Effects of Neutralization of Circulating Melatonin and N-acetylserotonin on Plasma Prolactin Levels

The effect of pinealectomy or immunization against melatonin (Mel) and N-acetylserotonin (NAS) on plasma prolactin (Prl) levels was studied in rats following pineal stimulation induced by blinding or exposure to short photoperiods (1 h light and 23 h darkness daily). Blinding alone or together with pinealectomy or immunization did not alter resting Prl levels or the response to novelty stimulation. Exposure to short photoperiods flattened the diurnal Rrl rhythm seen in control rats kept in 12 h of light and 12 h of darkness daily. Pinealectomy slightly lowered Prl levels but did not affect the diurnal rhythm. Immunization caused a significant reduction in Prl levels, although the diurnal Prl rhythm persisted. These data suggest that Mel and/or NAS may be involved in the maintenance of basal Prl levels.