Visualization of beta-adrenergic receptor sites with fluorescent beta-adrenergic blocker probes--or autofluorescent granules?

Synthesis and characterization of high-affinity 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene-labeled fluorescent ligands for human β-adrenoceptors

The growing practice of exploiting noninvasive fluorescence-based techniques to study G protein-coupled receptor pharmacology at the single cell and single molecule level demands the availability of high-quality fluorescent ligands. To this end, this study evaluated a new series of red-emitting ligands for the human β-adrenoceptor family. Upon the basis of the orthosteric ligands propranolol, alprenolol, and pindolol, the synthesized linker-modified congeners were coupled to the commercially available fluorophore BODIPY 630/650-X. This yielded high-affinity β-adrenoceptor fluorescent ligands for both the propranolol and alprenolol derivatives; however, the pindolol-based products displayed lower affinity. A fluorescent diethylene glycol linked propranolol derivative (18a) had the highest affinity (log K_D of −9.53 and −8.46 as an antagonist of functional β2- and β1-mediated responses, respectively). Imaging studies with this compound further confirmed that it can be employed to selectively label the human β2-adrenoceptor in single living cells, with receptor-associated binding prevented by preincubation with the nonfluorescent β2-selective antagonist 3-(isopropylamino)-1-[(7-methyl-4-indanyl)oxy]butan-2-ol (ICI 118551) (J. Cardiovasc. Pharmacol.1983, 5, 430–437.)

Synthesis and properties of fluorescent beta-adrenoceptor ligands

We describe the synthesis of bordifluoropyrromethene (BODIPY), fluorescein, and related fluorescent derivatives of the beta-adrenergic ligand CGP 12177. With these probes we screened insect (Sf9) cells stably transformed with the human beta 2-adrenoceptor gene and expressing (2-3.5) x 10(5) human beta 2-adrenoceptors per cell. Among these derivatives only BODIPY-CGP gave a receptor-specific signal sufficiently strong for measuring the on- and off-rate constants and the equilibrium dissociation constant of beta-adrenoceptor-specific binding by spectrofluorometry or photon counting. Similar KD values for BODIPY-CGP binding were obtained by kinetic measurements (approx. 250 pM) and under equilibrium conditions (400 +/- 180 pM), and these were in the same range as those obtained with [3H]CGP 12177 (200 +/- 32 pM). The cell-bound fluorescence could be quenched specifically with nonfluorescent CGP 12177 to near background levels. The disposition of the beta 2-adrenoceptors in BODIPY-CGP-stained Sf9 cells was mainly restricted to the cell surface at 4 and 30 degrees C. Hence, beta-adrenoceptor-expressing cells can be stained specifically with BODIPY-CGP, and beta-adrenoceptors on a single cell can be assessed by photon counting under the fluorescence microscope. Cells can also be scanned by fluorescence-activated flow cytometry.

Locus coeruleus and dorsal pontine reticular influences on the gain of vestibulospinal reflexes

Experimental anatomical and physiological studies have shown that noradrenergic locus coeruleus (LC) neurons, which are NE-sensitive due to inhibitory adrenoceptors, send inhibitory afferents to neurons of the peri-LC alpha and the adjacent dorsal pontine reticular formation (pRF); on the other hand these tegmental neurons, which are, in part at least, cholinergic as well as cholinoceptive, send excitatory afferents to the medullary inhibitory reticulospinal (RS) system. Experiments performed in precollicular decerebrate cats indicate that these pontine structures exert a regulatory influence on posture as well as on the gain of vestibulospinal (VS) reflexes. In particular, the increased discharge of dorsal pontine reticular neurons, and the related inhibitory RS neurons induced by microinjection of cholinergic agonists into the peri-LC alpha and the adjacent pRF of one side, decreased the postural activity, but greatly increased the response gain of the ipsilateral triceps brachii in response to stimulation of labyrinth receptors resulting from roll tilt of the animal (at 0.15 Hz, +/- 10 degrees). Similar results were also obtained when the discharge of these pontine and medullary reticular neurons was raised, either by local injection into the peri-LC alpha and the dorsal pRF of the beta-adrenergic antagonist propranolol, which blocked the inhibitory influence of the noradrenergic LC neurons on these structures, or by local injection into the LC complex of an alpha 2- or beta-adrenergic agonist (clonidine or isoproterenol) which led to functional inactivation of the noradrenergic neurons; in the latter case the effects were bilateral. Just the opposite results were obtained after microinjection into the LC of a cholinergic agonist, leading to activation of the corresponding neurons. Evidence was also presented indicating that the cholinergic excitatory afferents to the LC originated from the ipsilateral dorsal pRF. The effects described above were dose-dependent and site-specific, as shown by histological controls. Under given conditions, the decrease in postural activity induced either by direct activation of presumptive cholinergic and cholinoceptive pRF neurons or by inactivation of noradrenergic and NE-sensitive LC neurons was followed by transient episodes of postural atonia which lasted several minutes and affected the ipsilateral and sometimes also the contralateral limbs. In these instances, the EMG modulation of the corresponding triceps brachii to animal tilt was suppressed. These findings suggest two different ranges of operation for the noradrenergic and cholinergic structures located in the dorsolateral pontine tegmentum, leading either to a decrease or to an increase in gain of the VS reflexes. The cellular basis of these gain changes is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Glial localization of adenylate-cyclase-coupled beta-adrenoceptors in rat forebrain slices

Fluorocitrate (FC), a selective inhibitor of glial cell respiration, was used to estimate the extent to which glial cells contain adenylate cyclase-coupled beta-adrenoceptors in rat brain slices. The drug blocked 75-95% of the elevation of cyclic AMP caused by the beta-agonist, isoproterenol, in the 4 forebrain regions sampled (frontal and parietal cortex, caudate nucleus, olfactory tubercle). Intracellular recording of neurons in the treated slices confirmed that they were unaffected by FC. Treatment with the neurotoxin, kainic acid, eliminated all electrophysiological activity but did not affect the cAMP response. The results indicate that glial cells contain the preponderance of adenylate-cyclase-coupled beta-adrenoceptors in slices of the rat forebrain and may constitute an important target of the central noradrenergic system in vivo.

Adrenergic beta receptors are not uniformly distributed in the cerebellar cortex

The noradrenergic (NE) innervation of the cerebellar cortex is sparse, forming a broad plexus of radially oriented axons distributing throughout the granular and molecular layers. Autoradiographic studies of beta-adrenergic receptor distribution in the rat show the greatest density of silver grains in the molecular layer (Palacios and Kuhar, '82). In the course of studies of NE hyperinnervated structures, we found that beta receptors are nonhomogeneously distributed in the Purkinje cell layer, where they occur in "patches" overlying small groups of Purkinje cell somata. Tissue sections were incubated in 10 pM 125iodocyanopindolol (ICYP), which binds equally to beta1 and beta2 adrenergic receptors. Nonspecific binding was determined in sections incubated in 125ICYP and 1 microM dl-propranolol. Beta-adrenergic receptor patches are of irregular size and are most prominent in the vermis of lobules I-IX, although the medial cerebellar hemispheres also show areas of increased silver grains over Purkinje cells. In order to determine the subtype of beta receptors, adjacent sections were incubated with either 125ICYP and the beta 2-selective antagonist IPS-339, or 125ICYP and the beta 1-selective antagonist practolol. Patches were observed after each incubation procedure, indicating that they are composed of both beta1 and beta2 receptors. Patches are observed in normal animals and also in rats in which cerebellar NE content was increased 165% by neonatal treatment with 6-hydroxydopamine. This treatment does not alter the density of beta receptors. The cerebellar elements in which the beta receptors are located is not known. While silver grains accumulate over small groups of Purkinje cell somata, they are not coextensive with these cell bodies. The distribution of beta-adrenergic receptors does not parallel the arrangement of noradrenergic varicosities in the rat cerebellar cortex.

Autoradiographic localization of beta-adrenoceptor subtypes in guinea-pig kidney

The distribution of beta-adrenoceptor subtypes in slide-mounted sections of guinea-pig kidney has been examined by the technique of in vitro labelling combined with autoradiography. Binding of (-)-[125I]-cyanopindolol (Cyp) to kidney sections equilibrated and dissociated slowly, was saturable and stereoselective with respect to the isomers of propranolol and pindolol. These characteristics were appropriate for binding to beta-adrenoceptors. Delineation of beta-adrenoceptor subtypes was achieved by use of betaxolol (beta 1-adrenoceptors) and ICI 118,551 (beta 2-adrenoceptors) and computer assisted curve fitting techniques. Both beta 1- and beta 2-adrenoceptors were present in the proportions 1:2. 3H-Ultrofilm images of (-)-[125I]-Cyp binding to guinea-pig kidney sections showed localized patches of binding in the cortex and concentrated binding in the outer stripe of the medulla. Cortical receptors were of the beta 1 subtype and those associated with the outer stripe of the medulla were of the beta 2-adrenoceptor subtype. beta 1-Adrenoceptors were concentrated over glomeruli and beta 2-adrenoceptors over the straight portion of the proximal tubule.

Non-specific binding of the fluorescent beta-adrenergic receptor probe alprenolol-NBD

The fluorescent beta-adrenergic receptor probe alprenolol-NBD was found to exhibit a high affinity (Kd 3.2 nM) and a low capacity (10 fmol/mg protein) for the beta 2-adrenergic receptor on living Chang liver cells but also a high affinity (Kd 320 nM) for non-beta-adrenergic receptor binding sites with a very high capacity (28,000 fmol/mg protein). Calculations are presented which make clear that less than 3% of the binding of alprenolol-NBD during visualization experiments is beta-adrenergic receptor related. Furthermore, it is shown that besides the downregulation of beta-adrenergic receptors during incubation with isoproterenol, the high-affinity non-beta-receptor binding sites are also deminishing during incubation with isoproterenol. Based on our findings it is concluded that the results of Henis et al. who claimed the visualization of the beta-adrenergic receptor population on Chang liver cells by alprenolol-NBD must be interpreted as an almost completely non-specific fluorescence.

Beta-adrenoceptors in the rat kidney. Immunohistochemical study

The direct histochemical detection of beta-blocker binding sites was studied in sections of rat kidney using an immunohistochemical technique developed in our laboratory. Frozen sections of rat kidney were incubated in a solution of (-)alprenolol, washed, exposed to fluorescent (-)alprenolol antibodies (FAA) and then observed at a fluorescence microscope. Strong fluorescence was found within the wall of renal artery and vein, but primarly in the artery. At the level of blood vessels (-)alprenolol binding sites were located chiefly in the media and in the intima. The renal glomerulus, the loop of Henle and collecting tubules appear to be free of any fluorescence. Consequently they do not have beta-adrenoceptors. On the contrary, the glomerular afferent and afferent arterioles, the cellular elements of the juxtaglomerular apparatus, proximal and distal convoluted tubules, are rich in (-)alprenolol binding sites. At higher magnifications the immunoreactivity appears to be located in the basal membrane of cellular elements which indicates that (-)alprenolol binding sites are membrane receptors. The direct immunohistochemical detection of beta-blocker binding sites in the kidney may offer useful information concerning the site of action of beta-blockers at the level of an important target organ for this class of drugs.

Developmental changes in the sensitivity of the chick embryo ventricle to beta-adrenergic agonist during adrenergic innervation

As early as the 4th embryonic day, the ventricle of the chick embryo responded to isoproterenol (Iso) with an increase in the force of contraction; at all ages studied, this positive inotropic effect was accompanied by an increased rate of tension development. There was a transient, 10-fold decrease in the sensitivity (increase in ED50) of the right ventricle to Iso between the 16th and 21st embryonic day. This change in the sensitivity to Iso was not due either to an increased inactivation of Iso by non-neuronal cells or to a change in the thickness of the ventricle. It was found that adrenergic nerves were first capable of altering ventricular contractility on the 16th embryonic day. Whereas they interfered with the function of adrenergic nerves, injections of reserpine or 6-hydroxydopamine had no effect on the subsensitivity to Iso. Furthermore, these agents did not affect the normal developmental changes in heart weight. We conclude that the local release of transmitter from adrenergic nerves does not cause the transient subsensitivity of the ventricle of the chick embryo to beta-adrenergic agonists.

Effects of thyroid state on brain development: beta-adrenergic receptors and 5'-nucleotidase activity

The effect of thyroid status on beta-adrenergic receptor binding and 5'-nucleotidase activity was studied in the forebrain and the cerebellum of the rat during the first 5 postnatal weeks. The developmental increase in beta-adrenergic receptor binding was significantly depressed in thyroid deficiency in both the forebrain and the cerebellum. The effect was more pronounced in the cerebellum, where at day 35 the concentration and the total number of beta-adrenergic receptor sites were reduced by 35% and 50% respectively. In contrast, hyperthyroidism had no significant effect on the development of beta-adrenergic receptors in the brain. On the other hand, hyperthyroidism led to a sustained increase in the forebrain in the activity of 5'-nucleotidase, an enzyme which is also associated with plasma membranes and has been proposed to play some role in neurotransmission. In thyroid deficiency the enzyme activity was markedly depressed. The effect was significant from day 12 in the cerebellum and from day 21 in the forebrain, the maximal depression, at day 21, being 55% and 45% respectively. In comparison with these plasma membrane markers, the accretion of membranous proteins was less affected: although this was retarded in hypothyroidism and advanced in hyperthyroidism there was no residual effect at 35 days except those attributable to changes in organ size. The results indicated, therefore, that the biochemical specialization of cells, as reflected in certain plasma membrane constituents, are chatacteristically influenced in the developing brain by thyroid disorders.

Beta adrenergic receptors in the parathyroid glands

The location of beta-adrenoceptors in human parathyroid gland was studied using an immunohistochemical method. Frozen sections of human parathyroid glands, taken from surgical samples, were treated with (-)-alprenolol, washed and exposed to (-)-alprenolol antibodies conjugated with fluorescent dyes. The (-)-alprenolol was bound to the parathyroid principal cells and to the main blood vessels. On the contrary, adrenergic nerve fibres, demonstrated with formaldehyde fluorescence technique, were only located within the walls of main blood vessels. The findings are discussed.

Beta-adrenergic-receptor localization by light microscopic autoradiography

beta-Receptors were identified in rat brain by a light microscopic autoradiographic technique. The procedure involved binding 3H-labeled dihydroalprenolol to beta-receptors in intact slide-mounted tissue sections and generating autoradiograms by the apposition of emulsion-coated cover slips, Biochemical analysis of the binding indicated that these conditions provided a high degree of selective labeling of beta-receptors. High densities of receptors were found in superficial layers of the cerebral cortex, throughout the caudate-putamen, in the periventricular nucleus of the thalamus, in the molecular layer of the cerebellum, and in other areas. These results are in agreement with other electrophysiological and histochemical data. This radiohistochemical approach should be an important addition to other methods for mapping functional catecholamine neuronal pathways and sites of hormonal action.

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

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

Fluorescent probes of alpha- and beta-adrenergic and opiate receptors: biochemical and histochemical evaluation

Dansyl derivatives of the opiate antagonist naloxazone and the alpha-adrenergic blocking drug 2-[2',6'-dimethoxy)phenoxyethylamino]methyl-benzodioxan (WB-4101) were synthesized as potential in vivo fluorescent labels for opiate and alpha-adrenergic receptors, respectively. When assayed in vitro dansyl analogs display Ki-values for [3H]naloxone and [3H]WB-4101 binding of 8 nM and 80 nM, respectively. The same patterns of histofluorescence are observed in rat brain slices after the intravenous or intracerebroventricular injection of both dansyl drugs. The same patterns are also observed in dansyl-propranolol and 9-aminoacridine-propranolol (9-AAPN) treated animals as well as in untreated control rats. Accordingly, the observed fluorescence does not reflect the labeling of any receptor but is consistent with the distribution of lipofuscin, an endogenous fluorescent compound.