A preparation of decapitated Drosophila melanogaster has been used for direct application of drugs to the nerve cord. Serotonin, dopamine, and octopamine stimulate locomotion and grooming, showing distinguishable effects that often are potentiated by addition of the vertebrate monoamine oxidase-inhibitor hydrazaline. Many of the hydrazaline-induced effects are sexually dimorphic, with males showing greater responses than females. Behaviors similar to those induced by dopamine can be induced by application of the vertebrate dopamine D2-like receptor agonist quinpirole, whose effects are also sexually dimorphic. In contrast, vertebrate D2-like and D1-like dopamine antagonists result in akinesic states, and D1-like agonists selectively stimulate grooming. These data indicate that Drosophila nerve cord amine receptors are coupled to ref lexive behaviors similar to those stimulated by brain dopamine receptors in vertebrates.In both vertebrates and invertebrates, the basic neural oscillators controlling reflex behaviors and locomotion are contained within the spinal cord, or the nerve cord in the case of invertebrates (reviewed in refs. 1-6). Several nonhuman vertebrates show both locomotion and reflex scratching behaviors in response to irritants even after the spinal cord is cut. Similarly, decapitated insects show a basal level of spontaneous grooming as well as a normal grooming response when a sensory bristle is stimulated by gentle mechanical contact (5, 6). Studies in both Drosophila and larger insects show that the grooming response consists of a stereotyped series of leg, wing, and body movements that result in removal of debris from the legs and body of the fly (6-8).Many of these behaviors can be stimulated by application of biogenic amines. Injection of L-DOPA and 5-HTP, the precursors for dopamine͞noradrenaline and serotonin, respectively, can initiate walking motor patterns in spinal cats (9-11) and rabbits (12), although a more recent study shows that adrenergic agents are the most effective (13). Similarly, in partly dissected preparations of moth and locust, addition of dopamine or octopamine to the thoracic ganglia leads to stimulation of flight motor and stepping oscillators (14-16), and in cockroaches, dopamine can stimulate an escape response (17). These applications of exogenous biogenic amines likely mimic the roles of amines and͞or noradrenaline provided to the nerve or spinal cord from the brain by descending projections that are found in both vertebrates and insects (18,19). The invertebrate nerve cord differs from the higher vertebrate spinal cord in that the nerve cord contains aminergic cells bodies (20)(21)(22)(23). Presumably these cell bodies provide localized sources of amines to regions of the nerve cord neuropil that are not accessed by the descending aminergic projections.In vertebrates and invertebrates, dopamine and other amines act through families of G protein-linked seven transmembrane receptors (reviewed in refs. 24-26). Stimulation or inhibition of vertebrate dopamine...
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