Cytochrome P450 (CYP, P450) is the collective term for a superfamily of heme-containing membrane proteins responsible for the metabolism of approximately 70 - 80 % of clinically used drugs. Besides the liver and other peripheral organs, P450 isoforms are expressed in glial cells and neurons of the brain. To enlighten their function and significance is a topic of high interest, as most of the neuroactive drugs used in therapy today are not only substrates, but also inducers of brain P450s with far reaching consequences. First of all, brain P450s are regulated differentially from those in liver. The availability of the prosthetic heme group appears to be essential for correct membrane insertion and enzymatic functionality of brain P450s. Furthermore, although not contributing to body's overall drug metabolism, brain P450s fulfil particular functions within specific cell types of the brain. In astrocytes of brain's border lines P450 isoforms are expressed at very high level. They form a metabolic barrier regulating drugs' influx, modulate blood-flow regulation, and act as signalling enzymes in inflammation. In neurons, however, P450s apparently have different function. In specified brain regions such as hypothalamus, hippocampus and striatum they provide signalling molecules like steroids and fatty acids necessary for neuronal outgrowth and maintenance. Induction of these P450s by neuroactive drugs can alter steroid hormone signalling directly in drug target cells, which may cause clinically relevant side effects like reproductive disorders and sexual or mental dysfunction. The understanding of brain P450 function appears to be of major interest in long-term drug mediated therapy of neurological diseases.

Summary Purpose:  Many of the antiepileptic drugs (AED) used in therapy of temporal lobe epilepsy (TLE) are known as cytochrome P450 (CYP, P450) inducers. These AEDs are thought to modulate androgen and estrogen pathways in hippocampus, and therefore cause mental and reproductive disorders found in TLE patients. In the present study, we analyzed expression of androgen receptor (AR), estrogen receptor α (ERα), and CYP3A in the hippocampus of TLE patients and in murine hippocampal cell line HN25.1. Methods:  Patients and cell lines had been treated with P450‐inducing or noninducing AEDs, or with prednisolone, applied to prevent oedema formation prior to neurosurgical resection of the epileptic hippocampus. Human patient samples were analyzed by immunohistochemical approach, the HN25.1 cell line by quantitative RT‐PCR, CAT reporter gene assay, and immunoblot. Results:  In both, humans and cell lines, the expression of testosterone metabolising CYP3A4 (human) or CYP3A11 (mouse) and AR was up‐regulated when P450‐inducing AEDs and/or prednisolone had been applied. AR responsive CAT reporter gene assay indicated an increase of AR‐signalling after treatment of the HN25.1 cells with the P450‐inducers phenytoin and carbamazepine. ERα expression was increased only by the P450‐inducing AEDs, but not by prednisolone, which indicates that pathways different from CYP3A4/11 led to ERα enhancement. Discussion:  We conclude that P450‐inducing AEDs influence AR expression and signalling in hippocampus most likely via CYP3A4/11‐induction. The HN25.1 cell line holds promise to investigate the correlation between drug application and AR regulation, and to specifically address issues that are relevant to human TLE patients.

No abstract

Inactivation of testosterone by specific hydroxylations is a main function of cytochrome P450 (P450, CYP) in the brain. Recent data imply that induction of brain P450s by neuroactive drugs alters steroid hormone levels and endocrine signalling, giving rise to endocrine disorders. In this study, we investigated this drug–hormone crosstalk in mouse brain. Phenytoin led to a significant increase of 2α‐, 2β‐, 6β‐, 16α‐ and 16β‐hydroxytestosterones, while 6α‐ and 15α‐hydroxytestosterones showed no significant alteration of their metabolism compared with untreated controls. Inhibition of testosterone hydroxylation using the chemical inhibitors orphenadrine, chloramphenicol, ketoconazole and nifedipine as well as antibodies against CYP3A‐ and 2B‐isoforms pointed to major role of Cyp3a11 and an only minor function of Cyp2b9/10 in mouse brain. Cyp3a11 revealed to be the major isoform affected by phenytoin. There was considerable overlap of Cyp3a11 and AR expression in neuronal structures of the limbic system, namely the hippocampus, amygdala, hypothalamus and thalamus. Phenytoin treatment led to an increase of both, Cyp3a11 and AR expression in the limbic system. Additionally, the coherence between CYP3A and AR expression was analysed in PC‐12 cells. Inhibition of phenytoin‐induced endogenous CYP3A2 and AR by ketoconazole led a reduction of their expression to basal levels. We conclude that Cyp3a11 plays a crucial role in directing drug action to hormonal response within the limbic system of mouse brain in a so‐called drug–hormone crosstalk.

The drug-hormone crosstalk may be of considerable importance in the assessment of neuroactive drugs and future drug design.