Fas-acting memory.

Genetic and behavioral analysis points to a role for fasciclin II in controlling odor memory and alcohol sensitivity in Drosophila.

Learning in normal and mutant Drosophila larvae.

Adult Drosophila melanogaster have previously been conditioned with shock to avoid various odors. In these experiments, larvae also sensed airborne odorants, responded to electric shock, and learned. Larval behavior paralleled adult behavior for (i) a mutant, smellblind, which failed to respond to odorants; (ii) three mutants, dunce, turnip, and cabbage, which were deficient in olfactory learning ability; and (iii) a mutant heterozygote, turnip/+, which learned but also forgot rapidly.

A neuropeptide gene defined by the Drosophila memory mutant amnesiac.

Mutations in genes required for associative learning and memory in Drosophila exist, but isolation of the genes has been difficult because most are defined by a single, chemically induced allele. Here, a simplified genetic screen was used to identify candidate genes involved in learning and memory. Second site suppressors of the dunce (dnc) female sterility phenotype were isolated with the use of transposon mutagenesis. One suppressor mutation that was recovered mapped in the amnesiac (amn) gene. Cloning of the locus revealed that amn encodes a previously uncharacterized neuropeptide gene. Thus, with the cloning of amn, specific neuropeptides are implicated in the memory process.

cAMP-dependent protein kinase and the disruption of learning in transgenic flies.

A molecular genetic approach was used to test for a role of cAMP-dependent protein kinase (PKA) in learning and memory in Drosophila. We used genes encoding a peptide inhibitor of PKA, an N-terminal regulatory subunit fragment containing a pseudosubstrate inhibitory domain, and a wild-type catalytic subunit. These dominantly acting genes were placed under control of the hsp70 promoter and transformed into wild-type flies. Induction of the transgenes by 1 hr heat shock results in overproduction of their RNA in adult flies. The same heat shock treatment disrupts the ability of the flies to learn in an odor discrimination task reinforced with electric shock. The results demonstrate the involvement of PKA in the associative learning of Drosophila.

Isolation and characterization of the gene for Drosophila tyrosine hydroxylase.

A Drosophila cDNA homologous to a rat tyrosine hydroxylase (TH) probe has been isolated and sequenced. The deduced amino acid sequence predicts a 57,861 dalton protein with almost 50% identity with rat TH. In vitro transcription of the cDNA followed by in vitro translation yields a single protein species of approximately 58,000 daltons. The in vitro translation product, as well as a protein of the same molecular weight from wild-type Drosophila head protein extracts, is recognized by an antibody made against bovine TH. The presence of TH enzymatic activity in heads was demonstrated. In situ hybridization to polytene chromosomes localized the gene to 65B. The comapping of the mutant pale to this same region, as well as its phenotype, suggests that pale may be a TH mutation.

What can we teach Drosophila? What can they teach us?

A number of single gene mutations dramatically reduce the ability of fruit flies to learn or to remember. Cloning of the affected genes implicated the adenylyl cyclase second-messenger system as key in learning and memory. The expression patterns of these genes, in combination with other data, indicates that brain structures called mushroom bodies are crucial for olfactory learning. However, the mushroom bodies are not dedicated solely to olfactory processing; they also mediate higher cognitive functions in the fly, such as visual context generalization. Molecular genetic manipulations, coupled with behavioral studies of the fly, will identify rudimentary neural circuits that underly multisensory learning and perhaps also the circuits that mediate more-complex brain functions, such as attention.

A Drosophila CREB/CREM homolog encodes multiple isoforms, including a cyclic AMP-dependent protein kinase-responsive transcriptional activator and antagonist.

We have characterized a Drosophila gene that is a highly conserved homolog of the mammalian cyclic AMP (cAMP)-responsive transcription factors CREB and CREM. Uniquely among Drosophila genes characterized to date, it codes for a cAMP-responsive transcriptional activator. An alternatively spliced product of the same gene is a specific antagonist of cAMP-inducible transcription. Analysis of the splicing pattern of the gene suggests that the gene may be the predecessor of the mammalian CREB and CREM genes.

Reductionism in learning and memory.

This chapter examines the successes and (at least for now) failures of reductionist approaches in dealing with the problem of learning and memory. Beginning with the work of Pavlov on classical conditioning and the theoretical work of Hebb, the paper traces the contributions made by studies on Aplysia, Drosophila and long-term potentiation in the mammalian hippocampus.

Classical conditioning and retention in normal and mutant Drosophila melanogaster.

By changing the conditioned discrimination paradigm of Quinn et al. (1974) from an instrumental procedure to a classical (Pavlovian) one, we have demonstrated strong learning in wildtype flies. About 150 flies were sequestered in a closed chamber and trained by exposing them sequentially to two odors in air currents. Flies received twelve electric shock pulses in the presence of the first odor (CS+) but not in the presence of the second odor (CS-). To test for conditioned avoidance responses, flies were transported to a T-maze choice point, between converging currents of the two odors. Typically, 95% of trained flies avoided the shock-associated odor (CS+). Acquisition of learning was a function of the number of shock pulses received during CS+ presentation and was asymptotic within one training cycle. Conditioned avoidance increased with increasing shock intensity or odor concentration and was very resistant to extinction. Learning was best when CS+ presentations overlap shock (delay conditioning) and then decreased with increasing CS-US interstimulus intervals. Shocking flies immediately before CS+ presentation (backward conditioning) produced no learning. Nonassociative control procedures (CS Alone, US Alone and Explicitly Unpaired) produced slight decreases in avoidance responses, but these affected both odors equally and did not alter our associative learning index (A). Memory in wild-type flies decayed gradually over the first seven hours after training and still was present 24 h later. The mutants amnesiac, rutabaga and dunce showed appreciable learning acquisition, but their memories decayed very rapidly during the first 30 min. After this, the rates of decay slowed sharply; conditioned avoidance still was measureable at least three hours after training.

Flies, genes, and learning.

Flies can learn. For the past 25 years, researchers have isolated mutants, engineered mutants with transgenes, and tested likely suspect mutants from other screens for learning ability. There have been notable surprises-conventional second messenger systems co-opted for intricate associative learning tasks, two entirely separate forms of long-term memory, a cell-adhesion molecule that is necessary for short-term memory. The most recent surprise is the mechanistic kinship revealed between learning and addictive drug response behaviors in flies. The flow of new insight is likely to quicken with the completion of the fly genome and the arrival of more selective methods of gene expression.

Symmetric replication of the Bacillus subtilis chromosome.

Synchronously replicating chromosomes in germinating spores of Bacillus subtilis were labeled near the origin with [(3)H]bromouracil. The label appeared in heavy-light DNA. When cell growth and chromosome replication were continued in unlabeled bromouracil, nearly all the tritium label was transferred to heavy-heavy DNA while the terminus remained unreplicated. This implies that both origins of the replicating chromosome can undergo reinitiation. Therefore, during multifork replication the chromosome takes on the symmetric "dichotomous" form rather than the asymmetric configuration predicted by the rolling circle model.

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.

Conditioning of leg position in normal and mutant Drosophila.

Individual Drosophila melanogaster, with or without heads, can be trained to lift their legs to avoid electric shock. This behavior is similar to the operant conditioning previously demonstrated in intact and headless cockroaches. More than 90% of headless wild-type flies learned to our criterion. In contrast, three mutants (dunce, cabbage, and turnip), originally selected for failure to learn in an olfactory discrimination paradigm, tended to perform poorly in this new learning situation. The difference in learning behavior between normal and mutant flies is distinguishable in individuals and may be useful for mosaic analysis.

Radish, a Drosophila mutant deficient in consolidated memory.

We have characterized the behavior and genetics of the Drosophila mutant radish (rsh gene). Initial learning of radish flies in two olfactory discrimination tests is high, but subsequent memory decays rapidly at both early and late times after training. Anesthesia-resistant memory (consolidated memory) is undetectable in radish flies 3 hr after training. The mutant shows normal locomotor activity and normal sensitivity to the odor cues and electric-shock reinforcement used in the learning tests. The radish gene maps within a 180-kb interval in the 11D-E region of the X chromosome.

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.

Loss of calcium/calmodulin responsiveness in adenylate cyclase of rutabaga, a Drosophila learning mutant.

We have isolated and mapped an X-linked recessive mutation in Drosophila that blocks associative learning, and have partially characterized it biochemically. The mutation affects adenylate cyclase activity. Cyclase activity from mutant flies differed from the wild-type enzyme in that it was not stimulated by calcium or calmodulin. Mutant cyclase activity did respond to guanyl nucleotides, fluoride, and monoamines, which suggests that the defect is neither in the hormone receptor nor in either known GTP-binding regulatory protein. The mutation possibly affects the catalytic subunit directly. We postulate that there is at least one other type of adenylate cyclase activity that is unaffected by the mutation and insensitive to calcium/calmodulin.

Mutations in the dopa decarboxylase gene affect learning in Drosophila.

Fruit flies synthesize several monoamine neurotransmitters. Dopa decarboxylase (Ddc) mutations affect synthesis of two of these, dopamine and serotonin. Both transmitters are implicated in vertebrate and invertebrate learning. Therefore, we bred flies of various Ddc genotypes and tested their learning ability in positively and negatively reinforced learning tasks. Mutations in the Ddc gene diminished learning acquisition approximately in proportion to their effect on enzymatic activity. Courtship and mating sequences of the mutants appeared normal, except for one aspect of male courtship that had previously been shown to be experience dependent. In contrast, the effect on behavior patterns that do not involve learning--phototaxis, geotaxis, olfactory acuity, responsiveness to sucrose--was relatively slight under these conditions. Moderate Ddc mutations affected the acquisition of learned responses while leaving memory retention unaltered. This is in contrast to the mutations dunce , rutabaga , and amnesiac , which primarily affect short-term memory.

dunce, a mutant of Drosophila deficient in learning.

Normal Drosophilia learn to avoid an odorant associated with electric shock. An X-linked mutant, dunce, has been isolated that fails to display this learning in spite of being able to sense the odorant and electric shock and showing essentially normal behavior in other respects.

Deficient protein kinase C activity in turnip, a Drosophila learning mutant.

The Drosophila mutant turnip was initially isolated based on poor learning performance (Quinn, W.G., Sziber, P.P., and Booker, R. (1979) Nature 277, 212-214). Here we show that turnip is dramatically reduced in protein kinase C (PKC) activity. In addition, turnip flies are deficient in phosphorylation of a 76-kDa head membrane protein (hereafter pp76) which is a major substrate for protein kinase C in homogenates of wild-type flies. Reduced PKC activity, defective pp76 phosphorylation, and most of turnip's learning deficiency co-map genetically to a region on the X-chromosome, 18A5-18D1-2, spanned by the deletion Df(1)JA27. Apparently turnip+ is not a structural gene for PKC because Drosophila PKC genes map elsewhere in the genome. Our results suggest that turnip gene product is required for activation of PKC and that PKC plays a role in associative learning in Drosophila.