Neuromodulatory circuit effects on Drosophila feeding behaviour and metabolism.

Animals have evolved to maintain homeostasis in a changing external environment by adapting their internal metabolism and feeding behaviour. Metabolism and behaviour are coordinated by neuromodulation; a number of the implicated neuromodulatory systems are homologous between mammals and the vinegar fly, an important neurogenetic model. We investigated whether silencing fly neuromodulatory networks would elicit coordinated changes in feeding, behavioural activity and metabolism. We employed transgenic lines that allowed us to inhibit broad cellular sets of the dopaminergic, serotonergic, octopaminergic, tyraminergic and neuropeptide F systems. The genetically-manipulated animals were assessed for changes in their overt behavioural responses and metabolism by monitoring eleven parameters: activity; climbing ability; individual feeding; group feeding; food discovery; both fed and starved respiration; fed and starved lipid content; and fed/starved body weight. The results from these 55 experiments indicate that individual neuromodulatory system effects on feeding behaviour, motor activity and metabolism are dissociated.

Drosophila learn efficient paths to a food source.

Elucidating the genetic, and neuronal bases for learned behavior is a central problem in neuroscience. A leading system for neurogenetic discovery is the vinegar fly Drosophila melanogaster; fly memory research has identified genes and circuits that mediate aversive and appetitive learning. However, methods to study adaptive food-seeking behavior in this animal have lagged decades behind rodent feeding analysis, largely due to the challenges presented by their small scale. There is currently no method to dynamically control flies' access to food. In rodents, protocols that use dynamic food delivery are a central element of experimental paradigms that date back to the influential work of Skinner. This method is still commonly used in the analysis of learning, memory, addiction, feeding, and many other subjects in experimental psychology. The difficulty of microscale food delivery means this is not a technique used in fly behavior. In the present manuscript we describe a microfluidic chip integrated with machine vision and automation to dynamically control defined liquid food presentations and sensory stimuli. Strikingly, repeated presentations of food at a fixed location produced improvements in path efficiency during food approach. This shows that improved path choice is a learned behavior. Active control of food availability using this microfluidic system is a valuable addition to the methods currently available for the analysis of learned feeding behavior in flies.

Distinct pathways for rule-based retrieval and spatial mapping of memory representations in hippocampal neurons.

Hippocampal neurons encode events within the context in which they occurred, a fundamental feature of episodic memory. Here we explored the sources of event and context information represented by hippocampal neurons during the retrieval of object associations in rats. Temporary inactivation of the medial prefrontal cortex differentially reduced the selectivity of rule-based object associations represented by hippocampal neuronal firing patterns but did not affect spatial firing patterns. In contrast, inactivation of the medial entorhinal cortex resulted in a pervasive reorganization of hippocampal mappings of spatial context and events. These results suggest distinct and cooperative prefrontal and medial temporal mechanisms in memory representation.

Single neuron recordings in dorsal cochlear nucleus (DCN) of awake gerbil.

The auditory responses of neurons in the dorsal cochlear nucleus (DCN) are known to be sensitive to anesthesia, and consequently many studies have used an unanesthetized, decerebrate preparation. Decerebration, however, severs multiple descending pathways to the DCN and is traumatic to the brain, and so sensory responses may be influenced. It was possible, by combining sterile surgery, animal conditioning, and mild restraint to allow recordings in awake gerbils (Merionus unguiculatus). Response maps (RMs), post-stimulus time histograms (PSTHs), and responses to notch noise stimuli were recorded in awake gerbils. Some units' responses were compared to those from previous experiments in anesthetized and decerebrate gerbil preparations. All RM types observed in decerebrate gerbils were also observed in the awake gerbil, with the notable exception of type IV units. Type III units were still the most commonly recorded units. No significant changes were observed in either RM shape or notch noise responses. Although most awake DCN units had spontaneous activity (typically <20spikes/s), these rates were lower than those in decerebrate gerbils. Type III units' spontaneous activity rates were significantly different from those in the decerebrate prep (P<0.05). DCN neurons may be more inhibited in awake gerbils. Although type III units were most sensitive to spectral notches, different unit types seem to respond slightly differently to notch noise. This study indicates that responses of DCN units in awake and decerebrate gerbils are different, though the shapes of RMs are the same.