Sensitivity of Goats to a Light Pulse During the Night as Assessed by Suppression of Melatonin Concentrations in the Plasma

Abstract

This study investigates the ability of a 1 h light pulse of different intensities at night to suppress plasma melatonin in goats. Six female Saanen dairy goats, about 2 yr old, were housed in a light-tight shed. The goats were habituated for 1 wk to an 8L:16D photoperiod (40.70 +/- 4.16 microW/cm2; 137 +/- 14 lux), lights on 0800 h. A 1 h light pulse, of different intensity on each occasion, was given from 1900 to 2000 h. Light intensity was measured by using a lux meter (mean of 36 measurements at goat's eye level). Five different light intensities were given during December in the order 4.22 +/- 0.62 microW/cm2 (14.2 +/- 2.1 lux), 0.68 +/- 0.09 microW/cm2 (2.3 +/- 0.3 lux), 0.26 +/- 0.004 microW/cm2 (0.87 +/- 0.14 lux), darkness, 40.70 +/- 4.16 microW/cm2 (137 +/- 14 lux), with 1-3 d between treatments. The goats were bled hourly from 1500 to 1900 h and every 15 min from 1900 to 2100 h, and a last bleed occurred at 2200 h. Dark-phase samples were taken in dim red light (less than 0.03 microW/cm2; 0.1 lux). Plasma was assayed for melatonin by radioimmunoassay. Suppression of melatonin concentrations increased as light intensity increased as follows: Darkness, 0%; 0.26 +/- 0.004 microW/cm2; 0%; 0.68 +/- 0.09 microW/cm2; 43.1%; 4.22 +/- 0.62 microW/cm2, 71.1%; 40.70 +/- 4.16 microW/cm2, 81.2%. Suppression was significant (P less than 0.05) at light intensities greater than 0.68 microW/cm2, 2.3 lux. A hyperbolic relationship existed between percent suppression and light intensities.

Plasma melatonin and insulin‐like growth factor‐1 responses to dim light at night in dairy heifers

“…It is possible that cattle sensitivity to light changes throughout the night as endocrine responses of dairy cows to light provided at 13–15 hr after subjective dawn (6 hr/day) was different than that found when light was provided 10–12 or 16–18 hr after subjective dawn [22]. In ewes [4], a 1‐hr light pulse after 3 hr of darkness suppressed NML for 1 hr, but with a similar exposure at 10 hr of darkness, the NML level remained suppressed for the remainder of the night [23]. Similarly, in the hamster [24], NML returned to the normal night level when a 5‐s 10,900 lx pulse was applied at 8 hr, but not at 12 hr of darkness.…”

“…This is the first study to show that the light intensities at night of 5 and 10 lx have no effect on NML in cattle over a 14‐day treatment period. In sheep [3], a light intensity of 1.0 lx partially suppressed NML and in the goat [4], a light intensity of 2.3 lx suppressed NML, whereas intensities of 0.8 lx or less were ineffective. It appears that cattle are at least 5‐ to 10‐fold less sensitive to light than are sheep and goats, possibly because cattle are not seasonal breeders.…”

“…Melatonin of the mammalian pineal gland is synthesized and released into the circulatory system during the night when the inhibitory effect of light is absent [1]. The lowest intensity of light (threshold intensity) to cause a significant suppression in night pineal or plasma melatonin level (NML) varies among species: 1.0 lx in the Syrian hamster [2] and sheep [3]; 2.3 lx in the goat [4], and between 500 and 1500 lx in humans [5]. A light intensity of 50 lx suppressed NML by 70% in dairy heifers and higher light intensities had greater suppressive effects [6].…”

Inhibition of nighttime melatonin secretion in cattle: threshold light intensity for dairy heifers

“…The threshold light intensity for inhibition of melatonin secretion varies considerably among species and in small ruminants the threshold appears to be quite low. Deveson et al (1990) found that 2.3 ± 0.3 lx inhibited melatonin secretion in the goat, and Arendt and Ravault (1988) found that 0.15 lx was adequate to influence plasma melatonin concentration in sheep. Stanisiewski et al (1988) found that 525 lx abolished the melatonin surge that occurred at the onset of darkness in cattle, however, no lower intensities were examined.…”

Timing of melatonin treatment: differential effects on oestrus and coat growth in goats

“…21) jxW/cm 2 from 16 measurements at goat eye level) sufficient to suppress night time melatonin concentrations proportionately by more than 0-80 (Deveson et al, 1990). = 30) were born and raised at 52°14'N (Reading).…”