Mechanisms of Drugs Affecting the Parasympathetic and Sympathetic Nervous System

Our second week of physiology lecture ended with a laundry list of drugs whose mechanism of action we had to memorize (thankfully not too long). I feel grateful that I've seen some of these used in practice so have some context for their use. I also learned a fair bit about some of these drugs in tech school. Still, it gets tiresome having list after list of things to memorize (I know, welcome to vet school).

For anyone who cares to follow along here is a quick cheat sheet;
Nicotinic receptors are found at ganglia in the parasympathetic nervous system (PNS) and sympathetic nervous system (SNS) as well as at the neuromuscular junction. They are ionotropic channels.
Muscarinic receptors are found at the neuroeffector junction in the PNS and at the neuroeffector junction of sweat glands in the sympathetic nervous system (except in the horse, which has adrenergic receptors). They are metabotropic, or G-protein coupled receptors. There are five types of muscarinic receptors but their significance was not emphasized to us.
Adrenergic receptors are found at the neuroeffector junction in the SNS. They are also metabotropic receptors and come in several flavors, or subtypes.
α1: generally excitatory, contractile on postsynaptic membrane
α2: mostly on presynaptic membrane, part of negative feedback mechanism
β1: mostly found on heart
β2: blood vessels, bronchioles, liver (smooth muscle and metabolic)
β3: adipocytes

Cholinergics:

Atropine: Muscarinic antagonist. Competitive inhibitor at parasympathetic neuroeffector junctions. Can block nicotinic sites only at very high concentrations.
Heart: Tachycardia, but the degree depends on the species' vagal tone, which is high in horses and dogs, producing marked effects in those species.
Blood vessels: Very little effect. Will block vasodilation induced by choline esters.
GI and Urinary Tracts: Reduce tone, motility, secretions
Respiratory: Increase luminal diameter, decrease secretions of bronchioles
Eye: Mydriasis, cyclopegia, increase intraocular pressure
Sweating: Impaired (horse is an exception)
Salivation: Reduced (dry mouth)
CNS: Moderate doses stimulate medullary and higher centers, eventually death by medullary paralysis

Bethanecol: Muscarinic agonist. Parasympathomimetic. More resistant to hydrolysis by cholinesterases than acetylcholine, so it has a longer duration of action that Ach. Never heard of this drug and not sure why someone would use it.

Edrophonium: Anticholinesterase (inhibits enzyme that breaks down acetylcholine, the neurotransmitter at the muscarinic junction). Shortest acting anticholinesterase. Used to diagnose myasthenia gravis (an immune disease where the muscarinic receptors are attacked and subsequently decreased in number).

Neostigmine: Anticholinesterase. Lasts for a few hours. Can be used to treat myasthenia gravis.

Pilocarpine: Muscarinic agonist. Parasympathomimetic. Treatment for glaucoma, lowers intraocular pressure.

Adrenergics:

Albuterol: Selective β2 agonist. Bronchodilate while minimizing stiumlation of the heart. I saw terbutaline used more often to treat asthmatics than albuterol, not sure why from this class. (Maybe later, as a first year there is always later!)

Epinephrine: Directly acting adrenergic agonist. Non-selective for α or β receptors.

Phenylephrine: Directly acting adrenergic agonist. "Pure" α-receptor agonist. Not sure why I've never seen this used in practice, instead always saw use of epinephrine.

Isoproterenol: Directly acting adrenergic agonist. Non-selective for β-recptors.

Phenoxybenzamine: α-adrenergic antagonist. Irreversible non-competitive antagonist that covalently attaches to receptors. I saw this used in the treatment of UO (obstructed cats) with a urinary catheter in place. I always thought it decreased spasms in the urethra. Now I know it blocks the contraction of the urinary sphincters.

Phentolamine: α-adrenergic antagonist. Competitive antagonist. Not sure when this is used.

Propanolol: Non-selective β-adrenergic antagonist. Primarily used to decrease blood pressure, excitability and work of the heart (decrease in heart rate, force of contraction, and cardiac output). Since it blocks β2-receptors in addition to β1-receptors it also blocks bronchodilation and can cause hypoglycemia. Therefor it should not be used in asthmatics or in the case of allergic reaction. It is also contraindicated in diabetes with insulin treatment. Interestingly, this drug is now being used to treat stage fright in people since it prevents the racing heart without affecting the mood.

Metoprolol: Cardioselective β1 antagonist. More potent for cardiac effects. Avoids the side effects of blocking bronchodilation and potentially causing hypoglycemia associated with propanolol. Never heard of this or seen it used either but sounds useful in certain cases.