NEURAL ADAPTATIONS ACROSS DAILY TORPOR AND SLEEP IN DJUNGARIAN HAMSTERS (<EM>PHODOPUS SUNGORUS</EM>)

Daily torpor is a reversible hypometabolic state marked by reduced body temperature and metabolic rate, yet its effects on cortical synaptic organisation and molecular composition remain unclear. We examined neural adaptations across torpor, arousal, and subsequent sleep in Djungarian hamsters (Phodopus sungorus), which exhibit spontaneous daily torpor under short photoperiods. Animals were monitored using continuous non-invasive thermal imaging and sampled during euthermia, mid-torpor, arousal, and post-torpor sleep (n = 5-6 per group). Synaptic ultrastructure of layer 2/3 pyramidal neurons in primary motor cortex was analysed using serial block-face scanning electron microscopy. Behaviour was assessed using novel object recognition after arousal (n = 12), and molecular changes were examined using biochemical fractionation and western blotting (pilot analyses, n = 3 per group). Axon-spine interface area and spine density did not differ between mid-torpor and euthermic states (linear mixed-effects models, p = 0.785). In contrast, the proportion of synapses containing presynaptic mitochondria was reduced during mid-torpor (generalised linear mixed model, p = 0.0389). Novel object recognition performance did not differ between torpor and non-torpor animals (linear mixed-effects model, p = 0.61). Pilot molecular analyses indicated state-dependent protein localisation without widespread changes in synaptic abundance. CaMKIIα increased in the cytosolic fraction during torpor (t-test, p = 0.006), and MAP2 abundance increased in whole lysate (t-test, p = 0.009) and cytosol (t-test, p = 0.029), while most synaptic markers were unchanged. These findings indicate that daily torpor preserves synaptic structure and behaviour while inducing targeted, potentially reversible molecular and subcellular remodelling.