Retrograde amnesia in honeybees (Apis mellifera carnica)

Olfactory encoding within the insect antennal lobe: The emergence and role of higher order temporal correlations in the dynamics of antennal lobe spiking activity

In this review, we focus on the antennal lobe (AL) of three insect species - the fruit fly, sphinx moth, and locust. We first review the experimentally elucidated anatomy and physiology of the early olfactory system of each species; empirical studies of AL activity, however, often focus on assessing firing rates (averaged over time scales of about 100 ms), and hence the AL odor code is often analyzed in terms of a temporally evolving vector of firing rates. However, such a perspective necessarily misses the possibility of higher order temporal correlations in spiking activity within a single cell and across multiple cells over shorter time scales (of about 10 ms). Hence, we then review our prior theoretical work, where we constructed biophysically detailed, species-specific AL models within the fly, moth, and locust, finding that in each case higher order temporal correlations in spiking naturally emerge from model dynamics (i.e., without a prioriincorporation of elements designed to produce correlated activity). We therefore use our theoretical work to argue the perspective that temporal correlations in spiking over short time scales, which have received little experimental attention to-date, may provide valuable coding dimensions (complementing the coding dimensions provided by the vector of firing rates) that nature has exploited in the encoding of odors within the AL. We further argue that, if the AL does indeed utilize temporally correlated activity to represent odor information, such an odor code could be naturally and easily deciphered within the Mushroom Body.

Acute exposure to urban air pollution impairs olfactory learning and memory in honeybees

While the ecological effects of pesticides have been well studied in honeybees, it is unclear to what extent other anthropogenic contaminants such as air pollution may also negatively affect bee cognition and behaviour. To answer this question, we assessed the impacts of acute exposure to four ecologically relevant concentrations of a common urban air pollutant-diesel generated air pollution on honeybee odour learning and memory using a conditioned proboscis extension response assay. The proportion of bees that successfully learnt odours following direct air pollution exposure was significantly lower in bees exposed to low, medium and high air pollutant concentrations, than in bees exposed to current ambient levels. Furthermore, short- and long-term odour memory was significantly impaired in bees exposed to low medium and high air pollutant concentrations than in bees exposed to current ambient levels. These results demonstrate a clear and direct cognitive cost of air pollution. Given learning and memory play significant roles in foraging, we suggest air pollution will have increasing negative impacts on the ecosystem services bees provide and may add to the current threats such as pesticides, mites and disease affecting colony fitness.

Genetic Dissection of Aversive Associative Olfactory Learning and Memory in Drosophila Larvae

Memory formation is a highly complex and dynamic process. It consists of different phases, which depend on various neuronal and molecular mechanisms. In adult Drosophila it was shown that memory formation after aversive Pavlovian conditioning includes—besides other forms—a labile short-term component that consolidates within hours to a longer-lasting memory. Accordingly, memory formation requires the timely controlled action of different neuronal circuits, neurotransmitters, neuromodulators and molecules that were initially identified by classical forward genetic approaches. Compared to adult Drosophila, memory formation was only sporadically analyzed at its larval stage. Here we deconstruct the larval mnemonic organization after aversive olfactory conditioning. We show that after odor-high salt conditioning larvae form two parallel memory phases; a short lasting component that depends on cyclic adenosine 3’5’-monophosphate (cAMP) signaling and synapsin gene function. In addition, we show for the first time for Drosophila larvae an anesthesia resistant component, which relies on radish and bruchpilot gene function, protein kinase C activity, requires presynaptic output of mushroom body Kenyon cells and dopamine function. Given the numerical simplicity of the larval nervous system this work offers a unique prospect for studying memory formation of defined specifications, at full-brain scope with single-cell, and single-synapse resolution.

Learning and memory helps organisms to predict and adapt to events in their environment. Gained experience leaves traces of memory in the nervous system. Yet, memory formation in vertebrates and invertebrates is a highly complex and dynamic process that consists of different phases, which depend on various neuronal and molecular mechanisms. To understand which changes occur in a brain when it learns, we applied a reductionist approach. Instead of studying complex cases, we analyzed learning and memory in Drosophila larvae that have a simple brain that is genetically and behaviorally accessible and consists of only about 10,000 neurons. Drosophila larvae are able to learn to associate an odor with punishing high salt concentrations. It is therefore possible to correlate changes in larval behavior with molecular events in identifiable neurons after classical olfactory conditioning. We show that under these circumstances larvae form two parallel memory phases; a short lasting component (lSTM) that is molecularly conserved throughout the animal kingdom as it depends on the classical cAMP pathway. In parallel they establish a larval anesthesia resistant memory (lARM) that relies on a different molecular signal. lARM has not been described in larvae before.

A neonicotinoid impairs olfactory learning in Asian honey bees (Apis cerana) exposed as larvae or as adults

Xenobiotics such as the neonicotinoid pesticide, imidacloprid, are used globally, but their effects on native bee species are poorly understood. We studied the effects of sublethal doses of imidacloprid on olfactory learning in the native honey bee species, Apis cerana, an important pollinator of agricultural and native plants throughout Asia. We provide the first evidence that imidacloprid can impair learning in A. cerana workers exposed as adults or as larvae. Adults that ingested a single imidacloprid dose as low as 0.1 ng/bee had significantly reduced olfactory learning acquisition, which was 1.6-fold higher in control bees. Longer-term learning (1-17 h after the last learning trial) was also impaired. Bees exposed as larvae to a total dose of 0.24 ng/bee did not have reduced survival to adulthood. However, these larval-treated bees had significantly impaired olfactory learning when tested as adults: control bees exhibited up to 4.8-fold better short-term learning acquisition, though longer-term learning was not affected. Thus, sublethal cognitive deficits elicited by neonicotinoids on a broad range of native bee species deserve further study.

Effects of cold narcosis on memory acquisition, consolidation and retrieval in honeybees (Apis mellifera)

In learning and memory studies on honeybees (Apis mellifera), cold-induced narcosis has been widely used to temporarily immobilize honeybees. In this study, we investigated the effects of cold narcosis on the associative memories in honeybees by using the proboscis extension response (PER) paradigm. Severe impairments in memory acquisition was found when cold narcosis was performed 30 min, instead of 1 h before training. Locomotor activities were reduced when honeybees were tested 15 min, instead of 30 min after cold narcosis. These results indicate that cold narcosis impairs locomotor activities, as well as memory acquisition in a time-dependent manner, but by comparison no such effects on memory retrieval have yet been observed.[0].

Reward value determines memory consolidation in parasitic wasps

Animals can store learned information in their brains through a series of distinct memory forms. Short-lasting memory forms can be followed by longer-lasting, consolidated memory forms. However, the factors determining variation in memory consolidation encountered in nature have thus far not been fully elucidated. Here, we show that two parasitic wasp species belonging to different families, Cotesia glomerata (Hymenoptera: Braconidae) and Trichogramma evanescens (Hymenoptera; Trichogrammatidae), similarly adjust the memory form they consolidate to a fitness-determining reward: egg-laying into a host-insect that serves as food for their offspring. Protein synthesis-dependent long-term memory (LTM) was consolidated after single-trial conditioning with a high-value host. However, single-trial conditioning with a low-value host induced consolidation of a shorter-lasting memory form. For Cotesia glomerata, we subsequently identified this shorter-lasting memory form as anesthesia-resistant memory (ARM) because it was not sensitive to protein synthesis inhibitors or anesthesia. Associative conditioning using a single reward of different value thus induced a physiologically different mechanism of memory formation in this species. We conclude that the memory form that is consolidated does not only change in response to relatively large differences in conditioning, such as the number and type of conditioning trials, but is also sensitive to more subtle differences, such as reward value. Reward-dependent consolidation of exclusive ARM or LTM provides excellent opportunities for within-species comparison of mechanisms underlying memory consolidation.

Natural variation in learning rate and memory dynamics in parasitoid wasps: opportunities for converging ecology and neuroscience

Although the neural and genetic pathways underlying learning and memory formation seem strikingly similar among species of distant animal phyla, several more subtle inter- and intraspecific differences become evident from studies on model organisms. The true significance of such variation can only be understood when integrating this with information on the ecological relevance. Here, we argue that parasitoid wasps provide an excellent opportunity for multi-disciplinary studies that integrate ultimate and proximate approaches. These insects display interspecific variation in learning rate and memory dynamics that reflects natural variation in a daunting foraging task that largely determines their fitness: finding the inconspicuous hosts to which they will assign their offspring to develop. We review bioassays used for oviposition learning, the ecological factors that are considered to underlie the observed differences in learning rate and memory dynamics, and the opportunities for convergence of ecology and neuroscience that are offered by using parasitoid wasps as model species. We advocate that variation in learning and memory traits has evolved to suit an insect's lifestyle within its ecological niche.

Sparse odor representation and olfactory learning

Sensory systems create neural representations of environmental stimuli and these representations can be associated with other stimuli through learning. Are spike patterns the neural representations that get directly associated with reinforcement during conditioning? In the moth Manduca sexta, we found that odor presentations that support associative conditioning elicited only one or two spikes on the odor's onset (and sometimes offset) in each of a small fraction of Kenyon cells. Using associative conditioning procedures that effectively induced learning and varying the timing of reinforcement relative to spiking in Kenyon cells, we found that odor-elicited spiking in these cells ended well before the reinforcement was delivered. Furthermore, increasing the temporal overlap between spiking in Kenyon cells and reinforcement presentation actually reduced the efficacy of learning. Thus, spikes in Kenyon cells do not constitute the odor representation that coincides with reinforcement, and Hebbian spike timing-dependent plasticity in Kenyon cells alone cannot underlie this learning.

Rapid consolidation to a radish and protein synthesis-dependent long-term memory after single-session appetitive olfactory conditioning in Drosophila

In Drosophila, formation of aversive olfactory long-term memory (LTM) requires multiple training sessions pairing odor and electric shock punishment with rest intervals. In contrast, here we show that a single 2 min training session pairing odor with a more ethologically relevant sugar reinforcement forms long-term appetitive memory that lasts for days. Appetitive LTM has some mechanistic similarity to aversive LTM in that it can be disrupted by cycloheximide, the dCreb2-b transcriptional repressor, and the crammer and tequila LTM-specific mutations. However, appetitive LTM is completely disrupted by the radish mutation that apparently represents a distinct mechanistic phase of consolidated aversive memory. Furthermore, appetitive LTM requires activity in the dorsal paired medial neuron and mushroom body alpha'beta' neuron circuit during the first hour after training and mushroom body alphabeta neuron output during retrieval, suggesting that appetitive middle-term memory and LTM are mechanistically linked. Last, experiments feeding and/or starving flies after training reveals a critical motivational drive that enables appetitive LTM retrieval.

Serotonin modulation of moth central olfactory neurons

In the tobacco hornworm, Manduca sexta, 5-hydroxytryptamine (5HT) acting at the level of the antennal lobes contributes significantly to changing the moth's responsiveness to olfactory stimuli. 5HT targets K(+) conductances in the cells, increasing the excitability of central olfactory neurons and their responsiveness to olfactory cues. Effects of 5HT modulation are apparent not only at the single cell level, but also in the activity patterns of populations of neurons that convey olfactory information from antennal lobes to higher centers of the brain. Evidence suggests that 5HT-induced changes in activity within neural circuits of the antennal lobes might also drive structural plasticity, providing the basis for longer-term changes in antennal lobe function.

Effects of two bitter substances on olfactory conditioning in the moth Heliothis virescens

In nature, moths encounter nutritious and toxic substances in plants, and thus have to discriminate between a diversity of tastants. Whereas olfactory learning allowing memory of nutritious plants is well demonstrated, little is known about learning and memory of toxic items in adult lepidopterans. Moths may use bitter substances to detect and possibly learn to avoid noxious plants. We have studied the physiological and behavioural effects of two bitter substances, quinine and sinigrin, on the moth Heliothis virescens. Electrophysiological recordings showed responses to both compounds in gustatory receptor neurons on the antennae. The response patterns suggested a peripheral discrimination between quinine and sinigrin. We evaluated their putative aversive effect in an appetitive conditioning context where the moths learned to associate an odour with sucrose. We first aimed at enhancing olfactory conditioning of the proboscis extension response by testing the effect of the sucrose concentration on acquisition, retention and extinction. 2 mol l–1 and 3 mol l–1 sucrose concentration gave similar acquisition, retention and extinction performances. Experiments involving pre-exposure or facilitated extinction with an odour paired with quinine, sinigrin or no tastant showed a latent inhibitory effect,as well as an aversive effect of quinine and, to a lesser extent, of sinigrin. The results suggested that the two tastants may act as negative reinforcers in H. virescens.

The Drosophila radish gene encodes a protein required for anesthesia-resistant memory

Long-term memory in Drosophila is separable into two components: consolidated, anesthesia-resistant memory and long-lasting, protein-synthesis-dependent memory. The Drosophila memory mutant radish is specifically deficient in anesthesia-resistant memory and so represents the only molecular avenue to understanding this memory component. Here, we have identified the radish gene by positional cloning and comparative sequencing, finding a mutant stop codon in gene CG15720 from the Drosophila Genome Project. Induction of a wild-type CG15720 transgene in adult flies acutely rescues the mutant's memory defect. The phospholipase A2 gene, previously identified as radish [Chiang et al. (2004) Curr. Biol. 14:263-272], maps 95 kb outside the behaviorally determined deletion interval and is unlikely to be radish. The Radish protein is highly expressed in the mushroom bodies, centers of olfactory memory. It encodes a protein with 23 predicted cyclic-AMP-dependent protein kinase (PKA) phosphorylation sequences. The Radish protein has recently been reported to bind to Rac1 [Formstecher et al. (2005) Genome Res. 15:376-384], a small GTPase that regulates cytoskeletal rearrangement and influences neuronal and synaptic morphology.

Spontaneous recovery after extinction of the conditioned proboscis extension response in the honeybee

In honeybees, the proboscis extension response (PER) can be conditioned by associating an odor stimulus (CS) to a sucrose reward (US). Conditioned responses to the CS, which are acquired by most bees after a single CS-US pairing, disappear after repeated unrewarded presentations of the CS, a process called extinction. Extinction is usually thought to be based either on (1) the disruption of the stored CS-US association, or (2) the formation of an inhibitory "CS-no US" association that is better retrieved than the initial CS-US association. The observation of spontaneous recovery, i.e., the reappearance of responses to the CS after time passes following extinction, is traditionally interpreted as a proof for the formation of a transient inhibitory association. To provide a better understanding of extinction in honeybees, we examined whether time intervals during training and extinction or the number of conditioning and extinction trials have an effect on the occurrence of spontaneous recovery. We found that spontaneous recovery mostly occurs when conditioning and testing took place in a massed fashion (1-min intertrial intervals). Moreover, spontaneous recovery depended on the time elapsed since extinction, 1 h being an optimum. Increasing the number of conditioning trials improved the spontaneous recovery level, whereas increasing the number of extinction trials reduced it. Lastly, we show that after single-trial conditioning, spontaneous recovery appears only once after extinction. These elements suggest that in honeybees extinction of the PER actually reflects the impairment of the CS-US association, but that depending on training parameters different memory substrates are affected.

Neural substrates of memory: from synapse to system

One of the fundamental challenges of modern neuroscience is to understand how memories are acquired, stored, and retrieved by the brain. In the broadest terms, attempts to dissect memory can be broken down into four experimental disciplines: (1) identification of molecular components, (2) ex vivo and in vivo cellular analysis of neuronal function, (3) theoretical modeling approaches of neural systems, and (4) organismal-level behavioral analyses. Our objective here is to offer a conceptually unifying perspective and to discuss this perspective in relation to an experiment analysis of memory in Drosophila.

Operant conditioning of antennal movements in the honey bee

An operant learning protocol was developed for honeybees that are fixed in small tubes. The bees had to touch one or two small silver plates within the range of one antenna. The contacts of the antenna with the silver plates were registered electronically. Three conditioning protocols were analysed. In the first series the conditioned increase of the contact frequency was tested. The animals could touch one plate and received a reward (a small drop of sucrose) whenever the instantaneous frequency at this plate was more than one or two standard deviations above the spontaneous frequency. After conditioning the bees showed a significant increase of the contact frequency. No significant changes were found in a group of yoked controls. In the second series differential conditioning was tested. The animals could touch two silver plates. The spontaneous behaviour was measured and the animals received the reward upon touching the plate with the lower spontaneous frequency. The rewards were only applied whenever the instantaneous frequency exceeded a defined threshold. After ten conditioning trials the animals showed a significant increase in contact frequency for the conditioned plate compared to spontaneous behaviour. No significant changes were found in a group of yoked controls. In the third series reversal learning was tested. The animals were able to touch two silver plates. They were first conditioned to touch the plate which had the lower spontaneous contact frequency. After these conditioning trials they were tested for 10 min and subsequently conditioned to the alternative plate. The experiments demonstrated significant reversal learning compared to yoked controls. This new operant conditioning paradigm for the bee offers the possibility to analyse at the physiological level the mechanisms underlying different forms of learning in this insect.

Temporary amnesia induced by cold anesthesia and hypoxia in Drosophila

Memory consolidation in Drosophila was investigated using cold anesthesia- and hypoxia-induced amnesia. Individual flies were operantly trained to avoid the specific flight orientations with respect to the landmarks surrounding them when paired with heat reinforcement at a flight simulator. Cold anesthesia, introduced immediately after training, exerted a significantly diminishing effect on memory between 15 and 150 min after training. Hypoxia delivered immediately after training had a significantly diminishing effect on memory between 30 and 150 min after training. In addition, cold anesthesia disrupted memory only when introduced within the first 20 min, while hypoxia worked only when delivered within the first 2 min after training. When interpreted in the context of a four-phase model of memory consolidation, the results suggest that 1) cold anesthesia disrupts both short-term memory (STM) and anesthesia-resistant memory (ARM), 2) hypoxia disrupts ARM specifically, 3) both of them leave long-term memory (LTM) intact, and 4) LTM may be independent of availability of STM and ARM in flies.

Neuroanatomy: mushrooming mushroom bodies

A revolution is spreading in the study of mushroom bodies, structures within the insect brain that mediate learning and memory processes and pheromonal discrimination of the opposite sex.

Molecular cloning of linotte in Drosophila: a novel gene that functions in adults during associative learning

The linotte (lio) gene was identified in a screen for mutations that disrupted 3 hr memory after olfactory associative learning, without affecting the perception of odors or electroshock. The mutagenesis yielded a transposon-tagged gene disruption, which allowed rapid cloning of genomic DNA. The lio transcription unit was identified via rescue of the lio1 learning/memory defect by induced expression of a lio+ transgene in adults. The perception of odors or electroshock remained normal when the lio+ transgene was expressed in these lio1 flies. Learning/memory remained normal when the lio+ transgene was expressed in wild-type (lio+) flies. The lio gene produces only one transcript, the level of expression of which varies throughout development. Sequence analysis indicates that lio encodes a novel protein.

CREB as a memory modulator: induced expression of a dCREB2 activator isoform enhances long-term memory in Drosophila

Genetic studies of memory formation in Drosophila have revealed that the formation of a protein synthesis-dependent long-term memory (LTM) requires multiple training sessions. LTM is blocked specifically by induced expression of a repressor isoform of the cAMP-responsive element-binding protein (CREB). Here, we report an enhancement of LTM formation after induced expression of an activator isoform of dCREB2. Maximum LTM is achieved after one training session, and its formation depends on phosphorylation of the activator transgene. A model of LTM formation based on differential regulation of CREB isoforms is proposed.

Color learning and memory in honey bees are not affected by protein synthesis inhibition

The role of protein synthesis was tested for memory formation after color learning in honey bees. Free flying bees were trained to a feeding place. Before the color conditioning started, foragers accustomed to flying to the feeding place received an injection of either cycloheximide in bee ringer or bee ringer alone into the brain. Afterward, the animals were trained appetitively to a specific colored target by three training trials. During the test situation, two targets with different colors and no reward were presented. The choice behavior toward the two targets was evaluated. The first test immediately followed the last trial, the second test 24 h later. A comparison between cycloheximide- and ringer-injected bees showed no significant difference in choice behavior in either test. Although the injection of cycloheximide causes the inhibition of protein synthesis (> 95%) for a period of 3 h, the memory for the learned color signal is not affected. These results corroborate those found for the olfactory conditioning of the proboscis extension reflex in bees (Wittstock, Kaatz, & Menzel, 1993, Menzel, Gaio, Gerberding, Nemrava, & Wittstock, 1993).

Genetic dissection of consolidated memory in Drosophila

Behavioral and pharmacological experiments in many animal species have suggested that memory is consolidated from an initial, disruptable form into a long-lasting, stable form within a few hours after training. We combined these traditional approaches with genetic analyses in Drosophila to show that consolidated memory of conditioned (learned) odor avoidance 1 day after extended training consisted of two genetically distinct, functionally independent memory components: anesthesia-resistant memory (ARM) and long-term memory (LTM). ARM decayed away within 4 days, was resistant to hypothermic disruption, was insensitive to the protein synthesis inhibitor cycloheximide (CXM), and was disrupted by the radish single-gene mutation. LTM showed no appreciable decay over 7 days, was sensitive to CXM, and was not disrupted by the radish mutation.

Cooling-induced retrograde amnesia reflexes Pavlovian conditioning associations in Limax flavus

The relationships between cooling-induced retrograde amnesia and associations in Pavlovian conditioning in the terrestrial mollusk Limax flavus were studied. In the first experiment, the slugs were conditioned to avoid carrot odor and the experimental conditions required for amnesia induction were studied. Memory reactivation before cooling was found to be necessary for amnesia induction and the induced amnesia was selective for the reactivated memory. In the subsequent experiments, slugs were conditioned to avoid both carrot and cucumber odors using one of three Pavlovian conditioning paradigms, namely two-independent first-order conditioning, phase-2-sequential second-order conditioning and phase-2-simultaneous second-order conditioning, after which amnesia was induced by cooling immediately after presentation of one of the conditioning odors. The amnesia pattern induced differed depending upon the conditioning procedure used, which indicated that amnesia induction was related closely to stimulus associations in slugs. The possible role of cooling-induced retrograde amnesia as a tool for studying memory associations is also discussed.

Rapid dendritic spine stem shortening during one-trial learning: the honeybee's first orientation flight

Our initial study of honeybees using the rapid Golgi method showed that dendritic spines on calycal interneurons had shorter stems due to spine head enlargement in bees with greater cumulative experience. This study sought to determine if spine stem shortening could be induced rapidly during the first orientation flight, a one-trial place learning event. Newly emerged bees were reared in a small broodless hive with a virgin queen and allowed to take their first orientation flight at 6 and 8 days of age. Spine profiles of 5 flyers and 5 non-flyers were traced in large scale using light microscopy and a modified camera lucida. Overall spine length and stem length were measured on these tracings using a digitizing tablet. Additional measurements of maximum spine head width, profile area, and perimeter were made using computer image analyses. Examining group differences in spine stem length as a function of overall spine length, our results revealed a clear association between rapid spine stem shortening and the first orientation flight lasting several minutes. This effect, however, was restricted to only the long spines. Flight-induced stem shortening was accompanied by elongated swelling of the spine head without an appreciable expansion of the spine perimeter.

Behavioural access to short-term memory in bees

Memory formation proceeds in temporal phases which differ in their effectiveness in controlling subsequent behavior and in their susceptibility to amnestic treatment. The initial phase of memory formation, frequently termed short-term memory, is generally considered a necessary precursor to long-term memory. However, the course of short-term memory differs widely between animal species and is dependent on experimental procedure. Information may even bypass the short-term phase en route to the long-term one. Experiments reported here using honey bees in a behavioural learning situation suggest that the greatest significance of short-term memory is its function as a mode of memory storage which may be altered effectively by new and contradictory information. Freely flying honey bees were presented two colour alternatives and rewarded on first one and then the other in a reversal learning paradigm. Subsequent colour preference was dependent on the interval between the two trials. Several new features of short-term memory are described. It is concluded that a single mechanisms of short- to long-term memory transfer cannot account for the observed bimodal interval dependent behaviour. Two mechanisms are proposed.