Evidence for the reestablishment of copulatory behavior in castrated male rats with a brain-enhanced estradiol-chemical delivery system

New evidence for the selective, long-lasting central effects of the brain-targeted estradiol, Estredox

The present study examined the dose- and time-dependent central effects of an estradiol chemical delivery system cyclodextrin complex (E(2)-CDS-CD) on the reestablishment of copulatory behavior of castrated male and ovariectomized female rats with concomitant determination of the blood luteinizing hormone (LH) and E(2) levels. In orchidectomized males, Estredox, after single doses of 0.3 and 3.0 mg/kg iv, reestablished the mounting and intromission up to 4 weeks. The LH suppressive effect lasted to Day 7 and 28, respectively. After repeated administration for 10 days at a dose of 0.01mg/kg iv, significant effect was obtained by Day 14. Ovariectomized females were treated iv daily for 5 days either with E(2)-CDS-CD, estradiol benzoate (EB) or vehicle, and the lordosis quotient was determined. At a dose of 0.03 mg/kg the duration of EB's effect was 10 days shorter and only one-third of that of E(2)-CDS-CD. The LH suppression lasted to Day 18. On the other hand, after EB treatment there was no significant decrease in LH levels. The low plasma E(2) levels indicated fast rate of peripheral elimination in both males and females. The brain-targeting E(2) indicates better efficacy and increased safety in replacement therapies because of the reduced peripheral side effects.

Barriers to remember: brain-targeting chemical delivery systems and Alzheimer's disease

Brain-targeted chemical delivery systems (CDSs) represent rational drug design attempts not only to deliver but also to target drugs to their site of action. Using a sequential metabolism approach, the special bidirectional properties of the blood-brain barrier can be exploited to smuggle the precursors of therapeutic compounds across the barrier and lock them inside the brain ready for sustained release of the active drugs. Many potential therapeutic applications can be envisioned for such CDSs; here, the potential of brain-targeted estradiol for the prevention and treatment of Alzheimer's disease is reviewed in detail.

Distribution of gonadal steroid receptor-containing neurons in the preoptic-periaqueductal gray-brainstem pathway: a potential circuit for the initiation of male sexual behavior

The present study used anterograde and retrograde tract tracing techniques to examine the organization of the medial preoptic-periaqueductal gray-nucleus paragigantocellularis pathway in the male rat. The location of neurons containing estrogen (alpha subtype; ER alpha) and androgen receptors (AR) were also examined. We report here that injection of the anterograde tracer biotinylated dextran amine (BDA) into the medial preoptic (MPO) produced dense labeling within the periaqueductal gray (PAG); anterogradely labeled fibers terminated in close juxtaposition to neurons retrogradely labeled from the nucleus paragigantocellularis (nPGi). Dual immunostaining for Fluoro-Gold (FG) and ER alpha or FG and AR showed that over one-third of MPO efferents to the PAG contain receptors for either estrogen or androgen. In addition, approximately 50% of PAG neurons retrogradely labeled from the nPGi were immunoreactive for either ER alpha or AR. These results are the first to establish an MPO-->PAG-->nPGi circuit and further indicate that gonadal steroids can influence neuronal synaptic activity within these sites. We reported previously that nPGi reticulospinal neurons terminate preferentially within the motoneuronal pools of the lumbosacral spinal cord that innervate the pelvic viscera. Together, we propose that the MPO-->PAG-->nPGi circuit forms the final common pathway whereby MPO neural output results in the initiation and maintenance of male copulatory reflexes.

Targeting drugs to the brain by redox chemical delivery systems

Chemical delivery systems (CDSs) based on the redox conversion of a lipophilic dihydropyridine to an ionic, lipid-insoluble pyridinium salt have been developed to improve the access of therapeutic agents to the central nervous system. A dihydropyridinium-type CDS or a redox analog of the drug is sufficiently lipophilic to enter the brain by passive transport, then undergoes an enzymatic oxidation to an ionic pyridinium compound, which promotes retention in the CNS. At the same time, peripheral elimination of the entity is accelerated due to facile conversion of the CDS in the body. This review discusses chemical, physicochemical, biochemical, and biological aspects in relation to the principles and practical implementation of the redox brain-targeting approach to various classes of drugs. Representative examples to the brain-enhanced delivery of neurotransmitters, steroids, anticonvulsants, antibiotics, antiviral, anticancer and antidementia agents, and neuropeptides and their analogs are presented in detail. In vivo and in vitro studies and preliminary clinical data of several novel derivatives have been promising, which could lead to a practical use of the redox CDSs after proper pharmaceutical development. The investigations accentuate the need for considering physicochemical, metabolic, and pharmacokinetic properties in designing of carrier systems that are able to target drugs into the central nervous system.

Steroid sulfatase inhibitor alters blood pressure and steroid profiles in hypertensive rats

Our hypothesis is that the steroid sulfatase gene (Sts) may indirectly contribute to the modulation of blood pressure (BP) in rats with genetic hypertension. The steroid sulfatase enzyme (STS) catalyzes the conversion of estrone sulfate, dehydroepiandrosterone sulfate, cholesterol sulfate and glucocorticoid sulfates to their active nonconjugated forms. This causes the elevation of biologically active steroids, such as glucocorticoids, mineralcorticoids as well as testosterone, which may lead to increased BP. The main objective was to examine the effects of a steroid sulfatase inhibitor on blood pressure and steroid levels in rats with hypertensive genetic backgrounds. Three treatment groups, 5-15 weeks of age were used: controls, estrone and STS inhibitor (estrone-3-O-sulfamate), (n=8 per group). BP was taken weekly by tail cuff, and serum testosterone (T), estrogens (E), and plasma corticosterone (C) levels were measured by radioimmunoassay. BP was significantly reduced by the STS inhibitor in the strains with genetically elevated BP. Also the inhibitor alone significantly reduced plasma corticosterone in all strains compared to estrone treatment with a concomitant as well as significant rise in estrogens and reduction in testosterone and body weight.

Effect of sustained estradiol release in the intact male rat: correlation of estradiol serum levels with actions on body weight, serum testosterone, and peripheral androgen-dependent tissues

The differential effect of increasing serum estradiol on various parameters in the intact male rat was assessed through the use of subcutaneously implanted, hormone-laden pellets. The delivery systems were designed to release drug through bioerosion at a zero-order rate over a 12-day time-course. Male Sprague-Dawley rats (190 to 220 g) were given estrogen pellets at increasing labeled strenghts (0, 0.001, 0.01, 0.1, 1.0, 10, 50, and 100 mg). Animals were weighed at various intervals before and after implantation. At Day 6, 12, and 26 after drug administration, rats were examined for 4 additional parameters, including serum estradiol and testoterone concentrations and accessory organ weights (i.e., ventral prostate and seminal vesicles). Serum estradiol levels were consistent with pellet potency and lifetime. Increases in body weight were suppressed 50% by circulating estradiol levels of approximately 200 pg/mL at Day 6,250 pg/mL at Day 12, and 285 pg/mL at Day 26. On the other hand, suppression of serum testosterone was more sensitive and was decreased 50% by peripheral estrogen levels of 36, 43, and 51 pg/mL at Days 6, 12, and 26, respectively. Accessory organ weights essentially reflected serum testosterone levels as indicated by their similar ED50 values: 50.5, 50.5, and 44.3 pg/mL for the ventral prostate at Day 6, 12, and 26, respectively, and 48, 56, and 51.5 pg/mL for the seminal vesicle regression at Day 6, 12, and 26, respectively. The data indicate the pellet used provided sustained plasma levels of hormone and these constant peripheral levels exerted potent pharmacological action. Initial body weight changes seemed to be less sensitive to the action of estradiol than serum testosterone or derivative properties, such as accessory organ weight.

Suppression of plasma testosterone and prostate carcinoma size by a redox-based, brain-targeted estrogen delivery system in the rat

Studies were undertaken to examine the effects of an estradiol-chemical delivery system (E2-CDS) or castration (CAST) on plasma testosterone (T) and growth of the Segaloff 11095 carcinoma. Fischer 344 rats were implanted subcutaneously with the Segaloff 11095 tumor and tumor growth was monitored thereafter. After optimal tumor growth, when the average tumor size was approximately 25 x 15 mm (length x width; 4-5 g wet weight), rats were randomized into (1) testis-intact controls; (2) CAST; (3) intact+E2-CDS groups (rats received weekly injection of the E2-CDS at 0.5 mg/kg). Animals were killed 7 or 14 days after the initiation of treatments. Blood and tissue samples were collected for subsequent analysis. Plasma T levels were suppressed by 98% and 97% through 14 days after CAST or E2-CDS treatment. CAST increased plasma gonadotropin (LH) concentrations, while E2-CDS reduced LH compared to intact control levels. E2-CDS treatment increased plasma E2 levels to 24 (one injection) or 75 pg/ml (two injections) at 7 or 14 days, respectively. E2-CDS, given once a week for 2 consecutive weeks, resulted in a decreased growth of the prostate tumor by 61%, while CAST reduced the weights of these tumors by only 20%. In response to E2-CDS (one or two injections), weights of the in situ ventral prostate and seminal vesicles were significantly reduced by 70% and 50%, respectively, in tumor-bearing rats. Similarly, CAST reduced the weights of these tissues by 80% (prostate) or 52% (seminal vesicle) at 7 or 14 days after treatment. Pituitary weight increased, while testes weight decreased by 20% with two injections of E2-CDS, compared with intact control rats. Collectively, these data indicate that E2-CDS is effective in reducing the growth rate of prostatic tumors in the rat.

A redox-based chemical delivery system that enhances estradiol distribution to the brain: disposition studies in the rat

The disposition of a chemical delivery system for estradiol (E2-CDS) which is based on a redox dihydropyridine-pyridinium salt conversion was investigated in rats. Tissue and plasma concentrations of E2-CDS and the oxidized metabolite (E2-Q+) were evaluated at times ranging from 1 to 14 days after intravenous administration of E2-CDS formulated as a modified cyclodextrin inclusion complex. While E2-CDS levels were below HPLC assay detection limits for all samples by 1 day postdosing, E2-Q+ was readily quantified. The calculated half-life of E2-Q+ was longest in brain tissue, significantly shorter in heart, lung, and kidney tissues, and shortest in plasma. There was a linear relationship between administered E2-CDS dose and oxidized metabolite measured in brain as well as in other tissues collected 24 hr after drug administration. Coadministration of high doses of a similarly oxidizable dihydropyridine, 1-methyl-1,4-dihydronicotinamide (NMN), in a dimethylsulfoxide (DMSO) vehicle decreased E2-Q+ measured in brain and other tissues without significantly affecting the relative patterns of distribution in these tissues. Brain tissue E2Q+ levels were not detected after dosing with the oxidized metabolite.

Effects of a brain-enhanced estrogen delivery system on tail-skin temperature of the rat: implications for menopausal hot flush

The menopause results from the decreasing production of ovarian estrogens/progestins. This loss of ovarian hormones in 75-85% of women leads to a number of brain-mediated steroid-withdrawal symptoms, the most frequent being hot flushes. Thus, replacement therapy with a brain-enhanced estrogen delivery system (E2-CDS) with sustained release of estradiol (E2) in the brain may be more effective in the treatment of menopausal symptoms than currently used estrogens. The present study was designed to evaluate the effects of E2-CDS vs. E2, on the tail-skin temperature (TST) surge associated with administration of naloxone to morphine-dependent rats, an animal model for menopausal hot flush. Ovariectomized rats received a single or multiple doses of E2-CDS at 1.0 mg/kg body weight or E2 (0.5 mg pellet) weekly for 1 or 3 weeks before temperature recording. The mean maximal elevation in TST of the control animals was 6.4 +/- 0.2 degrees C. A single injection of E2-CDS attenuated the naloxone-induced rise in TST by 25%, while multiple injections resulted in significant attenuation of the rise in TST (3.4 +/- 0.6). By contrast, multiple implants of E2 pellet (3 pellets over 3 weeks) did not affect the surge of TST. Plasma E2 levels in animals treated with E2-CDS were slightly increased to 13 pg/ml for single-injected and to 44 pg/ml for multiple-injected rats. However, the E2-pellet treatment produced plasma E2 levels that were 2-fold greater than the E2 levels produced by multiple injections of E2-CDS. Plasma gonadotropins (LH and FSH) were significantly suppressed with the E2-pellet as well as the single and multiple E2-CDS treatment. Plasma prolactin levels were significantly elevated by E2 pellet and multiple injections of E2-CDS. The kinetic profiles of E2-CDS metabolites in plasma indicated an apparent t1/2 = 8 h for E2-Q+ and 3 h for E2. Collectively, these data support the view that E2-CDS may be potentially useful in the treatment of vasomotor hot flushes.

Dose and time-course evaluation of a redox-based estradiol-chemical delivery system for the brain. II. Pharmacodynamic responses

Clinically, brain-enhanced delivery and sustained release of estradiol (E2) are desirable for effective treatments of menopausal hot flushes and prostatic adenocarcinoma and for fertility regulation. Thus, we conducted studies to determine the dose- and time-dependent effects of a brain-enhanced estradiol-chemical delivery system (E2-CDS) on anterior pituitary hormones secretion in ovariectomized (OVX) rats. The E2-CDS has consistently demonstrated preferential retention of its intermediate metabolite (E2-Q+), with slow release of E2 in the brain but rapid clearance from peripheral tissues. Animals received a single iv injection of E2-CDS at doses of 0.01, 0.1, or 1.0 mg/kg or an E2 dose of 0.7 mg/kg on day 0. The responses of plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), and prolactin (PRL) were then evaluated at 1, 7, 14, 21, or 28 days after drug administration. The E2-CDS caused a dose- and time-dependent suppression of LH and FSH throughout the time course studied. The maximum LH and FSH reduction occurred at 7 days postinjection. Plasma LH and FSH were significantly suppressed by 86 and 58% on day 7, respectively, and were suppressed by 35% (LH) or were at preinjection levels (FSH) at 28 days following the single injection of a 1.0-mg E2-CDS dose. An equimolar E2 dose suppressed LH and FSH by only 29 and 20% on day 7, respectively which were not significantly different from time 0 values. Plasma PRL increased significantly on day 14 with the 1.0-mg E2-CDS dose but levels returned to preinjection values by 28 days after drug administration.(ABSTRACT TRUNCATED AT 250 WORDS)

A redox-based system that enhances delivery of estradiol to the brain: pharmacokinetic evaluation in the dog

The pharmacokinetics of a dihydropyridine-pyridinium salt-type chemical delivery system (CDS) for brain-targeted delivery of estradiol (E2) were examined in dogs. Parameters evaluated in vitro included stability in buffers and biological fluids and plasma protein binding. In vivo studies examined drug and metabolite concentrations in plasma, urine, and cerebrospinal fluid as well as in selected brain regions. The administered lipophilic E2-CDS disappeared very quickly from plasma and was not detected in urine. The oxidized drug form, E2-Q+, was excreted unchanged or as a conjugate in the urine for as long as 2 weeks. Plasma levels were below assay detection limits at later times. Pharmacokinetic analysis of urine E2-Q+ levels allowed estimation of a half-life of 2.2 days. Amounts of E2-Q+ excreted into the urine were proportional to the dose but averaged only 13.9% of the dose, indicating that other routes of excretion must be considered. CSF levels were below the limit of detection for both E2-CDS and E2-Q+, however, brain tissue concentrations of E2-Q+ were similar in several brain regions of individual animals examined 1 or 3 days after drug dosing.

Chronic inhibition of hypothalamic-pituitary-ovarian axis and body weight gain by brain-directed delivery of estradiol-17 beta in female rats

The effect of preferential delivery of estradiol (E2) into the brain on both the hypothalamic-pituitary-ovarian axis and weight gain was studied in female rats. When E2 was coupled to a lipoidal dihydropyridine-pyridinium carrier, the resulting carrier E2 complex (CE), upon a single intravenous administration to cycling female rats, caused a dose-dependent inhibition of ovulation which lasted 3 times longer than with uncoupled E2. The dose of CE that delayed ovulation for 4 days was one twentieth the amount of E2 needed to produce the same effect. Studies in ovariectomized (OVEX) rats indicated that the prolonged ovulation-blocking action of CE appeared to be related to a sustained storage and release of E2 in the brain, which in turn suppressed the release of hypothalamic luteinizing hormone-releasing hormone (LHRH) and pituitary luteinizing hormone (LH). Upon single intravenous administration in pubertal female rats, CE caused a dose-dependent reduction of body weight gain for a minimum period of 28 days. The inhibitory action of CE on body weight gain was more potent and longer lasting than that of E2 in pubertal rats. When administered in OVEX rats, CE produced a loss of body weight that lasted significantly longer than that produced by uncoupled E2 in these rats. These results suggest that the biological action of E2 can be potentiated by this novel chemical delivery system.

A rapid, sensitive method for the simultaneous quantitation of estradiol and estradiol conjugates in a variety of tissues: assay development and evaluation of the distribution of a brain-enhanced estradiol-chemical delivery system

A rapid and sensitive method that permits the simultaneous quantitation of estradiol (E2) and the E2 conjugate [estradiol 17-(1,4-dihydrotrigonellinate)] in biological tissues from rats is described. This method development was necessitated by the design of this E2-chemical delivery system (E2-CDS) which permits the predictable metabolism of the E2-CDS to a charged quaternary ion (E2-Q+) and its subsequent hydrolysis to slowly liberate E2. Based upon in vitro determinations of the rates of oxidation of E2-CDS to E2-Q+ and hydrolysis of E2-Q+ to E2, we anticipated that concentrations of E2-Q+ would be several orders of magnitude higher in tissues than E2. Three steps were used to extract and prepare samples for the radioimmunoassay (RIA) of estradiol. The first step is homogenization and extraction of the biological samples with an organic solvent; the second step is the base catalyzed hydrolysis of the E2 conjugate in 1 N NaOH; and the third step utilizes solid-phase extraction (SPE) with C18 reversed-phase columns which provide a means of achieving a rapid and precise extraction and separation of E2. The analysis of plasma samples does not require the initial solvent extraction. All purified E2 samples were then reconstituted in the assay buffer and assayed by RIA for E2. The application of this procedure to the determination of E2-Q+ and E2 in biological materials was assessed for specificity, reliability and recovery in vitro by using tissues and plasma from rats and in vivo evaluations of the distribution of E2 and E2-Q+ were done using rats treated with 1 mg E2-CDS/kg. Our in vitro analyses revealed that E2 and E2-Q+ could be differentially extracted with a high recovery and that both compounds could be specifically and reliably assayed in brain, plasma, anterior pituitary, liver, kidney, lung, heart and adipose tissue. In vivo analyses revealed that following a single i.v. injection of E2-CDS, brain levels of E2 exceeded serum levels by 30-, 41-, and 82-fold while brain levels of E2-Q+ exceeded serum levels by 33-, 70-, and 294-fold at 1, 7 and 14 days, respectively. Collectively, these data indicate that E2-Q+ and E2 can be reliably quantitated in a variety of tissues following the administration of an E2-chemical delivery system.

Effects of a brain-enhanced chemical delivery system for estradiol on body weight and food intake in intact and ovariectomized rats

Studies were undertaken to determine the effects on body weight of a brain-enhanced chemical delivery system for estradiol. This estradiol-chemical delivery system (E2-CDS) has a long half-life in the brain, where it slowly releases estradiol but is quickly cleared from peripheral tissues. We administered, by a single iv injection, E2-CDS (0.2, 1.0, or 5.0 mg/kg), equimolar doses of another 17-hydroxy-substituted estrogen, estradiol valerate (E2-VAL), or the dimethyl sulfoxide (DMSO) vehicle to female rats. Daily food intake and body weight was determined for 24 days thereafter. E2-CDS caused an initial dose-dependent suppression in body weight for up to 8 days and a suppression in food intake for up to 4 days. In response to E2-VAL, the initial declines in body weight and food intake were lower in magnitude, were shorter in duration, and showed no dose dependency. Following this period of weight loss, E2-CDS-treated rats gained weight at a rate greater than that of the DMSO controls, and at the 0.2- and 1.0-mg/kg doses, body weights achieved were greater than control levels. To determine the role of the ovaries on this biphasic response to E2-CDS, long-term ovariectomized rats were treated with E2-CDS (1.0 mg/kg) or the vehicle and parameters of body weight regulation were determined for 25 days. Ovariectomized rats responded to E2-CDS with a prompt and sustained decrease in body weight which did not recover over the 25-day course of the study. The body-weight loss in ovariectomized rats was associated with a marked reduction in food intake for 8 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a brain-enhanced chemical delivery system for estradiol on body weight and serum hormones in middle-aged male rats

We have developed a redox-chemical delivery system for brain-enhanced drug delivery of estradiol based on an interconvertible dihydropyridine in equilibrium pyridinium salt carrier. Estradiol, when combined with the carrier, readily crosses the blood-brain barrier and upon oxidation of the carrier is "locked" in the brain. The aim of this study was to evaluate the effects of the estradiol-chemical delivery system (E2-CDS) on body weight change and associated alterations in the secretion of anterior pituitary hormones in middle-aged, male rats. The data revealed that rats receiving E2-CDS exhibited a significant weight loss by 2 days which continued to day 14, the last observation day. A significant weight difference was observed between E2-CDS and DMSO-treated animals. Serum estradiol levels of rats treated with E2-CDS were elevated 100-fold by day 1 and decreased thereafter and serum prolactin concentrations were doubled by 24 hours and continued to increase to the completion of the experiment. Testosterone levels were markedly suppressed by 24 hours while serum levels of LH, TSH, T3, T4 and GH were not significantly altered. These data indicate that the E2-CDS causes a long-term reduction in body weight and testosterone secretion and that these changes are not mediated by alterations in the secretion of anterior pituitary hormones.

Chronic weight loss in lean and obese rats with a brain-enhanced chemical delivery system for estradiol

Studies were undertaken to determine the effects on body weight and food intake of a chemical delivery system which preferentially delivers estradiol (E2) to the brain and there serves as a source for the sustained release of the steroid. We injected intravenously various doses of this estradiol-chemical delivery system (E2-CDS), E2-valerate (E2-VAL) or the dimethyl sulfoxide (DMSO) vehicle to young lean male rats and monitored body weight and 24 hr food intake for 39 days postinjection. E2-VAL caused a transient reduction in food intake and body weight gain. By contrast, a single injection of E2-CDS caused a chronic, dose-dependent reduction in the rate of body weight gain. In these lean rats, the duration of reduced body weight gain was not correlated with the observed transient reduction in food intake. In aged, obese male rats, E2-CDS caused a marked and chronic dose-dependent reduction in body weight. In contrast to lean rats, E2-CDS caused a long-term reduction in food intake in obese rats. To evaluate the importance of the E2-CDS-induced reduction in food intake in the observed persistent weight loss in obese rats, E2-CDS was administered to a group of obese rats and a second group which received the DMSO vehicle was pair-fed an equivalent amount of food daily. The resulting weight loss in both groups was equivalent. These results show that the enhanced delivery of E2 to the brain with the E2-CDS causes sustained reduction in the rate of body weight gain in lean rats and persistent weight loss in obese animals.

Evidence for suppression of serum LH without elevation in serum estradiol or prolactin with a brain-enhanced redox delivery system for estradiol

We developed a redox system for brain-enhanced delivery of estradiol based on an interconvertible dihydropyridine in equilibrium pyridinium salt carrier. Estradiol (E2), when combined with the lipoidal carrier, readily crosses the blood-brain barrier. The carrier, when oxidized, reduces the rate of exit of the estradiol-carrier complex from the brain. Subsequent hydrolysis of the carrier provides sustained production of estradiol in the brain. The aim of the study was to evaluate the effects of single vs. multiple injections of the estradiol-chemical delivery system (E2-CDS) on both central and peripheral estrogen-responsive tissues. Ovariectomized Sprague-Dawley rats received an intravenous injection of E2-CDS at 10, 33, 100 or 333 micrograms/kg BW or the drug vehicle, dimethyl sulfoxide (DMSO; 0.5 ml/kg) every 2 days for 7 injections (2 weeks) or a single injection only at 2 days before sacrifice. With a single injection, E2-CDS did not affect serum luteinizing hormone (LH) levels at the 10 micrograms/kg dose but caused a dose-dependent reduction in serum LH of 39-52% at the dose range of 33 to 333 micrograms/kg. By contrast, multiple injections of E2-CDS caused a 32 to 76% reduction in serum LH levels at doses ranging from 10 micrograms/kg to 333 micrograms/kg. Additionally, multiple doses of E2-CDs caused a dose-dependent reduction in body weight at the 10 and 33 micrograms/kg doses with the higher doses causing no further weight reduction. For both single and multiple dosage groups, serum E2 levels remained unchanged after doses of E2-CDS of 10 and 33 micrograms/kg, then increased to 21 pg/ml for the single dosage group and to 23 pg/ml for the multiple dosage group at the 100 micrograms/kg dose, and to 59 pg/ml for singly-injected rats and 60 pg/ml for multiply-injected rats at the 333 micrograms/kg dose. Serum prolactin concentrations were closely correlated with serum E2 levels for both the single and multiple dose groups. These data reveal that a single or multiple doses of E2-CDS can reduce serum LH levels without elevating serum E2 or prolactin concentrations, supporting the concept of brain-enhanced delivery of estradiol with an estradiol chemical delivery system.