5.4: Receptors

Receptors are proteins that are capable of sending a signal to change the function or activity of a neuron. Most receptors that function in neurotransmission are large transmembrane proteins. On the extracellular surface is a specific series of amino acid residues called the active site. The active site, also called the orthosteric site, is shaped to allow molecules of neurotransmitter to bind to the receptor.

Figure 5.11 Ionotropic receptors (top) allow ion movement after receptor binding, while metabotropic receptors (bottom) trigger second messengers to induce signaling.

Receptors are classified into one of two main categories.

1. Ionotropic receptors. Physically, ionotropic receptors are transmembrane proteins with a large-diameter pore through which ions can pass. These channels only open when a molecule of chemical binds to the active site on the extracellular side of the protein. These chemicals are also called ligands, and so ionotropic receptors are also called ligand-gated ion channels. Once a neurotransmitter activates the ionotropic receptor, ions will move based on the electrochemical gradient for that ion. As a result of ion movement, the cell’s membrane potential will change. For example, nicotinic acetylcholine receptors are ligand-gated sodium channels, so when these receptors are activated by a ligand like acetylcholine, sodium ions enter into the cell causing depolarization and excitation.

Ionotropic receptors are able to induce a change in membrane potential very rapidly, on the scale of milliseconds.

Due to the nature of the amino acid residues that make up the pore of ionotropic receptors, they can be very selective for certain ions. For example, negatively charged residues lining the inside of the pore repel negatively charged Cl- ions while allowing positively charged cations to pass through the channel.

2. Metabotropic receptors. These receptor complexes cause the cell to change its metabolism in a way that leads to either excitation or inhibition. Ions do not pass through these receptors. Instead, metabotropic receptors use the actions of G proteins, proteins which induce changes in neuronal excitability through the action of second messenger signaling molecules.

Physically, metabotropic receptors are transmembrane proteins that contain 7 alphahelix motifs that pass through the cell membrane. The N-terminus of the protein is extracellular, and the protein “weaves” back and forth across the cell membrane, resulting in a protein with three extracellular loops and three intracellular loops. Because of this shape, these receptors are also called seven-transmembrane receptors (7-TM receptors.)

Another name for these receptors is G protein-coupled receptors (GPCRs). Metabotropic receptors are physically linked to proteins called G proteins, which exist on the inner surface of the cell membrane. Functionally speaking, these G proteins are capable of binding to molecules of guanosine triphosphate (GTP) or guanosine diphosphate (GDP). Chemically similar to ATP, GTP can function as a source of energy. G proteins themselves exhibit catalytic activity of GTP. This means that they are capable of breaking down GTP into the less-energetic GDP. When GTP is bound to the GPCR, the receptor is active. When this molecule is hydrolyzed into GDP, the receptor becomes inactive.

Some G proteins are heterotrimeric, meaning that they are made up of three different subunits, alpha, beta, and gamma. The GTP binding sites and catalytic sites are found on the alpha subunit, the largest of the three subunits. Usually, the alpha subunit becomes soluble after activation, while the beta / gamma complex stays embedded in the neuronal membrane. The G alpha subunit exists in different varieties.

Figure 5.12 GPCRs signal via activation of the G protein attached to the intracellular side of the receptor.

\(\mathbf{G_{\alpha \mathrm{s}}}\) When a neurotransmitter activates a GPCR coupled with the \(\mathrm{G}_{\mathrm{\alpha s}}\) protein, the \(\mathrm{G}_{\mathrm{\alpha s}}\) protein is excitatory (The “s” stands for stimulatory).

Binding of a ligand to the active site of Galphas-coupled GPCRs results in increased activity of the enzyme adenylate cyclase (AC). AC itself is an enzyme that creates a second messenger, a molecule called cyclic AMP (cAMP). Elevated levels of cAMP activate an enzyme called protein kinase A (PKA).

Figure 5.13 GPCRs that are coupled with \(\mathrm{G}_{\mathrm{\alpha s}}\) are excitatory through adenylate cyclase signaling.

PKA is a kinase, an enzyme that phosphorylates other proteins. The addition of a phosphate group onto a protein changes its properties dramatically. PKA phosphorylates protein targets that increase cell excitation. For example, one target of PKA activity is the intracellular side of certain glutamate receptors. Phosphorylation causes these receptors to stay open longer than normal when they are activated by a molecule of glutamate. This means that a single molecule of glutamate causes more excitation (passes more depolarizing current into the cell) in the presence of increased PKA activity.

Targets of PKA activity also include the intracellular store of glutamate receptors. When phosphorylated, these receptors are trafficked to the neuronal membrane. An increase of receptors at the postsynaptic side leads to increased excitatory neurotransmission over a period of minutes and hours, representing one form of plasticity.

An even longer-term action of PKA is its ability to change the transcription of various genes, which can trigger the synthesis of proteins. Some genes downstream of PKA activity include the structural protein actin, important for morphological changes in neuronal structure, or ion channels which change neuronal responses to neurotransmitter release.

\(\mathbf{G_{\alpha \mathrm{i}}}\). A GPCR that is coupled with \(\mathrm{G}_{\mathrm{\alpha i}}\) causes a decrease in excitability. In many ways, \(\mathrm{G}_{\mathrm{\alpha i}}\) proteins serve the opposite function as \(\mathrm{G}_{\mathrm{\alpha s}}\) proteins - the “i” stands for inhibitory.

Whereas activation of \(\mathrm{G}_{\mathrm{\alpha s}}\) increases the action of AC, \(\mathrm{G}_{\mathrm{\alpha i}}\) proteins decrease AC activity. Therefore, \(\mathrm{G}_{\mathrm{\alpha i}}\) activation decreases the intracellular concentration of cAMP, in turn decreasing PKA activity. Given the function of PKA as a kinase that increases cellular excitation as described above, decreased PKA activity inhibits cellular activity through multiple mechanisms, some of which include decreased current through glutamate receptors, decreased trafficking of glutamate receptors to the postsynaptic neuronal membrane, and decreased transcription of certain genes.

\(\mathbf{G_{\alpha \mathrm{q}}}\). Generally, \(\mathrm{G}_{\mathrm{\alpha q}}\) is an excitatory G protein. \(\mathrm{G}_{\mathrm{\alpha q}}\) uses a different signaling pathway compared to the PKA pathway that is downstream of \(\mathrm{G}_{\mathrm{\alpha s}}\) or \(\mathrm{G}_{\mathrm{\alpha i}}\). \(\mathrm{G}_{\mathrm{\alpha q}}\) protein activation leads to activity of the enzyme phospholipase C (PLC).

On a biomolecular level, PLC acts on the phospholipid membrane molecule phosphatidylinositol 4,5-bisphosphate (PIP2). PLC is a hydrolytic enzyme, and it breaks PIP2 into two separate second messenger molecules: the soluble inositol triphosphate (IP3) and the membrane-embedded diacylglycerol (DAG). One of the functions of IP3 is to liberate Ca²⁺ from intracellular stores, which elevates intracellular Ca²⁺ levels, depolarizing the cell and activating calcium-dependent processes, which are often excitatory. DAG activates protein kinase C (PKC), an enzyme with substrates that increase neurotransmitter release probability or decrease potassium channel conductance.

While the alpha subunits carry out a large part of GPCR-mediated changes in cellular excitation, the beta and gamma subunits also affect excitability. The beta and gamma subunits are bound together as a dimer that separates from the alpha subunit once the GPCR becomes activated. The beta-gamma complex can also function as a signaling molecule.

Compared to ionotropic receptors, metabotropic receptors affect neuron activity on a slower time scale, on the range of milliseconds to seconds, and possibly even longer depending on the downstream mechanisms that are activated.

Presynaptic receptors

In the discussion of receptors, it is common to think of receptors as being expressed at the dendrites, embedded within the postsynaptic membrane. However, not all receptors are located here. Some receptors are presynaptic, meaning they can be found at the axon terminal. Presynaptic receptors are often inhibitory and serve a self-regulatory function. Presynapticallyexpressed receptors that respond to the same neurotransmitter that is released are called autoreceptors.

Figure 5.14 \(\mathrm{G}_{\mathrm{\alpha q}}\) signals using PLC, which then produces two signaling molecules, IP3 and DAG.