Baroreflex-linked variations of catecholamine metabolism in the caudal ventrolateral medulla: an in vivo electrochemical study

Tyrosine hydroxylase phosphorylation in catecholaminergic brain regions: a marker of activation following acute hypotension and glucoprivation

The expression of c-Fos defines brain regions activated by the stressors hypotension and glucoprivation however, whether this identifies all brain sites involved is unknown. Furthermore, the neurochemicals that delineate these regions, or are utilized in them when responding to these stressors remain undefined. Conscious rats were subjected to hypotension, glucoprivation or vehicle for 30, 60 or 120 min and changes in the phosphorylation of serine residues 19, 31 and 40 in the biosynthetic enzyme, tyrosine hydroxylase (TH), the activity of TH and/or, the expression of c-Fos were determined, in up to ten brain regions simultaneously that contain catecholaminergic cell bodies and/or terminals: A1, A2, caudal C1, rostral C1, A6, A8/9, A10, nucleus accumbens, dorsal striatum and medial prefrontal cortex. Glucoprivation evoked phosphorylation changes in A1, caudal C1, rostral C1 and nucleus accumbens whereas hypotension evoked changes A1, caudal C1, rostral C1, A6, A8/9, A10 and medial prefrontal cortex 30 min post stimulus whereas few changes were evident at 60 min. Although increases in pSer19, indicative of depolarization, were seen in sites where c-Fos was evoked, phosphorylation changes were a sensitive measure of activation in A8/9 and A10 regions that did not express c-Fos and in the prefrontal cortex that contains only catecholaminergic terminals. Specific patterns of serine residue phosphorylation were detected, dependent upon the stimulus and brain region, suggesting activation of distinct signaling cascades. Hypotension evoked a reduction in phosphorylation in A1 suggestive of reduced kinase activity. TH activity was increased, indicating synthesis of TH, in regions where pSer31 alone was increased (prefrontal cortex) or in conjunction with pSer40 (caudal C1). Thus, changes in phosphorylation of serine residues in TH provide a highly sensitive measure of activity, cellular signaling and catecholamine utilization in catecholaminergic brain regions, in the short term, in response to hypotension and glucoprivation.

The caudal pressor area of the rat: its precise location and projections to the ventrolateral medulla

Investigators have demonstrated pressor areas in the medullas of various species. The present study precisely localized the pressor area in the caudal medulla of the rat and determined its projections to the caudal and rostral ventrolateral medulla. The caudal medulla first was mapped grossly in rats with injections (30 nl) of glutamate (30-, 15-, and 7.5-nmol doses) placed 0.5, 1.0, and 1.5 mm caudal to the calamus scriptorius, 1.0, 1.5, and 2.0 mm lateral to the midline, and 1.8, 1.7, and 1.6 mm ventral to the dorsal medullary surface, respectively, and their arterial pressures were recorded. One of these nine injections showed significant increases in arterial pressure. We micromapped this area with a total of 27 injections of glutamate (10 nl; 5 nmol) placed 300 microm apart at 3 different dorsoventral levels. This micromapping study pinpointed the precise location of caudal pressor area (CPA) neurons in a restricted region lateral to the caudal end of the lateral reticular nucleus and ventromedial to the medullary dorsal horn near the level of the pyramidal decussation. Injections of glutamate into this spot, 1.0 mm caudal to the calamus scriptorius, 2.0 mm lateral to the midline, and 1.7 mm ventral from the dorsal surface of the medulla, induced significant increases in arterial pressure. The neuroanatomic connections of neurons in the CPA to the ventrolateral medulla were then investigated with iontophoretic injections of either the anterograde tracer biotinylated dextran amine (BDA) made into the CPA or the retrograde tracer FluoroGold (FG) injected into either the caudal or rostral ventrolateral medulla. BDA injections resulted in bouton-laden fibers throughout both caudal and rostral portions of the ventrolateral medulla. Either of the FG injections resulted in numerous spindle-shaped neurons interspersed between the longitudinal fiber bundles running through the CPA area. The proximity of the CPA neurons to the A1 catecholaminergic cell group is discussed.

Activation of the rostral ventrolateral medulla in an acute anesthetized rodent strychnine model of allodynia

After the administration of intrathecal strychnine, allodynia is manifested as activation of supraspinal sites involved in pain processing and enhancement of cardiovascular responses evoked by normally innocuous stimuli. The objective of this study was to investigate the effect of strychnine-induced allodynia on adrenergic neuronal activity in the C1 area of the rostral ventrolateral medulla (RVLM), a major site involved in cardiovascular regulation. The effect of intrathecal strychnine (40 microg) or saline followed by repeated hair deflection to caudal lumbar dermatomes in the urethane-anesthetized rat was assessed by measuring voltammetric changes in the RVLM catechol oxidation current (CA x OC), mean arterial pressure (MAP), and heart rate (HR). After the administration of intrathecal strychnine, hair deflection evoked a significant and sustained increase in the RVLM CA x OC and MAP (peak 146.4%+/-5.6% and 159%+/-18.4% of baseline, respectively; P < 0.05). There was a nonsignificant increase in HR (peak 128%+/-8.2%). In the absence of hair deflection, there was no demonstrable change. Intrathecal saline-treated rats failed to demonstrate changes in RVLM CA x OC, MAP, or HR. In the present study, we demonstrated that, after the administration of intrathecal strychnine, innocuous hair deflection evokes temporally related neuronal activation in the rat RVLM and an increase in MAP. This suggests that the RVLM mediates, at least in part, the cardiovascular responses during strychnine allodynia.

IMPLICATIONS

Neural injury-associated pain, as manifested by allodynia, is resistant to conventional treatment. In a rat model of allodynia, we demonstrated activation of the brain region involved in sympathetic control. Innovative therapies that target this region may be successful in managing this debilitating condition.

Differential time- and dose-related effects of haemorrhage on tyrosine hydroxylase and neuropeptide Y mRNA expression in medullary catecholamine neurons

Hypotensive haemorrhage induces nuclear Fos expression and upregulates tyrosine hydroxylase (TH) mRNA in catecholamine-containing cell groups of the rat medulla oblongata. To shed light on the significance of the coexistence of neuropeptide Y (NPY) in aminergic neurons, the impact of graded levels of haemorrhage on temporal changes in the expression of TH and NPY mRNAs was compared; concurrent staining for Fos permitted comparisons between cells that ostensibly were and were not targeted by the stimulus. A 15% haemorrhage provoked increased NPY expression in all medullary catecholamine cell groups except the A2; these changes were detected predominantly in Fos-immunoreactive neurons (Fos-ir) at later (2-4 h) time points. Upregulation of TH and NPY mRNAs in Fos-ir neurons followed distinct time courses, with NPY responses peaking more rapidly, particularly in the C1 and C2 cell groups. Adrenergic cell groups displayed greater maximal increases in NPY expression than the A1 noradrenergic cell group while the converse was true of TH mRNA response. Increasing the severity of haemorrhage resulted in more pronounced increases in both mRNA responses in each aminergic region. These findings indicate that haemorrhage differentially affects TH and NPY expression in medullary catecholamine cell groups that participate in the maintenance of cardiovascular homeostasis. The differential nature of these responses suggests them not to be a simple consequence of metabolic alterations pursuant to increased synaptic activity. The prompt and robust NPY mRNA responses in adrenergic neurons suggests a mechanism by which peptide content of these cell groups' terminal projections is defended.

Catechol activation in rat rostral ventrolateral medulla after systemic isocapnic metabolic acidosis

The catechol signal recorded using in vivo voltammetry within the rat rostral ventrolateral medulla (RVLM) can be interpreted as a catechol-specific index of the integrated activity of RVLM adrenergic barosensitive bulbospinal and nonbulbospinal neurons. To test the hypothesis that systemic acidosis leads to the activation of RVLM adrenergic neurons, the RVLM catechol signal was observed in rats after mild systemic acidosis (pH 7.20-7.25 for 30 min) induced by 1 M HCl under halothane anesthesia, controlled mechanical ventilation, and continuous infusion of Ringer lactate. Particular attention was paid to ensure that changes in mean arterial pressure (MAP) were <15 mmHg during HCl challenge. Saline administration was not associated with any significant change in all considered variables (n = 5). Mild isocapnic systemic acidosis was associated with an increase in catechol signal (n = 5), irrespective of carotid sinus nerve section (n = 5). In keeping with the aim of the study, there were minor (<15 mmHg) but significant changes in MAP among saline, intact, and deafferented groups. Changes in heart rate were not significant. In conclusion, a catechol activation is observed in the RVLM when arterial pressure is maintained during isocapnic systemic metabolic acidosis. This catechol activation appears primarily centrally mediated. Therefore, adrenergic RVLM neurons may relay inputs from the central respiratory generator to the sympathetic system and/or act as chemosensors for H+ next to the surface of the ventrolateral medulla.

The neuroendocrinology of thirst and salt appetite: visceral sensory signals and mechanisms of central integration

This review examines recent advances in the study of the behavioral responses to deficits of body water and body sodium that in humans are accompanied by the sensations of thirst and salt appetite. Thirst and salt appetite are satisfied by ingesting water and salty substances. These behavioral responses to losses of body fluids, together with reflex endocrine and neural responses, are critical for reestablishing homeostasis. Like their endocrine and neural counterparts, these behaviors are under the control of both excitatory and inhibitory influences arising from changes in osmolality, endocrine factors such as angiotensin and aldosterone, and neural signals from low and high pressure baroreceptors. The excitatory and inhibitory influences reaching the brain require the integrative capacity of a neural network which includes the structures of the lamina terminalis, the amygdala, the perifornical area, and the paraventricular nucleus in the forebrain, and the lateral parabrachial nucleus (LPBN), the nucleus tractus solitarius (NTS), and the area postrema in the hindbrain. These regions are discussed in terms of their roles in receiving afferent sensory input and in processing information related to hydromineral balance. Osmoreceptors controlling thirst are located in systemic viscera and in central structures that lack the blood-brain barrier. Angiotensin and aldosterone act on and through structures of the lamina terminalis and the amygdala to stimulate thirst and sodium appetite under conditions of hypovolemia. The NTS and LPBN receive neural signals from baroreceptors and are responsible for inhibiting the ingestion of fluids under conditions of increased volume and pressure and for stimulating thirst under conditions of hypovolemia and hypotension. The interplay of multiple facilitory influences within the brain may take the form of interactions between descending angiotensinergic systems originating in the forebrain and ascending adrenergic systems emanating from the hindbrain. Oxytocin and serotonin are additional candidate neurochemicals with postulated inhibitory central actions and with essential roles in the overall integration of sensory input within the neural network devoted to maintaining hydromineral balance.

Integrative role of the lamina terminalis in the regulation of cardiovascular and body fluid homeostasis

1. Cardiovascular and body fluid homeostasis depends upon the activation and co-ordination of reflexes and behavioural responses. In order to accomplish this, the brain receives and processes both neural and chemical input. Once in the brain, information from sources signalling the status of the cardiovascular system and body fluid balance travels, and is integrated, throughout a widely distributed neural network. Recent studies using neuroanatomical and functional techniques have identified several key areas within this neural network. One major processing node is comprised of structures located along the lamina terminalis. 2. Structures associated with the lamina terminalis include the median preoptic nucleus (MePO) and two sensory circumventricular organs (SCVO), the subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT). Current evidence indicates that blood-borne signals, such as angiotensin II (AngII), reach SCVO (e.g. SFO) where they are transduced. This information is then carried via neural pathways to brain nuclei (e.g. MePO) where it is integrated with other inputs, such as those derived from systemic arterial blood pressure and volume receptors. 3. Because of their receptive and integrative functions, lamina terminalis structures are essential for the normal control of hormone release (e.g. vasopressin), sympathetic activation and behaviours (thirst and salt appetite), which collectively contribute to maintenance of cardiovascular and body fluid homeostasis.

Hemodynamic regulation of tyrosine hydroxylase messenger RNA in medullary catecholamine neurons: a c-fos-guided hybridization histochemical study

Medullary catecholamine cell groups are involved in multiple modes of cardiovascular regulation and display indices of functional activation, including widespread c-fos expression, in response to hypotensive hemorrhage. Assessments of the impact of such challenges on transmitter-related gene expression are complicated by the biochemical and connectional heterogeneity that characterize these cell groups. Quantitative hybridization histochemical methods were used to follow the effects of 15% hemorrhage on levels of messenger RNA encoding tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, in medullary aminergic neurons; concurrent staining for nuclear Fos-immunoreactivity permitted comparisons between cells that ostensibly were and were not targeted by the challenge. Increased levels of tyrosine hydroxylase messenger RNA were detected in Fos-immunoreactive neurons in all cell groups examined. Mean maximal increases ranged between 133 and 192% of control values, and were attained within 0.5-1 h post-hemorrhage in noradrenergic (A1 and A2) cell groups, and at 2 h in adrenergic ones (C1, C2, and C2d or dorsal strip). By 4 h after the challenge, tyrosine hydroxylase messenger RNA levels in Fos-immunoreactive neurons in all cell groups had returned to control values. By contrast, tyrosine hydroxylase messenger RNA in non-Fos-immunoreactive cells either did not change significantly over the course of the experiment (C2 and C2d), or showed a rapid and transient increase, whose magnitude tended to be less than that seen in Fos-immunoreactive cells. c-fos messenger RNA was prominently induced in catecholaminergic neurons in each of the medullary cell groups examined at 0.5 h after hemorrhage, suggesting that the early tyrosine hydroxylase messenger RNA response to hemorrhage in non-Fos-immunoreactive cells preceded the capacity of responsive neurons to manifest detectable Fos protein expression. These findings indicate that hemorrhage up-regulates tyrosine hydroxylase messenger RNA levels in medullary catecholaminergic cell groups which have access to adaptive neuroendocrine and/or autonomic control systems. The approach employed here should prove of general utility in assessing the impact of environmental events on messenger RNA expression in connectionally heterogeneous cell groups that share a common biochemical phenotype.

Noxious heat-evoked Fos-like immunoreactivity in the rat medulla, with emphasis on the catecholamine cell groups

The objectives of the present study were 1) to utilize Fos immunohistochemistry as a marker for neuronal activity in order to examine the population of neurons in the medulla that is engaged by activation of nociceptive peripheral afferents and 2) to determine whether catecholamine-containing neurons in the medulla also express noxious heat-evoked Fos-like immunoreactivity. Noxious heating of the hindpaw evoked specific patterns of Fos-like immunoreactivity in the medulla in regions known to be involved in both nociceptive processing and cardiovascular regulation. Noxious heating of the hindpaw significantly increased the mean number of neurons expressing Fos-like immunoreactivity in the contralateral ventrolateral medulla. Increased numbers of Fos-positive neurons also were observed in both the ipsilateral and the contralateral A1 catecholamine cell groups. Similarly, in the contralateral medullary dorsal reticular fields, noxious heating of the hindpaw significantly increased the mean number of neurons expressing Fos-like immunoreactivity. In contrast, in the paramedian reticular nucleus, noxious heating of the hindpaw resulted in a significant decrease in the mean number of neurons expressing Fos-like immunoreactivity. No significant differences in the mean numbers of neurons expressing Fos-like immunoreactivity were noted in the A2, C1, or C2/C3 medullary catecholamine cell groups. These results suggest that noxious stimuli affect pools of neurons in the medulla with multiple physiological functions.

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.

Catecholamine and NPY efferents from the ventrolateral medulla to the amygdala in the rat

Anatomical tracing studies have demonstrated an efferent pathway to central nucleus of the amygdala (CeA) from the ventrolateral medulla. The combined retrograde tracing/immunohistochemical method was used to test for the presence of catecholamines and neuropeptide Y (NPY) in ventrolateral medulla neurons that innervate the CeA. Numerous retrogradely labeled neurons were observed in the ventrolateral medulla caudal to the area postrema. Fewer retrogradely labeled neurons were also observed in the rostral ventrolateral medulla. Retrogradely labeled neurons immunoreactive for both tyrosine hydroxylase and NPY were found in the caudal ventrolateral medulla. Double-labeled phenylethanolamine-N-methyltransferase neurons were present at levels rostral to the area postrema. A substantial portion of the ventrolateral medulla projection to the CeA arises from adrenergic cells of the C1 group, because nearly 40% of the retrogradely labeled cells were also immunoreactive for phenylethanolamine-N-methyl-transferase. The high percentage of double-labeled NPY-immunoreactive neurons suggests NPY is colocalized in CeA-projecting catecholamine neurons, indicating that input from the ventrolateral medulla to the CeA primarily arises from C1 adrenergic neurons that also express NPY. This contrasts with previous data suggesting the catecholaminergic projection from the nucleus of the solitary tract to the CeA originates from the A2 noradrenergic cell group. Thus, the input to the CeA from catecholaminegic groups in the caudal medulla is chemically coded in terms of a dorsal noradrenergic and a ventrolateral adrenergic pathway.

The role of excitatory amino acids in the expression of precipitated acute and chronic clonidine withdrawal: an in vivo voltammetric study in the rat locus coeruleus

It has been previously shown that activation of excitatory amino acid (EAA) pathways contributes to hyperactivity of the locus coeruleus (LC) in antagonist precipitated opioid withdrawal. In this study, using differential normal pulse voltammetry to monitor catechol oxidation as an index of the activity of the LC, the role of EAA pathways in antagonist precipitated withdrawal after acute and chronic clonidine treatment was examined. Intracerebroventricular clonidine (10 micrograms i.c.v.) significantly reduced LC activity to 54.4 +/- 3.1% of baseline 45 minutes following the injection. Subsequent systemic injection of the selective alpha 2 receptor antagonist atipamezole (0.2 mg/kg i.v.) or yohimbine (0.5 mg/kg i.v.) resulted in a rapid reversal of the depressant effects and a significant increase in LC activity above baseline. Pretreatment with the non-selective EAA receptor antagonist gamma-D-glutamylglycine (DGG) (50 micrograms i.c.v.) attenuated the atipamezole-induced rebound response of the LC but not the reversal of clonidine action. However, both the yohimbine-induced rebound and reversal of clonidine effects were attenuated by DGG treated animals. In chronic clonidine treated animals (2, 5, 7, 10 micrograms/h i.c.v., 5 days), a challenge with atipamezole (0.2 mg/kg i.v.) produced an immediate increase in LC activity, blood pressure and heart rate. The magnitude of these responses was dependent on the dose of clonidine. The atipamezole-induced increase in LC activity and blood pressure was significantly attenuated by pretreatment with DGG (200 micrograms i.c.v.). These findings suggest that LC hyperactivity and blood pressure increases elicited during clonidine withdrawal are mediated in part by activation of EAA receptors. In this regard, the mechanisms underlying clonidine withdrawal closely resembles those underlying opioid withdrawal.

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.

Baroreceptor reflex-linked changes in catechol metabolism in the rat rostral ventrolateral medulla

1. Using in vivo voltammetry, this study relates catecholamine metabolism within the rat rostral ventrolateral medulla to the level of mean arterial pressure (MAP) under halothane anaesthesia. 2. A vasopressor region was circumscribed with electrical stimulations in an area located 1000-1700 microns rostral to the obex. A catechol signal was then ascertained within this area. The recording site was surrounded with phenyl-N-methyl-ethanolamine transferase immuno-positive cell bodies. 3. Three levels of decrease of arterial pressure were induced with nitroprusside infusion: -15, -35 and -55 mmHg (n = 5 in each group) from baseline for 30 min. This led to increases in the catechol signal which were inversely related to the degree of hypotension (P < 10(-4) vs. saline for the 35 and 55 mmHg groups, P < 0.05 for the 35 mmHg group as compared to the 15 and 55 mmHg groups following recovery from hypotension). 4. Following sino-aortic deafferentation, nitroprusside-induced hypotension (-35 mmHg) did not lead to any change in the catechol signal in the rostral ventrolateral medulla (n = 5). Furthermore, controlled hypotension induced in intact rats did not evoke any change in the catechol signal recorded in a dopaminergic area of the midbrain, the ventral tegmental area (A10 area; n = 5). 5. An infusion of phenylephrine increased MAP by 35 mmHg from a baseline pressure of 105 mmHg for 30 min and evoked a non-significant decrease in the catechol signal (n = 5). In another group of rats a lower baseline pressure (80 mmHg) was stabilized (n = 5) with a higher concentration of halothane. An identical increment in pressure was then produced by a phenylephrine infusion and led to a significant reduction in the catechol signal (P < 0.05 vs. saline under similar conditions; n = 5). 6. The new findings of this study are that the level of activity of the metabolism of catecholamine in the rostral ventrolateral medulla (i) is continuously related to the level of arterial pressure, (ii) functions close to its resting level under baseline conditions and is primarily engaged during hypotension and (iii) is baroreflex linked. 7. Given the lack of direct evidence for a link between unit activity and catechol metabolism, these changes in catechol activity, recorded continuously in vivo next to adrenergic cell bodies, may represent the biochemical-specific counterpart of changes in the level of electrical unitary activity of presumed adrenergic cardiovascular medullospinal sympathoexcitatory neurons. Therefore, it provides evidence that adrenaline-synthesizing neurons in the rostral ventrolateral medulla respond to baroreceptor inputs.

Stereoselective effects of central α2-adrenergic agonist medetomidine on in vivo catechol activity in the rat rostral ventrolateral medulla (RVLM)

The stereoselective central effects of a novel, highly potent and selective alpha 2-agonist medetomidine on adrenergic neuronal activity, reflected by changes in catechol oxidation current, in the rostral ventrolateral medulla of the halothane-anesthetized rat were examined using in vivo differential normal pulse voltammetry. Dexmedetomidine, the active isomer, significantly decreased catechol oxidation current to 33.4 +/- 4.5% of baseline when given centrally (1 microgram, i.c.v.) and to 10.3 +/- 3.9% of baseline when given systemically (50 micrograms/kg, i.v.). Dexmedetomidine also significantly reduced mean arterial blood pressure by 19.9% following central administration but significantly increased mean arterial blood pressure by 59.9% following systemic administration. Levomedetomidine, the inactive isomer, had no effect on catechol oxidation current or blood pressure. The depressant effects of dexmedetomidine on catechol oxidation current were reversed by the selective alpha 2-adrenoceptor antagonist atipamezole (2 micrograms, i.c.v. or 200 micrograms/kg, i.v.). The results of the present study demonstrate, to our knowledge, for the first time the central stereoselective effects of medetomidine and antagonism by atipamezole on rostral ventrolateral medulla activity in the anesthetized rat.

Hemorrhage-Induced Activations of Adrenocorticotropin Release and Catecholamine Metabolism in the Ventrolateral Medulla are Differently Affected by Glucocorticoid Feedback

We have compared the effects of increasing doses of dexamethasone on the hemorrhage-induced stimulation of the corticotropic axis and the metabolism of the catecholamines of the A1 group in the ventrolateral medulla. Adrenocorticotropin was measured in sequential samples of plasma while the metabolism of the catecholamines was recorded by in vivo electrochemistry in urethane-anesthetized rats. Combined intracerebroventricular injection of specific adrenergic blockers (α(1) -antagonist, prazosin and ß-antagonist, propranolol) prevented the stimulation of the adrenocorticotropin release by hemorrhage. Pretreatment with dexamethasone (1 mg/kg sc) fully blocked the hemorrhage-induced adrenocorticotropin release but did not affect the concomitant stimulation of the catecholamine metabolism in A1 cells. The latter was partially decreased only with the highest dose (10 mg/kg sc). While a central catecholaminergic input appears to be necessary for the hemorrhage-induced stimulation of the corticotropic axis, it does not seem to play a significant role in the feedback regulation by glucocorticoids.

Drug-Induced Changes in Blood Pressure Lead to Changes in Extracellular Concentrations of Epinephrine, Dihydroxyphenylacetic Acid, and 5-Hydroxyindoleacetic Acid in the Rostral Ventrolateral Medulla of the Rat

Neurochemical changes in the extracellular fluid of the rostral ventrolateral medulla (RVLM) were produced by changes in arterial blood pressure. Blood pressure was raised or lowered with systemic infusions of phenylephrine or nitroprusside and neurochemicals were recovered from RVLM by in vivo microdialysis. A dialysis probe 300 microns in diameter and 500 microns in length was stereotaxically implanted in the RVLM of the urethane-anesthetized rat. Sterile physiological Ringer's solution was perfused at a rate of 1.5 microliter/min. The perfusate was collected under ice-cold conditions every 15 min for the assay of epinephrine, dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA), ascorbic acid, and uric acid. After stable baseline neurochemical concentrations were achieved, animals were infused with phenylephrine or nitroprusside intravenously to raise or lower the blood pressure. Increasing blood pressure 50 mm Hg above the baseline value by phenylephrine led to a significant reduction in heart rate and a reduction in extracellular epinephrine and DOPAC concentrations. The 5-HIAA concentration was increased during the hypertensive drug infusion. There were no changes in the concentrations of ascorbic acid or uric acid. Hypotension produced by nitroprusside (-20 mm Hg) led to neurochemical changes which were the reciprocal of those seen during hypertension. During hypotension, heart rate increased as did the extracellular fluid epinephrine concentration. The 5-HIAA concentration fell with hypotension and remained depressed following the nitroprusside infusion. Ascorbic acid and uric acid concentrations did not change during hypotension but ascorbic acid did increase after the nitroprusside infusion stopped. These data provide direct evidence that epinephrine release in RVLM is linked to changes in systemic blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Central and peripheral biogenic amine effects of brain missile wounding and increased intracranial pressure

This study was performed to ascertain the acute effects of brain missile wounding on brain-stem and hypothalamic biogenic amines in a group of cats anesthetized with pentobarbital (40 mg/kg). Brain wounding is associated with a concomitant increase in intracranial pressure (ICP); to separate the effects of elevated ICP alone from the effects of wounding, a second group of cats had ICP artificially increased from a normal level of approximately 5 mm Hg to approximately 140 mm Hg by infusion of mock cerebrospinal fluid into the cisterna magna. In both groups, significant epinephrine depletions (47% to 74%) occurred in the nucleus tractus solitarius, area A1C1, locus ceruleus, raphe nuclei, and posterior hypothalamus. Epinephrine levels were also significantly decreased in the anterior hypothalamus in the wounded cats. In addition, both brain wounding and artificially induced ICP increases caused significant decreases of norepinephrine in the posterior hypothalamus, and of serotonin, 5-hydroxyindoleacetic acid, dopamine, and homovanillic acid in the raphe nuclei. Only brain wounding, however, caused significant reductions of norepinephrine, dopamine, and homovanillic acid in the nucleus tractus solitarius and area A1C1. The plasma catecholamine levels resulting from brain wounding or artificially induced ICP increases were dissimilar only in the amount of time required to attain maximum plasma levels, with the wounded animals responding faster. It is concluded that the hypothalamic and brain-stem biogenic amine changes resulting from either brain wounding or increased ICP alone are reflective of a stress response. Brain-stem distortion caused by brain wounding did not appear to be a factor and monoaminergic systems appeared to remain intact despite a severe and eventually lethal brain injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of catecholaminergic neurons in the ventral medulla by various stressors monitored with in vivo electrochemistry

Catecholaminergic metabolism in the A1 cell group of the ventrolateral medulla oblongata was followed after application of 3 interoceptive stressful stimuli by monitoring the extracellular concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) with an in vivo voltammetric approach. These stimuli provoked an increase of the DOPAC signal with different time course and amplitude. Histamine led to a maximal 200% increase that vanished within two hours. Insulin induced a long-lasting increase of up to 350% that could be reversed by glucose infusion. Electrical stimulation of the sciatic nerve triggered an immediate increase of up to 140% which stopped with the ending of the stimulation. The time-course of this activation is compatible with a possible involvement of catecholaminergic afferents from the A1 group projecting to the paraventricular nucleus in the stimulation of the hypothalamic neurosecretory cells elicited by the stressors.

Central noradrenergic neurons: the autonomic connection

Most CNS noradrenergic (NE) cell groups reside in portions of the medulla oblongata primarily involved in autonomic control (A1, A2, A5) and even the pontine locus coeruleus (A6) receives a major innervation from these medullary areas. This review examines the neuroanatomical and neurophysiological literature relevant to the issue of the role of CNS NE neurons in central autonomic control (with emphasis on cardiovascular control). It is concluded that NE cells, with the possible exception of certain A5 and A1 neurons, have relatively weak or no inputs from visceral cardiovascular afferents but provide a complex "open loop" control over non-aminergic circuits which are more specialized in the processing of cardiovascular and other autonomic reflexes. The question of whether the C1 "adrenergic" cells of the rostral medulla oblongata actually use noradrenaline as a neurotransmitter is also briefly addressed.

Changes in catecholamine metabolism in the rostral ventrolateral medulla following halothane and nitroprusside-induced hypotension: an in vivo electrochemical study

The objective was to observe changes in rostral ventrolateral medulla (RVLM) catecholamine metabolism using in vivo voltammetry following induced hypotension with halothane or nitroprusside (SNP). Rats anesthetized (halothane, metocurine) and ventilated were stereotaxically implanted with carbon microelectrodes in the RVLM. The catechol oxidation current (CA.OC, % baseline) was used as an index of RVLM catecholaminergic metabolism. Groups of rats (n = 5) were given (A) halothane 0.75% for 60 min; (B) halothane 2.75% plus phenylephrine infusion to maintain mean arterial pressure (MAP) for 30 min, then halothane 0.75% for 30 min; (C) halothane (2.5-3.0%) for 30 min (MAP 60 +/- 5 mmHg) then halothane 0.75% for 30 min; (D) halothane 0.75% and sodium nitroprusside (SNP) for 30 min (MAP 60 +/- 5 mmHg), then halothane 0.75% for 30 min. Halothane 0.75% produced no significant change in CA.OC or MAP (A), while halothane 2.5-3.0% produced a significant decrease in MAP and a symmetrical significant increase in CA.OC (ANOVA, P less than 0.5). This increase peaked at 30 min (180 +/- 28%) and reached 110 +/- 9% baseline at 60 min. The halothane and phenylephrine combination produced no significant change in CA.OC or MAP during the 30 min exposure (B). SNP (D) produced a significant increase in CA.OC (peak 48 min, 224 +/- 35%) which remained elevated at 60 min (198 +/- 32%). Thus, the induced hypotension produced activation of RVLM catecholaminergic neurons. SNP induced a prolonged significant increase in RVLM catecholamine metabolism which may relate to rebound hypertension following use of this drug.

In vivo monitoring of catecholaminergic metabolism in the C1 region of rat medulla oblongata: a comparative study by voltammetry and intracerebral microdialysis

In vivo voltammetry or microdialysis was used to monitor catecholaminergic metabolism in the C1 region of the ventrolateral medulla oblongata of anesthetized rats. In vivo voltammetry allowed the recording of a catechol oxidation current (CA.OC) peak in this region. This CA.OC was suppressed after inhibition of monoamine oxidase by pargyline or after inhibition of tyrosine hydroxylase by alpha-methyl-p-tyrosine and was markedly increased after blockade of dopamine-beta-hydroxylase by FLA 63. Similar results were found when intracerebral microdialysis coupled with HPLC and electrochemical detection was used to measure the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) in the dialysates obtained from the C1 region: The changes in CA.OC and DOPAC concentration in the dialysates exhibited very similar kinetic characteristics in the three pharmacological experiments. These results support the involvement of DOPAC as a major component of the electrochemical signal recorded by voltammetry in the C1 group of adrenergic neurons.

Pharmacological and functional evidence for extracellular 3,4-dihydroxyphenylacetic acid as an index of metabolic activity of the adrenergic neurons: an in vivo voltammetry study in the rat rostral ventrolateral medulla

Catecholamine metabolism was studied in vivo in the C1 adrenergic area of the rostral ventrolateral medulla oblongata in rats, using differential normal pulse voltammetry coupled with an activated carbon fiber microelectrode. Pharmacological evidence indicates that 3,4-dihydroxyphenylacetic acid, the major dopamine metabolite, is responsible for the electrochemical signal appearance in the C1 group, and that it reflects the catecholamine synthesis rate, as previously reported in the locus coeruleus. Indeed, 3,4-dihydroxyphenylacetic acid was estimated to be formed from 77% of the intracellular dopamine, since its synthesis was increased by only 23%, after blockade of the dopamine-beta-hydroxylase activity. Neuronal activation by retrograde electrical stimulation increased the electrochemical signal, as well as hemorrhage and hypotension, suggesting that the level of extracellular 3,4-dihydroxyphenylacetic acid is a good biochemical index of the C1 adrenergic cellular activity in baseline conditions and during cellular activation.

Continuous volume infusion improves circulatory stability in anesthesized rats

'Optimized' management (OM) was provided to chloralose-anesthetized rats for 12 h by combining continuous infusion (7 ml.kg-1). mechanical ventilation and strict control of acid-base equilibrium (n = 7). The chloralose-anesthetized rats managed conventionally (conventional management: CM, n = 9) received neither volume infusion, nor mechanical ventilation, nor correction of acid-base disturbances. All the OM rats completed the study while 6 out of 9 CM rats died before the end of the study period. Mean arterial pressure (MAP) remained at 100 mmHg for 12 h in the OM group. MAP stayed close to 70 mmHg in the CM group for 6 h and declined to very low levels thereafter (mean +/- S.E.M.: 46.0 +/- 3.9 mmHg at 12 h, P less than 10(-4) when compared to the other group). Central venous pressure and cardiac output remained close to baseline values for 12 h in the OM group. Acid-base equilibrium was preserved in the OM group in contrast to a severe metabolic acidosis in the CM group (pH = 7.14 +/- 0.03 at 12 h; P less than 10(-4). Such as 'optimized' management involving mechanical ventilation with oxygen, continuous infusion and acid-base monitoring may be of value to maintain circulatory stability in anesthetized rodent preparations during long periods of time, as in neurophysiological experiments.

In vivo voltammetry--prospects for the next decade

In vivo voltammetry has been in existence for more than ten years. The technique now benefits from smaller, more selective sensors-true 'chemical microelectrodes'. The scope of experimentation has widened dramatically both in vivo and in vitro as new applications are reported. The speed of measurements has increased giving high temporal and spatial resolution, approaching 'real time'. This article discusses the progress of the past decade and looks forward to the advances of the next ten years.

Fentanyl increases catecholamine oxidation current measured by in vivo voltammetry in the rat striatum

A proposed mechanism for fentanyl-induced muscular rigidity is the effect of opioids on dopaminergic transmission in the striatum. The objective of this study was to observe the effect of fentanyl on the rat striatal catechol oxidation current (CA.OC) which reflects extracellular DOPAC (3-4,dihydroxyphenylacetic acid) concentration (a major metabolite of dopamine), as measured by in vivo voltammetry. Male Sprague-Dawley rats, anaesthetized with halothane, were stereotaxically implanted with carbon fibre electrodes in the striatum and after an initial stabilization period of an hour were given a control saline IV injection followed 30 min later by fentanyl 10 micrograms.kg-1 IV over 10 min and at 70 min by the monoamine oxidase inhibitor pargyline 70 mg.kg-1 IP. Fentanyl produced a significant (P less than 0.05 Anova) increase in CA.OC in all animals. This reached a plateau 15 min following the administration of fentanyl and was at a maximum of 148 +/- 10.2 per cent of control 35 min after the administration of fentanyl. Pargyline produced a rapid decline in CA.OC peak height which went from 143 +/- 11.6 to 39 +/- 6.8 per cent of control over 30 min. There were no significant differences between the pH, PaO2 and PaCO2 during the saline and fentanyl injection periods and there was no significant variation of blood pressure throughout the experiment. This study shows that under stable physiological conditions, fentanyl produces a significant increase in CA.OC in the rat striatum.