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Zhu, J.W., Park, K.C., and Baker, T.C. (2003). of Neuroscience, Newcastle University, Identification of odors from overripe mango Newcastle upon Tyne NE1 7RU, UK. that attract vinegar flies, Drosophila E-mail: [email protected] References melanogaster. J. Chem. Ecol. 29, 899–909. 1. Soto-Vaca, A., Gutierrez, A., Losso, J.N., Xu, Z., 8. Grosjean, Y., Rytz, R., Farine, J.P., Abuin, L., and Finley, J.W. (2012). Evolution of phenolic Cortot, J., Jefferis, G.S., and Benton, R. (2011). compounds from color and flavor problems to An olfactory receptor for food-derived odours http://dx.doi.org/10.1016/j.cub.2014.12.052 Neuromodulation: Letting Sources of messenger pathways [5,6]. In larval zebrafish, for example, dopamine can Spinal Dopamine Speak for promote locomotion via activation of lower-affinity D1 or higher-affinity D4 Themselves receptors, and inhibit locomotion via higher-affinity D2 or D3 receptors [7,8]. To begin to place these observations A recent study of dopaminergic neurons in the brain of larval zebrafish has in a behavioral context, Jay et al. [1] important implications for interpreting the natural actions of neuromodulators focused on an evolutionarily in the spinal cord. conserved group of dopaminergic diencephalospinal neurons (DDNs) in Sandeep Kishore head on, as it were, using the zebrafish the forebrain, which provide the sole and David L. McLean* model system. source of spinal dopamine not only in One of the earliest demonstrations zebrafish [9,10], but also in mammals The surest way to get reliable that aminergic neuromodulators play a [11]. The authors took advantage of an information is to go to the source. In critical role in facilitating vertebrate enhancer trap transgenic line of vertebrates, key sources of aminergic locomotion was provided by zebrafish, Tg(ETvmat2:GFP), in which neuromodulators that help produce experiments in the 1960s, where these neurons are labeled by green locomotion are located in the brain. systemic application of a precursor to fluorescent protein (GFP) [12]. The However, much of our understanding catecholamine synthesis, L-DOPA, relatively large size and location of the of neuromodulation during locomotion rescued walking movements in cells made it possible to monitor not has come from studies where the spinalized cats [2]. This led to a focus on only their activity patterns, but also spinal cord is isolated, drugs are bath the actions of amines such as their excitatory and inhibitory synaptic applied, and changes in locomotor serotonin, noradrenaline and dopamine inputs, using patch-clamp recordings output due to changes in spinal neuron within the spinal cord, which contains in intact, chemically-immobilized fish excitability and connectivity are the circuitry necessary for executing capable of generating ‘fictive’ measured. Because of the difficulty of locomotion [3]. A body of work too large swimming (Figure 1A). By including recording from sources of to adequately cover here has fluorescent dye in the patch pipette, or neuromodulators in the brain of intact, subsequently described myriad actions by using a post-hoc stain, the authors locomoting animals, the behavioral in numerous species [4]. Of particular confirmed that their recordings were relevance of pharmacological relevance to the Jay et al. [1] study is the from spinal projecting neurons. manipulations in the spinal cord is still fact that dopamine can exert different Unexpectedly, they also found that unclear. In this issue of Current effects via receptor subtypes with DDN axons exited the central nervous Biology, Jay et al. [1] tackle this issue different affinities and second system and targeted auditory and Dispatch R147 somatosensory structures in the swimming could occur in the absence of A 4-day-old larval zebrafish periphery (Figure 1A). So what might bursting and vice versa. Consistent with Spinal cord these unusual neurons be doing? this idea, they found no relationship Diencephalon The authors first investigated the between the occurrence of a DDN burst firing behavior of DDNs and the and the duration of a swim bout, LL underlying synaptic drive. They suggesting spinal dopamine is unlikely OC identified two firing regimes that to play a role in patterning spontaneous occurred spontaneously within the locomotor output. Critically, however, Motor neurons same fish, namely asynchronous ‘tonic’ when combined with the patch-clamp or muscle fibers and synchronous ‘burst’ activity. The observations, these data reveal for the Dopaminergic diencephalospinal neurons (DDNs) tonic pattern was characterized by a first time that DDNs receive a bolus of relatively low, steady sequence of firing excitatory drive concomitant with B Tonic DDN activity (frequencies less than 6 Hz). In excitation to the spinal circuits mammalian midbrain dopamine responsible for locomotion. neurons, this form of tonic firing is The work described thus far is No motor activity largely a product of their intrinsic already a remarkable achievement, as electrical properties [13]. Using few studies have monitored the natural antibiotic-based perforated patch- firing patterns of spinal-projecting Burst DDN activity clamp methods that preserve the modulatory populations during integrity of the intracellular milieu and locomotion [16], and none have sequential pharmacological blocks of examined the synaptic inputs excitatory glutamate and inhibitory responsible for driving these patterns. Episodic motor activity GABA receptors, Jay et al. [1] However, Jay et al. [1] followed this up demonstrated that DDN neurons also by answering a major open question: have the inherent ability to spike what happens if you selectively remove tonically, and that variability in this the principal source of spinal C DDN intact DDN ablated pattern of firing is a product of dopamine? The authors took intermittent glutamate and GABA input. advantage of the fact that the DDNs are Dish The burst behavior, on the other the first dopamine neurons to develop hand, was characterized by higher in zebrafish [17] and are thus more incidences of firing (frequencies up to easily targeted. Using intense Larva 40 Hz), which resulted from a ultraviolet light to selectively ablate synchronous volley of glutamate input. GFP-positive DDNs in one-day-old Normal swim Reduced swim Again, behaviorally relevant switches ETvmat2:GFP embryos (the Current Biology between tonic and burst modes of effectiveness and selectivity of which firing have been reported in mammalian were confirmed using immunolabeling Figure 1. Anatomy, activity and ablation of dopamine neurons [14]. So what, if any, in four-day-old larvae), they then spinal sources of dopamine in larval zebra- is the behavioral distinction between assessed the behavioral consequences fish. tonic and burst firing in zebrafish? via semi-automated tracking of real (A) Schematic of the front half of a four-day- At this point, the advantage of swimming in four-day-old larvae within old larval zebrafish (head is to the left) conducting such experiments in an a multiwell dish (Figure 1C). In control illustrating the experimental set up for patch-clamp recordings. Projections from animal capable of autonomously and DDN ablated fish, the characteristic DDNs (in red) innervate the spinal cord by generating fictive locomotion becomes episodic locomotor pattern was still way of the midbrain and hindbrain (light apparent.