Changes in cortical noradrenergic axon terminals of locus coeruleus neurons in aged F344 rats.

The noradrenergic innervations and noradrenaline contents of the frontal cortex in two age groups (9 and 25 months) of male F344 rats have been quantified by electrophysiological and biochemical methods. In the electrophysiological study, the percentage of locus coeruleus (LC) neurons activated antidromically from the frontal cortex decreased with age. In contrast, the percentage of LC neurons showing multiple antidromic latencies, which suggests axonal branching of individual LC neurons, increased markedly between 9 and 25 months in the frontal cortex. In the biochemical study, we found no significant difference in noradrenaline levels in the cortical terminal fields of LC neurons during aging. These results suggest that LC neurons give rise to axonal branches to retain noradrenaline levels in their target fields in the aged brain. Our findings show that LC neurons preserve a strong capability for remodeling their axon terminals even in the aged brain.     

The mode of projections of single locus coeruleus neurons to the cerebral cortex in rats

Axonal distributions of single locus coeruleus neurons within the cerebral cortex were examined with antidromic stimulation technique combined with cortical lesions (frontal lobotomy and lobectomy). In urethan-anesthetized rats, stimulating electrodes were implanted in 10 points extending over nearly the entire cerebral cortex, and antidromic responses of single locus coeruleus neurons to stimulation of these stimulus sites were analysed. Fifty percent of locus coeruleus neurons examined were activated antidromically from at least one cortical point in the cerebral cortex. The pattern and extent of axonal distributions of single locus coeruleus neurons in the cortex appeared to vary from cell to cell. From the results obtained in rats with the cortical lesions, it is concluded that in addition to locus coeruleus neurons with intracortical axons running from rostral to caudal, there are the neurons projecting to the occipital cortex without innervating the frontal cortex and those projecting simultaneously to the frontal and occipital cortex with two axonal branches. There was no topographic order between the recording sites within the locus coeruleus and the projection sites in the cortex.     

A study of the organization of the locus coeruleus projections to the lateral geniculate nuclei in the albino rat

The lateral geniculate nuclei of the rat are known to receive an innervation from catecholamine-containing neurons. In the present study the origin, axonal projections and terminal distribution of this innervation was studied. The lateral geniculate nuclei contain a356 ± 20 ng norepinephrine/g and64 ± 7 ng dopamine/g tissue; the latter is within the range expected for dopamine as a precursor in a region innervated by a norepinephrine-containing terminal system. When separate norepinephrine-containing cell groups located at various brain stem levels are ablated or their axonal projections destroyed, only lesions in the locus coeruleus produce a significant decrease in the norepinephrine content of the lateral geniculate nuclei. Injections of horseradish peroxidase into the lateral geniculate nuclei result in retrograde transport of horseradish peroxidase only to the noradrenergic neurons of the locus coeruleus. The labelled neurons are pretent throughout the rostrocaudal and dorsoventral axes of both the ipsilateral (60%) and contralateral (40%) nucleus. Autoradiographic and fluorescence histo-chemical experiments indicate that axons that ascend from the locus coeruleus reach the lateral geniculate nuclei via the dorsal tegmental catecholamine-containing bundle and the medial forebrain bundle. These fibers enter the ventral lateral geniculate nucleus from the zona incerta and the dorsal lateral geniculate nucleus from the superior thalamic radiation, thalamic reticular nucleus, and lateral posterior nucleus. Contralateral fibers from the locus coeruleus cross in the posterior commissure, supraoptic and pontine decussations and join the ipsilateral projections to the lateral geniculate nuclei. The bilateral locus coeruleus innervation of the nuclei is comprised of a highly branched network of varicose axons. Neither the ipsilateral nor the contralateral projections appear to be topographically organized; instead, a single fiber may have collateral axons that branch throughout large areas of the nuclei. This innervation is moderately dense in the ventral, and very dense in the dorsal, lateral geniculate nucleus.The study indicates that both the dorsal and ventral lateral geniculate nuclei receive a diffuse catecholamine-containing innervation which arises solely from the norepinephrine-containing neurons of the locus coeruleus. The innervation of each lateral geniculate nucleus is bilateral, with noradrenergic neurons located throughout both the ipsilateral and the contralateral locus coeruleus contributing to ascending pathways that terminate as a diffuse, plexiform innervation interspersed among other afferents to the lateral geniculate nuclei. It is speculated that such a diffuse noradrenergic innervation might depress the spontaneous activity of neurons in the lateral geniculate nuclei, while preserving or enhancing their responsiveness to afferent optic stimulation.     

Survival and growth of neurons with enkephalin-like immunoreactivity in fetal brain areas grafted to the anterior chamber of the eye

Areas of fetal rat brain and spinal cord known to contain enkephalin-like immunoreactive cell bodies and/or terminal fields were transplanted to the anterior chamber of the eye of adult rats. Enkephalin-like immunoreactive neurons survive and produce an enkephalin-like immunoreactive fiber network within grafts of spinal cord, ventral medulla oblongata, ventrolateral pons, tectum, locus coeruleus, substantia nigra and the areas containing columna fornicis and globus pallidus. Although single intraocular grafts of neocortex do not apparently contain enkephalin-like immunoreactive fibers, such grafts contain a variable amount of sparsely distributed enkephalin-like fibers when sequentially grafted in oculo with either locus coeruleus or spinal cord. Combinations of locus coeruleus and globus pallidus contained a rich enkephalin fiber network in the locus coeruleus part and a sparse innervation of the globus pallidus part.

We conclude that enkephalin-like immunoreactive neurons in small areas of fetal rat brain can be successfully transplanted to the anterior chamber of the eye. They are able to survive and develop to maturity in complete isolation from the rest of the brain. In general, the enkephalin-like immunoreactive fiber density in the various single grafts approximated that of their brain counterparts in situ. Fiber formation can be reinitiated in mature enkephalin-like immunoreactive neurons by addition of new brain target areas. Thus, the technique permits establishment of isolated, defined enkephalin systems and pathways accessible to functional analysis.     

Descriptive and functional neuroanatomy of locus coeruleus-noradrenaline-containing neurons involvement in bradykinin-induced antinociception on principal sensory trigeminal nucleus

The present study was carried out in Wistar rats, using the jaw-opening reflex and dental pulp stimulation, to investigate noradrenaline- and serotonin-mediated antinociceptive circuits. The effects of microinjections of bradykinin into the principal sensory trigeminal nucleus (PSTN) before and after neurochemical lesions of the locus coeruleus noradrenergic neurons were studied. Neuroanatomical experiments showed evidence for reciprocal neuronal pathways connecting the locus coeruleus (LC) to trigeminal sensory nuclei and linking monoaminergic nuclei of the pain inhibitory system to spinal trigeminal nucleus (STN). Fast blue (FB) injections in the locus coeruleus/subcoeruleus region retrogradely labeled neurons in the contralateral PSTN and LC. Microinjections of FB into the STN showed neurons labeled in both ipsilateral and contralateral LC, as well as in the ipsilateral Barrington's nucleus and subcoeruleus area. Retrograde tract-tracing with FB also showed that the mesencephalic trigeminal nucleus sends neural pathways towards the ipsilateral PSTN, with outputs from cranial and caudal aspects of the brainstem. In addition, neurons from the lateral and dorsolateral columns of periaqueductal gray matter also send outputs to the ipsilateral PSTN. Microinjections of FB in the interpolar and caudal divisions of the STN labeled neurons in the caudal subdivision of STN. Microinjections in the STN interpolar and caudal divisions also retrogradely labeled serotonin- and noradrenaline-containing nucleus of the brainstem pain inhibitory system. Finally, the gigantocellularis complex (nucleus reticularis gigantocellularis/paragigantocellularis), nucleus raphe magnus and nucleus raphe pallidus also projected to the caudal divisions of the STN. Microinjections of bradykinin in the PSTN caused a statistically significant long-lasting antinociception, antagonized by the damage of locus coeruleus-noradrenergic neuronal fibres with (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) (DSP4), a neurotoxin that specifically depleted noradrenaline from locus coeruleus terminal fields. These data suggest that serotonin- and noradrenaline-containing nuclei of the endogenous pain inhibitory system exert a key-role in the antinociceptive mechanisms of bradykinin and the locus coeruleus is crucially involved in this effect.     

Noradrenergic terminal excitability: effects of opioids

The local infusion of morphine or D-Ala2, Met5-enkephalinamide into the frontal cortical terminal fields of noradrenergic neurons of the nucleus locus coeruleus resulted in a decrease in the excitability of the axon terminal regions to direct electrical stimulation. These effects were concentration dependent and could be blocked or partially reversed by the local infusion of naloxone. Some evidence was obtained for a differential antagonizing effect of naloxone upon the effects of morphine and D-Ala2, Met5-enkephalinamide. These results are discussed with respect to an effect of opioids on the polarization and/or ionic conductance of the terminal fields of locus coeruleus neurons, and to the possible regulation of neurotransmitter release by presynaptic opiate receptors.     

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 that dorsal locus coeruleus neurons can maintain their spinal cord projection following neonatal transection of the dorsal adrenergic bundle in rats

Summary In adult rats, locus coeruleus neurons which extend axons to the spinal cord are found only at mid-rostrocaudal levels of the nucleus, where they are essentially confined to its ventral, wedge-shaped half (Satoh et al. 1980; Westlund et al. 1983; Loughlin et al. 1986). However, during early postnatal development, coeruleospinal cells are found throughout the locus coeruleus (Cabana and Martin 1984; Chen and Stanfield 1987). This developmental restriction of the distribution of coeruleospinal neurons is due to axonal elimination rather than to cell death, since neurons retrogradely labeled through their spinal axons perinatally are still present in the dorsal portion of the locus coeruleus at survival periods beyond the age at which these cells lose their spinal projection (Chen and Stanfield 1987). I now report that if axons ascending from the locus coeruleus are cut by transecting the dorsal adrenergic bundle on the day of birth, a more widespread distribution of coeruleospinal neurons is retained beyond the perinatal period. These results not only indicate that the absence of the normally maintained collateral of a locus coeruleus neuron is sufficient to prevent the elimination of a collateral which would otherwise be lost, but also may imply that during normal postnatal development the presence of the maintained collateral is somehow causally involved in the elimination of the transient collateral.     

Bladder distention activates noradrenergic locus coeruleus neurons by an excitatory amino acid mechanism.

The present study was designed to determine the neurotransmitter(s) involved in activation of noradrenergic locus coeruleus neurons by urinary bladder distention. The spontaneous discharge rate of single locus coeruleus neurons was recorded from halothane-anesthetized rats during the physiological challenge of bladder distention. Intrabladder saline infusion (0.5 ml) increased bladder pressure by 77 +/- 9.7 mmHg (n = 19) and this was associated with an increase in locus coeruleus discharge rate of 53 +/- 4.8% (n = 29). Simultaneous recordings of cortical electroencephalographic activity demonstrated that electroencephalographic activation, characterized by a decreased amplitude and tendency to shift from low frequency activity to higher frequency activity, was also associated with bladder distention. The role of corticotropin-releasing factor and excitatory amino acid inputs to the locus coeruleus in activation by bladder distention was tested in rats pretreated with a corticotropin-releasing factor antagonist, or excitatory amino acid antagonists. Intracerebroventricular administration of the corticotropin-releasing factor antagonist did not alter locus coeruleus activation by bladder distention. In contrast, both locus coeruleus activation and electroencephalographic activation associated with bladder distention were prevented by intracerebroventricular administration of kynurenic acid. The same dose of kynurenic acid also prevented locus coeruleus activation by repeated sciatic nerve stimulation, as previously reported. Local administration of kynurenic acid into the locus coeruleus greatly attenuated, but did not completely prevent the increase in locus coeruleus discharge elicited by bladder distention.(ABSTRACT TRUNCATED AT 250 WORDS)     

Divergent projections of catecholamine neurons of the locus coeruleus as revealed by fluorescent retrograde double labeling technique

Divergent projections of catecholamine (CA) neurons of the locus coeruleus have been studied by fluorescent retrograde double labeling in conjunction with monoamine histofluorescence technique. The present results indicate that the coerulo-cortical CA system is composed of two types of neurons. A predominant type possesses few divergent axons innervating a restricted region, while the other type projects widely to various areas of the cerebral cortex. The existence of divergent axonal projections of single CA neurons in the locus coeruleus to the cerebellum and the spinal cord, to the frontal cortex and the cerebellum, is also demonstrated.     

Neuron numbers in locus coeruleus do not change with age in fisher 344 rat

Counts of neurons in locus coeruleus of Fisher 344 rats of ages from 12 to 32 months showed no loss with advancing age. Species comparisons of age changes in locus coeruleus neuron counts and measures of catecholamine systems suggest that the human and nonhuman primate locus coeruleus shows age changes not seen in rodent locus coeruleus. It is suggested that although rodent models of human aging have value under some circumstances, rodent and primate brain should not be considered equivalent with regard to phenomena related to noradrenergic mechanisms.     

Changes in noradrenergic terminal excitability induced by amphetamine and their relation to impulse traffic

The effects of amphetamine upon the terminal excitability of noradrenergic neurons of the nucleus locus coeruleus were studied in urethane anesthetized rats. Terminal excitability was measured by determining the stimulus currents necessary to evoke antidromic responses in locus coeruleus neurons from terminals in the frontal cortex. In most cases, terminal excitability was decreased following local infusion of amphetamine into the frontal cortex, while intravenous administration of the drug tended to increase terminal excitability. The decreased terminal excitability induced by local infusion of amphetamine appeared to be due to activation of alpha-adrenergic receptors located on the terminals of locus coeruleus neurons, since this effect mimics that of clonidine, a direct acting alpha-adrenergic agonist, and since the effect was abolished by pretreatment with alpha-methyl-p-tyrosine which disrupts the catecholamine liberating properties of amphetamine. Phentolamine, a direct acting alpha-adrenergic receptor antagonist was also found to block or reverse the effect of amphetamine. The changes in terminal excitability following intravenous injection of amphetamine appeared to be related to changes in the spontaneous activity of locus coeruleus neurons. A large decrease in spontaneous activity following intravenous administration of amphetamine was associated with increased terminal excitability, whereas when smaller changes in spontaneous activity occurred, terminal excitability was found to be decreased. These results are discussed with respect to the pharmacological properties of catecholaminergic neurons and the mechanisms of action of amphetamine.     

Effects of the noradrenaline neurotoxin DSP4 on the postnatal development of central noradrenaline neurons in the rat

The effect of the noradrenaline neurotoxin DSP4 on the postnatal development of central noradrenergic neurons in the rat has been investigated using neurochemical techniques. The results demonstrated a preferential effect of DSP4 on the locus coeruleus noradrenergic neuron system without any notable effects on the dopamine and adrenaline neurons and only a minor neurotoxic effect on the serotonin neurons. The effect of DSP4 on the serotonin neurons could be completely prevented by pretreatment with the uptake blocker zimelidine, without affecting the action of DSP4 on noradrenergic neurons. Neonatal DSP4 treatment systemically led to permanent depletions of noradrenaline in the cerebral cortex and spinal cord and marked increases of noradrenaline in the cerebellum and ponsmedulla. These effects of DSP4 were dose-dependent and could be blocked by pretreatment with the noradrenaline uptake blocker desipramine. The alterations in endogenous noradrenaline levels were quantitatively similar to changes observed in [ ³H]noradrenaline uptake in slices in vitro. There were no significant changes of these noradrenergic parameters when analysing the whole CNS after neonatal DSP4 treatment, in spite of marked regional changes in both directions. Administration of DSP4 to rats of different ages produced acutely marked depletions of noradrenaline in all regions including the pons-medulla and the cerebellum at all developmental stages. Marked and permanent depletions of noradrenaline were found in the distant noradrenergic nerve terminal projections after treatment at all ages, whereas increases in noradrenaline levels in the pons-medulla and cerebellum were only observed in rats treated with DSP4 up to the age of 3–5 days, whereas a DSP4 administration in older rats led to substantial and permanent depletions of noradrenaline in both these regions.The results indicate that the alteration of the postnatal development of noradrenergic neurons after treatment of rats up to the age of 3–5 days is mainly related to a ‘pruning effect’ of DSP4, in which prevention of the development of distant nerve terminal projections causes an increased outgrowth of nerves in collateral systems spared by the neurotoxin. The results indicate that DSP4 may be a useful denervation tool for studying various aspects of noradrenergic neurotransmission of developing locus coeruleus neurons.     

Activation of adenylate cyclase attenuates the hyperpolarization following single action potentials in brain noradrenergic neurons independently of protein kinase a

Afterhyperpolarizations that follow action potentials are a prominent mechanism for the control of neuronal excitability. Such afterhyperpolarizations in many neurons are modulated by a variety of second messenger systems. Here, we examined the regulation of afterhyperpolarizations in noradrenergic locus coeruleus neurons by the adenylate cyclase system. Although superfusion of the adenylate cyclase activator, forskolin, had no effect on hyperpolarizations following trains of action potentials, both forskolin and a membrane permeable analog of cyclic AMP, 8-bromo-cyclic AMP, attenuated the amplitude of afterhyperpolarizations which followed single action potentials of locus coeruleus neurons recorded intracellularly in brain slices. In contrast, superfusion of 1,9-dideoxyforskolin, the forskolin analog that does not activate adenylate cyclase, had no effect on these single action potential afterhyperpolarizations. Co-application of a protein kinase inhibitor (H8, KT5720, staurosporin or Rp-cAMPS) with either forskolin or 8-bromo-cyclic AMP failed to block the reduction of afterhyperpolarization amplitude, but blocked the cyclic AMP-dependent enhancement of opiate responses in the same locus coeruleus neurons. Furthermore, application of a membrane permeable analog of 5'-AMP, 8-bromo-5'-AMP, the cyclic AMP metabolite that does not activate a protein kinase, potently reduced the amplitudes of single action potential afterhyperpolarizations. The afterhyperpolarization amplitude was also reduced in locus coeruleus neurons taken from chronically morphine-treated rats, a treatment known to increase adenylate cyclase activity.These results indicate that elevation of intracellular cyclic AMP or 5'-AMP reduces the single action potential afterhyperpolarization in locus coeruleus neurons. This action may be mediated through a mechanism independent of protein kinase activation.     

Cell suspension grafts of noradrenergic locus coeruleus neurons in rat hippocampus and spinal cord: reinnervation and transmitter turnover

Fetal noradrenergic neurons from the brain stem locus coeruleus region can be successfully grafted as a dissociated cell suspension provided that the dissociation is done in the absence of any trypsin digestion step. The survival, fiber outgrowth and biochemical function of locus coeruleus neurons, taken from 13- to 15-day-old rat embryos, have been studied after injection into the dorsal hippocampal formation and the thoracolumbar spinal cord in adult rats. All rats were treated with an i.v. injection of 6-hydroxydopamine prior to grafting to remove the intrinsic locus coeruleus projections to these areas, and they were taken for fluorescence histochemical or biochemical analyses 2-7 months after transplantation. Up to 330 surviving noradrenaline neurons were found at each implantation site (injected with 2-3 microliters of cell suspension) which represents an estimated survival rate of about 40%. In the most successful cases the entire dorsal hippocampal formation, and an approximately 4 cm long segment of the thoracolumbar spinal cord, was supplied with a new noradrenaline-containing terminal network, which reached normal densities in the regions closest to the grafts. In the hippocampal formation, in particular, the ingrowing axons re-established a laminar innervation pattern which resembled that of the normal locus coeruleus afferents. In the hippocampus, two 2-microliters injections of locus coeruleus cell suspension restored the total hippocampal noradrenaline content to an average of 55%, and the noradrenaline synthesis rate (as assessed by the rate of DOPA accumulation after synthesis inhibition) was found to be close to normal in the graft-reinnervated specimens. In the spinal cord, two 3-microliters injections restored the noradrenaline level in the thoracolumbar cord (a 4.5 cm long segment) to an average of 22% of normal, with the highest individual levels being close to normal. Determinations of the noradrenaline metabolite 3,4-dihydroxy-phenylethyleneglycol indicated that the rate of noradrenaline metabolism in the graft-reinnervated spinal cord was close to that of the normal intact spinal cord. The results demonstrate the potential of the suspension grafting technique for extensive noradrenergic reinnervation of the hippocampal formation or large portions of the spinal cord. Fetal locus coeruleus neurons implanted in this way can re-establish fairly normal terminal innervation patterns and reinstate noradrenaline turnover and metabolism in a previously denervated central target.     

BDNF is necessary for maintenance of noradrenergic innervations in the aged rat brain

In the axon terminals of the locus coeruleus (LC) neurons, a high level of axonal branching was occurred in the middle-aged brain, and the increased branching was maintained in the aged brain. In the present study, we hypothesized that neurotrophic support is necessary for the morphological age-related changes seen in the noradrenergic innervations from the LC to frontal cortex. Through immunohistochemical and quantitative image analyses, we examined the age-dependent effects of brain-derived neurotrophic factor (BDNF) on the noradrenergic axon terminals in the frontal cortex of F344 rats. We continuously infused BDNF into the frontal cortex of young (6-months-old), middle-aged (13-months-old), or aged (25-months-old) rats. Exogenous BDNF infusion caused a marked increase in the density of noradrenergic axons in the aged brain, but no trophic action of BDNF was observed in the young and middle-aged brain. Neutralization of endogenous BDNF with a specific function-blocking antibody to BDNF led to a reduction in noradrenergic axons in the frontal cortex of 19-month-old rats. The present results suggest that BDNF is not involved in the augmentation of noradrenergic innervations in the aging brain, but it is necessary for the maintenance of noradrenergic innervations in the aged brain.     

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

Summary 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.     

Involvement of neurotrophic factors in aging of noradrenergic innervations in hippocampus and frontal cortex

In the present study, we investigated the age-dependent changes in the axon terminals of the locus coeruleus (LC) neurons in the frontal cortex and hippocampus, in which a high degree of axonal branching in the middle-aged brain was suggested to occur in our previous electrophysiological study. We used 6-, 13- and 25-month-old male F344/N rats, and performed Western blot analysis of the norepinephrine transporter (NET), brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF). NET expression level increased in the 13-month-old hippocampus, but was not altered by aging in the frontal cortex. BDNF expression level increased in the hippocampus, but did not change with age in the frontal cortex. On the other hand, GDNF expression level was increased with age in the frontal cortex, but was not in the hippocampus. These results suggest that the LC noradrenergic innervations may be locally regulated by different neurotrophic factors that exert their trophic actions at different target sites.     

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.