Effect of the blockade of μ<Subscript>1</Subscript>-opioid and 5HT<Subscript>2A</Subscript>-serotonergic/α<Subscript>1</Subscript>-noradrenergic receptors on sweet-substance-induced analgesia

Rationale Sweet-substance-induced analgesia has been widely studied, and the investigation of the neurotransmitters involved in this antinociceptive process is an important way for understanding the involvement of the neural system controlling this kind of antinociception.Objective The aim of this study was to investigate the involvement of opioid and monoaminergic systems in sweet-substance-induced analgesia.Methods The present work was carried out in an animal model with the aim of investigating whether acute (24 h) or chronic (14 days) intake of a sweet substance, such as sucrose (250 g/l), is followed by antinociception. Tail withdrawal latencies in the tail-flick test were measured before and immediately after this treatment. Immediately after the recording of baseline values, independent groups of rats were submitted to sucrose or tap-water intake and, after chronic treatment, they were pretreated with intraperitoneal administration of (1) naltrexone at 0.5, 1, 2 or 3 mg/kg; (2) naloxonazine at 5, 10, 20 or 30 mg/kg; (3) methysergide at 0.5, 1, 2 or 3 mg/kg; (4) ketanserin at 0.5, 1, 2 or 3 mg/kg; or (5) physiological saline.Results Naltrexone and methysergide at two major doses decreased sweet-substance-induced analgesia after chronic intake of a sweet substance. These effects were corroborated by peripheral administration of naloxonazine and ketanserin.Conclusions These data give further evidence for: (a) the involvement of endogenous opioids and a ₁-opioid receptor in the sweet-substance-induced antinociception; (b) the involvement of monoamines and 5HT_2A serotonergic/₁-noradrenergic receptors in the central regulation of the sweet-substance-produced analgesia.     

Molecular structure of the rabbit α<Subscript>2A</Subscript>-adrenoceptor: a contribution to the α<Subscript>2A</Subscript>-adrenoceptor versus I<Subscript>1</Subscript> imidazoline receptor controversy

In view of the high structural and pharmacological similarities between the alpha(2A)-adrenoceptors of humans and other mammalian species, it has been concluded, in particular, from experiments in rabbits that the (2A)-adrenoceptor is the exclusive site of action of central antihypertensive drugs, although the amino acid sequence of the alpha(2A)-adrenoceptor of just this species was unknown. Therefore, the aim of the present investigation was to determine the complete nucleotide sequence of the coding region of the rabbit alpha(2A)-adrenoceptor gene. Degenerate oligonucleotides corresponding to regions of the alpha(2A)-adrenoceptor conserved between rat and man were used in a polymerase chain reaction with genomic DNA prepared from rabbit. A 1,356-base pair product with an open reading frame of 1,353 base pairs was obtained that encodes a protein of 451 amino acids which is similar to the alpha(2A)-adrenoceptors of other mammals (man, pig, rat, mouse, guinea-pig and cattle) but not to their alpha(2B)- and alpha(2C)-adrenoceptor subtypes suggesting its classification as an alpha(2A)-adrenoceptor. However, the degree of amino acid sequence identity is, at best, only 80% and, thus, about 10% less than between the other mammalian species. Compared with the human sequence there are 81 substantial changes of amino acids. In conclusion, rabbit and human alpha(2A)-adrenoceptors substantially differ in their amino acid sequence which may explain the opposite pharmacodynamic properties of the central antihypertensive drug rilmenidine (alpha(2)-adrenoceptor agonism and antagonism, respectively) reported in the literature. Hence, the present study supports the view that experiments with central antihypertensive drugs in rabbits are not reliably predictive for the site of action of such drugs in man.     

Cross-talk between β<Subscript>1</Subscript>-adrenoceptors and ET<Subscript>A</Subscript> receptors in modulation of the slow component of delayed rectifier K<Superscript>+</Superscript> currents

Delayed rectifier K⁺ currents (I_K) play a critical role in determining cardiac action potential duration (APD). Modulation of I_K affects cardiac excitability critically. There are three components of cardiac delayed rectifier, and the slowly activating component (I_Ks) is influenced strongly by a variety of stimuli. Plasma levels of noradrenaline and endothelin are elevated in heart failure, and arrhythmias are promoted by such humoral abnormalities through modulation of ion channels. It has been reported that protein kinase A (PKA) and protein kinase C (PKC) modulate I_Ks from human minK in a complex manner. In the present study, we coexpressed human minK with the human ₁-adrenoceptor (h₁AR) and the endothelin receptor subtype A (hET_AR) in Xenopus oocytes and investigated the effects of receptor activation on the currents (I_Ks) flowing through the oocytes. ET-1 modulated I_Ks biphasically: a transient increase followed by a decrease. The PKC inhibitor chelerythrine completely inhibited the effects of ET-1. Intracellular EGTA abolished the transient increase by ET-1 and partially inhibited the subsequent decrease in the currents. When I_Ks was increased by 10^–6 M isoproterenol (ISO), ET-1 did not increase but rather decreased the current to an even greater extent than under control conditions. In addition, the effects of ISO on I_Ks were suppressed by ET_AR stimulation. These data indicate that I_Ks can be regulated by cross-talk between the ET_AR and ₁AR systems in addition to direct regulation by each receptor system.     

Discriminative stimulus effects of zolpidem in squirrel monkeys: role of GABA<Subscript>A</Subscript>/α<Subscript>1</Subscript> receptors

Abstract Rationale. The discriminative stimulus effects of zolpidem in squirrel monkeys trained at doses greater than or equal to 3.0 mg/kg differ from those of conventional benzodiazepines (BZs), but the extent to which these effects reflect the selectivity of zolpidem for GABA_A/α₁ receptors is not known. Objectives. The present study investigated the ability of GABA_A/α₁-preferring agonists to substitute for training doses of zolpidem greater than or equal to 3.0 mg/kg and the ability of GABA_A/α₁-preferring antagonists to block zolpidem's discriminative stimulus effects. Methods. Squirrel monkeys were trained to discriminate intravenous injections of zolpidem (3.0 or 5.6 mg/kg) from saline and tested with BZ agonists differing in selectivity and efficacy at GABA_A/α₁ receptors. Antagonism of the effects of zolpidem was studied using the GABA_A/α₁-preferring antagonists β-carboline-3-carboxylate-t-butyl ester (β-CCT) and 3-propyloxy-β-carboline (3-PBC). Results. Zolpidem and quazepam (GABA_A/α₁-preferring agonist) engendered full substitution for zolpidem, whereas CL 218,872 (GABA_A/α₁-preferring partial agonist) and the non-selective BZ agonists alprazolam and flunitrazepam engendered low and variable levels of zolpidem-lever responding (35–58%). Both β-CCT and 3-PBC antagonized the discriminative stimulus effects of zolpidem in a surmountable fashion. Conclusions. Our findings provide evidence for a key role of GABA_A/α₁ receptors in the discriminative stimulus effects of zolpidem at relatively high training doses, and suggest that selectivity and relatively high efficacy at GABA_A/α₁ receptors is required for BZ agonists to reproduce these discriminative stimulus effects. Electronic Publication     

Antiproliferative and cytotoxic effects of some σ<Subscript>2</Subscript> agonists and σ<Subscript>1</Subscript> antagonists in tumour cell lines

To establish the activity of ligands at ₁ and ₂ receptor, we chose two tumour cell lines, the human SK-N-SH neuroblastoma and the rat C6 glioma lines, which express ₂ receptors at a high density and ₁ receptors in their high-affinity or low-affinity state. We tested the ₂ receptor agonist PB28 and the ₂ antagonist AC927, and (+)-pentazocine and NE100 as agonist and antagonist, respectively, at ₁ receptors, with regard to antiproliferative and cytotoxic effects. In addition, 1,3-di(2-tolyl)guanidine (DTG) and haloperidol were tested as reference compounds displaying nearly equipotent affinity (₂>₁ and ₁>₂, respectively). In both SK-N-SH and C6 cells, PB28 and NE100 displayed the most potent results both in antiproliferative and cytotoxic assay while AC927 and (+)-pentazocine were inactive in both assays. The cytotoxic and antiproliferative effects of DTG and haloperidol reflected their ₁ antagonist activity and ₂ agonist activity. Moreover, our results in the tumour cell lines correlated well with those for ₂ activity found previously in a functional assay in the guinea-pig bladder. These findings establish a new model for evaluating both ₂ and ₁ receptor activity of ligands, which could be useful for developing new ligands having mixed ₂ agonist/₁ antagonist activity as potential antineoplastic agents.     

α<Subscript>1</Subscript>-Adrenergic and α<Subscript>2</Subscript>-adrenergic balance in the dorsal pons and gross behavioral activity of mice in a novel environment

Rationale Central α₁- and α₂-adrenoceptors in a number of different brain regions are known to have opposing actions on gross behavioral activity, with the former stimulating and the latter inhibiting activity. Therefore, blockade of α₁-receptors may induce inactivity by leading to unopposed α₂ activity.Objective The aim of this study was to test if central blockade of α₂-receptor function restores behavioral activity in α₁-receptor-blocked mice.Methods Dose-response studies were undertaken on the effects of α₁- and α₂-agonists and antagonists microinjected into the dorsal pons on gross behavioral activity in a novel cage test.Results The behavioral inactivity resulting from blockade of α₁-receptors in the pons with the antagonist, terazosin, was reversed by either a low dose of an α₂-antagonist, atipamezole, or a low dose of an α₂-agonist, dexmedetomidine, but was exacerbated by a high dose of the α₂-agonist.Conclusion The results support the hypothesis that blockade of α₁-receptors in the dorsal pons of mice produces inactivity by causing unopposed activity of α₂-receptors. This condition may be relevant to inactive states seen after stress or during depressive illness.     

Positive allosteric modulatory effects of ajulemic acid at strychnine-sensitive glycine α<Subscript>1</Subscript>- and α<Subscript>1</Subscript>β-receptors

The synthetic cannabinoid ajulemic acid (CT-3) is a potent cannabinoid receptor agonist which was found to reduce pain scores in neuropathic pain patients in the absence of cannabis-like psychotropic adverse effects. The reduced psychotropic activity of ajulemic acid has been attributed to a greater contribution of peripheral CB receptors to its mechanism of action as well as to non-CB receptor mechanisms. Loss of inhibitory synaptic transmission within the dorsal horn of the spinal cord plays a key role in the development of chronic pain following inflammation or nerve injury. Inhibitory postsynaptic transmission in the adult spinal cord involves mainly glycine. As we hypothesised that additional non-CB receptor mechanisms of ajulemic acid might contribute to its effect in neuropathic pain, we investigated the interaction of ajulemic acid with strychnine-sensitive α₁- and α₁β-glycine receptors by using the whole-cell patch clamp technique. Ajulemic acid showed a positive allosteric modulating effect in a concentration range which can be considered close to clinically relevant concentrations (EC₅₀ values: α₁ = 9.7 ± 2.6 μM and α₁β = 12.4 ± 3.4 μM). Direct activation of glycine receptors was observed at higher concentrations above 100 μM (EC₅₀ values: α₁ = 140.9 ± 21.5 μM and α₁β = 154.3 ± 32.1 μM). These in vitro results demonstrate that ajulemic acid modulates strychnine-sensitive glycine receptors in clinically relevant concentrations.     

Comparison of the α-adrenoceptor-mediated effects of β<Subscript>3</Subscript>-adrenoceptor ligands in rat pulmonary artery

We have recently shown that the -adrenoceptor ligands CGP 12177, bupranolol, and SR 59230A (aryloxypropanolamines), but not BRL 37344 and CL 316243 (phenylethanolamines), exhibit significant affinity for ₁-adrenoceptors and that CGP 12177 displays partial agonist properties at -adrenoceptors in rat pulmonary artery. In this study, bupranolol and SR 59230A were further evaluated for their potential -adrenoceptor mediated effects (i.e., agonist and/or antagonist properties) in rat intralobar pulmonary artery and compared with BRL 37344 and CL 316243. Bupranolol induced a relaxation in phenylephrine-precontracted arteries, but had no effect in prostaglandin -precontracted ones. SR 59230A also elicited a relaxation in phenylephrine-precontracted arteries. In -precontracted arteries, SR 59230A induced a contractile response that was insensitive to the irreversible -adrenoceptor antagonist phenoxybenzamine. BRL 37344 at high concentrations, but not CL 316243, produced slight relaxation in both phenylephrine- and -precontracted arteries. The contractile response to phenylephrine was antagonized by bupranolol and SR 59230A in a competitive manner (pA₂: 6.38 and 7.08 respectively). The concentration–response curve to phenylephrine was also shifted to the right by BRL 37344 (mean pK_b: 4.45), but not by CL 316243 (100 M). This study indicates that the aryloxypropanolamine derivatives bupranolol and SR 59230A exhibit competitive antagonist, but no agonist properties on ₁-adrenoceptors, SR 59230A also inducing -adrenoceptor-independent contraction. Among the phenylethanolamines, BRL 37344 but not CL 316243, also exerts an antagonist effect on ₁-adrenoceptors, with a much lower potency than the aryloxypropanolamines studied.     

Stimulatory effect of Coca-Cola<Superscript>TM</Superscript> on gastroduodenal HCO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> secretion in rats

We examined the effect of various carbonated beverages, especially Coca-Cola^TM, on the HCO₃⁻ secretion in the rat stomach and duodenum. Under urethane anaesthesia, a chambered stomach or a proximal duodenal loop was perfused with saline, and HCO₃⁻ secretion was measured at pH 7.0 using a pH-stat method and by adding 2 mM HCl. The amount of CO₂ contained in these beverages was about 4–7 g/mL. Coca-Cola^TM topically applied to the mucosa for 10 min significantly increased the HCO₃⁻ secretion in both the stomach and the duodenum. The HCO₃⁻ response in the duodenum was totally abolished by indomethacin and also partially inhibited by acetazolamide, an inhibitor of carbonic anhydrase. Likewise, the response in the stomach was also markedly inhibited by either acetazolamide or indomethacin. The mucosal application of Coca-Cola^TM increased the PGE₂ contents in both the stomach and the duodenum. Other carbonated beverages, such as sparkling water, Fanta Grape^TM or cider, also increased the HCO₃⁻ secretion in these tissues. These results suggest that Coca-Cola^TM induces HCO₃⁻ secretion in both the stomach and the duodenum, and these responses may be attributable to both the intracellular supply of HCO₃⁻ generated via carbonic anhydrase, and endogenous PGs, probably related to the acidic pH of the solution. Received 4 August 2006; accepted 10 November 2006     

Binding of [<Superscript>3</Superscript>H]prazosin to α<Subscript>1A</Subscript>- and α<Subscript>1B</Subscript>-adrenoceptors,and to a cimetidine-sensitive non-α<Subscript>1</Subscript> binding site in rat kidney membranes

[(3)H]Prazosin bound to alpha(1A)- and alpha(1B)-adrenoceptors, as well as to a cimetidine-sensitive non-alpha(1)-adrenoceptor binding site in rat kidney membranes. An experimental design is presented where the alpha(1)-adrenoceptors are selectively exposed by blocking the non-alpha(1) binding site with 60 microM cimetidine. Conversely, the non-alpha(1) binding site can be selectively exposed by blocking the alpha(1)-adrenoceptors with 600 nM metitepine. The identity of the non-alpha(1) binding site for [(3)H]prazosin in the rat kidney, herein pharmacologically characterized by 33 competing substances, is still unknown.     

Calcium channel function and regulation in β<Subscript>1</Subscript>- and β<Subscript>2</Subscript>-adrenoceptor transgenic mice

Cardiac effects of catecholamines on the L-type calcium channel depend on -adrenoceptor subtype (₁- vs. ₂-adrenoceptor). Chronic overexpression of these receptors leads to hypertrophy and early death at moderate (₁) or excessive (₂) levels of overexpression respectively. In order to examine the role of L-type calcium channels in altered cardiomyocyte calcium homeostasis found with ₁-adrenoceptor overexpression, and to understand the quantitative differences between -adrenoceptor subtypes regarding calcium channel regulation, we examined single channels in myocytes obtained from ₁- and ₂-adrenoceptor transgenic mice. The effects of the agonist isoproterenol were investigated and compared with acute receptor stimulation in the respective non-transgenic littermates.Channels from ₁-adrenoceptor transgenic mice have normal baseline activity, and channel number is not reduced. This contrasts to previous findings with ₂-adrenoceptor transgenic mice, where channel activity is depressed. Isoproterenol is unable to stimulate channel activity in both transgenic models.In conclusion, the L-type calcium channel is not likely to be involved in alterations of calcium handling of ₁-adrenoceptor transgenic myocytes. Furthermore, chronic ₁-adrenoceptor overexpression does not depress channel activity, giving another example of the difference between ₁- and ₂-adrenoceptor signal transduction.K.F. and T.K. equally contributed to this work     

Involvement of α<Subscript>1</Subscript> and β-adrenoceptors in adrenaline stimulation of the glucagon-secreting mouse α-cell

Stimulation of glucagon release and inhibition of insulin secretion from the islets of Langerhans are important for the blood-glucose-elevating effect of adrenaline. The mechanisms by which adrenaline accomplishes these actions may involve direct effects and indirect ones mediated by altered release of other islet hormones. In the present study we investigated how adrenaline affects the cytoplasmic Ca²⁺ concentration, which controls glucagon secretion from the pancreatic -cell. The studies were performed on isolated mouse -cells, which were identified by immunocytochemistry.The adrenaline effects consisted of initial mobilisation of intracellular Ca²⁺, accompanied by voltage-dependent influx of the ion. Part of the effect could be attributed to -adrenoceptor activation, as it was mimicked by the rise in cAMP and inhibited by the antagonist propranolol as well as the protein kinase A inhibitor adenosine 3,5-cyclic monophosphorothioate Rp-isomer. ₁-Adrenoceptors were also involved, since the antagonists phentolamine and prazosin completely abolished the effects of adrenaline. Experiments with clonidine and yohimbine gave little evidence of a role of ₂-adrenoceptors. The results indicate that ₁- and -adrenoceptors on the -cells mediate adrenaline-stimulated glucagon secretion. The complete inhibition of the adrenaline response after blocking ₁-adrenoceptors indicates an interaction with the -adrenergic pathway.Drs. Vieria and Liu contributed equally to the article     

Nandrolone treatment decreases the level of rat kidney α<Subscript>1B</Subscript>-adrenoceptors

Abuse of anabolic androgenic steroids (AAS) is associated with serious side effects, such as hypertension and fluid retention. Renal ₁- and ₂-adrenoceptors are implicated in the regulation of blood pressure and fluid balance. In the present study, the levels of renal _1A-, _1B-, _2A- and _2B-adrenoceptors, and spleen _1B-adrenoceptors, were quantified in tissue membranes from rats treated with the AAS nandrolone decanoate (15 mg/kg) for 14 days. The radioligands used were [³H]-prazosin and [³H]-RX821002. The nandrolone treatment caused a 50% reduction of kidney _1B-adrenoceptors (from 15 fmol/mg protein in control rats to 6.5 fmol/mg protein in treated rats). In contrast, the levels of kidney _1A-, _2A- and _2B-, and spleen _1B-adrenoceptors were unaffected. These results raise the possibility that a decreased level of kidney _1B-adrenoceptors may cause some of the effects observed on blood pressure and fluid balance in heavy abuse of AAS.     

The sigma<Subscript>1</Subscript> (σ<Subscript>1</Subscript>) receptor activation is a key step for the reactivation of cocaine conditioned place preference by drug priming

Rationale Cocaine-seeking behavior can be investigated in rodents using the conditioned place preference (CPP) paradigm, in which the drug-paired environment serves as a conditioned stimulus. Such approach allowed to previously demonstrate the importance of the neuromodulatory sigma₁ (₁) receptor in acquisition of cocaine-induced CPP. CPP can be extinguished and then reactivated, notably using a cocaine challenge (i.e., priming).Objectives and methods In order to examine the role of the ₁ receptor in reinstatement of Cocaine-seeking, Swiss mice acquired CPP with cocaine (30 mg/kg, ip) and then CPP was extinguished.Results A challenge cocaine priming (15 mg/kg) reactivated CPP up to 140% of the post-conditioning response. Pre-administration of the ₁ receptor antagonist BD1047 (330 mg/kg, ip) or repeated treatment with an antisense probe targeting the ₁ receptor prevented CPP reactivation. The ₁ agonist igmesine (1–10 mg/kg, ip) or the steroid dehydroepiandrosterone (DHEA, 10–40 mg/kg, sc) reactivated CPP, in a BD1047-sensitive manner. Moreover, the in vivo [³H](+)-SKF-10,047 binding levels to the ₁ receptor were increased after cocaine conditioning in numerous brain structures and these increases subsisted after extinction. Finally, cross-reactivation of cocaine-induced CPP was observed after phencyclidine (PCP), morphine, nicotine and ethanol administration. However, BD1047 blocked reactivation of CPP induced by PCP, morphine and nicotine but not ethanol.Conclusions Since activation of the ₁ receptor is not sufficient to sustain CPP in naive animals [Neuropsychopharmacology 26 (2002) 444], it is concluded that ₁ receptor activation is a key event for relapse to drug seeking. Activation may occur via sensitization due to enhanced in vivo available of receptors.     

The effects of hydrocortisone on rat heart muscarinic and adrenergic α<Subscript>1</Subscript>, β<Subscript>1</Subscript> and β<Subscript>2</Subscript> receptors,propranolol-resistant binding sites and on some subsequent steps in intracellular signalling

Glucocorticoids affect the expression and density of neurotransmitter receptors in many tissues but data concerning the heart are contradictory and incomplete. We injected rats with hydrocortisone for 1–12 days and measured the densities of cardiac muscarinic receptors, ₁-, ₁- and ₂-adrenoceptors and propranolol-resistant binding sites (formerly assumed to be the putative ₄-adrenoceptor). Some aspects of intracellular signalling were also evaluated: we measured adenylyl cyclase activity (basal, isoprenaline- and forskolin-stimulated and carbachol-inhibited), the coupling between muscarinic receptors and G proteins and basal and isoprenaline-stimulated heart rate. The density of cardiac muscarinic receptors increased (in both the atria and the ventricles). The density of ₁-adrenoceptors increased in the atria and was little changed in the ventricles. The density of ₂-adrenoceptors increased in both the atria and the ventricles. The number of ₁-adrenoceptors decreased initially, followed by a transient increase in the atria and did not change in the ventricles. The density of propranolol-resistant binding sites first increased and then diminished in the atria and did not change in the ventricles. Although there were noticeable changes in receptor densities, the stimulatory and inhibitory effects on adenylyl cyclase, basal and isoprenaline-stimulated heart rate and the coupling between muscarinic receptors and G proteins were not significantly altered. This may indicate that changes in receptor densities might be one of the mechanisms maintaining stable functional output. Deceased     

Influence of α<Subscript>2</Subscript>-autoreceptor stimulation on the facilitation by angiotensin II and bradykinin of noradrenaline release

The interaction between alpha(2)-autoreceptors and receptors for angiotensin II and bradykinin was studied in the heart of newborn rats. The tissues were labeled with [3H] noradrenaline and then superfused with cocaine-containing medium and stimulated electrically. Angiotensin II (30-1,000 nM) and bradykinin (10-300 nM) enhanced the evoked overflow of tritium, the maximum increase reaching 52.2 and 72.8%, respectively. In the presence of the selective alpha(2)-adrenoceptor agonist UK-14,304, the maximal effect caused by angiotensin II and bradykinin was enhanced reaching 92.1 and 87.1% for angiotensin II and bradykinin, respectively. On the contrary, in the presence of chelerythrine (a protein kinase C blocker), not only the increment in facilitation by UK-14,304, but to some extent also the basal facilitation caused by angiotensin II and bradykinin were markedly reduced (to 17.3 and 43.2%, respectively). We conclude that: 1.The stimulation of alpha2-autoreceptors enhances the facilitatory responses caused by angiotensin II and bradykinin, confirming that an ongoing alpha2-autoinhibition is required for the facilitatory influence of the peptides. 2.Protein kinase C is involved in the enhancement by angiotensin II and bradykinin of electrically-evoked release of noradrenaline from the nerve terminals.     

Functional characterisation of α<Subscript>1</Subscript>-adrenoceptors in denervated rat vas deferens

We investigated whether or not surgical denervation of the rat vas deferens changes the ₁-adrenoceptor subtypes involved in the contractions to noradrenaline. Denervated vas deferens was 22 times more sensitive to noradrenaline (pD₂=7.35±0.04) than control vas (pD₂=6.01±0.03). This difference in noradrenaline potency was eliminated when cocaine (6 M) was added to control vas (pD₂=7.22±0.04). The noradrenaline-induced contractions of control and denervated vas deferens were insensitive to the _1B/_1D-adrenoceptor alkylating agent chloroethylclonidine (100 M, 45 min). The concentration-response curves to noradrenaline in control and denervated vas deferens were competitively antagonised by prazosin (pA₂9.6), WB-4101 (pA₂9.5), 5-methyl urapidil (pA₂8.4), phentolamine (pA₂8.7), yohimbine (pA₂6.9), BMY 7378 (pA₂6.9) and indoramin (pA₂8.7). After the treatment of control and denervated vas deferens with phenoxybenzamine, the partial agonist oxymetazoline antagonised competitively the concentration-response curves to noradrenaline showing pA₂ values 7.4 in both groups. We conclude that noradrenaline-induced contractions in control and denervated rat vas deferens are mediated by _1A-adrenoceptors and that surgical denervation of the rat vas deferens is not able to change the ₁-adrenoceptor subtypes involved in the contractions to noradrenaline.     

Role of β<Subscript>3</Subscript>-adrenoceptors in regulation of retinal vascular tone in rats

The aim of this study was to determine the role of β₃-adrenoceptors in the action of endogenous catecholamines (adrenaline and noradrenaline) on rat retinal arterioles in vivo. Using an original high-resolution digital fundus camera, the rat ocular fundus images were captured. The diameter of retinal arterioles contained in the images was measured. Both systemic blood pressure and heart rate were recorded continuously. Adrenaline (0.3–5.0 μg/kg/min, i.v.) increased the diameter of retinal arterioles, mean blood pressure and heart rate in a dose-dependent manner. Under blockade of β₁/β₂-adrenoceptors with propranolol (2 mg/kg, i.v. bolus followed by 100 μg/kg/min infusion), adrenaline decreased the diameter of retinal arterioles. Similar observation was made under treatment with the β₃-adrenoceptor antagonist L-748337 (50 μg/kg, i.v.). The pressor response to adrenaline was enhanced by propranolol, but not by L-748337. The positive chronotropic action of adrenaline was markedly prevented by propranolol, whereas it was unaffected by L-748337. Noradrenaline (0.03–1.0 μg/kg/min, i.v.) decreased the diameter of retinal arterioles but increased the mean blood pressure and heart rate. The effects of noradrenaline on retinal arteriolar diameter and blood pressure were unaffected by propranolol or L-748337. The positive chronotropic action of noradrenaline was almost completely abolished by propranolol. These results suggest that β₃-adrenoceptors play crucial roles in vasodilator responses to adrenaline of retinal arterioles but have minor or no effect on noradrenaline-induced responses. The results also indicate that the functional role of β₃-adrenoceptors may be more important than that in peripheral resistance vessels.