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Biomedical subjects

J E Robinson

Publications and source records attributed to J E Robinson.

At least 91 records · Page 5Linked to original sources
PubMed1988

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.

Animals↗
PubMed1987

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.

Animals↗
PubMed1987

Photoperiodic history and a changing melatonin pattern can determine the neuroendocrine response of the ewe to daylength.

The reproductive neuroendocrine response of Suffolk ewes to the direction of daylength change was determined in animals which were ovariectomized and treated with constant release capsules of oestradiol. Two groups of animals were initially exposed to 16 or 10 h light/day for 74 days. On day zero of the study, when one group of ewes was reproductively stimulated (elevated LH concentrations) and the other reproductively inhibited (undetectable LH concentrations), half the animals from each group were transferred to an intermediate daylength of 13 h light/day. The remaining ewes were maintained on their respective solstice photoperiods to control for photorefractoriness. LH concentrations rose in animals experiencing a 3 h decrease in daylength from 16L:8D to 13L:11D while LH concentrations fell to undetectable values in those that experienced a 3 h increase in daylength from 10L:14D to 13L:11D. The photoperiodic response of the Suffolk ewe, therefore, depends on her daylength history. Such a result could be explained if the 24-h secretory pattern of melatonin secretion, known to transduce photoperiodic information to the reproductive axis, was influenced by the direction of change of daylength. Hourly samples for melatonin were collected for 24 h 17 days before and three times after transfer to 13L:11D. The melatonin secretory profile always conformed to daylength. Therefore, the mechanism by which the same photoperiod can produce opposite neuroendocrine responses must lie downstream from the pineal gland in the processing of the melatonin signal.

Animals↗
PubMed1986

Immunocytochemical localization of luteinizing hormone-releasing hormone (LHRH) pathways in the sheep brain during anestrus and the mid-luteal phase of the estrous cycle.

The luteinizing hormone-releasing hormone (LHRH) system of the sheep brain was examined by light microscopic immunocytochemistry with thick, unembedded sections. We compared the distribution and morphology of LHRH cells and their fibers in intact and ovariectomized anestrous ewes, and in breeding season ewes during the mid-luteal phase of their estrous cycle. In all animals, a majority of LHRH neurons were found in the medial preoptic area adjacent to the organum vasculosum of the lamina terminalis. These cells formed a continuum rostrally with immunoreactive neurons in the diagonal band of Broca and medial septum and caudally with cells in the ventrolateral anterior hypothalamus and lateral hypothalamus. Relatively few cells (1-2%) were seen in the arcuate nucleus or its vicinity. Preoptic LHRH neurons project to the tubero-infundibular sulcus of the median eminence by at least two routes: a major ventrolateral projection above the optic tract in the anterior and lateral hypothalamus, and a less prominent periventricular pathway along the third ventricle. LHRH fibers were also observed in a number of extrahypothalamic regions, including the medial amygdala and the accessory olfactory bulb. Immunoreactive LHRH neurons in the sheep exhibited a complex light microscopic morphology unlike that seen in LHRH cells of any other species to date. LHRH cells with extensive, branching processes were frequently found in clusters with close somatic appositions between neighboring cells. Multiple thin protuberances emanated from the soma of many immunoreactive neurons. Immunoreactive fibers with beaded varicosities often were intimately associated with both cell bodies and their dendritic processes. Morphometric analyses revealed that preoptic LHRH neurons in three of four mid-luteal phase ewes had a shorter total dendritic length than those neurons in either intact or ovariectomized anestrous ewes, but this difference between breeding season and anestrous ewes was not statistically significant. Evidence for possible seasonal and/or steroid-induced alterations in the morphology of LHRH neurons must be documented by further studies, including immunocytochemical observations at an ultrastructural level.

Animals↗
PubMed1986

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.

Animals↗
PubMed1986

Does melatonin alter pituitary responsiveness to gonadotropin-releasing hormone in the ewe?

The diurnal secretion of melatonin from the pineal gland transduces information about day length to the reproductive axis of many seasonal breeders including the ewe. In the sheep the target for melatonin is thought to be neural, such that the hormone acts through the GnRH pulse generator to produce seasonal alterations in the frequency of pulsatile LH secretion. These effects on the pulse generation mechanism take approximately 50 days to become evident. It is possible that melatonin also exerts direct effects at the level of the pituitary gland to alter responsiveness to GnRH. Such effects have been noted in other species. The site of action of melatonin to regulate pulsatile LH secretion was assessed in the ewe by determining whether the animal's endogenous melatonin acutely modifies pituitary responsiveness to sustained pulsatile administration of GnRH. Using an animal model in which endogenous GnRH was blocked, pituitary responsiveness to hourly pulses of exogenous GnRH was assessed under conditions of both high (dark period) and low (light period) melatonin. No evidence for acute effects of melatonin on pituitary response to GnRH was found. In another experiment, the amplitude and frequency of endogenously generated LH pulses in ovariectomized ewes was found not to change during the 24-hour light/dark cycle. These data lead to the conclusion that melatonin does not act at the pituitary gland to produce acute effects on LH secretion. Rather, our findings are consistent with the hypothesis that the action of melatonin, in this short-day breeder is long term, and is directed towards the neural elements of the hypothalamic pulse-generating mechanism.

Animals↗
PubMed1985

Far-field 2.45 GHz irradiation system for cellular monolayers in vitro.

A 2.45-GHz microwave exposure facility was developed for long-term TEM irradiation of cellular monolayers. Culture flasks with cells attached to the inside bottom surface were filled with medium, submerged in a 60 X 60 X 12-cm water bath on the field central axis, and exposed in the far-field 2 m below the ceiling-mounted antenna. A quarter-wave transformer plate increased the power transmitted into the water bath, and treatment temperatures were maintained by closed circulation with an external temperature control reservoir. Power density mapped below the quarter-wave plate indicated uniform TEM fields in the 25 X 25-cm region where flasks were located. With 1 kW of forward power to the antenna, the SAR [W/kg] = 45 exp(-0.607d) where d [cm] is the depth in water at any point within this area.

Animals↗
PubMed1985

Importance of duration of nocturnal melatonin secretion in determining the reproductive response to inductive photoperiod in the ewe.

The pineal gland, through its nocturnal melatonin secretion, mediates the effects of inhibitory (long) and stimulatory (short) photoperiod on reproduction in female sheep. Earlier studies revealed that duration of the nighttime melatonin rise is important in determining the inhibitory effect of day length on reproduction in the ewe. The present study tested whether the duration is also important in mediating the inductive response to short days. Pinealectomized ewes, housed under long days, received a short-day melatonin infusion (16-h duration) for 90 days. Reproductive status was monitored from the response to estradiol negative feedback of luteinizing hormone (LH) secretion. This short-day melatonin pattern led to unambiguous reproductive induction, despite the exposure to inhibitory long days. The increase in serum LH was comparable, in terms of latency and magnitude, to that in pinealectomized controls receiving the same short-day melatonin pattern under short days, and in pineal-intact controls transferred from long to short days. Since the reproductive status conformed to the length of time that melatonin was elevated each day rather than to photoperiod, these results support the conclusion that duration of the nighttime melatonin rise mediates the reproductive response of the ewe to an inductive photoperiod. In all, the melatonin rhythm is considered an integral component of the physiologic mechanism measuring day length; through duration of its nocturnal secretion, melatonin encodes both inhibitory and stimulatory photoperiods.

Animals↗
PubMed1985

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.

Animals↗
PubMed1985

Seasonal changes in pulsatile luteinizing hormone (LH) secretion in the ewe: relationship of frequency of LH pulses to day length and response to estradiol negative feedback.

Seasonal changes in pulsatile luteinizing hormone (LH) secretion in ovariectomized ewes were examined over the course of 2 yr in relation to annual changes in environmental photoperiod, shifts in response to estradiol negative feedback control of LH secretion, and timing of the breeding season. Under natural environmental conditions, the frequency of LH pulses in individual ovariectomized ewes changed gradually and in close association with the annual cycle of day length. As days became shorter in late summer and autumn, LH pulse frequency increased; conversely, as day length increased in late winter and spring, frequency declined. Under artificial conditions in which ovariectomized ewes were exposed to different photoperiods, a similar inverse relationship was observed between day length and LH pulse frequency. The seasonal changes in frequency of LH pulses in ovariectomized ewes, although symmetric with the annual photoperiodic cycle, were not temporally coupled to the dramatic shifts in response to estradiol feedback inhibition of LH secretion at the transitions between breeding season and anestrus. The feedback shifts occurred abruptly and at times when LH pulse frequency in ovariectomized ewes was at, or near, the annual maximum or minimum. The tight coupling between LH pulse frequency and photoperiod leads to the conclusion that there is a photoperiodic drive to the LH pulse-generating system of the ewe. The temporal dissociation between changes in this photoperiodic drive and the seasonal shifts in response to estradiol negative feedback support the hypothesis that the neuroendocrine basis for these two phenomena is not one and the same.

Animals↗
PubMed1985

Pineal melatonin mediates photoperiodic control of pulsatile luteinizing hormone secretion in the ewe.

Seasonal breeding in the ewe is regulated by photoperiod through a pineal-dependent mechanism. Changes in the ability of estradiol to inhibit tonic LH secretion are critical. During anestrus, this ovarian steroid gains the ability to slow the frequency of pulsatile LH secretion through an action on the brain. Exposure of ovariectomized, estradiol-implanted ewes to short photoperiods during summer anestrus revealed that daylength can control LH pulse frequency. After removal of estradiol, LH pulse frequency still differed between long- and short-day ewes, suggesting photoperiodic modulation of LH and presumably GnRH secretion independent of gonadal steroids. Significantly, the effects of daylength expressed both in the presence and the absence of estradiol failed to occur in pinealectomized ewes. Long-term infusions of melatonin, given in physiological patterns to pinealectomized ewes, mimicked the effects of photoperiod on pineal-intact ewes. Specifically, a pattern of melatonin characteristic of that in short days (16-hour night-time rise) led to an increase in LH pulse frequency to a breeding season rate. Conversely, melatonin infusions typifying a long-day pattern (8-hour night-time rise) produced an anestrous pulse pattern. Pituitary sensitivity to GnRH was not reduced in sheep which were reproductively suppressed by photoperiod or melatonin treatments. These observations support the conclusion that day-length acts through pineal melatonin secretion to regulate a neural LH pulse generator which, by changing the frequency of GnRH pulses, determines the ewe's seasonal reproductive state.

Animals↗
PubMed1984

Production of autoantibodies to cellular antigens by human B cells transformed by Epstein-Barr virus.

Lymphocytes obtained from the blood of three patients who lacked clinical and serologic evidence of autoimmunity were inoculated with Epstein-Barr virus (EBV) and plated in microwell cultures at low cell densities. Large numbers of the resulting transformed cell lines produced IgM autoantibodies which reacted by ELISA with antigens in cultured human fibroblasts. Precursor frequencies of autoantibody-secreting transformed cells ranged from 27 to 630 per 10(6) cells. Autoantibodies from 20 ELISA-positive cell lines were studied by indirect immunofluorescence and found to react with a variety of highly conserved cellular antigens. Eleven cell lines were tested for the production of multiple isotypes; six of these cell lines produced both IgM and IgG autoantibodies. Findings point to the existence in normal human beings of a sizable population of B cells committed to synthesize a broad spectrum of autoantibodies and demonstrate that EBV transformation provides a potent tool for exploring the normal human B-cell repertoire.

Antibody Formation↗
PubMed1984

Identification of Epstein-Barr nuclear antigen polypeptide in mouse and monkey cells after gene transfer with a cloned 2.9-kilobase-pair subfragment of the genome.

A new antigenic polypeptide was identified in mouse L cells and in monkey COS-1 cells in which Epstein-Barr nuclear antigen (EBNA) was expressed as the result of gene transfer with cloned fragments of Epstein-Barr virus (EBV) DNA. The same-size protein (Mr, approximately equal to 78,000) was seen in stably transformed mouse cells harboring only the BamHI K fragment [approximately equal to 5.2 kilobase pairs (kbp)] or its BamHI/HindIII subfragment, I1f (approximately equal to 2.9 kbp). Thus, the latter DNA fragment is sufficient to code for the protein. In transfected COS cells, a deletion mutant of the I1f fragment (approximately equal to 2.3 kbp) gave rise to a truncated protein (Mr, approximately equal to 52,000), whereas the BamHI K fragment yielded a full-sized Mr 78,000 species. This finding indicates that EBNA is encoded by viral genes. In Burkitt lymphoma lines or in immortalized lymphocytes, variation in the size of the I1f fragment correlated with the apparent molecular weight of the EBNA polypeptide. EBNA is truncated in two Burkitt lymphoma lines, Raji (Mr, 67,000) and P3JHR-1 (Mr, 70,000), which have deletion mutant I1f genes. EBNA in human lymphoid cells bearing a complete I1f fragment as part of the entire EBV genome is the same size (Mr, 78,000) as EBNA found after gene transfer of I1f alone into mouse or monkey cells. Therefore, these expression systems make an authentic EBNA after transfer of the appropriate EBV genes.

Animals↗
PubMed1984

Refractoriness to inductive day lengths terminates the breeding season of the Suffolk ewe.

The ambient photoperiod was recorded over an 8-yr period at the beginning and end of the breeding season in a flock of Suffolk ewes maintained outdoors. The transition into the breeding season in this short-day breeder occurred at a day length which was much longer (14.0 h, Sept. 3 +/- 5 days) than that at the onset of anestrus (11.5 h, Feb. 15 +/- 3 days). This produced a marked asymmetry of the annual cycles of photoperiod and reproduction. In an attempt to explain this asymmetry, we determined if ewes enter anestrus because increasing day lengths curtail breeding or because ewes lose the ability to respond to prevailing short day lengths (i.e., become photorefractory). On the winter solstice, 3 groups of 6 ovariectomized ewes bearing s.c. Silastic implants of estradiol were placed on different day length treatments: 1) natural environment; 2) artificial day length which simulated natural photoperiod; or 3) artificial day length equivalent to that of the winter solstice (10L). Transition into anestrus was determined from the precipitous drop in serum luteininzing hormone (LH) concentrations. The breeding season was not prolonged in ewes held on the winter solstice day length; LH did not remain elevated beyond the time that it plummeted in the other groups. These findings lead to the conclusion that Suffolk ewes normally cease breeding, not because they are actively inhibited by increasing day lengths, but because they become photorefractory. This can account, at least in part, for the asymmetry between the annual cycle of photoperiod and reproduction in this short-day breeder.

Anestrus↗
PubMed1983

A photoperiodic response in gonadectomized Japanese quail exposed to a single long day.

Gonadectomized male Japanese quail maintained on short daylengths (8 h light:16 h darkness) showed a reliable and repeatable response, in terms of increased gonadotrophin secretion, to a single long photoperiod (20 h light) followed by continuous darkness. Plasma levels of LH were significantly increased within 4 h of the end of the long day. Secretion continued to increase with time and LH and FSH concentrations reached a maximum some 24 h later, thereafter decreasing only slowly over the next 8-10 days. Quail could be used repeatedly at fortnightly intervals and gave a consistent response for at least 15 months. The size of the photoperiodic response and its considerable duration were much greater than found in intact quail. The birds showed a similar (though reduced) response if a short daylength was followed by a single 4-h light pulse interrupting the 16-h dark period. Using this system the time of maximum responsivity to the inductive effects of light lies 10-16 h from 'dawn'. The marked and rapid effects described provide an excellent experimental system in which to study how photoperiodic changes are recognized and how consequent neural activation is translated into enhanced secretion rates of hormones in the peripheral endocrine system.

Animals↗
PubMed1983

The pineal gland and the photoperiodic control of luteinizing hormone secretion in intact and castrated Japanese quail.

The effects of pinealectomy on a range of photoperiodic responses were investigated in male Japanese quail by measuring plasma LH concentrations in intact, sham-operated and pinealectomized birds in the following four experiments: (1) transfer of sexually quiescent birds from a short photoperiod of 8 h light: 16 h darkness (8L:16D) to a photostimulatory daylength of 16L:8D; (2) transfer of sexually mature birds from 16L:8D to 8L:16D; (3) castration in 16L:8D and exposure to 13L:11D; (4) castration in 8L:16D and exposure to 13L:11D. There was no evidence of effects of the pineal gland on the photoperiodically induced changes in LH secretion, the quantitative relationship between LH secretion and photoperiod in intact and castrated birds, or the induction of relative photorefractoriness by prolonged exposure to 16L:8D. This suggests that there is no pineal influence on the photoperiodic clock or its effectors in this bird.

Animals↗