Effect of experimentally induced hyperprolactinemia on growth hormone secretion

In order to study the existence of possible interrelation-ships between prolactin (PRL) and growth hormone (GH) secretions, adult male rats bearing an anterior pituitary graft under the kidney capsule since day 90 of life and their sham-operated controls were submitted to a single i.p. administration of L-dopa (50 mg/kg weight) or saline 30 days after the operation. Plasma PRL and GH levels were measured by using specific RIA methods. Dopamine (DA) and norepinephrine (NE) contents in the hypothalamus and in the in situ anterior pituitary gland were measured by using a specific radioenzymatic assay. An increase in plasma PRL levels and a decrease in plasma GH levels were shown in grafted rats. Hypothalamic contents of DA and NE were increased in these animals, while the anterior pituitary content of DA was not modified as compared to controls. The administration of a single injection of L-dopa led to decreases of plasma PRL and GH levels in both grafted and control rats, but while marked increases in hypothalamic and anterior pituitary contents of DA were shown in both groups, the hypothalamic content of NE was only increased in control animals. These data suggest that PRL and GH secretions were closely related. Dopamine could be mediating the action of PRL on GH, while NE would be less involved.     

Serotonin synthesis inhibition or receptor antagonism reduces pregnancy-induced nocturnal prolactin secretion

During early pregnancy, two surges of prolactin (PRL) designated as nocturnal (N) and diurnal (D) are displayed by the rat. We previously reported the positive influence of serotonin (5-HT) in regulating the D surge. Its role in the N surge remained inconclusive due to the contradictory results obtained with the 5-HT synthesis inhibitor parachlorophenylalanine (PCPA) and 5-HT2 receptor antagonists. This study further characterizes the involvement of 5-HT in regulating the N surge. The effectiveness of different doses of ketanserin (KET), a 5-HT2 receptor antagonist, to reduce plasma PRL levels during the surge was established. Sub-threshold (1 mg/kg BW) or just maximally effective (10 mg/kg BW) doses of KET were administered to rats that had been pre-treated with PCPA (250 mg/kg BW) for 24h. The lower dose of KET was ineffective in reducing the N surge even though less 5-HT was available due to PCPA treatment 24h earlier. The higher dose was effective in blocking the surge. Subsequently, the effect of one compared to two injections of PCPA 24 hours apart on plasma PRL levels and concentrations of 5-HT, dopamine (DA) and their respective metabolites 5-hydroxy-indoleacetic acid (5-HIAA) and dihydroxyphenylacetic acid (DOPAC) in the medial basal hypothalamus (MBH) and the medial dorsal hypothalamus (MDH) was studied. Two injections of PCPA but not one abolished the N PRL surge. Levels of 5-HT and 5-HIAA were significantly (p less than .005) reduced following either one or two injections of PCPA. Nevertheless, there was a greater (50 fold) decrease in 5-HIAA following 2 injections compared to one injection (10 fold), resulting in lower 5-HT turnover as indicated by lower 5-HIAA/5-HT ratios. Levels of DA in the MBH were reduced significantly only following two injections of PCPA, suggesting that the lack of effect of PCPA after one injection on the N surge was not due to a decrease in DA.     

Pituitary prolactin concentrations associated with daily prolactin surges in pseudopregnant rats

During pseudopregnancy (PSP) two surges of prolactin (PRL) secretion from the pituitary are observed, the nocturnal surge at dawn and the diurnal surge in the evening. An attempt was made to clarify the correlation between changes in serum and pituitary PRL concentrations on day 5-6 of PSP. During the nocturnal surge, pituitary PRL concentration decreased significantly from 0000 hr to 0300-0600 hr. On the other hand, the high pituitary PRL concentration remained unchanged during the diurnal surge from 1200 hr to 1800 hr. These findings suggest that the nocturnal and diurnal PRL surges are regulated by separate controlling mechanisms.     

Turnover rates of hypothalamic dopamine associated with the two types of prolactin surges during pseudopregnancy in rats.

Two surges of prolactin (PRL) are observed daily during pseudopregnancy (PSP) in the rat: the nocturnal (N) surge at dawn and the diurnal (D) surge in the evening. In order to clarify differences in the controlling mechanisms of the two types of PRL surges, we attempted to examine the turnover rates of dopamine (DA) and norepinephrine (NE) in the preoptic-anterior hypothalamus (PAH) and mid-posterior hypothalamus (MPH) on day 5-6 of PSP. The turnover rates of DA in the hypothalamus were inversely correlated with the serum PRL levels at the D surge when a significant increase in serum PRL was accompanied by not only a marked decrease in turnover rates of DA in the PAH, but also a slight decrease in turnover rates of the amine in the MPH. Contrarily, no significant decrease occurred in the turnover rates of DA in the hypothalamus at the N surge. There was no obvious correlation between the turnover rates of NE in the hypothalamus and the serum PRL levels at either PRL surge. These findings suggest that decreased turnover rates of DA in the hypothalamus are involved in the D surge, and it is most probable that the N PRL surge occurs under the control of another factor such as the hypothalamic PRL-releasing factor.     

Diurnal alterations of catecholamines, indoleamines and their metabolites in specific brain regions of the mouse

1. The diurnal variations of regional brain concentrations of dopamine (DA), norepinephrine (NE), serotonin (5-HT) and metabolites were determined in unperturbed male CD-1 mice. Determinations were made every 4 hr for 24 hr. 2. The most striking and significant variations in biogenic amines were seen in the hypothalamus, where concentrations of NE, DA and 5-HT varied in a rhythmic pattern and as much as two-fold during this period. 3. In some cases, daily alterations in parent biogenic amines were reflected by concurrent changes in their metabolites. 4. Since concentrations of neurotransmitters in the brain are often used as an indicator of stress and/or toxicity, these data should provide an accurate data base allowing for more accurate interpretation of results.     

Norepinephrine Turnover in the Hypothalamus of Adult Male Rats: Alteration of Circadian Patterns by Semistarvation

Norepinephrine (NE) turnover, as estimated by 3-methoxy-4-hydroxyphenylethyleneglycol concentration, was studied in the mediobasal hypothalamus of control and semistarved adult male rats at eight time points of a 24-h period. The marked circadian periodicity of NE turnover with a peak in the dark phase in control rats is completely suppressed in semistarved rats. The average 24-h concentration of the NE precursor tyrosine in brain and of tyrosine flow into brain (calculated from plasma amino acid concentrations) is reduced in semistarved rats, but both brain tyrosine and tyrosine flow show continuing circadian fluctuations.     

Developmental Changes in Biogenic Amine Levels in the Central Nervous System and the Haemolymph of the Eastern Death's Head Hawk Moth,Acherontia styx (Lepidoptera: Sphingidae)

In an effort to understand the role of biogenic amines in insect development, changes in the levels of octopamine (OA), dopamine (DA), epinephrine (E), norepinephrine (NE), and serotonin (5-HT) in the brain, the optic lobes and the haemolymph of different developmental stages of Acherontia styx were analyzed using HPLC with electrochemical detector. In the brain, OA was the most abundant monoamine. DA, OA, and E levels in larvae peaked around the wandering stage (W). A dramatic increase in DA, 5-HT, and E levels was observed in the brain of the adult as compared to the pupal stage. NE, however, was not detected in the brain of most stages of the insect, except in the brain of 20-day-old pupae and adults. A 3-fold increase in OA levels was observed in the optic lobes of the adult as compared to late pupal stage. No changes were observed for E, DA, and 5-HT. NE was not detected in the optic lobes. In the haemolymph of 5^th instar larvae, OA was also the most abundant amine. Both DA and OA peaked prior to the onset of the W stage. In contrast, E and NE concentrations decreased with development of the 5^th instar larvae. 5-HT was not detected in the haemolymph. Finally, different profiles for amine levels were observed for the two forms of the 5^th instar larvae (green vs brown). These results are interpreted in the light of the role of biogenic amines and their relation to development in the nervous system of lepidopteran insects.     

Time-dependent changes in brain biogenic amine dynamics following castration in male rats

—Alterations in whole-brain and hypothalamic levels of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), norepinephrine (NE), dopamine (DA) as well as the turnover rates of NE and DA of adult male rats were analysed fluorometrically at either 3 weeks or 6 weeks following castration. Significant increases were observed in whole-brain (minus hypothalamus) 5-HIAA levels and hypothalamic DA levels, fractional rate constants and utilization rates at the 3 but not the 6 week intervals. Elevated levels of 5-HT were observed at both time intervals while an increase in whole-brain DA was seen only at the 6 week interval. Whole brain NE turnover rates of castrated animals did not differ significantly from those of sham-castrate control animals at either test interval. However, a tendency toward increased hypothalamic NE turnover rates was seen in the castrated animals. These biochemical changes resulted in decreased NE/5-HT and DA/5-HT ratios for the castrate rats as compared to controls. The results are discussed in relation to emotional and aggressive behavior and are interpreted as being consistent with the hypothesis purporting an inhibitory role for 5-HT and excitatory role for NE and DA in sex-specific behavior patterns including aggression.     

Effect of aging on monoamines and their metabolites in the rat brain

Concentrations of dopamine (DA), norepinephrine (NE), serotonin (5-HT) and their acid merabolites were assayed in specific brain areas of Wistar rats of various ages. DA and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were significantly lower in striatum and mesolimbic areas of old (24 mos) rats than young adult (3 mos), but not mature (12 mos) rats. The decrease of homovanillic acid (HVA) was significant in mesolimbic areas but not in striatum. Neither cortical NE nor its metabolite methoxydroxyphenylglycol sulphate (MHPG-SO₄) were significantly changed by aging. 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the brainstem showed a tendency to a decrease and increase respectively in aged animals compared with young adults, but the differences were not statistically significant. However, the ratio of 5-HIAA to 5-HT concentrations was significantly higher in aged animals. The conclusion can be drawn that, in these brain areas, DA is more vulnerable to aging than NE and 5-HT, the metabolism of the latter being even enhanced.     

Circadian rhythm of serotonin binding in rat brain--II. Influence of sleep deprivation and imipramine

Sleep deprivation (SD) modified the circadian rhythm of specific high affinity serotonin (5-HT) binding to rat brain membranes. In control rats a 24-hr rhythm was evident with a trough at 1000-1200 and a nadir at 0000. During the last 26 hr of a 49 hr SD period, trough and peak values were delayed by 4-6 hr. The 24-hr mean binding was significantly (P less than 0.001) different from that of controls. If sleep deprivation was followed by recovery sleep (RS), the normal rhythm of 5-HT binding was obtained already within 1 hr after SD. The effects of SD and RS were ascertained by plasma ACTH and corticosterone assay. No significant change in the hormone rhythms were observed through the mean plasma level of ACTH and corticosterone were enhanced to about 180 and 150%, respectively. Chronic treatment with the antidepressant imipramine resulted in a decrease of the 24-hr mean 5-HT binding by about 50% and a 2-hr delay of peak and trough values. Imipramine treatment decreased the peak value of 5-HT concentration at 1000 to about 65% and appears to abolish the rhythm of 5-HT concentration.     

Lack of circadian rhythmicity of rat fetal intestinal enzymes as compared to the dams

The 24-hr activity patterns of intestinal maltase, lactase, leucylnaphthylamine hydrolyzing activity, γ-glutamyltransferase, and alkaline phosphatase were determined in pregnant rats maintained on a 12-12 light-dark cycle, with feeding during the dark period (1800-0600 hr, EST). The activities of these enzymes plus those of lysosomal maltase and lactase were followed during the same time period in 19- to 20-day-old fetuses. The activity patterns in the dams followed circadian rhythms, with peak activities occurring during the feeding-dark period. These rhythms are similar to the feeding schedule-cued rhythms observed in male rats and, therefore, are assumed to be feeding schedule cued also. In the fetuses, which obtained nutrients through the placenta, the activities increased in a somewhat nonlinear manner throughout the entire 24-hr period, but did not display a defined rhythm. It is concluded that endogenous intestinal enzyme rhythms do not exist in utero, and that oral and/or intermittent feeding is necessary for these rhythms to occur.     

Effects of pargyline on hypothalamic biogenic amines and serum prolactin. LH and TSH in male rats.

The time course effects of pargyline on hypothalamic biogenic amines and serum prolactin (PRL), LH and TSH were studied in adult male rats. The rats were killed at intervals of 1–6 hrs after pargyline injection. Hypothalamic dopamine (DA) rose 79% by 1 hr and was 41% above “0” time by 6 hrs. Norepinephrine (NE) increased 31% by 1 hr and remained at about this level through 6 hrs, whereas serotonin (5HT) increased from 42% by 1 hr and to 95% by 6 hrs. Serum PRL LH and TSH fell significantly during the first 2 hrs, but all had returned to pretreatment values by 4 hrs. Serum PRL was about 4-fold above pretreatment values by 6 hrs, but LH and TSH remained at pretreatment levels. Stimulation by pargyline of PRL release was potentiated by Lilly compound 110140, a serotonin reuptake inhibitor, and blocked by parachlorophenylalanine, a serotonin synthesis inhibitor. These results suggest that the inhibitory effects of pargyline on PRL, LH, and TSH release during the first 2 hrs were associated mainly with a rapid increase in DA, and subsequent elevation of PRL release was related to the increase in 5HT. Return of serum LH and TSH to pretreatment levels at 4 and 6 hrs appeared to be associated mainly with the decrease in DA and perhaps to elevated NE levels. These results suggest that changes in relative concentrations of hypothalamic amines are related to differential release of PRL, LH and TSH.     

Circadian Rhythm of Serotonin Binding in Rat Brain—II. Influence of Sleep Deprivation and Imipramine

Sleep deprivation (SD) modified the circadian rhythm of specific high affinity serotonin (5-HT) binding to rat brain membranes. In control rats a 24-hr rhythm was evident with a trough at 1000-1200 and a nadir at 0000. During the last 26 hr of a 49 hr SD period, trough and peak values were delayed by 4-6 hr. The 24-hr mean binding was significantly (P < 0.001) different from that of controls. If sleep deprivation was followed by recovery sleep (RS), the normal rhythm of 5-HT binding was obtained already within 1 hr after SD. The effects of SD and RS were ascertained by plasma ACTH and corticosterone assay. No significant change in the hormone rhythms were observed though the mean plasma level of ACTH and corticosterone were enhanced to about 180 and 150%, respectively. Chronic treatment with the antidepressant imipramine resulted in a decrease of the 24-hr mean 5-HT binding by about 50% and a 2-hr delay of peak and trough values. Imipramine treatment decreased the peak valueof 5-HT concentration at 1000 to about 65% and appears to abolish the rhythm of 5-HT concentration.     

Effect of barbitone sodium and thiopental sodium on brain dopamine, noradrenaline, serotonin and 5-hydroxyindoleacetic acid content in Arvicanthis niloticus

The quantitative estimation of total dopamine (DA), noradrenaline (NE), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content in the whole brain tissue of normal Nile grass rat, Arvicanthis niloticus, gives and average of 631 +/- 12 ng DA/g, 366 +/- 12 ng NE/g, 617 +/- 15 ng 5-HT/g and 431 +/- 10 ng 5-HIAA/g fresh brain tissue. The effect of barbitone sodium and thiopental sodium on the total DA, NE, 5-HT and 5-HIAA content in the brain tissue of the Nile grass rat, Arvicanthis niloticus, was studied. The total DA, NE, 5-HT and 5-HIAA contents were determined 5 hr after i.p. injection of different doses of barbitone sodium (20, 40 and 80 mg/ml/100 g body wt) and thiopental sodium (5, 10 and 20 mg/ml/100 g body wt). The effect of different time intervals (1, 10, 30 min, 1, 2.5, 5, 8, 16, 24 and 48 hr) on the total brain DA, NE, 5-HT and 5-HIAA content was investigated after i.p. injection of 40 mg of barbitone sodium and 10 mg of thiopental sodium/ml/100 g body wt. Both barbitone sodium and thiopental sodium caused an increase in DA, NE and 5-HT content and a decrease in 5-HIAA content in the brain tissue of Arvicanthis niloticus. The increase in the whole brain contents of DA, NE and 5-HT after the administration of barbitone sodium and thiopental sodium may be due either to inhibition of transmitter release by an action at the monoamine nerve terminal or to effects causing a decrease in nerve impulse flow. On the other hand, the decrease in 5-HIAA may be due to the decrease in the turnover of 5-HT.     

Circadian Assessment of Lipids in the Hyperphagic Obese Rat Compared with Lean Litter-Mates

Time and feeding influences on cholesterol, triglyceride, glucose and insulin levels, and serum cholinesterase activity were assessed in a genetically-hyperlipidemic hyperphagic obese rat model, and compared with its lean litter-mate. Following a 28-day acclimation to a 12-hr light/dark cycle, blood samples were obtained every 2 hr from rats via tail bleed for a 24-hr period. Synchronization with other animal studies was established by endogenous serum Cortisol levels [acrophase 18–20 hr after light onset (HALO) in both groups]. Triglycerides cholesterol, insulin and glucose levels were significantly elevated in obese versus lean rats. Obese rats were observed to feed throughout the 24-hr cycle, whereas lean litter-mates ate only during the dark cycle. No circadian rhythmicity was found in glucose levels with either rat group. Insulin levels were not correlated. Although triglyceride levels peaks at 13 HALO in lean rats, no pattern was observed in obese rats. Cholesterol levels were unchanged with time in either group. Cholinesterase activity followed a circadian rhythm in the lean, but not obese, rats with an acrophase estimated at 8 HALO. In contrast to previous reports, enzyme activity was not correlated with triglyceride levels in either rat group. Circadian similarities in insulin levels between rat groups suggest changes in insulin metabolism and/or secretion which are likely to be independent of feeding or activity. Conversely, triglyceride levels remained elevated throughout the 24-hr period in obese rats, whereas significant increases were observed in lean rats during the dark active cycle. These data suggest that triglyceride levels, and not insulin and cholesterol levels, are most likely dependent on feeding patterns.     

Circadian Assessment of Lipids in the Hyperphagic Obese Rat Compared with Lean Litter-Mates

Time and feeding influences on cholesterol, triglyceride, glucose and insulin levels, and serum cholinesterase activity were assessed in a genetically-hyperlipidemic hyperphagic obese rat model, and compared with its lean litter-mate. Following a 28-day acclimation to a 12-hr light/dark cycle, blood samples were obtained every 2 hr from rats via tail bleed for a 24-hr period. Synchronization with other animal studies was established by endogenous serum Cortisol levels [acrophase 18-20 hr after light onset (HALO) in both groups]. Triglycerides cholesterol, insulin and glucose levels were significantly elevated in obese versus lean rats. Obese rats were observed to feed throughout the 24-hr cycle, whereas lean litter-mates ate only during the dark cycle. No circadian rhythmicity was found in glucose levels with either rat group. Insulin levels were not correlated. Although triglyceride levels peaks at 13 HALO in lean rats, no pattern was observed in obese rats. Cholesterol levels were unchanged with time in either group. Cholinesterase activity followed a circadian rhythm in the lean, but not obese, rats with an acrophase estimated at 8 HALO. In contrast to previous reports, enzyme activity was not correlated with triglyceride levels in either rat group. Circadian similarities in insulin levels between rat groups suggest changes in insulin metabolism and/or secretion which are likely to be independent of feeding or activity. Conversely, triglyceride levels remained elevated throughout the 24-hr period in obese rats, whereas significant increases were observed in lean rats during the dark active cycle. These data suggest that triglyceride levels, and not insulin and cholesterol levels, are most likely dependent on feeding patterns.     

Effect of thyroidectomy on rhythmic gonadotropin release.

Nonstress blood samples were obtained from intact and thyroidectomized (TE) male rats at 3-hr intervals over a 24-hr period via rapid decapitation. The animals were thyroidectomized when 40 days old and used 6 weeks later. Intact animals showed periodicity in serum LH (P less than 0.01) and prolactin (P less than 0.01). Both gonadotropins began increasing after 8 PM and peak levels occurred at 11 PM. In contrast, 24-hr periodicity was not observed in serum FSH. Corticosterone levels in these same serum samples showed the expected circadian periodicity. After TE, the 24-hr pattern in all gonadotropins was altered significantly. Serum LH increased (P less than 0.01) and circadian periodicity appeared to be absent. FSH and prolactin levels were increased and decreased, respectively (P less than 0.01), with serum prolactin showing a 9-hr phase shift. Prolactin began increasing at 2 AM and reached a peak at 8 AM. Corticosterone in TE animals showed a 24-hr rhythm similar to that of intact rats. These findings confirm our previous observations that nonstress serum LH and prolactin levels fluctuate with a 24-hr periodicity and suggest that the level of, and the phase angle betweeen, these rhythms is markedly influenced by pituitary-thyroid activity.     

Endocrine effects of chronic intraventricular administration of delta9-tetrahydrocannabinol to prepuberal and adult male rats.

The daily intraventricular administration of Δ⁹-tetrahydrocannabinol (Δ⁹-THC) in microgram amounts for a week to prepuberal and adult rats had definite endocrine effects. Prostate weights were reduced and plasma and pituitary levels of growth hormone (GH) were increased in prepuberal rats. Pituitary levels of prolactin (PRL) were increased both in prepuberal and in adult animals while pituitary and adrenal weights and plasma corticosterone (B) levels were increased in adult rats. On the other hand, brain weights were significantly reduced by Δ⁹-THC in prepuberal and significantly increased in adult animals. No changes in brain levels of noradrenaline (NA), dopamine (DA) or serotonin (5-HT) were found in treated animals. These results indicate that Δ⁹-THC may modify some endocrine functions when injected directly into the brain in microgram amounts. They show on the other hand that young and adult animals may respond differently to the chronic administration of the psychoactive drug, although the difference may be due to a biphasic effect of different doses.     

Dissociation between increased growth hormone and prolactin secretion during the morning hours of early pregnancy in the rat

We investigated whether serum growth hormone (GH) concentration changes in association with the rise in serum prolactin (PRL) concentration known to occur during the early morning hours in the pregnant rat. Animals were kept in a room with the lights on from 0500 to 1900 hours (hr) daily and decapitated for the collection of trunk blood at 2200 or 2400 hr on Day 6 of pregnancy or at 0200, 0400, 0800 or 1000 hr on Day 6 of pregnancy. Serum GH concentration rose more than 4-fold from low levels at 2200 and 2400 hr to higher levels at 0400 and 0800 hr and then declined by 1000 hr. Serum prolactin (PRL) concentration followed a similar pattern except that it returned to low levels earlier, by 0800 hr. Serum luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone concentrations showed no significant changes. Serum GH levels at 0800 hr in pregnant rats were higher than those observed in cyclic rats (13 time periods sampled). The results demonstrate that serum GH concentration is elevated during a circumscribed period in the 6- to 7-day pregnant rat. The time of onset of the rise is similar to that for serum PRL but the elevation in GH levels persists longer than that for PRL.     

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.