Biomedical subjects

G A Lincoln

Publications and source records attributed to G A Lincoln.

At least 19 recordsLinked to original sources

PubMed1995

Effects of a one-hour light pulse on the timing of the circadian rhythm in melatonin secretion in rams.

The effects of a 1-hr light pulse on the timing of the circadian rhythm in the blood plasma concentration of melatonin were documented in Soay rams. Groups of 5 to 6 animals were transferred from short days (LD 8:16) to constant dim red light (DD) for 6 days, and were exposed to a 1-hr light pulse at one of 16 different times throughout 24 hr on day 3. Blood samples were collected hourly for 30 hr before (day 2-3) and after the light pulse (day 5-6), and the plasma concentrations of melatonin were measured by radioimmunoassay. The animals were allocated to experimental groups based on the circadian time (CT) when the light pulse was given using two hourly blocks through the circadian day; the onset of enhanced melatonin secretion (melatonin peak) was designated as CT 12. Under DD there was a clearly defined plasma melatonin rhythm in all animals. The mean duration of the melatonin peak was 13.24 +/- 0.16 hr (n = 91) and the mean period between the onset of successive melatonin peaks was 23.55 +/- 0.10 hr (n = 21). The effect of the 1-hr light pulse on the time of onset of the melatonin peak varied significantly with the circadian time when the light pulse was given (ANOVA, P = 0.031).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed1995

Light control of the duration of the daily melatonin signal under long and short days in the Soay ram. Role of inhibition and entrainment.

Light acts in two ways to control the duration of the nocturnal melatonin rhythm. It inhibits the production of melatonin from the pineal gland and it entrains the underlying circadian rhythm generators located in the suprachiasmatic nuclei. To investigate the role of these two mechanisms under long and short days, four experiments were carried out using groups of adult Soay rams (n = 6-8). The animals were housed in individual pens in light-controlled rooms and entrained to long (LD 16:8) or short (LD 8:16) days for at least 8 wk. The treatments were as follows: (i) dark period extended by 4 h under long days (L dark-delay), (ii) dark period advanced by 4 h under long days (L dark-advance), (iii) dark period extended by 4 h under short days (S dark-delay), and (iv) dark period advanced by 4 h under short days (S dark-advance). Each treatment was given on a single day and the animals were subsequently maintained in, or transferred to, constant dim red light (DD) for 24 h. A control group (C) was run in parallel with each treatment group. Blood samples were collected every 30 min for 6-9 h during the dark-shift to monitor the light-induced changes in the secretion of melatonin, and during DD to monitor any phase shift in the endogenous rhythm (phase markers provided by onset or offset of melatonin secretion). L dark-delay resulted in a significantly (p < 0.01, ANOVA) later offset of the melatonin peak (3.4-h delay) with no phase shift of the onset of the rhythm under DD.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Seasonal and photoperiod-induced changes in the secretion of alpha-melanocyte-stimulating hormone in Soay sheep: temporal relationships with changes in beta-endorphin, prolactin, follicle-stimulating hormone, activity of the gonads and growth of wool and horns.

Blood plasma concentrations of alpha-melanocyte-stimulating hormone (alpha-MSH), beta-endorphin (beta-END), prolactin and follicle-stimulating hormone (FSH), and associated changes in the size of the testes, and growth of the horns and pelage were measured in male (n = 8), castrated male (n = 5) and female (n = 9) Soay sheep. The animals were born in April and kept outdoors near Edinburgh (56 degrees N) during the first two years of life. In all groups there was a close association between the weekly changes in the plasma concentrations of alpha-MSH and beta-END; the molar ratio in mean concentrations was close to 1:1. The blood plasma concentrations of both hormones varied markedly with season with a 3- to 10-fold increase in concentrations from the minimum in winter to the maximum in autumn. The seasonal peak occurred in September in the first year of life as juveniles, and between July (males) and September (females) in the second year when the animals were sexually mature. The plasma concentrations of ACTH did not vary in parallel with the seasonal changes in the concentrations of alpha-MSH (measured only in males); the molar ratio for the concentrations of alpha-MSH:ACTH was 1:0.12. The seasonal increase in the concentrations of alpha-MSH occurred 1-3 months after the seasonal increase in the concentrations of prolactin and the associated growth in horns and pelage, and slightly before, or coincident with the seasonal increase in the concentrations of FSH and the growth in the testes. In a second experiment, the same parameters were measured in a group of adult male Soay sheep (n = 8) housed indoors under an artificial lighting regimen of alternating 16-week periods of long (16 h light: 8 h darkness) and short days (8 h light: 16 h darkness). In this situation, there was a clearly defined photoperiod-induced cycle in the plasma concentrations of alpha-MSH with a 25-fold increase from a minimum under long days to a maximum under short days. The concentrations of beta-END varied in close parallel with the changes in alpha-MSH, and the temporal associations with the changes in the other pituitary hormones were similar to those observed in animals housed outdoors. Overall, the results support the view that alpha-MSH is co-secreted with beta-END from the melanotrophs in the pars intermedia of the pituitary gland, and that the secretory activity of the melanotrophs changes markedly with season, increasing in summer and autumn, and decreasing in winter and spring.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1995

Effects of melatonin in the mediobasal hypothalamus on the secretion of gonadotrophins in sheep: role of dopaminergic pathways.

Previous studies have shown that treatment with micro-implants of melatonin in the mediobasal hypothalamus (MBH) of sexually inactive Soay rams exposed to long days induces an increase in the secretion of FSH and reactivation of the testicular axis, as normally occurs in response to short days. The current study was conducted to investigate the possible involvement of hypothalamic dopaminergic (DA) systems in this melatonin-induced effect. At 10 weeks under long days, sexually inactive Soay rams were treated in the MBH with micro-implants containing bromocriptine (DA agonist) or sulpiride (DA antagonist), given alone or in combination with melatonin, to establish whether the DA drugs would mimic or negate the effects of melatonin. All micro-implants were inserted bilaterally and left in place for 14 weeks; the study lasted a total of 28 weeks (14 weeks implant period and 14 weeks post-implant period) while the animals remained under long days. The ability of the micro-implants to release bromocriptine and sulpiride for 14 weeks was confirmed by incubating implants in vitro and testing for the presence of the compounds in the incubate using a pituitary cell bioassay. Profiles of FSH, determined in blood samples collected three times weekly, were significantly different among treatments (time x treatment interaction, P < 0.001, ANOVA). Melatonin in the MBH induced a marked increase in the concentrations of FSH during the implant period, and a decrease during the post-implant period (P < 0.001). Bromocriptine given alone in the MBH induced a decrease in the concentrations of FSH which became statistically different from the control during the post-implant period (P < 0.05). Treatment with sulpiride alone also resulted in a suppressive effect during the post-implant period (P < 0.01). When given in combination with melatonin, bromocriptine or sulpiride significantly reduced the melatonin-induced increase in the concentrations of FSH observed during the implant period (P < 0.001). The results support the view that DA pathways in the MBH play an important role in the inhibitory regulation of gonadotrophin secretion in the ram. The inhibitory effect of bromocriptine is likely to result from the direct activation of the hypothalamic DA receptors linked to GnRH neurones regulating the secretion of FSH. The apparent paradoxical inhibitory effect of sulpiride is thought to be due to enhanced gonadal steroid negative feedback resulting from blockade of the inhibitory DA pathways, as evidenced by significantly increased secretion of testosterone (P < 0.05) in the animals receiving sulpiride in combination with melatonin.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Photoperiodically-induced cycles in the secretion of prolactin in hypothalamo-pituitary disconnected rams: evidence for translation of the melatonin signal in the pituitary gland.

Long term changes in the secretion of prolactin were monitored in groups of hypothalamo-pituitary disconnected rams (HPD rams, n = 8) and control rams (HPD sham-operated and unoperated, n = 8) while exposed to an artificial lighting regimen of alternating 16-weekly periods of long days (16L:8D) and short days (8L:16D) for 72 weeks, and during a treatment with subcutaneous constant-release implants of melatonin under long days. The HPD rams showed all the clinical characteristics of complete pituitary disconnection (diabetes insipidus, gonadal regression and slight obesity), and were unresponsive to a range of provocation tests (exposure to a barking sheep dog, cannulation of the jugular vein, injection of serotonin and NMDA) which caused acute changes in the blood plasma concentrations of prolactin in the controls. Nevertheless, there was a clearly defined cycle in the blood concentrations of prolactin in the HPD rams related to the imposed lighting regimen with values 10-fold higher under long days compared to short days (HPD mean +/- SEM: 90.1 +/- 24.7 vs 9.4 +/- 2.0 micrograms/l, long vs short day respectively, P < 0.001). The temporal pattern was very similar to that observed in the controls, although the concentrations of prolactin were higher in the HPD rams and more variable (control mean +/- SEM: 55.6 +/- 3.6 vs 3.0 +/- 0.5 micrograms/l, long vs short day, P < 0.001). There was a corresponding cycle in the growth and moulting of the wool in the HPD rams consistent with a biological response to the photoperiodically-induced changes in the secretion of prolactin. The diurnal rhythm in the blood concentrations of prolactin was absent in the HPD rams, but there was a normal rhythm in the secretion of melatonin. The treatment of the animals with constant-release implants of melatonin under long days caused a marked decrease in the blood concentrations of prolactin in both the HPD and control rams. The overall conclusion is that the endogenously generated daily melatonin signal which encodes daylength acts directly in the pituitary gland to mediate the effects of photo-period on the secretion of prolactin. The photo-period transduction pathway thus by-passes the hypothalamus.

Aggression↗

PubMed1994

Effects of placing micro-implants of melatonin in the pars tuberalis, pars distalis and the lateral septum of the forebrain on the secretion of FSH and prolactin, and testicular size in rams.

Previous studies involving the placement of microimplants of melatonin in the brain in sheep exposed to long days have provided evidence that melatonin acts within or close to the mediobasal hypothalamus (MBH) to mediate the effects of daylength on cycles in reproduction, moulting and other seasonal characteristics. To extend these observations, groups of Soay rams have now been treated with micro-implants of melatonin placed in the pars tuberalis (PT) and pars distalis (PD) of the pituitary gland, and in the lateral septum of the forebrain (septum). The PT and septum are potential target sites for the action of melatonin based on the localized binding of iodomelatonin assessed by in situ autoradiography. The animals were initially exposed to alternating 16-week periods of long days (16 h light: 8 h darkness; 16L:8D) and short days (8L:16D) to entrain the seasonal cycles. The treatments were started at 10 weeks into a period of long days when the animals had a physiology normally observed in summer (low blood plasma concentrations of FSH and high concentrations of prolactin), and they remained under long days throughout the experiments. In experiment 1, animals received micro-implants of melatonin placed in the PT (n = 6) or PD (n = 4), or received empty implants in similar sites (n = 4) or no surgery (n = 4; total control, n = 8). In experiment 2, groups of animals received microimplants of melatonin placed in the lateral septum (septum, n = 7) or received corresponding control treatments (total control, n = 8). The micro-implants consisted of 22 gauge stainless-steel needles with melatonin fused inside the tip. They were inserted bilaterally in the selected sites and left in place for 14 weeks. The biological effects of the treatments were assessed by measuring the changes in the blood plasma concentrations of FSH and prolactin, growth of the teses and moulting of the pelage over a period of 28 weeks (14 weeks treatment and 14 weeks post-treatment). The administration of melatonin in the PT, but not in the PD or septum, affected the photoperiodically induced cycle in the secretion of FSH and prolactin. In the PT group there was no significant change in the plasma concentrations of FSH during the treatment with melatonin, but there was a significant (P < 0.001, ANOVA) decrease in the levels of FSH after the treatment associated with premature regression of the testes.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Photoperiodic modulation of the dopaminergic control of pulsatile LH secretion in sheep.

This study was conducted to investigate whether the photoperiodic regulation of the seasonal changes in pulsatile LH secretion in the ram involves changes in the activity of inhibitory hypothalamic dopaminergic (DA) pathways. To test this hypothesis, a series of experiments was carried out in Soay rams in which the effects of a DA-D2 receptor antagonist (sulpiride) or a DA-D2 receptor agonist (bromocriptine) on the pulsatile secretion of LH were determined under both long and short days. In each experiment blood samples were collected every 10 min for 8 h starting at the time of vehicle, sulpiride or bromocriptine injections to assess concentrations of LH. Sulpiride (0.59 mg/kg, s.c.) administered to rams under long days induced an immediate and sustained increase in the secretion of LH that lasted for approximately 4 h (P < 0.05; ANOVA); this LH response reflected both a rise in mean concentrations (0.247 +/- 0.03 vs. 0.452 +/- 0.1 microgram/l) and an increase in the frequency of LH pulses (0.5 +/- 0.5 vs. 2.33 +/- 0.42 pulses/8 h; P < 0.01). In contrast, under short days sulpiride had no effect. Bromocriptine (0.06 mg/kg, s.c.) administered to rams under long days, when LH concentrations were low, was without effect, but when given to rams under short days significantly (P < 0.05) suppressed mean LH concentrations (0.627 +/- 0.08 vs. 0.320 +/- 0.02 microgram/l) and LH pulse frequency (4.86 +/- 0.46 vs. 2.43 +/- 0.37 pulses/8 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

The role of prolactin in the reactivation of hair follicles in relation to moulting in cashmere goats.

The effects of the suppression or elevation of plasma prolactin concentrations in spring on the timing of the reactivation of the hair follicles and the timing of the spring moult were investigated in cashmere goats. Thirty eight adult female goats, housed under conditions of natural photoperiod at 55 degrees 55'N from mid-December until May, were allocated to four groups starting on 5 January: ten served as untreated controls, eight received 2 mg ovine prolactin subcutaneously every 12 h for 7 weeks (PRL), twelve received 35 mg bromocriptine intramuscularly every 14 days for 17 weeks (BCR) and eight received injections of both ovine prolactin and bromocriptine at the above dose rates for 7 weeks (PRL+BCR). In the PRL group there was an earlier reactivation of the secondary hair follicles (PRL vs control, proportion of secondary follicles in antigen, weeks 1-5, P < 0.01) associated with an earlier moult of secondary fibres (cashmere) but no significant difference in the activity of the primary hair follicles. In the BCR group there was a delay in the reactivation of both the secondary and primary hair follicles (BCR vs control, proportion of secondary and primary hair follicles in anagen, weeks 5-13, P < 0.01) and a delay in the moult. In the PRL+BCR group there was an early reactivation and moult similar to the PRL group. Voluntary food intake (VFI) and liveweight were also measured. Only in the BCR group was there a decrease in VFI compared with the controls but with no effect on liveweight.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Administration of melatonin into the mediobasal hypothalamus as a continuous or intermittent signal affects the secretion of follicle stimulating hormone and prolactin in the ram.

The biological effects of administering melatonin into the mediobasal hypothalamus (MBH) was documented in adult Soay rams using two delivery systems: (1) microimplants in the MBH delivering melatonin continuously and (2) microdialysis probes in the MBH delivering melatonin intermittently as a daily timed infusion. The experimental protocol was to precondition rams to long days (LD 16:8) for 10 to 12 weeks, and then introduce the exogenous source of melatonin by implantation or infusion. Sixteen rams were divided equally into four treatment groups: (a) microimplants in the MBH, (b) microdialysis probes in the MBH, (c) empty microimplants in the MBH to act as sham-operated controls, and (d) no surgery to act as unoperated controls. The microimplants consisted of 22-gauge stainless steel cannulae with melatonin fused inside the tip and were placed bilaterally in the brain for 14 weeks. These implants had previously been shown to release melatonin at a relatively constant rate when incubated in buffered saline at 37 degrees C (3.42 +/- 0.42 micrograms/24 hr, mean +/- SEM, 1-10 weeks) and to produce a localised concentration of melatonin when implanted in the brain (localised to within 1 mm of the center of the implant). The microdialysis probes were also 22-gauge cannulae with a 3 mm membrane (Biotech). They were placed bilaterally into the MBH, connected to two portable syringe drivers secured to a backpack. Melatonin was infused daily for 10 hr (estimated delivery: 0.5 microgram/hr) starting in the mid-light phase to produce a long-duration intermittent melatonin signal. Technical problems limited the period of infusions to 8-10 weeks with minor interruptions. Animals from all groups were maintained on long days, and the observations extended for a period of 28 weeks. The melatonin implants placed in the MBH induced a premature increase in the blood concentrations of FSH and growth of the testes. This treatment also induced a marked decrease in the plasma concentrations of prolactin and the earlier development of the long winter pelage. These changes were reversed after the end of treatment with a decline in the plasma concentrations of FSH and regression of the testes, and an increase in the concentrations of prolactin and moult of the winter pelage. Daily infusions of melatonin from the microdialysis probes in the MBH produced qualitatively similar, but less marked responses. The overall results illustrate that the administration of melatonin into the MBH, either continuously or intermittently, to extend the duration of the daily melatonin signal, induces multiple short-day responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Reproductive effects of placing micro-implants of melatonin in the mediobasal hypothalamus and preoptic area in rams.

The reproductive effects of placing micro-implants of melatonin in the mediobasal hypothalamus (MBH) and preoptic area (POA) were monitored in Soay rams. Groups of animals were initially conditioned to alternating 16 weekly periods of long days (16 h light: 8 h darkness; 16L:8D) and short days (8L:16D) for at least 9 months to entrain the seasonal reproductive cycle. All experiments were then initiated at 10 weeks under long days when the animals were sexually inactive. In experiment 1, rams were exposed to short days for 14 weeks or maintained on long days to illustrate the photoperiodically induced re-activation and regression of the reproductive axis. In experiments 2-4, rams received micro-implants of melatonin in the MBH or POA, or received control treatments (sham-operated or no surgery) for 12-14 weeks while maintained on long days (total of 12 animals/treatment). The melatonin implants consisted of 22-gauge stainless-steel cannulae with melatonin fused inside the tip and were placed bilaterally in the brain. Incubation of the implants in Tricine-buffered saline (pH 8.0) at 37 degrees C showed that the release rate of melatonin was relatively constant after an initial peak in week 1 (means +/- S.E.M.: 3.42 +/- 0.43 micrograms/24 h). Rams with melatonin implants placed in the MBH, but not in the POA, showed a consistently earlier re-activation of the reproductive axis compared with the control animals in all three experiments (12/12 for MBH vs 2/12 for POA). The mean time to maximum testicular diameter was 12.2 +/- 0.9, 21.6 +/- 1.8 and 22.3 +/- 1.2 weeks for the MBH, POA and combined control groups respectively (MBH vs control, P less than 0.01; analysis of variance). The premature growth of the testes in the MBH group was associated with an earlier increase in the blood plasma concentrations of FSH and testosterone, and the appearance of the sexual skin coloration. Removal of the implants resulted in a decline in all reproductive parameters. The melatonin treatments did not cause a detectable increase in the peripheral concentrations of melatonin, or affect the diurnal rhythm in melatonin which reflected the long-day photoperiod. When implants containing 125I-labelled melatonin were introduced into the brain the associated radioactivity was localized to within 1 mm of the implants. The overall results demonstrate that the constant administration of melatonin into the MBH blocks the effect of the endogenous long-day melatonin signal and induces gonadal redevelopment.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Effects of placing micro-implants of melatonin in the mediobasal hypothalamus and preoptic area on the secretion of prolactin and beta-endorphin in rams.

In a previous study, we showed that the local administration of melatonin in the mediobasal hypothalamus (MBH), but not the preoptic area (POA), caused a premature increase in the secretion of FSH and growth of the testes in sexually inactive Soay rams exposed to long days. To extend these observations, we have now measured blood concentrations of prolactin and beta-endorphin and the associated peripheral responses in the same animals, to establish whether the treatments produced multiple endocrine changes such as those which occur following exposure to short days. Groups of rams were initially exposed to alternating 16 weekly periods of long days (16 h light: 8 h darkness; 16L:8D) and short days (8L:16D) for at least 9 months to entrain the seasonal cycles in the secretion of the pituitary hormones. The treatments were started at 10 weeks under long days, when the animals had a physiology characteristic of the early summer with high blood plasma concentrations of prolactin (associated with growth of the summer pelage), and low concentrations of beta-endorphin (associated with low body weight). The animals were assigned at random to the following treatments: (i) micro-implants of melatonin in the MBH, (ii) microimplants of melatonin in the POA, (iii) empty implants in the MBH or POA to act as operated controls, and (iv) no surgery to act as unoperated controls (n = 12 rams/treatment). The micro-implants consisted of 22-gauge stainless-steel needles with melatonin fused inside the tip. The implants were inserted bilaterally in the brain, and left in place for 12-14 weeks. The observations continued for a total of 28 weeks while the animals remained under long days. The administration of melatonin in the MBH induced a rapid decreased in plasma concentrations of prolactin while in the POA it induced a less marked but significant effect. The mean times to minimum concentrations of prolactin were 7.4 +/- 0.4, 17.3 +/- 2.8 and 26.0 +/- 0.3 weeks for the MBH, POA and combined control groups respectively (MBH vs control, P < 0.001, POA vs control P < 0.01). In the MBH group, the concentrations of prolactin subsequently increased to a maximum 6 weeks after the end of melatonin treatment. The changes in prolactin were accompanied by changes in growth and moulting of the pelage; only animals in the MBH group showed a conspicuous moult associated with the change from low to high prolactin secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1990

Phase shifts in the circadian rhythm in plasma concentrations of melatonin in rams induced by a 1-hour light pulse.

The effect of a 1-hr light pulse, given at night, on the timing of the circadian rhythm in the plasma concentration of melatonin was examined in Soay rams to investigate the mechanisms involved in determining the duration of the nocturnal peak in melatonin secretion. Animals (n = 8) were housed under short days (LD 8:16) or long days (LD 16:8) and received a light pulse at various times of night. They were released into constant dim red light (DD) on day 1. Blood samples were collected hourly for 30 hr from 1000 hr on day 3, and the plasma concentration of melatonin was determined by radioimmunoassay to assess the timing of the melatonin peak. Control animals (n = 8) were maintained under the same conditions but received no light pulse. Under short days, a light pulse given early in the night caused a phase delay in the melatonin peak, and a light pulse given in the late night caused a phase advance. The mean duration of the melatonin peak was slightly reduced following a light pulse in the early or late night, and slightly increased following a pulse given near the middle of the night. Under long days, both light-pulse treatments given at night caused a phase delay in the melatonin peak, but there was no significant change in duration of the melatonin peak. The duration of the melatonin peak at day 3 under DD in the control animals was similar for all treatments, regardless of the previous entraining photoperiod (mean duration: 12.6-14.8 hr) and was similar to that under short days (14.6 hr), but was significantly longer than that under long days (8.2 hr). Information on the phase response curve in the Soay ram and on the period of the circadian oscillator governing the melatonin rhythm (c 23.0 hr under DD) predicts a close phase relationship between the end of the light phase and the onset of the melatonin peak as observed under normal 24-hr LD cycles. The current results also indicate that light acts to entrain the circadian rhythm influencing the onset and offset of melatonin secretion, and thus dictates the duration of the melatonin peak.

Animals↗

PubMed1990

Seasonal cycles in the blood plasma concentration of FSH, inhibin and testosterone, and testicular size in rams of wild, feral and domesticated breeds of sheep.

Seasonal cycles in testicular activity in rams were monitored in groups of wild (mouflon), feral (Soay) and domesticated breeds of sheep (Shetland, Blackface, Herdwick, Norfolk, Wiltshire, Portland and Merino) living outdoors near Edinburgh (56 degrees N). The changes in the blood plasma concentrations of FSH, inhibin and testosterone, and the diameter of the testis were measured every half calendar month from 1 to 3 years of age. There were significant differences between breeds in the magnitude and timing of the seasonal reproductive cycle. In the mouflon rams, the seasonal changes were very pronounced with a 6-15-fold increase in the plasma concentrations of FSH, inhibin and testosterone from summer to autumn, and a late peak in testicular diameter in October. In the Soay rams and most of the domesticated breeds, the seasonal increase in the reproductive hormones occurred 1-2 months earlier with the peak in testicular size in September or October. In the two southern breeds (Portland and Merino), the early onset of testicular activity was more extreme with the seasonal maximum in August. In cross-bred rams, produced by mating Soay ewes (highly seasonal breed) with Portland or Merino rams (less seasonal breeds), there was a seasonal reproductive cycle that was intermediate compared to that of the parents. A comparison between all 11 breeds showed a significant correlation between the timing of the seasonal cycle in plasma FSH concentration and testicular diameter (time of peak FSH vs testis, r = 0.95). The overall results in the rams are consistent with a primary role of FSH in dictating the seasonal cycle in testicular size and the secretion of inhibin. The earlier seasonal onset in the testicular cycle in the southern breeds of domesticated sheep, and the differences from the wild type, are taken to represent the effects of genetic selection for a longer mating season.

Animals↗

PubMed1990

Correlation with changes in horns and pelage, but not reproduction, of seasonal cycles in the secretion of prolactin in rams of wild, feral and domesticated breeds of sheep.

Seasonal cycles were monitored in groups of wild (mouflon), feral (Soay) and domesticated breeds of sheep (Shetland, Blackface, Herdwick, Norfolk, Wiltshire, Portland, Merino, Soay x Portland and Soay x Merino) living outdoors near Edinburgh (56 degrees N). Changes in the blood plasma concentrations of prolactin and FSH, and growth of the horns and pelage were measured every half calendar month from 1 to 3 years of age. In all breeds there was a clearly defined seasonal cycle in the plasma concentration of prolactin with an 18-66-fold increase in mean values from the nadir in November and December to the peak in May and June. The seasonal increase in prolactin was closely correlated with the seasonal increase in the growth of the horns, both within and between breeds (e.g. time of peak prolactin vs horn growth for 11 breeds, R = 0.62, P less than 0.05). In the mouflon, Soay and some of the domesticated breeds of sheep (Wiltshire, Herdwick and Shetland), the seasonal increase in prolactin was also temporally correlated with the resurgence of growth of the pelage in spring and a conspicuous moult. In the other breeds developed to produce fine wool (e.g. Norfolk, Portland and Merino), there was no clear seasonal change in the pelage and growth continued throughout the year. Comparison between breeds indicated that continuous growth of the pelage was associated with higher plasma prolactin concentrations in winter. The times of the seasonal changes in plasma concentrations of prolactin were not significantly correlated with the corresponding changes in the plasma concentrations of FSH. The overall results are consistent with a role for prolactin related to the growth of the horns and pelage rather than the seasonal cycle in reproduction. The differences between the wild-type and the domesticated breeds in the pelage represent the effect of selective breeding to produce a long fine fleece which has involved changes in both the seasonal pattern of prolactin secretion and the growth characteristics of the hair fibres.

Animals↗

PubMed1990

Regulation of the seasonal cycle of beta-endorphin and ACTH secretion into the peripheral blood of rams.

In a group of 12 adult Soay rams living outdoors near Edinburgh there was a conspicuous seasonal cycle in the peripheral plasma concentrations of beta-endorphin, ACTH and cortisol. The concentration of all three hormones increased 5- to 20-fold from winter to summer; the seasonal maximum occurring from May to July for ACTH and cortisol and in August for beta-endorphin. At the peak of the cycle the ratio of beta-endorphin to N-acetyl-beta-endorphin was 22:1. The regulation of the seasonal cycle was investigated in a series of five experiments involving treatments with arginine vasopressin (AVP), corticotrophin-releasing factor (CRF) and the synthetic glucocorticoid, dexamethasone. Injection of AVP i.v. induced a dose-dependent increase in the plasma concentration of beta-endorphin (AVP doses of 0, 0.07, 0.33 and 1.67 micrograms/kg). AVP (0.33 micrograms/kg) and CRF (1.67 micrograms/kg) given alone or in combination (equimolar doses), induced an increase in the plasma concentrations of beta-endorphin and ACTH in spring, summer, autumn and winter, and produced a synergistic response when given together. The responses varied with season and were greatest in summer and autumn at the time of the seasonal increase in endogenous secretion. Dexamethasone injected i.v. at 68.04 micrograms/kg produced a decrease in the plasma concentrations of beta-endorphin and ACTH, and the responses were also greatest in summer and autumn. A similar treatment with dexamethasone blocked the AVP-induced increase in the plasma levels of beta-endorphin, indicating an action of dexamethasone on the pituitary gland. Administration of ACTH (0.33 micrograms/kg; i.v.) to rams pretreated with dexamethasone stimulated an increase in the plasma concentration of cortisol; this response varied with season, being greatest in spring at the time of the peak in the seasonal cycle in cortisol secretion. The administration of beta-endorphin (0.33 micrograms/kg) failed to induce an increase in the plasma levels of cortisol at any season. Analysis of the hormone profiles in the control rams based on blood samples collected every 10 min for 8 h revealed pulsatile variations in the plasma concentration of ACTH; some of the spontaneous ACTH peaks were correlated with beta-endorphin peaks. From these results in the Soay ram, we conclude that beta-endorphin and ACTH are co-secreted from the pituitary gland following stimulation by AVP and CRF, and that adrenal glucocorticoids stimulated by ACTH can act in a negative feedback role at the level of the pituitary gland to inhibit the release of both beta-endorphin and ACTH.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenocorticotropic Hormone↗

PubMed1990

Regulation of the photoperiod-induced cycle in the peripheral blood concentrations of beta-endorphin and prolactin in the ram: role of dopamine and endogenous opioids.

In a group of adult Soay rams housed indoors under an artificial light cycle of alternating 16-week periods of long and short days, there was a conspicuous long-term cycle in the peripheral plasma concentrations of beta-endorphin and prolactin. The levels of beta-endorphin were highest under short days and lowest under long days (15-fold change), and inversely related to the changes in the plasma levels of prolactin (120-fold change). The role of dopamine in the control of beta-endorphin and prolactin was investigated in a series of experiments, conducted under both long and short days, in which rams were treated with dopamine receptor agonists (dopamine and bromocriptine) and antagonists (pimozide and sulpiride). Naloxone (opioid antagonist) was also administered to assess the additional involvement of endogenous opioids. Dopamine injected i.v. (6.6 mg/kg every 10 min) did not significantly affect the mean plasma concentrations of beta-endorphin and prolactin under either long or short days. Pimozide (0.08 mg/kg i.m. every 2 h) caused a large increase in the mean plasma concentrations of beta-endorphin and prolactin under long days but not short days. Naloxone (1.6 mg/kg, i.v.), administered alone or in combination with dopamine or pimozide, had no effect on the mean plasma concentrations of beta-endorphin and prolactin, except under short days when, combined with pimozide, it induced an increase in the plasma concentrations of the two polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Long-term reproductive cycles in rams after pinealectomy or superior cervical ganglionectomy.

Long-term cycles in diameter of the testes, colouration of the sexual skin and plasma concentrations of testosterone, FSH and prolactin were monitored in groups of pinealectomized (PINX), superior cervical ganglionectomized (SCGX), and control Soay rams living near Edinburgh (56 degrees N). In Exp. 1, PINX, SCGX and control rams were kept outside for 4 years, and well defined seasonal cycles in each of the reproductive parameters were evident in all 3 groups (e.g. testosterone cycle length assessed by sine-wave analysis: 12.08 +/- 0.17, 12.39 +/- 0.14 and 12.15 +/- 0.10 months for PINX, SCGX and control rams respectively). Qualitative differences, however, were apparent between the groups in the timing and amplitude of the reproductive cycle. The seasonal peak in reproductive function occurred from July to September in the PINX and SCGX rams, some 2 months earlier in the year than in controls, while the amplitude of the cycle was less marked in the PINX and SCGX rams. There were no significant differences between the experimental groups in the seasonal cycle in the plasma concentrations of prolactin. In Exp. 2, SCGX and control rams were kept indoors under an artificial environment with a 32-week light cycle and constant nutrition for 4 years. Compared to the controls, in which the reproductive changes were synchronized to the driving light cycle, the SCGX rams showed poorly defined reproductive cycles of lower amplitude and longer period (e.g. testosterone cycle length: 57.8 +/- 6.1 and 32.1 +/- 0.2 weeks for SCGX and control rams, respectively). There was evidence of a cycle in some of the reproductive parameters in the SCGX rams with a period close to 32 weeks during the second half of the study (e.g. testosterone cycle 32.4 +/- 0.8 weeks), which was taken to indicate social induction from the neighbouring control rams. In two further short-term experiments, SCGX rams showed a decline in testicular activity in response to receiving a restricted diet (60% of controls) and an increase in testosterone secretion in response to exposure to oestrous ewes. The overall results illustrate that PINX and SCGX rams can generate long-term synchronized cycles in pituitary and testicular activity. The animals are apparently unable to respond to changes in daylength due to the loss of the functional pineal gland but they remain competent to respond to other environmental cues such as changes in nutrition, temperature and social factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Test of ML23 as an antagonist to the effects of melatonin in the ram.

In Exp. 1, four groups of 8 yearling Soay rams were housed under long days (16L:8D) to induce reproductive quiescence and were treated daily for 12 weeks with: (I) vehicle (2 or 4 ml 50% ethanol/water), (II) ML23 (2 mg), (III) melatonin (2 mg) and (IV) melatonin and ML23 (2 mg of each). All treatments were given orally in the mid-light phase. In the rams receiving melatonin (Group III) there was an earlier increase in the plasma concentrations of FSH and testosterone and regrowth of the testes compared to the controls (time to maximum testicular diameter: 10.0 +/- 0.5 and 15.3 +/- 1.2 weeks). These differences were reversed after the end of the 12-week treatments when rapid testicular regression occurred in melatonin-treated rams but not in the controls. In the group receiving ML23 and melatonin (Group IV), there was early reactivation and regression of the reproductive axis as in the melatonin group (testis max. 9.9 +/- 0.7 and 10.0 +/- 0.5 weeks) while in the group receiving ML23 alone (Group II) there was a slower redevelopment and regression as in the controls (testis max. 15.7 +/- 1.1 and 15.3 +/- 1.2 weeks). The comparison between the 4 groups in the changes in the blood concentrations of prolactin, voluntary food intake and total body weight also indicated that the treatment with ML23 failed to modify the effect of melatonin (combined treatment vs melatonin) or the effect of the long day photoperiod (ML23 vs vehicle).(ABSTRACT TRUNCATED AT 250 WORDS)

5-Methoxytryptamine↗