- Atropine decreases salivary secretion primarily by blocking muscarinic receptors.
- Salivary glands are innervated by the parasympathetic nervous system, where acetylcholine (ACh) acts on muscarinic receptors (specifically M3 subtype) located on the glandular cells. Activation of these receptors leads to increased secretion of saliva.

- Atropine is a competitive antagonist of muscarinic receptors, meaning it binds to these receptors without activating them, thus inhibiting the action of acetylcholine.
- This blockade results in a reduction of parasympathetic stimulation, which in turn decreases the production and secretion of saliva.

To clarify why the other options are incorrect:
- Option 1: Blocking adrenergic receptors — Adrenergic receptors are part of the sympathetic nervous system, which plays a less significant role in salivary secretion compared to parasympathetic activation.
- Option 2: Inhibiting nicotinic receptors — Nicotinic receptors are typically involved in the ganglionic transmission of autonomic signals, not directly in salivary secretion at the glandular level.
- Option 4: Stimulating beta-2 receptors — Beta-2 receptors are part of the sympathetic receptors which, when stimulated, may cause minor modifications in secretion, but this is not the mechanism by which atropine acts.

In summary, atropine reduces salivation by competitively blocking M3 muscarinic receptors on salivary glands, preventing acetylcholine-mediated secretion.