Brain organic cation transporter 2 controls response and vulnerability to stress and GSK3β signaling

Interactions between genetic and environmental factors, like exposure to stress, have an important role in the pathogenesis of mood-related psychiatric disorders, such as major depressive disorder. The polyspecific organic cation transporters (OCTs) were shown previously to be sensitive to the stress hormone corticosterone in vitro, suggesting that these transporters might have a physiologic role in the response to stress. Here, we report that OCT2 is expressed in several stress-related circuits in the brain and along the hypothalamic-pituitary-adrenocortical (HPA) axis. Genetic deletion of OCT2 in mice enhanced hormonal response to acute stress and impaired HPA function without altering adrenal sensitivity to adrenocorticotropic hormone (ACTH). As a consequence, OCT2(-/-) mice were potently more sensitive to the action of unpredictable chronic mild stress (UCMS) on depression-related behaviors involving self-care, spatial memory, social interaction and stress-sensitive spontaneous behavior. The functional state of the glycogen synthase kinase-3β (GSK3β) signaling pathway, highly responsive to acute stress, was altered in the hippocampus of OCT2(-/-) mice. In vivo pharmacology and western blot experiments argue for increased serotonin tonus as a main mechanism for impaired GSK3β signaling in OCT2(-/-) mice brain during acute response to stress. Our findings identify OCT2 as an important determinant of the response to stress in the brain, suggesting that in humans OCT2 mutations or blockade by certain therapeutic drugs could interfere with HPA axis function and enhance vulnerability to repeated adverse events leading to stress-related disorders.

A wireless beta-microprobe based on pixelated silicon for in vivo brain studies in freely moving rats

The investigation of neurophysiological mechanisms underlying the functional specificity of brain regions requires the development of technologies that are well adjusted to in vivo studies in small animals. An exciting challenge remains the combination of brain imaging and behavioural studies, which associates molecular processes of neuronal communications to their related actions. A pixelated intracerebral probe (PIXSIC) presents a novel strategy using a submillimetric probe for beta(+) radiotracer detection based on a pixelated silicon diode that can be stereotaxically implanted in the brain region of interest. This fully autonomous detection system permits time-resolved high sensitivity measurements of radiotracers with additional imaging features in freely moving rats. An application-specific integrated circuit (ASIC) allows for parallel signal processing of each pixel and enables the wireless operation. All components of the detector were tested and characterized. The beta(+) sensitivity of the system was determined with the probe dipped into radiotracer solutions. Monte Carlo simulations served to validate the experimental values and assess the contribution of gamma noise. Preliminary implantation tests on anaesthetized rats proved PIXSIC's functionality in brain tissue. High spatial resolution allows for the visualization of radiotracer concentration in different brain regions with high temporal resolution.

Neurochemical characterization of pathways expressing plasma membrane monoamine transporter in the rat brain

Neurotransmitter transporters play an important role in the control of synaptic transmission by ensuring the clearance of transmitters liberated in the synaptic cleft. In the case of monoaminergic neurotransmitters, this clearance is carried out by high-affinity reuptake transporters located in the plasma membrane of the presynaptic terminals. Recently plasma membrane monoamine transporter (PMAT), a transporter from the SLC29 (equilibrative nucleoside transporter) family, was shown to transport in vitro monoaminergic neurotransmitters, in particular dopamine and serotonin, nearly as efficiently as the high-affinity transporters. This transporter, well expressed in CNS, represents an interesting candidate for the control and modulation of aminergic pathways. We performed an extensive study of the distribution of PMAT in the rat brain. Our results highlight PMAT expression in brain regions which play a pivotal role in significant CNS functions and human neuropathologies. Using in situ hybridization immunohistochemistry co-labeling, PMAT mRNA was found in various neuron subtypes, including glutamatergic neurons of the hippocampus, mitral cells of the olfactory bulbs and GABAergic neurons in the substantia nigra pars reticulata and hypothalamus. Paradoxically, rat PMAT mRNA was found in some but not all monoaminergic nuclei. It was on the contrary predominantly expressed in major cholinergic groups throughout the brain, including brainstem motor nuclei, components of the basal forebrain cholinergic system and cholinergic interneurons of the striatum. These systems, implicated in locomotion, associative and spatial memory and reward-related learning, are disrupted at early stages of Parkinson's and Alzheimer's disease. Taken together, our observations support a role for PMAT in monoamine uptake in cholinergic neurons.