Short days induce premature reproductive maturation in juvenile starlings, Sturnus vulgaris

Castration of juvenile and photorefractory adult starlings caused no immediate increase in circulating concentrations of LH. In castrated juveniles and adults exposed to natural changes in daylength, plasma LH increased between mid-October and mid-November, although the increase was more rapid in adults. In castrated photorefractory adults, plasma LH increased 3-5 weeks after transfer to artificial short days (8L:16D). In castrated juvenile starlings plasma LH increased 4-6 weeks after transfer to 8L:16D, irrespective of the age of the birds. Birds as young as 17 weeks had high LH concentrations. These results suggest that the reproductive system of juvenile starlings is in the same state as that of photorefractory adults, and therefore the activation of the hypothalamo-pituitary axis for the first time in juveniles is analagous to the termination of photorefractoriness in adults.     

Effect of neonatal castration on the content of hypothalamic LHRH, pituitary LH and plasma LH in developing male rats.

The content of hypothalamic LHRH and concentration of LH in pituitary and plasma were measured on day 5, 7, 10, 14, 17, 22, 25, 30, 45, 52 and 60 in male rats which were bilaterally castrated on day 2. The levels of plasma LH were significantly higher in all the groups of castrated rats than in normal male rats of corresponding ages. The concentration of plasma LH did not rise progressively but showed day to day fluctuation apparently due to alteration of sexual differentiation of the hypothalamus. The concentration of pituitary LH was significantly lower in neonatally castrated rats compared to normal male rats except on days 17, 25 and 30. The content of hypothalamic LHRH declined initially following castration, but from day 17 onwards significantly higher levels of hypothalamic LHRH were maintained in neonatally castrated rats than in intact control. Initial decline in the content of hypothalamic LHRH may be because of stimulation of release of LHRH which exceeds maximal rate of synthesis and subsequent increase in the content of hypothalamic LHRH may be due to enhanced LHRH synthesis as a result of castration.     

Photoperiodic control of the termination of reproduction in Japanese quail (Coturnix coturnix japonica)

Typically, birds come into breeding in the spring as a response to long days, and end reproduction some weeks later by becoming refractory to those long days. The refractory state is subsequently dissipated by the short days of autumn and winter, so producing once again a bird that can respond to long days. Bird species differ in the extent to which refractoriness is developed; the present experiments took advantage of the relative, rather than the absolute, refractoriness in quail to measure quantitatively the dissipation process. Quail were made refractory by exposure to long days, then transferred to short days and at various times thereafter photostimulated with longer daylengths, the degree of photoresponsiveness being assessed by measuring changes in luteinizing hormone (LH) secretion or cloacal gland size or both. The most clearcut results came from using 13L:11D as the test stimulus to measure photoresponsiveness, and this indicated, in both intact and castrated quail, no response to 13L:11D after one week of short days, a minor response after two weeks, a strong response after three weeks and a full response after five weeks. Thus refractoriness appears to be dissipated gradually under short days, and not in an all-or-none fashion. Confirmation of this conclusion came from experiments in which refractory quail were moved to short days and after one or two weeks transferred to a range of long daylengths. After one week of short days no responses were obtained to 13L:11D or 14L:10D and moderate responses only to 16L:8D, but after two weeks of short days the magnitudes of all the responses were increased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of photoperiod and melatonin on testis development and regression in wild European rabbits (Oryctolagus cuniculus)

Testis size in male wild rabbits (Oryctolagus cuniculus) kept in an enclosure in Cambridgeshire, U.K. was maximal during May and June and minimal between October and December. Regression occurred after the summer solstice and recrudescence occurred after the winter solstice. Rabbits kept in long days for 25 wk showed no sign of spontaneous testis regression. Hence, testis regression during the summer is probably not due to development of refractoriness to long days. Testis regression occurred in rabbits transferred from long (16L:8D) to short (8L:16D) days. Within 8 wk of the transfer spontaneous regrowth of the testes occurred and the rabbits moulted, and after 16 wk the testes had recovered to their size before the transfer. Subcutaneous implants of melatonin given to rabbits in long days mimicked the effect of a transfer to short days by causing testis regression followed by recrudescence. Moult occurred in rabbits immediately after short day- or melatonin-induced testis regression. The study demonstrates that seasonal reproduction in male wild rabbits in Britain is largely controlled by changes in photoperiod and that this is probably mediated via the pineal gland.     

Effects of short-term treatment with testosterone on the secretion of FSH and LH in castrated golden hamsters exposed to short days

Castrated hamsters which were transferred from long (14L:10D) to short (9L:15D) days and received testosterone-filled capsules for 1 week after transfer failed to show a significant suppression in the plasma levels of FSH and LH after capsule removal. In contrast, gonadotrophin concentrations were suppressed in hamsters in which the long-day castration response had been blocked with exogenous testosterone. After castration on long days and exposure to 10 weeks of short days pituitary gland weight and gonadotrophin content, as well as plasma FSH titres, were higher in control animals than in those that had received testosterone implants for 7 weeks of short days. The results suggest that failure of castrated hamsters to respond to the suppressive effects of short days reflects castration-induced changes in hypothalamo-pituitary physiology rather than a neuroendocrine mechanism by which photoperiod modulates gonadotrophin secretion.     

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.     

Effects of short photoperiod on the ability of golden hamster pituitaries to secrete prolactin and gonadotropins in vitro

Transfer of male golden (Syrian) hamsters from a 14L:10D (light:dark) to a 5L:19D photoperiod induced significant changes in pituitary function tested in vitro. Within 27 days after transfer to a 5L:19D photoperiod, basal prolactin (Prl) release was significantly depressed and response to dopamine (DA) was significantly enhanced as compared to Prl release by pituitaries from 14L: 10D hamsters. Follicle-stimulating hormone (FSH) release tended to be depressed after 9 or 27 days of 5L:19D exposure, but the effect was not significant. After 77 days of 5L:19D exposure, Prl release was further suppressed, while FSH release surpassed that seen in 14L:10D pituitaries. In vitro FSH response to luteinizing hormone releasing hormone (LHRH) was also enhanced at this time. After 15 weeks of exposure to a short photoperiod, FSH secretion was still elevated above control levels, but Prl release and Prl response to DA were no longer different from that of 14L: 10D controls. Secretion of luteinizing hormone (LH) in vitro, either basal or LHRH stimulated, was not affected by photoperiod at any time tested. From these results, we conclude that short photoperiod exposure does not reduce the pituitary's ability to secrete LH or FSH, although secretion of Prl is severely attenuated.     

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)     

Abolition of seasonal testis changes in the Ile-de-France ram by short light cycles: relationship to luteinizing hormone and testosterone release

The purpose of this study was to examine the relationships between testis weight and the luteinizing hormone (LH) and testosterone releases in rams subjected to 14 successive 2-mo artificial light cycles in which daylength increased from 8 to 16 h in one month and decreased from 16 to 8 h the following month. Testis weights were measured fortnightly. Serial bleedings were performed during 6 to 8 h the last three 2-mo light cycles, when daylengths were 8, 12 (increasing), 16 and 12 h (decreasing) and plasma LH and testosterone were measured by radioimmunoassay. The overall mean testis weight, continuously high and equal to 314 g per testis, was not correlated with daylength. Mean LH levels and LH pulse height varied significantly according to daylength (P < 0.05 and P < 0.01, respectively) and were maximal on short days (8 h). Both mean and maximal testosterone levels presented variations according to daylength (P = 0.05). LH and testosterone pulsatility were quite high (equivalent to 7 to 10.3 pulses/day); however, mean plasma testosterone levels remained low. It is proposed that in rams under 2-mo light cycles, frequent but short stimulation of LH release by decreasing daylength phases allows maximal testis weight, while the interruption of LH stimulation by increasing daylengths prevents overstimulation of testosterone that can inhibit the LH release by negative feedback. Rams a under 2-mo light regimen presented a persistent high testis weight, indicating that seasonality had been broken down in the Ile-de-France male which is normally a marked seasonal breeder.     

Neuroendocrine changes in male hamsters following photostimulation

Transfer of gonadally regressed male golden hamsters from a short (5 L:19 D) to a stimulatory (14 L:10 D) photoperiod elicits, within 24 hr, significant changes in hypothalamic dopamine, serotonin, and possibly norepinephrine metabolism. Hypothalamic LHRH content was significantly elevated in short-photoperiod animals, but within 24 hr of transfer to a 14:10 photoperiod, LHRH declined to levels not different from those in hamsters maintained continuously in a long photoperiod. Plasma FSH levels were also significantly elevated within 24 hr of transfer, but increases in plasma LH were somewhat slower. Chronic treatment with the tyrosine hydroxylase inhibitor, alpha-methyl tyrosine (alpha MPT), which inhibits catecholamine synthesis, blocked the effect of a stimulatory photoperiod on plasma FSH levels, while treatment of 5:19 hamsters with the catecholamine precursor, L-dopa, mimicked the effects of photostimulation on plasma FSH levels. Testicular weights were not affected by alpha MPT or L-dopa treatment for 1 week. From these data, it appears that endocrine events associated with photoperiod-induced testicular recrudescence are under the control of hypothalamic neurotransmitters.     

Influences of castration and testosterone on spring to summer changes in release of luteinizing-hormone-releasing hormone in rabbits.

Push-pull cannulae were implanted toward the tuberal region of the hypothalamus in ten intact New Zealand male rabbits. In the first experiment, rabbits were perfused at different times after castration: 5-10 days (n = 10), 22-31 days (n = 9) and 50-64 days (n = 8). The release, mean amplitude and mean frequency of luteinizing-hormone-releasing hormone (LHRH) signals from 37 perfusions in ten animals were analysed in intact rabbits and at different times after castration. No significant changes in release of LHRH and in amplitude were observed, but the frequency was significantly higher 22-31 days after castration than in intact rabbits (intact: 0.86 +/- 0.12; castrated: 1.20 +/- 0.13 pulses h-1, P < 0.035; n = 9). In Expt 2, testosterone and placebo Silastic capsules were implanted in the castrated rabbits. Perfusions were performed in the following four periods, defined by season and time after testosterone and placebo implants: (i) spring; before implants, (ii) late spring; 0-2 weeks after implants, (iii) summer solstice; 2-4 weeks after implants and (iv) summer; 4-6 weeks after implants. Castrated rabbits were perfused during spring; castrated rabbits with testosterone capsule implants were perfused during late spring, around summer solstice and in summer and castrated rabbits with placebo implants were perfused during periods (iii) and (iv). Castrated animals with placebo implants showed no significant changes in mean LHRH release and amplitude, although the frequency was significantly higher around the summer solstice period than in castrated rabbits perfused in the spring. In castrated rabbits with testosterone implants LHRH release was significantly higher in late spring than around the summer solstice and in the summer. In addition, the concentrations of LHRH in late spring were significantly higher than those of intact and castrated animals. In contrast, mean LHRH amplitude and frequency did not change. Mean amount of LHRH released and amplitude in castrated rabbits with testosterone implants were significantly lower around the summer solstice than in late spring or summer and compared with intact animals around summer solstice and in castrated rabbits in early spring. These data demonstrate that there were no significant changes in the mean amplitude and release of LHRH after castration from 5 and up to 64 days in rabbits with hypothalamic push-pull cannulae, in contrast to the well established dramatic effect of castration on gonadotrophin concentrations. However, there was a small, but significant, increase in the mean frequency of LHRH pulses 22-31 days after castration compared with values from intact rabbits.(ABSTRACT TRUNCATED AT 400 WORDS)     

Influence of photoperiod and 6-methoxybenzoxazolinone on the reproductive axis of inbred LSH/Ss Lak male hamsters

Adult male Syrian hamsters of the inbred LSH/Ss Lak strain were maintained under a 14L:10D light cycle until 13 weeks of age. At this point, they were implanted s.c. with elastomer capsules that were either empty or packed with 30-40 mg of 6-methoxybenzoxazolinone (6-MBOA), a compound found naturally in some monocotyledonous plants; half of the animals from each treatment group were then kept in long days (14L:10D) or transferred to short days (9L:15D). Testicular size was measured and blood samples collected from each hamster immediately before capsule implantation and again 2, 4, 6 and 8 weeks later. Within just 2 weeks of exposure to short days the mean plasma levels of LH and FSH had significantly declined, in both the control and 6-MBOA-treated animals, and were basal within 4 weeks. Testicular size closely followed these gonadotrophin changes; within 4-6 weeks the testes from all of the short-day hamsters had completely regressed to a prepubertal size. At the end of the experiment, at Week 8, the animals were killed and various components of the hypothalamo-pituitary-testicular axis were compared between the treatment groups. The pituitary content of FSH and LH, testicular weight, mean serum level of testosterone, but not hypothalamic LHRH content or pituitary gland weight, were considerably lower in the short-day than in the long-day hamsters, regardless of whether or not they had been chronically treated with 6-MBOA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reproductive refractoriness in the Welsh Mountain ewe induced by a short photoperiod can be overridden by exposure to a shorter photoperiod

The objectives were to determine if relative lengths of photoperiods that induce reproductive cycles in ewes affect the length of the subsequent breeding season, if duration of the refractoriness that terminates breeding is affected by photoperiod length, and if the resulting refractoriness to an inductive photoperiod is absolute. Groups of Welsh Mountain ewes were exposed to either 12L:12D (n = 12) or 8L:16D (n = 6) photoperiods beginning at the summer solstice when daylengths reach a maximum of 17.5 h at Bristol, England. A control group (n = 10) was exposed to natural daylengths. Ovarian cycles in the controls, as judged by monitored plasma progesterone levels, commenced in early October, about 1 mo later (p less than 0.001 in both cases) than in sheep exposed to 12L:12D or 8L:16D. The advancement in cycle onset was similar under 12L:12D and 8L:16D (69 +/- 2 and 77 +/- 4 days after the summer solstice compared with 102 +/- 2 days in the controls). Duration of the breeding season (100 +/- 4 days) in ewes exposed to 12L:12D was significantly shorter (p less than 0.001 in both cases) than in ewes exposed to natural daylengths or 8L:16D (153 +/- 3 and 133 +/- 5 days, respectively). Approximately 70 days after the ending of ovulatory cycles in the 12L:12D group, half of the animals (n = 6) were transferred to 8L:16D. This treatment greatly (p less than 0.001) reduced the duration of anestrus and cycles began again 62 +/- 4 days after transfer to 8L:16D, or about 90 days earlier than in ewes (n = 6) remaining in 12L:12D.(ABSTRACT TRUNCATED AT 250 WORDS)     

LH and testosterone responses to GnRH in red deer (Cervus elaphus) stags kept in a manipulated photoperiod

Six red deer stags from age 4 months were kept in a light-proof room under an artificial photoperiod consisting of 5.5 cycles of alternate 2-month periods of 16 h light and 8 h dark (16L:8D) and 8L:16D. At 2 or 3 weekly intervals from 10 months of age through 4 cycles, the stags were anaesthetized with xylazine and challenged i.v. with 10 micrograms GnRH. Blood samples were withdrawn immediately before and 10 and 60 min after injection. LH and testosterone concentrations were measured in all samples by RIA. Antler status was recorded daily. Peak LH values on each sampling day occurred in the sample taken 10 min after GnRH stimulation while peak testosterone occurred in the sample taken at 60 min. There were 4 cycles of LH and testosterone secretion accompanied by 4 antler cycles in the stags. The highest LH responses were detected during short days (8L:16D), and the highest testosterone responses were detected around the time of the change from short to long days. The responses of both hormones were lowest at the end of periods of long days or the beginning of short days. The increased pituitary LH response to GnRH was evident 4 weeks after the change to short days which are stimulatory for gonadal development. Antler casting occurred at the end of long days and cleaning at the end of short days. It is considered that antler cycles were due to the ability of the stags to vary release of LH and testosterone in response to changes in the artificial photoperiod.     

Modulation by sex steroids and [D-Trp6, Des-Gly-NH2(10)]luteinizing hormone (LH)-releasing hormone ethylamide of alpha-subunit and LH beta messenger ribonucleic acid levels in the rat anterior pituitary gland

LHRH and sex steroids play a major and direct regulatory role in the secretion of LH by the anterior pituitary gland. The aim of the present study was to investigate the interactions between sex steroids, more especially the potentiating effect of progesterone (P) in the presence or absence of a low dose of 17 beta-estradiol (E2) and/or dihydrotestosterone (D) on mRNA levels encoding the alpha- and beta-subunits of LH in both female and male rats. We also studied the effect of 2-week treatment with the LHRH agonist [D-Trp6, des-Gly-NH2(10)]LHRH ethylamide on the same parameters. After treatment with the LHRH agonist (5 micrograms daily), the accumulation of mRNA encoding the alpha-subunit was stimulated by approximately 3-fold while the LH beta mRNA concentration remained unchanged. Ovariectomy performed 14 days earlier, increased pituitary alpha and LH beta mRNA levels by 3.7- and 8.8-fold, respectively, while orchiectomy performed 14 days earlier increased alpha and LH beta mRNA levels by 6- and 6.5-fold, respectively. The present data demonstrate that although P alone exerts no effect on alpha and LH beta mRNA levels in castrated animals, treatment with P markedly potentiates the inhibitory effect of E2 on both mRNA levels in female as well as male rats. In addition, P potentiates the inhibitory effect of D on LH beta mRNA levels in castrated female rats. Furthermore, the present study illustrates the importance of the cumulative inhibitory effects of relatively low doses of E2 and D on mRNAs encoding both LH subunits. Moreover, the present observation of a differential modulation of alpha-subunit and LH beta mRNA levels after chronic treatment with an LHRH agonist offers an explanation for the high plasma levels of free alpha-subunit found in patients treated with LHRH agonists.     

Feedback regulation of gonadotropic hormone secretion in neonatal pigs

The effect of castration and of administration of charcoal-treated porcine follicular fluid (pFF) containing inhibin-like activity on plasma concentration of gonadotropic hormones was studied in neonatal pigs. Plasma follicle-stimulating hormone (FSH) concentration averaged 25.1 +/- 1.5 ng/ml (mean +/- SEM) in 1-wk-old females and gradually declined to 20.2 +/- 0.7 ng/ml 6 wk later. Ovariectomy did not significantly influence plasma FSH concentration. In males, concentration averaged 8.0 +/- 0.7 ng/ml before castration but rose significantly within 2 days after castration. Injection of luteinizing hormone-releasing hormone (LHRH) did not influence plasma FSH concentrations in intact males, but did in females and in 7-wk-old males castrated at 1 wk. Plasma luteinizing hormone (LH) concentrations in 1-wk-old females (2.2 +/- 0.4 ng/ml) gradually declined and were not influenced by castration. Concentrations of plasma LH in 1-wk-old male piglets (2.8 +/- 0.7 ng/ml) were not significantly influenced by castration within 2 days but were significantly higher 6 wk later. LHRH induced a significant rise in plasma LH concentrations in all animals. Injection of pFF resulted in a decline of plasma FSH concentrations in intact and castrated males and in intact females, but did not influence plasma LH concentrations. These data demonstrate a sex-specific difference in the control of plasma FSH, but not in plasma LH concentration in the neonatal pig. Plasma FSH concentrations, but not plasma LH concentrations, are suppressed by testicular hormones in 1-wk-old piglets. Plasma FSH concentrations can be suppressed in both neonatal male and female pigs by injections of pFF.     

Relative photorefractoriness, prolactin, and reproductive regression in a flexibly breeding sonoran desert passerine, the rufous-winged sparrow, Aimophila carpalis

We tested the hypothesis that adult male rufous-winged sparrows, Aimophila carpalis, exhibit relative photorefractoriness. This condition results in partial loss of sensitivity to photoperiod as a reproductive stimulus after prolonged exposure to long photoperiods and is similar to the mammalian condition called photoperiodic memory. Captive birds were exposed either to 8 h of light/16 h of dark per day (8L) or to 16L for 11 weeks and were then exposed either to 8L, 13L, 14L, or 16L. Testicular diameter, plasma luteinizing hormone (LH), and plasma prolactin (PRL) were measured to assess reproductive system activity in response to photostimulation. In free-living birds, testicular diameter, plasma LH, and PRL were compared in birds caught in September in a year when birds were breeding and in a year when birds were not breeding to further evaluate the role of PRL in the termination of seasonal breeding. Testes completely developed after transfer from 8L to 14L or to 16L and partially developed after transfer from 8L to 13L. However, after 11 weeks of 16L exposure, transfer to 14L caused partial regression and transfer to 13L caused complete regression of the testes. Plasma LH increased in all birds that were transferred from 8L to a longer photoperiod. PRL showed a weak response to longer photoperiod treatment and was elevated in birds after chronic 16L exposure in comparison to birds exposed to chronic 8L. These data indicate that male rufous-winged sparrows lose sensitivity to photoperiod after long photoperiod exposure consistent with the relative photorefractoriness and photoperiodic memory models. Lower PRL in birds that developed testes on 13L and 14L compared to birds that regressed testes on 13L and 14L are consistent with the hypothesis that PRL regulates relative photorefractoriness. However, PRL does not appear to regulate interannual differences in the timing of testicular regression.     

Thyroidectomy results in termination of photorefractoriness in starlings (Sturnus vulgaris) kept in long daylengths

Four groups of 10 male starlings were transferred from short daylengths (8 h light/day) to long daylengths (18 h light/day), which caused the tests to develop rapidly to maximum size and then to decrease to minimal size as birds became photorefractory. Birds were surgically thyroidectomized at 8, 16 or 28 weeks. A fourth group was left intact. Testicular volume and plasma FSH and prolactin concentrations were measured. After 42 weeks all birds were castrated and plasma FSH was measured during the next 6 weeks. Testicular growth began in all thyroidectomized birds between 4 and 8 weeks after thyroidectomy. By 42 weeks, the testes of all thyroidectomized birds were large, whereas those of intact birds were still of minimal size. Plasma FSH concentrations remained low in all birds and plasma prolactin values, originally elevated by long daylengths, decreased at a similar rate in thyroidectomized and intact birds. After castration at 42 weeks, plasma FSH values increased rapidly in all thyroidectomized birds but remained low in non-thyroidectomized birds. The results demonstrate that thyroidectomy of photorefractory starlings does not induce immediate testicular growth but may initiate a process which eventually terminates photorefractoriness in a way similar to that caused by return to short daylengths.     

Effects of a long-acting LHRH agonist preparation on plasma gonadotropin and prolactin levels in castrated male rats and on the release of prolactin from ectopic pituitaries

We have examined the effects of a single subcutaneous injection of an LHRH agonist, D-Trp-6-LHRH, in biodegradable microcapsules of poly(DL-lactide-co-glycolide) on plasma gonadotropin and prolactin (PRL) levels in castrated and in castrated-hypophysectomized-pituitary grafted (CAST-APX-GRAFT) male rats. The results were compared to the effects of daily injections of the same LHRH agonist dissolved in saline. In castrated rats, there were no significant alterations in plasma LH or PRL levels during the 10 days following the injection of LHRH agonist microcapsules, while FSH levels were generally reduced. In castrated males given daily injections of 6 micrograms of LHRH agonist in saline, plasma LH levels were significantly reduced while plasma PRL levels were not changed. In CAST-APX-GRAFT rats, both D-Trp-6-LHRH microcapsules and daily LHRH agonist injections appeared to increase plasma PRL levels. The pattern of changes in PRL release in both groups was similar, with levels on day 6 being significantly higher than those measured on days 1, 3 and 10 after onset of treatment. As expected, LH and FSH levels in these animals were extremely low. Immunoreactive D-Trp-6-LHRH was consistently detectable in the plasma of CAST-APX-GRAFT animals after microcapsule administration, whereas in animals given daily injections of this agonist in saline, its plasma concentrations were often below the detectability limit of the employed assay. These findings suggest that the LHRH agonist, D-Trp-6-LHRH, is capable of causing a short term stimulation of PRL release from ectopic pituitaries. Elevation of plasma LH levels is apparently not required for this effect.