Control of N-Acetyltransferase
20. Wiechmann, A. F.& Hollyfield, J. G. (1 987) J. Comp. Neurol. 258,253-260 21. Hubenik, G. A,, Hrown. C. M. & (;rota, I,. J. (1976) Hrain Kes. 118, 4 17-427 22. Vivien-Roels, H.. I’bvet. I’., 1)ubois. M. I’., Arendt, J. & Hrown, G. M. (1081) Cell Tiss. Kes. 217, 105-1 15 23. luvone, 1’. M., Avendano, G., Hutler, H. J. & Adler. K. (1000)J. Neurochem. 55,073-682 24. Wiechniann. A. F.* Hok. I). Kr Horwitz. J. (108h) Invest. Opthalniol. Vis. Sci. 27. 1.5.3-163 25. Cahill. C. M. & Hesharse. J. C. (1000) J. Neurochem. 54. 7 10-7 10 20, Ilnderwood, H.. Siopes. T. & Hxrett, K. K. (1088) J. Biol. Rhythms 3, 323-33 1 27. Kemb. C., Win-Justice. A. & Terman, M. (1991) J. Hiol. Rhythms 6, 5-20 28. Takahashi. J. S.. Murakami, N., Nikaido, S. S., I’ratt. 13. I,. & Robertson, 1,. M. ( I O X O ) Recent I’rog. klorm. Kes. 45,270-352 20, luvone, 1’. M. & Hcsharse, J. C, (1080) Hrain Kes. 369, 108- 170 30 Zawilska, J. & luvonc, 1’. M. (1080)J. I’harniacol. Exp. ”her. 250, 80-02 31. Hoatright, J. l1. & Iuvone. 1’. M. (1080) Neurochem. I n t . 15. .WI-.547 32. Hesharse. J. C. & Ihnis, I). A. (1983) Science 219, 1341- 1343 33. Ogino. N., Matsuniura, M., Shirakawa, [ I . & Tsukahara, 1. (1083) Opthalniic Kes. 15.72-89 34. Pierce, M. E. & Hesharse. J. C. (1085) J. Gen. I’hysiol. 86, 67 1-080
35. Pang, S. F. & Yew, 1). T. (1079) Experientia 35, 23 1-233 36. Nao-i, N.. Nilsson, S. E. G., Gallemore, K. 1’. & Steinberg, K. H.( 198’9) Exp. Eye Kes. 49, 573-589 37. Ihbocovich. M. I,. (1088) h o g . Retinal Kes. 8, 129-1 5 1 38. Hesharse, J. C., Iuvone, 1’. M. & Pierce. M. E. (19x8) I’rog. Retinal Kes. 7. 2 1-6 1 30. Wiechmann. A. F. Kr Wirsig-Wiechmann, C. K. (1991) J. Pineal. Kes. 10, 174-179 40. Laitinen, J. T. & Saavedra, J. M. (1090) Hrain Kes. 528.349-352 41. Hlazynski, C. Kr l)ubocovich, M. I,. (1091) J. Neurochem. 56, 1873-1 880 42. Ehinger, H. & I>owling,J. E. (10x7) in Handbook of Chemical Neuroanatomy (Hjorklund, A,, Hokfelt, T. & Swanson. I,. W.. eds.) vol. 5, pp. 389-440, Elsevier, Amsterdam 43. Gern, W. A,, Gorell. T. A. & Owens, 1). W. (IO81) in Melatonin-Current Status and Perspectives (Hirau, N. & Schloot. W., cds.), pp. 223-233, I’ergamon I’ress, Oxford 44. Dubocovich, M. I,. Kr Takahashi, J. S.(1087) I’roc. Natl. Acad. Sci. U S A . 84, 3Olh-3020 4.5. Chong, N. W. S. & Sugden, I). (1091) J. Neurochem. 57, 685-080 40. Cohen. M., Koselle, 1). & Chabner, H. (1078) Nature (Idondon)274,894-895 47. Vanecek, J. (1088) J. Neurochem. 51, 1436- I440 Received 7 January 1002
Melatonin: the clinical perspective in man Robert Silman Academic Unit of Obstetrics and Gynaecology, The London Hospital Medical College, Whitechapel, London E l IBB, U.K.
In amphibia melatonin is a skin-lightening hormone and is secreted i n response to light to assist the camoutlage of the animal. In mammals, it is a reproductive hormone and, paradoxically, it is secreted in response to dark.
The synthesis of melatonin Tryptophan, provided in the diet and brought to the pineal gland by the blood supply, is converted into 5-hydroxytryptamine (serotonin) and then to N-acetyl-5-hydroxytryptamine (N-acetylserotonin) and then to N-acetyl-5-methoxytryptamine(melatonin). Abbreviations used: HIOMT, hydroxyindole O-methyltransferase; NAT. N-acetyltransferase; CNS, central nervous system; GnKH. gonadotropin-releasing hormone; I,H, luteinizing hormone.
T h e pineal specific enzyme, hydroxyindole 0-methyltransferase (HIOMT) permits the conversion of N-acetylserotonin into melatonin. T h e enzyme N-acetyltransferase (NAT) permits the conversion of serotonin into N-acetylserotonin. NAT is the rate-limiting enzyme and, consequently, NAT controls the rate of synthesis (and secretion) of melatonin.
The control of N A T N A T is activated by dark and inhibited by light. The nervous pathway starts at the retina a n d passes through the central nervous system (CNS) via the supra chiasmatic nucleus (SCN) to the lateral horn of the thoracic spinal cord. It exits from the CNS as pre-ganglionic sympathetic fibres to synapse in the superior cervical ganglia, a n d re-enters the brain along the blood supply as post-ganglionic sympa-
I992
315
Biochemical Society Transactions
thetic fibres terminating at the nor-adrenergic receptors of the pinealocytes.
Melatonin, light and season 316
Since darkness activates N A T and since N A T promotes the synthesis and secretion of melatonin, it follows that melatonin is secreted principally at night, during darkness. I Iowever, nights are longer in winter than in summer. Consequently it also follows that the duration of the night-time secretion of melatonin is longer in winter than in summer.
Seasonal breeding and the biological activity of melatonin A change in the duration of the night-time release of melatonin, consequent upon a change of season, activates or inactivates the hypothalamic gonadotrophin-releasing hormone ((;nK€I) pulse generator, which in turn activates or inactivates the pituitary gonadal axis. In long-day breeding animals the sequence of events which follows a change of season is as follows: In summer the nights are short and the duration of the night-time secretion of melatonin is equally short. As a consequence the GnRH pulse generator is active, i.e. it is pulsing GnKH at a rate of approximately 6 min every 60 min. The pulsatile release of GnKlI causes a pulsatile release of the pituitary gonadotrophins, particularly luteinizing hormone (I,€{). The pulsatile release of Ll1 from the pituitary causes either spermatogenesis and the secretion of testosterone in the testis, or the cycle of oestrogen, progesterone and ovulation in the ovary. In winter, the nights are long and the duration of the night-time secretion of melatonin is equally long. As a consequence of this change in the duration of the night-time release of melatonin from summer to winter, the GnK€I pulse generator is inactivated and the pituitary gonadal axis is quiescent [ 1-51.
Man and the biological activity of melatonin Man is not a seasonal breeding animal and the hypotheses which could explain this are either: (a) darkness does not cause a night-time release of melatonin, i.e. there is an impairment in the neural control of NAT by light and dark; or (b) a change in the duration of the night-time release of melatonin does not control the GnRI-1pulse generator. ‘There are two pieces of evidence which support the first hypothesis. First, in seasonal breeding animals the night-time release of melatonin is universal, i.e. there are no substantial interanimal variations, and the amplitude of nocturnal
Volume 20
’
circulating melatonin exceeds 500 pmole/l. In adult humans, the night-time release of melatonin is erratic, i.e. there is substantial inter-individual variation, and the amplitude of nocturnal circulating melatonin is less than 500 pmole/l. This suggests that, in man, there is an impairment in the neural control of NAT by light and dark. Secondly, there are three examples in man where the nocturnal circulating concentration of melatonin exceeds 500 pmole/l. The three examples are: (i) children before puberty; (ii) hypogonadotropic hypogonadal infertility and (iii) high-dose melatonin as a contraceptive. In these examples there is also an inhibition of the GnRI I pulse generator, suggesting that melatonin may be able to control the GnRII pulse generator in man, as much as in seasonal breeding animals 161.
Puberty and melatonin From birth to puberty the GnRI 1 pulse generator is inactive and puberty can be defined as the moment at which it becomes activated. Hence the problem is: what causes the GnRH pulse generator to be activated? The secretion of melatonin may provide the clue. We now know that there is a dramatic fall in the nocturnal circulating concentration of melatonin throughout childhood. W e also know that the daily production rate of melatonin by the pineal gland is constant throughout life. Given that body mass increases as the child grows, it follows that the circulating concentration of melatonin must fall. In other words, a constant daily production of melatonin by the pineal gland plus an increasing body mass equals a decreasing circulating concentration of melatonin [ 7, 81. Consequently, the elements are in place for a ‘catastrophe’ theory of puberty [ 6 I. As body mass increases and as the circulating concentration of melatonin declines, melatonin levels eventually drop below a critical concentration (approximately 500 pmole/l) and the CnKII pulse generator is activated, thereby activating all the endocrine events which accompany sexual maturation.
Hypogonadotropic hypogonadal infertility and melatonin Hypogonadotropic hypogonadal infertility, or hypothalamic amenorrhoea, is a condition characterized by an inactive GnKII pulse generator leading to a failure of pituitary gonadotrophins and gonadal steroids. It can be treated by the pulsatile infusion of GnRII. Recently, there have been two studies investigating the levels of circulating melatonin in
Control of N-Acetyltransferase
this condition [ ( I , lo]. Both studies reported significantly higher levels of melatonin at night (exceeding 500 pmole/l) in this condition compared with normal controls. If the high melatonin levels are responsible for the inactivation of the GnRH pulse generator in this condition, it opens the possibility that the condition can be treated by inhibiting the secretion of melatonin.
Contraception and melatonin Melatonin is a comparatively cheap and readily synthesized drug which has been administered to human volunteers since the early 1960s without ill effect. It has also been without biological effect, which is unsettling if it is meant to control the GnKI 1 pulse generator. 1 lowever, past studies have been conducted using an inappropriate regimen of admininstration. Either the drug was administered for too short a period (one or two days) o r at too low a dose t o sustain high circulating levels for several hours each night. For example. the nightly ingestion of 2 mg of melatonin produced a maximum nocturnal concentration within 1 h of more than 4000 pmole/l, but by 3 h the circulating concentration had dropped below 500 pmole/l [ 1 1 I . Recently a melatonin-based contraceptive has been developed (AMK I’harm I Iolland HV, 25 14 1311 the I Iague, Netherlands) which uses a nightly dose of 75 mg of melatonin. Presumably the efficacy of this preparation is due to the Fact that the nocturnal concentration of melatonin is maintained above 500 pmole/l for 8-12 h.
Conclusion It is possible that melatonin controls the GnKtI pulse generator in humans as it does in seasonal breeding anirnals. I Iowever, melatonin is not released from the pineal gland in response to darkness as effectively in humans as it is in seasonal
breeding animals. Therefore two strategies are required if melatonin is to be used therapeutically: (1) there has to be a method for the administration of melatonin whereby it can be provided as a controlled-release preparation, ensuring a circulating concentration of more than 500 pmole/l for 10-1.1 h; (2) means should be sought to control the activation or inactivation of NA‘I’, the rate-limiting enzyme in the syntheses of melatonin, so that the circulating concentration of rnelatonin can be manipulated endogenously rather than exogenously.
1. Rittman, I
20. Wiechmann, A. F.& Hollyfield, J. G. (1 987) J. Comp. Neurol. 258,253-260 21. Hubenik, G. A,, Hrown. C. M. & (;rota, I,. J. (1976) Hrain Kes. 118, 4 17-427 22. Vivien-Roels, H.. I’bvet. I’., 1)ubois. M. I’., Arendt, J. & Hrown, G. M. (1081) Cell Tiss. Kes. 217, 105-1 15 23. luvone, 1’. M., Avendano, G., Hutler, H. J. & Adler. K. (1000)J. Neurochem. 55,073-682 24. Wiechniann. A. F.* Hok. I). Kr Horwitz. J. (108h) Invest. Opthalniol. Vis. Sci. 27. 1.5.3-163 25. Cahill. C. M. & Hesharse. J. C. (1000) J. Neurochem. 54. 7 10-7 10 20, Ilnderwood, H.. Siopes. T. & Hxrett, K. K. (1088) J. Biol. Rhythms 3, 323-33 1 27. Kemb. C., Win-Justice. A. & Terman, M. (1991) J. Hiol. Rhythms 6, 5-20 28. Takahashi. J. S.. Murakami, N., Nikaido, S. S., I’ratt. 13. I,. & Robertson, 1,. M. ( I O X O ) Recent I’rog. klorm. Kes. 45,270-352 20, luvone, 1’. M. & Hcsharse, J. C, (1080) Hrain Kes. 369, 108- 170 30 Zawilska, J. & luvonc, 1’. M. (1080)J. I’harniacol. Exp. ”her. 250, 80-02 31. Hoatright, J. l1. & Iuvone. 1’. M. (1080) Neurochem. I n t . 15. .WI-.547 32. Hesharse. J. C. & Ihnis, I). A. (1983) Science 219, 1341- 1343 33. Ogino. N., Matsuniura, M., Shirakawa, [ I . & Tsukahara, 1. (1083) Opthalniic Kes. 15.72-89 34. Pierce, M. E. & Hesharse. J. C. (1085) J. Gen. I’hysiol. 86, 67 1-080
35. Pang, S. F. & Yew, 1). T. (1079) Experientia 35, 23 1-233 36. Nao-i, N.. Nilsson, S. E. G., Gallemore, K. 1’. & Steinberg, K. H.( 198’9) Exp. Eye Kes. 49, 573-589 37. Ihbocovich. M. I,. (1088) h o g . Retinal Kes. 8, 129-1 5 1 38. Hesharse, J. C., Iuvone, 1’. M. & Pierce. M. E. (19x8) I’rog. Retinal Kes. 7. 2 1-6 1 30. Wiechmann. A. F. Kr Wirsig-Wiechmann, C. K. (1991) J. Pineal. Kes. 10, 174-179 40. Laitinen, J. T. & Saavedra, J. M. (1090) Hrain Kes. 528.349-352 41. Hlazynski, C. Kr l)ubocovich, M. I,. (1091) J. Neurochem. 56, 1873-1 880 42. Ehinger, H. & I>owling,J. E. (10x7) in Handbook of Chemical Neuroanatomy (Hjorklund, A,, Hokfelt, T. & Swanson. I,. W.. eds.) vol. 5, pp. 389-440, Elsevier, Amsterdam 43. Gern, W. A,, Gorell. T. A. & Owens, 1). W. (IO81) in Melatonin-Current Status and Perspectives (Hirau, N. & Schloot. W., cds.), pp. 223-233, I’ergamon I’ress, Oxford 44. Dubocovich, M. I,. Kr Takahashi, J. S.(1087) I’roc. Natl. Acad. Sci. U S A . 84, 3Olh-3020 4.5. Chong, N. W. S. & Sugden, I). (1091) J. Neurochem. 57, 685-080 40. Cohen. M., Koselle, 1). & Chabner, H. (1078) Nature (Idondon)274,894-895 47. Vanecek, J. (1088) J. Neurochem. 51, 1436- I440 Received 7 January 1002
Melatonin: the clinical perspective in man Robert Silman Academic Unit of Obstetrics and Gynaecology, The London Hospital Medical College, Whitechapel, London E l IBB, U.K.
In amphibia melatonin is a skin-lightening hormone and is secreted i n response to light to assist the camoutlage of the animal. In mammals, it is a reproductive hormone and, paradoxically, it is secreted in response to dark.
The synthesis of melatonin Tryptophan, provided in the diet and brought to the pineal gland by the blood supply, is converted into 5-hydroxytryptamine (serotonin) and then to N-acetyl-5-hydroxytryptamine (N-acetylserotonin) and then to N-acetyl-5-methoxytryptamine(melatonin). Abbreviations used: HIOMT, hydroxyindole O-methyltransferase; NAT. N-acetyltransferase; CNS, central nervous system; GnKH. gonadotropin-releasing hormone; I,H, luteinizing hormone.
T h e pineal specific enzyme, hydroxyindole 0-methyltransferase (HIOMT) permits the conversion of N-acetylserotonin into melatonin. T h e enzyme N-acetyltransferase (NAT) permits the conversion of serotonin into N-acetylserotonin. NAT is the rate-limiting enzyme and, consequently, NAT controls the rate of synthesis (and secretion) of melatonin.
The control of N A T N A T is activated by dark and inhibited by light. The nervous pathway starts at the retina a n d passes through the central nervous system (CNS) via the supra chiasmatic nucleus (SCN) to the lateral horn of the thoracic spinal cord. It exits from the CNS as pre-ganglionic sympathetic fibres to synapse in the superior cervical ganglia, a n d re-enters the brain along the blood supply as post-ganglionic sympa-
I992
315
Biochemical Society Transactions
thetic fibres terminating at the nor-adrenergic receptors of the pinealocytes.
Melatonin, light and season 316
Since darkness activates N A T and since N A T promotes the synthesis and secretion of melatonin, it follows that melatonin is secreted principally at night, during darkness. I Iowever, nights are longer in winter than in summer. Consequently it also follows that the duration of the night-time secretion of melatonin is longer in winter than in summer.
Seasonal breeding and the biological activity of melatonin A change in the duration of the night-time release of melatonin, consequent upon a change of season, activates or inactivates the hypothalamic gonadotrophin-releasing hormone ((;nK€I) pulse generator, which in turn activates or inactivates the pituitary gonadal axis. In long-day breeding animals the sequence of events which follows a change of season is as follows: In summer the nights are short and the duration of the night-time secretion of melatonin is equally short. As a consequence the GnRH pulse generator is active, i.e. it is pulsing GnKH at a rate of approximately 6 min every 60 min. The pulsatile release of GnKlI causes a pulsatile release of the pituitary gonadotrophins, particularly luteinizing hormone (I,€{). The pulsatile release of Ll1 from the pituitary causes either spermatogenesis and the secretion of testosterone in the testis, or the cycle of oestrogen, progesterone and ovulation in the ovary. In winter, the nights are long and the duration of the night-time secretion of melatonin is equally long. As a consequence of this change in the duration of the night-time release of melatonin from summer to winter, the GnK€I pulse generator is inactivated and the pituitary gonadal axis is quiescent [ 1-51.
Man and the biological activity of melatonin Man is not a seasonal breeding animal and the hypotheses which could explain this are either: (a) darkness does not cause a night-time release of melatonin, i.e. there is an impairment in the neural control of NAT by light and dark; or (b) a change in the duration of the night-time release of melatonin does not control the GnRI-1pulse generator. ‘There are two pieces of evidence which support the first hypothesis. First, in seasonal breeding animals the night-time release of melatonin is universal, i.e. there are no substantial interanimal variations, and the amplitude of nocturnal
Volume 20
’
circulating melatonin exceeds 500 pmole/l. In adult humans, the night-time release of melatonin is erratic, i.e. there is substantial inter-individual variation, and the amplitude of nocturnal circulating melatonin is less than 500 pmole/l. This suggests that, in man, there is an impairment in the neural control of NAT by light and dark. Secondly, there are three examples in man where the nocturnal circulating concentration of melatonin exceeds 500 pmole/l. The three examples are: (i) children before puberty; (ii) hypogonadotropic hypogonadal infertility and (iii) high-dose melatonin as a contraceptive. In these examples there is also an inhibition of the GnRI I pulse generator, suggesting that melatonin may be able to control the GnRII pulse generator in man, as much as in seasonal breeding animals 161.
Puberty and melatonin From birth to puberty the GnRI 1 pulse generator is inactive and puberty can be defined as the moment at which it becomes activated. Hence the problem is: what causes the GnRH pulse generator to be activated? The secretion of melatonin may provide the clue. We now know that there is a dramatic fall in the nocturnal circulating concentration of melatonin throughout childhood. W e also know that the daily production rate of melatonin by the pineal gland is constant throughout life. Given that body mass increases as the child grows, it follows that the circulating concentration of melatonin must fall. In other words, a constant daily production of melatonin by the pineal gland plus an increasing body mass equals a decreasing circulating concentration of melatonin [ 7, 81. Consequently, the elements are in place for a ‘catastrophe’ theory of puberty [ 6 I. As body mass increases and as the circulating concentration of melatonin declines, melatonin levels eventually drop below a critical concentration (approximately 500 pmole/l) and the CnKII pulse generator is activated, thereby activating all the endocrine events which accompany sexual maturation.
Hypogonadotropic hypogonadal infertility and melatonin Hypogonadotropic hypogonadal infertility, or hypothalamic amenorrhoea, is a condition characterized by an inactive GnKII pulse generator leading to a failure of pituitary gonadotrophins and gonadal steroids. It can be treated by the pulsatile infusion of GnRII. Recently, there have been two studies investigating the levels of circulating melatonin in
Control of N-Acetyltransferase
this condition [ ( I , lo]. Both studies reported significantly higher levels of melatonin at night (exceeding 500 pmole/l) in this condition compared with normal controls. If the high melatonin levels are responsible for the inactivation of the GnRH pulse generator in this condition, it opens the possibility that the condition can be treated by inhibiting the secretion of melatonin.
Contraception and melatonin Melatonin is a comparatively cheap and readily synthesized drug which has been administered to human volunteers since the early 1960s without ill effect. It has also been without biological effect, which is unsettling if it is meant to control the GnKI 1 pulse generator. 1 lowever, past studies have been conducted using an inappropriate regimen of admininstration. Either the drug was administered for too short a period (one or two days) o r at too low a dose t o sustain high circulating levels for several hours each night. For example. the nightly ingestion of 2 mg of melatonin produced a maximum nocturnal concentration within 1 h of more than 4000 pmole/l, but by 3 h the circulating concentration had dropped below 500 pmole/l [ 1 1 I . Recently a melatonin-based contraceptive has been developed (AMK I’harm I Iolland HV, 25 14 1311 the I Iague, Netherlands) which uses a nightly dose of 75 mg of melatonin. Presumably the efficacy of this preparation is due to the Fact that the nocturnal concentration of melatonin is maintained above 500 pmole/l for 8-12 h.
Conclusion It is possible that melatonin controls the GnKtI pulse generator in humans as it does in seasonal breeding anirnals. I Iowever, melatonin is not released from the pineal gland in response to darkness as effectively in humans as it is in seasonal
breeding animals. Therefore two strategies are required if melatonin is to be used therapeutically: (1) there has to be a method for the administration of melatonin whereby it can be provided as a controlled-release preparation, ensuring a circulating concentration of more than 500 pmole/l for 10-1.1 h; (2) means should be sought to control the activation or inactivation of NA‘I’, the rate-limiting enzyme in the syntheses of melatonin, so that the circulating concentration of rnelatonin can be manipulated endogenously rather than exogenously.
1. Rittman, I
