Some of the schemes developed, mainly for the STG of crustaceans located within a blood vessel, suggest that the whole network is exposed to a given neuromodulator. This may be true for this particular system, but may differ for other motor systems. Even in the STG the release may be more targeted within the neuropil. In insect and crustacean neuropils, glia cells may form effective compartments or "pools" in which neuromodulators act on all cells in possession of the respective receptor proteins but may leave other neurons outside these borders rather unaffected (see Figure 4.1). Similarly, Nusbaum (2002) has suggested that the action of co-released biogenic amines or peptides may be affected crucially by either the respective uptake mechanisms or the presence of extracellular peptidase activity. With respect to peripheral modulation, locusts provide a good example: the octopamine levels in the hemolymph increase within the first ten minutes of flight, yet the octopamine levels of individual flight muscles decrease significantly, as do substances that normally would increase if a muscle is bathed in salines containing high octopamine concentrations (Wegener 1996). This clearly indicates that the octopamine level in the hemolymph and the octopamine level within a muscle represent two different compartments in the intact animal. Preliminary work on the ultrastructure of DUM neuron terminals (Biserova and Pfluger, unpublished) indicates that octopamine may be released towards the basal lamina in some cases and directly to the muscular membrane in others. Similar mechanisms may apply to the CNS where glial cells can provide compartmental barriers, and thus induce a flow system that directs the modulators only into a certain neuropilar area.
As stated above, neuromodulator release within the CNS is not well understood. Evidence from immunocytochemistry suggests that neuromodulators, such as octopamine, dopamine, 5-HT, histamine, and numerous peptides, are present in all major neuropils of the brain (Homberg 1994). Some neuropilar regions (e.g., those of the optical ganglia, the antennal lobes, or the central complex of the insect brain) are often densely stained by the respective antibodies, whereas others (e.g., mushroom bodies) exhibit sparse but topographically distinct staining (Sinakevitch et al. 2001). This suggests that only a subpopulation of synapses of central neurons that are part of these neuropils may be exposed to the neuromodulator, and thus may undergo modulation, whereas synapses at other locations on the same neuron may not be affected at all. Caution, however, must be applied when evidence from anatomical data is interpreted functionally. Nevertheless, there maybe functional compartments for neuromodulator action within neuropils whose functional consequences are not yet understood. A similar situation may also apply to peripheral nerves. For example, in nerves innervating the STG, neuropilar structures exhibiting immunoreactivity to synaptic proteins and certain peptidergic transmitters were found (Skiebe and Ganeshina 2000); their function, however, remains to be identified.
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