Classical conditioning in the blowfly (Phormia regina): associative and excitatory factors

Remembering Components of Food in Drosophila

Remembering features of past feeding experience can refine foraging and food choice. Insects can learn to associate sensory cues with components of food, such as sugars, amino acids, water, salt, alcohol, toxins and pathogens. In the fruit fly Drosophila some food components activate unique subsets of dopaminergic neurons (DANs) that innervate distinct functional zones on the mushroom bodies (MBs). This architecture suggests that the overall dopaminergic neuron population could provide a potential cellular substrate through which the fly might learn to value a variety of food components. In addition, such an arrangement predicts that individual component memories reside in unique locations. DANs are also critical for food memory consolidation and deprivation-state dependent motivational control of the expression of food-relevant memories. Here, we review our current knowledge of how nutrient-specific memories are formed, consolidated and specifically retrieved in insects, with a particular emphasis on Drosophila.

Invertebrate learning and cognition: relating phenomena to neural substrate

Diverse invertebrate species have been used for studies of learning and comparative cognition. Although we have gained invaluable information from this, in this study we argue that our approach to comparative learning research is rather deficient. Generally invertebrate learning research has focused mainly on arthropods, and most of that within the Hymenoptera and Diptera. Any true comparative analysis of the distribution of comparative cognitive abilities across phyla is hampered by this bias, and more fundamentally by a reporting bias toward positive results. To understand the limits of learning and cognition for a species, knowing what animals cannot do is at least as important as reporting what they can. Finally, much more effort needs to be focused on the neurobiological analysis of different types of learning to truly understand the differences and similarities of learning types. In this review, we first give a brief overview of the various forms of learning in invertebrates. We also suggest areas where further study is needed for a more comparative understanding of learning. Finally, using what is known of learning in honeybees and the well-studied honeybee brain, we present a model of how various complex forms of learning may be accounted for with the same neural circuitry required for so-called simple learning types. At the neurobiological level, different learning phenomena are unlikely to be independent, and without considering this it is very difficult to correctly interpret the phylogenetic distribution of learning and cognitive abilities. WIREs Cogn Sci 2013, 4:561-582. doi: 10.1002/wcs.1248 For further resources related to this article, please visit the WIREs website.

Classical conditioning through auditory stimuli in Drosophila: methods and models

The role of sound in Drosophila melanogaster courtship, along with its perception via the antennae, is well established, as is the ability of this fly to learn in classical conditioning protocols. Here, we demonstrate that a neutral acoustic stimulus paired with a sucrose reward can be used to condition the proboscis-extension reflex, part of normal feeding behavior. This appetitive conditioning produces results comparable to those obtained with chemical stimuli in aversive conditioning protocols. We applied a logistic model with general estimating equations to predict the dynamics of learning, which successfully predicts the outcome of training and provides a quantitative estimate of the rate of learning. Use of acoustic stimuli with appetitive conditioning provides both an alternative to models most commonly used in studies of learning and memory in Drosophila and a means of testing hearing in both sexes, independently of courtship responsiveness.

A paradigm for operant conditioning in blow flies (Phormia terrae novae Robineau-Desvoidy, 1830)

An operant conditioning situation for the blow fly (Protophormia terrae novae) is described. Individual flies are trained to enter and reenter a hole as the operant response. Only a few sessions of contingent reinforcement are required to increase response rates. When the response is no longer followed by food, the rate of entering the hole decreases. Control procedures revealed that rate of responding is not a simple overall result of feeding or of aging. The flies entered into the hole only if the response was required to obtain the food.

Chemosensory conditioning in molluscs: II. A critical review

We critically review chemosensory conditioning studies with molluscs and find that, in many studies, the influence of nonassociative processes complicates, obscures, and renders ambiguous the unique contribution of associative learning. These nonassociative processes include sensory adaptation, habituation, sensitization, and changes in feeding motivation. They arise from both the food extracts that have often been used as conditioned stimuli and the aversive stimuli that have been used as unconditioned stimuli.

Classical conditioning of feeding in Aplysia: I. Behavioral analysis

A training protocol was developed to classically condition feeding behavior in Aplysia californica using tactile stimulation of the lips as the conditional stimulus (CS) and food as the unconditional stimulus (US). Paired training induced a greater increase in the number of bites to the CS than unpaired training or US-only stimulation. Memory for classical conditioning was retained for at least 24 hr. The organization of the reinforcement pathway that supports classical conditioning was analyzed in additional behavioral experiments. No evidence was found for the contribution to appetitive reinforcement of US-mediating pathways originating in the lips of the animals. Bilateral lesions of the anterior branch of the esophageal nerve, which innervates parts of the foregut, however, were found to attenuate classical conditioning. Thus, it appears likely that reinforcement during appetitive classical conditioning of feeding was mediated by afferent pathways that originate in the foregut. The companion paper () describes two neurophysiological correlates of the classical conditioning.

Effects of aging on the acquisition and extinction of excitatory conditioning in Drosophila melanogaster

Young (7-day-old), middle-aged (28-day-old), and old (49-day-old) Drosophila melanogaster were compared for acquisition, then extinction, of excitatory conditioning of the Proboscis Extension Response. The contribution of nonassociative processes (sucrose-induced Central Excitatory State, Pseudoconditioning) in the elicitation of conditioned responses was simultaneously assessed. Old flies displayed a faster and stronger acquisition than middle-aged ones, whereas no significant acquisition was stated in young flies. The influence of nonassociative processes on the acquisition was of minor importance, even though Central Excitatory State increased with age. Old flies proved also to be the slowest to extinguish. Because nonassociative contamination was not involved during extinction, one may wonder whether extinction was delayed in old flies mainly due to a higher level of acquisition or to a behavioral rigidity.

Discovery of genes involved with learning and memory: an experimental synthesis of Hirschian and Benzerian perspectives

The biological bases of learning and memory are being revealed today with a wide array of molecular approaches, most of which entail the analysis of dysfunction produced by gene disruptions. This perspective derives both from early "genetic dissections" of learning in mutant Drosophila by Seymour Benzer and colleagues and from earlier behavior-genetic analyses of learning and in Diptera by Jerry Hirsh and coworkers. Three quantitative-genetic insights derived from these latter studies serve as guiding principles for the former. First, interacting polygenes underlie complex traits. Consequently, learning/memory defects associated with single-gene mutants can be quantified accurately only in equilibrated, heterogeneous genetic backgrounds. Second, complex behavioral responses will be composed of genetically distinct functional components. Thus, genetic dissection of complex traits into specific biobehavioral properties is likely. Finally, disruptions of genes involved with learning/memory are likely to have pleiotropic effects. As a result, task-relevant sensorimotor responses required for normal learning must be assessed carefully to interpret performance in learning/memory experiments. In addition, more specific conclusions will be obtained from reverse-genetic experiments, in which gene disruptions are restricted in time and/or space.

Discriminative conditioning alters food preferences in the leech, Haemopis marmorata

The feeding behavior of the carnivorous leech, Haemopis marmorata, was aversively trained in a discriminative classical conditioning task. Two conditioned stimuli were used: One consisted of a food (chicken or liver) paired with an unconditioned stimulus of quinidine (bitter chemical); the other consisted of the alternate food presented in an unpaired relationship with the quinidine. Training consisted of alternating exposures to the two conditioned stimuli. Testing consisted of the simultaneous presentation of a conditioned stimulus food and a neutral food, beef. The percentages of responding to the conditioned stimuli were tabulated. Haemopis could discriminate between the conditioned stimuli. As a result of pairing a food with quinidine, the leeches selectively reduced their preference for that paired food, while they did not alter their preference for the unpaired food.

Chemically reinforced conditioned courtship in Drosophila: responses of wild-type and the dunce, amnesiac and don giovanni mutants

To test the hypothesis that associative learning is the basis for conditioned courtship, male Drosophila melanogaster were paired with virgin females in the presence of quinine, known to be a negative reinforcer in a learning paradigm independent of courtship. Such "experienced" wild-type males failed to court virgin females and remained refractory to them for 1-2 h. But experienced males from the learning-defective strain, dunce, continued to court females at high levels; and experienced males from the retention-defective strain, amnesiac, failed to demonstrate the wild-type refractory period. Finally, males from the don giovanni strain, defective with respect to fertilized-female conditioning, were conditioned by presentation of virgin females and quinine. Courtship-depressing effects of cis-vaccenyl alcohol, which can be recovered from fertilized females, were confirmed, but no evidence for its role as a negative reinforcer of male courtship was obtained.

Biometrical and chromosome analyses of lines of Drosophila melanogaster selected for central excitation

Lines of Drosophila melanogaster, bidirectionally selected for extreme and opposite expression of central excitatory state (CES), were subjected first to biometrical and then to chromosome analysis. The analyses revealed that low CES expression is partially dominant to high, at least two chromosomes (II and III) are correlated with CES expression, for the low line, both chromosomes, II and III, are necessary for low expression, cytoplasmic factors are involved with CES expression, and loci of minor effect on the X and Y chromosomes are correlated with CES expression.

Visual learning in walking blowflies, Lucilia cuprina

The Australian sheep blowflies Lucilia cuprina were trained by presenting droplets of sugar solution on a light spot of blue (460 nm wavelength) or green (520 nm wavelength). During the test, the searching behaviour was elicited by sugar stimulation. Then, the flies were allowed to walk in the arena where four coloured spots (two blue and two green) with light intensities similar to the training light were exhibited. Visits at these coloured spots were recorded. The flies visited preferably the light spot of the colour to which they had been trained. Next, the flies were trained to a light spot of blue or green displayed in various intensities, and later tested to discriminate between these two colours displayed in fixed intensities. The flies preferred the trained colour over the untrained one irrespective of the intensity used during training. It was only at the lowest intensity that they showed random orientation. These results suggest that the flies can learn to visit a coloured spot, and that they can discriminate between colours on the basis of wavelength rather than intensity. Training caused the flies not only to increase the probability of visiting the trained colour, but also to extend the proboscis and to elicit a characteristic searching behaviour once they had reached the coloured spot.

The regulation of feeding: locusts and blowflies are not so different from mammals

The regulation of feeding in two insect groups, the locusts and the blowflies, is reviewed. The patterning of feeding is discussed first, and then the underlying mechanisms are considered in detail. The concept of "central excitability" is the key to understanding the controls of feeding. It represents a central nervous mechanism for tuning the sensitivity of the insect to food-related stimuli. The level of central excitation is influenced positively by food and also non-food stimuli, both from within and outside the animal, and negatively by deterrent stimuli and by feedbacks from peripheral systems such as stretch receptors on the gut wall, hormones and blood composition. Sustained gustatory input is required if feeding is to continue, once initiated. The level of central excitation present as a meal begins influences ingestion rate and the amounts of negative feedback tolerated before feeding stops. The duration of intervals between meals is then a function of a number of excitatory stimuli and of feeding-induced changes which reduce the likelihood of further feeding, e.g., hormone release, changes in nutrient composition or osmotic pressure of the blood, and gut distension. Other areas reviewed include changes in the regulation of feeding during the life history and the control of drinking.

Three Drosophila mutations that block associative learning also affect habituation and sensitization

Drosophila melanogaster has been cultured with shock to avoid various odors. Mutants that failed to learn this task have been isolated. Here we report tests on these mutants for more elementary types of behavioral plasticity--habituation and sensitization of a reflex. Fruit flies have taste receptors on their feet. When a starved, water-satiated fly has sucrose applied to one foot, it usually responds by extending its proboscis. In normal flies this feeding reflex shows habituation: application of sugar to one foot depresses responsiveness through the contralateral leg for at least 10 min. The reflex also shows brief sensitization application of concentrated sucrose solution to the proboscis increases subsequent responsiveness to tarsal stimulation for 2-5 min. In three associative learning mutants , the proboscis-extension reflex is present with a normal threshold but behavioral modulation of the response is altered. The dunce, turnip, and rutabaga mutants all habituate less than normal flies. In addition, sensitization wanes unusually rapidly in dunce and rutabaga flies, lasting less than a minute in the case of dunce.

Selection for central excitatory state (CES) in the blow fly Phormia regina

Food-deprived but water-satiated blow flies (Phormia regina) were tested for their central excitatory state (CES) response. Bidirectional selection based on individual differences for CES expression resulted in separate high- and low-CES strains in only one generation in three replicate experiments. This suggest that differences in CES expression are correlated with allelic differences at a single locus. The use fo such selected lines in a component analysis of classical conditioning is discussed.

Conditioned behavior in Drosophila melanogaster

Populations of Drosophila were trained by alternately exposing them to two odorants, one coupled with electric shock. On testing, the flies avoided the shock-associated odor. Pseudoconditioning, excitatory states, odor preference, sensitization, habituation, and subjective bias have been eliminated as explanations. The selective avoidance can be extinguished by retraining. All flies in the population have equal probability of expressing this behavior. Memory persists for 24 hr. Another paradigm has been developed in which flies learn to discriminate between light sources of different color.