Effects of triiodothyronine and fluoxetine on 5-HT1A and 5-HT1B autoreceptor activity in rat brain: regional differences

The thyroid hormone triiodothyronine (T3) augments and accelerates the effects of antidepressant drugs. Although the majority of studies showing this have used tricyclics, a few studies have shown similar effects with the selective serotonin re-uptake inhibitor (SSRI) fluoxetine. In this study we investigated the effects of fluoxetine (5 mg/kg), T3 (20 μg/kg) and the combination of these drugs, each administered daily for 7 days, on serotonergic function in the rat brain, using in vivo microdialysis. Fluoxetine alone induced a trend towards desensitization of 5-HT_1A autoreceptors as shown by a reduction in the effect of 8-OH-DPAT to lower 5-HT levels in frontal cortex, and desensitized 5-HT_1B autoreceptors in frontal cortex. The combination of fluoxetine and T3 induced desensitization of 5-HT_1B autoreceptors in hypothalamus. Since there is evidence linking hypothalamic function and depression, we suggest that this effect may partly account for the therapeutic efficacy of the combination of an SSRI and T3.     

No association of C-1019G promoter polymorphism of 5-HT1A receptor gene with migraine

It is well known that migraine has a strong genetic component, although the type and number of genes involved remains unclear. There is evidence to suggest that serotonin-related genes may be involved in the pathogenesis of migraine. To investigate whether the 5-HT1A receptor gene contributes to the risk of migraine we performed an association study of C-1019G promoter polymorphism of the gene in 102 migraineurs and 93 controls. Subjects were of Han Chinese origin. No significant differences in allele (P=0.82) or genotype frequencies (P=0.71) were seen in migraineurs compared with the controls. When migraine with aura, without aura, with family history, without family history were analyzed separately, the frequencies did not vary significantly. Our results suggest that C-1019G in 5-HT1A is not a major genetic risk factor for migraine.     

Sleep/waking effects of a selective 5-HT1A receptor agonist given systemically as well as perfused in the dorsal raphe nucleus in rats

Sleep/waking stages and behavior were studied following the selective 5-HT_1A agonist 8-OH-DPAT given subcutaneously (s.c.) (0.010–0.375 mg/kg) as well as perfused continuously (10 μM) for 6 h into the dorsal raphe nucleus (DRN) using microdialysis. Given systemically, 8-OH-DPAT at 0.375 mg/kg s.c. induced 5-HT behavioral syndrome, increased waking to 149% and reduced slow wave sleep (SWS) to 86%, transition to 76% and rapid eye movement (REM) sleep to 73%. The effect on deep SWS (SWS-2) was biphasic, with an increase after 2 h. 8-OH-DPAT at 0.010 mg/kg did not have any vigilance effects. 8-OH-DPAT perfusion in DRN produced a fourfold increase in REM sleep compared to perfusion of artificial cerebrospinal fluid. This is consistent with the hypothesis that reduced 5-HT neurotransmission following 5-HT_1A autoreceptor stimulation will disinhibit cholinergic REM-promoting mesopontine neurons and thereby lead to a REM sleep increase. The other sleep/waking stages were not significantly affected by 8-OH-DPAT perfusion in DRN.     

Serotonin-induced head shaking behavior in rats does not involve receptors located in the frontal cortex

Serotonin-induced head shaking behavior, a response associated with 5-HT₂ receptors, was examined in rats with lesions of the frontal cortex because of the high density of 5-HT₂ receptors in this area. Head shaking behavior caused by the serotonin precursor, 5-hydroxy-L-tryptophan, or by the serotonergic agonist, quipazine, was unchanged following the complete ablation of the frontal cortex. Although 5-HT₂ receptors are associated with the head shake response, this behavior is probably not related to serotonin receptors located in the frontal cortex.     

Autoradiographic analyses of 5-HT1A and 5-HT2A receptors after social isolation in mice

Social isolation of rodents is used to model human psychopathological processes. In the present study, the effects of intermediate and long term isolation housing on postsynaptic 5-HT_1A and 5-HT_2A receptors were analyzed in male mice housed in groups or isolation for 4 and 12 weeks. [³H]8-OH-DPAT and [³H]ketanserin were used to label 5-HT_1A and 5-HT_2A receptors. Four representative sagittal sections (planes 1–4) were scored by in vitro autoradiography. Whereas after 4 weeks of housing both receptor densities were lowered significantly in isolated mice, after 12 weeks of housing only marginal isolation effects were seen. Intermediate isolation reduced 5-HT_1A receptors especially in the lateral frontal, parietal and entorhinal cortex (−63%), in the lateral CA1–3 and dentate gyrus region of the hippocampus (−68%), in the basolateral, basomedial, central and medial amygdaloid nuclei (between −38 and −66%), and in the hypothalamus (−28%). 5-HT_2A receptors were strongly reduced in the frontal cortex (between −47 and −74%), in the hippocampus (between −47 and −95%), in the striatum (between −66 and −76%), and in the accumbens nucleus (between −59 and −73%) in comparison to group housed control mice. After 12 weeks of isolation in the hippocampus continuously decreased 5-HT_1A receptor densities were demonstrated (between −24 and −61%). But increased 5-HT_2A receptor densities were seen in the lateral striatum (+86%) compared to control mice. Age-dependent effects were also found. After 12 weeks of group housing the 5-HT_1A and 5-HT_2A receptor densities were decreased (between −28 and −54%) in all analyzed brain regions in comparison to 4 weeks of group housing. Isolated animals showed diminished 5-HT_1A receptor densities in the cortex (−14%) and hippocampus (−15%), but increased 5-HT_1A receptor densities in the amygdala (+33%) after 12 weeks. The 5-HT_2A receptor densities were increased in all analyzed regions (between +31 and +96%) after 12 weeks of isolation compared to 4 weeks. To explain these dynamic, time-dependent pattern of isolation-induced changes different regulation processes are supposed regarding 5-HT_1A and 5-HT_2A receptors. Besides metabolism-related adaptation processes also neurotransmitter and hormonal (e.g., glucocorticoid) interactions especially in limbic regions have to be considered.     

Activation of 5-HTIB receptors suppresses low but not high frequency synaptic transmission in the rat subicular cortex in vitro

We have shown previously that activation of 5-HT_1B serotonin receptors mediates suppression of the amplitude of evoked potentials in the subiculum [2]. Here we show that after application of 5-HT (10 μM), excitatory postsynaptic potentials of subicular neurons have reduced amplitudes with no change in membrane potential, input resistance and postynaptic fiber volleys. These results suggest that activation of 5-HT_1B receptors reduces the release of glutamate from incoming fibers originating from CA1 pyramidal cells. In presence of 5-HT (10 μM), theta patterned stimulation still induced LTP, similar to that observed in control slices. Application of similar concentrations of 5-HT during double pulse stimulation (interval 10–15 ms) reduced the response to the first pulse (repetition interval 30 s), but the response to the second pulse of the pair was unaffected. We propose that 5-HT_1B receptor activity suppresses subicular transmission at low but not at high frequencies.     

Fluoxetine-induced plasticity in the rodent visual system

We studied the effect of fluoxetine, a selective serotonin reuptake inhibitor, in the development and lesion-induced plasticity of retinotectal axons in pigmented rats. Neonatal rats received a daily injection of either fluoxetine or vehicle from postnatal day 1 (PND 1) to PND 10 or from PND 14 to PND 28 (fluoxetine, 7.5 and 10.0 mg/kg, respectively). In the latter group, some animals received a single lesion at the temporal periphery of the left retina at PND 21. Unoperated animals were use as the control. At the end of the treatment, the animals received an intraocular injection of horseradish peroxidase (HRP) in the right (intact) eye to trace the uncrossed retinotectal pathway. Chronic fluoxetine treatment, induced, in unoperated rats, an expansion of the retinal terminal fields along the rostro-caudal axis of the tectum both in the PND 10 and PND 28 groups. Following a retinal lesion in the left eye at PND 21, the vehicle-treated group showed a small reorganization of the intact uncrossed projection. In this group only a few terminals were labeled invading the denervated tectal surface one-week after the lesion. Fluoxetine-treated animals on the other hand, showed a great amplification of plasticity with a conspicuous sprouting of the uncrossed retinal axons into denervated areas. The data suggest that fluoxetine induces extensive axonal rearrangements in neonatal and juvenile central nervous system and amplifies neuroplasticity following retinal lesions late in development.     

Serotonin increases the excitability of the hypothalamic paraventricular nucleus magnocellular neurons

Recent evidence that 5-hydroxytryptamine (5-HT or serotonin) enhances the release and the gene expression of vasopressin and oxytocin in the hypothalamic paraventricular nucleus (PVN) suggests that 5-HT can excite the PVN magnocellular neurons. The objective of this study was to examine the underlying mechanisms for such excitatory action in the electrophysiologically identified hypothalamic PVN magnocellular neurons in rats using whole-cell patch-clamp. We found that 5-HT weakly depolarizes 33.3% of PVN magnocellular neurons in the presence of tetrodotoxin. A minuscule inward current was produced by 5-HT in 48% of the cells, which was attenuated when the 5-HT(4) antagonist GR113808 or the 5-HT(7) antagonist SB269970 was added. In addition, 5-HT reduced the frequency of miniature inhibitory postsynaptic currents in a dose-dependent manner. This inhibition was mimicked by the 5-HT(1B) agonist CP93129, and reversed in the presence of 5-HT(1B) antagonists cyanopindolol and SB224289. Besides, 5-HT induced a biphasic effect on the frequency of miniature excitatory postsynaptic currents, comprising a transient inhibition and a delayed concentration-dependent excitation (onset latency approximately 5 min). The facilitation was mimicked by the 5-HT(2A/2C) agonist DOI and abolished in the presence of the 5-HT(2C) antagonist RS102221. Our findings reveal that 5-HT directly increases the excitability of the PVN magnocellular neurons via multiple receptor subtypes and mechanisms. This may help understanding the regulation of 5-HT-induced hormone release and feeding behavior in the PVN.     

No Effect of Serotoninergic Gene Variants on Response to Interpersonal Counseling and Antidepressants in Major Depression

Objective

Gene variants within the serotonin pathway have been associated with major depressive disorder (MDD) treatment outcomes, however a possible different modulation on pharmacological or psychological treatments has never been investigated.

Methods

One hundred sixty MDD patients were partially randomized to either inter-personal counseling (IPC) or antidepressants. The primary outcome was remission at week 8. Five serotonergic polymorphisms were investigated (COMT rs4680, HTR1A rs6295, HTR2A rs2224721, HTR2A rs7997012 and SLC6A4 rs421417).

Results

IPC (n=43) and antidepressant (n=117) treated patients did not show any difference in remission rates at week 8 (corrected for baseline severity, age and center). None of the studied gene variants impacted on response and remission rates at week 8 neither in the IPC nor in the antidepressant group. An analysis of the whole sample showed a trend of association between rs7997012 AA genotype and a better treatment outcome.

Conclusion

Our study confirms that IPC is an effective psychological intervention comparable to antidepressants in mild-moderate MDD. Polymorphisms related to the serotonin system did not exert a major effect on clinical outcomes in none of the treatment groups.     

西酞普兰对脑卒中后抑郁大鼠海马齿状回5-羟色胺1A受体表达的影响

目的 观察西酞普兰对拟脑卒中后抑郁(post-stroke depression,PSD)大鼠海马齿状回5-羟色胺1A(5-HT1A)受体表达的影响,探讨其治疗PSD可能的药理机制.方法 将雄性SD大鼠分为3组正常对照组、拟PSD组和西酞普兰干预组.左侧大脑中动脉阻塞(MCAO)联合慢性不可预见温和应激刺激(CMS)及孤养法建立拟PSD动物模型,同时予西酞普兰(10 mg·kg-1·day-1)干预4周,荧光实时定量PCR和Western印迹方法检测并比较CMS开始后第19、28天各组大鼠海马齿状回5-HT1A受体的基因(mRNA表达水平)和蛋白表达水平.结果 CMS开始后第19天,西酞普兰组5-HT1A受体的基因和蛋白表达均高于拟PSD组[(0.131±0.008)vs(0.012±0.001)和(0.95±0.06)vs(0.40±0.03),P均小于0.001].第28天,西酞普兰组5-HT1A受体的基因和蛋白表达均高于拟PSD组[(0.224±0.012)vs(0.013±0.001)和(0.52±0.06)vs(0.08±0.02),P均小于0.001].结论 西酞普兰促进拟PSD大鼠海马齿状回5-HT1A受体的基因和蛋白表达,从而可促进海马神经重塑,这可能为西酞普兰治疗PSD的分子机制之一.     

Postnatal treatment with NAN-190 but not with 5-HT1A receptor agonists retards growth of the rat brain

We investigated the influence of prolonged administration of the 5-HT1A receptor agonists (8-OH-DPAT or buspirone) or its antagonist, NAN-190 to rat pups on development of their cortical barrel field. Pups were injected daily with the drugs starting from the day of birth till either the 5th postnatal day or the 22-25th postnatal day and were perfused one day later. Square areas of their whisker barrel fields were measured on tangential sections of the cortex stained for cytochrome oxidase. Injections of 8-OH-DPAT or buspirone till the 5th postnatal day did not change any of the investigated parameters, while injections of NAN-190 resulted in 15% reduction of the pups' body and brain weight and proportional reduction of the square area of their barrel fields. Groups treated till the 22-25th postnatal day showed similar results. Some of these pups were injected with [C(14)]2-deoxyglucose to investigate the strength of responses of their cortical barrels to stimulation of corresponding vibrissae. The cortical area labeled with 2-deoxyglucose after stimulation of vibrissae of the row C was narrower in the NAN-190 injected rats. This functional deficit was more pronounced than the anatomical one, which resembled the effects of neonatal serotonin depletion (Neuroreport, 1997). Therefore, the results of injecting NAN-190 to the rat pups point to a deficit of trophic developmental influences of serotonin, adding new arguments for the hypothesis of a trophic role of 5-HT1A receptors in the brain development.     

5-HT1B基因A161T多态性与抑郁症的关联研究

目的探讨抑郁症患者与5-HT1B受体基因A161T多态性之间的关系。方法应用聚合酶链式反应（PCR）扩增技术及限制性片段长度多态性（arLP）分别检测365例抑郁症患者（病例组）、365名健康人（对照组）的5-HT1B受体基因A161T多态性。结果5-HT1B受体基因A161T多态性在病例组和对照组的基因型和等位基因分布频率无显著性差异；按照发病年龄（30岁为界）、有无家族史及有无自杀观念分层后，各亚组与对照组间基因型和等位基因分布也无显著性差异。经多因素分析控制年龄、性别因素后各组基因型分布仍无显著性差异。结论5-HT1B受体基因A161T多态性可能不是抑郁症及其各临床亚型发病的一个危险因素。     

Effects of serotonergic drugs in experimental brain ischemia: evidence for a protective role of serotonin in cerebral ischemia

We have examined the significance of the serotonergic system in the pathophysiology of ischemic brain damage. Permanent occlusion of the middle cerebral artery (MCA) was performed in male NMRI mice. After 48 h, the animals received a transcardiac injection of carbon black. The area of ischemia was restricted to the neocortex and its size was determined planimetrically by means of an image analyzing system. In control experiments, the NMDA antagonist dizocilpine (MK-801), the AMPA/kainate antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)-quinoxaline) and the L-type calcium channel blocker nimodipine all produced a significant reduction in ischemic injury of the mouse neocortex. Interestingly, all of the 5-HT_1A agonists tested (ipsapirone, CM 57493 [4-(3-trifluoromethylphenyl)-1-(2-cyanoethyl)-1,2,3,6-tetrahydropyridine] and urapidil) were equally efficacious in reducing ischemic injury. On the other hand, the 5-HT₂ antagonist naftidrofuryl failed to protect the brain tissue significantly against ischemic brain damage. Roxindole, a 5-HT_1A agonist and 5-HT uptake inhibitor, was the most potent serotonergic compound tested. In order to examine the effects of 5-HT_1A receptor activation in a different context, 10 min of forebrain ischemia was induced in male Wistar rats by a bilateral occlusion of the common carotid arteries combined with systemic hypotension. Administration of the 5-HT_1A agonist CM 57493 reduced the neuronal damage within the ventral hippocampus and the entorhinal cortex as assessed histologically 7 days after ischemia. Finally, we found that 5-HT_1A agonists are capable of reducing neuronal damage of cultured neocortical and hippocampal neurons subjected to a chemical hypoxia or glutamate in a dose dependent manner. These data suggest that 5-HT, released during ischemia, may have protective effects in the pathophysiology of ischemic brain damage through a direct action on neurons mediated via the inhibitory 5-HT_1A receptor subtype. The results obtained from different in vivo and in vitro models indicate that 5-HT_1A agonists are promising agents for the treatment of ischemic brain disorders.     

Further evidence for differential affinity states of the serotonin1A receptor in rat hippocampus.

The binding profile of [³H]8-hydroxy-2-(di-N-propylamino)-tetralin ([³H]8-OH-DPAT) to serotonin_1A (5-HT_1A) sites in rat hippocampal, frontocortical and striatal membranes has been compared. In these regions, [³H]8-OH-DPAT labels both a high and a low-affinity binding sites; the affinity values for each of the two sites are comparable in the different brain regions, but have different maximal capacity. By modifying the experimental conditions in a series of hippocampal membrane preparations, reciprocal changes in the proportion of the two sites were observed suggesting that they represent, at least in this region, different conformations or affinity states of a single receptor protein. In contrast to the lower affinity states, it appears that the high-affinity state is stabilized by coupling with a G-protein. Evidence supporting this statement is provided by addition of the guanine nucleotide Gpp(NH)p, breakage of labile disulfide bonds using N-ethylmaleimide and increasing membrane rigidity with ascorbate-induced lipid peroxidation, conditions which all reduced the density of receptors in the high-affinity state. Moreover, the high-affinity state appears to be stabilized at the expense of the lower affinity state in the presence of Mn²⁺. On the other hand, a complete shift to the low-affinity binding state was observed after a 24 h in vivo treatment with inhibitors of monoamine oxidase A (phenelzine or clorgyline) but not of monamine oxidase B (deprenyl). This disappearance of the high-affinity state with a concomitant increase in the binding capacity of the low-affinity state was reproduced by inhibiting monoamine oxidase A in vitro, as well as by reducing preincubation washout periods. Also, competitors of the [³H]8-OH-DPAT binding site, such as serotonin and unlabelled 8-OH-DPAT, display two affinity states while others like (±)-propranolol, tryptamine and spiperone recognize a single affinity component. These results suggest that the 5-HT_1A binding site may exhibit at least two different affinity states depending upon its microenvironment and the intrinsic activity of the ligand used.     

Vilazodone: A 5-HT1A Receptor Agonist/Serotonin Transporter Inhibitor for the Treatment of Affective Disorders

Vilazodone ( EMD 68843; 5-{4-[4-(5-cyano-3-indolyl)-butyl]-1-piperazinyl}-benzofuran-2-carboxamide hydrochloride) is a combined serotonin specific reuptake inhibitor (SSRI) and 5-HT_1A receptor partial agonist currently under clinical evaluation for the treatment of major depression. This molecule was designed based on the premise that negative feedback circuitry, mediated via 5-HT₁ receptors, limits the acute SSRI-induced enhancements in serotonergic neurotransmission. If the hypothesis is correct, combination of SSRI with 5-HT_1A partial agonism should temporally enhance the neuroplastic adaptation and subsequently hasten therapeutic efficacy compared to current treatments. Preclinical in vitro evaluation has confirmed vilazodone's primary pharmacological profile both in clonal and native systems, that is, serotonin reuptake blockade and 5-HT_1A partial agonism. However, in vivo and in contrast to combination of 8-OH-DPAT and paroxetine, vilazodone selectively enhanced serotonergic output in the prefrontal cortex of rats. Behavioral evaluations, in the ultrasonic vocalization model of anxiety in rats, demonstrated anxiolytic efficacy. In the forced swim test (a putative model of depression), vilazodone also showed efficacy but at a single dose only. In man, vilazodone abolished REM sleep and demonstrated clinical antidepressant efficacy equivalent to an SSRI. Ongoing clinical evaluations will hopefully reveal whether the founding hypothesis was valid and if vilazodone will produce a more rapid onset of antidepressant efficacy.     

Involvement of 5-HT1A receptor mechanisms in the inhibitory effects of methamphetamine on photic responses in the rodent suprachiasmatic nucleus

We examined the role of serotonin 1A (5-HT_1A) receptors in the inhibitory effects of methamphetamine (MA) on photic entrainment to the circadian pacemaker in the suprachiasmatic nucleus (SCN) of rodents. MA inhibited optic nerve stimulation-evoked field potential in the SCN, light-induced Fos expression in the SCN and light-induced phase shift of hamster wheel-running rhythm. NAN-190, a 5-HT_1A receptor antagonist, eliminated the inhibitory effects of MA. NAN-190 has also been reported to antagonize ₁ adrenergic receptors. However, prazosin, which selectively antagonizes ₁ adrenergic receptors, did not affect the inhibitory action of MA on light-induced Fos expression. In addition, parachloroamphetamine, which is known to be a 5-HT releaser, dose-dependently inhibited light-induced phase shift of wheel-running rhythm. These findings suggest that elevation of endogenous 5-HT levels by MA inhibits the photic entraining responses of the circadian pacemaker in the SCN via 5-HT_1A receptor stimulation of the 5-HT released by MA.     

Apparent absence of serotonin1B receptors in biopsied and post-mortem human brain

In order to determine if post-mortem delays could account for the apparent absence of 5-HT1B sites reported in human brain, 5-HT1B binding parameters were determined in fresh and fresh-frozen cortical biopsies and compared to results obtained in post-mortem tissues. Binding parameters and in vitro receptor autoradiography were performed by using two different ligands which have been shown to label 5-HT1B sites, namely [3H]5-HT, in presence of 100 nM 8-OH-DPAT, and [125I]cyanopindolol, in presence of 10 microM (-)isoproterenol. No specific binding was detected with [125I] cyanopindolol in either fresh cortical biopsies or post-mortem tissues, suggesting that the apparent absence of 5-HT1B receptor sites reported earlier was not related to long post-mortem delays. Some specific labelling was seen with [3H]5-HT, in presence of 8-OH-DPAT. This binding, which is unlikely to be to the 5-HT1B type, could represent labelling to the 5-HT1D or 5-HT1E sites.     

Serotonergic receptors in the brain of in utero undernourished rats

In this study, we report that 5-HT(1A) receptors are already present in fractions of axonal growth cones, from the normal rat fetal brain (E-17). Also, in utero undernourished (UN) rat pups at birth show a noteworthy enhancement in the B(max) of [3H]5-hydroxytryptamine (5-HT) and [3H]8-hydroxy-(2-N,N-dipropilamin)-tetralin (([3H])8-OH-DPAT), in the brainstem and cerebral cortex up to the second week after birth. Afterwards, there is a significant decrease in the binding of these ligands. [125I]Cyanopindolo binding in the cerebral cortex only showed a decrease in the same period. An elevation of brain serotonin in both regions was also present. These findings together, suggest that the mechanisms of regulation of serotonergic receptors' expression during the period studied, may not depend on the amount of neurotransmitter in the synaptic cleft, because in the early UN brain it would be expected only a lower receptor's density due to the chronic serotonin increase. On this basis, we propose that developmental activation of brain serotonin biosynthesis observed in early UN animals may disrupt the mechanism regulating the expression of 5-HT receptors during development.