Receptors Flashcards
neurotransmitter
-effect of a NT depends on the kind of channel that opens when the NT binds to a receptor
ion channels
- some allow positive ions (Na+ or Ca+) in, allowing an excitatory effect
- some allow negative ions (Cl-) allowing an inhibitory effect
ionotropic receptors
Ionotropic receptors are transmembrane molecules that can “open” or “close” a channel that would allow smaller particles to travel in and out of the cell. As the name implies, Ionotropic receptors allow different kinds of ions to travel in and out of the cell.
Ionotropic receptors are not opened (or closed) all the time. They are generally closed until another small molecule (called a ligand — In our case, a neurotransmitter) binds to the receptor.
As soon as the ligand binds to the receptor, the receptor changes conformation (the protein that makes up the channel changes shape), and as they do so they create a small opening that is big enough for ions to travel through.
Therefore, ionotropic receptors are “ligand-gated transmembrane ion channels”.
The ions that can travel through ionotropic receptors are generally limited to K+, Na+, Cl-, and Ca2+.
Metabotropic receptors
Metabotropic receptors do not have a “channel” that opens or closes. Instead, they are linked to another small chemical called a “G-protein.”
As soon as a ligand binds the metabotropic receptor, the receptor “activates” the G-Protein (it basically changes the G-Protein). Once activated, the G-protein itself goes on and activates another molecule. This new molecule is called a “secondary messenger.”
(A secondary messenger is a chemical whose function is to go and activate other particles).
So far, this process is common to all metabotropic receptors. What happens from there on is different for every metabotropic receptor.
In some cases, the secondary messenger travel until it binds to and opens ion channels located somewhere else on the membrane.
In some cases, the secondary messenger will go and activate other intermediate molecules inside the cell.
The important thing to remember is that metabotropic receptors do not have ion channels, and binding of a ligand may or may not result in the opening of ion channels at different sites on the membrane. But they will always activate a g-protein that will in turn activate secondary messengers.
agonist
any NT that facilitates the actions of a receptor (whether excitatory or inhibitory)
antagonist
any NT that diminishes the actions of a receptor (whether excitatory or inhibitory)
Partial agonist
a ligand that does not fully activate the receptor.
In some cases this can have a facilitating effect (some action is better than no action).
In other situations, such as when it blocks access to the receptor for a full agonist, the net effect is to reduce the overall activity of the receptor from what it could maximally be.
inverse agonist
activates a receptor but produces the opposite effect to that which it normally produces (e.g., hyper-activity versus sedation).
opioid receptors
Another example would be the opioid receptors. There are at least 3 different types of opioid receptors, that is, receptors to which opiates can bind: mu, delta, and kappa receptors.
Each type of receptor has different effects. For example, the mu and delta receptors are involved in reducing the activity of neurons that convey information about pain. They reduce pain by directly inhibiting the neurons that convey pain information (i.e., opening Cl- channels or allowing potassium to exit the cell via K+ channels).
Opioids also work by inhibiting the action of inhibitory neurons that release GABA. This is called disinhibition, and refers to the action of “taking off the brakes”.
dopamine
- stimulates the nucleus accumbens
- produced in the substantia nigra & ventral tegmental area
dopamine pathways: mesolimbic
– VTA to nucleus accumbens, then to prefrontal cortex. Important for reward and pleasure.
Disorders:Volkow and colleagues have argued that all addictive behaviors probably activate the mesolimbic DA pathway, although she and colleagues have also proposed a more elaborate model of addiction (e.g., Volkow, Fowler, & Wang, 2003)
dopamine pathways: tuberoinfundibular
dopamine is released from the hypothalamus into the pituitary gland. Regulates / inhibits production of prolactin (lactogenesis).
dopamine pathways: mesocortical
VTA to prefrontal and other cortical areas directly. Important in thinking.
Disruptions in this pathway can result it symptoms relating to schizophrenia and ADHD.
dopamine pathways: nigrostriatal
– substatia nigra (in the midbrain) to the striatum (basal ganglia).Involved in regulation of motor functions and procedural learning).
Disorders: These include neurological disorders affecting movement such as Parkinson’s disease and Huntington’s disease
