Spinomedullary pathways in the pigeon (Columba livia): differential involvement of lamina I cells.

The lamina I (marginal zone) of the spinal cord dorsal horn is an important site for pain processing. In mammals, lamina I neurons have been shown to constitute a heterogeneous population made up of four morphological groups with particular neurochemical nature, supraspinal connection patterns, and nociceptive response properties. In order to obtain a comparative view of the mechanisms of nociceptive processing, the analysis of the structural morphology and supraspinal connectivity of lamina I neurons was, in this study, extended to the avian family. Cholera toxin subunit B (CTb) was injected in the nucleus tractus solitarius (NTS), nucleus centralis medullae pars dorsalis (Cnd), and the dorsolateral portion of the nucleus reticularis lateralis (RLlat) of the pigeon (Columba livia), areas equivalent to the rat caudal medulla oblongata lamina I targets, which have been shown to receive differential projections from all cell groups present in lamina I of mammals. In the pigeon, lamina I cells project to the three medullary regions and present the same morphology of spinomedullary lamina I cells of mammals: the spinal-NTS and the spinal-RLlat pathways originated from fusiform, pyramidal, and flattened neurons, and the spinal-Cnd pathway from multipolar, pyramidal, and flattened neurons. Furthermore, the relative participation of each lamina I cell type in each pathway was found to be similar to that previously observed in the rat. The observed similarities on the anatomical organization of lamina I neurons in mammalian and avian species can be taken as a phylogenetic indication of the importance of the nociceptive circuitry centered in lamina I.