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Biology of Reproduction 66, 603-609 (2002)
© 2002 Society for the Study of Reproduction, Inc.

Regular Article

Estradiol Regulation of Luteinizing Hormone Secretion in Heifers of Two Breed Types That Reach Puberty at Different Ages1

H.D. Rodriguesa, J.E. Kinder3,,c, and L.A. Fitzpatrick2,,a,b

a Australian Institute of Tropical Veterinary and Animal Science b Department of Physiology and Pharmacology, James Cook University, Townsville 4811, Australia c Department of Animal Science, University of Nebraska, Lincoln, Nebraska 68583


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The working hypothesis was that 17ß-estradiol (E2) negative feedback on the hypothalamic-pituitary axis in regulation of LH secretion decreases during peripuberty in heifers of 2 different genotypes. We investigated whether Bos indicus heifers had a period postpuberty, as compared with prepuberty, of greater E2 inhibition of LH secretion at a time when heifers of this genotype have been reported to have a period of anestrus. Prepubertal heifers 9 mo of age of 2 genotypes (B. indicus and B. taurus) were assigned to 3 groups (6 animals/group) to either remain intact (control), be ovariectomized, or be ovariectomized and implanted with E2. Variables evaluated from 10 to 28 mo of age were circulating concentrations of progesterone (P4), presence of corpora lutea, and pulsatile pattern of LH release. Results confirmed that B. taurus heifers attained puberty at younger ages (P < 0.001) and at lower live weights (P = 0.015) than did B. indicus heifers (507 ± 37 days of age vs. 678 ± 7 days of age; 259 ± 14 kg vs. 312 ± 11 kg; respectively). There was cessation of E2 inhibition of LH pulses coincident with the onset of puberty in heifers of both breed types but at a much younger age in B. taurus heifers. There was no evidence of enhanced negative feedback of E2 on LH secretion subsequent to puberty in B. indicus heifers nor was there cessation of estrous cycles in control heifers of either breed type after puberty.

estradiol, female reproductive tract, hypothalamus, luteinizing hormone, neuroendocrinology


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Lambs of predominantly Suffolk breeding (reviewed by Foster et al. [1]) and heifers of Bos taurus genotypes (reviewed by Kinder et al. [2]) have been studied to evaluate the effects of 17ß-estradiol (E2) feedback on release of LH during the peripubertal period in domestic ruminants. These studies have revealed that the hypothalamus of these ruminant females is more sensitive to the inhibitory effects of E2 on the release of GnRH [1–3] during the prepubertal period than during the postpubertal period. Secretion of LH is regulated in these species in a manner consistent with the gonadostat theory [4]. During prepuberty, pulses of LH are infrequent (1 or 2 pulses every 6 h). However, as puberty approaches maturation of the hypothalamus results in a decreased negative feedback of E2 on LH secretion, which leads to an increased frequency of LH pulses (approximately 1 pulse every hour) [2, 3].

Cattle of the B. taurus genotype are better adapted to temperate regions of the world than are cattle of the B. indicus genotype [5–7], and the latter are better adapted to tropical regions [8–11]. Physiological differences seem to have resulted from this adaptation [12], e.g., puberty occurs at younger ages in B. taurus than in B. indicus. Postpubertal B. indicus heifers had a greater incidence of anestrus during periods of short day length [13, 14], i.e., during the winter months in northern Australia [15], than during other seasons of the year. However, the effects of season on pre- and postpubertal reproductive function have not been evaluated in heifers of both breed types in the same environment.

The working hypothesis was that E2 negative feedback effects on the hypothalamic-pituitary axis in regulation of LH secretion decrease during peripuberty in heifers of B. indicus and B. taurus genotypes. Because puberty occurs at younger ages in B. taurus heifers (reviewed by Kinder et al. [16]), we hypothesized that there would be decreased negative feedback effects of E2 on LH secretion at younger ages in heifers of this breed type than in heifers of the B. indicus breed. We also investigated whether heifers of the B. indicus genotype had a period of negative feedback effect of E2 on LH secretion postpuberty, as compared with peripuberty, at a time when heifers of this genotype have been previously reported to have a period of anestrus.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Experimental Animals and Location

Thirty-six prepubertal Brahman (3/4 to 7/8 B. indicus genotype, n = 18) and Shorthorn (>90% B. taurus genotype, n = 18) heifers 10 mo of age (born within a 3-mo period, November 1993 to January 1994) were used in the study. Throughout the study (until heifers were 28 mo of age), all heifers shared common grazing of native pasture (black spear grass Heteropogon contortus; northern blue grass Bothriochloa spp.) and improved pasture (buffel grass Cenchrus ciliaris; Indian couch Bothriochloa pertusa; urochloa Urochloa mosambicensis; seca stylo Stylosanthes scabra) at the Lansdown Research Station (19°06'S; 146°08'E) in the dry tropics of northern Australia. Prior to the commencement of the study, all heifers were vaccinated against tick fever (Babesia spp.) and treated for tick control using Cydectin (Cyanamid Aust. Pty. Ltd., Sydney, NSW, Australia; 5 g/L moxidectin) and were fitted with Optimiser insecticidal ear tags (Y-Tex; Flycan Pty. Ltd., Brisbane, QLD, Australia).

Experimental Procedures

The investigations were conducted in accordance with the Guiding Principles of the Care and Use of Research Animals issued by the Society for the Study of Reproduction and were approved by the Institutional Animal Care and Use Committee of James Cook University. Heifers were ranked based on live weight and were randomly assigned by weight to 1 of 3 groups (6 animals/group) within each genotype to remain intact (CONT), be ovariectomized (OVX), or be ovariectomized and implanted with E2 (OVXE). A polydimethylsiloxane implant (Silastic; Dow-Corning, Midland, MI; 3.35 mm inside diameter x 4.65 mm outside diameter x 270 mm long) filled with E2 (Sigma Chemical Co., St. Louis, MO) [17] was inserted s.c. caudal to the left shoulder blade of each animal between the third and fourth intercostal space. Ovariectomy (at 9 mo of age) of heifers using standard surgical procedures was performed through the left paralumbar fossa under local anesthesia [18]. Heifers in the OVXE groups received the E2 implants at the time of ovariectomy.

Blood Sampling

Jugular blood was collected from all heifers at monthly intervals from 6 to 9 mo of age and then weekly from 10 to 30 mo of age, the latter interval corresponding to the interval when puberty was expected to occur in heifers of both genotypes. Blood (10 ml) was collected into sterile evacuated glass tubes containing 143 000 units of lithium heparin (Vacutainer; Becton-Dickinson, Rutherford, NJ). Samples were stored on ice before being centrifuged. Plasma was then separated for subsequent assay for progesterone (P4) and E2 by RIA.

Blood sampling for the assessment of the pulsatile nature of LH secretion was facilitated by aseptic insertion of indwelling jugular polyethylene catheters (medical grade vinyl tubing; Dural Plastics, Auburn, NSW, Australia; 1 mm inside diameter x 2 mm outside diameter) approximately 30 min before the first sample collection. Heifers were individually placed in stanchions during periods of serial sampling, with water and hay provided ad libitum. Blood samples were collected through remote catheter lines (medical grade vinyl tubing; Dural Plastics; 2 mm inside diameter x 3 mm outside diameter) connected to the i.v. (jugular) catheters to minimize disturbance of animals.

Two volumes of the sampling catheter were discarded before the withdrawal of a 10-ml blood sample, and the catheter was flushed with heparinized saline solution (12.5 IU/ml sodium heparin in 0.9% saline) immediately following sample collection. Blood was collected into 10-ml glass tubes containing a small quantity of Kwik-Spin granules (Disposable Products, Archerfield, QLD, Australia) to which 100 µl of 12.5 IU/ml sodium heparin in 0.9% saline was added.

Serial blood samples were collected once every 2 mo throughout the study. On each sampling day, blood was collected every 15 min for 8 h from each heifer of each genotype in the OVX and OVXE groups. Samples were placed on ice, and the plasma was separated by centrifugation, decanted into 10-ml vials, and stored at -20°C for subsequent hormone analysis. After completion of each serial sampling period, catheters were removed and heifers were treated with long-acting antibiotics (oxytetracycline hydrochloride, 100 mg/ml) before being placed back in the paddocks.

Live Weight Measurements

The animals were weighed at monthly intervals from 6 to 9 mo of age and then weekly from 10 to 30 mo of age, the latter interval corresponding to the time interval when puberty was expected to occur in heifers of both genotypes.

Hormone Assays

Concentrations of P4 in plasma were measured by RIA as previously described [19]. The sensitivity of the P4 assay (90% zero binding) was 0.04 ng/ml, and quality control (1 ng/ml) intra- and interassay coefficients of variation were 7.1% and 11.4%, respectively. Concentrations of E2 in plasma were determined by RIA, following ether extraction, by one of us (J.E.K.) using a method that has been previously described [20]. Sensitivity of the E2 assay was 0.02 pg/ml, and intra- and interassay coefficients of variation were 8.1% and 12.4%, respectively. Plasma concentrations of LH were measured by double-antibody RIA, using purified bovine LH standards (USDA bLH-B-6) and a modification of the method described by Niswender et al. [21], and results were validated in our laboratory by the method of Rhodes et al. [22]. The sensitivity of the LH assay (90% zero binding) was 0.21 ng/ml, and intra- and interassay coefficients of variation were 13.5% and 16.5%, respectively.

Determination of Age at Puberty

Puberty has been defined as the first behavioral estrus accompanied by the development of a corpus luteum that is maintained for a period characteristic of a particular species [23]. In this study, age at puberty was taken to be the age of the animal on the day that a corpus luteum was first detected by transrectal ultrasonography (Aloka 210DX, 7.5 MHZ probe; Veterinary Medical and Surgical Supplies, Newcastle, NSW, Australia), confirmed to be functional by concentrations of P4 in plasma of >1 ng/ml [24], and maintained for at least 2 successive examinations.

Statistical Analyses

All data are presented as mean ± SEM unless otherwise stated. Differences in age and weight at puberty among heifers of the 2 breed types were analyzed using the general procedure in SPSS for Windows [25] for multivariate ANOVA (MANOVA). Mean concentration, pulse frequency, and pulse amplitude for LH measured in the serial blood samples were determined using Pulster [26] a pulse-analysis computer program. Repeated measures ANOVAs [25] were used to evaluate changes in LH pulse frequency across time (from 10 to 28 mo of age) for each genotype. Differences between groups for each genotype were analyzed using a one-way ANOVA, least significant difference test, or t-test.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Age and Weight at Puberty

Puberty occurred in B. taurus heifers at a younger age (P < 0.001) and lower weight (P < 0.015) than in B. indicus heifers (507 ± 37 days of age vs. 678 ± 7 days of age, and 259 ± 14 kg vs. 312 ± 11 kg, respectively; Table 1).


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  		TABLE 1. Age and weight at puberty for Bos indicus and B. taurus heifers

Changes in Weight During the Study

Data for changes in weight of B. indicus and B. taurus heifers over time are presented in Figure 1, a and b, respectively. Growth patterns were typical for heifers of B. indicus and B. taurus genotypes in the dry tropical region in which the cattle were located during the study. In heifers of both genotypes, there was an initial period of rapid growth (14–16 mo of age), which plateaued at 18 mo of age (corresponding to seasonally more harsh grazing conditions), and another period of rapid weight gain (corresponding to optimum grazing conditions) around the time of puberty. However, unlike B. indicus genotype heifers, B. taurus heifers attained puberty during the period of initial weight gain, at 16–18 mo of age. There were no differences in growth rates of heifers among treatments.



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  		FIG. 1. Live weights (kg; mean ± SEM) of heifers in the CONT, OVX, or OVXE groups for B. indicus (a) and B. taurus (b) genotypes, from 10 to 28 mo of age

Mean LH Concentrations

Data for mean concentrations of LH for B. indicus and B. taurus heifers throughout the study are summarized in Figure 2, a and b, respectively. In heifers of both genotypes, concentrations of LH were greater at 10 mo of age (P < 0.05) in heifers of the OVX group, in contrast to the lower concentrations in heifers of the same age in the OVXE group. In B. indicus heifers, this relationship continued until 22 mo of age, at which time the concentrations of LH in the OVX and OVXE groups were similar, coinciding with the time of attainment of puberty in heifers of the CONT group. In B. taurus heifers, the relationship of greater concentrations of LH of heifers in the OVX than in those of the OVXE group (P < 0.05) continued only until 14 mo of age (compared with 22 mo of age in B. indicus heifers), at which time the concentrations of LH increased in heifers from the OVXE group until they were either equal to or greater than (P < 0.05) those of heifers in the OVX group.



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  		FIG. 2. Concentrations of LH (mean ± SEM) in plasma of heifers in the OVX and OVXE groups of B. indicus (a) and B. taurus (b) genotypes from 10 to 28 mo of age. Heifers were ovariectomized (OVX and OVXE) and received E2 (OVXE) at 9 mo of age

Frequency of LH Pulses

In B. indicus and B. taurus heifers, frequencies of LH pulses at 10 mo of age were lower (P < 0.001) in heifers in the OVXE group than those of heifers in the OVX group (Fig. 3, a and b). In B. indicus heifers, the relationship of fewer LH pulses in heifers of the OVXE than in heifers of the OVX group continued until 22 mo of age, at which time the frequency of release of LH pulses increased to the extent that frequencies were similar in the OVX and OVXE groups. The age of heifers at which the frequency of LH pulses increased in the OVXE groups to the extent that they were similar to those of the OVX groups coincided with the age of puberty of heifers in the CONT group for both genotypes (Fig. 3, a and b).



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  		FIG. 3. Frequency of LH pulses (mean ± SEM) of heifers in the OVX and OVXE groups for B. indicus (a) and B. taurus (b) genotypes from 10 to 28 mo of age

Amplitude of LH Pulses

In both B. indicus and B. taurus heifers, amplitude of LH pulses did not differ among heifers in the OVX and OVXE groups at 10 mo of age or at 22 or 16 mo of age, the time of puberty in heifers of the CONT groups for B. indicus and B. taurus heifers, respectively (Fig. 4, a and b).



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  		FIG. 4. Amplitude of LH pulses (mean ± SEM) of heifers in the OVX and OVXE groups for B. indicus (a) and B. taurus (b) genotypes from 10 to 28 mo of age

Secretion of LH and Cyclic Luteal Function During the Postpubertal Period

Heifers in the CONT groups of both genotypes continued to have patterns of cyclic luteal function at regular intervals during the postpubertal phase of the study, as indicated by transrectal ultrasonographic identification of corpora lutea and concentrations of P4. In addition, no significant changes were observed in pulsatile secretion of LH in heifers in the OVXE or OVX groups during the period corresponding to postpuberty of age-matched heifers of the CONT group. During the period that corresponded to prepuberty in age-matched heifers in the CONT group, heifers in the OVX group had higher mean concentrations of LH throughout the serial sampling period (P < 0.05) than did heifers in the OVXE group (Figs. 5a and 6a). During the period that corresponded to postpuberty in age-matched heifers of the CONT group, mean concentrations of LH in heifers of the OVX and OVXE groups were similar (Figs. 5b and 6b).



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  		FIG. 5. Concentrations of LH (mean ± SEM) in plasma of prepubertal heifers (11 mo of age) (a) and postpubertal heifers (22 mo of age) (b) of the B. indicus genotype in OVX and OVXE groups during the periods of serial blood sampling (every 15 min for 8 h)

Plasma E2 Concentrations

Concentrations of E2 in plasma of CONT, OVX, and OVXE heifers of both genotypes from 12 to 23 mo of age are presented in Figure 7, a and b, for B. indicus and B. taurus heifers, respectively. The E2 implant in the OVXE heifers resulted in those animals having plasma E2 concentrations from 12 to 23 mo of age similar to those found in postpubertal intact (CONT) heifers.



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  		FIG. 7. Concentrations of E2 (mean ± SEM) in plasma of CONT, OVX, and OVXE heifers from 12 to 23 mo of age for B. indicus (a) and B. taurus (b) genotypes. Mean age of puberty is indicated ({downarrow})


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The results of this study confirm that genotype influences age and weight at puberty in cattle, with B. taurus attaining puberty at much younger ages than B. indicus, even when heifers of both genotypes were grazed on the same pasture in the dry tropics. The greater age at puberty of the B. indicus heifers has been attributed to adaptation of these cattle to the tropical regions of the world [27], suggesting that B. taurus heifers, if reared in tropical regions may be unable to express the genes that allow for puberty at younger ages.

Nutritional and environmental stresses of tropical regions could inhibit expression of the genes that initiate pubertal processes at younger ages in B. taurus than in B. indicus when these cattle are raised in temperate regions. Puberty, however, occurred at younger ages in B. taurus than in B. indicus in the present study because puberty occurred in heifers of the B. taurus genotype at lower weights compared with B. indicus heifers. This finding suggests that B. indicus heifers are more sensitive to the conditions typical of the dry tropics (e.g., type of forage) than are B. taurus heifers [28], and therefore, puberty occurs at younger ages in heifers of the B. taurus genotype than in B. indicus heifers. It also indicates that B. taurus heifers do not have delayed onset of puberty to the extent that B. indicus heifers do when they are raised in tropical regions of the world. Furthermore, in a previous study Shorthorn (B. taurus) cows conceived at lower weights than did Brahman (B. indicus) cows [29], suggesting that puberty occurred in these females at lower weights.

In the present study, we focused on the underlying endocrine mechanism(s) that result in puberty occurring later in B. indicus than in B. taurus heifers. During the period when age-matched heifers in the CONT group were prepubertal, E2 had a strong inhibitory influence on LH secretion, as evidenced by higher circulating concentrations of LH and higher frequency of LH pulses in heifers in the OVX group than in those in the OVXE group.

Coincident with the onset of puberty in age-matched heifers in the CONT group, there was a marked decrease in the inhibitory influence of E2 on release of LH pulses in heifers of both breed types. This finding is in agreement with results of previous research [17], where there was a significant increase in frequency of LH pulses after ovariectomy of prepubertal B. taurus heifers, and the acute response of LH secretion to ovariectomy was inhibited by treatment with E2 until the time of puberty in age-matched control heifers. This release from the inhibitory influence of E2 was reflected more in the frequency of LH pulses than in mean plasma concentrations of LH, in agreement with previous findings of Day et al. [30], who concluded that an increase in frequency of LH pulses was a better predictor of imminent puberty that mean concentrations of LH in plasma.

These data supported the gonadostat theory [4] of attainment of puberty in heifers of B. indicus genotype similar to that previously reported for heifers of B. taurus genotype [31]. Consistent with these findings, LH concentrations were elevated in prepubertal heifers of B. taurus genotype actively immunized against E2 [32]. The results of the present study confirm that the same endocrine regulation of pubertal processes is present in heifers of both genotypes, but puberty occurred later in heifers of the B. indicus genotype compared with B. taurus heifers; therefore, there is a significant difference in the age at which the endocrine processes involved in regulation of puberty were activated.

There were no apparent seasonal effects on the continuance of estrous cycles during the postpubertal period in the present study nor were there indications of increased E2 inhibition of LH secretion during specific periods subsequent to puberty. In agreement with results of the present study, Brahman (B. indicus) and Hereford-Shorthorn (B. taurus) cows (all pastured in paddocks with high-quality forage) had continued cyclic luteal function throughout the year in a study conducted in a dry tropical region similar to that in which heifers were located in the present study [33]. Only 20–30% of the cows of B. taurus genotype in paddocks with relatively poor quality forage (mimicking the subtropical dry season) had a continuance of cyclic luteal function, indicating that most of these cows had a cessation of estrous cycles during the period of the year when nutritional intake was limited. Results from this previous study suggested that seasonal variation in nutrient availability affected reproductive function in B. taurus but not B. indicus females when nutrient availability during the dry season of the year was limited. This finding differs from those of previous reports, which indicated that heifers of B. indicus genotype are more sensitive to seasonal influences than are those of B. taurus genotype, with B. indicus heifers showing an increased incidence of anestrus during periods of short day length [13–15]. Heifers in the present study were grazed on high-quality forage throughout the study; therefore, any effects of limited nutritional intake during the dry seasons of the year in the dry tropical regions may not have been expressed in the present study.

In addition, no obvious changes were observed in the pulsatile secretion of LH in heifers of the OVXE group during the period that corresponded to postpuberty in age-matched heifers of the CONT group, suggesting that seasonal effects on LH secretion did not occur in the heifers in the present study. This finding differs from those of another study in which ovariectomized Hereford-Shorthorn cows pastured in a subtropical region showed a distinct suppression of plasma LH, which was more pronounced in cows on poor quality pasture during the dry season [34].

The results of a series of studies indicate that treatment of ovariectomized female cattle with E2 during periods when age-matched females were prepubertal resulted in higher mean concentrations of LH in ovariectomized cows treated with E2 than in ovariectomized cows not treated with E2 (reviewed by Kinder et al. [35]). The higher mean concentrations of LH in ovariectomized cows treated with E2 compared with ovariectomized cows not treated with E2 result from an increase in amplitude of LH pulses. In the present study, mean concentrations of LH of heifers in the OVXE group remained below those of the heifers in the OVX group during the period when heifers of the age-matched CONT group were postpuberty. The inconsistency between results of the present study in this regard and those of previous studies may have resulted from continued inhibition of LH secretion by E2 during the initial postpubertal period. If comparisons of mean circulating concentrations of LH had been made later in life, we expect concentrations of LH would have been higher in heifers in the OVXE group than in those of the OVX group. The higher amplitude of LH pulses, which increases mean circulating concentrations of LH in ovariectomized female cattle treated with E2 as compared with ovariectomized female cattle not treated with E2 [3, 16, 36, 37], likely occurs because of increased sensitivity of pituitary gonadotropes to GnRH as a result of increased numbers of receptors for GnRH induced by E2.

Seasonal effects were not observed in the present study probably because seasonally favorable conditions provided adequate nutritional support for heifers during their pre- and postpubertal phases of development. The effects of photoperiod on reproductive function may be confounded with seasonal variation in nutrient availability [23]; therefore, because adequate feed was available during the postpubertal phase of the present study, cessation of cyclic luteal function did not occur and consequently there was a continuance of estrous cycles. Had dietary intake been limited to a greater extent during the postpubertal period, cessation of estrous cycles may have been observed, as has been previously reported to occur subsequent to puberty [13].

In this study, we demonstrated that among factors that affect age and weight of heifers at puberty, breed type has a greater influence than was anticipated when heifers were located in the dry tropical region of northern Australia. This factor can be exploited by using B. indicus x B. taurus crosses for beef production, if a reduction in age at puberty can be achieved by cattle producers and the resistance to stressors that are present in the dry tropics can be maintained by using these 2 breed types. We accept our working hypothesis that E2 negative feedback effects on the hypothalamic-pituitary axis in regulation of LH secretion decreases during peripuberty in heifers of the 2 breed types evaluated in the present study. We also accept our hypothesis that there is a decreased negative feedback effect of E2 on LH secretion at a younger age in heifers of the B. taurus than in heifers of the B. indicus type. There was no anestrual period in heifers of B. indicus type nor was there a period of greater E2 feedback effect on LH secretion postpuberty in heifers of B. indicus genotype during a period when heifers of this genotype have been reported to have an anestrual period.



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  		FIG. 6. Concentrations of LH (mean ± SEM) in plasma of prepubertal heifers (11 mo of age) (a) and postpubertal heifers (17 mo of age) (b) of the B. taurus genotype for OVX and OVXE groups during the periods of serial blood sampling (every 15 min for 8 h)


   ACKNOWLEDGMENTS
 
We thank C. Coleman, P. Finlay, J. Palpratt, and R. Dixon for their technical assistance.


   FOOTNOTES
 
First decision: 17 December 1998.

1 This work was supported by the Australian Meat Research Corporation. Back

2 Correspondence. FAX: 61 7 47791526; lee.fitzpatrick{at}jcu.edu.au Back

3 Current address: Department of Animal Science, Ohio State University, Columbus, OH 43210 Back

Accepted: October 9, 2001.

Received: November 20, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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