Neural systems underlying photoperiodic time measurement: a blueprint

The melanocyte photosensory system in the human skin

The pigment cells form the largest population of neural crest cells to migrate into the epidermis and hair follicle along each dermatomic area from the neural folds. The melanopsin system responsible for photoentrainment, was isolated from the photosensitive dermal melanophores of frogs Xenopus laevis responding to light. Melanocytes form a photoresponsive network which reads the environmental seasonal variations in the light cycles in the same manner. The present work was undertaken to study the organization of this system by: I. Experimental assessment of photoresponse and II. Evidence of an organized system of photoreception in the skin. Melanocytes, in whole skin organ cultures and epidermal strips, from margin of vitiligo in G₂ phase show prominent dendricity, and express pigment, biogenic amines and hormones on UV exposure. The photoresponse depends on the photosensitive enzymes NAT/HIOMT and dopaoxidase. Melanocytes interact with adjacent keratinocytes, dermal capillaries, and nerve endings. The melanocyte network reads the diurnal and seasonal photophase by the melatonin/serotonin switch like the pineal. Sleep disorders and winter depression are corrected by phototherapy utilising this mechanism. Melanocytes showing photoactivity, aplasia, hypoplasia and hyperplasia, and interactive keratinocytes occupy the trigeminal, brachial and lumbosacral dermatomes, zones of high embryonic induction, forming an ectodermal placodal system. Melanin units and hair follicles serve as photoreceptors. Migration of active melanocytes to defined areas is evident in pigment patterns in guinea pigs. This study identifies defined photoreceptor melanocyte/epidermal domains which read the seasonal photophase and control the sleep waking cycle in response to the environmental light. I. Whole skin organ cultures, and epidermal strips from margin of vitiligo in G₂ phase are exposed to UV and IR to study sequential and dose response of marginal melanocytes, using histochemistry, immunohistochemistry to assess pigment, biogenic amines and hormones on UV exposure. II. Dermatomic Distributions: Detailed maps of melanocyte photoresponse in 356 biopsies, lesions in 297 vitiligo, 100 melanosis, 165 melanomas 142 leprosy and 442 basal cell/keratinocytes lesions were assessed for patterns of dermatomic distribution. Embryonal melanocyte migration along dermatomes was assessed in 285 guinea pigs from an inbred colony having black, brown and white patches.

Climate change and seasonal reproduction in mammals

Seasonal reproduction is common among mammals at all latitudes, even in the deep tropics. This paper (i) discusses the neuroendocrine pathways via which foraging conditions and predictive cues such as photoperiod enforce seasonality, (ii) considers the kinds of seasonal challenges mammals actually face in natural habitats, and (iii) uses the information thus generated to suggest how seasonal reproduction might be influenced by global climate change. Food availability and ambient temperature determine energy balance, and variation in energy balance is the ultimate cause of seasonal breeding in all mammals and the proximate cause in many. Photoperiodic cueing is common among long-lived mammals from the highest latitudes down to the mid-tropics. It is much less common in shorter lived mammals at all latitudes. An unknown predictive cue triggers reproduction in some desert and dry grassland species when it rains. The available information suggests that as our climate changes the small rodents of the world may adapt rather easily but the longer lived mammals whose reproduction is regulated by photoperiod may not do so well. A major gap in our knowledge concerns the tropics; that is where most species live and where we have the least understanding of how reproduction is regulated by environmental factors.

Lesions in the bed nucleus of the stria terminalis, but not in the lateral septum, inhibit short-photoperiod-induced testicular regression in Syrian hamsters

The transfer of adult male hamsters from long days (LD) to short days (SD) (i.e. < 12 h of light per day) typically results in marked testicular regression and a decline in plasma testosterone concentrations. To help disclose key brain regions responsible for mediating this photoperiodic response male hamsters received either chemical (i.e. N-methyl-D-aspartate; NMDA) or radiofrequency current lesions in the bed nucleus of the stria terminalis (BNST), and were then exposed to SD for 15 or 12 weeks, respectively. Although body weights were similar between sham-lesioned controls and the NMDA-lesioned hamsters, the latter showed a significant attenuation of testicular regression; additionally, their plasma testosterone concentrations remained at typical LD levels. When radiofrequency current-lesioned hamsters were transferred from LD to SD they also failed to show significant signs of testicular regression, nor a decline in plasma testosterone concentrations, nor a complete arrest of spermatogenesis. In contrast, sham-lesioned controls or hamsters that were lesioned dorsally to the BNST at a site primarily involving the lateral septum all showed the expected degree of testicular regression, a decline in plasma testosterone concentrations, and complete arrest of spermatogenesis; body weights were similar in all of the experimental group. Taken together, these findings suggest that the BNST, a brain area traditionally not associated with reproductive function, may play an important role in mediating photoperiodic information to the neural circuits that control the reproductive axis.

Circadian variations of superoxide dismutase activity in the rat pineal gland

The 24 h profile of the activity of the antioxidant enzyme superoxide dismutase (SOD) in the pineal gland of rats was studied. Rhythmic analysis showed a significant 24 h rhythm with an amplitude of oscillation of 25% of the 24 h mean value, that was 100.34 +/- 1.6 U SOD (nitrite). An ultradian rhythm of 9 h was also detected. The diurnal profile of superoxide dismutase activity is discussed in relation to the oxidative metabolism of the pineal gland.

In vitro autoradiographical localization of melatonin binding sites in the caprine brain

The recent development of a specific 2-[125I]-iodo-melatonin ligand has led to the identification of 125I-melatonin binding sites in the brains of numerous mammalian species. The present study reports the localization of 125I-melatonin binding sites in the brain of the dairy goat. Six previously untreated female goats, aged 5-7 years, were culled under natural light between 0900 and 1100. Brains and pituitaries were immediately dissected out and frozen on dry ice. Both transverse and sagittal sections of frozen brain were cut 20 microns thick and thaw-mounted onto gelatin-coated slides. Three consecutive sections were cut at intervals throughout the brain, mounted onto three slides, labeled A, B, and C, and thusly treated: (A) incubated for 2 hr at room temperature in a 50 pM solution of 125I-melatonin; (B) incubated for 2 hr at room temperature in a 50 pM solution of 125I-melatonin plus 1 microM cold melatonin; (C) fixed in Clarke's fluid and stained with toluidine blue. After incubation, A (specific) and B (nonspecific) slides were washed three times in ice-cold Tris-HCl buffer (pH 7.7), air-dried, exposed to an X-ray film for 2 weeks at -20 degrees C, and then fixed and stained. Specific 125I-melatonin binding sites were found in the pars tuberalis (PT), the area of the suprachiasmatic nucleus (SCN), preoptic area (POA), fornix/mediolateral septal areas, hippocampus, and the cerebral cortex. 125I-melatonin did not bind in the hindbrain, midbrain, neurohypophysis, pars intermedia or pars distalis of the adenohypophysis, or the pineal.

Proposed role of septohippocampal and pallido-habenulo-raphe systems in photoperiodic time measurement

The neuroanatomic substrate for mammalian photoperiodic time measurement (PTM), a primary requirement for appropriately timed seasonal reproductive function, is unknown. It is suggested that the 'comparator' of successive day lengths previously postulated as necessary for PTM may be located within the septohippocampal system, output from which is then directed to reproductive-system response elements by way of several possible routes including the pallido-habenulo-raphe conduction line. PTM thus requires the integration of an invariant, context-free circadian function subserved by the well-known melatonin-generating system with a flexible, context-dependent extracircadian function subserved by the septohippocampal system. The context-dependent neuronal network endows reproductive-system responses with the ability to be modulated by previous photoperiodic history, an ability not possessed by the context-free circadian network proper. Several suggestions for testing the validity of the proposed model are offered.

Circasemidian sleep propensity and the phase-amplitude maintenance model of human sleep/wake regulation

Evidence for circasemidian sleep/wake regulation is briefly reviewed with respect to protocols used to quantify sleep propensity. Existing models of sleep/wake regulation are examined in view of their ability to accommodate data which demonstrate an afternoon sleep period. Finally, a modelling approach is briefly outlined which emphasizes the maintenance of the phase and amplitude characteristics of the circadian rhythm of body (and brain) temperature and predicts the circasemidian phenomena.

Occlusion of the melatonin-free interval blocks the short day gonadal response of the male Syrian hamster to programmed melatonin infusions of necessary duration and amplitude

Abstract Photoperiodic control of the neuroendocrine axis is mediated by changes in the duration of the nocturnal melatonin signal. This study tested the hypothesis that reading of the signal depends upon the presence of a period free of melatonin between successive signals. Adult male Syrian hamsters were pinealectomized and received chronic subcutaneous infusions of melatonin or saline for 6 weeks. Animals which received saline had large testes. Those which received a single daily infusion which lasted for 10 h (50 ng/h) followed by 14 h without infusion underwent gonadal atrophy. Other animals received a compound melatonin signal in which the melatonin-free interval was occluded by a continuous infusion (25 ng/h). Superimposed upon this was a 10 h phasic increase in infusion rate such that the maximum rate of infusion was equivalent to that observed in controls (25 ng/h increase, 50 ng/h peak rate), or the increase in rate over the baseline was the same as in controls (50 ng/h increase, 75 ng/h peak rate). In neither group did the animals undergo gonadal regression. Analysis of iodomelatonin binding sites by in vitro autoradiography failed to reveal any systematic difference between animals which did and did not respond to melatonin and so the absence of a response could not be attributed to loss of receptors. These data demonstrate that the photoperiodic system cannot identify the melatonin signal solely upon the features of nocturnal peak height or amplitude of the peak over baseline. They are consistent with the hypothesis that the melatonin-free interval plays a significant role in photoperiodic time measurement.