Pineal Serotonin N-Acetyltransferase Activity and Melatonin Concentrations in Prepubertal and Adult Syrian Hamsters Exposed to Short Daily Photoperiods

Abstract

Pineal serotonin N-acetyltransferase activity (NAT) and melatonin concentrations were determined at various intervals in prepubertal (35 days old) and adult male hamsters (74 day old) throughout a 24 hour period with the animals kept in a light:dark cycle of 6:18 (lights on at 0600 h and off at 1200 h). In prepubertal animals, daytime pineal NAT activity of 0.20-0.28 nmoles 14C-N-acetyltryptamine/pineal/hour was maintained for 8 hours after the initiation of darkness. Peak pineal NAT activity of 0.46 +/- 0.64 nmoles 14C-N-acetyltryptamine/pineal/hour occurred 13 hours after the onset of darkness and remained significantly elevated until 0400 h (p less than 0.001). Daytime pineal melatonin concentrations of 78-194 pg/pineal gland also were maintained for 8 hours after the initiation of darkness. At 13 hours into the dark period, pineal melatonin concentrations rose to 788 +/- 150 pg/pineal gland (p less than 0.01 vs all other time points except 0230 h and 0400 h). At one hour before the onset of light both the pineal NAT activity and pineal melatonin concentrations returned to daytime values. Adult male hamsters had diurnal pineal NAT and melatonin rhythms which are indistinguishable from those found in the prepubertal animals.

Norepinephrine or Isoproterenol Stimulation of Pineal N-Acetyltransferase Activity and Melatonin Content in the Syrian Hamster Is Restricted to the Second Half of the Daily Dark Phase

“…Typically, both pineal N A T activity and mela tonin levels [22,25] increase at night, and usually the two constituents are closely correlated [7,12,15]. This has led to the assumption that the N AT rate limits melatonin produc tion [9].…”

Influence of Morning Melatonin Injections on the Antigonadotrophic Effects of Afternoon Melatonin Administration in Male and Female Hamsters

“…This phenomenon may be effected either by a diminu tion in the receptor-ligand affinity or by an actual decrease in the number of receptors [26]. Therefore, since the synthe sis and secretion of endogenous melatonin occurs near the end of the dark period of the LD cycle [12][13][14], the target or gans become insensitive to additional melatonin given early in the light period. Not only would the above hypothesis ex plain the present findings but it would also account for previous observations on the apparent paradoxical effects of melatonin.…”

“…Endoge nously produced melatonin normally reaches a peak late during the dark phase [12][13][14]. When melatonin acts on its receptors [15][16][17] it is possible they become transiently re fractory to additional melatonin [18].…”

Melatonin Given in the Morning Prevents the Suppressive Action on the Reproductive System of Melatonin Given in Late Afternoon

“…In an attempt to explain this we theorized that when melatonin is exogenously injected or endogenously secreted it acts on a receptor and thereafter renders the receptor transiently insensitive to additional melatonin, i.e., it transiently down-regulates the receptor [14] or possibly reduces their number. Thus, since endogenously produced melatonin is normally secreted near the end of the dark phase of the LD cycle [12,13,19], the animals are naturally insensitive to additional melatonin given early in the light phase. However, by late in the day the receptors again become sensitive to the indoleamine.…”

“…However, we have con sidered the following possibility. The endogenous peak of melatonin occurs at 04.00 h during the dark phase of the LD cycle [12,13,19]. It is possible that when melatonin acts on its receptors it renders them insensitive (it downregulates them) to additional melatonin for a period of time [14].…”