Antidromic activation of the rat locus coeruleus neurons from hippocampus, cerebral and cerebellar cortices

Neuromodulation in circuits of aversive emotional learning

Emotional learning and memory are functionally and dysfunctionally regulated by the neuromodulatory state of the brain. While the role of excitatory and inhibitory neural circuits mediating emotional learning and its control have been the focus of much research, we are only now beginning to understand the more diffuse role of neuromodulation in these processes. Recent experimental studies of the acetylcholine, noradrenaline and dopamine systems in fear learning and extinction of fear responding provide surprising answers to key questions in neuromodulation. One area of research has revealed how modular organization, coupled with context-dependent coding modes, allows for flexible brain-wide or targeted neuromodulation. Other work has shown how these neuromodulators act in downstream targets to enhance signal-to-noise ratios and gain, as well as to bind distributed circuits through neuronal oscillations. These studies elucidate how different neuromodulatory systems regulate aversive emotional processing and reveal fundamental principles of neuromodulatory function.

Locus Coeruleus Optogenetic Light Activation Induces Long-Term Potentiation of Perforant Path Population Spike Amplitude in Rat Dentate Gyrus

Norepinephrine (NE) in dentate gyrus (DG) produces NE-dependent long-term potentiation (NE-LTP) of the perforant path-evoked potential population spike both in vitro and in vivo. Chemical activators infused near locus coeruleus (LC), the source of DG NE, produce a NE-LTP that is associative, i.e., requires concurrent pairing with perforant path (PP) input. Here, we ask if LC optogenetic stimulation that allows us to activate only LC neurons can induce NE-LTP in DG. We use an adeno-associated viral vector containing a depolarizing channel (AAV8-Ef1a-DIO-eChR2(h134r)-EYFP-WPRE) infused stereotaxically into the LC of TH:Cre rats to produce light-sensitive LC neurons. A co-localization of ~62% in LC neurons was observed for these channels. Under urethane anesthesia, we demonstrated that 5-10 s 10 Hz trains of 30 ms light pulses in LC reliably activated neurons near an LC optoprobe. Ten minutes of the same train paired with 0.1 Hz PP electrical stimulation produced a delayed NE-LTP of population spike amplitude, but not EPSP slope. A leftward shift in the population spike input/output curve at the end of the experiment was also consistent with long-term population spike potentiation. LC neuron activity during the 10 min light train was unexpectedly transient. Increased LC neuronal firing was seen only for the first 2 min of the light train. NE-LTP was more delayed and less robust than reported with LC chemo-activation. Previous estimates of LC axonal conduction times suggest acute release of NE occurs 40-70 ms after an LC neuron action potential. We used single LC light pulses to examine acute effects of NE release and found potentiated population spike amplitude when a light pulse in LC occurred 40-50 ms, but not 20-30 ms, prior to a PP pulse, consistent with conduction estimates. These effects of LC optogenetic activation reinforce evidence for a continuum of NE potentiation effects in DG. The single pulse effects mirror an earlier report using LC electrical stimulation. These acute effects support an attentional role of LC activation. The LTP of PP responses induced by optogenetic LC activation is consistent with the role of LC in long-term learning and memory.

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.

Projection specificity in heterogeneous locus coeruleus cell populations: implications for learning and memory

Noradrenergic neurons in the locus coeruleus (LC) play a critical role in many functions including learning and memory. This relatively small population of cells sends widespread projections throughout the brain including to a number of regions such as the amygdala which is involved in emotional associative learning and the medial prefrontal cortex which is important for facilitating flexibility when learning rules change. LC noradrenergic cells participate in both of these functions, but it is not clear how this small population of neurons modulates these partially distinct processes. Here we review anatomical, behavioral, and electrophysiological studies to assess how LC noradrenergic neurons regulate these different aspects of learning and memory. Previous work has demonstrated that subpopulations of LC noradrenergic cells innervate specific brain regions suggesting heterogeneity of function in LC neurons. Furthermore, noradrenaline in mPFC and amygdala has distinct effects on emotional learning and cognitive flexibility. Finally, neural recording data show that LC neurons respond during associative learning and when previously learned task contingencies change. Together, these studies suggest a working model in which distinct and potentially opposing subsets of LC neurons modulate particular learning functions through restricted efferent connectivity with amygdala or mPFC. This type of model may provide a general framework for understanding other neuromodulatory systems, which also exhibit cell type heterogeneity and projection specificity.

Characterization of neurochemically specific projections from the locus coeruleus with respect to somatosensory-related barrels

Tactile information from the rodent mystacial vibrissae is relayed through the ascending trigeminal somatosensory system. At each level of this pathway, the whiskers are represented by a unique pattern of dense cell aggregates, which in layer IV of cortex are known as "barrels." Afferent inputs from the dorsal thalamus have been demonstrated repeatedly to correspond rather precisely with this modular organization. However, axonal innervation patterns from other brain regions such as the noradrenergic locus coeruleus are less clear. A previous report has suggested that norepinephrine-containing fibers are concentrated in the center/hollow of the barrel, while other studies have emphasized a more random distribution of monoaminergic projections. To address this issue more directly, individual tissue sections were histochemically processed for cytochrome oxidase in combination with dopamine-beta-hydroxylase, the synthesizing enzyme for norepinephrine, or the neuropeptide galanin. These two neuroactive agents were of particular interest because they colocalize in a majority of locus coeruleus neurons and terminals. Our data indicate that discrete concentrations or local arrays of dopamine-beta-hydroxylase- or galanin-immunoreactive fibers are not apparent within the cores of individual barrels. As such, the data suggest that cortical inputs from the locus coeruleus are not patterned according to cytoarchitectural landmarks or the neurochemical identity of coeruleocortical efferents. While transmitter-specific actions of norepinephrine and/or galanin may not be derived from the laminar/spatial connections of locus coeruleus axons, the possibility remains that the release of these substances may mediate distinctive events through the localization of different receptor subclasses, or the contact of their terminals onto cells with certain morphological characteristics or ultrastructural components.

Age-dependent changes in projections from locus coeruleus to hippocampus dentate gyrus and frontal cortex

Age-dependent changes in noradrenergic innervations of the hippocampal dentate gyrus (DG) and the frontal cortex (FC) have been studied in male F344 rats. The projections from the nucleus locus coeruleus (LC) to DG or FC with advancing age (from 7 to 27 months) in rats have been quantified by electrophysiological and immunohistochemical methods. In the electrophysiological study, we observed that the percentage of LC neurons activated antidromically by electrical stimulation (P-index) of DG or FC decreased with age. We found that the percentage of LC neurons showing multiple antidromic latencies (M-index), which suggests axonal branching of individual LC neurons, increased markedly between 15 and 17 months in DG or FC. In DG, the M-index increased steadily between 15 and 24 months. In contrast, the increased M-index in FC was maintained until 24 months. The increased M-index in both targets declined at 27 months. These results suggest that LC neurons give rise to axonal branching following the loss of projections to DG or FC with age. In the immunohistochemical study, the density of dopamine-beta-hydroxylase-positive axonal varicosities was measured in molecular, granule cell and polymorphic layers of DG. The density in the polymorphic layer significantly decreased in the earlier stage of ageing (7-19 months), whilst the density in the molecular and granule cell layers decreased in the later stage (27 months). These findings suggested that a layer-specific decline occurred with age in the noradrenergic axon terminals in DG.

Potent excitatory influence of prefrontal cortex activity on noradrenergic locus coeruleus neurons

An influence of the prefrontal cortex on noradrenergic locus coeruleus neurons would have profound implications for the function of the locus coeruleus system. Although the medial prefrontal cortex does not substantially innervate the core of the nucleus locus coeruleus, evidence indicates that the medial prefrontal cortex projects to regions containing locus coeruleus dendrites; indirect medial prefrontal cortex-locus coeruleus projections are also possible. Here, we examined influences of prefrontal cortex activity on locus coeruleus firing rates by activating or inactivating the medial prefrontal cortex while recording impulse activity of locus coeruleus neurons extracellularly in anaesthetized rats. Most of our electrical stimulation experiments were conducted in rats which underwent lesions of the ascending dorsal bundle of noradrenergic fibres from the locus coeruleus to eliminate locus coeruleus projections to the prefrontal cortex, because antidromic activation of locus coeruleus from the prefrontal cortex affects even non-driven locus coeruleus neurons through collaterals. Single pulse stimulation (1 mA, 0.3-0.5 ms) of the dorsomedial (frontal region 2) or prelimbic region of the medial prefrontal cortex synaptically activated 13/16 (81%) or 16/56 (29%) locus coeruleus neurons, respectively. Train stimulation (20 Hz for 0.5 s) synaptically activated greater percentages of locus coeruleus cells, 11/12 cells (92%) for the dorsomedial prefrontal cortex, and 41/50 cells (82%) for the prelimbic cortex. No inhibitory responses in the locus coeruleus were obtained with dorsomedial prefrontal stimulation, and weak inhibition was found in 16% of locus coeruleus cells with prelimbic stimulation. Electrical stimulation of more lateral frontal cortex (Fr1 area) had no effects on locus coeruleus activity. Chemical stimulation of the dorsomedial prefrontal cortex with L-glutamate (10 or 100 mM) or D,L-homocysteic acid (10 mM) phasically activated 15/26 (55%) locus coeruleus cells, and 15/68 cells (22%) with prelimbic stimulation; such activation was sometimes followed by long-lasting oscillatory activity. No locus coeruleus cells exhibited purely inhibitory responses with chemical stimulation of any prefrontal cortex site. Inactivation of the dorsomedial or prelimbic region of the prefrontal cortex with lidocaine microinjection (2%, 180 or 300 nl) reduced locus coeruleus firing rates in 6/10 (60%) or 7/19 (37%) locus coeruleus cells, respectively. In no case did lidocaine in any prefrontal cortex site activate a locus coeruleus neuron. These results indicate that the medial prefrontal cortex provides a potent excitatory influence on locus coeruleus neurons. The fact that inactivation of the medial prefrontal cortex suppressed locus coeruleus firing indicates that the medial prefrontal cortex also provides a resting tonic excitatory influence on locus coeruleus activity.

Different direct pathways of locus coeruleus to medial prefrontal cortex and centrolateral thalamic nucleus: electrical stimulation effects on the evoked responses to nociceptive peripheral stimulation

Projections from the locus coeruleus (LC) to the centrolateral thalamus (Cl) and the medial prefrontal cortex (PfCx) were studied using orthodromic and antidromic stimulation techniques. The LC is a major noradrenergic source in the central nervous system, and its descending projections provide an important source of pain suppression at spinal level. Previously, the author has described a cortico-thalamic loop involved in pain modulation. The present paper reports on a study of the participation of LC as part of an ascending pain-control system acting on the cortico-thalamic loop.Rats were anaesthetized with halothane, and single unit recordings were made in LC using glass micropipettes. Stainless steel electrodes were placed in cortex and thalamus for electrical stimulation.Stimulation in PfCx or Cl produces antidromic responses in neurons in LC. The latencies, conduction velocity and location of neurons in LC projecting to PfCx or Cl structures are described. Separate projections to both structures have significantly different conducting velocities, arriving earlier at Cl (mean conduction velocities 0.27 and standard deviation +/-0.06 m/s) and then at PfCx (mean conduction velocities 0.20+/- 0.04 m/s). The presence of orthodromic responses suggests reciprocal connections. The paper also describes the suppression of spontaneous and nociceptive-evoked activity in the PfCx and Cl following electrical stimulation in LC.It is proposed that the LC innervation could be associated with an ascending noradrenergic system acting upon a Cl-PfCx pain-modulation mechanism. Copyright 1998 European Federation of Chapters of the International Association for the Study of Pain.

Developmental changes in the electrical activity of locus coeruleus neurons during cortical spreading depression

A previous study has demonstrated that during cortical spreading depression (CoSD), locus coeruleus (LC) neurons in adult rats reveal antidromic burst activity consisting of multiple initial segment (IS) spikes and IS-somatodendritic (SD) spikes with a distinct IS-SD break. In addition, the spontaneous firing rate of the neurons was reduced during CoSD. In the present experiments, we studied developmental changes in the electrical activity of LC neurons during CoSD. Since stable and repetitive DC shifts occurred in rats older than postnatal day 13 (PD13), the electrical activity of LC neurons at developmental stages later than PD13 was examined. The CoSD-related burst activity similar to that observed in adults was recorded at all stages of development, and the proportion of LC neurons showing the burst activity was nearly the same through all developmental stages and in adults. The frequency of IS spikes in the burst activity at PD13-P15 was not different from that in adults. However, the spontaneous firing rate of LC neurons at early developmental stages remained unchanged during CoSD. Based on these findings, it is conceivable that the mechanism of the generation of multiple IS spikes during CoSD at early developmental stages is different from that at later developmental stages and in adults.

Slow oscillations as a probe of the dynamics of the locus coeruleus-frontal cortex interaction in anesthetized rats

Multiunit or single unit activity recorded simultaneously from frontal cortex (FC) and locus coeruleus (LC) under ketamine anesthesia revealed that both regions show slow oscillatory activity, together or separately. If, however, both regions are engaged in this oscillatory activity, there is a systematic relationship between their phases with peak LC firing always following FC firing by 200-400 ms. This was confirmed by cross-correlational analyses, which indicated that the two structures temporarily form a resonant system. The FC-LC resonant state is, however, loose enough to remain open to other intrinsic or extrinsic influences, keeping the measured frequencies of oscillations at each site slightly different, as demonstrated by a detailed analysis of the autocorrelograms. An injection of lidocaine at the frontal cortex site, while sharply reducing the prefrontal activity to essentially zero, leads to an increase of the LC activity and to a modification of the shape of the LC autocorrelogram, but does not change appreciably the phase relationship between the activity in the two structures during the diminishing activity in FC.

Anatomical evidence for the presence of glutamate or enkephalin in noradrenergic projection neurons of the locus coeruleus

This paper reviews the anatomical evidence for the presence of glutamate (GLU) in noradrenergic neurons of the nucleus locus coeruleus (LC) and adjacent nuclei in the dorsolateral pontine tegmentum (DLPT) that project to the spinal cord, cerebellum, or cerebral cortex. Additionally, the evidence for the existence of methionine-enkephalin (ENK) in noradrenergic neurons of the DLPT that project to the spinal cord of the cat is reviewed. In these studies, we have combined the retrograde transport of either Fast Blue (FB), rhodamine labeled latex microspheres (MS), or rhodamine labeled dextran and indirect immunofluorescence histochemistry to determine whether the neurons that contain tyrosine hydroxylase (TH) and project to these terminal fields also contain GLU or ENK. The neurons of the cat that project to the spinal cord, cerebellum, and neocortex were observed in the nucleus LC and Kölliker-Fuse (KF) nucleus. They were also present, to a lesser extent, in the nucleus subcoeruleus (SC) and nuclei parabrachialis medialis (PBM) and lateralis (PBL). In the rat the majority of the neurons that project to the neocortex and hippocampus were located in the nucleus LC. Our data revealed a major proportion of these neurons to be immunostained for both GLU and TH (cat, rat), or ENK and TH (cat). Functional implications of such colocalized neurochemicals within individual LC projection neurons are discussed.

Existence of glutamate in noradrenergic locus coeruleus neurons of rodents

This study distinguished three types of immunolabeled neurons in nucleus locus coeruleus (LC) of the rat and mouse: cells single labeled either for tyrosine hydroxylase-like immunoreactivity (TH-LI) or glutamate (Glu)-LI, and those double labeled for both antigens. Although the double labeled neurons tend to be located in the middle and ventral thirds of the rat LC nucleus, throughout its rostrocaudal extent, such feature was not apparent in the mouse. Quantitatively a majority of neurons cocontaining TH- and Glu-LI were commonly observed in the rat (62%) and mouse (77%) LC. Additional studies utilizing the combined retrograde and immunohistochemical labeling revealed that such a high incidence of coexistence of the TH- and Glu-LI was also represented by coeruleocortical neurons in the rat (69% and 75% of all ipsilateral and contralateral projection cells, respectively). A possible role of coeruleocortical neurons involvement in Glu- and norepinephrine-mediated target neuron dysfunction is discussed.

Lateral olfactory tract input to dentate gyrus is depressed by prior noradrenergic activation using nucleus paragigantocellularis stimulation

Nucleus paragigantocellularis stimulation potentiates the medial perforant path population spike in the dentate gyrus via beta-receptor activation. In this study, the same paragigantocellularis stimulation preceding lateral olfactory tract pulses depressed the lateral perforant path mediated synaptic potential in dentate gyrus. Depression of the lateral olfactory tract input was blocked by a beta-antagonist. These in vivo results confirm in vitro reports that norepinephrine induces potentiation of medial perforant path input and depression of lateral perforant path input to dentate gyrus.

Topographic organization of rat locus coeruleus and dorsal raphe nuclei: distribution of cells projecting to visual system structures

Previous reports from this laboratory and elsewhere have provided evidence that the locus coeruleus (LC) and dorsal raphe (DR) nuclei are topographically organized with respect to their efferent targets. Whereas most of these previous studies have focused on relationships between these monoamine-containing brainstem nuclei and cerebral cortex, basal ganglia, and limbic structures, they have not systematically examined the distribution of LC and DR cells that project to multiple structures with common sensory or motor functions. The goal of the present study was to characterize and compare the distributions of LC and DR cells which project to different visual areas of the rat central nervous system. Long-Evans hooded rats received unilateral pressure injections of the retrograde tracer wheat germ agglutinin-horseradish peroxidase in either the dorsal lateral geniculate, ventral lateral geniculate, or lateral posterior nucleus of thalamus; superior colliculus, cortical area 17, cortical area 18a/b; cerebellar vermis (lobules VI and VII); or paraflocculus. Transverse sections through the midbrain and pons were examined by light microscopy after performing routine tetramethyl benzidine histochemical procedures. For all cases studied, retrogradely labeled cells were observed throughout the rostrocaudal extent of the LC and DR; however, labeling patterns which were distinctive for different injection sites were noted in each of these brainstem nuclei. The major conclusion drawn from this work is that subsets of LC and DR cells which project to different target structures within the rat visual system are found in overlapping but not necessarily coextensive zones within these nuclei. These studies provide further evidence of a rough topographic ordering within both the LC and DR nuclei, as well as support a new hypothesis that the outputs from each of these nuclei are organized with respect to the sensory related functions of their efferent targets.

Immunocytochemical and in situ hybridization evidence for the coexistence of GABA and tyrosine hydroxylase in the rat locus ceruieus

We have demonstrated the coexistence of GABA-like and tyrosine hydroxylase-like immunoreactivities (GABA-LI and TH-LI, respectively) in the same neurons of the rat locus ceruleus (LC). The profiles of these cells were labeled by alternately immunostaining adjacent sections for GABA-LI or TH-LI by the avidin-biotin-peroxidase complex method or the peroxidase-anti-peroxidase method after perfusion (either Zamboni's fixative or PPG), and observation at light and electron microscopic levels. For light microscopy, pairs of adjacent sections of more than 590 (Zamboni's) and 260 (PPG), and for electron microscopy, 40 ultrathin sections cut from adjacent semithin plastic sections (Zamboni's), were examined. GABA-LI was found in 80% (1,309/1,642 in total) of small and medium-sized neurons, uniformly scattered throughout the LC. Observations unequivocally show that the majority of GABA-ergic neurons are also noradrenergic. Several neurons are neither noradrenergic nor GABA-ergic, while other noradrenergic neurons do not show GABA-LI. It is shown that astrocytes, but not oligodendrocytes, contain GABA. In situ hybridization using a probe DNA fragment of the glutamic acid decarboxylase (GAD) cDNA, amplified by the polymerase chain reaction, detected GAD mRNA signals in many neurons throughout the LC, supporting the presence of a GAD/GABA system in the LC. Multiple "classical" transmitters, including GABA, serotonin, and noradrenaline, coexist in many LC neurons and may contribute to its widely diverging projections throughout the entire CNS.

Pyramidal neurons in immature rat hippocampus are sensitive to beta-adrenergic agents

The development of hippocampal neuronal sensitivities to the beta-noradrenergic agent, isoproterenol, was examined in tissue from immature rats. The in vitro hippocampal slice preparation was used to assess intracellularly recorded responses from hippocampal neurons to pressure-pulse and bath application of noradrenergic drugs. Effects of the drug on individual hippocampal CA3 pyramidal neurons were compared across several stages of development, ranging from postnatal day 4-5 (P4-5) to maturity. Isoproterenol, pressure-pulse applied to CA3c pyramidal cells, produced a depolarization of membrane potential and an increase in cell input resistance in tissue as young as P7. Spike frequency adaptation (in trains of action potentials triggered by depolarizing pulses) was reduced, as were the slow after-hyperpolarizations following the spike trains. All agonist effects were blocked by timolol, a beta-antagonist. Drug-induced changes in cell membrane and firing properties in immature tissue were qualitatively similar to beta-receptor-mediated noradrenergic effects in adult tissue. These results indicate that the beta-receptor-mediated component of the noradrenergic effect in rat hippocampus is physiologically functional by the seventh day of postnatal life; at earlier times (P4-5) these beta-receptor-mediated noradrenergic actions are, at best, equivocal.

Paragigantocellularis stimulation induces beta-adrenergic hippocampal potentiation

The nucleus paragigantocellularis (PGi) is the major excitatory glutamatergic input to the locus coeruleus (LC). Glutamate activation of the LC has previously been shown to produce beta-adrenergic-dependent potentiation of the perforant path- (PP) evoked population spike in the dentate gyrus (DG). The present study asks if electrical stimulation of the PGi, by activating the LC endogenously, can produce a parallel beta-receptor-dependent potentiation of the PP-evoked population spike. An optimal interstimulus interval (ISI) was determined for PGi-PP stimulation in urethane-anesthetized rats and propranolol was used to assess the role of noradrenergic beta-receptors. PGi stimulation potentiated the PP-evoked population spike at an optimal ISI of 30 ms. The population synaptic response slope and spike latency were not affected. Propranolol blocked the PGi-induced potentiation, as would be expected for beta-receptor-dependent modulation. The parallels between PGi- and LC-induced effects on the PP-evoked population spike suggest PGi stimulation offers an alternate method of LC activation for studies of LC's role in behavior.

Auditory-evoked response of the cortex after yohimbine administration: phase advance effect of central noradrenergic activation

The effects of central noradrenergic activation on an auditory-evoked cortical response were studied using systemic administration of yohimbine (2 mg/kg intravenously, IV), a noradrenergic stimulant, in 13 anesthetized rats. To analyze changes of the response, surface and intracortical evoked potentials (EP) as well as extracellular single-unit recordings with tungsten microelectrodes were employed. It was noted that the initial-positive wave of the surface EP corresponded to unit firing responses in a restricted area of the auditory cortex, where the surface EP was largest and a polarity inversion of the intracortical EP was observed. The following effects were produced by yohimbine: 1) The initial-positive surface potential (n = 10) and corresponding intracortical potential with inverted polarity (n = 6) both showed an increase in amplitude and a decrease in peak latency; 2) the unit firing response (n = 10) tended to show an increase in peak frequency and a decrease in peak firing latency; and 3) yohimbine produced an earlier ending of the firing period, and in paired stimulation experiments (n = 7) it prolonged the period during which the second response was suppressed, indicating an augmentation of postexcitation inhibition. Later histological examination suggested that most of the units recorded were pyramidal cells. These findings indicate that chemical stimulation of the central noradrenergic system by yohimbine enhances both the initial excitatory and following inhibitory processes in the auditory-evoked response of the cortical units (probably pyramidal cells), resulting not only in amplification of the response but also in advancement of the response phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical stimulation of the locus coeruleus: inhibitory effects on hemodynamics and renal sympathetic nerve activity

We examined the role of the locus coeruleus (LC) in the regulation of the hemodynamics and sympathetic nerve activity in anesthetized rats. Unilateral microinjection into the LC of the excitatory amino acid, L-glutamate (Glu), elicited dose-dependent decreases in arterial pressure (AP) and heart rate (HR). The bradycardic response was partially attenuated after intravenous injection of atropine sulfate, but the greater part of this response still remained. Interruption of the ascending projections of the LC by midbrain transection did not affect the depressor and bradycardic responses elicited by chemical stimulation. The renal sympathetic nerve activity showed transient but strong inhibition with this stimulation. Cardiac output was measured using an electromagnetic flowmeter implanted in the ascending aorta. The stroke volume and total peripheral resistance (TPR) were calculated. Microinjection of Glu elicited a significant decrease in TPR and slight decreases in cardiac output and stroke volume. Microinjection of the inhibitory amino acid, gamma-aminobutyric acid (GABA), or the alpha 2-adrenergic agonist, clonidine, exerted no effect on AP and HR. The present results therefore suggest that: (1) the LC neurons have an inhibitory influence on the sympathetic nervous system, and stimulation of these neurons can elicit depressor and bradycardic responses; (2) the depressor response was produced predominantly as a result of a decrease in vascular resistance, rather than a decrease in cardiac output; (3) these inhibitory responses may be provided not via the ascending projections of the LC; and (4) the LC neurons do not have a tonic influence on the cardiovascular system.

Electrophysiological evidence for axonal degeneration of locus coeruleus neurons following long-term forced running stress

Using electrophysiological methods, a change in the density of axon terminals of locus coeruleus (LC) neurons in the cerebral cortex of rats following long-term forced running stress was examined. The stressed animals were classified into two groups based on spontaneous running activity (SRA) measured for 2 weeks after the stress treatment: 1) animals showing early restoration of SRA (poststress active rat) and 2) animals showing little or no SRA (poststress inactive rat). To quantify the density of LC axon terminals in the cerebral cortex, the percentage of LC neurons antidromically activated by cortical stimulation (projection index, P-index) was assessed. The P-indices for the cortex decreased in the poststress inactive rats. Since the threshold currents for antidromic activation were not altered by the stress treatment, the observed change was considered to reflect a change of the density of LC axon terminals rather than physiological consequences. Therefore, when animals receive a prolonged, severe stress, LC neurons in a certain group of the animals may cause axonal retraction or degeneration in the cerebral cortex.

Axonal sprouting of noradrenergic locus coeruleus neurons following repeated stress and antidepressant treatment

Plastic changes in axon terminals of NA LC neurons following repeated stress and antidepressant treatments were examined using electrophysiological or morphological methods. For stress treatment, rats restrained in a small cage were immersed up to the neck in warm water for 10 min daily. Electrophysiological experiments were performed under urethane anesthesia on the day following the termination of stress treatment. To quantify the density of cortical axon terminals arising in the LC, the percentage of LC neurons activated antidromically from the cerebral cortex was assessed. The percentage of LC neurons showing antidromic response to cortical stimulation was increased in the animals stressed for two weeks but not for one week. Since threshold currents for antidromic activation were not changed by the stress treatment, the observed changes were interpreted as morphological (axonal sprouting) rather than physiological consequences in NA axon terminals of LC neurons. To test the ability of antidepressants to induce the regeneration of central NA axons, local injections of 6-OHDA were made bilaterally into the symmetrical sites of the FC. Two weeks after the 6-OHDA injections, the same cortical site of one hemisphere was infused with the antidepressant MPL, DMI, or MIA, and the corresponding site of the other hemisphere with SAL. The density of glyoxylic acid-induced catecholamine fibers was greater in the cortical hemisphere infused with the antidepressants than that infused with SAL. These findings indicate that repeated mild stress and antidepressant treatments induce sprouting of NA LC axons in the cerebral cortex. Axonal sprouting of LC neurons can explain both the delayed onset of the clinical response to antidepressants and subsensitivity of beta-adrenoceptors following repeated stress and antidepressant treatments, and may be a common mechanism for the clinical efficacy of antidepressant drugs and electroconvulsive shock. Furthermore, the findings suggest the possibility that axonal retraction or degeneration of central NA neurons may be involved, at least in part, in the pathology of clinical depression.

Noradrenergic and locus coeruleus modulation of the perforant path-evoked potential in rat dentate gyrus supports a role for the locus coeruleus in attentional and memorial processes

The perforant path-dentate gyrus synapse has provided a model system for functional neural plasticity in adult mammalian brain. NMDA-dependent long-term changes in neural connectivity occur at this synapse in response to high-frequency input. Norepinephrine (NE) applied exogenously or released endogenously can initiate both a short- and a long-term potentiation (LTP) of the dentate gyrus response to perforant path input. Triggering of the potentiated response depends on beta-receptor activation and does not require a high-frequency stimulus. An increase in locus coeruleus (LC) activity can initiate both short and LTP of the perforant path response, although a reduction in LC activity does not alter baseline perforant path responses. This chapter considers differences between NE modulation in vitro and in vivo, differences and similarities between NE-LTP and frequency-induced LTP, and the surprising specificity of NE effects at the perforant path synapse. Studies of NE in the dentate gyrus support a role for the LC in promoting both short- and long-term enhancement of responses to complex sensory inputs and are consistent with a role for the LC in memorial as well as attentional processes.

Duration-dependent effects of repeated restraint stress on cortical projections of locus coeruleus neurons

Using electrophysiological techniques, changes in noradrenergic fiber innervation in the cerebral cortex following repeated stress (restraint in a small cage for either 1 or 6 h daily) were examined by quantifying the density of cortical terminal axons of locus coeruleus (LC) neurons in the rat. After termination of the stress treatment, the single-unit activity of LC neurons was recorded extracellularly under urethane anesthesia, and antidromic activation from 7 cortical points covering nearly the entire cerebral cortex was examined. The percentage of LC neurons activated anti-dromically from the medial frontal cortex was higher in the animals stressed for 1 h daily for 2 weeks. In contrast, the percentage of LC neurons activated antidromically from the cerebral cortex decreased in the animals who received 6 h stress for 2 weeks. These results suggest that stress can cause dual effects, either sprouting or retraction of cortical LC axons depending upon the duration of stress treatment.

Locus coeruleus potentiation of dentate gyrus responses: evidence for two systems

Glutamate activation of the locus coeruleus (LC) and norepinephrine (NE) have both been shown to potentiate the perforant path (PP)-evoked population spike. This potentiation may be short-lasting, the population spike returning to baseline levels within minutes after NE-application or LC activation, or can be long-lasting, persisting 20 minutes or more after termination of the NE or glutamate manipulation. In the present study LC electrical stimulation (333 Hz, 15 msec) initiated 40 msec prior to a PP stimulus reliably caused short-lasting potentiation of the dentate gyrus population spike amplitude (mean maximal = 161%, N = 22). With 50 LC-PP pairings a long-lasting potentiation (greater than 30 min after offset of LC stimulation) was seen in 10/22 experiments. Propranolol (20-30 mg/kg IP) did not block the potentiating effect of LC electrical simulation but completely suppressed the potentiating effect of glutamate activation of the LC in the same animals (N = 5). The beta receptor dependence of short-and long-lasting hippocampal NE potentiation has been previously demonstrated. The inability of a beta receptor antagonist to attenuate the potentiation induced by LC electrical stimulation suggests there are two distinct systems. Both the beta-NE-dependent and the beta-NE-independent system are capable of inducing long-lasting potentiation of the PP-evoked potential.

The role of alpha 1-adrenoceptor-mediated collateral excitation in the regulation of the electrical activity of locus coeruleus neurons

The physiological role of two types of autoreceptors, alpha 1- and alpha 2-adrenoceptors, located on the somadendritic membranes of locus coeruleus neurons, was studied in the developing and adult rat brain. Animals from birth to adulthood were anesthetized with urethan, and single-unit activity was recorded extracellularly in the locus coeruleus. The spontaneous firing of most locus coeruleus neurons was inhibited by iontophoretic application of noradrenaline at a high concentration, while noradrenaline at a low concentration frequently caused excitation of the neurons, predominantly in the developing brain. A similar excitation was also produced by iontophoretic application of the alpha 1-agonist phenylephrine. These excitations were antagonized by the alpha 1-antagonist, 2-beta [4-hydroxyphenylethylaminomethyl]-tetralone, while this antagonist had little effect on glutamate-induced excitation. The noradrenaline- and phenylephrine-induced excitation occurred more frequently in the neurons having little or no spontaneous activity. Electrical stimulation of the dorsal noradrenergic bundle arising in the locus coeruleus produced both inhibition and excitation. The excitatory responses were manifest primarily in early developmental stages, and occurred predominantly when the neurons had little or no spontaneous activity. When the neurons began firing at relatively high rates, the effects of dorsal noradrenergic bundle stimulation became principally inhibitory. Since the excitation evoked by dorsal noradrenergic bundle of stimulation was blocked by the alpha 1-antagonist, the excitation was thought to result from activation of alpha 1-adrenoceptors by noradrenaline released from the terminals of recurrent axon collaterals of locus coeruleus neurons themselves.(ABSTRACT TRUNCATED AT 250 WORDS)

Locus coeruleus neurons projecting to the forebrain and the spinal cord in the cat

The intranuclear organization of the cat locus coeruleus neurons was investigated anatomo-physiologically. The locus coeruleus neurons project to the forebrain through the dorsal noradrenergic bundle and to the spinal cord. Horseradish peroxidase, a retrograde tracer, was pressure-injected into either the dorsal noradrenergic bundle or the ventrolateral funiculus of the high cervical cord (C1-C2). The cats (n = 12) were killed after a 2- or 3-day survival period. The frontal sections (100 micron) throughout the locus coeruleus were observed by light microscope after carrying out the diaminobenzidine reaction. The labeled locus coeruleus neurons were located predominantly in the rostral locus coeruleus proper and locus coeruleus alpha when horseradish peroxidase was injected into the dorsal noradrenergic bundle, whereas they were predominantly located in the caudal locus coeruleus alpha and subcoeruleus when horseradish peroxidase was injected into the spinal cord. In the electrophysiological experiments, cats (n = 30) were anesthetized with alpha-chloralose and two stimulating electrodes were placed stereotaxically in the dorsal noradrenergic bundle and the ipsilateral ventrolateral funiculus of the high cervical cord (C1-C2), respectively. Monophasic square-wave pulses (2.5 Hz, 100 microsecond duration, 800 microA) were delivered. A recording glass electrode, filled with 2 M NaCl saturated with Fast Green, was placed in the locus coeruleus. Neurons with different conduction velocities, which were evoked by the antidromic stimulation of the dorsal noradrenergic bundle and spinal cord, were verified in the locus coeruleus and the adjacent areas. The slow conductive neurons with a conduction velocity of less than 1 m/s had a slow firing rate (1.6 +/- 0.9/s). They were located predominantly in the rostral locus coeruleus proper and locus coeruleus alpha by the dorsal noradrenergic bundle stimulation. From the anatomical and electrophysiological experimental results, it was concluded that the conduction velocities of the horseradish peroxidase-labeled neurons observed in locus coeruleus proper and locus coeruleus alpha were mostly slow and less than 1 m/s. Most of the slow conductive neurons were considered to be noradrenergic. Neurons evoked antidromically by both the dorsal noradrenergic bundle and spinal cord stimulation were not observed.

Antidromic activation of rat dorsomedial hypothalamic neurons from locus coeruleus and median eminence

In rats anesthetized with urethane, single unit activity was recorded in the hypothalamic dorsomedial nucleus (DMH) to obtain antidromic response to stimulation of locus coeruleus (LC) and median eminence (ME). Ninety-two cells were activated antidromically from LC and/or ME. Antidromic latencies to LC stimulation ranged from 7 to 39 msec and those to ME stimulation ranged from 5 to 20 msec. Approximately 13% of the neurons recorded revealed antidromic responses simultaneously from LC and ME, and they were found to bifurcate near the soma. The majority of DMH neurons projecting to LC alone were not spontaneously active, while those projecting to ME tended to discharge spontaneously.

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.

Concentration-dependent suppression by beta-adrenergic antagonists of the shift in ocular dominance following monocular deprivation in kitten visual cortex

We showed that beta-adrenergic receptor antagonists blocked the shift in ocular dominance following brief monocular deprivation in young kittens. Localized microperfusion of propranolol into the kitten visual cortex reduced the expected shift in the ocular dominance approximately 2 mm away from the center of perfusion. The blocking effect, however, did not reach an area approximately 5 mm from the perfusion center, suggesting that beta blockers work in a concentration-dependent fashion in the present paradigm. We further studied the concentration-effect relationship by widely changing the concentration of beta blockers (propranolol and sotalol) stored in an osmotic minipump. The proportion of binocular cells increased from 0.13 to 0.67 when the concentration of propranolol was increased from 10(-6)M to 10(-2)M, giving the half-maximum effect (binocularity, 0.40) at about 10(-4)M propranolol. However, the maximum binocularity obtained with the sotalol perfusion under the comparable condition was apparently much lower (0.45) than that with propranolol. Accordingly, the half-maximum binocularity (0.30) was obtained at about 10(-5)M sotalol. We also noted the presence of a linear, inverse relation between the logarithmic concentration of the beta blockers and the extent of the shift in ocular dominance as measured by the proportion of monocular cells which responded exclusively to stimulation of the nondeprived eye. The latter decreased from 0.75 to 0.25, when the former was increased from 10(-6)M to 10(-2)M (in an osmotic minipump). The two beta blockers behaved similarly in this correlation. The intracortical spread of locally perfused [3H]propranolol was studied at the end of the cortical perfusion which lasted for a week. The radioactivity was highest at the perfusion center and rapidly declined with increasing distance, leveling off approximately 3 mm from the perfusion center. The average "dilution factor" of locally perfused [3H]propranolol was calculated as about 1/170 of the original solution in an area of physiological recordings (approximately 2 mm from the perfusion center). Applying the "dilution factor" of 1/170, we estimated the approximate concentration of beta blockers needed at the recording sites to obtain the half-maximum effect; it was about 5.8 X 10(-8)M for sotalol. Taken together, the present results were interpreted as suggesting that there is a positive correlation between the number of activated beta-adrenergic receptors within the visual cortex and the extent of changes in ocular dominance following monocular deprivation.(ABSTRACT TRUNCATED AT 250 WORDS)

A transmembrane sodium cycle in astrocytes

This study demonstrates that the Na+-K+ pump of mouse astrocytes in primary cultures is stimulated by increases of intracellular Na+. The data presented show that the coupling ratio of the pump varies as a function of intracellular Na+ but not of extracellular K+. A furosemide-sensitive K+ net uptake activated by increased external K+ was partly ouabain-sensitive and found to be dependent upon the Na+ driving force. These findings and those of other authors indicating that an increase of intracellular K+ of glial cells did not involve a concomitant decrease in intracellular Na+ are explained by a transmembrane Na+ cycle. Na+ would enter the cells by providing the driving force for the KCl carrier and would be pumped out by the Na+-K+ pump. The proposed Na+ cycle would function as a coupler and synchronizer of the Na+-K+ and KCl pump under conditions of physiologically elevated external K+.

Locus coeruleus neurons in the neonatal rat: electrical activity and responses to sensory stimulation

Newborn rats, 1-3 days of age, were anesthetized with urethane and single-unit activities were recorded extracellularly in the locus coeruleus. Ca. 35% of locus coeruleus neurons recorded were antidromically activated from stimulation of the frontal cortex. Although the majority of locus coeruleus neurons were not spontaneously active, non-noxious as well as noxious sensory stimuli such as touches to the skin, air puffs and tail pinches were very effective in exciting locus coeruleus neurons.

Amphetamine's effects on terminal excitability of noradrenergic locus coeruleus neurons are impulse-dependent at low but not high doses

The actions of amphetamine in the locus coeruleus and its terminal fields in the frontal cortex were studied using extracellular recording to measure terminal excitability, firing rate and the probability of antidromic action potential invasion of the somatodendritic region in urethane anesthetized rats. At low dose (0.25 mg/kg), amphetamine increased terminal excitability. In comparison, subsequent administration of the highest dose (5.0 mg/kg, i.v.) of amphetamine tested suppressed neuronal firing and blocked antidromic action potential invasion of the somatodendritic region. Despite the absence of impulse traffic, high dose amphetamine reversed the effect of low dose amphetamine in the terminal field and decreased terminal excitability. The alpha 2 antagonist, yohimbine (0.5 mg/kg, i.v.), reversed the effects of high dose amphetamine on terminal excitability and somatodendritic invasion without reinstating neuronal firing. Noradrenergic autoreceptor agonists are known to decrease terminal excitability, whereas antagonists are known to increase terminal excitability. Thus, since low dose amphetamine produces the same effect on terminal excitability that antagonists do, it appears that low dose amphetamine may reduce autoreceptor activation by reducing norepinephrine release in frontal cortex as a consequence of inhibiting locus coeruleus neuronal firing. In contrast, high dose amphetamine acts like autoreceptor agonists do and decreased terminal excitability. Hence high dose amphetamine may increase norepinephrine release, even in the absence of impulse traffic.

Impulse conduction properties of noradrenergic locus coeruleus axons projecting to monkey cerebrocortex

Antidromically driven action potentials were recorded from norepinephrine-containing locus coeruleus neurons in response to electrical stimulation of cerebrocortical and thalamic areas in anesthetized squirrel monkeys. These cells reliably conducted impulses from cortical sites of distances up to 100 mm from locus coeruleus. Monkey locus coeruleus neurons were found to exhibit several properties previously described for these cells in rat, including slow spontaneous discharge rates, characteristic impulse waveforms, antidromic activation from many target areas, a period of suppressed activity following either antidromic or orthodromic driving and responsiveness to noxious stimuli presented as subcutaneous electrical stimulation of a rear foot. However, a large population of monkey locus coeruleus neurons was found to exhibit more rapid conduction velocities than previously found for rat (e.g. approximately 34% were greater than 1 m/s), resulting in similar conduction latencies to distant target areas in the two species. This indicates that the conduction times required for locus coeruleus impulses to reach distant target areas may be conserved across different species and sizes of brains, suggesting that these latencies play an important role in the general function of the locus coeruleus system in brain and behavioral processes.

Partial damage to the noradrenergic bundle increases tyrosine hydroxylase activity in noradrenergic terminals of hippocampus but not cerebellum

Intracerebral administration of 6-hydroxydopamine (6-HDA) along the dorsal noradrenergic bundle produced a gradual and long-lasting decrease of norepinephrine (NE) in hippocampus. This decrease in NE levels initially was accompanied by a parallel decrease in the activity of the rate-limiting enzyme for NE biosynthesis, tyrosine hydroxylase (TH). However, by 14 days the decrease in enzyme activity was much less pronounced than that of NE levels, resulting in an 8-fold increase in the ratio of TH activity to NE content. This persisted for at least two months, the longest time examined. Depletion of NE and changes in TH activity were not observed in cerebellum after the 6-HDA treatment. The selective increase in the ratio of TH activity within denervated regions may reflect an adaptation to the lesion whereby residual noradrenergic neurons increase their synthesis and release of NE.

Changes in projection from locus coeruleus to lateral geniculate nucleus following ablation of visual cortex in adult rats

The visual cortex of adult rats was unilaterally ablated. A histofluorescence study revealed an increase of noradrenergic terminals in the lateral geniculate nucleus (LGN) ipsilateral to the decortication, confirming the previous report. Corresponding to this, the frequency for encountering neurons in the locus coeruleus (LC) activated antidromically from the LGN was increased. We suggest that LC neurons whose axon terminals were damaged by ablation of the visual cortex formed new axons or axon collaterals (pruning effect) in the LGN, thus contributing, at least partly, to the increase of noradrenergic terminals therein.

Increased projection from the locus coeruleus to the lateral geniculate nucleus and visual cortex in young adult rats following unilateral enucleation

Rats of 30 and 60 days of age were subjected to removal of one eye, and electrophysiological experiments were carried out to see if the density of the projection from the locus coeruleus to the visual centers such as the lateral geniculate nucleus and visual cortex increased. In rats with an eye removed at 30 days, the projection of locus coeruleus neurons increased in the lateral geniculate nucleus contralateral and visual cortex ipsilateral to the removed eye, whereas rats with an eye removed at 60 days did not show any notable change in the density of the projection.

Increased adrenergic projection from the locus coeruleus to the lateral geniculate nucleus of rats following one-eye-removal at birth

Electrophysiological studies were made on plastic changes in the projection from the locus coeruleus (LC) to the lateral geniculate nucleus (LGN). In adult rats with one eye removed at birth it was found that the chance for encountering LC neurons activated antidromically from the LGN was significantly larger in the side of eye removal than in control. This was thought to indicate that the amount of projection from the LC to the LGN had increased in response to eye removal at birth.

Physiological properties of ascending locus coeruleus axons in the squirrel monkey

Discharge activity was recorded extracellularly from individual neurons of the nucleus locus coeruleus in anesthetized squirrel monkeys. These cells exhibited long-duration (2-3 ms) action potentials and discharged spontaneously in a slow (0.2-2 Hz) irregular fashion. Stimulation of the lateral hypothalamus evoked antidromic responses at latencies of 10-20 ms, indicating conduction velocities of over 1 m/s in some cases. The mean refractory period for these axons was 2.6 ms. When the rate of hypothalamic stimulation was increased from 1 to 10 Hz there was a 15-20% increase in antidromic latencies. These properties are similar to those previously observed for rat LC neurons, except that conduction velocities are higher in monkey.

Identification of spinally projecting neurones in the A1 catecholamine cell group of the ventrolateral medulla

In anaesthetized rats, neurones were found in the ventrolateral medulla that responded to antidromic stimulation of their axons in the thoracic spinal cord. These neurones were identified as being antidromic, according to various established electrophysiological criteria. A total of 44 antidromically identified neurones were found, 23 had conduction velocities below 2.0 m/s. Many (70%) had ongoing activity with a slow firing rate (0.2-5.0 Hz). Catecholamine-containing cells were visualized in this ventrolateral region using a modified Mg+-catalyzed glyoxylic acid technique and revealed multipolar, small diameter cells (17-20 microns) which were diffusely scattered (as demonstrated in previous histofluorescence studies). The Pontamine sky blue-marked recording sites of 7 units (0.3-1.1 m/s) showed close apposition to a CA-fluorescent cell whilst a further 4 (2.0-2.7 m/s) could not be correlated with the presence of a fluorescent cell. The results are discussed in the light of recent data in the rat, suggesting that the spinal cord catecholamine innervation arises from brainstem cell groups other than A1.

The distribution of neocortical projection neurons in the locus coeruleus

The present study was conducted to examine the spatial organization of locus coeruleus (LC) neurons that project to rat cerebral cortex. Long-Evans hooded rats received unilateral pressure injections of horseradish peroxidase (HRP) in either frontal (n = 6) or sensorimotor (n = 11) or occipital (n = 7) cortex to determine the intranuclear location of LC neurons which project to specific neocortical regions. Coronal and sagittal sections (40-100 micron) through the LC were examined by light microscopy after carrying out the tetramethyl benzidine reaction and staining with neutral red. The locations of retrogradely labeled cells were recorded on a three-dimensional biological coordinate system maintained by a computer linked to the light microscope. LC neurons labeled from cerebrocortical injections of HRP were primarily located in the ipsilateral and to a lesser extent (fewer than 5% of total labeled cells) in the contralateral nucleus. Coeruleocortical projection neurons were concentrated in the caudal three-fifths of the dorsal division of the ipsilateral LC. Within this portion of the nucleus, HRP-filled neurons were distributed so that individual groups of cells projecting to occipital or sensorimotor or frontal cortex were coarsely aligned in a dorsal to ventral array, respectively. Moreover, in the sagittal plane of the nucleus the pattern of labeling was spatially graded so that the subset of neurons projecting to the occipital cortex was displaced more caudally in the LC than the groups of cells sending axons to sensorimotor or frontal cortex. Only the frontal area of the cortex received a projection from both dorsal and ventral divisions of the ipsilateral LC. Computer-assisted analysis of the data further suggested that neocortical projection neurons in the dorsal LC are loosely organized into two groups which run rostrocaudally through the core of the caudal nucleus. The zone of labeling resulting from injections confined to the neocortical gray matter overlapped with but was not coextensive with that observed following injections into the caudate, hippocampus, and cerebellum. These results suggest that partially overlapping subsets of LC cells might independently influence separate populations of neurons within noradrenergic terminal fields of the neocortex.

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.