Neural and neuroendocrine processing of a non-photic cue in an opportunistically-breeding songbird.

Recent studies of the onset of breeding in long-day photoperiodic breeders have focused on the roles of type 2 and 3 iodothyronine deiodinases (DIO2 and DIO3) in the conversion of thyroxine (T4) to triiodothyronine (T3) and subsequent activation of the reproductive axis. It has been hypothesized that an increase in DIO2 and a reciprocal decrease in DIO3 causes the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, setting off a reproductive cascade, and that this DIO mechanism for GnRH release is conserved across vertebrate taxa. We sought to test whether social cues that are known to stimulate reproductive behaviors can activate the DIO system to initiate reproduction in a non-photoperiodic bird, the zebra finch (Taeniopygia guttata). Isolation of males and subsequent presentation of females did not increase DIO2 or GnRH expression in the hypothalamus, nor did it decrease gonadotropin-inhibitory hormone (GnIH) or DIO3. Males receiving a female stimulus showed significantly higher mRNA expression and immunoreactive cell count of the immediate-early gene early growth response protein 1 (EGR-1) than isolated males, indicating hypothalamic activation in response to a female. Cells immunoreactive for EGR-1 were not co-localized with those immunoreactive for GnRH. Reproductive behaviors (singing, copulation attempts and overall activity) were significantly higher in males receiving a female stimulus. This study presents a social effect on behavior and EGR-1 expression in the hypothalamus of males in response to females, but more research is needed to determine whether the DIO2 system and the GnRH system are responsive to social stimulation in this species.