Sex and season influence the proportion of thin spike cells in the canary HVc

In songbirds, anatomical attributes of song nuclei exhibit sexual and seasonal differences. To extend these data to physiological correlates, neurons (n = 374) were recorded in the HVc of male and female canaries during and outside the breeding period. Surprisingly, a particular type of action potential waveforms was observed more frequently in breeding than in non-breeding birds and in males than in females. These neurons showed both a shorter action potential duration (<0.4 ms) and a higher firing rate (2.5 vs 1.4 spikes/s) than the other neurons. Such characteristics are usually associated with interneurons in the songbird HVc as well as in the mammalian neocortex. Thus, these results provide the first electrophysiological evidence for an alteration of the neuronal network of HVc across sexes and seasons.