THE DEVELOPMENT AND EVOLUTION OF INNATE OLFACTORY BEHAVIOUR IN <EM>MUS MUSCULUS</EM> AND <EM>MUS CAROLI</EM>

Instincts drive a key subset of adaptive behaviours, yet little is known about their neural representations, the developmental processes that form these circuits, or how they interact with memory engrams. To explore these processes we investigated the onset of the innately primed, experience-dependent olfactory attraction to trimethylamine (TMA). TMA is a putrefaction signal that is repulsive to most rodents, including rats and Mus caroli, but Mus musculus have evolved to be attracted to it. Through behavioural tests on young or TMA-depleted mice, we demonstrate that this attraction emerges post-weaning, and is dependent on developmental exposure to TMA. To identify the neuronal circuits involved in TMA attraction, we analysed brain-wide c-Fos expression in adult Mus musculus, identifying the anterior olfactory nucleus (AON) as a region of interest. We then induced activity-dependent expression of inhibitory DREADDs or channelrhodopsin to demonstrate the sufficiency and necessity of TMA-responsive AON ensembles for appetitive behaviour. To explore developmental changes in circuitry, we then analysed c-Fos expression before and after the onset of TMA attraction. Finally, we investigated the potential role of TMA as a social signal by measuring social preference in mice with different developmental experiences of TMA. By comparing the brains and behaviour of animals across different ages and developmental backgrounds, this study reveals the intricate interaction between experience-dependent and innately-driven mechanisms in shaping complex instinctive behaviour. This allows for cross-species comparisons as a method to investigate the evolution of the innate neuronal mechanisms that drive these plasticity changes.