Light entrainment and stimulation of the secretion of prolactin in rams

Light entrainment and stimulation of PRL release were studied in rams submitted to artificial light regimes: In a first experiment four groups of animals were submitted to light regimes with a period of 6, 4, 3 or 2 months respectively (T6 to T2 groups) and amplitude of 8-16 h of daylength. In all four groups PRL level was significantly correlated with daylength (r greater than 0.51; P less than 0.01). However in July, when temperatures were indeed high, an additional PRL increase was observed in the T6, T4 and T3 groups, although the role of temperature appears debatable. In a second experiment, animals were exposed daily to 8 hrs. of light given in two photofractions. The first fraction of 7 hrs. began at the same time as "dawn", and the second of 1 hr ended at the same time as "dusk" of the T6 group in the first experiment which served as control group. Correlation of plasma prolactin in rams receiving 8 h light in one or two photofractions was highly significant (r = 0.66; P less than 0.001). This suggests that rams measured daylength between two limits considered as "dawn" and "dusk" even if lights were turned off during part of this interval.     

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

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

Reproductive potential and endocrinological responses of sheep kept under controlled lighting. II. Pituitary and gonadal responses of ewes and rams to a six-monthly light cycle

Forty entire ewes of mixed breeds were kept in environmentally-controlled rooms with a 6-monthly light cycle. Six mature spayed Border Leicester × Merino ewes and four mature intact Poll Dorset rams were kept under the same conditions.Over a period of 2 years (four light cycles) estimates were made of ovarian, testicular and pituitary activity in response to the artificial light regime. Non-pregnant ewes were bled twice weekly: peripheral plasma progesterone levels > 1 ng/ml were taken as indicative of ovarian activity. Testicular activity was estimated by weekly tests for peripheral plasma testosterone and scrotal sac volume. Pituitary activity was estimated monthly by the response to the injection (i.v.) of 75 μg gonadotrophin releasing hormone (GnRH) to rams and of 18.75, 37.5 or 75 μg to ewes, using peripheral plasma luteinising hormone (LH) estimations from the time of GnRH injection.Data for ovarian and testicular measurements were classed into categories according to week and for pituitary response to month of the light cycle.Cubic regression analyses were conducted on the percentage of ewes with progesterone levels > 1 ng/ml and on ram testosterone and scrotal measurements.Despite the irregularity of the curve for the light cycle, the ovarian and testicular responses of rams and ewes followed an alternating curve with peaks and troughs of activity separated by approximately 13 weeks in the 26-week cycle. The peak of ovarian activity occurred during the long daylength period which followed a 22-week period of decreasing daylength and was preceded by 1 month by peak ram testosterone and scrotal sac volume.The pattern of pituitary response was related to that of the actual light cycle and the data were subjected to time-lag regression. This econometrical technique revealed that there was a delay in pituitary response to daylength changes of 2 months for rams, and between 2 and 3 months for the spayed ewes. The peak pituitary response to the GnRH test occurred one month earlier for rams than for the spayed ewes, and coincided with the corresponding troughs of gonadal activity of each sex.The results showed that the breeding season of sheep can be compressed into 6 months and that the pattern of pituitary response follows the daylength pattern more closely than do measurements of gonadal activity. Peak reproductive activity in rams, as measured by pituitary and gonadal activity, precedes that of ewes by approximately 1 month.     

The measurement of daylength in the Ile-de-France ram

Two groups of 12 adult Ile-de-France rams were exposed to artificial 6-month light cycles in which daily illumination was provided in one or two photofractions. In Group I, daylength increased linearly from 8 to 16 h in 3 months and decreased similarly from 16 to 8 h. The daily increment or decrement (5.33 min/day) was constant. In Group II, 8 h of light were given in two parts: the main one, 7 h, began at the time of dawn in Group I and an additional 1 h light pulse was coincident with the last hour of the former group. The onset of the pulse changed therefore each day and the interval between dawn of the first block and dusk of the second block of light increased from 8 to 16 h in 3 months and declined from 16 to 8 h the next 3 months. Testicular weight was estimated by an orchidometer every 2 weeks for 2 (N = 12/group) or 3 consecutive light cycles (N = 6/group). The testicular weight variations were identical in both groups. In the 6 rams of each group studied during 3 light cycles, variations of testicular weight were submitted to an harmonic regression analysis according to time and the computed values for the mean, amplitude, period and phase were, respectively, 260 g, 66 g, 185 days and 120 days in Group I and 262 g, 65 g, 181 days and 111 days in Group II. Analysis performed for each ram gave very similar values in all individuals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Contribution of increasing and decreasing daylength to the photoperiodic control of LH secretion in the Ile-de-France ram

Sexually mature Ile-de-France rams were exposed to an 8-month light regimen in which the daily light increment and decrement were constant and equal to 7 min/day. Daylength therefore varied from 6 to 20 h. The animals were allotted to two groups of 12 rams each and submitted to the same light regimen but 4 months out of phase. Blood was collected every 40 min for 10 h, on 15 occasions at various intervals during the second light cycle. Plasma LH and on some occasions plasma testosterone concentrations were measured by radioimmunoassay. In both groups the number of LH pulses increased significantly as daylength increased from 11:40 to 20:00 h (P less than 0.01) but because their amplitude was low the mean plasma LH increase, although significant, was moderate. As daylength started to decrease (from 20:00 to 18:30 h), the frequency of LH pulses further increased (P less than 0.05) and the pulse amplitude doubled so that mean plasma LH values increased abruptly and remained high until the decreasing light photoperiod reached 11:40 h. Thereafter, the frequency and amplitude of LH pulses decreased and the mean plasma LH dropped to one-fourth of its maximal value. Mean plasma testosterone concentrations were low during most of the 8-month cycle but increased steeply when daylength decreased from 11:40 to 6:00 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Circannual Growth Rhythm in a Brown Alga,Pterygophora californica

A circannual rhythm was found in the kelp Pterygophora californica which forms a new blade with a free running period of 7 ? 8 months under constant conditions. Individual plants exposed to cycles of daylength with T = 12, 6, or 3 months performed 1, 2, or 4 growth cycles, respectively, in one calendar year showing the entrainment of the endogenous circannual rhythm. The annual growth cycle also followed a phase shift of the annual cycle of daylength (T = 12 months) by 3 or 6 months.     

Endogenous circannual rhythms and photorefractoriness of testis activity, moult and prolactin concentrations in mink (Mustela vison).

Mink are seasonal photosensitive breeders; testis activity is triggered when days have less than 10 h light. Increasing and decreasing plasma concentrations of prolactin induce the spring and autumn moults. In a 5 year experiment, males were maintained under short days (8 h light:16 h dark) at 13 degrees C or long days (16 h light:8 h dark) at 21 degrees C, winter and summer conditions, respectively. Under winter and summer conditions, circannual cycles of prolactin secretion and moulting were observed at intervals of about 11 months. Recurrence of testis cycles was not evident. In a second experiment, males were maintained under an 8 h light:16 h dark cycle from the winter solstice or under 10 h light:14 h dark, 12 h light:12 h dark or 14 h light:10 h dark cycles from 10 February. Under 8 h light:16 h dark cycle, testis regression was slightly later than under natural conditions, indicating photorefractoriness. However, mink remained sensitive to light: the longer the photoperiod, the faster the testis regression. In a third experiment, males were transferred under 8 h light:16 h dark or 16 h light:8 h dark from 15 May (group 1), 12 June (group 2) or 4 July (group 3); males submitted to long days received melatonin capsules on the day of transfer. Increasing concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and testis volume were shown by half the males in group 2 and nearly all the males in group 3; the constant release of melatonin from implants was more efficient than short days; but in the three groups, prolactin concentrations decreased in the few days after short-day or melatonin treatment. Overall, the results demonstrate endogenous circannual rhythms of prolactin secretion, body weight and moulting. Although a refractory period to short days was observed, the annual cycle of testis activity totally relies on the annual changes in daylength.     

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.     

Circadian rhythm of sex pheromone-releasing behaviour in females of the dermestid beetle,Trogoderma glabrum: Regulation by photoperiod

The effects on sex pheromone-releasing, or calling behaviour, of diel photoperiods of varying daylength, of light cycle phase shifts, and of continuous illumination were investigated in Trogoderma glabrum females. On light régimes with 8 to 20 hr daylengths, calling maxima tended to centre close to photophase midpoints. Although influencing the time of day at which calling occurred, daylength had little effect on the amount of activity or the length of the calling period. When 16 : 8 LD light cycles were advanced or delayed by 4 hr, the time of day at which calling peaks were observed shifted within 2 to 4 cycles so that a constant phase relationship with photoperiod was maintained. Daily calling peaks were evident in groups of females exposed to between 1 and 5 days of continuous illumination, but mean calling time occurred earlier in the day as light exposures were lengthened. It was concluded that a circadian rhythm of calling behaviour exists in T. glabrum females. and that the rhythm can be entrained to 24 hr periodicity by photoperiod.     

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

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

Circadian participation in the photoregulation of testis activity and prolactin secretion in the mink, a short-day breeder

It has been demonstrated that an endogenous mechanism is involved in photoperiodic time measurement in the mink, a short-day-breeding mannal. A study of testicular activity (testicular volume, plasma testosterone concentration) and plasma prolactin level was carried out in sexually resting minks (the experiment began in November). Groups of minks were kept in the natural photoperiod or subjected to different resonance light-dark (LD) cycles (LD 4:8, LD 4:20, LD 4:32, LD 4:44); an additional group of animals was reared in an ahemeral photoperiod (LD 4:16). A rapid increase of testicular activity was observed in control animals or those kept in LD 4:20 (T 24) and LD 4:44 (T 48). In the other groups of animals, those kept in LD 4:8 (T 12), LD 4:32 (T 36), and LD 4:16 (T 20), testicular function remained at rest. Prolactin secretion was, in contrast, stimulated in the groups kept in LD 4:8 (T 12). LD 4:32 (T 36), and LD 4:16 (T 20), and remained low in the groups kept in LD 4:20 (T 24) and LD 4:44 (T 48). These results show that the effects of the different photoperiodic regimens do not depend on the duration of the photophase, but rather on the period of the LD cycles. The LD cycles that allow an increase of testicular function are those that are inhibitory to reproduction in birds and long-day-breeding mammals. To explain these results, it is suggested that in the mink exposure to light during the circadian photosensitive phase induces inhibition of testicular activity and stimulation of prolactin secretion. To explain the opposite effects of a single photoperiod on testicular function and secretion of prolactin, the hypothesis has been advanced that, in the mink, long days might simultaneously inhibit hypothalamic luteinizing-hormone-releasing hormone (LH-RH) activity and prolactin-inhibiting factor (PIF) activity.     

Effects of sexual stimulation on testicular size in the ram

Experiments were conducted to determine the influence of sexual stimulation on testicular size and morphology.Scrotal volume decreased by 18–26% in rams mated for 4–6 weeks on two commercial farms, with only part of the decrease being accounted for by the 4–12% decrease in live weight. Similar observations were made over 7–8 weeks on rams exposed to increasing intensity of sexual stimulation by having rams either isolated, non-mated with or without non-oestrous ewes or mated with oestrous ewes. Again live weight change could not account entirely for the decrease in scrotal volume. The decrease in testicular size was confirmed on castration of the rams at the end of the experimental period. In addition to the 15–42% reduction in testicular weight with increased sexual stimulation, there was a reduction in the number of spermatozoa in the testes (21–36%) and an increase in the volumetric proportion of intertubular tissue (16–58%). A decrease in the volumetric proportion of spermatocytes (18–47%) and of round spermatids (13–57%) in the testicular tissue indicated that sexual stimulation was affecting spermatogenesis and decreasing daily production of spermatozoa.A decrease in the number of spermatozoa in the epididymides (24–26%) with increased sexual stimulation reflected an increased outflow of spermatozoa and/or reduced production of spermatozoa.     

Effect of melatonin implants on secretion of luteinizing hormone in intact and castrated rams

Rams were treated with melatonin implants in 2 experiments designed to examine the control of reproductive seasonality. In Exp. 1, rams (n = 12) were allocated to 3 treatment groups: 2 groups were treated with 2 melatonin implants per ram for 4 months from 11 November (N) and 9 December (D) and the remaining group was untreated (C). The seasonal increase in luteinizing hormone (LH) pulse frequency and testes size was advanced in Groups N and D. A second seasonal cycle in LH secretion and testes size occurred in Groups N and D after melatonin implants became exhausted. In Exp. 2, rams (n = 20) were allocated to 4 treatment groups: 10 rams were castrated on 6 October and 1 group of entire rams (EM) and one group of castrated rams (CM) were treated with 2 melatonin implants per ram each month from 3 November until 8 January. The other group of entire rams (EC) and castrated rams (CC) was untreated. An increase in LH pulse frequency occurred after castration. Melatonin treatment increased LH pulse frequency in entire rams and reduced LH pulse frequency in castrated rams. The results demonstrated that the advanced reproductive development as a result of treatment with melatonin implants was due to an effect of melatonin on the hypothalamic pulse generator to increase LH pulse frequency. The ability of melatonin to influence LH pulse frequency in entire and castrated rams indicated that an effect of melatonin on the hypothalamic pulse generator is independent of testicular steroids.     

Disjunction between the daily duration of light and the plasma level of prolactin under hyperlong days in rams

Adult rams were submitted to an artificial light regime with increasing and decreasing daylengths between 6 and 20 hrs. over a period of 8 months. Blood samples occurred at different durations of daylengths for measuring plasma levels of prolactin. Levels were high for a daily duration of light of 14.20, 17 and 18.30 hrs. along the increasing light-phase and of 18.30 hrs. during the decreasing light-phase. On the other hand prolactin levels were low for short daylengths, but also for a daylength of 20 hrs. It clearly appears that hyperlong days (greater than 18 hrs.) do not stimulate (or do actively inhibit) the secretion of prolactin in ovines.     

Photoperiod entrainment of testosterone, luteinizing hormone, follicle-stimulating hormone, and prolactin cycles in rams in relation to testis size and semen quality

Adult rams were exposed to photoperiod treatments over 2 years to study the influence of light regimes on pituitary-testicular activity and semen quality. Initially, all rams (12 per group) were exposed to 3 months of long days (16L:8D). Group 1 was then exposed to a regime of continuous short days (8L:16D) and Groups 2, 3, and 4 were exposed to 4 months of short days alternated with 1, 2, or 4 months, respectively, of long days. Every 2 weeks, serum hormone levels and scrotal circumference were determined and semen quality was evaluated. Regular cycles in pituitary and testicular activities corresponding to the period of the lighting regime resulted in Groups 2, 3, and 4, but not in Group 1. In general, the change from long days to short days induced increases in follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone levels, scrotal size and sperm numbers and a decrease in prolactin. The reverse occurred after subsequent exposure to long days. After 4 months of long days, testicular regression was complete, but when long-day exposure was reduced, less regression occurred. With continuous exposure to short days, FSH and testosterone remained above basal levels, prolactin levels were depressed, scrotal size remained near the maximum, and elevated numbers of motile sperm were sustained.     

Semen characteristics, scrotal circumference and bacterial isolates of fine wool range rams

During 1983, 887 fine wool rams were subjected to breeding soundness evaluation to determine effects of age and scrotal circumference on semen characteristics. Of rams evaluated, 94.4, 84.8, and 81.9% of young (6 yr), respectively, were rated as satis-factory. Old rams had fewer (P < 0.05) motile cells and more (P < 0.05) abnormal cells than young rams. Among older rams there was a higher percentage (P < 0.05) with testicular lesions than among young or mature rams. Scrotal circumference was positively correlated (P < 0.01) with semen volume and percentage of motile normal cells. Motility was negatively correlated (P < 0.01) with percentage of abnormal cells. Mature and old rams with large (>/=36.7 cm) testis had more (P < 0.10) abnormal cells than those in the same age groups with smaller testis. Scrotal circumference was positively correlated (P < 0.10) to volume and motility in mature and old rams, while motility was negatively correlated (P < 0.01) to percentage of abnormal cells in all age groups. Previous semen testing reduced (P < 0.05) the percentage of mature rams with leucocytes. Vaccination against epididymitis reduced (P < 0.05) the incidence of mature rams with leucocytes and testicular lesions. Brucella ovis was recovered from 54 (67.5%) of 80 ejaculates cultured. Among rams infected with B . ovis , only three (5.6%) were vaccinated against B . ovis and had their semen tested previously.     

The influence of breed,season and photoperiod on semen characteristics,testicular size,libido and plasma hormone concentrations in rams

Twenty-seven adult rams (9 Suffolk, 9 Texel and 9 Dorset Horn) were raised under natural photoperiod and were trained to serve into an artificial vagina. On 1 April they were abruptly exposed to 3 different photoperiods as follows: (i) 8 hours light and 16 hours darkness (8L : 16D); (ii) 16 hours light and 8 hours darkness (16L : 8D); (iii) natural photoperiod. All rams were kept at pasture daily between 09.30 h and 16.00 h except when required indoors for experimental work. Rams on artificial photoperiod had appropriate supplemental lighting in an environmental chamber. Semen collection was attempted from each ram on alternate weeks during the experiment which lasted for 6 months. Semen was evaluated for volume, density, motility and abnormalities. Testicular length and circumference were recorded at 2-week intervals and libido was recorded at 4-week intervals. Three blood samples were collected from each ram at 30-min intervals on a weekly basis and the plasma was stored at ?20°C until assayed for testosterone and prolactin.Photoperiod had no significant effect on semen volume, motility and percentage dead or abnormal cells. Breed of ram had a significant effect on semen motility (P < 0.05) with Dorset Horn rams producing semen with the highest motility. Volume and motility scores both increased as the breeding season approached (P < 0.05), while the percentage of abnormal cells decreased (P < 0.01). Breed or photoperiod did not significantly affect scrotal measurements although animals exposed to 8L : 16D had the highest measurements. Month affected testicular measurements which generally increased from April to September. Suffolk rams had higher testosterone concentrations, and this breed also completed the highest number of mounts within an allocated test time (P < 0.05). Dorset Horn rams reached a peak in testosterone concentrations in June/ July whereas Suffolks and Texels reached a similar peak in August. Prolactin concentrations decreased from a maximum at the start and rams on natural photoperiod tended to have highest levels. These results show that month can have a bigger influence on semen characteristics than imposed artificial photoperiods in rams which have been exposed to increasing natural daylength for some months.     

Circadian phototransduction: phase resetting and frequency of the circadian clock of Gonyaulax cells in red light

Constant red light (RR) influences the Gonyaulax clock in several ways: (1) Phase resetting by white or blue light pulses is stronger under background RR than in constant white light (WW); (2) frequency of the rhythm is less in RR than in WW; and (3) the amplitude of the spontaneous flashing rhythm is greater in RR than in WW. The phase response curve (PRC) to 4-hr white or blue light pulses is of high amplitude (Type 0) for cells in RR, but is of lower amplitude (Type 1) for cells in WW. In all cases, the PRC is highly asymmetrical: The magnitude of advance phase resetting is far higher than that of delay resetting. Consistent with this PRC, Gonyaulax cells in RR (free-running period greater than 24 hr) will entrain to T cycles of between 21 and 26.5 hr. The bioluminescence rhythms exhibit "masking" by blue light pulses while entrained to these T cycles. The fluence response of phase resetting to light-pulse intensity is not linear or logarithmic--rather, it is discontinuous. This feature is consistent with a limit cycle interpretation of Type 0 resetting of circadian clocks. Light pulses that cause large phase shifts also shorten the subsequent free-running period. The phase angle difference between the clock and the previous LD cycle is within 2 hr of the same phase between 16 degrees C and 25 degrees C, as determined from the light PRCs at various temperatures. Several drugs that inhibit mitochondria and/or electron transport will partially inhibit the phase shift by light.     

The importance of glucose in the oxidative metabolism of the testis of the conscious ram and the role of the pentose cycle

1. [U-(14)C]Glucose was infused into one or both testicular arteries of ten conscious rams and the specific activity of the glucose taken up by the testis was compared with the specific activity of the carbon dioxide produced by the testis. 2. Equilibration had occurred after infusion for 3hr. when a mean of 68% of the carbon dioxide was being derived by the testis from blood glucose and 86% of the glucose taken up by the testis was being oxidized to carbon dioxide. After 5hr. infusion, these values were 71% and 83% respectively. 3. In four other conscious rams, [1-(14)C]glucose was infused into one testicular artery and [6-(14)C] glucose into the other and the ;specific yields' of carbon dioxide calculated for the two forms of glucose. 4. From these values, it was calculated that a mean of 9.3% of the glucose taken up by the testis was metabolized via the pentose cycle.