Interaction betweenα2- and β-adrenergic receptors in rat cerebral cortical membranes: clonidine-induced reduction in agonist and antagonist affinity for β-adrenergic receptors

The interaction betweenα₂- and β-adrenergic receptors was investigated in rat cerebral cortical membranes. Clonidine inhibition of [³H]dihydroalprenolol ([³H]DHA) binding resulted in biphasic competition curves with a mean Hill coefficient of 0.45. The addition of 1 μM yohimbine caused a rightward shift of the first portion of the clonidine inhibition curve. In the presence of 1 μM clonidine, the maximum concentration which did not inhibit [³H]DHA binding, inhibition curves of [³H]DHA binding by isoproterenol shifted to the right. A mean Hill coefficient increased from a control value of 0.63 to 0.76. Computer modeling analysis revealed that 1 μM clonidine decreased a β-adrenergic high-affinity state from 28% to 13%. However, the addition of 1 μM yohimbine completely prevented the clonidine-induced reduction in the β-adrenergic high-affinity state. In the presence of 200 μM GTP, the effect of clonidine was not observed. In addition,K_d andB_max values for[³H]p-aminoclonidine ([³H]PAC) binding were not significantly changed by the addition of 100 nM isoproterenol, the maximum concentration which did not inhibit [³H]PAC binding. Moreover, isoproterenol inhibition of [³H]PAC binding resulted in steep competition curves with a mean Hill coefficient of 0.97. The addition of 1 μM alprenolol did not affect the isoproterenol inhibition curve. These data demonstrated that clonidine caused a decrease in agonist and antagonist affinity for β-adrenergic receptors, while isoproterenol did not modulate the binding characteristics ofα₂-adrenergic receptors. Furthermore, these results suggest that regulation betweenα₂- and β-adrenergic receptors is not bidirectional, but is instead unidirectional fromα₂-adrenergic receptors to β-adrenergic receptors.     

Evidence for involvement of an adrenal catecholamine in the beta-adrenergic inhibition of oxytocin release in lactating rats

Adrenergic systems exert dual control over the release of oxytocin (OT) in rats, with stimulation of alpha-adrenergic receptors exciting, and stimulation of beta-adrenergic receptors inhibiting, release of this neurohormone. Because suckling stimulation also releases epinephrine from the adrenal medulla, the present experiments tested whether catecholamines of adrenal origin may participate in the adrenergic regulation of OT release during lactation. In two independent experiments, adrenal demedullation of rats in midlactation did not alter the basal plasma levels of OT, but markedly enhanced the suckling-induced release of OT, suggesting an inhibitory action of an adrenal catecholamine. The OT release induced by suckling in both sham-operated and adrenal demedullated rats was prevented by stimulation of beta-adrenergic receptors with isoproterenol. Conversely, blockade of beta-adrenergic receptors with propranolol prevented the inhibitory effects of isoproterenol, and when given alone, mimicked the effects of demedullation to enhance suckling-induced OT release. Stimulation of peripheral alpha-adrenergic receptors with phenylephrine did not affect either basal or suckling-induced OT release, but blockade of alpha-adrenergic receptors with phentolamine also completely prevented the release of OT by suckling. These data support the concept that stimulation of beta-adrenergic receptors inhibits OT secretion, and further suggest that this may be due, at least in part, to an action of an adrenal catecholamine, which may act centrally and/or directly on the neurohypophysis. The present results also provide further evidence that activation of central, but not peripheral, alpha-adrenergic mechanisms is necessary for suckling-induced OT release.     

β-Adrenergic agonists suppress chronic/relapsing experimental allergic encephalomyelitis (CREAE) in Lewis rats

Chronic/relapsing experimental allergic encephalomyelitis (CREAE) serves as an animal model for relapsing/remitting multiple sclerosis. Treatment with the β-adrenergic agonist isoproterenol or the β₂-adrenergic agonist terbutaline significantly suppressed both the first acute attack and the number of relapses in CREAE Lewis rats. The number of relapses was decreased even when treatment with β-adrenergic agonist was started after the onset of the first acute attack of CREAE. β-adrenergic receptor number was increased significantly on splenocytes from CREAE rats as compared to healthy controls or CFA-injected rats. Terbutaline treatment of CREAE rats lowered the splenocyte receptor number to normal values.     

Effects of unilateral decortication on β-adrenergic receptors in the remaining cortex and in the hypothalamus of female rats

Many experiments have been performed to evaluate the physiological role of catecholaminergic mechanisms in gonadotropin release. The purpose of the present study was to determine the concentration of beta-adrenoreceptors in the remaining (right) cerebral cortex and in right and left hypothalamic halves of hemi-decorticated female rats which exhibited elevated plasma gonadotropin levels as observed previously. The density of beta-receptors was measured using a high-affinity beta-adrenergic ligand, iodocyanopindolol (ICYP). Scatchard estimates were obtained for maximum binding (Bmax fmol/mg of tissues) from pooled cerebral cortical and hypothalamic tissue of animals under several experimental conditions after hemi-decortication and sham operation. There was an increase in beta-adrenoreceptor density in the remaining (right) cerebral cortex at all times examined in hemi-decorticate in comparison with the sham-operated animals (7 days, +10.9%; 21 days, +8.4%; 90 days, +22%; and 90 days plus ovariectomy, +34.8%). The number of beta-adrenoreceptors in the right hypothalamic half in hemi-decorticates decreased at 21 days (-42.20%) and then increased at 90 days (+76.63%) and 90 days plus ovariectomy (+51.75%) when compared with the left hypothalamic half. At the same time there were no significant changes in the sham-operated animals when comparing the receptor density in the right and left hypothalamic halves, respectively. Thus, our results suggest a direct or indirect adrenergic pathway by which the left cortex can influence the right cortex and a crossed pathway to the contralateral hypothalamus changing adrenergic activity which can alter the beta-adrenergic receptor binding capacity in the hypothalamus.     

Autoradiographic localization of β-adrenergic receptors in the rabbit pituitary

The localization and characterization of beta-adrenergic receptors in the rabbit pituitary have been studied using [125I]cyanopindolol as a specific ligand. Slide-mounted frozen sections were used for the autoradiographic localization and characterization of beta-adrenergic receptors. The displacement curves obtained from optical density of radioautograms or counting of scraped off sections demonstrated that beta-adrenergic receptors were mostly of the beta 2-subtype and highly concentrated in the intermediate lobe. Low concentrations of beta 2-adrenergic receptors were also found to be uniformly distributed in the anterior and posterior lobes. These results suggest that epinephrine and/or norepinephrine might play a physiological role in the regulation of the secretion of not only the intermediate lobe but also the anterior and posterior lobes of the rabbit pituitary.     

Expression of the α7, α4 and α3 nicotinic receptor subtype in the brain and adrenal medulla of transgenic mice carrying genes coding for human AChE and β-amyloid

Human AChE-enzyme (hAChE) enhances the over-expression of beta-amyloid (Abeta) containing plaques in the brain of transgenic mice (APP(SWE)/hAChE-Tg) carrying mutated genes for human amyloid precursor protein (APP(SWE)) and hAChE. In this study, we showed that interaction of hAChE with Abeta affects the plasticity of the alpha7 nicotinic acetylcholine receptors (nAChRs) both in the brain and adrenal medulla. An age-related increase in the (125)I-alphabungarotoxin ((125)I-alphaBTX) binding (specific to alpha7 nAChRs) was observed in the adrenal medulla of 3, 7 and 10 months old control mice. In contrast, a significant decrease in (125)I-alphaBTX binding was detected in the adrenal medulla of 10 months old APP(SWE)/hAChE-Tg. A significantly higher alpha7 nAChR mRNA level was observed in the brain of APP(SWE)/hAChE-Tg at 3 and 7 months of age and in the adrenal medulla at 3 and 10 months of age compared to those of the control mice. The alpha3 nAChR mRNA level was significantly higher in the brain of APP(SWE)/hAChE-Tg at 3 months of age and in the adrenal medulla at 10 months of age. The alpha4 nAChR mRNA level remained unchanged in the brain and adrenal medulla of APP(SWE)/hAChE-Tg for all age groups. Based on these observations, we conclude that a high load of Abeta and an over-expression of hAChE induce differences in the expression of the nAChR subtypes at various ages in the brain and in the adrenal medulla of hAChE/APP(SWE)Tg mice. The findings may have implications for a better understanding the underlying mechanism for AD-related pathogenesis.     

Lindane may enhance nocturnal pinealN-acetyltransferase activity via β-adrenergic receptors

Lindane, a chlorinated hydrocarbon pesticide, was previously shown to enhance the nighttime rise in pineal N-acetyltransferase (NAT) activity and melatonin as well as serum melatonin levels. The purpose of the present study was to test whether lindane acts on the pineal gland by means of a beta-adrenergic receptor mechanism. Whereas lindane (total dose 17.8 mg/kg b.wt. over 6 days) by itself significantly augmented the nocturnal levels of pineal NAT activity in otherwise untreated rats, the pesticide was ineffective in reference to this enzyme when it was given in conjunction with the beta-adrenergic receptor antagonist propranolol (20 mg/kg b.wt., one hour before lights off). The augmentation of NAT activity by lindane also caused significant reductions in pineal serotonin (5-HT) and 5-hydroxyindole acetic acid (5-HIAA); again, both these responses were blocked by propranolol treatment. Neither pineal 5-hydroxytryptophan nor pineal or serum melatonin levels were significantly changed as a result of either lindane or propranolol treatment. The results are consistent with the idea that lindane influences pineal 5-HT metabolism either at the level of the beta-adrenergic receptor or via the sympathetic innervation to the pineal gland.     

Δ-Tetrahydrocannabinol-induced changes in β-adrenergic receptor binding in mouse cerebral cortex

The effects of 3 cannabinoids, Δ⁹-tetrahydrocannabinol (Δ⁹-THC), 11-OH-Δ⁹-tetrahydrocannabinol (11-OH-Δ⁹-THC) and cannabinol (CBD) on the binding of [³H]dihydroalprenolol ([³H]DHA) to mouse brain β-adrenergic receptors were determined. In vitro, Δ⁹-THC and 11-OH-Δ⁹-THC increased the specific binding of [³H]DHA. The increased specific binding of [³H]DHA was due to an increase in receptor affinity as indicated by a decrease in the dissociation constant (K_d). CBD had no effect on binding. Chronic administration of Δ⁹-THC in vivo caused a decrease in the number of [³H]DHA binding sites with no change in K_d.     

β-Endorphin-induced hyperglycemia is mediated by increased central sympathetic outflow to adrenal medulla

Synthetic human β-endorphin increased plasma glucose concentration when administered intracisternally in chronically cannulated, conscious, unrestrained, adult male rats. This hyperglycemic effect of β-endorphin was blocked by prior systemic administration of naloxone, supporting mediation of the effect at opioid receptors in brain. Adrenal denervation blocked the β-endorphin-induced increase in plasma glucose, supporting a thesis that this effect is mediated at least in part by increased epinephrine secretion. The hyperglycemic response to intracerebral β-endorphin was also blocked by either intracerebral hemicholinium-3 or somatostatin, supporting both a cholinergic link and a somatostatin neuron in the brain mechanism regulating endorphin-induced stimulation of sympathetic outflow.     

Effect of age on β-adrenergic receptors on cerebral microvessels

The responsiveness of β-adrenergic receptors in cerebral microvessels was studied in aged rats by measuring cAMP formation induced by norepinephrine and [¹²⁵I]iodohydroxybenzylpindolol binding. The density of β-receptor sites is reduced by aging in the rat cerebral microvessels. These results suggest that the changes in brain circulation during aging may be at least partially due to a reduction of the β-adrenergic control of cerebral microvessels.     

α- and β-adrenergic binding sites in sheep cerebral cortex: Characterisation, effects of photoperiod and treatment with estrogen/progesterone

The radioligands [3H]-dihydroergocryptine and [3H]-dihydroalprenolol were used to characterise alpha- and beta-adrenergic binding sites, respectively, in membrane fractions of sheep cerebral cortex. In terms of affinity, density and specificity these sites possess properties similar to those previously characterised in rat brain. Further, in preliminary studies, these sites also appear to be responsive to treatment with estradiol/progesterone as well as to photoperiod. Thus, estrogen treatment can elevate both alpha- and beta-adrenergic binding sites in cortical tissue of sheep kept in natural light. In contrast, artificial light either has no effect or inhibits binding in response to estrogen.     

Presynaptic input regulates development of β-adrenergic control of rat brain ornithine decarboxylase: Effects of 6-hydroxydopamine or propranolol

beta-Adrenergic control of ornithine decarboxylase (ODC) activity is exerted only during a critical period in central nervous system development, playing an important role in neurotransmitter modulation of cell replication and differentiation. The current study examines the effects of lesions caused by 6-hydroxydopamine administration to neonatal rats, or of gestational exposure to propranolol, on the subsequent development of the ODC response to beta-adrenergic stimulation elicited by an acute intracisternal challenge with isoproterenol. 6-Hydroxydopamine treatment severely attenuated the ability of isoproterenol to stimulate ODC in the cerebellum, a tissue that shows a postnatal peak of ODC reactivity. In contrast, much smaller effects were seen in the cerebral cortex, which has an earlier (pre/perinatal) peak of ODC, despite the fact that norepinephrine depletion was more persistent in the cortex. On the other hand, blockade of fetal beta-receptors by maternal propranolol infusions resulted in immediate postnatal attenuation of the ODC response in cerebral cortex, but not cerebellum. These data suggest that early exposure of beta-receptors to norepinephrine "programs" the subsequent efficiency of the receptor linkage to ODC during a critical ontogenetic period that occurs prenatally in the cerebral cortex and postnatally in the cerebellum.     

Peripheral catecholamine administration does not alter cerebral β-adrenergic receptor density

Chronic peripheral administration of isoproterenol, epinephrine or norepinephrine to rats caused a decrease in β-adrenergic receptor density in heart and lung, but not in cerebral cortex, caudate or cerebellum. The results show that alterations in peripheral catecholamine levels have no direct or indirect effects on brain β-adrenergic receptors.     

β-Adrenoceptors regulate myoelectric activity in the small intestine of rats: stimulation by β2 and inhibition by β3 subtypes

Abstract Using β-adrenergic agonists and antagonists this study investigated the importance of three different adrenoceptor subtypes for the regulation of migrating myoelectric complexes (MMCs) of the upper small intestine in conscious, naive rats. After a control period of 60 min with four activity fronts, agonists were given as an intravenous infusion for another 60 min. The non-selective β-adrenoceptor agonist isoprenaline (1 μg kg^?1 min^?1) inhibited MMCs and induced irregular spiking during the infusion period. This effect was blocked by intravenous administration of a bolus dose of either the non-selective β-adrenoceptor antagonist propranolol (1 mg kg^?1), or the β₂-antagonist ICI 118 551 (1 mg kg^?1), both given prior to isoprenaline. However, acebutolol (1 mg kg^?1), a selective β₁-antagonist, failed to antagonize the effect of isoprenaline. Furthermore, prenalterol, a selective β₁-agonist (12.5–800.0 μg kg^?1 min^?1), had no effect on the MMC pattern, whereas the β₂-selective agonist ritodrine (25–100 μg kg^?1 min^?1) induced a myoelectric pattern similar to one induced by isoprenaline. The partial β₃-adrenoceptor agonist D7114 (50–100 μg kg^?1 min^?1), disrupted the MMCs and induced quiescence. Neither of the antagonists, i.e. propranolol (1 mg kg^?1), acebutolol (1 mg kg^?1) nor ICI 118 551 (1 mg kg^?1), given alone induced changes in the MMC pattern. In conclusion, β₂-adrenoceptors in particular but also β₃-adrenoceptors seem to be of importance in the regulation of small intestinal motility by disrupting the regular MMC pattern in rats.     

Differential effects of morphine withdrawal on cerebral β1- and β2-adrenergic receptors

Effect of morphine dependence and its withdrawal on the ³H-dihydroalprenolol (³H-DHA) binding for βadrenergic receptors,β₁ andβ₂, was examined by a computerized analysis of biphasic Hofstee plots. The relative density ofβ₁ andβ₂ receptors in the rat cerebral cortex was found to be approximately 70% and 30%, respectively. In rats rendered dependent on morphine by a subcutaneous implantation of a morphine pellet, the ³H-DHA binding toβ₁ andβ₂ receptors was not altered. During the stage of withdrawal induced by administration of naloxone, however, the ³H-DHA binding to the cerebral particulate fractions was increased, and this increase was due to the increased binding sites inβ₁ but not inβ₂ receptor. On the other hand, the apparent affinities ofβ₁ andβ₂ receptors for atenolol and salbutamol, selective antagonists forβ₁- andβ₂-adrenergic receptors, respectively, were not altered under these experimental conditions. These results suggest that an abrupt increase in cerebralβ₁-receptor binding sites occurs at morphine withdrawal, and the occurrence of such a supersensitivity in cerebralβ₁ receptor may be involved in the exhibition and/or maintenance of the abstinence syndrome in morphine-dependent subjects.     

Modulation of rat brain α- and β-adrenergic receptor populations by lesions of the dorsal noradrenergic bundle

Bilateral lesion of the ascending noradrenergic fibers in the dorsal bundle of adult Wistar rats with 4 μg 6-hydroxydopamine caused extensive depletion of norepinephrine in all forebrain areas, but led to a 54% increase in norepinephrine levels in the cerebellum. β-Adrenergic receptor binding of [³Hdihydroalprenolol was significantly increased in all forebrain areas depleted of norepinephrine except hypothalamus. The increase in [³Hdihydroalprenolol binding was due to 62% and 34% increases in the number of β-receptor sites in the frontal cerebral cortex and hippocampus respectively. Binding of [³HWB-4101 toα₁-adrenergic receptors after dorsal bundle lesion was augmented generally to a lesser extent than β-receptor binding, with significantly increased numbers of sites only in the frontal cortex (74%), thalamus (20%) and septum. Bothα₁-andβ-receptor binding sites were reduced in number by 25–28% in the cerebellum of dorsal bundle-lesioned rats, whereas intraventricular administration of 6-hydroxydopamine to adult rats, which depletes norepinephrine in the cerebellum by 96%, increased cerebellarα₁-andβ-receptor binding by 33–40%. Binding of [³Hclonidine to forebrainα₂-adrenergic receptors was significantly elevated in the frontal cortex, but reduced in the amygdala and septum, after dorsal bundle lesion.     

Ultrastructural localization of β-adrenergic receptor-like immunoreactivity in the cortex and neostriatum of rat brain

We sought to quantitatively examine the processes containing β-adrenergic receptor-like immunoreactivity (β-AR-LI) in the cerebral cortex and neostriatum using a previously characterized rabbit antiserum to frog erythrocyte β-ARs under optimized immunolabeling conditions. Quantitative assessments of the laminar distribution of β-AR-LI in the cortex was achieved by computer-assisted image analysis of immunoautoradiographs and by quantitative electron microscopic analysis of peroxidase-antiperoxidase (PAP) labeling in aldehyde-fixed sections and unfixed synaptosomes. In the somatosensory and anterior cingulate cortical areas, light microcopy of aldehyde-fixed sections immunolabeled by the PAP method revealed small (0.5–1.0 μm) punctate processes in all layers. In the deeper layers, rims of immunoreactivity around the plasmalemma of a population of neuronal perikarya and processes were also observed. By immunoautoradiography, labeling was seen in distinct, laminar distributions resembling the reported autoradiographic patterns using radioligands^{34, 40, 46}. By electron microscopy, the immunoreactive profiles in all cortical layers were primarily thick and thin postsynaptic densities (PSDs), comprising 4% of all identifiable PSDs in fixed sections and 12% in unfixed synaptosomal preparations. Also labeled were saccules of smooth endoplasmic reticulum and pinocytotic vesicles in dendrites, glial processes and lightly myelinated axons. In the neostriatum, the density of autoradiographic immunoreactivity was equivalent to the heavily labeled laminae of the cerebral cortex. Immunoreactivity detectable by light microscopy included punctate processes and rims of perikarya, as was seen in the cerebral cortex. The PAP reaction was shown by electron microscopy to be localized to the cytoplasmic surface of plasmalemma of a few proximal dendrites, but was most prominently associated with PSDs of dendritic spines. Preadsorption of the antiserum with a partially purified β-AR preparation abolished all detectable immunoreactivity. These results provide further support for the specificity of the antiserum for β-ARs, and are the first quantitative ultrastructural evidence for association of β-AR-LI with PSDs in the cerebral cortex. The neostriatum, whose major catecholaminergic innervation is dopaminergic, and not noradrenergic, is also confirmed to exhibit high levels of β-AR-LI within subcellular structures analogous to those seen in the cerebral cortex. The ultrastructural distribution of β-AR-LI in the two regions provides morphological support for the receptor's membrane-associated recycling, axonal transport, function at both excitatory and inhibitory synapses, and role in interactions between specific neuorons and glia.     

Taurine release from the pineal gland is stimulated via a β-adrenergic mechanism

Pineal gland were prelabeled in organ culture with [¹⁴Ctaurine, and then treated with biogenic amines. The rate of release of [¹⁴Ctaurine was rapidly elevated by low concentrations of norepinephrine, apparently acting through a β-adrenergic mechanism. The release was probably not due to a nonspecific change in membrane permeability, as a comparable release was not seen with [¹⁴Cglycine and [¹⁴Cα-aminoisobutyric acid. These findings suggest that the release of taurine may be related to or a reflection of the early events in adrenergic activation of the pineal gland, such as hyperpolarization of pinealocytes. The released taurine might also have an extracellular role as it has been recently shown that extracellular taurine can interact with pineal β-adrenergic receptors to stimulate melatonin production. Thus, the released taurine could possibly act as an extracellular feedback messenger.     

Stress and β-adrenergic receptor binding in the rat's brain

The effect of acute and chronic electric shocks on β-adrenergic receptor binding in the rat's brain was investigated. β-adrenergic receptor subsensitivity in the corter was induced by chronic shocks, but not by acute shocks. This reduction appears to be due to a decreased number of receptors. It seems that stress, by increasing intrasynaptic norepinephrine levels resulting from an accelerated turnover rate, causes β-adrenergic receptor subsensitivity.     

Effects of α-endorphin, β-endorphin and (des-tyr)-γ-endorphin on α-MPT-induced catecholamine disappearance in discrete regions of the rat brain

Following the intracerebroventricular administration of α-endorphin, β-endorphin and (des-tyrosine¹)-γ-endorphin in a dose of 100 ng, the α-MPT-induced catecholamine disappearance was found to be altered in discrete regions of the rat brain. In the regions in which α-endorphin exerted an effect, it without exception caused a decrease in catecholamine disappearance. Thus, in rats treated with α-endorphin the disappearance of noradrenaline was decreased in the medial septal nucleus, dorsomedial nucleus, central amygdaloid nucleus, subiculum, the ventral part of the nucleus reticularis medullae oblongatae and the A1 region, and that of dopamine in the caudate nucleus, globus pallidus, medial septal nucleus, nucleus interstitialis striae terminalis, paraventricular nucleus, zona incerta and central amygdaloids nucleus. β-endorphin was found to decrease noradrenaline disappearance in the ventral part of the nucleus reticularis medullae oblongatae, dopamine disappearance in the lateral septal nucleus and the disappearance of both amines in the rostral part of the nucleus tractus solitarii. Dopamine disappearance was increased in the medial septal nucleus and the zona incerta following β-endorphin treatment. Following treatment with (des-tyrosine¹)-γ-endorphin, noradrenaline disappearance was enhanced in the anterior hypothalamic nucleus, whereas dopamine disappearance was increased in the paraventricular nucleus, the zona incerta and the rostral part of the nucleus tractus solitarii. In addition to this the latter peptide also caused a decreased noradrenaline disappearance in the periventricular thalamus and the A7 region. The results fit well with the suggestion that endorphins act as modulators of catecholamine neurotransmission in particular brain regions. The pattern of effects of the endorphins differ from that previously observed following intracerebroventricular administration of methionine-enkephalin. This is in keeping with the notion that the enkephalin containing network in the brain and that containing β-LPH represent two independent systems with distinct differences in their projections to various brain regions.