Signal transduction and gene expression in cultured accessory olfactory bulb neurons.

Glutamate and norepinephrine (NE) are believed to mediate the long-lasting synaptic plasticity in the accessory olfactory bulb (AOB) that underlies pheromone recognition memory. The mechanisms by which these neurotransmitters bring about the synaptic changes are not clearly understood. In order to study signals that mediate synaptic plasticity in the AOB, we used AOB neurons in primary culture as a model system. Because induction of pheromone memory requires coincident glutamatergic and noradrenergic input to the AOB, and requires new protein synthesis, we reasoned that glutamate and NE must induce gene expression in the AOB. We used a combination of agonists that stimulate alpha1 and alpha2 adrenergic receptors in combination with N-methyl-d-aspartic acid and tested expression of the immediate-early gene (IEG) c-Fos. We found that the glutamatergic and noradrenergic stimulation caused significant induction of c-Fos mRNA and protein. Induction of c-Fos was significantly reduced in the presence of inhibitors of protein kinase C, mitogen-activated protein kinase (MAPK) and phospholipase C. These results suggest that glutamate and NE induce gene expression in the AOB through a signaling pathway mediated by protein kinase C and MAPK.

Differential metabolism of alprazolam by liver and brain cytochrome (P4503A) to pharmacologically active metabolite.

Cytochrome P450 (P450) is a superfamily of enzymes which mediates metabolism of xenobiotics including drugs. Alprazolam, an anti-anxiety agent, is metabolized in rat and human liver by P4503A1 and P4503A4 respectively, to 4-hydroxy alprazolam (4-OHALP, pharmacologically less active) and alpha-hydroxy alprazolam (alpha-OHALP, pharmacologically more active). We examined P450 mediated metabolism of alprazolam by rat and human brain microsomes and observed that the relative amount of alpha-OHALP formed in brain was higher than liver. This biotransformation was mediated by a P450 isoform belonging to P4503A subfamily, which is constitutively expressed in neuronal cells in rat and human brain. The formation of larger amounts of alpha-OHALP in neurons points to local modulation of pharmacological activity in brain, at the site of action of the anti-anxiety drug. Since hydroxy metabolites of alprazolam are hydrophilic and not easily cleared through blood-CSF barrier, alpha-OHALP would potentially have a longer half-life in brain.

Characterization and localization of cytochrome P450 mediated metabolism of MPTP to nor-MPTP in mouse brain: relevance to Parkinson's disease.

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) is a dopaminergic toxin which produces Parkinson's disease-like symptoms in primates and dopaminergic cell loss in mice. MPTP is bioactivated through monoamine oxidase to MPP(+) and detoxified by cytochrome P450 to nor-MPTP. We have examined metabolisms of MPTP to nor-MPTP by mouse brain microsomes and compared it with corresponding activity in liver. In brain, but not in liver, this biotransformation was completely abolished by quinidine, an inhibitor of P4502D. Northern blotting experiments demonstrated constitutive expression of cytochrome P4502D mRNA predominantly in neuronal cells within the cortex, hippocampus, thalamus, Purkinje and granule cell layers of the cerebellum and in the reticular neurons of midbrain. Striatal neurons were sparsely stained indicating a relative paucity of expression. These studies demonstrate for the first time that detoxification of MPTP to nor-MPTP occurs in mouse brain through cytochrome P4502D which is primarily localized in neuronal cells. Cytochrome P4502D6 is known to exhibit genetic polymorphism in humans, and a defect in this isoform could potentially lead to decreased detoxification of neurotoxins in certain neuronal sub-population, which in turn may have implications in pathogenesis of Parkinson's disease.

Cytochrome P4502E (CYP2E) in brain: constitutive expression, induction by ethanol and localization by fluorescence in situ hybridization.

Cytochrome P4502E (P4502E), the major ethanol-inducible P450 metabolizes ethanol to acetaldehyde and bioactivates procarcinogens to ultimate carcinogens. Metabolism of ethanol to acetaldehyde in the brain could be deleterious since it can react with cytoskeletal proteins, forming adducts. In the present study, rats were administered ethanol chronically to evaluate its effect on chlorzoxazone hydroxylation in rat brain regions. Chlorzoxazone hydroxylation in brains from the treated rats was induced in hippocampus and cortex, downregulated in brainstem, and unchanged in cerebellum, striatum, and thalamus. The presence of functionally active P4502E was also seen in human brain regions obtained at autopsy from traffic accident victims. Northern blot analysis of rat and human brain poly(A)(+) RNA hybridized with cDNA to rat CYP2E1 revealed the constitutive presence of a corresponding transcript in rat and human brain. Localization of CYP2E by fluorescence in situ hybridization demonstrated the constitutive expression of CYP2E preferentially in the neuronal cells in rat and human brain. CYP2E expression was seen in neurons within the cerebral cortex, Purkinje and granule cell layers of cerebellum, granule cell layer of dentate gyrus, and pyramidal neurons of CA1, CA2, and CA3 subfields of hippocampus in both rat and human brain. The present studies demonstrate constitutive expression of P4502E1 in brain, its differential induction in rat brain regions by chronic ethanol treatment, and its topographic distribution in rat and human brain.

Expression of multiple forms of cytochrome P450 and associated mono-oxygenase activities in rat brain regions.

Cytochrome P450 (P450) content and P450-mediated mono-oxygenase activities were measured in microsomes prepared from various regions of rat brain. The regional P450 content in brain varied between 0.1 and 0.15 nmol/mg of protein, with the brainstem and cerebellum showing the highest levels. NADPH cytochrome c reductase activity was highest in the cortex followed by cerebellum and brainstem as compared with the whole brain. Mono-oxygenase activities also varied among the various brain regions. Southern blot analysis of the cDNA synthesized from the poly(A)RNA isolated from rat brain regions and hybridized with cDNA to rat liver P4502B or P4502E1 revealed the presence of a transcript in untreated rat brain that had a molecular mass similar to that of the corresponding transcript from rat liver. Immunoblot analyses using antisera to purified rat liver P4502E1, P450(2B1/2B2), and a phenobarbital-inducible form of rat brain P450 revealed the presence of corresponding immunoreactive protein bands in all the brain regions examined. The present study demonstrated the diversity in the distribution of P450 and associated mono-oxygenase activities in brain and thus may reflect the differential capability of various regions of the brain to detoxify or bioactivate diverse xenobiotics.