Reduction of 5-hydroxytryptamine (5-HT)(1A)-mediated temperature and neuroendocrine responses and 5-HT(1A) binding sites in 5-HT transporter knockout mice

The aim of the present study was to determine whether alterations in 5-hydroxytryptamine (5-HT)(1A) receptors would be found in knockout mice lacking the serotonin transporter (5-HTT). Hypothermic and neuroendocrine responses to the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) were used to examine the function of 5-HT(1A) receptors. Initial studies evaluated the dose-response and time course of 8-OH-DPAT-induced hypothermia and hormone secretion in normal CD-1 mice (the background strain of the 5-HTT knockout mice). 8-OH-DPAT dose-dependently produced hypothermic responses that peaked at 20 min postinjection. 8-OH-DPAT-induced hypothermia was blocked by the 5-HT(1A) antagonist WAY-100635. 8-OH-DPAT dose-dependently increased the concentrations of plasma oxytocin, corticotropin, and corticosterone. In the 5-HTT knockout (-/-) mice, the hypothermic response to 8-OH-DPAT (0.1 mg/kg s.c.) was completely abolished. Furthermore, 5-HTT-/- mice had significantly attenuated plasma oxytocin and corticosterone responses to 8-OH-DPAT. No significant changes in the hypothermic or hormonal responses to 8-OH-DPAT were observed in heterozygous (5-HTT+/-) mice. [(3)H]8-OH-DPAT- and [(125)I]MPPI [4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-iodobenzamido]ethyl] pip erazine]-binding sites in the hypothalamus and [(125)I]MPPI-binding sites in the dorsal raphe were significantly decreased in 5-HTT-/- mice. The results indicate that lack of the 5-HTT is associated with a functional desensitization of 5-HT(1A) receptor responses to 8-OH-DPAT, which may be a consequence, at least in part, of the decrease in density of 5-HT(1A) receptors in the hypothalamus and dorsal raphe of 5-HTT-/- mice.     

5-hydroxytryptamine (5-HT)1A receptors and the tail-flick response. I. 8-hydroxy-2-(di-n-propylamino) tetralin HBr-induced spontaneous tail-flicks in the rat as an in vivo model of 5-HT1A receptor-mediated activity

This study pharmacologically characterizes a novel behavioral response as a potential in vivo model of serotonin (5-HT)1A receptor-mediated activity. In rats restrained in horizontal cylinders, the selective 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin HBr (8-OH-DPAT), dose-dependently (0.04-10.0 mg/kg s.c.) elicited spontaneous tail-flicks (STFs). This action was mimicked by other ligands possessing high affinity and high efficacy at 5-HT1A sites: RU 24969 [(5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole], lisuride, (+)-lysergic acid diethylamide and 5-methoxy-N,N-dimethyltryptamine hydrogen oxalate. The response could not be elicited by CGS 12066B [7-trifluormethyl-4-(4-methyl-l-piperazonyl)-pyrrolol- [1-2-a] quinoxaline dimaleate], mCPP 1-(3-chlorophenyl)-piperazine-2-HCl, TFMPPm-trifluromethylphenylpiperazine HCl, MK 212 [6-chloro-2-(l-piperzinyl)pyrazine], quipazine and DOI (+-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCl, which act in vivo as agonists at 5-HT1B, 5-HT1C and/or 5-HT2 receptors, or by the 5-HT3 agonist, 2-methyl-5-HT. p-chloroamphetamine, which releases endogenous 5-HT, also evoked STFs; in contrast, d-amphetamine, a preferential releaser of catecholamines, was inactive, as were agonists and antagonists at alpha-1, alpha-2, beta-1, beta-2, dopamine D1 and D2 sites. 8-OH-DPAT-elicited STFs were blocked by the 5-HT1/2 antagonist, methiothepin, but not by the 5-HT1C/5-HT2 antagonists, mianserin, ritanserin and ICI 169,369 [2-(2-dimethylaminoetheylthio)-3-phenylquinoline] nor by the 5-HT3 antagonists, GR 38032F [(1,2,3,9-tetrahydro-9-methyl-3-[(2-methyl-1H-imidazol-l-yl)methyl]-4H- carbazol-4-one HCl], ICS 205,930 [(3 alpha-tropanyl)-1H-indol-3-carboxylic acid ester] and MDL 72222 [(1 alpha H, 3 alpha, 5 alpha H)-tripan-3-yl-3,5- dichlorobenzoate]. beta-Blockers with 5-HT1A affinity i.e., (-)-alprenolol, (+/-)-isamoltane and, stereoselectivity, (-)-but not (+)-pindolol, blocked the action of 8-OH-DPAT. Spiperone and spiroxatrine, D2 antagonists with high 5-HT1A affinity, also inhibited 8-OH-DPAT-induced STFs. Selective beta-blockers and D2 antagonists with low 5-HT1A affinity were inactive. 5-HT1A partial agonists, the pyrimidinylpiperazines, buspirone, gepirone and ipsapirone, the halogenated phenylpiperazine, LY 165,163 [1-(2-(4-aminophenyl) ethyl-4-(3-trifluoromethylphenyl)-piperazine], and the benzodioxane, MDL 72832 [8-(4-(1,4-benzodioxan-2-yl-methylamino)-butyl-8-azaspiro-(4 ,5)-decane- 7,9-dione] did not elicit STFs and antagonized the effect of 8-OH-DPAT.(ABSTRACT TRUNCATED AT 400 WORDS)     

Antidepressant-like behavioral effects in 5-hydroxytryptamine(1A) and 5-hydroxytryptamine(1B) receptor mutant mice

The development of serotonin receptor knockout mice has provided an opportunity to study antidepressant drug effects in animals with targeted genetic deletion of receptors involved in antidepressant responses. In the current study, the effects of two types of antidepressant drugs, the selective serotonin reuptake inhibitors fluoxetine and paroxetine and the selective norepinephrine reuptake inhibitor desipramine, were examined in 5-hydroxytryptamine (5-HT)(1A) and 5-HT(1B) receptor mutant mice using the tail suspension test (TST). Under baseline conditions, the immobility of 5-HT(1A) receptor mutant mice, but not 5-HT(1B) receptor mutant mice, was significantly lower than that of wild-type mice. The decreased baseline immobility in 5-HT(1A) receptor mutant mice was reversed by pretreatment with alpha-methyl-para-tyrosine, but not by para-chlorophenylalanine, suggesting mediation by enhanced catecholamine function. In wild-type mice, fluoxetine (10.0--20.0 mg/kg i.p.) and desipramine (5.0--20.0 mg/kg i.p.) both significantly decreased immobility in the TST. In 5-HT(1A) receptor mutant mice, desipramine (20.0 mg/kg i.p.) significantly decreased immobility, whereas fluoxetine (20.0 mg/kg i.p.) and paroxetine (20.0 mg/kg i.p.) had no effect. The immobility of 5-HT(1B) receptor mutant mice was decreased similarly by desipramine (5.0--20.0 mg/kg i.p.). However, the effect of low doses of fluoxetine were significantly augmented in the 5-HT(1B) receptor mutant mice (2.5--20.0 mg/kg i.p.) compared with wild-type mice. Administration of selective 5-HT receptor antagonists in wild-type mice partially reproduced the phenotypes of the mutant mice. These results suggest that 5-HT(1A) and 5-HT(1B) receptors have different roles in the modulation of the response to antidepressant drugs in the TST.     

The 5-hydroxytryptamine1A receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin, increases cardiac output and renal perfusion in rats subjected to hypovolemic shock

The 5-hydroxytryptamine(1A) receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), raises blood pressure (BP) and venous tone in rats subjected to hemorrhagic shock. Here, BP, ascending aortic blood flow [i.e., estimate of cardiac output (CO)] and venous blood gases were measured to determine the hemodynamic effects of 8-OH-DPAT (30 nmol/kg i.v., n = 10), saline (n = 10), or an equipressor infusion of epinephrine (n = 10) in unanesthetized rats subjected to hemorrhagic shock (25 min of hypotensive hemorrhage, approximately 50 mm Hg). Renal and iliac blood flow were measured in separate groups of similarly hemorrhaged rats given the same dose of 8-OH-DPAT (n = 7) or saline (n = 6). Compared with saline treatment, 8-OH-DPAT produced a sustained rise in BP (+32 +/- 4 versus +9 +/- 2 mm Hg, 15 min after injection, P < 0.01) and CO (+27 +/- 5 versus +4 +/- 6 ml/min/kg, P < 0.01) but did not affect total peripheral resistance (TPR). Infusion of epinephrine reduced CO (-12 +/- 6 ml/min/kg, P < 0.01) and dramatically increased TPR [+0.37 +/- 0.11 versus +0.05 +/- 0.05 log (mm Hg/ml/min/kg), P < 0.01]. 8-OH-DPAT increased renal conductance (+7 +/- 1 versus +4 +/- 1 microl/min/mm Hg, P < 0.01) but did not significantly affect iliac conductance. 8-OH-DPAT attenuated further development of acidosis compared with either saline or epinephrine (-5.6 +/- 1.6 versus -13.0 +/- 2.0 versus -11.3 +/- 2.6 mmol/liter base excess 45 min after start of hemorrhage, both P < 0.01 versus 8-OH-DPAT). These data demonstrate that 8-OH-DPAT improves hemodynamics during circulatory shock, in part, through renal vasodilation and mobilizing of blood stores.     

Regulation of extracellular concentrations of 5-hydroxytryptamine (5-HT) in mouse striatum by 5-HT(1A) and 5-HT(1B) receptors

The ability of selective serotonin (5-HT) receptor agonists to reduce the extracellular concentration of 5-HT was examined in the striatum of awake, unrestrained mice by in vivo microdialysis. Systemic administration of either 8-OH-PIPAT (R-(+)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl)] aminotetralin), a novel 5-HT(1A) receptor agonist, or CP 94,253, a selective 5-HT(1B) receptor agonist, resulted in significant dose-related reductions of striatal 5-HT. The effect of 8-OH-PIPAT (1.0 mg/kg) was blocked by pretreatment with WAY 100635 (0.1 mg/kg), a selective 5-HT(1A) receptor antagonist, but it was not blocked by pretreatment with GR 127935 (0.056 mg/kg), a selective 5-HT(1B/1D) receptor antagonist. The effect of CP 94,253 (1.0 mg/kg) was blocked by pretreatment with GR 127935 (0.056 mg/kg) but was not blocked by pretreatment with WAY 100635 (0.1 mg/kg). Neither WAY 100635 nor GR 127935 altered extracellular 5-HT levels at the doses that were able to completely block the effects of either 8-OH-PIPAT or CP 94,253. The present findings suggest that, on systemic administration, both 8-OH-PIPAT and CP 94,253 are potent and selective agonists at the somatodendritic 5-HT(1A) autoreceptor and terminal 5-HT(1B/1D) autoreceptor, respectively, and are each able to cause decreases in extracellular levels of 5-HT in the mouse striatum by activating a distinct set of receptors.     

Temporal regulation of agonist efficacy at 5-hydroxytryptamine (5-HT)1A and 5-HT 1B receptors

Coactivation of purinergic (P 2Y) receptors reduces agonist efficacy at serotonin 1B (5-HT 1B), but not 5-HT 1A receptors. Herein, we report that pretreatment for 5 min with the P 2Y receptor agonist ATP reduced agonist responsiveness at the 5-HT 1A, but not at the 5-HT 1B, receptor. The effect of ATP pretreatment on the 5-HT 1A receptor response rapidly reversed within a 10 min time frame between P 2Y receptor and 5-HT 1A receptor activation. ATP pretreatment effects on 5-HT 1A agonist responsiveness were blocked by the protein kinase inhibitors staurosporine and bisindolylmaleimide, suggesting that the ATP-mediated temporal regulation involves activation of protein kinase C (PKC). Moreover, the temporal effect of ATP was blocked by incubation with 1% ethanol, suggesting that consequences of phospholipase D (PLD) activation play a role. ATP pretreatment blocked the inhibitory effect produced by 5-HT 2C receptor activation on the 5-HT 1A, but not the 5-HT 1B, receptor response, suggesting that the 5-HT 1A receptor itself was the target for PLD/PKC action. Finally, ethanol did not block the reduction in responsiveness of the 5-HT 1A receptor system produced by activation of PKC with phorbol ester treatment, suggesting that PKC activation lies downstream of PLD. Taken together, these data suggest that activation of P 2Y receptors can reduce responsiveness of the 5-HT 1A receptor system via a PLD/PKC-dependent mechanism that is highly dependent upon the temporal pattern of receptor activation. Moreover, this work underscores the importance of time as a variable in receptor signaling cross talk and serves to further illustrate differences between the 5-HT 1A and 5-HT 1B receptor systems.     

5-HT1A and alpha-2 adrenergic receptors mediate the hyperglycemic and hypoinsulinemic effects of 8-hydroxy-2-(di-n-propylamino)tetralin in the conscious rat

The ability of the 5-hydroxytryptamine (5-HT)1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to affect plasma glucose levels and insulin release was assessed in rats bearing chronic jugular catheters. The i.v. administration of 8-OH-DPAT (150 micrograms/kg) rapidly promoted a transient hyperglycemia. Despite high glucose levels, insulinemia remained constant. Dose-response curves revealed that maximal hyperglycemia was associated with hypoinsulinemia. Increased glycemia, which was also found to be induced by other 5-HT direct and indirect agonists, lasted longer in food-deprived rats. Evidence for a strong inhibitory effect of 8-OH-DPAT on insulin release was reported in rats submitted to i.v. glucose tolerance tests. Pretreatments with the dopaminergic blocker haloperidol, the alpha-1 adrenoceptor antagonist prazosin or the 5-HT2 blocker ketanserin were ineffective. In contrast, the alpha-2 adrenoceptor antagonist idazoxan and the unspecific 5-HT antagonist methiotepin prevented the hyperglycemic and the hypoinsulinemic effects of 8-OH-DPAT. Blockade of these changes by (-)-propranolol (a 5-HT1 blocker), but not by (+)-propranolol, indicated that 5-HT1 and alpha-2 adrenergic receptors mediated 8-OH-DPAT-induced hyperglycemia. Reserpine pretreatment did not prevent the effects of 8-OH-DPAT. Central injection of 8-OH-DPAT induced hyperglycemia, the amplitude of which was equivalent to that measured after i.v. administration. Selective degeneration of serotonergic nerve cells by 5,7-dihydroxytryptamine did not prevent 8-OH-DPAT-induced alterations, thus rendering a key role for presynaptic mechanisms unlikely.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible 5-hydroxytryptamine1 (5-HT1) receptor involvement in the stimulus properties of 1-(m-trifluoromethylphenyl)piperazine (TFMPP)

Male rats (N = 24) were trained to discriminate 1-(m-trifluoromethylphenyl)piperazine (TFMPP) (0.8 mg/kg) from saline in a two-lever, drug discrimination situation. 5-Hydroxytryptamine (5-HT) agonists such as fenfluramine (0.8-1.6 mg/kg), m-chlorophenylpiperazine (0.1-1.6 mg/kg) and RU 24969 (0.1-1.6 mg/kg) mimicked TFMPP; 8-hydroxy-2-(di-n-propylamino)tetralin (0.02-0.32 mg/kg) and quipazine (0.2-3.2 mg/kg) elicited saline lever responding; d-lysergic acid diethylamide (0.1-0.16 mg/kg) produced intermediate results. The 5-HT antagonists BC 105 (1.6-12.8 mg/kg), bromolysergic diethylamide (0.8-1.28 mg/kg), ketanserin (0.8-6.4 mg/kg), Ly 53857 (0.2-1.6 mg/kg) and pirenperone (0.08-0.64 mg/kg) failed to attenuate the TFMPP cue; metergoline (0.4-6.4 mg/kg) and spiperone (0.08-1.28 mg/kg) decreased drug lever responding by as much as 60%. These data suggest that 5-HT agonists are not identical and that drug discrimination procedures can differentiate among them. Given that there is strong evidence to support the existence of heterogeneous 5-HT receptors, the present results also suggest that TFMPP acts through mechanism(s) similar to those of the novel 5-HT1 agonists m-chlorophenylpiperazine and RU 24969; these actions can be differentiated from those underlying d-lysergic acid diethylamide, quipazine and 2,5-dimethoxy-4-methylamphetamine, which are attenuated by putative 5-HT2 antagonists. Thus, the authors propose a role for 5-HT1 receptors in mediating the stimulus effects of TFMPP, although further research is necessary to identify functional antagonists of such systems.     

Dual, hyperalgesic, and analgesic effects of the high-efficacy 5-hydroxytryptamine 1A (5-HT1A) agonist F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl}piperidin-1-yl]methanone, fumaric acid salt]: relationship with 5-HT1A receptor occupancy and kinetic parameters

The aim of the present study was to establish the relationship between the plasma and brain concentration-time profiles of F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl}piperidin-1-yl]methanone, fumaric acid salt] after acute administration and both its hyper- and hypoanalgesic effects in rats. The maximal plasma concentration (C(max)) of F 13640 after i.p. administration of 0.63 mg/kg was obtained at 15 min and decreased to half its maximal value after about 1 h. The amount of F 13640 collected by means of in vivo microdialysis in hippocampal dialysates could be measured reliably after 0.63 and 2.5 mg/kg, reached its maximum at about 1 h, and fell to half of its maximal value at about 3 h. 5-Hydroxytryptamine 1A (5-HT(1A)) receptor occupancy was estimated by ex vivo binding in rat brain sections. F 13640 inhibited [(3)H]8-hydroxy-2-[di-n-propylamino] tetralin binding ex vivo in rat hippocampus, entorhinal cortex, and frontal cortex (ED(50), 0.34 mg/kg i.p.). Maximal inhibition was reached at approximately 30 min after 0.63 mg/kg F 13640 and fell to half of its value after about 4 to 8 h. After injection (15 min) in the paw pressure test, F 13640 (0.63 mg/kg i.p.) induced an initial hyperalgesia that was followed 4 h later by a paradoxical analgesia that lasted until 8 h. In contrast, in the formalin test, F 13640 inhibited pain behaviors until 4 h after drug administration. F 13640 also produced elements of the 5-HT syndrome that lasted up to 4 h after administration. These results demonstrate that F 13640 induces hyperalgesia and/or analgesia with a time course that parallels the occupancy of 5-HT(1A) receptors and the presence of the compound in blood and brain.     

The effects of the 5-hydroxytryptamine(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin on spontaneous activity, cocaine-induced hyperactivity and behavioral sensitization: a microanalysis of locomotor activity

The influence of the 5-hydroxytryptamine(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT) on locomotor hyperactivity induced by the acute and chronic administration of cocaine was assessed. Horizontal activity was measured in the periphery and center of an open field test enclosure equipped with photobeams; vertical activity was also recorded. Peripheral hyperactivity induced by an acute administration of cocaine (10 or 20 mg/kg) was significantly enhanced by 0.2 mg/kg DPAT. In contrast, central and vertical activities were reduced in a dose-related manner by DPAT (0.1 and 0.2 mg/kg); DPAT also suppressed central (0.2 mg/kg) and vertical (0.1 and 0.2 mg/kg) activities when administered alone. Similar observations were made on day 1 of chronic treatment with DPAT (0, 0.1, or 0.2 mg/kg) injected 15 min before an injection of cocaine (0, 10, or 15 mg/kg) administered twice daily for 7 days. By day 7 of repeated DPAT treatment, sensitization of DPAT-evoked peripheral activity developed, which contrasted with tolerance to the central and vertical hypoactivity evoked by DPAT. Sensitization developed to the repeated treatment with 15 mg/kg cocaine but not 10 mg/kg cocaine. Interestingly, enhancements of all activity measures were observed between days 1 and 7 in rats cotreated with DPAT plus either dose of cocaine. This sensitization to DPAT plus cocaine was expressed on challenge with DPAT and cocaine but not with cocaine alone. The present study implies that the stimulation of 5-hydroxytryptamine(1A) receptors is capable of modulating the hyperactivity evoked by cocaine, possibly via modulation of the mesoaccumbens dopamine circuit thought to mediate the behavioral effects of cocaine.     

Emesis and defecations induced by the 5-hydroxytryptamine (5-HT3) receptor antagonist zacopride in the ferret

Three antiemetic compounds (zacopride, batanopride, granisetron [BRL43694]) were evaluated for the production of gastrointestinal side effects in the conscious ferret after i.v. or p.o. administration. Zacopride evoked multiple emetic and defecatory responses at clinically relevant doses (0.003-0.3 mg/kg) and in a dose-dependent manner. The oral route was the more potent one for eliciting emesis (ED50 0.033 mg/kg). At 0.3 mg/kg p.o., zacopride reliably evoked an 80 to 100% incidence of emesis and a 40 to 80% incidence of defecation. In contrast, batanopride and BRL43694 i.v. evoked a small (10%) incidence of these side effects, but only at 0.1 to 10 mg/kg doses. When given p.o. (0.00003-10 mg/kg), these latter compounds never evoked emesis and significantly reduced (P less than .05) the incidence of defecation below that of vehicle. Responses to zacopride (0.3 mg/kg p.o.) were challenged by i.p. pretreatment with the 5-hydroxytryptamine receptor agonist 2-methyl serotonin, the 5-hydroxytryptamine receptor antagonist BRL43694, the quaternary atropine derivative glycopyrrolate, the dopamine receptor antagonist domperidone or selective abdominal vagotomies. All compounds and either bilateral or dorsal vagotomy significantly reduced the incidence of emesis, but did not abolish it. Latency to first emesis was delayed by BRL43694, domperidone or dorsal vagotomy. The data suggest that the emetic response to p.o. zacopride is mediated in part by 5-hydroxytryptamine receptors residing on either enteric neurons or vagal afferents. However, the underlying pharmacology of the response may also include activation of cholinergic and dopaminergic pathways.     

The serotonin 5-Hydroxytryptaphan1A receptor agonist, (+)8-hydroxy-2-(di-n-propylamino)-tetralin, stimulates sympathetic-dependent increases in venous tone during hypovolemic shock

Adjuvant treatment of hypovolemic shock with vasoconstrictors is controversial due to their propensity to raise arterial resistance and exacerbate ischemia. A more advantageous therapeutic approach would use agents that also promote venoconstriction to augment perfusion pressure through increased venous return. Recent studies indicate that 5-hydroxytryptophan (5-HT)(1A) receptor agonists increase blood pressure by stimulating sympathetic drive when administered after acute hypotensive hemorrhage. Given that venous tone is highly dependent upon sympathetic activation of alpha(2)-adrenergic receptors, we hypothesized that the 5-HT(1A) receptor agonist, (+)8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), would increase venous tone in rats subject to hypovolemic shock through sympathetic activation of alpha(2)-adrenergic receptors. Systemic administration of 8-OH-DPAT produced a sustained rise in blood pressure (+44 +/- 3 mm Hg 35 min after injection, P < 0.01 versus saline) and mean circulatory filling pressure (+4.2 +/- 0.7 mm Hg, P < 0.01 versus saline) in conscious rats subjected to hypovolemic shock. An equipressor infusion of epinephrine failed to influence mean circulatory filling pressure (MCFP). Ganglionic blockade, alpha(1)-, or peripheral alpha(2)-adrenergic receptor blockade prevented the rise in MCFP observed with 8-OH-DPAT, but only alpha(1)-adrenergic receptor blockade diminished the pressor effect of the drug (P < 0.01). 8-OH-DPAT raises blood pressure in rats in hypovolemic shock through both direct vascular activation and sympathetic activation of alpha(1)-adrenergic receptors. The sympathoexcitatory effect of 8-OH-DPAT contributes to elevated venous tone through concurrent activation of both alpha(1)- and alpha(2)-adrenergic receptors. The data suggest that 5-HT(1A) receptor agonists may provide an advantageous alternative to currently therapeutic interventions used to raise perfusion pressure in hypovolemic shock.     

Binding characteristics of the gamma-hydroxybutyric acid receptor antagonist [(3)H](2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid in the rat brain.

Radioligand binding studies with [(3)H](2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene) ethanoic acid ([(3)H]NCS-382), an antagonist of gamma-hydroxybutyric acid (GHB) receptor, revealed specific binding sites in the rat cerebral cortex and hippocampus. However, there was very little binding in the rat cerebellum, heart, kidney, liver, and lung membranes. Binding was rapid and reached equilibrium in about 5 min. Scatchard analysis of saturation isotherms revealed two different populations of binding sites in the rat cerebral cortex (K(d1), 795 nM, B(max1), 25.4 pmol/mg of protein; K(d2), 21 microM; B(max2), 178 pmol/mg of protein) as well as in the rat hippocampus (K(d1), 441 nM; B(max1), 16.2 pmol/mg of protein; K(d2), 9.8 microM; B(max2), 255 pmol/mg of protein). (+/-)Baclofen (500 microM) and gamma-aminobutyric acid (100 microM) inhibited the binding only partially, whereas (+)bicuculline, muscimol, picrotoxinin, and phaclofen did not modify the binding. Interestingly, potassium chloride (100-300 mM) inhibited [(3)H]NCS-382 binding (34-56%), and this inhibitory effect was not affected by picrotoxinin. GHB and NCS-382 completely inhibited the [(3)H]NCS-382 (16 nM) binding in the rat cerebrocortical and hippocampal membranes, and NCS-382 was found to be about 10 times more potent than GHB in this regard. A variety of ligands for other receptors did not modify the [(3)H]NCS-382 binding, thereby suggesting selectivity of this radioligand for the GHB receptor sites in the brain. Based on these observations, [(3)H]NCS-382 seems to be a better radioligand than [(3)H]GHB for investigating the role of the GHB receptors in various pharmacological actions.     

Effects of serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibition plus 5-HT(2A) receptor antagonism on the firing activity of norepinephrine neurons

YM992 [(S)-2-[[(7-fluoro-4-indanyl)oxy]methyl]morpholine monohydrochloride] is a selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) and a potent 5-HT(2A) antagonist. The aim of the present study was to assess, using in vivo extracellular unitary recordings, the effect of acute and sustained administration of YM992 (40 mg kg(-1) day(-1) s.c., using osmotic minipumps) on the spontaneous firing activity of locus coeruleus (LC) norepinephrine (NE) neurons. Acute intravenous injection of YM992 (4 mg kg(-1)) significantly decreased NE neuron firing activity by 29% and blocked the inhibitory effect of a subsequent injection of the 5-HT(2) agonist DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride]. A 2-day treatment with YM992 decreased the firing rate of NE neurons by 66%, whereas a partial recovery was observed after a 7-day treatment and a complete one after a 21-day treatment. Following the injection of the alpha(2)-adrenoceptor antagonist idazoxan (1 mg kg(-1) i.v.), NE neuron firing was equalized in controls and 2-day YM992-treated rats. This put into evidence an increased degree of activation of alpha(2)-adrenergic autoreceptors in the treated rats. The suppressant effect of the alpha(2)-adrenoceptor agonist clonidine was significantly decreased in long-term YM992-treated rats. The recovery of LC firing activity after long-term YM992 administration could thus be explained by a decreased sensitivity of alpha(2)-adrenergic autoreceptors. Sustained SSRI administration leads to a gradual reduction of the firing activity of NE neurons during long-term administration, whereas YM992 produced opposite effects. The exact basis for the increased synaptic availability of NE by YM992 remains to be elucidated. This NE activity, resulting from 5-HT reuptake inhibition plus 5-HT(2A) receptor antagonism, might confer additional benefits in affective and anxiety disorders.     

Temperature-dependent 5-hydroxytryptamine (5-HT)-sensitive [3H]spiperone binding to rat cortical membranes: regulation by guanine nucleotide and antidepressant treatment

5-Hydroxytryptamine (5-HT)-sensitive binding of [3H]spiperone to rat cortical membranes was examined at 22 degrees C and 37 degrees C. At the lower temperature a significantly higher density of specific sites with identical affinity for [3H]spiperone (0.15 nM) was observed. Furthermore, a proportion of sites at 22 degrees C, although not at 37 degrees C, possessed high affinity for 5-HT and these were apparently converted to low affinity sites by GTP (100 microM). Examination of the apparent affinities of the 5-HT derivatives 5-methoxytryptamine, 5,6-dihydroxytryptamine and tryptamine revealed differences in the potencies of these analogs that were particularly evident for the high affinity proportion of sites observed at 22 degrees C. Subchronic treatment of rats with the antidepressants mianserin and iprindole reduced the density of [3H] spiperone binding sites when assays were performed at either temperature. However, antidepressant treatment appeared to preferentially reduce the number of sites that possess high affinity for 5-HT.     

Regional patterns of compensation following genetic deletion of either 5-hydroxytryptamine(1A) or 5-hydroxytryptamine(1B) receptor in the mouse

Plasticity in serotonergic transmission in serotonin or 5-hydroxytryptamine (5-HT) receptor mutants was examined by measuring the regulation of extracellular 5-HT levels in the striatum and ventral hippocampus of 5-HT(1A) and 5-HT(1B) receptor knockout mice using in vivo microdialysis. The efficacy of genetic deletion was verified by showing blunted regulation of extracellular 5-HT with selective 5-HT receptor agonists. 5-HT(1A) receptor knockout mice failed to demonstrate reduction of extracellular 5-HT in response to systemic administration of the 5-HT(1A) receptor agonist R-8-hydroxydipropylaminotetralin (R-8-OH-DPAT) and 5-HT(1B) receptor knockout mice failed to demonstrate reduction of extracellular 5-HT in response to systemic administration of the 5-HT(1B) receptor agonist CP 94,253. Plasticity also developed to deletion of the complementary autoreceptor. 5-HT(1A) receptor knockout mice demonstrated a significantly greater response to CP 94,253 in the striatum, but not the ventral hippocampus, suggesting the development of enhanced sensitivity of striatal 5-HT(1B) receptors. In 5-HT(1B) receptor knockout mice, R-8-OH-DPAT evoked a significantly diminished response in the ventral hippocampus, but not the striatum, suggesting the potential desensitization of 5-HT(1A) receptors in the median raphe nucleus. The pattern of regional compensations between somatodendritic and terminal autoreceptors was confirmed by pharmacological challenges using the selective serotonin reuptake inhibitor fluoxetine combined with either a 5-HT(1A) (WAY 100635) or a 5-HT(1B/1D) (GR 127935) receptor antagonist. The regional pattern of compensation may be determined by the preferential role of 5-HT(1A) or 5-HT(1B) receptors in regulating 5-HT release. Taken together, these results demonstrate the development of regional plasticity between complementary somatodendritic and terminal autoreceptors after the genetic deletion of 5-HT(1A) or 5-HT(1B) receptors.     

Attenuation of scopolamine-induced and age-associated memory impairments by the sigma and 5-hydroxytryptamine(1A) receptor agonist OPC-14523 (1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2[1H]-quinolinone monomethanesulfonate)

Sigma and 5-HT(1A) receptor stimulation can increase acetylcholine (ACh) release in the brain. Because ACh release facilitates learning and memory, we evaluated the degree to which OPC-14523 (1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2[1H]-quinolinone monomethane sulfonate), a novel sigma and 5-HT(1A) receptor agonist, can augment ACh release and improve learning impairments in rats due to cholinergic- or age-related deficits. Single oral administration of OPC-14523 improved scopolamine-induced learning impairments in the passive-avoidance task and memory impairment in the Morris water maze. The chronic oral administration of OPC-14523 attenuated age-associated impairments of learning acquisition in the water maze and in the conditioned active-avoidance response test. OPC-14523 did not alter basal locomotion or inhibit acetylcholinesterase (AChE) activity at concentrations up to 100 microM and, unlike AChE inhibitors, did not cause peripheral cholinomimetic responses. ACh release in the dorsal hippocampus of freely moving rats increased after oral delivery of OPC-14523 and after local delivery of OPC-14523 into the hippocampus. The increases in hippocampal ACh release were blocked by the sigma receptor antagonist NE-100 (N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)-phenyl]-ethylamine). Thus, OPC-14523 improves scopolamine-induced and age-associated learning and memory impairments by enhancing ACh release, due to a stimulation of sigma and probably 5-HT(1A) receptors. Combined sigma/5-HT(1A) receptor agonism may be a novel approach to ameliorate cognitive disorders associated with age-associated cholinergic deficits.     

The time-course for up- and down-regulation of the cortical 5-hydroxytryptamine (5-HT)2A receptor density predicts 5-HT2A receptor-mediated behavior in the rabbit

5-Hydroxytryptamine (serotonin; 5-HT)2 receptor agonists such as (+/-)-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) injected systemically or directly into frontal cortex, elicit stereotyped head movements that are mediated by 5-HT2A receptors. Chronic administration of 5-HT2A receptor antagonists can produce either a down-regulation, e.g., d-2-bromolysergic acid diethylamide (BOL) or an up-regulation, e.g., alpha-phenyl-10(2-phenylethyl)-4-piperidinemethanol (MDL11,939) of cortical 5-HT2A receptors in the rabbit with no change in the density of the 5-HT2C receptor. We examined the degree to which the time course for the onset and offset of changes in cortical 5-HT2A receptor density was correlated with functional changes as measured by the magnitude of DOI elicited, 5-HT2A receptor-mediated head movements (head bobs). First, the magnitude of DOI-elicited head bobs was measured over 1 to 8 days after chronic BOL (5.8 micromol/kg), MDL11,939 (10 micromol/kg), or vehicle administration. Second, rabbits were injected with BOL, MDL11,939, or vehicle once daily for 8 days, and then, 1 to 8 days after the cessation of drug or vehicle, DOI-elicited head bobs were determined. Samples of frontal cortex were obtained for each animal immediately following behavioral testing, and 5-HT2A receptor density was measured using [3H]ketanserin. Thus, each animal provided a value for receptor density and number of head bobs, and these two measures showed a high degree of correlation between 0.94 for BOL and 0.95 for MDL11,939. This study establishes that the density of 5-HT2A receptors in cortex reflects their functional status.     

Genetic regulation of extracellular serotonin by 5-hydroxytryptamine(1A) and 5-hydroxytryptamine(1B) autoreceptors in different brain regions of the mouse

The regulation of extracellular levels of 5-hydroxytryptamine (serotonin) (5-HT) in the striatum and ventral hippocampus was studied using in vivo microdialysis in awake, unrestrained wild-type 5-HT(1A) and 5-HT(1B) receptor knockout mice. Systemic administration of the selective serotonin reuptake inhibitor fluoxetine evoked a significant dose-dependent increase in extracellular 5-HT in both the striatum and hippocampus at both 2.5 mg/kg (i.p.) and 20 mg/kg (i.p.) in wild-type mice. In 5-HT(1A) receptor knockout mice, the response to 2.5 mg/kg fluoxetine was significantly augmented in the striatum but not the hippocampus, whereas the response to 20 mg/kg fluoxetine was significantly greater in both brain regions. In 5-HT(1B) receptor knockout mice, the increase of extracellular 5-HT was augmented in the hippocampus but not the striatum at both doses of fluoxetine. The response pattern to fluoxetine alone in 5-HT receptor mutant mice corresponded with the effects of fluoxetine given with either the 5-HT(1A) receptor antagonist WAY 100635 (0.1 mg/kg i.p.) or the 5-HT(1B/1D) receptor antagonist GR 127935 (0.056 mg/kg) in wild-type mice. These results indicate common topographical regulation of 5-HT release in different brain regions by genetic mutation and pharmacological challenges. The 5-HT(1A) autoreceptor plays a larger role in regulating 5-HT release in the striatum and possibly other brain regions innervated by the dorsal raphe nucleus, whereas the role of the 5-HT(1B) receptor is relatively greater in the hippocampus and possibly other brain regions innervated by the median raphe nucleus.     

The 5-hydroxytryptamine (5-HT2) receptor stimulates inositol phosphate formation in intact and broken WRK1 cells: determination of occupancy-response relationships for 5-HT agonists

5-Hydroxytryptamine (5-HT) stimulates the accumulation of inositol-trisphosphate in WRK1 cells, a cell line originating from a rat mammary tumor. 5-HT acts via a single receptor type for which it has an affinity constant estimated to be 1.27 microM. A series of agonists known to act at 5-HT2 receptors are partial agonists in this system and have a rank order of relative intrinsic efficacies corresponding to that seen in other systems possessing 5-HT2 receptors. There is an essentially linear occupancy-response relationship for 5-HT and other agonists indicating the absence of a strong amplification mechanism between receptor activation and inositol phosphate formation. The selective blockade of the 5-HT response by nanomolar concentrations of 5-HT2 selective antagonists but not by drugs acting at other 5-HT receptor subtypes suggest that the receptor in WRK1 cells is of the 5-HT2 type. Additionally, we demonstrate that in WRK1 membranes 5-HT acts via the 5-HT2 receptor to elicit a GTP dependent increase in the production of inositol-bisphosphate and inositol-trisphosphate. These properties of the WRK1 cell line indicate that it is a useful model with which to study the nature of 5-HT receptor coupling to the putative second messenger(s), the inositol phosphates.