Subtype-specificity of the presynaptic alpha 2-adrenoceptors modulating hippocampal norepinephrine release in rat

In vivo brain microdialysis and high-performance liquid chromatography with electrochemical detection were used to study the effect of different selective alpha 2-antagonists on hippocampal norepinephrine (NE) release in freely moving awake rat. Systemic administration (0.5 mg/kg i.p.) of either the alpha 2AD-antagonist BRL 44408 or the alpha 2BC-antagonist ARC 239 did not significantly change the basal release of NE. At a higher dose (5 mg/kg i.p.) ARC 239 was still ineffective, whereas BRL 4408 caused a significant increase of the extracellular level of NF. Similar results were obtained from in vitro perfusion experiments. Rat hippocampal slices were loaded with [3H]NE and the electrical stimulation-evoked release of [3H]NE was determined. The alpha 2-antagonists were applied in a concentration range of 10(-8) to 10(-6) M, ARC 239 was ineffective, whereas BRL 44408 significantly increased the electrically induced release of [3H]NE. In agreement with the data of microdialysis and perfusion experiments, BRL 44408 displaced [3H]yohimbine from hippocampal and cortical membranes of rat brain with high affinity whereas ARC 239 was less effective. The pKi values of eight different alpha 2-adrenergic compounds showed a very good correlation (r = 0.98, slope = 1.11 P < 0.0001) in hippocampus and frontal cortex have the alpha 2-adrenoceptors have been characterized as alpha 2d-subtype. Our data indicate that hippocampal NE release in rat is regulated by alpha 2D-adrenoceptors, a species variation of the human alpha 2A-subtype.

Effect of noradrenergic lesions on subtypes of alpha 2-adrenoceptors in rat brain

The binding of [3H]rauwolscine to alpha 2A- (also referred to as alpha 2D-) and alpha 2C-adrenoceptors in homogenates of rat cerebral cortex was measured by exploiting the selectivity of oxymetazoline for alpha 2A-adrenoceptors. Inhibition of [3H]rauwolscine binding by oxymetazoline was modeled best assuming binding to two sites (p < 0.001). Competition curves for oxymetazoline were shifted rightward by the addition of GTP (250 microM) but were still fit best by a two-site model (p < 0.001). A concentration of oxymetazoline was calculated that would optimally antagonize [3H]rauwolscine binding (with GTP present) to oxymetazoline-sensitive alpha 2A-adrenoceptors, minimally inhibiting binding to alpha 2C-adrenoceptors. Subsequently, [3H]rauwolscine binding to alpha 2A- and alpha 2C-adrenoceptors in cortex was examined 3 weeks after destruction of noradrenergic terminals. Binding to alpha 2C-adrenoceptors was increased significantly after treatment with 6-hydroxydopamine, (6-OHDA) compared with vehicle-treated controls, whereas binding to alpha 2A-adrenoceptors was unchanged. Pretreatment of rats with desipramine before 6-hydroxydopamine, to protect noradrenergic neurons, resulted in no changes in binding to either alpha 2A- or alpha 2C-adrenoceptors. Thus, alpha 2C-adrenoceptors are regulated by changes in synaptic availability of norepinephrine. alpha 2A-Adrenoceptors are either not regulated by synaptic norepinephrine or are located both post- and presynaptically so that up-regulation of postsynaptic alpha 2A-adrenoceptors is offset by a loss of presynaptic alpha 2A-adrenoceptors.

The relationship between density of alpha-adrenoceptor binding sites and contractile responses in several porcine isolated blood vessels

1. The aim of this study was to investigate constrictor alpha-adrenoceptors in three isolated blood vessels of the pig, the thoracic aorta (TA), the splenic artery (SA) and marginal ear vein (MEV) and then compare the functional response with the densities of alpha 1- and alpha 2-adrenoceptor binding sites in these and several other porcine vascular tissues, palmar common digital artery (PCDA), palmar lateral vein (PLV) and ear artery (EA). 2. Noradrenaline (NA), phenylephrine (PE) and UK14304 (all at 0.03-10 microM) elicited concentration-dependent contractions in the TA and MEV, with a rank order of potency of UK14304 > NA > PE. UK14304 produced maximal responses which were 58% (TA) and 65% (MEV) of that of NA. In the SA, UK14304 and PE produced maximal responses which were less than 10% and 50% of the NA-induced maximal response respectively, with an order of potency of NA > PE. In the SA, NA-induced contractions were competitively antagonized by prazosin (pA2 = 8.60 +/- 0.15). Further, rauwolscine (1-10 microM) antagonized NA-induced contractions with an apparent pKB of 6.09 +/- 0.11 (n = 6), indicating an action at alpha 1-adrenoceptors. The combination of the two antagonists at concentrations selective for alpha 1- (0.1 microM) and alpha 2-adrenoceptors (1 microM) had no greater effect than either antagonist alone. This suggests that the SA expresses only post-junctional alpha 1-adrenoceptors. 3. In the TA, prazosin produced non-parallel shifts in the NA-induced CRC and this was also observed with rauwolscine, where reductions in the maximal responses were also observed. In the MEV, prazosin was largely inactive in antagonizing NA-induced contractions. In both these vessels a combination of these two antagonists had a greater effect than either alone, indicating the presence of functional alpha 1- and alpha 2-adrenoceptors. The post-junctional alpha 2-adrenoceptors in all of these vessels were resistant to prazosin, suggesting the alpha 2-adrenoceptor to be of the alpha 2A/2D subtype. The expression of functional alpha 2-adrenoceptors was MEV > TA > PLV > PCDA > SA. 4. In radioligand binding studies using TA P2 pellet membranes, [3H]-prazosin and [3H]-RX821002 ([1,4-[6,7(n)-3H] benzodioxan-2-methoxy-2-yl)-2-imidazole) labelled different high affinity sites, and in competition studies using identical membranes corynanthine displaced [3H]-prazosin with 10 fold higher affinity than rauwolscine, indicating that [3H]-prazosin was selectively binding to alpha 1-adrenoceptor sites. Further, rauwolscine displaced [3H]-RX821002 with approximately 100 fold greater affinity compared to corynanthine, which is indicative of selective alpha2-adrenoceptor binding.5. Separation of the P2 pellet into plasma membrane and mitochondrial fractions was carried out using a differential sucrose density gradient. [3H]-prazosin and [3H]-RX821002 binding sites were found in both the plasma membrane and mitochondrial fractions.6. In saturation studies all tissues produced single site saturation curves with no difference in the Kd(range 0.13-0.20nM) of the alpha1-adrenoceptor sites for [3H]-prazosin. However, there was considerable variation in Bmax of alpha 1-adrenoceptor sites; the highest density was found in the TA (397.9 =/- 52.7 fmol mg-1, n = 4), followed by the PCDA (256.7 +/- 22.7 fmol mg-1, n = 4), the PLV and SA having approximately equal density (143.6 +/- 3.9 and 159.1 +/- 7.0 fmol mg-1 respectively, n = 4 for both), followed bythe EA (91.3 +/- 10.5 fmol mg-1, n = 3) and the MEV had the lowest density (48.9 +/- 11.4 fmol mg-1,n = 3).7. In saturation studies using [3H]-RX821002, all tissues produced single site saturation curves with no differences in the Kd values (range 1.31 +/- 2.16 nM) but the highest densities were found in the TA and MEV (545.3 +/- 36.2 and 531.0 +/- 40.9 fmol mg-1 respectively), followed by the PLV (418.4 +/- 39.4 fmol mg-1), then the EA (266.3 +/- 40.0 fmol mg-1), and low densities of [3H]-RX821002 binding being found in the PCDA and SA (155.9 +/- 18.1 and 117.5 +/- 19.3 fmol mg-1 respectively).8. The pattern of binding site distribution for alpha l- and alpha 2-adrenoceptors is in reasonable agreement with functional studies carried out in these porcine vascular tissues; the TA has the highest densities of alpha 1-and alpha2-adrenoceptors; in the SA and PCDA there is a predominance (although small) of alpha l-adrenoceptor binding sites, the reverse of which is observed both in the PLV and MEV (i.e. greater density of alpha2-adrenoceptor sites). Thus, it would appear that alpha 1- and alpha2-adrenoceptor densities play a role in the expression of functional responses via these receptor subtypes; although it is interesting to note that the SA did have a small density of alpha 2-adrenoceptor binding sites, no functional response was observed after alpha2-adrenoceptor activation.

Precoupling of alpha-2B adrenergic receptors and G-proteins in transfected PC-12 cell membranes: influence of pertussis toxin and a lysine-directed cross-linker

The ability of pertussis toxin (PTX) pretreatment to alter the binding of [3H]rauwolscine (RAU) to alpha-2B adrenergic receptors expressed in PC12 cells was examined. PTX caused a 30% increase in the Bmax for [3H]RAU and reduced its KD, whereas in the added presence of Na+ and Gpp(NH)p binding was increased to 75% above the level in untreated membranes. Because all three agents act to reduce receptor/G-protein affinity, the increased binding may reflect extensive precoupling of the alpha-2B receptor. The affinity of the agonist epinephrine in displacing [3H]RAU was normally reduced by both Na+ and Gpp(NH)p; however, in PTX-treated membranes the effect of Gpp(NH)p was eliminated, and Na+ remained effective. The lysine-directed cross-linking reagent ethyleneglycol bis(succinimidyl)succinate (EGS) was utilized in an attempt to cross-link precoupled receptor and G-protein. Maximal [3H]RAU binding was reduced by EGS in a time- and dose-dependent manner, and this action was reversed by prior incubation with Na+ and Gpp(NH)p, suggesting that EGS did indeed cross-link receptor and G-protein. RAU and epinephrine each provided protection against the effect of EGS. The inclusion of Na+ and Gpp(NH)p during [3H]RAU binding studies was able to restore maximal binding in EGS-treated membranes to the same level as untreated membranes. These results indicate that in the absence of Na+ and Gpp(NH)p at least 40% of the total alpha-2B adrenergic receptors in these membranes exist as a precoupled receptor/G-protein complex which fails to bind [3H]RAU.

Species orthologs of the alpha-2A adrenergic receptor: the pharmacological properties of the bovine and rat receptors differ from the human and porcine receptors

Four pharmacological subtypes of the alpha-2 adrenergic receptor have been identified; however, only three subtypes exist in any given species. Although the alpha-2A adrenergic receptor, as defined by the human platelet, and the alpha-2D receptor, as defined in the bovine pineal, have very different pharmacological characteristics, they are more similar to each other than either is to the alpha-2B or alpha-2C subtype. The human alpha-2-C10 clone (alpha-2A) and the rat RG20 clone have an 89% identity in their predicted amino acid sequence and are considered to be species orthologs. Although the expressed RG20 clone appears to have alpha-2D pharmacology, a careful comparison of its pharmacological characteristics with the bovine pineal has not been reported previously. Based on the pKi values of a panel of 13 alpha-2 adrenergic agents that have been used previously to compare the alpha-2A, alpha-2B and alpha-2C subtypes, the pharmacological characteristics of the bovine pineal alpha-2D receptor appear to be very similar to the rat RG20 clone (correlation coefficient, r, of 0.93). The porcine ortholog of the human alpha-2-C10 receptor has pharmacological characteristics identical to the human alpha-2A receptor (r = 0.99). Because of its higher affinity for the alpha-2D receptor, [3H]RX821002 is a better radioligand than [3H]rauwolscine for studying this receptor subtype.

Normal distribution of alpha 2-adrenoceptors in the rat spinal cord and its modification after noradrenergic denervation: a quantitative autoradiographic study

The distribution of alpha 2 (alpha 2)-adrenoceptors along cervical, thoracic, lumbar, and sacral segments of the spinal cord of normal rats has been studied by quantitative autoradiography using the specific alpha 2-antagonist [3H]rauwolscine as a ligand. In addition, the influence of noradrenergic (NA) denervation [obtained either by complete transection of the spinal cord at vertebrae level T8-T9 or by selective lesion of NA spinal cord system carried out by intracisternal injection of 6-hydroxydopamine (6-OHDA)] on eventual variations of alpha 2-adrenoceptor density at spinal cord target cells was studied in parallel. In control rats, the quantitative analysis of alpha 2-adrenoceptor densities revealed the presence of these receptors throughout the whole gray matter with a preferential location in the superficial dorsal horn. This pattern was the same at all rostro-caudal levels of the cord and appeared very well correlated with the distribution of NA terminals revealed by immunohistochemistry, particularly in the superficial layers of the dorsal horn. After total transection of the spinal cord (caudally to the section) and 6-OHDA-induced lesion, an increase of alpha 2-adrenoceptor density was mainly observed within the distal dorsal horn thus evidencing supersensitivity in this area, while modifications were not detectable in other regions of the spinal gray matter, except at the lumbar level where other dorsal, central, and intermediate zones were significantly enriched.

Use of a hollow fiber bioreactor for large-scale production of alpha 2-adrenoceptors in mammalian cells

Gene cloning has revealed the existence of receptors, which are structurally similar but pharmacologically distinct. One recent example is the alpha 2-adrenergic receptor (alpha 2AR) family with three members. Preparation of membrane-embedded G-protein coupled receptor subtypes in pure form is practically impossible from natural sources and only recombinant techniques have provided possibilities to study these receptors in great detail. In this respect, both yeast and insect cell hosts have been applied successfully but no good mammalian alternative has been described for large-scale production. We describe in this report the use of S115 mouse mammary tumor cells as an effective host for large-scale production of alpha 2-adrenoceptors. These cells can be easily adapted to grow in a hollow fiber bioreactor, with up to 2.8 g of total cellular protein produced in one 0.8 m2 casette. We also show that each recombinant alpha 2-subtype exhibits their expected ligand binding properties, and suggest therefore that this system could be generally applicable to other eukaryotic plasma membrane proteins.

Statistical analysis of ligand-binding experiments

After a brief review of commonly used methods for parameter estimation from ligand-binding data in the biochemistry literature, we propose some diagnostic checks and statistical tests of the underlying assumptions and develop methods for evaluating the biases and variances of the estimates and for constructing confidence intervals. Examples on the analysis of data from two radioligand-binding experiments are presented to illustrate these methods.

Expression of two alpha 2-adrenergic receptor subtypes in human placenta: evidence from direct binding studies

Adrenergic receptors may play an important role for mediating a variety of metabolic and haemodynamic effects of catecholamines including placental blood flow. The alpha-adrenergic receptors of the human placenta were characterized in vitro by the use of [3H]rauwolscine and [3H]prazosin as radioligands. Saturation experiments would suggest that the alpha-adrenoceptors in the human placenta are alpha 2. Comparative binding studies were performed, using recently synthesized compounds (Beecham Pharmaceuticals, UK) selective for alpha 2A (BRL-44408) and alpha 2B (BRL-41992) subtypes. The results indicate that human placenta contains at least two pharmacologically distinct alpha 2-adrenoceptor subtypes with approximately 60 per cent alpha 2A and 40 per cent alpha 2B receptors. In contrast with the pattern of increasing beta-adrenoceptor density, the concentration of alpha 2-adrenoceptors in term placentae is significantly lower than in placentae from the first trimester.

Characterization of imidazoline binding protein(s) solubilized from human brainstem: studies with [3H]idazoxan and [3H]clonidine

Imidazoline binding sites from the human brainstem were solubilized with 3-[(3-cholamido-propyl)-dimethylammonio]-1-propane-sulfonate (CHAPS). [3H]idazoxan and [3H]clonidine were used as ligands to characterize the solubilized binding sites. In both the soluble and membrane fractions, [3H]idazoxan binding was saturable, stereoselective, sensitive to imidazolines and insensitive to (-)norepinephrine and to amiloride. The affinities of [3H]idazoxan for the soluble and membrane sites were similar (KD = 25 +/- 11 nM and 20 +/- 3 nM). In both soluble and membrane fractions, the alpha 2-adrenoceptor binding being masked with (-)norepinephrine, [3H]clonidine bound to a low affinity site which was insensitive to (-)norepinephrine and which exhibited the same selectivity for various drugs as the [3H]idazoxan binding site. alpha 2-adrenoceptor binding was present in the membrane and the soluble fractions although it was difficult to detect in the soluble fraction because of inhibition of [3H]rauwolscine binding by the CHAPS detergent.

Agonist binding to alpha 2-adrenoceptors is elevated in the locus coeruleus from victims of suicide

The binding of an agonist, p-[125I]iodoclonidine, and an antagonist, [3H]yohimbine, to alpha 2-adrenoceptors was measured autoradiographically in the locus coeruleus from 10 pairs of antidepressant-free victims of suicide and age-matched controls. Agonist binding to alpha 2-adrenoceptors was significantly greater in the locus coeruleus from victims of suicide compared with control subjects. In contrast, antagonist binding to alpha 2-adrenoceptors in the locus coeruleus did not differ significantly between control and suicide subjects. HPLC analysis of norepinephrine in tissue sections of the locus coeruleus did not reveal any differences between control subjects and suicide victims, suggesting that differences in agonist binding are not a result of differences in retention of the endogenous agonist norepinephrine in tissue sections. The increase in agonist binding to alpha 2-adrenoceptors in the locus coeruleus of victims of suicide links an altered expression of the high-affinity state of autoinhibitory alpha 2-adrenoceptors with suicide.

Characterization of binding sites for [3H]-idazoxan, [3H]-P-aminoclonidine and [3H]-rauwolscine in the kidney of the dog

1. We characterized the binding of [3H]-rauwolscine, [3H]-p-aminoclonidine and [3H]-idazoxan in a dog kidney membrane preparation. Our aim was to determine the pharmacological nature of the alpha 2-adrenoceptor- and imidazoline-preferring binding sites in this organ. 2. [3H]-Rauwolscine bound to an apparent single site with an affinity (KD) of 2.2 nmol/L and a maximum density (Bmax) of 58.5 fmol/mg protein, when 10 mumol/L idazoxan defined non-specific binding. However displacement studies demonstrated that a number of compounds, including prazosin, inhibited [3H]-rauwolscine binding in a complex manner consistent with displacement from two distinct binding sites. The majority (69%) of the [3H]-rauwolscine binding sites had a relatively low affinity for prazosin (KI = 398 nmol/L), while the remainder had a relatively high affinity for prazosin (KI = 7.9 nmol/L). 3. [3H]-p-Aminoclonidine bound to an apparent single site (KD = 5.2 nmol/L; Bmax = 72.4 fmol/mg protein), when 10 mumol/L phentolamine defined non-specific binding. When 1 mumol/L of the potent and selective alpha 2-adrenoceptor antagonist 2-methoxyidazoxan was included in the incubate, no specific binding was detected. We therefore conclude that under the conditions of this experiment [3H]-p-aminoclonidine binds only to alpha 2-adrenoceptors in the dog kidney. 4. [3H]-Idazoxan bound to two sites, with a higher (KD = 0.95 nmol/L; Bmax = 43.9 fmol/mg protein) and lower (KD = 9.1 nmol/L; Bmax = 93.8 fmol/mg protein) affinity, respectively, when 1 mmol/L phentolamine defined non-specific binding. When 10 mumol/L GTP gamma S was included in the incubate, the low affinity site was unaffected but the maximum binding at the higher affinity site was reduced by 79%. 2-Methoxyidazoxan displaced [3H]-idazoxan in a monophasic manner and with low potency (IC50 = 11.5 mumol/L). Yohimbine, efaroxan, clonidine, rilmenidine, guanabenz and idazoxan itself displaced [3H]-idazoxan in a complex manner; the slope of the displacement curves being less than unity. 5. We conclude that the dog kidney contains a heterogeneous population of alpha 2-adrenoceptors that can be labelled either with [3H]-rauwolscine or [3H]-p-aminoclonidine. The dog kidney also contains a heterogeneous population of non-adrenoceptor imidazoline-preferring binding sites of the I2-subtype, that can be labelled with [3H]-idazoxan. The binding site for which [3H]-idazoxan has the highest affinity appears to be coupled to a guanine nucleotide binding regulatory protein.

Large-scale cultivation of Catharanthus roseus cells: production of ajmalicine in a 20-l airlift bioreactor

Bioreactor systems have been developed for the production of ajmalicine, an alkaloid used in the treatment of hypertension. Cell cultures of Catharanthus roseus produced higher levels of ajmalicine (323 micrograms g-1 dry weight) in a production medium enriched with tryptophan. The cell cultures were grown in medium prepared in tap water and market sugar with a view to minimise the costs of production. Large-scale cultivation of cell suspension was performed in a 20-l airlift bioreactor under controlled conditions. An ajmalicine production of 315 micrograms g-1 dry weight was achieved in the bioreactor after 14 d of cultivation.

Monoamine receptors in the amygdaloid complex of the tree shrew (Tupaia belangeri)

Although it is well known that the mammalian amygdala comprises a heterogeneous complex of cytoarchitectonically and histochemically distinct nuclei, the association of these nuclei with different monoamine systems has not been described in detail. We therefore investigated the pattern of receptors for monoamines in the amygdala of the tree shrew (Tupaia belangeri). Binding sites for the alpha 2-adrenoceptor ligand (3H)rauwolscine, the alpha 1-adrenoceptor ligand (3H)prazosin, the beta-adrenoceptor ligand (125I)iodocyanopindolol, and the serotonin1A-receptor ligand (3H)8-hydroxy-2(di-n-propylamino)tetralin were visualized by in vitro autoradiography, and anatomically localized by comparing the autoradiograms to Nissl- and acetylcholinesterase-stained sections. To characterize binding of the radioligands pharmacologically, displacement experiments with different specific competitors were performed. Whereas the highest number of alpha 2-adrenergic binding sites was detected in the medial and the central nucleus as well as in the intercalated nuclei, the majority of serotonin1A binding sites was found in the magnocellular basal nucleus and the accessory basal nucleus, demonstrating a clear difference in the anatomy of the alpha 2-adrenergic and the serotonin1A receptor systems. In contrast, the pattern of alpha 1-adrenoceptor binding partially overlaps with that of both former receptor types. While the number of alpha-adrenergic and serotonin1A binding sites is relatively high in the tree shrew amygdala, there is a low number of beta-adrenergic binding sites in most nuclei. However, in the cortical nuclei, moderate to high numbers of binding sites for all radioligands are present. Therefore, according to our data on the tree shrew amygdala, which is anatomically similar to the amygdala of cats and primates, alpha 2-adrenoceptors cover primarily the medial part of the amygdaloid formation and serotonin1A-receptors predominantly occupy the basal nuclei, whereas alpha 1-adrenoceptors are present in both parts of the formation.

Characterization of alpha 2-adrenoceptor binding sites in rabbit ciliary body membranes

PURPOSE

This study sought to identify and characterize subtypes of alpha 2-adrenoceptors in rabbit ciliary body.

METHODS

Radioligand binding assays were performed with the alpha 2-agonist ligand [125I]-p-iodoclonidine ([125I]PIC) and the antagonist ligand [3H]rauwolscine.

RESULTS

Both [125I]PIC and [3H]rauwolscine bound to a single population of receptors in membrane preparations of rabbit ciliary body. The densities for the two ligands were the same (about 640 fmol/mg). However, the affinity of [125I]PIC (dissociation constant, Kd = 1.91 +/- 0.24 nM) was about threefold higher than that of [3H]rauwolscine (dissociation constant = 6.79 +/- 1.5 nM), and binding of [125I]PIC exhibited guanine nucleotide sensitivity. Inhibition of [125I]PIC binding by epinephrine, idazoxan, and amiloride was examined to differentiate between alpha 2-adrenoceptors and the imidazoline-preferring receptor. Epinephrine and idazoxan competed for all of the [125I]PIC binding; relative potency was epinephrine > idazoxan >> amiloride. Subtypes of alpha 2-adrenoceptors were further studied by competition for [125I]PIC binding by subtype-selective compounds. [125I]PIC binding sites showed the pharmacologic characteristics of an alpha 2A-adrenoceptor (oxymetazoline > chlorpromazine >> prazosin), and competition by oxymetazoline and chlorpromazine was best fit by a one-site model. Likewise, the relative potency of inhibition of [3H]rauwolscine binding was oxymetazoline > chlorpromazine. However, inhibition of [3H]rauwolscine-binding by oxymetazoline was better fit by a two-site model, which was converted to a one-site model in the presence of 100 microM 5'-guanylimidodiphosphate.

CONCLUSIONS

A large number of alpha 2-adrenoceptors are present in rabbit ciliary body. They are not the imidazoline-preferring receptor but are alpha 2-adrenergic-specific receptors of the alpha 2A subtype.

Differential changes in presynaptic modulation of transmitter release during aging

The purpose of this study was to assess the functional role of presynaptic alpha 2-autoreceptors in noradrenergic transmission in the hippocampus and dopamine-2 heteroreceptors in cholinergic transmission in the striatum in young, adult, and senescent rats. Male and female Wistar rats (4, 12, and 24 months old) were used and the release of radioactivity from striatal and hippocampal slices that had been loaded either with [3H]choline or with [3H]norepinephrine was measured at rest and in response to field stimulation (2 Hz, 360 shocks). The release was challenged by sulpiride, a selective dopamine-2 receptor antagonist, and CH-38083, a selective alpha 2-adrenoceptor antagonist. The dissociation constant and the number of alpha 2-adrenoceptors was also determined by binding studies using [3H]yohimbine as ligand in crude membrane preparations of frontal cortex. There were an age-related changes in alpha 2-adrenoceptor-mediated negative feedback modulation of norepinephrine release and in the density and dissociation constant of alpha 2-adrenoceptors. They were reduced in senescent rats. In contrast the presynaptic modulation of striatal cholinergic transmission by dopamine-2 receptors was not altered during aging, but the storage capacity of and the release of acetylcholine from cholinergic interneurons was significantly lower.

Different sensitivity of alpha 2A-C10 and alpha 2C-C4 receptor subtypes in coupling to inhibition of cAMP accumulation

Two human alpha 2-adrenergic receptor subtypes, alpha 2A-C10 and alpha 2C-C4, were compared with respect to their ability to inhibit stimulated cAMP-production. The inhibition was transduced with about one order of magnitude higher sensitivity in the alpha 2C-C4 subtype than in the alpha 2A-C10 subtype. The phorbol ester, TPA, known to desensitize alpha 2-adrenergic receptor function, possible through phosphorylation of Gi, almost completely abolished the inhibition of cAMP-production in the alpha 2C-C4 subtype, while only a partial effect was seen in the alpha 2A-C10 subtype. These results suggest that the receptor subtypes differ with respect to their coupling efficiency to adenylyl cyclase.

Similar ligand binding in recombinant human alpha 2 C2-adrenoceptors produced in mammalian, insect and yeast cells

Ligand binding properties were investigated in recombinant human alpha 2C2-adrenoceptors expressed in three different host systems: Shionogi S115 mouse mammary tumour cells, Spodoptera frugiperda Sf9 insect cells and Saccharomyces cerevisiae yeast cells. The expected 43 kDa alpha 2C2 protein was visualized with immunoblotting using a polyclonal alpha 2C2-receptor antibody. [3H]Rauwolscine binding in cell homogenates or membranes (Bmax 3-11 pmol/mg protein; Kd approximately 5.5 nM) was inhibited by prazosin, oxymetazoline, RX821002, chlorpromazine and (-)-noradrenaline with and without the GTP-analogue Gpp(NH)p with similar Ki values in the different host systems. This indicates that alpha 2C2-adrenoceptors retain their binding characteristics irrespective of the host environment.

Characterization of [3H]RX821002 binding to alpha-2 adrenergic receptor subtypes

Alpha-2 adrenergic receptors have been divided into four pharmacological subtypes based on their differences in affinity for several drugs. Previous studies showed that [3H]RX821002 has a high affinity for the alpha-2A subtype. The current study characterized the binding properties of [3H]RX821002 [2-(2-methoxy-1,4- benzodioxan-2yl)-2-imidazoline] to the alpha-2A receptor in CHO-C10 cells, alpha-2B in neonatal rat lung, alpha-2C in OK cells and alpha-2D in bovine pineal gland. Membrane binding studies of [3H]RX821002 were done in 25 mM glycylglycine buffer at room temperature. The nonspecific binding rates at the KD concentration were 4.9%, 20%, 14% and 8.3% of the total for CHO-C10, neonatal rat lung, OK cells and bovine pineal, respectively, which were determined by adding 100 microM norepinephrine. Saturation curves indicate that [3H]RX821002 has a high affinity for all alpha-2 adrenergic subtypes. The KD values were 0.29, 1.05, 0.37 and 0.19 nM for CHO-C10, neonatal rat lung, OK cells and bovine pineal, respectively. [3H]Rauwolscine has affinities of 0.34, 0.55 and 0.24 nM for the alpha-2A, -2B and -2C subtypes. By contrast, [3H]rauwolscine has a much lower affinity for alpha-2D subtype with a KD value of 5.2 nM. The binding site density for [3H]RX821002 was significantly lower in the neonatal rat lung compared with [3H]rauwolscine. The correlation coefficients of pKi values of adrenergic compounds against [3H]RX821002 versus [3H]rauwolscine were close to unity for each tissue. These data clearly show that the two ligands label the same alpha-2 adrenergic receptor population.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of alpha 2-adrenergic receptor activation of G proteins: evidence for a precoupled receptor/G protein state

The ability of agonist-occupied alpha 2D-adrenergic receptors to activate G proteins was measured in membranes from PC-12 cells stably expressing the cloned receptor, using guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding as an endpoint. Epinephrine (EPI) stimulated [35S]GTP gamma S binding in a Mg(2+)-dependent manner, showing both micromolar and millimolar cation affinities. Prior treatment of cells with pertussis toxin completely eliminated the EPI stimulation. The presence of GDP decreased basal [35S]GTP gamma S binding and increased the proportion of EPI-stimulated binding. Increasing concentrations of Na+ also reduced basal [35S]GTP gamma S binding but had less effect on EPI-stimulated binding, such that the agonist response was proportionately greater at higher Na+ levels. In saturation binding studies with [35S]GTP gamma S only low affinity binding was observed in the presence of 100 mM Na+, whereas in the absence of Na+ a high affinity component was also present, indicating a Na(+)-regulated receptor/G protein interaction. EPI induced high affinity [35S]GTP gamma S binding in the presence of Na+ and increased the affinity of the high affinity component under Na(+)-free conditions. The selective alpha 2-adrenergic antagonist rauwolscine produced rightward shifts of EPI dose-response curves and decreased the basal level of [35-S]GTP gamma S binding across the same range of concentrations. The extent of decrease was dependent upon the alpha 2-adrenergic receptor expression level, indicating that alpha 2-adrenergic receptors contribute to basal G protein activation in the absence of agonist. The ability of rauwolscine to decrease basal [35S]GTP gamma S binding was diminished as the level of Na+ was increased, suggesting that both agents act to reduce receptor/G protein interaction, by distinctive mechanisms. alpha 2-Adrenergic receptor antagonists reduced basal G protein activation with a rank order for maximal effectiveness that was different from their receptor binding affinities. These results support the existence of precoupling between alpha 2D-adrenergic receptors and G proteins; coupling can be diminished by both Na+ and antagonists, whereas agonists increase the efficiency of receptor/G protein coupling.

Distribution and density of alpha- and beta-adrenergic receptor binding sites in the bovine mammary gland

Radioreceptor binding studies were designed to localize and determine the number of alpha- and beta-adrenergic receptors in the mammary gland of lactating cows. 3H-prazosin, 3H-rauwolscine and 3H-dihydroalprenolol were used for the regional characterization of alpha 1-, alpha 2- and beta-adrenergic receptors by competitive inhibition of binding of 3H-ligands with unlabelled adrenergic agonists and antagonists. The alpha 1-, alpha 2- and beta 2-adrenergic receptor subtypes could thus be demonstrated in the regions of the teats, large mammary ducts and parenchyma. Tissues of the teat wall, of the large mammary ducts above the gland cistern and of the mammary parenchyma were prepared to determine the density of alpha 1-, alpha 2- and beta-receptors by saturation binding assays using 3H-prazosin, 3H-rauwolscine and 3H-dihydroalprenolol respectively. Binding to high affinity sites was reversible within minutes and saturable. Equilibrium was reached within minutes. The number of alpha 1- and alpha 2-adrenergic receptors decreased from the teat to the mammary ducts to the parenchyma. Most of the alpha 1- and alpha 2-adrenergic receptors were found in the teat wall, whereas in the parenchyma alpha-adrenergic receptors were absent or barely detectable. The density of beta-adrenergic receptors was similar in the teat wall and the large mammary ducts, but much lower in the parenchyma. Thus, alpha 1-, alpha 2- and beta-adrenergic receptors were found mainly in the milk purging system and hardly at all in mammary parenchyma. Inhibition of milk removal by alpha-adrenergic stimulation is possibly due to constriction of teat wall and to constriction of the mammary ducts, whereas enhanced milk flow after beta-adrenergic stimulation is possibly due to relaxation not only of the teat sphincter and teat wall, but probably also of the large mammary ducts.

Effects of dicentrine, a novel alpha 1-adrenoceptor antagonist, on human hyperplastic prostates

The effects of dicentrine, an alpha 1-adrenoceptor antagonist, on human hyperplastic prostates were investigated by radioligand binding and in vitro isometric tension experiments. In human hyperplastic prostates, alpha 1-adrenoceptors were characterized by a binding assay using [3H]prazosin as a radioligand. Specific [3H]prazosin binding was saturable and of high affinity (Kd = 0.2 +/- 0.02 nM) with a maximal number of binding sites (Bmax = 55.2 +/- 3.2 fmol/mg protein). alpha-Adrenoceptor antagonists competed with [3H]prazosin for binding in the order: dicentrine > phentolamine > rauwolscine. Norepinephrine (0.3-100 microM) or phenylephrine (1-300 microM) produced gradual contractions of human hyperplastic prostates. The concentration-response curve of norepinephrine or phenylephrine was shifted in parallel to the right by dicentrine, consistent with a competitive blockade. The pA2 values of dicentrine against norepinephrine and phenylephrine were 8.04 +/- 0.09 and 8.33 +/- 0.11, respectively. These experiments were conducted to confirm that there was no interaction between alpha 1- and alpha 2-adrenoceptors in the tissue. Rauwolscine (1 microM) caused 1.7-fold, while dicentrine (0.1 microM) caused 15.8-fold shift of norepinephrine-induced contraction of human hyperplastic prostates. Combination of rauwolscine with dicentrine caused 17.8-fold shift of norepinephrine-induced prostatic tissue contraction. The contractile response to transmural field stimulation was abolished by pretreatment with tetrodotoxin, and suppressed concentration dependently by dicentrine or prazosin, whereas rauwolscine had little effect. It is concluded that dicentrine inhibits human hyperplastic prostate contractions in response to exogenous and endogenous adrenergic stimulation. Dicentrine may thus hold potential to relieve bladder outlet obstruction caused by benign prostatic hyperplasia via alpha 1-adrenoceptor blockade.

The 5-HT3 receptor agonist 1-(m-chlorophenyl)-biguanide facilitates noradrenaline release by blockade of alpha 2-adrenoceptors in the mouse brain cortex

We analyzed the facilitatory effect of the 5-HT3 receptor agonist 1-(m-chlorophenyl)-biguanide (mCPBG) on the electrically evoked noradrenaline release in superfused mouse brain tissue. In addition, we determined the affinities of mCPBG and two other 5-HT receptor ligands, namely 2-methyl-5-hydroxytryptamine (2-methyl-5-HT; also a 5-HT3 receptor agonist) and 5-carboxamidotryptamine (5-CT; a 5-HT1 receptor agonist) for alpha 2 binding sites. The latter two 5-HT receptor agonists were included because of the claimed involvement of alpha 2-adrenoceptors in their effects on noradrenaline release. In superfusion experiments on mouse brain cortex slices preincubated with 3H-noradrenaline, tritium overflow evoked by 2-min periods of electrical field stimulation (3 Hz) was facilitated by mCPBG and, in addition, by rauwolscine (alpha 2-adrenoceptor antagonist) and tetraethylammonium (K+ channel blocker) (which were examined for comparison). The effect of mCPBG was not affected by the 5-HT3 receptor antagonist tropisetron or by desipramine but was abolished by rauwolscine. In slices superfused with medium containing desipramine, the concentration-response curve of unlabelled noradrenaline for its inhibitory effect on the electrically (0.3 Hz) evoked overflow was shifted to the right by mCPBG and rauwolscine (apparent pA2 5.35 and 7.88, respectively). In another series of superfusion experiments, 4 electrical pulses, administered at 100 Hz, were used to evoke tritium overflow. Tritium overflow evoked by this stimulation procedure (under which an endogenous tone of noradrenaline does not develop) was not affected by mCPBG and rauwolscine but still increased by tetraethylammonium.(ABSTRACT TRUNCATED AT 250 WORDS)

2,4-D and alkaloid accumulation in periwinkle cell suspensions

Omission of 2,4-D from culture medium during one subculture of Catharanthus roseus cells, strain C20, resulted in an increased alkaloid accumulation, without effect on growth. Alkaloid accumulation, rather than growth, seemed to be more sensitive to 2,4-D. 2,4-D inhibited alkaloid accumulation essentially during growth phase, but its inhibitory effect during this period was partially reversible. As this reversibility was underlined only during the stationary phase, this suggested that this action could be situated upstream in a terpenoid non-specific pathway. 2,4-D feeding showed that inhibition is weaker and weaker as the alkaloid accumulation period proceeds. Auxin action during this period could take place downstream in specific alkaloid pathways. The lower alkaloid accumulation obtained after loganic acid feeding compared to that obtained with secologanin and loganin could indicate that loganic acid methylation should be one of the 2,4-D target(s).

Binding and functional characterization of alpha-2 adrenergic receptor subtypes on pig vascular endothelium

Alpha-2 adrenergic receptor subtypes were characterized in membranes of pig vascular endothelium using [3H]rauwolscine. Alpha-2 adrenergic receptor subtypes that mediate endothelium-dependent vascular relaxation were studied in vitro by using ring segments of pig epicardial coronary arteries. Specific [3H]rauwolscine binding in endothelial membranes was saturable and to a single class of high-affinity sites with a mean KD of 0.217 +/- 0.05 nM and Bmax of 156 +/- 28 fmol/mg of protein. Nonlinear regression analysis indicated that competition binding curves for drugs that distinguish the alpha-2A adrenergic receptor subtype from the alpha-2C adrenergic receptor subtype fit best to two-site binding models. Kl values for drugs in binding to endothelial alpha-2 adrenergic receptors correlated well with their Kl values for alpha-2A (r = .98) and alpha-2C (r = .97) adrenergic receptor subtypes identified in other tissues. Vascular endothelium contained 23% alpha-2A and 77% alpha-2C adrenergic receptors. In the presence of indomethacin, the rank order of potency for agonists that cause endothelium-dependent vascular relaxation was p-iodoclonidine > clonidine > UK-14,304 > guanabenz > epinephrine > norepinephrine. KB values for antagonist inhibition of epinephrine-induced, endothelium-dependent vascular relaxation correlated best with Kl values for antagonist binding at the alpha-2A adrenergic receptor subtype. These results suggest that the alpha-2A and alpha-2C adrenergic receptor subtypes are present on pig vascular endothelium and that the alpha-2A adrenergic receptor subtype mediates indomethacin-insensitive, endothelium-dependent relaxation of pig epicardial coronary arteries.

Suppressed renal response to 2,6-dimethyl clonidine but not clonidine in one kidney-one clip hypertensive rats

The disparate renal actions of clonidine and 2,6-dimethyl clonidine (2,6-DMC), recently reported by the authors, may be the result of stimulation of alpha-2 adrenoceptors and imidazoline-preferring sites, respectively. Studies found that in one kidney-one clip (1K-1C) hypertension, the density of renal binding sites labeled by [3H] idazoxan (imidazoline-preferring sites) but not [3H] rauwolscine (alpha-2 adrenoceptors) was decreased compared with the 1K control rats. The renal response to 2,6-DMC in 1K-1C hypertensive rats and 1K control rats was compared. In 1K control rats, 2,6-DMC infusion directly into the renal artery (0, 1 and 3 micrograms kg-1 min-1) produced a dose-related increase in the percent fractional excretion of water and sodium and osmolar clearance. This effect was similar to that reported for the imidazoline-preferring site agonist moxonidine. Conversely, in 1K-1C hypertensive rats, these infusion rates of 2,6-DMC failed to alter renal water and solute excretion. By contrast, clonidine (3 micrograms kg-1 min-1) produced a similar increase in the percent fractional excretion of water and sodium, free water clearance and osmolar clearance in 1K-1C hypertensive rats and 1K normotensive rats. In renal membranes from 1K-1C hypertensive rats, [3H] idazoxan binding (imidazoline-preferring site antagonist) was decreased, whereas the binding of [3H] rauwolscine (alpha-2 adrenoceptor antagonist) was unaltered. As proposed in a previous study, the decreased response to 2,6-DMC but not clonidine in 1K-1C hypertensive rats indicates that these two agonists, although structurally similar, are functioning at two unique sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes of noradrenergic-NPY interaction in rat brain: norepinephrine, NPY levels and alpha-adrenoceptors

Noradrenergic-neuropeptide Y interaction, which is implicated in different physiological functions, was studied in senescent rats. Norepinephrine (NE) and neuropeptide Y (NPY) levels were measured in brainstem and hypothalamus, and alpha-adrenergic binding was investigated in brainstem in young (4 months) and old (34 months) Wistar rats. NE concentration was the same in senescent rats, whereas NPY concentration was decreased both in brainstem and hypothalamus compared to levels in young rats. [3H]prazosin binding to alpha 1-adrenoceptors was not modified, but [3H]rauwolscine binding to alpha 2-adrenoceptors was altered with age. In fact, the density of alpha 2-adrenoceptors (Bmax) was lower, while the binding affinity (Kd) was increased in old compared to young rats. These results suggest that the decrease of NPY levels could be one of the possible reasons for changes in [3H]rauwolscine binding to alpha 2-adrenoceptors in old rats. The G-protein-adenylate cyclase system, which is impaired in senescent rats, could be involved in the disorganization of noradrenergic-NPY interaction.

The release of endothelium-derived relaxing factor via alpha 2-adrenergic receptor activation is specifically mediated by Gi alpha 2

The purpose of this study is to determine which pertussis toxin-sensitive guanine nucleotide-binding protein (Gi) mediates alpha 2-adrenergic receptor stimulation of endothelium-derived relaxing factor (EDRF) release. Bovine aortic endothelial cells were treated with pertussis toxin (0-100 ng/ml) for 16 h and stimulated with an alpha 2-adrenergic receptor agonist, UK14304, to release EDRF in a bioassay system. Pertussis toxin produced a concentration-dependent decrease in EDRF release with maximal inhibition (80%) occurring at 5 ng/ml. This correlated with a decrease in receptor-G protein coupling as measured by 87% loss of high affinity agonist binding sites and 94% decrease in agonist-stimulated GTPase activity. Immunoprecipitation of [32P]NAD-ribosylated membranes using specific Gi protein antisera demonstrated that complete ADP-ribosylation of Gi alpha 2 occurred at 5 ng/ml compared to 30 ng/ml for Gi alpha 3. When bovine aortic endothelial cell membranes were treated with carboxyl terminus-directed antisera to Gi alpha 2 (P4) and Gi alpha 3 (EC), the P4 antisera abolished 86% of the high affinity agonist binding sites and 93% decrease in agonist-stimulated GTPase activity, while the EC antisera had minimal effect (12%). These results indicate that Gi alpha 2 mediates most of the EDRF released via the alpha 2-adrenergic receptor.

Postnatal development of central nervous alpha 2-adrenergic binding sites: an in vitro autoradiography study in the tree shrew

The postnatal development of the alpha 2-adrenoceptor pattern was investigated by in vitro receptor autoradiography with the antagonist [3H]rauwolscine in the brains of tree shrews (Tupaia belangeri). At birth, high numbers of [3H]rauwolscine binding sites are diffusely distributed in the whole brain with exception of the neocortex which is very weakly labeled at this time. While the number of [3H]rauwolscine binding sites in the cerebellum decreases to low levels during the first three postnatal weeks, several brain regions show a significant increase in binding sites which are progressively concentrated in distinct nuclei. In the medulla oblongata, the diffuse labeling pattern changes so that binding sites become centralized in the dorsomedial nuclei. In the pons, similar changes can be observed with a moderate labeling of the locus coeruleus on postnatal day 10 and a strong labeling in the adult. In the thalamus, a transient appearance of high numbers of [3H]rauwolscine binding sites can be observed during the second and third postnatal week in specific nuclei. In the preoptic area and hypothalamus, there are only minor postnatal changes but the numbers of [3H]rauwolscine binding sites decrease between postnatal day 5 and adulthood. The high number of binding sites in the limbic system does not significantly change after birth. In the neocortex and the superior colliculus, the [3H]rauwolscine labeling pattern which is characteristic for the adult is achieved not before the third postnatal week. Competition experiments demonstrate that [3H]RAUW binds with high affinity to alpha 2-adrenoceptors in the postnatal as well as in the adult brain. Therefore, this study demonstrates region specific developmental profiles of the pattern of alpha 2-adrenoceptors in the postnatal tree shrew brain.

Characteristic localization of alpha 1- and alpha 2-adrenoceptors in the human kidney

1. The localization of alpha-adrenoceptors in the plasma membranes of human kidney were investigated by radioligand binding, using an alpha 1-antagonist, [3H]-bunazosin, and an alpha 2-antagonist, [3H]-rauwolscine. 2. Both the maximum binding (Bmax) and dissociation constant (Kd) of [3H]-bunazosin were greater in the cortex than in the medulla. The Bmax of [3H]-rauwolscine in the medulla was greater than in the cortex. 3. Thus, alpha 1-adrenoceptors appeared to be localized predominantly in the cortex, while the alpha 2-adrenoceptors were mainly present in the medulla of the human kidney.

Characterization of alpha 1-adrenoceptors which mediate chronotropy in neonatal rat cardiac myocytes

1. In the present study, we investigated the effect of culture on alpha 1-adrenoceptors that mediate chronotropy and on alpha 1-adrenergic signal transduction in neonatal rat cardiac myocytes. 2. The spontaneous beating rate of neonatal rat myocytes after 3 or 7 days in culture was 37.4 +/- 4.2 or 102.0 +/- 4.3 beats min-1, respectively. The alpha 1-adrenoceptor-mediated chronotropic effect of norepinephrine was positive at day 3 of culture. In contrast to day 3 of culture, the neonatal myocytes exhibited a negative chronotropic response to norepinephrine on day 7 of culture. Both of these effects of norepinephrine were completely abolished by prazosin. 3. The affinity (Kd) and/or density (Bmax) of alpha 1-adrenoceptors labeled with [3H]prazosin in membranes from cultured myocytes were not significantly different between day 3 and day 7 of culture. 4. The expression of Gs, Gi, Gq and Go alpha-subunits in membranes from cultured myocytes was found to be significantly increased with the passage of culture time by immunoblot analysis. In contrast, no significant differences in G beta-subunit expression were observed between day 3 and day 7 of culture. 5. Norepinephrine-stimulated inositol 1,4,5-trisphosphate production by radio-binding protein in neonatal myocytes after 7 days of culture was significantly higher than that of the day 3 counterpart. 6. No significant changes in phospholipid and cholesterol contents in membranes from neonatal myocytes were observed with longer culture times.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of alpha-2 adrenoceptors in pregnant rat uterus: identification of subtypes and autoradiographic distribution

Pharmacological characterization of alpha-2 adrenoceptors of the pregnant rat myometrium was assessed using the ability of various alpha adrenoceptor agonists and antagonists to inhibit [3H]rauwolscine or [3H]idazoxan binding to myometrial 50,000 x g fraction or to slide-mounted sections of the whole pregnant uterus. Saturation binding studies with both radioligands showed that the number of myometrial alpha-2 adrenoceptors is greatly increased on days 10 to 12 of pregnancy vs. cyclic rats. It then decreased from midpregnancy to term (about -75%; P < .01) with no change of the equilibrium dissociation constant (between 7-11 nM). Chemical sympathectomy by 6-hydroxydopamine significantly decreased (P < .01) the density of alpha-2 adrenoceptors at days 8 and 12 of pregnancy. Later, 6-hydroxydopamine administration did not alter Bmax or Kd values suggesting that the pregnancy decrease of alpha-2 adrenoceptors may be related to a loss of presynaptic receptors. In order to identify myometrial postsynaptic alpha-2 adrenoceptor subtypes, the inhibition of [3H]rauwolscine or [3H]idazoxan binding by oxymetazoline, prazosin and chlorpromazine was studied on days 20 and 21 of pregnancy. All inhibition curves were consistent with a model of two classes of binding sites: about 55% of the myometrial alpha-2 adrenoceptors, which had a higher affinity for oxymetazoline, may represent the alpha-2A subtype whereas the other 45% of the sites, which had a higher affinity for prazosin and chlorpromazine, may represent the alpha-2B subtype. Autoradiographic studies using [3H]rauwolscine revealed that both subtypes are colocalized in the longitudinal muscle. A high density of alpha-2A and alpha-2B subtypes was also detected in the chorioallantoic and yolk sac placenta and in the embryonic nervous system.

Alpha-2 adrenoceptors in medullary thick ascending limbs of the rabbit kidney

Marked differences in the characteristics of alpha-2 adrenoceptors in the kidney of various species have been reported. In addition, there are functional differences in alpha-2 adrenoceptors in various nephron segments of the same species. In this study we have characterized alpha-2 adrenoceptors in the medullary thick ascending limb (MTAL) isolated from the rabbit kidney. The equilibrium binding of [3H]rauwolscine to MTAL homogenates was measured after incubation for 45 min at 25 degrees C in the absence and presence of 10 microM phentolamine. The specific binding of [3H]rauwolscine was saturable with a Kd of 2.6 +/- 0.1 nM and maximal binding of 234 +/- 18 fmol/mg of protein. Adrenergic drugs competed with MTAL-bound [3H]rauwolscine with the following order of potency: 1) antagonists: rauwolscine = yohimbine > phentolamine >> prazosin = propranolol; and 2) agonists: oxymetazoline > clonidine > epinephrine > BHT 933 (azepexole) > alpha-methylnorepinephrine. Our results indicate that specific alpha-2 adrenoceptors are present in the rabbit MTAL. The high (> 1600) ratio of Ki of prazosin: oxymetazoline suggests that the alpha-2 adrenoceptors in the rabbit MTAL belong to an alpha-2A subtype. Guanabenz, iodoclonidine and epinephrine at high concentrations decreased forskolin- and vasopressin-stimulated cyclic AMP formation in the rabbit MTAL. These results suggest that alpha-2 adrenoceptors are negatively coupled to adenylate cyclase system in the MTAL.

Motor and cardiovascular effects of selective alpha 2-adrenoceptor antagonists in the decerebrated rabbit

In the decerebrated rabbit, intrathecal administration of the alpha 2-adrenoceptor antagonists idazoxan and yohimbine increases reflex responses of gastrocnemius motoneurones to electrical stimulation of the sural nerve. The maximum effect of idazoxan is about twice that of yohimbine, suggesting that part of the effect of idazoxan may be due to an action at non-adrenergic idazoxan or imidazoline binding sites. RX811059 and RX821002 are selective alpha 2-adrenoceptor antagonists with minimal affinity for imidazoline binding sites. In the present study it was found that intrathecal administration of these compounds in doses of 5-200 micrograms increased the sural-gastrocnemius reflex to the same level as idazoxan, confirming that this reflex is subject to powerful adrenergic inhibition mediated through spinal alpha 2-adrenoceptors. RX811059 and RX821002 also increased arterial blood pressure and heart rate. A retrospective analysis of data from earlier studies showed that whereas intrathecal yohimbine had increased blood pressure, idazoxan had not. It is possible that idazoxan acts at non-adrenergic idazoxan binding sites to cancel out the hypertensive effects which are associated with alpha 2-adrenoceptor blockade.

The alpha 2A-adrenergic receptor is targeted directly to the basolateral membrane domain of Madin-Darby canine kidney cells independent of coupling to pertussis toxin-sensitive GTP-binding proteins

The alpha 2-adrenergic receptor (alpha 2-AR) is a member of the seven transmembrane-spanning G-protein-coupled receptor superfamily. In the kidney, the alpha 2-AR is most abundant in the epithelial cells of the proximal tubule where it is important in enhancing Na+ reabsorption via the modulation of Na+/H+ exchange. Radioligand binding and physiological studies suggest that the alpha 2-AR residues primarily on the basolateral surface of these proximal tubule cells in vivo. To investigate the mechanisms underlying alpha 2-AR polarization in epithelial cells, we permanently expressed wild-type and an epitope-tagged version of the alpha 2A-AR in Madin-Darby canine kidney (MDCK) cells. Using a steady-state surface biotinylation assay, we observe that 80-90% of the alpha 2A-AR in MDCK cell clones is located on the basolateral membrane domain. Immunolocalization studies confirm the biotinylation results and demonstrate that the alpha 2A-AR is actually confined primarily to the lateral domain of the basolateral surface. Metabolic labeling experiments suggest that basolateral polarization of the alpha 2A-AR is achieved by direct targeting of the receptor to the basolateral domain. Targeting of the alpha 2A-AR to the basolateral surface is not perturbed by pertussis toxin-treatment of MDCK cells, suggesting that coupling of the alpha 2A-AR to GTP-binding proteins is not important for receptor polarization.

Alpha 2A adrenoceptors and non-adrenergic idazoxan binding sites in calf brain and retina are distinct from those in human brain

alpha 2 Adrenoceptors in membrane preparations of human and calf frontal cortex and of calf retina can be labelled by the antagonists [3H]idazoxan, [3H]rauwolscine and [3H]RX 821002. Present and previous data indicate that [3H]idazoxan possesses the highest affinity for the alpha 2 adrenoceptors in the calf tissues, whereas [3H]rauwolscine displays the highest affinity for those in the human frontal cortex. Competition binding experiments with adrenergic and serotonergic drugs further support the notion that the alpha 2 adrenoceptors in calf frontal cortex and retina are similar, but distinct from the receptors in human frontal cortex. The alpha 2 adrenoceptors in the three investigated tissues display low affinity for the antagonist prazosin, which suggests that they all belong to the alpha 2A subclass. The competition binding curves of the alpha 2A adrenoceptor subtype-selective agonist oxymetazoline are shallow, but undergo a rightward shift and steepening in the presence of GTP. The shallow curves can therefore be attributed to the coupling of the alpha 2 adrenoceptors to G proteins. The different binding characteristics of the alpha 2A adrenoceptors from the investigated human and bovine tissues are likely to reflect species-related differences in protein structure. [3H]Idazoxan binds also to non-adrenergic sites in membrane preparations from the three tissues. However, the affinity of [3H]idazoxan for these sites in calf cortex and retina is appreciably lower than for those in human cortex. The species-related differences of the non-adrenergic idazoxan binding sites may be due to differences in protein structure or even to differences in gene-product.

Mutations of the alpha 2A-adrenergic receptor that eliminate detectable palmitoylation do not perturb receptor-G-protein coupling

The alpha 2A-adrenergic receptor (alpha 2AAR) is coupled to a variety of effectors via pertussis toxin-sensitive GTP-binding proteins. Like most members of the G-protein-coupled receptor superfamily, the primary structure of the alpha 2AAR possesses a putative consensus sequence for palmitoylation in the COOH terminus at Cys-442. This study demonstrates that the alpha 2AAR incorporates [3H] palmitic acid in metabolic labeling studies and that mutation of Cys-442 to Ala or Ser eliminates detectable 3H-palmitoylation. However, mutation of Cys-442 does not alter adrenergic ligand specificity or allosteric modulation by amphipathic agents, such as amiloride analogs. Since reports in the literature suggest that a homologous mutation in the beta 2-adrenergic receptor attenuates coupling to Gs (O'Dowd, B. F., Hnatowich, M., Caron, M. G., Lefkowitz, R. J., and Bouvier, M. (1989) J. Biol. Chem. 264, 7564-7569) whereas chemical removal of palmitate from bovine rhodopsin enhances coupling to Gt (Morrison, D. F., O'Brien, P. J., and Pepperberg, D. R. (1991) J. Biol. Chem. 266, 20118-20123), we examined if mutation of Cys-442 and parallel loss of detectable palmitoylation alter alpha 2AAR coupling to G-proteins. Several independent cell lines of Madin-Darby canine kidney II cells expressing wild-type (Cys-442) or mutant (Ala-442 and Ser-442) alpha 2AARs were established. Metabolic labeling of Madin-Darby canine kidney cells expressing wild-type (Cys-442) or mutant (Ala-442) alpha 2AARs with [3H]palmitic acid indicated that only wild-type Cys-442-containing receptors incorporated [3H]palmitate, monitored following isolation of the alpha 2AAR detergent extracts using yohimbine-agarose chromatography. Receptor-G-protein coupling was assessed by evaluating sensitivity of receptor-agonist interactions to guanine nucleotides in competition for [3H]yohimbine antagonist binding, guanyl-5'-yl imidotrisphosphate sensitivity of pertussis toxin-sensitive p-[125I]iodoclonidine agonist binding, and agonist-stimulated guanosine 5'-O-(thiotriphosphate) binding. Using all three approaches, no detectable change in alpha 2AAR-G-protein coupling was apparent, in contrast to apparent opposite effects on the beta 2-adrenergic receptor-Gs and rhodopsin-Gt coupling reported previously by others. One interpretation is that this conserved cysteine may play differing roles at different receptor-G-protein interfaces. Alternatively, this shared structural motif may play a role in not yet investigated pathways, such as receptor expression, turnover, and localization.

Alpha 2-adrenoceptor antagonist potencies of two hydroxylated metabolites of yohimbine

1. The alpha 2-adrenoceptor antagonist capacities of two hydroxylated metabolites of yohimbine in man (10-OH-yohimbine and 11-OH-yohimbine) were investigated on the alpha 2-adrenoceptors of human platelets and adipocytes and compared to those of yohimbine. 2. Yohimbine and 11-OH-yohimbine exhibited similar alpha 2-adrenoceptor affinity in biological studies i.e. inhibition of adrenaline-induced platelet aggregation and inhibition of UK14304-induced antilipolysis in adipocytes. 3. Yohimbine and the two metabolites displaced [3H]-RX 821002 binding with equivalent affinities in platelet and adipocyte membranes with the following order of potency: yohimbine > 11-OH-yohimbine > 10-OH-yohimbine. However, when binding studies were carried out in binding buffer supplemented with 5% albumin, the apparent affinity of yohimbine was reduced about 10 fold and was similar to that of 11-OH-yohimbine. 4. Yohimbine and its metabolites were bound to different extents to plasma proteins, the bound fraction being 82%, 43% and 32% respectively for yohimbine, 11-OH-yohimbine and 10-OH-yohimbine. 5. These results show that the main hydroxylated metabolite of yohimbine in man (11-OH-yohimbine) possesses alpha 2-adrenoceptor antagonist properties. The discrepancies found in binding studies (i.e. 10 fold lower affinity of 11-OH-yohimbine than yohimbine for alpha 2-adrenoceptors but similar capacities in blocking biological alpha 2-adrenoceptor effects in cells) are attributable to the higher degree of binding of yohimbine to plasma protein.

Coexpression studies with mutant muscarinic/adrenergic receptors provide evidence for intermolecular "cross-talk" between G-protein-linked receptors

We have tested the hypothesis that guanine-nucleotide-binding-protein-coupled receptors may be able to interact with each other at a molecular level. To address this question, we have initially created two chimeric receptors, alpha 2/m3 and m3/alpha 2, in which the C-terminal receptor portions (containing transmembrane domains VI and VII) were exchanged between the alpha 2C-adrenergic and the m3 muscarinic receptor. Transfection of COS-7 cells with either of the two chimeric constructs alone did not result in any detectable binding activity for the muscarinic ligand N-[3H]methylscopolamine or the adrenergic ligand [3H]rauwolscine. However, cotransfection with alpha 2/m3 and m3/alpha 2 resulted in the appearance of specific binding sites (30-35 fmol/mg of membrane protein) for both radioligands. These sites displayed ligand binding properties similar to those of the two wild-type receptors. Furthermore, COS-7 cells cotransfected with alpha 2/m3 and m3/alpha 2 were able to mediate a pronounced stimulation of phosphatidylinositol hydrolysis upon stimulation with the muscarinic agonist carbachol (Emax approximately 40-50% of wild-type m3). A mutant m3 receptor (containing 16 amino acids of m2 receptor sequence at the N terminus of the third cytoplasmic loop) that was capable of binding muscarinic ligands but was virtually unable to stimulate phosphatidylinositol hydrolysis was also used in various cotransfection experiments. Coexpression of this chimeric receptor with other functionally impaired mutant muscarinic receptors (e.g., with an m3 receptor containing a Pro-->Ala point mutation in transmembrane region VII) resulted in a considerable stimulation of phosphatidylinositol breakdown after carbachol treatment (Emax approximately 40-50% of wild-type m3). Thus, these data suggest that guanine-nucleotide-binding-protein-coupled receptors can interact with each other at a molecular level. One may speculate that the formation of receptor dimers involving the intermolecular exchange of N- and C-terminal receptor domains (containing transmembrane domains I-V and VI and VII, respectively) may underlie this phenomenon.

Induction of ajmalicine formation and related enzyme activities in Catharanthus roseus cells: effect of inoculum density

In Catharanthus roseus cell cultures the time courses of four enzyme activities, tryptophan decarboxylase (TDC), strictosidine synthase (SSS), geraniol-10-hydroxylase (G10H) and anthranilate synthase (AS), and alkaloid accumulation were compared under two different culture conditions (low-inoculum density and high-inoculum density on induction medium) and a control on growth medium. In growth medium a transient increase in TDC activity was first observed after which G10H reached its maximum activity; only tryptamine accumulated, no ajmalicine could be detected. Apparently, a concerted induction of enzyme activities is required for ajmalicine formation. Cells inoculated in induction medium showed such a concerted induction of AS, TDC and G10H activities. After 30 days the low-density culture had accumulated six times more ajmalicine (in mumoles/g) than the high-density culture. Thus, increase in biomass concentration (high-density cultures) did not enhance the total alkaloid production. The major differences observed in enzyme levels between high- and low-density cultures were in the AS and TDC activities, which were two to three times higher in the low-density culture, indicating that there is a positive correlation between ajmalicine formation and AS and TDC activities.

Alpha-2 adrenergic receptors in inner medullary collecting duct cells of the rabbit kidney

The stimulation of alpha-2 adrenergic receptors in the kidney produces diuresis. In this study, alpha-2 adrenergic receptors were examined in inner medullary collecting duct (IMCD) cells isolated from the rabbit kidney. The equilibrium binding of [3H] rauwolscine to IMCD cell homogenate was measured after incubation for 45 min at 25 degrees C in the absence (total binding) and presence (nonspecific binding) of 100 microM phentolamine. The specific binding of [3H]rauwolscine was saturable with a Bmax of 170 fmol/mg of protein and Kd of 4.18 nM. The displacement of [3H]rauwolscine binding to IMCD cells by adrenergic antagonists and agonists displayed the following order of potency: phentolamine > yohimbine > clonidine > oxymetazoline > azepexole > propranolol > prazosin > alpha-methyl norepinephrine > epinephrine. Activation of alpha-2 adrenergic receptors by azepexole decreased vasopressin- and forskolin-stimulated cAMP formation in the IMCD cells. Because IMCD cells in the kidney have a hypertonic environment made up of urea and NaCl, we also determined the effects of a mixture of NaCl and urea (1200 mOsm/kg water) on the binding of [3H]rauwolscine and cAMP formation in the IMCD cells. The hypertonicity increased the Kd and Bmax of [3H]rauwolscine binding to 15.2 nM and 240 fmol/mg of protein, respectively. In addition, hypertonicity decreased forskolin-stimulated cAMP formation in IMCD cells. It is concluded that: 1) high-affinity specific alpha-2 adrenergic receptors are present in the rabbit IMCD; 2) selective alpha-2 agonists inhibit vasopressin- and forskolin-stimulated cAMP formation in rabbit IMCD and 3) the effects of adrenergic drugs on the IMCD in vivo will depend on the renal medullary osmolality that itself depends on the state of the hydration of the animal.

Characterization of subtypes of alpha-2 adrenoceptors in the human brain

Radioligand binding studies have demonstrated that oxymetazoline has a high affinity for alpha-2A receptors, but lower affinities for alpha-2B and alpha-2C receptors. In contrast, prazosin has high affinity for alpha-2B and alpha-2C receptors, but relatively lower affinity for alpha-2A receptors. We exploited the respective selectivity of oxymetazoline and prazosin for alpha-2A and alpha-2B/2C adrenoceptors, respectively, to describe pharmacologically and to locate neuroanatomically alpha-2 adrenoceptor subtypes in the human brain. Competition curves for the inhibition of [3H]yohimbine binding by oxymetazoline and prazosin in homogenates of human caudate nucleus were fit best by a model assuming binding to two sites (P < .005). A concentration (CONCopt) of oxymetazoline and prazosin was calculated from these curves that would optimally antagonize binding of [3H]yohimbine to high-affinity sites, minimally inhibiting low-affinity binding. In the presence of CONCopt of prazosin, competition studies of remaining [3H]yohimbine binding in cerebral cortex revealed a rank order potency and potency ratios of compounds which were characteristic of alpha-2A receptors. In the presence of CONCopt of oxymetazoline, competition studies of remaining [3H]yohimbine binding in caudate revealed a rank order potency and potency ratios of compounds which were characteristic of alpha-2C receptors. The existence of a small population of alpha-2B adrenoceptors in the caudate could not be ruled out because slopes of competition curves of compounds which distinguish alpha-2B and alpha-2C adrenoceptors were shallow at oxymetazoline-insensitive sites. The percentages of [3H]yohimbine binding that were inhibited by the CONCopt of oxymetazoline and prazosin were determined in numerous brain regions. Oxymetazoline-sensitive binding of [3H]yohimbine predominated in most regions except for the caudate nucleus, where prazosin-sensitive binding was greatest.

Heterogeneity of the specific imidazoline binding of [3H]idazoxan in the human cerebral cortex

The aim of the present study was to verify whether [3H]idazoxan can be considered as a highly selective ligand for imidazoline preferring receptors (IPR). In human frontal cortex membrane preparations [3H]idazoxan at a low concentration (2 nM) only labelled imidazoline sensitive, catecholamine insensitive sites. Binding was of high affinity, saturable and stereospecific. The rank order of potency of different compounds able to inhibit this binding was cirazoline > (+/-)-idazoxan > guanoxan > (-)-idazoxan > tolazoline > UK-14304 > clonidine. Amiloride, imidazol-4-acetic acid and histamine had no significant affinity for IPR labelled by [3H]idazoxan. [3H]idazoxan bound to 2 different sites (KD1 = 1 nM and KD2 = 16.4 nM). Clonidine behaved as a non competitive, non allosteric inhibitor of [3H]idazoxan binding. Both [3H]idazoxan binding sites were equally affected by clonidine. In membrane preparations obtained from the Nucleus Reticularis Lateralis region (NRL) of the brainstem, [3H]idazoxan binding was similar to that in cortical membranes, particularly with regard to specificity and kinetics. However, in the NRL region binding sites were 4-5 times more numerous than in the frontal cortex. Non linear analyses of saturation data obtained with NRL membrane preparations were compatible with both a one site and a two sites model. No significant effects of 1 mM MgCl2 alone or with 100 microM Gpp(NH)p were observed on either [3H]idazoxan binding or the competition with clonidine or rilmenidine. As in the cortical membrane, clonidine was a non competitive inhibitor of [3H]idazoxan binding to membranes from the NRL region. In conclusion, we show that when a low concentration is used, [3H]idazoxan binding to human brain involves sites almost completely insensitive to catecholamines and specific for imidazolines or related compounds. This binding involves two distinct sites. We also report that [3H]idazoxan imidazoline binding sites are not coupled with a G protein. Because of the non competitive interaction between clonidine and [3H]idazoxan for the binding sites of the latter, we are unable to conclude that the binding sites of the two drugs are identical. However, the non competitive, non allosteric interaction suggests a complex model of multiple binding sites.

Alpha-adrenergic receptors in human myometrium: changes during pregnancy

Myometrial biopsies from 80 women, delivered by cesarean section in 26-42 completed weeks of pregnancy, were analyzed using radiolabeled alpha 1-antagonist [3H]-prazosin and alpha 2-antagonist [3H]-rauwolscine. Statistical analysis was performed using Student's t test (two-tailed), where p < 0.05 was considered significant. The number of alpha 2-adrenergic receptors did not vary (mean value weeks 37-39 = 17.2 +/- 1.3 fmol/mg protein), while the dissociation constant (Kd) for alpha 2-receptors tended to increase with increasing gestational age, implying reduced affinity near term (p < 0.05, mean values 3.4 +/- 0.2 nM). alpha 1-Receptors, however, increased significantly (p < 0.01) in the pregnancy weeks 37-39 (mean value 29.2 +/- 2.9 fmol/mg prot) while Kd was unaltered. Increased number of alpha 1-receptors, along with a decreased affinity of alpha 2-receptors, could, in combination with other factors, play a role in the initiation of labor.

Inverse regulation of alpha- and beta-adrenoceptors during trinitrobenzenesulfonic acid (TNB)-induced inflammation in guinea-pig small intestine

The hypothesis has been raised that intestinal motor disturbances induced by inflammation of the digestive tract may reflect alterations in intestinal cell-membrane receptors. This question has been addressed herein for adrenoceptors by performing [3H]prazosin, [3H]rauwolscine and [3H]DHA binding studies on guinea-pig jejunal smooth-muscle membrane preparations from both healthy controls and 3, 6, and 10 days after TNB-induced intestinal inflammation. Each of the adrenoceptor subtype-selective radioligands used bound selectively to a single saturable class of sites, with no significant (p < 0.05) variation of dissociation constant (KD) values along the inflammatory process. In contrast maximal binding capacities (Bmax) for the different radioligands varied moderately but significantly (p < 0.05 and p < 0.01) according to the time after TNB injection. The alpha-adrenoceptors were significantly upregulated, respectively from (mean +/- SE in fmoles/mg of proteins) 27 +/- 3.8 (controls) to 91 +/- 3.1 (day 10) for the alpha 1-subtype, and from 26 +/- 2.7 (controls) to 102 +/- 5.9 (day 10) for the alpha 2-subtype. In contrast beta-adrenoceptors were down-regulated from 384 +/- 34.6 (controls) to 158 +/- 17.2 (day 10). These findings highlighted an apparently inverse pathological regulation of intestinal alpha- and beta- adrenoceptor densities. They suggest that changes in intestinal adrenoceptors, probably resulting from functional denervation of the inflamed bowel tissue, may contribute to the altered intestinal motility observed in inflammatory bowel diseases.

Autoradiographic localization of prazosin and rauwolscine binding sites in the human kidney

In the human kidney, binding sites for the alpha 1-adrenoceptor antagonist 3H-prazosin and the alpha 2-adrenoceptor antagonist 3H-rauwolscine were localized and quantified by in vitro autoradiography. 3H-prazosin binding was found predominantly in the renal cortex. In the medulla, tubular structures were also specifically labelled. No binding sites, however, were detected in association with glomeruli or large blood vessels. 3H-rauwolscine labelled the medullary vascular bundles intensively, but no binding sites were associated with glomeruli or other cortical structures. Thus, the binding pattern for 3H-prazosin is quite similar in both human and rat renal cortex. There are, however, distinct differences between human and rat kidneys in the distribution of the alpha 2-adrenergic binding sites visualized with the antagonist 3H-rauwolscine.

Postnatal development of 3H-rauwolscine binding sites in the dorsal lateral geniculate nucleus and the striate cortex of the tree shrew (Tupaia belangeri)

Noradrenaline has been shown to play an important role within the visual system of the brain. To analyze the postnatal development of alpha2-noradrenergic receptors in the visual system of tree shrews, we localized and quantified binding sites for the antagonist [3H]-rauwolscine by in vitro-autoradiography in the dorsal lateral geniculate nucleus and the striate cortex at different postnatal ages. At birth, the dorsal lateral geniculate nucleus is only slightly labeled by [3H]-rauwolscine. During the postnatal period, the number of binding sites increases to reach a maximum around postnatal day 20. Since the young tree shrews open their eyes at approximately day 19, it appears that this high concentration of alpha2-adrenoceptors is related to eye opening. In the adult animal, [3H]-rauwolscine labeling shows a laminated pattern in the dorsal lateral geniculate nucleus. Laminae 1, 2, and 3 are more strongly labeled than laminae 4, 5, and 6. In the striate cortex, the pattern of [3H]-rauwolscine-binding sites changes dramatically during the early postnatal period. Immediately after birth, there is only one layer, located within the subplate zone, which is labeled. From postnatal day 5 onwards, all cortical layers which can be distinguished on histologically stained sections reveal [3H]-rauwolscine-binding sites, but in layer IV, which is known to receive major inputs from the dorsal lateral geniculate nucleus, there is very little labeling during the first two postnatal weeks. In this layer, a large number of [3H]-rauwolscine-binding sites occurs between postnatal day 15 and 20, that is slightly before and around the time of eye opening.(ABSTRACT TRUNCATED AT 250 WORDS)

The alpha 2-adrenoceptors of the human retinoblastoma cell line (Y79) may represent an additional example of the alpha 2C-adrenoceptor

1. In agreement with the literature, correlation of the ability of a series of agonists and antagonists to displace [3H]-rauwolscine binding shows the alpha 2-adrenoceptors of HT29 cells, NG108-15 cells, OK cells and homogenates of rat sublingual gland to represent four distinct subtypes. 2. [3H]-rauwolscine also bound with high affinity (KD = 0.30 +/- 0.10 mM) to a human retinoblastoma cell line (Y79). Specific binding represents 73% of total binding, and a Bmax of 38 +/- 1 fmol mg-1 protein was determined. 3. Correlation of antagonist affinities against [3H]-rauwolscine with corresponding values in the other four tissue sources showed the Y79 cells to resemble most closely the OK cells, the prototype example of an alpha 2C-adrenoceptor, with a correlation coefficient of 0.90 and a regression slope of 1.01 being obtained for 10 antagonists in these two systems. 4. Comparison of KD values for [3H]-rauwolscine also showed a similarity between the OK cells (0.19 +/- 0.07 nM) and Y79 cells. 5. These data suggest that the human retinoblastoma cell line may represent an additional example of the alpha 2C-adrenoceptor subtype.

[3H]Rauwolscine binding sites in the brains of male tree shrews are related to social status

The aim of the present study was to investigate the influence of social status on central nervous alpha 2-adrenoceptors. Using the specific alpha 2-adrenoceptor antagonist [3H]rauwolscine, binding sites in the brains of dominant and subordinate male tree shrews were quantified by in vitro autoradiography. In 5 of the 14 brain structures investigated, subordinates had significantly lower numbers of binding sites than dominants. These structures were the solitary tract nucleus, the dorsal motor nucleus of the vagus, the periaqueductal gray, the perifornical region of the hypothalamus and the medial nucleus of the amygdala. These brain areas are all intimately involved in the regulation of autonomic functions and of emotional behavior. Also the affinities for [3H]rauwolscine differed between the groups. In 3 nuclei, the solitary tract nucleus, the periaqueductal gray and the medial nucleus of the amygdala, dominants had significantly higher Kd-values than subordinates. This demonstrates the presence of low affinity binding sites in dominants which do not exist in subordinates. It is suggested that the low number of [3H]rauwolscine binding sites in subordinates results from down-regulation of alpha 2-adrenoceptors by high levels of noradrenaline and/or adrenaline. The disappearance of low affinity [3H]rauwolscine binding sites may play an important role in the etiology of psychosocial stress.