Blockade of muscarinic acetylcholine receptor by scopolamine impairs the memory formation of filial imprinting in domestic chicks (Gallus Gallus domesticus)

Abstract

Filial imprinting in precocial birds is a useful model for studying memory formation in early learning. The intermediate medial mesopallium (IMM) in the dorsal telencephalon is one of the critical brain regions where the releases of several neurotransmitters increase after the start of imprinting training. Among the increased neurotransmitters, the role of acetylcholine in imprinting has remained unclear. Acetylcholine in the mammalian brain plays an important role in encoding new memories. The muscarinic acetylcholine receptor subtype 1 (M₁ receptor) and subtype 3 (M₃ receptor) in the hippocampus and cortex of mammalian brain have been shown to be necessary for memory encoding. In this study, we examined whether the imprinting acquisition in chick can be impaired by injecting muscarinic acetylcholine receptor (mAChR) antagonist scopolamine into the bilateral IMM. We show that the injection of scopolamine decreased the preference for the imprinting object in the test, but did not affect the number of approaches to the imprinting object during training. Immunoblotting and immunohistochemistry revealed that M₃ receptors were expressed in the IMM. Our data suggest that acetylcholine is involved in the memory formation of imprinting through M₃ receptors in the IMM. The scopolamine-injected chicks may be useful as an animal model for dementia such as Alzheimer's disease.

Keywords: Alzheimer’s disease; Domestic chicks; Filial imprinting; Intermediate medial mesopallium; Muscarinic acetylcholine receptor; Scopolamine.