The mechanism of cerebrovascular vasoconstriction in response to locus coeruleus stimulation

Stimulation of the locus coeruleus in 11 Macaca nemestrina monkeys resulted in a frequency-dependent increase in internal carotid resistance (vasoconstriction) and a frequency-dependent decrease in external carotid resistance (vasodilation). The internal carotid constrictor response is maximal at low frequency stimulation (5 s-1), and decreases as frequency of stimulation is increased or decreased. It is blocked by the non-specific alpha-adrenoceptor blocker phentolamine and the alpha-2 adrenoceptor blocker yohimbine, but is unaffected by the alpha-1 adrenoceptor blocker prazosin. It is concluded that the cerebral vasoconstrictor response to locus coeruleus stimulation is mediated by alpha-2 adrenoceptors.     

Lumbar dorsal root potentials elicited by stimulation of nucleus locus coeruleus

Lumbar dorsal root potentials (DRP) were elicited by nucleus locus coeruleus (LC) stimulation in the cat. Inhibition, by LC stimulation, of dorsal horn cells responding to noxious inputs corresponded in time with the DRPs evoked by LC stimulation. Comparing cutaneous stimulation-evoked DRPs with LC stimulation-evoked DRPs and their respective effects on dorsal horn single-unit activity suggested a shared segmental underlying mechanism and the possible involvement of the coeruleospinal system with that of a diffuse noxious inhibitory suprasegmental loop.     

Evidence against specificity of electrical stimulation of the nucleus locus coeruleus in activating the sympathetic nervous system in the rat

Electrical stimulation of the central nucleus locus coeruleus (LC) was previously shown to increase activity of the peripheral sympathetic nervous system (SNS) as measured by increases in plasma levels of the norepinephrine (NE) metabolite 3-methoxy-4-hydroxyphenthyleneglycol (MHPG) in the rat⁵. Four experimental approaches were designed to test the specificity of the LC cell group in activating the SNS in the stimulation paradigm. Varying the stimulation current amplitude, varying the site of stimulating electrode placement, and electrolytic lesions of the LC yielded results consistent with the hypothesis that the site of SNS activation was within the anatomical region of the LC cell group. Neurochemical lesioning with intraventricular 6-hydroxydopamine, however, did not effectively block the plasma MHPG increase observed after stimulation of the LC region. The possibility that non-noradrenergic cells, fibers of passage, or terminals in the LC region of the midbrain may be responsible for SNS activation when the LC is electrically stimulated is discussed. These studies are pertinent to all studies of LC function which employ electrical stimulation of the LC nucleus, including investigations of the role of the LC in social behavior, intracranial self-stimulation, and blood pressure regulation.     

Potentiation of GABA inhibitory action in cerebellum by locus coeruleus stimulation

In cerebellum, excitatory and inhibitory responses of Purkinje cells, produced both synaptically and by microiontophoresis of putative amino acid neurotransmitters, have been shown previously to be enhanced during NE iontophoresis. The influence of locus coeruleus conditioning stimulation on Purkinje cell responses to GABA iontophoresis was examined to determine whether endogenous NE, released from synaptic terminals, could exert similar modulatory effects. Locus coeruleus stimulation at current intensities which alone elicited no direct depression of Purkinje cell spontaneous discharge potentiated the inhibition produced by GABA. Iontophoretic application of sotalol, a specific β-adrenergic blocker, antagonized this enhancement of GABA inhibition. Repetitive activation of the classic non-adrenergic cerebellar afferents did not enhance the GABA response, despite causing a direct depression in spontaneous rate. A neuromodulatory role is suggested for tonic adrenergic input in the mammalian central nervous system.     

Effects of locus coeruleus stimulation on carotid vascular resistance in the cat

The locus coeruleus was stimulated in 62 cats in order to investigate the effect on cephalic blood flow and cephalic vascular resistance. Flow was measured by electromagnetic flow probes applied to the common carotid artery. Stimulation over a range of frequencies (0.2-200 s-1) produced a frequency-dependent fall in carotid vascular resistance, greater on the ipsilateral side. This response was not affected by either cervical sympathectomy or spinal cord section. The response was blocked by bilateral section of the facial nerve but was not abolished by classical cholinergic, histaminergic or adrenergic blocking agents. Stimulation of the locus coeruleus also resulted in a pressor response through spinal mechanisms in which coeruleo-hypothalamic projections were not involved. A post-stimulation constriction in the carotid vasculature followed the dilator response and was attributed to release of catecholamines from the adrenal medulla.     

Autonomic responses are elicited by electrical stimulation of the medial but not lateral frontal cortex in rabbits

Conscious rabbits received electrical stimulation of the anterior midline frontal cortex or lateral somatosensory and motor cortex, through chronically-implanted electrodes. Active sites for cardiovascular responses were found in the anterior midline cortex, but stimulation of the frontolateral sensory-motor cortex either did not elicit cardiovascular changes or elicited only small and variable changes when stimulated. The heart-rate response elicited was, in all cases, bradycardia. All blood pressure changes consisted of depressor responses. Stimulation of the lateral frontal cortex almost always resulted in increases in EMG activity, although many placements were observed in the medial frontal cortex that were unaccompanied by movement. In all cases in which depressor responses and bradycardia were elicited, increases in respiration rate and decreases in depth also occurred. The active area from which bradycardia and depressor responses were elicited forms the medial portion of the cortical projection area of the mediodorsal nucleus of the thalamus and thus may be involved in the autonomic accompaniments of the behavioral activities, i.e. learning and memory processes, associated with this nucleus.     

The action of adenosine on noradrenergic neuronal inhibition induced by stimulation of locus coeruleus

Stimulation of locus coeruleus (LC) has been used to induce a noradrenergic inhibition of neurones in the rat cerebral cortex. The local application of adenosine or adenosine monophosphate by microiontophoresis in the cortex was found to produce a reduction of the evoked inhibition if the purine application was begun 20 sec before LC stimulation, but an enhancement of the inhibition if applied up to 35 sec after the LC stimulation. GABA increased the duration of LC inhibition irrespective of time of application. Adenosine and GABA showed no mutual potentiation, but norepinephrine increased the size of responses to both iontophoretic adenosine and GABA. The adenosine-norepinephrine interaction was synergistic, irrespective of the order of application. It is concluded that adenosine may act both presynaptically to inhibit, and postsynaptically to enhance the effects of noradrenergic neurone activation, the dominant effect observed depending on the temporal relationship between LC activation and adenosine application.     

Coexistence of vasopressin, neurophysin and noradrenaline immunoreactivity in medium-sized cells of the locus coeruleus and subcoeruleus in the rat

Vasopressin- and neurophysin-immunoreactive cells have recently been demonstrated in the rat locus coeruleus (A6) and subcoeruleus (A7). Using consecutive 5 μm thick frozen sections, medium-sized cells throughout the locus coeruleus area, but predominantly in the posterior parts of the A6 displayed coexistence for vasopressin and noradrenaline or neurophysin and noradrenaline immunoreactivity. The putative projection areas of putative fibers from vasopressin-containing cells in the locus coeruleus still remain to be elucidated.     

蓝斑参与调节室旁核对心血管活动的作用

为探讨化学刺激室旁核对心血管活动的影响及蓝斑在这一效应中的作用 ,给乌拉坦和氯醛糖混合液麻醉的 Wistar系雄性大鼠室旁核微量注射 L-谷氨酸 ,观察了血压、心率和肾交感神经放电活动的变化以及用红藻氨酸损毁蓝斑对刺激室旁核效应的影响。另外 ,为进一步研究室旁核与蓝斑功能联系的机制 ,也观察了蓝斑核区微量注射血管升压素对心血管活动的影响。结果表明 :1室旁核内注射 L-谷氨酸引起血压下降、心率减慢和肾交感神经活动减弱 ;2损毁蓝斑后 ,刺激室旁核引起的心血管效应明显减弱 ;3蓝斑区注射血管升压素导致血压升高、心率加快、肾交感神经放电增加。提示 :1 L-谷氨酸兴奋室旁核抑制心血管活动 ;2室旁核的上述效应可能与交感神经紧张性降低有关 ;3蓝斑参与室旁核对心血管活动的调节过程 ;4血管升压素刺激蓝斑引起心血管活动加强。     

Regulation of locus coeruleus neurons and splanchnic, sympathetic nerves by cardiovascular afferents

The brain norepinephrine (NE) neurons in the nucleus locus coeruleus (LC) have been claimed to be involved both in the regulation of behavioral functions, e.g. vigilance and arousal reactions, and in cardiovascular control. Recent studies from this laboratory have also shown that cardiovascular, vagal afferents can participate in the regulation of the LC neurons in the rat.

Utilizing electrophysiological techniques, we have now studied the effects of activation of blood volume receptors or arterial baroreceptors on the firing rate of single cells in the LC and, parallelly, on splanchnic, sympathetic discharge in the chloral hydrate anesthetized rat.

Blood volume load (0.5–5 ml heparinized blood, intravenously administered) induced a reduction in both LC neuronal firing rate and splanchnic nerve activity (SNA), effects which were readily and completely reversed by withdrawal of the corresponding amount of blood. In comparison, the central LC neurons were more sensitive to blood volume expansion than the peripheral splanchnic nerves. The effects of blood volume load on LC and SNA remained unaffected after deafferention of arterial baroreceptors.

Blood pressure elevation, induced by slow intravenous infusion of NE or angiotensin (AII) (total dose 2 μg/kg), caused an immediate reduction in both the firing rate of most of the LC cells tested as well as in SNA. While the effect on SNA was abolished by deafferentation of arterial baroreceptors, the effect on central LC activity remained largely unaffected.

Consequently, these data strengthen the concept that brain NE neurons in the LC are subject to control by peripheral blood volume receptors, analogously to peripheral sympathetic nerves. Arterial baroreceptors may still participate in the control of central noradrenergic nerve activity, but in contrast to their function for SNA they are not critical for the inhibition of LC neurons by blood pressure elevation. Rather, these two cardiovascular afferent systems may participate in the physiological regulation of the LC activity in a complimentary and convergent fashion.     

Long lasting suppression of firing of cortical neurons and decrease in cortical blood flow following train pulse stimulation of the locus coeruleus in the cat

Single neuron activity and local cerebral blood flow were recorded simultaneously in the same spot of the gyrus proreus in cats. Train pulse stimulation (10–20 Hz, 30 sec) of the ipsilateral locus coeruleus induced long lasting suppression of firing in up to 78% of neurons and decrease in local flow, which lasted 1.9–5.6 min and 3.8–6.5 min, respectively. Single pulse stimulation evoked inhibition of firing in 55% of the neurons investigated.     

Differentiated cardiovascular afferent regulation of locus coeruleus neurons and sympathetic nerves

The activity of brain norepinephrine (NE) neurons in the locus coeruleus (LC) and peripheral sympathetic nerve activity (NE-SNA) in the splanchnic/renal nerve were recorded simultaneously during alterations of arterial blood pressure and circulating blood volume. Utilizing this experimental procedure we have previously found that both central and peripheral NE neurons are inhibited during blood pressure elevation. Furthermore, both neuronal systems were found to be inhibited during blood volume load, an effect apparently mediated by vagal afferents. In the present study both brain NE-LC activity and NE-SNA were increased during blood volume depletion. However, during prolonged hemorrhage the initial excitation of NE-SNA was followed by a marked inhibition. In contrast, the increase in NE-LC activity remained throughout the volume depletion period. The responses of central and peripheral NE neurons during hemorrhage were abolished in animals subjected to bilateral cervical vagotomy. Nitroprusside- or phenylephrine-induced blood pressure variations were associated with reciprocal changes in both central and peripheral NE neuronal activity. The NE-LC responses to blood pressure variations were abolished after bilateral vagotomy. NE-SNA responses, on the other hand, persisted after bilateral vagotomy. Our present and previous findings show that brain NE-LC neurons, similarly to peripheral NE neurons in the splanchnic/renal nerve, are regulated by tonically active cardiovascular afferents. Whereas peripheral NE-SNA is regulated by both arterial (high pressure) baroreceptors and cardiac volume (low pressure) receptors, the NE-LC neurons seem exclusively regulated by cardiac volume (low pressure) receptors.     

Effect of locus coeruleus stimulation on regional cerebral oxygen consumption in the cat

Regional cerebral oxygen consumption was determined during stimulation of the intra-axial noradrenergic pathway to quantitate the metabolic effects of this manipulation on cerebral oxygen extraction, cerebral blood flow (CBF) and its regional distribution. Regional arterial and venous oxygen saturation were examined microspectrophotometrically. Regional CBF was examined using radioactively tagged microspheres (15 +/- 3 microns in diameter). Oxygen consumption was calculated as the regional product of CBF and oxygen extraction. Bipolar concentric electrodes were stereotaxically implanted bilaterally in the locus coeruleus of alpha-chloralose anesthetized, artificially respired adult mongrel cats. The control group was killed after hemodynamic and CBF measurements were taken. The experimental group was sacrificed after these same measurements were taken before and during 10 min of bilateral locus coeruleus stimulation. The cats' heads were simultaneously sawed in 3 places and quickly frozen in liquid nitrogen-cooled propane. Systolic blood pressure was significantly increased during treatment. The heterogeneity of venous oxygen saturation was significantly reduced by stimulation. Average CBF and oxygen consumption were significantly decreased to 57% and 59% of control, respectively. Oxygen consumption was significantly reduced in the hypothalamus from 1.5 +/- 0.3 to 0.9 +/- 0.3 ml O2/min/100 g and from 3.5 +/- 0.9 to 1.2 +/- 0.4 ml O2/min/100 g in the cerebellum by treatment. Changes in the neuronal and/or synthetic cerebral activity produced regional decreases in cerebral oxygen consumption and secondarily altered CBF. These changes are probably due to interaction of the intraparenchymal noradrenergic pathways with other systems or processes in the brain.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition from locus coeruleus of nucleus accumbens neurons activated by hippocampal stimulation

Electrophysiological studies using rats were performed to examine the influence of locus coeruleus (LC) on nucleus accumbens (Acc) neurons. Spike generation by hippocampal stimulation was inhibited by both LC conditioning stimulation and iontophoretic application of noradrenaline, but spikes elicited by stimulation of parafascicular nucleus of thalamus were rarely affected by LC conditioning stimulation or noradrenaline. The LC-induced inhibition was antagonized by iontophoretic sotatol, but not by phentolamine, suggesting that noradrenaline derived from the LC inhibits the Acc neurons receiving input from the hippocampus, probably acting on a β-adrenergic receptor.     

Spinal projections from the nucleus locus coeruleus and nucleus subcoeruleus in the cat and monkey as demonstrated by the retrograde transport of horseradish peroxidase

The rostral pons of the cat and rhesus monkey were examined for the presence of labeled cells following injections of horseradish peroxidase (HRP) into the lumbar spinal cord. Labeled cells were found in the ipsilateral dorsolateral pontine tegmentum and in the contralateral ventrolateral pontine reticular formation. In both the cat and monkey, labeled cells were located in the nucleus locus coeruleus, nucleus subcoeruleus, in or near the Kölliker-Fuse nucleus, and in the ventral part of the lateral parabrachial nucleus. There is a striking similarity between the distribution of HRP-labeled cells in the dorsolateral pontine tegmentum of the cat and monkey and that of catecholamine-containing cells observed in this area in previous studies.     

Catecholamine metabolism in the rat locus coeruleus as studied by in vivo differential pulse voltammetry. II. Pharmacological and behavioral study

Following injection of the acylating reagent N-succinimidyl [3H]propionate into the optic nerve of goldfish, labeled protein appeared in the ipsilateral retina and contralateral tectum in a time-dependent manner. Autoradiography indicated the presence of the labeled material in the neuroplasm of the retinal ganglion cells and their projections. While most of the recoverable injected radioactivity was confined to the injection site even after 1 week, labeled proteins arriving in the retina by retrograde flow or in the tectum by anterograde flow had distinctly different patterns, a result suggesting specific transport processes rather than diffusion. In contrast to reported studies with the labeling agent in other species, a prominent 68,000 molecular weight component was not seen. The results are discussed in relation to the role of retrograde transport in regeneration.     

Release of GABA from the guinea-pig neocortex induced by electrical stimulation of the ‘locus coeruleus’ or by norepinephrine

GABA release from the cortical surface was measured in freely moving guinea-pigs using collecting cups and a mass-fragmentographic method. Stimulation of the locus coeruleus caused a prolonged sedation of the animals and a 60% increase of GABA output from their cerebral cortex. Similar results were obtained after intraventricular injections of norepinephrine. Phentolamine antagonized these effects. The results suggest that the noradrenergic innervation of the cortex modulates the function of cortical GABA neurons.     

Effect in cat locus coeruleus lesions on the response of cerebral blood flow and cardiac output to altered paCO2

In pentobarbital-anesthetized cats, over arterial p_aCO₂ values of 20–60 mm Hg, cerebral blood flow (CBF, Xenon) and cardiac output (CO, thermal dilution) show positively inflected curves with slopes significantly greater than zero. To examine the role of the locus coeruleus (LC) in these responses, bilateral stereotactic thermo-coagulation lesions of the LC were made. The effect of lesions confirmed to involve the LC bilaterally (n= 10), were compared with the effects of misdirected lesions placed in the cerebellum abd lateral to the LC (n= 10) and sham lesions (n= 10). Ted days after the lesioning procedure, the animals were re-anesthetized with pentobarbital and p_aCO₂ response curves were determined for CBF and CO prior to and following intravenous administration of propranolol (1 mg/kg, i.v.). The results obtained with the sham-operated animals and the animals with lesions outside of the LC were indistinguishable. Bilateral LC lesions had no significant effect on normocapnic CBF as compared to control animals. They did, however, significantly reduce the slope of the CBF p_aCO₂ response curve. Propranolol produced a significant reduction in CBF in lesioned and non-lesioned animals measured at all levels of pCO₂ and did not alter the slope of the pCO₂ response curve for any group as compared to predrug values. Bilateral lesions of the LC did not significantly alter either normacapnic CO or the slope of the CO-p_aCO₂ relationship, but did reduce the elevation in mean arterial blood pressure otherwise observed during hypercarbia. Measurement of norepinephrine levels in cortex indicate a close indicate a correlation between the ability of the lesion to reduce norepinephrine content and produce the observed physiological effects. The results of these experiments suggest that the hypercapnic response of CBF, but not CO to arterial p_aCO₂ is modulated by systems which traverse the dorsal brainstem. The role of the locus coeruleus-catecholamine cell bodies in this effect, however, must be considered speculative until further transmitter-selective interventions are carried out.     

Facilitation of lumbar monosynaptic reflexes by locus coeruleus in the rat

The present study was initiated to delineate whether species difference exists between cats and rats in the descending influence of locus coeruleus (LC) on spinal motoneuronal activity. In male Sprague-Dawley rats anesthetized with chloral hydrate (400 mg/kg, i.p.), localized activation of LC promoted an exclusive facilitation of lumbar spinal extensor and flexor monosynaptic reflexes (MSRs). Such LC-evoked potentiations may vary in degree (37.5-147.4%), duration (70.6-72.9 ms) and latency (3.0-5.5 ms) among different animals. While minimally affecting the control MSRs, the alpha 1-adrenoceptor blocker prazosin (20 micrograms/kg, i.v.) significantly antagonized the enhancing effect of the LC on MSRs, suggesting the participation of noradrenergic neurotransmission in the process. Since these results are in general agreement with previous observations from our laboratory on the cat, we conclude that the LC exerts similar facilitatory actions on both extensor and flexor motoneuron activity of the hindlimb in at least two animal species, rat and cat.