A glutamatergic circuit from the piriform cortex to the mediodorsal thalamus regulates trigeminal neuropathic pain and comorbid cognitive impairment in mice

Trigeminal neuropathic pain (TNP) is a chronic, debilitating pain disorder affecting the orofacial region. Patients often experience concomitant cognitive impairment, severely impacting their quality of life. The principal neural circuit mechanisms that link TNP to cognitive dysfunction remain unclear. The piriform cortex (PIR), primarily recognised for its role in olfaction, has not been systematically investigated in trigeminal neuropathic pain processing or pain-related cognitive dysfunction. This study utilised pseudorabies virus-mediated multisynaptic retrograde tracing to map whole-brain anatomical connectivity associated with orofacial sensory pathways. c-Fos immunofluorescence staining and in vivo fibre-optic calcium signalling recordings were used to detect alterations in glutamatergic neuronal activity within the PIR of TNP model mice. Chemogenetic and optogenetic techniques were employed to manipulate PIR glutamatergic (PIR Glu ) neurons in a bidirectional manner, validating their functional roles in pain and cognitive regulation. Anterograde viral tracing was integrated with circuit-specific chemogenetic intervention to elucidate downstream regulatory pathways. This study identified the PIR as a previously unrecognized brain region anatomically associated with trigeminal neuropathic pain-related pathways, providing evidence for its potential involvement in TNP. PIR Glu neurons in TNP model mice exhibited marked burst-like activation, with in vivo calcium activity intensity exhibiting considerable synchronisation with facial pain-sensitive behaviour. Chemogenetic inhibition of PIR Glu neurons significantly increased pain thresholds and ameliorated cognitive deficits in TNP mice, whereas selective activation of PIR Glu neurons in sham mice recapitulated the pain–cognition comorbidity phenotype, demonstrating the essential role of these neurons in mediating pain-related cognitive dysfunction. Viral tracing identified the mediodorsal thalamus (MD) as a critical downstream target of PIR Glu neurons. Electrophysiological recordings further confirmed that PIR Glu neurons establish direct excitatory monosynaptic connections with MD neurons, with fast glutamatergic transmission predominantly mediated by AMPA receptor-dependent signalling. Specific disruption of the PIR Glu –MD circuit simultaneously alleviated pain and cognitive impairment in model mice, while inhibition of PIR Glu neurons markedly reduced calcium activity in downstream MD neurons. This study highlights the PIR Glu –MD circuit as an important pathway contributing to TNP–cognitive impairment comorbidity. Furthermore, it augments our understanding of the non-olfactory physiological functions of the PIR, offering a novel potential target for investigating the mechanisms underlying the association of chronic pain with cognitive impairment.