Inhibition Of Noradrenaline Release Research Articles

Voltammetry in the spleen assesses real-time immunomodulatory norepinephrine release elicited by autonomic neurostimulation

BackgroundThe noradrenergic innervation of the spleen is implicated in the autonomic control of inflammation and has been the target of neurostimulation therapies for inflammatory diseases. However, there is no real-time marker of its successful activation, which hinders the development of anti-inflammatory neurostimulation therapies and mechanistic studies in anti-inflammatory neural circuits.MethodsIn mice, we performed fast-scan cyclic voltammetry (FSCV) in the spleen during intravenous injections of norepinephrine (NE), and during stimulation of the vagus, splanchnic, or splenic nerves. We defined the stimulus-elicited charge generated at the oxidation potential for NE (~ 0.88 V) as the “NE voltammetry signal” and quantified the dependence of the signal on NE dose and intensity of neurostimulation. We correlated the NE voltammetry signal with the anti-inflammatory effect of splenic nerve stimulation (SpNS) in a model of lipopolysaccharide- (LPS) induced endotoxemia, quantified as suppression of TNF release.ResultsThe NE voltammetry signal is proportional to the estimated peak NE blood concentration, with 0.1 μg/mL detection threshold. In response to SpNS, the signal increases within seconds, returns to baseline minutes later, and is blocked by interventions that deplete NE or inhibit NE release. The signal is elicited by efferent, but not afferent, electrical or optogenetic vagus nerve stimulation, and by splanchnic nerve stimulation. The magnitude of the signal during SpNS is inversely correlated with subsequent TNF suppression in endotoxemia and explains 40% of the variance in TNF measurements.ConclusionsFSCV in the spleen provides a marker for real-time monitoring of anti-inflammatory activation of the splenic innervation during autonomic stimulation.

Alpha-2 adrenoreceptor agonists for the intensive care physician

Clonidine and dexmedetomidine are two α2-adrenoreceptors agonists available for the intensivist in the clinical practice. The affinity of dexmedetomidine is eight times greater than clonidine affinity for the α2 receptors. Their main effect is sedation. They act by inhibition of noradrenaline release in the locus coeruleus in the brainstem. α2-agonists are used primarily for sedation, analgesia, and management of delirium. Nowadays, dexmedetomidine application is increasing in critically ill patients showing a good safety. Most frequent side effects include bradycardia and hypotension.

Alpha2-adrenergic receptor activation reinstates motor deficits in rats recovering from cortical injury.

Norepinephrine plays an important role in motor functional recovery after a brain injury caused by ferrous chloride. Inhibition of norepinephrine release by clonidine is correlated with motor deficits after motor cortex injury. The aim of this study was to analyze the role of α2-adrenergic receptors in the restoration of motor deficits in recovering rats after brain damage. The rats were randomly assigned to the sham and injury groups and then treated with the following pharmacological agents at 3 hours before and 8 hours, 3 days, and 20 days after ferrous chloride-induced cortical injury: saline, clonidine, efaroxan (a selective antagonist of α2-adrenergic receptors) and clonidine + efaroxan. The sensorimotor score, the immunohistochemical staining for α2A-adrenergic receptors, and norepinephrine levels were evaluated. Eight hours post-injury, the sensorimotor score and norepinephrine levels in the locus coeruleus of the injured rats decreased, and these effects were maintained 3 days post-injury. However, 20 days later, clonidine administration diminished norepinephrine levels in the pons compared with the sham group. This effect was accompanied by sensorimotor deficits. These effects were blocked by efaroxan. In conclusion, an increase in α2-adrenergic receptor levels was observed after injury. Clonidine restores motor deficits in rats recovering from cortical injury, an effect that was prevented by efaroxan. The underlying mechanisms involve the stimulation of hypersensitive α2-adrenergic receptors and inhibition of norepinephrine activity in the locus coeruleus. The results of this study suggest that α2 receptor agonists might restore deficits or impede rehabilitation in patients with brain injury, and therefore pharmacological therapies need to be prescribed cautiously to these patients.

Dexmedetomidine for Attenuation of Sympathomimetic Response to Laryngoscopy and Tracheal Intubation in Neurosurgical Patients

Background: Stress responses caused by noxious stimuli during laryngoscopy and tracheal intubation can incite harmful effects, especially in neurosurgical patients. Among various drug regimens and techniques tried for obtunding such consequences, dexmedetomidine, an α-2 receptor agonist mediating its action through α-2A receptors located in locus coeruleus, inhibits noradrenaline release thus decreases such sympathetic activity. Aim and Objective: To evaluate the effect of a single preoperative dose of dexmedetomidine 1 μg/kg slow infusion on hemodynamic responses to laryngoscopy and tracheal intubation. Materials and Methods: Sixty patients randomized into two groups of 30 each of age range 20–60 years, the American Society of Anesthesiologists physical Status I and II posted for planned neurosurgical procedures, to receive dexmedetomidine (Group D): 1 μg/kg diluted with 0.9% saline to a total volume of 20 mL, control Group (C): 20 ml 0.9% normal saline, both infused intravenously over 10 min, 3 min before induction. Changes in heart rate (HR), systolic, diastolic, mean arterial pressure (MAP), and any side effect associated with the drug during the study of 20 min of intubation, were observed and statistically analyzed. Results: Insignificant difference (P > 0.05) in demographic criteria. In comparison to the baseline (BL) value, during laryngoscopy, a fall in mean HR by 12.1% in dexmedetomidine group and a rise of 28.39% in control group, reduction in mean systolic blood pressure value 11.40% in dexmedetomidine group, a rise by 19.50% in control group (P < 0.05), a fall in mean diastolic blood pressure by 11.19% in dexmedetomidine, in control group a rise of 16.97%, the difference is statistically significant (P < 0.0001), rise in the mean MAP value in the control group to 18.12% above the BL value but in Group D, it was reduced by 11.29% (P < 0.05). Conclusion: Infusion of dexmedetomidine, in attenuation of hemodynamic response to laryngoscopy and tracheal intubation, is a safe and effective protective method.

Dexmedetomidine as an adjuvant for patients undergoing breast cancer surgery: A meta-analysis.

Background:The goal of this study was to comprehensively evaluate the analgesic and antiemetic effects of adjuvant dexmedetomidine (DEX) for breast cancer surgery using a meta-analysis.Methods:Electronic databases were searched to collect the studies that performed randomized controlled trials. The effect size was estimated by odd ratio (OR) or standardized mean difference (SMD). Statistical analysis was performed using the STATA 13.0 software.Results:Twelve published studies involving 396 DEX treatment patients and 395 patients with control treatment were included. Pooled analysis showed that the use of DEX significantly prolonged the time to first request of analgesia (SMD = 1.67), decreased the postoperative requirement for tramadol (SMD = −0.65) and morphine (total: SMD = −2.23; patient-controlled analgesia: SMD = −1.45) as well as intraoperative requirement for fentanyl (SMD = −1.60), and lower the pain score at 1 (SMD = −0.30), 2 (SMD = −1.45), 4 (SMD = −2.36), 6 (SMD = −0.63), 8 (SMD = −2.47), 12 (SMD = −0.81), 24 (SMD = −1.78), 36 (SMD = −0.92), and 48 (SMD = −0.80) hours postoperatively compared with the control group. Furthermore, the risks to develop postoperative nausea/vomiting (PONV) (OR = 0.38) and vomiting (OR = 0.54) were significantly decreased in the DEX group compared with the control group. The pain relief at early time point (2, 6, 12, 24 hours postoperatively) and the decrease in the incidence of PONV were especially obvious for the general anesthesia subgroup (P < .05) relative to local anesthesia subgroup (P >.05).Conclusion:DEX may be a favorable anesthetic adjuvant in breast cancer surgery, which could lower postoperative pain and the risk to develop PONV. DEX should be combined especially for the patients undergoing general anesthesia.

Ethanol abolishes vigilance-dependent astroglia network activation in mice by inhibiting norepinephrine release

Norepinephrine adjusts sensory processing in cortical networks and gates plasticity enabling adaptive behavior. The actions of norepinephrine are profoundly altered by recreational drugs like ethanol, but the consequences of these changes on distinct targets such as astrocytes, which exhibit norepinephrine-dependent Ca2+ elevations during vigilance, are not well understood. Using in vivo two-photon imaging, we show that locomotion-induced Ca2+ elevations in mouse astroglia are profoundly inhibited by ethanol, an effect that can be reversed by enhancing norepinephrine release. Vigilance-dependent astroglial activation is abolished by deletion of α1A-adrenergic receptor from astroglia, indicating that norepinephrine acts directly on these ubiquitous glial cells. Ethanol reduces vigilance-dependent Ca2+ transients in noradrenergic terminals, but has little effect on astroglial responsiveness to norepinephrine, suggesting that ethanol suppresses their activation by inhibiting norepinephrine release. Since abolition of astroglia Ca2+ activation does not affect motor coordination, global suppression of astroglial networks may contribute to the cognitive effects of alcohol intoxication.

Endothelin ETA- and ETB-Receptor-Mediated Inhibition of Noradrenaline Release From Isolated Rat Stomach

We examined the effect of endothelin-1, endothelin-3 and sarafotoxin S6c on the release of noradrenaline from gastric sympathetic nerve terminals using an isolated, vascularly perfused rat stomach. The release of noradrenaline evoked by electrical stimulation of the gastric postganglionic sympathetic nerves (at 2.5 Hz for 1 min) was inhibited by endothelin-1 (10-10 – 10-8 M), endothelin-3 (10-9 – 10-8 M) and sarafotoxin S6c (a highly selective agonist of endothelin ETB receptors) (10-9 – 10-8 M) in a concentration-dependent manner; the inhibitory potencies were as follows: endothelin-1 > endothelin-3 > sarafotoxin S6c. The inhibitory effect of endothelin-1 (3 × 10-9 M) on noradrenaline release was abolished by BQ-123 (a selective antagonist of endothelin ETA receptors) in a dose-dependent manner (10-7 and 10-6 M), but not influenced by BQ-788 (a selective antagonist of endothelin ETB receptors) (10-7 and 10-6 M). The endothelin-1-induced inhibition of noradrenaline release was attenuated by pretreatment with pertussis toxin (10 μg/animal, i.v., 4 days before experiments), but not influenced by indomethacin (3 × 10-6 M). These results indicate that endothelin ETA and ETB receptors located on the sympathetic nerve terminals play a role in the inhibition of noradrenaline release from the rat stomach: endothelin ETA receptor-mediated inhibition is carried out by pertussis toxin-sensitive and indomethacin-insensitive mechanisms.

Sevoflurane sparing effect of dexmedetomidine in patients undergoing laparoscopic cholecystectomy: A randomized controlled trial.

Background and Aims:Sevoflurane is an excellent but expensive anesthetic agent for laparoscopic cholecystectomy. To decrease sevoflurane consumption during surgery adjuvants like dexmedetomidine may be used. Dexmedetomidine is a recently introduced drug which alleviates the stress response of surgery, produces sedation and analgesia. We aimed to evaluate sevoflurane sparing effect of dexmedetomidine in patients undergoing laparoscopic cholecystectomy under entropy-guided general anesthesia (GA).Material and Methods:In this prospective randomized control study, 100 American Society of Anesthesiologists physical status I–II adult surgical patients scheduled to undergo laparoscopic cholecystectomy were enrolled. Patients were randomly divided into two groups (n = 50). In dexmedetomidine group, patients received intravenous (IV) dexmedetomidine 0.5 μg/kg over 10 min before induction followed by 0.5 μg/kg/h infusion while in control group, patients received the same volume of normal saline.Results:Sevoflurane consumption was 41% lower in dexmedetomidine group as compared to control group (7.1 [1.6] vs. 12.1 [1.9] ml, P <0.001). A 40% reduction was observed in induction dose of propofol (83.0 [19.1] vs. 127.6 [24.8] mg, P <0.001). Mean Riker sedation-agitation score, visual analog score for pain and Aldrete's score were significantly lower in dexmedetomidine group as compared to control group. None of the patients experienced any significant side effects.Conclusion:A 41% reduction in sevoflurane consumption was observed in patients receiving IV dexmedetomidine as an adjuvant in patients undergoing laparoscopic cholecystectomy under GA.

Macrophage‐Dependent Impairment of α2 Autoreceptor Inhibition of Ca2+ Currents in Sympathetic Neurons Contributes to Hypertension in DOCA‐Salt Rats

Norepinephrine (NE) release from perivascular sympathetic nerves is regulated by prejunctional alpha‐2 autoreceptors (α2AR), which inhibit nerve terminal N‐type Ca2+ channels. We showed previously that macrophage infiltration and superoxide production in mesenteric arteries (MA) impairs α2AR‐mediated inhibition of NE release. We tested the hypothesis that macrophages disrupt α2AR coupling to N‐type Ca2+ channels in sympathetic neurons.MethodsThe DOCA‐salt rat model of hypertension was used for this study. Immunohistochemistry was used to localize macrophages in MA. Ca2+ current inhibition was evaluated using whole‐cell patch clamp on cultured sympathetic neurons of the celiac ganglion. Liposome‐encapsulated clodronate (LEC) was used to deplete macrophages.ResultsMacrophage infiltration is increased in MA of DOCA‐salt hypertensive rats. The α2AR agonists NE and UK14304 inhibit Ca2+ currents in neurons from normotensive but not hypertensive rats. In all neurons, yohimbine (α2AR antagonist) attenuates the effects of NE and dialysis of GppNHp (non‐hydrolyzable GTP analog) inhibits Ca2+ current equally. In DOCA‐salt rats, LEC treatment prevents macrophage infiltration in MA, reduces blood pressure elevation, and preserves α2AR‐mediated inhibition of Ca2+ current in sympathetic neurons.ConclusionsIn DOCA‐salt hypertensive rats, α2AR dysfunction leads to impaired modulation of Ca2+ current that is independent of factors downstream of receptor activation. Macrophage depletion preserves α2AR function and protects against blood pressure elevation.Support or Funding InformationP01 HL070687

A search for presynaptic inhibitory histamine receptors in guinea-pig tissues: Further H3 receptors but no evidence for H4 receptors

The histamine H4 receptor is coupled to Gi/o proteins and expressed on inflammatory cells and lymphoid tissues; it was suggested that this receptor also occurs in the brain or on peripheral neurones. Since many Gi/o protein-coupled receptors, including the H3 receptor, serve as presynaptic inhibitory receptors, we studied whether the sympathetic neurones supplying four peripheral tissues and the cholinergic neurones in the hippocampus from the guinea-pig are equipped with release-modulating H4 and H3 receptors. For this purpose, we preincubated tissue pieces from the aorta, atrium, renal cortex and vas deferens with 3H-noradrenaline and hippocampal slices with 3H-choline and determined the electrically evoked tritium overflow. The stimulation-evoked overflow in the five superfused tissues was inhibited by the muscarinic receptor agonist oxotremorine, which served as a positive control, but not affected by the H4 receptor agonist 4-methylhistamine. The H3 receptor agonist R-α-methylhistamine inhibited noradrenaline release in the peripheral tissues without affecting acetylcholine release in the hippocampal slices. Thioperamide shifted the concentration–response curve of histamine in the aorta and the renal cortex to the right, yielding apparent pA2 values of 8.0 and 8.1, respectively, which are close to its affinity at other H3 receptors but higher by one log unit than its pKi at the H4 receptor of the guinea-pig. In conclusion, histamine H4 receptors could not be identified in five experimental models of the guinea-pig that are suited for the detection of presynaptic inhibitory receptors whereas H3 receptors could be shown in the peripheral tissues but not in the hippocampus.This article is part of the Special Issue entitled ‘Histamine Receptors’.

L-/N-type Calcium Channel Blockers and Proteinuria

The first-line depressor agents for hypertensive patients with chronic kidney disease are the renin-angiotensin system inhibitors because of their antiproteinuric and reno-protective effects. However, only one renin-angiotensin system inhibitor often cannot achieve target blood pressure in patients with injured kidney. Thus, second-line antihypertensives are required. Calcium channel blockers are frequently added on the renin-angiotensin system inhibitors in hypertensive patients with chronic kidney disease. However, they do not always show reno-protective effects because of their glomerular pressure-increasing action; Antihypetensive calcium channel blockers suppress L-type calcium channels, which exist in glomerular afferent but not efferent arterioles, and their afferent arteriole-specific vasodilation causes glomerular hypertension. The decrease in glomerular pressure due to their systemic hypotesive effect is counteracted by the glomerular pressure-incresing action. However, L-/N-type calcium channel blockers inhibits norepinephrine release from the sympathetic nerve terminal by blockade of N-type calcium channels, and dilate both afferent and efferent arterioles, which were innervated sympathetically, resulting in decrease in glomerular pressure. Actually, we have demonstrated that an L-/N-type calcium channel blocker cilnidipine decreased urinary protein more greatly than an L-type calcium channel blocker amlodipine in the renin-angiotensin system inhibitor-treated patients with chronic kidney disease. Thus, L-/N-type calcium channel blockers are one of suitable candidates for the second-line antihypertensives in the renin-angiotensin system inhibitor-treated hypertensive patients with proteinuria.

Purinergic modulation of norepinephrine release and uptake in rat brain cortex: contribution of glial cells

The pathogenesis of psychiatric and neurodegenerative diseases is often associated with a deregulation of noradrenergic transmission. Considering the potential involvement of purinergic signaling in the modulation of noradrenergic transmission in the brain cortex, this study aimed to identify the P2Y receptor subtypes involved in the modulation of neuronal release and neuronal/glial uptake of norepinephrine. Electrical stimulation (100 pulses at 5 Hz) of rat cortical slices induced norepinephrine release that was inhibited by ATP and ADP (0.01-1 mM), adenosine 5'-O-(2-thiodiphosphate) (ADPβS, 0.03-0.3 mM), and UDP (0.1-1 mM). The effect of ADPβS was mediated by P2Y1 receptors and possibly by A1/P2Y1 heterodimers since it was attenuated by the A1 receptor antagonist DPCPX and by the P2Y1 receptor antagonist MRS 2500 but was resistant to the effect of adenosine deaminase (ADA). UDP inhibited norepinephrine release through activation of P2Y6 receptors, an effect that was abolished by the P2Y6 receptor antagonist MRS 2578 and by DPCPX, indicating that it depends on the formation and/or release of adenosine and activation of A1 receptors. Supporting this hypothesis, the inhibitory effect of UDP was also prevented by inhibition of ectonucleotidases, by ADA and was attenuated by the inhibitor of nucleoside transporter 6-[(4-nitrobenzyl)thio]-9-β-d-ribofuranosylpurine (NBTI). Additionally, the inhibitory effect of UDP was attenuated when norepinephrine uptake 1 or 2 was inhibited. In astroglial cultures, ADPβS and UDP increased norepinephrine uptake mainly by activation of P2Y1 and P2Y6 receptors, respectively. The results indicate that neuronal and glial P2Y1 and P2Y6 receptors may represent new targets of intervention to regulate noradrenergic transmission in CNS diseases.

The α2-adrenoceptors mediating inhibition of the vasopressor sympathetic outflow in pithed rats: Pharmacological correlation with α2A, α2B and α2C subtypes

α2-Adrenoceptors were first described as presynaptic receptors inhibiting the release of various transmitters from neurons in the central and peripheral nervous systems. In vitro studies have confirmed that α2A, α2B and α2C subtypes inhibited noradrenaline release from postganglionic sympathetic neurons but no study has been reported their involvement in the vasopressor sympathetic outflow in vivo. Thus, this study analysed the subtype(s) involved in the inhibition produced by the α2-adrenoceptor agonist, B-HT 933, on the vasopressor sympathetic outflow. Male Wistar pithed rats were pre-treated with i.v. bolus injections of gallamine (25mg/kg) and desipramine (50µg/kg) and prepared to stimulate the vasopressor sympathetic outflow (T7–T9) or to receive i.v. bolus of exogenous noradrenaline. Sympathetic stimulation or exogenous noradrenaline produced, respectively, frequency-dependent and dose-dependent vasopressor responses. I.v. continuous infusion of B-HT 933 (30μg/kgmin) failed to modify the vasopressor responses to exogenous noradrenaline and inhibited those induced by preganglionic stimulation of the vasopressor sympathetic outflow at all frequencies of stimulation (0.03–3Hz). The sympatho-inhibition elicited by B-HT 933 was: (i) unaffected by vehicles (1ml/kg); (ii) partially antagonised by BRL44408 (300μg/kg; α2A), imiloxan (3000μg/kg; α2B) and/or JP-1302 (300μg/kg; α2C) given separately; and (iii) completely blocked by rauwolscine (300μg/kg) or the combination of BRL44408 (300μg/kg)+imiloxan (3000μg/kg)+JP-1302 (300μg/kg). The above doses of antagonists did not modify per se the sympathetically-induced vasopressor responses. These results suggest that the vasopressor sympatho-inhibition to B-HT 933 is primarily mediated by activation of α2A/2B/2C-adrenoceptors in pithed rats.

Impaired Function of Prejunctional Adenosine A1 Receptors Expressed by Perivascular Sympathetic Nerves in DOCA-Salt Hypertensive Rats

Increased sympathetic nervous system activity contributes to deoxycorticosterone acetate (DOCA)-salt hypertension in rats. ATP and norepinephrine (NE) are coreleased from perivascular sympathetic nerves. NE acts at prejunctional α2-adrenergic receptors (α2ARs) to inhibit NE release, and α2AR function is impaired in DOCA-salt rats. Adenosine, an enzymatic ATP degradation product, acts at prejunctional A1 adenosine receptors (A1Rs) to inhibit NE release. We tested the hypothesis that prejunctional A1R function is impaired in sympathetic nerves supplying mesenteric arteries (MAs) and veins (MVs) of DOCA-salt rats. Electrically evoked NE release and constrictions of blood vessels were studied in vitro with use of amperometry to measure NE oxidation currents and video microscopy, respectively. Immunohistochemical methods were used to localize tyrosine hydroxylase (TH) and A1Rs in perivascular sympathetic nerves. TH and A1Rs colocalized to perivascular sympathetic nerves. Adenosine and N(6)-cyclopentyl-adenosine (CPA, A1R agonist) constricted MVs but not MAs. Adenosine and CPA (0.001-10 µM) inhibited neurogenic constrictions and NE release in MAs and MVs. DOCA-salt arteries were resistant to adenosine and CPA-mediated inhibition of NE release and constriction. The A2A adenosine receptor agonist CGS21680 (C23H29N7O6.HCl.xH2O) (0.001-0.1 μM) did not alter NE oxidation currents. We conclude that there are prejunctional A1Rs in arteries and both pre- and postjunctional A1Rs in veins; thus, adenosine selectively constricts the veins. Prejunctional A1R function is impaired in arteries, but not veins, from DOCA-salt rats. Sympathetic autoreceptor dysfunction is not specific to α2ARs, but there is a more general disruption of prejunctional mechanisms controlling sympathetic neurotransmitter release in DOCA-salt hypertension.

Opposing local effects of endocannabinoids on the activity of noradrenergic neurons and release of noradrenaline: relevance for their role in depression and in the actions of CB1 receptor antagonists

There is strong evidence that endocannabinoids modulate signaling of serotonin and noradrenaline, which play key roles in the pathophysiology and treatment of anxiety and depression. Most pharmacological and genetic, human and rodent studies suggest that the presence of under-functioning endocannabinoid type-1 (CB(1)) receptors is associated with increased anxiety and elevated extracellular serotonin concentration. In contrast, noradrenaline is presumably implicated in the mediation of depression-type symptoms of CB(1) receptor antagonists. Evidence shows that most CB(1) receptors located on axons and terminals of GABA-ergic, serotonergic or glutamatergic neurons stimulate the activity of noradrenergic neurons. In contrast, those located on noradrenergic axons and terminals inhibit noradrenaline release efficiently. In this latter process, excitatory ionotropic or G protein-coupled receptors, such as the NMDA, alpha1 and beta1 adrenergic receptors, activate local endocannabinoid synthesis at postsynaptic sites and stimulate retrograde endocannabinoid neurotransmission acting on CB(1) receptors of noradrenergic terminals. The underlying mechanisms include calcium signal generation, which activates enzymes that increase the synthesis of both anandamide and 2-arachidonoylglycerol, while G(q/11) protein activation also increases the formation of 2-arachidonoylglycerol from diacylglycerol during the signaling process. In addition, other non-CB(1) receptor endocannabinoid targets such as CB(2), transient receptor potential vanilloid subtype, peroxisome proliferator-activated receptor-alpha and possibly GPR55 can also mediate some of the endocannabinoid effects. In conclusion, both neuronal activation and neurotransmitter release depend on the in situ synthesized endocannabinoids and thus, local endocannabinoid concentrations in different brain areas may be crucial in the net effect, namely in the regulation of neurons located postsynaptically to the noradrenergic synapse.

Involvement of presynaptic voltage-dependent Kv3 channel in endothelin-1-induced inhibition of noradrenaline release from rat gastric sympathetic nerves

We previously reported that two types of K+ channels, the BK type Ca2+-activated K+ channel coupled with phospholipase C (PLC) and the voltage-dependent K+ channel (Kv channel), are, respectively, involved in the prostanoid TP receptor- and muscarinic M2 receptor-mediated inhibition of noradrenaline (NA) release from rat gastric sympathetic nerves. In the present study, therefore, we examined whether these K+ channels are involved in endothelin-1-induced inhibition of NA release, using an isolated, vascularly perfused rat stomach. The gastric sympathetic postganglionic nerves around the left gastric artery were electrically stimulated twice at 2.5Hz for 1min, and endothelin-1 was added during the second stimulation. Endothelin-1 (1, 2 and 10nM) dose-dependently inhibited gastric NA release. Endothelin-1 (2nM)-induced inhibition of NA release was neither attenuated by PLC inhibitors [U-73122 (3μM) and ET-18-OCH3 (3μM)] nor by Ca2+-activated K+ channel blockers [charybdotoxin (0.1μM) (a blocker of BK type K+ channel) and apamin (0.3μM) (a blocker of SK type K+ channel)]. The endothelin-1-induced inhibitory response was also not attenuated by α-dendrotoxin (0.1μM) (a selective inhibitor of Kv1 channel), but abolished by 4-aminopyridine (20μM) (a selectively inhibitory dose for Kv3 channel). These results suggest the involvement of a voltage-dependent Kv3 channel in the endothelin-1-induced inhibition of NA release from the gastric sympathetic nerves in rats.

Alpha2A and Alpha2C Adrenergic Receptors are involved in the regulation of Bone Growth, Structure and Function

Sympathetic nervous system (SNS) activation causes osteopenia via β2 adrenoceptor (β2AR). We showed that sympathetic hyperactivity due to double knockout (KO) of adrenoceptors (AR) that inhibit norepinephrine release, α2A and α2C AR (α2A/α2CAR−/−), leads to an unexpected phenotype of high bone mass. It suggests that β2AR is not the single AR involved in bone mass regulation and that α2‐AR may also mediate the skeletal SNS actions. To investigate the role α2ARs in bone, the skeleton phenotype of 120 day old female α2AAR and α2CAR single KO mice (α2AAR−/− andα2CAR−/−), which exhibit normal sympathetic tone, was evaluated. α2AAR−/−mice shows decreased femur (9%, p&lt;0,001) and tibia (5%, p&lt;0,01) length and α2CAR−/− shows decreased femur length (4%, p&lt;0,001). This is in agreement with α2AAR and α2CAR protein expression in the epiphyseal growth plates. Surprisingly, a microtomography analysis showed that α2CAR−/−mice present 15% decrease (p&lt;0,01) in the femural trabecular bone volume, while there are no differences between wild‐type and α2AAR−/−mice. Accordingly, the three‐point bending test showed 35% decrease in the tibia maximum load in α2CAR−/−, but not in α2AAR−/−mice. These data suggest that α2AAR and α2CAR are involved in the longitudinal bone growth and that α2CAR may positively affect bone structure and function, while α2AAR may negatively regulate these parameters when sympathetic tone is elevated.

Activation of adenosine 2A receptors (A2ARs) enhances norepinephrine (NE) transport into perivenous sympathetic nerves in normotensive, but not DOCA‐salt hypertensive rats

NE and ATP are released from perivascular sympathetic nerves to control peripheral resistance and blood pressure. NE is cleared from the neuroeffector junction by the NE transporter (NET). Adenosine, a product of ATP hydrolysis, inhibits NE release via an action at adenosine A1 receptors (A1Rs). A2ARs are also expressed on perivascular sympathetic nerves, but their function is unclear. We determined the role of A2ARs in modulating NE release and tested if A2AR function is impaired in DOCA‐salt rats. Electrically‐evoked NE release was measured in vitro using amperometry. The amplitude, rise slope, and decay slope of NE oxidation currents were measured in the absence or presence of drugs. CGS21680 (1–100 nM, A2AR agonist) increased the decay slope in mesenteric veins, but not arteries. The effect was antagonized by an A2AR antagonist (SCH58261 400 nM), and a NET blocker (cocaine 10 μM). This effect was mimicked by adenylate cyclase activation (forkolin, 1μM), but not PKC activation (PDBu 100 nM, or PMA 100 nM). A PKA inhibitor, H‐89 (1 μM) blocked the CGS‐induced effect. In mesenteric veins from DOCA‐salt rats, forskolin but not CGS21680 increased the decay slope of NE current. These data indicate that A2AR activation enhances NE transport into perivenous sympathetic nerves through activation of adenylate cyclase/cAMP/PKA pathway and A2AR function is impaired in DOCA‐salt veins.

Physiology of quantal norepinephrine release from somatodendritic sites of neurons in locus coeruleus

Norepinephrine (NE) released from the nerve terminal of locus coeruleus (LC) neurons contributes to about 70% of the total extracellular NE in primates brain. In addition, LC neurons also release NE from somatodendritic sites. Quantal NE release from soma of LC neurons has the characteristics of long latency, nerve activity-dependency, and autoinhibition by α2-adrenergic autoreceptor. The distinct kinetics of stimulus-secretion coupling in somata is regulated by action potential patterns. The physiological significance of soma and dendritic release is to produce negative-feedback and to down-regulate neuronal hyperactivity, which consequently inhibit NE release from axon terminal of LC projecting to many brain areas. Recent discoveries about the LC somatodendritic release may provide new insights into the pathogenesis of clinic disease involving LC-NE system dysfunction, and may help developing remedy targeted to the LC area.

Indirect sympatholytic actions at β-adrenoceptors account for the ocular hypotensive actions of cannabinoid receptor agonists.

Intraocular pressure (IOP) is the primary risk factor for glaucoma, a blinding eye disease. Cannabinoid agonists have long been known to decrease IOP, suggesting they may be useful in glaucoma treatment. However, the specific mechanism by which cannabinoids generate this ocular hypotensive effect remains unknown. The current evidence suggests the cannabinoids reduce IOP through actions at cannabinoid 1 (CB(1)) receptors within the eye, and adrenergic receptors (ARs) may also contribute to this action of cannabinoids. Considering this, the present study aimed to elucidate the mechanism behind the ocular hypotensive properties of cannabinoids through the use of mice genetically lacking either cannabinoid receptors or βARs. Cannabinoid agonists, βAR antagonists, and βAR agonists decreased IOP in wild-type mice and CB(2)(-/-) mice. In contrast, none of these compounds were found to reduce IOP in βAR(-/-) or CB(1)(-/-) mice. Desensitization of the βARs and depletion of catecholamines in wild-type mice also eliminated the ability of the cannabinoid agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2) to reduce IOP, strongly implicating a role for both βARs and catecholamines in the ocular hypotensive properties of cannabinoids. Finally, CB(1) receptors were shown to colocalize with tyrosine hydroxylase, a marker for adrenergic neurons. Taken together, these findings suggest that βARs are required for the ocular hypotensive properties of cannabinoids, and cannabinoids reduce IOP by acting as indirect sympatholytics and inhibiting norepinephrine release within the eye.