Which of the following is not considered as a secondary messenger?

Acetylcholine is not considered as a secondary messanger. It is a common neurotransmitter present in the neuromuscular junctions, voluntary neural synapses, synapses of pre-ganglionic nerve fibres, synapses of post-ganglionic parasympathetic nerve fibres. Cholinergic nerve fibres release acetylcholine. It has excitatory effect on the skeletal muscles and excitatory or inhibitory effect at the other sites.