Influence of temperature and photoperiod on plasma melatonin in the mudpuppy, Necturus maculosus

Abstract

A melatonin (MEL) radioimmunoassay employed previously only in mammals was used to estimate, plasma MEL in a salamander, Necturus maculosus. Validation- procedures included thin-layer chromatography of plasma extracts, parallel inhibition curves of authentic MEL and serially diluted plasma and plasma extracts and quantitative recovery of authentic MEL added to pooled Necturus plasma. A diel cycle of plasma MEL was demonstrated in mudpuppies acclimatized for a minimum of 3 weeks under a 12L: 12D photoperiod and 15 ± 1° and sampled in late March. The MEL cycle persisted under a reversed photoperiod (lights on, 1800 hr), but the amplitude of the MEL peak was diminished, and the peak was more than 180° out of phase with the corresponding peak under a normal photoperiod. In animals acclimated to 5° in mid-June under a 12L:12D photoperiod, the diet cycle of plasma MEL continued, but both midphotophase and midphotophase concentrations were reduced compared with 15° controls. The diel cycle was also present in animals acclimated to 25°, but the decrease in MEL concentrations was less marked than that in 5° animals. Photoperiod is apparently the primary cue for cycles in plasma MEL in Necturus, but the cycle can be influenced by temperature.