The Intergeniculate Leaflet Plays a Key Role in Sex Differences in the Biological Rhythm of Locomotor Activity in M₄ Muscarinic Receptor Knockout Mice

Circadian locomotor activity rhythms are generally regulated by complex neural networks involving multiple brain regions and neurotransmitter systems. Previous research has established that the M4 muscarinic receptor (mAChR) subtype plays a crucial role in regulating the circadian rhythm across various brain structures. However, significant sex differences exist in how these rhythms respond to circadian disruption, with females showing changes in locomotor activity biological rhythms while males do not, yet the specific pacemaker structures responsible for maintaining consistent locomotor activity in wild-type and M4 muscarinic receptor knockout (M4 KO) males remain unclear. To address this gap, we compared mAChR densities between females and males under standard light/dark conditions across eight brain regions: the motor cortex, somatosensory cortex, striatum, thalamus, intergeniculate leaflet (IgL), suprachiasmatic nucleus, subparaventricular zone, and posterior hypothalamic area. We then examined the effects of constant darkness and constant light regimes on receptor density in locomotor rhythm-related brain areas in males only. Among all examined structures, only the IgL showed variations across the light/dark, constant darkness, and constant light regimes. Significant differences were observed between light/dark and constant light in wild-type, between light/dark and constant darkness in both wild-type and M4 KO, between constant darkness and constant light in M4 KO, and between wild-type and M4 KO under light/dark conditions. These findings indicate that the IgL is the key structure determining sex differences in the regulation of locomotor activity biological rhythms.