The fine structural organization of the locus coeruleus in the rat with reference to noradrenaline contents

Cocaine increases quantal norepinephrine secretion through NET-dependent PKC activation in locus coeruleus neurons

The norepinephrine neurons in locus coeruleus (LC-NE neurons) are essential for sleep arousal, pain sensation, and cocaine addiction. According to previous studies, cocaine increases NE overflow (the profile of extracellular NE level in response to stimulation) by blocking the NE reuptake. NE overflow is determined by NE release via exocytosis and reuptake through NE transporter (NET). However, whether cocaine directly affects vesicular NE release has not been directly tested. By recording quantal NE release from LC-NE neurons, we report that cocaine directly increases the frequency of quantal NE release through regulation of NET and downstream protein kinase C (PKC) signaling, and this facilitation of NE release modulates the activity of LC-NE neurons and cocaine-induced stimulant behavior. Thus, these findings expand the repertoire of mechanisms underlying the effects of cocaine on NE (pro-release and anti-reuptake), demonstrate NET as a release enhancer in LC-NE neurons, and provide potential sites for treatment of cocaine addiction.

Spatiotemporal characterization of cellular tau pathology in the human locus coeruleus-pericoerulear complex by three-dimensional imaging

Tau pathology of the noradrenergic locus coeruleus (LC) is a hallmark of several age-related neurodegenerative disorders, including Alzheimer's disease. However, a comprehensive neuropathological examination of the LC is difficult due to its small size and rod-like shape. To investigate the LC cytoarchitecture and tau cytoskeletal pathology in relation to possible propagation patterns of disease-associated tau in an unprecedented large-scale three-dimensional view, we utilized volume immunostaining and optical clearing technology combined with light sheet fluorescence microscopy. We examined AT8⁺ pathological tau in the LC/pericoerulear region of 20 brains from Braak neurofibrillary tangle (NFT) stage 0-6. We demonstrate an intriguing morphological complexity and heterogeneity of AT8⁺ cellular structures in the LC, representing various intracellular stages of NFT maturation and their diverse transition forms. We describe novel morphologies of neuronal tau pathology such as AT8⁺ cells with fine filamentous somatic protrusions or with disintegrating soma. We show that gradual dendritic atrophy is the first morphological sign of the degeneration of tangle-bearing neurons, even preceding axonal lesions. Interestingly, irrespective of the Braak NFT stage, tau pathology is more advanced in the dorsal LC that preferentially projects to vulnerable forebrain regions in Alzheimer's disease, like the hippocampus or neocortical areas, compared to the ventral LC projecting to the cerebellum and medulla. Moreover, already in the precortical Braak 0 stage, 3D analysis reveals clustering tendency and dendro-dendritic close appositions of AT8⁺ LC neurons, AT8⁺ long axons of NFT-bearing cells that join the ascending dorsal noradrenergic bundle after leaving the LC, as well as AT8⁺ processes of NFT-bearing LC neurons that target the 4th ventricle wall. Our study suggests that the unique cytoarchitecture, comprised of a densely packed and dendritically extensively interconnected neuronal network with long projections, makes the human LC to be an ideal anatomical template for early accumulation and trans-neuronal spreading of hyperphosphorylated tau.

The Locus Coeruleus- Norepinephrine System in Stress and Arousal: Unraveling Historical, Current, and Future Perspectives

Arousal may be understood on a spectrum, with excessive sleepiness, cognitive dysfunction, and inattention on one side, a wakeful state in the middle, and hypervigilance, panic, and psychosis on the other side. However, historically, the concepts of arousal and stress have been challenging to define as measurable experimental variables. Divergent efforts to study these subjects have given rise to several disciplines, including neurobiology, neuroendocrinology, and cognitive neuroscience. We discuss technological advancements that chronologically led to our current understanding of the arousal system, focusing on the multifaceted nucleus locus coeruleus. We share our contemporary perspective and the hypotheses of others in the context of our current technological capabilities and future developments that will be required to move forward in this area of research.

Neuropeptide and Small Transmitter Coexistence: Fundamental Studies and Relevance to Mental Illness

Neuropeptides are auxiliary messenger molecules that always co-exist in nerve cells with one or more small molecule (classic) neurotransmitters. Neuropeptides act both as transmitters and trophic factors, and play a role particularly when the nervous system is challenged, as by injury, pain or stress. Here neuropeptides and coexistence in mammals are reviewed, but with special focus on the 29/30 amino acid galanin and its three receptors GalR1, -R2 and -R3. In particular, galanin's role as a co-transmitter in both rodent and human noradrenergic locus coeruleus (LC) neurons is addressed. Extensive experimental animal data strongly suggest a role for the galanin system in depression-like behavior. The translational potential of these results was tested by studying the galanin system in postmortem human brains, first in normal brains, and then in a comparison of five regions of brains obtained from depressed people who committed suicide, and from matched controls. The distribution of galanin and the four galanin system transcripts in the normal human brain was determined, and selective and parallel changes in levels of transcripts and DNA methylation for galanin and its three receptors were assessed in depressed patients who committed suicide: upregulation of transcripts, e.g., for galanin and GalR3 in LC, paralleled by a decrease in DNA methylation, suggesting involvement of epigenetic mechanisms. It is hypothesized that, when exposed to severe stress, the noradrenergic LC neurons fire in bursts and release galanin from their soma/dendrites. Galanin then acts on somato-dendritic, inhibitory galanin autoreceptors, opening potassium channels and inhibiting firing. The purpose of these autoreceptors is to act as a 'brake' to prevent overexcitation, a brake that is also part of resilience to stress that protects against depression. Depression then arises when the inhibition is too strong and long lasting - a maladaption, allostatic load, leading to depletion of NA levels in the forebrain. It is suggested that disinhibition by a galanin antagonist may have antidepressant activity by restoring forebrain NA levels. A role of galanin in depression is also supported by a recent candidate gene study, showing that variants in genes for galanin and its three receptors confer increased risk of depression and anxiety in people who experienced childhood adversity or recent negative life events. In summary, galanin, a neuropeptide coexisting in LC neurons, may participate in the mechanism underlying resilience against a serious and common disorder, MDD. Existing and further results may lead to an increased understanding of how this illness develops, which in turn could provide a basis for its treatment.

Amyloid beta peptides, locus coeruleus-norepinephrine system and dense core vesicles

The evolution of peptidergic signaling systems in the central nervous system serves a distinct and crucial role in brain processes and function. The diversity of physiological peptides and the complexity of their regulation and secretion from the dense core vesicles (DCV) throughout the brain is a topic greatly in need of investigation, though recent years have shed light on cellular and molecular mechanisms that are summarized in this review. Here, we focus on the convergence of peptidergic systems onto the Locus Coeruleus (LC), the sole provider of norepinephrine (NE) to the cortex and hippocampus, via large DCV. As the LC-NE system is one of the first regions of the brain to undergo degeneration in Alzheimer's Disease (AD), and markers of DCV have consistently been demonstrated to have biomarker potential for AD progression, here we summarize the current literature linking the LC-NE system with DCV dysregulation and Aβ peptides. We also include neuroanatomical data suggesting that the building blocks of senile plaques, Aβ monomers, may be localized to DCV of the LC and noradrenergic axon terminals of the prefrontal cortex. Finally, we explore the putative consequences of chronic stress on Aβ production and the role that DCV may play in LC degeneration. Clinical data of immunological markers of DCV in AD patients are discussed.

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.

Looking at neurotransmitters in the microscope

This review article covers the early period of my career. I first summarize research initiated by the late Nils-Ake Hillarp, after his appointment in 1962 as professor in the Department of Histology at Karolinska Institutet. He only lived for three more years, but during this short period he started up a group of ten students who explored various aspects of the three monoamine transmitters, dopamine, noradrenaline and 5-hydroxytryptamine, using the new formaldehyde fluorescence method developed by Bengt Falck and Hillarp in Lund. This method allowed visualization of the cellular localization in the microscope of these monoamines, which introduced a new discipline in neurobiology-chemical neuroanatomy. I then deal with work aiming at localizing the monoamines at the ultrastructural level, as well as attempts to use radioactively labeled aminoacids, especially gamma-aminobutyric acid (GABA), and autoradiography, to identify, in the microscope, neurons using such transmitters. Finally, our immunohistochemical work together with Kjell Fuxe and the late Menek Goldstein, using antibodies to four monoamine-synthesizing enzymes is summarized, including some aspects on the adrenaline neurons, which had escaped detection with the Falck-Hillarp technique.

Long latency of evoked quantal transmitter release from somata of locus coeruleus neurons in rat pontine slices

The locus coeruleus (LC) harbors a compact group of noradrenergic cell bodies projecting to virtually all parts of the central nervous system. By using combined measurements of amperometry and patch-clamp, quantal vesicle release of noradrenaline (NA) was detected as amperometric spikes, after depolarization of the LC neurons. After a pulse depolarization, the average latency of amperometric spikes was 1,870 ms, whereas the latency of glutamate-mediated excitatory postsynaptic currents was 1.6 ms. A substantial fraction of the depolarization-induced amperometric spikes originated from the somata. In contrast to glutamate-mediated excitatory postsynaptic currents, NA secretion was strongly modulated by the action potential frequency (0.5-50 Hz). Somatodendritic NA release from LC upon enhanced cell activity produced autoinhibition of firing and of NA release. We conclude that, in contrast to classic synaptic transmission, quantal NA release from LC somata is characterized by a number of distinct properties, including long latency and high sensitivity to action potential frequency.

Functional role of alpha1-adrenoceptors in the locus coeruleus: A microdialysis study

The present study elucidates the role of alpha(1)-adrenoreceptors in the locus coeruleus (LC) using a dual-probe microdialysis in conscious rats. One probe sampled noradrenaline in the LC, whereas the second probe sampled noradrenaline in a main projection area, the prefrontal cortex (PFC). To investigate a possible tonic activation of LC neurons by alpha(1)-adrenoceptor, the alpha(1)-antagonist prazosin (10 microM) was infused into the LC. Extracellular noradrenaline in the LC decreased to about 50% of basal levels but no change of noradrenaline release was detected in the ipsilateral PFC. Next, the interaction between alpha(1)- and alpha(2)-adrenoceptors was investigated. Local administration of the alpha(2)-adrenoceptor antagonist idazoxan (100 microM) into the LC increased the noradrenaline release in the LC to about 400%, whereas noradrenaline release in the PFC rose to 150% of basal levels. A similar effect was seen when the specific alpha(2A)-adrenoceptor antagonist BRL 44408 (10 microM) was infused: extracellular noradrenaline in the LC and PFC increased to about 400 and 120% of the basal levels, respectively. When infusions of idazoxan (100 microM) or BRL 44408 (10 microM) into the LC were combined with prazosin (10 microM), the excitatory effects of the alpha(2)-adrenoceptor antagonists on the release of noradrenaline were strongly suppressed in the LC as well as in the ipsilateral PFC. It is concluded that alpha(1)-adrenoreceptors are involved in the regulation of LC activity. Apparently, alpha(1)- and alpha(2)-adrenoceptors have opposite roles in their function as autoreceptors on LC cells.

Characterization of noradrenaline release in the locus coeruleus of freely moving awake rats by in vivo microdialysis

RATIONALE

The origin and regulation of noradrenaline (NA) in the locus coeruleus (LC) is unknown.

OBJECTIVES

The neurochemical features of NA overflow (nerve impulse dependence, neurotransmitter synthesis, vesicle storage, reuptake, alpha2-adrenoceptor-mediated regulation) were characterized in the LC.

METHODS

Brain microdialysis was performed in awake rats. Dialysates were analyzed for NA.

RESULTS

NA in the LC decreased via local infusion of Ca2+-free medium (-42+/-5%) or the sodium channel blocker tetrodotoxine (TTX) (-47+/-8%) but increased (333+/-40%) via KCl-induced depolarization. The tyrosine hydroxylase (TH) inhibitor alpha-methyl-p-tyrosine (250 mg kg(-1), i.p.) and the vesicle depletory drug reserpine (5 mg kg(-1), i.p.) decreased NA. Therefore, extracellular NA in the LC satisfies the criteria for an impulse flow-dependent vesicular exocytosis of neuronal origin. Local perfusion of the alpha2-adrenoceptor agonist clonidine (0.1-100 microM) decreased NA (E(max)=-79+/-5%) in the LC, whereas the opposite effect (E(max)=268+/-53%) was observed with the alpha2A-adrenoceptor antagonist BRL44408 (0.1-100 microM). This suggests a tonic modulation of NA release through local alpha2A-adrenoceptors. The selective NA reuptake inhibitor desipramine (DMI) (0.1-100 microM) administered into the LC increased NA in the LC (E(max)=223+/-40%) and simultaneously decreased NA in the cingulate cortex, confirming the modulation exerted by NA in the LC on firing activity of noradrenergic cells and on the subsequent NA release in noradrenergic terminals.

CONCLUSION

Synaptic processes underlying NA release in the LC are similar to those in noradrenergic terminal areas. NA in the LC could represent local somatodendritic release, but also the presence of neurotransmitter release from collateral axon terminals.

The release of noradrenaline in the locus coeruleus and prefrontal cortex studied with dual-probe microdialysis

The present study was undertaken to investigate and compare the properties of noradrenaline release in the locus coeruleus (LC) and prefrontal cortex (PFC). For that aim the dual-probe microdialysis technique was applied for simultaneous detection of noradrenaline levels in the LC and PFC in conscious rats. Calcium omission in the LC decreased noradrenaline levels in the LC, but increased its levels in the PFC. Novelty increased noradrenaline levels in both structures. Infusion of the alpha(2)-adrenoceptor agonist clonidine decreased extracellular noradrenaline in the LC as well as in the PFC. Infusion of the alpha(2A)-adrenoceptor antagonist BRL44408, or the alpha(1)-adrenoceptor agonist cirazoline into the LC or PFC caused a similar dose-dependent increase in both structures. When BRL44408 or cirazoline were infused into the LC, few effects were seen in the PFC. Infusion of the 5-HT(1A)-receptor agonist flesinoxan into the LC or the PFC decreased the release of noradrenaline in both structures. When flesinoxan was infused into the LC, no effects were seen in the PFC. When the GABA(A) antagonist bicuculline was applied to the LC, noradrenaline increased in the LC as well as in the PFC. It is concluded that the release of noradrenaline from somatodendritic sites and nerve terminals responded in a similar manner to presynaptic receptor modulation. The possible existence of dendritic noradrenaline release is discussed.

Dendrites of locus coeruleus neurons extend preferentially into two pericoerulear zones

The intrinsic cytoarchitecture and neurochemical organization of the nucleus locus coeruleus have been characterized extensively, but there is little information about the organization of locus coeruleus neuronal processes extending outside of the nucleus proper. Light and electron microscopic immunocytochemical techniques were used to investigate the distribution of dopamine-beta-hydroxylase- or tyrosine-hydroxylase-labeled extranuclear processes in the rat pericoerulear region. The vast majority of these processes extended preferentially into two zones: (1) the pontine tegmentum medial and rostral to locus coeruleus, here termed the rostromedial pericoerulear region; and (2) a narrow region adjacent to the IVth ventricle caudomedial to locus coeruleus, designated here as the caudal juxtaependymal pericoerulear region. Far fewer labeled processes extended into the lateral and ventral pericoerulear regions. Seventy-seven percent of the labeled profiles in the pericoerulear region were dendrites. All labeled profiles in the rostromedial pericoerulear region and 94% of the labeled profiles in the caudal juxtaependymal zone were dendrites. By contrast, in the rostroventral pericoerulear region, 25% of the labeled profiles were axons. Locus coeruleus extranuclear dendrites were never presynaptic to other structures but were often contacted by several unlabeled presynaptic terminals. These results indicate that the dendrites of locus coeruleus neurons extend preferentially into two pericoerulear zones. Extranuclear dendrites in all pericoerulear regions receive extensive, nonnoradrenergic synaptic contacts. Thus, pericoerulear dendrites, particularly in the rostromedial and caudal juxtaependymal zones, are important sites for the integration of inputs to locus coeruleus neurons.

Afferent projections to the rat locus coeruleus demonstrated by retrograde and anterograde tracing with cholera-toxin B subunit and Phaseolus vulgaris leucoagglutinin

The aim of this study was to examine the afferents to the rat locus coeruleus by means of retrograde and anterograde tracing experiments using cholera-toxin B subunit and phaseolus leucoagglutinin. To obtain reliable injections of cholera-toxin B in the locus coeruleus, electrophysiological recordings were made through glass micropipettes containing the tracer and the noradrenergic neurons of the locus coeruleus were identified by their characteristic discharge properties. After iontophoretic injections of cholera-toxin B into the nuclear core of the locus coeruleus, we observed a substantial number of retrogradely labeled cells in the lateral paragigantocellular nucleus and the dorsomedial rostral medulla (ventromedial prepositus hypoglossi and dorsal paragigantocellular nuclei) as previously described. We also saw a substantial number of retrogradely labeled neurons in (1) the preoptic area dorsal to the supraoptic nucleus, (2) areas of the posterior hypothalamus, (3) the Kölliker-Fuse nucleus, (4) mesencephalic reticular formation. Fewer labeled cells were also observed in other regions including the hypothalamic paraventricular nucleus, dorsal raphe nucleus, median raphe nucleus, dorsal part of the periaqueductal gray, the area of the noradrenergic A5 group, the lateral parabrachial nucleus and the caudoventrolateral reticular nucleus. No or only occasional cells were found in the cortex, the central nucleus of the amygdala, the lateral part of the bed nucleus of the stria terminalis, the vestibular nuclei, the nucleus of the solitary tract or the spinal cord, structures which were previously reported as inputs to the locus coeruleus. Control injections of cholera-toxin B were made in areas surrounding the locus coeruleus, including (1) Barrington's nucleus, (2) the mesencephalic trigeminal nucleus, (3) a previously undefined area immediately rostral to the locus coeruleus and medial to the mesencephalic trigeminal nucleus that we named the peri-mesencephalic trigeminal nucleus, and (4) the medial vestibular nucleus lateral to the caudal tip of the locus coeruleus. These injections yielded patterns of retrograde labeling that differed from one another and also from that obtained with cholera-toxin B injection sites in the locus coeruleus. These results indicate that the area surrounding the locus coeruleus is divided into individual nuclei with distinct afferents. These results were confirmed and extended with anterograde transport of cholera-toxin B or phaseolus leucoagglutinin. Injections of these tracers in the lateral paragigantocellular nucleus, preoptic area dorsal to the supraoptic nucleus, the ventrolateral part of the periaqueductal gray, the Kölliker-Fuse nucleus yielded a substantial to large number of labeled fibers in the nuclear core of the locus coeruleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Locus coeruleus noradrenergic neurons as a micturition center

The micturition reflex arch is composed of an afferent pathway from the urinary bladder to the pontine micturition center via the pelvic nerve and spinal cord. The efferent pathway projects from the center to the bladder through the sacral parasympathetic center of intermediolateral column cells. The pontine micturition center is thought to be noradrenergic (NA) neurons in the locus coeruleus (LC) due to the following observations: (1) LC stimulation induces bladder contraction in cats, and this response is blocked by intrathecal application of alpha 1-adrenergic antagonist (prazosin), but not the alpha 2- nor beta-adrenergic antagonist. Although this contraction is not observed after NA depletion with reserpine, with subsequent i.v. injection of L-dopa a NA precursor induces recurrence of the response. (2) The micturition reflex induced by bladder distention is similarly reversed with alpha 1-adrenergic antagonist and chemical destruction of NA cells by injecting 6-hydroxydopamine in the LC. However, subsequently applied alpha 1-adrenergic agonist induces the contraction due to bladder distention. (3) LC stimulation elicits spike generation of sacral intermediolateral cells. Microiontophoretically applied alpha 1-adrenergic antagonist inhibits the LC stimulation-induced spikes of the neuron, which is not antidromically activated by pelvic nerve stimulation. However, spikes in neurons activated antidromically were not affected by the drug. This indicates that the former and latter neurons are interneuron and parasympathetic projecting neurons, respectively. The existence of NA terminals from the LC and alpha 1-adrenergic receptors in the intermediolateral column cells supports the concept that NA cells in the LC are units constituting the micturition center.

Locus coeruleus and dorsal pontine reticular influences on the gain of vestibulospinal reflexes

Experimental anatomical and physiological studies have shown that noradrenergic locus coeruleus (LC) neurons, which are NE-sensitive due to inhibitory adrenoceptors, send inhibitory afferents to neurons of the peri-LC alpha and the adjacent dorsal pontine reticular formation (pRF); on the other hand these tegmental neurons, which are, in part at least, cholinergic as well as cholinoceptive, send excitatory afferents to the medullary inhibitory reticulospinal (RS) system. Experiments performed in precollicular decerebrate cats indicate that these pontine structures exert a regulatory influence on posture as well as on the gain of vestibulospinal (VS) reflexes. In particular, the increased discharge of dorsal pontine reticular neurons, and the related inhibitory RS neurons induced by microinjection of cholinergic agonists into the peri-LC alpha and the adjacent pRF of one side, decreased the postural activity, but greatly increased the response gain of the ipsilateral triceps brachii in response to stimulation of labyrinth receptors resulting from roll tilt of the animal (at 0.15 Hz, +/- 10 degrees). Similar results were also obtained when the discharge of these pontine and medullary reticular neurons was raised, either by local injection into the peri-LC alpha and the dorsal pRF of the beta-adrenergic antagonist propranolol, which blocked the inhibitory influence of the noradrenergic LC neurons on these structures, or by local injection into the LC complex of an alpha 2- or beta-adrenergic agonist (clonidine or isoproterenol) which led to functional inactivation of the noradrenergic neurons; in the latter case the effects were bilateral. Just the opposite results were obtained after microinjection into the LC of a cholinergic agonist, leading to activation of the corresponding neurons. Evidence was also presented indicating that the cholinergic excitatory afferents to the LC originated from the ipsilateral dorsal pRF. The effects described above were dose-dependent and site-specific, as shown by histological controls. Under given conditions, the decrease in postural activity induced either by direct activation of presumptive cholinergic and cholinoceptive pRF neurons or by inactivation of noradrenergic and NE-sensitive LC neurons was followed by transient episodes of postural atonia which lasted several minutes and affected the ipsilateral and sometimes also the contralateral limbs. In these instances, the EMG modulation of the corresponding triceps brachii to animal tilt was suppressed. These findings suggest two different ranges of operation for the noradrenergic and cholinergic structures located in the dorsolateral pontine tegmentum, leading either to a decrease or to an increase in gain of the VS reflexes. The cellular basis of these gain changes is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Monoaminergic interaction in the central nervous system: a morphological analysis in the locus coeruleus of the rat

The locus coeruleus of the rat is richly innervated by many aminergic neurons varying in amine content and in site of origin. There are adrenergic and noradrenergic neurons originating in the medulla oblongata, dopaminergic from the hypothalamus, serotonergic from the mesencephalon and also intrinsic noradrenergic neurons in the locus coeruleus complex. Of these, adrenergic and dopaminergic inputs appear relatively specific and powerful.

3,4-Dihydroxyphenylacetic acid (DOPAC) as an index of noradrenaline turnover: effects of Hydergine and vincamine

Among the drugs commonly used in the treatment of memory disorders of the elderly, vincamine and hydergine have been shown to moderately increase the firing rate of noradrenergic locus coeruleus (LC) neurons. Since changes in electrical activity of noradrenergic neurons are generally reflected in corresponding alterations of the turnover of this transmitter, the effects of these drugs on the accumulation of 3,4-dihydroxyphenylacetic acid (DOPAC) and dopamine (DA) in the presence and absence of the dopamine-beta-hydroxylase inhibitor, FLA 63, were studied in the LC as well as in two of its projection areas, the hippocampus and the cerebellum. Characterization of this procedure with the alpha 2-adrenoceptor antagonist, idazoxan, the corresponding agonist, clonidine, the alpha 1-adrenoceptor antagonist prazosine, and haloperidol, suggested that--DOPAC changes are more suitable than those of DA or DOPAC/DA ratios in reflecting changes in noradrenaline (NA) turnover, inhibiting DBH is advantageous if NA turnover is to be measured in projection areas, but not in LC, and haloperidol and prazosine, in principle, did not affect NA turnover. Vincamine and hydergine at 10 mg/kg doses, at which they were reported to increase LC firing by 50%, did not induce a change in NA turnover in any of the areas. This, together with the data obtained with haloperidol, suggests that a minimal increase in the firing rate of LC cells (+140%) is required before it could influence the turnover of NA, as measured by DOPAC changes. Thus, the stimulating effect of nootropics on the central noradrenergic system may be more sensitively detected by electrophysiological techniques than by biochemical ones.

Ultrastructural localization of phenylethanolamine N-methyltransferase-like immunoreactivity in the rat locus coeruleus

Adrenergic afferents from the rostral ventrolateral medulla are known to modulate the activity of noradrenergic neurons of the locus coeruleus (LC). The light and electron microscopic localization of a polyclonal antiserum directed against the adrenaline synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT) was used to determine the identity and targets of the adrenergic afferents to the LC of the rat brain. By light microscopy, varicose processes showing intense PNMT-like immunoreactivity (LI) were seen throughout the neuropil surrounding neuronal perikarya which in adjacent sections were shown to contain immunoreactivity for the noradrenaline synthesizing enzyme, dopamine-beta-hydroxylase. Electron microscopy confirmed that these labeled varicose processes were primarily axon terminals. Terminals containing PNMT-LI constituted 30% (141 out of 464) of all identifiable terminals within the LC. These terminals were 0.5-1.8 micron in diameter and contained many small, clear and from 2 to 10 larger dense-core vesicles. The targets of the terminals with PNMT-LI were principally unlabeled (i.e. non-PNMT-containing) perikarya and dendrites. The synaptic junctions on perikarya were rare and exclusively symmetric; whereas, those on proximal (large) dendrites were somewhat more numerous and included symmetric as well as asymmetric membrane specializations. However, the vast majority (85% from a total of 141) of the terminals with PNMT-LI formed asymmetric synaptic junctions on unlabeled distal (small) dendrites and dendritic spines. In rare instances, the PNMT-immunoreactive terminals also formed synaptic junctions with other similarly labeled terminals. These findings provide the first ultrastructural evidence that adrenergic terminals in the LC (1) are one of the more prevalent synaptic inputs to the principally noradrenergic neurons; (2) have both symmetric and asymmetric synaptic specializations conventionally associated with inhibition and excitation, respectively; and (3) may modulate other adrenergic terminals through presynaptic mechanisms. In addition to the varicose processes, light microscopy revealed diffuse PNMT-LI throughout the LC. The ultrastructural correlate of this labeling was seen as patches of peroxidase product within the cytoplasm of a few perikarya and dendrites and throughout the cytoplasm of astrocytes identified by their discrete bundles of microfilaments. The detection of PNMT-LI in cells that are not known to synthesize adrenaline is surprising and suggests either a functional diversity for PNMT or amino acid sequence homologies with related enzymes which are enriched in the LC.

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.

Topographic relations of cholinergic and noradrenergic neurons in the feline pontomesencephalic tegmentum: an immunohistochemical study

The immunohistochemical localization of the neurotransmitter synthesizing enzymes choline acetyltransferase, tyrosine hydroxylase and dopamine-beta-hydroxylase was examined in the feline pontomesencephalic tegmentum. Examination of adjacent sections stained for either choline acetyltransferase, tyrosine hydroxylase or dopamine-beta-hydroxylase immunoreactivity, as well as individual sections doubly stained for both choline acetyltransferase and tyrosine hydroxylase immunoreactivity, unequivocally demonstrated that noradrenergic and cholinergic neurons were extensively intermingled in the brainstem tegmentum of the cat. This contrasts with the situation in various other species, where neurons utilizing these two neurotransmitters are discretely localized in distinct nuclei. Furthermore, the present studies demonstrate the existence of two types of choline acetyltransferase immunoreactive neurons in the feline tegmentum: the magnocellular neurons of the pedunculopontine and laterodorsal tegmental nuclei which stain histochemically for NADPH diaphorase, plus a population of small spindle-shaped neurons in the medial and lateral parabrachial nuclei which do not stain positively for NADPH diaphorase. The data are discussed with respect to several influential hypotheses of sleep cycle control.

Postnatal development of alpha-adrenoceptor-mediated autoinhibition in the locus coeruleus

Rats, from birth to postnatal day 34, were anesthetized with urethane and a neuropharmacological study was carried out of the autoreceptors located on the somadendritic membranes of locus coeruleus (LC) neurons. Iontophoretic application of noradrenaline (NA) caused inhibition of LC cell firing at all developmental stages, and such inhibition was totally blocked by the alpha 2-antagonist piperoxane. The sensitivity of LC neurons to iontophoretically applied NA appeared to become reduced with age. In LC neurons from birth to postnatal day (PD) 8, the prolonged period of suppressed firing after antidromic activation by stimulation of the dorsal noradrenergic bundle was not shortened by piperoxane. After PD 9, the proportion of LC neurons in which piperoxane could antagonize the postactivation inhibition increased with age. These results indicated that although LC neurons, even at birth, had alpha 2-adrenoceptors on the somadendritic membranes which were responsible for the NA-induced inhibition, inhibition of LC cell firing caused by NA released from the terminals of axon collaterals and/or possibly from dendrodendritic synapses did not occur until PD 9.

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.

Evidence for self- and neighbor-mediated postactivation inhibition of locus coeruleus neurons

Activation of locus coeruleus (LC) neurons is typically followed by inhibition of impulse activity lasting hundreds of ms. Previous studies have implicated two possible mechanisms for this postactivation inhibition: collateral synaptic interactions among LC neurons; and spike-induced, calcium-activated potassium conductance in the soma-dendritic membrane of LC cells. In the present study, antidromic or sensory stimuli were presented at near-threshold intensities for activation of LC neurons. A special computer program accumulated activity for trials yielding driven responses separately from that for trials of identical stimuli during the same train that failed to evoke activity. We found significant inhibition of LC impulse activity for antidromic or sensory stimuli that failed to excite the recorded cell as well as for stimuli that activated the recorded cell. The former result precludes an essential role of intrinsic inhibitory membrane currents (e.g. calcium-activated potassium conductance) in generating postactivation inhibition. Administration of the alpha antagonist piperoxane reduced the magnitude of inhibition on both driven and non-driven trials. Our findings indicate that inhibition on non-driven trials appears to be a synaptically mediated phenomenon, perhaps reflecting norepinephrine released from neighboring LC neurons that are activated. Furthermore, our data support the presence of a spike-dependent mechanism that also contributes substantially to postactivation inhibition in these cells. Thus, the overall results indicate the presence of two intracoerulear mechanisms that mediate postactivation inhibition characteristic of noradrenergic LC neurons.

Catecholamine inhibition of calcium action potentials in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus in a slice of tissue cut from the rat pons. Clonidine (100 nM-10 microM), noradrenaline (10 microM-1 mM) and adrenaline (10 microM-1 mM) all reduced the duration of the spontaneously occurring action potential of the neurones. This effect was also observed on the action potential in the presence of tetrodotoxin, which results from calcium entering the cell. These concentrations of clonidine, noradrenaline and adrenaline always hyperpolarized the membrane. This hyperpolarization was prevented by two procedures which block potassium currents--intracellular caesium and extracellular barium. In conditions of potassium current blockade, noradrenaline (100 microM-1 mM) and adrenaline (20 microM-1 mM) shortened the calcium action potential but clonidine was ineffective even at 10 microM. Adrenaline and noradrenaline also suppressed inward calcium and barium currents measured under voltage clamp. This action of noradrenaline and adrenaline was not prevented by yohimbine (10 microM), propranolol (20 microM) or prazosin (1 microM); it was reduced by a concentration of phentolamine about 100 times higher than its Ke for alpha 2-adrenoceptors on locus coeruleus neurones. It is concluded that noradrenaline and adrenaline can directly inhibit calcium action potentials in locus coeruleus neurones when applied in high concentrations, but that this does not involve an alpha 2-adrenoceptor.

Ultrastructural and histochemical evidence for differentiation of intraocular locus coeruleus grafts and invasion of the host iris by central neurites and glia

Intraocular grafts of dorso-lateral pons, including the noradrenaline-containing cell group locus coeruleus, have been studied with ultrastructural and histochemical techniques. Also, the invasion of neuronal and glial constituents from the grafts into the iris of the host animal is described. In mature brain grafts, aggregates of locus coeruleus neurons were easily discernible with monoamine histofluorescence. These cells had an ultrastructural appearance very similar to that in situ. Numerous somatic spines were frequently associated with synaptic specializations, and monoamine-containing vesicles could be found scattered in the cytoplasm of the locus coeruleus cells. Large neurons of the nucleus tractus mesencephalici nervi trigemini were also found. These cells were neurofilament-immunoreactive just as in situ, and were ultrastructurally characterized by size, distribution of the granular endoplasmic reticulum and abundant large terminals in synaptic contact with their somata and processes. All grafts showed a vigorous astroglial proliferation, evidenced both with immunohistochemistry of glial fibrillary acidic protein and electron microscopy. The astroglial cells were more numerous, larger and with more processes than in adult in situ counterparts. At the attachment site of the brain stem grafts, the iris dilator plate was entirely changed ultrastructurally by a vigorous invasion of neuronal and astrocytic processes. The normal, loose connective tissue stroma of the iris was replaced by layers of almost exclusively central nerve fibres and astrocytes respectively. Monoamine histofluorescence demonstrated an extreme adrenergic hyperinnervation of the iris at the attachment site of the graft, compared to the normal sympathetic ground plexus, whereas neurofilament immunohistochemistry did not visualize any substantial ingrowth of such positive central nerve fibres. Immunohistochemistry of glial fibrillary acidic protein strongly supported the ultrastructural evaluation, showing profound astroglial invasion deep into the iris stroma. Electron microscopic identification of central nerve fibres in the iris showed numerous adrenergic locus coeruleus fibres with small dense-core vesicles. Also, bundles of thin, central, unmyelinated axons were found deep in the iris as well as occasional dendrites. Both large dense-cored and small clear vesicles were encountered in the iris fibres of brain graft origin. Axo-dendritic synaptic specializations formed by locus coeruleus-derived adrenergic fibres were found in the iris.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrinsic regulation of locus coeruleus neurons: electrophysiological evidence indicating a predominant role for autoinhibition

Locus coeruleus neurons were antidromically activated and the resulting post-stimulation inhibition was compared to the interspike interval and examined for its dependency on antidromic invasion and stimulus intensity. The post-stimulation inhibition seen in these cells following antidromic activation approximated the interspike interval, was critically dependent on the antidromic invasion of the cell under study and was only weakly dependent on stimulus intensity. These results suggest that the post-stimulation inhibition following antidromic activation in the locus coeruleus is mediated principally by autoinhibition and not by hypothesized local inhibitory interactions between locus coeruleus neurons.

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.

The locus coeruleus and its possible role in ageing and degenerative disease of the human central nervous system

The central noradrenergic pathways with the mammalian brain are principally based on that group of nerve cells within the reticular substance of the upper pons known as the locus coeruleus. The physiological role of these nerve cells appears to be one of maintaining homeostasis within the central nervous system, whatever adverse conditions prevail in the rest of the body, through governing the flow of blood through, and degree of water permeability of, the capillary bed. The extensive ramifications of these noradrenergic terminals mean that the atrophy and loss of nerve cells from locus coeruleus that occurs in old age, and especially so in degenerative diseases of the central nervous system such as Alzheimer's disease and other conditions, will have widespread repercussions for brain function. The chain of physiological disturbances set up as a result of this cell loss may mean a progressive failure of homeostasis within the brain, which in the extreme may culminate in that pattern of mental breakdown which is usually termed dementia.

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

Using Rapid Golgi and Nissl techniques, 3 major cell types: fusiform, multipolar and ovoid shaped cells were identified in the nucleus locus coeruleus of male rats. Each cell type was described and quantitated as to age-related changes between 30 and 90 and between 90 and 220 days of age. The orientation and dendritic architecture of each type of cell in the locus coeruleus and relationship of these cells to blood vessels in the locus coeruleus and to surrounding structures is also described. One hundred neurons per age group were measured as to their maximal linear extent and the number of spines on the somal surfaces were counted. Dendritic number, linear extent, diameter and number of spines along a 50 microns segment near the mid-point of dendritic extensions in an equal number of primary and secondary dendrites were quantified for each age group and comparisons of these parameters between each cell group were made. Axons of each cell type were defined as to their origin and general orientation and trajectory. Axon collaterals of multipolar cells were shown to be recurrent in type projecting back onto the dendrites and soma of multipolar cells. One of the most striking findings was that between 30 and 90 days there were significant decreases in spine density on both primary and secondary dendrites in all three cell types in the locus coeruleus. This was followed by significant increases in spine density on both primary and secondary dendrites between 90 and 220 days in each of the 3 cell types. It is of interest that these age-related cell changes in spine density in the nucleus locus coeruleus are exactly out-of-phase with those of the nucleus raphe dorsalis.

Locus coeruleus projections to cortex: topography, morphology and collateralization

The relationship between individual cells of origin within the nucleus locus coeruleus (LC) and the geometry and distribution of terminal fields in cortex was examined in the albino rat. Computer-assisted 3-dimensional reconstructions of the Nissl-stained LC allowed the characterization of the spatial distribution of LC cells. Similar reconstructions of the distributions of labelled cells following cortical injections of horseradish peroxidase were created. Comparisons of such reconstructions revealed that LC cells projecting to cortex were distributed throughout the compact dorsal LC. These cells were predominantly medium sized multipolar cells. Significant labelling of other morphological sub-populations of LC did not occur following cortical injections. Simultaneous injections of multiple fluorescent retrograde tracers into different cortical regions allowed the characterization of LC axon collateralization in cortex. Individual LC cells innervate functionally and cytoarchitectonically distinct cortical regions simultaneously. LC cells arborize more extensively in the anterior-to-posterior axis of cortex and exhibit relatively minimal medial-to-lateral collateralization. Individual LC cells were also shown to innervate both superficial and deep layers of a cortical region.

Noradrenergic axon terminals in the substantia gelatinosa of the rat spinal cord: an electron-microscopic study using glyoxylic acid-potassium permanganate fixation

The noradrenergic terminals in the substantia gelatinosa of the dorsal horn of the cervical spinal cord of the rat were investigated by means of the histofluorescence technique and electron-microscopic cytochemistry using the glyoxylic acid-KMnO4 fixation technique. In accordance with the topographical distribution of fluorescent catecholaminergic fibers, noradrenergic terminals containing small granular vesicles were frequently observed electron microscopically in the outer layer of the substantial gelatinosa. These terminals were most frequently found to appose (without showing typical synaptic features, small-caliber dendrites, spine apparatus, and rarely, large caliber dendrites. Only in a few cases, the noradrenergic terminals exhibited typical synaptic contacts with dendritic elements of small size. In addition, noradrenergic terminals apposed non-noradrenergic terminals containing small agranular vesicles. In rats bearing surgical lesions of the dorsal roots, no noradrenergic terminal were found in contact with the degenerated axon terminals in the substantia gelatinosa. These findings suggest that the noradrenergic afferents to the substantia gelatinosa may exert their influence on sensory transmission via dorsal horn cells.