PHOTOPERIODICALLY DRIVEN CHANGES IN FOS EXPRESSION WITHIN THE BASAL TUBERAL HYPOTHALAMUS AND MEDIAN-EMINENCE OF JAPANESE-QUAIL

The rapid photoperiodic response in Japanese quail is so precise that it allows neural analyses of how photoperiodic information is transduc ed into an endocrine response. After transfer from short [SD; 6L:18D ( 6:18 hr light/dark cycle)] to long (LD; 20L:4D) days, luteinizing horm one (LH) first rises 20 hr after dawn. Using Fos immunocytochemistry, we examined the basal tuberal hypothalamus (BtH) to determine the rela tionship between brain cell activation and the first endocrine changes . Two separate cell populations within the BtH expressed Fos-like immu noreactivity (FLI) by hour 18 of the first LD. Importantly, this activ ation occurred before the LH rise. Median eminence activation appeared within glial cells, whereas activated infundibular nucleus cells were neuronal, providing support to the view that gonadotropin-releasing h ormone (GnRH) release can be controlled at the terminals by glia. The FLI induction parallels LH changes, suggesting that gene expression ma y be involved in events preceding photostimulation and is the earliest photoperiodically stimulated physiological change yet reported. Addit ional experiments provided further support for this hypothesis. First, photoperiodically induced activation is not a result peculiar to cast rates because intact birds displayed similar results. Second, the crit ical length of 14 hr of light had to be exceeded to cause both BtH act ivation and a LH rise 30 hr from dawn. Finally, valuable evidence of t he response specificity was provided by using a unique property of the quail photoperiodic clock in which exposure to 10L:26D, but not 10L:1 4D, causes photoinduction. The 36 hr paradigm increased both plasma LH and BtH activation.