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Using an immunocytochemical technique, we have studied in the alpaca brainstem the distribution of immunoreactive structures containing prodynorphin (alpha-neoendorphin)- and pro-opiomelanocortin (adrenocorticotrophin hormone (18-39) (ACTH), beta-endorphin (1-27))-derived peptides. No peptidergic-immunoreactive cell body was observed. Immunoreactive fibres were widely distributed, although in most of the brainstem nuclei the density of the peptidergic fibres was low or very low. In general, the distribution of the immunoreactive fibres containing the peptides studied was very similar. A close anatomical relationship occurred among the fibres containing alpha-neoendorphin, ACTH or beta-endorphin (1-27), suggesting a functional interaction among the three peptides in many of the brainstem nuclei. The number of fibres belonging to the prodynorphin system was higher than that of the pro-opio...