Beta-adrenergic receptors and adenylate cyclase in hypertrophic and hyperplastic rat salivary glands.

Isoproterenol induces both the secretion of protein and the stimulation of DNA synthesis and growth in rat salivary glands. The specific binding of the labelled beta-adrenergic antagonist [3H]dihydroalprenolol has been used to measure the number of beta-adrenergic receptors in rat parotid glands during isoproterenol-induced growth. Isoproterenol-enlarged glands display no change in the specific binding capacity per gland for [3H]-dihydroalprenolol compared with normal tissue. Catecholamine sensitive adenylate cyclase activity varies independently of the number of specific [3H]dihydroalprenolol binding sites during isoproterenol-induced growth. Previously-described di-ferences in optimal isoproterenol doses which produce protein secretion and stimulation of DNA synthesis may reflect different responses to various rates of receptor occupancy, or may be due to the presence of more than one type of beta-adrenergic receptor.     

The (--)[3H]dihydroalprenolol binding to rat adipocyte membranes: an explanation of curvilinear Scatchard plots and implications for quantitation of beta-adrenergic sites

In rat adipocyte membranes, both beta-adrenergic agonists and beta-adrenergic antagonists competed with (--)[3H]dihydroalprenolol for high affinity (KD 2-4 nM) and low capacity binding sites. The antagonists but not the agonists competed with (--)[3H]dihydroalprenolol for lower affinity and higher capacity sites. The present studies were performed in order to characterize the adipocyte beta-adrenergic receptor and distinguish it from low affinity, higher capacity sites which were heat-labile and not stereoselective. When isoproterenol was used to define the nonspecific binding, saturation studies showed a single binding site with a capacity of approximately 100 fmol/mg membrane protein (corresponding to approximately 50,000 sites/adipocyte). Binding was saturated by 10 nM (--)[3H]dihydroalprenolol. Approximate KD's of 204 nM were observed. Kinetic analysis of (--)[3H]dihydroalprenolol binding provided an independent measurement of KD between 0.75 and 1.1 nM. This binding site had the characteristics of a beta 1-adrenergic receptor with the potency of isoproterenol greater than norepinephrine greater than or equal to epinephrine as competitors of binding. Furthermore, the KD of inhibition of (--)[3H]dihydroalprenolol binding correlated with the Ki of inhibition by antagonists or Ka of activation by agonists of glycerol release in isolated adipocytes (r = 0.968, P less than 0.001). These results suggest that beta-adrenergic agonists compete with (--)[3H]dihydroalprenolol for the high affinity binding site which represents the physiological site. Furthermore, the use of antagonists (propranolol, alprenolol) to define specific beta-binding includes nonspecific site(s) as well as the beta-adrenergic site. Previous characterization and quantitation of beta receptors in rat fat cell membranes may have been in error by incorporating both types of binding in their measurement.     

Beta-adrenergic receptors and adenylate cyclase in hypertrophic and hyperplastic rat salivary glands

Isoproterenol induces both the secretion of protein and the stimulation of DNA synthesis and growth in rat salivary glands.The specific binding of the labelled beta-adrenergic antagonist [³H]dihydroalprenolol has been used to measure the number of beta-adrenergic receptors in rat parotid glands during isoproterenol-induced growth. Isoproterenol-enlarged glands display no change in the specific binding capacity per gland for [³H]-dihydroalprenolol compared with normal tissue.Catecholamine sensitive adenylate cyclase activity varies independently of the number of specific [³H]dihydroalprenolol binding sites during isoproterenol-induced growth.Previously-described differences in optimal isoproterenol doses which produce protein secretion and stimulation of DNA synthesis may reflect different responses to various rates of receptor occupancy, or may be due to the presence of more than one type of beta-adrenergic receptor.     

Characteristics of beta-adrenergic receptors in isolated cells and in crude membranes of brown adipose tissue

In relation to decreased metabolic sensitivity to catecholamines observed, in vitro, in brown fat of cold-acclimated rats, beta-adrenergic receptors were studied in isolated cells and in a crude membrane preparation from rat interscapular brown adipose tissue. [3H] dihydroalprenolol binding had the same characteristics in both types of preparation; competition studies of [3H] dihydroalprenolol binding led to the characterization of beta 1 subtype adrenergic receptors with a lower affinity of beta-adrenergic agonists for [3H] dihydroalprenolol binding sites in membranes than that found in isolated cells. Cold acclimation produced, in isolated cells only, a decrease of 41% in the [3H] dihydroalprenolol binding sites and a beta-adrenergic agonist affinity increase. It is concluded that beta-adrenergic receptor decrease could be a factor, at the hormone receptor interaction level, in the regulation of the transmission of biological action responsible for the cold-induced decrease in catecholamine responsiveness in brown adipose tissue. For a study of the desensitization process in brown fat, isolated cells seem to offer certain advantages over a crude membrane preparation.     

Do catecholestrogens interact with the in vitro binding of radioligands to catecholamine receptors?

Using a competitive binding assay the effects of 2-hydroxyestradiol-17 beta, 4-hydroxyestradiol-17 beta, estradiol-17 beta and progesterone on the binding of tritiated catecholaminergic ligands to membrane preparations from rat brain and pituitary gland were studied. Up to a concentration of 10(-5) M none of the steroids tested was able to displace [3H]spiroperidol, [3H]dihydroergocryptine or [3H]dihydroalprenolol. The data suggest that the catecholestrogens do not interfere directly with the binding of catecholaminergic ligands to dopaminergic, alpha-adrenergic or beta-adrenergic receptors in the central nervous system. The view that a catechol structure is not essential for the interaction with dopaminergic receptors was further supported by the results obtained from additional studies on the competition of O-methylated and deaminated dopamine metabolites with [3H]spiroperidol binding.     

In vitro characterization of skeletal muscle beta-adrenergic receptors coupled to adenylate cyclase.

[3H]Dihydroalprenolol, a potent beta-adrenergic antagonist, was used to identify the adenylate cyclase-coupled beta-adrenoceptors in isolated membranes of rat skeletal muscle. The receptor sites, as revealed by [3H]dihydroalprenolol binding, were predominantly localized in plasmalemmal fraction. That skeletal muscle fraction may also contain the plasmalemma of other intramuscular cells, especially that of blood vessels. Hence, the [3H]dihydroalprenolol binding observed in that fraction may be due partly to its binding to the plasmalemma of blood vessels. Small but consistent binding was also observed in sarcoplasmic reticulum and mitochondria. The level of [3H]dihydroalprenolol binding in different subcellular fractions closely correlated with the level of adenylate cyclase present in those fractions. The binding of [3H]dihydroalprenolol to plasmalemma exhibited saturation kinetics. The binding was rapid, reaching equilibrium within 5 min, and it was readily dissociable. From the kinetics of binding, association (K1) and dissociation (K2) rate constants of 2.21 . 10(7) M-1 . min-1 and 3.21 . 10(-1) min-1, respectively, were obtained. The dissociation constant (Kd) of 15 mM for [3H]dihydroalprenolol obtained from saturation binding data closely agreed with the Kd derived from the ratio of dissociation and association rate constants (K2/K1). Several beta-adrenergic agents known to be active on intact skeletal muscle also competed for [3H]dihydroalprenolol binding sites in isolated plasmalemma with essentially similar selectivity and stereospecificity. Catecholamines competed for [3H]dihydroalprenolol binding sites with a potency of isoproterenol greater than epinephrine greater than norepinephrine. A similar order of potency was noted for catecholamines in the activation of adenylate cyclase. Effects of catecholamines were stereospecific, (-)-isomers being more potent than (+)-isomers. Phenylephrine, an alpha-adrenergic agonist, showed no effect either on [3H]dihydroalprenolol binding or on adenylate cyclase. Known beta-adrenergic antagonists, propranolol and alprenolol, stereospecifically inhibited the [3H]dihydroalprenolol binding and the isoproterenol-stimulated adenylate cyclase. The Ki values for the antagonists determined from inhibition of [3H]dihydroalprenolol binding agreed closely with the Ki values obtained from the inhibition of adenylate cyclase. The data suggest that the binding of [3H]dihydroalprenolol in skeletal muscle membranes possess the characteristics of a substance binding to the beta-adrenergic receptor.     

(+/-)-[3H]Epinephrine and (-)[3H]dihydroalprenolol binding to beta1- and beta2-noradrenergic receptors in brain, heart, and lung membranes.

(+/-)-[3H]Epinephrine binds to beta-receptors in calf cerebellar and rat lung membranes in the presence of 1.0 mM pyrocatechol and 1.0 microM phentolamine, with dissociation constants at 4 degrees C of 11 nM and 24 nM, respectively. (+/-)-[3H]Epinephrine associates to equilibrium within 20 min in both tissues, and over 50% of the binding is rapidly dissociable. Inhibition of binding by agonists and antagonists is highly stereoselective, and the structure-activity relationships of adrenergic agents in inhibiting (+/-)-[3H]epinephrine binding suggest an interaction with beta2 type noradrenergic receptors. (-)-Isoproterenol has an apparent Ki of 2 nM, (-)-epinephrine is 1.5 to 3 times weaker, and (-)-norepinephrine is 30 to 60 times weaker. Salbutamol and terbutaline, selective beta2-agonists, are potent inhibitors of binding, as are several nonspecific antagonists. Properties of the sites labeled by (+/-)-[3H]epinephrine in calf cerebellum and rat lung are closely similar. (-)-[3H]Dihydroalprenolol binding in calf cerebellum and rat lung also shows beta2 characteristics. Antagonists have similar potencies in inhibiting (-)-[3H]dihydroalprenolol and (+/-)-[3H]epinephrine binding in both tissues, but agonists are in general more potent inhibitors of (+/-)-[3H]epinephrine. Sodium and lithium selectively lower the affinity of (+/-)-[3H]epinephrine at its binding sites and the affinities of agonists, but not antagonists, at the (-)-[3H]dihydroalprenolol site. Specific (+/-)-[3H]epinephrine binding was not detectable in calf cortex and rat heart, where (-)-[3H]dihydroalprenolol binding suggests a beta1-receptor. A physiological significance of (+/-)-[3H]epinephrine binding is suggested by the strong correlation for agonists and antagonists between affinities in inhibiting binding, and in stimulating or inhibiting a beta-receptor-coupled adenylate cyclase in frog erythrocytes.     

Uptake of L-[propyl-2,3-3H]dihydroalprenolol by intact HeLa cells

This report describes the uptake of L-[propyl-2,3-3H]dihydroalprenolol, a beta-adrenergic antagonist, by HeLa (human adenocarcinoma) cells. [3H]Dihydroalprenolol binds to sites of high capacity and low affinity in intact HeLa cells. The binding achieves equilibrium rapidly and is rapidly reversible. Bound [3H]dihydroalprenolol is displaceable by beta-adrenergic antagonists in a nonstereoselective fashion, but is not displaceable by isoproterenol, an adrenergic agonist. Phentolamine, an alpha-adrenergic antagonist, and chloroquine, a lysosomotropic amine, also compete for [3H]dihydroalprenolol binding sites. [3H]Dihydroalprenolol binding is inhibited by metabolic inhibitors, but not by cytoskeletal blocking agents. The binding is sensitive to extracellular pH (less binding at lower pH) and is temperature-sensitive (less binding at lower temperatures). The bound radioligand is rapidly reversed following hypotonic lysis of the cells. These [3H]dihydroalprenolol binding sites in intact HeLa cells therefore do not have the characteristics expected for beta-adrenergic receptors. Further studies showed that beta-adrenergic receptors could be detected in a HeLa membrane preparation using [125I]iodohydroxybenzylpindolol, and that chloroquine had very low affinity for these receptors. We conclude that [3H]dihydroalprenolol diffuses across the plasma membrane of intact HeLa cells and accumulates in acidic intracellular compartments.     

Alpha- and beta-adrenergic and muscarinic cholinergic binding sites in the bladder and urethra of the rabbit

The affinities of a number of alpha- and beta-adrenergic binding sites and muscarinic cholinergic binding sites in rabbit urethra and bladder have been determined, using specific radioligand receptor binding assays. There was a greater density of beta-binding sites than alpha-binding sites in the bladder, while, in the urethra, there was a greater density of alpha-binding sites than beta-binding sites. The number of alpha-binding sites was fourfold greater in the urethra, whereas there were fewer beta-binding sites in the urethra. There were fewer muscarinic binding sites in the urethra than in the bladder. The dissociation constant for [3H]dihydroalprenolol at the beta-binding site was 6.4 nM, for [3H]dihydroergocryptine at the alpha-binding site was 2.11 nM, and for 3H-labelled l-quinuclidinyl benzilate at the muscarinic binding site was 0.22 nM.     

Thyroid hormone regulation of beta-adrenergic receptor number.

The effects of exogenous thyroid hormones (thyroxine and triiodothyronine) on beta-adrenergic receptors in the rat myocardium were investigated. The potent beta-adrenergic antagonist, (-)-[3H]dihydroalprenolol, was used to directly estimate the number and affinity of beta-adrenergic receptors in rat heart membranes from control and hyperthyroid rats. Cardiac membranes from hyperthyroid rats contained 196 +/- 7 fmol of (-)-[3H]dihydroalprenolol binding sites/mg of protein which was significantly (p less than 0.005) greater than the number of binding sites (89 +/- 5 fmol/mg of protein) present in control membranes. The equilibrium dissociation constant (KD) for the interaction of receptors with dihydroalprenolol was the same (2 to 15 nM) in membranes from control and hyperthyroid rats. Similarly, there was no significant difference between the control and hyperthyroid membranes in the affinity of the beta-adrenergic receptor binding sites for the beta-adrenergic agonist isoproterenol. The results of this study demonstrate that thyroid hormones can regulate the number of cardiac beta-adrenergic receptors. The increased numbers of receptors may be responsible, at least in part, for the enhanced catecholamine sensitivity of beta-adrenergic-coupled cardiac responses in the hyperthyroid state.     

Effects of androgen on alpha- and beta-adrenergic receptors in membranes from the rat seminal vesicle.

The effects of castration and androgen-replacement on adrenergic receptors in membranes from the rat seminal vesicle were studied. Membranes from seminal vesicles showed saturable and high-affinity binding sites for the beta-adrenergic receptor antagonist, [3H]dihydroalprenolol ([3H]DHA), and the alpha 1-adrenergic receptor antagonist, [3H]prazosin. Castration markedly reduced beta-adrenergic receptors with decreasing the effect of GTP modulating the receptor-ligand affinity, suggesting defects in both the receptor per se and the guanine-nucleotides-regulating mechanism after castration. In contrast, castration increased alpha 1-adrenergic receptors and androgen-replacement reversed this change. The effects of GTP decreasing the alpha 1-receptor binding affinity to the radioligand were observed to a similar extent in the castrated and control membranes. These results demonstrate an inverse regulation by androgen on beta- and alpha 1-adrenergic receptors in membranes of the rat seminal vesicle.     

Adenylate cyclase in lean and obese (ob/ob) mouse epididymal white adipocytes

The results presented in this study indicate that the defect in catecholamine-stimulated adenylate-cyclase which is characteristic of the ob/ob mouse is associated with a decrease in the sensitivity of the system to guanine nucleotides (guanosine 5'-[beta gamma-imido]triphosphate and guanosine 5'-triphosphate). No difference in the beta-adrenergic receptor activity was found between the lean and obese mice on the basis of their capacity to bind the beta-adrenergic antagonist [3H]dihydroalprenolol. The data suggest that a defect in the activation of the adenylate cyclase by beta-adrenergic agents may reside in the guanyl nucleotide binding site(s).     

Prenatal Diazepam Exposure Affects β-Adrenergic Receptors in Brain Regions of Adult Rat Offspring

The postnatal development of [3H]dihydroalprenolol binding to beta-adrenergic receptors has been studied in frontal cortex, cerebellum, striatum, and hypothalamus of the rat after prenatal and perinatal exposure to diazepam. Dams were injected subcutaneously with single daily doses of 1 mg of diazepam/kg from day 7 to 20 of gestation or from day 15 of gestation to day 6 after birth. Prenatal exposure had no effect on litter size or length of gestation or on the postnatal development of body and brain weights of the progeny. However, a reduced mortality of the pups was observed in relation to vehicle-treated controls until postnatal day 10. Prenatal diazepam administration decreased [3H]dihydroalprenolol binding in frontal cortex, striatum, and hypothalamus but not in cerebellum. This decrease in beta-adrenergic receptor binding was due to a decrease in receptor density rather than in receptor affinity. In contrast, perinatal diazepam exposure led to a transient decrease in [3H]dihydroalprenolol binding limited to the frontal cortex. The permanent reduction in number of beta-adrenergic receptors, which depends on the scaling and duration of the drug application period, points to the necessity of a prolonged evaluation of effects of exposure to psychotropic drugs in early stages of brain development.     

Decreased number of beta-adrenergic receptors in hypertensive vessels.

Responsiveness to inotropic agents is altered in hypertension and may contribute to its initiation and maintenance. A biochemical basis for this change was provided by the observation that the number of beta-adrenergic receptors, as reflected in specific [3H]dihydroalprenolol binding, was diminished in both arteries and veins of spontaneously hypertensive rats. There was no change in the affinity of dihydroalprenolol for the binding sites or in the capacity of isoproterenol to displace dihydroalprenolol. The decline in beta-adrenergic receptor numbers is not secondary to blood pressure elevation but may, instead, contribute to the pathogenesis of hypertension.     

High number of high-affinity binding sites for (-)-[3H]dihydroalprenolol on isolated hamster brown-fat cells. A study of the beta-adrenergic receptors.

The beta-adrenergic receptors of hamster brown adipocytes have been characterised by binding of the radioactive ligand (-)-[3H]dihydroalprenolol, directly to isolated intact cells in suspension. The brown fat cell contains 57,000 specific and saturable binding sites which have a dissociation constant (Kd) for [3H]dihydroalprenolol of 1.4 nM as determined by Scatchard analysis. The kinetically derived Kd, determined from forward and reverse rate constants, is 5 nM. Both of these values are in agreement with the dissociation constant (Kd = 2.2 nM) for alprenolol, determined from competition studies with [3H]dihydroalprenolol in these cells. Beta-adrenergic agonists competed for the specific binding sites with a typical beta 1-adrenergic specificity. The order of potency of agonists agrees well with the ability of these agents to stimulate respiration in isolated brown adipocytes: 50% stimulation of respiration occurs with apparently less than 10% occupancy of binding sites. Both the high affinity and high number of specific binding sites of [3H]dihydroalprenolol in brown fat cells presumably reflect the generally accepted dominating role of catecholamines in the regulation of brown fat metabolism and non-shivering thermogenesis.     

Metabolism of glucose in hyper- and hypo-thyroid rats in vivo. Relation of catecholamine actions to thyroid activity in controlling glucose turnover.

1. In euthyroid rats, treatment with reserpine of 6-hydroxydopamine, which deprived neuronal terminals of catecholamines, resulted in increases in rates and rate coefficients for blood glucose turnover in the starved states as determined by decay of [U-14C,6-3H]-glucose. Conversely, the injection of adrenaline or noradrenaline into starved euthyroid rats caused a marked decrease in rate coeeficients for glucose turnover. There was no change in the percentage glucose recycling under these conditions. 2. Adrenaline and noradrenaline caused more pronounced hyperglycaemia in hyperthyroid than in euthyroid rats owing to the greater activation of hepatic glucose production. 3. The increase in glucose turnover characteristics of hyperthyroidism was observed even after treatment with an alpha- or beta-adrenergic antagonist, showing the insignificant role of the balance between alpha- and beta-adrenergic receptors in the thyroid-dependent metabolic changes. 4. Rate coefficients for glucose turnover were not affected by reserpine treatment or catecholamine injections when rats had been rendered hyperthyroid. 5. Thus catecholamines are direct determinants of glucose-turnover rates in the starved state, and depend to some extent on the prevailing thyroid state.     

A reliable assay for beta-adrenoceptors in intact isolated human fat cells with a hydrophilic radioligand, [3H]CGP-12177

The beta-adrenergic receptors of isolated human fat cells were identified using a new hydrophilic beta-adrenergic radioligand (+/-)[3H]CGP-12177. The results were compared with those from [3H]dihydroalprenolol binding to fat cells and membranes. [3H]CGP-12177 binding to isolated fat cells showed lower nonspecific binding (less than 15% of total binding) than the lipophilic [3H]dihydroalprenolol (40-60%) at 3 times the KD. At 37 degrees C, [3H]CGP-12177 binding was rapid, reversible, of high affinity (1.2 +/- 0.3 nM) and saturable. The total number of binding sites per cell in subcutaneous adipocytes was 25,000 +/- 6,000 and was equivalent to that found using membrane fractions. Displacement of [3H]CGP-12177 bound to adipocytes by propranolol was stereoselective, consistent with competition at a single site, and had the same characteristics as in membranes. The displacement curves of the beta 1-selective antagonists (atenolol and betaxolol) were biphasic, the high affinity displacement accounting for 70% of the total binding sites. Beta-adrenergic agonists also competed with [3H]CGP-12177 binding in the order of potency: (-) isoproterenol greater than (-) norepinephrine greater than (-) epinephrine, similar to that found in membranes and in in vitro studies on the lipolytic activity of isolated fat cells. This study demonstrates that the sites specifically labeled by [3H]CGP-12177 are the physiological beta-adrenoceptors and also shows that the ligand is better than [3H]dihydroalprenolol for the accurate identification of these receptors in intact human adipocytes. The methodology, which requires biopsies of less than 1 gram of adipose tissue, can be of potential interest for clinical studies investigating the status of fat cell beta-adrenoceptors in various pathophysiological situations.     

An assay for beta-adrenergic receptors in isolated human fat cells

The beta-adrenergic receptors have been characterized in isolated human adipocytes using a potent beta-adrenergic antagonist (-)-[3H]dihydroalprenolol. Binding of (-)-[3H]dihydroalprenolol to isolated fat cells was stereospecific and saturable, the maximum number of binding sites calculated being 7.8 +/- 2.2 pmol of bound ligand/10(7) cells, corresponding to 450,000 binding sites/cell. The dissociation constant was estimated to be 2.7 +/- 1.1 nM. The results with competition-inhibition experiments using beta-adrenergic agonists and antagonists indicated that the binding sites in isolated adipocytes were predominantly of the beta1-subtype; about 80% of the receptors were of this type. With the present method, specific beta-adrenergic receptor number and affinity in isolated human adipocytes could be determined in about 1 g of human adipose tissue.     

Differences between agonist and antagonist binding following beta-adrenergic receptor desensitization.

The specific beta-adrenergic agonist radioligand (+/-)-[3H]hydroxybenzylisoproterenol ([3H]HBI) was used to investigate alterations in the beta-adrenergic receptors of frog erythrocytes occurring during the process of agonist-induced, receptor-specific desensitization. There was close agreement between the percentage fall in [3H]HBI binding and that in catecholamine-stimulated adenylate cyclase activity following periods of preincubation of up to 7 h with 0.1 mM (-)-isoproterenol. Desensitization was maximal by 5 h, resulting in a 69% reduction in [3H]HBI binding and a 67% reduction in isoproterenol-stimulated adenylate cyclase activity. In contrast, binding of the beta-adrenergic antagonist (-)-[3H]dihydroalprenolol was significantly less affected by desensitization (p is less than 0.05 at 2 1/2, 5, and 7 h), showing a maximum reduction in binding of only 35% in these experiments. The consistent close agreement of reduction in agonist binding with that in hormone-stimulated adenylate cyclase activity, together with the significant difference observed between agonist and antagonist binding, implies that an alteration occurs during desensitization which preferentially interferes with agonist binding, while antagonist binding is less affected. The locus of this agonist-specific alteration may be the receptor binding site or a site involved in receptor-enzyme coupling. Agonist binding studies may now be used to assess more completely the desensitized state of beta-adrenergic receptors in systems in which marked desensitization of beta-adrenergic responses is associated with little or no reduction in antagonist binding.     

Characterization of Neurotransmitter Receptors in the Rat Hippocampal Formation

The hippocampal formation has been extensively research in terms of its putative neurotransmitters, anatomical connections, and behavioral relevance. An aspect of importance is the assessment of apparent neurotransmitter receptors by using receptor binding assays. In the present study, such assays were done in vitro to investigate alpha 1-adrenergic, alpha 2-adrenergic, beta-adrenergic, muscarinic cholinergic, benzodiazepine, and opiate receptors in the rat hippocampal formation. The corresponding radioligands for these receptors were [3H]prazosin, [3H]p-aminoclonidine, [3H]dihydroalprenolol, [3H]quinuclidinyl benzilate, [3H]flunitrazepam, and [3H]naloxone. An analysis of the binding parameters for the ligands indicated saturable binding of a high affinity and the following rank order of maximal binding capacities: [3H]flunitrazepam greater than [3H]quinuclidinyl benzilate greater than [3H]naloxone greater than [3H]p-aminoclonidine greater than [3H]prazosin greater than [3H]dihydroalprenolol. Competition experiments with pharmacologic agonists and antagonists confirmed the specificity of each ligand. The results are integrated with information on other types of receptors and with neurotransmitter concentrations, and discussed in terms of hippocampal function.