Effects of clenbuterol on central beta-1 and beta-2 adrenergic receptors of the rat

Repeated administration of the centrally acting beta adrenoceptor agonist, clenbuterol, to rats reduced the ability of isoproterenol to increase the concentration of cyclic AMP (cAMP) in slices of cerebellum. This reduced responsiveness to isoproterenol was accompanied by a marked reduction in the density of beta adrenoceptors as measured by the binding of the beta adrenoceptor antagonist [125I]iodopindolol. In addition, the agonist-binding properties of remaining cerebellar beta adrenoceptors were altered after clenbuterol treatment. The clenbuterol-induced reduction in the density of beta adrenoceptors in the cerebellum is in marked contrast to its inability to do this in cerebral cortex. Comparison of the ability of clenbuterol to that of isoproterenol to increase levels of cAMP in slices of cerebral cortex or cerebellum showed that clenbuterol is a weakly potent agonist in both brain regions. The increase in cAMP induced by isoproterenol in the cortex was significantly reduced in the presence of the selective beta-1 adrenoceptor antagonist, ICI 89,406. In contrast, the clenbuterol-induced increase in cortical cAMP was unchanged by ICI 89,406 but was reduced significantly by the beta-2 adrenoceptor antagonist, ICI 118,551. In cerebellum, both isoproterenol- and clenbuterol-stimulated accumulation of cAMP were antagonized much more potently by ICI 118,551 than by ICI 89,406. Furthermore, clenbuterol antagonized the cAMP response induced by isoproterenol in the presence of ICI 118,551 in a concentration-dependent manner. In terms of measurement of cAMP in brain slices, clenbuterol is weakly potent as an agonist at beta-2 adrenoceptors and has antagonist properties at beta-1 adrenoceptors.     

Chronic isoproterenol administration causes altered beta adrenoceptor-Gs-coupling in guinea pig lung

Chronic administration of isoproterenol caused major changes in the molecular and functional characteristics of lung beta adrenoceptors. The dose of isoproterenol delivered by the minipumps over 7 days corresponded to approximately 60 micrograms.kg-1h-1. The agonist treatment produced an 80% decrease in the number of beta adrenoceptors in lung membranes. However, a significant proportion of the remaining receptors could still form complexes with the nucleotide regulatory protein (Gs) in the presence of agonist. This was demonstrated by the persistence of high affinity sites (42% versus 53% in controls, P less than .01) in isoproterenol competition binding curves and of the tight binding of isoproterenol in the presence of the reagent N-ethylmaleimide (44% versus 60% in controls, P less than .01). However, part of the receptor population displaying high affinity toward the agonist was insensitive to guanosine 5'-triphosphate. These changes were associated with a decrease in the Gs activity, as demonstrated by the lower degree of cholera toxin-mediated adenosine diphosphate ribosylation. Moreover, chronic against treatment caused a 50% reduction in the maximal relaxing effect of isoproterenol in the isolated lung parenchymal strip, whereas the tissue sensitivity to beta adrenoceptor stimulation was unchanged. It is concluded that chronic administration of isoproterenol to guinea pigs caused marked changes in beta adrenoceptor function in the lung tissue. The molecular events involved in beta adrenoceptor desensitization by chronic in vivo administration of relatively low catecholamine doses appear to differ from those reported for cultured cells, as well as those mediated by acute in vivo catecholamine administration.     

Application of [125I]iodocyanopindolol to measure 5-hydroxytryptamine1B receptors in the brain of the rat

In this study, [125I]iodocyanopindolol ([125I]ICYP), in the presence of isoproterenol, was used to label 5-hydroxytryptamine1B (5-HT1B) receptors in homogenates of the cortex, substantia nigra and caudate-putamen of the rat. The determination of the appropriate concentrations of isoproterenol required to block optimally beta adrenoceptors whereas producing minimal occupancy of 5-HT1B receptors was achieved by generating isotherms for isoproterenol at multiple concentrations of [125I]ICYP. When different concentrations of isoproterenol were used with increasing concentrations of [125I]ICYP, a linear Scatchard transformation of the saturation curve was achieved, even with ligand concentrations about 6-fold greater than the KD for [125I]ICYP. Competition for [125I]ICYP (100 pM) labeled binding sites by 15 serotonin agonists or antagonists was adequately described by a single site model, and the affinity of these drugs for the site labeled by [125I]ICYP was similar to that determined previously when using indirect methods to label 5-HT1B receptors. Serotonin itself showed high affinity for this binding site as did two antagonists, metergoline and methiothepin. By contrast, drugs thought to be selective for the 5-HT1A receptor (e.g., 8-hydroxy-2-(di-n-propylamino)tetralin, buspirone and spiperone) showed very weak affinity for the binding site labeled with [125I]ICYP. The effect of nucleotide regulation on [125I]ICYP binding at 5-HT1B receptors also was evaluated. It was determined that GTP had little effect on the binding of [125I]ICYP, reducing total binding by only 15% and shifting the displacement curve of 5-HT by a factor of less than two. The regulation of 5-HT1B receptors, labeled by [125I]ICYP, also was evaluated. Intraventricular injections of 5.7-dihydroxytryptamine increased significantly the number of 5-HT1B receptors in the caudate-putamen; this treatment had no effect on 5-HT1B binding sites either in the cortex or substantia nigra. The regulatable binding site for [125I]ICYP in the caudate-putamen had a pharmacological profile very similar to that of the 5-HT1B binding site in the cortex. [125I]ICYP appears to be a useful ligand to measure 5-HT1B receptors in the brain of the rat. The localized increase in 5-HT1B receptors in the caudate-putamen after destruction of central serotonergic neurons might indicate that the majority of 5-HT1B receptors in this area of brain are not located on serotonergic nerve terminals.     

Quantitative autoradiography of central beta adrenoceptor subtypes: comparison of the effects of chronic treatment with desipramine or centrally administered l-isoproterenol

This study compares the regulation of the subtypes of central beta adrenoceptors produced by treatment of rats with desipramine (10 mg/kg i.p. twice daily for 10 days) to that caused by central infusion of l-isoproterenol (5 micrograms/hr for 7 days). Beta adrenoceptors were measured by quantitative autoradiography of the binding of [125I]iodopindolol ([125I]IPIN) to coronal sections of rat brain as well as the binding of this radioligand to homogenates of certain areas of brain. Administration of desipramine caused significant reductions in the total specific binding of [125I]IPIN in many areas of the brain, including regions of the amygdala, cerebral cortex, hippocampus, hypothalamus and thalamus. This reduction in the total specific binding of [125I]IPIN was primarily the result of a reduction in its binding to beta-1 adrenoceptors in most brain regions. With the exception of the paraventricular nuclei of the thalamus, no desipramine-induced change in the binding of [125I]IPIN to beta-2 adrenoceptors was observed in any region of the brain. Treatment of rats with desipramine did not alter the binding of [125I]IPIN to the caudate putamen, globus pallidus or the cerebellum. Intracerebroventricular fusion of the nonselective beta adrenoceptor agonist, isoproterenol, reduced the maximum binding site density of the binding of [125I]IPIN to homogenates of cerebellum and hypothalamus, but not to homogenates of cerebral cortex. Autoradiography of the binding of [125I]IPIN to brain sections from rats treated with isoproterenol revealed reductions in many subcortical areas and the cerebellum. In contrast to the effects produced by treatment with desipramine, reductions in the binding of [125I]IPIN after infusions of isoproterenol were a result of a decrease in the binding of [125I]IPIN to beta-2 adrenoceptors in most brain regions. Infusion of isoproterenol reduced the binding of [125I]IPIN to beta-2 adrenoceptors in 10 areas of brain in which the binding of the ligand to beta-1 adrenoceptors was not affected significantly. Inasmuch as there is a selective regulation of beta-1 adrenoceptors by the norepinephrine uptake blocker, desipramine, it may be inferred that these receptors are under the influence of endogenous norepinephrine. By contrast, the density of central beta-2 adrenoceptors are regulated more easily by an exogenous beta adrenoceptor agonist, even one like isoproterenol which is a full agonist at both beta-1 and beta-2 adrenoceptors.     

Effects of clenbuterol and antidepressant drugs on beta adrenergic receptor/N-protein coupling in the cerebral cortex of the rat

Repeated administration of the centrally acting beta adrenergic agonist clenbuterol to rats reduced the ability of isoproterenol to increase levels of cyclic AMP in slices of cerebral cortex. This lessened response to isoproterenol was not due to a reduction in beta receptor density but appeared to be due to diminished receptor/N-protein coupling. This was determined by measuring the ability of isoproterenol to inhibit the binding of the beta adrenergic antagonist [125I]iodopindolol to membranes prepared from cerebral cortex. Using membranes prepared from vehicle-treated rats, isoproterenol, in the absence of GTP, inhibited the binding of [125I]iodopindolol with an IC50 value of 85 nM and a Hill coefficient of 0.65. GTP (250 microM) increased the IC50 value to 290 nM and the Hill coefficient to 0.98. After repeated administration of 10mg/kg of clenbuterol to rats for 8 days, isoproterenol inhibited the binding of [125I]iodopindolol with an IC50 value of 125 nM and a Hill coefficient of 0.90; GTP increased the IC50 value to 170 nM and the Hill coefficient to 0.98. It was inferred from the results of modeling of the isoproterenol competition curves that the repeated administration of clenbuterol reduced or eliminated the high-affinity component of isoproterenol binding. These effects of clenbuterol were found to depend on dose and duration of treatment and were reversible. Repeated administration of the antidepressant drugs desipramine, imipramine, phenelzine, zimelidine and mianserin twice daily for 21 days, by contrast, did not affect receptor/N-protein coupling.(ABSTRACT TRUNCATED AT 250 WORDS)     

Agonist interactions with beta adrenergic receptors in rat brain

Agonist interactions with beta adrenergic receptors on membranes prepared from rat brain were examined by measuring agonist inhibition of [125I]iodopindolol binding in the absence or presence of GTP. When rat cerebral cortical membranes were prepared with 1 mM EDTA in the homogenization medium and 2.5 mM MgCl2 was included in the binding reaction, then 250 microM GTP increased the Hill coefficient for isoproterenol from 0.77 to 0.99 and increased the IC50 from 88 to 213 nM. By contrast, I-propranolol competition curves were steep (Hill coefficient = 0.98) and were not affected by GTP. It was inferred from the results of computer-modeling that, in the absence of GTP, isoproterenol bound to two states of the receptor; GTP converted isoproterenol binding to a single low-affinity state. I-Propranolol bound to a single state in the absence or presence of GTP. The effect of GTP on I-epinephrine inhibition of [125I]iodopindolol binding was essentially identical to its effect on isoproterenol inhibition. GTP and GDP were the most potent of all the nucleotides tested. Guanylylimidodiphosphate (1 mM) produced only partial shifts in the isoproterenol competition curves and GMP and ATP were inactive. In membranes prepared from rat hippocampus and hypothalamus, isoproterenol competition curves and GTP effects were qualitatively similar to those observed in cerebral cortex. However, GTP produced only partial shifts of I-isoproterenol competition curves in cerebellum and neostratium. It appears that agonists, but not antagonists, can stabilize a high-affinity ternary complex with the beta adrenergic receptor and the guanine nucleotide binding regulatory protein in membranes prepared from various regions of the rat brain.     

Interactions of flerobuterol, an antidepressant drug candidate, with beta adrenoceptors in the rat brain.

Interactions of dl-flerobuterol with central beta adrenoceptors were investigated. It inhibited the binding of [3H]CGP 12177, a selective beta adrenoceptor ligand, to membranes prepared from rat cerebral cortex, cerebellum, heart and lung. The affinity of dl-flerobuterol was very close in all tissues (Ki approximately 1 microM). In cerebral cortex, binding inhibition of [3H]CGP 12177 was stereospecific, l-flerobuterol (Ki = 483 nM) being 70-fold more potent than d-flerobuterol (Ki = 34 microM). Moreover, dl-flerobuterol (Ki = 926 nM) was 7-fold less potent than isoproterenol (Ki = 140 nM) to displace [3H]CGP 12177 binding, but 5-fold more potent than salbutamol (Ki = 4600 nM). Flerobuterol did not inhibit the radioligand binding to the other receptors at the highest concentration tested, thus leading to a very high beta adrenergic selectivity. Flerobuterol increased the concentration of cyclic AMP in slices of rat cerebral cortex in a dose-dependent manner; this effect was antagonized by atenolol and propranolol. Compared to isoproterenol or norepinephrine, which produced cyclic AMP maximal increases of 380 and 460%, respectively, it showed a weaker activity with a maximal stimulation obtained at 100 microM, corresponding to a cAMP increase of 140% over basal value (100%). These data revealed that flerobuterol possessed a beta adrenergic agonist activity. Moreover, it antagonized competitively the isoproterenol- or norepinephrine-stimulated accumulation of cAMP. At low concentrations of isoproterenol or norepinephrine, the stimulation of adenylate cyclase was only due to the action of flerobuterol, but at higher concentrations, the response of isoproterenol or norepinephrine was competitively blocked by flerobuterol. At 10 microM, isoproterenol surmounted fully this antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)     

An in vitro quantitative analysis of the alpha adrenoceptor partial agonist activity of dobutamine and its relevance to inotropic selectivity

In rat anococcygeus muscle, dobutamine produced concentration-related submaximal contractions which were antagonized competitively by phentolamine (pKB = 8.3) and dobutamine antagonized norepinephrine-induced contractions in a competitive manner with an equilibrium dissociation constant for the alpha adrenoceptor of 20 nM (pKB = 7.7). Therefore, dobutamine satisfied criteria for a partial agonist of alpha adrenoceptors having an affinity for alpha adrenoceptors 25 times that of norepinephrine (pKA = 6.3) in this tissue. An estimate of the relative efficacy of dobutamine showed one-fortieth the the efficacy of norepinephrine at the alpha adrenoceptors. Dobutamine contracted rabbit aorta and produced concentration-related relaxations at 1000 times greater concentrations after alkylation of alpha adrenoceptors by phenoxybenzamine. In noncontracted canine saphenous vein, dobutamine had no visible agonist activity but did produce contractions after propranolol. In partially contracted saphenous vein, dobutamine produced a small contraction which was converted to a propranolol-sensitive relaxation of tone after phentolamine. Dobutamine was a full beta adrenoceptor agonist in guinea-pig trachea under spontaneous tone but a partial agonist after strong contraction by bethanechol. This allowed measurement of the pKB of dobutamine at beta adrenoceptors (pKB = 5.35) and estimation of efficacy at beta adrenoceptors relative to isoproterenol (eDob/eIso = 1/20). No evidence for beta adrenoceptor selectivity was found in studies of potency ratios and relative efficacy using isoproterenol for comparison. Dobutamine showed a slight (2-fold) selectivity for inotropy in vitro when compared to isoproterenol in guinea-pig right and left atria. This selectivity was removed by phentolamine suggesting a cardiac alpha-like adrenoceptor effect; this finding was confirmed in propranolol-treated guinea-pig left atria. These results are discussed in terms of the in vivo effects of dobutamine and its use as a tool for classification of beta adrenoceptors, particularly the putative presynaptic beta adrenoceptor.     

Antagonist characterization of atypical beta adrenoceptors in guinea pig ileum: blockade by alprenolol and dihydroalprenolol

The present study was undertaken to further characterize the atypical beta adrenoceptor in guinea pig ileum. Tension was developed in isolated segments of ileum using transmural electrical stimulation of enteric cholingeric nerves. The ability of isoproterenol to relax the ileum, via beta-1 adrenoceptor and atypical beta adrenoceptor agonism, was measured. Propranolol (5 x 10(-6) M) and bromoacetylaprenololmetane blocked beta-1 adrenoceptors but, at the concentrations tested, were without affinity at atypical beta adrenoceptors. (-)-Alprenolol and (-)-dihydroalprenolol, however, acted as competitive antagonists at both sites (pA2 values of 8.2 and 8.81 at beta-1 adrenoceptors and 6.47 and 6.43 at atypical beta adrenoceptors, respectively). (-)-Alprenolol also exerted agonistic activity at the atypical beta adrenoceptor. [3H](-)-Dihydroproalprenolol failed to identify beta-1 adrenoceptors or atypical beta adrenoceptors but, instead, bound to a putative lipophilic site unrelated to ileal adrenoceptors. Before this study, nadolol (pA2 = 4.7) was the only documented antagonist at the atypical beta adrenoceptor in guinea pig ileum. Thus, the present results detail two additional pharmacological probes which exhibit about a 100-fold greater affinity than nadolol for the atypical site.     

(+/-)-3,4-Methylenedioxymethamphetamine administration to rats does not decrease levels of the serotonin transporter protein or alter its distribution between endosomes and the plasma membrane

We showed that the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) reduces brain tissue 5-HT, decreases expression of 5-HT transporter (SERT) protein, and increases expression of glial fibrillary acidic protein (GFAP). In contrast, doses of (+/-)-3,4-methylenedioxymethamphetamine (MDMA) that decrease brain tissue 5-HT fail to alter expression of SERT or GFAP. Using a new and highly sensitive anti-SERT antibody, we determined whether MDMA alters the subcellular distribution of SERT protein by measuring SERT expression in endosomes and plasma membranes 2 weeks after MDMA administration. Rat brain tissues (caudate, cortex, and hippocampus) were collected 3 days and 2 weeks after MDMA (7.5 mg/kg i.p., every 2 h x 3 doses) or 5,7-DHT (150 microg/rat i.c.v.) administration. Representative results from cortex are as follows. At both 3 days and 2 weeks postinjection, MDMA decreased tissue 5-HT (65%) and had no effect on GFAP expression. MDMA increased heat shock protein 32 (HSP32; a marker for microglial activation) expression (30%) at 3 days, but not 2 weeks. MDMA did not alter SERT expression at either time point and did not alter SERT levels in either endosomes or plasma membranes (2 weeks). 5,7-DHT decreased tissue 5-HT (80%), increased HSP32 expression at both time points (about 50%), and increased GFAP expression at 2 weeks (40%). 5,7-DHT decreased SERT expression (33%) at 2 weeks, but not at 3 days. These findings indicate that a dosing regimen of MDMA that depletes brain 5-HT does not alter SERT protein expression or the distribution of SERT between endosomes and the plasma membrane and does not produce detectable evidence for neurotoxicity.     

Pharmacological analysis of the cardiac actions of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, in guinea pig heart: evidence for involvement of adenylate cyclase system in its cardiac stimulant actions

The pharmacological effects of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, were examined in guinea pig cardiac preparations and compared with those of isoproterenol to assess possible mechanisms of its cardiac stimulant actions. Xamoterol produced a positive inotropic effect in the papillary muscles and a positive chronotropic effect in the spontaneously beating right atria in a concentration-dependent manner. The maximum inotropic and chronotropic effects of xamoterol were about 33 and 35% of those of isoproterenol, respectively. Although xamoterol failed to produce a consistent increase in contractile force in the left atria, the positive inotropic effect of the agent was observed clearly in preparations obtained from reserpine-pretreated animals. The positive inotropic and chronotropic effects of xamoterol were antagonized by atenolol, but not by ICI 118,551. On the other hand, xamoterol antagonized competitively the positive inotropic and chronotropic responses to isoproterenol. In papillary muscles the increases in contractile force induced by xamoterol and isoproterenol were depressed markedly in the presence of carbachol or adenosine. In all of left atria, right atria and papillary muscles obtained from reserpine-pretreated animals, xamoterol caused a significant elevation in cyclic AMP levels, while inhibiting the isoproterenol-induced increase in cyclic AMP levels. Computer-assisted analysis of concentration-response curves for the inhibition by xamoterol of the binding of [125I]iodocyanopindolol in the membranes from guinea pig ventricles showed the existence of the 5'-guanylylimidodiphosphate sensitive, highly affinity site of beta adrenoceptors for xamoterol, suggesting that xamoterol may induce the formation of a ternary complex with the beta adrenoceptor and a stimulatory guanine nucleotide regulatory protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Yohimbine as a serotonergic agent: evidence from receptor binding and drug discrimination.

Stimulus control was established in rats trained to discriminate either 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT) (0.2 mg/kg) or yohimbine (3 mg/kg) from saline. Tests of generalization were then conducted with a group of drugs thought to act via the 5-hydroxytryptamine1A (5-HT1A) receptor and a group of drugs thought to act as antagonists at alpha 2 adrenoceptors. In addition, each drug was characterized in terms of its affinity for 5-HT1A and alpha 2 adrenoceptors by means of radioligand binding techniques. It was observed that the stimulus effects of DPAT generalized fully to those of the alpha 2-adrenoceptor antagonists, yohimbine, rauwolscine and L-657,743, but not to idazoxan or atipamezole. The dissociation constants (Kd, nM) of the alpha 2-adrenoceptor antagonists at the 5-HT1A receptor were 74, 52, 80, 199 and 13,000, respectively. Thus, the discrimination data are explicable in terms of a direct action of yohimbine and some other alpha 2 adrenoceptor antagonists upon 5-HT1A receptors. In yohimbine-trained rats, full generalization to DPAT, flesinoxan and tandospirone was observed. In light of the negligible affinity of flesinoxan and tandospirone for the alpha 2 adrenoceptor (9000 and 8800 nM, respectively), and high affinity for the 5-HT1A receptor (0.3 and 43 nM, respectively), a mechanism mediated by the latter site is suggested. We would emphasize two implications of the present study. First, the present data suggest that rats trained with yohimbine as a discriminative stimulus generalize to drugs with minimal affinity for the alpha 2 adrenoceptor but with high affinity for 5-HT1A receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

The insulin-secreting cell line, RINm5F, expresses an alpha-2D adrenoceptor and nonadrenergic idazoxan-binding sites.

The pharmacological properties of alpha-2 adrenoceptors and the existence of nonadrenergic idazoxan-binding sites (NAIBS) were investigated in the insulin-secreting cell-line, RINm5F, using [3H]RX821002 and [3H]idazoxan. Analysis of [3H]RX821002 saturation isotherms revealed the presence of a single class of binding sites (Bmax = 47.5 +/- 3.5 fmol/mg protein) having high affinity (Kd = 1.26 +/- 0.18 nM). Inhibition of [3H]RX821002 binding by adrenergic compounds showed that the labeled sites displayed the properties expected for an alpha-2 adrenoceptor. Based on competition data with drugs having alpha-2 adrenoceptor subtype selectivity, the receptor from RINm5F is neither an alpha-2B nor an alpha-2C. It resembles the alpha-2A, but deviates from this subtype because of a weak affinity for yohimbine and rauwolscine. In this respect, RINm5F alpha-2 adrenoceptor is identical to the receptor previously described in rat intestinal mucosa and corresponds to a fourth subtype: alpha-2D. Agonist inhibition curves were better fitted by a two-site model and indicated that about half of the receptor population was under a high-affinity state corresponding to G protein-coupled receptors. [32P]ADP-ribosylation with pertussis toxin and immunodetection with specific antibodies permitted the identification of three distinct G proteins: Gi2, Gi3 and G0. Binding experiments with [3H]idazoxan showed that this imidazoline labeled two types of sites corresponding to alpha-2 adrenoceptors and NAIBS. Analysis of saturation isotherms under binding conditions allowing to discriminate between the two site populations indicated that the density of NAIBS (44 +/- 2 fmol/mg protein) was fairly identical to that of alpha-2 adrenoceptors. The pharmacological properties of NAIBS, as assessed by determining the relative affinity of imidazolinic and nonimidazolinic compounds, reasonably matched that reported in other tissues. Taken together, these data make the RINm5F cell-line 1) the first model in permanent culture known as expressing an alpha-2 adrenoceptor of the alpha-2D subtype; 2) a good system for studying in vitro the respective role of alpha-2 adrenoceptors and NAIBS in the regulation of insulin secretion by beta cells.     

Inhibition by 5-hydroxytryptamine of the beta adrenoceptor-mediated positive inotropic responses to catecholamines in rabbit papillary muscles: direct interaction with beta adrenoceptors.

The effects of 5-hydroxytryptamine (5-HT) on the positive inotropic responses to catecholamines were investigated in isolated rabbit papillary muscles. 5-HT produced a concentration-dependent positive inotropic effect, an effect which was antagonized by prazosin, but not by propranolol. The positive inotropic effect of 5-HT diminished greatly in muscles from rabbits pretreated with 6-hydroxydopamine. Thus, it is likely that 5-HT causes a release of norepinephrine and increases force of contraction indirectly through alpha-1 adrenoceptors. In the presence of prazosin, 5-HT exerted a concentration-dependent inhibition of the positive inotropic response to isoproterenol. The positive inotropic responses to tyramine and a beta-1 adrenoceptor agonist T-1583 were also inhibited by the addition of 5-HT. The inhibitory effect of 5-HT on the beta adrenoceptor-mediated responses was unaffected by methysergide, ketanserin, ICS 205-930 or atropine. Pretreatment with pertussis toxin did not block the inhibitory effect of 5-HT on the inotropic response to isoproterenol, while abolishing the cholinergic interaction against the isoproterenol response. In contrast to its antagonizing effect on the inotropic response to isoproterenol, 5-HT produced an additive effect on the positive inotropic response to norepinephrine. However, when neuronal amine uptake was blocked by cocaine, the positive intropic response to norepinephrine was suppressed by the addition of 5-HT. 5-HT inhibited (-)-[125I]iodocyanopindolol binding to the membranes from rabbit ventricles with a monophasic displacement curve.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cellular localization of serotonin 1A, 1B and uptake sites in cingulate cortex of the rat

Experimental lesions followed by binding of [3H]8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), [125I]cyanopindolol and [3H] paroxetine to cryostat sections and coverslip autoradiography were used to localize 5-HT1A, 5-HT1B and 5-HT uptake sites in rat posterior cingulate cortex. Ablations included: 1) undercutting for removal of all afferent axons; 2) destruction of the raphe nuclei; 3) cortical ibotenic acid injections for removal of neurons and 4) anterior thalamic and caudate nuclei injections of the immunotoxin OX7-saporin which destroys single classes of cortical projection neurons by retrograde axonal transport. Peak paroxetine binding was in layer Ia with low binding in layer Va and moderate amounts in other layers. Undercut lesions reduced binding only in layer Ia by 35%. Major loses were observed after raphe ablations with decreases of 40 to 72% across all layers. Cortical ibotenic acid injections did not alter paroxetine binding. Peak cyanopindolol binding was in layers Ia to Ic. Undercutting decreased binding significantly in layers Ia, Ib, III and IV, whereas after raphe lesions binding was decreased by 34 to 58% in layers Ia to IV. 5,7-Dihydroxytryptamine injection increased binding by 10 to 40% in layers Ib, II, III and IV. Cortical ibotenic acid injections reduced grain density in all layers with a range of 28 to 47%. Peak 8-OH-DPAT binding was in layer Vb. No change was observed after undercut lesions, whereas after cortical ibotenic acid injection, binding reductions of 44 to 75% were observed throughout all nine sublaminae. Thalamic OX7-saporin injections destroyed almost all layer VI neurons, which resulted in a 45% decrease in layer VI 8-OH-DPAT binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Maturation of sympathetic neurotransmission in the rat heart. IX. Development of transsynaptic regulation of cardiac adrenergic sensitivity

The number of beta adrenoceptors and the cardiac sensitivity to adrenergic stimulation increase substantially in the immediate postnatal period of the rat. To determine whether transsynaptic input influences this developmental process, the effects of a sympathomimetic and of agents which destroy noradrenergic nerve terminals on regulation of adrenergic postsynaptic sensitivity were compared in hearts from adult and developing rats. In mature animals, chronic exposure to the beta agonist isoproterenol (2.5 mg/kg s.c.) led to rapid onset (3-5 days) of chronotropic adrenergic subsensitivity accompanied by a loss of beta adrenoceptor binding sites; chemical sympathectomy by daily administration of guanethidine (50 mg/kg s.c.) or by 6-hydroxydopamine (100 mg/kg s.c. given once daily for 3 days) resulted in chronotropic adrenergic supersensitivity and increases in binding sites. These data in the adult agree with classical transsynaptic modulation of adrenergic postsynaptic reactivity. In contrast, identical drug treatments of immature rats beginning 1 day after birth failed to evoke changes in either chronotropic adrenergic sensitivity or in numbers of beta adrenoceptor binding sites until the 3rd to 4th week. Consequently, the initial development of beta adrenoceptors and responsiveness to catecholamines in the neonatal myocardium are not transsynaptically regulated; rather, other (e.g. hormonal) factors appear to control early maturation of cardiac adrenergic sensitivity.     

Effects of various serotonin receptor subtype-selective antagonists alone and on m-chlorophenylpiperazine-induced neuroendocrine changes in rats.

Administration of m-chlorophenylpiperazine [m-CPP, a serotonin (5-HT) agonist] to rats increases plasma concentrations of prolactin and corticosterone. Pretreatment with various doses of ritanserin (5-HT1C/5-HT2 antagonist), ICS 205-930 and MDL-72222 (5-HT3 antagonists), iodocyanopindolol or CG361A (beta adrenoceptor antagonists) and spiperone (5-HT1A/5-HT2 antagonist) did not attenuate m-CPP-induced increases in plasma concentrations of prolactin. In contrast, pretreatment with various doses of metergoline (5-HT1/5-HT2 antagonist), propranolol (beta adrenoceptor antagonist that also has binding affinity for 5-HT1A, 5-HT1B and 5-HT1C sites), mesulergine and mianserin (5-HT1C/5-HT2 antagonists) attenuated m-CPP-induced increases in plasma prolactin. On the other hand, m-CPP-induced increases in corticosterone concentrations were attenuated only by pretreatment with a low dose of mianserin and a high dose of spiperone. When administered without m-CPP, metergoline, mesulergine, ritanserin, ICS 205-930 and high doses of mianserin, spiperone and propranolol increased plasma corticosterone secretion. On the other hand, none of the antagonists used in the present study, except spiperone, had any significant effect on plasma prolactin secretion. These findings suggest that m-CPP-induced prolactin secretion is mediated by stimulation of 5-HT1C receptors while corticosterone secretion may be mediated either by an antagonistic effect at 5-HT3 receptor subtype or by nonserotonergic mechanisms. Alternatively, enhancement of corticosterone secretion by the 5-HT antagonists when administered alone may be responsible for their failure to block m-CPP-induced corticosterone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta adrenergic antagonists inhibit serotonin uptake by pulmonary vascular cells in culture.

Beta adrenergic antagonists have been demonstrated to show stereoselective affinity for serotonin (5-HT) receptors in the central nervous system, and competitively inhibit 5-HT transport of platelets, but have not previously been evaluated for their influence on either 5-HT receptors or 5-HT transport systems of vascular cells. We have reported stimulation of 5-HT uptake by subjection of endothelial (EC) and smooth muscle cells (SMC) to anoxia. We now report that (+/-)-propranolol inhibits 5-HT uptake by both room air- and anoxia-exposed EC and SMC in culture. The effect on SMC was more marked than that on EC and showed a similar inhibition as ketanserin. The relative inhibitory potencies of beta adrenergic antagonists and ketanserin on uptake of 5-HT by SMC were as follows: (+/-)-propranolol = ketanserin greater than alprenolol = pindolol greater than oxprenolol = atenolol. The beta adrenergic receptor agonist, isoproterenol, in the presence of isobutylmethylxanthine, an inhibitor of phosphodiesterase, produced a high elevation of cyclic AMP in SMC along with a cellular configurational change and partially inhibited uptake of 5-HT. Propranolol inhibited 5-HT uptake both in the presence and absence of isoproterenol plus isobutylmethylxanthine, suggesting that its inhibitory effect was not mediated through its interaction at the beta adrenergic receptor. Kinetic analyses of the effect of propranolol on 5-HT uptake indicated that propranolol reduced 5-HT uptake by noncompetitive inhibition. We conclude that beta adrenergic antagonists block vascular cell uptake of 5-HT through an action other than interaction with the beta adrenergic receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intrinsic sympathomimetic activity of the partial agonist prenalterol in relation to beta adrenoceptor interaction in various tissues, in vitro

The intrinsic sympathomimetic activity (ISA) of prenalterol was studied in isolated tissues from different species, including tissues containing predominantly beta-1 adrenoceptors (cardiac preparations from cat, rabbit, rat and guinea pig) and tissues characterized by beta-2 adrenoceptor predominance (cat skeletal muscle and rat uterus). The ISA of prenalterol, varying between 0 and 94% in the various tissues, was found to be positively correlated to the stimulatory potency (-log EC50) of isoproterenol and prenalterol. In the cardiac preparations from the rabbit there was an interindividual variation in the ISA of prenalterol, which was also positively correlated to the stimulatory potency of the beta agonists. The density of beta adrenoceptors in the tissues studied correlated neither to the variable ISA of prenalterol nor to the -log EC50 values of isoproterenol or prenalterol. The affinities of isoproterenol and prenalterol for the beta adrenoceptors were subject to less variation than were the stimulatory potencies of the agonists. The degree of separation between the concentration-effect curves for beta adrenoceptor occupancy and mechanical performance, expressed as the ratios Kd/EC50 for both agonists, were positively correlated to the corresponding ISA of prenalterol in various tissues. However, a considerably steeper relationship between occupancy/potency ratio and ISA was seen with prenalterol than with isoproterenol. The present data suggest that the level of ISA of the partial agonist, prenalterol, depends upon the efficiency of signal transmission from the activated receptor to the final end-organ response. The separation between the concentrations of the full agonist, isoproterenol, required for receptor occupancy and response serves as an index of the efficiency of coupling between the stimulus, elicited by activation of the receptor, and the response.     

Potentiation in phencyclidine-induced serotonin-mediated behaviors after intracerebroventricular administration of 5,7-dihydroxytryptamine in rats

Phencyclidine (PCP)-induced behaviors were compared with 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)- and p-chloroamphetamine-induced behaviors in rats pretreated with ritanserin or 5,7-dihydroxytryptamine (5,7-DHT) in order to investigate whether PCP interacts with 5-hydroxytryptamine2 (5-HT2) receptors. Head-twitch and wet-dog shake induced by p-chloroamphetamine, a 5-HT releaser, and head-twitch induced by PCP were blocked completely by pretreatment with ritanserin, a specific 5-HT2 receptor blocker, but other behaviors induced by p-chloroamphetamine, PCP and 5-MeODMT, a 5-HT agonist, were not. The intensity of head-weaving, turning, backpedalling and hind-limb abduction induced by 5-MeODMT and the intensity of head-weaving, turning and head-twitch induced by PCP were markedly greater in the rats 2 weeks after the 5,7-DHT, a 5-HT neurotoxin-injection. Contrarily, 5-HT-mediated behaviors induced by p-chloroamphetamine were attenuated in the 5,7-DHT-treated rats. 5,7-DHT-treatment increased the number of 5-HT1 ([3H]-5-HT), 5-HT2 ([3H]ketanserin) and PCP ([3H]PCP) binding sites in the synaptic membrane of rat brain, but decreased the brain level of 5-HT (41% of control). These results may indicate that PCP as a 5-HT2 agonist induces head-twitch via 5-HT2 receptors, and that PCP induces head-weaving and turning via 5-HT1 receptors and/or some other mechanisms in rats.