Metabolism of 3H-dopamine continuously perfused through push-pull cannulas in rats' brains: modification by amphetamine or prostaglandin F2 alpha

Physiologically based pharmacokinetic modeling to investigate regional brain distribution kinetics in rats

One of the major challenges in the development of central nervous system (CNS)-targeted drugs is predicting CNS exposure in human from preclinical data. In this study, we present a methodology to investigate brain disposition in rats using a physiologically based modeling approach aiming at improving the prediction of human brain exposure. We specifically focused on quantifying regional diffusion and fluid flow processes within the brain. Acetaminophen was used as a test compound as it is not subjected to active transport processes. Microdialysis probes were implanted in striatum, for sampling brain extracellular fluid (ECF) concentrations, and in lateral ventricle (LV) and cisterna magna (CM), for sampling cerebrospinal fluid (CSF) concentrations. Serial blood samples were taken in parallel. These data, in addition to physiological parameters from literature, were used to develop a physiologically based model to describe the regional brain pharmacokinetics of acetaminophen. The concentration-time profiles of brain ECF, CSF(LV), and CSF(CM) indicate a rapid equilibrium with plasma. However, brain ECF concentrations are on average fourfold higher than CSF concentrations, with average brain-to-plasma AUC(0-240) ratios of 121%, 28%, and 35% for brain ECF, CSF(LV), and CSF(CM), respectively. It is concluded that for acetaminophen, a model compound for passive transport into, within, and out of the brain, differences exist between the brain ECF and the CSF pharmacokinetics. The physiologically based pharmacokinetic modeling approach is important, as it allowed the prediction of human brain ECF exposure on the basis of human CSF concentrations.

Preclinical prediction of human brain target site concentrations: considerations in extrapolating to the clinical setting

The development of drugs for central nervous system (CNS) disorders has encountered high failure rates. In part, this has been due to the sole focus on blood-brain barrier permeability of drugs, without taking into account all other processes that determine drug concentrations at the brain target site. This review deals with an overview of the processes that determine the drug distribution into and within the CNS, followed by a description of in vivo techniques that can be used to provide information on CNS drug distribution. A plea follows for the need for more mechanistic understanding of the mechanisms involved in brain target site distribution, and the condition-dependent contributions of these mechanisms to ultimate drug effect. As future direction, such can be achieved by performing integrative cross-compare designed studies, in which mechanisms are systematically influenced (e.g., inhibition of an efflux transporter or induction of pathological state). With the use of advanced mathematical modeling procedures, we may dissect contributions of individual mechanisms in animals as links to the human situation.

Inhibitory effect of prostaglandins on dopamine release from the retina

1. Prostaglandins have been shown to modulate transmitter release from both central and peripheral neuroeffector junctions. In the present study, we examined the effect of prostaglandins on [3H]-dopamine release from isolated, superfused rabbit retina. 2. Both naturally occurring and synthetic prostaglandins produced concentration-dependent reduction of electrically evoked [3H]-dopamine overflow without affecting basal tracer efflux. The rank order of potencies of the agonists was: sulprostone > 16,16-dimethyl PGE2 > PGE2 >> 11-deoxy-PGE1 > PGF2 alpha. 3. The PGE2-mediated inhibition of field stimulated [3H]-dopamine release was not blocked by the selective EP1-receptor antagonist, AH6809 (5-30 microM). 4. The cyclooxygenase inhibitor, flurbiprofen (3 microM) had no effect on basal or evoked [3H]-dopamine overflow nor did it affect the inhibition caused by PGE2 suggesting that endogenous prostaglandins are not involved in the regulation of dopamine release in the retina. 5. The inhibition of [3H]-dopamine release produced by submaximal concentrations of PGE2, apomorphine and melatonin were not additive indicating that presynaptic PGE2, D2- and melatonin receptors coexist at sites for neurotransmitter release and may share a common mechanism for regulation of dopamine release. 6. We conclude that prostaglandin-induced inhibition of electrically evoked [3H]-dopamine release from the rabbit retina may be mediated by specific prostaglandin receptors of the EP3 subtype.

The serotonin2 antagonist ritanserin blocks quasi-morphine withdrawal at a time when mianserin is no longer effective

A quasi-morphine withdrawal syndrome (QMWS), produced in opiate-naive rats with an injection of isobutylmethylxanthine (IBMX) and the opioid antagonist naloxone, allows one to study the expression of opiate withdrawal in the absence of the acute or chronic effects of opiates and the adaptive processes termed dependence. The allegedly selective and long-acting serotonin2 (5-HT2) antagonist ritanserin attenuated the QMWS-induced suppression of fixed ratio (FR) operant responding, which is a sensitive measure of the expression of a QMWS. When administered 30 min prior to precipitation of the QMWS, the lowest dose of ritanserin tested (0.158 mg/kg) was the most effective in blocking the expression of withdrawal; however, there was not complete reversal of the behavioral suppression. Acutely, the two higher doses of ritanserin tested (2.5 and 10 mg/kg) suppressed responding when given alone. This may have masked their ability to attenuate a QMWS. At a dose of 2.5 mg/kg, ritanserin completely blocked the QMWS-induced suppression of responding 24 h post-administration, at a time when its actions at other receptors (e.g., alpha 2) have dissipated. At an equivalent dose, the shorter-acting 5-HT2 antagonist mianserin was unable to attenuate the QMWS-induced suppression of FR operant responding 24 h post-administration. The 5-HT2 antagonists reportedly produce a paradoxical down-regulation of 5-HT2 binding sites upon chronic treatment, rather than the expected supersensitivity. Chronic treatment with ritanserin (2.5 mg/kg/day for 7 days), but not mianserin (same regimen), attenuated a QMWS 24 h after the final injection, thus supporting with a functional measure, the down-regulation of such binding sites by ritanserin.(ABSTRACT TRUNCATED AT 250 WORDS)

Central administration of prostaglandin E2 facilitates while F2 alpha attenuates acute dependence upon morphine rats

The effects of prostaglandin D2 (PGD2), E2 (PGE2), and F2 alpha (PGF2 alpha) on acute dependence on morphine were investigated. Five mature, male Long-Evans rats were trained to lever press for food reinforcement on a fixed-ratio 30 schedule (FR 30 behavior) and implanted with permanent guide cannulas with the tips of the cannulas in their right lateral brain ventricles. The experimental protocol began with a 45 minute behavioral session and brain infusion (1 microliter/minute of a solution containing 2.3 mM CaCl2 in 0.9% saline, ICV). Fifteen minutes into the session the rats were injected with 7.5 mg morphine/kg (IP). Beginning 2.25 hours later the brain infusion was reinitiated during a second 45 minute behavioral session which was interrupted after 15 minutes to inject 1.0 mg naloxone/kg (IP). In several experiments a dose of PG, which did not in-and-of-itself affect behavior, was added to the infusion medium. Prior to the naloxone injection it was ascertained that the behavioral effects of morphine had dissipated. The injection of naloxone or saline did not alter behavior of the rats while they were being infused with a PG or PG vehicle. Injection of naloxone, 3 hours after the injection of morphine, resulted in a significant suppression of FR 30 behavior (withdrawal). A dose of PGE2, which did not alter the initial suppressant action of morphine, potentiated the naloxone effect. A dose of PGF2 alpha, which likewise did not alter the initial action of morphine, antagonized the naloxone effect. However, a higher dose of PGF2 alpha which enhanced the initial morphine effect, caused an enhanced naloxone effect as well.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative study of the effects of prostaglandins and d-amphetamine on the metabolism of 3H-dopamine continuously presented to rat brain in vivo

Unanesthetized rats with chronic indwelling cannulas, engaged in food reinforced operant behavior, were infused intracerebroventricularly with a solution containing a trace concentration of 3H-dopamine (3H-DA) with or without prostaglandins (PGs). Approximately 45 minutes after the infusion was started, the procedure was changed to a push-pull perfusion. Perfusate from the ventricles contained significant quantities of the 3H-DA metabolites 3H-3,4-dihydroxyphenylacetic acid (3H-DOPAC), 3H-3-methoxy-4-hydroxyphenylacetic acid (3H-homovanillic acid, 3H-HVA), 3H-3-methoxytyramine (3H-3-MT), and the 3H-noradrenaline (3H-NA) metabolite 3H-3-methoxy-4-hydroxy-phenylethyleneglycol (3H-MHPG). The presence of PGF2 alpha decreased the amount of 3H-DOPAC, 3H-HVA, and 3H-3-MT in perfusate, while PGE1 had the opposite effects. d-Amphetamine (0.5 mg/kg, 1P) affected the recovery of these metabolites from perfusate in a manner similar to PGF2 alpha and opposite to PGE1. PGF2 alpha and the highest (seizure-inducing) dose of PGE1 significantly decreased, while d-amphetamine significantly increased, the quantity of 3H-MHPG in perfusate. Therefore, PGs affect central dopaminergic and noradrenergic activity in vivo, as reflected by changes in their metabolic profiles, and may play a role in the response of the central nervous system to drugs which act through catecholaminergic mechanisms.

d-Amphetamine antagonizes prostaglandin E1-induced hyperthermia and suppression of fixed interval operant behavior in rats

The experiments reported herein were designed to study the effects of prostaglandin F2 alpha (PGF2 alpha) and PGE1 on operant behavior and rectal temperature of rats. A solution containing PGF2 alpha or PGE1 was infused intracerebroventricularly into rats trained to press a lever for food reward on a fixed interval 75 second (FI 75 sec) schedule. PGF2 alpha (10, 100 or 1000 ng/min) had no effect on FI 75 sec operant behavior. Only the highest dose increased temperature. PGE1 (100 ng/min) had no effect, whereas higher doses (250 and 500 ng/min) produced a rate-dependent effect on behavior, increasing low rates and decreasing high rates. The two higher doses also produced convulsions after about 25 min or 20 min infusions, respectively. PGE1 also increased temperature in a dose-dependent manner. Systemic administration of a low dose of d-amphetamine (0.5 mg/kg IP) had little or no effect on behavior or temperature. d-Amphetamine did not alter hyperthermia induced by the highest dose of PGF2 alpha, but antagonized the PGE1-induced hyperthermia. d-Amphetamine also antagonized all of the behavioral effects of PGE1, including convulsions. The results are discussed in relation to the actions of PGs and d-amphetamine on catecholamine neurons in the central nervous system.

Thyrotropin-releasing hormone and its analog MK-771 increase the cerebroventricular perfusate content of dihydroxyphenylacetic acid

The effects of thyrotropin-releasing hormone (TRH) and its synthetic analog, pyro-2-aminoadipyl-histidyl-thiazolidine-4-carboxamide (MK-771), were determined on the efflux of dihydroxyphenylacetic acid (DOPAC) collected from push-pull cannulae chronically implanted into the lateral cerebral ventricles of rats. Intracerebroventricular and intraperitoneal injections of both peptides increased the efflux of DOPAC. These results suggest that TRH and MK-771 increase the activity of dopaminergic neurons that terminate in periventricular regions.

Differential effects of d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate on the rate of dopamine synthesis in terminals of nigrostriatal and mesolimbic neurons and on the efflux of dopamine metabolites into cerebroventricular perfusates of rats

The in vivo effects of four psychomotor stimulants (d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate) were determined on: 1) the rate of dopamine (DA) synthesis, as measured by the accumulation of dihydroxyphenylalanine (DOPA) after aromatic L-amino acid decarboxylase inhibition, in the striatum (terminals of nigrostriatal neurons) and in the nucleus accumbens and olfactory tubercle (terminals of mesolimbic neurons) and 2) the efflux of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) into cerebroventricular perfusates of conscious, freely-moving rats. d-Amphetamine and beta-phenylethylamine produced biphasic responses with lower doses of each drug increasing both the efflux of DOPAC and the rate of DA synthesis in the striatum. Higher doses of each drug either had no effect or actually decreased the efflux of DOPAC and also decreased the rate of DA synthesis in the striatum. On the other hand, cocaine and methylphenidate only decreased the efflux of DOPAC and the rate of DA synthesis in the striatum. The effects of the drugs on the rate of DA synthesis in the nucleus accumbens and olfactory tubercle were similar to, but less pronounced than those seen in the striatum. These results are consistent with the following suggestions: 1) low doses of d-amphetamine and beta-phenylethylamine facilitate the neuronal release of DA while higher doses of both drugs facilitate release and inhibit neuronal reuptake of the amine, and 2) cocaine and methylphenidate preferentially block the neuronal reuptake of DA.

6-Hydroxydopamine and 5,7-dihydroxytryptamine selectively reduce dopamine and 5-hydroxytryptamine metabolites in cerebroventricular perfusates of rats

The efflux into the lateral cerebral ventricles of metabolites of dopamine (DA) and 5-hydroxytryptamine (5HT) was determined in unanesthetized rats bearing chronically implanted push-pull cannulae. Pretreatment with 6-hydroxydopamine (6-OHDA) reduced the basal efflux of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), but not of 5-hydroxyindoleacetic acid (5HIAA). The haloperidol-induced increase in the efflux of DOPAC and HVA was markedly attenuated in the 6-OHDA-pretreated rats. In rats treated with 5,7-hydroxytryptamine (5,7-DHT) the basal efflux of DOPAC and HVA was unaffected, while that of 5HIAA was markedly reduced; in these animals the ability of L-tryptophan to increase the perfusate content of 5HIAA was abolished. These results indicate that metabolites of DA and 5HT appearing in cerebroventricular perfusates of rats originate from DA and 5HT neuronal terminals in the brain.

Tolerance develops to the haloperidol-induced increase in the efflux of dopamine metabolites from the brains of unanesthetized, freely-moving rats

The lateral cerebral ventricles of freely moving rats were perfused by means of chronically implanted push-pull cannulae every second day for 2 weeks. Perfusates were analyzed for metabolites of dopamine [dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)] and of 5-hydroxytryptamine [5-hydroxyindoleacetic acid (5HIAA)] using high performance liquid chromatography and an amperometric detector. Rats received daily subcutaneous injections of haloperidol (1 mg/kg) or its vehicle. After the first injection of haloperidol the concentrations of DOPAC and HVA were markedly increased while that of 5HIAA was unchanged. Complete tolerance developed to the haloperidol-induced increased efflux of dopamine metabolites by day 9, although a higher dose of haloperidol (2 mgf/kg) on day 15 was still capable of eliciting a modest increase in the efflux of DOPAC and HVA.

Measurement of metabolites of dopamine and 5-hydroxytryptamine in cerebroventricular perfusates of unanesthetized, freely-moving rats: selective effects of drugs

A method was developed for measuring endogenous acid metabolites of dopamine (dihydroxyphenylacetic acid, DOPAC; homovanillic acid, HVA) and of 5-hydroxytryptamine (5-hydroxyindoleacetic acid, 5HIAA) in serial samples of cerebroventricular perfusates from unanesthetized, freely-moving rats by the use of high performance liquid chromatography with electrochemical detection. Systemic administration of probenecid increased while pargyline reduced the efflux of all of the acid metabolites. Intraperitoneal injection of L-tryptophan increased the perfusate content of 5HIAA, decreased HVA and was without effect on DOPAC. Injections of haloperidol increased the efflux of DOPAC and HVA but did not alter the efflux of 5HIAA. Administration of apomorphine decreased the concentration of all three compounds. These drug-induced changes are generally consistent with those reported to occur in brain tissues. The results indicate that this method can be employed to monitor the activity of dopamine and 5-hydroxytryptamine neurons in unanesthetized, freely-moving rats.