Which of these is a neurotransmitter?

A) Beta 1-receptors:
Beta 1-receptors are primarily found in the heart, and their stimulation leads to increased heart rate and contractility. These receptors are not typically involved in vasoconstriction, pupil dilation, or piloerection.

B) Beta 2-receptors:
Beta 2-receptors are primarily located in smooth muscles such as the bronchi and blood vessels. Their stimulation causes smooth muscle relaxation, leading to bronchodilation and vasodilation. Beta 2-receptors are not responsible for the effects of vasoconstriction, piloerection, or pupil dilation described in the question, making this option incorrect.

C) Alpha 2-receptors:
Alpha 2-receptors are involved in the inhibition of norepinephrine release and play a role in regulating sympathetic tone. They are not primarily responsible for vasoconstriction or the other physiological responses listed in the question. Alpha 2-stimulation typically results in decreased sympathetic activity, not the effects described here.

D) Alpha 1-receptors:
Alpha 1-receptors are located on smooth muscle, including the blood vessels, and their stimulation results in vasoconstriction. They are also involved in other actions such as piloerection (hair standing on end), pupil dilation (mydriasis), and closure of the salivary sphincter. Additionally, they play a role in male sexual emission (ejaculation). The physiological responses described in the question—vasoconstriction, piloerection, pupil dilation, and male sexual emission—are all consistent with alpha 1-receptor stimulation.

A) It decreases the activity of norepinephrine receptor sites: Cholinergic agonists specifically act on acetylcholine receptors, not norepinephrine receptors. Norepinephrine is associated with the sympathetic nervous system and its receptors (alpha and beta), whereas cholinergic agonists affect the parasympathetic system through acetylcholine receptors.
B) It increases the activity of acetylcholine receptor sites: This is the correct answer. Cholinergic agonists work by stimulating acetylcholine receptors (both muscarinic and nicotinic), thereby enhancing the action of acetylcholine. This can lead to various effects such as decreased heart rate, increased digestion, and muscle contraction, as part of the parasympathetic nervous system response.
C) It decreases the activity of GABA receptor sites: GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that acts on GABA receptors, not acetylcholine receptors. Cholinergic agonists have no direct effect on GABA receptor activity.
D) It increases the activity of dopamine receptor sites: Dopamine is a neurotransmitter primarily involved in the reward system, motor control, and mood regulation. Cholinergic agonists do not directly affect dopamine receptors. Instead, they target acetylcholine receptors, which are part of the parasympathetic nervous system.