Intramuscular injection of atropine causes initial bradicardia. The reason for this effective being seen is

Ref-KDT 6/e p107 Atropine is a non-selective antagonist of M1, M2, and M3 muscarinic receptors. M2, cholinergic receptors are responsible for bradycardia and blockade of these receptors can result in tachycardia. Atropine initially acts on presynaptic M1, receptors (normally decrease the release of ACh) and result in greater release of ACh which is responsible for bradycardia. Later on, blockade of M2 receptors will lead to tachycardia.