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.     

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.     

Modulation of rat brain alpha- and beta-adrenergic receptor populations by lesion of the dorsal noradrenergic bundle

Bilateral lesion of the ascending noradrenergic fibers in the dorsal bundle of adult Wistar rats with 4 micrograms 6-hydroxydopamine caused extensive depletion of norepinephrine in all forebrain areas, but led to a 54% increase in norepinephrine levels in the cerebellum. beta-Adrenergic receptor binding of [3H]dihydroalprenolol was significantly increased in all forebrain areas depleted of norepinephrine except hypothalamus. The increase in [3H]dihydroalprenolol binding was due to 62% and 34% increases in the number of beta-receptor sites in the frontal cerebral cortex and hippocampus respectively. Binding of [3H]WB-4101 to alpha 1-adrenergic receptors after dorsal bundle lesion was augmented generally to a lesser extent than beta-receptor binding, with significantly increased numbers of sites only in the frontal cortex (74%), thalamus (20%) and septum. Both alpha 1- and beta-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 cerebellar alpha 1- and beta-receptor binding by 33-40%. Binding of [3H]clonidine to forebrain alpha 2-adrenergic receptors was significantly elevated in the frontal cortex, but reduced in the amygdala and septum, after dorsal bundle lesion.     

Chronic ethanol administration alters the modulatory effect of 5α-pregnan-3α-ol-20-one on the binding characteristics of various radioligands of GABAA receptors

In this study, we investigated the modulatory effect of 5α-pregnan-3α-ol-20-one, a neurosteroid, on the binding characteristics of [ ]flunitrazepam (2 nM), [ ]muscimol (5 nM), and 4 nM [ ]t-butylbicyclophosphorothionate (TBPS) in cerebral cortex, cerebellum, and hippocampus of control, ethanol-dependent, and ethanol-withdrawn rats. 5α-Pregnan-3α-ol-20-one potentiated the binding of [ ]flunitrazepam and [ ]muscimol in all the rat brain regions investigated in this study. There was a significant increase in the maximal potentiation of [ ]flunitrazepam as well as [ ]muscimol binding (E_max) in the ethanol-dependent rat cerebellum as compared to control group (p<0.025). Furthermore, 5α-pregnan-3α-ol-20-one elicited a biphasic response, i.e., it potentiated the binding of [ ]TBPS at lower concentrations (100 nM) and inhibited the binding at higher concentrations (>100 nM). There was a significant higher inhibition of [ ]TBPS binding (−E_max) by 5α-pregnan-3α-ol-20-one in the hippocampus of ethanol-dependent as well as ethanol-withdrawn rats (p<0.025). These observations suggest that the neurosteroid binding site associated with the γ-aminobutyric acid_A (GABA_A) receptors in cerebellum and hippocampus plays an important role during ethanol-dependence and ethanol-withdrawal, and some of the changes following ethanol dependence and its withdrawal may be mediated through the neurosteroid binding site.     

Characterization of β-adrenergic receptors in rat brain and pituitary using a new high-affinity ligand, [I]iodocyanopindolol

The binding of [¹²⁵I]iodocyanopindolol (ICYP) to membrane preparations from rat cerebral cortex, hypothalamus and anterior pituitary gland was characterized in regard to specificity, density, and the proportion of β-adrenergic receptor subtypes. By employing a mixture of ligands specific for α-adrenergic, serotoninergic and dopaminergic receptors, it was possible to eliminate most of the less-specific contributions to ICYP binding profiles, which resulted in narrowing the range of measured dissociation constants to 35–50 pM for all neural tissues studied. These values corresponded well with constant for the ‘slow’ component discernible in ICYP association with cerebral cortical membranes at 37° C. The maximum binding values were 63, 29 and 5.6 fM/mg membrane protein in cortical, hypothalamic and anterior pituitary membrane fractions, respectively.Evaluation of the β-adrenergic receptor subtypes using 4 selective competitors indicated an average 19% content of the β₂-subtype in cortical membranes, while in hypothalamic membranes 47% of the receptors could be assigned to that subtype. In the anterior pituitary as well as in the cerebellum, the receptors were predominantly ofβ₂-subtype. These findings are discussed in terms of possible physiological functions of β-receptors in these tissues, including the regulation of the release of pituitary hormones.     

α-Adrenergic receptor-mediated depolarization of rat neocortical astrocytes in primary culture

The membrane potentials of rat neocortical astrocytes growing in primary cultures (mean resting potential; −79 mV at [K⁺]_o = 4.5 nM) were depolarized by up to 30 mV by 10⁻⁵ M norepinephrine added to the medium, or up to 11 mV by norepinephrine or phenylephrine applied by ionophoresis. This depolarization could be inhibited by the α-adrenergic receptor antagonist phentolamine (10⁻⁵ M) but not by the β-adrenergic antagonist propranolol (10⁻⁵ M). These results suggest that the norepinephrine-evoked depolarizations seen in these cells may be mediated through an α-adrenergic receptor.     

Presence of an endogenous factor which inhibits binding of α-bungarotoxin 2.2 to its receptor

Cerebral cortical membranes and supernatant from rat were prepared by centrifugation of tissue homogenates at 45,000 g for 10 min. The supernatant fraction thus obtained was found to significantly inhibit α-bungarotoxin binding to the membrane preparation. After a 3 min incubation period, the supernatant inhibited toxin binding by approximately 65%, while the inhibition declined to about 40% after 30 min of incubation, presumably due to the slow reversility of α-bungarotoxin binding. The choice of buffer was found to be an important determinant of the degree of inhibition observed, with 10 mM Tris pH 7.4 providing the most effective condition. This inhibition of toxin binding to cortical membranes by the 45,000 g supernatant was shown not to be due to adsorption of the radiolabeled compound to soluble or residual particulate material in the supernatant fraction. Specificity of the supernatant for the α-bungarotoxin site was demonstrated; a supernatant fr action could be prepared which inhibited α-bungarotoxin binding by 50% but had no effect on [³H]spiroperidol (DA₂ and 5-HT₂), [³H]prazosin, (α₁-adrenergic), [³H]5-hydroxytryptamine (5-HT₁) and [³H]quinuclidinylbenzilate (muscarinic cholinergic) binding. The inhibition of toxin binding also occurred in several other CNS regions including hippocampus, brainstem, spinal cord and cerebellum with an 80 to 90% inhibition of binding occuring in the latter two regions. In addition, the 45,000 g cortical supernatant completely prevented the binding of α-bungarotoxin to extrajunctional neuromuscular receptors and inhibited the binding to junctional receptors by 50%. Supernatants prepared from heart, liver and kidney or bovine serum albumin, at a concentration similar to the supernatant fraction, did not alter radiolabeled toxin binding to cortical membranes, while supernatant prepared from striated muscle tissue was effective. These results suggest there may be an endogenous ligand for the α-bungarotoxin 2.2 binding site in tissues which receive nicotinic cholinergic innervation.     

Characterization of [I]HEAT binding to α1-receptors in human brain: assessment in aging and Alzheimer's disease

The binding of [¹²⁵I]2-(β-4-hydroxyphenylethylamino-ethyltetralone ([¹²⁵I]HEAT), an α₁-adrenergic receptor antagonist, to human brain membranes was characterized and the binding assessed in tissue from subjects with Alzheimer's disease (AD) and aging controls. Under Na⁺-K⁺ phosphate buffer conditions, [¹²⁵I]HEAT bound to a single class of binding sites in prefrontal cortex (Brodmann area 10) with a K_d of about 120 pM. High binding capacities of [¹²⁵I]HEAT were evident in the hippocampus and neocortex but were low in subcortical areas and cerebral microvessels comparable to the regional distribution of [³H]prazosin binding reported previously. Displacement of [¹²⁵I]HEAT by various adrenergic drugs was consistent with its binding to α₁-adrenergic receptors. The specific binding was not affected by postmortem delay between death and freezing of tissue at autopsy. There was no correlation of [¹²⁵I]HEAT binding with age of subjects. In AD subjects, the binding was significantly decreased in prefrontal cortex by about 25% but not changed in hippocampus, putamen or cerebellum compared to age-matched controls. The reduced binding of [¹²⁵I]HEAT in prefrontal cortex may reflect a region-specific change in α₁-adrenergic receptors associated with neuronal loss in AD.     

Neurotransmitter receptor localizations: Brain lesion induced alterations in benzodiazepine, GABA, β-adrenergic and histamine H1-receptor binding

Selective neuronal lesions have been utilized in efforts to localize binding sites in rat brain for β-adrenergic, γ-aminobutyric acid (GABA), histamine H₁ and benzodiazepine receptors. The various receptors respond differentially to lesions both in extent of change and in time course. After kainate lesions in the corpus striatum, benzodiazepine receptors are depleted up to 45% at 45–78 days but are unaffected after 7 days. By contrast striatal GABA receptors are increased at 7 days but depleted at later times. Thus both striatal benzodiazepine and GABA receptors appear to be associated at least in part with intrinsic neurons.In the cerebellum both benzodiazepine and GABA receptors are reduced in kainate treated rats and in Nervous mice, mutants which lack Purkinje cells. The most pronounced dissimilarity between benzodiazepine and GABA receptors occurs in Weaver mice, which selectively lack granule cells and display a 60% reduction in GABA receptors but a 40% augmentation in benzodiazepine receptors. A major portion of cerebellar GABA receptors, therefore, appear to be localized to granule cells.Striatal β-adrenergic receptors are reduced following intrastriatal kainate injections but are unaffected by cerebral cortex ablation, suggesting an association with intrinsic neurons but not with axon terminals of the corticostriate pathway. While intraventricular injections of 6-hydroxydopamine enhance [³H]dihydroalprenolol binding to β-adrenergic receptors in the cerebral cortex and hippocampus, such binding is not augmented in the corpus striatum, brain stem, midbrain or thalamus-hypothalamus by this treatment. Moreover, medial forebrain bundle lesions, which destroy ascending adrenergic neurons, fail to alter cerebral cortical or striatal β-adrenergic receptors. Thus denervation-elicited increase in β-adrenergic receptors vary with brain region and the type of denervating lesion.Histamine H₁-receptors are the most resistant of all to neuronal lesions. In the corpus striatum [³H]mepyramine binding is unaffected by cerebral cortex ablation, nigral injections of 6-hydroxydopamine or brain stem hemisection. In the hippocampus, medial forebrain bundle lesions, intrahippocampal kainate injection, and fimbria and fornix transection largely fail to alter [³H]mepyramine binding. Accordingly, a major portion of these receptors may be associated with nonneuronal elements such as glia or blood vessels.     

The resolution of dopamine and β1- and β2-adrenergic-sensitive adenylate cyclase activities in homogenates of cat cerebellum, hippocampus and cerebral cortex

The stimulation of adenylate cyclase by dopamine and various β-adrenergic agonists has been investigated in homogenates from 3 areas of cat brain: the cerebral cortex, cerebellum and hippocampus. The purpose of the study was to determine whether the β-adrenergic receptors coupled to adenylate cyclase could be classified as either β₁ and β₂ subtypes in the different regions studied.The stimulation of adenylate cyclase by the β-adrenergic agonist, (−)isoproterenol (5 × 10⁻⁶M), was completely blocked by the specific β-adrenergic antagonist, (−)alprenolol (10⁻⁵ M), but not by the dopaminergic antagonist, fluphenazine (10⁻⁵ M), whereas the stimulation of adenylate cyclase by (−)epinephrine (10⁻⁴ M) was blocked to varying extents by these two drugs in each of the 3 regions studied. The (−)epinephrine effect was always blocked in the combined presence of (−)alprenolol and fluphenazine. The adenylate cyclase stimulation by (−)epinephrine which is not blocked by (−)alprenolol was due to interaction of (−)epinephrine with a dopaminergic-sensitive adenylate cyclase which has been characterized in cerebral cortex, hippocampus and cerebellum.Regional differences in the affinity of β-adrenergic-sensitive adenylate cyclase for various agonists were investigated in the presence of fluphenazine (10⁻⁵ M). In the cerebellum the potency order was (±)protokylol> (±)hydroxybenzylisoproterenol> (±)isoproterenol> (−)epinephrine> (±)salbutamol> (−)norepinephrine, indicating the presence of a β₂-adrenergic receptor. In the cerebral cortex the potency order was (−)isoproterenol> (±)protokylol> (±)hydroxybenzylisoproterenol> (−)epinephrine= (−)norepinephrine((±)salbutamol being inactive). A similar pattern was found in the hippocampus indicating the presence of a β₁-adrenergic receptor in these two regions. (±)Salbutamol was a partial agonist in the cerebellum and a competitive antagonist in the cerebral cortex.The ratio of the antagonist potencies of (±)practolol and (±)butoxamine preferential β₁- and β₂-adrenergic antagonists respectively, to block the stimulation of adenylate cyclase was 25 in the cerebellum, compared to 0.5 in the cerebral cortex and 1.6 in the hippocampus. These results confirm the presence of a β₂ subtype of receptor coupled to adenylate cyclase in the former and β₁ subtypes in the latter two regions. The comparison between the affinities of a series of β-adrenergic agonists and antagonists for the β-adrenergic receptors coupled with an adenylate cyclase in cerebral cortex and cerebellum with their affinities for well characterized β₂-adrenergic receptors in lung and β₁-adrenergic receptor in heart substantiated this conclusion.     

Reserpine and the role of axonal transport in the independent regulation of pre- and postsynaptic β-adrenoreceptors

The response of pre- and postsynaptic β-adrenoreceptors to depletion of brain norepinephrine (NE) with reserpine in the rat was characterized by studying the anterograde and retrograde axonal transport of presynaptic receptors and the receptor binding changes induced in postsynaptic frontal cortex cells. Anterograde transport was shown to occur by the linear accumulation of [³H]dihydroalprenolol ([³H]DHA) binding sites (by in vitro binding assay) proximal to a 6-hydroxydopamine (6-OHDA) lesion placed in the ascending pathway of the locus coeruleus and was blocked by more proximal lesions in the pathway. Retrograde transport was demonstrated by the accumulation of [¹²⁵I]iodocyanopindolol binding distal to similar lesions. Autoradiograms from sections of 6-OHDA injected brains were produced with [³H]DHA binding in the presence of theβ₂-agonist, zinterol, and suggested that the antegrade accumulation of binding sites was primarily of theβ₁-subtype. A single injection of reserpine (5 mg/kg, i.p.) produced a long lasting (6–8 weeks), biphasic decrease in cortical NE levels with nadirs and 4 and 28 days (10% and 45% of control, respectively). Frontal cortex binding of [³H]DHA increased to a maximum at 7–14 days and again at 28 days post-reserpine (230% and 167% of control, respectively). These increases were not prevented by the destruction of presynaptic noradrenergic nerve terminals with intraventricular administration of 6-OHDA 1 day prior to sacrifice and therefore appeared to take place solely in postsynaptic cells. Presynaptic, anterograde axonal transport of β-receptors was completely blocked from 4–14 days post-reserpine, increased to 323% of control at 21 days, was blocked again at 6 weeks and returned to control by 8 weeks. Retrograde transport of β-receptors followed a similar pattern suggesting that the presynaptic alterations in β-receptors in noradrenergic neurons of the locus coeruleus take place independently from those in postsynaptic cortical β-receptors as a response to NE depletion by reserpine.     

Quantitative autoradiography of μ-,δ- and κ1 opioid receptors in κ-opioid receptor knockout mice

Mice deficient in the κ-opioid receptor (KOR) gene have recently been developed by the technique of homologous recombination and shown to lack behavioural responses to the selective κ₁-receptor agonist U-50,488H. We have carried out quantitative autoradiography of μ-, δ- and κ₁ receptors in the brains of wild-type (+/+), heterozygous (+/−) and homozygous (−/−) KOR knockout mice to determine if there is any compensatory expression of μ- and δ-receptor subtypes in mutant animals. Adjacent coronal sections were cut from the brains of +/+, +/− and −/− mice for the determination of binding of [³H]CI-977, [³H]DAMGO ( -Ala²-MePhe⁴-Gly-ol⁵ enkephalin) or [³H]DELT-I ( -Ala² deltorphin I) to κ₁-, μ- and δ-receptors, respectively. In +/− mice there was a decrease in [³H]CI-977 binding of approximately 50% whilst no κ₁-receptors could be detected in any brain region of homozygous animals confirming the successful disruption of the KOR gene. There were no major changes in the number or distribution of μ- or δ-receptors in any brain region of mutant mice. There were, however some non-cortical regions where a small up-regulation of δ-receptors was observed in contrast to an opposing down-regulation of δ-receptors evident in μ-knockout brains. This effect was most notable in the nucleus accumbens and the vertical limb of the diagonal band, and suggests there may be functional interactions between μ- and δ-receptors and κ₁- and δ-receptors in mouse brain.     

Canine narcolepsy is associated with an elevated number of α2-receptors in the locus coeruleus

α₂-Receptors in the canine brain were pharmacologically characterized using [³H]yohimbine binding. Competition studies revealed a single class of binding sites in frontal cortex but two distinct subtypes in nucleus caudatus. The role of central α₂-receptors in narcolepsy was investigated in 5 normal and 5 narcoleptic Doberman pinschers. Scatchard analysis of [³H]yohimbine binding in different brain areas revealed an increase in the number of α₂-binding sites limited to the locus coeruleus. This suggests that altered autoinhibition of norepinephrine release may be associated with the narcoleptic symptomatology.     

Selective increases in the density of cerebellarβ1-adrenergic receptors

Destruction of noradrenergic neurons by 6-hydroxydopamine or chronic blockade of β-adrenergic receptors with propranolol increased the density ofβ₁-adrenergic receptors two-fold in rat cerebellum but had no effect on the density ofβ₂-adrenergic receptors. The results suggest that even thoughβ₁ receptors comprise only 5–10% of the total number of β-adrenergic receptors in the cerebellum they are the receptors specifically innervated by noradrenergic neurons and they may thus be the physiologically important receptors.     

Long-term treatment with clonidine and α-receptors in the brain of Normotensive rats

The aim of the present study was to investigate whether or not changes in rat brain α-adrenoceptors take place during chronic treatment with a low dose of clonidine. Male Wistar normotensive rats were treated with clonidine (0.1 mg/kg) i.p. twice daily for 12 days. This treatment caused a significant increase in [³H]clonidine and in [³H]WB4101 binding, respectively, to α₂- and to α₂-adrenoceptors of the frontal cortex; the levels were 30% for [³H]clonidine and 20% for [³H]WB4101. The Scatchard analysis of data obtained in binding studies indicated that the enhanced binding of two ligands to membranes prepared from chronically clonidine-treated animals, was due to an apparent increase in the number of binding sites. These changes were seen 4 h after administration of the last treatment, before the appearance of the withdrawal syndrome. However, noradrenergic α₂-autoreceptors of synaptosomes, from the frontal cortex and hypothalamus of treated animals, were sensitive to the regulatory action of clonidine or of noradrenaline on the [³H]noradrenaline overflow elicited by high K⁺ as well as on the control animals. On the contrary, the α₂-receptors on the serotoninergic nerve terminals from the frontal cortex of treated animals were more sensitive than those of control animals to the action of clonidine or of noradrenaline in counteracting the [³H]5-hydroxytryptamine overflow elicited by high K⁺. These results suggests that during treatment with clonidine no autoreceptor hyposensitivity to the regulatory action of clonidine or noradrenaline on [³H]noradrenaline overflow elicited by high K⁺ takes place, but, as a consequence of the diminished noradrenaline availability at the synaptic cleft, the binding of [³H]WBA101 to α₁-receptors and of [³H]clonidine to pre- and postsynaptic α₂-receptors were significantly elevated in the frontal cortex, a brain areas where the α-₂-receptors are mainly postsynaptic. Thus, the neurotransmitter concentration in the synaptic cleft may be responsible for the trans-synpatic modulation of the α₂-adrenoceptor postsynaptic population. In fact, the α₂-adrenoceptors which are presynaptically located on the serotoninergic terminals, but are postsynaptic in relation to the noradrenergic neurons, also show increased sensitivity after chronic clonidine treatment.     

Activation of lateral geniculate neurons by norepinephrine: Mediation by an α-adrenergic receptor

Adrenergic receptors in the vicinity of neurons in the lateral geniculate nucleus (LGN) of the rat were pharmacologically characterized using extracellular single-cell recording and microiontophoretic techniques. Application of norepinephrine (NE) at low iontophoretic currents (1–15 nA) produced a delayed activation of most LGN neurons. This activation was mimicked by various sympathomimetic amines. The relative potency series of agonists was typical of postsynaptic α-adrenergic receptors:epinephrine>NE>phenylephrine α-methylnorepinephrine>dopamine>isoproterenol. The α-antagonists phentolamine, piperoxane and WB-4101, when applied at low iontophoretic currents (<10nA), produced a selective, dose-dependent and reversible blockade of the response to NE. The β-antagonist sotalol had weak and variable effects at these currents. At low currents, the presynaptic α-agonist clonidine was also able to block the response to NE but, at higher currents, produced a partial activation of some units, suggesting that it is a weak agonist. The ability of sympathomimetic amines to activate LGN neurons correlates well with their reported affinities for brainα₁-adrenoceptors labeled with [³H]WB-4101. It is concluded that NE activates neurons in the LGN via a postsynaptic orα₁-adrenergic receptor.     

The role of β- and α-adrenoceptors in the regulation of the stages of the sleep-waking cycle in the cat

The effects of β-adrenergic drugs alone and in combination with α-adrenergic drugs on the stages of the sleep-waking cycle were studied in adult cats. Polygraphic sleep recordings of 16 h showed that prenalterol (20 and 40 mg/kg i.p.), a β₁-adrenoceptor-stimulating drug increased paradoxical sleep (PS) in a dose-related manner during 4–12 h. Salbutamol (40 mg/kg), a β₂-adrenoceptor-stimulating drug, decreased PS during the first 4 h. Metoprolol (10 and 50 mg/kg), a relatively selective β₁-adrenoceptor blocking drug, increased drowsy waking during the first 4 h. The larger dose also tended to decrease PS. Already at the lower dose metoprolol partially antagonized the PS increase produced by prazosin, an α₁-adrenoceptor blocking drug. Propranolol (5 mg/kg), a β₁-andβ₂-adrenoceptor blocking drug, which alone decreases PS, antagonized the PS increase induced by phentolamine, an α₁-andα₂-adrenoceptor drug. Atenolol (5 mg/kg), a poorly lipid-soluble β-adrenoceptor blocking drug, failed to counteract phentolamine in increasing PS. Metoprolol (10 and 50 mg/kg) and propranolol (5 mg/kg) clearly potentiated the increase in drowsy waking and decrease in deep slow wave sleep and PS induced by clonidine (0.01 mg/kg), an α₂-adrenoceptor-stimulating drug.The results support the involvement of β-adrenoceptors in the regulation of the sleep-waking cycle. A high level of β-adrenergic activity may facilitate the production of PS. A low level of β-adrenergic activity, especially in combination with a high level of α₂-adrenergic activity, may facilitate the production of drowsy waking. Central α₁-andβ₁-adrenoceptors may mediate opposite functions in the regulation of PS.     

Modulation of rat brainα2- and β-adrenergic receptor sensitivity following long-term treatment with antidepressants

After 7 days of treatment with a variety of antidepressant drugs (desipramine, imipramine, clomipramine, nortriptyline, nialamide), both an increase inα₂-receptor density and a decrease in β-receptor density were observed in the cerebral cortex but not limbic forebrain. However, mianserin caused a marked increase inα₂-receptors without any change in β-receptors. Nisoxetine did not produce any change in these two adrenergic receptors. It is suggested that intrasynaptic norepinephrine is important but that, in addition, other factors may be involved in the increase inα₂-receptors induced by antidepressant drugs.     

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.     

Retrograde axonal transport of β-adrenoreceptors in rat brain: Effect of reserpine

Retrograde axonal transport of β-adrenoreceptors was assessed by measuring the accumulation of binding sites for the β-receptor ligand [¹²⁵I]iodocyanopindolol ([¹²⁵I]ICP) distal to a unilateral 6-hydroxydopamine (6-OHDA) lesion placed in the ascending noradrenergic axons of the locus coeruleus. Accumulation of binding sites was linear over a 3 day period and was blocked by intracerebroventricular 6-OHDA given 1 day prior to sacrifice. A single dose of reserpine (5 mg/kg, i.p.) caused a long lasting (6–8 week) biphasic depletion of frontal cortex norepinephrine (NE) associated with increased frontal cortex binding of another β-receptor ligand. [³H]dihydroalprenolol ([³H]DHA), at 7–14 days, and again at 28 days post-reserpine. Unlike the changes in cortical β-receptors, retrograde transport of [¹²⁵I]ICP in presynaptic noradrenergic neurons was decreased or blocked completely at 7–14 days and at 6 weeks, and was increased to 470% and 240% of control at 21 days and 8 weeks after reserpine. Anterograde transport of [³H]DHA binding sites was measured by accumulation proximal to a 6-OHDA lesion in this pathway. This transport varied in a pattern similar to that seen for retrograde transport of [¹²⁵I]ICP binding sites. These data and others suggest that presynaptic β-receptors are regulated independently of frontal cortex β-receptors, which appear to be located primarily on postsynaptic cells. On the other hand, the regulation of both anterograde and retrograde transport appears to be interrelated since both types of transport were altered in a similar way in the face of long-term NE depletion by reserpine.