Species-specific modulation of pattern-generating circuits

Phylogenetic comparison can reveal general principles governing the organiz ation and neuromodulation of neural networks. Suitable models for such an a pproach are the pyloric and gastric motor networks of the crustacean stomat ogastric ganglion (STG). These networks, which have been well studied in se veral species, are extensively modulated by projection neurons originating in higher-order ganglia. Several of these have been identified in different decapod species, including the paired modulatory proctolin neuron (MPN) in the crab Cancer borealis [Nusbaum & Marder (1989) J. Neurosci., 9,1501-159 9; Nusbaum & Marder (1989), J. Neurosci., 9, 1600-1607] and the apparently equivalent neuron pair, called GABA (gamma-aminobutyric acid) neurons 1 and 2 (GN1/2), in the lobster Homarus gammarus [Cournil et al. (1990) J. Neuro cytol., 19, 478-493]. The morphologies of MPN and GN1/2 are similar, and bo th exhibit GABA-immunolabelling. However, unlike MPN, GN1/2 does not contai n the peptide transmitter proctolin. Instead, GN1/2, but not MPN, is immuno reactive for the neuropeptides related to cholecystokinin (CCK) and FLRFami de. Nonetheless, GN1/2 excitation of the lobster pyloric rhythm is similar to the proctolin-mediated excitation of the crab pyloric rhythm by MPN. In contrast, GN1/2 and MPN both use GABA but produce opposite effects on the g astric mill rhythm. While MPN stimulation produces a GABA-mediated suppress ion of the gastric rhythm [Blitz & Nusbaum (1999) J. Neurosci., 19, 6774-67 83], GN1/2 activates or enhances gastric rhythmicity. These results highlig ht the care needed when generalizing neuronal organization and function acr oss related species. Here we show that the 'same' neuron in different speci es does not contain the same neurotransmitter complement, nor does it exert all of the same effects on its postsynaptic targets. Conversely, a differe nt transmitter phenotype is not necessarily associated with a qualitative c hange in the way that a modulatory neuron influences target network activit y.