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Photoperiodism, melatonin and the pineal
Molecular and Cellulur Endocrinology, 50 (1987) 269-270 Elsevier Scientific Publishers Ireland, Ltd.
269
MCE 01621
Book Review
Photoperiodism, Melatonin and the Pineal. David Evered and Sarah Clark (Eds.), Ciba Foundation Symposium 117, Pitman, London, 1985, 323 pp. The pineal has a long and fascinating history which is highlighted in the first chapter of this exciting symposium held in March, 1985. Roger Short’s opening paper poses questions about how circadian and circannual rhythms can be timed, whether Nature has utilized one clock to predict these independent rhythms and if so what are the mechanisms ‘that makes this clock tick’. The story that unfolds in subsequent papers is compulsory (and compulsive) reading for all who are intrigued by questions such as the way that the onset of seasonal breeding is regulated with such precision and whether melatonin plays a direct, or indirect role in co-ordinating external stimuli and endogenous responses. The first series of papers deals with the nature of the circadian timing system in the detection and transduction of photoperiod (daylength), the involvement of neural elements such as the suprachiasmatic nuclei (SCN), the parvocellular portion of the paraventricular nucleus and the preganglionic sympathetic neurons of the thoracic spinal cord in the circadian rhythm of melatonin production. Neurobiological techniques applied to the structural complexity of the mammalian pineal gland show the presence of heterogeneous ‘synaptic’ ribbons on pinealocytes among which exist intracellular organelles that respond differently to melatonin or 5-hydroxytryptamine according to the time of day at least under in vitro conditions. Comparative studies from David Klein’s laboratory show that the melatonin rhythm-generating system differs among species, though the underlying features seem to include a nocturnal increase in melatonin synthesis, a neural pathway that includes the SCN and superior cervical ganglion and adrenergic control of cyclic AMP which plays a key role in the regulation of melatonin production. Whether species differences in the relative impor0303.7207/87/$03.50
Q 1987 Elsevier Scientific
Publishers
Ireland,
tance of (Y- or /?-adrenergic stimulation and in the kinetics of N-acetyltransferase provide selective advantages according to the photic environment is a question for the future. Michael Menaker contends that the title given to the symposium is incorrect since among vertebrates (including birds) there are other melatonin-forming tissues, particularly the retina, and he argues that the eyes could well be called second and third pineal glands. Brian Follett’s group report that in birds photoperiodic light is detected not with the retina but by brain photoreceptors that are rhodopsin-based and probably lie in the hypothalamus. Hence, a single 4 h light pulse has the ability to initiate a wave of gonadotrophin secretion lasting for 10 days. The report from Menaker’s laboratory that melatonin synthesis is the pineal of the mouse (non-photoperiodic?) can be rapidly abolished by the selection involved in producing inbred strains, provoked a lively discussion. The theme of the second series of papers is the influence of photoperiodic rhythms during development, seasonal breeding and puberty. An entrainable circadian clock that oscillates in the nuclei of cell in the SCN of fetal rats during late gestation is dependent on the presence of an intact maternal SCN. Although melatonin is transferred across the placenta it appears not to be the entraining signal. Work in ewes and rams seeks to elucidate the ability of light to entrain the daily endogenous melatonin rhythm and to suppress melatonin secretion. The target of melatonin remains elusive but appears to regulate the neural LH pulse generator which establishes the level of gonadal function according to the season of the year. The mink provides an example of a carnivore in which there are marked changes in body weight, gonadal activity, moulting periods and hormonal patterns that are synchronized by annual changes in day-length and evidence is presented that all photoperiodic signals in this species are conveyed by the pineal. Ltd.
270
The final section turns to relationships between melatonin, light and chronobiological disorders with reference to winter depressive patients, improved diagnosis and treatment of certain types of sleep and mood disorders, and problems associated with shift-work and jet-lag. Jo Arendt’s group describe a measurable increase in evening fatigue in human subjects given a low dose of melatonin daily for one month, though little, if any, influence on mood or endocrine variables (except for prolactin) could be detected. In humans, unlike sheep, there is an absence of a consistent, overt melatonin rhythm and quite a few normal people have remarkably low levels of the compound in circulation. Melatonin’s potential widespread effects, however, were illustrated by an examination of how it protects against DMBA-induced mammary
tumours in rats. The effects may be due to melatonin suppression of prolactin secretion or to an action on oestrogen receptors. Many other gems are to be found in this stimulating volume, particularly among the discussions which are well-edited and worth reading. There is a valuable penultimate discussion on melatonin assay technology which emphasizes some of the pitfalls. The book does much to vindicate Short’s opening statement that ‘it is only within the last decade that we have accumulated solid experimental evidence to accord melatonin its true role - the Hormone of Darkness, the Hourglass of Time’. R.B. HEAP
269
MCE 01621
Book Review
Photoperiodism, Melatonin and the Pineal. David Evered and Sarah Clark (Eds.), Ciba Foundation Symposium 117, Pitman, London, 1985, 323 pp. The pineal has a long and fascinating history which is highlighted in the first chapter of this exciting symposium held in March, 1985. Roger Short’s opening paper poses questions about how circadian and circannual rhythms can be timed, whether Nature has utilized one clock to predict these independent rhythms and if so what are the mechanisms ‘that makes this clock tick’. The story that unfolds in subsequent papers is compulsory (and compulsive) reading for all who are intrigued by questions such as the way that the onset of seasonal breeding is regulated with such precision and whether melatonin plays a direct, or indirect role in co-ordinating external stimuli and endogenous responses. The first series of papers deals with the nature of the circadian timing system in the detection and transduction of photoperiod (daylength), the involvement of neural elements such as the suprachiasmatic nuclei (SCN), the parvocellular portion of the paraventricular nucleus and the preganglionic sympathetic neurons of the thoracic spinal cord in the circadian rhythm of melatonin production. Neurobiological techniques applied to the structural complexity of the mammalian pineal gland show the presence of heterogeneous ‘synaptic’ ribbons on pinealocytes among which exist intracellular organelles that respond differently to melatonin or 5-hydroxytryptamine according to the time of day at least under in vitro conditions. Comparative studies from David Klein’s laboratory show that the melatonin rhythm-generating system differs among species, though the underlying features seem to include a nocturnal increase in melatonin synthesis, a neural pathway that includes the SCN and superior cervical ganglion and adrenergic control of cyclic AMP which plays a key role in the regulation of melatonin production. Whether species differences in the relative impor0303.7207/87/$03.50
Q 1987 Elsevier Scientific
Publishers
Ireland,
tance of (Y- or /?-adrenergic stimulation and in the kinetics of N-acetyltransferase provide selective advantages according to the photic environment is a question for the future. Michael Menaker contends that the title given to the symposium is incorrect since among vertebrates (including birds) there are other melatonin-forming tissues, particularly the retina, and he argues that the eyes could well be called second and third pineal glands. Brian Follett’s group report that in birds photoperiodic light is detected not with the retina but by brain photoreceptors that are rhodopsin-based and probably lie in the hypothalamus. Hence, a single 4 h light pulse has the ability to initiate a wave of gonadotrophin secretion lasting for 10 days. The report from Menaker’s laboratory that melatonin synthesis is the pineal of the mouse (non-photoperiodic?) can be rapidly abolished by the selection involved in producing inbred strains, provoked a lively discussion. The theme of the second series of papers is the influence of photoperiodic rhythms during development, seasonal breeding and puberty. An entrainable circadian clock that oscillates in the nuclei of cell in the SCN of fetal rats during late gestation is dependent on the presence of an intact maternal SCN. Although melatonin is transferred across the placenta it appears not to be the entraining signal. Work in ewes and rams seeks to elucidate the ability of light to entrain the daily endogenous melatonin rhythm and to suppress melatonin secretion. The target of melatonin remains elusive but appears to regulate the neural LH pulse generator which establishes the level of gonadal function according to the season of the year. The mink provides an example of a carnivore in which there are marked changes in body weight, gonadal activity, moulting periods and hormonal patterns that are synchronized by annual changes in day-length and evidence is presented that all photoperiodic signals in this species are conveyed by the pineal. Ltd.
270
The final section turns to relationships between melatonin, light and chronobiological disorders with reference to winter depressive patients, improved diagnosis and treatment of certain types of sleep and mood disorders, and problems associated with shift-work and jet-lag. Jo Arendt’s group describe a measurable increase in evening fatigue in human subjects given a low dose of melatonin daily for one month, though little, if any, influence on mood or endocrine variables (except for prolactin) could be detected. In humans, unlike sheep, there is an absence of a consistent, overt melatonin rhythm and quite a few normal people have remarkably low levels of the compound in circulation. Melatonin’s potential widespread effects, however, were illustrated by an examination of how it protects against DMBA-induced mammary
tumours in rats. The effects may be due to melatonin suppression of prolactin secretion or to an action on oestrogen receptors. Many other gems are to be found in this stimulating volume, particularly among the discussions which are well-edited and worth reading. There is a valuable penultimate discussion on melatonin assay technology which emphasizes some of the pitfalls. The book does much to vindicate Short’s opening statement that ‘it is only within the last decade that we have accumulated solid experimental evidence to accord melatonin its true role - the Hormone of Darkness, the Hourglass of Time’. R.B. HEAP