铁离子对黑质纹状体多巴胺神经元毒性作用的实验研究

目的 探讨铁离子对黑质纹状体多巴胺神经元的毒性作用。方法 采用立体定向偏侧大鼠黑质内注入50μg FeCl3和FeCl2，4周后用阿朴吗啡诱导动物行为学变化，高效液相色谱(HPLC)检测纹状体内多巴胺、去甲肾上腺素、肾上腺素递质含量的变化，免疫组化观察黑质多巴胺神经元和胶质细胞的改变。结果 FeCL3和FeCL2均可引起注射侧纹状体内DA含量选择性降低，而NA、A含量无显著改变；注射侧黑质内DA神经元显著缺失、胶质细胞显著增生；FeCL3组阿朴吗啡诱导大鼠向同侧旋转行为，FeCL2组大鼠于术后即出现特征性自发性对侧旋转行为，阿朴吗啡不能诱发其旋转。结论 铁离子对黑质纹状体多巴胺神经元具有毒性作用，Fe^3 作用最强，胶质细胞的增生可能参与了这一毒性作用过程。     

脑源性神经营养因子对帕金森病大鼠多巴胺能神经元的影响

目的观察脑源性神经营养因子对帕金森病(Parkinson’s disease,PD)大鼠模型黑质多巴胺能神经元的影响。方法选用Wistar种系大白鼠30只,体质量230~250g,随机分3组,通过左侧中脑黑质立体定向注射法,组1为生理盐水对照组(简称对照组)10只,注射相应量(5μL)的生理盐水;组2为注射6-OHDA制作帕金森病模型组(简称6-OHDA组)10只,注射6-OHDA,5μL(2μg/μL);组3为(6-OHDA+BDNF)组,在制成帕金森病模型后再向同侧中脑黑质注射BDNF 5μL(3μg/5μL),连续6d,1次/d。分别观察动物的旋转行为,免疫组化染色方法观察黑质酪氨酸羟化酶(tyrosine hydroxylase,TH)阳性神经元的数量,高效液相法测定纹状体部多巴胺(dopamine,DA)含量的变化。结果单侧黑质内注入6-OHDA制成帕金森病大鼠模型后,6-OHDA组与对照组比较,产生旋转行为,(6-OHDA+BDNF)组在观察旋转行为时,症状明显改善;镜下见TH阳性神经元主要见于对照组的黑质致密部,数量为(42.3±7.56)个/μm2,模型组黑质致密部TH阳性神经元数明显减少为(2.41±1.07)个/μm2,(6-OHDA+BDNF)组黑质致密部TH阳性神经元数为(15.36+3.04)个/μm2;纹状体部多巴胺含量:生理盐水组为(11.4±1.2)μg/g,6-OHDA组(3.6±0.5)μg/g,(6-OHDA+BDNF)组(5.5±0.6)μg/g。结论 BDNF能改善6-OHDA所致的帕金森病大鼠黑质多巴胺能神经元数目的减少;明显抑制6-OHDA引起的纹状体部多巴胺含量降低;并可抑制6-OHDA对黑质多巴胺能神经元的毒性作用。     

脂多糖对大鼠黑质纹状体多巴胺能系统的影响

目的　观察脂多糖(lipopolysaccharide , LPS)对大鼠黑质纹状体系统的神经毒性作用,探讨帕金森病(Parkinson disease,PD)与炎症反应的关系。方法　采用立体定向术向大鼠脑黑质注射5μg LPS,于注射后不同时间点(1、3、7、14、21、30 d)观察经腹腔注射阿朴吗啡后大鼠的旋转行为以及采用高效液相色谱(HPLC)测定黑质 纹状体单胺类递质含量的变化;通过免疫组化观察多巴胺(dopamine,DA)合成限速酶酪氨酸羟化酶(tyro sine hydroxylase,TH)的变化。结果　LPS注射后 14、21、30 d大鼠出现向注射侧的旋转行为,黑质 纹状体 DA及其代谢产物随时间延长呈不同程度下降(P<0 05),而 5 羟色胺(5 HT)仅有短暂下降,去甲肾上腺素(nora drenaline,NA)无变化。TH阳性细胞数在第3天开始下降为对照组的45%,14 d时为对照组的5%~10%,30 d时几乎完全消失。结论　LPS注射黑质后能特异性损害DA能神经元,可用于建立PD免疫炎症动物模型。     

神经节苷脂对帕金林病鼠旋转行为,纹状体多巴胺浓度及黑质…

观察神经节苷脂对帕金森病鼠模型旋转行为、纹状体多巴胺浓度及黑质病理的影响。方法将６－羟基多巴胺用立体定地注入大鼠－－侧中脑被盖腹侧区制作ＰＤ大鼠模型，并于同侧侧脑室注射混合型神经节苷脂，观察ＧＭ对由阿朴吗啡所诱发的旋转行为、受损侧纹状全多巴胺浓度及黑质病理的影响。结果 ＧＭ能减轻ＰＤ大鼠模型的旋转行为，、受损侧纹状体多巴胺浓度下降和黑质神经细胞减少。结论ＧＭ可减轻６－羟基多巴胺对黑质多巴胺了的听凭     

侧脑室微量注射左旋多巴治疗大鼠帕金森病

目的观察经侧脑室注射左旋多巴对帕金森病（PD）大鼠的影响。方法应用“羟基多巴胺立体定向脑内注射制备偏侧损毁的PD大鼠模型，并用阿扑吗啡（APO）皮下注射诱发大鼠向健侧旋转。将24只PD大鼠随机分为4组（n=6），经侧脑室注入生理盐水组为对照组，余3组分别经侧脑室注射浓度为0．1μg／μl、1μg／μl和5μg／μl的左旋多巴1μl，4μl／d，连续1周；观察在注射后不同时间大鼠旋转行为以及中脑黑质多巴胺能神经元数量的变化。结果经侧脑室注射1μg/μl和5μg/μl的左旋多巴后。与对照组相比，PD大鼠向健侧的旋转圈数明显减少（P〈0．01），左旋多巴效果在2h左右达到高峰，且中脑黑质多巴胺能神经元的数量也明显增多（P〈0．01）。结论经侧脑室注射适当剂量的左旋多巴可有效地改善PD大鼠的旋转行为，并增加中脑黑质多巴胺能神经元数量。     

帕金森病大鼠黑质NurrlmRNA表达的动态变化

目的　探讨帕金森病大鼠黑质核内受体相关因子 1(Nurr1)mRNA表达的动态变化。方法　通过脑立体定位注射 6 羟基多巴胺 (6 OHDA)的方法建立大鼠帕金森病 (Parkinson’sdisease,PD)模型 ,采用HE染色 ,酪氨酸羟化酶 (TH)免疫组织化学染色、原位杂交技术 ,选择 6 OHDA注射术后 1d、3d、5d、7d、2 1d为研究时点 ,观察大鼠PD模型形成过程中黑质TH 多巴胺细胞数量及Nurr1mRNA表达的改变。结果　与健侧比较 ,注射 6 OHDA 5d组损毁侧黑质TH 细胞显著减少 (P <0 .0 1) ,2 1d时仅为健侧的 15 %且出现明显的旋转行为 ;同时 ,注射 6 OHDA 1d组损毁侧黑质Nurr1mRNA表达即开始下降 ,且以 3d组最为显著 (P <0 .0 1) ,7d以后各组完全消失。结论　本实验研究结果表明 ,6 OHDA能下调大鼠黑质Nurr1mRNA的表达早于诱导多巴胺细胞的死亡     

黑质注射脂多糖致大鼠黑质纹状体多巴胺含量下降

目的 :探讨黑质内注射脂多糖对黑质纹状体系统单胺类递质的影响。方法 :将 2 0只SD大鼠随机分成A、B、C 3组和对照组D ,取各组大鼠纹状体和黑质组织 ,采用高压液相色谱 电化学法 (HPLC ECD)检测该部位的单胺类递质含量。结果 :A、B、C 3组纹状体、黑质部位多巴胺及其代谢产物含量明显并持续降低 ,较D组均有显著性差异 ,但纹状体、黑质部位 5 羟色胺含量仅有一过性变化。结论 :黑质内注射脂多糖能使纹状体、黑质多巴胺及其代谢产物含量显著下降     

递增负荷运动大鼠脑皮质神经元和脑脊液中去甲肾上腺素、5-羟色胺的动态变化*

背景：运动性疲劳的发生、恢复是否与脑脊液循环存在着某种联系？ 目的：观察递增负荷运动后大鼠大脑皮质和脑脊液中去甲肾上腺素、5-羟色胺的动态变化及其相关性。 方法：随机将30只雄性SD大鼠分为对照组10只,运动组20只,运动组采用6周递增负荷运动方案训练,游泳时间由第1周的10 min递增为第6周的60 min,负重由体质量的3%增至体质量的5%。对照组正常饲养。 结果与结论：高效液相色谱检测结果显示,递增负荷运动后即刻,大鼠大脑皮质和脑脊液中去甲肾上腺素、5-羟色胺的水平显著增高;递增负荷运动后24 h,大鼠大脑皮质中去甲肾上腺素、5-羟色胺水平继续增高(P < 0.01),而脑脊液中去甲肾上腺素、5-羟色胺降至正常水平。其中,递增负荷运动后即刻,大鼠大脑皮质中的去甲肾上腺素与5-羟色胺水平及呈显著正相关(P < 0.05),大脑皮质和脑脊液中的去甲肾上腺素及5-羟色胺水平高度相关(P < 0.01)。提示递增负荷运动后去甲肾上腺素的兴奋作用和5-羟色胺的抑制作用处于动态平衡中。递增负荷运动后的即刻,脑脊液中去甲肾上腺素、5-羟色胺的变化能够反映大脑皮质中去甲肾上腺素、5-羟色胺的变化。     

神经节苷脂对帕金森病鼠旋转行为、纹状体多巴胺浓度及黑质病理的影响

目的观察神经节苷脂对帕金森病（Ｐａｒｋｉｎｓｏｎｄｉｓｅａｓｅ，ＰＤ）鼠模型的旋转行为、纹状体多巴胺浓度及黑质病理的影响。方法将６－羟基多巴胺用立体定向法注入大鼠一侧中脑被盖腹侧区制作ＰＤ大鼠模型，并于同侧侧脑室注射混合型神经节苷脂（ａｍｉｘｅｄｇａｎｇｌｉｏｓｉｄｅｐｒｅｐａｒａｔｉｏｎ，ＧＭ），观察ＧＭ对由阿朴吗啡所诱发的旋转行为、受损侧纹状体多巴胺浓度及黑质病理的影响。结果ＧＭ能减轻ＰＤ大鼠模型的旋转行为、使受损侧纹状体多巴胺浓度下降和黑质神经细胞减少。结论ＧＭ可减轻６－羟基多巴胺对黑质多巴胺能神经元的损伤。     

一种帕金森病大鼠模型的建立及评价

目的探讨应用6-羟基多巴胺（6-OHDA）毁损大鼠黑质致密部制作偏侧帕金森病（PD）模型的方法和应用价值。方法采用立体定向微量注射6-OHDA于大鼠黑质致密部,观察经阿朴吗啡诱导后大鼠的行为及黑质多巴胺能神经元形态学变化。结果部分大鼠注射后即出现行动迟缓、少动、竖毛、躬身、尾部强直、肢体震颤、嗅探和易激惹等异常行为。术后4周时,共33只大鼠经阿朴吗啡诱导后在30min（P〈0．01）的平均旋转圈数〉7r／min,达到成功模型的标准,模型成功率为82．5％（33／40）。免疫组化观察发现模型组大鼠注射侧黑质区多巴胺能神经元较对侧和对照组注射侧区明显减少（P〈0．01）。结论利用6-OHDA毁损大鼠黑质致密部可以较快建立稳定的PD大鼠模型,方法简便实用,动物死亡率低,模型成功率高。     

Effects of capsaicin on central monoaminergic mechanisms in the rat

Summary The acute and chronic effects of capsaicin (s.c.) on the monoamines in the preoptic region + hypothalamus (RPO-H), spinal cord, substantia nigra and striatum were studied. Levels of DOPA, DA, DOPAC, HVA, 3-MT, NA, Trp, 5-HTP, 5-HT and 5-HIAA were determined by means of liquid chromatography (HPLC-EC). In response to acute capsaicin treatment, the levels of DA, DOPAC and DA synthesis rate (DOPA formation) were increased in a dose-dependent manner in the RPO-H and spinal cord. The disappearance rate of NA was accelerated in both regions. In substantia nigra, increased DOPAC levels were found whereas the levels of 3-MT were decreased in striatum after acute capsaicin treatment. Only minor changes on the levels of 5-HT and 5-HIAA in the regions studied were noted. Neonatal or adult capsaicin treatment failed to affect the levels of NA, DA and 5-HT (measured two months or five weeks after injection, respectively) in the regions studied. A capsaicin injection to rats pretreated with the drug as adults did not affect either the monoamines in the RPO-H and spinal cord or the body temperature. In contrast, in rats pretreated with capsaicin as neonates, a second injection of the drug to adult animals elicited hypothermia and changes in monoamines similar to those observed in naive animals.     

Dopaminergic projection from nucleus raphe dorsalis to neostriatum in the rat

The existence of a dopamine (DA) projection from nucleus raphe dorsalis (RD) to neostriatum was demonstrated in the rat by combined tyrosine hydroxylase (TH) immunohistochemistry and radioautography after retrograde axonal transport of [3H]noradrenaline ([3H]NA). Intrastriatal injections of [3H]NA were carried out in normal rats or after ipsilateral destruction of the nigrostriatal DA system by injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra. Some 1,000 TH-positive nerve cell bodies were counted within the confines of RD as defined by its content in serotonin (5-HT) neurons. These DA neurons occupied the upper third of the RD and they were part of its small cell population. In all cases, a small proportion of the TH-immunoreactive nerve cell bodies in RD were retrogradely radiolabeled. Radiolabeled but immunonegative cells were exceedingly rare. The double-labeled neurons were generally more numerous after elimination of the nigrostriatal DA innervation than in normal rats. They mostly lay within the ventral portion of the medial subdivision of RD and always predominated on the [3H]NA- injected side. Some were also present in nucleus linearis caudalis. It was concluded that [3H]NA had been taken up and retrogradely transported exclusively by catecholamine neurons; part of the DA cell group in RD projects to the neostriatum; and that most if not all non-5-HT neurons projecting from RD to neostriatum are likely to be dopaminergic.     

Serotonergic sprouting is induced by dopamine-lesion in substantia nigra of adult rat brain

We have previously extracted a serotonin (5-HT) neurotrophic supernatant from the 5,7-DHT lesioned hippocampus. The current study shows that a new 5-HT neurotrophic signal was monitored in the striatum and nigra after DA-denervation. Such a signal may be involved in the heterotypic sprouting. Dopaminergic neurotoxin, 6-hydroxydopamine (6-OHDA), was injected directly into the substantia nigra of adult rats. Two months after surgery, immunocytochemical staining showed that tyrosine hydroxylase (TH)-positive cell bodies had mostly disappeared in the substantia nigra, and TH-positive terminals in the striatum were almost completely depleted. Meanwhile, the 5-HT fibers, which exist in the same areas with low density, sprouted in the nigra as well as in the striatum and became dense. Normally 5-HT fibers innervate the striatum sparsely and the globus pallidus densely with sharp delineation (in the control side), and become dense across both areas with no appreciable delineation (in the lesion side). The increase of 5-HT fibers was more prominent in the posterior than in the anterior striatum. A significant increase in 5-HT and 5-HIAA levels was also evident in the posterior striatum when the decrease in DA level exceeded 90% in the nigra and striatum. In addition, we found that induction of 5-HT sprouting requires a greater than 90% decrease of DA level. Current data support that 6-OHDA injection in the substantia nigra of adult rats triggered a trophic signal or removed an inhibition for the growth of 5-HT neurons which responded with sprouting in the nigra as well as in the striatum.     

High-sensitive liquid chromatographic method for determination of neuronal release of serotonin, noradrenaline and dopamine monitored by microdialysis in the rat prefrontal cortex

A high-sensitive liquid chromatographic method based on precolumn derivatization and fluorescence detection allowing simultaneous determination of serotonin (5-HT), noradrenaline (NA) and dopamine (DA) in brain microdialysis samples is described. 5-HT, NA and DA were derivatized with benzylamine and 1,2-diphenylethylenediamine in the presence of potassium hexacyanoferrate(III) and glycine, which yielded to highly fluorescent and stable benzoxazoles. The derivatized samples were separated on a microbore column (150 mm × 1.0 mm i.d., packed with C18 silica, 5 μm) within 60 min. The mobile phase consisted of acetonitrile–Briton–Robinson buffer (pH 7.2) (32:68, v/v) containing 5 mM Na₂EDTA and 5 mM octanesulfonic acid sodium salt. The detection limits (signal-to-noise ratio of 3) for 5-HT, NA and DA were 76, 42 and 95 amol/10 μl injected on-column, respectively. Microdialysis samples were collected at 10-min intervals from the probes implanted in the prefrontal cortex of awake rats. The basal levels of 5-HT, NA and DA were 7.3 ± 0.7, 5.3 ± 0.31 and 8.1 ± 0.47 fmol/5 μl (mean ± S.E.M., n = 5). Following 90-min perfusion with tetrodotoxin (1 μM) or calcium-free Ringer solution, the DA and NA levels were reduced to about 15 and 20%, respectively and the 5-HT levels to 45 and 60% of the basal levels, respectively. Reserpine, 12 h after a dose of 5 mg/kg i.p., reduced the extracellular 5-HT, NA and DA concentrations to about 34, 39 and 32% of the basal levels, respectively. In conclusion, the preset microdialysis/analytical method enables simultaneous monitoring of basal and pharmacologically reduced neuronal release of 5-HT, NA and DA in the rat brain.     

Dopamine and noradrenaline content in fish retina: modulation by serotonin

The presence of noradrenaline (NA) and the possible interaction of serotonin (5-HT) with dopaminergic and noradrenergic neuronal elements was studied in the retina of the teleost Eugerres plumieri. By means of a histofluorescent technique, paired amacrine cells can be visualized after intravitreal injection of NA or 5,6-dihydroxytriptamine, suggesting their probable catecholaminergic and indoleaminergic nature. With a high-performance liquid chromatographic (HPLC) method and after pargyline treatment of the animal, 6.86 ng/mg protein of dopamine (DA) was detected, while NA content was 0.50 ng/mg protein. The NA levels of the retina increased, whereas the DA levels decreased in a significant manner after in vivo treatment with 5-HT. 5-methoxy-N,N-dimethyltryptamine, a 5-HT agonist, was able to mimic this effect partially, while the agonists tryptamine and quipazine did not affect the levels of DA and NA. The antagonists methysergide, metergoline, and cyproheptadine significantly blocked the 5-HT-induced NA increase, whereas only the first two antagonists were effective in inhibiting the 5-HT-induced DA decrease. The 5-HT modulation of NA and DA is apparently receptor mediated and is not due to a hetero-exchange, since imipramine was not able to block the 5-HT effect. These findings support the suggested interaction between serotoninergic and dopaminergic cells [Negishi et al: J Neurosci Res 5:621-635, 1980; Neurosci Lett 25:1-5, 1981]. Furthermore, they demonstrate the possible modulation which 5-HT exerts on the endogenous levels of NA.     

Quantification and pharmacological characterization of dialysate levels of noradrenaline in the striatum of freely-moving rats: release from adrenergic terminals and modulation by alpha2-autoreceptors

Information concerning striatal levels of noradrenaline (NA) remains inconsistent. Here we have addressed this issue using a sensitive method of HPLC coupled to amperometric detection. The NA reuptake-inhibitor, reboxetine, selectively elevated levels of NA versus dopamine (DA), and NA levels were also selectively elevated by the alpha2-adrenoceptor (AR) antagonist, atipamezole. The actions of atipamezole were mimicked by the preferential alpha2A-AR antagonist, BRL44408, while JO-1 and prazosin, preferential antagonists at alpha2C-ARs, caused less marked elevations in NA levels. In contrast to antagonists, the alpha2-AR agonist, S18616, decreased NA levels and likewise suppressed those of DA. Unilateral lesions of the substantia nigra with 6-hydroxydopamine depleted DA levels without affecting those of NA. Further, the D3/D2 receptor agonist, quinelorane, decreased levels of DA without modifying those of NA. However, the D3/D2 receptor antagonists, haloperidol and raclopride, and the DA reuptake-inhibitor, GBR12935, elevated levels of both DA and NA. Levels of 5-HT (but not of NA or DA) were increased only by the 5-HT reuptake-inhibitor, citalopram. They were decreased by S18616 and prazosin, reflecting the inhibitory and excitatory influence of alpha2- and alpha1-ARs, respectively, upon serotonergic pathways. In conclusion, NA in the striatum is derived from adrenergic terminals. Its release is subject to tonic, inhibitory control by alpha2-ARs, possibly involving both alpha2A- and alpha2C-AR subtypes, though their respective contribution requires clarification. A role of dopaminergic terminals in the reuptake of NA likely explains the elevation in its levels elicited by DA reuptake-inhibitors and D3/D2 receptor antagonists.     

Activation and blockade of dorsal hippocampal Serotonin6 receptors regulate anxiety-like behaviors in a unilateral 6-hydroxydopamine rat model of Parkinson’s disease

ABSTRACT

Objectives: This study aimed to investigate the effect of serotonin₆ (5-HT₆) receptors in the dorsal hippocampus (dHip) on the regulation of Parkinson’s disease (PD)-associated anxiety.

Methods: We examined whether intra-dHip injection of both 5-HT₆ receptor agonist and antagonist was involved in the regulation of anxiety-like behaviors in sham-operated rats and rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle by the open-field and elevated plus maze (EPM) tests. Four weeks after injection of 6-OHDA, the concentrations of dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) in relative related brain regions were measured by reverse-phase high-performance liquid chromatography.

Results: In sham-operated rats, intra-dHip injection of both 5-HT₆ receptor agonist WAY208466 (3 and 6 µg/rat) and antagonist SB258585 (4 µg/rat) increased the percentage of time spent in the center area in the open-field test and percentages of open arm entries and open arm time in EPM test, indicating that induced anxiolytic effects. In the lesioned rats, WAY208466 (1.5, 3 and 6 µg/rat) produced anxiolytic responses, whereas SB258585 (2 and 4 µg/rat) produced anxiogenic effects. Neurochemical results showed that intra-dHip injection of WAY208466 (6 µg/rat) decreased NA level in the amygdala, and SB258585 (4 µg/rat) increased DA levels in the dHip and vHip in sham-operated rats, whereas WAY208466 increased DA levels in the dHip, vHip, and amygdala in the lesioned rats.

Discussion: dHip 5-HT₆ receptors are involved in the regulation of anxiety-like behaviors, which may be mediated through different neurochemical mechanisms, and the dHip is an important site involved in these effects.

Abbreviation: PD: Parkinson’s disease;6-OHDA: 6-hydroxydopamine; dHip: dorsal hippocampus; vHip: ventral hippocampus; 5-HT: serotonin; MFB: medial forebrain bundle; DA: dopamine; NA: noradrenaline; EPM: elevated plus-maze; GABA: gamma-aminobutyric acid; BLA: basolateral amygdala     

Behavior, neurochemistry and histology after intranigral lipopolysaccharide injection

Inflammation and neuronal degeneration of the substantia nigra (SN) occur in Parkinson's disease (PD). We studied the effects of intranigral lipopolysaccharide (LPS) injection on adult Sprague-Dawley rats. Locomotor activity measurement, neurotransmitter determination and perfusion fixation for immunohistochemistry were done on the 7th day. Bilateral LPS injection increased locomotor activity 2- to 3-fold. In the SN, dopamine (DA) and serotonin (5-HT) decreased but the ratios dihydroxyphenylacetic acid (DOPAC)/DA, homovanillic acid (HVA)/DA and 5-hydroxyindole-acetic acid (5-HIAA)/5-HT increased. In the striatum, DA, DOPAC, HVA, 3-methoxytyramine and epinephrine decreased but HVA/DA and 5-HIAA/5HT ratios increased. Unilateral LPS decreased dopamineric neurons ipsilaterally but increased contralaterally. This study provides the first evidence of behavioral hyperactivity, epinephrine suppression and neuronal plasticity in the LPS model of PD.     

An evaluation of the role of 5-HT(2) receptor antagonism during subchronic antipsychotic drug administration in rats

A widely postulated mechanism of action for the atypical profile of many novel antipsychotic drugs (APDs) is their relatively high affinity for 5-HT(2) receptors. The present study investigated motor function and striatal dopamine (DA) efflux and metabolism in rats given 21 daily injections of drugs that differed in 5-HT(2) affinity. These drugs included: risperidone (high 5-HT(2A/2C)/high D(2)), clozapine (high 5-HT(2A/2C)/low D(2)), haloperidol (low 5-HT(2A/2C)/high D(2)), haloperidol+ritanserin (selective 5-HT(2A/2C)), or vehicle. Rats injected with haloperidol (0.5 mg/kg) or haloperidol+ritanserin (0.5 mg/kg and 1.0 mg/kg, respectively) showed extreme catalepsy on day 1, but significantly decreased catalepsy when tested again on days 7 and 21. Acute or subchronic risperidone (0.05 or 0.5 mg/kg), clozapine (20 mg/kg), or vehicle did not induce significant catalepsy. Microdialysis performed 24 h after the last injection demonstrated that rats treated with risperidone, clozapine, or vehicle showed similar increases in DA efflux and metabolism following an acute injection of a selective DA D(2/3) antagonist (raclopride, 0.5 mg/kg). DA efflux showed an attenuated response to raclopride in the haloperidol alone group; this effect was less apparent in the haloperidol+ritanserin group. However, both of these groups showed a similar tolerance effect to the raclopride-induced increase in DA metabolites. These results suggest that the profile seen after subchronic risperidone more closely resembles clozapine than haloperidol. While ritanserin reduced the tolerance-like effects of haloperidol on striatal DA efflux, the overall results demonstrate that potent 5-HT(2) blockade alone may not entirely account for the distinctive profile of novel APDs.     

Altered neuronal responsiveness to biogenic amines in rat cerebral cortex after serotonin denervation or depletion

To further investigate monoaminergic mechanisms in cerebral cortex, responsiveness of cortical neurons to microiontophoretic applications of serotonin (5-HT), dopamine (DA) or noradrenaline (NA) was examined in the frontoparietal region of control, 5,7-dihydroxytryptamine (5,7-DHT)- and p-chlorophenylalanine (PCPA)-treated rats anesthetized with urethane. As a rule, 100 nA applications of either one of these biogenic amines induced marked slowings or total interruptions of ‘spontaneous’ firing overlasting the 30s periods of ejection. Given the large amounts of monoamines ejected, it could be inferred that such microiontophoretic applications produced a maximal activation of receptors. In control rats, the responses to 5-HT, DA and NA were of approximately equal duration ( 5 min). Two to 4 weeks after denervation with 5,7-DHT, most neurons (75%) exhibited greatly prolonged responses to 5-HT( 14 min), and marked depressions of firing could be induced by small ejection currents (2 nA) having little or no effect in the controls. In addition, 85% of the units supersensitive to 5-HT showed considerably shortened responses to DA and NA( 1 min). After 2–14 days of depletion with PCPA, there was no change in the responsiveness to 5-HT in spite of a 91% lowering of cortical 5-HT content equivalent to that measured after denervation. Nevertheless, responsiveness to DA and NA was again diminished in a majority (80%) of the units tested. In control or PCPA-treated rats, acute administration of the 5-HT re-uptake blocker fluoxetine increased the duration of depressions induced by 100 nA applications of 5-HT but did not enhance responsiveness to low ejection currents. This suggested that, after 5-HT denervation, the suppression of re-uptake was mainly responsible for the prolongation of 5-HT responses (‘presynaptic’ component of supersentivity), whereas a modification of 5-HT receptors accounted for the greater efficacy of small doses of 5-HT (‘postsynaptic’ component). Responsiveness to the microiontophoretic application of phenylephrine (PHE), a noradrenergic α-agonist, was comparable with that to NA in PCPA- and 5,7-DHT-treated as well as in control rats. Therefore, the hyposensitivity to DA and NA appeared indicative of a desensitization of catecholamine receptors caused by the absence of 5-HT. Such a desensitization may be viewed as an adaptive change resulting from an increased release of endogenous DA and NA. This interpretation would in turn imply that, normally, 5-HT regulates catecholamine release in the neocortex.