6-Nitrodopamine potentiates contractions of rat isolated vas deferens induced by noradrenaline, adrenaline, dopamine and electric field stimulation

6-Nitrodopamine (6-ND) is a novel endogenous catecholamine that is released from the rat isolated vas deferens, and has been characterized as a major modulator of the contractility of rat isolated epididymal vas deferens (RIEVD). Drugs such as tricyclic antidepressants, α1 and β1β2 adrenoceptor blockers, act as selective antagonists of the 6-ND receptor in the RIEVD. In the rat isolated atria, 6-ND has a potent positive chronotropic action and causes remarkable potentiation of the positive chronotropic effects induced by dopamine, noradrenaline, and adrenaline. Here, whether 6-ND interacts with the classical catecholamines in the rat isolated vas deferens was investigated. Incubation with 6-ND (0.1 and 1 nM; 30min) caused no contractions in the RIEVD but provoked significant leftward shifts in the concentration-response curves to noradrenaline, adrenaline, and dopamine. Pre-incubation of the RIEVD with 6-ND (1 nM), potentiated the contractions induced by electric-field stimulation (EFS), whereas pre-incubation with 1 nM of dopamine, noradrenaline or adrenaline, did not affect EFS-induced contractions. In tetrodotoxin (1 μM) pre-treated (30 min) RIEVD, pre-incubation with 6-ND (0.1 nM) did not cause leftward shifts in the concentration-dependent contractions induced by noradrenaline, adrenaline, or dopamine. Pre-incubation of the RIEVD with the α2A-adrenoceptor antagonist idazoxan (30 min, 10 nM) did not affect dopamine, noradrenaline, adrenaline, and EFS-induced contractions. However, when idazoxan (10 nM) and 6-ND (0.1 nM) were simultaneously pre-incubated (30 min), a significant potentiation of the EFS-induced contractions of the RIEVD was observed. 6-nitrodopamine causes remarkable potentiation of dopamine, noradrenaline, and adrenaline contractions on the RIEVD, due to activation of adrenergic terminals, possibly via pre-synaptic adrenoceptors.

Alpha-2A but not 2B/C noradrenergic receptors in ventral tegmental area regulate phasic dopamine release in nucleus accumbens core

Adrenergic receptors (AR) in the ventral tegmental area (VTA) modulate local neuronal activity and, as a consequence, dopamine (DA) release in the mesolimbic forebrain. Such modulation has functional significance: intra-VTA blockade of α₁-AR attenuates behavioral responses to salient environmental stimuli in rat models of drug seeking and conditioned fear as well as phasic DA release in the nucleus accumbens (NAc). In contrast, α₂-AR in the VTA has been suggested to act primarily as autoreceptors, limiting local noradrenergic input. The regulation of noradrenaline efflux by α₂-AR could be of clinical interest, as α₂-AR agonists are proposed as promising pharmacological tools in the treatment of PTSD and substance use disorder. Thus, the aim of our study was to determine the subtype-specificity of α₂-ARs in the VTA capable of modulating phasic DA release. We used fast scan cyclic voltammetry (FSCV) in anaesthetized male rats to measure DA release in the NAc after combined electrical stimulation and infusion of selected α₂-AR antagonists into the VTA. Intra-VTA microinfusion of idazoxan - a non-subtype-specific α₂-AR antagonist, as well as BRL-44408 - a selective α_2A-AR antagonist, attenuated electrically-evoked DA in the NAc. In contrast, local administration of JP-1302 or imiloxan (α_2B- and α_2C-AR antagonists, respectively) had no effect. The effect of BRL-44408 on DA release was attenuated by intra-VTA DA D₂ antagonist (raclopride) pre-administration. Finally, we confirmed the presence of α_2A-AR protein in the VTA using western blotting. In conclusion, these data specify α_2A-, but not α_2B- or α_2C-AR as the receptor subtype controlling NA release in the VTA.

Hippocampal monoamine changes in the Flinders sensitive line rat: A case for the possible use of selective α2C-AR-antagonists in stress and anxiety disorders in companion animals

Non-selective α₂-adrenoreceptor (AR) stimulation delivers favourable sedative, analgesic, muscle relaxant and anxiolytic actions in companion animals, but is associated with cardiovascular and respiratory side effects. Anxiety conditions underscore monoamine disturbances amenable to α₂-AR modulation. We investigated sub-chronic (14 day s.c.) treatment with the selective α_2C-AR antagonist, ORM-10921 (0.03, 0.1, 0.3 mg/kg/d) on hippocampal noradrenaline (NA), dopamine (DA), serotonin (5-HT) and their turnover levels in stress sensitive Flinders Sensitive Line (FSL) rats versus Flinders Resistant Line (FRL) controls, using high performance liquid chromatography. The effects of ORM-10921 were compared to the non-selective α₂-AR antagonist, idazoxan (IDAZ; 3 mg/kg/d), and to imipramine (IMI; 15 mg/kg/d), a reference antidepressant in this model. FSL rats displayed significantly reduced 5-HT (p = 0.03) and DA (p = 0.02) levels vs. FRL controls, while NA levels showed a similar trend. ORM-10921 significantly increased NA (all doses p ≤ 0.02), 5-HT (0.1 and 0.3 mg/kg p ≤ 0.03) and DA levels (all doses p ≤ 0.03), which correlated with decreased monoamine turnover. In contrast, IDAZ significantly elevated NA (p < 0.005) and DA (p < 0.004) but not 5-HT levels. IMI also significantly increased 5-HT (p < 0.009), with a tendency to increase NA (p = 0.09) but not DA. ORM-10921 exerts similar albeit broader effects on hippocampal monoamines than IDAZ, explaining earlier established efficacy associated with α_2C-AR antagonism in animal models of depression and cognitive dysfunction. These and the current studies encourage application of ORM-10921 in depression in humans, as well as raise the intriguing possibility that selective α_2C-AR antagonists may be beneficial in anxiety and stress-related disorders in companion animals. Both warrant further study.

Effects of imidazoline I2 receptor agonists on reserpine-induced hyperalgesia and depressive-like behavior in rats

Pharmacotherapies for fibromyalgia treatment are lacking. This study examined the antinociceptive and antidepressant-like effects of imidazoline I2 receptor (I2R) agonists in a reserpine-induced model of fibromyalgia in rats. Rats were treated for 3 days with vehicle or reserpine. The von Frey filament test was used to assess the antinociceptive effects of I2 receptor agonists, and the forced swim test was used to assess the antidepressant-like effects of these drugs. 2-BFI (3.2-10 mg/kg, intraperitoneally), phenyzoline (17.8-56 mg/kg, intraperitoneally), and CR4056 (3.2-10 mg/kg, intraperitoneally) all dose-dependently produced significant antinociceptive effects, which were attenuated by the I2R antagonist idazoxan. Only CR4056 significantly reduced the immobility time in the forced swim test in both vehicle-treated and reserpine-treated rats. These data suggest that I2R agonists may be useful to treat fibromyalgia-related pain and comorbid depression.

Analgesic Effect of Melittin on Oxaliplatin-Induced Peripheral Neuropathy in Rats

Oxaliplatin is a chemotherapeutic agent used for metastatic colon and other advanced cancers. Most common side effect of oxaliplatin is peripheral neuropathy, manifested in mechanical and cold allodynia. Although the analgesic effect of bee venom has been proven to be effective against oxaliplatin-induced peripheral neuropathy, the effect of its major component; melittin has not been studied yet. Thus, in this study, we investigated whether melittin has an analgesic effect on oxaliplatin-induced allodynia. Intraperitoneal single injection of oxaliplatin (6 mg/kg) induced mechanical and cold allodynia, resulting in increased withdrawal behavior in response to von Frey filaments and acetone drop on hind paw. Subcutaneous melittin injection on acupoint ST36 (0.5 mg/kg) alleviated oxaliplatin-induced mechanical and cold allodynia. In electrophysiological study, using spinal in vivo extracellular recording, it was shown that oxaliplatin-induced hyperexcitation of spinal wide dynamic range neurons in response to peripheral stimuli, and melittin administration inhibited this neuronal activity. In behavioral assessment, analgesic effect of melittin was blocked by intrathecal α1- and α2- adrenergic receptor antagonists administration. Based on these results, we suggest that melittin could be used as an analgesic on oxaliplatin-induced peripheral neuropathy, and that its effect is mediated by activating the spinal α1- and α2-adrenergic receptors.

Microinjection of 26RFa, an endogenous ligand for the glutamine RF-amide peptide receptor (QRFP receptor), into the rostral ventromedial medulla (RVM), locus coelureus (LC), and periaqueductal grey (PAG) produces an analgesic effect in rats

26RFa is an endogenous ligand for the QRFP receptor. We previously found that intracerebroventricular injection of 26RFa produces an analgesic effect in a rat formalin test. In the present study, we directly tested the hypothesis that the analgesic effects of 26RFa in the formalin test are mediated in well-recognized regions of the descending inhibitory pain pathways, such as the rostral ventromedial medulla (RVM), locus coeruleus (LC), and periaqueductal grey (PAG) in rats. Injection cannulae were stereotaxically placed in the RVM, LC, or PAG through a burr hole. 26RFa (15 μg) or saline was delivered in a total volume of 0.5 μL. In a formalin test, 50 μL of 5% formalin was injected subcutaneously into the hind paw. In an antagonist study, idazoxan, an α-2 antagonist, or naloxone, an opioid receptor antagonist, was administered. Microinjection of 26RFa into the RVM had no effect compared with that in saline-injected rats. Microinjection of 26RFa into the LC contralateral, but not ipsilateral, to the formalin injection site significantly decreased the number of flinching behaviors compared with that of saline-injected rats. This effect was antagonized by intrathecal injection of idazoxan. Microinjection of 26RFa into the contralateral, but not ipsilateral, PAG produced an analgesic effect, and this effect was partly antagonized by intraperitoneal naloxone. These data suggest that 26RFa microinjected into the contralateral LC induced noradrenaline release in the spinal cord and produced an analgesic effect. In the contralateral PAG, 26RFa activated the opioid system, and some analgesic effects were mediated by opioid system activation.

Suppressive Effects of Bee Venom Acupuncture on Paclitaxel-Induced Neuropathic Pain in Rats: Mediation by Spinal α₂-Adrenergic Receptor

Paclitaxel, a chemotherapy drug for solid tumors, induces peripheral painful neuropathy. Bee venom acupuncture (BVA) has been reported to have potent analgesic effects, which are known to be mediated by activation of spinal α-adrenergic receptor. Here, we investigated the effect of BVA on mechanical hyperalgesia and spinal neuronal hyperexcitation induced by paclitaxel. The role of spinal α-adrenergic receptor subtypes in the analgesic effect of BVA was also observed. Administration of paclitaxel (total 8 mg/kg, intraperitoneal) on four alternate days (days 0, 2, 4, and 6) induced significant mechanical hyperalgesic signs, measured using a von Frey filament. BVA (1 mg/kg, ST36) relieved this mechanical hyperalgesia for at least two hours, and suppressed the hyperexcitation in spinal wide dynamic range neurons evoked by press or pinch stimulation. Both melittin (0.5 mg/kg, ST36) and phospholipase A2 (0.12 mg/kg, ST36) were shown to play an important part in this analgesic effect of the BVA, as they significantly attenuated the pain. Intrathecal pretreatment with the α₂-adrenergic receptor antagonist (idazoxan, 50 µg), but not α₁-adrenergic receptor antagonist (prazosin, 30 µg), blocked the analgesic effect of BVA. These results suggest that BVA has potent suppressive effects against paclitaxel-induced neuropathic pain, which were mediated by spinal α₂-adrenergic receptor.

[The anti-inflammatory effects of idazoxan on inflammatory mediator release in endotoxin-challenged mice in vivo and activated macrophages in vitro]

OBJECTIVE

To study the anti-inflammatory effects of idazoxan (IDA) on endotoxin lipopolysaccharide (LPS) challenged mice in vivo and activated macrophages in vitro, and explore its potential molecular mechanisms.

METHODS

To do the experiments in vivo,30 adult male C57BL/6 mice were randomly divided into control group, model group, and low,medium and high doses IDA groups (IDA-L,IDA-M, and IDA-H groups),n =6 in each group. The inflammatory model was reproduced by intraperitoneal injection of LPS 10 mg/kg, and the control group was injected with the same amount of normal saline. The IDA groups received LPS (10 mg/kg) and IDA 0.3,1.0 and 3.0 mg/kg, respectively. The blood samples of mice in each group were collected at 6 hours after the reproduction of the model .For the in vitro experiments, primary peritoneal macrophages were collected from 20 adult male C57BL/6 mouse cells and they were divided into control group, LPS group (10 mg/L) and LPS+IDA-L,IDA-M,IDA-H groups (10 mg/L LPS + 5,25,100 μmol/L IDA, respectively).Cell culture supernatants were collected at 24 hours after the reproduction of the model. Detection methods: enzyme linked immunosorbent assay (ELISA) was used to determine the levels of serum tumor necrosis factor-α (TNF-α),interleukin-6 (IL-6),monocyte chemotactic protein-1 (MCP-1) and nitric oxide (NO).Western Blot was used to determine the effect of IDA on the expression levels of nuclear factor-κB (NF-κB) in macrophages.

RESULTS

① For the in vivo experiment, the serum levels of TNF-α and IL-6 were significantly elevated in the model group as compared with those in the control group [TNF-o (ng/L):403.96 ± 40.98 vs.17.50 ± 8.68;IL-6 (ng/L):61 400.31 ± 7 826.61 vs.2 436.30 ± 448.89;both P ＜ 0.01].IDA treatment could inhibit the elevation of inflammatory cytokines in a dose-dependent manner, with the most significant decrease in LPS+IDA-H group [TNF-α (ng/L):170.09 ± 28.53 vs.403.96 ± 40.98,IL-6 (ng/L):16 570.81 ± 1 083.65 vs.61 400.31± 7 826.61;both P ＜ 0.01].② For the in vitro experiment, the levels of TNF-α,IL-6,MCP-1,and NO secreted by LPS-stimulated macrophages were distinctly higher in the LPS group than those in the control group [TNF-α (ng/L):7 259.14 ± 320.70 vs.28.50±27.08,IL-6 (ng/L):14809.60±5852.73 vs.1 113.47±465.53,MCP-1 (ng/L):20847.37± 1 788.33 vs.447.37± 395.69,NO (μmol/L):1 900.00 ± 144.31 vs.603.03 ± 102.18;all P ＜ 0.01]. However, IDA intervention could lower the secretion of TNF-α,IL-6,MCP-1 and NO in a dose-dependent manner, with the most notable decrease in the LPS+IDA-H group [TNF-α (ng/L):784.40±281.90 vs.7259.14±320.70,IL-6 (ng/L):1 802.96± 1 534.18 vs.14 809.60± 5 852.73,MCP-1 (ng/L):2005.26± 1 534.28 vs.20847.37 ± 1 788.33,NO (μ mol/L):654.54± 150.21 vs.1 900.00 ± 144.31;all P ＜ 0.05].In addition, IDA at the concentration of 100 μmol/L could promote the translocation of NF-κBp65 in macrophages into the nucleus 15 minutes early and lead to increased NF-κBp65 expression (gray value:18.70 ± 2.29 vs.1.09 ± 0.36,P ＜ 0.05),hut significantly reduce the expression levels of NF-κBp50 in the nucleus at 45 minutes after treatment (gray value:1.99 ± 0.14 vs.2.94 ± 0.54,P ＜ 0.05).

CONCLUSIONS

IDA could significantly reduce inflammation of mice challenged with LPS and inhibit inflammatory cytokines and mediators secreted by macrophage in a dose-dependent manner. High concentration of IDA (100 μmol/L) exhibited the greatest anti-inflammatory effects. The anti-inflammatory effect of IDA may be worked through NF-κB signaling pathway.

Interdependent adrenergic receptor regulation of Arc and Zif268 mRNA in cerebral cortex

Norepinephrine is a neurotransmitter that signals by stimulating the α1, α2 and β adrenergic receptor (AR). We determined the role of these receptors in regulating the immediate early genes, Activity Regulated Cytoskeleton Associated Protein (Arc) and Zif268 in the rat cerebral cortex. RX821002, an α2-AR antagonist, produced Arc and Zif268 elevations across cortical layers. Next we examined the effects of delivering RX821002 with an α1-AR antagonist, prazosin, and a β-AR antagonist, propranolol. RX821002 given with a prazosin and propranolol cocktail, or with each of these antagonists individually, decreased Arc and Zif268 to saline-treated control levels in most cortical layers. Arc and Zif268 levels were also similar to saline-treated control levels when rats were given a prazosin and propranolol cocktail alone, or when each of these antagonists were delivered individually. Taken together, these data reveal that α2-AR uniquely exert a tonic inibitory regulation of both Arc and Zif268 compared to α1 and β-AR. However, the ability of RX821002 to increase Arc and Zif268 is interdependent with α1 and β-AR signaling.

Clozapine reconstructed: Haloperidol's ability to reduce alcohol intake in the Syrian golden hamster can be enhanced through noradrenergic modulation by desipramine and idazoxan

BACKGROUND

Alcohol use disorder commonly occurs in patients with schizophrenia. Most antipsychotic drugs do not lessen alcohol use; although the atypical antipsychotic clozapine has been shown to reduce alcohol use in patients with schizophrenia, its toxicity severely limits its use in patients. With an eye toward creation of a safer clozapine-like drug, we have investigated the pharmacological basis of the clozapine's effects on alcohol drinking in the Syrian golden hamster. In this animal, as in patients with schizophrenia, clozapine reduces alcohol drinking while the typical antipsychotic haloperidol does not. We have suggested that clozapine decreases alcohol drinking due to its weak dopamine D2 receptor blockade, its potent norepinephrine α-2 receptor antagonism, as well as its ability to elevate plasma norepinephrine.

METHODS

We recreated a clozapine-like drug to reduce alcohol drinking in the Syrian golden hamster by combining low dose haloperidol with a norepinephrine α-2 receptor antagonist, idazoxan, and a norepinephrine reuptake inhibitor, desipramine. Hamsters were given free access to water and alcohol (15% v/v) and were treated daily with each drug or with the three-drug combination for 23 days.

RESULTS

The drug combination reduced alcohol drinking and preference significantly as compared to vehicle or to haloperidol, idazoxan or desipramine, while not altering food-intake or body-weight.

CONCLUSION

These findings suggest that that haloperidol, which does not reduce alcohol drinking in patients with schizophrenia or the hamster, if combined with idazoxan and desipramine (producing a drug combination that mimics aspects of clozapine's pharmacology) is able to reduce alcohol drinking in the hamster.

Spinal cord stimulation modulates supraspinal centers of the descending antinociceptive system in rats with unilateral spinal nerve injury

BACKGROUND

The descending antinociceptive system (DAS) is thought to play crucial roles in the antinociceptive effect of spinal cord stimulation (SCS), especially through its serotonergic pathway. The nucleus raphe magnus (NRM) in the rostral ventromedial medulla is a major source of serotonin [5-hydroxytryptamine (5-HT)] to the DAS, but the role of the dorsal raphe nucleus (DRN) in the ventral periaqueductal gray matter is still unclear. Moreover, the influence of the noradrenergic pathway is largely unknown. In this study, we evaluated the involvement of these serotonergic and noradrenergic pathways in SCS-induced antinociception by behavioral analysis of spinal nerve-ligated (SNL) rats. We also investigated immunohistochemical changes in the DRN and locus coeruleus (LC), regarded as the adrenergic center of the DAS, and expression changes of synthetic enzymes of 5-HT [tryptophan hydroxylase (TPH)] and norepinephrine [dopamine β-hydroxylase (DβH)] in the spinal dorsal horn.

RESULTS

Intrathecally administered methysergide, a 5-HT1- and 5-HT2-receptor antagonist, and idazoxan, an α2-adrenergic receptor antagonist, equally abolished the antinociceptive effect of SCS. The numbers of TPH-positive serotonergic and phosphorylated cyclic AMP response element binding protein (pCREB)-positive neurons and percentage of pCREB-positive serotonergic neurons in the DRN significantly increased after 3-h SCS. Further, the ipsilateral-to-contralateral immunoreactivity ratio of DβH increased in the LC of SNL rats and reached the level seen in naïve rats, even though the number of pCREB-positive neurons in the LC was unchanged by SNL and SCS. Moreover, 3-h SCS did not increase the expression levels of TPH and DβH in the spinal dorsal horn.

CONCLUSIONS

The serotonergic and noradrenergic pathways of the DAS are involved in the antinociceptive effect of SCS, but activation of the DRN might primarily be responsible for this effect, and the LC may have a smaller contribution. SCS does not potentiate the synthetic enzymes of 5HT and norepinephrine in the neuropathic spinal cord.

[Allosteric effect of serotonin and mianserin on the kinetics of specific [3H]-ligand binding to adrenergic and muscarinic receptors in the rat cerebral cortex membranes]

The effects of serotonin receptor activation (by serotonin) and inhibition (by mianserin) on the properties of the α1-, α2-adrenoreceptors, and muscarinic cholinergic receptors in subcellular membrane fractions from the rat cerebral cortex were studied. Experimental data on the kinetics of specific antagonists binding to adrenergic and muscarinic receptors were analyzed by graphical and mathematical methods. The results suggest the presence of allosteric (cross-talk) interaction. In the control, α1- and α2-adrenoreceptors were represented by a single pool, and muscarinic receptors, by two pools. Two pools of adrenoreceptors with different affinity were detected against the background of serotonin. It was found that mianserin induces the formation of two pools of only (α2-receptors and muscarinic receptors are represented by two pools differing in the main parameters, such as dissociation constants and adrenoreceptor concentrations, in the control and experimental groups. It was shown that the allosteric effect of serotonin and mianserin is manifested in the inhibition of muscarinic receptors. It was assumed that the adrenergic and cholinergic receptors exist as dimers. The interaction between the adrenergic, cholinergic, and serotonergic systems is likely to be implemented at the cell membrane level.

Imidazoline receptor antagonists idazoxan and efaroxan enhance locomotor functions in rats

Discovered in 1984, imidazoline receptors (I1, I2, I3) are located centrally and peripherally being involved in various physiologic and pathophysiologic processes in the body. Experimental and clinical investigations have suggested the interrelations between imidazoline, adrenergic, dopaminergic, glutamatergic and opioid systems, which may explain the influence of different substances acting on imidazoline receptors in cognitive disorders, behavioral disturbances and motor diseases pathways.

AIM

To investigate the effects of two imidazoline receptor antagonists on locomotor activity and endurance capacity in rats.

MATERIAL AND METHODS

The experiment was carried out with white male Wistar rats (200-250 g) divided into 3 groups of 7 animals each, treated intraperitonealy with the same volume of solution as follows: Group I (Control): distilled water 0.3 ml/100 g body weight; Group II (IDZ): idazoxan 3 mg/kbw; Group III (EFR): efaroxan 1 mg/kbw. Exercise capacity was evaluated using a locomotor PanLAB treadmill test. The data were presented as mean +/- standard deviation and significance was tested by SPSS Statistics for Windows version 17.0 and ANOVA method. Experimental protocol was implemented according to recommendations of the Gr.T. Popa" University Committee for Research and Ethical Issues.

RESULTS

Intraperitoneal administration of idazoxan and efarox- an resulted in a significant increase in running distance compared with the control group (p < 0.05). At the same time a reduction in the number and time of electric shocks delivered to motivate the animal to keep running was observed. In this experimental behavioral model the effects of idazoxan on the evaluated parameters were more intense than those of efaroxan.

CONCLUSIONS

In our experimental conditions we demonstrated the ability of imidazoline receptor antagonists idazoxan and efaroxan to improve fatigue resistance during forced running in rats.

Amphetamine and cocaine suppress social play behavior in rats through distinct mechanisms

RATIONALE

Social play behavior is a characteristic form of social behavior displayed by juvenile and adolescent mammals. This social play behavior is highly rewarding and of major importance for social and cognitive development. Social play is known to be modulated by neurotransmitter systems involved in reward and motivation. Interestingly, psychostimulant drugs, such as amphetamine and cocaine, profoundly suppress social play, but the neural mechanisms underlying these effects remain to be elucidated.

OBJECTIVE

In this study, we investigated the pharmacological underpinnings of amphetamine- and cocaine-induced suppression of social play behavior in rats.

RESULTS

The play-suppressant effects of amphetamine were antagonized by the alpha-2 adrenoreceptor antagonist RX821002 but not by the dopamine receptor antagonist alpha-flupenthixol. Remarkably, the effects of cocaine on social play were not antagonized by alpha-2 noradrenergic, dopaminergic, or serotonergic receptor antagonists, administered either alone or in combination. The effects of a subeffective dose of cocaine were enhanced by a combination of subeffective doses of the serotonin reuptake inhibitor fluoxetine, the dopamine reuptake inhibitor GBR12909, and the noradrenaline reuptake inhibitor atomoxetine.

CONCLUSIONS

Amphetamine, like methylphenidate, exerts its play-suppressant effect through alpha-2 noradrenergic receptors. On the other hand, cocaine reduces social play by simultaneous increases in dopamine, noradrenaline, and serotonin neurotransmission. In conclusion, psychostimulant drugs with different pharmacological profiles suppress social play behavior through distinct mechanisms. These data contribute to our understanding of the neural mechanisms of social behavior during an important developmental period, and of the deleterious effects of psychostimulant exposure thereon.

Anatomical and pharmacological characterization of catecholamine transients in the medial prefrontal cortex evoked by ventral tegmental area stimulation

Voltammetric measurements of catecholamines in the medial prefrontal cortex (mPFC) are infrequent because of lack of chemical selectivity between dopamine and norepinephrine and their overlapping anatomical inputs. Here, we examined the contribution of norepinephrine to the catecholamine release in the mPFC evoked by electrical stimulation of the ventral tegmental area (VTA). Initially, electrical stimulation was delivered in the midbrain at incremental depths of -5 to -9.4 mm from bregma while catecholamine release was monitored in the mPFC. Although catecholamine release was observed at dorsal stimulation sites that may correspond to the dorsal noradrenergic bundle (DNB, containing noradrenergic axonal projections to the mPFC), maximal release was evoked by stimulation of the VTA (the source of dopaminergic input to the mPFC). Next, VTA-evoked catecholamine release was monitored in the mPFC before and after knife incision of the DNB, and no significant changes in the evoked catecholamine signals were found. These data indicated that DNB fibers did not contribute to the VTA-evoked catecholamine release observed in the mPFC. Finally, while the D2-receptor antagonist raclopride significantly altered VTA-evoked catecholamine release, the α₂-adrenergic receptor antagonist idazoxan did not. Specifically, raclopride reduced catecholamine release in the mPFC, opposite to that observed in the striatum, indicating differential autoreceptor regulation of mesocortical and mesostriatal neurons. Together, these findings suggest that the catecholamine release in the mPFC arising from VTA stimulation was predominately dopaminergic rather than noradrenergic.

Both α1- and α2-adrenoceptors in the insular cortex are involved in the cardiovascular responses to acute restraint stress in rats

The insular cortex (IC) is a limbic structure involved in cardiovascular responses observed during aversive threats. However, the specific neurotransmitter mediating IC control of cardiovascular adjustments to stress is yet unknown. Therefore, in the present study we investigated the role of local IC adrenoceptors in the cardiovascular responses elicited by acute restraint stress in rats. Bilateral microinjection of different doses (0.3, 5, 10 and 15 nmol/100 nl) of the selective α1-adrenoceptor antagonist WB4101 into the IC reduced both the arterial pressure and heart rate increases elicited by restraint stress. However, local IC treatment with different doses (0.3, 5, 10 and 15 nmol/100 nl) of the selective α2-adrenoceptor antagonist RX821002 reduced restraint-evoked tachycardia without affecting the pressor response. The present findings are the first direct evidence showing the involvement of IC adrenoceptors in cardiovascular adjustments observed during aversive threats. Our findings indicate that IC noradrenergic neurotransmission acting through activation of both α1- and α2-adrenoceptors has a facilitatory influence on pressor response to acute restraint stress. Moreover, IC α1-adrenoceptors also play a facilitatory role on restraint-evoked tachycardiac response.

Evidences for the agmatine involvement in antidepressant like effect of bupropion in mouse forced swim test

Although bupropion has been widely used in the treatment of depression, the precise mechanism of its therapeutic actions is not fully understood. The present study investigated the role of agmatine in an antidepressant like effect of bupropion in mouse forced swim test. The antidepressant like effect of bupropion was potentiated by pretreatment with agmatine (10-20mg/kg, ip) and by the drugs known to increase endogenous agmatine levels in brain viz., l-arginine (40 μg/mouse, icv), an agmatine biosynthetic precursor, ornithine decarboxylase inhibitor, dl-α-difluoromethyl ornithine hydrochloride, DFMO (12.5 μg/mouse, icv), diamine oxidase inhibitor, aminoguanidine (6.5 μg/mouse, icv) and agmatinase inhibitor, arcaine (50 μg/mouse, icv) as well as imidazoline I1 receptor agonists, moxonidine (0.25mg/kg, ip) and clonidine (0.015 mg/kg, ip) and imidazoline I2 receptor agonist, 2-(2-benzofuranyl)-2-imidazoline hydrochloride, 2-BFI (5mg/kg, ip). Conversely, prior administration of I1 receptor antagonist, efaroxan (1mg/kg, ip) and I2 receptor antagonist, idazoxan (0.25mg/kg, ip) blocked the antidepressant like effect of bupropion and its synergistic combination with agmatine. These results demonstrate involvement of agmatine in the antidepressant like effect of bupropion and suggest agmatine and imidazoline receptors as a potential therapeutic target for the treatment of depressive disorders.

Intestinal fatty acid infusion modulates food preference as well as calorie intake via the vagal nerve and midbrain-hypothalamic neural pathways in rats

The intestine plays important roles in the regulation of feeding behavior by sensing macronutrients. Intestinal fatty acids strongly suppress food intake, but little is known about whether intestinal fatty acids affect food preference. We investigated the effects of jejunal fatty acids infusion on food preference by conducting two-diet choice experiments in rats fed a high-fat diet (HFD) and a high-carbohydrate diet (HCD). Jejunal linoleic acid (18:2) infusion reduced HFD intake dose-dependently, while HCD intake increased with the middle dose of the infusion we examined (100 μL/h) and reduced to the control level with the higher doses (150 and 200 μL/h). α-Linolenic acid (18:3), but not caprylic acid (8:0), altered the food preference and total calorie intake in the same manner as linoleic acid. Linoleic acid infusion dose-dependently increased plasma glucagon-like peptide-1, peptide YY and cholecystokinin levels, but not ghrelin levels. Subdiaphragmatic vagotomy or midbrain transection prevented the change in food preference and total calorie intake by linoleic acid infusion. Jejunal linoleic acid infusion increased norepinephrine turnover in the paraventricular hypothalamic nucleus, while intracerebroventricular injection of idazoxan, an α2-adrenergic receptor (AR) antagonist, suppressed the increased HCD intake, but did not affect the decreased HFD intake. These findings indicated that intestinal long-chain fatty acids modulated food preference as well as total calorie intake via the vagal nerve and midbrain-hypothalamic neural pathways. The effects of the α2-AR antagonist in the brain suggested that the brain distinctly controlled HCD and HFD intake in response to jejunal linoleic acid infusion.

Effects of (±)-Idazoxan Alone and in Combination with L-DOPA Methyl Ester in MPTP-Induced Hemiparkinsonian Monkeys

The effects of a combination of the alpha2-adrenergic receptor antagonist (+/-)-idazoxan with L-DOPA methyl ester were examined in three of four female adult monkeys (Macaca nemestrina) rendered hemiparkinsonian with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). (+/-)-Idazoxan (0.16, 0.63, and 1.0 mg/kg) was given i.m. 10 min before L-DOPA methyl ester (12.5 mg/kg). (+/-)-Idazoxan reduced the maximum peak of contralateral circling elicited during the first hour following injection of L-DOPA methyl ester, but prolonged the duration of the circling response up to 50% (p < 0.05). The data support a role for alpha2-adrenergic receptor mechanisms in modulating the effects of L-DOPA on nigrostriatal dopamine function in the MPTP monkey model of hemiparkinsonism.

Spontaneous spiking and synaptic depression underlie noradrenergic control of feed-forward inhibition

Inhibitory interneurons across diverse brain regions commonly exhibit spontaneous spiking activity, even in the absence of external stimuli. It is not well understood how stimulus-evoked inhibition can be distinguished from background inhibition arising from spontaneous firing. We found that noradrenaline simultaneously reduced spontaneous inhibitory inputs and enhanced evoked inhibitory currents recorded from principal neurons of the mouse dorsal cochlear nucleus (DCN). Together, these effects produced a large increase in signal-to-noise ratio for stimulus-evoked inhibition. Surprisingly, the opposing effects on background and evoked currents could both be attributed to noradrenergic silencing of spontaneous spiking in glycinergic interneurons. During spontaneous firing, glycine release was decreased due to strong short-term depression. Elimination of background spiking relieved inhibitory synapses from depression and thereby enhanced stimulus-evoked inhibition. Our findings illustrate a simple yet powerful neuromodulatory mechanism to shift the balance between background and stimulus-evoked signals.

Role of the bed nucleus of the stria terminalis in the cardiovascular responses to acute restraint stress in rats

The aim of this work was to test the hypothesis that the bed nucleus of the stria terminalis (BST) and noradrenergic neurotransmission therein mediate cardiovascular responses to acute restraint stress in rats. Bilateral microinjection of the non-specific synaptic blocker CoCl(2) (0.1 nmol/100 nl) into the BST enhanced the heart rate (HR) increase associated with acute restraint without affecting the blood pressure increase, indicating that synapses within the BST influence restraint-evoked HR changes. BST pretreatment with the selective alpha(1)-adrenoceptor antagonist WB4101 (15 nmol/100 nl) caused similar effects to cobalt, indicating that local noradrenergic neurotransmission mediates the BST inhibitory influence on restraint-related HR responses. BST treatment with equimolar doses of the alpha(2)-adrenoceptor antagonist RX821002 or the beta-adrenoceptor antagonist propranolol did not affect restraint-related cardiovascular responses, reinforcing the inference that alpha(1)-adrenoceptors mediate the BST-related inhibitory influence on HR responses. Microinjection of WB4101 into the BST of rats pretreated intravenously with the anticholinergic drug homatropine methyl bromide (0.2 mg/kg) did not affect restraint-related cardiovascular responses, indicating that the inhibitory influence of the BST on the restraint-evoked HR increase could be related to an increase in parasympathetic activity. Thus, our results suggest an inhibitory influence of the BST on the HR increase evoked by restraint stress, and that this is mediated by local alpha(1)-adrenoceptors. The results also indicate that such an inhibitory influence is a result of parasympathetic activation.

[5-HT1A/1B receptors, alpha2-adrenoceptors and the post-receptor adenylate cyclase activation in the mice brain are involved in the antidepressant-like action of agmatine]

This study is to explore the possible mechanisms of the antidepressant-like effect of agmatine. By using two traditional "behavior despair" model, tail suspension test and forced swimming test, we examined the effects of some monoamine receptor antagonists (including beta-adrenergic receptor antagonist propranolol, beta-adrenergic receptor antagonist/5-HT1A/1B receptor antagonist pindolol, alpha2-adrenergic receptor antagonists yohimbine and idazoxan and 5-HT3 receptor antagonist tropisetron) on the antidepressant-like action of agmatine in mice. Activity of adenylate cyclase (AC) in the synapse membrane from rat frontal cortex was determined by radioimmunoassay. Single dose of agmatine (5-40 mg x kg(-1), ig) dose-dependently decrease the immobility time in tail suspension test in mice, indicating an antidepressant-like effect. The effect of agmatine (40 mg x kg(-1), ig) was antagonized by co-administration of beta-adrenergic receptor antagonist/5-HT1A/1B receptor antagonist pindolol (20 mg x kg(-1), ip), alpha2-adrenergic receptor antagonists yohimbine (5-10 mg x kg(-1), ip) or idazoxan (4 mg x kg(-1), ip), but not beta-adrenergic receptor antagonist propranolol (5-20 mg x kg(-1), ip) and 5-HT3 receptor antagonist tropisetron (5-40 mg x kg(-1), ip). Agmatine (5-40 mg x kg(-1), ig) also dose-dependently decrease the immobility time in forced swimming test in mice. The effect of agmatine (40 mg x kg(-1), ig) was also antagonized by pindolol (20 mg x kg(-1), ip), yohimbine (5-10 mg x kg(-1), ip), or idazoxan (4 mg x kg(-1), ip). Incubation of agmatine (0.1-6.4 micromol x L(-1)) with the synaptic membrane extracted from rat frontal cortex activated the AC in a dose-dependent manner in vitro. While the effect of agmatine (6.4 micromol x L(-1)) was dose-dependently antagonized by pindolol (1 micromol x L(-1)) or yohimbine (0.25-1 micromol x L(-1)). Chronic treatment with agmatine (10 mg x kg(-1), ig, bid, 2 w) or fluoxetine (10 mg x kg(-1), ig, bid, 2 w) increased the basic activity, as well as the Gpp (NH)p (1-100 micromol x L(-1)) stimulated AC activity in rat prefrontal cortex. These results indicate that regulation on 5-HT1A/1B and alpha2 receptors, and activation AC in the frontal cortex is one of the important mechanisms involving in agmatine's antidepressant-like action.

Agmatine inhibits morphine-induced locomotion sensitization and morphine-induced changes in striatal dopamine and metabolites in rats

The effects of agmatine on morphine-induced locomotion sensitization and morphine-induced changes in extracellular striatal dopamine (DA) and DA metabolites were studied. The locomotor response to morphine challenge (3 mg/kg, s.c.) was enhanced in rats 3 days after repeated morphine administration, indicating development of locomotion sensitization. In vivo microdialysis demonstrated a significant increase in striatal basal levels of the DA metabolites DOPAC and HVA, but not in DA itself, and an increase in DA response to morphine challenge in rats 3 days after withdrawal. Agmatine (1, 10, 80 mg/kg) inhibited morphine-induced locomotion sensitization and the changes in DA noted above. Idazoxan attenuated the effects of agmatine on locomotion, suggesting that the effects are mediated by imidazoline receptors. In addition, repeated morphine also increased the expression of tyrosine hydroxylase mRNA in the VTA after 4 days of morphine pretreatment, while decreasing the expression of dynorphin mRNA at 3 days after withdrawal. Agmatine inhibited morphine-induced changes in dynorphin, but not in tyrosine hydroxylase mRNA expression. These data suggest that agmatine, likely by activating imidazoline receptors, inhibits morphine-induced locomotion sensitization and morphine-induced changes in extracellular DA and in dynorphin expression. Thus, agmatine deserves further study as an anti-opioid medication.

Study into a possible mechanism responsible for the antidepressant-like activity of the selective 5-HT6 receptor antagonist SB-399885 in rats

The mechanism of the antidepressant-like activity of the selective 5-hydroxytryptamine(6) (5-HT(6) receptor antagonist N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)benzenesulfonamide (SB-399885) was studied in the forced swim test in rats. SB-399885 administered intraperitoneally at a single dose of 10 mg/kg potently shortened the immobility time in rats. That potential antidepressant-like effect of SB-399885 was not modified in animals with a lesion of the 5-HT system produced by p-chloroamphetamine (p-CA, 2 x 10 mg/kg). The anti-immobility effect of SB-399885 was blocked by the dopamine D(1)- and D(2)-like receptor antagonists SCH 23390 (0.063 mg/kg) and sulpiride (10 mg/kg), respectively, as well as by the alpha(2)-adrenoceptor antagonist idazoxan (4 mg/kg), but it was not changed by the alpha(1)-adrenoceptor antagonist prazosin (1 mg/kg). Neither sulpiride (10 mg/kg) or idazoxan (4 mg/kg) nor SCH-23390 (0.063 mg/kg) administered jointly with SB-399885 (10 mg/kg) noticeably changed the exploratory locomotor activity of rats evaluated by the open field test. The results described in the present paper indicate that the anti-immobility activity of SB-399885 is not connected with 5-HT innervation, and that D(1)- and D(2)-like receptors and alpha(2)-adrenoceptors are involved in this action.

Noradrenergic stimulation within midbrain raphe increases electrolyte excretion in rats

The present study was carried out to assess the influence of noradrenergic stimulation of the midbrain dorsal (DRN) and median raphe nuclei (MRN) on urinary volume and electrolyte excretion in hydrated rats. Wistar rats were implanted with a guide cannula into the MRN or DRN and then submitted to two intragastric administrations of water in order to attain an increased diuresis. The following treatments were performed. (1) Intra-DRN microinjections of saline (0.2 microl), alpha(1)-adrenergic agonist phenylephrine (PHE, 0.49 and 4.9 nmol in 0.2 microl), alpha(2)-adrenergic antagonist idazoxan (IDZ, 0.42 and 4.2 nmol in 0.2 microl) or the alpha(1)-adrenergic antagonist prazosin (PRZ, 0.24 and 2.4 nmol in 0.2 microl). (2) Intra-MRN microinjections of saline, IDZ (4.2 nmol in 0.2 microl), PHE (4.9 nmol in 0.2 microl) or PRZ (2.4 nmol in 0.2 microl). Urine samples were subsequently collected over 120 min at 20 min intervals for photometric measurement of sodium and potassium. Intra-DRN administration of PHE and IDZ significantly increased the urinary volume, natriuresis and kaliuresis. Intra-DRN microinjection of a higher dose of PRZ reduced the urinary volume and both sodium and potassium excretion. Intra-MRN microinjections of PHE, IDZ or PRZ did not induce any significant effect on urinary volume or electrolyte excretion. These data suggest that the increase of tonic excitatory noradrenergic input conveyed to DRN influences the hydroelectrolyte homeostasis, possibly through 5-HTergic circuitry.

Receptor-mediated activation of nitric oxide synthesis by arginine in endothelial cells

Arginine contains the guanidinium group and thus has structural similarity to ligands of imidazoline and alpha-2 adrenoceptors (alpha-2 AR). Therefore, we investigated the possibility that exogenous arginine may act as a ligand for these receptors in human umbilical vein endothelial cells and activate intracellular nitric oxide (NO) synthesis. Idazoxan, a mixed antagonist of imidazoline and alpha-2 adrenoceptors, partly inhibited L-arginine-initiated NO formation as measured by a Griess reaction. Rauwolscine, a highly specific antagonist of alpha-2 AR, at very low concentrations completely inhibited NO formation. Like L-arginine, agmatine (decarboxylated arginine) also activated NO synthesis, however, at much lower concentrations. We found that dexmedetomidine, a specific agonist of alpha-2 AR was very potent in activating cellular NO, thus indicating a possible role for alpha-2 AR in L-arginine-mediated NO synthesis. D-arginine also activated NO production and could be inhibited by imidazoline and alpha-2 AR antagonists, thus indicating nonsubstrate actions of arginine. Pertussis toxin, an inhibitor of G proteins, attenuated L-arginine-mediated NO synthesis, thus indicating mediation via G proteins. L-type Ca(2+) channel blocker nifedipine and phospholipase C inhibitor U73122 inhibited NO formation and thus implicated participation of a second messenger pathway. Finally, in isolated rat gracilis vessels, rauwolscine completely inhibited the L-arginine-initiated vessel relaxation. Taken together, these data provide evidence for binding of arginine to membrane receptor(s), leading to the activation of endothelial NO synthase (eNOS) NO production through a second messenger pathway. These findings provide a previously unrecognized mechanistic explanation for the beneficial effects of L-arginine in the cardiovascular system and thus provide new potential avenues for therapeutic development.

Disruption of noradrenergic transmission and the behavioural response to a novel environment in NK1R-/- mice

The behaviour of neurokinin-1-receptor gene knockout (NK1R-/-) mice, which lack functional, substance P-preferring receptors, resembles that of NK1R+/+ mice treated with an antidepressant. Because all antidepressants increase central monoamine transmission, we have investigated whether noradrenergic transmission is increased in NK1R-/- mice and, if so, whether this could influence their behaviour. In anaesthetized subjects, the concentration of extracellular noradrenaline in NK1R-/- mice was two-fourfold greater than in NK1R+/+ mice. Systemic administration of the alpha2-adrenoceptor antagonist, 2-(2,3-dihydro-2-methoxy-1,4-benzodioxan-2-yl)-4,5-dihydro-1H-imidazoline (RX 821002), in anaesthetized or freely moving animals increased extracellular noradrenaline in NK1R+/+ mice only. This suggests that the function of alpha2a-autoreceptors, which modulate noradrenergic transmission, is impaired in NK1R-/- mice. Consistent with this, [35S]GTPgammaS binding to activated alpha2a-adrenoceptors was lower (-70%) in the locus coeruleus, but not the frontal cortex, of NK1R-/- mice compared with their NK1R+/+ counterparts. RX 821002-pretreatment, followed by retrodialysis of the noradrenaline reuptake inhibitor, desipramine, into the frontal cortex of anaesthetized mice increased extracellular noradrenaline to the same extent in the two genotypes. Western blots confirmed that there was no difference in the amount of noradrenaline transporter protein in NK1R-/- and NK1R+/+ mice. Finally, the effects of RX 821002 on certain behaviours in a light/dark exploration box were blunted in NK1R-/- mice, but there was no consistent effect on anxiety-like behaviour in the two genotypes. It is concluded that the greater basal efflux of noradrenaline in NK1R-/- mice is explained by increased transmitter release, coupled with desensitization of somatodendritic alpha2a-adrenoceptors. These changes could contribute to the difference in the behavioural phenotypes.

Presynaptic α-adrenoceptors in median preoptic nucleus modulate inhibitory neurotransmission from subfornical organ and organum vasculosum lamina terminalis

The median preoptic nucleus (MnPO) in the lamina terminalis receives a prominent catecholaminergic innervation from the dorsomedial and ventrolateral medulla. The present investigation used whole cell patch-clamp recordings in rat brain slice preparations to evaluate the hypothesis that presynaptic adrenoceptors could modulate GABAergic inputs to MnPO neurons. Bath applications of norepinephrine (NE; 20–50 μM) induced a prolonged and reversible suppression of inhibitory postsynaptic currents (IPSCs) and reduced paired-pulse depression evoked by stimulation in the subfornical organ and organum vasculosum lamina terminalis. These events were not correlated with any observed changes in membrane conductance arising from NE activity at postsynaptic α1- or α2-adrenoceptors. Consistent with a role for presynaptic α2-adrenoceptors, responses were selectively mimicked by an α2-adrenoceptor agonist (UK-14304) and blockable with an α2-adrenoceptor antagonist (idazoxan). Although the α1-adrenoceptor agonist cirazoline and the α1-adrenoceptor antagonist prazosin were without effect on these evoked IPSCs, NE was noted to increase (via α1-adrenoceptors) or decrease (via α2-adrenoceptors) the frequency of spontaneous and tetrodotoxin-resistant miniature IPSCs. Collectively, these observations imply that both presynaptic and postsynaptic α1- and α2-adrenoceptors in MnPO are capable of selective modulation of rapid GABAA receptor-mediated inhibitory synaptic transmission along the lamina terminalis and therefore likely to exert a prominent influence in regulating cell excitability within the MnPO.

Agmatine blocks morphine-evoked hyperthermia in rats

The present study investigated the effect of agmatine on morphine-evoked hyperthermia in rats. Morphine (4 mg/kg, s.c.) produced hyperthermia by activating mu opioid receptors. Agmatine (10 and 50 mg/kg, i.p.) was ineffective. For combined administration, agmatine decreased morphine-evoked hyperthermia. The effect was prevented by idazoxan (5 mg/kg, i.p.), an imidazoline/alpha(2)-adrenoeceptor receptor antagonist. Yohimbine, an alpha(2)-adrenoeceptor antagonist, did not prevent the attenuation of morphine-evoked hyperthermia by agmatine. The present data provide pharmacological evidence that agmatine blocks the hyperthermic effect of morphine by activating imidazoline receptors.

In Vivo Effect of Venlafaxine on Locus Coeruleus Neurons: Role of Opioid, α2-Adrenergic, and 5-Hydroxytryptamine1A Receptors

The locus coeruleus (LC) is involved in several neural pathways responsible for some somatic and emotional processes, such as pain and depression; its activity is regulated by several receptors, such as opioid, alpha(2)-adrenergic, and 5-hydroxytryptamine (5-HT)(1A) receptors. The present study investigates the in vivo effects of venlafaxine, an antidepressant with analgesic properties, on locus coeruleus neurons, and its modulation by opioid, alpha(2)-adrenergic, and 5-HT(1A) receptors. The results show that acute administration of venlafaxine produced a dose-dependent, complete inhibition of LC activity. This inhibitory effect was not reversed by the opioid receptor antagonist naloxone, but subsequent administration of idazoxan, an alpha(2)-adrenoceptor antagonist, did reverse it. The preadministration of the 5-HT(1A) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) (1 and 40 microg/kg) significantly enhanced the venlafaxine inhibitory effect, decreasing the ED(50) by 56 and 44%, respectively. A 14-day treatment with venlafaxine (40 mg/kg/day) induced a suppression of the firing activity of LC neurons. In these treated animals, venlafaxine produced an inhibitory effect similar to that in nontreated animals. This inhibitory effect was not reversed by naloxone, but it was reversed by idazoxan. In addition, the preadministration of 8-OH-DPAT (40 microg/kg) significantly enhanced the venlafaxine effect, decreasing the ED(50) by 60%. These results suggest that the effect of venlafaxine on LC neurons is modulated by alpha(2)-adrenergic and 5-HT(1A) receptors, and not by opioid receptors. These data could contribute to the further understanding of the antidepressant and analgesic mechanism of action of venlafaxine.

Enhanced efficacy of both typical and atypical antipsychotic drugs by adjunctive alpha2 adrenoceptor blockade: experimental evidence

Adjunctive treatment with the selective alpha2 adrenoceptor antagonist idazoxan augments the effect of conventional antipsychotics in treatment-resistant schizophrenics comparing favourably with clozapine. Clozapine has high affinity for alpha2 adrenoceptors. Previously, we found that adjunctive idazoxan treatment to the dopamine (DA) D2/3 antagonist raclopride enhanced raclopride-induced effects in an animal model of antipsychotic activity (conditioned avoidance response, CAR) and, similarly to clozapine, reversed the disruption of working memory induced by N-methyl-D-aspartate receptor blockade in rats with a concomitant increase in prefrontal DA efflux. To further investigate the significance of alpha2 adrenoceptor affinity for antipsychotic efficacy, we here investigated, in rats, the effects of adjunctive idazoxan treatment to low doses of a typical (haloperidol) and an atypical (olanzapine) antipsychotic drug, both lacking appreciable alpha2 adrenoceptor affinity, on (i) CAR; (ii) catalepsy; and (iii) DA output in the prefrontal cortex and the nucleus accumbens using microdialysis. Adjunctive treatment with idazoxan to haloperidol or olanzapine enhanced suppression of CAR to a level predicting sufficient antipsychotic activity, increased DA output preferentially in the prefrontal cortex, and reversed haloperidol-induced catalepsy. Our data confirm and extend our previous findings as well as clinical observations, and suggest that adjunctive alpha2 adrenoceptor blockade both typical and atypical antipsychotic drugs, lacking appreciable affinity for the alpha2 adrenoceptor, may contribute to a more advantageous therapeutical profile of these drugs in schizophrenia treatment, allowing for reduced DA D2 occupancy and reduction of unwanted side-effects.

Contribution of imidazoline receptors and alpha2-adrenoceptors in the rostral ventrolateral medulla to sympathetic baroreflex inhibition by systemic rilmenidine

OBJECTIVES

To determine whether the hypotensive and sympathetic baroreflex inhibition by rilmenidine administered systemically are mediated via imidazoline receptors in the rostral ventrolateral medulla (RVLM).

METHODS

Initial dose-response curves to rilmenidine were determined in urethane anaesthetized rabbits. Effects of a single intravenous dose of rilmenidine (445 microg/kg) on the renal sympathetic nerve activity (RSNA) baroreflex were examined before and after microinjection into the RVLM of the mixed imidazoline/alpha2-adrenoceptor antagonist idazoxan and the alpha2-adrenoceptor antagonist 2-methoxyidazoxan (2-MI).

RESULTS

Intravenous administration of rilmenidine lowered mean arterial pressure and RSNA, inhibited the RSNA baroreflex range by 33% and shifted the baroreflex curve to the left. Idazoxan injected into the RVLM reversed the hypotension and completely restored the baroreflex curve at doses that did not affect the hypotension produced by the selective alpha2-adrenoceptor agonist alpha-methylnoradrenaline. The alpha2-adrenoceptor antagonist, 2-MI also reversed the rilmenidine sympatho-inhibition suggesting that alpha2-adrenoceptors are activated as well.

CONCLUSIONS

The results of the present study show that the hypotensive and sympatho-inhibitory actions of systemic rilmenidine are primarily mediated via imidazoline receptors in the RVLM. However, alpha2-adrenoceptors are also involved, probably as a direct result of the imidazoline receptor action.

Adrenergic drugs modify the level of noradrenaline in the insular cortex and alter extinction of conditioned taste aversion in rats

We compared the effect of conditioned taste aversion in rats by measuring the amount of sucrose that they drunk after conditioning, which differed according to whether rats had drunk the sucrose freely (SD: self drinking) during the conditioning session, or had been forced to drink it (IO: intra-oral administration through a chronically implanted cannula). The SD procedure delayed the extinction of conditioned taste aversion. Enhanced arousal, alertness, awareness or attention in the SD condition may have strengthened the memory of the taste. Brain noradrenergic networks are involved in such processes. We administered two noradrenergic drugs that produce opposite effects on noradrenaline release in the brain, methoxy-idazoxan, RX821002 (1mg/kg, i.p.), and guanfacine (0.12mg/kg, i.p.). We evaluated their effect (i) on the level of noradrenaline in the gustatory cortex using microdialysis, (ii) on glycaemia that is an essential factor of taste learning and (iii) on the comparative SD versus IO conditioned taste aversion protocol mentioned above. Injecting RX821001 increased the level of noradrenaline in the gustatory cortex up to two-fold of the baseline. This effect lasted 1h. The same dose of RX821002 did not elicit any alteration of glycaemia. It enhanced extinction of conditioned taste aversion in the SD group of rats. Injecting 0.12mg/kg of guanfacine produced the opposite effect. The noradrenaline level of the gustatory cortex decreased, but only down to 20% of the baseline. This decrease lasted 2h. Guanfacine increased glycaemia. Extinction of conditioned taste aversion was only marginally decreased by guanfacine in the SD group of rats. These results fit with Aston-Jones' point of view that the role of the noradrenergic coeruleo-cortical system may be to enhance arousal, alertness, awareness or attention to an event by a transient increase of cortical noradrenaline.

Alpha-noradrenergic receptors modulate the development and expression of cocaine sensitization

The increased activity and stereotyped behaviors that result from repeated administration of cocaine is called cocaine sensitization. This sensitized response has been postulated as one of the basic pathophysiological mechanisms in drug addiction. Recent evidence indicates that noradrenergic neurotransmission might be implicated in some of the behavioral effects of cocaine. The present article examined the role of alpha-adrenergic receptor agonists and antagonists in the development and expression of cocaine sensitization. Rats were injected once per day, for 7 consecutive days, with the alpha-1 receptor antagonist prazosin (0.5 mg/kg, i.p.) 15 min before cocaine administration (15 mg/kg, i.p.). After 8 days, animals received a cocaine challenge (15 mg/kg, i.p.) and were tested for locomotion. Following a 7-day withdrawal period rats received a second cocaine challenge. One day after the last challenge, rats were reinstated to the initial protocol for 1 day. In another set of experiments, rats were injected twice per day with the alpha-2 receptor antagonists yohimbine (5 mg/kg, i.p.), idazoxan (0.25 mg/kg, i.p.), or with the alpha-2 agonist clonidine (0.025 mg/kg, i.p.), followed by cocaine injections (15 mg/kg, i.p.), for 7 consecutive days. Thereafter, the protocol was similar to that following prazosin administration. The results demonstrated that the alpha-1 receptor antagonist prazosin blocked the development and expression of cocaine sensitization. On the other hand, both alpha-2 antagonists failed to inhibit the development or the expression of cocaine sensitization. Instead, they produced an increase in locomotor activity during the first day of experimentation. The alpha-2 agonist clonidine attenuated the acute response to cocaine on day 1 and retarded the increased locomotor activity on the following 2 days. There was a dramatic increase in the level of sensitization after the first cocaine challenge. However, it inhibited the expression of cocaine sensitization during the reinstatement protocol. These results suggest that alpha adrenoreceptors play an important role in modulating different stages of cocaine sensitization and probably cocaine addiction.

The alpha2 adrenoreceptor agonist clonidine suppresses evoked and spontaneous seizures, whereas the alpha2 adrenoreceptor antagonist idazoxan promotes seizures in amygdala-kindled kittens

Microinfusion of alpha2 adrenoreceptor agonists and antagonists into amygdala has contrasting effects on evoked and spontaneous seizure susceptibility in amygdala-kindled kittens. Subjects were 14 preadolescent kittens between 3 and 4 months old at the beginning of kindling. The same protocol was followed except that half the kittens received microinfusions (1 mul) of the alpha2 agonist clonidine (CLON; 1.32 nmol), and half received the alpha2 antagonist idazoxan (IDA; 0.33 nmol). Infusions were made over 1 min through needles inserted into cannulae adjacent to stimulating electrodes in the kindled amygdala, and evoked seizures were tested 10-12 min later. The results were: (1) CLON elevated seizure thresholds obtained once at the beginning and end of kindling, but only when compared to sham control values (needle insertion only) in the same animals; IDA significantly reduced thresholds. (2) CLON retarded and IDA accelerated kindling rate, defined as the number of afterdischarges (ADs) required to achieve the first stage 6 seizure or generalized tonic-clonic convulsion (GTC). These effects were most pronounced on the emergence of seizure "generalization" stages (3-6) from "focal" seizure stages (1-2). (3) CLON prevented onset of spontaneous seizures, whereas IDA precipitated onset of spontaneous seizures in 100% of the animals before or during the 5-week post-kindling follow-up during which seizures were evoked once each work day. The study confirms previous findings in kindled rodents to show that CLON and IDA can have opposing effects on kindling development in kittens and is the first report to show contrasting effects on spontaneous epileptogenesis in kindled animals as well.

Spinal alpha(2)-adrenergic and muscarinic receptors and the NO release cascade mediate supraspinally produced effectiveness of gabapentin at decreasing mechanical hypersensitivity in mice after partial nerve injury

After partial nerve injury, the central analgesic effect of systemically administered gabapentin is mediated by both supraspinal and spinal actions. We further evaluate the mechanisms related to the supraspinally mediated analgesic actions of gabapentin involving the descending noradrenergic system. Intracerebroventricularly (i.c.v.) administered gabapentin (100 microg) decreased thermal and mechanical hypersensitivity in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve. These effects were abolished by intrathecal (i.t.) injection of either yohimbine (3 microg) or idazoxan (3 microg), alpha(2)-adrenergic receptor antagonists. Pretreatment with atropine (0.3 mg kg(-1), i.p. or 0.1 microg, i.t.), a muscarinic receptor antagonist, completely suppressed the effect of i.c.v.-injected gabapentin on mechanical hypersensitivity, whereas its effect on thermal hypersensitivity remained unchanged. Similar effects were obtained with pirenzepine (0.1 microg, i.t.), a selective M(1)-muscarinic receptor antagonist, but not with methoctramine (0.1 and 0.3 microg, i.t.), a selective M(2)-muscarinic receptor antagonist. The cholinesterase inhibitor neostigmine (0.3 ng, i.t.) potentiated only the analgesic effect of i.c.v. gabapentin on mechanical hypersensitivity, confirming spinal acetylcholine release downstream of the supraspinal action of gabapentin. Moreover, the effect of i.c.v. gabapentin on mechanical but not thermal hypersensitivity was reduced by i.t. injection of L-NAME (3 microg) or L-NMMA (10 microg), both of which are nitric oxide (NO) synthase inhibitors. Systemically administered naloxone (10 mg kg(-1), i.p.), an opioid receptor antagonist, failed to suppress the analgesic actions of i.c.v. gabapentin, indicating that opioid receptors are not involved in activation of the descending noradrenergic system by gabapentin. Thus, the supraspinally mediated effect of gabapentin on mechanical hypersensitivity involves activation of spinal alpha(2)-adrenergic receptors followed by muscarinic receptors (most likely M(1)) and the NO cascade. In contrast, the effect of supraspinal gabapentin on thermal hypersensitivity is independent of the spinal cholinergic-NO system.

α2-Adrenoreceptor mediated sympathoinhibition of heart rate during acute hypoxia is diminished in conscious prostacyclin synthase deficient mice

Acute hypoxia increases ventilatory drive in conscious animals, resulting in tachycardia. Sustained hypoxia changes the initial chemoreflex ventilatory increase to secondary ventilatory depression, which then evokes a gradual secondary heart rate (HR) reduction. Prostacyclin (PGI(2)) release is known to potentiate alpha(2)-adrenoreceptor (alpha(2)-AR) mediated inhibition of sympathoactivation during ischaemia and hypoxia. We examined whether alpha(2)-AR mediated sympathoinhibition was responsible for limiting hypoxic heart rate increases during initial sympathoactivation, and subsequent secondary HR depression, and if PGI(2) is required for sympathoinhibition of HR. The responses of unrestrained PGI(2) synthase deficient (PGID) and wild type (WT) mice to acute hypoxia (10% O(2) for 30 min) were investigated by simultaneous telemetry, whole body plethysmography and open-flow respirometry. PGID mice exhibited potentiated .V(E) (p < 0.007) after intraperitoneal vehicle injection (n = 8), but not so HR responses compared to WT mice during sustained hypoxia. Idazoxan (alpha(2)-AR antagonist, i.p. bolus 3 mg/kg) pretreatment did not change hypoxic ventilatory response in either group, but significantly elevated hypoxic HR in WT mice only (p < 0.013). Sodium meclofenamate (cyclooxygenase inhibition, i.p. bolus 25 mg/kg) pretreatment eliminated the potentiated .V(E) of PGID and caused significant basal hypotension that led to a transient hypertensive response to hypoxia. From these results, we suggest that alpha(2)-AR activation is required for coupling HR to central inspiratory drive during acute hypoxia, and that PGI(2) is required to enhance the inhibition of sympathoactivation.

Pharmacologic characterization of the cloned human trace amine-associated receptor1 (TAAR1) and evidence for species differences with the rat TAAR1

The hemagglutinin-tagged human trace amine-associated receptor1 (TAAR1) was stably coexpressed with rat Galpha(s) in the AV12-664 cell line, and receptor activation was measured as the stimulation of cAMP formation. After blockade of endogenously expressed alpha2- and beta-adrenoceptors with 2-[2-(2-methoxy-1,4-benzodioxanyl)]-imidazoline hydrochloride (2-methoxyidazoxan, RX821002) and alprenolol, respectively, the resulting pharmacology was consistent with that of a unique receptor subtype. beta-Phenylethylamine (beta-PEA), the putative endogenous ligand, gave an EC50 of 106 +/- 5 nM in the assay. For a series of beta-PEA analogs used to explore the pharmacophore, small substituents at ring positions 3 and/or 4 generally resulted in compounds having lower potency than beta-PEA, although several were as potent as beta-PEA. However, small substituents at ring position 2 resulted in a number of compounds having potencies as good as or better than beta-PEA. A number of nonselective antagonists known to share affinity for multiple monoaminergic receptors were evaluated for their ability to inhibit beta-PEA stimulation of the human TAAR1. None had an IC50 <10 microM. For comparison, the rat TAAR1 receptor was expressed in the AV12-664 cell line. A number of agonist compounds had significantly different relative potencies between the rat and human TAAR1, demonstrating a significant species difference between the rat and human TAAR1. The TAAR1 receptor exhibits a pharmacologic profile uniquely different from those of classic monoaminergic receptors, consistent with the structural information that places them in a distinct family of receptors. This unique pharmacologic profile suggests the potential for development of TAAR-selective agonists and antagonists to study their physiologic roles.

Involvement of I2-imidazoline binding sites in positive and negative morphine analgesia modulatory effects

Some studies, suggesting the involvement of I(2)-imidazoline binding sites (I(2)-IBS) in morphine analgesia modulation, prompted us to examine on mice antinociceptive assays the effect produced by 1 (phenyzoline), that in view of its high I(2)-IBS affinity and high I(2)-IBS selectivity with regard to I(1)-IBS, alpha(2)-adrenoreceptors and mu-opioid receptors might be considered the first interesting I(2)-IBS ligand. The study was also applied to its ortho phenyl derivative 2 (diphenyzoline), designed and prepared in order to produce a possible modification of the biological profile of 1. Diphenyzoline (2) retains a significant I(2)-IBS selectivity with regard to I(1)-IBS, alpha(2)-adrenoreceptors and mu-opioid receptors. Moreover, by the functional assays 1 and 2 proved inactive at all alpha(2)-adrenoreceptors subtypes up to 10(-3) M. As expected, phenyzoline and diphenyzoline, which are structurally related, highlighted an interesting "positive" or "negative", respectively, morphine analgesia modulatory effect. In fact, 1 (s.c. 10 mg/kg) enhanced morphine analgesia (60% and 40% in mouse tail-flick and mouse hot-plate, respectively), while 2 (s.c. 10 mg/kg) decreased it (-41% and -20%, respectively). The ability to decrease morphine analgesia had never been observed before in I(2)-IBS ligands. These effects were not affected by i.p. treatment of animals with yohimbine (a selective alpha(2)-adrenoreceptor antagonist, 0.625 mg/kg) or efaroxan (an I(1)-IBS/alpha(2)-adrenoreceptor antagonist, 1.0 mg/kg). In contrast, they were completely reversed by i.p. treatment of animals with idazoxan (an I(2)-IBS/alpha(2)-adrenoreceptor antagonist, 2 mg/kg). Moreover, compound 2, in mouse tail-flick test, was able to potentiate by 23% the naloxone-induced decrease of morphine analgesia. Therefore, the results of this study indicate the crucial involvement of I(2)-IBS in the morphine analgesia modulatory effects of 1 and 2.

Alpha2-adrenergic activation in the lateral parabrachial nucleus induces NaCl intake under conditions of systemic hyperosmolarity

The inhibition of sodium intake by increased plasma osmolarity may depend on inhibitory mechanisms present in the lateral parabrachial nucleus. Activation of alpha(2)-adrenergic receptors in the lateral parabrachial nucleus is suggested to deactivate inhibitory mechanisms present in this area increasing fluid depletion-induced 0.3 M NaCl intake. Considering the possibility that lateral parabrachial nucleus inhibitory mechanisms are activated and restrain sodium intake in animals with increased plasma osmolarity, in the present study we investigated the effects on water and 0.3 M NaCl intake produced by the activation of alpha(2)-adrenergic receptors in the lateral parabrachial nucleus in rats with increased plasma osmolarity. Male Holtzman rats with stainless steel cannulas implanted bilaterally into the lateral parabrachial nucleus were used. One hour after intragastric 2 M NaCl load (2 ml), bilateral injections of moxonidine (alpha(2)-adrenergic/imidazoline receptor agonist, 0.5 nmol/0.2 microl, n=10) into the lateral parabrachial nucleus induced a strong ingestion of 0.3 M NaCl intake (19.1+/-5.5 ml/2 h vs. vehicle: 1.8+/-0.6 ml/2 h), without changing water intake (15.8+/-3.0 ml/2 h vs. vehicle: 9.3+/-2.0 ml/2 h). However, moxonidine into the lateral parabrachial nucleus in satiated rats not treated with 2 M NaCl produced no change on 0.3 M NaCl intake. The pre-treatment with RX 821002 (alpha(2)-adrenergic receptor antagonist, 20 nmol/0.2 microl) into the lateral parabrachial nucleus almost abolished the effects of moxonidine on 0.3 M NaCl intake (4.7+/-3.4 ml/2 h). The present results suggest that alpha(2)-adrenergic receptor activation in the lateral parabrachial nucleus blocks inhibitory mechanisms, thereby allowing ingestion of hypertonic NaCl under conditions of extracellular hyperosmolarity. We suggest that during cell dehydration, circuits subserving sodium appetite are activated, but at the same time strongly inhibited through the lateral parabrachial nucleus.

Agmatine and a cannabinoid agonist, WIN 55212-2, interact to produce a hypothermic synergy

Agmatine blocks morphine withdrawal symptoms and enhances morphine analgesia in rats. Yet, the role of agmatine in the pharmacological effects of other abused drugs has not been investigated. The present study investigates the effect of agmatine administration on the hypothermic response to cannabinoids. Hypothermia is an effective endpoint because cannabinoid agonists produce a rapid, reproducible, and significant decrease in body temperature that is abolished by cannabinoid CB(1) receptor antagonists. WIN 55212-2, a cannabinoid agonist, was administered to rats by itself and with agmatine. WIN 55212-2 (1, 2.5, 5 and 10 mg/kg, i.m.) caused a significant hypothermia. Agmatine (10, 25 and 50 mg/kg, i.p.) was ineffective. For combined administration, agmatine (50 mg/kg, i.p.) enhanced the hypothermic effect of WIN 55212-2 (1, 2.5, 5 and 10 mg/kg, i.m.). The enhancement was strongly synergistic, indicated by a 2.7-fold increase in the relative potency of WIN 55212-2. The central administration of agmatine (25 and 50 mug/rat, i.c.v.) significantly increased the hypothermic effect of WIN 55212-2 (2.5 mg/kg, i.m.). This indicates that agmatine acts through a central mechanism to augment cannabinoid-evoked hypothermia. Idazoxan (2 mg/kg, i.p.), an imidazoline antagonist, blocked the enhancement by agmatine, thus suggesting that imidazoline receptor activation is required for agmatine to enhance cannabinoid-evoked hypothermia. The present data reveal that agmatine and a cannabinoid agonist interact to produce a hypothermic synergy in rats. These results show that agmatine acts in the brain and via imidazoline receptors to enhance cannabinoid-evoked hypothermia.

K(+)-evoked [(3)H]-norepinephrine release in human brain slices from epileptic and non-epileptic patients is differentially modulated by gabapentin and pinacidil

The modulation of K(+)-evoked [(3)H]-norepinephrine ([(3)H]-NE) release by gabapentin (GBP) and pinacidil (PIN), a known K(ATP) agonist, was examined in human brain slices. We compared the pharmacological effects on NE-release in human epileptic neocortex and epileptic hippocampus to non-epileptic neocortex. GBP (100 microM) decreased [(3)H]-NE release by 22% in non-epileptic neocortical slices, whereas this inhibition was absent in slices from epileptic hippocampus and epileptic neocortex. PIN (10 microM) also reduced [(3)H]-NE release by 30% in non-epileptic neocortical slices and only by 5% in epileptic hippocampal slices. The blockade of voltage-gated calcium channels by omega-conotoxins MVIIA and MVIIC (0.1 microM) reduced [(3)H]-NE release in epileptic and non-epileptic neocortical slices to the same extend. The data show a marked reduction in K(+)-evoked [(3)H]-NE release by GBP and PIN in epileptic hippocampus and neocortex, suggesting an alteration of K(ATP) channel function, whereas the effects of the calcium channel modulators omega-conotoxins MVIIA and MVIIC are similar in both epileptic and non-epileptic neocortex.

Prenatal cocaine exposure alters alpha2 receptor expression in adolescent rats

BACKGROUND

Prenatal cocaine exposure produces attentional deficits which to persist through early childhood. Given the role of norepinephrine (NE) in attentional processes, we examined the forebrain NE systems from prenatal cocaine exposed rats. Cocaine was administered during pregnancy via the clinically relevant intravenous route of administration. Specifically, we measured alpha2-adrenergic receptor (alpha2-AR) density in adolescent (35-days-old) rats, using [3H]RX821002 (5 nM).

RESULTS

Sex-specific alterations of alpha2-AR were found in the hippocampus and amygdala of the cocaine-exposed animals, as well as an upregulation of alpha2-AR in parietal cortex.

CONCLUSION

These data suggest that prenatal cocaine exposure results in a persistent alteration in forebrain NE systems as indicated by alterations in receptor density. These neurochemical changes may underlie behavioral abnormalities observed in offspring attentional processes following prenatal exposure to cocaine.

Orexin neurons are directly and indirectly regulated by catecholamines in a complex manner

We reported elsewhere that orexin neurons are directly hyperpolarized by noradrenaline (NA) and dopamine. In the present study, we show that NA, dopamine, and adrenaline all directly hyperpolarized orexin neurons. This response was inhibited by the alpha2 adrenergic receptor (alpha2-AR) antagonist, idazoxan or BRL44408, and was mimicked by the alpha2-AR-selective agonist, UK14304. A low concentration of Ba2+ inhibited NA-induced hyperpolarization, which suggests that activation of G protein coupled inward rectifier potassium channels is involved in the response. In the presence of a high concentration of idazoxan, NA induced depolarization or inward current. This response was inhibited by alpha1-AR antagonist, prazosin, which suggests the existence of alpha1-ARs on the orexin neurons along with alpha2-AR. We also examined the effects of NA on glutamatergic and GABAergic synaptic transmission. NA application dramatically increased the frequency and amplitude of spontaneous inhibitory synaptic currents (sIPSCs) and inhibited excitatory synaptic currents (sEPSCs) in orexin neurons; however, NA decreased the frequency of miniature EPSCs (mEPSCs) and IPSCs and the amplitude of evoked EPSCs and IPSCs through the alpha2-AR, because the NA response on mPSCs was inhibited by idazoxan. These results suggest that the NA-induced increase in sIPSC frequency and amplitude is mediated via alpha1-ARs on the somata of GABAergic neurons that innervate the orexin neurons. Calcium imaging using orexin/YC2.1 transgenic mouse brain revealed that NA-induced inhibition of orexin neurons is not altered by sleep deprivation or circadian time in mice. The evidence presented here revealed that orexin neurons are regulated by catecholamines in a complex manner.

Ontogenesis of mRNA levels and binding sites of hepatic alpha-adrenoceptors in young cattle

Catecholamines affect hepatic glucose production through (alpha- and beta2-) adrenoceptors (AR). We studied mRNA abundance and binding of hepatic alpha-AR in pre-term (P0) calves and in full-term calves at day 0 (F0), day 5 (F5) and day 159 (F159) to test the hypothesis that gene expression and numbers of hepatic alpha-AR in calves are influenced by age and associated with beta2-AR and selected traits of glucose metabolism. mRNA levels of alpha1- and alpha2-AR were measured by real time RT-PCR. alpha1- and alpha2-AR numbers (maximal binding, Bmax) were determined by saturation binding of (3H)-prazosin and (3H)-RX821002, respectively. alpha1- and alpha2-AR subtypes were evaluated by competitive binding. alpha1A-AR mRNA levels were lower in P0 than in F0, F5 and F159 and alpha(2AD)-AR mRNA levels were lower in F159 than in P0, F0 and F5, while alpha2C-AR mRNA levels increased from P0 and F0 to F5 and F159. Bmax of alpha1-AR increased from P0 to F5, then decreased in F159. Bmax of alpha2-AR decreased from F0 to F159. Bmax of alpha1-AR was positively associated with mRNA levels of alpha1A-AR (r = 0.7), Bmax of beta2-AR (r = 0.5) and negatively with hepatic glycogen content (r = -0.6). Bmax of alpha2-AR was negatively associated with Bmax of beta2-AR (r = -0.4). In conclusion, mRNA levels and binding sites of alpha1- and alpha2-AR in calves exhibited developmental changes and were negatively associated with hepatic glycogen content.

Long-lasting effects of yohimbine on the ejaculatory function in male dogs

Previous studies have demonstrated that systemic administration of a low dose of the alpha2-adrenoceptor antagonists stimulates the ejaculatory response of male dogs, when this function is analyzed using the amount of ejaculated semen in response to genital stimulation. The present study was designed to further examine the features of the stimulatory effects of the alpha2-adrenoceptor antagonists on ejaculation, especially the duration of action. Treatment with yohimbine (0.1 mg/ kg, i.p.) to male dogs, at 0.5, 1, 3, or 5 h before the testing, produced a significant stimulatory effects on the ejaculatory response elicited by manual penile stimulation; the amount of ejaculated semen was increased and the onset of ejaculation was shortened following each treatment. However, such effects were not observed in the treatment with yohimbine at 8 and 24 h before the testing, indicating that the ejaculatory stimulation induced by yohimbine lasted for a relative long period. By contrast, the stimulatory effects of RX821002 (0.1 mg/kg, i.p.), a selective alpha2-adrenoceptor antagonist, on ejaculation were observed only for 1 h after administration. To determine the contribution of the alpha2-adrenoceptor blockade for the long-lasting effect of yohimbine, we tested whether yohimbine can prevent the ejaculatory inhibition induced by clonidine, an alpha2-adrenoceptor agonist. The ejaculatory inhibition (a decrease in the amount of ejaculated semen and a delay onset of ejaculation) elicited by clonidine (0.05 mg/kg, i.p.; 1 h before testing) was completely blocked by pretreatment with yohimbine at 1 or 5 h before the testing, whereas the pretreatment with the drug at 24 h before the testing did not affect the clonidine-induced ejaculatory inhibition. These results indicate that yohimbine-induced ejaculatory stimulation is continued for a relative long period (at least 5 h after administration), and this long-lasting effects may be related to the alpha2-adrenoceptor blocking property of the drug.

Rho kinase inhibitors reduce neurally evoked contraction of the rat tail artery in vitro

The effects of Rho kinase inhibitors (Y27632, HA-1077) on contractions to electrical stimulation and to application of phenylephrine, clonidine or alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-mATP) were investigated in rat tail artery in vitro. In addition, continuous amperometry and intracellular recording were used to monitor the effects of Y27632 on noradrenaline (NA) release and postjunctional electrical activity, respectively. Y27632 (0.5 and 1 microM) and HA-1077 (5 microM) reduced neurally evoked contractions. In contrast, the protein kinase C inhibitor, Ro31-8220 (1 microM), had little effect on neurally evoked contraction. In the absence and the presence of Y27632 (0.5 microM), the reduction of neurally evoked contraction produced by the alpha-adrenoceptor antagonists prazosin (10 nM) and idazoxan (0.1 microM) was similar. The P2-purinoceptor antagonist, suramin (0.1 mM), had no inhibitory effect on neurally evoked contraction in the absence or the presence of Y27632 (1 microM). In the presence of Y27632, desensitization of P2X-purinoceptors with alpha,beta-mATP (10 microM) increased neurally evoked contractions.Y27632 (1 microM) and H-1077 (5 microM) reduced sensitivity to phenylephrine and clonidine. In addition, Y27632 reduced contractions to alpha,beta-mATP (10 microM). Y27632 (1 microM) had no effect on the NA-induced oxidation currents or the purinergic excitatory junction potentials and NA-induced slow depolarizations evoked by electrical stimulation. Rho kinase inhibitors reduce sympathetic nerve-mediated contractions of the tail artery. This effect is mediated at a postjunctional site, most likely by inhibition of Rho kinase-mediated 'Ca2+ sensitization' of the contractile apparatus.

Idazoxan increases perforant path-evoked EPSP slope paired pulse inhibition and reduces perforant path-evoked population spike paired pulse facilitation in rat dentate gyrus

Norepinephrine, acting via beta-adrenoceptors, enhances the perforant path-evoked potential in dentate gyrus. Using systemic idazoxan to increase norepinephrine, and paired perforant path pulses to probe early inhibition, previous investigators reported that idazoxan increased initial spike amplitude and increased somatic feedback inhibition. Here, feedback inhibition was re-examined in idazoxan-treated (5 mg/kg) rats under urethane anesthesia. To control for initial increased spike amplitude after idazoxan, evoked potentials were matched, pre- and post-idazoxan, on initial population spike. Input-output current profiles were also compared pre- and post-idazoxan. Saline- and timolol-filled micropipettes permitted evaluation of a contribution of local beta-adrenoceptors. As previously observed, initial spike amplitude was potentiated by idazoxan. Comparable spike potentiation was not seen on the timolol micropipette. Paired pulse inhibition of spike amplitude apparently increased, but input-output curve comparisons revealed a loss of feedback facilitation rather than an increase in feedback inhibition. Initial EPSP slopes were depressed after idazoxan in input-output curve data. EPSP slope feedback ratios were significantly reduced following idazoxan. These data suggest idazoxan has multiple effects on perforant path input to the dentate gyrus. Spike potentiation following idazoxan has previously been shown to depend on intact norepinephrine input. Here, the reduction in spike potentiation on the timolol pipette is consistent with other evidence that norepinephrine-mediated potentiation of the perforant path-evoked potential is dependent on local beta-adrenoceptor activation. The input-output data suggest a decrease in feedback facilitation after idazoxan is likely to account for the apparent increase in feedback inhibition previously reported. Decreased EPSP slope ratios with similar paired pulse intervals have been reported in novel environments. Since exposure to novel environments activates locus coeruleus neurons, norepinephrine may mediate the change in EPSP slope inhibition reported in awake rats. In summary, these results are consistent with the hypothesis that idazoxan potentiates granule cell responses to perforant path input in the dentate gyrus via increases in norepinephrine that lead to beta-adrenoceptor activation, and, further, that idazoxan reduces paired pulse feedback spike facilitation and enhances EPSP slope, but not spike, feedback inhibition.

Norepinephrine transporter knockout-induced up-regulation of brain alpha2A/C-adrenergic receptors

The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory alpha(2)-adrenergic receptors (alpha(2)ARs). Long-term inhibition of the NET by antidepressants has been reported to change the density and function of pre- and postsynaptic ARs, which may contribute to the antidepressant effects of NET inhibitors such as desipramine. NET-deficient (NET-KO) mice have been described to behave like antidepressant-treated mice. By means of quantitative real-time PCR we show that mRNAs encoding the alpha(2A)-adrenergic receptor (alpha(2A)AR) and the alpha(2C)-adrenergic receptor (alpha(2C)AR) are up-regulated in the brainstem, and that alpha(2C)AR mRNA is also elevated in the hippocampus and striatum of NET-KO mice. These results were confirmed at the protein level by quantitative autoradiography. The NET-KO mice showed enhanced binding of the selective alpha(2)AR antagonist [(3)H]RX821002 in several brain regions. Most robust increases (20-25%) in alpha(2)AR expression were observed in the hippocampus and in the striatum. Significant increases (16%) were also seen in the extended amygdala and thalamic structures. In an 'in vivo' test, the alpha(2)AR agonist clonidine (0.1 mg/kg) caused a significantly greater reduction of locomotor activity in NET-KO mice than in wild-type mice, showing the relevance of our findings at the functional level.

Pharmacological characterization and autoradiographic distribution of α2-adrenoceptor antagonist [3H]RX 821002 binding sites in the chicken brain

Knowledge about the noradrenergic system in birds is very scarce even though their biological diversity and complex social behavior make them an excellent model for studying neuronal functions and developmental biology. While the role of norepinephrine has been described in depth in a large number of central and peripheral functions in mammals, reports for avian species are limited. The radioligand [(3)H]RX 821002 ([(3)H]1,4-[6,7(n)3H]-benzodioxan-2-methoxy-2-yl)-2-imidazol) has been used to map and characterize alpha(2)-adrenoceptors through the chicken brain using in vitro autoradiography and membrane homogenates binding assays. [(3)H]RX 821002 showed a saturable and high affinity binding to a site compatible with alpha(2)-adrenoceptor, and to a serotonergic component. The autoradiographic assays displayed a similar alpha(2)-adrenoceptor distribution than those previously reported in birds using other radioligands such as [(3)H]UK 14304 ([(3)H]5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) or [(3)H]clonidine. [(3)H]RX 821002 binding pharmacological characterization was carried out in different chicken brain regions using membrane homogenates for competition assays with different alpha(2)-adrenoceptor agonists and antagonists drugs (oxymetazoline, BRL 44408 [2-(2H-(1-methyl-1,3-dihydroisoindole)methyl)-4,5-dihydroimidazole] ARC 239 [2-(2-4-(O-methoxyphenyl)-piperazin-1-yl)-ethyl-4,4-dimethyl-1,3-(2H,4H)-isoquinolindione], prazosin, UK 14304 and RX 821002). The results showed alpha(2A) as the predominant alpha(2)-adrenoceptor subtype in the chicken brain while alpha(2B)- and/or alpha(2C)-adrenoceptor subtypes were detected only in the telencephalon. RX 821002, serotonin (5-HT) and 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] competition assays, and competition binding assays performed in the presence of serotonin demonstrated that [(3)H]RX 821002 binds with higher affinity to a serotonergic component, probably 5-HT(1A) receptors, than to the alpha(2)-adrenoceptors. Similar pharmacological properties for the alpha(2)-adrenoceptor component were observed both in rat and chicken brain. The results demonstrate that the different alpha(2)-adrenoceptor subtypes are present in chicken brain and suggest that these receptors are highly conserved through evolution.