Adrenergic Receptor Agonist Clonidine Research Articles

Cognitive and Histopathological Alterations in Rat Models of Early- and Late-Phase Memory Dysfunction: Effects of Sigma-1 Receptor Activation.

Sigma-1 receptors are highly expressed in brain areas related to cognitive function and are a promising target for anti-amnesic treatments. We previously showed that activation of sigma-1 receptors by the selective agonist compound methyl(1 R,2 S/1 S,2 R)-2-[4-hydroxy-4-phenylpiperidin-1-yl)methyl]-1-(4-methylphenyl) cyclopropane carboxylate [(±)-PPCC] promotes a remarkable recovery in rat models of memory loss associated to cholinergic dysfunction. In this study, we sought to assess the role of (±)-PPCC on working memory deficits caused by noradrenergic depletion. Animals with a mild or severe working memory deficits associated to varying degrees of noradrenergic neuronal depletion were treated with the sigma-1 agonist just prior to the beginning of each behavioral testing session. While (±)-PPCC alone at a dose of 1 mg/kg/day failed to affect working memory in lesioned animals, its association with the α2 adrenergic receptor agonist clonidine, completely blocked noradrenaline release, significantly improving rat performance. This effect, distinct from noradrenaline activity, is likely to result from a direct action of the (±)-PPCC compound onto sigma-1 receptors, as pre-treatment with the selective sigma-1 receptor antagonist BD-1047 reversed the improved working memory performance. Despite such clear functional effects, the treatment did not affect noradrenergic neuron survival or terminal fiber proliferation. Future studies are thus necessary to address the effects of long-lasting (±)-PPCC treatment, with or without clonidine, on cognitive abilities and Alzheimer's disease-like histopathology. Considering the already established involvement of sigma-1 receptors in endogenous cell plasticity mechanisms, their activation by selective agonist compounds holds promises as possibly positive contributor to disease-modifying events in neurodegenerative diseases.

Prenatal LPS leads to increases in RAS expression within the PVN and overactivation of sympathetic outflow in offspring rats

The renin-angiotensin system (RAS) and the sympathetic nervous system (SNS) are two major blood pressure-regulating systems. The link between the renal and cerebral RAS axes was provided by reflex activation of renal afferents and efferent sympathetic nerves. There is a self-sustaining enhancement of the brain and the intrarenal RAS. In this study, prenatal exposure to lipopolysaccharide (LPS) led to increased RAS activity in the paraventricular nucleus (PVN) and overactivation of sympathetic outflow, accompanied by increased production of reactive oxygen species (ROS) and disturbances between inhibitory and excitatory neurons in PVN. The AT1 receptor blocker losartan and α2 adrenergic receptor agonist clonidine in the PVN significantly decreased renal sympathetic nerve activity (RSNA) and synchronously reduced systolic blood pressure. Prenatal LPS stimulation caused H3 acetylation at H3K9 and H3K14 in the PVN, which suggested that epigenetic changes are involved in transmitting the prenatal adverse stimulative information to the next generation. Additionally, melatonin treatment during pregnancy reduced RAS activity and ROS levels in the PVN; balanced the activity of inhibitory and excitatory neurons in the PVN; increased urine sodium secretion; reduced RSNA and blood pressure. In conclusion, prenatal LPS leads to increased RAS expression within the PVN and overactivation of the sympathetic outflow, thereby contributing to hypertension in offspring rats. Melatonin is expected to be a promising agent for preventing prenatal LPS exposure-induced hypertension.

Involvement of dopamine D2 and glutamate NMDA receptors in the antidepressant-like effect of amantadine in mice

The present study investigated the pharmacological mechanisms of the antidepressant-like effects of amantadine in mice and their influence on hippocampal neurogenesis. To improve the translational validity of preclinical results, reproducibility across laboratories and replication in other animal models and species are crucial. Single amantadine administration at doses of 50 and 75 mg/kg resulted in antidepressant-like effects in mice in the tail suspension test (TST), reflected by an increase in immobility time. The effects of amantadine were seen at doses that did not alter locomotor activity. The tyrosine hydroxylase inhibitor α-methyl-ρ-tyrosine did not influence the anti-immobility effect of amantadine in the TST. Pretreatment with the α1 adrenergic receptor antagonist prazosin, β adrenergic receptor antagonist propranolol, α2 adrenergic receptor antagonist yohimbine, and α2 adrenergic receptor agonist clonidine did not alter the antidepressant-like effect of amantadine. However, amantadine’s effect was blocked by the dopamine D2 receptor antagonist haloperidol and glutamate receptor agonist N-methyl-D-aspartate (NMDA). Repeated amantadine administration (50 mg/kg) also exerted an antidepressant-like effect, paralleled by an increase in hippocampal neurogenesis. The present results demonstrate that the antidepressant-like effects of amantadine may be mediated by its actions on D2 and NMDA receptors and likely involve hippocampal neurogenesis.

Cell-type specific expression and behavioral impact of galanin and GalR1 in the locus coeruleus during opioid withdrawal.

The neuropeptide galanin is reported to attenuate opioid withdrawal symptoms, potentially by reducing neuronal hyperactivity in the noradrenergic locus coeruleus (LC) via galanin receptor 1 (GalR1). We evaluated this mechanism by using RNAscope in situ hybridization to characterize GalR1 mRNA distribution in the dorsal pons and to compare galanin and GalR1 mRNA expression in tyrosine hydroxylase-positive (TH+) LC cells at baseline and following chronic morphine or precipitated withdrawal. We then used genetically altered mouse lines and pharmacology to test whether noradrenergic galanin (NE-Gal) modulates withdrawal symptoms. RNAscope revealed that, while GalR1 signal was evident in the dorsal pons, 80.7% of the signal was attributable to TH- neurons outside the LC. Galanin and TH mRNA were abundant in LC cells at baseline and were further increased by withdrawal, whereas low basal GalR1 mRNA expression was unaltered by chronic morphine or withdrawal. Naloxone-precipitated withdrawal symptoms in mice lacking NE-Gal (GalcKO-Dbh ) were largely similar to WT littermates, indicating that loss of NE-Gal does not exacerbate withdrawal. Complementary experiments using NE-Gal overexpressor mice (NE-Gal OX) and systemic administration of the galanin receptor agonist galnon revealed that increasing galanin signaling also failed to alter behavioral withdrawal, while suppressing noradrenergic transmission with the alpha-2 adrenergic receptor agonist clonidine attenuated multiple symptoms. These results indicate that galanin does not acutely attenuate precipitated opioid withdrawal via an LC-specific mechanism, which has important implications for the general role of galanin in regulation of somatic and affective opioid responses and LC activity.

Corticotropin-releasing factor (CRF) and α2 adrenergic receptors mediate heroin withdrawal-potentiated startle in rats

Anxiety is one of the early symptoms of opioid withdrawal and contributes to continued drug use and relapse. The acoustic startle response (ASR) is a component of anxiety that has been shown to increase during opioid withdrawal in both humans and animals. We investigated the role of corticotropin-releasing factor (CRF) and norepinephrine (NE), two key mediators of the brain stress system, on acute heroin withdrawal-potentiated ASR. Rats injected with heroin (2mg/kg s.c.) displayed an increased ASR when tested 4h after heroin treatment. A similar increase in ASR was found in rats 10-20h into withdrawal from extended access (12h) to i.v. heroin self-administration, a model that captures several aspects of heroin addiction in humans. Both the α 2 adrenergic receptor agonist clonidine (10μg/kg s.c.) and CRF1 receptor antagonist N,N-bis(2-methoxyethyl)-3-(4-methoxy-2-methylphenyl)-2,5-dimethyl-pyrazolo[1,5-a] pyrimidin-7-amine (MPZP; 20mg/kg s.c.) blocked heroin withdrawal-potentiated startle. To investigate the relationship between CRF1 and α 2 adrenergic receptors in the potentiation of the ASR, we tested the effect of MPZP on yohimbine (1.25mg/kg s.c.)-potentiated startle and clonidine on CRF (2μg i.c.v.)-potentiated startle. Clonidine blocked CRF-potentiated startle, whereas MPZP partially attenuated but did not reverse yohimbine-potentiated startle, suggesting that CRF may drive NE release to potentiate startle. These results suggest that CRF1 and α 2 receptors play an important role in the heightened anxiety-like behaviour observed during acute withdrawal from heroin, possibly via CRF inducing the release of NE in stress-related brain regions.

Neonatal Abstinence Syndrome: The Use of Clonidine as a Treatment Option

Infants exposed to opioids in utero and postnatally are at risk for developing withdrawal symptoms upon discontinuation of the drugs. This condition is known as the neonatal abstinence syndrome (NAS). Different medications have been used to ameliorate the symptoms of withdrawal, most commonly opioids. Clonidine has also been evaluated both as an additive and alternative option for the treatment of opioid withdrawal symptoms. Data evaluating the use of clonidine for the treatment of NAS are limited; only six studies have been published. The α-2 adrenergic receptor agonist clonidine is believed to reduce the excessive noradrenergic activity that results from opioid withdrawal. Clonidine has the potential to serve as an attractive option to treat NAS because it possesses a favorable adverse effect profile, is easy to administer, and does not require a long tapering period, unlike other agents currently used to treat NAS. Blood pressure and heart rate must be monitored with clonidine use.

Regulation of CCL2/MCP-1 production in astrocytes by desipramine and atomoxetine: Involvement of α2 adrenergic receptors

Having previously observed that noradrenaline activation of β adrenergic receptors induces the synthesis of the chemokine monocyte chemoattractant protein (CCL2/MCP-1) in astrocytes, it is our interest to analyze the mechanisms involved in this process, particularly the possible effect of noradrenaline-modulating drugs. The treatment of primary rat astrocyte cultures with the noradrenaline transporter inhibitors desipramine or atomoxetine induced the expression and synthesis of CCL2/MCP-1 in these cells. This effect of both drugs in vitro suggests that CCL2/MCP-1 expression could also be modulated by some mechanism independent of the elevation of brain noradrenaline levels. This was confirmed by measuring a reduction in CCL2/MCP-1 production by the treatment with the α2 adrenergic receptor agonist clonidine. Accordingly, the blockade of α2 adrenergic receptors with yohimbine potentiated the production of MCP-1 stimulated by the activation of β receptors. While the activation of β adrenergic receptors and the subsequent elevation of cAMP levels seem to be the main pathway for noradrenaline to induce CCL2/MCP-1 in astrocytes, our data indicate that the α2 adrenergic receptors also regulate CCL2/MCP-1 expression working as inhibitory mediators.

Protracted cognitive effects produced by clonidine in Macaca nemestrina performing a delayed matching task

The alpha(2)-adrenergic receptor agonist clonidine was examined for its ability to improve working memory in monkeys. Clonidine (0.116-34.8 microg/kg) was administered to six pigtail macaques in their performance of a computer-assisted delayed matching-to-sample (DMTS) task. During DMTS sessions initiated 1 hour after dosing, there was a slight improvement in mean task accuracy (long delay trials; 0.116-microg/kg). On the following day, there was continued and added improvement in accuracies associated with the long delay trials. On the day following 1.16-microg/kg, the entire memory retention curve was shifted to the right of vehicle. When the animals were again tested 48 hours after dosing (no pretreatment), these two patterns of task enhancement were continued and enhanced. Mean task accuracy associated with long delay trials was significantly increased by 14.2% trials correct when animals were originally treated with 0.116-microg/kg of clonidine. Mean task accuracy associated with medium delay trials was significantly increased by 11.8% trials correct when animals were treated with 1.16-microg/kg. On the sixth day after clonidine, task accuracies were still significantly improved during medium delay trials after 0.116-microg/kg. Median sample and choice latencies were not significantly influenced by clonidine treatment. These findings are consistent with the ability of clonidine to induce a protracted improvement in aspects of working memory. Early (attentional) and late (retention) components of memory appeared to be differentially sensitive to the dose of clonidine. Central alpha(2)-adrenergic receptors should be considered legitimate drug targets for future compound development for cognition enhancement.

Aminergic control of neuronal firing rate in thalamic motor nuclei of the rat.

The effects induced on neuronal firing by microiontophoretic application of the biological amines noradrenaline (NA) and 5-hydroxytryptamine (5-HT) were studied "in vivo" in ventral-anterior (VA) and ventrolateral (VL) thalamic motor nuclei of anaesthetized rats. In both nuclei the amines had a mostly depressive action on neuronal firing rate, the percentage of units responsive to NA application (88%) being higher than to 5-HT (72%). Short-lasting (less than 2 min) and long lasting (up to 20 min) inhibitory responses were recorded, the former mostly evoked by NA and the latter by 5-HT ejection. In some cases 5-HT application had no effect on the firing rate but modified the firing pattern. NA-evoked responses were significantly more intense in VL than in VA neurons. Short-lasting inhibitory responses similar to NA-induced effects were evoked by the alpha2 adrenergic receptor agonist clonidine and to a lesser extent by the beta adrenergic receptor agonist isoproterenol. Inhibitory responses to 5-HT were partially mimicked by application of the 5-HT(1A) receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) and of the 5-HT2 receptor agonist alpha-methyl-5-hydroxytryptamine (ALPHA-MET-5-HT). The latter evoked excitatory responses in some cases. Both 5-HT agonists were more effective on VA than on VL neurons. The effects evoked by agonists were at least partially blocked by respective antagonists. These results suggest that although both 5-HT and NA depress neuronal firing rate, their effects differ in time course and in the amount of inhibition; besides aminergic modulation is differently exerted on VA and VL.

The Effects of the α2‐Adrenergic Receptor Agonists Clonidine and Rilmenidine, and Antagonists Yohimbine and Efaroxan, on the Spinal Cholinergic Receptor System in the Rat

Cholinergic agonists produce spinal antinociception via mechanisms involving an increased release of intraspinal acetylcholine. The cholinergic receptor system interacts with several other receptor types, such as alpha2-adrenergic receptors. To fully understand these interactions, the effects of various receptor ligands on the cholinergic system must be investigated in detail. This study was initiated to investigate the effects of the alpha2-adrenergic receptor agonists clonidine and rilmenidine and the alpha2-adrenergic receptor antagonists yohimbine and efaroxan on spinal cholinergic receptors in the rat. Spinal microdialysis was used to measure in vivo changes of acetylcholine after administration of the ligands, with or without nicotinic receptor blockade. In addition, in vitro binding properties of the ligands on muscarinic and nicotinic receptors were investigated. It was found that clonidine and rilmenidine increased, while yohimbine decreased spinal acetylcholine release. Efaroxan affected acetylcholine release differently depending on concentration. Nicotinic receptor blockade attenuated the effect of all ligands. All ligands showed poor binding affinity for muscarinic receptors. On the other hand, all ligands possessed affinity for nicotinic receptors. Clonidine and yohimbine binding was best fit to a one site binding curve and rilmenidine and efaroxan to a two site binding curve. The present study demonstrates that the tested alpha2-adrenergic receptor ligands affect intraspinal acetylcholine release in the rat evoked by nicotinic receptor mechanisms in vivo, and that they possess binding affinity to nicotinic receptors in vitro. The binding of alpha2-adrenergic receptor ligands to nicotinic receptors might affect the intraspinal release of acetylcholine.

Alpha(2C)-Adrenergic receptors mediate spinal analgesia and adrenergic-opioid synergy.

The alpha(2A)-adrenergic receptor (AR) subtype mediates antinociception induced by the alpha(2)AR agonists clonidine, dexmedetomidine, norepinephrine, and 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14,304) as well as antinociceptive synergy of UK-14,304 with opioid agonists [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin and deltorphin II. Differential localization of alpha(2)-adrenergic (alpha(2A)-, alpha(2B)-(,) alpha(2C)-) and opioid (mu-, delta-, kappa-) subtypes suggests differential involvement of subtype pairs in opioid-adrenergic analgesic synergy. The present study applies a novel imidazoline(1)/alpha(2)-adrenergic receptor analgesic, moxonidine, to test for involvement of alpha(2B)- and alpha(2C)ARs in antinociception and antinociceptive synergy, because spinal antinociceptive activity of moxonidine shows minimal dependence on alpha(2A)AR. Intrathecal administration of moxonidine produced similar (2-3-fold) decreases in both mutant mice with a functional knockout of alpha(2A)AR (D79N-alpha(2A)AR) and alpha(2C)AR knockout (KO) mice. The potency of moxonidine was not altered in alpha(2B)KO mice, indicating that this subtype does not participate in moxonidine-induced spinal antinociception. Moxonidine-mediated antinociception was dose dependently inhibited by the selective alpha(2)-receptor antagonist SK&F 86466 in both D79N-alpha(2A) mice and alpha(2C)KO mice, indicating that alpha(2)AR activation is required in the absence of either alpha(2A)- or alpha(2C)AR. Spinal administration of antisense oligodeoxynucleotides directed against the alpha(2C)AR decreased both alpha(2C)AR immunoreactivity and the antinociceptive potency of moxonidine. Isobolographic analysis demonstrates that moxonidine-deltorphin antinociceptive synergy is present in the D79N-alpha(2A) mice but not in the alpha(2C)AR-KO mice. These results confirm that the alpha(2C)AR subtype contributes to spinal antinociception and synergy with opioids.

Influence of G protein type on agonist efficacy.

An agonist at a specific G protein-coupled receptor may exhibit a range of efficacies for any given response in a cell-specific manner. We report that the relationship between different states of agonism is regulated by the type of G protein expressed in the cell. In NIH-3T3 alpha(2)-adrenergic receptor (AR) transfectants, the alpha(2)-AR agonists clonidine, oxymetazoline, UK-14304, and epinephrine increased [(35)S]guanosine-5'-O-(3-thio)triphosphate binding in a dose-dependent manner from a basal value of 101.2 +/- 6. 5 fmol/mg to a maximal response (100 microM) of 196.6 +/- 9.8, 182.3 +/- 2, 328.1 +/- 11.2, and 340.6 +/- 3 fmol/mg, respectively. Thus, clonidine and oxymetazoline behaved as partial agonists. Receptor-mediated activation of G proteins in membrane preparations was blocked by cell pretreatment with pertussis toxin, indicating receptor coupling to the subgroup of pertussis toxin-sensitive G protein (Gialpha2,3) expressed in NIH-3T3 cells. Ectopic expression of Goalpha1 but not Gialpha1 increased the relative efficacy of clonidine and oxymetazoline such that the two ligands now behaved as close to full agonists in this assay system. The relationship between full and partial agonists in the different genetic backgrounds was not altered by progressive reduction in the amount of G protein available for coupling to receptor. The increased efficacy observed for clonidine in the Goalpha1 transfectants was not due to changes in the relative affinities or amounts of high-affinity, Gpp(NH)p-sensitive binding of agonist. These data suggest that there is little difference in the ability of clonidine to interact with or stabilize alpha(2)-AR-Gialpha2/Gialpha3 versus alpha(2)-AR-Goalpha1 complexes, but that the subsequent step of signal transfer from receptor to G protein is more readily achieved for the clonidine/alpha(2)-AR/Goalpha1 complex. Such observations have important implications for receptor theory and drug development.

Novelty seeking behavior in the rat is dependent upon the integrity of the noradrenergic system

These experiments were designed to investigate the role of the noradrenergic system in promoting investigation of novelty in rats. Behavior was monitored in a hole board equipped with photoelectric cells strategically placed so that locomotor activity, rearing and investigation of each of the holes could be quantified independently. Specially designed computer software permitted recording of the sequence and cumulative duration of the visits to specific holes throughout the session. Dose-response curves of the sedative effect of the α2 adrenergic receptor agonist clonidine were establised, a sedative effect being defined as a decrease in overall horizontal displacements, rearings and hole visits. After a one week interval, the rats were rerun in the holeboard, with novel objects placed in four of the nine holes. Previous experiments had shown that rats spend significantly more time investigating holes containing objects than empty holes in this apparatus and this was replicated here. Doses of clonidine which were below threshold for inducing any sedative effect (10 μg/kg) totally eliminated preference for holes with objects while having no effect on total time investigating the holes. A subsequent experiment showed that the β receptor antagonist propranolol (10 mg/kg) produced a similar effect. These results suggest that the noradrenergic system is implicated in stimulus seeking behavior and the post-synaptic β receptors are involved in mediating the behavior.

Differential Nerve Block by the a2 Adrenergic Receptor Agonists Clonidine and Guanfacine

Differential Nerve Block by the a2 Adrenergic Receptor Agonists Clonidine and Guanfacine

Suppression of feeding induced by phenylephrine microinjections within the paraventricular hypothalamus in rats

Rats were treated with the alpha-2 agonist clonidine (4, 20 and 50 nMol) and with the alpha-1 agonist 1-phenylephrine (50, 100, 200 and 400 nMol). Phenylpropanolamine (PPA) is a phenythylamine anorectic drug that exerts direct agonist effects predominantly on alpha-1 adrenergic receptors, with some alpha-2 adrenergic activity. We recently reported that injection of PPA significantly suppressed feeding in rats. Prior studies have noted that into the paraventricular hypothalamus (PVN) microinjections of the alpha-2 adrenergic receptor agonist clonidine into the PVN induced feeding behavior in satiated rats. However, the effect on feeding of administration of alpha-1 adrenergic agonists within the PVN remains unknown. In the present study, unilateral guide cannulae aimed at the PVN were surgically implanted in adult male rats. In an initial 60 min feeding test conducted under free-feeding (“non-deprived”) conditions, each rat was found to eat significantly more food after injection of 25 nMol norepinephrine (NE) into the PVN. In subsequent tests, the feeding increased significantly to 4 nMol clonidine; however, feeding was suppressed by 50 nMol clonidine. Food intake after 20 nMol clonidine was not significantly different from that recorded after vehicle. In contrast, phenylephrine (100–400 nMol) reliably suppressed feeding behavior. In the final phase of the study, the rats ate significantly less food after injection of 160 nMol PPA into the PVN but consumed significantly more food after a final injection of 25 nMol NE into the PVN. These results suggest that the anorexic action of PPA may be linked to activation of alpha-1 adrenergic receptors within the PVN.

Early clinical experience with clonidine in spinal spasticity.

The centrally active, alpha-2 adrenergic receptor agonist clonidine was given to 12 spinal cord injury patients with problematic spasticity not adequately controlled by recognized spasmolytic drug therapy. Five patients had an excellent reduction and 2 patients had some reduction in clinical spasticity (average dose 0.39 mg daily). Four of the 7 responders discontinued clonidine because of adverse reactions after an average of ten weeks of therapy. Three responders have continued to tolerate the drug well with excellent control of spasticity for 18 to 34 months. Five patients had no change in clinical spasticity (average dose of 0.24 mg daily). Three of the non-responders discontinued clonidine because of adverse reactions after an average of three weeks of therapy. Significant associated adverse reactions included syncopal seizures (3), cerebrovascular accident (1), deep vein thrombosis (1), autonomic hyperreflexia (3), lethargy/drowsiness (3), and nausea/vomiting (1). Possible mechanisms of action for clonidine to affect spasticity and the unstable cardiovascular system of quadriplegics is discussed. While spinal cord injured patients with severe spasticity may benefit from clonidine, great caution is recommended during its use until further study establishes safe parameters of administration and efficacy is confirmed on controlled studies.