Rapid action on neuroplasticity precedes behavioral activation by testosterone

Abstract

Testosterone has been shown to increase the volume of steroid-sensitive brain nuclei in adulthood in several vertebrate species. In male Japanese quail the volume of the male-biased sexually dimorphic medial preoptic nucleus (POM), a key brain area for the control of male sexual behavior, is markedly increased by testosterone. Previous studies assessed this effect after a period of 8-14 days but the exact time course of these effects is unknown. We asked here whether testosterone-dependent POM plasticity could be observed at shorter latencies. Brains from castrated male quail were collected after 1, 2, 7 and 14 days of T treatment (CX+T) and compared to brains of untreated castrates (CX) collected after 1 or 14 days. POM volumes defined either by Nissl staining or by aromatase immunohistochemistry increased in a time-dependent fashion in CX+T subjects and almost doubled after 14 days of treatment with testosterone while no change was observed in CX birds. A significant increase in the average POM volume was detected after only one day of testosterone treatment. The optical density of Nissl and aromatase staining was also increased after one or two days of testosterone treatment. Activation of male copulatory behavior followed these morphological changes with a latency of approximately one day. This rapid neurochemical and neuroanatomical plasticity observed in the quail POM thus seems to limit the activation of male sexual behavior and offers an excellent model to analyze features of steroid-regulated brain structure and function that determine behavior expression.

Figure 1

Bar graphs illustrating A. the frequencies of neck grabs and cloacal contact movements (CCM) and B. the size of the cloacal gland recorded just before brain collection in castrated birds killed on day 1 (CXD1) and day 14 (CXD14) and in castrated birds treated with exogenous testosterone for 1 day (CX+T D1), 2 days (CX+T D2), 7 days (CX+T D7) or 14 days (CX+T D14). In this and other bar graphs, the bars relative to the two CX groups are partly overlapping to indicate that data for these two groups are illustrated separately but they were pooled for all statistical analyses. Results of Post Hoc Fisher’s PLSD tests are presented as follows: a: p<0.05 vs pooled CX birds; b: p<0.05 vs CX+T D1; c: p<0.05 vs CX+T D2; d: p<0.05 vs CX+T D7.

Figure 2

Photomicrographs illustrating the POM as defined by Nissl staining (A–C) and ARO-ir cells (D–F) in castrates (A, D) and castrates treated with exogenous testosterone for 1 day (B, E) or for 14 days (C, F). The inserts represent higher magnification photomicrographs in the dorso-lateral part of the POM. CA: anterior commissure. The cale bar located in F represents 200 µm in the 6 main panels and 50 µm in the 6 inserts at higher magnification.

Figure 3

Bar graphs illustrating the POM volume defined by Nissl staining (A) and aromatase immunoreactivity (B) in castrated birds killed on day 1 (CXD1) and day 14 (CXD14) and in castrated birds treated with exogenous testosterone for 1 day (CX+T D1), 2 days (CX+T D2), 7 days (CX+T D7) or 14 days (CX+T D14). Results of Post Hoc Fisher’s PLSD tests are presented as follows: a: p<0.05 vs CX; b: p<0.05 vs CX+T D1; c: p<0.05 vs CX+T D2; d: p<0.05 vs CX+T D7.

Figure 4

Bar graphs illustrating the relative optical density of Nissl staining (A) and aromatase immunoreactivity (B) measured in the POM at the objective 10X in castrated birds killed on day 1 (CXD1) and day 14 (CXD14) and in castrates treated with exogenous testosterone for 1 day (CX+T D1), 2 days (CX+T D2), 7 days (CX+T D7) OR 14 days (CX+T D14). Measures at objective 40X were also performed in the ventro-lateral, dorso-lateral and medial periventricular regions of the POM in Nissl- (C) and aromatase-stained (D) sections. Results of Post Hoc Fisher’s PLSD tests are presented as follows: a: p<0.05 vs CX; b: p<0.05 vs CX+T D1; c: p<0.05 vs CX+T D2; d: p<0.05 vs CX+T D7.

Figure 5

Bar graphs illustrating the fractional area covered by aromatase-immunoreactive structures measured at objective 40X. The analyses were performed in the ventro-lateral, dorso-lateral and medial periventricular regions of the POM in castrated bird killed on day 1 (CXD1) and day 14 (CXD14) and in castrated birds treated with exogenous testosterone for 1 day (CX+T D1), 2 days (CX+T D2), 7 days (CX+T D7) or 14 days (CX+T D14). Results of Post Hoc Fisher’s PLSD tests are presented as follows: a: p<0.05 vs CX; b: p<0.05 vs CX+T D1; c: p<0.05 vs CX+T D2; d: p<0.05 vs CX+T D7.