Acetylcholine acting at nicotinic receptors is involved in some inhibitory circuits on motor neurones. Muscarinic receptors play a role in learning and memory. Experimental specific agonists for neuronal nicotinic receptors have some analgesic and sedative properties. Since animals do not smoke and do not seem to get Alzheimer’s disease, acetylcholine receptors are mainly important in poisoning in veterinary practice - many plants contain cholinergic drugs.
Histamine acting at H1, H2 and H3 receptors can be either excitatory or inhibitory but its physiological role is unclear. It may be involved in sleep. Several histamine antagonists are used in veterinary medicine for their central effects (mainly phenothiazine sedatives), but they all affect other receptor systems as well as histamine.
Purines, ATP (co-released with noradrenaline), AMP and adenosine, have only recently been recognised as neurotransmitters in the brain (as well as doing other things connected to metabolism). Adenosine acts at purinergic A receptors of which there are several subtypes, ATP acts at purinergic P receptors. P2x receptors are important ionotropic receptors (again there are several subtypes), P2y receptors are metabotropic. Expect more new functions for these receptors to be discovered soon. The stimulant effect of caffeine and similar drugs is probably due to an action at purinergic receptors.
