Alpha 1-receptor Density Research Articles

Distribution of the Noradrenaline Innervation and Adrenoceptors in the Macaque Monkey Thalamus.

Noradrenaline (NA) in the thalamus has important roles in physiological, pharmacological, and pathological neuromodulation. In this work, a complete characterization of NA axons and Alpha adrenoceptors distributions is provided.NA axons, revealed by immunohistochemistry against the synthesizing enzyme and the NA transporter, are present in all thalamic nuclei. The most densely innervated ones are the midline nuclei, intralaminar nuclei (paracentral and parafascicular), and the medial sector of the mediodorsal nucleus (MDm). The ventral motor nuclei and most somatosensory relay nuclei receive a moderate NA innervation. The pulvinar complex receives a heterogeneous innervation. The lateral geniculate nucleus (GL) has the lowest NA innervation.Alpha adrenoceptors were analyzed by in vitro quantitative autoradiography. Alpha-1 receptor densities are higher than Alpha-2 densities. Overall, axonal densities and Alpha adrenoceptor densities coincide; although some mismatches were identified. The nuclei with the highest Alpha-1 values are MDm, the parvocellular part of the ventral posterior medial nucleus, medial pulvinar, and midline nuclei. The nucleus with the lowest Alpha-1 receptor density is GL. Alpha-2 receptor densities are highest in the lateral dorsal, centromedian, medial and inferior pulvinar, and midline nuclei.These results suggest a role for NA in modulating thalamic involvement in consciousness, limbic, cognitive, and executive functions.

Subchronic treatment with prazosin improves passive avoidance learning in aged mice: possible relationships to alpha1-receptor up-regulation.

While many evidences indicate that deficits of central noradrenergic neurotransmission are involved in the age-related decline of cognitive functions in animals and man, very little is known about the specific role of central alpha1-adrenergic receptors. Therefore, this problem was specifically addressed in the present communication using the alpha1-adrenoceptor antagonist prazosin as pharmacological tool. While the acute administration of prazosin did not affect passive avoidance learning of young or aged mice, an improvement of learning capabilities was seen after subchronic treatment, but for the aged animals only. An U-shaped dose response curve was seen. 0.3 mg/kg prazosin was most effective. Very interestingly, only for this dose a significant up-regulation of the density of alpha1-adrenoceptors in the mouse brain was found in the aged animals only. The data suggest a possible causal relationship between effects of prazosin on passive avoidance learning and central alpha1-receptor density.

Function and distribution of autonomic receptors in canine ureteral smooth muscle

There have been only a few reports of the measurement of autonomic receptors in ureteral smooth muscle. Furthermore, it is so difficult to maintain stable spontaneous contractions in the ureter, that either electrical field stimulation or KCl at high concentrations are utilized to induce ureteral contractions in many in vitro ureteral pharmacologic examinations. We used the spiral ureteral strips which generate spontaneous contractions in the ureter in present experiments. Norepinephrine, phenylephrine, clonidine, carbachol, and prostaglandin F2 alpha enhanced the spontaneous contractile force and/or increased the contractile frequency of spontaneous rhythmic contractions in spirally-incised muscle strips from isolated canine ureters. In contrast, isoproterenol, terbutaline, a beta 2-adrenergic agonist, and prostaglandin E2 reduced the spontaneous contractile force and/or decreased the contractile frequency of spontaneous rhythmic contractions. Dobutamine, a beta 1-adrenergic agonist, did not affect significantly the spontaneous rhythmic contractions. The effects of the prostaglandins were not influenced by autonomic antagonists or tetrodotoxin. The existence of alpha 1-, alpha 2-, and beta-adrenoceptors and muscarinic cholinergic receptors were demonstrated in the canine ureter using radioligand techniques. The density of alpha 1-receptors binding sites was significantly greater than that of the other receptors examined. Our data show that the sympathetic nervous system is more involved than the para-sympathetic nervous system in canine ureteral contractile activities, and that alpha- and beta-receptors contained in canine ureteral smooth muscle are comprised mainly of the alpha 1- and beta 2-subtypes. It is also suggested that prostaglandins directly affect canine ureteral contraction.

Impaired inotropic response to alpha 1- but not to beta-adrenoceptor stimulation in isolated hearts from spontaneously hypertensive rats.

Hypertension in humans and experimental animals is known to be associated with an increase in left ventricular myocardial mass. The development of cardiac hypertrophy is not caused by increased blood pressure alone; the autonomic nervous system may also play an important role. The functional responses to the beta-adrenoceptor agonists isoprenaline, dobutamine, salbutamol and terbutaline, and the alpha 1-adrenoceptor agonists methoxamine, cirazoline and phenylephrine were studied in isolated (Langendorff) hearts from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls. The results were compared with data from radioligand binding experiments. There was no significant difference in the increase of left ventricular pressure induced by all beta-adrenoceptor agonists studied in SHR and WKY rat hearts. Although there was no significant difference in the response to phenylephrine, the inotropic responses to cirazoline and methoxamine proved to be significantly weaker in hearts from SHR than in those from WKY rats. Binding experiments with 3H-prazosin revealed no differences in density or affinity for cardiac tissues from SHR and WKY rats. Long-standing hypertension leads to an impaired response of the isolated heart to alpha 1-adrenoceptor stimulation, without changes in alpha 1-receptor density or affinity. It seems likely that changes in postreceptor events are responsible for the impaired inotropic response to alpha 1-adrenoceptor agonists in hearts from SHR.

Effect of Cimetidine on the Density of Adrenergic Alphal-1 and Alpha-2 Receptors in the Rat Whole Prostate.

In order to study the antiprostatic effect of cimetidine, we measured adrenergic alpha-1 and alpha-2 receptors of the whole prostate of cimetidine-administered rats using radioligand binding techniques. Saturation experiments using 3H-prazosin and 3H-yohimbine revealed that there were significant amounts of alpha-1 and alpha-2 receptors in the rat whole prostate. There were more alpha-1 receptors than alpha-2 receptors in both the control and treated groups. In the treated group, the density of alpha-1 receptors showed little change, while that of alpha-2 receptors showed a significant decrease in comparison to the control group without changes in KD values. These data suggest that cimetidine affects the prostatic alpha receptors and consequently decreases the density of adrenergic alpha-2 receptors in the prostate.

Reduced high‐affinity α1‐adrenoceptors in liver of senescent rats: implications of assessment at various temperatures

1. We investigated the changes occurring as a result of aging in alpha 1-adrenoceptors in the livers of Fisher 344 females rats. For comparison, we also measured beta-adrenoceptors in this tissue. Three age groups were studied, including young adults (aged 6 months), mature adults (aged 16 months) and senescent animals (aged 25 months). 2. The density of alpha 1-receptors was measured by use of [3H]-prazosin and was found to be reduced 39% (P less than 0.01) at 25 months compared with 6 months. The percentage of alpha 1-receptors displaying high affinity for adrenaline was also reduced from 85.6% at 6 months to 51.6% at 25 months (P less than 0.02). 3. In contrast, the density of beta-receptors, which was measured with [125I]-iodocyanopindolol, was increased 104% between 6 months and 25 months. The affinity of both alpha 1- and beta-adrenoceptors for antagonists was unchanged with age. 4. We found that receptor affinity for agonists may be measured accurately in binding studies conducted at 4 degrees C or 25 degrees C, but that the apparent affinity for agonist was artifactually reduced in studies conducted at 37 degrees C. This effect is poorly reversible, in that reduced agonist-affinity is also observed in tissue which has been incubated at 37 degrees C and then cooled to 4 degrees C before performing the binding studies. 5. It is concluded that liver alpha 1-adrenoceptor function is reduced and beta-adrenoceptor function increased in senescence.

Autoradiographic delineation of skeletal muscle alpha 1-adrenergic receptor distribution

We used light microscopic autoradiography to quantify the distribution of alpha 1-adrenergic receptors in vessels and muscle fibers of slow-twitch (type I), fast-twitch (types IIa and IIb), and mixed fiber muscles of the rat hindquarter. Frozen cross sections of soleus, vastus lateralis, and gastrocnemius muscles were incubated under equilibrium binding conditions with 10-200 pM [3H]prazosin with or without 10(-5) M phentolamine. Because of the low concentration of bound radioligand, specific binding could not be detected with scintillation spectrometry in whole tissue sections scraped from slides. However, quantitative autoradiographic analysis after extended intervals of emulsion exposure revealed a low but significant level of specific binding in muscle fibers. No difference in alpha 1-receptor density was observed among types I, IIa, and IIb fibers. Small blood vessels had a much greater alpha 1-receptor density than muscle fibers. Resistance arterioles (20-100 microns diam) and small arteries (100-500 microns diam) contained 5.8 +/- 0.9 and 31.6 +/- 7.6 (+/- SE) times more binding sites per unit section area, respectively, than did surrounding muscle fibers (both P less than 0.001). Ratios of specific grain densities in fibers and blood vessels did not vary with radioligand concentration, indicating that observed grain densities reflected differences in receptor concentration rather than radioligand affinity by fiber and vessel receptors. The densities of vascular alpha 1-receptors did not vary in slow- and fast-twitch muscles, but resistance arterioles were six and eight times more numerous in soleus than in gastrocnemius and vastus muscles, respectively (both P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Aging Progressively Decreases the Densities and Alters the Diurnal Rhythms of α1-Adrenergic Receptors in Selected Hypothalamic Regions*

Associated with the decline in reproductive function that occurs with age in female rats are a diminished ability to secrete LH in response to ovariectomy as well as alterations in the preovulatory LH surge mechanisms. The noradrenergic system, an important regulator of LH release, exhibits age-related alterations in the pattern of neurotransmitter release. In addition, the density of alpha 1-receptors changes in intact old rats. To determine whether age-related changes in the pulsatile pattern of LH secretion are associated with declines in the densities and/or a loss of diurnal rhythm of alpha 1-receptors, we measured the density of alpha 1-receptors at various times of day in young, middle-aged, and old ovariectomized rats. By middle-age, mean concentrations of alpha 1-receptors decline in the median eminence and suprachiasmatic nucleus. By old age, mean concentrations of alpha 1-receptors decline in all regions of the hypothalamus examined but not in the dorsal lateral septum. In addition, the diurnal rhythms in alpha 1-receptors, which occur in the medial preoptic nucleus and the suprachiasmatic nucleus of young rats, are lost by middle-age. These data indicate that progressive alterations in the mean concentrations and diurnal rhythms of alpha 1-receptors occur with age in brain regions important for the regulation of reproductive functions and may contribute to the age-associated deficits in LH secretion.

Distribution of alpha 1-adrenergic receptors in myocytic regions and vasculature of feline myocardium

The physiological and pathophysiological roles of myocardial alpha 1-adrenergic receptors are not clearly defined. To delineate the distribution of alpha 1-receptors in myocytic and vascular components of the heart, we characterized the binding of [3H]prazosin to alpha 1-receptors in unfixed, transmural slices of feline left ventricle. Specific binding ratios greater than 95% were achieved at radioligand concentrations near the dissociation constant (Kd). Binding of radioligand to receptors in transmural slices was rapid, reversible, saturable, stereoselective, and displaceable by subtype-selective antagonists with a rank order of potency characteristic of alpha 1-receptors. Analysis of binding isotherms indicated a Bmax of 9.1 +/- 1.9 fmol/mg protein and a Kd of 36.9 +/- 6.3 pM. Results of light microscopic autoradiography indicated that regions composed of closely arranged cardiac myocytes contained three to fourfold more alpha 1-receptors per unit section area than the resistance microvasculature, which in turn contained approximately twice the density of alpha 1-receptors observed in the medial smooth muscle of large conductance arteries. No differences in the density of alpha 1-receptors between subepicardial and subendocardial regions were observed for either myocytic regions or coronary arterioles. These results indicate that myocytic regions of the cat ventricle contain a high density of alpha 1-adrenergic receptors. The methods developed should be of value in characterizing the distribution and function of alpha 1-receptors and mechanisms of regulation of adrenergic responsiveness in intact myocyardium.

α1- and β-adrenergic receptors in brown adipose tissue of lean (Fa/?) and obese (fa/fa) Zucker rats. Effects of cold-acclimation, sucrose feeding and adrenalectomy

1. The populations of alpha 1- and beta-adrenergic receptors in brown adipose tissue (BAT) of genetically obese Zucker rats (fa/fa) were studied with [3H]prazosin and [3H]CGP-12177 respectively. 2. The density of alpha 1-adrenergic receptors in BAT was significantly lower in obese than in lean Zucker rats, both at 2-4 months of age and at 6 weeks of age. The density of beta-adrenergic receptors was identical in BAT of lean and obese 6-week-old Zucker rats. 3. Cold-acclimation increased the alpha 1-receptor density significantly in BAT of both lean and obese Zucker rats, and the number of beta-receptors was also somewhat increased. 4. Sucrose feeding did not affect the density of alpha 1-receptors in BAT of lean or obese Zucker rats, but it increased beta-receptor density. 5. Adrenalectomy restored the density of alpha 1-adrenergic receptors in BAT of obese Zucker rats to the value observed in lean rats. 6. It is concluded that there is a direct correlation between alpha 1-receptor density and tissue recruitment, and that alpha 1-receptor density is thus positively correlated with sympathetic activity. beta-Receptor density is apparently better correlated with feeding conditions.

Estrogen alters the diurnal rhythm of alpha 1-adrenergic receptor densities in selected brain regions.

Norepinephrine regulates the proestrous and estradiol-induced LH surge by binding to alpha 1-adrenergic receptors. The density of alpha 1-receptors may be regulated by estradiol, photoperiod, and noradrenergic neuronal activity. We wished to determine whether alpha 1-receptors exhibit a diurnal rhythm in ovariectomized and/or estradiol-treated female rats, whether estradiol regulates alpha 1-receptors in those areas of brain involved with LH secretion and/or sexual behavior, and whether the concentrations of alpha-receptors vary inversely relative to previously reported norepinephrine turnover patterns. Young female rats, maintained on a 14:10 light-dark cycle were ovariectomized. One week later, half of them were outfitted sc with Silastic capsules containing estradiol. Groups of animals were decapitated 2 days later at 0300, 1000, 1300, 1500, 1800, and 2300 h. Brains were removed, frozen, and sectioned at 20 micron. Sections were incubated with [3H]prazosin in Tris-HCl buffer, washed, dried, and exposed to LKB Ultrofilm. The densities of alpha 1-receptors were quantitated using a computerized image analysis system. In ovariectomized rats, the density of alpha 1-receptors exhibited a diurnal rhythm in the suprachiasmatic nucleus (SCN), medial preoptic nucleus (MPN), and pineal gland. In SCN and MPN, receptor concentrations were lowest during the middle of the day and rose to peak levels at 1800 h. In the pineal gland, the density of alpha 1-receptors was lowest at middark phase, rose to peak levels before lights on, and remained elevated during the day. Estradiol suppressed the density of alpha 1 binding sites in the SCN, MPN, median eminence, ventromedial nucleus, and the pineal gland but had no effect on the lateral septum. Estrogen treatment altered the rhythm of receptor densities in MPN, median eminence, and the pineal gland. In MPN, a transient suppression in the density of receptors occurred at 1500 h, a time associated with increased norepinephrine turnover and the onset of the LH surge. Therefore, we conclude that alpha 1-receptor densities undergo a diurnal rhythm in brain regions associated with entrainment to the photoperiod. Estrogen alters the rhythm of alpha 1-receptors in areas involved with the regulation of LH secretion and decreases the density in other estrogen-responsive regions.

Ischemia- and agonist-induced changes in alpha- and beta-adrenergic receptor traffic in guinea pig hearts.

We have used radioligand binding techniques and subcellular fractionation to assess whether changes in expression of myocardial alpha 1- and beta-adrenergic receptors are mediated by a redistribution of receptors between various membrane fractions. Three fractions were prepared from the left ventricles of guinea pigs that underwent either 1 h of ischemia or injection of epinephrine (0.25 mg/kg ip): a crude membrane, a purified sarcolemma, and a light vesicle fraction. In control animals alpha 1-adrenergic receptors ([3H]prazosin binding) in light vesicles was only 25% of the total alpha 1-receptor density found in sarcolemmal and light vesicle fractions as compared with 50% for beta-adrenergic receptors ([125I]iodocyanopindolol binding sites). Although ischemia was associated with a 53% decrease in the number of light vesicle beta-adrenergic receptors and a 42% increase in the number of sarcolemma beta-receptors (P less than 0.05), there was no change in the number of light vesicle alpha 1-receptors, even though the number of sarcolemmal alpha 1-receptors increased 34%. Epinephrine treatment promoted internalization of beta-adrenergic receptors; sarcolemma beta-receptors decreased 37% and light vesicle beta-receptors increased 28% (P less than 0.025). For alpha 1-receptors, epinephrine treatment decreased the number of sarcolemmal receptors 41% (P less than 0.025) but failed to increase the number of receptors in the light vesicle fraction. The changes in receptor binding to beta-adrenergic receptors in sarcolemmal fractions were mirrored by parallel changes in isoproterenol-stimulated adenylate cyclase activity. These results indicate that alpha 1- and beta-adrenergic receptors may undergo a different cellular itinerary in guinea pig myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal ontogeny of alpha 1-adrenergic receptor in rat liver is thyroid hormone dependent.

Postnatal ontogeny of rat liver alpha 1-adrenergic receptor was examined using alpha 1-specific radioligand [3H]prazosin in control and propylthiouracil-treated congenital hypothyroid rats at various ages. Partially purified rat liver membranes prepared by the Neville method had 8-fold purification of 5'-nucleotidase from the crude homogenates from postnatal day 5 to adulthood. [3H]Prazosin binding was typical of an alpha 1-adrenergic receptor, and (-)epinephrine affinity for the [3H]prazosin-binding sites was not altered in the presence of 10(-5) M guanylyl-imidodiphosphate. The receptor density was lower in 5- and 15-day-old rats than in 28-day-old or older rats in both control and hypothyroid groups. (P less than 0.01). At 28-34 days of age, hypothyroid pups had significantly lower alpha 1-receptor density than controls (399 +/- 10 vs. 869 +/- 40 fmol mg protein-1; P less than 0.01). Replacement therapy with daily T4 injection from postnatal days 16-27 restored 54% of the deficit in PTU-treated hypothyroid pups at 28 days. The dissociation constant of [3H] prazosin did not change with advancing age or with different treatment and was consistent at 0.1 nM. These findings indicate that the normal ontogeny of plasma membrane alpha 1-adrenergic receptors is dependent upon thyroid hormone and matures postnatally in rat liver.