Photoperiod and social cues influence the medial amygdala but not the bed nucleus of the stria terminalis in the Siberian hamster.

We investigated whether the posterodorsal nucleus of the medial amygdala (MePD) and the posteromedial nucleus of the bed nucleus of the stria terminalis (BSTpm) undergo structural changes in response to photoperiod or social environment in the Siberian hamster, a seasonally breeding rodent. Adult male hamsters were either kept in long days (LD; 15:9 h light:dark) from birth or were transferred at 12-16 weeks of age to short days (SD; 8:16) and housed with a male conspecific for 11 weeks. Other males were transferred to SD but were housed with an unrelated female conspecific from LD. Males transferred to SD without a female cagemate displayed testicular regression, but males transferred to SD with a female cagemate did not. The regional volume and average soma size of the BSTpm and the MePD were estimated using Nissl-stained brain sections. Neither photoperiod nor social condition modified either of the BSTpm measures. Among males housed in same-sex groups, the average soma size in the MePD was significantly smaller in SD males than in LD males. Cohabitation with a female resulted in MePD volumes indistinguishable from LD males. These results indicate that the MePD, a nucleus implicated in socio-sexual behavior, can respond to photoperiodic as well as to social cues.