A functional basis for classification of alpha-adrenergic receptors

Centrally induced impairment of the hypotensive effects of R 28935 and R 29814 by prazosin in anaesthetized cats

Prazosin (3 micrograms/kg) infused prior to erythro-1-(1-[2-(1,4-benzodioxan-2-yl)-2-hydroxyethyl]-4-piperidyl)-2-benzimidazolinone (R 28935) into the left vertebral artery (v.a.) of chloralose-anaesthetized cats diminished the hypotensive response to R 28935 (3 micrograms/kg, v.a.). A similar i.v. treatment did not alter the central hypotensive effect of R 28935 (micrograms/kg, v.a.). After prazosin (3 micrograms/kg, v.a.) there was a parallel shift to the right of the dose-response curve of R 28935 (v.a.). The central hypotensive effect of the threo isomer (R 29814; 30 micrograms/kg) was also diminished by the previous administration of prazosin (3 micrograms/kg, v.a.). These results indicate the involvement of central alpha 1-adrenoceptors in the mechanism of action of R 28935 and R 29814.

Activation of lateral geniculate neurons by norepinephrine: mediation by an alpha-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1-15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic alpha-adrenergic receptors: epinephrine greater than NE greater than phenylephrine greater than or equal to alpha-methylnorepinephrine greater than dopamine greater than isoproterenol. The alpha-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (less than 10 nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The beta-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic alpha-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brain alpha1-adrenoceptors labeled with [3H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic or alpha1-adrenergic receptor.

Hippocampal alpha- and beta-adrenergic receptors: comparison of [3H]dihydroalprenolol and [3H]WB 4101 binding with noradrenergic innervation in the rat

The relationship of alpha- and beta-adrenergic receptors to the noradrenergic innervation of the rat hippocampal formation was studied with histofluorescent, biochemical, and radioligand binding methods. The dentate gyrus received a major portion of the innervation and contained twice the norepinephrine content of the hippocampal gyrus. However, the density of beta-adrenergic receptors, determined by [3H]dihydroalprenolol (DHA) binding, was approximately equal in both gyri. By contrast, a presumed alpha-adrenergic binding site was relatively concentrated in the dentate gyrus, roughly correlating with the distribution of the noradrenergic innervation. Although a significant concentration of beta-adrenergic receptors in the pyramidal cell layer had been suggested in previous studies with fluorescent analogs of propranolol, direct microchemical measurements of [3H]DHA binding in stratum pyramidalis and stratum radiatum showed that beta-adrenergic receptors were uniformly distributed in the hippocampal gyrus. These data, demonstrating a ubiquitous distribution of beta-adrenergic binding in a brain region with a well-defined noradrenergic input, suggest that a portion, perhaps a majority, of beta-adrenergic receptors are not associated with noradrenergic nerve terminals.

Mechanisms involved in alpha-adrenergic phenomena: role of calcium ions in actions of catecholamines in liver and other tissues

Epinephrine and norepinephrine binding sites with the physiological characteristics of alpha-adrenergic receptors have been identified in the plasma membranes of liver and other cells. Interaction of catecholamines with these receptors causes a mobilization of calcium ions from mitochondria and perhaps other intracellular stores in liver cells. In other cells, there may also be influx of extracellular calcium ions. Evidence is presented in support of the hypothesis that the rise in cytosolic calcium ions resulting from these changes is responsible for many of the alpha-adrenergic actions of catecholamines. Possible mechanisms by which activation of alpha-adrenergic receptors causes changes in calcium and other aspects of cellular metabolism are discussed.

Effects of metoclopramide and isoprenaline in the rat vas deferens; interactions with alpha-adrenoceptors

1 Metoclopramide (2.8 to 280 microM) augmented contractions of rat vas deferens preparations induced by field stimulation (6 Hz for 1 s). This effect was antagonized by phentolamine (0.1 microM). Metoclopramide (2.8 to 280 microM) did not affect phenylephrine-induced contractions. 2 Metoclopramide (2.8 to 280 microM) antagonized the inhibitory effects of clonidine on the contractions induced by field stimulation, but not the inhibitory effects of purine nucleosides. 3 From these results it is concluded that metoclopramide (2.8 to 280 microM) is a presynaptic alpha-adrenoceptor antagonist in the rat vas deferens. 4 Following beta-adrenoceptor blockade with (+/-)-propranolol (3.3 microM), (-)-isoprenaline (0.47 to 14 microM) inhibited responses to field stimulation but not to phenylephrine. These propranolol-resistant effects of isoprenaline were antagonized by metoclopramide (2.8 to 280 microM) and by phentolamine (0.1 to 10 microM), indicating that isoprenaline may stimulate presynaptic alpha-adrenoceptors in this preparation.

Relation between central sympathoinhibitory and peripheral pre- and postsynaptic alpha-adrenoceptors as evaluated by different clonidine-like substances in rats

A series of 8 clonidine-like substances were tested in rats on central sympathoinhibition (heart rate decrease in vagotomized rats, D50 = dose which decreased heart rate by 50 beats/min), peripheral postsynaptic activity (blood pressure increase in spinal rats, PD30 = dose which increased blood pressure 30 mm Hg) and peripheral presynaptic activity (inhibition of tachycardia as elicited by electrical stimulation in the spinal canal of pithed rats, ID50 = dose which inhibited for 50%). The D50, PD30 and ID50 were converted in moles/kg and the negative logarithms thereof were correlated. There was a good correlation between the central effect (D50) and both peripheral effects (PD30 and ID50, respectively; r = 0.89 and 0.91, respectively) provided one drug with low lipoid affinity was omitted from calculation (St 600). There was also good correlation between both peripheral effects (PD30 and ID50; r = 0.94) and this could be extended to other imidazolines which are not acting centrally as antihypertensives (oxymetazoline, naphazoline, St 91, tramazoline; r = 0.90). The results were interpreted by (1) the agonistic activity of the clonidine-like drugs mainly at alpha 2-adrenoceptors and (2) the presence of alpha 2-adrenoceptors at peripheral presynaptic, peripheral postsynaptic as well as central cardiovascular sites.

Possible subdivision of postsynaptic alpha-adrenoceptors mediating pressor responses in the pithed rat

Additional evidence has been obtained indicating a possible subclassification of postsynaptic alpha-adrenoceptors into alpha 1 - and alpha 2 -subtypes. The pressor responses to the alpha-adrenoceptor agonists L-phenylephrine and guanfacine were quantified after i.v. administration to pithed rats. The alpha-sympatholytic drug yohimbine (1 mg/kg) displaced both dose-response curves to the right, but the effect was greatest for guanfacine. After prazosin (0.1 mg/kg) a 53-fold shift to the right was noticed for the dose-response characteristic of L-phenylephrine. Prazosin antagonized the effect of only the higher doses of guanfacine. The findings indicate that L-phenylephrine and prazosin preferentially interact with alpha 1 -adrenoceptors as agonist and antagonist, respectively. Yohimbine proved less selective than prazosin, but preferentially blocks postjunctional alpha 2 -adrenoceptors in the vascular wall. The results obtained with guanfacine may be interpreted to indicate that this drug acts on alpha 2 -adrenoceptors at lower doses and additionally stimulates alpha 1 -adrenoceptors at higher ones. Preliminary findings with corynanthine and rauwolscine support this interpretation.

Effects of ring substitution on the pre- and postjunctional alpha-adrenergic activity of aryliminoimidazolidines

The pre- and postjunctional alpha-adrenergic agonist potency of a series of aryliminoimidazolidines was determined in the isolated rabbit ear artery. This series included clonidine, an antihypertensive agent thought to act by stimulating brainstem alpha-receptors and known to be a preferentially prejunctional alpha-adrenergic agonist. Although all of the compounds acted preferentially on the prejunctional alpha-adrenoceptor, ring substitution had a dramatic effect on both potency and the degree of selectivity. 2-(3,4-Dihydroxyphenylimino) imidazolidine was both the most potent and most selective prejunctional alpha-agonist in this series.

Noradrenergic agonists and antagonists: effects on conditioned fear as measured by the potentiated startle paradigm

Clonidine (10-40 microgram/kg) produced a dose-dependent reduction of fear as measured by the potentiated startle effect (increased acoustic startle in the presence of a cue which had been previously paired with shock). The reduction of potential startle could not be accounted for entirely by a general depressant effect of clonidine on startle nor by an acceleration of extinction. Piperoxane and yohimbine, which are associated with anxiety in humans, increased potentiated startle, whereas propranolol and WB-4101 did not. These results provide further evidence that the potentiated startle paradigm in the rat is sensitive to drug that alter anxiety in humans. Moreover, they support the hypothesis that norepinephrine transmission is important for the expression of fear or anxiety.

Effects of clonidine, prazosin and phentolamine on heart rate and coronary sinus catecholamine concentration during cardioaccelerator nerve stimulation in spinal dogs

1 In spinal dogs, continuous electrical stimulation of the cardioaccelerator nerve produced a transient rise in aortic blood pressure and a sustained increase in both heart rate and coronary sinus blood flow. The latter effects were accompanied by a significant elevation in the coronary sinus plasma noradrenaline concentration without significant changes in the levels of dopamine and adrenaline. The concentrations of the three catecholamines in thoracic aorta plasma were not significantly changed by cardioaccelerator nerve stimulation.2 Clonidine (20 mug/kg, i.v.), given during cardioaccelerator nerve stimulation, increased both mean aortic blood pressure and coronary sinus blood flow and decreased heart rate and coronary sinus venous plasma noradrenaline overflow.3 Phentolamine (0.3 mg/kg, i.v.) completely antagonized these effects of clonidine. Prazosin (0.3 mg/kg, i.v.) inhibited by only 43 and 38% the respective reductions in heart rate and noradrenaline overflow elicited by clonidine.4 On termination of cardioaccelerator stimulation (about 10 min after either prazosin or phentolamine), heart rate and coronary sinus noradrenaline overflow returned to control prestimulation levels.5 Phentolamine or prazosin, administered alone during stimulation of the cardioaccelerator nerve, increased heart rate and noradrenaline overflow into the coronary sinus plasma. However, intravenous phentolamine and prazosin, in contrast to desipramine (0.3 mg/kg, i.v.) or tyramine (1.0 mg, i.a.), failed to change the tachycardia resulting from the local administration of noradrenaline into the sinus node artery (i.a.).6 These results show that in spinal dogs the clonidine-induced reduction in heart rate (elevated by electrical stimulation of the cardioaccelerator nerve) is accompanied by a fall in the quantity of noradrenaline overflowing into the coronary sinus plasma. The latter effect is presumably the result of an action of clonidine on cardiac presynaptic alpha-adrenoceptors, the activation of which is followed by a reduction in the release of noradrenaline per nerve impulse. Phentolamine and prazosin are both antagonists of cardiac presynaptic alpha-adrenoceptors in spinal dogs, as suggested by their action against clonidine and by their positive chronotropic effect when administered during stimulation of the cardioaccelerator nerve.

Alpha adrenoceptors in rat brain: direct identification with prazosin

Tritiated prazosin was used to characterize high affinity binding sites with characteristics similar to alpha 1 adrenoceptors in rat brain membranes. These sites were compared with alpha 2 adrenoceptors labeled with tritiated clonidine. The prazosin sites had an association constant of 2 nM-1 and bound to ligand optimal around pH 7.0. The density of the sites was 300 fmoles per mg of protein; the half time of dissociation of prazosin was 7 min at 30 degrees C. The order or potencies of agonists, determined from binding-inhibition experiments with labeled prazosin, was: naphazoline greater than clonidine greater than adrenaline greater than noradrenaline greater than phenylephrine greater than alpha-methylnoradrenaline greater than dopamine. The order of potencies of antagonists was: prazosin greater than phenoxybenzamine greater than phentolamine greater than clozapine greater than yohimbine. Sodium ions and divalent cations as well as guanyl nucleotides have little or no effect on the binding of the labeled antagonist. This is in contrast to the binding of the labeled agonist clonidine (Glossmann and Presek, 1979a, 1979b). Labeled prazosin may be a useful tool to characterize alpha 1 adrenoceptors.

The molecular structure of adrenergic and dopaminergic substances

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).

GABAergic and glycinergic mechanisms within the substantia nigra: pharmacological specificity of dopamine-independent contralateral turning behavior and interactions with other neurotransmitters

The pharmacological specificity of the GABA agonist muscimol-induced contralateral turning behavior after unilateral injection into substantia nigra pars reticulata (SNR) has been studied. Muscimol-induced turning was antagonized by intranigral bicuculline methochloride (BMC) and picrotoxin, whereas antagonists of glycine, morphine, dopamine, noradrenaline, and serotonin were ineffective. Glycine induced a qualitatively similar turning behavior which was strychnine-sensitive but relatively BMC and picrotoxin-insensitive. Other drugs, including substance P, kainic acid, clonidine, oxymetazoline, serotonin, and carbachol, induced turning that could be dissociated from the effect of muscimol. Muscimol-induced turning was dopamine-independent, indicated by resistance to haloperidol (1 mg/kg), to pretreatment with reserpine (7.5 mg/kg) plus alpha-methyl-p-tyrosine (200 mg/kg), to haloperidol injections into the SNR, striatum and nucleus accumbens, and finally to kainic acid lesions of the striatum. 6-Hydroxydopamine lesions increased the efficacy of intranigral muscimol, while kainic acid lesions of the SNR antagonized muscimol. Muscimol-induced turning was inhibited by oxotremorine (0.25 mg/kg), by intranigral carbachol, and by apomorphine (0.1--0.5 mg/kg), but only moderately by intranigrally injected apomorphine. These data suggest specificity of GABA-agonist-induced contralateral turning and indicate an interaction between nigral GABA and other neurotransmitters, particularly dopamine and acetylcholine.

The characteristics of [3H]-clonidine binding to an alpha-adrenoceptor in membranes from guinea-pig kidney

1. [3H]-clonidine binds to membranes prepared from guinea-pig kidney. 2. At 25 degrees C the binding is rapid and saturable. 3. Scatchard analysis of the binding data showed that the Kd for [3H]-clonidine binding in kidney membranes is 8.54 nM and the density of binding sites 12.5 pmol/g wet wt. tissue. 4. Hill plots of the binding data showed that there were no cooperative site interactions associated with binding. 5. [3H]-clonidine binding could be displaced by drugs, the most potent being drugs with a high affinity for the alpha-adrenoceptor. The neuroleptic drugs (+)-butaclamol, cis-clopenthixol and cis-flupenthixol at high concentration also displaced [3H]-clonidine binding. 6. Drugs acting as agonists or antagonists of beta-adrenoceptors, histamine receptors, acetylcholine receptors as well as prostaglandins E1, E2, F1alpha and F2alpha, angiotensin II, arginine vasopressin, naloxone, nalorphine and pargyline had little effect on binding. 7. It is likely that the binding site labelled by [3H]-clonidine in guinea-pig kidney membranes is an alpha-adrenoceptor similar in some pharmacological aspects to an alpha2-adrenoceptor.

Effect of catecholamine-receptor stimulating agents on blood pressure after local application in the nucleus tractus solitarii of the medulla oblongata

The effect of various catecholamines and alpha-mimetics, given by microinjection in the A2-region of the nucleus tractus solitarii (NTS), on blood pressure was investigated in anesthetized male rats. A dose-dependent decrease of blood pressure and heart rate was induced by adrenaline as the most effective drug, followed by noradrenaline, dopamine, alpha-methylnoradrenaline and octopamine. Ablation of the rostral or caudal part of the NTS, or removal of the area postrema did not diminish the effect of alpha-methylnoradrenaline. Higher doses of noradrenaline and alpha-methylnoradrenaline caused an initial rise of blood pressure, while the blood pressure lowering effect of noradrenaline was diminished, and that of alpha-methylnoradrenaline and dopamine delayed. Isoprenaline and the (+)-stereoisomers of noradrenaline and alpha-methylnoradrenaline were ineffective. The hypotensive effect of dopamine was not prevented by systemic injection of the dopamine beta-hydroxylase inhibitor FLA 63. Prior application of haloperidol, yohimbine and phentolamine antagonized the hypotensive response to dopamine and alpha-methylnoradrenaline. Application of peripherally effective alpha-mimetics into the A2-region had no or little effect, while high doses increased blood pressure. Tyramine and clonidine caused some decrease of blood pressure. Clonidine also decreased blood pressure when it was applied in the area of the locus coeruleus. Application of isoprenaline in the locus coeruleus also decreased blood pressure while in contrast adrenaline, noradrenaline, dopamine and alpha-methylnoradrenaline increased blood pressure. The present data suggest that the catecholaminergic receptors in the A2-region of the NTS differ from the classic vascular alpha-receptor and that the NTS also may contain structures which can antagonize the decrease in blood pressure.

Hyperpolarizing 'alpha 2'-adrenoceptors in rat sympathetic ganglia

1 Receptors mediating catecholamine-induced hyperpolarization of isolated superior cervical sympathetic ganglia of the rat have been characterized by means of an extracellular recording method.2 (-)-Noradrenaline (EC(50), 1.7 +/- 0.6 muM) produced an immediate low-amplitude (< 400 muV) hyperpolarization. The hyperpolarization was increased on removal of external Ca(2+) or on reduction of external K(+) from 6 to 2 mM. Hyperpolarization was unaffected by changing the temperature from 25 degrees to 37 degrees C.3 Hyperpolarization was also produced by the following agonists (potencies relative to (-)-noradrenaline): (-)-noradrenaline 1; (+/-)-isoprenaline 0.41; (-)-phenylephrine 0.40; (+)-noradrenaline 0.13; 2-amino-6,7-dihydroxy tetrahydronaphthalene (ADTN) 0.25; dopamine 0.1; methoxamine 0.012; amidephrine 0.0015.4 Responses were antagonized by phentolamine (1 muM) but not by (+/-)-propranolol (1 muM), haloperidol (10 muM) or alpha-flupenthixol (1 muM). This suggested that hyperpolarization was mediated solely through alpha-receptor stimulation not through stimulation of beta-receptors or dopamine-receptors.5 Dose-ratio shifts produced by phentolamine varied with different agonists. The shift increased in inverse proportion to the ability of the agonists to inhibit [(3)H]-(-)-noradrenaline uptake, suggesting that uptake of agonists limited the dose-ratio shift. Cocaine and nortriptyline reduced catecholamine-induced hyperpolarization in concentrations (10 muM and 1 muM respectively) necessary to inhibit [(3)H]-(-)-noradrenaline uptake.6 Clonidine (0.01 to 1 muM), oxymetazoline (0.01 to 1 muM) and ergometrine (0.1 to 10 muM) produced a persistent, low-amplitude hyperpolarization, as though they were partial agonists. Responses to the agonists were blocked by yohimbine (1 muM) but not be prazosin (1 muM).7 It is concluded that the adrenergic cell bodies in the ganglion were hyperpolarized through activation of the same type of alpha-receptor (;alpha(2)-receptors') as those present at adrenergic nerve terminals.

Blockade by WB 4101 of alpha-adrenoceptors in the rat vas deferens and guinea-pig taenia caeci

The effectiveness of WB 4101, a recently described alpha-adrenoceptor antagonist, in blocking an excitatory and two inhibitory responses to alpha-receptor activation was studied. One of the inhibitory responses was the reduction by the selective alpha-agonist amidephrine of carbachol contractures of isolated guinea-pig taenia caeci. WB 4101 antagonised this inhibition with a Schild plot slope of 0.99 and a pA2 of 8.9. The same pA2 value was obtained for blockade of the contractile effect of amidephrine and noradrenaline on the rat vas deferens. WB 4101 was, however, several hundred times less active in antagonising the inhibitory effect of clonidine on the twitch response of the vas deferens to field stimulation. Incidental observations were that the twitch was increased by low concentrations of amidephrine, and by relatively high concentrations of WB 4101. Because of its potency and postsynaptic selectivity, WB 4101 should be useful for adrenoceptor classification.

Catecholamine binding to CNS adrenergic receptors

The properties of 3H-catecholamine binding to alpha- and beta-adrenergic receptors in CNS are reviewed. 3H-epinephrine and 3H-norepinephrine label one class of alpha-receptors throughout the brain, with high affinities for agonists and some antagonists. Agonist affinities at this site are increased in low temperature conditions but are reduced by guanine nucleotides and monovalent cations. Divalent cations reverse both effects. This alpha-receptor may be coupled to adenylate cyclase by GTP and/or sodium, and uncoupled by divalent cations. 3H-epinephrine labels beta2, but not beta1, receptors in CNS, especially in bovine cerebellum. The same beta-receptor does not show agonist-specific GTP-sensitivity, but does exhibit Na+-sensitivity. This receptor appears to be linked to adenylate cyclase, and sodium rather than GTP may be the coupling agent.