IMPACTOS DE LAS EMISIONES DE LOS MOTORES DIESEL

Densely packed neuronal and glial cells are interconnected in systems and form a net: the cen- tral nervous system (CNS) - with over 1015synaptic and even more extrasynaptic connections. The CNS is an active assembly of cells that continously receives information, analyzes it, per- ceives it and makes decisions. In the brain, information is communicated between neurons in the form of electrical signals. This communication occurs at highly specialized sites of contact between a presynaptic nerve terminal and a postsynaptic neuron – the synapse.

A typical neuron has four morphologically defined regions: the dendrites, the cell body, the axon and the presynaptic terminals. The branched dendrites are the main apparatus for re- ceiving incoming signals from other cells. In contrast, the usually long axon is the main conducting unit for carrying signals to other neurons or target organs. The information is transmitted in the form of chemical transmitters released from synaptic vesicles in the presy- naptic terminal of the axon into the synaptic cleft (v25 nm wide). After rapid diffusion across the synaptic cleft, the neurotransmitters bind to their specific receptors (transmitter-gated ion channels) in the postsynaptic membrane of the dendrite, where the message is processed, inte- grated and propagated. Depending on the kind of transmitter and the ion channel, an electric signal can cause an inhibitory or excitatory synaptic input in the target cell. The postsynaptic site of many synapses is specialized in a thorny-like protrusion called a spine (Cajal, 1888).

4.1.1. Glutamate receptors

The amino acid glutamate is the most important and prevalent excitatory neurotransmitter in the mammalian brain and spinal cord and acts through the binding on different types of glutamate receptors: the ionotropic receptors and the metabotropic receptors. Ionotropic re- ceptors form ion channels (glutamate-gated cation channels) and are the most common of all transmitter-gated channels in the brain. According to their different sensitivities to glutamate analogues and sequence similarities, they can be divided into three functionally distinct sub- classes: N-methly-D-aspartate (NMDA) receptors, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors and kainate receptors (Hollmann and Heinemann, 1994; Dingledine et al., 1999). The channels are multimeric complexes composed of a combinatorial assembly of two to four subunits. These subunits NR1 and NR2A through NR2D are involved in the formation of NMDA receptors. AMPA receptors are assembled from another set: GluR1 through GluR4 (GluRA-GluRD) and kainate receptors are combinations of the subunits KA1- KA2 and GluR5-GluR7 (Wisden and Seeburg, 1993; Bettler and Mulle, 1995). The subunit com-

4.1. The synapse

position of glutamate receptors varies, creating great structural and functional heterogeneity. This heterogeneity is further increased by alternative splicing and RNA editing. AMPA re- ceptors in the adult hippocampus are composed of mainly GluR2 with either GluR1 or GluR3 (Wenthold et al., 1996). During postnatal development, NMDA receptor subunit composi- tion changes from mainly NR1/NR2B to mainly NR1/NR2A, which has a significant effect on the time course of the NMDA-mediated excitatory postsynaptic potential (EPSC) (Flint et al., 1997).

Postsynaptic AMPA receptors mediate the majority of the fast excitatory synaptic transmis- sion in the CNS. Kainate receptors contribute to the postsynaptic responses at some excitatory synapses (Castillo et al., 1997; Vignes et al., 1998) and can also modulate presynaptic neuro- transmitter release (Chittajallu et al., 1996; Contractor et al., 2000). These non-NMDA recep- tors open for Na+ _{or K}+ _{influx. The postsynaptic NMDA receptors are unique among the}

ionotropic channels in that they are in addition voltage-dependent. At resting membrane po- tentials, NMDA receptors are blocked by extracellular magnesium-ions and are only relieved from this Mg2+ block when the membrane is depolarized. Thus, both membrane depolariza- tion and glutamate binding are needed to open the NMDA receptors for Ca2+influx (= coin- cidence receptors), which contributes only to the late component of the EPSP. These inflow- ing calcium-ions can activate downstream calcium-dependent signal transduction processes, thereby modulating the excitatory synaptic transmission of the neuron. The metabotropic glutamate receptors link glutamate release to GTP-binding (G) protein-mediated signalling cascades in the postsynaptic membrane (Pin and Duvoisin, 1995).

4.1.2. The postsynaptic density

Glutamate receptors are an essential component of the postsynaptic density (PSD). Ultrastruc- tural studies of this disk-shaped postsynaptic membrane undercoat in excitatory synapses have revealed an electron-dense thickening that marshals more than seventy proteins (Husi et al., 2000; Walikonis et al., 2000). The PSD has been proposed to be a protein lattice that localizes and organizes the various ion channels, receptors, scaffold and adaptor proteins, cy- toskeleton proteins, kinases, phosphatases and other signalling proteins (Pawson and Scott, 1997; Ziff, 1997; Kennedy, 1998). Many of these proteins contain postsynaptic density-95/discs- large/zona occludens 1 (PDZ)-domains, which are protein-interaction domains that bind to the PDZ-binding motif usually at the cytoplasmic carboxyl (C-)termini of membrane proteins, including ionotropic glutamate receptors, Eph receptors, neuroligins, stargazin, thereby clus-

tering and anchoring proteins at high density in the PSD (Doyle et al., 1996; Songyang et al., 1997; Scannevin and Huganir, 2000; Sheng, 2001). Structurally, proteins in the PSD are in close proximity to glutamate receptors and other signalling proteins, which allow rapid responses of the synapse once glutamate has been released (Kennedy, 1998). The PDZ scaffold proteins are well-positioned to link the postsynaptic membrane with the underlying actin cytoskeleton and couple spine morphology and actin dynamics to postsynaptic receptor activation (Figure 4.1).

Figure 4.1. Ultrastructure of the synapse. (A) The PSDs (red asterisks) of chemical synapses are darkly stained and are in contrast with all other, much lighter structures in the mouse neocortex. Scale bar = 1 µm. (Ref. http://synapses.mcg.edu/atlas/1_6_1.stm) (B) Electron micrograph of a synapse between an

axon terminal (AR) and a dendritic spine (D) with polyribosomes (red arrow) and microtubules (blue arrows) present in the subsynaptic cytoplasm. The presynapse contains numerous synaptic vesicles. The PSD is vis- ible as a thick membrane undercoat postsynaptically in the rat hippocampus. Scale bar = 200 nm. (Ref. http://synapses.mcg.edu/atlas/1_6_37.stm)

The best characterized of the PDZ scaffold proteins is postsynaptic density-95 (PSD-95) pro- tein, an abundant component of the PSD. This multidomaine scaffold protein is capable of forming a macromolecular signalling complex by binding to subunits of the NMDA receptor and other molecules (Scannevin and Huganir, 2000; Sheng, 2001). Overexpression of PSD-95 promotes spine growth in cultures (El-Husseini et al., 2000) and potentiates AMPA receptor- mediated EPSCs in brain slices (Béïque and Andrade, 2003; Stein et al., 2003; Ehrlich and Malinow, 2004). The synaptic potentiation induced by the ectopical expression of PSD-95 in brain slices seems to mimic long-term potentiation (LTP), in that it converts silent synapses