Contraction-mediating α2-adrenoceptors in the mouse vas deferens

Summary The question of the existence of postjunctional, contraction-mediating ₂-adrenoceptors, in addition to the known ₁-adrenoceptors, was studied in the mouse isolated vas deferens. Both the ₁-selective agonist phenylephrine and the ₂-selective agonist 5-bromo-6-(2imidazolin-2-ylamino)-quinoxaline (UK 14,304) caused contraction of the vas deferens. In the presence of the ₁-selective antagonist prazosin (added in order to prevent an ₁ component in the effect of high concentrations of UK 14,304), the ₂-selective antagonists yohimbine and idazoxan shifted the concentration—response curve of UK 14,304 to the right in a manner compatible with competitive antagonism and with dissociation constants K_B indicating the involvement of ₂-adrenoceptors. The maximal contraction elicited by UK 14,304 (in the presence of prazosin) was much lower than the maximal contraction elicited by phenylephrine. The effect of UK 14,304 was not changed by the P₂-purinoceptor agonist ,-methylene-ATP and was reduced by neuropeptideY, but was markedly enhanced by relatively low concentrations of phenylephrine. When the sympathetic fibres of the vas deferens were stimulated by trains of ten widely spaced (0.5 Hz) electric pulses, the tissue responded with ten separate twitches in which purinergic and adrenergic components were isolated by prazosin and suramin, respectively. Prazosin reduced the first adrenergic twitch in these trains at concentrations close to its K_B value at ₁-adrenoceptors, whereas yohimbine and idazoxan reduced the first adrenergic twitch at concentrations far lower than their K_B values at ₁-adrenoceptors. The results indicate that the smooth muscle of the mouse vas deferens possesses contraction-mediating ₂-adrenoceptors. They are activated by UK 14,304 and probably also by noradrenaline of neural origin. Responses mediated by the ₂-adrenoceptors are enhanced by simultaneous ₁-receptor activation, an interaction that may increase the contribution of the ₂-adrenoceptors to the adrenergic phase of neurogenic contractions. Send offprint requests to R. Bültmann at the above address     

Involvement of presynaptic imidazoline receptors in the α2-adrenoceptor-independent inhibition of noradrenaline release by imidazoline derivatives

Summary An involvement of imidazoline recognition sites in the modulation of transmitter release was investigated in the rabbit pulmonary artery and aorta preincubated with [³H]noradrenaline and superfused with physiological salt solution containing cocaine, corticosterone and propranolol. Electrical impulses were applied transmurally at 0.66 or 2 Hz. In the absence of further drugs, rauwolscine as well as the imidazoline derivatives BDF 6143 [4-chloro-2-(2-imidazoline-2-ylamino)-isoindoline], idazoxan and phentolamine increased the ³H overflow from the pulmonary artery, evoked by electrical stimulation at 2 Hz; the effect was due to the ₂-autoreceptor blocking property of these drugs. The maximum increase in overflow obtainable with the imidazolines was considerably lower than with rauwolscine. The concentration-response curves of the imidazolines were bell-shaped. At 0.66 Hz, BDF 6143 did not facilitate, but concentration-dependently inhibited, whereas idazoxan failed to change the evoked ³H overflow. When, at the stimulation frequency of 2 Hz, presynaptic ₂-adrenoceptors were blocked by rauwolscine and/or pre-exposure to phenoxybenzamine, the electrically evoked ³H overflow from the pulmonary artery and/or aorta was inhibited by the following imidazoline derivatives: the ₂-adrenoceptor antagonists BDF 6143, idazoxan and phentolamine, the ₁-adrenoceptor agonist with ₂-blocking property cirazoline as well as the ₂-adrenoceptor agonists clonidine and moxonidine. The maximum inhibition caused by BDF 6143 was greater than that due to clonidine and moxonidine; the latter two, hence, behaved as partial agonists. At the stimulation frequency of 0.66 Hz, the imidazolines exhibited a higher potency than, but a similar intrinsic activity to that at 2 Hz. Noradrenaline did not affect the evoked ³H overflow. The BDF 6143-induced inhibition of evoked ³H overflow was not modified by metitepine, atropine, theophylline, dipyridamole and indometacin, but was counteracted by the partial agonists clonidine and moxonidine.The results exclude the possibility that ₁- and ₂-adrenoceptors, 5-HT₁ receptors, muscarine receptors, P₁ purinoceptors and prostaglandin receptors are involved in the imidazoline-induced inhibition of noradrenaline release. They provide evidence indicating that the inhibitory effect is mediated by imidazoline receptors on the postganglionic sympathetic nerve terminals of the rabbit pulmonary artery and aorta.     

Effect of α2-adrenoceptor agonists on the expression of morphine-withdrawal in rats

Summary Previous studies using clonidine indicate that ₂-adrenoceptors are involved in suppressing opiate-withdrawal symptoms. However, clonidine may act as a partial agonist at ₂-adrenoceptors and it also possesses significant ₁-receptor agonist activity.The aim of this study was to determine the role of ₂-adrenoceptors in the expression of opiate withdrawal signs using morphine-dependent rats. A range of agonists were selected for study on the basis of their differential preferences for -adrenoceptors.Hooded Wistar rats were made physically dependent on morphine (s.c. injection of an emulsion releasing a total of 250 mg/kg of morphine base over 48 h). Test drugs were injected s.c. followed by naloxone (10 mg/kg i.p.) 20 min later. The incidence of 5 selected withdrawal signs was recorded during the following 20 min. The ₂-adrenoceptor agonists displayed different profiles of activity. Azepexole (1–10 mg/kg) reduced all signs. Clonidine (80–800 g/kg) reduced all signs except paw shakes while guanfacine (25–250 g/kg) reduced all except jumping and diarrhoea. Talipexole (0.1–1 mg/kg) reduced all signs except diarrhoea which was not affected and jumping which was markedly enhanced. UK 14,304 (80–800 g/kg) reduced jumps, potentiated paw shakes but did not affect body shakes, teeth chattering or diarrhoea. The results suggest that there are subpopulations of ₂-adrenoceptors that modulate the expression of opiate withdrawal signs and/or that some of the drugs used affect receptors other than ₂-adrenoceptors.     

Increase by α-adrenolytic drugs of acetylcholine release evoked by field stimulation of the guinea-pig ileum

Summary The release of acetylcholine evoked by field stimulation of the guinea-pig ileum (3 Hz) is increased by yohimbine and tolazoline but not affected by phentolamine. It is proposed that yohimbine and tolazoline by blocking -adrenoceptors of the cholinergic nerves abolish the inhibition caused by endogenous noradrenaline, and thus facilitate the output of acetylcholine.     

Pertussis toxin does not attenuate α2-adrenoceptor mediated inhibition of noradrenaline release in mouse atria

Summary 1. The ₂-adrenoceptor agonist clonidine (0.03 and 0.1 ol/l) significantly inhibited stimulation-induced overflow of radioactivity from mouse isolated atria pre-incubated with [³H]-noradrenaline. This effect of clonidine was blocked by idazoxan (0.3 gmol/l) but not prazosin (0.3 ol/l), indicating that an ₂-adrenoceptor was involved. 2. In some experiments mice were injected with pertussis toxin (1.5 g/mouse) 4 days before their atria were removed and subsequently incubated with [³H]-noradrenaline. Alternatively, isolated atria from untreated mice were suspended in Krebs-Henseleit solution, incubated for 16 h with pertussis toxin (1.0 and 4.0 g/ml) or vehicle and subsequently incubated with [³H]-noradrenaline. The effectiveness of pertussis toxin pretreatment was assessed indirectly using carbachol. Carbachol caused a dose dependent fall in both the rate and force of contraction of isolated, spontaneously beating atria from mice pretreated with vehicle in vivo or in vitro. This effect of carbachol was not seen in atria from mice pretreated with pertussis toxin in vivo or in vitro, suggesting that active toxin penetrated the myocardium. 3. Pertussis toxin pretreatment, either in vivo or in vitro did not alter the inhibitory effect of clonidine (0.03 and 0.1 gmol/l), or the facilitatory effect of the -adrenoceptor antagonist phentolamine (1.0 mol/l), on the stimulation-induced overflow of radioactivity. These results suggest that ₂-adrenoceptor modulation of noradrenaline release from sympathetic nerve terminals is not dependent on an inhibitory guanine-nucleotide-binding protein. Send offprint requests to I. Musgrave     

The effect of selective α1- and α2-adrenoceptor stimulation on intraocular pressure in the conscious rabbit

Summary The influence of topically applied selective ₁- and ₂-adrenoceptor agonists on intraocular pressure and the diameter of the pupil was investigated in conscious rabbits. Selective stimulation of the ₁-subtype of receptors induced an elevation in intraocular pressure, accompanied by mydriasis, whereas stimulation of the ₂-subtype caused a marked and dose-dependent ocular hypotensive response, which was blocked by the selective ₂-adrenoceptor antagonist yohimbine. ₂-Agonists induced neither macroscopic ocular side effects, nor an effect on the pupil size. Possibly, the subclass of ₂-adrenoceptor stimulating drugs represent a group of new antiglaucomatous agents.     

α2-Adrenoceptor-mediated autoinhibition of sympathetic transmitter release in guinea-pig vas deferens studied by intracellular and focal extracellular recording of junction potentials and currents

Summary Excitatory junction potentials (e j.ps; intracellular electrodes) and excitatory junction currents (e.j.cs; extracellular electrodes) elicited by stimulation (20 pulses at 1 Hz every minute) of the hypogastric nerve trunk were recorded from guinea-pig isolated vas deferens. Intracellular recording. At a variety of stimulation intensities, bath-applied yohimbine (0.1–1 mol/l) did not change the first one to three e j.ps in a train but increased the amplitude of subsequent e j.ps. The effect of yohimbine was abolished in tissues from reserpinepretreated guinea pigs. Bath-applied desipramine (0.1 mol/l) diminished the amplitude of all but the first one to three e j.ps in a train. - Extracellular recording. Yohimbine (0.1–1 mol/l), when applied locally through the recording suction electrode, increased the number of e.j.cs per given number of stimuli, i. e., enhanced the probability of occurrence of e.j.cs. When desipramine (0.1 mol/l) was present both in the bath and in the recording electrode, the probability of the occurrence of e.j.cs was decreased. In the presence of desipramine, yohimbine (0.1–1 mol/l) increased the number of e j.cs even more markedly. Neither the nerve terminal impulse nor the number of spontaneous e j.cs was changed by yohimbine. A mixture of tetraethylammonium (2 mmol/l) and 4-aminopyridine (0.2 mmol/l), when applied locally, both increased the number of e.j.cs and changed markedly the shape of the nerve terminal impulse.These experiments demonstrate presynaptic ₂-autoinhibition at a high degree of resolution, i. e., when the intermittent release of transmitter from only a few varicosities along a single terminal axon is monitored by the e j.c. ₂-Autoinhibition is not due to a depression of impulse conduction but to a depression of stimulus-secretion coupling in varicosities reached by the impulse. Taken together with the low probability of transmitter release at the level of individual varicosities, the results support the idea of lateral inhibition by noradrenaline released from distant varicosities rather than inhibition due to noradrenaline released from the same varicosity. The mode of action of yohimbine differs from that of K⁺ channel blocking agents. Send offprint requests to K. Starke at the above address     

Effects of suramin and α,β-methylene ATP indicate noradrenaline-ATP co-transmission in the response of the mouse vas deferens to single and low frequency pulses

Summary Vasa deferentia from mice were field-stimulated by trains of 10 pulses delivered at 0.5 Hz. The pulses elicited separate twitches, the first of which (corresponding to a single pulse) exceeded the following ones in height and width and often was clearly biphasic. , -Methylene-ATP 1 mol/1 and suramin 100 mol/1 caused almost identical changes. They reduced the height of the first twitch in the train by about one half and also reduced its width in a manner indicating that only the second, slow phase remained, but reduced much more markedly the following twitches in which now a small second, slow phase also became detectable. Idazoxan 0.1 mol/1 or yohimbine 0.1 mol/l, when added in the presence of a, -methylene-ATP or suramin, further decreased the first twitch but enhanced twitches No. 2 to 10. These responses were then almost abolished by prazosin 0.1 mol/l. Successive addition of prazosin 0.1 mol/l and idazoxan 0.1 mol/1 to previously untreated vasa deferentia depressed the response to the first pulse by about one half in a manner indicating that only the first, rapid phase remained, but had comparatively little effect on the responses to the subsequent pulses. Suramin 100 mol/l almost abolished the contractions remaining in the presence of prazosin and idazoxan. The results indicate that the first, rapid phase of the neurogenic contractions elicited by single or low frequency pulses is mediated by ATP which substantially contributes to all responses in a train. The second, slow phase is mediated by noradrenaline which substantially contributes to the response to the first pulse only. The adrenergic contribution seems to be mediated by postjunctional ₁- as well as a ₂-adrenoceptors. Prejunctional ₂-adrenergic autoinhibition depresses the release of both ATP and noradrenaline. Send offprint request to I. v. Kügelgen at the above address     

Inhibition by α2-adrenoceptor agonists of the secretion of catecholamines from isolated adrenal medullary cells

Summary In adrenal medullary cells, carbachol evokes the secretion of catecholamines with simultaneous uptake of⁴⁵Ca. Highly selective agonists for ₂-adrenoceptors, clonidine, naphazoline, guanfacine, tramazoline and oxymetazoline inhibited carbachol evoked secretion of catecholamines in a dose-dependent manner. These ₂-agonists also inhibited the uptake of⁴⁵Ca evoked by carbachol with similar dose-response curve to inhibition of catecholamine secretion. On the contrary, highly selective agonists for ₁-adrenoceptors, phenylephrine and norfenefrine did not inhibit the secretion of catecholamines and cellular uptake of⁴⁵Ca. The inhibition by ₂-agonists was not restored either by the increase in carbachol, or Ca concentrations, suggesting that the mode of inhibition was distinct from competition at cholinergic receptors or Ca-channels. It is likely that ₂-agonists inhibited the secretion of catecholamines via the inhibition of Ca uptake which was probably caused through the activation of ₂-adrenoceptors.     

Effect of K+ channel blockers on the α2-adrenoceptor-coupled regulation of electrically evoked noradrenaline release in hippocampus

Summary The question whether presynaptic ₂-adrenoceptors regulating noradrenaline release in hippocampus directly couple to tetraethylammonium chloride (TEA) or -dendrotoxin (-DTX)-sensitive K⁺ channels was investigated. Hippocampal slices, prelabelled with [³H] noradrenaline, were superfused in the presence of (+)-oxaprotiline and electrically stimulated with 4 pulses delivered at 100 Hz, in order to avoid autoinhibition due to released noradrenaline.TEA enhanced the evoked [³H]noradrenaline release in rabbit hippocampus in a concentration-dependent manner, yielding an approximately 4-fold increase at 30 mmol/l, whereas the spontaneous outflow of tritium was only slightly affected at this concentration. The ₂-adrenoceptor agonist clonidine, at 10–100 nmol/l inhibited the evoked [³H]noradrenaline release between 77% and 96%. The inhibitory effect of the ₂-agonist was distinctly diminished in the presence of 30 mmol/l TEA but was restored in low Ca²⁺/high Mg²⁺ buffer. Therefore, the diminution of the ₂-agonist effect by TEA observed in experiments with normal Ca²⁺ can be explained by an increase of the Ca²⁺ availability for the release process due to the prolongation of action potentials. In rabbit hippocampus -DTX (10–200 nmol/l) did neither affect the evoked release of [³H]noradrenaline nor its ₂-agonist-induced modulation. However, in rat hippocampus -DTX significantly increased the evoked transmitter release and diminished the effect of clonidine.Taken together, the present data for the rabbit hippocampus exclude the possibility that activation of presynaptic ₂-adrenoceptors inhibits depolarization-evoked [³H]noradrenaline release by inducing an outward K⁺ current through TEA- or -DTX-sensitive K⁺ channels. However, there are species differences between the rabbit and the rat so that in the rat the ₂-adrenoceptors could actually be coupled to K⁺ channels — provided that the release-enhancing properties of -DTX do not account for the ₂-antagonism observed.Correspondence to C. Allgaier at the above address     

Relationship of metabolism of 2′-, 3′- and 5′-adenine nucleotides to presynaptic inhibition of transmitter release in rat vas deferens

Summary In the isolated rat vas deferens stimulated at 0.2 Hz, a series of 2, 3-, and 5-substituted adenine nucleotides all inhibited the twitch responses, their actions being potentiated by the nucleoside transport inhibitors, HNBTGR, NBMPR and dipyridamole.The metabolism of these nucleotides was examined utilising HPLC analysis of the bathing medium after exposure to 30 M nucleoside or nucleotide for 5 min. 5-AMP, 5-ADP, 5-ATP, and NAD⁺ were all partially hydrolysed to adenosine, the relative extent of this being 5-AMP>5-ADP=5-ATPNAD⁺. However, the other nucleotides examined were not detectably converted to adenosine or to adenosine deamination products.These results indicate that the 2-, 3- and 5-substituted nucleotides studied act at a P₁-purinoceptor in rat vas deferens to inhibit neurotransmission and, with the exception of 5-AMP, 5-ADP, 5-ATP and NAD⁺, all appear to act directly at this receptor. However, the 5-adenine nucleotides (AMP, ADP and ATP) and NAD⁺ all appear to act at least partially indirectly subsequent to their hydrolysis to adenosine.Abbreviations. The following abbreviations are used ADA adenosine deaminase (EC 3.5.4.4) - 5-ADP adenosine 5-diphosphate - 2,5-ADP adenosine 2,5-diphosphate - 3 5-ADP, adenosine 3,5-diphosphate - 2-, 3 or 5-AMP adenosine 2-, 3-, or 5-monophosphate - 5-ATP adenosine 5-triphosphate - cNADP⁺ -nicotinamide dinucleotide 2,3-cyclic monophosphate - CoA coenzyme A - HNBTGR 6-(2-hydroxy-5-nitrobenzyl)-thioguanosine - NAD⁺ -nicotinamide adenine dinucleotide - NADP⁺ -nicotinamide adenine dinucleotide phosphate - NBMPR 6-(4-nitrobenzylthio)-purine riboside     

Antagonism of the hypothermic effect of clozapine in mice by centrally-active α2-adrenergic antagonists and α1-adrenergic agonists

Hypothermia induced by either clozapine or clonidine in mice was blocked by the ₂-adrenergic antagonists yohimbine, idazoxan, CH-38083, SKF 86466, and L-657,743. These effects were dose related, and the ID₅₀ values for inhibition of clozapine- or clonidine-induced hypothermia were fairly comparable. The order of potency for blocking clonidine-induced hypothermia was: L-657,743>CH-38083>yohimbine>idazoxan>SKF 86466. A very similar blockade hierarchy for clozapine-induced hypothermia was observed, with the order of the two most effective compounds being reversed. Hypothermia induced by either compound was not blocked by the peripherally-acting, selective ₂-adrenergic antagonist, L-659,066, indicating that blockade by the other compounds occurred centrally. The centrally-acting, ₁-adrenergic agonists St 587, cirazoline, and SKF 89748 were very effective in blocking the response to clozapine, but ineffective in antagonizing clonidine-induced hypothermia. The ED₅₀ values for the blockade of this response to clozapine, however, did not correlate with their reported potencies in stimulating either peripheral or central ₁-adrenergic receptors. This indicates that clozapine-induced hypothermia in mice is not a suitable model for evaluating the properties of central ₁-adrenergic compounds. Moreover, since the clonidine-induced hypothermia is not influenced by ₁-adrenergic agonists, this paradigm is preferable to clozapine-induced hypothermia in the assessment of ₂-adrenergic antagonism. The ability of ₂-adrenergic antagonists to block clozapine-induced hypothermia may result from the central overflow of norepinephrine, which is known to be brought about by this group of compounds. The neurochemical mechanism responsible for the anti-clozapine effect of the ₁-adrenergic agonists in mice is not clear. An as yet unknown property of these compounds, unrelated to ₁-agonism, may have to be considered. An interaction of these compounds at the high-affinity clozapine binding site is a possibility. Discovery of antagonists to clozapine may help to elucidate the mechanism of action of this atypical neuroleptic.     

Activation of presynaptic α2-adrenoceptors attenuates the inhibitory effect of μ-opioid receptor agonists on noradrenaline release from brain slices

Summary ³H-noradrenaline release from rat neocortical slices induced by 15 mM K⁺ was concentration-dependently inhibited by morphine, [D-Ala²-D-Leu⁵] enkephalin (DADLE) and the calcium entry blocker Cd²⁺. Blockade of presynaptic ₂-adrenoceptors with phentolamine, almost doubling K⁺-induced ³H-noradrenaline release, slightly enhanced the relative inhibitory effects of morphine and DADLE, whereas that of Cd²⁺ remained unaffected. In contrast, activation of presynaptic ₂-adrenoceptors with clonidine (1 M) or TL-99 (1 M), inhibiting release by about 50%, completely abolished the inhibitory effects of morphine and DADLE without affecting that of Cd²⁺. When in the presence of 1 M clonidine adenylate cyclase was activated with forskolin (10 M), which restored release to the drug-free control level, the opioids still did not display their inhibitory effects. Therefore, -opioid receptor efficacy appears to be dependent on the degree of activation of ₂-adrenoceptors in central noradrenergic nerve terminals, probably through a local receptor interaction within the nerve terminal membrane.     

Protein kinase C activation increases noradrenaline release from the rat hippocampus and modifies the inhibitory effect of α2-adrenoceptor and adenosine A1-receptor agonists

Summary We have studied the effect of stimulating protein kinase C with phorbol esters on the release of [³H]-noradrenaline (NA) in the absence or presence of presynaptic ₂-adrenoceptor blocking agents and compared that to the elevation of cyclic AMP levels more than 10-fold by a combination of rolipram and forskolin. 4--Phorbol 12,13-dibutyrate (PDiBu) increased stimulated (3 Hz) [³H]-NA release markedly and in a concentration dependent manner. 4--Phorbol-12,13-didecanoate was ineffective. The effect of PDiBu was not significantly reduced by nifedipine (1 M), but was proportionally less in the presence of an ₂-adrenoceptor antagonist, yohimbine. PDiBu inhibited the presynaptic effect of ₂-adrenoceptor agonists clonidine and UK 14304. By contrast, the presynaptic effect of the adenosine analogue R-PIA was not reduced by PDiBu. PDiBu caused an increase in cyclic AMP that depended on adenosine receptor stimulation. Elevation of cyclic AMP had a limited effect on NA release from rat hippocampus, and did not significantly decrease the presynaptic inhibitory effect of UK 14304 (0.1 M), of morphine (1 M) or of the adenosine A₁-receptor agonist CHA (1 M). The effect of phorbol esters and several presynaptic inhibitors of NA-release in the rat hippocampus cannot be explained by changes in cyclic AMP levels in the tissue. Phorbol esters that stimulate protein kinase C appear to interact with a target that is the site of action ₂-adrenoceptors in this tissue. This site is not a dihydropyridine sensitive Ca-channel and is also different from the target of presynaptic adenosine receptors. Thus, activation of protein kinase C discriminates between apparently similar presynaptic mechanisms. Send offprint requests to B. Fredholm at the above address     

Electrophysiological evidence for an α2-adrenergic inhibitory control of transmitter release in the rabbit mesenteric artery

Excitatory junction potentials (e.j.p.s) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of the rabbit isolated mesenteric artery. Clonidine and B-HT 933 depressed all e.j.p.s. in the train. The percentage inhibition was inversely related to the number of pulses. Yohimbine, rauwolscine and tolazoline reduced the early e.j.p. amplitudes but enhanced the later ones. The percentage facilitation of e.j.p.s increased with the number of pulses until a maximum was reached. Prazosin and corynanthine did not influence the first few e.j.p.s but potentiated the subsequent ones; their effects were less pronounced than those of yohimbine and rauwolscine. All the drugs antagonized the inhibition by clonidine but the effects of yohimbine and rauwolscine were more marked than those of prazosin and corynanthine. Phenylephrine, St 587 and noradrenaline depressed the e.j.p.s. Yohimbine diminished the effects of these substances and was a stronger antagonist of phenylephrine than prazosin. We suggest that, in the rabbit mesenteric artery, noradrenaline and the neuroeffector transmitter (probably ATP) are co-released from the terminals of postganglionic sympathetic nerves. Noradrenaline activates presynaptic α₂-adrenoceptors and thereby depresses transmitter release. The degree of presynaptic inhibition depends on the number of pulses applied, i.e. on the biophase concentration of noradrenaline.     

In vivo potentiation of reboxetine and citalopram effect on extracellular noradrenaline in rat brain by α2-adrenoceptor antagonism

The therapeutic activity of noradrenaline reuptake inhibitors (NaRIs) and serotonin reuptake inhibitors (SSRIs) as antidepressant is based on their ability to increase monoamine concentrations in the synaptic cleft. α₂-Adrenoceptors inhibit noradrenaline (NA) release, which modulates antidepressant neurochemical activity. The present study assesses the influence of the addition of the selective α₂-adrenoceptor antagonist RS79948 to the NaRI reboxetine and the SSRI citalopram on brain extracellular NA. Dual-probe microdialysis technique in the locus coeruleus (LC) and prefrontal cortex (PFC) was performed in freely moving rats. Acute reboxetine (3 and 5 mg/kg i.p.) promoted a dose-dependent increase of NA in LC (164 ± 15%; 243 ± 24%) and PFC (140 ± 7%; 181 ± 30%). Acute citalopram (5 mg/kg i.p.) did not change NA in LC or PFC, but at 10 mg/kg i.p. increased NA in LC (144 ± 14%) and decreased it in PFC (− 42 ± 7%). An inactive dose of RS79948 (0.1 mg/kg i.p.) in rats pretreated with reboxetine (3 mg/kg i.p.) or citalopram (5 mg/kg i.p.) induced a significant enhancement of NA in LC (reboxetine: 462 ± 137%; citalopram: 142 ± 11%) and PFC (reboxetine: 281 ± 56%; citalopram: 130 ± 16%). The results indicate that co-administration of selective α₂-adrenoceptor antagonist drugs might improve the effects of NaRI or SSRI antidepressants by enhancing extracellular NA concentrations in the brain.     

A comparison of the effects of TMB-8 and nifedipine on pressor responses to α1- and α2-adrenoceptor agonists in pithed rats

TMB-8 has been characterized as an inhibitor of the release of Ca⁺ from intracellular pools. We have studied the modification of the pressor responses to selective _l-adrenoceptor agonists (methoxamine and phenylephrine), and to selective ₂-adrenoceptor agonists (B-HT 920 and B-HT 933) in pithed rats, produced by TMB-8. We have compared this modification with that produced by the calcium antagonist nifedipine. Nifedipine (100 g/kg, 300 g/kg, and 1000 g/kg) inhibited in a dose-dependent manner the pressor responses to the ₁- and ₂-adrenoceptor agonists, the dose-response curves to the ₂-adrenoceptor agonists being shifted further to the right. TMB-8 at a dose of 3000 g/kg did not modify the pressor effects of the _l-adrenoceptor agonists, and neither did it reinforce the inhibition of such responses produced by nifedipine. By contrast, TMB-8 pretreatment (0.03 g/kg, 0.3 g/kg, 3 g/kg, 30 g/kg, 300 g/kg and 3000 g/kg) inhibited the responses to both ₂-adrenoceptor agonists, the inhibition being more pronounced with B-HT 920. A similar effect was obtained with 0.03 g/kg TMB-8 and 0.3 g/kg TMB-8, particularly in the case of B-HT 920. It was stronger with higher doses, but similar for all doses over 3 g/kg. The inhibition of the pressor responses mediated by the stimulation of ₂-adrenoceptors by TMB-8 was less in rats treated with the Ca²⁺ entry promoter BAY K 8644 (300 g/kg), and could also be reduced by the continuous infusion of CaCl₂ (0.25 g/min). These results suggest that in pithed rats TMB-8 may also behave as an inhibitor of the Ca⁺ influx into vascular smooth muscle.     

Use of transgenic (knockout) mice reveals a site distinct from the α2A-adrenoceptors for agmatine in the vas deferens

The inhibitory effect of agmatine on electrically induced contractions was studied in vas deferens of Adra 2a transgenic mice lacking α_2A-adrenoceptors. Agmatine and clonidine caused a concentration-dependent inhibition of twitches. However, while agmatine showed a similar pIC₅₀ value in control and transgenic mice, the pIC₅₀ value for clonidine was about 30-fold lower in knockout mice. In both strains, yohimbine shifted the curve for clonidine, but not for agmatine, even when a 100-fold higher concentration of yohimbine was employed. Our results indicate that inhibition by agmatine in mouse vas deferens is not simply due to interactions with α₂-adrenoceptors in our experimental conditions.     

Opposite modulation of noradrenaline and ATP release in guinea-pig vas deferens through prejunctional β-adrenoceptors: evidence for the β2 subtype

Activation of prejunctional -adrenoceptors has been suggested to increase the release of noradrenaline but to decrease the neural release of ATP in the guinea-pig vas deferens. Experiments were carried out to determine the subtype of -adrenoceptor involved.In [³H]-noradrenaline-preincubated tissues superfused with medium containing prazosin and suramin, isoprenaline (1–100 nM), salbutamol (0.01–1 M) and terbutaline (0.1–10 M) increased the overflow of tritium but reduced the overflow of ATP elicited by electrical stimulation (210 pulses/7 Hz). The effects of isoprenaline were blocked by the ₂-selective antagonist 1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol (ICI 118,551; 100 nM). In prazosin- and suramin-free medium, isoprenaline (100 nM) did not change the overflow of ATP elicited by exogenous noradrenaline (10 M). Isoprenaline (1–100 nM), salbutamol (0.01–1 M) and terbutaline (0.1–10 M) reduced the initial twitch contraction elicited by electrical stimulation (210 pulses/7 Hz) in prazosin-and suramin-free medium as well as the isolated purinergic neurogenic contraction obtained by exposure to prazosin. They increased or tended to increase the secondary sustained contraction elicited by electrical stimulation in prazosin- and suramin-free medium as well as the isolated adrenergic neurogenic contraction obtained in the presence of suramin. The inhibition by isoprenaline of the isolated purinergic contraction was attenuated by ICI 118,551 (100 nM) but not by the ₁-selective antagonist 1-[2-((3carbamoyl-4-hydroxy)phenoxy)ethylamino]-3-[4-(1-methyl4-trifluoromethyl-2-imidazolyl)phenoxy]-2-propanol (CGP 20712A; 100 nM).The results confirm the opposite -adrenoceptor-mediated modulation of noradrenaline and neural ATP release in the guinea-pig vas deferens. They show that the prejunctional -adrenoceptor is of the ₂ subtype.