A sex-dependent influence of testosterone on the dorso-medial neuronal population of the Japanese quail intercollicular nucleus.

The nucleus intercollicularis in quail is implicated in the control of a number of vocalizations. It is also a target structure for steroids and includes high numbers of cells containing androgen and estrogen receptors. We recently demonstrated that in another steroid target, the medial preoptic nucleus, the effects of testosterone are paralleled by significant changes in the neuronal size. A morphometrical analysis of the nucleus intercollicularis was therefore undertaken in male and female Japanese quail that were either gonadectomized or gonadectomized and treated with testosterone or left intact as controls. This showed that, in males, testosterone increases the cellular size in the dorso-medial part of the nucleus: the mean neuronal area was larger in intact and testosterone-treated males than in castrated. Such an effect was not observed in females nor in the adjacent nucleus mesencephalicus dorso-lateralis which was taken as control structure because it is devoid of steroid receptors. The changes observed in the nucleus intercollicularis of males represent a morphological marker for the cellular actions of the steroid. They could provide a useful tool to analyze the mechanisms by which testosterone activates vocalizations in male quail.