D2-like receptors mediate the expulsion phase of ejaculation elicited by 8-hydroxy-2-(di-N-propylamino)tetralin in rats

The mechanism of action by which 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) facilitates ejaculation in conscious rats is not clearly established. The serotonin (5-HT) 1A agonist 8-OH-DPAT may actually act on cerebral dopaminergic receptors to exert its proejaculatory effect. The present work was undertaken to clarify this issue by testing various compounds i.c.v. delivered in an experimental model of the expulsion phase of ejaculation in anesthetized Wistar rats. Intracerebroventricular delivery of 8-OH-DPAT dose-dependently (ED(50) = 17 microg) induced rhythmic contractions of bulbospongiosus (BS) muscles, which are of paramount importance for the expulsion of semen, occurring in the form of cluster of bursts evidenced by the recording of BS muscle electrical activity. The 5-HT1A antagonist WAY100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide) (20 microg) i.c.v. coadministered with 8-OH-DPAT (20 microg) was unable to inhibit the effect of 8-OH-DPAT on BS muscle contractile activity. Conversely, raclopride (40 microg) and spiperone (10 microg), both dopamine D2-like receptor antagonists, i.c.v. coinjected with 8-OH-DPAT (20 microg), abolished BS muscle contractions. The involvement of D2-like receptors was further supported by the fact that the D2-like agonist quinelorane (20 microg i.c.v.) also induced BS muscle rhythmic contractions. Our data demonstrate that D2-like receptors mediate the induction by 8-OH-DPAT of rhythmic BS muscle contractions and suggest that i.c.v. delivery of D2-like receptor agonists to anesthetized rats represents a relevant experimental model to study the expulsion phase of ejaculation.     

5-hydroxytryptamine (HT)1A receptors and the tail-flick response. II. High efficacy 5-HT1A agonists attenuate morphine-induced antinociception in mice in a competitive-like manner.

This study examined the influence of s.c. administration of 5-hydroxytryptamine (HT)1A agonists upon the antinociceptive action of s.c. injected morphine in tail-flick tests to noxious heat and pressure. The selective 5-HT1A agonist, (+-)-8-hydroxy-diprolaminotetralin HBr (8-OH-DPAT), dose-dependently antagonized morphine-induced antinociception (MIA) without affecting the latency to respond when applied alone. In the presence of increasing doses of 8-OH-DPAT (0.16-0.63 mg/kg), the morphine dose-response curve was shifted progressively in parallel to the right and the maximal effect of morphine was not altered; Schild analysis yielded a slope of close to -1.0. 8-OH-DPAT both prevented and reversed the action of morphine. The action of 8-OH-DPAT was reversible (at 24 hr). In distinction, 8-OH-DPAT neither blocked morphine-induced Straub tail nor precipitated withdrawal in morphine-dependent animals; thus, it lacked opioid-antagonist properties. The antagonism of MIA by 8-OH-DPAT was mimicked by additional drugs acting as high efficacy 5-HT1A agonists: lisuride, 5-methoxy-N,N-dimethyltryptamine hydrogen oxalate, RU 24969 [methoxy-3-(1,2,3.6-tetrahydropyridin-4-yl)-1H-indole] and d-lysergic acid diethylamide. In contrast, the 5-HT1B/1C agonist, TFMPP m-trifluromethylphenylpiperazine HCl, and the 5-HT1C/2 agonist, DOI (+-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCI, were ineffective. The putative selective 5-HT1A antagonists, BMY 7378 [(8-[-[4-(2-,ethoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4]- decane-7,9-dione-2-HCL] and spiperone, did not reduce MIA. Indeed, BMY 7378 blocked the ability of 8-OH-DPAT to antagonize MIA. Under the present conditions, agonists and antagonists at adrenergic and dopaminergic receptors did not attenuate MIA. These data show that, over a certain range of doses, the systemic administration of 8-OH-DPAT and other high efficacy 5-HT1A agonists functionally antagonizes the antinociceptive action of systemically applied morphine in a competitive-like manner. It is suggested that 5-HT1A receptors play an important role in the modulation of opioidergic antinociceptive mechanisms.     

5-Hydroxytryptamine 1A receptors in the paraventricular nucleus of the hypothalamus mediate oxytocin and adrenocorticotropin hormone release and some behavioral components of the serotonin syndrome

Neuroendocrine responses to administration of serotonin releasing agents or 5-hydroxytryptamine (5-HT) 1A receptor agonists have been used as an index of serotonin receptor function in patients with depression and other mood disorders. However, the receptor population that mediates these responses has not been clearly identified. We tested the hypothesis that 5-HT1A receptors in the paraventricular nucleus of the hypothalamus (PVN) mediate the release of adrenocorticotropin hormone (ACTH) and oxytocin after administration of a selective 5-HT1A agonist in conscious rats. Low-dose infusion (1 nmol/100 nl/side) of the selective 5-HT1A antagonist, WAY100635 (WAY; [O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl) ethyl)-N-(2-pyridinyl)cyclohexanecarboxamidetrihydrochloride), into the PVN blocked the rise in ACTH and oxytocin stimulated by low-dose (30 nmol/kg) i.v. administration of the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 274 +/- 53 versus 70 +/- 20 pg/ml, P < 0.01 for ACTH and 10.7 +/- 3.4 versus 4.6 +/- 0.7 pg/ml, P < 0.05 for oxytocin after saline or WAY pretreatment, respectively). WAY did not influence the bradycardic effect of 8-OH-DPAT (-56 +/- 7 versus -54 +/- 6 beats per minute after saline or WAY). 8-OH-DPAT treatment also elicited locomotor activation followed by hind limb abduction and flat body posture. Surprisingly, WAY attenuated some aspects of locomotor activation and reduced the duration of hind limb abduction elicited by the agonist (5.1 +/- 0.9 versus 0.3 +/- 0.3 min for saline- or WAY-treated rats). These data indicate that 5-HT1A receptor stimulation in the PVN mediates the characteristic neuroendocrine response to serotonin agonist challenge. Moreover, they provide the first evidence that aspects of the behavioral serotonin syndrome are mediated by forebrain hypothalamic receptors.     

Desensitization of 5-HT1A receptors by 5-HT2A receptors in neuroendocrine neurons in vivo

An imbalance between serotonin-2A (5-HT2A) and 5-HT1A receptors may underlie several mood disorders. The present studies determined whether 5-HT2A receptors interact with 5-HT1A receptors in the rat hypothalamic paraventricular nucleus (PVN). The sensitivity of the hypothalamic 5-HT1A receptors was measured as oxytocin and adrenocorticotropic hormone (ACTH) responses to the 5-HT1A receptor agonist (+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide [(+)8-OH-DPAT] (40 microg/kg s.c.). The 5-HT(2A/2C) receptor agonist (-)DOI [(-)-1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane HCl] (1 mg/kg s.c.) injected 2 h prior to (+)8-OH-DPAT significantly reduced the oxytocin and ACTH responses to (+)8-OH-DPAT, producing a heterologous desensitization of the 5-HT1A receptors. Microinjection of the 5-HT2A receptor antagonist MDL100,907 [(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol; 0, 10, or 20 nmol, 15 min prior to (-)DOI] into the PVN dose-dependently prevented the desensitization of 5-HT1A receptors induced by the 5-HT2A receptor agonist (-)DOI. Double-label immunocytochemistry revealed a high degree of colocalization of 5-HT1A and 5-HT2A receptors in the oxytocin and corticotropin-releasing factor neurons of the PVN. Thus, activation of 5-HT2A receptors in the PVN may directly induce a heterologous desensitization of 5-HT1A receptors within individual neuroendocrine cells. These findings may provide insight into the long-term adaptation of 5-HT1A receptor signaling after changes in function of 5-HT2A receptors; for example, during pharmacotherapy of mood disorders.     

Central nervous actions of serotonin and a serotonin1A receptor agonist: cardiovascular excitation at low doses.

Studies were performed in conscious unrestrained rats to compare the cardiovascular effects of i.c.v. administration of serotonin (5-HT) and the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Low doses (less than 10 nmol) of 5-HT and 8-OH-DPAT elicited significant elevations of arterial pressure and heart rate. At doses greater than 10 nmol, 5-HT produced pressor responses attended by significant reductions of heart rate whereas 8-OH-DPAT produced significant depressor and bradycardiac responses. Concurrent i.c.v. administration of the nonselective 5-HT receptor antagonist, methiothepin, abolished the cardiovascular responses to a low (1 nmol) and a high (100 nmol) dose of 5-HT and a low (3 nmol) dose of 8-OH-DPAT. In contrast, methiothepin cotreatment did not prevent the reductions of arterial pressure and heart rate induced by a high (100 nmol) dose of 8-OH-DPAT. These results suggest that stimulation of central nervous system 5-HT1A receptors with low doses of 8-OH-DPAT produces cardiovascular activation.     

Inhibition of bladder activity by 5-hydroxytryptamine1 serotonin receptor agonists in cats with chronic spinal cord injury

The serotonin (5-hydroxytryptamine1A) 5-HT1A receptor agonist 8-OH-DPAT [(R)- (+)-8-hydroxy-2-(di-n-propylamino)tetralin] inhibits bladder activity under nociceptive but not innocuous conditions in cats with an intact spinal cord, suggestive of an effect on primary afferent C fibers or their targets. Because C fibers play a key role in reflex micturition in chronic spinal cord injury (SCI), we investigated the effect of 8-OH-DPAT on micturition in SCI cats. We also investigated GR-46611 (3-[3-(2-dimethylaminoethyl)-1H-indol-5-yl]-N-(4-methoxybenzyl)acrylamide), which has agonist activity predominantly at 5-HT1B and 5-HT1D receptors but also at the 5-HT1A receptor. Chloralose-anesthetized cats were catheterized through the bladder dome for saline-filling cystometry. Dose-response curves for i.v. 8-OH-DPAT (0.3-30 microg/kg) and GR-46611 (0.03-300 microg/kg) were followed in three cases each by 5-HT1A antagonist WAY-100635 [N-tert-butyl-3-(4-(2-methoxyphenyl)-piperazin-1-yl)-2-phenylpropanamide] at 300 microg/kg. Threshold volume, capacity, residual volume, micturition volume, and arterial pressure were measured. Intact cats showed few significant changes in cystometric variables. SCI cats responded to both 8-OH-DPAT and GR-46611 with dose-dependent increases in threshold volume, capacity, and residual volume, significant at > or =10 microg/kg for 8-OH-DPAT and at > or =3 microg/kg for GR-46611. Effects of 8-OH-DPAT but not GR-46611 were largely reversed by WAY-100635. Both 5-HT1A and 5-HT1B/1D agonists may offer a promising means of reducing bladder hyperactivity and increasing bladder capacity in patients with chronic SCI.     

5-Hydroxytryptamine(1A) receptor-stimulated [(35)S]GTPgammaS binding in rat brain: absence of regional differences in coupling efficiency

In hippocampal membranes, the selective 5-hydroxytryptamine (5-HT(1A)) receptor agonists 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and N,N-dipropyl-5-carboxamidotryptamine (N,N-DP-5-CT) stimulated guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPgammaS) binding by 130 to 140%; binding stimulated by nonselective agonists (5-HT and 5-CT) was approximately 30% greater. However, the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohex anecarboxamide (WAY100,635) completely abolished the increases produced by 8-OH-DPAT and N,N-DP-5-CT but only eliminated 70% of that elicited by 5-CT. The rank potency order of the tested agonists was identical with their rank order of affinity for 5-HT(1A) receptors [5-CT congruent with N,N-DP-5-CT > R-(+)-8-OH-DPAT > 5-HT > ipsapirone]. Racemic 8-OH-DPAT and the partial agonist ipsapirone exhibited lower intrinsic activity than R-(+)-8-OH-DPAT. R-(+)-8-OH-DPAT also stimulated [(35)S]GTPgammaS binding in cortex, but not in striatum, which lacks 5-HT(1A) receptors. Partial irreversible inactivation of 5-HT(1A) receptors, in vitro with phenoxybenzamine (0.3 or 1 microM) or in vivo with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (1 mg/kg), reduced the maximal response produced by R-(+)-8-OH-DPAT but did not alter its EC(50). In autoradiographic sections, R-(+)-8-OH-DPAT stimulated [(35)S]GTPgammaS binding in 5-HT(1A) receptor-rich regions (dorsal hippocampus, 123%; lateral septum, 111%; midhippocampus, 110%; dorsal raphe nucleus, 83%; medial prefrontal cortex, approximately 60%). The EC(50) of R-(+)-8-OH-DPAT did not vary significantly among brain regions (46-96 nM). Partial irreversible blockade of 5-HT(1A) receptors in brain sections (phenoxybenzamine, 10 microM) reduced the maximal response without altering the EC(50) in both the hippocampus and dorsal raphe. Despite prior evidence that dorsal raphe somatodendritic 5-HT(1A) autoreceptors exhibit high receptor/effector coupling efficiency (receptor reserve) compared with postsynaptic receptors in hippocampus, there was no evidence of a difference at the level of receptor/G protein coupling.     

Selective antiaggressive effects of alnespirone in resident-intruder test are mediated via 5-hydroxytryptamine1A receptors: A comparative pharmacological study with 8-hydroxy-2-dipropylaminotetralin, ipsapirone, buspirone, eltoprazine, and WAY-100635

The present study characterized the effects of the novel, selective, and potent 5-hydroxytryptamine1A (serotonin) (5-HT1A) receptor agonist, alnespirone [S-20499, (S)-N-4-[5-methoxychroman-3-yl)propylamino)butyl- 8-azaspiro-(4,5)-diacetamide, hydrochloride] on offensive and defensive resident-intruder aggression in wild-type rats and compared its actions with those of the prototypical full 5-HT1A agonist 8-hydroxy-2- dipropylaminotetralin (8-OH-DPAT), the partial 5-HT1A agonists ipsapirone and buspirone, and the mixed 5-HT1A/1B agonist eltoprazine. All five agonists exerted effective dose-dependent decreases of offensive aggressive behavior in resident rats; 8-OH-DPAT was the most potent (ID50 = 0.074 mg/kg), followed by eltoprazine (0.24), buspirone (0.72), ipsapirone (1.08), and alnespirone (1.24). However, in terms of selectivity of the antiaggressive effects as determined by the absence of decrements in social interest and general motor activity, alnespirone appeared to be superior. In the defensive aggression test, neither alnespirone nor any of the other four agonists changed defensive behaviors in the intruder rats. The involvement of 5-HT1A receptors in the antiaggressive actions of these drugs was confirmed by showing that the selective 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2- pyridinyl)cyclohexanecarboxamide trihydrochloride), which was inactive alone, fully prevented the antiaggressive effects of alnespirone, 8-OH-DPAT, and buspirone and partly reversed those of ipsapirone and eltoprazine. The data clearly indicate that alnespirone effectively suppresses offensive aggression with an advantageous profile of action compared with other full or partial 5-HT1A agonists. These selective antiaggressive actions of alnespirone are mediated by stimulating 5-HT1A receptors, presumably the somatodendritic autoreceptors at the raphe nuclei. Furthermore, the data provide evidence for a major involvement of these 5-HT1A receptors in the modulation of aggressive behavior by 8-OH-DPAT, ipsapirone, buspirone, and eltoprazine.     

Single exposure to a serotonin 1A receptor agonist, (+)8-hydroxy-2-(di-n-propylamino)-tetralin, produces a prolonged heterologous desensitization of serotonin 2A receptors in neuroendocrine neurons in vivo

We previously demonstrated colocalization of serotonin 1A (5-HT(1A)) and serotonin 2A (5-HT(2A)) receptors in oxytocin and corticotropin-releasing factor neurons in the hypothalamic paraventricular nucleus (PVN). Because a functional imbalance between hypothalamic 5-HT(1A) and 5-HT(2A) receptors has been implicated in several neuropsychiatric disorders, in this study we investigated whether acute in vivo activation of 5-HT(1A) receptors in the PVN results in desensitization of 5-HT(2A) receptor signaling. Functional desensitization of hypothalamic 5-HT(2A) receptors was assessed via a reduction in oxytocin and adrenocorticotropin (ACTH) responses to the 5-HT(2A/2C) receptor agonist (-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl [(-)DOI]. We report here that a single systemic injection of the 5-HT(1A) receptor agonist (+)-8-hydroxy-2-(di-n-propylamino)-tetralin [(+)8-OH-DPAT] (200 microg/kg) significantly reduced the 5-HT(2A) receptor-mediated oxytocin responses for at least 72 h. Direct intraparaventricular injection of (+)8-OH-DPAT (0.2 nmol) 24 h before a submaximal dose of (-)DOI (0.35 mg/kg) significantly inhibited the 5-HT(2A) receptor-mediated increases in both oxytocin and ACTH (-39 and -16%, respectively). In addition, the (+)8-OH-DPAT-induced desensitization of the 5-HT(2A) receptor-mediated oxytocin but not the ACTH response was inhibited in rats pretreated with either a systemic (0.1 mg/kg) or intraparaventricular (10 nmol) injection of the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY100635). This is the first in vivo demonstration of a prolonged heterologous intracellular desensitization of 5-HT(2A) receptors after acute activation of 5-HT(1A) receptors. These findings may provide insight into the long-term heterologous interactions between 5-HT(1A) and 5-HT(2A) receptor signaling that could occur in response to antidepressants, antipsychotics, or drugs of abuse that target these receptor subtypes.     

Involvement of 5-hydroxytryptamine1A receptors in the modulation of micturition reflexes in the anesthetized rat.

Intravenous administration of the selective 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-N-propylaminotetralin (8-OH-DPAT) and of a low doses of buspirone elicited the supraspinal micturition reflex (SMR) in urethane-anesthetized rats when the urinary bladder was filled with just a subthreshold volume of saline (threshold conditions). The effect of i.v. 8-OH-DPAT was abolished by hexamethonium or spiroxatrine. When SMR was elicited by bladder distension (suprathreshold conditions), i.v. 8-OH-DPAT increased the frequency of bladder contractions. In threshold conditions, stimulation of SMR was also induced by i.c.v. or by i.t. administration of 8-OH-DPAT and 5-HT but not by topical application of 8-OH-DPAT onto the bladder. Guanethidine pretreatment, which produced detrusor hyperreflexia, antagonized the effect of both i.c.v. and i.t. 8-OH-DPAT. In rats treated with capsaicin as adults, the response to 8-OH-DPAT was unchanged. In rats treated with capsaicin as newborns, instead, the response to i.t. 8-OH-DPAT was abolished and that to i.c.v. 8-OH-DPAT was shifted to higher doses. Pretreatment with 5,7-dihyroxytryptamine did not affect the response to i.t. 8-OH-DPAT but shifted to higher doses the response to i.c.v. 8-OH-DPAT. Intravenous administration of spiroxatrine, methysergide, NAN-190 [1-(2-methoxyphenyl)-4-[4-(2-phtalimido)butyl] piperazine] or high doses of buspirone but not of 1-sulpiride inhibited SMR in suprathreshold conditions. The inhibitory effect of spiroxatrine, NAN-190 and buspirone was not reduced by guanethidine pretreatment. In chronically spinalized animals, i.v. 8-OH-DPAT increased the amplitude of the reflex bladder contractions induced by bladder distension.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Effects of the potential antidepressant OPC-14523 [1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2-quinolinone monomethanesulfonate] a combined sigma and 5-HT1A ligand: modulation of neuronal activity in the dorsal raphe nucleus

OPC-14523 (OPC; [1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2-quinolinone monomethanesulfonate)] is a novel compound with high affinity for sigma and 5-HT(1A) receptors as well as for the 5-HT transporter. OPC has previously been shown to produce antidepressant-like effects in animal models of depression. This project set out to determine the effect of OPC on serotonergic neurotransmission and to shed light on its mechanism(s) of action. In an electrophysiological model of in vivo extracellular recordings in anesthetized rats, a 2-day treatment (1 mg/kg/day) with OPC induced a significant increase in dorsal raphe nucleus (DRN) putative 5-HT neurons' firing activity. This increase was blocked by the coadministration of NE-100 [N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy)phenyl)-thylamine], a selective sigma(1) antagonist (10 mg/kg/day). Furthermore, after 2-day treatments with OPC, the 5-HT(1A) autoreceptor response was altered, as demonstrated by the dramatically reduced response to an increase of endogenous 5-HT induced by the acute administration of paroxetine (500 microg/kg, i.v.). However, the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (4 microg/kg, i.v.) maintained its ability to decrease 5-HT firing activity, an effect that was reversible by the subsequent administration of the 5-HT(1A) antagonist WAY 100635 [N-[2-(4-[2-methoxyphenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexanecarboxamide] (100 microg/kg, i.v.). As 8-OH-DPAT has been shown to act preferentially through postsynaptic 5-HT(1A) receptors, our data suggests that this effect of OPC is mediated primarily by the 5-HT(1A) autoreceptor. The decreased response of the 5-HT(1A) autoreceptor to paroxetine was not blocked by the coadministration of NE-100 indicating that sigma(1) receptors are not involved in this effect. Thus, both sigma and 5-HT(1A) receptors play a role in the "antidepressant-like" effects produced by OPC, which is in keeping with previously published behavioral data. In addition, the current series of experiments suggest that OPC might have potential as an antidepressant with a rapid onset of action compared with selective serotonin reuptake inhibitor treatments, which initially suppress the firing activity of putative 5-HT neurons and require at least 2 to 3 weeks to restore the firing activity to baseline neuronal firing activity through a desensitization of the 5-HT(1A) autoreceptor.     

Alterations of central serotonin and dopamine turnover in rats treated with ipsapirone and other 5-hydroxytryptamine1A agonists with potential anxiolytic properties

Measurements of tissue levels of monoamines and their metabolites, and of the rates of 5-hydroxytryptophan and dihydroxy-phenylalanine accumulation after blockade of aromatic amino acid decarboxylase by benserazid indicated that ipsapirone (1-10 mg/kg i.p.) decreased 5-hydroxytryptamine (5-HT) turnover and accelerated dopamine (DA) turnover in various brain regions. The reduced 5-HT turnover probably resulted from the stimulation of 5-HT1A autoreceptors within the anterior raphe nuclei as in vitro tests [( 3H]-8-hydroxy-2-[di-n-propylamino]tetralin binding and adenylate cyclase assays) demonstrated that ipsapirone was a 5-HT1A agonist almost as potent as 8-OH-DPAT, and the same decrease in 5-hydroxytryptophan accumulation could be induced by the i.p. (5 mg/kg) or intraraphe (1 microgram) injection of ipsapirone. Ipsapirone-induced acceleration of DA turnover persisted after the selective degeneration of serotoninergic neurons by intraraphe 5,7-dihydroxytryptamine infusion, and could be reproduced by i.p. administration of other 5-HT1A agonists like buspirone and gepirone, but not 8-OH-DPAT. These results demonstrate that ipsapirone-induced acceleration of DA turnover did not result from the stimulation of 5-HT1A (auto)receptors, but involved additional target(s) of the drug. The possible participation of dopaminergic systems in the "anxiolytic" properties of ipsapirone should deserve further investigations.     

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)     

Role of spinal 5-HT(1A) receptors in morphine analgesia and tolerance in rats.

We here studied the involvement of spinally located 5-HT(1A) and opioid receptors, in the paradoxical effects that their activation can produce on nociception. Intrathecal (i.t.) injection of the 5-HT(1A) receptor agonist 8-hydroxy-2-[di-n-propylamino] tetralin (8-OH-DPAT) (1-10 microg) induced analgesic effects in the formalin model of tonic pain whereas in the paw pressure test, it decreased the vocalization threshold. In this latter test, i.t. 8-OH-DPAT also markedly reduced the analgesic effect of systemic morphine (5-10 mg/kg, s.c.). At 10 microg, 8-OH-DPAT totally abolished the effect of 5 mg/kg of morphine; this inhibitory effect was antagonized by pre-treatment with 0.63 mg/kg of the 5-HT(1A) antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)-cyclohexanecarboxamide-trihydrochloride). In contrast, the i.t. injection of WAY-100635 (1-10 microg) dose-dependently potentiated the antinociceptive activity of a dose of morphine (2.5 mg/kg, s.c.). Furthermore, WAY-100635 (10 microg, i.t.) potentiated morphine analgesia in morphine-tolerant rats. These findings demonstrate that 5-HT(1A) receptor agonists can act in the spinal cord to produce both hyper- and hypo-algesic effects and play a major role in the opioid analgesia and tolerance.     

Pindolol suppresses serotonergic neuronal activity and does not block the inhibition of serotonergic neurons produced by 8-hydroxy-2-(di-n-propylamino)tetralin in awake cats.

Clinical studies have shown that pindolol can enhance the effects of antidepressant drugs, presumably by acting as an antagonist at somatodendritic 5-hydroxytryptamine (5-HT)(1A) autoreceptors, which regulate the firing rate of central serotonergic neurons. The current study characterized the action of pindolol on the single-unit activity of serotonergic neurons in the dorsal raphe nucleus of freely moving cats. (+/-)-Pindolol produced a dose-dependent inhibition of neuronal activity after i.v. (ED(50) = 0.25 mg/kg) and s.c. (ED(50) = 1.23 mg/kg) administration. The active enantiomer (-)-pindolol (1 mg/kg i.v.) also suppressed neuronal activity (maximal decrease, 88%). Upon p.o. administration, (+/-)-pindolol (10 mg/kg) produced a marked, long-acting suppression of neuronal activity similar to that observed after s.c. administration. In all cases, the reduction in firing rate produced by pindolol was completely reversed by low doses of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)cycloh exanecarboxamide (WAY-100635) (0.1 mg/kg i.v. or 0.2 mg/kg s.c.), a selective 5-HT(1A) antagonist. Systemic administration of (-)-tertatolol (1-5 mg/kg i.v.), another beta-adrenoceptor blocker/putative 5-HT(1A) antagonist, had no significant effect on neuronal activity. The ability of i.v. (+/-)-pindolol (0.1-1 mg/kg) to reverse the suppression of serotonergic neuronal activity produced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10 microg/kg i.v.), a selective 5-HT(1A) agonist, also was examined. (+/-)-Pindolol had no appreciable effect on the action of 8-OH-DPAT. In contrast, the 5-HT(1A) antagonist drugs WAY-100635 (0.1 mg/kg i.v. ), 4-fluoro-N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl benzamide (0.1 mg/kg, i.v.), N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylprop anamid e [(S)-WAY-100135] (0.5 mg/kg i.v.), and (-)-tertatolol (1-5 mg/kg i. v.) reversed the effect of 8-OH-DPAT to varying degrees. Overall, these results indicate that pindolol acts as an agonist rather than an antagonist at 5-HT(1A) autoreceptors in awake animals.     

S 14671: a naphtylpiperazine 5-hydroxytryptamine1A agonist of exceptional potency and high efficacy possessing antagonist activity at 5-hydroxytryptamine1C/2 receptors.

The interaction at 5-hydroxytryptamine (5-HT) receptors of the novel naphtylpiperazine, S 14671 (1-[2-(2-thenoylamino)ethyl]-4[1-(7- methoxynaphtyl)]piperazine), was compared to that of the 5-HT1A ligands, 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), WY 50,324 [N-(29(4-(2-pyrimidinyl)-1-piperazinyl)ethyl)tricyclo(3.3.1.1(3,7) )- decane-1-carboxamide], (+)-flesinoxan, buspirone and BMY 7378 [(8-[2-[4-(2-methoxyphenyl)- 1-piperazinyl]ethyl]-8-azaspirol[-4-]-decane-7,9-dione 2HCl]. S 14671 showed a very high affinity for 5-HT1A sites (pKi, 9.3) as compared to the reference ligands (pKi values, 9.2, 8.7, 8.7, 7.9 and 8.7, respectively). S 14671 bound in an apparently competitive manner and, in distinction to the reference compounds, possessed a Hill Coefficient (1.4) significantly superior to 1. Although showing low affinity at 5-HT1B and 5-HT3 sites, S 14671 displayed significant affinity at both 5-HT1C and 5-HT2 sites; pKi, 7.8 in each case. Furthermore, S 14671 acted as an antagonist of 5-HT-stimulated phosphoinositide turnover in rat choroid plexus (5-HT1C) and cortex (5-HT2). In vivo, upon s.c. administration, S 14671 acted as a high efficacy agonist in models of 5-HT1A receptor-mediated activity: induction of flat-body posture, spontaneous tail-flicks, hypothermia and corticosterone secretion and inhibition of morphine-induced antinociception. In every test, S 14671 was the most potent compound: it was active at doses as low as 5 micrograms/kg s.c. Relative potency across all tests was S 14671 greater than 8-OH-DPAT greater than WY 50,324 greater than (+)-flesinoxan greater than buspirone with BMY 7378 too weak for comparison to be meaningful. The action of S 14671 in 5-HT1A tests was blocked by BMY 7378 and the 5-HT1A antagonist, (-)-alprenolol, but unaffected by the 5-HT1C/2 antagonist, ritanserin, and the 5-HT3 antagonist, ondansetron. Activation of postsynaptic 5-HT1A receptors was confirmed in 5,7-dihydroxytryptamine-lesioned rats, in which the potency of S 14671 to elicit spontaneous tail-flicks was potentiated. Activation of presynaptic receptors was demonstrated by inhibition of the electrical activity of the dorsal raphe nucleus with the following order of relative potency: S 14671 greater than 8-OH-DPAT greater than WY 50,324 greater than BMY 7378 greater than buspirone. Spiperone, which acts as a pure 5-HT1A antagonist at raphe 5-HT1A receptors, blocked the action of S 14671. In conclusion, S 14671 is a structurally novel ligand manifesting high efficacy and exceptional potency at both pre- and postsynaptic 5-HT1A receptors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Serotonin-induced reduction of the calcium-dependent plateau in frog dorsal root ganglion cells is blocked by serotonergic agents acting at 5-hydroxytryptamine1A sites

Intracellular recordings from the dorsal root ganglion cells of adult frogs in the presence of tetraethylammonium display action potentials with a prominent calcium-dependent plateau. These action potentials can be altered by serotonergic agents in one of two ways. The superfusion of 5-HT (0.1-1 microM) usually produces a dose-dependent reduction of the action potential duration, whereas 8-hydroxy dipropylaminotetralin (8-OH-DPAT) (10-50 microM) produces a dose-dependent increase in duration. A series of 5-HT antagonists were tested for their ability to block either the 5-HT or the 8-OH-DPAT effect. The antagonists were chosen for their reported selectivity in distinguishing receptors of the 5-HT1A, 5-HT2 and 5-HT3 subtypes. The antagonists' action on 5-HT narrowing [blockade by methiothepin, spiperone and spiroxitrine, but not by ketanserin or 3-tropyl-indole-3-carboxylate (ICS 205-930)] suggests that this response is mediated by 5-HT1A receptors. The widening effect produced by 8-OH-DPAT (a putative 5-HT1A agonist) was not blocked by any antagonist tested. At lower concentrations (0.1-2.5 microM) 8-OH-DPAT exhibited no agonist actions, but antagonized the 5-HT-induced narrowing. These results suggest the 5-HT receptors mediating 5-HT action potential narrowing in these cells are of the 5-HT1A subtype, but that they differ from the 5-HT1A receptors described in other tissues in which 8-OH-DPAT is an agonist or a partial agonist.     

Differential effects of antipsychotic drugs on serotonin-1A receptor-mediated disruption of prepulse inhibition

Serotonin-1A (5-HT(1A)) receptors have been implicated in the symptoms of schizophrenia. However, there is limited in vivo evidence for an interaction of antipsychotic drugs with 5-HT(1A) receptor-mediated behavioral effects. We therefore investigated in rats the action of several antipsychotic drugs on prepulse inhibition (PPI), a measure of sensorimotor gating that is deficient in schizophrenia. Disruption of PPI at the 100-ms interstimulus interval (ISI), but not the 30-ms ISI, was induced by treatment with 0.5 mg/kg 8-hydroxy-di-propylaminotetralin (8-OH-DPAT), the 5-HT(1A) receptor agonist. In rats pretreated with 0.25 mg/kg haloperidol (4-[-4-(p-chlorophenyl)-4-hydroxypiperidino]-4'-fluoro butyrophenone) or raclopride [3,5-dichloro-N-(1-ethylpyrrolidin-2-ylmethyl)-2-hydroxy-6-methoxybenzamide tartrate], the disruption of PPI was no longer significant. Of the atypical antipsychotic drugs clozapine (8-chloro-11-(4-methyl-1-piperazinyl)-5H-dibenzo[b,e][1,4]-diazepine), olanzapine (2-methyl-4-(4-methyl-1-piperazinyl)-10H-thieno[2,3-b][1,5]benzodiazepine), risperidone [3-[2-[-4-(6-fluoro-1,2-benzisoxazol-3-yl) piperidino] ethyl-6,7,8,9-tetrahydro-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one)], amisulpride (4-amino-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-(ethylsulfonyl)-o-anisamide), and aripiprazole (7-[4-[-4[-(2,3-dichlorophenyl)-1-piperazinyl]butoxy]-3,4-dihydrocarbostyrilor 7-[4-[4-(2,3-dichlorophenyl) piperazin-1-yl]butoxy]-1,2,3,4,-tetrahydroquinolin-2-one), only aripiprazole significantly reduced the effect of 8-OH-DPAT on PPI. This effect was mimicked by pretreatment with the 5-HT(1A) receptor partial agonist, buspirone [N-[4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5]decane-7,9-dione hydrochloride]. On the other hand, some of the antipsychotic drugs and other pretreatments showed complex, prepulse-dependent effects on their own. These data show little in vivo interaction of several atypical antipsychotic drugs with the disruption of PPI mediated by 5-HT(1A) receptor stimulation. The action of haloperidol and raclopride suggests a major involvement of dopamine D(2) receptors in this effect, possibly downstream from the initial serotonergic stimulation. The action of aripiprazole could be mediated by its partial agonist properties at 5-HT(1A) receptors or its dopamine D(2)-blocking properties.     

The partial 5-hydroxytryptamine1A receptor agonist buspirone does not antagonize morphine-induced respiratory depression in humans

Based on experiments in rats, serotonin receptor 5-hydroxytryptamine (5-HT)(1A) agonists have been proposed as a potential therapeutic strategy for the selective treatment of opioid-induced respiratory depression. We investigated the clinical applicability of this principle in healthy volunteers. Twelve subjects received 0.43 mg/kg morphine (30 mg for 70 kg body weight) administered intravenously (i.v.) over approximately 2 h. At the start of the morphine infusion, they received in a randomized, double-blind cross-over design 60 mg p.o. buspirone or placebo. Respiratory depression (hypercapnic challenge) and pain (electrical stimuli: 5 Hz sinus 0-20 mA; chemical stimuli: 200 ms gaseous CO(2) pulses applied to the nasal mucosa) were assessed at baseline, at the end of the morphine infusion, and a third time after antagonizing the opioid effects by i.v. administration of 2 mg naloxone. The linear relationship between the minute ventilation and the CO(2) concentration in the inspired air of 1.07+/-0.27 l/mm Hg CO(2) at baseline conditions became shallower (0.45+/-0.23 l/mm Hg CO(2)) after morphine administration (P<0.001), indicating respiratory depression, which was significantly reversed by naloxone (0.95+/-0.43 l/mm Hg CO(2); P=0.001). Co-administration of buspirone had no effect on morphine-induced respiratory depression (slope 0.45+/-0.23 l/mm Hg CO(2) under morphine plus placebo versus 0.38+/-0.25 l/mm Hg CO(2) under morphine plus buspirone; P=0.7). Significant morphine-induced analgesia was observed in both pain models and was reversed by naloxone but unaffected by buspirone. Buspirone significantly increased the nausea induced by morphine (P=0.011). Oral co-administration of a high dose of the clinically available 5-HT(1A) agonist buspirone cannot be advised as a remedy for opioid-induced respiratory depression. This is indicated by its lack of anti-respiratory depressive effects and by the buspirone-associated increase of morphine-induced nausea.