Development of a radioiodinated ligand for characterising alpha 1-adrenoceptors

Loss of [125I]-pindolol binding to beta-adrenoceptors on rat nodose ganglion after chronic isoprenaline treatment

The nodose ganglion contains the cell bodies of afferent nerves which convey predominantly sensory information from the viscera to the central nervous system (CNS). Autoradiographic studies show binding sites for beta-adrenoceptor ligands are present on sections of the rat nodose ganglion and also on the corresponding inferior vagal ganglion in humans, indicating the presence of beta-adrenoceptors in these ganglia. Since prolonged stimulation of beta-adrenoceptors in rats with the nonselective beta-adrenoceptor agonist isoprenaline (400 micrograms kg-1 day-1 s.c.) for 14 days results in desensitisation and/or down-regulation of receptors in peripheral tissues, such as heart, kidney and blood vessels, the effects of this treatment on the beta-adrenoceptor population on the nodose ganglion have been examined. Using [125I]-pindolol as a radioligand, autoradiographic studies revealed that specific binding was reduced by 74% in ganglia from isoprenaline-pretreated rats compared to that in ganglia from vehicle-pretreated rats, demonstrating down-regulation of receptors by isoprenaline. [125I]-Pindolol binding was sensitive to inhibition by ICI 118.551 (selective beta 2-adrenoceptor antagonist) but not to atenolol (selective beta 1-adrenoceptor antagonist), indicating receptors are predominantly of the beta 2-adrenoceptor subtype. No change in binding was apparent over the vagus nerve. The nodose ganglion appears to be an additional site at which beta 2-adrenoceptors may be down-regulated in vivo, possibly interfering with normal baro-, chemo- and sensory reflexes.

Functional dopamine D2 receptors on rat vagal afferent neurones

1. In the present study in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess dopamine D2 receptors. 2. Dopamine (10-300 microM) elicited a temperature- and concentration-dependent depolarization of the rat isolated nodose ganglion preparation. When applied to the tissue 15 min prior to agonist, raclopride (10 microM), clozapine (10 microM) or a mixture of raclopride and clozapine (10 microM each) all produced a threefold parallel shift to the right of the dopamine concentration-response curve. In contrast, SCH 23390 (100 nM), phentolamine and propranolol (1 microM each) failed to antagonize the dopamine-mediated depolarization. 3. [125I]-NCQ 298 (0.5 nM), a D2 selective radioligand, bound topographically to sections of rat brainstem. Densitometric quantification of autoradiograms revealed 93.8 +/- 0.5% specific binding of this salicylamide radioligand, as determined by raclopride (10 microM, n = 10 animals). Binding was highest in the nucleus tractus solitarius (NTS), particularly the medial and gelatinous subnuclei. In addition, specific binding was also observed in the interpolar spinal trigeminal nucleus and the inferior olive. 4. Unilateral nodose ganglionectomy caused a 36.6 +/- 3.0% reduction in specific binding in the denervated NTS compared to the contralateral NTS. Furthermore, the loss of binding was confined to the dorsal aspect of the medial subnucleus of the NTS. Sham surgery had no effect on the binding of [125I]-NCQ 298 in rat brainstem. 5. The present data provide evidence for the presence of functionally relevant dopamine D2 receptors on both the soma and central terminals of rat vagal afferent neurones. In addition, the majority of D2 receptors in the rat NTS appear to be located postsynaptically with respect to vagal terminals, and are presumably located either on ascending glossopharyngeal terminals, descending terminals from higher brain regions or on neuronal cell bodies within the NTS.

Characterization of [125I]HEAT binding to alpha 1-receptors in human brain: assessment in aging and Alzheimer's disease

The binding of [125I]2-(beta-4-hydroxyphenylethylamino-ethyltetralone ([ 125I]HEAT), an alpha 1-adrenergic receptor antagonist, to human brain membranes was characterized and the binding assessed in tissue from subjects with Alzheimer's disease (AD) and aging controls. Under Na+-K+ phosphate buffer conditions, [125I]HEAT bound to a single class of binding sites in prefrontal cortex (Brodmann area 10) with a Kd of about 120 pM. High binding capacities of [125I]HEAT were evident in the hippocampus and neocortex but were low in subcortical areas and cerebral microvessels comparable to the regional distribution of [3H]prazosin binding reported previously. Displacement of [125I]HEAT by various adrenergic drugs was consistent with its binding to alpha 1-adrenergic receptors. The specific binding was not affected by postmortem delay between death and freezing of tissue at autopsy. There was no correlation of [125I]HEAT binding with age of subjects. In AD subjects, the binding was significantly decreased in prefrontal cortex by about 25% but not changed in hippocampus, putamen or cerebellum compared to age-matched controls. The reduced binding of [125I]HEAT in prefrontal cortex may reflect a region-specific change in alpha 1-adrenergic receptors associated with neuronal loss in AD.

Upregulation of the vascular alpha-1 receptor in malignant DOCA-salt hypertension

In this investigation we describe regulation of the vascular alpha-1 receptor and functional properties of resistance vessels in malignant hypertensive DOCA-salt rats (DOCA-salt). Uninephrectomized control and DOCA-salt rats were maintained for 6 weeks; microscopic renal morphology provided an index of vascular injury. Radioligand binding studies indicated a striking increase in the density of mesenteric alpha-1 binding sites in DOCA-salt (542 +/- 44 fm/mg) vs. salt control (206 +/- 4 fm/mg) and water control (223 +31 fm/mg) P less than .01. The affinity of the receptor for the radioligand [125I] (+/-) BE 2254 was reduced in DOCA-salt rats. Electrical nerve stimulation and agonist dose response curves were performed on isolated perfused mesenteric arteries. A singular correlation between increased receptor density and vascular responses in DOCA-salt rats could not be demonstrated. The norepinephrine (NE) content of mesenteric arteries was reduced in DOCA-salt (1001 +/- 32 ng/g) vs. water control (1522 +/- 44 ng/g) and saline control (1538 +/- 30 ng/g) P less than .01. Our results indicate, upregulation of the mesenteric alpha-1 receptor occurs in DOCA-salt rats, however, additional factors participate in the vascular response to adrenergic stimulation in this model.

alpha 1-Adrenoceptors in the mammalian brain: similar pharmacology but different distribution in rodents and primates

We have used membrane binding assays and in vitro autoradiographic techniques to study the characteristics and distribution of alpha 1-adrenoceptors in the brain of the rat, mouse, guinea pig, cat, pig, monkey and man. 125I-BE 2254 was used as ligand. The binding characteristics of alpha 1-adrenoceptors in the cortex of the rat and pig and the human cortex and hippocampus were fully comparable as shown by the binding profile of a series of alpha 1-adrenergic compounds. Differences were, however, seen in the densities of sites in human and rat hippocampus. Contrasting with the similarities seen in the pharmacological characteristics of alpha 1-adrenoceptors in the brain of several mammals, autoradiographic results showed dramatic differences in the regional distribution of brain alpha 1-adrenoceptors. While the thalamus and cortex were rich in alpha 1-sites in all the species investigated, differences in the nuclear and laminar distributions were seen between rodents and primates. One of the most important differences was the high density of alpha 1-adrenoceptors in the human and monkey hippocampus, mainly in the CA3, CA4 and dentate gyrus, while in all the other species investigated, the hippocampal formation was poor in these sites. These results suggest that, while alpha 1-adrenoceptors appear to be similar in their pharmacology, their distribution differs among these 6 mammals. Thus, different patterns of central activity can be expected from alpha 1-adrenergic drugs in the rodent and in the primate and man.

Effect of chronic uremia on the cardiovascular alpha 1 receptor

Adrenergic dysfunction in uremia has been well described. Several lines of evidence suggest disorders of blood pressure regulation and myocardial response may occur secondary to adrenergic dysfunction; attenuated pressor response to norepinephrine (NE) in uremia; attenuated chronotropic responses during dialysis induced hypotension. Since the adrenergic receptors are the effector component of the adrenergic nervous system, we have employed the partially nephrectomized uremic rat, to examine the effect of chronic uremia (4-6 weeks) on the binding properties of alpha 1 receptors in rat mesenteric artery and myocardial tissue. The results indicate that moderate levels of uremia alter the binding properties of both the alpha 1 vascular and myocardial receptor.

Characterization of [3H]rauwolscine binding to alpha 2-adrenoceptor sites in the lumbar spinal cord of the cat: comparison to such binding sites in the cat frontal cerebral cortex

The binding of the selective alpha 2-adrenoceptor antagonist radioligand [3H]rauwolscine ([3H]RAUW) to homogenates of cat frontal cerebral cortex and cat lumbar spinal cord was investigated. Experiments were performed at 20 degrees C in 50 mM Tris HCl/l mM Na2EDTA buffer (pH 6.9 at 20 degrees C). At this temperature, specific [3H]RAUW binding, defined as the difference between the amount of [3H]RAUW bound in the absence and presence of 1 microM rauwolscine or 1 microM rauwolscine or 1 microM yohimbine, reaches equilibrium values by approximately 60 min and is reversible with a mean t1/2 of dissociation of 15 min in cortex and 20 min in spinal cord. The kinetically determined Kd of [3H]RAUW (mean K-1/mean K1) was 0.59 nM and 1.68 nM in cortex and spinal cord, respectively. The results of equilibrium saturation experiments, routinely performed at [3H]RAUW concentrations between 0.1 nM and 6.0 nM, indicate that [3H]RAUW binds to saturable sites in both CNS regions of the cat. Scatchard plots of saturation isotherm data were consistently linear and the mean Kd value determined from 10 such experiments was 0.72 nM in frontal cortex and 0.82 nM in lumbar spinal cord. A mean Bmax value of 230 fmol/mg protein was determined for saturable [3H]RAUW binding sites in the cat frontal cortex. In teh cat lumbar spinal gray, a mean Bmax value for saturable [3H]RAUW binding sites of 75 fmol/mg protein was obtained. Saturable [3H]RAUW binding sites in the cat lumbar spinal gray are present at apparently equal density in dorsal and ventral horns. Inhibition experiments, performed at 0.2 nM or 0.4 nM [3H]RAUW, indicate that the binding sites labeled by [3H] RAUW possess a pharmacology characteristic of alpha-adrenoceptors. Thus, rauwolscine, yohimbine, and phentolamine compete for specific [3H]RAUW binding with high affinity and are much more potent inhibitors than corynanthine, prazosin, and propranolol. Mean Hill coefficients, calculated from logit-log plots of competition data, were close to one for all antagonists examined. L-Epinephrine and L-norepinephrine were 15-20 times more potent inhibitors of specific [3H]RAUW binding than were their corresponding D-isomers. The agonist inhibitor potency series: p-aminoclonidine = clonidine = L-epinephrine greater than L-norepinephrine much greater than isoproterenol, is that expected of alpha 2-adrenoceptor sites. Mean Hill coefficients efficients for all agonists were considerably less than one.(ABSTRACT TRUNCATED AT 400 WORDS)

[3H]-rauwolscine binding to alpha 2-adrenoceptors in the mammalian kidney: apparent receptor heterogeneity between species

Binding of the alpha 2-adrenoceptor antagonist [3H]-rauwolscine was characterized in membrane preparations from the kidneys of mouse, rat, rabbit, dog, and man. In all species, binding reached equilibrium within 45 min and dissociated at a single exponential rate after addition of phentolamine 10 microM. Saturation studies showed that the affinity of [3H]-rauwolscine was similar in all species (2.33-3.03 nM) except man where it was significantly higher (0.98 nM). Marked differences were seen in the density of binding sites, increasing in the order: man less than dog less than rabbit less than rat less than mouse. In all cases, Hill coefficients were not significantly different from unity. [3H]-rauwolscine binds with low affinity (KD greater than 15 nM) to membranes prepared from guinea-pig kidney. The low affinity binding is not due to the absence of particular ions in the incubation medium or to receptor occupation by endogenous agonist. The binding in all species was found to be stereoselective with respect to the isomers of noradrenaline. However, differences were seen in the characteristics of agonist interactions with the binding site both between isomers and between species. Marked differences in affinity of particular alpha-adrenoceptor antagonists were observed for alpha 2-adrenoceptors labelled by [3H]-rauwolscine. These differences were most evident with the alpha 1-adrenoceptor selective antagonist prazosin which displayed inhibition constants (Ki values) of 33.2, 39.5, 261, 570 and 595 nM in rat, mouse, dog, man and rabbit, respectively. Differences are apparent in the characteristics of alpha 2-adrenoceptors labelled by [3H]-rauwolscine between species and it is suggested that the differences observed for alpha 1-selective antagonists such as prazosin may be related to binding to additional sites in the vicinity of the alpha 2-adrenoceptor.

Thermodynamics of agonist and antagonist binding to the alpha 1-adrenoceptor studied using [125I]BE 2254

The thermodynamics of binding of [125I]BE 2254 to the alpha 1-adrenoceptor in guinea pig brain membranes have been investigated at four different temperatures between 0 and 37 degrees C. The affinity and binding capacity of the radioligand did not vary with temperature. Thus, the change in enthalpy upon binding was close to zero whereas the change in entropy was large and positive (delta S degrees approximately 45 cal/mol-deg). In addition, [125I]BE 2254 has been used as a reporter ligand to probe the thermodynamics of the interaction of a variety of alpha-adrenoceptor agonists and antagonists with the alpha 1-adrenoceptor. Binding of all ligands was associated with large positive changes in entropy (delta S degrees between 18 and 48 cal/mol-deg) and little, or no, change in enthalpy, a finding that provides no convincing evidence for conformational rearrangement of alpha 1-adrenoceptors upon ligand binding.

Deafferentation of the rat hippocampus does not alter the affinity or binding capacity for the alpha 1-adrenoceptor radioligand [125I]BE 2254

Sectioning the dorsal afferents to the rat hippocampus led to an 88% decrease in the level of noradrenaline in the dorsal third of the structure, a 62% reduction in the medial region and no significant change in the ventral third. The lesion did not produce a change in either the affinity or maximum binding capacity of the alpha 1-adrenoceptors in any of the three regions of the hippocampus, as assessed by the binding of the radioligand [125I]BE 2254. This result indicates that reported increases in the response to noradrenaline in hippocampal slices after denervation is most likely due to a loss of noradrenaline uptake mechanisms.

Lack of alpha-adrenoreceptor binding of [125I]BE2254 to rat basilar artery membranes

Rat basilar arteries do not contain classical alpha- or beta-adrenoreceptors as assessed by electrophysiological techniques even though these arteries are innervated by catecholamine-containing perivascular nerves. These arteries were therefore examined for their ability to selectively bind an alpha-adrenoceptor radioligand, [125I]BE2254 (2/beta/4-hydroxyphenyl)-ethylaminomethyl)-tetralone). For comparison, rat tail arteries were also studied as these are known to contain functional alpha-adrenoreceptors. It was found that basilar artery membranes had only one-third of the specific binding of tail artery membranes and this finding collaborates the electrophysiological data.