Expression and localisation of CYP2D enzymes in rat basal ganglia

The potential impact of CYP and UGT drug-metabolizing enzymes on brain target site drug exposure

Drug metabolism is one of the critical determinants of drug disposition throughout the body. While traditionally associated with the liver, recent research has unveiled the presence and functional significance of drug-metabolizing enzymes (DMEs) within the brain. Specifically, cytochrome P-450 enzymes (CYPs) and UDP-glucuronosyltransferases (UGTs) enzymes have emerged as key players in drug biotransformation within the central nervous system (CNS). This comprehensive review explores the cellular and subcellular distribution of CYPs and UGTs within the CNS, emphasizing regional expression and contrasting profiles between the liver and brain, humans and rats. Moreover, we discuss the impact of species and sex differences on CYPs and UGTs within the CNS. This review also provides an overview of methodologies for identifying and quantifying enzyme activities in the brain. Additionally, we present factors influencing CYPs and UGTs activities in the brain, including genetic polymorphisms, physiological variables, pathophysiological conditions, and environmental factors. Examples of CYP- and UGT-mediated drug metabolism within the brain are presented at the end, illustrating the pivotal role of these enzymes in drug therapy and potential toxicity. In conclusion, this review enhances our understanding of drug metabolism's significance in the brain, with a specific focus on CYPs and UGTs. Insights into the expression, activity, and influential factors of these enzymes within the CNS have crucial implications for drug development, the design of safe drug treatment strategies, and the comprehension of drug actions within the CNS. To that end, CNS pharmacokinetic (PK) models can be improved to further advance drug development and personalized therapy.

Cytochrome P450 2D (CYP2D) enzyme dysfunction associated with aging and serotonin deficiency in the brain and liver of female Dark Agouti rats

Among the enzymes that support brain metabolism, cytochrome P450 (CYP) enzymes occupy an important place. These enzymes catalyze the biotransformation pathways of neuroactive endogenous substrates (neurosteroids, neurotransmitters) and are necessary for the detoxification processes. The aim of the present study was to assess changes in the CYP2D activity and protein level during the aging process and as a result of serotonin deficiency in the female brain. The CYP2D activity was measured in brain and liver microsomes of Dark Agouti wild type (WT) female rats (mature 15-week-old and senescent 18-month-old rats) and in tryptophan hydroxylase 2 (TPH2)-deficient senescent female rats. The CYP2D activity in mature WT Dark Agouti females was independent of the changing phases of the estrous cycle. In senescent WT females rats, the CYP2D activity and protein level were decreased in the cerebral cortex, hippocampus, cerebellum and liver, but increased in the brain stem. In the other examined structures (frontal cortex, hypothalamus, thalamus, striatum), the enzyme activity did not change. In aging TPH2-deficient females, the CYP2D activity and protein levels were decreased in the frontal cortex, hypothalamus and brain stem (activity only), remaining unchanged in other brain structures and liver, relative to senescent WT females. In summary, the aging process and TPH2 deficit affect the CYP2D activity and protein level in female rats, which may have a negative impact on the compensatory capacity of CYP2D in the synthesis of serotonin and dopamine in cerebral structures involved in cognitive and emotional functions. In the liver, the CYP2D-catalyzed drug metabolism may be diminished in elderly females. The results in female rats are compared with those obtained previously in males. It is concluded that aging and serotonin deficiency exert sex-dependent effects on brain CYP2D, which seem to be less favorable in females concerning CYP2D-mediated neurotransmitter synthesis, but beneficial regarding slower neurosteroid metabolism.

Cytochrome P450-mediated estrogen catabolism therapeutic avenues in epilepsy

Epilepsy is a neuropsychiatric disorder, which does not have any identifiable cause. However, experimental and clinical results have asserted that the sex hormone estrogen level and endocrine system function influence the seizure and epileptic episodes. There are available drugs to control epilepsy, which passes through the metabolism process. Cytochrome P-450 family 1 (CYP1A1) is a heme-containing mono-oxygenase that are induced several folds in most of the tissues and cells contributing to their differential expression, which regulates various metabolic processes upon administration of therapeutics. CYP1A1 gene family has been found to metabolize estrogen, a female sex hormone, which plays a central role in maintaining the health of brain altering the level of estrogen active neuropsychiatric disorder like epilepsy. Hence, in this article, we endeavor to provide an opinion of estrogen, its effects on epilepsy and catamenial epilepsy, their metabolism by CYP1A1 and new way forward to differential diagnosis and clinical management of epilepsy in future.

The catalytic competence of cytochrome P450 in the synthesis of serotonin from 5-methoxytryptamine in the brain: an in vitro study

Brain serotonin has been implicated in the pathophysiology of a wide spectrum of psychiatric disorders, as well as in the mechanism of action of psychotropic drugs. The aim of present study was to identify rat cytochrome P450 (CYP) isoforms which can catalyze the O-demethylation of 5-methoxytryptamine to serotonin, and to find out whether that alternative pathway of serotonin synthesis may take place in the brain. The study was conducted on cDNA-expressed CYPs (rat CYP1A1/2, 2A1/2, 2B1, 2C6/11/13, 2D1/2/4/18, 2E1, 3A2 and human CYP2D6), on rat brain and liver microsomes and on human liver microsomes (the wild-type CYP2D6 or the allelic variant 2D6*4*4). Of the rat CYP isoforms studied, CYP2D isoforms were the most efficient in catalyzing the O-demethylation of 5-methoxytryptamine to serotonin, but they were less effective than the human isoform CYP2D6. Microsomes from different brain regions were capable of metabolizing 5-methoxytryptamine to serotonin. The reaction was inhibited by the specific CYP2D inhibitors quinine and fluoxetine. Human liver microsomes of the wild-type CYP2D6 metabolized 5-methoxytryptamine to serotonin more effectively than did the defective CYP2D6*4*4 ones. The obtained results indicate that rat brain CYP2D isoforms catalyze the formation of serotonin from 5-methoxytryptamine, and that the deficit or genetic defect of CYP2D may affect serotonin metabolism in the brain. The results are discussed in the context of their possible physiological and pharmacological significance in vivo.

Cytochrome P450 mediates dopamine formation in the brain in vivo

The cytochrome P450-mediated synthesis of dopamine from tyramine has been shown in vitro. The aim of the present study was to demonstrate the ability of rat cytochrome P450 (CYP) 2D to synthesize dopamine from tyramine in the brain in vivo. We employed two experimental models using reserpinized rats with a blockade of the classical pathway of dopamine synthesis from tyrosine. Model A estimated dopamine production from endogenous tyramine in brain structures in vivo (ex vivo measurement of a tissue dopamine level), while Model B measured extracellular dopamine produced from exogenous tyramine (an in vivo microdialysis). In Model A, quinine (a CYP2D inhibitor) given intraperitoneally caused a significant decrease in dopamine level in the striatum and nucleus accumbens and tended to fall in the substantia nigra and frontal cortex. In Model B, an increase in extracellular dopamine level was observed after tyramine given intrastructurally (the striatum). After joint administration of tyramine and quinine, the amount of the dopamine formed was significantly lower compared to the group receiving tyramine only. The results of the two complementary experimental models indicate that the hydroxylation of tyramine to dopamine may take place in rat brain in vivo, and that CYP2D catalyzes this reaction.

Induction of the drug metabolizing enzyme CYP2D in monkey brain by chronic nicotine treatment

Cytochrome P450 (CYP) 2D6, an enzyme found in the liver and the brain, is involved in the metabolism of numerous centrally acting drugs (e.g. antidepressants, neuroleptics, opiates), endogenous neurochemicals (e.g. catecholamines) and in the inactivation of neurotoxins (e.g. pesticides, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)). Although CYP2D6 is essentially an uninducible enzyme in the liver, we show that smokers have higher CYP2D6 in the brain, especially in the basal ganglia. In order to determine whether nicotine, a component of cigarette smoke, could increase brain CYP2D, African Green monkeys were treated chronically with nicotine (0.05 mg/kg for 2 days, then 0.15 mg/kg for 2 days followed by 0.3 mg/kg for 18 days s.c., b.i.d.). Monkeys treated with nicotine showed significant induction of CYP2D in brain when compared to saline-treated animals as detected by western blotting and immunocytochemistry. No changes in liver CYP2D were observed in nicotine-treated monkeys. Induction was observed in various brain regions including those affected in Parkinson's disease (PD) such as substantia nigra (3-fold, p = 0.01), putamen (2.1-fold, p = 0.001) and brainstem (2.4-fold, p = 0.001), with the caudate nucleus approaching significance (1.6-fold, p = 0.07). Immunocytochemistry revealed that the expression of CYP2D in both saline- and nicotine-treated monkeys is cell-specific particularly in the cerebellum, frontal cortex and hippocampus. These results suggest that monkey brain expresses CYP2D, which is induced in specific cells and brain regions upon chronic nicotine treatment. Smokers, or those using nicotine treatment, may have higher levels of brain CYP2D6 that may result in altered localized CNS drug metabolism and inactivation of neurotoxins.

Potential role of cerebral cytochrome P450 in clinical pharmacokinetics: modulation by endogenous compounds

Cytochrome P450 (CYP) enzymes catalyse phase I metabolic reactions of psychotropic drugs. The main isoenzymes responsible for this biotransformation are CYP1A2, CYP2D6, CYP3A and those of the subfamily CYP2C. Although these enzymes are present in the human brain, their specific role in this tissue remains unclear. However, because CYP enzymatic activities have been reported in the human brain and because brain microsomes have been shown to metabolise the same probe substrates used to assess specific hepatic CYP activities and substrates of known hepatic CYPs, local drug metabolism is believed to be likely. There are also indications that CYP2D6 is involved in the metabolism of endogenous substrates in the brain. This, along with the fact that several neurotransmitters modulate CYP enzyme activities in human liver microsomes, indicates that CYP enzymes present in brain could be under various regulatory mechanisms and that those mechanisms could influence drug pharmacokinetics and, hence, drug response. In this paper we review the presence of CYP1A2, CYP2C9, CYP2D6 and CYP3A in brain, as well as the possible existence of local brain metabolism, and discuss the putative implications of endogenous modulation of these isoenzymes by neurotransmitters.

A STRATEGY FOR INVESTIGATING THE CYP SUPERFAMILY USING TARGETED ANTIBODIES IS A PARADIGM FOR FUNCTIONAL GENOMIC STUDIES

In this review we describe the use of targeted antibodies developed to facilitate studies on the expression of P450 proteins. The challenge of producing specific antibodies that distinguish between often highly related P450 proteins has led to the development of methods of antibody production to meet this need. Targeting antibodies toward the C terminus of P450 proteins has been found to be a particularly successful approach that is both rapid and efficient at producing specifically binding antibodies. Recent advances in genomic sequencing and proteomics now allow ready identification of expressed proteins. The levels and distributions of these proteins may be determined using antibody-based methods. However, for each protein to be studied, a unique antibody will be required. Consequently, some means of producing large numbers of well defined antibodies is needed. In this context, the potential of extending the approach used to produce specific antibodies against P450 proteins to the wider field of functional genomics is discussed.

Regional and cellular expression of CYP2D6 in human brain: higher levels in alcoholics

Cytochrome P450 (CYP) 2D6 is expressed in liver, brain and other extrahepatic tissues where it metabolizes a range of centrally acting drugs and toxins. As ethanol can induce CYP2D in rat brain, we hypothesized that CYP2D6 expression is higher in brains of human alcoholics. We examined regional and cellular expression of CYP2D6 mRNA and protein by RT-PCR, Southern blotting, slot blotting, immunoblotting and immunocytochemistry. A significant correlation was found between mean mRNA and CYP2D6 protein levels across 13 brain regions. Higher expression was detected in 13 brain regions of alcoholics (n = 8) compared to nonalcoholics (n = 5) (anovap < 0.0001). In hippocampus this was localized in CA1-3 pyramidal cells and dentate gyrus granular neurons. In cerebellum this was localized in Purkinje cells and their dendrites. Both of these brain regions, and these same cell-types, are known to be susceptible to alcohol damage. For one case, a poor metabolizer (CYP2D6*4/*4), there was no detectable CYP2D6 protein, confirming the specificity of the antibody used. These data suggest that in alcoholics elevated brain CYP2D6 expression may contribute to altered sensitivity to centrally acting drugs and to the mediation of neurotoxic and behavioral effects of alcohol.

Progesterone oxidation by cytochrome P450 2D isoforms in the brain

The existence of cytochrome P450 2D isoforms in the brain has been demonstrated, although their physiological functions remain to be elucidated. In this study we demonstrated that recombinant rat cytochrome P450 2D1 and 2D4 and human cytochrome P450 2D6 possess progesterone 6 beta- and 16 alpha- hydroxylation activities; 2 beta- and 21-hydroxylation activities; and 2 beta-, 6 beta-, 16 alpha- and 21-hydroxylation activities, respectively. Cytochrome P450 2D4 had the lowest K(m) value and the highest maximum velocity value toward these activities. Progesterone 2 beta- and 21-hydroxylation activities were also detected in rat brain microsomes, and these activities were completely inhibited by anticytochrome P450 2D antibodies. The presence of endogenous 2 beta- and 21-hydroxyprogesterones in rat brain tissues was also demonstrated. The mRNAs of cytochrome P450 2D4, CYP11A, and 3 beta-hydroxysteroid dehydrogenase were detected in the rat brain, suggesting that progesterone was generated from cholesterol by CYP11A and 3 beta-hydroxysteroid dehydrogenase and then underwent hydroxylation to hydroxyprogesterones by cytochrome P450 2D4 in rat brain. Collectively, our findings support the idea that cytochrome P450 2D may be involved in the regulation (metabolism and/or synthesis) of endogenous neuroactive steroids, such as progesterone and its derivatives, in brain tissues.

Cellular localization and regional distribution of CYP2D6 mRNA and protein expression in human brain

The cytochrome P4502D6 (CYP2D6) is involved in the biotransformation of many drugs which predominantly act in the central nervous system (CNS), including opioids, various psychotrophic drugs and neurotoxins. Until now, however, only controversial information is available regarding the presence of CYP2D6 in CNS. In this study, the regional and cellular expression of CYP2D6 transcripts and proteins in postmortem brain tissues of three individuals was analysed. A combination of in-situ hybridization coupled with immunohistochemistry on adjacent sections allowed simultaneous detection of CYP2D6 mRNA and protein. However, discrepancies existed in the results such that the mRNA was more widely distributed in the brain areas analysed compared to the protein. Neuronal cells, as well as glial cells, showed labelling for mRNA in brain regions such as the neocortex, caudate nucleus, putamen, globus pallidus, hippocampus, hypothalamus, thalamus, substantia nigra and cerebellum. In contrast, CYP2D6 protein was primarily localized in large principal neurons such as pyramidal cells of the cortex, pyramidal cells of the hippocampus, and Purkinje cells of the cerebellum. In glial cells, CYP2D6 protein was absent. These results provide clear evidence of CYP2D6 expression in certain regions of the CNS and may indicate the role CYP2D6 plays in a number of drug interactions that are of potential clinical importance for neurological diseases.

Expression and distribution of CYP2C enzymes in rat basal ganglia

The function and integrity of the basal ganglia is modulated by sex steroids whose activity may be controlled by P450 enzymes, such as members of the CYP2C subfamily. The expression of CYP2C enzymes in rat basal ganglia was examined by immunohistochemistry along with some of the factors that might control their expression. Whereas no CYP2C11 or CYP2C12 immunoreactivity was detected in the basal ganglia of either male or female rats, marked CYP2C13 immunoreactivity was evident in neurones of the subthalamic nucleus, substantia nigra, and interpeduncular nucleus. Strong CYP2C13 immunoreactivity was also expressed in the cortex, olfactory tubercle, hippocampus, dentate gyrus, hypothalamic nuclei, medial habenular nucleus, red nucleus, and medial forebrain bundle. Similar results were found in male and female rats. Following 6-hydroxydopamine lesioning of the nigro-striatal tract, tyrosine hydroxylase immunoreactivity was absent and CYP2C13 immunoreactivity was decreased markedly in the substantia nigra pars compacta, implying its presence in dopaminergic neurones. Modulation of sex steroids, using castrated rats, had no effect on the number of CYP2C13 positive neurones in the substantia nigra pars compacta. These results indicate that CYP2C13 protein is constitutively and widely expressed in rat brain. However, its expression is not sex-specific and is unaffected by castration. The role of CYP2C13 in brain is unknown but it may be involved in the generation of neurosteroids and catecholoestrogens.