In some situations, animals seem to ignore stimuli which in other contexts elicit a robust response. This attenuation in behavior, which enables animals to ignore a familiar, unreinforced stimulus, is called habituation. Despite the ubiquity of this phenomenon, it is generally poorly understood in terms of the underlying neural circuitry. Hungry fruit flies show a proboscis extension reflex (PER) when sensory receptors are stimulated by sugars. The PER is usually followed by feeding. However, if feeding is disallowed following sugar stimulation, PER is no longer robust, and the animal is considered to be habituated to this stimulus. Our results suggest that PER habituation requires an adenylate cyclase-dependent enhancement of inhibitory output of GABAergic neurons in the subesophageal ganglion (SOG), which mediates PER. GABA synthesis in and release from glutamic acid decarboxylase (GAD1) expressing neurons is necessary, and GABA A receptors on cholinergic neurons are required for PER habituation. The proposed inhibitory potentiation requires glutamate/NMDAreceptor signaling, possibly playing a role in stimulus selectivity. We explain why these data provide significant and independent support for a general model in which inhibitory potentiation underlies habituation in multiple neural systems and species.

Walking is a complex rhythmic locomotor behavior generated by sequential and periodical contraction of muscles essential for coordinated control of movements of legs and leg joints. Studies of walking in vertebrates and invertebrates have revealed that premotor neural circuitry generates a basic rhythmic pattern that is sculpted by sensory feedback and ultimately controls the amplitude and phase of the motor output to leg muscles. However, the identity and functional roles of the premotor interneurons that directly control leg motoneuron activity are poorly understood. Here we take advantage of the powerful genetic methodology available in Drosophila to investigate the role of premotor inhibition in walking by genetically suppressing inhibitory input to leg motoneurons. For this, we have developed an algorithm for automated analysis of leg motion to characterize the walking parameters of wild-type flies from high-speed video recordings. Further, we use genetic reagents for targeted RNAi knockdown of inhibitory neurotransmitter receptors in leg motoneurons together with quantitative analysis of resulting changes in leg movement parameters in freely walking Drosophila. Our findings indicate that targeted down-regulation of the GABA A receptor Rdl (Resistance to Dieldrin) in leg motoneurons results in a dramatic reduction of walking speed and step length without the loss of general leg coordination during locomotion. Genetically restricting the knockdown to the adult stage and subsets of motoneurons yields qualitatively identical results. Taken together, these findings identify GABAergic premotor inhibition of motoneurons as an important determinant of correctly coordinated leg movements and speed of walking in freely behaving Drosophila.alking is a complex rhythmic locomotor behavior that requires the coordinated control of movements among legs, leg segments, and leg joints (1-5). While the complete neural circuitry of the motor control networks that orchestrate this control is not known in any animal, significant progress has been made in understanding the general organization of the underlying networks. Studies of reduced preparations in vertebrates have revealed that premotor interneuronal circuits present in the spinal cord are capable of generating patterned rhythmic locomotor activity and distributing this activity to motoneurons (6-8). Furthermore, in several cases, the function of identified spinal interneuron types in this type of fictive locomotion has been elucidated (9, 10). However, the complexity and the difficulty of genetic intervention in vertebrate models have made the cellular identification and functional analysis of these premotor control circuits difficult.Invertebrate models have reduced complexity and for some of them, such as Drosophila, genetic reagents that enable visualization and perturbation of neural circuitry are available (11-13). In Drosophila, the neural connectivity of leg motoneurons, their structural organization, and their relationship with leg muscles has been well establish...

25Walking is a complex rhythmic locomotor behaviour generated by