Nucleus locus coeruleus: a morphometric Golgi study in rats of three age groups

Juxtacellular recordings from identified neurons in the mouse locus coeruleus

The locus coeruleus (LC) is the primary source of noradrenergic transmission in the mammalian central nervous system. This small pontine nucleus consists of a densely packed nuclear core-which contains the highest density of noradrenergic neurons-embedded within a heterogeneous surround of non-noradrenergic cells. This local heterogeneity, together with the small size of the LC, has made it particularly difficult to infer noradrenergic cell identity based on extracellular sampling of in vivo spiking activity. Moreover, the relatively high cell density, background activity and synchronicity of LC neurons have made spike identification and unit isolation notoriously challenging. In this study, we aimed at bridging these gaps by performing juxtacellular recordings from single identified neurons within the mouse LC complex. We found that noradrenergic neurons (identified by tyrosine hydroxylase, TH, expression; TH-positive) and intermingled putatively non-noradrenergic (TH-negative) cells displayed similar morphologies and responded to foot shock stimuli with excitatory responses; however, on average, TH-positive neurons exhibited more prominent foot shock responses and post-activation firing suppression. The two cell classes also displayed different spontaneous firing rates, spike waveforms and temporal spiking properties. A logistic regression classifier trained on spontaneous electrophysiological features could separate the two cell classes with 76% accuracy. Altogether, our results reveal in vivo electrophysiological correlates of TH-positive neurons, which can be useful for refining current approaches for the classification of LC unit activity.

Probing the structure and function of locus coeruleus projections to CNS motor centers

The brainstem nucleus locus coeruleus (LC) sends projections to the forebrain, brainstem, cerebellum and spinal cord and is a source of the neurotransmitter norepinephrine (NE) in these areas. For more than 50 years, LC was considered to be homogeneous in structure and function such that NE would be released uniformly and act simultaneously on the cells and circuits that receive LC projections. However, recent studies have provided evidence that LC is modular in design, with segregated output channels and the potential for differential release and action of NE in its projection fields. These new findings have prompted a radical shift in our thinking about LC operations and demand revision of theoretical constructs regarding impact of the LC-NE system on behavioral outcomes in health and disease. Within this context, a major gap in our knowledge is the relationship between the LC-NE system and CNS motor control centers. While we know much about the organization of the LC-NE system with respect to sensory and cognitive circuitries and the impact of LC output on sensory guided behaviors and executive function, much less is known about the role of the LC-NE pathway in motor network operations and movement control. As a starting point for closing this gap in understanding, we propose using an intersectional recombinase-based viral-genetic strategy TrAC (Tracing Axon Collaterals) as well as established ex vivo electrophysiological assays to characterize efferent connectivity and physiological attributes of mouse LC-motor network projection neurons. The novel hypothesis to be tested is that LC cells with projections to CNS motor centers are scattered throughout the rostral-caudal extent of the nucleus but collectively display a common set of electrophysiological properties. Additionally, we expect to find these LC projection neurons maintain an organized network of axon collaterals capable of supporting selective, synchronous release of NE in motor circuitries for the purpose of coordinately regulating operations across networks that are responsible for balance and movement dynamics. Investigation of this hypothesis will advance our knowledge of the role of the LC-NE system in motor control and provide a basis for treating movement disorders resulting from disease, injury, or normal aging.

Locus Coeruleus Optogenetic Modulation: Lessons Learned from Temporal Patterns

After reviewing seminal studies using optogenetics to interrogate the functional role of the locus coeruleus in behavior, we conclude that differences in firing rates and firing patterns of locus coeruleus neurons contribute to locus coeruleus nucleus heterogeneity by recruiting different output circuitry, and differentially modifying behavior. The outcomes initiated by different optogenetic input activation patterns and frequencies can have opposite consequences for behavior, activate different neurons in the same target structure, be supported by distinct adrenoceptors and vary with behavioral state.

The 'a, b, c's of pretangle tau and their relation to aging and the risk of Alzheimer's Disease

Braak has described the beginnings of Alzheimer's Disease as occurring in the locus coeruleus. Here we review these pretangle stages and relate their expression to recently described normal features of tau biology. We suggest pretangle tau depends on characteristics of locus coeruleus operation that promote tau condensates. We examine the timeline of pretangle and tangle appearance in locus coeruleus. We find catastrophic loss of locus coeruleus neurons is a late event. The strong relationship between locus coeruleus neuron number and human cognition underscores the utility of a focus on locus coeruleus. Promoting locus coeruleus health will benefit normal aging as well as aid in the prevention of dementia. Two animal models offering experimental approaches to understanding the functional change initiated by pretangles in locus coeruleus neurons are discussed.

The Locus Coeruleus Is a Complex and Differentiated Neuromodulatory System

Diffuse projections of locus coeruleus (LC) neurons and evidence of synchronous spiking have long been perceived as features of global neuromodulation. Recent studies demonstrated the possibility of targeted modulation by subsets of LC neurons. Non-global neuromodulation depends on target specificity and the differentiated spatiotemporal dynamics within LC. Here, we characterized interactions between 3,164 LC cell pairs in the rat LC under urethane anesthesia. Spike count correlations were near zero and only a small proportion of unit pairs had synchronized spontaneous (15%) or evoked (16%) discharge. We identified infra-slow (0.01-1 Hz) fluctuations of LC unit spike rate, which were also asynchronous across the population. Despite overall sparse population synchrony, we report the existence of LC ensembles and relate them to forebrain projection targets. We also show that spike waveform width was related to ensemble membership, propensity for synchronization, and interactions with cortex. Our findings suggest a partly differentiated and target-specific noradrenergic signal.

Two Subpopulations of Noradrenergic Neurons in the Locus Coeruleus Complex Distinguished by Expression of the Dorsal Neural Tube Marker Pax7

Central noradrenergic neurons, collectively defined by synthesis of the neurotransmitter norepinephrine, are a diverse collection of cells in the hindbrain, differing in their anatomy, physiological and behavioral functions, and susceptibility to disease and environmental insult. To investigate the developmental basis of this heterogeneity, we have used an intersectional genetic fate mapping strategy in mice to study the dorsoventral origins of the En1-derived locus coeruleus (LC) complex which encompasses virtually all of the anatomically defined LC proper, as well as a portion of the A7 and subcoeruleus (SubC) noradrenergic nuclei. We show that the noradrenergic neurons of the LC complex originate in two different territories of the En1 expression domain in the embryonic hindbrain. Consistent with prior studies, we confirm that the majority of the LC proper arises from the alar plate, the dorsal domain of the neural tube, as defined by expression of Pax7^Cre. In addition, our analysis shows that a large proportion of the En1-derived A7 and SubC nuclei also originate in the Pax7^Cre-defined alar plate. Surprisingly, however, we identify a smaller subpopulation of the LC complex that arises from outside the Pax7^Cre expression domain. We characterize the distribution of these neurons within the LC complex, their cell morphology, and their axonal projection pattern. Compared to the broader LC complex, the newly identified Pax7^Cre-negative noradrenergic subpopulation has very sparse projections to thalamic nuclei, suggestive of distinct functions. This developmental genetic analysis opens new avenues of investigation into the functional diversity of the LC complex.

León Cintra's contribution to the field of protein malnutrition effects on sleep and the brain

On June 19 2009, everyone who knew Leon Cintra was shocked by the terrible news of the automobile accident that took his life. The feeling within the scientific community was that his passing was not only a great loss for Mexican science but also the loss of a beloved friend. He will be missed and forever remembered for his brilliant mind and noble heart. His scientific career was focused, since the beginning, on the study of protein malnutrition effects on brain morphometry, somato-sensory transmission, sleep, circadian rhythms and behavior. His findings showed that malnutrition has long lasting adverse effects on morphometry of systems involved in sleep regulation such as locus coeruleus, nucleus raphe dorsalis and susprachiasmatic nucleus, and on hippocampal circuit implicated in theta activity generation. His results on spectral analysis of electrical field potential at every 4 sec from 24-h baseline recording and 72-h of recovery sleep after total sleep deprivation or selective REM sleep deprivation demonstrated that protein malnutrition induced alterations on homeostatic as well as on circadian sleep regulation; brain oscillations and theta coherent activity between left and right hemisphere and between hippocampus and cerebral cortex are also affected by malnutrition.

Topographical distribution of the locus coeruleus and raphe nuclei in the lizard Ctenosaura pectinata: Functional implications on sleep

The nature of sleep in reptiles has traditionally created intense discussion and has originated some controversy. Nevertheless, some authors have described a sleep phase analogous to sleep in endotherm vertebrates. It is known that in mammals, the locus coeruleus and raphe nuclei, located in the brain stem, are functionally related to the sates vigilance regulation. In contrast the presence of two sleep phases in the lizard Ctenosaura pectinata similar to slow wave sleep and rapid eye movement sleep have been described. Therefore we carried out studies of the brain stem of C. pectinata to search for cellular groupings related to the regulation of these sleep phases. We identified and described the topographical distribution of the locus coeruleus and raphe nuclei in the lizard C. pectinata. Results show that these nuclei that have been functionally related to vigilance states in mammals, are also present in C. pectinata. These nuclei are formed by fairly well defined cellular groupings placed in the brain stem.

Chronic exposure to cold stress alters electrophysiological properties of locus coeruleus neurons recorded in vitro

Chronic stress exposure can alter central noradrenergic function. Previously, we reported that in chronically cold-exposed rats the release of norepinephrine and electrophysiological activation of locus coeruleus (LC) neurons is enhanced in response to multiple excitatory stimuli without alterations in basal activity. In the present studies, we used in vitro intracellular recording techniques to explore the effect of chronic cold exposure on the basal and evoked electrophysiological properties of LC neurons in horizontal slices of the rat brainstem. Consistent with our findings from in vivo experiments, chronic cold exposure did not affect basal firing rate. Furthermore, gross morphology of LC neurons and spike waveform characteristics were similar in slices from control and previously cold-exposed rats. However, excitability in response to intracellular current injection and input resistance were larger in slices from previously cold-exposed rats. In addition, the accommodation of spike firing in response to sustained current injection was smaller and the period of postactivation inhibition appeared to be less in LC neurons from cold-exposed rats. These data demonstrate that the stress-evoked sensitization of LC neurons observed in vivo is at least in part maintained in the slice preparation and suggest that alterations in electrophysiological properties of LC neurons contribute to the chronic stress-induced sensitization of central noradrenergic function observed in vivo. Furthermore, the present data suggest that an alteration in autoinhibitory control of LC activity is involved in the chronic stress-induced alterations. The enhanced functional capacity of LC neurons following cold exposure of rats may represent a unique model to study the mechanisms underlying the alterations in central noradrenergic function observed in humans afflicted with mood and anxiety disorders.

The development of sex differences in the locus coeruleus of the rat

Development of sex differences in the locus coeruleus (LC) is investigated. The LC is a sexually dimorphic structure in which the female manifests a larger volume and greater number of neurons than do males. Male and female Wistar rats were sacrificed on prenatal days (E) 16 and 20 and postnatally (P) on days 1, 3, 7, 15, 35, 45, 60, and 90. Male and female rats show a continuous increase in the number of neurons after birth that stops in the males by P45 and in females by P60. These findings point out the existence of different patterns of development in male and female rats and may suggest that sex differences could be established because of the existence of a differential period of neurogenesis in both sexes in the postpubertal period.

Tyrosine hydroxylase expression within Balb/C and C57black/6 mouse locus coeruleus. II. Quantitative study of the enzyme level

The tyrosine hydroxylase phenotype expression was further investigated in the perikarya and pericoerulean areas of the locus coeruleus of two pure inbred mouse strains, Balb/C and C57Black/6, which the topological organization and phenotypic plasticity of the enzyme-containing cell population were previously studied. The tyrosine hydroxylase level and the mean protein quantity provided by each cell were significantly higher within the spaces delimited by the enzyme containing perikarya in the C57Black/6 strain as compared to the Balb/C strain. Three days after RU24722 administration, tyrosine hydroxylase tissue concentration and quantity were significantly increased in both strains. Two strain-dependant mechanisms of this pharmacologically induced protein modulation were demonstrated: the mean tyrosine hydroxylase quantity provided by each cell was increased in the C57Black/6 strain whereas the increase was obviously explained by the subset of additional tyrosine hydroxylase expressing cells previously reported in Balb/C strain. A comparable volume of pericoerulean immunolabeled neuropile, which contains a similar tyrosine hydroxylase level, was measured between the two strains. Within this tissue compartment, an undissociated RU24722 responsiveness was observed between the two strains: a significant increase in the protein level was measured principally resulting from a significant increase in the volume. These results revealed a strain-dependent difference in the response to the RU24722 treatment which may result from a genetic separation of two kinds of tyrosine hydroxylase phenotypic regulations within the perikarya area of the locus coeruleus; whereas the surrounding neuropile seemed to have a different mechanism of the phenotypic protein expression and modulation.

Neuronal regeneration and estrous cycle restoration after locus coeruleus-periventricular gray substance section

The locus coeruleus (LC) was anatomically separated from the periventricular gray substance (PVG) by means of knife cuts in the adult female rat presenting regular estrous cycling. This resulted in a transient suppression of the estrous cycling that lasted 10-13 days after surgery. After this period, irregular or regular cycling activity was observed. The regular cycling was restored 30-45 days after the knife cuts. Golgi impregnation of some of the brains of these rats revealed regenerative elements in the knife-cut-insulted area. Thus, blood vessels, macrophagic-like elements, and glial-like elements were observed in close relation with the knife-cut pathway. Additionally, well-defined stained neurons typical of the LC and PVG were observed in close proximity to the knife-cut pathway. Dendritic and axon projections towards the insulted area were observed. Well defined axons were seen across the knife-cut pathway. These data support, first, that the LC-PVG communication is part of a circuitry for the modulation of gonadotropic activity, and second, that in the restoration of the estrous cyclicity after the knife cut, regenerative processes leading to a LC-PVG functional reconnection occurred after the knife cut.

A Golgi study of human locus coeruleus in normal brains and in Parkinson's disease

The locus coeruleus (LC) of eight adults without neurodegenerative disease and eight patients with Parkinson's disease was investigated by means of the Golgi-Braitenberg method for formalin-fixed human autopsy material. As with Golgi studies in the rat and cat, two main neuronal classes could be demonstrated in the human LC: (i) medium-sized fusiform and multipolar LC neurons known to contain neuromelanin and (ii) smaller neurons of widely varying somatic shape and dendritic arborization which are considered to be intermingled neurons of adjacent brain stem nuclei not containing neuromelanin. In Parkinson's disease, the Golgi-impregnated medium-sized LC neurons were reduced in number. They showed marked reduction of dendritic length, severe loss of spines, dendritic varicosities and swollen perikarya. The last two findings could be due in part to Lewy-body inclusions. The smaller non-noradrenergic neurons did not show severe pathological changes by the Golgi impregnation technique, which is in line with the fact that only neuromelanin-containing LC neurons are affected in the pathological process of Parkinson's disease.

Sexual dimorphism of the dopamine-beta-hydroxylase-immunoreactive neurons in the rat locus ceruleus

Sex differences in the noradrenaline synthesizing neurons of the locus ceruleus (LC) in rat brain were investigated immunocytochemically using an antibody to dopamine-beta-hydroxylase. Female adult rats contained a greater structural volume and average somatic area in the anterior intermediate region of the nucleus compared with males. Whether this difference is related to the endocrine status of the animals, and consequently a functionally distinct population of neurons, is yet to be determined.

Efferent projections of different subpopulations of central noradrenaline neurons

Early anatomical studies of the projections of central noradrenergic (NA) neurons led to the widely accepted view of NA cells as a class of diffusely projecting neurons. This view greatly influenced the formulation of numerous hypotheses about the functional role of these neurons in the central nervous system (CNS). With the introduction of transmitter-specific retrograde and anterograde transport methods, two powerful tools became available to rigorously re-examine whether the projections of NA neurons are diffuse or topographically organized. This article summarizes some of the results of these studies in which retrograde transport of fluorescent tracers and anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA-L), respectively, were combined with immunohistochemical identification of NA neurons and their projections. The results of these studies revealed a remarkable degree of specificity in the projections of different subgroups of NA neurons. In the rat CNS, the differential distribution of NA axons of the locus coeruleus (LC) and non-coerulean NA cells is particularly striking in the spinal cord and brainstem. In these regions, NA axons of the LC are primarily distributed to sensory nuclei while NA axons of non-coerulean NA neurons are distributed to motor nuclei. The results support the proposition that NA neurons can be divided into subgroups which differ in their connections and hence represent separated anatomical entities with different functional capacities.

Afferent regulation of locus coeruleus neurons: anatomy, physiology and pharmacology

Tract-tracing and electrophysiology studies have revealed that major inputs to the nucleus locus coeruleus (LC) are found in two structures, the nucleus paragigantocellularis (PGi) and the perifascicular area of the nucleus prepositus hypoglossi (PrH), both located in the rostral medulla. Minor afferents to LC were found in the dorsal cap of the paraventricular hypothalamus and spinal lamina X. Recent studies have also revealed limited inputs from two areas nearby the LC, the caudal midbrain periaqueductal gray (PAG) and the ventromedial pericoerulear region. The pericoeruleus may provide a local circuit interface to LC neurons. Recent electron microscopic analyses have revealed that LC dendrites extend preferentially into the rostromedial and caudal juxtaependymal pericoerulear regions. These extracoerulear LC dendrites may receive afferents in addition to those projecting to LC proper. However, single-pulse stimulation of inputs to such dendritic regions reveals little or no effect on LC neurons. Double-labeling studies have revealed that a variety of neurotransmitters impinging on LC neurons originate in its two major afferents, PGi and PrH. The LC is innervated by PGi neurons that stain for markers of adrenalin, enkephalin or corticotropin-releasing factor. Within PrH, large proportions of LC-projecting neurons stained for GABA or met-enkephalin. Finally, in contrast to previous conclusions, the dorsal raphe does not provide the robust 5-HT innervation found in the LC. We conclude that 5-HT inputs may derive from local 5-HT neurons in the pericoerulear area. Neuropharmacology experiments revealed that the PGi provides a potent excitatory amino acid (EAA) input to the LC, acting primarily at non-NMDA receptors in the LC. Other studies indicated that this pathway mediates certain sensory responses of LC neurons. NMDA-mediated sensory responses were also revealed during local infusion of magnesium-free solutions. Finally, adrenergic inhibition of LC from PGi could also be detected in nearly every LC neuron tested when the EAA-mediated excitation is first eliminated. In contrast to PGi, the PrH potently and consistently inhibited LC neurons via a GABAergic projection acting at GABAA receptors within LC. Such PrH stimulation also potently attenuated LC sensory responses. Finally, afferents to PGi areas that also contain LC-projecting neurons were identified. Major inputs were primarily autonomic in nature, and included the caudal medullary reticular formation, the parabrachial and Kölliker-Fuse nuclei, the PAG, NTS and certain hypothalamic areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Mesencephalic projections to the thalamic centralis lateralis and medial prefrontal cortex: a WGA-HRP study

In order to provide anatomical information for a possible pathway involved in pain mechanisms, rats were injected with horseradish peroxidase wheat germ agglutinin (WGA-HRP) in the centralis lateralis nucleus of the thalamus (Cl) or in the medial prefrontal cortex (PFCx) from which originated retrogradely labelled cells in the dorsal raphe nucleus (DR), locus ceruleus (LC) and surrounding structures. The locations of the Cl and the PFCx injections were previously determined by the presence of evoked single neuronal responses to noxious stimulations. The present study gives evidence for ascending pathways which originated in DR and LC and project to the Cl and PFCx. LC and DR projections suggest a possible route to an ascending modulation pain system.

Electrophysiological evidences of a bidirectional communication between the locus coeruleus and the suprachiasmatic nucleus

To assess a possible relation between the suprachiasmatic nucleus (SCN) and the locus coeruleus (LC), a study with evoked potentials was performed in the rat. An evoked potential was recorded in the SCN area after electrical stimulation of the LC. Also, an evoked potential was recorded in the LC after electrical stimulation of the SCN area. The results indicate specificity of the projecting regions and of the activated regions, suggesting the existence of a possible bidirectional communication between the LC and the SCN. The pathways possibly involved are discussed.

Two types of locus coeruleus neurons born on different embryonic days in the mouse

Retrograde axonal tracing studies were performed in combination with tritiated thymidine cell birthday analyses in order to determine whether or not any hodologicotemporal gradients exist in neuron genesis within the murine locus coeruleus. Following injections of retrograde tracers within the forebrain or cerebellum in mice exposed in utero to the radiolabeled nucleoside on embryonic days 9-11 (E9-11), combined histochemical and autoradiographic preparations revealed: 1) Locus coeruleus neurons that give rise to long distance axonal projections to the cortices are born exclusively on E9 (other studies indicate that these cells are noradrenergic); and 2) Locus coeruleus cells born on E10 and E11 are a class of smaller cells which were never observed to project to distant structures. The transmitters of these apparent local circuit neurons have not yet been determined, but gamma aminobutyric acid is one possible candidate. These findings support the interpretation that monoaminergic neurons tend to arise earlier during development than non-monoaminergic neurons within the locus coeruleus, and that distinctly different connectional arrangements exist for these monoaminergic and non-monoaminergic cells.

Effects of sex steroids on the development of the locus coeruleus in the rat

The effects of postnatal (on day 1 (D1) after birth) male orchidectomy and female androgenization on the locus coeruleus (LC) are studied. Normal adult female rats show greater LC volume and cell number than males. Testosterone treatment of females on D1 eliminates these differences. However, D1 male orchidectomy does not affect either the volume or the number of cells.

Synchronous bursting of locus coeruleus neurons in tissue culture

Synchronous bursts of firing of locus coeruleus neurons have been observed in unanesthetized rats, particularly in response to various sensory stimuli. In explant tissue cultures, synchronous bursting activity of locus coeruleus neurons was also observed and the possible mechanisms responsible for this synchronous activation have been investigated. Barrages of depolarizing events apparently initiated and continued throughout spontaneous bursts of spikes in the cultured neurons. Simultaneous intracellular recordings from pairs of neurons show a very high degree of synchrony of such barrages between cells. On the basis of tests for electrical coupling in simultaneously recorded cell pairs, and tests for dye coupling with Lucifer Yellow, it was concluded that the synchrony is not due to electrical coupling of locus coeruleus neurons. Small non-synaptic interactions between cell pairs that may reflect elevated extracellular potassium levels have been observed on some occasions. Spontaneous and evoked depolarizations similar to those initiating the bursts appear to be synaptically mediated events, suggesting that locus coeruleus neurons are synchronously activated by a common excitatory input. It was concluded that the neurons providing this common excitation are located within or very close to the locus coeruleus, at least at birth. The synchronization of activation of many locus coeruleus neurons could result in almost simultaneous release of neurotransmitter in the widespread target areas of locus coeruleus projections.

Antemortem markers of Alzheimer's disease

This review examines various approaches to the development of antemortem markers of Alzheimer's disease. Among the procedures discussed are: neurochemical and histopathologic studies of the cholinergic system, concentrating on CSF and blood plasma; genetic studies; imaging and electrophysiological studies; and neuroendocrine studies.

Efferent projections of nucleus locus coeruleus: morphologic subpopulations have different efferent targets

This study quantitatively addresses the hypothesis that there is a systematic relationship between the morphologic characteristics of locus neurons and the particular target regions they innervate. Following horseradish peroxidase injections into selected terminal fields, locus coeruleus cell bodies are heavily labeled by retrograde transport so that somata size and shape, and in many cases primary dendritic pattern can be observed. This allows the classification of neurons as one of six cell types: large multipolar cells within ventral locus coeruleus, large multipolar cells in the anterior pole of locus coeruleus, fusiform cells in dorsal LC, posterior pole cells, medium-sized multipolar cells (termed core cells in this report), and small round cells. It was found that while core cells contribute to the innervation of all terminal fields examined, other cell types project to more restricted sets of targets. The contributions of each type to selected efferents are presented in detail. In particular, fusiform cells project to hippocampus and cortex, large multipolar cells in ventral locus coeruleus project to spinal cord and cerebellum, and small round cells in central and anterior locus coeruleus, as well as large multipolar cells in anterior locus coeruleus, project to hypothalamus. These results, in conjunction with those described in the preceding report, indicate that locus coeruleus is intrinsically organized with respect to efferent projections with much more specificity than has previously been evident. This high degree of organization is consistent with other recent demonstrations of functional specificity exhibited by locus coeruleus neurons.

Morphologic and functional abnormalities that develop in kitten Purkinje neurons during maintenance for months after maturation in organotypic cultures

The morphologic and functional properties of the Purkinje cells (P-cells) grown for 10-11 weeks in organotypic cultures from newborn kitten cerebella were studied and compared to cultures which had been grown for 4-5 weeks under the same standard conditions. Electrophysiological and morphological data were obtained from HRP iontophoretically labeled neurons and were quantified by means of computerized techniques. Extracellular recordings of spontaneous activity showed that the 10-11-week-old P-cells had a pacemaker-like firing rate whereas the P-cells aged 4-5 weeks in vitro displayed a bursting activity. The qualitative morphological data evidenced abnormal swellings both on dendritic and axonal processes of the 10-11-week-old P-cells which were not present on the 4-5-week-old P-cells. The quantitative data revealed a significant decrease in the overall size of the dendritic network of the 10-11-week-old P-cells mainly due to a reduction in the total dendritic length and in the total number of dendritic segments, whereas the individual segment lengths remained almost unchanged. Dendritic spine counts showed no decrease in the dendritic density of these older P-cells. Such data suggest that the changes observed in 10-11-week-old cultured P-cells may be compared to the age-related changes occurring in vivo and that such in vitro models could be useful tools in the study of the pathology of aging. However, alternative factors other than senescence are discussed since they may account for some degenerative changes observed in the older cultured P-cells.

The central neural connections of the area postrema of the rat

We applied the neuroanatomical tracers cholera toxin-horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase to investigate the neural connections of the area postrema (AP) in the rat. We find that the AP projects to the nucleus of the solitary tract (NTS) and dorsal motor nucleus of the vagus bilaterally both rostral and caudal to obex; the nucleus ambiguus; the dorsal aspect of the spinal trigeminal tract and nucelus and the paratrigeminal nucleus; the region of the ventrolateral medullary catecholaminergic column; the cerebellar vermis; and a cluster of structures in the dorsolateral pons which prominently include a discrete set of subnuclei in the lateral parabrachial nucleus. The major central afferent input to the area postrema is provided by a group of neurons in the paraventricular and dorsomedial hypothalamic nuclei whose collective dendrites describe a horizontally oriented plexus which encircles the parvocellular nucleus of the hypothalamus bilaterally. In addition, the caudal NTS may project lightly to the AP. The lateral parabrachial nucleus provides a very light input as well. These connections, when considered in the context of the known vagal afferent input and reduced blood-brain barrier of AP, place this structure in a unique position to receive and modulate ascending interoceptive information and to influence autonomic outflow as well.

Anatomical alterations in locus coeruleus neurons in the adult spontaneously hypertensive rat

A golgi-Cox examination of the locus coeruleus in the spontaneously hypertensive rat (SHR) has shown an increase in the number of primary dendritic branches, in the number of secondary branch points, in the mean length of the dendritic branches from the soma to the outer extent of the secondary branches, and in the longest extent of the dendritic domain in comparison with the locus coeruleus in the Wistar-Kyoto normotensive rats at sixteen weeks of age. We suggest that the anatomical changes may provide a substrate for altered afferent relationships in the neurochemical regulation of neurons of the locus coeruleus, although it remains to be shown whether such alterations are related in any way to blood pressure. It appears evident that the altered mechanisms resulting in hypertension in this genetic model are more complex than an anatomical or neurochemical modification in one of the central autonomic nuclei. However, the present finding that anatomical alterations in the dendritic arborizations of a key nucleus such as the locus coeruleus occur, points towards a need for further examination of the interrelationships of specific neurotransmitter systems in this nucleus.

Membrane properties of rat locus coeruleus neurones

Intracellular recordings were made from neurones in the locus coeruleus contained within a slice cut from rat pons and maintained in vitro. Most neurones fired action potentials spontaneously at frequencies of between 1 and 5 Hz; this did not arise from spontaneous synaptic input but appeared to result from endogenous properties of the membrane conductances. Under voltage clamp at potentials near threshold for action potential generation (-55 mV) there was a persistent inward calcium current. This current became less with membrane hyperpolarization and was abolished at about -70 mV. Two potassium currents were observed. The first had properties similar to that generally described as the "fast" potassium current (IK,A); it flowed transiently (for about 200 ms) when the membrane potential passed from about -65 to -45 mV, and was blocked by 4-aminopyridine. The second was a calcium-activated potassium current (IK,Ca); it flowed for several seconds following a burst of calcium action potentials. Spontaneous and evoked action potentials had both tetrodotoxin-sensitive and tetrodotoxin-resistant components. The latter was apparently due to calcium entry. The potential changes occurring during the spontaneous firing of locus coeruleus neurones could be reconstructed qualitatively from the ionic conductances observed. The membrane properties of the locus coeruleus neurones were remarkably uniform; however, about 5% of cells impaled within the region of the locus coeruleus were electrophysiologically distinct. These atypical cells had short duration action potentials, did not fire spontaneously and had large spontaneous depolarizing synaptic potentials.

Hyperpolarizing and age-dependent depolarizing responses of cultured locus coeruleus neurons to noradrenaline

The electrical activity and responses to noradrenaline (NA) of locus coeruleus (LC) neurons have been studied in organotypic cultures using intracellular recording. Most LC neurons were predominantly quiescent, though occasional bursts of activity were observed; a few cells were tonically active at rates of 0.5-5/s. In most cells tested, iontophoretic application of NA evoked responses which were initially hyperpolarizing, sometimes followed by a depolarizing phase and frequently followed by a period of increased excitatory synaptic activity. The enhanced synaptic activity appeared to be an indirect effect since it was blocked by bath application of tetrodotoxin (TTX). In the presence of TTX, responses to NA of all but one cell were simple hyperpolarizations or biphasic (hyperpolarization/depolarization) responses. The presence of the depolarizing component appeared to be age-dependent, since it was frequently observed in cultures grown in vitro for less than 26 days, while neurons in older cultures exhibited only hyperpolarizing responses. If such age-dependent depolarizing responses are present in vivo, they would represent a unique example of a transmitter response which is present only during a transient developmental phase.

Selective prefrontal cortical projections to the region of the locus coeruleus and raphe nuclei in the rhesus monkey

Descending cortical projections to the region of the noradrenergic locus coeruleus (LC) and the serotonergic dorsal and central superior raphe nuclei were analyzed in the rhesus monkey using anterograde labeling techniques. HRP pellets or tritiated leucine were injected into one of 7 cortical areas: the dorsolateral prefrontal cortex, the dorsomedial prefrontal cortex, the orbital prefrontal cortex, the parietal association cortex, somatosensory cortex, the anterior portion of the inferior temporal gyrus, and the posterior portion of the inferior temporal gyrus. Anterogradely labeled fibers were found in and adjacent to the LC and raphe nuclei only following the dorsolateral and dorsomedial prefrontal cortical injections. Terminal labeling was densest at rostral levels of the LC, particularly in the area directly medial to the nucleus. Labeled fibers could not be followed beyond caudal levels of the LC. The projections to the contralateral LC and raphe nuclei were similar to, but less dense than that to the ipsilateral region. Injections into cortical areas other than the dorsal prefrontal cortex resulted in anterograde labeling of the pontine nuclei or pyramids, but not the LC/raphe region. These data, in conjunction with studies in the rat, suggest that the dorsal prefrontal cortex may be the only cortical area to have direct influence on the LC and raphe nuclei and secondary influence on the monoaminergic innervation of large areas of cerebral cortex.

The effects of protein deprivation on the nucleus locus coeruleus: a morphometric Golgi study in rats of three age groups

In a previous morphometric Golgi study of the nucleus locus coeruleus we identified in rats fed a 25% casein diet 3 cell types, fusiform, multipolar and ovoid, and compared their age-related changes from 30 to 90 days and 90 to 220 days. In the present study we investigated the effects of an 8% casein diet, initiated prenatally and continued postnatally in the pups, using the same morphometric parameters at the same 3 ages. In these rats the majority of significant age-related changes were in primary and secondary dendritic spine density. On all 3 cell types between 30 and 90 days there was a decreased spine density followed by an increase between 90 and 220 days. These age-related changes closely followed those in controls and, as a result, when the two diet groups were compared at each age they showed only 4 significant differences out of 18 comparisons. In control rats these age-related changes in dendritic spine density in the nucleus locus coeruleus were diametrically out-of-phase with those found in the nucleus raphe dorsalis in a previous study. This is consistent with the postulated mutual inhibitory relationship between the nucleus locus coeruleus and nucleus raphe dorsalis. Comparison of these same events in the nucleus locus coeruleus and nucleus raphe dorsalis in the 8% casein diet rats show no evidence of an out-of-phase relationship. Thus, the relationship between these two closely related nuclei appears to be fundamentally altered by the 8% casein diet. In these comparisons of dendrite spine density and other parameters the 8% casein diet rats have shown in both the nucleus locus coeruleus and nucleus raphe dorsalis deficits and increases as compared to controls. In contrast, in morphometric Golgi studies of pyramidal and Purkinje cells undernutrition generally results in deficits or little change in the various parameters. This suggests that the adaptation of the non-pyramidal neurons in the present study is different from that shown by pyramidal and Purkinje cells.