Endogenous opioids are known to modulate motivated behaviors. Male zebra finches produce a highly motivated behavior (directed song) to court females and also sing in isolation (undirected song). We found that adult male zebra finches sang significantly fewer directed and undirected songs after administration of low doses (2.5 mg/kg body weight) of the general opioid antagonist naloxone, even though the order of syllables in songs was not altered. Surprisingly, high doses of naloxone (10 mg/kg body weight) dramatically decreased the production of undirected songs but had no significant effects on directed songs. There were no changes in the number of calls during directed or undirected song, movement, stereotyped behaviors including pecking and preening, feeding or drinking behaviors after naloxone administration. We also found that treating zebra finches with naloxone led to a decrease in tonality (goodness of pitch), frequency and amplitude modulation and an increase in the length of intersyllable intervals. Our results suggest that the opioid system can differentially modulate directed and undirected song as well as the acoustic characteristics of birdsong, perhaps by acting on different components of the song control system.
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