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Mozkový systém odměny u hmyzu
DVOŘÁČEK, Jiří
Animal behavior is not random; rather, it is primarily determined by the biological significance of environmental stimuli: stimuli essential for a survival are marked by the brain reward system, with a positive hedonic value and their achievement is associated with the pleasure (reward). The concept of the reward system emerged from research of the mammalian brain; early theories held that it was a system present only in the brains of higher animals (the mesolimbic dopaminergic system), and that reward was a manifestation of more complex neural networks and higher brain functions. The brain reward has evolved from a solitary phenomenon to a complex function that is divided into the components of ´liking´, ´wanting´, and ´learning´, and from the predominate role of dopamine to a more sophisticated idea that assigns important functions to other neurochemical systems. While dopamine still plays a significant part in the ´wanting´ function, the opioid system likely plays a larger role in the ´liking´ function. The distinction of stimuli into pleasant/unpleasant (attractive/aversive) has been described in insects, and it is widely believed that this principle applies throughout the animal kingdom. Mushroom bodies have been identified as the critical regions of reward functions in the brains of insects. The exact descriptions of the implicated neurotransmitters and modulators, as well as specific cellular and network structures, were also provided. Although the complexity of the brain networks in mammalian and insect reward systems differs, the general principles are the similar in both. The fly Drosophila melanogaster is a frequent laboratory model for investigating the principles of neural network functioning. When studying the brain reward system, it is not only appealing because it is a relatively simple organism with a transparent brain and a described genome, but it may also have the benefit for us that when thinking about its brain, we do not apply relatively old, complex concepts with unlimited meanings, which are a problem in interpreting the human brain study. In the case of the fruit fly, we can highlight that 1) the brain regions involved in associative learning and brain reward functions are surprisingly complex, despite the fact that it is a relatively simple and short-lived organism, 2) its brain almost certainly has a system that creates a motivational drive (´wanting´), and 3) there are indications of the potential existence of a hedonic component of pleasure or its evolutionary predecessor, based not on endogenous opioids. It is inspiring in many ways to compare the brain structures of two evolutionary distinct animal groupsinsects and mammals/humans. This comparison has several implications for a broad paradigm of animal reward, including: Reward principles are universal, and all species are likely fundamentally motivated by the need for rewards. The brain reward mechanisms appear to be hierarchically structured; rather than being centrally organized, they are distributed among other brain networks and mechanisms. The components of these mechanisms can operate independently of one another and concurrently. While the function of neuropeptides in the reward system is flexible, the function of monoamines in the reward system is likely to be conservative in evolutionary terms (the function of endogenous opioids in mammals may be at least partially regulated by another neuropeptide in insects). The neurotransmitter identity of dopaminergic neurons in the reward system is likely to be very context-dependent. Two other interesting concepts can be found in the bee: the sublimation of reward functions in individuals in favor of collective pleasure, and the implied integrated function connecting reward functions and social behavior into one continuum. The comparative study gives new scope for understanding disorders of the reward system, especially addiction, and may also have significant philosophical consequences.
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