Potency of lipid and protein formulation of 5 alpha-pregnanolone at induction of anaesthesia and the corresponding regional brain distribution

Allopregnanolone and perampanel as adjuncts to midazolam for treating diisopropylfluorophosphate-induced status epilepticus in rats

Combinations of midazolam, allopregnanolone, and perampanel were assessed for antiseizure activity in a rat diisopropylfluorophosphate (DFP) status epilepticus model. Animals receiving DFP followed by atropine and pralidoxime exhibited continuous high-amplitude rhythmical electroencephalography (EEG) spike activity and behavioral seizures for more than 5 hours. Treatments were administered intramuscularly 40 min after DFP. Seizures persisted following midazolam (1.8 mg/kg). The combination of midazolam with either allopregnanolone (6 mg/kg) or perampanel (2 mg/kg) terminated EEG and behavioral status epilepticus, but the onset of the perampanel effect was slow. The combination of midazolam, allopregnanolone, and perampanel caused rapid and complete suppression of EEG and behavioral seizures. In the absence of DFP, animals treated with the three-drug combination were sedated but not anesthetized. Animals that received midazolam alone exhibited spontaneous recurrent EEG seizures, whereas those that received the three-drug combination did not, demonstrating antiepileptogenic activity. All combination treatments reduced neurodegeneration as assessed with Fluoro-Jade C staining to a greater extent than midazolam alone, and most reduced astrogliosis as assessed by GFAP immunoreactivity but had mixed effects on markers of microglial activation. We conclude that allopregnanolone, a positive modulator of the GABAA receptor, and perampanel, an AMPA receptor antagonist, are potential adjuncts to midazolam in the treatment of benzodiazepine-refractory organophosphate nerve agent-induced status epilepticus.

GR3027 reversal of neurosteroid-induced, GABA-A receptor-mediated inhibition of human brain function: an allopregnanolone challenge study

RATIONALE

GR3027 is a novel small molecule GABA-A receptor-modulating steroid antagonist, which in non-clinical studies has shown promise for treatment of human disorders due to allosteric over-activation of GABA-A receptors by neurosteroids, such as allopregnanolone. We here studied its safety, pharmacokinetics, and ability to inhibit allopregnanolone effects in humans.

METHODS

Safety and pharmacokinetics were studied in healthy adult males receiving ascending single or multiple oral GR3027 vs. placebo. GR3027-mediated reversal of allopregnanolone effect on maximal saccadic eye velocity (SEV), and self-rated somnolence was studied in a double-blind, placebo-controlled, three-part cross-over study in which 3 or 30 mg oral GR3027 preceded 0.05 mg/kg of i.v. allopregnanolone.

RESULTS

GR3027 was well tolerated, adverse events were generally mild and transient, and no dose-limiting toxicity or grade 3 adverse events were observed up to the highest single (200 mg) or multiple (100 mg every 12 h for 5 days) doses. The maximum concentration (C_max) and systemic exposure (area under the plasma concentration-time curve from dose extrapolated to infinity [AUC_0-∞] and/or AUC during the dosing interval [AUC_τ]) varied linearly with dose; with dose-dependent accumulation ratios of 1.3-1.6. Allopregnanolone decreased SEV and induced somnolence in most, but not all subjects. By predefined analyses, 30 mg GR3027 significantly inhibited allopregnanolone-induced decrease in SEV (p = 0.03); 3 and 30 mg GR3027 non-significantly inhibited allopregnanolone-induced sedation. By post hoc analyses restricted to subjects with allopregnanolone-induced changes and the time period over which they occurred, GR3027 dose dependently inhibited allopregnanolone-induced decrease in SEV (p = 0.04 at 30 mg, non-significant at 3 mg) and allopregnanolone-induced sedation (p = 0.01/0.05 at 3/30 mg doses).

CONCLUSION

Oral GR3027 mitigates inhibition of brain function induced by allopregnanolone at doses which are clinically well tolerated and associated with linear pharmacokinetics.

Women with polycystic ovary syndrome have elevated serum concentrations of and altered GABA(A) receptor sensitivity to allopregnanolone

OBJECTIVE

Several studies have reported that γ-aminobutyric acid (GABA) ergic circuits are involved in the pathophysiology of polycystic ovary syndrome (PCOS). The progesterone metabolite allopregnanolone is a potent GABA(A) -receptor-modulating steroid, and patients may have increased concentrations of allopregnanolone or altered GABAA receptor sensitivity. We investigated both of these possibilities in this study.

PATIENTS

We enrolled 9 women with PCOS and 24 age-matched eumenorrhoeic controls, who were divided into two groups by body mass index (BMI) (16 normal weight and 8 overweight).

MEASUREMENTS

We investigated the effects of allopregnanolone injection on GABA(A) receptor sensitivity in both groups of women. All women received a single intravenous dose of allopregnanolone (0·050 mg/kg). GABA(A) receptor sensitivity was assessed with the saccadic eye velocity (SEV) over 30° (SEV30°), the SEV30°/allopregnanolone concentration ([Allo]) ratio, and sedation, which were measured together with serum allopregnanolone at intervals for 180 min after injection. The controls were tested in the follicular phase of the menstrual cycle.

RESULTS

Baseline allopregnanolone concentrations were higher in the PCOS women than in the normal-weight (P = 0·034) and overweight controls (P = 0·004). The allopregnanolone concentrations after injection were higher in the PCOS women (P = 0·006) and overweight controls (P = 0·037) than in the normal-weight controls. All groups showed a decline in the SEV30°/[Allo] ratio after injection. Allopregnanolone had a smaller effect on the SEV30°/[Allo] ratio in the overweight women (PCOS, P = 0·032; controls, P = 0·007) than in the normal-weight controls. The sedation score after allopregnanolone injection was lower in the PCOS patients than in the controls, but was not different between the two control groups.

CONCLUSIONS

PCOS women had elevated baseline allopregnanolone concentrations compared with follicular-phase controls. All overweight women (PCOS and controls) were less sensitive to allopregnanolone than normal-weight controls.

Isoallopregnanolone antagonize allopregnanolone-induced effects on saccadic eye velocity and self-reported sedation in humans

Allopregnanolone (AP) is an endogenous neurosteroid. It modulates the effect of γ-amino-butyric acid (GABA) on the GABA type A (GABAA) receptor, which leads to increased receptor activity. Since the GABA-system is mainly inhibitory, increased AP activity leads to modulation of neuronal activity. In vitro studies of GABAA receptor activity and in vivo animal studies of sedation have shown that AP-induced effects can be inhibited by another endogenous steroid, namely isoallopregnanolone (ISO). In this study we investigated if ISO can antagonize AP-induced effects in healthy female volunteers, via measurements of saccadic eye velocity (SEV) and self-rated sedation. With a single-blind cross-over design, 12 women were studied on three separate occasions; given AP alone or AP in combination with one of two ISO doses. Congruent with previous reports, AP administration decreased SEV and induced sedation and these effects were diminished by simultaneous ISO administration. Also, the ISO effect modulation was seemingly stronger for SEV than for sedation. These effects were observed already at an ISO dose exposure that was approximately half of that of AP. In conclusion, ISO antagonized AP-induced decrease in SEV and self-reported sedation, probably in a non-competitive manner.

Changes in functioning of mesolimbic incentive processing circuits during the premenstrual phase

The premenstrual phase of the menstrual cycle is associated with marked changes in normal and abnormal motivated behaviors. Animal studies suggest that such effects may result from actions of gonadal hormones on the mesolimbic dopamine (DA) system. We therefore investigated premenstrual changes in reward-related neural activity in terminal regions of the DA system in humans. Twenty-eight healthy young women underwent functional magnetic resonance imaging on 2 days during the menstrual cycle, once during the late follicular phase and once during the premenstrual phase, in counterbalanced order. Using a modified version of the monetary incentive delay task, we assessed responsiveness of the ventral striatum to reward anticipation. Our results show enhanced ventral striatal responses during the premenstrual as compared to the follicular phase. Moreover, this effect was most pronounced in women reporting more premenstrual symptoms. These findings provide support for the notion that changes in functioning of mesolimbic incentive processing circuits may underlie premenstrual changes in motivated behaviors. Notably, increases in reward-cue responsiveness have previously been associated with DA withdrawal states. Our findings therefore suggest that the sharp decline of gonadal hormone levels in the premenstrual phase may trigger a similar withdrawal-like state.

Cellular distribution of the GABAA receptor-modulating 3alpha-hydroxy, 5alpha-reduced pregnane steroids in the adult rat brain

The 3alpha-hydroxy,5alpha-reduced pregnane steroids, allopregnanolone and allotetrahydrodeoxycorticosterone, are the most potent endogenous positive modulators of GABA(A) receptor-mediated inhibition. This study presents the first immunohistochemical examination of the cellular distribution of 3alpha-hydroxy,5alpha-reduced pregnane steroids across the brain. We found a widespread distribution in the adult rat, with dense immunolabelling in the olfactory bulb, striatum and cerebral cortex, and lower density labelling in the brainstem reticular formation. In general terms, this distribution accords with the regional concentrations of 3alpha-hydroxy,5alpha-reduced steroids determined, in other laboratories, by brain region sampling and either gas chromatography-mass fragmentography or radioimmunoassay. However, immunohistochemistry allowed for a more detailed examination of regional distribution and cellular specificity. All immunoreactivity was confined to the cell bodies and thick dendrites of neurones; no identifiable glia were labelled. In most brain areas, the location and morphology of labelled cells identified them as excitatory neurones. In addition, cell populations known to be projecting GABAergic neurones (e.g. cerebellar Purkinje cells) were immunoreactive, whereas local inhibitory neurones generally were not. The cellular distribution of 3alpha-hydroxy,5alpha-reduced steroids suggests that sensory, motor, limbic and homeostatic systems can be influenced by neurosteroids at multiple stages of processing.

3Beta-hydroxysteroids and pregnenolone sulfate inhibit recombinant rat GABA(A) receptor through different channel property

3Beta-hydroxysteroids are pregnenolone sulfate-like GABA(A) receptor antagonists. The aim of the current study was to compare the functional differences between 3beta-hydroxysteroids and pregnenolone sulfate to inhibit GABA(A) receptors expressed in Xenopus oocytes. Recombinant rat GABA(A) receptors encoding wild type alpha1 beta2 gamma2L receptor, mutant alpha1V256S beta2 gamma2L and alpha1 beta2A252S gamma2L receptors were examined using a two-electrode voltage-clamp technique. A homologous mutation of the residue at 2'position closest to the cytoplasmic end of the M2 helix to serine on both alpha1 and beta2 subunit, alpha1V256S and beta2A252S, reduced the slow desensitization components of GABA-activated currents at saturating doses. Compared to the wild type receptor, the potency of GABA increased significantly in the alpha1V256S beta2 gamma2L receptor (P<0.05), whereas it decreased moderately in the alpha1 beta2A252S gamma2L receptor. We found that 5alpha-pregnan-3beta, 20(S)-diol (UC1019) and 5beta-pregnan-3beta, 20(R)-diol (UC1020) were the most effective blockers of maximal GABA responses among a panel of 3beta-hydroxysteroids. Pregnenolone sulfate, UC1019 and UC1020 were potent antagonists in the wild type receptor with calculated IC50s of 0.20+/-0.07 microM; 1.88+/-0.32 microM and 2.58+/-0.58 microM, respectively. The inhibitory effect of pregnenolone sulfate was significantly reduced in both mutants alpha1V256S beta2 gamma2L and alpha1 beta2A252S gamma2L receptors (P<0.05), whereas the inhibitory effects of UC1019 and UC1020 were reduced only in the mutant alpha1V256S beta2 gamma2L receptor. Pregnenolone sulfate promoted slow desensitization with prolonged GABA application in a dose-dependent manner in the wild type receptor, but not mutant receptors. On the contrary, UC1019 and UC1020 (< or = 20 microM) did not promote desensitization in both wild type and mutant receptors. In conclusion, the GABA(A) receptor inhibition by pregnenolone sulfate, but not 3beta-hydroxysteroids, was dependent on desensitization kinetics of the Cl- channels. A point mutation at M2 helix of the beta2-subunit (beta2A252S) can dramatically reduce the inhibitory effect of pregnenolone sulfate on the GABA(A) receptors without affecting the inhibitory properties of 3beta-hydroxysteroids. These results are consistent with the hypothesis that pregnenolone sulfate-inhibition does not share with 3beta-hydroxysteroids the coincident channel property at the GABA(A) receptor.

Neurosteroids Involved in Regulating Inhibition in the Inferior Colliculus

Fast inhibitory neurotransmission in the brain is largely mediated by the γ-aminobutyric acid-type A (GABAA) receptor. The 3α,5α-reduced neurosteroids (e.g., allopregnanolone) are the most potent endogenous modulators of the GABAA receptor. Although it is known that 3α,5α-reduced neurosteroid levels change during stress or depression and over the estrus cycle, a basic physiological role consistent with their pharmacological action remains elusive. We used the unique architecture of the auditory midbrain to reveal a role for 3α,5α-reduced neurosteroids in regulating inhibitory efficacy. After blocking the massive GABAergic projection from the dorsal nucleus of the lateral lemniscus (DNLL) to the contralateral central nucleus of the inferior colliculus (ICC) in anesthetized rats, a reactive increase in the efficacy of other inhibitory circuits in the ICC (separable because of the dominant ear that drives each circuit) was demonstrated with physiological measures—single-neuron activity and a neural-population-evoked response. This effect was prevented by blocking 3α,5α-reduced neurosteroid synthesis with a 5α-reductase inhibitor: finasteride. Immunohistochemistry confirmed that the DNLL blockade induced an increase in 3α,5α-reduced neurosteroids in the contralateral ICC. This study shows that when GABAergic inhibition is reduced, the brain compensates within minutes by locally increasing synthesis of neurosteroids, thereby balancing excitatory and inhibitory inputs in complex neural circuits.

Oral progesterone decreases saccadic eye velocity and increases sedation in women

The aim of this study was to investigate the neurophysiological and behavioural effects of a single dose of progesterone in women. Allopregnanolone is a metabolite of progesterone and a potent positive modulator of the GABA(A) receptor and produces sedative and anxiolytic effects. This study was designed to examine the effect of oral progesterone and the metabolite allopregnanolone in women. Women (n=15) in their follicular phase received oral progesterone (400mg) or placebo. Dependent measures included plasma levels of progesterone and allopregnanolone, saccadic eye velocity (SEV), subjective ratings (visual analogue scales), and reaction time. Administration of progesterone decreased SEV and increased sedation. This effect is probably due to enhanced GABA activity.

Separation and detection of neuroactive steroids from biological matrices

This review is based on a selection of research papers published mainly in the last decade and it describes various analytical aspects of separation and detection of neuroactive steroids in biological matrices.

Allopregnanolone inhibits learning in the Morris water maze

The progesterone metabolite allopregnanolone (3alpha-OH-5alpha-pregnane-20-one) inhibits neural functions, enhancing the GABA induced GABA(A) receptor activation. This effect is benzodiazepine like and benzodiazepines are known to impair memory. Acute effects of allopregnanolone on the hippocampus dependent spatial learning in the Morris water maze have not been studied. Adult male Wistar rats where injected (i.v.) with allopregnanolone (2 mg/kg), or vehicle, daily for 11 days. At 8 or 20 min after each injection, studies of place navigation were performed in the Morris water maze. Allopregnanolone concentrations in plasma and in nine different brain areas where analyzed by radioimmunoassay. The latency to find the platform was increased 8 min after the allopregnanolone injection, while normal learning was seen after 20 min. Swim speed did not differ between groups. A higher number of rats were swimming close to the pool wall (thigmotaxis) in the 8 min allopregnanolone group compared to the other groups. Allopregnanolone concentrations in the brain tissue at 8 min were 1.5 to 2.5 times higher then at 20 min after the allopregnanolone injections. After vehicle injections the brain concentrations of allopregnanolone were at control levels. Plasma concentrations of allopregnanolone followed the same pattern as in the brain, with the exception of an increase 8 min after vehicle injections. The natural progesterone metabolite allopregnanolone can inhibit learning in the Morris water maze, an effect not caused by motor impairment. The learning impairment might be due to a combination of changed swimming behavior and difficulties in navigation.

Neuroactive steroids and central nervous system disorders

Steroid hormones are vital for the cell life and affect a number of neuroendocrine and behavioral functions. In contrast to their endocrine actions, certain steroids have been shown to rapidly alter brain excitability and to produce behavioral effects within seconds to minutes. In this article we direct attention to this issue of neuroactive steroids by outlining several aspects of current interest in the field of steroid research. Recent advances in the neurobiology of neuroactive are described along with the impact of advances on drug design for central nervous system (CNS) disorders provoked by neuroactive steriods. The theme was selected in association with the clinical aspects and therapeutical potentials of the neuroactive steroids in CNS disorders. A wide range of topics relating to the neuroactive steroids are outlined, including steroid concentrations in the brain, premenstrual syndrome, estrogen and Alzheimer's disease, side effects of oral contraceptives, mental disorder in menopause, hormone replacement therapy, Catamenial epilepsy, and neuractive steroids in epilepsy treatment.

Evaluation and comparison of the pharmacokinetic and pharmacodynamic properties of allopregnanolone and pregnanolone at induction of anaesthesia in the male rat

We have evaluated and compared the pharmacokinetic and pharmacodynamic properties of allopregnanolone and pregnanolone at induction of anaesthesia in male rats. A threshold method was used, and the first burst suppression period of 1 s or more in the EEG was selected as the end-point after fairly slow infusions. An optimal dose of 4.0 mg kg(-1) min(-1) was noted for both steroids. Brain concentrations were low at low infusion rates, indicating that acute tolerance was not occurring. Significant positive correlations were noted between dose rate and serum concentrations of allopregnanolone (r = 0.94, P<0.001) and pregnanolone (r = 0.88, P<0.001). Such correlations were also seen in striatum, cerebellum, cortex and muscle for both steroids (P<0.01). Despite changing infusion rates, the concentrations of both steroids in brainstem, hippocampus and fat remained stable. Because no correlation between infusion rate and steroid concentration was noted in the brainstem and hippocampus, these two brain areas may be regarded as primary sites of action for allopregnanolone and pregnanolone. Pregnanolone concentrations in the brainstem and hippocampus were significantly higher than those of allopregnanolone, suggesting that allopregnanolone was more potent than pregnanolone in inducing anaesthesia.

High-performance liquid chromatography of the neuroactive steroids alphaxalone and pregnanolone in plasma using dansyl hydrazine as fluorescent label: application to a pharmacokinetic-pharmacodynamic study in rats

This report describes a rapid and sensitive analytical method for the quantification of the neuroactive steroids alphaxalone and pregnanolone in rat plasma using derivatization with dansyl hydrazine as fluorescent label. The method involves protein precipitation, alkaline derivatization and extraction of the compounds and internal standard pregnenolone with dichloromethane, followed by isocratic reversed-phase high-performance liquid chromatography on a 3-microm Microsphere C18 column with fluorescence detection at wavelengths 332 nm and 516 nm for excitation and emission, respectively. The mobile phase consists of a mixture of 25 mM acetate buffer (pH 3.7)-acetonitrile (45:55, v/v for alphaxalone and 40:60, v/v for pregnanolone) with a flow-rate of 1 ml/min. The total run time was approximately 35 min. In the concentration range of 0.010-10 microg ml(-1), the intra- and inter-assay coefficients of variation were less than 17% for both methods. In 50 microl plasma samples the corresponding limits of detection were 10 ng ml(-1) (signal-to-noise ratio=3). The utility of the analytical method was established by analyzing plasma samples from rats, which had received an intravenous administration of 5 mg kg(-1) alphaxalone or pregnanolone. Values for clearance, volume of distribution at steady state and terminal half life were 71.9 ml min(-1) kg(-1), 814 mg kg(-1) and 13.5 min for alphaxalone and 69.2 ml min(-1) kg(-1), 1,638 ml kg(-1) and 27.8 min for pregnanolone, respectively. Due to its simplicity and sensitivity this method can be used on a routine basis for pharmacokinetic analysis of neuroactive steroids.

Anxiolytic effects of the neuroactive steroid pregnanolone (3 alpha-OH-5 beta-pregnan-20-one) after microinjection in the dorsal hippocampus and lateral septum

The anxiolytic effects of the neuroactive steroid, 3 alpha-OH-5 beta-pregnan-20-one (pregnanolone), were determined after injection into the dorsal hippocampus or lateral septum in adult male rats. An increase in the proportion of time spent on the open arms of the elevated plus-maze was found after 2.5 and 5 micrograms of pregnanolone in the hippocampus, but not in the lateral septum. Intrahippocampal injection of 2.5 micrograms of the 3 beta-epimer of pregnanolone did not affect behavior in the plus-maze; a higher dose of 5 micrograms produced an anxiogenic effect. In the shock-probe burying test latency to burying behavior was increased by intrahippocampal or intraseptal injection of 2.5 and 5 micrograms of pregnanolone; the duration of burying behavior was decreased by 0.5, 2.5 and 5 micrograms of pregnanolone injection in the dorsal hippocampus or lateral septum. The number of contacts with the shock probe was not affected by any dose of pregnanolone in either intracranial site of injection. The anxiolytic effects of intrahippocampal or intraseptal injection of pregnanolone were blocked by intracranial pretreatment with 20 ng of picrotoxin, but not by microinjection of 5 micrograms of flumazenil or 200 ng of PK 11195. Thus, inhibition of the hippocampus, mediated by the pregnanolone's action at the GABAA receptor, produces a general anxiolytic effect. However, similar inhibition in the lateral septum attenuates active avoidance of anxiogenic stimuli (i.e., decreased burying behavior), but not passive avoidance of aversive stimuli (i.e., exploration of open arms of the plus-maze and number of shocks in the probe burying test).

Endocrine response to pregnanolone

Neuroendocrine effects of the neurosteroids, pregnanolone and allopregnanolone have been demonstrated in rats. The endocrine effects of pregnanolone in humans have so far not been fully elucidated. This study has evaluated the effects of pregnanolone administration on part of the hypothalamus-pituitary-gonadal (HPG) axis throughout the menstrual cycle in control subjects and patients with premenstrual syndrome (PMS). Intravenous pregnanolone and vehicle were given to eight women with, and eight women without, PMS during the mid-follicular and late luteal phase. Following the drug administrations, progesterone, estradiol, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin plasma levels were measured. Intravenous pregnanolone induced a rise in progesterone levels in the follicular phase. In the luteal phase progesterone levels decreased in response to pregnanolone provocation. Pregnanolone did not induce any changes in estradiol, LH, FSH or prolactin plasma levels in either cycle phase. PMS patients and control subjects did not differ with respect to the endocrine effects of pregnanolone. In conclusion, our data show that pregnanolone, in moderate doses, appears not to have any adverse effects on the HPG axis, irrespective of cycle phase.

Nongenomic effects of neurosteroids

This review summarizes the current knowledge about the synthesis, the mechanism of action, and the effects of neurosteroids in the central nervous system. Particular attention is paid to the nongenomic actions of neurosteroids, which are discussed in relation to their clinical relevance for physiological and pathological states.

The EEG burst suppression threshold test for the determination of CNS sensitivity to intravenous anesthetics in rats

The electroencephalographic (EEG) threshold test was developed to determine the central nervous system (CNS) sensitivity to several depressant drugs, mainly intravenous anesthetic agents. Test drugs were administered to the rats by continuous intravenous infusion until a defined EEG criterion indicating deep anesthesia was reached. The criterion was a burst suppression which lasted 1 s or more, the 'silent second' (SS). The dose of the drug needed to induce the SS, the threshold dose, was the dependent variable. In the intact animal, it is influenced by the potency of the drug and dose administration rate of the infusion (mg/kg/min). The method has several advantages over the commonly used anesthesia times (misnomer: sleeping time) where the duration is influenced by several pharmacokinetic properties such as metabolism and redistribution. With the method it is possible under in vivo conditions to monitor continuously the electrical changes in the CNS. No interruptions are necessary. The pharmacological end-point is well defined, easy to monitor and detect. The threshold doses of the drugs applied remained stable with repetitive testing of the same individual. Thus, it can be used effectively to determine the CNS sensitivity to a number of drugs and to follow changes in this sensitivity after several physiological, pharmacological or pathological interventions.

The regional brain distribution of the neurosteroids pregnenolone and pregnenolone sulfate following intravenous infusion

We have studied the distribution of the neurosteroids pregnenolone (Pe) and pregnenolone sulfate (PeS) in seven brain regions, and plasma and fat tissues in male adult rats following the intravenous infusion of 14 mg/kg Pe and 18 mg/kg PeS, respectively. After chromatographic separation of steroid sulfate esters and non-conjugated steroids by solid phase octadecyl C18 columns and celite column chromatographic separation of Pe from cross-reacted steroids, the concentrations of Pe and PeS were determined by radioimmunoassay. We found that both Pe and PeS concentrations were significantly increased in plasma, fat and brain compared to the vehicle controls after i.v. infusion of Pe and PeS. In the controls, Pe concentrations were highly correlated within brain regions and between fat and brain regions. Most correlations were lost after Pe and PeS infusions. The content of Pe and PeS was not uniformly distributed in the brain. The hypothalamus contained the highest level of Pe in controls, Pe-infused and PeS-infused rats (12 +/- 3.1, 3500 +/- 180 and 590 +/- 54 ng/g, respectively). The highest concentration of PeS was detected in the hypothalamus (26 +/- 8.2 ng/g) and striatum (17 +/- 4.1 ng/g) in controls, in the hypothalamus (200 +/- 24 ng/g) after PeS infusion as well as in the hypothalamus and medulla oblongata (57 +/- 9.6 and 55 +/- 7.6 ng/g, respectively) after Pe infusion. This study has yielded evidence that PeS injected i.v. can cross the blood-brain barrier without being hydrolysed to the more lipophilic Pe, and can thus be taken up by the brain.