Dopamine modulation of the in vivo acetylcholine response in the Drosophila mushroom body

Octopamine and Dopamine differentially modulate the nicotine-induced calcium response in Drosophila Mushroom Body Kenyon Cells

In Drosophila associative olfactory learning, an odor, the conditioned stimulus (CS), is paired to an unconditioned stimulus (US). The CS and US information arrive at the Mushroom Bodies (MB), a Drosophila brain region that processes the information to generate new memories. It has been shown that olfactory information is conveyed through cholinergic inputs that activate nicotinic acetylcholine receptors (nAChRs) in the MB, while the US is coded by biogenic amine (BA) systems that innervate the MB. In this regard, the MB acts as a coincidence detector. A better understanding of the properties of the responses gated by nicotinic and BA receptors is required to get insights on the cellular and molecular mechanisms responsible for memory formation. In recent years, information has become available on the properties of the responses induced by nAChR activation in Kenyon Cells (KCs), the main neuronal MB population. However, very little information exists on the responses induced by aminergic systems in fly MB. Here we have evaluated some of the properties of the calcium responses gated by Dopamine (DA) and Octopamine (Oct) in identified KCs in culture. We report that exposure to BAs induces a fast but rather modest increase in intracellular calcium levels in cultured KCs. The responses to Oct and DA are fully blocked by a VGCC blocker, while they are differentially modulated by cAMP. Moreover, co-application of BAs and nicotine has different effects on intracellular calcium levels: while DA and nicotine effects are additive, Oct and nicotine induce a synergistic increase in calcium levels. These results suggest that a differential modulation of nicotine-induced calcium increase by DA and Oct could contribute to the events leading to learning and memory in flies.

Evolution of the techniques used in studying associative olfactory learning and memory in adult Drosophila in vivo: a historical and technical perspective

Drosophila melanogaster behavioral mutants have been isolated in which the ability to form associative olfactory memories has been disrupted primarily by altering cyclic adenosine monophosphate signal transduction. Unfortunately, the small size of the fruit fly and its neurons has made the application of neurobiological techniques typically used to investigate the physiology underlying these behaviors daunting. However, the realization that adult fruit flies could tolerate a window in the head capsule allowing access to the central structures thought to be involved plus the development of genetically expressed reporters of neuronal function has allowed a meteoric expansion of this field over the last decade. This review attempts to summarize the evolution of the techniques involved from the first use of a window to access these brain areas thought to be involved in associative olfactory learning and memory, the mushroom bodies and antennal lobes, to the current refinements which allow both high-resolution multiphoton imaging and patch clamping of identified neurons while applying the stimuli used in the behavioral protocols. This area of research now appears poised to reveal some very exciting mechanisms underlying behavior.

KCNQ channels regulate age-related memory impairment

In humans KCNQ2/3 heteromeric channels form an M-current that acts as a brake on neuronal excitability, with mutations causing a form of epilepsy. The M-current has been shown to be a key regulator of neuronal plasticity underlying associative memory and ethanol response in mammals. Previous work has shown that many of the molecules and plasticity mechanisms underlying changes in alcohol behaviour and addiction are shared with those of memory. We show that the single KCNQ channel in Drosophila (dKCNQ) when mutated show decrements in associative short- and long-term memory, with KCNQ function in the mushroom body α/βneurons being required for short-term memory. Ethanol disrupts memory in wildtype flies, but not in a KCNQ null mutant background suggesting KCNQ maybe a direct target of ethanol, the blockade of which interferes with the plasticity machinery required for memory formation. We show that as in humans, Drosophila display age-related memory impairment with the KCNQ mutant memory defect mimicking the effect of age on memory. Expression of KCNQ normally decreases in aging brains and KCNQ overexpression in the mushroom body neurons of KCNQ mutants restores age-related memory impairment. Therefore KCNQ is a central plasticity molecule that regulates age dependent memory impairment.

Long-term enhancement of synaptic transmission between antennal lobe and mushroom body in cultured Drosophila brain

In Drosophila, the mushroom body (MB) is a critical brain structure for olfactory associative learning. During aversive conditioning, the MBs are thought to associate odour signals, conveyed by projection neurons (PNs) from the antennal lobe (AL), with shock signals conveyed through ascending fibres of the ventral nerve cord (AFV). Although synaptic transmission between AL and MB might play a crucial role for olfactory associative learning, its physiological properties have not been examined directly. Using a cultured Drosophila brain expressing a Ca(2+) indicator in the MBs, we investigated synaptic transmission and plasticity at the AL-MB synapse. Following stimulation with a glass micro-electrode, AL-induced Ca(2+) responses in the MBs were mediated through Drosophila nicotinic acetylcholine receptors (dnAChRs), while AFV-induced Ca(2+) responses were mediated through Drosophila NMDA receptors (dNRs). AL-MB synaptic transmission was enhanced more than 2 h after the simultaneous 'associative-stimulation' of AL and AFV, and such long-term enhancement (LTE) was specifically formed at the AL-MB synapses but not at the AFV-MB synapses. AL-MB LTE was not induced by intense stimulation of the AL alone, and the LTE decays within 60 min after subsequent repetitive AL stimulation. These phenotypes of associativity, input specificity and persistence of AL-MB LTE are highly reminiscent of olfactory memory. Furthermore, similar to olfactory aversive memory, AL-MB LTE formation required activation of the Drosophila D1 dopamine receptor, DopR, along with dnAChR and dNR during associative stimulations. These physiological and genetic analogies indicate that AL-MB LTE might be a relevant cellular model for olfactory memory.

Insights from honeybee (Apis mellifera) and fly (Drosophila melanogaster) nicotinic acetylcholine receptors: from genes to behavioral functions

Nicotinic acetylcholine receptors (nAChRs) are widely expressed throughout the central nervous system of insects where they supply fast synaptic excitatory transmission and represent a major target for several insecticides. The unbalance is striking between the abundant literature on nAChR sensitivity to insecticides and the rarity of information regarding their molecular properties and cognitive functions. The recent advent of genome sequencing disclosed that nAChR gene families of insects are rather small-sized compared to vertebrates. Behavioral experiments performed in the honeybee demonstrated that a subpopulation of nAChRs sensitive to the venom α-bungarotoxin and permeant to calcium is necessary for the formation of long-term memory. Concomitant data in Drosophila reported that repetitive exposure to nicotine results in a calcium-dependent plasticity of the nAChR-mediated response involving cAMP signaling cascades and indicated that ACh-induced Ca++ currents are modulated by monoamines involved in aversive and appetitive learning. As in vertebrates, in which glutamate and NMDA-type glutamate receptors are involved in experience-associated synaptic plasticity and memory formation, insects could display a comparable system based on ACh and α-Bgt-sensitive nAChRs.

Optical calcium imaging in the nervous system of Drosophila melanogaster

BACKGROUND

Drosophila melanogaster is one of the best-studied model organisms in biology, mainly because of the versatility of methods by which heredity and specific expression of genes can be traced and manipulated. Sophisticated genetic tools have been developed to express transgenes in selected cell types, and these techniques can be utilized to target DNA-encoded fluorescence probes to genetically defined subsets of neurons. Neuroscientists make use of this approach to monitor the activity of restricted types or subsets of neurons in the brain and the peripheral nervous system. Since membrane depolarization is typically accompanied by an increase in intracellular calcium ions, calcium-sensitive fluorescence proteins provide favorable tools to monitor the spatio-temporal activity across groups of neurons.

SCOPE OF REVIEW

Here we describe approaches to perform optical calcium imaging in Drosophila in consideration of various calcium sensors and expression systems. In addition, we outline by way of examples for which particular neuronal systems in Drosophila optical calcium imaging have been used. Finally, we exemplify briefly how optical calcium imaging in the brain of Drosophila can be carried out in practice.

MAJOR CONCLUSIONS AND GENERAL SIGNIFICANCE

Drosophila provides an excellent model organism to combine genetic expression systems with optical calcium imaging in order to investigate principles of sensory coding, neuronal plasticity, and processing of neuronal information underlying behavior. This article is part of a Special Issue entitled Biochemical, Biophysical and Genetic Approaches to Intracellular Calcium Signaling.