Changes in neurotransmitter turnover in locus coeruleus produced by changes in arterial blood pressure

Brainstem Nuclei Associated with Mediating Apnea-Induced Respiratory Motor Plasticity

The respiratory control system is plastic. It has a working memory and is capable of retaining how respiratory stimuli affect breathing by regulating synaptic strength between respiratory neurons. For example, repeated airway obstructions trigger a form of respiratory plasticity that strengthens inspiratory activity of hypoglossal (XII) motoneurons. This form of respiratory plasticity is known as long-term facilitation (LTF) and requires noradrenaline released onto XII motoneurons. However, the brainstem regions responsible for this form of LTF remain unidentified. Here, we used electrophysiology, neuropharmacology and immunohistochemistry in adult rats to identify the brainstem regions involved in mediating LTF. First, we show that repeated airway obstructions induce LTF of XII motoneuron activity and that inactivation of the noradrenergic system prevents LTF. Second, we show that noradrenergic cells in the locus coeruleus (LC), which project to XII motoneurons, are recruited during LTF induction. Third, we show that targeted inactivation of noradrenergic LC cells during LTF induction prevents LTF. And lastly, we show that the nucleus tractus solitarius (NTS), which has known projections to the LC, is critical for LTF because its inactivation prevents LTF. Our results suggest that both the LC and NTS are involved in mediating apnea-induced LTF, and we hypothesize that a NTS → LC → XII circuit mechanism mediates this form of respiratory motor plasticity.

The locus coeruleus and central chemosensitivity

The locus coeruleus (LC) lies in the dorsal pons and supplies noradrenergic (NA) input to many regions of the brain, including respiratory control areas. The LC may provide tonic input for basal respiratory drive and is involved in central chemosensitivity since focal acidosis of the region stimulates ventilation and ablation reduces CO(2)-induced increased ventilation. The output of LC is modulated by both serotonergic and glutamatergic inputs. A large percentage of LC neurons are intrinsically activated by hypercapnia. This percentage and the magnitude of their response are highest in young neonates and decrease dramatically after postnatal day P10. The cellular bases for intrinsic chemosensitivity of LC neurons are comprised of multiple factors, primary among them being reduced extracellular and intracellular pH, which inhibit inwardly rectifying and voltage-gated K(+) channels, and activate L-type Ca(2+) channels. Activation of K(Ca) channels in LC neurons may limit their ultimate response to hypercapnia. Finally, the LC mediates central chemosensitivity and contains pH-sensitive neurons in amphibians, suggesting that the LC has a long-standing phylogenetic role in respiratory control.

Purinergic modulation of cardiovascular function in the rat locus coeruleus

1 The purpose of the present study was to determine whether purines exerted a physiological role in central cardiovascular modulation at the level of the locus coeruleus (LC). 2 In pentobarbitone-anaesthetised Wistar-Kyoto rats, unilateral microinjection of ATP or alpha,beta-methyleneATP into the LC elicited dose-related decreases in blood pressure and heart rate. Unilateral microinjection of the P2 purinoceptor antagonists suramin and PPADS, caused pressor and tachycardic responses. Administration of the selective P2X(1) receptor antagonist NF-279 had no effect. While both ATP and L-glutamate (L-GLU) resulted in depressor responses after intra-LC microinjection, following intra-LC microinjection of P2 purinoceptor antagonists into the LC, the effects of subsequent administration of either ATP or L-GLU were functionally reversed, such that a pressor response ensued. 3 Microinjection of noradrenaline into the LC caused an increase in blood pressure and heart rate; however, the alpha(2)-adrenoceptor antagonist idazoxan had no cardiovascular effects, but did prevent the pressor response to PPADS or suramin. In addition, coinjection of idazoxan with either suramin or PPADS abolished the ATP and L-GLU mediated pressor responses observed following either suramin or PPADS administration. 4 The present data suggest that firstly, purines are capable of acting within the LC to ultimately modulate the cardiovascular system and secondly, that there is apparently a functional interaction between tonically active purinergic and noradrenergic systems within the LC of the rat.

Sinoaortic denervation abolishes blood pressure-induced GABA release in the locus coeruleus of conscious rats

Male Sprague-Dawley rats underwent sinoaortic denervation (SAD) or sham operation. We examined changes in the release rates of GABA, glutamate and arginine in the locus coeruleus (LC) elicited by experimental blood pressure increases (i.v. noradrenaline infusion for 3 min, 4 microg kg(-1)min(-1)) or decreases (i.v. sodium nitroprusside infusion for 3 min, 150 microg kg(-1)min(-1)). The release of the neurotransmitters was monitored by the push-pull superfusion technique. Mean blood pressure did not differ between sham-operated and SAD rats but blood pressure lability was greatly enhanced in SAD rats and accompanied by increased basal release of glutamate in the LC. GABA release was not affected. A rise in blood pressure induced by noradrenaline enhanced GABA release in the LC of sham-operated rats. This effect was abolished by SAD. Glutamate release did not respond to hypertension either in SAD or in sham-operated rats. Nitroprusside led to a fall in blood pressure which was more pronounced and lasted longer in SAD than in sham-operated rats. In SAD rats, glutamate release was enhanced by nitroprusside. The depressor response had no effect on glutamate release in sham-operated rats. GABA release did not respond to this stimulus in either SAD or sham-operated rats. SAD and blood pressure changes did not influence the release rate of arginine. In conclusion, experimental hypertension increases GABAergic activity in the LC by stimulating peripheral baroreceptors. In SAD rats, augmented blood pressure lability seems to be at least partly due to elevated glutamate outflow within the LC.

In vivo measurement of noradrenaline in the locus coeruleus of rats during the formalin test: A microdialysis study

The locus coeruleus is involved in the regulation of the sense of pain. To demonstrate the changes in noradrenaline level in the locus coeruleus during the formalin test, a microdialysis probe was implanted into the left locus coeruleus of rats. Formalin was subcutaneously injected into the plantar surface of the right hind paw and pain ratings were recorded. The concentrations of noradrenaline and its metabolite 3-methoxy-4-hydroxyphenylethylenglycol (MHPG) were measured. The results showed an almost four-fold elevation in noradrenaline release in the early phase of the formalin test; levels return to baseline in the late phase. Levels of MHPG changed in a similar fashion.

Peripheral arousal-related hormones modulate norepinephrine release in the hippocampus via influences on brainstem nuclei

The peripheral hormone epinephrine (EPI) is known to modulate memory for arousing experiences. The mnemonic effects of EPI are attributed almost exclusively to actions on amygdala noradrenergic (NE) systems. EPI also increases neuronal activity in the locus coeruleus (LC), the primary source of NE to other limbic structures that process memory such as the hippocampus (HIPP). The actions of EPI on the LC suggest that its mnemonic properties may also be mediated by influencing NE output in the HIPP. To test this hypothesis, dialysate levels of NE were collected from the HIPP of male rats given an i.p. injection of saline that was followed 100 min later by i.p. EPI (0.3 mg/kg). NE levels sampled 20 min after EPI injection were significantly larger than baseline and continued to show significant peaks for 60 min. Experiment 2 examined whether peripheral signals initiated by EPI influence the HIPP via the nucleus of the solitary tract (NTS) by inactivating this nucleus with lidocaine prior to EPI injection. EPI injection did not increase NE levels sampled from the HIPP of rats given lidocaine into the NTS. EPI injection did produce significant elevations in HIPP NE levels in animals given a control solution into the NTS prior to the EPI injection. These findings indicate that the mnemonic effects of EPI reported in a wide range of learning conditions may not be mediated solely by NE release in the amygdala, but may also involve coactivation of the HIPP NE system.

Cortical catecholamine secretion following intravenous nitroprusside infusion: a voltammetric study

Intravenous administration of sodium nitroprusside (NP) decreases blood pressure and activates noradrenergic neurons in the locus coeruleus (LC). Microdialysis studies have shown that NP infusion is accompanied by increased extracellular concentrations of norepinephrine (NE) in the medial prefrontal cortex. The present study used in vivo voltammetry to obtain a finer temporal analysis of the NP-induced changes in the extracellular concentrations of catecholamine-like compounds in the LC terminal fields in the rat medial prefrontal cortex. Intravenous infusion of rats with NP caused a rapid decrease in blood pressure that lasted for the duration of the infusion but rapidly reversed when the infusion was terminated. After a delay of between about 2 and 8 min (mean 5 min), there was an increase in extracellular concentrations of a NE-like substance. Presumed cortical release of NE lasted for several minutes but had almost returned to baseline by the time the NP infusion was terminated at 15 min. In many cases, the first peak was followed by a second one, usually of smaller amplitude but more prolonged than the first one. There was no clear response to the cessation of infusion of NP. The time course of the initial response is comparable to the previously reported electrophysiological response of LC-NE neurons to NP. In rats treated with DSP-4 to deplete cortical NE, blood pressure was reduced as in untreated rats, but no voltammetric response to NP infusion was observed. These results suggest that activation of the NE-LC neurons by NP results in a delayed synaptic release of NE in the cerebral cortex which attenuates within several minutes.

Release of serotonin in the rat locus coeruleus: effects of cardiovascular, stressful and noxious stimuli

To investigate the function of serotonergic neurons within the locus coeruleus, this brain nucleus of conscious, freely moving rats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the extracellular concentration of serotonin was determined in the superfusate. Serotonin release was increased by depolarization with veratridine (5 microM) or 80 mM K+, while superfusion with tetrodotoxin (1 microM) or systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin substantially diminished the release rate of serotonin in the locus coeruleus. The pressor response to intravenous infusion of noradrenaline (4 micrograms/kg/min) was associated with a pronounced increase in the release rate of serotonin. Superfusion of the locus coeruleus with tetrodotoxin (1 microM) abolished the increase in serotonin release evoked by the pressor response. A fall of blood pressure produced by intravenous administration of nitroprusside (150 micrograms/kg/min) or chlorisondamine (3 mg/kg) diminished the release rate of serotonin. Immobilization, noise (95 dB) or tail pinch increased the release of serotonin in the locus coeruleus and slightly elevated blood pressure. Chlorisondamine abolished the rise in blood pressure elicited by tail pinch without influencing the increased serotonin release. Tail pinch-induced serotonin release was abolished by superfusion with tetrodotoxin. The findings demonstrate that neuronal serotonin release in the locus coeruleus responds to cardiovascular and sensory stimuli, suggesting a function of serotonergic neurons in central blood pressure regulation, as well as in the modulation of locus coeruleus activity by stress and noxious stimuli.

Extracellular catechol and indole turnover in the nucleus of the solitary tract of spontaneously hypertensive and Wistar-Kyoto normotensive rats in response to drug-induced changes in arterial blood pressure

Drug-induced alterations in arterial blood pressure are reflected in the extracellular fluid neurotransmitter levels of the nucleus of the solitary tract (NTS). Urethane-anesthetized spontaneously hypertensive rats (SHRs) and Wistar-Kyoto normotensive (WKY) rats were used in this study. The extracellular neurochemical profile of the NTS was quantified using the in vivo microdialysis technique. In SHR, phenylephrine-induced hypertension produced no significant changes in the extracellular norepinephrine (NE) and dihydroxyphenylacetic acid concentrations, whereas a significant increase in the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) concentration was observed. Wistar normotensive rats, in response to phenylephrine-induced hypertension, showed a significant increase in extracellular NE and 5-HIAA concentrations. Hypotension produced by the intravenous infusion of nitroprusside failed to show significant changes in the extracellular neurotransmitters in both SHR and WKY rats. However, there was a significant increase in 5-HIAA concentration in SHRs during the rebound hypertension, which followed the nitroprusside-infused hypotension. No such change was observed in the case of the WKY rats. These results suggest the possible involvement of the serotonergic mechanisms of NTS in the regulation of normal arterial blood pressure in these two rat strains.

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.

Inhibition by locus coeruleus on the baroreceptor reflex response in the rat

We evaluated the modulation of baroreceptor reflex (BRR) response by locus coeruleus (LC) in adult, male Sprague-Dawley rats anesthetized with urethane (1.5 g/kg, i.p.). Under an electrical stimulation condition that did not appreciably alter the basal systemic arterial pressure and heart rate, the LC significantly suppressed the BRR response. Microinjection of L-glutamate (1 nmol, 50 nl) into the LC essentially duplicated this depressant effect. Intracerebroventricular (i.c.v.) administration of the alpha 1-adrenoceptor antagonist, prazosin (6.5 nmol), appreciably blunted the inhibition by LC on the BRR response. Yohimbine (6.5 nmol), the alpha 2-adrenoceptor blocker, however, was ineffective. Direct microinjection of prazosin (50 pmol), but not yohimbine (50 pmol), into the terminal site of baroreceptor afferents at the nucleus tractus solitarii (NTS) also significantly blunted the suppressive effect of LC on the BRR response. These results suggest that the LC may produce an inhibition on the BRR response by a process that involves the alpha 1-adrenoceptors located in the NTS.

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)

Atrial natriuretic peptide in the locus coeruleus and its possible role in the regulation of arterial blood pressure, fluid and electrolyte homeostasis

Atrial natriuretic factor (ANP) is present in neuronal cells of the locus coeruleus and its vicinity in the pontine tegmentum and moderate amount of ANP is detectable in this area by radioimmunoassay. The ANP (both peripheral and brain-born) is known as a neuropeptide which may influence the body salt and water homeostasis and blood pressure by targeting both central and peripheral regulatory mechanisms. Whether this pontine ANP cell group is involved in any of these regulatory mechanisms, the effect of various types of hypertension and experimental alterations in the salt and water balance on ANP levels was measured by radioimmunoassay in the locus coeruleus of rats. Adrenalectomy, as well as aldosterone and dexamethasone treatments failed to alter ANP levels in the locus coeruleus. Reduced ANP levels were measured in spontaneously hypertensive (both young and adult) rats, and in diabetes insipidus (Brattleboro) rats with vasopressin replacement. In contrast to these situations, elevated ANP levels were found in rats with DOCA-salt or 1-kidney-1-clip hypertension. These data suggest a link between ANP levels in the locus coeruleus and fluid volume homeostasis. Whether this link is causal and connected with the major activity of locus coeruleus neurons (noradrenergic influence on brain regulatory activities) needs further informations.

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.