Iain M. McIntyre

Bio: Iain M. McIntyre is an academic researcher from University of Melbourne. The author has contributed to research in topics: Melatonin & Serotonin. The author has an hindex of 21, co-authored 59 publications receiving 1709 citations. Previous affiliations of Iain M. McIntyre include Astra & Wayne State University.

Human Melatonin Suppression by Light is Intensity Dependent

Abstract: Five intensities of artificial light were examined for the effect on nocturnal melatonin concentrations. Maximum suppression of melatonin following 1 hr of light at midnight was 71%, 67%, 44%, 38%, and 16% with intensities of 3,000, 1,000, 500, 350, and 200 lux (lx), respectively. In contrast to some previous reports, light of 1,000 lx intensity was sufficient to suppress melatonin to near daytime levels, and intensities down to 350 lx were shown to significantly suppress nocturnal melatonin levels below prelight values. On the basis of these data, it is suggested that when examining the melatonin sensitivity of patient groups (such as bipolar affective disorders) to artificial light, an appropriate light intensity should be established in each laboratory. Light of less intensity (e.g., 200-350 lx) may be more suitable to dichotomize patient groups from control subjects.

Aging and cortisol resistance to suppression by dexamethasone: a positive correlation.

Abstract: Cortisol resistance to suppression by 0.5 mg of dexamethasone given at 11 p.m. was studied in 30 normal subjects, 17 to 78 years of age. Serum cortisol concentrations were determined by radioimmunoassay. A strong positive correlation was found between age and cortisol concentrations 9 hours after dexamethasone administration. The data suggest that aging, per se, might contribute to the increased cortisol resistance to suppression by dexamethasone reported in depression and dementia.

Melatonin Rhythm in Human Plasma Saliva

Abstract: Human plasma and saliva were collected at frequent intervals throughout the night and after a nocturnal challenge by exposure to 3,000 lx of light for 1 h in the middle of the night. Melatonin, as measured by radioimmunoassay, was found to correlate highly in plasma and saliva, described by a linear regression equation: y = 55x-2.6 (r = 0.90). The nocturnal melatonin rhythm in saliva was parallel to that observed in plasma. A good correlation was also observed between plasma and salivary melatonin on exposure to light. Melatonin in both fluids showed a significant fall during light exposure. Levels returned to normal nocturnal values within 2 h after returning to darkness. These results indicate that salivary melatonin, although lower than plasma melatonin, may be used as an index of pineal gland release of melatonin. It is suggested that saliva may be useful as a non-invasive technique for obtaining data on melatonin profiles, especially in pilot-test and screening situations.

Effects of pinealectomy and aging on the serum corticosterone circadian rhythm in rats.

Abstract: Male Sprague-Dawley rats were housed in alternate light/dark conditions (light on, 7:00 AM, light off, 7:00 PM). Corticosterone was determined by radioimmunoassay from blood samples that were obtained by tail clip at 4-h intervals. Pinealectomized animals have shown significant increase of corticosterone levels at 7:00 AM, 11:00 AM and 7:00 PM in comparison with 2-month-old intact rats. There were no differences in serum corticosterone rhythm between 24-month-old and pinealectomized animals. Twelve-month-old rats have shown significant increase of corticosterone levels at 7:00 and 11:00 AM in comparison with 2-month-old animals. The age-associated increase of serum corticosterone and the similarity between serum corticosterone circadian rhythm in aged and pinealectomized animals suggest that an age-related decrease in melatonin production [Reiter et al., 1981] may contribute to age related changes of hypothalamic-pituitary adrenal axis regulation.

Pineal Melatonin: Cell Biology of Its Synthesis and of Its Physiological Interactions*

Abstract: I Introduction UNTIL 35 yr ago, most scientists did not take research on the pineal gland seriously The decade beginning in 1956, however, provided several discoveries that laid the foundation for what has become a very active area of investigation These important early observations included the findings that, 1), the physiological activity of the pineal is influenced by the photoperiodic environment (1–5); 2), the gland contains a substance, N-acetyl-5-methoxytryptamine or melatonin, which has obvious endocrine capabilities (6, 7); 3), the function of the reproductive system in photoperiodically dependent rodents is inextricably linked to the physiology of the pineal gland (5, 8, 9); 4), the sympathetic innervation to the pineal is required for the gland to maintain its biosynthetic and endocrine activities (10, 11); and 5), the pineal gland can be rapidly removed from rodents with minimal damage to adjacent neural structures using a specially designed trephine (12) Since the mid 1960s, research on t

The Neuroendocrinology of Stress and Aging: The Glucocorticoid Cascade Hypothesis

Abstract: Over the past 5 yr, we have examined some of the sharpest edges of the pathology of aging. We have studied the capacity of aged organisms to respond appropriately to stress and the capacity of stre...

Action Spectrum for Melatonin Regulation in Humans: Evidence for a Novel Circadian Photoreceptor

Abstract: The photopigment in the human eye that transduces light for circadian and neuroendocrine regulation, is unknown. The aim of this study was to establish an action spectrum for light-induced melatonin suppression that could help elucidate the ocular photoreceptor system for regulating the human pineal gland. Subjects (37 females, 35 males, mean age of 24.5 +/- 0.3 years) were healthy and had normal color vision. Full-field, monochromatic light exposures took place between 2:00 and 3:30 A.M. while subjects' pupils were dilated. Blood samples collected before and after light exposures were quantified for melatonin. Each subject was tested with at least seven different irradiances of one wavelength with a minimum of 1 week between each nighttime exposure. Nighttime melatonin suppression tests (n = 627) were completed with wavelengths from 420 to 600 nm. The data were fit to eight univariant, sigmoidal fluence-response curves (R(2) = 0.81-0.95). The action spectrum constructed from these data fit an opsin template (R(2) = 0.91), which identifies 446-477 nm as the most potent wavelength region providing circadian input for regulating melatonin secretion. The results suggest that, in humans, a single photopigment may be primarily responsible for melatonin suppression, and its peak absorbance appears to be distinct from that of rod and cone cell photopigments for vision. The data also suggest that this new photopigment is retinaldehyde based. These findings suggest that there is a novel opsin photopigment in the human eye that mediates circadian photoreception.

Melatonin in humans.

Abstract: Three centuries ago, the French philosopher Rene Descartes described the pineal gland as “the seat of the soul,” but it was not until the late 1950s that melatonin, the principal substance secreted by the pineal gland, was identified.1 There is now evidence that melatonin may have a role in the biologic regulation of circadian rhythms, sleep, mood, and perhaps reproduction, tumor growth, and aging (Table 1). However, uncertainties and doubts still surround the role of melatonin in human physiology and pathophysiology. This review summarizes current knowledge about melatonin in humans and its clinical implications. Physiology and Pharmacology In humans, the . . .

An action spectrum for melatonin suppression: evidence for a novel non‐rod, non‐cone photoreceptor system in humans

Abstract: 1 Non-image forming, irradiance-dependent responses mediated by the human eye include synchronisation of the circadian axis and suppression of pineal melatonin production The retinal photopigment(s) transducing these light responses in humans have not been characterised 2 Using the ability of light to suppress nocturnal melatonin production, we aimed to investigate its spectral sensitivity and produce an action spectrum Melatonin suppression was quantified in 22 volunteers in 215 light exposure trials using monochromatic light (30 min pulse administered at circadian time (CT) 16-18) of different wavelengths (lambda(max) 424, 456, 472, 496, 520 and 548 nm) and irradiances (07-650 microW cm(-2)) 3 At each wavelength, suppression of plasma melatonin increased with increasing irradiance Irradiance-response curves (IRCs) were fitted and the generated half-maximal responses (IR(50)) were corrected for lens filtering and used to construct an action spectrum 4 The resulting action spectrum showed unique short-wavelength sensitivity very different from the classical scotopic and photopic visual systems The lack of fit (r(2) or =073) Of these, the best fit was to the rhodopsin template with lambda(max) 459 nm (r(2) = 074) 5 Our data strongly support a primary role for a novel short-wavelength photopigment in light-induced melatonin suppression and provide the first direct evidence of a non-rod, non-cone photoreceptive system in humans