Regional effect of estradiol on rat caudate-putamen dopamine receptors: lateral-medial differences

The impact of estradiol on serotonin, glutamate, and dopamine systems

Estradiol, the most potent and prevalent member of the estrogen class of steroid hormones and is expressed in both sexes. Functioning as a neuroactive steroid, it plays a crucial role in modulating neurotransmitter systems affecting neuronal circuits and brain functions including learning and memory, reward and sexual behaviors. These neurotransmitter systems encompass the serotonergic, dopaminergic, and glutamatergic signaling pathways. Consequently, this review examines the pivotal role of estradiol and its receptors in the regulation of these neurotransmitter systems in the brain. Through a comprehensive analysis of current literature, we investigate the multifaceted effects of estradiol on key neurotransmitter signaling systems, namely serotonin, dopamine, and glutamate. Findings from rodent models illuminate the impact of hormone manipulations, such as gonadectomy, on the regulation of neuronal brain circuits, providing valuable insights into the connection between hormonal fluctuations and neurotransmitter regulation. Estradiol exerts its effects by binding to three estrogen receptors: estrogen receptor alpha (ERα), estrogen receptor beta (ERβ), and G protein-coupled receptor (GPER). Thus, this review explores the promising outcomes observed with estradiol and estrogen receptor agonists administration in both gonadectomized and/or genetically knockout rodents, suggesting potential therapeutic avenues. Despite limited human studies on this topic, the findings underscore the significance of translational research in bridging the gap between preclinical findings and clinical applications. This approach offers valuable insights into the complex relationship between estradiol and neurotransmitter systems. The integration of evidence from neurotransmitter systems and receptor-specific effects not only enhances our understanding of the neurobiological basis of physiological brain functioning but also provides a comprehensive framework for the understanding of possible pathophysiological mechanisms resulting to disease states. By unraveling the complexities of estradiol's impact on neurotransmitter regulation, this review contributes to advancing the field and lays the groundwork for future research aimed at refining understanding of the relationship between estradiol and neuronal circuits as well as their involvement in brain disorders.

Striatal dopamine D2-type receptor availability and peripheral 17β-estradiol

Research using rodent models has established a relationship between the steroid hormone estrogen and dopamine function, by revealing changes throughout the estrous cycle and by directly manipulating neuroendocrine signaling through ovariectomy and administration of estrogen. However, a direct link between estrogen levels and dopamine signaling had not been established in humans. The goal of this study, therefore, was to assess the relationship between circulating 17β-estradiol and dopamine signaling in the human brain by testing for a relationship between two proxies for these variables: peripheral 17β-estradiol and striatal dopamine D₂-type receptor availability, measured with [¹⁸F]fallypride and positron emission tomography (PET). Sixteen (23-45 years of age) women were tested on 2 days of the menstrual cycle estimated prospectively to occur during (a) the early follicular phase, when estrogen levels are near their nadir, and (b) the periovulatory phase, when estrogen levels peak. PET scans with [¹⁸F]fallypride were performed on these 2 days, and serum 17β-estradiol was measured using radioimmunoassay. Dopamine D₂-type receptor availability did not differ significantly in the whole striatum or the caudate, putamen, or accumbens subregions during the high-estrogen vs. the low-estrogen phases of the menstrual cycle. We conclude that circulating estrogen levels do not affect dopamine D₂-type receptor availability in the human striatum although other indices of dopaminergic function may be affected.

Sex differences in dopamine release regulation in the striatum

The mesolimbic dopamine system-which originates in the ventral tegmental area and projects to the striatum-has been shown to be involved in the expression of sex-specific behavior and is thought to be a critical mediator of many psychiatric diseases. While substantial work has focused on sex differences in the anatomy of dopamine neurons and relative dopamine levels between males and females, an important characteristic of dopamine release from axon terminals in the striatum is that it is rapidly modulated by local regulatory mechanisms independent of somatic activity. These processes can occur via homosynaptic mechanisms-such as presynaptic dopamine autoreceptors and dopamine transporters-as well as heterosynaptic mechanisms, such as retrograde signaling from postsynaptic cholinergic and GABAergic systems, among others. These regulators serve as potential targets for the expression of sex differences in dopamine regulation in both ovarian hormone-dependent and independent fashions. This review describes how sex differences in microcircuit regulatory mechanisms can alter dopamine dynamics between males and females. We then describe what is known about the hormonal mechanisms controlling/regulating these processes. Finally, we highlight the missing gaps in our knowledge of these systems in females. Together, a more comprehensive and mechanistic understanding of how sex differences in dopamine function manifest will be particularly important in developing evidence-based therapeutics that target this system and show efficacy in both sexes.

Steroid 5α-reductase 2 deficiency leads to reduced dominance-related and impulse-control behaviors

The enzyme steroid 5α-reductase 2 (5αR2) catalyzes the conversion of testosterone into the potent androgen 5α-dihydrotestosterone. Previous investigations showed that 5αR2 is expressed in key brain areas for emotional and socio-affective reactivity, yet the role of this enzyme in behavioral regulation remains mostly unknown. Here, we profiled the behavioral characteristics of 5αR2 heterozygous (HZ) and knockout (KO) mice, as compared with their wild-type (WT) littermates. While male 5αR2 KO mice displayed no overt alterations in motoric, sensory, information-processing and anxiety-related behaviors, they exhibited deficits in neurobehavioral correlates of dominance (including aggression against intruders, mating, and tube dominance) as well as novelty-seeking and risk-taking responses. Furthermore, male 5αR2 KO mice exhibited reduced D₂-like dopamine receptor binding in the shell of the nucleus accumbens - a well-recognized molecular signature of social dominance. Collectively, these results suggest that 5αR2 is involved in the establishment of social dominance and its behavioral manifestations. Further studies are warranted to understand how the metabolic actions of 5αR2 on steroid profile may be implicated in social ranking, impulse control, and the modulation of dopamine receptor expression in the nucleus accumbens.

Dyskinesias and levodopa therapy: why wait?

Throughout the years there has been a longstanding discussion on whether levodopa therapy in Parkinson's disease should be started in early vs. later stages, in order to prevent or delay motor complications such as fluctuations and dyskinesias. This controversial topic has been extensively debated for decades, and the prevailing view today is that levodopa should not be postponed. However, there is still fear associated with its use in early stages, especially in younger patients, who are more prone to develop dyskinesias. Even though dyskinesias are linked to levodopa use in Parkinson's disease, it has been shown that starting with a different medication (such as dopamine agonists) will not significantly delay their onset once levodopa is introduced. Since levodopa provides better symptomatic control, and other drugs may be associated with notable side effects, it is our view that there is insufficient evidence to justify levodopa-sparing strategies. The physician should try to assess each patient individually, taking into account motor and non-motor demands, as well as risk factors for potential complications, finding the optimum treatment strategy for each one. The following article provides an historical narrative perspective, as well as a literature review of those intrinsic and modifiable risk factors that have been associated with levodopa-induced dyskinesias, which should be taken into consideration when choosing the therapeutic strategy in individual Parkinson's disease patients.

DAT1-Genotype and Menstrual Cycle, but Not Hormonal Contraception, Modulate Reinforcement Learning: Preliminary Evidence

Hormone by genotype interactions have been widely ignored by cognitive neuroscience. Yet, the dependence of cognitive performance on both baseline dopamine (DA) and current 17ß-estradiol (E2) level argues for their combined effect also in the context of reinforcement learning. Here, we assessed how the interaction between the natural rise of E2 in the late follicular phase (FP) and the 40 base-pair variable number tandem repeat polymorphism of the dopamine transporter (DAT1) affects reinforcement learning capacity. 30 women with a regular menstrual cycle performed a probabilistic feedback learning task twice during the early and late FP. In addition, 39 women, who took hormonal contraceptives (HC) to suppress natural ovulation, were tested during the "pill break" and the intake phase of HC. The present data show that DAT1-genotype may interact with transient hormonal state, but only in women with a natural menstrual cycle. We found that carriers of the 9-repeat allele (9RP) experienced a significant decrease in the ability to avoid punishment from early to late FP. Neither homozygote subjects of the 10RP allele, nor subjects from the HC group showed a change in behavior between phases. These data are consistent with neurobiological studies that found that rising E2 may reverse DA transporter function and could enhance DA efflux, which would in turn reduce punishment sensitivity particularly in subjects with a higher transporter density to begin with. Taken together, the present results, although based on a small sample, add to the growing understanding of the complex interplay between different physiological modulators of dopaminergic transmission. They may not only point out the necessity to control for hormonal state in behavioral genetic research, but may offer new starting points for studies in clinical settings.

Genetic alteration in the dopamine transporter differentially affects male and female nigrostriatal transporter systems

Female mice with a heterozygous mutation of their dopamine transporter (+/- DAT) showed relatively robust reductions in striatal DAT specific binding (38-50%), while +/- DAT males showed modest reductions (24-32%). Significant decreases in substantia nigra DAT specific binding (42%) and mRNA (24%) were obtained in +/- DAT females, but not +/- DAT males (19% and 5%, respectively). The effects of this DAT perturbation upon vesicular monoamine transporter-2 (VMAT-2) function revealed significantly greater reserpine-evoked DA output from +/+ and +/- DAT female as compared to male mice and the DA output profile differed markedly between +/+ and +/- DAT females, but not males. No changes in VMAT-2 protein or mRNA levels were present among these conditions. On the basis of these data, we propose: (1) a genetic mutation of the DAT does not exert equivalent effects upon the DAT in female and male mice, with females being more affected; (2) an alteration in the DAT may also affect VMAT-2 function; (3) this interaction between DAT and VMAT-2 function is more prevalent in female mice; and (4) the +/- DAT mutation affects VMAT-2 function through an indirect mechanism, that does not involve an alteration in VMAT-2 protein or mRNA. Such DAT/VMAT-2 interactions can be of significance to the gender differences observed in drug addiction and Parkinson's disease.

Circuit-based framework for understanding neurotransmitter and risk gene interactions in schizophrenia

Many risk genes interact synergistically to produce schizophrenia and many neurotransmitter interactions have been implicated. We have developed a circuit-based framework for understanding gene and neurotransmitter interactions. NMDAR hypofunction has been implicated in schizophrenia because NMDAR antagonists reproduce symptoms of the disease. One action of antagonists is to reduce the excitation of fast-spiking interneurons, resulting in disinhibition of pyramidal cells. Overactive pyramidal cells, notably those in the hippocampus, can drive a hyperdopaminergic state that produces psychosis. Additional aspects of interneuron function can be understood in this framework, as follows. (i) In animal models, NMDAR antagonists reduce parvalbumin and GAD67, as found in schizophrenia. These changes produce further disinhibition and can be viewed as the aberrant response of a homeostatic system having a faulty activity sensor (the NMDAR). (ii) Disinhibition decreases the power of gamma oscillation and might thereby produce negative and cognitive symptoms. (iii) Nicotine enhances the output of interneurons, and might thereby contribute to its therapeutic effect in schizophrenia.

Estrogen modulates learning in female rats by acting directly at distinct memory systems

Physiologically high levels of circulating estradiol enhance the use of place learning and impair the use of response learning to find food on a land maze. These two types of learning are impaired by lesions of distinct neuronal structures, i.e. the hippocampus and striatum, respectively. Moreover, it has been shown in male rats that compromising hippocampal function can promote the use of response learning, while compromising striatal function can promote place learning. These findings suggest an ongoing competition between the hippocampus and striatum during cognition, such that intact functioning of one structure somehow obstructs the relative participation of the other. The goal of this study was to determine if estrogen's opposing effects on place and response learning in female rats are due to direct actions, either independent or interacting, at the hippocampus and striatum. We infused 0.5 microM 17beta-estradiol 3-sulfate sodium or vehicle bilaterally into the dorsal hippocampus or dorsolateral striatum of ovariectomized young adult female rats, 48, 24 and 2 h before training. Rats were tested on one of three appetitive tasks in a Y-maze: place learning, response learning, or response learning with reduced visual cues (cue-poor condition). Intrahippocampal estradiol infusions enhanced place learning, reversing a cannula-induced impairment, whereas intrastriatal infusions had no effects on place learning. Estradiol infusions into neither structure significantly affected response learning when extramaze cues were visible. However, in the response task, cue-poor condition, intrastriatal but not intrahippocampal infusions impaired learning. These data demonstrate that estrogen modulates place and response learning at the hippocampus and striatum respectively, most likely through independent actions at these two structures.

Sex differences in neurochemical effects of dopaminergic drugs in rat striatum

Previous data indicate that dopamine neurotransmission is differently regulated in male and female rats. The purpose of the present study was to investigate the dopamine transporter and autoreceptor as potential loci responsible for this sex difference. Fast cyclic voltammetry at carbon-fiber microelectrodes was used to monitor changes in electrically evoked levels of extracellular dopamine in the striata of anesthetized male and female rats before and after administration of an uptake inhibitor, a dopamine D2 antagonist, or a D3/D2 agonist. Administration of 40 mg/kg cocaine ip increased electrically-evoked extracellular dopamine concentrations in both sexes, but to a significantly greater extent in female striatum at the higher stimulation frequencies. The typical antipsychotic, haloperidol, increased dopamine efflux in both sexes but the effect was twice as large in the female striatum. The D3/D2 agonist quinpirole induced an unexpected, transient increase in dopamine efflux following high-frequency stimulation only in females, and evoked dopamine was higher in females across this entire time course. More detailed analysis of cocaine effects revealed no fundamental sex differences in the interaction of cocaine with DAT in vivo or in synaptosomes. These results indicate that nigrostriatal dopamine neurotransmission in the female rat is more tightly regulated by autoreceptor and transporter mechanisms, perhaps related by greater autoreceptor control of DAT activity. Thus, baseline sex differences in striatal dopamine regulation induce different pharmacologic responses. These results contribute to understanding sex differences in stimulant-induced locomotor activity in rats and may have broader implications for neurologic disorders and their pharmacotherapies in humans.

Estrogen increases the sensitivity of ovariectomized rats to the disruptive effects produced by antagonism of D2 but not D1 dopamine receptors during performance of a response learning task

Estrogen impairs performance on some striatum-sensitive tasks of learning and memory. Evidence indicates that it may have these impairing effects by creating a bias to use hippocampally based strategies to solve tasks whether or not it is advantageous to do so. Estrogen may also exert direct effects in the striatum to affect performance on striatum-mediated procedural memory tasks. In spite of the robust effects that estrogen exerts on nigrostriatal dopaminergic neurons, the role of dopamine in the estrogen-induced effects on procedural memory tasks remains unexplored. The goal of the present study was to assess the independent and interactive effects of estrogen and dopamine antagonists on a striatum-mediated response learning task. Adult rats were ovariectomized and implanted with Silastic capsules containing 25% estradiol diluted in cholesterol or 100% cholesterol. Rats were trained to receive food rewards in an elevated plus maze by making a specified response (right or left turn). Following acquisition, dose-effect curves were determined for the D(1) dopamine receptor antagonist, SCH 23390, and the D(2) dopamine receptor antagonist, eticlopride. Estrogen did not significantly affect acquisition of the task and had no significant effect on the ability of SCH 23390 to disrupt performance on the task. However, estrogen significantly increased the sensitivity of the rats to the error-increasing effects of eticlopride. These results indicate that estrogen may differentially interact with D(1) and D(2) dopamine receptors to affect response learning. They also suggest that in addition to creating a bias to use hippocampally based strategies to solve tasks, estrogen may affect performance on procedural memory tasks through direct action on dopaminergic functioning.

Influence of oestrogenic compounds on monoamine transporters in rat striatum

Oestrogens have been reported to modulate rat membrane (DAT) and vesicular (VMAT(2)) dopamine transporters. A recent pilot study of postmenopausal women showed that chronic oestrogen replacement therapy increases striatal DAT. In the present study, we first investigated whether the oestrogen receptors alpha and beta mediate the effects of oestradiol on DAT and VMAT(2). Two days after ovariectomy, Sprague-Dawley rats were treated for 2 weeks with oestradiol or specific ligands for oestrogen receptor alpha, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) or oestrogen receptor beta, 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN). Ovariectomy caused a decrease in [(125)I]-3beta-(4-iodophenyl)-tropane-2beta-carboxylic acid isopropyl ester ([(125)I] RTI-121) specific binding to DAT transporters in the middle striatum compared to values for intact rats, and this was reversed by oestradiol replacement therapy. DPN, but not PPT, mimicked the effect of oestradiol. [(125)I] RTI-121 specific binding in the anterior and posterior striatum was not affected by ovariectomy or any of the drug treatments. Second, we investigated whether oestradiol increased DAT specific binding after a longer period of hormonal withdrawal (a model of hormonal withdrawal at menopause) and whether the selective oestrogen receptor modulators (SERMs), tamoxifen and raloxifene, could reproduce the oestradiol-induced increase of [(125)I] RTI-121 specific binding in long-term ovariectomised rats. Four months after ovariectomy, Sprague-Dawley rats were treated for 2 weeks with oestradiol, tamoxifen or raloxifene, and then killed. Ovariectomy decreased [(3)H] RTI-121 specific binding to DAT transporters in the middle striatum compared to values for intact rats. Treatment with oestradiol, tamoxifen and raloxifene reversed this effect. [(125)I] RTI-121 specific binding in anterior and posterior striatum was not affected by ovariectomy or treatment with oestrogen receptor ligands. In both experiments, neither ovariectomy nor the oestrogenic treatments modulated striatal [(3)H] tetrahydrobenazine specific binding to VMAT(2). Overall, these results suggest that oestrogen receptor beta mediates the oestradiol-induced increase of striatal DAT and that oestradiol can increase DAT density even after long-term steroid withdrawal. The results also support the premise that the SERMs tamoxifen and raloxifene exert oestrogenic agonist effects in the brain.

The number of cells expressing dopamine D2 receptor mRNA in rat brain caudate putamen is higher in oestrus

Dopamine D2 receptors (D2-Rs) in the central nervous system are involved in the control of locomotion, cognition, emotion and neuroendocrine secretion. The intensity of cellular responses to specific stimuli is dependent on the concentration of dopamine or its agonist, and the availability, as well as the concentration, of all the other components of the signalling pathway in the cell, including the receptors. Many factors can influence the level of mRNA encoding the receptors. In order to study the changes in the level of expression of the D2-R mRNA in the brain of female rats at different stages of the oestrous cycle, we used a quantitative in situ hybridization technique. Four groups of animals were analysed: rats in prooestrus (POE), oestrus (OE), dioestrus 1 (DOE1) and dioestrus 2 (DOE2). A 35S-labelled riboprobe was transcribed in vitro from the 1.5-kb D2-R cDNA. The caudate putamen of the rats, which shows the highest level of expression of D2-R mRNA in the brain, was examined. The number of silver grains per cell, representing hybridization of riboprobe, and the number of cells expressing the D2-R mRNA, were counted with the computer-assisted image analysis system Lucia-M. Our results show that the expression of the D2-R mRNA in the lateral striatum varies during the oestrous cycle, with the highest expression measured during DOE2. The number of cells expressing the D2-R mRNA also changes during the different phases, with the highest number being detected in OE. This indicates that during OE more cells transcribe the D2-R mRNA. The results suggest that the variations in the concentration of the D2-R mRNA in the caudate putamen of the rat brain at different stages of the reproductive cycle are caused by the combination of variable expression of the mRNA per cell and different number of the cells that express the mRNA.

Estrogenic properties of raloxifene, but not tamoxifen, on D2 and D3 dopamine receptors in the rat forebrain

The present study investigated the estrogenic specificity of the modulation of dopamine D(2) and D(3) receptors by comparing the effects of estradiol with tamoxifen or raloxifene. These compounds have estrogenic and/or antiestrogenic activity depending on the target tissue. Two weeks after ovariectomy of female rats, we observed a 60% decrease in the uterine weight, which was prevented by a replacement therapy of 2 weeks with 17beta-estradiol. A tamoxifen or raloxifene treatment of 2 weeks increased uterine weights by 35 and 15%, respectively, but significantly less than estradiol treatment. Ovariectomy decreased dopamine D(2) receptor specific binding (20%) in the dorsolateral part of the anterior striatum and these receptors were left unchanged in the other parts of the striatum as well as in the olfactory tubercle and the nucleus accumbens. 17beta-Estradiol and raloxifene, but not tamoxifen treatment prevented this decrease. Ovariectomy left dopamine D(3) receptor specific binding unchanged. However, estradiol and raloxifene treatment decreased dopamine D(3) receptor binding in the islands of Calleja, the core and shell of the nucleus accumbens and the dorsal part of the anterior striatum, compared with ovariectomized rats. Our results show that raloxifene, but not tamoxifen, has an agonist estrogenic activity on dopamine receptors. Furthermore, estradiol and raloxifene have opposite effects on specific binding to dopamine D(2) and D(3) receptors.

Dopaminergic activity in transgenic mice underexpressing glucocorticoid receptors: effect of antidepressants

Transgenic mice bearing a transgene coding for a glucocorticoid receptor antisense mRNA, which partially blocks glucocorticoid receptor expression, were used to investigate the long-term effect of hypothalamic-pituitary-adrenal axis dysfunction on brain dopamine transmission. Compared to control mice, the transgenic animals showed increased amphetamine-induced locomotor activity and increased concentrations of striatal dopamine and its metabolites dihydroxyphenylacetic acid and homovanillic acid. Binding of [3H]SCH 23390 and [3H]spiperone to, respectively, D1 and D2 dopamine receptors was increased in transgenic mice. In contrast, autoradiography of striatal [3H]GBR 12935 binding to the dopamine transporter was decreased and the mRNA levels of this transporter, measured by in situ hybridization, remained unchanged in the substantia nigra pars compacta. The effect of chronic treatment for two weeks with amitriptyline or fluoxetine was compared in control and transgenic mice. No significant changes were observed in control mice following antidepressant treatment, whereas in transgenic mice both antidepressants reduced striatal [3H]SCH 23390 and [3H]raclopride specific binding to D1 and D2 receptors. Amitriptyline, but not fluoxetine, increased striatal [3H]GBR 12935 binding to the dopamine transporter, whereas its mRNA level in the substantia nigra pars compacta was decreased in fluoxetine, compared to vehicle- or amitriptyline-treated transgenic mice. From these results we suggest that hyperactive dopaminergic activity of the nigrostriatal pathway controls motor activity in the transgenic mice. Furthermore, antidepressant treatment corrected the increased striatal D1 and D2 receptors and decreased dopamine transporter levels in the transgenic mice.

Selective colocalization of immunoreactivity for intracellular gonadal hormone receptors and tyrosine hydroxylase in the ventral tegmental area, substantia nigra, and retrorubral fields in the rat

Gonadal hormones influence brain functions, including motor and motivational behaviors, transmitter release, and receptor binding in midbrain dopamine systems. Much of this influence suggests genomic hormone action. To identify which midbrain cells may be targets of genomic influence, double label immunocytochemistry was used to map intracellular estrogen and androgen receptors and tyrosine hydroxylase (TH) in the ventral tegmental area (VTA), substantia nigra (SN), and retrorubral fields (RRF) in intact, adult rats. The distribution of estrogen and androgen receptor immunoreactivity was highly selective, similar in males and females, and largely nonoverlapping. Estrogen receptors were present within subpopulations of cells in the ventrolateral paranigral VTA and rostrolateral RRF; of these, only a few cells in the RRF were immunoreactive for TH. Cells immunoreactive for androgen receptors were numerous in the paranigral and parabrachial VTA, SN pars lateralis and dorsomedial pars compacta, and lateral RRF. Nearly every androgen receptor-bearing cell in the VTA and SN pars compacta, roughly half in the SN pars lateralis, and about one-third in the RRF were TH immunopositive. The localization of estrogen receptors approximates the distribution of subsets of cells labeled following neostriatal injections, whereas androgen receptors tend to occupy regions labeled by injections in cortical or limbic targets. These receptor-specific alignments with origins of nigrostriatal, mesolimbic, and mesocortical projections are consistent with identified estrogen influence over motor behaviors and androgen involvement in motivational functions and may hold clues for understanding hormone action in these and other functions and dysfunctions of midbrain dopamine systems.

Presence of catecholamines and serotonin in the rat vestibule

The concentrations of norepinephrine (NE), dopamine (DA) and its metabolites DOPAC and HVA, and serotonin (5-HT) and its metabolite 5-HIAA, were quantified in the rat vestibule. For this purpose, homogenates of vestibules, of albino and pigmented rats, were analyzed using HPLC with electrochemical detection. Vestibules of pigmented rats showed higher DOPAC and HVA concentrations than those of albino rats, and male pigmented rats also showed significantly more DA than male albino rats. These results could indicate that the rate of DA metabolism in vestibules was higher in pigmented than in albino rats. The vestibular concentrations of NE and 5-HT did not differ significantly between the two strains. In contrast, 5-HIAA concentration was higher in vestibules of pigmented rats than in those of albino rats, suggesting an increased 5-HT metabolism for the former strain. Differences in monoamine concentrations between the two sexes o the same strain were scarce. Only, a higher HVA concentration in vestibules of females could indicate a higher DA metabolism.

Effects of testosterone upon MPTP-induced neurotoxicity of the nigrostriatal dopaminergic system of C57/B1 mice

We have recently reported that treatment of gonadectomized female and male C57/B1 mice with the gonadal steroid hormone, estrogen, reduced nigrostriatal dopaminergic neurotoxicity resulting from the Parkinson's-like inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In the present report we examined whether the predominantly male gonadal steroid hormone, testosterone, would similarly modulate MPTP-induced neurotoxicity. Male C57/B1 mice were assigned to one of the following five treatment conditions: (1) Intact, (2) Orchidectomized, (3) Intact + MPTP, (4) Orchidectomized + Testosterone + MPTP and (5) Orchidectomized + MPTP. Corpus striatal and olfactory tubercle dopamine. DOPAC and norepinephrine concentrations were determined from the animals within each of the five treatment conditions. Orchidectomy alone failed to alter striatal dopamine and DOPAC concentrations, with levels obtained being similar to that of Intact animals. MPTP treatment significantly reduced striatal reduced striatal dopamine and DOPAC concentrations, regardless of hormonal condition of the animal. Similar results were obtained for olfactory tubercle determinations, with the exception that DOPAC levels from Orchidectomized mice were significantly greater than Intact males. No significant differences were obtained for norepinephrine within either brain area sampled. These results show that unlike estrogen, testosterone is devoid of any capacity to modulate nigrostriatal dopaminergic neurotoxicity resulting from MPTP. These findings may be related to the gender differences which exist in the prevalence of Parkinson's disease.

Behavioral and neurochemical changes induced by aging in dopaminergic systems of male and female rats

Changes induced by aging in dopaminergic activity of male and female rats were compared by behavioral and neurochemical methods. Young (3 months) and old (23 months) rats were used. Aging decreased animal activity in the open field and increased apomorphine-induced stereotyped behavior. No differences in open field data were observed between males and females. Young and aged female rats had higher striatal DA and HVA levels than males; aging induced a decrease in both striatal DA and HVA levels in males, but not in females. No changes in HVA/DA ratios were observed among the different groups. These results show that aging reduces nigrostriatal activity as well as nigrostriatal DA levels. Furthermore, they indicate that time course events related to aging differ between males and females.

Sex differences in the rapid and acute effects of estrogen on striatal D2 dopamine receptor binding

Regional changes in striatal D2 dopamine (DA) receptor binding in castrated (CAST) or ovariectomized (OVX) rats were investigated following administration of a low dose of estradiol benzoate (EB), repeated treatment with EB followed by progesterone, or vehicle. In two separate experiments, there was a significant decrease in striatal D2 DA receptor binding in caudal striatum from OVX, but not CAST rats 30 min after a single injection of EB. In CAST rats, there was a significant increase in striatal D2 DA receptor binding in rostral striatum 4 h after injection of EB. There was no effect of EB plus progesterone treatment in either OVX or CAST rats. In addition, CAST rats had significantly lower D2 DA receptor binding in the lateral region of the rostral striatum than did OVX rats. These results show sexually dimorphic and regionally specific effects of estrogen on striatal D2 DA receptor binding, and a significant sex difference in striatal D2 DA receptor binding in the absence of circulating gonadal hormones. The present findings are discussed in light of previous reports of sex differences in gonadal hormone influences on striatal DA mediated behaviors.

Modulatory effects of testosterone on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity

In this experiment, we examined the modulatory effects of testosterone on the parkinsonism-inducing drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in two strains of mice. Orchidectomized male CD-1 and C57/B1 mice were implanted with either empty Silastic capsules or capsules containing testosterone and subsequently treated with MPTP. A small area of the corpus striatum was removed for determination of dopamine (DA) content, whereas the remainder was superfused and used to measure L-DOPA (5 microM)-evoked DA release. In animals treated with MPTP, L-DOPA-evoked DA release was reduced significantly in CD-1 mice, but not in C57/B1 mice, treated with testosterone. No differences in L-DOPA-stimulated DA release between MPTP-versus vehicle-treated mice was observed in either the CD-1 or C57/B1 mice receiving empty Silastic capsules. Corpus striatum DA contents were more severely depleted in the MPTP-sensitive C57/B1 versus the CD-1 mouse strain irrespective of hormone treatment. These results confirm previous results demonstrating differences in these two mouse strains in response to the neurotoxic effects of MPTP upon corpus striatum DA content. More interestingly, they show an important differential modulatory effect of testosterone upon L-DOPA-evoked DA release as a function of MPTP treatment and indicate that testosterone significantly alters the neurotoxic effects of MPTP in the CD-1 mouse.

Modulation of brain dopamine transmission by sex steroids

Sex steroid hormones influence the dopaminergic systems of the hypothalamus as well as the extrahypothalamic regions of the brain in controlling movement and behavior in both humans and animals. This review focuses on the effects of sex steroids on dopaminergic activity in extrahypothalamic brain areas. Among sex steroids, estrogens have been most extensively investigated, and many studies report that estrogens affect behaviors mediated by the basal ganglia, such as in humans suffering from extrapyramidal disorders. Epidemiological and clinical evidence also suggests an influence of estrogens on the vulnerability threshold for schizophrenia and sex differences in the clinical expression of this disease. Clinical observations point to a role of androgenic hormones in Gilles de la Tourette's syndrome. In normal humans, sex steroids were also shown to influence motor and cognitive performance. Biochemical and behavioral studies in animals have also shown the effect of sex steroids on dopaminergic activity in the basal ganglia; however, both activating and inhibiting effects have been reported. This may partly be explained by effects of the dose, duration of treatment, interval between steroid administration and testing the behavior measured, and the part of the basal ganglia from which the behavior is elicited. In view of the numerous variables that influence net dopaminergic response to steroids, focus will be on the literature using similar experimental conditions to assess the effect of in vivo chronic steroid treatment, acute short-term steroid treatment and the estrous cycle as well as in vitro effects of steroids on dopamine receptors. These experimental paradigms point to two general mechanisms of action of steroids: a rapid short-term non-genomic membrane effect and a slower long-term possibly genomic effect of steroids on dopamine systems. Combining dopaminergic drugs with sex steroids could improve efficacy or reduce side effects associated with these drugs. Examples of such combined treatments in rats and monkeys are presented for delta 9-tetrahydrocannabinol, cocaine, neuroleptics, apomorphine and L-DOPA. A better understanding of steroid-dopamine interactions and the possible isolation of conditions to have only pro or anti dopaminergic activity could then be used to develop combined therapies or to optimize drug treatments that would take into account the patient's sex and endocrine status.

Effects of noise stimulation on cochlear dopamine metabolism

Dopamine (DA) appears to be one of the putative neurotransmitters of the lateral efferent olivocochlear fibers. However, its role in the cochlear physiology remains unknown. In this study, animals were exposed for 1 h to white noise at 70, 90 or 110 dB SPL or were kept in silence conditions. Afterwards, the cochlear content of DA and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were analyzed using HPLC coupled to electrochemical detection. Cochlear DA concentration decreased with the noise intensity, while cochlear DOPAC and HVA concentrations increased. Males presented higher cochlear DOPAC contents and lower HVA contents than females. This sexual dimorphism could be related to the link between DA and gonadal steroids. Present results show that DA, as other lateral efferent neurotransmitters, is released and metabolized in relationship with the noise stimulation, and suggest that DA could be involved in the modulation of the type I afferent fiber activity.

Effect of chronic estradiol and progesterone treatments of ovariectomized rats on brain dopamine uptake sites

Dopamine released from brain nerve terminals is mainly removed from the synaptic cleft by an uptake mechanism. Despite their functional importance, modulation of the dopamine uptake sites is still not well known. Steroid hormones were shown to modulate brain dopamine transmission. The aim of this study was thus to investigate in ovariectomized rats the effects of 17 beta-estradiol and progesterone treatments on brain dopamine uptake sites. Treatments consisted of 17 beta-estradiol (10 micrograms/0.2 ml), progesterone (0.72 mg/0.2 ml), 17 beta-estradiol + progesterone, or the vehicle (0.3% gelatin in saline solution) twice daily for 2 weeks. The steroid treatments left the affinity of [3H]GBR 12935 binding to striatal homogenates unchanged (ovariectomized rats, 0.823 +/- 0.028 nM), whereas the density was increased by these steroids alone or in combination to a similar extent of 16-23%. Chronic treatment of ovariectomized rats with 17 beta-estradiol, progesterone, or their combination increased to the same extent and uniformly [3H]-GBR 12935 binding in the striatum as measured by autoradiography; the increase was similar in the substantia nigra pars compacta, whereas no steroid effect was observed in the nucleus accumbens and in the substantia nigra pars reticulata. In summary, chronic exposure to 17 beta-estradiol and/or progesterone increased dopamine uptake site density in the nigrostriatal dopaminergic system, whereas the nucleus accumbens and the substantia nigra pars reticulata were unaffected.

Modulation by estradiol and progesterone of the GTP effect on striatal D-2 dopamine receptors

Agonist binding properties of rat striatal D-2 dopamine (DA) receptors were investigated after in vivo or in vitro estradiol or progesterone exposures in order to elucidate the mechanism of action of steroid hormones on DA receptors. Chronic estradiol treatment of ovariectomized rats (10 micrograms, twice each day, for 2 weeks) increased lateral striatum total receptor density and left unchanged the proportion and affinity of the agonist high- and low-affinity states of this receptor in the striatum. In addition, when GTP was added in DA competition for [3H]spiperone binding experiments, D-2 receptors in the medial part of the striatum from estrogen-treated animals were more sensitive to GTP than those in the lateral part, whereas GTP had equal activity in both parts of the striatum in vehicle-treated rats. With apomorphine, but not with DA competition for [3H]spiperone binding, addition of estradiol (1 nM) to striatal homogenates of intact male rats prevented the expected shift of the high- to the low-affinity state of D-2 receptors, normally induced by GTP (100 microM) under these conditions. This effect of estradiol was not observed in the presence of 4 mM MgCl2, while in vitro progesterone (100 nM) had no effect in either the absence or presence of MgCl2. In addition, in vivo chronic progesterone treatment of ovariectomized rats left striatal [3H]spiperone density and affinity unchanged. Moreover, 1 nM estradiol increased the IC50 of GTP for inhibition of [3H]N-propylnorapomorphine binding to the high-affinity state of striatal D-2 receptors. This effect was also observed but decreased by 2-fold in the presence of MgCl2. Our data suggest that estradiol in vivo and in vitro interferes with the effect of GTP on striatal D-2 DA receptors.

Lithium effects on estrogen-induced dopaminergic supersensitivity in rats

The effect of lithium (2.0 mEq/kg, IP) on dopaminergic supersensitivity induced by long-term estrogen treatment (0.25 mg/day, SC) was investigated. Ovariectomized rats were treated for 28 days with estrogen and/or lithium and 6 days after the last administration all animals were observed for apomorphine-induced stereotyped behavior (1.0 mg/kg, SC). The abrupt withdrawal from long-term estrogen treatment increased rats' sensitivity to apomorphine but lithium administration was able to prevent this increase. Lithium alone produced no effect on stereotyped behavior. It was suggested that lithium may antagonize the estrogen effect on the nigrostriatal dopamine system.

Chronic estradiol treatment increases anterior pituitary but not striatal D2 dopamine receptor mRNA levels in rats

The effect of chronic 17 beta-estradiol treatment (10 micrograms, twice each day, for 2 weeks) of ovariectomized rats on D2 dopamine (DA) receptor mRNA levels was investigated in striatum and anterior pituitary gland tissues. We used 32P-labeled probes specific for D2 receptor and beta-actin mRNAs in Northern blot analysis. The ratio of D2 DA receptor mRNA/beta-actin mRNA level was significantly increased in the anterior pituitary of estradiol-treated rats compared to vehicle-treated animals. The D2 DA receptor mRNA/beta-actin mRNA ratio in the striatum was not affected by estradiol treatment. However, the medial portion of the striatum showed a significantly lower ratio compared to the lateral portion of the striatum in both vehicle- and estradiol-treated rats. Thus, the estradiol effect on anterior pituitary D2 receptors may implicate transcriptional regulation, whereas our results do not support this hypothesis for the estradiol action on striatal D2 receptors.

Effects of estradiol on the ontogenesis of striatal dopamine D1 and D2 receptor sites in male and female rats

Since estradiol (E2) either increases or reduces the number of dopamine receptors in the corpus striatum of adult rats, depending on the dose and length of administration, the sensitivity of the two receptor subpopulations (D1 and D2) to E2 during ontogenesis was investigated. Rats of both sexes received either 10 micrograms/kg E2 for 3 days or 50 micrograms/kg for 6 days, and were sacrificed at the age of 15, 21, 40 and 120 days. D1 receptors (identified by [3H]SCH 23390 binding) displayed no changes in density and affinity in function of age, sex or E2 dose, whereas the D2 receptors (identified by [3H]spiperone binding) fell after the lower dose in all groups, and the higher dose resulted in supersensitivity in males of all ages, but only in the 15-day-old females. These findings show that the effect of E2 is bivalent on D2 density only. The effect of its brief administration at a low dose is not sex-dependent, whereas at higher doses administered for longer periods it appears to involve mechanisms linked to sexual differentiation after birth.

Effect of estrogen and progesterone on L-dopa induced dyskinesia in MPTP-treated monkeys

A group of 4 cynomolgus monkeys was rendered parkinsonian by the toxin MPTP and then treated daily with L-DOPA until all animals developed dyskinesia. At this point, 17 beta-estradiol, 17 alpha-estradiol and progesterone were injected s.c. singly or in combination for 7 days and the treatment with L-DOPA reinstituted. Our results show that the pretreatment with 17 beta-estradiol reduced the dyskinetic effect without altering the therapeutic response to L-DOPA, while 17 alpha-estradiol did not alter the dyskinetic and the antiparkinsonian effect.

Decrease of behavioral and biochemical denervation supersensitivity of rat striatum by nigral transplants

The effect of fetal mesencephalic transplants on dopamine receptor supersensitivity has been studied behaviorally and biochemically in rats with a unilateral lesion of the nigrostriatal pathway. Female rats were lesioned with 6-hydroxydopamine in the left substantia nigra. At least one month later they were tested with apomorphine (0.25 mg/kg, s.c.), amphetamine (5 mg/kg, s.c.), LY 171555 (D2 agonist) (0.5 mg/kg, i.p.) and CY 208243 (D1 agonist) (0.5 mg/kg, s.c.). A suspension containing approximately 1.5 x 10(6) cells from the ventral mesencephalon of rat embryos was distributed in three sites in a triangular fashion in the center of the denervated striatum. Six months later, grafted dopamine neurons reinnervated the medial part of the dorsal striatum, increased the dopamine level and reversed the rotational asymmetry evoked by amphetamine. Apomorphine given four months post-transplant still elicited contraversive circling but the number of turns was reduced. Circling evoked six months post-transplant by CY 208243 or LY 171555 was significantly less in grafted rats than in lesioned non-grafted rats. The density of dopaminergic receptors in the striatum of grafted and lesioned rats was examined by autoradiography by means of in vitro binding with [3H]SCH 23390 for D1 receptors and [3H] spiperone for D2 receptors. The results show that intrastriatal nigral transplants decrease the supersensitivity of the D2 receptors and to a lesser extent of the D1 receptors. Normalization of D2 receptors may explain the decrease of behavioral supersensitivity following administration of apomorphine and D2 agonist in grafted rats. D1 receptors were less affected by the lesion and also less normalized than D2 receptors by the transplants.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of chronic treatment of MPTP monkeys with bromocriptine alone or in combination with SKF 38393

Eight monkeys developed a severe parkinsonian syndrome after i.v. administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Treatment with bromocriptine (5 mg/kg) relieved the parkinsonian symptoms, but the efficacy of this treatment appeared to decrease slightly with time. The addition of SKF38393 (5 mg/kg) to the bromocriptine treatment in four monkeys prevented and even reversed the tendency to decreased efficacy with an increased response in three out of four animals. Neither of these two treatments induced dyskinesia in these monkeys. Dopamine D1 and D2 receptors, assayed in the caudate nucleus, the putamen and nucleus accumbens with [3H]SCH 23390 and [3H]spiperone binding respectively, were not significantly different in MPTP monkeys treated with bromocriptine or with bromocriptine plus SKF38393. Monkeys in the two treatment groups had a similar extent of denervation of the striatum and accumbens as assessed by the content of dopamine and its metabolites. These results suggest that in MPTP monkeys, the behavioral response to the D2 agonist bromocriptine can be enhanced by concomitant activation of the D1 receptors while this combination of agonists does not induce dyskinesia. The addition of the D1 agonist does not appear to cause further alteration of the D1 or D2 receptors.

Estradiol in the striatum: effects on behavior and dopamine receptors but no evidence for membrane steroid receptors

Estradiol was applied directly to the striatum of ovariectomized female rats by a unilateral intracerebral cannula for three hr or four days. Following four days of estradiol treatment, rats increased the number of rotations in the direction away from the side of the hormone treatment. Cholesterol-treated animals did not change their rotational behavior. Dopamine receptors were assayed in the same animals by autoradiography; D2 receptors increased on the hormone-treated side relative to the untreated side after four days of treatment, only in the lateral striatum. D1 dopamine receptors did not change. The D2:D1 receptor ratio was related to the direction of rotation. Measurements of membrane fluidity with a fluorescent probe revealed no effect of estradiol on striatal membrane fluidity. Membrane proteins were labeled with estrogen agonist and antagonist affinity labels and analyzed by gel electrophoresis, but no saturable membrane binding sites were detected. The results indicate that estradiol acts directly in the striatum to affect behavior and dopamine receptors, but the neurochemical mechanisms remain to be determined.

The influence of the estrous cycle on the activity of striatal neurons recorded from freely moving rats

Recordings from striatal neurons in conscious rats at various stages of the estrous cycle revealed differences in the activity of the neurons. Average firing rate was higher in diestrus than in animals in estrus or proestrus. The increase did not seem to depend on the motor activity of the animals nor was it present in ovariectomized rats. Since application of estrogen to ovariectomized rats did not change the firing rate of the neurons, we conclude that differences during the cycle in conscious rats are not mediated by an action of estrogen unlike those seen in anesthetized rats. Differing electrode sampling bias in the two situations, or an action of anesthetic in revealing the estrogen effect may explain the differences.

Effects of orchidectomy on nigro-striatal dopaminergic function: behavioral and physiological evidence

Abstract In the present experiments, we examined the effect of castration upon two indices of nigro-striatal dopaminergic function in the male rat. In Experiment I, differences in spontaneous locomotor behavioral activity between intact and castrated male rats were examined. The total distance traveled, horizontal activity and mean revolutions of castrated male rats were significantly greater than that of intact males. No significant differences between intact and castrated males were obtained for vertical activity. In Experiment II, the spontaneous in vitro dopamine release from the corpus striatum of intact and castrated rats as sampled during the light-phase (1500 h) and dark-phase (2400 h) of the photoperiod was examined. At both time periods, the spontaneous in vitro dopamine release of castrated males was significantly greater than that of intact males. Both intact and castrated males showed statistically significant increases in dopamine release at the 2400 h compared to the 1500 h time period. To examine if testicular hormones were responsible for these castration induced changes in dopamine release, in Experiment III we treated castrated male rats with testosterone propionate. Administration of testosterone propionate (0.1 mg/day x 5 days) significantly reduced in vitro dopamine release compared to untreated or castrated male rats receiving vehicle treatment. These results demonstrate that testicular hormones, most likely testosterone, have a markedly suppressive effect upon the nigro-striatal dopaminergic system as evidenced from changes in spontaneous behavioral activity and in vitro dopamine release.

Effects of acute and chronic treatments with clozapine and haloperidol on serotonin (5-HT2) and dopamine (D2) receptors in the rat brain

The effects of acute and chronic treatments with haloperidol or clozapine on the binding of [3H]spiperone to D2 and 5-HT2 receptors were examined in 6 discrete regions of the striatum, n. accumbens and frontal cortex using quantitative autoradiography. Acute treatment with haloperidol, 0.1-2.0 mg/kg, i.p., produced a dose-dependent reduction to 60% of control in the binding of [3H]spiperone to D2 receptors in the striatum and n. accumbens and no effect on the binding of [3H]spiperone to 5-HT2 receptors in the striatum, n. accumbens or frontal cortex. Acute treatment with clozapine, 10-40 mg/kg, i.p., produced a dose-dependent reduction in D2-specific binding in both the n. accumbens and the striatum and also significant reductions to 24% of control in the binding of [3H]spiperone to cortical 5-HT2 receptors. Chronic treatment with haloperidol, 1 mg/kg/day, i.p., significantly increased (40-65%) the maximal number of D2-specific [3H]spiperone binding sites in the n. accumbens and the dorsolateral and ventrolateral regions of the striatum, whereas small increases (20-29%) were seen in the ventromedial, dorsomedial, rostral and caudal regions of the striatum. Chronic treatment with clozapine, 20 mg/kg/day, i.p., did not change the maximal number of D2 receptors in the n. accumbens or any region of the striatum. Chronic treatments with clozapine produced a decrease in the maximal number of cortical 5-HT2 receptors to 55% of control whereas haloperidol had no effect. This study demonstrates regional differences in the up-regulation of striatal D2 receptors following chronic treatment with haloperidol and different effects of a typical and atypical neuroleptic on 5-HT2 receptors following acute and chronic treatments.

Striatal D1 dopamine receptor density fluctuates during the rat estrous cycle

Striatal D1 dopamine (DA) antagonist binding sites were investigated in intact female and male rats, ovariectomized (OVX) animals and during the 4-day estrous cycle. The affinity of the striatal D1 receptor as labelled with [3H]SCH 23390 remains unchanged in intact male, female rats during the estrous cycle and OVX animals. By contrast, ovariectomy decreased striatal D1 receptor density by 17% (P less than 0.01) compared to intact female rats while a small but significant higher density was observed in intact male compared to female rats (10% higher, P less than 0.05). The density of striatal D1 DA receptor was higher on the day of diestrus I (DI) and diestrus II (DII) (P less than 0.01 vs OVX) and fluctuates throughout the estrous cycle with a maximum on the day of DII (P less than 0.05 vs proestrus PM). Our results show for the first time that striatal D1 DA receptors fluctuate during the estrous cycle and can be modulated by gonadal steroids.

Estradiol and parental experience, but not prolactin are necessary for ultrasound recognition and pup-retrieving in the mouse

The effects of estradiol, prolactin and experience with pups on pup-retrieving and on the recognition of ultrasonic distress calls of the young by adult ovariectomized female mice were studied. Treatment with estradiol benzoate or experience with pups (for 5 days) induced pup-retrieving in 40% and 60% of the animals, respectively, as compared to 0% in inexperienced ovariectomized females. However, if pup-experience was achieved in the presence of estradiol, retrieving was shown by 90.5% of the animals. In addition, in this case models of the ultrasonic distress calls of the pups were significantly preferred ("recognized") in a discrimination task by the females. These females also showed a sharp increase in serum prolactin concentrations. Depleting prolactin from the blood by cysteamine, however, neither reduced the retrieving score nor disturbed ultrasound recognition. Thus prolactin is dispensable for maintaining maternal pup-retrieving and ultrasound recognition. The process of achieving pup-experience is enhanced by the presence of estradiol. Possible actions of estradiol in the brain are discussed.

Distribution of dopamine D1 and D2 receptors in rabbit cortical areas, hippocampus, and neostriatum in relation to dopamine contents

The densities of dopamine D1 and D2 receptors were measured by using [3H]SCH23390 and [3H]raclopride, respectively, in the rabbit cingulate, visual, sensorimotor, and entorhinal-piriform cortical areas; the dorsal and ventral hippocampus; and the putamen as well as the medial and lateral caudate. Endogenous dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), 4-hydroxy-3-methoxyphenylacetic acid (HVA), and 3-methoxytyramine (3-MT) were assayed by HPLC with electrochemical detection. The distributions of [3H]SCH23390 and [3H]raclopride binding were heterogenous with the greatest densities in the neostriatum. The concentrations of DA and its metabolites were also highest in this structure. Regions with low DA content, i.e., cortex and hippocampus, had lower densities of [3H]SCH23390 and [3H]raclopride binding. Furthermore, these sites were differentially localized within the various regions and there were substantially more D1 than D2 receptors. The functional significance and heterogeneities in the distribution of D1 and D2 receptors are discussed in relation to the dopaminergic innervation and the turnover estimated by the ratios between endogenous DA and its metabolites.

Chronic estradiol treatment increases ovariectomized rat striatal D-1 dopamine receptors

Striatal D-1 dopamine (DA) receptors were investigated following chronic 17 beta-estradiol (10 micrograms, b.i.d., s.c., for two weeks) to ovariectomized (OVX) female rats. This treatment initiated the day after ovariectomy has revealed that the maximal density in homogenates of striatal D-1 DA receptors (Bmax) labelled with [3H] SCH 23390 was increased (44% without and 28% with 120 mM NaCl in the assay buffer). Estradiol treatments initiated 2 or 4 weeks after ovariectomy did not induce D-1 DA receptor binding modifications. The affinity (Kd) of the ligand for the receptor remains unchanged by the steroid treatment while NaCl increased both the density and the affinity of [3H] SCH 23390 binding to striatal D-1 DA receptors. By autoradiography, the increase of striatal [3H] SCH 23390 binding to D-1 DA receptors after chronic estradiol treatment was found to be homogenously distributed in this brain region. Thus, chronic treatment with estradiol of ovariectomized rats leads to an increased density of striatal D-1 DA receptors but, this hormonal modulation of D-1 DA receptors is lost when treatment is started 2 weeks after ovariectomy or later.

Repeated stimulation of D-1 dopamine receptors increases the circling response to bromocriptine in rats with a 6-OHDA lesion

We have studied the effect of repeated injections of a specific D-1 agonist, (SKF 38393), on circling behaviour in response to a specific D-2 agonist (bromocriptine) and on striatal D-2 DA receptors in rats with a unilateral lesion of the nigrostriatal pathway. Two groups of rats were formed on the basis of their circling response to a first injection of SKF 38393: clear contralateral circling (SKF-positive) and no (or ipsiversive) circling (SKF-negative). A few days later, these rats were tested once for bromocriptine-induced circling. A repeated treatment of 8 injections of SKF 38393 over 4 days was started the next day. The animals were tested again for bromocriptine-induced circling at the end of the repeated treatment with the D-1 agonist. A third group of rats received only the 2 test injections of bromocriptine and a fourth group included intact control animals. During the repeated treatment, the D-1 agonist induced an increase of the circling response up to the fourth injection and a decrease thereafter in the SKF-positive group. This was followed by a pronounced increase of circling in response to the second injection of bromocriptine. This increased sensitivity appears to be due to the chronic treatment with SKF 38393 since animals that did not respond initially to the D-1 agonist (SKF-negative group) and those that received only 2 injections of bromocriptine showed no increase of their circling response. The treatments did not increase the affinity of [3H]spiperone binding for the striatal D-2 dopamine receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral and biochemical effect of chronic treatment with D-1 and/or D-2 dopamine agonists in MPTP monkeys

Monkeys developed a severe parkinsonian syndrome after intravenous administration of (MPTP). L-DOPA/carbidopa (D-1 and D-2) or bromocriptine (D-2) treatment relieved the parkinsonian symptoms, whereas SKF 38393 (D-1) was ineffective. No dyskinesia was seen in monkeys receiving bromocriptine or SKF 38393 as opposed to the L-DOPA-treated animals, in which the dyskinetic response appeared to increased with time. MPTP induced a significant increase (25%, P less than 0.01) in the number of [3H]spiperone binding sites (Bmax) in the caudate nucleus and in putamen. The Bmax of spiperone binding in the L-DOPA-treated monkeys was on average 18% lower (P less than 0.01) than that of the animals treated with MPTP alone. The Bmax for the bromocriptine-treated group was 29% (P less than 0.01) less than that in the MPTP-treated group or 11% (P less than 0.05) less than that in the L-DOPA-treated monkeys. The SKF 38393 treatment induced a 23% (P less than 0.01) decrease in the Bmax as compared to that of animals treated with MPTP alone, and no significant change compared to the L-DOPA- or bromocriptine-treated animals. These results suggest that stimulation of D-1 and D-2 dopamine receptors can differently influence the mechanisms controlling dopamine agonist-induced dyskinesia in MPTP-treated monkeys.

Relation between brain dopamine loss and D2 dopamine receptor density in MPTP monkeys

The relationship between dopamine (DA), dihydroxyphenylacetic acid (DOPAC) or homovanillic acid (HVA) concentrations and [3H]spiperone binding to D2 DA receptors in caudate nucleus, putamen and nucleus accumbens following DAergic lesion with MPTP in monkeys was investigated. The level of DA depletion varied from 37% to 100% while a mean elevation in [3H]spiperone binding density of 114.8 +/- 4.7% of control (P less than 0.01) was observed when DA depletion was at least 90% of control values. A logarithmic correlation (y = -7.19 In x +111.15; r = -0.54, P less than 0.01) between DA concentrations (x) and the density of [3H]spiperone binding sites (y) was observed in all brain regions. The correlation between DOPAC or HVA concentrations and [3H]spiperone binding was best represented by linear rather than logarithmic equations. These results indicate that supersensitivity of DA receptors develops after MPTP lesion in monkeys when the number of remaining nerve fibers becomes too few to compensate for the DA loss.

Striatal D-2 dopamine agonist binding sites fluctuate during the rat estrous cycle

Striatal D-2 dopamine (DA) antagonist and agonist binding sites were measured during the rat estrous cycle and compared to ovariectomized (OVX) rats. Dopaminergic D-2 antagonist binding sites were constant during the estrous cycle while agonist binding sites show a rapid and significant decrease of the ratio of high to low D-2 agonist binding sites from proestrus AM (PAM) to diestrus 1 (D1) and return to OVX value in diestrus 2 (DII). Thus, physiological fluctuations of hormones as occur during the estrous cycle can modulate extrahypothalamic biogenic amine activity, namely striatal DA systems which are not involved in the control of hormone secretion.