Ligands of Adrenergic Receptors: A Structural Point of View

Adrenergic receptors are G protein-coupled receptors for epinephrine and norepinephrine. They are targets of many drugs for various conditions, including treatment of hypertension, hypotension, and asthma. Adrenergic receptors are intensively studied in structural biology, displayed for binding poses of different types of ligands. Here, we summarized molecular mechanisms of ligand recognition and receptor activation exhibited by structure. We also reviewed recent advances in structure-based ligand discovery against adrenergic receptors.

The adrenergic receptor antagonist carvedilol interacts with serotonin 2A receptors both in vitro and in vivo

There is increasing support for the potential clinical use of compounds that interact with serotonin 2A (5-HT_2A) receptors. It is therefore of interest to discover novel compounds that interact with 5-HT_2A receptors. In the present study, we used computational chemistry to identify critical ligand structural features of 5-HT_2A receptor binding and function. Query of compound databases using those ligand features revealed the adrenergic receptor antagonist carvedilol as a high priority match. As carvedilol is used clinically for cardiovascular diseases, we conducted experiments to assess whether it has any interactions with 5-HT_2A receptors. In vitro experiments demonstrated that carvedilol has high nanomolar affinity for 5-HT_2A receptors. In vivo experiments demonstrated that carvedilol increases the ethanol-induced loss of the righting reflex and suppresses operant responding in mice, and that these effects are attenuated by pretreatment with the selective 5-HT_2A receptor antagonist M100907. Moreover, carvedilol did not induce the head-twitch response in mice, suggesting a lack of psychedelic effects. However, carvedilol did not activate canonical 5-HT_2A receptor signaling pathways and antagonized serotonin-mediated signaling. It also reduced the head-twitch response induced by 2,5-Dimethoxy-4-iodoamphetamine, suggesting potential in vivo antagonism, allosteric modulation, or functional bias. These data suggest that carvedilol has functionally relevant interactions with 5-HT_2A receptors, providing a novel mechanism of action for a clinically used compound. However, our findings do not clearly delineate the precise mechanism of action of carvedilol at 5-HT_2A receptors, and additional experiments are needed to elucidate the role of 5-HT_2A receptors in the behavioral and clinical effects of carvedilol.

Effect of Cortisol on Plasma Lactate Levels following Cortisolinduced Hyperglycaemia in Common African Toad, Bufo regularis

Previous studies in man have shown that cortisol induces hyperglycemia through gluconeogenesis. However,the metabolic substrates involved in cortisol-induced hyperglycemia and the role of adrenergic receptors in lactate productionin toads have not been well studied. This study investigated the effects of adrenergic receptor blockers in cortisol-inducedhyperglycemia and blood lactate levels in the common African toad (Bufo regularis). Each toad was fasted and anaesthetizedwith sodium thiopentone given intraperitoneally (50mg/kg/i.p). The animals (control) received 0.7% amphibian saline whileanimals (untreated) received cortisol intravenously (50µg/kg/i.v). In pre-treatment groups, animals received propanolol (0.5mg/kg/i.v), prazosin (0.2 mg/kg/i.v) and combination of propanolol (0.5mg/kg/i.v) and prazosin (0.2 mg/kg/i.v) respectivelyfollowed by administration of cortisol 50µg/kg/i.v. Thereafter, blood samples were collected for estimation of glucose andlactate using the modified glucose oxidase method and colorimetric method respectively. Cortisol caused significant increase in blood glucose level ((p<0.05) and reduction in blood lactate levels. Pre-treatment with Prazosin (0.2 mg/kg/i.v) causedsignificant (p<0.05) increase in blood glucose level and significant reduction in blood lactate levels while pre-treatment withPropanolol (0.5mg/kg/i.v) abolished cortisol-induced hyperglycemia and caused increase in blood lactate levels comparedwith the untreated group. The combination of both blockers abolished the hyperglycemic effect of cortisol and causedincrease in the blood lactate levels. The results of this study show that cortisol-induced hyperglycemia is a consequent ofgluconeogenesis and mediated through the beta-adrenergic receptors. The results also show that lactate is produced andused as a gluconeogenic substrate to induce cortisol hyperglycemia in the Common African toad bufo regularis. The betaadrenergic receptors are involved in the use of lactate to induce cortisol hyperglycemia in the Common African toad Buforegularis.

α₁-Adrenoceptor and serotonin 5-HT(1A) receptor affinity of homobivalent 4-aminoquinoline compounds: an investigation of the effect of linker length

α₁-adrenoceptor (α₁-AR) subtype-selective ligands lacking off-target affinity for the 5-HT(1A) receptor (5-HT(1A)-R) will provide therapeutic benefits in the treatment of urogenital conditions such as benign prostatic hyperplasia. In this study we determined the affinity of 4-aminoquinoline and eleven homobivalent 4-aminoquinoline ligands (diquinolines) with alkane linkers of 2-12 atoms (C2-C12) for α(1A), α(1B) and α(1D)-ARs and the 5-HT(1A)-R. These ligands are α(1A)-AR antagonists with nanomolar affinity for α(1A) and α(1B)-ARs. They display linker-length dependent selectivity for α(1A/B)-ARs over α(1D)-AR and the 5-HT(1A)-R. The C2 diquinoline has the highest affinity for α1A-AR (pKi 7.60±0.26) and greater than 30-fold and 600-fold selectivity for α(1A)-AR over α(1D)-AR and 5-HT(1A)-R respectively. A decrease in affinity for α₁-ARs is observed as the linker length increases, reaching a nadir at 5 (α(1A/1B)-ARs) or 6 (α(1D)-AR) atoms; after which affinity increases as the linker is lengthened, peaking at 9 (α(1A/1B/1D)-ARs) or 8 (5-HT(1A)-R) atoms. Docking studies suggest that 4-aminoquinoline and C2 bind within the orthosteric binding site, while for C9 one end is situated within the orthosteric binding pocket, while the other 4-aminoquinoline moiety interacts with the extracellular surface. The limited α(1D)-AR and 5-HT(1A)-R affinity of these compounds makes them promising leads for future drug development of α(1A)-AR selective ligands without α(1D)-AR and the 5-HT(1A)-R off-target activity.

Molecular modeling study of chiral separation and recognition mechanism of β-adrenergic antagonists by capillary electrophoresis

Chiral separations of five β-adrenergic antagonists (propranolol, esmolol, atenolol, metoprolol, and bisoprolol) were studied by capillary electrophoresis using six cyclodextrins (CDs) as the chiral selectors. Carboxymethylated-β-cyclodextrin (CM-β-CD) exhibited a higher enantioselectivity power compared to the other tested CDs. The influences of the concentration of CM-β-CD, buffer pH, buffer concentration, temperature, and applied voltage were investigated. The good chiral separation of five β-adrenergic antagonists was achieved using 50 mM Tris buffer at pH 4.0 containing 8 mM CM-β-CD with an applied voltage of 24 kV at 20 °C. In order to understand possible chiral recognition mechanisms of these racemates with CM-β-CD, host-guest binding procedures of CM-β-CD and these racemates were studied using the molecular docking software Autodock. The binding free energy was calculated using the Autodock semi-empirical binding free energy function. The results showed that the phenyl or naphthyl ring inserted in the hydrophobic cavity of CM-β-CD and the side chain was found to point out of the cyclodextrin rim. Hydrogen bonding between CM-β-CD and these racemates played an important role in the process of enantionseparation and a model of the hydrogen bonding interaction positions was constructed. The difference in hydrogen bonding formed with the -OH next to the chiral center of the analytes may help to increase chiral discrimination and gave rise to a bigger separation factor. In addition, the longer side chain in the hydrophobic phenyl ring of the enantiomer was not beneficial for enantioseparation and the chiral selectivity factor was found to correspond to the difference in binding free energy.

GPCR antitarget modeling: pharmacophore models for biogenic amine binding GPCRs to avoid GPCR-mediated side effects

G protein-coupled receptors (GPCRs) form a large protein family that plays an important role in many physiological and pathophysiological processes. However, the central role that the biogenic amine binding GPCRs and their ligands play in cell signaling poses a risk in new drug candidates that reveal side affinities towards these receptor sites. These candidates have the potential to interfere with the physiological signaling processes and to cause undesired effects in preclinical or clinical studies. Here, we present 3D cross-chemotype pharmacophore models for three biogenic amine antitargets: the alpha(1A) adrenergic, the 5-HT(2A) serotonin, and the D2 dopamine receptors. These pharmacophores describe the key chemical features present within these biogenic amine antagonists and rationalize the biogenic amine side affinities found for numerous new drug candidates. First applications of the alpha(1A) adrenergic receptor model reveal that these in silico tools can be used to guide the chemical optimization towards development candidates with fewer alpha(1A)-mediated side effects (for example, orthostatic hypotension) and, thus, with an improved clinical safety profile.

Pharmacological characterisation of novel α2-adrenoceptor antagonists as potential brain imaging agents

Development of suitable imaging ligands to facilitate in vivo characterisation of alpha(2)-adrenoceptors has been limited in its success. In the present study, a series of iodinated derivatives and a fluorinated derivative of the classical alpha(2)-adrenoceptor antagonist, idazoxan, have been evaluated as potential imaging ligands. These compounds are based on the structure of idazoxan but more closely resemble the selective alpha(2)-adrenoceptor antagonists 2-methoxy-idazoxan (RX821002) and 2-ethoxy-idazoxan (RX811059). Preliminary studies, investigating their affinities at alpha(2)-adrenoceptors, using brain membranes prepared from a variety of species, and their ability to antagonise UK14, 304-induced inhibition of twitch in mouse vas deferens highlighted 2-iodopropoxy-idazoxan and 2-fluoroethoxy-idazoxan as the most promising candidates. Further characterisation of these two compounds showed they had a good selectivity for alpha(2)-adrenoceptors compared with imidazoline(2)-binding sites and beta-adrenoceptors. Additional functional studies also showed a lack of intrinsic activity at alpha(2)-adrenoceptors. Following intravenous injection, both compounds were able to cross the blood brain barrier when tested using an ex vivo binding assay. These data show that both 2-iodopropoxy-idazoxan and 2-fluoroethoxy-idazoxan have binding and functional properties suitable for imaging ligands. Further studies using radiolabelled forms of these ligands and a more extensive characterisation of their binding profiles are necessary but these initial evaluations demonstrate their potential.

The effects of epiphyseal peptides on the release of immunoglobulins in Peyer's patches in rats in vitro

The effects of epiphyseal peptides (1 microg/ml) on the release of immunoglobulins into the incubation medium by isolated Peyer's patches from non-immunized mice and mice immunized orally against ovalbumin were studied during 40-min incubations. The possibility that epiphyseal peptides act on adrenoreceptors of cells in secondary lymphoid organs in the small intestine was assessed using alpha- and beta-adrenoreceptor blockers, i.e., phentolamine HCl (0.02 mg/ml) and anaprilin (0.06 mg/ml) respectively. Basal levels of secretory activity in control Peyer's patches from immunized rats were 2.4 times (p < 0.01) greater than for the lymphoid organs of non-immunized animals. The effects of epiphyseal peptides on the secretory activity of antibody-forming cells depended on the functional state of Peyer's patches. Application of epiphyseal peptides led to a 35% increase (p < 0.05) in the release of immunoglobulins from Peyer's patches in non-immunized rats and a 25% decrease (p < 0.05) in the release of antibody into the incubation medium from the lymphoid organs of immunized animals. These data lead to the suggestion that the activatory effect of epiphyseal peptides on antibody-forming cells in Peyer's patches from non-immunized animals is associated with alpha-adrenoceptors, while their inhibitory action on immunoglobulin release by the small intestine lymphoid organs from immunized animals is not mediated via adrenoceptors.

Effect of alpha 2-adrenoceptor blockade on lithium action in the rat brain

The inhibitory effect of different concentrations of lithium (0.15-10 x 10(-3) M) on cAMP production induced by isoprenaline (1 x 10(-4) M) after the blockade of alpha(2)-adrenoceptors in the rat cerebral cortex was investigated. Low lithium concentrations (0.3-0.6 x 10(-3) M) exerted a significant inhibitory effect after yohimbine (1 x 10(-5) M) addition, but had no effect when isoprenaline alone or prazosin (1 x 10(-7) M) was added. The recovery of [3H]yohimbine binding after irreversible inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was evaluated in cortical membranes to study how alpha(2)-adrenoceptor repopulation affects the action of lithium on the adenylyl cyclase-cAMP system. When the density of alpha(2)-adrenoceptors was lower than 21%, lithium showed a significant inhibitory effect at all concentrations tested. However, at higher densities, increased concentrations of lithium were required to inhibit cAMP production. Our results suggest that the inhibitory effect of lithium on cAMP levels in the rat brain is conditioned by alpha(2D)-adrenoceptors.

Influence of agonist efficacy and receptor phosphorylation on antagonist affinity measurements: differences between second messenger and reporter gene responses

The ability of an antagonist to bind to a receptor is an innate property of that ligand-receptor chemical interaction. Provided no change in the antagonist or receptor chemical nature occurs, this affinity should remain constant for a given antagonist-receptor interaction, regardless of the agonists used. This fundamental assumption underpins the classification of receptors. Here, measurements of beta2-adrenoceptor-mediated cAMP accumulation and cAMP response-element (CRE)-mediated reporter-gene transcription revealed differences in antagonist affinity that depended upon agonist incubation time and the efficacy of the competing agonist. In cAMP accumulation studies (10-min agonist incubation), antagonist affinities were the same regardless of the agonist used. The CRE-reporter gene assay (5 h of incubation) antagonist affinities were 10-fold lower in the presence of isoprenaline and adrenaline than when salbutamol or terbutaline were present (e.g., log KD propranolol -8.65 +/- 0.08, n = 22, and -9.68 +/- 0.07, n = 17, for isoprenaline and salbutamol-induced responses, respectively). Isoprenaline and adrenaline were more efficacious in functional studies, and their ability to internalize GFP-tagged human beta2-adrenoceptors. Longer-term cAMP studies also showed significant differences in KD values moving toward that seen with gene transcription. Agonist-dependent differences in antagonist affinity were reduced for reporter-gene responses when a phosphorylation-deficient mutant of the beta2-adrenoceptor was used. This study suggests that high-efficacy agonists induce a chemical modification in beta2-adrenoceptors (via phosphorylation) that reduces antagonist affinities. Because reporter-gene assays are used for high-throughput screening in drug discovery, less efficacious or partial agonists may be more reliable than highly efficacious agonists when reporter-gene techniques are used to estimate antagonist affinity.

Spinal gabapentin and antinociception: mechanisms of action

Spinal gabapentin has been known to show the antinociceptive effect. Although several assumptions have been suggested, mechanisms of action of gabapentin have not been clearly established. The present study was undertaken to examine the action mechanisms of gabapentin at the spinal level. Male SD rats were prepared for intrathecal catheterization. The effect of gabapentin was assessed in the formalin test. After pretreatment with many classes of drugs, changes of effect of gabapentin were examined. General behaviors were also observed. Intrathecal gabapentin produced a suppression of the phase 2 flinching, but not phase 1 in the formalin test. The antinociceptive action of intrathecal gabapentin was reversed by intrathecal NMDA, AMPA, D-serine, CGS 15943, atropine, and naloxone. No antagonism was seen following administration of bicuculline, saclofen, prazosin, yohimbine, mecamylamine, L-leucine, dihydroergocristine, or thapsigargin. Taken together, intrathecal gabapentin attenuated only the facilitated state. At the spinal level, NMDA receptor, AMPA receptor, nonstrychnine site of NMDA receptor, adenosine receptor, muscarinic receptor, and opioid receptor may be involved in the antinociception of gabapentin, but GABA receptor, L-amino acid transporter, adrenergic receptor, nicotinic receptor, serotonin receptor, or calcium may not be involved.

Antiplatelet activity of synthetic pyrrolo-benzylisoquinolines

Pyrrolo-benzylisoquinolines were prepared as target compounds and their antiplatelet aggregation activity, adreno-receptor affinity, and cytotoxicity were screened. Compounds 1d-9d showed specific antiplatelet aggregation activity induced by arachidonic acid and collagen. Among them, 8d and 9d exhibited better activity than the reference drug, aspirin and 9d also showed inhibition of platelet aggregation by all four inducers.

Modulatory effect of the adrenergic system upon fibroblast proliferation: participation of beta 3-adrenoceptors

1. The beta3-adrenoceptor agonist ZD 7114, like the non-selective beta-adrenoceptor agonist isoprenaline, but unlike the beta1-adrenoceptor agonist dobutamine and the beta2-adrenoceptor agonist salbutamol, produced an increment on mouse embryonic fibroblast proliferation. The half-maximal stimulation of cell growth occurred at substantially lower concentrations with the beta3-adrenoceptor agonist (EC50: 5.5 x 10(-8) m) than with isoprenaline (EC50: 1.25 x 10(-6) m). 2. The selective beta3-adrenoceptor antagonist SR 5923 OA prevented the beta3-stimulated fibroblast proliferation. Conversely, practolol and butoxamine did not prevent fibroblast growth. 3. Additionally, a decrease of cAMP was obtained in fibroblasts cells upon stimulation with isoprenaline and ZD 7114. 4. The expression of beta-adrenoreceptors on fibroblast cells was also studied by radioligand binding. The Ki values in the presence of beta1- and beta2-adrenoceptor antagonist was two-fold higher than the Ki values for beta3 adrenoceptor antagonist indicating the presence of A3-receptor subtype. 5. Inhibitors of different intracellular coupling pathways including phospholipase C (U 73122), protein kinase C (staurosporine), calcium/calmodulin (trifluoroperazine) and calcium channel (verapamil), prevented the stimulatory actions of the selective beta3-adrenoceptor agonist ZD 7114. 6. The presence of beta3-adrenoceptors on embryonic mouse fibroblast cells may play a role in the modulation of cell growth and biologic activity. The mechanism by which ZD 7114 triggers cell proliferation and function, involves the activation of phospholipase C, PKC, calcium/calmodulin and the influx of calcium.

Novel arylpiperazines as selective alpha1-adrenergic receptor antagonists

A novel series of arylpiperazines has been synthesized and identified as antagonists of alpha1a adrenergic receptor (alpha1a-AR) implicated in benign prostatic hyperplasia. These compounds selectively bind to membrane bound alpha1a-AR with K(i)s as low as 0.66 nM. As such, these potentially represent a viable treatment for BPH without the side effects associated with known alpha1-adrenergic antagonists.

Cocaine-induced alterations in the density of monoaminergic receptors in the embryonic guinea pig cerebral wall

Quantitative receptor autoradiography was used to examine the effect of chronic cocaine exposure on the density of alpha1-, alpha2- and beta-adrenergic, 5-HT1A- and 5-HT2-serotonergic, and D1- and D2-dopaminergic receptors in the fetal guinea pig cerebral wall which contained forming motor area of the cerebral cortex. The pregnant guinea pig received two daily subcutaneous injections of 20 mg/kg cocaine beginning on the 20th day of pregnancy (E20). The control animals received injections of equivalent volume of saline. The receptor densities were examined between days 5-30 of the treatment, which corresponds to E25-E50. By the fifth day of treatment (E25), cocaine produced downregulation of all receptors studied throughout the entire depth of the fetal cerebral wall. More extended treatment, however, resulted in recovery of receptor levels. Finally, from days 20-30 of treatment (E40-E50) there was a significant upregulation of noradrenergic and dopaminergic receptor sites. These findings demonstrate that exposure to cocaine in utero can influence adrenergic, serotonergic, and dopaminergic receptors in the embryonic cerebral wall, which may lead to alteration in corticogenesis. Furthermore, the present study reveals that, in the course of chronic treatment, cocaine may completely reverse its receptor regulatory activity in the fetal brain.

Interaction of metals with muscarinic cholinoceptor and adrenoceptor binding, and agonist-stimulated inositol phospholipid hydrolysis in rat brain

In vitro mercury (Hg) or lead (Pb) effectively inhibited the binding of 3H-quinuclidinyl-benzilate (QNB) (a muscarinic cholinoceptor antagonist) and 3H-prazosin (an alpha 1-adrenoceptor antagonist) to their receptors in cerebellar and cerebral cortex membranes in a concentration-dependent manner. Hg was more potent than Pb. When the rats were treated with Hg (5 mg/kg body wt) or Pb (25 mg/kg body wt) for 24 hr, a decrease in 3H-prazosin and an increase in 3H-QNB receptor binding were observed in cerebral cortex. There was no alteration in 3H-prazosin binding in cerebellum with the above treatment of metals, but 3H-QNB binding in cerebellum was significantly inhibited by Hg. However, both 3H-prazosin and 3H-QNB receptor bindings were significantly decreased in cerebellum of rats treated for 7 days with Hg (1 mg/kg body wt/day) or Pb (25 mg/ kg body wt/day). But in cerebral cortex of rats treated with these metals for 7 days, a decrease in 3H-prazosin and an increase in 3H-QNB receptor binding activities were noticed. There was a significant decrease in phospholipid content in cerebral cortex but not in cerebellum of rats treated with these metals for 7 days. At 100 microM concentration carbachol or acetylcholine or norepinephrine stimulated 3H-inositol incorporation and 3H-inositol phosphate (IP) formation in rat cerebral cortical slices. Hg or Pb in vitro though increased the agonist-stimulated 3H-inositol incorporation, 3H-IP formation was not significantly altered. The present investigation demonstrates the differential responses by alpha 1-adrenoceptor and muscarinic cholinoceptor in cerebellum and cerebral cortex of rat to in vitro and in vivo effects of Hg or Pb.

Expression of beta 2-adrenoceptors in halobacteria

Halobacteria are halophilic representatives of the recently defined domain, the Archaea. Halobacterium salinarium belongs to this group of microorganisms and contains large amounts of bacteriorhodopsin in its membrane. Bacteriorhodopsin is a seven-transmembrane protein that consists of bacterio-opsin (BO), and the chromophore retinal, which is covalently attached to BO. We have investigated whether the expression machinery for BO can be utilized for synthesis of the human beta 2-adrenoceptor (beta 2-AR), a protein with a similar seven-transmembrane-helix topology. An expression vector for BO synthesis was modified to express beta 2-ARs under the control of BO regulatory clements in H. salinarium. Homologous recombination into the genome was verified by polymerase chain reactions. Northern blots revealed transcripts of the calculated size and significant amounts of epitope-tagged beta 2-ARs were detected in Western blots. However, binding of the beta-AR antagonist 125I-cyanopindolol revealed low levels of functional receptors, and the ligand binding properties of these receptors were altered when compared to native receptors. Expression of chimeras containing larger amino terminal portions of BO did not result in higher receptor levels. Expression of beta 2-AR in Haloferax volcanii, another member of halobacteria, was achieved with a vector carrying the ferredoxin promoter. The levels of functional receptor as determined by 125I-cyanopindolol binding were 180 fmol/mg protein. The beta-AR ligands isoprenaline and propranolol showed affinities expected for functional beta 2-ARs. Thus, functional human beta 2-ARs were expressed in halobacteria, constituting a first approach for expression of a eukaryotic protein in the domain of Archaea.

Differences in ketanserin binding in the ventromedial hypothalamus of ewes responsive or refractory to short days

Participation of central 5HT receptors in the inhibition of LH pulsatility during refractoriness to short days (SD) in ewes has been suggested by previous in vivo studies using various 5HT-antagonist such as ketanserin. In the present study, binding of [3H]ketanserin in ewe brain sections was similar to that described in the brain of other species and could correspond with an interaction at 5HT2 receptors sites. Rosenthal analysis from the caudate nucleus was linear (Kd = 3 nM). The displacement studies from the cortex slices showed that the 5HT antagonists such as methysergide, ketanserin, cyproheptadine and spiperone competed with the labelled ligand at nanomolar concentrations whereas serotonin was less active. However, the first 3 drugs recognized different populations of binding sites. Prazosin, an alpha 1-adrenergic antagonist was inactive, but a slight inhibition of [3H]ketanserin binding was induced by pyrilamine, an H1 histaminic antagonist, within a nanomolar range. Methysergide (10(-6) M), which does not bind to H1 receptors, was therefore used to determine the nonspecific binding. Quantitative analysis of the binding of 3 nM [3H]ketanserin on sections of the ewe brain at the preopticohypothalamic level was then carried out by autoradiography. The highest binding densities were observed in the caudate nuclei (64.0 fmol/mg tissue Eq) and the mammillary bodies (52.7 fmol/mg tissue Eq) whereas intermediate or low densities were found in the other structures. The anatomical distribution of the labelling was similar to that described in other species for 5HT2 receptors. Ketanserin binding in these areas was compared between two groups of ovariectomized estradiol-treated Ile-de-France ewes, submitted to artificial short days (SD: 8L:16D), one group with a high LH pulsatility (responsive to SD) and the other one with a low LH pulsatility (photorefractory to SD). Binding densities were similar for each one of the studied regions between the two groups, except in the ventrolateral part of the mediobasal hypothalamus, where ewes exhibiting high LH pulsatility had a more than 2-fold higher binding density than those with a low LH pulsatility (mean +/- SEM, 14.6 +/- 1.4 vs. 5.7 +/- 1.0 fmol/mg tissue Eq, respectively; p < 0.0016). These results suggest that [3H]ketanserin binding sites in the ventromedial part of the mediobasal hypothalamus could be associated to the regulation of the photoperiodic inhibition of LH at the time of establishment of refractoriness to short days in the Ile-de-France ewe.