Evidence of facilitatory coerulospinal action in lumbar motoneurons of cats

Functional connectivity of the feline coerulospinal projection was delineated by utilizing the combined approaches of antidromic activation and electrical stimulation. We isolated 25 locus coeruleus (LC) neurons that were electrophysiologically identified and histologically verified and that could be driven by stimulating the spinal cord. Antidromicity of the spike potentials was confirmed by the constant latency, the high frequency (100 Hz) following, fractionation of the initial segment-somatodendritic potential, and collision between the antidromic and the spontaneous orthodromic spikes. The mean conduction speed was20 ± 8m/sec(range= 7to32m/sec). Intracellular studies revealed facilitatory LC actions in 22 lumbar motoneurons (MNs), In 13 MNs, LC activation alone produced slow-rising excitatory postsynaptic potentials (EPSPs) of3 ± 1mV amplitude that lasted 4–30 msec. Six of the 13 MNs discharged action potentials upon LC stimulation. In the remaining 9 MNs, no observable potential change was registered after LC activation. Antecedent LC stimulation consistently potentiated the synaptic efficacy of testing dorsal root shocks. The enhancement of synaptic activation was antagonized by systemic injection of phenoxybenzamine (3 mg/kg). These results suggest that facilitation of MNs by the LC is at least in part mediated by distal dendritic depolarization. Those MNs that exhibited augmented excitability but no demonstrable EPSPs may have been activated by norepinephrine-mediated synaptic modulation.     

Effect of adrenaline synthesis inhibition on brain noradrenaline neurons in locus coeruleus

The functional significance of the morphologically identified adrenaline (A)-mediated input to the noradrenergic nucleus locus coeruleus (LC) was pharmacologically analyzed. By means of single unit recording techniques the LC neurons in the rat brain were studied following administration of SK&F 64139 and DCMB, drugs which are both potent inhibitors of the A-forming enzyme phenylethanolamine-N-methyltransferase (PNMT). SK&F 64139 (2–200 mg/kg i.v.) caused an immediate, dose-dependent and long-lasting increase in firing rate of the LC neurons. The dose-response curve for the LC inhibitory effect of theα₂-receptor agonist clonidine was shifted in parallel to the right by pretreatment with SK&F 64139. All the above mentioned effects of SK&F 64139 were mimicked by SK&F 72223, a structurally analogous compound which is reported to lack PNMT inhibitory activity. Consequently, the activation of LC neurons by SK&F 64139 is probably not related to its capacity to inhibit the synthesis of A but rather to some other action of the drug, such as anα₂-receptor blocking effect. In contrast to SK&F 64139, the other PNMT inhibitor tested, DCMB (1–60 mg/kg), produced no significant activation of the LC neurons and but little clonidine antagonistic action. Thus, judging from these experiments, DCMB is devoid of significantα₂-receptor blocking properties. At the time for maximal brain A depletion after DCMB administration (4–6 h) the average firing rate of randomly encountered LC neurons was unaltered when compared with controls. In contrast, pretreatment with the tyrosine hydroxylase inhibitorα-methyl-p-tyrosine methylester, which causes depletion also of brain noradrenaline, significantly increased LC neuronal firing rates. These data indicate that if there exists a physiologically relevant A-mediated input to the LC, this is not of critical importance for the tonic activity of the LC neurons.     

Adenosine inhibits locus coeruleus neurons: An intracellular study in a rat brain slice preparation

Intracellular recording was used to study the effect of adenosine (3-100 microM) on rat locus coeruleus (LC) neurons in a brain slice preparation. Bath application of adenosine (100 microM) reduced the rate of spontaneous firing in 88% of LC neurons. In some LC neurons adenosine also caused membrane hyperpolarization (2-10 mV) and reductions in input resistance of 9-24%. Adenosine effects were dose-dependent and antagonized by the adenosine receptor antagonist theophylline.     

Cerebral blood flow and oxygen consumption in the rat brain after lesions of the noradrenergic locus coeruleus system

The effects of lesions of the locus coeruleus neuron system on cerebral metabolic rate for oxygen (CMR_O2) and blood flow (CBF) was evaluated in paralyzed and mechanically ventilated rats, using a¹³³xenon modification of the Kety-Schmidt inert gas technique. Bilateral electrothermic lesions of the nucleus locus coeruleus or bilateral 6-hydroxydopamine lesions of its ascending bindle caused no significant change in CBF or CMR_O2. The 6-hydroxydopamine lesions did not influence the CBF and CMR_O2 responses to hypercapnia and hypoxia.It is concluded that the locus coeruleus does not exert any resting tone on CBF and CMR_O2 and that no influence on the CBF and CMR_O2 responses to hypercapnia and hypoxia is mediated via its ascending projections.     

Time course of appearance and release of [35S]cysteine labelled neurophysins and peptides in the neurohypophysis

[35S]cysteine injected adjacent to the supraoptic nuclei (SON) of rats is rapidly incorporated into two macromolecular (both about 20,000 Daltons) common precursors of arginine vasopressin (AVP) and its associated neurophysin, and oxytocin (OT) and its neurophysin. Conversion of the larger precursor proteins to the smaller peptides appears to occur intragranularly during axonal transport to the neurohypophysis. The labelled products of this conversion (neurophysin, AVP, and OT) are released, in a Ca2+-dependent manner, from the posterior pituitary in response to depolarization by veratridine. Both the rates of biosynthesis and of processing of the precursors are greatly increased by increased functional activity (i.e. secretion) of the hypothalamo--neurohypophysial system.     

Catecholamine inhibition of calcium action potentials in rat locus coeruleus neurones

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

Diverse afferents converge on the nucleus paragigantocellularis in the rat ventrolateral medulla: retrograde and anterograde tracing studies

The nucleus paragigantocellularis in the ventrolateral medulla has been implicated in cardiovascular, pain, and analgesic functions; and it has also been found to be a major afferent to the pontine nucleus locus coeruleus. In the present study, afferents to the nucleus paragigantocellularis were identified in the rat by means of the retrograde tracers wheat germ agglutinin-conjugated horseradish peroxidase or Fluoro-Gold. Projections to the nucleus paragigantocellularis arise from a wide variety of nuclei with autonomic, visceral, and sensory-related functions. Major afferents with consistent and robust retrograde labeling include most laminae of the spinal cord, the caudal lateral medulla, the contralateral paragigantocellularis, the nucleus of the solitary tract, the A1 area, the lateral parabrachialis, the K?lliker-Fuse nucleus, the periaqueductal gray, and a preoculomotor nucleus in the ventral central gray, the supraoculomotor nucleus. Other notable afferents, seen only after large caudal injections into the nucleus paragigantocellularis, include the lateral hypothalamus, the paraventricular nucleus of the hypothalamus, and the medial prefrontal cortex. Minor afferents include the gigantocellular nucleus, the area postrema, the caudal raphe groups, the inferior colliculus, the A5 area, and the locus coeruleus. The projection from the supraoculomotor nucleus, not previously reported as an afferent to the ventrolateral medulla, was confirmed with anterograde tracing by means of Phaseolus vulgaris-leucoagglutinin. Iontophoretic deposits of Phaseolus vulgaris-leucoagglutinin into the nucleus of the solitary tract (commissuralis level) or into the periaqueductal gray also yielded terminal fiber labeling in the nucleus paragigantocellularis. Fibers from the supraoculomotor nucleus and the nucleus of the solitary tract were densest in the lateral aspect of the nucleus paragigantocellularis (corresponding to the rostroventrolateral reticular nucleus), while fibers from the periaqueductal gray were more medially located. Previous studies have defined inputs to the rostral ventrolateral medulla from the cochlear nucleus as well as from the colliculi. In the present study, deposits of wheat germ agglutinin-conjugated horseradish peroxidase or Phaseolus vulgaris-leucoagglutinin into the cochlear nucleus or the superior colliculus yielded only sparse anterograde labeling in the nucleus paragigantocellularis, but heavily labeled adjacent areas. The inferior collicular injections yielded strong but restricted anterograde labeling in the rostromedial paragigantocellularis, medial to the facial nucleus. These results indicate that the paragigantocellularis area receives inputs from diverse brain structures. Neurons in the nucleus paragigantocellularis afferent to the locus coeruleus, being distributed throughout this region, may provide a channel where several types of information are integrated and transmitted to the extensive locus coeruleus noradrenergic efferent network...     

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.     

Supernumerary locus coeruleus neurons as a determinant of inherited epilepsy in the convulsive mutant mouse quaking

In quaking mice (a genetic model of epilepsy with an increased number of noradrenergic neurons) bilateral electrolytic coagulation of locus coeruleus (LC) in adult mice inhibited the convulsions elicited by somatic stimulations while neonatal 6-hydroxydopamine (6-OHDA) treatment remained ineffective upon the convulsions. Biochemical effects of the two treatments differed only in the brainstem where electrolytic lesion decreased while 6-OHDA treatment increased noradrenaline (NA) and 3-methoxy 4-hydroxyphenylethyleneglycol (MHPG) levels. Our results suggest that supernumerary LC neurons mediate the convulsions of the mutants through an action presumably restricted to the brainstem.     

The role of the posterior hypothalamic area in the pathogenesis of hypertension in the spontaneously hypertensive rat

To examine the hypothesis that abnormalities of noradrenergic innervation of hypothalamic pressor areas influence the pathogenesis of hypertension in the spontaneously hypertensive rat (SHR), the norepinephrine content of individual hypothalamic and brainstem nuclei was determined in SHR and control Wistar-Kyoto (WKY) rats at 5, 7, and 11 weeks of age. At 5 and 7 weeks of age, the norepinephrine content of the posterior hypothalamic area (PHA) of SHR was significantly greater than that of WKY controls. These changes occurred at a time when blood pressure was not significantly different between the two groups. The increase in the norepinephrine content of the PHA was accompanied by increases in the norepinephrine content of the A1 region and locus coeruleus in 5-week SHR. In seven week old animals, studies of norepinephrine turnover using alpha-methyl-p-tyrosine revealed no differences between SHR and WKY in turnover in the PHA. The increase in norepinephrine content of the PHA in the absence of changes in turnover is interpreted to indicate increased noradrenergic input to the PHA of SHR and supports the hypothesis that noradrenergic pathways to the PHA are important in the development of hypertension in this model.