Effect of estradiol and haloperidol on hypophysectomized rat brain dopamine receptors

Abnormally rapid reversal learning and reduced response to antipsychotic drugs following ovariectomy in female rats

Epidemiological and clinical life cycle studies indicate that favorable illness course and better response to antipsychotic drugs (APDs) in women with schizophrenia are positively correlated with estrogen levels. Accordingly, the estrogen hypothesis of schizophrenia proposes a neuroprotective role of estrogen in women vulnerable to schizophrenia. Previously we demonstrated in the rat that low levels of estrogen induced by ovariectomy led to disruption of latent inhibition (LI) reflecting impairment of selective attention, a core deficit of schizophrenia. LI disruption was reversed by 17β-estradiol and the atypical APD clozapine, whereas the typical APD haloperidol was ineffective unless co-administered with 17β-estradiol. Here we aimed to extend these findings by testing ovariectomized rats in another selective attention task, discrimination reversal. Ovariectomy led to a loss of selective attention as manifested in abnormally rapid reversal. The latter was normalized by high dose of 17β-estradiol (150 μg/kg) and clozapine (2.5mg/kg), but not by haloperidol (0.1mg/kg) or lower doses of 17β-estradiol (10 and 50 μg/kg). However, co-administration of haloperidol with 17β-estradiol (50 μg/kg) was effective. In sham rats low 17β-estradiol (10 μg/kg) produced rapid reversal, while high 17β-estradiol (150 μg/kg), haloperidol alone, or haloperidol-17β-estradiol combination reduced reversal speed. Clozapine did not affect reversal speed in sham rats. These results strengthen our previous results in suggesting that schizophrenia-like attentional abnormalities as well as reduced response to APDs in female rats are associated with low level of gonadal hormones. In addition, they support the possibility that estrogen may have an antipsychotic-like action in animal models.

Contrasting effects of increased and decreased dopamine transmission on latent inhibition in ovariectomized rats and their modulation by 17beta-estradiol: an animal model of menopausal psychosis?

Women with schizophrenia have later onset and better response to antipsychotic drugs (APDs) than men during reproductive years, but the menopausal period is associated with increased symptom severity and reduced treatment response. Estrogen replacement therapy has been suggested as beneficial but clinical data are inconsistent. Latent inhibition (LI), the capacity to ignore irrelevant stimuli, is a measure of selective attention that is disrupted in acute schizophrenia patients and in rats and humans treated with the psychosis-inducing drug amphetamine and can be reversed by typical and atypical APDs. Here we used amphetamine (1 mg/kg)-induced disrupted LI in ovariectomized rats to model low levels of estrogen along with hyperfunction of the dopaminergic system that may be occurring in menopausal psychosis, and tested the efficacy of APDs and estrogen in reversing disrupted LI. 17beta-Estradiol (50, 150 microg/kg), clozapine (atypical APD; 5, 10 mg/kg), and haloperidol (typical APD; 0.1, 0.3 mg/kg) effectively reversed amphetamine-induced LI disruption in sham rats, but were much less effective in ovariectomized rats; 17beta-estradiol and clozapine were effective only at high doses (150 microg/kg and 10 mg/kg, respectively), whereas haloperidol failed at both doses. Haloperidol and clozapine regained efficacy if coadministered with 17beta-estradiol (50 microg/kg, an ineffective dose). Reduced sensitivity to dopamine (DA) blockade coupled with spared/potentiated sensitivity to DA stimulation after ovariectomy may provide a novel model recapitulating the combination of increased vulnerability to psychosis with reduced response to APD treatment in female patients during menopause. In addition, our data show that 17beta-estradiol exerts antipsychotic activity.

Estrogen, but not testosterone, attenuates methamphetamine-evoked dopamine output from superfused striatal tissue of female and male mice

The gonadal steroid hormone, estrogen, has the capacity to function as a neuroprotectant against methamphetamine (MA)-induced neurotoxicity of the nigrostriatal dopaminergic system within female, but not male, mice. In an attempt to understand some of the bases for this effect of estrogen, the incipient effects of MA upon evoked dopamine output from superfused striatal tissue fragments of gonadectomized female and gonadectomized as well as intact male mice were evaluated under conditions where estrogen (or testosterone) was present in the medium. The amount of dopamine evoked by MA was significantly reduced when estrogen was co-infused with MA. This attenuation was obtained with striatal tissue fragments of gonadectomized female and gonadectomized and intact male mice. In contrast to estrogen, co-infusion of testosterone failed to produce an overall statistically significant change in MA-evoked dopamine output within superfused striatal tissue fragments of gonadectomized female and male mice. In this way, the gonadal steroid hormones, estrogen and testosterone, exert differential modulatory effects upon MA-evoked dopamine output from superfused striatal tissue fragments. However, similar effects to these gonadal steroid hormones were observed between gonadectomized female and gonadectomized or intact male mice. These data reveal an absence of a sexual dimorphism in striatal responsiveness with regard to estrogen's ability to alter MA-evoked DA output. Accordingly, the sexually dimorphic capacity for estrogen to function as a neuroprotectant may involve a composite of actions upon the nigrostriatal dopaminergic system involving events/sites other than the initial stimulation of dopamine output.

Psychoneuroendocrinology of schizophrenia

The study of psychoneuroendocrinology of schizophrenia has yielded an extensive but inconclusive body of data. Investigations to date have been limited by several factors, including the confounding effects of neuroleptic drugs, methodological limitations, and lack of appreciation for the heterogeneity of the illness. Previously, the focus of research has been on the measurement of anterior pituitary hormones, guided by the assumptions that these hormones are regulated by the central nervous system (CNS) to a significant degree and that the unique anatomic relationship of the pituitary gland to the hypothalamus and the CNS is potentially relevant. Patients with schizophrenia do appear to have distinct endocrinologic profiles. However, although the hormonal differences between patients with schizophrenia and the general population appear to be subtle in magnitude. Nonetheless, investigation, and the exploration of the possible effect of gonadal and posterior pituitary hormones merits particular attention.

Women and schizophrenia: a review

OBJECTIVE

This is a selective review article which aims to explore current areas of research involving women with schizophrenia and to identify areas requiring future research.

METHOD

Research studies conducted both nationally and internationally are reviewed with an emphasis on a gender-based approach.

RESULTS

The review is divided into three sections. First there is an introduction including a brief consideration of the possible reasons for the noted lack of studies focussing on women with schizophrenia and a summary of the epidemiological evidence that has prompted interest in gender-based schizophrenia research. Second, there is a review of some studies that utilise gender as a paradigm to understand the pathophysiology of schizophrenia, and third, a discussion of the role of gender in management approaches is presented.

CONCLUSIONS

The main conclusion is that women have not been well represented in schizophrenia research. Female-specific management issues and a better understanding of the impact of schizophrenia on women require more gender-based research.

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.

Serum gonadal steroid hormones in young schizophrenic patients

Psychosis is reported to show a later age of onset in women than in men and its nature and course in women may also differ. The purpose of this study was to determine if levels of four steroid hormones at the start of early onset psychosis differ from the levels of other groups of young people and if predicted low levels of estrogen (E2) are a feature of female psychosis. Two blood samples from 22 young psychotic patients were analysed by radioimmunoassay for E2, progesterone (PROG), testosterone (TE), and dehydroepiandrosterone sulphate (DHEAS). Female psychotic patients showed E2 levels lower than matched healthy cycling controls but higher than those on a contraceptive pill; they also showed higher TE levels than controls. Male psychotic patients had higher DHEAS levels than healthy or obsessive compulsive disorder (OCD) subjects. We suggest that illness-related changes of steroids can be measured superimposed on medication-induced changes and that lower E2 levels in psychotic women may increase their vulnerability to psychosis. Changes of TE in female and DHEAS in male psychotics may be more a consequence of the illness.

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.

From an animal model of an attentional deficit towards new insights into the pathophysiology of schizophrenia

The paper presents an animal model of schizophrenic-like attentional deficit, consisting of an inability to ignore irrelevant stimuli. It is based on the paradigm of latent inhibition (LI), in which animals learn to ignore repeatedly presented stimuli not followed by meaningful consequences. In a series of experiments it was demonstrated that the capacity to ignore irrelevant stimuli is lost in rats treated with systemic or intra-accumbens injections of amphetamine, in normal volunteers given amphetamine, in high "psychosis-prone" persons, in acute schizophrenic patients and in untreated male adult rats that were raised until weaning under conditions of extremely restricted stimulation. In addition, LI is lost following the disruption of the hippocampal input to the nucleus accumbens. In all of the above conditions tested for antagonism by anti-psychotic drugs a loss of LI is reversed. On the basis of these results we propose an animal model which accommodates a neurodevelopmental dysfunction, hippocampal pathology, mesolimbic DA overactivity, vulnerability to stress, and gender differences, all of which have been postulated as factors in the pathophysiology of schizophrenia.

Effect of sex steroids on cocaine lethality in male and female mice

1. Endogenous sex steroid levels were altered in mice via gonadectomy, via physiological or supraphysiological doses of testosterone and/or estradiol, and via tamoxifen dosing to antagonize estrogens. 2. The role of sex hormones in susceptibility to cocaine lethality was examined via the response of mice after endocrine alterations to an intraperitoneal (i.p.) cocaine HCl (75 mg/kg). Incidence of deaths was significantly decreased only in sham-operated males receiving estradiol or tamoxifen and in ovariectomized or sham-operated females receiving doses of estradiol. 3. The levels of estradiol in both sexes appeared to be more influential than were levels of testosterone as a determinant of susceptibility to cocaine.

Leu-enkephalin, tamoxifen and ethanol interactions: effects on motility and hepatic ethanol metabolizing enzymes

1. Short-term treatment with tamoxifen (a nonsteroidal antiestrogen) decreased mouse spontaneous locomotor activity compared to controls. 2. Short-term pretreatment with tamoxifen prior to an acute sedative dose of ethanol potentiated ethanol-medicated behavioral depression in the mouse. 3. Injection of a small dose of Leu-enkephalin, which is devoid of effect on mouse motility, prior to an acute sedative dose of ethanol to tamoxifen pretreated female mice counteracted ethanol-produced suppression of motor activity. 4. Mouse liver aldehyde dehydrogenase was inhibited by the short-term administration of tamoxifen when given alone or preceding acute dosages of Leu-enkephalin. Concomitantly, there was an increase in blood plasma ethanol concentration from corresponding control. 5. The results of the behavioral performance test used suggest that tamoxifen possesses depressant property and exerts synergestic effect with Leu-enkephalin in antagonizing ethanol-produced behavioral depression in the mouse. 6. The enzymatic part of the study indicates an adverse metabolic influence by tamoxifen on hepatic metabolism of ethanol-derived acetaldehyde which could contribute to the potentiation of the sedative effect of ethanol.

Reduction of motor behavioural deficits in senescence via chronic prolactin or estrogen administration: time course and putative mechanisms of action

The effects of chronic estrogen (E2), rat prolactin (rPRL), modified ovine prolactin (mPRL) administration on motor behavior (inclined screen performance) and striatal dopamine (DA) (D2subtype) receptor concentrations were examined in senescent (greater than 24 months of age) female rats, mPRL possesses no lactotrophic activity. Administration of either E2 or rPRL was effective in improving both inclined screen performance (increased time that the animal could remain on the screen by 95 and 413 s, respectively, compared to highest pre-injection performance) and striatal D2 receptor concentrations (14 and 20% respectively). These were indications, however, from separate analyses that improvements in inclined screen performance were seen prior to any increases in striatal D2 receptor concentrations. These early performance increases seemed instead to be the result of improved muscarinic receptor control over striatal DA autoreceptor function. Later improvements in inclined screen performance (at 6-7 days after the E2 injections were begun) were more dependent on increased striatal DA receptor concentrations. A second set of experiments which involved the injection of E2 into senescent male as well as female rats indicated that there were no sex differences in improvements in inclined screen performance, and that once the E2 injections were discontinued, performance returned to preadministration levels. The results are discussed in terms of two important processes that may be involved in mediating enhanced inclined screen performance following E2 administration: (1) enhancement of muscarinic receptor regulation of DA autoreceptor function; and (2) increases in striatal DA receptor density.

Gender differences in the brain: are they relevant to the pathogenesis of schizophrenia?

Gender differences are present in the clinical expression of schizophrenia, age of onset, course of illness, and response to pharmacologic treatment. These differences are not surprising in view of the normal gender differences in brain growth, differentiation, adult brain structure, and neurochemistry. The present review examines what is presently known about brain gender differences, and whether this information is consistent with the published reports of brain functional and morphological abnormalities in schizophrenia. Whether gender differences in the brain can explain the gender differences in clinical aspects of the disorder remains unknown.

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.

Reproductive hormones control striatal tyrosine hydroxylase activity in the male rat

The effects of castration, hypophysectomy and testosterone treatment on striatal tyrosine hydroxylase (TOH) activity were examined in male rats. Enzyme activity was measured by means of high-performance liquid chromatography (HPLC) determination of L-3,4-dihydroxphenylalanine (DOPA) formed. Serum levels of both testosterone and luteinizing hormone (LH) were measured by radioimmunoassay (RIA). Castration, but not hypophysectomy, reduced TOH activity in the striatum. The administration of testosterone propionate (TP) to castrated animals in a dose of 10 micrograms/100 g b.wt. during the two days previous to sacrifice, completely prevented the castration-induced reduction of striatal TOH activity. In orchidectomized rats treated with different doses of testosterone propionate (TP) up to 20 micrograms/100 g b.wt., the levels of striatal TOH activity were apparently related to either the dose-related increase of serum testosterone or the decrease of serum LH. Higher doses of the androgen failed to further modify striatal TOH activity, in spite of the dose-related elevation of serum testosterone concentration. These results suggest that circulating levels of gonadal and/or pituitary hormones partially control dopaminergic synthesis in striatal terminals, which in turn may account for some behavioral effects of reproductive hormones.

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

The effect of a chronic 17 beta-estradiol (10 micrograms, b.i.d.) treatment of 2 weeks to ovariectomized rats was investigated on lateral and medial caudate-putamen dopamine receptors. Dopamine D2 receptors were assayed with [3H]spiperone binding to caudate-putamen homogenates or by autoradiography of forebrain sections. Estradiol treatment leads to a significant increase in the density of lateral caudate-putamen dopamine receptors while for the medial dopamine receptors the increase is non-significant. This effect is observed when plasma levels of 17 beta-estradiol and prolactin are increased. These results indicate that the effect of estradiol on striatal dopamine receptors is heterogeneous and that those in the lateral part are more susceptible to this steroid.

Elevation of striatal dopamine receptors by estrogen: dose and time studies

Administration to male rats of a single dose of 17 beta-estradiol valerate (8-500 micrograms/rat) or implantation of a pellet containing 17 beta-estradiol (0.5-50 mg/rat) increased serum 17 beta-estradiol levels in a dose-dependent relationship when measured on the sixth day after administration. At the same time, after these doses, the serum rat prolactin (rPRL) levels were doubled and the striatal 3,4-dihydroxyphenylethylamine (DA, dopamine) receptor densities were increased 20%. A single dose of 17 beta-estradiol valerate of 4 micrograms/rat or less did not alter serum 17 beta-estradiol or rPRL levels or the striatal DA receptor density. After the single injection of 17 beta-estradiol valerate (125 micrograms/rat) the serum 17 beta-estradiol levels peaked at 1 day, the serum rPRL levels peaked at 2 days, and the striatal DA receptor density elevation peaked from 4 to 8 days. Implantation of a pellet containing 17 beta-estradiol (25 mg/rat) produced a constant elevation of serum 17 beta-estradiol levels from 1 to 10 days. Whereas the serum rPRL levels were continuously elevated about two-fold, the densities of the striatal DA receptors were increased significantly by 20-25% only from 4 to 8 days after pellet implantation. These results indicate that striatal DA receptor density rises and returns to control levels during the constant elevation of serum 17 beta-estradiol and rPRL levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term effects of MPTP on central and peripheral catecholamine and indoleamine concentrations in monkeys

5 Macaca fascicularis monkeys developed a severe parkinsonian syndrome in the days following intravenous administration of the toxin MPTP. One monkey remained untreated while two groups of two animals were treated daily for 5 months with supramaximal oral doses of either Sinemet or bromocriptine. Both drugs relieved the parkinsonian symptoms. Plasma prolactin concentrations were elevated in MPTP-treated monkeys compared to intact monkeys. MPTP caused a rapid decrease of homovanillic acid (HVA) concentrations in the CSF of these monkeys within days of the toxin injection and these values remained low until sacrifice of the animals 5 months later. By contrast, CSF 5-hydroxyindoleacetic acid (5-HIAA) concentrations were elevated a few days after the start of MPTP treatment and these values returned to control levels by 5 months. Five months after the start of MPTP treatment, epinephrine (E) and dopamine (DA) levels were decreased in the adrenal medulla while the norepinephrine (NE) concentration remained unchanged. Catecholamines were assayed in the caudate putamen, nucleus accumbens, amygdala and frontal cortex of these monkeys. NE concentrations were decreased in the frontal cortex of MPTP-treated monkeys while a decrease of E concentrations after MPTP was only observed in the n. accumbens. Dopamine and its metabolites dihydroxyphenylacetic acid (DOPAC) and HVA were reduced in the caudate, putamen, n. accumbens and frontal cortex. Our results show that MPTP treatment in the long-term (5 months) not only affects the dopaminergic system of the caudate-putamen but also has effects on dopaminergic systems in other regions as well as on noradrenergic and adrenergic systems in the brain and the periphery.

Chronic treatment with L-DOPA, but not bromocriptine induces dyskinesia in MPTP-parkinsonian monkeys. Correlation with [3H]spiperone binding

A group of 5 monkeys developed a severe parkinsonian syndrome after intravenous administration of the toxin MPTP. One remained untreated while two animals were treated daily for 5 months with supramaximal doses of Sinemet and two with bromocriptine orally. Both drugs relieved the parkinsonian symptoms but the animals on Sinemet developed after 2 weeks prominent lingual dyskinesia which remained visible after each dose until the end of the experiment. In the two animals on bromocriptine no dyskinesia was observed. After sacrifice, the levels of dopamine and [3H]spiperone binding were studied bilaterally in the anterior and posterior caudate nucleus, anterior and posterior putamen and in the nucleus accumbens. The loss of dopamine was equivalent in the Sinemet and the bromocriptine treated animals (more than 90%) and there was a complete disappearance of the substantia nigra pars compacta. In all structures studied, the Bmax for [3H]spiperone binding was on average 10% higher in the Sinemet than in the bromocriptine-treated animals. We therefore believe that L-DOPA and bromocriptine affect denervated postsynaptic dopamine receptors differently, that bromocriptine is less likely to induce agonist supersensitivity and that this probably explains the lesser tendency to induce dyskinesia after chronic treatment.

Estradiol can suppress haloperidol-induced supersensitivity in dyskinetic monkeys

We have studied a monkey model of lingual dyskinesia, due to a midbrain lesion, which is markedly increased by apomorphine. In such animals a single large intramuscular dose of haloperidol (HAL), 1 mg/kg, almost completely abolishes the apomorphine potentiation after 24 h, but 15 days later there is a 5-fold increase in the response to apomorphine which we attribute to supersensitivity. Estradiol benzoate (0.15 mg/kg, subcutaneously) on days 3, 6, 9 and 12 after HAL completely suppresses the expected rebound supersensitivity to apomorphine. However, the suppression is not seen if the animals have received HAL and estradiol together in the initial treatment.

17 beta-Estradiol at a physiological dose acutely increases dopamine turnover in rat brain

Ovariectomized rats injected with 17 beta-estradiol (30 ng, s.c.) showed an increase of the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with no change of dopamine concentrations in the striatum and nucleus accumbens. This increase was observed 30 min after the steroid injection and coincided with peak plasma 17 beta-estradiol concentrations. Plasma prolactin concentrations were not significantly elevated after 30 ng of 17 beta-estradiol. In ovariectomized rats with a unilateral lesion of the entopeduncular nucleus, the same dose of 17 beta-estradiol induced a postural deviation to the lesioned side with a maximum at 30 min. Thus, very small doses of estradiol were able to increase dopamine turnover. This effect was seen within minutes and was relatively short. It is probably non-genomic, presynaptic and similar to the effect of a small dose of a dopamine releasing agent.

Modulation of brain and pituitary dopamine receptors by estrogens and prolactin

Ovariectomized rats were treated for 2 weeks with 17 beta-estradiol (0.0002-100 micrograms/day). [3H]spiperone striatal dopamine receptor binding was maximally increased by 30% after 0.05 micrograms/day of 17 beta-estradiol and a similar increase is observed at higher doses. By contrast, plasma prolactin concentrations of these rats are unchanged after 0.05 micrograms/day and increased after 100 micrograms/day. A chronic estradiol treatment at very low doses (0.0002-0.001 microgram/day) leads to increases in pituitary dopamine receptor binding while plasma prolactin levels are unchanged. At higher doses (1-100 micrograms/day) binding is decreased and plasma prolactin concentrations are elevated. [3H]spiperone striatal dopamine receptor binding is elevated in lactating female rats compared to intact or ovariectomized female rats. Anterior pituitary dopamine receptor concentrations fluctuate during the estrous cycle while striatal dopamine receptors are unchanged. An injection of 30 ng of estradiol, which reproduces the estradiol proestrus surge, leads as in proestrus, to a decrease of anterior pituitary dopamine receptors.