Alterations of central serotoninergic and dopaminergic neurotransmission in rats chronically treated with ipsapirone: biochemical and electrophysiological studies.

Inasmuch as sustained treatment with the 5-hydroxytryptamine1A (5-HT1A) agonist, ipsapirone, is necessary for inducing anxiolytic and antidepressant effects in the clinic, investigations were performed for assessing the possible changes in serotoninergic and dopaminergic neurotransmission in the brain of rats 24 hr after a 2-week treatment with this drug. Receptor binding assays with membranes and quantiative autoradiography indicated that the twice-daily administration of ipsapirone (5 mg/kg i.p.) for 14 days did not alter the characteristics of 5-HT1A sites in the hippocampus, septum and dorsal raphe nucleus. In contrast, significant decreases in the Bmax values for 5-HT2 sites (-24%) and 5-HT3 sites (-19%) were found in the frontal and posterior cortex, respectively. As expected from unchanged postsynaptic 5-HT1A receptors, inhibition of forskolin-stimulated adenylate cyclase by 5-HT1A agonists (8-hydroxy-2-(di-n-propylamino)tetralin, ipsapirone) exhibited the same characteristics in hippocampal homogenates from both control and ipsapirone-treated animals. An acute administration of 8-hydroxy-2-(di-n-propylamino)tetralin (0.5 mg/kg i.p.) or ipsapirone (1 or 5 mg/kg i.p.) 24 hr after the last injection for the chronic treatment produced a similar decrease in the rate of 5-HT turnover in various brain areas in rats treated for 2 weeks with saline or ipsapirone. At the highest dose (5 mg/kg i.p.), acute ipsapirone also increased the rate of dopamine turnover in the striatum and cerebral cortex approximately to the same extent in both treatment groups. In vitro recording of the firing of serotoninergic neurons in brain stem slices revealed a desensitization of the somatodendritic 5-HT1A receptors, which might be responsible for the increased 5-HT turnover in the brain stem and striatum of rats chronically treated with ipsapirone as compared with controls. These data demonstrated that chronically administered ipsapirone produces adaptive changes in central serotoninergic neurotransmission which might account for the anxiolytic and antidepressant properties of this drug after sustained treatment.     

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.     

Evaluation of the central and peripheral components for induction of postural hypotension by guanethidine, clonidine, dopamine2 receptor agonists and 5-hydroxytryptamine1A receptor agonists.

Previously, it was observed that the severity of postural hypotension in rats after i.v. administration is: guanethidine (severe) greater than clonidine greater than dopamine2 (DA2) receptor agonists greater than 5-hydroxytryptamine1A (5-HT1A) receptor agonists (none). In this paper we investigated central and peripheral mechanisms involved in postural hypotension induced by these drugs. Intracerebroventricular or intracisternal doses of the above agents produced a fall in mean arterial pressure of 32 to 42 mm Hg and were used for evaluation of the central component. Intracerebroventricular, but not intracisternal, clonidine induced postural hypotension. DA2 or 5-HT1A receptor agonists did not induce postural hypotension after either i.c.v. or intracisternal administration. Intracerebroventricular guanethidine did not lower arterial pressure dose-dependently, or did it produce postural hypotension. A peripheral action postulated to be involved in the postural hypotension was inhibition of sympathetic neurotransmission; to evaluate this inhibition, drug effects on pressor and tachycardiac responses elicited by electrical stimulation in pithed rats were determined. Inhibition of heart rate changes by each of the drugs was: guanethidine, 60 to 100% at 0.1 to 8.0 Hz; clonidine, 20 to 40% at 0.1 to 2.0 Hz; DA2 receptor agonists, 10 to 45% at 0.1 to 2.0 Hz; and 5-HT1A receptor agonists, 20 to 40% at less than or equal to 0.5 Hz. These data indicate that the frequency selective inhibition in the peripheral sympathetic nervous system may explain the likelihood of postural hypotension for guanethidine, clonidine DA2 and 5-HT1A receptor agonists. Clonidine has some central component for induction of postural hypotension in addition to the frequency-related peripheral component.     

Discriminative stimulus properties of D1 and D2 dopamine agonists in rats

Rats were trained to discriminate 8.0 mg/kg of SKF 38393 (SKF) or 1.0 mg/kg of piribedil (PIR) from saline. Drugs were given 10 min before each session in a two-lever, food-reinforced (FR 30) drug discrimination paradigm. SKF (2.0-8.0 mg/kg i.p.) produced a dose-related increase in drug-appropriate responding in the SKF group but not in the PIR group. PIR (0.06-1.0 mg/kg i.p.) produced a dose-related increase in drug-appropriate responding in the PIR group but not in the SKF group. Apomorphine (0.03-0.5 mg/kg i.p. also produced a dose-related increase in PIR-appropriate responding, whereas dopamine (DA; 4.0-16 mg/kg i.p.), which does not readily cross the blood-brain barrier, did not. When pretreatment time was varied, SKF-appropriate responding was maximal when 8.0 mg/kg of SKF was injected 30 min before the session. PIR (1.0 mg/kg i.p.) occasioned maximal PIR-appropriate responding when injected 1 or 10 min before the session but did not when injected 30 or 60 min before the session. In rats trained to discriminate SKF (8.0 mg/kg) using a 30-min pretreatment time, the D1 antagonist SCH 23390 blocked the SKF discriminative stimulus (DS) but did not alter the PIR DS in PIR-trained rats. The D2 antagonist pimozide blocked the PIR DS but did not alter the SKF DS. Thus, the DS properties of D1 and D2 agonists are functionally distinct in rats and are antagonized by DA antagonists selective for D1 or D2 receptors, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin agonists inhibit synaptic potentials in the rat locus ceruleus in vitro via 5-hydroxytryptamine1A and 5-hydroxytryptamine1B receptors

Intracellular recordings were made from rat locus ceruleus neurons in the slice preparation in vitro. Depolarizing synaptic potentials (DSP)2 elicited by electrical stimulation were typically 10 to 15 mV in amplitude and 200 msec in duration. Superfusion with 5-hydroxytryptamine (5-HT, serotonin) or the 5-HT1 receptor agonist 5-carboxamidotryptamine (5-CT), produced an inhibition of the DSP. The maximal inhibition was 55 +/- 2% (mean +/- S.E.M.). The EC50 for 5-CT was 60 nM, whereas for 5-HT it was 12 microM. Cocaine (10 microM) shifted the 5-HT concentration-response curve to the left and the EC50 to 320 nM. 8-Hydroxy-2-(di-n-propylamino)tetralin, a selective 5-HT1A receptor ligand, also inhibited the DSP, but only produced about 65% of the maximal 5-CT or 5-HT response (EC50 = 50 nM). A relatively selective 5-HT1B ligand (65-fold 5-HT1B greater than 5-HT1A), 1-(m-trifluoromethyl-phenyl)-piperazine, acted as a full agonist (EC50 = 110 nM). None of these compounds had any effects on the membrane properties of the cell at the doses tested. The response to 8-hydroxy-2-(di-n-propylamino) tetralin was antagonized by pretreatment with the 5-HT1A antagonist spiperone (1 microM). The estimated KD for spiperone was 16 nM. At this same concentration, however, there was no effect on the 5-CT-induced inhibition. The antagonist 4-(3-ter-butyl-amino-2-hydroxy-propoxyl)-indol-2-carbonic acid isopropyl ester (LM 21-009, 100 nM) was found to be a partial agonist producing a 26 +/- 4% inhibition of the DSP.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of 5-hydroxytryptamine1A and 5-hydroxytryptamine1B receptors in modulating spinal nociceptive transmission in normal and carrageenan-injected rats

Single unit extracellular recordings from the dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 115 wide dynamic range (WDR) neurons were studied in 94 rats. In normal rats, the size of nociceptive receptive fields (RFs) of WDR neurons was approximately 123.3 +/- 8.21 mm2 (n = 88). Following carrageenan-induced inflammation, the RFs were markedly enlarged (332.4 +/- 30.1 mm2, n = 27, P < 0.001). The frequency of background activity of the WDR neurons in carrageenan-injected rats (11.3 +/- 2.1 imp/s, n = 27) was greater than that in normal rats (7.1 +/- 0.8 imp/s, n = 88, P < 0.05). In 82% of WDR neurons in normal rats, there was a separation between the A- and C-responses. In contrast, in 67% of the neurons in carrageenan-injected rats, the response to suprathreshold electrical stimuli was a long train with no separation between the A- and C-responses. In carrageenan-injected rats, the magnitude and duration of the nociceptive responses were significantly increased compared to those in normal rats, and the average C-response threshold (7.7 +/- 1.1 mA, n = 27) was lower than that in normal rats (10.4 +/- 0.7 mA, n = 88, P < 0.05). Intrathecal injection of the 5-hydroxytryptamine(1A) (5-HT1A) receptor agonist 8-hydroxy-DPAT hydroxybromide (8-OH-DPAT) (0.305, 1.525, 3.05, and 15.25 mM) dose-dependently increased Adelta- and C-responses and post-discharge in most of the WDR neurons. Following carrageenan-induced inflammation, the 8-OH-DPAT-induced facilitatory effect on Adelta- and C-responses and post-discharge was significantly enhanced (P < 0.05). Intrathecal injection of the 5-hydroxytryptamine1B (5-HT1) receptor agonist CGS12066A (0.222, 1.11, 2.22, and 11.1 mM) dose-dependently enhanced the C-response and post-discharge without influencing the Adelta-response. In carrageenan-injected rats, CGS12066A not only enhanced the facilitatory effect on the C-response and post-discharge, but also facilitated the Adelta-response. Intrathecal injection of the 5-HT(1A) receptor antagonist NAN-190 (0.2 mM) alone did not influence Adelta- and C-responses and post-discharge of WDR neurons in normal rats. When 0.2 mM NAN-190 was co-administered with 3.05 mM 8-OH-DPAT, the facilitatory effect of 8-OH-DPAT on Adelta- and C-responses and post-discharge was completely antagonized, whereas CGS12066A-induced facilitation on the C-response and post-discharge was not influenced by co-administration of 0.2 mM NAN-190 and CGS12066A. These data suggest that 5-HT1A and 5-HT1B receptor subtypes mediate the facilitatory effect of 5-HT on nociceptive processing in the spinal cord of rats. The excitability of dorsal horn WDR neurons and the sensitivity of the neurons to intrathecal 5-HT1A and 5-HT1B receptor agonists might increase following carrageenan-induced inflammation.     

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.     

Contrasting contribution of 5-hydroxytryptamine 1A receptor activation to neurochemical profile of novel antipsychotics: frontocortical dopamine and hippocampal serotonin release in rat brain

Several novel antipsychotics, such as aripiprazole, bifeprunox, SSR181507 [(3-exo)-8-benzoyl-N-(((2S)7-chloro-2,3-dihydro-1,4-benzodioxin-1-yl)methyl)-8-azabicyclo(3.2.1)octane-3-methanamine], and SLV313 [1-(2,3-dihydro-benzo[1,4]dioxin-5-yl)-4-[5-(4-fluorophenyl)-pyridin-3-ylmethyl]-piperazine], activate serotonin 5-hydroxytryptamine (5-HT)1A receptors. Such activity is associated with enhanced treatment of negative symptoms and cognitive deficits, which may be mediated by modulation of cerebral dopamine and serotonin levels. We employed microdialysis coupled to high pressure liquid chromatography with electrochemical detection to examine 5-HT1A receptor activation in the modulation of extracellular dopamine in medial prefrontal cortex and serotonin in hippocampus of freely moving rats. The above compounds were compared with drugs that have less interaction with 5-HT1A receptors (clozapine, nemonapride, ziprasidone, olanzapine, risperidone, and haloperidol). Hippocampal 5-HT was decreased by bifeprunox, SSR181507, SLV313, sarizotan, and nemonapride, effects similar to those seen with the 5-HT1A agonist, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)8-OH-DPAT], consistent with activation of 5-HT1A autoreceptors. These decreases were reversed by the selective 5-HT1A antagonist, WAY100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide]. In contrast, haloperidol, risperidone, clozapine, olanzapine, ziprasidone, and aripiprazole did not significantly modify hippocampal serotonin levels. In medial prefrontal cortex, dopamine levels were increased by SSR181507, SLV313, sarizotan, and (+)8-OH-DPAT. These effects were reversed by WAY100635, indicating mediation by 5-HT1A receptors. In contrast, the increases in dopamine levels induced by clozapine, risperidone, olanzapine, and ziprasidone were not blocked by WAY100635, consistent with predominant influence of other mechanisms in the actions of these drugs. Haloperidol, nemonapride, and the D2 partial agonists, aripiprazole and bifeprunox, did not significantly alter dopamine release. Taken together, these data demonstrate the diverse contribution of 5-HT1A receptor activation to the profile of antipsychotics and suggest that novel drugs selectively targeting D2 and 5-HT1A receptors may present distinctive therapeutic properties.     

A review of the receptor-binding and pharmacokinetic properties of dopamine agonists

BACKGROUND: Dopamine agonists (DAs), which can be categorized as ergot derived and non-ergot derived, are used in the treatment of Parkinson's disease. OBJECTIVES: This review describes the pharmacologic and pharmacokinetic properties of selected DAs and relates these characteristics to clinical outcomes, with an emphasis on adverse events. METHODS: Relevant articles were identified through a search of MEDLINE (to May 2006) using the terms dopamine agonists (or each individual drug name) and pbarmacokinetics, metabolism, drug-drug interaction, interactions, CYP450, fibrosis, valvular heart disease, tremor, clinical trials, reviews, and meta-analyses. Abstracts from recent sessions of the International Congress of Parkinson's Disease and Movement Disorders were also examined. Clinical studies with <20 patients overall or <10 patients per treatment group in the final analysis were excluded. All DAs that were graded at least possibly useful with respect to at least 3 of 4 items connected to the treatment/prevention of motor symptoms/complications in the most recent evidence-based medical review update were included. This resulted in a focus on the ergot-derived DAs bromocriptine, cabergoline, and pergolide, and the non-ergot-derived DAs pramipexole and ropinirole. RESULTS: Bromocriptine, cabergoline, pergolide, and ropinirole, but not pramipexole, have the potential for drug-drug interactions mediated by the cytochrome P450 (CYP) enzyme system. The occurrence of dyskinesia may be linked to stimulation of the dopamine D(1) receptor, for which cabergoline and pergolide have a similar and relatively high affinity; bromocriptine, pramipexole, and ropinirole have been associated with a lower risk of dyskinesias. The valvular heart disease (VHD) and pulmonary and retroperitoneal fibrosis seen with long-term use appear to represent a class effect of the ergot-derived DAs that may be related to stimulation of serotonin 5-HT(2B) (and possibly 5-HT(2A)) receptors. The incidence of valvular regurgitation was 31% to 47% with ergot-derived DAs, 10% with non-ergot-derived DAs, and 13% with controls. CONCLUSIONS: As reflected in the results of the clinical trials included in this review, dyskinesia associated with DA therapy may be linked to stimulation of the D(1) receptor. Fibrosis (including VHD) seemed to be a class effect of the ergot-derived DAs. Each of the DAs except pramipexole has the potential to interact with other drugs via the CYP enzyme system.     

5-hydroxytryptamine (HT)1A receptors and the tail-flick response. III. Structurally diverse 5-HT1A partial agonists attenuate mu- but not kappa-opioid antinociception in mice and rats.

This study examined the effects of structurally diverse 5-hydroxytryptamine (HT)1A partial agonists upon opioid-induced antinociception against noxious heat and pressure stimuli in rats and mice. The pyrimidinylpiperazines, buspirone, ipsapirone and gepirone, the halogenated phenylpiperazine, LY 165, 163 [1-(2-(4-aminophenyl)ethyl-4-(3-trifluoromethylphenyl)-piperazine], the heterobicylic arylpiperazine, (+/-)-flexinoxan, and the benzodiaxane, MDL 728328-[(4-(1,4-benzodioxon-2-ylmethylamino)butyl-8-azasp iro-(4,5)-decane-7,9-dione], exerted little or no effect upon basal latencies. In both mice and rats, each dose-dependently attenuated the antinociceptive action of the mu-opioid, morphine, against heat and pressure. In their presence, the morphine dose-response curve was shifted in parallel to the right with no loss of maximal effect. In mice, Schild analysis of the action of ipsapirone and gepirone yielded slopes of close to -1. In contrast to the partial agonists, the buspirone metabolite, 1-pyrimidinylpiperazine, which lacks 5-HT1A affinity, and the putative 5-HT1A antagonists, methiothepin, spiperone, BMY 7378 [(8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol [4]-decane-7,9-dione) 2HCl] and alprenolol, did not reduce the action of morphine. In rats, the antagonistic effect of buspirone, gepirone and ipsapirone could be blocked by BMY 7378. The 5-HT1A partial agonists also antagonized the antinociception-induced by the mu-opioid, sufentanil, but were virtually inactive against the selective kappa-opioid agonists, U69,593 (5 alpha,7 alpha,8 beta-(+)-N-methyl-N-[7-(l-pyrrolidinyl)-1-oxaspirol-(4,5)-dec-8-yl ] benzene-acetamide) and U50,488H (trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide methane sulfonate hydrate.(ABSTRACT TRUNCATED AT 250 WORDS)     

射频消融治疗前后大鼠树突状细胞的变化

目的探讨肝脏射频消融(RFA)对正常大鼠树突状细胞(DC)的影响。方法26只健康SD大鼠随机分为治疗7d组(9只)、治疗14d组(9只)和对照组(8只),对照组不做治疗直接处死取材,后两组分别在RFA治疗后7d和14d处死取材。取外周血、消融区周边0.5cm内肝组织(对照组取正常肝组织)及脾脏,采用流式细胞术检测树突状细胞表面OX62、OX6、CD86表达水平。结果对照组大鼠OX62 细胞占外周血单个核细胞比率为(0.37±0.11)%,治疗7d组增高为(0.59±0.23)%,两组相比差异有显著性(P<0.05)。对照组肝脏、脾脏OX62 细胞占单个核细胞比率分别为(17.50±9.51)%、(10.36±3.21)%,射频消融后分别增高至(34.94±15.14)%、(18.03±5.70)%(P<0.05);射频后肝组织局部和脾脏DC表面OX6阳性表达率明显增高,与对照组大鼠相比差异有显著性(P<0.05)。结论射频消融后大鼠外周血DC前体细胞数量明显增多,肝脏局部和脾脏DC数量增多,OX6 表达率增高。射频治疗后机体DC的数量及功能发生变化,为进一步研究射频治疗肝肿瘤对机体抗原递呈功能的影响奠定了基础。     

Differential inhibition of cholinergic and noncholinergic neurogenic contractions by 5-hydroxytryptamine1A receptor agonists in guinea pig ileum.

Previous studies have shown that 5-hydroxytryptamine1A (5-HT1A) receptors are localized only to a subset of myenteric neurons in guinea pig ileum; the purpose of this study was to determine the possible functional significance of this differential receptor localization. Nerve-mediated contractions of the longitudinal muscle, myenteric plexus of guinea pig ileum were studied in vitro. Contractions were evoked by single transmural electrical stimuli (0.5-msec duration, at 0.1 Hz; cholinergic) or trains of stimuli (10 or 20 Hz for 1 sec; noncholinergic; scopolamine, 1 microM present). The 5-HT1A agonist, 8-hydroxy-2-(n-dipropylamino)tetralin (DPAT), reduced cholinergic contractions by no more than 14% in the concentration range of 0.001 to 0.3 microM. At high concentrations (1 to 100 microM), DPAT inhibited longitudinal muscle, myenteric plexus contractions; the EC50 was 6 microM. 5-Hydroxytryptamine (5-HT) and 5-carboxamidotryptamine (5-CT) reduced longitudinal muscle, myenteric plexus contractions by a maximum of 35% and 24% at 100 and 30 microM, respectively. Spiperone and 1-(2-methoxyphenyl)-4-[4-(2-phthalimidobutyl]piperazine (NAN-190), two 5-HT1A receptor antagonists, did not block DPAT-induced inhibition of cholinergic contractions. DPAT (3, 10 and 30 microM) shifted histamine concentration-response curves to the right in an apparently competitive manner; Schild analysis yielded a pA2 of 5.2. DPAT (3, 10, 30 and 100 microM) also shifted bethanechol concentration-response curves to the right in an apparently competitive manner; Schild analysis yielded a pA2 of 5.5. These data indicate that DPAT blocks histamine and muscarinic receptors on longitudinal muscle.(ABSTRACT TRUNCATED AT 250 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.     

Modulation of bone turnover in orchidectomized rats treated with raloxifene and risedronate

Osteoporosis is a reduction in bone mineral density (BMD). It develops less often in men than in women. This study aimed to evaluate the bone protective effects of raloxifene (RAL), risedronate (RIS), and their combination on osteoporotic male rats. Forty male Wister rats (12 weeks) were randomly divided into five groups: sham‐operated group (n = 8), orchidectomized (ORX) group (n = 7), RAL group (n = 9), RIS group (n = 7) and RAL + RIS group (n = 7). RAL was orally administered at 3 mg/kg three times/week, and RIS was given subcutaneously at 5 μg/kg, twice weekly. After 6 weeks of treatment, serum cathepsin‐K, alkaline (ALP) and acid phosphatase activities, serum osteocalcin, serum Ca²⁺, and Pi were determined. Urinary Ca²⁺ and deoxypyridinoline levels, BMD, and Ca²⁺ content of femur ash were estimated. Histochemical localization of ALP activity of tibia and histomorphometry was examined. As compared to sham, ORX rats showed a significant increase in bone turnover markers, and histochemical activity of ALP was increased markedly in proximal tibia of ORX rats, whereas BMD and Ca²⁺ content of femur ash were reduced after ORX. These changes were modulated after treatment with RAL and RIS or both to ORX rats; BMD of femur was improved by each treatment, and bone turnover markers were reduced as compared to ORX vehicle group. We concluded that orchidectomy induced osteoporosis and increased bone turnover in male rats because of withdrawal of sex hormones. Both RAL and RIS could treat osteoporosis in ORX rats; they reduced bone turnover markers and maintained BMD.     

Regulation of 5-hydroxytryptamine1A receptor function in rat hippocampus by short- and long-term administration of 5-hydroxytryptamine1A agonist and antidepressants.

Single s.c. injections of the 5-hydroxytryptamine (5-HT)1A receptor agonists buspirone at 4 mg/kg, 8-hydroxy-2-(di-n-propylamino)tetralin at 1 or 4 mg/kg or ipsapirone at 10 mg/kg did not affect 5-HT inhibition of forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes. However, a single injection of buspirone at 8 mg/kg, and daily injections of each of the agonists for 8 days, resulted in a reduction in the degree of enzyme inhibition by 5-HT. Chronic administration of the antidepressants fluoxetine, zimelidine and maprotiline by i.p. injections at 15 mg/kg for 3 weeks also resulted in a decreased degree of enzyme inhibition. Chronic iprindole at the same dose had no effect. It is concluded that the antidepressant-like properties of 5-HT1A receptor agonists may be mediated partly by a postsynaptic action at the level of serotonergic second messenger transduction in the hippocampus.     

Benzimidazolone derivatives: a new class of 5-hydroxytryptamine4 receptor agonists with prokinetic and acetylcholine releasing properties in the guinea pig ileum.

The influence of three azabicycloalkyl benzimidazolone derivatives, DAU 6236, BIMU 1 and BIMU 8, which act as agonists at central 5-hydroxytryptamine (5-HT)4 receptors, has been investigated on cholinergic neuromuscular transmission and peristalsis in the guinea pig small intestine. In the longitudinal muscle myenteric plexus preparations, these compounds caused a concentration-dependent (range 1-300 nM) enhancement of the amplitude of nerve-mediated cholinergic submaximal contractions to electrical stimulation. In comparison to the potentiating effect of 5-methoxytryptamine (a reference 5-HT4 receptor agonist), the rank order of agonist potency was BIMU 8 = BIMU 1 greater than DAU 6236 = 5-methoxytryptamine. In whole ileal segments, DAU 6236, BIMU 1 and BIMU 8 increased markedly (maximum increase, 200%) the frequency of peristalsis within the range of 0.1 to 3 microM. Micromolar concentrations of ICS 205-930, which is a low affinity antagonist of 5-HT4 receptors, were required to antagonize the facilitatory effect on cholinergic transmission caused by benzimidazolone derivatives and 5-methoxytryptamine (pA2 values, 6.5 in average) and to reverse the increase in the frequency of peristalsis induced by DAU 6236, BIMU 1 and BIMU 8. By contrast, the potent and selective 5-HT3 receptor antagonist ondansetron (1 microM) was ineffective. Our findings indicate that benzimidazolone derivatives act as agonists in the guinea pig ileum causing enhancement of acetylcholine release and peristaltic activity. The neural receptor site involved in the action of benzimidazolone derivatives and which showed low affinity for ICS 205-930 is probably identical to the putative 5-HT4 receptor subtype agonized by indoleamines and substituted benzamide derivative prokinetic agents.     

Coadministration of 5-hydroxytryptamine(1A) antagonist WAY-100635 prevents fluoxetine-induced desensitization of postsynaptic 5-hydroxytryptamine(1A) receptors in hypothalamus

Treatment with selective serotonin reuptake inhibitors induces a desensitization of hypothalamic postsynaptic 5-hydroxytryptamine (5-HT)(1A) receptors in humans and rats. This study investigated whether fluoxetine-induced desensitization is due to overactivation of postsynaptic 5-HT(1A) receptors; whether blockade of somatodendritic 5-HT(1A) autoreceptors accelerates this desensitization; and whether desensitization is associated with a reduction of Gz proteins, which couple to 5-HT(1A) receptors. WAY-100635 was tested at low doses (0.03-0.3 mg/kg), which antagonize somatodendritic 5-HT(1A) autoreceptors in the raphe nuclei, and at a higher dose (1 mg/kg), which completely blocks postsynaptic 5-HT(1A) receptors. Plasma levels of oxytocin and adrenal corticotrophic hormone (corticotropin) were measured as peripheral indicators of hypothalamic 5-HT(1A) receptor function. Daily injections of fluoxetine (10 mg/kg/day i.p.) for 2 days did not desensitize 5-HT(1A) receptors but three daily injections of fluoxetine produced a partial desensitization of the hormone responses to (+/-)-8-hydroxy-2-dipropylaminoetetralin (50 microg/kg s.c.). WAY-100635 (0.03-0.3 mg/kg) did not accelerate or potentiate the fluoxetine-induced desensitization of 5-HT(1A) receptors. However, WAY-100635 at a dose that completely blocks postsynaptic 5-HT(1A) receptors (1.0 mg/kg) completely prevented the fluoxetine-induced desensitization of 5-HT(1A) receptors. These data demonstrate that at least 3 days of fluoxetine exposure is required to produce a homologous desensitization of hypothalamic 5-HT(1A) receptors. Although previous studies indicate that injections of fluoxetine for 14 days produce a reduction of Gz protein levels in the hypothalamus, the levels of Gz proteins were not affected by either fluoxetine or WAY-100635. Alternative mechanisms mediating the initial stages of 5-HT(1A) receptor desensitization could involve post-translational modifications in the 5-HT(1A) receptor-Gz protein-signaling cascade.