The effect of amphetamine on blood pressure will be blunted if an animal has first been treated with a. reserpine b. terbutaline c. trimethaphan d. physostigmine e. botulinum toxin

Before answering the question, let's briefly explain the mechanism of action of each substance: a. Reserpine: It is an antihypertensive drug that depletes the levels of norepinephrine (and other monoamines) in the nerve terminals. b. Terbutaline: A Beta-2 adrenergic agonist primarily used for bronchodilation in asthma, it has minimal effects on alpha-adrenergic receptors and therefore limited effects on blood pressure. c. Trimethaphan: A ganglionic blocker that inhibits the transmission between sympathetic and parasympathetic nervous system to decrease blood pressure. d. Physostigmine: A cholinesterase inhibitor that increases acetylcholine levels at synaptic clefts with potential bradycardic effects. e. Botulinum toxin: A potent neurotoxin that blocks acetylcholine release, causing muscle paralysis and is not related to blood pressure regulation.

Amphetamines increase blood pressure by inducing the release of norepinephrine in the nerve endings and promoting alpha-adrenergic receptor stimulation, resulting in vasoconstriction and an increase in blood pressure.

Considering the mechanism of action of each substance, we can identify that reserpine (option a) is the most likely substance to blunt the effect of amphetamine on blood pressure. This is because reserpine depletes the levels of norepinephrine in the nerve terminals, which is essential for increased blood pressure induced by amphetamines. With low levels of norepinephrine, the amphetamine will not be able to exert its effect on blood pressure significantly. So, the correct answer is: a. Reserpine