic synapses on DA neurons through an inositol triphosphate receptor-mediated local rise in postsynaptic Ca(2+) and CaN activation accompanied by PKA inhibition, which requires AKAP150 as a bridging signaling molecule. Our data also illuminate a requirement for a clathrin-mediated internalization of GABA(A) receptors in expression of LTD(GABA). Moreover, disruption of AKAP-PKA anchoring does not affect glutamatergic synapses onto DA neurons, suggesting that the PKA-AKAP-CaN complex is uniquely situated at GABA(A) receptor synapses in VTA DA neurons to regulate plasticity associated with GABA(A) receptors. Drug-induced modulation of GABAergic plasticity in the VTA through such novel signaling mechanisms has the potential to persistently alter the output of individual DA neurons and of the VTA, which may contribute to the reinforcing or addictive properties of drugs of abuse.