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

Enhanced tail pinch-induced activation of catecholamine metabolism in the pericerulean area of RU 24722-treated rats

Our study was devoted to determine in freely moving rats whether the increase in tissue concentration of tyrosine hydroxylase (TH) elicited by a single administration of RU 24722 could modify the catecholaminergic reactivity of neuronal processes present in the rostrolateral part of the pericerulean area (r-lPCA) in response to tail pinch. Catecholaminergic activity was monitored by measuring in vivo the concentration of dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) using microdialysis coupled to HPLC detection. In this study, the microdialysis probe was implanted at a sufficient distance from the lateral border of rostral nucleus locus ceruleus (LC) to avoid a large contribution of the noradrenergic cell bodies in the measurements performed. We first evidenced that DOPAC measured in the r-lPCA indicated the functional state of catecholaminergic metabolism in neuronal processes (dendrites and fibers) laying in this region. We also showed that the enhancement of TH protein concentration in the r-lPCA following RU 24722 treatment supported an increased in vivo catecholaminergic metabolism in this region. Furthermore, catecholaminergic metabolism response to tail pinch was potentiated in animals with greater TH tissue concentration. Thus, our study reveals that the modulation of both TH concentration and catecholaminergic metabolism in the r-lPCA may be critical in the functioning of cells and neuronal elements present in this region, notably in adaptive responses to noxious stimuli.

Controlled targeting of tyrosine hydroxylase protein toward processes of locus coeruleus neurons during postnatal development

Dendrites of locus coeruleus (LC) neurons laying within the pericoerulean neuropil (PCA) organize the major site where tyrosine hydroxylase (TH) is present throughout postnatal development. Those dendrites constitute the neuronal compartment in which TH levels increase beyond postnatal day (P) 21 or after RU24722-induced TH expression. Distal LC dendrites are present in the PCA by at least P20 but are devoid of TH and can rapidly accumulate TH protein when gene induction is triggered. Contrasting with the increase in TH levels within LC perikarya and dendrites, TH-mRNA concentration remains constant in LC perikarya from P4 to P42. Thus, supposing TH synthesis and degradation are also constant, any change in TH levels targeted toward axons might be balanced by a shift in the TH deposition within LC dendrites. This mechanism may be crucial in functions that the different processes of LC neurons have at critical steps of postnatal ontogeny.

Effects of long-term hypoxia on tyrosine hydroxylase protein content in catecholaminergic rat brainstem areas: a quantitative autoradiographic study

The nucleus tractus solitarius (NTS), the ventrolateral medulla (VLM), the dorsal motor vagus nucleus (DMnX) and the locus coeruleus (LC) are catecholaminergic brainstem areas involved in ventilatory and cardiovascular responses to hypoxia and tyrosine hydroxylation is the rate limiting step of cathecholamine biosynthesis in the central nervous system. The aim of this study was to evaluate the effects of long-term hypoxia on tyrosine hydroxylase (TH) content in these different areas using a quantitative autoradiographic technique. Two experimental groups of rats were studied: Group I (9 males, 8 females) was submitted to normobaric hypoxia (10% O2-90% N2) for 21 days and compared to 12 (6 males, 6 females) normoxic control rats (Group II). Coronal tissue sections from fresh-frozen rat brains, obtained along the caudo-rostral axis, were incubated in the presence of a TH monoclonal antibody, and the reaction was revealed by a 35S-labelled secondary antibody. TH levels were quantified in the NTS, VLM, DMnX and LC by measuring optical density on autoradiographic films using an automatic image analyser system. Regional antigen quantification was assessed by computer-assisted image analysis. Chronic hypoxia led to body weight decrease until day 5, haematocrit increase (65 +/- 2% vs. 44 +/- 2%, P < 0.01) and right ventricle hypertrophy (35 +/- 0.5% vs. 23 +/- 0.1% of the weight of the two ventricles, P < 0.01). TH protein contents expressed as percentage of controls were as follows. In males, in the rostral part of the NTS 132 +/- 9% (P < 0.02), in the caudal part of the NTS, 117 +/- 5% (P < 0.04). In female rats, the TH quantity reached a value of 124 +/- 4% (P < 0.01) in the rostral part and 126 +/- 6% (P < 0.01) in the caudal part of the NTS. In females, TH content was significantly increased in the VLM, 124 +/- 6%, P = 0.01, whereas in males there was only a non-significant trend to increase, 122 +/- 11%. In females, there was a significant increase in the DMnX, 127 +/- 9%, P = 0.05, whereas in males there was only a trend to increase, 120 +/- 5%. This study shows that long-term hypoxia induces a persistent increase in TH protein content both in the caudal and rostral part of the NTS, which are known to receive respectively chemo- and barosensory inputs, and in other catecholaminergic areas involved in baroreflex activity. Our data clearly demonstrate the implication of neurochemical mechanisms in the central relationship between chemo- and baroreflex which are responsible for changes in systemic arterial pressure and oxygen partial pressure as required for maintaining an adequate oxygen supply to the tissues.

Pharmacological modulations of adrenergic phenotype in medullary C2 and C3 cell groups of adult rat

The adrenergic phenotype was analysed in the rat's rostral dorsomedial medulla under normal conditions and 3 days after a single intraperitoneal injection of an eburnamine derivative, RU 24722, which increases tyrosine hydroxylase protein expression in the rostral portion of the nucleus tractus solitarius. This approach was investigated by a double immunofluorescence labelling of tyrosine hydroxylase and phenylethanolamine N-methyltransferase proteins. Under normal conditions, most adrenergic cell bodies are anatomically distributed in the dorsal and rostral medulla oblongata between the rostral part of the dorsal motor nucleus of the vagus nerve and the medial longitudinal fasciculus. Adrenergic neurons detected in this medullar region were distributed between both cell groups. Three days after the pharmacological RU 24722 treatment, an upregulation in tyrosine hydroxylase and phenylethanolamine N-methyltransferase protein expression was detected in both cell groups characterized by a highly increased number of tyrosine hydroxylase- and phenylethanolamine N-methyltransferase-containing cell bodies. The number of TH-mRNA containing neurons was also increased, indicating the transcriptional level of this regulation. These results demonstrated a particular neuronal plasticity of adrenergic phenotype in the medullary cell groups of adult rat.

Tyrosine hydroxylase expression within Balb/C and C57black/6 mouse locus coeruleus. II. Quantitative study of the enzyme level

The tyrosine hydroxylase phenotype expression was further investigated in the perikarya and pericoerulean areas of the locus coeruleus of two pure inbred mouse strains, Balb/C and C57Black/6, which the topological organization and phenotypic plasticity of the enzyme-containing cell population were previously studied. The tyrosine hydroxylase level and the mean protein quantity provided by each cell were significantly higher within the spaces delimited by the enzyme containing perikarya in the C57Black/6 strain as compared to the Balb/C strain. Three days after RU24722 administration, tyrosine hydroxylase tissue concentration and quantity were significantly increased in both strains. Two strain-dependant mechanisms of this pharmacologically induced protein modulation were demonstrated: the mean tyrosine hydroxylase quantity provided by each cell was increased in the C57Black/6 strain whereas the increase was obviously explained by the subset of additional tyrosine hydroxylase expressing cells previously reported in Balb/C strain. A comparable volume of pericoerulean immunolabeled neuropile, which contains a similar tyrosine hydroxylase level, was measured between the two strains. Within this tissue compartment, an undissociated RU24722 responsiveness was observed between the two strains: a significant increase in the protein level was measured principally resulting from a significant increase in the volume. These results revealed a strain-dependent difference in the response to the RU24722 treatment which may result from a genetic separation of two kinds of tyrosine hydroxylase phenotypic regulations within the perikarya area of the locus coeruleus; whereas the surrounding neuropile seemed to have a different mechanism of the phenotypic protein expression and modulation.

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.

In vivo voltammetry and microdialysis monitoring of monoamine metabolism in the rat brainstem neurons

Two "in vivo" techniques allow the monitoring of extracellular levels of monoamines and related compounds in selected rat brainstem regions: voltammetry and microdialysis. "In vivo" voltammetry has a high regional selectivity: for example, we have been able to perform a subregional study and to show that the increase in extracellular DOPAC induced by 30 min-hypotension was twice as larg in the rostral as in the caudal rat locus coeruleus. The spatial resolution, as expressed by x 1/2 (see text), is 4 times better for voltammetry (50 microns) than for microdialysis (190 microns). Another advantage of voltammetry is its excellent time resolution. However, microdialysis has a much better biochemical specificity than voltammetry. Furthermore it allows some enzymatic activities, such as tyrosine hydroxylase, to be measured almost continuously in catecholaminergic brain nuclei. From a functional point of view, the results of our experiments (alpha 2 ligand administration, arterial hypotension or stress) illustrate the respective interest and complementarity of these two "in vivo" techniques. Their current developments will lead to a better temporal and biochemical resolution combined with an increase in the number of substances analyzed "in vivo", including peptides and nitric oxide.

Relationship between tyrosine hydroxylase content and noradrenergic cell reactivity to piperoxane: an in vivo voltammetric approach in the rat locus coeruleus

A previous electrochemical study showed that the increase in the tyrosine hydroxylase (TH) content of the locus coeruleus (LC) produced by RU24722 administration was associated with a relative decrease in the catecholaminergic metabolic reactivity of this nucleus to a hypotensive stimulus. Since alpha 2 receptors participate in the regulation of the activity of both LC neurons and TH, the aim of the present work was to evaluate the possible involvement of the autoinhibition mediated by alpha 2 autoreceptors in the inverse relationship between the reactivity of the LC and its TH content. Our study was divided into two successive steps: (i) the electrochemical measurement of the in vivo metabolic activation of LC cells in response to alpha 2-adrenergic receptor blockade, and (ii) the evaluation of the quantity of TH every 100 microns along the caudorostral axis in each recorded LC. The capacity of TH protein to be activated was evaluated by the measurement, using differential normal pulse voltammetry, of the in vivo variations of the extracellular 3,4-dihydroxyphenylacetic acid concentrations in response to six cumulated doses of the alpha 2-antagonist piperoxane. The corresponding dose-response curves, determined in control- and RU24722-treated rats, were expressed as a function of the quantity of TH contained either in the whole recorded LC or in the 100 microns-wide coronal interval surrounding the recording site. It was established that the slopes of the dose-response curves were significantly (P < 0.01) and inversely related to the quantity of TH at the level of the recording site. This result suggests that the negative control of the catecholaminergic metabolic reactivity in a restricted area of the LC could be directly or indirectly dependent on the level of expression of TH protein in this particular area.

Regional specificity of the long-term variation of tyrosine hydroxylase protein in rat catecholaminergic cell groups after chronic heat exposure

The present study was carried out to investigate the effect of chronic heat exposure on tyrosine hydroxylase (TH) protein content in catecholaminergic rat brain-stem areas such as the anterior (LCA) and posterior (LCP) locus coeruleus, the substantia nigra (SN), the ventral tegmental area, and the dorsomedial (DMM) and the ventrolateral medulla and in the adrenal gland (AG). Male Sprague-Dawley rats were exposed to 34 degrees C during 3, 7, or 14 days. Controls were kept at 25 degrees C for the same period. In the LCA, TH content was decreased on day 7 (-34%) and 14 (-37%) of heat exposure. In the SN, TH protein content was decreased on day 7 (-25%) and 14 (-20%) after 34 degrees C. In the DMM cell group, 14 days at 34 degrees C produced a decrease (-20%) of TH content. In all of these structures, TH content variations were correlated with body temperature variations. In the AG, TH content increased progressively to peak (+31%) after 14 days of chronic heat exposure. This increase was also associated with body temperature modification. The selective and body temperature-related response to long-term TH protein content variations following chronic heat exposure observed in the LCA, SN, DMM, and AG could represent an adaptive physiological response of these catecholaminergic cells.