Neurotransmitters

Neurotransmitters:
Chemicals substance for communication between neurons.

Acetylcholine (ACh):
major transmitter in the peripheral nervous system
released by all neurons that stimulate skeletal muscles and some in ANS and CNS
binds to post-synaptic receptors briefly – released and degraded to acetic acid and choline by the enzyme acetylcholinesterase. Choline is recaptured by presynaptic terminal  resynthesised to acetylcholine
excitatory to skeletal muscle or inhibitory to visceral effectors (depends on receptors bound)

Biogenic amines:
includes the catecholamines (dopamine, norepinephrine, epinephrine) and the indolamines (serotonin and histamine)
broadly distributed in brain  play role in emotional behaviours and regulation of the biological clock
catecholamines are released by some motor neurons in ANS

Amino acids:
include gamma-aminobutyric acid (GABA), glycine, aspatate and glutamate
so far only reported in CNS; glycine is only in spinal cord; up to 70% of brain synapses are glutamate
GABA and glycine is inhibitory; antianxiety drugs (eg benzodiazepines) try to mimic action of GABA
glutamate is generally excitatory – main transmitter in CNS;

Peptides:
endorphins, dysnorphin, enkephalins – generally inhibitory
tachykinins, substance P, neurokinin A (NKA) – generally excitatory
somatostatin – generally inhibitory

Action of neurotransmitters:
Excitatory or inhibitory:
Some are excitatory  cause depolarisation; some are inhibitory  cause hyperpolarisation; some are both (depends on receptor type)

Direct or indirect:
Direct – open ion channels; provoke a more rapid response in postsynaptic cells (eg ACh, amino acid neurotransmitters)
Indirect – have a longer lasting effect; act through intra-cellular second messengers (eg biogenic amines, peptides)

Neurotransmitter Receptors:
Two types:
1) Channel linked or ionotropic receptors:
mediate the faster direct transmitter action
after binding, receptor proteins change shape  opens ions channel  ions pass into cell  alters membrane potential

2) G protein linked receptors (metabotrophic receptors):
mediate the slower and more prolonged indirect transmitter action (by allowing the ion channel to stay open longer)
after neurotransmitter binds to G protein, it is activated  produces affect by releasing second messenger (eg cyclic AMP, cyclic GMP, Ca+)  these regulate ion channels or activate enzyme controlled pathways.