Psychoneuroendocrtnology, Vol. 4t pp. 29 to 35. 0306-4530/79/0101-0029502.00/0 © Pergamon Press Ltd. 1979. Ptanted in Great Britain.

L-TRYPTOPHAN TREATMENT A N D THE EPISODIC SECRETION OF PITUITARY HORMONES A N D CORTISOL

MARKKU T. HYYPPA, TAPANI JOLMA, JUHA LIIRA, VIVI-ANN L~NGV1K and OssI KYTOM.~KI The Third Clinical Institute and Institute of Biomedicine, Department of Neurology,

University of Turku, SF-20520 Turku 52, Finland

(Received 25 May 1978, revised 2 October 1978)

SUMMARY

(1) L-Tryptophan, the natural precursor of brain serotonin, was administered to healthy subjects. Plasma free and total tryptophan, somatotropin (growth hormone, GH), follitropin (FSH), lutropin (LH), prolactin and cortisol were analysed after the oral administration of 2 g and 100 mg/kg of L-tryptophan or after 1"28 g of L-leucine, at 08:30 and 11:30. (2) Plasma levels of free and total tryptophan were markedly increased after the oral administration of L-tryptophan. (3) Plasma somatotropin levels were significantly elevated after L-tryptophan treatment at different times of day, but this elevation was not dose-dependent. (4) L-Trypto- phan or L-leucine treatment did not affect the pulsatile secretion of follitropin and lutropin. (5) Plasma prolactin was not significantly elevated after the oral administration of L-trypto- phan. (6) The morning decline of plasma cortisol was significant with or without L-tryptophan. However, no decline was noticed after L-tryptophan in the middle of day.

Key wotda--L-tryptophan; pituitary hormones; cortisol; episodic secretion.

INTRODUCTION

PREVIOUS studies have shown that either the infusion or the oral administration of L- tryptophan, the serotonin precursor, caused significant changes in the secretion of some pituitary hormones (McIndoe & Turkington, 1973; Miiller, Brambilla, Gavagnini, Peracchi & Panerai, 1974; Hyypp/i, Falck, G/ivert, Kyt6m/iki, Rautakorpi & Syv/ilahti, 1976; Hyypp/i, Jolma, L/tngvik, Kyt6m/iki & Syv/ilahti, 1977) and of cortisol (Woolf & Lee, 1977). These effects have been connected with the serotoninergic transmission mediating

• neuroendocrine functions (Kordon & Glowinski, 1972; Kamberi, 1973; De Wied & De Jong, 1974). Neuroendocrine regulation of pituitary hormones and cortisol is rhythmi- cally controlled via the central nervous system in human subjects (Weitzm n, Boyar, Kapen & Hellman, 1975). It is difficult to study the effects of changed brain serotoninergic tone on the rhythmic hormone secretion. Recent results, (Faber, Hagen, Kirkegaard, Birk Lauridsen & Moiler, 1977; Wieb¢, Handwerger & Hammond, 1977) did not confirm the earlier findings of MacIndoe & Turkington (1973) who found a marked increase in serum prolactin after i.v. administration of L-tryptophan. On the other hand, the decline of plasma cortisol due to L-tryptophan treatment (Woolf & Lee, 1977) was not found in our study (Hyypp/i et aL, 1977). In order to elucidate the effects of increased brain serotonin tone on the episodic and pulsatile secretion of pituitary hormones and cortisol, we chose two times of day as well as two different doses for oral L-tryptophan administration.

29

30 MARKKU T. HYYPPA, TAPAN1 JOLMA, JUHA LIIRA, V1vI-ANN L.g, NGVIK and OssI KYT(3MhK[

L-Leucine was given as a con t ro l a m i n o ac id a s suming tha t b o t h a m i n o acids use s imi lar

ab so rp t i on mechan i sms . [It has been shown, however , tha t L-leucine c o m p e t e s wi th L-

t r y p t o p h a n fo r u p t a k e in to the b ra in (Kie ly & Sourkes , 1972; F e r n s t r o m & W u r t m a n , 1972)

and it can inh ib i t t r y p t o p h a n t r a n s p o r t across the b l o o d - b r a i n bar r i e r ( Y o u n g & Sourkes ,

1977.] T h e s tudy was p e r f o r m e d on n o r m a l , hea l thy subjects .

METHODS

Subjects Five volunteers--4 healthy men (aged 24-42 yr) and 1 woman (26 yr)--were chosen from the research

group of the Department of Neurology, University of Turku, Finland. They were fully informed about the nature of the study and all were free of medical problems and medication of any kind. Six healthy subjects served as controls for concentrations of endogenous plasma tryptophan during testing hours.

Procedure Experiments were carried out at 08:30 or 11:30. Tests wcre repeated with 2 different doses of L-tryptophan

and L-leucine, in a randomized manner, at intervals of several days. An intravenous catheter was inserted in a forearm vein 45 min before the administration of the test solution, the catheter being kept open with a slow infusion of 200 ml 0.85 % NaCI. Experiments were performed after a 12-hr fast and 1 hr of bed rest at the in-patient department of the clinic. L-Tryptophan (0.5 g) or L-leucine (0.32 g) (Leiras, Finland) were administered orally in 4 capsules with a glass of tap water, or 100 mg/kg of L-tryptophan was given orally in suspension. The fixed doses of L-tryptophan and L-leucine were 31 ± 2 and 20 ~ 1 mg/kg respectively, when calculated per body weight for 5 subjects (mean ± S.E.). Likewise, the second dose of L-tryptophan, 100 mg/kg was 6.5 ~ 0.4 g calculated as a fixed dose for 5 subjects (mean J= S.E.). Six milliliters of blood were withdrawn at 15-30 rain intervals for 150 min into heparinized glass tubes. Plasma was immediately separated and stored at -20°C until assay.

Plasma free tryptophan was separated from albumin-bound tryptophan (Knott & Curzon, 1972). Both free and total plasma tryptophan were determined (Hyypp/i, Cardinali, Baumgarten & Wurtman, 1973). Immunoreactive levels of plasma follitropin and lutropin were assayed by a double-antibody solid phase technique (Den Hollander, Shuurs & Van Hell, 1972). Radioimmunoassay for plasma somatotropin was performed by the method of Wid6 (1969). Purified gonadotropins and somatotropin, their antibodies and reference standards were donated by N.P.A., N.I.H., Betbesda, MD, U.S.A. Prolactin was assayed by radioimmunoassay kit from CIS (CEA-IRE-SOR1N, Gif-sur-Yvctte, Fleurs, Saluggia, EEC). A fluori- metric method was used for plasma total cortisol (Nordstr6m & Peltola, 1974). All samples from the subject were run in duplicate in the same assay. The intra-assay and interassay coefficients of variation and minimum sensitivity for hormones are given in Table 1.

TABLE l. SPECIFICATIONS ]FOR HORMONE ASSAYS

Coefficients of variation (%) Minimum sensitivity

Intra-assay Inter-assay

Cortisol - - 7.1 - - Follitropin 2.6 14.7 1.5 i.u./l Lutropin 4"7 17.0 I "5 i.u./1 Somatotropin 2.4 5.5 0.5 ng/ml Prolactin 3'6 12" 1 1.0 ng/ml

For statistical analysis of the results the Mann-Whitney (M-W) test was calculated between baseline and treatment values of hormones after the loading with amino acids. Since the measured maximal increase in somatotropin varies between subjects as well as does the time at which the maximum takes place, the increase was also estimated planimetrically and given in area units (one unit = 1 ng of GH/60 min). Two-way analysis of variance (ANOVA) was used to test the differences in plasma cortisol values between different doses and times after the loading. Student's t-test was utilized for other statistical calculations when the normal distribution was confirmed.

L-TRYPTOPHAN, PITUITARY HORMONES AND CORTISOL 31

RESULTS

A f t e r the o ra l l oad ing w i t h L - t r y p t o p h a n , p l a s m a c o n c e n t r a t i o n s o f free a n d to ta l

t r y p t o p h a n inc reased m a r k e d l y (Tab le II) . T h e ra t io b e t w e e n free a n d to ta l t r y p t o p h a n

TABLE I1. MEAN PLASMA LEVELS OF TRYPTOPHAN AFTER L-TRYPTOPHAN TREATMENT IN 4 SUBJECTS

100 mg/kg of L-tryptophan 2 g of L-tryptophan 1"28 g of L-leucine

a b c a b c a b c

Time (rain) - 3 0 8.5 0.7 8 6.7 1.2 18 6.0 1.1 18 - 1 5 8.7 0.7 8 7.1 0.9 13 6.2 0.9 15 4- 0 9-3 1.1 12 7.0 1.5 21 6.7 0.8 12 +30 28-7 7.9 28 18.0 3.1 17 6.0 0.9 15 4-60 41.8 12-4 30 19.5 2.9 15 5.3 1.4 26 +90 59.4 20.0 34 27.1 5-6 21 5.6 1.1 20 +120 69.0 17.7 26 28.5 6-4 22 5.1 0.9 18

a = plasma total tryptophan (g/l). b = plasma free tryptophan (g/l). c = ratio ( ~ ) between free and total tryptophan.

inc reased m o s t a f t e r 100 m g / k g o f L - t ryp tophan , whi le L-leucine t r e a t m e n t d id n o t signifi-

can t ly a l te r p l a s m a t r y p t o p h a n levels. N o e n d o g e n o u s v a r i a t i o n o f p l a s m a t r y p t o p h a n

levels was seen b e t w e e n 08 :00 a n d 11:30 in 6 hea l t hy subjects (pa i red t-test). B o t h t r y p t o p h a n

doses inc reased p l a s m a s o m a t o t r o p i n levels s ignif icant ly a t b o t h t imes o f day. T h e effect

TABLE III . MAXIMAL BASE-LINE AND MAXIMAL TREATMENT VALUES OF PLASMA SOMATOTROPIN ( n g / m ] 4- S .E . ) DURING L-TRYPTOPHAN LOADING

Dose and treatment Maximal base-line Maximal treatment

2 g of L-tryptopban (5) 6"0 4- 0"8* 12-4 4- 2'5t 100 mg/kg of L-tryptophan (4) 3'1 4- 0"7 12"5 4- 7"2:~ 2 g of leucine (3) 3.6 4- 0.9 3.4 4- 1.0

Two grams of L-tryptophan were given at 08:30 and 100 mg/kg of L-tryptophan and 1-28 g of L-leucine were given at 11:30. For maximal base-line value plasma samples were collected at - 30, - 15 and 4-0 min before the administration of amino acids, and the mean of maximal levels was used. Thereafter samples were collec- ted at +30, +60, 4-90, and +120 min. Number of subjects is given in brackets.

Statistically significant difference was found in base-line levels between 8 and 11 a.m. (* p <0.05) and between maximal base-line and treatment independent of times of day (t P <0.05); (~: p <0.02) (M-W- test).

TABLE IV. BASE-LINE, MAXIMAL AND MINIMAL TREATMENT VALUES or PLASMA FOLLITROPIN (FSH) AND LUTROPIN (LH) (i.u./l 4- S.E.) DURING THE L-TRYPTOPHAN LOADING

Dose and treatment Base-line Maximal Minimal

2 g of L-tryptophan (5)

100 mg/kg of L-tryptophan (4)

1 '28 g of L-leucine (3)

FSH 7"9 ± 0.7 8.9 4- 0.9 5"8 ± 0-9 LH 6.0 ± 1-2 8.2 ± 5.6 4"2 4- 2"1 FSH 6"7 -4- 0"8 7.2 4- 1"0 5"9 -4- 0.9 LH 4.5 4- 1-0 6.4 4- 2-5 4-1 4- 2'5 FSH 7.8 4- 1"1 8.5 4- 1"8 7.1 4- 1.4 LH 6"7 4- 1.4 8.6 ± 3-9 4"5 4- 2'1

Two grams of L-tryptophan were given at 08:30, and 100 mg/kg of L-tryptophan and 1.28 g of L-leucine were given at 11:30. For base-line value plasma samples were collected at - 30, - 15 and 4-0 min before the administration of amino acids and thereafter at +30, +60, +90, and +120 rain. Number of subjects is given in brackets. No significant variations were seen.

32 MARKKU T. HYYPP.~, TAPAN1 JOLMA, JUHA LIIRA, VIvI-ANN L,~NGVIK an d OssI KYTOM.~KI

TABLE V. MAXIMAL BASE-LINE AND MAXIMAL TREATMENT VALUES OF PLASMA PROLACTIN (ng/ml ± S.E.) DURING THE L-TRYPTOPHAN LOADING

Dose and treatment Maximal base-line Maximal treatment 2 g of L-tryptophan (5) 13'5 d: 2"5 10"0 % 3"0 100 mg/kg of L-tryptophan (4) 4'5 ± 0"3 6"0 ~ 1"1" 1"28 g of L-leucine (3) 11'4 ± 3"5 11"8 ~ 4"1

For legend, see Table IV. * An almost significant difference (p ~< 0.05; M-W-test).

TABLE VI. EFFECT OF L-TRYPTOPHAN LOADING ON PLASMA CORTISOL (nmole/l :~: S.E.) LEVELS AT TWO DIFFERENT TIMES OF DAY

Dose and treatment

2 g of L-tryptophan at 08:30 (3) 100 mg/kg of L-tryptophan at 11:30 (4) 1"28 g of L-leucine at 11:30 (3)

Time (min) 0 30 60 90 120

432 ±46* 363 ± 30 303 ± 2 6 293 2:38 252± 30t 313 ±465 242 ± 2 3 252 ± 35 294± 18 2 8 4 ± 2 3 358 5:59 356 ± 39 323 d: 33 317 ± 2 0 295 ± 15

0-value is averaged from -30, -15 and ±0 values before treatment. * A significant difference was found between 08:00 and 11:00 (p <0-05; M-W-test). t A significant decrease of plasma cortisol (p <0"01 ; M-W-test) was found. Two-way ANOVA did not

show any difference between times or treatments, A significant difference between the two times of day was reached at the 90 ~ confidence limit, if the

natural decline of plasma cortisol, i.e. at - 30, - 15 and ±0 min before treatment was included in the two-way ANOVA.

was transient, having a peak at 60 min. L-Leucine had no effect on plasma somatotropin levels (Table IlI). When expressed as area units, the responses to different doses of L- t ryptophan did not show significant differences between 2 g (31 i 2 mg/kg) or 100 mg/kg. Respective values in 5 subjects were 8.3 ± 3.3 and 12.0 ~_ 8.0 area unit /hr (mean ± S.E; M-W-test) . The individual pulsatile secretion o f gonadotropins was found after different treatments. No statistical differences were noticed between base-line and treatment values (Table IV). Plasma prolactin levels were not changed after 2 g (31 ~ 2 mg/kg) o f L- t ryptophan or 1.28 g (20 + 1 mg/kg) o f L-leucine. An almost significant increase o f plasma prolactin was seen (p ~< 0.05; M-W-tes t ) after 100 mg/kg of L-tryptophan (Table V).

Evaluation o f the presence o f plasma curt±sol rhythm was carried out. Without any treatment plasma curt±sol showed higher base-line values at 08:00 than at 11:00 (p <0.05; M-W-test) . L-Tryptophan treatment at 08:30 seemed to reduce plasma curt±sol levels, but this decline was not found at 11:30, even though the higher dose o f L-tryptophan (100 mg/ kg) was used (Table VI). (L-Leucine was also given at 08:30 to 2 subjects, and again the morning decline o f plasma curt±sol was noted). When the natural morning decline o f plasma curt±sol was included in the two-way A N O V A , a significant difference was reached between the two times o f day at the 95 ~ confidence limit showing a decline o f plasma curt±sol after 08:00, i.e. even before the initiation of the treatment (Table VI).

DISCUSSION

L-Tryptophan loading accelerates brain serotonin turn-over in human subjects (Eccleston, Ashcroft. Crawford, Stanton, W o o d & McTurk, 1970; Young & Sourkes, 1977). The free

L-TRYPTOPHAN, ~ A R Y HO~O~.S ~ COR'rISOL 33

portion of plasma tryptophan is important for brain serotonin synthesis (Knott & Curzon, 1972; Young & Sourkes, 1977). In the present study plasma free tryptophan was markedly increased after the oral administration of L-tryptophan, and therefore it could be expected that brain serotonin turn-over also would be accelerated. An equimolar dose of L-leucine did not change plasma free tryptophan levels. However, it may be possible that L-leucine inhibits the transport of tryptophan across the blood-brain barrier (Young & Sourkes, 1977). Hence, its use as a control for r-tryptophan treatment may not be theoretically adequate without further studies of its effect in rive on tryptophan transport into brain.

In the present study doses of either 2 g (31 4- 2 mg/kg) or 100 mg/kg of L-tryptophan each produced similar responses in somatotropin secretion, and the rise of plasma somatotropin was rather constant in every individual at different times of day, although the peak showed individual time variations. The base-line level of plasma somatotropin was somewhat higher at 08:00 than at 11:00. These results are in agreement with our earlier findings on neurologic control patients (Hyypp/i et al., 1977).

Plasma follitropin and lutropin showed no response to L-tryptophan loading, which is contrary to an earlier investigation (Maclndoe & Turkington, 1973), but in agreement with our previous findings (Hyypp/i et aL, 1977). Different doses of L-tryptophan were used in these studies. High doses of L-tryptophan may alter various other neurotransmission mechanisms (Smith, Lane, Shea & McBride, 1977), which can lead to the decline of gonado- tropin levels. MacIndoe & Turkington, (1973) injected 10 g of L-tryptophan over a 20 min period while in the present study the maximal oral dose was only 6.5 4- 0.4 g (mean -b S.E.). Normal pulsatile secretion patterns of these hormones were present during L-tryptophan as well as during L-leucine treatment.

Only the higher dose of L-tryptophan (100 mg/kg) was just able to elevate plasma prolac- tin levels. This is in disagreement with earlier observations (Maclndoe & Turkington, 1973; Woolf & Lee, 1977). We also report here a lower dose of L-tryptophan than was used in earlier investigations. In agreement with our results, the long-term oral administration of I.-tryptophan (6 g daily) did not elevate serum prolactin levels (Faber et aL, 1977). An acute L-tryptophan load (90 mg/kg) had no effect on prolactin secretion (Wiebe et al., 1977).

A recent study on the effects of L-tryptophan loading on plasma corticotropin and cortisol concentrations suggested that L-tryptophan has an immediate inhibitory effect on cortisol secretion in human subjects (Woolf & Lee, 1977). Since this loading test and our earlier study (Hyypp/i et aL, 1977) were performed during the morning hours when the natural decline of plasma cortisol takes place (Weitzman et al., 1975), we decided to study the effects of oral L-tryptophan at two different times of day (08:30 and 11:30). Tests were performed in a cross-over manner to avoid the possible 'first' experiment effect on cortisol secretion (Sabshin, Hamburg, Grinker, Persky, Basowitz, Korchin & Chevalier, 1957). Evaluation of the presence of a plasma cortisol rhythm before treatment indicated higher base-line values at 08:00. After the administration of the lower dose of L-tryptophan (2 g) and L-leucine (1-28 g), a decline of plasma cortisol was seen at 08:30, but neither these doses nor the higher dose of L-tryptophan (100 mg/kg) were capable of reducing cortisol secretion at 11:30. In our earlier study we found an absence of the morning decline of plasma cortisol in patients with hypothalamic lesion with or without L-tryptophan treatment (Hyypph et al., 1977).

34 MARKKU T. HYYPP.~., TAPANI JOLMA, JUHA LIIRA, VIVI.-ANN L.~NGVIK and Ossl KYT~M.g.KI

The discrepancy between our present study and that of Wool f & Lee (1977) can be explained by a lower concentra t ion of t ryptophan in our studies. Woolf & Lee (1977) used 10 g of L-tryptophan orally or infused at 800-1600 mg/hr. High doses of L-tryptophan are

able to cause other neurochemical changes (Smith et al., 1977) which may lead to mis- interpretat ion of the role of brain serotonin in the neuroendocr ine regulat ion of pituitary hormones.

We are grateful to The Academy of Finland, Medical Research Council for financial support (Grant 63/1975-77). We also thank the National Pituitary Agency, N.I.A.M.D.D., Bethesda, Md., U.S.A. for pituitary hormones and their antiserums. We should like to thank Ms. Eija Lehtovirta for technical help.

REFERENCES

DEN HOLLANDER, F. G., SHUURS, A. H. dk VAN HELL, H. (1972) Radioimmunoassays for human gonado- trophins and insulin employing a 'double-antibody solid-phase' technique. J. Immunol. Meth. 1, 247-262.

ECCLESTON, D., ASHCROFT, G. W., CRAWFORD, T. B.:, STANTON, J. B., WOOD, D. & MCTURK, P. H. (1970) Effect of L-tryptophan administration on 5-HIAA in cerebrospinal fluid in man. J. Neurol. Neurosurg. Psychiat. 33, 269-272.

FABER, J., HAGEN, C., KIRKEGAARD, C., BIRK LAURIDSEN, O. • MOLLER, S. E. (1977) Lack of effects of L-tryptophan on basal and TRH-stimulated TSH and prolactin levels. Psychoneuroendocrinology 2, 413-415.

FERNSTROM, J. O. 8£ WURTMAN, R. J. (1972) Brain serotonin content: physiological regulation by plasma neutral amino acids. Science 178, 414-416.

HYYPP.~, M., CARDINALI, D., BAUMGARTEN, H. ~6 WURTMAN, R. J. (1973) Rapid accumulation of H 3- serotonin in brain of rats receiving intraperitoneal 3H-tryptophan: effects of 5,6-dihydroxytryptamine on female sex hormones. J. Neurol. Transm. 34, 111-124.

HYYPP.~, M. T., JOLMA, T., L.~NGVIK, VIvI-ANN, KYT6M~,KI, O. & SYV.g, LAHTI, E. (1977) L-Tryptophan and neuroendocrine regulation in neurologic patients: hormone response to L-tryptophan loading in patients with hypothalamic lesion. Psychoneuroendocrinology 2, 349-357.

HYYPP~, i . T., FALCK, S., G.~VERT, J., KYT6MgKI, O., RAUTAKORPI, I. ,~r SYV.~LAHTI, E. (1976) L-Trypto- phan administration in man: effects on gonadottophin, growth hormone and cortisol secretion and on sexual motivation. Monogr. Neural. Sci. (Kargel, Basel) 3, 102-108.

KAMBEm, I. A. (1973) The role of brain monoamines and pineal indoles in secretion of gonadotrophins and gonadotrophin-releasing factors. Prog. Brain Res. 39, 261-278.

KmLY, MAUREEN & SOURKES, T. L. (1972) Transport of L-tryptophan into slices of rat cerebral cortex. J. Neurochem. 19, 2863-2872.

KNox-r, P. J. & CURZON, G. (1972) Free tryptophan in plasma and brain tryptophan metabolism. Nature 239, 452-453.

KOROON, C. & GLOWINSKI, J. (1972) Role of hypothalamic monoaminergic neurons in the gonadotrophin release regulating mechanisms. Neuropharmacology 11, 153-162.

MAClNDOE, J. W. & TURKINGTON, R. W. (1973) Stimulation of human prolactin secretion by intravenous infusion of L-tryptophan. J. clin. Invest. 52, 1972-1978.

MOILER, E. E., BRAMalLLA, F., GAVAGNINI, F., PERACCm, M. & PANERAI, A. (1974) Slight effect of L- tryptophan on growth hormone release in normal human subjects. J. clin. Endocr. Metab. 39, 1-5.

NORDSTR6M, C. G. & PELTOLA, O. (1974) Determination of 1 l-hydroxycorticosteroids in plasma. Scand. J. Reumat. 3, 143-144.

SAaSnTN, M., HAMBURG, D., GRINKER, R., PERSKY, H., BASOWITZ, H., KORCHIN, S. & CHEVALIER, J. (1957) Significance of preexperimental studies in the psychosomatic laboratory. Archs Neurol. Psychiat. (Chicago) 78, 207-219.

SMITH, J. E., LANE, J. D., SHEA, P. A. & McBRIDE, W. J. (1977) Neurochemical changes following the administration of precursor of biogenic amines. Life Sci. 21, 301-306.

WEITZMAN, E. D., BOYAR, R. M., KAPEN, S. ~ HELLMAN, L. (1975) The relationship of sleep and sleep stages to neuroendocrine secretion and biological rhythms in man. Recent Prog. Horm. Res. 31, 399-441.

WID~, L. (1969) Radioimmunoassays employing immunosorbents. Acta Endocr. (Copenh.) 63, Suppl 142, 207-218.

L-TRYPTOPnAN, PrrurrARY HORMONES AND CORTISOL 35

W~aE, R. H., ~ W E R G e R , S. & HAMMOND, C. B. (1977) Failure of L-tryptophan to stimulate prolactin secretion in man. J. din. Endocr. Metab. 45, 1310-1312.

De WrieD, D. & De JONG, W. (1974) Drug effects and hypothalamic-anterior pituitary function..4. Rev. Pharmac. 14, 389-412.

WOOLF, P. D. & LEE, L. (1977) Effect of the serotonin precursor, tryptophan, on pituitary hormone secretion. J. din. Endocr. Metab. 45, 123-133.

YOUNG, S. N. & SOURKES, T. L. (1977) Tryptophan in the central nervous system: regulation and significance. Adv. Neurochem. 2, 133-191.