Types Of Receptors And Potential Sites Of Plasticity

Neurotransmitters can be broadly categorized into two types, based on the types of receptors they bind to and their effects on the postsynaptic cell. Some neurotrans-mitters directly mediate neuron-neuron communication by binding to and opening ligand (neurotransmitter)-gated ion channels. A second major category generally serves a more subtle role—modulating neuronal function by eliciting intracellular second-messenger generation.

The first type of receptors form neurotransmitter-regulated pores that can open upon binding of neurotransmitter, allowing ions to flux across the cell membrane and resulting in an electrical change (generally depolarizing or hyperpo-larizing) in the cellular membrane. Neurotransmission of this sort is typically how one neuron excites (or inhibits) another follower neuron within a neuronal circuit. The two predominant types of ligand-gated ion channels in the hippocampus and elsewhere in the CNS bind glutamate (excitatory) or gamma-amino-butyric acid (GABA, inhibitory). The major glutamate receptor subtype is named for a selective agonist at this receptor, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (mercifully abbreviated AMPA). The AMPA subtype of receptors are glutamate-gated cation channels that when opened lead to membrane depolarization. Another subtype of glutamate receptor that is very similar is activated by kainic acid (KA, kainate), and this subtype is referred to as the kainate receptor. The typical EPSP in a hippocampal neuron is mediated by ion flux through the AMPA subtype of glutamate receptor. Inhibitory postsynaptic potentials (IPSPs) are mostly mediated by the GABA-A subtype of GABA receptors.

GABA-A receptors are GABA-gated chloride channels. Opening these channels moves the membrane potential in a negative direction, toward the hyperpolarized chloride ion equilibrium potential. This tends to hyperpolarize the membrane and clamp it there.

The second major category of neuro-transmitter receptor doesn't directly produce electrical changes in the postsynaptic neuron but rather elicits biochemical changes within the postsynaptic neuron. Typically, these types of neurotransmitters couple to second-messenger-generating enzymes that can lead to alterations in a wide variety of cellular chemical processes. These types of neurotransmitters are referred to as modulatory because their effects typically (but by no means exclusively) sculpt and fine-tune the electrical and cellular responses to the neurotransmitters that open ligand-gated ion channels. Almost all neurotransmitters have specific subtypes of receptors that act in this fashion, including specific receptors for glutamate, GABA, norepinephrine, dopamine, serotonin, and acetylcholine, in addition to a large number of different neuropeptides.

The second-messenger-generating enzymes that these modulatory neuro-transmitter receptors couple to are also quite diverse. A partial listing includes adenylyl cyclase, which makes cyclic AMP (cAMP) and activates the cAMP-dependent protein kinase (PKA); phospholipase C (PLC), which makes diacylglycerol (DAG) and activates protein kinase C (PKC); PLC also makes inositol tris-phosphate (IP3), which mobilizes intracellular calcium and can activate the calcium/calmodulin-dependent protein kinase type II (CaMKII);

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