Pharmacokinetic/pharmacodynamic feedback modelling of the functional corticosterone response in rats after acute treatment with escitalopram

The objective of this study was to characterize the pharmacokinetic/pharmacodynamic (PK/PD) relationship of the drug-induced corticosterone response after administration of escitalopram in rats. To achieve this, a mechanistic feedback turnover model mimicking the acute mechanism of action of selective serotonin reuptake inhibitors was assessed. Conscious and freely moving rats received constant rate infusions of 2.5, 5 or 10 mg/kg escitalopram or vehicle over 60 min. Automated serial blood sampling was conducted to determine escitalopram and corticosterone concentrations. The PK/PD model consisted of a turnover model of escitalopram-evoked changes in response, which included an inhibitory feedback moderator function. Accordingly, response acted linearly on the production (k(tol)) of the moderator, which acted inversely on the production (k(in)) of response. The escitalopram plasma kinetics served as input to an inhibitory function acting on the loss (k(out)) of response. The corticosterone responses were successfully described using the model by fitting responses from all doses simultaneously resulting in estimation of drug parameters (I(max), IC(50) and n) in addition to system parameters (k(in), k(out) and k(tol)) for the whole exposure range. Thus, the applicability of the model for analysis of the acute selective serotonin reuptake inhibitor-induced corticosterone response including acute auto-inhibitory feedback was demonstrated.     

Consequences of changes in BDNF levels on serotonin neurotransmission, 5-HT transporter expression and function: studies in adult mice hippocampus

In vivo intracerebral microdialysis is an important neurochemical technique that has been applied extensively in genetic and pharmacological studies aimed at investigating the relationship between neurotransmitters. Among the main interests of microdialysis application is the infusion of drugs through the microdialysis probe (reverse dialysis) in awake, freely moving animals. As an example of the relevance of intracerebral microdialysis, this review will focus on our recent neurochemical results showing the impact of Brain-Derived Neurotrophic Factor (BDNF) on serotonergic neurotransmission in basal and stimulated conditions. Indeed, although the elevation of 5-HT outflow induced by chronic administration of selective serotonin reuptake inhibitors (SSRIs) causes an increase in BDNF protein levels and expression (mRNA) in the hippocampus of rodents, the reciprocal interaction has not been demonstrated yet. Thus, the neurochemical sight of this question will be addressed here by examining the consequences of either a constitutive decrease or increase in brain BDNF protein levels on hippocampal extracellular levels of 5-HT in conscious mice.     

Mechanistic model of acute autoinhibitory feedback action after administration of SSRIs in rats: Application to escitalopram-induced effects on brain serotonin levels

This study presents the development and evaluation of a feedback turnover model that mimics drug-induced effects on brain extracellular levels of serotonin (5-HT) after acute administration of the selective serotonin reuptake inhibitor (SSRI) escitalopram (S-citalopram) in rats. The extracellular 5-HT output in the hippocampus was continuously monitored by intracerebral microdialysis in conjunction with serial arterial blood sampling for evaluation of escitalopram pharmacokinetics. 5-HT levels were significantly increased following administration of 2.5, 5 and 10 mg/kg of escitalopram and the 5-HT levels gradually declined to its baseline value within 360 min. However, at 5 and 10 mg/kg, the response–time curves were almost identical. This might be explained by activation of serotonergic autoreceptors exerting negative feedback, leading to a reduced release of new 5-HT into the synapse. The dynamics of escitalopram-evoked changes of 5-HT response was characterized by a turnover model, which included an inhibitory feedback moderator component. Thus, the response acted linearly on the production of the moderator, which acted inversely on the production of response. The plasma kinetics served as input to an inhibitory function acting on the loss of response. Simultaneous fitting of the model after three constant rate infusions demonstrated the flexibility of the system. The efficacy (I_max) and potency (IC₅₀) of inhibition of reuptake were 0.9 ± 0.03 and 4.4 ± 1.4 ng/ml, respectively, corresponding to an EC₅₀ of escitalopram about 30 ng/ml. In conclusion, the model lends itself to ‘what–if’ predictions at different drug exposure scenarios, and has potential for extrapolation of the pharmacodynamics of SSRIs in man.     

Pharmacokinetics and pharmacodynamics of norfluoxetine in rats: Increasing extracellular serotonin level in the frontal cortex

Norfluoxetine is the most important active metabolite of the widely used antidepressant fluoxetine. Although the pharmacokinetics/pharmacodynamics (PK/PD) relationship and neurochemical profile of fluoxetine is well characterized in human and in animals, little is known about the effect of its metabolite. The aim of this study was to characterize extracellular level of serotonin (5-hydroxytryptamine, 5-HT)-time profile of norfluoxetine after acute administration over 18 h post dose and to establish the relationship between this pharmacodynamic (PD) profile and its pharmacokinetic (PK) properties. Following subcutaneous administration of fluoxetine in rats, plasma and brain PK of fluoxetine and norfluoxetine were monitored respectively by liquid chromatography/tandem mass spectrometry (LC/MS/MS). The extracellular level of 5-HT in the frontal cortex was measured by microdialysis as a PD endpoint. Norfluoxetine when directly administrated to rats caused a significant increase in extracellular level of 5-HT in the frontal cortex and maintained for 18 h. This result is correlated well with higher plasma and brain concentration and longer plasma and brain retention time of norfluoxetine. Our results showed that norfluoxetine contributes to 5-HT transporter inhibition and extends fluoxetine efficacy.     

Pharmacokinetic modelling of blood-brain barrier transport of escitalopram in rats

This study examined the pharmacokinetics and distribution of escitalopram in the brain extracellular fluid in rats by the concurrent use of intracerebral microdialysis and serial blood sampling. Following three constant intravenous infusions, drug concentrations in the hippocampus and plasma were monitored for 6 h. To estimate the integrated pharmacokinetics and intercompartmental transport parameters, including blood-brain barrier (BBB) transport over the entire dose range, unbound brain and plasma escitalopram concentration data from all doses were simultaneously analysed using compartmental modelling. The pharmacokinetic analysis revealed that systemic clearance decreased as a function of dose, which was incorporated in the integrated model. Escitalopram was rapidly and extensively transported across the BBB and distributed into the brain extracellular fluid. The modelling resulted in an estimated influx clearance into the brain of 535 microl/min/g brain, resulting in an unbound brain-to-plasma AUC ratio of 0.8 independent of escitalopram dose. The model may be applied for preclinical evaluations or predictions of escitalopram concentration-time courses in plasma as well as at the target site in the CNS for various dosing scenarios. In addition, this modelling approach may also be valuable for studying BBB transport characteristics for other psychotropic agents.     

An automated blood sampler for simultaneous sampling of systemic blood and brain microdialysates for drug absorption, distribution, metabolism, and elimination studies

INTRODUCTION: A major problem in preclinical drug development where blood sampling from small animals is a routine practice is the time and labor involved in the serial sampling of small blood volumes from small animals such as rats for the duration of pharmacokinetic/pharmacodynamic (PK/PD) studies. The traditional method of manually drawing blood from the animal requires the animal to be anesthetized or restrained with some device, both of which cause stress to the animal. METHODS: An automated blood sampler (ABS) was developed to simultaneously collect blood and brain microdialysate samples at preprogrammed time points from awake and freely moving animals. The samples are delivered to fraction collectors and stored at 4 degrees C until use. The lost blood volume during collection is replaced with sterile saline to prevent fluid loss from the animal. In addition, the system is capable of collecting urine and feces for metabolism studies and monitoring the animal activity for behavioral studies. In the present study, blood samples were collected for 24 h after dosing rats orally with a 5 mg/kg dose of olanzapine (OLAN). Brain dialysates were collected for the same duration from a microdialysis probe implanted in the striatum. RESULTS: The pharmacokinetic parameters, obtained after an oral dose, are in good agreement with reported values in literature. The pharmacodynamic information obtained from brain dialysates data show that OLAN elevates the concentration of dopamine (DA) in the brain and remains in the brain even after it is cleared from the plasma. DISCUSSION: The ABS described here is a very useful tool in drug development to accelerate the pace of preclinical in vivo studies and to simultaneously provide pharmacodynamic and physiological information.     

Prediction of clinical response based on pharmacokinetic/pharmacodynamic models of 5-hydroxytryptamine reuptake inhibitors in mice

Background and purpose:Bridging the gap between preclinical research and clinical trials is vital for drug development. Predicting clinically relevant steady-state drug concentrations (Css) in serum from preclinical animal models may facilitate this transition. Here we used a pharmacokinetic/pharmacodynamic (PK/PD) modelling approach to evaluate the predictive validity of 5-hydroxytryptamine (5-HT; serotonin) transporter (SERT) occupancy and 5-hydroxytryptophan (5-HTP)-potentiated behavioral syndrome induced by 5-HT reuptake inhibitor (SRI) antidepressants in mice.Experimental approach:Serum and whole brain drug concentrations, cortical SERT occupancy and 5-HTP-potentiated behavioral syndrome were measured over 6 h after a single subcutaneous injection of escitalopram, paroxetine or sertraline. [(3)H]2-(2-dimethylaminomethylphenylsulphanyl)-5-methyl-phenylamine ([(3)H]MADAM) was used to assess SERT occupancy. For PK/PD modelling, an effect-compartment model was applied to collapse the hysteresis and predict the steady-state relationship between drug exposure and PD response.Key results:The predicted Css for escitalopram, paroxetine and sertraline at 80% SERT occupancy in mice are 18 ng mL(-1), 18 ng mL(-1) and 24 ng mL(-1), respectively, with corresponding responses in the 5-HTP behavioral model being between 20-40% of the maximum.Conclusions and implications:Therapeutically effective SERT occupancy for SRIs in depressed patients is approximately 80%, and the corresponding plasma Css are 6-21 ng mL(-1), 21-95 ng mL(-1) and 20-48 ng mL(-1) for escitalopram, paroxetine and sertraline, respectively. Thus, PK/PD modelling using SERT occupancy and 5-HTP-potentiated behavioral syndrome as response markers in mice may be a useful tool to predict clinically relevant plasma Css values.British Journal of Pharmacology (2008) 155, 276-284; doi:10.1038/bjp.2008.243; published online 16 June 2008.     

The R-enantiomer of citalopram counteracts escitalopram-induced increase in extracellular 5-HT in the frontal cortex of freely moving rats

The selective serotonin (5-HT) reuptake inhibitor, citalopram, is a racemic mixture of an S(+)- and R(-)-enantiomer, escitalopram and R-citalopram, respectively. The present study compares the effects of escitalopram, R-citalopram and citalopram on extracellular levels of 5-HT in the frontal cortex of freely moving rats. In addition, co-injection of escitalopram and R-citalopram (ratios 1:2 and 1:4) were assessed. In some experiments escitalopram and R-citalopram were infused into the frontal cortex by reverse microdialysis. Finally, the extracellular level of escitalopram in the frontal cortex was studied after administration of escitalopram alone or in combination with R-citalopram. Escitalopram (1.0-3.9 mg/kg, s.c.) produced a greater maximal increase in extracellular 5-HT than citalopram (2.0-8.0 mg/kg, s.c.). R-citalopram (15.6 mg/kg s.c.) did not affect the 5-HT levels. When co-injected, R-citalopram counteracted the escitalopram-induced increase in extracellular 5-HT levels. Local infusion of the two enantiomers into the frontal cortex produced a similar inhibitory response. R-citalopram did not influence the extracellular levels of escitalopram and therefore does not exert its effect via a pharmacokinetic interaction with escitalopram. In conclusion, the 5-HT-reuptake inhibitory activity of citalopram resides in escitalopram, and the R-enantiomer counteracts this effect. This observation would predict an improved clinical profile of escitalopram compared to citalopram.     

Peripheral tissue microdialysis technique in unrestrained, conscious animals

The microdialysis procedure had been developed in the past 2 to 3 decades to determine levels of drug and endogenous compounds in several organs under physiological conditions. Advantages of microdialysis include: minimal stress on the experimental animals; studies may be done in unrestrained, conscious animals, and multiple determination can be made without concern for excess blood loss from small animals; and measurements can be made of drug and/or metabolites at multiple sites in the animal. By employing the microdialysis technique, I developed the method of multiple sampling for a long term period from an animal under the freely moving condition. In addition, I developed a novel microdialysis probe that was applied to several peripheral tissues and/or organs. In this paper, I will describe the fundamental procedure for peripheral tissues and/or organs such as the jugular vein and liver, using subcutaneous and ocular microdialysis sampling in unrestrained, conscious animals.     

Cerebral microdialysis in clinical studies of drugs: pharmacokinetic applications

The ability to deliver drug molecules effectively across the blood–brain barrier into the brain is important in the development of central nervous system (CNS) therapies. Cerebral microdialysis is the only existing technique for sampling molecules from the brain extracellular fluid (ECF; also termed interstitial fluid), the compartment to which the astrocytes and neurones are directly exposed. Plasma levels of drugs are often poor predictors of CNS activity. While cerebrospinal fluid (CSF) levels of drugs are often used as evidence of delivery of drug to brain, the CSF is a different compartment to the ECF. The continuous nature of microdialysis sampling of the ECF is ideal for pharmacokinetic (PK) studies, and can give valuable PK information of variations with time in drug concentrations of brain ECF versus plasma. The microdialysis technique needs careful calibration for relative recovery (extraction efficiency) of the drug if absolute quantification is required. Besides the drug, other molecules can be analysed in the microdialysates for information on downstream targets and/or energy metabolism in the brain. Cerebral microdialysis is an invasive technique, so is only useable in patients requiring neurocritical care, neurosurgery or brain biopsy. Application of results to wider patient populations, and to those with different pathologies or degrees of pathology, obviously demands caution. Nevertheless, microdialysis data can provide valuable guidelines for designing CNS therapies, and play an important role in small phase II clinical trials. In this review, we focus on the role of cerebral microdialysis in recent clinical studies of antimicrobial agents, drugs for tumour therapy, neuroprotective agents and anticonvulsants.     

Blockade of the high-affinity noradrenaline transporter (NET) by the selective 5-HT reuptake inhibitor escitalopram: an in vivo microdialysis study in mice

BACKGROUND AND PURPOSE

Escitalopram, the S(+)-enantiomer of citalopram is the most selective 5-HT reuptake inhibitor approved. Although all 5-HT selective reuptake inhibitors (SSRIs) increase extracellular levels of 5-HT ([5-HT]_ext). some also enhance, to a lesser extent, extracellular levels of noradrenaline ([NA]_ext). However, the mechanisms by which SSRIs activate noradrenergic transmission in the brain remain to be determined.

EXPERIMENTAL APPROACH

This study examined the effects of escitalopram, on both [5-HT]_ext and [NA]_ext in the frontal cortex (FCx) of freely moving wild-type (WT) and mutant mice lacking the 5-HT transporter (SERT^−/−) by using intracerebral microdialysis. We explored the possibilities that escitalopram enhances [NA]_ext, either by a direct mechanism involving the inhibition of the low- or high-affinity noradrenaline transporters, or by an indirect mechanism promoted by [5-HT]_ext elevation. The forced swim test (FST) was used to investigate whether enhancing cortical [5-HT]_ext and/or [NA]_ext affected the antidepressant-like activity of escitalopram.

KEY RESULTS

In WT mice, a single systemic administration of escitalopram produced a significant increase in cortical [5-HT]_ext and [NA]_ext. As expected, escitalopram failed to increase cortical [5-HT]_ext in SERT^−/− mice, whereas its neurochemical effects on [NA]_ext persisted in these mutants. In WT mice subjected to the FST, escitalopram increased swimming parameters without affecting climbing behaviour. Finally, escitalopram, at relevant concentrations, failed to inhibit cortical noradrenaline and 5-HT uptake mediated by low-affinity monoamine transporters.

CONCLUSIONS AND IMPLICATIONS

These experiments suggest that escitalopram enhances, although moderately, cortical [NA]_extin vivo by a direct mechanism involving the inhibition of the high-affinity noradrenaline transporter (NET).     

Effect of 5-HT1A receptor activation on hypothalamic glucose.

The somatodendritic 5-HT1A agonist 8-OH-DPAT reduces serotonergic activity and stimulates feeding in freely feeding rats. Interactions between circulating glucose and 5-HT1A receptor expression related to feeding have been described. The aim of the present microdialysis study was to (1) describe the relation between feeding and glucose in the LH, (2) to investigate if peripherally administered 8-OH-DPAT itself has an effect on extracellular glucose in the lateral hypothalamus (LH) of conscious rats. Baseline glucose concentrations were significantly different in microdialysis samples obtained from food deprived rats compared to freely feeding rats. After re-feeding, a significant rise in glucose levels by 45% was observed in the formerly food deprived rats. In freely feeding rats, 8-OH-DPAT (0.3 mg/kg, i.p.) reduced glucose level in the LH significantly. The effect of 8-OH-DPAT on brain glucose was antagonized by pre-treatment with the 5-HT1A antagonist WAY 100635 (3 mg/kg i.p.) which had no effect on its own. The data indicate, therefore, that the effect of 8-OH-DPAT on hypothalamic glucose is mediated by 5-HT1A receptors. In contrast, the same dose of 8-OH-DPAT proven effective in the brain had no effect on peripheral glucose. Only a very high dose of the 5-HT1A agonist (1.8 mg/kg i.p.) had a hyperglycaemic effect in the periphery. In conclusion, the present results show for the first time, that glucose in the lateral hypothalamus increases with a meal. The data demonstrate furthermore 8-OH-DPAT-induced changes of hypothalamic glucose level, implicating 5-HT1A receptors being involved not only in the control of hypothalamic 5-HT as shown before, but also in the control of hypothalamic glucose.     

Do plasma concentrations obtained from early arterial blood sampling improve pharmacokinetic/pharmacodynamic modeling?

In pharmacokinetic/pharmacodynamic (PK/PD) modeling the first blood sample is usually taken 1 to 2 min after drug administration (late sampling). Therefore, investigators have to extrapolate the plasma concentration to Time 0. Extrapolation, however, erroneously assumes instantaneous and complete mixing of drug in the central volume of distribution. We investigated whether plasma concentrations obtained from early arterial blood sampling would improve PK/PD modeling. In 14 pigs, one of five neuromuscular blocking agents (NMBAs) was administered into the right ventricle within 1 sec and arterial sampling was performed every 1.2 sec (1st min). The response of the tibialis muscle was measured mechanomyographically. The influence of inclusion of data from early arterial sampling on PK/PD modeling was determined. Furthermore, the concentrations in the effect compartment at 50% block (EC50) derived from modeling were compared to the measured concentration in plasma during a steady state 50% block. A very high peak in arterial plasma concentration was seen within 20 sec after administration of the NMBA. Extensive modeling revealed that plasma concentrations obtained from early arterial blood sampling improve PK/PD modeling. Independent of the type of modeling, the EC50 and KeO based on data sets that include early arterial blood sampling were, for all five NMBAs, significantly higher and lower respectively, than those based on data sets obtained from late sampling. Early arterial sampling shows that the mixing of the NMBA in the central volume of distribution is incomplete. A parametric PD (sigmoid Emax) model could not describe the time course of effect of the NMBAs adequately.     

High plasma corticosterone levels persist during frequent automatic blood sampling in rats

Corticosterone levels in blood may be used as a marker of stress in rodents, provided that the blood sampling procedure itself is non-stressful. Automated blood sampling equipment (Accusampler) allows blood sampling without any interference with the animal and might be useful as a tool for an on-line measurement of stress markers in blood. However, the impact of the blood sampling itself on the corticosterone levels in blood is unknown. The present study was designed to evaluate whether the frequency of blood sampling influences the plasma corticosterone levels in male and female rats. During anaesthesia, a catheter was placed in the jugular vein and attached to an Accusampler. Blood samples (200 microl) were withdrawn with a high (24 samples) or low frequency (3 samples) during a six-hour period immediately after the catheter insertion. The corticosterone levels in the plasma were quantified with ELISA. The corticosterone levels persisted at high post-operation concentrations when blood was collected frequently, while the levels steadily declined significantly during low-frequency sampling. The corticosterone levels were higher in female than in male rats, but the curves were similar. The present study elucidates the importance of considering the frequency of blood withdrawal during automated blood sampling. This parameter may have an impact on the experimental results when using blood corticosterone levels as a stress marker, but also during any in vivo study where blood is collected, since high corticosterone levels may affect the normal physiology of the animals.     

Model-based approaches for ivabradine development in paediatric population,part II: PK and PK/PD assessment

The objectives of this work were first to describe the pharmacokinetic (PK) of ivabradine and its active metabolite in a paediatric patient population after repeated oral administration of ivabradine using a population PK approach, and secondly to assess whether the blood/plasma ratio and the pharmacokinetic/pharmacodynamic (PK/PD) relationship are preserved in the paediatric population in comparison to adult. PK data for 70 patients were obtained after blood sampling using dried blood spot and one plasma sample in order to assess the relationship between blood and plasma concentration. In order to describe ivabradine and its metabolite blood concentrations in children, a joint population PK model was developed taking into account weight & age effects on PK parameters. Plasma PK exposure parameters were calculated in children using plasma PK profiles. In order to assess the PK/PD relationship in children, an adult PK/PD model was used. The relationship between blood and plasma concentrations was described using linear mixed effect models. Two and one—compartment models best described parent and metabolite dispositions. Weight effects were fixed to the allometric values of ¾ on clearance (CL) and 1 on volume. A maturation function was added on metabolite formation clearance (CL _PM ) reflecting enzyme maturation. Plasma exposure comparison indicated that higher dose/kg were necessary to achieve a similar exposure between younger and older children. No differences between age classes were observed in terms of range of exposure at the maintenance dose. The PK/PD relationship in adult patients is conserved in children.     

Do Plasma Concentrations Obtained from Early Arterial Blood Sampling Improve Pharmacokinetic/Pharmacodynamic Modeling?

In pharmacokinetic/pharmacodynamic (PK/PD) modeling the first blood sample is usually taken 1 to 2 min after drug administration (late sampling). Therefore, investigators have to extrapolate the plasma concentration to Time 0. Extrapolation, however, erroneously assumes instantaneous and complete mixing of drug in the central volume of distribution. We investigated whether plasma concentrations obtained from early arterial blood sampling would improve PK/PD modeling. In 14 pigs, one of five neuromuscular blocking agents (NMBAs) was administered into the right ventricle within 1 sec and arterial sampling was performed every 1.2 sec (1st min). The response of the tibialis muscle was measured mechanomyographically. The influence of inclusion of data from early arterial sampling on PK/PD modeling was determined. Furthermore, the concentrations in the effect compartment at 50% block (EC₅₀ ) derived from modeling were compared to the measured concentration in plasma during a steady state 50% block. A very high peak in arterial plasma concentration was seen within 20 sec after administration of the NMBA. Extensive modeling revealed that plasma concentrations obtained from early arterial blood sampling improve PK/PD modeling. Independent of the type of modeling, the EC₅₀ and K_eO based on data sets that include early arterial blood sampling were, for all five NMBAs, significantly higher and lower respectively, than those based on data sets obtained from late sampling. Early arterial sampling shows that the mixing of the NMBA in the central volume of distribution is incomplete. A parametric PD (sigmoid E_max ) model could not describe the time course of effect of the NMBAs adequately.     

Depression and poor sleep: the effect of monoaminergic antidepressants in a pre-clinical model in rats

The effects of five antidepressants (escitalopram, paroxetine, duloxetine, venlafaxine, and reboxetine) on the sleep architecture were investigated in freely moving rats in the light phase of a 12:12 h light:dark cycle following a single i.p. dose of antidepressant. Overall, paroxetine and escitalopram exhibited the least sleep disruptive profiles, whereas duloxetine, venlafaxine, and reboxetine increased the time spent awake and suppressed paradoxical sleep. Analysis of the EEG at 1 h intervals revealed only subtle differences from the overall picture. The effect of venlafaxine on disruption of sleep architecture could not be readily explained by its in vitro serotonin (5-HT) and noradrenaline (NA) reuptake inhibitory potencies. In vivo microdialysis experiments in the ventral hippocampus of freely moving rats revealed that venlafaxine affected the 5-HT and NA systems equally at the doses tested. Duloxetine (7.7 mg/kg) induced maximal blockade of the 5-HT transporter and duloxetine 7.7 mg/kg also modulated the noradrenaline system. Thus, in this animal model, the enhancement of noradrenergic activity is more disruptive on the sleep architecture than enhancement of serotonergic activity.     

Functional effects of corticosterone on 5-HT(1A) and 5-HT(1B) receptor activity in rat brain: in vivo microdialysis studies

Glucocorticoid hormones are known to be elevated in depression, and to interact with serotonin 5-HT(1A) receptors at both the presynaptic and postsynaptic levels. Since one of the presumed mechanisms of action of antidepressant drugs is induction of changes in sensitivity of 5-HT(1A) and also 5-HT(1B) receptors, the effects of repeated administration of corticosterone (50 mg/kg s.c. b.i.d. for 10 days) on activities of these receptors were determined using in vivo microdialysis in freely moving rats. Presynaptic 5-HT(1A) receptor activity, as measured by the effect of a challenge dose (0.2 mg/kg s.c.) of the 5-HT(1A) agonist 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) to reduce 5-HT levels in the hypothalamus, was not affected by corticosterone administration. Presynaptic 5-HT(1B) receptor activity, as measured by the effect of the 5-HT(1B) receptor antagonist (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide (GR 127935) (5 mg/kg s.c.) to increase 5-HT levels, was increased in hypothalamus but not hippocampus of corticosterone-treated rats. Postsynaptic 5-HT(1A) receptor activity, as measured by the effect of 8-OH-DPAT to increase cyclic AMP levels in the hippocampus, was not affected by corticosterone administration. The decrease in presynaptic 5-HT(1B) receptor activity after chronic administration of antidepressant drugs complements the increases in 5-HT(1B) receptor number observed in animal models of depression.     

Peritoneal microdialysis in freely moving rodents: an alternative to blood sampling for pharmacokinetic studies in the rat and the mouse.

By performing microdialysis in the peritoneal cavity, we studied the pharmacokinetics of Tramadol in awake, freely moving small laboratory animals. The systemic exposure to Tramadol was determined using both microdialysis sampling and collection of whole blood following a single intravenous injection (10 mg/kg) or a single oral dose (100 mg/kg) of Tramadol HCl. The sampling frequency of the dialysate was 10 min (mouse study) or 20 min (rat study). In rats and in mice, intraperitoneal microdialysis sampling gets reliable pharmacokinetic results without taking blood. The concentration-time curves obtained from peritoneal microdialysis were parallel to the concentration-time curves obtained from classical blood sampling. Accordingly, dose independent pharmacokinetic parameters were similar. A scaling factor, however, has to be introduced (e.g. peritoneal versus plasma AUC ratio) in order to obtain comparable pharmacokinetic results also with dose-dependent parameters. As there was no blood loss during the experiment, peritoneal microdialysis allowed the investigation of complete concentration-time curve profiles. Thus, the number of animals could be kept to a minimum. In conclusion, in vivo peritoneal microdialysis is a unique tool to obtain a complete set of free drug concentrations to determine reliable pharmacokinetic parameters from awake, freely moving rodents. Therefore, we suppose that the technique will have relevance for pharmacokinetic studies in future.