Neuromodulators have been shown to affect all aspects of a neuronally generated motor program: the priming of the state of premotor networks necessary to generate a specific behavior or a particular motor output, the initiation and the maintenance of motor programs and, finally, the intensity, cycle period, and phasing of the motor output within motor programs. These influences on the systems level arise from actions of neuromodulators on sensory neurons, central neurons, and muscle properties. For example, biogenic amines, such as octo-pamine and serotonin, injected into the hemolymph can determine the posture of an organism by altering the magnitude of neural output to the leg muscle control system in crayfish (Kravitz et al. 1980). Elevated levels of octopamine within the central nervous system are capable of initiating and maintaining the generation of locomotor programs, like the flight motor pattern in the locust CNS (Stevenson and Kutsch 1988). Similarly, elevated levels of octopamine can activate the leg muscle control system of the stick insect and induce active leg movements (Buschges et al. 1993). The specific effects of octopamine on motor networks led to the formation of the "orchestration hypothesis" (Sombati and Hoyle 1984; Hoyle 1985), in which a particular neuromodulator organizes more complex sequences of behavior. This is in agreement with the findings ofneuro-modulator effects on more complex behaviors, such as aggression (see below) or peptidergic cascades in ecdysis (see above discussion on metamodulation).
Serotonin has been shown to be of importance for the initiation and maintenance of swimming in the leech (Kristan and Nusbaum 1982), although recent experiments, where various modulators were applied to the brain of the leech, question some of those initial results (Crisp and Mesce 2003). Differential actions of various neuromodulators, such as peptides (e.g., the crustacean cardioactive peptide, CCAP, andproctolin) and biogenic amines (e.g., octopamine and 5-HT), are reported for the swimmeret system of crustaceans (Mulloney et al. 1987; Acevedo et al. 1994). Some interesting behavioral aspects of neuro-modulator action come from studies on crayfish and insects where the level of neuromodulators, such as 5-HT and octopamine, may vary depending on the social status and may control the level of aggression (Kravitz and Huber 2003). One network that is particularly affected by varying 5-HT levels is the crayfish escape system, where some of the effects seen in dominant and submissive animals may be due to different expression of 5-HT receptors in component neurons of the escape network (Edwards et al. 2002). Studies on aggression in wild-type and neuromodulator-deficient Drosophila melanogaster will soon provide important new insights into how particular neuromodulators may be responsible for controlling whole sequences of behavior (Baier et al. 2002). As recording from the Drosophila CNS becomes increasingly feasible (Wilson et al. 2004), studies combining electrophysiology with genetics will undoubtedly add a new quality to understanding the behavioral role of neuromodulators.
Other interesting aspects of biogenic amines are described for honeybees in which octopamine levels in some neuropilar areas, such as the antennal lobes, are increased only in certain behavioral contexts. For example, octopamine levels increase only when foraging behavior is anticipated; they remain high during subsequent foraging, in contrast to nonforaging flights, which have no effect on octopamine levels (Schulz et al. 2002). During foraging, olfactory associative learning occurs, which itself is influenced by octopamine, most likely through the VUMmx neuron (Hammer 1993). The whole cascade of neuromodulatory events is not yet known; however, circulating hormones (e.g., juvenile hormones) most likely play a role in orchestrating these events.
Whereas for some vertebrate motor systems it is clear that neuromodulators play a decisive role in development, maturation, and maintenance of motor patterns (see Grillner 2003; Sillar and Grillner, this volume), evidence for these types of effects in invertebrate systems is largely lacking, except for the stomatogastric ganglion (STG) of crustaceans where neuromodulators regulate the sequential expression of behavior during maturation. Neuromodulators also play most relevant roles throughout adult life in processes ofplasticity: 5-HT exerts long-term structural and functional effects on neural circuits during Aplysia learning (Bailey et al. 2000).
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