Synthesis and characterization of a radioiodinated photoaffinity probe for the alpha 1-adrenergic receptor

Advances in the design and synthesis of prazosin derivatives over the last ten years

INTRODUCTION

Mechanistic, translational and pharmacological studies led to the identification and discovery of the preferred localization, binding characteristics, structure and functional properties of α1-adrenoceptor (α1-AR) subtypes in the bladder neck, bladder and prostate gland. The evidence gathered on α1-ARs, provided a molecular platform for the development of subtype-selective antagonists, resulting in more effective approaches targeting those receptors for the treatment of outlet bladder obstruction and benign prostate hyperplasia.

AREAS COVERED

Advances over the last decade in the design and optimization of Prazosin, Doxazosin and Terazosin quinazoline-based derivatives as α1-AR antagonists. Evidence on the metabolic and growth interference action by these agents, in addition to their smooth-muscle-relaxing effects. The new action recognition emerges from data on the inhibitory effect of quinazoline-based antagonists on primary tumor growth and progression to metastasis. In addition to the cellular findings in the prostate, functional validation and therapeutic effects of selected lead pharmaceutically optimized derivatives in the context of impairing vascularity and triggering tumor apoptosis.

EXPERT OPINION

Knowledge on targeting intracellular signalling pathways driving the cellular response via an α1-AR-dependent and independent antagonistic action, must be invested towards the optimization of new agents that while bypassing AR, exhibit improved pharmacological efficacy against human cancer.

Androgens alter brain catecholamine content and blood pressure in the testicular feminized male rat

Androgens interact with catecholamines in the central nervous system (CNS) to regulate many physiological processes including blood pressure (BP). To test the hypothesis that testosterone (T) and 5a-dihydrotestosterone (DHT) modulate CNS catecholamines and BP through androgen receptor (AR)-dependent and independent mechanisms, we used the testicular feminized male (Tfm) rat. Females that carry the AR mutation (Tfm mutation) on the X chromosome were bred with spontaneously hypertensive rat (SHR) males. The normal AR male and Tfm offspring were divided into groups: control, castrated, castrated, and T or (DHT) replacement. In both AR normal and Tfm males, BP was reduced by castration, but T restored BP in both groups. In the amygdale, castration decreased dopamine (DA) in both strains and both T and DHT restored it. In the bed nucleus of the stria terminalis castration increased DA which was further increased by DHT and reduced to normal by T in both strains. In the frontal cortex, castration reduced DA content in both strains but only T restored it to normal in SHR but not in Tfm. Brain norepinephrine (NE) content showed a significant strain effect for the preoptic area (POA), but no treatment effect. Although castration did not change NE in the amygdala or POA in either strain, both T and DHT increased NE in the Tfm castrates. Blood pressure was influenced by T manipulation and correlated most significantly with DA content in the amygdala, frontal cortex, and stria terminalis. These data demonstrate an action of androgen on brain catecholamines and BP, which is independent of the classic androgen receptor.

Hypotensive effects of neuromedin U microinjected into the cardiovascular-related region of the rat nucleus tractus solitarius

Neuromedin U (NMU) is a brain-gut peptide with potent contractile effects on the uterus and smooth muscle. Intracerebroventricular injection of NMU reportedly decreased food intake and body-weight gain in the rat. We evaluated the effects of NMU delivered by microinjection into the rat nucleus tractus solitarius (NTS) on cardiovascular responses. At the concentrations used (5, 10 or 50 pmol), the intra-NTS injection of NMU in artificial cerebrospinal fluid produced a significant reduction in both the mean arterial pressure and heart rate. The hypotensive responses were dose dependent. Our findings suggest that NMU may act as a neurotransmitter or neuromodulative substance that causes excitation of neurons in the NTS and that it may play a role in cardiovascular regulation in vivo.

Nitric oxide and central antihypertensive drugs: one more difference between catecholamines and imidazolines

NO is known to be involved in the peripheral and central regulation of the cardiovascular function. It plays a neuromodulatory role via a direct action on presynaptic nerve terminals, stimulating the release of gamma-aminobutyric acid, glutamate, and norepinephrine. Our aim was to study the possible role of NO in the cardiovascular effects of the central antihypertensive drugs clonidine, rilmenidine, and alpha-methyl-norepinephrine (alpha-MNA). Sites and mechanisms of the hypotensive action of these drugs were different; clonidine and rilmenidine acted on imidazoline receptors in the nucleus reticularis lateralis, whereas alpha-MNA acted upon alpha(2)-adrenoceptors in the nucleus tractus solitarius. The influence of N:(G)-nitro-L-arginine, an NO synthase inhibitor, on the central hypotensive effects of these drugs was investigated in pentobarbital-anesthetized rabbits. The intracisternal (IC) administration of alpha-MNA (30 microg/kg) induced hypotension (79+/-2 versus 103+/-4 mm Hg) and bradycardia (222+/-8 versus 278+/-4 bpm) (P:<0.05) (n=5). Clonidine (0.07 microg/kg IC) also induced hypotension (69+/-5 versus 99+/-4 mm Hg) and bradycardia (266+/-7 versus 306+/-10 bpm) (P:<0.05) (n=5). In addition to clonidine, rilmenidine (1 microg/kg IC) induced hypotension (64+/-4 versus 97+/-4 mm Hg) and bradycardia (264+/-11 versus 310+/-4 bpm) (P:<0.05) (n=5). Pretreatment with N:(G)-nitro-L-arginine (900 microg/kg IC) completely prevented the hypotensive effect of alpha-MNA but influenced the cardiovascular effects of neither clonidine nor rilmenidine. These results confirm that imidazoline drugs, such as clonidine, rilmenidine, and the catecholamine alpha(2)-adrenoceptor agonist alpha-MNA, have distinct mechanisms of action.

Relative importance of medullary brain nuclei for the sympatho-inhibitory actions of rilmenidine in the anaesthetized rabbit

OBJECTIVE

To determine the contribution of the rostral ventrolateral medulla and the nucleus of the solitary tract in mediating the attenuation of the renal sympathetic baroreflex produced by administration of rilmenidine to anaesthetized rabbits and to examine the relative contribution of alpha2-adrenoceptors and imidazoline receptors at these sites to the cardiovascular effects of rilmenidine.

METHODS AND RESULTS

Rilmenidine micro-injected into the rostral ventrolateral medulla produced hypotension and inhibition of renal sympathetic nerve activity with doses an order of magnitude lower than those required in the nucleus tractus solitarius. Alpha-methylnoradrenaline, however, was similarly potent at producing hypotension when it was injected into the rostral ventrolateral medulla or nucleus tractus solitarius but, unlike rilmenidine, did not lower renal sympathetic nerve activity when it was injected into the nucleus tractus solitarius. The alpha2-adrenoceptor antagonist 2-methoxyidazoxan partially reversed the hypotension and renal sympathetic nerve activity inhibition due to alpha-methylnoradrenaline when it was administered into the rostral ventrolateral medulla, whereas the mixed alpha2-adrenoceptor/imidazoline receptor antagonists, idazoxan and efaroxan, did not. 2-Methoxyidazoxan, but not idazoxan, also reversed the hypotension when alpha-methylnoradrenaline was administered into the nucleus tractus solitarius. The hypotension induced by rilmenidine in the rostral ventrolateral medulla was completely reversed both by 2-methoxyidazoxan and by idazoxan, as was the sympathetic inhibition. To assess any interaction between the nucleus tractus solitarius and the rostral ventrolateral medulla in mediating the baroreflex effects of rilmenidine, we injected rilmenidine into the rostral ventrolateral medulla, the nucleus tractus solitarius or both nuclei and determined renal baroreflex responses of sympathetic nerve activity using drug-induced changes in blood pressure. Injection of 0.5 nmol rilmenidine into the rostral ventrolateral medulla reduced mean arterial pressure and basal renal sympathetic nerve activity as well as renal sympathetic baroreflex range (by 27%) and gain (by 35%). In contrast, injection of rilmenidine into the nucleus tractus solitarius had no effect on basal renal sympathetic nerve activity and renal sympathetic baroreflex parameters. The effect of combined injection was similar to that of administration into the rostral ventrolateral medulla alone.

CONCLUSION

Our results show that the rostral ventrolateral medulla, rather than the nucleus tractus solitarius, is the major site involved in the hypotension and inhibition of the renal sympathetic baroreflex by rilmenidine. Comparison of the actions of alpha2-adrenoceptor and imidazoline receptor antagonists on the effects of rilmenidine and alpha-methylnoradrenaline suggests that these agents are acting at different receptors, presumably imidazoline and alpha2-adrenoceptors receptors, respectively, and that both are important in lowering sympathetic tone and blood pressure in the rostral ventrolateral medulla.

Cardiac baroreflex in hypertension: role of the heart and angiotensin II

Sigmoid logistic function curves provide a powerful means of characterizing the baroreceptor-heart rate reflex. In hypertension the operating range of the reflex is reset in the direction of the elevated resting BP; this can be accounted by rapid resetting of the threshold of the arterial baroreceptors. In addition, there is a deficit in the vagal component of the heart rate (HR) range. Reduction in gain occurs in moderate/severe hypertension, but may be absent in young primary hypertensives. All the changes are reversible, and reversibility of HR range and gain is related to reducing left ventricular hypertrophy or central blood volume rather than to reduction in BP. High plasma angiotensin II can further accentuate the vagal deficit. An input-output model has been developed from comparison of perivascular cuff and drug methods for eliciting the reflex, which place different loads on the heart; the greater load changes simulate many of the alterations in reflex properties observed in hypertension. We conclude that during changes in vasomotor tone in normal animals, about 70% of the drive for the cardiac baroreflex comes from arterial baroreceptors and about 30% from low threshold cardio-pulmonary baroreceptors. In hypertension, the vagal deficit in HR range is due to afferent interactions involving arterial and low and high threshold cardio-pulmonary baroreceptors.

Neurogenic hypertension related to basilar impression. Case report

The authors report the resolution of essential hypertension following transoral odontoidectomy and medullary decompression in a 39-year-old woman with basilar invagination. Current understanding of central regulation of the cardiovascular system is discussed and the pertinent neuroanatomy illustrated. Experimental and clinical evidence supporting the role of neurogenic mechanisms in the pathogenesis of hypertension is reviewed.

Alpha-1 blockers. A new generation of antihypertensive agents

Alpha-1 blockers have certain disadvantages over conventional and antihypertensive therapies in their haemodynamic profile and metabolic effects. This paper reviews the development of alpha-blockade, the therapeutic efficacy of prazosin, the prototype alpha-1 blocker, and the rationale for the once-daily antihypertensive compounds, terazosin and doxazosin. These drugs offer a useful alternative to first- or second-line therapy in suitable hypertensive patients, particularly with their potentially beneficial effects on serum lipids.

Limitation of the alpha-methylnorepinephrine hypothesis in the hypotensive effect of alpha-methyldopa

To test the hypothesis that alpha-methylnorepinephrine (MNE) is the principal active metabolite involved in the hypotensive action of alpha-methyldopa (MD), we determined the relationship between MD's depressor response and tissue levels of MD metabolites in critical sites. After administration of 250 mg/kg MD intraperitoneally to Sprague-Dawley rats (300 +/- 50 g), we studied both heart (left ventricle) and brainstem MD, MNE and endogenous NE levels using HPLC with electrochemical detection. We also measured systolic blood pressure before and during MD (25-250 mg/kg i.p.) treatment using the tail-cuff method. Our results indicate that: (1) peak MD hypotensive response was dose-dependent. (2) Central NE concentration was maximally reduced by 2 hours whereas peripheral NE was maximally reduced by 18 hours. The maximal hypotensive effect was closer to the central peak distribution of MNE than MD. (3) The MD concentrations and NE concentrations in brainstem and heart showed counterclockwise hysteresis while MNE showed clockwise hysteresis. Furthermore, the area of MNE hysteresis in brainstem was larger than that of NE. We conclude that MD's depressor effect can not be completely explained by the assumption that MNE is the sole active metabolite; alternate metabolites or mechanisms would appear to be operative.

Synthesis and characterization of arylamine derivatives of rauwolscine as molecular probes for alpha 2-adrenergic receptors

The selective alpha 2-adrenergic receptor antagonist rauwolscine was structurally modified to yield a series of arylamine carboxamide derivatives, which were investigated as potential molecular probes for the localization and structural characterization of alpha 2-adrenergic receptors. The arylamine carboxamides differ in the number of carbon atoms separating the reactive phenyl moiety from the fused ring structure of the parent compound, rauwolscine carboxylate. Competitive inhibition studies with [3H]rauwolscine in rat kidney membranes indicate that the affinity for the carboxamide derivatives is inversely related to the length of the carbon spacer arm with rauwolscine 4-aminophenyl carboxamide (zero carbon spacer arm; rau-AMPC) exhibiting the highest affinity (Kd = 2.3 +/- 0.2 nM). Radioiodination of rau-AMPC yields a ligand, 125I-rau-AMPC, which binds to rat kidney alpha 2-adrenergic receptors with high affinity, as determined by both kinetic analysis (Kd = k2/k1 = 0.016 min-1/2.1 X 10(7) M-1 min-1 = 0.76 nM) and equilibrium binding studies (Kd = 0.78 +/- 0.16 nM). 125I-rau-AMPC was quantitatively converted to the photolabile arylazide derivative 17 alpha-hydroxy-20 alpha-yohimban-16 beta-(N-4-azido-3-[125I]iodophenyl) carboxamide (125I-rau-AZPC). In a partially purified receptor preparation from porcine brain, this compound photolabels a major (Mr = 62,000) peptide. The labeling of this peptide is inhibited by adrenergic agonists and antagonists with a rank order of potency consistent with an alpha 2-adrenergic receptor binding site. Both 125I-rau-AMPC and the photolabile arylazide derivative, 125I-rau-AZPC, should prove useful as molecular probes for the structural and biochemical characterization of alpha 2-adrenergic receptors.

Baroreflex sensitivity alteration following transient hemispheric ischaemia in rats: protective effect of alphamethyldopa and guanfacine

The reflex tachycardia to arterial vasodilation was analysed in anaesthetized and ventilated Long Evans rats. Nicardipine was given by slow intravenous injection. The decrease in blood pressure was accompanied by a sympathetically mediated reflex tachycardia. The slope of the mean blood pressure-pulse period relationship was considered as an index of baroreflex sensitivity. Two injections of nicardipine were given during a single experiment. For studying the suprapontine control of the baroreflex arc, rats were subjected to transient (10 min) bilateral hemispheric ischaemia. These rats exhibited a blood pressure drop following recirculation and baroreflex sensitivity was impaired. Pretreatment with alpha-methyldopa, guanfacine and enalapril lowered mean blood pressure to a similar extent. In those rats pretreated with alpha-methyldopa and guanfacine transient hemispheric ischaemia did not alter baroreflex sensitivity while enalapril pretreated rats exhibited an impaired baroreflex. The protective effect of alpha-methyldopa and guanfacine against the cardiovascular consequence of hemispheric ischaemia may depend on reduced monoamine turnover resulting from central alpha-adrenoceptor stimulation.

Para-azidoclonidine: a novel photoaffinity ligand for the alpha 2-receptor

Reversible and irreversible interactions of the photoreactive clonidine analogue p-azidoclonidine (PAZC) with brain alpha 2-adrenergic receptors were examined. In the absence of light, PAZC displayed selective, high affinity, competitive interactions with sites labeled by the alpha 2-agonist 3H-p-aminoclonidine (3H-PAC). Reversible binding characteristics resembled those of other alpha 2-agonists. Preincubation of bovine frontal cortex membranes with 100 nM PAZC followed by ultraviolet irradiation and thorough washing decreased alpha 2-receptor density 42% relative to controls receiving irradiation alone. The loss of receptors could be prevented by inclusion of 500 nM phentolamine in the preincubation medium. Alpha 1- and beta-adrenergic receptors were relatively unaffected. PAZC is a potential photoaffinity ligand for the alpha 2-adrenergic receptor.