Nightly duration of pineal melatonin secretion determines the reproductive response to inhibitory day length in the ewe

The pineal controls the reproductive response of ewes to both stimulatory (short) and inhibitory (long) day lengths. Melatonin, a pineal hormone whose nocturnal secretion is entrained by photoperiod, mediates the effect of stimulatory photoperiod. We now report that melatonin also mediates the effect of inhibitory day length, monitored as response to estradiol negative feedback on luteinizing hormone (LH) secretion. Ovariectomized, estradiol-implanted ewes were pinealectomized and intravenously infused with melatonin to restore the nightly melatonin rise. Following transfer from short to long days, and a concurrent switch from short- to long-day melatonin patterns, LH dropped precipitously in pinealectomized ewes, matching the photoinhibitory response of pineal intact controls. LH dropped similarly in pinealectomized ewes when long-day melatonin was infused under short days. Pinealectomized ewes transferred from long to short days displayed a marked LH rise, provided melatonin was also switched to the short-day pattern. LH remained suppressed if long-day melatonin was infused following transfer to short days. These data indicate the nighttime melatonin rise mediates reproductive responses to inhibitory, as well as stimulatory photoperiods; they further suggest the duration of this rise controls suppression of LH under long days. Rather than being strictly pro- or antigonadal, the pineal participates in measuring day length.     

Reproductive refractoriness of the ewe to inductive photoperiod is not caused by inappropriate secretion of melatonin

The 24-h pattern of melatonin secretion was evaluated in Suffolk ewes during prolonged exposure to an inductive photoperiod to assess whether altered secretion of melatonin could account for the eventual loss of response to stimulatory photoperiod (photorefractoriness). Secretory patterns of melatonin were determined approximately every two weeks in samples obtained hourly for 24-48 h. Sampling was begun one week before the switch from inhibitory (long) to inductive (short) photoperiod and continued for 150 days, by which time all but one of the ewes were unresponsive to that stimulatory day length. Melatonin was measured in two different radioimmunoassay systems. Reproductive state was monitored by luteinizing hormone secretion in ovariectomized ewes bearing constant-release estradiol implants. No evidence for disruption of the melatonin pattern was observed on any occasion. The duration and the phase of the melatonin elevation relative to the light/dark cycle did not vary with time of exposure to short days. These findings indicate that refractoriness of the Suffolk ewe to an inductive photoperiod is not caused by an inappropriate secretory pattern of melatonin.     

How does melatonin code for day length in the ewe: duration of nocturnal melatonin release or coincidence of melatonin with a light-entrained sensitive period?

The main objective of the study was to test the hypothesis that the phase of melatonin release with respect to the light-dark cycle mediates the effects of photoperiod on the reproductive response of the ewe. To test the phase hypothesis, we eliminated endogenous melatonin secretion by pinealectomy and then restored physiological levels of serum melatonin with rises of the same duration but at different phases of the light-dark cycle (either at night or in the middle of the day). Serum melatonin patterns were determined by radioimmunoassay in samples taken hourly for 24 h. The reproductive state was monitored by measuring serum luteinizing hormone (LH) in ovariectomized ewes treated with constant-release estradiol implants. Infusion of a long-day pattern of melatonin was equally effective in maintaining reproductive suppression when given during the night or the middle of the day. LH remained low for approximately 150 days and then rose as ewes became refractory to the inhibitory melatonin signal. These results do not support the phase hypothesis. Rather, they are consistent with the hypothesis that the duration of the nocturnal secretion of melatonin codes for day length.     

Melatonin and photorefractoriness: loss of response to the melatonin signal leads to seasonal reproductive transitions in the ewe

Experiments were conducted to examine whether the refractoriness of the Suffolk ewe to the reproductive effects of day length is associated with a deficit in the generation of the circadian rhythm of melatonin secretion or in the postpineal processing of this photoperiodic message. Using serum luteinizing hormone (LH) concentrations in ovariectomized ewes bearing constant-release estradiol implants as a marker of reproductive induction, ewes with intact pineal glands were found to become unresponsive to fixed artificial photoperiods that initially had been either inductive (short days) or inhibitory (long days). The loss of the photoperiodic response was not associated with notable changes in the 24-h secretory pattern of melatonin, which remained characteristically low throughout the day and rose at night. In pinealectomized ewes, nightly infusion of a stimulatory pattern of melatonin (simulating that seen on short days) initially provoked reproductive induction; this response then lessened over much the same time course that pineal intact ewes became refractory to short days. These results support the hypothesis that photorefractoriness reflects a deficit in the postpineal processing of the photoperiodic message. Further, in view of recent evidence that photorefractoriness normally leads to both onset and cessation of the breeding season in Suffolk ewes maintained outdoors, these findings suggest that the loss of response to the melatonin signal contributes to at least one of these reproductive transitions, the cessation of the breeding season, under natural environmental conditions.     

Importance of changing photoperiod and melatonin secretory pattern in determining the length of the breeding season in the Suffolk ewe

Three groups of ovariectomized Suffolk ewes bearing s.c. Silastic implants of oestradiol were subjected to a 90-day priming treatment of an inhibitory long photoperiod (16 h light/day; 16L:8D). On Day 0 of the experiment, they were moved to stimulatory photoperiods. One control group was transferred to 12L:12D and a second control group was transferred to 8L:16D; both groups remained in those photoperiods to determine the timing of reproductive induction and refractoriness. The experimental group was transferred to 12L:12D on Day 0 and then to 8L:16D on Day 55 to determine whether the further reduction in daylength could delay the development of refractoriness. Reproductive neuroendocrine condition was monitored by serum concentrations of LH and FSH. Both gonadotrophins remained elevated for a longer period of time in the experimental group receiving the second reduction in daylength than in either control group, indicating that the second photoperiodic drop delayed the onset of photorefractoriness. Measurement of 24-h patterns of circulating melatonin suggests that the prolonged stimulation of reproductive neuroendocrine activity in the experimental group resulted from a lengthening of the nocturnal melatonin rise. These findings indicate that refractoriness to an inductive photoperiod can be temporarily overcome by exposure to a shorter daylength, and that the change in duration of the nocturnal increase in melatonin secretion is important in photoperiodic signalling. Thus, in natural conditions, the decreasing autumnal daylength, and the resulting expansion of the nocturnal elevation in melatonin secretion, may be utilized to produce a breeding season of normal duration.     

Amplitude modulation of the nightly melatonin rise in the neonatal lamb and the subsequent timing of puberty

Spring-born female lambs require a decrease in day length for the normal timing of puberty the following autumn. If this decrease occurs early in postnatal life (i.e. 0-10 weeks), puberty is delayed. This study tested the hypothesis that failure of the neonatal lamb to respond to the critical long-day to short-day signal is due to inadequate nocturnal melatonin secretion. The approach was to artificially increase, to adult levels, the low nighttime rises of melatonin during the early postnatal period. Eight female lambs served as controls; they were raised on short days until 17 wk of age, and then exposed to 5 wk of long days, after which they were returned to short days. This alternating sequence of photoperiods during mid-development would be expected to induce normal puberty. Sixteen experimental females were exposed to the critical block of long days much earlier; they were placed in long days between 2 and 7 wk of age and in short days thereafter. Half (n = 8) received no further treatment. The other half (n = 8) were infused nightly with melatonin during the 8-h dark phase of the 5-wk, long-day photoperiod. This increased the amplitude of the natural nighttime melatonin rises 3- to 4-fold, well into the adult range.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of short days in timing the onset and duration of reproductive activity in ewes under artificial photoperiods

The objective of this work was to evaluate the role of short photoperiod in timing the onset and duration of reproductive activity in ewes. The perception of photoperiod was disrupted by pinealectomy following transfer from long (17L:7D) to short (8.5L:15.5D) photoperiod and the subsequent reproductive response was monitored. Ovariectomized ewes given Silastic implants containing estradiol-17 beta were exposed to long days until Day 0 (May 24) and then were allocated to the following groups (n = 5-6/group): Group 1) short-day control--moved to short days; Groups 2 to 5) pinealectomy after 0, 30, 60, or 90 short days, respectively; Group 6) long-day hold--kept on long days; Group 7) long days after 60 short days--moved to short days on Day 0 and returned to long days on Day 60. Six ewes kept outdoors served as additional controls. Reproductive neuroendocrine activity was assessed from plasma LH concentrations, high values being indicative of the breeding season and low values indicative of anestrus. Time of reproductive neuroendocrine activity onset (LH rise) did not differ among animals in the 7 groups kept indoors, but was advanced (p less than 0.05) relative to that of ewes outdoors. In contrast, duration of the LH elevation differed among ewes in groups kept indoors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endogenous circannual cycles of ovarian activity and changes in prolactin and melatonin secretion in wild and domestic female sheep maintained under a long-day photoperiod

The present study examines the ovulatory activity of wild and domesticated ewes subjected to either a constant photoperiod of long days (16L:8D) or natural changes in daily photoperiod for 16 mo. The aim was to determine whether an endogenous reproductive rhythm controls seasonal reproductive activity in these sheep, and how the photoperiod might affect this. The effects of long-day photoperiods on long-term changes in prolactin and melatonin secretion were also evaluated. The two species showed changes in reproductive activity under the constant photoperiod conditions, suggesting the existence of an endogenous rhythm of reproduction. This rhythm was differently expressed in the two types of ewe (P < 0.05), with the domestic animals exhibiting much greater sensitivity to the effects of long days. A circannual rhythm of plasma prolactin concentration was also seen in both species and under both photoperiod conditions, although in both species the amplitude was always lower in the long-day animals (P < 0.01). The duration of the nocturnal melatonin plasma concentrations reflected the duration of darkness in both species and treatments. The peak melatonin concentration did not differ between seasons either under natural or long-day photoperiods.     

Kisspeptin and the seasonal control of reproduction in hamsters

Reproduction is a complex and energy demanding function. When internal and external conditions might impair reproductive success (negative energy balance, stress, harsh season) reproductive activity has to be repressed. Recent evidence suggests that these inhibitory mechanisms operate on Kiss1-expressing neurons, which were recently shown to be implicated in the regulation of GnRH release. Hamsters are seasonal rodents which are sexually active in long photoperiod and quiescent in short photoperiod. The photoperiodic information is transmitted to the reproductive system by melatonin, a pineal hormone whose secretion is adjusted to night length. The photoperiodic variation in circulating melatonin has been shown to synchronize reproductive activity with seasons, but the mechanisms involved in this effect of melatonin were so far unknown. Recently we have observed that Kiss1 mRNA level in the arcuate nucleus of the Syrian hamster is lower in short photoperiod, when animals are sexually quiescent. Notably, intracerebroventricular infusion of Kiss1 gene product, kisspeptin, in hamsters kept in short photoperiod is able to override the inhibitory photoperiod and to reactivate sexual activity. The inhibition of Kiss1 expression in short photoperiod is driven by melatonin because pinealectomy prevents decrease in Kiss1 mRNA level in short photoperiod and melatonin injection in long photoperiod down regulates Kiss1 expression. Whether melatonin acts directly on arcuate Kiss1 expressing neurons or mediates its action via interneurons is the subject of the current investigations.     

Refractoriness to inhibitory day lengths initiates the breeding season of the Suffolk ewe

Recent evidence indicates that the breeding season of the Suffolk ewe ends because of loss of response to a day length that was previously inductive. This condition of photorefractoriness could potentially also initiate reproduction, as is the case in several long-day breeding rodents. In this study we determined if ewes enter their breeding season because they become refractory to the long ambient photoperiods of late summer. On the summer solstice, 3 groups of ovariectomized ewes (n = 6) bearing s.c. Silastic implants of estradiol (OVX + E) were placed in different day length treatments: 1) natural photoperiod; 2) artificial photoperiod, stimulating natural day lengths; or 3) artificial photoperiod equivalent to that of the summer solstice (16.25L). Entry into the breeding season is associated with a striking (greater than 30-fold) increase in circulating levels of luteinizing hormone (LH). Timing of the onset of the breeding season was not delayed in ewes maintained on the summer solstice photoperiod; LH levels rose simultaneously in all groups. We conclude that ewes normally begin breeding not because they are actively driven to do so by decreasing or short days, but because they become refractory to prevailing long days. Because the pattern of circulating melatonin, which is known to transduce the photoperiodic message, remained entirely appropriate to day length, we believe that the mechanism responsible for photorefractoriness resides in the postpineal processing of the melatonin signal.     

What photoperiodic signal is provided by a continuous-release melatonin implant?

The purpose of this study was to evaluate whether the insertion of a continuous-release melatonin implant into ewes provides a short-day photoperiodic signal or acts as a functional pinealectomy (provides no specific photoperiodic signal but renders ewes incapable of responding to changes in photoperiod). Ewes primed with 60 long days (18L:6D) during the spring were moved to intermediate day length (13L:11D) for 66 days and then given one of five treatments: 1) short-day control, second drop in photoperiod to 8L:16D; 2) intermediate-photoperiod control, kept on 13L:11D; 3) pinealectomy and kept on 13L:11D; 4) melatonin implant and kept on 13L:11D; 5) melatonin implant and moved to 8L:16D. Mean number of estrous cycles per group and total duration of reproductive activity were determined. Ewes in all groups began to exhibit estrous cycles after the initial reduction in photoperiod. The number of estrous cycles and duration of reproductive activity differed among groups. The number of estrous cycles and duration of reproductive activity was extended in ewes receiving the second drop in photoperiod compared to that of the intermediate-photoperiod controls. Pinealectomized ewes had a number of estrous cycles and duration of reproductive activity similar to those of ewes maintained on the intermediate photoperiod. Melatonin implants increased the number of estrous cycles and prolonged reproductive activity in ewes maintained on the intermediate photoperiod; melatonin implants did not prevent the extension of reproductive activity in ewes receiving the second photoperiodic drop to the short daylength.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photoperiodic regulation of pulsatile luteinizing hormone secretion and adenohypophyseal gene expression in female golden hamsters.

LH concentrations were measured in serum collected at 10-min intervals from chronically ovariectomized female Syrian hamsters that had been maintained for 9 wk in stimulatory (long) or inhibitory (short) photoperiods. Short days reduced the number of detectable LH pulses during both the morning and the afternoon. Most short-day hamsters experienced a gradual afternoon rise in serum LH concentrations; this rise was not composed of multiple pulses. In separate groups of similarly treated hamsters, pituitary LH-beta mRNA abundance was significantly reduced by short-day exposure at both times of day even though serum LH concentrations rose in the afternoon. Estradiol treatment induced an afternoon surge of serum LH in both photoperiods, and eliminated the effect of photoperiod on LH-beta mRNA abundance in the afternoon. Serum prolactin (PRL) concentrations were not consistently influenced by day length in castrated hamsters with or without estrogen treatment, but PRL mRNA abundance was significantly suppressed by short-day exposure in all groups. The results indicate that day length exerts profound steroid-independent effects upon hypophyseal gene expression, and that the regulation of LH-beta mRNA abundance may be due to photoperiodic control of the neural GnRH pulse generator.     

Pineal melatonin rhythms in female Turkish hamsters: effects of photoperiod and hibernation

Daily rhythms of pineal and serum melatonin content were characterized for adult female Turkish hamsters (Mesocricetus brandti) exposed to long days (16L:8D, 22 degrees C) or after transfer to short days (10L:14D, 22 degrees C). The nocturnal peak of pineal melatonin content was found to be approximately 3 b greater in duration on short than on long days. Changes in levels of serum melatonin closely paralleled those of pineal melatonin. Thus, an effect of photoperiod on synthesis and secretion of pineal melatonin was demonstrated. In a separate experiment, female hamsters were induced to hibernate by exposure to a short-day, cold environment (10L:14D, 6 degrees C). During the 4 to 5-mo hibernation season, Turkish hamsters are known to display 4 to 8-day hours of torpor (body temperature = 7-9 degrees C) alternating with 1 to 3-day intervals of euthermia (body temperature = 35-37 degrees C). Little evidence of nocturnal synthesis or secretion of pineal melatonin was detected in females sampled during torpor. However, animals sampled during the first day after arousal from a torpor bout displayed melatonin rhythms no different in phase or amplitude from those seen in females held at 22 degrees C. Thus, despite the absence of pineal melatonin output during torpor, the pineal gland of hibernating Turkish hamsters produces an appropriately phased, rhythmic melatonin signal during intervals of euthermia.     

Ontogeny of the daily profile of plasma melatonin in European starlings raised under long or short photoperiods

Photoperiodic manipulation of young European starlings suggests that their reproductive physiology is incapable of responding to a short photoperiod until they are fully grown. This study aimed to determine whether the lack of response to a short photoperiod is reflected in the daily profile of plasma melatonin concentrations. Five-day-old starlings taken from nest boxes showed a significant (p < 0.0001) rhythm in plasma melatonin concentrations, with high values during night. In nestlings hand-reared from 5 days of age on a long photoperiod (LD 16:8), equivalent to natural photoperiod at the time, the amplitude of the daily rhythm in melatonin increased significantly (p < 0.01) with age until birds were fully grown (20 days old). In nestlings reared on a short photoperiod (LD 8:16), the daily melatonin profile remained almost identical to that of long photoperiod birds until they were fully grown. However, after 20 days old, the duration of elevated nighttime melatonin began to extend to encompass the entire period of darkness. In contrast, fully grown starlings transferred from a long to a short photoperiod had partially adapted to the short photoperiod after 5 days; by 10 days, the daily melatonin profile was identical to that of birds held chronically on a short photoperiod. Thus, consistent with responses of reproductive physiology, the pineal of young birds appears to be incapable of perceiving, or adapting to, a short photoperiod.     

Photoperiodic regulation of glutamatergic stimulation of secretion of luteinizing hormone in male Syrian hamsters.

It has been suggested that changes in endogenous glutamatergic stimulation of secretion of luteinizing hormone (LH) induced by photoperiod play a role in regulating seasonal cycles of reproductive activity. The aim of this study was to test the hypothesis that the glutamatergic control of the secretion of LH in the male Syrian hamster is sensitive to photoperiod, by determining whether the glutamate agonist N-methyl-D-aspartate (NMDA) could stimulate LH secretion in this species and, if so, to determine whether the response varied among animals exposed to different daylengths. In the first experiment, adult male hamsters were housed in either short day (8 h light: 16 h dark) for 6 weeks to induce testicular regression, or long days (16 h light: 8 h dark) to maintain testicular function, and the effects of systemic administration of NMDA on serum LH concentrations were determined. In the short-day hamsters, all s.c. doses of NMDA (25-75 mg kg-1 body weight) produced a robust rise in serum LH concentrations within 15 min. In the long-day hamsters, basal LH concentrations were higher than in short-day hamsters, but only the highest dose of NMDA produced a significant increase in LH concentrations, and the magnitude of this increment was less than those observed in short days. In hamsters in long days, the low doses of NMDA that did not significantly alter LH concentrations nevertheless significantly suppressed serum prolactin concentrations, demonstrating the efficacy of the drug. In hamsters in short days, serum prolactin concentrations were at the limit of detection of the assay, so no inhibitory effect of NMDA on prolactin secretion could be determined on this photoperiod. In the second experiment, the effects of a fixed dose of NMDA (50 mg kg-1 body weight) was tested at intervals in hamsters exposed to short days for a prolonged period such that their testes initially regressed, but then became scotorefractory and testicular recrudescence occurred. After 6 and 12 weeks in short days, NMDA stimulated LH secretion. However, after 24 weeks in short days when testicular recrudescence was complete, the response to NMDA was lost. A third experiment determined whether the reduced response to NMDA in hamsters on long days relative to those in short days might result from higher concentrations of circulating testosterone. Hamsters in long days were castrated to remove the influence of gonadal feedback, and the response to NMDA tested 3 weeks later when endogenous LH concentrations had risen to levels characteristic of the chronically castrated condition.(ABSTRACT TRUNCATED AT 400 WORDS)     

Does seasonal reproductive state affect the neuroendocrine response of the ewe to a long-day pattern of melatonin?

This study examined whether or not the reproductive response of female sheep to photoperiod varies with seasonal reproductive state. The specific objective was to test the hypothesis that the reproductive response to a long-day pattern of melatonin varies with the reproductive state of the ewe. The response examined was the synchronization of reproductive neuroendocrine induction (rise in serum luteinizing hormone, or LH) following nocturnal infusion of melatonin into pinealectomized ewes for 35 consecutive nights. This infusion restored a pattern of circulating melatonin similar to that in pineal-intact ewes maintained in a long photoperiod (LD 16:8). The ewes had been pinealectomized and without melatonin replacement for 16-25 months prior to the study. They were in differing reproductive states at the start of the infusion, as their endogenous reproductive rhythm had become desynchronized among individuals and with respect to time of year. Noninfused pinealectomized ewes served as controls. Regardless of the reproductive state at the start of the 35-day infusion of the long-day pattern of melatonin, all treated ewes exhibited the same reproductive neuroendocrine response after the infusion was ended. This consisted of a synchronized rise in LH some 6-8 weeks after the infusion was terminated, the maintenance of a high level of serum LH for some 15 weeks, and a subsequent precipitous fall in LH to a very low level. These results provide evidence that a long-day pattern of melatonin can synchronize reproductive neuroendocrine induction in the ewe, regardless of reproductive condition, and thus do not support the hypothesis that this response differs with seasonal reproductive state.     

Plasma and pineal melatonin levels in female ferrets housed under long or short photoperiods

Ovohysterectomized female ferrets were housed in controlled environment rooms in which the daily lighting schedule was either 15L:9D (long days) or 9L:15D (short days). After 2 weeks some ferrets in each group were given an intrajugular catheter: beginning 1 week later, a blood sample was taken daily at one of eight different clock times over an 8 to 10 day period. One additional blood sample plus the pineal gland were collected from these animals and from uncathetarized animals in each group after decapitation at different clock times. Both plasma melatonin concentrations and pineal melatonin content were elevated in a square-wave pattern during the dark hours, with the duration of elevation being longer in ferrets kept under the short days. These results suggest that differences in the duration of nocturnal increments in melatonin secretion may mediate the stimulatory and inhibitory effects of long and short days, respectively, on ovarian activity in female ferrets.     

Pineal-independent regulation of photo-nonresponsiveness in the Siberian hamster (Phodopus sungorus)

The pineal hormone melatonin influences circadian rhythms and also mediates reproductive responses to photoperiod. The authors tested whether pinealectomy influences circadian oscillators responsible for induction of nonresponsiveness to short day lengths by preventing normal short-day patterns of circadian entrainment. Adult male Siberian hamsters were pinealectomized or sham operated, maintained in either 18 h light per day (18L) or 15L for 10 weeks, and then tested for responsiveness to 10L. Because pinealectomized hamsters do not show gonadal regression in short day lengths, responsiveness was assessed by measuring phase angle of entrainment and the length of the nightly activity period following transfer to 10L. The incidence of nonresponsiveness was significantly higher in 18L hamsters than in 15L hamsters but was unaffected by pineal status. Fully 88% of 18L hamsters failed to entrain to 10L in the normal short-day manner; the duration of nightly activity remained compressed, and the phase angle of entrainment was large and negative relative to lights off. The 15L hamsters entrained normally to 10L. Exposure to constant light after 10L treatment was equally effective in inducing arrhythmicity in pinealectomized and intact hamsters. Changes in the period of morning and evening circadian oscillators subsequent to 18L treatment did not predict circadian responsiveness to short photoperiod. Long-day induction of photo-nonresponsiveness, which prevents winter responses to short day lengths, occurs independently of pineal melatonin feedback on the circadian system.     

Effects of Superior Cervical Ganglionectomy and Melatonin Replacement on Intra-hypothalamic LHRH Content and Pulsatile Luteinizing Hormone Release in the Mink

Long-term effects of subcutaneous melatonin implants on intrahypothalamic LHRH content and on pulsatile luteinizing hormone release have been investigated in ganglionectomized male mink. Animals were submitted to bilateral removal of the superior cervical ganglion in mid-April. A preliminary study revealed that plasma LH concentrations remain at a basal level throughout the year following ganglionectomy. In a second experiment, one month after ganglionectomy and transfer from the natural photoperiod environment to short daylengths (LD 4:20), melatonin pellets were subcutaneously implanted to overcome deafferentation of the pineal. Progressive effects of treatment were studied 7 days, 15 days, and one, two and three months after insertion of the melatonin implants. The intra-hypothalamic LHRH content in ganglionectomized mink was at a basal level similar to that observed during seasonally sexual quiescence, or after exposure to inhibitory long days (LD 20:4). A significant and transient elevation in LHRH content was observed already after fifteen days, and also one month after insertion of melatonin implants. This resulted in mean values similar to those observed during the breeding season, or after exposure to stimulatory short days (LD 4:20). A decrease in hypothalamic LHRH content started after two months. No pattern of pulsatile LH secretion was recorded in ganglionectomized untreated mink. A significant increase in all parameters of pulsatile LH secretion was observed fifteen days after the elevation of LHRH content induced by melatonin treatment, and maximum values were reached after two months. Pituitary activity tended to decrease after three months, characterized in particular by a significant decrease in the mean frequency of LH pulses. In addition, the increase in pulsatile characteristics of LH release occurred two months before the peripheral renewal of testicular activity. Apparently, the reproductive endocrine function in ganglionectomized mink treated with melatonin implants is restored more rapidly at the hypothalamic level than at the pituitary or testicular levels.     

Effects of Superior Cervical Ganglionectomy and Melatonin Replacement on Intra-hypothalamic LHRH Content and Pulsatile Luteinizing Hormone Release in the Mink

Long-term effects of subcutaneous melatonin implants on intrahypothalamic LHRH content and on pulsatile luteinizing hormone release have been investigated in ganglionectomized male mink. Animals were submitted to bilateral removal of the superior cervical ganglion in mid-April. A preliminary study revealed that plasma LH concentrations remain at a basal level throughout the year following ganglionectomy. In a second experiment, one month after ganglionectomy and transfer from the natural photoperiod environment to short daylengths (LD 4:20), melatonin pellets were subcutaneously implanted to overcome deafferentation of the pineal. Progressive effects of treatment were studied 7 days, 15 days, and one, two and three months after insertion of the melatonin implants. The intra-hypothalamic LHRH content in ganglionectomized mink was at a basal level similar to that observed during seasonally sexual quiescence, or after exposure to inhibitory long days (LD 20:4). A significant and transient elevation in LHRH content was observed already after fifteen days, and also one month after insertion of melatonin implants. This resulted in mean values similar to those observed during the breeding season, or after exposure to stimulatory short days (LD 4:20). A decrease in hypothalamic LHRH content started after two months. No pattern of pulsatile LH secretion was recorded in ganglionectomized untreated mink. A significant increase in all parameters of pulsatile LH secretion was observed fifteen days after the elevation of LHRH content induced by melatonin treatment, and maximum values were reached after two months. Pituitary activity tended to decrease after three months, characterized in particular by a significant decrease in the mean frequency of LH pulses. In addition, the increase in pulsatile characteristics of LH release occurred two months before the peripheral renewal of testicular activity. Apparently, the reproductive endocrine function in ganglionectomized mink treated with melatonin implants is restored more rapidly at the hypothalamic level than at the pituitary or testicular levels.