Catecholamine Metabolism in Locus Coeruleus Neurons: A Study of its Activation by Sciatic Nerve Stimulation in the Rat

Where does a migraine attack originate? In the brainstem

Migraine is a common, paroxysmal, highly disabling primary headache disorder. The origin of migraine attacks is enigmatic. Numerous clinical and experimental results suggest that the activation of distinct brainstem nuclei is crucial in its pathogenesis, but the primary cause of this activation is not fully understood. We conclude that the initialization of a migraine attack can be explained as an altered function of the neuronal elements of the brainstem nuclei. In light of our findings and the literature data, we can assume that migraine is a subcortical disorder of a specific brainstem area.

Kynurenine metabolites and migraine: experimental studies and therapeutic perspectives

Migraine is one of the commonest neurological disorders. Despite intensive research, its exact pathomechanism is still not fully understood and effective therapy is not always available. One of the key molecules involved in migraine is glutamate, whose receptors are found on the first-, second- and third-order trigeminal neurones and are also present in the migraine generators, including the dorsal raphe nucleus, nucleus raphe magnus, locus coeruleus and periaqueductal grey matter. Glutamate receptors are important in cortical spreading depression, which may be the electrophysiological correlate of migraine aura. The kynurenine metabolites, endogenous tryptophan metabolites, include kynurenic acid (KYNA), which exerts a blocking effect on ionotropic glutamate and α7-nicotinic acetylcholine receptors. Thus, KYNA and its derivatives may act as modulators at various levels of the pathomechanism of migraine. They can give rise to antinociceptive effects at the periphery, in the trigeminal nucleus caudalis, and may also act on migraine generators and cortical spreading depression. The experimental data suggest that KYNA or its derivatives might offer a novel approach to migraine therapy.

In vivo monitoring of evoked noradrenaline release in the rat anteroventral thalamic nucleus by continuous amperometry

Continuous amperometry coupled with untreated carbon-fibre electrodes was used in anaesthetized rats to measure the noradrenaline release evoked in the anteroventral thalamic nucleus by electrical stimulation of the dorsal noradrenergic bundle. As expected, the variations in the oxidation current detected in the anteroventral thalamic nucleus exhibited the characteristics of the in vivo noradrenaline release. They were closely correlated with stimulation and consistent with the anatomy of the noradrenergic system involved. They were abolished by the ejection of tetrodotoxin in the vicinity of the carbon-fibre electrode, diminished by clonidine, an alpha-2 agonist, and restored by yohimbine, an alpha-2 antagonist. Furthermore, the time course of these variations was dramatically increased by desipramine, a specific noradrenaline reuptake blocker. In contrast, neither dopamine nor serotonin reuptake blockers, nor the monoamine oxidase inhibitor pargyline were able to alter them. The main advantage of the present approach is its excellent time resolution. We show here for the first time that after single pulse stimulation, noradrenaline is released and eliminated in 118 milliseconds, this time lapse corresponding to the maximal period beyond which subsequent noradrenaline releases could not add up. These observations are in good agreement with the physiological relationship previously observed between impulse flow and noradrenaline overflow.

In situ examination of tyrosine hydroxylase activity in the rat locus coeruleus using (3',5')-[(3)H(2)]-alpha-fluoromethyl-tyrosine as substrate of the enzyme

Tyrosine hydroxylase (TH) activity can be modified by changes in the specific activity of the enzyme (SA(TH)) or in the levels of active enzyme. We developed a methodology making it possible to measure with excellent anatomical resolution TH enzymatic activity and TH protein quantity by quantitative autoradiography and immunoautoradiography, respectively, from adjacent sections taken at serial intervals along the longitudinal extent of a same brain. SA(TH) was estimated by the slope of linear regressions established between TH activity and TH quantity measured at each anatomical plane. To evaluate TH activity, we used (3',5')-[(3)H(2)]-(D, L)-alpha-fluoromethyl-tyrosine [(3)H(2)]-MFMT, which is transformed by TH to [(3)H]-MFM-dopa, a potent and irreversible substrate for aromatic amino acid decarboxylase. We found that the SA(TH) in the cell body area of the LC (PKA) was 48% lower than that evaluated in the surrounding pericoerulean neuropil (PCN). In the PCN, 22% only of TH level exhibited a level of enzymatic activity above threshold. We also examined how SA(TH) was distributed in the LC 15 min and 3 days after RU 24722 treatment, a potent phasic and tonic activator of TH enzyme in noradrenergic neurons. Two distinct mechanisms have been observed: the short-term effect was due to an increase in the SA(TH) in the PKA only, while the long-term effect was mainly caused by an increase in the number of active TH proteins in the PCN. These results suggest that the fine regulation of TH activity which occurs in the different compartments of LC neurons may be critical in the functions involving the LC.

Release of excitatory and inhibitory amino acids from the locus coeruleus of conscious rats by cardiovascular stimuli and various forms of acute stress

The release of amino acids in the locus coeruleus (LC) of conscious, freely moving rats was studied in time periods of 3 min by use of push-pull superfusion under basal conditions and during application of various experimental stimuli known to influence the activity of the LC-noradrenergic system. Tail pinch for 3 min led immediately to a pronounced tetrodotoxin-sensitive increase in the release rates of the excitatory amino acids (EAA) glutamate (Glu) and aspartate (Asp) and to moderate increases in GABA and taurine (Tau) outflow. Immobilization stress for 9 min elevated the release of the EAA Glu and Asp, as well as that of the inhibitory amino acid GABA to a similar extent. A fall of blood pressure (BP) by nitroprusside or haemorrhage slightly enhanced the release rates of Glu and Asp. Noradrenaline-induced rise in BP, as well as hypervolaemia increased the release rate of GABA, but did not influence the release rates of Glu, Asp, Tau and arginine (Arg). The results provide direct evidence that the amino acid release pattern in the LC of conscious rats differs in response to various stimuli, according to the modality of the stimulus. A functional significance of excitatory and inhibitory amino acids in the regulation of LC activity during stress and haemodynamic changes is suggested.

In vitro study of the catecholaminergic metabolism of locus coeruleus neurones by differential normal pulse voltammetry

The aim of the present work was to measure, by voltammetry, the catecholaminergic metabolic activity of rat locus coeruleus (LC) neurones in brain slices. For this new experimental approach, we used an optimized protocol of slice preparation intended to prevent neuronal damages due to brain ischaemia. Our results show that the LC neurones exhibit in vitro a stable spontaneous catecholaminergic metabolic activity and that, as in vivo, 3,4-dihydroxyphenylacetic acid (DOPAC) is likely to be the main contributor to the recorded signal. This catecholaminergic metabolic activity can be pharmacologically altered by administering carbachol and clonidine to the superfusion fluid. We also determined the values of bath temperature and superfusion flow rate providing, in our methodological conditions, an optimal catecholaminergic metabolic activity. Finally, we took advantage of both the direct accessibility to the LC and the compactness of this nucleus to determine the spatial resolution of differential normal pulse voltammetry. In conclusion, the study of the subregional mechanisms controling the catecholaminergic metabolism in LC neurones can be performed in brain slices by differential normal pulse voltammetry.

Effects of neuroactive compounds, noxious and cardiovascular stimuli on the release of amino acids in the rat locus coeruleus

The release of excitatory amino acids (glutamate, aspartate), inhibitory amino acids (GABA, taurine) and arginine was determined in the locus coeruleus (LC) of anaesthetized rats. The neuronal origin of stimulated amino acid release was verified by superfusion with neuroactive compounds. Electrical stimulation of the sciatic nerve, as well as mechanical footshock, enhanced LC release rates of glutamate and aspartate without influencing those of taurine and arginine. GABA release rate was increased slightly after some delay. Excitatory amino acid release was not influenced by changes in blood pressure. The results provide direct neurochemical evidence that noxious stimuli activate LC neurons via the glutamate and aspartate input into this nucleus.

The habituation of brainstem catecholaminergic groups to chronic daily restraint stress is stress specific like that of the hypothalamo-pituitary-adrenal axis

It has previously been shown that immobilization and ether stress induce activation of the hypothalamo-pituitary-adrenal (HPA) axis and that this activation occurs subsequent to activation of brain stem catecholaminergic neurones. In the present study we have investigated whether the brain stem catecholaminergic (CA) neurons show habituation to chronic daily intermittent exposure to the same restraint stress comparable to that of the HPA axis. The level of activity of the brainstem CA groups was estimated by measurement in tissue punches of content of 3,4-dihydroxyphenylacetic acid (DOPAC), a side metabolite of noradrenaline and adrenaline biosynthesis which has been shown to be a reliable index of the stress-induced activation of the CA groups. The level of activity of the HPA axis was determined by measurement of plasma corticosterone and adrenocorticotropic hormone (ACTH) levels. The animals were submitted to a 15 min restraint stress daily. They were sacrificed at the end of the stress session on day 3, 5 and 10. The ACTH response to the acute restraint stress whilst unchanged on day 3 was significantly decreased on day 5 (-54%) and day 10 (-70%) compared to the response in naive rats. The approximately twofold increase in DOPAC level induced by acute restraint stress in the so-called CA medullary group A1/C1 of naive rats was reduced in daily restraint rats on day 5 (-22%) and day 10 (-30%) but was unchanged on day 3. A small (-20%) decrease of the stress-induced DOPAC response in the A2/C2 CA group and locus coeruleus was also observed on day 10.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the involvement of excitatory amino acid pathways in the development of precipitated withdrawal from acute and chronic morphine: an in vivo voltammetric study in the rat locus coeruleus

Previous studies have demonstrated that activation of excitatory amino acid (EAA) pathways projecting to the locus coeruleus may be involved in the increased firing of locus coeruleus (LC) neurons during opioid withdrawal. Using differential normal pulse voltammetry to monitor catechol oxidation current (CA.OC), an index of neuronal activity in the LC, the role of EAA pathways in naloxone precipitated withdrawal after acute and chronic morphine treatment was examined. Acute morphine treatment (10 micrograms i.c.v.) significantly reduced the CA.OC signal in the LC to 54.3 +/- 3.1% of baseline. Naloxone challenge (1 mg/kg i.v.) completely reversed the morphine effect and produced a significant increase in the CA.OC signal above baseline, peak 145.4 +/- 10.1% of baseline. This naloxone-induced rebound response was attenuated by pretreatment with the EAA receptor antagonists gamma-D-glutamylglycine (DGG) (2, 20, 200 micrograms i.c.v.) and (-)-2-amino-7-phosphonoheptanoic acid (D-APH), but not L-APH (25 micrograms i.c.v.). In chronically morphine-treated rats (25 micrograms/h i.c.v., 5 days), naloxone challenge (1 mg/kg i.v.) produced a significant increase in CA.OC signal, peak 466.5 +/- 112.7% of baseline. This naloxone-induced response was attenuated by pretreatment with DGG (200 micrograms i.c.v.) or D-APH (25 micrograms i.c.v.). To the extent that CA.OC reflects locus coeruleus neuronal activity, the present findings further suggest that increases in locus coeruleus activity during naloxone precipitated withdrawal after both acute and chronic morphine treatment are mediated at least in part by activation of EAA pathways.

High sensitivity measurement of brain catechols and indoles in vivo using electrochemically treated carbon-fiber electrodes

The combination of electrochemically treated carbon-fiber electrodes with DPV, DNPV or DPA represents a wide range of possibilities. As shown in this review, the choice of treatment and measurement technique depends on the purpose. As regards in vivo monitoring of 5-HIAA or DOPAC from very small brain nuclei, electrochemically treated carbon-fiber electrodes appear very potent and inexpensive. The main limitation of the established electrochemical techniques, including those discussed here, is that the unequivocal measurement of the basal extracellular neurotransmitter level cannot be achieved unless animals are treated with pargyline. On the other hand, this monitoring is feasible with in vivo dialysis. Therefore, electrochemical techniques, on the one hand, and in vivo dialysis, on the other hand, present different advantages. The former are much more potent than the latter in two respects. First, due to the much smaller size of the sensor, electrochemical techniques are more suitable for studying small brain nuclei. Second, since electrochemical techniques exhibit a better temporal resolution, they are recommended for investigating the relationship between impulse flow and neurotransmitter release. However, when high anatomical or temporal resolution is not required, in vivo dialysis is more suitable for recording the basal monoamine release.

Differential effects of desipramine on direct- and sensory-evoked noradrenaline release in thalamic locus coeruleus terminals

The effects of desipramine on sensory-evoked (sciatic nerve stimulation) activation of locus coeruleus neurones were investigated in vivo by using treated carbon-fibre electrodes in conjunction with either differential normal pulse voltammetry or differential pulse amperometry. Firstly, the amplitude of the sensory-evoked increase in extracellular noradrenaline, monitored in thalamic locus coeruleus terminals, was not modified by desipramine (10 mg/kg), whereas that evoked by direct activation of locus coeruleus neurones was greatly increased: +143% for dorsal noradrenergic bundle stimulation and +761% for glutamate ejection in the locus coeruleus. Secondly, desipramine administered at the same dose significantly reduced (-48%) the activation of locus coeruleus neurones (monitored at the somato-dendritic level) evoked by prolonged sciatic nerve stimulation. Our results indicate that acute treatment with desipramine does not potentiate overall noradrenergic transmission by locus coeruleus neurones during sensory stimulation.

Decrease in the reactivity of locus coeruleus neurons to hypotension after an increase in their tyrosine hydroxylase content: a subregional in vivo voltammetry study in the rat

The aim of the present work was to determine if noradrenergic neurons of the anterior and the posterior subregions of the locus coeruleus exhibit a difference in reactivity in response to sodium nitroprusside-induced arterial hypotension, and if the pharmacological induction of tyrosine hydroxylase by RU24722 modifies the reactivity of locus coeruleus neurons to this hypotensive stimulus. Previous findings have demonstrated that administration of RU24722 increases the concentration of tyrosine hydroxylase in the rat locus coeruleus by two different mechanisms in the anterior and in the posterior locus coeruleus subregions. The goal of the present study was to measure in vivo the changes in catecholaminergic metabolism in the locus coeruleus after treatment with RU24722 using differential normal pulse voltammetry (DNPV). In vehicle-treated rats, arterial hypotension increased catecholaminergic metabolism with the same pattern in the two locus coeruleus subregions. However, the changes in the magnitude of the catechol oxidation current throughout the recording period were significantly smaller in the posterior subregion (P < 0.001). In the RU24722-pretreated rats, there was a 39% increase in tyrosine hydroxylase and dihydroxyphenylacetic acid in the locus coeruleus. The functional reactivity to hypotension measured by DNPV was significantly decreased (P < 0.001) in both the anterior and posterior locus coeruleus subregions with RU24722 treatment. Therefore, this study suggests that the response of locus coeruleus cells to a hypotensive stimulus depends upon the intracellular tyrosine hydroxylase concentration both in the basal condition and during pharmacological induction of tyrosine hydroxylase gene expression.

In vivo noradrenaline release evoked in the anteroventral thalamic nucleus by locus coeruleus activation: an electrochemical study

The anteroventral thalamic nucleus is innervated by noradrenergic terminals exclusively originating in the locus coeruleus, a densely packed cell group located in the dorsotegmental part of the pons. In urethane-anaesthetized rats, electrical stimulations of locus coeruleus axons (dorsal noradrenergic bundle; 14 Hz, 20 s) evoked a rapid increase in the signal (catechol oxidation current) measured within the anteroventral thalamic nucleus by the use of carbon fibre electrodes combined with electrochemistry. This effect was reproducible and immediately reversible. Evoked changes in this current were found to be due to oxidation of noradrenaline released from terminals. The amplitude of the evoked noradrenaline release varied non-linearly with the frequency of stimulation. We investigated the influence of locus coeruleus activation on noradrenaline release measured in the anteroventral thalamic nucleus every second by means of differential pulse amperometry: (i) chemical activation of locus coeruleus by local injection of glutamate (0.2-0.8 nmol) immediately and consistently evoked noradrenaline release in a dose-dependent manner; and (ii) peripheral stimulation of the sciatic nerve (20 s)--known to enhance the firing rate of locus coeruleus neurons-evoked a noradrenaline release similar to that produced by a stimulation of the dorsal noradrenergic bundle at 8-10 Hz. Pharmacological and kinetic characteristics of the noradrenaline release were the same for central or peripheral stimulation of locus coeruleus neurons. Our results indicate that in vivo electrochemistry, because of its sensitivity and its high space and time resolution, is well suited for studies of evoked noradrenaline release from locus coeruleus terminals. This approach allowed us to describe the characteristics of central noradrenaline release evoked by central and peripheral stimulations of short duration. In particular, we observed a very close relationship between impulse flow and evoked noradrenaline release.

Lack of glucocorticoids enhances the early activation of the medullary catecholaminergic cell groups triggered by restraint stress

We have investigated whether the stress-induced activation of the medullary catecholaminergic neurons, that was shown previously to provide the main central activation input to the hypothalamo-pituitary-adrenocortical axis during an immobilization stress, is sensitive to circulating corticosteroids. Experiments were carried out on adrenalectomized rats that were first maintained on corticosterone in the drinking water for 5 days following surgery and then switched to corticosterone-free water 15 h before stress application. Some of the latter animals were injected with dexamethasone. Activation of the brainstem Catecholaminergic neurons was estimated by assaying 3,4-dihydroxyphenylacetic acid (DOPAC), a side metabolite of the noradrenaline and adrenaline biosynthesis pathway that was established previously as a reliable index of the activity of these neurons. In the so-called A(1) C(1) and A(2) C(2) Catecholaminergic groups of the medulla, lack of corticosterone under these experimental conditions did not modify the basal level of DOPAC but led to a further enhancement (+ 35% to 40%) of the approximately 2-fold increase in DOPAC content observed 15 min after the onset of the 5-min immobilization stress. No significant further enhancement of the stress-induced DOPAC increase was observed in the locus coeruleus. In these adrenalectomized rats, dexamethasone pretreatment prevented the enhancement of the stress-induced increase in DOPAC level observed in the medullary cell groups but did not abolish the response to stress. Lack of endogenous corticosteroids led to a 10-fold enhancement of the adrenocorticotropin increase following immobilization stress. Pretreatment with dexamethasone fully abolished the stress-induced increase in adrenocorticotropin plasma level. Our results show that circulating corticosteroids reduce the stress-induced activation of the medullary A(1) and A(2) C(2) groups i.e. those that contribute mainly to the catecholaminergic innervation of the hypothalamo-pituitary-adrenocortical axis within the hypothalamic paraventricular nuclei. However, since the feedback regulation of the central catecholaminergic systems is much less efficient than the feedback actually observed on the adrenocorticotropin secretion, we suggest that it is likely to play a minor physiological role in the overall feedback regulation exerted by circulating corticosteroids on the hypothalamo-pituitary-adrenocortical axis.