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

Sleep and local field potential effect of the D2 receptor agonist bromocriptine during the estrus cycle and postpartum period in female rats

BACKGROUND

Pituitary lactotrophs are under tonic dopaminergic inhibitory control and bromocriptine treatment blocks prolactin secretion.

METHODS

Sleep and local field potential were addressed for 72 h after bromocriptine treatments applied during the different stages of the estrus cycle and for 24 h in the early- and middle postpartum period characterized by spontaneously different dynamics of prolactin release in female rats.

RESULTS

Sleep changes showed strong dependency on the estrus cycle phase of the drug application. Strongest increase of wakefulness and reduction of slow wave sleep- and rapid eye movements sleep appeared during diestrus-proestrus and middle postpartum treatments. Stronger sleep-wake effects appeared in the dark phase in case of the estrus cycle treatments, but in the light phase in postpartum treatments. Slow wave sleep and REM sleep loss in case of estrus cycle treatments was not compensated at all and sleep loss seen in the first day post-injection was gained further later. In opposition, slow wave sleep loss in the light phase after bromocriptine injections showed compensation in the postpartum period treatments. Bromocriptine treatments resulted in a depression of local field potential delta power during slow wave sleep while an enhancement in beta and gamma power during wakefulness regardless of the treatment timing.

CONCLUSIONS

These results can be explained by the interplay of dopamine D2 receptor agonism, lack of prolactin release and the spontaneous homeostatic sleep drive being altered in the different stages of the estrus cycle and the postpartum period.

Role of estrogen in treatment of female depression

Depression is a neurological disorder that profoundly affects human physical and mental health, resulting in various changes in the central nervous system. Despite several prominent hypotheses, such as the monoaminergic theory, hypothalamic-pituitary-adrenal (HPA) axis theory, neuroinflammation, and neuroplasticity, the current understanding of depression's pathogenesis remains incomplete. Importantly, depression is a gender-dimorphic disorder, with women exhibiting higher incidence rates than men. Given estrogen's pivotal role in the menstrual cycle, it is reasonable to postulate that its fluctuating levels could contribute to the pathogenesis of depression. Estrogen acts by binding to a diversity of receptors, which are widely distributed in the central nervous system. An abundance of research has established that estrogen and its receptors play a crucial role in depression, spanning pathogenesis and treatment. In this comprehensive review, we provide an in-depth analysis of the fundamental role of estrogen and its receptors in depression, with a focus on neuroinflammation, neuroendocrinology, and neuroplasticity. Furthermore, we discuss potential mechanisms underlying the therapeutic effects of estrogen in the treatment of depression, which may pave the way for new antidepressant drug development and alternative treatment options.

Estradiol and progesterone in female reward-learning, addiction, and therapeutic interventions

Sex steroid hormones like estradiol (E2) and progesterone (P4) guide the sexual organization and activation of the developing brain and control female reproductive behavior throughout the lifecycle; importantly, these hormones modulate functional activity of not just the endocrine system, but most of the nervous system including the brain reward system. The effects of E2 and P4 can be seen in the processing of and memory for rewarding stimuli and in the development of compulsive reward-seeking behaviors like those seen in substance use disorders. Women are at increased risk of developing substance use disorders; however, the origins of this sex difference are not well understood and therapeutic interventions targeting ovarian hormones have produced conflicting results. This article reviews the contribution of the E2 and P4 in females to functional modulation of the brain reward system, their possible roles in origins of addiction vulnerability, and the development and treatment of compulsive reward-seeking behaviors.

Dysregulation of dopamine neurotransmission in the nucleus accumbens in immobilization-induced hypersensitivity

Cast immobilization causes sensory hypersensitivity, which is also a symptom of neuropathic pain and chronic pain. However, the mechanisms underlying immobilization-induced hypersensitivity remain unclear. The present study investigated the role of dopamine neurotransmission in the nucleus accumbens shell (NAcSh) of rats with cast immobilization-induced mechanical hypersensitivity using in vivo microdialysis. Cast immobilization of the hind limb decreased the paw withdrawal threshold (PWT). Mechanical stimulation of the cast-immobilized hind limb induced a decrease in dopamine in the NAcSh, and this decrease was associated with the upregulation of presynaptic D2-like receptors. A D2-like receptor antagonist infused into the NAcSh reversed the decrease in PWT in rats with cast immobilization, whereas a D2-like receptor agonist infused into the NAcSh induced a decrease in PWT in control rats. In addition, the expression of the D2 receptor (Drd2) mRNA in the NAcSh was increased by cast immobilization. Importantly, systemic administration of the D2-like receptor antagonist reversed the decrease in PWT in rats with cast immobilization. As dopamine levels regulated by presynaptic D2-like receptors did not correlate with the PWT, it is presumed that the D2-like receptor antagonist or agonist acts on postsynaptic D2-like receptors. These results suggest that immobilization-induced mechanical hypersensitivity is attributable to the upregulation of postsynaptic D2-like receptors in the NAc. Blockade of D2-like receptors in the NAcSh is a potential therapeutic strategy for immobilization-induced hypersensitivity.

Sex Differences in Opioid and Psychostimulant Craving and Relapse: A Critical Review

A widely held dogma in the preclinical addiction field is that females are more vulnerable than males to drug craving and relapse. Here, we first review clinical studies on sex differences in psychostimulant and opioid craving and relapse. Next, we review preclinical studies on sex differences in psychostimulant and opioid reinstatement of drug seeking after extinction of drug self-administration, and incubation of drug craving (time-dependent increase in drug seeking during abstinence). We also discuss ovarian hormones' role in relapse and craving in humans and animal models and speculate on brain mechanisms underlying their role in cocaine craving and relapse in rodent models. Finally, we discuss imaging studies on brain responses to cocaine cues and stress in men and women.The results of the clinical studies reviewed do not appear to support the notion that women are more vulnerable to psychostimulant and opioid craving and relapse. However, this conclusion is tentative because most of the studies reviewed were correlational, not sufficiently powered, and not a priori designed to detect sex differences. Additionally, imaging studies suggest sex differences in brain responses to cocaine cues and stress. The results of the preclinical studies reviewed provide evidence for sex differences in stress-induced reinstatement and incubation of cocaine craving but not cue- or cocaine-induced reinstatement of cocaine seeking. These sex differences are modulated in part by ovarian hormones. In contrast, the available data do not support the notion of sex differences in craving and relapse/reinstatement for methamphetamine or opioids in rodent models. SIGNIFICANCE STATEMENT: This systematic review summarizes clinical and preclinical studies on sex differences in psychostimulant and opioid craving and relapse. Results of the clinical studies reviewed do not appear to support the notion that women are more vulnerable to psychostimulant and opioid craving and relapse. Results of preclinical studies reviewed provide evidence for sex differences in reinstatement and incubation of cocaine seeking but not for reinstatement or incubation of methamphetamine or opioid seeking.

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.

D-Amphetamine Rapidly Reverses Dexmedetomidine-Induced Unconsciousness in Rats

D-amphetamine induces emergence from sevoflurane and propofol anesthesia in rats. Dexmedetomidine is an α₂-adrenoreceptor agonist that is commonly used for procedural sedation, whereas ketamine is an anesthetic that acts primarily by inhibiting NMDA-type glutamate receptors. These drugs have different molecular mechanisms of action from propofol and volatile anesthetics that enhance inhibitory neurotransmission mediated by GABA_A receptors. In this study, we tested the hypothesis that d-amphetamine accelerates recovery of consciousness after dexmedetomidine and ketamine. Sixteen rats (Eight males, eight females) were used in a randomized, blinded, crossover experimental design and all drugs were administered intravenously. Six additional rats with pre-implanted electrodes in the prefrontal cortex (PFC) were used to analyze changes in neurophysiology. After dexmedetomidine, d-amphetamine dramatically decreased mean time to emergence compared to saline (saline:112.8 ± 37.2 min; d-amphetamine:1.8 ± 0.6 min, p < 0.0001). This arousal effect was abolished by pre-administration of the D₁/D₅ dopamine receptor antagonist, SCH-23390. After ketamine, d-amphetamine did not significantly accelerate time to emergence compared to saline (saline:19.7 ± 18.0 min; d-amphetamine:20.3 ± 16.5 min, p = 1.00). Prefrontal cortex local field potential recordings revealed that d-amphetamine broadly decreased spectral power at frequencies <25 Hz and restored an awake-like pattern after dexmedetomidine. However, d-amphetamine did not produce significant spectral changes after ketamine. The duration of unconsciousness was significantly longer in females for both dexmedetomidine and ketamine. In conclusion, d-amphetamine rapidly restores consciousness following dexmedetomidine, but not ketamine. Dexmedetomidine reversal by d-amphetamine is inhibited by SCH-23390, suggesting that the arousal effect is mediated by D₁ and/or D₅ receptors. These findings suggest that d-amphetamine may be clinically useful as a reversal agent for dexmedetomidine.

Ovarian Hormones and Reward Processes in Palatable Food Intake and Binge Eating

Ovarian hormones are associated with risk for binge eating in women. Recent animal and human studies suggest that food-related reward processing may be one set of neurobiological factors that contribute to these relationships, but additional studies are needed to confirm and extend findings.

The therapeutic potential of exercise for neuropsychiatric diseases: A review

Exercise is known to have a myriad of health benefits. There is much to be learned from the effects of exercise and its potential for prevention, attenuation and treatment of multiple neuropsychiatric diseases and behavioral disorders. Furthermore, recent data and research on exercise benefits with respect to major health crises, such as, that of opioid and general substance use disorders, make it very important to better understand and review the mechanisms of exercise and how it could be utilized for effective treatments or adjunct treatments for these diseases. In addition, mechanisms, epigenetics and sex differences are examined and discussed in terms of future research implications.

Influence of Acute Exercise on DNA Repair and PARP Activity before and after Irradiation in Lymphocytes from Trained and Untrained Individuals

Several studies indicate that acute exercise induces DNA damage, whereas regular exercise increases DNA repair kinetics. Although the molecular mechanisms are not completely understood, the induction of endogenous reactive oxygen species (ROS) during acute exhaustive exercise due to metabolic processes might be responsible for the observed DNA damage, while an adaptive increase in antioxidant capacity due to regular physical activity seems to play an important protective role. However, the protective effect of physical activity on exogenously induced DNA damage in human immune cells has been poorly investigated. We asked the question whether individuals with a high aerobic capacity would have an enhanced response to radiation-induced DNA damage. Immune cells are highly sensitive to radiation and exercise affects lymphocyte dynamics and immune function. Therefore, we measured endogenous and radiation-induced DNA strand breaks and poly (ADP-ribose) polymerase-1 (PARP1) activity in peripheral blood mononuclear cells (PBMCs) from endurance-trained (maximum rate of oxygen consumption measured during incremental exercise V'O_2max > 55 mL/min/kg) and untrained (V'O_2max < 45 mL/min/kg) young healthy male volunteers before and after exhaustive exercise. Our results indicate that: (i) acute exercise induces DNA strand breaks in lymphocytes only in untrained individuals, (ii) following acute exercise, trained individuals repaired radiation-induced DNA strand breaks faster than untrained individuals, and (iii) trained subjects retained a higher level of radiation-induced PARP1 activity after acute exercise. The results of the present study indicate that increased aerobic fitness can protect immune cells against radiation-induced DNA strand breaks.

Rapid effects of ovarian hormones in dorsal striatum and nucleus accumbens

Contribution to Special Issue on Fast effects of steroids. Estradiol and progesterone rapidly induce changes in dopaminergic signaling within the dorsal striatum and nucleus accumbens of female rats. In ovariectomized females, estradiol rapidly enhances dopamine release and modulates binding of dopamine receptors. Progesterone further potentiates the effect of estradiol on dopamine release. The effects of both estradiol and progesterone are time course dependent, with increases in dopamine release immediately after acute hormone administration followed by later inhibition of dopamine release. Importantly, these changes are also seen in naturally cycling females, indicating their importance for normal physiological states and relevant reproductive behaviors. Here, we summarize the literature establishing the rapid effects of estradiol and progesterone on dopamine release and receptor expression in dorsal striatum and nucleus accumbens of both males and females. Integrating this literature with the larger body of work focusing on dopamine regulated behaviors, we propose hypotheses for adaptive reasons (i.e., ultimate causes) as to why changes in ovarian hormones modulate dopamine release. Finally, we note the importance of these studies for understanding sex differences in vulnerability to drug addiction. Research on how dopaminergic systems regulate behavior in both males and females is crucial for developing a full appreciation of dopamine's role in both natural and drug-induced behaviors.

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.

Using positron emission tomography to investigate hormone-mediated neurochemical changes across the female lifespan: implications for depression

Ovarian hormones, particularly oestrogen and progesterone, undergo major fluctuations across the female lifespan. These hormone transition periods, such as the transition from pregnancy to postpartum, as well as the transition into menopause (perimenopause), are also known to be times of elevated susceptibility to depression. This study reviews how these transition periods likely influence neurochemical changes in the brain that result in disease vulnerability. While there are known associations between oestrogen/progesterone and different monoaminergic systems, the interactions and their potential implications for mood disorders are relatively unknown. Positron Emission Tomography (PET) allows for the in-vivo quantification of such neurochemical changes, and, thus, can provide valuable insight into how both subtle and dramatic shifts in hormones contribute to the elevated rates of depression during pre-menstrual, post-partum, and perimenopausal periods in a woman's life. As one better understands how to address the challenges of PET studies involving highly vulnerable populations, such as women who have recently given birth, one will gain the insight necessary to design and individualize treatment and therapy. Understanding the precise time-line in younger women when dramatic fluctuations in the hormonal milieu may contribute to brain changes may present a powerful opportunity to intervene before a vulnerable state develops into a diseased state in later life.

17β-Estradiol infusions into the dorsal striatum rapidly increase dorsal striatal dopamine release in vivo

Systemic injections of 17β-estradiol (E2) in ovariectomized (OVX) female rats rapidly enhance dorsal striatal dopamine (DA) release in response to amphetamine (AMPH). Additionally, a single injection of E2 rapidly (within 30min) enhances amphetamine-induced DA release. In situ studies show that this rapid effect of E2 occurs specifically within the dorsal striatum (DS). The present study investigated the in vivo effects of E2 infused into the DS, medial prefrontal cortex (mPFC) or the substantia nigra (SN) on dorsal striatal DA release. Rats were OVX and implanted with a silastic tube containing 5% E2 in cholesterol, previously shown to mimic low physiological serum concentrations of 18-32pg/ml. Single-probe microdialysis was used to measure extracellular DA levels in the DS. In addition, DA release was measured subsequent to systemic injections of the indirect DA agonist, AMPH (0.5mg/kg SC), administered simultaneously with E2 (0.544μg/100μl) or its vehicle, cyclodextrin (VEH) (0.520μg/100μl). Local infusions of E2 into the DS resulted in a greater amphetamine-induced dorsal striatal DA release in comparison to vehicle. Local infusions of E2 into the mPFC or the SN did not result in an enhancement of amphetamine-induced DA levels in the DS. These studies suggest that increases in dorsal striatal DA release in response to systemic E2 are a consequence of E2 actions within the DS itself.

Variability in paralimbic dopamine signaling correlates with subjective responses to d-amphetamine

Subjective responses to psychostimulants vary, the basis of which is poorly understood, especially in relation to possible cortical contributions. Here, we tested for relationships between participants' positive subjective responses to oral d-amphetamine (dAMPH) versus placebo and variability in striatal and extrastriatal dopamine (DA) receptor availability and release, measured via positron emission tomography (PET) with the radiotracer (18)F-fallypride. Analyses focused on 35 healthy adult participants showing positive subjective effects to dAMPH measured via the Drug Effects Questionnaire (DEQ) Feel, Like, High, and Want More subscales (Responders), and were repeated after inclusion of 11 subjects who lacked subjective responses. Associations between peak DEQ subscale ratings and both baseline (18)F-fallypride binding potential (BPnd; an index of D2/D3 receptor availability) and the percentage change in BPnd post dAMPH (%ΔBPnd; a measure of DA release) were assessed. Baseline BPnd in ventromedial prefrontal cortex (vmPFC) predicted the peak level of High reported following dAMPH. Furthermore, %ΔBPnd in vmPFC positively correlated with DEQ Want More ratings. DEQ Want More was also positively correlated with %ΔBPnd in right ventral striatum and left insula. This work indicates that characteristics of DA functioning in vmPFC, a cortical area implicated in subjective valuation, are associated with both subjective high and incentive (wanting) responses. The observation that insula %ΔBPnd was associated with drug wanting converges with evidence suggesting its role in drug craving. These findings highlight the importance of variability in DA signaling in specific paralimbic cortical regions in dAMPH's subjective response, which may confer risk for abusing psychostimulants.

Stress, sex, and addiction: potential roles of corticotropin-releasing factor, oxytocin, and arginine-vasopressin

Stress sensitivity and sex are predictive factors for the development of neuropsychiatric disorders. Life stresses are not only risk factors for the development of addiction but also are triggers for relapse to drug use. Therefore, it is imperative to elucidate the molecular mechanisms underlying the interactions between stress and drug abuse, as an understanding of this may help in the development of novel and more effective therapeutic approaches to block the clinical manifestations of drug addiction. The development and clinical course of addiction-related disorders do appear to involve neuroadaptations within neurocircuitries that modulate stress responses and are influenced by several neuropeptides. These include corticotropin-releasing factor, the prototypic member of this class, as well as oxytocin and arginine-vasopressin that play important roles in affiliative behaviors. Interestingly, these peptides function to balance emotional behavior, with sexual dimorphism in the oxytocin/arginine-vasopressin systems, a fact that might play an important role in the differential responses of women and men to stressful stimuli and the specific sex-based prevalence of certain addictive disorders. Thus, this review aims to summarize (i) the contribution of sex differences to the function of dopamine systems, and (ii) the behavioral, neurochemical, and anatomical changes in brain stress systems.

Iron and mechanisms of emotional behavior

Iron is required for appropriate behavioral organization. Iron deficiency results in poor brain myelination and impaired monoamine metabolism. Glutamate and γ-aminobutyric acid homeostasis is modified by changes in brain iron status. Such changes produce not only deficits in memory/learning capacity and motor skills, but also emotional and psychological problems. An accumulating body of evidence indicates that both energy metabolism and neurotransmitter homeostasis influence emotional behavior, and both functions are influenced by brain iron status. Like other neurobehavioral aspects, the influence of iron metabolism on mechanisms of emotional behavior is multifactorial: brain region-specific control of behavior, regulation of neurotransmitters and associated proteins, temporal and regional differences in iron requirements, oxidative stress responses to excess iron, sex differences in metabolism, and interactions between iron and other metals. To better understand the role that brain iron plays in emotional behavior and mental health, this review discusses the pathologies associated with anxiety and other emotional disorders with respect to body iron status.

Effects of SCH-23390 in combination with a low dose of 17β-estradiol on anxiety-like behavior in ovariectomized rats

The aim of this study was to explore effects on anxiety-like behavior of D₁ dopamine receptor agonist, SKF-38393, and of D₁ dopamine receptor antagonist, SCH-23390, given alone or in combination with a low dose of 17β-estradiol (17β-E₂) to ovariectomized (OVX) rats. Two weeks after surgery, OVX rats began 14 days of treatment with the vehicle, a low dose of 17β-E₂ (5.0 μg/rat, s.c.), SKF-38393 (0.1 mg/kg, i.p.), SCH-23390 (0.1 mg/kg, i.p.), SKF-38393 plus 17β-E₂, or SCH-23390 plus 17β-E₂. The animals were tested in the black and white model (BWM) and the open field test (OFT). SCH-23390 (0.1 mg/kg, i.p.) alone or in a combination with a low dose of 17β-E₂ (5.0 μg/rat, s.c.) resulted in anxiolytic-like effect in OVX rats in the BWM. Repeated treatment with SCH-23390 and 17β-E₂ profoundly increased anxiolytic-like effect of single substances exerted per se. Coadministration of SCH-23390 with 17β-E₂ increased frequency of rearing and grooming in OVX rats in OFT. SKF-38393 (0.1 mg/kg, i.p.) treatment failed to alter anxiety-like behavior in OVX rats in the BWM. The results of the present study suggest that 17β-E₂ and SCH-23390 interact to exert anxiolytic-like action and that each of these drugs can potentiate effects of each other.

Sex, hormones and neurogenesis in the hippocampus: hormonal modulation of neurogenesis and potential functional implications

The hippocampus is an area of the brain that undergoes dramatic plasticity in response to experience and hormone exposure. The hippocampus retains the ability to produce new neurones in most mammalian species and is a structure that is targeted in a number of neurodegenerative and neuropsychiatric diseases, many of which are influenced by both sex and sex hormone exposure. Intriguingly, gonadal and adrenal hormones affect the structure and function of the hippocampus differently in males and females. Adult neurogenesis in the hippocampus is regulated by both gonadal and adrenal hormones in a sex- and experience-dependent way. Sex differences in the effects of steroid hormones to modulate hippocampal plasticity should not be completely unexpected because the physiology of males and females is different, with the most notable difference being that females gestate and nurse the offspring. Furthermore, reproductive experience (i.e. pregnancy and mothering) results in permanent changes to the maternal brain, including the hippocampus. This review outlines the ability of gonadal and stress hormones to modulate multiple aspects of neurogenesis (cell proliferation and cell survival) in both male and female rodents. The function of adult neurogenesis in the hippocampus is linked to spatial memory and depression, and the present review provides early evidence of the functional links between the hormonal modulation of neurogenesis that may contribute to the regulation of cognition and stress.

Allopregnanolone modulates striatal dopamingergic activity of rats under different gonadal hormones conditions

OBJECTIVES

Progesterone modulates dopamine (DA) release in corpus striatum. Our objective was to evaluate the effect of the i.c.v injection of the neurosteroid allopregnanolone (ALL), a progesterone metabolite on dopaminergic activity in the corpus striatum of rats under different gonadal hormonal conditions.

METHODS

We have measured the concentrations of DOPA, DA and DOPAC (main metabolite of DA) in the corpus striatum in estrus and diestrus rats and in ovariectomized rats without hormonal replacement (OVX group) and primed with estrogen and progesterone (OVX(i) group). Additionally, we have used the aromatic acid decarboxylase inhibitor NSD in order to evaluate the function of tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis.

RESULTS

ALL significantly decreased the striatal concentrations of both DA and DOPAC in the estrus. On the other hand, ALL increased significantly the levels of DA in the OVX(i) group. The DOPA accumulation in OVX(i) after NSD treatment in the ALL-treated groups was greater than in the vehicle group. However, the estrus group did not modify the DOPA accumulation after NSD injection.

DISCUSSION

Our results suggest that ALL could modulate the dopaminergic transmission in the corpus striatum by causing changes in the activity of TH and/or in the pre- and post-synaptic dopaminergic terminals in the corpus striatum. This neurosteroidal mechanism could be a new kind of neurotransmitter systems modulation accomplished on TH activity itself and/or on the second messengers not related to ionic channels. Additionally, our results reinforce the idea of a close relationship between the fast non-genomic mechanism of ALL and the genomic actions of estrogen and progesterone.

Distal pup cues evoke dopamine responses in hormonally primed rats in the absence of pup experience or ongoing maternal behavior

During the early postpartum period or following estrogen/progesterone administration, pups elicit maternal behavior accompanied by a robust dopamine (DA) response in the nucleus accumbens (NAC) of female rats (Afonso et al., 2009). To determine whether DA responds to ostensibly "salient" stimuli in the absence of consummatory behaviors, we examined NAC shell DA responses during restricted (stimuli placed in a perforated box), and unrestricted access to pup and food stimuli. Microdialysis samples were collected from female rats that were either cycling and postpartum (Experiment 1), or after ovariectomy and treated with empty and hormone-filled capsules (Experiment 2). Relative to nonprimed controls, hormonally primed females had suppressed basal DA concentrations and facilitated pup-evoked DA responses, regardless of stimulus access condition. In contrast, food-evoked DA responses were unchanged by hormonal priming and were greater when females consumed food compared with distal (restricted) exposure to food. During pup and food restriction conditions, the lack of any "appetitive" behavioral differences, even in pup experienced postpartum females, was surprising. In Experiment 3, we confirmed that postpartum dams allocated time equivalently to restricted pup and food stimuli, even after pup deprivation. This was in sharp contrast to the effects of deprivation during the unrestricted access phase. Together, our data demonstrated that, in hormonally primed females, distal pup cues could evoke DA responses without prior stimulus experience, ongoing maternal (behavioral) responses, or clear evidence of robust pup saliency. The results suggest that NAC DA response reflects a state of responsiveness related to basal DA suppression in the hormonally primed female rat.

Accumbal dopamine function in postpartum rats that were raised without their mothers

Postpartum rats that had been previously raised in an artificial rearing (AR) apparatus, without their mothers or siblings during the preweaning period, show altered maternal responses towards their own offspring in adulthood. In mother-reared (MR) rats, nucleus accumbens (NAC) dopamine (DA) responses to pups evoke a robust sustained rise during the postpartum period and following treatment with estrogen/progesterone parturient-like hormones (Afonso et al., 2009). These MR females had siblings that received AR rearing with varying amounts of preweaning tactile stimulation (ARmin; ARmax). The present study examined NACshell DA responses to pup and food stimuli in these AR rats, and statistically compared them to their MR siblings. Microdialysis samples were collected from adult (90 days postnatal) AR females in different parity states (cycling vs. postpartum, Exp. 1), or after ovariectomy with different hormone treatments (sham vs. hormone, Exp. 2. After basal sample collection, pup and then food stimuli were individually presented to the females in the dialysis chamber. As with their MR siblings, basal DA concentrations were lower and pup-evoked DA responses greater in hormonally-primed AR females than in non-primed AR controls. Compared to their postpartum MR sisters (Exp. 1), AR rats had increased basal DA levels, reduced pup related DA elevations, and disrupted maternal behavior. The postpartum AR impairment in pup-evoked DA was reversed by additional pre-weaning tactile stimulation. Exogenous hormones (Exp. 2) eliminated AR impairments on pup-evoked DA responses. Although MR and AR siblings had comparable DA responses to food stimuli, upon reanalyzing MR data it was found that only postpartum dams had DA responses to pups greater than to food. These data suggest that that the hormonally induced suppression of basal DA levels may reflect saliency of pups which was greater in MR than in AR dams. Preweaning tactile stimulation could partially reverse these effects only in naturally cycling or parturient animals.

Females are more vulnerable to drug abuse than males: evidence from preclinical studies and the role of ovarian hormones

Human and animal research indicates the presence of sex differences in drug abuse. These data suggest that females, compared to males, are more vulnerable to key phases of the addiction process that mark transitions in drug use such as initiation, drug bingeing, and relapse. Recent data indicate that the female gonadal hormone estrogen may facilitate drug abuse in women. For example, phases of the menstrual cycle when estrogen levels are high are associated with enhanced positive subjective measures following cocaine and amphetamine administration in women. Furthermore, in animal research, the administration of estrogen increases drug taking and facilitates the acquisition, escalation, and reinstatement of cocaine-seeking behavior. Neurobiological data suggest that estrogen may facilitate drug taking by interacting with reward- and stress-related systems. This chapter discusses sex differences in and hormonal effects on drug-seeking behaviors in animal models of drug abuse. The neurobiological basis of these differences and effects are also discussed.

Gonadectomy and hormone replacement affects in vivo basal extracellular dopamine levels in the prefrontal cortex but not motor cortex of adult male rats

Gonadectomy in adult male rats is known to impair performance on dopamine (DA)-dependent prefrontal cortical tasks and selectively dysregulate end points in the mesoprefrontal DA system including axon density. In this study, in vivo microdialysis and high-pressure liquid chromatography were used to determine whether short (4 day)- and/or long-term (28 day) gonadectomy and hormone replacement might also influence the more functionally relevant metric of basal extracellular DA level/tone. Assessments in medial prefrontal cortex revealed that DA levels were significantly lower than control in 4-day gonadectomized rats and similar to control in 4-day gonadectomized animals supplemented with both testosterone and estradiol. Among the long-term treatment groups, DA levels were significantly higher than control in gonadectomized rats and gonadectomized rats given estradiol but were similar to control in rats given testosterone. In contrast, extracellular DA levels measured in motor cortex were unaffected by long- or short-term gonadectomy. The effects of gonadectomy and hormone replacement on prefrontal cortical DA levels observed here parallel previously identified effects on prefrontal DA axon density and could represent hormone actions relevant to the modulation of DA-dependent prefrontal cortical function and perhaps its dysfunction in disorders such as schizophrenia, attention deficit hyperactivity disorder, and autism where males are disproportionately affected relative to females.

Hormones that increase maternal responsiveness affect accumbal dopaminergic responses to pup- and food-stimuli in the female rat

The present study investigated hormonal mediation of maternal behavior and accumbal dopamine (DA) responses to pup-stimuli, as measured in microdialysis samples collected from the nucleus accumbens shell of female rats in non-homecage environment. In Experiment 1, samples were collected before and after continuous homecage pup experience from either intact postpartum or cycling females. In Experiment 2, samples were collected before and after responding maternally in homecage from ovariectomized females given either parturient-like hormone or sham treatments. After baseline sample collection in the dialysis chamber, pup and food stimuli were individually presented to females. Upon sampling completion, all animals were placed back into their homecage with donor pups for several days, and then the sample collection procedure was repeated. Prior to stimulus presentation, postpartum and hormone-treated females had decreased basal DA release compared to their controls. In response to pup stimuli, only postpartum and hormone-treated females had increased DA release compared to basal release (both sampling days). In response to food stimuli, all females had increased DA responses from basal; although there were group differences on the initial day of sampling. Findings suggest that hormones associated with inducing maternal behavior in the postpartum rat play a significant role in modifying accumbal dopaminergic responses on first exposure to pup stimuli in the rat. However, the postpartum experience provides further modifications to this brain region to promote DA responses to pup stimuli.

Effect of menstrual cycle phase on dopamine D2 receptor availability in female cynomolgus monkeys

Sex differences have been reported in a variety of affective and neurodegenerative disorders that involve dysfunctional dopamine (DA) neurotransmission. In addition, there is evidence for differences in sensitivity to the abuse-related effects of psychostimulants across the menstrual cycle which may result from effects of ovarian hormones on DA function. The goal of the present study was to extend previous work examining menstrual cycle-related changes in DA D2 receptor availability in humans to drug-naive female cynomolgus monkeys (n=7) using the selective D2-like receptor ligand [(18)F]fluoroclebopride (FCP) and a high-resolution microPET P4 scanner. Menstrual cycle phase was characterized by daily vaginal swabs and measurements of serum progesterone levels. PET studies were conducted once during the luteal phase and once during the follicular phase. Regions of interest in the caudate nucleus, putamen, and cerebellum were defined on coregistered MRIs. Distribution volumes were calculated for FCP in each structure and the distribution volume ratio (DVR) for both brain regions relative to the cerebellum was used as a measure of D2 receptor availability. FCP DVRs were significantly higher in the luteal phase compared to the follicular phase in both the caudate nucleus (11.7% difference, p=0.02) and putamen (11.6% difference, p=0.03). These findings extend earlier work in humans and suggest that changes in DA receptor availability may be involved in the variation in symptoms of various neuropsychiatric disorders across the menstrual cycle, including differences in sensitivity to the abuse-related effects of stimulants.

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.

Sex differences in drug abuse

Sex differences are present for all of the phases of drug abuse (initiation, escalation of use, addiction, and relapse following abstinence). While there are some differences among specific classes of abused drugs, the general pattern of sex differences is the same for all drugs of abuse. Females begin regularly self-administering licit and illicit drugs of abuse at lower doses than do males, use escalates more rapidly to addiction, and females are at greater risk for relapse following abstinence. In this review, sex differences in drug abuse are discussed for humans and in animal models. The possible neuroendocrine mechanisms mediating these sex differences are discussed.

Neonatal immune system activation with lipopolysaccharide enhances behavioural sensitization to the dopamine agonist, quinpirole, in adult female but not male rats

Administration of the bacterial cell wall component, lipopolysaccharide (LPS), stimulates the immune and endocrine systems inducing an acute phase of sickness and stress responses in adult and neonatal rats. Neonatal LPS exposure has been shown to alter a variety of behavioural and physiological processes in the adult animal. Early developmental stress, such as maternal separation, causes similar acute as well as long-term behavioural changes in adults, including altered sensitivity to drugs of abuse. Moreover, results of studies have shown evidence of a direct link between immune activation and sensitivity to dopamine-based drugs of abuse. The current study examined the effects of neonatal LPS treatment on subsequent locomotor sensitization to the dopamine (D(2)/D(3)) agonist, quinpirole, in adult rats as an index of drug sensitivity. Male and female Long-Evans rats were treated systemically with either LPS (50microg/kg) or saline (0.9%) on postnatal days 3 and 5. Locomotor sensitization was then examined in the adult rats (postnatal day 70). Animals were injected with quinpirole (0.5mg/kg, s.c.) or saline every other day for a total of 10 injections and locomotor activity was assessed for 60min immediately following injections 1, 2, 4, 6, and 10. Animals also received a 'challenge' injection of 0.5mg/kg quinpirole 28 days after injection 10, to assess persistence of behavioural sensitization. Locomotor activity progressively increased with repeated administration of quinpirole, indicating locomotor sensitization in all of the drug-treated groups. There was an overall sex difference, with females showing significantly greater sensitization than males. Moreover, neonatal LPS treatment potentiated both the level and the rate of development of locomotor sensitization to quinpirole administration in females, but not in males. Thus, the current study revealed that neonatal exposure to bacterial infection increases dopamine (D(2)/D(3)) agonist sensitivity in a sex-specific manner. These findings have important implications for the sexually dimorphic development of addictions to both natural and artificial rewards.

Gender differences in Parkinson's disease

Because estrogen has numerous effects on dopamine neurotransmission, many researchers are interested in its possible use to either slow the progression or reduce the risk of Parkinson's disease (PD). The incidence of PD is greater in men than in women. Gender differences in neurotoxicity have been observed, and basic research in experimental animals indicates that estrogen protects neurons from various forms of injury. However, the results of retrospective surveys of the neuroprotective effects of estrogen replacement in PD have been mixed, with some showing no effect on risk and others showing a reduction in risk. A mildly significant gender difference in disability and quality-of-life reporting has been noted, with women citing greater disability and reduced quality of life. Gender differences have been shown in response to treatment of PD, for example, in how levodopa is metabolized--women have greater levodopa bioavailability. In the Parkinson's Disease on Estrogen Therapy Replacement in the Menopause Years (POETRY) study, participants were found to have improved scores on the Unified Parkinson Disease Rating Scale. Based on the POETRY results, it is hypothesized that estrogen replacement therapy (ERT) may lead to improvement in PD symptoms and provide an opportunity to reduce the dosage of antiparkinsonian medication in women.

Interactions among ovarian hormones and time of testing on behavioral sensitization and cocaine self-administration

Ovarian hormones play a role in the use of drugs of abuse in women. In female rats estradiol has been shown to enhance acquisition of cocaine self-administration and behavioral sensitization induced by repeated cocaine treatment. Experiments were conducted to determine the effects of estradiol and/or progesterone on cocaine self-administration and behavioral sensitization to cocaine (10mg/kg; in animals with unilateral 6-hydroxydopamine lesions). Five groups of ovariectomized females were tested: (1) oil vehicle; (2) estradiol (E); (3) progesterone (P); (4) estradiol and progesterone given concurrently (EPC); (5) estradiol and progesterone given sequentially (EPS: 3 days of estradiol, 1 day progesterone, 1 day oil). All animals were tested during the dark phase of the light:dark cycle at ZT1600 and ZT2000-2100. Behavioral sensitization results: there was substantial conditioned turning throughout the habituation periods, and all animals exhibited behavioral sensitization with repeated cocaine treatment. Multivariate analysis indicated a significant effect of hormone treatment, time of day and day of testing. When individual groups were compared, however, only at ZT1600 did the E-treated and the EPS-treated animals show a trend (p<0.06) for greater behavioral sensitization to cocaine relative to the oil-treated animals. Self-administration results: all groups showed rapid acquisition of cocaine self-administration at 0.3 mg/kg/infusion, so we did not see an effect of ovarian hormones on acquisition, or a difference between groups tested at ZT1600 versus ZT2100 (p<0.05). There was, however, enhanced total intake of cocaine at 0.75 mg/kg/infusion in the E and the EPS groups. Concurrent administration of progesterone with estradiol counteracted the effect of estradiol on cocaine intake at 0.75 mg/kg/infusion, while progesterone alone did not enhance cocaine self-administration.

Estrous cycle-dependent changes in basal and ethanol-induced activity of cortical dopaminergic neurons in the rat

The influence of the estrous cycle on dopamine levels in the rat medial prefrontal cortex under basal and ethanol-stimulated conditions was evaluated by microdialysis. The basal dopamine concentration in the dialysate varied markedly during the estrous cycle, being highest in estrus and lowest in proestrus. Furthermore, a challenge intraperitoneal administration of ethanol (0.5 g/kg) induced a significant increase in dopaminergic output (+50%) during estrus but had no effect in diestrus or proestrus. Ovariectomy or pretreatment with either finasteride (a 5alpha-reductase inhibitor) or clomiphene (an estrogen receptor antagonist) prevented this ethanol-induced increase in dopamine concentration. The effect of ethanol was restored in ovariectomized rats by pretreatment with estrogen but not by that with progesterone. Our results thus show that the basal levels of dopamine in the prefrontal cortex are dependent on the phase of the estrous cycle. Furthermore, this dependence appears to be attributable to the effects of ovarian steroid hormones and results in a differential sensitivity of the dopaminergic neurons to ethanol. The hormone-induced changes in the activity of these neurons might contribute to the differences in drug sensitivity and mood state apparent among phases of the estrous cycle and between the sexes.

Changes in the content of estrogen alpha and progesterone receptors during differentiation of mouse embryonic stem cells to dopamine neurons

Embryonic stem cells (ESC) can differentiate to derivatives of the three embryonic germ layers. Dopamine neurons have been produced from mouse and human ESC. This in vitro induction mimics the developmental program followed by dopaminergic cells in vivo. Production of dopamine neurons might have clinical applications for Parkinson's disease, which has a higher incidence in men than in women, suggesting a protective role for sex hormones, particularly progesterone and estradiol. These hormones exert many of their effects through the interaction with their nuclear receptors. In this study, we used a described 5-stage protocol for dopamine neuron differentiation of ESC, allowing neuronal commitment as evidenced by specific markers and by behavioural recovery of hemiparkinsonian rats after grafting. We studied the expression of steroid hormone receptors by immunoblot during this procedure and found an increase in the content of both A and B isoforms of progesterone receptor (PR) and a decrease in estrogen receptor alpha (ER-alpha) when cells were at the neural/neuronal stages, when compared with the amount found in initial pluripotent conditions. We also found the same pattern of PR and ER-alpha expression by immunocytochemistry. Ninety-two percent of dopamine neurons expressed progesterone receptors and only 19% of these neurons co-expressed tyrosine hydroxylase and ER-alpha. These results show a differential expression pattern of ER-alpha and PR isoforms during neuronal differentiation of ESC.

Sex differences in striatal dopamine release in healthy adults

BACKGROUND

Sex differences in addictive disorders have been described. Preclinical studies have implicated the striatal dopamine system in these differences, but human studies have yet to substantiate these findings.

METHODS

Using positron emission tomography (PET) scans with high-specific-activity [11C] raclopride and a reference tissue approach, we compared baseline striatal dopamine binding potential (BP) and dopamine release in men and women following amphetamine and placebo challenges. Subjective drug effects and plasma cortisol and growth hormone responses were also examined.

RESULTS

Although there was no sex difference in baseline BP, men had markedly greater dopamine release than women in the ventral striatum. Secondary analyses indicated that men also had greater dopamine release in three of four additional striatal regions. Paralleling the PET findings, men's ratings of the positive effects of amphetamine were greater than women's. We found no sex difference in neuroendocrine hormone responses.

CONCLUSIONS

We report for the first time a sex difference in dopamine release in humans. The robust dopamine release in men could account for increased vulnerability to stimulant use disorders and methamphetamine toxicity. Our findings indicate that future studies should control for sex and may have implications for the interpretation of sex differences in other illnesses involving the striatum.

Neonatal isolation alters estrous cycle effects on ventral striatal extracellular monoamine levels

We demonstrated that neonatal isolation (ISO) enhances cocaine self-administration in male and female adult rats and alters ventral striatal extracellular levels of serotonin (5-HT) and dopamine (DA) basally or in response to psychostimulants in infant rats. Now, we examine basal 5-HT, DA, and norepinephrine (NE) levels in nucleus accumbens (NAc) using in vivo microdialysis in adult male and female rats with or without ISO experience. NAc shows estrous cycle-dependent effects as do behavioral responses to cocaine. Because our prior work showed ISO eliminated estrous-cycle effects on behavior, we now test separate groups of females in proestrus, estrus, or diestrus stages. Litters were assigned to the ISO (1-h isolation; postnatal days 2-9) or non-handled (NH) condition. During adulthood (postnatal day 70-90), microdialysis probes were implanted and aimed at NAc core. Ten samples were collected over 150-min and measures of 5-HT, DA, and NE were analyzed via HPLC. ISO did not affect 5-HT levels in males. However, ISO modified estrous stage effects on 5-HT. The pattern of 5-HT levels in NH females (higher in diestrus and proestrus vs. estrus) was reversed in ISO females. DA levels were unaffected by ISO, similar to our findings at other ages, and did not differ by gender or estrous stage. None of these factors affected NE levels. Because 5-HT modulates DA and levels of both transmitters are increased by cocaine, this neurochemical effect of ISO may contribute to the ability of ISO to alter the behavioral responses to cocaine as we showed previously.

Estrogen prevents neuroprotection by caffeine in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson's disease

Epidemiological studies have strongly linked caffeine consumption with a reduced risk of developing Parkinson's disease (PD) in men. Interestingly, in women, this inverse association is present only in those who have not taken postmenopausal estrogens, suggesting an interaction between the influences of estrogen and caffeine use on the risk of PD. To explore a possible biological basis for this interaction, we systematically investigated how the neuroprotective effect of caffeine is influenced by gender, ovariectomy (OVX), and then exogenous estrogen in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD. (1) Caffeine treatment produced a dose-dependent attenuation of MPTP-induced striatal dopamine loss in both young and retired breeder (RB) male, but not female, mice. (2) In female mice (both young and RB), caffeine was less potent or altogether ineffective as a neuroprotectant after sham surgery compared to OVX or after OVX plus estrogen replacement compared to OVX plus placebo treatment. (3) Estrogen treatment also prevented the protection of caffeine against dopamine loss in young male mice. (4) Consistent with the putative protective effect of estrogen, female and OVX plus estrogen mice were relatively resistant to MPTP toxicity compared to male and OVX plus placebo mice, respectively. (5) There was no overall difference in brain levels of caffeine and its metabolites between OVX plus placebo and OVX plus estrogen mice. Together, these results suggest that estrogen can occlude and thereby prevent the neuroprotective effect of caffeine in a model of PD neurodegeneration, supporting a biological basis for the interaction between estrogen and caffeine in modifying the risk of PD.

Sex differences and hormonal influences on acquisition of cocaine self-administration in rats

Men and women differ in their response to cocaine, and a woman's response varies with the menstrual cycle. For example, women have greater subjective responses to cocaine in the follicular phase of the menstrual cycle when estradiol is predominant, than they do during the luteal phase when both estradiol and progesterone are elevated. Similarly, female rats show significantly more cocaine-induced locomotor behavior and cocaine self-administration during behavioral estrus, shortly after estradiol peaks, than during other stages of the cycle, and estradiol administration to ovariectomized (OVX) females enhances the acquisition of cocaine self-administration. The purpose of this study was to expand upon these findings by studying the effects of progesterone administration to females, and estradiol administration to males, on acquisition of cocaine self-administration. We report here that there are both sex differences in and effects of circulating ovarian hormones on acquisition of cocaine self-administration. We demonstrate that although estradiol administration enhances acquisition of cocaine self-administration in OVX female rats, concurrent administration of progesterone with estradiol inhibits this effect of estradiol. In a separate experiment, we demonstrate that estradiol administration does not enhance acquisition of cocaine self-administration in castrated male rats. We conclude that (1) there is a sex difference in the effects of estradiol on cocaine self-administration: it facilitates acquisition in female, but not male rats; and that (2) in females concurrent progesterone treatment counteracts the facilitory effect of estradiol on cocaine self-administration.

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.

Estrogen mediated inhibition of dopamine transport in the striatum: regulation by G alpha i/o

In the current study, the interaction between estrogen priming and dopamine D2 receptor activation on dopamine uptake in the striatum of ovariectomized female rats was investigated. Basal ADP-[(32)P(i)]ribosylation of G(i/o) was examined in synaptosomal membranes prepared from ovariectomized, estrogen primed or N-p-(isothiocyanatophenethyl) spiperone (NIPS) treated rats. [(32)P(i)]-incorporation was significantly increased (141%) in tissue from NIPS treated animals but attenuated (57%) in tissue from estrogen primed animals. Dopamine uptake kinetics were measured in vivo following manipulation of the heterotrimeric G-protein by pertussis toxin (0.5 microg, 48 h). Pertussis toxin significantly inhibited dopamine uptake at all concentrations of dopamine examined. Co-treatment with estrogen and pertussis toxin resulted in a further attenuation of dopamine transport at high but not low dopamine concentrations. These data are consistent with an estrogen mediated alteration of G-protein activity and support the hypothesis that estrogen may alter transporter activity through a modulation of dopamine D2 autoreceptor/G alpha(i/o) protein coupling.