An autoradiographic demonstration of prazosin binding to arterial vessels in the rat

3-D Image Analysis of Fluorescent Drug Binding

Fluorescent ligands provide the means of studying receptors in whole tissues using confocal laser scanning microscopy and have advantages over antibody- or non-fluorescence-based method. Confocal microscopy provides large volumes of images to be measured. Histogram analysis of 3-D image volumes is proposed as a method of graphically displaying large amounts of volumetric image data to be quickly analyzed and compared. The fluorescent ligand BODIFY FL-prazosin (QAPB) was used in mouse aorta. Histogram analysis reports the amount of ligand-receptor binding under different conditions and the technique is sensitive enough to detect changes in receptor availability after antagonist incubation or generic manipulations. QAPB binding was concentration dependent, causing concentration-related rightward shifts in histogram. In the presence of 10 microM phenoxybenzamine (blocking agent), the QAPB (50 nM) histogram overlaps the autofluorescence curve. The histogram obtained for the 1D knockout aorta lay to the left of that control and 1B knockout aorta, indicating a reduction in 1D receptors. We have shown, for the first time, that it is possible to graphically display binding of a fluorescent drug to a biological tissue. Although our application is specific to adrenergic receptors, the general method could be applied to any volumetric, fluorescence-image-based assay.

Potent alpha-adrenoceptor blocking action of SGB-1534, a new quinazoline antihypertensive agent in vitro experiments

SGB-1534 inhibited the contractile response to norepinephrine (NE), methoxamine (MO) and electrical transmural stimulation in the rabbit aorta, basilar and cat coronary arteries and only in the rabbit aorta its inhibitor action was a competitive manner. In the rabbit aorta, the potency (pA2) of SGB was greater than that of prazosin, whereas in the other preparations, the effect of SGB was comparable to that of prazosin. In the rabbit aorta, both SGB and prazosin abolished the contraction induced by transmural stimulation. In the guinea-pig vas deferens, SGB failed to affect the inhibitory effect of clonidine on the response to electrical field stimulation. SGB had no inhibitory effects on the contractile response to potassium, 5-hydroxytryptamine, PGF2 alpha and an excess Ca2+ and the relaxing response to isoproterenol in the vascular tissues. In the electrically driven left atria of guinea-pigs, SGB had no effect on the electromechanical parameters and the isoproterenol induced inotropic effect. On the specific binding of [3H]prazosin in the rabbit aorta, the inhibitory effect of SGB was greater than that of prazosin and phentolamine.

The alpha-adrenergic receptor: radiohistochemical analysis of functional characteristics and biochemical differences

The partial agonist [3H]para-aminoclonidine was used to label alpha 2-adrenergic binding sites in intact sections of the rat central nervous system using in vitro labeling receptor autoradiographic techniques. The distribution of alpha 2-agonist binding sites closely parallels the reported distribution of noradrenergic and adrenergic cell groups and their terminal fields, particularly the projections of the medullary catecholamine neurons. This distribution of alpha 2 binding sites confirms physiological studies which indicate that the anti-hypertensive actions of alpha 2-agonist compounds are mediated centrally in medullary and spinal centers involved in the control of parasympathetic and sympathetic outflow. Further, the high concentrations of alpha 2 binding sites in pontine and limbic areas such as the locus coeruleus, parabrachial nucleus, dorsal raphe, hypothalamus, amygdala, bed nucleus of the stria terminalis, septum and entorhinal cortex offer an anatomical basis for understanding the anxiolytic and antidepressant actions of drugs like clonidine. The antagonists [3H]prazosin and [3H]WB4101 were used to study the distribution of alpha 1-adrenergic binding sites in the rat forebrain and biochemical studies were performed to analyze the marked differences that were initially seen in the distribution of [3H]prazosin and [3H]WB4101 binding sites. Several pieces of evidence derived from both biochemical and autoradiographic studies suggest that [3H]prazosin and [3H]WB4101 act at distinctly different binding sites. However, both sites may represent components of an alpha 1-adrenergic receptor-effector complex since a high degree of overlap was seen in the binding site distribution of these two ligands and since kinetic interactions could be demonstrated in at least one region of the brain, the hippocampus. Differences noted in the relative displacements of [3H]prazosin and [3H]WB4101 binding in various forebrain regions could reflect differences in the coupling efficiency of the [3H]prazosin and [3H]WB4101 component of the hypothesized complex. Further, in some regions, [3H]WB4101 labeled a binding site that is different from the alpha 1-receptor. Thus, [3H]prazosin and [3H]WB4101 binding sites seen in forebrain regions such as lamina V of the cortex, thalamic nuclei and dorsal raphe probably represent alpha 1-adrenergic receptors and confirm electrophysiological and biochemical studies which demonstrate that adrenergic transmission in these regions can be mediated through an alpha 1-receptor.

Central and peripheral prazosin binding: an in vitro autoradiographic study in the rat

Using an in vitro autoradiographic technique the binding of [3H]prazosin to brain, spinal cord, heart and vascular tissue was studied. Histologically prepared tissue sections were incubated in 5 nM [3H]prazosin or 5 nM [3H]prazosin in the presence of 100 microM unlabelled prazosin base in order to establish sites exhibiting specific prazosin binding. Dense prazosin binding sites were found in all arterial tissue examined except mesenteric artery. Dense prazosin binding was also found in cardiac tissue. Prazosin binding to veins was present sometimes but often absent. Prazosin does not readily pass the blood brain barrier and in this in vitro study the only area, within the central nervous system, showing specific prazosin binding sites was the olfactory bulb. The selective binding of prazosin, an alpha 1-adrenoceptor antagonist, to arterial and cardiac tissue would suggest that the antihypertension induced by prazosin is mediated by a peripheral action.