Pinealectomy Decelerates the Circadian Food Intake Rhythm of Cervically Sympathectomized Rabbits

Long-term records of the free-running food intake rhythms were obtained from 26 young adult blinded rabbits that were subjected in the course of recording to bilateral superior cervical ganglionectomy (sympathectomy). This always resulted in acceleration of the rhythm (mean λ r= - 0.35 h). Some 4 months afterwards, 12 rabbits were pinealectomized and after another 4 months sham pinealectomized also. In the other 14 animals, these operations were performed in the reverse order. It appeared that suctioning away part of the ventricular walls during sham pinealectomy was followed by a slight reduction in r (mean = - 0.07 h) that could be largely attributed to the spontaneous gradual reduction of r that occurred throughout all experiments. On the other hand, total pinealectomy in these already sympathectomized blinded rabbits always resulted in a substantial deceleration of the rhythms (mean λ r = + 0.23 h). These observations on blinded rabbits suggest that a sympathetically denervated pineal gland releases appreciable amounts of melatonin or of another hormone with a melatonin-like accelerating effect on the circadian pacemaker (SCN).     

The role of the pineal and the retinae in the expression of circadian locomotor rhythmicity in the ruin lizard,Podarcis sicula

Summary A marked interspecific variability in the role played by the pineal and the retinae characterizes the circadian system of lizards. I examined the role played by these structures in a new model species, the ruin lizard, Podarcis sicula. In constant temperature and darkness pinealectomy as well as bilateral removal of the retinae produced significant changes (both lengthening and shortening) in the freerunning period of locomotor rhythms. Circadian activity time was also affected by pinealectomy. Circadian locomotor rhythmicity persisted in all cases even when both operations were combined in the same individuals. This demonstrates in Podarcis sicula the existence of an oscillatory system outside the pineal and the retinae which can drive locomotor rhythms. The period changes recorded after pinealectomy as well as after bilateral removal of the retinae specifically suggest that both the pineal and the retinae play a modulating role on circadian oscillators located elsewhere in the system, with the final effect of stabilizing the overt rhythms.Abbreviations DD constant darkness - LL constant light - PIN-X pinealectomy - RET-X bilateral removal of the retinae - SHAM sham pinealectomy - circadian activity time - freerunning circadian period     

Circadian organization in the pigeon,Columba livia: the role of the pineal organ and the eye

Summary The roles of the pineal organ and the eye in the control of circadian locomotor rhythmicity were studied in the pigeon (Columba livia). Neither pinealectomy nor blinding abolished the circadian rhythms in constant dim light conditions (LLdim). All the pinealectomized birds and the blinded birds entrained to light-dark (LD) cycles with no discernible anticipatory activity. However, the birds which had been both pinealectomized and blinded showed no circadian rhythms in prolonged LLdim. These birds entrained to LD cycles with anticipatory activity and showed residual rhythmicity for a while after transfer from LD cycles to LLdim. Continuous administration of melatonin induced suppression of the circadian rhythms and reduced total amount of locomotor activity in LLdim. These results suggest that not only the pineal organ but also the eye (perhaps the retina) is involved in the pigeon's circadian system.Abbreviations NAT N-acetyltransferase - LLdim constant dim light - cadian period - SCN suprachiasmatic nucleus - circadian activity time - LD light-dark     

The effects of pinealectomy and blinding on the circadian locomotor activity rhythm in the Japanese newt,Cynops pyrrhogaster

We examined the effects of pinealectomy and blinding (bilateral ocular enucleation) on the circadian locomotor activity rhythm in the Japanese newt, Cynops pyrrhogaster. The pinealectomized newts were entrained to a light-dark cycle of 12 h light and 12 h darkness. After transfer to constant darkness they showed residual rhythmicity for at least several days which was gradually disrupted in prolonged constant darkness. Blinded newts were also entrained to a 12 h light/12 h dark cycle. In subsequent constant darkness they showed free-running rhythms of locomotor activity. However, the freerunning periods noticeably increased compared with those observed in the previous period of constant darkness before blinding. In blinded newts entrained to the light/dark cycle the activity rhythms were gradually disrupted after pinealectomy even in the presence of the light/dark cycle. These results suggest that both the pineal and the eyes are involved in the newt's circadian system, and also suggest that the pineal of the newt acts as an extraretinal photoreceptor which mediates the entrainment of the locomotor activity rhythm.Abbreviations circadian period - DD constant darkness - LD cycle, light-dark cycle - LD 12:12 light-dark cycle of 12 h light and 12 h darkness     

Influence of food intake on the 24-hr variations of plasma iron concentration in the rabbit

Circadian variations in plasma iron levels were first reported in humans in 1937. Influences of the sleeping pattern and of plasma cortisol and adrenaline levels on these variations as well as the reproducibility of the phenomenon itself are discussed controversially in the literature. The influence of food intake, however, was not considered in most of the studies and is therefore subject of this investigation. Circadian plasma iron and plasma transferrin variations were determined in rabbits and compared under free access to food and under starvation (caecotrophy was not prevented). Population-mean-cosinor analysis of circadian plasma iron concentrations showed similar variations in the fed and starved condition (mesor: 116.6 micrograms/dl vs 118.1 micrograms/dl, acrophase 0752 hr vs 0728) except for a significant increase of the circadian amplitude under free access to food (30.9 micrograms/dl vs 22.3 micrograms/dl, P less than 0.05). There was no variation in plasma transferrin, which shows that 24 hr variations in plasma iron are not caused by modulation of plasma transferrin. These findings demonstrate a circadian rhythm for plasma iron, the amplitude of which is increased by food intake.     

Ultradian, circadian and seasonal variations of plasma progesterone and LH concentrations during the luteal phase

The circadian variations in plasma progesterone (P) and LH concentrations were investigated in six women, aged 23-40 years. All were studied in the mid-luteal phase (7 +/- 2 days after LH mid-cycle surge). Experiments were conducted in autumn and in spring. Blood samples were obtained every 15 min for 24 hr. Plasma P and LH concentrations were measured by RIA. Each subject's time-series was analysed using three methods; visual inspection (chronogram), spectral analysis to estimate component periods of rhythms (tau) and cosinor analysis to quantify the rhythms parameters. Marked temporal variations in plasma P concentration were observed in each subject. The maximal variations over a 24-hr period, ranged between 13-58.5 mmol/l. Differences related to sampling time were statistically validated by ANOVA (p less than 0.00001). Significant harmonic periods were detected by spectral analysis but differed among subjects. In all subjects but one, a circadian rhythm was detected. The acrophase location was similar (about 0700 hr) in the four subjects studied in autumn, but ranged from 1940 to 0320 hr in those studied in spring. An ultradian rhythm with tau = 8 hr was also validated in six time-series with similar acrophases (about 0200, 1000, and 1800 hr). Cosinor analysis of pooled data revealed that the 24-hr, 12-hr, and 8-hr rhythms were statistically significant (p = 0.001) in autumn. algebraic sum of these three cosine functions yielded a circadian waveform with peak-times occurring near 0300 and 1130 hr and a trough-time about 2200 hr. In spring, the circadian pattern appeared quite different, and peak-times were found near 0700 and 2000 hr, and trough-times near 0300 and 1500 hr. Furthermore, the 24-hr mean of P was higher in autumn (28.9 +/- 0.4 nmol/l) than in spring (17.2 +/- 0.4 nmol/l), p from ANOVA less than 0.00001. The evidence for a similar circadian LH pattern is not as strong. Seasonal, circadian and ultradian rhythms characterize the physiologic time structure of plasma P concentration in mid-luteal phase.     

The internal synchronization of five circadian functions of the rabbit

Five different physiological functions of the rabbit (hard faeces and urine excretion, food and water intake and locomotor activity) were registered during LD 12:12 and during continuous light conditions (LL).

(1) In LD 12:12 a strong synchronization of the five parameters existed. The minima of all functions consistently occurred during the hours of light. The nocturnal percentage of overall 24-hr events was increased significantly in 'hard faeces excretion' (66±8 (S.D.) %), 'water intake' (64±15 (S.D.) %) and 'urine excretion' (58±10 (S.D.) %). The nocturnal percentage of locomotor activity was significantly increased during the dark-hours in 9 out of 14 animals. In the other five individuals prominent peaks were present even during the photoperiod. On the average of all 14 animals 5S±13 (S.D.) % of the 24 hr events of locomotor activity occurred during the night. Despite a trough during the cessation of hard faeces excretion the events of food intake were not elevated significantly during the dark hours.

(2) During LL the synchronization of the five functions within each animal persisted during the complete 90-day LL period. A free-running circadian rhythm with-: = 24.8±0.5 (S.D.) hr was present in the four rabbits kept in LL conditions within 5-16 days after the withdrawal of the zeitgeber.

(3) In addition to the circadian period the power spectrum analysis of data obtained during LD 12:12 revealed significant ultradian periods of an average period length of 11,6 hr (hard faeces and urine excretion), 8 hr (food and water intake, locomotor activity) and 4 hr (food intake, locomotor activity). During the free-run ultradian periods of 8 and 3.2-4.2 hr were significant in almost all parameters.

(4) During LL the level of locomotor activity was reduced for 13±16 (S.D.) %, the events of food intake were increased for 17±12 (S.D.) %.

(5) The reinserted LD 12:12 zeitgeber re-entrained the circadian rhythms within 25-45 days.

(6) These results provided evidence of a predominant nocturnality of the rabbits under investigation.     

Air-breathing rhythm in Clarias batrachus (Linn.): modulatory role of eyes, pineal and exogenous melatonin

Effects of enucleation followed by pinealectomy and administration of exogenous melatonin on air-breathing activity rhythm in a fresh water catfish, C. batrachus maintained at LD 12:12 and laboratary temperature during its prepratory phase, were examined. Results of cosinor analysis clearly reveal that most of the intact individuals exhibited circadian rhythm in their air-breathing activity and such rhythm persists even after enucleation followed by pinealectomy and then melatonin administration. However, the period (tau) of the activity obtained by power spectrum analysis was prominent 24 hr in most of the intact individuals, but it was increased (tau > 24 hr) after enucleation in most of the individuals. In most of the enucleated + pinealectomized individuals tau was less than 24 hr, and after receiving melatonin treatment tau was shifted to prominent 24 hr in most of the individuals. In addition, visual analysis of the actograms depicted that in intact individuals air-breathing activity is entrained with the timings of lights on/off with elevation of activity during dark period and decreased activity during light hours. However, enucleated and enucleated + pinealectomized individuals showed free run in their activity rhythm. The treatment of melatonin reestablished the entrainment of activity at least with the timing of lights off, in most of the studied individuals. Further, daily mean of the air-breathing activity was decreased in enucleated + pinealectomized individuals as compared with other studied groups (intact, enucleated, enucleated + pinealectomized + melatonin receiving). It could be speculated that there may be existence of extraretinal and extrapineal photoreceptors in C. batrachus. However, eyes play an important role in regulating air-breathing activity rhythm in such species. In addition, exogenous melatonin may also have some modulatory effect on such rhythm.     

Activity feedback to the mammalian circadian pacemaker: influence on observed measures of rhythm period length.

In the mouse, activity is precisely timed by the circadian clock and is normally most intense in the early subjective night. Since vigorous activity (e.g., wheel running) is thought to induce phase shifts in rodents, the temporal placement of daily exercise/activity could be a determinant of observed circadian rhythm period. The relationship between spontaneous running-wheel activity and the circadian period of free-running rhythms was studied to assess this possibility. With ad libitum access to a running wheel, mice exhibited a free-running period (tau) of 23.43 +/- 0.08 hr (mean +/- SEM). When running wheels were locked, tau increased (23.88 +/- 0.04 hr, p less than 0.03), and restoration of ad libitum wheel running again produced a shorter period (tau = 23.56 +/- 0.06 hr, p less than 0.05). A survey of free-running activity patterns in a population of 100 mice revealed a significant correlation between the observed circadian period and the time of day in which spontaneous wheel running occurred (r = 0.7314, p less than 0.0001). Significantly shorter periods were observed when running was concentrated at the beginning of the subjective night (tau = 23.23 +/- 0.04), and longer periods were observed if mice ran late in the subjective night (tau = 23.89 +/- 0.04), F (1, 99) = 34.96, p less than 0.0001. It was previously believed that the period of the circadian clock was primarily responsive to externally imposed tonic or phasic events. Systematic influences of spontaneous exercise on tau demonstrate that physiological and/or behavioral determinants of circadian timekeeping exist as well.     

Food intake in captive leverets before weaning and the composition of the milk of the brown doe-hare (Lepus europaeus)

Daily milk and solid food intakes were studied in leverets before weaning and during 52 lactations in captive doe-hares. Changes in milk composition were studied in parallel during 11 other lactations. With the mode of feeding used (one nursing per 24 hr), milk and solid food intakes were recorded for the first 30 neonatal days. The profile of milk intake had three phases: it showed an increase between D0 and D12, a plateau between D12 and D22, and a decrease between D22 and D30. Solid food intake began at D7 and increased significantly from D14. The mean levels of dry matter, fat and protein in doe hare's milk were similar to those in rabbit's milk The amino acid composition of total protein resembled that of rabbit's milk. There were fewer medium-chain fatty acids (8:0-14:0) in here's milk than in rabbit's milk and the levels of some major minerals (Ca, P, K) were also different. The levels of protein and medium-chain fatty acids (except 14:0) increased significantly with time. On the other hand, the levels of long-chain fatty acids (except 18:0 and 18:3, N = 3) decreased significantly. There was no significant change in the other factors studied.     

Photoperiodic and pineal effects on food intake, food retrieval, and body weight in female Syrian hamsters

Food intake, food retrieval, and body weight changes were measured in female Syrian hamsters maintained under long and short photoperiods, and in animals sustaining pinealectomy or sham surgery. Animals maintained under short photoperiods gained more weight, ingested more food, and brought more food from more distant compartments into their nest compartments than did animals maintained under long photoperiods. Pinealectomy prevented the short-day elevations in body weight and food retrieval during the period of gonadal recrudescence, but it did not produce significant changes during the period of gonadal regression; in contrast, pinealectomy prevented the short-day increases in food intake only during initial weeks of the period of gonadal regression: The effects of photoperiod or pinealectomy were not evident for any of the dependent measures during pregnancy. The role of the pineal gland in mediating short-day effects is discussed.     

Effects of feeding cycles on circadian rhythms in squirrel monkeys

Squirrel monkeys (Saimiri sciureus) were housed singly in cages equipped with a tree for climbing to measure locomotor activity, and with a movable food cup that could be arrested automatically. The animals were kept in continuous dim illumination (LL), twice interrupted by several weeks of entrainment by a light-dark (LD) 12:12 cycle. Apart from three control sections in which the food cups were unlocked continuously (ad libitum feeding), food was accessible for 3 hr per day only, with interfeeding intervals varying from 23 to 26 hr (periodic restricted feeding, or RF). During LD entrainment, the imposition of an RF schedule resulted in anticipatory behaviors, represented by increased tugs at the food cup and a pause in locomotor activity preceding the feeding time. In LL, the animals showed free-running circadian rhythms of locomotor and "feeding" activity that nearly always persisted when ad libitum feeding was replaced by RF. The period (tau) of the free-running rhythm was slightly modulated in relation to the varying interfeeding intervals (T), but entrainment was never achieved except in one test with an animal whose tau was very close to T. It is concluded that periodic availability of food represents an extremely weak zeitgeber, if any, for the circadian pacemaker of squirrel monkeys.     

Period and phase control in a multioscillatory circadian system (Iguana iguana)

The circadian system of the lizard Iguana iguana is composed of several independent pacemakers that work in concert: the pineal gland, retinae of the lateral eyes, and a fourth oscillator presumed to be located in the hypothalamus. These pacemakers govern the circadian expression of multiple behaviors and physiological processes, including rhythms in locomotor activity, endogenous body temperature, electroretinogram, and melatonin synthesis. The numerous, easily measurable rhythmic outputs make the iguana an ideal organism for examining the contributions of individual oscillators and their interactions in governing the expression of overt circadian rhythms. The authors have examined the effects of pinealectomy and enucleation on the endogenous body temperature rhythm (BTR) and locomotor activity rhythm (LAR) of juvenile iguanas at constant temperature both in LD cycles and in constant darkness (DD). They measured the periods (tau) of the circadian rhythms of LAR and BTR, the phase relationships between them in DD (psiAT), and the phase relationship between each rhythm and the light cycle (psiRL). Pinealectomy lengthened tau of locomotor activity in all animals tested and abolished the BTR in two-thirds of the animals. In those animals in which the BTR did persist following pinealectomy, tau lengthened to the same extent as that of locomotor activity. Pinealectomy also delayed the onset of activity with respect to its normal phase relationship with body temperature in DD. Enucleation alone had no significant effect on tau of LAR or BTR; however, after enucleation, BTR became 180 degrees out of phase from LAR in DD. After both pinealectomy and enucleation, 4 of 16 animals became arrhythmic in both activity and body temperature. Their data suggest that rhythmicity, period, and phase of overt circadian behaviors are regulated through the combined output of multiple endogenous circadian oscillators.     

Circadian and circahemidian rhythms in plasma prolactin of the ram: seasonal changes

The existence of a circadian rhythm in plasma prolactin of the ram is controversial. Differences among authors can be related to both data sampling (e.g. interindividual changes, time of day, time of year, sampling interval among others) and statistical analyses. To test this hypothesis six adult "Préalpes du Sud" rams were studied individually during 72 hr in January (8 hr of light-16 hr of darkness), April (13L-11D), June (16L-8D) and September (13L-11D). Blood was sampled (vacutainer) from a jugular vein every hour, centrifuged and plasma samples stored at -20 degrees C until prolactin determinations (radioimmunoassay) were made. Individual time series were analysed according to three complementary methods: display of raw data (chronogram), best fitting cosine functions with different period tau (iterative cosinors) and power spectra. Seasonal changes in the 24 hr mean (peak time in June) were confirmed. A circahemidian rhythm (tau = 12 hr) and a circadian rhythm (tau = 24 hr) were validated, respectively in January and April while time series documented in June and September exhibited no rhythmic organization. It seems, therefore, that animals adjusted their rhythmic patterns of prolactin secretion to the increasing (January, April) rather than decreasing (June, September) photofraction (duration of the light span/24 hr).     

Pineal and retinal melatonin is involved in the control of circadian locomotor activity and body temperature rhythms in the pigeon

Summary Although pinealectomy or blinding resulted in loss of the clarity of the free-running rhythm of locomotor activity and body temperature and reduced the peak level of circulating melatonin rhythms to approximately a half in intact pigeons, neither pinealectomy nor blinding abolished any of these rhythms. However, when pinealectomy and blinding were combined, the rhythms of locomotor activity and body temperature disappeared in prolonged constant dim light, and melatonin concentration was reduced to the minimum level of detection. In order to examine the role of melatonin in the pigeon's circadian system, it was administered either daily or continuously to PX + EX-pigeons in LLdim. Daily administration of melatonin restored circadian rhythms of locomotor activity which entrained to melatonin injections, but continuous administration did not induce any remarkable change of locomotor activity. These results suggest that melatonin synthesized in the pineal body and the eye contributes to circulating melatonin and its rhythmicity is important for the control of circadian rhythms of locomotor activity and body temperature in the pigeon.Abbreviations LD Light-dark - LLdim constant dim light - LLbright constant bright light - PX pinealectomy - EX blinding - SCN suprachiasmatic nucleus     

Alteration of neonatal rat parotid gland acinar cell proliferation by guandethidine-induced sympathectomy.

Newborn rats were injected with guanethidine-sulfate (20 micrograms/g body weight) every 48 hr from 12 hr after birth until day 14 (eight injections per animal). The guanethidine treatment resulted in an 86% absolute reduction in cell number in the superior cervical ganglia of 15 day old rats. The cells which remained after guanethidine treatment showed destruction of mitochondria and an extensive decrease in endoplasmic reticulum. Chemical sympathectomy with guanethidine induced a 3.1 hr lengthening of the acinar cell generation cycle time (17.4 hr to 20.5 hr), resulting from a longer G1 period (6.9 hr in the control group as compared to 10.5 hr in the guanethidine-treated group), as well as a cecrease in the mean percentage of [3H]thymidine-labeled acinar cells (22.3 +/- 0.5% to 19.3 +/- 0.5%) and mean acinar cell mitotic index (2.6 +/- 0.2% to 2.1 +/- 0.1%). A circadian rhythm was found to exist in parotid gland acinar cell mitotic activity of 15 day old rats and the amplitude of the rhythm was reduced from 26.5% to 14.9% in guanethidine-treated rats. This study indicates that the diminution of sympathetic influence on the developing parotid gland results in a slight, but significant alteration in acnar cell proliferation.     

Circadian rhythms of food and 1% NaCl intake, urine and electrolyte excretion, plasma renin activity and insulin concentration in adrenalectomized rats

The circadian rhythms of food and 1% NaCl intake, and urine, Na+, Cl- and K+ excretion were followed up in male Wistar rats before and one week after bilateral adrenalectomy at 4-hour intervals during two consecutive days. The circadian rhythms of plasma renin activity (PRA) and plasma immunoreactive insulin (IRI) were evaluated after decapitation of both intact and adrenalectomized rats at 08, 16 and 24 h. To all rats 1% NaCl was offered instead of drinking water. Adrenalectomy did not cause any significant phase shift in the cosine curves approximating the data collected at 4-hour intervals. The circadian rhythms showed the same relationships before and after the operation: the rhythms of food intake, K+ excretion and saline intake preceded significantly the rhythms of urine, Na+ and Cl- excretion. Adrenalectomy induced an increase in mean PRA and shifted its minimal value from 08 to 24 h. After the operation mean IRI decreased and the minimal value shifted from 16 to 24 h. It was concluded that adrenal glands do not play an important role in the synchronization of the circadian rhythms of food and 1% NaCl intake, urine and synchronization of the circadian rhythms of food and 1% NaCl intake, urine and electrolyte excretion with the illumination cycle, but play a relevant role in the synchronization of the circadian rhythms of PRA and IRI in the rat.     

Damped oscillation of the lateral hypothalamic multineuronal activity synchronized to daily feeding schedules in rats with suprachiasmatic nucleus lesions.

In male Wistar rats with chronically implanted electrodes, multiple-unit activity (MUA) was recorded from the lateral hypothalamus (LH) and ventromedial hypothalamus (VMH). Blinded rats with bilateral suprachiasmatic nucleus (SCN) lesions showed no circadian rhythm in MUA or motor activity when food was available ad libitum. However, under a restricted-feeding schedule (food was available from 1400 to 1600 hr; water was always available) lasting for 10 days, a gradual increase of MUA of the LH developed, starting 3-4 hr prior to the feeding time. The elevated MUA lasted up to 6-7 hr after feeding and subsequently returned to the baseline level. This circadian rhythm of MUA of the LH persisted up to 4 days under total food deprivation, with quickly decreasing amplitude after termination of the schedule. MUA rhythm in VMH was less obvious than that in LH. Also, general motor activity showed a rhythm comparable to that of MUA, but it was less prominent. The elevated MUA in the LH prior to the feeding time may have been neural substrate of anticipatory activity appearing under the restricted-feeding schedule. These findings may suggest the existence of a quickly damping oscillator mechanism in the brain, presumably in the LH, which can be induced by daily feeding cues in the absence of the SCN.     

Inhibitory effects of intestinal electrical stimulation on food intake, weight loss and gastric emptying in rats

The aim was to investigate the effects of intestinal electrical stimulation (IES) on food intake, body weight, and gastric emptying in rats. An experiment on food intake and weight change was performed in 22 rats on a control diet and 10 diet-induced obese (DIO) rats for 4 wk with IES or sham IES. The effect of IES on gastric emptying was performed in another 20 rats in the control group. We found that 1) in control rats, 4-wk IES resulted in a reduction of 18.2% in the total amount of food intake compared with sham-IES (P = 0.02); the rats treated with IES had a weight change of -1 +/- 7.8g (P = 0.03), which was equivalent to a weight loss of 6.2% due to IES when adjusted for normal growing. 2) Acute IES delayed gastric emptying by 20% in the control rats (P < 0.01). 3) In the DIO rats, 1-wk IES with the same parameters as those used in the control rats resulted in a significant reduction in the total amount of food intake (126.6 +/- 6.3 g vs. 116.9 +/- 3.2 g, P < 0.01). More reduction in food intake was noted, and a significant weight change was also observed when stimulation energy was increased. 4) No adverse events were observed in any of the experiments. In conclusion, IES delays gastric emptying, reduces food intake, and decreases weight gain in control growing rats. These data suggest that it is worthy to explore therapeutic potentials of IES for obesity.     

Circadian and circannual study of the hypophyseal-gonadal axis in healthy young males

Circadian and circannual variations of Testosterone, FSH and LH secretions, other than Oral Body Temperature (OBT) have been studied in four healthy males. OBT showed a constant circadian rhythm with an acrophase located in the afternoon. Plasma Testosterone exhibited both a circadian (acrophase = hr 09,28) and a circannual rhythm (acrophase = 22 february); plasma FSH also showed a circannual rhythm (acrophase = 13 february). By mean chronogram +/- SEM we documented the highest LH levels in December and the lowest in February. These observations would suggest the hypothesis that the winter could be the period in which the hypophysis-gonadal axis in young males exhibits its maximal activity as previously documented for other hormones.