Intro to CNS Pharmacology Flashcards
How have proteins evolved in function to allow ionic passage across the membrane?
- ATPase driven pumps
- Transporters
- Ion channels
Ion channels: some generalities
- Integral membrane proteins
- Multiple membrane-spanning domains
- Form a hydrophobic channel in the center
- Selective for ions and regulated by changes in the cellular environment
- Multiple gene products; multiple subunits
- Glycosylated on the extracellular side
- Consensus sequences for kinases
- Exhibit specificity for the ion(s) that permeates the channel
- Ionic movement is driven by its electrochemical gradient
Functional classification of ion channels based upon the gating mechanism:
- Passive: non-gated, always open
-
Active: are gated
- i.e. the closed and open states of the channel are regulated
Some types of gating include:
- membrane potential difference (voltage gated)
- small extracellular molecules (i.e. neurotransmitters)
- other membrane proteins
- e.g. beta-gamma subunits of G proteins
- Intracellular molecules
- e.g. ions, ATP
What is a leak channel?
channel is open at resting membrane potential
- Can be either active or passive
- All passive channels are leak channels
What is the resting membrane potential in neurons?
Em (or Vm) = -60 mV
What factors that give rise to the resting membrane potential?
- Intracellular proteins are predominantly anions
- Leak channels are present in the plasma membranes that allow for potassium and chloride movement across the membrane
- Conductance (g) of the membrane to K is 20 times greater than the conductance to Na
- As a result, there is an unequal distribution of Cl, K and Na across the membrane
Describe the distribution of Na+, K+ and Cl- across the membrane:
-
K+:
- high inside and low outside
-
Na+ and Cl-:
- high outside and low inside
Nernst potentials:
membrane potentials at which the ion is in
electrochemical equilibrium across the membrane
- EK = -75 mV
- ENa = +55 mV
- ECl = -69 mV
What can oppose the leak channels?
Na-K ATPase pump that moves Na ions
out of the cell and K ions into the cell
What causes action potentials in CNS neurons?
voltage operated sodium channels open in the membrane in response to localized depolarization
Action Potentials:
Properties
- Since V=IR, increased sodium current results in change in V
- Voltage gated potassium channels also open
- opening is more gradual
- inactivation is slower than sodium channels
- Action potentials are all or none
- Amplitude of about 100 mV
- 1-10 msec in duration
- Propagated through cycles of depolarization and repolarization
Synaptic potentials:
Properties
- Small, graded potentials that can lead to the initial depolarization that causes an action potential
- Local
- Can summate in time and space
- Only a few mV in size and a few msec in duration
Synaptic potentials:
Two Types
-
Excitatory, postsynaptic potential (EPSP)
- membrane potential becomes more positive
- if it increases enough, threshold will be reached
-
Inhibitory postsynaptic potential (IPSP)
- Membrane potential moves to more negative values
- Impacts a summating EPSP which will now not reach threshold
Two mechanisms by which an EPSP can occur:
-
Increased conductance
- Open a ligand gated ion channel for sodium or calcium
- Nicotinic cholinergic receptor
- Glutamate receptor
- Open a ligand gated ion channel for sodium or calcium
-
Decreased conductance
- Close a leak channel for potassium
- Usually due to changes in the phosphorylation of the channel
- regulated by second messenger cascades; GPCRs
- Close a leak channel for potassium
Mechanisms for the production of IPSPs:
-
Increased conductance of the membrane to either potassium or chloride
- Ligand gated chloride channel
- e.g. GABA receptor
- Ligand gated chloride channel
-
G protein coupled receptor activation can result in the opening of K channels
- Via direct interactions between the channel protein and G protein
- As a result of changes in phosphorylation state of closed K channels
- mediated by second messenger cascades
Norepinephrine:
CNS Distribution & Physiological Roles
- Noradrenergic neurons are located in the medulla oblongata, pons and midbrain
- Called the reticular activating system
- Very important in arousal (wakefulness) and in regulation of autonomic functions like breathing and blood pressure
How is norepinephrine synthesized?
- Precursor: tyrosine
- Tyrosine ⇒ 3,4-dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase
- DOPA is converted into dopamine by DOPA decarboxylase
- low substrate specificity
- Dopamine ⇒ norepinephrine by dopamine beta hydroxylase
- will oxidize almost any phenyl-ethylamine to the corresponding phenylethanolamine
Norepinephrine:
Regulation of synthesis
Primary regulation of norepinephrine synthesis occurs via tyrosine hydroxylase (TH)
- TH is normally saturated with tyrosine so its activity is the rate limiting step for DOPA synthesis under basal conditions
- TH has an essential co-factor tetrahydrobiopterine, BH4
-
Short-term regulation of tyrosine hydroxylase activity occurs via:
- phosphorylation at four different serine residues
-
end-product (i.e. norepinephrine) inhibition of BH4 binding to the enzyme
- detects over-filled vesicles
-
Long-term regulation occurs via new protein synthesis
- primarily increased amounts of tyrosine hydroxylase
Norepinephrine:
Regulation of storage and release
- Norepinephrine is found in vesicles along with the enzyme dopamine betahydroxylase
- Dopamine is taken into the vesicle, the last step of synthesis occurs there
- Vesicular monoamine transporters are called VMAT; VMAT2 is found in the brain
- Three mechanisms of norepinephrine release
- Release is regulated by presynaptic receptors (autoreceptors)
Three mechanisms of norepinephrine release:
- calcium-dependent exocytosis of vesicles
- reversal of plasma membrane transporters
- dendritic release that is not calcium-dependent
What are the autoreceptors that regulate norepinephrine release?
- alpha-2 receptor inhibits release
- beta receptor increases release
Norepinephrine:
Regulation of Inactivation
- All transmitters can be inactivated by diffusion
- Reuptake by the presynaptic neuron is the most important: neuronal not astrocytic
- Enzymatic inactivation
Describe the reuptake of norepinephrine in the presynaptic neuron:
- High affinity carrier proteins move norepinephrine from extracellular to intracellular compartments
- Energy requiring ⇒ sodium co-transporter
- Binding site for norepinephrine that is the site of action of inhibitors
- Once norepinephrine is intracellular, it can be re-packaged
Describe the enzymatic inactivation of norepinephrine:
-
Monoamine oxidase (MAO)
- deaminates norepinephrine
- two forms: MAOA and MAOB
- MAOB is found in the brain
-
Catechol-O-methyl-transferase (COMT)
- transfers a methyl group from S-adenosylmethionine to the meta OH of norepinephrine
- These two can act on the same molecule, in either order
Norepinephrine:
Receptors
- All of the receptors for norepinephrine are G protein coupled
- **alpha and beta designation **
Norepinephrine:
alpha receptors
bind norepinephrine slightly better than epinephrine
-
alpha 1 subtypes:
- predominately post-synaptic in the periphery
- tend to function as excitatory receptors:
- increase calcium
-
alpha 2 subtypes:
- presynaptic, release modulating alpha receptor is alpha2
- inhibit adenylyl cyclase
Norepinephrine:
beta receptors
bind epinephrine better than norepinephrine
- unlike the alpha receptor subtypes, the beta subtypes are all very similar to each other with regard to function and signal transduction mechanisms
- All beta receptors couple to an increase in adenylyl cyclase activity
- What does differ is the tissue distribution of these receptor subtypes
What is an example of a neuroactive peptide?
neurotensin
Neurotensin:
Nervous system distribution & physiological roles
- Neurotensin is a 13 amino acid peptide
- Found in the prefrontal cortex (limbic cortex) and in the hypothalamus and midbrain
- Always is co-localized with the dopamine
- Peptides in general are co-localized with other neurotransmitters and affect and are affected by these NTs:
- Therefore, it is hypothesized that the neuropeptides add complexity, (“color”) to the simple signaling that could occur with a single transmitter
Neurotensin:
Regulation of synthesis
- Gene encoding a precursor to neurotensin is transcribed, spliced and translated by ribosomes
- Precursor: neuromedin N
- Packaged at the Golgi into secretory granules
- Granules also contain peptidases that process the precursor protein into neurotensin
- Granules are dense core
Neurotensin:
Regulation of storage and release
- Stored in different type of vesicles: dense core
- Transported to the terminals via axonal transport
- Concentrations at the terminal are low and can be variable
- Release is calcium dependent, but require high intensity, rapid firing of neurons to be released
-
Release is not necessarily at the active zone
- can be released anywhere along the membrane
How is neurotensin inactivated?
- Nonselective peptidases cleave the peptide
- Diffusion
-
No reuptake by transporters
- However, peptide bound to receptor can be internalized along with the receptor
- As a result, peptides tend to “last longer”
- more persistent at receptors than norepinephrine
Neurotensin:
Receptors
G protein coupled
- Neuropeptide is large compared to glutamate, its binding results in many bonds being formed between the receptor and ligand
- Results in a higher affinity binding
- (i.e. receptor activation occurs at lower concentrations of neurotensin compared to glutamate or NE
- This could compensate for the lower amounts released and the distance needed to travel
