Neuron metabotropic receptors

The excitatoiy amino acids (EAA), glutamate and aspartate, are the principal excitatory neurotransmitters in the brain. They are released by neurons in several distinct anatomical pathways, such as corticofugal projections, but their distribution is practically ubiquitous in the central nervous system. There are both metabotropic and ionotropic EAA receptors. The metabotropic receptors bind glutamate and are labeled mGluRl to mGluRB. They are coupled via G-proteins to phosphoinositide hydrolysis, phospholipase D, and cAMP production. Ionotropic EAA receptors have been divided into three subtypes /V-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA), and kainate receptors (Nakanishi 1992). 

FIGURE 2.3 Glutamatergic synapse. Glutamate binds to ionotropic receptors (1) and metabotropic receptors (2). The glutamate transporter (3) pumps glutamate back into the glutamatergic neuron. 

Whereas membrane-delimited actions occur within microdomains in the membrane, second messenger-mediated effects can occur over considerable distances. Finally, an important consequence of the involvement of G proteins in receptor signaling is that, in contrast to the brief effect of ionotropic receptors, the effects of metabotropic receptor activation can last tens of seconds to minutes. Metabotropic receptor predominate in the diffuse neuronal systems in the CNS (see below). 

Information transfer between two neurons or between neurons and effector cells involves the release of chemical substances, which then act on the target cell by binding to appropriate receptors embedded in the plasma membrane. This process, as originally described by Otto Loewi (Loewi 1921), is termed chemical neurotransmission and occurs at contact sites known as synapses. Neurotransmitters exert their effects via members of two major families of receptors ionotropic and metabotropic neurotransmitter receptors. Activation of ionotropic receptors leads to an increase in the ion conductance of the membrane within a time scale of milliseconds or even less, whereas activation of metabotropic receptors results in slow effects (within seconds or even minutes) which involve more or less complex signaling cascades. Accordingly, information transfer via ionotropic receptors is called fast synaptic transmission, whereas the slow counterpart is called neuromodulation (Kaczmarek and Levitan 1987). 

All receptors for NE (adrenoceptors) are G-protein-coupled metabotropic receptors. However, they vary by their location (presynaptic autoreceptors and post-synaptic hetreroreceptors), pharmacology (three families au a2 and (3 with three subtypes in each family a1A/B/D, ot2A/B/D and P1/2/3), signal transduction (GQ, i -rx, family, Gra2, and Gs-(3), and physiology (excitatory of inhibitory effect on the postsynaptic neuron). 

Accumulating findings indicate that nucleotides play an important role in neuron-to-glia communication through P2 purinoceptors. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (seven types P2X]-P2X7) contain intrinsic pores that open... 

Kawasaki, Y., Kohno, T., Zhuang, Z. Y., Brenner, G. J., Wang, H., Van Der Meer, C., Befort, K., Woolf, C. J., and Ji, R. R. (2004). Ionotropic and metabotropic receptors, protein kinase A, protein kinase C, and Src contribute to C-fiber-induced ERK activation and cAMP response element-binding protein phosphorylation in dorsal horn neurons, leading to central sensitization. J. Neurosci. 24, 8310-8321. 

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