Catecholamines

I. Classification

A. Dopamine
B. Norepinephrine
C. Epinephrine

II. Distribution

A. Norepinephrine
1. Peripheral Location
Transmitter in postganglionic sympathetic neurons
2. Central Distribution
Cell bodies are present in pons and medulla; axons descend to innervate various levels of the spinal cord (spinal motor neurons, sympathetic preganglionic cells). Axons ascend to innervate vast region of the brain.
Dorsal projection: locus coeruleus (A6 cell group to ctx, cbl, hipp, olf bulb.
Ventral projection: Tegmental nuclei (A1 cell group) and dorsla medularry group (A2) project foreward to hypothalamus primarily; also innervate central autononomic nuclei
NOTE: There are no NE projections to telencephalic basal ganglia structures

B. Dopamine

1. Peripheral location: in SIF cells in sympathetic ganglia
2. Central distribution Cell bodies in midbrain
A9 cells in substantia nigra project to neostriatum and nuc. accumbens
A10 cells (more medial to A9) project to limbic cortex, hipp.
Tuberoinfundibular, arcuate nuc. in hypothalamus to median eminence

C. Epinephrine

Projection of C1 and C2 cell groups in medulla similar to the A1 and A2 NE projections.

II. Synthesis

A. Begins with uptake of the amino acids tyrosine from the circulation via carrier mediated transport at level of capillary endothelial cell. Tyrosine is produced in the liver from phenylalanine. Must compete with other large, neutral Amino acids for the endothelial carrier.
B. Steps in Synthetic Pathway
1. Initial step is hydroxylation of tyrosine by tyrosine hydroxylase producing DOPA
2. Dopa is decarboxylated to DA via nonspecific decarboxylase enzyme.
cDNA for amino acid decarboxylase has been cloned and protein produced acts on several substrates. mRNA formed from cDNA hybridizes with mRNA in both CA and 5-HT containing cells. Production of DA is a two-step process. DA is produced in cytoplasma, must be pumped into vesicles if it is to serve as a transmitter.
3. In non-Dopaminergic neurons, DA is converted to NE via DBH, which is associated with membrane of synaptic vesicle.
4. In adrenal medulla chromaffin granu;es and in select neurons in brain, NE is converted to epinephrine via PNMT.
5. All enzymes require cofactors
6. In some neurons, multiple enzymes will be expressed. Ty-OH will be present in all CA-containing neurons.

C. Rate-limiting step

1. Because there are multiple steps to synthesis of these amines, the limiting step must be identified.
2. Ty-OH is rate limiting in CA synthesis

                Km x 10-5M         Vmax
      Ty-OH   5              150
      AADC   40           33,000
      DBH         580             50,000

Ty-OH has highest affinity but Vmax is limiting. Reasons:

a. Not due to substrate availability
Km of tyr for enzyme is 5 x 10-5M
(Tyr) ~ 10-4M
b. Availability of tetrahydrobiopterin cofactor is limiting

 D. Regulation of synthesis to maintain transmitter availability 1. End product inhibition a. Most effective in NE neurons
Clear that NE feeds back onto Ty-OH and competes with PtH4 for enzyme. 2. Enzyme activation linked to neural activity a. CA levels stay constant over range of neural activity
b. Neural stimulation of CA neurons has been shown to increase Ty-OH activity.
Increase associated with increase in affinity for PtH4 cofactor (decrease Km) and decrease in affinity for transmitter (increase in Ki) thus reducing feedback inhibition. Kinetic changes probably brought about by phosphorylation perhaps mediated by Ca++ or other second messengers. 3. Long term activation leads to enzyme induction a. Prolonged stimulation or certain drug treatment leads to production of new molecules of enzyme in cell body and transport to terminals; considered trans-synaptic mechanism
b. Enzyme induction preceded by increase in mRNA.

III. Inactivation of transmitter action

A. CA are cleared from synaptic cleft mainly by reuptake of transmitter back into the presynaptic terminal via a Na+ and energy dependent process. Energy requirement probably maintains sodium gradient across cells rather than drives carrier mechanism. Reaccumulated transmitter can be repackaged for release.
B. Neither extracellular enzymatic degradation nor diffusion play a major role in regulating synaptic levels of transmitter. IV. Enzymatic degradation A. Does not terminate synaptic action of released transmitters
B. Intracellular degradation by monoamine oxidase (mainly MAOA) localized to outer membrane of mitochondria. Threat to cytoplasmic amines.
C. Extracellular degradation by COMT; more prominent in periphery than in CNS.
D. Serves to prevent oversupply of transmitter V. Vesicular Storage A. Serves Two Functions
B. Protects amines from enzymatic degradation thus making transmitter available for release
C. Catecholamine uptake into vesicles is ATP dependent; pump mechanism is not selective, thus other amines can compete with CAs. CA exist in complex with ATP and chromogranins (proteins).
E. Different populations of vesicles
1. Large dense core vesicles contain DBH activity and synthesize NE; newly synthesized transmitter is released into cytoplasm where it is taken up and stored in SDV.
2. SDV responsible for releasing NE under physiologic conditions. LDV may become important under demanding conditions; then DBH will be released. LDV may also store peptides.

VI. Release

A. Calcium dependent
B. Related to firing rate of neuron
C. Vesicles mediate transmitter release 1. Upon depolarization of nerve terminal, Ca++ enters cell and this leads to fusion of vesicles with cell membrane. When CA terminals depolarized, NE, ATP as well as DBH may be discharged; the release of DBH with NE establishes that release is by exocytosis, since proteins would not be expected to diffuse across membrane. D. Influenced by presence of autoreceptors on presynaptic terminal than can either facilitate or inhibit release, thereby sculpturing release. Certain DA projections do not have autoreceptors (mesolimbic).
Last modified: 9/10/97