Exposure to a short photoperiod increases sensitivity to dopamine inhibition of prolactin release from Siberian hamster pituitary

Prolactin (PRL) secretion from the anterior pituitary is tonically controlled by the activity of the hypothalamic tuberoinfundibular dopaminergic (TIDA) system. Changes in TIDA activity have been associated with the decrease in basal PRL levels shown by hamsters exposed to a short-day photoperiod. Potential concurrent changes in the sensitivity of pituitary lactotropes to the inhibitory actions of dopamine on PRL release were investigated in tissue harvested from animals that had been preexposed to either a stimulatory, long-day 16-hour-light:8-hour-dark (16L:8D) photoperiod, or to a nonstimulatory, short-day(10L:14D) photoperiod for 8-10 weeks. Tissue was maintained in a perifusion tissue culture system and received a 1-hour pulse of varying concentrations of dopamine (DA) shortly after the equilibration period. Fractions of the media were collected every 1/2 h and PRL concentrations in each sample determined via radioimmunoassay. At the highest concentration of DA (10(-6) M), a highly significant inhibition of PRL levels was seen for both 16L and 10L pituitaries. At both mid range concentrations of DA (10(-7) and 10(-8) M), both tissue types showed a significant suppression of PRL, but the reduction was greater for tissue that had been obtained from IOL animals. At the lowest concentration of DA (10(-9) M), a significant suppression of PRL occurred for tissue harvested from 10L animals, but not for tissue taken from 16L animals. These results indicate that the responsiveness of anterior pituitary tissue to the inhibitory effects of DA are altered by photoperiod exposure, and this change is likely a consequence of changes in the characteristics of the D2 receptor population on pituitary lactotropes. Short-day-induced increases in sensitivity to the inhibitory effects of DA may be due to increases in receptor number, or to increased affinity of the D2 receptor for its ligand.