Multifocal sites of action involved in dopaminergic-cholinergic neuronal interactions in yawning

Abstract

Bromocriptine (BRC), a dopamine D-2 receptor agonist, physostigmine, an anticholinesterase agent and pilocarpine, a muscarinic cholinergic receptor agonist, produced yawning in rats, with the most effective doses being 2.5 mg/kg, 0.2 mg/kg and 4 mg/kg, respectively. BRC-induced yawning was inhibited by high doses of SK&F38393 (5 and 10 mg/kg), a selective D-1 receptor agonist. BRC or SK&F38393 alone did not induced stereotyped behaviors. However, when BRC was administered after SK&F38393 (5.0 and 10 mg/kg), stereotyped behaviors occurred; i.e., mainly sniffing at 2.5 and 5.0 mg/kg BRC, and mainly licking and biting 10 and 20 mg/kg BRC. A high dose of apomorphine (4 mg/kg IP) completely inhibited physostigmine-induced yawning (physostigmine yawning) but did not affect pilocarpine-induced yawning (pilocarpine yawning). BRC (2.5–20 mg/kg) increased physostigmine yawning in an additive fashion. Pilocarpine yawning was completely blocked by either low or high doses of BRC. The inhibitory effect of BRC on pilocarpine yawning was reversed by sulpiride (20 mg/kg). α-Methyl-p-tyrosine (α-MPT; 100 and 200 mg/kg) did not affect physostigmine yawning but diminished pilocarpine yawning. Furthermore, physostigmine (0.2 mg/kg) inhibited apomorphine (4.0 mg/kg)-induced hyperlocomotion and sniffing but not licking and biting, whereas pilocarpine (4.0 mg/kg) had the opposite effect. These results suggest that activation of postsynaptic dopamine receptors (primarily D-1 receptors) may inhibit cholinergic neurons activated by cholinesterase inhibition, i.e., endogenous acetylcholine, and that the postsynaptic sensitivity of cholinergic neurons to a direct acetylcholine agonist (muscarinic M-1 receptor agonist) may be reduced by stimulation of presynaptic dopamine receptors (D-2 receptor) and/or inhibition of dopamine synthesis.