A circannual cycle in pinealocyte synaptic ribbons in the hibernating and seasonally reproductive 13-lined ground squirrel (Spermophilus tridecemlineatus)

Seasonal and post-trauma remodeling in cone-dominant ground squirrel retina

With a photoreceptor mosaic containing ∼85% cones, the ground squirrel is one of the richest known mammalian sources of these important retinal cells. It also has a visual ecology much like the human's. While the ground squirrel retina is understandably prominent in the cone biochemistry, physiology, and circuitry literature, far less is known about the remodeling potential of its retinal pigment epithelium, neurons, macroglia, or microglia. This review aims to summarize the data from ground squirrel retina to this point in time, and to relate them to data from other brain areas where appropriate. We begin with a survey of the ground squirrel visual system, making comparisons with traditional rodent models and with human. Because this animal's status as a hibernator often goes unnoticed in the vision literature, we then present a brief primer on hibernation biology. Next we review what is known about ground squirrel retinal remodeling concurrent with deep torpor and with rapid recovery upon re-warming. Notable here is rapidly-reversible, temperature-dependent structural plasticity of cone ribbon synapses, as well as pre- and post-synaptic plasticity throughout diverse brain regions. It is not yet clear if retinal cell types other than cones engage in torpor-associated synaptic remodeling. We end with the small but intriguing literature on the ground squirrel retina's remodeling responses to insult by retinal detachment. Notable for widespread loss of (cone) photoreceptors, there is surprisingly little remodeling of the RPE or Müller cells. Microglial activation appears minimal, and remodeling of surviving second- and third-order neurons seems absent, but both require further study. In contrast, traumatic brain injury in the ground squirrel elicits typical macroglial and microglial responses. Overall, the data to date strongly suggest a heretofore unrecognized, natural checkpoint between retinal deafferentiation and RPE and Müller cell remodeling events. As we continue to discover them, the unique ways by which ground squirrel retina responds to hibernation or injury may be adaptable to therapeutic use.

Cluster headache: evidence for a disorder of circadian rhythm and hypothalamic function

This article reviews the literature for evidence of a disorder of circadian rhythm and hypothalamic function in cluster headache. Cluster headache exhibits diurnal and seasonal rhythmicity. While cluster headache has traditionally been thought of as a vascular headache disorder, its periodicity suggests involvement of the suprachiasmatic nucleus of the hypothalamus, the biological clock. Normal circadian function and seasonal changes occurring in the suprachiasmatic nucleus and pineal gland are correlated to the clinical features and abnormalities of circadian rhythm seen in cluster headache. Abnormalities in the secretion of melatonin and cortisol in patients with cluster headache, neuroimaging of cluster headache attacks, and the use of melatonin as preventative therapy in cluster headache are discussed in this review. While the majority of studies exploring the relationship between circadian rhythms and cluster headache are not new, we have entered a new diagnostic and therapeutic era in primary headache disorders. The time has come to use the evidence for a disorder of circadian rhythm in cluster headache to further development of chronobiotics in the treatment of this disorder.

Dissimilarity of slow-wave activity enhancement by torpor and sleep deprivation in a hibernator

Sleep regulation processes have been hypothesized to be involved in function and timing of arousal episodes in hibernating ground squirrels. We investigated the importance of sleep regulation during arousal episodes by sleep deprivation experiments. After sleep deprivation of 4, 12, and 24 h, starting 4 h after onset of euthermy, a duration-dependent enhancement of slow-wave activity (SWA) of the cortical electroencephalogram during non-rapid eye movement sleep was found, as expected for normal sleep regulation. When sleep deprivation was applied during the initial phase of the arousal episode, in which effects of prior torpor were present in undisturbed recordings, no subsequent recurrence of SWA was found. In addition, prior torpor induced a reduction in the spectral activity of the sigma frequency range (7-14 Hz), which was not observed after sleep deprivation. The effects of torpor and sleep deprivation on subsequent SWA appear qualitatively different. This indicates that effects of deep torpor on sleep are dissimilar to normal sleep regulation.

The neuroendocrine system in hibernating mammals: present knowledge and open questions

The present review describes the distribution and the function-dependent reactivity pattern of those peptidergic and aminergic components of the neuroendocrine system of hibernating mammals that have been studied by histological, pharmacological and physiological techniques. Particular attention has been paid to the intrinsic connectivity of the peptidergic apparatus and its input systems. Since the reactivity patterns of the neuroendocrine system show remarkable fluctuations in relation to the various stages of hibernation and euthermia, these fluctuations have been analyzed with respect to (1) their causative role in the regulation of hibernation and (2) their secondary response to physiological changes during hibernation. The author's investigations described in this review have mainly been performed in European hedgehogs (Erinaceus europaeus), European and golden hamsters (Cricetus cricetus, Mesocricetus auratus), dormice (Glis glis), and in Richardson's and Columbian ground squirrels (Spermophilus richardsonii, Spermophilus columbianus), by the use of light- and electron-microscopic immunocytochemistry and histochemistry, in situ hybridization, radioimmunoassays and stereotaxically guided application techniques. These experiments were also performed in hypothermic animals. The (partially published) results obtained by the author and his associates are reviewed with reference to the body of evidence found in the recent literature. With respect to their reactivity patterns, several neuropeptide and transmitter systems can be regarded as candidates for control systems of hibernation. Neuronal complexes immunoreactive for endogenous opiates, in particular enkephalin, and also for vasopressin, somatostatin, substance P, corticotropin-releasing factor and serotonin are probably involved in the neuroendocrine control of hibernation.

Annual variations in the vasopressin neuron population of the human suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus is considered to be the major component of the biological clock, involved in the temporal organization of a wide variety of physiological and behavioral processes. The present study was conducted to investigate whether there are diurnal or annual variations in the morphology of the vasopressin-containing neuron population of the suprachiasmatic nucleus in human beings. To that end, the brains of 48 human subjects were investigated. A marked annual variation was observed in the volume and vasopressin cell number of the human suprachiasmatic nucleus: the volume of the vasopressin cell population was, on average, 2.5 times larger in October-November than in May-June and contained 2.7 times as many vasopressin-immunoreactive neurons. In general, the annual cycle of the human suprachiasmatic nucleus showed a non-sinusoidal pattern with a maximum in early autumn, a lower plateau in winter and a deep trough in late spring and early summer. In contrast, no such seasonal variations could be detected in suprachiasmatic nucleus vasopressin numerical cell density or cell-nuclear diameter. The number of vasopressin-immunoreactive neurons in the paraventricular nucleus of the hypothalamus, on the other hand, did not show any significant periodic changes over the year, indicating the specificity of the suprachiasmatic nucleus rhythm. In contrast with the annual cycle of the suprachiasmatic nucleus, no significant diurnal variations were observed in any of these parameters. In conclusion, the findings indicate that photoperiod may be considered a potential environmental factor controlling the activity of the vasopressin system of the human suprachiasmatic nucleus.

Pinealocyte synaptic ribbons and neuroendocrine function

A comparative study of pinealocyte synaptic ribbons (SR) revealed two predominant populations exhibiting either a rod/ribbon shape (SRr) or a spherical/punctate shape (SRsp). Species-specific differences were found in the abundance of SR, the ratio of SRr/SRsp, and the occurrence of SR in ribbon fields. The close topographical relationship of SR to the plasma membrane and the numerical changes that occurred with changes in metabolism of the pinealocytes suggest that SR have important vesicle-mediated interactions with the cell membrane. Experiments designed to clarify the relationship between SR and pineal neuroendocrine function revealed a positive correlation between SR numbers and indole intermediates during pineal development in the rat, and increased SR frequency after denervation of the rat pineal gland or administration of the beta-adrenergic agonist, isoproterenol. These data are consistent with the hypothesis that SR function is linked to receptor mechanisms regulating indoleamine production in the pineal gland.

Repeated changes of dendritic morphology in the hippocampus of ground squirrels in the course of hibernation

Quantitative Golgi study of hippocampal pyramidal neurons of ground squirrels showed rapid and profound transformation of their apical dendrites in the course of hibernation. The dendrites were significantly shorter, less branched and had fewer dendritic spines in the middle of hibernation bout than in the active euthermic ground squirrels between bouts. After arousal from torpor, within 2 h dendrites completely restored their structure. During hibernation, season remodelling of the hippocampal dendrites occurs repeatedly during each torpor-activity cycle.

The human hypothalamus: comparative morphometry and photoperiodic influences

The concept of the hypothalamus as a distinct neurological entity concerned with a variety of regulatory processes dates back to the end of the 19th century. Before 1900 there were only vague intimations of the function of the brain surrounding the third ventricle and these were based primarily on various pathological and assorted clinical observations. Since then a large body of evidence has been derived implicating that the hypothalamus contains the control systems which are critically involved in many physiological, endocrine and behavioral processes. Among these are feeding and drinking, reproduction and the regulation of the sleep-wake cycle and temperature. Although the human hypothalamus accounts for only 4 cm3, or 0.3% of the adult brain volume, it contains the integrative systems critical for all these processes. A comparative morphometric analysis of the hypothalamus among mammals revealed that the volume of this part of the brain is highly correlated with brain size, irrespective of the ecological strategy or evolutionary history of the species considered. It appears that the human hypothalamus has just the size we may expect of such a large-brained mammal, but it is considerably larger than would be predicted from its body size. In mammals the preoptic region of the hypothalamus is implicated in the neural control of endocrine functions and in the temporal organization of a wide spectrum of biological rhythms. In recent years, the pivotal role of two hypothalamic cell groups have been considered in this context: the sexually dimorphic nucleus (SDN-POA) as part of the neural circuitry underlying masculine sexual behavior and reproductive functions and the suprachiasmatic nucleus (SCN) as the principal component of the central clock mechanism. Consistent with its role in the temporal organization of circadian processes, investigations in rodents and non-human primates suggest that the SCN is also involved in the seasonal control of reproductive and metabolic phenomena. Since the environmental light-dark cycle is the main Zeitgeber for circadian and seasonal rhythms in most species, including man, photic information could have substantial effects, not only on the neural activity of the biological clock, but also on its underlying structure. Our observations on the human SCN in relation to photoperiod indeed revealed a marked seasonal variation in the morphology of the human SCN. The volume of the SCN was, on average, twice as large in the autumn as in the summer and contained more than twice as many vasopressin immunoreactive neurons.(ABSTRACT TRUNCATED AT 400 WORDS)