Locus Coeruleus Degeneration Induces Forebrain Vascular Pathology in a Transgenic Rat Model of Alzheimer's Disease.

Noradrenergic locus coeruleus (LC) neuron loss is a significant feature of mild cognitive impairment and Alzheimer's disease (AD). The LC is the primary source of norepinephrine in the forebrain, where it modulates attention and memory in vulnerable cognitive regions such as prefrontal cortex (PFC) and hippocampus. Furthermore, LC-mediated norepinephrine signaling is thought to play a role in blood-brain barrier (BBB) maintenance and neurovascular coupling, suggesting that LC degeneration may impact the high comorbidity of cerebrovascular disease and AD. However, the extent to which LC projection system degeneration influences vascular pathology is not fully understood. To address this question in vivo, we stereotactically lesioned LC projection neurons innervating the PFC of six-month-old Tg344-19 AD rats using the noradrenergic immunotoxin, dopamine-ß-hydroxylase IgG-saporin (DBH-sap), or an untargeted control IgG-saporin (IgG-sap). DBH-sap-lesioned animals performed significantly worse than IgG-sap animals on the Barnes maze task in measures of both spatial and working memory. DBH-sap-lesioned rats also displayed increased amyloid and inflammation pathology compared to IgG-sap controls. However, we also discovered prominent parenchymal albumin extravasation with DBH-sap lesions indicative of BBB breakdown. Moreover, microvessel wall-to-lumen ratios were increased in the PFC of DBH-sap compared to IgG-sap rats, suggesting that LC deafferentation results in vascular remodeling. Finally, we noted an early emergence of amyloid angiopathy in the DBH-sap-lesioned Tg344-19 AD rats. Taken together, these data indicate that LC projection system degeneration is a nexus lesion that compromises both vascular and neuronal function in cognitive brain areas during the prodromal stages of AD.

Pupil diameter covaries with BOLD activity in human locus coeruleus.

The locus coeruleus-noradrenergic (LC-NA) neuromodulatory system has been implicated in a broad array of cognitive processes, yet scope for investigating this system's function in humans is currently limited by an absence of reliable non-invasive measures of LC activity. Although pupil diameter has been employed as a proxy measure of LC activity in numerous studies, empirical evidence for a relationship between the two is lacking. In the present study, we sought to rigorously probe the relationship between pupil diameter and BOLD activity localized to the human LC. Simultaneous pupillometry and fMRI revealed a relationship between continuous pupil diameter and BOLD activity in a dorsal pontine cluster overlapping with the LC, as localized via neuromelanin-sensitive structural imaging and an LC atlas. This relationship was present both at rest and during performance of a two-stimulus oddball task, with and without spatial smoothing of the fMRI data, and survived retrospective image correction for physiological noise. Furthermore, the spatial extent of this pupil/LC relationship guided a volume-of-interest analysis in which we provide the first demonstration in humans of a fundamental characteristic of animal LC activity: phasic modulation by oddball stimulus relevance. Taken together, these findings highlight the potential for utilizing pupil diameter to achieve a more comprehensive understanding of the role of the LC-NA system in human cognition.

Locus coeruleus stimulation recruits a broad cortical neuronal network and increases cortical perfusion.

The locus coeruleus (LC), the main source of brain noradrenalin (NA), modulates cortical activity, cerebral blood flow (CBF), glucose metabolism, and blood-brain barrier permeability. However, the role of the LC-NA system in the regulation of cortical CBF has remained elusive. This rat study shows that similar proportions (∼20%) of cortical pyramidal cells and GABA interneurons are contacted by LC-NA afferents on their cell soma or proximal dendrites. LC stimulation induced ipsilateral activation (c-Fos upregulation) of pyramidal cells and of a larger proportion (>36%) of interneurons that colocalize parvalbumin, somatostatin, or nitric oxide synthase compared with pyramidal cells expressing cyclooxygenase-2 (22%, p < 0.05) or vasoactive intestinal polypeptide-containing interneurons (16%, p < 0.01). Concurrently, LC stimulation elicited larger ipsilateral compared with contralateral increases in cortical CBF (52 vs 31%, p < 0.01). These CBF responses were almost abolished (-70%, p < 0.001) by cortical NA denervation with DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride] and were significantly reduced by α- and ß-adrenoceptor antagonists (-40%, p < 0.001 and -30%, p < 0.05, respectively). Blockade of glutamatergic or GABAergic neurotransmission with NMDA or GABA(A) receptor antagonists potently reduced the LC-induced hyperemic response (-56%, p < 0.001 or -47%, p < 0.05). Moreover, inhibition of astroglial metabolism (-35%, p < 0.01), vasoactive epoxyeicosatrienoic acids (EETs; -60%, p < 0.001) synthesis, large-conductance, calcium-operated (BK, -52%, p < 0.05), and inward-rectifier (Kir, -40%, p < 0.05) K+ channels primarily impaired the hyperemic response. The data demonstrate that LC stimulation recruits a broad network of cortical excitatory and inhibitory neurons resulting in increased cortical activity and that K+ fluxes and EET signaling mediate a large part of the hemodynamic response.

Self-amplification of nigral degeneration in Parkinson's disease: a hypothesis.

This review analyzes current evidence regarding possible mechanisms of nigral damage in idiopathic Parkinson's disease (iPD). In normal brain, a specific interplay among the blood-brain barrier (BBB), substantia nigra (SN), and locus coeruleus (LC) creates the condition for a self-accelerating damage to the SN. Three vicious circles involving SN-BBB, LC-SN-BBB, and histamine-BBB-SN interactions are described. In iPD, a self-accelerating loss of nigral cells can be triggered by brain hypoperfusion and by an increased blood histamine level. iPD-associated factors such as decreased CSF levels of substance P, somatostatin, and glutamate can aggravate the vicious-circle-induced damage to the SN.

Localization of copper to afferent terminals in rat locus ceruleus, in contrast to mitochondrial copper in cerebellum.

We examined copper localization in the locus ceruleus and cerebellum of rat brain by Timm's sulfide-silver staining, as modified by Danscher. Dense silver particles revealing copper localization were observed in sections of the locus ceruleus and cerebellum after pre-treatment with trichloroacetic acid. In the locus ceruleus, copper appeared to be distributed to neuropil rather than glial or neuronal cell bodies, and at the ultrastructural level copper was mainly localized on synaptic membranes of afferent terminals in contact with somatic spines or dendrites of locus ceruleus neurons, whereas copper was distributed to mitochondria in the granular layers of cerebellum and fine, sparse silver particles were observed throughout ependymal cells and epithelial cells of blood vessels. The specific localization of copper to afferent terminals in the locus ceruleus was confirmed by X-ray microanalysis, which showed a significant level of copper, but not zinc, in synaptic membranes. These results suggest a distinct role of copper depending on its regional distribution. Copper or copper protein may be involved in neurotransmission in the locus ceruleus but in mitochondrial activity in the cerebellum.

Intracerebral vascular changes induced by cold pressor test: a model of sympathetic activation.

Cerebral vascular changes seem to play a fundamental role in the pathogenesis of various functional disturbances, (i.e. those suggested for migraine pathogenesis). However the exact role of single regulatory aspects (metabolic-neuronal-mechanic) are not completely understood and easily investigated in man. In particular, the role of autonomic nervous system is widely debated and recently the stimulation of tegmental noradrenergic nuclei (locus coeruleus in particular) has proved capable of inducing, in the animal, both the reduction and the increase of extracerebral blood flow. In order to evaluate the vascular effect of locus coeruleus stimulation in man, we investigated intracerebral vascular changes induced by the cold pressor test (CPT) (a well standardized method for activating both nociceptive and sympathetic pathways) by means of transcranial Doppler sonography. The examinations were performed in 14 healthy controls. CPT induced a constant and evident reduction in mean arterial velocity of the middle cerebral artery. The response was triggered during the first minute following hand immersion in ice water and reached its maximum level by the 3rd minute. Pretreatment with the alfa2-agonist clonidine caused a marked reduction of the cerebrovascular response. These data suggest that: a) intracerebral vascular response induced by CPT may be attributed to a central noradrenergic mechanism (possibly modulated at the locus coeruleus level) and b) transcranial Doppler monitoring of CPT effect is a potential tool for investigating peculiar patterns of functional disturbances of cerebral circulation.

The effect of locus ceruleus lesion on water, sodium and potassium content of the rat cerebral cortex.

Locus ceruleus lesion decreases the density of ouabain binding sites, and presumably Na+, K+-ATPase, in brain microvessels. To determine if this decrease affects the transport of Na+, K+ or water across the blood-brain barrier, we studied the influence of unilateral locus ceruleus lesion on Na+, K+ and water content of the ipsilateral cerebral cortex. Unilateral locus ceruleus lesion depleted norepinephrine in the ipsilateral cerebral cortex but had no effect on tissue Na+, K+ or water under steady-state conditions. When the Na+/K+ exchange pump of the blood-brain barrier was stressed by hyperkalaemia, K+ content in the ipsilateral cerebral cortex rose to higher levels than in the contralateral cortex, but the difference did not reach statistical significance. Locus ceruleus lesion also did not cause significant differences in the cerebral cortical content of water, Na+ or K+ in hyponatraemia. The results suggest that brain water and ion homeostasis are tightly controlled, probably by multiple mechanisms with biological redundancies, so that even a 50% decrease in the density of ouabain binding sites in brain endothelium does not result in significant alterations in brain water, Na+ or K+ content.

A combined vascular-catecholamine fluorescence method reveals the relative vascularity of rat locus coeruleus and the paraventricular and supraoptic nuclei of the hypothalamus.

Vascular perfusion with a fluorescent dye, Pontamine sky blue, was combined with glyoxylic acid-induced catecholamine fluorescence to produce simultaneous staining of intracerebral blood vessels and catecholamine-containing cells and fibers. Quantitative measurements of blood vessels per unit area revealed that the vascular density of rat locus coeruleus did not differ from the surrounding neuropil but was significantly lower than that of the hypothalamic supraoptic and paraventricular (PVN) nuclei. Moreover, the density of blood vessels in the magnocellular portion of PVN was not uniform throughout its rostrocaudal extent but was dense only in a specific subdivision of lateral PVN, which is predominantly vasopressinergic and is also heavily innervated by catecholamine terminals.

Neuronal-vascular relationships in the raphe nuclei, locus coeruleus, and substantia nigra in primates.

A fluorescence histochemical and electron microscopic study of the monoaminergic cell groups in the squirrel monkey and Rhesus monkey brains has revealed the direct apposition of blood vessels to perikarya and dendrites of monoaminergic neurons. Capillaries and small arterioles or venules, ranging from 8-50 microns in diameter, showed perikarya and dendrites abutting the basement membrane without evidence of glial interposition. This neuronal-vascular relationship was present in 20% to 30% of the small vessels in the serotonergic nuclei raphe dorsalis and centralis superior and in the noradrenergic locus coeruleus. Such contacts were clearly present but observed less frequently in the dopaminergic substantia nigra pars compacta and in the serotonergic nuclei raphe obscurus, pallidus, magnus, and pontis. We postulate that monoamine-containing neurons apposed to blood vessels in certain regions of the brain may be influenced directly by hormones or other substances in blood.