Properties of [3H]prazosin-labeled alpha 1-adrenergic receptors in rat brain and porcine neurointermediate lobe tissue

Synthesis of various analog derivatives of ORG13514 as 5-HT1A ligands

In connection with the development of new potential 5-HT1A ligands, multistep synthesis of N-substituted-3-aminomethyl-2,3-dihydro-1,4-dioxinol[2,3-b]pyridin e derivatives as ORG13514 analogs are described. Their biological activity as 5-HT1A type ligands is reported and compared with ORG13514 affinity and selectivity for 5-HT1A receptors.

Investigation of alpha1-adrenoceptor subtypes mediating vasoconstriction in rabbit cutaneous resistance arteries

1. Cutaneous resistance arteries (c.r.a.) (internal diameter=240.94+/-5.42 microm, n=67/25 (number arteries/number animals)) from New Zealand white rabbits were mounted in wire myographs and a normalization procedure followed. 2. Cumulative concentration-response curves (CCRCs) were constructed for the alpha-adrenoceptor agonists noradrenaline (NA), (R)A61603 and phenylephrine (PE) in the presence of cocaine (3 microM), propranolol (1 microM) and corticosterone (10 microM). The effects of competitive alpha1-adrenoceptor antagonists, prazosin, WB4101, 5-methyl-urapidil, HV723, BMY7378 and the irreversible alpha1B selective compound chloroethylclonidine (CEC) were examined versus the potency and maximum response of the c.r.a.s to noradrenaline. 3. The high potency of A-61603 relative to PE has been shown to differentiate both functional and binding site alpha1A- or alpha1B-adrenoceptors from alpha1D-adrenoceptors: A-61603 was 944 times more potent than phenylephrine (at EC50) suggesting the presence of a functional alpha1A or alpha1B as opposed to an alpha1D-subtype. 4. Exposure to chloroethylclonidine (CEC; 100 microM) decreased the maximum response to noradrenaline but did not significantly change noradrenaline sensitivity indicating that a substantial part of noradrenaline-induced vasoconstriction in rabbit cutaneous arteries is CEC-insensitive. 5. The potencies of prazosin (pA2=9.14) and WB4101 (pA2=9.30) indicate the involvement of prazosin-sensitive functional alpha1-adrenoceptors. The slopes of corresponding Schild plots for prazosin and WB4101 did not include negative unity which implies the possible involvement of more than one functional alpha1-adrenoceptor subtype in noradrenaline-induced vasoconstriction in rabbit cutaneous resistance arteries. In contrast to this, in the case of 5-methyl-urapidil and HV723, the Schild plot slope parameters were not significantly different from negative unity over the range of concentrations used; the low pA2 value for 5-methylurapidil (7.27) suggests the non-involvement of an alpha1A- or an alpha1D-adrenoceptor; the low pA2 value for HV723 (8.47) was similar to that against responses postulated as alpha1L. 6. We conclude that rabbit cutaneous resistance arteries express a prazosin-sensitive functional alpha1-adrenoceptor resembling the alpha1B and another low affinity site for prazosin which on the basis of the functional antagonism produced by HV723 most closely resembles the alpha1L-adrenoceptor; the low pA2 value for HV723 (8.47) is similar to that against responses postulated as alpha1L.

N-arylpiperazinyl-N'-propylamino derivatives of heteroaryl amides as functional uroselective alpha 1-adrenoceptor antagonists

Novel arylpiperazines were identified as alpha 1-adrenoceptor (AR) subtype-selective antagonists by functional in vitro screening. 3-[4-(ortho-Substituted phenyl)piperazin-1-yl]propylamines were derivatized with N,N-dimethyl anthranilamides, nicotinamides, as well as carboxamides of quinoline, 1,8-naphthyridine, pyrazolo[3,4-b]pyridine, isoxazolo[3,4-b]pyridine, imidazo[4,5-b]pyridine, and pyrazolo[1,5-a]pyrimidines. Strips of rabbit bladder neck were employed as a predictive assay for antagonism in the human lower tract. Rings of rat aorta were used as a "negative screen" for the test antagonists. Binding to alpha 1-ARs was relatively sensitive to size and electronic features of the arylpiperazine portion of the antagonists and permissive to these features on the heteroaryl carboxamide side. These structure-affinity findings were exploited to produce nicotinamides (e.g. 13ii and 25x) and pyrazolo[3,4-b]pyridines (e.g. 37f and 37y) ligands with nanomolar affinity at the alpha 1-AR subtype prevalent in the human lower urinary tract(pA2 values: 8.8, 10.7, 9.3, and 9.9, respectively) and displaying 2-3 orders of magnitude selectivity over the alpha 1D-AR.

Analysis of the activity of alpha 1-adrenoceptor antagonists in rat aorta

1. In this study, the effect of seven alpha 1-adrenoceptor antagonists (tamsulosin, phentolamine, prazosin, WB-4101, 5-methylurapidil, spiperone and HV723) have been examined on the contractile response to noradrenaline (NA) and phenylephrine (PE) in rat isolated aorta. 2. NA and PE, when administered using a cumulative dosing schedule, both produced concentration-dependent contraction of aortic rings. It was possible to fit the individual concentration-effect (E/[A]) curve data to the Hill equation to provide estimates of the curve midpoint location (p[A]50 = 7.74 +/- 0.10 and 7.14 +/- 0.18), midpoint slope (nH = 0.82 +/- 0.03 and 0.99 +/- 0.10) and upper asymptote (alpha = 3.2 +/- 0.3 and 3.1 +/- 0.2 g) parameters for NA and PE, respectively. However, the Hill equation provided a better fit to the E/[A] curve data obtained with another contractile agent, 5-hydroxytryptamine (5-HT) (p[A50] = 6.09 +/- 0.08, nH = 1.49 +/- 0.09, alpha = 2.6 +/- 0.3 g), as judged by calculation of the mean sum of squares of the differences between the observed and predicted values. 3. All of the antagonists investigated produced concentration-dependent inhibition of the contractile responses of the aorta to NA and PE. Although no significant effects on the upper asymptotes of the E/[A] curves of any of the antagonists tested were detected, only tamsulosin and 5-methylurapidil did not have a significant effect on the slope (nH) of the NA and PE E/[A] curves. The other antagonists produced significant steepening of the curves obtained with NA and/or PE. 4. Notwithstanding the fact that one of the basic criteria for simple competitive antagonism at a single receptor class was not always satisfied, the individual log [A]50 values estimated in the absence and presence of antagonist within each experiment were fitted to the competitive model. The Schild plot slope parameters for the antagonism of NA and PE by phentolamine and HV723 were found to be significantly less than unity. The Schild plot slope parameters for the other antagonists were not significantly different from unity. 5. In the absence of evidence to suggest that the deviations from simple competitive antagonism were due to failure to satisfy basic experimental conditions for quantitative analysis, an attempt was made to see whether the data could be accounted for by an existing two-receptor model (Furchgott, 1981). The goodness-of-fit obtained with the two-receptor model was significantly better than that obtained with the one-receptor model. Furthermore, with the exception of the data obtained with phentolamine, the pKB estimates for the two receptors were independent of whether NA or PE was used as agonist. 6. To determine which alpha 1-adrenoceptor subtypes may be associated with those defined by the two receptor model, the mean pKB estimates obtained from the two-receptor model fit were compared with affinities measured by Laz et al. (1994) for rat cloned alpha 1-adrenoceptor subtypes expressed in COS-7 cells. The sum of squared differences of the data points from the line of identity was smallest for both pKB1 and pKB2 in the case of the alpha 1a/d-adrenoceptor (now referred to as alpha 1d-adrenoceptor; Hieble et al., 1995). Therefore, the complexity exposed in this study may be due to the expression of closely-related forms of the alpha 1d-adrenoceptor. However, relatively good matches were also found between pKB1 and alpha 1c and between pKB2 and alpha 1b. Therefore, on the basis of these data, it is not possible to rule out the involvement of all three alpha 1-adrenoceptors. The conflicting reports concerning the characteristics of the alpha 1-adrenoceptor population mediating contraction of the rat aorta may, at least in part, be due to the lack of highly selective ligands and to between-assay variation in the expression of multiple alpha 1-adrenoceptors.

Alpha 1B-receptors and intracellular calcium mediate sympathetic nerve induced constriction of rat irideal blood vessels

The present study has investigated the receptors involved in the non-cholinergic nerve mediated constriction of the larger blood vessels (30-50 microns) within the rat iris. This response was blocked by the alpha-adrenoceptor antagonist, benextramine (10(5) M). Furthermore, the response was more sensitive to blockade by the alpha 1 antagonist, prazosin (IC50 9 x 10(-10) M), than to blockade by the alpha 2 antagonist, yohimbine (IC50 2 x 10(-7) M), or the adrenergic antagonist, WB4101 (IC50 2 x 10(-8) M), and was abolished by chloroethylclonidine (10(-5) M). These results suggest the involvement of alpha 1B-adrenoceptors. The nerve mediated constriction was not blocked by the voltage-dependent calcium channel blocking drugs, nifedipine (10(-6) M), verapamil (10(-6) M) or diltiazem (10(-6) M), but was completely abolished by the intracellular calcium mobilizer, caffeine (10(-3) M), supporting the hypothesis that alpha 1B-adrenoceptors are activated following nerve stimulation. Dantrolene (10(-4) M), which interferes with calcium release from the sarcoplasmic reticulum, reduced the nerve mediated constriction by 40% as did thapsigargin (2 x 10(-6) M), which inhibits the calcium ATPase responsible for uptake of calcium into intracellular stores. When influx of calcium was blocked by verapamil (10(-6) M), thapsigargin, but not dantrolene, completely abolished the response. Noradrenaline (10(-5) M) produced a vasoconstriction in the presence or absence of external calcium although the latter response was significantly smaller than the former. Vasoconstriction produced by a submaximal concentration of noradrenaline (10(-6) M), was completely prevented by pretreatment with chloroethylclonidine. The data indicate that noradrenaline released from sympathetic nerves causes a constriction of arterioles in the iris by activating alpha 1B-adrenoceptors and releasing calcium from dantrolene sensitive and insensitive intracellular stores, followed by inflow of calcium through verapamil sensitive calcium channels. Applied noradrenaline also activates chloroethylclonidine sensitive receptors on the arteriolar surface.

Decreased alpha 1-adrenergic receptors after experimental brain injury

The magnitude of neuronal damage in central nervous system (CNS) injury may be related, in part, to alterations in the balance between excitatory and inhibitory neurotransmitters. Previous studies have implicated a role of central inhibitory noradrenergic mechanisms in the pathophysiologic sequelae of traumatic brain injury. In the present study, we examined alpha 1-adrenergic receptor binding after parasaggital lateral fluid percussion (FP) brain injury of moderate severity (2.3 atm) in the rat. At 30 min following injury, the specific binding of [3H]prazosin to membranes isolated from left cortex (injury site) was reduced by 37% in brain-injured animals when compared to sham-operated noninjured animals (p < 0.05). However, there were no significant differences in [3H]prazosin binding to membranes of either contralateral (right) cortex or left and right hippocampi between brain-injured and sham-operated animals. Conversely, at 24 h posttrauma, specific binding to membranes of left cortex, cortex adjacent to injury site, contralateral (right) cortex, and left hippocampus was reduced by 25%, 16%, 27%, and 24%, respectively (all p < 0.05). Scatchard analysis revealed that a reduction of [3H]prazosin binding to membranes of injured animals resulted from a decrease in alpha 1-receptor binding density (B-max) but not from changes in ligand affinity. Histopathologic assessment of neuronal damage at 24 h postinjury revealed neuronal loss within injury site cortex and left hippocampus but no clearly discernible cell loss in contralateral right cortex, suggesting that the decrease in B-max might be a consequence of early pathophysiology of trauma rather than of neuronal cell loss. We suggest that alterations in alpha 1-adrenergic receptors after brain injury may result in decreased inhibitory neurotransmitter action of norepinephrine and may thus contribute to the pathophysiology of traumatic brain injury.

Buffer effects on high affinity [3H]-prazosin binding in brain and spinal cord

[3H]-Prazosin binding was characterized in cortical and spinal membranes from Fischer 344N and Sprague-Dawley rats. Estimates of Bmax and Kd values were comparable with earlier studies of these regions in the central nervous system (CNS). However, the Kd obtained using Tris buffer system was greater than when HEPES or phosphate buffer was used. These data indicate that high affinity [3H]-prazosin binding in the homogenates of tissue from the CNS is affected critically by buffer selection.

Hallucinogenic drug interactions at human brain 5-HT2 receptors: implications for treating LSD-induced hallucinogenesis

It has been shown that the hallucinogenic potencies of LSD, the phenylisopropylamines, such as DOB (4-bromo-2,5-dimethoxyphenylisopropylamine) and DOI (4-iodo-2,5-dimethoxyphenylisopropylamine), and the indolealkylamines, such as DMT (dimethyltryptamine) and 5-OMe-DMT (5-methoxy-dimethyltryptamine), strongly correlate with their in vitro 5-HT2 receptor binding affinities in rat cortical homogenates. In order to ascertain if this correlation applies to human 5-HT2 receptors as well, we examined the affinities of 13 psychoactive compounds at 3H-ketanserin-labelled 5-HT2 receptors in human cortical samples. Both radioligand binding and autoradiographical procedures were used. As in rat brain, d-LSD was the most potent displacer of 3H-ketanserin specific binding with a Ki of 0.9 nM. The phenylisopropylamine DOI also displayed high affinity (Ki of 6 nM). Stereospecific interactions were found with DOB; (-) DOB had a Ki of 17 nM while (+) DOB had a Ki of 55 nM. The behaviorally active compound DOM (4-methyl-2,5-phenylisopropylamine) had an affinity of 162 nM while its behaviorally less active congener iso-DOM had an affinity of 6299 nM. The indolealkylamines 5-OMe-DMT and DMT competed with moderate affinities (207 and 462 nM, respectively). In general, Hill coefficients were significantly less than unity which is consistent with an agonist interaction with 5-HT2 receptors. MDMA, a substituted amphetamine analog was inactive with a Ki of greater than 10 microM. A strong correlation was found for the hallucinogen affinities and human hallucinogenic potencies (r = 0.97). Also, human and rat brain 5-HT2 receptor affinities were strongly correlated (r = 0.99). These results strongly support the hypothesis that the hallucinogenic effects of these drugs in humans are mediated in whole or in part via 5-HT2 receptors. Furthermore, these studies imply that treatment with 5-HT2 receptor antagonists may be effective in reversing the hallucinogenic effects caused by the ingestion of LSD and LSD-like drugs.

Alpha-adrenergic stimulation of growth hormone release in perifused rat anterior pituitary reaggregate cell cultures

It has been previously shown that beta-adrenergic agonists stimulate growth hormone (GH) release from perifused rat anterior pituitary cell aggregates cultured in serum-free defined medium for 5-6 days. The present data show that the natural beta-agonist epinephrine (EPI) stimulates GH release through an additional alpha-adrenergic mechanism. The involvement of this mechanism was suggested by the following findings. The intrinsic activity of EPI, at concentrations greater than 10 nM, was significantly higher than that of isoproterenol (ISO) in stimulating GH release. Phenylephrine (PHE), a specific agonist of alpha 1-adrenergic receptors, provoked a significant rise of GH release. The effect was concentration dependent between 100 nM and 10 microM. The stimulatory effect of EPI and PHE was lowered, respectively blocked, by low concentrations of the potent alpha 1-adrenergic antagonist prazosin. The EPI-induced GH release could be totally blocked only by administration of both alpha- and beta-receptor antagonists. Under the same conditions and using concentrations up to 1 microM, the alpha 2-agonist clonidine had no or only a slight stimulatory effect; dopamine had no effect. Administration of both PHE and ISO resulted in a more than additive stimulation of GH release. The effectiveness of PHE but not clonidine, together with the high potency of the alpha 1-blocker prazosin suggest that the alpha-adrenergic receptor is predominantly of the alpha 1-subtype. When tested on days 1, 3, 6 and 8 in culture, both alpha- and beta-adrenergic responses appeared to be higher in the presence of the glucocorticoid dexamethasone compared to the responses in the absence of dexamethasone.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Identification of subtypes of [3H]prazosin-labelled alpha 1 receptor binding sites in rat brain

Detailed antagonist competition curves for [3H]prazosin-labelled binding sites in rat cerebral cortex membranes reveal anomalous binding characteristics. Dihydroergocryptine and indoramine compete in a steep, monophasic manner while WB4101 and phentolamine exhibit shallow competition curves. Computer-assisted analysis of binding data indicate that both WB4101 and phentolamine discriminate identical subpopulations of [3H]prazosin binding sites, which each comprise approximately 50% of specific [3H]prazosin binding. These data suggest the presence of subtypes of [3H]prazosin-labelled alpha 1 adrenergic receptors in rat cerebral cortex.

Alpha 1 adrenergic receptors in the porcine pituitary neurointermediate lobe: detection with [3H]ketanserin

[3H]Ketanserin, a serotonin receptor antagonist, labelled high affinity, saturable sites in homogenates of porcine neurointermediate lobe tissue. Cinanserin, a potent and selective serotonin receptor antagonist, inhibited the specific binding of 5 X 10(-10)M [3H]ketanserin with a high affinity component representing 20% of the total binding. Prazosin, a potent and selective alpha 1 adrenergic antagonist, inhibited [3H]ketanserin binding with a high affinity component representing 60% of total binding. The prazosin-specific component was demonstrated to be distinct from the cinanserin-specific component. 10(-7)M cinanserin was co-incubated with [3H]ketanserin to eliminate the serotonergic component of the binding and allow pharmacological characterization of the remaining prazosin-specific component. The prazosin-specific binding of [3H]ketanserin binding closely resembled the results of experiments using [3H]prazosin to label alpha 1 receptors in neurointermediate lobe tissue homogenates. Ketanserin was found to be seven-fold more potent in inhibiting [3H]prazosin binding to alpha 1 adrenergic receptors in the neurointermediate lobe tissue than in brain tissue. This observation explains why low concentrations of [3H]ketanserin can selectively label serotonin receptors in the brain but will label both adrenergic and serotonin receptors in the neurointermediate lobe.