Evaluation of SSR161421, a novel orally active adenosine A3 receptor antagonist on pharmacology models

The effects of a novel adenosine A(3) receptor antagonist, SSR161421, were examined on both antigen per se and adenosine receptor agonist-increased airway responses in antigen-sensitized guinea pigs. Adenosine (10(-5)M) and AB-MECA [N6-(4-aminobenzyl)-adenosine-5'-N-methyl-uronamide dihydrochloride] (10(-7)M) increased the antigen response up to 61 ± 3.0% and 88 ± 5.2% of maximal contraction, respectively. The agonists of adenosine A(1) and A(2) adenosine receptors NECA [1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-b-d-ribofuranuronamide-5'-N-ethylcarboxamidoadenosine], R-PIA [N(6)-R-phenylisopropyladenosine], and CGS21680 (10(-7)M) were ineffective. In vivo intravenous adenosine (600 μg/kg) and AB-MECA (30 μg/kg) increased the threshold antigen dose-induced bronchoconstriction by 214 ± 13.0% and 220 ± 15.2%, respectively. SSR161421 in vitro (IC(50)=5.9 × 10(-7)M) inhibited the AB-MECA-enhanced antigen-induced airway smooth muscle contractions and also in vivo the bronchoconstriction following either intravenous (ED(50)=0.008 mg/kg) or oral (ED(50)=0.03 mg/kg) administration in sensitized guinea pigs. Antigen itself could evoke tracheal contraction in vitro and bronchoconstriction in vivo in antigen-sensitized guinea pigs. SSR161421 (3 × 10(-6)M) decreased the AUC of the antigen-induced contraction-time curve to 20.8 ± 5.4% from the 100% control level. SSR161421 effectively reversed the antigen-induced bronchoconstriction, plasma leak and cell recruitment with EC(50) values of 0.33 mg/kg p.o., 0.02 mg/kg i.p. and 3 mg/kg i.p., respectively.

Interaction of SSR161421, a novel specific adenosine A(3) receptor antagonist with adenosine A(3) receptor agonists both in vitro and in vivo

A novel adenosine A(3) receptor antagonist (SSR161421) was characterized by both receptor binding assays and pharmacological tests. Binding studies on cloned human adenosine receptors showed that SSR161421 has high affinity for adenosine hA(3) receptors (K(i)=0.37 nM) with at least 1000-fold selectivity compared to hA(1), hA(2A) and hA(2B) receptors. The receptor antagonist nature of SSR161421 was determined in a functional study on Chinese hamster ovarian cells (CHO) cells expressing human adenosine A(3) receptors. SSR161421 competitively antagonized the effect of 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide (Cl-IB-MECA) on cAMP production with a pA2 value in a luciferase reporter gene construct. In mice, intravenously administered SSR161421 inhibited the N6-(4-aminobenzyl)-adenosine-5'-N-methyl-uronamide dihydrochloride (AB-MECA) induced increase in plasma histamine levels (ED(50)=2.0mg/kg) and the Cl-IB-MECA evoked plasma extravasation (ID(50)=2.9 mg/kg) and oedema formation (ID(50)=4.6 mg/kg) in mouse ear.

The central cannabinoid CB1 receptor is required for diet-induced obesity and rimonabant's antiobesity effects in mice

Cannabinoid receptor CB1 is expressed abundantly in the brain and presumably in the peripheral tissues responsible for energy metabolism. It is unclear if the antiobesity effects of rimonabant, a CB1 antagonist, are mediated through the central or the peripheral CB1 receptors. To address this question, we generated transgenic mice with central nervous system (CNS)-specific knockdown (KD) of CB1, by expressing an artificial microRNA (AMIR) under the control of the neuronal Thy1.2 promoter. In the mutant mice, CB1 expression was reduced in the brain and spinal cord, whereas no change was observed in the superior cervical ganglia (SCG), sympathetic trunk, enteric nervous system, and pancreatic ganglia. In contrast to the neuronal tissues, CB1 was undetectable in the brown adipose tissue (BAT) or the liver. Consistent with the selective loss of central CB1, agonist-induced hypothermia was attenuated in the mutant mice, but the agonist-induced delay of gastrointestinal transit (GIT), a primarily peripheral nervous system-mediated effect, was not. Compared to wild-type (WT) littermates, the mutant mice displayed reduced body weight (BW), adiposity, and feeding efficiency, and when fed a high-fat diet (HFD), showed decreased plasma insulin, leptin, cholesterol, and triglyceride levels, and elevated adiponectin levels. Furthermore, the therapeutic effects of rimonabant on food intake (FI), BW, and serum parameters were markedly reduced and correlated with the degree of CB1 KD. Thus, KD of CB1 in the CNS recapitulates the metabolic phenotype of CB1 knockout (KO) mice and diminishes rimonabant's efficacy, indicating that blockade of central CB1 is required for rimonabant's antiobesity actions.

Adiponectin is required to mediate rimonabant-induced improvement of insulin sensitivity but not body weight loss in diet-induced obese mice

The increase in adiponectin levels in obese patients with untreated dyslipidemia and its mRNA expression in adipose tissue of obese animals are one of the most interesting consequences of rimonabant treatment. Thus, part of rimonabant's metabolic effects could be related to an enhancement of adiponectin secretion and its consequence on the modulation of insulin action, as well as energy homeostasis. The present study investigated the effects of rimonabant in adiponectin knockout mice (Ad(-/-)) exposed to diet-induced obesity conditions. Six-week-old Ad(-/-) male mice and their wild-type littermate controls (Ad(+/+)) were fed a high-fat diet for 7 mo. During the last month, animals were administered daily either with vehicle or rimonabant by mouth (10 mg/kg). High-fat feeding induced weight gain by about 130% in both wild-type and Ad(-/-) mice. Obesity was associated with hyperinsulinemia and insulin resistance. Treatment with rimonabant led to a significant and similar decrease in body weight in both Ad(+/+) and Ad(-/-) mice compared with vehicle-treated animals. In addition, rimonabant significantly improved insulin sensitivity in Ad(+/+) mice compared with Ad(+/+) vehicle-treated mice by decreasing hepatic glucose production and increasing glucose utilization index in both visceral and subcutaneous adipose tissue. In contrast, rimonabant failed to improve insulin sensitivity in Ad(-/-) mice, despite the loss in body weight. Rimonabant's effect on body weight appeared independent of the adiponectin pathway, whereas adiponectin seems required to mediate rimonabant-induced improvement of insulin sensitivity in rodents.

Different transcriptional control of metabolism and extracellular matrix in visceral and subcutaneous fat of obese and rimonabant treated mice

BACKGROUND

The visceral (VAT) and subcutaneous (SCAT) adipose tissues play different roles in physiology and obesity. The molecular mechanisms underlying their expansion in obesity and following body weight reduction are poorly defined.

METHODOLOGY

C57Bl/6 mice fed a high fat diet (HFD) for 6 months developed low, medium, or high body weight as compared to normal chow fed mice. Mice from each groups were then treated with the cannabinoid receptor 1 antagonist rimonabant or vehicle for 24 days to normalize their body weight. Transcriptomic data for visceral and subcutaneous adipose tissues from each group of mice were obtained and analyzed to identify: i) genes regulated by HFD irrespective of body weight, ii) genes whose expression correlated with body weight, iii) the biological processes activated in each tissue using gene set enrichment analysis (GSEA), iv) the transcriptional programs affected by rimonabant.

PRINCIPAL FINDINGS

In VAT, "metabolic" genes encoding enzymes for lipid and steroid biosynthesis and glucose catabolism were down-regulated irrespective of body weight whereas "structure" genes controlling cell architecture and tissue remodeling had expression levels correlated with body weight. In SCAT, the identified "metabolic" and "structure" genes were mostly different from those identified in VAT and were regulated irrespective of body weight. GSEA indicated active adipogenesis in both tissues but a more prominent involvement of tissue stroma in VAT than in SCAT. Rimonabant treatment normalized most gene expression but further reduced oxidative phosphorylation gene expression in SCAT but not in VAT.

CONCLUSION

VAT and SCAT show strikingly different gene expression programs in response to high fat diet and rimonabant treatment. Our results may lead to identification of therapeutic targets acting on specific fat depots to control obesity.

Antidepressant-binding sites in brain and platelets

[3H]Imipramine and [3H]paroxetine label with high affinity a site associated with the serotonin transporter in brain and platelets. The maximum binding capacity (Bmax) of [3H]imipramine in platelets is reduced in untreated depressed patients, and it may represent a useful biological marker in depression. The existence of an endogenous ligand acting on the [3H]imipramine-recognition site to modulate the serotonin transporter has been proposed by several laboratories. 5-Methoxytryptoline inhibits [3H]imipramine binding and [3H]serotonin uptake in the nanomolar range. This compound has been reported to occur in the pineal gland, but probably only in trace amounts. While the physiological relevance of 5-methoxytryptoline or a close analogue remains an open question, the possibility exists that the 'endocoid' for the [3H]imipramine-recognition site plays a role in the pathogenesis of depression.

Involvement of 5-hydroxytryptamine (HT)7 receptors in the 5-HT excitatory effects on the rat urinary bladder

OBJECTIVE

To investigate the in vitro and in vivo effects of 5-hydroxytryptamine (5-HT) on the rat urinary bladder and to characterize the receptors involved in mediating these pharmacological effects by using selective antagonists.

MATERIALS AND METHODS

Female Wistar rats (250-350 g) were used for all studies. In vitro, detrusor muscle strips were mounted between two platinum electrodes in organ baths filled with a modified Krebs' solution bubbled with 95% O(2) and 5% CO(2) at 37 degrees C. After equilibration and a contraction to 80 mmol/L KCl, strips were exposed to electrical field stimulation for 30 min and incubated with the antagonist or vehicle for a further 30 min, then a 5-HT concentration-response curve (CRC) was obtained. In vivo, rats were anaesthetized with pentobarbital, and the ureters and urethra ligated, the bladder catheterized and infused with saline. 5-HT (3-100 microg/kg intravenous) dose-dependently increased intravesical pressure (IVP). After administering 5-HT at 30 microg/kg three times at 10 min intervals (controls), one dose of antagonist was perfused for 5 min and, after a further 5 min, 30 microg/kg 5-HT was tested again. This cycle was repeated four times using increasing doses of the antagonist to be tested.

RESULTS

In vitro, 5-HT (0.01-100 micromol/L) induced a concentration-dependent enhancement of the neurogenic response, with a mean (sd) pEC(50) of 6.36 (0.15) and E(max) of 41.1 (4.6)% KCl (eight rats). In unstimulated tissues, 5-HT induced no contractile effect. Selective 5-HT(4), 5-HT(3) and 5-HT(1A) receptor antagonists had no effect on the 5-HT potentiating effects. The potentiating effect of 5-HT was antagonized by mesulergine at 0.3 micromol/L, R(+)lisuride at 0.3 micromol/L and the selective 5-HT(7) receptor antagonist SB-258741 at 0.3 micromol/L. In vivo, in anaesthetized rats, IVP increases induced by repeated doses of 30 microg/kg 5-HT were reproducible. R(+)lisuride (3-100 microg/kg) dose-dependently inhibited the 5-HT-induced increase of IVP. At the maximum dose tested, R(+)lisuride almost totally inhibited the 5-HT effect.

CONCLUSIONS

In rat isolated detrusor muscle the 5-HT(7) receptor antagonists SB-258741, R(+)lisuride and mesulergine blocked the 5-HT potentiating effect with the expected potency. Moreover, in anaesthetized rats, R(+)lisuride abolished 5-HT effects on IVP at doses that antagonize physiological effects known to be mediated by 5-HT(7) receptor activation in several animal species. These results suggest the involvement of 5-HT(7) receptors in the modulation of rat bladder contraction both in vitro and in vivo.

Role of cannabinoid CB1 receptors and tumor necrosis factor-alpha in the gut and systemic anti-inflammatory activity of SR 141716 (rimonabant) in rodents

(1) We investigated the effect of the cannabinoid CB1 receptor antagonist, SR 141716, on indomethacin-induced small intestine inflammation and Escherichia coli lipopolysaccharide (LPS)-induced plasma TNF-alpha (TNF) release in comparison to the cannabinoid CB2 receptor antagonist, SR 144528, in rodents. (2) In rats, indomethacin induced significant ulcer formation in the small intestine; this was accompanied by an increase in tissue TNF levels and myeloperoxidase (MPO) activity. SR 141716 prevented the ulcers and the rise in TNF levels (ID50 3.3, 0.4 mg kg-1, respectively) and MPO activity. SR 144528 prevented intestinal ulcers only. (3) The effect of SR 141716 against indomethacin-induced ulcers and increase of plasma TNF levels after LPS was also studied in wild-type and CB1 receptor knockout mice. Indomethacin induced intestinal ulcers in mice, but not tissue TNF production and MPO activity. SR 141716 reduced the ulcers to a similar extent in wild-type and CB1 receptor knockout mice. In rats and wild-type mice, but not in CB1 receptor knockout mice, SR 141716 inhibited the LPS-induced increase in plasma TNF levels. (4) These findings provide evidence that the indomethacin model of intestinal lesions differs in rat and mouse and support the existence of several mechanisms for the antiulcer activity of SR141716, the most important involving the inhibition of TNF production. The potent anti-inflammatory activity of SR141716 in rodents indicated its potential therapeutic interest in chronic immune-inflammatory diseases.

Synthesis and SAR of 3- and 4-substituted quinolin-2-ones: discovery of mixed 5-HT(1B)/5-HT(2A) receptor antagonists

Quinolin-2-ones bearing a heteroaryl-piperazine linked by a two carbon chain at the 3- or 4-position were synthesised and evaluated as mixed 5-HT(1B)/5-HT(2A) receptor antagonists. Potent mixed antagonists were obtained with thieno[3,2-c]pyridine derivatives. In this series, compound 2.1 (SL 65.0472) proved to be functional antagonist at both the 5-HT(2A) receptor (rat in vivo 5-HT-induced hypertension model) and the 5-HT(1B) receptor (dog in vitro saphenous vein assay).

Effects of SL 65.0472, a novel 5-HT receptor antagonist, on 5-HT receptor mediated vascular contraction

5-hydroxytryptamine (5-HT) contracts vascular smooth muscle and pharmacological and molecular biological data suggest that these effects are mediated primarily by stimulation of 5-HT(1B) and 5-HT(2A) receptor subtypes. We have studied the properties of 7-fluoro-2-oxo-4-[2-[4-(thieno[3,2-c] pyridin-4-yl) piperazin-1-yl] ethyl]-1,2-dihydroquinoline-1-acetamide (SL 65.0472 ), a novel 5-HT receptor antagonist, in isolated vascular preparations contracted by 5-HT or sumatriptan. In canine isolated saphenous vein strips (putatively 5-HT(1B)-mediated contraction), SL 65.0472 antagonised sumatriptan-induced contractions in a competitive manner (pA(2) 8. 17+/-0.36). 5-HT contracts rabbit aorta by stimulation of 5-HT(2A) receptors. SL 65.0472 displaced the 5-HT concentration response curve in rabbit aorta rightwards with a significant reduction in maximum. The apparent pK(B) value was 8.58+/-0.18. 5-HT-induced contractions of human coronary arteries are mediated by a mixed population of 5-HT(1B) and 5-HT(2A) receptors. SL 65.0472 produced rightward parallel shifts of the 5-HT concentration response curves in all tissues studied (pA(2) 8.8+/-0.14, n=7). In conclusion, SL 65. 0472 is a potent antagonist of vascular smooth muscle contraction in vitro mediated by 5-HT receptor stimulation.

Functional characterization of alpha-1-adrenoceptor subtypes in the prostatic urethra and trigone of male rabbit

The effects of alpha-1-adrenoceptor antagonists on the concentration-response curves (CRC) to phenylephrine and oxymetazoline have been studied in prostatic urethra and trigone of male adult rabbits (28 wk old). WB4101 and phentolamine, at low concentrations which should preferentially antagonize the alpha-1A-adrenoceptor subtype, did not modify the oxymetazoline-induced contraction of both prostatic urethra and trigone, suggesting that alpha-1A-adrenoceptors are not activated under these conditions. In urethra, pretreatment with 50 microM chloroethylclonidine (CEC), significantly reduced the maximal contraction to both agonists to 60 and 70% of control, respectively. In trigone, CEC decreased the maximum contraction to phenylephrine, but not to oxymetazoline, by 50%. In addition, CEC shifted to the right the CRC to both agonists. These results suggest the presence of an alpha-1B-adrenoceptor in both rabbit urethra and trigone. Exposure to prazosin (0.01-1 microM) significantly shifted to the right the CRC to phenylephrine (pA2 or affinity values without CEC treatment: 7.77 and 7.96 in urethra and trigone respectively; with CEC pretreatment: 7.49 and 7.42, respectively). When oxymetazoline was used as an agonist and in the presence of CEC, prazosin was unexpectedly weak in urethra with an affinity value of 6.70, although the antagonist potency was not modified in trigone (affinity 7.38). These values suggest that alpha-1-adrenoceptor agonists contract rabbit urethra and trigone through activation of alpha-1-adrenoceptors displaying low affinity for prazosin. Whether this receptor coincides with the alpha-1L- or alpha-1N-subtype remains to be clarified.

Pharmacological characterization of a specific binding site for angiotensin IV in cultured porcine aortic endothelial cells

This study demonstrated the existence of a specific binding site for angiotensin IV in porcine aortic endothelial cells. Non-equilibrium kinetic analyses at 37 degrees C allowed the calculation of a kinetic Kd of 0.44 nM. Pseudo-equilibrium saturation binding studies at 37 degrees C for 90 min indicated the presence of a single high-affinity site (Kd = 3.87 +/- 0.60 nM), saturable and abundant (Bmax = 9.64 +/- 1.44 pmol/mg protein). Competitive binding studies demonstrated the following rank order of effectiveness: angiotensin IV > angiotensin III > angiotensin II > angiotensin I > angiotensin II-(1-7), while 2-n-butyl-4-chloro-5-hydroxymethyl-1 [(2'-(1H-tetrazol-5-yl) biphenyl-4-yl) methyl] imidazol (DuP 753: losartan), 1-(4-amino-3-methyl-phenyl) methyl-5-diphenylisoethyl-4,5,6,7-tetrahydro-1H-imidazo [4,5-C] pyridine-6-carboxylic acid (PD 123177) or nicotinic acid-Tyr-(N alpha -benzyl-oxycarbonyl-Arg) Lys-His-Pro-Ile-OH (CGP 42112A) were inactive at the concentration of 100 microM. This binding site is, therefore, distinct from angiotensin II receptors, AT1 and AT2. Addition of the divalent cations Mg2+, Mn2+ or Ca2+ to the incubation buffer resulted in 90-95% inhibition of the [125I]angiotensin IV-specific binding to porcine aortic endothelial cells. Furthermore, the chelator, EGTA, at 5 mM increased the number of binding sites (Bmax = 17.8 +/- 2.5 pmol/mg protein), with no change in affinity (Kd = 5.7 +/- 1.3 nM). Exposure of porcine aortic endothelial cell membranes to the non-hydrolyzable GTP analog, GTP gamma S, had no effect on [125I]angiotensin IV binding. The presence of a high concentration of binding sites for angiotensin IV in porcine aortic endothelial cells suggests that this peptide may play an important role in the modulation of the cardiovascular system.

Characterization of the angiotensin II AT1 receptor subtype involved in DNA synthesis in cultured vascular smooth muscle cells

1. This study was undertaken in cultured vascular smooth muscle cells to characterize the angiotensin II (AII) AT1 receptor subtype involved in DNA synthesis because (i) the AII receptor involved in vascular proliferation has previously been characterized in vitro in rat aortic cells and identified as an AT1 subtype and (ii) molecular cloning and biochemical studies have provided evidence for the existence of different AT1 receptor subtypes. 2. In cultured rat aortic vascular smooth muscle (VSMC), exposure to AII (0.1 to 100 nM) resulted in a concentration-dependent increase in [3H]-thymidine incorporation with an EC50 of 1.41 +/- 0.51 nM. Maximal stimulation was observed in the presence of 100 nM AII and corresponded to 271 +/- 40% of basal [3H]-thymidine incorporation. 3. To characterize the AII AT1 receptor subtype involved in this effect, cells were exposed to AII (3 nM) in the absence or presence of increasing concentrations of various AII receptor antagonists. The stimulatory effect of AII (3 nM) on [3H]-thymidine incorporation in VSMC was antagonized by the non-selective AT1/AT2 receptor antagonist, [Sar1, Ile8]-AII (IC50 = 5.6 nM), by the AT1A/AT1B receptor antagonist, losartan (IC50 = 10.5 nM) and the AT1 receptor antagonist, L-158809 (IC50 = 0.20 nM). The selective AT2 receptor ligand, CGP 42112A, antagonized AII-induced [3H]-thymidine incorporation with an IC50 of 6.3 +/- 1.3 microM while the AT2/AT1B receptor antagonist, PD 123319, was found to be almost inactive (IC50 > 10 microM). 4. Under the same experimental conditions, angiotensin III (AIII) was found to be at least 50 times less potent than All with an apparent EC50 of 81.6 +/- 7.7 nM. At the highest concentration tested (10 microM),the effect of AIII corresponded to 327 +/- 61% of basal [3H]-thymidine incorporation.5. These results confirm that All can stimulate DNA synthesis in VSMC through an AT, receptor.Furthermore, the pharmacological characterization of this AT1 receptor is compatible with the ATlA receptor subtype recently described on cultured mesangial cells since (i) the ATIA/ATIB receptor antagonist losartan is active at nanomolar concentrations, (ii) micromolar concentrations of the AT2/AT1B receptor antagonist PD 123319 are ineffective at antagonizing the AII-induced [3H]-thymidine incorporation and (iii) All is at least 50 times more potent than AIII in stimulating DNA synthesis.

SL 84.0418: a novel, potent and selective alpha-2 adrenoceptor antagonist: in vitro pharmacological profile

One novel, potent and selective alpha-2 adrenoceptor antagonist is 2-(4,5-dihydro-1H-imidazol-2-yl)-1,2,4,5-tetrahydro-2- propylpyrrolo[3,2,1-hi]-indole hydrochloride (SL 84.0418). It inhibits with high affinity the radioligand binding to rat cortical alpha-2 adrenoceptors, as well as to human platelet alpha-2 adrenoceptors labeled with [3H]idazoxan (Ki = 7 nM). SL 84.0418 has low affinity for alpha-1 adrenoceptors labeled with [3H]prazosin (Ki = 3.3 microM). In vitro, SL 84.0418 has no alpha agonist properties, whereas it is a potent alpha-2 adrenoceptor antagonist at both pre- and postsynaptic alpha-2 adrenoceptors. In contrast, it possesses low potency as an antagonist at postsynaptic alpha-1 adrenoceptors demonstrating a more than 1000-fold selectivity toward alpha-2 compared with alpha-1 adrenoceptors. In the same tests, the alpha-2 adrenoceptor antagonist idazoxan had a selectivity ratio of 200. SL 84.0418 is the racemic mixture of two enantiomers, SL 86.0715 [(+) enantiomer] and SL 86.0714 [(-) enantiomer]. The alpha-2 adrenoceptor blocking activities reside with SL 86.0715. Similar to idazoxan, SL 84.0418 increases in a concentration-dependent manner the electrically evoked release of [3H]norepinephrine from rat hypothalamic slices through the blockade of the presynaptic inhibitory alpha-2 adrenoceptors. In isolated hamster adipocytes, SL 84.0418 potently antagonizes the inhibition of lipolysis induced by UK 14,304. In addition, SL 84.0418 inhibits epinephrine-induced aggregation of rabbit platelets, effects mediated by postsynaptic alpha-2 adrenoceptors. SL 84.0418 does not inhibit (IC50 > 1,000 nM) radioligand binding to other receptors or recognition sites, nor does it inhibit calcium, sodium or potassium channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the 5-Hydroxytryptamine Receptor Modulating the Release of 5-[3H]Hydroxytryptamine in Slices of the Human Neocortex

In the rat brain, the presynaptic 5-hydroxytryptamine (5-HT) autoreceptors located on 5-HT terminals correspond to the 5-HT1B subtype. The presence of a 5-HT receptor probably located on 5-HT nerve endings and modulating transmitter release in the human neocortex has been reported, but its detailed pharmacological characterization is not yet available. On the other hand, receptor binding and autoradiographic results indicate that the 5-HT1B receptor subtype is not present in the human brain. We, therefore, studied the modulation of the electrically evoked release of [3H]5-HT by various 5-HT receptor agonists and antagonists in preloaded slices of human neocortex obtained from 18 patients undergoing neurosurgery. The nonselective 5-HT1A/1B/1D receptor agonist 5-carboxamidotryptamine produced a potent inhibition (70% at 0.03 microM) of the electrically evoked release of [3H]5-HT which was blocked by 5-HT receptor antagonists with the following relative order of potency: methiothepin greater than metergoline = methysergide greater than propranolol. The selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin at 0.1 microM did not modify the electrically evoked release of [3H]5-HT. The 5-HT1A/1B receptor agonist RU 24969 was 10 times more potent at inhibiting [3H]5-HT overflow in the rat frontal cortex than in the human neocortex. The potent 5-HT1B receptor antagonist cyanopinodolol did not modify the 5-carboxamidotryptamine-induced inhibition of the electrically evoked release of [3H]5-HT in slices of the human neocortex, but produced by itself a small inhibition of [3H]5-HT overflow.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between serotonin uptake inhibitors and alpha-2 adrenergic heteroreceptors in the rat hypothalamus

The effectiveness of presynaptic receptor agonists to inhibit the electrically evoked release of [3H]monoamines from brain slices is attenuated in the presence of blockade of neuronal uptake for the serotonin (5-HT) and the norepinephrine (NE) systems. There is controversy, however, as to the existence of a functional link between the presynaptic receptors and the neuronal uptake carriers. An alternative hypothesis involves competition for the presynaptic receptor sites between the exogenous agonist and the released neurotransmitter. In order to examine the proposed functional interaction, we studied the alpha-2 adrenoceptor-mediated inhibition of the electrically evoked release of [3H]-5-HT from slices of the rat hypothalamus, a model in which endogenous NE does not activate the alpha-2 heteroreceptors located on 5-HT terminals. The inhibitors of 5-HT uptake, citalopram (0.01-1 microM) and paroxetine (1 microM), which by themselves did not modify [3H]-5-HT release, antagonized the inhibition of [3H]-5-HT overflow produced by UK 14.304, an alpha-2 adrenoceptor agonist. The inhibition of the electrically evoked release of [3H]-5-HT by exogenous NE (0.1-1 microM) was also attenuated in the presence of citalopram. In contrast, citalopram did not modify the electrically evoked release of [3H]-NE or the inhibition of [3H]-NE release mediated by UK 14.304. When the 5-HT autoreceptor was blocked by cyanopindolol, the inhibitory effect of UK 14.304 on [3H]-5-HT release was unaltered in the presence of citalopram.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of adenylate cyclase and protein kinase C in the alpha 2-adrenoceptor-mediated inhibition of noradrenaline and 5-hydroxytryptamine release in rat hypothalamic slices

In superfused rat hypothalamic slices prelabelled with [3H]-noradrenaline, the alpha 2-adrenoceptor agonist UK 14304 inhibited in a concentration-dependent manner the electrically-evoked release of tritium. This inhibition was antagonized by the alpha 2-adrenoceptor blocking agent idazoxan, which by itself increased the electrically-evoked tritium overflow. Exposure to forskolin, an adenylate cyclase activator, increased the electrically-evoked release of [3H]-noradrenaline. In the presence of forskolin (1 mumol/l), both the inhibitory effect of UK 14304 and the increasing effect of idazoxan on the electrically-evoked release of [3H]-noradrenaline were less pronounced than in the absence of the adenylate cyclase activator. Exposure to forskolin and to the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine shifted to the right the concentration-effect curve for UK 14304 in a similar manner as that observed in the presence of forskolin alone. Exposure to phorbol-12,13-dibutyrate (0.01-10 mumol/l), a drug which activates protein kinase C, increased the electrically-evoked release of [3H]-noradrenaline. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mumol/l), the concentration effect curve for UK 14304 on tritium overflow was significantly shifted to the right. The increasing effect of idazoxan on tritium overflow was significantly less pronounced in the presence of 1 mumol/l phorbol-12,13-dibutyrate. In superfused rat hypothalamic slices prelabelled with [3H]-5-hydroxytryptamine, the alpha 2-adrenoceptor agonist UK 14304 significantly inhibited the electrically-evoked release of tritium. Exposure to forskolin increased in a concentration-dependent manner [3H]-5-hydroxytryptamine overflow, but did not modify the UK 14304-mediated inhibition. Exposure to 3-isobutyl-1-methylxanthine enhanced the electrically-evoked release of [3H]-5-hydroxytryptamine. In the presence of both forskolin (1 mumol/l) and 3-isobutyl-1-methylxanthine (1 mmol/l), the concentration-response curve for UK 14304 was significantly shifted to the right. Exposure to phorbol-12,13-dibutyrate (0.01-10 mumol/l) enhanced in a concentration-dependent manner the electrically-evoked overflow of [3H]-5-hydroxytryptamine. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 mumol/l), UK 14304 was significantly less potent to inhibit tritium release than in the absence of the protein kinase C activator. It is concluded that both cyclic AMP and phosphoinositide turnover are involved in the modulation of noradrenaline and 5-hydroxytryptamine release by presynaptic alpha 2-adrenoceptors in rat hypothalamic slices. However, these interactions do not represent definitive proof for a cause-effect relationship for the second messengers mediating the alpha 2-adrenoceptor induced inhibition of transmitter release either as autoreceptor or as heteroreceptor.

Light-dark differences in [3H]-paroxetine binding to rabbit platelet membranes

[3H]-Paroxetine binding to rabbit blood platelet membranes from samples obtained under light and dark conditions was examined. Animals were kept on a 14 h light (L) - 10 h dark (D) schedule and blood samples were collected at L + 7 and D + 5 h. Significant differences were found for Bmax values of [3H]-paroxetine binding, with low Bmax values during the light period and high Bmax values during the dark period. The Kd values were not significantly different. These results confirm previous observations on light-dark differences of [3H]-imipramine binding in rabbit blood platelets suggesting the existence of a circadian rhythm for the 5-HT transporter complex.

Platelet [3H]-imipramine binding is not modified in Alzheimer's disease

Platelet [3H]-imipramine binding was studied in patients with Alzheimer's disease and control subjects matched to the patients for age and sex. There were no differences in the binding parameters of [3H]-imipramine on platelet membranes from patients with Alzheimer's disease, when compared with the control group. These results suggest that [3H]-imipramine binding could be a useful tool to discriminate between demented and depressive patients in elderly populations.

Diurnal variation in the function of serotonin terminals in the rat hypothalamus

The high-affinity binding of [3H]imipramine is associated with the serotonin (5-hydroxytryptamine; 5-HT) transporter in the brain and in platelets. In the rat hypothalamus it has been reported that the density of these sites is increased in the dark period of the day, and this could result in an alteration in the release of 5-HT. The electrically evoked release of [3H]5-HT was thus studied in preloaded hypothalamic slices prepared from rats kept under 12:12 h light/dark or dark/light schedules. The fractional release of [3H]5-HT evoked by electrical stimulation, but not by the 5-HT releasing agent fenfluramine, was significantly decreased during the dark period when compared with the light period. The effects of the 5-HT reuptake blocker citalopram, of the two 5-HT autoreceptor agonists 5-methoxytryptamine and RU 24969, and of the 5-HT autoreceptor antagonist methiothepin on the release of [3H]5-HT were the same in both groups of rats. In conclusion, the release of [3H]5-HT from prelabelled rat hypothalamic slices is decreased during the dark period of the day. This modification is not reflected by changes in the effects of citalopram, an inhibitor of 5-HT reuptake, to modify the overflow of [3H]5-HT. The sensitivity and efficacy of agonists of the 5-HT autoreceptor are the same during the light and dark periods of the day.

Phorbol-12,13-dibutyrate antagonizes while forskolin potentiates the presynaptic autoreceptor-mediated inhibition of [3H]-5-hydroxytryptamine release in rat hypothalamic slices

In superfused rat hypothalamic slices prelabeled with [3H]-5-hydroxytryptamine [( 3H]-5-HT), the 5-HT autoreceptor agonists 5-methoxytryptamine and RU 24969 inhibited in a concentration-dependent manner the electrically evoked release of [3H]-5-HT. Exposure to phorbol-12,13-dibutyrate increased in a concentration-dependent manner the stimulation-evoked overflow of [3H]-5-HT and shifted to the right the 5-methoxytryptamine inhibition curve. Exposure to forskolin, a potent activator of adenylate cyclase, increased the stimulation-evoked [3H]-5-HT overflow and shifted to the left the 5-methoxytryptamine or RU 24969 inhibitory curves on transmitter release. A similar interaction was observed in the presence of 1-isobutyl,3-methylxanthine (IBMX), a phosphodiesterase inhibitor, or 8-bromo-cyclic AMP and the serotonin autoreceptor agonist 5-methoxytryptamine. In the presence of phorbol-12,13-dibutyrate or forskolin, the enhancing effect of the 5-HT autoreceptor antagonist methiothepin on the stimulation-evoked [3H]-5-HT overflow was significantly less pronounced than in the absence of these compounds. These results indicate that the presynaptic 5-HT autoreceptors that modulate the release of [3H]-5-HT in rat hypothalamic slices may be coupled to the phosphoinositide cycle and protein kinase C-dependent mechanisms. In addition, the increase in intracellular level of cyclic AMP by forskolin, IBMX, and 8-bromo-cyclic AMP potentiates the inhibitory effects of the autoreceptor agonist 5-methoxytryptamine on [3H]-5-HT release, although the mechanism of the interaction remains to be elucidated.

Frequency-dependence of serotonin autoreceptor but not alpha 2-adrenoceptor inhibition of [3H]-serotonin release in rat hypothalamic slices

The overflow of tritium from stimulated rat hypothalamic slices preincubated with [3H]-serotonin (5-HT) was significantly enhanced by reducing the frequency of stimulation from 3 Hz to 1 Hz while keeping the number of impulses constant. The 5-HT receptor agonist 5-methoxytryptamine inhibited in a concentration-dependent manner the electrically-evoked release of [3H]-5-HT with IC50 values of 560 nmol/l and of 34 nmol/l when the stimulations were delivered at 3 Hz and 1 Hz, respectively. The terminal 5-HT autoreceptor antagonist methiothepin enhanced in a concentration-dependent manner the electrically-evoked release of [3H]-5-HT and this effect was greater at a frequency of stimulation of 3 Hz than at 1 Hz. In the same paradigm, the 5-HT reuptake inhibitors citalopram and paroxetine did not alter the overflow of radioactivity elicited by stimulation at 3 Hz but significantly decreased it at 1 Hz. In the presence of 5-HT autoreceptor blockade achieved with methiothepin, citalopram increased the overflow of [3H]-5-HT to the same extent at 1 Hz and at 3 Hz. The IC50 values for inhibition of [3H]-5-HT release by the selective alpha 2-adrenoceptor agonist UK 14.304 were 35 nmol/l at 3 Hz and 30 nmol/l at 1 Hz. It is concluded that modulation of 5-HT release by 5-HT autoreceptors, but not by alpha 2-adrenoceptors is dependent on the synaptic concentration of 5-HT as a function of the frequency of depolarization.

Day-night rhythm of 5-methoxytryptamine biosynthesis in the pineal gland of the golden hamster (Mesocricetus auratus)

5-Methoxytryptamine is a potent agonist of presynaptic 5-hydroxytryptamine autoreceptors modulating serotonin release in the central nervous system. This methoxyindole can be synthesized in the pineal gland, but its presence in vivo is still controversial, probably because of rapid catabolism by monoamine oxidase. An improved high-pressure liquid chromatography method, with coulometric detection, has been developed for the simultaneous measurement of melatonin, 5-methoxytryptamine, 5-methoxytryptophol and 5-methoxyindolacetic acid. We have demonstrated a day-night rhythmicity in the amount of 5-methoxytryptamine in the pineal gland of golden hamsters (Mesocricetus auratus) maintained under a long photoperiod (14 h light: 10 h darkness) and pretreated with the monoamine oxidase inhibitor pargyline. Levels of 5-methoxytryptamine were highest at 16.30 h and lowest at 00.30 h. The rhythm for 5-methoxytryptamine appears to be the same as for serotonin (opposite in phase to that of melatonin). The identification of 5-methoxytryptamine has been confirmed by analysis with gas chromatography-mass spectrometry.

Short-term lithium administration to healthy volunteers produces long-lasting pronounced changes in platelet serotonin uptake but not imipramine binding

Platelet [3H]-5HT uptake, [3H]-imipramine binding and endogenous 5HT levels were measured in healthy volunteers during short-term (20 days) administration of lithium, and following its withdrawal. The Vmax of [3H]-5HT uptake was significantly decreased during lithium treatment. Following lithium withdrawal, platelet [3H]-5HT uptake (Vmax) remained decreased and was followed by a pronounced rebound effect in some of the subjects for up to 3 months. The affinity constant (Km) of [3H]-5HT uptake was not modified. Binding of tritiated imipramine during the same period and platelet 5HT levels measured till 14 days after withdrawal was not affected by lithium treatment. As lithium is devoid of in vitro effects on both 5HT uptake and imipramine binding, it is concluded that the effects of lithium on the 5HT transporter do not reflect a direct effect on the transporter complex. Our results indicate that lithium-induced changes at the level of 5HT uptake in platelets are not correlated with concomitant variations in platelet 5HT content and can be dissociated from modifications at the level of imipramine binding sites within the macromolecular complex of the 5HT transporter. Moreover, platelet 5HT uptake is apparently modulated by lithium, with a similar pattern in healthy volunteers and in manic-depressive patients.

Methiothepin enhances the potassium-evoked release of [3H]-noradrenaline in rat pineal gland

The 5-hydroxytryptamine (5-HT) autoreceptor antagonist methiothepin increased in a concentration-dependent manner the K+-evoked release of [3H]-noradrenaline in pineal glands from normal and parachlorophenylalanine (PCPA)-treated rats. However, 5-HT and the 5-HT receptor agonists, LSD and 5-methoxytryptamine, were inactive at modulating the K+-evoked release of [3H]-noradrenaline in pineal glands from normal and PCPA-treated rats. When tested on the uptake of [3H]-noradrenaline in the pineal gland, methiothepin was found to be a potent inhibitor (IC50 = 10.6 nmol/l). Exposure to methiothepin failed to increase the K+-evoked release of [3H]-noradrenaline when tested in the presence of cocaine. While the K+-evoked release of [3H]-noradrenaline was shown to be modulated through inhibitory presynaptic alpha 2-adrenoceptors in pineal glands from normal and PCPA-treated rats, no evidence was obtained for a presynaptic modulation through 5-HT receptors of [3H]-noradrenaline release. The facilitation by methiothepin of the K+-evoked release of [3H]-noradrenaline in rat pineal gland appears to be due to the inhibition of noradrenaline uptake by this compound.

Circadian rhythm of the Bmax of [3H]-imipramine binding in rabbit platelets

[3H]-imipramine binding was measured in rabbit blood platelet membranes on a 24 h cycle. Animals were kept on a 14 h light (L) 10 h dark (D) schedule, and blood samples were collected at L + 2, L + 8, D + 2, D + 8 and L + 2 h on a following cycle. Significant differences were found for Bmax values of [3H]-imipramine binding, with highest values during the dark phase and lowest during the light phase. No significant differences were found in Kd values. These results suggest the existence of a circadian rhythm for the Bmax of [3H]-imipramine binding in blood platelets.

Interaction between neuronal uptake inhibitors and presynaptic serotonin autoreceptors in rat hypothalamic slices: comparison of K+ and electrical depolarization

In rat hypothalamic slices prelabeled with [3H]-5-hydroxytryptamine ([3H]-5-HT), exposure to the 5-HT receptor agonist lysergic acid diethylamide (0.1-1 microM) or 5-methoxytryptamine (0.1-10 microM) decreased in a concentration-dependent manner the release of 3H-transmitter elicited by high K+ or electrical stimulation. Exposure to the 5-HT autoreceptor antagonist methiothepin (0.1-1 microM) increased in a concentration-dependent manner the K+ stimulation-evoked overflow of [3H]-5-HT and a similar increase was observed under conditions of electrical stimulation. In contrast, exposure to the nontricyclic 5-HT uptake inhibitor citalopram (0.1-1 microM) did not modify by itself the electrically evoked overflow of [3H]-5-HT, but increased in a concentration-dependent manner the release of 3H-transmitter elicited by K+ stimulation. This effect of citalopram on transmitter release was potentiated when the endogenous stores of 5-HT were depleted by pretreatment with para-chlorophenylalanine methyl ester (300 mg/kg i.p.). Citalopram was shown previously to antagonize the inhibition by lysergic acid diethylamide of the electrically evoked release of [3H]-5-HT in rat hypothalamic slices. Yet, this inhibitor of neuronal uptake of 5-HT did not antagonize the effects of lysergic acid diethylamide when the release of [3H]-5-HT was evoked by K+ depolarization. Electrical stimulation represents a more physiological experimental model for transmitter release than exposure to high K+, and therefore the interaction between 5-HT uptake blockade and presynaptic inhibitory 5-HT autoreceptors, observed in the hypothalamus with electrical stimulation but not with K+ depolarization, remains of biological relevance.

Pitfalls in demonstrating an endogenous ligand of imipramine recognition sites

The recognition sites for the 5-hydroxytryptamine (5-HT) uptake inhibitors imipramine and paroxetine may represent receptors for a presently unknown endogenous ligand, whose function would be to modulate 5-HT uptake. Attempts to isolate such a factor from rat brain tissue are described, following a published procedure. It is shown that chromatographic fractions found to inhibit the binding of [3H]imipramine and [3H]paroxetine to rat brain membranes consisted of material essentially unretained by the reverse-phase HPLC column, and they were of high osmolarity. Thus, the activity was probably unspecific in nature, and the presence in rat brain of the factor has not been unequivocally demonstrated.

Changes in [3H]5-HT uptake and [3H]imipramine binding in platelets after chlorimipramine in healthy volunteers. Comparison with maprotiline and amineptine

In the platelets of normal healthy volunteers (n = 8) taking chlorimipramine (50 mg/day) for 1 week, the saturable uptake of [3H]5-hydroxytryptamine (5-HT) was fully inhibited at the end of the week, but returned to control values after 2 weeks washout. The Bmax of [3H]imipramine binding was decreased by 63% at the end of the treatment and remained significantly decreased below control values after 1 week washout, whereas the Kd values were increased at the end of the treatment, but had returned to baseline values after 1 week washout. The time course of recovery following the administration of chlorimipramine showed some variation between subjects, but it was necessary to wait up to 4 weeks of washout before the Bmax of [3H]imipramine returned to baseline levels. In contrast, neither 1-week treatment with maprotiline (50 mg/day) nor with amineptine (100 mg/day) changed the parameters of [3H]5-HT uptake or [3H]imipramine binding in platelets from healthy volunteers. These results support the following conclusions. (1) [3H]Imipramine binding in platelets can be down-regulated by relatively low, subtherapeutic doses of chlorimipramine. (2) It is possible to dissociate [3H]imipramine binding parameters from [3H]5-HT uptake because the time course of recovery was clearly different, indicating that [3H]imipramine labels a site linked with, but different from, the 5-HT recognition site in the transporter complex. (3) A washout of antidepressants of 4 weeks may be needed when studying the parameters of [3H]imipramine binding in platelets from depressed patients if the previous medication involved chlorimipramine. For antidepressants like maprotiline or amineptine, that act through mechanisms other than inhibition of 5-HT uptake, the time of washout appears to be less critical, although it is not possible to rule out the existence of some secondary modifications influencing the 5-HT transporter complex.

Association of [3H]-imipramine and [3H]-paroxetine binding with the 5HT transporter in brain and platelets: relevance to studies in depression

[3H]-Imipramine and [3H]-paroxetine label with high affinity a site which is associated with the serotonergic transporter in brain and platelets. The pharmacological profile of inhibition by drugs of [3H]-imipramine and [3H]-paroxetine binding is highly correlated with the potency of the drugs to inhibit the uptake of 5HT. Denervation of serotonergic neurons by electrolytic lesions or with 5,7-dihydroxytryptamine produces marked decreases in the density of [3H]-imipramine as well as [3H]-paroxetine binding. Dissociation kinetic experiments support the view that the substrate recognition site for 5HT is different from the modulatory site which is labelled by [3H]-imipramine or [3H]-paroxetine. The existence of an endogenous ligand acting on the [3H]-imipramine recognition site to modulate the 5HT transporter was proposed by several laboratories. [3H]-Imipramine binding in platelets appears to be a biological marker in depression. Studies carried out in several laboratories report a significant decrease in the Bmax of platelet [3H]-imipramine binding without changes in Kd, when severely depressed untreated patients are compared with healthy volunteers matched for age and sex. The Bmax of platelet [3H]-imipramine binding appears to be a state-dependent biological marker in depression. It is tempting to speculate that the endocoid of the [3H]-imipramine recognition site may play a role in the pathogenesis of depression.

Lack of seasonal variation in platelet [3H]imipramine binding in humans

The Bmax of [3H]imipramine (IMI) binding has been reported to be reduced in platelets of depressed untreated patients as compared with normal controls. However, it has also been suggested that this difference could be related to the failure to take into account seasonal variations in the binding parameters for [3H]IMI recognition sites in platelets. For this reason, [3H]IMI binding was studied throughout 1 year in platelet membranes from 11 control volunteers, with blood samples collected once a month. The Bmax and Kd values of [3H]IMI binding showed no significant variation throughout the 12-month period of the study. These results indicate that in the control population, the platelet [3H]IMI binding parameters remain stable, and that the decrease in Bmax observed in depressed untreated patients reflects a genuine difference, which may be considered to be a biological marker in depression.

Potentiation by deprenyl of the autoreceptor-mediated inhibition of [3H]-5-hydroxytryptamine release by 5-methoxytryptamine

The 5-hydroxytryptamine (5HT) receptor agonist, 5-methoxytryptamine, inhibited in a concentration-dependent manner the electrically-evoked release of 3H-5HT from superfused rat hypothalamic slices, with an IC50 of 560 nmol/l, without affecting the spontaneous outflow of radioactivity. In the presence of the selective monoamine oxidase B (MAO B) inhibitor, (-)-deprenyl (1 mumol/l), the concentration-effect curve for 5-methoxytryptamine was shifted significantly to the left, and the IC50 was decreased to 25 nmol/l. Under the same experimental conditions, the potency of the 5HT receptor agonist lysergic acid diethylamide (LSD) at inhibiting the electrically-evoked release of 3H-5HT was the same in the presence as well as in the absence of (-)-deprenyl. The IC50 values for LSD were 34 nmol/l in the absence of deprenyl, and 31 nmol/l in the presence of the MAO B inhibitor. It is concluded that deprenyl potentiates the inhibition by 5-methoxytryptamine of 3H-5HT release, by preventing its inactivation through MAO B. Since 5-methoxytryptamine may be present in the pineal gland of some species, the potent effects of this 5-HT receptor agonist on serotoninergic neurotransmission may be of physiological relevance.

Reduced Bmax of [3H]-imipramine binding to platelets of depressed patients free of previous medication with 5HT uptake inhibitors

The high-affinity binding sites for [3H]-imipramine (IMI) present in human platelets are associated with the neuronal uptake system for 5HT. It was recently demonstrated that previous antidepressant therapy with drugs which inhibit 5HT uptake could down-regulate [3H]-IMI binding and that this effect could persist up to 1 month after the end of treatment. We therefore re-examined the reported differences in Bmax of [3H]-IMI binding in platelets between control and depressed untreated patients, to evaluate the residual influence of previous antidepressant medication. The saturation characteristics of [3H]-IMI binding were compared in platelets from 17 depressed patients carefully selected according to previous antidepressant therapy and washout period, who were closely matched, for age and sex, with a group of control healthy volunteers. The results reveal a significant decrease by 47% in the Bmax of [3H]-IMI binding in platelets of untreated depressed patients when compared with controls. There was no significant modification of Kd values for platelet [3H]-IMI binding between the depressed and the control groups. Our results support the view that platelet [3H]-IMI binding is a useful tool as a biological marker in depression.

Platelet 3H-imipramine binding and steroid hormones serum concentrations during the menstrual cycle

The presence of high affinity binding sites for 3H-imipramine (3H-IMI) in human platelets is by now well established. This recognition site is associated with the transporter for 5HT, and may be a biological marker in depression. Fluctuations of other putative biological markers of depression (i.e. platelet MAO activity) have been demonstrated and shown to be correlated with variations in steroid hormones. Therefore, the KD and Bmax of 3H-IMI binding was determined in platelets of young women during the menstrual cycle. Our results indicate that within the limits of intraindividual variations, neither the KD or the Bmax of 3H-IMI binding in platelets is significantly modified during the menstrual cycle.

Interaction between tricyclic and nontricyclic 5-hydroxytryptamine uptake inhibitors and the presynaptic 5-hydroxytryptamine inhibitory autoreceptors in the rat hypothalamus

In slices of the rat hypothalamus prelabeled with [3H]-5-hydroxytryptamine [( 3H]-5-HT), exposure to lysergic acid diethylamide or 5-methoxytryptamine decreased, in a concentration-dependent manner, the release of 3H-transmitter elicited by electrical stimulation. These inhibitory effects were antagonized by the 5-HT receptor antagonist methiothepin (1 microM). Exposure to methiothepin on its own increased in a concentration-dependent manner the electrically evoked overflow of [3H]-5-HT. Exposure to tricyclic antidepressants, like imipramine and amitriptyline, and to nontricyclic 5-HT uptake inhibitors, like paroxetine and citalopram, did not modify by themselves the electrically evoked overflow of [3H]-5-HT. Yet, the four inhibitors of neuronal uptake of 5-HT, antagonized the inhibition by lysergic acid diethylamide or 5-methoxytryptamine of the electrically induced release of [3H]-5-HT. After depletion of endogenous stores of 5-HT by pretreatment with para-chlorophenylalanine (300 mg/kg i.p.), the inhibitors of 5-HT uptake increased the electrically evoked release of [3H]-5-HT in a concentration-dependent manner. Their order of potency to enhance 5-HT overflow after pretreatment with parachlorophenylalanine paralleled their potency at inhibiting neuronal uptake of 5-HT (paroxetine = citalopram greater than imipramine greater than amitriptyline). In para-chlorophenylalanine-treated rat hypothalamic slices, these inhibitors of 5-HT uptake antagonized the inhibition by 5-HT autoreceptor agonists of the electrically evoked release of [3H]-5-HT to a similar extent than was observed in control rats. It is concluded that inhibition of 5-HT uptake reduces the effectiveness of 5-HT autoreceptor agonists to inhibit the electrically evoked release of [3H]-5-HT, irrespective of the chemical structure of the uptake inhibitor or of the levels of endogenous 5-HT achieved in the synaptic gap.

Inhibition by 5,6-dihydroxy-2-dimethylaminotetralin (M7) of noradrenergic neurotransmission in the rabbit hypothalamus: role of alpha-2 adrenoceptors and of dopamine receptors

Rabbit hypothalamic slices were prelabeled with [3H]norepinephrine and transmitter release elicited by electrical stimulation. In the presence of 10 microM cocaine and in a low Ca++ medium (0.65 mM), exposure for 8 min to exogenous dopamine (0.01-1 microM) inhibited, in a concentration-dependent manner, the electrically evoked release of [3H]norepinephrine. This inhibitory effect of dopamine on [3H]norepinephrine release was antagonized by the dopamine receptor antagonist S-sulpiride (1 microM), but remained unchanged in the presence of the alpha-2 adrenoceptor antagonists idazoxan (1 microM) or yohimbine (0.1 microM). These results indicate that, in a low Ca++ medium, exposure to dopamine decreased [3H]norepinephrine overflow in rabbit hypothalamic slices through the exclusive activation of presynaptic inhibitory dopamine receptors. M7 (5,6-dihydroxy-2-dimethylaminotetralin) is a potent agonist at central presynaptic dopamine autoreceptors and at peripheral alpha-2 adrenoceptors. Exposure to M7 in a normal Ca++ medium, inhibited in a concentration-dependent manner the electrically evoked release of [3H]norepinephrine without affecting the spontaneous outflow of radioactivity. The slope of the concentration-effect curve for these inhibitory effects of M7 was rather flat and the maximal inhibition obtained was 80%. The selective D2 receptor antagonist S-sulpiride (1 microM) failed to produce a significant shift to the right in the concentration-effect curve for the inhibitory effects of M7 on [3H]norepinephrine release. The preferential alpha-2 adrenoceptor antagonist yohimbine (0.1 microM) significantly antagonized the inhibition of [3H]norepinephrine release elicited by 0.01 microM M7, but not for higher concentrations of this aminotetraline.(ABSTRACT TRUNCATED AT 250 WORDS)

[3H]Imipramine binding and [3H]5HT uptake in human blood platelets: changes after one week chlorimipramine treatment

In platelets of normal volunteers taking chlorimipramine (50 mg/day) for one week, the saturable uptake of [3H]5HT was fully inhibited at day 8, but returned to control values at day 15. The Bmax of [3H]imipramine binding was decreased by 65% at day 8 and remained significantly below control values at day 15. If the present findings can be extrapolated to other antidepressants, the reported decreases in [3H]imipramine binding in depression may partly reflect residual treatment effects. It cannot be excluded that, in depression, the platelet [3H]imipramine receptor already is down-regulated maximally which would preclude a further down-regulation due to antidepressant drug therapy.

Evidence that exogenous but not endogenous norepinephrine activates the presynaptic alpha-2 adrenoceptors on serotonergic nerve endings in the rat hypothalamus

In superfused rat hypothalamic slices, clonidine, norepinephrine (NE) and 6-fluoronorepinephrine reduced the electrically evoked release of recently taken up [3H]-5-hydroxytryptamine (5-HT). The inhibitory action of these drugs involves the activation of presynaptic alpha-2 adrenoceptors and it was antagonized by the alpha adrenoceptor antagonists phentolamine or RX 781094. In contrast to the facilitating effect of alpha-2 adrenoceptor antagonists on the electrically evoked [3H]NE overflow in rabbit hypothalamic slices, neither phentolamine nor RX 781094 modified the stimulation-evoked release of [3H]-5-HT at concentrations which completely antagonized the inhibitory action of NE, 6-fluoronorepinephrine and clonidine on 5-HT neurotransmission. In the presence of cocaine, which inhibits the neuronal uptake of NE and increases the concentration of this neurotransmitter in the synaptic gap, alpha-2 adrenoceptor antagonists were still unable to modify the electrically evoked release of [3H]-5-HT. It is concluded that presynaptic alpha-2 adrenoceptors present on serotonergic nerve endings in the hypothalamus are not activated by endogenous NE and do not seem to play a physiological role in the regulation of serotonergic neurotransmission. However, presynaptic inhibitory alpha-2 adrenoceptors can be acted upon by exogenous agonists to inhibit 5-HT release. On the other hand, the presynaptic alpha-2 adrenoceptors on noradrenergic nerve terminals in the rabbit and rat hypothalamus are acted upon by released NE because the alpha-2 adrenoceptor antagonists by themselves increase the electrically evoked release of [3H]NE.

Amphetamine inhibits the electrically evoked release of [3H]dopamine from slices of the rabbit caudate

The effects of d-amphetamine on the spontaneous and electrically evoked release of [3H]dopamine in slices of the rabbit caudate nucleus were investigated. At a concentration of 0.1 microM amphetamine did not modify the spontaneous outflow of radioactivity, but significantly inhibited the release of [3H]dopamine elicited by electrical stimulation. At a 10-fold higher concentration (1 microM) amphetamine enhanced the spontaneous outflow of radioactivity and also inhibited the stimulation-evoked release of [3H]dopamine. The inhibition by amphetamine of electrically evoked release of [3H]dopamine was also observed under conditions in which monoamine oxidase was inhibited by pargyline. At concentrations of 0.1 and 0.5 microM amphetamine there was no inhibition of neuronal uptake and retention of [3H]dopamine in slices of the rabbit caudate. In the presence of 100 microM l-3-iodotyrosine, the inhibition by amphetamine of [3H]dopamine release was still obtained. The dopamine receptor antagonists, haloperidol and sulpiride, were not able to antagonize the inhibition by amphetamine of the electrically evoked release of [3H] dopamine at concentrations which effectively blocked apomorphine-induced inhibition of stimulation-evoked release of the labeled neurotransmitter. Exposure to serotonin in the presence of an inhibitor of neuronal uptake did not modify the spontaneous outflow of radioactivity or the electrically evoked release of [3H] dopamine. Nomifensine, an inhibitor of neuronal uptake of dopamine prevented the release of [3H]dopamine induced by exposure to 10 microM amphetamine and antagonized the inhibitory effects of lower concentrations of amphetamine on the electrically evoked release of [3H]dopamine. Tyramine and amfonelic acid in low concentrations enhanced the spontaneous outflow of radioactivity and, similarly to amphetamine, inhibited the electrically evoked release of [3H]dopamine. Exposure to bretylium (1 and 10 microM) inhibited the release of [3H]dopamine elicited by electrical stimulation. In the presence of bretylium, the inhibition by amphetamine of the stimulation-evoked release of [3H]dopamine was still present. In contrast to its inhibitory action on the release of [3H]dopamine, exposure to amphetamine (0.1-1.0 microM) enhanced in a concentration-dependent manner the electrically evoked release of [3H]norepinephrine from the rabbit hypothalamus. These results indicate that the inhibition by amphetamine of the electrically evoked release of [3H]dopamine does not involve the activation of presynaptic inhibitory dopamine autoreceptors possibly located on dopaminergic nerve terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in sensitivity of release modulating dopamine autoreceptors after chronic treatment with haloperidol

The release of recently taken up [3H]dopamine ([3H]DA) elicited by electrical stimulation (3 Hz, 2 min, 16 mA) from slices of the rabbit caudate nucleus is inhibited by apomorphine (0.01-0.1 microM) in a concentration-dependent manner. This action is mediated through the activation of presynaptic inhibitory DA autoreceptors. The inhibition of [3H]DA release by apomorphine (0.1 microM) was antagonized 2 hr, but not 24 hr after the single administration of haloperidol (1 mg/kg s.c.). After 2 days of withdrawal after 28 days of chronic treatment with haloperidol (1 mg/kg s.c.) once daily, apomorphine (0.01-0.1 microM) was more effective in inhibiting [3H]DA release elicited by electrical stimulation when compared with rabbits injected chronically with either the vehicle for haloperidol or with saline. In superfused slices of the rabbit caudate nucleus, exposure to S-sulpiride (0.1 and 1 microM) increased in a concentration-dependent manner the release of [3H] DA elicited by electrical stimulation. After 28 days of chronic treatment with haloperidol, the facilitation of [3H]DA release by S-sulpiride was significantly reduced when compared with the controls. The inhibition of central noradrenergic transmission by DA receptor agonists was studied in hypothalamic slices prelabeled with [3H]norepinephrine ([3H-NE]). Apomorphine (0.01-1 microM) inhibited the electrically evoked (5 Hz, 2 min, 26 mA) release of [3H]NE from hypothalamic slices of untreated rabbits. The sensitivity to the inhibitory effect of apomorphine on [3H]NE overflow remained unaffected after 2 days of withdrawal following 28 days of chronic treatment with haloperidol. In summary, our results indicate that chronic haloperidol administration induces changes in sensitivity of the DA autoreceptors regulating dopaminergic neurotransmission but does not affect the sensitivity of DA receptors modulating NE release in the central nervous system. These results suggest that the DA autoreceptors that regulate dopaminergic neurotransmission may play a physiological role in the modulation of transmitter release and consequently are susceptible to the development of changes in sensitivity after chronic receptor blockade. The possible implication of changes in sensitivity of the DA autoreceptor during the treatment of schizophrenia with neuroleptics is discussed.

Presynaptic alpha 2-adrenoceptor antagonism by verapamil but not by diltiazem in rabbit hypothalamic slices

1 Rabbit hypothalamic slices prelabelled with [3H]-noradrenaline and superfused with Krebs solution were stimulated electrically at a frequency of 5 Hz. Exposure to verapamil (0.1 to 10 microM) significantly increased, in a concentration-dependent manner, the electrically-evoked overflow of tritium, without affecting the spontaneous outflow of radioactivity. 2 Exposure to diltiazem in concentrations up to 100 microM had no effect on the electrically evoked release of [3H]-noradrenaline, but increased the basal outflow of radioactivity at 10 and 100 microM. 3 The preferential alpha 2-adrenoceptor antagonist, yohimbine (0.1 microM) significantly antagonized the inhibitory effect of clonidine or adrenaline on [3H]-noradrenaline overflow elicited by electrical stimulation. Verapamil (3 microM) also antagonized this inhibitory effect of the alpha 2-adrenoceptor agonists on [3H]-noradrenaline release. In contrast to these results, exposure to diltiazem (10 microM) was ineffective in blocking the action of the alpha 2-adrenoceptor agonist. 4 These results suggest that the two Ca2+-antagonists verapamil and diltiazem differ in their ability to affect central noradrenergic neurotransmission. While verapamil is a relatively potent alpha 2-adrenoceptor antagonist, diltiazem is devoid of presynaptic alpha 2-adrenoceptor antagonist properties.

[3H]Imipramine is accumulated but not released from slices of the rabbit caudate and hypothalamus

Slices of rabbit caudate and hypothalamus take up and accumulate [3H]imipramine. In superfused slices of both structures electrical stimulation or exposure to tyramine failed to release recently taken up [3H]imipramine. Depolarization by exposure to 30--60 mM-potassium caused only a small release of [3H]imipramine that was not concentration-dependent. The release of [3H]imipramine by high potassium was independent of the presence of calcium ions in the superfusion medium. These results contrasted with those obtained for the release of [3H]dopamine from the caudate and [3H]noradrenaline from the hypothalamus, where tyramine, electrical stimulation, and high potassium caused a significant release of the labeled neurotransmitters. The release of [3H]dopamine from the caudate and [3H]noradrenaline from the hypothalamus elicited by electrical stimulation or high potassium was entirely calcium-dependent. It is concluded that [3H]imipramine is taken up into the two brain regions and is accumulated in a nonvesicular site from which it is not released by calcium-dependent depolarizing stimuli.