Pilocarpine treatments differentially affect alpha2-adrenoceptors which modulate the firing rate of locus coeruleus neurones and the synthesis and release of noradrenaline in rat brain.

We have investigated the effect of treatments with the muscarinic acetylcholine receptor agonist, pilocarpine, on the sensitivity of central alpha2-adrenoceptors that regulate the firing activity of rat locus coeruleus, the tyrosine hydroxylase activity in the rat cortex, hippocampus and hypothalamus, and the K(+)-evoked release of [3H]noradrenaline from rat cortical and hippocampal synaptosomes. Short-term (4 days), but not acute, treatment with pilocarpine caused a small but statistically significant increase in the inhibitory effect of the alpha2-adrenoceptor agonist clonidine on the firing rate of locus coeruleus neurones, with a decrease in the ED50 of 290% (P<0.001). However, no change in the effect of clonidine on the locus coeruleus was observed after longer pilocarpine (11 days) treatment. In the rat cerebral cortex, but not in hippocampus or hypothalamus, chronic (19 days) treatment with pilocarpine caused a decrease in the inhibitory effect of clonidine on tyrosine hydroxylase activity (55%, P<0.05), but did not change the stimulatory effect of the alpha2-adrenoceptor antagonist idazoxan. Moreover, treatments (4, 11 and 19 days) with pilocarpine did not alter the inhibitory effect of clonidine [10(-8)-10(-5) M] on the K(+)-evoked release of [3H]noradrenaline from rat cortical and hippocampal synaptosomes. These results indicate that administration of pilocarpine slightly potentiates some but not all the functional responses mediated by brain presynaptic alpha2-adrenoceptors. In conclusion, these results do not support the hypothesis that chronic treatments with pilocarpine lead to a suitable model of alpha2-adrenoceptor supersensitivity.     

Determination of the somatodendritic alpha2-adrenoceptor subtype located in rat locus coeruleus that modulates cortical noradrenaline release in vivo.

The regulation of extracellular noradrenaline levels in the cingulate cortex by somatodendritic alpha2-adrenoceptors located in the locus coeruleus was evaluated in the rat by using dual-probe microdialysis. The concentration of noradrenaline in the cingulate cortex was decreased (37%-40%) by administration into the locus coeruleus (1microM) of the agonists clonidine and UK14304 (bromoxidine), whereas it was increased by similar administration of the nonselective antagonist RX821002 (2-methoxyidazoxan) (+ 103%) and the selective alpha2A-adrenoceptor antagonist BRL44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidaz ole) (+ 148%). The selective alpha2B/C-adrenoceptor antagonist ARC239 (2-[2[4-(o-methoxyphenyl)piperazin-1-yl]ethyl]-4,4-dimethyl-1,3-(2 H,4H)-isoquinolimedione) did not induce changes. In the presence of BRL44408, the effects of clonidine and UK14304 were abolished, but they were not modified in the presence of ARC239. The data demonstrate that noradrenaline release in terminal areas is tonically modulated by somatodendritic alpha2A-adrenoceptors.     

In vivo tonic modulation of the noradrenaline release in the rat cortex by locus coeruleus somatodendritic alpha(2)-adrenoceptors

The regulation of noradrenaline release in the rat cingulate cortex by somatodendritic alpha(2)-adrenoceptors placed in the locus coeruleus was evaluated by dual-probe microdialysis. The alpha(2)-adrenoceptor antagonists BRL44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole), RS79948 ((8,12,13)-decahydro-3methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine) and RX821002 (2-methoxyidazoxan) administered by reverse dialysis into the locus coeruleus increased concentration-dependently (0.01-100 microM) noradrenaline release in the cortex (maximal effects 170+/-30%, 543+/-17%, 195+/-26%, respectively). Administration of the alpha(2)-adrenoceptor antagonist idazoxan increased at lower (0.1-10 microM) but decreased at the highest dose (100 microM) noradrenaline in the cortex. These data demonstrate that somatodendritic alpha(2)-adrenoceptors in the locus coeruleus exert an inhibitory tonic modulation on noradrenaline release in noradrenergic terminal areas.     

Chronic reboxetine desensitizes terminal but not somatodendritic alpha2-adrenoceptors controlling noradrenaline release in the rat dorsal hippocampus.

The slow onset of antidepressant drugs' effects is thought to reflect the time required for the development of adaptive changes such as desensitization of presynaptic autoreceptors controlling the release of neurotransmitters. Using in vivo microdialysis in conscious rats, we studied the effect of a continuous infusion of the selective noradrenaline (NA) reuptake inhibitor reboxetine on extracellular concentrations of NA. Doses of 10 mg/kg/day reboxetine through subcutaneous osmotic pumps for 2 days increased extracellular NA by 272% in the dorsal hippocampus (DH) of rats. NA rose significantly more in rats given reboxetine for 7 (469%) and 14 (437%) days. Intraperitoneal injection of 30 microg/kg clonidine, an alpha2-adrenoceptor agonist, reduced the release of NA to 49% of basal levels in rats given vehicle or reboxetine for 2 days, but this effect was markedly less in rats given reboxetine for 7 and 14 days. Likewise, the effect of intrahippocampal infusion of clonidine (0.05 and 0.2 microM) on extracellular NA was significantly attenuated in rats given reboxetine for 7 and 14 days, whereas the injection of 0.6 nmol clonidine into the locus coeruleus caused similar reductions of extracellular NA in the DH and prefrontal cortex (PFC) of rats infused with vehicle (DH -64%; PFC -42%) and reboxetine (DH -45%; PFC -28%) for 14 days. The results indicate that chronic treatment markedly enhances the effect of reboxetine on extracellular NA in the DH and suggest that this effect may be due to the desensitization of hippocampal alpha2-adrenoceptors.     

Stress-induced release of substance P in the locus coeruleus modulates cortical noradrenaline release

Several lines of evidence implicate the neuropeptide substance P (SP) in the modulation of emotional behavior. Interaction between SP and noradrenergic systems has been proposed to be important in the regulation of stress, depression, and anxiety mechanisms; however, most evidence so far is based on studies in unchallenged and/or anesthetized animals. Thus, by using a dual-probe microdialysis approach in freely moving animals, the aim of the present study was to investigate whether a relevant stressor can trigger the release of SP in the locus coeruleus (LC) and whether and how this response modulates noradrenaline (NA) transmission both in the LC and in the medial prefrontal cortex (mPFC), an important LC terminal region involved in emotional processing. While confirming previous reports that neurokinin 1 receptor (NK1R) antagonists activate cortical noradrenergic transmission under resting conditions, we present evidence that this interaction is opposite during stress challenge. Our results show that exposure to forced swimming considerably enhanced the release of SP and NA in the LC. Administration of a selective NK1R antagonist into the LC potentiated this NA response within the LC but abolished the stress-induced increase in NA release within the mPFC. These findings demonstrate stress-induced increase in endogenous extracellular SP levels within the LC exerting a facilitatory effect on the noradrenergic pathway to the mPFC. The attenuation of stress-induced hyperactivation of this pathway by NK1R antagonists, presumably via enhancing NA and autoinhibition in the LC, may contribute to the therapeutic efficacy of these drugs known to ameliorate symptoms of stress-related disorders.     

Alpha 2-receptor control over release of noradrenaline in rat thalamus

Rat brain thalamic tissue was examined for the presence of alpha 2-receptor control over noradrenaline (NA) utilization. Systemically administered clonidine (0.1 mg/kg i.p.) decreased NA turnover, yohimbine (10 mg/kg i.p.) increased NA turnover, and prazosin (0.5 mg/kg i.p.) had no effect. In vitro, yohimbine increased K+-stimulated overflow of endogenous NA in a dose-dependent fashion. It appears that locus coeruleus neurons which project to thalamus exhibit the same type of alpha 2-receptor control as those terminating in other forebrain regions.     

Noradrenaline release in rat locus coeruleus is regulated by both opioid and alpha(2) -adrenoceptors.

Extracellular fluid levels of noradrenaline (NA) in the locus coeruleus (LC) during naloxone-precipitated morphine withdrawal with pretreatment of yohimbine (1 mg kg (-1), s.c.) or clonidine (1 mg kg (-1), s.c.) were measured in rats. There was a significant increase in the NA level after the injection of naloxone (2 mg kg (-1), i.p.) in the morphine-dependent rats. Moreover, the NA levels in the LC markedly increased during the 30-60 min following the naloxone (i.p.) challenge in the morphine-dependent rats pretreated with yohimbine. In contrast, the naloxone challenge in morphine-dependent rats pretreated with clonidine notably decreased the levels of NA in the LC. Behavioral signs of withdrawal were observed following the naloxone challenge in the morphine-dependent rats pretreated with yohimbine, with minimal signs in the morphine-infused rats pretreated with clonidine, and none in the saline-infused controls. These results directly suggest that NA increased within the LC after the naloxone challenge in morphine-dependent animals pretreated with yohimbine may be, at least in part, regulated by alpha(2) -adrenoceptors in the LC.     

Pharmacological evidence that alpha2A- and alpha2C-adrenoceptors mediate the inhibition of cardioaccelerator sympathetic outflow in pithed rats

It has been suggested that the alpha(2)-adrenoceptors mediating cardiac sympatho-inhibition in pithed rats closely resemble the pharmacological profile of the alpha(2A)-adrenoceptor subtype. However, several lines of evidence suggest that more than one subtype may be involved. Thus, the present study has pharmacologically re-evaluated the receptor subtype(s) involved in the inhibitory effect of the alpha(2)-adrenoceptor agonist, B-HT 933, on the tachycardic responses elicited by selective cardiac sympathetic stimulation (0.03, 0.1, 0.3, 1 and 3 Hz) in desipramine-pretreated pithed rats. I.v. continuous infusions of B-HT 933 (30 microg/kg min), which failed to modify the tachycardic responses to exogenous noradrenaline, inhibited those induced by preganglionic (C(7)-T(1)) stimulation of the cardiac sympathetic outflow at all frequencies of stimulation (0.03-3 Hz). This cardiac sympatho-inhibitory response to B-HT 933 was: (1) unaltered by saline (1 ml/kg) or the antagonists BRL44408 (100 microg/kg; alpha(2A)) or imiloxan (3000 and 10,000 microg/kg; alpha(2B)); (2) partially antagonized by BRL44408 (300 microg/kg) or MK912 (10 microg/kg; alpha(2C)) given separately; and (3) completely antagonized by rauwolscine (300 microg/kg; alpha(2)), MK912 (30 microg/kg) or the combination of BRL44408 (300 microg/kg) plus MK912 (10 microg/kg). Moreover, the above doses of antagonists, which are high enough to block their respective receptors, failed to block per se the tachycardic responses to sympathetic stimulation. These results suggest that the cardiac sympatho-inhibition induced by B-HT 933 in pithed rats is mainly mediated by stimulation of alpha(2A)- and alpha(2C)-adrenoceptors.     

Neuropeptide Y potentiates via Y2-receptors the inhibitory effect of noradrenaline in rat locus coeruleus neurones

Summary Intracellular recordings were carried out in a pontine slice preparation of the rat brain containing the locus coeruleus (LC). Pressure application of noradrenaline with various pulse durations inhibited the spontaneous frequency of action potentials and hyperpolarized the membrane. Neuropeptide Y (NPY), its C-terminal fragment NPY(16–36) and peptide YY (PYY), at a concentration of 0.1 µmol/l all, potentiated the effect of noradrenaline, while [Leu³¹, Pro³⁴]NPY (0.1 µmol/l) was inactive. These results are compatible with the presence of Y₂-type NPY-receptors at the cell somata of LC neurones. Correspondence to: P. Illes at the above address     

Presynaptic alpha 2-adrenoceptors modulate the release of [3H]noradrenaline from rat spinal cord dorsal horn neurones

Slices of the dorsal half of the rat spinal cord were used to investigate the existence of a noradrenergic feedback modulation of noradrenaline release. After crude preparation of the vertebral column, the spinal cord was ejected by hydraulic pressure and transverse slices were cut. These were preincubated with [3H]noradrenaline during 0.1 Hz electrical stimulation and then superfused and stimulated electrically for two periods. The stimulation-evoked release of [3H]noradrenaline was Ca2+-dependent and tetrodotoxin-sensitive. Pretreatment of the animals with the noradrenergic neurotoxin, DSP-4, reduced the tritium content in the slices and the stimulation-evoked release to less than 10% of the controls. Clonidine (0.01-1 microM) inhibited the evoked overflow by 60% maximally and yohimbine (0.1-1 microM) enhanced it by 160% maximally. The effects of clonidine were antagonized by yohimbine. These results provide evidence that noradrenaline release from spinal cord slices is controlled by an alpha 2-adrenoceptor-mediated, negative feedback mechanism.     

(-)-Deprenyl inhibits tyramine-induced noradrenaline release, but not tyramine-induced dopamine release or potassium-induced noradrenaline release, from rat brain synaptosomes.

The effect of (-)-deprenyl (selegiline), a therapeutic agent for Parkinson's disease, on the tyramine-induced release of catecholamine from rat brain synaptosomes was studied using a superfusion system. Tyramine (10(-7) to 10(-5)M) enhanced the release of [3H]noradrenaline (NA) and [3H]dopamine (DA) from forebrain and striatal synaptosomes in a dose-dependent manner. (-)-Deprenyl (5x10(-5)M) had no effect on spontaneous catecholamine release, suggesting that it has no tyramine-like catecholamine releasing effect. Pretreatment with (-)- or (+)-deprenyl (5x10(-5)M) significantly prevented the tyramine (10(-6)M)-induced NA release, but not DA release. The inhibitory action of (-)-deprenyl was not observed on potassium (15mM)-induced NA release. (-)-Desmethyldeprenyl (5x10(-5)M), a metabolite of (-)-deprenyl, and a monoamine oxidase-A (MAO-A) inhibitor, clorgyline (5x10(-5)M), failed to block the tyramine-induced NA and DA release. Although (+)-deprenyl, a potent DA uptake inhibitor, did not inhibit tyramine-induced DA release, a catecholamine uptake inhibitor nomifensine (5x10(-5)M) did. In summary, (-)-deprenyl at a dose inhibiting tyramine-induced NA release did not have any effect on tyramine-induced DA release or potassium-induced NA release.     

Alpha 2C-adrenoceptor-modulated release of noradrenaline in human right atrium.

1. The aim of the present study was to characterize the presynaptic alpha 2-autoreceptors in human right atrium in terms of the alpha 2A-D system. Segments of atrial appendages were preincubated with [3H]-noradrenaline and then superfused in the presence of cocaine and stimulated electrically. pEC30% values of eight alpha-adrenoceptor antagonists with discriminatory power were determined. pEC30% is the negative logarithm of the antagonist concentration that increased the stimulation-induced overflow of tritium by 30%. For four antagonists, the dissociation constant KD was determined, in addition to pEC30%, against the overflow-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under autoinhibition-free conditions. 2. pEC30% and KD values yielded identical rank orders of antagonist affinity (rauwolscine > WB 4101 > phentolamine > prazosin) suggesting that both released noradrenaline and the exogenous agonist UK 14,304 activated the same receptor to inhibit release. 3. The eight antagonist pEC30% values obtained in right atrium correlated significantly with their pEC30% values, reported in the literature, at the presynaptic alpha 2C-autoreceptors in human kidney (r = 0.817; slope of the regression line 1.03). No significant correlation was obtained between pEC30% values at atrial autoreceptors and pKD values at previously characterized alpha 2A-autoreceptors in rabbit and alpha 2D-autoreceptors in rat, mouse and guinea-pig tissues. 4. Comparison of antagonist pEC30% values with their pKD values at native alpha 2 binding sites in cells or tissues that express a single subtype only, and with pKD values at alpha 2 binding sites in membranes of COS cells transfected with human alpha 2 subtype genes confirms the alpha 2C character of the atrial autoreceptors: significant correlations were obtained exclusively with the alpha 2C binding sites. 5. Ratios of KD values were computed for alpha 2-autoreceptors in human right atrium and for binding sites in COS cells transfected with human alpha 2 subtype genes. The autoreceptor ratios corresponded well with the respective ratios for the alpha 2C binding sites (maximal three fold deviation) but were, in part, markedly different from ratios calculated for alpha 2A and alpha 2B binding sites (up to 166 fold deviation). This outcome supports the alpha 2C designation of the autoreceptors. 6. In conclusion, the presynaptic alpha 2-autoreceptors in human right atrium are alpha 2C. In this they agree with the previously characterized alpha 2-autoreceptors in human kidney. The alpha 2C classification possibly separates, in general, human alpha 2-autoreceptors from those in lagomorph (rabbit) and rodent (rat, mouse, guinea pig) species that have been proposed to be predominantly alpha 2A or alpha 2D.     

Characterization of noradrenaline release in the locus coeruleus of freely moving awake rats by in vivo microdialysis

Rationale The origin and regulation of noradrenaline (NA) in the locus coeruleus (LC) is unknown.Objectives The neurochemical features of NA overflow (nerve impulse dependence, neurotransmitter synthesis, vesicle storage, reuptake, ₂-adrenoceptor-mediated regulation) were characterized in the LC.Methods Brain microdialysis was performed in awake rats. Dialysates were analyzed for NA.Results NA in the LC decreased via local infusion of Ca²⁺-free medium (–42±5%) or the sodium channel blocker tetrodotoxine (TTX) (–47±8%) but increased (333±40%) via KCl-induced depolarization. The tyrosine hydroxylase (TH) inhibitor -methyl-p-tyrosine (250 mg kg^–1, i.p.) and the vesicle depletory drug reserpine (5 mg kg^–1, i.p.) decreased NA. Therefore, extracellular NA in the LC satisfies the criteria for an impulse flow-dependent vesicular exocytosis of neuronal origin. Local perfusion of the ₂-adrenoceptor agonist clonidine (0.1–100 M) decreased NA (E_max=–79±5%) in the LC, whereas the opposite effect (E_max=268±53%) was observed with the _2A-adrenoceptor antagonist BRL44408 (0.1–100 M). This suggests a tonic modulation of NA release through local _2A-adrenoceptors. The selective NA reuptake inhibitor desipramine (DMI) (0.1–100 M) administered into the LC increased NA in the LC (E_max=223±40%) and simultaneously decreased NA in the cingulate cortex, confirming the modulation exerted by NA in the LC on firing activity of noradrenergic cells and on the subsequent NA release in noradrenergic terminals.Conclusion Synaptic processes underlying NA release in the LC are similar to those in noradrenergic terminal areas. NA in the LC could represent local somatodendritic release, but also the presence of neurotransmitter release from collateral axon terminals.     

Modulation of noradrenaline release in rat isolated stomach by prostanoids,but not by histaminergic mechanisms

Several gastric functions are modulated by the sympathetic nervous system, but local mechanisms involved in the control of noradrenaline release are largely unknown.Overflow of endogenous noradrenaline was studied from isolated rat stomach incubated in Ussing chambers allowing the separate determination of mucosal and serosal overflow. Spontaneous noradrenaline overflow was similar at the mucosal and serosal side, but electrical field stimulation caused a frequency-dependent increase in noradrenaline overflow selectively at the serosal side. Evoked noradrenaline overflow was blocked by tetrodotoxin, not affected by indometacin and markedly enhanced (by about 250%) by yohimbine. In the presence of indometacin and yohimbine, sulprostone (an agonist at EP₁/EP₃ receptors) and misoprostol (an agonist at EP₂/EP₃ receptors) reduced the noradrenaline overflow evoked by stimulation at 3 Hz maximally by about 80% (EC₅₀: 6 nmol/l and 11 nmol/l, respectively). The EP₁ receptor selective antagonist AH 6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) did not antagonize the inhibition by sulprostone. Noradrenaline overflow evoked by stimulation at 1 Hz and 3 Hz was increased by scopolamine by about 50% and almost completely inhibited by oxotremorine. Neither, histamine nor the H₃ receptor selective agonist (R)--methyl-histamine, nor the H₁, H₂ and H₃ selective receptor antagonists mepyramine, cimetidine and thioperamide significantly affected noradrenaline overflow evoked by stimulation at 1 Hz or 3 Hz.In conclusion, impulse-induced noradrenaline release in the rat stomach is controlled by multiple presynaptic mechanisms involving ₂-adrenergic autoreceptors, EP₃ prostanoid and muscarine heteroreceptors, whereas histaminergic mechanisms do not appear to be significant.     

Chronic dexmedetomidine, a selective alpha 2-agonist, decreases serotonin but not noradrenaline turnover in rat brainstem nuclei.

The effects of a chronic treatment with dexmedetomidine, the active d-isomer of a selective alpha 2-agonist, medetomidine, on the metabolism and turnover of catecholamines and indoleamines in discrete rat brain stem and mesencephalic nuclei were investigated. Chronic dexmedetomidine (3 or 10 micrograms/kg per h for 14 days, delivered by osmotic minipumps) did not change the basal concentrations of noradrenaline, dopamine or 3,4-dihydroxyphenylacetic acid, or the rates of accumulation of 3,4-dihydroxyphenylalanine after inhibition of decarboxylase by NSD 1015. In contrast, the concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and/or 5-HIAA/5-HT were significantly reduced in the nucleus tractus solitarii, raphe dorsalis, locus coeruleus, A1-C1 area, A9 and A10 areas after 10 micrograms/kg per h of dexmedetomidine. The accumulation of 5-hydroxytryptophan was significantly reduced in the locus coeruleus and nucleus tractus solitarii. It is concluded that chronic dexmedetomidine treatment significantly decreases the turnover of 5-hydroxytryptamine in discrete brainstem and mesencephalic nuclei, while the rate of synthesis of noradrenaline in the same areas is not changed.     

5-HT1B receptors and alpha 2A/2C-adrenoceptors mediate external carotid vasoconstriction to dihydroergotamine

Dihydroergotamine produces external carotid vasoconstriction in vagosympathectomized dogs by 5-HT(1B/1D) receptors and alpha(2)-adrenoceptors. This study identified the specific subtypes involved in this response. One-minute intracarotid infusions of dihydroergotamine (5.6-10 microg/min) dose-dependently decreased external carotid blood flow without affecting blood pressure or heart rate. This response was: (1) partly blocked in dogs pretreated intravenously with the antagonists SB224289 (5-HT(1B); 2,3,6,7-tetrahydro-1'-methyl-5-[2'-methyl-4' (5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-carbonyl]furo[2,3-f]indole-3-spiro-4'-piperidine hydrochloride), rauwolscine (alpha(2)), BRL44408 (alpha(2A); 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole) or MK912 (alpha(2C); (2S,12bS)-1'3'-dimethylspiro(1,3,4,5',6,6',7,12b-octahydro-2Hbenzo[b]furo[2,3-a]quinazoline)-2,4'-pyrimidin-2'-one); (2) markedly blocked after SB224289 plus rauwolscine; and (3) unaffected after BRL15572 (5-HT(1D); 1-(3-chlorophenyl)-4-[3,3-diphenyl (2-(S,R) hydroxypropanyl) piperazine] hydrochloride) or imiloxan (alpha(2B)). Therefore, the above response involves 5-HT(1B) receptors and alpha(2A/2C)-adrenoceptors.     

Modulation of noradrenaline release in slices of rat kidney cortex through alpha 1- and alpha 2-adrenoceptors

Slices of rat kidney cortex were incubated in [3H]noradrenaline, then placed in a flow cell and subjected to electrical field stimulation. At a stimulation frequency of 5 Hz, both the alpha 2-adrenoceptor antagonist idazoxan (0.1 microM) and the alpha 1-adrenoceptor antagonist prazosin (0.1 microM) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity from the slice. However, neither clonidine (0.1 microM) nor methoxamine (10 microM), alpha 2- and alpha 1-agonists respectively, affected the S-I outflow of radioactivity at this stimulation frequency. At a lower stimulation frequency (1 Hz), the S-I outflow of radioactivity was not affected by idazoxan or prazosin, but was inhibited by both clonidine and methoxamine. The effect of clonidine was prevented by idazoxan (0.1 microM), but not by prazosin (0.1 microM). The effect of methoxamine was abolished by prazosin (0.1 microM), but not by idazoxan (0.1 microM). The inhibitory effect of methoxamine was not prevented by the prostaglandin synthesis inhibitor indomethacin (10 microM) or the adenosine receptor antagonist 8-phenyltheophylline (1 microM) and thus was not mediated by either prostaglandins or adenosine. The results suggest that both prejunctional alpha 1- and alpha 2-adrenoceptors are directly involved in modulation of noradrenaline release from the renal sympathetic nerves of the rat.     

Sympathectomy reveals alpha 1A- and alpha 1D-adrenoceptor components to contractions to noradrenaline in rat vas deferens

We have previously demonstrated that contractions of rat vas deferens to exogenous noradrenaline involve predominantly alpha(1A)-adrenoceptors, but that contractions to endogenous noradrenaline involve predominantly alpha(1D)-adrenoceptors. In this study, we have examined the effects of sympathectomy on the subtypes of alpha(1)-adrenoceptor in rat vas deferens in radioligand binding and functional studies. In vehicle-treated tissues, antagonist displacement of [(3)H]prazosin binding to alpha(1)-adrenoceptors was consistent with a single population of alpha(1)-adrenoceptors. Binding affinities for a range of alpha(1)-adrenoceptor antagonists were expressed as pK(i) values and correlated with known affinities for alpha(1)-adrenoceptor subtypes. The correlation was significant only with alpha(1A)-adrenoceptors. In tissues from rats sympathectomised with 6-hydroxy-dopamine (2 x 100 mg kg(-1) i.p.), binding affinity for the alpha(1D)-adrenoceptor antagonist BMY 7378 fitted best with a two-site model. In functional studies, the potency of noradrenaline at producing total (phasic plus tonic) but not tonic contractions was increased in tissues from sympathectomised rats. Results obtained from sympathectomised rats suggest that phasic contractions are mainly alpha(1D)-adrenoceptor mediated, whereas tonic contractions are mainly alpha(1A)-adrenoceptor mediated, based on the effects of BMY 7378 and the alpha(1A)-adrenoceptor antagonist RS 100329. It is concluded that the predominant alpha(1)-adrenoceptor in vehicle-treated rat vas deferens is the alpha(1A)-adrenoceptor, both in terms of ligand binding and contractions to exogenous agonists. The alpha(1D)-adrenoceptor is only detectable by ligand binding following chemical sympathectomy, but is involved in noradrenaline-evoked contractions, particularly phasic contractions, of rat vas deferens.     

Different alpha-adrenoceptors modulate the release of 5-hydroxytryptamine and noradrenaline in rat cortex.

1 The potassium-evoked release of [3H]-noradrenaline from slices of rat occipital cortex and the potassium-evoked release of [3H]-5-hydroxytryptamine from slices of rat frontal cortex were measured using a superfusion system. 2 The rank order of potency for a number of alpha-adrenoceptor agonists was different for the two neuronal systems, clonidine and azepexole being the most potent inhibitors of noradrenaline release and methoxamine and phenylephrine being the most potent against 5-hydroxytryptamine release. 3 The rank order of potency for a series of alpha-adrenoceptor antagonists in reversing the inhibition of noradrenaline release produced by clonidine was: phentolamine greater than rauwolscine = yohimbine = corynanthine much greater than WB4101, whereas against methoxamine-inhibition of 5-hydroxytryptamine release the rank order of potency was: WB4101 greater than phentolamine greater than corynanthine greater than yohimbine greater than rauwolscine. 4 The results suggest that the alpha-adrenoceptors which modulate potassium-evoked 5-hydroxytryptamine release are not identical with the alpha 2-adrenoceptors which modulate potassium-evoked 5-hydroxytryptamine release are not identical with the alpha 2-adrenoceptors located on noradrenergic nerve terminals and may more closely resemble alpha 1-than alpha 2-adrenoceptors.